Abstract

For purposes of this discussion, mass loss refers to loss due to fungal degradation. Thermal modification improves advantageous properties in wood, including an attractive darker color, reduced equilibrium moisture content, and degradation of water-binding hemicelluloses (Sinoven et al. 2002, Hakkou et al. 2005, Repellin and Guyonnet 2005, Kocaefe et al. 2008). The result is a product with increased moisture resistance, decreased swelling and shrinkage, and increased resistance to biological degradation (Syrjänen and Kangas 2000). Thermal treatment has also been reported to be the most environmentally sound method for increasing the dimensional stability of wood (Olarescu et al. 2014).

The increased resistance to biological decay is potentially due to the loss of hemicelluloses and other sugars (Ibach 2010), reduced water absorption, release of extractives with antifungal properties, generation of modified wood polymers that become unrecognizable to fungal enzymes, and a decrease in cell wall porosity that retards the penetration of fungal enzymes (Lekounougou et al. 2009).

Previous soft-rot durability results with ash, oak, beech, pine, spruce, and fir utilizing a closed, pressurized thermal modification process showed mass loss reductions ranging from 13 to 97 percent, depending on temperature, when subjected to 32 weeks of soil contact exposure (Ohnesorge et al. 2009). The investigators also reported a maximum 10 percent mass loss for pine thermally modified at 180°C when subjected to a brown and white rot fungi monoculture test. In addition, when thermally modified at 180°C, all species had increased durability sufficient to reach at least Durability Class 2 (Durable) when classified according to CEN/TS 15083-1 (British Standards Institution 2005).

For basswood thermally modified at 210°C, Donahue et al. (2011) reported a drop in mass loss of 22 and 60 percent compared with unmodified basswood exposed to Trametes versicolor and Gloeophyllum trabeum fungi, respectively. A study by Santos and Del Menezzi (2012) revealed no durability improvement against T. versicolor for tropical pine boards undergoing thermomechanical treatments. Resistance to termites and other insects has not been proven. Long-term ground-contact applications are not recommended owing to severe losses in mechanical strength.

While there is an increasing amount of durability data for thermally modified solid wood, there is limited information on the impacts of thermal modification processing, especially using the closed, pressurized process as used in this study, on the performance of engineered wood products (Aro et al. 2014). Therefore, the objective of the current study was to investigate the impacts that thermal modification processing has on the durability of commercial plywood, oriented strand board (OSB), laminated strand lumber (LSL), and laminated veneer lumber (LVL).

Donahue and Aro (2010) found that OSB panels thermally modified at 190°C had 6.5, 24.1, and 39.7 percent improvements in width, length, and thickness swell, respectively. There was a 6.2 percent reduction in modulus of rupture but larger decreases in split resistance and tensile strength perpendicular to the surface. Others discovered improved swelling and water absorption properties as well as increased tensile strength for particleboard panels made from thermally modified Scots pine and Norway spruce chips (Boonstra et al. 2006). Similar results were found by Borysiuk et al. (2007). For OSB made from Cupressus glauca, Okino et al. (2007) found that thermal treatment caused a significant decrease in the mass loss for the fungi Pycnoporus sanguineus and G. trabeum while remaining almost unchanged for Ganoderma applanatum and Neolentinus lepideus. Mechanical properties were reduced for board bonded with 8 percent urea-formaldehyde resin, while boards bonded with 5 percent resin were not significantly different.

Chotchuay et al. (2008) studied oriented strand lumber from parawood strands thermally modified at 190°C and found that tensile strength parallel to the grain (36 MPa), compression (39 MPa), and edgewise bending (61 MPa) were significantly higher than untreated controls. There was no significant difference in compression parallel to the grain or internal bond strength.

Poncsak et al. (2007) studied laminated lumber from bonded thermally modified yellow poplar, Scots pine, jack pine, and aspen lamellas. Shear strength of most samples was reduced 30 to 50 percent, but Scots and jack pine had only moderate decreases of 5 and 11 percent, respectively. The interfacial bonding for jack and Scots pine was much stronger than yellow poplar and aspen. In addition, Sernek et al. (2007) studied thermally modified spruce lamellas and found no significant decrease in shear strength. Also, shear strength did not vary significantly with treatment temperature. More results are described in Aro et al. (2014).

Methods and Materials

Material preparation

The plywood, OSB, LVL, and LSL utilized in this study were obtained from commercial sources. In addition, a laboratory-manufactured OSB made from sapwood or heartwood eastern larch (OSB-EL) was included. Details for the materials are given in Table 1.

