Aerospace sealant

TECHNOLOGY ASSESSMENT ASSOCIATES P.O. Box 1362, Amherst NH 03031-9939, (603) 673-0312. A number of studies on Adhesives in Electronics, Bonding and Joining Composites, Markets for Adhesive Tapes and Films, Conductive Adhesives, Aerospace Sealants, and Growth Markets for Adhesives. 

In the following sections/ we shall discuss adhesion chemistry/ adhesion physics / radiation-curable adhesiveS/ high-temperature adhesiveS/ anaerobic and structural adhesiveS/ hot-melt adhesives/ film adhesiyes/ waterborne adhesives/ aerospace structural adhesiveS/ conventional sealants/ advanced aerospace sealants/ and adhesives and sealants for solar collectors. 

For intermediate low temperatures, e.g.,-70 C (-94 F), several polymers (7) have been developed for advanced aerospace sealants ... 

Because of its excellent range of properties and reliability, poly(fluoroalkoxyphosphazene) elastomers are used as seals, gaskets, and shock mounts in demanding military, aerospace, petroleum and industrial applications. In addition, applications under development for this elastomer include fuel hoses for artlc use, coated fabrics for protective clothing, sealants, coatings and medical devices. 

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. 

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... 

Epoxy-based primers are commonly used in the aerospace and automotive industries. These primers have good chemical resistance and provide corrosion resistance to aluminum and other common metals. Primer base resins, curing agents, and additives are much like adhesive or sealant formulations except for the addition of solvents or low-viscosity resins to provide a high degree of flow. 

Use High-temperature coatings, laminates and composites for aerospace vehicles, ablative materials, oil sealants and retainers, adhesives, semiconductors, valve seats, bearings, insulation for cables, printed circuits, magnetic tapes (high- and low-tem-perature-resistant), flame-resistant fibers, binders in abrasive wheels. 

MAJOR PRODUCT APPLICATIONS bowling balls, cast polyester, foam, caulk, explosives, putties, sealants, pipe insulation, potting compounds, speckling compounds, reflective paints, golf balls, pultrusion, aerospace, marine, automotive, composites, and many more... 

In the aerospace industry, resinous polymers encompass a wide variety of hardware applications for aircraft, missiles, and space structures. In aircraft, resins are used as a matrix material for primary (flight-dependent) and secondary fiber-reinforced composite (FRC) structures, adhesives for the bonding of metal and composite hardware components, electronic circuit board materials, sealants, and radomes. Missile applications include equipment sections, motor cases, nose cones, cartjon-carbon composites for engine nozzles, adhesive bonding, and electronics. As the exploration of outer space intensifies, applications will become even more exotic. FRC will be used to construct telescopes, antennas, satellites, and eventually housing and other platform structures where special properties such as weight, stiffness, and dimensional stability are important. 

This chapter will deal with the chemistry and applications of epoxies, phenolics, urethanes, and a variety of current vogue high-temperature polymers. Applications in fiber-reinforced plastics will be discussed in the individual sections on resin chemistry where appropriate. Separate sections will deal with adhesives and sealants. Adhesives are most important because, as early history demonstrates, they led the way to the application of resins in aerospace. A section is also included on silicone and polysulfide sealants. Although these materials are elastomers rather than resins, no discussion of aerospace polymers would be complete without some mention. Some major thermosetting polymers have been omitted from this review. Among these are the unsaturated polyesters, melamines, ureas, and the vinyl esters. Although these products do find their way into aerospace applications, the uses are so small that a detailed discussion is not warranted. 

Polymeric sealants are important materials in the aerospace industry, "irtually every aircraft or space craft employs them in one form or another. The most important resins used for this purpose are the polysulfides and the polydimethyl siloxanes. Of the two materials, sealants based on the polysulfides are the most widely used. 

The history of the modern adhesives and sealants industry is closely tied to the development of the aircraft and aerospace industries. From the earliest flights to the most modem aerospace equipment, light weight has been one of the most vital considerations. Adhesive bonding was an ideal joining method for the early wood and textile aircraft, and today is the most important joining method for aluminium, titanium, and other metals in advanced military air- and spacecraft and some advanced commercial airplanes. 

This company s new 2600-093 Micro-Kiss series mix-dispense valves are designed for the low-flow mixing and dispensing of two-component adhesives and sealants such as silicones, epoxies, urethanes, and acrylics. The valves can be used to apply two-part adhesives and sealants for bonding, gasketing, potting and filling processes in aerospace, appliance, contractor, defense, electronics, filters, marine. 

