School chemistry revision 14-16 GCSE level chemistry notes: Issues to do with using & recycling plastic materials

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7H to 7J - More on polymer uses, issues with using plastics, solutions and recycling and how to choose a polymer/plastic for a particular use and application

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7H More general comments on the uses of polymer plastics

7I Problems with using polymer plastics: methods of recycling, disposing, pollution e.g. PET, PVC etc.

7J Highlighting three specific problems associated with using and disposing of polymers or plastic materials

7K Biodegradable and compostable polymers - these should break down due to bacterial activity

7L The recycling case of PET/PVC and other plastics - examples of reclamation

7M Summary of advantages and disadvantages of recycling plastic polymers - the 'pros and cons'!

7N Logistic exercise - choosing a plastic for a particular use

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Introduction to addition polymers: poly(ethene), poly(propene), polystyrene, PVC, PTFE - structure, uses

More on the uses of plastics, issues with using plastics, solutions and recycling methods

Introducing condensation polymers: Nylon, Terylene/PET, comparing thermoplastics, fibres, thermosets

Extra notes for more advanced level organic chemistry students

Polymerisation of alkenes to addition polymers - structure, properties, uses of poly(alkene) polymers

The manufacture, molecular structure, properties and uses of polyesters

Amides chemistry - a mention of polyamides

The structure, properties and uses of polyesters and polyamides involving aromatic monomers

The chemistry of amides including Nylon formation, structure, properties and uses

Stereoregular polymers -  isotactic/atactic/syndiotactic poly(propene) - use of Ziegler-Natta catalysts

7H. More general points on the USES OF POLYMERS - Plastics

Polymers have many useful applications and new uses and new polymers are being developed all the time

  • There are lots of examples! - some described in section 7B and more listed below (and described above) are on many school syllabuses.

  • A wide range of polymers are available for use in an even wider range of applications, it seems endless these days from when I was a chemistry student in the 1960s when some of the properties of these new polymers were unthinkable! e.g. electrical conducting, light sensitive etc.

    • Its important to realise that polymer plastics are alternatives to traditional materials like wood and metals and in many cases, if not completely, replacing them on a large scale e.g. water pipes, window frames.

  • There physical properties can range from being very rigid or flexible-stretchy-elastic, very strong, easily moulded if gently heated and pressed (thermoplastic) or set hard after moulding and very heat resistant (thermoset).

  • Polymers vary considerably in strength e.g. polyamides or nylons are much stronger than poly(ethene) and can even be manufactured to provide a variety of strengths and flexibilities (see example of LDPE and HDPE commercial products of 'polythene').

    • Polyamides like Nylon and poly(propene) can be manufactured to make very strong fibres for fabrics and ropes.

  • Many metals have become quite expensive so things made from plastics derived from oil and quite cheap at the moment, but prices will rise as cheap oil gets scarcer.

  • New packaging materials which are light and stretchable.

  • Waterproof coatings for fabrics

  • Dental polymers for tooth fillings

  • Wound dressings made from hydrogels - polymers that hold water and keep wounds moist.

  • Smart materials (including shape memory polymers) eg 'memory foam' is a polymer that gets softer when it gets warmer so you can make a mattress which adopts your body shape for comfortable sleeping!

    • Elastic fibres that are very stretchable to make tight fitting clothing eg Lycra fibres for tights and sports clothing.

    • Heat resistant polymers are usually thermosets e.g. like melamine resin (plastic plates), but even thermoplastics like poly(propene) can be used in hot situations e.g. plastic electric kettles.

    • Plastics are widely used in the manufacture of cars and other road vehicles because they are cheap to make of varied composition for a wide variety of uses, they are light, durable and can be dyed any colour, they can be flexible or rigid, and so can be used for e.g. used for internal fittings e.g. dashboard cover, floor covers (can be rubber too), door coverings, transparent and coloured covers over headlights and brake lights, and the insulating sheathing for all the electrical wiring.

    • to redo photo

    • Plastics can be coated with all sorts of materials. This plastic knife and fork are coated with a material that gives the impression of being a silvery metal and is as bright and shiny as the brass case of the old metal pocket watch.

  • Some polymer materials are relatively soft and 'springy', these are called elastomers e.g. rubber which can be stretched and deformed, but on release of the tension, the material goes back to its original shape.

    • Most polymer plastics are not elastomers and under high tension will permanently deformed in shape.

  • -

7I. Problems with using polymer plastics: methods of recycling, disposing and pollution

  • Some problems associated with using and disposing of polymers include ....

