Aramid Kevlar fibres

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 aramid fibres (Kevlar and Nomex are the trade names for two of the most common) are especially desirable for their strength-to-weight ratios being superior to metals. They are most often used in composites having polymer matrices (generally epoxies and polyesters). Since the fibres are relatively flexible and somewhat ductile, they may be processed by most common textile operations. 

The epoxy resin used was Shell s Epon 828/MNA/BDMA. Reinforcement was with aramid fibres Kevlar 49 and Kevlar Hm (du Pont). 

Thornel is made by Amoco, AS4 by Hercules and the aramid fibre, Kevlar 49, by du Pont. 934 and 3501-6 are aerospace epoxy resins from Fiberite and Hercules, respectively. 

Regular viscose rayon has quite low crystallinity and it is possible that more highly ordered polymers may yield activated chars with alternative adsorptive properties. Carbon fibres have been produced (ref. 4) from the high-performance aramid fibre, Kevlar , but no attempt has been made to develop porosity in such chars by gaseous activation. We report here the preparation of activated carbons from Kevlar and an assessment of their adsorptive properties. 

Many composite overwrap repair systems have been introduced to the pipeline industry, their differences being in fibres, adhesive, resin and method of application. Carbon fibre, glass fibre and aramid fibre (Kevlar reinforcement) are the usual fibres used in these composite systems to provide for the required strength and stiffness. In general, two types of composite repair, namely layered system and wet lay-up system, are currently used for pipeline repair (Figure 10.1). 

Fig. 7. Stress-strain diagram for two types of Kevlar (PPT) aramid fibre...

In the case of PPT, the main commercial outlet has been in the form of aramid fibres having the trade names Kevlar and Arenka whereas the development of PBT is still at the experimental stage and evaluation trials, although encouraging, are not very comprehensive. 

A well-known example of a lyotropic LCP is the aramide fibre (Twaron or Kevlar), an aromatic polyamide with the structure ... 

Source From DuPont Technical Guide for Kevlar Aramid Fiber, H-77848, 4/00 DuPont Technical Guide for Nomex Brand Aramid Fiber, H-52720, 7/01 Teijin Ltd., Teijinconex Heat Resistant Aramids Fiber 02.05 Teijin Ltd., High Tenacity Aramids Fibre Technora TIE-05/87.5 Akzo Nobel, Twaron—Product Information Yarns, Fibers and Pulp. 

Kevlar 49 (aramid) fibres (parallel to fibre axis) -2... 

Properties and uses of Kevlar-49 aramid fibre and of reinforced plastics of Kevlar 49 , DuPont Trade Literature Bulletin K-1, June 1974. 

Glass and carbon/graphite fibres do not absorb significant quantities of moisture, although glass fibres are eventually attacked, the rate of attack being signihcant with hot water. Aramid fibres (e.g. Kevlar, Nomex, Twaron) in contrast are polymers, just like the matrix resins and therefore they absorb water. The amount is typically about 5% w/w. The fibre/matrix interface is weak, even in the dry condition. 

For areas with special application requirements, specifically modified polyester fabrics, as well as fabrics from aramid fibres, fluorine polymers and arylamides like Kevlar (ref. DuPont), have proved to be satisfactory. The membranes show different characteristics depending on the coatings used. Fluorine polymers such as PVDF (polyvinylidene fluoride) are used on PES fabrics (refs Mehler and Ferrari) a PTFE (polytetrafluoroethylene) coating is very suitable for fibreglass fabrics (ref Verseidag) and there is a newly developed composite membrane with THV (ret Dyneon), a polymeric blend of tetrafluoroethylene, hexafluoropropylene and vinylidenefluorine, used as a coating on PES fabrics, of which VALMEX vivax (ret Mehler) is one example. 

Staple fibre Kevlar yams with two different linear densities were sourced from Aramex-Game, Germany, and one of the yams was doubled locally without any twist, coded as K2. This is mainly to increase the linear density of the yam to enable the use of thicker yam. Tilsa is a para-aramid yam from Tilsatec, with a similar linear density to that of the Kevlar yams from Aramex-Game. Several dope-dyed para-aramid yams were sourced from Tilsatec in various linear densities. 

Kevlar 29 is included as the typical para-aramid fibre used in tyres. ... 

Anon. Technical guide for Kevlar Aramid Fibre, Technical Report H-77848 4/00, 2000, DuPont, USA. 

A failure in 1983 when aramid fibres began to be used illustrates the problems that can arise. A ship. Ocean Builder, was to be used to erect an oil rig in the Gulf of Mexico. Five weeks before, 12 buoys were secured to anchors by Kevlar ropes. When the Ocean Builder arrived and the mooring lines were picked up, four ropes broke reportedly at 20% of break load. The up-and-down movement of the buoys had led to axial compression fatigue of the Kevlar fibres. 

Aramid fibres have the highest strength to weight ratio compared to the other fibres but broadly similar tensile strength to glass fibre. There are a number of manufacturers but Kevlar and Nomex are the familiar brand names. 

Aramide fibres. Man-made fibres of aromatic polyamides characterized by high strength, high elastic modulus, good chemical resistance, high thermal stability. Applications tyre cords, protective garments, electric insulators. Trade names Arenka (NL), Kevlar (USA), Nomex (USA). 

To date, only one class of polymer has been found in which this problem can be overcome and highly successful reinforcement fibres produced. These are the aramld polymers, so-called because they contain both aromatic and amide groups in the molecular chain. Indeed, it is this combination which is the key to their success. Kevlar 49 is currently the most widely used aramid fibre for reinforcement of plastics, and it illustrates the point well. It consists of the polymer poly(paraphenylene terephthalamide) ... 

Aramids, such as Kevlar 49, are chemically quite stable and have high resistance to neutral chemicals. But they are susceptible to attack particularly by strong acids and also by bases. However, Technora aramid fibre has extremely high strength retention in both acids and alkalis. 

In view of a comment on fracture to be mentioned later, I will contrast in Fig. 1 two chemical types the para-aramid of Kevlar and Twaron and one of the experimental polyamide-hydrazide X500 fibres made by Monsanto, whose lack of commercial utility led to the disclosure of extensive technical detail in Black and Preston (1973). A critical difference is the greater number of -CO.NH- groups in the Monsanto polymer, which will give stronger intermolecular bonding. 

An early view of fracture of para-aramid fibres was given by Yang (1993, p. 97), who refers to three basic forms. The caption to his fig. 3.28 describes fracture morphology of Kevlar aramid fibre in tensile breaks as Type (a), pointed break type (b) fibrillated break type (c) kink-band break. The kink-band breaks, which extend over a length approximately equal to a fibre diameter can be attributed to fibres that have been weakened by axial compression and will be discussed in a later section. 

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