IntroductIon

Many challenges face today’s coatings formula-tors. Some of these challenges involve removing cost from a coating formulation or improving performance, but many are related to producing coatings with im-proved environmental profiles. Lowering volatile organic compound (VOC) levels in architectural and industrial coatings is a chief concern in many new product de-velopment and reformulation projects. Balancing good performance and low VOC is critical, as a low-VOC coat-ing with poor performance is of little value. Coatings formulators and end-users are provided with several technology options to address the desire for lower VOC, including the use of solventborne coatings with high solids or VOC-exempt solvents (such as acetone or t-bu-tylacetate), powder coatings, and waterborne technolo-gies. Waterborne acrylic latex technology, in particular, offers a number of advantages such as low toxicity, lower risk of fire from handling flammable solvents, easy one-component use, easy and safe cleanup with water, and less hazardous disposal. However, many la-tex coatings still utilize a significant level of coalescing solvents to aid in film formation, so the challenge of go-ing to even lower VOC levels is still present in many ap-plications currently using or moving towards waterborne latex coatings.

Lowering VOC in a waterborne latex coating would be as simple of an exercise as lowering polymer glass transition temperature (Tg) or increasing pigment vol-ume concentration (PVC), if there were not also the requirement to maintain a certain balance of key film properties such as the quality of film formation or film hardness. The quality of film formation, while perhaps

Designing Latex Film Morphology

To Optimize Wood Coating PerformanceBy Leo Procopio, Laura Vielhauer, Eric Greyson, and Andrew Hejl The Dow Chemical Company, Spring House Technical Center

February 201232 COATINGSTECH

sometimes regarded as a slightly mysterious feature, is reflected in many film properties crucial for architectural and industrial coatings, including barrier properties (such as water, corrosion, stain, and efflorescence resistance), tensile strength, and elongation. Good film formation involves not only the deformation of latex particles as the coating dries, but ultimately the diffusion of polymer chains across particle boundaries to form a tightly knit, cohesive film. Film hardness is reflected in important resistance properties such as block, print, scratch, mar, abrasion, and dirt pickup. Although decreasing the Tg of an acrylic binder is a simple method for lowering the co-alescent demand of the film while also maintaining good film formation, the resulting film is often too soft to func-tion properly in many settings.

A more elegant approach to lowering VOC while main-taining hardness properties in latex coatings is to incor-porate hard polymer domains into a continuous matrix of softer polymer. Traditionally, this has been accomplished by either blending hard and soft latexes,1 or by producing latex particles with internal morphologies such as core shell or multi-domain2 (Figure 1). Both approaches have been successful in lowering the overall level of coales-cent needed to produce a good film, but still have limita-tions when it comes to providing an optimal solution for balancing film formation, VOC, and film hardness. For example, when formulating a hard/soft latex blend, it is inevitable that a portion of the coalescent partitions to the soft phase and a portion of the coalescent parti-tions to the hard phase. The exact ratios of how the coalescent partitions in the wet paint relies no doubt on a number of factors such as polymer hydrophobicity and relative amounts of the different phases. However, the soft and hard latex polymers have different Tg values and correspondingly have different coalescing requirements for good film formation. Some coalescent is “wasted” by its location in the soft phase, where less is needed for the various stages of film formation such as particle deformation and polymer diffusion. On the other hand, the hard phase may end up being under-coalesced, and may not participate equally in the film formation process, thus affecting final film properties. Similar film formation issues will occur with latex particles containing the soft and hard phases in the same latex particle, such as for the core/shell morphology.

Both hard/soft blends and multi-domain latex poly-mers are methods to improve the hardness properties of acrylic polymers at lower VOC levels. However, hard-ness development is dependent not only on the polymer Tg, but also on the relative rate of coalescent diffusion from the acrylic film. An example is a hard acrylic film coalesced with 2,2,4-trimetyl-1,3-pentanediol monoiso-butyrate (Texanol™). Although its ultimate hardness may be quite high, it will take a very long time at ambient con-ditions to reach that level because diffusion and evapo-ration of Texanol from the film is very slow. Hardness de-velopment for acrylic latex polymers is generally reduced by the slow diffusion of coalescent out of the drying film, particularly under ambient drying conditions. Hard/soft latex blends and multi-domain particle morphologies may address the ultimate hardness reached, but they do not necessarily affect the rate at which coalescent is released from the film, and thus hardness development is not optimized.

