Rubber chemistry

Rubber chemistry is complex and forms the very basis for ultimately being able to vulcanize rubber. In this chapter, an overall description of rubber chemistry, the various blocks of which it consists and its impact on the final result is given.

A polymer is usually named with the prefix "poly" followed by the name of the monomer. The polymer polychloroprene is thus obtained from the monomer chloroprene. This designation is used above all when you want to describe the chemical structure of the polymer molecule. In addition to polymers, the rubber materials used in practice also contain varying amounts of additives. For these composite materials, it is preferred to use a designation consisting of the name of the monomer plus the suffix "rubber". With the polymer polychloroprene, for example, a rubber material called chloroprene rubber is obtained.

Below you can delve into the raw materials, cross-linking and vulcanization that make up the building blocks of rubber chemistry.

Raw materials

Raw materials
Monomers and thus also polymers are usually organic compounds, i.e. based on carbon. The raw materials for the monomers must therefore consist of substances that contain carbon in some form. The most important are petroleum and coal.

At refineries, the crude oil is distilled, whereby a product called naphtha is obtained, among other things. This naphtha is the starting product for a large part of the chemical industry.

In polymerization, small molecules (monomers) are joined to form large molecules (polymers). The polymerization reactions can be divided according to different bases. Most common is division by reaction mechanism. Thereby, two main groups can be distinguished, chain polymerization and stepwise polymerization.

Chain polymerization
In chain polymerization, the reaction usually takes place through the addition of monomer molecules to growing chains, without by-products being formed (poly-addition).

Above, only polymers formed from a single monomer, so-called homopolymers, have been treated. However, it is not uncommon for polymers to form during chain polymerization from two or more monomers, so-called copolymers. This applies especially to rubber polymers. Thus, the most common type of rubber is SBR, a copolymer of styrene and butadiene. In some cases, more than two monomers can be copolymerized, for example the rubber polymer EPDM, which is a copolymer of ethylene, propylene and a diene.

Stepwise polymerization
Stepwise polymerization usually means that two different functional groups react with each other during the splitting off of a low molecular weight substance such as water. In stepwise polymerization, cross-linking can take place alongside the polymerization, so that a three-dimensional network is formed.

Molecular structure
Polymer molecules are rarely completely linear, but are often more or less branched. The chains can be of very different lengths. The degree of branching can be of great importance for the properties of the polymer, as it affects the forces between the molecules, among other things.

Molecular weight
The molecular weight of a polymer is usually very high, as it is composed of a very large number of mores. The so-called degree of polymerization indicates how many more are linked together. If we take (poly)isoprene (molecular weight of the monomer = 68) as an example, then a polymer with a degree of polymerization of 10,000 has a molecular weight of 68 x 10,000 = 68,000. Most polymers, useful as plastics, rubbers and fibers, have molecular weights between 10 000 and 1,000,000.


Cross-linking means that the chains of the polymer molecules are joined at several points into a three-dimensional network. The purpose of cross-linking is primarily to prevent the molecular chains from sliding over each other during deformation, i.e. to increase the elasticity of the polymer materials and reduce their plasticity or viscous deformation. Cross-linking is mainly used for rubber and thermoplastics, but occurs to a limited extent also for the modification of thermoplastics and fiber materials.
The effects of the crosslinks can be briefly summarized as follows:

1. As already mentioned, the crosslinks prevent the molecular chains from sliding over each other under load, i.e. the elastic deformation of the material increases and its plastic or viscous deformation decreases. This also applies to long-term loading, which is why cross-linking reduces tendencies towards creep and relaxation.

2. With an increasing number of crosslinks, that is, with increasing bond density, the mobility of the molecules gradually decreases. This means that hardness and stiffness increase.

3. As the number of crosslinks increases, the solubility of the polymer in solvents decreases. Above a certain crosslink density, the polymer becomes insoluble. However, it still swells, but the swelling decreases with increasing crosslink density.

4. Permeability to liquids and gases decreases with increasing crosslink density.

5. Mechanical damping decreases with increasing crosslink density.

6. Fatigue properties deteriorate with increasing crosslink density.

7. While hardness and stiffness increase with increasing crosslink density, the material's extensibility or elongation at break decreases.

Chemical cross-linking of linear polymers
In a large number of cases, especially in the case of rubber polymers, it is preferred to build up the molecular structure in two steps. In the first step, which is usually carried out in the chemical industry, an essentially linear polymer is produced. This is plastically malleable upon moderate heating. After forming, which is usually carried out in the rubber industry, the shape of the part is fixed by introducing chemical cross-links. Technically, this is achieved by mixing the polymer with the chemicals that will create cross-linking and other additives before shaping, after which the mass is shaped plastically. The molded part is then exposed to elevated temperature, usually in the range of 100-200°C, whereby cross-linking reactions occur.
The process usually takes place under pressure to avoid blistering in the product. A general problem in this context is that during processing the pulp must not be exposed to such high temperatures that the cross-linking reaction starts.

Even with a very small degree of cross-linking, the material loses its ability to be plastically formed.
As little as the cross-linking possibilities of the molecular chains are used to one hundred percent, just as little can be used to one hundred percent of the vulcanizing agent's reaction possibilities. Some vulcanizing agents can give a yield of crosslinks that approaches one hundred percent of the theoretical. This applies, for example, to peroxides, while, for example, sulfur sometimes does not give more than 20% of the theoretical yield.

Vulcanization of rubber
Earlier it was mentioned how the increase in the number of cross-links affects the properties of the cross-linked polymer. It was stated, for example, that modulus and hardness increase, swelling and solubility decrease, creep and relaxation decrease, and that fatigue strength decreases. In addition, it can be shown that the wear resistance increases with a moderate increase in the crosslink density. The tensile strength first rises, but then passes through a maximum and drops again.
You normally study the vulcanization process by vulcanizing the material at a certain temperature for a number of different times and plotting the properties as a function of the vulcanization time. Here, the effect of the increase in the number of cross-links during the vulcanization reaction can be seen.

In peroxide vulcanization of rubber, the obtained cross-linking density within a fairly large range is directly proportional to the amount of peroxide used. Even in the case of sulfur vulcanization of rubber, the crosslink density can be affected by variation of the sulfur content. Usually, however, there is no direct proportionality between these two factors. Normally, you work with sulfur contents of up to about 3%. With further rising sulfur content, the strength properties first deteriorate and then increase again at very high sulfur levels. At the same time, the cross-linking density has become so great that the mobility of the molecular segments has been lost. The material then loses its rubber character. Instead, a hard, thermoplastic-like material has been obtained, so-called hard rubber or ebonite.