Block copolymers as low-VOC coatings for wood: characterization and tannin bleed resistance

Abstract

Replacement of solvent-borne primers with low-VOC water-based coatings has created the problem of tannin bleed for cedar siding finishers. Three amphiphilic block copolymers, poly(styrene- b-acrylic acid) (PS-PAA), poly(styrene- b-ethylene oxide) (PS-PEO), and poly(styrene- b-methacrylic acid) (PS-PMAA), were investigated as potential water-based coating components that could create a hydrophobic barrier to block tannin extraction. Resistance to extractive bleed was determined by measuring topcoat color change under ambient and humid exposure of coated wood panels. Contact angles of water on the copolymer-coated surfaces were measured to reveal the relative hydrophobicity of the coating surface. Adhesion characteristics of the experimental coatings on wood substrates were determined with a standard tape test. The block copolymer coatings were found to have less darkening and yellowing of the topcoat than the unprimed control and commercial oil-based primer. They also exhibited bleed resistance similar to that of a commercial water-soluble tannin-blocking polymer, but did not perform as well as a commercial water-based tannin-blocking coating. Most copolymers had contact angles close to or greater than 90°, indicating hydrophobic surfaces, with four coatings exhibiting surfaces as hydrophobic as a commercial oil-based primer. Adhesion of the copolymer coatings was found to be adequate under ambient conditions but poor for substrates exposed to humidity. Thus, the amphiphilic block copolymers appear to be potential waterborne tannin-blocking agents with other properties suitable for wood coatings.