Лако-красочные материалы — производство

Table 8 describes an example of the use of an accelerator which distinctly increases the gelling rate of a core layer glue mix, hence enabling a significant shortening of the neces­sary press time. The quick reaction of the accelerator with the hardener salt generates the acid for the acid-induced hardening reaction of the resin. The accelerator is mixed with the resin just prior to use. Since it does not contain any hardener or acid, there is no limiting pot life of this premix. To compensate for the additional formaldehyde, small amounts of formaldehyde catchers are recommended for addition to the glue mix. [18]
low reactivity, causing a low capacity of the production line. In these systems the hardener is premixed in greater proportion with the adhesive resin. The limitation of these is the too short pot life obtained after hardener addition, causing early gelling of the glue mix in the storage vessel. Using smaller glue mixes increases the chances of improving the resin reactivity. With this presupposition very reactive hardeners can be used. They decrease the gel time of the glue mix and hence the necessary press times. Special aminoplastic resin systems with distinctly higher reactivity have therefore been developed to fulfill the requirements of each customer in terms of saving time, energy, and costs. The higher the reactivity, the higher is the capacity of the production line, or the lower is the necessary press temperature at a given press time. Lower temperatures are beneficial for the quality of the wood itself as well as for saving energy costs.

It has been shown that such very reactive hardeners perform favorably also when in liquid form. This enables the use of various acids or acidic substances in the formulations of these hardeners. Such reactive adhesive systems usually consist of two liquid compo­nents, one being a high viscosity resin and the other a high viscosity liquid hardener. The hardener contains some inorganic fillers or organic thickeners. The mixing of these two components is performed just prior to the application of the resin mix to the roll coater. The liquid-liquid two-component mixer is installed preferably above the roll coater, in order to reduce considerably the amount of each batch of prepared glue mix. If a long stop of the production occurs the lost amount of the ready-to-use glue mix is rather small. Another advantage of this system is that both components can be pumped directly from the storage vessels to the mixer, without the use of any powder. The disadvantage of these two-com­ponent systems is the fixed ratio between the extender and the hardener. If the amount of extender should be changed, the amount of the hardener itself is also changed and hence the glue mix reactivity and pot life are changed too.

Because of the well known marked influence of the temperature on the pot life, cooling of the whole system is necessary using a chilled water cooler. The raw adhesive resin should have a temperature not higher than 15° C prior to use, which is especially important in summer due to the higher room temperature. Cooling of the adhesive resin can be performed in a small vessel with cooling coils, which is installed between the storage tank and the mixer. Additionally the roll coater itself also needs cooled cylinders in order to stabilize the temperature at approximately 15°C. Figure 2 shows the scheme of a liquid — liquid two-component mixing station. [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]