Page 360 - Fiber Fracture

342 J.W.S. Hearle
a b C d

Fig. 9. Four views suggesting the fine structure of nylon or polyester fibres: (a) from Hearle and Greer
(1970); (b) from Prevorsek et al. (1973); (c) from Hearle (1977); (d) from Heuvel and Huisman (1985).
Note that these diagrams, which were drawn to indicate the authors' views of particular features of the
structure, are grossly inadequate representations of reality. They are pseudo-two-dimensional views of
three-dimensional structures, and nylon and polyester molecules are inadequately represented by lines.

Table I. Alternating sequences in nylon and polyester
flexible inert sequences interactive sequences
Nylon 6 (XH2-15 40-NH-
Nylon 66 (-CH2-)4 and (-CH2-)6 -CO-NH-
Polyethylene terephthalate -0-CO-CH2-CH2-CO-0- benzene ring

order of 10 nm. Another contrast is that the natural fibres are laid down under genetic
control from solution and thus have well-defined structures, which vary only in specific
details and are far from a liquid state, whereas melt-spun fibres are processed close
to their molten form, and the structure varies with the crystallisation conditions and
subsequent thermo-mechanical treatments. There is no single type of structure. These
problems are discussed in a recent book, Salem (2001).
A feature of nylon and polyester, which makes them good textile fibres, is that their
molecules have long repeats (7 to 14 units) with the different chemical groups shown
in Table 1. Above about -1OO"C, the flexible inert sequences are free to rotate in a
rubbery state between each unit, but up to about +lOO°C the interactive groups stiffen
the amorphous regions by hydrogen bonding in nylon or phenyl interaction in polyester.
This combination gives the required limited extensibility to the fibres. In polypropylene,
there is a single transition around 20°C, and the tendency of the molecules to take up
a helical form is an important factor. Much more could be written on structure and
thermo-mechanical responses, including the influence of water absorption on nylon and
the stiffening effect of the benzene rings in polyester, but this brief account is sufficient
as a basis for a discussion of mechanical properties (see Morton and Hearle, 1993).
Fibres that are extruded and cooled slowly solidify in an unoriented state. When
tension is applied, there is a small amount of elastic extension but then the fibre yields