A simple rheological method for the experimental assessment of the fiber percolation threshold in short fiber biocomposites

The identification of the percolation threshold (Phi(c)) in short fiber composites is a challenging problem in Composite Science. Above Phi(c) the fibers form a continuous network that causes substantial changes in mechanical and transport properties. Besides, percolation of natural fibers in biodegradable polymer matrices allows water and other pro-degradative species to access the inner parts of the material from the external environment, accelerating biodegradation. Whether such a speeding up is desired or not, assessing Phi(c) in composites is of utmost importance. Unfortunately, natural fibers are not conductive and exhibit highly variable shape and physical properties. This prevents the use of many experimental and theoretical approaches for the estimate of Phi(c). Here we propose an original rheological approach borrowed from the viscoelastic modelling of polymer nanocomposites. The method was applied to two systems made of poly(lactic acid) filled with hemp or kenaf fibers (average length <500 mu m, average length-to-diameter ratio <5). The estimate of Phi(c) (similar to 10.1 and 19.5 vol% for the hemp- and kenaf-based composite, respectively) required a single set of simple linear viscoelastic measurements, and the computed values were in good agreement with those obtained through time-consuming (measurement times >3 weeks) dielectric spectroscopy analyses (similar to 10.1 and 18.5 vol%).