Лако-красочные материалы — производство

For adhesive uses, the particle size of ground casein is normally controlled within the range of about 250-500 pm [56]. Particles coarser than 500 pm (30 mesh) may not dissolve and disperse completely during glue preparation. Those much finer than 250 11 pm (60 mesh) tend to form immediate lumps on wetting, even if oiled. For single-package casein glues (by far the most widely used type), preliminary oiling of the dry ingredients is a very important manufacturing step. It helps prevent the pickup of atmospheric moisture by alkaline salts in the dry composition followed by premature attack on the casein during storage. Oiling also slows down the solution of these salts in water at the time of glue mixing, thus allowing the casein particles to become wetted and lump-free in reasonably neutral water.

The lime content of casein glues is similarly important. A high percentage of lime (above 30% of dry casein weight) ensures maximum water resistance of the cured glue film but sharply reduces mixed adhesive working life. A lime content below 10% provides a long working life and adequately strong dry bonds on wood but significantly reduces moisture resistance. Most commercial adhesive formulations balance these properties by utilizing lime additions in the range 15-25% [57].

As with blood and soybean flour, the maximum adhesive capability of casein is attained only by complete aqueous dispersion of the folded protein molecules with a strongly alkaline inorganic salt such as sodium hydroxide [56]. Since sodium hydroxide cannot be incorporated successfully into a dry adhesive composition, it is quickly pro­duced on mixing through a double decomposition reaction between calcium hydroxide and strongly ionized but less alkaline salts such as sodium fluoride, sodium carbonate, and trisodium phosphate. (The residues from this reaction are insoluble calcium compounds.)

The viscosity and consistency of casein glues can be altered substantially by reaction with most of the classic protein denaturants such as sulfur compounds, formaldehyde donors, and complexing metal salts [6,56]. One or more of these are frequently used as manufacturing control to offset the natural variability of casein and produce glues of uniform properties. The water resistance of cured casein glues is also improved by mod­erate denaturing.

Finally, to provide mold resistance adequate for interior and covered exterior struc­tural requirements, a fungicide must be added to casein glues [40,58]. In this case there is no excess of sodium hydroxide in the glue composition to convert a water-insoluble fungicide to its soluble sodium salt. Therefore, it is added as a prepared soluble salt in order not to upset the fairly precise alkaline balance in the dry glue composition needed to fully disperse the casein. Sodium orthophenylphenate and sodium pentachlorophenate are examples.

The casein adhesive formulation described in Table 15 embodies all the foregoing technology. The dry ingredients are intensively blended in an appropriate dry powder

mixer while the defoamer is sprayed in to provide uniform distribution. The dimethylol urea addition, a protein denaturant, is variable for glue viscosity control. The small adjustment is made in the defoamer. Mixing directions are provided in Table 15. The finished glue viscosity should be in the range 4000 to 8000 cP at room temperature, thickening gradually over several hours and attaining a firm gel overnight.

In a totally different area of application, casein adhesives for paper sizing, chipboard laminating, and label gluing are more nearly ‘‘casein solutions’’ [59,60]. They are simple dispersions with ammonia or borax at moderate pH and low viscosity. They are frequently combined with latexes or soluble rosin derivatives for special performance improvements [59].