Silicon extraction

Santoprene extracts contained higher concentrations of inorganic carbon and little, if any, extractable silicon. On the other hand, the amount of silicone in extracts of silicone polymers was very high. From Table 37, it is possible to see that these materials, though all made of silicone, are very different each other. While about lOOpg/g Si was found in the aqueous extract from sample 1, no measurable Si was detected in the same extracts from samples 2 and 6. As could be expected, silicone extract-ables were much higher in ethanolic extracts, being that 739 pg/g Si from sample 6 was the lowest amount leached. 

The Freon TA Soxhlet extraction of the RTV silicone sample at different curing times with Freon reveals the degree of curing of the material. At time zero, when the RTV material was first coated for room temperature cure, almost 100% extractables were obtained (except residue of fillers). This 100% extractable indicates a 0% cure of the material. After 16 hrs. of room temperature cure at 50% relative humidity, most of the RTV material (>90%) was cured. After the second day, almost all of the RTVs studied were fully cured, and they seem to reach an extraction equilibrium. Further curing time shows no noticeable change in amount of extractables. For the fully cured RTV silicone, the level of extractables at their equilibrium extraction was a good indication of the unreactive cyclics present in this material. In Figure 5, the RTV silicone extractable is approximately 4%. 

Figure 9 Fourier Transform-Infrared Spectrum (FT-IR) of Silicone Extractables Fluids...

Figure 10 Gas Chromatography/Mass Spectrometer (GC/MS) of Silicone Extractables...

Table III. GC/MS Analysis of Low Volatiles from Cured RTV Silicone Extractables ...

Mesoporous Fe-MFI zeolites have been successfixlly prepared by treatment of isomorphously substituted Fe-silicalite and ion-exchanged Fe-ZSM-5 in alkaline medium. Iron in framework positions directs the silicon extraction towards mesoporosity development, whereas iron in non-framework positions inhibits silicon dissolution and limits mesoporosity development. 

The relatively high newly developed mesopore surface area of 120 m g is however in contrast to alkaline-treated silicalite-1, in which despite the high degree of unselective silicon dissolution hardly any mesoporosity has been observed. Due to the uncontrolled silicon extraction in the absence of framework aluminum, mostly large macropores were obtained in alkaline-treated silicalite [18]. We have previously reported a correlation between mesopore surface area development and framework Si/Al ratio of the parent... 

Alkaline treatment of Fe-Z15, its Si/Al ratio of 16 being outside the optimal range for mesopore formation as shown in Fig. 1, leads to a significantly lower Si extraction of 290 mg F as compared to the iron-free system (570 mg f ), while a similar (limited) mesoporosity (50 m g ) has been measured (Table 1). The slight increase of 10 m g" in mesopore surface area must be attributed to the relatively high framework aluminium concentration in Z15, which stabilizes the zeolite framework against desilication and suppresses silicon extraction [18]. Similarly to s-FeS-at, the presence of non-framework iron does not facilitate sihcon dissolution. 

Alkaline treatment of Fe-MFI zeolites prepared via ion exchange or isomorphous substitution leads to combined micro- and mesoporous Fe-MFI structures. The preparation method highly determines the impact of the alkaline treatment. Iron in framework positions (iron sihcahte) directs the silicon extraction towards controlled mesopore formation similarly to framework aluminium in ZSM-5. Iron in non-framework positions inhibits leaching of silicon to the filtrate. Higher concentrations of non-framework iron, as obtained in ion-exchanged Fe-ZSM-5 catalysts, lead to a lower mesoporosity development. TTie iron nanoparticles appear to remain unchanged upon Eilkaline treatment and speculatively isolated and oligonuclear iron species are held responsible for the restricted silicon dissolution and mesoporosity development. 

Silicones have been extracted from environmental samples with solvents such as hexane, diethyl ether, methyl isobutylketone, ethyl acetate, and THF, using either sequential or Soxhlet techniques (690-695). Silicones of a wide range of molecular weights and polarities are soluble in THF. This feature, coupled with its volatility and miscibility with water, makes THF an excellent solvent for the extraction of silicones from wet samples, ie, soils and sediments. Trace levels of silicones extracted from environmental samples have been measured by a number of techniques, including atomic absorption spectroscopy (AA), inductively coupled plasma-atomic emission spectroscopy (ICP-AES), pyrolysis GC-MS, as well as H and Si NMR spectroscopy (674,684,692,696-700). The use of separation techniques, such as gel permeation and high pressure liquid chromatography interfaced with sensitive, silicon-specific AA or ICP detectors, has been particularly advantageous for the analysis of silicones in environmental extracts (685,701-704). 

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