Лако-красочные материалы — производство

The strength of a bond in a wood panel increases with the value of wood density for the range of approximately 0.7 to 0.8g/cm3. Above this density a decrease of the bond strength occurs. The performance and properties of wood-based panels are strongly influ­enced by the properties of the wood used. Thus, wood anisotropy as well as its hetero­geneous nature, the variability of its properties, and its hygroscopicity have to be taken into account in all bonding processes. Equally, the orientation of the wood fibers and the grain angle in bonding solid wood have to be considered.

Particles as raw material for particleboards show a great variety in wood species, origin, method of preparation, age, and especially size and shape. If wood is ground into particles, a mixture of particles of very different sizes and shapes is always obtained. Particles can be described in a simple way as squared flat pieces with certain values for length / (mm), width b (mm), thickness d (mm), and slenderness ratio s = //d. The volume of a particle is then given as

V = /bd (mm3)

Considering particles with / ^ d, the effective gluing surface area is

F = lib (mm[31])

The area form factor [384] can be considered as measure of the effective gluing surface area based on the volume. It is inversely proportional to the thickness of the particles:

F _ 2 _ 2s

V = d = 7

The influence of particle size and shape on mechanical and hygroscopic properties of boards is well described in several papers in the literature [385-390]. The central statement of these papers is an increase of bending strength, and compression and tension strength in the board plane, but a decrease of internal bond strength with increasing particle length. In particleboards the particles overlap, and thus the overlapping areas must be large enough to guarantee the transmission of the wood strength to the strength of the whole assembly.