Лако-красочные материалы — производство

In the case of the application of phenolic adhesives to the manufacture of exterior-grade particleboard, the closest attention must be focused on the application of the resin rather than on its formulation. A good phenolic resin for plywood can be used successfully for the manufacture of particleboard once the various conditions of application have been understood. The pressing time of the board varies according to the type of adhesive, its reactivity, and the moisture content of the glued particles. In many cases a light water spray is applied to the top surfaces of the board before prepressing to shorten the pressing time. The light film of water covering the surface is vaporized when it comes in contact with the hot caul sheet of the press and migrates from the surfaces toward the core of the panel, causing a faster increase in temperature and a faster cure.

The water spray prevents precuring of the adhesive on the surface of the board during closure of the press before contact with the hot top caul sheet. The wood undergoes a partially irreversible plastic deformation during pressing, caused by the combined action of pressure and heat. Different products can be obtained by varying the type of pressing cycle of the board. Different pressing procedures and diagrams are available. A diagram for industrial three-layer particleboards may read as follows:

1. Maximum Pressure A pressure of 23 to 27kg/m2 is reached as fast as possible after pressure closure (i. e., after 35 to 50 s; other processes use pressure as high as 35 kg/cm2).

2. Contact with the Gauge Bars As a rule, contact is made after 60 to 120 s from the start of the press closure. The higher the density, the longer the time it takes. This, however, can be reduced by increasing the moisture content of the glued mat.

3. Steam Escape This is expected to begin 1 to 3 min after making contact with the gauge bars.

4. Pressure Decrease After approximately 1% to 2 min of maximum pressure, the pressure is slowly decreased until the final pressure on the panel is as low as 2 to 3 kg/cm2. This takes place toward the end of the cycle, just before press opening.

This pressing diagram produces a board with high-density face layers and the short­est possible pressing time, at a given temperature and a low power consumption. The main properties of panels with high-density face layers are the stiffness of the panel; better warp resistance; high dimensional stability; hard, glossy, and shockproof surfaces that need less adhesive for subsequent veneering; and narrow thickness tolerances.

Considerable variation in the properties of the final board can be obtained by vary­ing the moisture contents of surface and core layers, and by using faster resins in the core layer and slower reacting resins in the surface layer; these variations, among others, intending to increase the board core density and to improve the density profile of the panel as a function of its thickness. This is one of the contributory factors to improving properties based on the adhesive application technology rather than on the characteristics of the adhesive itself. In wood particleboard manufacture, factors derived from the appli — cation/pressing technology contribute as much as 50% to the final performance, the rest being due to how good the adhesive itself is. This can also be achieved by varying the geometry and sizes of the wood chips, the density of the board, and so on. Small variations in the manufacture and characteristics of the phenolic resin used do not affect the property of the finished particleboard as extensively as do the factors listed above. Experiments [51] on the correlation of curing and bonding properties of particleboard glued with resol-type phenolic resins by differential scanning calorimetry show that resols tend to reach two endotherm peaks; the first at 65 to 80°C and the second at 150 to 170°C. Resols used for particleboard have been shown to begin curing at lower temperatures than those for novolak resins. Resol-glued particleboard shows no bond formation at 120°C. At 130°C the resol-glued panels show internal bond strengths of 0.55 to 0.7 Mpa. The internal bond strength for the wet tests increases as the board core temperature goes over 120°C during pressing. The normal press platen temperatures for 12- to 13-mm-thick board glued with phenolic adhesives are 170 to 230°C. The pressing time is 18 to 12s/mm for standard PF resins but today PUFs [31], and ester-accelerated [31,32], tannin-accelerated [51], and urea — drowned PF resin [50] adhesives can reach pressing times as fast as 5 s/mm at 190-210°C in industrial applications [30,31]. Typical results obtained using PF adhesives for particle­board are shown in Table 2.