Лако-красочные материалы — производство

The coated-abrasive industry consumes a large volume of animal glue in such applications as sandpaper and cloth in rolls, sheets, and various specialties. Animal glue is also used for ‘‘set up’’ abrasive wheels, belts, disks, cones, and bobs, as well as in greaseless abrasive composition, which is basically a mixture of animal glue, water, and abrasive in stick form for application to polishing wheels. Abrasives are involved in the production and finishing of wood products and for fine metal finishing, which is of importance in the production of machinery, automobiles, household equipment, appliances, and similar products.

In the production of coated abrasives, the backing material, especially in the case of cloth, is sized as required with low — to medium-grade animal glue or glue and starch, and is applied in the concentration range 30 to 40% to impart body and strength. The base or ‘‘make’’ coat is applied to the paper or the presized cloth by a roller applicator which deposits the glue film in a thickness determined by the grit size: the larger the grit, the thicker the film. The base coat takes advantage of the gelling property of the animal glue. Glue concentration is in the range of 25 to 40% and may contain extenders such as calcium carbonate. Higher-grade glues are used for silicon carbide and aluminum oxide, medium grades for emery and garnet, and the lower grades for flint.

The adhesive grain is applied uniformly to the tacky glue film on the moving backing sheet from a gravity-fed hopper. The grain is held in place as the glue gels and can be electrostatically oriented into a vertical position as the sheet passes through the initial drying stage. The partially dried sheet is passed through a second coating machine, which applies a thin film of animal glue (or in some cases a resin), usually in the concentration range 10 to 15%, to lock the grit firmly in position. The abrasive cloth or paper is then dried in a controlled hot-air dryer and wound into jumbo rolls for storage and subsequent slitting and cutting to the desired shape and form.

Closely allied with coated abrasives is the use of animal glues by the end user for set up, polishing wheels, belts, disks, and bobs. The user prepares these by applying an animal glue base coat and an appropriate abrasive grain, and sometimes a top coat, followed by drying. The principles, grade selection, and handling are similar to those for coated abrasives. Applications include metal finishing, glass grinding, granite finishing, and leather finishing.

A major advantage of animal glues over phenolic resins used for the same purpose is film resilience. Phenolic films tend to be brittle, which can result in scratching and other undesirable finishing problems, especially with the finer grades of abrasives. A common practice is to use an animal glue make coat and a phenolic resin size coat, to take advan­tage of the adhesive and film properties of animal glue while imparting moisture resistance.