RU2837866C1 - Method of steam thermal treatment of wood - Google Patents

Abstract

FIELD: woodworking industry.

SUBSTANCE: invention relates to woodworking, particularly, to steam-thermal treatment of wood. Method of steam thermal treatment of wood includes a low-temperature step at temperature of 60-100 °C – heating of air medium with temperature gradient of 5-8 °C/h to temperature of 60-65 °C with duration of 15-40 minutes, heating of air medium with temperature gradient of 3-5 °C/h to temperature of 95-100 °C with duration of 10-30 minutes. Then steam is supplied for 90-150 minutes and excess pressure is formed. After that, steam-gas medium is heated to 160-170 °C with temperature gradient 3-8 °C/h for 20-30 minutes; heating of steam-gas medium with temperature gradient of 5-8 °C/h to temperature of 180-190 °C and pressure relief for 40-60 min; heating of steam-gas medium with temperature gradient of 3-5 °C/min to temperature of 200-240 °C and pressure relief for 60-90 minutes; effect on wood with water vapour with duration of 90-120 minutes; cooling wood to room temperature.

EFFECT: preventing cracking, deformation and warping of the treated wood, increasing resistance to decay, resistance to fungi, microorganisms and termites.

1 cl, 3 tbl

Description

The invention relates to the woodworking industry and can be used for steam-thermal treatment of wood, wood materials, and timber: boards, bars, parquet, blanks and products of various shapes, etc.

A method of wood processing is known, including its stepwise heating with subsequent cooling (patent FR No. 2786426A1, published 25.01.2002). The process is carried out at least during the cooling stage in the complete absence of oxygen, in a nitrogen atmosphere. The disadvantage of this method is the nitrogen treatment, which does not allow the wood to acquire some useful properties that it could have after treatment with water vapor.

A method for thermal treatment of wood is known, which includes loading wood into a chamber, heating and cooling it, in which heating is carried out by heating the wood with air in the chamber at a rate of 30-45 °C/h to a temperature of 130-165 °C, followed by exposure of the wood to water vapor for 0.5-1.75 h, after which the resulting steam-gas environment in the chamber is heated at a rate of 4-8 °C/h to a temperature of 160-200 °C, then the wood is exposed to water vapor for 0.5-1.75 h, after which the steam-gas environment in the chamber is heated at a rate of 4-8 °C/h to a temperature of 160-200 °C and the wood is maintained at this temperature for 2.5-6 h, then the wood is exposed to water vapor for 0.5-1.75 h, after which heating is carried out steam-gas environment in the chamber at a rate of 4-8 °C/h to a temperature of 160-200 °C, and after 1-3.5 hours the wood is exposed to water vapor for 0.5-1.75 hours, wherein the wood is exposed to water vapor by feeding water vapor with a temperature of 120 to 160 °C into the chamber, and cooling is carried out by removing the steam-gas environment from the chamber (RU patent No. 2235636, published 10.09.2004). The disadvantages of this method are the complexity of the process due to its multi-stage nature, as well as high energy costs.

A method of thermal treatment of wood is known, which includes loading wood into an autoclave, heating and cooling it, in which the wood is heated to a temperature of 140-150 °C in an air atmosphere for 2-3 hours, then in an atmosphere of water vapor to a temperature of 210-220 °C for 2-3 hours by constant metered injection of water, then heated to a temperature of 230-240 °C for 30-60 minutes with subsequent injection of water, and cooling is carried out in an atmosphere of 100% supersaturated water vapor by controlled injection of water (patent RU No. 2277045, published 27.01.2006). A disadvantage of the known method is that the wood obtained as a result of the treatment has a sharp smell of burnt wood for a long time.

A method for producing heat-treated wood is known, which prevents cracking, deformation and warping even of heat-treated wood having a large cross-section, and increases resistance to decay, resistance to termites and safety (JP patent No. 2009172787A, published on 06.08.2009). Wood with a moisture content of 15% or more and a large cross-section is selected from logs with a diameter of 50 mm or more in cross-section, beams with a minimum cross-sectional side of 50 mm or more, as well as plank materials with a minimum cross-sectional side of 50 mm or more. The first stage of drying the wood to a moisture content of 12% or less while maintaining the temperature of the wood at a level of from 50 to 130 °C using at least one heat carrier selected from superheated steam, microwaves and heating oil; and drying at the first stage, the second stage of heat treatment of wood for 4 to 24 hours while maintaining the temperature of the wood at a level of 180 to 250 °C using at least one heating agent selected from superheated steam and furnace fuel. The disadvantages of the known method are splitting, twisting and warping of the wood due to different shrinkage rates in the longitudinal, radial and tangential directions.

