Mechanical and morphological characterization of wood plastic composites based on municipal plastic waste

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Mechanical and morphological characterization of wood

plastic composites based on municipal plastic waste

Mémoire

Yasamin Kazemi

Maîtrise en Génie chimique

Maître ès sciences (M. Sc.)

Québec, Canada

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Résumé

Les développements récents de la législation associée aux impacts environnementaux des déchets plastiques d’origine post-consommation ont mené à des efforts sur le développement de techniques viables de recyclage. Ainsi, le but de cette recherche était de produire des composites bois-plastique (WPC : wood plastic composites) à partir de la fraction légère des déchets plastiques municipaux (post-consommation) et de résidus de transformation du bois (sciure). Afin d’améliorer la compatibilité et l’adhésion entre le polyéthylène (PE) et le polypropylène (PP), un copolymère d’éthylène-octène (EOC: ethylene-octene copolymer) a été utilisé pour développer la compatibilité entre les phases polymères tout en agissant comme modificateur d’impact. L’ajout de PE et PP maléatés (MAPE: maleated polyethylene; MAPP maleated polypropylene) a permis de fournir une meilleure compatibilité entre la matrice polymère et la farine de bois. Les effets combinés de tous les composants ont mené à la production de composites présentant des propriétés morphologiques (dispersion et adhésion) et mécaniques (traction, torsion, flexion et impact) intéressantes après l’optimisation de l’ensemble des additifs (mélanges d’agents couplants). Dans un second temps, des composites structuraux à trois couches ont été produits à partir des matériaux composites mentionnés plus haut afin d’étudier l’effet des paramètres de design sur les performances en flexion et à l’impact. Les paramètres étudiés incluent la teneur en bois, l’épaisseur des couches individuelles de composite, ainsi que la séquence et la configuration d’empilement des différentes couches (structures symétriques et asymétriques). Enfin, la théorie classique des poutres a été utilisée avec succès pour prédire le module en flexion et ce, avec un maximum de 10% de déviation pour ces structures complexes.

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Abstract

Recent legislations associated with environmental impacts of post-consumer plastic wastes have driven substantial attention toward developing viable recycling techniques. Therefore the aim of this research was to produce wood plastic composites (WPC) from the light fraction of municipal plastic wastes (post-consumer) and wood processing residues (sawdust). In order to improve compatibility and adhesion between polyethylene (PE) and polypropylene (PP), an ethylene-octene copolymer (EOC) was used to compatibilize the polymer phases and also to act as an impact modifier. Addition of maleated polyethylene (MAPE) and maleated polypropylene (MAPP) provided improved compatibility between the polymer matrix and the wood flour. The combined effect of all the components was found to produce composites with interesting morphological (dispersion and adhesion) and mechanical properties (tension, torsion, flexion and impact) after optimization of the additive package (blend of coupling agents).

In the second phase, three-layered structural composites were produced from the aforementioned composites to investigate the effects of design parameters on their flexural and impact performance. The studied parameters include wood content, thickness of individual composite layers, as well as stacking sequence and configuration (symmetric and asymmetric structures). In addition, the classical beam theory was successfully used to predict the flexural modulus within 10% of deviation for these complex structures.

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Foreword

This dissertation is written in a manuscript-based format and consists of five chapters. The first chapter includes a brief introduction to environmental issues, obstacles and possibilities of plastic recycling. The current plastic recycling techniques are discussed and subsequently mechanical recycling is proposed as the most beneficial technique. The light fraction of recycled plastics (mainly composed of polyethylene and polypropylene) is introduced as the main contributor of plastic waste streams and therefore compatibilization techniques between PE and PP are reviewed in this chapter.

In the second chapter, natural fiber composites are discussed as a proposed application for recycled plastic materials. Fabrication and modification techniques of wood-plastic composites (WPC) are discussed and WPC literature based on polyethylene and polypropylene blends is reported. In addition, structural designing is introduced to improve the mechanical properties of these composite materials. The objectives of this research work are presented at the end of this chapter.

The following two chapters present the experimental results in the form of submitted articles. My contribution in these manuscripts was to perform all the experimental work, data collection and analysis (including calculations) and to write their first drafts.

Chapter 3 discusses the fabrication and characterization of wood-plastic composites of recycled origin. The results of this work are submitted in the following manuscript:

[1] Kazemi, Y., Cloutier, A. and Rodrigue, D., Mechanical and morphological properties of wood-plastic composites based on municipal plastic waste, Polymer Composites, Submitted in October 2012.

