Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix

doi:10.1016/j.polymer.2005.02.018

Polymer

Volume 46, Issue 10, 25 April 2005, Pages 3462-3471

https://doi.org/10.1016/j.polymer.2005.02.018 Get rights and content

Abstract

Polypropylene/organoclay nanocomposites have been prepared via direct melt intercalation in an internal mixer. Maleic anhydride grafted polypropylene (PP-g-MA) was used as a compatibilizer to improve the dispersability of the clay. The structures of nanocomposites have been characterized by X-ray diffraction, transmission electron microscopy and rheometry in small amplitude oscillatory shear. The effects of concentration of PP-g-MA and processing parameters were investigated. Wide angle X-ray diffraction shows that the interlayer spacing increases with the concentration of PP-g-MA, but is not significantly influenced by processing conditions. The study of linear viscoelastic properties shows that the storage modulus G′ is very sensitive to the microstructure of the nanocomposite. A Carreau–Yasuda law with a yield stress is proposed to describe the rheological behavior of these materials. Applications to the twin screw extrusion process are also presented.

Introduction

Organically modified layered silicates (organoclay) are increasingly used for reinforcement of polymeric materials. It has been reported that the dispersion of such minerals at the level of a few nanometers induces a significant improvement in mechanical properties, flame resistance and barrier properties, compared with the pure polymer [1], [2]. In addition, these improvements may be obtained with low clay loading (typically less than 5%). There are several techniques used for dispersing organoclay at a nanoscopic scale, including the addition of organoclay during polymerization (in situ method), or to a solvent swollen polymer (solution blending), or to a polymer melt (melt intercalation method), as described in recent reviews [1], [2], [3].

According to these techniques, two ideal structures defined as intercalated nanocomposite and exfoliated nanocomposite are commonly used to describe the state of dispersion [1], [2]. In the former, the polymer chains are intercalated between the silicate layers, resulting in a well ordered alternating layered silicates and polymer chains. In contrast, in the exfoliated structure, the individual clay layers are dispersed in the polymer matrix. The term conventional composite or microcomposite is also used to describe the structure of nanocomposite containing the clay tactoids with the layers aggregated in unintercalated form.

One of the most commonly used organically layered silicates is derived from montmorillonite (MMT). Its structure is made of several stacked layers, with a layer thickness around 0.96 nm and a lateral dimension of 100–200 nm [4], [5]. These layers organize themselves to form the stacks with a regular gap between them, called interlayer or gallery. The sum of the single layer thickness (0.96 nm) and the interlayer represents the repeat unit of the multilayer material, called d-spacing or basal spacing (d₀₀₁), and is calculated from the (001) harmonics obtained from X-ray diffraction patterns.

The clay is naturally a hydrophilic material, which makes it difficult to exfoliate in a polymer matrix. Therefore, the surface treatment of silicate layers is necessary to render its surface more hydrophobic, which facilitates exfoliation. Generally, this can be done by ion-exchange reactions with cationic surfactants, including primary, secondary, tertiary and quaternary alkylammonium cations [6], [7]. This modification also leads to expand the basal spacing between the silicate layers due to the presence of alkyl chain intercalated in the interlayer. For polymer containing polar functional groups, an alkylammonium surfactant is adequate to promote the nanocomposite formation. However, in the case of polypropylene, it is frequently necessary to use a compatibilizer, such as maleic anhydride modified polypropylene (PP-g-MA). Kawasumi et al. [8], Kato et al. [9] and Hasegawa et al. [10] showed that there are two important factors to achieve the exfoliation of the clay layer silicates: (1) the compatibilizer should be miscible with the polypropylene matrix, and (2) it should include a certain amount of polar functional groups in a molecule. Generally, the polypropylenes modified with maleic anhydride (MA) fulfill the two requirements and are frequently used as compatibilizer for polypropylene nanocomposites. However, they have mechanical properties lower than the native polypropylene, due to chain scission during grafting. Therefore, the addition of PP-g-MA can result in lower mechanical properties of the final composite. Hence, it is important to investigate the effect of PP-g-MA on the degree of dispersion, to optimize its concentration.

The object of this study is to examine the effect of PP-g-MA concentration in the system polypropylene/organoclay and to characterize the influence of processing conditions on clay dispersion. The polypropylene-organoclay composites were prepared by direct melt intercalation method, both in internal mixer and twin screw extruder. The state of dispersion was analyzed by X-ray diffraction, transmission electron microscopy as well as melt rheometry.

Section snippets

Materials

The organoclay used in this study (Cloisite®20A) is obtained from Southern Clay Products (Gonzales, TX). It is a Na⁺-montmorillonite, chemically modified with dimethyl dihydrogenated tallow quaternary ammonium chloride, where N⁺ denotes quaternary ammonium chloride and HT denotes hydrogenated tallow (Fig. 1). HT is made of approximately 65% C₁₈H₃₇, 30% C₁₆H₃₃ and 5% C₁₄H₂₉.

All polymers used in this study are obtained from Atofina. The homopolymer polypropylene (PPH5060) has a melt flow index of

Effects of PP-g-MA concentration

Fig. 2 shows the series of X-ray diffraction spectra of original Cloisite®20A and PP/PP-g-MA/Cloisite®20A composites, in which the concentration of PP-g-MA varies from 0 to 40 wt%. We recall that the Cloisite®20A concentration is constant and equal to 5% wt. The interlayer spacing of Cloisite®20A is 2.51 nm before compounding. For the uncompatibilized system (95/0/5), the XRD pattern exhibits no significant increase in interlayer spacing (2.56 nm after mixing). This indicates that the

Application to twin screw extrusion

We have until now studied the dispersion of layered silicates using an internal mixer, to elucidate the effects of various processing parameters. In this section, we will present the results of a preliminary study on the dispersion of layered silicates using an industrial self-wiping co-rotating twin screw extruder. The formulation used is 80/15/5.

Fig. 17 shows XRD patterns of composites extruded at different feed rates. Compared to the native clay, it is observed that the peak d₀₀₁ is shifted

Conclusion

Polypropylene nanocomposites have been prepared via direct melt intercalation by using an internal mixer and a co-rotating twin screw extruder. The degree of dispersion is improved by incorporating a maleic anhydride grafted polypropylene (PP-g-MA). However, this improvement is obtained for concentrations of PP-g-MA higher than 10 wt%. The clay aggregates become smaller and silicate layers are finely dispersed, as the ratio of PP-g-MA increases. However, no further improvement on the

Acknowledgements

Polymers used in this study (polypropylene, PP-g-MA) were kindly provided by ATOFINA, which is gratefully acknowledged. We also thank M.Y. Perrin (CEMEF) for his precious help in Transmission Electron Microscopy.

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Influence of compatibilizer and processing conditions on the dispersion of nanoclay in a polypropylene matrix

Abstract

Introduction

Section snippets

Materials

Effects of PP-g-MA concentration

Application to twin screw extrusion

Conclusion

Acknowledgements

Mater Sci Eng

Prog Polym Sci

Adv Polym Sci

Ind Eng Chem Res

X-Ray diffraction and the identification and analysis of clay minerals

Polymer

J Adh