POLYMERS A world of plastic How many different uses of plastic can you spot? What are polymers? Polymers are very large molecules made when hundreds of monomers join together to form long chains. The word polymer comes from the Greek words poly (meaning many) and meros (meaning parts).
Plastics are synthetic polymers that can be shaped by heat or pressure. What keeps the chain together? The monomers in a polymer are joined together by covalent bonds between atoms. In a covalent bond, each atom shares one or more electrons with another atom. The bonds are sometimes shown as sticks. covalent
bond What are polymers made from? Many polymers are formed from alkenes, which are a family of hydrocarbon molecules with the general formula CnH2n. Alkenes contain at least one double covalent bond between carbon atoms. The double bond makes them very reactive. The simplest alkene is ethene (C2H4). double covalent
bond The second simplest alkene is propene (C3H6). Making polymers How are monomers turned into polymers? Addition polymerization Polyethene (sometimes called polythene) is a polymer made from ethene. The process by which polyethene and other polymers is
made is called addition polymerization. This is because many monomers (ethene molecules) are added together. polymer monomers addition polymerization
How is polyethene made? Drawing polymers shorthand formulae Polymers contain thousands of molecules, so how can their structures be easily drawn? Part of the polymer molecule can be drawn: A better way is to show a shorthand formula: The n means that the polymer contains a very large number of the repeating unit shown in the brackets.
Whats the polymer? What is the shorthand formula for polypropene? The monomer is propene (C3H6): which can be drawn as: 1. Draw two C atoms that were in the double bond with a single
covalent bond. 2. Draw the brackets and the n. 3. Add the links outside the brackets. 4. Add the atoms that were attached to each C atom of the double bond. polypropene Whats the monomer? What is the monomer of polyvinylchloride (PVC)?
1. Draw two C atoms joined with a double covalent bond. 2. Add the atoms attached to each C atom. 3. Draw the brackets and n. The equation for the reaction can be drawn as:
What are the properties of plastics? Plastics are all different, but they show a few general properties: they do not conduct electricity and are poor conductors of heat they are unreactive most are not affected by water or air, and many are not affected by chemicals. Why is the unreactivity of plastics both useful and problematic? Their unreactivity makes plastics durable and able to safely
contain and protect many substances. However, it also means that they persist in the environment for a long time. What makes plastics different? The properties of a plastics depend greatly on how the polymer chains are arranged: branching chains make plastics light, soft and easy to melt (e.g. low-density
polyethene) lined-up chains make plastics dense, rigid and harder to melt (e.g. high-density polyethene). Changing the properties of plastics What factors might determine the properties of a plastics? Factor
Effect reaction conditions Temperature, pressure and catalysts affect the length and branching of the polymer chain. monomer
The type of monomer used affects the type of forces between polymer chains. additives Additives can lubricate polymer chains, join them together with crosslinks, or preserve them from decay. What are thermosoftening plastics? What do polymer chains and spaghetti have in common?
Cooked spaghetti is solid when cold, but soft when warm. The strands can slide past each other. It is the same with many polymers. weak intermolecular forces these let the chains slide past each other Plastics made of these polymers are stretchy and have a low melting point. They are called thermosoftening plastics.
Uses of thermosoftening plastics Thermosoftening plastics (also called thermoplastics) do not contain cross-links. This means they are flexible, stretchy and have a low melting point. It also means they can be moulded and shaped after they have been made, many times. What are some examples of thermoplastics? polyethene
natural rubber What are thermosetting plastics? Some polymer chains cannot slide past each other. strong intermolecular forces (cross-links) these hold the chains firmly in place
Plastics made of these polymers cannot be stretched, are rigid and have a high melting point. They are called thermosetting plastics (or thermosets). Uses of thermosetting plastics Thermosetting plastics contain cross-links. This means that they: are rigid will break when bent have a high melting point (they char rather than melt) must be moulded into shape when they are being made,
What type of objects might you make from thermosetting plastics? uPVC chains uPVC is perfect for window frames as it is strong, light and durable. The u stands for unplasticized, and means the polymer is rigid.
chains packed tightly together Oiling the chains? Adding a plasticizer lets the chains slide. plasticizer How will the plasticizer change the polymers properties? The polymer will be flexible, not rigid.
Matching polymers to their uses How much waste plastic? In the UK, 3.5 million tonnes of plastic packaging is thrown away each year! There are three ways to dispose of waste plastics: landfill incineration (burning) recycling Each method of disposal has its own advantages and
disadvantages. Why has the issue of dealing with waste plastic in a cheap and environmentally-friendly way become more important? Disposing of plastics What happens to plastics in landfill sites? Plastic bags are a major source of waste at landfill. British shoppers use over 8 billion of them a year! The UK has 4,000 landfill sites and it is
predicted that the largest of these will become full in less than 5 years. Landfill is a convenient method of waste disposal but it is only designed to bury rubbish, not to break it down. Most plastics are made up of tightly bonded molecules that cannot be decomposed by micro-organisms. These will remain buried at landfill sites for thousands of years without rotting.
How are plastics identified for recycling? Most plastic products carry a symbol that shows which type of polymer they are made from. Usually, the only types of plastic to be recycled are PET, PVC and HDPE. Many plastic items look and/or feel similar to each other but they are actually made from different materials, e.g. margarine tubs (polystyrene) and plant pots (polypropene).
If different polymers are mixed together during recycling, it can reduce the quality and value of the final recycled plastic. Recycling symbols for plastics What is the effect of recycling plastics? Recycling plastic uses less water and energy resources than in producing new plastics, and produces fewer greenhouse gases. One problem with recycling, however,
is that is reduces the strength and versatility of the plastic over time. This is because the polymer chains become damaged or contaminated with food or other types of plastic. What are biodegradable plastics? One of the problems with traditional plastics is that they do not break down when thrown away. Biodegradable plastics are
plastics that can be broken down. They are converted into carbon dioxide, water and minerals by micro-organisms. Biodegradable plastics are increasingly being used in carrier bags, bin bags and food packaging. Biodegradable plastics, such as polylactide, are plant-based polymers. They are often made from starch that has been modified to become more stable.
Dealing with waste Dealing with waste is important, but there are many issues involved: what can businesses and individuals do to reduce the amount of waste they produce? how many products could be made from biodegradable plastic?
if more products are made of biodegradable plastics, how will the management of landfill sites change? what will happen to closed landfill sites in future? Glossary biodegradable A substance that can be naturally broken down by micro-organisms. cross-link A chemical bond that joins one polymer chain to another. monomer A molecule that is the building block of a polymer.
polymer A long chain molecule formed from many monomers joined together. polymerization The reaction used to convert monomers into a polymer. thermosetting A type of plastic that is hard, rigid and has a high melting point. thermosoftening A type of plastic that is flexible, stretchy and has a low melting point.
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