Clay-polymer nanocomposites thermal stability

The procedure to obtain nanocomposites based on unsaturated polyester resins leads to improvements in the order of 120% in the flexural modulus, 14% in flexural strength and 57% increase in tensile modulus with 4.7% of clay slurry content. Thermal stability augments and the gelation temperature increases to 45 °C, as compared to that of the resin (Fig. 31.6). It seems that adding water to the MMT allows better intercalation of polymer chains into the interlamellar space. Because clay is first suspended in water, this improves dispersion and distribution of the particles in the resin matrix. Longer gelation times lead to more uniform and mechanically stronger structures and to yield stresses (Fig. 31.7). Enhanced polymer-clay interactions are revealed by XPS in this case (Fig. 31.8). 

The mechanism of the improvement of thermal stability in polymer nanocomposites is not fully understood. It is often stated [126-129] that enhanced thermal stabihty is due to improved barrier properties and the torturous path for volatile decomposition products, which hinders their diffusion to the surface material where they are combusted. Other mechanisms have been proposed, for example, Zhu et al. [130] recently proposed that for polypropylene-clay nanocomposites, it was the structural iron in the dispersed clay that improved thermal stability by acting as a trap for radicals at high temperatures. 

More recently, the same research group also reported that in hydro-calcite nanoparticle modified PE fibers [21] the incorporation of clay improved the thermal stability and induced heterogeneous nucleation of polyethylene crystals. Hydrocalcite exhibited good dispersion into the polymer matrix, and hence positively affected the mechanical properties in terms of both stiffness and strength. The toughness of the nanocomposite as spun fibers was also increased up to 30% with respect to neat polymer. 

Diagne, M., Gueye, M., Vidal, L., and Tidjani, A. 2005. Thermal stability and fire retardant performance of photo-oxidized nanocomposites of polypropylene-grayi-maleic anhydride/clay. Polymer Degradation and Stability 89 418-426. 

J. G. Zhang, D. D. Jiang, and C. A. Wilkie, Thermal and flame properties of polyethylene and polypropylene nanocomposites based on an oligomerically-modified clay. Polymer Degradation and Stability, 91 (2006), 298-304. 

In nanoparticle-polymer composites, thermal stability is one of the most important property enhancements. Recently, some theoretical efforts have been made to predict the thermal stability of such composites. For example, FEM and the theory of Chow have been used to predict the thermal expansion of clay-polymer nanocomposites. The results indicate that it is possible to considerably reduce and eventually match the thermal expansion of metal and polymer parts by dispersing a small amount of exfoliated muscovite mica platelets into a polymer matrix. Moreover, reduction is controlled by the product of aspect ratio and volume fraction of the platelets. 

Because of the multifaceted features of clay as a nanoparticle, the benefits in polymer-clay nanocomposites range from increased mechanical performance, barrier performance, and thermal stability. A systems approach to the design of polymer-clay nanocomposites with excellent flame retardancy will provide superior solutions in relation to formulating existing flame retardants with polymer-clay nanocomposites. Providing surface modifications for the clay with higher thermal stability that will not compromise the mechanical and barrier performance of... 

Polyimide-clay nanocomposites constitute another example of the synthesis of nanocomposite from polymer solution [70-76]. Polyimide-clay nanocomposite films were produced via polymerization of 4,4 -diaminodiphenyl ether and pyromellitic dianhydride in dimethylacetamide (DMAC) solvent, followed by mixing of the poly(amic acid) solution with organoclay dispersed in DMAC. Synthetic mica and MMT produced primarily exfoliated nanocomposites, while saponite and hectorite led to only monolayer intercalation in the clay galleries [71]. Dramatic improvements in barrier properties, thermal stability, and modulus were observed for these nanocomposites. Polyimide-clay nanocomposites containing only a small fraction of clay exhibited a several-fold reduction in the... 

In the case of 34NBR, the polymer chains have H-bonding interactions with the clay along with van der Waals interactions. These in turn improve the thermal stability of the nanocomposite. 

Although the nanoadditive can enhance both the thermal stability and the fire performance of the matrix polymer, it has been noted that the enhancement on the fire performance is not parallel to that on the thermal stability, i.e an observation about the reduction of the peak HRR of the resulting PN does not necessarily mean that an enhancement of the thermal stability of the PN can be observed. A typical example is PA-6. It has been seen that the PA-6/clay nanocomposite shows a significantly reduced peak HRR,98 but it does not show enhanced thermal stability. 

Zhu, J., Start, R, Mauritz, K. A., and Wilkie, C. A. Thermal stability and flame retardancy of poly(methyl methacrylate)-clay nanocomposites, Polym. Degrad. Stab. (2002), 77, 253-258. 

Gilman, J. W. Flammability and thermal stability studies of polymer layered-silicate (clay) nanocomposites, Appl. ClaySci. (1999), 15, 31 19. 

Incorporation of modified clays into thermosetting resins, and particularly in epoxy35 or unsaturated polyester resins, in order to improve thermal stability or flame retardancy, has been reported.36 A thermogravimetric study of polyester-clay nanocomposites has shown that addition of nanoclays lowers the decomposition temperature and thermal stability of a standard resin up to 600°C. But, above this temperature, the trend is reversed in a region where a charring residue is formed. Char formation seems not as important as compared with other polymer-clay nanocomposite structures. Nazare et al.37 have studied the combination of APP and ammonium-modified MMT (Cloisite 10A, 15A, 25A, and 30B). The diluent used for polyester resin was methyl methacrylate (MMA). The... 

For polyester, the reported work82 done in Sichuan University of China, involves adding MMT clay in a copolymer of poly(ethyleneterephthalate), which with a phosphorus-containing monomer could produce PET with higher thermal stability and char-forming tendency. However, fibers were not produced from this PET-nanocomposite polymers. 

Polymer Degradation and Stability 79, No.2, 2003, p.319-24 XPS INVESTIGATION OF THERMAL DEGRADATION AND CHARRING ON POLY (VINYL CHLORIDE)-CLAY NANOCOMPOSITES... 

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