Use of new synthetic talc as reinforcing nanofillers for polypropylene and polyamide 6 systems: Thermal and mechanical properties

Highlights

• Physico-chemical characterization of nanotalcs.

• Nanostructuring of new synthetic nanotalc in polyamide 6 and polypropylene matrices.

• Improvement of thermal and mechanical properties of nanocomposites.

Abstract

New layered synthetic nanotalc prepared at a lab-scale by a conventional hydrothermal process and commercial natural fine talc were used in order to establish a comparative study in terms of their contributions on the improvement of the final properties of two different polymers: a nonpolar polyolefin matrix and a polyamide. All samples were prepared by melt extrusion in a co-rotating microcompounder. The surface properties of talc – surface energy and isoelectric point – were probed. The particles’ crystalline structure and the distribution/dispersion within the polymer matrix were performed using transmission electron microscopy and X-ray diffraction. The effect of talc particles on the crystallinity, the thermal and mechanical properties was highlighted as a function of the surface properties of talc. In the case of talc-filled PP systems, it seems that the incorporation of both natural and synthetic talc greatly improves the thermal stability of polypropylene matrix. The highest elastic modulus was obtained in presence of highly nucleating natural talc. Oppositely, the best ductility was observed for the synthetic talc-filled PP systems. For PA6/talc nanocomposites, a remarkable improvement in the dispersion of talc layers was shown and a significant increase in Young’s modulus was determined due to the closer affinity between the hydrophilic nanotalc lamellae and the polar PA6 matrix.

Introduction

Traditionally, polymeric materials have been filled with synthetic or natural inorganic compounds in order to improve their properties or simply to reduce cost. The use of fillers with reduced dimensions in the nanosize range allowed designing and creating new materials called nanocomposite with unprecedented enhancements in physical property such as mechanical strength and modulus, barrier or fire resistance, and this with a low amount of nanoparticles substantially less than 5 wt% [1]. Among all the potential nanocomposite precursors, those based on clay and layered silicates have been most widely investigated, probably because the starting clay materials are easily available [2], [3]. However, talc particles are widely used as plate-like mineral filler because of its cheap cost [4]. But natural talc also presents a number of disadvantages. Specifically, natural talc ores consist in a mixture of several minerals, and natural talc structure presents some cationic substitutions [5] and, consequently, are inhomogeneous in the chemical structure, crystalline phase, and size distribution [6]. It is interesting to note that because the natural talc consists fundamentally of particles in the micrometer scale, many researchers have tried to produce fine ground particles by mechanical means to reach the nanometer scale [7]. But quickly it is appeared that this method had its limits and even it was difficult to obtain mechanically a nanometric talc. Indeed, several of these studies came to the conclusion that intense mechano-chemical effects occurred during grinding leading to significant crystal structural and lamellar alteration of talc particles [7], [8], [9]. The controlled synthesis of talc layered nanoparticles is one of the soft methods that provide a mean for producing materials for which the crystallinity, the composition, the particle size, and the layer thickness can be tuned in a controlled way. For example, by varying of a few tens of degrees the temperature of hydrothermal reaction, the average particle size can vary by several hundreds of nanometers [10]. Typically, the resulting physical properties of polymers that contain such nanosized talc particles are superior to those containing micron-sized talc particles. Therefore, the synthesis of talc of a well defined composition with a narrow particle size distribution is of considerable scientific and practical interest in view of its application as filler in polymer nanocomposites. In addition, one advantage of the synthetic talc filler is the possibility of its packaging in the form of aqueous suspension (output of the synthesis reactor). Thus, the particles contained in the slurry are more stable. In fact, as compared to their corresponding dried counterparts, these can be easily agglomerated during the drying step and induce an unavoidable technical complexity in the development of these composite materials.

Despite all the benefits of synthetic talc previously cited, a literature search reveals that little attention has focused on the use of this type of synthetic talc. We can just cited a very recent work of Joncoux-Chabrol et al. [11] which have studied the effect of different synthetic talc-like phyllosilicates of nanometric size on the barrier properties of the sol–gel coatings deposited on carbon steel. They suggested that the better corrosion protection was linked to the better dispersion of these hydrophilic fillers in the sol–gel coatings compared to the hydrophobic natural counterpart. It is worth keeping in mind that the synthetic talc nanoparticles usually display a more marked hydrophilic character due to the exacerbated surface providing from layer edges. Dispersion of particles into discrete monolayers is depending on the nature of the matrix. Hydrophobic polymers like polyolefins cannot wet or interact interfacially with the filler due to the difference in surface energies which leads generally to face-to-face stacking in agglomerated tactoïds [12], [13]. Synthetic talc particles need to be organically modified to become more compatible toward polymer matrix. A wide variety of compounds such as titanates [14] or silanes [15], [16] has been developed to promote the filler/polymer affinity.

In this work, we investigated the possibility of using new synthetic talc phyllosilicates as nanofillers for polymer nanocomposites prepared by melt blending that it is considered as a very attractive industrial process. A comparative study was established between natural and synthetic talc in terms of their contributions on the improvement of mechanical and thermal properties of two polymers different by their surface properties: a nonpolar polypropylene matrix and a polar polyamide matrix. The surface properties of synthetic talc particles as well as the extent of exfoliation will be tuned as a function of the matrix used. The relationship between the final properties obtained and talc’s surface characteristics will be detailed throughout the paper. The bond strength between the filler and the matrix is probably one of the most important tunable parameters that greatly influence the final properties of nanocomposite.