Polymers polysulfides

Polysulfide elastomer Polysulfide polymers Polysulfide process Polysulfidepulping Polysulfide rubbers Polysulfides... 

Lead dioxide is electrically conductive and is formed ia place as the active material of the positive plates of lead-acid storage batteries. Because it is a vigorous oxidizing agent when heated, it is used ia the manufacture of dyes, chemicals, matches (qv), pyrotechnics (qv), and Hquid polysulfide polymers (42) (see Polypous containing sulfur). 

Polysulfide polymers have the foUowiag general stmcture ... 

Typical specifications for the polysulfide polymers are summarized in Table 1. Specifications for the sealants vary widely depending on the specific apphcation and the needs of the appHcators. Standards for sealant testing vary in different countries. Ultimately, the tests should simulate the environment the sealants will be exposed to and measure their performance under these conditions. 

The Hquid polysulfide polymers, as manufactured, are slightly acidic. In this condition they are very stable. In an alkaline, oxidi2ing medium they... 

The second catalyst paste of the two-paste product is a curing agent. A wide variety of materials convert the Hquid polysulfide polymers to elastomeric products. Alkalies, sulfur, metallic oxides, metallic peroxides, organic peroxides, and many metal—organic salts, ie, paint driers, are all potential curing agents. 

Many curing systems bring about aHquid—to—soHd conversion of the polysulfide polymers. Most curing agents produce an initial soHd mass characterized by a high degree of plasticity and poor elasticity. The development of elastic properties is so slow that the materials are not suitable for the accurate reproduction of undercut areas found ia oral stmctures. 

Only two types of systems have found appHcation ia dentistry. Lead peroxide is the curing agent most frequently used for the polysulfide polymers that serve as dental impression materials. Lead peroxide converts the Hquid polymer to an elastic soHd within a time short enough for oral appHcations. 

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)... 

Howard B.J. Alley, Determination of Ferric Oxide, Aluminum, and Magnesium Oxide in Polysulfide Propellants , ARGMA TR-1D2R (1960) 6) Anon, Polysulfide, Polymer,... 

The dry peroxide, added to cause cross-linking in liquid polysulfide polymers with pendant thiol groups, caused sparking or ignition, depending on the scale of the... 

Polysulfide polymers provide inherent resistance to fluel and quite good resistance to alkali. In contrast to the silicone polymers, they have low gas... 

Two of the more recently developed polysulfide polymers are the mercaptan-terminated polyoxypropylene urethane polymer and the polythioether polymer. The urethane-backbone-based polymer is used in many sealant formulations for insulating glass applications. The thioether backbone contains sulfur, but no S—S bonds, which are the weakest part of the conventional polysulfide polymer. This polymer improves the thermal stability and reduces the gas—liquid permeability. 

Chemistry of Polysulfide Polymers. Propellant chemistry based on chemically crosslinked binders had its beginning at the Jet Propulsion Laboratory in the winter of 1946 when potassium and/or ammonium perchlorate were mixed into Thiokol LP-3 polysulfide liquid polymer, to which had been added an oxidative curative, p-quinone dioxime. This polysulfide polymer, as described by Jorczak and Fettes (13), is prepared... 

In addition to the historic position in the development of a science of propellants, experiences with the chemistry of polysulfide polymers and propellants will be presented in greater detail than the other types of propellants because of the clarity with which the areas of chemistry can be described. 

Polysulfide polymers Polysulfide polymers increase the reactivity of some explosives and propellants. They are not compatible with PETN, Pentolite, Composition B, Amatol or Tetryltol. Polysulfide rubbers absorb some NG from DB and triple-base ( ) propellants. However, some propellants show little or no reactivity in the presence of polysulfides. 

The liquid polymer is converted to the rubbery state by reagents that react with mercaptan (-SH) and side groups of the polymer segments by oxidation, addition or condensation to effect sulfide (-S-S-) bond formation. The oxidation reactions are exothermic and accelerated by an alkaline environment. The most commonly employed oxidizing agents which are suitable for curing liquid polymers are cobalt or manganese or lead octoate, p-quinonedioxime and di- or tri-nitrobenzene. Epoxy resin also reacts with liquid polysulfide polymers by addition in the presence of an aliphatic or aromatic amine and polyamide activator as shown in Equation 5.8 ... 

Reactions of the Disulfide Group. Besides the thiol end groups, the disulfide bonds also have a marked influence on both the chemical and physical properties of the polysulfide polymers. One of the key reactions of disulfides is nucleophilic attack on sulfur (eq. 4). The order of reactivity for various thiophiles has been reported as (C2HsO)3P > R , HS-, C2H5S > C6H5S > C6H5P,... 

This reaction also plays a role in the degradation of polysulfides. A back-biting mechanism as shown in equation 6 results in formation of the cyclic disulfide (5). Steam distillation of polysulfides results in continuous gradual collection of (5). There is an equilibrium between the linear polysulfide polymer and the cyclic disulfide. Although the linear polymer is favored and only small amounts of the cyclic compound are normally present, conditions such as steam distillation, which remove (5), drive the equilibrium process toward depolymerization. 

Disulfides are susceptible to attack by strong oxidizing agents and this can result in decomposition of polysulfides. For example, nitric acid causes violent decomposition of polysulfide polymers. 

Polysulfide polymers are made commercially according to the reactions shown in equations 9—12. Details of the process and alternative approaches have been described (1,7). Although other dihalides can be used, its favorable economics, minimal competition with ring formation, and the desirable physical properties of the resulting polymer have made bis-chloroethylformal the monomer of choice. Only occasionally are other dihalides used in special applications. 1,2,3-Trichloropropane [96-18A] is sometimes added as a branching agent. 

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. 

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