MASARYK UNIVERSITY

Faculty of Natural Sciences

DISSERTATION THESIS

Sharbel LUZURIAGA Brno, 2009

MASARYK UNIVERSITY IN BRNO Faculty of Natural Sciences

Sharbel Luzuriaga

UTILIZATION OF COMPATIBILIZATION AND RESTABILIZATION METHODS IN THE RECYCLING OF COMMINGLED MUNICIPAL PLASTIC WASTE

Dissertation Thesis

Tutor: doc. RNDr. Ivan Fortelný, DSc Brno, 2009

BIBLIOGRAPHY

Author: Sharbel Eduardo Luzuriaga Pesantez

Disertáční Práce: Využití kompatibilizace a stabilizace při recyklaci komunálního směsného plastového odpadu.

Dissertation Thesis: Utilization of compatibilization and stabilization methods in the recycling of commingled municipal plastic waste

Study: Chemistry

Specialization: Macromolecular Chemistry

Tutor: doc. RNDr. Ivan Fortelný, DSc

Prague 2009

Klíčová slova: Urychlené stárnutí, kompatibilizace, stabilizace, simulovaná recyklace, polyolefiny, styrenové plasty, degradace, opětovné zpracování

Key words: Accelerated weathering, simulated recycling, compatibilization, stabilization, polyolefins, styrenics, degradation, reprocessing

COPYRIGHT DECLARATION

I declare that the present dissertation thesis has been elaborated independently and is my own account of my research and contains as is main content work which has not previously been submitted. All sources are appropriately quoted.

I do agree with the further publication of the present work in accordance with ordinance 111/1998/Sb., of University law of the Czech Republic and the Czech copyright act 121/2000/Sb.

In Prague,

© Sharbel Luzuriaga, Masaryk University in Brno, 2009 i

Acknowledgements

I owe my deepest gratitude to the all persons listed in the following lines. Without their constant support, advice and motivation this thesis would not been possible; particularly considering all vicissitudes occurred in the course of the present work.

I am heartily thankful to my supervisors, Dr. Ivan Fortelný, and Dra. Kovařová, their encouragement, guidance and support from the initial to the final level enabled me to develop an understanding of the subject. In the same way I am very grateful to Prof. Milán Potáček in Brno for the coordination of all the formalities related to this doctoral study. Especial thanks to Danuše Michálková for the technical advice, and in general to all the staff in Machova and Petřiny workplaces for providing a friendly working environment.

Lastly, I offer my regards and blessings to all of those who supported me in any respect during the completion of the project. In particular I am indebted to my mother Mrs. Mappy Pesantez and Petr Valenta for the support, encouragement and motivation.

Sharbel E Luzuriaga Pesantez

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ABSTRACT

The present dissertation thesis is primarily focused on the optimization of an efficient compatibilization and stabilization method applied during material recycling of commingled municipal plastic waste stream in the Czech Republic which basically consist in polyolefins, namely low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and high-impact polystyrene (HIPS), since other material are sorted separately. The first part of this work was devoted to the study on the mechanism of various degradation processes of each component.

Accelerated thermal and photo-aging of four aforementioned homopolymers (LDPE/HDPE/PP/HIPS) was performed and the impact of subsequent reprocessing conditions on their properties studied. Polymer samples oven aged at 100ºC for varying periods of time or UV irradiated in a Weather-O-Meter (WOM) at λ = 340 nm were reprocessed in a Brabender plasticorder at 190º C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical properties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI, tensile impact strength). It was shown that LDPE and HIPS are more vulnerable to thermo-oxidation than HDPE and PP, whereas HDPE and PP were affected to a greater extent by UV exposure.

Knowledge obtained in the preceding study had been further applied on the interpretation and understanding of the complex processes occurring in quaternary model blends from virgin and pre-aged components. The next part of the work has been devoted to the study of the influence of the degree of previous degradation of each of the components of a model of municipal plastic waste (LDPE/HDPE/PP/HIPS) on the toughness and stability of the resulted recyclates, whilst attempting to improve the observed mechanical performance by means of an upgrading system on the basis of a cooperative compatibilization-stabilization system (a mixture of ethylene-propylene statistical and styrene-butadiene block copolymers with a secondary amine-based stabiliser). It was shown that good impact strength was achieved for recyclates having the components with a low or medium degree of degradation. Mechanical properties of the recyclates having the components with a high degree of degradation are deteriorated. The addition of the cooperative compatibilization system leads to a high thermo-oxidative stability of recyclates irrespective degree of degradation of their components. Photo-oxidative stability of the recyclates is low but an acceptable improvement to satisfactory level by the addition of carbon black or a commercial photo-stabiliser was achieved.

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SOUHRN

Předložena disertační práce je především zaměřená na optimalizace vhodné kompatibilizační-stabilizační metody, používané při recyklaci směsného komunálního plastového odpadu v České Republice, který se v podstatě se skládá z různých polylefinů, a sice: polyethylenu (LDPE, HDPE), polypropylenu (PP) a v menší míře ze styrenových plastů (jako representant byl vybrán houževnatý polystyren – HIPS). Zastoupení dalších polymerních materialů je malé z důvodů používaného způsobu třídění (separace PET lahví) a potlačení PVC v obalových materiálech. První část této práce se zabývá studiem degradačních mechanismů probíhajících v jednotlivých složkách.

Pokusy týkající se urychleného tepelného a světelného starnuntí byly provedené u všech čtyř polymerů (LDPE/HDPE/PP/HIPS). Přitom byl zkoumán vliv opětovného zpracování na jejich vlastnosti. Vzorky polymerů byly bud´ podrobeny urychlenému stárnutí v horkovzdušné sušárně při teplotě 100 °C anebo vystaveny UV ozařování v přístroji Weather-O-Meter (WOM) při λ = 340 nm, a to v různých časových intervalech. Stárnuté materiály byly následně přepracovány v laboratorním hnětači Brabender plasticorder (190 °C, 60 ot/min., po dobu10 min). Změny v chemické struktuře a vývoj reologických a mechanických vlastností související s postupnou degradací jednotlivých složek byly předmětem analysy a monitorování pomocí různých analytických metod( DSC, FTIR, kolorimetrické metody, index toku, rázová houževnatost). Získané výsledky prokázaly, že LDPE a HIPS jsou náchylnější k tepelné degradaci než HDPE a PP, na druhé straně HDPE a PP byly méně oddolné vůči UV záření.

Poznatky získané v předchozí práci byly následně aplikovány při interpretaci a zkoumání komplexních pochodů odehrávajících se při zpracování kvarternárních modelových směsí připravených jak z panenských, tak i ze stárnutých polymerů. V této druhé části práce je věnovaná pozornost jek vlivu degradačního poškození jednotlivých složek modelové směsi(LDPE/HDPE/PP/HIPS)-která se svým složením bliží reálným směsným plastovým odpadům - na vlastnosti výsledného materiálu, tak zlepšení mechanických vlastností a stabilizaci pomocí kooperativního kompatibilizačního-stabilizačního systému založeného na kombinaci ethylen-propylenového statistického kopolymeru (EPM), styren-butadien- styrenového blokového kopolymeru (SBS) a diaminového stabilizátoru (DUS).

Ukázalo se, že modelové směsi vystavené mírným degradačním podmínkám prokázaly přijatelné mechanické vlasnosti. Účinnost kooperativního kompatibilizačně- stabilizačního systému (EPM/SBS/DUS) byla prokázána nejen zlepšením mechanických vlastností, ale i preventivním potlačením dalších degradačních pochodů při přepracování. Foto-oxidační stabilita výsledných recyklátů se ukázala být spíše nízka. Použitím vhodného světelného stabilizátoru a/nebo sazí bylo dosaženo výrazného zvýšení světelné stablity, což je považován za klíčový parametr pro praktické použítí recyklátů v druhotné aplikaci.

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Table of Contents ABSTRACT

SOUHRN

LIST OF ABBREVIATIONS AND SYMBOLS

Introduction...... 1 Aims...... 4 1. STATE­OF­THE ART ...... 5 1.1 Degradation and aging...... 5 1.1.1.Classification of degradation processes ...... 5 1.1.1.1 Auto-oxidation ...... 6 1.1.1.2 Photo-oxidation...... 7 1.1.2 Aging...... 9 1.1.2.1 Accelerated (artificial) contra natural aging of polymers...... 9 1.1.2.2 Oven aging experiments ...... 10 1.1.2.3 Accelerated UV irradiation‐ UV experiments...... 12 1.1.3 Functional groups formation and degradation profile of aged polymers . 14 1.1.4 Degradation of commercial polymers (polyolefins and PS)...... 15 a) Polyethylene: LDPE and HDPE...... 15 b) Polypropylene ...... 16 c) Polystyrene (PS) and High Impact Polystyrene (HIPS) ...... 16 1.1.5 Impact on the mechanical and morphology properties...... 17 a) PE ...... 17 I) Impact on the mechanical properties ...... 17 II) Changes in morphology ...... 18 b) PP ...... 19 I) Impact on mechanical properties...... 19 II) Changes in morphology ...... 20 c) PS...... 21 I) Changes in mechanical properties ...... 21 II) Changes in morphology ...... 21

1.2 Stabilization...... 22 1.2.1 Types of Stabilization ...... 22 1.2.2 Mechanism of stabilization...... 22 1.2.3 Re-stabilization of recyclates...... 23

1.3 Compatibilization ...... 26 1.3.1 Methods of Compatibilization ...... 26 1.3.1.1 Non-reactive compatibilization...... 26 1.3.1.2 Reactive “ in situ” compatibilization ...... 27

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1.4 Recycling of plastic waste stream ...... 28 1.4.1 Definition of plastic waste ...... 28 1.4.2 Classification of plastic recycling methods ...... 29 1.4.2.1 Material Recycling...... 30 a) Major end-user industries generating plastic waste and applied recycling methods...... 31 b) Material Recycling of various plastic commodities ...... 32 Polypropylene (PP)...... 32 High-density polythylene (HDPE) ...... 32 Low-density polyethylene (LDPE) ...... 32 Polystyrene (PS)...... 33 1.4.3 Recycling of commingled plastic waste...... 33

2.EXPERIMENTAL ...... 37 2.1. Materials...... 37 2.2 Sample preparation ...... 37 2.2.1.Oven aging...... 37 2.2.2.Photo-oxidation...... 37 2.2.2.3 Blend preparation ...... 38 2.3.Methods ...... 39 2.3.1.Determination of hyroperoxide content by Spectrocolorimetry ...... 39 2.3.2.Crystallinity, melting enthalpy and oxidative stability...... 40 2.3.3 MFI ...... 40 2.3.4 Tensile impact strength...... 42

3.RESULTS AND DISCUSSIONS...... 43 3.1 Thermo-oxidation- Oven aging Experiment ...... 43 3.1.1 Photo-oxidation: exposure in Weather-o-meter (WOM) at λ = 340 nm...... 48 3.1.2 Formation of oxygenated groups ...... 49 3.1.3 Onset Temperature...... 50 3.1.4. Crystallinity variation ...... 51 3.1.5 Mechanical and rheological properties ...... 52 3.1.6. MFI ...... 55 3.2. Upgrading mechanical properties and stability of mixed plastic waste…………..58 3.3. Enhancement of light stabilization of mixed plastic waste...... 65 3.4. Processing stability- Melt flow index ...... 69

4.CONCLUSIONS ...... 71 4.1 Thermo-oxidation ...... 71 4.2 Photo-oxidation...... 71 4.3 Upgrading of polymer blends: Compatibilization, processing and light re- stabilization...... 71

5.REFERENCES...... 73

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LIST OF ABBREVIATIONS AND SYMBOLS

Abbreviations of polymers

CPE chlorinated polyethylene EPM ethylene-propylene statistical copolymer EVA ethylene-vinyl-acetate HDPE high-density polyethylene HIPS high-impact polystyrene LDPE low-density polyethylene LLDPE linear low-density polyethylene PB polybutadiene PET polyethylene terephthalate PO polyolefin PP polypropylene PS polystyrene PVC polyvinyl chloride SB styrene-butadiene block copolymer SEP styrene-ethylene-propylen copolymer

Other abbreviations and symbols aε tensile impact strength ARA Alstoff Recycling Austria ATR-FTIR attenuated total reflection Fourier transformation infrared spectroscopy BQDI N,N’-disubstituted 1,4-benzoquinonediimine CI carbonyl index DSC differential scanning calorimetry DSD Duales System Deutschland DUS Dusantox Ea activation energy EMMA equatorial mount with mirrors for acceleration ESRI electro spin resonance imaging FTIR Fourier transformation infrared spectroscopy

ΔGmix Gibbs energy of mixing HALS hindered amine light stabilizer HAS hindered amine stabilizer ΔHmix enthalpy of mixing hν light quantum IS impact strength λ wavelength Mw molecular weight OIT oxygen induction time PAS photo acoustic spectroscopy PD 1,4- phenylenediamines SEM scanning electron microscopy vii

ΔSmix entropy of mixing TGA thermogravimetric analysis Tg glass transition temperature Tonset onset temperature WOM weather-o-meter

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USE OF COMPATIBILIZERS AND RE-STABILIZATION DURING RECYCLING OF THE MUNICIPAL PLATIC WASTE STREAM

Introduction

Polymers, and plastics in particular, transformed society in recent decades. A wide range of products composed on the basis of polymers are omnipresent in our daily lives. Polymers are enhanced by particular properties, which make them very attractive, and in many cases, irreplaceable materials in construction, automotive, packaging and other technologies. Unfortunately, with all the advantages and contributions ascribed to plastics, collateral concerns regarding the proper disposal of plastic scrap have also appeared.

The increasing world production of plastic, originating huge amounts of waste, the financial costs of landfilling, and landfill capacities reaching limits, are becoming unbearable. On the other hand, growing concern for global climatic change and its negative environmental impact has raised public awareness on this issue. Public opinion is changing consciousness and different ecological movements and green organizations are pressuring and compelling local governments and policy-makers to take immediate actions in this matter.

In many developed countries a number of environmental regulations have been adopted. In particular, Europe is recognized for its very strict and stringent environmental legislation regulating all aspects of human activity and their environmental implication. Europe represents a major plastic manufacturing region, producing about 25 % of the total estimate of plastic production worldwide (260 million tons), thus about 65 million tons [1]. It’s worth mentioning that packaging remains the largest end-use industry in Europe, accounting for nearly half of all polymers processed in 2007.

Facing this challenge, regulatory bodies within the European Union set up ambitious targets which are expected to be accomplished by implementing a system of recycling quotas. The current regulation stipulates a quota of 15 %, which is subject to further progressive increase. This system of quotas is not new, but has already been applied during the last century. The system was initially adopted due to a debilitating lack of raw materials between the First and Second World Wars and a second wave occurred as a consequence of unstable fluctuations in crude-oil prices during the 70’s.

Polymer recycling can be carried out in a number of ways, including material, chemical and thermal recycling. Part of the stored energy recovered from waste decreases from material to thermal recycling. Chemical recycling consists of depolymerization of the post-consumer resin into monomers, which are then used as raw material for synthesis of virgin polymers. At thermal recycling (incineration), plastic waste is not necessarily separated from other organic waste (paper, wood, etc.). Therefore, the temperature at incineration must be carefully controlled to avoid the harmful emission of polluting chemicals.

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Material recycling consists in the collection and reprocessing of commingled/separated industrial or municipal scrap that would otherwise become waste. The overall material recycling rate of EU post-consumer plastics in 2007 was 20.1 %, one of the fastest growing recycling methods in most of the EU countries.

Since chemical recycling is only applicable to a certain group of polymers (produced by polycondensation) and thermal treatment is a rather controversial method, due to the emission of harmful gases into the atmosphere, material recycling seems a more reasonable solution to the question of waste management of household commingled plastic scrap. These mainly consist of polyolefins (high density polyethylene-HDPE, low density polyethylene-LDPE, polypropylene) and polystyrene, including high impact polystyrene-HIPS, PET and PVC. If polyethylene terephtalate (PET) bottles are not taken into account -since they are sorted manually [2] for further chemical or material recycling- and amounts of PVC are considered to be negligible, as contemplated by a legislative mandate, which had suppressed its usage for packaging, then the largest amount of material to be recycled in the Czech Republic is composed of polyolefins and HIPS.

Nevertheless, some technical challenges arise, related to different chemical structure and rheology, when blending commingled plastic waste. Most polymers are not only thermodynamically immiscible but also incompatible with each other [3, 4, 5]. This incompatibility is manifested as poor interphase adhesion and a tendency to phase separation, which consequently leads to a deterioration of the mechanical properties of such blends. Products based on these recyclates are less competitive in the market, with limited ranges of application.

Finding applications for recyclates in a broader end-use market, is an aspect of utmost importance for the incipient plastic recycling industry to definitely take off without governmental subsidies; therefore, by making recycling a more cost effective and efficient process, added value products at lower processing costs will be ensured

Post-consumer materials are degraded to a different degree depending on its inherent chemical structure, level of stabilization, etc. Formed chemical moieties such as carbonyl or hydroperoxide groups act as oxidation sensitizers, imparting recycled materials higher susceptibility towards degradation during the second service life.

Hence, a strategy of successful material recycling procedures must be based on application of suitable compatibilizing and stabilizing systems, ensuring both acceptable mechanical performance and enhanced stability.

In this regard, significant progress has been achieved in the field, so far a considerable number of works devoted to the recyclability of polyolefins based on model binary or ternary blends, using various types of block or graft copolymers and processing stabilizers had been reported. In spite of this, only few works dealt with the recycling of model quarternary blends and their posterior overall stability, a critical parameter for its second life-time application.

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Fortelny et al reported a method by which a mixture of ethylene-propylene statistical copolymer EP(D)M, a styrene-butadiene block copolymer (SB) and a disubstituted amine stabilizer evidenced an efficient procedure for recycling commingled plastic waste. This system was successfully applied in quarternary model blends -LDPE/HDPE/PP/HIPS (6/2/2/1) - and in real plastic waste scrap from local containers [6]. The efficiency of the method was corroborated by increased tensile impact strength. For further practical utilization of such materials, it is necessary to establish also their long-term stability.

