Friction of PTFE

Isobaric cells are preferable for experimentation, primarily because their constraints are the intrinsic thermodynamic variables of pressure and temperature. One such modern facility has been described by Hikosaka et al. (Fig. 4.41). But if there is no internal pressure sensor it is still necessary, for a precise knowledge of the internal pressure, to know the friction in the system, or at least to ensure that it retains the same sense so that it is not a factor when the pressure is changed. The Hikosaka cell relies upon the low friction of PTFE to keep the internal pressure close to the controlled applied pressure. There is no reason why larger isobaric cells could not be made with internal sensors for... 

Figure 9-1. Friction of PTFE against clean aluminum in vacuum. 1 First revolution of the disk. 2 Second revolution. Load 2.48 N. Sliding velocity 0.07 cm/s. Data by Brainard and Buckley [7].

Polytetrafluoroethylene is a slippery material with a smooth surface due to its low coefficient of friction. Numerous mechanical applications have been developed for PTFE with slight or without lubrication, particularly at low velocities and pressures above 35 kPa. Table 3.26 contains values for coefficient of friction as a function of velocity. D5mamic coefficient of friction of PTFE is larger than its static coefficient of friction and grows with increasing speed until the motion is destabilized. Static coefficient of friction remains unchanged in the temperature range of 27°C-327°C which is important in applications where a polytetrafluoroethylene part may experience heat buildup and temperature increase. 

PTFE is a crystalline polymer consisting of twisted zigzag spirals with at least 13 repeating units per turn. This nonpolar polymer has a solubility parameter of 6.2 H, a high (327 C), and a heat deflection temperature of 121 C PTFE is a tough, flexible polymer which retains its ductility at extremely low temperatures (-269 0. The coefficient of friction of ptfe is the lowest of any known solid material (see Table 13.4). Films of ptfe can be bonded by adhesives to other surfaces if the polymer surface is treated with sodium. It also bonds to diamonds and graphite whose surfaces have been fluorinated. Liquid sodium removes fluoride ions from the surface and leaves free radicals on the polymer surface, ptfe is resistant to almost all corrosives and solvents, but it can be dissolved in hot perfluorinated kerosene, ptfe is difficult to mold or extrude. 

Brainard, W.A. Buckley, D.H. "Adhesion and Friction of PTFE in Contact With Metals as Studied by Auger Spectroscopy, Field Ion and Scanning Electron Microscopy," WEAR, 26, pp. 75-93,... 

Fig. 7 Effect of the Irradiation by Fast Helium Atom on Friction of PTFE.

The paper deals mainly with the friction of PTFE and high density polyethylene sliding over hard clean flat surfaces. The behavior of these polymers is somewhat anomalous and is contrasted with the behavior of "normal" polymers that is virtually all other polymers. The friction and transfer behavior falls into two groups. (i) At low speeds the friction is low (y - 0.05) a thin film of polymer is drawn out of the slider and adheres to the counter surface. Its thickness is of... 

Fig. 10. Coefficient of friction of PTFE on aluminum disk and tungsten disk in vacuum. Siding velocity, 0.07 centimeter per second load, 250 grams.

The coefficient of friction of PTFE is lower than almost any other material and it has excellent temperature and mechanical properties although it does have a tendency to creep. The outstanding chemical resistance and electrical properties mean that it is often used in applications that require long-term performance in extreme service environments. 

There is also another form of transfer which is thought to occur under purely isothermal conditions this is sometimes called cold transfer Figure 13(bXii). PTFE is the best example and its transfer mechanisms have been studied extensively. During sliding the adhesion-induced interface tractions first produce extensive interfacial reorientation of the polymer s molecular chains of morphology. The polymer s oriented interface is then drawn out onto the counterface as a coherent, thin and highly oriented layer. This interfacial reorientation is the major reason why the friction of PTFE is small. Many polymers produce transfer films but not all have the same pronounced orientation as the PTFE films. 

The surface of PTFE articles is sHppery and smooth. Liquids with surface tensions below 18 mN/m(=dyn/cm) are spread completely on the PTFE surface hence, solutions of various perfluorocarbon acids in water wet the polymer (78). Treatment with alkafl metals promotes the adhesion between PTFE and other substances (79) but increases the coefficient of friction (80). 

Mech nic lProperties. Extensive Hsts of the physical properties of FEP copolymers are given in References 58—63. Mechanical properties are shown in Table 3. Most of the important properties of FEP are similar to those of PTFE the main difference is the lower continuous service temperature of 204°C of FEP compared to that of 260°C of PTFE. The flexibiUty at low temperatures and the low coefficients of friction and stabiUty at high temperatures are relatively independent of fabrication conditions. Unlike PTFE, FEP resins do not exhibit a marked change in volume at room temperature, because they do not have a first-order transition at 19°C. They ate usehil above —267°C and are highly flexible above —79°C (64). 

