Modification clays

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

Clay modification Cation Designation Alkyl Chain Length... 

To date only few dielectric relaxation studies have been reported on thermosetting nanocomposite systems. Kanapitsas et al. [109] reported isothermal dielectric relaxation studies of epoxy nanocomposite systems based upon three different clay modifications, a low viscosity epoxy resin based on the diglycidyl ether of bisphenol-A type (Araldite LY556, CIBA) and an amine hardener in a temperature range of 30-140 °C. Whilst details on the epoxy system investigated and the nanocomposite morphology were vague, it was reported that the overall mobility is reduced in the nanocomposite compared to the neat matrix resin. 

The effect of added nanoclays to the morphological characteristics and the macroscopic properties in a blend of isotactic PP and PEO was examined. It was shown that strong interactions between the surfactant used for clay modification and the binary matrix effectively controlled the spatial organization of the suspended polymer droplets. The incorporation of a small amount of organically modified nanoclay induced a dramatic transformation from an opaque to a transparent system (Kelarakis and Yoon 2008). 

Ollier, R., Rodriguez, E., Alvarez, V. Unsaturated polyester/bentonite nanocomposites influence of clay modification on final performance. Compos. A Appl. Sci. Manuf. 48,137-143 (2013)... 

Ollier et al. reported the effects of modified and unmodified bentonite loading on the decomposition behavior of unsaturated polyester thermosets [71]. It was shown that the addition of bentonite increased the thermal stability of the resin. Furthermore, the clay modification did not significantly influence the degradation temperatures of the nanocomposites. 

Zheng, H., Zhang, Y., Peng, Z., Zhang, Y.J. Inlluence of the clay modification and compatibilizer on the structure and mechanical properties of ethylene-propylene-diene rubber/montmorillonite composites. J. Appl. Polym. Sci. 92, 638-646 (2004)... 

T. Nazari, H. Garmabi, A. Arefazar, Effect of clay modification on the morphology and the mechanical/physical properties of ABS/PMM A blends. Journal of Applied Polymer Science 126 (5) (2012). 

Flammabihty studies using cone calorimetry at 50 kW m heat flux showed that incorporation of 5 wt% organoclay reduced peak heat release rate (PHRR) by 23-27% and total heat release (THR) values by 4—11%, depending on the clay modification. However, no simple correlation was observed between the FR efficiency and the degree of day exfoliation. The synergistic effect was observed for the combination of ammonium polyphosphate and 5 wt% amount of nanoday, which resulted in the total reduction of the PHRR of polyester resin in the range 60-70%. 

The benefit of exchanging polymeric initiators onto the clay surface is that the growing chains are anchored to the clay surface, in a surface initiated polymerization and the growing PS chains may help to push the clay platelets further apart. The clay modification itself however still needs to be compatible with the styrene, in order to allow monomer to swell the platelets. This approach is further discussed in Section 13.4, Figure 13.9. 

Bulk polymerization initiated via more novel methods have also been used to form bulk PS-MMT nanocomposites. Zhang et al. [19] used gamma irradiation to initiate the polymerization of PS-MMT nanocomposites with different surface modifications (3, 34) and successfully prepared exfohated morphologies when reactive clay modifications were used. Uthirakumar et al. [51-54] modified MMT with a cationic radical initiator which was used to initiate the bulk polymerization of styrene. Because the polymerization was initiated from the clay surface and the monomer and the clay were suitably compatible, exfoliated morphologies were formed. 

These studies show that while surface initiated polymerization can be beneficial to the dispersion of the clay in the PS, other factors such as the compatibility of the styrene and the clay modification, and the interaction of the clay modification with the crystal structure of the clay itself can affect the final morphology. 

These studies have helped to develop a level of understanding of the importance of the interplay between the clay modification, the polymerization technique, the... 

Figure 2.8 Effect of clay modification degree on the onset decomposition temperature (estimated at 2% weight loss) of PLEA hybrids. Reprinted from Ref. [102], with permission from Wiley.

D. Garcia-Lopez, I. Gobernado-Mitre, J. F. Fernandez, J. C. Merino, and J. M. Pastor, Influence of clay modification process in PA6-layered silicate nanocomposite properties. Polymer, 46 (2005), 2758-65. 

Wilkie and co-workers [69, 70] synthesized two organically modified clays to produce nanocomposites of PS, HIPS, and ABS terpolymer. They used the following copolymers to modify clay vinylbenzyl chloride (COPS) and methyl methacrylate and vinylbenzyl chloride (MAPS). The cation head for clay modification with these compounds was ammonium. After melt-blending, styrene copolymer-modified clays yielded exfoliated nanocomposites, whereas the methacrylate copolymer clays yielded a mixture of immiscible and intercalated nanocomposites. In general, all nanocomposites exhibited improved thermal stability and mechanical properties, in addition to improvements in flame retardancy, depending on the quality of clay dispersion. 

Sen et al. [75] prepared PS nanocomposites by free radical polymerization in the presence of organically modified MMT with low-molecular weight quartemized poly(styrene-b-4-vinylpyridine) (SVP) (Table 3.6). Clay modification was carried out in different compositions of THF and water. Copolymer intercalation and thermal stability of the resulting organoclays depended on the THF/water proportion. Greater distances were obtained... 

Clay modification with thermally stable additives that would promote deagglomeration, dispersion, and exfoliation of agglomerated nanoclays, particularly in high-temperature thermoplastic matrices. 

Clay modification with thermally stable ionic liquids... 

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