1 Introduction
1.6 Problem statement and research aims
Many bio-based building materials are (to some extent) biodegradable, an excellent quality at the end of a material’s service life as it solves waste issues, but a less desirable feature during use. When an organic material is exposed to favourable moisture and temperature conditions as well as to degrading organisms, its functional and aesthetic service life can decrease. The risk of fungal decay depends on the environment in which a material is applied and the material resistance.
A key aspect is how specific environmental conditions affect mycelial growth.
Therefore, the following research aim is stated:
Aim 1. Assess the influence of temperature and relative humidity on fungal growth dynamics at mycelial level.
Most studies assessing the influence of environmental conditions on fungal growth rely on simple experimental set-ups where all but one environmental condition are fixed and the fungus grows on an optimal growth medium. Most of the studies assessing detailed fungal growth dynamics focus on small areas or the germination phase, tracking growth of a few hyphae not representative for the entire mycelium. In Chapter 2, our aim is to assess detailed fungal growth dynamics at mycelial level, including the mycelial area, number of tips and branching angle, at different environmental conditions. This chapter will build on the work of Guillermo Vidal-Diez de Ulzurrun on automated image-based analysis of spatio-temporal fungal dynamics (Vidal-Diez de Ulzurrun et al., 2015) A material’s resistance to fungal decay depends on the material’s natural durability and its hygroscopicity. Wood protection used to focus mainly on naturally durable wood species, often from tropical regions, or applying non-durable wood species that are treated with fungicidal wood preservatives. Due to the biocidal products directive (Directive 98/8/EC, 1998), many of the active ingredients of wood preservation products were banned in the EU. General awareness of the negative impact of biocidal
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products on the environment initiated a new way of thinking about wood protection.
Other material characteristics that could extend service life, such as a material’s moisture dynamics and structure, are gaining importance. These are especially interesting when it comes to bio-based building materials, as there are many opportunities to alter material structure and moisture dynamics of engineered wood products and bio-based insulation materials. Unfortunately, in-depth knowledge on the intricate relationship between material characteristics and fungal behaviour is still lacking.
To contribute to the abovementioned potential, the following four research aims focus on unraveling the influence of material chemistry, moisture dynamics and structure on fungal susceptibility:
Aim 2. Develop a method suited to assess the influence of material chemistry on fungal susceptibility.
Usually, assessment of fungicidal components is done by extracting components of interests (fatty acids, organic acids, terpenes, tannins, and other benzenoid compounds) from a wood sample and assessing the effect of the extractive on the development of pure fungal cultures growing in or on a nutrient solution or agar. While effective, extraction procedures are quite complex, requiring several steps and various solvents, specific to the type of component targeted. It also does not allow an assessment of the fungicidal components in their entirety, as only specific components are targeted.
Therefore, we aim to develop a method suited to assess the influence of fungicidal components, in their entirety, on the natural durability of a material in Chapter 3. The importance of material chemistry on the natural durability of 10 reference wood species, with durability classes ranging from I (Very durable) to V (Not durable), will be assessed. This method will also be used to assess the presence of fungicidal components in a selection of commonly used bio-based building materials.
Aim 3. Develop a method to assess the influence of RH and material moisture content on fungal decay in the presence of a moisture source.
The influence of moisture dynamics on the fungal susceptibility of wood, and the moisture dynamics of wood in general, has been a topic of great interest in wood research. Many efforts have been made to assess the minimum moisture threshold at which wood is at risk of fungal decay. Existing methods either assess the influence of RH on decay, without the presence of a liquid moisture source, or the level of decay in function of a moisture gradient, always at a RH of 100%, which is often insufficient and hence we aim to develop a method that is able to assess the influence of RH on decay in the presence of a moisture source, over time (Chapter 4). Whereas in Chapter 2 the influence of relative
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humidity on mycelial growth is assessed, this chapter discusses a method to assess the influence of relative humidity and moisture content on actual wood degradation. Wood samples are to be exposed to brown rot at relative humidity conditions ranging from 11-100%, with the prerequisite that the fungus has access to a moisture source.
Aim 4. Apply state-of-the-art methods to better understand material moisture dynamics of commonly used wood-based panels and wood fibre insulation products.
Since a material’s hygroscopicity affects the risk of fungal decay, substantial research has been done to assess and understand the moisture dynamics of solid wood, and especially thermally and chemically modified wood. However, research into the moisture dynamics of engineered wood products and bio-based insulation materials is limited. In Chapter 4, the materials will be exposed to liquid water in the so-called floating test, and water absorption and desorption will be measured over time to compare the materials’ moisture dynamics. Then, three state-of-the-art techniques will be applied to better understand the differences in moisture dynamics. Since Low-Field Nuclear Magnetic Resonance (LNFMR) spectroscopy has been successfully applied to interpret changes in pore size distributions and hydrophilicity of wood due to chemical and thermal modification (Beck et al., 2018; Cai et al., 2020), a similar approach will be applied here to determine differences in water distribution between various bio-based building materials. X-ray CT, a method used to visualize internal material structure in a non-destructive way, will be used to gather complementary information on the pore size distribution of the materials in dry state. Additionally, Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) absorbance spectra will be obtained to assess the hydrophilicity of the bio-based building materials.
Aim 5. Develop a method to assess the influence of material structure on moisture dynamics and progress of fungal decay.
The current knowledge on the impact of material structure on fungal decay is mostly related to hyphae interacting with specific wood anatomical features, such as pits. Observations are usually made with light microscopy or SEM, for which samples are taken from a fungal decay experiment for examination at high resolution. A non-destructive method that has successfully been applied to assess fungal activity over time is isothermal calorimetry, in which heat production due to biochemical processes is related with fungal activity.
Nevertheless, this technique does not offer any spatial information. To gain more insight into how a material’s structure and moisture properties affect the degradation process, we propose the use of X-ray CT. X-ray CT is a promising technique for fungal decay research as it is fast, non-destructive and provides 3D images of the internal structure of a material. It has been applied to assess
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decay before, yet by removal of samples from fungal cultures and drying before assessment with X-ray CT for easier analysis. Here, we aim to develop a method to non-destructively assess the degradation process with X-ray CT scanning. In that way, decay of individual wood specimens, which each have a unique anatomy, can be monitored over time.
Figure 1-13 Thesis outline
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