Edexcel A Level Chemistry:复习笔记3.3.7 Waste Polymers

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.:

polymer-recycling-pet-symbol

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 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

 

7.8.1-The-Twelve-Principles-of-green-chemistry

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

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