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

Abstract

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 first 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 effects of said factors. A batch of virgin cork was boiled, dried, ground and graded following standard granule classification 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 significant effects of particle size, density class, type of cork and first 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 differences between virgin and standard cork granulates are small and should have no effect on less demanding applications. Image analysis is likely to prove useful in further, more in-depth studies.