Polyurethane is ubiquitous in contemporary life, a material that we are all very familiar with, even if unaware of its existence. From binding pigments within modern paints, providing cushioning in the soles of shoes, to giving structure to the infamous 1980’s shoulder pad, this polymer has found wide application. Such materials and objects are present in a variety of different forms, owing to the numerous polyurethane formulations that are available on the market today.
So, what is polyurethane? Well, in chemistry terms, a urethane is a generic name for the molecule formed via the reaction of an isocyanate and an alcohol:
R and R’ represent an organic molecule
N is nitrogen
C is carbon
O is oxygen
H is hydrogen
This type of reaction is called an addition reaction, where no smaller molecules are lost in the process; all of the original atoms are incorporated within the new material.
In relation to polyurethane we have many repeat units of the urethane group; poly being derived from the Greek word for many, polus. Polyurethane is formed through the polymerization reaction of the initial molecules above, but key to its formation is the use of isocyanates and alcohols that can react at both ends of the molecule, thus enabling many single units of the urethane group to join onto each other:
How can a manufactured, inanimate material exhibit unpredictable movement and lifelike qualities? How have artists and designers animated the inanimate, producing works that seemingly have neither defined form nor apparent structure?
This has been achieved through a process called nucleation, where gaseous bubbles of carbon dioxide develop within the polymer solution, which in turn causes the expansion of the polyurethane and the formation of a foam structure. As carbon dioxide is produced via a secondary reaction in the material, the pressure that each bubble imparts on it’s neighbor causes a matrix of voids, each taking on a shape reminiscent of an old fashioned football; a pentagonal dodecahedron. The expansion of the foam continues until both reactions have come to completion, with no further carbon dioxide being produced (Eq. 3) and the urethane fully polymerized (Eq 2).
Figure 1 shows a magnified image of the three dimensional structure of an open cell foam. The voids are caused by the agglomeration of gas bubbles during the reaction and separated from each other by thin struts of the host medium; polyurethane. One of the key features characterizing such foam is the very large surface area of the material, which is a key issue in relation to the stability; such a large amount of surface material in contact with the atmosphere accelerates the deterioration of the synthetic polymer.