On 2018-08-08 14:48:54
On 2018-08-08 14:47:24
On 2018-08-08 11:45:46
It is generally accepted that the presence and variation of moisture is the root cause of wood size instability. High temperature heat treatment of wood is refers to the use of 160 ~ 230 ℃ (180 ~ 210 ℃) is commonly used in the temperature heat treated wood, the creatures with good durability, weather resistance, dimensional stability and safety, environmental protection and a series of prominent advantages, is increasingly widely used in floor, indoor and outdoor decoration, wooden staircase, park facilities, furniture manufacturing, and many other aspects.
Wood is a kind of natural biological material. As a decorative material for buildings and indoor furniture, wood has many advantages, such as light weight and high strength. Easy to process, low energy required for processing, not easy to pollute the environment; Good insulation of heat and electricity; Having good acoustic properties; Wood material has a strong cushioning effect on vibration; Timber has a warning before it is destroyed; Woodiness resource material has natural and beautiful pattern, burnish and color, have special adornment effect. As indoor use material, lumber has the function of adjusting the relative humidity of the bedroom.
However, as a naturally generated organism, wood also has some undesirable inherent characteristics, among which, the wet expansion and shrinkage of wood and the warping, cracking and other deformation caused by the inconsistent changes in the dimensions of each direction are the most intractable problems encountered in wood processing and utilization. Many researchers have been trying to study and actively explore ways to stabilize wood size, accumulating experience and achieving certain results. It is generally accepted that the presence and variation of moisture is the root cause of wood size instability.
High temperature heat treatment of wood is refers to the use of 160 ~ 230 ℃ (180 ~ 210 ℃) is commonly used in the temperature heat treated wood, the creatures with good durability, weather resistance, dimensional stability and safety, environmental protection and a series of prominent advantages, is increasingly widely used in floor, indoor and outdoor decoration, wooden staircase, park facilities, furniture manufacturing, and many other aspects.
1. High temperature heat treatment equipment
At present the domestic wood heat treatment equipment, according to the shape of the treatment room, can be divided into rectangular and cylindrical; According to the pressure of the treatment environment, can be divided into atmospheric pressure and pressure type. Among them, the structure of the atmospheric high temperature heat treatment equipment is similar to that of the wood drying room, the main differences include: large thermal power, large heat dissipation area of the indoor radiator, high airflow circulation speed and uniform, in addition, the equipment (tank) shell and internal equipment should be corrosion-resistant, the inner wall needs to be welded as a whole, not riveting or bolt connection. The air tightness and heat preservation of the shell are good to prevent air leakage and waste of heat energy; When working, the equipment should be filled with (overheated) water vapor as a protective gas, almost no air, to protect the wood from oxidation and fire.
As shown in the picture, it is a photo of the wood heat treatment equipment developed by weifang fushun energy saving technology co., LTD. The equipment adopts cylindrical structure, fast door opening, maximum load of 15 cubic meters, normal pressure operation to ensure the safety of operation, can be remotely controlled system (PC terminal; Mobile phone) save time and labor; It also reduces the disadvantage of relying too much on technicians; The whole system is made of stainless steel and heated by electricity. At the same time, the whole system should be equipped with a steam generator to meet the process requirements of high temperature heat treatment.
2.High temperature heat treatment process
As shown in the figure, the processing of high-temperature heat-treated wood is mainly divided into the following three stages:
The drying stage is the longest stage of the whole heat treatment process. This stage is also known as the high temperature drying stage. The temperature of the drying chamber rose rapidly to about 100℃, after which the temperature gradually rose to 130℃. In this stage, the moisture content of wood is almost reduced to 0, and then the real wood pyrolysis stage.
The drying period mainly depends on the initial moisture content of the wood, the species and the thickness of the wood. Whether the raw material has been dried can be. Proper drying is important to prevent cracking inside the wood.
