Synthetic abrasives

Abrasives have evolved iato an essential component of modem iadustry. Sandstone, emery, and comndum were the abrasives of choice until the late 1800s when artificial materials were developed. Today synthetic abrasives offer such improved performance that the natural ones have been largely replaced except for jobs where cost is paramount. In 1987 U.S. statistics (4) showed natural abrasive production to be about 7 million while that of cmde manufactured abrasives was over 182 million. Total value of abrasives and abrasive products worldwide is estimated to be over 6 biUion dollars. 

Naturally occurring abrasives are still an important item of commerce, although synthetic abrasives now fill many of thek former uses. In 1987 about 156 million metric tons of natural abrasives were produced in the United States. Production was up from 1986 because of increased nonabrasive uses and increased use of garnet in sandblasting (4). 

In 1885, Charles Martin Hall invented his aluminum process and Hamilton Young Castner in 1890 developed the mercury-type alkali-chlorine cell, which produced caustic (sodium hydroxide) in its purest form. Edward G. Acheson in 1891, while attempting to make diamonds in an electric furnace, produced silicon carbide, the first synthetic abrasive, second to diamond in hardness. Four years later, Jacobs melted aluminum oxide to make a superior emeiy cloth. Within two decades, these two abrasives had displaced most natural cutting materials, including naturally occurring mixtures of aluminum and iron oxides. 

As shown in Figure 2.33, the mill consists of a flat rotor and stator manufactured in a chemically inert synthetic abrasive material, and the mill can be set to operate at clearances from virtually zero to 1.25 mm, although in practice the maximum clearance used is about 0.3 mm. When duty demands, steel working surfaces may be fitted, and in such cases the minimum setting between rotor and stator must be 0.50-0.75 mm, otherwise pick up between the steel surfaces occurs. 

Uses. Production of aluminum synthetic abrasives refractory material... 

The general properties of natural and synthetic abrasives are given in Table I. Both natural and synthetic abrasives must be crushed to small particle size before bonding to cloth or paper for mounting on various tools See Table 2. 

The chief contaminant is 0.3-0.5% sodium oxide, which fortunately does not affect electrolysis, with <0.05% calcium oxide, <0.025% of silica or iron oxide, and <0.02% of any other metallic oxide [4]. Apart from metal production, some of this high temperature alumina is used for the manufacture of synthetic abrasives and refractory materials. Activated alumina destined for adsorptive uses is produced in the same way, except that more moderate calcining temperatures of about 500°C are employed, which produces a highly porous product with excellent surface activity. The volume of alumina from the world s major producers is listed in Table 12.3. Australia has been the largest producer for many years (Table 12.3). 

Examination of the Smithsonian skull similarly showed marks produced by high-speed rotary cutting and also retained trace amounts of the abrasive material. Although only traces were found, this was nonetheless enough to perform an X-ray diffraction analysis and identify it as silicon carbide, a synthetic abrasive invented at the end of the nineteenth century. 

There has also been a modest use of the sillimanite minerals as abrasives. This was originally a matter of convenience, perhaps even mistaking them for corundum. However, the recent globalization of trade has practically eliminated this application, given the modest hardness of the sillimanites, only about 7 on the mineralogical Mohs scratch hardness scale. This hardness level cannot compete with corundum and the recently developed and quite superior synthetic abrasives. 

The abrasive grains used for Grinding Wheels today are synthetic. Natural abrasives are rarely used because they provide low stabiUty (Klocke and Konig 2005). Similar to the Grinding Wheel itself, the synthetic abrasive grains can be subdivided into ... 

Modem day abrasive grits have also progressed from natural minerals to synthetic abrasives, which include alumina-zirconia arid ceramic aluminum oxide abrasives. Conventional aluminum oxides are still widely used however, these new abrasive materials are... 

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Rubber. Both natural and synthetic rubber are used as bonding agents for abrasive wheels. Rubber-bond wheels are ideal for thin cut-off and slicing wheels and centerless grinding feed wheels. They are more flexible and more water-resistant than resinoid wheels. 

