Polysulfides mercaptans

Desorption Time. One-half to one hour with good agitation is usually sufficient desorption time for most systems. Once optimum desorption has taken place, the system is generally stable however, some exceptions have been observed Some compounds react or are readsorbed after an optimum desorption time as shown in Fig. 5. The active surface of the sorbent may act as a catalyst. Methanol has been shown to react with carbon disulfide in the presence of charcoal to form polysulfides, mercaptans and polyether-thioether compounds Decanting the solvent from the sorbent usually corrects this problem. 

Several types of hardeners are used in the curing Aliphatic amines Amidoamines Aromatic amines Cycloaliphatic amines Imidazoles Polyamides Anhydrides Polysulfides Mercaptans Dicyandiamide (DiCy)... 

Sulfur reacts with mercaptans ia the presences of basic catalysts at temperatures of 75—105°C, forming sulfides. These sulfides are usually light ia color and are formed without cross-linking. The sulfurization of mercaptans leads to di-, tri-, or higher polysulfides, depending on the mole ratio used (eqs. 5 and 6). An extensive Hst of references to the sulfurization of mercaptans is available (8). 

Sulfurization of unsaturated compounds ia the presence of hydrogen sulfide also affords polysulfides (9). It is postulated that this reaction forms the mercaptan in situ which then further reacts to form the polysulfide (see Sulfurcompounds). 

Polysulfide Impression Materials. In 1953 the first nonaqueous, elastic dental impression material based on the room-temperature conversion of a Hquid polymer, a polyfunctional mercaptan (polysulfide), to a strong, tough, dimensionally accurate elastomer, was introduced. The conversion of the Hquid polymer to an elastic soHd has been achieved in most products by lead peroxide [1309-60-0]. Significant improvements in strength, toughness, and especiaHy dimensional stabiHty of the set polysulfide elastomers over the aqueous elastic impression materials made these materials popular. 

The polysulfide base material contains 50—80% of the polyfunctional mercaptan, which is a clear, amber, sympy Hquid polymer with a viscosity at 25°C of 35, 000 Pa-s(= cP), an average mol wt of 4000, a pH range of 6—8, and a ntild, characteristic mercaptan odor. Fillers are added to extend, reinforce, harden, and color the base. They may iaclude siUca, calcium sulfate, ziac oxide, ziac sulfide [1314-98-3] alumina, titanium dioxide [13463-67-7] and calcium carbonate. The high shear strength of the Hquid polymer makes the compositions difficult to mix. The addition of limited amounts of diluents improves the mix without reduciag the set-mbber characteristics unduly, eg, dibutyl phthalate [84-74-2], tricresyl phosphate [1330-78-5], and tributyl citrate [77-94-1]. 

Polysulfide Propellant Cured with a Mono-functional Mercaptan , USP 3706610 (1972)... 

Two of the more recently developed polysulfide polymers are the mercaptan-terminated polyoxypropylene urethane polymer and the polythioether polymer. The urethane-backbone-based polymer is used in many sealant formulations for insulating glass applications. The thioether backbone contains sulfur, but no S—S bonds, which are the weakest part of the conventional polysulfide polymer. This polymer improves the thermal stability and reduces the gas—liquid permeability. 

The liquid polymer is converted to the rubbery state by reagents that react with mercaptan (-SH) and side groups of the polymer segments by oxidation, addition or condensation to effect sulfide (-S-S-) bond formation. The oxidation reactions are exothermic and accelerated by an alkaline environment. The most commonly employed oxidizing agents which are suitable for curing liquid polymers are cobalt or manganese or lead octoate, p-quinonedioxime and di- or tri-nitrobenzene. Epoxy resin also reacts with liquid polysulfide polymers by addition in the presence of an aliphatic or aromatic amine and polyamide activator as shown in Equation 5.8 ... 

These directions have been used equally successfully with twice, one-third, and one-fifth the amounts specified. The reaction of chloro- or bromoacetal with sodium disulfide results in the formation of a considerable quantity of the corresponding monosulfide which is not subsequently reduced to mercaptan. Polysulfides are, however, easily reduced to mercaptans. 1,1,11 -Tetramethoxyethyl poly sulfide has been prepared from commercially available dimethyl chloroacetal in a similar fashion. A 10-hour heating period and the addition of 5 g. of potassium iodide per 100 g. of acetal are recommended in the latter preparation. 

The polysulfide then reacts by a condensation polymerization with the difunctional and trifunctional chloro compounds. It is also possible to react cyclic S8 with a mercaptan-capped molecule or dithiol in a condensation reaction in the presence of a basic catalyst. An example is the reaction with ethanedithiol (reaction (10)) 56... 

Arretz, E., Lopez, F. Method for preparing organic disulfides and polysulfides from mercaptans and sulfur in the presence of heterogeneous styrene-divinylbenzene copolymer catalysts having pendant guanidine or amidine groups. 1996, WO 9721673. 

Polysulfides and mercaptan Moisture resistance Quick set time Flexible Odor Poor elevated-temperature performance Poor tensile strength Adhesives and sealants Civil engineering Casting and encapsulation Coatings... 

Polysulfide resin, on the other hand, does not have fast and low-temperature curing properties. It is used as more of a flexibilizer than a curing agent. However, polysulfide resins do have mercaptan groups, and these enter into the epoxy cure reaction. 

The feedstocks (straight-mn naphtha (SRN) and a blend of SRN and hydrocracked naphtha) and hydrotreated products were analysed by ASTM methods for density, carbon, hydrogen, hydrocarbon and boiling point distribution. Total sulfur was determined by ASTM D-4045 method, mercaptan sulfur by the potentiometric method (ASTM D-3227 and UOP-212), disulfides by the UOP-202 method, polysulfides by polarography [1], and elemental sulfur by the UOP-286 method. The Perkin-Elmer gas chromatograph (Model 8700), equipped with a flame photometric detector (GC/FPD) and a DB-1 fused silica capillary column (30 m x 0.53 mm), was used for identification of individual sulfur compounds [2-6]. The sensitivity of the GC/FPD technique was maximized by optimizing the gas flow rates and temperature programming as presented elsewhere [1]. 

To describe the full range of thiochemicals is beyond the scope of this entry, which will, therefore, place emphasis on the most common thiochemicals. This entry will cover mercaptans, sulfides, and polysulfides, while the next entry will cover sulfonic acids and its derivatives, some sulfoxides, mercaptoacids and their derivatives. The safety, health, and environmental issues for both entries are discussed at the end of this chapter. 

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