A method of thermal treatment of wood is known, which includes loading wood into an autoclave, heating and cooling it, in which after loading, vacuuming is carried out to 0.2 atm, followed by supplying water vapor to a pressure in the autoclave of 0.7-0.8 atm, and heating of the wood is carried out with water vapor at a temperature of 180-220 °C, followed by holding for 2-5 hours, and cooling is carried out at least three times by vacuuming for 15-20 minutes, followed by supplying saturated water vapor to a pressure in the autoclave close to atmospheric, and holding at this pressure for 5-10 minutes (patent RU No. 2453425, published 20.06.2012). The disadvantages of the known method are the low rate of heating of wood, thermal inertia, heat losses.

The closest known method for thermal treatment of wood to the claimed one includes the following stages: a) placing a batch of processed wood in the treatment chamber, b) changing the atmosphere inside the treatment chamber by pumping out air, replacing the pumped out air with an inert gas atmosphere in a gaseous state at a pressure of 0.8-1.2 MPa, c) heating the inert gas atmosphere to 165-175 °C, d) increasing the pressure in the inert gas atmosphere to 1.4-1.6 MPa, e) maintaining the temperature at stage c) and the pressure at stage d) for 90-150 min, f) cooling the inert gas atmosphere to a temperature of 20-35 °C, g) removing the batch of processed wood (RU patent No. 2735769, published 06.11.2020). The disadvantage of the known method is the nitrogen treatment, which does not allow the wood to acquire some of the useful properties that it could have after treatment with water vapor.

The purpose of the present invention is to develop a method for steam-thermal treatment of wood, improve the performance characteristics of wood, which prevents cracking, deformation and warping of wood, and also significantly increases resistance to decay, resistance to the effects of many fungi, microorganisms and termites.

Current methods of heat treating wood are performed using wood that has been dried to a moisture content of approximately 6 to 15% to relieve internal stresses, which subsequently eliminates internal cracks and warping that may occur during heat treatment.

The set goal is achieved by using a number of effects controlled by automation, the control points are selected in the following order:

1. Low-temperature stage of the process - processing, occurs at a temperature of 60-100 °C. The wood loses free water, the humidity decreases. The duration of the processing phase depends on the initial moisture content in the wood, the type of wood, as well as the thickness of the workpiece, and consists of the following stages:

- Heating of the air environment at a rate of 5-8 °C/h to a temperature of 60-65 °C. After reaching the set temperature, a "plateau" is reached lasting 15-40 minutes, this process equalizes the moisture content of the wood and prevents cracking, deformation and warping of the wood.

- Heating of the air environment at a rate of 3-5 °C/h to a temperature of 95-100 °C. Exit to a plateau lasting 10-30 minutes, to remove extractive substances: terpenes, waxes, phenols and fats released during the heating process.

Water vapor is supplied for 90-150 minutes and excess pressure is created.

2. Wood processing is carried out at elevated temperatures in the complete absence of air, but in the presence of supersaturated water vapor of high temperatures, obtained as a result of controlled injection of steam at certain stages of the processing cycle, and consists of the following stages:

- Heating of the steam-gas medium in the chamber to 160-170 °C with a temperature gradient of 3-8 °C/h. An inert processing environment is created from 100% supersaturated water vapor, pumped under excess pressure of 0.8 atm, which leads to complete displacement of air from the processing chamber. The pressure is maintained at a level of 0.4-0.8 atm by releasing the pressure. Reaching the plateau is accompanied by a pressure release lasting 20-30 minutes, during which acetic, formic and propionic acids, formaldehydes, furfural and turpentine are removed.

- Heating of the steam-gas medium in the chamber with a temperature gradient of 5-8 °C/h to a temperature of 180-190 °C. An inert processing environment is created from 100% supersaturated water vapor pumped under excess pressure of 0.8 atm, which leads to complete displacement of air from the processing chamber. The pressure is maintained at a level of 0.4-0.8 atm by pressure relief. Reaching the plateau is accompanied by a pressure relief lasting 40-60 min, during which cellulose hydrolysis begins, cellulose crystallinity increases due to the decomposition of amorphous cellulose, as a result of which the availability of hydroxyl groups to water molecules decreases, which helps to reduce the equilibrium moisture content and, as a consequence, the ability of wood to change geometric dimensions by 2-15%.

- Heating of the steam-gas medium in the chamber with a temperature gradient of 3-5 °C/h to a temperature of 200-240 °C. An inert processing environment is created from 100% supersaturated water vapor pumped under an excess pressure of 0.8 atm, which leads to complete displacement of air from the processing chamber. The pressure is maintained at 0.4-0.8 atm by releasing the pressure. Reaching the plateau is accompanied by a pressure release lasting 60-90 min, during which the hemicellulose chains are broken, the concentration of water-absorbing hydroxyl groups decreases, compression resistance increases and the level of internal stresses decreases. Lignin is partially modified and destroys its lignocarbohydrate bonds. Polycondensation of lignin with other components of the cell wall occurs, which leads to further crosslinking and contributes to an increase in the lignin content. As a result of heat treatment, the lignin content increases by 20%. The lignin molecules become less elastic and the cellulose microfibers have less ability to expand and absorb water, which explains the decrease in equilibrium moisture content and improved dimensional stability.

