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We use a lot of polymers each day. From the fabric of your clothes to the plastic of your pen, they make up an important part of our everyday lives. But what happens to polymers when we are finished with them? We have a few different options for polymer disposal. This…
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Jetzt kostenlos anmeldenWe use a lot of polymers each day. From the fabric of your clothes to the plastic of your pen, they make up an important part of our everyday lives. But what happens to polymers when we are finished with them?
We have a few different options for polymer disposal. This largely depends on whether the polymer is biodegradable or not.
Biodegradable substances are ones that can be broken down naturally by living organisms.
Take a polymer. It is made up of lots of different parts called monomers. Ideally, when we are finished with a polymer, we want to reclaim as many of the monomers and the atoms they are made up from as possible so that nothing is wasted. This saves money and resources. But to reclaim these monomers and atoms, we need to break the bonds holding them in place in the polymer. One way of doing this is through biodegradation.
Biodegradation involves getting other living organisms, called decomposers, to do the hard work for us and break down a substance into smaller pieces. Substances that can be broken down in this way are said to be biodegradable. Some polymers commonly encountered in chemistry are biodegradable, but some aren't. We'll start by looking at biodegradable polymers.
In chemistry, you'll come across two main types of polymers: addition polymers, and condensation polymers. Addition polymers aren't biodegradable, and we'll look at why later on. First, let's look at polymers that are biodegradable: condensation polymers.
To learn more about these structures, have a look at Polymerisation Reactions and Condensation Polymers.
Condensation polymers are produced in condensation reactions between monomers with two different functional groups. This reaction forms a long polymer chain and releases a small molecule in the process, commonly water. We can break a condensation polymer back into its individual monomers through the reverse of this reaction, known as hydrolysis.
Hydrolysis involves adding water to a condensation polymer. By itself, it is a very slow reaction. But if you add a catalyst, typically an acid or an alkali, the reaction happens a lot more quickly. Some living organisms provide these catalysts and are thus able to biodegrade condensation polymers.
Let's now focus on two main groups of condensation polymers, polyesters and polyamides. We'll also look at a specific example of a biodegradable polyamide, nylon.
Polyamides are a type of condensation polymer made in a condensation reaction between an amine and a carboxylic acid. This reaction releases water. The resulting polymer contains the Amide functional group, -NHCO-. The polymer can be broken back down into its monomers in a hydrolysis reaction like the one we described above, using an acid or an alkali as a catalyst.
Fig. 1 - Polyamide synthesis
Nylon is a polyamide. It is made industrially from 1,6-diaminohexane and hexane-1,6-dicarboxylic acid (also known as hexanedioic acid). Like all polyamides, it is biodegradable and breaks down in a hydrolysis reaction. The reaction is slow, but is dramatically sped up using an acid catalyst.
Fig. 2 - Nylon synthesis
Polyesters are another type of condensation polymer made in a condensation reaction between an alcohol and a carboxylic acid. They contain the ester functional group, -COO-. Once again, the condensation reaction releases water. The polymer can be broken down in a hydrolysis reaction with an acid or alkali catalyst.
Fig. 3 - Polyester synthesis
Polyesters and polyamides also degrade under the influence of light. This is known as photodegradation, and is all thanks to UV radiation from the sun. The radiation interacts with the bonds in the polymer, creating highly reactive free radicals which react with oxygen in the atmosphere. This weakens the polymer, making it more brittle. Eventually, the polymer breaks down into lots of little pieces.
But just because polyamides and polyesters can biodegrade, doesn't mean that the process is quick or easy. The living organisms responsible for biodegradation by hydrolysis require certain conditions, such as specific light, water, oxygen, and temperature levels. If you throw a piece of nylon fabric on the floor outside and leave it for a couple of months, the chances are it will still be there when you come back. Even biodegradation by light is a slow process.
We've learnt that condensation polymers are biodegradable. However, another type of polymer, known as addition polymers, is not biodegradable. Let's look at why this is so.
Addition polymers are made from lots and lots of the same monomer, which join together in a long chain. They typically have a C-C backbone. The C-C bond is both strong and non-polar and can't be broken down easily by any living organism - not even with a catalyst. This means that addition polymers are resistant to biodegradation.
Polyalkenes are a type of addition polymer. They're made from alkene monomers and are based on many strong C-C and C-H bonds. Because of this, they can't be broken down easily and don't biodegrade.
Fig. 4 - Poly(ethene) - an example of polyalkene synthesis
Learning about polyalkenes and their resistance to biodegradation brings up one big question: how else can we dispose of polymers? Getting rid of them can be done in one of three main ways.
Unfortunately, none of these methods are ideal, and they each have their pros and cons. Let's start with landfill sites.
Did you know that in 2016, 24 percent of the UK's waste was sent to landfill sites? These are large, deserted areas of land where rubbish is dumped, either in huge heaps or in holes in the ground. They're useful because they keep the waste together in one area, and because polyesters and polyamides will slowly biodegrade there. However, the conditions are anaerobic; this results in lots of methane being released by the decomposers. Landfill sites are also ugly, smelly, and take up a lot of land. Furthermore, toxic chemicals found in the waste polymers sometimes leach into the environment, polluting the surrounding soil, water, and air.
Another way of getting rid of waste is by burning it: incineration. Incineration is quick, saves on space, and even releases useful energy, but has its drawbacks as well. For example, burning polyalkenes releases carbon dioxide, a greenhouse gas, whilst burning other polymers can release toxic chemicals such as styrene vapour or hydrochloric acid.
In the article Cracking (Chemistry), we explore how long-chain alkanes can be broken down into shorter, more useful alkanes and Alkenes in a process called cracking. This is one use of polyalkenes - after all, they are essentially just very long alkane chains!
Cracking has its benefits. Scientists estimate that the process is more energy-efficient than incineration, and unlike recycling, can be used for mixtures of dirty or impure polymers.
Finally, let's look at recycling. This involves melting and remoulding polymers, turning them into a new product. It is often done with plastics.
Recycling is beneficial because it means that we don't have to produce more polymers, which typically come from the finite resource, crude oil. This saves raw materials. It also releases less carbon dioxide than incineration. However, most polymers can only be recycled a handful of times, as their polymer chains break down with each subsequent melting and remoulding. On top of that, sorting and processing waste for recycling is an expensive and time-consuming process.
Fig. 5 - The well-known recycling motif
Biodegradable materials are materials that can be naturally broken down due to their chemical structures, which allow their bonds to be easily broken. Examples include food refuse and condensation polymers.
Nylon is biodegradable because it is a condensation polymer that has polar bonds which can be attacked by nucleophiles.
Polyethylene is not biodegradable. The covalent bonds that hold it together are non-polar, and are therefore very stable and unreactive.
Biodegradable means that a compound can be broken down through natural processes such as decay by microorganisms.
An example of a biodegradable product is food refuse. This can be broken down by microorganisms.
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SIGNUP SIGNUPFlashcards in Biodegradability17
Start learningWhat does biodegradable mean?
It can be broken down naturally by living organisms.
Which of the following are biodegradable?
Just polyalkenes
Polyesters are broken down in a _____ reaction.
Hydrolysis
Polyamides are broken down in a _____ reaction.
Hydrolysis
Why are polyesters and polyamides biodegradable?
They can be broken down in a hydrolysis reaction.
True or false? Adding water to a polyamide breaks it down rapidly.
False
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