Adhesives in Wood Industry

47Adhesives in the Wood Industry

Manfred DunkyDynea Austria GmbH, Krems, Austria

I. INTRODUCTION

Progress in research and development within the wood-based industry and within theadhesive industry has shown many successes during the past few decades.Notwithstanding this, the industrial requirements of the wood industry still inducetechnical improvement in the adhesives and their application in this area. What drivesthis technical development is the search for ‘‘cheaper,’’ ‘‘faster-curing,’’ and ‘‘more com-plex’’ adhesives. The first two requirements are caused by the heightened competitionwithin the wood industry and efforts to minimize costs at a certain level of product qualityand performance. The requirement ‘‘more complex’’ stands for new and specializedproducts and process. Adhesives play a central role within wood-based panels production.The quality of bonding and hence the properties of the wood-based panels are determinedmainly by the type and quality of the adhesives. Development in wood-based panels,therefore, is always linked to development in adhesives and resins.

Both the wood-based panels industry and the adhesive industry shown a high com-mitment to and great capability towards innovation. The best evidence for this is theconsiderable diversity of types of adhesives used for the production of wood-basedpanels. Well known basic chemicals have been used for a long time for the productionof adhesives and their resins, the most important ones being formaldehyde, urea, mela-mine, phenol, resorcinol, and isocyanate. The greater part of the adhesive resins andadhesives currently used for wood-based panels is produced with these few raw materials.The ‘‘how to cook the resins’’ and the ‘‘how to formulate the adhesive’’ therefore becomemore and more complicated and sophisticated and are key factors to meet today’s require-ments of the wood-based panels industry.

The quality of bonding and hence the properties and performance of the wood-basedpanels and beams are determined by three main parameters:

the wood, especially the wood surface, including the interface between the woodsurface and the bondline

the applied adhesivethe working conditions and process parameters.

Copyright © 2003 by Taylor & Francis Group, LLC

Good quality bonding and adequate properties of the wood-based panels can beattained only if each of these three parameters contributes to the necessary extent to thebonding and production process.

In this chapter are then covered the types of adhesives used in the wood industry andtheir characteristics. The influences on their performance of the adhesives’ physicochem-ical characteristics, of their application parameters, of the wood itself, and of the woodcomposite process parameters are also described. In wood adhesives the application par-ameters other than the characteristics of the adhesive itself account for around 50% ofperformance.

II. TYPES OF WOOD ADHESIVES

In the wood-based panels industry a great variety of adhesives are currently is use.Condensation resins based on formaldehyde represent the biggest volume within thewood adhesives field. They are prepared by the reaction of formaldehyde with variouschemicals such as urea, melamine, phenol, resorcinol, or combinations thereof. At deliverythese adhesive resins are mainly liquid and consist of linear or branched oligomers andpolymers in aqueous solution or dispersion. During hardening and gelling they convert tothree-dimensionally crosslinked and, therefore, insoluble and nonmeltable networks. Thehardening conditions used can be acidic (for aminoplastic resins), highly alkaline (forphenolic resins), or neutral to lightly alkaline (for resorcinol resins). Isocyanates [especiallypolymeric 4,40-diphenyl methane diisocyanate (PMDI)] are another important chemicalgroup used for various applications in the wood industry, especially for water resistantbonds. In Table 1 are reported the main wood adhesives in use today with their mainapplications.

III. OVERVIEW ON REQUIREMENTS CONCERNING WOOD ADHESIVES

Table 2 summarizes the general parameters of importance for wood adhesives. Researchand development in adhesives and resins are mainly driven by the requirements of thebonding and production processes and by the intended properties of the wood-basedpanels. These requirements are summarized in Table 3.

The necessity to achieve shorter press times is omnipresent within the woodworkingindustry, to keep production costs low. An increased production rate gives the chance toreduce production costs. This is only valid when the market is able to absorb such a highlevel of production. Shorter press times within a given production line and for certaintypes of wood-based panels can be achieved by, among others:

highly reactive adhesive resins possessing rapid gelling and hardening and steepincrease in bonding strength even at a low degree of chemical curing

highly reactive adhesive glue mixes obtained by the addition of accelerators, specialhardeners, crosslinkers, and others

the optimization of the pressing process, e.g., by increasing the effect of the steamshock by (i) increased press temperatures, (ii) a more marked difference in themoisture content between the surface and the core layer of the panel before hotpressing, or (iii) an additional steam injection step.

constancy of as many parameters of the production process as possible.


Table 1 Fields of Application for Various Wood Adhesives

Adhesive type V20 V100 V313 FP MDF PLW HLB MH ven. furn.

UF x x x x x xa xa

MUF xb x

MF/MUF x x x x x x

MUPF x x x

PF/PUF x x x x x

RF x

PMDI x x x

PVAc x x x

old nat.adhesives x

nat.adhesives x x x x x

inorg.adhesives x xc

activation x

UF, urea–formaldehyde resin; MUF, melamine fortified UF resin; MF/MUF, melamine and melamine–urea

resins (MF resins are only used mixed/coreacted with UF resins; MUPF, melamine–urea–phenol–formaldehyde

resin; PF/PUF, phenol and phenol–urea–formaldehyde resin; (P)RF, resorcinol–(phenol–)formaldehyde resin;

PMDI, polymeric methylenediisocyanate; PVAc, polyvinylacetate adhesive; old nat.adhesives, old (historic) nat-

ural adhesives (e.g., starch, glutin, casein adhesives); nat.adhesives, natural adhesives (e.g., tannins, lignins,

carbohydrates); inorg.adhesives, inorganic adhesives (e.g., cement, gypsum); activation: activation constituents

of wood to function as adhesives (i.e., lignin).

V20, particleboard according to DIN 68761 (parts 1 and 4, FPY, FPO), DIN 68763 (V20) and EN 312-2 to 4 and

312-6; V100, particleboard according to DIN 68763 and EN 312-5 and 312-7, option 2 (internal bond after boil

test according to EN 1087-1); V313, particleboard according to EN 312-5 and 312-7, option 1 (cycle test accord-

ing to EN 321); FP, hardboard (wet process) according to EN 622-2; MDF, medium density fiberboard according

to EN 622-5; PLW, plywood according to EN 636 with various resistance against influence of moisture and water;

HLB, laminated beams; MH, solid wood panels according to OeNORM B 3021 to B 3023 (prEN 12775, prEN

13353 part 1 to 3, prEN 13017-1 and 2, prEN 13354); ven., veneering and covering with foils; furn., production of

furniture.aPartly powder resins.bBoards with reduced thickness swelling, e.g., for laminate flooring.cSpecial production method.

Table 2 General Requirements for Wood Adhesives

Composition, solids content, viscosity, purity

Color and smell

Sufficient storage stability for given transport and storage conditions

Easy application

Low transport and application risks

Proper gluing quality

Climate resistance

Hardening characteristic: reactivity, hardening, crosslinking

Compatibility for additives

Cold tack behavior

Ecological behavior: Life cycle analysis (LCA), waste water, disposal, etc.

Emission of monomers, Volatile organic compounds (VOC), formaldehyde during production of the

wood-based panels and during their use


Cheaper raw materials are another way to reduce production costs. This includes, forexample, the minimization of the melamine content in a MUF resin, to produce boardswith reduced thickness swelling or increased resistance against the influence of water andhigh humidity of the surrounding air. Impeding factors (often temporary) can be theshortage of raw materials for the adhesives, as was the case with methanol and melamineduring the 1990s.

Life cycle analysis and recycling of bonded wood boards also concerns the adhesiveresins used, since adhesives and resins are one of the major raw materials in the productionof wood-based panels. This includes, for example, the impact of the adhesives on variousenvironmental issues such as waste water and effluent management, noxious gas emissionduring panel production and from the finished boards, or the reuse of panels to burn forenergy generation. Furthermore, for certain recycling processes the type of resin has also acrucial influence on their feasibility and efficiency.

Gas emission from wood-based panels during their production can be caused bychemicals inherent to wood itself, such as terpenes or free acids, as well as by volatilecompounds and residual monomers coming from the adhesive. The emission of form-aldehyde especially is a matter of concern, but so are possible emissions and dischargesof free phenols or other materials. The formaldehyde emission noted only after panelmanufacture and adhesive resin hardening is due, on the one hand, to the residual,unreacted formaldehyde present in urea–formaldehyde (UF)-bonded boards, or as gastrapped in the wood or dissolved in the moisture still present in the panel. On the otherhand, in aminoplastic resins the hydrolysis of weakly bonded formaldehyde from N-methylol groups, acetals, and hemiacetals as well as in more severe cases of hydrolysis(e.g., at high relative humidity) from methylene ether bridges, increases again the contentof emittable formaldehyde after resin hardening. In contrast to phenolic resins, a per-manent reservoir of potentially emittable formaldehyde is the consequence of the pres-ence of these weakly bonded structures. This explains the continuous, yet low, release offormaldehyde from UF-bonded wood-based panels even over long periods. However,the level of emission depends on the environmental conditions, a fact which may bedescribed by the resin hydrolysis rate which indicates if this formaldehyde reservoir willor will not lead to unpleasantly high emission values [1–4]. The higher this hydrolysisrate is, the higher is the potential reservoir of formaldehyde which contributes to sub-sequent formaldehyde emission. The problem of formaldehyde emission after adhesivehardening in panel manufacture can fortunately be regarded today as solved, due toclear and stringent emission regulations in many European and other countries and tosuccessful long term R&D investement by the chemical industry and the wood workingindustry.

The so-called E1-emission class regulations shown in Table 4 for different panelproducts describe the level of formaldehyde emission which is low enough to prevent

Table 3 Actual Requirements in the Production and in the Development of Wood Adhesives

Shorter press times, shorter cycle times

Better hygroscopic behavior of boards (e.g., lower thickness swelling, higher resistance against the

influence of humidity and water, better outdoor performance)

Cheaper raw materials and alternative products

Modification of the wood surface

Life cycle assessment, energy and raw material balances, recycling and reuse

Reduction of emissions during the production and the use of wood-based panels


any danger, irritation, or inflammation of the mucous membranes in the eyes, nose, andmouth. However, it is important that not only the boards themselves, but also veneeringand carpenters’ adhesives, lacquers, varnishes, and other sources of formaldehyde becontrolled, since they also might contribute to a close environment formaldehydesteady-state concentration [1–4].

IV. AMINOPLASTIC ADHESIVE RESINS (UREA RESINS,MELAMINE RESINS)

The various aminoplastic resins are the most important class of adhesives in the wood-based panels industry, especially for the production of particleboards and medium densityfibreboard (MDF), and partly also for oriented strandboard (OSB), plywood, block-boards, and some other types of wood panels. They are also used in the furniture industryas well as in carpenters’ shops.

Aminoplastic adhesive resins are formed by the reaction of urea and/or melaminewith formaldehyde. Based on the raw materials that are used various types of resins can beprepared, namely:

UF urea–formaldehyde resinMF melamine–formaldehyde resinMUF melamine–urea–formaldehyde cocondensation resinmUF melamine fortified UF resinsMFþUF mixture of an MF and a UF resinMUPF, PMUF melamine–urea–phenol–formaldehyde cocondensation resin.

The most important parameters for the aminoplastic resins are:

(a) The type of monomers used.(b) The relative molar ratio of the various monomers in the resin:

F/U molar ratio of formaldedhyde to ureaF/M molar ratio of formaldehyde to melamineF/(NH2)2 molar ratio of formaldehyde to amide or amine groups,

whereby urea counts for two NH2 groups, and melamine forthree NH2 groups.

(c) The purity of the different raw materials, e.g., the level of residual methanol orformic acid in formaldehyde, biuret in urea, or ammeline and ammelide inmelamine.

Table 4 Actual Regulations Concerning Formaldehyde Emission from Wood-Based Panels

According to the German Regulation of Prohibition of Chemicals (formerly Regulation of

Hazardous Substances) for E1 Emission Class (the Lowest Emission Types panels)

(a) Maximum steady state concentration in a climate chamber:

0.1 ppm (prEN 717-1; 1995)

(b) Laboratory test methods (based on experimental correlation experiences):

Particleboard: 6.5mg/100 g dry board as perforator value (EN 120; 1992)

MDF: 7.0mg/100 g dry board as perforator value (EN 120; 1992)

Plywood: 2.5mg/h-m2 with gas analysis method (EN 717-2)

Particleboard and MDF: correction of the perforator value to 6.5% board moisture content


(d) The reaction procedures used, e.g.

the pH variation sequencethe temperature variation sequencethe types and amount of alkaline and acidic catalyststhe sequence of addition of the different raw materialsthe duration of the different reaction steps in the cooking procedures.

The production of aminoplastic adhesive resins is usually a multistep procedurewhere both alkaline and acidic steps occur. Aminoplastic resins can be prepared in avariety of different types for all the different needs in wood bonding. This can be achievedby just using the three main monomers mentioned above and varying the preparationprocedure.

A. UF Resins

Urea–formaldehyde resins [1–9] are based on a series of consecutive reactions of urea andformaldehyde. Using different conditions of reaction and preparation a practically endlessvariety of condensed UF chemical structures is possible. UF resins are thermosettingresins and consist of linear or branched oligomers and polymers always admixed withsome amounts of monomers. The presence of some unreacted urea is often helpful toachieve specific effects, e.g., a better storage stability of the resin. The presence of freeformaldehyde has, however, both positive and negative effects. On the one hand, it isnecessary to induce the subsequent hardening reaction while, on the other hand, itcauses a certain level of formaldehyde emission during the hot press, resin hardeningcycle. Even in the hardened state, low levels of residual formaldehyde can lead to thedispleasing odor of formaldehyde emission from the boards while in service. This fact haschanged significantly the composition and formulation of UF resins during the past 20years.

After hardening, UF resins consist of insoluble, three-dimensional networks whichcannot be melted or thermoformed again. In their application stage UF resins are used aswater solutions or dispersions or even in the form of still soluble spray dried powders.These, however, in most cases have to be redissolved and redispersed in water forapplication.

Despite the fact that UF resins consist of only the two main components, namelyurea and formaldehyde, a broad variety of possible reactions and resin structures can beachieved. The basic characteristics of UF resins can be ascribed at a molecular level to:

their high reactivitytheir waterborne state, which renders these resins ideal for use in the woodworking

industrythe reversibility of their aminomethylene bridge, which also explains the low resis-

tance of UF resins to water and moisture attack, especially at higher tempera-tures; this is also one of the reasons for the hydrolysis leading to subsequentformaldehyde emission.

The reaction of urea and formaldehyde is basically a two-step process, usually con-sisting of an alkaline methylolation (hydroxymethylation) step and an acid condensationstep. The methylolation reaction, which usually is performed at a high molar ratio(F/U¼ 1.8 to 2.5), is the addition of up to three (four in theory) molecules of bifunctionalformaldehyde to one molecule of urea to give methylolureas; the types and the proportions


of the formed methylol groups depend on the molar ratio F/U. Each methylolation stephas its own rate constant ki, with different values for the forward and the backwardreactions. The formation of these methylol groups mostly depends on the molar ratioF/U. The higher the molar ratio used, the higher the molecular weight the methylolatedspecies formed tends to be. The UF resin itself is formed in the acid condensation step,where still the same high molar ratios as in the alkaline methylolation step is used (F/U¼ 1.8 to 2.5): the methylol groups, urea and the free formaldehyde react with linear andpartly branched molecules with medium and even higher molar masses, forming thepolydisperse molar mass distribution pattern characteristic of UF resins. Molar ratioslower than approximately 1.8 during this acid condensation step tend to cause resinprecipitation.

The final UF resin has a low F/Umolar ratio obtained by the addition of the so-calledsecond urea, which might also be added in several steps [8,9]. The second urea process stepneeds particular care. It is important for the production of resins with good performance,especially at the very low molar ratios usually in use now in the production of particle-boards and MDFs. This last step also includes the distillation of the resin solution tousually 66% resin solids content, which is performed by vacuum distillation in the reactoritself or in a thin layer evaporator. Industrial manufacturing procedures usually are pro-prietary and are described in depth in the literature only in rare cases [7–11].

The type of bonding between the urea molecules depends on the conditions used: lowtemperatures and slightly acid pHs favor the formation of methylene ether bridges (–CH2–O–CH2–) and higher temperatures and lower pHs lead preferentially to the formation ofmore stable methylene bridges (–CH2–). Ether bridges can be rearranged to methylenebridges by splitting off formaldehyde. One ether bridge needs two formaldehyde moleculesand additionally it is not as stable as a methylene bridge, hence it is highly recommendedto follow procedures that minimize the formation of such ether groups in UF resins. In theliterature other types of resin preparation procedures are also described. Some of theseyield uron structures in high proportion [12–15] or triazinone rings in the resins [15–17].The latter are formed by the reaction of ammonia or an amine, respectively, with urea andan excess of formaldehyde under alkaline conditions. These resins are used, e.g., toenhance the wet strength of paper.

The following chemical species are present in UF resins:

free formaldehyde, which is in steady state with the remaining methylol groups andthe post-added urea

monomeric methylol groups, which have been formed mainly by the reaction of thepost-added urea with the high content of free formaldehyde at the still highmolar ratio of the acid condensation step

oligomeric methylol groups, which have not reacted further in the acid condensationreaction or which have been formed by the above-mentioned reaction of post-added urea

molecules with higher molar masses, which constitute the real polymer portion of theresin.

The condensation reaction as well as the increase in the molar mass can also bemonitored by gel permeation chromatography (GPC) [18,19]. At longer acid condensationsteps, molecules with higher molar mass form and the GPC peaks shift to lower elutionvolumes.

Because of the necessity to limit the subsequent formaldehyde emission, the molarratio F/U has been decreased constantly over the years [20]. The main differences between


the UF resins with high and low formaldehyde content are the reactivity of the resin due tothe different contents of free formaldehyde and the degree of crosslinking in the curednetwork. The main challenge has been to reduce the content of formaldehyde in the UFresins and to achieve this without any major changes in the performance of the resins. Intheory this is not possible, because formaldehyde is the reactive partner in the reaction ofurea and formaldehyde during the condensation reaction as well as curing. Decreasing themolar ratio F/U means lowering the degree of branching and crosslinking in the hardenednetwork, which unavoidably leads to a lower cohesive bonding strength. The degree ofcrosslinking is directly related to the molar ratio of the two components.

The UF resin formulators have revolutionized UF resin chemistry in the past 30years. For example, in a straight UF resin for wood particleboard the above mentionedmolar ratio F/U was approximately 1.6 at the end of the 1970s. It is now 1.02–1.08, but therequirements for the boards (e.g., internal bond strength or percent thickness swelling inwater) as given in the quality standards are still unaltered. Also the reactivity of the resinduring hardening, besides the degree of crosslinking of the cured resins, depends on theavailability of free formaldehyde in the system.

