S. Poncsak

Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera)

The thermal treatment of wood is an alternative to the chemical treatment for preservation purposes. The heat treatment process improves wood’s resistance to decay and its dimensional stability. However, mechanical strength decreases as a result of heat treatment. Therefore, the treatment parameters have to be optimized to keep this loss at a minimum while improving other properties. Thermal treatment is new in North America, and its parameters are not yet adjusted for the Canadian species. Carrying out the parameter adjustment in an industrial furnace requires many trials which are costly in terms of material and man-power. A laboratory study was carried out to determine the effect of different parameters of the heat treatment on the mechanical properties of birch in order to optimize this process. A thermogravimetric analyzer was built to carry out the laboratory tests. The impact of the process parameters–such as maximum treatment temperature, holding time at this temperature, heating rate, and gas humidity–on the mechanical properties of birch was investigated. Temperature distributions in wood and in gas as well as the weight loss of wood were measured during the experiments. Afterwards, hardness, modulus of elasticity, modulus of rupture, and resistance to screw withdrawal of the samples were measured. The relation between the process parameters and the resulting mechanical properties was examined.

Effect of heat treatment on the mechanical properties of North American jack pine: thermogravimetric study

Heat treatment improves dimensional stability of wood, reduces its decay, and darkens its color. However, mechanical properties of wood can deteriorate during the heat treatment. The effect of heat-treatment conditions (maximum treatment temperature, heating rate, exposure time at the maximum heat-treatment temperature, and the gas humidity) on the mechanical properties of North American jack pine (Pinus banksiana) was studied using thermogravimetric analyzer. This type of study permits the identification of the best treatment conditions which will minimize reduction of mechanical properties of jack pine. The results showed that the degree of change in bending strength, hardness, screw withdrawal strength, and dimensional stability of jack pine during heat treatment depends strongly on the treatment conditions used. Therefore, great care should be taken to select the treatment conditions. Thermogravimetric analysis can be used as a first step for selection.

Effect of thermal treatment of wood lumbers on their adhesive bond strength and durability

Wood used in outdoor applications needs to undergo either chemical or thermal treatment to improve its decay resistance. Thermal treatment permits to avoid the use of toxic chemicals, increases the dimensional stability and gives a dark color to the wood. However, this process deteriorates the mechanical properties of wood, i.e., the wood becomes more fragile and rigid. The chemical transformation of wood that takes place during the heat treatment changes the interaction between the wood surface and the adhesive. In this work, the interfacial bonding strength (the resistance to the shear stress by compression in parallel direction to the glued interface) and cyclic delamination (resistance to delamination during accelerated exposure) for different wood species and adhesives were tested in accordance with the ASTM D2559 standard. Four wood species: scott pine (Pinus sylvestris), aspen (Populus tremuloides), yellow poplar (Liriodendron tulipifera) and jack pine (Pinus banksiana) both treated and non-treated, and two structural adhesives, phenol resorcinol formaldehyde (PRF) and polyurethane (PUR), were used in the testing. Among the studied species, jack pine is found to be the easiest to bond, while aspen is found to be the most difficult. With the wood species and adhesives evaluated in this study, non-treated wood is found to provide a better bonding strength than the treated wood.

Improvement of the heat treatment of Jack pine (Pinus banksiana) using ThermoWood technology

