How is an elastic material synthesized? Summary of the process

A summary of the steps involved in the synthesis of elastic polymers.

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

In the following we are going to see how to synthesize an elastic materialWe will explain what are its components, the polymers, in addition to indicating its molecular properties and some indexes that are taken into account in the industry.

What are elastic polymers?

The elastic materials, known as elastic polymers, are those that can be deformed by the application of a force while the force 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 mankind since time immemorial, since they exist in nature. However, although polymers are naturally present in objects such as rubber, human beings have found it necessary to create some of them synthetically, i.e. 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.which would be the monomers. Normally these macromolecules are organic, i.e. 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 according to their mechanical response to elevated temperatures. Thus, there are two types of polymers.

1. Thermoplastic polymers

Thermoplastic polymers soften when subjected to high temperatures, even melting.They may even melt. At low temperatures 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 covalent bonds.

These materials are normally manufactured with simultaneous application of high temperature and pressure. As the temperature increases, the strength of the secondary bonds weakens, facilitating the relative movement of the polymer chains.This facilitates 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. Thermosetting polymers

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

When they begin to be subjected to heat, covalent crosslinking occurs between adjacent molecular chains. Because of this, movements between the polymer monomers are limited, preventing vibration and rotation of the monomers. However, if the temperature is excessively high, the crosslinks break and polymer degradation occurs.

Thermoset polymers are generally harder compared to thermoplastics. Examples of such polymers are epoxy, vulcanized rubber and phenolic polyester resins.

How are elastic materials synthesized?

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

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

When these substances are mixed, they react with each other by different polymerization mechanisms.These include condensation, addition or the free radical pathway.

Molecular characteristics of elastomers

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

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

The second requirement is that that the polymer is not too polar, i.e., that it does not have too much charge of one sign or the other.If this is the case, the intermolecular interactions will be stronger and there will be greater rigidity due to attraction (just like a positive magnet with a negative one).

The third requirement is that these polymers should be flexibleThe third requirement is that these polymers be flexible, that they admit some deformation when some kind of force is applied to them. If these polymers meet these three requirements, then the perfect situation is created for the synthesis of an elastomer.

Synthesis of elastomers

The polymers that will ultimately result in an elastomer must undergo a series of physical and chemical processes.

1. Crosslinking

In this process the molecular chains are joined to each other by means of bridgeswhich are capable of forming two or more strong covalent bonds.

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

Vulcanization

Although this is a process that would be found within crosslinking, it is interesting to give it a more detailed explanation separately.

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

3. After obtaining the elastomer

Once the elastomers have 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, which is why it will also receive different treatments.The process can include heating, molding or other types of physical curing, i.e., shaping.

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

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

Tg is the temperature at which thermodynamic pseudotransformation occurs in glassy materials.. The polymer decreases in density, stiffness and hardness at this temperature. This can be observed in glasses and amorphous inorganic materials.

The yield stress refers to the maximum stress that an elastic material can withstand without irreversible deformation. that an elastic material can withstand without irreversible deformation.

Once these indices have been checked and the elastomer is functional, it is then usually called rubber of all kinds: silicone, nitrile, urethane, butadiene-styrene...

Some elastic materials

Here are some elastic materials and what they are made of.

1. Polyester

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

Nylon

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

3. Lycra

Lycra is a synthetic fiber known to be a very elastic and resistant substance. It is a urethane-urea copolymer, consisting of about 95% of segmented polyurethanes.. A great variety of raw materials are mixed in its production, such as prepolymers, which constitute the main structure of this fiber.

Bibliographical references.

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