Sepiolite composition

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. 

Certain polymers of interest for potential use as ion-conductors, such as PEO, largely studied in layered clay nanocomposites [229, 232-235, 237], could also be assembled with sepiolite. Thus, PEO can be assembled with sepiolite either from solution in acetonitrile or from the melt by microwave irradiation, giving rise to nanocomposites in which a partial penetration of the polymer chains takes place [17, 237]. Further attempts recently reported focus on the modification of the physical characteristics of PEO-sepiolite composites by controlled modulation of the silicate interphase afforded by the incorporation of polyethylene glycol and other additives [246]. However, to our knowledge, no studies to date have addressed the use of PEO-fibrous clays as ion-conducting systems. 

Shafiq M, Yasin T (2012) Effect of gamma irradiation on linear low density polyethylene/ magnesium hydroxide/sepiolite composite. Radiat Phys Chem 81 52-56... 

Ekici S, Isikver Y, Saraydin D (2006) Poly(acrylamide-sepiolite) composite hydrogels preparation, swelling and dye adsorption properties. Polym Bull 57 231-241... 

Beauger C, Laine G, Burr A, Taguet A, Otazaghme B, Rigacci A (2013) Nafion -sepiolite composite membranes fra improved proton exchange membrane fuel cell performance. J Memb Sci 430 167-179... 

The raw materials used in this study were a hydroxylated gel of titania of about 50% water content with an average particle size of 90% <40 /tm, supplied by Tioxide (UK) and a natural a-sepiolite of >80% purity supplied by Tolsa S.A. (Spain). Monoliths were prepared to seven titania sepiolite compositions 1(X) 0, 80 20, 65 35, 50 50, 35 65, 20 80 and 0 100 wt% respectively. The production method used for all of these monoliths has been reported elsewhere [6]. In this study the monoliths were configured with 7.5 square cells cm and a 1 mm wall thickness on initial extrusion. 

Zhang, Y., Wang, D., and Zhang, G. (2011) Photocatalytic degradation of organic contaminants by Ti02/sepiolite composites prepared at low temperature. Chem. Eng. /., 173,1 10. 

Figure 24.3 compares the calculated composition of the evaporated water, concentrated 100-fold and 1000-fold, with analyses of waters from six saline alkaline lakes (compiled by Garrels and Mackenzie, 1967). The field for the modeled water overlaps that for the analyzed waters, except that Ca++ and Mg++ are more depleted in the model than in the lake waters. This discrepancy might be explained if in nature the calcite and sepiolite begin to precipitate but remain supersaturated in the fluid. 

Bearing in mind the difficulties of interpretation because of the lack of precise crystal structural detail characteristic of the diffraction data obtained from fibrous materials, chemical analyses indicate that the composition of sepiolite can be expressed as... 

Figure 1. X-ray diffractogram and chemical composition of the Spanish sepiolite (from TOLSA S.A.) used in this work.

Choudhury et al. [86] have studied the effect of polymer-solvent and clay-solvent interaction on the mechanical properties of the HNBR/sepiolite nanocomposites. They chose nine different sets of solvent composition and found that chloroform/methyl ethyl ketone (Qi/MEK) (i.e., HNBR dissolved in Ch and sepio-lite dissolved in MEK) is the best solvent combination for improvement in mechanical properties. XRD, AFM, , and UV-vis spectroscopy studies show that the dispersion of clay is best in the Ch/MEK solvent combination and hence polymer-filler interaction is also the highest. images shown in Fig. 14a, b clearly elucidate the aforementioned phenomena. Consequently, the tensile strength and modulus are found to be higher (5.89 MPa and 1.50 MPa, respectively) for the Ch/MEK system due to the minimum difference in interaction parameter of HNBR-solvent (xab) and sepiolite-solvent (Xcd)- Choudhury et al. have also studied the effect of different nanoclays [NA, , 15A, and sepiolite (SP)] and nanosilica (Aerosil 300) on the mechanical properties of HNBR [36]. The tensile... 

