By Laurie Morris, senior chemist, and Dr. Terri Carson, technical director, Alberdingk Boley, Inc.
Introduction
Waterborne (WB) UV chemistry has shown significant growth in interior industrial wood markets because the technology provides excellent performance, low solvent emissions and increased production efficiency. These same advantages can be beneficial for factory-applied exterior applications including window and door frames, siding and other millwork. These market segments conventionally utilize acrylic emulsions and polyurethane dispersions because they have excellent gloss and color retention and demonstrate superior durability. WB UV coatings are a viable alternative for the exterior market and have excellent performance on multiple substrates. This study will investigate WB UV coatings according to Window and Door Manufacturers Association (WDMA) TM 12-20 which specifies test methods for factory-applied pigmented finish coatings for wood and wood cellulosic composites used for millwork. Data will be presented discussing the features and performance advantages of WB UV coatings compared to commercially available 2K polyurethane coatings.
Benefits of UV Coatings
UV coatings systems offer the end user the benefits of outstanding chemical and scratch resistance, excellent block resistance, very low voltatile organic compounds (VOCs) and a small equipment footprint with less storage space required. These systems have properties that compare favorably with two-component urethane systems without the complications of hazardous cross-linkers and pot life concerns. The overall system is cost effective because of increased production speeds and lower energy costs. Additionally, waterborne UV coatings based on polyurethane dispersions (PUDs) have many inherent advantages. While 100% solid UV oligomers are typically high in viscosity and must be diluted with reactive diluents, WB UV PUDs are low in viscosity, and the viscosity can be adjusted with traditional WB rheology modifiers. WB UV PUDs have an initially high molecular weight and do not build molecular weight as they cure as dramatically as 100% solids UV coatings. Because they have little or no shrinkage as they cure, WB UV PUDs have excellent adhesion to many substrates. The gloss of these coatings is controlled easily with traditional matting agents. These polymers are quite versatile, and the polymer backbone can be designed to produce very hard polymers that are also extremely flexible, making them potential candidates for exterior wood coatings.
Traditional Exterior Wood Coatings
Environmentally friendly exterior wood coatings traditionally are based on waterborne acrylic emulsions and aliphatic polyurethane dispersions. Both one-component (1K) and two-component (2K) coatings are used, with the 2K coatings providing the best performance properties. WB acrylic emulsions have excellent UV stability and very good weathering resistance. These dispersions, however, can have limited flexibility depending on the morphology, leading to poor grain crack resistance. Flexibility is necessary for an exterior wood coating because wood is a dimensionally unstable substrate. The mechanical properties of polyurethane dispersions can be beneficial, leading to good flexibility while maintaining the surface hardness required for block resistance, scratch resistance, mar resistance and dirt pick-up resistance. The morphology plays a key role, and factors such as phase separation, domain structure and hard and soft segment ratios dictate the polymer characteristics.
Project Plan
The performance of four WB UV products produced by Alberdingk Boley Inc. (see Table 1) has been benchmarked against WB 2K products, including 2 hydroxy functional resins from Alberdingk and two commercial offerings for exterior industrial wood applications (see Table 2). The hardness development, adhesion, abrasion resistance, block resistance, chemical resistance, freeze-thaw resistance, impact resistance, detergent resistance, humidity resistance and xenon arc exposure were evaluated. Coatings were formulated using both UV absorbers and hindered amine light stabilizers (HALS). All products were formulated into pigmented coatings, and the WB UV coatings were cured using both mercury and gallium light sources.
Panel Preparation
Birch plywood and pre-primed composite siding panels were cut into 4 x 4-inch pieces, and the sides and backs of the panels were sealed with a 2K coating. The WB UV and WB 2K coatings were applied to the panels (see Tables 3 and 4). WB UV coatings were air dried for 10 minutes, then force dried for 10 minutes at 50° C. The panels were cured with both gallium and mercury bulbs at 800 mJ/cm2. The panels were sanded, and a second coat was applied and cured. WB 2K coatings were applied to the panels, allowed to air dry overnight, sanded, and a second coat was applied.
