Proceedings IRG Annual Meeting (ISSN 2000-8953)
© 2020 The International Research Group on Wood Protection
IRG/WP 20-20666
THE INTERNATIONAL RESEARCH GROUP ON WOOD PROTECTION Section 2 Test Methodology and Assessment
Performance of bio-based building materials – durability and moisture dynamics
Liselotte De Ligne1, Jordy Caes1, Salah Omar1, Jan Van den Bulcke1, Jan M Baetens2, Bernard De Baets2, Joris Van Acker1
1 Ghent University (UGent), Laboratory of Wood Technology (UGent-Woodlab) – Dept. Environment, Coupure links 653, 9000 Ghent Belgium
2 Ghent University (UGent), Research Unit Knowledge-based Systems (KERMIT) – Dept. Data Analysis and Mathematical Modelling, Coupure links 653, 9000 Ghent Belgium
Paper prepared for the IRG51 webinar on Wood Protection 10-11 June 2020
IRG SECRETARIAT Box 5604 SE-114 86 Stockholm
Sweden www.irg-wp.com
Disclaimer
The opinions expressed in this document are those of the author(s) and are not necessarily the opinions or policy of the IRG Organization.
Performance of bio-based building materials – durability and moisture dynamics
Liselotte De Ligne1,a, Jordy Caes1b, Salah Omar1c, Jan Van den Bulcke1,d, Jan M Baetens2,e, Bernard De Baets2,f, Joris Van Acker1,g
1 Ghent University (UGent), Coupure links 653
2 Ghent University (UGent), Coupure links 653, 9000 Ghent Belgium
{a Liselotte.DeLigne, bJordy.Caes, cSalah.Omar, dJan.VandenBulcke, eJan.Baetens, fBernard.DeBaets,
gJoris.VanAcker}@UGent.be
ABSTRACT
When exposed to conditions favourable for decay, bio-based building materials can be susceptible to degradation. Their ability to withstand deterioration over time (performance) depends on the intrinsic or enhanced durability of the material as well as its wetting and drying behaviour. The effect of fungicidal components in wood is known since long. Other material characteristics, such as the material’s moisture dynamics and structure, are crucial as well in prolonging a material’s service life in outdoor exposure conditions. The importance of these other material characteristics should not be underestimated, as there are many opportunities to alter a material’s moisture dynamics and to optimize the structural design of engineered wood products and bio-based insulation products. In order to do so, it is necessary to understand how different material characteristics influence the performance. In this paper, we assess the moisture dynamics of oriented strand board (OSB), porous bituminized wood fibre board (PBF), radiata pine plywood (PL), thermally modified spruce (TMT) and two wood fibre insulation boards (WF-A and WF-B).
With the ‘paste test’, we assess whether these materials contain fungicidal components affecting decay. Additionally, we assess how they perform in an adapted mini-block test. We are able to show that fungicidal components are not always of major importance for the durability of a bio- based building material. Some of the assessed materials have a remarkable moisture performance.
We need to work towards specific moisture performance criteria and consider including them in performance classification.
Keywords: durability, material resistance, moisture dynamics, floating test, chemistry, structure 1. INTRODUCTION
The resistance of a bio-based building material against degradation by decay fungi, depends on its intrinsic or enhanced biological durability and its wetting and drying behaviour. For many wood- based products, often produced from non-durable wood, the moisture dynamics are a key element in the product’s performance. The importance of the latter has been acknowledged and a standardized method is under development to determine the wetting ability of wood and wood products (CEN TS 16818, 2018). The laboratory method proposed in standard CEN/TS 16818 was found to be a promising tool to estimate the outdoor moisture performance of solid wood (Brischke et al., 2014). In 2018, De Windt et al. published their extensive research on the moisture dynamics of uncoated plywood, and found good correlations between laboratory tests and outdoor exposure tests. The different plywood types were classified over Use class 1 to 4, allowing for recommendations on end use and service life based on moisture dynamics. Furthermore, Kržišnik
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et al. (2020) showed there was a clear correlation between the floating test values and outdoor moisture performance of wood applied vertically and as decking.
A product’s performance can be improved by changing the material characteristics, such as the material’s chemistry and structure. A material’s chemistry, either intrinsic or enhanced, can influence durability not only by having fungicidal effects, but also by inducing hydrophobicity.
