Heat setting of polyester:
Heat-setting
is a heat treatment by which shape retention, crease resistance, resilience and
elasticity are imparted to the fibres. It also brings changes in strength,
stretchability, softness,
dyeability and sometimes on the colour of the material. All these changes are
connected with the structural and chemical modifications occurring in the
fibre.
Heat setting is the
process applicable to fabrics made from synthetic fibres like nylon, polyester
in which the fabric is subjected to the action of high temperature for a short
time to make it dimensionally stable so that the garments made from such
fabrics retain their shape on washing and ironing.
The main aim of heat
setting process is ensure that fabric do not alter their dimensions during use.
This is process for
the stabilisation of synthetic fibres so they do not shrink on heating.
Polyester shrinks -7%
in boiling water and 20% at 220 degree C.
Nylon-12% in boiling water
Higher the pressure
higher the shrinkage. So it is necessary to stablise dimensional stability of
such fabrics so that they do not shrink during processing and during washing. The
stablisation of dimensions of synthetic fibre/ fabrics by exposure to heat is
possible, because these fibres resists shrinkage at high temperatures, if they
are held tight at this temperatures for a short period. All synthetic fibres
whether they are produced by melt, dry and wet spinning process are deficient
in strength in their freshly formed condition.
To give them strength
they are stretched several times to their original length. This high degree of
stretching causes orientation of chain molecules. But this high degree of
stretching is not sufficient to make these fibre stable. The single chain held
together by vander waals force and
hydrogen bond, are not completely stretched. These chains start vibrating if
energy is supplied to them in the form of heat. As further heat is supplied the
intensity of vibrations increase and when it reached at a certain level the vibration become so strong that some of
hydrogen bonds are broken, thus there is breakdown of intermolecular hydrogen
bonds so that the molecules are freed from the restraints which maintain
orientation. This relaxation causes contraction of chains and therefore
longitudinal shrinkage occurs . As the temperature increases, shrinkage also
increase till a point is reached when the fibres melts. If the supply of energy
is stopped as soon as the maximum potential energy is reached and the fibres
are cooled rapidly, the hydrogen bonds gets stabilized. Heat set temperature
should be above the glass transition temperature.
TG- transition
temperature –the temperature at which the molecular movement starts in amorphous
region of the polymer.
Glass
Solid – Rubbery Solid
Melting point: at
this point the forces holding the molecules in the crystalline regions of the
fibre are overcome by the thermal energy and the polymer melts. In both
polyester and nylon these temperatures are separated by about 150°C.
Three different
stages of heat setting:
Heat-setting can be
carried out at three different stages in a processing sequence i.e.
- in grey condition (scarcely
applied)
- after scouring (Frequently
applied); and
- after dyeing (Scarcely applied) .
The stage of
heat-setting depends on extent of contaminations and types of fibres or yams
present in the fabric. Heat setting after dyeing could lead to the sublimation
of disperse dyes (if not accurately selected).
The process grants
excellent dimensional stability and good crease-proof properties. As far as
operating conditions are concerned, the fabric must be treated in accurately
controlled moisture and temperature conditions.
Heat setting of Some
Fibers
Stenter
Machine
Stenters
are widely used for stretching, drying, heat-setting and finishing of
Fabrics. The stenter frame is usually 80-100 feet long and 70-100 inches
wide. The speed ranges from 10-45 m/min with a maximum setting time in the
setting zone 30 sec at temperature ranging from 175 to 250~ depending upon the
thickness and type of the material.
Iodine absorption test: