Elsevier

Polymer

Volume 45, Issue 17, 5 August 2004, Pages 5985-5994
Polymer

Crystallization and impact energy of polypropylene/CaCO3 nanocomposites with nonionic modifier

https://doi.org/10.1016/j.polymer.2004.06.044Get rights and content

Abstract

Isotactic polypropylene (PP) and calcium carbonate (CaCO3) nanocomposites were prepared by melt extrusion in a twin screw extruder. The commercial CaCO3 nanoparticles had a poor dispersion in PP matrix. The addition of a small amount of a nonionic modifier during melt extrusion greatly improved the dispersion of CaCO3 nanoparticles. The influence of CaCO3 nanoparticles on the crystallization of PP was studied by wide angle X-ray diffraction and polarized optical microscopy. The introduction of CaCO3 particles resulted in small and imperfect PP spherulites, decreased spherulite growth rate and induced formation of β-form PP. The yield strength of PP decreased gradually while its Young's modulus increased slightly with increasing CaCO3 loading. By adding 1.5 wt% of nonionic modifier to PP/CaCO3 (85/15) nanocomposite these tensile properties were not changed much but the notched Izod impact energy of the composites was significantly increased.

Introduction

Isotactic polypropylene (PP) is a most common commodity plastic, which is of practical use in many areas such as home appliances, automotive, construction, and other industrial applications. However, PP is notch sensitive and brittle under severe conditions of deformation, such as at low temperatures or high impact rates, which has limited its wider range of engineering usage. Blending PP with rubber is an efficient way to increase its toughness, but one drawback of rubber toughening is the significant loss of both tensile strength and stiffness of PP. Incorporation of rigid inorganic particles is a promising approach to improve both stiffness and toughness of plastics simultaneously. Some positive results have been achieved in high density polyethylene (HDPE) and PP [1], [2], [3], [4], [5], [6], [7].

In 1992, Fu et al. [1] reported that HDPE could be toughened by micron-scale CaCO3 particles pre-treated with a phosphate modifier before melt blending. When notched Izod impact strength was plotted against matrix ligament length (τ), a master curve of brittle–ductile transition was obtained with a critical ligament (τc) of 5.2 μm. Bartczak et al. [2] toughened HDPE with a notched impact strength of 50 J/m by using calcium stearate modified CaCO3 particles with average diameters of 3.50, 0.70 and 0.44 μm, and obtained a critical ligament length 0.6 μm for the onset of brittle–ductile transition. SEM observations showed that the fracture process zone morphology in HDPE/CaCO3 composites was very similar to that in the HDPE/EPDM blends [3] indicating both had the same deformation and failure mechanisms. The only difference was that the cavities in the HDPE/EPDM blends originated from internal cavitation of EPDM particles while in the HDPE/CaCO3 composites they were mainly due to debonding at the particle-matrix boundaries.

PP has also been toughened with rigid inorganic particles. Thio et al. [4] used three types of CaCO3 particles with average diameters of 0.07, 0.7 and 3.5 μm to toughen PP. It was reported that the 0.7 μm diameter particles improved Izod impact strength up to four times that of the unfilled matrix. The major toughening mechanisms were interfacial debonding and plastic deformation of inter-particle ligament and crack deflection. The other particles, 0.07 and 3.5 μm, had either adverse or no effect on impact toughness due to their poor dispersion. Dispersion quality of CaCO3 particles played a crucial role in toughening efficiency. Gaymans et al. [5] reported that stearic acid coated particles showed a large positive effect on impact strength caused by the improved dispersion in PP. But particle sizes less than 0.7 μm tended to aggregate and showed very poor dispersion, which had a detrimental effect on impact strength. Agglomeration became worse as particle size was decreased.

