Лако-красочные материалы — производство

During the hot press cycle the hardening of the resin and possible reactions of the adhesive with the wood substance take place. The influential parameters are especially the press temperature and the moisture content in the mat. Additional parameters are the wood density, porosity, swelling and shrinking behavior of the wood, structure at the surface, and wetting behavior. During the press cycle several processes take place:

transport of heat and moisture

densification, increasing internal stresses, followed by relaxation processes

adhesion between the particles or fibers

increase of the bond strength in the glue line (cohesion).

Models describing what occurs in a panel during hot pressing have been published [437-443]. These take into consideration various conditions occurring during the hot press cycle such as heat transfer, temperature gradients, moisture content, steam pressure, bond strengths, and presence or absence of postcuring [437-443].

Different particle structures: coarser in the core, finer in the face layer.

Press temperature:

As high as possible, to enable a quick heating up of the core layer

due to an optimal steam shock effect. In continuous lines press temperatures

decrease from the entrance to the outlet of the press. In the last zone of the press

even active cooling in a few cases is possible (decreasing steam pressure in the core layer).

Moisture content of the glued particles:

Core layer as dry as possible (ca. 6-7% in the case of UF resins), face layer as high as possible (11-14%, depending on the proportion of the face layer in the board). Too high a moisture content can cause blistering.

Spraying of water onto both surfaces in order to enhance the steam shock, amount ca. 20-40 g/m2.

Press pressure profile:

The variation of pressure during hot pressing can follow different sequences. Quick densification with pressure maximum to enable a high density of the face layer and hence high modulus of elasticity (MOE). Sometimes a second densification step is used.

Table 22 Press Strategy for Production of MDF

Despite the uniform fiber material, a certain density profile is created due to the action of heat and compression.

Two-step pressure profile with quick densification at the start of the hot press cycle and a second densification step for the inner layer.

Uniform moisture content of the glued and dried fibers across the thickness of the mat. Higher moisture content in the outer layer would require a three-layer mat or spraying of water.

Tables 21 and 22 summarize the usual press strategies for the production of particleboards and MDF. The warming up of the mat is performed by the so-called steam shock effect [442-447]. The precondition for this is the high permeability to steam and gases of the particle or fiber mat [442,443,448,449]. High moisture contents of the face layers and spraying of water on the surface layers sustain this effect. The press temperature influences the possible press time and by this the capacity of the production line. The minimum press time has to guarantee that the bond strength of the still hot board can withstand the internal steam pressure as well as the elastic springback in board thickness at press opening.

XIII. CONCLUSIONS

Wood is a very complex material. Wood adhesives technology is an advanced science which blends the technology of adhesive preparation and formulation with a multitude of advanced application technologies to different wood products. In many fields other than wood, good bonding depends mainly on the use of a good adhesive. The situation is not as straightforward in wood gluing: in general one can obtain excellent wood panels when using a decidedly poor adhesive if the parameters governing the technology of manufacture of the wood product are well mastered. This indicates the extent to which a high level application technology can play a predominant role in this field. This is not

valid for all wood products. Of course, good results are better or easier to obtain if one uses an excellent adhesive. However, just the use of a good adhesive gives no assurance of good bonding in this field. It is the essential interaction of the equally important adhesive and its application technology that this chapter has tried to describe. It is exactly this interaction that is so important in a field that comprises more than 60% by volume of all the adhesives used today in the world for any application. Without mastering this inter­action between adhesive technology and wood product manufacturing technology there cannot be wood bonding of any consequence.