This article describes a new way of thinking about the roles of film morphology and coalescent on the film formation process.3 Specifically, it describes a blend system consisting of a hard acrylic latex and a soft poly-urethane dispersion (PUD), which allows a unique bal-ancing of lower VOC, good film formation, and improved hardness properties relative to a hard acrylic by itself. In this system, the hard polymer is not present simply to reinforce the soft continuous phase, as described in the examples above. Rather, the hard acrylic is now the major continuous phase. The soft polyurethane phase is present to aid in coalescent release. The unique film morphology generated by the blend leads to increased and rapid diffusion of coalescent from the drying film, resulting in greatly enhanced hardness development. The drying mechanism for this new technology will be discussed, and wood coatings based on the acrylic/PUD blends will be compared with commercially available systems designed for applications including kitchen cabi-nets, flooring, and architectural trim paints.

ExpErImEntal

Two acrylic/PUD blends based on the new technol-ogy were used in the evaluations, and are designated as APU-1 and APU-2. Each is a blend of a hard acrylic

Multi-Domain Core/Shell Blends

Figure 1—Types of heterogeneous latex morphologies that are used to affect film hardness. The two polymer phases represented by the colors blue and pink would have different hardness properties, as dictated by their Tg.Multi-Domain Core/Shell Blends

Multi-Domain Core/Shell Blends

COATINGSTECH 33February 2012

latex and a soft aliphatic polyester polyurethane dispersion (PUD-1). APU-1 was prepared at 39.0% weight solids and pH 7.6, and has a minimum film formation temperature (MFFT) of less than 0°C. APU-1 is based on hard acrylic AC-1 (Tg = 43°C, 40.5% solids, pH 7.0), and is compared to AC-1 throughout this study. APU-2 was prepared at 42.0% solids and pH 8.3, and has a MFFT of 10°C. APU-2 is based on hard acrylic AC-2 (Tg = 35°C, 45.0% solids, pH 9.0), to which it is compared in this study. Both APU-1 and APU-2 contain approximately 2.9% of dipropylene glycol dimethyl ether (DMM) on total, which gives them a calculated VOC of approximately 84 g/L as supplied. Both hard acrylics AC-1 and AC-2 contain ambient self-crosslinking functional-ity which aids in chemical resistance and durability. The polyurethane dispersion PUD-1 is an essential component to the coalescent release technology, and is significantly softer than the acrylics with a Tg well below 0°C. The acrylics and PUD-1 are produced by standard synthetic procedures for acrylic latex and polyurethane dispersions, respectively. Because synthesis of PUD-1 is performed in solvent (DMM) before it is dispersed in water, the blends contain some VOC content, as already mentioned. DMM acts as a coalescent and aids in film formation for the blends APU-1 and APU-2.

Polymers were tested in various experimental wood coating formulations. Tables 1 and 2 show the clear spray formulations based on APU-1 and AC-1, used as both sealer and topcoat for testing over wood substrates, and detail the coalescent packages and additives. APU-2 and AC-2 were tested in pigmented gloss white brushing formu-lations, detailed in Table 3. Commercial coating systems were obtained from their manufacturers and are described briefly in the Discussion section.

Panel PreparationThe key properties affected by the new technology are

hardness properties that are related to the rate of release of coalescent. For hardness, scratch/mar, and block and print resistance tests, coatings were applied by drawdown bar to a treated aluminum panel (Q Panel Type AL-412 chromate pretreated aluminum, 10 cm x 30 cm) to yield approximately 1.5 mil (40 mm) dry film thickness. Panels were placed in a constant temperature and humidity room (75°F and 50% RH) for the amount of time listed below for each test prior to testing.

Some tests were performed on wood panels to dem-onstrate that other properties of coatings based on the acrylic/PUD blend technology are comparable to current wood coatings systems. Those properties are typically not considered to be highly dependent on the rate of coales-cent release. For these tests, clear coating systems were applied to maple panels (10 cm x 30 cm) by conventional air spray at approximately 1 mil (25 mm) dry film thickness (DFT) per coat, with one coat of sealer and two coats of topcoat. Maple panels were sanded with 220 grit sandpa-per prior to use, with sanding dust removed by compressed air spray and wiping with a clean lint-free cloth. For each

table 1—Clear Formulation Based on Acrylic/Polyurethane Binder APU-1

table 2—Clear Formulation Based on Acrylic Binder AC-1

Material Name Pounds Gallons APU-1 704.00 79.91

Slowly add the following blend of coalescents: Premix DOWANOL® PnB 7.50 1.02 DOWANOL® DPM 7.50 0.94 DOWANOL® DPnB 5.00 0.66 PARAPLEX® WP-1 5.00 0.59 Water 75.00 8.99

Premix Sub-total 100.00 12.19 Tego Foamex™ 805 4.40 0.52 TegoGlide™ 410 2.00 0.24 Surfynol™ 104DPM 2.75 0.33 Michem™ Emulsion 39235 17.50 2.10 Water 39.50 4.73

Totals => 870.15 100.02 Levels w/o Additives Volume Solids: 28.49% Density: 8.70 lb/gal Weight Solids: 31.58% VOC (g/L): 145 Levels with Additives Volume Solids: 30.33% Weight Solids: 33.35%

