THERMAL PROPERTIES
Thermal
Properties:
The
behaviour shown by a textile material when it is subjected to heat is known as
thermal property. Followings are the thermal properties of a textile material-
v Thermal conductivity
v Glass transition
temperature
v Melting temperature
v Thermal expansion
v Heat of wetting or heat
of absorption
v Heat setting
v Thermal conductivity:
Thermal conductivity is the rate of
transfer of heat in calorie along the body of a textile material by conduction.
Higher conductivity of a material indicates that the heat will pass through the
material very easily.
Woolen dresses are comfortable to
wear during winter season due to its lower conductivity and cotton dresses are
comfortable to wear in summer season because of its higher conductivity.
Typical
values of thermal conductivity for some fibres:
Fibre Thermal
conductivity (mWm-1K-1)
Cotton 71
Wool 54
Silk 50
(Above thermal conductivity of
fibres with a bulk density of 0.5 gm/cm3)
PVC 160
Cellulose acetate 230
Nylon 250
Polyester 140
Polyethylene 340
Polypropylene 120
v Glass transition
temperature:
The temperature up to which a
textile material behaves hard as like glass and after which it behaves soft as
like rubber is known as glass transition temperature and it is expressed by Tg.
The range of Tg lies between -1000 C to 3000 C.
Factors
influence the Tg value of polymers:
v
Higher
the flexibility of chain bond, lower will be the Tg value.
v
Composition
of ring structure in molecular chain raises the value of Tg.
v
Bulky
side groups raise the value of Tg.
v
Flexibility
of side groups decreases the value of Tg.
v
Tg
increases with molecular weight upto 20,000.
v
Polarity
of side groups increases the value of Tg.
v
Co-polymers
have lower value of Tg than homo-polymers.
v
Increase
of orientation restrict the chain movement and increase the value of Tg. (Tg of
undrawn polymer fibre is 1100C whereas Tg of fully drawn polymer
fibre is 1500C)
v
Melting
temperature:
The temperature at which a textile
material melts is known as melting temperature and it is expressed by Tm. At
melting temperature a polymer losses its identity and change into viscous
liquid. It losses its strength and some molecular weight at melting
temperature. Cellulose and protein fibres decompose before melting.
Typical
values of Tg and Tm for some MMF:
Fibre Tg
(0C) Tm (0C)
Nylon-6 50 215
Nylon-6.6
50 260
Polyester
69 260
PVC 81 310
PAN 97 314
Rubber -73 36
Cellulose
tri-acetate -
300
v Thermal expansion:
Thermal
expansion is the increment of length of a textile material after heating.
Thermal expansion is measured by co- efficient of thermal expansion.
Co-efficient of thermal expansion can be defined as the fractional increase in
length of a material due to rise in temperature by 10 C.
Co-efficient of thermal expansion = Increase
in length x100
Initial
length of a textile material
v Heat of wetting or heat
of absorption:
When textile materials
absorb water they show their ability to leave off small amount of heat which is
known as heat of wetting or heat of absorption.
If 1 gm dried material is completely wetted, then heat
in calorie/gm involved in that material is known as heat of wetting.
v Heat setting:
Heat setting is the
process of stabilizing the form, size and dimension of the material by drying
and cooling in successive dry and wet condition.
