This is the seventh installment in Eric Larson?s series, "True confessions: A plastics engineer gets personal with recycling." You can read all of the previous columns, starting with part one, . Part eight will be published at the beginning of the new year.

In product design, there is a concept known as end of life, which may range from weeks to decades. Once a product has reached its end of life, using it becomes problematic: Warranties are over, customer service is no longer available, replacement parts become hard to find. The product itself may still be useful, but at some point the product is basically dead. That?s when you have a choice to make. You can scrap the product (toss it in the trash) or try to recycle it in some way. If the product was designed for disassembly, you can break it down into its component parts and re-use, recycle or re-process them.

This end of life is inherent in every product. Sadly, end-of-life planning is rarely addressed in the design phase. It is usually an afterthought, or not considered at all. The concept of recycling itself is fairly new. The Universal Recycling Symbol was not even developed until 1970.

There are different ways of recycling, and they go by different names: Upcycling, downcycling, long cycling, short cycling, etc. The key world, though, is cycle. A cycle is a series of events that are repeated in a circular manner, over and over and over. The design of the Universal Recycling Symbol was supposedly based on a Mobius strip, a closed loop known as a homeomorphic shape, also known as a bio-continuous function.

There are many biological cycles. Plants grow, drop seeds, die and decompose. The cycle is then repeated by new plants. Animals eat and mate, give birth, raise their young and then die and decompose (or get eaten). The cycle is then repeated by new animals. The same with people. With biological materials?wood, stone, fibers, bone, tissue?there are life cycles, as well. Ashes to ashes, dust to dust. With synthetic materials?steel, ceramics, plastics?the life cycle is often incomplete.

In 2018, U.S. production of polyethylene was . A metric ton is 1,000 kg, or 2,205 lbs. So that is 21.36 billion kilograms, or 47 billion pounds. According to the American Chemistry Council, in the United States in 2018, so polyethylene accounts for 41.5% of all plastics produced in the USA.

In the context of recycling, you can only re-use a plastic material so many times. There is a limit. With each melt cycle the material goes through, the molecular weight is affected; the average molecular weight goes down, and the distribution goes up. At a certain point, the material has reached the end of its useful life.

When a plastic material comes to the end of its useful life, there are basically three things you can do with it: Burn it, bury it or break it down.

I have no problem with burning polyethylene. Actually, I think incineration, done properly, is a great idea. After all, polyethylene is composed of two elements, carbon and hydrogen, with the chemical formula of (C2H4)n. Chemically, it is similar to paraffin wax, which has the general formula of CnH2n+2.

Throughout human history, fire has been a symbol of purification. It's part of the cycle of life. We light candles to celebrate the sacred, and to remember our loved ones. Why can?t we do this with polyethylene? Perhaps the plastics industry could create an ad campaign around this, feauturing the music of the Doors (?Come on, baby, light my fire?) or better yet, Johnny Cash, singing ?Ring of Fire.? I doubt that will happen any time soon.

I do have a problem with burying polyethylene in our trash. It?s what I call the Ostrich Syndrome. You?ve heard the story of how ostriches bury their heads in the sand to escape danger. The story is not based on fact?burying your head in the sand does nothing to protect you from danger. I would say the same thing is true about burying trash. Throughout history, humans have been digging holes to bury their trash. Out of sight, out of mind. As our villages got larger, so did the size of the holes we dug. We used to call them dumps, today we call them landfills. How many holes are we going to dig? Can we dig enough holes each and every year to store 47 billion pounds of used polyethylene? What about the remaining plastics? And all the other synthetic materials? Where are we going to put all of our trash?

The third option for a plastic material at its end of life is to break it down. One way to do this is via chemical recycling, turning polymers back into monomers or other basic feedstocks to be used in other areas, as fuel or in the polymerization of new materials.

You might even say that chemical recycling is the missing link in plastic?s cycle of life.

Image: Chan2545/Adobe Stock

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Eric R. Larson is a mechanical engineer with over 30 years' experience in designing products made from plastics. He is the owner of Art of Mass Production, an engineering consulting company based in San Diego, CA. Products he has worked on have been used by millions of people around the world.

Larson is also moderator of the blog site , where he writes about the effective use of plastics. His most recent book is Poly and the Poopy Heads, a children?s book about plastics and the environment. It is available on?.

» Publication Date: 20/12/2019

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