Wear resistance

Wear is the removal of surface material by one of three mechanisms erosion, abrasion, or cavitation. Erosion is the removal of a polymer s surface by abrasive materials carried in a fluid medium. We see this type of wear in plastic pipes used to transport waterborne slurries of minerals in mining operations and in vacuum transfer pipes used to convey powders in a stream of air. Abrasion is the result of two surfaces sliding against each other. We commonly observe abrasion of polymers in the fabrics of our clothes and upholstery. Cavitative wear is caused by voids in a liquid medium coUapsing against a surfece. It is essentially an impact process. Cavitation is a relatively uncommon cause of wear in polymers. Pump impeUers are one of the few applications where polymers must resist this type of wear. 

Abrasive wear of polymers has two components material can be removed by the rasping action of a countersurface or it can be sheared off viscoelastically by a countersurface to which it adheres. The precise balance of mechanisms depends on the characteristics of the counterface and the conditions under which the abrasion takes place. Many polymers exhibit excellent wear resistance, which in combination with their low coefficients of friction suit them for applications where lubrication is either impossible or undesirable. We use wear resistant polymers in such diverse applications as bushings in business machines, pump seals, and replacement hip and knee joints. 

We employ two basic approaches to rate a polymer s wear resistance. In the first, we expose a polymer surface to a standard set of abrasive or erosive conditions and examine the surface for visual evidence of wear. We primarily use this method to qualitatively rank materials. In the second approach, we expose samples to wear inducing conditions and determine wear resistance in terms of weight loss as a function of time. 

COMPARATIVE DRY ABRASION RESISTANCE OF POLYURETHANES VERSUS OTHER COMMON RUBBERS, PLASTICS AND METALS 

Materia Type Relative abrasion using mild steel as unity 

Source P. Wright A. P. C. Gumming, Solid Polyurethane Elastomers. Maclaren Sons, London, 1969. 

These comments apply to abrasion of dry surfaces and must be modified when considering abrasion under wet conditions. In these cases the water or other fluid lubricates the surfaces and reduces the coefficient of friction. Further, any heat generation that does occur is more easily dissipated. It is under these conditions that the polyurethanes are outstanding, since then their inherent abrasion resistance is not diminished by the adverse effect of heat build-up. 

For sliding abrasive resistance, the tough polyesters are the best suited. This also must be balanced against the hardness of the material. The older style PPG materials do not have the physical properties to provide outstanding resistance to abrasive wear. 

When there is impinging erosive wear, MDI-based polyethers are normally classified as the most suited because hydrolysis resistance is also taken into account. Certain of the more sophisticated esters have better erosive wear than the high-cost ethers (PTMEG) under hydrolysis conditions. The temperature limitation, however, is still just below that of the ether-based materials. The angle of impingement also must be taken into account. 

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It is noteworthy, however, that traces of sulfur can have beneficial effects on the anti-wear resistance of fuel injection pumps. It is thus undesirable to reduce the sulfur content to extremely low values unless additives having lubricating qualities are added. Independently from total sulfur content, the presence of mercaptans that are particularly aggressive towards certain metal or synthetic parts is strictly controlled. The mercaptan content is thereby limited to 0.002% (20 ppm) maximum. The analysis is performed chemically in accordance to the NF M 07-022 or ASTM D 3227 procedures. 

Heightened wear resistance of the pickup working surface thanks to the use of a protective plate of stainless steel. 

Experiments witli chemically grafted SAMs displayed much larger wear resistance tlian films produced by tire LB technique [188]. Also it was found tliat wear properties of SAMs can be furtlier improved by chemically grafting CgQ molecules onto SAM surfaces [189]. 

Beryllium is added to copper to produce an alloy with greatly increased wear resistance it is used for current-carrying springs and non-sparking safety tools. It is also used as a neutron moderator and reflector in nuclear reactors. Much magnesium is used to prepare light nieial allo>s. other uses include the extraction of titanium (p. 370) and in the removal of oxygen and sulphur from steels calcium finds a similar use. 

Ruthenium is a hard, white metal and has four crystal modifications. It does not tarnish at room temperatures, but oxidizes explosively. It is attacked by halogens, hydroxides, etc. Ruthenium can be plated by electrodeposition or by thermal decomposition methods. The metal is one of the most effective hardeners for platinum and palladium, and is alloyed with these metals to make electrical contacts for severe wear resistance. A ruthenium-molybdenum alloy is said to be... 

Rhenium is also used as an electrical contact material because it has good wear resistance and withstands arc corrosion. Thermocouples made of Re-W are used for measuring temperatures up to 2200C, and rhenium wire is used in photoflash lamps for photography. 

It resembles polytetrafiuoroethylene and fiuorinated 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. 

It has good weathering resistance and does not support combustion. It is resistant to most chemicals and solvents and has greater strength, wear resistance, and creep resistance than the preceding three fluorocarbon resins. 

Waxes hydrocarbon Wax printing Wax sizing materials Wax sweating Waxy corn WB4101 [613-67-2] W341C [110368-36-0] Weak interfaces Weapons Wear factors Wear resistance Weather balloons Weathering... 

A linear relationship exists between the cohesive energy density of an abrasive (10) and the WoodeU wear resistance values occurring between comndum H = 9) and diamond H = 42.5). The cohesive energy density is a measure of the lattice energy per unit volume. 

Static friction decreases with an increase in load, and the static coefficient of friction is lower than the dynamic coefficient. The tendency to creep must be considered carefliUy in FEP products designed for service under continuous stresses. Creep can be minimized by suitable fillers. Fillets are also used to improve wear resistance and stiffness. Compositions such as 30% bronze-fiUed FEP, 20% graphite-filled FEP, and 10% glass-fiber-filled FEP offer high PV values ( 400(kPa-m)/s) and are suitable for beatings. 

Cellular mbber has been used extensively as shoe soles, where its combination of cushioning abiUty and wear resistance, coupled with desirable economics, has led to very wide acceptance. In this case the cushioning properties are of minor importance compared with the abrasion resistance and cost. Other significant cushioning appHcations for cellular mbbers and latex foam mbbers are as carpet underlay and as cushion padding ia athletic equipment. 

In other areas, POD has been used to improve the wear resistance of a mbber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this appHcation a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Textile Finishing. Polyethyleneimine-A/-methylolurea derivatives improve the crease and wear resistance of cotton (429,430). The adhesion between individual wool fibers is improved by pretreatment with amines, which leads to improved shrink resistance (431). An antimicrobial finish can be appHed to cotton by using a combination of PEI and ureas to bind zinc pyrithione to the fabric (432). After wool has been provided with a flameproof finish using fluorozirconate or fluorotitanate, the wool can be neutralized with PEI (433). Conventional neutralizing agents caimot be used for this purpose since they impair the flameproof characteristics of the impregnated fabric. 

Aesthetic properties are of greatest concern in decorative laminates. These include gloss, appearance, cleanabiUty, wear resistance, stain resistance, and other surface properties. Physical properties are of most importance for industrial laminates. These include strength, electrical and thermal properties, expansion coefficient, and punchabiUty. The definitions of the laminate grades in these standards foUow. 

High wear type is also a general-purpose HPDL with increased surface wear resistance. 

The wide range of soHd lubricants can generally be classified as either inorganic compounds or organic polymers, both commonly used in a bonded coating on a matching substrate, plus chemical conversion coatings and metal films. Since solid-film lubricants often suffer from poor wear resistance and inabihty to self-heal any breaks in the film, search continues for improved compositions. 

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