An investigation on the permeability of different wood furnish materials

摘要

In wood-based composite mats, mass and heat transfer are substantially influenced by their gas permeability. Thus, reliable permeability data are needed if one wants to model and simulate those processes taking place prior, during, and after hot-pressing operations of the mat. In this study, wood particles of surface and core layer type were obtained from two particleboard manu-facturers, using impact mills and knife ring flakers, respectively. The permeability measurements were per-formed on the initial materials, as well as on subsets of it obtained by laboratory sieving. Additionally, medium density fibreboard (MDF) fibres from three species, namely pine (Pinus sylvestris), spruce {Picea abies) and rubberwood (Hevea Brasiliensis) were investigated. Cross-sectional and within-plane permeability was meas-ured for densities from 250 to 1000 kg m~(-3). The perme-ability of the core particle materials was determined on solidified samples. On the other hand, the surface parti-cles and MDF fibres were investigated by means of a rapid testing method on loose furnish materials (without solidification). MDF mats exhibited lower permeability values than particle mats at low densities, but values were found in the same order of magnitude for high den-sities. Mainly, the small particles determine the pore structure and permeability of particle mats. Thickness and in-plane size of particles affect the mat permeability, with the in-plane size becoming primarily effective at ele-vated densities. The permeability of MDF fibre mats depends more on the anatomy of the raw material than on its size distribution. We suggest, based on computer simulations, that only the within-plane permeability has a strong effect on gas pressure and tem-perature conditions during hot pressing. The cross-sec-tional permeability does not play an essential role in this regard.