Page 285 - Fiber Fracture

268 J.W.S. Hearle

Fig. 1. (a) Para-aramid, polyphenyleneterephthalamide (Kevlar). (b) Polyamide-hydrazide (X500).

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Fig. 2. Fibre strength and modulus from Smit et al. (2000). Specific stress equals 'stress/density'. N/tex
equals GPa/(g/cm3). If values in GPa were plotted, aramid with a density of 1.44 g/cm3 and PBO at 1.57
g/cm' would appear about 50% higher in comparison with polyethylene (Dyneema) at 0.97 g/cm3. PES is
high-tenacity polyester as used in tyre cords, etc.

highest strengths are achieved by gel-spinning, in which the coagulant from a concen-
trated solution can be extended to a high draw ratio. This is the method adopted by Allied
Fibers (now Honeywell) for Spectra and by DSM for Dyneema. There are differences in
properties among the various grades depending on process conditions. For example, pro-
duction is helped, but properties are less good, if some lower-molecular-weight polymer
is included. A second stage of slow processing under tension close to the melting point
increases the modulus and reduces creep. Melt-extrusion followed by super-drawing
or solid-state extrusion of compacted powder are two other methods used to make
high-modulus polyethylene fibres, but the strengths are not as great as gel-spun fibre.
In order to give a quantitative context, Fig. 2 shows a DSM presentation by Smit et al.
(2000) of values on a weight basis for strength and modulus of some HM-HT polymer
fibres compared to other materials.