Crystalline polymers friction

Poly acetals are highly crystalline polymers. The number of repeating units ranges from 500 to 3,000. They are characterized by high impact resistance, strength, and a low friction coefficient. 

For a semi-crystalline polymer, the coefficient of friction depends on temperature as schematically indicated in Figure 7.29 ... 

The relation of abrasion to friction appears from Figure 7.31, in which the abrasion is given as a function of temperature for some crystalline polymers. Temperature increase causes, from a certain temperature, a drastic increase in abrasion due to sticking together of the surfaces. 

The yield depends on the pressure according to crsh/y = crshyo I gP 50.3 + 0.204P MPa such a relationship has been found for many amorphous, glassy polymers g, i.e. the coefficient of internal friction, usually shows values between 0.1 and 0.25, depending on the polymer, whereas for semi-crystalline polymers this coefficient is smaller... 

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

In plastics, wear depends on adhesion and deformation of soft material. Frictional forces are proportional to velocity rather than load as in the case of metals. A number of additives can improve wear and slip properties. PTFE has lower friction coefficient than any other material. Additionally, its particles form a film on shearing surfaces thus decreasing friction. Its addition is very effective in high-pressure applications. Optimum additions are 20% in crystalline polymers and 15% in amorphous ones. Molybdenum disulfide is primarily used in PA compounds. It works as a nucleating agent, promoting increased crystallinity in PA, thus providing harder, more wear... 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

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. 

Tanaka, K. Uchiyama, Y., "Friction, Wear and Surface Melting of Crystalline Polymers," Advances in Polymer Friction and Wear, L. H. Lee, Ed., Polymer Sci. Techn., Vol. 5B, Plenum Press, New York, 1974, pp. 499-531. 

In order to study the effect of the degree of crystallinity on friction and wear of PET, the friction measurements in which the steel sphere was slid on flat PET surfaces were carried out. Measurements of the friction and wear rate were also made on the PET pins sliding against a smooth steel surface at a speed 0.1 m/s under a load 10 N by means of a pin-on-disk type apparatus. The PET specimens of four different crystallinity (8, 39, 55 and 75 % ) were used in the present work. The specimen of the lowest crystallinity had a structure similar to an amorphous polymer, while two specimens of crystallinity, 39 % and 55 % had a spherulite-like structure. The highest crystallinity had a fiber-like structure. Conclusions obtained are as follows ... 

FIFE was first commercialized by DuPont, under the brand name Teflon. PTFE is a very highly crystalline polymer, extremely inert, an excellent barrier, and exhibits a very low coefficient of friction. Its glass transition temperature (Tg) is about -100°C, and its melt temperature is about 327 C (621°F). Flowever, its very high viscosity makes it very difficult to process. PTFE is used most often as a component in packaging equipment, such as providing a nonstick surface on heat sealers, rather than in packages themselves. 

Nylons were one of the early polymers developed by Carothers. Today, nylons are an important thermoplastic, with consumption in the United States of about 1.2 billion lb in 1997 88 Nylons, also known as polyamides, are synthesized by condensation polymerization methods, often an ahphatic diamine and a diacid. Nylon is a crystalline polymer with high modulus, strength, and impact properties, and low coefficient of friction and resistance to abrasion. Although the materials possess a wide range of properties, they all contain the amide (-CONH-) linkage in their backbone. Their general structure is shown in Fig. 2.8. 

The above materials are crystalline polymers, and such materials have generally superior frictional, wear and fatigue properties, compared with amorphous polymers. However, amorphous polymers show lower mould shrinkage and where dimensional accuracy is paramount they may be preferred. Suitable engineering types are polycarbonate (PC) and noryl (styrene modified polyphenylene oxide). 

Acetal homopolymers and copolymers are crystalline polymers displaying physical properties that compare with die-cast metal. They are strong, stiff, tough, hard, and have excellent frictional properties over a wide range of temperature, humidity, and solvent exposures. 

Annealing drawn crystalline polymer mobilizes the almost fully extended amorphous tie molecules that try to assume the thermodynamically required random conformations. The oriented crystalline sample shrinks if annealed with free ends, which permits the crystalline blocks in different microfibrils and connected by almost extended TTMs to move toward the position that they occupied before plastic deformation. Hence, the end-to-end distance or the fraction of TTMs in the amorphous region is reduced. Shrinkage rate of the samples is determined by competition of retractive forces of TTMs with the frictional forces or the van der Waals forces among neighbor fibers... 

The method of reduced variables as outlined in Section B above, including fitting the function flr(r) to the WLF equation, is appropriate for data in the transition, plateau, and terminal zones of time scale, with the provision that all contributions to the measured viscoelastic properties involve the same friction coefficient ftfand moreover that the internal structure of the system does not change with changing temperature. The latter restrictions are inherent in the development of Section A. In glassy and crystalline polymers, and in situations where two or more different classes of molecular motions with different temperature dependences are involved, some modifications in the treatment are necessary. 

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