Influence of tribological additives on friction and impact performance of injection moulded polyacetal

Tribological additives are used to improve frictional properties of injection moulded thermoplastics. The additives might however also affect the mechanical properties of the material. The influence of processing conditions on both frictional and mechanical properties is highly relevant in the development of tribologically modified grades. In the present study we investigate how two commonly used tribological additives, polydimethylsiloxane and polytetrafluoroethylene, affect friction and impact properties of polyacetal (polyoxymethylene). A new injection mould provides test specimens for both friction and impact testing. Friction is evaluated through ring-on-disc testing against polypropylene simulating use-conditions in a particular medical device. The impact testing is based on instrumented three-point bending of single-edge notched beams of component-realistic thickness. Both frictional and mechanical properties are benchmarked against those of the base polyacetal resin. High-speed video recording of the crack propagation during impact testing reveals potential differences in fracture mechanisms among the blends. Fractography by scanning electron microscopy is used for comparison of fracture surface characteristics. The tested concentration of the additives is found to effectively reduce friction, yet are not found critical with respect to the impact properties evaluated. A noticeable interaction with respect to friction is found between the additives speaking in favour of their use in combination.