Effect of shearing on the orientation, crystallization and mechanical properties of HDPE/attapulgite nanocomposites

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Abstract

High density polyethylene (HDPE)/attapulgite (AT) nanocomposites, prepared by conventional injection molding (CIM) and dynamic packing injection molding (DPIM), were investigated with focus on AT-induced crystallization and orientation under shear. Infrared spectroscopy (FTIR) analysis showed there is no special chemical interaction between HDPE and AT, but shear induced significant changes on the material structure and properties. Differential scanning calorimetry (DSC) analysis showed strong nucleation effect by AT especially under shear. And more, shear will induce much better dispersion of AT in the DPIM sample vs. CIM. AT nanorods and lamellae of HDPE are more organized in the DPIM sample while there is only random distribution in the CIM sample. Most AT nanorods embed in the HDPE lamellae and form a brush-like hybrid structure due to shear. The shear-induced orientation will be enhanced with higher AT loading. The mechanical performance of the composites was significantly improved via DPIM.

Introduction

Polymer nanocomposite is an active research field. One of the focuses is to use nanoparticles to achieve high performance materials based on various polymeric systems. For such a system, strong interfacial adhesion, homogeneous dispersion of fillers, as well as proper topological structure, play crucial roles in enhancement of mechanical and physical properties [1], [2], [3]. So far, most study focused on using inorganic nano-fillers like clay, carbon nanotube and graphene.

Although carbon nanotubes have been used for industrial application to prepare polymer composites, the cost is fairly high. Researchers in both academia and industry are looking for promising alternatives to take the place of carbon nanotube in some applications. One of the substitutes is the natural nano-fiber, which has attracted massive interests in industry applications in recent years [4], [5], [6]. Attapulgite (AT) is a kind of crystalloid hydrous magnesium–aluminum silicate mineral with the shape of needles, fibers or fibrous clusters. Its structure was first proposed by Bradley in 1940s. A single attapulgite fibril is about 20 nm in diameter and several microns in length. Though the interest in attapulgite is in their absorptive, rheological and catalytic properties, compared with various silicates such as montmorillonite, the unique structure of AT, and its low cost, have attracted much interest as a filler for enhancement of mechanical properties of polymers [7], [8]. Moreover, unlike layered silicates, only weak physical adsorption exists between AT single crystals, which makes it possible that shear force could break the AT bundles into single rods without organic modification [9].

Generally, the alignment of fibrillar fillers in polymer matrix may happen via shear, stretching or electric field [7], [10], [11]. In our previous work, it was found that the layered clays could be oriented in the matrix along flow direction under shear force during injection molding [12], [13]. The enhanced orientation of fillers in the composites is an effective way to improve the mechanical properties. Moreover, the dispersion (exfoliation) of nano-fillers in polymeric matrices could also be significantly improved with the aid of shear [2], which will have great influence on the mechanical properties of the composites. On the other hand, shear field will also have strong impact on the crystallization and molecular orientation of polymers during the injection molding. The most notable changes in structure and properties are usually associated with the shear induced transition from a relatively isotropic morphology to a highly oriented morphology, which can markedly enhance stiffness of the products [14], [15].

In this study, HDPE/AT composites were prepared by melt blending and then fabricated by conventional injection molding (CIM) and dynamic packing injection molding (DPIM), where repeated shearing was imposed on the melts during the solidification stage. The dispersion and orientation of the filler, the interfacial crystallization and the orientation of HDPE matrix were investigated. The influences of different AT contents on HDPE molecular orientation as well as final mechanical properties of composites were examined, and the relationship between the structure and mechanical properties obtained was discussed.

Section snippets

Materials

High density polyethylene (HDPE) is available from Fushun Petrochemical Corp., with a MI of 20 g/10 min and molecular weight (Mw) of 1.2 × 105. Attapulgite (AT) is available from Jiangsu Autobang International Co. Ltd. (Xuyi County, Jiangsu Province, PRC) with the average aspect ratio of about 20 and was used as received.

Preparation of HDPE/AT composites

HDPE/AT composites with different AT contents (0%, 1%, 3%, 5% weight ratio) were melt mixed using a TSSJ-25 co-rotating twin-screw extruder. Barrel temperature was maintained at 130

Shear-induced dispersion of AT in HDPE matrix

The effect of shear on the dispersion of AT particles was investigated by checking the morphology and rheological properties of the composites. SEM micrographs of both the static and dynamic samples with 5 wt.% AT content are given in Fig. 2. AT appears as bright spots in the images. In the static samples (Fig. 2a), AT seems to form large agglomerates of several microns and only a small amount of AT are exfoliated, indicating a poor dispersion of the filler in the polymer matrix. However, in the

Conclusion

HDPE/attapulgite (AT) nano-composites prepared by conventional injection molding (CIM) and dynamic packing injection molding (DPIM) showed different structure–property relationships. Morphological studies showed that AT nanorods and lamellae of HDPE were randomly distributed in the samples prepared by CIM, while they were well-organized in the sample produce with DPIM. SEM and rheological analysis indicated that the dynamic shear field afforded in DPIM induced better dispersion of AT in HDPE.

Acknowledgement

The author would like to thank The Dow Chemical Company for the financial support to the project.

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