The difference and advantages between solid wood layered compression and overall compression

Funded project: National Forestry Administration Forestry Science and Technology Achievements National Promotion Project "Promotion and Application of Solid Wood Layered Compression Technology ([2017] No. 25)".

Author profile: Huang Rongfeng (1963-), female, researcher at the Institute of Wood Industry, Chinese Academy of Forestry.

Background

Wood compression technology first emerged in the United States and Germany in the 1930s. Initially, compressed wood was used to replace metal in military aircraft to prevent them from being detected by radar. It wasn't until the 1990s that research into wood compression technology, aimed at improving the properties of softwood and expanding the applications of plantation wood, began to attract significant attention from scholars and industry worldwide.

After more than 20 years of research, wood compression technology, particularly wet-heat softening wood compression technology, has been gradually refined in terms of wood softening, plastic deformation, changes in compressed wood properties, compression deformation fixation and its mechanism, compression methods, and compression processes. This has led to a comprehensive system of compressed wood processing technologies, including wet-heat softening log shaping compression, sawn timber overall compression, veneer compression, sawn timber surface compression, and high-frequency heating softening and deformation fixation. These technologies are being promoted and applied in the wood industry. However, to date, overall wood compression technology has not yet achieved industrial application.

Note: The dark part is the compression layer. From left to right are the compressed wood with surface compression of 5 mm, surface compression of 10 mm, middle layer compression of 5 mm, middle layer compression of 10 mm and the control material.

The main difference between solid wood layered compression and overall compression

The low-density Japanese cedar can be compressed to a density of over 0.80 g/cm³, significantly improving its mechanical properties. Based on this research, the densities of my country's major plantation species, poplar, fir, and cedar, are approximately 0.40, 0.38, and 0.28 g/cm³, respectively. If compression were used to increase hardness to the level of commonly used hardwoods for furniture, flooring, and other applications, the overall compression rate would need to reach 50%-140%. This not only increases manufacturing costs, but also makes it difficult to fix compression deformation due to the high compressive stress. Although saturated steam pressure can achieve permanent fixation of compression deformation, high equipment and production costs make it difficult to achieve industrial application.

The layered compression of solid wood is achieved by regulating the distribution of water and heat. Only the layers or parts that need to be compressed and strengthened are compressed, and the rest of the parts are hardly compressed. This achieves selective directional compression of the wood and improves the mechanical properties of the wood by controlling the amount of compression.

3 Comparison of the thickness of compressed wood after layered compression and before compression

Due to the small compression amount in the layered compression process, the required pressure is only about 50% of the overall compression. After hot steam pressurized heat treatment, the compression deformation rebound rate is reduced to below 1.50%.

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Advantages of laminated compression of solid wood

The layered compression method of solid wood realizes the selectivity and directionality of compression densification. On the premise that the material performance meets the product requirements, it can minimize the compression rate, effectively save wood, and reduce the manufacturing cost of compressed wood.

1) Layered compression technology can significantly improve the hardness and strength of wood at low compression rates, effectively reducing production costs;

2) The pressure required for layered compression is small, and the rebound rate of compression deformation is low;

3) Layered compression is achieved through moisture and heat regulation. The processing cost mainly includes heat consumption and labor costs. The production process is simple, low-cost, and environmentally friendly.

4) The product is lightweight, has high surface hardness, and is high strength. Solid wood flooring made from surface compressed wood has a high surface density and a low center density, making the floor elastic and softer and more comfortable to the touch.

5) Wide adaptability to tree species. All coniferous and broadleaved wood species with a density below 0.7 g/cm³ can be compressed and reinforced using layered compression technology.

Note: From left to right: Cryptomeria, Cunninghamia lanceolata, Pinus sylvestris var. mongolica, Poplar, Cannolum, and Okoume.

After years of basic research and production experiments, solid wood layered compression technology has formed a series of patented technologies (zl201010522617.1, zl201310044738.3, zl201020580419.6, zl201720651024.2) and products. The intellectual property system is relatively complete. The two companies have successfully completed production trials with an annual output of 100,000 square meters and 400,000 square meters. The technology is highly mature and relatively sufficient preliminary research and production practice have been carried out for the promotion and application of the technology.

Solid wood layered compression technology is a wood modification technology that does not add any chemical agents. It can improve the quality of low-density plantation wood, so that it meets the requirements of high-quality solid wood panels for the processing of wood products such as furniture and flooring. It opens up new paths for the utilization of low-density wood and has broad application prospects in the wood products processing industry.