It is a normal practice in oriented strandboard (OSB) production to store logs outdoors for a period of time prior to the flaking process. The duration of yard storage depends on harvest season and sources of log supply. Outdoor log storage without protection could change the mechanical and chemical properties of wood due to attack by various fungi. To understand how outdoor log storage affects the wood and subsequently the strandboard quality, two piles of aspen logs were set up outside and stored for a period of four months (July 9 to November 14, 2001). One of the piles was treated with a biological solution to prevent fungal growth. The other one was stored without such treatment. Both piles contained non-debarked and partially debarked logs. Evaluation of sap stain development indicated that all logs had been colonized by staining fungi with an average stain coverage of 9.37 to 57.18% and maximum stain penetration of 3.58 to 7.27 cm over the log cross-section. The variation of fungal colonization depended on log treatment and bark condition. The most effective way to prevent stain growth was with the combination of biological treatment and partial debarking. This was followed in effectiveness by biological treatment and no debarking, no treatment and partial debarking, and, finally, no treatment and no debarking. A series of strandboard was prepared from fresh and aged aspen logs. It was observed that all boards made from stored logs were statistically comparable to or superior to the control boards made from fresh aspen logs. The boards made from treated/partially debarked and untreated/non-debarked logs were statistically comparable to each other except for the higher wet MOR for the former. In addition, both board types were stronger than other boards in terms of IB and water resistance. Compared to control boards, the stronger boards in terms of water resistance were also made from biologically treated/non-debarked and untreated/partially debarked logs. Some individual stained strands were observed on the finished board surface, which could affect board appearance if the wood had been highly attacked by fungi. Less staining was found in the boards prepared from biologically treated and partially debarked logs, as compared to other stored logs.
Shavings stored outdoors may be susceptible to attack by fungi, which in turn could influence the quality of shavings, the resulting fiber quality, and the final medium density board (MDF) performance. Three piles of shavings were constructed as hemispheres (11 feet in height and 11 feet in diameter), which contained roughly 250 ft3 (7 m3) or 1,480 lb (675 kg) of shavings obtained from a local MDF plant. The shavings in Pile 1 and Pile 2 were protected with a biological agent and by a shelter, respectively; Pile 3 was built without protection during storage.
It was observed that outdoor storage of shavings influenced the fiber physical and chemical properties. In most cases, stored shavings resulted in a high percentage of longer fibers being retained on a 14-mesh screen (1.190 mm) compared to fresh shavings (control). Storage also resulted in less acidic fibers, as indicated by higher pH values and acid buffer capacity. The hot water solubility of the fibers was higher for the shelter-protected shavings and lower in both the biologically protected and unprotected shavings, compared to the control. In addition, the refining process made the wood more acidic. Converting the shavings to fibers decreased the pH in the control by 19% and by 17 to 27% in the stored shavings. The conversion also reduced the buffer capacity by 44% in the control and by 25 to 50% in the stored shavings.
The outdoor storage of shavings also affected the mechanical and physical properties of the resulting MDF board. In general, the shelter-protected shavings fibers yielded the strongest board, followed by the biologically protected and unprotected shavings fibers, while the least strong board was made from fresh shavings fibers. With regard to location of the shavings in the piles, in most cases, fibers made from exterior shavings resulted in stronger boards than fibers made from interior shavings. These results imply that outdoor storage of shavings for a selected time period (4 months) did not seem to negatively influence board performance, but instead seemed to significantly improve bond quality.
An apparent correlation existed between board performance and fiber chemical property. Fibers prepared from shelter-protected shavings not only showed higher hot water solubility, but also yielded overall stronger board, compared with other stored shavings fibers and the control fibers. This study also indicated that the physical or mechanical and chemical properties of the fibers were important to board quality. Therefore, the best quality fibers for high quality MDF could be obtained from a given wood raw material by optimizing the fiber refining process.
