Process for rigid foams

Polyisocyanate composition comprising the reaction product of a stoichiometric excess of an organic polyisocyanate and (a) substantially fluorinated isocyanate-reactive compound(s) characterized in that the organic polyisocyanate is diphenylmethane diisocyanate in the form of its 2,4'-, 2,2'- or 4,4'-isomer, or an oligomeric polymethylene polyphenylene polyisocyanate, or any mixture thereof, and the substantially fluorinated isocyanate-reactive compound is a compound of formula (I):

• [H--Y--(CH.sub.2).sub.m -(Z').sub.q' [.sub.p --A--(Z).sub.q -(CH.sub.2).sub.n --X--H (I)
• wherein n is an integer of from 1 to 11;
• m is 1 to 11;
• p is 0 or 1;
• q is 0 or 1;
• q' is 0 or 1;
• Y is oxygen, sulphur, the group of formula ##STR19## or a group of formula ##STR20## --SO.sub.2 --NR-- or --NR--, where R is hydrogen, C.sub.1-12 alkyl, or C.sub.1-12 fluorinated alkyl;
• X is oxygen, sulphur, the group of formula ##STR21## or a group of formula ##STR22## --SO.sub.2 --NR'-- or --NR'--, where R' is C.sub.1-12 alkyl, C.sub.1-12 fluorinated alkyl, hydrogen or the group of formula --(CH.sub.2).sub.n -(Z).sub.q --A--[(Z').sub.q' -(CH.sub.2).sub.m --Y--H].sub.p ;
• Z and Z' each independently are groups of formula ##STR23## or --SO.sub.2 NR" where R" is hydrogen, C.sub.1-12 alkyl, C.sub.1-12 fluorinated alkyl or the group of formula --R'"--Y--H where R'" is C.sub.1-12 alkylene;
• and either
• (i) when p is 0, A is substantially fluorinated or perfluorinated straight or branched chain alkyl group containing from 2 to 10 carbon atoms;
• or
• (ii) when p is 1, A is substantially fluorinated or perfluorinated, straight of branched chain alkylene group containing from 2 to 10 carbon atoms.