Submitted by DI in Leila Maringer

Submitted at Institute of Analytical Chemistry

Supervisor and First Examiner Micro- and macroscopic o.Univ.-Prof. DI Dr. Wolfgang Buchberger

Second Examiner approaches to polymer Univ.-Prof. DI Dr. Christian Paulik stabilizer analysis for November 2017 solar thermal systems

Doctoral Thesis to obtain the academic degree of

Doktorin der technischen Wissenschaften in the Doctoral Program

Technische Wissenschaften

JOHANNES KEPLER UNIVERSITY LINZ Altenberger Str. 69 4040 Linz, Austria

www.jku.at DVR 0093696

STATUTORY DECLARATION

I hereby declare that the thesis submitted is my own unaided work, that I have not used other than the sources indicated, and that all direct and indirect sources are acknowledged as references. This printed thesis is identical with the electronic version submitted.

Linz, November 2017

Leila Maringer

DANKSAGUNG

„Great things in business are never done by one person. They’re done by a team of people“ Steve Jobs

Am Ende meiner Dissertation nun angekommen, möchte ich mich bei all jenen bedanken, ohne die der erfolgreiche Abschluss meiner Arbeit nicht möglich gewesen wäre. Mein Dank gilt meinen Professoren Prof. Wolfgang Buchberger und Prof. Christian Klampfl für das persönliche Engagement am Institut, die fachliche Unterstützung und das ungezwungene und offene Arbeitsklima. Besonders geschätzt habe ich stets die Freiheit, die uns Dissertanten bei der Umsetzung unserer Arbeit entgegengebracht wurde. Weiters bedanken möchte ich mich bei meinen Projektpartnern Prof. Gernot Wallner, DI David Nitsche, DI Jürgen Link, DI Andreas Höllerbauer und DI Michael Grabmann für die gute Zusammenarbeit und die fruchtbaren Projektmeetings.

Ein großes Danke geht auch an meinen Bürokollegen Markus Himmelsbach, der mit seinem Witz und Humor meinen Arbeitsalltag sooft bereichert hat. Ohne sein unübertroffenes Talent für die Behebung von Instrumenten- und Computerfehlern wäre ich manchmal ernsthaft verzweifelt.

Auch bedanken möchte ich mich bei allen Kolleginnen und Kollegen am Institut für die lustigen Gespräche und Momente. Im Speziellen möchte ich mich jedoch bei unserem „Dissertationskolloquium“ Lisa Emhofer und Georg Kreisberger bedanken. Zusammen haben wir eine grandiose Zeit hier am Institut verbracht. Die gemeinsamen Konferenzen mit euch haben bleibende Erinnerungen hinterlassen!

Apropos Spaß, danke auch an dich liebe Susi für unsere legendären Donnerstagabende im Herberstein. Mit deiner lustigen Art und ansteckenden Lebensfreude hast du mich bei meiner Arbeit immer wieder inspiriert und motiviert.

Besonderer Dank gilt auch meiner Familie für das Vertrauen und die große Wertschätzung meiner Person. Vieles von dem, was ich bis jetzt erreicht habe, verdanke ich den Erfahrungen, die ich zuhause sammeln konnte.

Lieber Georg, was ich dir sagen möchte, lässt sich in Worte kaum fassen. Du bist in den guten aber auch in den schwierigen Momenten immer bedingungslos zu mir gestanden, hast mich mit deinen verrückten Ideen und Aktionen oft zum Lachen gebracht und mir stets eine starke Schulter zum Anlehnen geboten. Dafür schätze und liebe ich dich!

i

TABLE OF CONTENTS

Abstract ...... 1

Kurzfassung ...... 3

Statement of Co-Authorship ...... 5

1. Introduction ...... 6

1.1. Solar thermal systems based on polymer materials ...... 6

1.2. Polymer degradation and stabilization ...... 8

1.2.1. Polymer degradation pathways ...... 8

1.2.2. Polymer stabilizer classes ...... 10

1.2.2.1. Primary antioxidants...... 10

1.2.2.2. Secondary antioxidants ...... 13

1.2.2.3. Hindered amine light stabilizers (HALS) ...... 15

1.2.2.4. UV-absorbers ...... 16

1.3. Analytical methods for polymer stabilizer analysis ...... 18

1.3.1. Direct methods ...... 18

1.3.2. Chromatographic methods ...... 20

2. Investigation of stabilizer interaction mechanisms ...... 22

2.1. Methods for rating of stabilizer performances ...... 22

Research paper 1: Structure elucidation of photoluminescent degradation products from polyolefins and evaluation of stabilizer performances ...... 24

2.2. Interactions of HALS and phenolic antioxidants in squalane ...... 33

Research paper 2: The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants ...... 35

2.1. Interactions of HALS and phenolic antioxidants in polypropylene ...... 44

2.1.1. Results from HALS-phenol test series ...... 44

2.1.2. Results from HALS-phenol ox. test series...... 45

2.1.3. Influence of the specimen thickness ...... 47

3. Investigation of the polymer additive distribution ...... 49

Research paper 3: Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy ...... 50

ii

4. Conclusions ...... 59

5. References ...... 62

Curriculum Vitae ...... 70

Abstract 1

ABSTRACT

Due to the easy accessibility and availability, the interest in solar energy as renewable energy source has grown rapidly within recent years. Solar energy can be stored either indirectly as electric energy using photovoltaics or directly as thermal energy in solar thermal systems. To be able to compete with energy prices from non-renewable energy sources, commercially available solar systems have to be optimized in terms of the costs. Within the SolPol-4/5 project that provides the framework of this thesis, inexpensive polymeric materials used for the absorbers and the hot water heat stores were developed and evaluated according to their mechanical stability and degradation behavior. To guarantee a required durability of at least 20 years, various polymer stabilizers and additives are usually employed. In the best case, stabilizers with different mechanisms of actions complement each other, thereby contributing to an enhanced material stability. However, there are certain stabilizer combinations leading to unpredictable antagonistic interactions, making it necessary to investigate interaction reactions before preparing real polymer materials. To obtain such results within an acceptable time, accelerated aging experiments of polymer samples containing the stabilizers of interest are usually performed. To save time and reduce costs even further, within this thesis the polymer-mimicking solvent squalane was employed and tested in terms of applicability and validity. The major advantage of squalane arises from the fact that it is liquid at ambient conditions allowing to simply dissolve the stabilizers in the matrix without requiring a complex and time-consuming polymer extrusion process. The first publication presented in this thesis demonstrates that squalane may be perfectly suited for rating a large number of polymer stabilizers and combinations thereof using an approach based on high performance liquid chromatography (HPLC) and fluorescence detection. Similar to polypropylene, squalane reveals aging-induced photoluminescence emissions, which correlate with the extent of the matrix degradation and may therefore be used to discover synergistic and antagonistic stabilizer interaction phenomena. Investigation of the photoluminescent squalane signal applying high-resolution Orbitrap mass spectrometry (MS) revealed a formation of unsaturated carbonyl squalane compounds with different chain lengths. The extraction of the stabilizer-containing squalane samples with organic solvents additionally allows the investigation of the various reaction and degradation products of stabilizers, which is presented in the second research paper of this thesis. The study of the degradation products of hindered amine light stabilizers (HALS) and phenolic antioxidants using HPLC coupled to UV and high-resolution quadrupole time of flight MS detection enabled the elucidation of the observed synergism under UV-light. It was shown that the phenolic antioxidant Irganox 1330 is oxidized to a conjugated quinoid derivative, thereby developing UV-light-absorbing abilities. The oxidized antioxidant acts like a UV-absorber preventing or slowing down the degradation of the squalane matrix. However, without the protection of the HALS, the phenol may not develop its activity since

Abstract 2

it is degraded by the UV-light, so that this interaction comprises a synergistic effect. The stabilizer formulations applied in squalane were additionally tested in real polymer samples. The polymer films were analyzed with respect to stabilizer concentrations and changes of molecular weight during accelerated aging experiments. Assuming that stabilizers are distributed homogeneously within polymer materials, a monitoring of the concentrations during aging may provide useful information about the degradation behavior. It can however be expected that material failures like crack formation are caused by stabilizer inhomogeneities. The third paper of this thesis is dedicated to the imaging of polymer additives within a polypropylene matrix applying confocal fluorescence microscopy with a fluorescent whitening agent as stabilizer model compound. The superior method sensitivity enabled the imaging of samples containing less than 0.5 mass percent (wt%) additive without sacrificing time and lateral resolution as it is the case for less sensitive imaging techniques like IR or Raman microscopy.

Kurzfassung 3

KURZFASSUNG

Aufgrund der einfachen Zugänglichkeit sowie Verfügbarkeit ist das Interesse an Solarenergie als erneuerbare Energiequelle in den letzten Jahren rasant gewachsen. Solarenergie kann entweder indirekt als elektrische Energie unter Verwendung von Photovoltaik oder direkt als thermische Energie in solarthermischen Anlagen gespeichert werden. Um mit den Energiepreisen aus nicht erneuerbaren Energiequellen konkurrieren zu können, müssen handelsübliche Solaranlagen im Hinblick auf die Anschaffungskosten optimiert werden. Im Forschungsprojekt SolPol-4/5, welches den Rahmen dieser Arbeit liefert, wurden kostengünstige polymere Materialien für den Einsatz als Absorber und Warmwasserspeicher entwickelt und hinsichtlich ihrer mechanischen Stabilität und ihrem Abbauverhalten bewertet. Um eine erforderliche Haltbarkeit von mindestens 20 Jahren gewährleisten zu können, werden üblicherweise verschiedene Polymerstabilisatoren und Additive eingesetzt. Im besten Fall ergänzen sich Stabilisatoren unterschiedlicher Klassen und tragen so zu einer verbesserten Materialstabilität bei. Allerdings gibt es bestimmte Stabilisatorkombinationen, die zu unvorhersehbaren antagonistischen Wechselwirkungen führen, weshalb deren Untersuchungen vor Verarbeitung unverzichtbar ist. Üblicherweise werden die zu untersuchenden Polymere beschleunigten Alterungsverfahren unterzogen, wodurch es möglich ist Ergebnisse innerhalb einer akzeptablen Zeit zu erhalten. Um Zeit und Kosten noch weiter zu reduzieren wurde im Rahmen dieser Arbeit anstelle eines Polymers die Modellsubstanz Squalan eingesetzt und hinsichtlich Anwendbarkeit und Gültigkeit überprüft. Der Hauptvorteil von Squalan ergibt sich aus der Tatsache, dass es bei Umgebungstemperatur flüssig ist und Stabilisatoren somit in der Matrix gelöst werden können, ohne dass ein komplexes und zeitaufwändiges Extrusionsverfahren erforderlich ist. Die erste Publikation, die in dieser Arbeit vorgestellt wird, zeigt, dass sich Squalan besonders gut zum Screening einer großen Anzahl an Stabilisatoren unter Verwendung von Hochleistungsflüssigkeitschromatographie (HPLC) und Fluoreszenzdetektion eignet. Ähnlich wie Polypropylen zeigt Squalan alterungsinduzierte Photolumineszenzemissionen, die mit dem Ausmaß des Matrixabbaus korrelieren und somit als Parameter zur Feststellung etwaiger synergistischer oder antagonistischer Effekte herangezogen werden können. Die Untersuchung der Squalan-Abbauprodukte mit hochauflösender Orbitrap- Massenspektrometrie (MS), zeigte, dass die Bildung ungesättigter Squalan- Carbonylverbindungen in unterschiedlichen Kettenlängen für die auftretende Photolumineszenz verantwortlich ist. Die Extraktion der stabilisatorhaltigen Squalanproben mit organischen Lösungsmitteln ermöglicht zusätzlich die Untersuchung der verschiedenen Reaktions- und Stabilisator-Abbauprodukte, was in der zweiten Publikation dieser Arbeit diskutiert wird. Mittels Kopplung von HPLC-UV und hochauflösender Quadrupol time of flight MS konnte somit die UV-induzierte synergistische Wechselwirkung sogenannter Lichtschutzstabilisatoren (HALS) und phenolischer Antioxidantien aufgeklärt werden. Es wurde gezeigt, dass das phenolische Antioxidans Irganox 1330 zu einer

