Polysulphide, properties

Polysulphide rubbers. Ethylene dichloride and excess of sodium tetrasulphide when heated together give a polymeric polysulphide, Thiokol A, with properties resembling those of rubber ... 

Prepare a saturated solution of sodium sulphide, preferably from the fused technical sodium polysulphide, and saturate it with sulphur the sulphur content should approximate to that of sodium tetrasulphide. To 50 ml. of the saturated sodium tetrasulphide solution contained in a 500 ml. round-bottomed flask provided with a reflux condenser, add 12 -5 ml. of ethylene dichloride, followed by 1 g. of magnesium oxide to act as catalyst. Heat the mixture until the ethylene dichloride commences to reflux and remove the flame. An exothermic reaction sets in and small particles of Thiokol are formed at the interface between the tetrasulphide solution and the ethylene chloride these float to the surface, agglomerate, and then sink to the bottom of the flask. Decant the hquid, and wash the sohd several times with water. Remove the Thiokol with forceps or tongs and test its rubber-like properties (stretching, etc.). 

Further variations in the properties of the polysulphides were also achieved by the following means ... 

The polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications. Their value in casting and encapsulation lies mainly with their low curing shrinkage and flexibility in the cured state. Their tendency to corrode copper and the somewhat inferior electric insulation properties of the blends does lead to certain limitations. 

Bouroushian M, Karoussos D, Kosanovic T (2006) Photoelectrochemical properties of elec-trodeposited CdSe and CdSe/ZnSe thin films in sulphide-polysulphide and ferro-ferricyanide redox systems. Solid State Ionics 177 1855-1859... 

Cl Sulphur Black 1, which is produced from the relatively simple intermediate 2,4-dinitrophenol and aqueous sodium polysulphide. A similar product (Cl Sulphur Black 2) is obtained from a mixture of 2,4-dinitrophenol and either picric acid (6.148 X = N02) or picramic acid (6.148 X = NH2). A black dye possessing superior fastness to chlorine when on the fibre (Cl Sulphur Black 11) can be made from the naphthalene intermediate 6.149 by heating it in a solution of sodium polysulphide in butanol. An equivalent reaction using the carbazole intermediate 6.150 gives rise to the reddish blue Cl Vat Blue 43 (Hydron blue). This important compound, which also possesses superior fastness properties, is classified as a sulphurised vat dye because it is normally applied from an alkaline sodium dithionite bath. Interestingly, inclusion of copper(II) sulphate in the sulphurisation of intermediate 6.150 leads to the formation of the bluish black Cl Sulphur Black 4. 

Surface treatments of CD CdSe films deposited from selenosulphate/NTA solutions have a pronounced effect on various optical, electrical, and optoelectronic properties of the films, due to interaction with or modification of such surface states. Mild etching (dilute HCl) of the films reverses the direction of current flow both in CdSe/polysulphide photoelectrochemical cells [108] and in Kelvin probe surface photovoltage (SPV) measurements in air [109], These studies are discussed in more detail in Chapter 9, in Section 9.2 on photoelectrochemical cells. At this point, it is sufficient to state that the effect is believed to be due to preferential trapping of either electrons or holes at surface states that are modified by the etching process. 

In Ref. 53, the main pnrpose of the study was to investigate PEC properties of CD CdSe films rather than to optimize the actnal solar cell (e.g., CdSe annealing was carried ont at the relatively low temperatnre of 280°C and the film thickness was only ca. 0.9 p.m), hence the cell parameters are not as high as they could be. The CdSe/polysulphide junction was characterized by a nnmber of techniques. The effects of an snrface layer of CdS, dne to exchange of Se by S from the polysulphide, were considered. 

General Properties.—All the known hydrogen polysulphides, as well... 

This property can be improved by the addition of antiozonants. Finer fillers give better flexing characteristics. Low sulphur or sulphurless cures, though good for heat ageing, are very bad for increasing flex life. Polysulphide links are preferred to mono or di-sulphide cross links. 

The properties of polysulphide polymers are summarised in Table 7.2. Cured polysulphide systems possess excellent resistance to most solvents and mild inorganic acids. The solvent resistance depends on the sulphur content of the polymer. Table 7.3 compares the resistance of various liquid polysulphide polymers to a variety of solvents and chemical solutions. However, these data cannot be considered as absolute because the resistance is to some extent dependent on the efficiency of cure. Volume swell values below 40% illustrate excellent resistance. Those between 40% and 90% illustrate fair to good resistance whereas values above 90% may be only fair or even unsatisfactory (Thiokol Chemical Co., 1969). 