Certain wood species, such as gaboon (Aucoumea klaineana Pierre) and sapele (Entandrophragma cylindricum Sprague), contain insufficient amounts of active ingredients to prevent decay. Instead, their high natural durability against the brown-rot fungus Coniophora puteana is accredited to their hydrophobic character (De Ligne et al., subm.). A material’s structure can influence performance directly. Wood anatomical structure, for instance, affects degradation and degradation patterns, as (the ratio of) different cell types influences the penetration and colonization ability of fungal hyphae, and determines whether the fungus can tap into the nutrient sources in the wood structure (Bravery, 1975; Daniel, 2007; Schwarze, 2007; Antwi-Boasiako and Atta-Obeng, 2009). Material structure can also affect material performance indirectly, by its influence on moisture dynamics. The moisture dynamics of plywood are, for instance, affected by the choice of top veneer and wood type (Li et al., 2016). For a more in-depth understanding of the influence of moisture dynamics on fungal susceptibility of wood, Brischke and Alfredsen (2020) suggest combining techniques from wood science, mycology, biotechnology, and advanced analytics, such as DVS, DSC, LFNMR, fungal transcriptome and secretome, microspectroscopy and chemometrics with sub-cell wall spatial resolution.
The application potential of engineered wood and bio-based insulation products can be increased by optimizing their structural design or by altering the material characteristics during the manufacturing process. In order to do so, it is necessary to understand how different material characteristics influence the material performance. In this paper, we assess whether the selected engineered wood and bio-based insulation products contain fungicidal components. Additionally, we performed an adapted mini-block test, to assess the durability of the selected materials in optimal moisture conditions. With the floating test (CEN/TS 16818), we aim to compare the moisture dynamics of the selected bio-based materials. We assess their moisture performance and investigate the potential for service life classification based on moisture performance.
2. MATERIAL AND METHODS 2.1 Materials
We made a selection of eight commonly used bio-based building materials. Table 1 gives an overview of the materials, their main components and density.
Table 1: Materials tested
Material Abbreviation Components and/or treatment Density (kg/m³) Oriented strand board OSB Scots pine fibres, PUR resin,
formaldehyde-free gluing 600
Porous bituminized wood
fibre board PBF Norway spruce/Scots pine fibres, bitumen
emulsion 270
Radiata pine plywood PL Radiata pine veneers, glue type: non-
specified 550
Thermally modified spruce TMT
Process: 1) Hydrothermolysis up to 170°C 2) drying 3) heated again to up to 180⁰C in dry conditions without oxygen
414
4
Wood fibre insulation board –
type A WF-A Norway spruce/Scots pine fibres, PUR
resin, paraffin 140
Wood fibre insulation board –
type B WF-B
Norway spruce/Scots pine fibres, PUR resin, paraffin, aluminium sulphate, silicates, hydrophobic products
270
Cellulose insulation Cell Cellulose fibres, boric acid, magnesium
sulphate 20-55
Scots pine (Pinus sylvestris
L.) Sp 550
2.2 Paste test
In the paste test (De Ligne et al., subm.), a bio-based material is grinded to a fine powder with a particle size smaller than 0.1 mm, thus limiting the influence of the material’s structure on fungal growth. By mixing the powder with agar and water, we ensure that the powder is water saturated, thereby also limiting the influence of hydrophobic components. The resulting paste is inoculated with a wood degrading fungus and mycelial growth is assessed over time with a flatbed scanner, after which the logistic growth rate for each paste is inferred. The growth rate on each bio-based material paste is represented relative to the growth rate of C. puteana on a pure malt agar paste (8.4 cm² day-1). The paste test thus gives an indication of how the nutritional value and fungicidal components influence the fungal susceptibility of a bio-based material.
2.3 Adapted mini-block test
The most widely used laboratory method for determining the natural durability of solid wood against wood-destroying fungi is the CEN TS 15083-1 test method (CEN TS 15083-1, 2015).
