3. Thermoplastic Fabrics
1. Heat-setting or thermosetting is a heat treatment applied to fabrics
made of thermoplastic fibers such as polyester or Nylon to impart
dimensional stability.
2. Heat-setting affects the dye-ability of the fiber. Usually it decreases its
dye-ability, and therefore when performed before dyeing it is
extremely important to apply the heat- setting uniformly.
3. Uneven temperatures in the oven may cause differences in the fabric
from selvage to selvage and/or from selvage to center which will show
later as unlevel dyeing.
4. HEAT SETTING
This operation is crucial for fabrics made of synthetic fibres (PE, PA, elastomers),
for triacetate, and partly for PAC fibres (setting), since it grants excellent
dimensional stabilisation and crease- proof properties, maintained till the fabric is
exposed (by air blowing) to temperatures exceeding the heat setting one (after
being treated with water at a temperature above the second order glass transition
temperature, i.e. 80-85°C for acrylics).
Heat setting is carried out on gray fabrics (scarcely applied), on scoured fabrics
(frequently applied) and on dyed fabrics (scarcely applied).
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
5. The setting temperature used is above
Tg. In heat setting, inter-chain bonds,
such as hydrogen and dipole bonds,
break. This allows the molecular chains
to move and adopt new, stress-free
positions. New intermolecular bonds
then form with the fabric in a relaxed
condition at the setting temperature.
After cooling, the polymer molecules in
the filaments become frozen in place.
The new bonds are stable up to the heat
setting temperature. The reorganized
internal polymer structure, and the
material’s dimensions, will be stable.
5
HEAT SETTING MECHANISOM
6. Temperature Range
Fibre Min T. °C Max. T. °C Time (in seconds)
Polyester (PE) 170 210 15-50
Polyamide PA 6.6 170 210 15-40
Polyamide PA 6 160 180 15-40
Triacetate 160 180 15-40
Acrylic (PAC) 160 180-200 15-40
Elastomers 170 180-200 15-40
8. Heat setting: this process is crucial to give the fabric an optimum dimensional stability.
It is recommended to carry out a heat setting treatment before any further wet
treatment in order to avoid the formation of possible creases and folds. An
optimum heat setting requires a temperature ranging between 180°-200°C, which
must be maintained constant for at least 45 minutes. An optimum heat setting
also requires the use of an indirect-air heating stenter, allowing more uniform
temperatures and no-gas conditions, which could lead to fibre yellowing. The fabric is
weighed at the entry of the stenter and then subjected to steaming. Since the fabric
shrinks during the heat setting treatment, the fabric width on the stenter must exceed
the desired width by 5-10%. An excessive heat setting could decolorise the fabric while
an insufficient heat setting will result in poor fabric stability.
Heat setting: Elastic Fabrics
9. PURPOSE OF HEAT SETTING
The purpose of heat setting is to dimensionally stabilize fabrics containing
thermoplastic fibers.
Polyester and nylon are the principal fibers involved. Blended
polyester/cotton fabrics are produced in large quantities. These fabrics may
shrink, or otherwise become distorted either during wet processing or in
the consumer's hands. Heat setting is a way of reducing or eliminating these
undesirable properties.
The process is relatively simple - pass the fabric through a heating zone for a
time and at a temperature that resets the thermoplastic fiber's morphology
memory.
10. PURPOSE OF HEAT SETTING
The new memory relieves the stresses and strains imparted to the fiber by
the yarn - making and weaving processes, and makes stable the
configuration it finds itself in flat smooth fabric.
This newly imparted memory allows the fiber to resist fabric distorting
forces and provides a way to recover from them.
The time and temperature needed for the heat treatment depend on fabric
density and previous heat history of the polyester. Time and temperature
must exceed that imparted by previous heat treatments.
Usually 15 - 90 seconds at temperatures of 385 - 4150 F. will suffice. The
heat setting equipment can be hot air in a tenter frame, or surface contact
heat from hot cans. While the process is simple, careful control is required.
11. POINTS OF CONCERN
Heat setting reduces polyester's dye up take. Heat -set goods dye lighter
and slower than non heat -set good.
For uniform shades, side to center, front to back and beginning to end
exposure to heat must be controlled and uniform, otherwise these
differences will show up in the dyed cloth.
Heat - setting can be done either at the end of wet processing or at the
beginning.
At either point, the goods must be free of wrinkles and other distortions
otherwise the distortions will be permanently set.
Care must be taken when heat -setting greige goods sized with polyvinyl
alcohol.
12. POINTS OF CONCERN
At high temperatures, PVA will dehydrate, becoming discolored and
insoluble in water.
While greige heat -setting may be particularly beneficial when
preparing easily distorted fabrics, one may have to settle for less than the
full heat -setting benefit in order to still remove the size.
For these cases, alternate mechanical handling preparation equipment
may be the answer.
Heat -setting harshen the hand and stiffens the fabric. The condition is
more prone to happen when contact heat is used.
The fabric may become flatter and develop an unwanted gloss or sheen.
13. Machines used: Stenters.
1. Fluctuating temperatures inside the stenter cause a consistent variation of
crystallinity in the fibre structure, which leads to different affinity for dyes.
2. The moisture in the fibre produces soft hand, but variable moisture
percentages in the different fabric sections create the above mentioned defect
(variable crystallinity).
3. Too low temperatures do not allow a good setting while too high
temperatures and too long
4. setting times cause yellowing (PA and elastic fibres), stiff hand (acrylics), and
loss of elasticity (elastic fibres).
Machines Used For HEAT SETTING
14. Machines Used For HEAT SETTING
5. The presence of combustion gas (NOx) produces a yellowing of the
elastomers.
6. The heat setting process carried out before scouring could fix the stains on
the fabric or make the scouring process more difficult due to the
modification of the lubricating products (cracking with emission of polluting
gas).
7. Heat setting after dyeing could lead to the sublimation of disperse
dyes (if not accurately selected).
