How does hydration affect the mechanical properties of wine stoppers?

Data related to the comparison of the mechanical properties of the different stoppers used in the wine industry are scarce. This study aims at comparing the effect of hydration (from 0 to 100 % relative humidity at 25 °C) on the mechanical properties of four widely used types of stoppers: natural corks, agglomerated corks, technical stoppers and synthetic (co-extruded) stoppers. For both natural and agglomerated corks, the Young’s modulus was significantly and similarly affected by hydration, with a constant plateau value up to 50 % relative humidity (RH) and a mean value around 22 and 14 MPa, respectively. For higher RH, the increase in water content leads to a decrease in the material rigidity (Young’s modulus <10 MPa), which is attributed to water clusters formation between polymer chains. Technical stoppers revealed a similar profile, but with a much smaller impact of the water content and with overall lower Young’s moduli values, around 5 MPa, throughout the RH range. The stiffness of synthetic closures was not affected by hydration, in agreement with the hydrophobic behavior of polyethylene. Differential scanning calorimetry and dynamic mechanical thermal analysis allowed us to identify a glass transition temperature (Tg) in cork (around 0 °C), and another one in agglomerated cork and technical stoppers (close to −45 °C, corresponding to additives). All together, for the first time the data highlight the comparative mechanical properties of such materials of the wine industry, and the progressive loss of the “cork-like” behavior of cork composites when other components are mixed with cork.