Although the most familiar types of coated abrasives are probably the individual sheets of sandpaper with which home woodworkers prepare furniture or crafts for painting, the trade term "coated abrasives" actually encompasses a much wider array of products for both individual and industrial use. While these products assume many forms, all are essentially a single layer of abrasive grit attached to a flexible backing. In addition to their best-known form, sandpapers are also available to consumers on belts, rolls, and disks. However, the biggest users of coated abrasives are manufacturers who employ large-scale abrasives in various phases of industrial production. For example, coated abrasives are critical in both the furniture and automotive industries.

Coated abrasives date as far back as the thirteenth century, when the Chinese used crushed shells and seeds glued with natural gum to parchment. By 1769 coated abrasive paper was being sold on the streets of Paris. An 1808 article describes a process for making coated abrasives, and in 1835 a United States patent was issued for a machine that produced coated abrasives.

Not always a highly versatile tool, coated abrasives were originally restricted to finishing applications such as polishing or preparing surfaces for painting or plating. Through improvements in the strength of backings and the properties of abrasive minerals, coated abrasives now can be used for heavy-duty applications. Today, industrial uses for coated abrasives range from hand polishing with sheets of coated abrasive to grinding steel with large machines that use 300-horse-power electric motors to drive belts several feet wide.

Currently, approximately forty companies manufacture or import jumbo rolls in the United States. The size of the industry is limited because it requires a substantial investment in equipment, raw materials, energy, and labor. A larger number of companies convert the jumbo rolls into useable products such as disks and belts.

Raw Materials

The name "sandpaper" is actually a misnomer, as most coated adhesive products contain neither sand nor paper. Generally, they consist of some type of abrasive mineral, which can be organic or synthetic; flexible backings; and adhesives. Other materials may be added for special applications. Most companies that manufacture jumbo rolls of coated abrasives purchase minerals and backing materials from independent companies that specialize in making these items. Natural minerals come from companies that mine and process the minerals, synthetic minerals come from companies that specialize in such refractory materials, and most backings come from fabric manufacturers.

The abrasive grain, the key part of coated abrasive products, may be either a natural or synthetic mineral. Due to their extreme hardness, natural minerals such as garnet or emery (corundum with iron impurities) find limited use in products for wood-related applications, while crocus mineral (natural iron oxide) is limited to use as a polishing agent because of its softness. However, such natural minerals comprise less than one percent of the abrasives market. Metalworking applications require synthetic minerals exclusively because such minerals offer consistent quality and can be specially manufactured with an elongated structure that bonds well to flexible backings.

The use of a particular coated abrasive product determines the mineral that will be used in that product. Aluminum oxide is the most common abrasive, followed by silicon carbide. Because silicon carbide is harder and sharper, it is used for applications involving glass and other nonmetal materials. Aluminum oxide, which is the tougher abrasive, is used for metalworking applications where high forces are common. Minerals containing zirconium alumina and alumina are typically used where extremely rugged abrasives are needed, such as in foundries. Expensive and extremely hard minerals such as diamond or cubic boron nitride are restricted to special polishing processes.

The sizes of abrasive grains range from fine particles that look like flour (2,000 grit) to large particles that look like granulated sugar (60 grit). Finer grains are used for surface finishing applications and larger grains for shaping and material removal applications. Recent developments in making uniform and extremely small grain abrasives with particles the size of air-born particulate in smoke have created applications in fine polishing known as superfinishing. Other improvements include patented technology to cluster fine minerals into small hollow spheres or conglomerates the size of conventional grains. Such refinements have improved cutting ability and extended the useful life of coated abrasive products.

The backing is the flexible platform to which the abrasive mineral is attached. The development of coated abrasives as a versatile manufacturing tool can in part be attributed to improvements in backing materials. Without a strong and flexible backing, coated abrasives could not survive rough handling or the effects of liquids that are often used as grinding aids.

Backings come in four basic materials, each with unique attributes. Paper is the lightest of the backing materials and also the weakest. Although its lack of material strength limits paper's usefulness for hand applications, its flexibility makes it ideal for applications in which the coated abrasive must fit closely to the contour of a work piece. Graded on a scale that increases with the physical weight of a ream, paper backings come in weights rated A to F. Unless specially treated, paper cannot be used with water or other fluids.

Backings made from woven fibers come in progressively heavier weight designations of J, X, Y, M, and H and are typically made of cotton, polyester, or rayon. The pattern of weave in the backing varies from fibers woven at 90 degree angles to fibers overlaid at 90 degree angles and stitched together. A less-common mesh or screen pattern is used for backings in materials needed in wet, low-pressure applications. Fiber backings are made of multiple layers of resin-impregnated cloth fibers that are used in some dry, high-pressure applications. Film backings, a recent development, have improved the effectiveness of coated abrasives in precision finishing. Uniformly thick synthetic film can be used with special micron-sized minerals to produce highly reflective finishing and precision dimensions on parts.

The bond or adhesive is applied to the backing in two layers, each of which serves a different purpose. The first layer of adhesive, called the make coat, holds the abrasive mineral to the backing. After the first layer of adhesive and grain have been applied, a second adhesive, the size coat, is applied in varying thicknesses depending upon the kind of product being manufactured. A thin layer of size coat leaves more of the abrasive mineral exposed, yielding a product that cuts more aggressively. Thicker layers of size coats, which cover more of the mineral, create a product that cuts less aggressively but creates finer finishes.