Test Methods and Data
Hardness development
WB UV coatings: Make a 150 micron draw-down on a glass panel. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Cure the panels with gallium and mercury bulbs at 800 mJ/cm2. Measure Konig Pendulum hardness at 1, 4 and 7 days after cure (see Figure 1).
WB 2K: Make a 150 micron drawdown on a glass panel. Air dry. Measure Konig pendulum hardness after 1, 4 and 7 days (see Figure 1).
Crosshatch Adhesion
All adhesion was tested on birch plywood panels.
Dry adhesion: Age the panels for 24 hours. Make crosshatch cuts. Apply Intertape® 51596 to the cut surface. Sharply pull the Intertape off at a right angle to the plane of the surface being tested and record results.
Wet Adhesion: Age the panels for 24 hours. Immerse the panels in water at 100° F for 24 hours. Allow them to air dry for 24 hours. Apply Intertape 51596 to the cut surface. Sharply pull the Intertape off at a right angle to the plane of the surface being tested and record results.
Boiling Water Adhesion: Age the panels for 24 hours. Immerse the panels in boiling water for 20 minutes. Allow them to air dry for 24 hours. Apply Intertape 51596 to the cut surface. Sharply pull the Intertape off at a right angle to the plane of the surface being tested and record results (see Table 5).
Taber Abrasion Resistance
Abrasion resistance was evaluated using a Taber Abrader with CS 17 wheels, 1000 grams weight and 1000 cycles.
WB UV Coatings: Make a 10 mil drawdown on a black scrub chart. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Cure the panels with gallium and mercury bulbs at 800 mJ/cm2. Age the panels for 7 days.
WB 2K: Make a 10 mil drawdown on a black scrub chart. Air dry. Age the panels for 7 days
(see Figure 2).
Chemical Resistance
WB UV coatings: Make a 4 mil drawdown on maple plywood. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Cure the panels with gallium and mercury bulbs at 800 mJ/cm2. Sand the panels and repeat. Age the panels for 7 days.
WB 2K coatings: Make a 4 mil drawdown on maple plywood. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Sand the panels and repeat. Age the panels for 7 days.
Chemicals tested: 1% detergent, Formula 409, 70% isopropyl alcohol, vinegar and Windex – 1-hour open spot test. Glass cleaner – 24-hour covered spot test.
Evaluate staining, softening and film failure based on the following scale (see Figures 3 and 4):
5 = No impact on film
4 = Very light discoloration and no softening or film deterioration
3 = Moderate discoloration and/or moderate film softening and no film deterioration
2 = Heavy discoloration and/or moderate film softening allowing easy deformation
1 = Heavy film softening with little loss of adhesion
0 = Complete deterioration of the film
Freeze/Thaw Resistance
Allow birch plywood panels to age for 7 days. Place the panels in a humidity cabinet at 40° C and > 95% Relative Humidity for 24 hours. Transfer the panels to a freezer at -23° C for 20 hours. Remove the samples and allow them to remain at room temperature for 4 hours. Rate the panels for cracking, loss of adhesion and blistering. Repeat for a total of 5 cycles.
Block Resistance
WB UV coatings: Make a 4 mil drawdown on maple plywood. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Cure the panels with gallium and mercury bulbs at 800 mJ/cm2. Sand the panels and repeat. Age the panels for 24 hours.
WB 2K coatings: Make a 4 mil drawdown on maple plywood. Air dry for 10 minutes, then force dry for 10 minutes at 50° C. Sand the panels and repeat. Age the panels for 24 hours.
Cut panels to 4 square inches of coated surface. Place 2 panels with coated surfaces, face-to-face, in a hydraulic press at 50 psi for 1 hour. Rate for block resistance.
Impact Resistance
Allow birch plywood panels to age for 7 days. Using a Gardner Impact tester with a 0.5-inch diameter ball, test the effect of a 20-inch-pound impact. Measure and record any paint removal.
