Лако-красочные материалы — производство

Blends of soybean flour with ground and screened casein also yield a very useful series of protein adhesives, in this case, mostly cured cold [38]. While alkaline-dispersed casein yields strong, water-resistant cold-cured bonds in wood, its sticky dispersed consistency does not permit the rapid water loss needed for quick-clamping procedures. By combining it with an appropriate amount of soybean flour, the cold-press no-clamp process can be used. Formulations of this type have proved so successful for bonding plywood faces onto wooden flush-door cores and frames in a short cold-pressing cycle that the entire industry has employed these protein adhesives from about 1950 to the present [39]. As a special performance property, the bonds of soybean-casein door glues maintain strong adhesion in a fire until the glue lines are literally charred away. Thus glues of this type are widely used to bond flush-design fire doors. They are also excellent adhesives for millwork in general [40]. Some current formulations also contain minor amounts of blood.

The formulation of the typical soybean-casein blend glue listed in Table 7 is quite different from any protein adhesive described thus far in that all ingredients, dispersing

Table 7 Soybean-Casein Dry Glue: Ingredients and Mixing Procedure

Amount

(kg)

Adhesive-grade soybean floura 29.0

250-pm Lactic acid caseinb 9.5

Fresh hydrated lime 3.5

74-p. m Wood flourc 2.5

Granular sodium carbonate 2.5

Granular sodium fluoride 1.0

Granular trisodium phosphate 0.5

Pine oil or diesel oil defoamer 1.5

Water at 16-21°C 100

Dry glue: mix 2min, until smooth and thick; let stand 15 min or 50

until thinning has occurred; mix until smooth

Water at 16-21°C: mix 2min or until smooth 25

a44% protein, specific surface 3000-6000 cm2/g. b0.250mm (60 mesh). c0.074mm (200 mesh) and finer.

agents included, are dry-blended into a single packaged composition that requires only the addition of water to prepare. The uniform oiling of al1 ingredients during the blending operation is a key, since it slows down the solution of the alkaline ingredients long enough for the soybean flour and casein to wet out under reasonably neutral conditions. Then the alkaline agents dissolve. Highly alkaline dispersing conditions are provided by reaction of the sodium salts with lime to yield sodium hydroxide in situ plus insoluble calcium salts [6]. (The sodium hydroxide needed for this strongly alkaline dispersion step could not be included in a one-package composition because of its hygroscopic behavior). These sequential solution and dispersion reactions require some time for completion, which is the reason for the 15-min pause in glue preparation.

The dry ingredients are blended intensively in an appropriate dry powder mixer while the defoamer is sprayed in to provide uniform distribution. Mixing directions are given in Table 7. The second water addition may be increased or decreased to obtain the final viscosity desired. (A normal range is 4000-8000 cP.) Working life is 4-6 h at inside temperatures. Application rates must be determined by experience but will generally range from 245 to 345 g of mixed glue per square meter of a single glue line.

While the soybean-casein blend glue can be used according to the short-cycle no­clamp process on dry softwood, it will require 4-6 h of clamp time to cure to machining strength when used on dense hardwoods. Water removal from the glue film is simply too slow on hardwoods to develop adequate early gelation. However, the ultimate bond strength is excellent. Note that this formulation represents about the maximum casein content at which short-cycle clamping is possible for softwood flush-door or millwork assembly. Above this level water loss is too slow, interfering with normal production rates through the press.