Mechanical recycling of high density polyethylene/flax fiber composites

Mechanical recycling of high density polyethylene/flax

fiber composites

Thèse

Nathalie Benoit

Doctorat en génie chimique

Philosophiæ doctor (Ph. D.)

Québec, Canada

Mechanical recycling of high density polyethylene/flax

fiber composites

Thèse

Nathalie Benoit

Sous la direction de :

Denis Rodrigue, directeur de recherche

Rubén González-Núñez, codirecteur de recherche

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Résumé

Ce travail de doctorat est consacré à la production, au recyclage mécanique long-terme et à la caractérisation de matériaux polymères et composites à base de polyéthylène haute densité (HDPE) et de fibre de lin. L’objectif est de déterminer l’aptitude au recyclage long-terme de ces composites et de leur matrice, tout en évaluant la perte de performance subie. Le recyclage est réalisé ici par une extrusion en boucle fermée, durant 50 cycles, sans ajout intermédiaire de matières vierges et sans prise en compte de la détérioration et de la contamination subies lors du cycle de vie des produits.

Dans la première partie, une revue de littérature présente l’état de l’art concernant le recyclage mécanique des composites thermoplastiques. Les différents types de recyclage de composites sont présentés, ainsi que les différents travaux réalisés sur le recyclage de composites thermoplastiques à base de fibres naturelles ou inorganiques. Enfin, les différentes limitations rencontrées lors du recyclage de ces composites sont mises en lumière et des solutions sont présentées. Au cours de cette revue, des lacunes importantes sur le recyclage mécanique long-terme de ces composites sont observées.

Dans la seconde partie de ce travail, le polyéthylène haute densité est étudié et recyclé seul afin de connaître ses propriétés et son comportement au recyclage, tout en servant de base de comparaison pour les composites produits par la suite. L’étude des propriétés physique, thermique, moléculaire et mécanique permet d’analyser les différents mécanismes de dégradation induits par le recyclage mécanique. Les résultats montrent une diminution de la contrainte au seuil d’écoulement et une forte augmentation de l’élongation à la rupture avec le recyclage, indiquant que des phénomènes de rupture de chaînes ont lieu dans le polymère. La plupart des autres propriétés demeurent constantes et confirment le maintien des performances du polymère avec le recyclage.

Dans la dernière partie de cette thèse, deux séries de composites sont produites à partir du polyéthylène haute densité et de la fibre de lin (15% en masse), avec et sans polyéthylène greffé d’anhydride maléique (MAPE) comme agent couplant. Toutes deux seront caractérisées similairement au polymère afin d’évaluer l’effet de la présence de fibre dans le polymère. Une analyse de la distribution de fibres est aussi réalisée afin d’observer l’effet du recyclage mécanique sur la taille des fibres. L’analyse mécanique révèle que la fibre fournit un renfort efficace au polymère, en particulier avec l’agent couplant, mais les propriétés à la rupture diminuent. Cet effet diminue avec le recyclage, alors que les propriétés à l’élongation augmentent, du fait de la réduction de longueur des fibres. L’effet de l’agent couplant disparaît aussi au cours du recyclage. Toutefois, la majorité des performances mécaniques après recyclage restent supérieures à celles du polymère.

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Abstract

This thesis focuses on the production, the mechanical recycling and the characterization of polymers and composites based on high density polyethylene (HDPE) and flax fibers. It aims to determine the materials potential towards long-term recycling and to evaluate the resulting loss of performance. The recycling is realized by closed-loop extrusion, and repeated up to 50 times, without any addition of new material, and without any consideration of the possible degradation and contamination undergone during the life-cycle of the products. In the first part, a literature review presents the state of the art concerning the mechanical recycling of thermoplastic composites. The various types of composites recycling are introduced, as well as the various works conducted on the recycling of thermoplastic composites reinforced with both natural and inorganic fillers. Finally, the various limitations to the composites recycling are presented and some solutions are suggested. During this review an important lack of knowledge on the long-term mechanical recycling of these composites is observed.

In the second part of this work, the high density polyethylene is studied and recycled in order to know its properties and its behavior towards recycling, as well as to be used as a comparison basis for the further parts. The study of the mechanical, thermal, molecular and physical properties leads to the better understanding of the various degradation mechanisms induced by mechanical recycling. The results show a decrease of the yield stress and an important increase of the strain at break with recycling, indicating that chain scissions take place in the polymer during recycling. Most of the other properties remained stable, and confirmed the conservation of the polymer performances with recycling.

In the last part of this work, high density polyethylene is used to produce two series of composites with 15% wt. of flax fiber, with and without maleic anhydride grafted polyethylene (MAPE) as a coupling agent. Similar characterizations as for the matrix are conducted on both composites as to evaluate the effect of the fibers in the polymer matrix. A complete analysis of the fiber distribution is also performed to observe the effect of mechanical recycling on the fiber dimensions. The mechanical analysis reveals that the fibers provides an efficient reinforcement to the matrix, and especially with coupling agent, but the properties at break decrease. Nevertheless, this effect decreases with recycling, while the elongation properties increase due to the fiber size reduction. The effect of the coupling agent disappears with recycling. However, most mechanical properties remain higher for the composites after recycling than for the neat matrix.

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Table of content

Résumé ... iii

Abstract ... iv

Table of content ... v

List of Tables ... viii

List of Figures ... ix Abbreviations ... xi Symbols ... xiii Acknowledgments ... xiv Forewords ...xv Chapter I. Introduction ... 1

I.1 Natural fiber and thermoplastic based composites ... 1

I.1.1 Basic notions about composites ... 1

I.1.2 Polymer matrices ... 2

I.1.3 Natural fibers ... 3

I.1.4 Natural fibers vs. synthetic fibers ... 5

I.1.5 Interface properties and coupling agent ... 6

I.1.6 Applications ... 7

I.2 Recycling ... 9

I.2.1 Polymer recycling ... 9

I.2.2 Composite recycling ... 10

I.3 Thesis objective and organization ... 10

Chapter II. Mechanical recycling of thermoplastic composites ... 13

Résumé ... 13

Abstract ... 14

II.1 Introduction ... 15

II.2 Recycling methods... 18

II.2.1 Thermal recycling ... 19

II.2.2 Chemical recycling ... 20

II.2.3 Mechanical recycling ... 20

II.2.4 Conclusion... 21

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II.3.1 Wood ... 23

II.3.2 Cellulose ... 28

II.3.3 Flax ... 29

II.3.4 Sisal and hemp ... 30

II.3.5 Rice hulls and kenaf ... 31

II.3.6 Nettle ... 32

II.3.7 Conclusion... 32

II.4 Recycling of thermoplastic composites reinforced with inorganic fillers ... 35

II.4.1 Glass ... 35

II.4.2 Carbon ... 38

II.4.3 Talc ... 39

II.4.4 Conclusion... 39

II.5 Limitations and solutions ... 41

II.6 Conclusion... 44

Acknowledgements ... 46

Chapter III. Long-term recycling of high density polyethylene and characterization of its closed-loop degradation………..47 Résumé ... 47 Abstract ... 48 III.1 Introduction ... 49 III.2 Materials ... 52 III.3 Experimental... 52

