Expert: Reza Foudazi - 2/4/2009

Question
Salam! Shoma chetur hastin? Shoam mitooni ke komakam konid?

I am sorry to bother you, and thank you for your willingness to answer
questions. In science fiction, I have read a fictional idea for a new kind of
sprayable foam, for emergency services and for building semi-permanent
structures by spraying foam. This futuristic idea seems very interesting to
me. I would like to quote the idea that I read in the science fiction book, and
ask how feasible you think the foam would be. Here is the quote.

"Construction foam is a liquid polymer with suspended
nanoparticles, it foams up in the presence of nitrogen and
cures in the presence of oxygen. When exposed to air, it expands and
hardens to form a durable substance. When it is sprayed by a worker, a
barrier or other basic three-dimensional shape can be created. The strength
and thickness of the object will depend on how much foam is used and how
much the foam weighs. Construction foam is usually combined
with additional chemicals, so that it cures rapidly. Under ordinary conditions,
it is used to create temporary structures, to seal electronic components,
and to quickly make casts for injured limbs. Riot police and soldiers also use
construction foam for making temporary walls and bunkers, usually by
forming barriers and filling them with water or earth. It is hard to start
construction foam burning, but once this happens, it forms a foul black
sludge and releases toxic fumes. The foam will not cure if it
stays wet, but it will float, and it is waterproof once hardened.

Eleven liters of the polymer can make 14 cubic centimeters of hardened
foam; a barricade three meters long by one meter high by 30 centimeters
thick will take about 60 liters. In liquid form, it is about 0.6 liters per
kilogram."

I would be interested to know how feasible this kind of sprayable
construction foam would be, because I have read about amazing ways that
certain foams can be used today -- such as making emergency firebreaks
out of foam, or using foam as a military weapon to force an attacker to halt. I
have even read about using very thick panels of polystyrene, sandwiched
with wood, which provides a fast way to build very strong replacement
houses in Iraq and Afghanistan. These new houses are then covered with
adobe. This way, nothing looks strange about them, so people are
comfortable living there.

Thank you very much for our time and attention.

Answer
Hi,
This question is very difficult to answer, because always dreams cause inventions!
Ok, foam production is very sensitive and its optimization is very difficult, especially the cured type that you asked. Because, two processes are involved in it: nucleation and growth of gas bubbles, and curing. If you don't have enough nucleation, your foam will be very coarse (doesn't have good quality and properties). The growth of gas bubbles depends on the viscosity of your polymer. Your curing process also increases the viscosity with time up to infinity.
If your curing is very slow, the gas will be released and won't trap in the polymer matrix. If the curing is very fast, also no bubbles will be formed.
Now you see how much the controlling of this process is difficult!
Another point: to have a proper foamed geometry, you need a mold. A foam can not be shapen itself. Consider when you mix egg with kitchen blender, in this process you make a foam, but how is its shape!?
The foaming process involves un-controllable expansion which will have an irregular shape.
The other thing is the expansion in the presence of nitrogen does not seem realistic, because the only way for this process is a chemical reaction, and nitrogen itself is very stable and doesn't like to react with any substance!
Curing in the presence of oxygen is not impossible, although we have many curing agents which don't need even oxygen to start curing process.
Finally, polymeric materials strength is not very high in comparison to metals, although some exceptions may be found.
All in all, one of the scientists' objectives is to come to the point to be able to produce such a material, but in reality we are still far.
I wish it was helpful.
Regards,
Reza