Manufacturing technique and mechanical properties of plastic nanocomposite

Abstract

This research first produced the 20A140 nano-composite by melt-blending polyamide 6 (PA6) and one type of montmorillonite—(Cloisite 20A) one to six times, after which the nano-composites were evaluated by mechanical properties. The results showed that the exfoliated nano-composite displayed the best tensile modulus of 4.1 GPa; however, both its elongation at break and its notch Izod impact strength were inferior to those of the intercalated nano-composite. When melt-blended for six times, the exfoliated nano-composite displayed a tensile modulus of 3.9 GPa and a yield stress of 80 MPa. The intercalated nano-composite’s tensile modulus and yield stress were both on the increase when silicate layers were more dispersed. Finally, for the conventional nano-composites, the tensile modulus and yield stress rose slightly with an increase in melt-blending cycles; however, the elongation at break and notch Izod impact strength were not influenced by the number of melt-blending cycles.

Introduction

The most widely used polymer nano-composite is an inorganic layered material, which could be divided into artificially-synthesized clay and natural clay. The former contains fewer impurities but is more costly. The latter contains more impurities but is more abundant in nature. With proper purification, the natural clay features greater application possibility [1].

Montmorillonite (MMT) possesses a unique layered structure and a good intercalation ability. It can also be chemically-modified to increase its compatibility with a polymer, making it a good candidate for polymer nano-composites. MMT does not exist in nature in the form of nano-particles, but it can be exfoliated by a suitable method, thereby dispersing its internal silicate layers on a nano-scale. The exfoliated silicate layers have a thickness of 1 mm, an aspect ratio (L/D) of 100–200 [2], and a surface area of 700–800 m²/g [3]. The MMT used in this study has an aspect ratio of 75–150 and a surface area of 750 m²/g. Compared to an ordinary polymer, the silicate layers of MMT possess extremely high strength (178 GPa) and rigidity [4], [5] so a very small amount of MMT could reinforce the polymer matrices as much as the traditional reinforcing materials like glass fiber, talc, and calcium carbonate. As the silicon-oxygen bonds in silicate layers have polarity, rendering MMT hydrophilic, MMT is not compatible with ordinary hydrophobic polymers. Therefore, surface modification is crucial for making polymer/MMT nano-composite to turn hydrophilic MMT into hydrophobic organoclay. Some previous studies used ordinary polymer processing facilities for multiple melt-blending cycles to simulate and explore the influence of recycle process on the materials’ structure and properties. Lozano-González et al. found that after virgin polyamide 6 was processed with an injection molding machine ten times, its mechanical properties decreased by 10–15% and its elongation at break by 70% [6]. The result of gel permeation chromatograph (GPC) also indicated that the average molecular weight of polyamide 6 increased with the increase in processing cycles (Mw = 17%, Mn = 14%). This implies that during multiple melt-blending cycles, the molecular chains that broke because of the degeneration recombined, and their gel content also increased with the increase in melt-blending cycles [6].

This research prepared nano-composites with polyamide 6 (PA6) and three MMTs separately, during which the screw speed was changed in order to explore the intercalation behavior of the MMTs in the PA6 matrices; in addition, multiple melt-blending cycles were employed to simulate the recycling of nano-composites and to acquire the mechanical properties of the recycled nano-composites [7].