Oriented strand board (OSB) is widely used in house construction in North America. In Canada, OSB panels are commonly made of aspen strands and are susceptible to mould and decay when they get wet. Building envelope failures due to mould, decay or poor construction practices can negatively impact the image of wood. This can lead to product substitution that in turn can affect the wood industry’s overall competitiveness. To ensure durability of OSB panels, the most important consideration is the use of mould- and decay-resistant panels to prevent fungal attack. Using low environmental impact technology to improve the durability of OSB products could have market-related advantages over using chemical protection products. This project aimed to develop technologies for protecting OSB raw materials from biodegradation and to explore biological pre- or post-treatments to increase the durability of panels so they would better resist mould, stain and decay.
The project was divided into three parts. Part one involved developing a biological technology to protect OSB raw materials from biodegradation. In this part, aspen, red maple and yellow birch trees, which are commonly used to make OSB in Canada, were felled in May and cut into 4-foot logs. These logs were then equally divided into two groups (16 logs each) with one group keeping its bark and the other having it removed. These debarked and “bark-on” logs were further divided into two groups, each containing 8 logs. One group of logs was treated with a bioprotectant and another group served as a control. The treated and untreated logs were stored separately in Forintek’s yard. Two inspections were conducted, one at the end of the growth season (in October after a 5-month storage period) and the other after one year. During each inspection, four logs from each test group were examined for fungal degradation (mould, stain and decay), and then cut into strands to be used for manufacturing panels. The panels’ physical and mechanical properties and mould resistance were evaluated.
The second part involved developing a biological pre- or post-treatment technology by using naturally resistant wood species to increase the durability of panels so they would better resist mould, stain and decay. In this part, a series of tests were conducted using various wood species. These tests included a) determining the antifungal properties of bark from various wood species; b) using white cedar to improve panel durability; c) optimizing manufacturing conditions for producing durable panels with white cedar; d) using other wood species to produce mould-resistant panels; and e) post-treating panels with extracts of durable wood species.
The third part consists of developing a biological pre- or post-treatment technology by using fungal antagonists to increase the durability of panels against mould, stain and decay. This part will be conducted in the 2004-2005 fiscal year, and results will be included in next year’s report.
The results of the first part on the protection of raw materials showed that all untreated logs, with or without bark, were seriously degraded by moulds, stain and decay fungi after a summer storage period of five months. The logs with bark were more degraded than the debarked logs, and the log ends were more degraded than the middle sections. After summer storage, 55% to 83% of the wood was degraded in untreated logs. The biological treatment was effective, only 4% to 16% of the wood in treated logs was affected by various fungi after a five-month storage period. Furthermore, the biological treatment was more effective on logs without bark than logs with bark, and more effective on yellow birch and aspen than on red maple. After one year in storage, the total infection rates of untreated logs ranged from 68% to 91%, whereas the rate for biologically treated logs ranged from 27% to 49%. Among these treated logs, the logs ends were degraded from 31% to 62%, whereas the middle sections were degraded from 7% to 26%. Strands cut from untreated logs contained 50% to 75% of grey or blue stained strands, whereas those cut from biologically treated logs contained 10% to 25% of such strands. Panels made using biologically treated logs had the lowest TS and WA values compared with panels made using fresh-cut logs and untreated stored logs. The other physical and mechanical properties of the various panels made for this test were comparable.
The antifungal properties of bark from six wood species (aspen, red maple, yellow birch, balsam fir, white spruce and white cedar) were investigated in the second part of this research project. Based on the colony growth rate of moulds, stain and decay fungi on bark-extract-agar media, white spruce bark was the best at inhibiting growth of these fungi, followed by red maple bark. White cedar and balsam fir bark somewhat inhibited certain fungi tested. Aspen and yellow birch bark did little or nothing at all to inhibit fungal growth. The research also showed that the white cedar heartwood-extract-agar medium not only inhibited decay fungi growth, but also inhibited the growth of moulds and staining fungi. The bark-extract-agar medium of this wood species was less effective in inhibiting fungal growth than the heartwood was.
Three-layer panels made using white cedar heartwood strands in the face layers and aspen strands in the core layer at a ratio of 25:0:25 were mould and decay resistant, but the panels “blew” easily during manufacturing and their mechanical properties were not satisfying. The overall mould infection rate on white cedar heartwood-faced panels was 0.8, which indicated that the panel was mould resistant. White spruce heartwood-faced panels were highly mould resistant and moderately decay resistant. The overall mould infection rate on white spruce heartwood-faced panels was only 0.2 after 8 weeks of exposure to high humidity environmental conditions. In addition to being mould resistant, white spruce heartwood-faced aspen panels also had better IB, MOR and MOE properties, compared with aspen panels. The panels with black spruce in surface layer had mechanical and mould-resistance properties that were similar to those with white spruce in surface. The panels with surface layer of Eastern larch heartwood were non-resistant to moulds and slightly resistant to decay, but they had better IB, TS and WA properties compared with the other types of panels. The overall mould infection rate on the panel with surface layer of Eastern larch heartwood was 3.7, which was similar to the rate for aspen control panels. Aspen panels (serving as control panels) were seriously affected by moulds with overall mould infection rates ranging from 3.8 to 4.9.
