Bio-based PET: bio-MEG and bio-PTA

PET is made from two monomers, and the bio-based versions of them are at completely different stages. Understanding why is a lesson about what biology gives cheaply and what it does not.

The molecule is unchanged, and that is the point

Bio-PET is a drop-in. The polymer is identical to fossil PET: same repeat unit, same melting point, same clarity, same behaviour in an existing recycling stream. Nothing about its end of life changes, and it is no more biodegradable than any other PET.

That is the deliberate design goal. A drop-in requires no new bottle plant, no new sorting infrastructure and no reformulation — it substitutes carbon and nothing else. The trade-off is that all the benefit is upstream, in the feedstock and its life-cycle accounting, and none of it is in the product.

Why the two monomers are so different

Monoethylene glycol is straightforward. Bioethanol dehydrates to ethylene, ethylene oxidises to ethylene oxide, and hydration gives MEG. Each step is existing industrial chemistry running on a renewable input, and bio-MEG has been commercial for over a decade.

Purified terephthalic acid is not. PTA is a benzene ring carrying two carboxylic acids at opposite positions. Conventionally it comes from para-xylene, oxidised in acetic acid over a cobalt–manganese–bromide catalyst — and para-xylene comes from petroleum reforming, which produces aromatics abundantly.

Living systems do not. Cells build molecules from acetyl-CoA and sugars, and their routes to aromatic rings — the shikimate pathway to phenylalanine, tyrosine and tryptophan — are tightly regulated because those products are needed in small, precise amounts. Nothing in metabolism resembles a bulk aromatics stream. Renewable aromatic carbon is therefore genuinely scarce, and where it exists in quantity it is lignin, which is heterogeneous and difficult to depolymerise cleanly.

The routes under development reflect that difficulty. Bio-para-xylene can be made from bio-isobutanol by dehydration, oligomerisation and dehydrocyclisation, or from sugar-derived furanics by Diels–Alder cycloaddition — multi-step sequences building the ring from aliphatic pieces.

The 30 per cent figure, explained

A bottle described as “up to 30% plant-based” is being accurate, and the number is mass, not marketing. In PET’s repeat unit, the ethylene glycol contributes roughly 30% of the mass and terephthalic acid roughly 70%. Bio-MEG alone therefore caps the bio content near 30%, and reaching higher requires bio-PTA.

This is worth stating precisely because it is where the claim is most often misread. Thirty per cent renewable carbon is a real figure about the feedstock. It says nothing about degradability, recyclability or toxicity, all of which are identical to conventional PET.

PEF is the other answer, and it is a different polymer

Rather than reconstructing terephthalic acid, PEF replaces it. 2,5-furandicarboxylic acid (FDCA) is made from sugars via hydroxymethylfurfural, and its furan ring is a five-membered oxygen heterocycle rather than a benzene ring — a structure biology reaches far more readily.

Polymerised with MEG it gives polyethylene furanoate, which is fully bio-based and has genuinely better barrier properties than PET: substantially lower oxygen and carbon dioxide permeability, attributed to the furan ring’s restricted rotation, which suppresses the chain motions that let small molecules hop through.

The cost is compatibility. PEF is not PET. It has a different melting point and different processing behaviour, and it is a contaminant in the PET recycling stream — above a low threshold it degrades recycled PET quality. So the drop-in and the improved polymer are alternative strategies, not stages of one, and choosing between them is a choice between an existing infrastructure and a better material.