Influence of Natural and Artificial Weathering on the Colour Change of Different Wood and Wood-Based Materials

Boštjan Lesar,Miha Humar,Davor Kržišnik,Nejc Thaler

doi:10.3390/f9080488

Abstract

The importance of the aesthetic performance of wood is increasing and the colour is one of the most important parameters of aesthetics, hence the colour stability of twelve different wood-based materials was evaluated by several in-service and laboratory tests. The wood used for wooden façades and decking belongs to a group of severely exposed surfaces. Discolouration of wood in such applications is a long-known phenomenon, which is a result of different biotic and abiotic causes. The ongoing in-service trial started in October 2013, whilst a laboratory test mimicking seasonal exposure was performed in parallel. Samples were exposed to blue stain fungi (Aureobasidium pullulans and Dothichiza pithyophila) in a laboratory test according to the EN 152 procedure. Afterwards, the same samples were artificially weathered and re-exposed to the same blue stain fungi for the second time. The purpose of this experiment was to investigate the synergistic effect of weathering and staining. The broader aim of the study was to determine the correlation factors between artificial and natural weathering and to compare laboratory and field test data of fungal disfigurement of various bio-based materials. During the four years of exposure, the most prominent colour changes were determined on decking. Respective changes on the façade elements were significantly less prominent, being the lest evident on the south and east façade. The results showed that there are positive correlations between natural weathering and the combination of artificial weathering and blue staining. Hence, the artificial weathering of wood-based materials in the laboratory should consist of two steps, blue staining and artificial weathering, in order to simulate colour changes.

Highlights

During their service life, buildings and building components are exposed to a wide variety of environmental conditions
Norway spruce was studied in more detail, as it is one of the most frequently used wood species in central Europe
Lightness values (L*) for south exposure exhibited the lowest change, being the most light-coloured after four years of exposure, and decking had the highest rate of change; the latter samples were the darkest of the group at the end of the exposure

Summary

Introduction

Buildings and building components are exposed to a wide variety of environmental conditions. For wood-based materials, moisture stress and biological factors like mould, blue-stain, and decay fungi are often critical, especially for cladding and decking applications in exterior use conditions, representing two commodities where wood is frequently used [1,2]. The aesthetic service life is often a decisive criterion for these applications [3,4]. The service life of different building products and commodities is determined by very different criteria, e.g., colour stability of coated or uncoated surfaces; cracking and checking; the occurrence of moulds, stain, or fungal decay; damage by insects or marine borers; resistance to abrasion and wear, Forests 2018, 9, 488; doi:10.3390/f9080488 www.mdpi.com/journal/forests

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