Лако-красочные материалы — производство

Soybean flour will wet and swell in plain water but will not disperse to yield useful adhesive properties. For this purpose, treatment with a soluble alkaline material is neces­sary. Almost any organic or inorganic alkali will disperse wetted soybean flour to some degree. However, soybean wood glues of maximum bonding efficiency require dispersion with several percent of a strong alkali such as sodium hydroxide or trisodium phosphate

[20] . The effect of this strongly alkaline treatment is to break the internal hydrogen bonds of the coiled protein molecules, literally unfolding them and making all their complex polar structure available for adhesion to wood. Although essential for adhesion, this alkaline dispersion process exposes the protein structure to gradual destruction by alkaline hydrolysis. Thus a dispersed soybean glue has a definite useful life, slowly losing viscosity and adhesive functionality over a storage period of 6-12 h.

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

[21] . 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

Amount

(kg)

Water at 16-21°C Adhesive-grade soybean floura

Pine oil or diesel oil defoamer: mix 3 min or until smooth in a counterrotating mixer Water at 16-20°C: mix 2min or until smooth Fresh hydrated lime (as a slurry in)

Water at 16-21°C: mix 1 min

50% Sodium hydroxide solution: mix 1 min

Sodium silicate solution: mix 1 min

Orthophenyl phenol or other preservative: mix 10 min

a44% protein, specific surface 3000-6000 cm2/g. bNormally dry-blended for easier handling and dust control. c8.90% Na2O, 28.70% SiO2, 41° Baume.

Water at 16-21 °C Adhesive-grade soybean floura

Pine oil or diesel oil defoamer: mix 3 min or until smooth in a counterrotating mixer Water at 16-21°C: mix 2min or until smooth Fresh hydrated lime (as a slurry in)

Water at 16-21°C: mix 5min

a44% protein, specific surface 3000-6000 cm2/g. bNormally dry-blended for easier handling and dust control.

The additions of hydrated lime and sodium silicate solution in the high-alkali mix (Table 1) accomplish two purposes: (1) they help maintain a level glue viscosity for a longer adhesive working life, and (2) they improve the water resistance of the cured glue film by forming some insoluble proteinates [22].

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 limiting the performance of soybean glues with respect to water resistance [23].

The final addition of preservative shown in this formulation is essential in virtually all protein glues to provide mold resistance in high-humidity service. Without this protec­tion, even heat-cured soybean adhesives will mold as the moisture content of the bonded wood approaches 20% [24]. Copper-8-quinolinolate, and copper naphthenate are among the few remaining preservatives permitted to be used in the United States at this time for wood products. Where the use of chlorinated phenols or orthophenyl phenol is still permitted, they are also very effective preservatives for protein glues at the addition level shown. In this case, the sodium hydroxide content of the glue formulation converts the water-insoluble chlorinated fungicides to their soluble sodium salts.

Large quantities of this high-alkali soybean glue formulation were used to bond interior grades of softwood plywood between about 1940 and 1960 [13]. It was also used to some extent for assembling prefabricated wooden building components [11]. Its primary advantages were very low cost and the capability to bond almost any dry wood surface. It also offered real versatility in a bonding process because it could be hot pressed or cold pressed to promote cure. Appropriate pressing schedules for each curing mode are provided in tables that appear later in this chapter.

The low-alkali formulation (Table 2) has been used widely as a briquetting binder for wood, charcoal, and other absorbent particles. It is particularly suitable for paper and softboard laminating, where a colorless glue line and minimum swelling of the glue film on high-humidity exposure are desired [25]. It is not recommended for structural uses such as sheathing plywood because of its lower degree of protein dispersion and thus lower bond­ing strength. Substituting borax or monosodium phosphate for the hydrated lime disper­sing agent will yield similar nonstaining glues.

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.

Each of these groups of compounds appears to react with the starchy constituents of whole soybean flour as well as the dispersed protein molecules. Formaldehyde itself acts too rapidly and thus is difficult to control. Instead, such compounds as dialdehyde starch, dimethylol urea, sodium formaldehyde bisulfite, and hexamethylenetetramine have been

used successfully to toughen the cured glue film and improve its water resistance [26,27]. Similarly, carbon disulfide, thiourea, and ethylene trithiocarbonate, among the sulfur com­pounds [28,29], and the soluble salts of cobalt, chromium, and copper have been used to improve soybean glue working properties and adhesive performance [30,31]. These modi­fiers are generally added last when preparing the glue. The range of addition of all such denaturants is 0.1-1.0% based on the weight of soybean flour. Also, 5-20% of an aliphatic epoxy resin has been added to soybean glues, yielding significantly improved durability, but the cost is high [32]. Similarly, alkaline phenol-formaldehyde (PF) resins have been incor­porated for both durability and mold resistance. Proportions have ranged from a straight soybean glue with a minor resin addition to a primarily PF resin glue containing a moderate quantity of soybean flour [33,34]. Soybean protein is presently being evaluated as a cost — reducing adhesive constituent in phenolic and urea-formaldehyde resin binders for wood particle and cellulosic waste reconstituted products.