It has, however, to be considered that it is neither the content of free formaldehydeitself nor the molar ratio which should be taken as the decisive and only criterion for theclassification of a resin concerning its subsequent level of formaldehyde emission. Inreality the composition of the glue mix as well as the various process parameters duringboard production also determine the level of formaldehyde emission. Depending on thetype of board and the process of application, it is sometimes recommended to use a UFresin with a low molar ration F/U (e.g., F/U¼ 1.03), hence presenting a low content of freeformaldehyde; while sometimes the use of a resin with higher molar ratio (e.g., F/U¼ 1.10)to which a formaldehyde catcher has been added in the glue mix will give better results.Which of these two possible ways is the better one in practice can only be decided by trialand error in each case.

The higher the molar ratio F/U, the higher is the content of free formaldehyde in theresin. Assuming stable conditions in the resins, which means that, e.g., post-added ureahas had enough time to react with the resin, the content of free formaldehyde is verysimilar even for different manufacturing procedures. The content of formaldehyde in astraight UF resin is approximately 0.1% at F/U¼ 1.1 and 1% at F/U¼ 1.8 [19–21]. It alsodecreases with time due to aging reactions where this formaldehyde reacts further. Table 5summarizes the various influences of the molar ratio F/U on various properties of wood-based panels. Table 6 summerizes the influence of the molar rations F/U and F/(NH2)2,

Table 5 Influence of the Molar Ratio on Various Properties of

UF-Bonded Wood-Based Panels

Decreasing the molar ratio leads to

a decrease of the formaldehyde emission during the production of the

wood-based panels

the subsequent formaldehyde emission

the mechanical properties

the degree of hardening

an increase of the thickness swelling and the water absorption

the susceptibility of hydrolysis


respectively, of pure and melamine fortified UF resins currently in use in the wood-basedpanels industry.

The molar mass distribution of UF resins is determined

by the degree of condensation andby the addition of urea (and sometimes also other components) after the condensa-

tion step; this again shifts the resin mass distribution towards lower averagemolar masses.

For this reason the molar mass distribution is much broader than for other poly-mers: it starts at the low molar mass monomers (the molecular weight of formaldehyde is30, for urea it is 60) and goes up to more polymerized structures. It is not clearly known,however, what are really the highest molar masses in a UF resin. Molar masses of up to500,000, determined by light scattering, have been reported [18,22]. The conditions ofmolecular level shear within the chromatographic columns [23] should guarantee thatall physically bonded clusters, caused by the interaction of the polar groups present inthe resins and which might simulate too high a molar mass, are separated and that thesehigh numbers between 100,000 and 500,000, measured using low angle laser light scatter-ing (LALLS) coupled to GPC, really do describe the macromolecular structure of a UFresin in the right manner. A second important argument for this statement is the fact thatup to such a high molar mass the on-line calibration curve determined in the GPC–LALLSrun is stable and more or less linear. It does not show any sudden transition as would bethe case of a too sharp increase in apparent molar mass if molecular clustering occurredagain after the material has passed through the column.

The molar mass distribution (and the degree of condensation) is one of the mostimportant characteristics of the resin and it determines several properties of the resin.Consequence of highly condensed resin structures (high molar masses) are:

the viscosity at a given solids content increases [19,24]the flowing ability is reduced

Table 6 Molar Ratios F/U and F/(NH2)2, Respectively, of Pure and Melamine Fortified

UF Resins Currently in Use in the Wood-Based Panels Industry

F/U or F/(NH2)2molar ratio Resin type

1.55 to 1.85 Classical plywood UF resin, also cold setting; use is only possible with

special hardeners and additives, e.g., melamine containing glue mixes for

an enhanced water resistance

1.30 to 1.60 UF plywood resin; use for interior boards without special requirements

concerning water resistance; to produce panels with low subsequent

formaldehyde emission, the addition of formaldehyde catchers is necessary

1.20 to 1.30 Plywood or furniture resin with low content of formaldehyde; also

without addition of catchers, products with a low subsequent formaldehyde

emission can be produced

1.00 to 1.10 E1 particleboard and E1 MDF resins; especially in MDF production

further addition of catchers is necessary. Modification or fortification

with melamine can be done

below 1.00 MDF resins and special glue resins for boards with a very low

formaldehyde emission; in most cases modified or fortified with melamine


the wetting behavior of a wood surface becomes worse [24]the penetration into the wood surface is reduced [25,26]the distribution of the resin on the furnish (particles, fibers) worsensthe water dilutability of the resin becomes lowerthe portion of the resin that remains soluble in water decreases [22]

Diluting the resin with a surplus of water causes precipitation of parts of the resin.These parts preferably contain the higher molar mass molecules of the resin and theirrelative proportion increases at higher degrees of condensation [22]. Information on cor-relations between the molar mass distribution (degree of condensation) and mechanicaland hygroscopic properties of the boards produced, however, is rather rare and oftenequivocal [7,19,27–29].

The influence of the degree of condensation is mostly felt during the application andthe hardening reaction (wetting behavior and penetration into the wood surface whichdepend on the degree of condensation). At higher temperatures, during the curing hotpress cycle, the viscosity of the resin drops, before the onset of hardening again leads to anincrease of viscosity. With this temporary lowering of the viscosity the adhesive wettingbehavior improves significantly, but its substrate penetration behavior also changes. Thereactivity of an aminoplastic resin seems to be independent of its viscosity (degrees ofcondensation), at parity of molar ratio. Ferg [30] mentioned that the bonding strengthincreased with the degree of condensation of the applied UF resin. The higher molarmasses (higher viscosity resin fractions) give a more stable glue line and determine thecohesive properties of the hardened resin [7]. Also Rice [29] and Narkarai and Wantanabe[28] reported that the resistance of a bondline against water attack and redrying increasedwith the viscosity of the resin. The reason again might be that resins with an advanceddegree of condensation remain to a greater extent in the glue line, avoiding resin over-absorption by the substrate and hence avoiding starving of the bondline. Rice [29] foundan increase of the thickness of the glue line with an increased viscosity of the resin,obviously due to its lower penetration into the wood substrate. However, it must betaken into consideration that the strength and stability of a glue line decrease withincreased glue-line thickness [31]. According to the findings of Sodhi [32] the bondingstrength decreases the longer is the waiting time before application of the glue mix.Once the hardening reaction has started and, therefore, the average molar mass has startedto increase, the worse the resin wetting behavior and its penetration in the wood surfaceappears to be.

1. Cold Tack Properties of UF Resins

Cold tack means that the particle mat has attained some strength already after the pre-press at ambient temperature, without any hardening reaction having occurred. This‘‘green’’ strength is necessary for better handling of the particle mat during transfer onthe production line. This can well be the case in multiopening presses, in special formingpresses, or in plywood mills, where the glued veneer layers are prepressed to fit into theopenings of the presses. At least a low level of cold tack is also necessary to avoid blowingout and loss of the fine wood particles from the surface when panels enter a continuouspress at high belt speeds. On the other hand, cold tack can lead to agglomeration of finewood particles and fibers in the forming station.

Cold tack is generated during the dry out of glue line, and reaches a maximum after acertain period of time. After this point the cold tack decreases again, when the glue linestarts to dry out. Both the intensity of the cold tack as well as the optimum length of time


in which it develops after application of the adhesive can be adjusted by the degree ofcondensation of the resin as well as by using special resin preparation procedures [33–35].Also various additives can increase the cold tack of the adhesive resins, e.g., some thermo-plastic polymers such as poly(vinyl alcohol).

2. Isocyanate (PMDI) as Accelerator and Fortifier for UF Resins

Polymeric methylenediisocyanate (PMDI) can be used as an accelerator and as a specialcrosslinker for UF resins. UF resins and PMDI can be sprayed separately without priormixing onto the particles [36,37] or for improved performance the two resins can bepremixed and then applied [8,38,39]. In the usual mixing procedure PMDI is pumpedunder high pressure into the UF resin [40,41]. Usually 0.5 to 1.0% PMDI based on dryparticles is used, whereas at the same time the UF gluing factor might be reduced slightly.The specific press time is said to be reduced by up to 1 s/mm.

Addition of PMDI to UF resins with a very low molar ratio was also recommendedto achieve low formaldehyde emission. The poor properties of the UF resin due to its verylow molar ratio can then be improved by the addition of PMDI [42–45].

B. Improvement of the Hygroscopic Behavior of Boards by MelamineFortified UF Resins (MUF, MUPF and PMUF Resins)

The resin used has a crucial influence on the properties of wood-based panels. Dependingon the requirements, different resin types are selected for use. Whereas UF resins aremainly used for interior boards (for use in dry conditions, e.g., in furniture manufactur-ing), a higher water resistance can be achieved by incoroporating melamine and also somephenol into the resin (melamine fortified UF resins, MUF, MUPF, PMUF). The level ofmelamine addition and especially the resin manufacturing sequence used in relation tohow melamine is incorporated in the resin can be very different. The different types ofthese resins which exist today are given in Table 7. The different resistances of these resinsagainst hydrolysis are based on their differences at the molecular level. The methylenebridge linking the nitrogens of amido groups can be split rather easily by water attack inUF resins. The same is not so easy in the case of M(U)F resins, mainly due to the muchlower water solubility of melamine itself which is a consequence of the water repellency

Table 7 Molar Ratios F/(NH2)2 of MUF/MUPF Resins Currently in Use in the Wood-Based

Panels Industry

F/(NH2)2 molar ratio Resin type

1.20 to 1.35 Resins for water resistant plywood, in the case of the addition of a

formaldehyde catcher

0.98 to 1.15 E1 particleboard resin and E1 MDF resin for water resistant boards

(PB: EN 312-5 and 312-7; MDF: EN 622-5). For particleboards according

to option 1 (V313 cycle test) MUF resins can be used; for boards

according to option 2 (V100 2 h boiling test, tested wet) MUPF or MUF

with a special approval is necessary. In this case, especially for the MDF

production, formaldehyde catchers are added

1.00 Special resins for boards with very low formaldehyde emission during board

service [81,82]


characteristic of the triazine ring of melamine. The equivalent methylene bridge is insteadvery stable to hydrolytic attack in phenolic resins. The melamine fortified products, how-ever, are much more expensive due to the much higher price of melamine compared tourea. Therefore, the content of melamine in these resins is as high as strictly necessary butalways as low as possible.

A MUF resin, at parity of all other conditions, yields a lower pH drop after additionof the hardener than a UF resin [46]. This lower drop of the pH due to the buffer capacityof the triazine ring of melamine, however, also causes a decrease of the hardening rate ofthe resin and, therefore, a lengthening of its gel time [1], hence a lengthening of the hotpress time is necessary. This is also seen in the shifts of the exothermic differential scanningcalorimetry (DSC) peak of hardening which are observed in thermal experiments [47].

The deterioration of a bondline and hence its durability under conditions of weath-ering is determined essentially by:

The failure of the resin (low hydrolysis resistance, degradation of the hardened resincausing loss of bonding strength).

The failure of the interface between the resin and the wood surface (replacementof physical bondings between resin and reactive wood surface sites by wateror other nonresin chemicals). The adhesion of UF resins to cellulose is sensitiveto water not only due to the already mentioned lability to hydrolysis of themethylene bridge and of its partial reversibility, but also because theoreticalcalculations have shown that on most cellulose sites the average adhesionof water to cellulose is stronger than that of UF oligomers [8,48]. Thus,water can displace hardened UF resins from the surface of a wood joint.The inverse effect is valid for PF resins [8,49].

The breaking of bondings due to mechanical forces and stresses: water causesswelling and, therefore, movement of the structural components of thewood-based panels (cyclic stresses due to swelling and shrinking, includingstress rupture).

The durability of a glue line can be enhanced by the incorporation of hydrophobicchains into the hardened network. This was done by introducing urea-capped di- andtrifunctional amines containing aliphatic chains into the resin structure or by using thehydrochloride salts of some of these amines as a curing agent [50–54]. By this approachsome flexibility is introduced into the hardened network, which should decrease internalstresses.

In UF resins the aminomethylene link is susceptible to hydrolysis and, therefore, it isunstable at higher relative humidity, especially at elevated temperatures [55,56]. Water alsocauses degradation of the UF resin with greater devastating effect the higher is the tem-perature of the water in which the boards are immersed. This different behavior of boards atdifferent temperatures also is the basis for standard tests on which is based the classificationof bondlines, resins, and bonded wood products. These classes include the lowest require-ments (interior use) for the normal production of UF-bonded boards up to water andweather resistant boards (V100 boiling test, V313 cycle test, water and boil proof (WBP),and others) according to various national and international standard specifications.

Hardened UF resins can also be hydrolyzed by moisture or water, due to the relativeweakness of the bond between the nitrogen of the urea and the carbon of the methylenebridge, and this is especially so at higher temperatures. During this reaction the methylenebridge is eliminated as formaldehyde [57,58]. The amount of liberated formaldehyde canbe taken under certain circumstances as a measure of the resistance of the resin against


hydrolysis. The main parameters influencing the rate and extent of the hydrolysis aretemperature, pH, and degree of hardening of the resin [59]. The acid which has inducedthe hardening of the resin can also and especially induce such a hydrolysis and hence lossof bonding strength.

Another approach to increase the resistance of UF resins against hydrolysis is there-fore, based on the fact that the resin acid hardening causes acid residues in the glue line.Myers [60] pointed out that in the case of such an acid hardening system the decrease in thedurability of adhesive bonds could be initiated both by the hydrolysis of the wood cell wallpolymers adjacent to the glue line as well as in the case of UF-bonded products by acid-catalyzed resin degradation. A neutral pH glue line, therefore, should show a distinctlyhigher hydrolysis resistance. The amount of hardener (acids, acidic substances, latent hard-eners) therefore should always be adjusted to the desired hardening conditions (presstemperature, press time, and other parameters) and never follow ‘‘the more the better.’’Thus, too high an addition of hardener can cause brittleness of the cured resin and a veryhigh acid residue in the glue line. However, glue-line neutralization must not take place aslong as the hardening reaction is ongoing, otherwise this would delay or even preventcuring. This aspect is quite a challenge which in practice has not yet really been solved.Higuchi and Sakata [61] found that a complete removal of acidic substances by soakingplywood test specimens in an aqueous sodium bicarbonate solution resulted in considerableincrease in water resistance of UF glue lines. Another attempt was made by these authors[62,63] using glass powder as an acid scavenger, which reacts only slowly with the remainingacid of the glue line and, therefore, does not interfere with acid hardening of the resin.Dutkiewicz [64] obtained some good results in the neutralization of the inherent acidity of ahardened UF-bonded glue line by the addition of polymers containing amino or amidogroups. All these solutions, however, are not used as yet in broader industrial applications.

Laminate floorings require a very low, long term (24 h) thickness swelling of theMDF/high density fiberboard (HDF) or particleboard cores of which they are composed.Requirements usually are a maximum value of 8 or 10%, sometimes a maximum value of6% or even lower, all figures based on the original thickness of the board. Such lowpercentage thickness swelling results cannot usually be obtained by just using straightUF resins, whereas the incorporation of melamine in the resin is a suitable way to achievethe desired results. Other possibilities could be a pretreatment of the particles or the fibers(e.g., acetylation) or a special posttreatment of the board. The necessary melamine contentin the resin depends on various parameters, e.g., the type of wood furnish, the pressingparameters (pressure profile, density profile), and on resin consumption which can varybetween a few percent up to more than 30%, based on liquid adhesive resin. Due to theconsiderable cost of melamine itself the content of melamine must always be only as highas necessary but as low as possible. Other important parameters are the resin manufactur-ing procedure, which considerably influences the thickness swelling of the boards even atthe same adhesive solids content and at the same content of melamine.

Melamine fortified UF resins and MUF resins can be manufactured in a variety ofways, for example:

(i) By cocondensation of melamine, urea, and formaldehyde in a multistep reac-tion [65–69]. In this regard a comprehensive study of the various reaction typeswas done by Mercer and Pizzi [70]. They especially compared the sequence ofthe additions of melamine and urea.

(ii) By mixing of an MF resin with a UF resin according to the desired compositionof the resin [71–73].


(iii) By addition of melamine in various forms (pure melamine, MF/MUF powderresin) to a UF resin during the application of the glue mix. In the case of theaddition of pure melamine, a UF resin of a higher molar ratio must be used,otherwise there is not enough formaldehyde available to react with the mela-mine in order to incorporate it into the resin.

(iv) Melamine also can be added in the form of melamine salts such as acetates,formates, or oxalates [74–78], which decompose in the aqueous resin mix onlyat higher temperatures and enable some savings of melamine for the samedegree of water resistance compared to original MUF resins. Additionallythey act as a hardener. Some of the reasons why melamine salts yield asaving in melamine content have also been identified [74].

The higher the content of melamine, the higher is the stability of the hardened resintowards the influence of humidity and water (hydrolysis resistance) [79,80]. Resins con-taining melamine can be characterized by the molar ratio F/(NH2)2 (Table 7) or by thetriple molar ratio F:U:M. The mass portion of melamine in the resin can be describedbased on (i) the liquid resin, (ii) the resin solids content, or (iii) the sum of urea andmelamine in the resin.

One of the most interesting tasks is to clarify if there is a real cocondensation withinMUF resins or if two independent networks are formed, which only penetrate each other.The application of MUF resins is very similar to the UF resins, with the difference that thelevel of hardener addition is usually much higher.

MUPF resins are mainly used for the production of so-called V100 exterior gradeboards according to DIN 68763 and EN 312-5 and 312-7, option 2. They contain smallamounts of phenol. Production procedures are described in patents and in the literature[83–87] and a coreaction has been demonstrated here, although often not contributing toresin effectiveness [83,84,88,89].

PMF/PMUF resins, in which the amount of phenol is much higher than in MUPFresins, usually contain only little or no urea at all. The analysis of the molecular structureof these resins has shown that either there is no cocondensation between the phenol andthe melamine, but that there exist two distinct networks [90–93], or that cocondensationcan indeed occur [88]. The reason for this is the different reactivities of the phenol methy-lols and the melamine methylols, depending under which pH conditions the reaction iscarried out.

C. Reactivity and Hardening Reactions

During the curing process a three-dimensional network is built up. This leads to aninsoluble resin which is no longer thermoformable. The hardening reaction is the conti-nuation of the acid condensation process during resin production. The acid hardeningconditions can be adjusted (i) by the addition of a hardener (usually ammonium salts suchas ammonium sulfate or ammonium nitrate) or (ii) by the direct addition of acids (maleicacid, formic acid, phosphoric acid, and others) or of acidic substances, which dissociate inwater (e.g., aluminum sulfate). Ammonium chloride has not been in use in the particle-board and MDF industry for several years because of the generation of hydrochloric acidduring combustion of wood-based panels causing corrosion problems and because of thesuspected formation of dioxins [94].

