Analysis and control of emissions arising from stabilised polypropylene and the incorporated additives

Arshad, Aneela (2015) Analysis and control of emissions arising from stabilised polypropylene and the incorporated additives. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Thermo-oxidative degradation of polypropylene (PP) stabilised with a range of antioxidants of commercial applications is investigated by a simultaneous analysis of the chemical changes in the slid polymer mass, both in solid and molten state, as well as in the headspace of the samples as they degraded. Provided the thermo-oxidative degradation of un-stabilised PP has already been discussed in literature in breadth, the current work focuses on the degradation of the polymer stabilised with a range of antioxidants of commercial importance. Both the degradation of the polymer and the non-polymeric constituents i.e. the antioxidants and other additives, is critically evaluated and discussed in the context of contemporary research. The antioxidants targeted in the current study belong to a range of families of stabilisers with different structural features and stabilisation mechanisms, and were mixed to give fifteen formulations of different composition. A systematic way of mixing was adopted so that the experimental observations could be interpreted in more systematic and simpler way, and maximum possible information could be extracted from them. Experimental analysis of thermo-oxidative degradation was conducted by analysing structural changes inside the solid polymeric mass, both in solid and molten state, by attenuated total reflectance infrared (ATR-IR) and chemiluminescence (CL) spectroscopic methods, respectively. The emissions generated by the aged polymeric formulations were carefully and thoroughly analysed by chromatographic-mas-spectrometric (GC-MS) methods. The GC-MS and ATR-IR analysis has been conducted at 90°C, 120°C and 150°C, over multiple ageing periods, in presence of air. It is important to mention that while the spectroscopic methods were comparatively simpler, it took an extensive method development work for the GC-MS work, especially on the account of pre-analytical extraction and trapping required for the emissions. The results of method development stage proved useful in assessing the potential of various sampling techniques i.e. static headspace extraction (SHE), solid-phase micro-phase extraction (SPME), thermal desorption (TD) and micro-scale-sealed-vessel extraction (MSSV), eventually finding SPME and MSSV as more appropriate methods for the actual analytical testing work for the current project. Through a comparison of multiple analytical techniques, it has been emphasised that conventional and so-called standard testing methods (SHS and TA) are surpassed by the methods not so commonly used in the field of polymer analysis (SPME and MSSV). It is interesting that, despite the variations in their working principles and the physical states of the samples used in each individual method, a significant correlation was found among the results of the three experimental methods (ATR-IR and CL spectroscopic, and GC-MS methods). The correlation became more pronounced for the emissions at 150°C and the CL results of the samples. In general, the emissions measured by GC-MS analysis proved to a more sensitive method to study degradation than the conventionally used spectroscopic methods. Addition, the emissions data was more value-added both in qualitative and quantitative aspects, and was more reflective of the changes in the degradation/experimental conditions i.e. time and temperature, and composition of the samples i.e. stabilisation packages. A review of the spectroscopic and chromatographic data revealed that thermo-oxidative degradation chemistry of PP changes significantly in response to any changes in the stabilisation packages. The changes are so significant that they tend to limit the relevance of important established kinetic and mechanistic explanations (proposed for the degradation behaviour of un-stabilised form of PP) for the polymer in its stabilised form. Among the emissions, a significantly low proportion of the functionalities characteristic of tertiary alkyl radical oxidation pathways, and substantially high proportion of the compounds commonly associated with oxidation chemistry that begins with primary and secondary alkyl radicals, for example, does not support the so-called tertiary carbon chemistry that tends to prevail PP degradation literature. The apparently low representation of the methine among the products in PP emissions can be due to their preferred suppression by stabiliser. Similarly, almost negligible presence of alcohols among the emissions as well as in the corresponding IR spectra of the degraded polymer, reduced the role of the homolytic hydro-peroxide decomposition. It is important because, although significantly undermined for thermo-oxidative degradation in PP, homolytic decomposition of hydroperoxides still forms an important part of many kinetic and mechanistic schemes associated with PP degradation. Further, it has been observed that a range of compounds that have widely been reported and discussed in the context of PP degradation form either only a minor proportion of the total emissions from the stabilised PP formulations, or are too negligible to be detected even with a sensitive method like SPME-GC-MS. It has been observed that, the hydrocarbons that form the major part of the PP emissions (70-90 % of the total emissions, in general) have been previously under-represented in discussions on degradation of PP. A general trend of higher hydrocarbon emissions accompanied with relatively lower oxidation products has been observed across all formulations suggesting a relation between the two groups of compounds. Such a trend has also been observed to bear a correlation with the stabilisers used in a formulation, e.g. higher hydrocarbon and low oxidation content vi were characteristic of the formulations containing the phosphite-type antioxidant, while the reverse has been observed for the formulations with hydroxylamine replacing the phosphite. In stabilised PP, unlike the un-stabilised form of the polymer, any non-polymeric components of the formulations i.e. antioxidants/additives, catalytic residues and contaminants etc., initiate a parallel regime of degradation and stabilisation chemistry. The current study has revealed interesting synergist and antagonistic relations between a range of antioxidants from commonly used families of stabilisers, e.g. phosphites, hydroxylamine, hindered phenols, thioesters, light stabilisers and natural antioxidants etc. Such interactions, unlike the common understanding in polymer research community, were specific to the antioxidant families as well as individual antioxidants, e.g. while phosphite showed a general synergy for the hindered phenols, the interaction of the individual thioesters varied with the type of an individual hindered phenol. The effect of physical factors, e.g. thermal degradation of the parent antioxidant structure, on the stabilisation capacity of the antioxidants has been also been demonstrated through the analysis of the antioxidant degradation products. In short, the role of emissions, in conjunction with the conventional spectroscopic methods of analysis, has been appreciated as an enriched way to study early stages of PP degradation in the stabilised form of the polymer. Previously used to corroborate the macromolecular degradation mechanisms and fit into the existing theories, in the current study the emissions data has indeed been at the basis of the key questions related to PP degradation chemistry. It has also effectively been used as a framework to design more logical and fulfilling research targets and recommendations for any further works on the subject.