Polytetrafluoroethylene wear resistance

Carbon reduces creep, increases hardness and elevates thermal conductivity of polytetrafluoroethylene. Wear resistance of carbon filled compounds improves, particularly when combined with graphite. Carbon-graphite compounds perform well in non-lubricated applications such as piston rings in compressor cylinders. Carbon-filled PTFE has some electrical conductivity. Close tolerances can be achieved... 

It resembles polytetrafluoroethylene and fluorinated ethylene propylene in its chemical resistance, electrical properties, and coefficient of friction. Its strength, hardness, and wear resistance are about equal to the former plastic and superior to that of the latter at temperatures above 150°C. 

Polytetrafluoroethylene incorporation in polymers to yield multiple traversal wear resistant blends is such an example. Microfiber formation via extrusion of highly immiscible blends is another example [Robeson and Axelrod, 1992 Robeson et al., 1994] (see Figure 17.2). 

The third design feature is the polymer microstructure. Morphology of polymer can influence wear resistance of polymers. For example, in a semicrystalline polymer, both amorphous and crystalline phases coexist. The amorphous phase has been shown by Tanaka (8) to be weaker than the crystalline phase, thus the former wears faster than the latter. In addition to the difference in phases, the size of the spherulites and the molecular profile can also influence the wear rates. Thus, a control of the morphology through crystallization can improve the wear resistance of a polymer such as polytetrafluoroethylene (11). 

Polymers and polymer matrix composites are increasingly replacing metals in bearings, cams, gears, and other sliding components. Polytetrafluoroethylene (PTFE) is an example of a self-lubricating polymer that is widely used for its wear resistance. Fiber reinforcement of PTFE improves other mechanical properties without sacrificing the wear performance. 

Liu and co-workers [16] investigated the wear behaviour of ultra-high molecular weight polyethylene (UHMWPE) polymer. They concluded that the applied load is the main parameter and the wear resistance improvement of filler reinforced UHMWPE was attributed to the combination of hard particles, which prevent the formation of deep, wide and continuous furrows. Bijwe and co-workers [17] and Xu and Mellor [18] tested polyamide 6 (PA), polytetrafluoroethylene (PTFE) and their various composites in abrasive wear under dry and multi-pass conditions against SiC paper on a pin-on-disc tribometer. They concluded that the polymers without fillers had better abrasive wear resistance than their composites. 

The macromolecule of perfluorinated alkoxy (PFA) or simply perfluoroalkoxy is based on the monomer unit [—(CFj) —CF(0—C F, )—(CFj) —] . Perfluoroalkoxy is similar to other fluorocarbons such as polytetrafluoroethylene and fluorinated ethylene propylene regarding its chemical resistance, dielectric properties, and coefficient of friction. Its mechanical strength. Shore hardness, and wear resistance are similar to PTFE and superior to that of FEP at temperatures above ISO C. PFA has a good heat resistance from -200 C up to 260°C near to that of PTFE but having a better creep resistance. 

Materials used such as stifFer plastics can reduce hysteresis heating. Crystalline TPs for example (the popularly used acetal and nylon) can be stiffened by 25 to 50% with the addition of fillers and reinforcements. Others used include ABS, polycarbonates, polysulfones, phenylene oxides, polyurethanes, and thermoplastic polyesters. Additives, fillers, and reinforcements are used in plastics gears to meet different performance requirements (Chapter 1), Examples include glass fiber for added strength, and fibers, beads, and powders for reduced thermal expansion and improved dimensional stability. Other materials, such as molybdenum disulfide, polytetrafluoroethylene (PTFE), and silicones, may be added as lubricants to improve wear resistance. 

Upon addition of 3% UDD to polytetrafluoroethylene its wear resistance increased almost 30-fold at an insignificant increase of the friction coefficient. 

Polytetrafluoroethylene parts have good wear properties, as seen from the data in Table 3.27. The resistance of unfilled PTFE to wear is less than that of filled compositions. Data from tests measuring wear rate are presented in Tables 3.28-3.30. They should be viewed with an understanding that none of the techniques represent an actual wear situation. In all three methods, a new surface is exposed to abrasion during the repeated motion of the abrading surface. 

Fluoropolymers have outstanding chemical resistance, low coefficient of friction, low dielectric constant, high purity, and broad use temperatures. Most of these properties are enhanced with an increase in the fluorine content of the polymers. For example, polytetrafluoroethylene, which contains four fluorine atoms per repeat unit, has superior properties compared to polyvinylidene fluoride, which has two fluorine atoms for each repeat unit. Generally, these plastics are mechanically weaker than engineering polymers. Their relatively low values of tensile strength, deformation under load or creep, and wear rate require the use of fillers and special design strategies. 

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