Aerospace structural applications, thermoset

Polymers and composites are finding ever-increasing use as structural components in aerospace systems because of their attractive strength/weight characteristics. During the past 20 years, their use in military aircraft has increased from essentially zero in F-4 jets to over 25% in the advanced AV8-B Harrier II. Thermosetting resins, particularly epoxies and polyimides, are of prime interest for use in structural applications. 

Materials that come close to the ideal ladder structure are thermosetting polyimides and polybenzimidazoles. These are used primarily in high-temperature aerospace applications as composites and adhesives. 

Tape and film adhesives are most often used to bond large areas, such as for applications in the aerospace industry. For example, the joining of aluminum honeycomb structure to flat metal skins is often accomplished with thermosetting epoxy film adhesives. These films (Fig. 13.1) can easily be applied without the need to mix, meter, or apply a liquid coating. 

The purpose of this paper is to present a technique whereby manufacturing process dynamics for structural polymers can be accurately defined through efficient laboratory rheological characterization. Structural polymers, in this paper, refer principally to the thermosetting epoxides, phenolics and polyimides. This type of test pattern, however, is generally applicable to the production and utilization of most polymers. The engineering applications associated with these polymers involves primary and secondary aerospace articles. In this situation, failure to meet performance criteria could result in catastrophic loss of the vehicle and associated cargo. 

Organic matrices are divided into thermosets and thermoplastics. The main thermoset matrices are polyesters, epoxies, phenolics, and polyimides, polyesters being the most widely used in commercial applications (3,4). Epoxy and polyimide resins are applied in advanced composites for structural aerospace applications (1,5). Thermoplastics Uke polyolefins, nylons, and polyesters are reinforced with short fibers (3). They are known as traditional polymeric matrices. Advanced thermoplastic polymeric matrices like poly(ether ketones) and polysulfones have a higher service temperature than the traditional ones (1,6). They have service properties similar to those of thermoset matrices and are reinforced with continuous fibers. Of course, composites reinforced with discontinuous fibers have weaker mechanical properties than those with continuous fibers. Elastomers are generally reinforced by the addition of carbon black or silica. Although they are reinforced polymers, traditionally they are studied separately due to their singular properties (see Chap. 3). 

It is the purpose of this chapter to discuss the types and uses of resins for aerospace and also to document aerospace contributions to the science and understanding of structural polymers. Thermoplastics will not be a part of this discussion. They do have aerospace applications, most notably, in the interior furnishings of commercial aircraft. However, it is the thermoset resins that have been the major contributor to aerospace hardware technology. 

Compounds are short (/w 0.3 mm) or long (> 0.6 mm) fibre reinforced thermoplastic or thermoset polymeric materials, which are processed automatically (injection or compression moulding), have good (mechanical) properties (for automotive, electric and electronic applications) and are relatively cheap. Composites contain continuous fibres (rovings, fabrics or mats), usually combined with thermosets, have excellent mechanical (structural) properties, but are very expensive because lack of an industrial process (mainly used in aerospace and aircraft industry). 

Thermosetting plactics are often reinforced by as much as 100 phr of various fillers, for example, wood flour, cotton flock asbestos and synthetic fibre used with phenolic thermosets, and silica flour, mica or talc with epoxies. Glass fibre reinforcement of polyester resins for structural, automotive, aerospace and marine applications is too well known to require comment. 

These adhesives are important as structural adhesives, especially in metal bonding. They are comprised of a thermosetting and a thermoplastic polymer, including certain elastomers. Although each component has adhesive properties by itself, on the whole, the conjoint system forms a stronger and more versatile adhesive. The two-polymer systems have been particularly successful as film and tape adhesives, with applications ranging from building construction to aerospace. ... 

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