Ammonium sulfate reacts with the free formaldehyde in the resin to generate sulfuricacid, which decreases the pH; this low pH and hence the acid conditions enable the


condensation reaction to restart and finally the gelling and hardening of the resintakes place. The pH decrease takes place with a rate depending on the relativeamounts of available free formaldehyde and hardener and is greatly accelerated byheat [46,61].

UF resins differ from other formaldehyde resins (e.g., MF, MUF, and PF) due totheir high reactivity, and hence the short hot-press times which are achievable. Hot presstimes shorter than 4 s/mm board thickness are possible in the production of particleboardswith modern, long continuous press lines. This requires highly reactive UF resins, anadequate amount of hardener, as high a press temperature as possible, and a markeddifference in moisture content of the glued wood particles in the surface and the core ofthe mat before hot pressing. This moisture gradient induces the so-called steam shockeffect even without the additional steam injection often used in North American plants.The optimal moisture content of the glued particles is 6 to 7% in the core and 11 to 13% inthe surface. The lower the moisture content in the core, the higher the surface moisturecontent can be. However, a critical total moisture content in the mat must not be exceededas this might cause problems with steam ventilation and even steam blisters in the panel.For this it is necessary to have low moisture content of the glued core particles and it isnecessary to be thrifty with any extra addition of water in the mat core. The lower the resinsolids content on the wood, the lower is the amount of water applied to the wood furnishand hence the lower is the moisture content of the glued core particles. For the surfacelayers, on the other hand, additional water is necessary in the glue mix to increase themoisture content of the glued particles. This additional water, however, cannot be replacedby a higher moisture content of the dried particles themselves before blending, because thiswater must be available quickly for a strong steam shock effect. This would not be the caseif the water would still be present in the wood furnish as the internal wood cell wallmoisture content.

The mechanism of the hardening reaction of a MUPF/PMUF resin is not reallyclear. MUF resins harden is the acid range, whereas phenolic resins have their minimum ofreactivity under these conditions. There is then the possibility that the phenolic portion ofthe resin might not really be incorporated into the aminoplastic portion of the resin duringhardening. Different opinions and confusing reports have been advanced as regards PMFresin hardening. During the hardening of PMF resins either no cocondensation occurs [95]and in the hardened state two independent interpenetrating networks exist, or somecocondensation is reported to occur [88]. Only in model reactions between phenolmethy-lols and melamine have indications for a cocondensation via methylene bridges betweenthe phenolic nucleus and the amino group of the melamine been found by 1H nuclearmagnetic resonance (NMR).

In order to increase the capacity of a production line, especially by shortening thepanel hot press times, adhesive resins with a reactivity as high as possible should be used.This includes two parameters: a short gel time and a rapid and instantaneous bondstrength development, even at a low degree of chemical curing.

The reactivity of a resin at a certain molar ratio F/U or F/(NH2)2 is determinedmainly by its preparation procedure and the quality of the raw materials used. Figure 1shows the comparison of two straight low formaldehyde emission (E1) UF resins with thesame molar ratio, but prepared according to different manufacturing procedures. Thedifferences between the two resins are clearly evident by their different rates of strengthincrease obtained in the so-called ABES (Automatic Bonding Evaluation System) test [96].Resin A shows a distinctly quicker increase in bond strength than resin B, a fact which alsohas been verified in the industrial scale production of boards.


1. Glue Mixes with Enhanced Reactivity

Table 8 describes an example of the use of an accelerator which distinctly increases thegelling rate of a core layer glue mix, hence enabling a significant shortening of the neces-sary press time. The quick reaction of the accelerator with the hardener salt generates theacid for the acid-induced hardening reaction of the resin. The accelerator is mixed with theresin just prior to use. Since it does not contain any hardener or acid, there is no limitingpot life of this premix. To compensate for the additional formaldehyde, small amounts offormaldehyde catchers are recommended for addition to the glue mix.

2. Highly Reactive Adhesive Resins in Plywood, Parquet Flooring,and Door Production

Plywood, parquet flooring, and doors are usually produced using aminoplastic adhesives.The press time necessary for these applications depends on the press temperature, the totalthickness of the wood layers which have to be heated through, and the reactivity of theresin glue mix. Traditional adhesive resin systems need rather long press times due to their

Figure 1 Comparison of two UF resins with the same molar ratio F/U, but with different

reactivities, due to different preparation procedures, tested by means of the Automatic Bonding

Evaluation System (ABES) according to Humphrey [96,97]. UF-resin A, UF resin with

F/U¼ 1.08 and special preparation procedure for higher reactivity; UF-resin B, traditional UF

resin with F/U¼ 1.08.

Table 8 Acceleration of Aminoplastic Resins by Addition of an Accelerator [98]

Standard glue mix

(parts by weight)

Glue mix with

accelerator

(parts by weight)

Component

liquid UF resin (F/U¼ 1.05) 100 100

accelerator — 2.5

hardener solution (ammonium sulfate 20%) 10 10

formaldehyde catcher (urea) — 2

Property

calculated molar ratio F/U of the glue mix 1.05 1.05

gelation time at 100�C (s) 44 36


low reactivity, causing a low capacity of the production line. In these systems the hardeneris premixed in greater proportion with the adhesive resin. The limitation of these is the tooshort pot life obtained after hardener addition, causing early gelling of the glue mix in thestorage vessel. Using smaller glue mixes increases the chances of improving the resinreactivity. With this presupposition very reactive hardeners can be used. They decreasethe gel time of the glue mix and hence the necessary press times. Special aminoplasticresin systems with distinctly higher reactivity have therefore been developed to fulfill therequirements of each customer in terms of saving time, energy, and costs. The higherthe reactivity, the higher is the capacity of the production line, or the lower is the necessarypress temperature at a given press time. Lower temperatures are beneficial for the qualityof the wood itself as well as for saving energy costs.

It has been shown that such very reactive hardeners perform favorably also whenin liquid form. This enables the use of various acids or acidic substances in the formulationsof these hardeners. Such reactive adhesive systems usually consist of two liquid compo-nents, one being a high viscosity resin and the other a high viscosity liquid hardener. Thehardener contains some inorganic fillers or organic thickeners. The mixing of these twocomponents is performed just prior to the application of the resin mix to the roll coater. Theliquid–liquid two-component mixer is installed preferably above the roll coater, in order toreduce considerably the amount of each batch of prepared glue mix. If a long stop of theproduction occurs the lost amount of the ready-to-use glue mix is rather small. Anotheradvantage of this system is that both components can be pumped directly from the storagevessels to the mixer, without the use of any powder. The disadvantage of these two-com-ponent systems is the fixed ratio between the extender and the hardener. If the amount ofextender should be changed, the amount of the hardener itself is also changed and hence theglue mix reactivity and pot life are changed too.

Because of the well known marked influence of the temperature on the pot life,cooling of the whole system is necessary using a chilled water cooler. The raw adhesiveresin should have a temperature not higher than 15�C prior to use, which is especiallyimportant in summer due to the higher room temperature. Cooling of the adhesive resincan be performed in a small vessel with cooling coils, which is installed between the storagetank and the mixer. Additionally the roll coater itself also needs cooled cylinders in orderto stabilize the temperature at approximately 15�C. Figure 2 shows the scheme of a liquid–liquid two-component mixing station.

Figure 2 Scheme of a liquid–liquid two-component glue mixing station.

1. glue resin tank

2. hardener tank

3. twin pumps

4. filter unit

5. flow sensor

6. mixer

7. switchbox

8. distributor

9. level sensor

10. roll coater

11. ventilation valve


D. Correlations Between the Composition of Aminoplastic Resins andthe Properties of the Wood-Based Panels

Not much work has been done up to now concerning the prediction of bond strengths andother board properties based on the results of the analysis of the adhesive resin in its liquidstate. What has been investigated and derived up to now are correlation equations thatcorrelate the chemical structures in various UF resins having different molar ratios F/Uand different types of preparations with the achievable internal bond strengths of theboards as well as the formaldehyde emission measured after resin hardening.

The basic aim of such experiments is the prediction of the properties of the wood-based panels, hence of the adhesive resin in its hardened state, based on the compositionand the properties of the liquid resins used before their hardening. For this purposevarious structural components were determined by means of NMR spectroscopy and theratios of the amounts of the various structural components were calculated, for example:

(i) for UF resins:free urea related to total ureamethylene bridges with crosslinking related to total sum of methylenebridgessum of methylene bridges in relation to sum of methylols

(ii) for MF resins:unreacted melamine to monosubstituted melamineunreacted melamine to total melaminenumber of methylene bridges in relation to the number of methylol groupsdegree of branching: number of branching sites at methylene bridges inrelation to total number of methylene bridges

(iii) for MUF resins:sum of unreacted melamine and urea to sum of substituted melamine andureanumber of methylene bridges in relation to number of methylol groups orto the sum of methylene bridges and methylol groups

These ratios then are correlated to various properties of the wood-based panels, e.g.,internal bond strength or subsequent formaldehyde emission. Various papers in the lit-erature describe examples of such correlations and present workable predictive equations.For UF resins: Ferg [30], Ferg et al. [99,100]; for MF resins: Mercer and Pizzi [101]; forMUF resins: Mercer and Pizzi [102], Panamgama and Pizzi [103].

For certain boards, some good correlations exist. Even these equations, however,cannot predict all properties for all types of UF resins. This is because it must be assumedthat a general correlation for various resins and various panels cannot exist. Other corre-lation equations might have to be used sometime. However, the types of equations thathave already been published describe how a universal equation for this task might look.Only the coefficient need to be changed from case to case. These results are of someimportance, because they show that at least for a certain combination of resin type andboard type, correlations do indeed exist. It will be the task for chemists and technologiststo evaluate in further detail all possible parameters as well as their influence on theperformance of the resins and the wood-based panels. It can also be assumed that thevarious parameters mentioned above will also be decisive for other combinations, even ifthe numerical values of the coefficients within individual equations might differ. The rangeof the molar ratio under investigation in the papers mentioned above was rather broad.


It would not appear to be possible to use these equations for predictions withinnarrow ranges of molar ratio, e.g., the usual range of an E1 UF resin with approximateF/U¼ 1.03 to 1.10. A method showing how different resin preparation procedures, forequal molar ratio resins, can be included in these correlation equations also needs to bedeveloped.

E. Glue Resin Mixes

Table 9 summarizes some resin glue mixes for different applications in the production ofparticleboard and MDF. Table 10 summarizes various resin glue mixes for differentapplications in the production of plywood, parquet flooring, and furniture.

V. PHENOLIC RESINS

Phenolic resins [phenol–formaldehyde (PF) resins] show complete resistance to hydrolysisof the C–C bond between the aromatic nucleus and the methylene bridge and, therefore,are used for water and weather resistant glue lines and boards such as water and weatherproof particleboards, OSB, MDF, or plywood for use under exterior weather conditions.Another advantage of phenolic resins is the very low formaldehyde emission in service,after hardening, also due to the stability of the methylene bridges between aromatic nuclei.The disadvantages of phenolic resins are the distinctly longer press times necessary forhardening when compared to UF resins, the dark color of the glue line and of the boardsurface as well as a higher equilibrium moisture content of the boards due to the hygro-scopicity of the high alkali content of the board.

The preparation procedure of a phenolic resin is a multistage process, characterizedby the time, sequence, and amount (in the case of several steps) of the additions of phenol,formaldehyde, and alkali as the most important raw materials. Similarly to all otherformaldehyde condensation resins two main reactions predominate:

Methylolation: there is no special preference for ortho or para substitution,preference which, however, could be achieved using special catalysts [104–106].

Table 9 UF Resin Glue Mixes for the Production of Particleboard and MDF, Parts by Weight

Components/resin mixes PB-CLa PB-FLa PB-CLb PB-FLb MDFa

PB–UF resina,c 100 100 — — —

MDF–UF resind — — — — 100

MUF resine — — 100 100 —

Water — 10–20 — 10–20 30–80

Hardener solutionf 8 2 15 6 2

Urea solutiong up to 5 up to 5 up to 5 up to 5 15

PB, particleboard; CL, core layer; FL, face layer.aFor use in dry conditions.bFor use in moist conditions.cUF resin with molar F/U¼ 1.03 to 1.08.dUF resin with molar F/U� 0.98 to 1.02.eMUF resin with molar F/(NH2)2� 1.03 to 1.08.fAmmonium sulfate solution (20%).gUrea solution (40%).


Methylolation is strongly exothermic and includes the risk of an uncontrolledreaction [107].

Condensation methylene and methylene ether linkages are formed; the latter do notexist at high alkaline conditions. During this stage chains are formed, stillcarrying free methylol groups. The reaction is stopped just by cooling downthe preparation reactor thus preventing resin gelling.

Phenolic resins contain oligomeric and polymeric chains as well as monomeric methylol-phenols, free formaldehyde, and unreacted phenol. The contents of both monomers haveto be minimized by the proper preparation procedure. Various preparation procedures aredescribed in the literature and in patents [108–117].

Special PF resins consisting of a two-phase system of a highly condensed and inso-luble PF resin and a lower condensation standard PF resin have also been prepared [118]and used industrially. Another two-phase resin consists of a highly condensed PF resin stillin an aqueous solution and a PF dispersion [119]. The purpose of such special resins is thegluing of panel products of higher moisture content wood, where the danger of over-penetration of the resin into the wood surface would cause a starved glue line and otherserious problems.

The properties of the resins are determined mainly by the F/P molar ratio, theconcentration of phenol and formaldehyde in the resin, the type and amount of thepreparation catalyst (in most cases alkaline), and the reaction conditions. The reaction

Table 10 UF Resin Glue Mixes for the Production of Plywood, Parquet Flooring, and

Furniture, Parts by Weight

Components/resin mixes A B C D E

UF resina 100 100 100 — —

UF resinb — — — 100 —

UF resinc — — — — 100

Extenderd 20 40 10 — —

Water — 10–20 — — —

Hardener solutione 10 — — — —

Hardener solutionf — 20 — — —

Powder hardenerg — — 3 — —

Powder hardenerh — — — 25 —

Liquid hardeneri — — — — 10–20

Glue mix A: standard glue mix. Glue mix B: containing higher proportion of fillers than in A. Glue mix C: high

solids content, gives an enhanced water resistance to the glue line. Glue mix D: two-component glue mix: liquid

resinþ ready-to-use hardener in powder form, no addition of other components necessary. Glue mix E: two-

component glue mix: high viscosity liquid resinþhigh viscosity liquid hardener.aUF resin with molar F/U� 1.3.bUF resin with molar F/U� 1.5 to 1.6.cHigh viscosity UF resin with molar F/U� 1.3 to 1.4.dExtender: rye- or wheat-flour. In some cases some inorganic fillers are also included.eFor example, ammonium sulfate solution (20%).fFor example, ammonium sulfate–urea solution (20%/20%).gFor example, ammonium sulfate in powder form.hReady-to-use powder hardener, containing powdered hardener, formaldehyde catcher, extenders, and other

additives.iHigh viscosity filled hardener, containing inorganic fillers or organic thickeners, hardener, and sometimes some

formaldehyde catcher and other additives.


itself is performed in an aqueous system without addition of organic solvents. The higherthe F/P molar ratio, the higher is the reactivity of the resin hence the higher its hardeningrate [120], the degree of branching, and the three-dimensional crosslinking. At lower F/Pmolar ratios linear molecules are formed preferably. Chow et al. [121] found an increase inthe bonding strength of plywood with increasing F/P molar ratio; however, the bondingstrength remained constant for molar ratios higher than 1.4. This value is still distinctlylower than the common industrial molar ratios of PF resins for wood adhesives.

Usually sodium hydroxide is used as a catalyst, in an amount up to one mole per molephenol (molar ratio NaOH/P), which corresponds to a portion of alkali in the liquid resinof approximately 10% by weight. The pH of a phenolic resin is in the range of 10 to 13. Thepreponderant part of the alkali is free NaOH, and a smaller part is present as sodiumphenate. The alkali is necessary to keep the resin water soluble via the phenate ion forma-tion in order to achieve a degree of condensation as high as possible at a viscosity that stillcan be used in practice. Additionally the alkali content significantly lowers the viscosity ofthe reaction mixture. Thus, the higher the alkali content of the resin, the higher is itspossible degree of condensation, hence the greater is the reactivity of the resin and thehigher its hardening rate and, therefore, the shorter is the necessary press time.

High alkali contents have also some disadvantages. The equilibrium moisture contentin humid climates increases with the alkaline content as do some hygroscopic-dependentproperties (longitudinal stability, thickness swelling, water absorption), and some mechan-ical properties (creep behavior) become worse. The alkali content also causes a cleavage ofthe acetyl groups of the hemicelluloses. This leads to an enhanced emission of acetic acidcompared to UF-bonded boards. The higher the alkali content, the higher is the emission ofacetic acid. In European Norms EN 312-5 and 312-7 the content of alkali is limited to 2.0%for the whole board and 1.7% for the face layer, both figures being based on oven-driedmass of the board.

Besides NaOH, other basic catalysts can be and are used, such as Ba(OH)2, LiOH,Na2CO3, ammonia, or hexamine. However, with some notable exceptions, these are notused in practice. The type of catalyst significantly determines the properties of the resins[122–124]. Replacing alkali in PF-bonded boards could give some advantages. Ammoniabeing a gas evaporates during the hot press process and does not therefore contribute to thealkali content and the hygroscopicity of the boards. It is important to hold a fairly high pHas long as possible during hot pressing in order to guarantee a high reactivity and hence ashort press time [125,126].

The condensation process of PF resins can be followed by monitoring the increase inviscosity and by gel permeation chromatography (GPC) to measure the molar massdistribution. Chromatograms have been obtained by Duval et al. [122], Ellis and Steiner[127], Gobec et al. [128], Kim et al. [129], and Nieh and Sellers [130].

The penetration behavior strongly depends on the molar masses present in the resin:the higher the molar masses (approximately equivalent to the viscosity of the resin at thesame solid content), the worse is the wettability and the lower is the penetration into thewood surface [131,132]. The lower molar masses are responsible for the good wettability,however, too low a molar mass can cause overpenetration and hence starved glue lines.Contact angles of phenolic resins on wood increase strongly with the viscosity of the resin,which increases with the molar masses [133]. The higher molar masses remain at the woodsurface and form the glue line, but they will not anchor as well in the wood surface.Depending on the porosity of the wood surface, a certain portion with higher molarmasses must be present to avoid an overpenetration into the wood, causing a starvedglue line; this means a certain ratio between low and high molar masses is necessary


[127,130,134–141]. Gollob and co-workers [109,142] found a decrease in the wood failurewith increased molar mass averages of PF resins.

