Ambient ionisation mass spectrometry of polymer additives in surface coating applications

Hindered amine light stabilisers (HALS) are one class of additive known to be effective in retarding polymer degradation and have been deployed in a range of polymer-based surface coating applications. The use of HALS significantly improves gloss and colour retention of coatings in addition to maintaining surface integrity – resulting in a superior commercial product. Despite their demonstrable efficacy, the precise mechanism by which HALS protect coatings remains an open discussion. It is widely believed that HALS operate as chain-breaking antioxidants whereby, initially, the parent compound undergoes sacrificial oxidation of a heterocyclic amine to form an aminoxyl radical. It is this persistent aminoxyl radical that acts as a free radical scavenging intermediate and is thought to be involved in converting harmful free radicals to less harmful evenelectron species. In theory, this process could repeat indefinitely but empirical evidence suggests HALS become less effective over time suggesting that further investigation of their mechanism of action is required. The overarching goal of this research was to identify molecular-level changes in HALS within polymer systems. To achieve this required the development of new analytical methods that were sensitive to changes in molecular structure and abundance of HALS present in polymer-based surface coatings. The scope of this PhD project encompassed two major objectives. Firstly, to develop ambient ionisation mass spectrometric methods capable of interrogating additives within polymer-based surface coatings. Traditional mass spectrometry (MS) methods have long been employed for the characterisation of both polymers H vi and polymer additives but recent developments in the field suggest new approaches could provide distinct advantages for polymer analysis. Notable among these technological advancements are a class of desorption/ionisation methods capable of analysing solid and liquid material in its native state, under ambient conditions. These ambient ionisation MS methods are distinct from traditional MS in that they permit direct desorption and ionisation of analytes from a wide variety of substrates with minimum sample preparation, i.e., extraction, pre-concentration, and chromatographic separation are not required. Uncovering the true potential of these ambient sampling and ionisation methods for MS of synthetic polymers and their components is a major focus of the research undertaken for this thesis. The second objective was to use these optimised techniques to characterise changes in molecular structure and abundance of HALS in different polymers - mostly polyester and polyacrylate surface coatings - induced by different preparations and in-service conditions Results reported herein accomplish the objectives of method development and application. Three ambient ionisation mass spectrometry methods have been developed for polymer and polymer additive analysis. These are desorption electrospray ionisation (DESI), liquid extraction surface analysis (LESA), and paint spray ionisation – a new technique developed as a part of this PhD project. All three techniques are capable of detecting HALS directly from within the bulk polymer, removing laborious sample preparation steps required with traditional MS. Each technique is able to provide complementary information on the spatial distribution and change in molecular structure of HALS compounds. Applying these methods to polyester paint samples led to the discovery that all N-functionalised HALS (N-CH3, N-C(O)CH3, and N-OR) generate a substantial population of secondary amine (N-H). Detection of this molecular-level change represents key experimental evidence for a major role for secondary amines as intermediates in mechanisms of HALS stabilisation of polymers. These findings are consistent with the results of recent high-level computational studies that also identify secondary amines as a critical reaction intermediate and repository of active HALS (G. Gryn\u27ova, K. Ingold and M. L. Coote, J. Am. Chem. Soc., 2012, 134, 12979-12988). The study suggested formation of secondary amines from N-OR functionalised HALS would primarily occur by hydrogen abstraction and subsequent β-scission under normal service temperatures (25-80 °C). At low radical concentrations or at high temperatures associated with curing (260 °C), secondary amine formation via direct N-OR bond homolysis may become competitive. Both mechanisms are consistent with the observed experimental presented data and the combination of these results represents a significant advancement in understanding the mechanisms of protection of polymers by HALS