Nitrile mbber finds broad application in industry because of its excellent resistance to oil and chemicals, its good flexibility at low temperatures, high abrasion and heat resistance (up to 120°C), and good mechanical properties. Nitrile mbber consists of butadiene—acrylonitrile copolymers with an acrylonitrile content ranging from 15 to 45% (see Elastomers, SYNTHETIC, NITRILE RUBBER). In addition to the traditional applications of nitrile mbber for hoses, gaskets, seals, and oil well equipment, new applications have emerged with the development of nitrile mbber blends with poly(vinyl chloride) (PVC). These blends combine the chemical resistance and low temperature flexibility characteristics of nitrile mbber with the stability and ozone resistance of PVC. This has greatly expanded the use of nitrile mbber in outdoor applications for hoses, belts, and cable jackets, where ozone resistance is necessary. 

Modem furniture pohshes are designed for a wide variety of surfaces, eg, plastics, metals, and synthetic and natural resin coatings. Furniture pohshes impart shine and provide protection from abrasion, marring, and spills. The formulations clean weU in many cases. In common with most other pohshes, furniture pohshes are characterized by ease and speed of apphcation and of buffing, and by either the absence of objectionable odors or the addition of pleasing ones. 

These new synthetic mbbers were accessible from potentially low cost raw materials and generated considerable woddwide interest. For a time, it was hoped that the polysulftde mbbers could substitute for natural mbber in automobile tires. Unfortunately, these original polymers were difficult to process, evolved irritating fumes during compounding, and properties such as compression set, extension, and abrasion characteristics were not suitable for this apphcation. 

Pulpstones. Improvements have been made in the composition and speed of the grinding wheel, in methods of feeding the wood and pressing it against the stone, in control of power to the stones, and in the size and capacity of the units. The first pulpstones were manufactured from quarried sandstone, but have been replaced by carbide and alumina embedded in a softer ceramic matrix, in which the harder grit particles project from the surface of the wheel (see Abrasives). The abrasive segments ate made up of three basic manufactured abrasive siUcon carbide, aluminum oxide, or a modified aluminum oxide. Synthetic stones have the mechanical strength to operate at peripheral surface speeds of about 1200—1400 m /min (3900 to 4600 ft/min) under conditions that consume 0.37—3.7 MJ/s (500—5000 hp) pet stone. 

There is no question that the bane of textile chemists in the area of cross-linking for smooth-dry performance is the loss of abrasion resistance. This has been a continuing problem when durable press is pushed to high levels of performance. Numerous approaches to this problem have been explored (32). However, the simplest solution has been to blend cotton with synthetic fibers. A 50—50 cotton—polyester fabric can have exceUent smooth-dry performance and yet be able to endure numerous launderings. 

Two factors emerged to turn the focus of durable press the discovery that incorporation of a level of nylon or polyester in the fabric can substantially increase the garments abrasion resistance, and the reali2ation that the marketplace preferred cotton—polyester blends in delayed cure operations, even though 85% cotton—15% nylon fabric yields a suitable product. The 50% cotton—50% polyester fabric seemed particularly appropriate because it contained sufficient ceUulosic to benefit from a chemical finish and sufficient synthetic to provide strength and abrasion resistance. 

Property Modifiers. Property modifiers can, in general, be divided into two classes nonabrasive and abrasive, and the nonabrasive modifiers can be further classified as high friction or low friction. The most frequently used nonabrasive modifier is a cured resinous friction dust derived from cashew nutshell Hquid (see Nuts). Ground mbber is used in particle sizes similar to or slightly coarser than those of the cashew friction dusts for noise, wear, and abrasion control. Carbon black (qv), petroleum coke flour, natural and synthetic graphite, or other carbonaceous materials (see Carbon) are used to control the friction and improve wear, when abrasives are used, or to reduce noise. The above mentioned modifiers are primarily used in organic and semimetallic materials, except for graphite which is used in all friction materials. 

Fig. 8. (a) Synthetic diamond grit for resinoid or vitreous bond (free-cutting) abrasive wheels, and (b) synthetic diamond grit for metal bond abrasive... 

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