Microstructure and properties of highly filled rubber/clay nanocomposites prepared by melt blending

Abstract

A series of highly filled rubber/clay nanocomposites (RCNs) based on ethylene-propylene diene rubber (EPDM), styrene butadiene rubber (SBR) and epichlorohydrin rubber (ECO) were prepared by melt blending with traditional rubber processing technique. Wide-angle X-ray diffraction (WAXD) characterization shows that the highly filled RCNs (up to ∼60 wt%) have intercalated silicate structures. TEM observations reveal that the dispersion homogeneity of clay layers improves with increasing content of organically modified clay (OMC). It was shown by dynamic mechanical thermal analysis (DMTA) for the first time that the melt-like thermal transition of alkyl chains of the surfactant of OMC still occurs in the intercalated OMC. Addition of large amount OMC to rubber greatly improves the modulus of material. Highly filled RCNs also possess outstanding gas barrier properties when compared to neat rubbers.

Introduction

In the past decade, polymer/clay nanocomposites (PCNs) have attracted much attention from both academic and industrial researchers, since these nanocomposites commonly show significantly improved mechanical properties, reduced moisture adsorption, superior flame retardancy and decreased permeability, when compared to their micro- and macrocomposite counterparts and their neat polymer matrices [1], [2], [3], [4], [5], [6], [7]. In general, there are two idealized morphologies that can be developed using nano-silicate fillers: (1) exfoliated (i.e., silicate layers are totally delaminated and disordered), and (2) intercalated (i.e., silicate layers are partially separated by polymer chains but ordered structure is still retained), which are shown in Fig. 1a and b, respectively. The spatial distribution of clay particles in exfoliated PCNs is much more homogeneous than that in intercalated nanocomposites. In the exfoliated PCNs, the individual clay layer acts as the basic reinforcing unit. In contrast, in the intercalated PCNs, the reinforcement unit is the intercalated clay stacks, the equivalent aspect ratio of which is much lower than that of an individual clay layer [8], [9]. As a result, an exfoliated structure is desirable to obtain best enhancements of properties. However, exfoliation is challenging, because it requires fine-tuning of clay surface chemistry, synthesis and processing conditions.

We expect almost all the polymer chains would be inserted into the intra-galleries of silicate layers when the concentration of organically modified clay (OMC) reaches a very high level, thereby the entire polymer matrix would be full of intercalated silicates as shown schematically in Fig. 1c. In this highly filled intercalated PCN, the dispersion homogeneity of the individual silicate layers would be much better than that in the conventional intercalated PCNs with low OMC contents, and akin to that in the exfoliated PCNs. Thus, it has been predicted that PCNs with the above mentioned microstructure, hereafter called the “highly-filled intercalated structure”, should possess very much improved mechanical and gas barrier properties.

Based on the theory of polymer melt intercalation in OMC [10], [11], it should be thermodynamically possible to obtain highly filled PCNs by melt blending. However, there are very few experimental studies on highly filled PCNs due to processing difficulties caused by the high viscosity of polymer melt filled with large amounts of nano-silicates [12], [13], [14]. To prepare highly filled PCNs with homogeneous dispersion of silicate platelets, both long processing time and high mixing torque are required. Hence, for thermoplastic polymer matrices, long processing times under high-temperature would result in polymer degradation and reduction of the resultant nanocomposite properties. Also, the processing equipment for plastics (i.e., screw extruder) hardly sustains such large mixing torques. To overcome these problems, Koo et al. [12], [13] used maleated polyethylene and maleated polypropylene with low molecular weights to prepare highly filled PCNs with the melt intercalation method. Zerda et al. [14] also prepared highly filled polymethyl methacrylate/clay intercalated nanocomposites using in situ polymerization intercalation in supercritical carbon dioxide. In these studies, however, very limited properties of these highly filled PCNs were measured. Compared to thermoplastics, the processing temperature of rubbers is relatively low (i.e., less than 60 °C), thus allowing long processing times. Moreover, the processing facility for rubbers (i.e., inner mixer and two-roll mill) can provide and bear large mixing torques.

In this work, we showed for the first time that a series of highly filled PCNs based on several rubber matrices could be successfully processed by melt blending. These fabricated nanocomposites filled with high clay contents (up to ∼60 wt%) display great improvement in storage modulus at ambient and high (e.g., 100 °C) temperatures, and large reduction in gas permeability. Furthermore, some novel and interesting phenomena were observed.