Para-aramid fibres, properties

A copolymeric derivative of the para-aramid fibres was introduced by Teijin in 1985 under the tradename Technora . This fibre is based on the copolymeric structure copoly-(paraphenylene/3,4 -oxydiphenylene terephthalamide) and is claimed to have a much higher chemical resistance than PPTA as well as increased abrasion and steam resistance, which are useful properties in many protective applications. Technora has a decomposition temperature of about 500 °C, and other properties are comparable with PPTA although its LOI value is slightly lower at 25 vol%. 

Although the uses of ceramic fibres in composite structures lie mainly in ceramic-matrix and metal-matrix composites, where their outstanding chemical and thermal resistance are important, there are a few applications in organic polymers. Their relevant properties are low thermal expansion, low electrical conductivity, low dielectric constant, high stiffness, good compressive strength, and in most cases complete resistance to combustion. On the other hand they are very brittle, hard to process, and mostly considerably more expensive than carbon and para-aramid fibres. They have, for example, been used in hybrid structures with carbon and para-aramid and in electronic circuit boards. The fibres available or potentially available include alumina, combinations of alumina with... 

There are two types of aramid (fully aromatic polyamide) fibre those with high thermal stability and flame resistant properties but mechanical properties similar to those of standard textile fibres such as polyester, sometimes referred to as meta-aramid fibres because the main commercial products are of this chemical structure the others with additionally exceptionally high tensile strength and modulus, sometimes referred to as para-aramid fibres on similar grounds. The former type is not widely used in composite structures, but some of the most important uses of the latter lie in this area. 

Para-aramid fibres inherently have relatively poor surface adhesion properties unless pre-treated, but are available from the producers with enhanced adhesive properties. A particularly important feature of their mechanical behaviour is that beyond a certain flexural couple they undergo failure by fibrillation on the strained outer side and by a crushing mechanism, involving formation of kink bands in the structure, on the compressed inner side. This behaviour does not result in rupture, as observed with a brittle fibre, and therefore permits retention of some mechanical strength in the reinforcing material. 

We have developed different nonwoven stmctures with para-aramid fibres, and produced thick 3D stmctures by associating several nonwoven monolayers with a consolidation treatment. Porous composites were afterwards manufactured by impregnating the fibrous stmctures with an epoxy resin. Both dry and impregnated materials were characterized in terms of stmcture (density, fibre volume fraction), and compression tests were used to evaluate mechanical properties. Equivalent pore size and distance between fibre contacts were determined using theoretical models. They help to provide some insight on the mechanical behaviour of the different stmctures. 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

The first verification of the theoretical predictions of Onsager and of Flory that rod-like molecular chains might exhibit liquid-crystalline properties (see section 1.3.2 and chapter 12) was obtained in the 1960s and fibres from para-aramid polymers were commercialised under the name of Kevlar in 1970. These materials are very stiff and have excellent thermal stability many other materials of this class of rigid main-chain liquid-crystal polymers have been developed. They cannot, however, be processed by the more conventional processing techniques and this led to the development in the 1980s of another group of liquid-crystal polymers, the thermoplastic co-polyesters. 

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