Catalytic action of natural clay on polyolefins during reactive processing

Abstract

Natural and modified clays based on the montmorillonite structure are widely used for the preparation of polymer composites and nanocomposites. However, these clays can also cause changes in the properties of the polymer matrix after thermal treatment. The aim of the present study was to evaluate the changes in structure and properties of linear low-density polyethylene (LLDPE), polypropylene (PP) and LLDPE/PP blend after thermomechanical processing on high shear rate and controlled atmosphere in the presence of natural clay. Thus, natural bentonite clay in powder form was previously incorporated at 5 wt% to the LLDPE, PP and LLDPE blend in the proportion 70/30 wt%, respectively. Afterward, the materials were processed in an internal mixer MH-600 at 4500 rpm under nitrogen flow, maintaining the shearing by 90 s after polymer melting. Clay caused severe changes in the rheological, thermal and mechanical properties of the polymers, mainly for PP and LLDPE/PP blends, increasing the melt flow rate by 400% and decreasing elongation-at-break to 20% of the initial values. The presence of tertiary carbon in polymer chains of PP should lead to a higher sensibility to the degradation, showing that the stability of the polymer determines the structural changes in the materials. The use and control of these changes can be useful for the processing and compatibilization of complex polymer mixtures, such as polymer waste during mechanical recycling operations. On the other hand, in addition to the reinforcement action, the catalytic and deleterious effects of the clays should also be considered for the preparation of polymer composites.

Graphical abstract