Polysulfide formation

Early tests [37] utilized a cell design similar to that of early MCFC experiments. The assembled cell, machined from graphite blocks, is shown as Fig. 24. The electrodes and current collectors were machined from graphite and dense carbon, respectively. The electrolyte was a mixture of 63% Na2S, 37% Li2S, believed to melt near 850 °C the melting point after several days of operation was below 700 °C, probably because of polysulfide formation. The electrolyte was immobilized in a matrix of MgO, the whole formed by hot-pressing a mixture of electrolyte and ceramic powders. 

The warm solution (60 C) containing ca. 200 g/L barium sulfide is filtered and immediately pumped to the precipitation stage. Further purification is not necessary. Unreacted gangue and heavy metals are collected as insoluble sulfides in the filter cake. The almost clear solution can be stored only for a short period. Longer storage leads to undesirable polysulfide formation. 

The average concentrations of reduced inorganic sulfur species in the anoxic zone of the Black Sea measured using a new colorimetric method developed by Volkov [61,62] are summarized in Table 3. Presented elemental sulfur data refer to the stun of elemental sulfur allotropes (zero-valent sulfur) and the zero-valent sulfur derived from some fraction (n - 1) of the original polysulfide S 2. Thiosulfate data in the table represent the total amount of thiosulfate, sulfite, and polythionates. At some stations in the Black Sea, Volkov [61] observed a concentration maximum of elemental sulfur at the oxic/anoxic interface associated with sulfide oxidation by dissolved oxygen and/or Mn oxyhydroxides. Increasing with depth, elemental sulfur concentrations are probably explained by the ongoing process of polysulfide formation... 

Elemental sulfur, in the form of highly reactive colloid (28), can be the nucleus of additional pyrite reformation following polysulfide formation on the surface of the colloidal particles and... 

Polysulfide Formation by the Dimethyl Sulfoxide Oxidation of Dimercapto-... 

Interfacial aspects and adhesion of polysulfides have been studied extensively and reported by us [3,17]. The epoxy-modified poly sulfide has improved adhesion due to chemical reactions that increase electronic attraction forces. Water has been found to be the most potent debonding agent in cured polysulfides. Formation of thiourethane is responsible for excellent adhesion of polysulfide onto polyurethane coatings. 

Following reaction, the addition of alcohols converts excess B2S3 to soluble trialkyl borates and in favourable cases precipitates the desired product. Unfortunately, reduction and polysulfide formation complicate reactions directed at dioxo-Mo(vi) complexes. For example, Tp MoX(S4) (X = halide, NCS"), [Tp MoOSX]" (X = halide, S-donor anion) and [Tp MoO]2(p-0) (14-82) observed as the products of attempted OSCRs involving Tp Mo02X and B2S3. 

Kinetic studies conducted by DeBerry (1989) suggest that the polysulfide formation reaction mechanism can be represented by the following equation ... 

The sulfur then reacts to form the polysulfide according to equation 12. The key is the use of a catalyst to promote the formation of elemental sulfur. Commercial systems are based on the use of air with an activated carbon catalyst (41). The need for additional sulfur is eliminated, but the sulfur level is... 

The stmcture of the sulfuri2ed vat dyes is uncertain but in the presence of amino or methyl groups, thia2o1e-ring formation is possible. Examples have been confined to the dyes whose intermediates have been subjected to the conventional sulfur or polysulfide bake (Tables 2—5). 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

Monomer conversion (79) is followed by measuring the specific gravity of the emulsion. The polymerization is stopped at 91% conversion (sp gr 1.069) by adding a xylene solution of tetraethylthiuram disulfide. The emulsion is cooled to 20°C and aged at this temperature for about 8 hours to peptize the polymer. During this process, the disulfide reacts with and cleaves polysulfide chain segments. Thiuram disulfide also serves to retard formation of gel polymer in the finished dry product. After aging, the alkaline latex is acidified to pH 5.5—5.8 with 10% acetic acid. This effectively stops the peptization reaction and neutralizes the rosin soap (80). 

The Auger depth profile obtained from a plasma polymerized acetylene film that was reacted with the same model rubber compound referred to earlier for 65 min is shown in Fig. 39 [45]. The sulfur profile is especially interesting, demonstrating a peak very near the surface, another peak just below the surface, and a third peak near the interface between the primer film and the substrate. Interestingly, the peak at the surface seems to be related to a peak in the zinc concentration while the peak just below the surface seems to be related to a peak in the cobalt concentration. These observations probably indicate the formation of zinc and cobalt complexes that are responsible for the insertion of polysulfidic pendant groups into the model rubber compound and the plasma polymer. Since zinc is located on the surface while cobalt is somewhat below the surface, it is likely that the cobalt complexes were formed first and zinc complexes were mostly formed in the later stages of the reaction, after the cobalt had been consumed. 

