It seems I've been seeing a lot of uses for foam on this site, from a weapon to a spaceship defense mechanism. However, the details on what the foam is made of and how it works always seem lacking to me; thus, I worry that many of these foams are just science fiction.

My question is is there a foam that can be used to quickly construct airtight walls solid enough to keep humans out? I imagine using this foam on a spaceship to trap the enemy, or alternately to fix hull breaches. Perhaps there is a fast-acting variant that can be used as a grenade to immobilize enemies. As it is a foam, it could be carried around much more easily than other wall-building materials, thus I could outfit my space marines with a few cans apiece.

So, does this foaming material exist, or is there a theoritcal way to produce it? If so, how big would the initial substance be, and how long would it take to dry/harden?

The foam need not be activated by air; on the contrary, it would be great if it contains its own reactants, and can be activated in a vacuum.

It need not last; it should only be functional for a minimum of a couple hours. If it breaks down after that, that's fine.

I would like it to be as strong as possible. If it can get to concrete levels, that would be best, but as long as it's very difficult for a human to burrow through that'll work.

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    $\begingroup$ When you say "very difficult for a human to burrow through", what kind of tools does this human have access to? $\endgroup$ – Doug Warren Aug 10 '15 at 15:14
  • $\begingroup$ @DougWarren Let's assume just their hands for now, and if you can do better than go ahead. $\endgroup$ – DaaaahWhoosh Aug 10 '15 at 15:18

Foam is, by definition, simply a lot of bubbles.

Bubbles don't work in a vacuum. (Theoretically they do, but their behavior isn't interesting since they explode — emphasis on explode — immediately.)

So this gives you a few upsides and a few downsides. I could see a viable 'cement-like' foam that hardens because the water or other liquid with which it is inflated immediately evacuates and vaporizes in outer space (e.g. when repairing the wall of a spacecraft) leaving only the igneous-rock-like matrix behind.

@AndyD273 points out quite correctly that foam in a vacuum expands far more than it does in atmospheric pressure. But remember that this also means that you have far thinner bubbles in much more space, leaving you with a much weaker structure.

Your wall material would have to be pretty (1) flexible and (2) strong to begin with, even before spraying.

But this gets more complicated when working in atmospheric pressure. You're limited at the very fastest by the entropy-fueled diffusion and expansion of gas. This leads me to believe that a foam used as a quick-spray detainment weapon would need to have highly reactive gaseous compounds that are stored separately and then injected together into the foam as it's released, causing that sort of high-speed expansion you're looking for. (Side-benefit: Such a quick expansion would either release or absorb a lot of heat, depending on the mechanism. I could viably see your foam curing due to its own high heat, or freezing due to its below-zero temperatures.)

Side-note: Anyone who's tried to use their hands to tear that foam insulation stuff knows from experience that, even though it's possible, it's an enormous pain to actually do. Easy to dent it, not easy to rip.

[EDIT]: As @JDługosz points out, a foam in a vacuum can cure from the outside in... But that would lead to a pressurized interior. And we all know what happens when brittle materials harden on the outside and leave a still-liquid, still-cooling interior...

  • $\begingroup$ Interesting point about the heat transfer, it would be interesting if it got hot enough to weld doors closed... though that's probably even more unfeasible. $\endgroup$ – DaaaahWhoosh Aug 10 '15 at 18:57
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    $\begingroup$ It can work in a vacuum; it just needs more engineering. The inside of a sample doesn't know about vacuum. The surface needs to sealed and prevent bubbles from opening a channel. So have the sample dry out or change chemically where it hits vacuum, and the bubbles form and grow slowly. $\endgroup$ – JDługosz Aug 11 '15 at 2:29
  • $\begingroup$ Heat: oh yea. I've had epoxy melt the container and cure in seconds before I could even stir it, when the parts were too warm from the garrage. $\endgroup$ – JDługosz Aug 11 '15 at 2:30

The only foam I have much experience with is the expanding-foam insulation that comes in a spray can. From that, I find it fairly believable that you could plug a hole in the side of a ship with enough of it, though I'd imagine it would also unavoidably plug up a good deal of the interior of the ship. I've also unfortunately never been able to test it in vacuum.

The hard part is making it durable enough to prevent tunneling. But if the human-to-be-trapped lacks access to digging tools, you might be able to keep them in by embedding hand-lacerating caltrops of some sort in the foam mixture. (Maybe these caltrops "grow" there through rapid crystallization?) There's not much point in digging a tunnel out of prison if you bleed out halfway through.

Actually, if you use enough foam, and/or if it sets quickly enough, you could just trap the intruder like an insect in amber. That would also suffocate them almost immediately, unless they had a suit with an independent air supply, in which case you'd just have to wait until that supply runs out.

  • $\begingroup$ For availability, go to your favorite e-commerce site and do a search on "expanding foam". I was just quoted a price of $10.29 for a 12-ounce can. $\endgroup$ – Doug Warren Aug 10 '15 at 15:46
  • $\begingroup$ +1 for the crystallizing caltrops in the foam. That's a very nasty surprise for someone digging through the foam. $\endgroup$ – Green Aug 10 '15 at 17:35
  • $\begingroup$ @DougWarren Is that 12 ounces before or after? :P $\endgroup$ – Mason Wheeler Aug 10 '15 at 18:07
  • $\begingroup$ @MasonWheeler Before, obviously. And I assume that's ounces of weight, not "fluid ounces" of volume. I don't know if it gains or loses mass after application--at this point I'd have to weigh my whole house to know. : ) $\endgroup$ – Doug Warren Aug 10 '15 at 18:11

A foaming quick set epoxy grenade might work.
Basically a weaponized, military version of this expanding epoxy foam.
Inside the grenade would be multiple chambers containing either the epoxy and the hardener.
When the grenade triggers, a CO2 canister could cause the contents to mix and then be sprayed out in all directions through multiple nozzles.

Foam in a vacuum seems to be extra effective as the low pressure causes the cells to expand a lot. If the epoxy was very quick setting then the cells probably wouldn't burst before it hardened.

As @j6m8 points out, the cell walls would thin during the expansion into vacuum, which cause them to cure faster. The cells on the surface would burst, but when their walls collapsed surface tension would pull them back, thickening the outer surface, making it tougher and slowing the rate that the bubbles burst.

To further weaponize it, mix a breathable or contact anesthetic into the gas mix or hardener, so that if the cells are broken open in atmosphere the anesthetic would be released and potentially knock out the target.
In vacuum this wouldn't be effective, but the anesthetic could be switched out for something that could attack the targets suit like an acid compound, making it dangerous to attack the wall.


Shotcrete https://en.wikipedia.org/wiki/Shotcrete would probably be one of the best known structural foams currently available but there are certainly others available and is certainly able to set creating a gas tight surface and more than capable of keeping humans in or out. Good for making a permanent repair to that hole in your lunar bio-dome, probably heavier than you would want for a spaceship repair, and certainly too slow setting for foe suppression. For your emergency hull repair you would want a lighter weight substrate to quickly fill the gap and then use a heavier solid foam for the permanent repair For foe suppression just stick to sticky foam https://en.wikipedia.org/wiki/Sticky_foam.


Dental cement

Spray a foam version of dental cement onto the assailant. Use a high-powered laser to cure it. Alternatively don't even use foam. Just spray them with the liquid and use the laser. You will cover them in a thin but incredibly hard outer shell that will immobilise them.

Influence of argon laser curing on resin bond strength

The main point about using lasers is (1) the ability to aim them (2) they act at a distance and (3) they cause fast setting even if the strength isn't increased.


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