Rubber Chemistry
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 and air.
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 useful.
"How can I make this substance useful?" That’s the question facing Charles Goodyear and Thomas Hancock when they independently invented vulcanization in 1839.
Vulcanization
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 sulfur atom.
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 cure system.
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 (see below.)
| Conventional | Semi-EV | EV | |
|---|---|---|---|
| Poly and Disulfidic % | 95 | 50 | 20 | Monosulfidic % | 5 | 50 | 80 |
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.
