It is estimated that approximately 6300 Mt of plastic waste has been generated since the 1950s, of which around 9% has been recycled, 12% incinerated and 79% is accumulating in landfills or the natural environment. Plastic waste is so pervasive in the environment that it has been suggested as a key indicator of the Anthropocene era in which humans are having a substantial impact on the climate and ecosystems of Earth.
The very properties that have made polymers indispensable to modern life (durability, resilience to chemicals) are now research problems we need to find answers for – how to break down the structures of polymer materials already in circulation and how to design new polymers that are disposable in environmentally acceptable ways.
Recycling is not the answer it would seem. Traditional recycling in which plastics are sorted, shredded and melted is hampered by the inclusion of additives (many of which are toxic) such as plasticisers, flame retardants and UV stabilisers which are hard to remove. The resulting recycled plastic material is often of low quality and these plastics invariably end up in landfill – recycling simply defers the inevitable environmental problems of disposal.
Chemical recycling, in which the plastics are heated at very high temperatures in the absence of oxygen, aims to turn the plastic waste into its basic building blocks for high quality new products. However in a recent report the authors note that ‘most chemical recycling facilities in operation today produce very little usable plastic feedstock despite creating no shortage of hazardous waste that damages the environment and threatens human health and safety’.
However there is some good news. In 2016 Japanese scientists discovered a microbe that uses enzymes to break the ester bonds in poly(ethylene terephthalate), PET, a polyester commonly used to make clothing fibres and food containers.
How can biodegradable plastics and bioplastics help?
Firstly we need to be clear on what these terms mean …
Bioplastics are made from renewable biomass sources e.g. cellulose, shellac, corn-starch and soy proteins and they are often blended with polymers made from fossil fuels to improve their properties. They may or may not be biodegradable.
PLA (polylactic acid) is a polyester bioplastic made from fermented starch and it is biodegradable under industrial composting conditions where key conditions such as the microbial communities, oxygen and water concentrations, temperature and pressure are carefully controlled.
Biodegradable plastics decompose into carbon dioxide, water and biomass as a result of microbial action. The polymers may have been made from fossil fuels or they may be bioplastics.
The key to whether or not a polymer is biodegradable depends greatly on its molecular structure. Polymer chains are processed into fibres and plastics with varying percentages of amorphous and crystalline regions.
In the amorphous regions the polymer chains are tangled like noodles with intermolecular bonds between the chains (side branches and plasticisers can be used to prevent the chains packing closely together). Polymers with a high percentage of amorphous regions tend to have low heat resistance, they are tough and brittle at low temperatures but flexible or rubbery at higher temperatures (they don’t have a sharp melting point) and they are often transparent. Poly(styrene) and poly(chloroethene) are good examples of amorphous polymers.
In the crystalline regions, the polymer chains are aligned to pack closely together in a regular structure. There are very strong intermolecular bonds between the chains and often the chains are covalently bonded together to form a 3D network. Polymers with a high percentage of crystalline regions tend to have distinct melting points, high strength and they are more opaque. Nylons tend to have a high degree of crystallinity.
Polymers with a high degree of crystalline regions are less likely to be biodegradable simply because the close packing of the polymer chains makes it difficult for the microbial enzymes to get to specific chemical bonds.
The disposal of biodegradable plastics into landfill, rivers and oceans remains an environmental concern since true decomposition may only occur in industrial composting conditions (see above). In addition, some plastics labelled biodegradable only break down into microplastics rather than fully decompose into carbon dioxide, water and biomass.
The most widely used fossil fuel derived plastics such as poly(ethene), poly(propene) and poly(styrene) are not biodegradable but there are some notable exceptions:
- PGA, polyglycolic acid, is a polyester and thermoplastic fibre used for medical sutures.
It takes 4 weeks for the sutures to lose all their strength.
- PVA, polyvinyl alcohol, is unusual as it is soluble in water and it is commonly used in hydrogels for contact lenses, paper making and as a stabiliser in polyvinyl acetate adhesives.
Practice questions
- Poly-3-hydroxybutyrate, PHB, is a compostable bioplastic made by certain bacteria which has similar properties to poly(propene).
(a) Define the term ‘bioplastic’.
(b) Draw the monomer from which PHB is formed and give its systematic name.
(c) Poly(propene) can be described as a non-biodegradable petroplastic. Explain the environmental advantages of using PHB to make food packaging compared with using poly(propene).
- Nylon-2,6 is a biodegradable polyamide made from two amino acids.
(a) Write an equation for the hydrolysis of nylon-2,6 under acidic conditions.
(b) Explain how the hydrolysis of condensation polymers such as nylon-2,6 lead to their biodegradation in suitable environmental conditions.
Answers
- (a) A bioplastic is a a plastic or polymer which is made from renewable biomass sources such as maize or corn.
(b)
(c) Poly(propene) is made from the monomer propene which is obtained from the fractional distillation of crude oil. The heavier fractions undergo cracking to produce alkenes such as propene. Crude oil is a fossil fuel and is a non-renewable resource. Each stage of the production of poly(propene) is energy intensive – high temperatures and pressures are needed – and so contribute to global warming. At the end of its useful life, poly(propene) must either be recycled, incinerated or it goes into landfill. It will not degrade naturally for many thousands of years.
PHB is a bioplastic made from the action of bacteria on renewable plant material. Production of PHB has a lower carbon footprint (although this may depend on processing of the feedstock used). At the end of its useful life, PHB will biodegrade into non-toxic products (the CO2 produced approximately equates to that used by the plants in photosynthesis).
2. (a)
(b) Hydrolysis of an amide link / bond is the reverse of the condensation reaction that forms the polymer from its monomers.
The C-N bond is broken by the addition of water to form a dicarboxylic acid monomer and a diamine monomer, or in the case of nylon-2,6, two amino acids are formed.
These hydrolysis reactions are catalysed by aqueous acids, H+(aq), aqueous alkalis, OH–(aq) or specific microbial enzymes in the case of biodegradation.
The greater the number of amide links in the polymer chain, the greater the rate of biodegradation, although polymers with a higher percentage of crystalline regions will degrade more slowly as as the amide links are less accessible to water and enzymes. The environmental conditions for optimum enzyme activity are important in terms of pH, temperature, oxygen and water concentrations.