# Biodegradable polyurethane foams

The gel and blow reactions and why their balance decides whether a foam stands or collapses, why only polyester polyurethanes can biodegrade, and what embedding a thermostable cutinase in the matrix is meant to achieve.

Water is the blowing agent, and it blows by reacting with the isocyanate — so the gas that makes the foam and the network that holds it are made by two reactions competing for the same reagent.

Source: https://en.bioecon.ru/docs/biochem-industrial/polymers-materials/biodegradable-polyurethane-foams/
Updated: 2026-08-25



A polyurethane foam is a polymer network and a gas expansion happening at once in the same vessel. Almost everything about foam processing, and the limits on making one degradable, follows from that.

## Two reactions, one isocyanate

The isocyanate group reacts with any active hydrogen, and in a foam two such reactions run together.

**The gel reaction** — isocyanate plus polyol hydroxyl — builds the urethane network, raising viscosity and molecular weight until the material can support itself.

**The blow reaction** — isocyanate plus **water** — is the source of the gas. Water attacks the isocyanate to give an unstable carbamic acid, which decomposes to **carbon dioxide** and an amine; the amine then reacts with more isocyanate to form a urea linkage. So water is the blowing agent, and the CO₂ that expands the foam is generated chemically rather than added.

Note the implication: **water consumes two isocyanate groups per molecule of CO₂**, so blowing and gelling draw on the same reagent, and a formulation is a balance rather than two independent choices.

**The balance decides whether the foam survives.** If gelling runs ahead of blowing, the network stiffens before the gas has expanded it and the foam is dense and under-risen. If blowing runs ahead, cell walls thin and rupture while the polymer is still too weak to hold, and the foam collapses. Catalysts are chosen in pairs — tertiary amines favouring the blow, organometallics the gel — precisely to tune this race, and silicone surfactants stabilise the thin cell walls meanwhile.

The urea linkages matter too: they form hard segments that phase-separate from the soft polyol segments, and that microphase separation gives flexible foam its resilience.

## Only polyester polyurethanes can degrade

The polyol supplies most of the polymer's backbone, and there are two main families.

**Polyether polyols** — polypropylene glycol and its relatives — have C–O–C **ether** linkages. Ethers are hydrolytically very stable and are not substrates for common extracellular enzymes, so a polyether polyurethane is effectively non-biodegradable.

**Polyester polyols** have **ester** linkages — hydrolysable, and the substrate of widespread esterases, lipases and cutinases.

**So biodegradable polyurethane is necessarily polyester-based**, and that constrains the product before any additive is considered: polyester PUs are already more susceptible to hydrolysis and microbial attack in service, a durability liability as well as an end-of-life feature. The same bond does both jobs.

## Embedding the enzyme, and what it must survive

The more recent approach puts the degrading enzyme **inside** the material during manufacture. A thermostable **cutinase** — a fungal or bacterial esterase evolved to attack the polyester cutin of plant cuticles, already the workhorse enzyme for PET depolymerisation — is compounded into a thermoplastic polyurethane during melt processing.

Two problems must be solved at once. The enzyme must **survive melt processing** at up to around 200 °C, which is why thermostability is the selection criterion rather than raw activity. And it must stay **inactive during service** and act at end of life, which in practice means it is starved of the water it needs until the material reaches a wet, warm environment.

The argument for embedding rather than adding externally is diffusion. An enzyme applied to a surface acts only there and cannot penetrate a solid; an enzyme already dispersed through the matrix attacks from within, so degradation is not limited to the outer surface. Published work reports substantially accelerated degradation under activated-sludge conditions relative to externally added enzyme. **Activated sludge is a named, managed, warm and wet environment** — the claim is about that setting, and does not by itself extend to soil, home compost or seawater.