The Relative Thermal Stability of Polyurethane Elastomers: 3. Influence of Chain Extender Structure

Abstract

A method of using high melting point chain extenders in the synthesis of castable polyurethane elastomers (PU) based on the system polycaprolactone diol (Capa 225) and trans 1,4–cyclohexane diisocyanate (CHDI) was developed based on a semi-prepolymer (quasi) technique in which the high melting point chain extender was first dissolved in polyol before mixing with the diisocyanate. Using this technique it was found possible to carry out a bulk polymerisation in the normal PU reaction temperature range of 100–130°C.Thermal stability properties were measured by means of DMTA and DSC and the temperature at which the value of dynamic storage modulus (log E') changed significantly in the rubber region used to indicate the limit of thermal stability of the polyurethanes. As the molecular symmetry of the chain extender increases so does the thermal stability of the resulting PU.The best thermal stability was obtained by the use of diamine chain extender, trimethylene glycol di-p-aminobenzoate which results in a polymer thermally stable to 230°C.Using an odd molar ratio of polyol/diisocyanate, with diisocyanate being present in excess, also enhances thermal stability by the formation of isocyanurate crosslinks: such PU elastomers retain their characteristically high elongation at break.