Лако-красочные материалы — производство

The UF adhesives for plywood generally contain less than 2 mole of formaldehyde per mole of urea, and most of them are condensed to a slightly viscous, hydrophilic stage and are quite soluble in water. The degree of polymerization, and hence the viscosity under comparable conditions of UF resins for plywood is generally higher than those of UF resins for particleboard.

The application of UF resins for gluing purposes is based on the excellent control that can be exercised on the condensation reaction by varying the pH, a procedure easily applicable to a production scale. A small amount of an acid as hardener is added at ambient temperature. This produces no visible change at first, or possibly for a few hours; finally, thickening sets in, and the resins gels and hardens into an insoluble material. While the adhesive is still in liquid form, it can be spread on the wood surfaces which have to be glued and joined under pressure. These have to be cured either at room temperature for a few days or at elevated temperature for a few minutes. Solutions of ammonium salts, usually ammonium chloride or ammonium sulfate, or mixtures of ammonium chloride with urea, are generally used as resin hardeners. Often, ammonia solution is added to lengthen the usable life of the glue mix. Hydraulic presses with multiple openings are generally used for the production of plywood or flat veneer work. They can operate at pressures of 10 to 16 kg/cm2, but mostly operate in the range 12 to 14kg/cm2, according to the wood species, to avoid destruction of the porous structure of the wood.

The temperature is usually in the range of 120 to 160° C according to the type and moisture content of the veneers. It is chosen according to its capacity for the fastest pressing time and its ability to produce a good joint without blisters. Different pressing conditions are used in different countries, and the resin must be manufactured keeping the differences in the conditions of application in mind. There is quite a difference, for exam­ple, between a UF adhesive and glue mix which is capable of giving good joints at 5 to 8% moisture content of the veneer and a pressing temperature of 120° C, and a UF resin and glue mix usable at a veneer moisture content of 0 to 1% and pressing temperatures of 140 to 160° C. The former needs better flow characteristics and faster curing under standard measured conditions than does the latter if optimum pressing times and production schedules are to be maintained. Lower temperatures lengthen the curing time of the resins considerably but have the advantage that when the cured plywood sheets are taken out of the hot press, they tend to warp less on cooling or drying.

The use of fillers with plywood UF adhesives has important economical consequences and is necessary for technical reasons, because the fillers produce ‘‘body’’ in the glue solution and therefore prevent joint ‘‘starvation’’ in porous wood. Without filler it would be difficult to prevent part of the adhesive from flowing away or flowing into the open pores of the wood; or in the gluing of medium to thin veneers, from flowing through them to the other side, thereby causing undesirable resin patches on the outer veneer surfaces. As a rule, 20 to 50% filler is used for joinery and up to 100% for plywood. The most common fillers are wheat flour, corn flour, rye flour, very fine hardwood flour, and gypsum. If gypsum is used, it must be free of calcium hydroxide, because this interferes with the acid curing agent.