III.3.1 Sample Production ... 52

III.3.2 Physical Properties ... 54

III.3.3 Thermal Properties ... 54

III.3.4 Mechanical Properties ... 55

III.4 Results ... 55

III.4.1 Density ... 55

III.4.2 Gel Permeation Chromatography ... 55

III.4.3 Melt Flow Index ... 59

III.4.4 Thermogravimetric Analysis ... 59

III.4.5 Differential Scanning Calorimetry... 59

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III.4.7 Flexural Properties ... 64

III.5 Conclusion ... 64

Acknowledgements ... 65

Chapter IV. Long-term closed-loop recycling of high density polyethylene/flax composites. ... 66

Résumé ... 66

Abstract ... 67

IV.1 Introduction ... 68

IV.2 Materials ... 71

IV.3 Experimental ... 72

IV.3.1 Sample Production ... 72

IV.3.2 Physical Properties ... 74

IV.3.3 Thermal Properties ... 75

IV.3.4 Mechanical Properties ... 75

IV.3.5 Morphology ... 76

IV.4 Results and Discussion ... 76

IV.4.1 Density ... 76

IV.4.2 Gel Permeation Chromatography ... 76

IV.4.3 Thermogravimetric Analysis ... 80

IV.4.4 Differential Scanning Calorimetry ... 83

IV.4.5 Morphology ... 85

IV.4.6 Tensile Properties ... 91

IV.4.7 Bending Properties ... 96

IV.4.8 Impact Properties ... 96

IV.5 Conclusions ... 97

Acknowledgements ... 99

Chapter V. Conclusions and recommendations ... 100

V.1 General conclusion ... 100

V.2 Perspectives ... 102

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List of Tables

Table I-1: Classification of natural fibers according to their origin. ... 4

Table I-2: Comparison between the physical and mechanical properties of usual natural and synthetic fibers [26, 27, 28, 29]... 6

Table I-3: Plastics codes in Canada [52]. ... 9

Table I-4: Plastic waste recovered in Quebec during 2008 [52]. ... 10

Table II-1: Recycling methods for thermoplastics composites and their characteristics. ... 22

Table II-2: Overview of the investigations published on the mechanical recycling of natural organic fillers reinforced composites with their main parameters. ... 34

Table II-3: Overview of the works considering the mechanical recycling of inorganic fillers reinforced composites with their main characteristics. ... 40

Table II-4: Main limitations and solutions for the mechanical recycling of thermoplastic composites. ... 44

Table III-1: Injection molding parameters. ... 53

Table IV-1: Injection molding parameters. ... 74

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List of Figures

Figure I-1: Examples of potential and current commercial applications for natural fiber based composites [28,

42, 43, 44, 45, 46, 47, 48, 49, 50]. ... 8

Figure III-1: Virgin HDPE thermogram from DSC. ... 52

Figure III-2 : Molecular weight distribution for generations 0 (PG0) and 50 (PG50). ... 56

Figure III-3: Number average molecular weight as a function of generation number. ... 56

Figure III-4: Weight average molecular weight as a function of generation number. ... 57

Figure III-5: Polydispersity index as a function of generation number. ... 57

Figure III-6: Intrinsic viscosity as a function of generation number... 58

Figure III-7: Typical tensile stress-strain curves for different generation. ... 60

Figure III-8: Stress at break as a function of generation number. ... 61

Figure III-9: Strain at break as a function of generation number. ... 61

Figure III-10: Yield stress as a function of generation number. ... 62

Figure III-11: Yield strain as function of generation number. ... 62

Figure III-12: Young's modulus as a function of generation number. ... 63

Figure III-13: Energy at break as a function of generation number. ... 63

Figure III-14: Flexural modulus as a function of generation number. ... 64

Figure IV-1: Initial fiber L/D aspect ratio range distribution. ... 71

Figure IV-2: Initial fiber average length range distribution. ... 72

Figure IV-3: Average molecular weights as a function of generation number for the CS composites. ... 78

Figure IV-4: Average molecular weights as a function of generation number for CA composites. ... 78

Figure IV-5: Number average molecular weight (Mn) as a function of generation number. ... 79

Figure IV-6: Weight average molecular weight (Mw) as a function of generation number. ... 79

Figure IV-7: Polydispersity index as a function of generation number. ... 80

Figure IV-8: TGA results for the matrix. ... 81

Figure IV-9: TGA results for the composites. ... 82

Figure IV-10: Peak temperature obtained from the weight derivative curves for the matrix and the composites as a function of generation number... 82

Figure IV-11: Onset degradation temperature for the matrix and the composites as a function of generation number. ... 83

Figure IV-12: Melting point as a function of generation number. ... 84

Figure IV-13: Melting peak width as a function of generation number... 84

Figure IV-14: Crystallinity as a function of generation number. ... 85

Figure IV-15: Average fiber length as a function of generation number. ... 86

Figure IV-16: Average fiber diameter as a function of generation number. ... 86

Figure IV-17: Fiber length distribution for the CS composites. ... 87

Figure IV-18: Fiber length distribution for the CA composites. ... 87

Figure IV-19: Fiber L/D ratio distribution for the CS composites. ... 88

Figure IV-20: Fiber L/D ratio distribution for the CA composites. ... 88

Figure IV-21: Average L/D ratio as a function of generation number. ... 89

Figure IV-22: Typical tensile stress-strain curves for different samples... 92

Figure IV-23: Stress at break as a function of generation number. ... 92

x

Figure IV-25: Yield stress as a function of generation number. ... 93

Figure IV-26: Yield strain as a function of generation number. ... 94

Figure IV-27: Young's modulus as a function of generation number. ... 94

Figure IV-28: Energy at break as a function of generation number. ... 95

Figure IV-29: Bending modulus as a function of generation number. ... 96

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Abbreviations

ABS Acrylonitrile butadiene styrene

AFM Atomic-force microscopy

ATR Attenuated total reflectance

BLS Bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate

CMC Ceramic matrix composite

DSC Differential scanning calorimetry

ELV End-of-Life Vehicles

FBC Fluidized-bed combustion

FRTP Fiber reinforced thermoplastics FTIR Fourier transform infrared spectroscopy

GMT Glass mat thermoplastic

GPC Gel permeation chromatography

HDPE High density polyethylene

HT-DTA High temperature dynamic-thermal analysis HT-GPC High temperature gel permeation chromatography

LALS Low angle light scattering

LDPE Low density polyethylene

LLDPE Linear low density polyethylene

MA Maleic anhydride

MAPE Maleic anhydride-grafted polyethylene MAPP Maleic anhydride-grafted polypropylene

MIR Mid infrared

MMC Metal matrix composite

MSW Municipal solid waste

NIR Near infrared

PA Polyamide PA12 Polyamide 12 PA66 Polyamide 66 PC Polycarbonate PE Polyethylene PEEK Polyetheretherketone

