Using vacuum conductive drying to improve the environmental friendliness of materials

To minimize the risk of rotting damage to wood materials, special antiseptic preparations with biocidal, fungicidal and insecticidal properties are widely used. Many of them contain highly volatile chemicals that are toxic to humans and harmful to the environment. The development of approaches to reduce the emission of harmful substances from wood materials is an important problem in improving the environmental friendliness of construction and finishing materials. The paper describes a method for increasing the biostability of chipboard based on polyvinyl alcohol binder by chemical-free high-temperature treatment (thermal modification). The paper presents the results of experimental studies of vacuum-conductive drying of wall panels made of thermally modified filler. Drying curves for wall panels were plotted depending on various parameters: temperature conditions, different chipboard thickness, and the residual pressure in the vacuum chamber. The work performed revealed that panels with a thermally modified filler dry much faster than those with untreated filler and without the release of chemical compounds harmful to humans and the environment.


Introduction
The development of eco-friendly chipboard materials has long been a hot topic. However, research is mainly aimed at reducing the toxicity of the adhesive component of this composite, and therefore solutions are proposed to reduce phenol emissions by introducing various additives or replacing phenol-containing resins with other more environmentally friendly binders [2][3][4].
Previously published research was aimed at developing the chipboard technology, where polyvinyl alcohol was used as a binder [1]. This technology can be used in the furniture board production. At the same time, it is reasonable to produce wood-chip panels for heat insulation and soundproofing. In this case, to prevent the growth of mold fungi and various wood-destroying microorganisms, a necessary condition is the biological protection of wood particles. For this purpose, modification with various chemical compositions is usually used [2][3][4]. However, chemical modification is dangerous both for humans and for *Correspondingauthor:joker775.87@mail.ru the entire environment. In this regard, in recent years, research in the field of hightemperature treatment (thermal modification) of wood has come to the fore, which makes it possible to increase biostability without the use of chemicals [2].
So, creating eco-friendly wall materials should involve considering this type of pretreatment of wood filler and its impact on the technological process of production of chipboards based on a polyvinyl alcohol binder.

Methods and Materials
Initially, the temperature conditions for processing wood material (sawdust) were selected. The heat treatment temperature was 220°C. Thermal modification was carried out in a dense bed in a container placed in a chamber furnace. The camera and the container are shown in Figure 1. 2 to 5 mm sawdust were fed to special sections, which were loaded into the chamber furnace. After that, the furnace was hermetically closed, and the processing temperature was set to 220°C. After the set temperature was reached inside the material layer, determined using a pre-installed thermocouple, the wood material was kept under these conditions for 3 hours. Then the unit was switched off, and the thermally modified wood material was self-cooled in the closed furnace until the temperature inside the layer fell below the self-ignition temperature.
Further, samples of chipboards were prepared by mixing thermally modified wood particles and an aqueous solution of polyvinyl alcohol in the ratios of components provided in Table 1. The prepared samples were tested for drying speed according to the previously developed vacuum technology [1]. The levels of variation of various drying factors are shown in Table 2. Similarly to [1], to improve the accuracy of the mathematical description of the drying rate of wood composites, the technology was considered in two humidity intervals: 70 -40% and 30 -10%.

Results
Due to the fact that thermal modification results in a decrease in hygroscopicity and water absorption of wood, no significant moisture content of wood particles was observed during the formation of samples. This explains the higher rate of drying of composites with thermally modified filler compared to samples based on untreated wood particles [1]. Based on the experiments, the heat-treated wood filler-based wall board drying rate curves were plotted (Figure 2

Conclusion
The research is aimed at creating eco-friendly wall materials. The authors have studied the technology of high-temperature treatment (thermal modification) of wood and its influence on the technological process of production of chipboard panels based on a polyvinyl alcohol binder. The paper presents the results of experimental studies of vacuumconductive drying of wall panels made of thermally modified filler. Drying curves for wall panels were plotted depending on various parameters: temperature conditions, different chipboard thickness, and the residual pressure in the vacuum chamber. In the course of this work, it was found that panels with a thermally modified filler dry much faster than those with an untreated filler.
Planning and conducting experiments on vacuum-conductive drying of chipboard samples from thermally modified wood particles and polyvinyl alcohol, enabled us to create a mathematical model of the drying rate during molding and pressing chipboard depending on the press heating temperature, chamber pressure, thickness and density of the molded products.
The work was supported by a grant from the President of the Russian Federation to support young Russian scientists who have a Ph.D. degree (MK-2246.2020.8).