To synthesize a elastic material, First, you must have knowledge about what type of polymers make it up; since, otherwise, the elaboration of a plastic or a fiber would be formulated. Knowing this, the polymers that should be considered are those called elastomers.
So, elastomers make up elastic materials; But what are they? How are they different from other polymers? How to know if the synthesized material really has elastic properties?
One of the simplest examples of an elastic material is the elastic bands (or rubber bands) that tie newspapers, flowers, or a wad of bills. If they are stretched, it will be observed that they deform longitudinally, and then return to their original shape.
But, if the material is permanently deformed, then it is not elastic, but plastic. There are several physical parameters that allow discriminating between these materials, such as their Young's modulus, their elasticity limit, and the glass transition temperature (Tg)..
In addition to these physical qualities, chemically elastic materials must also meet certain molecular criteria to behave as such..
From here arises a wide range of possibilities, mixtures and synthesis, subject to endless variables; all this to converge on the "simple" characteristic of elasticity.
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As mentioned at the beginning, elastic materials are made of elastomers. The latter in turn require other polymers or smaller "molecular pieces"; that is, elastomers also merit their own synthesis from pre-polymers.
Each case requires a meticulous study of the process variables, the conditions and why with these polymers the resulting elastomer "works" and, therefore, the elastic material..
Without going into details, here are a series of polymers used for this purpose:
-Polyisocyanate
-Polyester polyol
-Ethylene propylene copolymers (i.e. blends of polyethylene and polypropylenes)
-Polyisobutylene
-Polysulfides
-Polysiloxane
In addition to many others. These react with each other through different polymerization mechanisms, among which are: condensation, addition, or via free radicals.
Therefore, each synthesis implies the need to master the kinetics of the reaction, in order to guarantee the optimal conditions for its development. Likewise, where the synthesis will be made comes into play; that is, the reactor, its type and the process variables.
What do all the polymers used for the synthesis of elastomers have in common? The properties of the former will synergize (the whole is greater than the sum of its parts) with that of the latter..
To begin with, they must have asymmetric structures, and therefore be as heterogeneous as possible. Their molecular structures must necessarily be linear and flexible; that is, the rotation of the single bonds should not cause steric repulsions between the substituent groups.
Also, the polymer must not be very polar, otherwise its intermolecular interactions will be stronger and it will show greater rigidity..
Therefore, polymers must have: asymmetric, nonpolar and flexible units. If they meet all these molecular characteristics, then they represent a potential starting point for obtaining an elastomer.
Having selected the raw material and all the process variables, the synthesis of the elastomers continues. Once synthesized, and after a subsequent series of physical and chemical treatments, the elastic material is created.
But, what transformations must the selected polymers undergo to become elastomers??
They have to undergo crosslinking or curing (crosslinking, in English); that is, its polymeric chains will connect with each other by molecular bridges, which come from bi or polyfunctional molecules or polymers (capable of forming two or more strong covalent bonds). The image below summarizes the above said:
The purple lines represent the polymer chains or the "stiffer" blocks of elastomers; while the black lines are the most flexible portion. Each purple line can consist of a different polymer, more flexible or rigid with respect to the one that precedes or proceeds..
What function do these molecular bridges play? That of allowing the elastomer wound on itself (static mode), to unfold under a stretching pressure (elastic mode) thanks to the flexibility of its links.
The magic spring (Slinky, for example, from Toystory) behaves slightly similar to how elastomers do.
Among all the crosslinking processes, vulcanization is one of the best known. Here, the polymer chains are interconnected by sulfur bridges (S-S-S…).
Returning to the image above, the bridges would no longer be black, but yellow. This process is essential in the manufacture of tires.
Once the elastomers have been synthesized, the next steps are to treat the resulting material to give them their unique characteristics. Each material has its own treatment, among which are heating, molding or grinding, or other physical "curing".
In these steps, pigments and other chemical substances are added to ensure its elasticity. Likewise, its Young's modulus, its Tg, and its elasticity limit are evaluated as quality analysis (in addition to other variables)..
It is here then where the term elastomer is buried by the word 'rubber'; silicone rubbers, nitrile, natural, urethanes, butadiene-styrene, etc. Rubbers are synonymous with elastic material.
Finally, a brief description of the elastic band synthesis process will be given..
The source of polymers for the synthesis of its elastomers is obtained from natural latex, specifically from the Hevea brasiliensis tree. This is a milky, resinous substance, which undergoes purification and is then mixed with acetic acid and formaldehyde..
From this mixture a slab is obtained, from which water is extracted by squeezing it and giving it the shape of a block. These blocks are cut into smaller pieces in a mixer, where they are heated and pigments and sulfur are added for vulcanization..
Then, they are cut and subjected to extrusion, to obtain hollow rods, inside which they will occupy an aluminum rod with talcum as support..
And finally, the rods are heated and removed from their aluminum support, to be squeezed one last time by a roller before being cut; each cut generates a garter, and countless cuts generate tons of them.
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