Development history
The prototype products of wood-based panels can be traced back to ancient times. In ancient Egypt in 3000 BC, veneers were made of selected precious wood and bonded with materials such as metal flakes or ivory to make high-grade furniture for kings and royalty. Some countries in Europe and America have produced a large number of plywood in the middle of 19 century, and a large-scale fiberboard industry appeared in the 1920s. Since then, various kinds of wood-based panels have been developed all over the world. Because wood-based panels can use small wood, inferior wood and waste wood as raw materials, their properties are separated and combined through processing technology, and some auxiliary raw materials are added, so as to remove the natural defects of wood, improve the original properties of wood, and make its application range much larger than that of solid wood; Wood is made into wood-based panels through industrial processing, which has obvious economic benefits, so the wood-based panel industry has developed very rapidly in the world (table 1). Since 1970s, the world's output of wood-based panels has remained at around 654.38 billion cubic meters, which has become an indispensable material for human life. At the same time, the world sawn timber output has been maintained at around 400 million cubic meters. This reflects the shortage of wood resources in the world, which limits the substantial increase in the output of wood products.
Table 1 plywood output has been fluctuating with market demand for many years, showing a stagnant trend, mainly due to the increasing shortage of high-quality wood resources. High-priced and high-quality wood is mostly used to make decorative materials. The decline in fiberboard output is mainly due to the lack of competitiveness with particleboard, foam plastic or mineral wool.
Since 1970s, new varieties of wood-based panels with certain characteristics have emerged one after another. Non-veneer wood-based panels such as particleboard, waffle particleboard, oriented particleboard and medium density fiberboard gradually replace plywood, blockboard and wood board; Gypsum board, cement particleboard and other mineral boards are used in buildings to replace traditional concrete boards, asbestos boards and other mineral boards; Veneer laminated timber, composite board, finger joint timber and other parts replace sawn timber as building materials such as keel and light truss.
The wood-based panel industry in China first appeared in 1920, but until 1949, there were few small plywood factories in China. After 1949, the wood-based panel industry developed rapidly, and 1960 began to take shape. Since the end of 1970s, the output has increased rapidly (Table 2), and the "three boards" all increased at the same time. This is because China's wood-based panel industry is in its infancy, and the wood supply is far from meeting the needs of rapid social growth. No matter what kind of wood-based panel is in short supply.
Table 2 Advantages of Wood-based Panel
There are mainly the following points:
Improve wood properties
As a material, wood has its inherent shortcomings, mainly as follows: ① The performance difference in three directions is too great. The mechanical properties in the length direction are much better than those in the chord direction and the radial direction. For example, the elastic modulus in the longitudinal direction is 8 ~ 10 times that in the chord direction and 17 ~ 70 times that in the radial direction. The wet expansion rate and dry shrinkage rate in the three directions are also very different. When the wood changes from wet to dry, the longitudinal shrinkage rate is only 1/30 ~ 1/70 in the chord direction and115 ~/40 in the radial direction. Therefore, wood is easily warped, deformed and cracked due to the change of air humidity in use. Figure 1 shows the deformation of boards taken from different parts of wood after drying shrinkage. (2) The knots, irregular textures, biological variation and inevitable defects of wood, such as insect eyes and decay, greatly limit its utilization. The starting point of processing wood into wood-based panels is to improve the properties of wood without losing its original advantages.
The plywood in figure 1 consists of more than three odd-numbered veneers with mutually perpendicular grain (figure 2a), which largely balances the performance differences of wood in longitudinal direction, radial direction and chord direction. But also removes various natural defects, and the board surface is flat, the width is enlarged, and the use is convenient. Especially in the aspect of mechanical properties, the shear strength has been greatly improved, which makes plywood have unique and excellent properties as the skin material of components. 9.5 mm thick plywood can replace 25.4 mm thick wooden board when making exterior skin. Because plywood is not easy to crack and has strong nail grip, 6.4 mm thick plywood can be used instead of 19 mm thick wooden board when making packing boxes. When making wooden floors, in view of the impact force often needed in use, slats have to be used for splicing; Because of its strong impact resistance, the width of plywood can be extended to 1220 mm, which brings great convenience to construction. When wood is used in wood structure, due to natural defects, biological variation and various problems caused by long-term load, the safety factor used in structural design is much greater than plywood.
Fig. 2 The wood board will be deformed and warped after drying and shrinking (fig. 1, figs. 3a and b). If the board is sawed and spliced, its warping can be significantly improved (Figure 3c). Therefore, the dimensional stability of blockboard (Figure 2b) is better than that of solid wood. Fiberboard and particleboard are made of smaller "unit" materials, and the warping deformation is reduced accordingly (Figure 3d).
Fig. 3 According to requirements, wood-based panels can be endowed with anti-corrosion, fire prevention, sound insulation, heat insulation, reinforcement and other properties by different processing methods or adding auxiliary materials. Wood-based panels are also easy to reprocess various surface decorations, so that they have various decorative effects.
With the rapid development of industry, especially since the early 1950s, the price of wood has been rising. Before 1970s, its rising speed far exceeded other raw materials. On the other hand, there is an increasing demand for wood products. Under the contradiction between the shortage of wood resources and the increase of demand, the development of wood-based panel industry has been strongly stimulated. According to calculation, using 2.5 cubic meters of logs to produce 1 cubic meter of plywood can replace the board produced by 4.3 cubic meters of logs; Fiberboard and particleboard can support 1 ~ 5 times of logs or sawn timber in use value. In addition, through scientific and reasonable utilization, the performance is improved and the service life is prolonged. Therefore, the development of wood-based panel industry can achieve the effect of doubling the yield of wood.
Fiberboard and particleboard can also use lignocellulose plants such as waste materials, small materials, inferior materials and processing residues as raw materials. With the development of technology and industry, the range of available raw materials is expanding.
It is convenient for the efficient and large-scale production of wood products and meets the social consumption demand. Because wood-based panel production is more convenient for mechanization and automation, the productivity is much higher than that of wood-based panel production. Moreover, the surface decoration processing of wood-based panels and the final wood products processing process are also convenient for mechanized and automatic production, such as the production of panel furniture and assembled houses. The production and consumption of wood-based panels are closely related to social economic development and consumption level (Table 3).
Table 3