Rubber chemistry
Rubber chemistry is complex and forms the very basis for being able to vulcanize rubber in the end. This chapter provides an overall description of rubber chemistry, the different blocks it consists of and its impact on the final result.
A polymer is usually named with the prefix "poly" followed by the monomer's name. Thus, the polymer polychloroprene is obtained from the monomer chloroprene. This naming convention is primarily used when describing the chemical structure of the polymer molecule. The rubber materials used in practice contain, in addition to polymers, varying amounts of additives. For these composite materials, it is preferred to use a name consisting of the monomer's name plus the suffix "rubber". For example, the polymer polychloroprene yields a rubber material called chloroprene rubber.
Below, you can delve into raw materials, cross-linking, and vulcanization, which are the building blocks of rubber chemistry.
Building blocks of rubber chemistry
Raw materials
Monomers, and therefore also polymers, are usually organic compounds, i.e., based on carbon. The raw materials for the monomers must thus consist of substances containing carbon in some form. The most important are petroleum and coal.
At refineries, crude oil is distilled, yielding a product called naphtha, among others. This naphtha is the starting material for a large part of the chemical industry.
Polymerization
During polymerization, small molecules (monomers) are joined together to form large molecules (polymers). Polymerization reactions can be classified on different bases. The most common classification is by reaction mechanism. Two main groups can be distinguished: chain polymerization and step-growth polymerization.
Chain polymerization
During chain polymerization, the reaction typically occurs through the addition of monomer molecules to growing chains, without the formation of byproducts (polyaddition).
Copolymerization
So far, only polymers formed from a single monomer, so-called homopolymers, have been discussed. However, it is not uncommon for polymers formed by chain polymerization to consist of two or more monomers, so-called copolymers. This is especially true for 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 terpolymer of ethylene, propylene, and a diene.
Step-by-step polymerization
Step-growth polymerization usually involves two different functional groups reacting with each other with the elimination of a low molecular weight substance such as water. In step-growth polymerization, cross-linking can occur alongside polymerization, forming a three-dimensional network.
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 polymer's properties, as it affects, for example, the forces between the molecules.
Molecular weight
The molecular weight of a polymer is usually very high, as it is composed of a very large number of mers. The so-called degree of polymerization indicates how many mers are linked together. If we take (poly)isoprene (monomer molecular weight = 68) as an example, a polymer with a degree of polymerization of 10,000 has a molecular weight of 68 x 10,000 = 680,000. Most polymers, useful as plastics, rubbers, and fibers, have molecular weights between 10,000 and 1,000,000.
Recipe - how it works
Rubber is a polymer produced through a process called vulcanization. However, to vulcanize rubber and simultaneously give it specific properties, several different components are required, which together form a recipe. Each recipe is unique and varies depending on the desired properties as well as the manufacturing process to be used during the vulcanization process itself, for example, injection or compression molding.
But fundamentally, you always start with some basic components in the recipe structure, all of which have different tasks.
Raw material
The base for rubber is usually natural rubber or synthetic rubber such as butadiene rubber or styrene rubber. Natural rubber is extracted from the latex sap of rubber trees, while synthetic rubber is oil-based and produced industrially.
Fillers
Fillers are used to reinforce rubber and improve its mechanical properties. Common fillers include carbon black, chalk, and kaolin. Fillers are often added to increase rubber's stiffness, abrasion resistance, and tear strength.
Aging protection
Stabilizers are used to prevent the degradation of rubber by protecting it from oxidation or thermal degradation. Common stabilizers include antioxidants and antiozonants. These help to protect the rubber and increase its lifespan.
Plasticizer
Plasticizers are used to increase the flexibility and softness of rubber. This results in both a softer final product and also helps to enable the molding and processing of the rubber. Plasticizers can be different types of oils or waxes that reduce friction during mixing and processing.
Vulcanizing agent
Vulcanizing agents are used, as the name suggests, to vulcanize rubber. The most common vulcanizing agents are sulfur, peroxides, and metal oxides. The vulcanization process itself involves heating the rubber to a certain temperature, which leads to the formation of sulfur bonds that give rubber its elastic properties.