Kurzfassung 4

konjugierten quinoiden Verbindung oxidiert und dadurch UV-absorbierende Eigenschaften entwickelt. Das oxidierte Antioxidans wirkt somit als UV-Absorber und verhindert bzw. verlangsamt den Abbau der Squalan Matrix. Dieser Effekt tritt jedoch nur bei Zugabe von HALS auf, da das Phenol ansonsten durch UV-Licht abgebaut wird, bevor es seine Wirkung entfalten kann. Die verwendeten Stabilisatorformulierungen wurden zusätzlich in realen Polymerproben getestet. Die Polymerfilme wurden während beschleunigter Alterungsverfahren hinsichtlich Stabilisatorkonzentration und Molmassenänderung analysiert. Geht man von einer homogenen Stabilisatorverteilung in einem Polymer aus, so kann deren Konzentrationsbestimmung während des Alterungsprozesses wichtige Rückschlüsse auf das Abbauverhalten eines Polymers liefern. Es wird jedoch vermutet, dass genau diese Stabilisatorinhomogenitäten zu Rissbildung und in Folge zu Materialschäden führen. In der dritten Publikation wird daher die konfokale Fluoreszenzmikroskopie als Methode zur Bestimmung der Verteilung von Additiven in Polypropylen vorgestellt. Polymerproben werden dafür mit einem fluoreszierenden Weißmacher versetzt, welcher als Modellsubstanz stellvertretend für nicht fluoreszierende Stabilisatoren eingesetzt wird. Durch die hohe Messempfindlichkeit der Fluoreszenzmikroskopie war es möglich die Additivverteilung in Proben mit weniger als 0,5 Massenprozent (Ma%) Additiv abzubilden, ohne dabei Messzeit oder Auflösung zu opfern, wie es bei weniger empfindlichen bildgebenden Verfahren wie IR- oder Raman-Mikroskopie der Fall ist.

Statement of Co-Authorship 5

STATEMENT OF CO-AUTHORSHIP

Paper 1: Maringer L, Himmelsbach M, Nadlinger M, Wallner G, Buchberger W. 2015. Structure elucidation of photoluminescent degradation products from polyolefins and evaluation of stabilizer formulations. Polym. Degrad. Stab., 121: 378-84.

Concept development, experimental work, data evaluation and interpretation as well as writing of the paper were done by L. Maringer. M. Himmelsbach helped with the operation of the devices that were included in the work. M. Nadlinger was involved in the structure elucidation of the photoluminescent squalane degradation products. W. Buchberger offered analytical expertise and was involved in final corrections of the manuscript. G. Wallner as a member of the Institute of Polymeric Materials and Testing (JKU Linz) provided the polymer samples for the work.

Paper 2: Maringer L, Roiser L, Wallner G, Nitsche D, Buchberger W. 2016. The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants. Polym. Degrad. Stab., 131: 91-7.

Concept development, experimental work, data evaluation and interpretation as well as writing of the paper were done by L. Maringer. L. Roiser as a member of the Institute of Organic Chemistry (JKU Linz) synthesized the oxidized phenolic model compound used in the work. W. Buchberger offered analytical expertise and was involved in final corrections of the manuscript. D. Nitsche as an employee of AGRU Kunststofftechnik GmbH (Bad Hall) and SolPol-4/5 project partner created the idea of the paper. G. Wallner as a member of the Institute of Polymeric Materials and Testing (JKU Linz) was involved in selecting the stabilizer formulations for the experiments.

Paper 3: Maringer L, Grabmann M, Muik M, Nitsche D, Romanin C, Wallner G, Buchberger W. 2017. Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy. Int. J. Polym. Anal. Charact., DOI: 10.1080/1023666X.2017.1367120.

Concept development, sample preparation, data evaluation and interpretation as well as writing of the paper were done by L. Maringer. M. Grabmann and G. Wallner both members of the Institute of Polymeric Materials and Testing (JKU Linz) prepared the polarized optical microscopy images. M. Muik and C. Romanin both members of the Institute of Biophysics (JKU Linz) prepared the confocal fluorescence microscopy images of the polymer samples. W. Buchberger offered analytical expertise and was involved in final corrections of the manuscript. D. Nitsche as an employee of AGRU Kunststofftechnik GmbH (Bad Hall) and SolPol-4/5 project partner was involved in the selection of the polymer materials used for preparation of the samples.

Introduction 6

1. INTRODUCTION

1.1. Solar thermal systems based on polymer materials

According to the statistics of the International Energy Agency (IEA) [1], in 2009 thermal energy used for the preparation of heat accounted for the biggest share of the total energy consumption worldwide. As can be seen from Fig. 1, the majority of this energy, needed for industrial use and residential building (housing), is gained from fossil fuels. In 2014, the share of renewable energies in the total primary energy supply (TPES) in Austria accounted for 30%, involving the sum of energy produced and imported subtracting exports and storage changes. The number of the TPES provides a good basis for comparison and is used for ranking different countries.

Energy end use sector for heat Fuel mix for production of heat

Commercial Combust. renew. Energy consumption worldwide heat 1% Industry &waste Coal& 7 Residential Peat Transport building 26 27 Solar, Wind, 20 Geothermal 42 47 44 % Heat % %

17 Electricity 9 19 27 Non-energy 5 9 use Agriculture Commercial Petroleum Gas building

Fig. 1: Left: Energy consumption worldwide, middle: Energy end use sector for heat, right: Fuel mix for production of heat; data relate to 2009 collected by the International Energy Agency (IEA).

As can be seen from Fig. 2 [2], in terms of renewable energies Austria ranks in the upper middle range of the European countries only outperformed by Iceland. Compared with the past, renewable energies have grown rapidly within recent years, however, the share of solar energy in the production of heat with less than 1% [1] is still very low. Due to the easy accessibility, the potential of solar energy is especially high, even though there are certain challenges to be faced. Solar thermal systems are available in various configurations [3-11]. While small-scale systems, usually employed for households, provide solar fractions (amount of energy provided by the solar heating system divided by the total energy demand) of 10% to 20% of the annual heat load, centralized large-scale solar thermal plants can raise this number up to 50%. With both systems a liquid medium is heated by a collector through absorption of sun light and then used for heating water in a secondary circuit for domestic hot water or space heating. The main challenge thereby lies in the storage of the excess energy during the summer months to refer to it in winter or

Introduction 7

whenever the system does not provide enough energy for the daily consumption. To cover the gaps between energy availability and demand, seasonal heat store solutions have been developed using pit, tank, aquifer and borehole storages [3-11]. Due to the high heat capacity, either water in its pure form or mixed with gravel or soil is commonly used as storage medium. In principal, thermal energy can be stored long-term (seasonal) as well as short-term (diurnal) with the difference that short-term storage requires less solar collector area and storage volume resulting in lower prices. Further, the water temperature in short-term storages is higher (95 °C) allowing the direct feed into the heating distribution network while long-term systems (< 50 °C) require an auxiliary heating system with heat pumps. The major problem in seasonal storage clearly lies in the heat loss of the water storage system, which depends on storage medium, elapsed time, temperature gradient and storage volume. Thus, low storage temperatures are usually applied and supporting equipment such as heat pumps are installed to increase the temperature to a useful level when the water is transferred back to the heating circuit. The storage tanks of such large-scale solar thermal plants often provide a couple of 10 to 100,000 m 3 [3-11]. Solar energy obviously has a great potential as alternative to non-renewable energies, however, research work is still needed to improve the overall performance of such systems in order to be able to compete with other heating supply technologies usually based on fossil fuels or electricity. To reduce the costs of solar thermal systems, approaches focusing on the development of novel collectors and storage tanks have been introduced within the last years [12]. One approach was based on the replacement of metal parts by enhanced plastic materials to cut the prices for the solar collectors. In 2009, the research project “Solar Energy Technologies based on Polymeric Materials (SolPol)” [13] was conceived aiming at the design, modelling, production and testing of novel plastics-based model collectors. The focus thereby was put on low-cost pumped systems as well as high-quality non-pumped integrated storage systems based on polymeric materials.

Fig. 2: Share of renewables in the total primary energy supply (TPES) in 2014 given in %. The picture is taken from the website of the International Energy Agency (IEA) [2].

Introduction 8

1.2. Polymer degradation and stabilization

1.2.1. Polymer degradation pathways

During service lifetime polymer materials are subjected to environmental influences like thermal radiation or UV-light, leading to polymer degradation reactions and in turn to material failures. Bolland and Gee [14, 15] were the first to report about the fundamental mechanism of polymer degradation, which was divided into an initiation, propagation and termination step. Up to now, it is not fully understood what causes the formation of the alkyl radicals in the first step, however, it is assumed that free radicals formed from impurities or catalyst residues in the polymer are responsible. The polymer degradation pathway with its single reactions, summarized in Fig. 3 and Fig. 4, is termed as autoxidation since it proceeds automatically whenever organic materials are exposed to the atmosphere. Alkyl radicals that are formed in the initiation step (Fig. 3, eq. 1) react with molecular oxygen to give alkylperoxy radicals (Fig. 3, eq. 2), whereby the reaction rate strongly depends on the type of carbon centered radical. Tertiary alkyl radicals react 10 times faster than methyl radicals, wherefore branched hydrocarbons are oxidized more easily than comparable linear compounds. In the second step the alkyl radicals abstract a hydrogen atom from the polymer backbone to give hydroperoxides and further alkyl radicals (Fig. 3, eq. 3), thereby entering the self-propagating pathway. Under elevated temperatures during processing for instance, the lifetime of these hydroperoxides is presumed to be rather short leading to monomolecular or bimolecular decomposition reactions (Fig. 3, eq. 4,5) yielding again more radical species. Chain termination may either occur by a disproportionation of two alkyl radicals into an olefin and an alkane (Fig. 3, eq. 6) or by recombination reactions of alkylperoxy, alkoxy or alkyl radicals (Fig. 3, eq. 7-9). Besides, crosslinking (Fig. 3, eq. 10-13) and chain scission reactions (Fig. 3, eq. 14,15) take place. Chain scission decreases the molecular weight primarily by formation of ketones, while crosslinking causes an enlargement. Both reactions take influence on the molecular weight distribution of the polymer and thus lead to changes in the material properties [14-16]. To avoid or slow down oxidation-related degradation reactions, various polymer stabilizers are usually applied. The overall goal of the stabilization is to convert reactive radical species as shown in Fig. 4 into stable compounds. Depending on the working principle, stabilizers are divided into primary and secondary antioxidants, alkyl radical scavengers and UV-absorbers, whereby the working principle of UV-absorbers does not involve the deactivation of radical species or such that would decompose to radicals. Primary antioxidants inhibit the hydrogen abstraction of a peroxy radical from the polymer (rate-determining step in the propagation reaction) by donation of an easily abstractable hydrogen atom. Secondary antioxidants work as hydroperoxide decomposers, preventing the formation of alkoxy, alkylperoxy and hydroxyl radicals. Unlike that, alkyl radical scavengers trap alkyl radicals, whereby they directly interfere with the self-propagating degradation cycle and stop autoxidation [16].

Introduction 9

Autoxidative polymer degradationpathway

Alkyl Peroxy Hydro- Alkoxy radicals radicals peroxides radicals

Initiation

RH, RR  R· (1)

Propagation

R· + O 2  ROO· (2) ROO· + RH  ROOH + R· (3) ROOH  RO· + ·OH (4)

2ROOH  RO· + ROO· + H 2O (5) Termination 2R·  RH + Olefin (6) R· + ROO·  ROOR (7) R· + R·  R-R (8) R· + RO·  ROR (9) Crosslinking R· + ROO·  ROOR (10) R· + R·  R-R (11) R· + RO·  ROR (12) R· + Olefin  R-R· (13) Chain scission

R-(CH 2)2-R  R-H2C· + ·CH 2-R (14)

R3C-O·  R2C=O + R· (15)

Fig. 3: Polymer degradation pathway, R: polymer backbone. Reactive (radical) species are given in colors, red: alkyl radical, purple: peroxy radical, grey: hydroperoxide, orange: alkoxy and hydroxyl radical.

