1. Field of the Invention
The invention generally relates to a process for the production of high density wood from low-density wood. In particular, the invention provides a continuous viscoelastic thermal compression (VTC) process for the production of VTC wood with high density, strength and dimensional stability.
2. Background of the Invention
Wood is widely used as a material for many manufacturing endeavors, including the construction of buildings, furniture, tools, decorative objects, composites, etc. The continual utilization of virgin forests has reduced the available supply of wood from large old growth logs. Further, the “green revolution” has increased public awareness regarding the efficient utilization of timber, and protection of forest lands, particularly of old growth forests. As a result, a shift in the available resource base has occurred, from old-growth mature forests to intensively managed, short-rotation, forest plantations. Many species of trees are now grown in plantations where conditions are manipulated to encourage rapid growth of the trees. The time to harvest for a tree grown on a plantation is typically less than 20 years, compared to 50-60 or more for trees in a naturally generated forest. Unfortunately, the demand for certain types of wood products cannot be met with trees that are so rapidly grown. Although these tree “crops” are adequate for such products as paper, a high percentage of the available wood is of low density and has mechanical properties that are inadequate for structural products.
Wood with inadequate mechanical properties can be modified by various combinations of compressive, thermal and chemical treatments. It can be densified by impregnating its void volume with polymers, molten natural resins, waxes, sulfur, and even molten metals, with subsequent cooling to solidify the impregnant. On the other hand, wood can be compressed in the transverse direction under conditions that do not cause damage to the cell wall (Kollmann et al. 1975). The compression of solid wood has been done in Germany since 1930 under the trade name of Lignostone. Laminated compressed wood has been made under the trade name Lignofol. Similar materials, Jicwood and Jablo, have been in production in England for some years (Rowell and Konkol, 1987). In the United States, patents on methods of densifying wood (such as Sears, U.S. Pat. No. 646,547, Apr. 3, 1900; Walch and Watts, U.S. Pat. No. 1,465,383 Aug. 21, 1923; Olesheimer, U.S. Pat. No. 1,707,135, Mar. 26, 1929; Brossman, U.S. Pat. No. 1,834,895, Dec. 1, 1931) date back to the 1900s. These patents did not adequately consider plasticization of the wood or stabilization of the final product; for this reason, the methods described therein have not been adopted by the industry (Kollmann et al. 1975).
Another densified wood product created in the United States is Compreg (Stamm and Seborg 1941). Compreg is resin-treated compressed wood. It is normally made by treating solid wood or veneer with water-soluble phenol formaldehyde resin and compressing it to the desired specific gravity and thickness. Compreg is much more dimensionally stable than non-impregnated compressed wood. However, treating resins harden within the cell wall making the treated wood brittle. Thus, if a tough, compressed product is desired, a brittle polymer should not be impregnated in the wood. A similar resin-treated compressed wood has been made in Germany under the name of Kunstharzschichtholz (Kollmann et al. 1975).
Unfortunately, untreated, compressed solid wood and veneer tend to undergo irreversible “springback” or recovery from compression when exposed to moisture. To eliminate springback wood should be pressed under conditions that cause sufficient flow of the lignin. A second compressed wood product developed in the U.S. that is not treated with resin is Staypak (Seborg et al. 1962a). Staypak is produced by compressing wood at a moisture content equal to or below that which it will have in service. One of the problems associated with making of Staypak is that the panels must be cooled to 100° C. or less while under the full pressure. Due to the thermoplastic nature of the lignin, and because the moisture content of the wood is only slightly less after compression than prior to pressing, considerable springback will occur if the product is removed while still hot (Kollmann et al. 1975). This necessity and other disadvantages of Staypak prevented this product from being adopted by the industry.
There have been many studies relating to wood stabilization by various treatments. Hillis (1984) reviewed the literature about stabilization of wood by a heating process. The effect of steam pretreatment on wood was investigated by Hsu et. al. (1988); Inoue et al. (1993); Inoue et al. (1996) and Kawai et al. (1992). Lately, the effect of heat on the dimensional stability of compressed wood has been evaluated by Dwianto et al. (1996). Tomme et al. (1998) performed thermo-hygromechanical treatment in order to produce densified wood with stable deformation.
Dwianto et al. (1996) found that preheating had a great influence on permanent fixation. According to their results, the permanent fixation of compressive deformation in wood resulted from the release of stresses stored in microfibrils and the matrix substance of the cell wall due to their degradation.
Hsu et al. (1988) developed a steam pretreatment process to produce highly dimensionally stable wood-based composites. They found that steam pretreatment causes partial hydrolysis of hemicelluloses for both hardwoods and softwoods, which greatly increases the compressibility of wood (i.e., reduces the tendency of internal stresses to build up in composites during hot pressing).
Inoue et al. (1993) found that almost complete fixation can be achieved by post-steaming compressed wood for 1 min. at 200° C. or 8 min. at 180° C. There was a large increase in hardness and only a slight decrease in bending stiffness (MOE) and bending strength (MOR). Inoue et al. (1996) also investigated the effect of pre-steaming. They found that the degree of recovery decreases if the press time and temperature increase. Pre-steaming increases the compressibility of wood and reduces the amount of stored stress due to the viscous flow of wood substances.
Kawai et al. (1992) produced laminated veneer lumber (LVL) by steam-injection pressing. They found that MOR and MOE of compressed LVL increased with increasing density. The dimensional stability of LVL has been improved considerably. They also have proposed the mechanism responsible for the fixation of compression set by steam treatment. They hypothesize that relaxation of the stresses stored in the microfibrils and fixation of the compressive set is due to: rapid hydrolysis of hemicellulose and partial degradation of lignin; partial hydrolysis of cellulose of amorphous and paracrystalline region, and reorientation in the crystalline region by steam treatment.
Another process to enhance the strength and stiffness of low-density wood species using steam, heat and mechanical compression has been termed Viscoelastic Thermal Compression, (VTC) (Kultikova, 1999; Kamke et al., 2000; Kamke and Sizemore; 2001). However, previous descriptions of this process have been limited to batch processes which utilize constant environmental conditions to produce flat, densified materials. Thus, previous VTC procedures are not suitable for the industrial manufacture of densified wood products. In addition, previous VTC methodology dealt only with whole wood and did not address the manufacture of laminae from veneer or composite panels for use in structural laminated composites.
The prior art has thus far failed to provide an industrially applicable method for treating low density wood to produce a wood product of high density, strength, and dimensional stability. In particular, the prior art has not provided a method to produce, from veneer or composite panels, laminae of high density, strength, and dimensional stability for use in structural laminated composites.