Mechanism of photostabilization of perfluoropolyether coatings by hindered amine stabilisers

Structural identification of hindered amine light stabilisers in coil coatings using electrospray ionisation tandem mass spectrometry

“…Like the product ion at m/z 123 for the HALS (1-4), the m/z 156 and 140 product ions appear to form irrespective of the substituents R 1 and R 2 , however, these ions were not consistently abundant for both (5) and (6), i.e. formation of m/z 140 was favoured during CID of (5), whereas formation of m/z 156 was favoured during CID of (6). As such these product ions while useful are less likely to provide such a robust target for detection of N-ether HALS in complex matrices.…”

“…The MS 3 spectrum shows loss of the alkylether group as 1-octene (m/z 358, −112 Da) by heterolytic β-elimination (Scheme 4(a)) and no product ions are observed at m/z 357 arising from the homolytic dissociation pathway. On the contrary, CID of the mono-functional product ion at m/z 520, formed from protonated (6) (Fig. 6(b)) reveals fragmentation via the homolytic pathway giving rise to the nitroxyl radical at m/z 437 (Scheme 4(b)) with no corresponding hydroxylamine observed at m/z 438 -this species is most likely protonated on the highly basic triazine moiety.…”

“…The C 9 H 15 + ion (m/z 123) that was abundant in the MS/MS spectra of protonated HALS (1-4), was observed to a much lesser extent in the MS/MS spectra of the N-ether HALS (5) and (6). For the N-ether HALS, this fragment arises from tertiary dissociation of unsubstituted piperidines, that are themselves secondary fragments.…”

“…This suggests that the processes are stepwise, where the loss of the first ether-moiety via a heterolytic cleavage of the C-O bond (cf Scheme 4(a)) is followed, somewhat surprisingly, by a homolytic cleavage of the N-O bond on the remaining aminoether moiety resulting in loss of a neutral alkoxyl radical [i.e. −129 Da, -C 8 H 17 O • for HALS (5) and −99 Da, -C 6 H 11 O • for HALS (6)]. While this mechanism leads directly to an aminyl radical cation as indicated in Scheme 4(c), it seems likely that this is followed by rapid hydrogen atom transfer between the two piperidine functional groups giving rise to a stabilised nitroxyl radical and a protonated secondary amine as indicated in Fig.…”

“…Gas chromatography-mass spectrometry (GC-MS) has been used to examine the effect of HALS on polymerisation (polyurethane cross linking) and aging of polymer formulations exposed to compost water and ambient environments. [6,15] In the studies of both groups, the products arising from modification and degradation of HALS were identified by matching experimental electron ionisation (EI) mass spectra to those in a reference database. While this is an excellent general approach for identifying degradation products from organic coatings, the coverage of HALS analogues and degradation products in these databases can be limited.…”