Introduction 10

Polymer degradation and stabilization strategies

Shear stress, UV radiation, heat O2

RH R· ROO· +RH

H-donors

Radical scavengers

HD

-H2O ROH RO·+·OH ROOH

+RH

Fig. 4: Strategies of polymer stabilization within the autoxidative degradation pathway, R: polymer backbone, HD: hydroperoxide decomposer. Reactive (radical) species are given in colors, red: alkyl radical, purple: peroxy radical, grey: hydroperoxide, orange: alkoxy and hydroxyl radical.

1.2.2. Polymer stabilizer classes

1.2.2.1. Primary antioxidants

Within the group of the primary antioxidants, there are two representatives, the sterically hindered phenols and the secondary aromatic amines with their structures and working principles depicted in Fig. 5 and Fig. 6. Phenolic antioxidants are by far the most commonly used stabilizer class for long-term heat protection during processing and service lifetime [16]. They easily abstract a hydrogen atom, which gave them also the name of H-donors. The main protection mechanism [17-20] consists of the conversion of alkylperoxy (ROO·) and alkoxy radicals (RO·) into hydroperoxides (ROOH) and alcohols (ROH), thereby inhibiting the hydrogen abstraction from the

Introduction 11

polymer itself (Fig. 5, A). The stability of the resulting phenoxy radical increases with the use of bulky substituents like tertiary butyl groups in the 2,6-position. After formation of the phenoxy radical, further reactions proceed. Either the phenol is regenerated via a disproportionation reaction yielding the parent phenol and the quinonid analog (Fig. 5, B) or addition reactions of alkylperoxy radicals (Fig. 5, C) and alkyl radicals (Fig. 5, D) take place.

Fig. 5: Stabilization mechanism of phenolic antioxidants. Reactive (radical) species are given in colors, purple: peroxy radical, grey: hydroperoxide, red: alkyl radical.

Similarly to the phenols, hindered amines scavenge radicals by hydrogen donation in the first step (Fig. 6, A). Unlike the phenoxy radical (from phenols), the aminyl radical is oxidized to a nitroxide radical, which may undergo two different stabilization mechanisms [21, 22] depending on the present temperature . As shown in Fig. 6 part B, below 120 °C the low temperature mechanism proceeds with two more peroxy radicals trapped, yielding in the formation of a nitrosobenzene and 1,4-benzoquininone. For the high temperature mechanism (Fig. 6, C), alkyl radicals are scavenged by a nitroxide radical, which is regenerated to its active antioxidative form within a catalytic cycle.

Introduction 12

Fig. 6: Stabilization mechanism of aminic antioxidants, A: Hydrogen donation reaction, B: Low temperature (T<120 °C) reaction pathway, C: High temperature (T>120 °C) reaction pathway. Reactive (radical) species are given in colors, red: alkyl radical, purple: peroxy radical, orange: alkoxy radical, grey: hydroperoxide.

Introduction 13

1.2.2.2. Secondary antioxidants

The main activity of secondary antioxidants is the conversion of hydroperoxides into non-radical, non-reactive and thermally stable products to avoid the decomposition into alkylperoxy, alkoxy and hydroxyl radicals. The two important representatives within the group of hydroperoxide decomposers are the trivalent phosphorus based stabilizers like the phosphites (P(OR) 3) as well as phosphonites (P(OR) 2R) and the sulfur based compounds like the thiosynergists. The stabilization mechanism of both stabilizer groups is based on a simple oxidation-reduction reaction. As shown in Fig. 7 part A, the main mechanism of phosphites is the oxidization to a phosphate, thereby converting a hydroperoxide into a stable alcohol [23-27]. Since this reaction proceeds stoichiometrically, the effect on the long-term thermal stability of the polymer is rather small. The standard application of aryl phosphites/phosphonites usually lies in the stabilization of the polymer melt during processing since they are active at high temperatures of 150 to 200 °C. To improve the overall melt stability of the polymer, phosphites are usually combined with alkyl radical scavengers like phenolic antioxidants. As can be seen from Fig. 7 part B, C and D, they may also work as primary antioxidants trapping alkylperoxy (ROO·) and alkoxy radicals (RO·). As reported by Schwetlick et al. [25], the activity as chain-breaking antioxidant however strongly depends on both the rate of their reactions with alkylperoxy radicals and on the way they react with alkoxy radicals. As can be seen from Fig. 7 part C and D, only those phosphites that react with alkoxy radicals by substitution to give an isomeric phosphite and chain-terminating aroxyl radicals may act as primary antioxidants. The oxidation reaction (Fig. 7, D) however leads to chain-propagating alkyl radicals (R·), which in turn may undergo disproportionation or fragmentation reactions under oxygen deficient conditions. It has been further reported that the chain-breaking efficiency of phenyl phosphites depends on the chemical nature of the substrate to be stabilized. While they exhibit a pronounced primary antioxidant activity in saturated aliphatic hydrocarbons and polyolefins, their chain-breaking activity in cyano and phenyl substituted polymers is very low. Another reaction pathway of phosphites is shown in Fig. 7 part E. The hydrolysis takes place at higher temperatures and results in the formation of phenols that contribute to the stabilization of the polymer. While hydrolysis is reported to take place at temperatures below 150 °C, oxidation is the preferred reaction at temperatures above 200 °C during processing.

Introduction 14

Fig. 7: Stabilization mechanism of phosphites/phosphonites. Reactive (radical) species are given in colors, red: alkyl radical, purple: peroxy radical, orange: alkoxy radical, grey: hydroperoxide.

The basic working principle of thiosynergists [28-31] is displayed in Fig. 8 and involves an oxidation of the sulfur atom to a sulfoxide or a thiosulfinate, thereby converting the hydroperoxide into a stable alcohol. Due to thermal instability, the formed oxidation products degrade to sulfenic or thiosulfoxylic acids, which constitute the true peroxidolytic antioxidants. Starting from these acids a set of oxidation reactions occurs, leading to further hydroperoxide decomposition. Since the reaction of thiosynergists and hydroperoxides proceeds over-stoichiometrically, they are perfectly suited for the long-term thermal stabilization of a polymer.

Introduction 15

Fig. 8: Stabilization mechanism of thiosynergists. The reactive hydroperoxide species is given in grey color.

1.2.2.3. Hindered amine light stabilizers (HALS)

Hindered amine light stabilizers act as radical trapping agents against thermal and light-induced degradation. By scavenging of an alkyl radical they directly interfere with the self-propagating degradation cycle, which immediately stops the autoxidation. They are usually offered in oligomeric structures with high molecular weights to avoid migration from the polymer matrix. Irrespective of their size, all HALS have the same basic structure, which is a tetramethyl piperidine group. HALS are especially famous for their excellent protection against thermal- as well as light- induced degradation [16]. The stabilization mechanism has been widely discussed in the literature [32-41] and still there are different suggestions for the detailed reaction mechanism. According to the Denisov cycle [32, 33], which is roughly depicted in Fig. 9, the parent amine is oxidized to the corresponding nitroxide radical by a hydroperoxide or an oxygen-centered radical. Nitroxide radicals further react with alkyl or alkoxy radicals to form alkoxyamines, which may be regenerated via radical trapping mechanisms.

Introduction 16

Fig. 9: Stabilization mechanism of HALS. Reactive (radical) species are given in colors, red: alkyl radical, purple: peroxy radical, grey: hydroperoxide.

1.2.2.4. UV-absorbers

The basic working principle [16] of UV-absorbers lies in the uptake of harmful UV-radiation and a subsequent dissipation to avoid photosensitization of the polymer material. UV-absorbers usually have a high absorption themselves so that they are not consumed in non-stabilizing secondary reactions. However, to develop their full activity, they need a certain absorption depth (sample

Introduction 17

thickness), which is why they provide only limited protection to thin samples such as fibers or films. The most prominent representatives are the benzophenone-type and the benzotriazole-type UV- absorbers with their basic structures and working mechanisms depicted in Fig. 10. UV-absorbers usually have electron-donating substituents to push the absorption towards wavelengths between 300 and 400 nm since the main absorption is situated in a less relevant wavelength range. A major disadvantage of UV-absorbers is the yellowing caused by the tailing absorption above 400 nm often leading to discoloration effects of the plastic materials. By variation of the alkyl group R in the alkoxy group, the performance of the stabilizers and their compatibility with the plastic substrate may be optimized. The protection mechanism of benzophenone-type UV-absorbers is based on a light-initiated keto-enol tautomerization, proceeding via intersystem crossing and vibrational relaxation transitions. As soon as the enol reacts back to the keto form, thermal energy is released to the medium. The energy transformation involves fast radiationless transitions exclusively, whereby the strength of the hydrogen bond influences the effectiveness of the respective UV-absorber. It turned out that the stronger the formed hydrogen bond, the better the resulting stabilizer performance. The stabilization mechanism of benzotriazole-type UV-absorbers also involves a proton transfer reaction, however no intersystem crossing transitions occur. The efficient conversion of the photon energy is mainly assigned to internal conversion transitions rather than proton transfer reactions as it is the case for the benzophenone-type UV-absorbers [16].

H H O O O O +hv A -kT OR OR Tautomeric form

R R R N N N B N N N N N N HO H O HO

Mesomeric forms

R N N N H O

Tautomeric form

Fig. 10: Proton transfer reactions of A: benzophenone-type and B: benzotriazole-type UV-absorbers.

Introduction 18

1.3. Analytical methods for polymer stabilizer analysis

Over the last years, a large number of techniques and approaches for the analysis of polymer stabilizers has been presented in the literature [42, 43] and still new methods are developed due to the constant innovation and improvement in the field of polymer stabilization. Especially within the group of HALS, there is a trend towards the design of high molecular weight stabilizers with masses often ranging up to 7000 Da, making it necessary to adapt existing methods. In principle, there are two different approaches employed in stabilizer analysis, the direct methods and the chromatographic techniques. The direct methods are mainly characterized by no requirements of chromatographic separation, leading to very short analysis times. Further, with a few minor exceptions (TLC-spray-MS and FI-MS), they do not require a sample preparation step allowing a straightforward analysis directly from the untreated sample. The chromatographic techniques on the other hand require an extraction of the stabilizers out from the polymer matrix prior analysis.

Within the group of the direct methods, the following techniques are included:  Desorption electrospray ionization mass spectrometry (DESI-MS)  Liquid extraction surface analysis mass spectrometry (LESA-MS)  Direct analysis in real time mass spectrometry (DART-MS)  Matrix assisted laser desorption ionization mass spectrometry (MALDI-MS)  Time of flight secondary ion mass spectrometry (TOF-SIMS)  Thin layer chromatography spray mass spectrometry (TLC-spray-MS)  Flow injection mass spectrometry (FI-MS)

The most prominent representatives in the group of chromatographic methods are:  Gas chromatography mass spectrometry (GC-MS)  Thermodesorption gas chromatography mass spectrometry (TD-GC-MS)  Pyrolysis gas chromatography mass spectrometry (Pyr-GC-MS)  High performance liquid chromatography (HPLC) with UV or MS detection  Capillary electrophoresis (CE) with UV or MS detection

1.3.1. Direct methods

The most important characteristics of the direct methods is the rapid analysis with no or just little sample preparation and a primary focus on the identification rather than the quantitation of the stabilizers. Since there is no chromatographic separation, the direct techniques cannot be coupled to nonspecific detection such as UV but require an analyte-specific MS detection allowing the assignment of the measured masses to molecular formulas. There are different ways how the analytes are ionized for detection in the MS.

