Cellulose Structural Changes during Mild Torrefaction of Eucalyptus Wood.

Ana Lourenço,Jorge Gominho,Dmitry Evtuguin,Solange Araújo

doi:10.3390/polym12122831

Abstract

The changes in the cellulose structure of eight Eucalyptus species (E. botryoides, E. globulus, E. grandis, E. maculata, E. propinqua, E. rudis, E. saligna and E. viminalis) in a mild torrefaction (from 160 °C to 230 °C, 3 h) were studied in situ and after cellulose isolation from the wood by solid-state carbon nuclear magnetic resonance (13C NMR), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR) and by analytic pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS). Changes in molecular weight were assessed by viscosimetry. A small decrease in cellulose crystallinity (ca. 2%–3%) was attributed to its amorphization on crystallite surfaces as a result of acid hydrolysis and free radical reactions resulting in the homolytic splitting of glycosidic bonds. The degree of the cellulose polymerization (DPv) decreased more than twice during the heat treatment of wood. It has been proposed that changes in the supramolecular structure of cellulose and in molecular weight during a heat treatment can be affected by the amount of lignin present in the wood. The limitations of FTIR and Py-GC/MS techniques to distinguish the minor changes in cellulose crystallinity were discussed.

Highlights

Wood is a natural complex of plant polymers, constituted mainly by cellulose, lignin and hemicelluloses, which provide it with unique physical properties
A summary of the chemical analyses of the eight Eucalyptus species showed that the heat treatment promotes an average mass loss of ca. 11% among the woods, with the major loss being observed for the E. propinqua, where the loss was as higher as 12%, followed by E. viminalis and E. saligna (Table 1)
The results of this study showed a small decrease in the crystallinity of cellulose in eucalyptus woods, during the mild torrefaction performed in the range of 160–230 ◦ C (3 h), within the trend of Thermowood® technology

Summary

Introduction

Wood is a natural complex of plant polymers, constituted mainly by cellulose, lignin and hemicelluloses, which provide it with unique physical properties. Wood has increased its consumer value due to being a sustainable natural resource, some of its shortcomings, such as high hydroscopicity, cause dimensional instability of wood-made products [1]. Another weak point of wood as a material is its biodegradability, i.e., susceptibility to either biotic or abiotic factors [2,3,4], creating some constrains/limitations when applying woody materials outdoors. Anaerobic heat treatment has been known since the 1920s as an alternative method of wood protection, providing a radical improvement in the dimensional stability of heat-treated wood [6,7]

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