From this article:
http://securitysolutions.com/mag/security_stuck_security/
"Sticky foam is a hydrocarbon solution held in pressurized containers built into steel doors. Should attackers attempt to break through a sticky foam door, they will rupture the container and expose the solution to air. Air transforms the liquid solution into a solid, and the sticky foam explodes out of the door, expanding by a 40-to-1 ratio."
They even point out that the defender can deploy it by shooting a hole in the door with a rifle. I'd watch where I stood when I did that, tho'...