Evaluation by torque-rheometer of suspensions of semi-rigid and flexible natural fibers in a matrix of poly(vinyl chloride)

The flow properties of three different PVC-natural fiber suspensions were evaluated using a torque-rheometer. The fibers used were extracted from henequen leaves, coconut husk, and bovine leather. They have distinct mechanical properties that produce suspensions with different flow behaviors. Nevertheless, the experimental results show that, at the rates employed, the behavior of the suspensions can be described by a power-law (sigma = m gamma(.n)). The flow (n) and consistency (m) indexes of each suspension were evaluated using an approximate technique previously developed. The evolution of these indexes as a function of the volume fraction of fibers (Phi) are studied and discussed. It was found that, at diluted concentrations, the value of the flow index unexpectedly increases with the concentration of fibers, especially in those suspensions containing the more flexible fibers. However, at elevated concentrations it substantially diminishes. To model this behavior an empirical function for n(Phi) is proposed. The index of consistency (m) presents a continuous increment with the concentration of fibers. To model the evolution of this parameter with the content of fibers, we have defined a relative consistency index m(r)(m(r) = m(s)/m(o), where m(s) and m(o) are the suspension and matrix indexes) in a similar form to that established for the relative Newtonian viscosity. The function m(r)(Phi) is modeled using several equations used to describe the behavior of this last viscosity (Maron and Pierce, Mooney, Thomas, Sudduth and Polynomial models). Among them, the Sudduth and Polynomial equations were those that better fit the behavior of these type of suspensions. Finally, to test the previous methodology, the theoretical torque values obtained were compared with experimental data for each suspension. Excellent agreement among them was found.