The penetration behavior of resins into the wood surface also is influenced byvarious other parameters, such as wood species, amount of glue spread, press temperature,and pressure and hardening time. The temperature of the wood surface and of the glue lineand hence the viscosity of the resin (which itself also depends on the degree of advance-ment of the resin at the time of measuring) influences the penetration behavior of the resin[143]. Table 11 summarizes the properties of various PF resins.

The contents of free monomers (formaldehyde, phenol) depend on the type of theresin and the preparation procedure. Usual values are <0.3 mass% for the free formal-dehyde and <0.1 mass% for free phenol.

The storage stability of liquid PF resins ranges from a few weeks up to severalmonths, depending on the degree of condensation, the content of alkali, and the viscosity.An important parameter for the length of the possible storage time is the viscosity of theresin, with regards to both the proper application onto the wood surface during blendingas well as the danger that the resin might gel in its storage tank. The lower the alkalicontent, the lower the storage stability. The aging behavior can also be monitored usingGPC [144].

The molecular characterization of PF adhesive resins is done in similar way to that ofall the other condensation resins by determining:

the molar ratios of the main components: F/P/NaOH; F/P; NaOH/Pthe composition of the resins, based on liquid form of deliverythe degree of condensation and molar mass distribution, molar mass averagesthe content of reactive sites and functional groups and their distribution in the resin,

type of bridges between the aromatic rings of the phenol molecule, branchingsites and others.

Due to the hydrolysis-resistant C–C bonding between the aromatic ring and themethylene group it is not possible to determine the molar ratio in the usual chemicalway. This is only possible by 13C-NMR. The F/P molar ratio of PF resins is usuallybetween 1.8 and 2.5, depending on the type of resin. The higher the molar ratio, thehigher its reactivity as well as its storage stability. However, the hardened resin is morebrittle due to its higher level of crosslinking.

A. Reactivity and Hardening Reactions

Phenol–formaldehyde core layer resins usually have the highest molar masses and henceshow a high reactivity and quick gelation. They contain higher amounts of alkali than face

Table 11 Properties of PF Adhesive Resins

Particleboard CL Particleboard FL AW100-Plywood

Solids content (%) 46–48 ca. 45 46–48

Total alkali (%) 7–9 3–4 7–10

Free alkali (%) 6–8 2–3 6–9

Viscosity (mPa s) 300–700 300–500 500–800

Density (g/ml) ca. 1.23 ca. 1.18 ca. 1.23

CL, core layer; FL, face layer; AW100-plywood according to DIN 68705.


layer resins in order to keep the resin soluble even at higher degrees of condensation. Thehigher the degree of condensation during the production process (the higher the viscosity),the shorter the gel time [145]. The upper limit of the degree of condensation of the resinduring its production process is given by (i) the viscosity of the resin (the resin must be ableto be pumped, and a certain storage stability as well as a proper distribution of the resin onthe particles during blending is required) and (ii) the flow behavior of the resin under heat,guaranteeing wetting of the unglued matching second wood surface and a sufficient pene-tration into the wood surface. Too high a moisture content of the glued particles limits thelevel to which it is possible to dilute the resin and its solids content.

The hardening of a phenolic resin can be seen as the transformation of molecules ofdifferent sizes via chains lengthening, branching, and crosslinking to a three-dimensionalnetwork with theoretically an endlessly high molar mass. The hardening rate depends onvarious parameters, such as molar mass, molecular structure of the resin, the portions ofvarious structural elements as well as possible catalysts and additives.

Alkaline PF resins contain free reactive methylol groups in sufficient number and canharden even without any further addition of formaldehyde, a formaldehyde source, orcatalysts. The hardening reaction is initiated by heat only. The methylol groups react toform methylene and methylene ether bridges. Under high temperatures methylene etherbridges can rearrange to methylene bridges. The lowest possible temperature for a suffi-ciently fast gel rate is approximately 100�C. In some cases to improve this, potash in theform of a 50mass% solution is added in the core layer resin mix in an amount of about 3to 5% potash solid based on resin solids content.

Pizzi and Stephanou investigated the dependence of the gel time on the pH of analkaline PF resin [146]. Surprisingly they found an increase in the gel time in the region ofvery high pH values (above 10). Standard commercial PF adhesive resins with a content ofNaOH of 5 to 10mass% have exactly such pHs. A decrease of the pH in order to accel-erate the hardening process is not possible, because a spontaneous precipitation wouldoccur with such standard PF resins. A change in pH of the resin, however, might occurwhen the resin comes into contact with a wood surface. Wood is generally acidic incharacter, and especially with rather acidic wood species, the pH of the resin could sig-nificantly drop when in contact with the wood surface [147].

Lu and Pizzi [148] showed that lignocellulosic substrates had a distinct influence onthe hardening behavior of PF resins, whereby the activation energy of the hardeningprocess was much lower than for the resin alone [149] and the hardening rate muchfaster [149]. The reason is a catalytic activation of the PF condensation by carbohydratessuch as crystalline and amorphous cellulose and hemicellulose. Covalent bondings betweenthe PF resin and the wood, especially lignin, play only a minor role, however.

The gelling process can be monitored via DSC, ABES, or dynamic mechanicalanalysis (DMA). The chemical hardening can be followed also by solid state NMR,looking (i) at the increase of the amount of methylene bridges based on the amount ofaromatic rings [123,150,151], (ii) at the portion of 2, 4, 6-three-substituted phenols [151], or(iii) at the ratio between methylol groups and methylene bridges [152,153]. This degree ofhardening, however, is not equal to the degree of hardening as monitored by DSC.Plotting one of these degrees of chemical hardening versus the degree of mechanicalhardening, as measured, e.g., via ABES or DMA, reveals the hardening pattern of aresin [151,154,155].

An acid- rather than alkali-induced gelling reaction of PF adhesive resins can causesevere deterioration of the wood substrate at the interface and, therefore, its use has lost itssignificance in the application of PF resins to bond wood. Pizzi et al. [156] describe,


however, an effective procedure for the self-neutralization of acid hardened PF glue lines.The system is based on a mixture of a complex formed by morpholine and a weak acid inthe presence of para-toluene sulfonic acid. The complex decomposes with heat and re-forms on cooling to a complex in which the weak acid has been exchanged with the weakbase, yeilding an almost neutral glue line. The system prevents, to a considerable extent,the acid deterioration of the wood substrate. Several other attempts based instead onincorporating the acid chemically into the resin or fixing the hardeners physically in theglue line have failed [157].

The acceleration of the hardening reaction is possible by using as high a degree ofcondensation as possible. Another approach is the addition of accelerating esters[146,158], among which, for example, is propylene carbonate [158,159]. The mechanismof this acceleration is not yet completely clear; it might be due to the hydrogen carbonateion after hydrolysis of propylene carbonate [160] although this has been shown to beunlikely [146,159] or due to the formation of hydroxybenzy alcohols and temporary aro-matic carbonyl groups in the reaction of the propylene carbonate with the aromatic ring ofthe phenol as in the Kolbe–Schmidt reaction of CO2 with phenol to give salicylic acid[146]. The higher the addition of esters such as propylene carbonate, the shorter the geltime of the PF resin [146]. Other accelerators for PF resins are potash (potassium carbo-nate), sodium carbonate [95,161], guanidines, or sodium and potassium hydrogencarbonate. Also chemicals inherent to wood might have an accelerating influence on thehardening reactivity of PF resins [161].

Since phenolic resins for wood bonding harden only thermally, postcuring duringhot stacking is very important. In contrast to UF-bonded boards, PF-bonded boardsshould be stacked as hot as possible to guarantee a maximum postcuring effect. Thestrength of the panel improves during hot stacking due to continuous slow curing ofthe PF resin. On the other hand, very high temperatures during stacking might causepartial deterioration of the wood, seen as discoloration.

B. Modification of Phenolic Resins

1. Post-Addition of Urea

The addition of urea to a phenolic resin causes several effects:

decrease of the content of free formaldehydedecrease of the viscosity of the adhesive resinacceleration of the hardening reaction via the possible higher degree of condensation

of the resinreduction of the costs of the resin.

The urea can be added to the finished PF resin or during its manufacture. Thedistinct decrease of viscosity observed when urea is added to the finished PF resin iscaused by the cleavage of hydrogen bonds [162] and by the dilution effect. There isobviously no cocondensation of this postadded urea with the phenolic resin. Ureareacts only with the free formaldehyde of the resin to form methylols which, however,do not react further due to the high pH [163]. Only at high temperatures did Scopelitis andPizzi [164] suppose some phenol–urea cocondensation occurs, but in their case the phenolused was the much more reactive resorcinol.

The higher the amount of postadded urea, the worse the properties of the boards. Areason for this might be urea’s diluting effect on the PF resin. Surprisingly Oldoerp and


Marutzky [165] found enhanced board properties at higher degrees of addition of uncon-densed urea. Since, however, in these experiments the postadded urea could be extractedcompletely from the boards, no significant cocondensation between the urea and thephenolic resin could have occurred. Using such PUF resins, the adhesive solids contentshould be calculated based only on the PF resin solids content in the PUF resin.

2. Cocondensation Between Phenol and Urea

A real concondensation between phenol and urea can be performed in three ways:

Reaction of methylolphenols with urea [166–169].Acidic reaction of urea–formaldehyde concentrate (UFC) with phenol followed by

an alkaline reaction [170,171].Reaction exclusively under alkaline conditions of urea and phenol in competition

with each other leading to reaction of methylol ureas with phenol and PFoligomers, and to reactions of methylolphenols with each other, as well asreactions of methylolphenols with urea [117,172]

The kinetics of the cocondensation of monomethylolphenol model compounds andurea under alkaline conditions is reported by Pizzi et al. [173] and Yoshida et al. [174].The same kinetics but under acidic conditions is described by Tomita and Hse[166,170,175]. The interaction of esters and copolymerized urea on the fast advancementand hardening acceleration of low condensation alkaline PUF resins is reported byZhao et al. [117,172].

3. Addition of Tannins

The purposes of the addition of tannins are to accelerate the hardening reaction[110,116,176–178] and to replace phenol or a part of the PF resin [179–185].

4. Addition of Lignins

Lignins can be added to phenolic resins (i) as an extender, e.g., in order to increase the coldtack or to reduce costs, or (ii) to achieve a chemical modification of the resin, whereby thelignin is chemically incorporated into the phenolic resin [186–190]. The idea behind this isbased on the chemical similarity between the phenolic resin and lignin or between phenoland the phenylpropane unit of the lignin. The lignin can be added at the beginning, duringthe cooking procedure, or at the end of the condensation reaction. It is not clear whetherthe lignin is really always incorporated into the phenolic resin or not. In practice lignin isused at present in just a few North American mills, only as a neutral filler/extender inadhesive resins.

5. Addition of Isocyanates

Isocyanates [polymeric MDI (PMDI)] as a fortifier for phenolic resins have only been usedin the past in rare cases. Deppe and Ernst [41] reported a precuring reaction between theisocyanate and the phenolic resin, even if both components had been applied separately tothe particles. Hse et al. [36] also found good results with an isocyanate and a PF resinadded separately to wood particles. Pizzi and Walton [191] reported on the reactions andtheir mechanisms of PF resins premixed in the glue mix with nonemulsifiable water-baseddiisocyanate adhesives for exterior plywood. Pizzi et al. [192] reported on the industrialapplications of such systems (PFþPMDIþ sometimes tannin accelerator; UFþPMDI)


[192–194], on the marked curing acceleration of the PF resin by the isocyanate, and on theexcellent results which were obtained industrially with these systems. Very recently thisapproach, due to its excellent performance, lower cost, and ease of preparation hasreceived intense attention, and several other studies on the subject have been recentlypublished [195–198].

C. Correlation Between the Composition of a Resin and the Propertiesof Wood-Based Panels Bonded with the Resin

Similar to the investigations described above for the aminoplastic resins, NMR results ofthe liquid phenolic resins can be correlated with certain board properties [199]. For thispurpose various structural components are determined by means of 13C-NMR and theratios of the amounts of the various structural components are calculated, e.g.:

methylol groups to methylene bridgesratio of free ortho and para sites in relation to all possible reaction sitesmethylol groups in relation to all possible reaction sitesmethylene bridges in relation to all possible reaction sitesether bridges in relation to all reaction sites

These ratios are then correlated to various properties of the wood-based panels,e.g., internal bond strength after boiling or after boilingþ redrying, and subsequentformaldehyde emission.

Also for phenolic resins it is not clear whether universally valid correlation equationswill exist or if they will differ for different types of resins and boards, although thecorrelations obtained appear to have a lower coefficient of variability than for theaminoplastic resins. Nevertheless, it can be safely assumed that the various parametersused with at least be the same in most cases, even if the actual numerical values ofthe coefficients within the individual equations might differ.

D. Adhesive Resin Glue Mixes

Table 12 summarizes the various glue mixes for different applications.

Table 12 Examples of PF Glue Mixes for Particleboard, Oriented Strandboard, and Plywood

Parts by Weight

Components

Particleboard

core layer

Particleboard

face layer OSB OSB AW100-Plywood

PF-resin A 100 — 100 — —

PF-resin B — 100 — — —

PF-resin C — — — — 100

PF-powder resin — — — 100 —

Water — — — — —

Potash 50% 6 — 6 — 6

Extender — — — — 10–15

PF-resin A, medium alkali content (8–10%); PF-resin B, low alkali content (3–5%); PF-resin C, medium alkali

content (6–8%); PF-powder resin, no addition of water, no dissolving of the powder before blending the strands;

Extender: e.g., coconut shell flour.


VI. ISOCYANATES

A. Adhesives for Wood-Based Panels (Polyisocyanates)

Adhesives based on isocyanates (especially PMDI) have been used for more than 25 yearsin the wood-based panel industry [40,41,200–203], but still have a relatively low consump-tion volume compared to systems based on UF, MUF, or PF resins. The main applicationis the production of waterproof panels, but there is also the production of panels from rawmaterials that are difficult to glue, such as straw, bagasse, rice shells, or sugar canebagasse. PMDI can be used as an adhesive for wood-based products such as exteriorparticleboard, exterior OSB, laminated strand lumber (LSL), MDF, or other speciallyengineered composites. During hot pressing the viscosity of PMDI is lowered, allowingit to flow across and penetrate below the surface, locking in the wood subsurface as hasbeen shown by Roll [204]. The low wetting angle of PMDI compared to water-basedcondensation resins allows a rapid penetration into the wood surface; however, this alsomight result in starved bondlines [205].

PMDI is produced during the manufacturing of monomeric MDI. The PMDI pro-duced industrially by phosgenation of di-, tri-, and higher amines contains a mixture of thethree different MDI isomers, triisocyanates, and different polyisocyanates, and thus thestructure and the molar mass depend on the number of phenyl groups. This distributioninfluences to a great extent the reactivity, but also the usual characteristics such as vis-cosity, flow, and wetting behavior as well as the penetration behavior into the woodsurface. The structure and the molar mass depend on the number of aromatic rings[206]. For PMDI the distribution of the three monomeric isomers has a great influenceon the quality, because the reactivities of the various isomers (4,40-, 2,40-, and 2,20-MDI)differ significantly [207]. The greater the portion of the 2,20- and 2,40-isomers, the lower thereactivity. This can lead to different bonding strengths as well as to residual low reactiveisomers in the wood-based panels produced. In the monomeric form (MDI) the function-ality is 2 and the NCO content is 33.5%, while PMDI has an average functionality of 2.7with an NCO content of approximately 30.5%. The HCl content is usually below 200 ppm.PMDI is cheaper than pure MDI and has a lower melting point (liquid at room tempera-ture) due to the increased asymmetry. It is less prone to dimerization and, as a conse-quence, it is more stable during storage than pure MDI. PMDI is used whenever the colorof the finished adhesive is not of concern [208].

The excellent application properties of PMDI and of the wood-based panels pro-duced with it are based on the special properties of PMDI, especially the excellent wettingbehavior of a wood surface when compared to waterborne polycondensation resins. Dueto this fact surfaces with poor wetting behavior such as straw can also be bonded.According to Larimer [209] the wetting angles for PMDI on various surfaces are muchlower than for UF resins. Additionally, these resins show a good penetration behavior intothe wood surface, which seems to be determined by the small molar mass of PMDI whencompared with polycondensation resins. Marcinko et al. [210] found in their measure-ments, using solid state 13C-NMR, DSC, fluorescence microscopy, and DMA, that PMDIcould penetrate 5–10 times further into wood than PF resins. PMDI not only penetratesthe macroscopic hollows of the wood substance, but even penetrates the polymer structureof the wood. This enables good mechanical anchoring. The good wetting and penetrationbehavior of PMDI can sometimes cause starved glue lines. Due to PMDI’s high reactivityand its low molar mass, a special interfacial layer between the wood surface and theadhesive appears to form. If hardening is quicker than the thermodynamically induceddesorption during the hardening reaction, then a polyurea/biuret network might form


interpenetrating the wood constituents network. Covalent bondings as well as secondaryforces can help to avoid desorption reactions during hardening.

Johns [211] showed that isocyanates spread easily on a wood surface; 2 to 3% ofisocyanate was enough to form a film completely covering the wood strands, which is notpossible even with 6% of a phenolic resin. The good mobility of MDI is based on severalparameters [211]:

MDI contains no water; it cannot lose its mobility during adsorption on the woodsurface

its low surface tension (ca. 50mN/m) compared to water (76mN/m)its low viscosity.

The impossibility of diluting PMDI with water was solved by the introduction ofemulsified PMDI, often called EMDI, which allows an even distribution of the adhesiveduring the gluing process. EMDI is a product of the reaction of PMDI with polyglycols.EMDI is manufactured under high pressure and dispersed in water.

The isocyanate group in PMDI is characterized by high reactivity towards all sub-stances which contain active hydrogens. The main hardening reaction proceeds via reac-tion with water to the final amide group, while at the same time CO2 is split off. The waternecessary to induce the hardening reaction is applied together with the PMDI (sprayingtogether with the PMDI or spraying of an aqueous dispersion of PMDI in water) or ispresent in wood in sufficient amount. The amine group then reacts further with anotherisocyanate group to form a polyurea structure:

R—N——C——OþH2O ! R—NH2 þ CO2

R—NH2 þO——C——N�R0 ! R—NH—ðC——OÞ—NH—R0

The reaction of an isocyanate group with a hydroxyl group leads to the so-called urethanebonding:

R—N——C——OþHO—R0 ! R—NH—ðC——OÞ—O—R0

Such a reaction can theoretically also occur between an isocyanate group and an –OHgroup of cellulose or lignin to form a covalent bond. These bonds are usually of greaterdurability than purely physical bonds. If one could manage to force such a reaction tooccur in an industrially useful short curing time, when the reaction of the isocyanategroups of the PMDI with water is suppressed, the probability of the formation of suchcovalent bonds and with this the quality of the bonding should increase, leading to higherbond strengths and especially a higher resistance against the influence of humidity.