    • The availability of starting materials to make polymers (oil is a finite resource) and geopolitics and wars and economic factors.

    • Unfortunately, once used and discarded, persistence in landfill sites - plastics are chemically relatively stable due to non-biodegradability - they will take many years to degrade away, they are synthetic materials not found in nature, so organisms don't have the enzymes to break them down - can't biodegrade them.

    • Poisonous gases are produced during disposal by combustion in incinerators, unless extreme precautions are taken - e.g. high temperature and monitoring exhaust gases in the chimney.

      • This problem is prevalent with plastics like PVC, which give off toxic fumes of hydrogen chloride gas.

    • The difficulty and cost in sorting out polymers in recycling centres so that they can be melted and reformed into a new product.

    • We use polymer plastic for some many things we are creating a huge waste mountain of unwanted items, many of them littering the environment from waste tips to the oceans!

    • The non-biodegradability of most polymers makes it imperative to design means of recycling and disposing of plastic waste.

    • This is no easy, the waste plastic needs sorting and these materials also contain additives such as colorants, plasticisers and stabilisers, some of which are poisonous substances.

    • Legislation will help to control further use of plastics, but that takes time for companies to adapt their production methods.

  • Many polymers are not biodegradable - they don't rot easily! ...

    • ... so they are not broken down by microbes and this can lead to problems with waste disposal - see also section 7c on pollution-environmental problems when disposing of plastic materials.

    • Because of lack of biodegradability by microorganisms, you get environmental problems with landfill sites and street litter and it may take years before the plastic materials break down - ie decompose!

      • Most plastic don't rot away because synthetic polymers are chemically very molecules and most microorganisms don't have the enzymes to break up the bonds of the polymer chain and break the plastic down.

      • This is fortunate enough when we want to use plastics, but not so good when we want to dispose of waste plastics!

      • There is also a growing problem with vast quantities of plastic debris circulating around our seas and oceans.

      • You can bury waste plastics in landfill sites BUT even after many years most plastics will not have rotted.

      • Burning waste plastics produces all sorts of polluting gases like hydrogen chloride, hydrogen cyanide and sulfur dioxide, incineration seems a good idea, but would you like to live down-wind of the combustion plant!

      • The latest bad news comes from the discovery of huge amounts of microplastic particle particles everywhere in water systems, including deep in oceans on the seabed.

        • Particles < 1mm are considered to be 'microplastics'.

        • Microplastic items include fibres from clothing and other synthetic textiles, and tiny fragments from larger objects that had broken down over time e.g. plastic bags or bottles.

        • One quote was ~2 million plastic particles per square meter.

        • We don't know whether microplastics have adverse health effects on humans as they move through the marine food chains - and we are an end consumer!

        • Microplastics both absorb and give off chemicals and harmful pollutants.

        • Plastics like polystyrene used in packaging, is chemically very stable and a potential pollutant, so, hopefully, food packaging will be minimised and polystyrene packaging replaced with biodegradable materials - maybe a return to paper and cardboard!

    • However, all is not lost and chemistry and technology can come to the 'rescue' and lots of new improved polymers for existing applications as well as new polymers for new applications are constantly being developed.

    • eg plastic bags are being made from polymers derived from cornstarch making biodegradable plastics that break down more easily via microorganisms in the environment.

    • AND, wherever possible, recycle and reuse! (see section below).

7J Highlighting three problems associated with using and disposing of polymers or plastic materials

  1. Polymers or plastics cannot be easily broken down by micro-organisms

    • i.e. most, at the moment, most are NOT biodegradable (non-biodegradable) which leads to waste disposal and other environmental problems eg

    • 'Non-rotting' litter around the environment.

    • Land-fill sites are getting full and recycling isn't as easy as it may seem. (see point 3.below)

    • Incineration i.e. using waste plastic as fuel must be very efficient to avoid any other pollution problem. (see point 2.)

    • Even our lakes, seas and oceans are carrying waste plastic materials causing harmful effects on wildlife.

  2. Can we burn waste plastic?

    • When plastic materials burn they can produce highly toxic gases such as carbon monoxide, hydrogen cyanide and hydrogen chloride - particularly from PVC and other plastics containing chlorine and nitrogen.

    • Combustion of waste plastics contributes to global warming since carbon dioxide produced.

      • In a recycling-disposal centre, you need very high tech incinerators with sophisticated facilities to remove any harmful gases or particles, plus monitoring technology - and then you can release the smoke into the environment.

