What are plastics?


What are plastics?

All plastics are composed of carbon atoms connected to one another in a chain-like structure, with mostly or exclusively hydrogen atoms attached to the carbon atoms.   Gasoline, candle wax and plastics all have this same structure; gasoline has about 8 carbon atoms connected together, candle wax about 30 carbon atoms and a plastic on the order of 10,000 carbon atoms.  Plastics, when stretched, will stretch some without breaking but also do not return to their original dimensions when the stretching force is removed.   Some plastics stretch a great deal without breaking, e.g. plastic wrap, while some do not, e.g. plexiglass.  This stretching property is because of the long carbon atom chain. 

How are plastics made?

Carbon Monomers and Polymers - Practice 1For most plastics, oil or natural gas is processed, with some chemical reactions occurring, to make what are called “monomers.”   Monomers are small molecules (the smallest being 2 carbon atoms) which in turn are chemically reacted to make “polymers.”  In other words, monomers are the individual units that are linked together to make the polymer chain.

For a molecule to be used as a monomer, this molecule usually has one of two characteristics.  Either the monomer has (1) a double bond between carbon atoms or (2) at both ends of a short carbon chain there is a functional group that can react with a complement functional group that is connected to the end of another short carbon chain; this second carbon chain also has the complement functional group at both ends. Regarding (1), ethane has two carbon atoms connected together but is not a monomer because it has a single bond between the carbon atoms with three hydrogen atoms connected to each carbon atom.   Ethylene has the same structure at ethane, but ethylene is a monomer because in this case only two hydrogen atoms are connected to each carbon and a double bond exists between the two carbons.   Polyethylene (#2 and #4 in recycling numbers, milk jugs and clear plastic wrap respectively) is formed from the polymerization of ethylene monomer, i.e. the sequential chemical reactions that create a long chain of monomers.   For case (2), ethanol (yes, the stuff some us drink!) has two carbon atoms connected together with one of the hydrogen atoms replaced with a hydroxyl group (a hydrogen atom bonded to an oxygen atom).  Ethanol is not a monomer, but if you put an alcohol group on the other carbon as well, then you have ethylene glycol, which is one of the monomers used to make polyethylene terephthalate (#1 in recycling number, most clear plastic beverage containers).   

The fundamental determination of the cost of a plastic is the monomer cost; even for the cheapest plastics the cost of the monomer is ~75% of the total cost of the polymer.  The cheapest monomers then are those that are the least costly to produce from natural gas or oil.  

Monomers will usually not spontaneously react, so heat, pressure and/or some other chemical compound are necessary to start the reaction.   To profitably make plastics, large facilities are required to achieve economies of scale.   Large companies are also much more capable of handling the substantial safety requirements.   Many polymerization reactions are prone to explosions; many of the major industrial accidents that result in loss of life in the US are a result of something going wrong in a polymerization.   Monomers themselves are also typically hazardous to human health, and oftentimes a dangerous chemical release involves a monomer.   Most plastics do not contain any monomer in the final product. 

Some facts and figures about the use of plastics

The graph below illustrates the growth in population relative to the growth in the per year production of plastics; both are for the entire world.   The graph shows that over this time scale plastics production is growing exponentially while population is growing linearly.  To put some numbers on the graph, in 2018 about 100 pounds of plastic/per year per person were produced worldwide; in 2030, if both graphs can be extrapolated, that number will be 125 pounds.  Developed countries use plastic at a much higher level than underdeveloped countries.  In 2015, the amount of plastics used in North America and Western Europe was ~300 lbs per person/per year.  A direct correlation exists between per capita GDP and plastics use, and I suspect (but cannot verify) that the world per capita GDP is growing in a manner more similar to the growth in production of plastics as opposed to the growth in population.

To a good first approximation, plastics were invented by humans (as were aluminum* and steel for example).  Most plastics that we use today were discovered between 1930 and 1960, and there was a great deal of serendipity in their discovery.   If all plastics were invented by ~1960, why such a slow rise in use?   I can’t pinpoint one thing, but some factors were better control/processes for polymerization reactions, improvements in processing plastics into usable forms, better control of polymer chain characteristics because of better understanding of the procedures used to synthesize these molecules (involving polymer length, and how the carbon atoms are hooked together, for example), and a better understanding of what shape of objects work best for a given application (examine a 2-liter soda bottle from the 1980’s vs. today!).  

Why are plastics used?

