Assessing diferences in the mechanical behaviour of loose cork granulates from virgin and reproduction cork

Resumen

Virgin cork is a little-known, sustainable and relatively scarce raw material. However, its global output is expected to increase substantially as recent cork oak plantations are stripped for the frst time. The work described here aimed to examine the factors underlying the mechanical properties of granulated cork, most particularly the type of cork (virgin or reproduction), and to develop a technique to deliver fast and accurate assessments of the efects of said factors. A batch of virgin cork was boiled, dried, ground and graded following standard granule classifcation procedures according to size and density. The resulting granulates were then compared with equivalent commercial-grade reproduction cork granulates. Physical variables (tapped density and moisture content) were measured and elastic recovery and Young’s modulus were used as proxies for mechanical properties. Image analysis was used to study the size, shape and colour of the cork particles. ANOVA results show signifcant efects of particle size, density class, type of cork and frst and second order interactions between most variables. Density class clearly reached the highest level of significance, whereas the type of cork was less critical. A very strong correlation was found between granulates’ elastic recovery and their tapped density (R2=0.98; RMSE<1%). Likewise, greyscale imaging revealed a good adjustment between tapped density and grey level (R2=0.84; RMSE=24 g·l−1). The primary conclusion was that the diferences between virgin and standard cork granulates are small and should have no efect on less demanding applications. Image analysis is likely to prove useful in further, more in-depth studies.