Introduction 19

In DESI-MS [44, 45] an electrospray is placed in front of the sample surface. The analytes are desorbed and ionized by the charged liquid droplets arriving from the electrospray. In DART-MS [46] a gas stream (helium, argon or nitrogen) is excited by applying an electrical potential. The gas atoms/molecules react with the ambient water vapor to give protonated water clusters, which in turn work as proton donors for the analytes on the sample surface. LESA-MS [47] represents a fully automated microextraction including a robotic pipette tip system, which is positioned onto the sample surface. Through the pipette a solvent droplet is released, the analytes are extracted, the droplet is sucked into the pipette again and finally injected into a nano electrospray ionization (ESI) system, where the ions are formed via the ESI process. In TOF-SIMS [48] the sample surface is sputtered with a focused primary ion beam, secondary analyte ions are formed, which are analyzed with a TOF-MS analyzer. For MALDI-MS [49] the polymer sample is grinded and merged with a matrix (in excess) that is excited by a laser, thereby transferring the energy to the analytes, which are desorbed, ionized and introduced to the MS. In TLC-spray-MS [50], the solid polymer (or a stabilizer extract) is positioned on a triangle shaped TLC plate, which is placed in front of the MS inlet. Solvent is added and a voltage is applied, which results in the formation of an ESI spray leading to an ionization of the analytes. The FI-MS [51, 52] constitutes a simplification of a standard HPLC-MS analysis with the big advantage that the analysis time is significantly reduced. The liquid stabilizer extract is injected into a HPLC system, where the separation column is removed. For detection in the MS a multiple reaction monitoring mode is selected, allowing the simultaneous identification and quantitation of several analytes without a chromatographic separation. Depending on the respective analyte, it may however be necessary to use other ionization techniques like atmospheric pressure photoionization (APPI) or atmospheric pressure chemical ionization (APCI) due to the possibly occurring ion suppression effects in the ESI process. The most obvious advantage of the listed direct methods clearly lies in the reduction of the total analysis time through the missing sample preparation or separation unit. Moreover, they allow the analysis of high molecular weight stabilizers like the oligomeric HALS, which are neither accessible to normal GC-MS due to the low volatility nor to HPLC-UV/MS due to the amino groups, which may cause irreversible adsorption to silica-based columns. The direct methods are usually applied for gaining qualitative information, however, fail when it comes down to quantitation for reasons of irreproducibility caused by inhomogeneous analyte distribution or sample positioning. Since there is no separation of the analytes before detection, identification of the analytes via standards, using the retention time for comparison, is not possible, too. Stabilizers having the same sum formulas like isomers for instance cannot be differentiated in the MS. In general, it can be stated that the direct methods are perfectly suited for an initial analysis of a polymer sample to get an idea about the contained stabilizers and their molecular weights to choose an appropriate chromatographic method for a subsequent comprehensive analysis.

Introduction 20

1.3.2. Chromatographic methods

Due to the fact that GC-MS is limited to the analysis of small and volatile stabilizers like the monomeric HALS Tinuvin 770 for instance [53], it only plays a minor role in routine analysis. The use of specially designed high temperature GC columns that are suited for analysis of high molecular weight analytes and an optimized temperature program may increase the number of analyzable stabilizers as shown in the doctoral thesis of Sternbauer [54], however requires extensive method development. For non-volatile additives there is also the possibility of chemical modification with the aim of enhancing the volatility either by saponification with potassium hydroxide [55] or by derivatization via silylation [56], but again this requires an additional sample preparation step. Alternatively, TD-GC-MS and Pyr-GC-MS [57-67] may be employed. Both techniques are mainly based on a separation according to the boiling point of the analytes or analyte fragments. While for normal GC-MS stabilizers need to be extracted from the polymer matrix prior analysis, no sample preparation is needed for TD-GC-MS and Pyr-GC-MS. For both techniques the solid sample is heated with the difference that the temperatures used in Pyr-GC- MS are much higher, thereby leading to an analyte degradation (pyrolysis). As a result also high molecular weight stabilizers may be analyzed, which are not vaporizable with normal GC-MS or TD-GC-MS. However, in the literature it is also mentioned that Pyr-GC-MS is not able to discriminate between stabilizers belonging to the same chemical class due to unspecific fragmentation, wherefore quantitation is not possible despite the separation step in the GC columns so that HPLC-UV/MS often remains the method of choice. Reversed phase (RP) HPLC [59, 68-81] is by far the most common technique for analysis of stabilizers. Unlike GC, in RP-HPLC the analytes are not separated according to volatility but polarity, which does not necessarily depend on the molecule size, thereby increasing the range of available stabilizers significantly. With the exception of high molecular weight HALS, most antioxidants are accessible to HPLC, which is usually coupled to UV detection if the analyte shows a UV-absorption or otherwise to MS detection. Common mobile phases for RP-HPLC are mixtures of organic solvents like acetonitrile or methanol with water. For separation and quantitation of oligomeric HALS, mobile phase gradients consisting of water, methanol and cyclohexane with ammonia as addition were successfully employed [81]. Since the analysis time for such a separation lasts about 90 minutes, an approach based on a pH gradient elution with only a pre- column, lasting only 10 minutes, was recently presented in the literature [82]. The method is based on a concentration step of the HALS on the column under alkaline conditions and a subsequent single peak elution using an acidic mobile phase leading to an increased water solubility due to a protonation of the HALS amino groups. The approach is restricted to the analysis of polymer samples containing only one HALS, however in industry combinations of different HALS are hardly used.

Introduction 21

CE coupled to UV or MS detection is a further separation technique suitable for identification and quantitation of oligomeric HALS [83, 84]. It is based on a separation according to the charge to radius ratios of the respective HALS oligomers in an electric field using an electrolyte system. Although it has been shown that oligomeric HALS can be separated as well as quantified, CE-MS is not a technique for routine analysis due to the insufficient robustness but also due to the tricky MS coupling.

Investigation of stabilizer interaction mechanisms 22

2. INVESTIGATION OF STABILIZER INTERACTION MECHANISMS

2.1. Methods for rating of stabilizer performances

Stabilizer performances can be rated either by characterization of the solid polymer or by analysis of the contained stabilizers and their formed degradation products. Accelerated material aging tests are performed to obtain results in acceptable periods, since polymeric materials with a proper stabilization may be stable over years. Aging tests are usually done at elevated temperatures in dry or humid atmospheres, but also under UV-radiation to simulate real-life conditions as best as possible. Other approaches applied to accelerate the aging speed include the decrease of the sample thicknesses [85] or the use of an increased oxygen pressure in the aging environment [86]. For lifetime assessment, the experimental data (failure times) obtained are extrapolated to service temperatures ranging from -20°C to 95°C (for solar thermal systems) applying a Miner’s rule approach [87]. Other polymer characterization methods allowing the monitoring of the material degradation before the endurance limit is reached involve laser confocal microscopy, UV-VIS and infrared (IR) spectroscopy, differential scanning calorimetry (DSC), tensile testing, high temperature gel permeation chromatography (HT-GPC) and photoluminescence spectroscopy [85-95]. Laser confocal microscopy is usually applied to view optical changes on the polymer surface including material abrasion or crack formation. With IR spectroscopy carbonyl-containing degradation products are detected and carbonyl-indices may be calculated. UV-VIS spectroscopy is applied to measure degradation-induced yellowing of polymer materials, which may be caused by oxidation of stabilizers like phenolic antioxidants resulting in the formation of light-absorbing chromophores. Changes in the polymer morphology such as the degree of crystallinity are characterized by DSC, the mechanical stability is determined via tensile testing, and changes of the molecular weight distribution are detected using HT-GPC. A relatively new polymer characterization method is the photoluminescence spectroscopy [86, 88] with the main advantage of its superior sensitivity. Oxidation-induced changes are detected already in the early stages of the degradation process, long before they are noticeable with other polymer characterization methods. Stabilizer performances may be rated by subjecting the polymer samples to an accelerated aging in the first step and a subsequent material characterization by one of the listed methods. Comparison of the respective samples allows performing a stabilizer efficacy rating, even though, the sample preparation is rather time consuming. Therefore, an approach based on the use of the PP-mimicking solvent squalane as testing medium was developed and summarized in the following research paper [96]. Squalane is liquid at ambient conditions, wherefore stabilizers may be easily dissolved, not requiring an extrusion process as it is the case for solid polymers. The key advantage of the use of squalane clearly lies in the simple sample preparation that saves time as well as costs and allows the screening of a great number of samples in a reasonable time. The advantage use of squalane has been described in the literature for studies of stabilizer interaction

Investigation of stabilizer interaction mechanisms 23

mechanisms under thermo-oxidative [53] and high-energy irradiation [97] as well as for studies using a chlorinated water medium [98] to simulate the influence of chlorine (contained in tap water) on the stability of polymer materials. It is, however, not reported in the literature if the results obtained in the PP-mimicking solvent squalane reflect the situation in real polymer materials. Besides, it is not clear if the kind of aging (thermo-oxidative, UV-irradiation, chlorine exposure) has an impact on the correlation of the results obtained for PP and squalane. These issues will be discussed in the following chapters. However, within the current work, the focus was put on the development of a method for characterization of the squalane matrix after accelerated aging experiments. Therefore, thermally aged squalane samples were extracted with organic solvents and analyzed with HPLC coupled to photoluminescence detection in a single peak elution. Similar to PP, squalane reveals aging- induced photoluminescence emissions, which correlate with the extent of degradation, so that it may be used as aging parameter for rating of stabilizer performances.

Investigation of stabilizer interaction mechanisms 24

Research paper 1:

Structure elucidation of photoluminescent degradation products from polyolefins and evaluation of stabilizer performances

Published in Polymer Degradation and Stability

Investigation of stabilizer interaction mechanisms 25

Graphical presentation of the formation of photoluminescent emissions during the aging of squalane

Investigation of stabilizer interaction mechanisms 26

Investigation of stabilizer interaction mechanisms 27

Investigation of stabilizer interaction mechanisms 28

Investigation of stabilizer interaction mechanisms 29

Investigation of stabilizer interaction mechanisms 30

Investigation of stabilizer interaction mechanisms 31

Investigation of stabilizer interaction mechanisms 32

Investigation of stabilizer interaction mechanisms 33

2.2. Interactions of HALS and phenolic antioxidants in squalane

Stabilizer interaction mechanisms may be both beneficial but also unfavorable regarding the material stability. Since it is not possible to determine such interactions in advance using theoretical considerations, the respective stabilizer formulations have to be tested before they are used for preparation of polymer master batches. By definition, an interaction is termed as synergistic if the combination leads to a better performance than would be expected from the additivity law. Conversely, if the overall stabilizer efficiency is lower than those obtained from both single formulations, the interaction is termed as antagonistic. In between, stabilizers with complementary action mechanisms may also show so-called heterosynergistic interactions as it is observed for combinations of hydroperoxide decomposers and chain-breaking antioxidants like phosphites and phenols for example [99, 100]. Phosphites degrade hydroperoxides, which would otherwise form radicals leading to a consumption of the phenol. In return, phenols work as chain- breaking antioxidants, avoiding radical reactions with oxygen, thereby reducing the hydroperoxide formation and in turn the degradation of the phosphites. The study of stabilizer interactions is important to avoid antagonistic reactions, but also to find synergistic combinations to enhance the material stability. Some stabilizers like the HALS cannot be applied in single formulations since they do not provide any oxidation protection during polymer processing, wherefore the study of their interaction reactions with other commonly used stabilizers is of particular interest. For combinations of HALS with phenolic antioxidants, both synergistic as well as antagonistic reactions are described in the Iiterature [40, 53, 101-108], whereby the former only occur under UV-light, while the latter are mainly observed under thermal aging conditions. As can be seen from Fig. 11 part A, HALS and phenols may interact antagonistically via hydrogen transfer from the phenol to a nitroxide radical, followed by a radical coupling reaction of the remaining phenoxy radical and a new nitroxide radical, leading to useless phenol and HALS consumption. Another suggested pathway is described via the formation of a nitrosonium ion in the first place and a subsequent reaction of this ion with a phenol also leading to useless stabilizer consumption (Fig. 11, B). Since both the hydrogen transfer reaction as well as the nitrosonium pathway lead to the same degradation species, it is not proven yet which interaction is most likely to take place. A synergistic reaction mechanism of HALS and phenols was described by Ohkatsu et al. [40] and involves the regeneration of the hydroxylamine by an oxidized phenol, which is excited by UV-light (Fig. 11, C). Within the current work (see following research paper [109]), a different synergistic mechanism for HALS and phenols is presented based on a comprehensive study of the formed degradation products in the PP-mimicking solvent squalane. Using HPLC-UV-MS, it was shown that oxidized phenolic antioxidants with Irganox 1330 based structures work as UV-absorbers, thereby most likely preventing the polymer material from undergoing degradation. Ohkatsu’s theory of a regeneration of HALS and phenol may not be supported since there was no increase in the phenol concentration observed as it should be the case. In addition, tests done in the PP-mimicking

Investigation of stabilizer interaction mechanisms 34

solvent squalane proved an increase in the stabilizer efficiency of HALS after addition of an oxidized phenolic model compound, which supports the suggested theory.

