---
title: "Plastics and Synthetic Materials"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 15
exercises: 12
source: https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 15 — Plastics and Synthetic Materials

Count the [plastic](#def-g8-plastics-plastic) objects you touch in one morning: a toothbrush, a bottle, a raincoat, a phone case, the seat of a bus, the bag of the shopping. Hardly any of them existed a hundred and twenty years ago, and none of them grew on a tree or was dug out of the ground. [Plastics](#def-g8-plastics-plastic) are materials invented by chemists — light, cheap, waterproof, shaped into anything — and that very success has become a problem.

**You already know.**

[Natural materials](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-natural-material) are used much as they are found in nature; [manufactured materials](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-natural-material) are made from [raw materials](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-raw-material) by changing them, and [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) turns the material of old objects into new ones ([Chapter 7](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#ch-g5-raw-materials-and-recycling)). A [combustion](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) of a [fuel](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-fuel) made of carbon and hydrogen gives carbon dioxide and water ([Chapter 14](https://one-course.com/books/chemistry/1/en/chapter/14-combustion-fuels-products-and-greenhouse-gases#ch-g8-combustion-and-fuels)).

![Everyday plastic objects: a bottle, a pot, a bag, a pipe, a cup, a toothbrush.](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/img-1d1d716db35c.jpg)

*Everyday [plastic](#def-g8-plastics-plastic) objects: a bottle, a pot, a bag, a pipe, a cup, a toothbrush.*

## 15.1 Natural, artificial and synthetic materials

**Definition 15.1 (Artificial and synthetic materials).**

Among [manufactured materials](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-natural-material), an *artificial material* is made by chemically transforming a [natural material](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-natural-material): paper and viscose (a fabric) from the cellulose of wood. A *synthetic material* is made from substances built by chemists, most often from crude oil, and has no natural counterpart: nylon, polyester, the [plastics](#def-g8-plastics-plastic).

![Natural, artificial and synthetic materials.](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/fig-e127648e8306.svg)

*Natural, artificial and [synthetic materials](#def-g8-plastics-artificial).*

## 15.2 What plastics are

**Definition 15.2 (Plastic).**

A *plastic* is a [synthetic material](#def-g8-plastics-artificial) made of very long [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), which can be shaped by moulding — most often while it is hot and soft — and keeps its shape when cool. Most plastics are made from crude oil or natural gas.

**Remark 15.3 (Very long molecules).**

A water [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) holds three [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom); a [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of a [plastic](#def-g8-plastics-plastic) can hold tens of thousands, joined in a long chain, like a necklace of identical beads. How such chains are built is the subject of a chapter at the end of this book.

![A picture, not a model: the molecule of a plastic is a very long chain of identical units.](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/fig-003259547acc.svg)

*A picture, not a model: the [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) of a [plastic](#def-g8-plastics-plastic) is a very long chain of identical units.*

**History — Bakelite, 1907.**

The first fully synthetic [plastic](#def-g8-plastics-plastic) was made by the chemist Leo Baekeland in 1907, from phenol, obtained from coal tar, and formaldehyde. Bakelite, hard, dark and heat-resistant, became the material of telephones, radios and electric switches. Nylon followed in the 1930s, then the [plastics](#def-g8-plastics-plastic) of bottles and bags after the Second World War.

![A Bakelite telephone of 1947. Photo: Holger Ellgaard, CC BY-SA 3.0.](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/img-3f4705700bf8.jpg)

*A Bakelite telephone of 1947. Photo: Holger Ellgaard, CC BY-SA 3.0.*

## 15.3 The main plastics and their codes

**Definition 15.4 (Resin identification code).**

The *resin identification code* is a number from 1 to 7, printed inside a triangle of arrows on a [plastic](#def-g8-plastics-plastic) object, that names the [plastic](#def-g8-plastics-plastic) it is made of, so that it can be sorted for [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling). The triangle does not mean that the object will be recycled.

**Proposition 15.5 (Seven codes).**

The codes, with the commonest uses of each [plastic](#def-g8-plastics-plastic):

| code | [plastic](#def-g8-plastics-plastic) | typical objects |
| --- | --- | --- |
| 1 | PET, polyethylene terephthalate | water and soda bottles, fleece fibres |
| 2 | HDPE, high-density polyethylene | milk and shampoo bottles, caps |
| 3 | PVC, polyvinyl chloride | pipes, window frames, cables |
| 4 | LDPE, low-density polyethylene | bags, films |
| 5 | PP, polypropylene | yogurt pots, bottle caps, car bumpers |
| 6 | PS, polystyrene | foam cups and trays, CD cases |
| 7 | other [plastics](#def-g8-plastics-plastic) | [mixtures](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture), multilayer packaging |

![The seven resin identification codes, each with the short name of its plastic.](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/fig-ae7aacf19d70.svg)

*The seven [resin identification codes](#def-g8-plastics-code), each with the short name of its [plastic](#def-g8-plastics-plastic).*

## 15.4 Thermoplastics and thermosets

**Definition 15.6 (Thermoplastic and thermoset).**

A *thermoplastic* softens each time it is heated and hardens again when it cools: it can be melted and moulded again and again (PET, polyethylene, PVC, polypropylene, polystyrene). A *thermoset* sets for good when it is first heated and moulded: heated again, it does not soften but chars (Bakelite, the resins of glues and of boat hulls).