        • I've visited such a recycling plant near Harrogate in North Yorkshire, England.

        • Its an impressive operation and the 'organic' waste that cannot be recycled is burnt to provide energy to generate electricity.

        • The plant generates 30MW of electricity, the recycling centre runs off 5MW of electricity, so 25 MW of power is put into the National Grid electricity supply.

        • Nothing is wasted, all residues are found some use for!

        • Allerton Waste Recovery Park: https://www.northyorks.gov.uk/allerton-waste-recovery-park

    • The burning of plastic waste as a fuel is sometimes referred to as 'energy recovery' and is appropriate if you can do nothing else with combustible waste!

    • The toxic fumes from plastic materials cause deaths in house fires and controversial problems with alleged inefficient waste incinerators as they will definitely cause environmental problems if burned on waste tips!

  3. doc b oil notesHow can we recycle plastics? What ways are used to recycle plastics? issues?

    • It is highly desirable to recycle plastics for several reasons

      • Less environmental impact i.e. less unsightly pollution, reduction of volume of waste in land-fill sites.

      • It conserves valuable oil based resources, especially as oil is becoming increasingly expensive.

    • It is difficult to recycle plastics because of separation into the different types of plastics and their different physical properties.

      • BUT this should not prevent us from trying and it would be beneficial to prolong the life of the finite crude oil reserves AND reduce pollution and space in land-fill sites.

      • Research is ongoing to devise methods of recycling plastics as 'new' chemical feedstock from which to make useful organic chemical products.

    • There are problems in trying to sort out the different plastics into useful categories, they are not easy separate, you can't just use any old mixture of polymers.

      • At the moment, a lot may have to be sorted by hand - lack of automation makes the sorting more costly.

      • You also have the cost of collecting domestic rubbish or industrial waste, transporting it and sorting it all out, all of which costs money.

      • Manual sorting of plastic waste is inefficient and labour intensive, but methods of sorting are improving including optical scanning techniques to separate the different types of plastic waster e.g. PET from HDPE and other technologies can separate PVC from other plastics

    • However, people are coming up with ideas. A company in Scarborough, England, is collecting waste plastic. This is shredded and compressed into porous pads and used for good 'underground' drainage layers for footpaths, golf greens and sand bunkers etc. and has a good working life because the material isn't biodegradable!

    • Clear soft drink bottles are made from PET (polyethene/polyethylene terephthalate) which can be recycled as fibre-fill for pillows and carpets.

      • This save 90% of the energy costs compared to the original manufacturing process.

      • Energy costs are a big economic recycling factor, its not just about making naturally occurring resources like oil last longer.

      • However, it takes about 20,000 drinks bottles to make a tonne of recycled PET.

    • In 1988 Australia issued bank notes made from recycled poly(propene). These plastic notes apparently have the advantage of being more difficult to forge and they last longer!

      • Waste poly(propene)/polypropylene can be ground up and recycled to make pipes, compost bins and flower pots

    • Other ideas include making more durable plastic bags that can be used many times for shopping.

    • Ideally some recycled thermoplastics and scrap material from a plastic product manufacturing process, can be heated and remoulded in the same process or another product.

    • You can also manufacture plastics whose break down is initiated by exposure to sunlight. This is particularly useful in agriculture where plastic bags of fertiliser are in common use.

7K Biodegradable and compostable polymers - these should break down due to bacterial activity

Degradable polymers will break down in the environment from the effects of weathering or light (particularly uv radiation), but very slowly due to their great chemical stability.

Biodegradable polymers can be broken down by the enzymes in microorganisms.

They often based on a polyester structure (but NOT PET/Terylene) and are relatively easily hydrolysed and broken down by the metabolic chemistry of microorganisms.

  • New bioplastics are being developed which are more biodegradable or can be recycled, so will the paper bag and cardboard package make a comeback? (in Ireland you have to bring your own bag or buy one, and not necessarily a plastic one!), this isn't a recycling process BUT it does reduce environmental pollution.

    • Hydrocarbon polymers like poly(ethene) or poly(propene) containers have strong C-H and C-C bonds are strong and non-polar so these polymers are not susceptible to chemical attack.

    • The same argument applies to PTFE with its strong C-C and C-F bonds.

    • In fact the vast majority of synthetic polymers are NOT biodegradable, because enzymes have not yet evolved in microorganisms to break them down.

  • Polymers can be made more degradable by including an additive that promotes breakdown by oxidation - makes the 'oxo-biodegradable'.

