Liquid polysulfide polymers, properties

Formulation. Polysulfide-based sealants are formulated with appropriate ingredients to obtain the desired properties for a particular application. A typical formulation contains liquid polysulfide polymer, curing agent, cure accelerators (bases) or retarders (acids), fillers, plasticizers, thixotropes, and adhesion promoters. 

For adhesive systems, the liquid epoxy resins most widely used with LP-3 polymers are liquid unmodified and diluent-modified bisphenol A resins and liquid blends of bisphenol A and bisphenol F resins. Solid bisphenol A, multifunctional, and aliphatic diepoxy resins have also been used. Ratios of liquid polysulfide polymer to epoxy are in the range of 1 2 to 2 1. The effect of various degrees of polysulfide on cure properties of a DGEB A epoxy is shown in Table 7.7. An increase in elongation and impact strength is the result of increased amounts of the liquid polysulfide polymer. 

The variety of fillers compared in Table IV are selected from various classes used in compounding liquid polysulfide polymers. It appears that particle size has the greatest effect on physical properties within a single type of filler. Other properties such as surface area and surface coatings will effect the viscosity and thixotropy of polysulfide polymer compound, while high pH fillers can result in increased viscosity after a period. 

The unique functional properties of the polysulfide liquid polymers have been put to various demanding uses, in some areas of application the liquid polysulfide polymer compound continues to be the current choice after more than thirty years of use. 

These adhesives are products of reachon between an epoxy resin and liquid polysulfide polymer, usually catalyzed by an additional tertiary amine.They are available as two-part liquids or pastes that are usually cured at room or higher temperatures to rubbery solids that provide bonds with excellent flexibility and chemical resistance. Epoxy-polysulfide adhesive forms satisfactory bonds to different substrates. Shear strength and elevated-temperature properties are low, but resistance to peel and low temperature is acceptable. ... 

Flexiblization. Aliphatic diepoxide reactive diluents enhance the flexibility or elongation by providing chain segments with greater free rotation between cross-links. Polyaminoamide hardeners, based on aliphatic polyamines and dimerized fatty acids, perform similarly. Liquid polysulfide polymers possessing terminal mercaptan functionality improve impact properties in conjimction with polyamine hardeners. 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

The basic determinant of the final properties of a cured polysulfide polymer/epoxy resin compound is the weight ratio of liquid polymer to epoxy resin. By varying this ratio, the formulator is able to develop the optimum properties for a specific application. The effect of varying LP/EP ratios on the properties of a typical compound is shown in the following table. 

The term polysulfide polymers referred at one time exclusively to the high-sulfur-containing polymers as manufactured by the Thiokol Chemical Corp. From 1928 to 1960 they were the only high-sulfur polymers available. The solid polysulfide polymers contained 37-82% bound sulfur, while the liquid polymers contain approximately 37%, which gives them their unique chemical properties. 

Table 5.7 Properties of some commercial polysulfide liquid polymers.

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. 

Polysulfide sealants are two-component systems based on liquid polymers compounded with additives as part (A) and an oxidizing curing agent as part (B). They have elastomeric properties and are distinguished... 

This discussion will be limited to adhesive compounds. In formulations developed for adhesives, the epoxy resin is the major component. However, modification of the epoxy resin with polysulfide liquid polymers gives compositions which in many cases have unique physical and chemical properties. 

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