Chain extension studies of water-borne polyurethanes from methyl ethyl ketoxime/ε-caprolactam-blocked aromatic isocyanates

A series of water-based blocked aromatic polyurethane dispersions (BPUDs) were prepared by prepolymer mixing process, using toluene 2,4-diisocyanate (TDI), 4,4′-di-p-phenylmethane diisocyanate (MDI), poly (oxytetramethylene) glycol (PTMG), dimethylol propionic acid (DMPA), methyl ethyl ketoxime (MEKO) and ε-caprolactam (CL). Particle size, viscosity, pH, molecular weight, storage stability, and deblocking temperature of the BPUDs were measured and compared. Chain extension of BPUD was carried out with three types of chain extenders, hexamethylene diamine (HMDA), isophorone diamine (IPDA) and tetraethylene pentamine (TEPA), at various de-blocking temperatures. Tensile and thermal properties of the chain extended BPUD films were investigated and results confirmed that MEKO-BPUDs de-blocked at lower temperature than ε-caprolactam-BPUDs. The T_g values of the chain extended BPUDs were higher than those of the BPUDs and pure PTMG. Improvement in the thermal stability confirmed the de-blocking and chain extension reaction of blocked PU prepolymers.