cium carbonate correlate with morphology, filler level, and the effect of .... W. C. J. Zuiderduin, C. Westzaan, J. Huétink, and R. J. Gaymans, Toughening of.
10.1002/spepro.002997
Nanofillers improve the mechanical properties of recycled polypropylene Sylvie Pimbert, Ahmed Elloumi, and Alain Bourmaud
The impact and modulus of polypropylene reinforced with nanocalcium carbonate correlate with morphology, filler level, and the effect of recycling on the resin matrix. Recycled resins have been used for many years, but without much concern for maintaining their mechanical properties. However, recent environmental, economic, and petroleum crises have induced the plastic industry to increasingly turn to reprocessed polymers as one way to improve sustainability. Recycled polypropylene (PP) is of great interest, particularly for the automotive industry.1 One problem with recycled resins is that tensile toughness and impact strength usually decrease during recycling and restoration requires additives. Nanosized additives can confer significant property improvement with very low loading levels compared to traditional microsized additives, which require much higher loading levels to achieve similar performance. Nanoparticles of calcium carbonate (CaCO3 /, an inexpensive filler obtained from limestone, appear promising for improving the mechanical characteristics of recycled PP.2 We injection molded virgin PP five times to simulate the thermal and shear history that a polymer undergoes during recycling. Using compounding extrusion, we prepared nanocomposites with 3, 10, and 20% nanosized CaCO3 filler in both virgin and recycled PP from each molding cycle. We examined the effects of recycling on the thermomechanical and rheological properties of the samples. Figure 1 shows a drop in complex viscosity of unfilled PP with successive injection-molding cycles. This indicates some polymer degradation, which correlates with decreased molecular weight and chain length, and also with slight changes in small-strain properties (yield stress and modulus).3 We used a scanning-electron microscope to observe the morphology of fractured surfaces of the composites. These tests revealed nanoparticle agglomeration (see Figure 2), which creates a weak structure that
Figure 1. Complex viscosity of unfilled polypropylene (PP) decreases with successive molding cycles, from one to five cycles (PP-1 to PP-5).
facilitates debonding between the filler and matrix. We also observed fibrillated and brittle regions, and detachment of particles from the matrix. These indicate poor nanofiller dispersion, which limits the mechanical-property improvement provided by the rigid nanoparticles. Rigid fillers typically increase stiffness, which can be measured with Young’s (tensile) modulus. Figure 3 shows the change in Young’s modulus with filler level for samples with recycled PP matrices. We measured an increase in Young’s modulus from 1025MPa for unfilled, recycled PP to 1122MPa for recycled PP with 10% CaCO3 , compared to 969MPa for unfilled, virgin PP and 1136MPa for virgin PP with 10% CaCO3 .4 The modulus is mainly influenced by filler level, while the effect of the type of PP matrix (virgin or recycled) is minor. Impact behavior is quite different for nanocomposites with a virgin compared to those with a recycled PP matrix. Figure 3 shows that nanocomposites with a recycled PP matrix increase in impact strength from 8kJ/m2 without fillers to 18kJ/m2 with 20% CaCO3 . In contrast,
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Author Information Sylvie Pimbert and Alain Bourmaud University of South Brittany Lorient, France Sylvie Pimbert received her PhD in materials science in 1989. She is associate professor in the Materials Engineering Laboratory of Brittany (LIMATB) and focuses on characterization and processing of polymer blends and nanocomposites.
Figure 2. Scanning-electron micrographs of the fractured surface of recycled PP nanocomposites containing 10% calcium carbonate. (A) and (D): Fibrillation. (B) and (E): Brittle deformation. (C): Nanoparticle agglomeration.
Alain Bourmaud received his MSc in polymer engineering at the Jean Monet University in Saint-Etienne (France) in 1993. He is a research engineer and is exploring polymer processing, biodegradable composites, and mechanical properties of polymers. Ahmed Elloumi National Engineering School of Sfax (ENIS) Sfax, Tunisia Ahmed Elloumi has a mechanical-engineering degreee from ENIS and is working on his PhD. His current research explores polymer composites with mineral fillers. References
Figure 3. Calcium carbonate (wt%: percent by weight) affects Izod impact strength () and Young’s modulus (N) of nanocomposites with recycled PP matrices.
1. J. Toch´acˇ ek, J. Janˇca´ ˇr, J. Kalfus, P. Zboˇrilov´a, and Z. Bur´anˇ , Degradation of polypropylene impact-copolymer during processing, Polym. Degrad. Stabil. 93 (4), pp. 770–775, 2008. 2. W. C. J. Zuiderduin, C. Westzaan, J. Hu´etink, and R. J. Gaymans, Toughening of polypropylene with calcium carbonate particles, Polymer 44 (1), pp. 261–275, 2003. 3. H. M. Da Costa, V. D. Ramosa, and M G. de Oliveira, Degradation of polypropylene (PP) during multiple extrusions: thermal analysis, mechanical properties and analysis of variance, Polym. Test. 26 (5), pp. 676–684, 2007. 4. P. Brachet, L. T. Høydal, E. L. Hinrichsen, and F. Melum, Modification of mechanical properties of recycled polypropylene from post-consumer containers, Waste Manag. 28 (12), pp. 2456–2464, 2008. 5. C. G. Ma, Y. L. Mai, M. Z. Rong, W. H. Ruan, and M. Q. Zhang, Phase structure and mechanical properties of ternary polypropylene/elastomer/nano-CaCO3 composites, Compos. Sci. Technol. 67, pp. 2997–3005, 2007.
composites with virgin PP show a dramatic decrease in impact toughness with increasing filler level. This may be explained by the lower viscosity of recycled PP, allowing better dispersion of the nanoparticles. In summary, recycling has a significant effect on the properties of PP. Nanocomposite properties are influenced by the dispersion of fillers and the presence of aggregates. Our next step will be to prepare and characterize ternary blends by adding a rubber to recycled PP-CaCO3 nano- or microcomposites to obtain composites with higher impact strength and rigidity. The dispersion of rigid fillers in a rubber phase is a good way to improve the compatibility between fillers and the matrix.5
c 2010 Society of Plastics Engineers (SPE)
