Crystalline structure in isocyanate reactive hot melt adhesives

Isocyanate reactive hot melt adhesives (RHMA) are low molecular isocyanate-capped prepolymers prepared from polyester and or polyether glycols and diisocyanates. After application, RHMA exhibit green strength by solidification while cooling to room temperature. Polarizing optical microscope observation revealed that the crystal of isocyanate prepolymer consisted in spherulites. RHMA prepolymer with higher crystal content showed better green strength but slower adhesion development as a result of hindrance to moisture diffusion by the crystalline structure. Crystalline structure was disrupted by the penetration of crosslinking reactions. Heat of crystallization and onset crystallization temperature in cooling traces of cured RHMA melt was reduced by the interruption of crosslinked bonding. Crystalline structure was proved to play an important part in the curing process of the isocyanate RHMA.