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February 10 - February 17

Question of the Week

Name: Andrea
Status: other
Grade: other
Country: USA
Date: Summer 2014


I have learned from reading your archives questions that many plastics are difficult to recycle, you cannot incinerate them because of the toxic byproduct gasses, and it takes many years for them to break down naturally. Is there a way to break down the components of plastic (by plastics I mean those used as common items, like food containers) completely back into their original form? If so, would this be a viable option for removing harmful and ubiquitous plastics from our waste stream?

Answers from Our Expert Staff
Hi Andrea,

The "original form" of most polymers (plastics) are their original monomers. With the exception of Polystyrene (recycling type #6), there is no practical way to reverse the original polymerization and end up with the original monomer (or monomers, where two or more monomers are used to make plastics like ABS, epoxies, etc).

The above exception to this (polystyrene) can be de-polymerized easily by heating, resulting in the original monomer "Styrene", which can be repolymerized to make new polystyrene again.

Another common way to recycle "thermoplastics" (those types that can be melted with heat), is to shred old plastic articles then put the resulting plastic pellets into a standard injection molding machine. This machine heats and melts the plastic pellets and injects the molten plastic into a mold to make new plastic articles.

This process works best when the scrap plastic being chopped up and remelted, can be easily identified as to what type it is. For example, 4 liter (1 gallon) milk jugs are well known to be made of pure polyethylene with no colorant (dyes) added, and can be identified at a glance. This is important, because you cannot just throw chunks of any old type of plastic into the feed hopper of an injection molding machine, and expect to get quality plastic parts out of the mold !

Your comment that incineration will result in the release if toxic waste products, is not always correct. Some common plastics such as polyethylene and polypropylene will burn cleanly. Most others, if burned in specially designed incinerators, will not result in release of toxins.

Bob Wilson

In principle, your thinking is on target. However, there are some problems with implementation. Of all the polymers used in making consumer goods, few consist of only one component, so some sort of separation process needs to be considered, keeping in mind that there may be other trash components such as paper, scrap metal, etc.

One step is to somehow isolate styrene for the trash mix. Heated under vacuum polystyrene "unzips" and the styrene monomer could be recovered.

Polyesters at high temperature and high pH tends to revert to the component acid and alcohol. Possibly this "witches brew" could treated to separate out the components. I do not know.

Your thinking is on target, but I think that you need to isolate your target substances.

See what larger consumer companies like Mac Donald's are doing.

What about changing packaging materials all together? For example, what about reusing paper fiber? That is the largest waste material. Keep "digging."

Vince Calder

Hi Andrea,

Thanks for the question. The formation of plastics, or polymers, from monomers releases heat energy. Thus in order to break down polymers into monomers, you will need to add energy. Yes, plastics can be incinerated as long as the flue gasses are appropriately scrubbed to remove gasses like hydrogen cyanide and hydrogen chloride. The technology for scrubbing is available, but implementation does increase the operating costs. It is possible that some plastics such as poly-lactic acid can be broken down by boiling in an acidic solution. However, plastics such as polypropylene or polyethylene cannot be broken down by this method. There may be enzymatic methods which could be used to break down some plastics, but the development of these methods are in their infancy. Another important class of polymers, rubber tires, may be broken down by using catalysts used in the production of ultra-low sulfur diesel fuel. If I recall correctly, there is a company that is working on this method at present.

I hope this helps.
Jeff Grell

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