Then, the composites produced in Chapter 3 were exploited in Chapter 4 to study the effect of design parameters on mechanical performance of three-layered structural composites. The manuscript was submitted as:

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[2] Kazemi, Y., Cloutier, A. and Rodrigue, D., Design analysis of three-layered structural composites based on post-consumer recycled plastics and wood residues, Composites part A, Submitted in January 2013.

The last chapter includes a general conclusion on the work performed and recommendations for future works.

Nevertheless, this research work includes more results presented in other manuscripts or conference presentations as follows:

[3] Kazemi, Y., Cloutier, A. and Rodrigue, D., Natural fiber composites based on post-consumer polyolefins and wood fiber residues: Effect of coupling agent addition, PPS Americas Conference 2012, Niagara Falls, ON, Canada, May 21-24 (2012).

[4] Ramezani Kakroodi, A., Kazemi, Y. and Rodrigue, D., Mechanical, rheological, morphological and water absorption properties of maleated polyethylene/hemp composites: effect of ground tire rubber addition, Composites Part B, in press (2012).

[5] Ramezani Kakroodi, A., Kazemi, Y. and Rodrigue, D., Impact modification of waste plastic/wood flour composites via structural modification, Submitted to ICCM19, Montreal, QC, Canada, July 28-August 2 (2013).

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Acknowledgments

This dissertation would not be accomplished without the advices and supports of people whom I am greatly indebted.

First and foremost, I wish to express my profound gratitude to Prof. Denis Rodrigue, my supervisor, for his invaluable guidance and assistance. I am incredibly grateful and

appreciative for his incessant kindness, patience and understanding. He gave me the confidence to pursue my ideas in this research work which is the most valuable achievement for me.

I would like to sincerely thank my co-supervisor, Prof. Alain Cloutier, for his helps, supportive direction and scientific insight.

I would like to express my sincere appreciation to my family who have been the source of encouragement and inspiration to me throughout my life. I would like to especially express my gratitude to my husband, Adel, for all his kindness and supports as a husband, colleague and a teacher.

I also appreciate the technical assistance of Mr. Yann Giroux, who is not only a capable technician, but also a very good friend. I would like also to thank my colleagues and friends of the chemical engineering department for their amity and supports which made great memories throughout my M.Sc. program.

Finally, I acknowledge the financial and technical support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and FPInnovations, as well as Centre de Recherche sur le Bois (CRB) and Centre Québécois sur les Matériaux Fonctionnels (CQMF) for technical and financial help.

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“Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.”

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List of Tables

Table ‎1.1: Calorific values of major plastic waste compared with common fuels. ... 3

Table ‎1.2: Common separation techniques for plastic recycling. ... 6

Table ‎2.1: Energy consumption (MJ/kg) for production of different fibers. ... 15

Table ‎2.2: Chemical composition of lignocellulosic fibers (weight %) ... 17

Table ‎2.3: Applications of multi-layered structural composites. ... 31

Table ‎2.4: Advantages and disadvantages associated with application of multilayered structures.. ... 31

Table ‎3.1: Mechanical properties of the composites without compatibilizer and coupling agent. ... 49

Table ‎3.2: Mechanical properties of the composites with compatibilizer and coupling agent. ... 50

Table ‎3.3: Hardness and density results for compatibilized and uncompatibilized composites. ... 53

Table ‎4.1: Flexural and impact properties of symmetric three-layered structural composites. ... 67

Table ‎4.2: Flexural and impact properties of asymmetric three-layered structural composites with equal layer thickness ... 74

Table ‎4.3: Flexural and impact properties of asymmetric three-layered structural composites with different thickness of layers ... 75

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List of Figures

Figure ‎1.1: MSW generation and recycling in USA from 1960 to 2010. ... 1

Figure ‎1.2: Life cycle of plastic materials including waste management options . ... 2

Figure ‎1.3: Common processes used for chemical recycling of plastic waste and their main products. ... 4

Figure ‎1.4: Schematic representation of plastics separation by a floatation technique. ... 7

Figure ‎1.5: LV-STEM of low density polyethylene/PP (80/20) blends (frame width: 9.7 μm) . ... 8

Figure ‎1.6: Common functional groups for chemical compatibilization. ... 10

Figure ‎1.7: SEM micrographs from fractured surfaces of fractured impact specimens of PE/PP: 50/50 blend (a) without and (b) with 10% EPDM. ... 12

