Plastics are very versatile materials that we use every day. Understanding plastics materials help us to select the right one for a given application and it also helps to develop improved plastic materials. Unfortunately, plastic training is uncommon and most of the people working with plastics have had no formal training. Furthermore, plastic training is usually in the form of a degree course which means it takes years and involves a great deal of theory.
I wanted to help the masses of people who need to understand plastics in a quick and easy way. This page aims to explain plastic materials in the simplest terms possible. Sometimes people ask whether I am “dumbing it down” for them because I have a PhD in plastics and the explanations are so simplistic. They are amazed to learn that I am not making it simple for them and that I actually use these same analogies every day when I am conceptualizing and designing new materials for the Fortune 500. Many years ago, my professor showed me that real experts are able to explain complex topics in a simple way whereas false “experts” hide behind long words and complex theory. So, if you’re ready to understand plastics in ten minutes, let’s get started. It’s much simpler than you expect! First, we explain some concepts in simple terms and then (below) there’s a free 1-hour video with even more information that expands on what you’ve learned.
Polymer Chains are Very Long
A polyethylene chain made from 10,000 monomer units joined together would be about 2 Ångströms across and 25000Å long (2.5μm). An Ångström is one ten billionth of a meter, which is too small to imagine, so let’s go back to our spaghetti analogy. If the polymer chain were as thick as a piece of spaghetti, how long would the spaghetti strand have to be in order to have the same proportions as a polymer chain? The answer is about 25 meters (or 25 yards) long. So, visualize a piece of spaghetti as long as two school buses and you have the right relative dimensions. It’s easy to understand why polymer chains get tangled up so easily. If the polymer chain were a strand of human hair then the hair would be about 3 feet long. I think anyone who has had hair that long knows how easily it gets tangled up. Polymer chains can be even longer. For example, ultra-high molecular weight polyethylene has a molecular weight of several million Daltons such that the molecule is centimeters long.
Some polymers, like polycarbonate, are famous for excellent impact resistance. Others, like polystyrene are hard and brittle so they can easily break when dropped. Compared to the other properties, impact resistance is a little harder to understand and predict. As a rule of thumb, polymers with flexible chains resist impact well. When they are exposed to an impact, the chains simply move and then bounce back, like a rubber ball would. Rubbers (also called elastomers) are a type of plastic with very flexible chains. Polystyrene has stiff chains that cannot deform when dropped so the only way for them to deal with the energy is for the chains to break and the part shatters.
Based on the above discussion, it’s clear that anything that makes the polymer chains more flexible will help impact resistance. For example, adding plasticizer or increasing the temperature. It is also possible to add rubber particles to a brittle polymer. That is commonly done. High impact polystyrene is simply polystyrene with tiny rubber particles added to improve impact performance. ABS is just SAN plastic with rubber particles dispersed inside of it. Lego® bricks are made of ABS for example.
On a side note, you will hear people talking about “impact strength” but that is a misnomer. Impact resistance is the energy needed to break the sample whereas strength is the force needed to break the part. Therefore it is incorrect to say “impact strength” even though almost everyone does.
Plastics and the Environment
Plastics are materials that have changed and improved our lives in innumerable ways. It’s hard to imagine life without them. For all the positives they bring, many people assume that plastics are a “necessary evil”. However, it turns out that plastics do not deserve that reputation at all. Life cycle analysis (LCA) has shown that plastics are actually greener than traditional materials like wood, metal and glass. Similarly, people assume that plastics use a great deal of oil in their production when the facts show the opposite. Although oil is indeed used to make plastics, the net affect of using plastics is to reduce oil consumption. How is that possible? Plastics make automobiles lighter which improves mpg and thereby reduces oil usage. Similarly, plastic insulation is very effective which reduces the amount of oil needed to heat our homes. So, although 4% of oil is used to make plastics, the two effects just mentioned combine to result in a reduction of oil usage. Of course, plastics can contribute to litter but that is not the fault of the plastics. Plastics do not throw themselves on the floor or in the ocean. People do that. Anyone blaming plastics for litter just needs to look in the mirror to see whether the real problem is.
Biodegradable polymers is in vogue at present but it should be remembered that PHB, PHA and PLA have been around for a long time (decades) and have gained very little commercial traction. Such polymers tend to have a worse cost to performance balance and they turn out to be less green that synthetic polymers like PE. I recently read an article saying that PHB would be the next big thing. I had to point out that it was first introduced by ICI in the 1980s and it still has not taken off for the reasons just described.
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