PTFE is outstanding in this group. In thin films it provides the lowest coefficient of friction (0.03—0.1) of any polymer, is effective from —200 to 250°C, and is generally unreactive chemically. The low friction is attributed to the smooth molecular profile of PTFE chains which allows easy sliding (57). Typical apphcations include chemical and food processing equipment, electrical components, and as a component to provide improved friction and wear in other resin systems. 

For wear resistance and low friction, coatings of PTFE or M0S2 generally have been satisfactory. Use of low thermal expansion filler in PTFE helps minimise cracking and loss of adhesion from metal substrates with their lower coefficients of expansion. 

The commercial polymers are mechanically similar to PTFE but with a somewhat greater impact strength. They also have the same excellent electrical insulation properties and chemical inertness. Weathering tests in Florida showed no change in properties after four years. The material also shows exceptional non-adhesiveness. The coefficient of friction of the resin is low but somewhat higher than that of PTFE. Films up to 0.010 in thick show good transparency. 

The frictional properties of PTFE are unique Its unusually low static coefficient of fnction decreases with mcreasing load and is lower than the dynamic coefficient of fnction. This precludes stick-shp behavior. The low surface energy also prevents wetting by liquids other than low-surface-tension fluids Like fluorocarbons. 

Composite deposits of electroless nickel and PTFE are characterised by a very low coefficient of friction . They contain 18-25% by volume of PTFE, and have found considerable application in the automobile, textile machinery and paper-making industries, among others. 

PTFE is a white solid with a waxy appearance. It is very tough and flexible, with a good electrical insulation properties. The surface energy and coefficient of friction are both very low, the latter being the lowest of any solid. The combination of low surface energy and low coefficient of friction cause PTFE to have excellent non-stick characteristics, a feature which underlies many of the everyday uses of this polymer. 

Fig. 10—Dependence of friction force of PTFE, Si3N4, and PTFE/Si3N4 film on load.

During the friction and wear tests of PTFE Him, two zones can be classified according to the load. One is the load below 70 nN, the friction force which was created in friction and wear tests is too small to make the PTFE film to shear. Within this zone the friction force increases linearly with the load, and there are no transfer of atoms and no worn marks. The second zone is when the load is above 70 nN, and the friction force created in the friction and wear tests is large enough to force the PTFE molecular atom to slip. So there were obvious worn mark and projection in the film, and the friction force stayed almost constant with load. 

There are two zones in the friction and wear tests of PTFE film. When the load was less than 70 nN, the micro friction force increases with the load. When the load is greater than 70 nN, the friction force of PTFE film is almost constant, and there is obvious worn mark in the PTFE film. 

PTFE/Si3N4 multilayers not only have the property of PTFE s low friction coefficient but also have the property of Si3N4 s high wear resistance. 

The DSC technique enables crystalline materials to be characterised by their melting point. Additives that come into this category include hydrocarbon waxes and poly(tetrafluoroethylene) (PFTE) lubricant. The presence of PTFE in low friction acetal mouldings can be established from the detection of the PTFE melting endotherm at 320°C. 

RE Gill, G Hadziioannou, P Lang, F Gamier, and JC Wittmann, Highly oriented thin films of a substituted oligo(para-phenylenevinylene) on friction-transferred PTFE substrates, Adv. Mater., 9 331-338, 1997. 

Nonstickiness, low friction, low wettability, and high thermal- and chemical-resistance are the major properties of PTFE, which was accidentally discovered in the DuPont laboratories in 1938, and these properties are more or less typical of other fluoropolymers that have been developed since. 

When a glass plate which is heated e.g. at 130°C is rubbed with a polytetrafluoroethylene (PTFE) briquette, a highly oriented thin film (2— 100 nm thick) of PTFE can be made on the plate such a film is termed a friction-transfer layer [24]. This method is applicable to make thin oriented layers of other polymers. Various kinds of organic compormds can be oriented on the PTFE friction-transfer layer from vapor phase, from solutions and from the melts [24]. 

Pentacene and tetracene films can be oriented by using friction-transferred PTFE polymer substrates (see Section 3.3). The films consist of both crystallographic... 

Because of the presence of the larger chlorine atom on the backbone which reduces intermolecular forces and the degree of crystallinity, this polymer is more readily processed and has a lower Tm than polytetrafluoroe thylene (ptfe). Thin films of pctfe are transparent and are resistant to corrosives and solvents. The coefficient of friction of pctfe is low but somewhat higher than that of ptfe ... 

The limited contact area between the chains in the crystal lattice results in weak intermolecular interactions and a high chain mobility. The latter, in turn, is responsible for the outstanding low friction characteristics of PTFE, and, somewhat... 

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