Page 95 - Handbook of Properties of Textile and Technical Fibres

76 Handbook of Properties of Textile and Technical Fibres

linear density can be determined by vibroscopy, before they are subjected to a tensile
test.
There are no clear guidelines on how to treat single fiber tensile data; however, the
most sensible seems to be to use the cross-sectional area at the position of break to
normalize the data (Viney, 2002). This position is likely to, but not required to, coin-
cide with the minimum initial cross-sectional area of the fiber. For ease of use it is
likely that many researchers will in future opt to use automated systems such as the
Favimat, generating specific stress values based on linear density. Whatever method
is used, however, it needs to be used consistently for all samples that are to be
compared, and thought needs to go into the consequences of the nonuniformity of
the fiber and clamping issues.

3.5 Tensile failure

There are a number of parameters such as specimen shape and size, rate and temper-
ature of test, and degradation in service that affect the tensile failure properties of all
polymeric materials. For a natural material such as wool, which has the ability to
absorb large quantities of water, the relative humidity of the environment is also of
utmost importance. During its lifetime, wool needs to perform under a range of
different conditions. During scouring it is wet and deformed slowly, whereas during
early stage processing (carding and combing) deformation is rapid and the fiber is
dry. During wear, fibers undergo torsional/abrasive deformation, mostly in a dry state
but occasionally wet during laundering. During formation of the fiber and for many
applications it is exposed to sunlight.

3.5.1 Effect of moisture, temperature, and rate of test

The ease with which molecules can rotate about their backbone, and through this allow
effective cooperative segmental motion, has a dramatic effect on the tensile properties
of the material. The more mobility the molecules possess and the more time they have
for cooperative segmental motion, the less stiff the material will be. Some polymers,
such as rubber, are naturally very mobile at room temperature. Others, such as
PVC, are stiff at room temperature but can be made flexible by the addition of a plas-
ticizer that acts as an internal lubricant. For wool, water is an excellent plasticizer,
dramatically altering the physical properties of the fiber. The best indicator of molec-
ular mobility is the glass transition temperature, T g . If the T g is below room tempera-
ture then the material exhibits rubbery behavior, and if the T g is above room
temperature then the material is more glasslike and brittle. For wool fibers the T g is

about 60 C under standard conditions but changes dramatically as the fiber dries
out or takes up more moisture in response to changes in the environment
(Fig. 3.17). This results in a decrease in strain at break and an increase in stress at break
as the fiber goes from wet to completely dry (Fig. 3.18(a))(Speakman, 1927; Aksakal
and Alekberov, 2009). The modulus also increases by about three times as the fiber