This report summarizes the research project carried out at Forintek’s Eastern Laboratory entitled Properties of MDF in Relation to Wood fibre Characteristics and Processing. This project included two phases. In the first phase, 10 types of raw materials, including wood, bark, and tops, were selected to cover large range of wood characteristics. The samples were classified by wood species (e.g., the less desirable red pine species), fast growing and short rotation (e.g., hybrid poplar), residue type (e.g., tops and bark), and forest management regime (e.g., commercial thinnings). The basic properties of raw materials, such as basic density, pH, and buffering capacity, were determined. The effects of raw material acidity on the curing behaviours of UF resins were investigated, and the properties of MDF panels manufactured using each sample at fixed manufacturing conditions were compared. The impact of bark content on MDF properties was also studied. In the second phase, two types of raw materials (black spruce tops and bark) were selected for the optimization of process parameters, including refining and hot pressing. The effects of refining conditions on pH and buffering capacity of refined fibres as well as on the properties of MDF panels were investigated. Optimal refining and hot-pressing parameters were established for manufacturing MDF products with black spruce tops and bark. Based on the results of this study, the following conclusions can be made:
Buffering capacity and pH differ among species and type of raw materials. Bark has higher acid and alkaline buffering capacities and a lower pH value than wood of the same species due to their extractives. Ten-year-old poplar wood has a higher pH than six year old poplar wood, tops, and bark.
The pH value of the raw fibre materials studied decreases with increased absolute and relative acid buffering capacity due to the increased absolute acidity mass in the solution.
At lower levels of added catalyst, the effect of raw materials pH on UF resin gel time is significant, while it is insignificant at higher catalyst contents. This may be due to the acidity of wood, which is the main source of acid catalyst in mixtures with lower levels of added catalyst. In contrast, at higher levels of added catalyst, the catalyst is the main source of acid catalyst. With higher catalyst contents, all studied raw materials mixed with UF resin result in a longer gel time than with UF resin alone. The gel time of UF resin/wood mixture does not correlate to acid buffering capacity or alkaline buffering capacity, but there is a strong relationship between gel time and both absolute acid buffering capacity and relative acid buffering capacity.
The reaction enthalpy of UF resin increases with catalyst content. The activation energy and peak temperature of curing UF resin generally decrease with increased catalyst content at lower catalyst levels. However, with further increases in catalyst content, the changes in activation energy and peak temperature are insignificant. The hydrolysis reaction of cured UF resin occurs during the latter stages of the curing process at both lower-level (<0.2%) and higher-level (>0.7%) catalyst contents. This indicates that there is an optimal range of catalyst content for UF resin. The curing enthalpy of UF resin decreases with increased amounts of wood raw materials; this is due to the effect of diffusion induced by wood materials and the changes in the phase of curing systems. This suggests the curing reactions reach a lower final degree of conversion for wood/resin mixtures than for UF resin alone.
Hybrid poplar MDF panels show better mechanical properties than jack pine panels; the mechanical properties of jack pine panels are better to those of red pine and white spruce panels. The dimensional stability is the best in jack pine panels among the materials studied (poplar, red pine, and white spruce).
The effect of refining on wood pH is significant, but it is insignificant on bark pH. The pH values of the wood from all species studied reduced after refining.
The effect of bark on the modulus of rupture (MOR) and modulus of elasticity (MOE) of MDF panels is more significant than the effects on IB and other physical properties, such as thickness swelling (TS), water absorption (WA), and linear expansion (LE). All properties of MDF panels (except TS and, in one case, MOR) made with up to 40% bark meet the ANSI standard.
When black spruce tops are used as raw material for MDF, steam pressure is more important than retention time during the refining process in order to achieve better panel properties. The panels show very good mechanical properties and dimension stability under optimal refining conditions even without the addition of wax.