Лако-красочные материалы — производство

Reference was made to the historical use of collagen glues derived from the gelatin extracts of animal bones and hides. This does not properly indicate the true importance to the wood industry of these materials. From ancient times to the present, animal glues have in fact remained one of the primary assembly adhesives for wooden furniture, cabinets, and musical instruments [66]. Applied as a hot, viscous solution to furniture joints, they rapidly develop gel strength on cooling that permits the prompt removal of clamping pressure. On subsequent drying, these glues cure to resilient, high-strength bonds between wood sur­faces, especially those involving end grain. Animal glue bonds are strong and permanent as long as they are kept dry and reasonably cool but are subject to softening and fungicidal attack when moistened or heated. Water and temperature resistance can be improved through the incorporation of most of the protein denaturants listed earlier [67,68].

Animal glues are used widely in a variety of ways with paper. For example, they have been the dominant adhesive for rewettable gummed paper tapes, labels, and envelope seals [69]. They are an important coadditive with synthetic wet-strength resins and rosin sizes for coated paper products [70]. They have been a primary binder for the grit that forms sandpaper [71].

In contrast to their widespread use in furniture and paper products, animal glues have not proved useful as structural adhesives for wood. When used as the principal protein constituent, their water sensitivity is excessive compared with other available proteins. When combined with soybean, blood, or casein, animal gelatin glues are com­pletely hydrolyzed and destroyed by the strong alkalies required to disperse these proteins. In addition, they soften when severely heated, which, by law, prohibits their use in struc­tural wood products [5].

Although in recent years animal glues have been replaced substantially by the newer synthetic adhesives, particularly the vinyl and acrylic emulsions, large quantities are still sold in dry and stable liquid forms for furniture assembly or repair and paper bonding applications.

Generally similar comments can be made with respect to fish skin adhesive extracts regarding these and other wood gluing applications. Fish skin glues are normally prepared in stable liquid form through mild acid hydrolysis and are frequently combined with animal glues for improved rewettability, tack, and adhesion of paper to glass or metal surfaces [72]. They differ from animal glues in one important respect: namely, they will not soften at elevated temperatures, especially when treated with aldehydic or polyvalent metal ion cross-linkers. Thus, in addition to the more conventional paper bonding applications, this property has created a major field of use for fish gelatin extracts as durable but temporary protective coatings and light-convertible photoresist films [73].

Other vegetable protein sources are occasionally mentioned as substitutes for soy­beans. These have included cottonseed meal, peanut flour, Alaska pea, and rapeseed meal, to name just a few. Although they do contain 25-35% useful protein substance, they have never made significant inroads on soybean flour for wood-gluing applications on the basis of comparable performance. However, they can be used and are prepared for adhesive purposes in the same manner as soybean flour itself.