High-temperature resins applications

Thiobisphenols are less effective than hindered phenols in terminating peroxy radicals. They also function as peroxide decomposers (secondary antioxidants) at temperatures above lOO C. Typically, thiobisphenols are chosen for use in high-temperature resin applications. Users generally prefer hindered phenolics over thiobisphenols where high-temperature service is not involved. 

Sulfur cross-links have limited stability at elevated temperatures and can rearrange to form new cross-links. These results in poor permanent set and creep for vulcanizates when exposed for long periods of time at high temperatures. Resin cure systems provide C-C cross-links and heat stability. Alkyl phenol-formaldehyde derivatives are usually employed for tire bladder application. Typical vulcanization system is shown in Table 14.24. The properties are summarized in Tables 14.25 and 14.26. 

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. 

Although they are not elastomers, other phosphazenes also offer potential In areas requiring heat and fire resistance. Mixed fluoroalkoxy - aryloxy substituted cyclic phosphazenes are currently attracting Interest for fire resistant fluid applications (46). In other studies, cyclophosphazene matrix resins have been prepared which have potential for high temperature adhesive applications (47) and as composite matrix materials (48). These studies serve to further demonstrate the enhanced thermal stability and fire resistance which can be achieved with these phosphorus - nitrogen systems. 

MAJOR APPLICATIONS Interlayer dielectrics, high-temperature resins. 

Aliphatic and cycloahphatic epoxy resins have less tendency to yellow and can even be used as a basis for epoxy gelcoats. Such gelcoats are not widely available due to difficulties in application and adhesion to the substrate, as well as high costs. Use of non-aromatic anhydride curing agents such as hexahydrophthalic anhydride will produce further improvements in colour stability and these systems are suitable for high temperature curing applications. 

Melamine fibres are primarily known for their inherent thermal resistance and outstanding heat blocking capability in direct flame applications. This high stability is due to the cross-linked nature of the polymer and the low thermal conductivity of melamine resin. The dielectric properties and its cross-sectional shape and distribution make melamine ideal for high temperature filtration applications. It is sometimes blended with aramid or other high strength fibres to increase final fabric strength. 

The most important resins available for use as adhesives in high-temperature structural applications are polyimides (Pis) and polybenzimidazoles (PBIs), both of which are described later (see Sections 5.35 and 5.33). These resins are supplied as prepolymers containing open heterocyclic rings, which are soluble and fusible. At elevated temperatures, the prepolymers undergo condensation reactions leading to ring closure and the formation of insoluble and infusible cured resins. 

For a general discussion of these high-temperature adhesives, compared with PBI, see Section 5.5. These adhesives are synthetic thermosetting resins formed by the reaction of a diamine and a dianhydride. As with PBIs, they were developed specifically for high-temperature aerospace applications. PI adhesives are superior to PBIs for long-term strength retention, as shown in Figure 5.4. ... 

Although polyimides are the largest class of high temperature resin systems, other resins have been developed that are used for specialist applications or, alternatively, were never developed commercially, perhaps due to cost considerations. 

The blend comprised at least two sulfone polymers, e.g., PES and PSF, and at least one non-sulfone polymer (e.g., PS, PPE, PEI, PC, PA, PEST, PP, or PE). The nucleating agent was either talc, mica, silica, Zn-stearate, Al-stearate, Ti02, or ZnO. The foams were used as insulation for high temperature structural applications. Since in the preceding part PPS blends with PSF were described, in Table 1.71 examples of PSF blends with other specialty resins are listed. 

In thermosets, where POSS is reacted into the material, the primary advantage is excellent high temperature modulus retention beyond 300 °C [8]. For example, octa functional POSS epoxies or POSS amines can be added to conventional epoxy or BMI resins to provide very high temperature resins. While traditional resins lose rigidity at and above the glass transition temperature, the POSS modified materials retain modulus even under extreme temperature conditions. Such materials are in demand for oil well applications where the drive to deeper, hotter wells pushes existing... 

However, the difficulty in processing such a high-temperature resin has inhibited its widespread application and commercialization. PPSU is available in extrusion and injection molding grades from Amoco (Radel). 

Miscellaneous Applications. Ben2otrifluoride derivatives have been incorporated into polymers for different appHcations. 2,4-Dichloroben2otrifluoride or 2,3,5,6-tetrafluoroben2otrifluoride [651-80-9] have been condensed with bisphenol A [80-05-7] to give ben2otrifluoride aryl ether semipermeable gas membranes (336,337). 3,5-Diaminoben2otrifluoride [368-53-6] and aromatic dianhydrides form polyimide resins for high temperature composites (qv) and adhesives (qv), as well as in the electronics industry (338,339). 

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