13. Abrasives

The historical development of the abrasives industry is noteworthy considering its evolution from a rudimentary beginning to a relatively mature industry. Presently, hardly a material exists that has not been exposed to a grinding operation. It can be stated without exaggeration that the abrasives industry represents a basic building block for all other branches of industry.

There are two basic types of abrasives: bonded abrasives or grinding wheels and coated abrasives. As one currently examines the abrasives market, the global market for abrasives is showing a downward trend for bonded abrasives while coated abrasives are increasing globally. In developing countries like China, India, and Thailand that are fostered by a robust economy, abrasives are increasing in use and are produced for industries such as construction, machinery, and automotive. Parallel to this activity, more abrasive production is moving from industrial nations such as US, Japan, and some European nations to these developing countries.

The performance of bonded abrasives and coated abrasives as they are used industrially is recognized by their physical behavior, stress, and heat generated during use. By examining the microstructure of both types of abrasives, abrasive performance characteristics are improved by a judicious evaluation of abrasive components. Bonded abrasive structure consists of two basic components such as grain and bond for most wheels, some voids or pores. The grain must be self-renewing during operation; bond created by phenolic resin provides strength and heat resistance as well as uniformity in bond strength to facilitate regeneration of grain during grinding, and finally the pores of the bonded abrasive draw out the grinding dust or debris. Too weak or too strong bond strength either facilitates rapid wheel wear or the pores are filled with debris and the grinding efficiency is reduced. “Trouble-shooting” suggestions are provided in the manufacture of bonded abrasives as they relate to plant environment (humidity/temperature) and adjustments of specific characteristics of liquid and powdered resins.

Coated abrasives are grain bonded on a backing material. The latter can be paper, vulcanized paper, cloth, or a non-woven. Coated abrasives are single layers of grain on backing material and quite different from bonded abrasives which are bonded aggregates of grains in a circular configuration. Grains of coated abrasives are not replaced; they just become eroded or possibly expelled. Coated abrasives lifetime ceases when the grain is nearly or completely eroded. The key phenolic resin bond requirement is to provide high bond strength between the grain and the backing material. Different resins both phenolic and non-phenolic are used in either the make coat or the size coat. Manufacturing conditions, various phenolic resins, and different backing materials are mentioned for the production of coated abrasives.

Attractive schematics within the chapter provide relationships of both liquid (viscosity, MW, water/solvents) and powdered resins (MW, hexa content, particle size) characteristics in guiding the manufacture of various grinding wheels with attractive wheel features such as good wear resistance, good self sharpening, good water dilutability, and good handling. Similar manufacturing features are mentioned for coated abrasives.