If another isocyanate group reacts with an amide hydrogen within the polyureastructure formed, a branching point is formed (biuret group):

R00—N——C——OþR—NH—ðC——OÞ—NH—R0 ! R—N—ðC——OÞ—NH—R0j

ðC——OÞ—NH—R00

During the hardening of PMDI Frazier et al. [212] have found the formation of urethanes,polyureas, biurets, and triurets/polyurets. The proportions of the various compoundsdepend on the working and hardening conditions. The forming of the network is especiallyinfluenced by the ratio between the isocyanate and water. The formation of a urethaneseems to be possible for low molar mass isocyanates, as e.g., the usual industrial PMDIs,under slightly alkaline conditions. It can also be assumed that the forming of a urethane


especially occurs by reaction with lignin. This bond, however, seems not to be stable athigher temperatures (120�C) for longer times.

Hydrophobic polyols should be able to repel and eliminate water from the woodsurface and, therefore to fortify the reaction of the isocyanate group with the hydroxylgroups of the wood surface [213]. Umemura et al. [214] and other workers [27] comparedthe reaction of isocyanate with water and small amounts of polyols using DMA. Thebonding strength and the thermal stability increased by adding dipropylene glycol withmolar mass in the 400–1000 range.

Usually no hardeners are added during the production of wood-based panels (par-ticle board, MDF, OSB, engineered wood products) using PMDI as adhesive. With specialadditives a distinct acceleration of the hardening reaction and hence shorter press times orlower press temperatures can be achieved [209]. This fact is especially interesting for cold-setting systems as well as for the production of particleboards. Possible catalysts aretertiary amines (e.g., triethanol amine, triethylamine, N,N-dimethylcyclohexylamine)and metal catalysts, based on organic compounds of tin, lead, cobalt, and mercury[208,215–218].

Compared to other adhesives, PMDI possesses various advantages, but also somedisadvantages (Table 13). For the production of plywood the addition of extenders isrecommended [221–226] or the mixing with other resins [191,192] as alone PMDI cannotbe used for plywood. In the production of OSB (especially for the two types OSB/3 andOSB/4 according to European Norm EN 300) often PMDI is used in the core layer.

B. Polyurethane Adhesives

Polyurethane adhesives are formed by the reaction of various types of isocyanates withpolyols. The polar urethane group enables bonding to various surfaces. Depending on theraw materials used, glue lines with either rubberlike behavior or elastic-to-brittle hardbehavior can be achieved. The end groups determine the type of the adhesive, whetherit is a reactively or a physically hardening adhesive.

Table 13 Advantages and Disadvantages of PMDI Compared to Other Adhesives, Especially

UF Resins

Advantages Higher storage stability

Formaldehyde-free gluing, despite the fact that formaldehyde is used in the

production of MDI/PMDI

Higher reactivity

Higher bonding strength

Higher tolerance against humidity

Lower consumption of adhesive

Disadvantages Higher price, but this is compensated by the low adhesive consumptions and

sometimes shorter press times

Adhesion to all other surfaces, e.g., also press platens. This imposes the use of

(i) special internal or external release agents [219], (ii) special types of PMDI

[220] or (iii) the use in the board surface of adhesives other than isocyanates

The necessary use of special emulsifiers (EMDI) or special dosing and gluing

systems

Greater worker protection requirements due to the toxicity and the low but

nevertheless existing vapor pressure of monomeric MDI, which need special

precautions during use


One-component isocyanate adhesive systems consist of chains with isocyanategroups on chain ends or on branching sites. These isocyanate groups can react with themoisture content of the surfaces to be bonded, and a hardened system forms from thisaddition reaction. Thus, at least one of the two surfaces must contain the amount of waternecessary for hardening. Due to the high viscosity of these adhesives, dilution with organicsolvents or higher temperatures are necessary. Additionally, the adhesive may containvarious other components, such as flowing agents, fillers, antioxidants, bactericides, ordyes. The bondline reaches the necessary green strength within a few hours and hardensover a few days. During the reaction of the isocyanate group with the moisture content ofthe wood, CO2 is formed, which causes some foaming of the bondline. The bondlinesthemselves are more or less resistant against humidity and water.

The two-component systems consist of (i) a polyol or polyamine and (ii) an iso-cyanate. The hardening starts with the mixing of the two components. Due to the lowviscosities of the two components they can be used without addition of solvents. Theweight ratio between the two components determines the properties of the bondline.Linear polyols and low amounts of isocyanates give flexible bondlines, whereas branchedpolyols and high amounts of isocyanates lead to hard and brittle bondlines. The pot life ofthe two-component systems is determined by the reactivity of the two components, thetemperature, and the addition of catalysts, and can vary between 0.5 and 24 h. At roomtemperature hardening occurs within 3 to 20 h.

VII. WOOD ADHESIVES BASED ON NATURAL RESOURCES

Bio-based adhesive resins have been under investigation for a long time; however, exten-sive industrial application, at least in Europe, has not yet occurred. The use and applica-tion of adhesives based on natural and renewable resources is often thought of by theindustry as well as the general public as a new approach that requires novel technologiesand methods to implement. Despite the increasing trend toward the use of syntheticadhesives, processes based on the chemical modification of natural products offer oppor-tunities for producing a new generation of high performance, high quality products. Thedistinct advantages in the utilization of natural materials, e.g., lower toxicity, biodegrad-ability, and availability, need to be paralleled by more efficient and lower cost methods ofproduction. Factors such as regional and species variation have to be considered in select-ing the optimum feedstock for a particular process; additionally cost-effective manufactur-ing techniques have to be developed that will enable these materials to capture a widerpercentage of the world market. Manufacturers need to have confidence that a continualuninterrupted supply of raw material can be sustained throughout the life cycle of aproduct. It is of equal importance that the feedstock should not be restricted by geogra-phical and climatic conditions or that yield should not dramatically vary when harvestedin different locations and at different times of the year. The key to an increased usage ofnatural products by industry is in the control of the above variables so that the endperformance by the industry remains consistent [227].

A. Tannins

Tannins are polyhydroxyphenols of vegetable origin, which are soluble in water, alcohols,and acetone and can coagulate proteins. They are obtained by extraction from wood,bark, leaves, and fruits. Other components of the extraction solutions are sugars, pectins


and other polymeric carbohydrates, amino acids, and other substances. The nontanninsubstances can reduce wood failure and decrease water resistance of glued bonds [176].The polymeric carbohydrates especially increase the viscosity of the extracts.

The basic structures of condensed or polyflavonoid tannins are [176]

in the A-ring: resorcinol, phloroglucinolin the B-ring: pyrogallol, catechol, and more rarely phenol.

Depending on the chemical structure of the A-rings two main types can be distin-guished:

resorcinol type: in mimosa/wattle, quebracho, Douglas fir, spruce tannin extractsphloroglucinol type (pine type): most pine species, e.g., Pinus radiata, Pinus patula,

Pinus elliotti, Pinus taeda, Pinus pinaster, Pinus halepensis, Douglas fir, andPinus echinata. Pinus brutia and Pinus ponderosa are mixed types with predo-minant resorcinol character.

The disadvantage of the phloroglucinol type is the distinct lower yield during extraction aswell as the much higher reactivity of the A-ring towards formaldehyde, which if uncon-trolled can cause extremely short pot lives of the glue mix.

The disadvantages of these polyphenols are the high viscosity of the solutions in therange of the concentrations of industrial application, due to the polymeric carbohydratesand high molecular weight tannins [228,229], and in some cases their short pot life. Themaximum usable concentration of tannin solutions is approximately 40% by mass, exceptfor mimosa where it can be as high as 50%. By selectively removing the polymeric carbo-hydrates the viscosity can be decreased and with this the possible concentration can beincreased. Such purification steps using an ultracentrifuge [229–233] and an acid precipita-tion followed by filtration or centrifugation have been described [185,234]. However, theyhave not yet been introduced in industrial practice; they are only available at laboratoryscale. A further possibility is the optimization of the conditions during the extraction inorder to minimize the content of nontannins in the extract.

The viscosity of tannin solutions usually increases at higher pHs [185,235,236], but forsome tannin types no clear dependence of the viscosity on the pH is shown. The viscosity ofan extract increases with the solids content, especially if carbohydrates are present from theextraction step. There are several ways to decrease the viscosity of tannin extracts:

Dilution (lower solids content): this leads to increased moisture content of the gluedparticles (which is not necessarily a disadvantage, since tannins need highmoisture contents of the glued particles to guarantee proper flow during press-ing) as well as to a decreased content of active adhesive [176].

Degradation of the high molecular carbohydrates, e.g., by NaOH [237,238].Addition of hydrogen bond breakers, e.g., urea [228,239,240].Modification of the extract by sulfite or bisulfite [241]: this modification of the

extracts will especially decrease the sometimes high viscosity to achieve abetter performance, but also a longer pot life and a better crosslinking will beachieved; however, it can give poor results if too high a level of sulfite is used.

Modification by treatment with acetic anhydride or maleic acid anhydride as well asNaOH to decrease the viscosity [228,242–244].

Tannins are used mostly in the southern hemisphere [176]; applications in Europe areonly for niche products with special properties. Depending on the resin content appliedto wood, tannins can be used for interior or exterior boards. The necessary crosslinking


is often done by addition of formaldehyde. This, however, can lead to some formalde-hyde emission, but this is low due to the phenolic nature of the tannin. Sometimescrosslinking is performed by the addition of isocyanate. Hardening by tannin autocon-densation without any aldehyde addition is also possible [245–247].

Tannins from mimosa, quebracho, and pine (Pinus radiata) are actually used on anindustrial scale for wood gluing. The extraction itself is only performed industrially inthe southern hemisphere. The tannins are produced by water extraction of the wood orof the bark. Suitable solvents are water, alcohols [248], or acetone. Some of the para-meters which influence tannin extraction are:

temperature [240,248–252]addition of various chemicals, e.g., NaOH or sodium carbonate [185,233,234,

248,249,253–261], sodium sulfite or bisulfite [241,248,250,262], and sulfite/bisulfite with sodium carbonate with or without urea [240]

duration of the extraction [240,258,260]concentration of the extraction solution: ratio of the amount of dry bark to the

amount of extraction solvent [260]properties of the raw material: wood species, age, time span between harvesting and

extraction, storage conditions, particle sizes [176].

Usually concentrated solutions or spray dried powders are sold [176]. A purification stepusually is not done at industrial scale level [176].

As tannins contain many ‘‘phenolic’’ type subunits one may be tempted to think thatthey will exhibit a similar reactivity potential to that of phenol and, therefore, proceduresused in standard PF production can be transferred to those containing tannin.This, however, is not the case; the real situation is that tannin is far more reactive thanunsubstituted phenol due to the resorcinol and phloroglucinol rings present in the tanninstructure [263,264]. This increase in hydroxyl substitution on the two aromatic ringsimparts an increase in reactivity to formaldehyde 10 to 50 times greater as comparedto simple phenol. This whilst initially sounding promising creates additional problemswith respect to producing an industrially applicable resin, due to limited pot lives of theready-to-use formulations [227], although these problems have been solved and solved welleven at industrial level [263,264].

Besides tannin autopolymerization, crosslinking usually is achieved via methyleneor other bridges in a polycondensation reaction with formaldehyde or isocyanates.Tannins react with formaldehyde similarly to phenol, whereby the nucleophilic sites ofthe A-ring are more reactive than those of the B-ring. Formaldehyde reacts with atannin in an exothermic reaction forming methylene bridges, especially between thereactive sites of the tannin A-rings. The reactive sites of the B-ring need a pH of at least10 [265,266] to react. However, at such a high pH the reactivity of the A-ring becomes sohigh that no useful pot lives of the glue mix are obtained any more. Due to their size andshape the tannin molecules become immobile already at rather low degrees of condensa-tion, so that formation of further methylene bridges is impeded or hindered, causing a lowdegree of hardening (crosslinking) [266]. The higher the molar mass of the tannin, the earlierthis effect occurs.

At neutral pH a rapid reaction of formaldehyde with the sites 6 and 8 on the A-ringtakes place. This leads to the advantage that no (high) alkaline pH as for the phenolicresins is necessary to achieve rapid gelling and that a neutral glue line is obtained. A minordisadvantage is the necessary exact adjustment of the pH, because the gelation time variesstrongly with the pH [266,267].


From a purely technological point of view the gel time may not be reduced below acertain limit. Decisive factors are the pot life, the viscosity of the tannin solution, and therate of the steam escaping from the mat and the board during hot pressing. One possibleway is the separate addition of the crosslinker, e.g., by dosing paraformaldehyde via a smallscrew conveyor directly to the particles in the blender. Also a liquid crosslinker, e.g., a urea–formaldehyde concentrate (UFC), can be mixed with the tannin solution in a static mixerjust prior to the blender. The higher viscosity of the tannin solution at higher pHs, evenwithout addition of the crosslinker, can be overcome by warming to 30–35�C or by addingwater. A higher moisture content of the glued particles is no disadvantage in tannin adhe-sives, on the contrary it helps to guarantee a proper flow of the tannin during hot pressing.

Possible crosslinkers are formaldehyde as aqueous solution [268], paraformaldehyde[263,265,267,269], UFC [270,271], UF resins [272], aqueous formaldehyde solution emul-sified in an oil [273], dimethylolurea [274] or urea and phenol methylols with longer chainsto overcome steric hindrance. Tannins can also be hardened by addition of hexamethy-lenetetramine (hexamine) [275], whereby these boards show a very low formaldehydeemission [269,275–281]. The autocatalytic hardening of tannins without any addition offormaldehyde or other aldehyde as crosslinker is possible, if alkaline SiO2 is present as acatalyst at high pH or just as a consequence of the catalysis of the reaction induced by alignocellulosic surface [282].

1. Application of Tannins as Adhesives

Themain parameter for the application of tannins as adhesives for wood-based panels is thecontent of reactive polyphenols and the reactivity of these components towards formalde-hyde. Tannins can be used as adhesives alone (with a formaldehyde component as cross-linker) or in combination with aminoplastic or phenolic resins. These resins can reactchemically with the tannin component in a polycondensation reaction, form only twointerpenetrating networks, or both. The simplest adhesive mix formulation consists ofthe tannin solution and powdered paraformaldehyde as crosslinker [283]. The additionof paraformaldehyde can cause in the short term a relatively high level of formaldehydeemission. Glue mixes using paraformaldehyde for the production of particleboards withlow formaldehyde emission are described and used industrially [284]. In the literature a largenumber of papers describe the combinations of tannins with synthetic resins (Table 14).

B. Lignins

Lignins are large three-dimensional polymers produced by all vascular terrestrial plants;they are second only to cellulose in natural abundance and are essentially the ‘‘naturalglue’’ that holds plant fibers together. Lignins are phenolic materials. They are primarilyobtained as a byproduct in wood pulping processes with estimates exceeding 75 milliontonnes per annum. Therefore great interest exists for possible applications. Lignins of verydifferent chemical composition and possible applications in the wood-based panels indus-try (adhesives, additive for part replacement of adhesives, raw material for syntheticresins) have been described in a large number of papers and patents. Research intolignin-based adhesives dates back more than 100 years with many separate examples ofresins involving lignin being cited. In reality, existing applications are very rare. Noindustrial use as a pure adhesive for wood is currently known despite the fact that con-siderable research has been directed toward producing wood adhesives from lignins. Bythemselves lignins offer no advantages in terms of chemical reactivity, product quality, or


Table 14 Combinations of Tannins and Synthetic Resins

Combination Description References

1. Tanninsþ aminoplastic reins

(a) UF resins UF resins with tannin molecules as

end groups

272,285

Addition of formaldehyde-rich

UF resins

228,272,274

TanninþUF resin mix 272,286,287

(b) UF–resorcinol–tannin Reaction of UF–methylols,

resorcinol, and tannin

288

UF resins with resorcinol end

groupsþ tannin

288

Reaction of UF–methylolþtannin followed by

addition of resorcinol

288

(c) Pine tanninþMF/MUF

resins

289

2. Tanninsþ phenolic resins

(a) Cocondensation of

tannins with phenol and

formaldehyde

Replacement of various amounts

of phenol of tannin extract

179,180,249,253,

254,290–295

(b) Tannins as hardening

accelerator for alkali

hardening PF resins

Addition of 10–20% 184,248,266,271,

296

(c) Low molar mass

polymethylolphenols (PMP)

The crosslinking molecules are of

greater size than formaldehyde

and can, therefore, bridge

better the gaps between

the reactive crosslinking sites

228,266,274,297,

298

(d) mixes of tannins and

PF resins; replacement

of phenolic resins by

tannins

Since the reaction of tannins with

PF–resols and formaldehyde is

accelerated strongly in the

alkaline region, the pot life is

reduced significantly, so PF resins

with low content of alkali are used

228,234,266,274,

287,289,297,

299–301

3. Tannins with enhanced

resorcinol content

(a) Opening of the

heterocyclic ring of

the tannin for warm-setting

resorcinol–tannin resins

Reduction of the necessary resorcinol

addition by modification with

sulfite (forming of resorcinol end

groups from the tannin molecule

by sulfite-induced cleavage of the

heterocyclic ring). No free methylol

groups present, addition of

formaldehyde as crosslinker is

necessary

302–304

(continued)


color when compared to conventional wood adhesives. The greatest disadvantages oflignins in their application as adhesives are (i) their low reactivity and, therefore, slowhardening compared to phenol due to the lower number of reactive sites in the molecule,causing increased press times, and (ii) the concern over the chemical variation of thefeedstock. The chemical structure of lignin is very complex with the added difficultythat unlike tannin the individual molecules are not fixed to any particular structure, there-fore no true generic molecule exists for lignin from softwood, hardwood, or cereals.Lignosulfonates can be added to synthetic glue resins as extenders (by partial replacementof resin). The partial replacement of phenol during the cooking procedure of PF resins hasno real industrial importance.

1. Use of Lignins as Adhesives Without Adding Other Synthetic Resins

The application of lignins as adhesives is, in principle, possible. Initial attempts neededvery long press times due to the low reactivity of lignin (Pedersen process) [317,318]. Thisprocess was a condensation under strong acidic conditions, which led to considerablecorrosion problems in the plant [318]. The particles are sprayed with spent sulfite liquor(pH 3 to 4) and pressed at 180�C. After this step the boards are tempered in an autoclaveunder pressure at 170–200�C, whereby the sulfite liquor becomes insoluble by splitting offwater and SO2. Shen [319–321], Shen and Fung [323] as well as Shen et al. [322,324]modified this process by spraying the particles with spent sulfite liquor containing sulfuricacid and pressing them at temperatures well above 210�C.

