
332 Engineering Materials Technology
Polymers have higher thermal expansivities than metals, in general about 2
to 10 times greater. Polypropylene has a linear thermal expansivity of about 9
x 10~
5
K"
1
while copper has a value of 1.7 x 10"
5
K"
1
.
Solid polymers have specific heat capacities two to five times those of metals.
For example, low density polythene has a specific heat capacity of 1.9 kj
kg"
1
K"
1
, copper 0.38 kj kg"
1
K"
1
.
Polymers are electrical insulators, having resistivities which are about 10
20
times greater than those of
metals.
If
a
piece of polymer is rubbed with a piece
of cloth it will become electrically charged, the hig