A

H-transfer

Coupling

B

C

Fig. 11: Interaction mechanisms between HALS and phenolic antioxidants, A: Antagonistic recombination between HALS nitroxide radical and phenol, B: Antagonistic interaction between HALS nitrosonium ion and phenol, C: UV-light- induced synergistic interaction between hydroxylamine HALS and oxidized phenol. HALS nitroxide radical is given in red color, intact phenol in blue color.

Investigation of stabilizer interaction mechanisms 35

Research paper 2:

The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants

Published in Polymer Degradation and Stability

Investigation of stabilizer interaction mechanisms 36

Graphical presentation of the UV-light-induced synergistic interaction of HALS with phenolic antioxidants

HPLC-UV-MS analysis of HALS-phenol-squalane samples

Intact Oxidized phenol phenol

Squalane aging Response Response

16 17 18 19 20 21 22 23 24 Retention time / min

UV-aborber

1500 1500 O

1200 1200

900 900 O

600 600 O

300 300 Absorption Absorption mAU / Absorption mAU /

0 0 150 200 250 300 350 400 450 150 200 250 300 350 400 450 Abs. wavelength / nm Abs. wavelength / nm

Investigation of stabilizer interaction mechanisms 37

Investigation of stabilizer interaction mechanisms 38

Investigation of stabilizer interaction mechanisms 39

Investigation of stabilizer interaction mechanisms 40

Investigation of stabilizer interaction mechanisms 41

Investigation of stabilizer interaction mechanisms 42

Investigation of stabilizer interaction mechanisms 43

Investigation of stabilizer interaction mechanisms 44

2.1. Interactions of HALS and phenolic antioxidants in polypropylene

2.1.1. Results from HALS-phenol test series

Within the above discussed research paper, the performance of various formulations of the phenolic antioxidant Irganox 1330 and different HALS were investigated using the PP-mimicking solvent squalane. Irrespective of the use of monomeric or oligomeric HALS, it turned out that the two stabilizer classes exhibit a strong synergism. Comparison of the dissolved stabilizer degradation products in the single formulation with the mixed formulations showed that Irganox 1330 was completely degraded in the phenol single formulation while the formulation with the additional HALS contained the one, two and three fold oxidized phenolic degradation product. The investigation of the UV-spectra of Irganox 1330 and its oxidized forms revealed an oxidation- induced absorption shift towards higher wavelengths into an area that is typical for UV-absorbers. The synergistic interaction of HALS and phenolic antioxidants was therefore assigned to the formation of a UV-absorber preventing the squalane matrix from harmful UV-light, thereby contributing to an enhanced overall stabilizer performance. This assumption was confirmed in an experiment with an oxidized Irganox 1330 model compound. Formulations containing HALS and oxidized phenol showed better performances than the respective HALS single formulations and also oxidized Irganox 1330 prepared in a single formulation showed a much better performance than the non-oxidized form. It can therefore be concluded that the HALS protects the phenol from undergoing degradation. Without the addition of HALS the phenol is degraded before it may oxidize and unfold its UV-absorbing activity as it was observed in the phenol single formulation. To see if the results obtained in the PP-mimicking solvent squalane match with real-life conditions, 100 µm PP films (commercially available PP) containing three of the tested HALS as well as mixtures with a phenolic antioxidant were prepared. Similar to squalane the samples were subjected to an accelerated aging under UV-light and analyzed with respect to their molecular weight as well as the stabilizer content. To monitor the changes in the molecular weight distributions as well as stabilizer concentrations, samples were taken after different aging times of 0, 125, 246, 969 and 2992 hrs. The average molecular weight distribution was determined using a high-temperature gel permeation chromatography instrument (HT-GPC) from Polymer Char equipped with an IR5 detector. The stabilizer concentrations in the samples were determined with HPLC-UV and GC-MS on a 1260 Agilent HPLC and a 6890N Agilent GC system using an extraction based on a dissolution-precipitation approach published previously [80, 110]. Contrary to what was observed in squalane, an unequivocal antagonism was determined in the PP samples as can be seen in Fig. 12. Both the molecular weight of the polymer as well as the HALS stabilizer concentrations decreased much faster in combination with the phenol. Further, no oxidized phenol was detected neither in the phenol single formulation nor in the HALS-phenol mixture although the phenol was not fully degraded. Moreover, the degradation rate of the phenol in the single formulation was almost the same as in the combination formulation. These

Investigation of stabilizer interaction mechanisms 45

phenomena were observed for all tested HALS. The formulation with the most significant effect is depicted in Fig. 12. It can be seen that the difference in the aging behavior of the HALS single formulation to the HALS-phenol formulation is most significant within the first 246 hrs of aging. The decrease of the molecular weight of the HALS-phenol samples is much faster in the beginning where they still contain intact phenol. After 246 hrs the molecular weight curves almost run parallel, which indicates that the antagonism is caused by the addition of phenol, since only a very low amount of intact phenol was detected in the samples at 500 hrs of aging.

Fig. 12: Relative molecular weight (upper part) and relative HALS stabilizer concentrations (lower part) of a PP sample after different accelerated aging times. Single HALS formulation: grey line with squares, formulation of HALS and phenol: purple line with dots.

2.1.2. Results from HALS-phenol ox. test series

Since no oxidized phenolic antioxidant was detected in any sample of the HALS-phenol test series after different aging times, it is not clear if the phenol is either degraded before it may oxidize or if the oxidized phenol is formed but is not stable. To find out if the oxidized phenol would contribute to the stabilization of the polymer if it was contained, 100 µm PP films with an oxidized phenolic model compound in a single formulation and in combination with a HALS were prepared. Additionally, PP films containing only HALS as well as a mixture of HALS and phenol were prepared for comparison. The stabilizer concentrations of HALS and phenol in the samples were

Investigation of stabilizer interaction mechanisms 46

reduced in order to accelerate the degradation, which is why the relative concentrations after the same aging times differ from the HALS-phenol test series presented in Fig. 12. The PP sample films were subjected to the same accelerated aging under UV-light as before. Samples were taken after 0, 25, 50, 75, 100, 259 and 544 hrs and analyzed using the same methods as described for the HALS-phenol test series above. As can be seen from Fig. 13, the addition of the oxidized phenolic model compound leads to a slower material degradation than observed for the intact phenol. The same trend can be seen in combination with the HALS. As depicted in Fig. 14, the formulation of HALS with the added oxidized phenol shows a better performance than the formulation containing HALS and intact phenol. However, both formulations are still outperformed by the HALS single formulation. Comparison of the material and HALS degradation rate of the three formulations reveals a clear trend. The fastest degradation is observed for the combination formulation of HALS and intact phenol, followed by the combination formulation of HALS and oxidized phenol, while the HALS single formulation hardly shows any drop in the molecular weight nor in the HALS content. Further, it can be seen that the concentration decrease of HALS in the formulation containing HALS and oxidized phenol is the strongest between 100 and 259 hrs of aging. The determination of the relative concentration of the oxidized phenol model compound at 100 and 259 hrs revealed a stabilizer drop from 53 to 8 % referred to the unaged material. These data clearly show that the HALS degradation proceeds faster with the beginning decomposition of the oxidized phenolic model compound. A possible explanation for these findings could be that the phenol notwithstanding if it is added in its intact or oxidized form is degraded after a certain aging time, which results in the formation of radical species that accelerate the degradation of the HALS and in turn the polymer material. However, it is still unclear why the oxidized phenolic model compound is degraded in the polymer matrix while it is stable in squalane.

Fig. 13: Relative molecular weight of PP samples after different accelerated aging times. PP samples containing phenol: grey line with squares, samples containing oxidized phenol model compound: orange line with dots.

Investigation of stabilizer interaction mechanisms 47

Fig. 14: Relative molecular weight (upper part) and relative HALS stabilizer concentrations (lower part) of PP samples after different accelerated aging times. Single HALS formulation: grey line with squares, formulation of HALS and intact phenol: purple line with dots, formulation of HALS and oxidized phenol model compound: red line with triangles.

2.1.3. Influence of the specimen thickness

A possible explanation for the difference in the stability of the oxidized phenolic model compound in PP and squalane could lie in the sample preparation. Although the concentration of the stabilizers were the same in the squalane and PP samples, the specimen thickness differed considerably. While the PP films had a thickness of 100 µm, the squalane matrix in the glass vials was about one centimeter high. The sample thickness that was irradiated by UV-light was therefore many times smaller for the PP films. Additionally, the sample surface to volume ratio was not the same for PP and squalane, resulting in a faster aging of the PP films. It must be assumed that the antagonistic effect observed in the PP films most probably results from the thin specimen thicknesses. To prove this assumption, PP films containing a mixture of HALS and the oxidized phenolic model compound were prepared with different sample thicknesses of 100, 200, 300, 400 and 500 µm and treated as described for the samples of the two test series. Surprisingly, as can be seen from Fig. 15, the specimen thickness in the range of 100 to 500 µm did not have any influence on the material degradation nor on the HALS degradation rate. This result however does not necessarily mean that the assumption made above is wrong, but may show that the

Investigation of stabilizer interaction mechanisms 48

specimen thickness was still too thin. There is not much published in the literature, however it is mentioned that the fundamental disadvantage of UV-absorbers lies in the fact that they need a certain absorption depth (sample thickness) to protect the plastic well, which is why they provide only limited protection to thin samples like fibers or films [16]. It would have been of great interest to investigate in how far the antagonistic effect is related to the PP specimen thickness, however this was outside the framework of this doctoral thesis.

Fig. 15: Relative molecular weight (upper part) and relative HALS stabilizer concentrations (lower part) of PP samples with different specimen thicknesses containing HALS and oxidized phenolic model compound. 166 hrs UV-aging: grey line with squares, 1000 hrs UV-aging: purple line with dots.

Investigation of the polymer additive distribution 49

3. INVESTIGATION OF THE POLYMER ADDITIVE DISTRIBUTION

Polymers used as high-performance materials for solar systems require proper stabilization to avoid early degradation reactions. To achieve the required lifetimes, the determination of the stabilizer concentrations and the identification of the formed degradation products are just as important as the question how they are distributed within the polymer material. Information on the homogeneity of the distribution of stabilizers is of particular interest in cases of material failures like crack formation for instance. In general, investigations on the additive distribution require microscopy techniques that offer a high spatial distribution. On the other hand, the imaging technique should additionally offer a high sensitivity since polymer additives are usually applied in concentrations less than 0.5 mass percent referred to polymer (wt%). UV and IR microscopy methods published in the literature [111, 112] offer sufficient resolutions, however suffer from poor detection limits. Within the following research paper [113] it was shown that confocal fluorescence microscopy is capable to fulfill both requirements, high resolution imaging at low concentration levels. Additive distribution patterns were recorded for polymer films containing a fluorescent additive as model compound. The high instrument sensitivity enabled the analysis of polymer samples containing less than 0.5 wt% additive. Comparison of the fluorescence images with those obtained from polarized optical microscopy provided useful information on the relation of the additive distribution behavior and the polymer morphology. It was shown that additives are distributed on a spherulitic scale with the majority being found at the boundary and only traces in the crystalline center. Further, it could be demonstrated that varying the quenching temperature within the crystallization process of the polymer melt affects the additive distribution patterns due to the changing polymer morphology. The big advantage of the presented confocal fluorescence microscopy approach lies in superior sensitivity allowing a sample imaging within only a few seconds. Unlike confocal fluorescence microscopy, image recording using less sensitive techniques like IR microscopy for instance increases the analyzing times significantly. Resolution and imaging time are indirectly proportional to each other. Consequently, achieving a comparable resolution like in fluorescence microscopy requires measuring times of a couple of hours for the same sample section. On the other hand, IR microscopy is applicable to a larger number of polymer stabilizers, while fluorescence imaging is limited to the use of fluorescent additives. However, Taniike et al. [112] have shown that the chemical structure of the stabilizer has no impact on the distribution behavior within the polymer. To this end, for the investigations presented in the following paper a fluorescent whitening agent was applied as model compound representing other types of polymer additives.

