Composite boards from isocyanate bonded pine needles

Composite Boards from Isocyanate Bonded Pine Needles

M. Gupta, Monika Chauhan, Naseeba Khatoon, B. Singh

Polymers, Plastics and Composites Division, Central Building Research Institute, A constituent Establishment ofCSIR, Roorkee 247 667, Uttarakhand, India

Received 27 August 2009; accepted 25 April 2010DOI 10.1002/app.32703Published online 14 July 2010 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The effect of alkali treatment on the wett-ability of pine needles was assessed by contact anglemeasurements. The lower critical surface energy of thetreated needle (42%) favors its superior wetting behaviorover the control. The composite boards were preparedfrom these treated needle furnishes and isocyanate prepol-ymer resin adhesive (3–20%). The results indicate that in-ternal bond strength, modulus of rupture, tensile strength,and screw withdrawal load increased with the increase ofresin adhesive content. At lower humidity, the boardswith varying resin adhesive contents exhibited 2–7% thick-ness swelling at equilibrium moisture content whereas athigher humidity, the thickness swelling in the boardsranged between 13–23%. Under immersed water, the com-posite boards swelled 2–3 times as much as the samples

exposed at 98% RH. After aging, the internal bondstrength of boards was reduced by 41–67% in acceleratedwater and 54–78% in cyclic exposure respectively. Fracto-graphic evidences such as pulled out of needle fibers, fiberfracture and debonding, because of swelling of fibers inthe aged samples could be used to explain the loss ofstrength. The screw withdrawal load of the high resin ad-hesive content boards (20%) was comparable with the nat-ural wood. The developed composite board satisfies therequirements of Standard Specification: IS: 3087-2005 / EN312-2003. VC 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118:3477–3489, 2010

Key words: pine needles; isocyanate prepolymer;adhesion; swelling; microstructure

INTRODUCTION

In recent years, use of nonwoody materials such asagricultural residues and forest wastes for makingpanel products1–8 has become more attractive becauseof ligno-cellulosic similarity to wood particles andalso limited wood supply. Several attempts have beenmade on bonding of these lignocellulosics with com-monly used formaldehyde based condensates2,5,6 andisocyanate resin adhesive2–4 to produce specificationgrade particle/ fiber boards. It has been shown thatwide variations in the nature and density of agro/for-est residues, moisture content of flakes/chips andresin binder flow, and penetration and spread ratesignificantly affect the properties of boards. Theboards made from isocyanate adhesives exhibitedimproved porosity and water resistance to thosebonded with either urea or phenolic resins.9 However,the main limitation of isocyanate bonded boards istheir tendency to adhere to the caul plates and pressparts.10,11 This adhesion can damage the boards andsubsequent clean-up of the contaminated platens isdifficult and time-consuming. This situation necessi-tates the use of a self-releasing type of isocyanatebased binders during pressing operation to eliminate

their sticking from platens and produce boards of ac-ceptable properties such as surface heterogeneity,strength, and dimensional stability.In earlier studies,12–16 utilization of pine needles

has been reported as a principal raw material in themanufacturing of particle boards, hard boards, andbiocomposites because of their renewableness andhuge availability in the Western part of Himalayas. Inthese, it was shown that boards fail to meet the per-formance requirements mentioned in Code/Specifica-tion set forth by National Organizations.13–15 Becauseof waxy coating, needle furnish does not present aneasy surface for bonding of adhesives such as ureaformaldehyde, phenol formaldehyde, and urea resin-isocyanate resin blend. The resin coverage on the inte-rior of needle will be nonexistent causing weak inter-nal bond strength and high-thickness swelling of theresultant boards. In the case of urea resin-isocyanateblend,17 rapid reaction between the isocyanate groupsand water takes place, because of alkaline propertiesof urea. This results in a very narrow processing lati-tude and poor stability of binder. Improvement inthese properties of boards could be achieved by pre-treatment of pine needles and also by using theirblend with other fibers such as bamboo, wood resi-dues, and other ligno cellulosics. Pine needles (40 wt%) bonded with magnesium oxychloride cement wasalso reported for use as a thermal insulation boards(density <0.4 g cm�3).18 In previous work, isocyanatebased binders have not received much attention for

Correspondence to: B. Singh ([email protected]).

