Abstract
This study aimed to investigate the effect of the thermal treatment technique under two conditions-autoclave combined with electric oven and electric oven only-on the hygroscopic properties of wood from Pinus taeda L. and Eucalyptus grandis W. Hill ex Maiden, comparing them with results obtained for untreated wood. Three trees at age 25 years were sampled, with boards being removed at DBH level in order to make specimens 2.5 x 2.5 x 5.0 cm in size in the radial, tangential and axial direction respectively. In the combined treatment, specimens were subjected to thermal treatment in an autoclave set at 130°C/± 3°C and kgf/cm² pressure for 3 hours, then to a conditioning period, followed by heat in an electric oven set at 160°C/±1°C for the same time. Properties being evaluated included water absorption rate, volumetric swelling, water repellency effectiveness and anti-swelling efficiency. Results revealed that the thermal treatments being adopted were effective in reducing hygroscopicity and in increasing the dimensional stability of both species, revealing also that the combined autoclave and electric oven treatment provides best results.
Highlights
Due to some peculiarities, such as dimensional movement, use of wood is restricted to specific applications
In an attempt to break new ground in research about methods of applying heat to wood, this study aims to evaluate the effect of thermal treatment using autoclave and electric oven combined and using electric oven only on the hygroscopic properties of wood from Pinus taeda and Eucalyptus grandis
The pinewood samples had a large number of cracks from the very end of cycle 2 of water immersion and subsequent oven drying, precluding the continuity of observations
Summary
Due to some peculiarities, such as dimensional movement, use of wood is restricted to specific applications. Modifying processes are designed to improve wood properties, that way restricting water exchange between the wood material and the surrounding environment. These modification processes can be divided into four types: chemical, thermal, surface and impregnation, among which thermal modification is the method that has evolved the most in recent years in commercial terms, mainly due to the low cost involved in the process. The underlying technique is to use heat in the modifying processes (‘thermal rectification’), which consists in subjecting wood samples to a range of 120°C to 200°C, from high drying temperature to torrefaction temperature, that way promoting considerable chemical changes in cellulose, hemicellulose and lignin polymers and obtaining a differentiated material similar to wood in normal conditions, yet with lower hygroscopicity but higher dimensional stability and biological resistance.
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