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PLA Poly lactic acid

PLLA Poly-L-lactic acid

PMC Polymer matrix composite

PMPI Polymethylene polyphenyl isocyanate

PP Polypropylene

PS Polystyrene

PUR Polyurethane

PVC Polyvinyl chloride

RI Refractive Index

SEM Scanning electron microscope

TCB 1,2,4-trichlorobenzene

TDA Triple detector array

TGA Thermogravimetric analysis

UV Ultraviolet

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Symbols

CA Composite with coupling agent

CAGN Composite with coupling agent at generation “N”

CS Composite without coupling agent

CSGN Composite without coupling agent at generation “N”

d Fiber diameter (mm)

E Young’s modulus (MPa)

Eb Bending modulus (MPa)

Fbk Impact strength (kJ/m²)

IV Intrinsic viscosity (dl/g)

L Fiber length (mm)

L/D Aspect ratio (-)

MFI Melt flow index (g/10 min)

MFR Melt flow rate (g/10 min)

Mn Number average molecular weight (kDa)

Mw Weight average molecular weight (kDa)

PDI Polydispersity index (-)

PGN Polymer at Generation “N”

TEB Tensile energy at break (J)

Tg Glass transition temperature (°C)

Tm Melting point (°C)

wt.% Percentage by mass (%)

Xcr Degree of crystallinity (%)

∆Hf Melting enthalpy (J/g)

∆H100 Heat of fusion of 100% crystalline material (J/g)

σb Stress at break (MPa)

σy Yield stress (MPa)

εb Strain at break (MPa)

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Acknowledgments

First and foremost, I would like to thank my supervisor Professor Denis Rodrigue for his trust, his support and help all along this project. I thank him for his advice and insight in this project, as well as for his availability. I am also grateful for the opportunity to go to Guadalajara and Guelph. I also would like to thank Professor Rubén González-Núñez, my codirector, for his help, his support, and for his kind welcome during my stay for the project in Guadalajara.

All those years would not have been the same without the people I worked with during this time. First of all, I would like to thank Yann Giroux for his training and help on different equipment, but above all, for his priceless help and support all along this project. Thanks to him for being there for me day after day. Then, I would like to thank all my colleagues and the staff in the Chemical Engineering Department of Université Laval. At last, I want to thank all the group member of the Proyectos and the Ingeniería Química departments of the Centro Universitario de Ciencias Exactas e Ingenierías for their help, their warm welcome and their support during my stay in Guadalajara.

I acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC), the Centre Québécois sur les Matériaux Fonctionnels (CQMF), the Centre de Recherche sur les Matériaux Renouvelables (CRMR) and the Centre de Recherche sur les Systèmes Polymères et Composites à Haute Performance (CREPEC).

Last, but not least, I want to thank all my friends and family members, especially my parents, my boyfriend and my best friends for their love, support, patience understanding and encouragement.

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Forewords

This thesis is composed of five chapters. The first chapter is a general introduction on natural fiber reinforced composites. It presents different aspects of these materials such as their properties, applications and recycling. The constituents and their specificities are also presented, as well as general notions and statistics on their recycling. The second chapter is a review on the mechanical recycling of thermoplastic composites published as a book chapter. In this chapter, the different recycling methods for composites were presented briefly. Then, the different works and studies on the mechanical recycling of organic and inorganic fiber reinforced thermoplastics were reviewed. Finally, the limitations associated with composites recycling and their possible solutions were presented in the last part of this work. This chapter is accepted for publication as:

Benoit, N., González-Núñez, R. and Rodrigue, D. Mechanical recycling of thermoplastic composites, in Thermoplastic Composites: Emerging Technology, Uses and Prospect. E. Ritter Ed., Nova Science Publishers,New York, Chapter 3, pp. 95-142, ISBN: 978-1-53610-727-2 (2017).

Chapters III and IV present experimental results in the form of published or submitted journal papers. In chapter III, high density polyethylene was recycled up to 50 times by closed-loop extrusion cycles. The physical, thermal, mechanical and molecular properties were analyzed to understand the effect of long-term recycling on the material structure and performances. This paper is accepted as:

Benoit, N., González-Núñez, R. and Rodrigue, D. High density polyethylene degradation followed by closed-loop recycling, Progress in Rubber, Plastics & Recycling Technology, 33, 17 (2017).

Chapter IV investigates the long-term recycling of composites. These composites were made from the same high density polyethylene reinforced with 15% wt. of flax fiber, with and without the addition of maleic anhydride grafted polyethylene as a coupling agent. Up to 50 closed-loop reprocessing cycles were conducted on the composites to analyze their effect on such materials and their constituents. Thermal, physical, morphological, mechanical and molecular characterizations were performed to evaluate the potential of such materials towards recycling. This paper has been submitted as:

Benoit, N., González-Núñez, R. and Rodrigue, D. Long-term recycling of high density polyethylene/flax fiber composites and characterization of its closed-loop degradation, Progress in Rubber, Plastics and Recycling Technology, submitted, 2016.

Finally, the fifth chapter is a general conclusion about the above-mentioned works followed by recommendations for future works. It should be mentioned that for all the publications, my contribution was performing the

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experimental works, collecting and analyzing the data, writing the first draft of the manuscripts and making all the corrections. The publications were then revised by all co-authors.

More results obtained from this work were also presented in the following conference presentation:

Nathalie Benoit, Denis Rodrigue, Effect of recycling and weld-lines on the properties of injection

molded high density polyethylene reinforced with flax fibers, 14th _{International Symposium on Bioplastics,}

Biocomposites & Biorefining, May 31st _{- June 3}rd _{2016, Guelph, ON, Canada.}

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Chapter I.

Introduction

I.1 Natural fiber and thermoplastic based composites

I.1.1 Basic notions about composites

Composites are constituted of at least two phases, one being the reinforcement (the fibers or particles), and the other one the binder (the matrix). The association of these two materials leads to interesting properties resulting from a synergetic combination of both components properties. The matrix binds the fibers in order to distribute the stresses undergone, protect the fibers and provide the cohesion, the shape and the non-structural properties and of the composite material. The reinforcement increases the rigidity and the mechanical performances of the matrix [1, 2]. It can be found in a wide diversity of shapes and configurations: powder, particles, fibers (log and short), fabric, layers, etc. Composite materials exhibit some typical characteristics allowing them to be distinguished from blends. First, the composite components should be immiscible and an interface should exists between them. They generally have higher properties than the bulk matrix material. Depending on the matrix nature, composites can be classified in three main categories [2]:

• Polymer matrix composites (PMC) are the most commonly used composites nowadays due to their easy processing, as well as the availability and low cost of their components. The presence of the reinforcement in the polymer matrix increases most of its mechanical properties, thus allowing them to be used in more demanding applications. Polymer based composites can be processed through similar methods than neat polymers, such as extrusion, injection, compression molding, rotomolding, etc. They can be divided in two main categories: the common composites, which have low cost and moderate properties, and the high performances ones, which are more expensive and essentially used in aeronautics and sports for more demanding applications.

