Resistance of thermally modified and pressurized hot water extracted Scots pine sapwood against decay by the brown-rot fungus Rhodonia placenta

Michael Altgen,Suvi Kyyrö,Olli Paajanen,Lauri Rautkari

doi:10.1007/s00107-019-01482-z

Abstract

The thermal degradation of wood is affected by a number of process parameters, which may also cause variations in the resistance against decay fungi. This study compares changes in the chemical composition, water-related properties and decay resistance of Scots pine sapwood that was either thermally modified (TM) in dry state at elevated temperatures (≥ 185 °C) or treated in pressurized hot water at mild temperatures (≤ 170 °C). The thermal decomposition of easily degradable hemicelluloses reduced the mass loss caused by Rhodonia placenta, and it was suggested that the cumulative mass loss is a better indicator of an actual decay inhibition. Pressurized hot water extraction (HWE) did not improve the decay resistance to the same extent as TM, which was assigned to differences in the wood-water interactions. Cross-linking reactions during TM caused a swelling restraint and an effective reduction in moisture content. This decreased the water-swollen cell wall porosity, which presumably hindered the transport of degradation agents through the cell wall and/or reduced the accessibility of wood constituents for degradation agents. This effect was absent in hot water-extracted wood and strong decay occurred even when most hemicelluloses were already removed during HWE.

Highlights

Most European wood species have a low resistance against the decomposition by decay fungi, which shortens their service life in exterior applications
The decrease in hemicellulose sugars and the increase in lignin as a function of the mass loss by heat treatment (HT) was larger after Thermal modification (TM) than after hot water extraction (HWE)
Since the samples were subjected to a water-leaching procedure prior to the determination of the dry mass after HT, residual degradation products in the wood did not cause an underestimation of the mass loss by HT

Summary

Introduction

Most European wood species have a low resistance against the decomposition by decay fungi, which shortens their service life in exterior applications. This has been overcome by impregnating wood products with preservatives that are toxic to fungi. The underlying modes of action in increasing the wood’s resistance against fungal decay by heat exposure as well as the link between these modes of Thermal modification (TM) of solid wood explicitly aims at improving the decay resistance to enhance the service life of wood products. Pressurized hot water extraction (HWE) of wood does not aim at improving the decay resistance, but at removing hemicelluloses as substrates for bioethanol production (Kemppainen et al 2012) or for conversion into high-value added products (Ebringerová and Heinze 2000). The solid residue can be used as a feedstock for the pulp and paper production (Yoon and Van Heiningen 2008), but may be utilized as construction material and in engineered wood products, such as oriented strand boards (Sattler et al 2008), particleboards (PelaezSamaniego et al 2014), or wood polymer composites (Hosseinaei et al 2012)

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Results

Conclusion