A method for stabilizing a synthetic thermoplastic material against thermal degradation, which comprises incorporating in said material one or more compounds of the formula (I) ##STR37## in which R.sub.1 is phenyl unsubstituted or substituted by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy and OH;
- n is 1, 2, 3 or 4 and, when n is 1, R.sub.2 is one of the groups of the formulae (IIa)-(IId) ##STR38## in which R.sub.3 is hydrogen, C.sub.1 -C.sub.18 alkyl, C.sub.5 -C.sub.12 cycloalkyl unsubstituted or mono-, di- or tri-substituted by C.sub.1 -C.sub.4 alkyl;
- C.sub.2 -C.sub.18 alkenyl, C.sub.7 -C.sub.9 phenylalkyl unsubstituted or substituted on the phenyl by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl and OH;
- phenyl unsubstituted or substituted by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy and OH;
- or a 5-membered to 6-membered heterocyclic group, R.sub.4 is C.sub.1 -C.sub.18 alkyl, C.sub.5 -C.sub.12 cycloalkyl unsubstituted or mono-, di- or tri-substituted by C.sub.1 -C.sub.4 alkyl;
- C.sub.3 -C.sub.18 alkenyl or C.sub.7 -C.sub.9 phenylalkyl unsubstituted or substituted on the phenyl by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl and OH;
- R.sub.5 and R.sub.6 which can be identical or different are hydrogen, C.sub.1 -C.sub.18 alkyl, C.sub.5 -C.sub.12 cycloalkyl unsubstituted or mono-, di- or tri-substituted by C.sub.1 -C.sub.4 alkyl;
- C.sub.3 -C.sub.18 alkenyl, C.sub.7 -C.sub.9 phenylalkyl unsubstituted or substituted on the phenyl by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl and OH;
- or phenyl unsubstituted or substituted by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy and OH;
- or ##STR39## is a 5-membered to 7-membered heterocyclic group and X and Y which can be identical or different are a group --OR.sub.7, --SR.sub.7 or ##STR40## where R.sub.7, R.sub.8 and R.sub.9 which can be identical or different are hydrogen, C.sub.1 -C.sub.18 alkyl, C.sub.5 -C.sub.12 cycloalkyl unsubstituted or mono-, di- or tri-substituted by C.sub.1 -C.sub.4 alkyl;
- C.sub.3 -C.sub.18 alkenyl, C.sub.7 -C.sub.9 phenylalkyl unsubstituted or substituted on the phenyl by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl and OH;
- phenyl unsubstituted or substituted by 1, 2 or 3 radicals selected from the group consisting of C.sub.1 -C.sub.4 alkyl, C.sub.1 -C.sub.4 alkoxy and OH;
- tetrahydrofurfuryl or C.sub.2 -C.sub.4 alkyl substituted in the 2-, 3- or 4-position by C.sub.1 -C.sub.8 alkoxy or by di-(C.sub.1 -C.sub.4 alkyl)amino;
- or ##STR41## is a 5-membered to 7-membered heterocyclic group, and, when n is 2, R.sub.2 is carbonyl or one of the groups of the formulae (IIIa)-(IIId) ##STR42## in which R.sub.10 is a direct bond, C.sub.1 -C.sub.12 alkylene, C.sub.2 -C.sub.12 alkylene interrupted by 1, 2 or 3 oxygen atoms or by 1 or 2>N--R.sub.13 groups with R.sub.13 being as defined above for R.sub.5 and R.sub.6 ;
- C.sub.2 -C.sub.20 alkylidene, phenyl-(C.sub.1 -C.sub.14 alkylidene), cyclohexylene, methylcyclohexylene, cyclohexenylene, phenylene, C.sub.2 -C.sub.18 alkenylene or a 5-membered to 6-membered heterocyclic group, R.sub.11 is C.sub.2 -C.sub.12 alkylene, C.sub.4 -C.sub.12 alkylene interrupted by 1, 2 or 3 oxygen atoms, cyclohexylene, cyclohexylenedimethylene, methylenedicyclohexylene, isopropylidenedicyclohexylene, phenylene, isopropylidenediphenylene or xylylene, R.sub.12 is as defined above for R.sub.11 or is tolylene, methylenediphenylene or a group ##STR43## and Z is as defined above for X and Y, and, when n is 3, R.sub.2 is aliphatic C.sub.4 -C.sub.18 triacyl, aliphatic C.sub.6 -C.sub.18 triacyl containing a trisubstituted nitrogen atom;
- aromatic C.sub.9 -C.sub.18 triacyl, heterocyclic triacyl containing up to 18 carbon atoms or 1,3,5-triazine-2,4,6-triyl, and, when n is 4, R.sub.2 is aliphatic C.sub.6 -C.sub.18 tetraacyl, aliphatic C.sub.10 -C.sub.18 tetraacyl containing 2 trisubstituted nitrogen atoms;
- aromatic C.sub.10 -C.sub.18 tetraacyl or cycloaliphatic C.sub.10 -C.sub.22 tetraacyl.