Possibility of production of binderless boards using wood powder by rolling

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1 Possibility of production of binderless boards using wood powder by rolling T. Miki 1, N. Takakura 1, T. Iizuka 1, K. Yamaguchi 1 & K. Kanayama 2 1 Kyoto Institute of Technology, Japan 2 National Institute of Advanced Industrial Science and Technology, Japan Abstract Recently, it has become more and more important to minimize environmental pollution. As an attempt to alleviate the environmental problems in materials, new wood based materials without using any adhesives and binders have been investigated. From this point of view, wood powder can be useful because it can be solidified and bonded under optimum pressure and temperature. This is because the component of wood, especially lignin and hemicelluloses, becomes soft and fills among components during heating and compacting. In addition, the powder form causes effective bonding of each particle. From the viewpoint of powder forming, Near Net Shape forming actually can be performed using plastics and metal powder. Therefore, the development of this technology enables the production of binderless wood products with arbitrary shapes and this leads to effective use of waste woods such as construction scraps. In this study, as fundamental tests for rolling, hot-pressings of wood powder of cedar were carried out at various forming conditions, temperature and compaction speed in order to confirm the possibility of solidification without using petroleum adhesive, and also to produce useable binderless board. Effects of the forming conditions on the mechanical properties, bending strength and bulk density of the hot pressed board are discussed. The experimental results show that the mechanical properties increase with increasing temperature during the hot press, and then, the maximum bending strength is about 30 MPa. However, when the temperature is too high, the mechanical properties tend to decrease due to burning. Finally, general rolling was conducted based on hot press results to produce boards continuously. However, it was necessary to push the powder between rolls in order to obtain a sound board. Key words: wood powder, binderless, hot press, rolling, compaction, temperature, moisture content, bending strength, bulk density

2 412 High Performance Structures and Materials II 1 Introduction Wood based materials are good resources for the environment, because they are finally resolved into carbon dioxide and water in the disposal stage. Problems, however, arise regarding productivity and workability: it takes long years for a tree to grow to a useable size, and the plastic forming of wood materials are more difficult than that of metal and plastic materials. To overcome these problems, the compaction or solidification techniques of crushed wood chips were developed, which enabled the production of new wood based materials such as plywood, fiber-board and particle-board. However, since an adhesive or bond made from petroleum is used in the production process of these materials, an environmental pollution problem remains when they are disposed of or incinerated. In an attempt to achieve the compaction of wood powders without any adhesive and binder, the effect of compaction conditions on the mechanical properties of product such as strength and density have been experimentally investigated [1] [2] [3]. As the results of the compaction of wood powders, optimum conditions exist where the density and bending strength of wood powder compacted product become max. And also, the maximum bending strength of product is higher than that of natural wood. Therefore, it was confirmed that a binderless compacted product can be obtained using only wood powder. However, large compacted products are difficult to produce since a huge compaction pressure is needed. Powder rolling is a method to produce long metal sheet from metal powder continually. In this process, the compaction pressure exists only under the roll. Thus, it is possible to obtain a long sheet with small pressure. In this study, the fundamental hot press tests for rolling were conducted to apply the powder rolling to wood powder. Effects of hot press conditions on the mechanical properties of product are experimentally investigated. The possibility and problems in producing a wood board by rolling of wood powder are also discussed. 2 Material and experimental methods 2.1. Wood powder Cedar wood powder was used in the experiment. The powder was prepared by crushing wood chips and screened (φ300 µm). The powder particle size distribution and powder morphology are shown in Table 1 and Fig. 1 respectively. The moisture content of wood powder 12% was adjusted based on the oven-dried weight Hot press process Fig. 2 shows hot press process schematically. The hot press mould consists of an upper square punch and a die. The punch and die can be heated by stick heaters. Wood powder is filled in the die. Lubricants are not used at all throughout the

3 High Performance Structures and Materials II 413 experiment. The wood powder is compressed by the punch up to a target stroke. In this stage, a pre-compacted product which has initial bulk density 0.40 g/cm 3 is obtained. The die and punch then are heated at a target temperature T, while the pre-compacted product is kept removing. The pre-compacted product is compressed at the punch speed V until it becomes target thickness 1.15 mm. An unloading process is carried out in same punch speed V without keeping the thickness. The final product is removed after unloading Rolling process As an attempt to produce a wood board continuously, rolling tests were performed. Fig. 3 illustrates rolling device schematically. In general, there are two types of rolling. One is a vertical rolling, the other is a horizontal rolling. In the experiment, it was difficult to fill the wood powder between the roll directly when the vertical rolling was applied. While, in a case of the horizontal rolling, Table 1 Powder size distribution. Powder particle size (μ m) Less than 75 Weight % μ m Fig. 1 Woodpowder. 30 Wood powder (a) Powder filling Temperature T (b) Compaction Pre-compacted product ρ = 0.40 g/ cm 3 Mechanical tests portion Hot pressed product (c) Heating mould (d) Hot pressing (e) Removal 1.15 mm Fig. 2 Hot press process.

4 414 High Performance Structures and Materials II the wood powder could fill directly through a hopper. The rolling process is conducted as Fig.4. The wood powder is filled in the hopper. In this stage, the powder in the hopper can be compacted by a punch. The powder then is compacted by heated rolls. A wood board will be obtained after the wood powder passes between the rolls. The rolling conditions such as a roll temperature were based on the hot press results Measurements of mechanical property of hot pressed product Bulk density ρ, bending strength σ were measured as mechanical properties of the hot pressed products. These tests were applied in central portion (about 25 x 25 mm, Fig. 2) of product. These values were calculated via the equations below. G t (2) 3FL max (3) 2 2wt Roll Heating tools Stick heater φ mm 1.0 mm Hopper (a) Vertical rolling Fig. 3 Rolling devices. (b) Horizontal rolling Punch ( a ) F i l l ing p o w d e r i n t he h o p p e r (b) Compaction and rolling Fig. 4: Rolling process. Woo d b o a r d

High Performance Structures and Materials II 415 Where, w, t and G are width, height and weight of the product,

In the hot press, the final product thickness was not same as the target due to an expansion.

equation. t 1.15 E (4) 1.15 3 Effects of hot press conditions on the mechanical properties of products Fig.

