Requirements, Polysulfide Polymers

The US Mii Spec covering iiq polysulfide polymers for use in the 3.0-inch, Mark 32 Mod 0 Rocket Motor, MlL-P-23702 (WEPl, (10 May 1963) contains the following requirements and criteria (Tables 1 and 2)... 

Caicium Peroxide H.O, Amines, Bases Moiecuiar Sieves CaO, requires moisture for activation and can provide a white cured system. Amount recommended per 100 parts of polysulfide polymer 10.0... 

The most common system for the outer or secondary sealant used is based on polysulfide polymers cured with manganese dioxide where the physical properties have been modified by the use of a low-volatility plasticizer such as a higher phthalate. Recent developments have seen an increase in the use of polyurethanes because of their higher adhesion potential. All the systems are required to have oil resistance when used in timber windows (from the glazing putt and preservatives) (see Selection of joint sealant). 

The initial sulfur copolymer that is formed is often high conversion and gelled. Molecular weight is reduced to the required level by cleaving some of the polysulfide Linkages, usually with tetraethylthiuram disulfide. An alkaU metal or ammonium salt (30) of the dithiocarbamate, an alkaU metal salt of mercaptobensothiasole (31), and a secondary amine (32) have all been used as catalysts. The peptization reaction results in reactive chain ends. Polymer peptized with diphenyl tetrasulfide was reported to have improved viscosity stabiUty (33). 

The polysulfidic moieties of the silanes are unstable, and cleavage of the sulfur groups results in active sulfur species. A notorious problem with this kind of coupling agents is the balance between its reactivity towards the silica, requiring a temperature of at least 130°C to obtain an acceptable speed, and its eagerness to react with the rubber polymer, which starts to become noticeable at temperamres above 145°C. Furthermore, the primary and secondary reactions are chemical... 

Various organic dihalides are employed in a reaction with sodium polysulfide to produce organic polysulfides (Thiokols). Ethylene dichloride, from the direct chlorination of ethylene, dichloroethyl formal, and /3,0 -dichlorodiethyl ether are the principal dihalides that have been employed in the process (44). These elastomeric polymers have been commercially available for a number of years, and many applications have been developed for them. They have excellent oil resistance and one of their principal uses has been in hose and tank linings in which that property is required. 

OTHER COMMENTS used as a solvent and extractant for resins, oils, fats, waxes, and chlorinated rubber used as a paint and varnish remover, and as a commercial solvent for degreasing metal parts useful in the synthesis of hexafluoropropylene has also been used in the synthesis of thiokol polysulfide elastomers if some branching of the polymer structure is required. 

LP-3 and LP-33. Formulations with epoxy generally utilize ratios of liquid polymer to epoxy of 1/2 to 1/1. Some amine catalyst is required to speed the reaction. The ratio of 10 to 15 parts of amine per 100 parts of epoxy is normally used. The most common amine for this application is DMP-30 or EH-330. Typical properties of compounds of epoxy/polysulfide liquid polymers are listed in Table XII. 

The TMDSC results are shown for the Coe-Flex polysulfide impression material and modulation condition (a) in Figure 19. There is a glass transition near -55 °C and an apparent weak crystalline polymer melting peak near 70 °C. No evidence of an exothermic crystallization peak can be seen on the bottom nonreversing heat-flow curve. The peak near 190 °C requires further study, but is not clinically relevant for the properties of the impression material. 

Sealants based on polysulfide liquid polymers originally found wide acceptance for applications requiring a flexible, adhering, chemically resistant composition of matter. Since they were the first liquid polymers available that could be cured at room temperature, they were soon specified for a number of military applications. Their use as aircraft sealants for fuel tanks still remains as a major outlet. Other military applications included a quick hose repair compound, a sealant for bolted steel tanks for quick assembly on the battle front, electrical... 

ST polysulfides are produced in a molecular weight range that is suitable for normal factory processing, hence do not need to be peptized. Highly filled Thiokol ST compounds may require some softening, hence the replacement of 3-7.5 phr of the polymer with Thiokol LP-3 is often done. 

---