Core layer

Lay-up proceeds by laying down the veneer which is to be the back surface of the panel. Then a sufficient number of pieces of core veneer are passed through the glue spreader to form the next layer of cross-oriented veneer. The glue spreader commonly used in hardwood plywood manufacture is a roU coater in which a pair of opposing mbber roUs are coated with a thin layer of adhesive. As the veneer is passed between the roUs, the adhesive is transferred to the surfaces of the veneer. Adhesive is appHed only to the cross-pfles and in sufficient quantity to provide a continuous layer on both opposing faces of veneer. Thus, in the case of a three-ply panel, only the core layer is spread with adhesive and in that of a five-ply panel, the second and fourth layers both of which are cross-pfles, are spread with adhesive. Then the top surface veneer, which is normally the decorative surface, is placed on the assembly. 

The process can be used to recover scrap or low quaUty resins by using them as the core layer, and using outer layers of virgin resins designed for the specific functional needs of the product such as sHp or gloss and appearance. The inner core may be a foamed resin with surface layers of supedor finish resins. Coextmded films often eliminate the need for cosdy lamination processes. 

Optimization of the pressing process, e.g. by increasing the effect of the steam shock by (1) increased press temperatures, (2) additional steam injection, or (3) an increased gradient in the moisture content difference between surface and core layer. 

Because of the reasons described above, the core layer and face layers are glued separately, that is, the core layer contains rather coarse particles, but the face layers contains rather fine particles. However, the two distributions might overlap to some extent. This separate gluing enables one to use different compositions of the glue resin mixes (e.g. different addition of water and hardener) and different gluing factors for the individual layers. 

The moisture content of the glued particles is the sum of the wood moisture content and the water which 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 6.5-8.5% in the core layer and 10-13% in the face layer for UF, and 11-14% in the core layer and 14-18% in the face layer for PF. 

The higher the moisture content of the particles, the easier can the face layer be densified at the start of the press cycle this leads to a lower density in the core layer. 

Fig. 8. Temperature increase as a function of pressing time in the core layer of an industrial OSB of 2440 X 1220 x 12 mm dimensions. The highest temperature is reached in the region of the panel comprised between the panel center and 500 mm from the board edge. The fastest initial increase in temperature but lowest max. temperature is for the region nearest to the board edge (100 mm from the board edge).

Phase C corresponds to a decreasing rate of temperature rise in the board core at about 100°C. It is followed by stabilization of the temperature, namely phase D. This latter being due to decrease of the steam gradient due to moisture escape from the board edges, and the increase and decrease of the wood wetting heat, respectively, near the board surface and in the core layer. Phase E, a new slow increase in temperature is observable in standard moisture content boards [225] but is absent at high moisture content (due to the maintenance of some moisture in the core layer), contrary to the case of traditional moisture content adhesives, as shown in Fig. 8. 

Fig. 9. Effect of press closing rate variation of counterpressure and temperature increase as a function of pressing time in the core layer of an OSB panel at a press closing lime of 10 s.

The press closing time also influences the relative densifications of the surface and core layers of the wood mat during pressing (Figs. 9 and 10). Fig. 11 details the density profile of the particleboard panels prepared at short and longer press closing times [226]. The two cases differ in several aspects. (1) A short... 

The temperature of pressing has also a noticeable effect [226,227] as it does influence the surface/core temperature gradient and has a direct influence on the temperature rise in the board core layer. In short, the higher the press temperature, the faster the heat conduction and the faster the development of the steam gradient across the wood mat. The press temperature will influence the steam front transfer time to the core layer. The higher the initial temperature, the faster the steam front enters the mat core. Increasing the press temperature will cause the maximum steam pressure peak to appear earlier but does not result in a higher core temperature. 

Overall board density will strongly affect core layer plasticization and density profile (Fig. 12), as at the highest overall board density a steep density gradient appears between the surface and core layers of the board. This is due to the greater difficulty encountered by the steam to penetrate and plasticize it. At lower density, the greater mat permeability enables a faster steam throughflow of the board, comparable to a steam injection process. The final result is similar as the overall board density is closer in value to both core and surface densities. 

The initial moisture content is a determinant factor in the rate of heat transfer to the center of the core mat [226,227]. At short press closing times the rapid temperature rise occurs at the same time for both lower and higher moisture content percentages indicating that the steam condensation front reaches the core at the same rate and that this is then determined more by local permeability rather than local moisture content. The slope of the rise is similar as it is the balance of horizontal and vertical permeability which controls the rate of steam flow to the core layer. Furthermore, water remains in the surface layer in a quantity such as... 

Figure 12 shows the dependence of the average aspect ratio and the TLCP volume fraction on the relative sample thickness for the four processing conditions in the core layer, transition layer and skin layer, respectively, by a morphological examination [13]. Generally, the aspect ratio increases from core to skin layer, whereas the situation is reversed for the volume fraction. An average volume fraction about 20% can be clearly seen. 

In all cases of the processing conditions, TLCP domains were well dispersed and deformed to droplets in the core layer, but there was only a narrow distribution of their aspect ratio (about Hd 6) and less orientation. In both transition and skin layers, the domains were also well dispersed, but more oriented and fibrillated in the flow direction. From this reason, we give the distribution of aspect ratio Ud) and fiber number (N) versus fiber length class in Fig. 22, only for skin and transition layers, respectively. 

This constant value can be taken in turn into the composite functions to calculate the composite properties. A calculation result is illustrated in Fig. 25. For the four sample groups, the calculated layer moduli , are uneven in the cross-section within a composite sample group. The lowest value is still located in the core layer due to the lower deformation and, therefore, the lower... 

Note that part of the field pattern is propagating (along the z-direction) outside the core layer and that its magnitude is decaying exponentially with the distance to the corresponding plane boundary. These fields outside the... 

Fig. 10.3 Total internal reflection in a three layer waveguide structure. n, (i C, F, S) indicates the refractive index of layer i of the waveguide structure, % > nc s dF is the thickness of the core layer...

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