Page 278 - Handbook of Plastics Technologies

ELASTOMERS

4.70 CHAPTER 4

sulfide. The polymer precipitates in the form of small particles that are washed, coagu-
lated, and dried. By varying the organic dihalide and the alkali polysulfide, a wide variety
of products can be obtained. A general chemical structure for these rubbers can be given as
follows:

where R is a divalent organic radical. There has been some question as to whether the sul-
fur atoms are connected one to another, in a row, as:

or in some other conformation such as:

where the sulfide rank (i.e., the subscript x) is about 4.
The polysulfide rubbers have been commercial products for a longer time than any
®
other synthetic rubber (since 1929). The products are sold as Thiokol rubbers in the fol-
lowing grades:
• Thiokol A, where the dihalide is ethylene dichloride
• Thiokol ST, where the dihalide is di-2-chloroethyl formal
• Thiokol FA, where the dihalide is a mixture of di-2-chloroethyl formal and ethylene
dichloride
Type A was the first commercial grade but has been largely superseded by type FA.
Type ST is prepared with a small percentage of 1,2,3 trichloropropane to provide a branch
point for improving the cure state, thus reducing compression set. It has a lower molecular
weight than Thiokol A or FA, and its molecules contain mercaptan, -SH, groups.
There is another type, Thiokol LP, grades of which are liquid polymers used in sealant
and mastic applications. They are formed by breaking down a high-molecular-weight
polymer in a controlled way. These grades also have mercaptan end groups.
Both Thiokol A and FA require peptization (e.g., in the presence of MBT or DPG) to
enable easy processing.
Types A and FA can be cured by the addition of ZnO alone at about 10 phr. Additions
of small amounts (up to 1 phr) of sulfur accelerate the curing.
Properties of Polysulfide Rubbers. Vulcanizates of polysulfide elastomers are better
than all other elastomers with respect to resistance to aromatic and chlorinated hydrocar-
bons, and to ketones. They have good weather and ozone resistance, and they are relatively
inexpensive. However, they have somewhat lower level of mechanical properties and par-
ticularly poor compression set.
Uses of Polysulfide Rubbers. Polysulfide elastomers are used in roller covering ap-
plications, hose liners, and solvent- and oil-resistant molded goods. The sealants are used
in construction and in aerospace industries.

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