Recently, CaCO3 nanoparticles were commercially available for toughening polymers. Although small and uniform particles are believed to be more effective in toughening than large ones, nano-particles, even if they were surface-modified, often agglomerate and cause a significant decrease in toughness [4], [5], [6]. Hence, failure to obtain uniform nanoparticles dispersion in the polymer matrix is a first major challenge for effective toughening. Chan et al. [7] prepared PP/CaCO3 nanocomposites by melt mixing in a Haake mixer. It was found that the dispersion of the CaCO3 nanoparticles in PP was good when the loading was lower than 9.2 vol%. The incorporation of CaCO3 nanoparticles increased the notched impact strength of PP by about 2-fold.

Compared to mixer, twin screw extruder is practically more convenient to compound polymer blends and composites in large batch sizes in the plastics industry. We used a twin screw extruder to compound PP with CaCO3 nanoparticles. The purposes of this study are to toughen and stiffen PP with these rigid inorganic particles and to explore the mechanism of nanoparticle-toughening. The CaCO3 nanoparticles, treated with fatty acid, were purchased from the same company with the same tradename (CCR) as those used in Ref. [7]. However, it was found by SEM observations that the CaCO3 nanoparticles exhibited a poor dispersion in PP matrix after the melt extrusion process. The fatty acid on the surface of nanoparticles did not ensure a uniform dispersion of these nanoparticles in PP. Thus, we chose a nonionic modifier to further improve dispersion of CaCO3 nanoparticles. The effects of this modifier on the dispersion quality and toughening efficiency of PP/CaCO3 nanocomposites were investigated. The interaction between CaCO3 particles and PP was evaluated and the toughening mechanism discussed.

Section snippets

Sample preparation

PP was provided by ICI Australia Operations under a tradename of GWM 22. Calcium carbonate nanoparticles were supplied by Guang Ping Nano Technology Group (Hong Kong) under a tradename of CCR. Chan et al. [7] reported that the CaCO3 size was in a range 30–90 nm with an average value of 44 nm. A nonionic modifier, polyoxyethylene nonyphenol (PN), was obtained from Tianjin Feejy Trade Co., Ltd (China) and used to improve the dispersion of these CaCO3 nanoparticles in the PP matrix.

The CaCO3

Dispersion of CaCO3 nanoparticles

Fig. 1 shows the CaCO3 nanoparticles, which form many aggregates because of their high specific surface areas. The strong tendency to form agglomerates makes it difficult to disperse uniformly the nanoparticles in the matrix. Chan et al. [7] confirmed by TEM observation that, for CaCO3 content lower than 20 wt%, most CaCO3 aggregates were broken down into primary particles in the PP melt by mixing them for 0.5 h in a Haake mixer. However, at a high CaCO3 loading of 27 wt%, more aggregates were

Conclusions

PP/CaCO3 nanocomposites were prepared by melt compounding in a twin screw extruder. The dispersion quality of CaCO3 particles in PP was greatly improved by the addition of the nonionic modifier. The CaCO3 particles caused small and imperfect PP spherulites, decreased the spherulite growth rate and induced formation of β-form PP. The modifier decreased the activation energy for the transport of PP molecular segments, leading to higher spherulite growth rates. There were only slight improvements

Acknowledgements

We wish to thank the Australian Research Council (ARC) for the continuing financially support of this project on ‘Polymer Nanocomposites’. Y.-W. Mai acknowledges the award of an Australian Federation Fellowship by the ARC tenable at the University of Sydney. Z.Z. Yu also appreciates the support of a Sydney University Sesqui Postdoctoral Fellowship.

References (22)

  • Z. Bartczak et al.

    Polymer

    (1999)
  • Z. Bartczak et al.

    Polymer

    (1999)
  • Y.S. Thio et al.

    Polymer

    (2002)
  • A.S. Argon et al.

    Polymer

    (2003)
  • C.M. Chan et al.

    Polymer

    (2002)
  • S.C. Tjong et al.

    Polymer

    (1996)
  • P.M. McGenity et al.

    Polymer

    (1992)
  • F.J. Torre et al.

    Polymer

    (2003)
  • Q. Fu et al.

    Polym Eng Sci

    (1992)
  • W.C.J. Gaymans et al.

    Polymer

    (2003)
  • S.C. Tjong et al.

    Polym Eng Sci

    (1997)
  • Cited by (0)

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