Effect of Polymer Matrix Recycling on the Mechanical, Thermal, and Flammability Properties of Recycled Polypropylene/OCC Pulp/Graphene Nanocomposites

Background and Objectives
The increasing consumption of polymers and the growing accumulation of plastic waste have intensified the need for the utilization of recycled polymers and the development of sustainable composite materials. In this context, the incorporation of cellulosic fibers into polymer matrices represents an effective approach to reducing the consumption of virgin polymers. However, the deterioration of mechanical properties caused by repeated recycling limits the broader application of such materials. Graphene nanoplatelets, as two-dimensional nanofillers with exceptional mechanical and thermal characteristics, offer considerable potential for compensating for these property losses. Therefore, the present study aimed to investigate the combined effects of polypropylene condition (virgin and three-times recycled) and graphene nanoplatelet content on the mechanical performance, thermal stability, and flammability behavior of polypropylene-based nanocomposites reinforced with recycled cellulosic fibers.
Materials and Methods
A 2 × 3 factorial experimental design was employed to manufacture rPP/OCC nanocomposites. Polypropylene was considered at two levels (virgin and three-times recycled), while graphene nanoplatelets were incorporated at three loading levels (0, 0.5, and 1 wt%) as the main experimental variables. The contents of OCC pulp (50 wt%) and maleic anhydride-grafted polypropylene (MAPP) compatibilizer (3 wt%) were kept constant in all formulations. Compounding was performed using a co-rotating twin-screw extruder, and test specimens were subsequently produced through injection molding. Mechanical characterization included tensile, flexural, and notched impact tests conducted according to relevant ASTM standards. Flammability behavior was evaluated using the Limiting Oxygen Index (LOI) test, while thermal stability was assessed through thermogravimetric analysis (TGA). Statistical analyses were performed using analysis of variance (ANOVA) and mean comparison tests at a 95% confidence level.
Results
The results demonstrated that graphene nanoplatelets significantly influenced most of the mechanical properties and the flammability behavior of the nanocomposites. Increasing graphene content to 1 wt% enhanced tensile strength and tensile modulus by 9.9% and 7.3%, respectively. Similarly, flexural strength and flexural modulus increased by 9.8% and 7.7%, respectively, while notched impact strength improved by approximately 2%. Furthermore, the incorporation of 1 wt% graphene increased the Limiting Oxygen Index by approximately 5.7%, indicating improved flame resistance. Thermogravimetric analysis revealed that graphene promoted higher degradation temperatures and greater char residue, thereby enhancing the thermal stability of the composites.
Conversely, recycling polypropylene three times adversely affected the mechanical performance of the materials. Tensile strength and tensile modulus decreased by 4.9% and 21.9%, respectively, while flexural strength and flexural modulus declined by 5.23% and 22.92%, respectively. In addition, notched impact strength decreased by 11.7%. The effect of polypropylene recycling on the Limiting Oxygen Index was not statistically significant, with only a marginal increase of approximately 0.3% being observed. Moreover, the use of recycled polypropylene reduced char residue formation and resulted in lower thermal stability. TGA results confirmed that graphene increased both degradation temperature and residual mass, thereby improving thermal performance, whereas recycled polypropylene exhibited comparatively poorer thermal behavior.
Conclusions
The findings of this study indicate that graphene nanoplatelets act as an effective reinforcing nanofiller capable of significantly enhancing the mechanical properties, flame resistance, and thermal stability of polypropylene-based composites. The incorporation of graphene up to an optimum level of 1 wt% successfully improved the overall performance of the composites and partially compensated for the property deterioration associated with recycled polypropylene. These results suggest that combining recycled polypropylene, recycled OCC cellulosic fibers, and graphene nanoplatelets provides a promising strategy for developing high-performance, environmentally sustainable composite materials.