7.1 Some aspects of the science of plastics

Let’s spend a few moments looking at some of the science that underlies plastics and their production.

Plastics are comprised of so-called polymer molecules, where a long molecular chain is formed from a repeating molecular unit. (This name derives from the Greek: poly + meros = ‘many’ + ‘parts’). Furthermore, the term ‘polymer’ explains the use of the term ‘poly’ in the chemical names of plastics that you met in the previous section.

As an example, let us consider the relatively simple structure of polyethene, which is, at a basic level, the molecule (CH₂CH₂)_n, where n is a large number resulting in a molecule with a long chain (see Figure 3).

Figure 3  Illustration of a section of the chemical structure of polyethene.

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This figure illustrates the chemical structure of polyethylene, i.e. (CH₂CH₂)_n, where n is a large number. Sections of the polyethylene structure have been drawn out to better show how the repeating CH₂ units are linked together by a bond that is illustrated by a line. This starts to show the chain structure of polyethylene and also that there are so-called cross-linking groups connecting these chains together, in this case simply another CH₂ unit.

Figure 3  Illustration of a section of the chemical structure of polyethene.

The value of n, and how the individual chains are connected to each other (which is known as cross-linking; see the CH–CH₂–CH arrangement next to the blue label in Figure 3), largely determines the properties observed for the plastic. These properties include the plastic’s density and melting temperature, which can be varied by altering the chemical production process.

This makes plastics like polythene highly versatile, and some everyday variations include:

• low density polyethene (LDPE), which is used in food packaging trays, wire insulation
• medium density polyethene (MDPE), which is used in carrier bags and shrink film
• high density polyethene (HDPE), which is used in milk bottles, soft drink bottle caps, pipes and some surgical implants.

Other chemicals may be added to the plastic to change the colour, act as antioxidants, improve how it wears or increase its plasticity or fluidity, where the latter chemicals are called plasticisers. However, additives in plastics sometimes cause health and environmental concerns if they leach out of the material, and this is an active area of research.

Question 2 Plastic degradation

Timing: Allow about 2 minutes

From your own observations, how easily do plastics degrade in the environment?

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Answer

You will probably realise from the amount of plastic litter that is often observed in the outdoors (see Figure 4) that plastics are slow to degrade in the environment.

Figure 4  Plastic materials in the environment.

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This is a photograph of several mostly coloured plastic materials that have been discarded in the environment. These plastic items are mixed within some seaweed on a beach. Several of the items are broken but it is possible to identify a water bottle and some clear plastic film.

Figure 4  Plastic materials in the environment.

Polyethene is rather chemically inert; this is a property that may be either beneficial or problematic during the lifetime of a product made from it. Video 4, dating from September 2015, considers some aspects of polyethene in the environment, and one way that scientific research is progressing for plastics.

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Transcript: Video 4 New biodegradable materials could replace plastic bags.

[MUSIC PLAYING]

CARL BOARDMAN

At the moment in the United Kingdom, when we shop at supermarkets like this, we are currently using over eight billion single-use carrier bags a year, which equates to approximately 60 000 tonnes of plastic – or about 130 bags per person.

Most single-use carrier bags – like this one here – are made out of fossil fuel derived polyethylene, and was subject to a five pence levy in Scotland, Wales, and Northern Ireland – and in the near future England, too. The UK government is expected to amend this policy to include an exemption for new, innovative biodegradable carrier bags, which is where our research at The Open University comes in where we develop biodegradable polymer films, such as this one here.

Here at The Open University, we’re helping UK industry develop new biodegradable plastic carrier bags and packaging materials. When developing biodegradable plastics, our target is for materials to lose 90% of their carbon content within less than one year whilst at the same time having no toxic properties.

In our labs, we’re working in partnership with DEFRA and a UK polymer company to undertake a series of biodegradability and ecotoxicology experiments and tests. To do this, we use instruments, such as this respirometer, which measures the breakdown of plastic materials through the evolution of carbon dioxide. This setup is currently simulating idealised composting conditions.

Within each of these vessels, we have a compost type material and plastic carrier bag film cut up into tiny pieces so the two mediums can interact with one another. Because it’s idealised, we have high temperatures and constant aeration through these inlet and outlet tubes here, which feed directly into our gas analysers. In this instance, measuring a high amount of CO₂ from the compost and plastic mix indicates biological breakdown and a positive result.