Journal ofAppliedPolymerScience,Vol. 118, 3477–3489 (2010)VC 2010 Wiley Periodicals, Inc.

bonding of the pine needle furnish. Therefore, anunderstanding on this binder will be required interms of tolerance of high-flake moisture content, sur-face absorptivity, low-resin application, press precuretime to maintain superior performance, and quality ofthe panel products.

In this study, we aimed at discussing the use ofprepolymer based on a diphenyl methane diisocya-nate (MDI) as a binder in the pine needle boards.The effectiveness of this binder is expected mainly,because of its high cross-linking and increased plas-ticity (better spreading rate on the surface of needlefurnish), which results in improved strength and du-rability of the boards. Using isocyanate prepolymernot only to provide superior adhesive characteristicsbut also minimizing unwanted adhesion to the plat-ens used in the board manufacturing when com-pared with conventional polyisocyanates. This couldhelp to increase usefulness of pine needles as a con-stituent of panel products to be used as wood substi-tutes in buildings. In this article, we report the prop-erties of composite boards made with variouspercentage of isocyanate prepolymer bonding agent.Dimensional changes in the boards are recorded as afunction of humidity and immersed water condi-tions. The internal bond strength of boards is dis-cussed in terms of their cohesive microstructuralfailure under both aged and unaged conditions. Thesuitability of composite boards is also assessed withrequirements of the existing Standard Specification.

EXPERIMENTAL

Materials

The pine needles of 300–380 mm length were collectedfrom the Indian forests (density, 0.22 g cm�3; moisturecontent, � 20%; water absorption, � 45%). The needleswere comprised of cellulose (40–43%), hemicellulose(20–24%), lignin (36–40%), extractives (25–28%), andash content (2–4%). Aromatic polyisocyanate prepoly-mer was obtained from M/s Bayer Material Science,Germany (Desmodur E 23, NCO content, 15.4 6 0.4%;viscosity, 1800 6 250 mPa.s; density, 1.13 g cm�3). Thepull-off adhesive strength of this prepolymer is 0.61MPa when tested as per ASTM D 4541. Under peel ad-hesion, the lattice squares are intact satisfying the 5 Bcriteria of ASTM D 3359. The single-lap joint shearstrength is 12.37 MPa tested as per ASTMD 1002. Com-mercial grade sodium hydroxide was used for diges-tion of the pine needles. Paraffin wax (m.p. 60�C) wasprocured fromM/s Fine Organics, Mumbai (India).

Sample preparation

Pine needles were cut to a desired length (30 mm).Thereafter, they were digested with 2% aqueous sodiumhydroxide solution (wt % of needles) as optimized in

earlier work.19 The treated mass was washed with run-ning water to remove its extractives and residual alkali.The initial weight loss in the pine needles was 15% aftertreatment. Subsequently, the samples were dried at 1036 2�C to a constant moisture content and hammermilled to an average furnish size of� 2 mm.The needle furnishes were placed in a rotary drum

blender and treated them by spray application of iso-cyanate prepolymer resin adhesive. The binder level isranged between 3–20 wt % of furnish. This range isselected because board manufacturing industries areusually applied resin adhesives to a level of 15–18 wt %for bonding of lignocellulosic particles to maintain theireconomical viability. It is also noted that above 20 wt %resin adhesive content, the excess of adhesive issqueezed out from the sides of molds during pressing.The treated mass was then placed on the bottom caulplate and transferred to a single daylight press withplaten temperature of 140�C. The press was closed at aram pressure of 10 MPa for 10 min. After pressing, thesamples were demolded and were allowed to cool atroom temperature. The pressing time was selected onthe basis of boil swell test (thickness swelling < 30%). Itis observed that the surfaces of demolded boardsshowed no adhesion with the caul plates.

