Page 47 - Plastics Engineering

30 General Properties of Plastics

L4,4 Special Properties

Thermal Properties. Before considering conventional thermal properties such

as conductivity it is appropriate to consider briefly the effect of temperature on

the mechanical properties of plastics. It was stated earlier that the properties

of plastics are markedly temperature dependent. This is as a result of their

molecular structure. Consider first an amorphous plastic in which the molecular

chains have a random configuration. Inside the material, even though it is not

possible to view them, we know that the molecules are in a state of continual

motion. As the material is heated up the molecules receive more energy and

there is an increase in their relative movement. This makes the material more

flexible. Conversely if the material is cooled down then molecular mobility

decreases and the material becomes stiffer.

With plastics there is a certain temperature, called the glass tramition

temperature, Tg9 below which the material behaves like glass Le+ it is hard and

rigid. As can be seen from Table lm8 the value for Tg for a particular plastic is

not necessarily a low temperature. This immediately helps to explain some of

the differences which we observe in plastics. For example, at room tempera+

ture polystyrene and acrylic are below their respective Ts values and hence we

observe these materials in their glassy state Note, however, that in contrast, at

room temperature, polyethylene is above its glass transition temperature and so

we observe a very flexible materialm When cooled below its Tg it then becomes

a hard, brittle. solid, Plastics can have several transitions.

The main Tg is called the glass-rubber transition and signifies a change from

a flexible, tough material to a glassy state in which the material exhibits stiff-

ness, low creep and toughness although with a sensitivity to notches. At lower

temperatures there is then a secondary transition characterised by a change to

a hard, rigid, brittle statet

It should be noted that although Table 1.8 gives specific values of Tg for

different polymers, in reality the glass-transition temperature is not a material

constant, As with many other properties of polymers it will depend on the

testing conditions used to obtain it.

In the so-called crystalline plastics the structure consists of both crystalline

(ordered) regions and amorphous (random) regions. When these materials are

heated there is again increased molecular mobility but the materials remain

relatively stiff due to the higher forces between the closely packed molecules.

When the crystalline plastics have their temperature reduced they exhibit a

glass transition temperature associated with the amorphous regions. At room

temperature polypropylene, for example, is quite rigid and tough, not because

it is below its T, but because of the strong forces between the molecules in the

crystalline regions. When it is cooled below -10°C it becomes brittle because

the amorphous regions go below their Tg'

In the past a major limitation to the use of plastics materials in the engi-

neering sector has been temperature. This limitation arises not only due to the