The only "true" rubber is natural rubber from the latex of nearly 2000 different plants. However, the latex
from the Hevea brasiliensis tree is the only important commercial source of natural rubber. All synthetic imitations
and variations that are used in the "rubber" industry are elastomers but they are like rubber in their degree of elasticity.
An elastomer is a macromolecular material that at room temperature can be stretched under low stress to at least twice
its original length and, after release of stress, will return to its approximate original dimensions and shape. Elastomers
as a class possess some basic characteristics. They are elastic, flexible, tough, and relatively impermeable in both water
What makes rubber and other elastomers unique? Certainly, we have other materials that are flexible, tough and impermeable;
that leaves elasticity!
What gives rubber its elastic properties? That is intricately linked to its molecular structure. For purposes of this
discussion, we will examine the structure of natural rubber, realizing that most synthetic rubbers are similar in an
essential manner that will be discussed later.
Natural rubber is made up of macromolecules. What does that mean? The natural rubber molecule is made up of thousands
of repeating units called isoprene units (see image at right), hence, we call this polyisoprene. The molecular formula for the
isoprene unit is C5H8. These units are strung together as a "chain" to form the molecule and the correct formula for
polyisoprene (the entire molecule) is (C5H8)X where X equals from 10,000 to 20,000.
With tens of thousands of carbon and hydrogen atoms, it is obvious why they are called macromolecules. However,
while this molecule is thousands of times larger than the molecules of an ordinary chemical substance (i.e. H2O)
it is still much too small to be visible in the most powerful microscopes available.
A good analogy to help visualize how these molecules interact is to think of each macromolecule as a single strand
of spaghetti in plate full of spaghetti. This, in essence, is the way the molecules are arranged in rubber when in its
uncured state. In this state, they can be "slipped" apart without much effort. In this state rubber isn’t particularly
"How can I make this substance useful?" That’s the question facing Charles Goodyear and Thomas Hancock when they
independently invented vulcanization in 1839.
What Goodyear "discovered," was that when he mixed sulfur and heat with his natural rubber, he changed it. He changed
it from a substance that became sticky and soft on hot days and that wouldn’t return to its original shape when stretched
or pressed, to a substance that returned to shape and remain relatively unchanged in the hottest of weather.
So how do sulfur molecules combine with polyisoprene molecules to cause the changes Goodyear observed? Now we will
discuss that essential feature that most elastomers have in common. Even when an elastomer is a macromolecule made up
of units other than isoprene, its repeating unit has one distinct feature in its molecular structure that enables it to
be an elastomer.
Notice that in the middle of the isoprene unit there are two carbon atoms linked together with two bonds while all
the other bonds are singular. We refer to these as single or double bonds. These bonds allow elastomers to be elastic.
They do this in two ways. First it allows the units to rotate about the single bonds and that gives the molecule flexibility.
Secondly, the double bond is not very stable. With sufficient heat energy, one of the bonds can be "disconnected" from one
of the carbon atoms. We refer to macromolecules with these double bonds as being "unsaturated"
These double bonds are common to most synthetic elastomers. When one of the bonds is disconnected from one of the
carbon atoms the "loose end" is an open "site" that is available to attach with a different atom. All the double bonds
are considered "potential sites." When we combine elemental sulfur with rubber, the disconnected site will attach to a
Of what value is this attachment to changing this uncured and relatively useless material into the very useful and
unique material that we work with every day? The answer is that it is of no value until that same sulfur atom attaches
to a similar site on another polyisoprene macromolecule. This reaction of "crosslinking" of two polyisoprene macromolecules
with sulfur atoms is referred to as vulcanization.
Now we have come to our explanation of how the double bond helps give elastomers their elasticity. Think of that plate
of spaghetti now; where those strands cross one another vulcanization occurs. This gives elastomers their elasticity. As
we try to slip those stands apart the crosslinks resist the movement and forces the strands back to their original position
after the stress is relieved. Vulcanization occurs at approximately one out of every 200 potential sites in an average sulfur
When X = 1, we refer to it as Monosulfidic.
When X = 2, we refer to it as Disulfidic.
When X > 2, we refer to it as Polysulfidic.
All vulcanized rubber will contain some of all of the above types of vulcanized crosslinks. The ratio of one type of
crosslink versus the others varies from cure system to cure system. We name these cure systems according to their ratio
Cure Systems: Conventional vs. Efficient Vulcanization
|Poly and Disulfidic %
Briefly speaking, we use EV and semi-EV systems to gain heat aging and compression set resistance but give up low
temperature crystallization resistance and higher extension ratios.
Sulfur is not the only vulcanizing agent that is used and not all elastomers can be crosslinked with sulfur. The most
common alternative to sulfur is organic peroxide. Peroxide is also used to crosslink "saturated" (not having a double
carbon bond) macromolecules. Peroxide releases a free radical that enables the formation of a carbon-carbon bond between
molecules. These vulcanizates have even greater thermal stability than sulfur EV systems. This reaction is often
erroneously referred to as vulcanization but only a sulfur to carbon link can be referred to as vulcanization.
The carbon to carbon bond is simply known as a crosslink.