Development of a coating technology for wood plastic composites

Barun Shankar Gupta

Wood plastic composites (WPCs) are gaining increasing popularity in exterior applications. However, the weatherability of WPCs drives the need for developing surface finishes that would offer both protection and aestheticism. Developing a suitable coating system for WPCs requires a thorough understanding of the surface properties, surface chemistry and wettability in particular, since these properties directly relate to paint adhesion to WPCs. The first objective of this thesis is therefore to understand the surface properties of WPCs and their relationship to the adhesion of a standard coating. The adhesion of coatings to plastics is greatly improved by surface treatments. The second objective of this research is then to evaluate the impact of common plastic surface treatments on the surface properties of WPCs and on the adhesion of a standard coating to WPCs. In performing these objectives, 8 WPC formulations are manufactured according to a 23 full-factorial, so that the effect of formulations on the surface chemistry, wettability and primer adhesion can be determined. Surface chemistry and wettability are evaluated using attenuated-total-reflection FTIR (ATR-FTIR) spectroscopic measurements and dynamic contact angle analysis with water. Primer adhesion to WPC is determined from an 180o Peel test. Untreated WPCs showed high contact angle (95-105o ) with water and low surface energy (31.5 mJ/m 2 ) similar to neat polyolefins. As a result, the primer adhesion to untreated WPCs (177-309 N/m) is intermediate to that of neat polyolefins (126-48 N/m) and neat wood (526 N/m). The primer adhesion is linearly related to WPC surface roughness and heterogeneity as determined from contact angle hysteresis. Moreover, the four surface treatments implemented, oxygen plasma, flame, ultraviolet (UV)-Benzophenone (BP) and chromic acid increase the primer adhesion to WPCs by 1.5-2.5 fold. The chromic acid treated (637 + 88 N/m) and oxygen plasma treated (516 + 116 N/m) WPCs show the highest primer adhesion, greater than that with neat wood. Surface analyses further suggest that different adhesion mechanisms are responsible for the enhancement in coating adhesion. In the case of plasma treated WPCs, surface oxidation and therefore primary bonding and secondary interactions likely play an important role. On the other hand, chromic acid treatment increases surface roughness significantly resulting in greater interfacial contact for adhesion mechanisms, Finally, to validate the results obtained on bulk WPCs, the extruded 'skin' surface and the bulk 'core' material of WPCs is compared. Concentration of lubricants and plastic in the 'skin' compared to the core yield lower peel load (135 + 24 N/m)

Development of a coating technology for wood plastic composites

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