Wood, due to the combination of such unique natural properties as availability, ease of processing and chemical resistance has found wide application in a variety of areas and spheres of human life.
However, wood as a structural material, as well as filler in composite materials, is not without drawbacks, which include significant volumetric moisture deformations, the development of swelling pressure, pronounced anisotropy and water absorption.
At the same time, hygroscopicity and wettability should be attributed to the physical properties of wood, which have a significant effect on the development of the listed disadvantages. Various methods of chemical modification are used to eliminate the existing shortcomings of wood, however, they are not devoid of a negative effect: the strength of the treated wood decreases, the processed material does not adhere well, and its toxicity increases.
In this regard, the methods of physicochemical action on wood raw materials, based on thermal modification of the wood structure, which leads to numerous reactions occurring at different stages of this process without losing the main components (cellulose and lignin) have attracted great interest of researchers in the last two decades.
Heat treatment of wood occurs at temperatures of 160−250 deg C without the access of air oxygen, which causes a change in the physical properties and chemical composition of wood. During heat treatment, irreversible changes occur in the structure at the molecular level, which are a consequence of the decomposition of hemicellulose, a decrease in the level of cellulose and lignin polymerisation.
Heat Treatment And Thermally Modified For Wood
The most active developments in the field of technologies for thermal modification of lumber and equipment are carried out in such leading countries in woodworking as Finland, Germany, Canada, and France, where the distinctive features are in the approaches to the process of thermal modification: in the use of various media to create oxygen-free conditions-the use of water vapor, nitrogen, flue gases, vacuum or various liquids.
Saturated water vapor in addition to the targeted physicomechanical properties is often used in the production of plywood, bent furniture, pressed wood, or in case of colour changing of wood. At the same time, all the researchers pursue the same goals: changes in the chemical composition of wood as a result of thermal modification in order to reduce sensitivity to moisture, entailing an increase in bio-stability and dimensional stability.
Researchers consider that an important aspect of the use of preliminary heat treatment of wood filler in the production of composite materials is a decrease in swelling pressure with an increase in processing temperature, as well as a decrease in shrinkage and swelling coefficients in proportion to an increase in the temperature of thermal modification.
The results of studies of thermal characteristics of thermally modified wood (TMW) are presented in the work of Safin et al. It has been established that with an increase in the processing temperature, the values of the thermal conductivity and thermal diffusivity of the material significantly decrease, which is explained by the author by a decrease in the density of wood and a change in the chemical composition of wood.
This contributes to a change in the cellular structure of the wood. At the same time, the structure of cellulose remains unchanged. In the process of heat treatment, pentoses decompose and the moisture content in the wood decreases, which, in turn, increases the wood's resistance to decay.
The influence of technological regime parameters of thermal modification of wood on its mechanical characteristics after a year′s exposure in moist soil was revealed in work. It has been found that the strength indicators of natural wood fall much more intensively than that of thermo modified wood, which leads to the fact that after a year of aging in the soil surpasses natural wood in strength, although initially it was inferior to it in this indicator.
The increase in the biostability of TMW was explained not only by chemical changes occurring in wood, but also by the influence of physical factors: a decrease in water absorption with a simultaneous increase in the drying rate of moist TMW in comparison with natural wood leads to a reduction in the residence time of thermal wood in the humidity zone (30−70%), most favourable for wood-destroying fungi.
The results of experimental studies of the energy-power parameters of the process of longitudinal and transverse cutting on a pile driver and surface roughness after shaping on a thickness planer of TMW, depending on the processing temperature are presented in the works.
The authors conclude that the value of the specific work of cutting decreases with an increase in the temperature of thermal modification, which is explained by a decrease in the strength properties of TMW.
At the same time, there is a significant increase in surface quality during shaping: there is a decrease in roughness by more than two times. Shaikhutdinova et al. explains this phenomenon by a decrease in the resilient-elastic properties of wood, which, in turn, suggests an increase in the fineness of grinding when grinding thermally modified wood in the production of composite materials. The cutting power when milling heat-treated wood was up to 50 percent lower than that of untreated wood.
During the operation of products made of heat-treated wood, the important characteristic is the adhesion of paint and varnish coatings to the wood surface, which depends significantly on the ability of the glue to wet the surface. This ability is determined by the value of the contact angle of wetting.
It has been experimentally established that an increase in the temperature of impact on wood during its thermal modification leads to an increase in the contact angle θ of the paint and varnish coating from 59 degrees (at 60 deg C) to 75 degrees (at 225 deg C), what increases the water repellence of wood.
However, there are also negative consequences, such as a decrease in strength, hardness, etc. This is due to decomposition of hemicellulose and depolymerisation of cellulose. Measurements of the contact angle before and after treatment showed a significant increase in the hydrophobicity of the wood. The wetting angles of the water droplet were in all cases systematically higher for heat-treated wood than for untreated wood.
The inactivated wood surface weakly binds to adhesives is due to physical and chemical modifications of the surface, which lead to a decrease in the ability of the adhesive to penetrate deeply and be absorbed into wood.
