In order to investigate the mix of softwood (SPF) and hardwood (yellow birch) furnishes as raw materials for manufacturing high-density fibreboard (HDF), a series of panels were manufactured from single softwood and hardwood species and softwood/hardwood mixes. The wood raw materials included sawdust and chips for both species. The panels were evaluated for internal bond (IB) strength, modulus of rupture (MOR), modulus of elasticity (MOE), 24-h thickness swelling (TS), 24-h edge TS, 24-h water absorption (WA), and linear expansion (LE).
First of all, the test results show that the sawdust fibres produced overall stronger panels than the mixed sawdust/chip fibres, regardless of wood species. Of the sawdust fibres, the softwood fibres resulted in panels with overall better static bending and water resistant properties compared with both hardwood alone and softwood/hardwood mixes (50/50). However, the hardwood fibres and softwood/hardwood mixed fibres produced higher IB panels than did the softwood fibres. For the mixed softwood/hardwood furnishes, fibres refined at 8.5- and 10-bar steam pressure produced high-quality panels compared to fibres refined at 12-bar steam pressure.
For the mixes of 50/50 chip/sawdust fibres, the panel made from softwood sawdust/hardwood chips performed slightly better than the panel made from softwood chips/hardwood sawdust with respect to MOE, TS, edge TS and WA. However, the opposite result was observed in terms of MOR and LE properties. These two panels were stronger than that made from hardwood chip/hardwood sawdust fibre with respect to static bending and water resistant properties. All three mixed chip/sawdust fibres yielded similar IB panels.
Pre-treating some of the furnish in the softwood/hardwood or hardwood/hardwood mix (50/50) with steam for 30 minutes at 120oC improved the performance for the panel made from the mixed hardwood chip (pre-treated)/hardwood sawdust fibre in terms of all panel properties; however, this pre-treatment had a negative impact on the overall performance for other mixed furnishes, i.e., softwood sawdust/ hardwood sawdust (pre-treated), softwood chip (pre-treated)/hardwood sawdust, and softwood sawdust/ hardwood chip (pre-treated). It was speculated that the pre-treatment conditions for the raw materials with higher moisture content (65-99%) might have been too severe, which could have negatively affected resulting wood strength properties via hydrolysis. Further study will be needed to understand why the pre-treatment had a negative impact on the performance of HDF panel made from the softwood/hardwood mixes and not the panel made from the hardwood mix.
To investigate hardwood species as a substitute for spruce/pine/fir (SPF) softwoods in particleboard manufacturing, a series of three-layer particleboards were prepared from various hardwood species including white birch (WB), sugar maple (SM), and red oak (RO) as well as their mixtures (WB/SM, WB/RO, and SM/RO at mixing ratios of 25/75, 50/50, and 75/25, respectively, and WB/SM/RO at mixing ratios of 25/25/50, 25/50/25, and 50/25/25, respectively). For each single and mixed species, two substitution levels of hardwood species for SPF were evaluated: overall 22% (10% in face and 30% in core) and 40% (10% in face and 60% in core). After conditioning at 65% RH/20oC, all boards were evaluated for mechanical and physical properties including internal bond (IB) strength, modulus of rupture (MOR), modulus of elasticity (MOE), 24-h thickness swelling (TS) and water absorption (WA), and linear expansion (LE).
All panels made from single and mixed hardwood species at 22% and 40% SPF substitution levels, with one exception, met ANSI A208.1 standard requirements for Grade M2 particleboard in terms of IB, MOR, MOE, and LE. The panel made with sugar maple at the 40% substitution level failed to meet the MOR requirement. This failure was attributed to the lower board density or compression ratio.
For the panels made from hardwood species at the 40% SPF substitution level, the average values for IB, MOR, and MOE exceeded the standard requirements by 177%, 21%, and 23%, respectively, while the average LE value was lower than the standard requirement by 30%. These results imply that more hardwoods could be used as substitutes for SPF softwoods in particleboard manufacturing, especially in the core layer.
With respect to overall panel performance, the following hardwood species appeared to produce panels that were comparable to the SPF control: (1) 100% red oak at both substitution levels (22% and 40%); (2) 25/75 WB/SM at the 40% substitution level; (3) 25/75 WB/RO at both substitution levels; and (4) 25/75 and 50/50 SM/RO at the 40% substitution level.
This study shows that wood species and density, original form of raw material, and moisture content (before grinding) influence the particle size distribution, and, consequently, influence the resin efficiency in terms of resin coverage over particle surfaces. This implies that the disadvantage of using hardwoods, in terms of their higher density and lower compression ratio (board density over wood density), as a substitute for SPF softwoods for manufacturing high-quality particleboard could be overcome by improving resin efficiency via optimizing particle size distribution during grinding and screening processes.
Seven B.C. species, namely, interior Douglas fir, coastal second growth Douglas fir, western larch, lodgepole pine, western white spruce, trembling aspen and white birch were evaluated for their laminating properties using different adhesive formulations and pressing conditions.
Using optimized gluing and pressing conditions, six of the B.C. species showed excellent bond quality when laminating with radio-frequency (RF) heating and either cross-linked polyvinyl acetate (PVAC) or phenol-resorcinol-formaldehyde (PRF) adhesive. These laminates easily passed the shear block wood failure requirement in the ASTM-D-2559 standard and the delamination requirement in the ASTM-D-1101 standard. Because white birch which has a high density showed the highest block shear strengths for the optimum PRF adhesive formulation, this species showed the lowest average percent wood failure of the seven B.C. species and did not meet the ASTM-D-2559 wood failure requirement of 75%.
Using conventional platen pressing at 20 or 25°C, laminates were prepared with different PRF adhesive formulations and the seven B.C. species. Using an optimized PRF adhesive formulation, the laminates for the seven BC species met the above ASTM standard requirements for wood failure and delamination.
Overall, the percent wood failure was higher for the laminates made at 25°C indicating more resin cure. Hence, for laminates made with the optimum PRF formulation, PRF-C, the average percent wood failure for western larch at 20°C was 78% compared to 98% at 25°C.
Laminated products - Manufacture - British Columbia