Making Rubber Stronger Through Vulcanization

Rubber is a polymer of isoprene that is formed with double bonds between each of the individual monomers (or units). In simple terms, one of these bonds is flexible and can allow the polymer to take up many shapes and sizes, which is what basically gives rubber its property of elasticity. But, that being the case, that flexible bond is also what makes rubber in its natural form somewhat imperfect (for example, perhaps natural rubber is too elastic for some applications), because if that bond were stronger, it would create a better material that could be more widely used. There is a process that can strengthen that bond called rubber vulcanization. This process involves heating natural sheet rubber and rubber products with sulfur, and it is a widely-used method for the creation of commercial rubber parts. While the process on how to vulcanize rubber is in fact a complex one, it can be broken down into simpler terms for everyone to understand. Rubber supplies and parts are now generally vulcanized, unless they are made out of tire crumb in which case they are molded.

Vulcanization was discovered accidentally by Charles Goodyear in 1839, and it involved altering the chemistry of the compound; it was named after Vulcan, the Roman god of fire. As stated previously, the molecular structure of rubber is that of a chain comprised of a very large amount of units of isoprene that are linked together with a double bond. Because of the free bond between the units, rubber is very flexible, perhaps too flexible for some commercial rubber applications. So, sulfur is introduced into the compound through intense heating, and the free bond is no longer free, creating a crosslink between chains. While this reduces natural rubber’s elasticity, it also makes for a substance that is far superior in its strength and resistance to abrasion and harsh conditions. It also makes the rubber non-stick and eliminates its thermoplastic nature, meaning that it will not return to its original shape after it cools. In truth, there are other elements that can be used for this process, such as selenium or tellurium, but sulfur is by far the element of choice. It can then be processed into sheets or rolls, then cut or customized into products for both simple and specialized uses.

There are two basic rubber vulcanization processes that will both contribute to the formation of polysulphide crosslinks between chains. The first process, called Pressure Vulcanization involves heating the rubber with sulfur under pressure, with a temperature of 150C. The second process, called Free Vulcanization, simply involves passing very hot steam or air through the rubber. Eight hours of curing time is necessary for vulcanization to complete, although, chemical “activators” can be used to control when the curing process begins, and “accelerators” can be used to speed up the curing time, as well as shorten the sulfur links down to a few atoms, making the bonds even stronger. These things must be used carefully though, for if vulcanization occurs too quickly, before the material has shaped, “scorching” will occur, effectively ruining the substance for commercial use.

Other agents can be used in the process and will produce different results. For example, fillers can be used to dilute or reinforce the sheet rubber and rubber products (increasing tensile strength and abrasion resistance), or carbon black can be introduced into the mix to reinforce the substance. Vulcanization can also involve the use of pigments to create colored products. Protective agents will prevent aging and deterioration (mainly from oxidation), and softeners or lubricants can be added to plasticize or increase the self-adhesion of the rubber mix.

When it comes down to it, rubber vulcanization produces a product that has far more uses than natural rubber, and so, we can be thankful that this process was discovered. Had it not been, we may have never seen the rise of commercial rubber, and there may have been things we could have never safely or effectively done with natural rubber and rubber products used in our lives. Now that you know the quick and easy of how to vulcanize rubber, you know that you’re getting superior rubber supplies that can do well for any of your needs. But we do not recommend you try it at home; leave it to the experts!