Viscolelastic behavior of agglomerated cork

Abstract

Cork (bark of Quercus suber L.) is a natural and renewable material with a unique combination of properties, like elasticity, resilience, impermeability, low density, and very low conductivity of heat, sound and vibration. Among many current uses, wine bottle stoppers are the leading application. These provide an effective and removable sealing, allowing preservation or even improvement of wine properties over time [1]. Cork waste from natural stopper manufacturing is granulated and combined with adhesives to form an agglomerated material with many uses, namely as the so-called technical cork stoppers. In this study, the viscoelastic properties of cork agglomerated with polyurethane adhesive (PUR) are investigated and compared to natural cork. Dynamic mechanical analysis (DMA) of the agglomerated material revealed two distinct thermal transitions, one at -45 °C, related to the glass transition of PUR, and one at 3 °C, expected to be associated with some of corks components. On the other hand, DMA results for natural cork showed a transition at 10 to 25 °C, depending on the direction of tensional testing relative to the cork cell structure. This relaxational process was found to be consistent with melting of suberin, a natural polyester that is corks main constituent. The apparent decrease in the transition temperature observed in the agglomerated material may be due to the process of agglomeration of the cork granules, which implies compression and heating to temperatures close to 120 °C. Upon cooling, re-crystallization under tension may lead to different structures, with lower melting temperature. The storage modulus, E, of agglomerated cork was found to be similar to that of natural cork. Creep-recovery experiments were well described by Burgers model and Weibull distribution function. Agglomerated cork exhibited better strain recovery than natural cork. This was also observed in multi-cycle tests and can be explained by the high elasticity of the PUR component.