Structure–property behavior of moisture-cure polyurethane wood adhesives: Influence of hard segment content

One-part moisture-cure polyurethane (PUR) wood adhesives and the effects of hard phase content were studied at three levels: liquid prepolymer, cured neat film, and cured adhesive bondline. Three PURs were prepared from polymeric methylenebis(phenylisocyanate) and poly(propylene glycol) with variable hard phase content (53.5–72.5%). At the prepolymer level, increasing hard phase content increased intermolecular interactions, liquid viscosity, and the soft phase glass transition temperature. When cured as neat films, greater hard phase content caused higher hard phase softening temperature, and greater quantities of hydrogen-bonded urea structures. When bonded within wooden dual cantilever beams, adhesive penetration and bondline thickness varied appreciably. Regardless of these many differences, the measured mode-I fracture toughness was rather insensitive to the adhesive hard phase content. This suggested that a critical hard phase content had been exceeded such that a variety of bonding variables became ineffectual. It was hypothesized that wood bonding performance is compromised when the PUR hard segment content exceeds about 51%, near the composition when the hard phase becomes continuous in the dual-phase PUR morphology. It was suggested that morphological effects should be studied for wood-bonding polyurethanes having hard segment contents near about 50%, near the composition associated with phase inversion or with the loss of a co-continuous morphology.