METHODS

Physical tests

Density and void content of the pine needle compositeboards were measured as per ASTM D 792-2008 andASTM D 2734-2009, respectively. Water absorption ofsamples was measured as per ASTM D 570-2005 after 2and 24 h immersion in cold water. The tensile proper-ties of dumbell shaped samples-Type I were deter-mined according to ASTM D 638-2008 at a cross-headspeed of 5 mm min�1 on a Hounsfield Material TestingMachine (H 25 KS). The flexural properties of sampleswere tested at a cross-head speed of 5 mm min�1 andspan-to-depth ratio of 16 : 1 according to ASTM D 790-2002. The internal bond strength of samples was deter-mined by testing tensile strength perpendicular to thesurface as per IS: 2380-81 at a cross-head speed of 0.08mm min�1 of thickness per min.20 The samples wereprepared by fixing 50 mm square board to 50 mmsquare and 25 mm thick aluminum loading blocks withan epoxy adhesive. The screw holding test was carriedout according to IS: 2380-81 at a loading rate of 1.5 mmmin�1. The wood screws No 8 and 50 mm length werethreaded into the specimen at right angle to the faceupto a half of their length in a prebore of 2.5 mm. Allresults were the average value of five measurements.

Surface examination

The surface morphology of untreated and alkalitreated needles was recorded on a Scanning electron

3478 GUPTA ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

microscope (SEM-Leo 435). Examination of split ten-sile fractured surfaces of both aged and unagedboards was carried out to know the failure behavior.Before SEM examination, the samples were vacuumcoated with a thin film of gold/palladium to render

them conductive. Atomic force microscope (AFMNTEGRA, TS, 150) was used to measure surface ho-mogeneity of the boards. The sample of size 12.50�8.0� 5.0 mm3 was mounted on a metal disk, whichis then held magnetically under the probe. A 100x

Figure 1 SEM of untreated and NaOH treated pine needles at different treatment time like, (a) Control, (b) 30 min, (c) 60min, and (d) 120 min.

TABLE ISurface Energetic Properties of Untreated and Alkali Treated Pine Needles

Treatment time(min) (NaOH: 0.08 M)

Contact angle(�)Critical surfaceenergy (mJ m�2)

Surface-freeenergy (mJ m�2)

Polar component(mJ m�2)

Dispersive component(mJ m�2)Water Xylene

Control 80.14 6 4.0 12.45 6 o.6 16.50 6 0.83 28.00 6 1.4 10.30 6 0.5 17.70 6 0.9030 81.39 6 4.2 14.16 6 0.7 16.10 6 0.8 27.30 6 1.3 9.70 6 0.45 17.60 6 0.8560 85.70 6 4.3 17.35 6 0.9 15.30 6 0.76 24.90 6 1.2 7.60 6 0.38 17.30 6 0.82120 86.45 6 4.3 29.17 6 1.5 9.50 6 0.47 23.70 6 1.2 7.80 6 0.41 15.90 6 0.75

Surface tension of water, 72.8 mN m�1; Surface tension of Xylene, 18.10 mN m�1; Needle diameter, 0.87–1.09 mm.

COMPOSITE BOARDS FROM ISOCYANATE BONDED PINE NEEDLES 3479


microscope is used to view surface texture of thesamples.

The contact angles of untreated and alkali treatedpine needle surfaces were measured using sessiledrop technique with the help of Dynamic contactangle analyzer (VCA Optima XE, AST Products,USA). The pine needle specimen of size 10 mm wasmounted on the microscope stage and adjusted to thehorizontal position. An aliquot of distilled water (0.5lL) was dropped onto the surface with the help of amicro syringe. A photograph was taken 10 s after thewater had been dropped. AutoFAST analysis imaging

Figure 2 Zisman plots of (a) untreated pine needle and(b) alkali treated pine needle.

TABLE IIProperties of Composite Boards Made from Untreated

and Alkali Treated Pine Needles

Property

Untreatedpine needlecompositeboard

Alkali treatedpine needlecompositeboard

Density (g cm�3) 0.90 6 0.045 0.86 6 0.043Water absorption (%)

2 h 34.81 6 1.7 19.11 6 0.9524 h 79.13 6 3.9 45.44 6 2.2

Thickness swelling (%)2 h 27.11 6 1.3 12.60 6 0.6024 h 47.32 6 2.5 37.40 6 1.8

Linear expansion 2 h soaking (%)Length 0.54 6 0.027 0.19 6 0.02Width 0.51 6 0.025 0.27 6 0.03

Internal bond strength (MPa) 0.59 6 0.029 1.12 6 0.06Screw withdrawal load (N) 637 6 32 1270 6 65

TABLEIII

Physico-M

ech

anicalPropertiesofPineNeedle

Composite

BoardsatDifferentResinAdhesiveContents

Resin

adhesive

content(w

t%)

Den

sity

(gcm

�3)

