Kévin Candelier

Control of wood thermal treatment and its effects on decay resistance: a review

Key messageAn efficient use of thermal treatment of wood requires a depth understanding of the chemical modifications induced. This is a prerequisite to avoid problems of process control, and to provide high quality treated wood with accurately assessed properties to the market. Properties and structural anatomy of thermally modified woods are slightly different than un-processed woods from a same wood species. So it is necessary to create or adapt new analytical methods to control their quality.ContextHeat treatment as a wood modification process is based on chemical degradation of wood polymer by heat transfer. It improves mainly the resistance of wood to decay and provides dimensional stability. These improvements, which come at the expense of a weakening of mechanical properties, have been extensively studied. Since a decade, researches focused mainly on the understanding of wood thermal degradation, on modelling, on quality prediction and quality control.AimsWe aimed at reviewing the recent advances about (i) the analytical methods used to control thermal treatment; (ii) the effects on wood decay resistance and (iii) the advantages and drawbacks of a potential industrial use of wood heating.MethodsWe carried out a literature review of the main industrial methods used to evaluate the conferred wood properties, by thermal treatment. We used papers and reports published between 1970 and 2015, identified in the web of science data base..ResultsApproximately 100 papers mostly published after 2000 were retrieved. They concentrated on: (i) wood mass loss due to thermal degradation determination, (ii) spectroscopic analyses of wood properties, (iii) colour measurements, (iv) chemical composition, (v) non-destructive mechanical assessments and (vi) use of industrial data.ConclusionsOne of most interesting property of heat-treated wood remains its decay resistance. Durability test with modified wood in laboratory are expensive and time-consuming. This review displays data from different analytical methods, such as spectroscopy, thermogravimetry, chemical analyses or mechanical tests that have the potential to be valuable indicators to assess the durability of heat treated wood at industrial scale. However, each method has its limits and drawbacks, such as the required investment for the equipment, reliability and accuracy of the results and ease of use at industrial scale.

Utilization of temperature kinetics as a method to predict treatment intensity and corresponding treated wood quality: Durability and mechanical properties of thermally modified wood

Wood heat treatment is an attractive alternative to improve decay resistance of wood species with low natural durability. However, this improvement of durability is realized at the expense of the mechanical resistance. Decay resistance and mechanical properties are strongly correlated to thermal degradation of wood cells wall components. Mass loss resulting from this degradation is a good indicator of treatment intensity and final treated wood properties. However, the introduction of a fast and accurate system for measuring this mass loss on an industrial scale is very difficult. Nowadays, many studies are conducted on the determination of control parameters which could be correlated with the treatment conditions and final heat treated wood quality such as decay resistance. The aim of this study is to investigate the relations between kinetics of temperature used during thermal treatment process representing heat treatment intensity, mass losses due to thermal degradation and conferred properties to heat treated wood. It might appear that relative area of treatment temperature curves is a good indicator of treatment intensity. Heat treatment with different treatment conditions (temperature-time) have been performed under vacuum, on four wood species (one hardwood and three softwoods) in order to obtain thermal degradation mass loses of 8, 10 and 12%. For each experiment, relative areas corresponding to temperature kinetics, mass loss, decay resistance and mechanical properties have been determined. Results highlight the statement that the temperature curves’ area constitutes a good indicator in the prediction of needed treatment intensity, to obtain required wood durability and mechanical properties such as bending resistance and Brinell hardness.

Heat treatment of Tunisian soft wood species: Effect on the durability, chemical modifications and mechanical properties

Last decades, wood was promoted as building material. Wood heat treatment by mild pyrolysis has been reported to improve biological durability and dimensional stability of the material and constitutes an attractive « non biocidal » alternative to classical preservation treatments. Previous studies have shown that conferred properties strongly depend on the heat treatment intensity. A quality control marker based on mass loss has been developed. For several years, the increased development of Tunisian wood industry provides a significant capacity of wood production and transformation. Forests in Tunisia consist essentially of coniferous species [Aleppo pine (Pinus halepensis), Radiata pine (Pinus radiata), Maritime pine (Pinus pinaster), Stone pine (Pinus pinea)], characterised by a weak natural durability. Improved durability and fungal resistance should allow the use of Tunisian species in the wood industry. Import limitation of European species and the use of local species allow the conservation of economic value added in the country and improve the economic balance. For this reason, several Tunisian softwood species (Aleppo pine, Radiata pine and Maritime pine) have been heat-treated under vacuum atmosphere at 230°C to obtain a thermal degradation with mass losses of approximately 8, 10 and 12%. The oven device allows recording the dynamic Mass Loss (ML) during the treatment and following the thermodegradation kinetic. The chemical composition of the studied wood samples was determined before and after heat treatment. For each wood species and treatment intensity, wood chemical and mechanical analyses were performed by measuring O/C ratio, bending and hardness tests. Afterward, tests of decay resistance were performed according to the EN 113 Standard, with different fungal attacks (Poria Placenta, Coriolus Versicolor) at 22°C and 70% of humidity for 16 weeks. Results were related to the mass loss. Furthermore, intensity of thermal degradation was evaluated by TD-GC-MS. Treated and untreated wood samples were maintained during 15 minutes at 230 °C under nitrogen in the thermodesorption tube in order to analyse and compare resulting from the wood thermodegradation volatile compounds.

Improvement of the durability of heat-treated wood against termites

Thermal modification is an attractive alternative to improve the decay durability and dimensional stability of wood. However, thermally modified wood is generally not resistant to termite attacks, limiting the field of application of such materials. One way to overcome this drawback is to combine thermal modification treatment with an additional treatment. One such treatment is the impregnation of a boron derivative associated with appropriate vinylic monomers, which takes advantage of the thermal treatment to polymerise these monomers for boron fixation. Using this strategy, we recently showed that an impregnation of borax (2 or 4% boric acid equivalent) dissolved in a 10% aqueous solution of polyglycerolmethacrylate followed by thermal treatment under nitrogen at 220°C protects wood from both termite and decay degradations, even after leaching. Additionally, wood samples treated with a 10% polyglycerolmethacrylate aqueous solution and subjected to thermal treatment at 220°C presented improved resistance to termites while avoiding boron utilization. Based on these results, we investigate the effect of impregnation with two types of vinylic monomers, which are already used in the presence of boron, followed by thermal treatments at different temperatures. We evaluate termite and decay durability of wood to evaluate if thermal modification associated with light chemical modification could be a solution for utilization of thermally modified materials in termite-infested areas.