Effect of reprocessing and clay concentration on the degradation of polypropylene/montmorillonite nanocomposites during twin screw extrusion

https://doi.org/10.1016/j.polymdegradstab.2012.12.009Get rights and content

Abstract

An evaluation was made of the degradation of polypropylene (PP) and montmorillonite (MMT) nanocomposites. The nanocomposites were compatibilized with polypropylene grafted with maleic anhydride (PP-g-MA) and with acrylic acid (PP-g-AA). The concentration of MMT varied from 1 to 5% (w/w). Nanocomposites were processed in a twin screw extruder up to five times. X-ray diffraction showed a decrease in the refraction angle with reprocessing steps and for PP-g-MA based nanocomposites, indicating an increase in the interplanar distance due to clay intercalation and exfoliation. Infrared analysis showed an increase in the concentration of carbonyl and unsaturated groups with the increase in the number of reprocessing cycles, and reduction as the MMT concentration increased. Colorimetry assays indicated a decrease in the luminosity and degree of redness due to the oxidation of the iron ions present in MMT to Fe2O3. The intensity of the yellow color of the b* coordinate was augmented with the number of reprocessing cycles but decreased with MMT concentration, confirming the FTIR analysis. Rheological measurements showed no crossing point between storage (G′) and loss (G″) moduli versus frequency, which is characteristic of a percolated nanocomposite networks. Within a range of frequencies, complex viscosity decreased with the number of reprocessing cycles, but the presence of montmorillonite was found to be responsible for lower differences between viscosity of nanocomposites reprocessed one and five times.

Introduction

Numerous researches have focused on the study and development of polymeric materials. One important aspect to be considered is the degradative process that can take place in polymer synthesis, processing, and use. The degradation leads to decrease [1], [2] or increase [1], [3] in molecular weight, but the fact is that these changes also influence materials properties. For example, polypropylene is prone to undergo chain scission during processing in extruder machines due to thermoxidative and mechanical degradation, which results in decreasing of molecular weight [4], [5].

In this regard, polymer nanocomposites are of particular interest, considering that they enhance the properties of polymers when compared with neat materials and regular composites [6], [7], [8]. Factors such as the nanometric size of dispersed phase and the addition of low concentrations of such filler (usually less than 5%) improve all the mechanical, physicochemical and thermal properties, contributing to the increasing use and study of these materials [9]. Ramos et al. [10] and Gutiérrez et al. [11] observed increase in aging resistance for polypropylene-based nanocomposites.

Montmorillonite (MMT) clay is currently one of the nanofillers most used in polymeric matrices and one of the most studied. Polypropylene (PP) and MMT nanocomposite is widely produced and studied, and its degradation reactions can be enhanced by the addition of these nanofillers due to reactions that occur above 180 °C with surfactants used to chemically modify the clay [7], [11]. In addition, the MMT structure contains numerous ionic metals that start and propagate degradative reactions [12], [13], [14], [15], [16], [17].

Lonkar et al. [9], who studied the photo-oxidation of polypropylene, layered double hydroxide (LDH) and MMT nanocomposites, reported that after 20 h of exposure, photodegradation due to ultraviolet radiation is more severe in samples containing LDH. Oxidation rates were influenced by nanoparticles and were dependent upon the divalent cations in LDH layers. Shah [14] prepared low density polyethylene (LDPE)/MMT nanocomposites at various processing temperatures to evaluate surfactant degradation and concluded that the surfactant in the clay structure evaporates rapidly at 180 °C. A recent study by Touati et al. [18] showed the effects of reprocessing on the structure and properties of iPP and MMT nanocomposites. These authors found that thermal and mechanical properties of these nanocomposites were significantly reduced after the first processing, but remained almost unchanged after subsequent reprocessing. Their results indicated that nanocomposite recycling is far more complex because the resulting structure affects its performance.

In view of the increase use and studies of nanocomposites, this study focused on the degradation of PP/MMT nanocomposites subjected to multiple reprocessing cycles in a twin screw extruder. In an attempt to achieve exfoliated structures, two types of compatibilizers were used: traditional polypropylene grafted with maleic anhydride (PP-g-MA) and grafted with acrylic acid (PP-g-AA). The former is largely used for compatibilization of MMT and polypropylene matrix. The latter was applied considering some relevant results obtained in other works [19], [20].

Section snippets

Materials

The materials used in the present work were polypropylene H301 (Braskem), with melt flow index (MFI) 10 g/10 min (ASTM 1238, 230 °C, 2.16 kg) and Cloisite® 15A montmorillonite clay (Bun Tech). The compatibilizers were polypropylene grafted with maleic anhydride (PP-g-MA) and polypropylene grafted with acrylic acid (PP-g-AA), from Chemtura, known by the trade names of Polybond 3200 and Polybond 1001, respectively. The concentration of reactive groups was performed by titration [21] and it was

X-ray diffractometry

Clay lamellae showed displacement due to structural changes that occur during intercalation and exfoliation processes. This separation can be measured by the characteristic peak displacement: according to Bragg's law, the increase in interplanar distance is followed by a decrease in the diffraction angle. Fig. 2 shows diffractograms of pure Cloisite® 15A and its mixtures with polypropylene and PP-g-MA, all of them taken after the first extrusion. Note the diffraction peak for MMT at 2.9°,

Conclusions

This paper reports an investigation of the degradation of PP/MMT nanocomposites. X-ray diffractometry revealed a decrease in the refraction angle due to an increase in the interplanar distance resulting from intercalation and exfoliation processes, mainly when using PP-g-MA as compatibilizer. After the data were normalized to eliminate the effect of uneven film thickness and the initial concentration of functional groups in the compatibilizer, the FTIR results indicated that the concentration

Acknowledgments

The authors thank Braskem, Chemtura and Southern Clay for their donations of the polypropylene, compatibilizers, and montmorillonite used in this work. The financial support of CAPES, CNPq and Fundação Araucária (Brazil) is also acknowledged.

References (32)

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