Mechanism of synergistic stabilization by hindered amines

Abstract

Tetramethyl piperidine (TMP) and several derivatives have been studied for their activities in various modes of polymer stabilization. The derivatives are tetramethyl piperidinoxyl (TMPO •), N-hydroxy tetramethyl piperidine (TMPOH), N-butoxy tetramethyl piperidine (TMPOR) where R is n-butyl and t-butyl, and N-methyl tetramethyl piperidine (TMPMe moiety of Tinuvin 292) as well as some commercial HALS (hindered amine light stabilizer). Inhibition of AIBN initiated cumene oxidation was used to determine free radical scavenging activity; decomposition of t-BuOOH was monitored by iodimetry, gas chromatography, and high pressure liquid chromatography; the rate constant for quenching of carbonyl excited states was measured by inhibition of diethyl ketone initiated photo-oxidation of propylene tetramer; singlet oxygen quenching was determined from the fluorescence intensity of rubrene; the effect on photo-oxidation initiated by photolysis of di- t-butyl peroxide was also studied. TMPOH and TMPMe each scavenges about one peroxyl radical, a combination of the two is strongly synergistic in this action. Further inclusion of TMPO is beneficial. The activity of hydroxyperoxide decomposition decreases in the order TMPOH > TMP > TMPO • > TMPMe. Evidence for Cope reaction between TMPMe and ROOH was seen by ESR. The combination of TMP/TMPO • shows high efficiency in the quenching of diethyl ketone excited states. Other synergistic systems for this process are TMPO •/TMPMe and TMPO •/TMPOR. TMPOH quenches singlet oxygen with an efficacy comparable to that of nickel chelates; it also interferes with di- t-butyl peroxide initiated photo-oxidation. TMPOR shows little or no activity in any of the above tests though it has been proposed to be a key ingredient in a well-known catalytic stabilization mechanism. The results showed that all the TMP derivatives, with perhaps the lone exception of TMPOR, exhibit individually only modest stabilization of some kind. For certain stabilizer functions there are combinations of compounds which are strongly synergistic. If these derivatives were produced during processing or aging of polyolefins containing HALS, their combined actions could explain the outstanding effectiveness of HALS.