This test method was developed by modifying a standard torsional rheometer (RSM 800 from Rheometrics), so that the torque required to rotate one polymer surface across another contacting polymer surface could be measured and converted into coefficients of friction [18].

The results showing the variations in coefficients of friction with temperature as measured by the two test techniques are illustrated in Figs.

8, the coefficients of friction are quite low at lower temperatures and only change significantly above 100[degrees]C; thereafter, the values rise extremely sharply.

In general, the measured coefficients of friction with PP are slightly higher at lower temperatures and do not exhibit the sharp rises evident with PS around 100[degrees]C.

But the change of values for elongation and reduction of area measured at different coefficients of friction is even smaller (less than 5%) than the change of tensile and yield stresses.

One of the reasons for very small difference in mechanical properties when using different lubricators with different coefficients of friction could be the fact that we have done our experiments with lubricant with rather low coefficients of friction [mu] (between 0,05 and 0,16).

The coefficients of friction decreased with increasing pressure for all resins.

Relationships between polymer structures and the coefficients of friction are possible, but they are beyond the scope of this paper.

The coefficients of friction for these resins increase with increasing velocity, and thus for a given pressure and temperature the coefficient will be higher at the barrel interface than at the screw root interface.

These materials have lower

coefficients of friction and higher useable service temperatures than engineered thermoplastic materials.