The role of resol fortifiers in latex wood adhesives

Abstract

Static and dynamic rheological analyses using time/temperature equivalence were applied to freestanding poly(vinyl acetate-co-NMA) adhesive films and to wood-bonded films (composites), which were completely dry in all cases. Films and composites were prepared with two types of cross-linking: (1) cross-linking through AlCl₃ catalysis of N-methylolacrylamide (NMA) comonomer; and (2) additional cross-linking using a resol phenolic (PF) additive, in addition to AlCl₃ catalysis. Rheological experiments revealed that accelerated weathering (AW) significantly modified the mechanical response of films and composites lacking the PF additive. For samples lacking PF fortifier, AW caused a new thermal transition appearing as a major mechanical softening in the long time domain (creep master curves) or in the low-frequency region (dynamic master curves). This new transition correlated to a performance loss found with fracture testing in a previous publication. Here, the static and dynamic rheological data indicated that the AW-induced softening was a reversible transition. Differential scanning calorimetry analysis and the manipulation of physical aging effects demonstrated that the AW-induced softening was a glass transition. The calorimetric weakness and temperature of this transition indicated that it corresponds to the T_g of poly(vinyl alcohol) (PVOH), which is the emulsion polymerization interfacial agent. In contrast, all PF-formulated samples displayed only the base polymer T_g. Therefore, the PF additive prevented weathering effects that lead to PVOH softening. Atomic force and scanning electron microscopy provided visual evidence of the action of PF on PVOH at interparticle boundaries. We suggest that the PF fortifier enhances latex durability through the formation of hydrolytically stable PVOH cross-links at the interparticle boundaries.