WILEY - Kirk-Othmer Encyclopedia of Chemical Technology, All Volumes
|Publication Date:||1 March 2004|
An abrasive is a substance used to abrade, smooth, or polish an object. If the object is soft, such as wood, then relatively soft abrasive materials may be used. Usually, however, abrasive connotes very hard substances ranging from naturally occuring sands to the hardest material known, diamond.
Abrasives were literally as old or older than the Egyptian pyramids; in ancient times, humans used a variety of materials to refine or polish surfaces. For example, the Chinese used corncob skins for polishing. Shark skin, with its dermal denticles known as placoid scales, naturally provided abrasive properties. Humans also harvested the most common and abundant minerals in the earth's crust, namely, the quartz family of minerals. The ancient Greeks called quartz ''crystal'' and this quartz took the form of sandstone, loose sand, and flint. This crystal material was used in the abrading of stone, wood, metal and grinding grains, and limestone. Additionally, the ancient Greeks developed the use of corundum (naturally occurring aluminum oxide) and garnet, which were superior to quartz. Other abrasive materials known were hematite, now known as Jeweler's rouge, in 325 BC by Theophratus (1). Diamond as a polishing material was referenced in India in 800 BC (2) and its exceptional hardness was referred to in ancient Hindu proverbs (3) and in the Bible (4).
During the Industrial Revolution of the 1800s, the development of abrasive articles went hand in hand with the metal-working industry. In early years, sandstone rocks were mined out of the earth and carved into grinding wheels. Flint and naturally occurring corundum were bonded to paper to form sandpaper. The abrasive grains used were primarily mined materials such as sandstone, quartz, naturally occurring corundum, and garnet. These minerals had significant amounts of impurities, including iron, silica, and silicates, which lowered their abrasive grain hardness and hindered their performance. During the late 1800s, and early 1900s, synthetically manufactured abrasive grains revolutionized the abrasive industry. The synthetic abrasives tended to be harder, tougher and purer than mined abrasive grains. Edward Acheson is credited with inventing synthesized silicon carbide in 1891 (5). Excluding diamond, silicon carbide was the hardest abrasive grain available for years to come. Even today, silicon carbide is produced under essentially the same process and furnace that Acheson invented. The silicon source (typically from very pure sand) and carbon source (usually graphite) are reacted at temperatures in excess of 2000 C to cause the reduction of silica by carbon.
Around the turn of the twentieth century, synthetically manufactured fused alumina was invented. In this process, alumina-based raw materials are heated above its melting point, typically 20008C, and subsequently cooled to form fused alumina. This basic process was originally patented by Werlein (6) in France in 1893 and by Hasslacher (7) in Germany in 1894. This process was further advanced by C. M. Hall through the addition of iron borings into the fusion melt to remove metallic impurities (8). The resulting aluminous abrasive grain was purer. A. C. Higgins then developed an improved furnace design that involved the use of a water-cooled shell container (9). This new furnace design used a solid, thin alumina coating on the furnace walls that prevented the molten alumina from attacking the steel furnace walls. This basic Higgins furnace design is still widely used today in manufacturing many fused alumina grains.
Over these multitude of centuries, abrasive articles were employed, quite simply, to change a surface. These abrasive articles relied on a broad range of technologies including ceramics, inorganic chemistry, paper, textiles, organic chemistry, polymer science, and related process technologies. During the past 150 years, as these technologies grew, so did the advancement of abrasive technologies to create abrasive articles with even higher efficiencies.
There are four major forms of abrasive articles. A bonded abrasive is a three-dimensional (3D) composite of abrasive grains dispersed in a bond system. This bond system may be organic (eg, resinoid wheels) glassy inorganic bond (vitrified wheels), or metallic. Bonded abrasives are commercially available in a wide variety of forms including wheels (most popular), stones, mounted points, saws, segments, and the like. Coated abrasives are generally described as a plurality of abrasive grains bonded to a backing. Nonwoven abrasives comprise a plurality of abrasive grains bonded into and onto a porous non-woven web substrate. Nonwoven and coated abrasives are also available in a wide variety of converted forms of belts, sheets, disks, cones, flap wheels, etc. Loose abrasive slurries comprise a plurality of abrasive grains dispersed in a liquid medium, such as water. Loose abrasive slurries are typically employed in polishing type applications where a very fine surface finish is desired.
These abrasive articles are used in a plethora of different refining processes including metal degating, grinding, shaping, cutting, deburring, finishing, sanding, cleaning, polishing, and planarizing. Today abrasive articles are employed in some aspect in most manufactured goods sold. The abrasive article may be used in the supporting equipment to make the manufactured good and/or the direct process to produce the manufactured good. For examples, resinoid grinding wheels are used to remove metal gates from castings, where cut rate is measured in mm3/s. Coated abrasive belts grind away parting lines from a forged hand tool and consequently reshape the surface of the hand tool. Metal bonded saws are employed to cut through concrete in road construction. Nonwoven flap wheels debur the surface of metal components. Coated abrasive belts sand wood boards to a finished surface such that these wood boards are ready for staining. Nonwoven abrasives are popular for cleaning the surface of metal components including cooking utensils. Loose abrasive slurries are utilized in precision polishing, eg, in semiconductor planarization or polishing glass to optical clarity.