Table 1 List of materials used in the study.^a

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Prior to thermal modification, the plywood and OSB master panels were cut to 2.4-m-long by 0.4-m-wide specimens and then weighed. All panels were equilibrated at approximately 21°C ± 5°C and 50 ± 5 percent relative humidity to constant mass prior to thermal modification. The ovendry moisture content of the plywood and OSB was then calculated according to ASTM D4442 (ASTM International 2007). The average density of the plywood and OSB was 743 and 717 kg/m³, respectively. All LSL and LVL specimens were equilibrated to constant mass at approximately 23°C ± 2°C and 35 ± 5 percent relative humidity prior to thermal modification. The average density of the LSL and LVL was 735 and 503 kg/m³, respectively.

To prepare feedstock for the tamarack (Larix laricina) OSB panels, six mature (90-yr-old) and six juvenile (30-yr-old) trees were selected from the Thunder Bay, Ontario, Canada, area. Details on the processing and manufacture of OSB produced from this feedstock can be found in detail elsewhere (Aro et al. 2014).

Durability testing

The test included 38 combinations of rated plywood, OSB, LSL, LVL, OSB-EL, chromated copper arsenate (CCA-C), and untreated southern pine (Pinus spp.) and sweetgum (Liquidambar styraciflua) controls for a total of 41 combinations (Table 2). Groups marked as matched controls are samples taken from the same board or panel as the corresponding temperature prior to heat treatment. Replicates for each combination were taken from different boards or panels. Samples were generated from board remnants from other studies.

Table 2 Treatments for samples evaluated using American Wood Protection Association Standard E10.^a

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The decay test was carried out in accordance with the AWPA E10-12 “Standard Method of Testing Wood Preservatives by Laboratory Soil-Block Cultures” (American Wood Protection Association 2012). G. trabeum (ATCC 11539) and T. versicolor (ATCC 12679) were used in this test. Five replicates plus an operational loss sample were tested for each treatment combination shown in Table 2.

Results and Discussion

Average mass loss data for the treated and untreated samples after exposure to G. trabeum for 12 weeks are shown in Table 3. Average mass loss data for the treated and untreated samples after exposure to T. versicolor for 24 weeks are also shown in Table 3. The data show that the untreated controls had considerable decay, as shown by the mass losses for G. trabeum and T. versicolor, indicating that these fungi were very active. In most cases, the heat-treated samples' mass loss was less than the respective matched untreated control sets exposed to fungi.

Table 3 Average adjusted mass loss data for the treated and untreated samples after exposure to Gloeophyllum trabeum for 12 weeks and Trametes versicolor for 24 weeks.^a

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Looking at the differential between heat-treated samples and controls by substrate (Fig. 1), this differential increased as temperature increased. This is indicative of the effectiveness of heat treatment in reducing mass loss from decay. However, only the LVL heat treatments yielded mass losses of the order of magnitude as southern pine treated to an aboveground CCA retention of 4 kg/m³.

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For G. trabeum, the differential (untreated − heat treated/untreated) ranged from −0.54 to 49.2 percent, averaging 19 percent (Fig. 1). Two sample groups had slightly higher mass losses for the heat-treated material. For T. versicolor, the differential ranged from −6.67 to 28.86 with an average of 10.82. There was no visual evidence of fungal colonization on the sterile controls, indicating that the mass loss was most likely due to leaching of extractives and not actual decay. These data indicate that all heat treatments–substrate combinations tested, except the six matched control comparisons, show some resistance to the fungi tested. For T. versicolor, the differential values were more variable, with four combinations showing higher loss for the heat-treated samples.

The effect of heat treatment on the mass loss of various substrates is shown in Figure 2. Excellent correlation between heat treatment temperature and mass loss is shown in Figure 2a for G. trabeum. LSL and LVL showed the best reduction in mass loss. For LVL, mass losses of <10 percent were achieved at temperatures >150°C. For LSL and rated plywood, the temperature required was around 180°C. The outlier value for rated plywood at 150°C was not included in the curve-fitting equation. For T. versicolor (Fig. 2b), all substrates showed a decrease in mass loss with increasing temperature, with the exception of OSB, the mass loss of which increased at 180°C. No treatment reached 10 percent mass loss.

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Figure 3 indicates significant reduction in mass loss for the 180°C treatment of heartwood and the 160°C and 180°C treatments of sapwood tamarack OSB. While heat treatment resulted in a lower mass loss for both heartwood and sapwood tamarack OSB, none fell below 10 percent (Fig. 3a).

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With T. versicolor, mass loss reduction in the heartwood tamarack OSB was not consistent with temperature, while a downward trend was noted for the sapwood OSB (Fig. 3b). The data suggest that heat treatment requires a higher temperature to effect a reduction in mass loss for heartwood panels.

Conclusions

For the most part, heat treatment caused a reduction in mass loss for all substrates and both fungi. Some variation in the differential in mass loss between treated and untreated samples was noted. For engineered wood products, heat treatment seems an effective way to reduce susceptibility to fungal deterioration. The impact of heat treatments on other properties should be investigated, as should susceptibility to termite attack.