However, this standard is found to be inadequate for the qualification of new wood products, whose durability is not enhanced with biocides but by new technologies, such as chemically modified wood (acetylation, furfurylation, etc.), thermally treated wood, glue-laminated wood, wood-based panels and wood treated with water repellents (Candelier et al., 2016; Jones and Brischke, 2017; Kutnik et al., 2014; Ringman et al., 2014; Ormondroyd et al., 2015). A standard does exists for wood-based panels, which allows for bigger samples (5 cm x 5cm x panel thickness) to be tested (ENV 12038, 2002). For example, laboratory tests are performed using conditions optimal for the test fungus. A growth medium is used and serves two purposes: nutritional support for the fungus and to maintain a moisture content sufficient for fungal activity. If the wood does not reach the required moisture content, the test may be invalid according to the strict interpretation of the standard (CEN TS 15083-1: 2015). However, the moisture dynamics of engineered wood products and modified wood may have changed in such a way that abovementioned criteria are not met. For instance, thermal and chemical modifications change the wood-water interactions in such a way that the resulting equilibrium moisture content (EMC) of modified wood is lower than the equivalent non-modified wood (Candelier et al., 2016; Mai et al., 2010; Ormondroyd et al., 2015; Ringman et al., 2014). The modified wood samples might not moisten fast enough during the 16 weeks testing period for the fungus to reach significant degradation. It does not mean, however, that these materials cannot eventually become wet enough for degradation. Kamden et al. (2002), confirmed that after a period of six weeks exposure, the modified wood samples did become wet and noted moisture contents varying between 72% and 156%. We therefore need to contemplate whether the set-up of the standard needs to be adapted, for instance by prolonging the test duration or increasing the initial moisture content of the modified wood, or whether fungal degradation of modified wood needs to be observed under the same humidity conditions as for the non-modified wood (Ormondroyd et al., 2015). As a preliminary experiment, we decided to bring all samples to a moisture content of 20-30% MC, before exposure to brown-rot fungus Coniophora puteana.
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Coniophora puteana is grown in Petri dishes (diameter 9 cm), filled with 20 ml malt agar medium (3% malt, 2% agar). Mini-blocks (3 × 1 x 0.5 m³) of Scots pine sapwood, OSB, PBF, PL, TMT and WF were ovendried, weighed and sterilized under steam at 121°C. After sterilisation, the mini- blocks were placed in a vacuum dessicator and water saturated under vacuum. The mini-blocks are then removed from the dessicator to condition in the laminar flow. The mini-blocks are regularly weighed until a MC in the range of 20-30% MC is reached. Then, they are wrapped in aluminium foil and stored in the fridge until the start of the experiment. At the start of the experiment, the mini-blocks are taken out of the fridge and all MCs are checked, before placing the mini-blocks on the fungal cultures. After 8 weeks of exposure to C. puteana, the mini-blocks are cleaned, weighed, oven dried and weighed again, to determine the MC at the end of the test and the mass loss due to fungal degradation.
2.4 Floating test (CEN/TS 16818)
With the floating test (CEN/TS 16818), the water uptake and moisture release of materials is determined in a laboratory setting. The edges of the specimens (5 x 5 x 2.5 cm³) are sealed, to prohibit water from entering through the sides. Five specimens per material are placed in a conditioning room (65%RH, 20°C) for two weeks and weighed (mi, in g). Containers with demineralized water are placed in a conditioning room (65%RH, 20°C). The samples are placed on the water surface for 144 hours and weighed (mx) at several time intervals (5s, 30s, 1min, 10min, 5min, 10min, 30min 1h, 4h, 8h, 24h, 48h, 72h, 144h) during the absorption cycle. We decided to have a high temporal resolution at the beginning of the experiment, as some of the selected materials take up water very fast. During the desorption cycle, the materials are put on drying racks and are weighed again after 1h, 4h, 8h, 24h, 48h, 96h and 168h to assess the desorption rate. After 168 hours of desorption, the samples are ovendried at (103 ± 2)°C and weighed again (m0).
The water uptake W (g/m²) is calculated over time, using Eq. (1), with A being the test surface area in m² and mi the initial mass of the sample.
W =𝑚𝑥− 𝑚𝑖
A (1)
Absorption and desorption curves are fitted based on Eq. (2) and (3) respectively, as described in Van Acker et al. (2014).
