Nano-clays polymer composites

Figure 10.2 Some processing strategies for making nano-clay polymer composites...

The relative importance of these factors will vary markedly from system to system and even from one property to another. As an example, not all thermoplastics contain crystallinity, and filler polymer interaction will be more important in high strain situations. They also make the interpretation of much of the present experimental data relating to nano-clay filled composites difficult. It is thus appropriate to look at theoretical predictions first. 

Clays have long been used as fillers in polymer systems because of low cost and the improved mechanical properties of the resulting polymer composites. If all other parameters are equal, the efficiency of a filler to improve the physical and mechanical properties of a polymer system is sensitive to its degree of dispersion in the polymer matrix (Krishnamoorti et ah, 1996). In the early 1990s, Toyota researchers (Okada et ah, 1990) discovered that treatment of montmorillonite (MMT) with amino acids allowed dispersion of the individual 1 nm thick silicate layers of the clay scale in polyamide on a molecular. Their hybrid material showed major improvements in physical and mechanical properties even at very low clay content (1.6 vol %). Since then, many researchers have performed investigations in the new field of polymer nano-composites. This has lead to further developments in the range of materials and synthesizing methods available. 

Lan and Pinnavaia [6] showed that hybrid organic-inorganic composites exhibited mechanical properties superior to those for their separate composites. Dependence of tensile strength and modulus of epoxy nano-clay composites on the chain length of the clay-intercalated alkylammonium ions is shown in Fig. 28. The presence of the organoclay substantially increased both the tensile strength and modulus relative to the pristine polymer. The mechanical properties increased with any exfoliation in the order ... 

Zhang and coworkers [27] reported the synthesis of SBR/clay nanocomposites by anionic polymerization, as shown in Scheme 11.2. The clay used is an OC modified by the intercalation of a quaternary long ammonium salt The fiUer is stirred into a 51 polymerization kettle filled with both styrene and butadiene monomer, THF, and cyclohexane. After stirring for 3 h, n-BuLi (THF/n-BuLi = 25) was added and the polymerization was carried out at 50 °C for 3 h. The authors found that the addition of the inorganic filler to the reaction mixture did not change the total conversion of both monomers, leading to the synthesis of SBR nano-composites with almost the same polymer composition as that of the materials... 

Nano-clay composites contain very thin, high aspect ratio, alumino-silicate platelets, derived from the stacks present in the parent clay. Two types of stmcture can be recognised, intercalated and delaminated. In the intercalated form, the stacks of platelets are still present, but with polymer chains present between clay platelets, pushing them apart. In the delaminated form, the original stacks are no longer present, and the platelets are fully dispersed. 

The primary effect of the nano-clays seems to be related to char formation. The workers at NIST have found that a reduction in mass loss and heat release rate only starts once the surface of the polymer is at least partly covered by char. Beyer reported that, while no char was produced by burning unfilled EVA, the filled composite formed a strong char early in the process [54]. Once the amount of clay is taken into account, final char levels are often similar to unfilled polymer, indicating that while a stronger, more insulating, char may form and retard combustion, it is eventually consumed in this test. 

In general, the dispersion of clay particles in a polymer matrix can result in the formation of three general types of composite materials (Figure 1). Conventional composites contain clay tactoids with the layers aggregated in unintercalated face - face form. The clay tactoids are simply dispersed as a segregated phase. Intercalated clay composites are intercalation compounds of definite structure formed by the insertion of one or more molecular layers of polymer into the clay host galleries and the properties usually resemble those of the ceramic host. In contrast, exfoliated polymer-clay nanocomposites have a low clay content, a monolithic structure, a separation between layers that depends on the polymer content of the composite, and properties that reflect those of the nano-confmed polymer. 

K. M. Dean, S. A. Bateman, and R. Simons, A comparative study of UV active silane-grafted and ion-exchanged organo-clay for apphcation in photocurable urethane acrylate nano-and micro-composites. Polymer 48, 2231-2240 (2007). 

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