(256a) Deformation of Clay-Filled Epoxy Nanocomposites

Polymer based nano-composites reinforced with a small amount of nanosize clay particles (<5 vol.%) significantly improve the mechanical, thermal and barrier properties of the pure polymer matrix. These improvements are achieved through conventional processing techniques and without affecting substantially the density of the matrix.

Improvement of properties of clay nano-composites is directly related to the complete exfoliation of silicate layers in the polymer matrix. In nano-indentation of cross-linked polymers, entanglements physical and chemical affect reduced modulus and hardness dependence on strain. Strain softening and strain hardening as well as dynamic frictional response are applied to indented polymer films consisting of surface, intermediate, and interface layers. Linear to non-linear deformation is found to be dependent on the density and spacing of the silicate layers. In Crosslinked polymers, interface due to confined surface leads to strain hardening depending on the stiffness of the layer and gel fraction in the layer.

Rheology of cured and uncured epoxy nano-composites were first investigated using a rheometer. Epoxies were mixed with organo-clays, Cloisites. The matrix used in the present study was a two-component epoxy resin system where epoxy resin DGEBA was mixed with Cloisite, commercially available from Southern Clay Products, Inc., USA. It is a natural montmorillonite (MMT) that is organically modified.. Before mixing with epoxy, Cloisite 10A was washed with ethanol and water to remove excess of benzyl dimethyl stearyl ammonium chloride and benzyl dimethyl cetyl ammonium chloride from 7.5% to 0.4% and from 4.0 to 0.2%, respectively.  Similarly, Cloisite 30A was washed with ethanol and water to remove bis(2-hydroxyethyl ) methyl stearyl ammonium chloride from 7.4 % to 0.4.   Vauum was applied to remove bubbles. Epoxies were crosslinked using a crosslinker, hexahydrophthalic anhydride (75%) and Polyether polyol (25%) with 50/50 proportion of crosslinker to epoxy at 90C, 110C and 130C. Coatings of epoxy cross-linked and linear on aluminum surface and Hysitron pyramid indenter was used for different depths.

Results showed nano-clay with higher basal spacing improves the Tg of the composites. SEM was used to distinguish dispersion between different nanoclays in epoxy. Cured epoxy in a

nanocomposite can lead to exfoliated morphology and increase in temperature increases the possibility of exfoliation.  A three phase model of epoxy, exfoliated clay layers and nano layer clusters developed previously was used to characterize and model the young’s modulus of the nanocomposites. Young’s modulus was obtained using Mori–Tanaka method. as a function exfoliation ratio, clay layer and clay cluster aspect ratios, d spacing and intragallery modulus, matrix modulus and matrix Poisson’s ratio. The composite modulus was found to increases with increase in the degree of dispersion, with increase in exfoliation ratio, high aspect of the clay platelets and with larger d spacing.