Wood heat treatment refers to the short-term pyrolysis of wood in a protective gas environment at the temperature of 160 ~ 250℃. It is an environment-friendly physical protection technology for wood, which can improve the dimensional stability, durability and color of wood. The change of wood properties after heat treatment largely depends on the difference of wood species and process conditions. Heat treatment wood is usually referred to as deep carbonization, in order to distinguish it from ordinary dry wood, surface carbonization wood or packaging board after heat treatment in import and export quarantine.
In the last 10 years, the Netherlands, France, Germany and Finland in Europe. The industrial application of wood heat treatment technology has been realized. In addition, Japan, Canada and other countries and China's Taiwan region, also conducted a variety of research on wood heat treatment, and achieved some results.
Heat treatment can improve the dimensional stability of wood and remove or slow release the internal stress of wood. The reason is that hemicellulose, especially polysaccharide aldosides, becomes weak hygroscopic monomer after chemical changes. In addition, the hydroxyl groups in the molecular chain of cellulose combine with each other to form hydrogen bonds.
The possible reason for heat treatment to improve the anti-corrosion performance of wood is that, during the heat treatment, the wood components have changed, cutting off the source of nutrients needed for the survival of fungi. At the same time, the decrease of water content inhibited the growth of fungi, so that the corrosion resistance can be improved.
The color change of heat treated wood is mainly caused by the degradation of hemicellulose, lignin and some extractive compounds.
At present, the main problems in China's wood heat treatment market are: lack of specialized heat treatment equipment; Relevant supporting technologies are not yet mature; Most use the traditional kiln drying method. The main defects of the product are tree joint fall off, surface crack and internal crack, color difference, restricting the price of the product. Due to the various heat treatment wood sales on the market are intermingled, and lack of clear quality evaluation indicators, users often confuse the surface carbonized wood and heat treatment wood, time actually the size of the two stability, corrosion resistance difference.
Recently, a kind of anti-corrosive relief wood (mostly known as carbonized wood) appeared on the market, which is to conduct anti-corrosive treatment by means of colorless preservative liquid vacuum pressure, and then carry out surface carbonization treatment after air drying, which has not fundamentally solved the problem of moisture absorption and deformation of wood, and at the same time, there are higher potential pollution problems.
2.1 existing typical processes
At present, European countries have established the following five typical wood heat treatment process, and obtained relevant patents.
1) Dutch Plato process. In the temperature of > 160℃, pressurized conditions, the full water wood heat treatment; When the wood is dried to 10% moisture content, it is solidified at 170 ~ 190℃ under normal pressure and without oxygen.
2) French Retification. The wood with a moisture content of 12% was treated above 210℃ with high nitrogen and oxygen content < 5%.
3) French LesBois technology. After drying, the raw wood is heated using steam generated from the moisture in the wood.
4) German hot oil treatment process. Hot oil bath is conducted above 160℃. At present, an oil treatment experiment combining chemical modification and heat treatment is being carried out by adding chemical reaction groups.
5) ThermoWood technology, Finland. The raw or pre-dried wood is dried and then steam treated. At present, it has made a great breakthrough in business and is most widely used.
The process prevents wood from catching fire during high temperature treatment by keeping oxygen levels low. But the treatment cycle is long, the energy consumption is high, the product quality is uneven; In particular, the temperature and pressure control requirements are higher, even if the difference of 1℃ or 0.4MPa, can lead to the occurrence of wood internal crack.
2.2 factors affecting the effect of heat treatment
1) equipment types are divided into tank type and kiln type. Germany's Reulbach wood thermal oil treatment is the hot oil pump into the closed tank, around the wood to achieve high temperature cycle. The type of oil, heating temperature and heating time are determined according to the end use. If the goal is to obtain the best durability and achieve the expected oil load, the sawn wood should be treated at 240℃; In order to obtain the best durability and maximum strength, it should be above 220℃.
The process in Sweden USES high-temperature steam as a protective gas, and the design and properties of the heat treatment drying kiln are similar to those of the ordinary sawn kiln. The steam generated by the steam generator is mixed with the water in the water spraying device above the timber pile. After electric heating, the wind will send it to the timber pile. By changing the air flow, the wood is dried and heat treated evenly.
2) the most commonly used heat treated wood species for raw materials are: pine, spruce, birch, European poplar, etc. Generally, conifer wood used for waterproof structure is treated more deeply; Hardwood for interior finishes is lightly treated to ensure color and surface quality. In addition, high density resin treatment is relatively difficult, easy to crack during heat treatment, mechanical properties.
Pine in the heat treatment of resin will flow out, easy to stain equipment; Spruce's tree nodes and rings tend to become loose after heat treatment, resulting in the rings protruding during planing; Birch wood shall be heat treated at low temperature to avoid the distortion of the plate; Poplar is not uniform after heat treatment and is liable to split.
3) sawing method for conifer wood, the traditional cutting method with the growth rings parallel to the surface is adopted, which is likely to cause the spalling of the growth rings. If cutting, the Angle between the tree ring and the wood surface > 45°, the deformation can be reduced, the surface hardness increased, the performance is better.
