Certain Wood-Based Composite Materials
1. Introduction
Although various definitions are possible, products obtained by combining wood (fibre, chip or particle) with other materials such as plastics, synthetic fibres, adhesives, fillers, functional compounds, etc. under temperature and pressure are briefly known as "Wood Composite Material".
In this respect, wood is held within a matrix structure in wood composite materials.
Over time, the decrease in forest resources globally and rising timber prices, along with the development of the chemical and adhesive industries, have led to the advancement of wood composite materials.
These materials have become widely used as engineered panel products made of solid wood material with its defects eliminated, more homogeneous, more durable and meeting consumer demands.
Because wood has many advantages over other materials (cost, ease of processing, lightness, etc.), demand for wood-based composite materials is expected to increase in the future.
2. Classification of Wood Composite Material
Wood composite materials can be classified in general according to the type of raw material used and the production methods applied, as shown in Figure 1.
Figure 1. Wood Composite Materials (Youngquist, 1988)
Another classification method, as shown in Figure 2, can be made according to density, raw material type and production method.
Figure 2. Wood composite material classes (Suchsland and Woodson, 1987)
More detailed information on wood-based composite materials classified briefly in Figures 1 and 2 can be obtained from other sources (Suchsland and Woodson, 1987; Youngquist, 1988).
Below, Table 1 provides some standards and properties related to wood-based composite materials in our country.
Table 1. Standards used in wood composite materials (intweb.tse.org.tr)
2.1. Chipboard Composites
Wood chipboards; according to TS EN 309 (1999) standard are defined as "panels obtained by hot pressing chips obtained from wood particles (wood particles, chips, sawdust, planer shavings, etc.) and/or lignocellulosic materials (flax, hemp fibre, jute fibre, drained sugar cane bagasse, etc. from lignified plants) after gluing". The classification of chipboards according to TS EN 309 is specified in Table 2.
Although different types of chipboards are produced depending on the size of the particles used in chipboard production and the existence of additional production techniques, the basic operations are the same in all production methods used (Güller, 2001). Figure 3 shows the typical production stages for chipboard.
• Chip geometry: normal chipboards (Particle board, PB),
• Wafer boards (Wafer board, WP),
• Flake boards (Flake board, FB),
• Oriented strand boards (Oriented structural board, OSB).
In addition to almost all types of wood material being usable in chipboard production, the use of waste from primary forest product industries (sawdust, waste and scrap wood, thin branches and stem wood, etc.) has enabled this product to be widely produced worldwide.
Furthermore, chipboards have the necessary physical and mechanical properties for many uses, have smooth surfaces, can be produced in desired thicknesses, have a homogeneous structure, and different properties can be imparted as a result of treatment with fire-retardant and water-repellent materials.
Chipboards do not have growth defects found in natural wood material such as fibre waviness, knots, rot, etc., making them both more economical and more practical in many fields.
Chipboards are classified in many ways. Below, Table 2 provides an example of the classification of chipboards according to TS-EN 309.
Table 2. Classification of Chipboards According to TS-EN 309
2.2. Fibreboard Composites
Fibreboards; according to TS EN 622-1 (2005) standard are defined as "material obtained by applying heat and/or pressure to a sheet of fibres obtained from wood or other lignocellulosic materials, utilizing their natural adhesion and felting properties, or with the addition of adhesive substances and in some cases other additives".
The use of fibreboard dates back to the 6th century BC in historical process, and a type of fibreboard in heavy paper form was used in the walls of small houses in Japan. In 1850 in England, boards obtained by gluing multiple layers of cardboard were used in the construction sector.
Particularly from the 1980s onwards, the most important factor driving the rapid increase in medium-density fibreboard (MDF) production in the world can be attributed to the broader range of raw material requirements compared to chipboard, greater use due to being able to be processed like solid wood material, and satisfactory physical and mechanical properties (Erinç, 2002).
Fibreboards can be classified according to criteria such as production processes, thickness, density, special properties, conditions of use or application purposes. The classification of fibreboards according to TS 3635 EN 316 (2005) is shown in Table 3.
There are two main production technologies: wet and dry methods. In the wet method, the sheet is formed in an aqueous environment and lignin is used as a binder. In the dry method, the fibres obtained are dried, the sheet is formed in a dry environment and synthetic resins are used as a binder.
Dry process fibreboard production is similar to chipboard production (Maloney, 1993).
The most important property providing superiority in MDF board application is its homogeneous structure. This structure is achieved through fibre fineness and pressing technology. As the specific gravity of the boards increases, their mechanical strength also increases (Youngquist, 1988).
These superior technological properties increase the usage volume of MDF. The increase in demand for MDF leads to raw material shortages and therefore the exploration of alternative raw material sources becomes important. Table 3 shows the classification of fibreboards according to TS 3635.
