Лако-красочные материалы — производство

Since soybean-blood blend glues were covered in the preceding section, in this portion of the chapter we deal only with all-blood adhesive compositions.

A. Raw Material Source and Preparation

Historically, animal blood could be used for adhesives only in reasonably fresh liquid form. These glues performed well on wood. However, the very rapid spoilage rate of liquid blood imposed real limitations on the general availability and use of this adhesive raw material. It was not until about 1910 that techniques were developed for drying whole blood in commercial quantities without denaturing its protein content, thus maintaining its water solubility [41]. As a result, blood could be collected, processed, and stored indefinitely for later use. The effect of this development was to stimulate rapid growth in the technology of blood-based adhesives, especially for wood, about the time of World War I.

Virtually all the proteins in animal blood can be dispersed into useful adhesive form. These include the serum albumin and globulin and even the red cell hemoglobin [42]. The fibrin clotting substance is sometimes removed before drying (by agitation or acidification) because of its instability in solution. Thus, except for residual moisture content, dried blood is essentially 100% active adhesive protein.

The principal North American bloods sold in quantity for adhesive uses are beef and hog, with lesser amounts from sheep and horses. Because of its high lysine content, poultry
blood is utilized almost exclusively as a feed additive or binder and is seldom available otherwise. For adhesive purposes, there are significant viscosity differences relating to species among these dried bloods, beef being highest and poultry lowest [43]. Viscosity and water-holding properties are also influenced by animal age, diet, activity, and other factors. As a result, industrial-grade dried soluble blood is generally blended in large quantities to provide average and reproducible properties for adhesive formulating.

The method employed for drying blood is now entirely spray drying. (Formerly, a certain amount of vacuum pan-dehydrated blood was also available.) Spray-drying con­ditions relating to temperature, dwell time, and humidity can be adjusted to produce a wide range of blood solubilities [44]. Also, chemical denaturants such as glyoxal can be added to the blood solution prior to drying to further modify its adhesive characteristics [45]. Solubilities from about 20 to 95% can be prepared with ± 5% control. (Dried bloods below 20% solubility can only be redissolved in strongly alkaline solutions, which destroy a significant portion of the adhesive proteins.) This controllable range of solubilities permits the formulation of blood glues with a variety of handling and performance properties.

Generally speaking, the lower the solubility of a dried blood product, the more granular and water holding is its alkaline-dispersed form [46]. For instance, blood glues of 20-40% solubility make excellent cold-press formulations (which must have a granular consistency) [42]. They also yield the most water — and mold-resistant (near-exterior) glue bonds when cured hot. By comparison, highly soluble bloods in the range 85-93% yield very slick and livery alkaline dispersions of somewhat lower water-holding capacity. Soybean flour is normally blended with these highly soluble bloods to produce appropri­ately granular glues. If soybean flour is not used, they must be combined with a particulate cellulosic filler such as wood flour or nutshell flour to develop this functional consistency. Examples of these glue types are provided in the following section.