Void

content

(%)

Ten

sile

strength

(MPa)

Ten

sile

modulus

(MPa)

Energyto

break

(J�

103)

Internal

bond

strength

(MPa)

Flexuralstrength

(MPa)

Screw

withdrawal

load

(N)

Contact

angle

(�)

30.75

60.035

31.876

1.5

0.78

60.04

3236

16.50

9.20

60.5

0.36

60.018

14.756

1.4

3826

1864

63.2

50.86

60.043

21.926

1.1

3.33

60.30

4406

2282.106

4.10

1.12

60.06

20.206

2.20

1270

665

73.506

3.6

70.91

60.05

17.436

1.0

5.94

60.55

5626

28172.20

68.6

6.01

60.31

25.506

1.3

1350

668

80.806

4.0

100.98

60.05

11.156

0.5

6.24

60.32

5776

29194.10

69.9

9.09

60.5

31.086

1.6

1766

688

85.706

4.3

151.06

60.10

4.02

60.2

8.91

60.50

5836

30301.20

615.12

13.726

1.4

44.116

2.0

2138

6107

90.846

4.5

201.12

60.12

2.26

60.11

18.236

1.82

2935

6150

404.90

620.20

28.606

2.9

49.336

2.5

2511

6125

92.956

5.0

3480 GUPTA ET AL.


system is used to capture a droplet image and auto-matically calculates the contact angle with no userintervention. Five measurements were made for eachsample. Surface energy software (SE-2500) was usedfor calculation of critical surface energy of untreatedand alkali treated pine needles using Zisman plots.

Accelerated aging test

The samples were exposed to various relative hu-midity (75% RH, 98% RH, and 98% RH at 50�C) and

immersion in cold water for 60 days. The relativehumidity was maintained in a desiccator accordingto the method described in ASTM C 427-64. Weightgain and thickness swelling of the exposed sampleswere recorded at a regular interval of time. The in-ternal bond strength of samples was determined byexposing them under accelerated water aging andcyclic aging according to IS: 12,406-2003.21 Underaccelerated aging, the samples were subjected toimmerse in cold water and water is brought to boil-ing and kept at boiling temperature for 2 h followed

Figure 3 AFM Images of pine needle composite boards with various resin adhesive contents, (a) 3%, (b) 5%, and (c) 7%.[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]



by cooling in water. In cyclic aging, the sampleswere immersed in water at 27 6 2�C for a period of72 h followed by drying in air at 27 6 2�C for 24 hand then kept at 70�C for 72 h. The cycle wasrepeated thrice as mentioned in the Standard. Theaged samples were examined for dimensionalchanges and internal bond strength.

RESULTS AND DISCUSSION

Effect of alkali treatment

Figure 1 shows SEM of the untreated and alkali treatedpine needles. The surface of untreated pine needles issmooth and fully covered with a thick waxy cuticlelayer that may inhibit their adequate bonding withresin adhesive [Fig. 1(a)]. However, the alkali treatedneedles showed a rough surface with clear fibrillar andless compact structure because of swelling [Fig. 1(b–d)].The progressive destruction of surface waxy layer fromthe needles along with partial removal of noncellulosicconstituents such as lignin, hemicellulose, and extrac-tives occurred with increasing treatment time. The nee-dles lost 15% of these materials after 2 h treatment lead-ing to better separation of the individual cells/ thinnerfibers. This needle fibrillation increases the effectivesurface area available for wetting by the resin adhesive.The improvement in the wettability of these treatedneedles can be explained on the basis of contact angledata (Table I). The differences in the surface analysis ofuntreated and alkali treated needles are visible. Unex-pectedly, the higher contact angle of treated needles

