NONCIRCULAR CROSS-SECTION FIBERS

Synthetic fibers like PET and nylon are normally round in cross section, however no natural
fiber has a circular cross section. Wool is irregular, cotton is ‘‘dogbone’’ shaped, and silk is
triangular. In the early 1970s, people began to study the effect of noncircular cross-section
(NCCS) fibers on yarn and fabric esthetics, which is a subjective topic involving such arcane
terms as ‘‘feel,’’ ‘‘drape,’’ and ‘‘handle.’’ Fortunately, a melt-spun fiber lends itself NCCS
well to the production of (NCCS) fibers by varying the shape of spinneret orifice, provided
the melt viscosity is high enough so that surface tension does not cause the filament to resume
a circular shape. Since the holes had to be very small (about 0.015 in. overall), machining a
multiplicity of holes at a uniform size and shape was a major engineering problem, particularly
in the hard metal alloys used for spinneret plates. Laser etching is one technique used.
A hole shaped like a T gave trilobal filaments. In the pioneering days, much of this work was
entirely heuristic, but gradually emerged some rules of thumb. Multilobed yarn cross sections
(trilobal and octalobal) can give quite different appearances. Trilobal is glittery as the incident
light reflects off the fiber surface, while octalobal gives an opaque matte effect, as the light is
effectively absorbed by multiple reflections from the many acute angles. Sharp-edged filaments
have the prized rustle and high frictional characteristics of pure silk, where it is called
‘‘scroop.’’ Flat rectangular filaments give fabrics an unpleasant ‘‘slimy’’ handle. Gradually,
these principles were applied to commercial yarns, and many filament yarns for the apparel
and BCF carpet markets now use NCCS fibers.