Лако-красочные материалы — производство

Soybean glues are very easy to mix, provided that they are wetted with plain water as a first step. (If any form of alkali is present in the first mixing water, the dry soybean flour will form permanent lumps.) As with all protein glues, the first mix is kept thick to break down any lumps of dry powder that may be present. The division of water additions in both formulations listed earlier demonstrates this mixing procedure. Once the soybean flour particles have been uniformly wetted, further dilution and dispersion steps can follow without difficulty. Water additions are adjusted to yield a mixed glue viscosity in the broad range of 500-25,000 cP at 25°C, depending on purpose. Briquetting and paper laminating glues would typically be 500-1000 cP, while cold-press plywood glues should be 10,000-20,000 cp for best performance [35]. Hot-press formulations would be midrange. Particularly because of the heavy first mixing stage and the high final viscosities, an appropriate soybean glue mixer should have relatively strong, slow-turning blades plus a counterrotating scraper for continuous removal of glue buildup from the mixer walls.

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.

One of the real advantages of protein glues generally is their ability to be cured (under pressure) either hot or cold. Typical commercial schedules for each mode of cure are given in Tables 3 and 4. The cold-pressing schedule in Table 4 is the result of an interesting laboratory observation and subsequent industry-wide patent [35]. Alkaline protein glues, particularly soybean glues, lose water quite rapidly into adjacent dry wood surfaces. As a result, they gain sufficient gel strength in 15-20 min to permit removal of a glued wood assembly from its clamping device without loss of intimate contact between the glued surfaces. Cure is then completed over the next 6-12 h simply by placing the bonded products in storage at ambient temperatures with minimum handling. This method of cold pressing, called the no-clamp process, was used throughout the softwood plywood industry for many years. Prior to its introduction, all protein-bonded cold-press plywood was clamped for 6-8 h with bulky steel beams and turnbuckles.

Because protein glues develop bond strength primarily by water loss over time, roll pressing has proved unsuccessful as a clamping method for wood products. The short, intense period of pressure simply squeezes the still-fluid mix off the glue line without

affording sufficient time for water loss and gelation. An exception is the soft rubber roll or brush roll lamination of paper to paper or paper to wood [36]. This can be accomplished at production speeds because of the extreme rapidity with which dry paper removes water from a protein glue film.