Characterization and Modeling of Strength Enhancement Mechanisms in a Polymer/clay Nanocomposite

Abstract

The compressive failure of continuous fiber reinforced composites has received considerable attention in the recent years because it typically occurs at a stress level that is 40-50 % below the tensile strength of the composite. This paper is primarily focused on enhancing the compressive strength of polymer matrix composites (PMC) through a series of modifications in material composition and thermoplastic pultrusion manufacturing process variables. Low-cost commodity resins such as polypropylene (PP), suffer primarily due to low compressive strength. Enhancement of the compressive strength of pultruded thermoplastic composites is achieved by improving the yield strength of the surrounding matrix in shear and reducing fiber misalignment in the composite through optimization of manufacturing process variables. The dispersed platelets are typically one micron in length but only a nanometer in thickness. A single-screw extruder was used to facilitate nanoclay dispersion in PP. This new family of materials exhibits enhanced stiffness and strength of the matrix material, through the inclusion of exfoliated nano-scale montmorillonite particles in the fabrication of resin pre-impregnated (prepreg) glass fiber filaments. After the prepreg was pultruded to form a composite laminate, uniaxial compression tests were performed to determine compression strength of the laminate. Scanning Electron Micrographs (SEM) were taken to examine the failure surfaces. Transmission Electron Microscopy (TEM), were also employed to reveal all nano-scale platelet morphologies, namely exfoliated, intercalated and stacked structures within the samples. Dramatic improvement in compressive strength and compressive modulus were observed with relatively low nanoclay loadings. Compressive strength tests were also performed on aged pultruded PP composite with 0% and 1 % nanoclay loadings, and the data were compared with compressive strength data for unaged specimens. A significant increase (12-20%) in compressive strength was observed after the specimens have been aged for 15 months under ambient conditions. However, a 30% decrease in compressive modulus due to aging was recorded for specimens with 1% clay loading. Multi-scale simulations of nanoclay/polymer interface behavior are currently in progress in order to understand the strength enhancement mechanism.