Figure ‎1.8: AFM micrographs of PP/HDPE compounds (a) without compatibilizer and (b) with multi-block EOC (frame width: 20 μm) ... 13

Figure ‎2.1: Generic life cycles of (a) glass fiber and (b) natural fiber reinforced composites. ... 16

Figure ‎2.2: Chemical structure of a repeating unit in cellulose molecule. ... 18

Figure ‎2.3: SEM micrographs from fractured surfaces of hemp filled PP. ... 19

Figure ‎2.4: Effect of different fillers on water uptake of an epoxy composite. ... 19

Figure ‎2.5: Schematic representation of interaction between LDPE and MPS-modified fiber. ... 21

Figure ‎2.6: Effect of cellulosic fibers surface treatment with MPS: a) untreated and b) treated fiber. ... 21

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Figure ‎2.7: Surface treatment of natural fibers with maleated polypropylene. ... 23 Figure ‎2.8: SEM micrographs of (a) untreated and (b) alkali treated hemp fibers. ... 24 Figure ‎2.9: SEM micrographs of PP/agro-fiber composites (a) before and (b) after

compatibilization with MAPP. ... 26

Figure ‎2.10: Current and emerging matrices for natural fiber composites and their

biodegradability. ... 27

Figure ‎2.11: Elastic modulus (E), tensile strength (TS), elongation at break (EB) and

impact strength (IS) of light fraction (LF) based composites with different fillers. .... 30

Figure ‎2.12: Effect of different cross-sectional designs on properties of composites. Lines

depict corresponding fiber placement. Superscript groups are not statistically different for each test. ... 32

Figure ‎2.13: Three point flexion test when the composite layer is in extrados or intrados. 33 Figure ‎2.14: Fractured areas of specimens from flexion test with composite layer placed in

the (a) intrados side and (b) extrados side. ... 34

Figure ‎2.15: Flexural modulus of two and three layered systems with dissimilar materials.

... 35

Figure ‎2.16: Flexural modulus of symmetric and asymmetric systems with similar volume

fraction of phases (MMC in black and 834 in white). ... 36

Figure ‎3.1: DSC curve of the municipal plastic waste light fraction used. ... 43 Figure ‎3.2: FTIR spectrogram of the municipal plastic waste light fraction used. ... 44 Figure ‎3.3: Typical SEM micrographs of the recycled light fraction plastics: (a) without

compatibilizer and (b) with 5 wt.% of EOC. The arrows indicate typical domain sizes in SEM micrographs. ... 47

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Figure ‎3.4: SEM micrograph of composites with 40 wt.% wood flour: (a,c) without

coupling agents and (b,d) with additives (5 wt.% of EOC and 5 wt.% (MAPE/MAPP : 80/20)) at different magnifications. ... 48

Figure ‎4.1: Load distribution in a three-layered composite under three point bending test.

... 61

Figure ‎4.2: SEM micrographs of: a) 40-0-40 and b) 0-20-10 samples. The arrows indicate

the position of the interface between two composite layers. ... 64

Figure ‎4.3: Optical micrographs of fractured samples for: a) 0(3)-40(4)-20(2) and b)

20(4)-0(3)-40(2). ... 65

Figure ‎4.4: Three-point bending of a beam with two different directions of flexural load: a)

40(2)-20(3)-0(4) for 3.3% deformation and b) 0(4)-20(3)-40(2) for 4.8% deformation. ... 71

Figure ‎4.5: Typical flexural stress-strain curves for different configurations of

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Nomenclature

AFM Atomic force microscopy

C1 Wood content of layer 1

D Ductility

DCP Dicumyl peroxide

Dev. Model standard deviation

DSC Differential scanning calorimetry

E Tensile modulus

E1 Flexural modulus of layer 1

Ee Experimental modulus of flexion

Ef Flexural modulus

EOC Ethylene-octene copolymer

EPDM Ethylene-propylene-diene-monomer

EPM ethylene-propylene copolymer

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Et Theoretical modulus of flexion

Etm Torsion modulus

EVA Ethylene-vinyl-acetate

FTIR Fourier transform infrared

HDPE High-density polyethylene

HDS hexadecyltrimethoxy-silane

I1 Moment of inertia for layer 1

MA-EPDM Maleated ethylene-propylene-diene monomer

MAPE Maleated polyethylene

MAPP Maleated polypropylene

MIR Mid infrared

MPS Methacryloxypropyltrimethoxy

MRPS Mercaptoproyltrimethoxy

MSW Municipal solid waste

NIR Near infrared

PE Polyethylene

PET Polyethylene terephthalate

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PLA Poly(lactic acid)

