Hydroxyaryl-tetramethyl-piperidines

A compound of the formula I ##STR20## or a salt thereof, wherein the substituent X has one of the formulae ##STR21## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are the same or different and each is hydrogen, a straight- or branched-chain alkyl residue having from 1 to 9 carbon atoms, a cycloalkyl residue having from 5 to 14 carbon atoms, an aralkyl residue having from 7 to 14 carbon atoms or an aryl or alkaryl residue having from 6 to 14 carbon atoms,

• Y is hydrogen, O, a straight- or branched-chain alkyl residue having from 1 to 12 carbon atoms, an alkenyl or alkynyl residue having from 3 to 12 carbon atoms or an aralkyl residue having from 7 to 12 carbon atoms.
• Z is hydrogen, an alkyl residue having from 1 to 20 carbon atoms, an alkenyl or alkynyl residue having from 2 to 20 carbon atoms, a cycloaliphatic residue having from 5 to 12 carbon atoms, an araliphatic residue having from 7 to 12 carbon atoms, an aromatic residue having from 6 to 12 carbon atoms or a .beta.-hydroxyethyl, .beta.-cyanoethyl, furan or thiophene residue or the group --COZ.sub.1 wherein Z.sub.1 has the same significance as Z, or Z.sub.1 is a group having the formula --NR.sub.5 R.sub.6 wherein R.sub.5 is hydrogen or an alkyl residue having from 1 to 4 carbon atoms, and R.sub.6 is hydrogen, an aliphatic residue having from 1 to 20 carbon atoms, a cycloaliphatic residue having from 5 to 12 carbon atoms or an aromatic residue having from 6 to 12 carbon atoms.