4) the heating medium currently used includes: air, water, oil and water vapor. The wood can also be used as the evaporating water as the heating medium. The results showed that the termite resistance of wood treated by steam was superior to that of wood treated by air at 160℃. Compared with vacuum heat treatment, under the condition of the same treatment temperature and time, the bright spot of the sample after air heat treatment is smaller and the color difference is larger. If natural vegetable oil is used as heating medium, the heat-resistant capacity should reach 230℃, otherwise, in the process of treatment, the wood decomposition of the material will be deposited into the oil, so that the oil color deepened and thickened.
5) temperature and time heat treatment temperature is too low (60℃), can not effectively improve the wood corrosion resistance. In a certain range, the increase of heat treatment temperature can improve the wood corrosion resistance. The results show that the wood treated at 180℃ has a certain corrosion resistance, but it will still rot in the long term use.
Heat treatment time is usually determined according to the end use of the product. The results show that the hygroscopic property of wood treated with 4h is better than that treated with 2h.
2.3 process focus
On the basis of the previous research results, the author studied the new processing technology according to the characteristics of Chinese tree species and obtained good results.
1) the initial moisture content should be controlled at 10% ~ 15%. If it is too high, the drying time will be prolonged. Too low will increase the generation of internal fissure;
2) in the heating and cooling stage, the internal and external temperature of the wood should be kept at about 5℃, which can effectively reduce cracking;
3) it is more ideal to use the medullating core plate
4) use the mild drying standard for early drying, and control the color change within 1 hour before the second stage of drying to make the color meet the expected requirements. The previous technology could not control the color by controlling the temperature and time, and at the same time meet the requirements of dimensional stability, corrosion resistance and strength.
5) shorten the time of high temperature heat treatment to avoid or reduce the obvious decrease in wood strength and hardness caused by degradation of cellulose;
6) the second stage of drying is the peak pressure stage, as well as the sensitive stage for the occurrence of internal fissure and the key stage for quality control. By strictly controlling the time and steam pressure in this stage, it is more effective and reasonable than the traditional water content control and the high temperature heat treatment stage control.
7) adjust the pH value in the enclosed tank. In the process of heat treatment, the formation of acid directly affects the pH value of the whole system, the service life of the equipment, and the mechanical properties, chromaticity and brightness of the product. The addition of appropriate alkaline buffer solution can delay the degradation reaction, avoid the reduction of wood mechanical properties and color, and improve a number of performance indicators of the product.
3.1 environmental advantages
In Europe, CTBA (wood and furniture technology center) based on the test results of EN84 on spruce showed that the leached material of heat-treated spruce does not contain toxic substances and is itself an environmentally friendly product. It can be used as a substitute for orthopedic materials in medical treatment.
The industrial waste water after heat treatment includes: acetic acid, ketone, diphenylmethane, terpene, fat, wax, phenol, etc., which can be treated by combustion, dissolution, extraction and other methods to meet the national environmental protection standards. Spruce treated with 1m3 can leach up to 100 ~ 130g/m3 of acetic acid and 20 ~ 40kg/m3 of other different compounds.
3.2 performance advantages
Heat treated wood has the function of anti-corrosion and insect protection, the service life is prolonged, the service performance is greatly improved, such as water absorption, moisture absorption and swelling ability decreased, the dimensional stability is improved, its products almost no deformation, no cracking, especially suitable for the manufacture of floor, door, window products.
The surface of heat-treated wood is light brown to dark brown, with even and soft color. The color difference is smaller than that of natural wood, and the surface is smooth for a long time. It can partly replace valuable hardwood wood and be used for making indoor and outdoor furniture.
High temperature treatment, the wood becomes brittle, bending strength and nail-holding force reduced by 10% ~ 30%, but the compression strength, impact strength, surface hardness change little, often used for cost structural materials.
The heat conductivity of the treatment material is reduced by 10% ~ 30%, which can be used in sauna room, bathroom and other occasions.
Compared with similar wood modified products, heat treatment wood also has: good processing performance, easy planing, milling, drilling; Gluing finishing effect is better than similar products; After the special uv protective coating, when used outdoors for a long time, can still maintain the original color; Can process the production of carbonized wood veneer, used for furniture surface decoration: in the protective agent can be added a variety of natural pigments, to meet different needs.
3.3 price advantage
The cost of heat treating wood using Plato process in the Netherlands was 972 yuan/m3, the cost of Retification process in France was 1,459 yuan/m3, and the cost of thermal oil treatment in Germany was 2,576 yuan/m3. The cost of domestic heat treatment is about 2000 yuan /m3, which is lower than the price of ACQ anti-corrosion treatment.
As a new wood modification technology, wood heat treatment should be further explored in the following aspects in the future.
1) expand the range of wood species for heat treatment. The adaptability of wood of main plantation to heat treatment process was studied and the optimum process conditions were found. Explore the use of low-end wood to simulate the color of precious hardwood wood technology, increase the added value of low-end wood, expand the use range.
2) on the basis of obtaining mature heat treatment process of sawn timber, the process is promoted to products such as logs and plywood. Product application from primary products to high-end products, such as: outdoor flooring, wallboard, container materials, indoor furniture, bathroom appliances, etc.
3) establish the quality standards of heat treatment wood products in China, clarify the evaluation indicators and standardize the product market. At the same time, improve the internal quality control procedures of heat treatment enterprises.
4) non-destructive testing technology for the quality of heat-treated wood; Development of portable testing equipment to promote the large-scale production of heat treated wood.
On 2018-08-08 11:45:46
