Study of the flow behavior of polymer-natural fiber suspensions in the power law validity range

The melt flow properties of PVC suspensions with henequen, coconut and bovine leather, in their raw condition, and after extraction of all their low molecular weight substances were studied. The experimental results show that, at the strain rates employed, the behavior of the suspensions is described by a power law (η = mγ^n–1). It is important to mention that in the present work we do not restrict our analysis to the evolution of the suspension Newtonian viscosity with the fiber concentration (as commonly is reported in the literature), but a more complete analysis is performed by describing the evolution of the consistency (m) and the power law (n) indices with the fiber concentration. Both indices were evaluated using a technique previously developed. The flow index (n) presents an anomalous behavior, it increases when the fibers are incorporated into the melted matrix at low fiber concentrations. This unexpected behavior is very different from that commonly reported in the literature and is apparently due to the flexibility of fibers. Moreover, the influence of the fiber extractable materials on the behavior of these suspensions was also evaluated. It was found that these latter materials apparently influence the value of the consistency index (m) of suspensions, at fiber concentrations above the critical pigment volume fraction; however, they do not have a significant effect on the power law index (n). Concerning the mechanical behavior of composites molded from the previous suspensions, it was found that when the fiber content augments, their elastic modulus increases, but their tensile strength falls. This behavior, however, could be improved by increasing the adhesion between the fibers and matrix.