S-triazine derivatives as light stabilizers

Compounds of formula (I):

• ##STR26## in which R is C.sub.1 -C.sub.18 straight or branched alkyl, C.sub.5 -C.sub.12 cycloalkyl optionally substituted with one or more C.sub.1 -C.sub.4 alkyl;
• X is oxygen or the --NH-- group;
• R.sub.1 has the same meanings as R or it is hydrogen, an alkali metal, an ammonium group or a group of formula (II):
• ##STR27## in which A is C.sub.1 -C.sub.18 straight or branched alkyl, C.sub.5 -C.sub.8 cycloalkyl, aryl optionally substituted with one or more C.sub.1 -C.sub.4 alkyl, R.sub.3 is hydrogen or methyl and n can be an integer from 1 to 10;
• R.sub.2 has the same meanings as R when X is the --NH-- group;
• and it has the same meanings as R.sub.1 when X is oxygen.