Page 284 - Fiber Fracture

FRACTURE OF HIGHLY ORIENTED, CHAIN-EXTENDED POLYMER FlBRES 267

INTRODUCTION
Fibre types

a
As Black and Preston (1973) point out in their symposium proceedings - book
which concentrates on experimental Monsanto fibres and, like Hamlet without the
Prince of Denmark, mentions only in passing the successful para-aramid Kevlar - it
was in the earliest days of acceptance of the polymer hypothesis that Mark (1936)
made theoretical calculations indicating “that synthetic organic fibres were capable of
very high Young’s moduli.” It was 30 years later, before such fibres were made and,
in addition to high-modulus, “tended to be on the order of twice as strong as high-
tenacity nylon or polyester.” These organic high-performance fibres, as well as others,
are described in Hearle (2001).
Three features are needed in high-modulus, high-tenacity (HM-HT) linear polymer
fibres: (1) very long chains, i.e. high molecular weight; (2) highly oriented chains;
(3) fully extended chains without crystallographic or irregular folds. Fibres with these
characteristics can be made by two routes that differ greatly in their molecular type
and production method, but give properties that have many similarities, though some
significant differences.
The first route to be developed uses rigid polymer molecules with fairly strong
interactions that will form liquid crystals in solution (or, for Vectran, in the melt).
Dry-jet, wet spinning, through an air-gap into a coagulating bath (or stretching
in melt-extrusion of Vectran) orients the liquid crystals. Monsanto concentrated on
polyamide-hydrazides and polyoxadiazole-amides. DuPont with Kevlar and later AKZO
(now Acordis) with Twaron combined chemical features of nylon and polyester in
a para-aramid, polyphenylene-terephthalamide; this polymer links benzene rings with
-CO.NH- groups, which form hydrogen bonds between chains in one crystallographic
plane. Teijin’s Technora and the Russian fibre Terlon are different copolymer variants.
There are also Russian heterocyclic aromatic polyamides, SVM and Armos. Vectran
is a fully aromatic copolyester, which needs a slow heat treatment of the solid fibre
to generate high molecular weight. Subsequently the USAF made polymers containing
benzoxazole and benzothiazole groups, having benzene rings between 5-membered
rings on either side; commercialisation of PBO occurred in 1999 with Zylon from
Toyobo. A more recent development by Sikkema (2001), which it is hoped LO commer-
cialise, is PIPD or M5; the important feature of this polymer is hydrogen bonding by
-OH in both transverse directions, which increases the shear strength and compressive
yield stress.
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.
The second route uses a flexible inert molecule, ultra-high-molecular-weight poly-
ethylene, which can be highly stretched to give highly extended, oriented chains. The