At the wikipedia are:

Spray foam (expanding foam in the UK) is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place.

Does the mixture of isocyanate and polyol produce polyurethane? How can a mixture of only isocyanate and polyol change its volume by 30-60 times? enter image description here

Perhaps water is additionally needed there? Does the mixture of isocyanate and water produce carbonic acid?

$$\begin{array}{l} \ce{ {R-N=C=O} + H2O ->[\ce{step}\ 1] R1-\underset{ | \atop \displaystyle H}{N}-\overset{\displaystyle O \atop \| }{C}-O-H ->[\ce{step}\ 2][\ce{-CO2}] {R-NH2} + {R-N=C=O} ->[\ce{step}\ 3] -R-\underset{ | \atop \displaystyle H}{N}-\overset{\displaystyle O \atop \| }{C}-\underset{ | \atop \displaystyle H}{N}-R}{-} \end{array}$$

However, serial disposable cylinders have a simple single-component construction. enter image description here

And their injection into the bottle forms a solid foam without significant access to water. For example: https://www.youtube.com/watch?v=uSygJ97_NkU https://www.youtube.com/watch?v=L8h3y1c20ZU

Does this mean that serial disposable foam balons already contain isocyanate and polyol and water in their composition, but high pressure shifts the reaction equilibrium towards the original components without polymerization and gas evolution? And when the mixture exits to atmospheric pressure, polymerization reactions and the release of carbonic acid begin. Or do serial disposable foam balons have nothing to do with foam based on isocyanate and polyol?

  • $\begingroup$ Some details are in the safety info online. Clearly the cans are under pressure they use propane, isobutane and dimethyl ether for this and an active ingredient diphenylmethanediisocyanate and isomers and homologues which seems to be quite a mixture. The gas clearly get the liquid out of the can and 'atomising ' it in the nozzle makes the foam. Shaking first helps increase the pressure. The foam clearly continues to react when outside the can. What role air/water vapour have I don't know. $\endgroup$
    – porphyrin
    Apr 15 at 12:38
  • 2
    $\begingroup$ For ingredients and rough composition, check sample SDS. $\endgroup$
    – Karsten
    Apr 15 at 12:50
  • $\begingroup$ Note that there are also liquid, non-pressurised polyurethane glues that foam much less (Gorilla probably being the best-known manufacturer), as well, of course, as non-foaming PU glues $\endgroup$
    – Chris H
    Apr 16 at 11:03

1 Answer 1


Single component spray foams work differently to two-component spray foams

The Wikipedia article on spray foams is somewhat misleading because it mostly talks about the characteristics of industrial two-component spray foams not the consumer variety single component foams.

Two component polyurethane processes allow better control of the properties of the resulting foams as the polyol, isocyanate and catalyst can be carefully controlled.

But that is hard to do in a simple consumer product. These usually consist of mixtures of relatively reactive isocyanates with solvents and, sometimes, blowing agents. For example this SDS of one product shows the main ingredient to be a mix of aromatic diisocyanates.

This type of product relies on the reaction with water to trigger the polymerisation. Indeed the product instructions often say the surfaces need to be wet for the foam to be effective. This one says:

"Surfaces to be bonded must be firm, clean, dry and free from dust, grease or contaminants that may hinder adhesion. They must be moistened well with water."

The reaction involves several steps. The isocyanate group reacts with water to form a carbamic acid which decomposes into CO2 and an amine. The amine, in turn, reacts with another isocyanate group to form a polymer with a disubstituted urea as the link. The carbon dioxide can act as a blowing agent to create a gas-filled foam.

The need for water explains why the mix doesn't polymerise in the can.

So, it seems, that these foams don't use polyols and rely on water. And this is what that Youtube video demonstrates.

  • 2
    $\begingroup$ See this blog for a discussion of humidity as a requirement. Some foams come with a straw applicator, the contents of which do not harden. This supports the hypothesis that pressure changes alone do not cause the reaction. $\endgroup$
    – Karsten
    Apr 15 at 12:49
  • $\begingroup$ Do serial disposable single component foam balons can not to create polyurethane? And an independent question: Is the phrase "two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times" a error at the wikipedia article? Because there needs to be at least a third substance for expand. $\endgroup$
    – Imyaf
    Apr 16 at 5:55
  • $\begingroup$ @Imyaf The products of single pack foams are polyurethanes but not the same type as two-component systems. And the expansion in volume is not a feature of all polyurethanes but a desirable feature in some which requires some blowing agent in the mix or generated by the reaction. The Wiki description correct for two-component systems but is wrong to ignore other types. $\endgroup$
    – matt_black
    Apr 16 at 8:45

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