Material Name Pounds Gallons AC-1 663.08 75.22 Slowly add the following blend of coalescents: Premix DOWANOL PnB 15.23 2.06 DOWANOL DPM 15.01 1.89 DOWANOL DPnB 9.85 1.29 PARAPLEX WP-1 10.25 1.22 Water 95.13 11.40

Premix Sub-total 145.47 17.86 Tego Foamex 805 4.33 0.51 TegoGlide 410 2.08 0.25 Surfynol 104DPM 2.77 0.33 Michem Emulsion 39235 17.35 2.08 Water 30.36 3.64

Totals => 865.44 99.89

Levels w/o Additives Volume Solids: 27.99% Density: 8.66 lb/gal Weight Solids: 31.03% VOC (g/L): 138

Levels with Additives Volume Solids: 30.46% Weight Solids: 33.42%

February 201234 COATINGSTECH

coat applied, the coating was dried at ambient temper-ature for 10 min, and then placed in a 60°C oven for an additional 10 min. The sealer was sanded with 280 grit sandpaper prior to application of the topcoat. After sealer and topcoats were applied, the panels were placed in a constant temperature and humidity room (75°F and 50% RH) for two weeks prior to testing.

Testing ProceduresSeveral tests were performed on just the topcoat,

including hardness, scratch/mar, and block and print resistance. These tests were evaluated for topcoats applied to treated aluminum panels.

Konig Hardness: Evaluated according to ASTM D4366 using a TQC SP0500 Pendulum Hardness Tester, and reported in seconds. Measurements were taken at various intervals over the course of two weeks. Ultimate hardness was determined for panels baked overnight at 60°C to drive off residual coalescent.

Block Resistance: Panels were dried for one or seven days, and then two 4-cm wide strips were cut from the aluminum panel, and placed face-to-face forming a cross. A #8 rubber stopper was placed on the cross section of the strips, and a 1 kg weight placed on top of the rubber stopper. Block resistance was rated under two conditions: (1) after 30 min at 60°C, and (2) after 24 hr at room temperature. The fol-lowing 0 to 10 scale was used to rate the coatings for tack and film damage: 10, no tack/perfect; 9, trace tack/excellent; 8, slight tack/very good; 7, slight tack/good; 6, moderate tack/good; 5, moderate tack/fair; 4, severe tack, no seal/fair; 3, 5–25% seal/poor; 2, 25–50% seal/poor; 1, 50–75% seal/poor; 0, complete seal/very poor.

Print Resistance: Panels were dried for one or seven days. A piece of cheesecloth (4 cm x 4 cm) was then placed on the panel, a #8 rubber stopper was placed on top of the cheesecloth, and a 1 kg weight placed on top of the rubber stopper. Print resistance was rated under two conditions: (1) after 30 min at 60°C, and (2) after 24 hr at room temperature. Damage to the film from the imprint of the cheesecloth was rated on a 1 to 10 scale, with 10 being no imprint or damage, and 1 being very poor print resistance.

Scratch/Mar Resistance: Scratch and mar resis-tance can be measured by several methods. A new method was employed in this study which insults the film with a combination of scratch, mar, and impact damage. Coatings were applied to a treated aluminum panel and dried for two weeks prior to testing. A sec-tion of the panel was placed on the bottom of a quart container with the film facing up. Various types of impact media were placed on top of the film, the container was sealed and placed on a paint shaker. The container was agitated for two minutes, and then the sample was removed and evaluated visually. Different types of impact media can lead to different types of damage,

corresponding to differing real world insults. This study used two different sets of impact media: (a) two 1/2 in. diameter hexagonal steel nuts, and (b) one metal lid for a 2-oz paint can. In general, impact media with more corners and with corners having smaller radii of curva-ture lead to more damage on most films. More massive media also tend to produce greater damage. The type of damage also changes with media shape; for example, metal lids create mainly a marring type of damage, and sharper objects (such as screws) can create mainly a gouging type of damage.

Coatings were evaluated over wood panels accord-ing to the test methods for kitchen and vanity cabinet coatings outlined in the Kitchen Cabinet Manufacturers Association (KCMA) standard,4 including:

Detergent and Water Resistance (Edge Soak): All six sides of the maple panels were coated as described above. A #8 cellulose sponge was soaked in a 0.5% Palmolive dish detergent solution, and the panel edge was placed on the sponge for 24 hr. Panels were rated for delamination, swelling, color change, checking, and other forms of film failure. A pass is reported for no change in the film and panel.

Hot and Cold Check Resistance: The panel was cycled between hot (49°C), ambient (25°C), and cold (–21°C) temperatures, and was run as described in the KCMA standard using a Thermotron environmen-tal chamber model S-8S-SL. A pass is reported for no change in the panel or coating.