may consider mainly because of poor liquid flow overthe rough surface as observed in SEM to fill completelyevery cell cavity, pore, or crevice. Because of this, wett-ability of the needles was characterized by measuring acritical surface tension through Zisman’s rectilinearrelationship between surface tension and cosine contactangle (Fig. 2). The 2 h alkali treated needles had a lowercritical surface tension (9.5 mJ m�2) than the untreatedones (16.5 mJ m�2) showing their superior surface wett-ability. The physical meaning of critical surface tensionis that surface tension below which a liquid will com-pletely wet the solid.22 It is found that pine needlebehaves as low energy surface (28 mJ m�2) as com-pared with the wood (56–97.6 mJ m�2). The surface-free energy of pine needles was reduced by 15% after 2h alkali treatment. The reduced polar component ofsurface-free energy of these treated needles indicatessuperior spreading of adhesive onto their surfaces ascompared with control samples. In the case ofuntreated pine needles, poor wettability may interferewith the spreading and penetration of resin adhesive,thereby affecting the bond formation between the resinadhesive and needles. On alkali treatment, the emer-gence of fibrillar and rough morphology is believed toprovide good bonding with the resin adhesive and con-sequently, to improve strength and dimensional stabil-ity of the composite boards. This can be explained onthe basis of properties of untreated and alkali treatedneedle composite boards (Table II). Under immersedwater for 2 h and 24 h, the alkali treated pine needleboards exhibited less water absorption (43–45%) andthickness swelling (21–54%) than the water absorptionand thickness swelling of untreated pine needle boards.The linear expansion of alkali treated pine needle

Figure 4 Tensile stress-strain curves of pine needle com-posite boards as a function of various resin adhesivecontents.

Figure 5 Screw withdrawal load of pine needle compos-ite boards as a function of resin adhesive contents.

3482 GUPTA ET AL.


boards is also 2–3 times less than that of untreatedboards showing their superior dimensional stability.The cohesion of interparticles in the untreated needleboards is also insufficient in terms of internal bondstrength. Its internal bond (split tensile strength) wasreduced by 90% compared with alkali treated boards.The withdrawal resistance of screws of untreated nee-dle boards was also 95% less than the alkali treated nee-dle boards along with embossing at the point of screwwithdrawal.

Properties of composite boards

The physico-mechanical properties of compositeboards are shown in Table III. It is observed thatboards properties improve with increasing quantities

of prepolymer resin adhesive. The presence of pol-yol in the isocyanate prepolymer may inhibit pene-tration of isocyanate into inner cavities of needleskeeping it more available for interparticle adhesion10

and consequently, the improvement in the strengthof composite boards. Evidence of this adhesion phe-nomenon can be viewed on AFM images of the fin-ished board surfaces where both bonded anddebonded regions coexist (Fig. 3). At low-resin adhe-sive content (3%), decohesion of particles was domi-nant onto surface whereas at higher resin adhesivecontent, a smooth surface morphology was observedin which particles are well bonded. It is noted thatdimensions of the smooth region increase withincreasing resin adhesive content. The uniformity inthe distribution of particles clearly indicates

Figure 6 Thickness swelling of pine needle composite boards as a function of exposure time under different humidityand when immersed in water.



adequate spreading of binder onto particle surfacesas evidenced also in their low surface-free energyvalue during contact angle measurement. Because ofgood spreading, resin availability even at low-levelis sufficient to wet out the needle furnishes duringintense mixing with resin adhesive. The compositeboards containing 3–7% resin adhesive contentbelong to medium density particle boards category(0.7–0.9 g cm�3) as per IS: 3087-2005.23 It is observedthat increase of resin adhesive content increases den-sity and reduces void content of the compositeboards. The boards with 3% resin adhesive contain31.87% void content. The presence of excessive voidsreduces board’s mechanical properties.24 Theyabsorb water more readily causing development ofswelling stresses at the interface, the consequentresin microcracking and loss of properties upon par-tial drying.

The difference in the slope of the tensile stress-strain curves of composite boards can be explainedon the basis of interparticle adhesion (Fig. 4). In thecase of low-resin adhesive content boards, lack ofknee point in the curves demonstrates synergisticinvolvement of needle furnish and resin adhesive inthe load carrying process, whereas the initial load inthe higher resin adhesive content boards was carriedout by resin alone showing a demarcated knee point.The tensile and flexural strengths of compositeboards increased as the resin adhesive content wasincreased. This could be attributed to the good inter-particle adhesion which in turn, provides better effi-ciency of stress transfer between needle furnish andresin adhesive. The isocyanate prepolymer appliedonto needle furnish reacts with hydroxyl groups oflignocellulosics to form urethane bonds in the pro-cess of pressing. Additionally, this improvement isalso supplemented to the formation of cross-linkedinterpenetrating network following the reactionbetween moisture and isocyanate that contributes tothe panels mechanical strength. The marginal changein the tensile modulus of composite boards wasnoticed except in the case of 20% resin adhesive con-tent boards. Energy to break of the boards showedan increasing trend as the resin adhesive contentwas increased. The adequate interparticle adhesioncreates satisfactory interfacial regions, which may in-hibit frictional pull-out of needle fiber bundles fromthe resin adhesive. This, in turn improves the abilityof the composite boards to absorb more energy dur-ing fracture propagation. The withdrawal resistanceof screws from the face panel increases from thelow-resin adhesive content boards to the high-resinadhesive content boards (Fig. 5). Embossing wascommon in all the samples at a point of screw with-drawal. Increasing the resin adhesive content from3–5% resulted in boards of sufficient screw with-drawal load to pass the requirement of IS: 3087-2005.

