Polysulphide elastomers

Two types of sulphur-containing polymers are of commercial significance, namely polysulphide elastomers and polysulphones. These materials have very different characterisitics and are considered separately in this chapter. 

Polysulphide elastomers are based on alkyl polysulphide polymers which can be cured by various means to give products with rubbery characteristics. The structure of the base polymer may be represented as follows ... 

The first poly sulphide elastomer was discovered in 1924 by Patrick during an attempt to prepare ethylene glycol by hydrolysis of ethylene dichloride. Commercial production was begun in 1929 by the Thiokol Chemical Corp. (U.S.A.). The current consumption of polysulphide elastomers, compared to that of other elastomers, is very small, use being restricted mainly to special applications involving resistance to oils and solvents. 

The sodium polysulphide solutions used for the preparation of polysulphide elastomers generally have a rank of approximately 2 for elastomers with a high sulphur content which have especially good solvent resistance, a rank of about 4 is used. In general, the rank of a polymer is close to that of the sodium polysulphide solution from which it is formed. 

As indicated earlier, polysulphide elastomers are distinguished by outstanding resistance to oils and solvents. The resistance improves as the sulphur content of... 

A characteristic of polysulphide elastomers, especially those with high sulphur content, is poor compression set resistance. This does not arise through oxidative degradation, as is the case with most elastomers, but throu interchange reactions at the polysulphide bonds [1]. Interchange reactions may proceed through both radical and ionic mechanisms. In the free radical process, initiation depends on the homolytic cleavage of a polysulphide link, e.g., a tetrasulphide link ... 

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

Polymerisation resulting from a chemical reaction involving condensation. The synthetic elastomers produced by condensation polymerisation include polysulphide rubbers, silicone rubbers and the ester and isocyanate rubbers. 

This group of synthetic elastomers is better known under the trade name ThiokoP. Polysulphide rubbers are condensation polymers of sodium poly sulphide and dichloro-compounds they have outstanding resistance to swelling by oils and solvents but tensile strength is... 

The high mechanical strength of natural and organic rubbers as used in tires is due to the incorporation of pyrogenic carbon blacks as active fillers. Elastomers of a more polar polymer backbone, such as polyacrylates, polyurethanes or polysulphides, require fillers of higher polarity. In particular the performance of polydimethylsiloxane elastomers (silicone rubber) is basically related to the addition of fumed silica. 

Elastic sealants based on synthetic elastomers such as polysulphides, silicones or polyurethanes allow the periodically alternating movements of building structures caused by temperature fluctuations to be followed in a consistent manner. It has gone unnoticed by some that owing to the development of modem elastomaic sealants, a new field of application has emerged - the load-bearing adhesive joint... 

Ionic interchange reactions may also contribute to stress relaxation and these can occur at disulphide bonds as well as at higher polysulphide bonds. Such reactions may be initiated by ionic impurities in the elastomer or by mercaptides formed by the use of metallic compounds as curing agents. In the latter case, the following reaction may be envisaged ... 

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