Abstract

The thermal modification of wood has become the most-commonly commercialised wood modification process globally, with the ThermoWood® process currently being the most dominant. As with all commercial processes, there is a need to have a robust quality control system, with several small–scale studies undertaken to date investigating quality control using a range of analytical methods, culminating in a multi-year assessment of colour as a means of quality control. This study, as an extension to this multi-year assessment, further explores the colour of Norway spruce and Scots pine commercially modified by the ThermoWood® S and D processes, respectively, along with the mechanical properties and structural characterisation by Fourier transform infrared (FT–IR) spectroscopy and principal component analysis (PCA) to ascertain further correlations between colour and other measurable properties. Infrared spectroscopy indicated modifications in the amorphous carbohydrates and lignin, whereas the use of PCA allowed for the differentiation between untreated and modified wood. Colour measurements indicated reduced brightness, and shifting toward red and yellow colours after thermal modification, hardness values decreased, whereas MOE and MOR values were similar for modified wood compared to unmodified ones. However, by combining the colour measurements and PC scores, it was possible to differentiate between the two modification processes (Thermo–S and Thermo–D). By combining the mechanical properties and PC scores, it was possible to differentiate the untreated wood from the modified ones, whereas by combining the mechanical properties and colour parameters, it was possible to differentiate between the three groups of studied samples. This demonstrates there is a degree of correlation between the test methods, adding further confidence to the postulation of using colour to ensure quality control of ThermoWood®.

Highlights

  • As a natural renewable resource, wood is in general a non-toxic, accessible, and inexpensive biomass–derived material

  • Wood modification has become the chosen method used to overcome some of its perceived weak points that are mainly related to low resistance to bio–deterioration against fungi, insects, termites, marine borers, moisture sensitiveness, low dimensional stability, low hardness and wear resistance, low resistance to UV irradiation and weathering in general, and aesthetic properties

  • Despite the advances in using colour as a key parameter for quality control of thermally modified wood [38,39], additional evidence to support quality control would enhance the commercial properties of products such as ThermoWood®

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Summary

Introduction

As a natural renewable resource, wood is in general a non-toxic, accessible, and inexpensive biomass–derived material. Wood modification has become the chosen method used to overcome some of its perceived weak points that are mainly related to low resistance to bio–deterioration against fungi, insects, termites, marine borers, moisture sensitiveness, low dimensional stability, low hardness and wear resistance, low resistance to UV irradiation and weathering in general, and aesthetic properties. This increased interest in wood modification has led to several key publications [1,2,3]. In parallel to the establishment of the ThermoWood® process, there have been other commercial developments across Europe, including the Netherlands [16,17,18], France [19,20,21], Germany [22,23] and Denmark [24]

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