Лако-красочные материалы — производство

Casein glues for wood pass through an early thick-consistency stage that requires fairly strong agitation to reduce them to a uniform and lump-free state. The mixer should be equipped for sidewall scraping to work thickened glue continuously back into the stirred composition. Counterrotating paddle mixers and bread dough mixers have proved ideal for this purpose.

Because of their thick, sticky consistency, casein glues are generally applied to only one of a mating pair of wood surfaces by roller, knife, or extrusion. Adequate adhesive wetting and transfer occur when the wood surfaces are brought together.

The stickiness of alkaline-dispersed casein glue provides two of its best performance attributes: long assembly-time tolerance and wipe resistance (difficulty of removal). A film of casein glue on dry lumber, for example, may allow an open/closed assembly time of 1-2 h before clamping is required. This property is especially useful in the timber laminat­ing industry, where it permits many pieces of lumber to be stacked over each other, adjusted for position, and assembled into large, complex laminated beams [61]. This long assembly tolerance plus the gap-filling and wipe-resistant capabilities of casein glues made them the outstanding choice for laminated structural wood products from the mid-1930s onward. Today’s phenol-resorcinol-formaldehyde laminating adhesives, which ultimately displaced casein glues on the basis of exterior durability, could still use a large measure of these working properties of casein glues.

While casein glues can be heat cured and were employed in the past to make hot — press plywood, most of the high-volume bonding applications have involved cold pressing. Casein glue films are adequately cured by water loss and insolubilizing of the proteins through various chemical reactions at room temperature [62]. Heating does not yield significantly improved water resistance. Except for soybean-casein blend glues, which take on the granular consistency of the soybean constituent, the inherent stickiness of straight casein glues dictates a fairly long clamping time to bring about water loss and adhesive hardening. Progressive shear tests have shown that these glues develop about half their dry strength in 3 h and substantially all of it in 6-8 h at room temperature. However, moisture resistance continues to improve for several days [63].

As mentioned previously, another performance attribute of casein glues that recom­mends their use in structural wood laminates is fire resistance [64]. While all three of the proteins discussed in this chapter burn to a char before losing bond strength, casein adhesives appear particularly durable in this respect. Thus casein glues remain the adhe­sives of choice for the economical assembly of wood-based fire doors of flush and panel designs. In yet another attribute, the combined adhesive strength and toughness (as opposed to brittleness) of casein glue films has made them an ideal bonding agent for wooden sporting equipment and other applications required to withstand flexing, vibra­tion, and shock, such as racquets, hockey sticks, and fishing rods [6,65].