Mass diffusivity of low-density fibreboard determined under steady- and unsteady-state conditions: Evidence of dual-scale mechanisms in the diffusion

Abstract

The mass diffusivity of low-density fibreboard (<300 kg m⁻³) was measured under steady-state conditions (cup method) and under unsteady-state conditions (sorption in the relative humidity range 37–71%). The diffusivity was measured parallel and perpendicular to the sheet plan, for different sample thicknesses (~24, 20, 10, 4 and 2 mm) and different densities (160 and 270 kg m⁻³). The change in mass was measured with a conventional balance for thick samples and a magnetic suspension balance for thin samples. The results indicated that the diffusivity of fibreboard thicker than 20 mm was similar in steady- and unsteady-state measurements. For boards less than this dimension, diffusivity in transient sorption (unsteady state) depended on the sample thickness. In this case, the simplified continuum representation of the physical medium must be discarded. The discussion emphasizes the effect of the double porosity of the material to explain this diffusion behaviour.

Keywords:

• Cup method
• dual porosity
• low-density fibreboard
• mass diffusivity
• sorption method

Acknowledgements

This research was supported by the ANR TransBatiBois (ANR-07-PBAT-008) and Conseil Régional de Lorraine.