DEVELOPMENT OF WEAR-RESISTANT THERMOPLASTIC POLYURETHANES BY BLENDINGWITH POLY(DIMETHYL SILOXANE) .2. A PACKING MODEL

It has been shown in a previous article that melt blending of low leve ls of commercial poly(dimethyl siloxane) (PDMS) fluid with commercial thermoplastic polyurethanes has a significant positive impact on the c oefficient of friction (CoF) and on the mechanical and wear properties of the polyurethanes. The improvements in CoF and wear resistance wer e expected due to surface modification of the polymer; however, the im provements in the mechanical properties were much more significant tha n expected. Evidence presented in the earlier publication suggests tha t the changes in the wear and mechanical properties are not due to sur face modification alone, but are largely due to modification of the bu lk by PDMS. In this article a model is presented that accounts for the observed relationship between PDMS content and the properties of the blends. It is proposed that the addition of PDMS facilitates an improv ed packing efficiency (antiplasticization) in the polyurethane soft do main, leading to improved material performance. Beyond an optimum PDMS concentration of 1.5-2.0%, phase separation of PDMS becomes significa nt, plasticization sets in, and mechanical properties then begin to di minish rapidly. This model has been rigorously investigated and has pr oven to be highly robust. (C) 1997 John Wiley & Sons, Inc.