Investigation of the polymer additive distribution 50

Research paper 3:

Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy

Published in International Journal of Polymer Analysis and Characterization

Investigation of the polymer additive distribution 51

Graphical presentation of confocal fluorescence imaging of polymer additives

Investigation of the polymer additive distribution 52

Investigation of the polymer additive distribution 53

Investigation of the polymer additive distribution 54

Investigation of the polymer additive distribution 55

Investigation of the polymer additive distribution 56

Investigation of the polymer additive distribution 57

Investigation of the polymer additive distribution 58

Conclusions 59

4. CONCLUSIONS

In this thesis, analytical methods for the investigation of high-performance polymer materials designed for the use in solar thermal systems were developed and evaluated with respect to their reliability. Polymers used in such demanding applications require proper stabilization to avoid decomposition reactions and thus material failures. One of the many challenges thereby lies in the selection of the right stabilizers. Contrary to what one would expect, the indiscriminate use of stabilizers may cause enormous economic damage due to possibly arising antagonistic interactions leading to an accelerated material degradation. Therefore, the investigation of the stabilizers and their interactions in advance is of major importance. Within this thesis, two different concepts for the analysis of polymer stabilizers and additives were developed. The first approach is focused on an investigation of the stabilizers on a macroscopic scale, while the other approach is based on a microscopic imaging of the additives within the polymer material. Polymers that are designed for solar applications are subjected to sunlight, which is why UV- stabilizers like the hindered amine light stabilizers (HALS) are usually applied to such materials. From the literature, it is known that HALS have to be combined with primary and secondary antioxidants since they fail as processing stabilizers making it necessary to investigate the possibly occurring interaction mechanisms with these stabilizer classes. Since the preparation of the stabilized polymer materials for screening is rather time-consuming, experiments were performed in the polypropylene-mimicking solvent squalane. Squalane is a non-polar short-chain hydrocarbon that is liquid at room temperature allowing the dissolution of the stabilizers instead of a complex and labor-intensive extrusion process. The first publication of this doctoral thesis describes the use of high-performance liquid chromatography (HPLC) coupled to photoluminescence detection for rating of stabilizer performances in squalane. Experiments have shown that squalane reveals aging-induced photoluminescence emissions that correlate with the extent of degradation. The value of the intensity of these emissions is therefore dependent on the added antioxidants so that it may serve as parameter for rating of the stabilizing capacity of a respective stabilizer formulation. The results obtained with this photoluminescence approach were compared to the rating that was performed according to the intensity of the four most abundant carbonyl degradation products formed in squalane and showed a perfect correlation. The second part of the publication was dedicated to the investigation of the photoluminescent squalane degradation products using high-resolution Orbitrap mass spectrometry. Since we were not able to separate the respective photoluminescence compounds chromatographically resulting in a single peak elution, the identification of the degradation products turned out to be quite challenging. The calculated sum formulas however indicated the formation of certain carbonyl degradation series with different numbers of double bond equivalents. As already suggested in the literature, the

Conclusions 60

photoluminescence was therefore assigned to such unsaturated carbonyl compounds, even though it is not clear yet at which extent of unsaturation the photoluminescence may be observed. The second publication of this thesis deals with the investigation of the interaction of HALS and phenolic antioxidants under the exposure of UV-light. Experiments were again performed in the polymer-mimicking solvent squalane. Measured in terms of the squalane degradation, the combination of all tested HALS with the phenolic antioxidant Irganox 1330 lead to a strong synergistic interaction. Investigation of the formed stabilizer degradation products allowed the assignment of the synergism to the formation of quinoid derivatives. Due to the oxidation of Irganox 1330, the absorption maximum of the resulting quinoid is shifted into a region that is typical for UV-absorbers. Thus, through absorption of the harmful UV-light the oxidized phenol contributes to the stabilization of the squalane matrix. Experiments with an oxidized phenolic model compound confirmed this stabilizing effect in the single formulation as well as in combination with HALS. Three of the selected HALS-phenol formulations were additionally tested in real polypropylene samples to check the reliability of the squalane model. Surprisingly, the same formulations that exhibited a synergism in squalane developed an antagonistic interaction in the polymer fibers. Investigations of the stabilizer degradation products revealed that no oxidized phenol was contained in the UV-aged samples. Experiments however proved that if the oxidized phenol model compound was added solely, the performance of these fibers was improved compared to the samples containing the intact phenol. One the other hand, the study also showed that the combination of the oxidized phenolic model compound and HALS again leads to an antagonistic interaction. It was therefore assumed that the degradation of the phenol irrespective of the use of the oxidized or non-oxidized form causes the formation of radical species promoting the degradation of HALS, that leads to an antagonistic interaction. The reason for the absence of the degradation of phenolic antioxidant in squalane was ascribed to the difference in the specimen thicknesses of the squalane and polymer samples. While the absorption depth in squalane was about a centimeter, the sample thickness of the polymer fibers was between 100 and 500 µm. It is stated in the literature that UV-absorbers show only limited protection when used in thin samples such as films or fibers, so that this explanation seems to be the most likely. For future projects, it would be interesting to repeat these experiments with macro-sized polymer samples to see if different results are obtained. Macroscopic studies as described above may provide information on the concentration of stabilizers and their degradation products, however no statement can be made about their distribution within the material. The second part and third paper of this thesis deals with the investigation of polymer additives on a microscopic scale using confocal fluorescence microcopy imaging. Preparation of polymer films containing a fluorescent whitening agent as stabilizer model compound allowed the imaging of samples with less than 0.5 wt% (mass percent referred to polymer) additive. The high instrument resolution enabled the imaging of the additive distribution

Conclusions 61

on a spherulitic scale. The majority of the additive was found at the boundary and only traces in the amorphous regions inside the spherulites.

References 62

5. REFERENCES

[1] Beerepoot M, Marmion A. 2012. Policies for Renewable Heat: An Integrated Approach. International Energy Agency Insights Series 2012. [2] International Energy Agency. http://energyatlas.iea.org/#!/tellmap/-1076250891/3. (August 17, 2017). [3] Gao L, Zhao J, Tang Z. 2015. A Review on Borehole Seasonal Solar Thermal Energy Storage. Energy Procedia, 70: 209-18. [4] Hesaraki A, Holmberg S, Haghighat F. 2015. Seasonal thermal energy storage with heat pumps and low temperatures in building projects-A comparative review. Renewable Sustainable Energy Rev., 43: 1199-213. [5] Novo AV, Bayon JR, Castro-Fresno D, Rodriguez-Hernandez J. 2010. Review of seasonal heat storage in large basins: Water tanks and gravel-water pits. Appl. Energy, 87: 390-7. [6] Bertram E, Glembin J, Scheuren J, Zienterra G. 2009. Soil regeneration by unglazed solar collectors in heat pump systems. ISES Solar World Congress. Johannesburg, South Africa. [7] Kjellsson E, Hellström G, Perers B. 2010. Optimization of systems with the combination of ground-source heat pump and solar collectors in dwellings. Energy, 35: 2667-73. [8] Socaciu LG. 2011. Seasonal sensible thermal energy storage solutions. Leonardo Electron. J. Pract. Technol., 19: 49-68. [9] Terziotti LT, Sweet ML, McLeskey JT. 2012. Modeling seasonal solar thermal energy storage in a large urban residential building using TRNSYS 16. Energy and Buildings, 45: 28-31. [10] Ucar A, Inalli M. 2008. Thermal and economic comparisons of solar heating systems with seasonal storage used in building heating. Renewable Energy, 33: 2532-9. [11] Xu J, Wang RZ, Li Y. 2014. A review of available technologies for seasonal thermal energy storage. Solar Energy, 103: 610-38. [12] W. Lang R, Wallner G, Fischer J. 2016. Novel Solar-Thermal Systems Based on Polymeric Materials: A Comprehensive Science-Driven Research Effort. World congress on civil, structural and environmental engineering. Prague, Czech Republic. [13] Solar Energy Technologies based on Polymeric Materials. www.solpol.at. (August 17 2017). [14] Bolland JL, Gee G. 1946. Kinetic studies in the chemistry of rubber and related materials. II. The kinetics of oxidation of unconjugated olefins. Trans. Faraday Soc., 42: 236-43. [15] Bolland JL, Gee G. 1946. Kinetic studies in the chemistry of rubber and related materials. III. Thermochemistry and mechanisms of olefin oxidation. Trans. Faraday Soc., 42: 244-52. [16] Zweifel H, Maier RD, Schiller M. 2009. Plastics Additives Handbook. 6th ed. Cincinnati, Ohio: Hanser Publications. [17] Pospíšil J. 1988. Mechanistic action of phenolic antioxidants in polymers-A review. Polym. Degrad. Stab., 20: 181-202.

References 63

[18] Pospíšil J. 1993. Chemical and photochemical behaviour of phenolic antioxidants in polymer stabilization: A state of the art report, part II. Polym. Degrad. Stab., 39: 103-15. [19] Pospíšil J. 1993. Chemical and photochemical behaviour of phenolic antioxidants in polymer stabilization-a state of the art report, Part I. Polym. Degrad. Stab., 40: 217-32. [20] Pospíšil J, Nešpůrek S. 1995. Chain-breaking stabilizers in polymers: the current status. Polym. Degrad. Stab., 49: 99-110. [21] Mortier RM, Fox MF, Orszulik ST. 2010. Chemistry and Technology of Lubricants. 3rd ed. Dordrecht: Springer Science+Business Media. [22] Rudnick LR. 2009. Lubricant additives: Chemistry and applications. 2nd ed. Boca Raton, FL: CRC Press. [23] Neri C, Costanzi S, Riva R, Farris R, Colombo R. 1995. Mechanism of action of phosphites in polyolefin stabilisation. Polym. Degrad. Stab., 49: 65-9. [24] Schwetlick K, Habicher WD. 1995. Organophosphorus antioxidants action mechanisms and new trends. Angew. Makromolek. Chem., 232: 239-46. [25] Schwetlick K, König T, Rüger C, Pionteck J, Habicher W. 1986. Chain-breaking antioxidant activity of phosphite esters. Polym. Degrad. Stab., 15: 97-108. [26] Schwetlick K, Pionteck J, Winkler A, Hähner U, Kroschwitz H, Habicher W. 1991. Organophosphorus antioxidants: Part X-Mechanism of antioxidant action of aryl phosphites and phosphonites at higher temperatures. Polym. Degrad. Stab., 31: 219-28. [27] Walling C, Rabinowitz R. 1959. The Reaction of Trialkyl Phosphites with Thiyl and Alkoxy Radicals. J. Am. Chem. Soc., 81: 1243-9. [28] Hawkins W, Sautter H. 1963. Synergistic antioxidant combinations. Mechanism of stabilization with organo-sulfur compounds. J. Polym. Sci., Part A: Polym. Chem., 1: 3499-509. [29] Kúdelka I, Misro P, Pospíšil J, Korbanka H, Riedel T, Pfahler G. 1985. Antioxidants and stabilizers: Part IC-Hydroperoxide deactivation by aliphatic sulfides. A model study. Polym. Degrad. Stab., 12: 303-13. [30] Shelton J, Harrington E. 1976. The Role of Certain Organic Sulfur Compounds as Preventive Antioxidants. II. Activity as Peroxide Decomposers and Effect of Bases. Rubber Chem. Technol., 49: 147-57. [31] Shelton JR. 1974. The Role of Certain Organic Sulfur Compounds as Preventive Antioxidants. Rubber Chem. Technol., 47: 949-59. [32] Denisov E. 1989. Mechanism of regeneration of hindered nitroxyl and aromatic amines. Polym. Degrad. Stab., 25: 209-15. [33] Denisov E. 1991. The role and reactions of nitroxyl radicals in hindered piperidine light stabilisation. Polym. Degrad. Stab., 34: 325-32. [34] Gijsman P, Hennekens J, Tummers D. 1993. The mechanism of action of hindered amine light stabilizers. Polym. Degrad. Stab., 39: 225-33.