• Ceramic matrix composites (CMC) are mainly used for structural and non-structural applications at very high temperatures and very high level of stresses. As ceramics exhibit a fragile behavior, especially towards impact stresses, they are often reinforced with ductile material as to deviate the crack propagation. Due to their matrix properties, they also exhibit high porosity, high chemical resistance and high rigidity. They are also lighter than most of the metals used for similar applications. However, due to the materials used and the complexity of processing, those materials are usually expensive and are mainly used for spatial and aeronautic applications.

• Metal matrix composites (MMC) are high performance materials. However, they are also still expensive, thus limiting their potential applications. They are generally constituted of a light metal matrix, such as aluminum, magnesium or titanium, reinforced with a ceramic or metal reinforcement (fibers, particles or powders). If short ceramic fibers are used, then traditional metal processing methods can be used. On the other hand, if long

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ceramic or metal fibers are used, the processing methods are more complex and expensive. Another major drawback to this type of composite is the potential high reactivity of the components, which limits the combination of matrix and reinforcements that can be used.

Natural fiber thermoplastic composites are formed by a thermoplastic phase as the matrix and natural fibers as reinforcement. They are more and more used due to their wide range of properties and their low cost. This thesis will focus on a thermoplastic matrix, and especially on high density polyethylene (HDPE).

I.1.2 Polymer matrices

Polymers can be classified in three main categories, defined by their thermomechanical properties: thermoplastics, thermosets and elastomers [2, 3, 4]. In thermoplastic polymers, the molecular chains are generally not crosslinked. These polymers often exhibit an elastic-plastic behavior, some ductility and are thermoformable. They can be melted and then molded, formed or welded when submitted to high temperature. This process is repeatable and can be done each time the polymer is submitted to high temperatures. They can be used with simple processing methods and exhibit some flexibility. Before processing, they can also be stored at ambient conditions for long time periods. They can be amorphous or semi-crystalline. Crystallinity can be up to 80% and grants the polymer opacity, high thermal expansion coefficient and high wear and failure resistance, as well as higher mechanical properties. On the contrary, the thermosets are constituted of a three dimensional tight-meshed crosslinked network between the molecular chains. They are processed through a curing reactive step and cannot be reshaped after hardening, even at high temperatures, making their recycling very difficult. Before processing, they have to be stored at low temperatures, and for limited time periods, if they already contained the hardener. Finally, elastomers are characterized by their ability to have important elasticity and relaxation behaviors despite their wide-meshed crosslinking of the chains. They are malleable under stress, but, as thermosets, they generally cannot be melted. Elastomers can be thermosets or thermoplastics depending on their nature, but due to their special properties, they are often considered as a specific category of polymers [2, 3, 4]. Most of the polymers can be used as matrices for the production of natural fiber composites. However, crosslinked elastomers and thermosets cannot be easily recycled.

Thermoplastics are often classified in three categories depending on their price and their volume of production [5]. Commodity plastics are the polymers used in high volume with a wide range of applications. They have an annual production of several hundred million tons. They have relatively good mechanical properties and their cost is generally less than a few dollars per kg. PE, PP, PVC and PS are typical examples of such thermoplastics. Technical thermoplastics are used for more technical applications. They present slightly higher mechanical properties, but their cost is higher, reaching five to ten dollars per kg, and their annual production volume is around ten million tons. ABS, PA, PC and PUR are examples of technical thermoplastics. Finally, specialty

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thermoplastics are polymers that are used for very specific and demanding applications. They exhibit much higher mechanical properties, but are also much more expensive as their price ranges from ten to several hundreds of dollars per kg. Due to their high cost, their production is less than one million tons per year. Examples of such polymers are PPS, PEEK, PEI and LCP.

All these types can be used for thermoplastic composites, but, due to their low cost and their availability, commodity plastics are more often used. However, due to the temperature limitation induced by the presence of natural fibers, not all thermoplastics can be used as the matrix. Considering their low melting point and their good mechanical properties, polyolefins are often used to produce natural fiber based composites, contrarily to PS and PVC, which both exhibit a low impact resistance at ambient temperature, and, in the case of PS, a higher melting point [6]. Polyethylene is one of the most produced and used thermoplastics. As most synthetic polymers, it is derived from petroleum. It has several advantages such as low price, simple structure, broad availability and ability to be reprocessed various times and recycled. It is a semi-crystalline thermoplastic with linear or branched structures. It is used in several applications in a wide range of fields such as cables, transportation, construction, packaging, leisure, sports, electronics, etc. Among all the different grades of polyethylene, HDPE presents higher mechanical properties, as well as higher density (around 0.965 g/cm3_{). However, it has the main drawback}

of degrading slowly. Due to its synthetic origin, its non-renewability, its slow degradation and its high volume of applications, its recycling became a major issue [4, 7].

I.1.3 Natural fibers

Natural fibers can be of different types, origins and natures. However, vegetal fibers are the most widely used for the production of thermoplastic based composites, mainly due to their availability and lower cost [1, 8, 9, 10]. Their worldwide production is estimated at about 4 billion tons [11]. They are generally classified in three main categories, as seen in Table I-1. First, the vegetal fibers include the fibers extracted from plant stems (rattan, flax, jute, hemp, ramie, wood, etc.), the fibers extracted from the seeds (mainly cotton and kapok) and the hard fibers extracted from the stalks (wheat, rice, barley, grass, bamboo, etc.), the fruits (coco), or the leaves (agave, banana, sisal, etc.) of the plants. Then, the animal fibers, which come from animal fur, fleece and secretions (silk). Finally, there are mineral fibers such as asbestos and basalt [1, 8, 9, 12]

4 Table I-1: Classification of natural fibers according to their origin.

Type Origin Examples

vegetal

stems flax, jute, hemp, ramie, rattan, wood

seeds cotton, kapok

stalks wheat, rice, barley, grass, bamboo

leaves agave, banana, sisal

fruits coco

animal

fleece wool

fur alpaca

secretions silk

mineral basalt, asbestos

Vegetal fibers are bio-composites mainly composed of cellulose, hemicellulose and lignin, and at much lower extent secondary components such as extracts, water, sugars, starches, pectins, proteins, and inorganic compounds [6, 12]. Most vegetal fibers can be assimilated to a natural composite constituted of a lignin matrix reinforced with cellulose fibrils and hemicellulose acting as a as coupling agent [13]. Each constituent has a specific function. The cellulose is mainly responsible for the fiber rigidity, resistance and structural stability, as well as for the polar aspect. The lignin ensures cohesion and other properties, while the hemicellulose improves the compatibility between lignin and cellulose. Each natural fiber presents unique, but variable properties depending on many factors such as the species, age, geographical origin, size and shape, environment and climate [6]. They are generally hydrophilic and can contain up to 13% wt. of humidity [14]. They tend to agglomerate as the hydroxyl groups create hydrogen bonds with other cellulose molecules and with water molecules [12, 14].