MAJOR APPLICATIONS Antifoam fluids, lubricants, protective gels, and elastomers and sealants in applications exposed to hydrocarbon fuels and oils and organic solvents in the automotive and aerospace industries. Longer fluorocarbon side-chain fluorosilicones are available with developing use as release coatings for silicone-based adhesives. 

In the field of aerospace applications, the reliability of materials under extreme exposure conditions is of prime importance. The high- and low-temperature properties of the fluoroelastomers have permitted them to give reliable performance in a number of aircraft and missile components, specifically manifold gaskets, coated fabrics, firewall seals, heat-shrinkable tubing and fittings for wire and cable, mastic adhesive sealants, protective coatings, and numerous types of O-ring seals. 

Recently, heavy coatings that act as sealants for crevices have been developed for the aircraft and aerospace industries. These coatings contain proprietary inhibitor formulations that are especially effective in minimizing corrosion associated with dissimilar metal fasteners (Moiseeva 2005). 

Chem. Descrip. Calcium sulfonate/carbonate complex in mineral spirits Uses Corrosion inhibitor in aerospace, automotive (body frame, sheet steel, sealant caulk), military, and storage coatings Features Exc. water repellency, low vise, and light color qualifies as military vehicle corrosion preventive compd. under MIL-C-62218 Properties Tan liq. or paste dens. 7.7-7.9 Ib/gal vise. 5,000-9,000 cps flash pt. (PMCC) > 105 F 52-56% solids... 

The elastomers are based on polysiloxanes that contain trifluoropropyl methyl siloxane units. The materials are used as sealants, elastomers, and fluids for aerospace applications. The monomers are prepared according to the following scheme ... 

This review emphasizes new materials for adhesives and sealants. Thus, we discuss both new adhesives and new forms of adhesives. As a result of socio-economical demands, energy-reduction and pollution-control are two main reasons for the development of radiation-curable, hot-melt, film, and waterborne adhesives. We shall examine some of these new forms of adhesives in detail. The applications of new adhesive materials to aerospace and solar-energy industries will also be briefly mentioned. 

In addition to adhesives, the aerospace industry also needs conventional sealants and special sealants to withstand the temperature extremes of the aerospace environments. Both these subjects will be discussed in the following sections. 

Though thioether sealants are somewhat more thermally stable than polysulfides, they are still not qualified as the advanced sealants for aerospace applications in severe environments. In the following section, we shall discuss some of the advanced sealants being considered for future applications. 

For aerospace applications, several advanced sealants have been mentioned cyanosilicones (123),fluoroalkyl-arylenesiloxanylene (FASIL) (124), phosphonitrilic fluo-roelastomers (PNF ) (125,126), flexible polyimide (127), tetrafluoroethylene oxide phenylquinoxaline elastomer (FEX) (128), perfluoroalkyl ethers (129), etc. These advanced sealants have been used for fuel tank sealing, channel sealing, filleting, aerodynamic smoothing, and other assemblies. Basically, they should be high-temperature serviceable, fault-tolerant, and adhere well to metals and/or composites. 

Since the structure contains very few weak links, it is oxidative and ozone resistant. The unique mechanical properties are low compression set, high modulus, excellent flex fatigue, and good abrasion resistance. Thus, it has been evaluated as an advanced sealant for aerospace applications. 

Adhesives and sealants used for the aerospace structures must endure severe environments. Another family of adhesive materials has also been developed for solar collectors, presumably based on the same ability to endure severe environments. Though there are different kinds of severe environments, conventional polymers generally do not survive in the solar collector environment. We shall discuss those adhesive materials used for solar applications in the following section. 

In this brief review, we have attempted to demonstrate the importance of chemistry to the developments of new (or new forms of) adhesive materials. Much success has been achieved in the syntheses of polymers for aerospace adhesives and sealants. New or modified structural adhesives have been developed with improved mechanical properties and durability. However, there is still room for innovation for developing new adhesive materials for use under severe environments. 

For aerospace adhesives and sealants, we need to consider not only extremely high temperatures, but also extremely low temperatures. Adhesives for cryogenic engines should maintain their mechanical performances between -184° and -196°C (-300° to -320°P), but this seems to be beyond the limits of polymer capability. To meet this need, an experimental composite or a blend of adhesives in a multilayer form has been developed. Some work has been carried out at Hughes Aircraft, McDonnell Douglas, or NASA-Lewis, but is not very well known to the public. 

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