Information on Rubber - 2
Low temperature behaviour. What is the 'glass
When different elastomers are being described, a fundamental property
which is often quoted is the 'glass transition temperature, Tg',
which differs from one elastomer to another. For example, for natural
rubber Tg is -70°C (-95°F). Broadly
this means that above -70°C the material behaves as a rubber,
but below -70°C the material behaves more like a glass. When
glassy, natural rubber is about one thousand times as stiff as it
is when rubbery. When glassy a hammer blow on natural rubber will
cause it to shatter like a glass; when rubbery the hammer is likely
just to bounce off.
Of course in practice the dividing line between the glassy and rubbery
behaviour just described is not as sharp as this. In fact the transition
is spread over some tens of degrees - but it is centred around -70°C.
Thus, although a Tg can be accurately defined
(although varying somewhat with the precise test conditions), for
practical purposes we have to consider a glass transition region
within which the properties are slowly changing from rubbery to glassy
or vice versa (the processes are completely reversible). The broadness
of this transition region varies from elastomer to elastomer.
Strictly speaking we should only use the term 'elastomer' to describe
a material when it is above its glass transition temperature, but
such a distinction would generally be regarded as pedantic. Definitions
of this type (such as 'elastomer' or 'glassy polymer') are accepted
as applying to the state of the polymer at ambient temperature.
The reason for the existence of a glass transition temperature can
be understood in terms of the molecular model of rubber which has
been described above. It has been seen that, at normal temperatures,
the molecular chains are in a constant state of thermal motion, that
they are constantly changing their configuration, and that their
flexibility makes them reasonably easy to stretch. It is not difficult
to appreciate that as the temperature is lowered the chains become
less flexible and the amount of thermal motion decreases. Eventually,
a low temperature, the glass transition temperature, is reached,
where all major motion of the chains essentially ceases. The material
no longer has the properties which make it an elastomer, and it behaves
as a glass.
For all practical engineering uses of elastomers we require good
flexibility - so it is essential that we use them only at temperatures
which are comfortably above the glass transition. This is generally
no problem for natural rubber with a Tg of
-70°C, or cis-butadiene rubber with a Tg of
-108°C (-160°F). But many elastomers, especially those which
have been designed to be highly heat or oil resistant, have much
higher Tgs, and this must be borne in mind
when selecting them for service applications. For example some fluoroelastomers,
which have excellent oil and heat resistance, have a Tg not
far below normal room temperature. This can result in problems if
a component required to work at high temperature also has to serve
the same function on cooling down, and must be considered in design.
High temperature behaviour.
The limit to the upper temperature at which an elastomer can be
used is generally determined by its chemical stability, and will
thus vary for different elastomers. Elastomers can be attacked by
oxygen or other chemical species, and because the attack results
in a chemical reaction, its potency will increase with temperature.
Degradative chemical reactions are generally of two types. The first
are those which cause breakage of the molecular chains or crosslinks,
softening the rubber because they weaken the network. The second
are those which result in additional crosslinking, hardening the
rubber - and often characterized by a hard, degraded, skin forming
on the rubber component. Selection of a suitable elastomer and the
use of chemical antidegradants can reduce the rate of chemical attack.