Moisture-curing polyurethane hot melt adhesives continue to gain market acceptance over liquid adhesives because they permit more rapid assembly processes. This study set out to understand how the performance of these adhesives is affected by the method by which they are synthesized, the ratio of primary ingredients, and the conditions under which they are applied. These hot melt adhesives are complex mixtures of a broad distribution of urethane prepolymers with an acrylic copolymer. Surprisingly, many formulations exhibited liquid-liquid phase separation. It was found that the phase behavior was dependent on both the formulation and the method used to synthesize the polyurethane prepolymers. The presence of small amounts of block copolymer dramatically improved overall system miscibility. Miscible systems crystallized (set) more slowly, but cured more rapidly. Crystallization rate decreased dramatically at slightly elevated temperatures (simulating summer use conditions). Cure rate increased rapidly with ambient temperature and humidity, and for thick sections became limited by moisture diffusion. Final crosslink density was extremely low and showed surprisingly little dependence on formulation.
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