Paint based on: AC-2 APU-2 Material Name Pounds Gallons Pounds Gallons

AC-2 563.00 63.98 APU-2 615.04 69.89 Water 70.00 8.39 68.00 8.15 Ammonia (15%) 3.00 0.38 3.00 0.38 Tego Foamex 1488 2.00 0.24 2.00 0.24 Ti-Pure R-746 slurry TiO2 275.00 14.15 275.00 14.15 Texanol 35.47 4.47 14.20 1.79 Propylene Glycol 5.00 0.58 5.00 0.58 Water 44.00 5.27 19.00 2.28 ACRYSOL® RM-2020NPR 20.00 2.30 20.00 2.30 ACRYSOL® RM-8W 2.00 0.23 2.00 0.23

Totals => 1019.47 99.99 1023.24 99.98

Levels w/o Additives Density (lb/gal): 10.20 10.23 PVC: 19.0% 19.2% Volume Solids: 33.19% 32.87% Weight Solids: 45.49% 45.32% VOC (g/L): 125 123

Levels with Additives Volume Solids: 33.67% 33.35% Weight Solids: 45.97% 45.80%

table 3—Gloss White Trim Paint Formulations based on APU-2 and AC-2

COATINGSTECH 35February 2012

Shrinkage and Heat Resistance: This test places the panel at 49°C and 70% RH for 24 hr and was run using a Hotpak Environmental Chamber model 417532. A pass is reported for no change in the condi-tion of the coating.

Chemical Resistance: The coating was subjected to a variety of household chemicals to determine its resistance to staining and discoloration. The chemicals included lemon, orange, and grape juices; vinegar; ketchup; coffee; olive oil; 50% ethanol; 0.5% Palmolive dish detergent; and mustard. Chemicals were applied to the panel in a vertical position, and allowed to stand for 24 hr before cleaning and rating of the panel. In the case of mustard, cleaning was performed after a one-hour exposure. The exposed coating was evaluated for discoloration, staining, and other obvious changes, and rated on a 1 to 10 scale, with 10 equal to no change.

Chemical resistance tests were also run on maple substrate to evaluate use of the coating systems as wood furniture and general wood coatings, as follows:

Chemical Spot Resistance: Resistance to common chemicals was evaluated by applying spots of the chemi-cal to the coated panel for a specified time, then clean-ing the panel with clean water and a sponge, and drying prior to rating. Chemicals were applied to the surface by saturating a 2.3 cm grade 3 Whatman filter and covering with a watchglass to prevent evaporation. One-hour spot tests were done using ethanol, isopropyl alcohol, butyl acetate, and acetone. Overnight exposures for 16 hr were done with water, hot coffee, 50% ethanol, Formula 409 cleaner, isopropyl alcohol, 7% ammonia, Scheaffer brand red ink, and grape juice. Rating of film damage and discoloration/staining is done on a 1 to 10 scale, with 10 being no change.

rEsults and dIscussIon

Depending on the application, interior wood coatings have a variety of performance requirements related to hardness. Kitchen cabinet coatings need excellent water and chemical resistance to withstand the kitchen envi-ronment, but must also provide great block resistance if they are being applied in a factory setting. Wood floor coatings need superior mar and abrasion resistance. Trim paints for the architectural DIY market need excel-lent block and print resistance to prevent the sealing of doors and windows and marring of painted surfaces. For waterborne acrylic latex coatings, the development of these properties is closely related to the release of coalescent from the drying film. The new technology represented here by the acrylic/PUD blends APU-1 and APU-2 provides a new mechanism for the accelerated diffusion and evaporation of coalescents and co-solvents from a coating film. The result is a dry film which offers faster hardness development and improvement in other film properties that rely on hardness such as block, print, scratch, mar, and dirt pickup resistance. The faster co-alescent release also presents the possibility for lowering the TVOC (total VOC) of factory-applied wood coatings, which is the amount of VOC released from a coated part after it has been dried for a specified period of time.5,6 Early water resistance can also be improved by remov-ing hygroscopic volatile materials such as hydrophilic coalescents and co-solvents from the drying film before it is exposed to water. The mechanistic details of the new acrylic/urethane blend technology has been described previously,3 but will be recounted briefly here to aid the reader in understanding the results of the current study.

The new technology depends on the presence of two very different polymer phases. The dominant phase is a hydrophobic, hard acrylic polymer. The best hardness

water evaporation

particle deformation

polymer diffusion

coalescent transfer tosoft polyurethane phase

accelerated evaporation

hard, high Tg acrylic soft PUD coalescent molecule

Figure 2—Mechanism of film formation and accelerated coales-cent release from acrylic/PUD blends.