**Remark 15.7 (Which can be recycled).**

[Thermoplastics](#def-g8-plastics-thermoplastic) can be melted and remoulded: they are the [plastics](#def-g8-plastics-plastic) that are recycled into new objects. [Thermosets](#def-g8-plastics-thermoplastic) cannot be melted again.

## 15.5 Plastics in the environment

**Proposition 15.8 (Plastics last).**

The world used about 460 million tonnes of [plastics](#def-g8-plastics-plastic) in 2019 and threw away about 353 million tonnes of [plastic](#def-g8-plastics-plastic) waste; recycled [plastic](#def-g8-plastics-plastic) made up only about $6\,\%$ of the [plastic](#def-g8-plastics-plastic) used. [Plastics](#def-g8-plastics-plastic) do not rot: in nature they break into smaller and smaller pieces, the microplastics, which are found in rivers, oceans, soils and the bodies of animals.

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/img-97a3810960dc.jpg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g8-plastics/img-344dd18aeb3e.jpg)

*[Plastic](#def-g8-plastics-plastic) litter washed up on a beach.*

*A sorting line in a [plastics](#def-g8-plastics-plastic) [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) plant.*

**Remark 15.9 (Burning plastics).**

[Plastics](#def-g8-plastics-plastic) burn, and they make carbon dioxide when they burn completely. But burnt in a garden fire they burn incompletely and release soot, carbon monoxide and other harmful substances; PVC, which contains chlorine, gives off hydrogen chloride, a corrosive gas. [Plastic](#def-g8-plastics-plastic) waste is burnt only in incinerators built to clean their fumes.

## 15.6 Exercises

**Exercise 15.1 ★.**

Natural, artificial or synthetic? Cotton, nylon, paper, wool, polystyrene, viscose.

**Solution of Exercise 15.1.**

Natural: cotton, wool. Artificial: paper, viscose. Synthetic: nylon, polystyrene.

**Exercise 15.2 ★.**

From which [raw material](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-raw-material) are most [plastics](#def-g8-plastics-plastic) made?

**Solution of Exercise 15.2.**

From crude oil (or natural gas).

**Exercise 15.3 ★.**

Which [plastic](#def-g8-plastics-plastic) has the code 1? Give one object made of it.

**Solution of Exercise 15.3.**

PET (polyethylene terephthalate): water and soda bottles.

**Exercise 15.4 ★.**

What is the difference between a [thermoplastic](#def-g8-plastics-thermoplastic) and a [thermoset](#def-g8-plastics-thermoplastic)?

**Solution of Exercise 15.4.**

A [thermoplastic](#def-g8-plastics-thermoplastic) softens each time it is heated and can be remoulded; a [thermoset](#def-g8-plastics-thermoplastic) sets for good the first time and chars instead of softening.

**Exercise 15.5 ★★.**

A yogurt pot carries the code 5. Name its [plastic](#def-g8-plastics-plastic), and say whether it could in principle be melted and remoulded.

**Solution of Exercise 15.5.**

Polypropylene (PP), a [thermoplastic](#def-g8-plastics-thermoplastic): it can be melted and remoulded.

**Exercise 15.6 ★★.**

Why does a triangle of arrows on an object not mean that the object will be recycled?

**Solution of Exercise 15.6.**

The triangle only names the [plastic](#def-g8-plastics-plastic), to help sorting. Whether the object is recycled depends on its being collected and on a plant that recycles that [plastic](#def-g8-plastics-plastic).

**Exercise 15.7 ★★.**

Why is the handle of a frying pan often made of a [thermoset](#def-g8-plastics-thermoplastic) rather than a [thermoplastic](#def-g8-plastics-thermoplastic)?

**Solution of Exercise 15.7.**

A pan gets hot: a [thermoplastic](#def-g8-plastics-thermoplastic) would soften, a [thermoset](#def-g8-plastics-thermoplastic) does not.

**Exercise 15.8 ★★.**

Using the figures of this chapter for 2019, what percentage of the [plastic](#def-g8-plastics-plastic) used became waste that year? Round to the nearest whole number.

**Solution of Exercise 15.8.**

$\frac{353}{460} \approx 0.77$: about $77\,\%$.

**Exercise 15.9 ★★.**

Why must PVC never be burnt in a garden fire?

**Solution of Exercise 15.9.**

PVC contains chlorine: [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) it gives off hydrogen chloride, a corrosive gas, besides soot and carbon monoxide from the [incomplete combustion](https://one-course.com/books/chemistry/1/en/chapter/14-combustion-fuels-products-and-greenhouse-gases#def-g8-combustion-and-fuels-complete) of a garden fire.

