The most important property for
synthetic fibres from processing point of view is their thermal behaviour. The
knowledge of this enables to select the proper processing conditions. The
temperature dependent properties of the fibres undergo major changes at
transition points: first order transition at the crystalline melting (at
melting point) and second order transition at glass transition temperature.
With the processing of fibres the transition at glass transition temperature is
important.
The glass transition temperature
is defined as the temperature below which the amorphous region is in glassy
(frozen) state and above which it become rubbery.
At very low temperatures chain
atoms undergo low amplitude vibratory motion but as the temperature rises,
amplitude and the nature of these vibrations motion of atom increases and when
the thermal energy is sufficient high to overcome the barrier of for movement,
segmental movement becomes possible and material becomes rubbery. Thus, above Tg, the chain
segments undergo rotational, translational and diffusional motions. As
temperature is further raised larger and larger chain segments undergo motions
until the amorphous material become like a viscous liquid.
Actually a simple explanation
of the Tg is given by the free volume theory, where the free volume is assumed be
the difference in the total volume and the occupied volume. According to this
model above Tg enough free volume becomes available for rotation of
molecular segments about 25 carbon atom long, leading to more rapid increase in
specific volume. This makes polymer soft and flexible.
The factors like thermal
history (conditions of heat-setting) of fibres, heating rate, presence of low
molecular weight compounds etc influenced the Tg. Since the fibre is not all
amorphous polymers and contains a substantial proportion of crystalline
fraction, the Tg of the fibres is influenced by its crstallinity,
crystal size and orientation. eg. The Tg of PET fibre increases with crystallinity from 80 °C
to 120 °C.
Tg of some
synthetic fibers
Fibers
|
Tg
|
Nylon 6
|
50-60
|
Nylon 66
|
47
|
Polyester
|
80
|
Acrylic
|
85
|
Polypropylen
|
-35
|
Polyethylene
|
-100
|
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