Desorption electrospray ionisation mass spectrometry of stabilised polyesters reveals activation of hindered amine light stabilisers

Abstract

. (2014). Desorption electrospray ionisation mass spectrometry of stabilised polyesters reveals activation of hindered amine light stabilisers. Polymer Degradation and Stability, Desorption electrospray ionisation mass spectrometry of stabilised polyesters reveals activation of hindered amine light stabilisers AbstractThe use of hindered amine light stabilizers (HALS) to retard thermo-and photo-degradation of polymers has become increasingly common. Proposed mechanisms of polymer stabilisation involve significant changes to the HALS chemical structure; however, reports of the characterisation of these modified chemical species are limited. To better understand the fate of HALS and determine their in situ modifications, desorption electrospray ionisation mass spectrometry (DESI-MS) was employed to characterise ten commercially available HALS present in polyester-based coil coatings. TINUVIN® 770, 292, 144, 123, 152, and NOR371; HOSTAVIN® 3052, 3055, 3050, and 3058 were separately formulated with a pigmented, thermosetting polyester resin, cured on metal at 262 °C and analysed directly by DESI-MS. High-level ab initio molecular orbital theory calculations were also undertaken to aid the mechanistic interpretation of the results. For HALS containing N-substituted piperidines (i.e., N-CH3, N-C(O)CH3, and N-OR) a secondary piperidine (N-H) analogue was detected in all cases. The formation of these intermediates can be explained either through hydrogen abstraction based mechanisms or direct N-OR homolysis with the former dominant under normal service temperatures (ca. 25-80 °C), and the latter potentially becoming competitive under the high temperatures associated with curing (ca. 230-260 °C).

Experimental evidence for competitive N O and O C bond homolysis in gas-phase alkoxyamines

“…Therefore, aminyl radicals and secondary amines observed in the degradation of alkoxyamines at normal service temperatures (up to 80 o C) are likely not formed directly from N-O bond homolysis, but by alternative processes, such as those outlined in Scheme 1(a). [12,13] …”

“…While direct cleavage of the O-C and N-O bonds directly connects the alkoxyamine with both the nitroxyl and aminyl radicals, under typical service conditions homolysis pathways are not energetically competitive with an alternative mechanism that involves a β-hydrogen elimination step and connects these intermediates within the catalytic cycle. [12][13][14] Nevertheless, the impact of the R 3 -substituent, which varies widely depending on the polymer substrate and breakdown mechanism, on the relative energetics of the O-C and N-O bonds remains of considerable interest. [15] Scheme 1: (a) Polymer stabilisation activity of HALS by regenerative cycling of nitroxyl radicals, as described by contemporary computational cycle involving β-hydrogen abstraction from an alkoxyamine and an aminyl radical intermediate; [12] (b) Simplified mechanism of nitroxide mediated polymerisation, highlighting reversible oxygen-carbon bond formation and homolysis (k comb , k diss , and k p represent the rates of combination, dissociation, and propagation, respectively).…”

A DFT mechanistic study on the N‐acyloxyamine‐initiated controlled degradation of polypropylene

“…43,44 NOR and nitroxide-based HAS are excellent polymer antioxidants. 12,45,46 In the antioxidant process, NO˙radicals are of importance. 34 Figure 8 shows the mechanism of the transformation between N˙and NO˙radicals, and the regeneration of NO˙radicals.…”

Rapid determination of oligomeric hindered amine light stabilizers in polymeric materials

“…Although the different HALS can vary significantly in their structure and size, all of them typically contain 2,2,6,6‐tetramethyl piperidinyl groups (see Fig. ), which are responsible for the stabilizing effect .…”