Chemical resistance polysulphides

Table 7.9 Chemical-resistant polysulphide-sealant (specimen according to DIN 52 455, part 1, with acid-resistant clinker) (a) thixotropic (b) self-levelling. Source Morton International Ltd, Coventry, UK...

This is an important and growing application for chemical-resistant and oil-resistant polysulphide sealants. As a result of a survey commissioned in Germany by the Association for Mineral Oil, Natural Gas and Coal (Deutsche Wissenschaftliche Gesellschaft fiir Erdbl, Erdgas und Kohle, 1989, 1991) it was found that less volatile diesel fuels permeate into the ground and contaminate the water-table. It was roughly estimated that one litre of diesel can spoil a million litres of water. The problem is more serious in areas where the water-table is near to the surface. 

Chemical resistance. Unmodified epoxy resins have excellent resistance to dilute acids, alkalis and certain solvents. The incorporation of an LP component into the epoxy resin extends this resistance to a wide variety of oils, aromatic and aliphatic hydrocarbons, esters and ketones. Coatings and adhesives based on polysulphide-epoxy resins exhibit reduced swell and maintenance of flexibility and adhesion when immersed in these fluids. In view of this, LP-modified epoxies are used on oilrigs, fuel-storage installations and chemical plants (Figure 10.4). 

Secondary and tertiary amines. Tertiary amines can be used as sole curing agents but in adhesive formulations they are more normally found in blends, for example as accelerators with polyamide systems(4) and as an essential part of polysulphide systems. The cured resins tend to be less temperature and chemical resistant than the amines described above. Secondary amines react first like primary then as tertiary amines. 

EHT86 Modified polysulphide/cyclo-aliphatic amine 1.07 0.3-0.6 1 0.65 1 - 2 hrs Casting laminating. Good impact chemical resistance... 

The high cost, poor physical properties and, to some extent, the unpleasant odour of the polysulphides prevent them from consideration as general purpose rubbers. On the other hand they do have very good oil, solvent and chemical resistance which firmly indicates their use in certain areas such as printers rollers, seals and gaskets. In order to improve their performance in these areas attention has been paid to three particular areas, namely ... 

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). 

Thiokol Chemical Co. (1969) Solvent Resistance of LP Liquid Polysulphide Polymers, bulletin 565 M, May, Thiokol Chemical Corporation, Trenton, NJ. 

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. 

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