TABLEIV

Dim

ensionalStabilityofPineNeedle

Composite

Boards

Resin

adhesive

content(w

t%)

Water

absorption(%

)Thicknesssw

elling(%

)Linearexpan

sion(len

gth)(%

)Equilibrium

moisture

content(%

)

2h

24h

2h

24h

2h

75%

RH

98%

RH

98%

RH

50� C

Immersionin

water

337.7

61.8

77.016

3.8

29.796

1.5

66.026

3.2

0.22

60.01

6.60

60.38

26.016

1.3

25.306

1.2

96.5

65.0

519.1

60.95

45.446

2.2

12.606

0.6

37.406

1.8

0.19

60.02

4.90

60.25

23.066

1.2

22.106

1.1

79.026

4.0

710.206

0.5

37.706

1.8

7.80

60.4

20.306

1.1

0.0936

0.005

4.20

60.21

19.606

0.95

19.206

0.95

64.206

3.2

105.54

60.25

22.706

1.1

3.40

60.17

16.806

0.80

0.0826

0.004

3.00

60.15

18.606

0.9

17.106

0.80

58.006

3.2

153.70

60.19

14.706

0.7

3.10

60.15

15.706

0.75

0.0796

0.004

2.19

60.12

17.906

0.85

16.406

0.82

44.206

2.2

202.50

60.12

9.93

60.45

2.50

60.12

13.706

0.68

0.0686

0.003

2.00

60.10

17.506

0.8

16.016

0.80

38.506

1.90

3484 GUPTA ET AL.


The high resin adhesive content boards possessscrew holding power comparable with conventionaltimber according to Indian National Building Code2005, Sec 3.

Effect of accelerated aging

Figure 6 shows thickness swelling of the compositeboards under different humidity and whenimmersed in water. During exposure, the thicknessswelling increased rapidly, and then it leveled off(15–20 days). But a further increase of exposure, thesamples experienced insignificant swell variations.The equilibrium moisture content of compositeboards increases with increasing humidity levels. Atlow-humidity, the variation in thickness swellingwas found to be 2–7% only at equilibrium moisturecontent whereas at higher humidity levels, the sam-ples exhibited these changes ranged between 13–23%. Compared with 98% RH, a decrease in thedimensional changes at 98% RH at 50�C is consid-ered mainly because of the formation of cross-linkedinterphase by the post curing of the resin adhesivebinder at 50�C. It is observed that the compositeboards swelled in immersed water 2–3 times asmuch as the samples exposed to 98% RH because ofless stable bonds formed at the interface in presenceof water by the reaction between isocyanate andhydroxyl groups of needles.

As expected, increase of resin adhesive contentreduces thickness swelling, linear expansion, andwater absorption of the composite boards (Table IV).The low-resinadhesive bonded boards exhibitedhigh-water absorption and thickness swelling after 2h and 24 h water soaking and do not meet commer-cial Standard requirements. These boards alsoquickly absorbed surrounding moisture and warped.This adverse effect might be attributed to the insuffi-

cient bond between needle furnish. As a result,many gaps/voids occur in the boards that allowwater to enter resulting in large swelling and waterabsorption. The addition of 1% paraffin wax in thecomposite board formulation (3% resin adhesivecontent) reduces 61.06% thickness swelling and67.03% water absorption respectively after 2 h watersoaking (Table V). The corresponding linear expan-sion of the boards was 0.1%. The surface absorptionwas also reduced significantly when wax is added inthe board formulation. The internal bond strength ofthe composite boards containing 5% resin adhesivecontent was reduced by 20% after adding paraffinwax in the formulation. This suggests that the waxinterfered with isocyanate resin adhesive bondingduring pressing resulting in reduced bond quality. Itis also reported that thickness swelling of the particleboards depend on their density. High-density boardsexhibited 4–8% more thickness swelling than the low-