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Aims

The aim of this work is to find a suitable recycling process by mean of compatibilization method, imparting enhanced mechanical properties to recyclate, whereas for successful utilization of this method in practice, also stability of recycled material during further processing and second life-cycle must be known and eventually enhanced.

The methodology used to achieve these goals, are summarized below. The scope of the present dissertation comprises three sections.

1. Assessment of the degradation behavior of individual plastic components. Taking into consideration the complexity of quaternary blends, it was found necessary and convenient to carry out a previous study monitoring and evaluating the response of single polymers when exposed to artificial thermal and photochemical aging, this, in terms of changes of mechanical properties, crystallinity and oxidative stability. This step avoided a transfer of knowledge from previous works in the field, as the studied polymer grades are specific to the Czech market.

2. Upgrading mechanical properties in order to meet second application requirements. To achieve this, model quaternary blends (LDPE/HDPE/PP/HIPS) based on pre-aged homopolymers were compatibilized by a mixture of commercially available styrene-butadiene block copolymers (SB) with ethylene- propylene elastomers (EPR). In order to ensure the processing and long-term stability a secondary amine stabilizer: trade name Dusantox (Dus) was added. The resulting upgraded mechanical properties of formed blends are studied, as well as the different mechanisms of cooperative effects of the components of the compatibilization systems and the stability of model blends during further aging.

3. Enhancing photo-stability in model quarternary pre-aged blends that are subjected to posterior aging. Here, the effect of commercial HALS stabilizer Tinuvin 770DF and of carbon black on the properties of the blends is evaluated.

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1. STATE­OF­THE ART

1.1 Degradation and aging

For proper selection of stabilizers and formulation of stabilizer systems, it is essential to understand the factors that cause degradation and the mechanism of various degradation processes.

The term polymer degradation is understood as a process involving a wide range of reactions, leading to irreversible chemical and mechanical changes in the polymer structure, such as apparition of chemical irregularities, molecular weight distribution, polydispersity, etc. The degradation of polymers is manifested macroscopically by a deterioration of mechanical, electrical and optical properties in cracking, erosion, tackiness, discoloration or embrittlement.

Reactions inducing polymer degradation are triggered by chemical, physical or biological factors which attack the polymer during processing (heat, mechanical stress) and further during their use in outdoor environments by atmospheric agents such as solar radiation, oxygen, humidity, pollutants, or bacterial attack.

Polymer degradation occurred via main-chain scission and cross-linking, both phenomena involving a free-radical chain reaction character and taking place simultaneously in a competitive way. The tendency of a given linear or branched polymer to predominantly degrade either by scission or cross-linking is influenced by the character of its chemical structure. It is well known that the presence of a less stable tertiary carbon atom in polypropylene main chain, leads to a chain scission reaction rather than to cross-linking. By contrast, linear long-chain polymers (such as PE) are more likely to branch/cross-link when submitted to thermal or photochemical stress.

Chain scission leads to a narrowing of the molecular weight distribution [7], whereas chain branching and cross-linking result in a broader distribution of molecular weight [8] and take place mostly in oxygen deficient environments.

1.1.1.Classification of degradation processes

Summarizing the main concepts under which we understand the definition of polymer degradation: Thermal degradation Mechanical degradation Thermomechanical degradation Auto-oxidation Photo-oxidation

Thermal degradation initiated by thermal energy occurs mostly during processing, when the absorbed heat energy starts covalent bond dissociation, leading to bond rupture and

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formation of shorter chains. In the case of mechanical degradation, the chain rupture is caused by the actions of mechanical work and can be considered a phenomenon exclusive for macromolecules. During extrusion processing, where high temperatures and shear stress conditions are applied, the polymer tends instead to suffer from thermomechanical degradation. In studies related to polyolefins, it was proved that mechanical stress has an accelerated influence on degradation during processing [9]. It was found that a series of polyolefins undergo thermomechanical degradation in the range of temperatures where they are unaltered by thermal treatment alone.

Furthermore, in real processing conditions, small amounts of oxygen have to be accounted for, since oxygen traces get through the extruder apparatus to polymer granules and, during processing, react with formed free alkyl radicals. This reaction initiates a chain, radical, autocatalytic, reaction, better known as “auto-oxidation” [10].

Therefore, proper processing stabilization plays a key role, reducing as much as possible, the extent of degradation during processing. Because the deterioration of polymer materials during processing and service life-time are ascribed mainly to photo and auto- oxidation, the following paragraphs describe their mechanism.

1.1.1.1 Auto-oxidation

The auto-oxidation mechanism includes initiation, propagation, chain branching and termination steps.

The initiating step begins when molecular atmospheric oxygen is activated by collision with alkyl macroradicals formed during production, processing, indoor or outdoor applications.

In the second propagating step, those radicals react with oxygen producing alkoxy radicals ROO•, which are very unstable and immediately react by attacking the most labile bond, preferentially hydrogen atoms in tertiary carbons. This hydrogen atom abstraction leads to the hydroperoxide formation ROOH. Hydroperoxides are formed quantitatively from oxygen absorbed during the induction period, which is when hydroperoxide accumulations in polymer bulk take place. The linear increase of hydroperoxide concentration during auto-oxidation shows an accumulative character. After reaching a plateau, hydroperoxide concentration begins to decay in thermal and photochemically triggered reactions generating free radicals. This decomposition is accompanied by chemiluminiscence.

The classic Bolland and Gee scheme for auto-oxidation of polymers accounts for all oxygen-containing groups formed and the molecular enlargement and diminishment observed at elevated temperatures.

Free radicals lead to further backbone scission, narrowing of molecular weight distribution and, ultimately, to formation of alcohol/carbonyl groups. Recombination of

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R• and ROO• or addition of R• radical to double bonds are also involved and account for formation of branched structures and cross-linking, hence to a molecular weight enlargement.

Recombination of radicals leads to the termination of the kinetic length of auto-oxidation. Russel [11] suggested a model using the cage-effect theory. According to it, the interaction of peroxy radicals in the cage may lead to alkoxy radicals or inactive products.

Scheme 1. The Russel model of free radical termination

2RO• + O2 propagation cage 2ROO• → [RO•O2 •OR]

ROOR + O2 termination

Polymer resistance to oxidation depends on properties inherent to the chemical structure of polymers- macromolecules, with a chemical skeleton containing double bonds, tertiary carbons and polar groups are more prompt to oxidize. [12] Therefore, on the facility of oxygen to diffuse across the polymer mass, amorphous polymers are more particularly vulnerable to auto-oxidative degradation.

Hydroperoxide and ketone groups are the initial point to a sequence of further and more complex reactions, where numerous oxygenated groups are formed. As shown by Gugumus [13, 14, 15], who proved that the reaction of aldehydes with secondary hydroperoxides in PE, transforms the aldehydes into acids and hydroperoxides into ketones.

1.1.1.2 Photo-oxidation

Terrestrial sunlight spectrum ranges from 290 to 3000 nm. Less than 10 % correspond to the UV light region, 50 % is visible light and 40 % infrared light. The actinic range of solar radiation which penetrates earth surfaces after atmosphere filtration is 6 %. The spectrum of UV light lies in that wavelength region between 290-400 nm. Although the quantum energy contained in this radiation is sufficient to dissociate covalent bonds in polymers, photolysis only occurs after light absorption by the material. This absorbed energy leads to excited states and subsequent energy dissipation by radiative (fluorescence and phosphoresce) or radiation-less transition or energy transfer to the acceptor.

From the point of view of chemical structure, a considerable number of polymers, in particular polyolefins, would be unable to absorb light in that wavelength. Then, the deterioration observed in weathered packaging materials implies an induced photo- oxidation by impurities present in the polymer.

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In this regard, polymers have been classified according to their intrinsic photoliability into [16]: a) absorbing polymers, containing inherent chromophores in their basic structural constitution. This group of polymers includes aromatic polyesters, polyamides, polyimides, aromatic urethanes and phenoxy resins. b) In non-absorbing polymers, such as polyolefins, aliphatic polyamides, polydienes, polyacrylics, light absorption is effectuated by chomophoric impurities [17].

The aforementioned division was established for the sake of illustration, as mechanisms involved in the photochemistry of polymers are rather complicated, since photo-oxidative changes of absorbing polymers may also be eventually induced by photosensitizing impurities originated from a wide range of processes.

Special attention was addressed to the role of chromophore groups; namely carbonyl and hydroperoxides in the photochemistry of non-absorbing polymers. Some authors argue that hydroperoxides are crucial during PP photo-oxidation [18], whereas it is believed that carbonyl groups have a dominant effect in PE [19].

However, an increasing number of researchers are arriving at a consensus that carbonyls are indispensable in the photo-oxidative processes of polyolefins [20]. Hydroperoxides are considered to be very effective free-radical initiators. Photolysis of ketone and aldehyde groups proceeds by involving Norrish I and II reaction mechanisms; they are referred to as the dominating process during PE photo-oxidation [21]. These reactions account for the formation of unsaturations and free-radical generation, as shown in Scheme 2.

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Scheme 2. The Norrish I and II reaction mechanism

1.1.2 Aging

The process of polymer aging is regarded as the gradual deterioration of useful and important material properties, as a result of long-lasting detrimental interaction of physical, chemical and biological stress during polymer service-life. The process of aging has significant practical importance as it determines the time after which polymers can no longer meet material or technical criteria for their proper application.

The longevity of polymers varies depending upon structural parameters and service conditions. As a matter of fact, determination of the life expectancy of polymers and its eventual enhancement is part of the integral manufacturing chain. Producers of commodities based on polymer devote such efforts to the development of new and more effective stabilization packages, as well as researching new substances.

Long-term durability prediction tests are a useful assessment that provides an estimate of longevity and the intrinsic qualities of a given material to withstand the negative effects of outdoor exposure. As said before, these are provided in order to develop both novel, high quality materials and more efficient stabilizers. To achieve this goal, long-term durability and recyclability are tested during natural and artificial weathering extremes.

1.1.2.1 Accelerated (artificial) contra natural aging of polymers

Nowadays natural and artificial weathering (aging) tests are widely applied methods in both academic and industrial fields [22, 23, 24, 25]. These long-term durability tests show changes of material properties as a function of exposure time.

Natural aging tests are considered more reliable, as they provide more precise and accurate information. From this point of view, a hierarchical list in the long-term durability forecast methodology was established. According to that classification, the best predictor of plastic material longevity consists in the real-time exposure in Florida at 45 degrees centigrade, facing south [26]. This method is considered a de facto standard.

A second effective methodology refers to a real-time exposure in the location where the polymer will be installed, followed by a third method based on artificially accelerated weathering tests. UV-A, Fresnel-type [27] and xenon arc merry-go-round devices are among the particularly recommended and numerous artificial weathering devices available.

It must be pointed out that methods based on natural aging tests are becoming rather obsolete and offer less perspective, as in practice a considerable number of works devoted to the prediction of the service life of polymer materials opted for tests based on artificial weathering. The reason is that under natural conditions, external factors such as an exposure to sunlight, day and night temperatures, seasonal variations, humidity or

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atmospheric pollution are difficult to control or even record. This reflects in the low reproducibility of the method.

Another drawback is that this methodology lacks universal validity, as data arising from measurements located at different geographical latitudes do not allow correlations among each other. As an example, a couple of cases can be cited from the literature identifying changes in mechanical properties of samples achieved from works carried out in northern latitudes [28]. These showed deterioration to a lesser extent than those observed during natural aging in tropical zones, due to the different intensity in the climatic agents [29, 30]. For comparison, HDPE, which had been exposed to extreme temperature conditions separately, first during Canadian winter [28], and a second work was on HDPE aging in Rio de Janeiro summer [29]. There are evident and remarkable differences in measured impact-resistance values. Other papers reported results where the lifetime of films exposed under dry conditions [30] are expected to be relatively longer than that of the same films exposed in rainy regions [22, 24].

From the practical point of view, another considerable disadvantage of natural aging tests is that they are time-consuming. For stabilized polymers in particular, it may take several years to obtain results.

The artificial methods of aging polymers appear as a reasonable solution for the life- service prediction of most commercially used polymers, since they are carried out in a considerably shorter time period and results are reproducible, even though less accurate. Regarding the running costs of both processes, the expenses of artificial weathering are higher, compared with free sunlight radiation in natural aging.

For artificial aging of polymers, two methods are conducted: oven aging and accelerated UV radiation.

1.1.2.2 Oven aging experiments

Oven aging experiments proceed by enhancing the stress level of one of the degradation factors present in the service environment, at specific temperatures.

Long-term heat aging is a widely used procedure providing information on the performance and durability of polymer stabilizers as well as the thermo-oxidative stability of a given polymer over a reasonable period of time. Here the response of polymeric materials to elevated temperatures, in terms of mechanical and chemical changes, is the subject of monitoring and analysis

In the particular case of long-term heat oven aging, the polymer items are exposed to increased temperatures in a circulating heated-air ambient, simulating the outdoor environment.

For the determination of the rate of oxidation, and the oxygen induction time (OIT), several well known analytical methods are applied. One of them is based on the direct

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measurements of oxygen uptake. In certain experimental sets, the stabilizers consumption can be indirectly determined by Differential Scanning Calorimetry (DSC) performed in air, where the oxidation induction time (OIT) is evaluated. Chemiluminescence, colorimetric and spectroscopic techniques are used to detect the generation of oxidation products such as hydroperoxide and carbonyl groups. Other useful complementary analytical techniques, such as rheological tests (MFI, viscosimetry),. thermogravimetric analysis (TGA), mechanical (tensile and impact tests) and physicochemical studies (GPC) are also applied.

Solid-state polymer samples are prepared mostly as pellets, granules or plates, to eliminate or at least reduce oxidation heterogenity. Samples are placed in heated-air ventilated ovens at temperatures below their melting point (at temperatures in the range between 70-180 °C) with the purpose of reproducing as closely as possible practical real service conditions. The pressure inside the oven and the temperature of exposure may vary, depending on the specific goals and priorities of a particular research.

Oven-aging experiments conducted either under vacuum, air, nitrogen (or increased pressure of one of these gases) can be found in literature [31]. They provide evidence of the importance of properly selecting exposure conditions and their impact on the durability and performance of a given material. For illustration, it was proved that the elongation-at-break decrease of PE-based non-woven material following oven-aging treatment, was greater in samples placed in ovens with high air speed circulation, indicating an antioxidant loss by evaporation. [32]

The selection of experimental conditions must also take into account the presence of stabilizers added during processing and the temperature under which their decomposition takes place. Pospisil et al [33] has emphasized the crucial role of testing conditions in long-term durability tests. According to him, oven aging experiments should select a range of temperatures at which stabilizers are still active, since the failure point in the material mechanics starts after the consumption of stabilizers during sacrificial reactions.

For that reason, phenol-stabilized polyolefins are mostly tested at 135 and 150 °C, whereas HAS-stabilized polymers are recommended to be tested below 120 °C [34]. Experiments in solid state, and in particular oven aging tests performed at moderate temperatures allow the monitoring of stabilizer efficiency on key mechanical properties of given materials, because under these conditions the course of both oxidation and stabilization mechanisms are considered almost identical with those involved in natural thermal aging.

The very well known Arrhenius equation represents a useful tool quoted in a number of works [35], for the determination of the equivalence between residence time in accelerated conditions, and under natural ageing. That correlation is determined by solving a time transformation function of the Arrhenius equation of temperature- dependence by some constant, as for instance the rate of oxygen absorption [35].

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Results permit the extrapolation of data obtained from short-time exposure at elevated temperatures to long-term in-use conditions. According to these calculations, one month of artificial ageing at 70°C corresponds to the lifetime of 5.5 years of outdoor ageing at 20 °C for a material with an activation energy of Ea = 70 kJ/mole [36].

In practice, laboratories and investigators are using the so-called “rule of thumb” in the selection of time and temperature for accelerated aging tests. According to this rule, if temperature is increased by 10 K, the rate of degradation reaction is doubled, consequently the time is halved. Moreover, it is recommended to use lower or conservative activation energy values during the calculation of the accelerating factor in order to avoid an over-estimation of service lifetime.

Following that methodology, Boldizar and co-workers [37] calculated and applied a residence time for oven-aging exposure of 48 h at 110 °C in order to simulate 2-3 years of indoor use of PP at 20 °C. Similarly, a period of time of 48 h at 130 °C, was used to simulate 10 years of natural indoor use [38].

For those reasons, the design of a life-time testing experiment must be done with utmost caution and considering all the factors that could alter the course and rate of degradation.

1.1.2.3 Accelerated UV irradiation­ UV experiments

Accelerated weathering experiments are carried out by devices which simulate terrestrial sunlight and other atmospheric agents. Because the UV light component of solar radiation is the main precursor of photodegradation reactions in polymers (such as in poorly photostable polyolefins), the selection of an appropriate source of UV light with emission spectra similar to natural sun radiation is crucial.

Table 1. Fragmentation of wavelengths in UV region and the corresponding light source of emitting radiation

UV Region Wavelength (nm) Equipment UV-A 315-400 Xenon arc (WOM, xenotest)

UV-B 280-315 Mercury lam (QUV tester)

UV-C 200-280

So far the best simulation was obtained by weather-o-meter (WOM) devices equipped with filtered xenon arcs. This artificial weathering equipment consists of a merry-go- round appliance with rotating racks designed for specimen placement and a light source emitting high concentration of UV-A radiation, which is placed in the middle of the device. Moreover the WOM apparatus is provided with misting spray nozzles for the simulation of rain and can be set up to alternate light and dark periods, simulating

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day/night cycles. Many authors agree that this type of radiation correlates well with natural terrestrial sunlight [39].

Fig 1. Natural ageing and accelerated aging of polymers

A second type of equipment utilizes florescent lamps as a source of irradiation. These devices are called QUV testers and are capable of irradiating light at a wavelength of 254 nm, in the region of UV-B light. QUV testers are a preferred alternative in the commercial sphere, due to their low running costs, simplicity and mainly because they obtain results in a considerably shorter period of time [27]. As conditions in QUV testers are more severe than in WOM [40], this is a main reason why results from QUV experiments can seldom be correlated with those arising from natural aging. Therefore, correlating degradation events taking place during QUV photo-oxidation is expected to be difficult, compared to those triggered under natural sunlight weathering.