Aspen panels with surface layer from whole-wood strands (using both sapwood and heartwood) from white cedar, in a ratio of 25:50:25 and pressed at 220°C for 150 seconds, were well bonded and had IB, TS, WA and MOE values that were similar to those of aspen control panel, but with a higher MOR. All the panels’ properties met the requirements of the standard. This type of panel also was the least infected by moulds, especially in the face layers which rated a 0.2. The panel sides were moderately infected, rating a 2.6, this occurring mostly in the middle layer of aspen strands. The overall rate of this type of panel was 1.0, which indicated that the panels were resistant to mould infection. This type of panel was also highly resistant to brown rot and moderately resistant to white rot.
Panels made of steam-treated white cedar whole-wood strands and aspen strands at a ratio of 3:7 based on oven-dry weight also had low infection rates: the average face infection rate was 1.2; the side infection was 2.4 and the overall rate was 1.6. Compared with aspen panels, this type of panel also had high MOR and MOE values and low TS and WA values.
In the case of white cedar whole-wood strands faced aspen panels, when the pressing time was increased from 160 seconds to 180 seconds at 200°C, the panels’ IB strength and MOE increased whereas the panels’ TS, WA and MOR decreased. By increasing the pressing temperature from 200°C to 240°C and pressing for 160 seconds, the panels’ IB strength, MOR and MOE increased and the panels’ TS and WA decreased sharply. At a pressing temperature of 240°C and a pressing time of 180 seconds, the panels’ IB strength, MOR and MOE increased significantly and the panels’ TS and WA decreased significantly. These data showed that aspen panels with surface layer from white cedar whole strands at a ratio of 25:50:25 and pressed at 240°C for 180 seconds had the best mechanical and physical properties. All panel samples were slightly infected by moulds on the faces. A fair amount of mould occurred on the sides of panels pressed at 200°C for 160 seconds and 180 seconds and those pressed at 240°C for 180 seconds. The panels pressed at 240°C for 160 seconds were the least infected by mould (with an infection rate of 0.3). Panels pressed at 200°C had a white-yellowish colour, whereas those pressed at 240°C were yellow-brownish and darker than those pressed at 200°C. Panels pressed at 200°C for 160 or 180 seconds and those pressed at 240°C for 160 seconds were highly decay resistant, especially to brown rot. The decay resistance of panels pressed at 240°C for 180 seconds was lower compared with the other panels.
Compared with aspen panels, panels with surface layer from steam-treated white cedar strands and aspen strands at a ratio of 7:3 based on oven-dry weight had higher TS, WA, MOR and MOE values and a similar IB value. Panels with surface layer from steam-treated white cedar strands and aspen strands at a ratio of 4:6 based on oven-dry weight had the highest IB value. A reduction in mould and decay resistance corresponded to a reduction in the proportion of white cedar strands in the face layers. The overall mould growth rate was 1.27 on panels with surface layer from steam-treated white cedar strands and aspen strands at a ratio of 4:6, 0.6 on panels with surface layer from steam-treated white cedar strands and aspen strands at a ratio of 7:3, and 0.4 on panels faced with 100% white cedar whole strands, respectively.
Panels made from 100% white cedar whole-wood strands or a mixture of whole-wood strands of white cedar and aspen (50:50) in the core layer were “blown” after pressing. Panels made from a mixture of white cedar and aspen strands at a ratio of 25:75 in the core layer and aspen strands in the face layers had superior IB, MOR and MOE values than other panels. However, their TS and WA values were also higher than those of white cedar-faced panels. Panels made from a mixture of white cedar and aspen strands at a ratio of 25:75 in the core layer and white cedar strands in the face layers had the worst physical and mechanical properties among all the panels made for this test. The tests results for mould showed that panels made with a mixture of white cedar and aspen strands at a ratio of 25:75 in the core layer and aspen strands in the face layers ware seriously attacked by moulds and had an overall mould growth rate of 4.2. No mould infection was found on panels made from 100% white cedar strands. Panels made from a strand mixture of white cedar (50%) and aspen (50%) in the core layer and white cedar strands in the face layers had little mould infection. The overall mould growth rate on this type of panel was 0.2.
Compared with the control aspen panels, aspen panels with surface layer from white cedar whole-wood strands at a ratio of 15:70:15 had similar IB and TS values, a lower WA value and higher MOR and MOE values. When the white cedar strand proportion in the face layer was increased from 15% to 25%, the panels’ IB strength and WA decreased, but their MOR and MOE values increased. Panels with surface layer from white cedar strands at a ratio of 15:70:15 had little infection from moulds on the face and bottom layers, but had an increased infection rate on all four sides. The average overall infection rate of this type of panel was 0.5. When the white cedar in the panels’ face layer was increased from 15% to 25%, the average infection rate on the panels’ faces was still 0.1, but the infection rate of the panels’ sides dropped from 1.2 to 1.0. The overall rate was 0.4. In terms of decay resistance, panels with surface layer from 25% white cedar strands performed better than those with surface layer from 15% white cedar.