Quality Control of Thermally Modified Timber Using Dynamic Vapor Sorption (DVS) Analysis

Miha Humar,Davor Kržišnik,Rožle Repič,Boštjan Lesar,Romana Cerc Korošec,Christian Brischke,Lukas Emmerich,Gregor Rep

doi:10.3390/f11060666

Abstract

The importance of thermal modification is increasing worldwide. Increased use of thermally modified timber (TMT) has resulted in a need for reliable quality control, comprising control of variation of the production within defined limits, allowing third-party control in the case of certification and the regulation of customer complaints and claims. Techniques are thus needed to characterise the modification of quality in terms of improved target properties of TMT during industrial production, and of TMT products that have been in service for an arbitrary time. In this study, we aimed to utilise dynamic vapor sorption (DVS) for this purpose. Norway spruce (Picea abies) and European beech (Fagus sylvatica) samples were thermally modified at different temperatures according to different heat treatment techniques: (1) the Silvapro process based on an initial vacuum; (2) an air heat treatment, whereby samples were wrapped in aluminium foil; (3) thermal modification of wood samples in the ambient atmosphere in a laboratory oven. Wood samples from closed processes were analysed for validation. TMT was characterised with respect to mass loss, colour and density. Mass loss of wood due to modification (MLTM) was correlated with factors derived from DVS analysis. The present DVS measurements suggest that the equilibrium wood moisture content (EMC95% RH), the time to reach 10% wood moisture content (t10% MC), and the elongation factor, c, derived from a logarithmic function, can serve as alternative parameters to characterise the quality of several thermal modification processes. Further studies are recommended using other wood species, different modification processes and further parameters gained from DVS measurements to understand the robustness and the predictive power of the applied technique.

Highlights

Wood in structural applications is often exposed to weathering and degrading organisms [1].It needs to be protected to slow the natural process of biodeterioration [2,3]
The experiments were performed with Norway spruce (Picea abies) and European beech (Fagus sylvatica), representing commonly used softwoods and hardwoods in Europe that are frequently subject to thermal modification
We primarily focused on the modification of wood in the open system and not on the material modified in the closed system

Summary

Introduction

Wood in structural applications is often exposed to weathering and degrading organisms [1]. It needs to be protected to slow the natural process of biodeterioration [2,3]. Biodegradation is fundamental and desirable, but when the wood is used for structural purposes, degradation needs to be inhibited [4]. Degradation was predominately controlled by the use of biocides [5]. The importance of wood preservatives has decreased significantly during the past two decades, not at least due to the changed attitude of consumers and the ban of the most. Forests 2020, 11, 666 common and effective preservatives, especially in Europe [6]. Thermal modification is the most important in terms of traded volumes.

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