Nimz [317,325] describes the crosslinking of lignin after an oxidation of the phenolicring in the lignin molecule using H2O2 in the presence of a catalyst, especially SO2 [326].This leads to the formation of phenoxy radicals and with this to radical coupling (but notto a condensation reaction), whereby inter- and intramolecular C–C bonds are formed.

Table 14 Continued

Combination Description References

(b) Reaction of tannin with resorcinol Replacement of resorcinol in a

traditional PRF

305

(c) Cold-setting tannin–resorcinol

resins (TRF)

Forming of resorcinol by

intermolecular rearrangement

of the tannins

285,302,306,308

Replacement of resorcinol in a

traditional PRF

302,307

(d) Cold-setting, honeymoon,

separate application structural

exterior adhesives (with and

without resorcinol)

PRF/tannin and TRF/TRF

separate-application fast-set

glulam and fingerjointing

adhesives

309–311

4. Tanninsþ isocyanate (PMDI)

Isocyanate as crosslinker

for polyflavonoid tannins

Distinct amelioration of the

properties, partial reaction

of the isocyanate group with the

OH groups of the tannins; for

a sufficient hardening of the

tannin the addition of a

formaldehyde component

seems to be necessary

263,264,266,284,

289,312–316


This reaction does not necessarily need heat or acidic conditions, but is accelerated byhigher temperatures (maximum 70�C) as well as lower pHs. In this way the disadvantagesof the processes mentioned above (high press temperatures, long press times, use of strongacids) can be avoided [317,326]. An oxidative activation of the lignin also can be achievedby biochemical means, e.g., by adding enzymes (phenoloxidase laccase) to the spent sulfiteliquor, whereby a polymerization via a radical mechanism is initiated. The enzymes areobtained from nutrient solutions of white fungi [327]. The two-component adhesive isprepared by mixing the lignin with the enzyme solution (after filtration of the mycelium).At the beginning of the press cycle the enzyme still works, since it is stable up to atemperature of 65�C. If this temperature is surpassed, the enzyme is deactivated. Atsuch time, however, the number of quinone methides is already high enough to initiatea crosslinking reaction [86,87,327–334]. This system, however, is not capable of keeping upwith the demands of modern day panel hot press times. To achieve viability this problemwas solved by the addition of a smaller than usual amount of isocyanate adhesives. Theuse of an adhesive thus denies this system any advantage [335]; the enzymatic approachalone only achieves results and pressing times comparable to those of nonenzyme treatednonglued hardboard, a long-existing process and product.

VIII. THERMOPLASTIC WOOD ADHESIVES

A. Hot Melts

Hot melts are 100% solid thermoplastic compounds, which are compounded and appliedin the molten state at elevated temperature, the resultant properties being obtainedby cooling. Due to the quick cooling, bonds can be established in a very short time. Alsoa hot melt can be melted again, when already in the glue line. The advantages of hotmelts are:

100% solid, contain no organic solvents; no water or solvent to be evaporated; lowrequirements concerning working and environmental safety

easy to use, short set time allows high speed operation (up to 100m/min)rapid bond strength increasehigh bond strengtheffective bonding even of difficult-to-bond surfaces: polyethylene (PE), poly-

propylene (PP), varnishes, and otherscombination of flexibility and toughnessadhesion to a wide variety of substrates even without a primerhigh variability in formulation (color, viscosity during application, temperature, and

others)practically unlimited storage life, easy storageno time limitations in application, hence no pot life problemsno pollution of machinery and adherends, because of exactly metered applicationgood temperature control during application, easy to use in automated production

systems

The disadvantages of hot melts are:

cold flow: hot melts creep under mechanical load, even far below the meltingtemperature; bonds can open slowly, this effect being accelerated by highertemperatures


low heat resistance at elevated temperatures due to thermoplastic behavior; loss ofbond strength

sensitivity of certain substrates to the required application temperaturedegradation at elevated temperature (color, viscosity)

1. Composition of Hot Melts

Polymer. The polymer determines the properties of the hot melt. Variations arepossible in molar mass distribution and in chemical composition (copolymers). Thepolymer is the main component and backbone of the hot melt adhesive blend as itgives strength, cohesion, and mechanical properties (filmability, flexibility). Ethylenevinyl acetate (EVA) is the most used type (approximately 80%). It can be varied inviscosity (melt index) and content of acetate within a broad range of values, and oncehardened it presents a predominantly amorphous structure. The vinyl acetate groupsimpart good adhesion ability towards many materials. The low heat stability, however,limits its areas of application. With increasing content of the vinyl acetate comonomerthe adhesion ability, the wetting behavior, and the flexibility increase, but also the settingtime and the price. Heat resistance and cohesion properties become worse. The higherthe average molar mass of the polymer, the worse its wetting behavior, but the better thecohesion properties, the heat resistance and temperature resistance, and the higher themelting viscosity at a given temperature.

Ethylene–acrylic acid ester copolymers show high heat resistance and high elasticityat low temperatures. Amorphous poly-a-olefins (APAOs) are also used as the basic poly-mer and their main component monomer is propylene. They present better heat resistancethan EVA. APAO shows good adhesion properties to nonpolar surfaces, good flexibility,and a high resistance to temperature and moisture.

Polyamides give the fastest setting speed, good cohesion and very high heat resis-tance. They are oil and solvent resistant. Due to the narrow melting region (sharptransition between the elastic and plastic areas) a short setting time during cooling isallowed. Depending on their type the melting temperature is between 105 and 190�C.Advantages are the low melt viscosity, high bond strength, and a high green tack.Disadvantages are the high price and the susceptibility for carbonization at high tem-peratures in the presence of oxygen. Thermoplastic polyurethanes have no reactive iso-cyanate groups and cannot crosslink. Thermoplastic, linear, and saturated polyestersgive, depending on their chemical composition, hard or elastic and tacky bondlines.They have relatively high melt viscosities, and the bondlines are resistant against moist-ure, water, and ultraviolet (UV) radiation.

Tackifiers. Tackifiers usually are hydrocarbon resins (aliphatic C5, aromatic C9)or natural resins (polyterpenes, rosin and rosin derivatives, tall oil rosin ester). Theyimprove hot tack, wetting characteristics and open time, and enhance adhesion. Thecontent of tackifiers in a hot melt can be in the region of 10–25% of total material.

Other Components. Waxes increase the resistance against water and moisture(hydrophobization) and improve flow and lubricate during application. Inorganicfillers (CaCO3 and/or BaSO4) improve cohesion (small particle size) and adhesion,decrease sagging, and improve the price of the product. Pigments are also used, oftenin the case of white colored hot melts, the most common pigment being TiO2. Plasticizersdecrease the viscosity and the heat resistance; they ameliorate the wetting behavior andthe flexibility of the bondline; however, cold flow can occur. Stabilizers improve thethermooxidative behavior of the hot melt (heat and aging stability).


2. Curing Hot Melts

Curing hot melts are easily meltable polyurethane prepolymers (polyaddition of polyva-lent alcohols and isocyanate) with reactive isocyanate end groups (–N¼C¼O), whichreact with the moisture content of the wood under hardening. This leads to the formationof a crosslinked polyurethane network. Therefore, as thermoplasticity is no longer present,they cannot melt and are insoluble and show good mechanical and chemical resistance.During application a two-step bonding process takes place, the two steps running inparallel, but at different rates:

(i) quick physical solidification due to cooling: high green strength for furtherrapid processing

(ii) slower chemical hardening by crosslinking: the reaction of the free isocyanategroups is initiated by the moisture content of the surrounding air and of theadherend.

The advantages of the curing hot melts are:Higher resistance against heat, moisture and steam, good aging and long term

stability.Higher mechanical bond strength.Lower application temperatures: lower molar masses and lower softening and

melting temperatures. Processing of heat susceptible adherends, e.g., PVCfoils, is possible, for example at a processing temperature of 70�C. The heatresistance of the bondline is up to 120�C.

Good aging resistanceThe disadvantages of curing hot melts are:

They contain monomeric isocyanate, which is toxic, and thus working safety mustalways be taken into account.

They have stricter requirements concerning packaging and application, namelypreventing the access of water during storage and application is necessary.

They are expensive.

Two component curing hot melts consist, for example, of (i) polyamideþ epoxy, or (ii) apolyol componentþ isocyanate. After the mixing of the two components, they possess onlylimited pot life.

B. Poly(Vinyl Acetate) Adhesives

Poly(vinyl acetate) (PVAc) adhesives are another important type of thermoplastic adhe-sive, especially in furniture manufacturing and carpentry. They form the bondline in aphysical process by losing their water content to the two wooden adherends. PVAc adhe-sives are ready to use, have a short setting time, and give flexible and invisible joints. Theyare easy to clean and show long storage life. Limitations are their thermoplasticity andtheir creep behavior. Due to the manifold variations available (homo- or copolymerizationproducts, unmodified or modified, with or without plasticizers) PVAc adhesives show agreat variety of processing and bonding properties. The various formulations differ inviscosity, drying speed, color of the bondline, flexibility or brittleness, hardness or smooth-ness, and other characteristics.

The bonding priniciple of PVAc adhesives is based on the removal of the water bypenetration into the wood substrate or by evaporation to the surrounding air. The forming


of the bondline also requires the application of proper pressure. The final bond strength isreached after migration of the residual water away from the bondline. The minimumtemperature of film formation (or white point) is 4–18�C, depending on the type of theadhesive and the addition of plasticizers. This temperature is determined mainly by theglass transition temperature Tg of the polymer used which for PVAc is approximately28�C. Parameters influencing the drying time are the type of the adhesive, the type of woodsurface, the wood substrate absorption behavior, the wood moisture content, relativehumidity and temperature of the surrounding air, the amount of adhesive applied, andthe temperature of the adhesive and the wood surfaces.

Depending on the formulations, various grades of water resistance can be achieved.For the two-component PVAc adhesives, crosslinking and hence a thermosetting behavioris obtained by addition of hardening resins (e.g., based on formaldehyde), complex form-ing salts [based on chromium (Cr III), e.g., chromium nitrate, or aluminum (Al III), e.g.,aluminum nitrate] or isocyanate. The bondlines are then resistant against high tempera-tures and the influence of water.

The addition of comonomers during polymerization enables a higher flexibility to beobtained compared to PVAc homopolymers. This causes also a lower glass transitiontemperature and a lower minimum film formation temperature. Possible comonomersare acrylic acid esters (butylacrylate, 2-ethylhexylacrylate), dialkylfumarates, ethylene,and others.

Plasticizers soften the film and increase both adhesion and setting rate. The mostcommon are phthalates, adipates, and benzoates. The amount added can be in a broadrange of 10–50% by weight. They affect swelling and softening of the PVAc particles andhence ensure the film-forming capabilities at room temperature, the tack of the still wetand of the dried bondline, and a better water and moisture resistance of the bondline.Disadvantages are the lower resistance of the bondline against heat, possible migration ofthe plasticizers, and an enhanced cold flow.

Fillers (calcium carbonate, calcium sulfate, aluminum oxide, bentonites, wood flour)increase the solid content of the dispersion, and they are added up to 50%, based onPVAc. The purpose of their addition is the reduction of the penetration depth, a thixo-tropic behavior of the adhesive, gap filling properties, and the reduction of the adhesivecosts. Disadvantages can be the increase of the white point and possibly the more markedtool wear rate due to greater hardness of the adhesive. Other components in PVAc for-mulations are defoamers, stabilizers, filler dispersants, preservatives, thickeners (hydro-xyethylcellulose, carboxymethylcellulose), poly(vinyl alcohols), starch, wetting agents,tackifiers, solvents (alcohols, ketones, esters), flame retardants, and others.

The PVAc bond strength decreases at higher temperatures due to the thermoplasticbehavior of the adhesive itself. The higher the average molar mass of the polymer, thesmaller this temperature-dependent loss of strength. Under long term load, PVAc bon-dlines are susceptible to cold flow, especially when plasticizers are included in the formula-tion. Both effects limit the heat resistance of a PVAc bondline and generally the long termstrength under load at higher temperatures (>40�C) as well.

IX. INFLUENCE OF THE ADHESIVE ON THE BONDING PROCESSAND THE PROPERTIES OF WOOD PRODUCTS

For the production of wood-based panels various adhesives are in use, includingaminoplastic resins (UF, MUF, MUPF), phenolic resins (PF), and isocyanate (PMDI).


The proper choice of the adhesive depends on the required properties of the wood-basedpanels, on the working conditions during production as well as on the cost of the adhesivesystem. This does not only include the net price of the adhesive but also the overall cost ofthe gluing system including glue spread, capacity of the line (necessary press time), andother parameters (Table 15). Environmental aspects can also have a significant influenceon the choice of the adhesive system.

A. Viscosity

The viscosity of a glue mix is determined by the viscosity of the resin (mainly depending onthe degree of condensation and the resin solids content) and the composition of the gluemix. If the viscosity or the degree of condensation of the resin is too low, a large portion ofthe resin might penetrate into the wood, causing a starved glue line. In such a case no trueglue line can be formed and hence no bonding strength can be obtained. Conversely, at atoo high viscosity there might be a lack of proper wetting by the adhesive of the woodsurface opposite to that surface where the adhesive was applied, consequently with no orvery low penetration into the word surface and hence no mechanical interlocking of theadhesive into the substrate. Poor bond strength will also be obtained in such a case.

Besides the viscosity of the adhesive resin itself, the viscosity of the glue mix alsoplays an important role in the final result. A higher dilution of the resin gives a highervolume to be spread and with this a better distribution of the resin on the particles orfibers, and thus better bonding strength [337]. This also saves on costs.

B. Flow Behavior

The flowability of a resin depends on its viscosity and the solids content as well as thechanges in the viscosity at elevated temperatures in the hardening glue line. A low flow-ability causes poor penetration of the resin into the wood surface and low bondingstrengths. A too high flowability, on the other hand, leads to overpenetration of theresin into the wood and hence to starved glue lines. Flowability and hardening act againstone another during the hot press curing process.

C. Surface Tension and Wetting Behavior

Aqueous adhesive resins behave similarly to water regarding surface tension and wettingbehavior. For UF resins the wetting behavior strongly depends on their molecular

Table 15 Evaluation of the Three Adhesive Types UF, PF, and PMDI with

Regard to Various Parameters

Property UF PF PMDI

Price low medium high

Necessary hardening temperature low high low

Susceptibility to wood species high low low

Efficiency low medium to high high

Manipulation easy easy difficult

Resistance against boiling water no high high

Source: Ref. 336.


composition [24]. The higher the F/U molar ratio the lower the surface tension, which alsocan be decreased by adding a detergent [24] (a practice well known in other wood adhe-sives too, such as resorcinol cold sets [317]) or a few percent of a PVAc adhesive [24]. Theproper wetting of the wood surface is a precondition for achieving high adhesion strengthbetween the resin and the wood surface.

D. Reactivity

The objective of the development of adhesive resins is to achieve as high reactivities aspossible, while maintaining within acceptable limits other properties such as the storagestability of the resin or the pot life of the glue mix. The reactivities of the resin and of theglue mix are determined by various parameters:

type of resincomposition and preparation proceduretype and amount of hardenersadditives which might accelerate or retard the hardening processhardening temperature (press temperature, temperature in the glue line, temperature

in the core layer)properties of the wood surfaces.

E. Liquid and Powdered Resins

In the production of particleboards and MDF only liquid resins are used. In OSB produc-tion in Europe liquid resins are more often used, while rather in North America powderresins are used. The advantages and disadvantages of liquid and powder resins are sum-marized in Table 16.

F. Combination of Various Adhesives

For the purpose of obtaining special gluing effects and results, combinations of adhesivesor resins might be used, for example:

addition of PVAc to UF resins in order to obtain better wetting of the wood surface[24] and a more elastic glue line [338]

UF/MUFþPMDI (as accelerator, crosslinker and/or fortifier) [8,9,192,193]combination of adhesives in particleboard or OSB production: e.g., core layer of

PMDI and face layer of MU(P)F resin or PF resinproduction of an MUF resin by mixing a UF and an MF resin or a UF resin with an

MF powder resin.

X. ANALYSIS OF WOOD ADHESIVES BASED ON FORMALDEHYDECONDENSATION RESINS

There has been considerable progress in the characterization of formaldehyde condensa-tion resins in the past two decades. It is now possible to analyze the polydisperse nature ofthe resins as well as the individual structural elements in the resins, even semi quantita-tively. The curing reaction can also be monitored by means of adequate methods. Themain topics of analysis are: curing reaction and building up of bonding strength; evalua-


tion and monitoring of the degree of condensation and the molar mass distribution;analysis of the chemical composition of the resins and of their structural components.

The characterization of formaldehyde condensation resins was for several decadesonly possible with basic chemical methods, including elemental analysis [339]. The appli-cation of modern spectroscopic and chromatographic methods started as late as the 1970s.One of the reasons for this delay certainly is the fact that condensation resins themselvesare still systems that might change during their preparation for analysis or during theanalysis itself. Furthermore, the resins’ polar character as well as their relatively lowsolubility often render their analysis problematic. Notwithstanding this, the chemicaland structural composition of condensation resins is today well known. The validity ofeach analytical method (Table 17) can be compared and correlated with the informationderived from the resins’ technological behavior and from the properties of the wood panelsbonded using these resins.

Table 17 Overview of Various Analysis Methods for Formaldehyde Condensation

Adhesive Resins

Chemical tests:

purity of raw materials

content of free formaldehyde during resin preparation

and in the finished resins

content of formaldehyde in different forms in the resins (total formaldehyde, methylol groups)

content of urea and melamine

content of free and total alkali

determination of various molar ratios: F/U; F/(NH2)2; F/P; F/P/NaOH

Physical analysis:

spectroscopic methods: IR, 1H-NMR, 13C-NMR, 15N-NMR

thermal analysis methods for monitoring gelling and hardening processes:

DTA, DSC, TMA, DMTA, ABES

Physicochemical methods:

determination of the molar mass distribution and the average molar masses of the resins

[GPC/SEC, GPC-LALLS, vapor pressure osmometry

(VPO), light scattering, intrinsic viscosity]

chromatographic methods (HPLC, TLC) for the determination of low molar mass species and

residual monomers in the resins

Table 16 Advantages and Disadvantages of Liquid and Powder Resins

Type Advantages Disadvantages

Liquid resin Low costs Short storage stability

No dust-related problems OSB: higher resin load on wood

needed because of the poorer

adhesive distribution

Powder resin Lower resin load on wood and

better resin distribution on OSB strands

Higher price due to costs for spray

drying and packaging

Lower contamination of OSB resin

application blenders

Dust-related problems

Longer resin storage stability

Quicker gelling as no evaporation

of water is necessary


A. Laboratory Test Results

The properties of a resin which can be determined by simple test methods are shown inTable 18. The solids content of a resin usually is determined by the so-called dish method at120�C for 2 h [different times and sometimes lower temperatures (105�C) are often used asseveral variations of this method exist]. Even if it is a rather simple test, some deviations inthe results might occur because not only does all water present as solvent in the liquidadhesive resin evaporate, but also a further condensation reaction with further waterelimination takes place. Both liberate condensation water and this additional water isevaporated as well. The more severe the conditions during drying, the lower the solidscontent measured. Also some details of the test, such as the type of oven, the number ofdishes in the oven at the same time, or recirculation of air or not, can influence the results ofthe test. The refractive index can be used as a quick method for the determination of solidscontent, however, the correlation between these two characteristic resin values is sometimesrather poor and not the same for all resins. The density is only important when usingvolumetric adhesive dosing systems, but not as a quality parameter of the adhesive.

