A long chain
The plastic materials, like many other molecules of organic chemistry, exist thanks to the remarkable characteristics of the carbon atom to bind to other carbon atoms to form long chains.
Each carbon atom has four valences, as chemists say, that is it is able to bind up to four other atoms.
In a chain of carbon atoms, each of them has 2 valences involved in bonds with its similar, as in a train, each car is preceded and followed by another car. Of the total 4 valences, 2 remain available to bind to the chain other atoms, or functional groups, which give different characteristics to the overall molecule .
The starting point is to create a plastic material having a certain amount of monomer.
The name monomer identifies a generic organic substance (a substance in which a certain amount of carbon atoms are linked to each other) in which, thanks to some chemical process, it is possible to force the individual molecules to break some bonds between internal atoms and form bonds with adjacent molecules. In this way, the individual molecules of the monomer remain ringed with each other to form the chain that we can now call the polymer.
To clarify things better, consider a particularly simple polymerization reaction: the one that gives rise to the formation of polyethylene, one of the most common polymers.
n CH2=CH2 → [-CH2-CH2-]n
What you see in the image on the left is a representation of ethylene, the monomer that is used to create this polymer. In the Ethylene molecule there are two carbon atoms linked by a double bond (thus each carbon has two valences engaged with two hydrogen atoms and two with the other carbon).
In the polymerization reaction, one of the two links is broken so that each carbon has a valence available to bind to the carbon of another monomer, which has suffered the same fate.
This kind of polymerization is said chain polymerization.
There is another very important type of polymerization, the polycondensation. In this type of polymerization the union between the monomers takes place by means of some functional groups between which takes place a reaction that ends with the binding of the two groups and the release of a molecule of low molecular weight, such as a water molecule.
Part of the polymers obtained by polycondensation are the polyamides. The polyamide is a very important material, commercially available under trade names such as Nylon, Kevlar or Nomex.
Most of the polymers are categorized into two major families: the thermoplastic polymers and the thermosetting polymers.
- The thermoplastic polymers are polymers formed by linear or little branched chains, not linked with each other. As their name suggests, it is sufficient to raise the temperature to bring them to a viscous state in order to shape them.
- The thermosetting polymers are polymers in which vice versa, during the polymerization process, are created some transverse chemical bonds between a polymer chain and the other. This gives the material superior mechanical characteristics, hardness, tensile strength, etc.. but generally makes the material more brittle. In addition, this kind of plastic is no longer meltable since even a substantial energy intake would not allow the polymer chains to slide over each other giving plasticity of the material. The chains are retained by the cross bonds and the heat would degrade both the cross-links and the polymer chains themselves, leading to the carbonization of the entire mass.