Natural rubber is a polymer (macromolecule) formed from isoprene molecules (methylbut-1,3-diene). It is usually obtained by extracting latex from the rubber tree (Havea brasiliensis), as shown below:
Latex (natural rubber) is extracted from the rubber tree (Hevea brasiliensis)
This latex is strained and added with ammonia (NH3) to preserve it and also from various preservative acids or salts so that it goes through a process of coagulation and separate from the liquid, forming a white pasty mass, which is ground and processed to remove possible contaminants and dry.
However, rubber, thus raw, has some undesirable characteristics for its use by industry, such as low tensile strength, solubility in solvents organic, easy to be oxidized and low resistance to heat and temperature variation, because on hot days it becomes soft and sticky, while on cold days it becomes hard and brittle.
To remedy these problems, rubber undergoes a process called vulcanization, which was discovered by accident in 1838 by
Charles Goodyear (1800-1860), when he dropped a mixture of rubber and sulfur on the hot stove and he noticed that this mixture had burned a little, but not melted.
Charles Goodyear
Thus, Vulcanization is the addition of sulfur to rubber, under heating and with the use of catalysts. Note in the diagram below that the polyisoprene (rubber polymer) double bonds are broken and sulfur bridges are formed, that is, side bonds between the chains, making the polymer three-dimensional:
Rubber Vulcanization Process
These sulfur bridges cause the rubber to have low hysteresis and low permanent deformation. Hysteresis refers to the delay in the response of a system when an external request is made. For example, if you squeeze a piece of raw rubber, it will take a while to return to its initial shape, which means it has high hysteresis. Vulcanized rubber will quickly return to its original shape, as the sulfur bridges between the chains are very flexible and allow these chains to easily slide over one another. others. The material is also more resistant because the sulfur bridges make it difficult for the rubber to break when it is stretched.
The proportion of sulfur added to rubber in vulcanization varies between 2 and 20%, depending on what is desired, and the more sulfur added to the rubber, the greater its hardness. Look:
Common rubbers: 2 to 10% sulfur content;
Rubbers used in tires: 1.5 to 5% sulfur content;
Rubbers used in protective coatings for machinery and equipment in chemical industries: approximately 30% sulfur content.
By Jennifer Fogaça
Graduated in Chemistry
Source: Brazil School - https://brasilescola.uol.com.br/quimica/vulcanizacao-borracha.htm