One of the most important characteristics is the reactivity of the adhesive resin. Withsome methods also the start and the end point of the gelling process, the duration of its timespan, the behavior of the resin during the test as well as the shape and strength of the gelledplug obtained are essential features of the gel time test. Gelling can occur within one or twoseconds (as is usual for UFs) or gelation can span ten or more seconds (as is usual formelamine fortified resins). A long gel time can indicate a slow generation of cohesive bond-ing strength in the actual application of the resin. The behavior of the resin in the test tube(e.g., foaming) and also the consistency and strength of the gelled plug can be evaluated.

The temperature used for the gel time test should always be adjusted to the tem-perature of application of the resin. If the maximum temperature in a glue line duringpressing is not higher than 70�C, then the gel test should be performed at such a tempera-ture and not at water boiling point. This is recommended in order to better interpret thebehavior of the resin or the resin glue mix under its conditions of industrial application.

B. Chemical Composition of Adhesive Resins

The various components and raw materials of the resins can be determined using differentchemical methods (Table 19). The content of total formaldehyde is accessible by hydrolysisof an aminoplastic resin; this process, however, is not possible for PF resins. Urea can bedetermined in the easiest way from the resin nitrogen content. However, other possible

Table 18 Basic Technological Tests

Property Test method description

Solids content Drying the sample for 2 h at 120�C; results can be influenced by the test

parameters

Viscosity Using a rotation viscometer or Ford cup (DIN cup)

pH Direct measurement using pH electrodes

Gel time and pot life Simplified method to determine the resin’s gel time

Gel time at 100�C or at 70�CPot life at 20�C or at 30�CB-time for PF resins at 100 to 140�C*

*Chapter 26, page 556.


sources of nitrogen have to be taken into account. Melamine is measured via a UV methodafter hydrolysis in dilute hydrochloric acid. The content of phenol and of the total form-aldehyde in PF resins can only be determined by NMR. Residual monomers such as freeformaldehyde, unreacted urea, and residual phenol or methanol as a residual product offormaldehyde production can be determined by various methods, e.g., free phenol via highperformance liquid chromatography (HPLC).

C. Structural Components

Using different spectroscopic methods such as infrared (IR), 1H-NMR, 13C-NMR, or15N-NMR, analysis of the adhesive structural compounds enables a deep insight intothe structural composition of resins. These results are the basis for correlations of resinstructural composition with their molar composition, their preparation procedure, and theproperties of the panels produced and hence to development and production of tailor-made resins. Extensive information is available on the basic nature of resins and on thecontent of the various structural elements, including, e.g., data concerning the type ofbridges between the monomers or the degree of branching.

D. Molecular Weight Distribution and Molar Mass Averages

The molecular weight distribution (MWD) can be determined by means of GPC [or sizeexclusion chromatography, (SEC)]. This method divides the molecules according to theirhydrodynamic volume, which is proportional to their molar mass. The most importantconsideration in the chromatography of formaldehyde condensation resins is the poorsolubility of the resins in most solvents usually used in GPC and hence the proper choiceof the solvent and the mobile phase. This choice influences the solubility of the resin, thebehavior of the chromatographic columns, and the effectiveness of detection. For lowermolar mass PF resins, tetrahydrofuran (THF) is a suitable solvent [128], while for highermolar mass phenolics and for MF resins, dimethylformamide (DMF) can be recom-mended, sometimes modified e.g., by addition of small amounts of ammonium formateor other salts such as LiCl [128,340]. UF resins are only soluble in DMF (withsome undissolved higher molar mass portions) and dimethylsulfoxide (DMSO). DMSOshortens the lifetime of the chromatographic columns and causes problems with highpressures because of its higher viscosity in comparison to other organic solvents and lowrefractive index increments [341]. The high reactivity of the functional groups of the resins

Table 19 Parameters to Be Determined in Adhesive Resins Analysis

Component urea

melamine

phenol

formaldehyde (total formaldehyde, methylol groups)

alkali (free alkali, total alkali, ash)

Analysis of residual monomers free formaldehyde

unreacted urea

free phenol

Molar ratios F/U for a straight UF resin

F/M for an MF resin

F/(NH2)2 for an MUF resin

F/P or F/P/NaOH for a PF resin


additionally requires the use of the correct solvent and mobile phase, especially concerningsample preparation, in order to obtain a satisfactory reproducibility of the results.

Another problem with GPC of condensation resins is the calibration of the columns.Because in the oligomeric and polymeric regions of the resins no compounds with a specialand singular molar mass and a clear molecular structure are available, similar or chemi-cally related substances have to be used as calibration standards. However, differences inthe hydrodynamic volumes even at the same molar mass cannot be excluded totally. Thisuncertain calibration of the columns also induces a great uncertainty in the calculation ofmolar mass averages on the basis of the chromatograms obtained.

Molar mass distributions of UF resins have been reported by several authors[22,125,340–345], as have mass distributions of MUF resins [71,346–348]. The molecularcharacterization of PF resins can also be performed without any major problems by GPC[128,134,139,349,350]. Due to newer GPC methods, modification of the PF resin beforethe analysis is no longer necessary.

Figure 3 shows chromatograms of two PF resins, one with a distinct high molecularweight portion, and the other with rather lower molar masses [128]. The averages of themolar mass can be (i) calculated from the gel chromatograms, taking into considerationthe above-mentioned problems with the calibration of the columns, and (ii) measured by

(a) vapor pressure osmometry for the number average molar mass (UF resins[19,22,341,344], MF resins [351], PF resins [121,352,353]) and

(b) light scattering for the weight average molar mass (UF resins [19,22,341], PFresins [354]).

The weight average molar mass at each elution volume can also be monitoreddirectly during each GPC run using GPC–LALLS. If the weight average molar mass inthis case is determined directly in the eluent by light scattering, no standard calibration of

Figure 3 GPC plot of two PF resins: (��) PF resin with a distinct high molecular weight portion;

(- - - -) PF resin with rather low molar masses. Column set: Merck HIBAR LiChrogel PS1þPS4þPS20þPS400. Solvent and mobile phase: THF. Detection: UV–VIS, 254 and 280 nm, respectively.

Concentration of samples: 1mg/ml. Flow rate: 0.5ml/min. (After Ref. 128.)


the column is necessary (GPC–LALLS). The eluent with the dissolved molecules passes alight scattering cell and the weight average molar mass is measured directly during eachchromatographic run. However, this method is laborious and, therefore, described only ina few cases in the literature (UF [18] Fig. 4; PF [109,142,355,356]). During each run twocurves are obtained: one is the concentration peak, and the other the light scattering peak,which is directly related to the actual molar mass average in the detection cell at eachmoment. Using these two curves, an individual calibration curve can be derived for eachrun. However, it must be taken into consideration that the light scattering signal can onlybe evaluated in the higher molar mass region and, therefore, the calibration curve is validwith sufficient accuracy only in this part of the chromatogram.

E. Monitoring of Gelling and Hardening

During gelling and hardening of the condensation resins in the hot press one can distin-guish between the chemical advancement of the condensation reaction during curing of thethermosetting resin (build up of the three-dimensional network) and the progressive devel-opment of the mechanical strength of the joint (increase in cohesive bond strength). Thetwo quantities do not progress at the same rate. The test methods that are used to followthe progression of the hardening of the resin are shown in Table 20.

The extent of chemical curing can be monitored using DTA and DSC. The exother-mic behavior of the curing process is then measured as a temperature difference or directlyas heat flow. Figure 5 shows a DSC plot of a PF resin [2]. The DSC run was done withpressure sealed capsules at a heating rate of 10�C/min.

Figure 4 GPC coupled with low angle laser light scattering (GPC–LALLS) of a UF resin:

e(V)¼ concentration signal; E(V)¼ normalized response of the LALLS detector; log

Mw(V)¼E(V)/e(V)¼measured weight average molar mass as a function of the elution volume V.

Column set Varian Toyo Soda TSK G4000 H8þ G3000 H8þ G2000 H8þ G1000 H8. Solvent and

mobile phase: 0.01m solution of LiBr in DMF. Temperature: 40�C, flow rate: 1.1ml/min, concen-

tration of samples: 10–15mg/ml. (After ref. 18.)


Table 20 Test Methods Used to Follow Building Up of Bonding Strength

Test method Description References

Differential

thermal

analysis (DTA)

Measures the difference in temperature between

two cells, these two cells are heated at a certain

heating rate; one ofthe two cells contains the

sample under investigation.

357–359

Differential

scanning

calorimetry (DSC)

Uses a similar type of instrument as DTA, but

measures directly the heat flow of the exothermic

and endothermic reactions occurring. The data

obtained that are of interest are: shape of the

curve, temperatures of the onset and the top

of an exothermic or an endothermic peak, slope

of the upcurve, width of the peak.

15,360–363

Differential

mechanical

analysis (DMA)

DMA uses a small sheet of glass fiber mats as a

substrate, which is impregnated with the resin.

This sample then undergoes periodic oscillations,

at the same time the sample is heated following

a special temperature program. The curing of the

resin leads to an increase in the strength of the

sample which then can be correlated with the

increase of the cohesive bonding strength.

364–367

Thermomechanical

analysis (TMA)

Similar to DMA but follows the adhesive

hardening in situ on the real wood substrate

(rather than on glass fiber). Thin wood strips

are used to sandwich a liquid glue line which

is then hardened. The curing of the resin leads

to an increase in the strength of the sample which

can then be correlated with the increase of the

cohesive bonding strength as well as with the

internal bond strength of wood particleboard

using the same adhesive. It has been used both

at constant heating rate and in isothermal mode.

368–377

Torsional

braid analysis

(TBA)

The damping behavior of the torsion of a glass

fiber probe impregnated with the resin is

characteristic for the increase of stiffness.

169,171,378

Automatic Bonding

Evaluation System

(ABES)

The ABES consists of a small press

and a tiny testing machine in a single unit.

It enables bonds to be formed under highly

controlled conditions; the joints that contain

the bonds which are to be measured are pressed

against heated blocks for a certain time,

cooled within a few seconds, and pulled immediately

thereafter in shear mode. Repetition of this procedure

at different curing times and temperatures

yields the points (a point for each specimen)

of a near-isothermal strength development curve.

96,97


During the curing of the resin the cohesive bonding strength develops step by step.Monitoring the effective strength increase (defined as the degree of mechanical curing)enables conclusions to be drawn about the suitability or not of a resin for a certainapplication. The best methods to use for this purpose are DMA (Fig. 6), TMA (Fig. 7)and ABES [96] (Fig. 1).

In the TMA plot in Fig. 7 it is possible to note the interactive nature of the substrateon the curing of the PF adhesive. For example, the modulus of elasticity (MOE) increasecurve shows two sections (and a two peak first derivative curve). This indicates formationof entanglement networks of the resin in wood which is not possible on noninteractivesubstrates such as glass as in Fig. 6. Of course DMA and TMA give equally good resultswhen used on the same wood substrate [379,380]. The ABES technique is also linearlycorrelated with TMA and DMA results as has been demonstrated by the linear relation-ship that has been found for both MUF and tannin–formaldehyde adhesives in the resultsof TMA and ABES [381].

In board manufacturing, when the press opens, a certain level of mechanical hard-ening and with this a certain bond strength is necessary to withstand the internal steampressure in the pressed board. The full chemical curing, however, can be attained outsideof the press during hot stacking. Advanced formation of the bond strength already at thesame degree of chemical curing will enable shorter press times and will, therefore, increasethe production capacity and reduce production costs. Plotting the chemical and mechan-ical degrees of curing in an x–y diagram shows the different hardening behaviors ofvarious resins; such a correlation plot of the degree of chemical cure (e.g., measured byDSC) and the increase of mechanical strength (e.g., measured by TMA, DMA, or ABES)can be regarded as a fingerprint of the curing behavior of a resin [154].

Figure 5 DSC plot of a PF resin. Pressure sealed capsules, heating rate 10�C/min. (From Ref. 2.)


Figure 6 DMA plot of a PF resin on glass fiber. Heating rate 10�C/min. (After Ref. 2.)

Figure 7 TMA plot of the curing of a PF resin on beech wood. Heating rate 10�C/min. Numbers in

the figure are temperatures in �C. (After ref. 370.) (*) MOE curve; (4) first derivative curve.


XI. WOOD AS AN INFLUENTIAL PARAMETER IN WOOD GLUING

The properties of wood-based panels are determined, in principle, by three parameters:wood, adhesive, and processing conditions. Only if all of these three parameters are correctand well balanced in the wood bonding process, can proper bonding results be achieved.The influence of the first parameter, wood, involves several factors. Bonded wood often isdescribed as a chain of several links: wood (substance), wood surface, interface betweenwood and adhesive, surface of the glue line (boundary layer), and glue line itself. As is truefor all such chains, the weakest link determines the strength of the chain, and in woodgluing this is in most cases the interface.

The strength of an adhesive bond depends on various parameters:

strength of the glue line and its behavior against stresses;influence of humidity, wood moisture content, and wood preservatives added;wood properties, which can influence the strength of the glue line and might cause

internal stresses; andmechanical properties of the wood material.

Hence wood, especially the wood surface and its interface with the bondline plays acrucial role in the quality of bonding and therefore the quality of the wood-based panels.Low or even no bonding strength can be caused by unfavorable properties of the woodsurface, e.g., low wettability.

A. Influence of Wood Species on the Properties of Wood-Based Panels

In the wood-based panels industry a great variety of wood species are used as raw materi-als. The choice of the wood species used is often determined just by the availability and theprice of the raw material. Furthermore, large amounts of wood residues from the primarywood processing industry (e.g., saw mill waste) as well as old (recycled) wood are used. Itis more than a proverb to say that the quality of a wood-based panel has already beenestablished, to a great extent, before the wood reaches the wood storage area of the panel-producing mill. The mills generally try to maintain as constant in time as possible thecomposition of the wood species mix as well as the mix of wood origins and preparationmodes for a certain board type. For various board types, different wood mixes (species,shape and size of the particles) are used. This is rather based on practical and empiricallong term experience and often not on any reasoned thinking. Economic reasons (avail-ability of special wood, price) can also play an important role in the choices made.

Many papers deal with special wood species in the production of wood-based panels,but the total knowledge available on this subject is not really satisfactory. Neusser andcoworkers [382,383] are two of the rare examples in the literature giving a broader over-view on this aspect: they have compared 18 different Austrian wood species by producingand testing laboratory particleboards. The test results obtained allowed adjustments forproperties and density of laboratory boards. The best results were found for ash, followedby white beech and oak. However, these results may not be valid for all types of wood andall types of boards.

B. Wood Particle Size and Shape Before Pressing

The strength of a bond in a wood panel increases with the value of wood density for therange of approximately 0.7 to 0.8 g/cm3. Above this density a decrease of the bond


strength occurs. The performance and properties of wood-based panels are strongly influ-enced by the properties of the wood used. Thus, wood anisotropy as well as its hetero-geneous nature, the variability of its properties, and its hygroscopicity have to be takeninto account in all bonding processes. Equally, the orientation of the wood fibers and thegrain angle in bonding solid wood have to be considered.

Particles as raw material for particleboards show a great variety in wood species,origin, method of preparation, age, and especially size and shape. If wood is ground intoparticles, a mixture of particles of very different sizes and shapes is always obtained.Particles can be described in a simple way as squared flat pieces with certain values forlength l (mm), width b (mm), thickness d (mm), and slenderness ratio s¼ l/d. The volumeof a particle is then given as

V ¼ lbd ðmm3Þ

Considering particles with l� d, the effective gluing surface area is

F ¼ 2lb ðmm2Þ

The area form factor [384] can be considered as measure of the effective gluing surface areabased on the volume. It is inversely proportional to the thickness of the particles:

F

V¼ 2

d¼ 2s

l

The influence of particle size and shape on mechanical and hygroscopic properties ofboards is well described in several papers in the literature [385–390]. The central statementof these papers is an increase of bending strength, and compression and tension strengthin the board plane, but a decrease of internal bond strength with increasing particle length.In particleboards the particles overlap, and thus the overlapping areas must be largeenough to guarantee the transmission of the wood strength to the strength of the wholeassembly.

C. Chemical Composition of Wood

Extractives contained in wood can influence the gluing process in the physical as well aschemical sense. Several authors [391–393] have indicated that the chemical composition ofa wood surface after processing might be different due to the concentration on it of polarand apolar substances coming from the wood itself. Even the fiber direction of the woodsurface (longitudinal, radial, tangential) can influence this composition. Extractives solu-ble in water or steam can migrate during the drying process to the wood surface and candecrease its wettability. In particular fatty substances and waxes might cover the woodsurface. As a consequence of this, chemical weak boundary layers (CWBLs) are formed[394,395]. A chemical-induced effect can also occur if the wood extractives have a strongacidic or alkaline behavior. This might cause acceleration or retardation of the hardeningprocess of the adhesives based on polycondensation resins.

Different wood species can show great differences in pH as well as in the bufferingcapacity. Even within a single wood species differences might occur due to seasonalvariations, position of origin within the tree log, pH of the soil, age of the tree, timespan after cutting, and drying and processing parameters.


D. Wood Surfaces

The wood surface is a complex and heterogeneous mixture of polymeric substances such ascellulose, hemicellulose, and lignin. It is also influenced by factors such as polymer mor-phology, wood extractives, and processing parameters. During the processing of wood andthe generation of new surfaces, damage to the wood material and to the surface can occur,which might cause low quality bonding and low bond strengths. This often shows as lowpercent wood failure or only as a thin fiber layer. The reason for this can be a mechanicaldestruction of the uppermost wood layer, usually described as a mechanical weak bound-ary layer (MWBL) [396–398]. This layer consists of damaged wood cells caused by proces-sing. A fracture of a bond at the interface between the wood and the adhesive can becaused by a cohesive fracture of such a weak boundary layer [399] or by a real adhesionfailure at the interface [396,400].

