Mechanical properties of polypropylene composites based on natural fibers subjected to multiple extrusion cycles

MORAN JUAN; ALVAREZ VERA; VAZQUEZ ANALÍA

Buenos Aires, Argentina

Congreso; III International Conference on Science and Technology of Composite Materiales; 2005

Universidad de Peruggia, Italia; Universidad de mar del Plata, Argentina

In this work, the feasibility of re-use of composite materials based on polypropylene and natural fibers was studied. Composites were made using a reinforcement weight fraction of 20 % of flax fibers. The composite was obtained by twin screw extrusion. The recycling process was simulated applying multiple extrusion steps. In order to characterize the resulting materials, compression molded plates were made.  The aim of this work was to study the thermo – mechanical degradation suffered by the compounding materials separately (matrix and fibers) and to show a relationship between that and the resulting materials final properties. In order to estimate the mechanical damage suffered by the natural fibers, its final dimensions (length and diameter) were measured by means of optical microscopy. Mechanical properties of polypropylene and the composite material were measured after each extrusion cycle. It was observed that the elastic modulus rose due to the addition of natural fibers, showing its reinforcement capabilities. For the case of polypropylene, the elastic modulus increased during the first process cycle, when compared to the virgin material, mainly due to intensive chain scission (also observed in Melt Index values). For the following steps, the modulus kept constant. In the other hand, for the case of the composite material, the measured elastic modulus for a single extrusion was lower than that predicted applying the Halpin – Tsai model due to a poor mixing. The modulus rose for the following two cycles, while a much better fiber dispersion was observed. For the following extrusion cycles (higher than three), the modulus decreased slowly. This correlates well with a darkening and poor organoleptic properties observed due to thermal degradation. The elastic modulus of the different composite materials obtained was correlated using the Halpin – Tsai model. This model was applied using two correction factors. The first one was applied to the fiber reinforcement volume and could be related to mixing quality. The second one was applied to the Young modulus of the fibers and could be related to the thermal degradation of the fibers. Finally, the mechanical damage influence on the fiber´s properties was considered by means of the Griffith model.