Лако-красочные материалы — производство

Basic manufacturing procedures for animal glues generally involve alkaline pretreatment (for hide glues) or acidic pretreatment (for bone glues). The raw materials for hide glues

include salted, limed, or pickled hide trimmings or splits, and chrome-tanned leather scrap. Tanned leather scrap requires special processing because of the chrome tannage.

Hide glues from hide trim and splits are prepared by initial washing with water. The stock is then soaked in lime (calcium hydroxide) and water for a period of weeks, which dissolves and removes extraneous protein-related materials, as well as conditioning the collagen for subsequent glue extraction by hydrolysis. The condi­tioned collagen is then washed with water, followed by acidulation with dilute acid such as sulfuric, hydrochloric, or sulfurous, for pH adjustment, followed by a final water rinse.

Chrome-tanned leather scrap for hide glues may be treated initially with lime or caustic, followed by a strong acid bath to remove the tannage. The stock is then soaked in magnesium hydroxide and rinsed prior to extraction. Alternatively, the chrome stock may be treated with a magnesium hydroxide soak only, prior to extraction, in which case the chromium tanning salts remain in the residue after extraction. The treated collagen is transferred to extraction kettles or tanks, where it is heated with water to convert the collagen and extract the glue. Several hot water extractions at progressively higher tem­peratures are made under carefully controlled conditions. Separate, successive dilute glue solutions are removed from the stock until the glue is completely extracted, usually in four extractions.

The dilute glue extractions, ranging from 2 to 9% glue solids, are filtered and con­centrated by vacuum evaporation to 20 to 50% concentration prior to drying. In some plants the glue is chilled until it will gel, then dried in tunnel dryers which circulate heated air over gelled sheets stacked on wire nets with air space between, taking up to 48 h to dry. In newer installations, the concentrated glue solutions are cooled to the gelling point and are extruded in noodle form into a continuous dryer which completes the drying in 2 to 2|h by circulating conditioned, filtered, heated air. The dried product, at 10 to 15% moisture content, is then ground to the desired particle size.

Bone glues fall into two categories, green bone or extracted bone. Green bone glues are prepared from fresh or ‘‘green’’ bones, which come primarily from the meat-packing industry. The bones are crushed, washed, and normally treated with dilute acid, either sulfuric or sulfurous, prior to extraction. Extracted bone glues are prepared from dry bones that have had a preliminary degreasing treatment with solvent prior to conditioning for extraction.

The glue is extracted from the conditioned bones by hydrolysis in pressure tanks by the successive application of steam pressure and hot water. Separate successive dilute glue solutions are removed from the tanks, followed by filtering or centrifuging to remove free grease and suspended particles. The dilute solutions are vacuum evaporated to high con­centration, followed by drying and grinding, as described for hide glues. Modern animal glues, whether of hide or bone origin, contain adequate preservatives for protection against bacteria or mold growth under normal conditions of use, and may contain defoaming agents where foam control is desired in the end application.