Composite elements have the following layer structure:
(i) from 2 to 20 mm of metal,
(ii) from 10 to 300 mm of polyisocyanate polyaddition products obtainable
by reacting (a) isocyanates with (b) compounds which are reactive toward
isocyanates in the presence of from 1 to 50% by volume, based on the
volume of the polyisocyanate polyaddition products, of at least one gas
(c) and also, if desired, (d) catalysts and/or (f) auxiliaries and/or
additives,
(iii) from 2 to 20 mm of metal.
