Polymer filler frictional effects

Belyi et al. W have discussed various methods for decreasing wear and/or friction in polymer-metal sliding. These involve modification of the supramolecular structure of the polymer, structural transformations by gamma-irradiation, thermo-activation effects from the decomposition of filled materials under sliding conditions, and selective transfer by incorporating specific fillers in the polymeric material. The use of cuprous oxide as a filler in polytetra-fluoroethylene (PTFE) has been shown to reduce the wear rate by a factor of 3. It is believed to be due to the selective transfer of copper to the steel surface because cuprous oxide is reduced to pure copper at the temperatures produced in sliding 9). 

The presence of dispersed fillers in the polymer material in low amounts may intensify electrization, increase the residual charge and change the friction coefficient. Introduction of the filler in the electret state exerts a still stronger effect on polymer electrization on frictional interaction with metals. Depending on the direction of the field intensity vector formed by the filler particles, the field generated by triboelectrization can be attenuated or intensified. This means that the principle of the electret-triboelectrization superposition is realized [49], which can be used to regulate the parameters of frictional interactions. Thus, by the introduction of the electret filler, e.g. mechanically activated F-3 powder, it is possible to decrease the friction force (Fig. 4.9). 

Figure 13. 9 shows that, in addition to slip agents, also incorporation of fillers may contribute to coefficient of friction. The effect of filler depends on polymer. In majority of cases filler lowers the coefficient of friction, but there are excep-... 

The concept that friction between filler particles and breakdown of filler network is heat generating is applied to an understanding of the fusion behaviour of PVC. It is shown that the treatment of a PVC melt as a filler-containing polymer provides information on the anomalous effects observed in the rheological properties of PVC and that differences in the fusion behaviour of PVC compounds containing different impact modifiers and lubricants can be explained with the aid of this approach. 17 refs. 

PTFE is a unique polymer in the formulation of composites, since it may be either the material of the matrix or a friction-reducing filler. It is a very soft polymer, which in the absence of reinforcement will wear rapidly, and it will cold flow under load. As a matrix material it must therefore be effectively reinforced. As a friction-reducing filler it may be used in the form of particles or fibres. Table 12.6 shows the effect of different fillers on the properties of PTFE. Spengler et aP reported an... 

Polytetrafluoroethylene, molybdenum disulfide, graphite, and aramid fibers reduce the frictional coefficient. These may be used as single friction additive, in combination with other fillers, and in combination with silicone oil. Table 5.17 illustrates effect of PTFE on the frictional properties of different polymers. 

Shang et al. (1995) show that the work of adhesion between a silica filler surface and a polymer matrix is directly related to the dynamic viscosity and moduli. Additionally, at lower frequencies there is a greater influence of the work of adhesion. The influence is shown to be described well by an effective increase in interphase thickness due to the increase in the work of adhesion, such that polymer chains are effectively immobilized around the filler, and the friction between the immobilized layer and the polymer then governs the dynamic rheology. It was noted that the immobilized layer could be reduced in extent at higher frequencies. 

In order to investigate the effect of free sulfur in the wear process, high density polyethylene was filled with 2% S and 2% 2-mercurobenzothiazole. This composition provided almost the same wear rate as that of the unfilled material and the coefficient of friction was also the same. It proves that the strengthening of the polymer from the filler material is important for the reduction of wear. 

Their mechanism can be presented as follows. During thermal treatment, FO contact the polymer melt and the oxides partially reduce to metals [72]. This leads to the generation of microchains M1-P-M2 in the lubricant bulk and formation of an MPE as a result of electrochemical interactions of unlike metal components. As a consequence, adhesive interactions between the filler and binder intensify. In our view, the high level of antiscoring properties and low friction coefEcient can be attributed to the catalyzing effect of the reduced metals on the formation of the friction polymers from the products of mechanical destruction of organic components of the lubricant. 

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