Information on Rubber - 1
What is rubber and why does it stretch?
The terms elastomer and rubber are scientifically identical and
interchangeable, although the latter is used in some areas to refer
only to natural rubber which comes from the latex contained by some
trees and other plants - as opposed to synthetic rubber which is
generally an oil by-product. Some Standards attempt to reserve the
term elastomer for a crosslinked material (see below for an explanation
of crosslinking), but there is no general agreement on this. In our
literature we use the terms elastomer and rubber as synonyms.
Elastomers are a class of materials which differ quite obviously
from all other solid materials in that they can be stretched, easily
and almost completely reversibly, to high extensions. An ordinary
postal rubber band illustrates this behaviour. It will generally
be made from natural rubber, and can be stretched perhaps 600% (i.e.
to seven times its original length), after which - before reaching
its ultimate breaking elongation - it can be released and will rapidly
recover to almost exactly the original length it had before stretching.
The material is said to be elastic.
Most synthetic elastomers are not as elastic as natural rubber,
but all can be stretched (or otherwise deformed) in a reversible
manner to an extent which easily distinguishes them from all other
solid materials. (n.b. a metal spring exhibits high reversible elasticity,
but this is a feature of its wound shape. The actual metal itself
of which the spring is made only deforms slightly, by twisting locally,
at any particular point - nothing like the high deformations of which
elastomers are capable.)
Elastomers are a special case of the wider group of materials known
as polymers. Polymers are not made up of discrete compact molecules
like most materials, but are made of long, flexible, chain-like or
string-like, molecules. At this scale the inside of a piece of rubber
can be thought of as resembling a pile of cooked spaghetti. In spaghetti,
however, the chains, though intertwined, are all separate. But in
most practical elastomers each chain will be joined together occasionally
along its length to one or more nearby chains with just a very few
chemical bridges, known as crosslinks. So the whole structure forms
a coherent network which stops the chains from sliding past one another
indefinitely - although leaving the long sections of chain between
crosslinks free to move. The process by which crosslinks are added
is known as vulcanization. To achieve vulcanization the raw rubber
is mechanically mixed with a number of compounding ingredients carefully
chosen to give the properties required for the particular application.
The reason why elastomers behave as they do is associated with the
type of molecular structure described above.
Against this background the reason why rubber can stretch so much
is that, at normal temperatures, each long chain-like molecule (like
any molecule) is in a constant state of agitation (thermal motion).
For these flexible long-chain molecules the movement is considerable,
and the molecule is agitated so much that it can take up a highly
kinked shape. Because of this kinking, the distance between the two
ends of the chain is very much less than its fully stretched length.
This gives the rubber its flexibility. When a rubber band is stretched
some of the highly kinked chains are simply being stretched out.
Stretching can then continue until many of the chains are fully extended,
or until the rubber breaks.