# Bio-based polyols and polyurethanes

Why polyurethane needs hydroxyls and vegetable oils mostly lack them, the dangling-chain problem that follows from mid-chain functionalisation, castor oil's natural advantage, and why the isocyanate half stays fossil.

A vegetable oil's hydroxyl sits in the middle of the fatty chain, not at its end — so the chain past it hangs loose, carries no load and softens the network. That geometry is the field's defining limitation.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/bio-based-polyols-biopolyurethanes/
Updated: 2026-08-25



Polyurethane is made by reacting a **polyol**, bearing hydroxyl groups, with an **isocyanate**, bearing –N=C=O groups. The two add together to form urethane linkages with no byproduct. Everything in bio-based polyurethane concerns the polyol half.

## Vegetable oils have the wrong functional groups

A triglyceride is three fatty acid chains esterified to glycerol. Its reactive features are the **carbon–carbon double bonds** in unsaturated chains and the ester groups — and crucially, **no hydroxyls**. An oil cannot react with an isocyanate as supplied.

So bio-polyols are made by installing hydroxyls. The dominant route is **epoxidation followed by ring-opening**: peracid converts the double bonds to epoxide rings, then an alcohol or acid opens each ring to leave a hydroxyl. Others include ozonolysis, which cleaves the double bond and gives a terminal group, hydroformylation, and transesterification with a polyol such as glycerol.

## The dangling chain is the structural problem

Here is the consequence that shapes the whole field. In a fatty acid, the double bonds sit **partway along the chain** — oleic acid's at carbon 9 of eighteen. Epoxidising and opening it therefore places the hydroxyl mid-chain, and the remaining segment beyond that point is attached to the network at only one end.

That segment is a **dangling chain**: it occupies volume, it moves freely, and it **bears no load**. Its effects are exactly those of an internal plasticiser — reduced effective crosslink density, a lower and broader glass transition, lower modulus, and reduced strength relative to a petrochemical polyol of the same nominal functionality.

This explains a pattern that otherwise looks like immaturity. Bio-polyols are readily used in **flexible foams** and cushioning, where softness is wanted, and are harder to use in **rigid** applications demanding high modulus and dimensional stability. It is not a purity problem; it is where the hydroxyl sits.

Two ways around it exist. **Terminal functionalisation** by ozonolysis places the hydroxyl at a chain end and removes the dangling segment, at the cost of a shorter chain and a more expensive process. And **castor oil needs no functionalisation at all** — ricinoleic acid carries a natural secondary hydroxyl at carbon 12, which is why castor is the one oil usable as a polyol directly. It still has a dangling six-carbon tail beyond the hydroxyl, so it illustrates the constraint as much as it escapes it.

**Lignin** offers something different: aromatic rings and phenolic hydroxyls, giving rigidity and thermal stability that aliphatic oils cannot. Its obstacles are its own — heterogeneity between sources and processes, limited solubility, and hydroxyls of varying reactivity.

## The other half is still fossil, and still the hazardous half

**Isocyanates remain almost entirely petrochemical.** MDI and TDI are made by phosgenating the corresponding amines, and phosgene is acutely toxic; the isocyanates themselves are potent respiratory sensitisers and a leading cause of occupational asthma.

A "bio-based polyurethane" at, say, 30% renewable content is therefore usually bio on the polyol side and conventional on the isocyanate side. **Non-isocyanate polyurethanes** avoid the chemistry entirely, reacting cyclic carbonates with amines to give poly(hydroxyurethane) — the CO₂-derived cyclic carbonate route makes this attractive on two counts at once. The materials do not yet match conventional PU across the property range, and the reaction is slower, but the approach removes phosgene and isocyanate rather than substituting a feedstock.