Besides wood, flax is one of the most common fiber used in North America and Europe. In 2015-2016, Canada produced about 940,000 tons of flax seeds and this amount should increase every year with the increasing interest for flax reinforced composites [15]. It is broadly available and is strongly used in the automobile sector to make various parts and boards [16]. Flax fibers are extracted from the stems of Linum usitatissimum, an annual plant from the Linaceae family that can be found in Asia and Europe [17]. They present high tenacity and thermal conduction, but they also have low elasticity and high absorption [11]. They have higher yield and are twice to three times more resistant than cotton [1, 6]. Depending on the applications, they can be of different sizes and shapes, ranging from long fibers to powder, and can also be used woven. Long fibers are used to create canvases, whereas shorter are used to create ropes and papers. All sizes and shapes can be used to

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produce flax fiber based composites, depending on the properties needed, and to fit different applications, from panels to construction materials [18, 19].

I.1.4 Natural fibers vs. synthetic fibers

Natural fibers (wood, flax, hemp, jute, kenaf, coco, etc.) are gaining interest over synthetic reinforcement (glass, carbon, aramid, talc, etc.) due to their numerous advantages such as low cost, lower density, high specific stiffness and strength, availability, as well as electrical and acoustic insulation properties [6, 10, 20]. They are non-toxic, non-corrosive and are much less abrasive for tooling and equipment. They can be easily used with traditional processing methods. They also have a good notoriety due to their natural source, as well as their renewable, sustainable and degradable aspects [20, 21]. They appear as an interesting alternative to more traditional reinforcements despite of their lower mechanical properties. However, they generally have lower resistance, lower durability, poorer fire resistance and higher quality variability than synthetic fibers. Their use also induces some processing issues due to their hydrophilic nature and their sensitivity to temperature, leading to low interface bonding, parasite foaming and void creation [21, 22, 23, 24]. Volatiles and water are released during the processing of natural fiber composites, to an extent depending on their preliminary drying, and leading to irregularities in the composites structure. Natural fibers also tend to degrade at temperature above 230°C [24]. Despite all these limitations, natural fibers are widely used in the composite industries and many of their issues can be overcome by careful selection of the matrix, the processing parameters and by preliminary drying. In order to find applications and to replace inorganic fiber composites in some applications, natural fibers should exhibit interesting and competitive properties. Table I-2 compares the main properties of natural fibers and synthetics ones. Except for the carbon fiber, which has very high rigidity, it appears that, when considering specific properties, some natural fibers have properties close to most of the other synthetic fibers, and especially to glass fibers which are the most used synthetic fibers. Considering that these fibers generally have lower density and cost than inorganic fibers, they thus appear as an interesting alternative to more traditional reinforcements such as glass fibers [25].

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Table I-2: Comparison between the physical and mechanical properties of common natural and synthetic fibers [26, 27, 28, 29].

Fiber type Density (g/cm3₎ Elongation at break (%) Tensile strength (MPa) Elastic modulus (GPa) Cotton 1.5-1.6 3.0-10.0 290-600 3-14 Jute 1.3-1.5 1.5-3.0 390-800 10-50 Flax 1.4-1.5 1.5-4.0 345-1500 25-90 Hemp 1.5 1.6-4.0 400-900 25-70 Kenaf 1.4 1.6-2.1 220-940 25-55 Ramie 1.5 2.0-3.8 290-700 45-130 Sisal 1.3-1.5 2-14 175-593 9-38 Coir 1.2 15-30 15.0-30.0 4-6

Softwood Kraft pulp 1.5 4.4 1000 40

Bamboo 1.4 2.0 500-740 30-50 Coconut 1.2 20-40 1200-1800 4-6 Spider silk 1.3 28-30 1300-2000 30 E-glass 2.5 0.5-3.0 1200-3500 70-75 S-glass 2.5 2.8 2000-4570 86 Aramid 1.4 3.3-3.7 3000-3150 63-67 Carbon 1.4 1.4-2.0 4000 230-240

I.1.5 Interface properties and coupling agent

Composite properties essentially depend on the materials capacity to distribute and transfer loads and stresses between both constituents [23, 24]. This is governed by the interface quality. However, for most natural fiber composites, interface quality is known to be poor due to the incompatibility and difference in polarity between the fiber and the matrix. Most natural fibers such as flax are hydrophilic, while polyolefins such as high density polyethylene are hydrophobic. This leads to a low interfacial adhesion and to low fiber wetting by the matrix due to water absorbed at their surface. Due to the poor wetting of the fibers by the polymer, they tend to agglomerate, leading to an inhomogeneous distribution and to low mechanical properties. Moreover, due to the poor interface properties, stress transfer is limited and stresses are not distributed ideally in the composite [23, 30, 31]. To limit this effect and improve the interface quality, three main solutions exist. First, a coupling agent can be used to improve the interface adhesion and the compatibility between the constituents. Coupling agents are polymers grafted with functional groups that can react with both the matrix and the fiber. They have characteristics and

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properties of both constituents and create a chemical bridge between them, thus increasing the interfacial adhesion. The coupling agent has to be chosen depending on the matrix and fiber nature and properties, as to have the maximum compatibility with both of them. For example, for natural fiber reinforced polyethylene, maleic anhydride grafted polyethylene (MAPE) is often used as a coupling agent because the polyethylene part entangles with the matrix and the maleic anhydride groups react and bond with the hydroxyl groups on the fiber surface. Then, it is also possible to treat the fiber to improve its adhesion or limit the water absorption. Depending on the type of treatment, different effects can be obtained such as the cleaning of the fiber, the modification of the fiber surface or of its chemistry. Two main types of treatment exist: physical and chemical. The chemical treatments modify the surface chemistry to improve the compatibility with the matrix, while physical treatments modify the surface structure and properties by physical, mechanical, thermal or electromagnetic effects, and influence bonding with the matrix. However, the last ones are complex, often harmful for the environment, and thus still uncommon [22, 23, 24, 32, 33, 34]. The use of one of these solutions, which is preliminary drying, substantially decreases the harmful effect of natural fibers making them suitable for the production of thermoplastics based composites.

I.1.6 Applications

Over the last years, the interest for natural fiber composites has grown strongly with the development of new materials and the optimization of the ones already on the market. These composites are used in a great diversity of fields for both structural and non-structural applications such as [16, 24, 35, 36, 37, 38, 39]:

• Construction (floor, roof, panels, railing)

• Transportation (car parts, train parts)

• Furniture (chairs, tables, decks)

• Leisure (toys, CD, phone and computer cases)

• Sport (kayaks, boat parts, snowboards)

• Music (guitars, violins, harmonicas)

• Decoration and home (jars, clocks, scales)

Some examples are presented in Figure I-1. Natural fiber composites are widely used in the automotive and construction industries because of the cost and weight reduction generated by the use of natural fibers instead of more traditional fibers and materials [26, 27, 38, 39, 40]. Lately, natural fiber use increased by 20% per year

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in the automotive sector [41]. Outdoors and sports applications also increased strongly in the last years, but remain limited due to the moisture sensibility of such composites. In order to find new applications and markets, these composites constantly need to be competitive and performant compared to more traditional materials such as inorganic fiber reinforced composites and wood. Besides their environmentally friendly image, they should have improved properties and performances, as well as durability in order to increase their market share [6].

Figure I-1: Examples of potential and current commercial applications for natural fiber based composites [28, 42, 43, 44, 45, 46, 47, 48, 49, 50].