PP Polypropylene

PS Polystyrene

PVC Polyvinyl chloride

SBS Styrene-butylene-styrene block copolymer

SEBS Styrene-ethylene-butylene-styrene tri-block copolymer

t1 Thickness of layer 1

WPC Wood plastic composite

y0 Neutral axis position

ΔG mix Energy of mixing

ΔH mix Heat of mixing

ΔS mix Entropy of mixing

Ε Tensile elongation at break

ε max Flexural strain at maximum stress

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Chapter 1. Plastic Recycling

1.1 Importance of plastic recycling

During the past decades, production of plastic based materials has attracted increasing attention due to their relatively low price, low density, easy processing conditions, durability and good mechanical characteristics. Plastics are used in a number of applications such as coating, wiring, packaging, as well as automotive and construction industries. In 2011, around 280 million tons of plastic were produced and almost 50% of this material was for single-use disposable applications such as packaging, agricultural films and disposable consumer items [1]. As a result, recycling of polymers has recently emerged as a global concern. Figure 1.1 shows the trends of municipal solid waste (MSW) generation and recycling in USA during the past decades. Plastics represent a significant portion of MSW. In USA, plastic waste represents 12.4% (31 million tons) of the waste stream in 2010 [2]. However, only 8% of plastic waste generated in 2010 was recycled.

Figure 0.1: MSW generation and recycling in USA from 1960 to 2010 [2].

0 50 100 150 200 250 300 1960 1970 1980 1990 2000 2010 M ass (m il li on tonn es) MSW Production MSW Recycling

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In the past, landfill disposal used to be one of the most common methods to deal with plastic wastes. However, increasing cost and diminishing spaces in landfills have driven some considerations to find alternative methods [3]. Furthermore, landfill disposal creates health and safety hazards, as well as damage to the environment. This is why the next section of this document is devoted to different recycling methods in opposition to landfill disposal.

1.2 Methods for recycling of thermoplastics

Several plastic recycling routes can be used regarding type and quality of plastic waste and market demands. Some methods include energy recovery from waste plastic, while in others plastic materials are recovered. Figure 1.2 shows a schematic representation for the life cycle (including fabrication, service life and recycling) of plastic materials [4].

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1.2.1 Re-use and primary recycling

Considering the short service life of most plastic materials, especially in packaging applications, re-use is an option to extend the life cycle of such materials. Re-using consumes less energy compared to other recycling techniques, which makes it a more preferable approach [4].

Primary recycling (re-extrusion) includes introduction of single polymer plastic waste to an extrusion process in order to fabricate similar plastic parts. This type of recycling is only available when the waste is clean (or semi-clean) or made of plastic parts with similar formulations. These criteria made primary recycling an unpopular option for the industry. One example of primary recycling is re-extrusion of plastic parts that do not meet the desired specifications (quality control) of final products in industries.

1.2.2 Energy recovery

This method includes combustion for energy production in the form of heat, steam and electricity. Currently, incineration is considered as the prevailing outlet of waste management in plastic recycling. Table 1.1 presents the calorific value of some plastics in comparison with common fuels. It is shown that waste plastics have high calorific values which introduces them as a convenient energy resource. It is also reported that incineration of plastic wastes leads to significant (90-99%) reduction in their volume. However, environmental dilemma associated with this approach, mainly emission of certain air pollutants such as CO2, NOx and SOx, is an incentive to find other options for material

recycling [5-6].

Table 0.1: Calorific values of major plastic waste compared with common fuels [7,8].

Item Caloriﬁc value (MJ/kg)

Polyethylene 43.3–46.5 Polypropylene 46.50 Polystyrene 41.90 Kerosene 46.50 Gas oil 45.20 Heavy oil 42.50 Petroleum 42.3 Household PSW mixture 31.8

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1.2.3 Chemical recycling (Feedstock recycling)

Chemical recycling includes advanced technology processes that are used to convert plastics into lower molecular weight materials such as liquids or gases. The products can be used as feedstock for production of new plastic materials or as fuel. In this method, the chemical structure of polymers is altered through de-polymerization process which results in minimum amount of waste, as well as high product yield [3]. De-polymerization processes include pyrolysis, hydrolysis, gasification, liquid-gas hydrogenation, viscosity breaking and steam or catalytic cracking. Figure 1.3 presents common processes for chemical recycling of plastic waste and their main products. The main advantage of chemical recycling is the ability to use several (heterogeneous) polymers with limited need for pre-treatment.

