Utilization of inorganic mineral filler material as partial replacement for wood fiber in medium density fiberboard (MDF) and its effect on material properties

The influence of inorganic mineral filler addition on the material properties of medium density fiberboard (MDF) was investigated at varying densities. Fibreboards with 10, 20 and 30% fibers replaced by calcium carbonate weight basis (dry/dry) and with target densities of 550, 700 and 850 kgm⁻³, respectively, were manufactured in a laboratory scale environment. The fiberboards were prepared by adding the mineral filler in dry powder form to the wood fibers to which urea formaldehyde UF resin in liquid state had previously been added. The obtained fiber-filler furnish, with the filler distributed uniformly by sticking to the UF resin on the fibers, was then pressed into panel forms. The effect of inorganic filler on elastic and strength properties, thickness swelling, permeability, as well as formaldehyde and volatile organic compound (VOC) emissions was quantified. The results demonstrate that the addition of mineral filler to MDF up to levels of 10 wt.% only have a small effect, if at all, on the material properties. The addition of calcium carbonate, and the associated increase in pH, does not have any effect on the strength development in the cured MDF, even though the results clearly demonstrate the negative effect on the curing kinetics of urea formaldehyde (UF) resin. Likewise, thickness swelling and formaldehyde and volatile organic compounds (VOC’s) emissions are not affected by filler addition levels of up to 10 wt.%. However, exceeding this level leads to a significant decline in strength, which is believed to be caused mainly by a reduction in the number of fiber-to-fiber stress transfer points. Further, increased emissions in VOC’s are expected due to a combination of the catalytic effect of the calcium carbonate that causes a breakdown of fatty acids to VOC’s and also increased emission rates due to a higher permeability of the MDF with filler addition levels above 10 wt.%.