Biodegradable polyurethane foams

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.