Face 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. 

The warming up of the mat is performed by the so-called steam shock effect [173,219]. A prerequisite for this is the high permeability of the particle or fiber mat. Higher moisture contents of the face layers and spraying of water sustain this effect. 

This low pressure process, also known as elastic reservoir molding, consists of making basically a sandwich of plastic-impregnated open-celled flexible polyurethane foam between the face layers of fibrous reinforcements. When this plastic composite is placed in a mold and squeezed, the foam is compressed, forcing the plastic outward and into the reinforcement. The elastic foam exerts sufficient pressure to force the plastic-impregnated reinforcement into contact with the heated mold surface. Other plastics are used. 

Fig. 2.—The arrangement of ions in cube-face layers of alkali halide crystals with the sodium chloride structure.

The sm-face layer on lumps of sodium is removed with a large knife, the clean sodium is rapidly weighed out on a few large Alter papers and immediately trans. ferred to a beaker containing sodium-dried ether. The sodium may then be removed at leisure, cut into small pieces and transferred to the flask. 

We often describe the structure of this coating as a bi-layer, with the inner (oil-facing) part made up of water-repelling hydrocarbon chains, and the outer (water-facing) layer comprising the sul-phonic acid groups. 

The macropolycycle 43 possesses a crystallographic center of symmetry. Consequently, the two aromatic residues are parallel with an interplanar separation between them of 7.9 A. Examination of the unit cell provides an explanation for the high instability of the crystals. The macropolycyclic rings are arranged in parallel edge-to-face layers with 12 disordered chloroform molecules sandwiched... 

Figure 9-41. The arrangement of ions in cube-face layers of alkali halide crystals with the sodium chloride structure. Adaptation from Pauling [61], Copyright (1960) Cornell University. Used by permission of the publisher, Cornell University Press.

C (lll)-H (diamond) (lxl) Hydrogen terminated diamond, sur- MEIS/2/ face layer spacing relaxed by -0.05 0.05 A. 

The investigation of Dr Duffek collabs continued and, on the strength of their suggestions a process was developed at the Njetallgesell-schaft AG, Frankfurt a/Main, (Refs 1, 2 3), which permitted the deposition of thicker sui> face layers of phosphate crysts due to deeper penetration of the phosphate solns into sintered iron objects. The process (called in Ger "Tief-bonder-Verfahren ) may be conducted by one of three methods, but the following procedure was preferred by Dr Duffek ... 

In Western Europe 250 000 tons of rigid polyurethane foam were used for the production of sandwich panels in 1994. Owing to their outstanding mechanical and physical properties, these panels, produced in either a continuous or a discontinuous process and provided with flexible or rigid facing layers, have found a wide range of applications in the building industry (Esser, 1996). 

Using four types of wood furnish, i.e., Particle-a, -b, -cl, and -c2, particleboards were made in the laboratory. Eight types of board were made from these particles six types of homogeneous board and two types of three-layer board which had a 15 weight percent of each face layer and 70% of core layer. Furnish type and construction of PBs are shown in Table 2. Specifications for board manufacturing were as follows ... 

In this section, a new type of structural panel manufactured with a combination of these two wood resources is discussed. Waste wood chips were processed to particle and used as a core material. Small diameter logs were cut by a disk flaker to produce wafer or strand , which was used in Wafer-board or OSB production in North America. These strands were used as face layers of three-layer structural board in this experiment. Strength properties of the composite board made from Sugi-strand and recycled wood particle were evaluated. 

For the evaluation of the fundamental properties of composite board named Japanese OSB , no orientation was given in the face layers in this experiment. 

Fig. 6. Effect of face layer thickness on MOR of three-layer composite made from Sugistrand and recycled wood particle. Table 3. Furnish type and construction of composite board tested.

Board type Strand (Face) Face layer ratio (%) Particle (Core) Weight percent of each layer (%) Strand (Face) ... 

Figure 7 shows the relationship between the face layer ratio and the modulus of elasticity (MOE) in bending of the composite boards. It was obvious that the MOE increased with an increasing face layer ratio of the composite board, because the elastic modulus of the strand layer was higher than that of the core layer with recycled wood particle. If a simple combination of different elastic bodies was introduced, the MOE of three-layer board can be predicted easily by a cubic equation of face layer ratio. It was thought that the MOE of the composite board shown in Figure 7 was predictable from the MOE data of recycled wood particleboard and strand-board. 

From the results of the bending tests as shown in Figure 6 and Figure 7, it could be concluded that a preferable face layer ratio was 30% to 40%. In the case of board-3, which had 17% by weight of strand layer in both faces, MOR and MOE were 4.04 MPa and 34.8 GPa, respectively. 

Fig. 7. Effect of face layer thickness on MOE of composite board.

The bending test results showed that a preferable face layer thickness in each face was 15% to 20% of the total thickness. 

There are two major disadvantages often cited in the use of isocyanate wood binders. The first is a higher raw material cost as compared to conventional binders and the second is a tendency to adhere to metal transfer plates or press platens. The latter problem has been solved on a commercial scale by producing a multilayer product with solid veneers or phenolic-bound particles as face layers. All isocyanate-bound non-veneered board can be made by treating the metal surfaces with a release material or, more recently, using a self-releasable isocyanate. 

Binder Content Face layers - 5 solids on 0D wood wt. 

Due to the force applied, the fastener is shifted under shear between the two sheets, and the eccentricity of the force introduces a considerable momentum locally into the joint. The sheets undergo local bending. When the load is increased, the rivet holes are additionally deformed into an oval shape, especially in the thin aluminum face layer of a sandwich panel. The deflection of the joint even leads to small separations between the sheets in the overlapping region. 

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