Coffee silverskin and cocoa pod husk modified with methacrylic acid as fillers for the urea-formaldehyde resin in plywood production

The aim of the study was to investigate the effect of using various amounts of coffee silverskin and cocoa pod husk (5%, 10%, 15%), both unmodified and modified with methacrylic acid, as the urea-formaldehyde adhesive fillers on the properties of plywood. The attenuated total reflectance Fourier transform infrared spectroscopy showed that the performed modification was effective. Based on the contact angle measurement and the analysis of phenolic compounds content using ultra performance liquid chromatography, it was found that the modified fillers were less hydrophilic and demonstrated lower content of phenolic acids and flavonoids. The viscosity and reactivity of the adhesive mixtures were influenced solely by the filler loading, rather than by the type or modification of the filler. The use of the tested fillers enabled the achievement of the wet shear strength level required by the standard. Furthermore, it was demonstrated that modifying the fillers with methacrylic acid positively influenced the strength of the bond lines. The negative effect on density, bending strength and modulus of elasticity was observed only for variants with the lowest filler content. Furthermore, the results of formaldehyde content in the cured adhesive and formaldehyde emission of plywood demonstrated the formaldehyde-scavenging ability of cocoa pod husk.