Biodegradable Polymers and Plastics

Most plastics (polyethylene, polypropylene, PVC) are made from long synthetic polymer chains with strong C–C and C–H bonds. Microbial enzymes cannot break these bonds → the plastic persists in the environment.

A polymer is biodegradable if it contains bonds that enzymes can hydrolyse (break using water):

• Ester bonds (-COO-) — broken by esterases
• Amide bonds (-CO-NH-) — broken by proteases
• Bonds made from natural monomers (sugars, amino acids, lactic acid) that microbes recognise

1. PLA — Polylactic Acid

• Made from lactic acid (fermented from corn starch or sugarcane)
• Decomposes in industrial composting conditions (high temperature + humidity)
• Used for: food packaging, disposable cups, 3D printing filament

2. PHB — Polyhydroxybutyrate

• Produced inside bacteria as an energy storage molecule
• Fully biodegradable in soil and water
• Expensive to produce — not widely used yet

3. Starch-based polymers

• Starch mixed with polyesters → partially biodegradable packaging

4. Natural biodegradable polymers

• Cellulose (cotton, paper), starch, protein (wool, silk)
• Already found in nature — completely biodegradable

• ~8 million tonnes of plastic enter oceans each year
• Plastic photodegrades (breaks into microplastics under UV) but does NOT biodegrade
• Microplastics (<5 mm) are ingested by marine animals → enter the food chain → reach humans
• Plastic bags take ~400 years; PET bottles ~450 years to decompose

1. Replace single-use plastics with biodegradable alternatives
2. Recycling: PET bottles can be recycled to make polyester fibre
3. Reduce and reuse: best environmental option
4. Bioplastics: PLA, PHB as packaging material
5. Extended Producer Responsibility (EPR): manufacturers take back plastic

• Biodegradable = broken down by microorganisms
• PLA is the main biodegradable synthetic polymer to know (made from lactic acid)
• Why plastics aren't biodegradable: synthetic C–C backbone, no enzyme can break it
• Microplastics: physical fragmentation without biodegradation
• Natural polymers (cellulose, starch, protein) ARE biodegradable

1. Why doesn't ordinary polythene biodegrade?
2. What is PLA made from, and why is it biodegradable?
3. Distinguish between biodegradable and non-biodegradable with two examples of each.
4. What are microplastics and how do they form?
5. Stretch: If PLA biodegrades, why do scientists say it still has environmental drawbacks?