February 201236 COATINGSTECH

results are found with an acrylic having a Tg greater than 25°C, because the ultimate hardness depends on the polymer Tg. The second, minor phase is a soft polyure-thane, which in the wet state is more hydrophilic than the acrylic due to its swollen, hydroplasticized state. In the wet state of the formulated polymer blend, the coalescent partitions mainly to the more hydrophobic acrylic phase, where it is most needed and aids in lower-ing the MFFT. This partitioning of coalescent has been demonstrated through computer modeling studies and experimentally using nuclear magnetic resonance (NMR) techniques.3 After application of the coating, water evap-orates and polymer particles will begin to deform as film formation proceeds, as shown in Figure 2. After most of the water has left the film, the polyurethane phase changes from being hydrophilic to hydrophobic, and this shift acts as a trigger for the coalescent to partition differently between the two phases. Coalescent trans-fers from the acrylic phase to the polyurethane phase. Changes in how small molecules partition between poly-mer phases of different composition after film formation have been previously demonstrated in related acrylic latex systems, where the small molecules were nonvola-tile low molecular weight acrylic oligomers.7 In the pres-ent study, the small molecules are volatile coalescents. It is known that small molecules, such as coalescents, will diffuse more rapidly through a soft polymer phase due to the effect of greater free volume, and thus the coalescent will diffuse and evaporate out of the film via the soft polyurethane phase more rapidly than it would if the hard acrylic phase were by itself. Gas chromatogra-phy mass spectrometry (GC/MS) analysis of the volatiles remaining in the film over time has demonstrated that coalescent is indeed being released more rapidly from films based on the new technology, and not simply trans-ferring from one polymer phase to another and somehow affecting hardness properties in that manner.5 One re-quirement of the mechanism shown in Figure 2 is that the polyurethane phase must form a continuous network through which the coalescent can diffuse. The amount of polyurethane needed to form this “percolation network” depends on the relative particle sizes of the polyure-

thane dispersion and acrylic latex employed and the ge-ometry of particle packing, and in general a minimum of about 20–30% by weight of the total polymer is required to be polyurethane.

The improved hardness development is detailed in Figure 3, which plots Konig pendulum hardness versus drying time under ambient conditions for acrylic/polyure-thane APU-1 and the corresponding hard acrylic AC-1. The clear formulations in this experiment were fairly straight-forward, consisting of binder, water, coalescent at 20% on polymer solids, wetting aid, and thickener. A single coalescing aid was used in each case, including Texanol, dipropylene glycol butyl ether (DPnB), and the fast-evap-orating ethylene glycol butyl ether (EB). For each coales-cent, APU-1 develops hardness much more quickly than the acrylic AC-1. The ultimate hardness for both polymers, which is measured after baking the panels to drive off all volatiles, is approximately 130 sec. Even after seven days, neither polymer has come close to its ultimate hard-ness when coalesced with Texanol, although APU-1 has

table 4—Block Resistance of Clear Formulations

Note: Coalescents added at 20% on total polymer solids. EB is ethylene glycol butyl ether. Block resistance rated on a 1–10 scale, 10 = best.

Figure 3—Plot of Konig hardness development for acrylic/polyurethane APU-1 and hard acrylic AC-1 in clear formulations with various coalescent packages. Coalescents in each case were present at 20% on total polymer solids.

0

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(sec

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AC-1 / TexanolAPU-1 / TexanolAC-1 / DPnBAPU-1 / DPnBAC-1 / EBAPU-1 / EB

0 1 2 3 4 5 6 7 8

Block conditions 24 hr @ RT 30 min at 60°C 24 hr @ RT 30 min at 60°C

Coalesced with 20% TexanolAC-1 3 6 9 9

APU-1 5 4 10 8

Coalesced with 20% EBAC-1 6 3 10 9

APU-1 8 8 10 9

1 day dry 7 day dry

COATINGSTECH 37February 2012

table 5—Results for Coating Systems Tested on Maple Substrate according to the KCMA Standard for Kitchen Cabinets

Sealer APU-1 AC-1Water-reducible sanding sealer

SB vinyl-modified nitrocellulose sealer

(uncatalyzed)

SB nitrocellulose sealer

Topcoat APU-1 AC-1 Water-reducible acrylic lacquer

SB CAB/Acrylic lacquer

SB nitrocellulose lacquer

VOC (g/L)Sealer < 150 < 150 < 240 < 550 < 600

Topcoat < 150 < 150 < 240 < 700 < 600

Edge Soak - Detergent and Water Resistance

pass pass passslight cracking,

whitening pass

Chemical Resistance (24 hr exposure except where noted)Vinegar 10 10 10 10 10

Lemon juice 10 10 10 10 9Orange juice 10 10 10 10 10Grape juice 10 10 10 10 10

Ketchup 10 10 10 10 10Coffee 10 10 10 10 10

Olive oil 10 10 10 10 10Ethanol 10 10 10 5 5

0.5% Palmolive 10 10 10 10 10Mustard (1 hr) 3 9 5 8 8

Hot and Cold Check Resistancepass pass pass pass pass

Shrinkage and Heat Resistancepass pass pass pass pass

Coating System Acrylic/Polyurethane Hard Acrylic COM-1 COM-2 COM-3

table 6—Chemical Resistance of Coating Systems Tested on Maple Substrate according to Wood Furniture MethodsHard Acrylic COM-1 COM-3