The influence of wood chemical characteristics on UF resin bonding was examined by evaluating the mechanical and physical properties of particleboards made with different amounts of white cedar, balsam fir or their mix as a substitute for commercial furnish in the core layer. Test results showed that the commercial core furnish could be replaced with up to 100% white cedar, balsam fir or a mix of the two species without adversely affecting panel performance; in fact, these compositions resulted in stronger panels. The optimum substitution level in the core was 15 to 20% for white cedar and 40 to 60% for balsam fir in terms of overall panel performance. The most significant improvement was observed for IB strength. For example, IB increased from 0.658 to 0.886 MPa (a 35% increase) when 5% white cedar was used and from 0.658 to 1.068 MPa (a 62% increase) when 20% white cedar was used; IB increased from 0.658 to 0.903 MPa (a 37% increase) when 40% balsam fir was used and from 0.658 to 0.941 MPa (a 43% increase) when 60% balsam fir was used in the core. For the panels made with 20/80 white cedar/commercial furnish in the core, a 10-second decrease in pressing time (from 150 to 140 seconds) and a 0.5% decrease in the amount of catalyst (from 1.0 to 0.5%) still produced panels stronger than the control in terms of overall panel properties. The density of panels made with 100% white cedar in the core was 4% lower than in the control; nonetheless, the cedar panel still performed better than the control. Furthermore, using up to 100% white cedar, balsam fir or their mix in the core did not cause any mats to blow during pressing. For all panels, the maximum mat gas pressure was less than 8 psi during pressing.
This study indicates that white cedar, balsam fir or their mix are preferable as substitutes for commercial core species containing both hardwood (e.g., yellow birch and red maple) and softwood species (e.g., balsam fir and black/white spruce) in particleboard manufacturing. Among the core furnishes examined, white cedar seemed to be the most suitable for use in particleboard manufacturing, probably because of its relatively low pH (4.49) and acid buffer capacity (1.81 mEq). It appears that the use of white cedar allowed for shorter pressing times and lower catalyst levels; it also decreased board density somewhat without having a detrimental effect on board performance. Compared to white cedar, balsam fir seemed to be less acidic (pH and acid buffer capacity were 4.97 and 6.93 mEq, respectively); however, this did not seem to cause bonding problems. Thus, increased catalyst levels are unnecessary in order to overcome the potential adverse effect of wood chemistry when balsam fir is used in the core.
Sixteen fresh and recycled particleboard furnishes were characterized for pH, acid buffer capacity, base buffer capacity and total buffer capacity. Mechanical and physical properties of particleboard prepared from heat-treated (kiln drying) particles were also correlated with the wood chemical properties. It was found that the wood chemical characteristics of both fresh and recycled materials were influenced by heat treatment time (4 minutes and 24 hours), temperature (105o and 150oC), and particle size (coarse face and fine core particles). In general, the recycled material resulted in lower pH values than fresh material at 105oC, which was probably attributed to the residual acids from cured urea-formaldehyde resin in the recycled particles. The pH values of both fresh and recycled materials increased as treatment time increased at 105oC. Increasing the treatment temperature from 105o to 150oC at 24-hour treatment time did not seem to affect the pH of recycled material but reduced the pH of fresh material. The decrease in pH of fresh material at 150oC/24 hours might be related to the release of organic acids from the wood particles via the initial decomposition (hydrolysis and/or pyrolysis) of wood extractives and components. No influence of treatment temperature on the pH of recycled material observed from 105o to 150oC at 24 hours could be resulted from the interaction between wood and cured resin after heat treatment. The acidity of wood due to the initial decomposition of wood extractives and chemical components might offset the alkalinity of wood caused by the generation of ammonium hydroxide via decomposition of cured UF resin. A higher temperature and longer heat treatment time generally resulted in higher acid, base, and total buffer capacities; the extent of the increase depended on face and core particles.
Particleboard was disintegrated using both BTCA (butanetetracarboxylic acid) treatment and cold water soaking (24 hours overnight), combined with hammer milling. BTCA treatment of particleboard resulted in decreased pH values in the particles due to the acid contained in the BTCA solution. Particles disintegrated with 1.0% BTCA solution yielded pH levels similar to the pH levels in particles treated with water soaking. In addition, BTCA treatment of particleboard yielded higher wood acid and base buffer capacities compared to water soaking.
A regression analysis indicated that some correlations existed between wood chemical characteristics and particleboard properties: internal bond (IB) strength strongly correlated with the pH value, but modulus of elasticity (MOE), thickness swelling (TS), and water absorption (WA) correlated with the base buffer capacity of core material. In addition, MOE and TS well correlated with the pH values of face materials, while modulus of rupture (MOR) and formaldehyde emission (FE) well correlated with the base buffer capacity of core material.
A total of 24 hybrid poplar clones grown in Windsor and St-Ours in southern Quebec, Canada, were characterized for chemical properties (pH, acid buffer capacity, and base buffer capacity). This test group of 10-year-old poplar clones (spring of 1993 to winter of 2002) was comprised of 12 clones grown in Windsor and 12 clones grown in St-Ours. At both locations, there were four hybrids: P. deltoides (D); P. deltoides x P. nigra (D x N); P. trichocarpa x P. deltoides (T x D); and P. maximowiczii x P. balsamifera (M x B). Thus, each hybrid included three clones. In addition, 3 replicates of two clones derived from D and D x N hybrids grown in Windsor were tested to examine the block (sub-location) influence of clone type on wood chemical characteristics.
