Photostability of Polyurethane Elastomers Based on Two Types of Diisocyanates

Abstract

The effect of UV irradiation on the mechanical and ther momechanical properties of polyurethane (PU) elastomers and their morphol ogy was studied. The chemical changes in the PU elastomers exposed to UV radiation were studied by means of Fourier Transform Infrared Spectroscopy (FTIR). The PU elastomers were based on an aromatic isocyanate (4,4' diphenylmethane diisocyanate) and a cycloaliphatic isocyanate (4,4' dicyclohexylmethane diisocyanate). Poly(caprolactone) glycol of 1250 molecular weight was the polyol used and 1,4-butanediol was the chain extender. The elastomers were prepared in a two-step procedure. First, prepolymers were pre pared from poly(caprolactone) glycol by reacting with the corresponding iso cyanate at NCO/OH ratios of 2/1 and 4/1. The prepolymers were then chain- extended with 1,4-butanediol and the mixture was press-molded in a platen press at 100°C. The results showed that the stability of the investigated PU elastomers depended on the chemical structure of the diisocyanate and the hard/soft segment ratio. The processes of photooxidative degradation are more prevalent in PU elastomers based on cycloaliphatic isocyanate than those based on the aromatic isocyanate. In the case of PU elastomers based on the cycloaliphatic isocyanate it was found that the photodegradative processes were enhanced with increasing hard segment concentration. The degree of pho tooxidative degradation of PU elastomers, as studied by FTIR, correlated very well with the extent of morphological changes and changes in mechanical prop erties. Although PU elastomers based on the aromatic isocyanate undergo color changes, they exhibited better retention of mechanical properties com pared to those based on the aliphatic isocyanate.