Chemical Spot Resistance (rated 1 - 10, 10 = best)

1 hour exposureEthanol 8 8 3 1 1

Isopropyl Alcohol 8 7 3 1 3Butyl Acetate 8 6 1 1 1

Acetone 10 7 1 1 1

16 hour exposureWater 10 10 10 10 10

Hot coffee 7 9 9 10 1050% Ethanol 9 9 9 10 10409 Cleaner 6 5 5 10 10

Isopropyl Alcohol 9 9 1 1 37% Ammonia 5 5 5 10 7

Red Ink 5 5 3 10 10Grape juice 6 10 9 10 10

Sealer APU-1 AC-1Water-reducible sanding sealer

SB vinyl-modified nitrocellulose sealer

(uncatalyzed)

SB nitrocellulose sealer

Topcoat APU-1 AC-1 Water-reducible acrylic lacquer

SB CAB/Acrylic lacquer

SB nitrocellulose lacquer

Coating System Acrylic/Polyurethane Hard Acrylic COM-1 COM-2 COM-3

February 201238 COATINGSTECH

a significant advantage. This demonstrates an important point regarding the new technology presented here. The presence of the polyurethane polymer phase in the dry film only provides a pathway for the coalescent to diffuse from the film more rapidly than through the acrylic phase. The ultimate hardness of the system relies mainly on the hardness (and Tg) of the acrylic phase. The rate of release is also dependent on the volatility of the coalescent. Thus, when a nonvolatile coalescent or plasticizer is used, the polyurethane-based pathway is ineffective at improv-ing hardness development or related properties, because the plasticizer never leaves the film. However, when a more volatile coalescent such as EB is utilized, hardness development can be greatly accelerated. In Figure 3, the APU-1 film containing EB has reached its ultimate hard-ness in less than three days, compared to the acrylic AC-1 which is still not there after seven days. Significant acceleration is also observed with a coalescent of more moderate volatility, such as DPnB.

Another benefit of the faster coalescent release is improved block resistance, as shown in Table 4 for clear formulations based on APU-1 and AC-1. Formulations were similar as described previously, and no waxes or mar/slip aids were included in order to see the polymer effect on block resistance. Coatings were coalesced with either a slow coalescent (Texanol) or a fast coalescent (EB) at 20% on polymer solids. Coatings were dried at room temperature for one or seven days before evalu-ating both 24-hr room temperature and 30-min oven (60°C) block resistance. The data shows an advantage for the acrylic/polyurethane blend technology, especially at early dry times. Block resistance is an important prop-

erty for factory-applied coatings such as those applied to kitchen cabinets, and it must develop before coated items are stacked or packaged to prevent damage to the new film. It should be noted that in the examples above, APU-1 and AC-1 were formulated at equivalent VOC lev-els, to show that hardness properties are truly developed due to faster coalescent release, and not to any differ-ences in the amount of volatile coalescents used in the formulations. Because a significant amount of the poly-mer in APU-1 is a soft PUD, it can actually be formulated at lower VOC compared to AC-1 and still maintain other film properties such as flexibility and barrier properties.

Coatings based on APU-1 and AC-1 at equivalent VOC were also evaluated over maple according to test methods used for kitchen and bath cabinets and wood furniture. The clear formulations used are shown in Tables 1 and 2. Two commercially available solventborne systems, based on nitrocellulose sealers and either CAB/acrylic (COM-2) or nitrocellulose (COM-3) topcoats were used for comparison. Also included was a commercially available waterborne system (COM-1) using an acrylic topcoat over a waterborne sealer of an undisclosed polymer type. All commercial systems were recommended by their manufacturers for kitchen cabinet and furniture application. Results of testing the self-sealed coating systems according to the Kitchen Cabinet Manufacturers Association standards are shown in Table 5. Overall, APU-1 performed very well versus the much higher VOC commercial systems, only showing some staining in the mustard spot resistance. Similar testing typi-cal for furniture coatings was also carried out, and results are included in Table 6. Again, APU-1 performs very well, with excellent solvent resistance compared to the industry

Figure 4—Scratch and mar damage for wood floor fin-ishes. Coatings were applied to aluminum panels, and impacted with steel hex bolts for 2 min. Photographs are a close-up of a 1 sq.in. area and representative of the entire panel. The wood finishes are: A—waterborne oil-modified polyurethane (<190 g/L); B—waterborne acrylic (<250 g/L); C—solventborne polyurethane (<350 g/L); and D—APU-1 (< 150 g/L).

