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Thermosetting polymers

The word polymer refers to a macromolecule composed of multiple monomers or repeating units. The various properties of polymers depend on a wide variety of factors such as

● The monomer unit
● The forces (intramolecular intramolecular) between the monomers.
● The linkages between each monomer.

Polymers are mainly of two types - thermosoftening polymers and thermosetting polymers. In this article, we will be learning the properties of thermosetting polymers and the thermosetting process.

Thermosetting polymers

Thermosetting polymers do not soften upon heating and are cross-linked to one another. The presence of cross-links in the polymer hardens the overall structure. These cross-linkages are the reason why thermosetting polymers do not soften when heated. Cross-linking takes place where the individual polymers connect with strong bonds. These strong bonds cannot be broken easily by the simple application of heat.

One of the finest examples of thermosetting polymers is the vulcanized rubber. Rubber tapped from para rubber trees is a polymer of isoprene monomers. It is a running liquid that can be further processed to prepare latex, party balloons, gloves, and erasers. It can also be used to make bicycle and car tires though it has to be vulcanized first. For the vulcanization process, sulphur is added because of which the disulfide bridges link the polymers together. The presence of cross-linkages significantly increases its overall strength. Hence, it does not soften quickly upon heating.

The other examples of thermosetting polymers include certain kinds of solid glue and substances used in old TV sets.

Properties of thermosetting polymers

The general characteristics and properties of thermosetting polymers are as follows.

● Thermosetting polymers are generally much stiffer to colour than thermoplastic polymers. This is primarily because of          the presence of cross-linkages between different polymers.
● Thermosetting polymers are not transparent, unlike thermoplastics. Obtaining a transparent thermoset is quite                       challenging because of the 3D lattice occurring while setting the thermoset polymer.
● The cross-linking seen in thermosets makes them more brittle. As a result, they generally become more brutal and can           withstand high forces before breaking. However, while breaking, it does not undergo bending. Hence, in the presence of      sufficient force, the maximum tensile strength will break and does not stretch as a thermoplastic would.
  When it comes to costs, thermosets are more expensive compared to thermoplastics. The reasons for the high costs of            thermosets include:
● Thermosets are extremely used-case specific.
● Thermosets cannot be remelted and recycled. Hence, the processing of thermosets may be more complex.
● Thermosets generally have high resistance to chemicals which is much better than thermoplastics.

Process of thermosetting

The processing of thermosetting polymers involves three stages which are as follows.

Stage 1 - The first stage of the thermosetting process is referred to as resole. This is a stage where resins exist in a fusible and insoluble condition.

Stage 2 - The second stage of the thermosetting process is where a thermoset becomes partially soluble. The material begins to exhibit the characteristics and properties of a thermoplastic upon the reversal of changes.

Stage 3 - The third stage of the thermosetting process is crucial because this is when cross-linking occurs. In this stage, strong bonds are formed between different polymers, thereby giving rise to cross-linkages. This is a moulding phase that is carried out in the presence of controlled temperature and pressure. This phase yields the end product of the cross-linking reaction, i.e. thermoset.

The end product obtained from the cross-linking reaction between different polymers contains a 3D internal network structure. This internal network structure is characterized by the presence of highly cross-linked chains of the polymer. Therefore, at this stage of the thermosetting process, the product obtained cannot be deformed in the presence of heat.

The other methods of processing a thermosetting polymer are as follows.
1. Reaction injection moulding - Reaction injection moulding, also known as the RIM, involves a single step similar to the        above-described method. The only difference is that this process involves a polymerization reaction where a permanent        chemical bond is formed between different polymers.
2. Extrusion moulding is a thermosetting process used to make insulation for pipes, electrical cables, and fabric threads.
3. Spin casting - Spin casting is another type of thermosetting process mainly used for the manufacturing of figurines. This      method is also applied in fishing jigs and lures, replacement parts, and emblems as well.
4. Compression moulding is a special kind of thermosetting process that is primarily used to provide a proper shape to              SMC and BMC thermosetting polymers.

Advantages of thermosetting polymers

As mentioned above, thermosetting polymers are extremely used-case specific. Therefore, these thermosetting polymers have a wide range of applications and advantages. Below mentioned are the various advantages of thermosetting polymers.

1. Thermosets have excellent dielectric strength.
2. Thermosets are highly resistant to water and the corrosive effects of chemicals.
3. The tooling and setup costs of thermosets are economical.
4. The overall performance and strength-weight ratio of thermosetting polymers are significantly high.
5. Thermosets offer a wide range of choices of surface finishes and colour.
6. The production costs of thermosetting are highly economical.

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