Structure of Wood-Based Materials

The quality of the final product, which is a glued wooden board, to a large extent depends on the quality of the technological process. Making board furniture ele­ments from solid wood requires gluing slatted wooden panels or laths together. The choice of the dimensions of these elements directly affects the form of distortion or twisting of finished boards. The mechanism of the formation of these phenomena can be briefly presented as follows. As a result of the hygroscopy of the material of individual elements, the boards absorb or expel steam in order to strive to the state of hygroscopical balance. Moisture content changes are accompanied by the phe­nomenon of shrinking and swelling of particular component parts of the board, causing the formation of stresses inside the material and glue-lines connecting the wood. The differences in the construction of individual elements of the board constitute the direct cause of differentiating strains and stresses, which consequently leads to distortions, twisting, cracks inside and on the surface of the wood and cracks in glue-line (Fig. 4.12).

Fig. 4.12 Change of the dimensions and shape of the board element

The moisture content of the wood, from which strips or laths are made, is shaped at the stage of their drying. Manufacturers of glued wooden boards are aware of the dependencies that occur between the quality of drying and later changes in the shape of finished boards. There is, however, a lack of general understanding of the impact of the parameters of air in objects, where semi-finished products and boards are stored or finished products are used, on changes in the moisture content of the wood, and thus, the change in the stresses and strains occurring in them. Assuming that the change of the dimension of the wood in the function of moisture content, between 0 and 30 %, is a linear dependency, changes of the dimensions of linear cross sections of board elements can be easily tracked. In order to do this, the following equation has to be used:

Nk = O. OlNp (bw(o)DWw(o) + 10q) , (4.36)

where

Nk final measurement,

Np initial measurement,

DWw(o) moisture content increase and

ew(o) linear expansion of the wood calculated from the equation.

bw(o) = Pr cos2 a + br sin2 a, (4.37)

where

вг linear expansion of the wood in the tangential direction, eR linear expansion of the wood in the radial direction and a the angle between the tangent to the growth ring and the direction of extensibility (Fig. 4.13).

Figure 4.14 shows the effect of changes in the moisture content of the wood and the angle of inclination of growth rings on the change of thickness of individual elements forming the board. It should be noted that a change in the moisture content of the wood by 2 %, at the simultaneous change of the angle of inclination of growth rings from 20° to 90°, causes changes in the element by 0.1 mm. The variability of dimensions in the area of one element translates into the change of the dimensions and shape of the whole board element. In effect, for the worktop of a

table this results in the impression of an uneven surface, on which rays of light are reflected in an disorderly manner.

Carpentry boards are one of the oldest semi-finished products used for board elements of furniture. They are built from the middle layer with single or double layers of veneers, plywood or sheets of hard fibreboards glued from both sides. The middle layer of the board may contain elements that are not connected or glued together. These elements can be laths, strips, veneer, cardboard or paper reinforced with synthetic resins (Onisko 1994) (Figs. 4.15 and 4.16). Depending on the construction of the middle layer, carpentry boards can be divided into:

• solid (Fig. 4.15) and

• honeycomb (Fig. 4.16).

The frame of the honeycomb frame board can be made of wood, chipboard, fibreboard and possibly of another wood-based material. However, it cannot show defects, which would have a negative effect on the strength of the material or cause strains. The elements of the frame are connected using glue or clasps. A frameless honeycomb consists of only the middle layer and lining (Fig. 4.17). In the process of the production of this board, the glue is to fix the middle layer with outer layers,

Fig. 4.15 Types of carpentry boards of full middle layers: a lath middle, b cut lath middle, c strip middle and d middle from strips of veneer (Onisko 1994)

Fig. 4.16 The construction of carpentry honeycomb (frame) boards: a with the middle made of hard fibreboard, b with the middle made of cardboard, c with the middle made of veneer and d with the middle made of strips (Szczuka and Zurowski 1995)

which is why the glue-line created should exhibit great resistance to stretching and bending (Zawierta 2007).

An important benefit resulting from the use of full carpentry boards and frameless honeycomb, in contrast to frame honeycomb, is the possibility of freely shaping the dimensions of board elements of furniture, including curved elements.

