Structure-property relationships of lignin-based isocyanate and amine adhesives for wood

Abstract

Hydroxyakyl lignin derivatives were reacted with polymeric methylene diphenyl diisocyanate (PMDI) and hexamethoxy-methyl-melamine (HMMM) to form polyurethane and polyether wood adhesives respectively.

Adhesive performance in shear block tests indicated: (a) that the combination of lignin and PHDI reduced the adhesive strength shown by neat PMDI. The HMMM failed to produce an acceptable wood adhesive in the absence of lignin, requiring 50-60% lignin derivative co-substrate for peak performance; (b) adhesive performance was related to molecular weight, if an organic solvent was the carrier, or solubility if the formulation was emulsified; (c) adhesive performance for the lignin based adhesives was better than a urea formaldehyde reference.

Structure property relationships were determined by correlating data obtained by the analysis of (in vivo) cured adhesive films and (in vitro) adhesive strength data resulting from shear block testing. The results indicated that: (a) glass transition temperatures of the in vivo cured adhesives were inversely related to the strength of the adhesives cured in vitro; (b) variations in infrared analysis of the in vivo cured adhesives were used to determine the levels of products from the cross linking reaction. In vitro adhesive strength was directly related to the level of reaction products determined to be present in the in vivo wood adhesives; (c) the relationships between the analysis of in vivo and in vitro cured adhesives indicated that the lignin component may act as a soft segment blocks or domains in a more rigid polymer matrix.

Particle board was produced with the lignin adhesives with: (a) properties equal to those produced with commercial OF resins; (b) spray application greatly reducing the effects of carrier compatibility; (c) none of the lignin based adhesives were water resistant.