High-temperature adhesives and sealants

Alkenyloxystyrene Monomers for High-Temperature Adhesives and Sealants... 

However, natural-based materials like starches and soybean oils are used in direct competition to animal or human food consumption. Technically, they are not at all suitable for high modulus adhesive and sealant joints. They usually are sensitive to hydrolysis and oxidization. Exposure to atmospherie eonditions or chemical attack can drastically limit their service life expectancy. They are sensitive to elevated temperatures and need to be protected against microbiological decay with more or less toxic preservatives. In light of those features and based on today s knowledge some important conclusions can be drawn ... 

Adhesives and sealants have become inseparable parts in mechanical and technological devices. Automobiles, airplanes and other transportation media cannot be built without structural adhesives, high-temperature adhesives and various sealants. Many mechanical devices are held together with bolts, nuts, and adhesives. More adhesives are going to replace metallic fasteners. 

Other pSOFC joining techniques that have been considered include the use of high-temperature cements and sealants formed by reaction bonding. Although commercial ceramic adhesives such as Duco and Sauereisen cements have been... 

Classification with respect to the methods of reaction is discussed in Sect. 12.3 to include thermosets, thermoplastics, high-temperature adhesives, elastomers, sealants, anaerobics, moisture curing adhesives, and primers. 

Room-temperature-vulcanizing silicone rubber (General Electric and Dow Corning) is available at hardware stores and is very useful as an adhesive and sealant. Atmospheric moisture is necessary to effect the cure, so broad areas of impermeable materials should not be cemented with these preparations. The uncured material evolves acetic acid, and the cured material appears to lose some weight in high vacuum but if used with moderation, it can be considered a satisfactory vacuum sealant for most chemical vacuum systems. 

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... 

Aliphatic amines Convenience Room temperature cure, fast elevated-temperature cure Low viscosity Low formulation cost Moderate chemical resistance Critical mix ratios Strong skin irritant High vapor pressure Short working life, exothermic Poor bond strength above 80°C Rigid, poor peel and impact properties Adhesives and sealants Casting and encapsulation Coatings... 

Fibers. At one time asbestos fibers were used as the primary thixotrope in the adhesive and sealant industry. Asbestos fiber filled materials are very resistant to sag under high temperatures—an important property for many adhesives and sealants. However, asbestos abruptly fell from the formulator s arsenal of additives primarily due to... 

Pyrolysis is simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes chain scission and decreased molecular weight of the bulk polymer. This results in reduced cohesive strength and increased brittleness. Resistance to pyrolysis is predominantly a function of the intrinsic heat resistance of the polymers used in the adhesive formulation. As a result, many of the aromatic and multifunctional epoxy resins that are used as base resins in high-temperature adhesives are rigidly crosslinked or are made of a molecular backbone referred to as a ladder structure, as shown in Fig. 15.4. 

Many applications for adhesives and sealants require high strength and durability at low temperatures. Many of these same applications also require resistance to thermal cycling between high and low operating temperatures. Unfortunately, the properties of adhesives and sealants at low temperatures are not as well studied or documented as they are at high temperatures. 

Epoxy and polyurethane adhesives are not appreciably affected by 10-9 torr for 7 days at room temperature. However polyurethane adhesives can exhibit significant outgassing when aged under 10-9 torr at 107°C.75 At room temperature a high vacuum does not generally cause significant weight loss in commercial adhesive and sealant materials. 

S-EB-S block copolymers have a saturated midblock and so can be compounded with suitable resins and oils to make adhesives and sealants that have improved stability during long-term or extreme temperature exposure. In addition, hot-melt adhesives formulated from these polymers are more stable under melt processing conditions and can be applied at high temperatures. Nitrogen-blanketing is not required. Stable adhesives and sealants with large amounts of oils and resins can be formulated from these polymers. 

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. 

Development of planar solid oxide fuel cells (SOFCs) and other electrochemical devices, such as oxygen generators and sensors, makes it necessary to elaborate sealant materials for hermetization in high-temperature zone. The sealants should satisfy numerous requirements, including chemical stability, good adhesion and thermal expansion. 

Silicone adhesives were introduced commercially in 1944 [5, p. 213]. In 1960 the silicone sealants were introduced to the construction industry [11, p. 86]. Silicones are useful at both high and low temperatures and are available today as solvent-based moisture-curing adhesives, one-part moisture-curing adhesives and sealants, two-part curing adhesives and sealants, and pressure-sensitive 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. 

C except for small area bonding. In addition, flexible bagging material and sealants to operate at these temperatures are a problem. Those companies that have a special need for a high temperature adhesive often develop a material for that particular application and do not rely upon a commercial source. These special needs vary from the joining of electronic elements to the bonding of ceramic components in a missile. It is unlikely that a single... 

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. 

The toughness properties of anaerobic adhesives can be enhanced by the addition of a reactive elastomer [178]. The heat resistance of anaerobic adhesives and sealants can be enhanced by the addition of a bis-maleimide [179]. These maleimide additives appear to be relatively unreactive during the initial anaerobic cure. As the adhesive is exposed to high temperatures, the methacrylate backbone degrades and the methacrylates can then copolymerize with the maleimides to form a more heat resistant matrix [180],... 

Room-temperature-vulcanizing (RTV) silicone-rubber adhesives and sealants form flexible bonds with high peel strength to many substrates. These resins are one-component pastes that cure by reacting with moisture in the air. Because of this unique curing mechanism, nonporous substrates should not overlap by more than 1 in. 

In addition to these major market segments, plasticizers are used in a variety of smaller segments. Adhesives and sealants generally use lower molecular weight phthalate esters, benzoate esters and diesters, and dibutyl terephthalate because of improved solvency and lower fusion temperatures. One major use of plasticized PVC is automotive underbody coatings these products are highly filled plastisols that are mostly commonly based upon GP plasticizers often with a fast fusing plasticizer added to improve processability however, plasticizer volatility can be a major concern. 

Sealants and adhesives are used widely in automotive applications. One of the advantages of an automotive production line is that the high-temperature (160-205 °C) paint bake ovens can be used to cure the adhesives and sealants. 

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