Sorption of water–ethanol mixtures by poplar wood: swelling and viscoelastic behaviour

Abstract

The swelling of wood upon impregnation by aqueous solutions of organic solvents is larger than swelling by pure water. This phenomenon, described as “hyperswelling”, results from specific interactions with components of the complex wood microstructure. To understand the combined effect of mixed solvents on wood properties, the specific influence of each solvent on wood biopolymers and structure must be first characterized. In this study, the effect of the impregnation of aqueous solutions of ethanol with concentrations from 0 to 100% on the physico-mechanical properties of poplar wood was investigated. For each solution, the sorption behaviour of veneer sapwood samples and the effect of sorption on wood viscoelasticity and microstructure were measured through vapour sorption gravimetry, dynamic mechanical analysis and optical microscopy monitoring. Pure water and pure ethanol showed contrasting sorption behaviour, ethanol leading to a much lower swelling and a very limited softening compared to water, despite comparable sorbed amounts. This result suggests different affinities of water and ethanol for biopolymers within wood microstructure. With mixed solutions, larger swelling and stronger variations in viscoelastic behaviour than in pure solvents were observed, confirming the synergistic effect of water/ethanol mixtures on structure and properties of wood cells. Microscopic observations evidenced that ethanol, both alone and in aqueous solutions, generates intercellular decohesion and disbonding of the different wood cell wall layers. These observations are consistent with a mechanism of partial solubilization by ethanol of lignin-related phenolic. This could lead to a release of the constraints which limit the swelling of the polysaccharide fraction of the cell wall by water. Such phenomenon is likely to be at the origin of the hyperswelling observed in mixed water/organic solvent systems.