Mechanical properties of cork: Effect of hydration

• The mechanical properties of cork highly depend on its hydration state.
• Young’s modulus of cork decreases for RH > 50% due to a clustering of water molecules.
• A glass transition at 0 °C and a β relaxation around −80 °C were identify by DMTA.
• This glass transition temperature is strongly affected by the moisture content in cork.

Cork is known to exhibit low permeability to liquids and gases, imputrescibility and good mechanical properties, with a remarkable elasticity. These properties make this material particularly interesting for sealing wine. We focused in this study on the compression properties of cork along the radial and tangential direction at 25 °C under atmospheric pressure when cork is stored in various relative humidity environments, from 0% to 100%.The direction of compression significantly affected the Young’s modulus, with a higher value for the radial direction. This corresponds to the orientation of the lenticels which reinforce the rigidity of the material when the strain is applied along their growth direction. More surprising is the effect of water sorption in cork on its mechanical property. Both radial and tangential directions exhibit the same behavior when the relative humidity is increasing. First the Young’s modulus is constant up to 50% relative humidity (RH) with mean values around 37 MPa and 22 MPa for radial and tangential directions, respectively. Then, above this point, the increase in water content leads to a decrease in material rigidity which is attributed to water clusters formations. For high moisture contents, the anisotropy of cork is reduced: Young’s moduli are of 10.5 MPa and 6.6 MPa for radial and tangential orientations, respectively.Differential scanning calorimetry and dynamic mechanical thermal analysis (DMTA) allowed to identify a glass transition temperature (Tg) in cork over a broad range of temperatures, depending on the moisture content and giving a Tg-midpoint from −8 °C to 3 °C. Moreover, a secondary transition was observed by DMTA at approximately −80 °C, for 50% RH.

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