Transient water vapor diffusion in paper treated with methyltrimethoxysilane modified starch

The interaction of cellulose paper with water is a major hindrance to its broader application. This study, which introduces a novel approach to understand water vapor diffusion in both untreated and treated paper, aims to identify the diffusion coefficient, a crucial property in improving the hydrophobicity of paper. The treatment process utilized an aqueous solution of starch or starch modified with methyltrimethoxysilane (MTMS). While the initial sorption method is frequently used to determine the diffusion coefficient, this study found that it could lead to significant errors due to the non-Fickian behavior exhibited by lignocellulosic materials. This behavior causes that the hygroscopic equilibrium is not instantly obtained by surface of paper. It also induces slowing down moisture diffusion in its final stage due to molecular relaxation. For the first time, the modified convective boundary condition was introduced into the moisture diffusion model in paper materials. The results from vapor sorption experiments demonstrated this non-Fickian behavior, particularly at high values of air relative humidity. The study also revealed that the commonly applied first kind boundary condition is not applicable, even for thin paper samples, inhibiting the use of the initial sorption method for determining the diffusion coefficient. While the treatment with starch and MTMS significantly improved the hydrophobic properties of paper, it didn’t alter substantially its hygroscopic properties, potentially due to not blocking active sorption sites of cellulose fibers. This research underscores the need for further investigation into the chemical modification of cellulose fibers to improve the hydrophobicity of paper.