Then the heating elements are switched off and the heating of the steam-gas medium in the chamber is stopped. 30-150 minutes after the heating of the steam-gas medium is stopped, the wood is exposed to water vapor for the last time for 90-120 minutes to stabilize the wood structure.

3. Cooling the wood to room temperature. This leads to the consolidation of the formed bonds, as a result of which the wood acquires new properties associated with irreversible polymerization processes of new polymers. The final moisture content of the wood should be 2-6%. This stage takes 2-10 hours.

The visual manifestation of changes in the structure and chemical composition of wood that occur during steam-thermal treatment is a change in color throughout the entire thickness of the assortment. The color range from natural to dark brown is mainly determined by the duration and temperature of heat treatment. Compliance with the technological process ensures unique homogeneity of the structure and color change of the original sawn timber.

It is important to note that before steam thermal treatment the wood is not treated with any chemical reagents or methods that cause wood staining (boiling, steaming, soaking, etc.).

When using this method, wood acquires geometric stability. This property of wood occurs due to the structuring of molecular chains of wood and the acquisition of additional rigidity due to the evaporation and decomposition of polymer compounds (resins).

Wood treated in this way also becomes resistant to many biological damages. High processing temperatures destroy polysaccharides in wood and create almost absolute resistance to the effects of many fungi and microorganisms against the background of very low residual moisture of 2-6%.

The temperature and duration of processing depend on the need to obtain a particular property of the material. The maximum temperature, as well as the schedule of its change, depends on the type and quality of the original material.

The technical result of the invention is the prevention of cracking, deformation and warping of even heat-treated wood with a large cross-section, and an increase in resistance to decay, resistance to the effects of many fungi, microorganisms and termites.

Three samples were prepared for testing: ash, beech and pine. Table 1 shows the physical and mechanical characteristics, Table 2 shows the content of extractive and main components in heat-treated wood, Table 3 shows the content of nitrogen, carbon, hydrogen and oxygen in heat-treated wood.

Table 1 - Physical and mechanical characteristics of heat-treated wood.

Properties Ash Beech Pine before heat treatment after heat treatment before heat treatment after heat treatment before heat treatment after heat treatment 1 Compressive strength, MPa 50 99 60 90 48 70 2 Bending strength, MPa 105 105.2 120 111.4 86 96.8 3 Tensile strength, MPa 130 1339 135 1092 100 700

Table 2 - Content of extractive and main components in heat-treated wood.

Content, % Ash Beech Pine before heat treatment after heat treatment before heat treatment after heat treatment before heat treatment after heat treatment 1 Extractive substances 5.0 1.8 6.0 2.4 7.6 2.9 2 Cellulose 58.3 46.7 41.5 47.7 50.6 45.2 3 Hemicellulose 29.1 6.3 26.0 10.1 9.6 8.9 4 Lignin 23.8 30.9 32.6 29.6 27.5 32.5

Table 3 - Final analysis of heat-treated wood.

Element content, % Ash Beech Pine before heat treatment after heat treatment before heat treatment after heat treatment before heat treatment after heat treatment 1 Nitrogen 0,1 0.06 0,1 0.05 0.2 0.05 2 Carbon 49.2 50.3 49.0 51.6 50.2 51.2 3 Hydrogen 6.3 6.0 6.1 6.0 6.1 6.1 4 Oxygen 43.9 42.4 44.3 42.1 43.3 41.2

Claims (1)

A method of steam-thermal treatment of wood, including the following effects controlled by automatic equipment: low-temperature stage at a temperature of 60-100 °C - heating the air environment with a temperature gradient of 5-8 °C/h to a temperature of 60-65 °C for 15-40 min, heating the air environment with a temperature gradient of 3-5 °C/h to a temperature of 95-100 °C for 10-30 min, then water vapor is supplied for 90-150 min and excess pressure is formed, after which the steam-gas environment is heated to 160-170 °C with a temperature gradient of 3-8 °C/h for 20-30 min; heating the steam-gas environment with a temperature gradient of 5-8 °C/h to a temperature of 180-190 °C and pressure relief to 0.4-0.8 atm for 40-60 min; heating of a steam-gas medium with a temperature gradient of 3-5 °C/min to a temperature of 200-240 °C and pressure relief to 0.4-0.8 atm for a duration of 60-90 min; exposure of wood to water vapor for a duration of 90-120 min; cooling of wood to room temperature.