Kevlar thermal properties

Mathematical Modeling. The results obtained by various dilatomet-ric techniques were compared to theoretical predictions of thermal expansion using a simple mathematical model. Thermal properties of Kevlar fabric/epoxy lamina were simulated by considering the fabric to be made of two consecutive 0° and 90° lamina of... 

A major use of Kevlar is in rigid and soft body armour protective applications. Kevlar fabric is bullet and fragment resistant, lightweight, flexible and comfortable, has excellent thermal properties, is resistant to cuts and chemicals and is flame resistant and self-extinguishing. The uses include bulletproof vests, chainsaw leg protection and military uses for helmets and armoured vehicles, cargo containers, armour shields and cockpit doors. 

Polyamides can also be made by the reaction of amines with acyl halides, where the condensate is hydrochloric acid. This process is used to make aromatic polyamides, notably Kevlar and Nomex (Scheme 1.7). The reaction of p-phenylenediamine with terephthaloyl chloride results in the high performance p-aramid Kevlar. While Kevlar is expensive because processing requires the use of anhydrous sulfuric acid as solvent, its outstanding mechanical and thermal properties led to its use in demanding applications, including personal armor, bicycle tires, and racing sails. 

XA and HM carbon fibres are Courtaulds products note that Courtaulds have ceased making carbon fibres but information on their material has been included here since other manufacturers produce fibres with similar properties, see Figure 3.1, and the thermal properties of Courtaulds materials may be relevant to these other fibres. Aramid fibres used were Kevlar 29, Kevlar 49, Kevlar Hm (du Pont). E-, R- and D-glass fibres were used and Tyranno SiC and Nicalon SiC ceramic fibres (Ube Industries and Nippon Carbon, respectively). 934 (Fiberite) and MY720/HT976 (Ciba Geigy) epoxy resins were used (among others), as was polyester resin and acid cure phenolic resin. 

A numerical heat transfer model of thin fibrous materials under high heat flux eonditions (bench-top burner) was developed by Torvi and Dale [37]. The model is applicable to two common, flame resistant fabrics, Nomex IIIA and Kevlar /PBI. A fabric-air gap-test sensor system (Figure 12.4) is used in which heat transfer is assmned to be one-dimensional. The fabric s thermal properties represent the average thermal property values of the fibrous stmcture. Mass transfer, hot gas flow and fabrie stmctural changes are not considered. The fabric s thermal properties are taken as fimetions of temperature only. The authors use energy balance equations and models of heat transfer modes to develop a differential equation (equation 12.26), and initial and boimdary conditions ... 

In most of the previous work with polyimide fibers, the fibers were spun from poly(amic acid) precursors, which were thermally imidized in the fiber form. However, high degrees of imidization were not achieved. Thus, tensile properties of these polymers were not as good as those of high-performance fibers. Work in our laboratories has shown that when the fibers are spun directly from preimidized polymers, it is possible to achieve tensile properties that are as good or even better than those of poly(p-phenyleneterephthalamide) (PPTA or Kevlar ) fibers. For example, fibers have been prepared from m-cresol solutions of BPDA-PFMB using a dry-jet wet-spinning method. The as-spun fibers were then extensively drawn and annealed above 400°C to achieve excellent mechanical properties. 

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. 

Aerogels are quasi-stable, low-density, three-dimensional assemblies of nanoparticles, which have usually poor mechanical properties. A facile one-pot synthesis of Kevlar-like aerogels based on the reaction of multifunctional isocyanates and carboxylic acids has been reported [57]. The materials exhibit an ultimate compressive strength, a high specific energy absorption, and a thermal conductivity like foamed PS. By a pyrolysis process at 800 °C, the materials can be converted to a porous, electrically conducting carbon with a high surface area. 

Table 2.36 Thermal and Electrical Properties of Kevlar 49 Fabric/Epoxy...

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