BP Educational Service. Science at Work Polymers for life?

Transcription

1 BP Educational Service Science at Work Polymers for life?

2 What is a polymer? Products made from oil and gas are all around you. They are the building blocks of modern life. You will find them in: furniture, cars and electronics fibres used to make clothes and textiles bags, boxes and trays used in packaging. All these products are made from a family of chemicals known as polymers. Polymers get their name from two Ancient Greek words: poly means many mer comes from meros, which means parts. Poly-mer = many parts Polymers are made from molecules called monomers. Mono comes from the word for one : Mono-mer. Bottles made from polymers 1. ow might these words explain how monomers become polymers? Polymers are made from oil and gas. These are limited resources. Society will need to change how it makes, uses and disposes of plastics and other polymers. Scientists must find ways to re-use polymers to make new products. This is a technological challenge. Scientists are already discovering new solutions, but technology alone cannot solve the problem: our attitudes towards using and recycling resources will need to change too. A new recycling technology will not benefit anyone unless people save their plastics, so that recycling can take place. 1

3 Alkanes and alkenes Two important types of hydrocarbon are alkanes and alkenes. Alkanes are made of one or more carbon atoms joined by single covalent bonds, and surrounded by hydrogen atoms. They have no spare bonds that can open up and allow other molecules to join on. This means that alkanes are saturated. Alkenes are also chains of carbon atoms with hydrogen atoms attached, but they contain at least one double covalent bond. These double bonds can open up, allowing new atoms or molecules to join on. Alkenes are therefore unsaturated. Ethane model Ethane structural formula Ethene model Ethene structural formula Alkanes have the formula n 2n+2. The first four alkanes are: All alkenes with one double bond have the formula n 2n. The first three alkenes are: Methane 4 Ethane 26 Propane 38 Butane 410 Ethene 2 4 Propene 3 6 Butene Pentane has five carbon atoms. What is pentane s chemical formula? Alkanes burn cleanly to produce carbon dioxide and water. They are useful as fuels. But because they are saturated, they cannot be used as monomers to make plastics and other polymers. 3. Why can t a saturated molecule join onto other molecules? Alkenes do not burn cleanly; they produce a lot of soot. They are not useful as fuels. But because they are unsaturated, alkenes can react with other molecules to form new substances. Alkenes can be joined together in long chains to make polymers. 4. Why is there no alkene that contains just one carbon atom? 2

4 Alkanes and alkenes Like other hydrocarbons, alkanes and alkenes have properties that depend on their size. Short chain alkanes and alkenes boil at lower temperatures are more flammable are less viscous and flow easily are more volatile Long chain alkanes and alkenes boil at higher temperatures are less flammable are more viscous are less volatile This link between properties and chain length is very useful. It allows refineries to separate crude oil into different components. Each component, or fraction, contains hydrocarbons with similar properties. 5. Why do you think BP uses its refineries to isolate components of crude oil that have similar properties? One fraction is naphtha. Naphtha is processed to produce a mixture of alkenes. Manufacturers use these as monomers to make a variety of plastics. Packaging made from polymers Another important fraction is the lightest one: a mixture of gases. This mixture includes ethane, which can be separated from the other gases. This ethane is processed to produce ethene, the most important building block for plastics and other polymers. 6. Why does ethane need to be changed into ethene to make polymers? 3

5 Making polymers Polymers are made when many small molecules join together to form a single, long chain molecule. The small molecules, or monomers, each have a double covalent bond. Under certain conditions, these bonds open up. This allows many monomers to join together. The ethene molecules join in an addition reaction. This is an example of addition polymerisation. 7. The polyethene molecules must end at some point. What atom do you think attaches to the single covalent bond at the ends of the polymer molecule? Propene ( 3 6) can polymerise to form polypropene (also known as polypropylene). n n Many monomers join......to form a section of polymer 3 propene 3 polypropene Monomers joining to form a polymer Polymers are extremely long. The longest hydrocarbons in crude oil can contain up to 60 carbon atoms. A polymer molecule may be thousands of carbon atoms long. Ethene ( 2 4) is the simplest monomer. Ethene polymerises to form polyethene (sometimes also known as polyethylene). When polymers form, the double bonds between carbon atoms in the monomers are replaced by single bonds. This means that simple polymers like polyethene cannot easily bond with other molecules. 8. Polymers are often used to make containers to store reactive chemicals. Why do you think this is? n ethene polyethene n Some polymers form branches on their long chains. Other polymers have cross-links that join chains together. These branches and cross-links change the properties of the polymer. By using polymers with different properties, chemists can devise plastics that have many different uses. 4

6 Using and recycling polymers Polymers have different physical properties. These properties depend on the monomer used, and on the type of bonds between polymer chains. There are two types of polyethene: Low density polyethene (LDPE) igh density polyethene (DPE) Polyethene pellets in the recycling process ow it is made LDPE igh temperature and very high pressure DPE Low temperature, and slightly raised pressure; a catalyst controls the reaction Type of polymer chain Include many branches Relatively straight and rarely branched ow molecules pack together annot pack closely together an pack closely together Intermolecular forces between chains Weak Strong Properties Soft and pliable ard and strong Uses Drink bottles and plastic bags Garden furniture, food storage containers and pipes 9. Why do the branches in LDPE mean the intermolecular forces are weak? 10.ow does the structure of DPE make it harder and stronger than LDPE? 5

7 Using and recycling polymers Polymers have many different uses. ow they are used depends on their properties. 11.Match the polymers in the table below to their uses. Name Property Use Polystyrene Low density polyethene Polytetrafluoroethene Polyvinylchloride Polypropene an be expanded to make a rigid foam full of trapped air Light and stretchable Extremely slippery, heat resistant an be made rigid and light resistant eat resistant, strong and hard Non-stick saucepan coatings Window frames Plastic bags Electric kettles Insulated packaging Despite their usefulness, polymers present two challenges for the future: ow we will make them ow we should dispose of them Polymers are made from oil and gas. These are finite resources and one day they will run out. The price of oil and gas is rising, because more people around the world need the energy these fuels can provide. 12.Why will the availability of oil and gas affect how plastics are made and used in the future? Polymers are extremely durable. They can take many hundreds of years to break down in landfill sites. One alternative to landfill is incineration. The other is to recycle. Recycling waste like plastic packaging has several advantages: It reduces the amount of oil and gas needed to make new plastics It means less material goes into landfill or must be incinerated It helps reduce carbon emissions because making recycled plastic generally uses less energy than producing plastic from new crude oil Even recycling has its limitations. Not all plastics can be recycled and some waste plastics are contaminated with substances that can be harmful. 13.ow can recycling reduce the amount of oil and gas needed to make plastics? 6

8 Glossary Addition polymerisation a chemical reaction in which unsaturated molecules join to form a chain. ovalent bond a chemical bond formed by a shared pair of electrons. Monomer a molecule that can be bonded to other identical molecules to form a polymer. Polymer a large chain molecule formed by many identical smaller molecules bonded together. Saturated a hydrocarbon molecule that contains no carbon carbon double bonds, and therefore has the greatest possible number of hydrogen atoms. Unsaturated a hydrocarbon molecule that contains one or more carbon carbon double bonds, and therefore has fewer than the maximum possible number of hydrogen atoms. BP Educational Service PO Box 105, Rochester, Kent, ME2 4BE Tel: +44 (0) Fax: +44 (0) bpes@bp.com Website: BP International Ltd 2008