Aircraft sealant

Worldwide production capacity of Hquid polysulfides is about 33,000 t with manufacturing sites in the United States, Japan, and Germany. Total consumption is about 28,600 t. Approximately 50% is for insulating glass sealants, 30% for constmction appHcations, and - 10% for aircraft sealants. In addition, - 909 t of the soHd polysulfide mbbers are sold each year. 

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. 

Aircraft canopies Aircraft engines Aircraft fittings Aircraft fuels Aircraft sealants... 

Barbier, J.W., Hanhela, P.J., Huang, R.H. and Paul, D.B. (1990) Effect of temperature on storage life of polysulfide aircraft sealants. Polymer Testing, 9(5), 291-313. 

Table 2 Chemical Composition of a Typical Aircraft Sealant ...

The issue is complicated by the parallel existence of aerospace committees within BSI, and in Europe within the Association Europeene des Constructeurs de Materiel Adrospatial (AECMA). This is the European organization responsible for standardization related to aerospace materials and, hence, covers many structural applications of adhesives, as well as aircraft sealants. Standards published by AECMA, like normal European standards, are required to be adopted by member states. 

Most of the polysulfide sales are in Europe, the United States, and Japan. Polysulfide sealants have 83% of the market share of insulating glass sealants in Europe, 35% in the United States, and a total of 60% worldwide. Polysulfides have only a modest share of the building sealant market. Polysulfides are virtually the only sealant specified for aircraft integral fuel tanks worldwide. In addition, they comprise about 90% of other aircraft sealant applications. The total volume used in the aircraft sealant market fluctuates with aircraft construction and repair. 

The results are reported of a study of the disparate swell behaviour in hot water of polysulphide sealants produced with different curing agents (manganese dioxide and ammonium dichromate). Sealants tested were commercial aircraft sealants PR-1750, Pro-Seal 899 and PR-1422 and laboratory formulations prepared using Thiokol liquid... 

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... 

Aircraft sealants, where plasticizer content is kept at a minimum to reduce extraction by jet fuels. 

Other curing agents have included inorganic chromates in aircraft sealants requiring better heat resistance. Some manganese has also been used for aircraft sealants. Cumene hydroperoxide has found a place in a few casting compounds but in building sealants, it interferes with adhesion. 

Where adhesion is required, all sealants need an adhesion additive. For two-part building sealants, a selected phenolic resin (Methylon AP-108, General Electric) has been very satisfactory. A silane monomer is used for the one-part sealants. Aircraft sealants have used a selected phenolic resin which is more resistant to the jet fuels. A Durez resin, 10694 (Occidental Chemical Corporation) has been most satisfactory. 

During the 1930s gradual improvements in the product and processing overcame some of the drawbacks of this material. Nonetheless, the apphcations were limited and Thiokol Corp. stmggled to remain solvent. The first year Thiokol reported a profit was in 1941, 13 years after its foundation. This was realized when the U.S. Air Force discovered that the aUphatic polysulftdes were usehil as a fuel-resistant sealant for aircraft tanks and hoses. Polysulftdes also began to be used as sealants for boat hulls and decks. 

Thermosets A number of thermosets have been used as adhesives. Phenolic resins were used as adhesives by Leo Baekeland in the early 1900s. Phenolic resins are still used to bind together thin sheets of wood to make plywood. Urea resins have been used since 1930 as binders for wood chips in the manufacture of particle board. Unsaturated polyester resins are used for body repair and PUs are used to bond polyester cord to rubber in tires, and vinyl film to particle board, and to function as industrial sealants. Epoxy resins are used in the construction of automobiles and aircraft and as a component of plastic cement. 

Unsaturated polyester resins have replaced lead for auto body repair, and polyurethanes (PUs) are being used to bond polyester cord to rubber in tires, to bond vinyl film to particleboard, and as industrial sealants. Epoxy resins are used in automotive and aircraft construction and as a component of plastic cements. 

Perhaps the biggest thrust for the development of high performance polymers over the next 10 years will be in the aerospace industry where materials will be required for a fleet of high speed civil transports (supersonic transports). At a speed of Mach 2.4, an aircraft surface temperature of about 150 to 180°C will be generated. The life requirement of materials at these temperatures will be about 60000 hours. Many different types of materials such as adhesives, composite matrices, fuel tank sealants, finishes and windows will be needed. These materials must exhibit a favorable combination of processability, performance and price. The potential market for these materials total several billions of US dollars. 

Since then, uses have shifted more toward civilian applications. Polysulfides have unusually good resistance to solvents and to the environment and good low temperature properties. This makes them particularly useful in a variety of sealant applications. For example, the outstanding resistance of polysulfides to petroleum (qv) products has made them the standard sealant for virtually all aircraft integral fuel tanks and bodies. Another important application is in insulating glass window sealants (qv). Sealants based on liquid polysulfides have had an excellent record since the 1950s and are the worldwide market leader in this application. 

Medium-molecular-weight PMTFPS with vinyl or hydroxyl end blocks are used for adhesives and sealants. They are cured either at ambient temperature (RTV-room temperature vulcanization) or at elevated temperature. One-part moisture-activated RTV sealants have been available commercially for many years. Because of then-very high resistance to jet engine fuels, excellent flexibility at very low temperatures, and high thermal stability, they have been used in both military and civilian aerospace applications.78 Two-part, heat-cured fluorosilicone sealants have been used in military aircraft applications and for sealing automotive fuel systems.79 Special class of fluorosilicone sealants are channel sealants or groove injection sealants, sticky, puttylike compounds, which do not cure. They are used to seal fuel tanks of military aircraft and missiles.75... 

Sealing Compound, One Part Silicone, Aircraft Firewall Adhesive/Sealant, Fluorosilicone Aromatic Fuel Resistant, One Part Room Temperature Vulcanizing... 

Another oil-derived class of organic chemical business includes adhesives, sealants, coatings, and so on, with companies like Ciba-Geigy, Dow, Monsanto, and Laporte in the lead. Nowadays aircraft... 

Epoxy is a synthetic polymer which is used in a variety of ways. Strengthened with some form of filler it is used for such purposes as aircraft manufacture. In connection with organics, it is used for items such as imitation scrimshaw, as it can be shaped in pliable moulds which can be peeled off after the polymer has set, minimising the marks left by the mould (Fig. 3.10). It is also used for coatings, sealants and adhesives. It is thermosetting. 

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