Лако-красочные материалы — производство

Commercially available animal glues are sold in granular or pulverized form, and are dry, hard, odorless materials that vary in color from light amber to brown. Animal glues may be stored indefinitely in the dry form. The density of animal glues is approximately 1.27. A moisture content of 8 to 15% is considered commercially dry. An inorganic ash content of

2.0 to 5.0%, and a grease content of 0.2 to 3.0%, are in the normal range for commercial animal glues. The pH range of commercial glues is 5.5 to 8.0.

Animal glues are hydrophilic colloids and are soluble only in water. In cold water, the glue particles absorb water and swell, resulting in a jellylike sponge. Upon application of heat, the particles dissolve, forming a solution. Upon cooling, the solutions set to an elastic gel. The gelation is a thermally reversible reaction, and on application of heat the gel reverts to liquid form. The melting or gelling point will vary from below room tem­perature to over 120°F, depending on glue concentration, grade, and possible presence of modifiers.

An important characteristic of animal glues is their film forming and bonding prop­erties. Dried films are continuous, noncrystallizing, permanent, and possess great strength and resilience. Tensile strengths in excess of 10,000 psi have been reported [12]. Animal glues are insoluble in oils, greases, waxes, alcohols, and other solvents. Being soluble only in water, the continuous films are ideally suited as barriers against these materials. With suitable practical methods, the films may be made moisture resistant.

Sensitivity to the effects of moisture may be reduced by the use of various insolubi- lizing agents (sometimes referred to as ‘‘tanning’’ or ‘‘hardening’’) which cross-link the protein molecules, rendering them less susceptible to hydration and solution. These agents include formaldehyde, paraformaldehyde, hexamethylene-tetramine, glyoxal, and dialde­hyde starch. Metal salts will thicken, coagulate, and sometimes precipitate animal glue solutions. A degree of moisture resistance and raising of solution melting point may be obtained with these salts. Salts of aluminum, chromium, and iron have this effect on animal glue solutions.

Animal glues are readily compatible with and are frequently modified by water — soluble plasticizers such as glycerine, sorbitol, glycols, and sulfonated oils to increase film flexibility. They are also compounded with many other materials, such as dextrins, starches, sugars, various salts, pigments, poly(vinyl alcohols), and acetates, as well as some water-soluble solvents such as butyl cellosolve acetate for specific properties. Viscosities can be modified by compounding with thickeners, including compatible natural gums, alginates, and synthetic thickeners such as carboxymethyl cellulose.

Because of their amphoteric properties, animal glues possess electrical charges which unmodified or with suitable modification by simple chemical additives are highly effective as colloidal flocculents and as protective colloids in such applications as paper manufac­turer, rubber compounding, ore and metal refining, and for water and industrial waste­water treatment.

A wide range of viscosities is possible, from almost water thin to in excess of

70.0 cP by variation of the dry glue concentration and test grade. Animal glues are available in a number of grades from low to high, varying in inherent viscosity and gelling properties. The gel property determines the grade and is the controlling factor in speed of set of a glue film for adhesive applications.