The diffusion of moisture in paper is a complex phenomenon with pore diffusion dominating at low moisture contents and diffusion through ﬁbres dominating at high moisture contents. Vapor diffusion through the pore space depends on the topology of the pore connections. Recently available three dimensional digital reconstructions of the pore space using X Ray Micro computed tomography (XμCT) enable us to determine the impact of the pore and ﬁbre tortuosities and connectivity to moisture diffusion in an explicit manner.
In this study, moisture diffusion was simulated through XμCT reconstructions of paper structures using a hybrid random walk algorithm that was developed to allow simultaneous diffusion in both the pore and ﬁbre spaces with differential ‘intrinsic diffusivities’. The algorithm is speciﬁcally applied to simulate simultaneous diffusion under low and high relative humidity conditions where diffusion occurs predominantly through one medium i.e. pore space and high humidity conditions where both media (i.e. ﬁbre and pore spaces) are highly conductive. The ‘intrinsic diffusivity’ of moisture through ﬁbres was determined by using numerical simulation and experimental results. This intrinsic diffusivity a fundamental ﬁbre characteristics is found to be independent of reﬁning level but depends only the ﬁbre moisture content under the conditions studied.
The algorithm also allowed the determination of the anisotropy in diffusivity. One interesting result is that the anisotropy in diffusion is most signiﬁcant at low moisture contents when diffusion through the pore space dominates. At high relative humidities (i.e. at high moisture contents), ﬁbre conduction provides an alternative diffusion path, homogenizing diffusion to a large extent. As a result, diffusion becomes more isotropic with increased moisture contents in paper.