Transparent polyurethanes : synthesis, characterisation and mustard gas resistance

Transparent Polyurethanes based upon Polycaprolactone and H 12MDIisocyanate (Desmodur W) have been synthesised and tested with regard to permeation by Mustard Gas (HD). Variations in soft segment molecular weight, reaction ratio, and chain extender, as well as the effect of crosslinking have been investigated. It was found that the principal factor governing permeation behaviour was soft segment molecular weight. However as hard segment content increased the determinant role of the soft segment diminished. Morphological Analysis of the synthesised polyurethanes was carried out using DSC, CMI'A, GPC, and WAXD techniques. The physical properties (Hardness, Tensile Strength, etc) of these materials were also determined. Results showed that the use of mixed diol chain extenders or trifunctional crosslinking elements produced increased phase mixing in the polyurethanes as a consequence of poorer hard segment domain organisation. This had the effect of reducing soft segment restrictions such that poorer permeation performance was obtained. . GPC and DSC Analysis of the various polycaprolactones used in the synthesis programme suggested that the lower molecular weight polyols (550-2000) were composed of at least two different molecular weight species. Work with mixed polyol formulations indicated this may affect the physical properties of the polyurethanes. GPC Analysis of the synthesised polyurethanes themselves showed a wide variety of molecular weights were obtained; these were seen to be very dependent upon polyol molecular weight. The CAPA 210 (1000 Mol.Wt.) based elastorners had the highest Mn values. It is believed these variations in molecular weight are related to the presence of different catalysts, activators, etc in the original polyols. Injection Moulding trials were carried out on the formulation considered to be the most promising candidate for the face mask material. Slight changes in physical properties were obtained as processing conditions were varied. Most significantly the polyurethane began to shown signs of yellowing at high injection temperatures (190C and above).