Today’s carrier bags mainly end up in landfill sites, but many evade waste treatment and recycling processes altogether and end up littered all over the countryside, or perhaps more worryingly, in the world’s oceans. The world’s oceans are currently estimated to contain over five trillion pieces of plastic – or put another way, about 270 000 tonnes. Here, plastics represent a threat to animals through entanglement, choking, and poisoning.

So in the future, bags that we’ve helped develop will avoid the five pence levy and have a reduced environmental footprint as well.

End transcript: Video 4 New biodegradable materials could replace plastic bags.

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Video 4 New biodegradable materials could replace plastic bags.

Question 3 Chemical inertness

Timing: Allow about 5 minutes

Considering the examples of everyday use given above, and the other information you've read so far, can you suggest when the chemical inertness of polyethene might be useful, and when it might be a problem?

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In food packaging, chemical inertness is a benefit as it means there will be no unwanted reactions that alter the taste of the products. Similarly, when used as a medical implant, there are generally no unwanted reactions to the implant in the patient’s body.

However, at the end of a product’s lifetime, this inertness may become a real problem, depending on how it is treated by society. If it is simply discarded, its lack of reactivity means that it will exist in the environment for a significant time, either as litter on land or pollution in the ocean (see Figure 5).

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Figure 5  Decomposition of common materials (many of them plastics) in the ocean.

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This is a poster entitled ‘How long until it’s gone?’ showing the estimated decomposition rates of common marine debris items many of which are plastic items. The items and their decomposition rates are a paper tissue (3–4 weeks); newspaper (6 weeks); apple core (2 months); cardboard box (2 months); waxed carton (3 months); photo-degradable beverage holder (6 months); cotton shirt (2–5 months); plywood (1–3 years); wool socks (1–5 years); cigarette butt (1–5 years); plastic grocery bag (10–20 years); styrofoam cup (50 years); foamed buoy (50 years); tin can (50 years); aluminium can (200 years); plastic beverage holder (400 years); plastic bottle (450 years); disposable diaper (450 years); fishing line (600 years) and a glass bottle (undetermined).

Figure 5  Decomposition of common materials (many of them plastics) in the ocean.

Video 4 suggested there is estimated to be 5 trillion pieces of plastic, or about 270 000 tonnes, in the oceans. It is noteworthy that this figure only refers to the plastic floating in the ocean (Eriksen et al., 2014). It is also noteworthy that other studies have estimated different quantities of plastic in the oceans. For example, Jambeck et al. (2015), proposed that in 2010 around 4.8 to 12.7 million tonnes entered the oceans. The range represented by the estimates from these two papers illustrates the extent to which we need to further develop our knowledge of exactly what impact plastics are having on our environment. Additionally, we need to develop better waste management for plastic wastes and more sustainable alternatives.

If a waste plastic is to be recycled, then the ease with which this can be performed depends on how many other types of plastic and other materials are present in the complete waste sample. The disposal of waste plastics, as it links to plastic pollution, is currently an important scientific challenge for society, particularly in terms of the efficiency of any proposed recycling processes (Carné Sánchez and Collinson, 2011).

The environmental and human health risks posed by discarded plastics are subject to much debate and research. Undertaking further work in this area could produce a more thorough risk–benefit analysis of the use of plastics in society, but the viewpoint and/or the scope of analysis would have to be carefully defined.

The scope might be narrow when considering the use of a particular plastic item in an application, such as polyester polyethylene terephthalate (PET) in single-use water bottles. Alternatively, it could be much wider, considering the science behind the formation, use and disposal of a particular plastic item. (For example: the use of polyester fibres (including PET) comprising a mix of both new and recycled materials; their application in textiles; and their collection, and the pros and cons of sorting and recycling such materials versus their disposal to landfill or for energy recovery.)

Strong supporting evidence needs to be provided, in the form of references to critically reviewed research and published data, for any such studies. This is often necessary when scientists produce reports for stakeholders, such as the public or government.

Shortly you will examine an example of an online article and some discussion of research literature on plastics. An important aspect of reading new materials to aid your understanding is to develop a glossary of new terms. You will now consider briefly how you might go about preparing such a glossary.