Elsevier

Bioresource Technology

Volume 144, September 2013, Pages 669-674
Bioresource Technology

Short Communication
Effects of thermal treatment on chemical, mechanical and colour traits in Norway spruce wood

https://doi.org/10.1016/j.biortech.2013.06.110Get rights and content

Highlights

  • Total colour difference, the content of lignin and extractives increased with the temperature.

  • DP of cellulose, modulus of rupture, and lightness difference decreased with the treatment.

  • Temperatures ranging from 20 to 187 °C formed a compact cluster in the multivariate space.

  • Chemical, mechanical and colour traits of thermally modified wood showed close relationships.

Abstract

In several different branches of the wood industry heat treatment is a growing application as it changes the chemical, mechanical, physical and biological properties of wood. Investigations using wet chemical analyses, Fourier transform infrared spectroscopy, size exclusion chromatography, and CIELab colour system have been conducted to study the changes in Norway spruce wood subjected to temperature up to 270 °C over a 30 min time period. The results showed that mass loss (ML), total crystallinity index (TCI) of cellulose, total colour difference (ΔE), and the content of lignin and extractives increased with the temperature, whereas degree of polymerization (DP) of cellulose, modulus of rupture (MOR), modulus of elasticity (MOE), lightness difference (ΔL), and the content of holocellulose, cellulose and hemicelluloses all decreased with the thermal treatment. Relationships between temperature and the examined wood traits were all fitted by exponential curves. Power law relationships were found to fit the trends for DP of cellulose with ΔE, ΔL, and TCI of cellulose. Also found were power law regressions for the content of hemicelluloses with MOE, MOR, ΔL, and ML. Temperatures ranging from 20 to 187 °C formed a compact cluster, clearly separated from the higher examined temperatures in the multivariate wood trait space.

Introduction

The effect of thermal treatment on wood is studied for two main reasons. The first one is that thermal modification improves wood durability, dimensional stability, resistance to fungi, and also other properties of the wood. The second reason is to examine changes in the properties of wood when it is exposed to fire, in particular, mechanical properties, since wood is used extensively as a construction material.

Windeisen et al. (2009) and Poletto et al. (2012) have provided an explanation for the decomposition of cellulose, hemicelluloses, lignin and extractives, and their resulting influence on heat-treated wood properties. Relationships between chemical changes and the mechanical and physical traits of heat-treated wood have been examined in several studies. Some authors have found several close relationships including those between total colour changes and mechanical traits of wood (Bekhta and Niemz, 2003); weight loss and the heating period, as well as weight loss and MOR (Kim et al., 1998); the content of lignin or cellulose and compression strength (Yildiz et al., 2006); plus the degradation of hemicelluloses, ramification of lignin, cellulose crystallization and the mechanical traits of thermally treated wood (Kocaefe et al., 2008). Alternatively, some results have shown strong correlations between selected physical and chemical traits for hardwood samples, whereas only weak correlations have been established for softwoods (Niemz et al., 2010).

The literature, however, lacks more comprehensive studies of the chemical, mechanical and optical traits of thermally treated wood, and their mutual linkages. Thus, the objective of this study was to determine changes in the chemical, mechanical and colour traits of Norway spruce wood after thermal treatment, and their mutual relationships.

Section snippets

Heat treatment

Test samples (8 groups of 30 samples with dimensions 8 × 10 × 120 mm) sawn from a mature Norway spruce (Picea abies L.) wood, were thermally treated in an oven at eight temperatures (113, 134, 158, 187, 221, 237, 253, and 271 °C) for 90 min at atmospheric pressure in the presence of air. One group of control samples was left untreated (denoted 20 °C) and compared to thermally modified samples.

Chemical analyses

Separated wood was mechanically disintegrated into sawdust, and a fraction size of 0.5–1.0 mm was extracted in

Chemical changes

The results of chemical analyses (Table 1) showed an increase in the amount of Klason lignin, and a decrease in the content of cellulose, holocellulose and hemicelluloses with an increase in loading temperature. A noticeable mass loss (ML) at temperatures above 220 °C suggested there was intensive decomposition of the wood matter. The ML of wood is one of the most important features in heat treatment, and it is commonly referred to as an indicator of quality (Esteves and Pereira, 2009). Popescu

Conclusions

Heat treatment of Norway spruce wood resulted in an increase of ML, TCI of cellulose, ΔE, and the content of lignin and extractives, as well as a decrease of DP of cellulose, MOR, MOE, ΔL, and the content of holocellulose, cellulose and hemicelluloses. Power law relationships were found to fit the trends for DP of cellulose with ΔE, ΔL, and TCI of cellulose. Also found were power law regressions for the content of hemicelluloses with MOE, MOR, ΔL, and ML. Chemical, mechanical and colour

Acknowledgements

The authors thank professor D. Gömöry and Dr. V. Vacek for statistical advice, Dr. I. Čaňová for technical assistance, and Mrs. E. Ritch-Krč for language revision. This work was supported by funding from the Slovak scientific grant agency VEGA, Grant No. 1/0446/12.

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