Natural fiber-induced degradation in PLA-hemp biocomposites in the molten state

The effect of small amounts (≤3 wt%) of short hemp fibers on stability of a poly(lactic acid) (PLA) in the molten state is investigated through time-resolved rheology. The decrease over time of the zero-frequency complex viscosity was correlated to severe hydrolytic degradation of the PLA matrix. A kinetic model that accounts for thermal degradation and hydrolysis was fitted to the experimental viscosity data. The significant fiber-induced hydrolysis is due to traces of residual water bound to the fibers inside the composites after the processing steps. A significant mitigation of the fiber-induced PLA hydrolysis can be obtained by carefully drying the hemp fibers before melt compounding and (to a lesser extent) by subjecting the fibers to an alkaline treatment. On the other hand, if the interest is a shorter persistence in the environment, natural fibers can be exploited to accelerate degradation of biocomposites.