Figure 5—Scratch and mar damage for wood floor fin-ishes. Coatings were applied to aluminum panels, and impacted with metal paint can lids for 2 min. Photographs are a close-up of a 1 sq.in. area and representative of the entire panel. The wood finishes are: A—waterborne oil-modified polyurethane (<190 g/L); B—waterborne acrylic (<250 g/L); C—solventborne polyurethane (<350 g/L); and D—APU-1 (< 150 g/L).

C D

A B

A B

C D

COATINGSTECH 39February 2012

standards. We have found that most of the resistance properties for the acrylic/polyurethane blends are dictated by the resistance properties of the acrylic component. In this case, AC-1 is already known to perform well in kitchen cabinet and furniture coatings.

An evaluation of the scratch and mar resistance was carried out to determine if the new technology might be suitable for wood floor finishes. The clear formulation of APU-1 shown in Table 1 was compared to several commercial wood floor coatings. The commercial floor finishes included a waterborne acrylic (< 250 g/L), a waterborne oil-modified polyurethane (< 190 g/L), and a solventborne polyurethane (< 350 g/L), and all are rec-ommended for wood sports floors. Scratch and mar was tested according to the new method described in the Experimental section, and involved impacting a coated

0

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AC-2

APU-2

Commercial WB Latex

Commercial WB Alkyd

0 2 4 6 8 10 12 14 16

Figure 6—Plot of hardness development vs. time for experimental and commercial gloss white trim paints.

aluminum panel with various types of media. Figure 4 shows pictures of representative sections of panels that were impacted with steel bolts, and Figure 5 shows the same coatings after impact from a metal paint can lid. Damage to the coatings is greater when using the more angular steel hex bolts, and although the coating based on APU-1 shows some damage, it demonstrates a slight to moderate advantage versus all of the commercial coatings. The damage inflicted by the metal paint can lids is more of a marring effect, compared to the goug-ing and scratching observed with the steel bolts. Still, the coating based on APU-1 shows a definite advantage compared to the two waterborne commercial finishes, and is equivalent to the high-performing solventborne urethane finish. We have also observed better perfor-mance for APU-1 in this test compared to the hard acrylic AC-1 in formulations both with and without waxes and mar aids.5 The excellent mechanical durability suggests that the new technology might be useful in applications where scratch and mar resistance are critical properties.

Pigmented wood coatings are used in a variety of ap-plications, including architectural painting and factory fin-ishing of articles such as moldings, joinery, and cabine-try. The acrylic/polyurethane technology was evaluated in pigmented coatings designed to mimic architectural DIY gloss trim paints. The acrylic/polyurethane APU-2, based on the hard acrylic AC-2 (Tg = 35°C), was used for this study, and compared to AC-2 and some commercially available trim paints. To demonstrate the lower VOC capability of the new technology, both APU-2 and AC-2 were formulated into 18 PVC gloss white paints at vari-ous VOC levels, and MFFT values measured. Results are given in Table 7, showing that even at a VOC reduction of approximately 40%, comparable film formation was achieved with APU-2.

For the comparison with the commercial trim paints, APU-2 and AC-2 were formulated, at equivalent VOC lev-els, into 19% PVC/33% volume solids gloss white coat-ings. Formulations are shown in Table 3. Commercially available gloss white interior trim paints based on wa-terborne latex and waterborne alkyd technologies were obtained from the manufacturers. Figure 6 shows the Konig hardness development of the coatings over a two-week period. The acrylic/polyurethane blend technology displays a significantly higher rate of hardness develop-ment compared to both the hard acrylic AC-2 and the commercial trim paints, which barely increases over the course of two weeks. Pencil hardness measurements at two weeks also showed an advantage for the new technology of a few pencil units (HB vs. values of 2B and 3B). The improvements in hardness are also reflected in print resistance, which is shown in Table 8 for the same paints. Print resistance was measured after both one– and seven–day drying periods, and tested for two conditions: (1) 24 hr at RT and (2) 30 min at 60°C. The acrylic/polyurethane APU-2 showed superior print resis-tance under all conditions.

Binder AC-2 APU-2

Coalescent (% on polymer solids)Texanol 17.0 8.3

DPM 8.0DMM 2.9

VOC (g/L) 209 126VOC reduction -40%

MFFT (°C)Visual -3.3 -3.3

Tape pull 8.2 7.7

Note: DPM = dipropylene glycol methyl ether DMM = dipropylene glycol dimethyl ether

table 7—MFFT for Formulated 18 PVC Gloss White Coatings

Note: DPM = dipropylene glycol methyl ether; DMM = dipropylene glycol dimethyl ether.

