Wood is a natural biological material. As a decorative material for architecture and indoor furniture, wood has many advantages, such as light wood and high strength; Easy to process, low energy required for processing, not easy to pollute the environment; Good thermal and electrical insulation; Have good acoustic properties; Wooden materials have a strong cushioning effect on vibration; There are omens before the destruction of wood; Wood resource materials have natural and beautiful patterns, luster and color, and have special decorative effects.
As an indoor material, wood has the function of adjusting the relative humidity of the room. As our good friend, wood has always been regarded as gentle and soft. Because of these feelings, wood has become the material of love in architecture, furniture, home furnishing, and other industries for ten minutes.
High temperature heat treatment
In fact, there is a way to process wood that will completely change your view.
Through high temperature heat treatment, the dimensional stability of wood is improved by more than 30%, and the corrosion resistance is significantly improved.
Wood heat treatment is also known as wood heat modification. The EU standard (CEN/TS 15679:2007) defines heat treated wood as the wood whose cell wall composition and physical properties change after being treated in a low oxygen environment at a temperature higher than 160 ℃.
Before the wood is processed into wood products, certain treatment shall be carried out to reduce the moisture content of the wood to the national standard, and ensure that the wood will not crack and deform. Wood heat treatment is a method to improve the dimensional stability of wood that has been used since ancient times. It is used to treat wood in a low oxygen environment at 160 ℃. At this time, the cell wall and physical properties of wood will change.
Since ancient times, China has used open fire to directly spray and bake wood surface for carbonization to improve wood durability and prolong the service life of wood. Nordic countries began to use this method to treat wood as early as the tenth century, but this method of surface carbonization cannot be uniformly treated.
From the 19th century to the 20th century, Germans and Canadians used heat treatment to treat wood with hot oil as the treatment medium. The French also invented “Torrfaction” and “Renovation” heat treatment processes. Up to now, the heat treatment process has been developed maturely and applied in the treatment steps before many wood products are manufactured
It shows two key technological conditions of wood heat treatment:
Hypoxic environment
The purpose is to avoid the influence of excessive oxidation on wood properties and prevent safety accidents caused by wood burning. There are many ways to reduce the oxygen content in the treatment environment. The treatment environment can be filled with inert gases such as water vapor or nitrogen, the wood can be immersed in water or heat transfer oil, or the air in the treatment tank can be pumped away to create a negative pressure environment with low oxygen content. These different oxygen isolation methods are also the main criteria for distinguishing different heat treatment processes.
The temperature is higher than 160 ℃.
160 ℃ is an important node in the wood heat treatment process. Below this temperature, the wood mainly volatilizes water and extracts. When the temperature reaches 150~160 ℃, an exothermic reaction begins to appear inside the wood, and the content of extracts in the wood increases, indicating that the cell wall components begin to pyrolysis, and the physical properties of the wood change accordingly
Technological parameters of wood heat treatment
2.1 Temperature
Temperature is the first technological parameter that affects the degree of wood thermal modification. The concept of heat treated material is defined by temperature. 160 ℃ is the starting temperature of heat treatment (CEN/TS 15679:2007).
However, as far as the treatment effect is concerned, the temperature generally needs to reach about 180 ℃ to achieve a more adequate modification effect. 200 ℃ is a turning point of heat treatment. Since cellulose begins to degrade after exceeding this temperature, the degradation level of wood is significantly improved, and the strength index decreases more.
When the treatment temperature was further increased to 240 ℃, the content of extractives in wood decreased, indicating the end of the main degradation reaction and the volatilization of degradation products at the heat treatment temperature level.
Chemical analysis shows that at this temperature level, only a small amount of galactose, xylose, mannose, etc. are left in the wood, indicating that hemicellulose has been fully decomposed.
At this time, if the treatment temperature is raised again, the wood will lose most of its application value. Therefore, this temperature level is also the upper limit temperature of most wood heat treatment.
2.2 Pressure
Some heat treatment processes are completed under pressure. The pressure can promote the degradation of chemical components in wood, reduce the initial temperature of thermal degradation, and make the heat treatment achieve more significant performance changes than under normal pressure at the same temperature level.
In terms of technology, this means lower treatment energy consumption and shorter treatment cycle. In addition, the high humidity environment also increases the final moisture content of the treated wood, which is helpful to shorten the conditioning time after treatment.
Wood heat treatment can also be completed in a negative pressure environment, and the pressure range is generally 0.15 × 105～0.35 × 105Pa, the negative pressure heat treatment generates an inert treatment environment through air extraction, without the need for additional protective gas.
Wood degradation
The acidic organic gas generated in the wood degradation process is discharged from the device, making the treatment conditions more mild, reducing the impact of heat treatment on wood mechanical properties, reducing the corrosion of the treatment device, and helping to eliminate the smell of the treated wood. However, due to the loss of catalytic effect of organic volatiles on pyrolysis, the temperature of negative pressure heat treatment is generally slightly higher than that of other heat treatment technologies (see Table 1).
The effect of pressure level on heat treatment is essentially to change the concentration of acidic volatile matter in the treatment environment. The promotion of high-pressure environment on wood pyrolysis is mainly due to the fact that the acid volatiles released from wood during heat treatment cannot be discharged from the treatment system, which increases the acidity of the treatment environment and promotes the degradation of hemicellulose in wood, while the negative pressure environment is just the opposite.
The process and product characteristics of heat treatment in high-pressure and negative-pressure environments are different from those in normal-pressure conditions.
However, due to the high requirements for the airtightness of the treatment environment, the treatment capacity of a single system is relatively limited, and the application scale cannot be compared with that of normal pressure treatment processes.
2.3 Processing time
Prolonging the treatment time at the highest heat treatment temperature can achieve the same effect as raising the treatment temperature. Guo et al. showed that treatment at 200 ℃ for 3h and 215 ℃ for 1h had similar effects on wood properties. Yildiza et al. believed that the effect of temperature level on the mechanical properties of treated wood was greater than that of treatment time. Johansson et al. predicted by the model that, as far as the treatment effect is concerned, every time the treatment temperature decreases by 1 ℃, the 50 min heat treatment time needs to be extended to compensate.
Although there is a deviation between the model prediction and the actual test results, it still shows that the temperature level has more influence on the heat treatment results than the treatment time.
Three Wood Heat Treatment Processes
According to different heat treatment media, wood heat treatment mainly includes three methods: gas phase medium heating method, hydrothermal method and heating in oil medium.
Gas phase medium heating method.
The gas medium heating method is divided into steam heating and inert gas heating. The use of inert gas is because oxygen will make the fiber more easily degraded, which is easy to cause wood burning at high temperatures. According to different tree species, the treatment temperature used varies, generally between 150 ℃ and 250 ℃. Generally speaking, the higher the temperature is, the better the durability and the darker the color of the wood will be, but its mechanical strength will decrease, so the appropriate treatment temperature should be selected.
Hydrothermal method. Hydrothermal method is divided into three stages.
The first stage is water pyrolysis, which is pressurized at 160 ℃~190 ℃ to split cellulose in wood; The second stage is to reduce the wood moisture content to about 10% to prepare for the follow-up work; The third stage is the heat treatment stage, and the treatment temperature is 170 ℃~190 ℃.
Oil thermal method. Oil medium is used to treat wood.
The oil can be rapeseed oil, flaxseed oil, sunflower seed oil and other vegetable oils. In order to ensure uniform heat transfer, one oil should be used instead of mixed use. Generally, the heat treatment temperature is 220 ℃. In order to ensure uniform heat transfer, it is better to isolate the wood from the air for 2-4 hours.