It is found that a surface color of the product becomes black when a mould temperature increases.

This is because the wood powder is compacted and solidified successfully as the mould temperature increases and there

And also, the black central portion expands with decreasing the punch speed.

This leads to the fact that the thermal conductivity increases when the bulk density of wood material increases.

5 High Performance Structures and Materials II 415 Where, w, t and G are width, height and weight of the product, respectively, and F and L are maximum bending load and the distance between supporting point in the bending test. In the hot press, the final product thickness was not same as the target due to an expansion. Therefore this difference in thickness expresses as an expansion rate of board thickness E and calculated from this equation. t 1.15 E (4) Effects of hot press conditions on the mechanical properties of products Fig. 5 shows examples of hot pressed wood powder products. It is found that a surface color of the product becomes black when a mould temperature increases. Especially a central portion is blacker. This is because the wood powder is compacted and solidified successfully as the mould temperature increases and there is larger friction between powders on the central portion during compaction. And also, the black central portion expands with decreasing the punch speed. This is related to a thermal conductivity of wood powder. This leads to the fact that the thermal conductivity increases when the bulk density of wood material increases. The longer time wood powder is in the mould, the more heat is transmitted to an inside of pre-compacted product. Fig. 6 illustrates the effects of mould temperature and punch speed on the expansion rate of board thickness of hot pressed product. The expansion rate T = 260? T =280? T =300? E f fect of mould temperature T 10 mm T = 2 8 0? 1 0 m m V = m m /s V = mm/s V= mm/s V= mm /s Effect of punch speed V F i g. 5 E x amples of hot pressed product.

6 416 High Performance Structures and Materials II tends to decrease with increasing mould temperature regardless of the punch speed. However the wood powder begins to burn at the temperature of more than 290 C. When the pre-compacted product is compressed at lower speed, the expansion rate becomes smaller. Fig. 7 shows the effects of mould temperature and punch speed on the bulk density of hot pressed product. The bulk density tends to increase as temperature increases except of the punch speed V of mm/s. In a case of lower punch speed, a hot pressed product having high bulk density can be obtained until the pre-compacted product begins to burn. Fig. 8 illustrates the effects of mould temperature and punch speed on the bending strength of hot pressed product. It is seen that the tendencies are similar to those of the bulk density for temperature. However a temperature exists in which the bending strength becomes max to the punch speed. For example, when the pre-compacted product is compressed at the punch speed V of mm/s, the bending strength of hot pressed product obtained from it reaches maximum at mould temperature T of 280 C. In cases of punch speed V of and mm/s, hot pressed products start to burn before they reach their maximum. 180 Expansion rate of board thickness (%) Punch speed (mm/s) Burning Mould temperature ( ) Fig.6 Effects of mould temperature and punch speed on expansion rate of board thickness.

7 High Performance Structures and Materials II Bulk density (g/cm 3 ) Punch speed (mm/s) Burning Mould temperature ( ) Fig.7 Effects of mould temperature and punch speed on bulk density of hot pressed product. 30 Burning Bending strength (MPa) Punch speed (mm/s) Mould temperature ( ) Fig.8 Effects of mould temperature and punch speed on bending strength of hot pressed product.

8 418 High Performance Structures and Materials II 10 mm Fig. 9 Examples of wood board made by rolling. 4 Possibility of rolling of wood powder As an attempt to produce a wood board, the rolling was carried out using a vertical rolling device. However it was impossible to obtain a wood board without using a punch. In this case, the wood powder cannot be supplied continuously between the rolls. On the other hand, a wood board can be obtained using punch as shown in Fig.9. This board was made after several powder replenishment-rolling cycles under following conditions; roll temperature 280 C, punch pressure 84 KPa (where, the bulk density of wood powder body is g/cm 3 ), rolling speed 2mm/s. Further investigation such as the powder supply method and the punch pressure for supply is necessary in this rolling process in order to make sound boards continuously. 5 Conclusions For successful rolling of wood powder, it is necessary to heat the roll, and to keep the supply of powders during the rolling process. It is expected from hot press results that the mechanical properties of binderless board such as bulk density and bending strength increase when the mould temperature increases. However the temperature becomes over 290 C, these values tend to decrease due to burning of wood powder. The rolling speed is also important factor in order to solidify the wood powder successfully. Since heat can be transmitted to an inside of powder body, wood board having high mechanical properties can be produce when the rolling speed is lower. Acknowledgment This study was supported by the Industrial Technology Grant Program 2002 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

9 High Performance Structures and Materials II 419 References [1] T. Miki, N. Takakura, K. Kanayama, K. Yamaguchi and T. Iizuka, Effects of Forming Conditions on Compaction Characteristics of Wood Powders, In: Transactions of the Japan society of mechanical engineers, volume C, (2003), pp [2] T. Miki, N. Takakura, T. Iizuka, K. Yamaguchi and K. Kanayama, Mechanical Properties of Binderless Compacted Product only using Wood Powders, Proc. of the 2 nd Int. Work shop on Green Composites, (2004), pp [3] T. Miki, N. Takakura, T. Iizuka, K. Yamaguchi and K. Kanayama, Possibility of Extrusion of Wood Powders, In: JSME International Journal, vol.46 No.3(2003), pp

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