Thermal treatment in inert atmosphere is used to preserve wood without utilisation of toxic chemical agents. In addition, this process increases the dimensional stability of the wood matrix and results in attractive dark colour. On the other hand, it can deteriorate the mechanical strength and the flexibility of wood. For this reason, heat treatment parameters (such as maximum temperature, heating rate, the duration of the first plateau at constant temperature (100–120°C) and the second plateau at the maximum treatment temperature, humidity, final cool down rate) must be optimised in order to benefit from advantages of thermal treatment without deteriorating significantly the mechanical properties of wood. Correlation between wood properties and thermal treatment parameters depends not only on the wood species but also the environment (climate, soil) where the given species grow. This paper presents a study on thermal treatment of Canadian Jack pine (Pinus banksiana) using a medium size prototype furnace. The aim of this study was to optimize the set of parameters used during industrial treatments. The possibility of shortening the process time without causing any deterioration in wood quality was also investigated. The results show that increasing the maximum heat-treatment temperature increased the dimensional stability of Jack pine and darkened its colour. This parameter did not affect the modulus of elasticity but it decreased the modulus of rupture of Jack pine. A slight reduction in gas humidity during the initial warming up period permitted to shorten the drying period and at the same time increased the mechanical strength. This improvement helps save energy and increase productivity.ZusammenfassungDie Wärmebehandlung von Holz unter Schutzgasatmosphäre wird zur Verbesserung seiner Dauerhaftigkeit ohne Verwendung giftiger chemischer Mittel angewandt. Zusätzlich wird mit diesem Verfahren die Dimensionsstabilität der Holzmatrix verbessert und eine attraktive Dunkelverfärbung erzielt. Andererseits kann sich dieses Verfahren jedoch negativ auf die mechanische Festigkeit und die Steifigkeit des Holzes auswirken. Deshalb müssen die Wärmebehandlungsparameter (wie zum Beispiel Maximaltemperatur, Aufheizrate, Dauer der ersten Temperaturstufe bei 100–120°C und der zweiten Stufe bei der Maximaltemperatur, Feuchte, Abkühlrate) optimiert werden, um die Vorteile einer Wärmebehandlung nutzen zu können ohne dabei die mechanischen Eigenschaften von Holz signifikant zu verschlechtern. Der Einfluss der Wärmebehandlungsparameter auf die Holzeigenschaften hängt nicht nur von der Holzart ab, sondern auch von den Wuchsbedingungen (Klima, Boden). In dieser Studie wird eine Wärmebehandlung von kanadischem Jack Pine Holz (Pinus banksiana) in einer mittelgroßen Prototypanlage vorgestellt. Ziel dieser Untersuchung war es, die Parameter bei der industriellen Behandlung zu optimieren. Zusätzlich wurde die Möglichkeit untersucht, die Verfahrensdauer zu verkürzen ohne die Holzqualität zu verschlechtern. Die Ergebnisse zeigen, dass eine Erhöhung der maximalen Behandlungstemperatur zu einer Verbesserung der Dimensionsstabilität von Jack Pine Holz sowie zu einer dunkleren Verfärbung führte. Dies hatte keinen Einfluss auf den Elastizitätsmodul, reduzierte jedoch die Biegefestigkeit von Jack Pine Holz. Mit einer geringfügigen Reduktion der Gasfeuchte während der Aufheizphase konnte die Trocknungszeit verkürzt und gleichzeitig die mechanische Festigkeit verbessert werden. Diese Verbesserung hilft, Energie zu sparen und die Produktivität zu steigern.

A Diffusion-based Model for Transient High Temperature Treatment of Wood:

A three-dimensional (3D) mathematical model describing simultaneous unsteady heat and moisture transfer between a gas phase and a solid phase during high temperature treatment of wood has been developed. The model is based on a diffusion equation with variable diffusion coefficients. The governing equations representing the heating process in a 3D rectangular object are discretized using an explicit finite-difference approach, and a computer code is developed to predict the temperature and moisture distributions inside the wood sample. The sample was subjected to high temperature treatment under different operating conditions. The model predictions are compared with experimental results obtained for temperature and average moisture content during high temperature treatment of birch wood. Satisfactory agreement is obtained over a range of heating conditions. A parametric study was also carried out to determine the effects of several parameters such as initial moisture content, heat and mass transfer coefficients, and the sample thickness on the temperature and moisture content distributions within the samples during heat treatment.

Evolution of Extractive Composition During Thermal Treatment of Jack Pine

Abstract The thermal treatment of wood has many benefits such as better dimensional stability and attractive dark color and does not use toxic chemicals. The resistance against biological decay can be improved when wood is not in contact with ground. On the other hand, after thermal transformation, wood becomes more fragile. The changes of the wood properties are related to the modification of the wood composition. During the thermal treatment, the evaporation of the moisture content is not the only event. Volatile extractives are evacuated from the wood, while new products and by-products of different chemical reactions appear. The comparison of the extracts obtained from untreated and treated wood can help to identify thermo-chemical reactions, taking place during the heat treatment. This article presents the analysis by Gas Chromatography–Mass Spectroscopy (GC-MS), High Performance Liquid Chromatography (HPLC), and Thin Layer Chromatography (TLC) of polar and non-polar extracts of untreated and heat-treated North American Jack pine (Pinus banksiana). The study of the impact of maximum heat treatment temperature on the composition of the Jack pine extracts showed that the major part of extractives leaves the wood under 200°C whereas most of the new products appear only above 200°C. While the extractives of the untreated Jack pine are dominated by non-polar components, the thermo-transformation seems to generate mainly polar compounds. However, presence of water vapor increases the portion of polar extractives in wood. Interestingly, an important decrease of concentration of phenolic compounds (such as pinosylvin, pinosylvin monomethyl ether, and pinobanksin) in Jack pine wood was observed between 160–200°C. On the other hand, 4-hydroxy-methylfurfural and vanillin have been identified as compounds generated by the heat treatment above 200°C. The identification of other by-products will be presented in a later paper.

