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.