Phil’s Finding’s #13: Plastic Fantastic
22 August 2011
Phil Howes, the Institute of Making’s post-doc researcher, shares his thoughts and findings as he delves into the sensoaesthetic world of materials. Check this space every Monday for Phil's latest posting.
The first man made plastic was patented by Alexander Parkes in the UK in 1856. Since then, plastic has gone on to become a dominate force in our material world, being used in everything from spectacles to spaceships. They can be moulded, cast, blown, pressed or extruded into sheets, films fibers, tubes, bottles, and much more. New plastics are still being actively developed, with the likes of biodegradable plastics looking to improve the environmental credentials of the material. The versatility and durability of plastics is rooted in their molecular make-up, and the way in which these molecules conduct themselves in each other’s company.
Many solid materials, like metals, have their atoms neatly arranged like stacked spheres with regular spacing and arrangement. In plastics, however, the atoms are arranged in long molecular chains which tangle and twine around each other like spaghetti on a plate. When plastics are hot, they are soft because these chains are able to twist and turn, and slip past each other. However, as the plastic cools, the chains no longer move around, and are happy to lie with each other in a big tangle. This tangled arrangement is what gives plastics their strength, because the long molecules are all stuck together and unable to move. Some polymers can form a partially crystalline arrangement, however the packing of the chains is much more like the threads in a blanket than the neat stacking of spheres in a metal crystal.
The long strings of atoms are called polymers, and they are the fundamental basis of all plastics. Polymer chains are composed of small repeating units referred to as monomers, usually stretching into the thousands. The polymer backbone usually consists of carbon atoms bound together in a chain, with elements like hydrogen, oxygen, nitrogen, chlorine and sulphur bound to it to give a vast variety of different structures and chemistries. The properties of a polymer can be finely tuned by changing the shape, size and atomic constituents of groups which hang of the side of the carbon backbone.