Another group of weathering testers, consisting of mercury lamps, can be mentioned: SEPAP SAIREM 12.24 and CEMP. Acceleration of weathering has also been achieved using devices containing mirror concentrators such as Equatorial Mount with Mirrors for Acceleration (EMMA, EMMAQUA)

Results obtained from artificial photo-oxidation are in some cases compared with those obtained from natural aging experiments in an attempt to establish helpful correlations. In previous works it has been determined that, 500 hours of accelerated weather conditions corresponds to 6 months of environmental aging in New Jersey [41]. A recent work concerning the characterization of the weathering extent of LLDPE/LDPE thin film was also reported. It established the equivalence of 1 h of accelerated weathering to 10.73 h of natural weathering [42].

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1.1.3 Functional groups formation and degradation profile of aged polymers

Chemical irregularities such as functional oxygenated groups, unsaturations and transformation products are incorporated into the polymer structure during their synthesis, processing and usage.

The mechanism of their formation has been widely studied and a variety of works devoted to the study of polyolefins, PS and PVC provides detailed information regarding their degradative pathways. Cross-linking and chain scission has been recognized as the main mechanism involved in the formation of these new non-saturated structures. The appearance of double bonds: trans and cis vinylene and vinylidene groups are attributed to cross-linking and branching during studies in PE processing degradation. Conjugated and isolated polyvinyl sequences were detected in aged PVC and PS [43]

Cleavage of peroxy radicals and β-scission of alkoxy radicals in the polymer main-chain lead to the formation of aldehyde and ketone groups, which at the same time may have a sensitizing effect and may initiate further photo-oxidation. These reactions are believed to occur principally at the early stages of degradation, when no hydroperoxides have been accumulated yet. In posterior stages, the chemistry of polymer degradation will be dominated by hydroperoxide decomposition, producing a wide range of free-radical species, such as peroxyls, alkoxyls and acylperoxyls. Fourier-transformation infrared spectroscopy (together with chemical treatment of aged polymers) has been successfully applied [44]. This approach made possible the detection of a large number of polymer- bound oxygenated products such as alcohols, γ-lactones, esters, peresters, carboxylic acids, peracids and phenyl-conjugated carbonyls.

The distribution of these groups across the polymer mass is not homogeneous. It displays a concentration gradient in the different layers due to the heterogenic character of polymer oxidation. Indeed, the formation of local variations of oxidation in semicrystalline polymers has been recognized as a diffusion controlled process, governed by the rate of oxygen diffusion throughout polymers, the migration of low-molecular weight free radicals, degradation spread from heavily oxidized sites via energy transfer and, ultimately, by a heterogeneous distribution of non-polymeric impurities (catalysts residues).

By means of sophisticated analytical methods such as attenuated total reflection Fourier transformation spectroscopy (ATR) FTIR, photo acoustic spectroscopy (PAS), electron spin resonance imaging (ESRI), energy dispersive spectroscopy [40], it was possible to monitor the space distribution of concentration gradients of these products in the different layers of polymer bulk.

Terselius reported results in which a carbonyl concentration gradient was observed, demonstrating the diffusion-controlled character of auto-oxidation processes [45].

Carlsson et al. also corroborated that weathering/photo-oxidation is principally a surface process [46] showing a gradient diffusion of oxygen, UV light and stabilizer.

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All these considerations should be taken into account when interpreting the changes in mechanical properties observed in aged polyolefins, as mechanisms of material failure are inherently linked to material surface irregularities.

Next, a brief summary of different oxygenated groups formed during the natural and artificial aging of polyolefins and PS will be described.

1.1.4 Degradation of commercial polymers (polyolefins and PS)

a) Polyethylene: LDPE and HDPE

The degradation process in PE involved cross-linking and main-chainscission. They occurred simultaneously and in a competitive manner, the predominance of one of the two mentioned degradation mechanisms depending upon external conditions and the presence of non-polymeric impurities, among others. Phillips type HDPE is more prone to degradation via cross-linking mechanisms, whereas HDPE prepared by the Ziegler method has a tendency toward chain scission [47]. The susceptibility of different PE types is also associated to the grade of crystallinity of polymer materials. As shown by Gulmine [40] in works related to PE submitted to artificial accelerated weathering; the stability of different PE types decreases in the order HDPE>LLDPE>LDPE.

The appearance of trans-vinylene groups is considered characteristic for HDPE processing. The progressive concentration decay of these unsaturations is attributed to the addition of free alkyl radicals to double-bond with low steric hindrance, resulting in branched and cross-linked structures [48].

The increasing intensity of C=O stretching vibration, with increasing period of exposure, is accompanied by band broadening, indicating the appearance of new functionalities. Deconvolution of this broadened band region was achieved after chemical derivatization. It was found in a wide scale of oxygenated functional groups, arising from the splitting of the carbonyl band. They correspond to aldehydes (1733 cm-1), carboxylic acid groups (1700 cm-1) and γ-lactones (1780cm-1) and 3500 cm-1, which belong to hydroxyl groups.

Comparative studies dealing with natural and artificial PE aging reported interesting results concerning the quantitative and qualitative differences between both processes. EL-Awady et al. [30] observed that carbonyl bands from LDPE (aged by sunlight) form more carboxylic groups, but less ester and vinyl alkene than that photo-oxidized under accelerated conditions. . The spatial distribution of oxidative products concentrate mainly in the surface layers of materials. With higher accessibility to oxygen, this concentration diminishes in the deeper, oxygen-lacking layers, in which the auto-oxidative process is less accentuated. At this region, non-oxidative processes, such as chain cleavage and branching are dominant. FTIR ATR revealed numerous unsaturated formations such as: vinylene, vinylidene and vinyl groups arising from the beta scission of the PE polymer chain [49].

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b) Polypropylene

PP fulfills all the presumptions that it is prone to deteriorate more quickly than other polyolefins, mainly due to its primary structure containing labile tertiary hydrogen [50, 51], which is vulnerable to radical attack. A frequently used analytical approach to PP aging comprises the FTIR method. There can be found two characteristic absorption regions corresponding to carbonyl (1715 cm-1) and hydroxyl groups (3500 cm-1) which are characteristics of PP degradation.

Works dealing with artificial and natural aging of PP evidence differences in the oxidation profile of both processes. Tidjiani and co-workers [11] found tert- hydroperoxide to be the dominant photoproduct of natural aging, whereas under accelerated conditions sec-hydroperoxide groups were produced at a higher rate.

A widely used assessment represents the so-called carbonyl index, which is defined as the ratio between the integrated band absorbance of the carbonyl around 1714 cm-1 and that of the PP-polymer band (1470cm-1). In this regard, in many studies the carbonyl index CI shows a linear increase with exposure time [24]. Further studies found detailed composition of ketone groups after the deconvolution of peaks corresponding to C=O absorption bands [49]. A complicated mixture of oxidation products was found: ketones conjugated to alkene, non-conjugated ketones, aldehydes, esters, carboxylic acids, peresters and peracids [48].

The decay of carbonyl index from surface to bulk in aged PP car bumpers, referred to oxidative processes occurring principally in the surface of material, proved that oxidation localized in the surface is one order of magnitude larger that in the bulk [23].

Moreover, the oxidation rate is also influenced by the morphology and type of applied aging methods. Under accelerated conditions, the rate of oxidation of PP low crystallinity is found to be higher than that of PP high-crystallinity. Under natural conditions, the oxidation rate did not differ between low and high-crystallinity PP, but showed a higher amount of tert-hydroperoxides than in PP aged at accelerated conditions [11]. Thus we recognize the dual character of C=O and OOH groups, as they are inter-converted in the course of aging.

Isotactic, highly crystalline PP is more resistant to photo-oxidation in experiments carried out in natural and artificial aging in the presence of air, because of the lower diffusion rate of oxygen through the crystalline phase. It was reported that the range of degradation in PP also depends on the degree of tacticity.

c) Polystyrene (PS) and High Impact Polystyrene (HIPS)

The rate of natural weathering depends upon both external and internal parameters. In aging of HIPS samples conducted in India, the largest increase in absorption in the carbonyl and hydroxyl regions were detected during the months April to March. During

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this time, the average temperature was approximately 40-44 ˚C and the humidity was 40- 50% [52].

Results arising from analysis of pure polystyrene, aged under artificial conditions, give evidence of the formation of different types of polystyryl radicals, which add oxygen to form peroxyradicals –CH2C(Ph)(OO( )CH2 – and alkoxy macroradical – CH2C(Ph)(O·)CH2. The β-scission of macroalkoxy radicals gives rise to an acetophenone type structure [53]. Norrish reactions are assumed to participate in the generation of carboxylic acids, esters and many other chemical irregularities, which are inserted to the polymer chain.

The intensive yellowing observed in aged samples of modified PS, is according to some authors, ascribed to the formation of a structure similar to benzacetophenone, which derives from hydroperoxide decomposition. The formation of stilbene, quinones, polydienes, and dicarbonyls, produced by ring opening reactions via singlet oxygen attack, is also attributed to be the origin of HIPS yellowing . The yellowing index (YI) is a useful criterion applied in studies concerning the extent of aging in PS and its derivatives [53]. Additionally, different types of acetophenone structures –CH2 C(O)Ph, chain ketones –CH2C(O)CH2-, molecular groups with the structure of dibenzoyl methane, C(O)(Ph)CH2 C(O)(Ph) were identified in aged PS [54].

However, the photochemistry of PS modified with polybutadiene (HIPS), takes a different course. Here, the rate of aging is directly related to the polybutadiene (PB) content. Double bonds in PB are considered to be the infectious site of oxidation, which sensitize further photo-oxidation of the polystyrene matrix. Works in this field demonstrated that the photo-oxidation of polystyrene in HIPS starts after 50 h irradiation, whereas pure polystyrene does not show any sign of oxidation before 150 h [55].

In anterior works, hydroperoxides (rather than carbonyl compounds) were claimed to be directly responsible for the initiation of photo-oxidation of polymers containing polybutadiene. Nowadays a smaller number of researchers are convinced of that. They focus their interest on α, β-unsaturated ketones, which are considered to be crucial in HIPS photo-degradation [56, 57].

1.1.5 Impact on the mechanical and morphology properties a) PE I) Impact on the mechanical properties

The effect of aging on polymer materials is manifested by the gradual deterioration of mechanical properties. It was observed that even small alterations in the polymer structure caused significant changes in the mechanical performance of materials, such as elasticity, hardness, strength. Tensile impact strength (IS), in particular, is very sensitive to degradation and to incompatibility of polymers.

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From previous works it is known that molecular weight has a direct or indirect (changes in morphology) impact on the IS. Narrowing of molecular weight distribution reduces IS whereas a broadening leads to the opposite effect. Some changes in the morphology also negatively influence the performance of the material. It was proved that bigger spherolites and increased crystallinity lead to tensile impact strength reduction. On the other hand, the IS of a material is increased by increasing amorphous fraction or rubber stereoblocks, although they also lead to molecular weight reduction.

As shown by Tavares et al [25], a direct link exists between carbonyl group formation and changes in hardness of aged PE. Carrasco [58] reported an increase of Young modulus, which perfectly coincides with the increasing formation of carbonyl groups detected by FTIR analysis.

Changes of branched polyethylene, exposed to sunlight in an aerated warm environment, showed tensile strength reduction by 40% [59].

Naturally weathered HDPE became fragile, less ductile and also suffered from loss of gloss and discoloration. Works carried out under natural exposure conditions of non- stabilized HDPE (Rio de Janeiro, Brazil) indicated that oxidative degradation causes significant reduction of molecular weight which is reflected in a drop of around 50% in impact resistance and elongation-at-break [29]. Samples of HDPE aged in the Arizona desert broke down more readily after experiencing less elongation and increased embrittlement [60]. Conversely, similar studies carried out in northern latitudes, during Canadian winter months, showed moderate deterioration of impact energy, tensile strength and elastic properties.

LDPE films, weathered in desert regions for 30 months, showed a slight increase in tensile strength during the early stage of exposure, followed by a rapid decrease due to a predominance of chain scission at further exposure times. Micro-cracks over the surface of artificially weathered LDPE samples were observed in SEM micrographs, the profile of the changes in the mechanical properties obeying the same pattern as that for carbonyl formation [40, 25]. As was shown by La Mantia [61], ketone groups strongly increase the photo-oxidation kinetics leading to a decrease of elongation at break and tensile strength.

Similar results were obtained from aged samples of materials subjected to artificial weathering. Accelerated weathering of HDPE leads to a loss of elasticity, thus increasing material embrittlement. After exposure to UV light for 120 days, the elongation at break of HDPE samples decreased drastically, the strain reduced from 231 % to 7.4 % [58]. Tensile strength showed a slight decrease with irradiation time. Changes in the mechanical properties were well correlated to carbonyl index increase.

II) Changes in morphology

Aging leads to denser and harder HDPE resulting from chain scission. Shorter molecules show higher chain mobility, which can easily crystallize into a more compact structure. It

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has been proven that scission reactions take place in an amorphous phase, which is in turn more accessible to oxygen diffusion. This contributes to local concentration of oxidative products as a function of depth in samples of aged polymers, with a decreasing concentration of oxygenated products from surface to bulk.

Density increase can also be ascribed to other phenomena such as chemocrystallization, annealing effects [62] and changes in lamellar orientation, as reported in different studies on artificial aging [63] of HDPE. The annealing effect comprises an enhancement of spherullite size within a material after its thermal treatment. Results from analysis of the aged surface indicate that the loss of gloss observed in weathered material is produced by a surface contraction, due to internal stress arising from chemocrystallization [64]. These surface contractions provoke the appearance of micro-cracks, which gradually coalescence, initiating crack expansion. This is one of the reasons for embrittlement of ductile semi-crystalline polymers.

DSC measurements provide a very effective tool for gathering information on changes taking place in polymer morphology. According to Gulmine [40], observed broadening in DSC thermograms of photoaged HDPE and LDPE was provoked by changes in crystallite size. Zhao et. al. [42] claimed that the drop in melting point of artificially and naturally aged LDPE films is attributed to the loss of connection between the crystalline and the amorphous region. According to him, the rupture of tie molecules between crystalline blocks has a substantial effect upon resistance of polymers to deformation under load. Torikai arrived at a similar conclusion, when studying factors affecting PE photostability [65]. He found a correlation between the deterioration of mechanical properties of PE samples (of different degrees of crystallinity) and the rupture of tie molecules that hold together crystalline lamellae.

The disappearing of tie molecules between crystalline blocks and amorphous phases was also suggested by El–Awady [30] in an attempt to explain severe deterioration in mechanical properties exhibited by unstabilized LDPE after 250 hours of artificial photo- oxidation. The author referred to previous results reported by Hawkins [67], who proposed that the crystalline fraction is regulated by the amorphous region, which restricts the crystallization process.

Last but not least, the orientation of phases is also involved in the course of material aging. According to reported studies, stabilizer consumption is slower in orientated phases [67]. b) PP I) Impact on mechanical properties

One of the most affected properties in PP aged by outdoor weathering is elongation at break. PP 0.1 mm thick films, exposed 10 days, showed a reduction of elongation at break by 88 % from the original value. Studies of old used-car bumpers on the basis of PP, showed reduced values of impact strength. Many authors [23] explain those changes on the basis of fracture mechanics. According to this approach, cracks are rapidly

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generated in the brittle layer, with subsequent propagation into inner layers, provoking a premature fracture with severe loss of elongation at break and fracture properties.

Tidjani [11] detected failure in the elongation at break of 0.1mm thick films during the first week of exposure to outdoor weathering in Tsukuba, Japan. In other studies carried out in more severe locations [24], aged 2mm thick plates showed a decrease of important mechanical properties after the second month of exposure.

The yield stress of PP samples aged in Nigeria during the rainy season, worsened slowly during the first week of exposure. In later stages the measured values increased up to the third month of aging. The initial modulus, plastic strain, yield strain, and work of yield attained double maxima for the 48- and 240- h exposed films [22].

II) Changes in morphology

The increase of crystalinity and density in PP aged samples can be regarded as a result of simultaneous effects, including chemocrystallization and annealing, among others. The evolution of crystalline formation may be observed as a function of exposure time. In the early exposure of PP to natural and accelerated aging [27], a crystallinity decrease was observed due to the formation of structure irregularities, such as oxygenated groups which make crystallization difficult.

However, at later stages of exposure, chain scission is believed to be the predominant mechanism, due to the accelerated effect of carbonyl and hydroperoxide groups formed previously. The re-organization process of shorter polymer chains account for the crystallinity increase [68].

Spherollite distribution is also affected by processes involved in aging. Microscopic monitoring demonstrated changes in the distribution of spherolitic structures. It was observed that before aging, spherollites were localized in polymer bulk, whereas in samples of aged PP the formation of spherollites was detected in surface layers, being significant in the exposed face of the specimen [69].

Results arising from work dealing with the photostability of two types of PP [11] observed a different behavior in the kinetics of oxidation of high and low crystalline PP. Under accelerated conditions, the rate of oxidation of the latter was found to be higher, whereas results from natural aging showed almost the same evolution of formed functional groups in both high and low crystalline PP. However, low crystalline PP retained mechanical properties for longer periods of time than high crystalline PP. Once again, the assumption of the existence of tie molecules seems as a consistent explanation of such behavior.

Discussions around the real role of crystalline/amorphous ratios on PP photostability were intensified by reports, at which PP samples with different crystallinity content (iPP with 50 % crystallinity and sPP with 28 % crystallinity) showed the same degree of

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photo-oxidation [70]. Those results demonstrate not only the role played by the crystalline or amorphous region of a given polymer but is also conditioned by the type of exposure and, tentatively, the theory of tie molecules (similarly to PE) could also be applied for an explanation of such behavior.

c) PS

I) Changes in mechanical properties

The character of changes in mechanical performance of PS and its derivates are related to seasonal factors, the mode of aging and the polybutadiene content in HIPS. Samples subjected to xenon and mercury lamps exhibited a slight increase in tensile strength during the early stages of irradiation of PS and HIPS. A loss of elasticity as well as of toughness was reported in natural and accelerated aging of PS and HIPS after 180 days of exposure. This was attended by a loss of 50 % of the original toughness, owing to chain scission taking place in a large extent during the period of time from April to August at the Pune region in India [52].