The first large scale use of plastics was in electrical insulation for wires and tires (yes, I am leaving out nylon stockings!).   I believe these are just about the only two applications where another type of material could not, in theory, be used.   For example, cars were once made without any plastics.   Now, plastics comprise about 25% of the weight on average of a new car (and much more than 50% of the volume!).   Automobiles are of course just one of the countless applications where plastics are used.   The question is, why?  The primary reason is cost.  Metals and glass cost less to manufacture per pound but many more pounds of these materials are needed for most applications where plastics have replaced metal or glass.  Fewer pounds are required both because plastics are less dense, and because plastics are more easily formed with thinner walls.  

This reduction in the cost of producing goods has led to substantial changes in the way humans live.  To pick one example; metals and refrigeration allowed for almost all food to be transported across large distances to its final destination, think of refrigerated cars on trucks or trains.   Of course, in some cases, the food transported was of substantially lower quality than fresh food and/or higher spoilage rates resulted.   Although changes in agricultural methods, e.g. using fertilizer to increase productivity, have certainly had a substantial impact, the ability to inexpensively put food into plastic containers has allowed for food to be shipped much more cheaply all over the world.  Improved and cheaper shipping of food has been an important contributor to improvements in people’s lives, especially in the developing world.    Other examples of how plastics have changed the way humans live exist in medicine, transportation, communication, and virtually every aspect of our lives.

The afterlife of plastic

The low cost of plastics has made them pervasive in modern life. Because plastics are so inexpensive and the cost of disposal is relatively low, there is very little disincentive to just throwing plastic materials away when we are done using them—packaging materials and shopping bags are obvious examples. With an increased emphasis on minimizing human impacts on nature, many people now see plastic waste as a major environmental problem, and even prompted the idea that plastic waste will get taken up in geological processes to become a marker of a human-dominated geological epoch, the Anthropocene (see the previous post on this blog).

In fact, when exposed to the elements, most plastic waste will eventually decompose back into carbon dioxide and water in a process that takes hundreds of years.  However, such timescales are far too long with regards to environmental issues. Plastics in waterways and oceans is currently the most visible environmental issue with plastics.   Plastic waste that is not either recycled, incinerated or in landfills is actually a very small percentage of what has been thrown away, but because the volumes are so large, a small percentage of the total is still a great deal of material.  Cumulatively about 9% of all plastic has been recycled, 12% has been incinerated and 79% still exists somewhere on Earth, the vast majority in landfills.  Some plastic waste does escape proper disposal, and researchers have tracked its sources with respect to material that ends up in the ocean.   About 60% of the plastic waste in the ocean comes from Southeast Asia, the amount from Europe and North America is about 2% of the total.   Developing countries contribute the vast majority of the remaining 38%.  Countries that contribute the most to ocean plastics are not the countries that use the most plastic by any means; the difference is the much less developed waste-disposal infrastructure in the former. 

So why is more plastic not recycled? If all plastic waste from a household were gathered together, melted, and made into a plastic product then the resulting product would be black and extremely brittle, and therefore not very useful.  In order for the product that results from recycling to be essentially equivalent to as originally synthesized material, two things would be needed. First, the collected waste would have to be perfectly clean (e.g. all non-plastic material removed) prior to final processing.  Second, the collected waste would have to be perfectly sorted into different types of plastic. The former requirement disqualifies many plastic products, because color is a contaminant.  The latter requirement represents a good portion of the cost of current recycling efforts.  Even with those constraints a market does exist that makes recycling economically viable.  Incompletely cleaned recycled material, if close to perfectly separated, can be used to make a product at about half of the cost of virgin plastic.   Increasing the rate of recycling therefore depends on improving the infrastructure and processes for handling plastic waste as well as manufacturers adopting strategies to simplify recycling of their products. 


*Aluminum essentially does not exist on Earth in its elemental form. However, aluminum is the most abundant metal in the earth’s crust, and in the 19th century human beings invented processes to manufacture elemental aluminum. [back]

5 thoughts on “What are plastics?

    • Great connection, Pete! As noted in our discussion, this reinforces the idea of plastic as an “emblem” of the Anthropocene (at least as much of one as the other indicators of the “Great Acceleration”).

  1. The timescales of the graphs in the paper are very different. World population, for example, is clearly a hockey stick (the mathematical term is exponentially increasing!) on the timescale given in the paper, but is not on the 60 year timescale I used in the plot. To overlay on my plot what you would like to see over this timescale is pretty easy; give me a few categories and I will be happy to do it and we can see if something is a hockey stick on this timescale.

  2. nice post for the plastic, Plastics come from the Greek word plastic skis, which means to form or to mold. However, plastics do not have a uniformly defined term. Some people define plastics in a narrower sense in which they focus on specific properties, others take a broader approach that includes various manufacturing processes. Plastics materials are composed of large molecules (polymers) that are synthetically made or modified.

  3. You are certainly correct in terms of the definition of plastics. Are automobile tires plastic? In terms of my post “synthetic polymers” is really what I am referring to.

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