Before World War II, the Germans once sold compressed solid wood with the trade name "Lignite". As early as 1900, the United States had a patent for manufacturing compressed wood. In 1932, the Soviet Union has developed two different compression processes: dry compression and wet compression. As early as 1947, China carried out research on compressed wooden shuttles, shuttle boards and other products in the wood engineering room of Shanghai Central Industrial Research Institute, and mass-produced them in Yangzi Timber Factory. At the end of 1950s and the beginning of 1960s, our country made compressed wood anchors for coal mines as compaction materials, which were popularized in production. In the early and middle 1960s, compressed wooden shuttles were widely used in China to replace Cyclobalanopsis glauca.
Wood compression methods can be divided into three categories: uniaxial compression, conformal compression and comprehensive compression. Uniaxial compression can be divided into end-to-end compression and transverse compression. Wood bending mainly depends on uneven end-to-end compression; The production of compressed wooden shuttles and anchors belongs to uniaxial transverse compression.
Wood is impregnated with phenolic resin and pressed into compressed wood, which is called impregnated pressed wood.
Compared with natural wood, compressed wood with bulk density of 65,438+0.4 g/cm3 has higher mechanical strength. Compared with natural wood, the transverse compressive strength is increased by 65,438+04 times, the parallel compressive strength by 65,438+0 times, the hardness by 2 times (end face) to 6-7 times (side face) and the wear resistance by 65,438+07 times. Compressed wood made of common tree species (such as birch, maple and poplar). ) can successfully replace hard hardwood (such as guaiac). ). See table for comparison of mechanical strength indexes of hardwood and compressed wood.
In addition, compressed wood has good impact toughness and wear resistance, which is beyond the reach of many materials. For example, the friction coefficient of copper is 0.05 (7.5 ~ 1 1 MPa load), while the friction coefficient of compressed birch is only 0.002 ~ 0.0005. Using this characteristic of compressed wood, compressed wood can be made into bearing bush and shaft sleeve.
The dimensional stability of compressed wood is not good. If it is soaked in water or placed in a high humidity environment, some products can expand to the volume before compression. The compressed wooden shuttle for textile requires high dimensional stability, so it is necessary to have reasonable compression technology and compression parameters (such as compression rate, temperature, time, etc.). The dimensional stability of compressed wood is also related to the density, moisture content, sapwood and wood source of compressed wood. In order to improve the dimensional stability of compressed wooden shuttles for textile, the finished products still need moisture-proof and waterproof treatment to avoid moisture absorption and expansion of the finished products.
Because the wood is impregnated with phenolic resin before compression, the cured phenolic resin can expand the cell wall of the wood, so the dimensional stability is greatly improved, but the impact toughness is lower than that of compressed wood without resin. The wooden shuttle made of wood treated with silicone resin has good dimensional stability, wear resistance and impact toughness, but the cost is too high to be popularized and applied.
The production process of compressed wood in China adopts wet method. That is, the wood is softened by steam cooking first, and then hot pressed in a hot press according to the specified wood compression ratio (the wood should be placed in a steel mold). After hot pressing, take it out of the hot press after it is fully cooled. In order to eliminate the internal stress produced in the hot pressing process, the product should also be heat treated.