Friction properties

Friction is the resistance that two surfaces experience as they slide or try to slide past each other. Friction can be dry (i.e., direct surface-surface interaction) or lubricated, where the surfaces are separated by a thin film of a lubricating fluid. 

The force that arises in a dry friction environment can be computed using Coulomb s 

Coefficients of friction between several polymers and different surfaces are listed in Table 2.19 [49]. However, when dealing with polymers, the process of two surfaces sliding past each other is complicated by the fact that enormous amounts of frictional heat can be generated and stored near the surface due to the low thermal conductivity of the material. The analysis of friction between polymer surfaces is complicated further by environmental effects such as relative humidity and by the likeliness of a polymer surface to deform when stressed, such as shown in Fig. 2.65 [49], The top two figures illustrate metal-metal friction, wheareas the bottom figures illustrate metal-polymer friction. 

Temperature plays a significant role for the coefficient of friction p as demonstrated in Fig. 2.66 for polyamide 66 and polyethylene. In the case of polyethylene, the friction first decreases with temperature. At 100°C, the friction increases because the polymer surface becomes tacky. The friction coefficient starts to drop as the melt temperature is approached. A similar behavior can be seen in the polyamide curve. 

As mentioned earlier, temperature increases can be caused by the energy released by the frictional forces. A temperature increase in time, due to friction between surfaces of the same material, can be estimated using 

Coulomb s law applies to the friction between two solid surfaces in most cases as a good approximation friction force iv is proportional to the normal force Fn which presses the surfaces together  

The friction force, which must be overcome between two surfaces at rest, is greater lhan the friction force, which develops between the siufaces sliding on each another. One speaks of static friction and sliding friction. The respective coefficients of proportionality // and //q are called coefifi- 

The interloeking effect changes with the normal force. The relationship Eq. 3.15 with a fixed adhesive force therefore does not apply over the entire range of normal force. The strength envelope can therefore only be linear over a more or less wide range of normal force according to Eq. 3.15. 

Dividing the forces by the contact area, the equation of friction strength envelope is obtained in the following form  

Normal stress loading device Qeosynthelic clamping deuice 

The application of higher loads increases the frictional force, although there is a tendency for the coefficient of friction to fall with time. This effect is possibly due to lubrication of the interface by abrasion debris. 

Fillers are available with a range of frictional properties from self-lubricating through severely abrasive which permits applications which range from slide bearings to brake pads. 

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. 

Polymer PTFE, % Wear factor Dynamic coefficient of friction  

The coefficient of friction and wear are substantially reduced by the incorporation of PTFE powder. Molybdenum disulfide has an even broader range of application temperatures than PTFE (-150 to 300°C, PTFE up to 260°C) and provides even better performance under high load. For this reason it is used either in combination 

Many fillers play a prominent role in brake pads and clutch linings. These include fibers such as aramid, glass, carbon, steel, and cellulose low cost fillers such as barites, calcium carbonate and clay frictional modifiers such as alumina, metallic flakes and powders. The combination of these materials with binders gives a broad range of brake pad materials. 

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This thermoplastic shows good tensile strength, toughness, low water absorption, and good frictional properties, plus good chemical resistance and electrical properties. 

We shall discuss three types of phenomena for polymer solutions thermodynamic properties in Chap. 8, frictional properties in Chap. 9, and lightscattering properties in Chap. 10. A common feature of virtually all phenomena in these areas is that they all depend on the molecular weight of the solute. Thus observations of these properties can be interpreted to yield values for M we shall use this capability as a unifying theme throughout these chapters. 

As a tme thermoplastic, FEP copolymer can be melt-processed by extmsion and compression, injection, and blow molding. Films can be heat-bonded and sealed, vacuum-formed, and laminated to various substrates. Chemical inertness and corrosion resistance make FEP highly suitable for chemical services its dielectric and insulating properties favor it for electrical and electronic service and its low frictional properties, mechanical toughness, thermal stabiUty, and nonstick quaUty make it highly suitable for bearings and seals, high temperature components, and nonstick surfaces. 

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