Clay surface compatibility with polymers

The discussion of clay surface compatibility with polymers in this section will focus primarily on montmorillonite as the example clay. The characteristics discussed will only vary by degree for other smectic clays. 

The exchange of inorganic cations by organic surfactant ions in the clay galleries not only makes the organoclay surface compatible with monomer or polymer matrix, but also de-... 

Besides their role of enhancing solvent-clay surface compatibility already discussed above, some organic modifier molecules may provide functional groups that participate in catalyst supporting reactions [62, 103] or even become copolymerized with the polymer during in-situ polymerization [70]. 

Figure 6.5 shows various functional groups which may be detected on silica, talc, and clay surfaces. The surface character of carbon black differs in that it is mostly nonpolar whereas the surface of silica is polar. Thus carbon black is more compatible with hydrocarbon polymers which are also nonpolar. Silica and other similar fillers (talc, clay) have more affinity to each other than to nonpolar polymers. This is a major factor in the inferior performance in rubber applications where interfacial adhesion is reduced. 

Natural, unmodified montmorillonite-Na (MMT-Na) has cation exchange capacity, typically 80-90 mequiv/100 g. Although some polymers, such as polyethylene oxide or polyvinylpyrrolidone, are of sufficient polarity to be able to directly exfoliate unmodified MMT-Na, organic modification of the layered clay is usually required to render the hydrophilic surface of the clay more hydrophobic and thus more compatible with most polymers, thereby improving the wettability and dispersibility of the clay in the polymer matrix. 

The compatibilizer as such may be compatible with the clay surfaces and hence easily intercalates into the interlayer spacing of the clay platelets and separates the clay platelets, and with bulk polymer it leads to the formation of well-dispersed nanocomposites. The compatibilizer widely used for polypropylene (PP) is maleic anhydride grafted PP (PP-g-MA) [15-16], Other compatibilizers used are diethyl maleate (DEM-g-PP) and polyolefin elastomer grafted maleic anhydride (POE-g-MA),... 

Nanoclay is the term generally used when referring to a clay mineral with a phyllosilicate or sheet structure with dimensions of the order of 1 nm thick and surfaces of perhaps 50-150 nm. The mineral base can be natural or synthetic and is hydrophilic. The clay surfaces can be modified with specific chemistries to render them organophilic and therefore compatible with organic polymers. Surface areas of nanoclays are very large, about 750 m /g. When small quantities are added to a host polymer, the resulting product is called a nanocomposite. 

In order to convert the montmorillonite clay into a nanoday compatible with organic polymers, an ion exchange process is performed to treat the day surfaces. Generally, an organic cation, such as from a quaternary ammonium chloride, is used to change the hydrophilic/hydrophobic characteristics of the clay (Figure 9.3). Typical characteristics of montmorillonite clays are as follows ... 

RTP, based in Winona, MN, USA, was probably the first independent compounder to go into commercial production with nanocompounds. Most nanocomposites currently use nanoclay, although carbon nanotubes are becoming much more widespread. In the US, Southern Clay Products of Texas is a leading producer of nanoclays with products like Cloisite w hich are high aspect ratio additives based on montmorillonite, consisting of layered magnesium aluminium silicate platelets that have been surface treated to improve their compatibility with the polymer. 

In this contribution, we intend to introduce recent work dedicated to polymer-clay nanocomposites based on sepiolite and palygorskite fibrous silicates. We will consider as a priority the role of the interface between the mineral surface and the polymer matrix. In fact, this type of clay is markedly hydrophilic because their surface is covered by hydroxyl groups, mainly silanol groups (=Si-OH) [17, 22], and therefore they are compatible with many polar polymers. However, chemical modification of the silicate surface could be necessary for adjusting their... 

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