Hectorite Composition

Transmission electron micrographs show hectorite and nontronite as elongated, lath-shaped units, whereas the other smectite clays appear more nearly equidimensional. A broken surface of smectite clays typically shows a "com flakes" or "oak leaf surface texture (54). High temperature minerals formed upon heating smectites vary considerably with the compositions of the clays. Spinels commonly appear at 800—1000°C, and dissolve at higher temperatures. Quartz, especially cristobalite, appears and mullite forms if the content of aluminum is adequate (38). 

R. B. Carpenter, J. B. Bloys, and D. L. Johnson. Cement composition containing synthetic hectorite clay. Patent WO 9902464,1999. 

Table I Approximate composition of hectorite based on chemical analysis.

The layered silicates hectorite and magadiite were synthesized using hydrothermal procedures based on those described in 111. For hectorite which contains lithium and magnesium atoms in octahedral sites between the silicate sheets, this synthesis procedure was modified. The crystallizations were carried out at temperatures ranging from 100°C to 200°C. The compositions of the reaction mixtures (expressed as molar ratios of the oxides) were ... 

Figure 15.11. Tensile modulus of composites made from nylon and different fillers (montmorillonite, saponite, hectorite and mica) vs. N-NMR chemical shifts of model compounds of fillers. [Adapted, by permission, from Usuki A, Koiwai A, Kojima Y, Kawasumi M, Okada A, Kurauchi T, Kamigaito O, J. Appl. Polym. Sci., SS, No.l, 1995, 119-23.]...

Figure 16.24 shows the schematic representation of dispersed clay particles in a polymer matrix. Conventionally dispersed clay has aggregated layers in face-to-face form. Intercalated clay composites have one or more layers of polymer inserted into the clay host gallery. Exfoliated polymer/clay nanocomposites have low clay content (lower than intercalated clay composites which have clay content -50%). It was found that 1 wt% exfoliated clay such as hectorite, montmorillonite, or fluorohectorite increases the tensile modulus of epoxy resin by 50-65%. ... 

The microparticles that make up the coating can be of any desired substance composition wise which can be reduced to a colloidal state of subdivision however, they must be dispersible in a medium as a colloidal dispersion. Water is the best medium for dispersions of particles of varying ionic charges. Examples of suitable aqueous sols are amorphous silica, iron oxide, alumina, thoria, titania, zirconia, zircon, and alumina sihcates, including colloidal clays such as montmorillonite, colloidal kaolin, attapul-gite, and hectorite. Silica is preferred material because of its low order of chemical activity, its ready dispersibility, and the easy availabihty of aqueous sols of various concentrations. 

Other than montmorillonite, synthetic mica, saponite and hectorite were used to synthesize a nylon 6-clay hybrid. The nano composites fabricated by using each of these types of clay were called NCH, NCHM, NCHP and NCHH. 

Silicate layers were uniformly dispersed in nylon 6 in NCH, NCHM, NCHP and NCHH at the molecular level. The thicknesses of the silicate layers were 1 nm in all of these nanocomposites, but their widths varied depending on the type of clay used. An examination of each photograph revealed that the width of the nano composites fabricated using montmorillonite and synthetic mica were about 100 nm and those of the nanocomposites fabricated using saponite and hectorite were about 50 nm. 

Use of nanoclay can be an initiative for preparation of PVC composites. Walid et al. have studied the preparation of poly(vinyl chloride) nanocomposites (Figure 6.11) using organically modified clays such as lecto-rite and bentonite and concluded that addition of a calculated amount of modified bentonite (or) hectorite nanoclay material leads to smoke reduction as well as enhances the time span of smoke evolution [31,32]. 

Kurokawa et al. [258-260] developed a novel but somewhat complex procedure for the preparation of PP/clay nanocomposites and studied some factors controlling mechanical properties of PP/clay mineral nanocomposites. This method consisted of the following three steps (1) a small amount of polymerizing polar monomer, diacetone acrylamide, was intercalated between clay mineral [hydrophobic hectorite (HC) and hydrophobic MMT clay] layers, surface of which was ion exchanged with quaternary ammonium cations, and then polymerized to expand the interlayer distance (2) polar maleic acid-grafted PP (m-PP), in addition was intercalated into the interlayer space to make a composite (master batch, MB) (3) the prepared MB was finally mixed with a conventional PP by melt twin-screw extrusion at 180°C and at a mixing rate of 160 rpm to prepare nanocomposite. Authors observed that the properties of the nanocomposite strongly dependent on the stiffness of clay mineral layer. Similar improvement of mechanical properties of the PP/clay/m-PP nanocomposites was observed by other researchers [50,261]. 

Laponite RD is a fnlly synthetic clay similar in structure and composition to natural hectorite of the smectite group (Scheme 1). Each layer is composed of three sheets two outer tetrahedral sihca sheets and a central octahedral magnesia sheet. Isomorphous substitution of magnesium with Uthium in the central sheet creates a net negative charge compensated by intralayer sodium ions located between adjacent layers in a stack. The cation exchange capacity of Laponite is 0.75 meq The dimen-... 

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

The two end members of this group with mainly tetrahedral substitutions are beidellite and saponite, which are di- and trioctahedral smectites, respectively. The corresponding end members with mainly octahedral substitutions are mont-morillonite and hectorite. Another common smectite, nontronite, is an iron-rich mineral. Chemical compositions of various smectite samples are provided in Table 2. Montmorillonite is the most common mineral of this group it is named for its location in Montmorillon, France. Figures 3d and 3e provide SEM views of the textural morphology of two montmorillonites. A common industrial mineral is bentonite, which is actually a montmorillonite of volcanic ash origin that contains a significant amount of impurities, such as cristobalite (a-quartz), that is intimately mixed with the clay. 

Pinnavaia et al. described the bonding of the pillars to the hectorite clay sheets, upon calcination, as a layer cross-linking mechanism [57]. Si and Al MAS-NMR data on Al-pillared smectite clays indicated the existence of two mechanisms for the linking of the pillars to the clay sheets. It was shown that the layer composition of the host clay plays a very important role in determining the layer reactivity upon heating. 

Synthetic hectorite, which closely resembles the natural clay mineral hectorite in both structure and composition, forms clear gels. It is free from natural clay impurities and is compatible with anionic and nonionic surfactants. The thickening efficiency of synthetic hectorite (commercially available under the trade name Laponite from Southern Clay Products, Inc.) is better than natural hectorite and natural bentonite in tap water [61]. 

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