Polyethylene friction coefficient

Fig. 8. Self-diffusion coefficients of polyethylene chains as a function of molecular mass. The measurements were carried out at the same value of the monomeric friction coefficient. (Reprinted with permission from [48]. Copyright 1987 American Chemical Society, Washington)...

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. 

Example 2.2 Consider an impact between a polyethylene particle (dp = 1 cm) and a copper wall. The incident velocity is 2 m/s, and the incident angle is 30°. The friction coefficient of the interface is 0.2. The densities of polyethylene and copper are 950 and 8,900 kg/m3, respectively. What is the contact time duration for the collision Estimate the rebound velocity of the particle. Repeat the problem for a copper particle colliding with a polyethylene wall. 

The ultra-high molecular weight polyethylene system exhibits the highest wear rate although it has the lowest friction coefficient [90]. 

Miyata and Yamaoka [152] used scanning probe microscopy to determine the microscale friction force of silicone-treated polymer film surfaces. Polyurethane acrylates cured by an electron beam were used as polymer films. The microscale friction obtained by scanning probe microscopy was compared with macroscale data, such as surface free energy as determined by the Owens-Wendt method and the macroscale friction coefficient determined by the ASTM method. These comparisons showed a good linear relationship between the surface free energy and friction force, which was insensitive to the nature of polymer specimens or to silicone treatment methods. Good linearity was also observed between the macroscale and microscale friction force. It was concluded that scanning probe microscopy could be a powerful tool in this field of polymer science. Evrard et al. [153] reported coefficient of friction measurements for nitrile rubber. Frictional properties of polyacetals, polyesters, polyacrylics [63], reinforced and unreinforced polyamides, and polyethylene terephthalate [52] have also been studied. 

Polytetrafluoroethylene is a white solid with a waxy appearance and feel. It is a tough flexible material of moderate tensile strength with a tendency to creep under compression. Comparative values for some properties of various fluoro-polymers are given in Table 7.1. The electrical insulation properties are outstanding and are nearly as good as those of polyethylene. The coefficient of friction is unusually low and stated to be lower than that of any other solid the non-stick properties are also excellent. 

Sedimentation experiments may also be used to test the scaling theory in another way. A small number of inert spheres with diameters of 2R 100-200 A (e.g., bovine serum albumin) is added to an aqueous solution of synthetic polymer (such as polyethylene oxide). If 2R >c the spheres should move easily and / (the frictional coefficient) is related to (the viscosity of solvent). If 2R <, the spheres will be trapped and/ is related to q (the viscosity of entangled solution). To... 

Slip agents are added to polyethylene films to reduce their friction coefficient. Figure 11.17 shows that friction coefficient depends on the amount of erucamide added to LLDPE film. Increased amounts of erucamide gradually become less effective as the coefficient of friction reaches the equilibrium value characteristic of the composition. The performance of erucamide depends on the amount migrating to the surface, as Figures 11.18 and 11.19 show. The more erucamide in the polymer bulk, the higher its concentration on the film surface and the lower the coefficient of friction. 

W. Brown, P. Stilbs, and R. M. Johnsen. Friction coefficients in self-diffusion, velocity sedimentation, and mutual diffusion for polyethylene oxide in aqueous solution. J. Polym. Sci. Polym. Phys., 21 (1983), 1029-1039. 

Figure 1.6 Friction coefficient versus chain length N of polyethylene, as obtained from the Rouse relation between and diffusion coefficient D ( d, full symbols) and from the Rouse relation between and the longest relaxation time ti (I, open symbols). After Reference [194].

This is a standard friction problem. A glance at Fig. 25.5 shows that, when polymers slide on metals and ceramics, x can be as low as 0.04. Among the polymers with the lowest coefficients are PTFE (Teflon ) and polyethylene. By coating the ski or sledge runners with these materials, the coefficient of friction stays low, even when the temperature is so low that frictional heating is unable to produce a boundary layer of water. Aircraft and sports skis now have polyethylene or Teflon undersurfaces the Olympic Committee has banned their use on bob-sleds, which already, some think, go fast enough. 

Many polymer items are designed specifically to make contact with other materials. Where surface contact is concerned, two key properties are coefficient of friction and abrasion resistance. Polymers used in such applications include ultra high molecular weight polyethylene, polyacctal, fluorinated polymers, and natural and synthetic rubbers. Examples that we routinely come across include furniture upholstery, bushings and gears in office equipment, and bicycle tires. Industrial uses include the outer cover of electrical cables, and pipes that convey abrasive liquids such as slurries and powders. 

Liquid-liquid solvent extraction, 21 399 Liquid lithium, 15 131 Liquid low density polyethylene, 20 205 Liquid lubricants, for extreme environments, 15 256 Liquid lubricated system, coefficient of friction in, 15 209 Liquid magnesium, 15 336 Liquid manometers, 20 646-647 Liquid MDI, 25 462. See also MDI [4,4 -methylenebis(phenyl isocyanate)] Liquid melamine resins, 15 773 Liquid membrane extraction, 10 766 Liquid membranes, 15 800, 814-815 supported, 16 28... 

High density and UHMW polyethylenes are used to make antifriction parts. The coefficients of friction are low but the moduli, hardnesses and softening temperatures are weak, which limits the loads and PV factors. 

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