    • As well as this additive, thin films of poly(ethene) and poly(propene) polymer formulations also include a controlled amount of antioxidant that prevents the plastic breaking down too soon! Once the antioxidant is used up, the polymer molecules will begin to break down. This non-oxidation lifetime can be designed to be months or years followed by the oxidative degradation in contact with the oxygen in air, which itself may take months or years.

  • Where possible, these plastics are made from renewable raw materials such as cellulose (from wood), lactic acid and starch.

    • Bioplastics are manufactured by non-hazardous methods and so better for the environment.

    • Waste bioplastic materials should naturally degrade in the environment to water and carbon dioxide from bacterial activity and not cause pollution and damage to the environment e.g. avoiding unsightly litter and dangers to animals.

  • Compostable plastics have to meet strict criteria  and must break down to water, carbon dioxide, harmless inorganic compounds and biomass residue i.e. degrade in the same way as any other organic material added to a compost heap!

  • You can mix starch granules (very biodegradable) with plastics like poly(ethene) which enables microorganisms to grow and feed in the plastic and may eventually help biodegrade the plastic itself?

    • By breaking down the starch e.g. by hydrolysis, the microorganisms also break down the poly(ethene) into tiny particles making less conspicuous litter, BUT, there is growing concern about the effects of micro plastic particles on organisms in the environment.

  • You can make biobags and cutlery from renewable and biodegradable cornstarch to replace non-biodegradable polythene bags. Many supermarket bags are made from plant starch and since they are compostable, they can be used as bin liners for collecting food waste - so they both compost together!

  • Similarly, tableware such as plates, cups and food bags can be manufactured from sugar cane fibre (a biodegradable natural polymer) replacing non-biodegradable plastics like polystyrene.

    • For hot drinks, biodegradable polymers are being developed that can withstand temperatures up to 100oC.

  • Polylactic acid is biodegradable, can be used for cold drink cups and biodegrades in 180 days.

  • Polyglycolic acid is also biodegradable, it is quite easily broken down by hydrolysis and decomposes in less than a month on a compost heap.

Extra advanced level note

Photodegradable polymers

These are synthetic polymers specially designed to become weak and brittle when exposed to sunlight for a long time - uv light of higher energy photons, has more effect than visible light photons.

This can be facilitated by incorporating light sensitive additives into the polymer formulation that catalyse the breakdown of the polymer on absorption of uv radiation.

Another way to achieve this effect is to incorporate carbonyl bonds (>C=O) into the polymer chain structure.

The carbonyl groups in the polymer chain absorb uv photons and break the polymer chain at that point.

The initial product from photodegradable polymers exposed to uv light, is a waxy mixture which ultimately breaks down into carbon dioxide and water by bacteria.

So the photodegradable polymer is converted to biodegradable products!

7L The recycling case of PET/PVC and other plastics - examples of reclamation

Recycling PET ('poly(ethylene terephthalate)')

PET is a polyester used for making plastic drinking bottles and made from the non-renewable resource of crude oil.

If it can be recycled, you save lots of energy and uses less of a valuable resource of organic chemicals (oil) than if you were making a new quantity of PET.

To make PET, you have to extract oil, fractionally distil it, chemically modify fractions to produce molecules from which you can then make the monomers to polymerise to make PET.

PET bottles can be recycled to make other products including packaging materials, carpets and new bottles themselves!

The PET objects must be sorted out from other plastic waste.

They can then be shredded and melted down to be moulded into other useful objects.

If PET wasn't recycled, it would end up in a polluting space occupying land-fill site.

PET bottles can be recycled e.g. in poorer countries, they can be filled with water unfit for cooking and drinking, left out in bright sunlight. PET lets uv radiation through which sterilises the heated water - the heat and uv radiation kills many pathogens (bacteria, viruses, protozoa and worms).

Recycling PVC poly(chloroethene), 'polyvinyl chloride'

The recycling of PVC is particularly awkward problem because its high content of chlorine in the polymer structure and the process is uneconomic i.e. it costs more to recycle than making 'new' PVC from the petrochemical industry.

To avoid a build-up of PVC in landfill sites, it has been incinerated, but this produces harmful fumes that may enter the air environment e.g. fumes of highly acidic fumes from hydrogen chloride, which are costly to remove from the waste combustion gases.

However, there is a method being developed that involves dissolving and separating PVC using a solvent. High quality PVC is precipitated from the solvent for reuse and the used solvent is recovered and recycled to be reused to extract more PVC from waste.