II) Changes in morphology

One aspect which is not totally clear and explained so far, is the issue concerning the role played by the polybutadiene component in HIPS and the subsequent effects of its degradation on the mechanical properties of aged samples. The polybutadiene component is directly related to changes in the mechanical properties of modified polystyrene during aging. As the weakest link, the oxidation process begins in the rubber region, in the PB inclusions formed during processing; a following step involves oxidation transfer to the PS matrix, thus modifying tensile and impact properties of the material. This degradation pattern was demonstrated in various works, at which HIPS was subjected to heat treatment in the presence of air, where photolysis of hydroperoxides present in the PB segments are responsible for the photo-initiation process [57, 55].

Similarly, cross-linked PB inclusions and, PS sub-inclusions inside the salami structure of HIPS are believed to regulate the HIPS dispersion size during HIPS blending with other materials. Some researchers explained the reduction in elongation and loss of impact strength observed in cross-linked, photo-oxidized HIPS, based on the cavitation theory of rubber particles in HIPS [71]. According to that theory, the formation of voids in the rubber domain causes multiple crazing in the PS matrix, whereas void formation within the salami particles, results in the formation of load-bearing fibrils, connecting adjacent PS sub-inclusions and leading to an increase in stiffness.

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1.2 Stabilization

During the elaboration of thermoplastics, a wide range of processing stabilizers are incorporated in order to conserve the chemical integrity of these materials and, therefore, to prolong their application life-time.

For this purpose, phosphate, phenol, amine, and sulfur compounds are used. One general classification divides processing stabilizers into antioxidants, light stabilizers and flame retardants..

1.2.1 Types of Stabilization

Depending on the mechanism, stabilizers are divided into the following groups:

Antioxidants - minimize the concentration of initiating steps Screeners - prevent damaging light from reaching the polymer Quenchers - quench excited states before photochemistry can occur UV absorbers - minimize the damage from each photochemical event

Addition of antioxidants is the most commonly used method of stabilization. Antioxidants comprise a large group of additives acting as inhibitors of auto-oxidative chain-reactions thermally triggered during processing. Over 80_% of the total amount of antioxidants are used in three polymer types only, polypropylene, polyethylene and styrenic polymers (HIPS, ABS).

1.2.2 Mechanism of stabilization

According to the inhibition mechanism, there are two types of antioxidants: free-radical scavengers or chain-breaking antioxidants (which terminate the chain-reaction through a proton-donor mechanism). This group comprises different sterically hindered phenols and amines that are widely used during the stabilization of polymers. The second group is denominated preventive antioxidants and they decompose hydroperoxides formed at the propagation step of auto-oxidation into inactive species. For this purpose, sulfides, disulfides and phosphites are efficient. Significant and practical synergetic effects were observed when phenolic antioxidants were incorporated together with phosphites or sulfides.

Regarding photostability, the UV stabilizers are often classified according to their mechanism of stabilization. UV absorbing pigments are used mainly in coating technology. They act by shielding the polymer from UV light. There are inorganic pigments that can be used as UV absorbers, such as TiO2, ZnO or carbon black. Another group of UV absorbers form organic compounds such as triazine, benzotriazole and acethophenone derivatives . Excited states of chromophores in polyolefins are quenched by nickel stabilizers.

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A wide group of light stabilizers represent compounds based on 2,2,6,6- tetramethylpiperidine, commercially known as HALS (Hindered Amine Light Stabilizers). They operate by catalyzing the termination step of oxidation as shown in Scheme 3.

Scheme 3. Mechanism of light stabilizing effect of HALS

In order to obtain high performance the stabilization package has to fulfill three criteria: - A high solubility and compatibility with the polymer bulk. For this purposes polar stabilizers are grafted into long-chain alkyl groups. - Minimal diffusion; it is crucial that they can not be removed, leached out or volatilize from plastic - High distribution homogenity throughout polymer bulk.

1.2.3 Re-stabilization of recyclates

During their first life-time, polymers are stabilized by adding small amounts of the aforementioned processing and light stabilizers, that are progressively consumed during the period of usage in sacrificial reactions. Other processes account for the consumption of stabilizers during service life, namely energetic radiation, atmospheric pollutants and residues of polymer catalysts, among others [72].

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Therefore the residual concentration of stabilizers is not sufficient to prevent material from further degradation during a second processing step. Aged polymer material will be more prone to degradation since the rate of auto-oxidation will be faster in recyclates than in virgin material. This effect is caused by the sensitizing effect of oxygenated functionalities formed during first service life-time.

Harmful light absorbing chromophores triggering photodegradation and hydroperoxide groups promoting thermo-oxidation are accumulated in significant levels in aged polymers. Their concentration depends on the inherent resistance of a given plastic and the character, intensity and duration of its environmental stress and on the character of primary stabilization. For instance, LDPE used for packaging, lacks light stabilization, indicating that recyclates from different sources deteriorate to very different degrees.

Material susceptibility to photo and thermal degradation is also influenced by transformation products from stabilizer decomposition. Different phenolic antioxidants react with hydroperoxide radicals and generate quinine methides, cyclohexadienones and benzoquinones [73]. Hydrolitic acid products such as (HO)2POR, H3PO3 or H3PO4 are formed from both trivalent and pentavalent phosphorous containing compounds, salts and other transformation products arising from HAS [74].

Also observed in recyclates, was the formation of structures arising from stabilizer decomposition initiated by reactions different from those involved in sacrificial consumption. They resulted from interaction with nonpolymeric impurities such as ozone, residues of polymer catalysts and oxidizing metallic impurities [75].

Therefore, one important aspect of an integral plastic waste recycling process is the proper replenishment of consumed stabilizers, if the aim of material recovery is to provide recyclates with mechanical properties as similar as possible to those of virgin material. This is particularly important for the final application of the recyclate. Park benches and highway barriers made from recycled resins should contain sufficient amounts of processing, light and heat stabilizers. Many authors suggested that re- stabilization of aged polymers can be achieved by replacement of “sacrificed” stabilizers [70, 75] by addition of the same stabilizing system used during manufacturing of virgin material, since both materials suffer from similar analogous degradation mechanisms. Combinations of phosphates and phenol antioxidants (to retain the heat stability of recycled polyolefins) are recommended.

In this regard, the re-stabilization of polyolefins recyclates in outdoor applications was successfully achieved after addition of 0.1% of an HAS light stabilizer. The sensitizing effect of aged HDPE fraction was suppressed by re-stabilization with 0.1% of P- 1(phosphate)/AO-1(phenol) (2:1) combined with 0.1% HAS-1. Similarly, good results were obtained by R Pfaendner et al [76] in suppressing degradation of used PP by addition of stabilizer combinations of Irganox 1010 and Irgafos 168. A hydroperoxide decomposer (TTP), together with a zinc stearate was found to be efficient during upgrading of PP recyclate [77]. Improved ultimate properties of reprocessed LDPE were

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reported after incorporation of aromatic phosphonite or a combination of hindered phenol, aromatic phosphite, and a lactone [78].

Furthermore a brand new line of re-stabilization packages specially designed for upgrading of recyclates has been developed and launched to the market. A wide range of product lines specifically designed for upgrading post-consumer resins has been introduced to the market under the following trade names:

Recyclostab 451® : processing stability, guaranted long-term thermal stability. It contains different types of antioxidants and co-stabilizers. Recyclossorb 550® : specially designed for light stabilization. It is a mixture of antioxidants, co-stabilizers and light stabilizers. Recyclobend 660® : created to suppress the sensitizing effect of impurities such as paint or ink residues, during processing and heat aging.

The efficiency of selected restabilization systems is evaluated in terms of the capability of certain materials to retain important application properties, for example: - melt flow rate (MFR) - mechanical performance (tensile and impact properties) - time until embrittlement.

The aforementioned tests are carried out after natural or accelerated weathering.

Satisfactory results reported by Kartalis [79], proved that restabilization systems based on the formulation 0.2% w/w Recyclostab® 421 suppress cross-linking reactions during recycling of PE packaging. Changes in MFR values are lower than those showed by unstabilized recyclates. From recycled PP filled garden chairs, it is also proven that a proper formulation of Recyclostab® 451 achieves excellent long-term thermal stability [80]. Results of bending tests demonstrate that the period to embrittlement was enhanced to 100 days, in comparison with 22 days to embrittlement showed by the recyclate restabilized by 0.2% Irganox B

From additional closed-loop recycling of bottle crates and PP-filled chairs [81, 82], it was apparent that re-stabilization remarkably meliorates light stability. In particular, the addition of 0.2 % Recyclossorb 550 improved tensile impact strength retention for over 5000h of exposure under artificial weathering. The same experiment reveals that in the absence of re-stabilization, the tensile impact strength of the material decreases considerably after only 1000h of exposure.

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1.3 Compatibilization

For purposes of material recycling of plastics, all polymers present in the MPW are reprocessed in order to obtain post-consumer raw material for a cyclic use.

One of the first obstacles to overcome is the incompatibility of most polymers. The phase behavior of polymer mixtures is controlled by the Gibbs energy of mixing, ΔGmix. ΔGmix is determined by the equation:

ΔGmix = ΔHmix -TΔSmix

where ΔHmix is the enthalpy of mixing, T is the absolute temperature and ΔSmix is the entropy of mixing.

Due to large molecular weights, entropic contribution to ΔGmix for polymer pairs is much smaller than for low-molecular weight liquids and ΔHmix practically controls polymer miscibility. As a consequence, only a small number of polymer pairs showing specific interactions form truly miscible blends characterized by a single Tg [83-88]. For most polymer blends, ΔGmix is positive and rather high. These blends are referred to as incompatible. Incompatible polymer blends mostly show rough phase structure and poor interphase adhesion. These irregularities in the interphase are the reason for mechanical failure propagating across polymer layers, leading to local fracture and premature crazing [90].

For this reason, blends from incompatible polymer couples are not suitable for material application. For that purpose a third component is added to the mixture, in order to increase the cohesive effect. This procedure is called compatibilization and the cohesive agent is called a compatibilizer.

1.3.1 Methods of Compatibilization

There are two main procedures for compatibilization according to the mechanism:

Non-reactive compatibilization Reactive compatibilization

1.3.1.1 Non-reactive compatibilization

Here a third component is added, mostly block or graft copolymers. They contain blocks, which are identical, miscible or at least compatible with the related blend components. These agents act as interphase regulators by increasing interphase stability. This is achieved owing to the ability of block copolymers to increase the degree of dispersion among phases and improve adhesion between phase boundaries, resulting in a finer dispersion, and to a greater adhesion of the interphase boundaries [90] as shown in SEM [91, 92]. They also suppress (or fully eliminate) coarsening of the phase structure during annealing of compatibilized blends. In some cases, statistical copolymers can also serve

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as quite efficient compatibilizers. However, they are less efficient at stabilization of the blend phase structure during annealing than those of block or graft.

1.3.1.2 Reactive “ in situ” compatibilization

In this procedure, the polymer compatibilization is achieved via chemical modification of the blend and compatibilizers are thus created in situ. Formation of appropriate graft or block copolymers can be achieved by spontaneous reaction of the blend components during melt mixing, by addition of low-molecular-weight free-radical initiators, mostly peroxides. [93]. Results from binary LDPE/HDPE blends demonstrate that the addition of 2wt.-% of dicumyl peroxide and its posterior thermal decomposition, substantially increased the tensile strength and elongation at break in comparison with unmodified PE blends. Analogous results were achieved at works, in which waste LDPE and PP were blended in the presence of different organic co-agents, such as hydroquinone and organic fillers. The higher strength results attribute to a co-cross-linking on the phase boundary and in-situ formation of a compatibiliser [94].

Another method involves creation of covalent bonds between components by functionalizating chain-ends of one or both blend components. Compatibilization systems on the basis of low-molecular weight liquid polybutadiene (PB) and dialkyl peroxide were successfully developed for virgin polyolefin blends. On the other hand, maleinized liquid polybutadiene [95, 96] proved more efficient during blending of already damaged waste polyolefins.

Last but not least, a third method is based on grafting polymers via free radicals produced by high shear forces during mixing at low temperatures [97].

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1.4 Recycling of plastic waste stream

1.4.1 Definition of plastic waste

Depending upon the site of origin, plastic waste streams can be defined into three categories: • Technological • Industrial • Communal or Municipal

Technological plastic waste originates during various technological operations: edge cuts from film production, neck and bottom scrap, cut-off pieces of tubes and pipes from installations, etc. Technological plastic waste is uncontaminated, well defined and appropriate for recycling purposes.

Industrial plastic waste is defined by large, relatively easily collectable plastic items, such as reusable crates and containers, automotive parts (bumpers, battery, shock absorbers), drums and pails. Just as technological plastic waste, so industrial plastic waste has been successfully recycled at large scale, for a long time and without any legislative pressure due to relatively high economic returns.

Municipal plastic waste represents 60% of overall plastic production. It comprises mostly plastics used in food packaging technology. Bottles for sparkling beverages are made from PET, milk, cream and juice bottles from HDPE, etc. According to the latest statistics, on average within the EU 517 kg of municipal waste per person was produced in 2006 [1].

Plastics represent around 25% of the materials used in packaging industry. Plastic packaging materials used per capita in European countries averaged about 145 kg in 2005, and this amount is continuously increasing. In the Czech context, there are plans to increase the recycling of municipal solid waste by 50% compared to 2000 rates. In 2006, the per capita production of municipal waste was approximately 296 kg, of which 29.6% was recycled. It is estimated that plastics account for 11% of the Czech municipal solid waste stream. In this specific area, the Czech Republic is considered the leading country in plastic packaging recycling, with 44% of such packaging waste being recycled in 2006, as shown in table below [98]:

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Table 2 Quantities (in tons) of waste generated in the Czech Republic in 2006 and recovered via material recycling

Packaging Waste Material Recycling Material Generated Recycling rate Glass 184,4 130,0 70.5 Plastic 204,1 90,4 44.3 Paper & board 335,3 304,3 90.8 Metals 46,8 22,0 47.1 Wood 99,3 21,0 21.2 Other 28,5 1,7 6.2 Total 898,6 569,7 63.4

Several industrialized countries implemented systems intended for more efficient and effective solid waste management. A good example of efficiency can be found in the German Duales System Deutschland (DSD), better known by its symbol “Green Dot”. This approach consists in the collection of recycling fees from packaging producers, the fees then used to offset collection and sorting costs [99]. A similar system was adopted in Austria, with the creation of the ARA system (Alstoff Recycling Autria) which organized an effective recovery, sorting and recycling system for waste packaging materials [100].

The economic return on recycling might be greatly stimulated by the production of recycled materials with upgraded properties, more suitable to a wider range of application. In order to enhance economic viability and consequently, the environmental contribution of recycling, the right choices of cost-effective recycling technologies must be rigorously selected. In the following chapter will describe the major recycling technologies currently available and their classification.

1.4.2 Classification of plastic recycling methods

According to the degree of aging, recycling has been classified into two categories: primary and secondary. Primary recycling comprises the reuse of unaged industrial and technological plastic waste. These technologies have been known for many years and initially were implemented in industry, as virgin wastage was collected and re-inserted to the production process. Secondary recycling consists in the re-insertion of post-consumer materials to the cycle of production. It is rather a more novel concept and developed in response to the green movement of the middle 70’s.

There are three main methods of secondary, plastic waste recycling: a) energy recovery (incineration); b) chemical (feedstock) recycling; and c) material (mechanical) recycling.

Energy recovery and material recycling are the most commonly used methods in Europe. From 52.5 million tons of plastics consumed in EU27 during 2007, 24.6 million tons

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ended up as post-consumer waste. 50% of it was landfilled and 50% (12.2 million tons) recovered via energy recovery (29.2%), chemical recycling (0.3%) and material recycling (20.1%).

During energy recovery (incineration), plastic streams are submitted to thermal decomposition (thermolysis) with and without the addition of oxygen (pyrolysis), under increased hydrogen pressure, or gasification. This method recovers only a small portion of energy stored in the waste.

The second method consists in chemical or thermal decomposition of single-sorted material into low-molecular compounds and monomers (depolymerization). The chemical recycling of sorted PET bottles has been very successful, where the recovered material was suitable for contact with food products [101]. Other procedures applied during chemical recycling involve solvolysis, hydrolysis, alcoholysis and glycolysis. Monomers from polymers produced via condensation reactions such as polyurethanes, polysters and polycarbonates are obtained by these methods.

Material recycling consists of the physical treatment of post-consumer resins resulting in the re-pelletization of the material. For a long time, this recycling method was used successfully for single-sort industrial and technological waste. At present, material recycling is an expanding recycling method, offering a variety of benefits. Unlike incineration, environmental implications are minimal and investment costs are much lower compared with feedstock recycling.

Recycling of mixed plastic from municipal waste streams is a little more complicated. Municipal plastic waste being a heterogeneous stream, containing different plastics that are incompatible with each other, the resulting mechanical properties will be mediocre. This is the main reason for limiting markets and applications for this kind of material.

In the next chapter a closer and more detailed description of this method will be discussed.

1.4.2.1 Material Recycling

Sometimes material is also referred as mechanical recycling. During this recycling method, the major part of energy stored in the material is recovered. A number of papers estimate a boom of material recycling in Europe within the next few years [102]. This enormous increase will be driven by high polymer prices and improved collection and sorting technology, along with more stringent environmental regulation. The growth rate for material recycling in EU 27 has been stable, showing a 10% increase over the period 1996-2007. Despite this increase, the rate of landfill disposal remained stable, due to higher consumption of plastics, driven by economic growth and technological development.

Single-polymeric post-industrial scrap is the most suitable raw material for material recycling. Here only reprocessing and stabilizing the material is required in order to

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ensure acceptable mechanical properties which can easily compete with virgin material in high value applications.