Fig. 43 Fitting of composite models on introduction of IAF, for the sepiolite-filled NR nanocomposites symbols represent predicted values and the line indicates the best fit of the experimental data. YM Young s modulus...

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

High pressure studies using natural sepiolite and palygorskite (Frank-Kameneckiji and Klockova, 1969) indicate that these minerals can contain variable quantities of silica because they exsolve quartz while retaining their basic structural and mineral identity. These experiments also demonstrate that the natural minerals are compositionally intermediate between talc or montmorillonite and quartz. These latter phases are formed upon the thermal breakdown of sepiolite and palygorskite under conditions of 1 and 2Kb total pressure. Both sepiolite and palygorskite appear to remain stable in sequences of buried rocks, at least up to the depth where fully expandable dioctahedral montmorillonite disappears (Millot, 1964). 

Since neither mineral is conspicuously alkali-rich, the M R -2R -3R plot is not appropriate to represent their bulk compositions. Both sepiolite and palygorskite contain small but variable amounts of Ca, Mg,... 

Figure 39. Compositions of natural sepiolites and palygorskites as a function of the variables Si-R -R where R Mg + Fe + Ca, R = Fe + + Al. Crosses = sepiolite, dots = palygorskite.

Given the chemical data for natural mineral compositions, it should be possible to construct a phase diagram including those phases that are likely to occur with sepiolite and palygorskite in a system where the mass of the components is an extensive variable of the system (a closed system). 

Figure 41. Phase diagram for the extensive variables R -R -Si combining the data for synthetic magnesian chlorites and the compositional series of natural sepiolites and palygorskites. Numbers represent the major three-phase assemblages related to sepiolite-palygorskite occurrence in sediments. Chi = chlorite M03 = trioctahedral montmorillonites M02 = dioctahedral montmorillonite Sep = sepiolite Pa = palygorskite Kaol = kaolinite T = talc.

Fig. 2. Logarithmic activity diagram depicting equilibrium phase relations among aluminosilicates and sea water in an idealized nine-component model of tire ocean system at the noted temperatures, one atmosphere total pressure, and unit activity of H20. The shaded area represents (lie composition range of sea water at the specified temperature, and the dot-dash lines indicate the composition of sea water saturated with quartz, amotphous silica, and sepiolite, respectively. The scale to the left of the diagram refers to calcite saturation foi different fugacities of CO2. The dashed contours designate the composition (in % illite) of a mixed-layer illitcmontmorillonitc solid solution phase in equilibrium with sea water (from Helgesun, H, C. and Mackenzie, F T.. 1970. Silicate-sea water equilibria in the ocean system Deep Sea Res.).

The commercial active carbon (C) from Panreac and a natural sepiolite (S) from Vallecas (Madrid) supplied by Tolsa S.A. were used as received. The nominal chemical composition of the sepiolite is Si02 62.0, MgO 23.0, AI2O3 1.7, Fe203 0.5, CaO 0.5, K2O 0.6, 0 0.3, weight loss from 293 to 1273 K 10.5 %. 

When the relative amount of large cations (Fe3 +, Fe2+, Mg) in the octahedral sheet is increased (80 to 100% of the octahedral cations), the layer strain is slightly reduced and the octahedral sheet can extend to 8 or 9 positions before forcing the tetrahedral inversion (sepiolite). It seems likely, that the octahedral sheets of attapulgite and sepiolite have a much wider compositional range than indicated by the present limited analyses and that they occupy the intermediate position between the diocta-hedral and trioctahedral clays. 

Sepiolite from Vallecas (Spain) with a chemical composition given in Table I, was used as starting material. Following the procedure described previously (8), between 1.7 and 2 meq. g 1 of Na+, La3+, A1, Cr3+ and H+ were introduced in the octahedral sheet of the silicate, replacing the magnesium ions located at the edges of the fibres in natural sepiolite (Figure 1). 

Table I. Chemical composition of natural Vallecas-type sepiolite (wt%) ...

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