Results showed that both hybrid and geographic location affected wood chemical characteristics, based on the average values of the same hybrid. In general, the fast-growing poplar clones (as determined by tree diameter) in St-Ours showed higher pH and acid buffer capacity, but lower base buffer capacity than those grown in Windsor. For clones within the same hybrid, observations showed that clone type had more influence on acid and base buffer capacities than on pH value, and the extent of the influence depended on each individual hybrid. In terms of variations in wood chemical properties within the same clone, replicate measurements of two clones (one from D hybrid and one from D x N hybrid) showed greater variations in base buffer capacity than in pH and acid buffer capacity. This study implies that differences in the wood chemical properties examined depend not only on wood genotype, but likely also on site quality, which correlates well to wood physical properties such as tree diameter, basic wood density and fiber length.
A total of 16 wood species were characterized for pH, acid buffer capacity, and base buffer capacity. These species are used in the manufacture of particleboard, medium density fiberboard (MDF), and oriented strand board (OSB) in Canada. The effect of wood characteristics on the curing behaviour of phenol-formaldehyde resin (PF), methylene diphenyl diisocyanate (MDI), and urea-formaldehyde (UF) resin were investigated by means of differential scanning calorimetry (DSC) analyses and gel tests.
First of all, it was observed that wood chemical characteristics influenced PF resin curing behaviour in terms of onset curing temperature (Te), exothermic peak temperature (Tp), and exothermic reaction heat measured by DSC. The extent of this influence on resin curing also depended on resin properties. High viscosity phenol-formaldehyde resin (PF H) cured faster than low viscosity phenol-formaldehyde resin (PF L) in the absence of wood, in terms of lower Te, Tp, and reaction heat. This is because PF H is a more advanced resin than PF L. In the presence of wood, including aspen, white birch, yellow birch, red maple or a mix of these species, PF H cured faster than PF L. However, the resin curing behaviour was influenced by wood species. For PF H, adding white birch and the mixed species seemed to retard resin curing as indicated by higher Te and Tp. This was likely due to the low pH and/or high base buffer capacity of these species. In the case of PF L, adding wood did not seem to interfere with resin curing, but rather promoted resin curing, regardless of wood species. This result can probably be attributed to the higher pH value of PF L, which made PF-L more tolerant of the influence of the wood species or wood chemical characteristics than PF-H. Statistical analysis also indicated that the difference in Te between PF-L and PF-H decreased with increased wood base buffer capacity or decreased wood acid buffer capacity.
Next, the effect of the above-mentioned four wood species and their mix on MDI curing behaviour was evaluated by DSC analysis. The study showed that wood chemical characteristics significantly influenced MDI curing. Aspen, having the highest acid buffer capacity and the lowest base buffer capacity, yielded the highest Tp and the lowest reaction heat; white birch, having the lowest pH and highest base buffer capacity resulted in the lowest Te. In addition, yellow birch, with the lowest acid buffer capacity, yielded the highest reaction heat. Statistical analysis showed that a strong correlation exists between pH and Te, and an adverse correlation exists between acid buffer capacity and reaction heat. Moreover, an apparent correlation was observed between pH and Tp, while an adverse correlation was observed between base buffer capacity and both Tp and Te. These results suggest that an increase in wood acidity promotes MDI curing.
Of the 16 wood species characterized, pH ranged from 3.74 to 6.32, acid buffer capacity ranged from 1.62 to 6.93 mEq, and base buffer capacity ranged from 4.44 to 16.22 mEq. Both gel test and DSC analysis indicated that an increase in wood pH and acid buffer capacity or a decrease in base buffer capacity appeared to retard UF resin curing. The effect of wood chemical characteristics on UF resin curing using two different catalysts was also evaluated by DSC. Ammonium chloride (NH4Cl, 10% aqueous solution) is a salt of strong acid and weak base, while acetic acid (CH3COOH) is a weak organic acid. Ammonium chloride seemed to be more effective than acetic acid in promoting resin curing in the presence of wood. At least 1.50% acetic acid was required in order to achieve a level of resin curing similar to that observed for 0.25% ammonium chloride. Increasing the catalyst content from 1.50 to 5.0% did not seem to influence the resin curing rate with ammonium chloride catalyzed resin but continue accelerating the resin curing rate with acetic acid catalyzed resin.