Material recycling is a process which basically comprises the following steps: collection, separation, cleaning and palletizing. Each company uses their own version of this process, depending on the technological step sequence or in the construction of the applicable apparatus [103]. In this respect, one has to distinguish between plastics that are separated according to type, and those mixed with other plastics. Secondary recycling of homogeneous industrial material, has been more successful at both achieving acceptable mechanical properties and finding markets. For instance, crates and pallets are two streams where recycling rates are well above 90% This is mostly allowed by combining PCR (post-consumer resin) with virgin resin at a proper and reasonable ratio, where the final product retaines required properties. The most effective systems are blends of 25%- 30% [104] recycled bottle material (with virgin resins) or a mixture of selected virgin resins.

a) Major end-user industries generating plastic waste and applied recycling methods

Packaging industry: The packaging application has the longest tradition of recovery, accounting for approximate 63% recovery. Bottles and industrial film streams are being recovered by material recycling approaching 40% across all countries of the European Union (EU27). An effective closed-loop process was achieved in the case of crates, pallets and boxes, which are recycled at 90%. However, recycling rates for the remaining mixed plastics (in particular those coming from household stream) are still low, below 10% across EU 27. Some countries within the EU are looking for ways to improve this situation. For example, countries like the UK are seeking increased recycling from mixed plastic streams (excluding bottles). At a European level, various national recycling organizations are assessing the feasibility of mixed plastic recovery by different aspects; i.e. increasing efficiency of sorting operations, as well as analyzing potential end-user markets.

Agricultural films:- Plastic waste originated during agricultural activities such as silage. Film is a good source for material recycling, as it is made mostly from polyolefins.

Automotive: - The recycling rate for automotive plastic waste continued to increase to just under 10% in 2007. A novel technology able to mechanically extract usable secondary raw material from shredded vehicles has been developed in Germany.

Electrical and electronics:- Material recycling from this sector is rather limited due to the complexity of sorting operations from technical and financial viewpoints. Recycling inner liners of refrigerator may be cited as an example of growing markets in the electrical and electronic sector. Nevertheless, for most waste streams, feedstock recycling is believed to be the most appropriate procedure.

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Construction: - Increased recycling of window profiles and pipes has been observed, the recycling rate being above 13%, although plastics used in construction are intended for long-term use, and hence do not produce much waste.

The following briefly discusses the recycling of polymers under study. b) Material Recycling of various plastic commodities

Polypropylene (PP)

Mechanical properties of recyclates from polypropylene copolymers present in batteries have been successfully tested. Upgrading of collected samples was based on the addition of restabilizing systems, especially designed for repairing aged, used material.

Kartalis and co-workers [79-82] focused their interest in upgrading PP from post-used chairs. This work team so far published detailed and extensive works related to the recycling of homogeneous PCR, and in particular PP. Their method is based on remelting-restabilization procedures. They demonstrate that pellets from post-used PP- filled garden chairs fulfill all requirements for use in high-grade uses after the application of suitable restabilization systems. These guarantee both their weatherability and acceptable mechanical performance.

High-density polythylene (HDPE)

The new concept of plastic upgrading, based on molecular “repair” operates in the recovery of all important commodity polyolefins. HDPE is not an exception. HDPE packaging, milk, cream, fruit juice and detergent bottles are currently recycled. Plastic blends from high-density polyethylene/polyamide (HDPE/PA) film are suitable for injection molding and extrusion. Similarly, it is possible to stabilize the melt flow index (MFI) of HDPE old material from transportation containers and waste bins, obtaining a post-consumer resin able to retain constant MFI values after multiple extrusions [105, 106].

Small amounts of remelted-reprocessed HDPE from old bottle crates are added to virgin material for fabrication of new crates [107]. High-quality resin systems, containing 10 to 100 percent postconsumer ethylenes, have been developed suitable for blown or cast films and injection molding. In all these applications, a case by case approach was established, taking into consideration the thermal history and periods of outdoor exposure.

Low-density polyethylene (LDPE)

LDPE is the most abundant polymer in waste, due to its predominance in packaging applications, accounting for a substantial part of the total plastics waste collected as municipal solid waste in Europe.

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Recyclates from LDPE showed decreased MFI, and in severe cases were accompanied by gel formation, since the vast majority of post-used LDPE comes from food packaging, bags, etc. and hence is provided with poor or no light stabilizer. LDPE films, which are widely used in cover and construction materials for agriculture, are recovered by the used of stabilizers specially designed for LDPE recyclates [107].

By this method it was possible, to increase the recyclate proportion in recyclate/virgin material blends. The closed-loop assessment was also efficient at maintaining stable MFI values of LDPE packaging films after multiple extrusion steps, as well as retaining required mechanical properties (such as elongation at break and tensile strength) which are mandatory for long-term service life.

Polystyrene (PS)

Polystyrene and their copolymers suffer mostly from discoloration, (yellowing) and reduced molecular weight. PS recyclates can not be designated for the same processes as virgin material, since PS mostly serves as raw material for food contact usage, i.e. yogurt cups and small containers and trays for meat and foods.

1.4.3 Recycling of commingled plastic waste

Comingled plastic waste comes mainly from household collection, and it consists predominantly of plastics used in the packaging industry, i.e. polyolefins, with smaller fractions of PS, PVC, PET and others.

Recycling of commingled municipal plastic waste is a complex process, taking into consideration that household plastic waste streams contain different types of plastics damaged to different degrees and reciprocally incompatible. Further processing without previous sorting leads to blends with detrimental mechanical performance. This is the reason why products based on recyclates are intended for low-value applications. They have found uses in some low technology markets, such as garden goods, sound barriers along highways, carpet and park benches, among other outdoor applications.

As previously stated, a common approach being used in practice during plastic recycling is based on compatibilization and a proper re-stabilization of the post-consumer resin.

In the specific case of commingled plastic waste, both compatibilizing methods were proven to be efficient while conserving important material properties.

Commercially available block copolymers are successfully used during material recycling of commingled plastic waste. Commercial statistical poly (ethylene-co- propylene) copolymers are recognized as efficient compatibilizers for binary blends consisting of LDPE, HDPE or LLDPE with PP. However, the vast majority of works found in available literature are devoted to the compatibilization of binary blends,

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whereas fewer studies reported results concerning the effect of particular compatibilizers on the properties of ternary blends [108].

A mixture of styrene-ethylene-propylene copolymer (SEP) with chlorinated polyethylene (CPE) and ethylene-propylene rubber (EPR) was tested during upgrading of ternary PE/PS/PVC blends from Korean plastics mixture waste. Chank-Sik studied the effect of compatibilizer for binary blends on the properties of ternary blends [109]. Ternary blends (HDPE/PP/PS or PVC) in ratio 8/1/1 from virgin polymers with simulated waste fraction were compatibilized with ethylene-polypropylene rubber (EPR), and chlorinated PE (CPE), and styrene-ethylene-propylene block copolymer (SEP) in a ratio 1/1 (w/w). It was demonstrated that SEP was a better impact modifier for the ternary blends than the CPE or SEP/CPE mixture.

The impact strength of PE-PP and PE-PET [110] blends was also improved by addition of elastomeric triblock copolymer SEBS. In general, EPDM was recognized as a very effective universal compatibilizer for blends of all kinds of polyolefins. Fortelný and co- workers [91] demonstrated that the addition of EPDM, which was localized at the interface between LDPE and PP phases, considerably increased the impact strength of LDPE/PP blends. Reported results on additive compatibilization for recycling of municipal plastic waste, indicated EPDM efficiency was successfully increased by the synergetic action of EPDM with elastomeric triblock SBS copolymer. This synergetic effect was even greater in pre-aged model blends where a substituted diamine stabilizer was added. [6].

Table 3 Some most common use block and statistical copolymers used in blending of MPW are summarized

Polymer 1 Polymer 2 Compatibilizer PS PET P(ET-S) [111] PS LDPE P(S-E)

PS HDPE SEBS HIPS PP P(S-P) [112] HDPE PP EPDM LDPE PP SBS, EPDM [108]

Reactive compatibilization methods were satisfactorily applied as a procedure for municipal plastic waste recycling. Krulis and co-workers [95, 96] proved that a system composed of liquid polybutadiene and dialkyl peroxide can be used as an efficient blending procedure for recycling of mixed polyolefin waste. An improvement in tensile impact strength was observed.

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Additional improvement in blend stiffness was achieved by the incorporation of small amounts of elemental sulfur and Mg (OH)2 filler. Moreover, it was found that the compatibilizing efficiency of the aforementioned compatibilizing systems was greater in a mixture of pre-aged polyolefins than in the virgin mixture. Maleinized 1-PB acts as a more efficient compatibilizer [96].

The concentration of functional groups in a reactive compatibilizer is as important as the processing conditions. Pracella showed [101] the compatibilizing effect depends on the type and concentration of functional groups, while work devoted to the reactive blending of PET and polyolefin functionalized with maleic anhydride, acrylic acid, and glycil methacrylate showed a large increase of elongation at break (from 110% to about 370%), along with a higher stress at break, with increasing E-GMA concentration. On the other hand, Viksne and co-workers [93] investigated the effect of addition of 2 wt.-% of dicumyl-peroxide during blending LDPE/HDPE waste. By this procedure, an improvement of both tensile and elongation properties was obtained.

Another critical parameter during reactive compatibilization is the selection of optimal reaction conditions. Krulis and co-workers [95, 96] demonstrated the positive influence of higher processing temperatures on the impact strength of final LDPE/HDPE/PS blends processed at low mixing rates. During reactive compatibilization of PE/PP, Chodak reported that a rather high processing temeperature of polypropylene leads to complications as a result on premature peroxide decomposition and consequent cross- linking [94].

A second, widely used recycling procedure utilizes block, and/or statistical copolymers with structure analogous to one of the components of the blends. It was proved that the performance of a given compatibilizing system will depend on factors inherent to structural parameters of the compatibilizer and selected processing conditions.

For instance the selection of a block copolymer with appropriate molecular weight is critical during the blending of a polymer mixture. Molecular weight plays a crucial role in the efficiency of compatibilizers, when the molecular weight of the homopolymer is much higher than that of the corresponding arm of the copolymer, the block copolymer cannot act as an emulsifier [88]. Di-blocks copolymers are generally considered as more efficient compatibilizers that tri-block or grafted copolymers [114]. In the same way, symmetrical copolymers can more easily saturate the interphase, whereas asymmetrical copolymers have a higher tendency to form micelles. Graft copolymers are less efficient, since their blends are less ductile [88].

Interesting results were published by Blom et al. [90] who was concerned over compatibilization and characterization of blends of post-consumer resins with virgin PP and HDPE. Results from this comparative study showed that used EPDM and ethylene/vinyl acetate (EVA) copolymers have remarkably different compatibilizing efficiencies. The EPDM was effective at improving impact properties, whereas the EVA was more effective at improving tensile properties. In general, it was concluded that

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EPDM has a higher efficiency than EVA. In general, the efficiency of both agents was acceptable for recycling purposes although only in blends with less than 25% of post- consumer resin content.

Another important parameter affecting the compatibilizing efficiency is the blend composition. Compatibilization efficiency of SB copolymers for PE/HIPS blends with various compositions was studied. It was shown [108] that for PE/HIPS 1/1 ratio, an improvement in impact strength was substantially greater in HDPE/HIPS than for LDPE/HIPS blends. For other PE/HIPS ratios, the difference between blends containing HDPE and LDPE was less pronounced.

In the same way, it was reported that efficiency of EPDM is driven by the time and intensity of mixing. Fortelny et al. [91] reported that higher intensity and longer mixing time leads to an increase of impact strength in binary LDPE/PP blends.

Follow-up work from the same working team focused on the study of compatibilizing procedure aimed at applying it to the recycling of local municipal plastic waste stream. The addition of a synergetic compatibilization system based on the mixture of ethylene/propylene/diene statistical terpolymer (EPDM) and styrene/butadiene block copolymer (SB) to a model blends based on the real waste stream, thus LDPE/HDPE/PP/HIPS (54/18/18/18), showed to be very efficient improving toughness of the final material [129]. It has been assumed that this effect might be caused by the correlation between the location of SB and EPDM in the blend; i.e. they are found in the PO/PS interface [6].

Pospišil et al reported surprising results obtained when substituted diamine processing stabilizer was incorporated to a recyclate or pre-aged LDPE/HIPS (70/30) blends compatibilized with SB/EPDM. The synergetic effect between SB/EPDM compatibilizers and the disubstituted amine stabilizer was very efficient not only improving the tensile impact strength, but the phase structure as well [113].

This compatibilization/stabilization earlier described was successful in the recycling of real municipal plastic waste as well [6,129].

Despite the remarkable progress on this field achieved in the recent years, there is a lack of papers dealing with the efficiency of above discussed compatibilization/stabilization system (SB/EPDM/DUS) on model blends based on different compositions than the ones described above. Another aspect not deeply studied referred to the re-stabilization of such blends during both processing and second service-life time. Besides mechanical properties, light stability of recyclate at acceptable levels is mandatory to obtain high- value products that will easily find applications in the end-use market; this point is crucial for the promotion of the recycling industry. The present thesis is devoted to the study of stability of quaternary model blends, and in the second part of this work, an attempt to enhance the light stability of recycled model blends is also carried out.

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2.EXPERIMENTAL

2.1. Materials

LDPE: Low-density polyethylene Bralen RA-2-19 (Slovnaft Bratislava, Slovakia), 3 density 918 kg/m , Mw = 120 000 (unstabilized) PP: Isotactic polypropylene Mosten 52522 (Chemopetrol Litvinov, Czech Republic) HDPE: High-density polyethylene Liten BB 29 (Chemopetrol Litvinov, Czech Republic) Mw = 420 000. HIPS: High-impact polystyrene Krasten 562E (Kaučuk a. s, Kralupy n/V, Czech Republic) Mw = 190 000, 7% polybutadiene Secondary amine stabilizer (Dus): trade name Dusantox L, Duslo Šalla, Slovakia: mixture of 60 wt.-% of N= 1, 3 dimethylbutyl-N‘-phenyl-1, 4-phenylenediamne and 40 wt.-% of N-[4-(α, α´-dimethylbenzyl) phenyl]-N´-(1, 3-dimethylbutyl)-1, 4- phenylenediamine. Commercial phenolic antioxidant: Irganox 1010 (Irg.1010), Ciba Specialty Chemicals. Styrene-butadiene block copolymer (SB): Europrene SOL 6414, Polimeri Europa, Italy, triblock S-B-S copolymer with diblock content 22 %, Mw = 84 000, styrene content 41.6 wt. %. Ethylene-propylene statistical copolymer (EPM): Dutral Co 034, Polimeri Europa, Italy with a propene content 28 %. Commercial hindered amine stabilizer (HALS): Tinuvin 770DF, of the chemical formula bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, Ciba Specialties Chemicals. Carbon black VULCAN CX-72, Cabot Corp., USA.

2.2 Sample preparation

2.2.1.Oven aging

Polymer granules were oven aged at 100 °C for various periods of time. Aged granules were kneaded in the W50EHT chamber of a Brabender plasticorder at 190°C for 10 minutes and the hot molten bulk was pressed at 200°C for 4 minutes in a Fontijne table press and then cooled in a cold press. Tensile strength, crystallinity, rheology and thermo- oxidative stability were monitored on specimens formed from these plates

2.2.2.Photo-oxidation

Thin polymer plates (150x150x2mm) of virgin single polymer were prepared by compression molding in a Fontijne table press after kneading in a chamber Brabender plasticorder at 190ºC/60 rpm for 6 min. They were then exposed to UV irradiation in a Xenon Weather-O-meter device at λ= 340 nm for one or two weeks. In the following step, the irradiated plates were reprocessed (Branbender plasticorder at 190 °C/60rpm for 10 min), and reformed into new compression-molded specimens. Reformed samples were again evaluated in terms of rheological, thermal and tensile properties.

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Virgin Granules

Thermo and Analysis photo-oxidative ageing

Compression moulding

Analysis

Test specimens (plates)

Scheme 4. The oven/Wheather-O-meter accelerated aging model: samples are analyzed after ageing step and after processing.

2.2.2.3 Blend preparation

Thin polymer plates (150x150x2mm) of virgin single polymer were prepared by compression molding in a Fontijne table press after kneading in a chamber Brabender plasticorder at 190ºC/60 rpm for 10 min. They were exposed to UV irradiation in a Xenon Weather-O-Meter device at λ= 340 nm for one or two weeks. In the subsequent step the granules of virgin polymers or irradiated plates of single polymers were blended (Brabender plasticored at 240 °C/90rpm for 6 min), then were reformed into new compression-molded specimens and subsequently submitted for a second aging UV exposure for one or two weeks. Neat LDPE/HDPE/PP/HIPS (25/25/25/25) blends (comp. 1) and blends with this composition and addition of EPM/SB (2.5/2.5) (comp.2), EPM/SB/DUS (2.5/2.5/0.5) (comp. 3) or DUS (0.5) (comp. 4) was prepared. In order to increase the photo-stability of the model recyclate, 0.5% of commercial HALS stabilizer Tinuvin 770DF or 1% of carbon black was applied. The numbers are weight per cent related to the weight of the whole blend.

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Virgin granules

Plates UV ageing Aged plates Blending Blends UV ageing Aged blends

Analysis Analysis

Scheme 5. Compatibilization and Stabilization model during upgrading of pre-aged blends

2.3.Methods

2.3.1.Determination of hyroperoxide content by Spectrocolorimetry

Hydroperoxide content was determined by means of spectrocolorimetric measurement of aged samples [115]. This analytical method involved stoichiometric oxidation of Fe2+ions by the present hydroperoxides leading to Fe3+ ions. They react with thiocyanate, giving rise to the intensive color of the solution, with the measured intensity being proportional to the hydroperoxide content.

2+ + 3+ ROOH + Fe + H → RO + Fe + H2 O ROH + Fe2+ + H+ → ROH + Fe3+ 3+ 3+ Fe + 6SCN → Fe(SCN) 6 intensive colour

The concentration of hydroperoxides in aged samples is calculated according to the expression: 7 2.5 x 10 ΔE1

CROOH = mmole kg e . ε . w where ΔE1 is the absorbance of a sample in a 1 cm cell as measured in the blank solution, w is the weight in milligrams, ε is the extinction coefficient of the measured complex and e is the stoichiometric coefficient of hydroperoxide interaction with the reagent .

