Thermal degradation of urethanes based on 4,4′-diphenylmethane diisocyanate and 1,4-butanediol (MDI/BDO)

Differential scanning calorimetry (d.s.c.) and gel permeation chromatography (g.p.c.) were used to study the thermal degradation of 4,4′-diphenylmethane/1,4-butanediol based monodisperse urethane model compounds. It was found that the urethane bond was unstable above 170°C in the crystalline state as well as in the molten liquid state. Significant degradation occurred above 200°C. The annealing time was found to be as important as the annealing temperature. The degradation species present were identified by g.p.c. using an absolute molecular weight-retention volume calibration curve. Polymerization as well as depolymerization occurred during the degradation process, so that the molecular weight distribution tended to broaden. The degradation mechanism was found to be thermal dissociation of the urethanes into free isocyanates and alcohols as confirmed by infra-red spectroscopy and gas liquid chromatography. The degradation rate fits well with first order kinetics with respect to the urethane. The triblocks and oligomeric multiblocks of MDI/BDO/poly(propylene oxide) polyol/polyurethanes were also found to be unstable at elevated temperature.