𝑓(𝑡) = 𝑎𝑡𝑏 (2)
𝑓(𝑡) = 𝑎 + 𝑏𝑒−𝑡𝑐 (3)
The residual moisture content (rm) is calculated after 168 hours of desorption (Eq. 5), with MC168, being the moisture content (Eq. 4) after t hours desorption and MC0 the initial moisture content of the sample. Additionally, the residual moisture content is calculated after 48h (rm48) and 96 hours (rm96) of desorption, for better comparison with literature data.
MC = 𝑚𝑥− 𝑚0
𝑚0 (4)
rm168 = MC168− MC0 (5)
The residue (res168) represents the moisture left in the test specimen after 168 h desorption as a percentage of the absorbed moisture after 144 h, and is calculated using Equation (6). Additionally, the residue is calculated after 96 hours (res96) of desorption.
6 res168 = rm168
MC144−MC0 (6)
The absorption and desorption phases are classified (Table 1) based on the classes proposed by Van Acker et al. (2014). For desorption, the water release (g/m²) at 96 and 168 hours of desorption is included, since 144 hours of desorption was not included as a weighing moment in the desorption phase.
Table 2: Classification of absorption and desorption based on water uptake and release after 144 h (from Van Acker et al., 2014).
Class upper limit Floating test (g/m²) Absorption Desorption
1 750 250
2 950 400
3 1150 500
4 1350 600
5 1750 750
6 2750 1000
7 5000 2000
8 ∞ ∞
3. RESULTS AND DISCUSSION
3.1 Assessment of fungicidal properties – paste test
Fungal growth on the material pastes is presented relative to the median growth rate of C. puteana growing on a paste of pure malt agar without wood powder (8.7 cm² day-1). Coniophora puteana grows faster on a paste of porous bituminized wood fibre board (PBF), wood fibre insulation board (WF-A and WF-B), and thermally modified spruce (TMT) than on a pure malt agar paste (Figure 1). For these materials, the powder has a growth promoting effect on C. puteana. This is a typical phenomenon, also found on pastes of beech sapwood, where the growth rate is more than twice as fast as on pure 2% malt agar medium (De Ligne et al., subm). Growth on radiata pine plywood (PL) is similar as on pure malt agar, while for oriented strand board (OSB), the growth rate is 20%
lower. However, these lower growth rates are most probably because the mycelium grows much denser on these materials and not due to a growth-inhibiting effect. Typical display of a growth- inhibiting effect on the mycelial development of C. puteana can be seen on a paste of cellulose insulation material (Cell), which contains boric acid (Figure 2, b). The difference in growth rate between WF-A and WF-B is probably due to a growth-decelerating effect caused by aluminium sulphate. It does not, however, impede growth that much, as the growth rate is still higher than on OSB and PL and the Petri dish is completely covered by mycelium at the end of the experiment (Figure 2, h). We therefore conclude that fungicidal components do not play a significant role in the material resistance of OSB, PBF, PL, TMT, WF-A and WF-B.
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Figure 1: Growth rate of C. puteana growing on pastes of cellulose insulation (Cell), oriented strand board (OSB), porous bituminized wood fibre board (PBF), radiata pine plywood (PL), thermally modified
spruce, wood fibre insulation type A (WF-A) and B (WF-B), relative to the growth rate of C. puteana on a paste of pure malt agar (8.4 cm² day-1). The red dotted line corresponds to the median growth rate on
malt agar medium without wood powder (8.7 cm² day-1).
Figure 2: Mycelium of C. puteana 9 days after inoculation on pastes of a) pure malt agar b) Cellulose insulation c) OSB d) PBF e) PL f) TMT g) WF-A h) WF-B
3.2 Assessment of degradation - adapted mini-block test
After 8 weeks of degradation by C. puteana, all samples were degraded up to 30% mass loss and more, except for thermally modified spruce (Figure 3). The results for OSB correspond with the findings of Amusant (2009) and Fojutowksi (2009), who found mass losses of 20-45% for different OSB-panels after degradation by C. puteana in a ENV12083 test set-up. With the paste test (see Section 3.1), we already showed that the bitumen emulsion in PBF is not fungicidal for C. puteana.