TABLE VEffect of Wax Content on the Dimensional Stability of the Pine Needle Composite Boards

Property

Pine needle boards

Control Wax containing boards

3% 5% 3% 5%

Water absorption (%)2 h 37.77 6 1.8 19.11 6 0.95 12.43 6 0.60 9.11 6 0.524 h 77.01 6 3.8 45.44 6 2.20 44.7 6 2.20 31.21 6 1.5

Thickness swelling (%)2 h 29.79 6 1.5 12.60 6 0.60 11.60 6 0.55 7.84 6 0.424 h 66.02 6 3.2 37.40 6 1.8 47.77 6 2.8 40.73 6 2.6

Surface absorption (%)2 h 10.81 6 0.50 3.70 6 0.03 4.02 6 0.22 2.70 6 0.1424 h 37.56 6 1.90 17.10 6 0.90 21.96 6 1.1 14.15 6 0.75

Linear expansion (%)2 h 0.22 6 0.01 0.19 6 0.02 0.10 6 0.005 0.02 6 0.00224 h 1.02 6 0.10 0.94 6 0.045 1.30 6 0.10 0.69 6 0.04

Internal bond strength (MPa) 0.36 6 0.018 1.12 6 0.06 0.28 6 0.015 0.89 6 0.045

Figure 7 Internal bond strength of pine needle compositeboards under dry and wet conditions.



density boards. This difference is mainly because ofthe release of greater compression stress at a laterstage in the high-density boards.25 Increasing resinadhesive content level from 3 to 5%, the compositeboards passed the tests. The thickness swelling andwater absorption reduced from 29.80 to 12.60% and37.70 to 19.11% respectively after 2 h soaking in waterand this is attributed to the good needle furnish-resinadhesive bonding. The composite boards with 5%resin adhesive content satisfied water absorptionrequirement prescribed in the standard after 24 hsoaking. The contact angle of the composite boards

was increased from 64� for 3% resin adhesive to92.95� for 20% resin adhesive content. This indicatesthat the boards have adequate surface moisture prob-ably due to availability of unreacted isocyanategroups.26 Their higher moisture content at the surfacemay encourage greater surface plasticization, whichresulted in less water absorption and hence betterdimensional stability of the boards.The internal bond strength of the fresh and the

aged composite boards is shown in Figure 7. It isnoticed that, the location of the plane of failureoccurs mainly in the upper or middle line of the

Figure 8 (a) SEM of split tensile surface of unaged pine needle composite boards with various resin adhesive contents:(i) 3% (ii) 5% (iii) 7% (iv) 10% (v) 15% (vi) 20%, (b) SEM of split tensile surface of pine needle composite boards with vari-ous resin adhesive contents under cyclic aging (i) 7% (ii) 10% (iii) 15% (iv) 20%, and (c) SEM of split tensile surface ofpine needle composite boards with various resin adhesive contents under accelerated water aging (i) 7% (ii) 10% (iii) 15%(iv) 20%.

3486 GUPTA ET AL.


board thickness. The internal bond strengthincreased significantly with an increase of resin ad-hesive content in the boards. To know cohesion of

the panels, the samples were aged in acceleratedwater and cyclic (alternate wetting and drying).Under cyclic aging, reduction in the internal bond

Figure 8



strength was between 54–78% whereas in acceleratedwater a decrease in strength ranged between 41–67%. The severity of cyclic aging effect was morethan that of an accelerated water aging. The high-resin adhesive content boards experienced morestrength loss during cyclic (21%) and acceleratedwater (8%) than that of the low-resin adhesive con-tent boards. The low-resin adhesive content bondedboards (<5%) possess insignificant internal bondstrength after both cyclic and accelerated wateraging. As expected, cohesive loss in the boardsaffected considerably by the combined action ofwater and temperature. SEM micrographs of splittensile surfaces of the control and the aged samplesare shown in Figure 8(a–c). In the case of unagedsamples, the split surface of low-resin bondedboards consists of unbonded pulled-out fiberswhereas higher resin adhesive content boards exhib-ited smooth cohesive failure surfaces dominatedwith embedded fibers. At some places, splitting ofneedle furnish is also viewed because of the exis-tence of strong interfacial regions and also inherentweakness of needle furnish itself. Under cyclic expo-sure, the split surfaces showed a loosen texture inwhich swelled and pulled out needle furnishes wereobserved. The splitting of needle furnishes, and theirbreakage was also noticed contrary to the unagedsamples [Fig 8(b)]. In the case of accelerated wateraging, the split surface exhibited relatively moreswelled, pulled out fibers with fractured/ breakagesurfaces than that of cyclic aging [Fig. 8(c)]. How-ever, it is surprisingly noticed that the compositeboards retained more internal bond strength afteraccelerated water aging than the samples aged to