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This method is very well known, and ongoing works have developed and improved its accuracy. Petruj [115] reported excellent reproducibility of this colorimetric method on the determination of hydroperoxide content in LDPE. It is worth mentioning that this method is suitable not only for the analysis of film but also for the hydroperoxide determination in pellets (in this case longer swelling time is recommended). The same author also found that commonly used stabilizers do not interfere with the determination.

Despite certain limitations, this method is simple, quite sensitive, is capable of detecting hydroperoxides at levels as low as 10-3 mol kg-1 and provides reliable results. For these reasons is a very suitable method for the analysis of hydroperoxides in polymers.

2.3.2.Crystallinity, melting enthalpy and oxidative stability

DSC measurements were performed in nitrogen at a heating rate 10°C/min. on a Perkin- Elmer DSC-7 apparatus, equipped with Pyris 1 software, and calibrated by indium standard. Melting temperature Tm and heat of fusion Hf were evaluated from the first and second heating runs for each sample. The crystallinity content was calculated according with the following relation: Crystallinity (%) = (ΔHs /Δ H0). 100

Δ H0 is the melting enthalpy of the 100% crystalline polymer, and Δ Hs, is the measured melting enthalpy of the sample.

Oxidation exotherms were obtained in air at a heating rate 3°/min. in a temperature range 130°C to 300°C, on about 5 mg of the sample. The onset temperature was determined from this thermogram, which is the temperature at which oxidation reaction in polymers start. The onset temperature is defined as the first derivate of heat flux curves in temperatures higher than 160°C. In aged samples, with products containing accumulated oxygen, the exothermic oxidative reactions begin at lower temperatures. Under isothermal conditions, the period of time before the onset temperature is called oxidation induction time (OIT).

2.3.3 MFI

Melt Flow Index is one of the most widely used methods to evaluate the primary rheological properties of polymers. The method is based on the principle of capillary viscosimetry. MFI corresponds to the mass of polymer that passes through a standard capillary over 10 min, at a given applied pressure (load). The MFI value then is calculated according to:

MFI (A) = 600 x average cut off weight (grams) / time interval in seconds

There are a variety of methods according to the polymer type and its molecular weight.

Fig. 2 shows important pieces of the equipment.

40

Fig 2. MFI device

The cylinder, made of steel is surrounded by a heating appliance, which can reach temperatures up to 300ºC ± 5ºC. The dimension of the cylinder (in particular its diameter) is meticulously specified. The piston is made of soft steel and the diameter of the head of piston is 0.075 ±0.015 mm smaller than the inner diameter of the cylinder. The weight of the load can be 2160 g or 5000 g. In the same way 2 alternatives jets can be used, one with inner diameter 2,095 mm and other 1.180 mm, both of them are 8.00 mm long.

The exact description on how to proceed during flow rate determination is specified in a wide range of local and international norms; namely ČSN 64 0321, DIN 53735, ISO/R 1133, and ASTM D 1238-65T.

In order to obtain reliable results, the piston and jet must previously be carefully cleaned. Before measurement begins, the device must be warmed up for at least 15 minutes. After this, a 5 g shot is poured into the cylinder and the piston is immediately inserted again. The speed of extrusion is determined by the cutting of the extruded string at convenient time intervals. First cut-offs should be ignored as they may contain air bubbles. The last 3 cut offs are weighed in analytical balances and the average calculated

In this work the melt flow index (MFI) of the polymers was monitored according to ISO 1133 at a temperature of 190ºC and weight 10 kg (method F); by PP method D, 190ºC and a weight of 2.16 kg was selected.

41

2.3.4 Tensile impact strength

Toughness and tensile impact strength are the most valuable material properties determining the suitability of given materials for a specific application.

In the present work, tensile impact strength, aε was measured at room temperature using a Zwick tester, equipped with a special fixture for the test specimens according to DIN 53 448. Plates were cut into dog-bone shape specimens with the short narrow section from the plates 1.6 mm thick, and the width of narrow section 3 mm. The maximum pendulum energy was 2J. The values obtained are presented as arithmetic values of the measurements on 10 specimens.

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3. RESULTS AND DISCUSSIONS

3.1 Thermo-oxidation- Oven aging Experiment

Municipal plastic waste streams consist of a mix of various polymer materials displaying different levels of chemical and physical damage.

As mentioned earlier, the main components of the Czech domestic commingled waste are polyolefins LDPE, HDPE, PP and HIPS. The susceptibility of given polymers to undergo degradation is not only inherent to their chemical structure but also to the degree (extension) of the stabilization package added during processing. Polymer stabilization is intended to protect polymers during processing and during material service-life. During this period, the concentration of stabilizers is being depleted via numerous sacrificial reactions. Therefore, in order to achieve a material with acceptable material properties, it is mandatory to replenish the residual amount of stabilizers present.

However, this step is a complex operation, taking into consideration the intricacies of the degradation process, with single polymers undergoing diverse degradation patterns. This is considerably magnified, due to the interaction of new formed chemical entities, if various degraded material is blended during material recycling.

For that reason, investigation demands a separate approach. The first step adopted in our study is to understand degradation processes in single polymers, while considering levels of original stabilization and the impact of aging in the deterioration of important material properties. The second part focuses on the study of aging in quarternary blends and the correlation between aging and mechanical failure and chemical transformation. A particular emphasis is put on the upgrading of blends by means of compatibilization methods.

To this point, we exposed single polymers to the effects of thermo and photo-oxidative accelerated aging in an attempt to reproduce processing and service-life conditions. Results and observations arising from this portion of our study will be a helpful tool in the following stages of this work.

Hydroperoxide concentration was used as an indicator of the extent of polymer degradation in relation to exposure time in oven-aging experiments performed at 100 ºC. Exposure times (Table 4) correspond to the time period within which most significant structural changes of the studied materials were detected.

LDPE has poor stabilization, since it is mostly intended for short-life applications in the packaging industry. This was evidenced by enhanced sensitivity of oven-aged LDPE granules to thermal degradation. LDPE granules were the first showing extensive symptoms of degradation (Table 5). Granules displayed considerable yellowing, whereas at a molecular level, hydroperoxide formation was detected during the early stage of exposure (as shown in Fig. 3). Here, the concentration of hydroperoxide in the exposed polymer is presented as a function of time of thermo-oxidation.

43

Structural factors, such as morphology, are helpful for the interpretation of such degradation profiles. It is known that oxidation of polyolefins is mostly restricted to the amorphous phase because oxygen diffusion across the compact crystalline domains is almost impossible. Thus, according to the expectations, highly amorphous LDPE suffers from oxidation reactions to a greater extent than the more crystalline HDPE. Indeed, colorimetric measurements of hydroperoxide, showed that after two weeks of LDPE exposure, the hydroperoxide concentration curve displayed a plateau (Fig 3), indicating a steady state hydroperoxide concentrations within which the rate of formation and decomposition are in equilibrium [115].

A mechanism accepted by numerous researchers is that at later stage of exposure the concentrations of hydroperoxide begin to decay in an autocatalytic manner. Particular attention has been paid to an explanation of the kinetics governing this step. It is accepted that the decomposition of formed hydroperoxides can occur via homolysis of the hydroperoxide bond or bimolecularly. It has been shown that in PE, true monomolecular hydroperoxide decomposition can be disregarded at temperatures below 300ºC [116]. Bimolecular mechanism is considered the main process during hydroperoxide decomposition, which may involve intramolecular hydroperoxide groups placed along the polymer chain, or between a hydroperoxide and an alcohol group [13], being that this is the only mechanism leading to hydroperoxide decomposition in the advance stages of degradation.

On the other hand, hydroperoxide formation in aged HIPS samples was lower in the later stage of oven exposure, as shown below in Table 4.

In contrast, HDPE and PP remained very stable. After 10 months of oven exposure, no hydroperoxide formation could be detected in these materials. In both polymers, the stabilization system comprising a combination of sterically hindered phenol and organic phosphite provided the studied material with long-term thermal resistance. Earlier works [37] devoted to long-term accelerated thermo-oxidative aging also reported that HDPE was well suited to withstand severe recycling processes. The same is applicable for PP, where a proper stabilization package is critical due to its chemical instability. It is obvious that the presence of stabilizers affects degradation behavior of polymers substantially, and must be considered during interpretation of experimental results.

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Table 4. Hydroperoxide formation in LDPE and HIPS as a function of exposure time

Exposure LDPE Exposure HIPS time ROOH c (mmol/l) time ROOH c (mmol/l) (days) (days) 2 0.2 8 0.7 5 1.1 18 2.7 8 1.2 29 2.6 12 2.1 56 2.7 15 4.3 91 2.9 20 4.5 142 2.9 30 4.3 157 2.3

LDPE OVEN AGING AT 100 C

5

4

3

mmol/l 2

1 hydroperoxide con. 0 0 5 10 15 20 25 30 35 exposure time (days)

Fig.3. Hydroperoxide formation in LDPE granules at 100°C during oven-aging

Table 5. Changes in polymer characteristics with increasing oven-aging exposure time at 100°C.

Material Exposure Crystallinity Tonset MFI Time (%) (Cº) (g/10min.) (days) GRANULES LDPEa 0 32.7 191 3.8 7 33.0 190 b HDPEa 0 53.2 237 2.8 311 67.8 229 2.9

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PPc 0 52.2 212 2.2 334 55.2 212 2.2 HIPS 0 181 9.4 20 179 8.2 93 175 8.9 159 181 9.3

a. 190ºC/10 kg b It was not possible to measure the MFI of the specimens due to the significant material degradation c 190 ºC/2.16 kg

During thermal aging of both PEs, cross-linking and chain scission reactions occur simultaneously in a competitive way.

One of the most useful criteria to determine the mechanical performance of plastic materials, consists in impact-strength tests. In the subsequent experimental set, granules from the various oven-aged polymers were kneaded in a Brabender plasticorder at 190°C for 10 minutes and further processed into plates, which are primarily intended for impact strength measurements. In order to determine the effect of processing conditions over degraded material, additional rheological, crystallinity and onset measurements were also conducted as shown in Table 6.

Table 6. Changes in polymer characteristics with increasing oven-aging exposure time at 100°C and after a processing step.

Material Exposure Impact Crystallinity Tonset MFI Time Strength (%) (Cº) (g/10min.) 2 (days) ae (kJ/m )

PLATES LDPE a 0 127 29.0 190 3.2 LDPE+0.5%Dus 7 76 34.4 192 * LDPE+0.5%Dus 7 62 31.7 208 * +0.5%Irg1010 HDPE a 0 129 52.9 232 2.9

311 98 61.2 219 3.4 PP b 0 51 49.6 210 2.5 334 49 45.4 207 2.7 HIPS a 0 26 - 178 9.4 20 39 - 176 10.7 93 36 - 167 8.4 159 28 - 169 9.9

a 190ºC/10 kg

b 190 ºC/2.16 kg

46

Crystallinity increase observed in oven-aged LDPE plates and HDPE indicate that scission prevails over cross-linking. Crystallinity increase probably also contributed to a drop in original impact strength (IS) observed in LDPE and HDPE plates prepared from oven-aged granules (Table 6).

Nevertheless, the crystallinity of the oven-aged HDPE plate is lower than in the respective granules, probably because of the difference in thermomechanical history. PP samples seemed to be very stable under thermal aging. Only slight decrease in IS were observed, while MFI and oxidative onset temperature (Tonset) remained almost constant. On the other hand, LDPE granules subjected to thermal aging at 100°C for 7 days were not further processible without additional stabilization.

Phenolic and amine based stabilizers are widely used during processing and service-life stabilization. Previously reported in numerous researches on recycling, that degradation of post-consumer plastic material follows the same pattern mechanism as virgin polymers, but at different reaction rates. Rates of degradation in post consumer materials are much higher due to the presence of moieties formed during the first life cycle, which have sensitizing effects [76,105].

For that reason the stabilization of post-consumer resins is based on the same types of stabilizers mentioned earlier. We used a phenylenediamine processing stabilizer, Dusantox L, but even more satisfying results (in terms of oxidative stability) were achieved by the application of a mixture of 0.5% Dusantox L and 0.5% Irganox 1010 (Table 6), Despite this improvement in oxidation stability, the damage caused to the rheology, and other material properties is irreversible, as can be seen in Table 6.

High-impact polystyrene (HIPS) consists of a blend of polystyrene matrix and a polybutadiene phase, present in a low concentration (2-8%). The HIPS used in our study contains 7 percent cis-1,4 polybutadiene. As known from the literature, degradation of HIPS is believed to proceed as a two-phase oxidation, in which the rate depends on the content of the polybutadiene (PB) component [52-55]. The yellowing observed in our exposed granules is caused by the presence of chromophores and their interaction with the aromatic ring of the PS matrix.

It was found from low temperature DSC measurements that degradation in the HIPS system occurs at different levels. In early stages of aging, the PS matrix remains almost intact. On the other hand, a slight increase of MFI observed after 20 days of exposure (Table 6) might be attributed to chain scission reactions reported earlier by other authors [117]. Conversely, the shift of the glass transition temperature (Tg) towards higher values demonstrates that, after an initial induction period, a series of degradation events follows cross-linking reactions in the PB phase, hence contributing to the overall mobility restriction of PB chains [118]. (Table 7).

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Similar behavior was observed by FT-IR measurements of photo and thermo-oxidized high-impact polystyrene samples. An induction period was detected, followed by fast oxidation of polybutadiene and then a slow oxidation rate corresponding to the degradation of the polystyrene matrix [54, 55]. The polybutadiene phase is the more sensitive component towards oxidation, due to the unsaturated double bonds in the structure, this is evidenced by the greater shift of the glass transition temperature (Tg) in the PB component compared to PS as shown in Tab. 7. HIPS first undergoes degradation associated with an increase in tensile strength in aged specimens, observed after the second week of oven exposure. This could result partly from cross-linked PB inclusions and (tentatively) from their additional grafting to the PS matrix via hydroperoxides formed by oxidation reactions on PB.

Table 7. Polybutadiene glass-transition temperature (Tg) in HIPS oven aged for different exposure time

Oven-aging exposure time PB Tg PS Tg (days) (°C) (°C)

HIPS 0 -97.5 94.9 HIPS 93 -95.8 94.2

HIPS 159 -91.1 94.3

3.1.1 Photo-oxidation: exposure in Weather-o-meter (WOM) at λ = 340 nm

Along with thermal stress, the second factor contributing to deterioration of polymers during service-life is photo-oxidation. Polyolefins are unable to absorb energy in the visible region of light due to their chemical structure. Photochemically triggered reactions in polymers are ascribed to photo-sensitizing chemical impurities formed during processing.

In the next section, the photo-oxidative behavior of single polyolefins and high-impact polystyrene is examined. For this purpose we selected an experimental set-up based on accelerated conditions provided by a weather-O-meter device. We opted for this particular approach mainly due to its reproducibility, reliability and fast results. Although natural aging experimental measurements are proved to be best approached when studying outdoor environmental polymer aging, it’s an extremely time consuming process.

For this purpose, thin polymer plates were prepared by compression molding after kneading in a Brabender plasticorder at 190ºC/60 rpm for 6 min. They were then exposed to UV irradiation in a Xenon Weather-O-meter device at λ= 340 nm for one or two weeks. In the following step, the irradiated plates were reprocessed (Branbender plasticorder at 190 °C/60rpm for 10 min), and reformed into new compression-molded specimens. Reprocessed samples (RE) were again evaluated in terms of rheological, thermal and tensile properties.

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3.1.2 Formation of oxygenated groups

In general, photo-oxidation is defined as oxidation process enhanced by photochemical reactions. Hydroperoxide group formation and its decomposition in the course of photo- aging or subsequent reprocessing plays a crucial role in the degradation chemistry of plastics, which may justify, among others, the faster degradation of materials subjected to simulated recycling [16,17].

Hydroperoxides are already quantitatively formed during processing of polymers, whereas carbonyl groups are often considered to play a crucial role during the service life-time of polyolefins.[20]. In the presence of ketones in the polymer backbone, the very well Norrish I and Norrish II reactions (Scheme 2) are more likely to occur. Indeed, results reported by Mellor and co-workers [119] indicated that during the early stage of ultraviolet exposure, hydroperoxide groups which are already present in the polymer material, were the precursors of photodegradation, whereas, at later stages of photooxidative degradation carbonyl initiation was found to be a crucial autoaccelerating factor.

By mean of spectrocolorimetric measurements, we’ve been able to detect and monitor the evolution of hydroperoxide groups during exposure of polymers to UV accelerating conditions (Table 8, the hydroperoxide evolution in photoaged and reprocessed samples, in which the basic stabilization did not comprise light stabilizers. UV light exposure leads to increased hydroperoxide formation in PP and HIPS samples.

There are indications that WOM exposure of PE favors cross-linking over scission [19] and since, in contrast to PP, polyethylene hydroperoxides readily photolyze on UV exposure, the detected hydroperoxide concentration is lower in LDPE and is not detectable in HDPE photoaged samples, confirming very good light stabilization of HDPE [29] (Table 8). On the other hand, higher hydroperoxide concentration in PP was expected, due to structural factors that favor scission over cross-linking [50].

Table 8 Hydroperoxide concentration and oxidation stability changes with increasing time of accelerated photo-aging of polymers in WOM

Exposure time(days) ROOH c (mmol/l) Oxidtion onset Tonset (ºC) LDPE 0 0.01 190 6 0.06 185 6(RE)a 0.05 191 14 0.05 189 14(RE)a 0.05 185 HDPE 0 - 232 6 - 197 6(RE)a - 194

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14 - 195 14(RE) - 192 PP 0 0.03 210 6 0.06 186 6(RE)a 0.06 191 14 0.48 187 14(RE)a 0.09 183 HIPS 0 3.41 171 6 3.54 170 6(RE)a 1.78 166 14 3.46 168 14(RE)a 1.83 166 a After re-extrusion.

The surprisingly high values of ROOH concentration found in the original virgin sample of HIPS, prior to aging, probably originate from the technological manufacturing process, as it was earlier stated; organic peroxides serve as reactive initiating species in grafting PS on PB particles.

The concentration of hydroperoxides generated decreases significantly in the course of reprocessing (190ºC) and subsequent molding (200ºC). This is obviously due to their decomposition under high-temperature processing conditions, as can be seen from the values corresponding to reprocessed samples (Table 8).

