24
1
v Answer
D = D + D
G
H T S
The total free energy, ΔG, is the function of stability of any
system, where ∆H, T, and ∆S are enthalpy or total heat content,
temperature after post curing, and configurational entropy,
respectively. Therefore, for the stability of cured composite, the
value of ΔG needs to be minimal.
ΔH is negligible for composites. Hence,
D = D
G T S
Therefore, the stability of the composite will depend upon the
entropy, ΔS, while temperature T is variable. The residual
energy is stored in the form of entropy. At post curing above T g ,
translational movement relaxes the polymeric chains, and
residual energy is released so it brings down the configuration
entropy.
DS » 0
Therefore,
ΔG = 0, and the post cured composite is dimensionally stable.
1.7.3 Electrical Behaviour
Electrical properties of polymeric composites are important to a
wide range of industries, such as automotive, aerospace, electrical
appliances, marine, packaging, and consumer goods [19]. Electrical
tests, in general, are measurements of the resistance, conductivity,
or charge storage either on the surface or through the composite
material. Typical electrical properties of polymeric composites are
given in . Table 1.12.
1.7.4 Hygrothermal Behaviour
According to Fick’s law, most composites absorb liquid. The hygrothermal behaviour of any material is its response to its contact with
Point to Ponder…
Low dielectric constant and
low loss tangent of polymeric
composites make it transparent
to electromagnetic waves. This
property of polymeric composites
makes it useful for radar
transparent applications in the
nosecones of aircrafts and missiles.
. Table 1.12 Typical electrical properties of unidirectional (UD) polymeric composites [10–12]
Serial no.
UD composites
Dielectric
constant
Loss tangent
Volume resistivity
(10 7 Ohm.cm)
Surface resistivity
(Ohm)
1.
Carbon/epoxy
(V f = 60%)
–
–
V1
175
2.
Boron/epoxy (V f = 50%) –
–
V0
175
3.
Glass/epoxy (V f = 45%)
4.4
0.02
V0
180
4.
Kevlar 49/epoxy
(V f = 60%)
3.5
0.05
V1
180
Chapter 1 · Introduction
1
v Answer
D = D + D
G
H T S
The total free energy, ΔG, is the function of stability of any
system, where ∆H, T, and ∆S are enthalpy or total heat content,
temperature after post curing, and configurational entropy,
respectively. Therefore, for the stability of cured composite, the
value of ΔG needs to be minimal.
ΔH is negligible for composites. Hence,
D = D
G T S
Therefore, the stability of the composite will depend upon the
entropy, ΔS, while temperature T is variable. The residual
energy is stored in the form of entropy. At post curing above T g ,
translational movement relaxes the polymeric chains, and
residual energy is released so it brings down the configuration
entropy.
DS » 0
Therefore,
ΔG = 0, and the post cured composite is dimensionally stable.
1.7.3 Electrical Behaviour
Electrical properties of polymeric composites are important to a
wide range of industries, such as automotive, aerospace, electrical
appliances, marine, packaging, and consumer goods [19]. Electrical
tests, in general, are measurements of the resistance, conductivity,
or charge storage either on the surface or through the composite
material. Typical electrical properties of polymeric composites are
given in . Table 1.12.
1.7.4 Hygrothermal Behaviour
According to Fick’s law, most composites absorb liquid. The hygrothermal behaviour of any material is its response to its contact with
Point to Ponder…
Low dielectric constant and
low loss tangent of polymeric
composites make it transparent
to electromagnetic waves. This
property of polymeric composites
makes it useful for radar
transparent applications in the
nosecones of aircrafts and missiles.
. Table 1.12 Typical electrical properties of unidirectional (UD) polymeric composites [10–12]
Serial no.
UD composites
Dielectric
constant
Loss tangent
Volume resistivity
(10 7 Ohm.cm)
Surface resistivity
(Ohm)
1.
Carbon/epoxy
(V f = 60%)
–
–
V1
175
2.
Boron/epoxy (V f = 50%) –
–
V0
175
3.
Glass/epoxy (V f = 45%)
4.4
0.02
V0
180
4.
Kevlar 49/epoxy
(V f = 60%)
3.5
0.05
V1
180
Chapter 1 · Introduction