Biopolymers and bioplastics

Almost every confusion in this field comes from treating two questions as one. This page sets out the distinctions the rest of the cluster depends on.

Two independent axes

Where the carbon came from and what happens at end of life are unrelated. All four combinations are commercial.

Bio-based and durable: bio-PE from sugarcane ethanol, bio-PET, bio-PA11 — chemically identical to their fossil counterparts, recyclable in the same stream, and just as persistent in the environment. Their case is a feedstock and carbon-accounting case.

Fossil-based and biodegradable: PBAT and PCL are petrochemical and certified compostable. Degradability is a property of the chain chemistry, not the feedstock.

Bio-based and biodegradable: PLA, PHA, starch blends. Neither: conventional polyolefins.

A polymer degrades because microorganisms have enzymes that cleave its backbone and can physically reach it. The C–C backbone of polyethylene offers no hydrolysable bond; an ester backbone does. Feedstock is invisible to an enzyme.

Biodegradation is enzymatic, and the first step is the slow one

Mineralisation runs in two stages: extracellular enzymes cut the chain into fragments small enough to cross a cell membrane, then the microorganism metabolises them to CO₂, water and biomass. Only the second stage happens inside a cell, so the first must happen at the solid surface, by enzymes that cannot enter the material.

Three consequences follow. Degradation is a surface phenomenon — the same polymer as a film and as a moulded lid degrade on different timescales. Amorphous regions go first, because crystalline domains pack chains too tightly for an enzyme to engage — crystallinity is a rate variable. And the environment must contain organisms carrying the right enzyme.

PLA and PHA differ fundamentally, and the difference is the lesson

PLA is the counter-intuitive case. Its rate-limiting first step is not enzymatic at all but chemical hydrolysis of the ester bonds, and that requires water to penetrate the solid. Below PLA’s glass transition of roughly 55–60 °C the chains are frozen and water barely enters. Above it they gain mobility and hydrolysis accelerates sharply.

This is exactly why industrial composting works at 58 °C and nothing else does. In a home compost heap, in soil or in the sea, PLA sits below its glass transition and persists for years. PLA is genuinely compostable and genuinely not marine-degradable, and both statements are about the same physics.

PHA behaves oppositely. It is an intracellular carbon store that bacteria evolved to accumulate and then re-consume, so PHA depolymerases are widespread in soil, freshwater and marine sediment. PHA degrades at ambient temperature in unmanaged environments — the property PLA lacks.

What the standards certify, and what fragmentation is not

Every honest degradability claim names an environment and a timescale, because the standards do.

Industrial composting — EN 13432, ASTM D6400, ISO 17088 — requires roughly 90% conversion to CO₂ within 180 days under thermophilic conditions, disintegration to under 2 mm within 12 weeks, heavy-metal limits and an ecotoxicity test on plant growth. It certifies behaviour in a managed facility at elevated temperature.

Home composting (TÜV Austria OK compost HOME, the French NF T51-800 scheme), soil (EN 17033, ISO 17556) and marine (ISO 22403, ISO 19679) are separate certifications at lower temperatures over longer windows. Passing one says nothing about the others.

Finally, fragmentation is not biodegradation. Oxo-degradable plastics contain transition-metal salts that promote chain scission under UV and heat, breaking an item into invisible pieces of the same polymer. The EU Single-Use Plastics Directive (EU) 2019/904 prohibits them, and the reasoning generalises: mineralisation is the endpoint, and anything short of it is a change in particle size.