Utilization of commercially available resource for particleboard and MDF. Part VI. Evaluation of SPF/yellow birch mixture as raw materials for HDF manufacturing
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.