Utilization of commercially available resource for particleboard and MDF. Part V. Evaluation of white birch, sugar maple, and red oak as raw materials for particleboard manufacturing
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.