1. Contact Angles of Wood Surfaces

A precondition for the gluing of two wood surfaces is the wetting of these surfaces by theliquid or liquified adhesive. Wetting here includes the value for the contact angle, thespreading of the liquid on the surface, and the partial penetration of the liquid intothe porous adherend. Good wetting enables the creation of high adhesion forces betweenthe wood surface and the adhesive. However, direct correlations between the contact angleand the bonding strength achieved are rather rare [401] or seem not to be universal [402].Low contact angles (�<45�) indicate good wetting behavior. Contact angles greater than90� lead to incomplete wetting, which might cause low bond strengths.

The main parameters that influence the surface tension of the adhesive, when on thesubstrate, and therefore the possible bond strengths are:

wood species [24–26,403]roughness of the surface [404–406]cutting direction (radial/tangential) [24–26]earlywood, latewood [24–26,407,408]direction of the spreading of the droplet during measurement of the contact angle

(along or lateral to the direction of the fibers) [409]wood moisture content [410–412]fiber angle [413]age of the wood surface [414,415]pH of the wood surface [416–418]type and amount of wood extractives [401,419,420]pretreatment of the surface, e.g., by extraction with various solvents [421]type of adhesive: UF resins [24–26]; PF resins [406–408]

During the production of wood-based panels a certain portion of the adhesivepenetrates into the wood surface. An overpenetration causes starved glue lines, whereastoo low a penetration limits the contact surface between the wood and the adhesive; lowpenetration often is the consequence of a poor wetting behavior.

2. Modifications of the Wood Surface

Modifications of the wood surface can be implemented using various physical, mechanical,and chemical treatments. Chemical treatments are performed in particular to enhancedimensional stability of the panel, but also to improve physical and mechanical properties


or to yield a higher resistance against physical, chemical, and biological degradation. Torender the wood substrate hydrophobic, e.g., by acetylation, decreases the number ofhydrophilic sites [422]. The OH groups of the cellulose react with acetic anhydride formingan ester. The hygroscopicity of the wood substrate decreases, and hence swelling andshrinking of the panel can be lowered [423]. Use of acetylated fibers for the productionof MDF boards showed marked reduction in their thickness swelling [424,425]. It has alsobeen reported that wood acetylation can yield reactions of the anhydride with the aromaticring of the lignin, although the exact reaction paths are not known [426]. This chemicalattack at the aromatic rings can yield some crosslinking of the constituents of the woodsubstrate and can, therefore, contribute to the improved wood dimensional stability.

3. Seasonal Variations of Wood Quality in the Wood Panelboard Industry

Hanetho [427] has discussed the experiences of the particleboard industry regarding theinfluence of seasonal variations of the wood quality. Some problems do occur using woodthat has been harvested in the winter time and which goes into board production immedi-ately. When these logs or chips have been stored for some time, these problems disappear.The contact angles of water and adhesive on wood are higher in the case of freshly har-vested wood compared to stored chips. This means that the surface of the wood particlesobtained from such a fresh wood is more hydrophobic, influencing negatively the wettingand penetration and thus the substrate gluability. It has been determined that the reason forthe lower wettability of freshly harvested wood is the higher content of wood extractives.These results, however, must not be confused with the better wettability of a freshly pre-pared surface, independently of whether it is freshly harvested or stored wood.

Hydrophobic wood extractives and components oxidize or polymerize duringstorage after harvesting, as also can be seen from their lower extractibility [428].Because of this effect the ability of wood extractives to migrate to a new surface isalso reduced. Figure 8 shows this effect by plotting contact angles versus time after the

Figure 8 Contact angles of a UF resin on the surfaces of wood particles, as a function of the contact

time, hence the time elapsed after the application of the droplet. The surfaces have been cut from a

freshly harvested log and from a log stored for 3 months. (After ref. 428.) Water extracts from the

particles made from freshly harvested wood have higher pH values, but lower buffer capacities than

the surfaces made from stored chips. The lower buffer capacity might lead to prehardening if the usual

amount of hardener is used, with a consequent decrease of the board strength.


application of the urea resin droplet onto the surfaces of freshly harvested wood andstored wood.

XII. PROCESSING CONDITIONS DURING PRODUCTION ASPARAMETERS INFLUENCING WOOD GLUING

A. Adhesive Consumption and Glue Spread in the Production ofParticleboards

Several aspects regarding the proportion of adhesive to be used in the production ofparticleboards must be evaluated to obtain good results:

proportion of adhesive on individual particlesproportion of adhesive in particle mixtures and fractionsproportion of adhesive in the total particle mixdistribution of the adhesive on the surface of the particles, and proportion of the

particles’ surface area covered with adhesive.

The resin load content on wood as a measure of the consumption of adhesive is oneof the more important parameters to consider during the production of particleboards.From a technological standpoint a certain minimum amount of resin is necessary to obtainthe desired properties of the boards resulting in sufficient bonding of the individual par-ticles. However, an excessive resin load imparts some technological disadvantages, such ashigh moisture content and hence possible problems with high vapor pressure during hotpressing. Furthermore, for economical reasons, the consumption of adhesive should be aslow as possible as the resin contributes considerably to the costs of the finished boards.The resin load, however, is only an overall average on the total mixture of particles,without considering differences in particle size distribution and the shape of the individualparticles. Moreover, the resin load gives no direct indication of the area-specific consump-tion of the adhesive, which is the amount of resin solids content based on the surface areaof the particles. The expression ‘‘resin-robbing by the fines’’ is well known and describesthe exceedingly high consumption of adhesive based on mass of particles owing to thegreat surface area of the fine particles [429,430].

The resin load on wood chips can be described in the following two ways:

mass resin load (percent or grams of resin solids content per 100 g dry particles) andsurface-specific resin load (grams of resin solids content per square meter of

surface area).

If one of these two terms is known, the other can be calculated assuming a uniformdistribution of the resin on the particle surfaces and estimating the total surface area of theparticles.

In the production of particleboards mixtures of particles are always used as rawmaterial, and thus the particles differ in size and shape. A size grading of the particles canbe performed by sieving, where two of the three dimensions of the particle must be smallerthan the standard measure of the actual sieve mesh to be passed. An exact sieving of theparticles according to their size, therefore, is only possible for particles of rather similarshapes. Particles can differ widely in shape. A simplification to describe their shape is toassume that they are squared, flat with length l, width b, and thickness d for medium andcoarse particles and rather cubic for the fines. Since the sieve mesh is usually graduatedaccording to a logarithmic scale, for the theoretical calculations of the particle size


distribution this was also assumed to be logarithmic and similar to a gaussian distribution.Distributions on an industrial scale might differ from this model.

Each particle fraction has a certain relation to its resin load according to the size ofthe particles. Because of the great surface area of the fine particles their resin load increasesstrongly (linearly with the term d�1). Even if there is only a small proportion of a massfraction of very fine particles in the mixture, the high consumption of resin solids contentof this fraction has a negative impact on the resin load of the coarse particles.Figure 9 shows an example of a particle size distribution with the calculated mass resinloads and the distribution of the resin solids content on the different fractions ofthe particle size distribution. Particle length was assumed to vary from 25mm for thecoarsest particles to 0.6mm for wood dust, according to experience with industrial particlemixtures.

Because of the reasons discussed above, usually core layers and face layers are gluedseparately. In the core layer rather coarse particles predominate and in the face layerrather fine particles predominate. This separate gluing enables the use of different com-positions of the glue mixes (e.g., different addition of water and hardener) and differentresin loads (gluing factors) for the two layer types. An example of separate gluing is shownin Fig. 10, with separate gluing of the core layer CL (6.5% mass gluing factor) and in theface layer FL (11.0% mass gluing factor). The mass ratio of the layers CL:FL is 60:40.Figure 10 shows the particle size distributions and the mass gluing factors of the individualparticle size fractions for this example of separate gluing.

Samples of industrial core layer and face layer particles, before and after gluing, canbe fractionated by sieving, and thus sampling has to be done at the same time before andafter blending. In the case of aminoplastic adhesives each particle fraction, glued or not,can be investigated for its nitrogen content. By knowing (i) the content of nitrogen as wellas the resin solids content in the glue mix and (ii) the moisture content of the particlesglued or not in the various fractions, the mass gluing factor of each glued particle sizefraction can be calculated. Figure 11 shows the results of one such calculation for gluedcore layer and face layer particles. Even if the absolute values may differ from the calcu-lated ones, the resin load (by weight) and the particle size show that the same shape ofdistribution curve is obtained.

Figure 9 Example of a particle size distribution, the calculated mass resin load (gluing factor), and

the distribution of the resin solids content. The overall adhesive resin consumption was assumed to

be 8% resin solids content/dry wood. (After ref. 429.)


Assuming that the gluing of particles of different sizes is performed randomly withtheir surface area as the decisive parameter, for various homogeneous particle size frac-tions and for different particle size mixtures the theoretical mass gluing factors and thedistribution of the resin solids content can be calculated and correlated with the samevalues obtained experimentally, by analysis. There are some indications [431–433], how-ever, that glue distribution is not exclusively influenced by the surface area of the particles,but has a certain preference for coarser particles. This may be due to the effectiveness ofthe adhesive application, thus to the separation and distribution of resin droplets, or to themixing action in the blender after application of the resin on the wood particles (wipingeffect). The concept that the particle surface area exclusively influences gluing is quiteclearly invalid, if glue droplets and the surface to be glued have similar size. Meineckeand Klauditz [431] mentioned diameters of glue droplets of 8 to 110 mm, depending on thetype of spraying and Lehmann [434] mentioned up to 200 mm. The latter values are of thesame order of magnitude as the size of the finest particles used for the calculations above.

Besides the surface area of the particles several other parameters also have someinfluence on the necessary resin consumption, e.g., type of boards, thickness of the sanding

Figure 10 Particle size distribution and mass gluing factor of the individual particle size fractions

for the separate gluing example ‘‘CLþFL’’. The resin consumption was assumed to be 6.5% resin

solids content/dry wood in the core layer (CL) and 11.0% in the face layer (FL). The mass propor-

tions are CL:FL¼ 60:40. (After ref. 429.)

Figure 11 Fractionated mass gluing factors of industrially glued core layer particles. The mass

gluing factor during blending was 9.5% resin solids/dry particles. (After ref. 429.)


zone, type and capacity of the blenders, separation and spraying of the resin (depending onif only the wiping/spreading effect occurs during blending or if instead spraying of theresin is used), shape of the particles for the same particle sizes, dependence of the slender-ness ratio on particle length, concentration and viscosity of the glue resin, or a partial sizedegradation of the coarser particles in the blender.

New strategies in blending take into account the reality of the higher resin con-sumption by the finer particles, e.g., by removing the dust and the finest particles fromthe particle mix before blending. Also an exact screening and classifying of the particlesbefore blending can improve the distribution of the resin on the particle surfaces and canhelp to spare some resin. A lower consumption of resin not only means lower costs forthe raw materials, but also helps to avoid various technological disadvantages. With theresin, water is also applied to the particles; as long as this amount of water is lowenough, especially in the core layer, no problem should occur with a too high vaporpressure during hot pressing. Often, however, the moisture content of the glued coreparticles is too high, due to an excessive gluing factor. The high vapor pressure in theboard at the end of the press cycle tends to expand the fresh board; if venting is notdone very carefully, blistering of the boards at the end of the continuous press or afterthe opening of the press might occur. Additionally, the heat transfer by the steam shockcan be delayed if the vapor pressure difference betweeen the face layer and the core layeris small. If the moisture content of the glued core layer particles is high, the moisture inthe glued face layer particles must be reduced. Also spraying water onto the belt beforethe forming station and onto the surface of the formed mat cannot be done due to theproblems with the too high moisture content in the mat and hence with the too highvapor pressure.

Gluing of particles is usually done in quickly rotating blenders by spraying theresin mix into the blender. Due to the rotation of the blender a partial degradation inthe size of the particles can occur. While blending OSB strands this degradation must beavoided; this is done by using slowly rotating big blender drums with a diameter ofapproximately 3m. The liquid adhesive is distributed by several atomizers in this blenderdrum.

Gluing of fibers in MDF production is usually done in the so-called blowlinebetween the refiner and the dryer. The advantage of this method is that it avoids resinspots at the surface of the board. The disadvantage, however, is the fact that the resinpasses the dryer and can suffer part precuring. This causes some loss of usable resin(approximately 0.5 to 2% in absolute figures); therefore the glue consumption in blowlineblending is higher than in the mechanical blending. Due to this fact mechanical blendershave lately been installed again in a few factories. The theory of turbulent flow blowline-gluing is not yet clearly defined [435,436]. However, some equations attempting to describeit have been recently presented [436].

B. Wood Moisture Content

The wood moisture content influences several important processes such as wetting, flow ofthe adhesive, penetration into the wood surface, and hardening of the adhesive in thegluing and production of wood-based panels. In bonding solid wood usually a woodmoisture content of 6 to 14% is seen as optimal. Lower wood moisture contents cancause a quick dryout of the glue spread due to a strong absorption of the water intothe wood surface as well as wetting problems. High moisture contents can lead to ahigh flow and an enhanced penetration into the wood, causing starved glue lines.


Additionally a high steam pressure can be generated which might give problems of blister-ing when the press opens or at the end of the continuous press. Also the hardening of acondensation resin might be retarded or even hindered.

During the hot press cycle of the particleboard or MDF production, quick changesof temperature, moisture content, and steam pressure occur. The gradients of temperatureand moisture content determine significantly the hardening rate of the resin and hence theboard properties. These gradients together with the mechanical pressure applied to densifythe mat are decisive for generating the density profile and hence for the applicationproperties and performance of the boards. The higher the moisture content of the gluedface layer particles, the steeper the moisture gradient between the surface and the core ofthe mat and the quicker the heating up of the mat occurs. In the fiber mat in MDFproduction no differences are seen in the moisture content of the outer layer and theinner layer due to the temperature applied to the mat, nevertheless a vapor pressuregradient occurs.

The moisture content of the glued particles is the sum of the wood moisturecontent and the water that is part of the applied glue mix. Therefore, the moisture contentof the glued particles mainly depends on the gluing factor. Usual moisture contentsof glued particles are: (a) for UF, 6.5–8.5% in the core layer and 10–13% in the facelayer; (b) for PF, 11–14% in the core layer and 14–18% in the face layer. The optimalmoisture content of the glued and dried MDF fibers in the mat before the press is inthe region of 9–11%. The higher the moisture content of particles, the easier the facelayer can be densified at the start of the press cycle; this leads to a lower density in thecore layer.

Blistering at the end of the press cycle or at the end of the continuous press occurs ifthe steam pressure within the fresh, and still hot, board exceeds the internal bond strengthof the board. It should be noted that the bond strengths at higher temperatures are alwayslower than after cooling the board. If blistering occurs using resins with low formaldehydecontent, press time should be shortened instead of prolonged, because a longer press timewould not increase the bond strength but certainly would increase the steam pressure inthe board. Careful venting as well as decreasing the moisture content of the glued particlesand reducing the press temperature will help.

C. Press Cycle

During the hot press cycle the hardening of the resin and possible reactions of the adhesivewith the wood substance take place. The influential parameters are especially the presstemperature and the moisture content in the mat. Additional parameters are the wooddensity, porosity, swelling and shrinking behavior of the wood, structure at the surface,and wetting behavior. During the press cycle several processes take place:

transport of heat and moisturedensification, increasing internal stresses, followed by relaxation processesadhesion between the particles or fibersincrease of the bond strength in the glue line (cohesion).

Models describing what occurs in a panel during hot pressing have been published[437–443]. These take into consideration various conditions occurring during the hotpress cycle such as heat transfer, temperature gradients, moisture content, steam pressure,bond strengths, and presence or absence of postcuring [437–443].


Tables 21 and 22 summarize the usual press strategies for the production ofparticleboards and MDF. The warming up of the mat is performed by the so-calledsteam shock effect [442–447]. The precondition for this is the high permeability to steamand gases of the particle or fiber mat [442,443,448,449]. High moisture contents of the facelayers and spraying of water on the surface layers sustain this effect. The press temperatureinfluences the possible press time and by this the capacity of the production line.The minimum press time has to guarantee that the bond strength of the still hot boardcan withstand the internal steam pressure as well as the elastic springback in boardthickness at press opening.

XIII. CONCLUSIONS

Wood is a very complex material. Wood adhesives technology is an advanced sciencewhich blends the technology of adhesive preparation and formulation with a multitudeof advanced application technologies to different wood products. In many fields otherthan wood, good bonding depends mainly on the use of a good adhesive. The situation isnot as straightforward in wood gluing: in general one can obtain excellent wood panelswhen using a decidedly poor adhesive if the parameters governing the technology ofmanufacture of the wood product are well mastered. This indicates the extent to whicha high level application technology can play a predominant role in this field. This is not

Table 21 Press Strategy for Production of Particleboards

Different particle structures: coarser in the core, finer in the face layer.

Press temperature:

As high as possible, to enable a quick heating up of the core layer

due to an optimal steam shock effect. In continuous lines press temperatures

decrease from the entrance to the outlet of the press. In the last zone of the press

even active cooling in a few cases is possible (decreasing steam pressure in the core layer).

Moisture content of the glued particles:

Core layer as dry as possible (ca. 6–7% in the case of UF resins), face layer as high as

possible (11–14%, depending on the proportion of the face layer in the board). Too high

a moisture content can cause blistering.

Spraying of water onto both surfaces in order to enhance the steam shock, amount

ca. 20–40 g/m2.

Press pressure profile:

The variation of pressure during hot pressing can follow different sequences. Quick

densification with pressure maximum to enable a high density of the face layer and

hence high modulus of elasticity (MOE). Sometimes a second densification step is used.

Table 22 Press Strategy for Production of MDF

Despite the uniform fiber material, a certain density profile is created due to the action of

heat and compression.

Two-step pressure profile with quick densification at the start of the hot press cycle and

a second densification step for the inner layer.

Uniform moisture content of the glued and dried fibers across the thickness of the

mat. Higher moisture content in the outer layer would require a three-layer mat or

spraying of water.


valid for all wood products. Of course, good results are better or easier to obtain if oneuses an excellent adhesive. However, just the use of a good adhesive gives no assurance ofgood bonding in this field. It is the essential interaction of the equally important adhesiveand its application technology that this chapter has tried to describe. It is exactly thisinteraction that is so important in a field that comprises more than 60% by volume of allthe adhesives used today in the world for any application. Without mastering this inter-action between adhesive technology and wood product manufacturing technology therecannot be wood bonding of any consequence.

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Adhesives in Wood Industry