Figure 0.3: Common processes used for chemical recycling of plastic waste and their main

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1.2.4 Mechanical recycling

This type of recycling includes the process of plastic waste recovery for reprocessing and production of new plastic materials through mechanical methods. Many products (grocery bags, pipes, gutters, window and door panels, etc.) are currently being produced using this type of recycling. Mechanical recycling of plastics involves several treatments and steps. For example, Aznar et al. [9] developed one of the most general schemes including the following steps:

- Cutting/shredding of large plastic parts to form small flakes,

- Separation of dust and other contaminants which is usually done in a cyclone, - Separation of different types of plastic wastes,

- Washing and drying of plastic flakes. Washing is usually performed in water (chemicals can also be used in case of contaminants such as glue, oils, etc.),

- Gathering and sorting of the products (for further processing), - Extrusion, quenching and pelletizing of recycled plastics.

In mechanical recycling, the plastic waste is preferred to contain as few types of polymers as possible to provide products with good homogeneity and characteristics. In case of waste streams with high contamination contents or high diversity of plastics, it is more difficult to use mechanical recycling. Therefore, separation is a critical step in fabrication of high-quality products via mechanical processing. In this case, different separation techniques are developed including manual sorting: triboelectric, mid infrared (MIR), near infrared (NIR), selective dissolution/precipitation technique, and density segregation [10-11]. Nevertheless, industrial applications of these techniques appear unlikely to meet the ideal separating expectations in terms of precision and economic efficiency. Some of the most important separation techniques are summarized in Table 1.2.

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Table 0.2: Common separation techniques for plastic recycling [5].

Method Separation property Comments

Manual sorting Only for large items Very labor intensive, bad working environment

Triboelectric Based on electrostatic charge

Only for clean, dry and non-surface-treated products

Mid infrared (MIR) Fundamental vibrations Surface sensitive, can measure black items, expensive

Near infrared (NIR) Fundamental vibrations Not applicable for dark or black products, expensive

Density sorting Large difference in density Fillers may alter density. Low cost

For the moment, sorting techniques based on density segregation in a float-sink tank or hydrocyclone are considered as the most industrially efficient methods in recycling process [12]. Using this technique, the plastic flakes are separated in two fractions with respect to water density. Generally, the light fraction comprises polyethylene and polypropylene, while the heavy fraction mainly contains polymers with higher densities including polyvinyl chloride, polystyrene and polyethylene terephthalate (Figure 1.4). Floatation is a suitable method for waste plastic separation for three reasons: 1) it is an easy and fast procedure, 2) separated plastics (especially PE and PP in light fraction) have chemical similarity and 3) absence of solvents and simplicity of this technique makes it a favorable method from environmental and economic points of view. Nevertheless, owing to the similar properties of PE and PP, it is technically uneconomical to perform additional separations on the light fraction [13].

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Figure 0.4: Schematic representation of plastics separation by a floatation technique [14].

1.3 Compatibility of polyethylene and polypropylene

Although the chemical structure of the plastics in the light fraction is considered relatively similar, the mechanical properties of the recycled blend (of PE and PP) are still lower than for neat plastics [15]. Blends of PE and PP are reported to have low tensile elongation at break and impact strength. This behavior is related to the fact that most polymers are basically immiscible (or have limited miscibility) which is caused by differences in their chain configuration [16]. It is reported that even different grades of polyethylene do not show complete miscibility when blended [17]. Lednicky [18] reported that introduction of

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20% PP in low density polyethylene led to high phase separation in the blend. Low voltage scanning transmission electron microscopy (LV-STEM) of such blends (Figure 1.5) showed that the PP phase formed spherical domains which proved high level of interfacial tension.

Figure 0.5: LV-STEM of low density polyethylene/PP (80/20) blends (frame width: 9.7

μm) [18].

Clemons [19] studied the effects of different concentrations of virgin polypropylene and polyethylene on the mechanical properties of their blends. He reported that inclusion of only 25% of polypropylene reduced tensile elongation at break of polyethylene from over 1200% to less than 550%. Notched Izod impact energy of PE also decreased from 125.8 J/m to 35.3 J/m after incorporation of PP. These observations were considered as a result of low compatibility between both phases in the blend.

