Influence of the length of the chain extender on the properties of thermoplastic polyurethanes

Three polyurethane elastomers (PUs) were prepared using MDI (diphenylmethane-4,4'-diisocyanate), polyadipate of 1,4-butanediol, and three linear chain extenders with different lengths (ethylene glycol, 1,4-butanediol, and 1,6-hexanediol). The prepolymer procedure was used to prepare the PUs. The PUs were characterized using gel permeation chromatography, differential scanning calorimetry, wide angle X-ray diffraction, dynamic mechanical thermal analysis, and stress-controlled rheometry. The surface properties were evaluated from contact angle measurements. The PUs were used as raw materials for solvent-based adhesives, the adhesion properties of which were assessed from T-peel tests of solvent-wiped poly(vinyl chloride) (PVC)/polyurethane adhesive joints. The use of a short-chain extender produced phase separation, high crystallinity, and adequate rheological properties in the PUs. The length of the chain extender used in this study did not affect the surface properties of the PUs nor the T-peel strength of PVC/polyurethane adhesive joints but a change in the locus of failure was obtained: the decrease in the length of the chain extender, i.e. the improved crystallinity and the high degree of phase separation in the PUs, favoured the interfacial failure.