Clay surface modification

Ha, SR, Ryu, SH, Park, S J, Rhee, KY. 2007. Effect of clay surface modification and concentration on the tensile performance of clay/epoxy nanocomposites. Mater Sci Eng A-Struct 44S 264—268. 

Mechanical Properties Toyota Central Research Laboratories in Japan was the first to obtain significant mechanical improvement of a PA matrix by adding as little as about 2 wt% of montmorillonite (MMT) [Kojima et ah, 1993 Usuki et ah, 1993 Okada and Usuki, 2006], Improvement in the mechanical properties on the vitreous and rubbery plateau by layered silicate nanoparticles depends on several factors, including clay surface modification, polymer chemistry, processing method, level of exfoliation, and clay orientation. In this section we present an overview of the influence of these factors on the dynamic mechanical properties of PLSN. 

Although a straight comparison between these different systems would be appealing, a comprehensive discussion on structure-property relationships will not be attempted in this review. Indeed, the diversity of systems in terms of type of clay, surface modification, and poiymerization methods renders this exercise quite risky. Therefore, we will not try to compare the properties of the different systems described, and only discuss those whose properties bring a clear added-value to the nanocomposite material. 

Tartaglione et al. [62] have recently studied the thermal behavior of melt-compounded PP/sepiolite composites (3 wt% loading) and the effect of the clay surface modification. In inert atmosphere, the catalytic activity of sepiolite on PP pyrolysis was confirmed, except for the begiiming of the degradation process (less than 20% of mass loss) where the presence of the clay improved the polymer stability (Figure 12.21a). This was explained by the adsorption of PP volatile products by the sepiolite external surface and internal zeolitic channels. 

Clay Surface Modification Methods for In-situ Polymerization... 

Unlike melt intercalation, a layered silicate is mixed with monomer before polymerization takes place with in situ polymerization. This method was developed by Toyota researchers [27,28], in which electrostatically held 1-nm thick layers of layered alumina silicates were dispersed in a polyamide matrix on a nanometer level, which led to an exponential growth in the research endeavors, in layered silicate nanocomposites. These nanocomposites were based on the in situ synthesis approach in which a monomer or monomer solution was used to swell the filler interlayers, followed by polymerization. With this process, one can control the nanocomposite morphology through the combination of reaction conditions and clay surface modification. The in situ polymerization method is especially important for insoluble and thermally unstable polymers, which solution blending or melt blending technique cannot process. 

Greesh et al described an interesting study on the effect of clay surface modification on the microstructure and properties of polymer-clay nanocomposites synthesized by using free radical emulsion polymerization. Copolymers of styrene and butyl acrylate were synthesized in the presence of montmorillonites modified with various surface modifiers in order to relate their chemical structure, degree of interaction within the clay gallery surface... 

Tombacz, E., Szekeres, M., Barany, L., and Micheli, E. (1998). Surface modification of clay minerals by organic polyions. Coll. Surf. A 141, 379-384. 

The catalytic role of metal-doped clay surface in fire-induced degradation could be proven and an epoxy-based interlayer has been developed for controlling this activity. As a result of the modification, an increased amount of char residue was formed in fire-retarded PP system. 

Special surface modifications are available to further improve reinforcement. The objective of the surface treatment is to increase filler loading and/or improve physical properties without loss of rheological characteristics. A variety of surface-modified kaolins have been introduced including clays treated with silane, titanate, polyester, and metal hydroxide. Silane-treated kaolin is used in applications requiring maximum aging characteristics in the service environment. 

Zeolite membranes are amenable by surface modification with a variety of chemical functional groups using simple silane chemistry, which may provide alternative surface chemistry pathways for enzyme immobilization. In this context, Shukla et al. [238] have recently used a chemically modified zeolite-clay composite membrane for the immobilization of porcine lipase using glutaraldehyde to provide a chemical linkage between the enzyme and the membrane. The effects of pH, temperature, and solvent on the performance of such biphasic zeohte-membrane reactors have been evaluated in the hydrolysis of olive oil to fatty acids. 

PEO/OMMT mixture. Based on Flory-Huggins interaction parameters between polymers and silicate layers in a PEO/PMMA/OMMT mixture, the PMMA molecules have a larger affinity for the hydrophobic surfaces of OMMT than hydrophilic PEO. The larger layer thickness and broadness of the peak in the PMMA/ OMMT mixture are indicative of an improved intercalation for PMMA matrix over the OMMT surfaces. The relatively weak interaction between PEO and OMMT is because of the hydrophobic modification of the clay surface. 

Pioneer investigations in chemical modification of mineral surface were performed by Kiselev and his colleagues [1]. The observed irreversible adsorption of methyl alcohol vapours on silica was associated with substitution of surface silanols with methoxy groups. At the same time, Deuel and co-workers [2-4] have performed surface modification of some clays. They used reactive organic compounds, which can readily react with surface hydroxyls. It is important that even in the early studies the goal of surface chemical modification was the directed changes of adsorptional and adhesive properties of solids. 

Similarly to the case of the hydrophobic clay minerals described above, isotherms determined on HDP-palygorskite are S-shaped (Fig. 18). Surface modification by HDP" -cations has a double effect. On increasing the amount of HDP-cations the liquid sorption capacities decrease, since the micropores get clogged. On the other hand, the polarity of the surface decreases and the azeotropic composition indicates the displacement of the alcohol on the surface[50]. 

Braggs, B. et al.. The effect of surface modification by an organosilane on the electrochemical properties of kaolinite. Clays Clay Miner., 42, 123, 1994. 

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