3.1.3 Onset Temperature

As described earlier in the experimental part of this work, in order to determine the onset temperature of aged materials, oxidation exotherms were obtained in air at a heating rate 3°/min. in a temperature range 130°C to 300°C, on about 5 mg of the sample. Onset temperature is defined as the first derivate of the heat flux curves in the region of temperatures higher than 160°C. In aged samples, with products containing accumulated oxygen, the exothermic oxidative reactions begin at lower temperatures.

The shift of the onset temperature Tonset (Table 8) to lower values indicates deterioration of the oxidative stability of aged polymers. In PP and HIPS this shift is associated mainly with the presence and formation of oxygenated groups and consequent increased stabilizer consumption. In the case of HDPE, this decrease is related to the depletion of the stabilizer, because no formation of ROOH was detected in the HDPE samples after photo-aging. Moreover, sterically hindered phenolic antioxidants, applied as basic stabilizers in polyolefins and HIPS, are not active as light stabilizers, because they decompose and deactivate under the influence of UV light.

Certain irregularities observed in the measured onset temperatures (Table 8) can be attributed to the heterogeneous character of photo-oxidation, where photo-degradation is

50

restricted mainly to the surface of the samples [64]. Similarly, irregular distribution of stabilizers resulting from a second reprocessing, can be attributed to different spatial distribution of oxygenated products.

On the other hand, relatively smaller changes in oxidation onset temperature in unstabilised LDPE are associated with structural changes of the unprotected aged samples. Similar behavior was also highlighted by Gulmine et al. [19,40] during the study of the degradation profile of three different type of polyethylene (LDPE, LLDPE and HDPE) under artificial weathering. A rapid decrease of oxidation temperatures at the early stage of degradation was observed from the DSC thermograms. This from 0 to 100 hours under a WOM device, whereas for longer exposure times, the onset temperature remained practically unchanged (from 200 to 800 hours), which is in complete accordance with our results, considering the long exposure time at our experimental set up, i.e. 6 days or 144 and 14 days or 336 hours). For this reason the determination of onset temperature, in the case of polyethylene, is not recommended as a suitable parameter for evaluation of levels of degradation of polyethylene exposed to aging.

3.1.4. Crystallinity variation

The reorganization [120,121] of smaller and more mobile chains resulting from PP scission into more compact structures contributes to the increase in the crystallinity of PP specimens aged in WOM for 14 days (Table 9). The contribution of other phenomena, such as annealing effects and lamellar thickening, can also be taken into consideration [24]. The presence of chemical irregularities formed by hydroperoxide decomposition during reprocessing of PP probably makes re-crystallization processes more difficult [122] and consequently leads to a decrease of crystallinity within reprocessed PP samples (Table 9).

Crystallinity content in aged HDPE samples is very irregular, displaying a crystallinty decrease after 6 days of exposure followed by an increase in the later stage of photo- aging and, after each re-processing step. A certain irregular course in the measured values of crystallinity may be expected due to differential cooling, leading to local stress variations [67].

The role played by carbonyl groups in the photo-stability of HDPE is fundamental in order to understand and interpret the irregular results coming from photo-aged HDPE. In the literature [130, 131], can be found numerous works referring to carbonyl groups as the initiators of crosslinking and/scission. Moreover, it has been also reported that carbonyl are more likely to cause chain scission in an oxygen atmosphere [132]. Valles- Lluch and co-workers [122] reported a reduction of carbonyl groups in HDPE when submitted to intensive radiation, which they had linked to a significant number of chain scission occurred in HDPE.

Based on the aforementioned premises, the observed crystallinity decrease might be justified assuming that during the first week, bulky chemical impurities, mainly oxygen

51

containing groups (indicated by the substantial Tox decrease shown in Table 8) prevent secondary crystallization, therefore, it is most probably that both, reduced chemical regularity, and crosslinking lead to the crystallinity decrease in photo-aged HDPE. However, at the posterior stage of photo-degradation, the accelerated decomposition of carbonyl groups might lead to an increased number of chain scission reactions, which is reflected in the higher crystalline content. This interpretation is just a tentative approach to understand the arising results; this fact only corroborates the complexity of structural modifications and chemical changes taking place during polymer photo-degradation. This is a quite common problem commonly met by other researchers. Carrasco et al.[58] arrived to the conclusion that UV irradiation has a negligible influence on the HDPE crystallinity. According to him, the crystallinity can increase or decrease depending on the relative importance of the structural and chemical changes.

A certain irregular course in the measured values of crystallinity may be expected due to differential cooling, leading to local stress variations [67].

It was experimentally demonstrated in high and low-density PE, that moderate experimental conditions led to a decrease in melting temperature and crystalline content resulted from an increase in the number of molecular defects (cross-linking, carbonyl groups, and unsaturations) in the polymer chain [122]. More severe experimental (irradiation) conditions led to chain scission, with subsequent recrystallization and crystalline content increase. The variation in the crystalline content of LDPE, (Table 9) corroborates the aforementioned statement. At an earlier stage of LDPE irradiation, the predominating cross-linking led to a continuous crystallinity decrease. Conversely, reprocessing triggers main chain cleavage, which ultimately leads to an increase of crystallinity.

Table 9. Crystallinity variation in polymer samples exposed to WOM aging (UV) for various periods of time and after reprocessing (RE)

Exposure time PP crystallinity HDPE crystallinity LDPE crystallinity (days) (%) (%) (%) 0 53.0 58.2 34.3 6UV 53.7 53.8 32.8 6UV(RE) 47.2 58.1 35.2 14UV 57.2 60.1 24.2 14UV(RE) 44.4 63.3 33.2

3.1.5 Mechanical and rheological properties

Even the smallest event taking place at a molecular level may result in tremendous changes in the macromolecular features of a polymeric material. Mechanical properties (and impact strength in particular) are important material parameters and useful indicators of the material history of a given polymer.

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As can be seen in Fig.4, UV irradiation negatively affects the tensile impact strength of WOM-exposed LDPE and HDPE. As pointed out earlier, the photo-oxidation of a semi- crystalline polymer can be regarded as a diffusion-controlled process. The degradation process starts at the outer layers and penetrates progressively into the bulk of the material, as a direct function of the diffusion rate of oxygen across the material. This leads to formation of concentration gradients of oxygenated products, with highest concentration detected at the surface [19, 123]. Conversely, higher concentrations of stabilizers in aged polymer are found in the bulk, as surface concentrations are depleted during sacrificial reaction.

Therefore, deterioration of mechanical properties is caused by the influence and interaction of two events: physical changes leading to surface defects [46] (cracks propagation) and chemical transformations resulting in molecular weight changes (cross- linking-chain scission).

During the first week of irradiation, surface degradation counterbalances the positive effect of molecular-weight increase (due to LDPE and HDPE cross-linking) on tensile impact properties, the final results being an overall decrease in the tensile impact strength, as seen in Fig. 4a and b, respectively.

During further prolonged irradiation times, the contribution of crystallinity becomes crucial at interpreting impact strength changes of LDPE and HDPE. This behavior is demonstrated by the difference in the course of photo-oxidation of LDPE and HDPE samples. The observed increase of crystallinity (Table 9) indicates that HDPE -evidently a Ziegler type grade- undergoes scission from the second week of irradiation on, unlike the more amorphous LDPE, which displays signs of cross-linking occurring predominantly in the amorphous region corroborating the well known heterogeneous character of polyolefin oxidation [124]. The progressive greater brittleness of aged and reprocessed HDPE samples results from increasing cristallinity content.

Consequently, the progressive greater brittleness of aged and reprocessed HDPE samples results from the simultaneous action of surface degradation [25] and higher content of crystallinity; whereas LDPE impact strength displays a slight improvement due to increasing molecular weight (crosslinking). The behavior of HDPE during prolonged exposures to accelerated aging conditions has also been explained based on the tie molecule theory, first suggested by Oswald and Turi [50] and later studied in detail by Torikai and co-workers [65].

This principle basically assumes the existence of tie molecules linking crystalline lamellae and bearing a disproportionate part of applied stress. Therefore, even a low level of oxidation of these molecules provokes substantial embrittlement. Hence, in HDPE, due to its higher crystalline content, a small amount of oxidative products cause great damage in tie molecules and ultimately in mechanical properties. On the other hand, results from mechanical tests shown below, indicate the high level of light stabilization package added

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to PP. The initial decay of the tensile impact strength observed after (Fig. 4 c) a week of irradiation remained almost unaltered during the later stages of exposure.

LDPE HDPE

135 140

120 120 105

90 100

75 80 Impact strength 60 60 (kJ/m2) 45 IS (kJ/m2)

30 40

15 20 0 0 6 6RE 14 14RE Exposure time (days) 0 0 6 6 RE 14 14 RE Exposure time (days)

( a ) ( b)

( c ) ( d )

PP

HIPS 60

30

45 25

20 Impact strength 30 Im p act streng th 15 (kJ/m2) (kJ/m2)

10 15 5

0 0 0 6 6 RE 14 14 RE 0 6 6 RE 14 14RE Exposure time (days) Exposure time (days)

Fig. 4. Changes in tensile impact strength of plates exposed for different period of time in WOM and after reprocessing (RE): (a) LDPE, (b) HDPE, (c) PP, (d) HIPS

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The tensile impact strength of irradiated reprocessed samples follows a zig-zag course, especially in HIPS (Fig 4). This can be attributed to surface degradation during photo- aging and to the subsequent impact-strength recovery after the second processing, resulting from the remixing of the degraded surface into the bulk, as was observed in previous work dealing with simulated recycling of post-consumer plastic material [38].

It is worthy of mention the different behaviors displayed by HIPS when exposed respectively to thermo and photo-oxidation. For instance, when comparing impact properties values of HIPS subjected to both processes, it was observed that while the former lead to an increase of the impact strength properties, the latter resulted in a continuous deterioration of mechanical performance. Some researchers explained this fact on the basis of hydroperoxide photo-decomposition, assuming that hydroperoxide content is already generated during processing [56, 57]. Israeli et al. claimed that α,β- unsaturated ketones generated from polybutadiene oxidation can be considered as another initiator of photo-instability. It was observed by FT-IR measurements that these formations accumulate under thermo-oxidative conditions. Conversely, they were very labile to photo-oxidation [55].

3.1.6. MFI

Cross-linking reactions prevail in HDPE and LDPE, evidenced by lower melt flow indices after 6 days of photo-aging in WOM (Fig. 5 and 6).

On the other hand, during consecutive reprocessing of pre-aged LDPE and HDPE samples, decomposition of hydroperoxides, chain scission, and other chemical irregularities appear, as manifested in higher values of MFI. The hydroperoxide decomposition in the course of the second processing also significantly enhanced the rate of chain cleavage in PP, as observed in Fig. 7. Chain scission reactions triggered by photo-oxidative mechanisms led to a decrease of the molecular weight in HIPS. This effect is reflected on the increased MFI values observed after the first week of exposure. These values remained almost unchanged for the rest of the exposure time, even in reprocessed samples (Fig. 8). The same rheological behavior has been reported by other researchers [52, 54, 56, 118].

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LDPE

18

16

14

12

10

8 MFI (g/10min.) MFI

6

4

2

0 0 6 6RE 14 14RE Exposure Time (days) Fig.5. MFI of photoaged and consecutively reprocessed LDPE specimens

HDPE

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0 066RE1414RE Fig.6. MFI of photoaged and consecutively reprocessed HDPE specimens

56

PP

25

20

15

MFI (g/10min) MFI 10

5

0 066RE1414RE Exposure time (days) Fig.7. MFI of photoaged and consecutively reprocesed PP specimens. (Method D at 190ºC/2.16 kg load)

HIPS

14

12

10

8

6

4

2

0 0 6 6RE 14 14RE

Fig. 8. MFI of photoaged and consecutively reprocesed HIPS specimens

In general, it can be said that the recyclability of the material studies previously is very high, especially in the case of HDPE and HIPS and to a lesser extend PP and LDPE. PP withstands thermo oxidative degradation very well, but when exposed to photo-oxidation

57

seems to be most vulnerable. For that reason, LDPE was the least suitable for recycling purposes in the packaging industry.

3.2. Upgrading mechanical properties and processing stability of mixed plastic waste by cooperative compatibilization system

In the previous part of the present work, four single polymers (which are considered the main components of Czech municipal plastic waste stream) were studied.

Each of those materials was submitted to accelerated thermo and photo-oxidative aging, followed by subsequent re-processing steps. Obtained results indicated the occurrence of chain scission, cross-linking and formation of chemical irregularities, which take place simultaneously in a competitive way. It was shown that photo-oxidative degradation led to more drastic deterioration of important material properties, such a tensile impact strength and oxidative stability.

It was stated earlier that for practical, economic, and technical reasons, material recycling represents a thoughtful and feasible method for dealing with increasing municipal plastic waste and collapsing landfills. Material recycling consists of remelting and reprocessing various and diverse polymer materials that display outdoor degradation at different grades and, in the vast majority of cases, are incompatible. Hence, acceptable mechanical performance and ulterior oxidative stability are achieved by means of compatibilizing and restabilizing steps.

In this part of the work, model quaternary blends are upgraded by compatibilization and restabilization methods. Model blends are composed by polymers present in the Czech municipal waste stream, i.e. LDPE, HDPE, PP, and HIPS.

Each individual component was previously exposed to accelerated UV irradiation for 6 and 14 days, in order to simulate first service-life. Therefore, the first issue to address is related to differences in degradation levels and their impact on the upgrading potential of the recyclates. Results from preceding chapters reveal the heterogenic character of oxidative aging, which is an oxygen-diffusion controlled event. Thermo and photo- oxidation lead to formation of oxygenated structures (carbonyl, hydroperoxides), cross- linking, and chain scission.

During melt processing of recyclates, oxygenated groups gradients are homogenized trough the bulk polymer mass, resulting in a recovery of impact tensile strength, as observed in reprocessed HIPS and partially PP, or they lead to further, accelerated degradation as in the case of LDPE and HDPE, with a consequent drop in mechanical performance. The melt processing of a pre-aged quarternary blend, represents a very complex task, due to the additivity of the various degraded components and their interaction, with the consequent effects on the interphase adhesion and compatibility of the resulted blend.

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Another crucial aspect to be taken into account concerns the ongoing stability of such a material. Chemical (oxygenated) impurities formed during first service life and reprocessing will display a photo-sensitizing effect during the second service-life. It has been demonstrated that carbonyl groups act as a photosensitizing impurity, triggering photo-oxidative reactions, accounting for continuous hydroperoxide formation and mechanical properties deterioration.

Hence, the present experimental set up addresses three key aspects when upgrading recyclates from commingled municipal plastic waste:

• The need of a processing stabilization • Mechanical properties enhancement of the resulting blend • Photo-stability enhancement of blends for a second service-life application.

The first step was to select the most appropriate and effective system, consisting of processing stabilizer and suitable compatibilizers. Chain breaking processing stabilizer on the basis of 1,4- phenylenediamines (PD) were successfully used during material recycling of model blends and real municipal mixed waste [6, 91, 108], while the application of a mixture of commercial copolymers: ethylene-propylene statistical copolymer (EPM) and styrene-butadiene block copolymer (SB) were beneficial retaining and enhancing the mechanical properties of model quarternary blends from virgin resins.

Tensile impact strength was chosen as a measure of efficiency of compatibilisation procedures because the lack of toughness is frequently the most important reason recyclates cannot be used in certain applications. The advantage of tensile impact strength is that it can be measured for the same conditions on materials with very different toughness. Moreover, this method is recognized as highly sensitive to changes in the phase structure of material, as well to changes in the interphase adhesion of heterogeneous systems.

The values of aε for blends with all four compositions, with the components aged for 0, 6 or 14 days in weather-O-meter are shown in Fig. 9. It is clear that, irrespective of the aging time of the component, the blend compatibilized with cooperative compatibilization system (composition 3, mixture EPR/SB/Dus (2.5/2.5/0.5)) exhibits superior toughness, which is ascribed to the synergetic effect among EPR/SB and the diamine stabilizer.

For blends with virgin components and components aged for 6 days, addition of neat Dus has no effect on their impact strength. Addition of a mixture EPM/SB enhanced the impact strength but less than the addition of EPM/SB/Dus. The same effect was found for samples of real plastic waste [6] and for model LDPE/HIPS blends [125]. For blends with components aged for 14 days, the impact strength increases in the following order: neat blend, blend with EPM/SB and with EPM/SB/DUS. In contrast to previously discussed blends, blend containing only Dus has substantially higher aε than the neat blend and is also superior to the blend compatibilized with EPM/SB.

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35,00

30,00 0 WOM 25,00 6 WOM 14 WOM 20,00 Impac strength Impac 15,00

10,00

5,00

0,00 1234

Figure 9. Dependence of the tensile impact strength of model recyclates on pre-aging of the blend components. 1 – neat blends, 2 – blends compatibilised with 5% EPR/SB, 3 – blends compatibilised with 5% EPR/SB + 0.5% Dus, 4 - blends stabilised with 0.5% Dus only

Comparison of the impact strength of blends with virgin components and components aged for 6 and 14 days show superiority of blends with the components aged for 6 days, irrespective of additives (see Fig. 9). It is a surprising fact at first and interpretation is not easy.

It was found that the cooperative effect of EPM/SB with Dus in LDPE/HIPS blend is stronger if pre-aged LDPE is used [113]. It was shown in previous works that the synergetic effect of the system, based on composition 3, involves formation of polyolefin-SB (PO-SB) graft terpolymers. It was also demonstrated that this cooperative compatibilization is operative especially in the presence of pre-aged polyolefin.

Screening data revealed that SB (which is localized at the interphase of polyolefins-SB blend) plays a key rule in the cooperative compatibilization mechanism. During melt processing, pre-aged materials are subjected to thermo and mechano-chemical stress events, favoring an easy formation of alkyl radicals P• and alkoxy radicals PO•, POO• (by reaction of traces of oxygen during melt processing and/or hydroperoxides

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decomposition). Alkyl and alkoxy radicals convert into radical P´• radicals by chain transfer from the SB component, in the α-position to its double bond as shown in the scheme below:

Under oxygen deficient environment cross-recombination reactions prevail, accounting for in situ formation of graft PO-SB terpolymer.

