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. UV coatings systems offer the end user the benefits of outstanding chemical and scratch resistance, excellent block resistance, very low 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 crosslinkers and pot life concerns. The overall system is cost effective because of increased production speeds and lower energy costs. 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. In this study, polyurethane-acrylic resins with UV functionality have been evaluated according to industry specifications for both interior and exterior industrial wood applications.
Three types of solvent-based coatings are commonly used in industrial wood applications. Nitrocellulose lacquer is typically a low-solids blend of nitrocellulose and oils or oil-based alkyds. These coatings are fast drying and have high gloss potential. They are typically used in residential furniture applications. They have the disadvantage of yellowing with time and can become brittle. They also have poor chemical resistance. Nitrocellulose lacquers have very high VOCs, usually at 500 g/L or higher. Pre-catalyzed lacquers are blends of nitrocellulose, oils or oil-based alkyds, plasticizers and urea-formaldehyde. They use a weak acid catalyst such as butyl acid phosphate. These coatings have a shelf life of approximately four months. They are used in office, institutional and residential furniture. Pre-catalyzed lacquers have better chemical resistances than nitrocellulose lacquers. They also have very high VOCs. Conversion varnishes are blends of oil-based alkyds, urea formaldehyde and melamine. They use a strong acid catalyst such as p-toluene sulfonic acid. They have a pot life of 24 to 48 hrs. They are used in kitchen cabinet, office furniture and residential furniture applications. Conversion varnishes have the best properties of the three types of solvent-based coatings typically used for industrial wood. They have very high VOCs and formaldehyde emissions.
Water-based self-crosslinking acrylic emulsions and polyurethane dispersions can be excellent alternatives to solvent-based products for industrial wood applications. Acrylic emulsions offer very good chemical and block resistance, superior hardness values, outstanding durability and weatherability, and improved adhesion to non-porous surfaces. They have fast dry times, enabling the cabinet, furniture or building products manufacturer to handle the parts soon after application. PUDs offer excellent abrasion resistance, flexibility, and scratch and mar resistance. They are good blending partners with acrylic emulsions to improve mechanical properties. Both acrylic emulsions and PUDs can react with crosslinking chemistries such as polyisocyanates, polyaziridine or carbodiimides to form 2K coatings with improved properties.
Waterborne UV-curable coatings have become popular choices for industrial wood applications. Kitchen cabinet and furniture manufacturers choose these coatings because they have excellent resistance and mechanical properties, excellent application properties and very low solvent emissions. WB UV coatings have excellent block resistance immediately after cure, which allows the coated parts to be stacked, packaged and shipped right off the production line with no dwell time for hardness development. The hardness development in the WB UV coating is dramatic and occurs in seconds. The chemical and stain resistance of WB UV coatings is superior to that of solvent-based conversion varnishes.
WB UV coatings 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% solid 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 easily controlled with traditional matting agents. These polymers can be very hard but also extremely flexible, making them ideal candidates for exterior wood coatings.
Post time: Mar-07-2024