It is reported in the literature that low compatibility between PE and PP is due to their positive energy of mixing (ΔGmix). Energy of mixing can be calculated as [20]:

ΔGmix = ΔHmix - T ΔSmix (1.1)

When blending two high molecular weight polymers, the gain in entropy (ΔSmix) is

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9 negative (the mixing must be exothermic). Negative heat of mixing only occurs in case of specific interactions between the blend components. Three types of blends can be distinguished regarding phase compatibility:

- Completely miscible blends: in this case, the heat of mixing is negative due to high level of interaction between the phases and homogeneity is observed at the nanometer scale or even molecular scale.

- Partially miscible blends: a small part of polymers is dissolved in the other. In this type of blends, the homogeneity (and characteristics) of the blend are relatively high, an interface is observed between phases and interfacial adhesion is good. - Fully immiscible blends: this type of blends has low homogeneity and poor

interfacial adhesion between the phases. The interface is sharp and easy to distinguish, and mechanical properties are poor. PE/PP blends are well-known examples of such blends.

1.4 Compatibilization methods for PE/PP blends

Compatibilization of plastic blends includes modification of the interface to reduce the interfacial tension and phase separation in the melt. Several compatibilizers and impact modifiers can be used to upgrade waste plastics blend during mechanical recycling. Such compatibilizers can reduce the interfacial tension between each plastic through chemical (reactive) and physical (nonreactive) effects [13].

1.4.1 Reactive compatibilization

In reactive compatibilization of plastic blends, functional or reactive additives are used to interact with both components. Usually a polymer, which is chemically identical with one of the components, is functionalized to create chemical reactivity with the other phase. Chemicals with such chemical groups are presented in Figure 1.6. Chemical bonding between phases in this technique allows the components to be held together by covalent bonds. PE, PP and ethylene-propylene-diene monomer functionalized by maleic anhydride or acrylic acid or poly(ethylene-co-propylene) grafted with succinic anhydride are

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examples of this technique. However, these grafted polymers are preferred for compatibilization of hydrocarbon polymers (such as PE and PP) with polar polymers (such as polyethylene terephthalate) [21].

Figure 0.6: Chemicals with functional groups for chemical compatibilization [4].

Another approach for reactive compatibilization of plastic blends is through incorporation of suitable monomers and initiators to perform compatibilization reactions by reactive extrusion. Free radical initiators such as organic peroxides are a common example for this method [22]. During processing of PE/PP blends with peroxides, PE tends to crosslink while PP degrades. In this condition, introduction of co-reactants is believed to promote production of PE-PP copolymers. Cheung et al. [23] studied the effects of compatibilization of linear low density polyethylene/PP (50/50) blends through injection of an organic peroxide (2,5-dimethyl-2,5-bis-(t-butyl-peroxy1)hexyne-3) during extrusion. Using scanning electron microscopy, they reported that maximum domain size of the dispersed

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1.4.2 Nonreactive compatibilization

In this method, compatibilization is achieved through addition of graft or block copolymers containing segments identical or compatible with blend components. Efficiency of such compatibilizers depends on their preferential location at the interface. Some examples of nonreactive compatibilizers include styrene-ethylene-butylene-styrene tri-block copolymers (SEBS) [24], ethylene-propylene-rubber (EPR) [25-28], ethylene-propylene-diene-monomer (EPDM) [19,29,30] and ethylene-vinyl-acetate (EVA) [24,31]. According to the literature, diblock copolymers containing segments identical to the blend components are more suitable in comparison with triblock and graft copolymers [5]. Nonreactive compatibilizers have been extensively studied for modification of PE/PP blends regarding their low cost, easy processing conditions and high performance.

Clemons [19] studied the effects of EPDM inclusion as a compatibilizer for blends of virgin PE with different concentrations of PP. From SEM observations, he reported that blends of PE/PP (50/50) had a co-continuous morphology in which the PE domains could be easily distinguished from PP in blend without compatibilizer (Figure 1.7). Inclusion of 10% EPDM was shown to increase the compatibility between PE and PP significantly. The domains were not distinguishable for compatibilized blends. Incorporation of 10% EPDM also resulted in 43% enhancement in tensile elongation at yield of the blend. It was also reported that tensile strength and modulus of PE/PP (50/50) blends decreased by 17% and 27%, respectively, due to the elastomeric nature of the EPDM phase (rubber).