By virtue of their solvent-resistant property, polysulphide polymers are used extensively as integral fuel tank sealants. The construction industry utilises this property mostly in sealants in jet fuel aprons, laboratory floors, airport pavements and for use in petrol stations. Specific applications will be discussed separately in section 7.4. 

The use of bis (2-chloroethyl) formal as the predominant monomer in the manufacture of polysulphides results in the formation of polymers with very good low-temperature properties. The glass transition temperature of these disulphide polymers is -59°C. Furthermore, there is no tendency in these polymers to crystallise at a temperature above the glass transition point. A glass transition temperature of -59°C applies to all the commercial liquid polysulphide (LP) polymers. 

The tensile modulus of polysulphide polymers is enhanced with additional cross-linking agent and filler loading. It decreases with plasticisation. The presence of suitable reinforcing pigments gives adequate tensile and elongation properties. 

This property is useful in the case of joint sealants where a movement accommodation factor is the main concern. Elongations of 100% are acceptable. However, it is possible to formulate polysulphides with ultimate elongations in excess of 1000%. 

The properties of polysulphide coatings are given in Table 7.11. A volume swell of less than 150% after 72 h is acceptable (Lowe, 1994a). 

Table 7.11 Properties of polysulphide coatings. Source Morton International Ltd, Coventry, UK...

The beneficial properties of polysulphides and epoxies can be combined to impart a synergistic effect to the system. Individual details of the polymers have been discussed in Chapters 3 and 7. The advantages of incorporating a liquid polysulphide (LP) polymer into an epoxy resin have already been highlighted (section 10.1). Of these, the development... 

Adducts can be manufactured from either liquid or solid epoxy resins by using whatever level of LP is appropriate for the properties required for the cured product. The only limitation is that a molar excess of epoxy resin must always be maintained (Rees et al, 1994 Morton International, undated a). Properties of epoxy-polysulphide adducts manufactured by the above route include (Morton International, undated a) ... 

Epoxy-polysulphide adducts or prepolymers are commercially available under the trade name ELP (a trade mark of Morton International). Properties of one such adduct, ELP-3 are presented in Table 10.1. 

Adhesion. Owing to improved wetting properties, excellent adhesive characteristics are exhibited for coatings and adhesives with relatively small additions of liquid. Polysulphides are required to produce significant improvements in the lap shear strength of the system (Rees et al., 1994). 

Evidence of the formation of multilayer or subsurface sulfides under conditions where bulk sulfides are not expected has been reported [24-28]. For both gold [26] and platinum [27] it was concluded that in aqueous acid medium, sulfur species formed from hydrogen sulfide, which are likely to be composed mainly of polysulphide and/or element sulfur layers on the metal sulfide. Buckley et al. [26] concluded that the multilayer sulfur deposit does not have the properties of bulk elemental sulfur, suggesting substantial interaction occurs between the multilayer sulfur and the underlying metal sulfide and substrate metal. Two models account for the characteristics of the deposit. The sulfur could grow as an extension of the initial metal sulfide lattice with the possible movement of some metal atoms from the substrate into the sulfur layer. Alternatively, the sulfur layer could develop as a conglomerate of chains, from the underlying metal sulfide by the formation of polysulphide bonds with surface sulfur atoms. 

Sulphur (S, at. mass 32.06) occurs in the oxidation states -II (in sulphide), IV (in SO2 and sulphite), and VI (in H2SO4 and sulphate). Sulphur dissolves in certain organic solvents (e.g., CS2 and CeHe) when dissolved in an alkali metal sulphide solution, it yields yellow polysulphides (S + nS —> Sn+i j. When heated with an alkaline solution of sulphite, sulphur forms thiosulphate. Sulphide, sulphite, and thiosulphate are reducing agents. Persulphate has strongly oxidizing properties. Complexes are formed by sulphide e.g., with As, Sb, Mo), and also by thiosulphate [e.g., with Ag, Cu, and Fe(lII)]. The most stable sulphur species is sulphate, which forms sparingly soluble compounds, for example with Ba and Pb, and stable soluble complexes with Zr and Th. 

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