Study of the synthesis and bonding properties of reactive hot-melt polyurethane adhesive

Reactive hot-melt polyurethane adhesive (RHMPA) is moisture-curing polyurethane (PU) adhesive whose main component is an isocyanate-terminated PU prepolymer that can be cured after application by reaction with ambient moisture. Two series of RHMP-containing poly(tetramethylene glycol) (PTMG) of different molecular weights and different degrees of polymerization of the hard segment have been successfully synthesized using a two-step bulk polymerization with 4,4′-diphenylmethane diisocyanate, PTMG, and 1,4-butanediol (1,4-BDO). The structure and basic properties of the RHMPA film have also been characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, atomic force microscopy, gel permeation chromatography (GPC), shore hardness (A/D), and tensile tests. The results show that the initial decomposition temperature is more than 250°C, and the tensile strength of the RHMPA film was greater than 15 MPa. The adhesive performance of the RHMPA with different hard-segment content was studied through studies of the single lap-shear strength and tensile strength of butt joints. The results show a general trend of the bonding strength increases with the hard-segment content at 25°C. The adhesive performance of RHMPA in different environments was also investigated. It was found that the RHMPA exhibited excellent bonding strength to thermoplastic polyurethane elastomer (TPU) when placed in the fridge at -20°C, hot water at 80°C, 1 M HCl solution (acid) at 25°C, and 1 M NaOH solution (alkaline) at 25°C for 10 h.