This study aimed to investigate how resin type (powder, liquid, powder/liquid combination) in conjunction with knife (sharpening) angle (22o, 32o) in strand preparation, and strand-drying temperatures (114oC, 160oC, 195oC) influenced the performance of aspen/white birch strand boards. Strands were characterized for curvature, surface roughness, and resin coverage. Board performance was evaluated by internal bond (IB), dry modulus of rupture (MOR), dry modulus of elasticity (MOE), wet MOR, thickness swelling (TS), and water absorption (WA).
Strand characteristics showed that white birch strands curled more than aspen strands for all experimental conditions. An increase in strand-drying temperature generally increased the extent of strand curvature. The 32o knife resulted in less curled strands than the 22o knife for aspen at 114oC, 160oC, 195oC, while the opposite result was found for birch at 114oC and 195oC. White birch strands showed a higher degree of surface roughness than aspen strands for all strand flaking and drying conditions. For aspen/white birch mixed strands, the 22o knife seemed to yield higher resin coverage than the 32o knife at 160oC and 195oC, while the opposite result was observed at 114oC for powder resin (PPF) and powder/liquid combination (PPF/LPF). In terms of wood species, birch strands normally showed higher resin coverage than aspen strands. Higher resin coverage was found on the loose side rather than the tight side of strands for both aspen and white birch.
Among 15 board manufacturing conditions (in terms of knife angle, strand-drying temperature, and resin type), overall stronger boards were produced with 32o knife-cut strands compared with 22o knife-cut strands. These results imply that the 32o knife seems to produce strands more suitable for making strand boards with PPF or PPF/LPF resin. With 32o knife-cut strands, the PPF/LPF combo yielded overall better quality boards than did PPF; however, both PPF/LPF and PPF resins produced much stronger boards than LPF resin. The poor performance by LPF resin can probably be attributed to a relatively higher resin viscosity and the resulting poor resin distribution. Preheating resin at 35oC may not be enough to atomize the resin through the nozzle spraying system. These results imply that the use of a liquid and powder combination binder system seems to improve bond quality even though the liquid resin has a higher level of viscosity than is normally required for OSB manufacturing. Resin types (PPF, PPF/LPF) had less influence on board performance for strands flaked at 22o.
This study showed that strand preparation conditions (knife angle, strand-drying temperature, and resin type) influenced strand characteristics (strand curvature, surface roughness, and resin coverage). However, the influence was not considered significant enough to be able to draw a clear conclusion on the impact of strand characteristics on board performance. It is believed that an increase in strand width (1.25-in wide aspen and birch strands were used in this study) would have a greater impact, especially concerning strand curling and folding problems and variation in resin distribution within and among strands.
Acknowledgements
Many thanks to all of the participating member mills for their technical support. A special thanks is extended to Dr. When-Huan (Bill) Man, visiting scientist from Beijing Forestry University, Beijing, China, for his assistance in the measurements and analyses of strand curling, strand surface roughness, and resin coverage on strand surfaces. The help of Ms. Johanne Emard, secretary in the Composite Wood Products Department at Forintek, for progress report formatting is also appreciated.