Wood is one of the most important renewable resources, and the comprehensive utilization level of wood is related to the sustainable development of the global economy and society. However, in recent years, natural forest timber resources have been in short supply worldwide, mainly due to the shift of timber resources to plantations. Due to the short rotation period of plantation, the proportion of juvenile wood in wood is large, the content of hemicellulose and lignin is high, the material is poor, the density is low, the dimensional stability and durability are poor, thus limiting the way of its use. .
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Researchers have tried various ways to improve the dimensional stability of wood, such as the method of overcoating or pasting the surface of the wood, and more by using the chemical infusion method to seal the hydroxyl of the wood, thereby reducing the wood. The hygroscopicity improves the dimensional stability.
However, in the past 20 years, with the improvement of the quality of life, there have been various reports abroad, and the safety of the use of chemical treatment materials has been questioned. At present, the United States and the European Union are in the process of setting restrictions on the use of chemicals in the treatment of wood, especially playgrounds, decks, picnic tables, fences, patio furniture, scenic spots and children's easy access. The European Union will fully implement the Directive on Restricting the Entry of Arsenic-Resistant Wood into the Market, covering the export of wood and various wood products. Similar restrictions apply in Japan and Australia. Strict protection regulations in various countries have promoted research and development in safety and target adaptation. Among various processing technologies, non-chemically treated wood products are gaining more and more market space, and the ultra-high temperature heat treatment technology of wood also occupies an important position.
Wood ultra-high temperature heat treatment and process
Generally, in wood drying, the temperature is <100 degrees for conventional drying, 100--150 degrees for high temperature drying, and >150 degrees for ultra-high temperature drying. Wood heat treatment is to use wood in an ultra-high temperature and low oxygen content environment close to or higher than 200 degrees. In the process of continuous treatment for a certain period of time, the hemicellulose in the wood is degraded, the hydroxyl groups in the wood cell wall are reduced, the hygroscopic property of the wood is decreased, the dimensional stability and the biodegradability are improved. Therefore, the ultra-high temperature heat-treated wood is used as a New materials are widely concerned.
The impact of the treatment process on the material is critical, such as processing temperature, processing time, heating rate, wood species, weight and size of the test piece, initial moisture content and other parameters, will affect the final performance of the product. The determination of the treatment process is mainly based on the purpose of the product, optimized between the improvement of water absorption performance and the reduction of mechanical properties, and finds an acceptable balance point. There are three main treatment processes depending on the medium used for the treatment.
1 steam treatment process
Finland's research on ultra-high-humidity heat-treated wood began earlier. After more than a decade of development, production technology has become relatively mature. During the treatment, water vapor is used to prevent wood burning, and the oxygen content in the treatment environment is controlled below 3%--5%. The treatment process is divided into three steps: 1) heating process, including preheating, high temperature drying and reheating stage; 2) actual heat treatment stage; 3) cooling and equilibrium stage.
Wood species processed include pine, spruce, birch and poplar. Because of the different species, their chemical composition and cell structure are different, so the choice of heat treatment parameters and the final effect are different.
1). Pine is one of the more suitable materials for heat treatment, usually used outdoors, and the heat treatment process is more severe. The problem is that the rosin that flows out after the pine treatment is troublesome for the processing equipment and post processing. At the same time, however, the use of skim pine has expanded.
2). The purpose of birch treatment is to obtain good color and surface quality.
3). Poplar can be used indoors after heat treatment, especially for sauna decoration.
The most significant change in the heat treated wood compared to the untreated material is the reduction in equilibrium moisture content, which in turn is associated with improved shrinkage. Tests have shown that when the treatment temperature exceeds 200 degrees, the weather resistance and corrosion resistance of pine are better. The biggest change after treatment of birch and poplar is that the influence of wood moisture content change on dimensional stability is significantly improved. The overall results show that the material with relatively uniform material has better treatment effect, while the treatment with knot or string material has more defects.
The steam treatment process used in the Netherlands is different, including two stages: 1) pyrolysis process, processing the raw material or steam dry material in a high pressure environment of 160-190 degrees, and drying it with a conventional kiln to make the treatment The wood afterwards reaches a lower moisture content (about 10%); 2) the dried product is again heated to 170-190 degrees, and the treatment time varies depending on the type of wood, thickness and the like.
The test results of the wood corrosion resistance showed that the corrosion resistance was significantly improved; in addition, the hygroscopicity of the material decreased after treatment, and the distance between the moisture absorption curve and the desorption curve was significantly larger than that of the untreated material, thereby reducing the climatic conditions. Under the chord, the radial expansion and contraction difference causes the deformation to occur.
France directly uses raw materials for steam treatment. In the drying kiln, the wood is heated to 230 degrees. Steam is mainly derived from the moisture in the wood. After ultra-high temperature treatment, the hygroscopicity of wood is lower than that of untreated materials. Under normal wet conditions, the moisture content is only 4%---5%, while the untreated material is about 10%--12% in the same environment. .
2 inert gas treatment process
The ultra-high temperature heat treatment of wood in France was originally designed to improve the dimensional stability of wood under different climatic conditions. The ultra-high temperature heat treatment has changed the hygroscopicity of wood. The treatment is carried out in a special treatment chamber filled with nitrogen. The indoor oxygen content is required to be less than 2%, and the wood having a moisture content of about 12% is slowly heated to 210-240 degrees for treatment. The results show that the moisture absorption performance of the treated wood is significantly reduced, and the high temperature treatment (230--240 degrees) process can destroy the nutrients required by the decay bacteria, and the corrosion resistance is better; but at the same time, the treatment will also cause the wood color to darken. The strength is reduced, the MOR loss is 40%, and the brittleness of the wood is increased.
3 hot oil treatment process
Wood heat treatment in Germany and Canada is carried out in hot oil (plant crude oil, such as rapeseed, flaxseed, sunflower oil, etc.), so that the wood is sufficiently isolated from oxygen during processing and has high heat transfer efficiency. The results of the study show that at a treatment temperature of 220 degrees, better durability and minimum oil absorption can be obtained. At the same time, it is found that in order to obtain the highest durability and maximum strength of the wood, the treatment temperature should be between 180 and 200 degrees, and the oil absorption is also within a small range of the control. The wood species treated were mainly spruce and pine. After 220 ° for about 4 h, the wood fiber saturation point was reduced to 14%, while the usual fiber saturation point was 29%. The disadvantage of this method is that the processing cost is high, and there is a problem of purification and disposal of waste oil.
4 Conclusions and prospects
Products made with environmentally friendly, safe and non-polluting materials are increasingly sought after by the world and in line with the international environment and development theme. Ultra-high temperature heat treated wood is such a non-polluting. Environmental protection. Safe and sustainable materials are one of the effective ways to expand the utilization of solid wood in plantation wood, which should be paid enough attention. Ultra-high temperature heat treatment technology can solve the problem of dimensional stability of wood, which will greatly promote the use of wood of plantation forest and reveal more potential market opportunities for plantation wood. Therefore, whether it is from foreign trade, or from people's health and environmental protection, we should vigorously develop wood products without chemical treatment. I believe that in the near future, this new material that meets the concept of environmental and development data sources is China’s economic development will play an important role.
The
raw materials are coated paper, special paper, powdered ash, kraft
paper, corrugated paper, which are used to hold items and can be
recycled.
Paper product packaging is the most widely used type in packaging industrial products. Carton
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September 06, 2019