Cross-linking reaction of waterborne polyurethane adhesives containing different amount of ionic groups with hexamethoxymethyl melamine

Two series of cross-linkable waterborne polyurethane (WBPU) adhesives were synthesized by a conventional prepolymer process using 2,2-bis(hydroxymethyl) propionic acid (DMPA) (13.85 and 25.00 mol%) and hexamethoxymethyl melamine (HMMM) as a hydrophilic agent and cross-linking agent, respectively. Using infrared (IR) and ¹H NMR spectroscopy analysis, the cross-linking reaction between DMPA (carboxyl acid salt group) and HMMM (methoxy group) was identified, and it was found that two methoxy groups per HMMM molecule, containing six methoxy groups, participated in the cross-linking reaction. Adhesive strength significantly increased by cross-linking, up to the optimum content of HMMM (molar ratio of HMMM:DMPA=1:2) and then showed a small decrease with excess HMMM. Adhesive strengths of samples containing 25.00 mol% DMPA (i.e. S-series) were higher than those of samples containing 13.85 mol% DMPA (i.e. P-series). It was found that the optimum pressing conditions for joint values and optimum HMMM content gave the highest failure temperature and thermal stability of adhesives.