Heat-Treated Wood as a Substrate for Coatings, Weathering of Heat-Treated Wood, and Coating Performance on Heat-Treated Wood

Vlatka Jirouš-Rajković,Josip Miklečić

doi:10.1155/2019/8621486

Vlatka Jirouš-Rajković, Josip Miklečić

Open Access

https://doi.org/10.1155/2019/8621486

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Abstract

Heat treatment is a method of wood modification with increasing market acceptance in Europe. The major patented European commercial heat treatment processes have trade names ThermoWood, Platowood, Retiwood, Le Bois Perdure, and Oil-Heat-Treated Wood (OHT). To what extent modification of wood affects the resistance of wood to weathering is also an important aspect for wood applications, especially where appearance is important. Unfortunately, heat-treated wood has poor resistance to weathering, and surface treatment with coatings is required for both protection and aesthetic reasons. As a substrate for coating, heat-treated wood has altered characteristics such as lower hygroscopicity and liquid water uptake and changed acidity, wettability, surface free energy, and anatomical microstructure. Various wood species, heat treatment method, treatment intensity, and treatment conditions exhibited a different extent of changes in wood properties. These altered properties could affect coating performance on heat-treated wood. The reported changes in acidity and in surface energy due to heat treatments are inconsistent with one another depending on wood species and temperature of the treatments. This paper gives an overview of the research results with regards to properties of heat-treated wood that can affect coating performance and weathering of uncoated and coated heat-treated wood.

Highlights

Heat treatment is one of the wood modification methods for improving wood properties such as dimensional stability, water resistance, and biological durability without using harmful chemicals
Heat treatment or thermal modification is controlled pyrolysis of wood being treated at high temperatures between 180°C and 240°C under an oxygen-free atmosphere to avoid burning, involving either steam, nitrogen, or oil [1]
Different processes give different heat-induced chemical changes in wood [2]. e main commercial heat treatment processes in Europe are covered by patents, and wood products are treated under names such as ermoWood, Platowood, Retiwood, Le Bois Perdure, and Oil-Heat-Treated Wood (OHT) [3]

Summary

Introduction

Heat treatment is one of the wood modification methods for improving wood properties such as dimensional stability, water resistance, and biological durability without using harmful chemicals. Heat treatment or thermal modification is controlled pyrolysis of wood being treated at high temperatures between 180°C and 240°C under an oxygen-free atmosphere to avoid burning, involving either steam, nitrogen, or oil [1]. E main commercial heat treatment processes in Europe are covered by patents, and wood products are treated under names such as ermoWood, Platowood, Retiwood, Le Bois Perdure, and Oil-Heat-Treated Wood (OHT) [3]. Besides improved stability, reduced hygroscopicity, and dimensional changes, heat-treated wood has some shortcomings, such as loss of toughness, reduced tensile and bending strengths, unstable colour in exterior exposure, and appearance of surface cracking [14]. It has been established that the resistance of heat-treated wood against weathering (UV light and moisture changes) is not changed largely when compared to untreated wood, making a surface treatment with coatings necessary [15, 16]

Heat-Treated Wood as a Substrate for Coatings

Coating Systems for Heat-Treated Wood

Weathering of Uncoated Heat-Treated Wood

Performance of Coatings on HeatTreated Wood

Findings

Conclusions