Synthetic hydrotalcite

Hydrotalcite. Synthetic hydrotalcite minerals are gaining commercial acceptance for thein abiHty to costabiHze PVC ia the presence of other primary stabilizers (see Table 2). The performance of the mixed metal stabilizers are particularly boosted when an equal part level, about 2—3 phr, of hydrotalcite is added to the PVC formulation. These minerals function by trapping HCl within the layered lattice arrangement of atoms. The formula. 

Tetrabutylammonium benzoate has been used as a catalyst for the polymerization of PO over the temperature range 40—108°C and the yield of polymer was typically low (2—78%) a large amount of unsaturation was present due to chain transfer (95). When synthetic hydrotalcite,... 

Obviously, use of such databases often fails in case of interaction between additives. As an example we mention additive/antistat interaction in PP, as observed by Dieckmann et al. [166], In this case analysis and performance data demonstrate chemical interaction between glycerol esters and acid neutralisers. This phenomenon is pronounced when the additive is a strong base, like synthetic hydrotalcite, or a metal carboxylate. Similar problems may arise after ageing of a polymer. A common request in a technical support analytical laboratory is to analyse the additives in a sample that has prematurely failed in an exposure test, when at best an unexposed control sample is available. Under some circumstances, heat or light exposure may have transformed the additive into other products. Reaction product identification then usually requires a general library of their spectroscopic or mass spectrometric profiles. For example, Bell et al. [167] have focused attention on the degradation of light stabilisers and antioxidants... 

Fig. 4 Diffraction patterns of a a synthetic hydrotalcite [Mg6Al2(0H)i6](C03)-4H20 which has a 3R polytype and b the mineral manasseite which has the same formula as hydrotalcite but a 2H polytype. Reprinted with permission from [107]. Copyright Kluwer Academic Publishers...

The interlayer Cl" anions in a synthetic hydrotalcite-like material, Mg6Al2(0H)i5Cl2 4H20, react with butyl bromide or benzyl bromide in a non-polar solvent of toluene. Almost all the chloride ions in the interlayer space... 

Douhova et al. (2003) found that after mixing for 24-72 hours at 20 °C, calcinated synthetic hydrotalcite and calcinated natural boehmite were much more effective in removing As(V) from water than oxihumolite (i.e. humic-rich low-rank coals) (Douhova et al., 2003, 259). Dosages of 1 gL-1 of hydrotalcite or 2.6 gL-1 of boehmite removed 70.5 and 97.1 %, respectively, of 2 mmol L-1 (150 mg As L-1) of As(V) (Dou.vova et al., 2003, 265). The arsenic probably bonded with oxygens on the hydrotalcite and boehmite. Infrared spectra detected As-0 bonds in both inorganic sorbents (Dousova etal., 2003, 261, 263). In contrast, As-0 bonds were absent in the infrared spectra of the oxihumolite (Dou. ovd et al., 2003, 263). 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

Miyata, S. (1980). Physico-chemical properties of synthetic hydrotalcites in relation to composition. Clays Clay Miner. 28, 50. 

Costantino, U., Coletti, N., Nocchetti, M., Aloisi, G. G. and Elisei, F. (1999). Anion exchange of methyl orange into Zn-Al synthetic hydrotalcite and photophysical characterization of the intercalates obtained. Langmuir 15, 4454. 

Constantino, U., Casciola, M., Massinelli, L., Nocchetti, M. and Vivani, R. (1997). Intercalation and grafting of hydrogen phosphates and phosphonates into synthetic hydrotalcites and a.c.-conductivity of the compounds thereby obtained. Solid State Ionics 97, 203. 

Intercalation and grafting of n-alkyl phosphonates into synthetic hydrotalcites. Mol. Cryst. Liq. Cryst. 311, 207. 

The replacement of conventional bases, such as NaOH, KOH and NaOMe, by recyclable solid bases, in a variety of organic reactions, is also a focus of recent attention [27, 40]. For example, synthetic hydrotalcite clays, otherwise known as layered double hydroxides (LDHs) and having the general formula Mg8 xAlx (0H)16(C03)x/2 nH20, are hydrated aluminum-magnesium hydroxides possess-... 

Table 2. Surface area and porosity of a synthetic hydrotalcite after calcination at 723 K and further rehydration at room temperature for different periods.

Giannelis, E.P., Nocera, D. G., and Pinnavaia, T. J. 1987. Anionic photocatalysts supported in layered double hydroxides intercalation and photophysical properties of a ruthenium complex anion in synthetic hydrotalcite. Inorg. Chem. 26 203-05. 

The empirical formulae for some synthetic hydrotalcites are as follows [13,14] ... 

Synthetic hydrotalcites employed in plastics applications are finely divided, free-flotving, odorless, amorphous powders. For polyolefin applications, hydrotalcites with an average particle size of 0.5 (tm and a specific surface area ofless than 20 m /g (BETmethod) are recommended. Their density varies around 2.1 g/cm depending on the composition. Sodium, zinc, and calcium stearate-coated hydrotalcites are available, which show enhanced compatibility with polyolefins [13]. Figure 20.3 is a scanning electron micrograph of a typical submicron-size synthetic hydrotalcite [20]. 

Figure 20.3 Scanning electron micrograph of synthetic hydrotalcite submicron particles [20].

Traditional substances used as co-stabilisers include polyols, epoxidised soya bean oil, phosphites and certain antioxidants. Arkema markets Stavinor calcium stearate co-stabilisers for use with calcimn-zinc systems. Akcros Chemicals sells P24D, which stands for pyrrolidene-2,4-dione. Siid-Chemie is now marketing a synthetic hydrotalcite, Sorbacid 911, while the PQ Corporation is selling two grades of aluminosilicate (zeolite) imder the name Advera 401. They act as scavengers for acids and metal chlorides, and enable users of tin mercaptides to use less tin, saving on material costs. They also allow stabiUsation to take place with the very low tin levels required by law in food contact applications. 

JA Kosin, BW Preston, DN Wallace. Modified Synthetic Hydrotalcite. US Patent 4,883,533, November 28, 1989. 

K Kaneda. Ru-Al-Mg Type Synthetic Hydrotalcite, Oxidation Catalyst and Production of Carbonyl Compound Using Same. Jap Patent 2,000,070,723, March 7, 2000. 

Compatibility with the polymer matrix, which is for example given by many metallic stearates or other fatty acid derivatives. These are miscible under processing conditions when molten together with (or prior to) the addition of the polymer. For the non-melting inorganic salts (e.g. synthetic hydrotalcites), a coating is recommended for better compatibilization. 

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