Effect of crosslinking on mechanical and viscoelastic properties of semiinterpenetrating polymer networks composed of poly(vinyl chloride) and isocyanate crosslinked networks

Semiinterpenetrating polymer networks (SIPNs) of PVC/isocyanate/polytriol w ere prepared by premixing small (150 mu m dia.) porous (30% voids) unplasti cized PVC particles, 10% by weight of isocyanate, and a triol at different OH/NCO mol ratios. Three types of isocyanates (methylene bis-phenyl diisocy anate (MDI), oligomeric NIDI isocyanates (PAPI), and toluene diisocyanate ( TDI) prepolymer/polytriol) were used. Two-roll milling was followed by hot- press curing. The tensile, flexural, and impact strengths increased when sm all amounts of crosslinked isocyanate networks were created in PVC. The iso cyanate/polyol hydroxyl stoichiometry was varied, and the effects of crossl inking on the tensile, impact, and flexural strengths of PVC/isocyanate/tri ol SIPNs were examined. The strength increments were greater when the OH/NC O mole ratio went from 0 to 0.25, than when it went from 0.25 to 1.0. In ma ny cases, increasing OH/NCO mol ratio from 0.5 to 1.0 decreased tensile, im pact, and flexural strengths. Both PAPI and MDI (30% NCO content) gave bigg er improvements in the these mechanical strengths than the TDI (only 9.7% N CO). These SIPN blends exhibited lower tan delta peak temperatures and a si ngle distinct loss modulus, E", peak values at lower temperatures than thos e of PVC that had been exposed to the same processing temperatures. Substan tial amounts of isocyanate networks exist in SIPN phases according to DMTA studies. The OH/NCO ratio did not generally correlate with the decreases in the glass transition temperatures in these three sets of blends. (C) 2000 John Wiley & Sons, Inc.