How is an elastic material synthesized? Process summary

Elastic materials are something that is present in our daily life. There are them for everything, such as elastic bands to tie bags, rubber bracelets, balloons, tires ...

Then let's see how an elastic material is synthesized, explaining what its components are, polymers, in addition to indicating their molecular properties and some indices that are taken into account in the industry.

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What are elastic polymers?

Elastic materials, known as elastic polymers, are those that can be deformed by applying a force while it is applied. As soon as the elastic object is no longer subjected to this force, it will return to its original shape. Otherwise, if the material is permanently deformed, we would not speak of something elastic, but of a plastic material.

Elastic materials have been known to humans since time immemorial, since they exist in nature. However, even though polymers are naturally present in objects like rubber, the human being has seen the need to create some of them synthetically, that is, in the laboratory.

Some examples of elastic materials, apart from the one already mentioned, we have elastic bands to close food bags, balloons, rubber bracelets, latex ...

What are polymers?

Polymers are macromolecules formed by the union of covalent bonds of one or more of the simple units, which would be the monomers. Normally these macromolecules are organic, that is, they contain carbon atoms in their structure. These chains are usually long, and are linked by Van der Waals forces, hydrogen bonds, and hydrophobic interactions.

One way to classify polymers is based on their mechanical response to elevated temperatures. That is why there are two types of polymers.

1. Thermoplastic polymers

Thermoplastic polymers soften when subjected to high temperatures, even getting to melt. When the temperature is low they harden. These processes are fully reversible and can be repeated over and over again.

However, if a very high temperature is reached, irreversible degradation can occur, since the Molecular vibrations between the monomers of the substance are so violent that they can break their bonds covalent.

These materials are normally manufactured with simultaneous application of high temperature and pressure. When the temperature increases, the strength of the secondary bonds weakens, facilitating the relative movement of the chains that make up the polymer.

Most linear polymers and those with branched structures, with flexible chains, are thermoplastics, which are soft and ductile.

2. Thermoset polymers

Thermoset polymers are those that remain hard regardless of how much temperature is applied to them.

When they begin to be subjected to heat, covalent crosslinks occur between the contiguous molecular chains. Due to this, the movements between the polymer monomers are limited, preventing their vibration and rotation. However, if the temperature is excessively high, the cross-links are broken and polymer degradation occurs.

Thermoset polymers are generally harder compared to thermoplastics. Some examples of polymers of this type are in epoxy, vulcanized rubber and phenolic polyester resins.

How are elastic materials synthesized?

Elastic materials are made of elastomers, which are generally thermoplastic polymers, which gives them their main characteristics: easy but not permanent elasticity and deformation.

There are many substances that make it possible to make an elastic material. Some of the polymers that are used to synthesize elastics are: polyol-polyester, polyisocyanate, copolymers of ethylene and propylene, polyisobutylene, polysulfides and polysiloxane, just to say a few many.

When these substances are mixed, they react with each other through different polymerization mechanisms., among which are the condensation, the addition or the free radical pathway.

Molecular characteristics of elastomers

For the combination of certain polymers to ultimately generate an elastomer or elastic material, it is necessary that the combination of them make some kind of synergy, resulting in something greater than the simple sum of its parts.

The first requirement is that they have asymmetric structures and, therefore, that they are as different as possible. Their structures at the molecular level must be linear and flexible, allowing, as we already mentioned with thermoplastic polymers, that the chains of molecules can vibrate without breaking bonds.

As a second requirement is the that the polymer is not very polar, that is, that it does not have too much charge of one or the other sign, since if this is the case, the intermolecular interactions will be stronger and there will be greater rigidity due to the attraction (as happens with a positive magnet with a negative one).

The third requirement is that these polymers are flexible, that admit some deformation when some type of force is applied to them. If these polymers meet these three requirements, then the perfect situation will be generated for the synthesis of an elastomer.

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Synthesis of elastomers

The polymers that will result in an elastomer must be subjected to a series of physical and chemical processes.

1. Crosslinking

In this process it is achieved that the molecular chains are united with each other by means of bridges, which are capable of forming two or more strong covalent bonds.

These molecular bridges allow the elastomer to roll on itself when it is in rest or static mode, while, when subjected to some type of stretching, it could be in elastic mode thanks to the flexibility of these links.

2. Vulcanization

Although it is a process that would be found within the crossovers, it is interesting to mention a more detailed explanation separately.

Vulcanization is one of the best-known processes for obtaining elastomers. In this process, polymeric chains are interconnected by sulfur bridges (S-S-S ...).

3. After obtaining the elastomer

When the elastomers have already been synthesized, the next steps consist of subjecting them to different treatments to give them certain characteristics.

Each material will be used for a different purpose, that is why it will also receive various treatments, among which can be found heating, molding or other types of physical curing, that is, giving them shape.

It is in this phase of the process where pigments are added to give color to the resulting elastic object, in addition to incorporating other chemicals that will ensure its elasticity. It is also at this stage that three fundamental aspects are evaluated to ensure that the elastic material is of quality: Young's modulus, glass transition temperature (Tg) and limit of elasticity.

Young's modulus is an index that indicates how an elastic material behaves according to the direction in which a force is applied.

The Tg is the temperature at which a thermodynamic pseudotransformation occurs in glassy materials. The polymer decreases its density, stiffness and hardness at that temperature. This can be seen in glass and amorphous inorganic materials.

The yield point refers to the maximum stress that an elastic material can support without becoming irreversibly deformed.

Having checked these indices and seeing that the elastomer is functional, this is when it is usually called rubber of all kinds: silicone, nitrile, urethane, butadiene-styrene ...

Some stretchy materials

Next we are going to see some elastic materials and what they are made of.

1. Polyester

Polyester is a manufactured fiber, and it is composed of any polymer of synthetic origin that is long chain. In this polymer about 85% of the compound is a terephlalic acid ester.

2. Nylon

Nylon is an artificial polymer, belonging to the group of polyamides. It is generated by the polycondensation of an acid such as a diamine. The best known is PA6.6.

3. Lycra

Lycra is a synthetic fiber known for being a very elastic and resistant substance. It is a urethane-urea copolymer, made up of about 95% segmented polyurethanes. In its elaboration, a great variety of raw materials are mixed, such as prepolymers, which constitute the main structure of this fiber.

Bibliographic references.

• Hate G. (1986) Introduction to Synthesis of Elastomers. In: Lal J., Mark J.E. (eds) Advances in Elastomers and Rubber Elasticity. Springer, Boston, MA