Relative modulus-relative density relationships in low density polymer-clay nanocomposite foams

Polymer-clay nanocomposite (PCN) foams represent an important class of new materials in structural engineering, biomedical fields and packaging. This paper reports the relative modulus-relative density relationship, a crucial correlation in cellular solids, for low-density PCN foams. Polyurethane (PU)-natural clay nanocomposite foams with a porosity of 97% were used for studies of such relationship. The foam structures were characterised by Scanning Electron Microscopy and X-ray Micro-Computed Tomography and the modulus was obtained from compressive testing. It was found the relative modulus-relative density relationship of low-density PCN foams with porosities higher than 95% closely followed the normalised Gibson-Ashby models for open cells and closed cells, and in the case of PU-clay nanocomposite foams the geometric constant of foam C₁ was determined to be approximately 0.45-0.88 in the well-established model for conventional open-cell foams, namely E _f/E_s = C₁(ρ_f/ρ_s) ² where E and ρ refer to modulus and density and subscripts f and s stand for foam and solid. The effects of clay, clay content and mixing sequence on the cell structure, physical and mechanical properties of the polymer foam were also discussed.