Abstract
In this study, percolation theory is applied to explain the transitions in mechanical properties in sand-filled polyethylene composites. It is well known that the mechanical properties of polymers are highly dependent on the presence of fillers. When the particle content in a polymer matrix is increased, particles will eventually touch and form a continuous path of connected particles, called the percolation point. A lesser studied phenomenon is the rigidity percolation point in particulate composites. At this volume fraction filler, there is just enough resin present in the composite to yield a rigid structure. At a volume fraction polymer less than the rigidity percolation point, the particles may very well form a continuous network, but this network has no rigidity and hence no mechanical strength. In this study, it is found that the rigidity percolation point occurs at approximately 20-25 vol% polymer, depending on which mechanical property is tested. The particle percolation point or the second transition at higher volume fraction polymer occurs over a wider range of resin volume fraction and falls between 65 and 85 vol% polymer, depending on which mechanical property is used for its measurement. It is found that the mechanical properties change dramatically at both these points, although not in the order of magnitude as has been reported for changes in electrical conductivity near the percolation point for polymers filled with conductive fillers.