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I.2 Recycling

I.2.1 Polymer recycling

Recycling is associated to the collect, sorting and treatment of waste to provide these materials a new lifecycle by their direct use or their reintroduction into the processing cycle of other products. It is applied for various materials such as plastics, metals, cardboard and paper, wood, solvents and textile. The recycling methods depend on the material considered. The main idea is to decrease the waste volumes while simultaneously preserving the natural resources by reusing materials and limiting the greenhouse gases emitted when burning the materials. Even if the amount of petroleum used for the production of plastics is currently relatively low (less than 10%), this number should increase with time as plastic demand and waste constantly increases, mainly due to the actual life and consumer styles [51]. Currently, almost 80% of the polymers are petroleum based. It is thus important to limit the waste and the consumption of new materials at best.

Plastic recycling takes place in consecutive steps. First, the materials should be collected, gathered and transported to the sorting center to be sorted out by resin type. To facilitate the sorting of polymers, code numbers have been attributed to the main plastics families used in everyday life. These codes are given in Table I-3. Polyethylene and polyethylene terephthalate (PET) are often sorted out, due to their high volume, while others are gathered. Industrial wastes are often easier to recycle than post-consumer ones due to their high volume and low contamination [52]. Then, the sorted polymers are conveyed to the various treatment centers. Various separation methods have been developed, but they remain expensive [7, 53]. However, they appear unavoidable as the inhomogeneity of polymer waste leads to poor properties of the recycled materials. Three main types of plastic recycling methods exist: mechanical, thermal (energetic) and chemical. The different types of recycling, their characteristics and specificities will be detailed in the next chapter. It is also possible to reuse directly some products after cleaning. However, for sanitary reasons, this method remains quite limited. Table I-4 summarizes the plastic waste recovered in Quebec in 2008.

Table I-3: Plastics codes in Canada [52].

Plastic code Type of polymer

1 Polyethylene Terephthalate (PET) 2 High Density Polyethylene (HDPE)

3 Polyvinyl Chloride (PVC)

4 Low Density Polyethylene (LDPE)

5 Polypropylene (PP)

6 Polystyrene (PS)

10 Table I-4: Plastic waste recovered in Quebec during 2008 [52].

Polymer category Total (in metric tons)

PET 27 976 HDPE 35 687 PVC 9 516 LDPE 19 048 PP 5 954 PS 655 Mixed plastics (#1-#7) 3 288 Mixed plastics (#2-#7) 4 974 Mixed plastics (#3-#7) 1 662 Others 11 289 Total 120 050

I.2.2 Composite recycling

Natural fiber composites are widely used nowadays and production/waste volumes constantly increase. Moreover, according to Faruk et al., the production volume of natural fiber reinforced composites should increase from 2.33 million tons to about 3.45 million tons between 2013 and 2020 [6]. The recycling of thermoplastic based composites is quite similar to the recycling of thermoplastics, even if some specifics and limitations appear due to the presence of fibers. This will be reviewed in the next chapter with a focus on the mechanical recycling of thermoplastic based composites and will not be detailed further here.

I.3 Thesis objective and organization

Due to the very high volume of thermoplastic based composites used, their recycling becomes a major issue. Although several studies have been done on the mechanical recycling of natural fiber reinforced thermoplastics, there are still lacks of knowledge concerning the effect of such recycling on the performances and properties of the final material, as most works reported contradictory trends. Moreover, there is even less information on the long-term and intensive aspects of such recycling, as most of the studies considered less than ten processing cycles. Thus, the main objective of this thesis is to study the recyclability of HDPE/flax composites during long-term intensive (up to 50 cycles) mechanical recycling. Two composite series were realized, with and without coupling agent, and the HDPE matrix alone was also studied to complete the knowledge on the intensive mechanical recycling of thermoplastics. A complete characterization (in terms of physical, thermal, molecular,

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morphological and mechanical properties) of the recycled samples was performed to evaluate the degradation and the loss of performance undergone after long-term mechanical recycling.

To achieve the main objective, this thesis tries to understand the degradation undergone by the materials during their long-term recycling through the following secondary objectives:

• Link all results to understand and quantify the degradation mechanisms taking place in the material and its constituents during recycling.

• Comparison between the short- and long-term recycling behaviors.

• Comparison between the neat matrix and the composite behaviors and properties as to understand the effect of fibers on the recycling behavior.

• Evaluation of the effect of coupling agent on the recycling behavior and the degradation undergone during recycling.

• Evaluation of the recycling potential of both the neat matrix and the composites after 50 close-loop reprocessing cycles.

Since the thesis is a paper-based document, it is composed of five main chapters:

In the first chapter, a brief introduction on natural fiber composites, their properties, their applications, their issues and their recycling was presented. General notions and definitions on composites and their constituents were also reported. The materials used in the following parts (high density polyethylene and flax fiber) were also described briefly, with their main characteristics and applications. Finally, the context of this work was also explained and general ideas on recycling were also presented.

The second chapter presents a literature review on the mechanical recycling of thermoplastic composites. The various types of composites recycling are briefly introduced. Then, the various works conducted on the recycling of natural fiber reinforced thermoplastic composites are presented, followed by the works on the mechanical recycling of composites reinforced with inorganic fibers. Finally, the various limitations to the composites mechanical recycling are considered and some possible solutions to these limitations are suggested. This review confirms the lack of knowledge on the long-term mechanical recycling of thermoplastic polymers and composites.

The third chapter is the first step to understand the behavior of the thermoplastic materials towards mechanical recycling. In this part, the HDPE neat matrix was produced and recycled by extrusion up to 50 cycles. The effect

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of intensive mechanical recycling on the polymer structure and properties (physical, thermal, molecular and mechanical) is reported, thus giving information on the matrix behavior during recycling.

The fourth chapter is the second step to understand the thermoplastic composite behavior towards mechanical recycling. It focuses on the intensive mechanical recycling of HDPE/flax composites, considering composites with and without MAPE as a coupling agent. A complete characterization (physical, thermal, morphological, molecular and mechanical) is also conducted and the processing parameters are chosen to be the same as for the neat matrix study in the third chapter. For each composite formulation, a total of 50 closed-loop reprocessing cycles are carried out and compared to the neat matrix to understand the effect of recycling on the composite properties, as well as to evaluate the effect of fiber and coupling agent on the recycling behavior.

Finally, the fifth chapter is an overall conclusion briefly reviewing the main results, observations and conclusions of the different chapters. It also presents several recommendations for future works.

