Soybean glue

In this chapter we address three of the most widely used families of protein-based adhesives for wood soybean, blood, and casein. The technology presented is drawn primarily from the years 1930 through about 1960, when the consumption and technical refinements of these adhesives were at their peak. Soybean glues are discussed first because they were often utilized in combination with blood or casein to yield adhesives of intermediate performance properties as well as being used alone. 

Although these strongly alkaline soybean glues are nearly colorless in an applied film, they cause a reddish-brown stain on wood surfaces as they cure, due to alkali burn of the cellulose itself [5], If a colorless glue line on wood is desired, the wetted soybean flour must be dispersed with a less strongly alkaline material, such as hydrated lime or ammonia... 

However, the adhesive bond strength of these low-color, mildly dispersed soybean glues is considerably less than that obtained with fully dispersed, highly alkaline formulations. Typical high- and low-alkali soybean glue formulations are listed in Tables 1 and 2. 

Table 1 High-Alkali Soybean Glue Ingredients and Mixing Procedure...

The starchy constituents of soybean flour also disperse in the presence of strong alkali to become useful adhesive molecules contributing to dry bond strength. However, this starchy fraction also retains its well-known sensitivity to water and is considered primarily responsible for Hmiting the performance of soybean glues with respect to water resistance [23]. 

Over the years a number of denaturants or cross-linkers have been added to soybean glues to improve their water resistance, working life, and consistency. These may be roughly categorized as formaldehyde donors, sulfur compounds, and inorganic complexing salts. 

Table 3 Soybean Glue Hot-Pressing for Interior Douglas Fir Plywood (a) Pressing schedule... 

Low-viscosity soybean briquetting adhesives are generally applied by spray. Paper and softboard laminating glues are usually applied by curtain coater, knife, or indirect roller. High-viscosity plywood and lumber assembly formulations are fairly well limited to application by spreader roll or extrusion. For very small assembly jobs, soybean glue can easily be applied by brush. 

Table 4 Soybean Glue Cold-Pressing (No-Clamp) for interior Douglas Fir Plywood... 

As mentioned earlier, the single largest commercial use of soybean flour in wood glues during the recent past has been as a blend with other adhesive proteins, mainly blood and casein, for bonding interior-grade plywood, doors, and millwork. These blended formulations exploit several unique properties of the soybean glues themselves and incorporate useful adhesive characteristics from the other protein materials. 

As with straight soybean glues, the low-blood-content formulation requires a mold-inhibiting ingredient to meet plywood performance standards, whereas the high-blood-content glue does not [24]. 

As a point of interest, blood glues are not affected by many of the protein denatur-ants used to improve the performance of soybean glues specifically, sulfur compounds and complexing salts [49]. However, they are very sensitive to aldehyde-acting compounds, and... 

Blood and soybean-blood blend glues of appropriate high viscosity and granular consistency can be pressed cold according to the schedule shown for soybean glues. For this purpose they must contain a terminal addition of about 5% preservative based on dry glue weight in order to meet product standards for mold resistance [24]. 

Soybean glue Basic salt (Sorel cements)... 

These materials will biodeteriorate under humid conditions unless inhibitors are used. Their resistance to heat and weathering is poor therefore, they are restricted to indoor applications. Soybean glues are filled for paper and paperboard lamination, cardboard and box fabrication, and particle binders. ... 

Formula No. 4 taken from Lambuth, Soybean Glues, in Handbook of Adhesives, 2nd Ed., I. Skeist, ed.. Chapter 10, p. 176, New York, Van Nostrand Reinholds Co., 1977. 

Soybean flour can be used as the main component in adhesive formulations, but it is commonly added as extender for phenolic systems, or blended with casein or other adhesives. They are packaged as dry powders that contain both proteins and carbohydrates. The flour is dispersed in aqueous sodium hydroxide for preparing the adhesive, and also calcium hydroxide is added to extend the open time and to improve water resistance. Soybean glues have a limited water resistance, but recover their strength on drying. They are also susceptible to mold growth, and some fungicides need to be included in the commercial formulations. Fillers are used to reduce cost, but they also lower the performance of the adhesive. Some commonly used fillers are wood and walnut-shell flours, and also clay. 

Vegetable glues Starch Dextrins Soybean glue Rosin... 

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