Surface modifi cation | additives
Whether to reduce long term wear or to ease one-off activation, tribological modifi cation of plastics is an increasing demand. Jennifer Markarian reviews the latest additive and compound developments
Formulating to reduce friction and wear
Wherever moving parts come into contact, friction and wear can be a concern. Polymeric materials used in applications such as gears, bearings, conveyors and seals must resist wear, sometimes under high tempera- tures and pressures, for extended service periods. And the trend toward lighter-weight and thinner parts only intensifi es this need for improved management of wear. But surface properties are not only a concern in long term applications. Wear many not be an issue in short-term or single-use applications such as medical devices, but low surface frictional properties can be vital. A wide variety of polymers are used in wear applica-
tions today. While certain polymers display inherently better tribological properties than others (see Table 1 on page 42), lubricating additives also play a signifi cant role in differentiating compounds and in enhancing compounds for specifi c application needs. There are certain key basics to take into account when designing a part where tribological properties (wear and friction) are a concern. These include consideration of the two materials that will be in contact, the surface roughness of the parts, the usage parameters (continuous or intermittent, sliding speed,
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contact pressure), ambient conditions (temperature and humidity), and other application-related factors (such as whether foreign debris will be present and whether external lubricants will be used), explains Cliff Watkins, marketing director at distributor PolySource. The most demanding applications are those with the
highest pressure-velocity (PV) load, which is a measure of the contact pressure multiplied by the sliding speed. Frictional heat generated during use can be a concern that must be managed, says Watkins. High friction can also cause energy losses, so a low coeffi cient of friction is desired for applications such as bearings or gear wheels. Common anti-friction additives that are compounded
into a polymer to act as internal lubricants include those that act immediately by migrating to the surface (such as perfl uoropolyether [PFPE] synthetic oil and silicone) and those that are distributed throughout the polymer and begin to lubricate when they are exposed after a “wear in” period, such as polytetrafl uoroethylene (PTFE), fi bres (aramid, carbon, and glass, for example), and other solid additives (such as molybdenum disulfi de and graphite powder). RTP Company also offers its all-polymeric wear additive (APWA), which is an
November 2015 | COMPOUNDING WORLD 41
Low friction and wear are prime require- ments in
polymer gear applications
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