7.7.5 Degradabiity of Polymers

• Many of the polymers in use have been produced through addition polymerisation of alkenes
• The (poly)alkene chains are non-polar and saturated
• This makes them chemically inert and, therefore, non-biodegradable
• (Poly)alkenes can be melted and recycled for new uses
  • However, even in the new applications, the (poly)alkenes are not biodegradable

• Recycling plants can burn used plastic materials
  • The energy released from burning can be used to generate electricity
  • Burning plastics in oxygen releases carbon dioxide and water (complete combustion) which can contribute to global warming

• Polyesters and polyamides are biodegradable polymers for a number of reasons
  • One such reason is their ability to breakdown with the use of light
• Carbonyl groups (C=O) along polymer chains are able to absorb energy from the Electromagnetic Spectrum, in particular ultraviolet (UV) light
  • Absorbing UV light weakens the carbonyl areas of polymers and breaks them down into smaller molecules

Disadvantages of photo degradability

• Despite this ability being a great advantage of polyesters and polyamides, it may pose problems when the polymers are repurposed
• When applied to a new use, the biodegradability could give a weaker polymer
• Breaking down polymers also poses another challenge
  • Once used, polymeric materials are taken to landfill sites where many other materials are piled on top of each other
  • This could mean that photodegradable polyesters or polyamides do not have access to UV light in order to break down naturally

Biodegradable polymers

• Both polyesters and polyamides 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

Hydrolysis of polyamides

• Hydrolysis is the breakdown of molecules using water
• In acidic hydrolysis, an acid (such as hydrochloric acid) acts as the catalyst
  • Polyamides such as Kevlar are heated with dilute acid
  • This reaction breaks the polyamide into a dicarboxylic acid and ammonium ions

• Alkaline hydrolysis
  • The polyamide is heated with a species containing hydroxide ions (eg. sodium hydroxide)
  • This breaks the polymer into the sodium salts of its monomers (dicarboxylic acid salt and diamines)

Hydrolysis of Kevlar, a polyamide

Hydrolysis of polyesters

• Ester linkages can also be degraded through hydrolysis reactions
  • The acidic and alkaline hydrolysis of polyethylene terephthalate (PET) is shown below
• Acid hydrolysis forms the diol and dicarboxylic acid that were used to form the polyesters
• Alkaline hydrolysis forms the diol and dicarboxylic acid salt 

 

Hydrolysis of polyethylene terephthalate (PET), a polyester