Composite Advanced

Advanced composites [43] are engineering materials that offer similar mechanical properties to metal alloys but are less dense. The materials consist of fibres embedded in a polymer matrix and there is a need for spectroscopic techniques that can examine the cured resins in the presence of the fibres to aid the understanding of the cure chemistry. The products are often highly cross-linked and thus insoluble, and the presence of the fibre matrix makes them difficult to study spectroscopically. INS has considerable potential in this regard since two common fibre t)q)es, glass and carbon are invisible to neutrons. 

A wide variety of polymers have been used including epoxies, bismaleimides and polyimides. One of the most common polyimides is PMR-15 [44]. The chemistry is complex, see Fig. 10.23, but consists essentially of two stages imidisation to give a norbomene end-capped 

INS spectra of the composites cured at 270, 330 and 330°C are shown in Fig. 10.24 [46]. Differences between the three samples are apparent bands at 1031 and 1114 cm have diminished in intensity and there are indications of changes in the region 200—400 em and at 638, 720 and 1273 cm. Comparison with the spectra of model compounds [47,48] suggests that the deerease in the 1114 cm and the changes in the 200— 400 and 600—800 cm regions can reasonably be assigned to loss of the endcap. The 1031 cm band does not fit this pattern and may represent a different type of cross-link. 

The example shown here is particularly difficult because by the cure temperatures employed here, a signifieant proportion of the end-groups 

Researchers developed relatively few synthetic composites throughout history. One exception was the discovery by French inventor Joseph Monier (1823-1906) that implanting steel rods in concrete after it had been laid greatly increases the strength of the final product. That discovery introduced the age of reinforced concrete, making available a product that is still used extensively throughout the world. 

During preceding decades, the discovery of new products such as Bakelite, nylon, rayon, celluloid, polyvinyl chloride, polyethylene, Saran , and Teflon convinced chemical corporations that such products held the key to an exciting and profitable future based on a host of amazing new miracle materials. Research departments around the world began the search for new materials with properties designed to meet a variety of special needs. One chemist who succeeded in this kind of project was Stephanie Kwolek. 

Kwolek s task at DuPont was to find a new kind of fiber that was resistant to acids and bases and that would remain stable at high temperatures. In 1964, she discovered such a product, an aromatic polyamide that was five times as strong as steel with half the density of fiberglass. The material was given the name aramid. Aramid was later marketed under the trade names of Kevlar and Nomex . Today, aramid is one of the most widely used substances in polymer matrix composites. 

Kwolek remained with DuPont for the rest of her working life, retiring in 1986. She has remained active in the field of chemistry since her retirement. She serves as a consultant to DuPont and to the National Research Council of the National Academy of Sciences. Kwolek was inducted into the National Inventors Hall of Fame in 1995, only the fourth woman to be so honored, and received the National Medal of Technology in 1996. She was also awarded the Perkin Medal in 1997, only the second woman to receive that award. 

Carbon fibers are the most rigid and strongest of commonly used reinforcements. They are produced by the pyrolysis (high-temperature decomposition) of natural and synthetic materials, such as rayon, polyacrylonitrile (PAN), and pitch (the tacky residue left from the distillation of petroleum or coal tar). Carbon fibers are commercially available in a variety of formats, including single strands and bundles (known as tows). They are midway in density between glass and polymer fibers and are the most expensive of commonly used reinforcements. 

Composite materials are made up of two or more materials that have different properties. 

These materials are combined (in many cases involving chemical bonding) to produce a new material that has properties that are superior to either material alone. An example of this type of composite material is fiberglass, in which glass fibers are held together by a polymeric resin. 

Because carbon fibers of different diameters are available and because construction parameters can be varied, it is possible to engineer composites having desired characteristics. By varying the orientation, concentration, and type of fiber, materials can be developed for specific applications. The fibers can be layered at different angles to minimize directional characteristics. Also, layers of fibers can be impregnated with epoxy resin to form sheets that can be shaped before resin polymerization. 

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Bisphenol A. One mole of acetone condenses with two moles of phenol to form bisphenol A [80-05-07] which is used mainly in the production of polycarbonate and epoxy resins. Polycarbonates (qv) are high strength plastics used widely in automotive appHcations and appHances, multilayer containers, and housing appHcations. Epoxy resins (qv) are used in fiber-reinforced larninates, for encapsulating electronic components, and in advanced composites for aircraft—aerospace and automotive appHcations. Bisphenol A is also used for the production of corrosion- and chemical-resistant polyester resins, polysulfone resins, polyetherimide resins, and polyarylate resins. 

Advanced composites and fiber-reinforced materials are used in sailcloth, speedboat, and other types of boat components, and leisure and commercial fishing gear. A ram id and polyethylene fibers are currentiy used in conveyer belts to collect valuable offshore minerals such as cobalt, uranium, and manganese. Constmction of oil-adsorbing fences made of high performance fabrics is being evaluated in Japan as well as the constmction of other pollution control textile materials for maritime use. For most marine uses, the textile materials must be resistant to biodeterioration and to a variety of aqueous pollutants and environmental conditions. 

The expiration of Phillips basic PPS patent in 1984 ushered in a large interest from the industrial sector. Companies, based largely in Europe and Japan, began acquiring patents worldwide for both the synthesis of PPS and a wide variety of appHcations, including compounds, blends, alloys, fiber, film, advanced composite materials, as well as end use products. 

