What is the chemistry of Plastics?
CHEMISTRY OF PLASTICS Like all chemical compounds, plastics are made up of atoms of various elements linked together to form molecules. Unlike materials such as glass and steel which can, similarly to plastics, be moulded by the application of heat and pressure, the molecules of plastics are relatively long, chain-like structures. The basic chemicals, which provide the building blocks of the various plastics material, are relatively small molecules, and are known as monomers. Under the correct conditions in specially designed plants, these single molecules of the monomers are made to link together to form long chain-like molecules which may or may not have side branches. These materials are known as polymers, whilst the processes whereby they are produced are known as polymerisation. (note – the word “mer” is a Greek word that means “part”; “mono” means “one” and “poly” means “many; hence monomer and polymer). The monomers are held together in a polymer chain by very strong attractive forces between molecules; but much weaker forces hold the polymer chains together.
Plastics encompass a large and varied group of materials consisting of different combinations or formulations of carbon, oxygen, hydrogen, nitrogen and other elements. Most plastics are a solid in a finished form; however, at some stage of their existence, they are made to flow and may be formed into various shapes. The forming is usually done through the application, either single or together, of heat and pressure. There are over 50 different, unique families of plastics in commercial use today and each family may have dozens of variations.
It is becoming more common in modern technology to modify the properties of polymers by linking together, in various proportions, two monomers. This process is known as co-polymerisation.
MONOMERS: Examples of monomers are ethylene, styrene, vinyl chloride and propylene.
HOMOPOLYMERS: Homopolymers are polymers constructed from joining like monomers. Some examples of these are polyethylene, polystyrene, polyvinyl chloride (PVC) and polypropylene (there is also a copolymer version).
COPOLYMERs: Copolymers are polymers constructed from two different monomers. They can be of the alternating type such as ethylene acrylic, and ethylene-ethyl acrylate or of the graft types such as styrene-butadiene, styrene-acrylonitrile and some acetals.
TERPOLYMERS: Terpolymers are polymers constructed from three different materials. An example of a terpolymer is acrylonitrile-butadiene-styrene (ABS).
The two monomers in a copolymer are combined during the chemical reaction of polymerisation. Materials called “alloys” are manufactured by the simple mixing of two or more polymers with a resulting blending of properties which are often better than either individual material. There is no chemical reaction in this process. Some examples of “alloys” are Polyphenylene oxide –high impact styrene, polycarbonate-ABS and ABS-PVC.
MOLECULAR WEIGHT: It is important to know how long the polymer chains are in a material. Changing the length of the chains in a thermoplastic material will change its final properties and how easily it can be shaped. The number of the “repeating units” or molecular group in the homopolymer, the group of molecules in the copolymer and in the terpolymer is called the “degree of polymerisation”. If the repeating unit has a molecular weight of 80 and the chain or polymer has 1,000 repeating units the polymer has an average “molecular weight” of 80,000. The molecular weight is a way of measuring the length of the polymer chains in a given material. The molecular weight of plastics is usually between 10,000 and 1,000,000. It becomes increasingly difficult to form or mould the plastic with the application of heat and pressure as the molecular weight increases. A molecular weight of 200,000 is about the maximum for a polymer to permit reasonable processability. Some higher molecular weight materials, like ultra high molecular weight polyethylene (UHMW-PE), which has a molecular weight from 5,000,000 to 8,000,000, can be cast using processes specially designed to shape it.