High temperature adhesion

Formed commercially by the polymerization of S, NajCOj and PhX in a sealed container at 275--370 C, substituted derivatives are known but not fully evaluated. Used as high-temperature adhesives, for laminates and in coatings. 

Polyphenylquinoxalines (PPQ) are easier to make than the polyquinoxalines and offer superior solubiHty, processibiHty, and thermooxidative stabiHty (65). The PPQs exhibit excellent high temperature adhesive, composite, and film properties. However, to increase the use temperature of PPQs, acetylene... 

PBI is being marketed as a replacement for asbestos and as a high temperature filtration fabric with exceUent textile apparel properties. The synthesis of whoUy aromatic polybenzimidazoles with improved thermal stabUities was reported in 1961 (12). The Non-MetaUic Materials and Manufacturing Technology Division of the U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, awarded a contract to the Narmco Research and Development Division of the Whittaker Corp. for development of these materials into high temperature adhesives and laminates. 

Their main applications have been in heat-resistant structural laminates, electrical laminates resistant to solder baths, chemical-resistant filament-wound pipe and high-temperature adhesives. 

Despite all precautions, urethanes can be used most effectively within certain thermal and oxidation limits. Outside these parameters, other adhesives, such as certain epoxies, cyanate esters, and other high-temperature adhesives, should be considered. 

Applications such as the noise attenuation panels that line engine inlets and exhausts require a special form of high temperature adhesive known as a reticulating adhesive. These bond honeycomb sandwich assemblies have inner facesheets with numerous small holes (Fig. 13). The holes and underlying honeycomb core... 

Maturation as a technology does not mean that advancement and innovation has ceased. Adhesive bonding is so essential to the aerospace field that as long as there is a desire to go higher, faster and farther more efficiently, there will be an incentive to develop new materials and processes for adhesive bonding. Areas of particular interest for future applications are high-temperature adhesives, fiber-reinforced metal laminates and more efficient bond assembly techniques. 

If one amino group in o-phenylenediamine is converted to an amide group by formic acid, the intermediate benzimidazole is formed. This reaction, conducted with a wide range of reactants, produces resins (polybenzimidazoles) used as high-temperature adhesives for laminates in the aerospace industry. Heat insulation is made by including tiny bubbles of silica and all... 

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

The semi-IPN concept has also been used in order to improve the high-temperature adhesive properties of a flexible polyimide. Polyimide prepared with isophthaloyldiphthalic anhydride and mefa-phenylenediamine presents good adhesive properties but is limited to 270 °C. By blending it with a bisnadimide... 

Tetraglycidyl ether of tetraphenolethane is an epoxy resin that is noted for high-temperature and high-humidity resistance. It has a functionality of 3.5 and thus exhibits a very dense crosslink structure. It is useful in the preparation of high-temperature adhesives. The resin is commercially available as a solid (e.g., EPON Resin 1031, Resolution Performance Polymers). It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

Glycidyl amine epoxy resins are reaction products of aromatic amines and epichlorohy-drin. They have high modulus and high glass transition temperature. These resins find use in aerospace composites and high-temperature adhesive formulations. 

PMDA can be reacted with glycols to produce an adduct having the general structure shown in Fig. 5.8. These adduct resins form in the presence of solvent, under dry nitrogen. The reaction is continued until a clear solution is obtained. Such PMDA adducts are used in the formulation of high-temperature adhesive films. 

Epoxy-phenolic adhesive was the first true high-temperature adhesive. It was developed in the early 1950s as a high-temperature aircraft adhesive. An example formulation is provided in Table 13.4, but these adhesives are discussed predominantly in Chap. 15. 

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. 

High-temperature adhesives are usually characterized by a rigid polymeric structure, high glass transition temperature, and stable chemical groups. The same factors also make... 

Additives and Modifiers Commonly Used in High-Temperature Adhesives... 

Oxidation Resistance. Oxidation in high-temperature adhesive joints involves reaction of the adhesive polymer with oxygen in the air as well as reaction with certain metal surfaces (e.g., ferrous metals). Oxidative degradation is initiated by the action of highly... 

The only high-temperature resin family that retains a moderate amount of flexibility is the polysiloxanes. A significant amount of research has been devoted to trying to marry the properties of siloxanes with epoxy resins to obtain less brittle, high-temperature adhesives. However, these efforts have yet to result in commercial adhesives systems. 

Reducing Internal Stress. Internal stresses are common in joints made with high-temperature adhesives. These stresses can be due to... 

Flexibilizers generally cannot be used to counteract internal stress in high temperature adhesive because of their relatively low glass transition temperature and thermal endurance properties. However, most high-temperature adhesive systems incorporate metallic fillers (generally aluminum powder) to reduce the coefficient of thermal expansion and degree of shrinkage. 

Metal fillers for high-temperature adhesives must be carefully selected because of their possible effect on oxidation, as indicated in the previous section. Carrier films, such as glass cloth, are generally used to facilitate the application of the adhesive, but they also provide a degree of reinforcement and lowering of the coefficient of thermal expansion. Thus, they reduce the degree of internal stress experienced at the joint s interface. 

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