Fundamentals of MDF Panel Dimensional Stability: Analysis of MDF High-Density Layers

The inherent resin and fiber chemistry of medium density fiberboard (MDF) panels has been compared with panel cold water soak properties. In hot-pressed panels it was observed that resin type and hydrophobicity significantly impact panel swell behavior. Resin affects both fiber hydrophobicity and equilibrium moisture content (EMC), but does not prevent water uptake. It is the influence of resin type on high-density layer thickness swell which suggests resin efficacy is important to retaining bonding in the fiber network and resisting fiber spring-back and panel swell. Fiber contact angles were found to be of less importance, only manifesting when comparing E0 and E1 urea formaldehyde (UF) resins. The E0 panel was considerably more hydrophilic, impacting its wetting behavior, and contributing to greater panel layer thickness swells. For cold-pressed panels, the absence of a high-density layer and relative fiber hydrophobicity were important. Cold-pressed polymeric methylene diisocyanate (pMDI) panels had greater hydrophobicity than E1 UF resin panels which, together with the greater bonding of pMDI, led to improved panel water soak properties. In the case of the cold-pressed E1 UF resin, the relatively hydrophilic fiber and relatively even density profile contributed to an extremely poor water soak performance. Overall, results show that the interactions of contributing factors to panel swell are complex and all may significantly affect panel water soak behaviors.