Posterior works revealed that the presence of the 1,4-phenylenediamine (PD) processing stabilizer has a beneficial effect of the mechanical properties [113, 125]. The phenilenediamine stabilizer during melt processing undergoes complex sacrificial transformation leading to a wide range of transformation products.

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In the ultimate phase of the sacrificial mechanism, PD is converted to N,N’-disubstituted 1,4-benzoquinonediimine (BQDI), which react with carbon-centered free radicals leading to the formation of ring-bound structures. These newly formed structures preserved secondary amines, hence retain their inherent antioxidant function. A bifunctional feature is attributed to these structures; they are believed to act as a stabilizer and simultaneously as an “in-situ” compatibilizer, where phenylendiamine plays an intermediating role during the compatibilization and formation of grafted structures, as shown below:

PO-SB terpolymers are recognized to be more efficient compatibilisers than SB copolymer alone. Commercial SB copolymers are typically used as thermoplastic elastomers in the tire industry. Impact modifiers have blocks substantially shorter than compatibilized polymer chains.

On the other hand, it is widely recognized that the highest compatibilization performance (measured as the ability to retain or improved mechanical properties and to lead to finer morphology) have block copolymers with block lengths comparable to the length of compatibilized polymers. Results reveal that the effect of block copolymers with block substantially shorter than compatibilized polymer chains, is sensitive not only to molecular structure of the applied block copolymer and the compatibilized polymers, but also to other many variables, like concentration of the blend component and mixing and processing conditions [6].

The higher compatibilization efficiency observed in situ graft PO-SB terpolymers compared to original SB copolymer terpolymers, might be ascribed to the length of PO grafts in PO-SB terpolymers, which is identical with polyolefin chains in compatibilized

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blends. Also more-less random grafting of PO chains to double bonds in butadiene blocks, which leads to variations in PO-SB structure, favors PO-SB as a compatibiliser.

Although this mechanism has satisfactorily explained the enhanced compatibilization efficiency of the EPM/SB system observed in model blends from pre-aged components, it cannot explain why blends with 6 days pre-aged components without a compatibiliser or compatibilised with EPM/SB or Dus only have higher impact strength than the related blends with virgin components.

Degradation during aging of individual blend components was studied in our preceding paper [126]. It was found that chain branching (cross-linking) and scission compete in this case in dependence of the polymer molecular structure and aging conditions. Reactive groups formed during aging can play important roles in degradation of pre-aged polymer during their mixing. For individual polymer, branching (cross-linking) mostly leads to an increase and chain scission to a decrease in impact strength. Chain branching and scission change rheological properties of the polymers and, therefore, the phase structure of their blends have substantial effects on the blend impact strength.

Therefore, evaluation of the effect of the component aging on impact the strength of blends is a difficult task. It is reasonable to assume that a mixture of LDPE with HDPE forms the matrix in blends under study. It follows from our preceding study of the degradation of the blend components, that LDPE and HDPE have higher tendencies to branch and crosslink than PP and HIPS. Apparently a mixture of LDPE and HDPE forms matrix in the blends under study in which particles of PP and HIPS are dispersed. It should lead to a finer phase structure of the blends. Moreover, branching or slight cross- linking of the matrix enhances toughness of the blend. Previous works indicated that the cross-linked PE component of the blend had a beneficial effect on the strength properties of the final recyclate [93, 94].

Results from rheological measurements corroborate cross-linking as the predominant degradation process taking place in blends from pre-aged (6 days) components. The melt flow index measurements of uncompatibilized blends from unexposed plates and blends from pre-aged components for 6 and 14 days respectively are shown below. On the other hand, samples from material aged for a longer period of time undergo chain scission, leading to a reduction of viscosity, which is reflected in higher MFI values.

Interesting findings were reported by Craig et. al. [127] during studies dealing with the recyclability of LDPE and HDPE samples that were photodegraded prior to reclamation. It was shown that the blends containing 25% recycled polymer had lower crystallinities that the corresponding virgin polymers. Those morphological modifications were ascribed to cross-linking and chemical defects formed during the UV exposure period and to its sensitizing effect during reprocessing.

Generally, we assume that the sum of the contributions from individual components and the effect of the components on the blend phase structure leads to an increase in the impact strength of the blends with 6 day pre-aged components with respect to virgin

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blend components. We can speculate that thermolysis of accumulated oxygenated groups (peroxides,carbonyl groups) formed in the course of previous photochemical aging triggers a series of irreversible, chemical transformations, involving changes in molecular weight and cross-linking reactions between the PE matrix and the PP/HIPS dispersed phases during high-temperature reprocessing.

This partly explaines enhanced impact properties observed in non-compatibilized blends from pre-aged components, compared to virgin blends. Besides, reactive compatibilization through hydroperoxide decomposition is a widely used method, during which copolymers and grafted polymers are formed. Previous studies indicated that the amount of accumulated oxygenated groups in aged materials has a substantial impact on the properties of resulting blends.

Martinez [128] and co-workers carried out a series of grafting procedures with three different PE types (HDPE/LDPE/LLDPE). Unlike traditional grafting, where various hydroperoxide concentrations are added, the material was irradiated with UV light as a preliminary step for hydroperoxide production. It was demonstrated that aged PE samples in the presences of maleic anhydride are suitable for grafting reactions. LDPE displays the highest grafting value.

A number of authors [94, 95] reported significant improvement of impact and tensile properties in blends based on polyolefin waste (PE/PP) via reactive compatibilization. It is assumed that co-cross-linking and grafting reactions across the phase boundaries account for this unexpected upgrade.

On the other hand, polymers pre-aged for 14 days are probably strongly damaged, as shown in Fig. 10. Reactive groups formed during pre-aging cause intensive chain scission during blend mixing, which leads to the formation of a substantial low-molecular weight fraction in the blend components [126] and, eventually, formation of domains with strongly variable molecular structure. As a consequence, a decrease in the blend impact strength appears. This assumption is supported by relatively high impact strength of blends containing only diamine-based stabilizer (without compatibilizer), which suppresses thermal degradation during mixing.

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40,00

35,00

30,00 x WOM - 0

25,00 x WOM - 6 x WOM - 14 20,00

Impact strength 15,00

10,00 5,00

0,00 123

Figure 10 Dependence of the tensile impact strength of blends, compatibilised with the cooperative compatibilisation system, on the time of their agingaging in Wheatherometer. 1 – blends with virgin components, 2 – blends with the components pre-aged for 6 days, 3 - blends with the components pre-aged for 14 days.

3.3. Enhancement of light stabilization of mixed plastic waste

Besides mechanical properties immediately after preparation, stability of recyclates during their posterior processing and ulterior service life is of major importance. Jansson and co-workers [38] reported that the extent of degradation depends upon the order in which extrusion and accelerated aging are performed. It was shown in studies dealing with simulated recycling of post-consumer resins that a repeated extrusion followed by accelerated aging more negatively affects the material properties than aging followed by extrusion.

The present experimental set up of simulated recycling involves molding extrusion, followed by aging of single material, then blending of aged plates. In the last stage, formed blends are subjected to a final aging exposure. Therefore, in order to obtain material with acceptable properties and durability, it is critical to ensure the processing and light stability of recyclates.

Processing restabilization involves replenishment of initial amounts of stabilizers depleted in the course of sacrificial transformations. Processing restabilization ensures the durability and properties of the recyclate to be adequate for the second life-cycle.

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Onset temperatures of the thermo-oxidative degradation of blends containing the components pre-aged in a Wheatherometer for 0, 6 and 14 days without additives and compatibilized with cooperative system EPM/SB + Dus are compared in Table 10. The table shows that the onset temperature, To, of all blends decreases with increasing times of pre-aging of their components, indicating the enhanced tendency to undergo thermo- mechanical degradation (observed in pre-aged material) can be correlated to the accumulation of oxygenated moieties formed during aging as shown in preceding works [61].

The onset temperature strongly increases with the addition of Dus and slightly decreases with the addition of SB/EPR mixtures. The latter might be explained partly because a certain amount of the processing stabilizer is consumed throughout the interaction of the amine-based stabilizer and the compatibilizing system as described earlier.

Still, To of blends with virgin components compatibilized with cooperative system is higher by 60°C than To of the related neat blend. The difference between the onset temperatures of the compatibilized and related neat blends is still more pronounced if blends with pre-aged components are compared. The blend containing 14 days pre-aged components and compatibilized with EPR/SB+Dus, shows To almost 50 °C higher than the neat blend containing virgin components. Thus, the results in Table 10 clearly demonstrate that the addition of cooperative compatibilization system EPM/SB + Dus not only improves toughness of recyclates but also substantially enhances their thermo- oxidative stability during melting.

Photo-oxidative stability of recyclates is also important for their application in practice. Therefore, the effect of photo-aging of the blends on their impact strength was studied, compatibilized with the cooperative compatibilization system and prepared from the components, pre-aged for various times in a Weatherometer.

Figure 10 shows that the impact strength of all blends under study quickly decrease with increasing exposition time. It should be mentioned that this decrease is stronger for blends containing pre-aged components – apparently degradation of the components enhances photo-oxidative stability of the recyclates.

Kartalis and co-workers [80-82] demonstrated the induced degradation during reprocessing of nonrestabilized post-consumer PP material significantly impacts the light stability of the reclaimed material. The appropriate combination of different types of antioxidants, co-stabilizers and light stabilizers efficiently delay the deterioration of the tensile impact strength of the recycled PP material.

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Table 10 Onset temperatures of model blends with various additive compositions

1 2 3 4

LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset PP/HIPS PP/HIPS PP/HIPS PP/HIPS (25/25/25/25) (25/25/25/25) (25/25/25/25) (25/25/25/25) EPM/SB EPM/SB/DUS DUS 0WOM-0 208 0WOM-0 200 0WOM-0 267 0WOM-0 268

0WOM-6 198 0WOM-6 196 0WOM-6 255 0WOM-6 262

0WOM-14 198 0WOM-14 195 0WOM-14 237 0WOM-14 248

1 2 3 4

LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset PP/HIPS PP/HIPS PP/HIPS PP/HIPS (25/25/25/25) (25/25/25/25) (25/25/25/25) (25/25/25/25) EPM/SB EPM/SB/DUS DUS 6WOM-0 193 6WOM-0 192 6WOM-0 263 6WOM-0 270

6WOM-6 190 6WOM-6 187 6WOM-6 265 6WOM-6 269

6WOM-14 191 6WOM-14 190 6WOM-14 260 6WOM-14 268

1 2 3 4

LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset LDPE/HDPE/ Tonset PP/HIPS PP/HIPS PP/HIPS PP/HIPS (25/25/25/25) (25/25/25/25) (25/25/25/25) (25/25/25/25) EPM/SB EPM/SB/DUS DUS 14WOM-0 181 14WOM-0 183 14WOM-0 255 14WOM-0 255

14WOM-6 185 14WOM-6 185 14WOM-6 224 14WOM-6 259

14WOM-14 187 14WOM-14 189 14WOM-14 202 14WOM-14 245

An innovative and cost-effective recycling method (which has been successfully proven) consists of a combination of light and thermal stabilizers with low amounts of carbon black [107]. Therefore, in order to increase the photo-stability of the model recyclates, 0.5% of commercial HALS stabilizer Tinuvin 770DF or 1% of carbon black were applied. Blends containing components aged for 6 days in a Weatherometer and

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compatibilized with cooperative compatibilization systems were chosen for these experiments.

The results are summarized in Fig. 11. Comparison with Fig. 9 shows that the addition of Tinuvin 770 does not change the impact strength of the compatibilized blend, which was not aged in the Weatherometer. On the other hand, some loss of the impact strength was detected in the blend photo-stabilized with carbon black. This can tentatively be explained by insufficient dispersion of the carbon black (e.g. presence of some aggregates) at chosen mixing conditions or by other unfavorable effects on the recyclate morphology.

Moreover, it should be mentioned that values of the tensile impact strength show rather high relative errors. Figure 11 clearly demonstrates that both Tinuvin 770 and carbon black efficiently protect model recyclates against substantial decreases in impact strength after their aging in the Weatherometer for 6 days (cf. Fig. 10). It proves that with the addition of the cooperative compatibilization system and a photo-stabilizer recyclates with mechanical properties and stability sufficient for large scale applications can be obtained.

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30,00 6 WOM0 25,00 6 WOM6 20,00 Impact strength Impact 15,00

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0,00 12

Figure 11. Dependence of the tensile impact strength of blends from 6 days pre-aged components compatibilised with cooperative compatibilisation system on the time of aging and addition of photo-stabiliser. 1 – 1% of carbon black, 2 – 0.5% of Tinuvin 770.

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3.4. Processing stability- Melt flow index

Rheological measurements of the melt flow index of virgin and aged neat blends showed that in earlier stages of irradiation (6days) the viscosity of model neat blends increased due to a predominance of cross-linking and branching reactions. It is very well known that this viscosity increase is caused by formation of less mobile, entangled long chain branches (taking place mainly in the PE matrix) during aging procedures and follow-on processing. [106].

On the other hand, blends from longer periods of exposure already display significant deterioration of rheological properties, reflected in higher MFI values as shown in Fig. 12

Due to structural considerations, the polypropylene and the high-impact strength polystyrene phases are mainly expected to undergo chain scission reactions during melt reprocessing.

20 18 16 14 12 10 8

MFI (g/10min) MFI 6 4 2 0 0WOM 6WOM 14WOM

Fig 12 Melt flow index measurement of neat model blends from virgin and pre-aged components (LDPE/HDPE/PP/HIPS) during various exposure periods

Concerning processing stability, it was demonstrated that in all measured samples a remarkable MFI retention was achieved by diamine substituted stabilizer, thus confirming the crucial role of stabilization during multiple extrusions, as shown in Fig 13

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14 12 10 8 6

MFI (g/10 min) 4 2 0 0WOM 6WOM 14WOM

Fig 13 Melt flow index measurement of model blends from virgin and pre-aged components (LDPE/HDPE/PP/HIPS) modified by the cooperative compatibilization- stabilisation system (EPM/SBS/DUS) during various exposure periods.

Additionally, it is worth mentioning that a lower performance of the diamine stabilizer was observed when the diamine stabilizer was added together with the compatibilizing system EPM/SB. In this case, the onset temperature dropped substantially compared to Tonset values from blends which had been only modified with dusantox, as shown in blend composition 3 and 4 in Table 10. The greatest variation in the onset temperature was measured in blends from pre-aged components (14 days), being the resulting blend subjected to further irradiated for additional 14 days (14WOM14). Onset temperature of blend only containing diamine stabilizer is 43.5 ºC higher than blend containing EPM/SB/DUS system (Tab 10).

This accentuated consumption of diamine stabilizer supports the mechanism of reactive cooperative synergism, which has been demonstrated is intermediated by an in-situ reactive LDPE-g-SBS-Dus compatibilizer, resulting from grafting of LDPE radicals formed during previous aging into SBS double bonds.

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4. CONCLUSIONS

4.1 Thermo-oxidation

Basic stabilization applied in the studied polyolefins and HIPS have different efficiencies, depending on aging conditions. Because it was unprotected by stabilizers, LDPE was rather sensitive towards degradation during oven-aging. It was shown that LDPE, oven- aged for seven days at 100ºC could not be used for reprocessing because of too low viscosity; a suitable combination of diamine and phenol stabilizers suppressed degradation during reprocessing. Modified polystyrene showed two-phase oxidation, where oxidation of polybutadiene components took place initially, leading to cross- linking, whereas the polystyrene matrix remained almost intact over longer periods of time.

On the other hand, HDPE and PP were well stabilized against thermo-oxidation, retaining acceptable mechanical properties and thermo-oxidative stability, even after 10 months of oven- aging. Hence, such materials qualify as suitable for recycling purposes.

4.2 Photo-oxidation

During accelerated photo-aging in the Weather-O-meter (WOM) device, PP was most prone to deterioration, due to structural factors, and a lack of stabilization against light. Photo- aging of PP and HIPS in the WOM led to chain scission. In the case of HDPE and LDPE a cross-linking reaction prevailed in the early stage of irradiation. However, with increasing time of exposure, the more crystalline HDPE underwent scission, whereas LDPE still displayed signs of cross-linking. The formation of hydroperoxides during the aging phase, and their further decay, was shown to play a significant role in faster chain scission in materials subjected to simulated recycling, showing a pronounced increase in melt flow indices after reprocessing in most cases.

In general, it can be said that structural inhomogeneities built up in thermo and photoaged samples have an accelerating role during subsequent reprocessing. It is therefore mandatory to apply a proper re-stabilization package prior to reprocessing.

It should be mentioned that the present results and conclusions are valid for the grades of polymers applied in this work, and hence their transfer to other grades of these polymers should be made with caution.

4.3 Upgrading of polymer blends: Compatibilization, processing and light re- stabilization

The cooperative compatibilization system on the basis of a mixture of EPM/SB in addition of a secondary amine-based stabilizer was proved a very efficient procedure for upgrading quaternary model blends (LDPE/HDPE/PP/HIPS) (1/1/1/1), aiming to apply this procedure for municipal plastic waste recycling.

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The cooperative effect is caused by the synergetic interaction between the compatibilizing system and the di-amine based stabilizer at which free polyolefin radicals are grafted to butadiene blocks in SB copolymers throughout the intermediation of amine-based stabilizer. This “in situ” formed structure acts as a better compatibilizer which leads to improvement of impact strength of prepared blends.

Moreover, the system based on EPDM/SB compatibilizers and di-substituted amine stabilizer was proved to be efficient method suppressing thermo-oxidative degradation triggered during remelting of quaternary model blends.

It was also found that reactive groups formed during previous aging (service life-time) might play an important role at degradation of pre-aged polymer during their mixing, with further implications on the mechanical performance of the formed blends. Ultimately, the final mechanical properties will result from the additivity and/or the antagonic interaction of each individual component of the blend.

On the other hand, an integral and effective light stabilization was achieved by addition of carbon black or a commercial photo-stabilizer in addition to the cooperative compatibilization system.

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