Waste Polymers (Edexcel A Level Chemistry): Revision Note

Disposal of Polymers

• Polymers provide a readily available, cheap alternative to many metal, glass, paper and cardboard materials in everyday use

• The low reactivity of many polymers makes them ideal for certain uses, e.g. food packaging, but at the same time creates problems with their disposal as a lot of polymers are non-biodegradable

  • There are many published articles about the environmental problems of waste plastic killing marine animals 

• One method of polymer disposal continues to be the use of landfill sites

  • This is not ideal and various initiatives are being introduced aiming to reduce this method of waste disposal in general as well as with specific regard to polymers

Recycling

• Polymer recycling reduces the amount of waste that it going to landfill sites

  • Newer landfill sites can have a recycling point where the new waste is brought before going to into the actual landfill - this is in an effort to reduce the amount of polymers (and other recyclable materials) unnecessarily going into the waste site

• The recycling of polymers can also reduce the use of finite resources

  • Lots of polymers are made from the products of cracking crude oil and it's fractions

• Recycling polymers is a time-consuming process as they have to be sorted into the different categories

  • These categories are usually shown somewhere on the plastic / polymer product with the recycling symbol and numbers or abbreviations for the different polymers, e.g.:

The recycling symbol for the polyethylene terephthalate polymer

• After sorting, the polymers are chopped, washed, dried, melted and then cast into pellets ready for use

  • However, mixed polymers can mean that this process is wasted as its produces an unusable mix of polymers 

• Certain polymers can cause problems when recycling due to their chemical composition, e.g. PVC contains a large amount of toxic chlorine which can be released

  • Modern techniques are overcoming this PVC problem by dissolving the polymer and precipitating out the recycled material 

Incineration

• Some petroleum / natural gas derived polymers are still difficult to recycle

• Since they have a large amount of energy stored within the polymer chains, these polymers can be incinerated

  • This process can then be used to boil water and use the water vapour to turn turbines inside a power station, in a  similar fashion to coal-fired power stations

• This process still causes environmental pollution as the carbon within the polymer can be released as carbon dioxide contributing to global warming

  • Other toxic waste products include hydrogen chloride and other chlorinated molecules from the combustion of PVC

Feedstock recycling

• Feedstock recycling is where waste polymers are broken down, by chemical and thermal processes, into monomers, gases and oils

• These products are then used as the raw materials in the production of new polymers and other organic chemicals

• The major benefit of feedstock recycling, compared to other methods of polymer disposal, is that it works with unsorted and unwashed polymers

Sustainability

• Chemists will often use the principles of green chemistry when designing a sustainable polymer manufacturing process

The twelve principles of green chemistry

• They will aim to:

  • Use chemicals (reactants, solvents and catalysts) that are as safe and environmentally friendly as possible

    • They also aim to reduce the amount of chemicals used, in terms of physical quantities as well as actual number of chemicals

  • Use renewable feedstock chemicals where possible

  • Reduce the energy requirements for the reaction as well as increase the energy efficiency, which has both an environmental and financial bonus

  • Improve atom economy / reduce the amount of waste byproducts

  • Consider the lifespan of the polymer, which will incorporate the idea of the polymer being suitable for its use

Limiting Problems of Polymer Disposal

• Chemists have designed ways to remove toxic waste products like HCl before they are emitted into the atmosphere

• The waste gase from the incinerator are scrubbed/reacted with a base or carbonate

• The base reacts with the acidic HCl gas, neutralising it

  • eg. CaO (s) + 2HCl (aq)→ CaCl₂ (aq) + H₂O (l) 

• Chemists have also developed biodegradable and compostable polymers

Biodegradable polymers

• Biodegradable polymers can be broken down over time by microorganisms

  • Common products from this process include carbon dioxide, water and other organic compounds

• The polyester and polyamide condensation polymers are considered to be biodegradable as they can be broken down using hydrolysis reactions

  • This is a major advantage over the polymers produced using alkene monomers (polyalkenes)

  • When polyesters and polyamides are taken to landfill sites, they can be broken down easily and their products used for other applications

Compostable polymers

• Compostable polymers are commonly plant based

  • Plant starch is being used in the production of biodegradable bin liners

  • Sugar cane fibres are replacing polystyrene in the production of disposable plates and cups

• Compostable polymers degrade naturally leaving no harmful residues