Youâ€™ve been told about the unusual property of carbon.Â It is one of the few elements can bond to itself. And Â because of that property, life developed (or in science, organic chemistry).Â But there is a similar reaction that can happen with molecules.Â Under the right conditions, molecules can bond to themselves to form long chains of molecules.Â These long chains of molecules are polymers.Â Common plastics, rubber, and silicones fit in this category.
So how does this work?Â Letâ€™s take a look at one common example, polyethylene.Â Â The starting block for polyethylene is ethylene.Â This is called the monomer and has the chemical structure of
In the above diagram, the H and the C each represent an atom of hydrogen and carbon. There is a bond between each carbon and solid line represents a chemical bond between the atoms.Â In the case of the carbon atoms, there is a double bond (equivalent to two single bonds) between the two carbon atoms.
You can get molecules of ethylene to react with each other.Â This forms a long chain of ethylene molecules.Â The quick chemical structure is shown below:
[Â -CH2-CH2-CH2-CH2-CH2-CH2- ] n
The Â â€œnâ€ shows the number of times the group is repeated.
I used polyethylene as a simple example.Â Many other polymers are possible, such as polyisoprene, polyvinyl chloride, polypropylene, acrylic, neoprene, silicones, etc.Â And that is before you talk mixtures of materials, such as copolymers, terpolymers, block copolymers, star branched polymers, and so on.Â You can look those up, if you want.
When you rub some polyethylene between your fingers, do you ever notice itâ€™s slightly waxy feel?Â Polyethylene is very similar in chemical structure to paraffin (candle wax). When polyethylene is analyzed by some techniques (infrared analysis, for example) it appears identical to polyethylene wax.
Polyethylene is a thermoplastic.Â Heat polyethylene and it will melt.Â Cool it down, it resolidifies.Â But since polyethylene melts at such a high temperature, polyethylene also oxidizes.Â The white plastic will start to turn yellow, and if enough heat is applied, it will turn black.Â In thermoplastics, as a cost saving, regrinding is often done.Â There is always scrap plastic that is reground, Â remelted, and reused in the molding process. Â Recycled plastics such as polyethylene are often black, since black hides a variety of sins (including oxidation).
There are other kinds of polymer.Â One other category is thermoset polymers.Â Think of these polymers as a raw egg.Â In the raw state, a raw egg is fluid and easily flows.Â However, if you heat the egg, it turns to a solid.Â Changing it back to a fluid is almost impossible.Â Vulcanization of rubber is one form of thermoset polymers.
These are basic principles of polymers, and yet are not understood.Â Here isÂ what I mean.
I was working in the quality department for a company that did both thermoplastic and thermoset work in the pharmaceutical industry.Â The new quality director had extensive experience with thermoplastics.Â There is a constant push for cost reductions. His idea: regrind the thermoset rubber and reuse it as a pharmaceutical polymer.Â The difference between thermoset and thermoplastic was explained to the quality director.Â His idea wouldnâ€™t work. Â But the quality director insisted, claiming a multimillion dollar cost savings.
Being relatively low on the totem pole, I was chosen to experiment with the reground rubber.Â The rubber lab, politely but mirthfully, gave permission to use their equipment. Â Â I was to remold the rubber, using presses and mills at different conditions. Â The result was as predicted.Â The rubber could not be used.
The directorâ€™s fallback position was to use a small amount of the reground rubber in the unvulcanized thermoset rubber.Â Â Which didnâ€™t work.Â For one, our regulatory department said the reground rubber would be a new ingredient, which is against FDA regulations.Â Â Then the added reground rubber dramatically changed the physical properties.Â It was no longer strong enough.Â Finally, adding the reground rubber changed the chemical extractables of the pharmaceutical compound, making it unacceptable for drug use.
The overall result?Â The most important, correct science was upheld.Â However, a multimillion dollar cost savings didnâ€™t happen.Â And the quality director was embarrassed. Which caused the quality director to quit and move to a new company.Â Â And the rubber chemists were amused for months.
As a scientist, be careful about what you claim.