Commercial thermoplastic poly(urethane)s (TPU) have been melt-blended on a standard laboratory extruder with low levels of commercial poly(dimethyl siloxane) (PDMS) fluid. The resultant poly(urethane)s show improvements in wear resistance of up to 25% (cf. pure poly(urethane)) for an optimal PDMS concentration of 1.5 -2.0%, beyond which the properties diminish rapidly. Unexpectedly, the mechanical properties of the blends (as measured by an Instron tensile testing machine) have been even more significantly enhanced, by up to 150% for their energy to break. Surface studies of the blends are also reported for x-ray photoelectron spectroscopy (XPS), contact angles and coefficient of friction (p). The XPS data showed that the PDMS preferentially concentrates on the surface of the blends. This leads to an increased surface hydrophobicity (contact angle studies) and a decrease in the kinetic coefficient of friction by as much as 55%.
The improvements in the coefficient of friction and the wear resistance were expected due to surface modification of the polymer, however, the improvements in the mechanical properties were much more significant than expected. Evidence is also presented that suggests that the changes in the wear and mechanical properties are not due to surface modification alone, but are mainly due to modification of the bulk by PDMS. A model is presented which accounts for the observed relationship between PDMS content and the properties of the blends. It is proposed that the addition of PDMS facilitates an improved packing efficiency (antiplasticisation) in the poly(urethane) soft domain, leading to improved material performance. Beyond the optimum PDMS concentration of 1.5 - 2.0%, phase-separation of PDMS becomes significant, plasticisation sets in, and mechanical properties then begin to diminish rapidly. This model has been rigorously investigated and has proven to be highly robust.
Given the excellent frictional and wear properties of the blends, data is also presented that indicate that the PDMS-modified poly(urethane)s have physical properties that may make them candidates to be used as replacement materials for ultra high molecular weight poly(ethylene) (UHMWPE) in high slip, wear resistant applications.
The dry sand - rubber wheel abrasion testing apparatus has been used predominantly for all of this study. This is an ASTM test primarily designed for metal specimens, therefore the test conditions have been optimised for use with polymers. The modified test has proven very useful in the assessment of the wear of poly(urethane). Using the test, the wear performance of poly(urethane)s have been assessed over a range of Shore hardness, from Shore 85A to 65D, and a relationship between wear and hardness is presented. Poly(urethane) elastomers can be separated into three classes according to their hardness. The wear resistance of each of these classes shows a different dependency on the specimen temperature. This work has implications for accelerated laboratory wear testing and for the prediction of field performance based on such data.