Montmorillonite exfoliated

Figure 9.4 Morphologies of bentonite rock and exfoliated montmorillonite.

An important result of having exfoliated montmorillonite composites in plastics is the increase in modulus, or stiffness of the material. Because the stiffness of the montmorillonite plates remains substantially constant on heating the composite, the relative stiffening effect increases as the polymer softens above its glass transition temperature. 

Figure 13.35 Storage modulus vs. temperature tor polyamide-6 nanocomposites. Note that the heat distortion temperature increases very significantly with addition of exfoliated montmorillonite.

Proposed applications include films for food packaging, making use of the reduced permeability feature of exfoliated montmorillonites. In the packaging industry, for instance, certain foods, (e.g. tomato products and beer) are sensitive to oxygen owing to the permeability of the plastic. The fire retardant property of these composites is also frequently mentioned (93). Other proposed applications include thin-walled medical tubing, abrasion-resistant and chemical-resistant coating, especially for transparent substrates. 

Newer methods are based on nanocomposites. Two materials of great interest include exfoliated montmorillonite-type clay and multi-walled carbon... 

Corresponding experiments using exfoliated montmorillonite clays, also dispersed in polypropylene, were carried out by Barthohnai and Schartel... 

Figure 14.34 Heat release rate and total heat release for maleic anhydride bonded polypropylene with various concentrations of exfoliated montmorillonite added.

Borodin O, Henderson WA, Eox ET, Berman M, Gobet M, Greenbaum S (2013) Anchoring effect of exfoliated-montmorillonite-supported Pd catalyst for the oxygen reduction reaction. JPhys Oiem B 117 10581-10588... 

The difficulty in preparing aligned and exfoliated montmorillonite composites in epoxy was again apparent. This complication was further compromised with the preferred association at the epoxy-elastomer interface of the intercalated organomontmorillonite bundles. The result was a greater randomization of the montmorillonite dispersed phase in the epoxy. 

Another approach to the orientation of exfoliated montmorillonite in polymers is the utilization of block copolymers. Work by Yung-Hoon Ha et al. [36] prepared oriented, montmorillonite nanocomposites in a styrene-butadiene-styrene (SBS) block copolymer (Vector 4461-D). Attempts to prepare exfoliated montmorillonite in block copolymers have been unsuccessful until the advent of this work. The montmorillonite was surface treated by two methods. 

The SBS polymer has layers of polybutadiene sandwiched between layers of polystyrene. The orientation of the layers is orthogonal to the direction of roll casting. When a polymer nanocomposite was prepared with SBS that contained 2% montmorillonite that was surface modified with polystyrene at 6000 mol. wt. prepared by surface initiated polymerization, an excellent exfoliated montmorillonite product was prepared. The orientation of the SBS layers was now parallel to the direction of roll casting. 

This approach to prepare fully exfoliated montmorillonite-polymer nanocomposites has yet to be fully exploited. One might imagine the unique mechanical performance of this type of block copolymer with different concentrations of montmorillonite in each layer. 

Full exfoliation of montmorillonite in polystyrene has not been achieved. Hence, the full realization of the reinforcing benefit of montmorillonite with regard to the mechanical property enhancement of polystyrene has not been exploited. The definitive discussion that relates to the complex and confounding problem relating to the inability to exfoliate montmorillonite into polystyrene is found in the work of Stretz and Paul [31]. The factors that relate to improved dispersion and exfoliation of montmorillonite in polyolefins that are discussed above do not provide a satisfying response with polystyrene. The conclusion found in this work is the result of extensive investigations of acrylonitrile (SAN) and maleic anhydride (SMA) modified polystyrene [32,33,34]. 

The Halpin-Tsai prediction for the modulus at 2 wt.% loading of fully exfoliated montmorillonite with an aspect ratio of 100 in this polymer is duplicated by the ammonium ion-exchanged montmorillonite that has an experimentally determined aspect ratio of approximately 53.3. The enhanced reinforcing efficiency of this organomontmorillonite in SAN as a function of aspect ratio was not anticipated. 

The Mori-Tanaka model for an aspect ratio of 100 of perfectly exfoliated montmorillonite aligned in the direction of stress predicts a slope of 10.8 for the same copolymer. When the r/min of the extruder is increased to 200 and 380, the slope increased to about 8.4 for the copolymer-montmorillonite composite with 25 wt.% acrylonitrile content. 

WAXS indicated an intercalated composite. The modulus values for the 5% loaded composite are indicative of an intercalated composite (a 12% increase above the pure polymer value of 8.66x10 Pa). A Halpin-Tsai calculation for a 3% loaded composite predicts a 63% increase in modulus for a fully exfoliated montmorillonite composite with the particles ahgned in the direction of applied stress. The unexpected result is the dramatic increase in percent elongation to failure... 

The second significant independent variable that layered silicates provide to increase thermal stability of the polymer in polymer-clay nanocomposites is an increase of the melt viscosity. If thermal degradation of the polymer is diffusion controlled, an increase in viscosity of the polymer melt will slow the mass loss associated with gas escaping from the composite during TGA evaluations. The increase in viscosity of dispersions is a function of the surface area of the dispersed phase. For example, water-based dispersions will increase in viscosity as the particle size of the dispersed phase decreases at constant total volume of the dispersed phase. This is the result of an increase in total surface area of the dispersed phase. Particle-particle interaction has increased as a function of increased total particle surface area. The surface area [17] of fully exfoliated montmorillonite is approximately 750 m /g. This enormous number results in a significant increase in polymer-montmorillonite melt viscosity at low concentration of montmorillonite and low shear rates [18]... 

Enhancement of mechanical properties by exfoliated montmorillonite clays in polyamide-6... 

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