Polymer/Clay Nanocomposites Produced by Dispersing Layered Silicates in Thermoplastic Melts

Introduction

Polymer/clay nanocomposites (n-PCM) contain a small amount of a layered clay silicate (LCS) arbitrarily dispersed in a polymer matrix. In the molded state, mechanical, thermal, barrier and other properties of n-PCM are significantly improved compared to traditional composites. Some of the first results in the field published in (Okada & Kawasumi, 1987) were amazing and, at least, initially considered as surprising, due to the “nano-effect.” They became widely known since the publication of several reviews on technology, structure and properties of n-PCM (Okada & Usuki, 2006; Pavlidou & Papaspyrides, 2008; Paul & Robeson, 2008).

A significant interest in these materials is stimulated, primarily, by the desire to prepare materials, featuring a low cost combined with a superior set of performance characteristics compared to traditional composites (Gerasin et al., 2013; Pesetskii, Bogdanovich & Myshkin, 2013).

Homogeneous distribution of LCS nanoparticles in a polymer matrix produces a vast interphase boundary and intense interphase interactions can be expected. For the nanoparticles to maintain such a state it is important that direct interparticle contact is avoided. To estimate the upper concentration of a nanofiller Okada and Uzuki assumed that the shape of the particles is tetrahedrical measuring 1×100×100 nm. In that case, full interparticle contact along all the planes is achieved in nanocomposites at ≈7.5 wt.%, while in case when the particles contact with all the vertices of the tetrahedron the upper concentration limit is achieved at 1.5 vol.% (3.8 wt.%). It led them to a conclusion that the upper limit of a clay content in n-PCM should not exceed several percent.

Methods of preparing n-PCM are divided into three main categories: in-situ polymerization (polycondensation) and intercalation in a polymer solution or in the melt. Melt intercalation possesses evident advantages. It was first implemented by Vaia et al. (Vaia, Ishii & Giannelis, 1993). Melt intercalation is not applicable for thermosetting matrices because of accompaning crosslinking processes, so n-PCM based on thermosets are most often prepared using the clays pre-swelled in resin solvents or using water suspensions of clays in a resin solvent (Domenech, Peuvrel-Disdier & Vergnes, 2011). Elastomer-based n-PCMs may be produced via similar routes, although the more promising techniques in this case are latex intercalation and melt compounding (Finnigan, 2004; Fischer, 2003). It should be marked that due to the specific technological equipment used in preparation of rubber compounds and the molecular structure of rubber (low shear effects on the melt, high molecular weight and melt viscosity of rubber), it is difficult to produce an exfoliated LCS in the final rubber (Finnigan, 2004).

It should be noted that despite a large number of original publications reviewing some aspects of technology, structure and properties of n-PCM prepared by melt compounding, the up-to-date overview information in this area is rather limited and focuses mainly on particular types of n-PCM (Frontini & Pouzara, 2015; Ray, 2015; Lu, Wang & Chua, 2008), which makes it difficult to analyze the overall situation and prospects for the development of this field. The present review attempts to analyze and generalize the published results of research and original results of the authors, and consider the most important directions of the development of the technology of n-PCM prepared by compounding LCS in thermoplastic melts.