Elsevier

Polymer Degradation and Stability

Volume 99, January 2014, Pages 223-232
Polymer Degradation and Stability

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

https://doi.org/10.1016/j.polymdegradstab.2013.10.026Get rights and content

Abstract

The 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).

Introduction

Many contemporary synthetic polymers require one or more chemical additives to enable them to carry out an intended function effectively. In the surface coatings sector, for example, the polymer provides the binder for a coating and the pigment for the aesthetic, but several types of functional additive are also required for a successful formulation. Thus, additives for rheology control, pigment dispersion, wetting, levelling etc. are commonly found in a wide range of coating types. Many of these additives have performed the role for which they were designed after the coating has been applied and dried (or cured). However, additives such as UV-absorbers and the so-called hindered amine light stabilisers (HALS) function during the service lifetime of the coating, and their role is to retard the degradation of the coating caused by the continuous barrage of environmental insults which can lead to compromised performance. HALS have been commonly employed in automotive, wood and plastic coatings for decades [1], [2], [3], [4], [5], [6], and the last 10 years has seen an increase in their use in coil coatings. Coil coating is a large-scale process for continuous painting of steel strip at speeds of up to 200 m min−1. The pre-painted steel strip thus produced is used in many different applications, the most severe of which is that employed in roofing, where the product needs to retain good appearance in service for 20 years or more. In turn, this places considerable emphasis on HALS to preserve the aesthetic and functional roles of the surface coating by protecting the polymer from degradation. Therefore, the optimisation of these compounds for such applications is of considerable interest; however, this first requires a thorough understanding of the chemistry associated with the protection of polymers by HALS. It is widely believed that HALS operate as chain-breaking antioxidants, undergoing oxidation of a heterocyclic amine to an aminoxyl radical, although the exact mechanisms by which this occurs is still the subject of investigation. It is this persistent aminoxyl radical that acts as a free radical scavenging intermediate and is thought to be involved in converting deleterious free radicals to less harmful even-electron species. As a result, regeneration of the aminoxyl radical occurs, theoretically allowing the process to repeat indefinitely [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. However, empirical evidence suggests that the protective effects are finite and the use of HALS only delays the failure of polymers rather than denying it. Thus stabilisation via HALS must consist of a more complex mechanism.

Recently, Hodgson and Coote [17] deployed high-level quantum chemical calculations to compare the kinetics and thermodynamics of a dozen different reaction pathways comprising over 30 individual reactions. This allowed critical assessment of all the previously suggested mechanisms. Hodgson and Coote's analysis shows most of the mechanisms are kinetically and/or thermodynamically disfavoured with even the most favourable mechanism subject to a large activation barrier (∼150 kJ mol−1) for one of its key steps [17]. Furthermore, this mechanism does not account for previous experimental observations that suggest in situ conversion of an alkoxyamine functional group (N–OR) – analogous to an intermediate expected in an aminoxyl radical regenerative mechanism – to a secondary piperidine (N–H). This phenomenon was observed following high temperature curing of the polymer-based coating as well as subsequent exposure of the coating to accelerated weathering conditions [18]. Concordant results have also been reported in the literature for the decomposition of 2,2,6,6-tetramethylpiperidine-based HALS under thermo- and photo-oxidative conditions [19], [20].

The inability to account for these observations by any of the commonly accepted mechanisms sparked a follow-up computational study by Coote and co-workers in which a new mechanism was proposed to explain the catalytic free radical scavenging by HALS in organic materials [21]. In this proposed cycle, an aminoxyl radical traps a carbon-centred substrate radical to form an alkoxyamine, and is then regenerated in a cascade of reactions triggered by hydrogen atom abstraction at the β-position of the alkoxyamine via another substrate-derived radical. The resulting species rapidly undergoes β-scission to form a ketone and an aminyl radical, and the aminyl radical can then either be oxidised back to the aminoxyl or abstract a hydrogen atom to form a secondary amine (see Scheme 1) [21]. This secondary amine can re-enter the catalytic cycle via hydrogen abstraction with any number of substrate-derived radicals, depending on the relative concentrations. In species that degrade via tertiary substrate-derived radicals, for which β-hydrogen abstraction is not possible, alternative catalytic cycles were proposed depending on whether direct N–OR homolysis was possible or not (see Scheme 1) [21]. These were shown to be much less energetically favourable, thus providing an explanation for the lower catalytic efficiency of HALS in such cases. The activation of the HALS was also studied, and shown to vary depending on whether the starting material was a secondary amine, the N-methyl derivative or an alkoxyamine (see Scheme 2) [21].

This recent computational study, supported by previous experimental observations suggests there may be other major repositories for HALS outside of the traditional regenerative cycles. Therefore, the aim of this work is to investigate the changing functionalisation of the piperidine nitrogen by characterising the structural changes occurring to a range of HALS compounds in polymer-based coatings. In this study, the focus is on the changes that occur specifically during curing under typical curing conditions and what impact they have on the chemical structure of the HALS. This in turn has implications for its activation (and hence protective action) under subsequent in-service conditions.

We have employed desorption electrospray ionisation mass spectrometry (DESI-MS) for the analysis of ten polyester-based coil coatings each containing a different commercially available HALS compound (Fig. 1). The compounds selected are structurally diverse providing the four most common piperidinyl moieties (i.e., N–H, N–CH3, N–C(O)CH3, and N-OR). It is noted that basic HALS (N–H, N–CH3; pKa 7.5–9.7) are not typically used in acid-catalysed, cross-linked polyesters as they interfere with the curing process. They are included here however, to provide insight into the changes in functionality of HALS that are associated with curing conditions. HALS compounds retained within the coating after cure are detected in situ, characterised by tandem mass spectrometry and the results are rationalised with the aid of high-level electronic structure calculations.

Section snippets

Reagents

Methanol and formic acid were HPLC grade (Crown Scientific, Minto NSW, Australia). Chloroform and acetone were AR grade (Crown Scientific, Minto NSW, Australia). The hindered amine light stabilisers; bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (TIN770), bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate (TIN292), bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)butylmalonate (TIN144), bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate

Positive ion DESI-MS of polyester-based coatings containing HALS

The ambient ionisation technique desorption electrospray ionisation-mass spectrometry (DESI-MS) has been employed herein for the detection of polymer additives in polyester-based coil coatings. To allow detection of HALS from thermoset coatings by DESI-MS a simple, non-destructive sample preparation method was developed by our research group that exposes the coatings to acetone vapour, partially swelling the coating and mobilising the additives to the surface for detection [18]. The samples

Conclusion

The detection of ten hindered amine light stabilisers within polyester-based coil coatings by DESI-MS has proven to be rapid, straightforward and able to be performed under ambient conditions. DESI-MS/MS also provides enough qualitative information to enable the characterisation of structural changes occurring to HALS within polyester-based coil coatings. HALS containing a 2,2,6,6-tetramethylpiperidine moiety functionalised at the nitrogen (TIN292, TIN144, HOST3058, TIN123, TIN152, and TIN

Acknowledgements

M.R.L.P. and S.J.B. acknowledge funding from the Australian Research Council (ARC) and BlueScope Steel (LP0775032) and S.J.B. and M.L.C. are supported by the ARC Centre of Excellence for Free Radical Chemistry and Biotechnology (CE0561607). G.G. and M.L.C. gratefully acknowledge the generous allocations of computing resources on the National Facility of the Australian National Computational Infrastructure, as well as helpful discussions with Dr Peter Nesvadba. M.R.L.P. is the holder of an

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