NMR and visible spectra have established that a number of S-N anions are present in such solutions.The primary reduction products are polysulfides Sx, which dissociate to polysulfur radical anions, especially the deep blue 83 ion (/Imax 620nm). In a IM solution the major S-N anion detected by NMR spectroscopy is cycZo-[S7N] with smaller amounts of the [SSNSS] ion and a trace of [SSNS]. The formation of the acyclic anion 5.23 from the decomposition of cyclo-Sjl is well established from chemical investigations (Section 5.4.3). The acyclic anions 5.22 and 5.23 have been detected by their characteristic visible and Raman spectra. It has also been suggested that a Raman band at 858 cm and a visible absorption band at 390 nm may be attributed to the [SaN] anion formed by cleavage of a S-S bond in [SSNS]. ° However, this anion cannot be obtained as a stable species when [SsN] is treated with one equivalent of PPhs. 

The red [SSNO] anion (9.2) (2max 448 nm) is produced by the reaction of an ionic nitrite with elemental sulfur or a polysulfide in acetone, DME or DMSO. ° The formation of 9.2 probably proceeds via an intermediate such as the [S2NO2] anion. This process is thought to occur in the gunpowder reaction, which also entails the reaction of potassium nitrite (produced by reduction of potassium nitrate with charcoal) and sulfur. 

The high resistivity of Inconel 600 (11 OjtI 0 8 Dm) demanded the application of this material as a composite with a central aluminum core. The aluminum was totally enclosed in Inconel 600 so that the Inconel was only exposed to sulfur and polysulfides. In a test over more than three years, cells with a composite current collector of this kind suffered from a high capacity decline. Post-test analysis showed that Inconel sustained polysulfide attack with the formation of a duplex nickel and chromium sulfide layer on the current collector surface. 

On the other hand, the large activation energy for the formation of sulfate from 8g and water makes it possible to prepare polysulfides as well as other reduced sulfur compounds as metastable products in aqueous solution at ambient conditions. 

The reaction at Eq. (12) allows the preparation of Na2S4 and K2S5 from the alkali metals, hydrogen sulfide and sulfur in anhydrous ethanol (ROH). First the metal is dissolved in the alcohol with formation of ethanolate (MOR) and hydrogen. Bubbling of H2S into this solution produces the hydrogen sulfide (MHS). To obtain the polysulfide the solution is refluxed with the calculated amount of elemental sulfur. After partial evaporation of the solvent and subsequent cooling the product precipitates. 

The chemistry of elemental sulfur and sulfur-rich molecules including polysulfides in liquid ammonia [82] and in primary as well as secondary amines [83] is complex because of the possible formation of sulfur-nitrogen compounds. Therefore, polysulfide solutions in these solvents will not be discussed here. Inert solvents which have often been used are dimethylfor-mamide (DMF) [84-86], tetrahydrofuran (THF) [87], dimethylsulfoxide (DMSO) [87], and hexamethylphosphoric triamide (HMPA) [86, 88]. 

The yellow disulfide radical anion and the briUiant blue trisulfide radical anion often occur together for what reason some authors of the older Hterature (prior to 1975) got mixed up with their identification. Today, both species are well known by their E8R, infrared, resonance Raman, UV-Vis, and photoelectron spectra, some of which have been recorded both in solutions and in solid matrices. In solution these radical species are formed by the ho-molytic dissociation of polysulfide dianions according to Eqs. (7) and (8). 8ince these dissociation reactions are of course endothermic the radical formation is promoted by heating as well as by dilution. Furthermore, solvents of lower polarity than that of water also favor the homolytic dissociation. However, in solutions at 20 °C the equilibria at Eqs. (7) and (8) are usually on the left side (excepting extremely dilute systems) and only the very high sensitivity of E8R, UV-Vis and resonance Raman spectroscopy made it possible to detect the radical anions in liquid and solid solutions see above. 

The reaction of dihydrostibine TbtSbH2 with Ss at room temperature resulted in the formation of mono- and dinuclear polysulfides (Scheme 14) [54], On the other hand, the dropwise addition of a THF solution of TbtSbH2 to a suspension of excess of Ss in THF at -30 °C led to quite a different re-... 

Composite Particles, Inc. reported the use of surface-modified rubber particles in formulations of thermoset systems, such as polyurethanes, polysulfides, and epoxies [95], The surface of the mbber was oxidized by a proprietary gas atmosphere, which leads to the formation of polar functional groups like —COOH and —OH, which in turn enhanced the dispersibility and bonding characteristics of mbber particles to other polar polymers. A composite containing 15% treated mbber particles per 85% polyurethane has physical properties similar to those of the pure polyurethane. Inclusion of surface-modified waste mbber in polyurethane matrix increases the coefficient of friction. This finds application in polyurethane tires and shoe soles. The treated mbber particles enhance the flexibility and impact resistance of polyester-based constmction materials [95]. Inclusion of treated waste mbber along with carboxyl terminated nitrile mbber (CTBN) in epoxy formulations increases the fracture toughness of the epoxy resins [96]. 

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