An investigation on the permeability of different wood furnish materials

Abstract

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 manufacturers, using impact mills and knife ring flakers, respectively. The permeability measurements were performed 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 measured for densities from 250 to 1000 kg m-3. The permeability of the core particle materials was determined on solidified samples. On the other hand, the surface particles 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 densities. 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 elevated 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 temperature conditions during hot pressing. The cross-sectional permeability does not play an essential role in this regard.

Effect of Density and Fiber Size on Porosity and Thermal Conductivity of Fiberboard Mats

“…Furthermore, the porosity decreases differently in mats made with fine, medium and coarse fibers due to the high impact of fiber size on the porous structure of the mat. Previous works performed by Bolton and Humphrey [29], Thoemen and Klueppel [15], Sliseris et al [28] have suggested a particle size effect on the mat structure. This is in agreement with the current results which indicated that fiber size had an impact on void spaces.…”

“…This allows increasing the heat conduction during hot pressing. Nevertheless, small fibers may have an impact on the structure of the fiberboard mat, improving its compression efficiency [15,16].…”

Air permeability in longitudinal and radial directions of compression wood of Picea abies L. and tension wood of Fagus sylvatica L.

“…In addition, due to the requirements of sap flow in living trees, wood permeability is highly anisotropic (Milota et al 1995). Permeability is well known to be an influential parameter for many practical purposes of wood, such as impregnation and preservation (Perré and Karimi 2002;Rhatigan et al 2002;Bradic and Avramidis 2007;Thoemen and Klueppel 2008).…”

3D simulation of macroscopic heat and mass transfer properties from the microstructure of wood fibre networks

“…Finally, the permeability coefficient can directly be computed after rearranging equation 1. A complete description of the sample preparation and the permeability measurement is given by Thoemen and Klueppel [3].…”

“…Direct measurements of the permeability as a function of density have been reported, among others, by von Haas et al [1], Garcia and Cloutier [2] and Thoemen 3 and Klueppel [3], and experimental work to determine the thermal conductivity of wood-furnish mats was done by Shao [4], von Haas [5] and Klueppel [6]. Please note that only those works are listed here where a homogeneous density distribution within the samples were used, which is a requirement to obtain a quantitative relationship between the respective property and density.…”