In the mini-block set-up, the fungus does not seem to be affected by the bitumen emulsion either, although the amount of mass loss is lower than for the wood fibre insulation board, type A (WF- A). Possibly, this is related to a lower amount of wood fibres (presence of a bitumen fraction), but could be impacted by accessibility as well. The radiata pine plywood (PL) was severely degraded as well. Do note that in this mini-block set-up, the edges of the plywood were not sealed and also
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the sample size did not allow for the glue layers to have much impact on the degradation process.
When plywood edges are sealed, glue layers have been shown to significantly restrict decay progress (Van den Bulcke et al., 2011). The moisture content of the thermally modified spruce samples (TMT) ranged from 25 to 30 % MC at the start of the experiment and had increased up to 60-90% MC after 8 weeks. Although similar moisture contents led to 30% mass loss for Scots pine, the TMT showed mass losses of only 3%. From the paste test (Section 3.1), we know that there is no influence of fungicidal components. It seems that, although the moisture content is sufficiently high, the fungus is not able to degrade the thermally modified wood.
A mini-block test set-up is not suited for assessing the durability of all wood-based panel types against Basidiomycetes, as demonstrated by the plywood samples for which the size is too limited to allow for the glue-layers and structure to have an influence on the degradation process. It does show, however, that these materials are not durable when they become and remain water saturated and when the hyphae can enter from the sides. In the case of modified wood, bringing the samples to a MC of 20-30% did not make the samples susceptible to decay by C. puteana. Since thermal modification influences the equilibrium moisture content by limiting the amount of water that can bind to the cell wall, it is expected that the water in the thermally modified wood samples is merely present as capillary or loosely bound water. It would be interesting to find out whether the fungus is able to degrade modified wood using capillary or loosely bound water, although probably slowly, and whether a longer duration of wetting would make the modified wood reach a minimum threshold of cell wall bound water that does allow for decay to occur.
Figure 3: Mass loss and moisture content of Scots pine heartwood (Sp), oriented strand board (OSB), porous bituminized wood fibre board (PBF), radiata pine plywood (PL), thermally modified spruce
(TMT) and wood fibre insulation – type A (WF-A) after 8 weeks of degradation by C. puteana.
3.3 Assessment of moisture dynamics - floating test
For each material, the water uptake over 144 hours of absorption and the water release over 168 hours of desorption (g/m²), as well as the moisture content over time are presented in Fig. 4.
Various indicators of outdoor moisture performance are calculated, with the classification and the parameters of fitted absorption and desorption curves in Table 3 and the residual moisture contents and residues in Table 4.
Thermally modified spruce (TMT) and porous bituminized wood fibre board (PBF) perform very well on the moisture performance indicators. TMT is classified as Absorption class 1, PBF as 2 based on the water uptake after 144 hours of absorption, indicating a low amount of water uptake during the absorption phase (Table 3). These values correspond to those found by Van Acker et al. (2014) for ipé, teak, walaba and thermally modified spruce, poplar and pine wood. Additionally, TMT and PBF have low c-values for desorption, indicating that they are fast-drying materials. The residue (res) represents the moisture content left in the sample after desorption as a percentage of
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the absorbed moisture after 144 h. TMT retains 18.8% of the absorbed moisture after 168 hours of desorption, while PBF retains only 12.0%. However, TMT takes up less water (g/m²) in total than PBF during absorption (Fig. 4, Table 3). The residual moisture content (rm) for TMT and PBF after 48 hours is below 2.5%. De Windt et al. (2018) classified plywood with a rm below 2.5%
after 72 hours of desorption, as a product that is expected to have a long service life in Use class 1 to 3, and even 4. Assuming that the same criteria are applicable to TMT and PBF, our results indicate that these particular products can be used up to Use class 3.2 (above ground, exposed to prolonged wetting conditions). The durability of PBF against basidiomycetes is low (see Section 3.2), so it should not be recommended in water or ground contact (Use class 4). Nevertheless, it is a good example of how a material can have a good resistance in practice, because of its moisture dynamics.
The OSB product assessed in this paper has an absorption classification of 6 (Table 3), indicating a high amount of water uptake, while it classifies better on desorption. The residue indicates that 25.7% of the absorbed moisture is retained after 168 hours of desorption. The rm is 5.7 after 96 hours of desorption. If we were to follow the recommendations for plywood, this would qualify this OSB product for use in Use class 1 (dry conditions) for a long period of time, or Use class 2 (covered) for a shorter period of time, based on only the moisture dynamics.