In addition, some shredded wood fibres may not come into contact with the adhesive due to the high degree of surface energy.
The authors carried out a study of the adhesive strength of the glue to TMW by the normal peel method using a portable PosiTest®AT adhesiometer. The research results showed that the adhesion strength of the covering coatings on the samples modified at temperatures up to 180 deg C was more than five MPa. With an increase in the heat treatment temperature to 200 deg C, the adhesion strength of these specimens decreased to 0.8 MPa.
Thus, the technology of heat treatment of wood provides a material with predictable consumer properties, the most important of which are prolongation of the service life, stability of geometric dimensions, rich colours, and high biostability, which leads to the expansion of areas of application of wood products.
However, researchers point to a significant decrease in the adhesion of glued compositions to TMW, which complicates the creation of high-strength glued structures.
The work of Khasanshina et al. is devoted to the problem of increasing the surface wettability of unmodified wood by ultraviolet treatment, due to the exhaustion of the possibilities of modifying materials by traditional methods.
Research On The Possibility Of UV To Activate Wood Properties
Ultraviolet (UV) radiation accelerates the surface oxidation process, which leads to the adding of a functional (carboxyl) group and contributes to an increase in the wettability and an increase in the free surface energy of wood.
Thus, Khasanshina et al. found that the modification of the surface layer of the material by the action of ultraviolet radiation contributed to an increase in the wettability of wood.
An increase in the contact angle was observed with an increase in the power of UV radiation and the processing time: for example, with an ultraviolet irradiation of 62 W/sq cm, the contact angle of pine specimens decreases by an average of 36 percent; with an irradiation of 375 W/sq cm, the contact angle is no longer more than 35 percent of the initial value.
It was found that the efficiency of UV treatment was also influenced by the density of wood, since the maximum value of the radial surface wettability for pine specimens was achieved with irradiation of 187 W/cm2 after 1.5 h of UV treatment, while for beech specimens treatment for at least 2.5 h at irradiation of 311 W/sq cm was required to achieve maximum wettability.
The tangential surfaces of both wood species showed similar results. It was also concluded that UV irradiation for pine samples provided surface cleaning, pore opening, changes in the surface structure, and, to a certain extent, changes in the surface chemical composition.
Moreover, the results showed that the extractive content of wood plays an important role in colour change not only during heat treatment, but also during light exposure. It was found that compared to thermally untreated samples, heat treatment at 200 deg C reduced the red colour change due to photodegradation.
The change in yellow colour during photodegradation was practically not affected by the applied heat treatments, which shows that heat treatments cannot reduce the light decomposition of lignin.
Therefore, we evaluate the possibility of UV exposure to activate thermally modified pine wood to increase surface roughness, enhance wettability of TMW and adhesive strength of the glue in the production of wood block furniture panels.
The study of the influence of ultraviolet treatment on thermally modified wood was carried out on the samples of Pinus sylvestris, common in the European part of Russia. The choice of pine is due to the fact that it is one of the most widely used softwood species in the industry in Russia.
By its structure, wood is characterised by significant unevenness of density, porosity and a number of other biological factors that affect the process under study. Therefore, in order to obtain the most homogeneous starting wood, careful selection of samples is necessary.
Samples were made from one log two m long. For this purpose, the logs were selected without any defects or damage. Logs were cut from a tree trunk at a distance of two m from the stump. The heartwoods were discarded when cutting logs. Further, samples of certain sizes were made on a milling machine. In this case, the deviation from the nominal dimensions of the samples did not exceed ±0.5 mm. The linear dimensions of the samples were measured with a micrometre with an error of ±0.01 mm.
As samples for experimental studies of the processes of thermal modification and ultraviolet treatment, wood blocks with a cross section (mm) of 20 × 100, 50 × 50 (the last dimension along the fibres) with thicknesses of 7 and 22 mm and initial moisture content of 6–2 percent were prepared. The total number of wood samples tested was 120.
Blocks with dimensions with a width of 20 mm and a length of 100 mm were used for testing the strength of the glutinous seam. For this, 48 wood blocks were used, subjected to thermal modification and UV treatment.
Blocks with dimensions with a width of 50 mm and a length of 50 mm were used in research to determine moisture absorption, wettability and adhesive strength of treated wood. The total number of wood samples was 24 pieces for each study.
Preliminary thermal modification of wood materials was carried out by vacuum-contact method in a laboratory setup. The choice of this method for modifying the material is due to the fact that with conductive heat supply, it is possible to achieve high quality heat treatment and precisely achieve the required degree of thermal modification.
For thermal modification, absolutely dry wood was used, dried at a temperature of 105 deg C to constant weight. The parameters of the wood heat treatment process varied within the following limits: the temperature of the environment during the high-temperature treatment was set in the range of 180−240 deg C, depending on the specified degree of treatment, the absolute pressure in the chamber was 20 kPa.
In order to ensure the uniformity of heat treatment over the section of wood, the rate of rise of the heating plate to the processing temperature was carried out at a rate of no more than two deg C/min. Soak at a constant temperature of thermal modification was 45−120 min, depending on the size of the processed material and the degree of heat treatment.