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Chapter II. Mechanical recycling of thermoplastic

composites

Résumé

Les composites thermoplastiques ont progressivement fait leur place sur le marché ces dernières années. Leur principal avantage, en comparaison avec les composites thermodurcissables, est leur possibilité d’être mis en forme de manière répétée par de simples procédés de fusion et moulage. Ceci permet aux matériaux d’être récupérés et remis en œuvre s’ils sont recalés au contrôle qualité (post-industriel) ou tout simplement à la fin de leur cycle de vie (post-consommation). Cette caractéristique apparaît très intéressante considérant que le recyclage est un des aspects primordiaux du développement durable, d’autant plus que la production et l’utilisation de composites thermoplastiques augmentent constamment. Ces composites se retrouvent en effet pour toutes sortes d’applications telles que l’emballage, l’automobile, l’aéronautique, l’ameublement, la construction et le bâtiment, ou encore les sports et loisirs. La demande constamment croissante pour ce type de produits engendre un très grand nombre de pièces produites, utilisées puis jetées. En raison de cet important volume de composites produits et utilisés, de nouvelles lois et directives toujours plus strictes sont constamment développées en faveur du développement durable et du recyclage. C’est pourquoi il est important d’étudier et de comprendre l’état de tels matériaux en fin de vie, ainsi que de développer des applications et des méthodes pour réintroduire ces grandes quantités de matières dans les lignes de production. Afin de limiter les quantités de matière consommées et mises en décharge, les déchets de composites thermoplastiques doivent être considérés comme une source de matières premières pour la production de nouveaux produits composites par recyclage et par remise en œuvre. Toutefois, afin d’obtenir de bonnes propriétés finales, il est essentiel de comprendre les mécanismes de dégradation et le comportement des matériaux lors leur cycle de vie et leur recyclage. Ce chapitre est une revue de littérature sur les différentes possibilités de recyclage de composites thermoplastiques, et notamment sur les techniques de recyclage mécaniques disponibles. Quelques chiffres sur la production et le recyclage de composites seront présentés et discutés, puis une brève présentation des différentes méthodes de recyclage disponibles pour les composites thermoplastiques, de leurs avantages et limitations sera faite. Finalement, une vue d’ensemble des différents travaux sur le recyclage mécanique de composites thermoplastiques à renforts organiques et inorganiques est présentée. Dans tous les cas, le comportement du matériau face au recyclage et les propriétés résultantes sont considérés, ainsi que les solutions développées pour améliorer les performances et la qualité des matériaux recyclés.

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Abstract

Thermoplastic composites have found several applications over the years, but their main advantage when compared with thermoset composites is their possibility to be reshaped after processing through further melting and remolding processes. This allows the materials to be recovered and reprocessed if they fail quality control (post-industrial) or after their end of life (post-consumer). This is very attractive as recycling is one of the main principles of sustainable development. This is even more appropriate as thermoplastic composites are more and more produced and consumed in a wide range of applications such as packaging, automotive and aeronautics, furniture, building and construction, as well as sport and leisure goods. Thus increasing demands leads to a high number of parts being produced and discarded. With this high volume of composites production and use, the emergence of constantly new and stronger policies and laws towards sustainable development and recycling were developed. This is why it becomes important to study and understand the conditions of the materials at their end of life and develop applications like recycling to reintroduce these high amounts of materials into production lines. To limit both the amount of material consumed and landfilled, thermoplastic composite waste can be considered as a source of raw material for the manufacture of products through recycling and reprocessing. However, to obtain good final properties, it is essential to understand the degradation processes and the materials behavior during their life cycle and their recycling. In this chapter, a review of the different possibility to recycle thermoplastics composites is presented, with a focus on the mechanical recycling techniques available. Some figures about composites production and recycling are presented and discussed, followed by a brief presentation of the different recycling methods available for thermoplastic composites with their advantages and limitations. Then, an overview of mechanical recycling is made considering both organic and inorganic fillers. For all cases, the material behavior towards recycling and the resulting properties are considered, as well as the solutions developed to improve the performance and quality of the recycled materials.

Keywords: Thermoplastics, composites, recycling, foams, properties.

N. Benoit, R. González-Núñez, and D. Rodrigue. Mechanical recycling of thermoplastic composites, in

Thermoplastic Composites: Emerging Technology, Uses and Prospect. E. Ritter Ed., Nova Science Publishers, New York, Chapter 3, pp. 95-142, ISBN: 978-1-53610-727-2 (2017).

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II.1 Introduction

Composite materials are composed of two phases, one being the matrix, and the other one, the reinforcement (fiber or particles). The matrix protects the reinforcement, provides cohesion to the material and binds the reinforcing particles together, thus distributing the stresses under load. The reinforcement increases the rigidity and the performances of the material. The association of both phases leads to interesting properties, combining the properties of both components. The performances of the resulting composites depend on the capacity of the material to transfer the stresses between both constituents, and thus on the interface quality. These materials have been produced continuously for a few decades now. The high demand for composites created a new category of plastic-based waste that has to be taken care of. Nowadays, 8.7 million tons of polymer composites are produced each year, thus generating high volumes of waste [54]. For example, in France, 30,000 tons of composites waste are collected every year. As they will reach their end of life soon and the quantities will increase in the future, their recycling should be studied and understood as to recover the materials and their value, as well as simultaneously avoid their accumulation in landfill [55]. Recycling of polymer based materials is environmentally very interesting as it leads to the reduction of waste simultaneously with saving of virgin resources, and especially petroleum [55]. However, due to the heterogeneous nature between the matrix and the reinforcement, and to the lowering of the properties of the recycled materials, thermoplastic composites are currently rarely recycled at the industrial scale, as the existing techniques are often expensive [55, 56, 57]. Thus, most composites are currently converted to energy and fuel or fiber recovery. However, in the latter, the fibers recovered show an important drop of performance (up to 90% for glass fibers) and suffer from a lack of cost competitiveness, which usually make them unsuitable for most applications and markets [58, 59].

Thermoplastic composites are widely used nowadays. Their matrices are characterized by their aptitude to be repeatedly melted by heating and solidified by cooling, and this in a reversible way. The reinforcement add rigidity and strength to the matrix, thus making a material combining the properties of both constituents. Thermoplastic composites have become an area of increased interest due to their relatively low cost, wide range of mechanical properties, toughness, problem free storage, resistance to chemical attack, ease of processability, and above all their better recyclability. This is related to their fundamental ability to be reprocessed and reshaped when reheated leading to much simpler and direct recycling paths [54, 56, 60, 61, 62]. Because they have a melting point, they can thus be remelted, reshaped and reharden multiple times. This is not the case for thermosets having highly reticulated networks which cannot be melted after their first shaping [63, 64]. Although thermosets are currently the most common composites representing about 90% of the composite market in 2014, thermoplastic composites are continuously increasing their market share due to their numerous benefits [54, 56, 62]. Over the last decades, the demand for thermoplastic composites increased with the development of new applications in several fields such as decking, outdoors equipment, household, leisure, construction,

16

transportation, packaging, sports, leisure, domestic applications, energy, automotive and aerospace [7, 54, 56, 62, 65]. This increased demand on composites produced and used led to an increase of waste generated, thus increasing the pressure for the development of efficient and viable recycling methods for such materials [7, 32, 66]. It was projected that in the next few years, the total global production of composites will exceed 10 million tons, thus occupying a volume of over 5 million cubic meters [59]. Among these composites, more than 90% are glass reinforced (GR) composites, and about 40% of this volume is associated to thermoplastics composites. With constantly new environmental directives and legislations being established, both producers and manufacturers are asked to consider the environmental impact of their products. This is particularly true for thermoplastics composites, which are gaining interest over thermosetting composites due to their several advantages like design flexibility, easier processing and shorter processing time [32, 63, 64, 66]. Moreover, most of them usually decompose slowly, thus requiring to be taken care of at the end of their life cycle.