Table 3. Classification of Fibreboards According to TS 3635
2.3. Plywood
Plywood; according to TS EN 313-27 standard is defined as "panel materials produced by peeling/cutting wood material with a certain diameter and length and smooth grain, gluing thin sheet layers with their fibre directions perpendicular to each other and hot pressing at high temperature".
Since the fibre directions in each thin layer forming the structure are perpendicular to each other, plywood contains 3 or more odd-numbered single layers.
Because plywood has a layered structure, a single wood species can be used for inner and outer layers depending on the intended use and raw material characteristics, and wood with different patterns and aesthetic properties can also be used, particularly on outer layers (Çolakoğlu, 2005).
Furthermore, because the defects and aesthetic appearance of the panels forming the surface layers are used in the classification of plywood, the use of panels with high aesthetic value and no defects that reduce quality on outer layers improves quality.
The classification of plywood according to TSE 3103 EN 313-1 is shown in Table 4.
Table 4. Plywood Classes According to TSE 3103 EN 313-1 (1998)
The wood material to be used in the core layer need not have excessive colour, pattern or aesthetic properties, and evenly grown wood material of a certain diameter suitable for peeling is sufficient for use.
Generally, plywood has two main areas of use: decorative and structural purposes. Those produced from softwoods, having sufficient strength and physical properties, are more preferred for structural purposes in building and construction work.
Those produced from hardwoods with higher decorative properties are used in places such as furniture where aesthetic properties are paramount.
Despite recent developments in plywood production and the production of products with different properties, prices are higher than for oriented strand board (OSB), for example, for the same uses, due to their production from thick-diameter, evenly grown and quality wood material suitable for peeling, with intensive labour and costs (Maloney 1996).
For this reason, in North America and European countries, structural and industrial plywood has begun to be replaced by OSB, which has lower production costs and easier production.
2.4. Laminated Solid Wood Materials
Because wood material defects such as knots, checks, fibre waviness, rot, etc., cause problems in single-piece form, particularly over long spans, the use of large-dimension laminated solid wood material created by joining sawn timber free of such defects has become widespread, particularly in the construction sector as a new and technological material.
After sawn timber is sorted to remove obvious defects and combined together, structural construction lumber can be grouped in different types. Some laminated solid wood materials are briefly given below:
PSL Parallam (Parallel Strand Lumber): Lumber obtained by compressing peeling veneers of certain dimensions, free from defects.
LSL (Laminated Strand Lumber): A type of wood-based material resistant to outdoor conditions, obtained by pressing long wood strips with water-resistant adhesives (e.g. diisocyanate) (Biron and Koch, 2014).
OSL (Oriented Strand Lumber): While similar in structure to LSL, it has lower density. OSL blocks are produced using flake strands, from which lumber of desired dimensions is cut from the material for use.
LVL Microlam (Laminated Veneer Lumber): Used mainly as beams in roof construction and in door and window frames and as flooring material in truck, ship and vessel decking. Its production technology is similar to plywood but has a much higher adhesive ratio (Stark et al., 2010).
2.5. Inorganic Binder Wood Composites
The purpose of combining a wood-based material and other raw materials is primarily to have properties that minimize wood defects, maintain recycling properties, not contain chemicals harmful to the environment such as formaldehyde and asbestos in their structure, and have lower total costs (Youngquist, 1995).
Gypsum Binder Wood Composite: Gypsum composites are sensitive to outdoor conditions and particularly to moisture; therefore, their use indoors is widespread. Like other inorganic binder composites, their fire, heat and sound insulation are above recommended standard values (Maloney, 1996).
Portland Cement Binder Wood Composite: Cement-wood composites show high strength and dimensional stability against outdoor conditions or rapid aging. These materials have high fire, sound and heat insulation as well as high resistance to biological factors. Although heavier than resin-based boards, they are lighter than concrete. For this reason, they are particularly preferred in the prefabricated building sector in non-load-bearing sections (Wolfe and Gjinolli, 1997).
Magnesium Cement Binder Wood Composite: Magnesium-based composites, although not as much as gypsum composites, have weak characteristics against moisture and should be used particularly indoors. The most important factor in the durability of magnesium cement boards is that they are least affected by sugars found in woody structures (Güller, 2001).
3. Conclusion and Recommendations
Wood has been one of the materials mankind has intensively utilized since earliest times. The main reason for this is the advantages found in wood that are not present in many other materials.
However, as population increased over time and demand for wood materials rose, forest resources decreased, resulting in wood material becoming more valuable.
In this situation, wood-derived composite materials were developed as an alternative to wood material, and through engineering design, many different wood materials were produced for desired uses. High-value-added products were produced from wood material and made available to people's use.
Today, wood composite materials, by making changes in the physical and chemical structure of raw materials, using effective reinforcing elements and enriching their structure in terms of sound, heat and fire insulation, have a wide range of applications in the furniture industry and in the construction sector for indoor and outdoor spaces.
Assoc. Prof. Ali İhsan Kaya / Burdur Technical Sciences Vocational School / Mehmet Akif Ersoy University