Recycling Poly(ethene), 'polythene'

HDPE can be re-used to make hard wearing tough plastic materials like waste bins, water butts and plastic boxes/crates.

LDPE waste is recycled to make plastic refuse sacks.

Recycling Poly(propene), 'polypropylene'

Poly(propene) can be recycled by mechanical means. It can be sorted from other types of waste plastic by hand - but, unfortunately, this is inefficient in time/cosy and labour intensive.

However, the sorting can be done automatically using infrared spectroscopy or flotation - the latter uses the different densities of plastics to sort them.

After the separation, the poly(propene) is shredded into flakes and processed into granules which can be melted down and moulded into new items.

This process can be repeated several times with significant degradation to the polymer structure e.g. no loss of strength.

It is possible, in the absence of air (oxygen), to thermally degrade waste poly(propene) at ~500oC to produce a chemical feedstock similar to naphtha from the fractional distillation of crude oil,

example of recycling door mat made from recycled plastic-rubber flip-flops present from the FAIRTRADE retail organisation

A nice example of recycling, a door mat made from recycled plastic-rubber flip-flops, a present from a friend who runs an outlet for the FAIRTRADE retail organisation.

See section 11. for the chemistry of making polyesters like PET

7M Summary of advantages and disadvantages of recycling plastic polymers - the 'pros and cons'!

Advantages Disadvantages
1. Reduces the quantity of oil needed - a finite resource 1. Its not easy, and expensive, to separate the different types of polymer in a recycling centre - prior to be reformed in some way to make a useful material
2. Reduces the quantity of non-biodegradable waste in land-fill sites 2. Mixing recycled plastics together reduces the quality of the plastic material.
3. Reduces emissions of greenhouse gases if burned 3. Unlike with metals, its difficult to keep on recycling the same polymer material e.g. the structure gets weaker
4. Recycling usually reduces the energy and resources needed to replaced redundant materials 4. Processing waste plastic can produce obnoxious harmful gases - not good for us or the environment - surely we have all smelt overheated or burned plastic!
5. Recycling reduces costs, because of 4. above.  

See also Pollution due to carbon monoxide etc.

7N. Logistic exercise - choosing a plastic for a particular use

(this section is repeated on the other GCSE condensation polymer notes page)

How we use polymer compositions depends on their properties, some are quoted in the data table below

So, below is a decision making exercise on choosing a plastic for a particular job!

Note: NOT ALL the properties are necessarily relevant to make the decision.

At the moment A to F match questions (a) to (f) once only, but I may add further questions!

This exercise should provide a good challenge and discussion for a class, any feedback comments appreciated.

Polymer product Production cost Chemical resistance Melting point Strength (rigidity) Transparency can be made into fibres?
A high high high high good no
B low low high moderate poor no
C low low low low opaque yes
D low low low low poor no
E high low high very high opaque yes
F low high high high poor no

(a) Which plastic could be used as for disposable tableware like plates for hot meals or coffee cups?

(b) Which plastic could be used as a moving plastic component in a machine?

(c) Which plastic could used as containers for high volume production line of acids or alkalis?

(d) Which plastic could be used as laboratory volume measuring instrument e.g. a syringe or measuring cylinder

(e) Which plastic is suitable for clothing fabrics?

(f) Which plastic would be used for super-market carrier bags?

Answers near the end of the page

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Answers to the Choosing a Plastic for a Particular Use Exercise in decision making

(a) Plastic B, to be disposable it must be cheap, chemical resistance doesn't matter (only in contact with food/drinks once), needs to be heat resistant, transparency doesn't matter?, needs to be reasonably rigid.

(b) Plastic E, firstly, it must be very strong to withstand the physical movement and friction in a working machine, it withstand heat from friction, chemical resistance need not be high, high cost acceptable for a specialised non-disposable part of a machine.

(c) Plastic F, needs to be reasonably cheap but heat and chemical resistant.

(d) Plastic A, it needs to be transparent to read the calibration marks accurately, it needs to be chemically and heat resistant to safe to use in the laboratory, it needs to be rigid to retain its accuracy, the high cost would be acceptable for a valuable measuring instrument in the laboratory, its not disposable.

(e) Plastic C, most important that it can be made into fibres, chemical and heat resistance is the responsibility of the owner/wearer!, needs to opaque or it becomes a 'see through'!

(f) Plastic D, the plastic needs to cheap, flexible and disposable (low cost), heat and chemical resistance are not important - responsibility of user!

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