cyclic exposure. The debonded regions developed incyclic aging are caused by expansion and contractionof the needle furnishes because of water absorption/desorption during wetting and drying stages. It isalso reported that a high content of water solublesubstances leached during immersion in hot waterled to the initiation of debonding between the fiberand the matrix via creation of an osmotic pressure atthe fiber surface. This has resulted in an early me-chanical failure of samples.27

A comparison was made between properties ofthe isocyanate prepolymer bonded pine needleboards and the commercial resin bonded wood/lignocellulosic particle boards23,28 (Table VI). Thecomposite boards at 5% resin adhesive level exhib-ited 40% higher internal bond strength than thespecified value of the Standard for phenolic/ureabonded boards because of its superior adhesiveproperties even at low-resin content. The screwwithdrawal load was comparable with the commer-cial particle boards. The water absorption of the pineneedle boards is 23.56 and 9.12% less than the phe-nolic/urea bonded boards after 2 and 24 h soaking.On contrary, thickness swelling of the pine needleboards after 2 h water immersion is 26% higher thanthe commercial resin bonded boards. After addingparaffin wax (1%), the thickness swelling in theboards is reduced from 12.62 to 7.84% (37.88%),which pass the requirement of the commercialStandard. Linear expansion of the pine needleboards is also 80% less than that of phenolic/ureabonded boards after 2 h water immersion.

CONCLUSIONS

The results indicate that mercerized pine needle fur-nishes bonded with isocyanate prepolymer can beused as panel products for wood substitute applica-tion in buildings. In making composite boards, it isdesirable to use wettable needle furnishes for theirgood bonding with resin adhesive. The preparedboards/panels at 5% resin adhesive content levelsatisfy the exterior grade requirements of IS: 3087-2005 / EN 312-2003 in terms of physico-mechanicalproperties and dimensional stability. At low-resinadhesive content, use of hydrophobic compound isnecessary in the board formulations to achievedesired dimensional stability to meet the Standardrequirements. Under wet conditions, loss in thestrength and dimensional stability can be minimizedby the use of isocyanate prepolymer with an opti-mum level of equivalent ratios within which the pol-yol can interact with the isocyanate to produce astrong bond by cross-linking. Blending of needle fur-nishes with other fibers at low-isocyanate prepoly-mer content would probably produce a strong com-posite board suitable for outdoor use especially in

TABLE VIProperties of Pine Needle Composite Boards as per IS:

3087–2005 / EN 312

Property

StandardSpecification

Pine needlecomposite

board (� 5% resinadhesive content)IS: 3087 EN 312

Density variation (%) 6 10 - 6 0.9Water absorption (%)

2 h Soaking 25 - 19.11 6 0.9524 h Soaking 50 - 45.44 6 2.20

Linear expansion 2 h Soaking (%)Length 0.5 - 0.19 6 0.02Width 0.5 - 0.27 6 0.03

Thickness swelling 2 hwater soaking (%)

10 16 12.6 6 0.6

Modulus of rupture(N mm�2)

11 14 20.20 6 2.20

Tensile strengthperpendicularto surface (N mm�2)

0.8 0.28–0.45 1.12 6 0.060

Screw withdrawalstrength Face (N)

1250 - 1270 6 65

3488 GUPTA ET AL.


moist places. Work is continued on decay resistanceand flammability behavior of composite boards.

The article forms part of a Supra Institutional Project of CSIRR & D program (Govt. of India) and is published with thepermission of Director, Central Building Research Institute,Roorkee.

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Composite boards from isocyanate bonded pine needles