The radiata pine plywood (PL) product assessed in this paper has an rm of 11.1 after 96 hours of desorption. Based on the plywood recommendations on end use of De Wind et al. (2018), this particular plywood type is qualified for Use class 1 (dry conditions) for a long period of time, or Use class 2 (covered) for a shorter period of time, based on the moisture dynamics only. It has an absorption classification of 7 (Table 3), indicating a high amount of water uptake during absorption. The desorption classification is 7 after 96 hours of desorption, while only 4 after 168 hours. The residue indicates that 25.7% of the absorbed moisture is retained after 168 hours of desorption.
Two types of wood fibre insulation boards (WF) were tested. WF-A and WF-B originate from the same production plant, using wood fibres from Norway spruce and Scots pine as a basis for wood fibre insulation boards. In addition to PUR resin and paraffin, WF-B contains silicates and hydrophobic products and has a higher density (Table 1). These factors clearly have an influence on the moisture dynamics. WF-B absorbs much less water than WF-A (Absorption class 3 versus 8), and dries out faster (desorption parameter c of 19 versus 162). WF-B has a rm lower than 2.5%
after 48 hours, therefore presumably suited for Use class 3.2. WF-A should only be used in Use class 1 for a long period of time or Use class 2 (covered) for a shorter period of time, based on only the moisture dynamics.
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Figure 4: Weight gain in g/m² (up) and moisture content (down) over 144 hours absorption and 168 hours desorption in a floating test.
Table 3: Classification, values at 144 hours of absorption and 96 and 168 hours of desorption, and parameters of fitted curves for absorption and desorption phases. An x indicates when fitting (f(t)=a*t^b)
was unsuccessful.
Absorption Desorption
Class g/m² 144h
f(t)=a*t^b Class 96h/168h
g/m² 96h
g/m² 168h
f(t)=a+b*exp(-t/c)
a b a b c
OSB 6 1863 115 0.57 5/3 624 476 458 1357 42
PBF 2 873 330 0.20 1/1 111 105 117 729 8
PL 7 4307 936 0.3 7/4 1037 591 463 3774 51
TMT 1 582 59 0.46 1/1 117 109 117 463 8
WF-A 8 5413 1021 0.37 7/1 1486 194 -2775 8028 162
WF-B 3 1140 x x 1/1 182 166 185 898 19
Table 4: Residual moisture content (rm) after 48, 96 and 168 hours of desorption, and residue (res) after 96 and 168 hours of desorption.
rm48 (%) rm96 (%) rm168 (%) res96 (%) res168(%)
mean std mean std mean std mean std mean std
OSB 8.05 2.23 5.74 1.62 4.37 0.96 33.44 4.17 25.69 2.93
PBF 2.07 0.13 1.81 0.11 1.71 0.06 12.78 0.79 12.04 0.56
PL 20.98 4.37 11.12 2.05 6.34 0.67 24.44 3.09 14.2 2.69
TMT 2.25 0.2 2.03 0.14 1.89 0.12 20.23 1.85 18.81 1.35
WF-A 20.64 4.91 9.52 3.93 1.24 0.2 26.98 10.2 3.58 0.54
WF-B 2.35 0.67 1.49 0.54 1.37 0.48 16.87 8.14 15.45 7.3
11 3. CONCLUSIONS
Fungicidal components do not play a significant role in the durability of the selected engineered wood products and insulation materials. However, there are other material characteristics that influence the performance of these materials in service. The floating test and related moisture performance indicators provided us with useful insights into the moisture dynamics of bio-based building materials. It is clear that thermally modified spruce, wood fibre insulation board containing hydrophobic products and porous bituminized wood fibre board have a remarkable moisture performance. This quality should not be overlooked. We would therefore like to stress the need to work towards specific moisture performance criteria, allowing performance classification towards service life in relation to use classes, so these materials can be applied in optimal conditions.
5. ACKNOWLEDGEMENT
The authors gratefully acknowledge Nolan Bonne, Piet Dekeyser, Judith Stecklina and Stijn Willen for their contributions to this project. This work was carried out with the financial support from the Research Foundation Flanders (FWO SB grant 1S53417N).
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