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Figure 0.7: SEM micrographs from fractured surfaces of fractured impact specimens of

PE/PP: 50/50 blend (a) without and (b) with 10% EPDM [19].

Souza and Demarquette [24] also added different compatibilizers including EPDM, EVA and SEBS to the blends of PE/PP. SEM micrographs revealed that all blends showed droplet dispersion morphology type. Small amplitude oscillatory shear analysis was also performed to determine interfacial tension between PP and PE. Increase in compatibilizer concentration led to an exponential decrease in both interfacial tension between the phases and average droplet radius in the blends. EPDM was also shown to be more efficient in comparison with other compatibilizers.

Recently, ethylene-octene-copolymer (EOC) has been synthesized by chain shuttling technology. These copolymers consist of crystallizable ethylene/α-olefin blocks with very low comonomer content and high melting temperature, as well as amorphous ethylene/α-olefin blocks with high comonomer content and low glass transition temperature. It is believed that the crystalline and amorphous segments of EOC are compatible with polyethylene and polypropylene phases, respectively. Such characteristic has introduced EOC as a promising compatibilizer for PE/PP blends [32].

Lin and co-workers [33] investigated the effects of incorporation of several compatibilizers on blends of PP/HDPE (70/30). The compatibilizers used were a multi-block ethylene– octene copolymer (EOC), two statistical octene copolymers, two

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13 propylene rubber (EPR) and a styrenic block copolymer (SBC). They reported that addition of multi-block EOC resulted in best combination of low brittle-to-ductile transition and high toughness. Morphological observations through atomic force microscopy (AFM) showed that the compatibilizer was preferentially located at the interface between polypropylene and high density polyethylene (Figure 1.8). Dark regions around HDPE domains indicate the presence of the compatibilizer at the interface.

Figure 0.8: AFM micrographs of PP/HDPE compounds (a) without compatibilizer and (b)

with multi-block EOC (frame width: 20 μm) [33].

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Chapter 2. Natural Fiber Composites

Polymers usually have relatively low mechanical properties, modulus and strength, compared to other solids. One of the most acceptable methods for enhancement of their properties is through inclusion of a dispersed phase with high stiffness and strength. The dispersed phase is usually in form of long/short fibers or even small (micro or nano size) particles [34-36].

2.1 Natural vs. artificial reinforcements

Artificial reinforcements (glass, talc, calcium carbonate, etc.) are proven to have higher mechanical properties compared to natural reinforcements (specifically plant or vegetable fibers). Natural fibers, on the other hand, are less expensive and are considered eco-friendly materials. Table 2.1 presents the energy consumption for production of selected natural fibers in comparison with glass fiber [37]. It is clear that production of natural fibers, such as flax, consumes significantly less nonrenewable energy (9.55 MJ/kg) than glass fiber (54.7 MJ/kg).

Table 0.1: Energy consumption (MJ/kg) for production of different fibers [37].

Glass fiber mat Flax fiber mat China reed fiber

Raw materials 1.7 Seed production 0.05 Cultivation 2.50 Mixture 1.0 Fertilizers 1.00 Transport plant 0.40 Transport 1.6 Transport 0.90 Fiber extraction 0.08 Melting 21.5 Cultivation 2.00 Fiber grinding 0.40 Spinning 5.9 Fiber separation 2.70 Transport fiber 0.26 Mat production 23.0 Mat production 2.90

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Joshi et al. [37] reviewed the comparative life cycle assessment studies of selected natural fiber and glass fiber composites. The study covered the environmental aspects of both composites through their life cycle considering their production, use and end of life management options (i.e. recycling, incineration and disposal). They suggested two different generic life cycles for glass fiber and natural fiber reinforced composites as presented in Figure 2.1. They concluded that natural fibers are environmentally superior compared to glass fiber because: 1) production of natural fibers leads to lower environmental impact compared to glass fiber, 2) natural fibers can be used at high concentrations (due to their low price and density) which results in lower amount of polymers (petroleum based) in the final products, 3) lower density of natural fibers results in better fuel efficiency in automotive applications, and 4) end of life incineration of natural fibers leads to energy recovery.

Figure 0.1: Generic life cycles of (a) glass fiber and (b) natural fiber reinforced composites

[37].

