# Biopolymers and bioplastics

The two-axis distinction that organises the whole field, why PLA needs 58 °C to degrade and PHA does not, what the composting standards actually certify, and why fragmentation is not biodegradation.

Bio-based and biodegradable are independent properties. Bio-PE is renewable and permanent; PBAT is fossil and compostable — and a label that gives one without the other has told you nothing useful.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/biopolymers-bioplastics/
Updated: 2026-08-25



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.