References 64

[35] Gugumus F. 1991. Mechanisms and kinetics of photostabilization of polyolefins with N- methylated HALS. Polym. Degrad. Stab., 34: 205-41. [36] Gugumus F. 1993. Current trends in mode of action of Hindered Amine Light Stabilizers. Polym. Degrad. Stab., 40: 167-215. [37] Hodgson JL, Coote ML. 2010. Clarifying the mechanism of the Denisov cycle: how do hindered amine light stabilizers protect polymer coatings from photo-oxidative degradation? Macromolecules, 43: 4573-83. [38] Klemchuk PP, Gande ME. 1988. Stabilization mechanisms of hindered amines. Polym. Degrad. Stab., 22: 241-74. [39] Kurumada T, Ohsawa H, Oda O, Fujita T, Toda T, Yoshioka T. 1985. Photostabilizing activity of tertiary hindered amines. J. Polym. Sci., Part A: Polym. Chem., 23: 1477-91. [40] Ohkatsu Y. 2008. Search for unified action mechanism of hindered amine light stabilizers. J. Jpn. Pet. Inst., 51: 191-204. [41] Step E, Turro N, Gande M, Klemchuk P. 1994. Mechanism of polymer stabilization by hindered-amine light stabilizers (HALS). Model investigations of the interaction of peroxy radicals with HALS amines and amino ethers. Macromolecules, 27: 2529-39. [42] Klampfl CW. 2013. Mass spectrometry as a useful tool for the analysis of stabilizers in polymer materials. TrAC, Trends Anal. Chem., 50: 53-64. [43] Dopico-García MS, Noguerol-Cal R, Castro-López MM, Cela-Pérez MC, Piñón-Giz E, López- Vilariño JM, González-Rodríguez MV. 2012. Determination of polyolefin additives by reversed- phase liquid chromatography. Cent. Eur. J. Chem., 10: 585-610. [44] Paine MRL, Barker PJ, Blanksby SJ. 2011. Desorption electrospray ionisation mass spectrometry reveals in situ modification of a hindered amine light stabiliser resulting from direct N-OR bond cleavage. Analyst, 136: 904-12. [45] Reiter SM, Buchberger W, Klampfl CW. 2011. Rapid identification and semi-quantitative determination of polymer additives by desorption electrospray ionization/time-of-flight mass spectrometry. Anal. Bioanal. Chem., 400: 2317-22. [46] Haunschmidt M, Klampfl CW, Buchberger W, Hertsens R. 2010. Rapid identification of stabilisers in polypropylene using time-of-flight mass spectrometry and DART as ion source. Analyst, 135: 80-5. [47] Paine MRL, Barker PJ, Maclauglin SA, Mitchell TW, Blanksby SJ. 2012. Direct detection of additives and degradation products from polymers by liquid extraction surface analysis employing chip-based nanospray mass spectrometry. Rapid Commun. Mass Spectrom., 26: 412-8. [48] Schnöller J, Pittenauer E, Hutter H, Allmaier G. 2009. Analysis of antioxidants in insulation cladding of copper wire: a comparison of different mass spectrometric techniques (ESI-IT, MALDI- RTOF and RTOF-SIMS). J. Mass Spectrom., 44: 1724-32.

References 65

[49] Schwarzinger C, Gabriel S, Beißmann S, Buchberger W. 2012. Quantitative Analysis of Polymer Additives with MALDI-TOF MS Using an Internal Standard Approach. J. Am. Soc. Mass Spectrom., 23: 1120-5. [50] Kreisberger G, Himmelsbach M, Buchberger W, Klampfl CW. 2015. Identification and semi- quantitative determination of anti-oxidants in lubricants employing thin-layer chromatography- spray mass spectrometry. J. Chromatogr. A, 1383: 169-74. [51] Beißmann S, Reisinger M, Reimann A, Klampfl CW, Buchberger W. 2014. High-throughput quantification of stabilizers in polymeric materials by flow injection tandem mass spectrometry. Rapid Commun. Mass Spectrom., 28: 939-47. [52] Beißmann S, Reisinger M, Toelgyesi L, Klampfl C, Buchberger W. 2013. Fast screening of stabilizers in polymeric materials by flow injection-tandem mass spectrometry. Anal. Bioanal. Chem., 405: 6879-84. [53] Barret J, Gijsman P, Swagten J, Lange RFM. 2002. A molecular study towards the interaction of phenolic anti-oxidants, aromatic amines and HALS stabilizers in a thermo-oxidative ageing process. Polym. Degrad. Stab., 76: 441-8. [54] Sternbauer L. 2014. Multimethods for the advanced analysis of additives in polyolefins. Dissertation, Johannes Kepler University, Linz. [55] Farajzadeh MA, Nasserzadeh A, Ranji A, Feyz E. 2007. Saponification then GC for Determination of Irganox 1010 and Irganox 1076 in a Polymer Matrix. Chromatographia, 65: 223- 7. [56] Proestos C, Sereli D, Komaitis M. 2006. Determination of phenolic compounds in aromatic plants by RP-HPLC and GC-MS. Food Chem., 95: 44-52. [57] Bart JCJ. 2001. Polymer/additive analysis by flash pyrolysis techniques. J. Anal. Appl. Pyrolysis, 58: 3-28. [58] Bart JCJ. 2001. Direct solid sampling methods for gas chromatographic analysis of polymer/additive formulations. Polym. Test., 20: 729-40. [59] Coulier L, Kaal ER, Tienstra M, Hankemeier T. 2005. Identification and quantification of (polymeric) hindered-amine light stabilizers in polymers using pyrolysis-gas chromatography- mass spectrometry and liquid chromatography-ultraviolet absorbance detection-evaporative light scattering detection. J. Chromatogr. A, 1062: 227-38. [60] Di Pasquale G, Galli M. 1984. Determination of additives in polyolefins by capillary gas chromatography. J. High Resolut. Chromatogr., 7: 484-6. [61] Herrera M, Matuschek G, Kettrup A. 2003. Fast identification of polymer additives by pyrolysis- gas chromatography/mass spectrometry. J. Anal. Appl. Pyrolysis, 70: 35-42. [62] Jansson KD, Zawodny CP, Wampler TP. 2007. Determination of polymer additives using analytical pyrolysis. J. Anal. Appl. Pyrolysis, 79: 353-61. [63] Kawamura Y, Watanabe K, Sayama K, Takeda Y, Yamada T. 1997. Simultaneous determination of polymer additives in polyethylene by GC/MS. Food Hyg. Saf. Sci., 38: 307-18.

References 66

[64] Manabe N, Toyoda T, Yokota Y. 1999. Determination of a hindered phenol-type antioxidant by pyrolysis-GC/MS in the presence of tetramethyl ammonium hydroxide. Bunseki kagaku, 48: 449-56. [65] Sobeih KL, Baron M, Gonzalez-Rodriguez J. 2008. Recent trends and developments in pyrolysis-gas chromatography. J. Chromatogr. A, 1186: 51-66. [66] Trimpin S, Wijerathne K, McEwen CN. 2009. Rapid methods of polymer and polymer additives identification: Multi-sample solvent-free MALDI, pyrolysis at atmospheric pressure, and atmospheric solids analysis probe mass spectrometry. Anal. Chim. Acta, 654: 20-5. [67] Wang FC-Y. 2000. Polymer additive analysis by pyrolysis-gas chromatography. J. Chromatogr. A, 891: 325-36. [68] Ashraf-Khorassani M, Nazem N, Taylor LT. 2003. Feasibility of supercritical fluid extraction with on-line coupling of reversed-phase liquid chromatography for quantitative analysis of polymer additives. J. Chromatogr. A, 995: 227-32. [69] Block C, Wynants L, Kelchtermans M, De Boer R, Compernolle F. 2006. Identification of polymer additives by liquid chromatography-mass spectrometry. Polym. Degrad. Stab., 91: 3163- 73. [70] Dopico Garcia MS, López V JM, Bouza R, Abad MJ, González Soto E, González Rodrıgueź MV. 2004. Extraction and quantification of antioxidants from low-density polyethylene by microwave energy and liquid chromatography. Anal. Chim. Acta, 521: 179-88. [71] Duderstadt RE, Fischer SM. 2008. Effect of organic mobile phase composition on signal responses for selected polyalkene additive compounds by liquid chromatography-mass spectrometry. J. Chromatogr. A, 1193: 70-8. [72] El Mansouri H, Yagoubi N, Ferrier D. 1998. Extraction of polypropylene additives and their analysis by HPLC. Chromatographia, 48: 491-6. [73] Farajzadeh MA, Eskandar SG, Ranji A, Feyz E. 2007. HPLC technique for quantitation of Chimassorb 944, and its evaluation in analysis of real and standard samples of polyolefins. Microchim. Acta, 159: 363-9. [74] Himmelsbach M, Buchberger W, Reingruber E. 2009. Determination of polymer additives by liquid chromatography coupled with mass spectrometry. A comparison of atmospheric pressure photoionization (APPI), atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI). Polym. Degrad. Stab., 94: 1213-9. [75] Lundbäck M, Strandberg C, Albertsson AC, Hedenqvist MS, Gedde UW. 2006. Loss of stability by migration and chemical reaction of Santonox® R in branched polyethylene under anaerobic and aerobic conditions. Polym. Degrad. Stab., 91: 1071-8. [76] Marcato B, Vianello M. 2000. Microwave-assisted extraction by fast sample preparation for the systematic analysis of additives in polyolefins by high-performance liquid chromatography. J. Chromatogr. A, 869: 285-300.

