Лако-красочные материалы — производство

During the curing process a three-dimensional network is built up. This leads to an insoluble resin which is no longer thermoformable. The hardening reaction is the conti­nuation of the acid condensation process during resin production. The acid hardening conditions can be adjusted (i) by the addition of a hardener (usually ammonium salts such as ammonium sulfate or ammonium nitrate) or (ii) by the direct addition of acids (maleic acid, formic acid, phosphoric acid, and others) or of acidic substances, which dissociate in water (e. g., aluminum sulfate). Ammonium chloride has not been in use in the particle­board and MDF industry for several years because of the generation of hydrochloric acid during combustion of wood-based panels causing corrosion problems and because of the suspected formation of dioxins [94].

Ammonium sulfate reacts with the free formaldehyde in the resin to generate sulfuric acid, which decreases the pH; this low pH and hence the acid conditions enable the condensation reaction to restart and finally the gelling and hardening of the resin takes place. The pH decrease takes place with a rate depending on the relative amounts of available free formaldehyde and hardener and is greatly accelerated by heat [46,61].

UF resins differ from other formaldehyde resins (e. g., MF, MUF, and PF) due to their high reactivity, and hence the short hot-press times which are achievable. Hot press times shorter than 4 s/mm board thickness are possible in the production of particleboards with modern, long continuous press lines. This requires highly reactive UF resins, an adequate amount of hardener, as high a press temperature as possible, and a marked difference in moisture content of the glued wood particles in the surface and the core of the mat before hot pressing. This moisture gradient induces the so-called steam shock effect even without the additional steam injection often used in North American plants. The optimal moisture content of the glued particles is 6 to 7% in the core and 11 to 13% in the surface. The lower the moisture content in the core, the higher the surface moisture content can be. However, a critical total moisture content in the mat must not be exceeded as this might cause problems with steam ventilation and even steam blisters in the panel. For this it is necessary to have low moisture content of the glued core particles and it is necessary to be thrifty with any extra addition of water in the mat core. The lower the resin solids content on the wood, the lower is the amount of water applied to the wood furnish and hence the lower is the moisture content of the glued core particles. For the surface layers, on the other hand, additional water is necessary in the glue mix to increase the moisture content of the glued particles. This additional water, however, cannot be replaced by a higher moisture content of the dried particles themselves before blending, because this water must be available quickly for a strong steam shock effect. This would not be the case if the water would still be present in the wood furnish as the internal wood cell wall moisture content.

The mechanism of the hardening reaction of a MUPF/PMUF resin is not really clear. MUF resins harden is the acid range, whereas phenolic resins have their minimum of reactivity under these conditions. There is then the possibility that the phenolic portion of the resin might not really be incorporated into the aminoplastic portion of the resin during hardening. Different opinions and confusing reports have been advanced as regards PMF resin hardening. During the hardening of PMF resins either no cocondensation occurs [95] and in the hardened state two independent interpenetrating networks exist, or some cocondensation is reported to occur [88]. Only in model reactions between phenolmethy — lols and melamine have indications for a cocondensation via methylene bridges between the phenolic nucleus and the amino group of the melamine been found by 1H nuclear magnetic resonance (NMR).

In order to increase the capacity of a production line, especially by shortening the panel hot press times, adhesive resins with a reactivity as high as possible should be used. This includes two parameters: a short gel time and a rapid and instantaneous bond strength development, even at a low degree of chemical curing.

The reactivity of a resin at a certain molar ratio F/U or F/(NH2)2 is determined mainly by its preparation procedure and the quality of the raw materials used. Figure 1 shows the comparison of two straight low formaldehyde emission (E1) UF resins with the same molar ratio, but prepared according to different manufacturing procedures. The differences between the two resins are clearly evident by their different rates of strength increase obtained in the so-called ABES (Automatic Bonding Evaluation System) test [96]. Resin A shows a distinctly quicker increase in bond strength than resin B, a fact which also has been verified in the industrial scale production of boards.