Detergent Resistance
Allow birch plywood and pre-primed composite panels to age for 7 days. Prepare detergent solution as directed in the WDMA TM12-20 standard. Immerse the panels in the detergent solution at room temperature for 24 hours. Remove the panels and rinse with water. Allow samples to recover for 24 hours. Record any blistering. Apply tape along the entire surface of the panel. Sharply pull the tape off at a right angle to the plane of the surface being tested. Record any loss of adhesion.
Discussion
Overall, the performance of the WB UV coatings was comparable to the 2K coatings, with some improvements in properties. As anticipated, the WB UV coatings exhibit much faster hardness development than the WB 2K coatings due to the rapid curing after UV exposure. The hardnesses of the UV 1 and UV 2 formulas were higher than those of UV 3 and UV 4, as these polymers do not incorporate any softer acrylic segments in the polymer network. The reduced hydroxyl number for 2K-1 accounts for the lower hardness compared to 2K-2. Quicker hardness development is advantageous and could enable faster production speeds, leading to better productivity. Further, it eliminates the need for forced drying and curing, which require specialized drying equipment to accelerate cure for 2K systems. All the Alberdingk coatings had significantly better abrasion resistance compared to the commercial controls. Such properties are beneficial for long-term protection and could minimize physical wear and tear and damage of the parts caused by impacts and marring in high -traffic areas. It should also be noted that waxes were included in the formulas, which also can have a positive impact on wear resistance. The impact and block resistance were comparable to the 2K formulas indicating high durability. Other properties such as detergent, freeze/thaw resistance and adhesion were comparable as well. Robust adhesion is critical for protection considering wood movement. The freeze/thaw test is an extreme environment for wood and wood coatings. Wood swells during periods of high humidity, and it shrinks during periods of low humidity. This instability puts severe stress on the coating. Coatings that do not have adequate flexibility can show whitening or cracking when exposed to this punishing environment. All the coatings showed excellent performance despite exposure to fluctuating harsh conditions (see Table 6).
One of the UV coatings (UV 2) showed inferior resistance to alkaline media, but the remaining products showed high resistance to common household agents. Increasing the crosslink density or selecting a different polyol could improve performance.
The low VOCs (≤ 50 g/L) of the WB UV coatings should also be noted, recognizing that 2K WB systems generally are formulated at approximately 250 g/L, restricting certifications for programs like Green Guard. The 2K formulas also have significant health hazards, including possible skin sensitization and respiratory issues. Two-component formulas also have narrow formulation latitude due to the pot life limitations and precise mixing ratios required of the resin to hardener for optimal performance. Any minor deviations of mixing ratios can lead to incomplete curing and cause application failures and visible defects. There is no possibility of reclaiming unused coating, considering reactions begin at the start of mixing. WB UV coatings are formulated with a photoinitiator that typically has unlimited pot life.
Conclusions
Water-based UV coatings have become the industry standard for interior industrial wood applications. The excellent hardness, flexibility and chemical resistance of these coatings make them good candidates for factory-applied finishes for exterior wood applications. WB UV coatings 1, 3 and 4 have equal or better performance compared with the WB 2K coatings. This technology combines the eco-benefits and ease of WB formulas, yielding an increase in productivity. Humidity and xenon arc exposure testing is ongoing. Future work will also focus on exterior exposures according to WDMA TM12-20 specifications.
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Laurie Morris is a senior chemist at Alberdingk Boley, Inc. She has worked in the coatings industry for over 45 years. She began her career in the general industrial lab at PPG Industries in 1979. She later worked as a chemist for The Permite Corporation and Eagle Bridges Paints. She became production manager at Eagle Bridges Paints in 1997 and continued in that role for eight years. She has been at Alberdingk Boley, Inc. for 20 years, where she works in the applications lab.
Dr. Terri Carson is an alumna of the University of North Carolina at Chapel Hill. Post-graduation, she spent six years at The Dow Chemical Company in Freeport, Texas, where she served as a functional development specialist within the epoxy and polyurethane business. In 2006, Dr. Carson joined Alberdingk Boley Inc. as the product development manager. She has since advanced to the role of technical director, where she currently oversees both the technical service team and quality control.
