At least for the Shuttle ET:
The tank’s foam is a polyurethane-type foam composed of five primary
ingredients: polymeric isocyanate, a flame retardant, a surfactant, a
blowing agent, and a catalyst. A surfactant controls the surface
tension of a liquid and thus cell formation. The blowing agent, HCFC
141b, creates the foam’s cellular structure by making millions of
tiny bubbles or foam cells.
There are four specially engineered closed-cell foams used on the
tank. The larger sections of the tank are covered in NCFI 24-124,
which accounts for 77 per cent of the total foam used on the tank.
NCFI 24-57, which has a slightly different formulation than NCFI
24-124, is used on the aft dome, or bottom, of the liquid hydrogen
tank. PDL 1034, hand-poured foam used for filling odd-shaped
cavities, and BX 250/265 foam is used on the tank’s “closeout”
areas. During the early days of the External Tank’s development,
PDL was an acronym for Product Development Laboratory, the first
supplier of that foam. NCFI 24-124 and NCFI 24-57 are mechanically
sprayed foams; BX 250/265 are applied mechanically on the feedlines
and domes inside of the intertank and manually-applied, or
hand-sprayed in the closeout areas.
Note: NCFI stands for North Carolina Foam Industries.