Thermal modification is carried out until a predetermined degree of heat treatment is achieved. The choice of the density of the material to determine the degree of heat treatment is explained by the fact that this parameter is the most striking indicator, changing in the process of thermal modification. The density of thermally modified wood was measured immediately after heat treatment, which prevented the sample from moistening due to hygroscopicity.
Moisture Absorption Of Treated Wood
To determine the moisture absorption of treated wood samples, a standardised method was used, which was carried out in a desiccator while keeping the samples in a humid environment. The studies were carried out at a constant high relative humidity.
The method for determining moisture absorption of wood materials is as follows. Thermally modified and subjected to subsequent surface UV-treatment pine samples in the form of a rectangular prism with a width of 50 mm and a length of 50 mm are dried to dryness and weighed on an electronic weighing system.
After that, distilled water is poured into the desiccator at the bottom. The samples are placed with their lateral surface on special inserts so that they do not touch each other and the walls of the desiccator and closed with a cover.
The experiment is carried out at a temperature of 20 deg C. Samples are periodically weighed with a drawback of not more than 0.01 g. The first weighing is carried out from the moment the samples are placed in a desiccator, followed by 2, 3, 6, 9, 13, 20 and 30 days.
Investigations of the wettability of pine thermo wood, treated with ultraviolet light, were carried out on a special microscope by the method of a resting “lying” drop, as the most simple and convenient.
Influence Of Ultraviolet Treatment On Wood
A preliminary analysis of the required power of UV irradiation was carried out to change the properties of the surface of thermo wood by studying the change in the contact angle of wetting of pine thermo wood with water.
The conducted studies of the influence of the power of ultraviolet treatment on the contact angle of wood wetting showed that the process of surface inactivation proceeds most actively during the first 60 min, at which the total UV radiation reaches 125 W/sq cm.
In this case, taking into account the measurement errors with a probability of 0.95, it follows that a further increase in the processing time does not give a significant effect.
At the same time, taking into account the relative measurement error of 12.4 percent with a confidence interval of 0.95, a further increase in the processing time does not give a significant effect. As a result, further studies of the effect of UV treatment on the properties of thermal wood were carried out on samples subjected to total UV irradiation at a rate of 125 W/sq cm.
Analysis has also shown that UV treatment of thermally modified pine samples causes a slight increase in the hygroscopicity of the material.
This is due to the fact that as a result of ultraviolet treatment of the lignocellulosic material, surface changes occur: the surface becomes loose, as a result of which liquid vapours actively penetrate into the surface layers of the material.
The results obtained are consistent with the studies of Kubovský et al., where the wood wettability also increases due to UV treatment. At the same time, the inner layers of the material, due to the high-temperature processing of wood contribute to an integral decrease in the hygroscopicity of the material in proportion to the degree of heat treatment.
As a result of the study of the wettability of pine thermo wood treated with ultraviolet light by the method of ‘lying’ drop, it was found that UV irradiation of thermally modified wood increased its wettability.
Rapid spreading of the drop over the surface of the thermally modified sample treated with UV is noted. A drop of water applied to the surface of the sample completely spread over the surface of the material by the 90th second, covering it with a thin layer. In the case of the control sample, the drop remained in the formed form on the surface for at least 10 min.
In addition, ultraviolet irradiation contributes to an increase in the adhesive interaction of the binder with the surface: wherein the combined action of two-stage wood treatment, including preliminary thermal modification followed by surface UV treatment, causes persistent adhesion of the binder to wood even after boiling compared to the use of natural wood.
It can be used to improve the technology for the production of furniture boards, characterised by increased moisture and water resistance. At the same time, taking into account the measurement errors with a probability of 0.95, we can assume that UV treatment contributes to an increase in the adhesion properties of the surface of thermally modified wood by more than 13 percent. The relative error of the studies is 3.6 percent.
Ultraviolet Treatment Increased Adhesion Of Wood
As a result of the research, it was revealed that the thermal modification of wood affects the decrease in adhesion, which can be explained by the deterioration of the wettability of the thermo wood, while the ultraviolet irradiation of thermally modified wood increases the adhesion strength of the binder to the surface of the material.
Based on the studies and analysis of visual measurements of the contact angle of wettability of thermally modified wood samples, it can be concluded that the UV treatment process contributes to an increase in the adhesion properties of the surface layer of wood due to the reactivity of UV rays to oxidise and destroy ligno-containing wood products.
This allows further reducing the consumption of glue when gluing samples, thereby reducing the emission of harmful volatile substances.
It is revealed that the most active process of surface inactivation occurs when the total irradiation is not less than 125 W/sq cm. A further increase in processing power does not give the desired effect.
Thus, in connection with the results obtained, an improved technology for the production of furniture boards for the manufacture of moisture-resistant wood products is proposed, in which ultraviolet treatment of the lamellas is carried out on both sides along the glued layers, which ensures high adhesion strength.
After furniture board assembly, the main layers are subjected to mechanical treatment, as a result of which the zones modified by UV rays are removed and the board has the properties of glued thermally modified wood.