By combining and dosing the right amount of the different base components, the rubber gets different properties such as elasticity, flexibility, wear resistance, and chemical resistance.
The recipe structure for rubber can vary depending on the type of rubber being manufactured and the specific properties disired.
Crosslinking & Properties
Crosslinking means that the chains of polymer molecules are tied together at multiple points to form a three-dimensional network. The purpose of crosslinking is primarily to prevent the molecular chains from sliding over each other during deformation, i.e., to increase the elasticity of polymer materials and reduce their plasticity or viscous deformation. Crosslinking is mainly used for rubber and thermosets, but also occurs to a limited extent for modifying thermoplastics and fiber materials.
The effects of cross-linking can be briefly summarized as follows:
1. As already mentioned, the cross-links prevent the molecular chains from sliding over each other under load, meaning the material's elastic deformation increases and its plastic or viscous deformation decreases. This also applies to long-term loading, and therefore cross-linking reduces tendencies for creep and relaxation.
2. With increasing cross-linkages, i.e., with increasing cross-linking density, the mobility of the molecules gradually decreases. This leads to an increase in hardness and stiffness.
3. As the number of cross-links increases, the polymer's solubility in solvents decreases. Above a certain cross-linking density, the polymer becomes insoluble. However, it still swells, but the swelling decreases with increasing cross-linking density.
4. Permeability to liquids and gases decreases with increasing crosslinking density.
5. Mechanical damping decreases with increasing crosslinking density.
6. Fatigue properties deteriorate with increasing crosslinking density.
7. At the same time as hardness and stiffness increase with rising crosslinking density, the material's ductility or elongation at break decreases.
Chemical cross-linking of linear polymers
In a large number of cases, especially concerning 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, a substantially linear polymer is produced. This is plastically formable upon moderate heating. After molding, which is usually carried out in the rubber industry, the shape of the part is fixed by introducing chemical cross-links. Technically, this is done by mixing the polymer before molding with the chemicals that will cause cross-linking and other additives, after which the mass is plastically molded. The molded part is then subjected to elevated temperatures, usually in the range of 100-200 °C, whereby cross-linking reactions occur.
The process usually occurs under pressure to avoid blistering in the product. A general problem in this context is that the mass must not be exposed to temperatures high enough to initiate the cross-linking reaction during processing.
Even with very slight cross-linking, the material loses its ability to be shaped plastically.
Just as the cross-linking potential of the molecular chains is never fully utilized, the reaction potential of the curing agent is also never fully utilized. Certain vulcanizing agents can yield a cross-linking yield approaching 100% of the theoretical value. This applies, for example, to peroxides, whereas sulfur, for instance, sometimes yields no more than 20% of the theoretical yield.
Rubber vulcanization
Previously, it was discussed how the increase in the number of crosslinks affects the property profile of the crosslinked polymer. For example, it was stated that modulus and hardness increase, swelling and solubility decrease, creep and relaxation decrease, and fatigue resistance decreases. Furthermore, it can be shown that wear resistance increases with a moderate increase in crosslinking density. Tensile strength first increases, but then passes through a maximum and decreases again.
The vulcanization process is typically studied by vulcanizing the material at a specific temperature for a number of different times and plotting the properties as a function of vulcanization time. This approach reveals the effect of the increase in cross-links during the vulcanization reaction.
In peroxide vulcanization of rubber, the resulting cross-link density is directly proportional to the amount of peroxide used within a fairly wide range. In sulfur vulcanization of rubber, the cross-link density can also be influenced by varying the sulfur content. However, there is usually no direct proportionality between these two factors. Typically, sulfur contents of up to approximately 3–3.5% are used. As the sulfur content increases further, the strength properties initially deteriorate, only to increase again at very high sulfur levels. At the same time, the cross-linking density has become so high that the molecular segments lose their ability to move. The material then loses its rubber-like character. Instead, a hard, thermoset-like material is obtained, known as hard rubber or ebonite.