Experimental and Numerical Investigation of Heat and Mass Transfer during High-Temperature Thermal Treatment of Wood

In this article, a three-dimensional mathematical model has been used to analyze the transient heat and moisture transfer during high thermal treatment of Aspen wood. The conservation equations for the wood sample are obtained using diffusion equation with variable diffusion coefficients and the three-dimensional incompressible Reynolds averaged Navier-Stokes equations have been solved for the flow field. Temperature distributions in wood and in gas as well as the moisture content of wood were measured during the experiments. Afterwards, hardness, modulus of elasticity, and modulus of rupture were measured. The experimental results and model predictions showed good agreement. Increasing heating rate is beneficial for the modulus of rupture and the modulus of elasticity heat treatment seems to increase for the range of parameters considered here. However, hardness of Aspen increases in the axial direction but does not change in the radial and tangential directions.

A high-temperature thermal treatment of wood using a multiscale computational model: application to wood poles.

The present study is devoted to a numerical study with experimental validation of the high-temperature thermal treatment of three-dimensional wood pole. During the heat treatment process, the heat and mass transfer takes place between the solid and the drying medium, and the moisture evaporation occurs within the solid due to the capillarity action and diffusion. The development of the model equations, taking into account both bulk phases and interfaces of the multiphase system is described, starting from the microscopic scale. Fundamental to this model is the ability to quantify the effects of key material and geometric properties of the pole. The three-dimensional and unsteady-state mathematical model equations are solved numerically by the commercial package FEMLAB for the temperature and moisture content histories under different treatment conditions. A detailed discussion of the computational model and the solution algorithm is given. Heat treatment was applied on the test samples in an oven for three final temperatures (180, 200 and 220 degrees C). A series of experimental tests aimed at determination of heat treatment schedules kinetics curves and the temperature and moisture profiles and there time evolution were carried out. A very good agreement between the experimental and predicted results was obtained, implying that the proposed numerical algorithm can be used as a useful tool in designing high-temperature wood pole treatment processes.

Numerical and experimental validation of the transient heat and mass transfer during heat treatment of pine wood

Abstract In the current work, the three-dimensional Navier-Stokes equations along with the energy and concentration equations for the fluid coupled with the energy and mass conservation equations for the solid (wood) are solved to study the transient heat and mass transfer during the heat treatment of wood. The model for wood is based on Luikovs approach and solves a set of coupled heat and mass transfer equations. The model equations are solved numerically for the temperature and moisture content histories under different treatment conditions. The simulation of the proposed conjugate problem allows the assessment of the effect of the heat and mass transfer within wood on the transfer in the adjacent gas, providing good insight on the complexity of the transfer mechanisms. To generate data for comparison, measurements of temperature and moisture content of wood samples in a thermogravimetric system were conducted under different operating conditions. It is shown that the predicted and measured values compared very favourably, implying that the proposed numerical algorithm can be used as a useful tool in designing high-temperature wood treatment processes.

Thermal treatment of electrical poles.

Thermal treatment is used to preserve the wood without any addition of any toxic chemicals. This process increases the dimensional stability and darkens the color of the wood. The improvement of the resistance to decay of wood by thermal treatment is also often suggested in the literature. However, some latest works contested if the durability of heat-treated wood is improved when it is used in contact with ground. The objective of this study was to investigate the possibility of thermally treating electrical poles which are larger compared to the standard wood lumber. One of the applications for thermally treated wood poles could be their use in environmental sensitive areas (along rivers, for example) as a replacement for untreated western red cedar (WRC) poles which are more expensive. Green and pre-dried red pine (Pinus resinosa) and jack pine (Pinus banksiana) poles, both with circular and square cross-sections, were heated to high temperatures under humid and inert atmosphere. Operating parameters such as maximum treatment temperature, maintenance time at this temperature, heating rate and gas humidity were varied in order to find most suitable treatment conditions for the poles. The tests showed that most of the cracks are formed during the drying process while thermal treatment only widened already existing cracks. The circular shape seems to promote crack formation during the drying period since the directional dependence of shrinking creates more stresses in circular poles compared to the square poles. A slight decrease in flexibility of the wood with increasing temperatures was observed. The protecting effect of gas humidity against oxidation of wood and the importance of the application of a moderate heating rate for poles with large cross-sections are also demonstrated in this article. The impact of the heat treatment on the resistance to decay of electrical wood pole will be presented in a future publication.