February 201240 COATINGSTECH

conclusIons

Coatings scientists have been looking for good meth-ods of balancing VOC level, quality of film formation, and hardness properties in acrylic latex coatings since VOC restrictions came into effect. By manipulating film and latex particle morphology through hard/soft blends or multi-domain structures such as core/shell latexes, for-mulators have been able to lower VOC while maintaining some hardness properties, but these approaches are not optimal for insuring good film formation. The approach presented in this article entails designing the final film structure such that a more optimal balance of film forma-tion and hardness properties are maintained at low VOC levels. Incorporation of a “percolation network” of a spe-cially designed soft polyurethane polymer within a continu-ous phase of a hard, hydrophobic acrylic provides a film structure that allows the rapid diffusion and evaporation of coalescent from the drying film. Volatile coalescents will diffuse faster through the soft polyurethane phase due to free volume effects, and the continuous network provides a pathway for the coalescent to escape more quickly compared to the hard acrylic by itself. When tested in vari-ous clear and pigmented wood coatings, the benefits are improved hardness development and related properties such as block, print, scratch, and mar resistance at lower VOC levels. The new acrylic/polyurethane technology de-scribed here offers these qualities in an environmentally advanced, one-component, low-VOC waterborne coating, and provides the coatings industry with a powerful tool for both architectural and industrial wood coatings.

acknowlEdgmEnts

The authors would like to thank Mr. Hank Bernacki for his assistance with paint formulation and testing.

references

1. For example, see: (a) Winnik, M.A., and Feng, J., “Latex Blends: An Approach to Zero VOC Coatings,” J. Coat. Technol., 68 (852), 39-50 (1996); (b) Geurts, J., Bouman, J., and Overbeek, A., “New Waterborne Acrylic Binders for Zero VOC Paints,” J. Coat. Technol. Res., 5 (1), 57-63 (2008).

2. For example, see: (a) Heuts, M.P.J., le Fêbre, R.A., van Hilst, J.L.M., and Overbeek, G.C., “Influence of Morphology on Film Formation of Acrylic Dispersions,” Chapter 18 in Film Formation in Waterborne Coatings, Provder, T., Winnik, M.A., and Urban, M.W. (Eds.), 1996; (b) Schuler, B., Baumstark, S., Kirsch, S., Pfau, A., Sandor, M., and Zosel, A., “Structure and Properties of Multiphase Particles and Their Impact on the Performance of Architectural Coatings,” Prog. Org. Coat., 40, 139-150 (2000); (c) Kirsch, S., Pfau, A., Stubbs, J., and Sundberg, D., “Control of Particle Morphology and Film Structures of Poly(n-butylacrylate)/Poly(methylmethacrylate) Composite Latex Particles,” Colloid Surface A, 183-185, 725-737 (2001).

3. Fu, Z., Hejl, A., and Swartz, A., “Designed Diffusion Technology: A Paradigm Shift in Film Formation,” Eur. Coat. J., 6, 26-33 (2009).

4. ANS/KCMA A161.1-2000, Performance and Construction Standard for Kitchen and Vanity Cabinets, Kitchen Cabinet Manufacturers Association, 2001.

5. Procopio, L., Vielhauer, L., and Greyson, E., “Novel Hybrid Technology Accelerates VOC Release from Wood Coatings,” Paint & Coatings Industry, 27 (5), 48-62 (2011).

6. (a) Greenguard Indoor Air Quality (IAQ) Standard for Building Materials, Finishes and Furnishings, Standard GGPS.001, Greenguard Environmental Institute, 2009; (b) ANSI/BIFMA M7.1-2007, Standard Test Method for Determining VOC Emissions from Office Furniture Systems, Components and Seating, BIFMA International, 2007; (c) Standard Method for Measuring and Evaluating Chemical Emissions from Building Materials, Finishes and Furnishings Using Dynamic Environmental Chambers, Standard GGTM.P066, Greenguard Environmental Institute, 2008.

7. Fasano, D.M., Fitzwater, S.J., Lau, W., and Sheppard, A.S., “Diffusion of Oligomers in Latex Systems—A Route to Low Volatile Organic Compound (VOC) Coatings,” Can. J. Chem., 88, 500-513 (2010).

Print conditions 24 hr @ RT 30 min at 60°C 24 hr @ RT 30 min at 60°C

AC-2 6 4 5 4APU-2 8 6 10 9

Commercial latex 4 4 3 5Commercial WB alkyd 3 3 1 2

1 day dry 7 day dry

table 8—Comparison of Print Resistance for Experimental and Commercial Gloss White Trim Paints

autHorsLeo Procopio, Laura Vielhauer, Eric Greyson, and Andrew Hejl, The Dow Chemical Company, Spring House Technical Center, 727 Norristown Rd., Spring House, PA 19477, USA; LProcopio@dow.com.

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COATINGSTECH 41February 2012