**Exercise 15.10 ★★.**

A school uses 300 bottles of $25\,\mathrm{g}$ each per week, during 36 weeks of the year. What mass of [plastic](#def-g8-plastics-plastic) is that in a year?

**Solution of Exercise 15.10.**

$300 \times 36 = 10\,800$ bottles; $10\,800 \times 25 = 270\,000$ g, that is $270\,\mathrm{kg}$.

**Exercise 15.11 ★★★.**

Polyethylene is made of carbon and hydrogen only. Write the products of its [complete combustion](https://one-course.com/books/chemistry/1/en/chapter/14-combustion-fuels-products-and-greenhouse-gases#def-g8-combustion-and-fuels-complete). Explain why [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) it adds carbon dioxide to the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) like a [fossil fuel](https://one-course.com/books/chemistry/1/en/chapter/14-combustion-fuels-products-and-greenhouse-gases#def-g8-combustion-and-fuels-fossil).

**Solution of Exercise 15.11.**

Carbon dioxide and water. Its carbon comes from crude oil, stored underground for millions of years: [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) it adds carbon dioxide to the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), as [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) a [fossil fuel](https://one-course.com/books/chemistry/1/en/chapter/14-combustion-fuels-products-and-greenhouse-gases#def-g8-combustion-and-fuels-fossil) does.

**Exercise 15.12 ★★★.**

Explain how a [plastic](#def-g8-plastics-plastic) bottle lost at sea can end up, years later, as microplastics inside a fish.

**Solution of Exercise 15.12.**

The bottle does not rot: sun and waves break it into smaller and smaller pieces. The tiny pieces float or sink in the water, and fish swallow them with their food.

## 15.7 Problem: The Sorting Line

**Problem 15.1.**

Weekend problem — a tonne of used bottles, their codes, and the clean plastic flakes that come out of the plant

A [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) plant receives bales of used drink bottles. A typical bale of $1\,\mathrm{t}$ ($1000\,\mathrm{kg}$) holds, by mass: $80\,\%$ of bottle bodies in PET, $12\,\%$ of caps (HDPE and PP), $5\,\%$ of labels (PP film) and $3\,\%$ of dirt and leftover liquid. Machines separate the caps and labels from the bodies, the PET is ground into flakes, and the flakes are washed; the washing loses $2.5\,\%$ of the PET. The clean flakes are melted and spun into the fibres of fleece jackets.

**Part I — The [plastics](#def-g8-plastics-plastic).**

1. Which code is printed on the bottle bodies? On the caps?
2. Are these [plastics](#def-g8-plastics-plastic) natural, artificial or synthetic?
3. Can PET be melted and spun again? What kind of [plastic](#def-g8-plastics-plastic) does this make it?
4. Why are the caps and labels separated from the bodies before the PET is melted?

**Part II — The bale.**

5. What mass of PET does a bale contain?
6. What mass of caps? Of labels?
7. What mass of the bale is not [plastic](#def-g8-plastics-plastic) ?
8. What percentage of the bale is [plastic](#def-g8-plastics-plastic) ?

**Part III — The flakes.**

9. What mass of PET is lost in the washing?
10. A fleece jacket needs $0.39\,\mathrm{kg}$ of PET fibre. Why is it better to make it from flakes than from new PET?
11. The plant could burn the caps and labels for heat instead of [recycling](https://one-course.com/books/chemistry/1/en/chapter/7-from-raw-materials-to-recycling#def-g5-raw-materials-and-recycling-recycling) them. Name the gas this would add to the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) .
12. Compute the mass of clean PET flakes that one bale gives.

**Solution of Problem 15.1.**

**1.** Bodies: 1 (PET). Caps: 2 (HDPE) or 5 (PP).

**2.** Synthetic: they are made from substances built by chemists, mostly from oil.

**3.** Yes: PET is a [thermoplastic](#def-g8-plastics-thermoplastic).

**4.** Each [plastic](#def-g8-plastics-plastic) is recycled on its own: melted together, a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) of [plastics](#def-g8-plastics-plastic) gives a poor material.

**5.** $0.80 \times 1000 = 800\,\mathrm{kg}$.

**6.** Caps: $0.12 \times 1000 = 120\,\mathrm{kg}$. Labels: $0.05
\times 1000 = 50\,\mathrm{kg}$.

**7.** $0.03 \times 1000 = 30\,\mathrm{kg}$ of dirt and liquid.

**8.** $80 + 12 + 5 = 97\,\%$.

**9.** $0.025 \times 800 = 20\,\mathrm{kg}$.

**10.** The flakes save crude oil and the energy of making new PET, and they turn waste into a useful object.

**11.** Carbon dioxide (and, if the [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) is poor, carbon monoxide and soot).

**12.** $800 - 20 = 780\,\mathrm{kg}$ of clean PET flakes per bale.