References 67

[77] Munteanu D, Isfan A, Işfan C, Tincul I. 1987. High-performance liquid chromatographic separation of polyolefin antioxidants and light-stabilisers. Chromatographia, 23: 7-14. [78] Reingruber E, Himmelsbach M, Sauer C, Buchberger W. 2010. Identification of degradation products of antioxidants in polyolefins by liquid chromatography combined with atmospheric pressure photoionisation mass spectrometry. Polym. Degrad. Stab., 95: 740-5. [79] Buchberger W, Stiftinger M. 2012. Analysis of Polymer Additives and Impurities by Liquid Chromatography/Mass Spectrometry and Capillary Electrophoresis/Mass Spectrometry. M. Hakkarainen, Mass Spectrometry of Polymers – New Techniques: 39-67. Berlin, Heidelberg: Springer Berlin Heidelberg. [80] Stiftinger M, Buchberger W, Klampfl CW. 2013. Miniaturised method for the quantitation of stabilisers in microtome cuts of polymer materials by HPLC with UV, MS or MS2 detection. Anal. Bioanal. Chem., 405: 3177-84. [81] Hintersteiner I, Reisinger M, Himmelsbach M, Buchberger W. 2016. Separation and characterization of oligomeric hindered amine light stabilizers using high-performance liquid chromatography with UV and quadrupole time-of-flight mass spectrometric detection. J. Sep. Sci., 39: 1056-66. [82] Kreisberger G, Buchberger WW. 2017. Rapid determination of oligomeric hindered amine light stabilizers in polymeric materials. J. Sep. Sci., 40: 2366-73. [83] Hintersteiner I, Himmelsbach M, Klampfl C, Buchberger WW. 2014. Characterization of hindered amine light stabilizers employing capillary electrophoresis coupled to quadrupole time- of-flight mass spectrometry. Electrophoresis, 35: 1368-74. [84] Hintersteiner I, Schmid T, Himmelsbach M, Klampfl CW, Buchberger WW. 2014. Quantitative analysis of hindered amine light stabilizers by CZE with UV detection and quadrupole TOF mass spectrometric detection. Electrophoresis, 35: 2965-71. [85] Grabmayer K, Beißmann S, Wallner GM, Nitsche D, Schnetzinger K, Buchberger W, Schobermayr H, Lang RW. 2015. Characterization of the influence of specimen thickness on the aging behavior of a polypropylene based model compound. Polym. Degrad. Stab., 111: 185-93. [86] Grabmayer K, Wallner GM, Beißmann S, Braun U, Steffen R, Nitsche D, Röder B, Buchberger W, Lang RW. 2014. Accelerated aging of polyethylene materials at high oxygen pressure characterized by photoluminescence spectroscopy and established aging characterization methods. Polym. Degrad. Stab., 109: 40-9. [87] Wallner GM, Povacz M, Hausner R, Lang RW. 2016. Lifetime modeling of polypropylene absorber materials for overheating protected hot water collectors. Solar Energy, 125: 324-31. [88] Grabmayer K, Wallner GM, Beißmann S, Schlothauer J, Steffen R, Nitsche D, Röder B, Buchberger W, Lang RW. 2014. Characterization of the aging behavior of polyethylene by photoluminescence spectroscopy. Polym. Degrad. Stab., 107: 28-36. [89] Kahlen S, Jerabek M, Wallner GM, Lang RW. 2010. Characterization of physical and chemical aging of polymeric solar materials by mechanical testing. Polym. Test., 29: 72-81.

References 68

[90] Kahlen S, Wallner GM, Lang RW. 2010. Aging behavior of polymeric solar absorber materials - Part 2: Commodity plastics. Solar Energy, 84: 1577-86. [91] Oreski G, Wallner GM. 2009. Evaluation of the aging behavior of ethylene copolymer films for solar applications under accelerated weathering conditions. Solar Energy, 83: 1040-7. [92] Oreski G, Wallner GM, Lang RW. 2009. Ageing characterization of commercial ethylene copolymer greenhouse films by analytical and mechanical methods. Biosystems Engineering, 103: 489-96. [93] Wallner GM, Weigl C, Leitgeb R, Lang RW. 2004. Polymer films for solar energy applications- thermoanalytical and mechanical characterisation of ageing behaviour. Polym. Degrad. Stab., 85: 1065-70. [94] Kahlen S, Wallner G, Lang R. 2010. Aging behavior of polymeric solar absorber materials- Part 1: Engineering plastics. Solar Energy, 84: 1567-76. [95] Kahlen S, Wallner G, Lang R, Meir M, Rekstad J. 2010. Aging behavior of polymeric solar absorber materials: Aging on the component level. Solar Energy, 84: 459-65. [96] Maringer L, Himmelsbach M, Nadlinger M, Wallner G, Buchberger W. 2015. Structure elucidation of photoluminescent degradation products from polyolefins and evaluation of stabilizer formulations. Polym. Degrad. Stab., 121: 378-84. [97] Soebianto YS, Katsumura Y, Ishigure K, Kubo J, Koizumi T, Shigekuni H, Azami K. 1993. Model experiment on the protection effect in polymers: radiolysis of liquid squalane in the presence and absence of additives. Polym. Degrad. Stab., 42: 29-40. [98] Azhdar B, Yu W, Reitberger T, Gedde UW. 2009. A new method for assessing the efficiency of stabilizers in polyolefins exposed to chlorinated water media. Polym. Test., 28: 661-7. [99] Pospíšil J, Nešpůrek S, Zweifel H. 1996. The role of quinone methides in thermostabilization of hydrocarbon polymers -II. Properties and activity mechanisms. Polym. Degrad. Stab., 54: 15- 21. [100] Ray WC, Isenhart K. 1975. Phosphites in polyolefin process stabilization. Polym. Eng. Sci., 15: 703-7. [101] Lucki J, Rabek JF, Rånby B. 1984. Photostabilizing effect of hindered piperidine compounds: Interaction between hindered phenols and hindered piperidines. Polym. Photochem., 5: 351-84. [102] Ohkatsu Y, Fujiwara T. 2007. Interaction between Nitroxide of Hindered Amine Light Stabilizers and Phenol. J. Jpn. Pet. Inst., 50: 87-93. [103] Ohkatsu Y, Yamacuchi K, Sengoku T, Sato T. 1994. Interactions of the Hydroxylamine Derived from Bis-(2, 2, 6, 6-tetramethyl-piperidinyl)-sebacate with Quinones. J. Jpn. Pet. Inst., 37: 405-10. [104] Ohkatsu Y, Yamaguchi K, Yamaki A, Maruyama Y, Kusano T. 1991. Influence of Hindered Amine Light Stabilizers on Antioxidative Activities of Quiones. J. Jpn. Pet. Inst., 34: 280-3. [105] Yamaguchi K, Ohkatsu Y, Kusano T. 1991. Oxidation Inhibition by Interaction of Quinones with HALS (hindered amine light stabilizers). J. Jpn. Pet. Inst., 34: 458-63.

References 69

[106] Yamashita H, Kawaguchi AW, Ohkatsu Y. 2006. New Antagonism of Hindered Amine Light Stabilizers with Acidic Compounds Including Phenolic Antioxidants (Part 2) Formation Mechanism of Active Species of Peroxide Decomposition Reaction. J. Jpn. Pet. Inst., 49: 294-300. [107] Yamashita H, Ohkatsu Y. 2003. A new antagonism between hindered amine light stabilizers and acidic compounds including phenolic antioxidant. Polym. Degrad. Stab., 80: 421-6. [108] Beißmann S, Reisinger M, Grabmayer K, Wallner G, Nitsche D, Buchberger W. 2014. Analytical evaluation of the performance of stabilization systems for polyolefinic materials. Part I: Interactions between hindered amine light stabilizers and phenolic antioxidants. Polym. Degrad. Stab., 110: 498-508. [109] Maringer L, Roiser L, Wallner G, Nitsche D, Buchberger W. 2016. The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants. Polym. Degrad. Stab., 131: 91-7. [110] Beißmann S, Stiftinger M, Grabmayer K, Wallner G, Nitsche D, Buchberger W. 2013. Monitoring the degradation of stabilization systems in polypropylene during accelerated aging tests by liquid chromatography combined with atmospheric pressure chemical ionization mass spectrometry. Polym. Degrad. Stab., 98: 1655-61. [111] Ryan T, Calvert P. 1982. Diffusion and annealing in crystallizing polymers. Polymer, 23: 877- 83. [112] Taniike T, Umemori M, Kobayashi F, Terano M. 2012. Critical Role of Spherulite Structure on Behavior of Stabilizers in Poly(propylene) Stabilization. Macrom. Symp., 312: 146-56. [113] Maringer L, Grabmann M, Muik M, Nitsche D, Romanin C, Wallner G, Buchberger W. 2017. Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy. Int. J. Polym. Anal. Charact., DOI: 10.1080/1023666X.2017.1367120.

70

Leila Maringer Aubergstraße 10, 4040 Linz Phone: +43 664 75090224, E-Mail: [email protected]

CURRICULUM VITAE

 Personal Data

Name: Leila Maringer Birth date: 09.01.1991 Citizenship: Austria

 Education

Since 10/2014 Doctoral Program at the Johannes Kepler University (JKU) Linz 09/2014 Master examination passed with distinction 09/2009-09/2014 Bachelor’s and Master’s Degree Program Technical Chemistry at the JKU Linz 06/2009 School-leaving examination passed with distinction 09/2001-06/2009 Bundesrealgymnasium (BRG) Vöcklabruck 09/1997-07/2001 Elementary school in Vöcklabruck

 Further education

07/2016-02/2017 “Quality assurance in chemical laboratories” at the Montan University Leoben, final examination passed with distinction

 Occupations

Since 10/2014 Project Assistant in the research project SolPol-4/5 02/2014-07/2014 Project Assistant; Institute of Analytical Chemistry at the JKU Linz 09/2013-12/2013 Project Assistant; Institute of Analytical Chemistry at the JKU Linz 07/2012-08/2012 Employee; Analytical laboratory Lenzing AG 08/2011 Employee; Analytical laboratory Lenzing AG

71

07/2011 Employee; Arbeitsmarktservice Vöcklabruck 07/2010-09/2010 Employee; Arbeitsmarktservice Vöcklabruck 07/2009-09/2009 Employee; Arbeitsmarktservice Vöcklabruck 09/2008 Employee; Arbeitsmarktservice Vöcklabruck 08/2008 Employee; Eternit Vöcklabruck 07/2008 Employee; Arbeitsmarktservice Vöcklabruck 07/2007-08/2007 Employee; Eternit Vöcklabruck 08/2006 Internship; Apotheke am Salzburger Tor Vöcklabruck

 Publications

[1] Causon TJ, Maringer L, Buchberger W, Klampfl CW. 2014. Addition of reagents to the sheath liquid: A novel concept in capillary electrophoresis-mass spectrometry, J. Chromatogr. A, 1343:182-7.

[2] Maringer L, Ibáñez E, Buchberger W, Klampfl CW, Causon TJ. 2015. Using sheath-liquid reagents for capillary electrophoresis-mass spectrometry: Application to the analysis of phenolic plant extracts. Electrophoresis, 36:348-54.

[3] Maringer L, Himmelsbach M, Nadlinger M, Wallner G, Buchberger W. 2015. Structure elucidation of photoluminescent degradation products from polyolefins and evaluation of stabilizer formulations. Polym. Degrad. Stab., 121:378-84.

[4] Maringer L, Roiser L, Wallner G, Nitsche D, Buchberger W. 2016. The role of quinoid derivatives in the UV-initiated synergistic interaction mechanism of HALS and phenolic antioxidants. Polym. Degrad. Stab., 131:91-97.

[5] Maringer L, Grabmann M, Muik M, Nitsche D, Romanin C, Wallner G, Buchberger W. 2017. Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy. Int. J. Polym. Anal. Charact., DOI: 10.1080/1023666X.2017.1367120.

 Poster Presentations

[1] ANAKON; Graz (Austria); 2015; Characterisation of plant extracts using capillary electrophoresis with sheath-flow chemistry

[2] DVSPM; Gmunden (Austria); 2015; Characterisation of polymer aging using the polypropylene- mimicking solvent squalane with HPLC coupled to fluorescence detection

[3] ISCC; Riva del Garda (Italy); 2016; HPLC-MS as a tool for studying interactions between stabilizers used in plastic materias

 Oral Presentations

[1] JunganalytikerInnen Forum (JAF); Tulln (Austria); 2014; Sheath-flow chemistry for CE-MS: Analysis of phenolic compounds in rosemary extracts

[2] Polymer Degradation and Discussion Group (PDDG); Stockholm (Sweden); 2015; Evaluation of different polymer stabilizer formulations in squalane as model for polypropylene using high performance liquid chromatography with photoluminescence detection

72

[3] CE Forum; Tübingen (Germany); 2015; Identification of plant extracts using CE-MS with sheath-flow chemistry

[4] JunganalytikerInnen Forum (JAF); Graz (Austria); 2016; Study of polymer stabilizers and their interaction mechanisms using HPLC-MS

[5] International Symposium on Polymer Analysis and Characterization (ISPAC); Linz (Austria); 2017; Analytical approaches to polymer additive analysis from a macroscopic to a microscopic scale

[6] Polymer Degradation and Discussion Group (PDDG); Taormina (Sicilia, Italy); 2017; Macro- and microscopic approaches to analysis of stabilizers and their degradation products

 Awards

Master Thesis Award 2015 of the Austrian Chemical Society (GÖCH)

Springer Oral Presentation Award – ISPAC 2017

Linz, November 2017

Leila Maringer