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A new strategy to control the cellular anisotropy ratio in natural rubber foams

Thursday (25.10.2018)
12:10 - 12:30
Part of:

Natural rubber (NR) based foams are generally produced from chemical foaming processes in which the expansion of the polymer is isotropic and hence, their properties are not directionally dependent. Nevertheless, obtaining anisotropic cellular structures could be important for applications in which for instance, different mechanical properties depending on the load direction would be required or if mechanical properties in one direction need to be improved. On the other hand, the comprehension of the effect of anisotropy ratio on the mechanical properties of foams is still a challenge, due to the difficult task of producing foams with the same density, same polymeric matrix and clearly different anisotropy ratios. In this work, elastomeric foams based on natural rubber with a medium relative density (around 0.3) and with varied cellular structures in terms of the shape anisotropy ratio of the cells were produced by a chemical foaming process in which expansion was restricted to only one direction inside a mould. The use of solid precursors with the same volume but with different diameter/height ratios, the elastomeric properties of natural rubber and the crosslinking by sulphur of the polymer matrix during foaming allowed us to produce foams with a wide range of anisotropy ratios between 0.90 and 2.48 at the same density and with the same properties of the polymer matrix. Therefore, anisotropy ratio was the only parameter able to affect the mechanical behavior of the foams obtained, which allowed to establish a very accurate relationship anisotropy ratio-compressive modulus by means of performing compressive tests at low strains in three different directions, one parallel to the cells elongation direction and the other two perpendicular to it. In addition, the results obtained were evaluated by means of analytical models generally used to describe the mechanical behavior of anisotropic polymer foams. Finally, it is fair to point out that a model based on geometrical considerations was built, which allowed to obtain a theoretical anisotropy ratio which fit reasonably well with the experimental results obtained. This model would allow predicting the anisotropy ratio of polymer foams produced under the same circumstances of the natural rubber-based foams produced in this work.

Dr. Leandra Oliveira Salmazo
University of Valladolid
Additional Authors:
  • Dr. Alberto López-Gil
    CellMat Technologies
  • Prof. Dr. Miguel Ángel Rodríguez-Pérez
    University of Valladolid