(2)Heat treatment stage
The heat treatment phase begins immediately after the high temperature drying phase. The internal temperature of the equipment gradually increased to 180 ~ 210℃. The steam used during drying and heat treatment carbonization is used as protective steam. These protective vapors prevent wood from burning and affect the chemical changes that take place inside the wood. When the required heat treatment temperature is reached, the product is kept warm for 2 to 3 hours depending on the end use of the product.
(3) cooling and moisture content regulation stage
In this stage, the cooling speed should be adjusted reasonably to prevent the appearance of defects such as wood surface crack, internal crack and split crack. In addition, the wood should be re-humidified to the appropriate moisture content according to the end use of heat treated wood. When the temperature drops to 80 ~ 90℃, the final moisture content of the treated material can be controlled at 4 ~ 7% by adjusting the humidity of the environment. The final moisture content has an important effect on the properties of wood. The wood with too low moisture content is not suitable for processing and application. Depending on the temperature of the heat treatment and the wood being treated, the conditioning phase takes 5 to 15 hours.
3. high temperature heat treatment of wood characteristics
Some of the chemical and physical properties of wood have changed permanently after heat treatment. These changes in properties are caused by the pyrolysis of hemicellulose. The desired reactions occur at a temperature of approximately 150℃ and continue as the heat treatment progresses and the temperature increases. The results show that the wood after heat treatment is reduced by drying shrinkage and wet expansion due to different humidity, the biological corrosion resistance is improved, the color of the wood is deepened, the extract is extracted from the wood with heat treatment, the weight of the wood becomes lighter, the equilibrium moisture content decreases, the PH value decreases, and the heat insulation performance is improved. In addition, to needle material, grease after treatment is all cloth, be helpful for paint.
The density of heat treatment wood is slightly lower than that of untreated wood, and the density of wood decreases slightly with the increase of heat treatment temperature, which is mainly caused by chemical changes in the wood in the process of heat treatment.
The strength of wood and the density of wood have a high degree of correlation, wood after heat treatment density will be reduced. Therefore, heat treated wood in some cases slightly lower strength. However, the strong weight ratio did not change. Wood strength is also strongly related to moisture content (below the fiber saturation point). Because of the lower equilibrium moisture content, heat treated wood has better strength.
(3) equilibrium moisture content
Compared with untreated wood, the equilibrium moisture content of heat treated wood decreases under different relative humidity. When the treatment temperature is 220℃, the equilibrium moisture content of the wood is half of that of the untreated wood.
(4) Dimensional stability
Heat treatment of wood can significantly reduce the instability of radial chord dimension of wood, effectively reduce the dry shrinkage and wet expansion of wood, thus making the final wood products have high dimensional stability. In addition, unlike other wood, heat treated wood has no drying stress, which is an obvious advantage, especially when chopping wood and making woodwork.
(5) Thermal conductivity
The thermal conductivity of treated wood is reduced by 20 to 25 percent compared to untreated material. Therefore, heat-treated wood is ideal for use in flooring, exterior doors, escalators, Windows and Finnish steam bath interiors.
The color of heat treated wood is affected by heat treatment temperature and heat treatment time. The higher the temperature of heat treatment, the darker the color after treatment. For all coniferous wood, the consistency of color is affected by the density, the normal variation of morning and evening wood.
Fir 180, 200, 220℃ heat preservation after 3 hours of color change
Birch 160, 180, 200℃ heat preservation after 3 hours of color change
The greatest amount of energy is consumed during the wood drying phase, which accounts for about 80% of the energy used to heat the wood. The total energy consumption required for the entire process of heat treatment is only 25% higher than that required for normal wood drying. The same is true of the electricity consumed. Because wood heat treatment does not need any chemical reagents, only water and heat, so heat treatment wood is completely non-toxic environmental protection materials. The heat treatment process also does not produce a large amount of waste water, the generated waste water in the solid substance is dissolved in a special container, the remaining part of the waste water treatment.
To sum up, the high temperature thermal modification technology of wood is still the most practical, economic and environmentally friendly modification method among all kinds of wood modification methods.