Лако-красочные материалы — производство

The wood moisture content influences several important processes such as wetting, flow of the adhesive, penetration into the wood surface, and hardening of the adhesive in the gluing and production of wood-based panels. In bonding solid wood usually a wood moisture content of 6 to 14% is seen as optimal. Lower wood moisture contents can cause a quick dryout of the glue spread due to a strong absorption of the water into the wood surface as well as wetting problems. High moisture contents can lead to a high flow and an enhanced penetration into the wood, causing starved glue lines.

Additionally a high steam pressure can be generated which might give problems of blister­ing when the press opens or at the end of the continuous press. Also the hardening of a condensation resin might be retarded or even hindered.

During the hot press cycle of the particleboard or MDF production, quick changes of temperature, moisture content, and steam pressure occur. The gradients of temperature and moisture content determine significantly the hardening rate of the resin and hence the board properties. These gradients together with the mechanical pressure applied to densify the mat are decisive for generating the density profile and hence for the application properties and performance of the boards. The higher the moisture content of the glued face layer particles, the steeper the moisture gradient between the surface and the core of the mat and the quicker the heating up of the mat occurs. In the fiber mat in MDF production no differences are seen in the moisture content of the outer layer and the inner layer due to the temperature applied to the mat, nevertheless a vapor pressure gradient occurs.

The moisture content of the glued particles is the sum of the wood moisture content and the water that is part of the applied glue mix. Therefore, the moisture content of the glued particles mainly depends on the gluing factor. Usual moisture contents of glued particles are: (a) for UF, 6.5-8.5% in the core layer and 10-13% in the face layer; (b) for PF, 11-14% in the core layer and 14-18% in the face layer. The optimal moisture content of the glued and dried MDF fibers in the mat before the press is in the region of 9-11%. The higher the moisture content of particles, the easier the face layer can be densified at the start of the press cycle; this leads to a lower density in the core layer.

Blistering at the end of the press cycle or at the end of the continuous press occurs if the steam pressure within the fresh, and still hot, board exceeds the internal bond strength of the board. It should be noted that the bond strengths at higher temperatures are always lower than after cooling the board. If blistering occurs using resins with low formaldehyde content, press time should be shortened instead of prolonged, because a longer press time would not increase the bond strength but certainly would increase the steam pressure in the board. Careful venting as well as decreasing the moisture content of the glued particles and reducing the press temperature will help.