In the last few years, the society has constantly increased its concern about sustainable development leading the actual governments to consider new laws and directives towards resource management and waste revalorization [67]. The plastic production continuously increase to fit the increasing demand for such materials. Around the world, the general plastic production increased by 38% in ten years, reaching 311 million tons in 2014, with an important part of thermoplastics materials [4]. In Europe, thermoplastics materials represent more than 72% of the total production [4, 67]. Thus, polymers represent an important part of the municipal solid waste (MSW) in many countries such as the United States, Canada and in Europe. For example, they represent up to 20% of the MSW in the United States, and only a few percent were recycled [32, 68]. Moreover, thermoplastics are generally the main constituents of the polymer fraction of MSW [69]. In Europe, 7.7 mT of plastic wastes were recovered in 2015. Among this waste, only 30% of the total volume was recycled, 39% was energetically revalorized, while the remaining 31% was simply landfilled [4]. In Canada, in the province of Quebec, plastics waste constantly increased over the last 20 years. Their revalorization only accounts for about 5% of the total revalorization for all materials, and represents only 16% of the total plastics waste collected from MSW [52]. These low recycling rates are essentially due to the lack of techniques available and problems associated with the collect and sorting processes. This shows that more efforts must be done in these fields to increase the revalorization rate of such materials. However, it should be noted that, over the last few years, both energy recovery and recycling increased, while landfilling rate decreased [4]. For example, in Europe, between 2006 and 2014, for the 25 million tons of the global plastic waste produced, the landfilled fraction decreased by 38%, while the energy recovery and recycling shares increased by 46% and 64% respectively, thus reaching 8 million tons of polymers discarded to landfill, 10.2 million tons of polymers energetically recovered, and 7.7 million tons of polymer waste recycled [4]. Thus, recycling of thermoplastics and their derivatives is fundamental in the actual social and political context, especially since they have a slow decomposition rate, as well as a low density, but represent very important volumes of materials [68]. Considering these facts, and especially the important volume

17

generated daily, some efforts still need to be done to manage resources, as well as to manage, treat and revalorize this kind of waste. However, if the waste volumes and recycling rate of thermoplastics are usually known, very few statistics can be obtained about thermoplastics composites [69]. Although some companies are already using recycled thermoplastics to produce wood plastic composites (WPC), no use of recycled composites was reported. In any case, the composition of the waste stream depends on the geographical location, time of the year (season), current trends and several other parameters [69].

Recycling consists in the collect, the sorting and the treatment of waste. Materials sorting is an important issue as the presence of impurities and contaminants can lead to unsuitable or poor properties of the recycled materials. Three main approaches can be considered for the recycling of composite waste [70, 71]. The simplest way is to dispose into landfill. However, this is also the worse scenario from an environmental and economical point of view, as it can lead to space problems if the volume of waste is too important [57, 66]. The second option is the incineration of the material, with possible energy recovery or heat generated, but this also lead to air pollution and environmental problems. This option is also known as thermal recycling and is often considered as a part of the third category, which is recycling. The last, but most important option, is recycling. It is based on the partial or total recovery of the constituents of the global material. It is sustainable, economically viable and appears to be the best environmental and technical choice [68, 72, 73]. It reduces the energy used and the waste volume disposed to landfill, while simultaneously reducing the raw material consumption. However, it induces to some extent some changes in the mechanical, physical and chemical properties of the final materials, that may affect the material processing conditions and the quality of the end products [56, 66, 68, 74]. It often gives, not always justified, a “low quality” image to the products, thus limiting its attractiveness. Compared to virgin composites, recycled composites often meet problems to satisfy the quality and economical requirements, thus limiting the choice of suitable market [57, 68]. Several works considered the partial addition of virgin materials to counter this drawback. Coupling agents are also often used to improve the composites properties, but their sensitivity towards recycling is not well known [55]. It is thus essential to know how the properties of thermoplastics composites are modified by recycling, and especially how the reinforcement modify polymer degradation [73]. This should be done by the study of the recycling effect on the materials properties, by the development of new, cheaper and more efficient recycling methods, as well as better separation techniques. But it should also be done by increasing the recyclability of the produced composites. Besides thermal recycling, two other categories of recycling methods exist: chemical recycling and mechanical recycling [55, 56, 67, 73]. Chemical recycling leads to reusing the constituents for the production of new composites, while mechanical recycling is often performed on the whole composite.

The recycling of thermoplastics is well known and was the focus of several authors. The extensive long-term recycling of such polymers was studied by Oblak et al. [74], Jin et al. [36, 37] and Benoit et al. [75]. However,

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the recycling of thermoplastics composites, and especially of their extensive or long-term recycling, is much more limited. This chapter aims at reviewing the various works on the mechanical recycling of thermoplastics composites to present the state of the art in this field. To do so, the main recycling methods for this type of composites are presented first with their advantages and drawbacks. Then, the recycling of natural organic fillers reinforced composites will be considered, followed by the study of the recycling of inorganic fillers reinforced composites. Finally, the main limitations to the recycling of thermoplastics composites will be presented, as well as their possible solution. Finally, this review will summarize and conclude about the actual state and viability of the mechanical recycling of thermoplastics composites.

II.2 Recycling methods

Due to the lack of knowledge, the complexity of recycling and the predominance of thermosets until recently, the majority of composites materials were disposed to landfill [76]. But the same situation is still going on. However, due to the constantly increasing market of composites materials and the important volumes of waste generated, as well as the high cost of the reinforcements and additives used in these materials, other alternatives must be considered [64, 77]. Nowadays, due to their numerous advantages, thermoplastics composites are gaining interest and gradually taking the place of thermosets composites in various applications, making their recycling easier [77]. They present lower manufacturing time, lower sensitivity to impact damage, easier manufacturing, and better recyclability due to their aptitude to be remelted [77]. These advantages allow to consider other ways of managing composite waste.

Composting can be conducted for biodegradable materials, but in this case, both matrix and reinforcement should be biodegradable to be sure not to contaminate the environment with permanent residues [78]. For very specific cases, reuse of materials after cleaning or basic operations can be considered. If this method is environmentally respectful, however, in the polymer and composite fields, reuse is quite limited due to health and safety reasons [64, 67]. Because of their inhomogeneous nature, recycling is a great challenge for polymer composites. The presence of reinforcement leads to technical challenges such as equipment wear, incompatibility, restrictions on temperature and other processing parameters, special attention during processing or other technical constraints. The waste stream has to be consistent and sufficient (in quality and quantity), and the price low enough to make the recycling process viable and profitable [52, 78].

Various recycling methods exist for polymer composite materials. These methods can be divided in three main categories: thermal recycling, chemical (or feedstock) recycling and mechanical (or physical) recycling [52, 56, 58, 67, 76, 77, 78, 79, 80, 81]. Each one has its own list of advantages and drawbacks [78, 82]. The choice of the method depends on several parameters including the nature and quality of materials, their initial state, the final state and quality desired for the materials, as well as their constituents, the nature of the composite