Composites. High molecular weight PPS can be combiaed with long (0.6 cm to continuous) fiber to produce advanced composite materials (131). Such materials having PPS as the polymer matrix have been developed by usiag a variety of reinforcements, including glass, carbon, and Kevlar fibers as mat, fabric, and unidirectional reinforcements. Thermoplastic composites based on PPS have found application ia the aircraft, aerospace, automotive, appliance, and recreation markets (see Composite materials, polymer-matrix). 

More than 95% of current carbon fiber production for advanced composite appHcations is based on the thermal conversion of polyacrylonitrile (PAN) or pitch precursors to carbon or graphite fibers. Generally, the conversion of PAN or pitch precursor to carbon fiber involves similar process steps fiber formation, ie, spinning, stabilization to thermoset the fiber, carbonization—graphitization, surface treatment, and sizing. Schematic process flow diagrams are shown in Eigure 4. However, specific process details differ. 

Currendy, epoxy resins (qv) constitute over 90% of the matrix resin material used in advanced composites. The total usage of advanced composites is expected to grow to around 45,500 t by the year 2000, with the total resin usage around 18,000 t in 2000. Epoxy resins are expected to stiH constitute about 80% of the total matrix-resin-systems market in 2000. The largest share of the remaining market will be divided between bismaleimides and polyimide systems (12 to 15%) and what are classified as other polymers, including thermoplastics and thermoset resins other than epoxies, bismaleimides, cyanate esters, and polyimide systems (see Composites,polymer-matrix-thermoplastics). 

Eor more demanding uses at higher temperatures, for example, in aircraft and aerospace and certain electrical and electronic appHcations, multifunctional epoxy resin systems based on epoxy novolac resins and the tetraglycidyl amine of methylenedianiline are used. The tetraglycidyl amine of methylenedianiline is currently the epoxy resin most often used in advance composites. Tetraglycidyl methylenedianiline [28768-32-3] (TGALDA) cured with diamino diphenyl sulfone [80-08-0] (DDS) was the first system to meet the performance requirements of the aerospace industry and is still used extensively. 

Fig. 2. Epoxy matrix resius for advanced composites (a) TACTIX 558 (Courtesy of The Dow Chemical Company), (b) EPON HPT Resia 1071 (Courtesy...

Applications. Epoxy resias constitute over 90% of the matrix resia material used ia advanced composites. In addition, epoxy resias are used ia all the various fabrication processes that convert resias and reinforcements iato composite articles. Liquid resias ia combiaation, mainly, with amines and anhydride are used for filament winding, resia transfer mol ding, and pultmsion. Parts for aircraft, rocket cases, pipes, rods, tennis rackets, ski poles, golf club shafts, and fishing poles are made by one of these processes with an epoxy resia system. 

Resins for advanced composites can be classified according to their chemistry typical resins are polyaryletherketones, polysulfides, polysulfones, and a very broad class of polyimides containing one or more additional functional groups (Table 2) (see also Engineering plastics). 

The polyimide shown is a tme thermosetting resin, but the general reaction procedure, coupling the dianhydride with the diamine, is extremely important throughout polyimide chemistry. The intermediate polyamic acid polymers form the basis for many of the polyimide resins used in advanced composites. 

Advanced Composite Materials Corp. (ACMC), Greer, S.C. Los Alamos National Lab, Los Alamos, N. Mex. 

Special materials Turbine-blade alloys, advanced composites (CFRP, BRFP), etc. UK 5,000-50,000 US 7,500-75,000... 

Ting J-M., and Lake, M.L. Processing, fabrication, and applications of advanced composites, Ed., ASM International, Materials Park, OH, 1993, pp 117 127. 

Pilato, L.A. and Michno, M.J., Advanced Composite Materials. Springer Verlag, Berlin, 1994, pp. 18-23. 

Over the years there have been several studies examining electron-curable adhesives [9-12]. Off-the-shelf acrylate adhesives were the primary focus of the studies. These adhesives, that have potential use for the repair of advanced composites using high-energy electron accelerators, offer several advantages over conventional repair systems, including [6] ... 

The type of damage an advanced composite sustains plays an important role in the type of repair that can be implemented. Damage to advanced composites can be... 

Saunders, C.S. and Lopata, V.J., Electron beam processing of advanced composites DREP Final Report (1994) Contract No. W7708-2-0001/01-XSA Department of National Defence Defence Research Establishment Pacific, FMO Victoria, Canada, 1994. 

Because of this continued emphasis on adhesive bonding technology development over the years, the airframes of modem front-line aircraft such as the B-2 bomber and the F-117 and F-22 fighters are largely structurally bonded advanced composites. They tend to be comprised of materials that are more advanced (expensive) than commercial aircraft such as carbon and boron fiber reinforcements with cyanate esters, bismaleimides, polyimides or other high-temperature resin matrices and adhesives. 

In order to describe completely the state of triaxial (as opposed to biaxial) stress in an anisotropic material, the compliance matrix will have 36 terms. The reader is referred to the more advanced composites texts listed in the Bibliography if these more complex states of stress are of interest. It is conventional to be consistent and use the terminology of the more general analysis even when one is considering the simpler plane stress situation. Hence, the compliance matrix [5] has the terms... 

Phillips, L.N. (ed.) Design with Advanced Composite Materials, Design Council, London (1989). Strong, B.A. High Performance Engineering Thermoplastic Composites, Technomic Lancaster, PA (1993). 

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