Impregnated aramids

Figure 8.44. Evolution of reflection coefficient (dB) vs frequency for a polj yrrole impregnated aramid honeycomb. The curve (a) is related to basic material and the curve (b) is related to the same one impregnated with an epoxy resin.

Mayo Jr., J. B. etal., 2009. Stab and puncture characterization of thermoplastic-impregnated aramid fabrics. International Journal of Impact Engineering, 36(9), pp. 1095-1105. 

S. Linder and B. Brailsford, Phenolic Resin-Impregnated Aramid Paper Honeycomb Core Program, Navy Man tech Program, Office of Naval Research, FY96-98, 1998. 

Fiber-reinforced composites have high stiffness-to-weight ratios, but this ratio may be increased if, instead of a monolithic structure, two thin composite facesheets of the same weight (for example, plastics reinforced with phenolic glass or carbon fibers), separated by a cellular core, are used. A phenolic resin-impregnated aramid paper honeycomb (Nomex honeycomb) is commonly used in the core structure. These structures are appropriate for applications in which weight reduction and fire safety are critical, such as in ground and air transportation. 

Towflex (Hexcel Composites) is based on carbon, glass or aramid fibres impregnated with polypropylene, polyamide, PPS, polyetherimide, PEEK. Product forms include flexible Towpreg, woven fabric, braided sleeving, UD tape, chopped compression moulding compound, moulded plates, thermoformable laminates. 

Tepex (Bond-Laminates) is delivered as sheets of 0.25-3 mm thickness based on carbon, glass or aramid fibres impregnated with polyamide or thermoplastic polyester. Fibre levels range from 35% up to 85%. 

SUPreM, Plytron, Quadrax (Gurit Suprem) consolidated tapes or fabrics are made from continuous fibres impregnated with thermoplastic powder. The fibres can be glass, aramid, carbon, steel and the matrices are polyethylene, polypropylene, polyamide, PPS, polyetherimide, PEEK, thermoplastic polyimide, or fluorothermo-plastic. High levels of fibres can be obtained, 65% in volume, for example. 

The electrically conductive composites prepared by these methods are not sufficiently conductive for shielding of electromagnetic interference. Still more effective in producing conductive aramids is electroless plating. A high electrically conductive fiber is obtained. The fiber is impregnated with metal complexes using supercritical carbon dioxide. 

An advanced reinforcements system identifies QBT. It is the name for a biaxial thermoplastic (PA, PBT, PET, PP/PPS, PEI, APC-2, Radel), carbon, aramid, and glass pre-impregnated tape. It is unidirectional, interlaced in biaxial form, in continuous lengths and very large width (up to 10 ft). It maintains and improved properties of unidirectional cross-ply laminates, giving the benefit of unidirectional tape in larger and more easily processed formats. It is claimed to be first real alternative to a woven composites compromise, with good drape. 

Manual transmissions use a clutch facing made from a resin impregnated wound structure composed of staple yarns, p-Aramid in yam form provides more strength and durability than a pulp-based paper sheet in this more demanding application. Although aramid reinforced facings have sufficient thermal stability for this application, compositions based on glass and metal fibers dominate this market. 

Aramid (e.g. Kevlar 49 ) fibres do not melt, but they degrade on heating in vacuum above 375°C and will oxidize at 400°C. The tensile strength of commercial Kevlar fibres declines with temperature, to about 75% of the ambient value by the time that 1TC is reached. Unprotected yarns lose 25% of their tensile strength and 7% of their modulus, measured at room temperature, after heating for 96h in air at 205°C. Data on bare fibres can be misleading impregnation with a resin protects the fibres, and there is no observed adverse effect [3] when a normal cure cycle is carried out for Kevlar/epoxy at 180°C. 

The Japanese are also active in the application of fibre reinforced plastics to reinforced concrete structures to improve their earthquake resistance. Kabatake et al [22] reported on 14 cases in which chimneys were retrofitted with fibre reinforced plastics and were judged by the Building Disaster Prevention Association of Japan to be one of the most effective retrofitting methods. In this retrofitting programme, carbon fibre tapes impregnated with an epoxy resin were wound and adhered to the exterior of the chimney. Sumida et al [ 23] have described experiences with aramid reinforced... 

Pre-impregnated sheet materials called prepregs are available, where the carbon fibres are impregnafed with reactive resins, usually modified epoxy resins. These can be worked on, layed-up and moulded fo shape with heat applied to cure the resins. They are also available with glass fibre/phenol and aramid fibre/phenol combinations. 

The use of fiber-reinforced polymer (FRP) composite materials in the reinforcement of concrete structures has shown important results. These interventions are based on the application of carbon fiber, glass, or aramid impregnated with thermosetting polymers. The effectiveness of these interventions is demonstrated both by extensive research in the laboratory and by applications to existing structures. 

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