What are Plastics?
When we talk of a substance being “plastic” (the adjective) most people would agree on its meaning. It can flow or be moulded, it is ductile or it can be shaped, but when we turn to defining “plastics” (the noun) we have problems. Almost any simple definition will exclude materials which everyone would agree should be included so we have to turn to scientific terms and start with the comment that plastics are all polymers (poly = many).
Natural polymers have been with us since time began but synthetic polymers are much more recent, their origins being traced to Alexander Parkes and his exhibits of Parkesine at the International Exhibition of London in1862. Natural polymers include shellac, tortoiseshell and horn, as well as many resinous tree saps. All these have been processed with heat and pressure into articles such as hair combs and items of jewellery for many centuries.
A polymer is simply a very large molecule made up of many smaller units joined together, generally end to end, to create a long chain. The smallest “building block” of a polymer is called a monomer (mono = one) and if all the monomers are chemically the same, then that polymer is called a homopolymer.
Monomers generally contain carbon and hydrogen with, sometimes, other elements such as oxygen, nitrogen, chlorine or fluorine. Perhaps the most common example of a synthetic homopolymer today is polyethylene or “polythene” whilst other common materials are polypropylene, polystyrene and poly(vinyl chloride), more commonly know as “PVC”. Sometimes two monomers are used, monomer “A” and monomer “B” which join together chemically in an alternating sequence: -A-B-A-B-A-B- etc. to give a copolymer. Examples of this type are the nylon family, the most common member being called nylon 66 for the simple reason that “A” and “B” both contain 6 carbon atoms in their respective monomers. From here on there are a number of possibilities. “A” and “B” could couple randomly, short chains of “A” could couple to short chains of “B” and there is even the possibility of the inclusion of a third monomer, monomer “C”. These can be very simply visualised by thinking of Christmas “paper chains” with each link being one monomer and different colours used to illustrate each type. Another example would be the classic plastic necklace made of “poppets”.
Polymers are divided into two distinct groups: thermoplastics and thermosets. The thermoplastics are those which, once formed, can be heated and reformed over and over again. This property allows for easy processing and facilitates recycling.
Thermosets cannot be reformed or remoulded. Once these polymers are formed in a particular shape, that is it! The earliest of all synthetic plastics, Parkesine, was of the former type whilst the first of all truly synthetic plastics, Bakelite, belongs to the latter category. Thermosets differ from thermoplastics chemically in that heating the former introduces a three-dimensional network to the long chains so that they are no longer able to flow freely past one another like they can in the case of the thermoplastics.
Whilst all plastics are polymers, not all polymers are plastics. Rubbers, more properly today called “elastomers” are also polymers and everything written above about the structure of plastics applies to elastomers. Whilst everyone would claim to be able to distinguish between a rubber and a plastic, there are many plastics on the market which have some elastic character and the distinction becomes blurred at the interface. Plasticized PVC is elastic enough to be used in applications where rubbers would be an equally valid choice whilst the Kratons are polymers of the (“A-A-A-A-A-A-” coupled to “B-B-B-B-B-B-B-”) type where the “A”’s are plastic segments and the “B”’s rubbery ones. These behave as thermoplastic materials on heating but like vulcanized rubbers at ambient temperature. Raw elastomers are of course thermoplastic but become thermosets after vulcanization.