The Perpetual Plastic Puzzle BY KERRY MILLER
GRAPHICS BY TERRA PLUMB
Picture it – it is Friday afternoon, the sun is shining, and you have finally put the finishing touches on your Jacobson’s Catalyst report for organic chem, so you decide to take a nice, peaceful stroll down Owen’s Beach. The birds are singing, the Sound smells fresh – everything is as it should be. But then, out of the blue, it happens. An assault on your senses. The peace is shattered. A plastic water bottle on the beach? A discarded Chaco tangled in the seaweed? A vape pen laying smashed under a rock? What is going on?
molecules with repeating units called monomers (Fig. 1). As seen in table 1, each of these common plastics have a different monomer, and that change is enough to give each plastic its own set of properties. But whatever their differences, for the most part they are pretty similar: they don’t dissolve in water, they are bendy, and all of them can be melted down and reshaped for recycling purposes. However, melting down the plastic degrades its purity, and each piece of plastic can only be recycled so many times before it is too unstable to be used. Because of this, scientists have been looking for alternate ways of degrading existing plastics since the late 70s (2). There are now hundreds of new ways of recycling plastics, all with their pros and cons, and the research into them being used on an industrial scale continues.
Fig. 1: Structure of a polymer. We have learned about the environmental harm that comes with littering since we were kids, and know that the solution lies in ‘reduce, reuse, recycle’, but with plastics, things aren’t all that simple. Most plastic made since the 1950s still exists today, polluting rivers and oceans and mucking up beaches and forests. Only about 10% of the plastic produced every year ends up getting recycled due to both chemical and economic difficulties (1). Luckily, scientists have been working hard the last few decades to come up with some solutions to this puzzle, and hopefully soon, we will be able to walk on the beach with no plastic in sight.
What is Plastic? Have you ever picked up a plastic bottle and seen the recycle symbol on it with a number inside? Those numbers can tell you which type of plastic you have! There are 6 plastics that are most commonly used in consumer products (number 7 means “other”), and these six are most responsible for creating the bulk of the plastic waste in landfills and the ocean. But what is a plastic exactly? Plastics are polymers. A polymer is a long chain of
Part 1: Re-ref ining Plastic is made from crude oil products, specifically naphtha (a mix of small carbon molecules). Depending on which hydrocarbon is used, a different plastic will form. In theory, another way of recycling plastic is to turn it back into these small hydrocarbons (polyethylene to ethene instead of ethene to polyethylene). There are a few ways this has been accomplished, the two most common are incinerating the plastic and converting the resulting gasses back into the starting reagents, or heating and pressurizing the plastic until the chains broke apart (4, 5). This takes an immense amount of energy and requires gas capture – a notoriously difficult process to perfect. A plastic chain is very chemically stable and does not like to react with other chemicals, and a lot of heat is needed to break that stability. But if the plastic is simply burned (oxidizing the carbon-carbon bonds in the backbone), the resulting products will be CO2 and CO (along with solid carbon and water), which we do not want to be releasing. If these gasses are captured, they can be converted to methanol and methyl chloride, which can be converted back into ethylene, but not very easily (Fig. 2). Breaking the chains with heat and pressure is not much better – so much heat has to be used (Fig. 3). Both of these processes
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