Chipboard is produced in the process of pressing piles of glued wood shavings in conditions of high temperature and pressure. Usually, they are produced as multi-layer boards. The inner layer of the board is made of chips of a thicker

Fig. 4.18 Structure of a chipboard

fraction, while the external layers are made of very fine and thin chips, so-called microchips (Fig. 4.18). Thanks to this, the surface of the board is characterised by low roughness and high adhesion strength. The chipboard is a direct raw material for the production of furniture units. In subsequent treatment processes, their wide and narrow planes are subjected to veneering with natural or synthetic veneers. This is a typical technological process used in the manufacture of house furniture. For public institutions, and in particular offices, banks, post offices, employee furniture is made of laminated boards. Laminated boards are a product made on the basis of a three-layer chipboard or MDF board covered unilaterally or bilaterally with papers saturated in thermosetting resins. In the process of lamination, the decorative film is pressed onto the board, while giving its surface the desired structure, which does not require further treatment. Chipboards in every form enable the design and production of rail, board and panel elements of furniture, giving them any shape in one, and sometimes even two planes.

Fibreboards are wood-based products made as the effect of pressing wood fibres with an organic addition of adhesive and hardening compounds in conditions of high pressure and temperature. They are a material of similar density and com­position in the entire cross section, thanks to which they have an excellent work­ability in the process of cutting. Fibreboards produced nowadays can be divided into the following:

• Medium density fibreboard (MDF)—this is a medium density board of wood fibres. It constitutes the primary raw material for the production of furniture elements, including ornamental strips, worktops, grilles, panels and door frames and milled drawer fronts. It is the perfect substitution for wood in covering or dark finishes, and it goes well with finishes of transparent dark natural veneers. Due to the qualities of the surface, they are suitable for upgrading by covering
with thin melamine films, painting and veneering with natural and synthetic veneer.

• Low density fibreboard (LDF)—a board, which is characterised by a smaller, in relation to MDF, density and is not usually used in furniture. However, this board is a base material for the production of wall panels used in dry areas.

• High density fibreboard (HDF)—a board of great hardness and increased den­sity. HDFs are intended mainly for the production of floor panels. They are also a base material for coating with HPL and CPL laminates, resin papers and natural veneers. Lacquered HDFs are ideal for rear walls and bottoms of drawers of case furniture.

Like chipboards, fibreboards enable the design and production of rail, board and panel elements in furniture of shapes contained in one or two planes. Unlike chipboards, fibreboards are suitable for milling wide and narrow surfaces. Thanks to this, in certain applications they are the perfect substitute for wood both in aesthetics and construction terms.

Plywood is a wood-based material built of layers as a result of bonding, usually at an angle of 90°, most commonly with an odd number of veneers (Fig. 4.19). Glues based on synthetic resins are used for bonding, that is, urea, melamine, phenolic and resorcinol. Veneers for plywood are mainly made of the wood of birch, alder, beech, pine and spruce. Usually, a sheet of plywood is made of different species of wood, e. g. pine/alder and birth/pine. The type of wood used to build the outer layers determines the qualification of the plywood as a coniferous or deciduous type. The inner layers can be made of wood of the same or other type than the outer layer. Depending on the adhesive mass used, distinguished are moisture resistant plywood, waterproof plywood and so-called waterproof plywood with a clear glue-line (Table 4.4).

In furniture, plywood is used for:

• door panels, rear walls, bottoms of drawers and slides of case furniture;

• skeletons and casings of frames, containers and strips of upholstery frames of upholstered furniture; and

• seats, backrest boards, rails and worktops of skeletal furniture.

Glue-line type

PN-EN

636:2013-03

DIN

68705-2:2003-10

BS1

203:2001

PN-83/D 97005.11 BN-73/7113-06

Moisture resistant: urea-

For use in

IF 20,

INT

Moisture resistant

formaldehyde resin-based adhesive

dry

conditions

BFU 20

MR

Waterproof: with clear glue-line: urea- melamine-formaldehyde resin-based adhesive

For use in

humid

conditions

A 100

BR

Semi-waterproof

Waterproof: phenol – formaldehyde resin-based adhesive

For use in

exterior

conditions

BFU100 AW 100

WBP

Waterproof