Static friction polymers

The existence of a static friction stress, corresponding in fact to the flow rate q on flow curves. Thus in the conditions used in this study, the polymer melts only slip above a critical stress threshold. They adhere when at rest. 

CZlCHOS Contact Deformation and Static Friction of Polymers... 

Sliding of the Steel Sphere on Flat PET Surfaces. Figure 2 shows typical friction traces in the sliding of a steel sphere at a speed 0,25 mm/s under a load 8 N. It is seen that the static friction is considerably higher than the kinetic friction and there is no stick-slip phenomenon. Comparing the friction of PET with that of other polymers obtained in the sliding experiment ( ) similar to that in the present work, it was found that PET exhibited relatively lower... 

Another approach to static friction is to measure friction at a number of very low-velocities and then to extrapolate to zero velocity. This gives a static friction" value for zero contact time and is likely to be much lower than the former method, although it may be argued that extrapolation in these circumstances is wholly inappropirate and that no real measurement of static friction has been made at all. However, there is no doubt that this approach to measurement has given a much greater insight into the nature of the friction of rubber-like polymers. 

The frictional behaviour of polymers differs somewhat from that of perhaps more familiar materials. The frictional force tends to be proportional not to load (as in the classical case) but to speed. The coefficient of friction is very dependent on the nature of the two surfaces in contact, but is generally low, when suitable pairs are selected. This means that plastics gears can usually be run without external lubricants. Often the static friction coefficient is lower than the dynamic, which helps to explain the absence of slip-stick phenomenon exhibited by some plastics systems in motion this is especially marked with PTFE, which has an exceptionally low coefficient (around 0.02). The non-classical response of plastics materials results from their much lower modulus. Their frictional response is characterized by adhesion and deformation. 

Polymer Coefficient of Static Friction (Against Self) (Dimensionless)... 

The above defects are due to different reasons. In the article (41), the stick-slip effect is related to self-excited oscillations initiated by the dependence of static friction-stress between the billet and the die at the time when they are in contact. The latter is connected with the viscoelastic properties of the rough billet surface and by lubricant squeezing out from the region of contact. Ward and co-workers (1) explain the stick-slip effect by the heating of billet dining the deformation. The pulsatory flow is assumed to be due to a competition between the viscosity and high elasticity of polymers (42). 

In a research program (3,4) sponsored by another manu cturer, a fiiction tester was developed with which fiiction force and relative velocity between the test specimens were measured. The friction-time and velocity-time data were combined to produce friction-relative velocity plots during the stick-slip events. Of the several fiiction parameters which were measured, such as the static friction, the maximum drop in friction when sliding commenced, the maximum relative velocity during stick slip, the slope of the friction-velocity plot, and the number of stick-sl cycles, the last was chosen as the figure of merit for polymer pairs. Of the ten material pairs tested (PP was not included) polycarbonate (PC) had the most number of tests with no stick-slip cycles. The fiiction response of the PC was least affected by doubling the normal load and quadrupling the test machine stiffiiess than that of the other polymer pairs (5). 

Effect of Critial Surface Tension on Static Friction Coefficient of Polymers Against Steel (This data from NRL (Ref. 27))... 

Fig. A. Load dependence of the coefficient of static friction Pg observed between the steel hemisphere, 0.24 cm in radius and the plate of polymers.

In the presence of a lubricant, the adhesion component of the sliding friction force Fg generally diminishes. Thus, surface energetics of the polymeric substrate do not affect friction. However, surface energetics of a lubricant can influence the friction. Presumably, the weakly held film reduces the shearing of the polymer surface and hence the interfacial adhesion. Owens found a linear correlation between critical surface tension of lubricants incorporated in vinylidene chloride-acrylonitrile copolymer and static friction of cellulose films (Fig. 12). 

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