Sealing polysulfides

Aircraft Sealants. Polysulfides have been used for sealing fuel tanks and aircraft stmctural components since the 1940s. There are stringent requirements for these sealants. They must have outstanding resistance to fuels and exceUent adhesion to many different materials such as various metal aUoys and protective coatings used in aircraft constmction. The sealants must also perform in extremely variable weather conditions (10). The polysulfides have an exceUent performance record. 

Batsanov et al. 23) reacted sulfur with PtCU and PtBr2 by heating mixtures of the reactants in evacuated, sealed ampoules. At 100 -200°C after 12-24 h, sulfide chlorides PtCljS (1.70 < x < 2 0.6 s y < 3.35) and sulfide bromides PtBr S (1.87 < x 2.06 0.84 y s 1.80) were formed. The compositions depended on the initial PtX2 S ratio, and the temperature. At 320-350°C, loss of chlorine led to the compounds PtClS (1.7 y 1.9). According to their X-ray powder patterns, all of these products retained the main structural features of the original platinum halides. From considerations of molar volumes, the authors deduced the presence of polysulfide anions. 

The solvothermal reaction between metal halides and polysulfide anions is also a useful method for the synthesis of metal-polysulfide clusters. Hydrothermal reaction of K2PtCl4 with K2S4 (5 eq) at 130 °C in a sealed tube... 

Thiokol elastomers possess fairly low tensile and tear properties. However, they have excellent resistance to both aliphatic and aromatic solvents at room temperature and slightly elevated temperatures. The Thiokol division of Morton International Corporation is the supplier of polysulfide elastomers in the United States. It is estimated that 1360—1600 t are used annually in the United States. The primary use of polysulfide is in seals, gaskets, rolls, and diaphragms where solvent resistance and low permeability are useful. 

The history of polysulfides began over 150 years ago. In 1838 chemists in Switzedand reported that the reaction of chloraetherin (1,2-dichloroethane) with potassium polysulfide gaveambivalent a rubbery, intractable, high sulfur semisolid. Subsequently there were reports of similar products obtained by various methods, but the first useful products were developed from studies in the late 1920s. This led to the formation of Thiokol Corp. which began production of the ethylene tetrasulfide polymer Thiokol A in 1928, the first synthetic elastomer manufactured commercially in the United States. One of the first successful applications of Thiokol A [14807-96-6] was for seals where its resistance to solvents justified its relatively high price. 

Many different adhesives have been tested and used in weldbonding. These include both high-strength structural adhesives, such as modified epoxies, and relatively low-strength adhesives, such as vinyl plastisols or epoxy-polysulfides that are commonly used for sealing and vibration damping. Table 14.10 shows that a variety of adhesives and substrates are compatible with the weldbonding process. 

Lee, T, Rees, T. and Wilford, A. (1992) Polysulfide sealants in water-retaining structures, in Science and Technology of Building Seals, Sealants, Glazing and Waterproofing, ASTM STP 1168, American Society of Testing and Materials, Philadelphia, PA, pp. 47-56. 

Willgerodt reaction. In its original form this reaction involved heating an aryl alkyl ketone in a sealed tube at 210-230° with an aqueous solution of yellow ammonium polysulfide, prepared by dissolving sulfur in ammonium sulfide solution. The product is an aryl substituted aliphatic aciil amide, together with some of the corresponding carboxylic acid and often the hydrocarbon. An example is the... 

The second subsample (2 to 5 mL) is placed in an ampoule and spiked with an excess of freshly prepared 0.01 M sodium sulfite and covered with a septum. The ampoule is later sealed under nitrogen and heated to 60 C for 2 hours. Sulfite reacts with Inorganic zerovalent polysulfide, S(0), to form thiosulfate. Thiosulfate is measured again by LSV or DPP. The difference between the values of thiosulfate in this subsample and the first subsample equals the total S(0) from polysulfides. 

Another, exploratory version of the Na-S system (aimed at present at stationary applications) is under development at Dow Chemical Company. It operates at 300°C and utilizes hollow sodium borate glass fibers as electrolyte in the form of thousands of thin hollow fibers sealed at one end and open at the other to a common reservoir of molten sodium. The fibers are wrapped in Mo-coated aluminum foil which serves as a positive electrode for the polysulfide melt, which fills the space between the fiber wall and the Al. 

Cal-Seal. [Calbar] Butyl, acrylic, polysulfide sealants. 

The construction of operational, hermetically sealed sodium-sulfur cells requires container materials, which are mechanically suitable and compatible with sodium and sulfur-sodium polysulfide, current leads to the sulfur-graphite electrode, and several kinds of seals. These requirements of course are in addition to those for the solid electrolytes and sulfur electrodes described earlier. The problem of satisfying these requirements has been summarized by Gratch and co-workers (3). Particular applications—utility load-leveling, traction power, and military— impose further design constraints dictated by performance, capacity, size and weight, life, cost, and safety requirements. 

The first polymerization of isoprene in sealed bottles was reported in 1884 by Tilden. Methyl rubber was thermally polymerized at 70°C — the reaction required 3 to 6 months, giving poor quality products. In 1926 BASF developed sodium-initiated polymerization of butadiene known as Buna (for BUtadiene -I- Natrium). The first successful, general purpose rubbers were copolymers of butadiene with either styrene, Buna-S, or acrylonitrile, Buna-N [Tschunkur and Bock, 1933 Komad and Tschunkur, 1934]. Poly(2-chlorobutadiene), chloroprene [Carothers et ah, 1931], was introduced in 1931 by DuPont. Elastomeric polysulfides [Patrick, 1932] were... 

B. F. Goodrich, 2) Pro Seal 890 (a polysulfide) from Essex Chemical Corporation, 3) SARAN (polyviny1idene chloride) from Dow Chemical Company, and 4) KRATON (a styrenebutadience block copolymer) from Shell Chemical Company. 

A variety of two component adhesives and sealants are currently available for applications requiring the ultimate in bonding and sealing efficiency and performance. Included among these conventional systems are epoxies, urethanes, polyesters, silicones, polysulfides and some modified phenolics and acrylics. Although their performance can be and usually is quite satisfactory, most are limited by pot life and fixturing requirements - and the need for thorough substrate treatment and cleanliness. 

---