Dynamic mechanical analysis and creep-recovery behavior of agglomerated cork

Abstract

The mechanical behavior of agglomerated cork, made of cork granules bound with polyurethane moisture-cured adhesive is investigated and compared to natural cork in the small strain regime (strain < 5%). Dynamic mechanical analysis (DMA) of the agglomerated material revealed two distinct thermal transitions, one at -45 A degrees C, related to the glass transition of polyurethane, and one at 3 A degrees C, associated with melting of suberin, a natural polyester that is the main component of cork's cell walls. Natural cork showed the latter transition to occur at a higher temperature range, between 10 and 25 A degrees C, probably due to a different crystalline arrangement being formed upon cooling the cork granules under pressure in the mold. The storage modulus of agglomerated cork was found to be similar to that of natural cork. Creep and recovery experiments were well described by Burgers model and Weibull distribution function, respectively. Agglomerated cork showed higher instantaneous creep strain and viscous flow than natural cork, probably due to relative displacement and slippage of the granules being allowed by the binder. In all cork materials, not all the instantaneous creep strain was instantaneously recovered. A fraction underwent delayed recovery and another turned into permanent strain. This behavior was associated with the deformation of corrugations in the cork cell walls. Cyclic creep-recovery tests showed for all cork materials recoveries above 90%, except for the first cycle.