It is notable that some natural fibers (such as pineapple and banana leaf fibers) are not even considered as products. Such materials are agricultural waste which can be purchased at

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17 very low prices. Such advantages, combined with low density and low abrasiveness, have recently driven a considerable number of researchers to develop engineered products containing naturally made fillers [38-40].

2.2 Characteristics of natural fibers

Lignocellulosic fibers are usually grouped into three types depending on their source: 1) seed hair (cotton), 2) bast fibers (jute and flax fiber), and 3) leaf fibers (sisal and abaca). Mechanical properties of natural fibers vary with chemical composition which depends on their type, age and climatic conditions. Components of natural fibers include cellulose, hemi-cellulose, lignin, waxes, water soluble components and pectin. Each natural fiber is a bundle of cellulose fibers which are covered with hemi-cellulose and lignin as matrix. Table 2.2 shows chemical compositions of selected lignocellulosic materials [41].

Table 0.2: Chemical composition of lignocellulosic fibers (weight %) [41].

Fiber Cellulose Lignin Hemicellulose Pectin Ash

Hemp 57-77 3.7-13 14-22.4 0.9 0.8

Jute 41-48 21-24 18-22 - 0.8

Flax 71 2.2 18.6-20.6 2.3 -

Sisal 47-78 7-11 10-24 10 0.6-1.0

Kenaf 37-49 15-21 18-24 - 2-4

Cellulose is considered the main component of all natural fibers. The chemical structure of a typical cellulose molecule is shown in Figure 2.2 [42].

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Figure 0.2: Chemical structure of a repeating unit in cellulose molecule [42].

Thermosets, thermoplastics and even elastomers are commonly used as matrices for natural fiber composites. Among these, thermoplastics have attracted more attention because of their ease of reprocessing and recycling. Other advantages of thermoplastics include their design flexibility and simple processing methods. Thermosets and elastomers, on the other hand, have crosslinked structures which do not allow them to be reprocessed by conventional methods. Among thermoplastics, however, only a handful can be used as matrix for natural fibers. Since natural fibers are prone to thermal degradation at high temperature, thermoplastics with high processing temperatures (higher than 200°C) cannot be used easily as matrix. This is why polyethylene and polypropylene (low melting points) are the most commonly used matrices for natural fiber composites [43].

Presence of high concentrations of hydroxyl groups on different components of natural fibers (especially cellulose) results in their hydrophilic behavior which leads to low surface adhesion with hydrophobic polyolefin matrices (such as polyethylene and polypropylene). Main disadvantages of low surface interaction are reduced mechanical properties (due to phase separation and also low homogeneity in composite) and high water absorption in natural fiber composites [44-46].

Rachini and coworkers [47] studied the effects of hemp fibers in polypropylene. Via morphological observations of fractured surfaces, they concluded that hemp has low surface interaction with PP. It is clearly shown in Figure 2.3 that the surface of natural fibers is completely clean from matrix and gaps exist at the interface between both phases.

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Figure 0.3: SEM micrographs from fractured surfaces of hemp filled PP [47].

Sgriccia et al. [48] reported that adding only 15% of natural fibers led to significant increase in water absorption of epoxy based composites, while glass filled composites showed lower water uptakes (Figure 2.4). Hydrophilic behavior of natural fibers results in higher water uptake. Low compatibility between matrix and fibers also increases the ability of water molecules to penetrate through the composite.

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2.3 Modification of natural fiber composites

Several methods have been developed to modify the surface interaction between cellulosic fibers and polyolefins: 1) surface modification of the fiber, 2) modification of the matrix phase (through grafting active groups), and 3) addition of a third (compatibilizer) phase [40].

2.3.1 Surface modification of natural fibers

Surface of natural fibers can be modified via different (chemical or physical) methods.

2.3.1.1 Chemical surface treatment of natural fibers

Silane treatment is one of the most frequently used chemical modifications to increase surface adhesion between lignocellulosic fibers and polymers. Silanes are chemical compounds (with SinH2n+2) and are also commonly used to compatibilize glass fiber with

polymers. In the presence of moisture, silanols are produced through hydrolyzation of alkoxy groups. Silanol groups then react with hydroxyl groups on the surface of natural fiber and create stable covalent bonds with the cell walls. Therefore, hydrophilic behavior of the fiber decreases which leads to enhanced fiber/matrix interaction. An example of silane treatment of natural fibers is as follows [41]:

CH2CHSi(OC2H5)3 + H2O → CH2CHSi(OH)3 + 3 C2H5OH (2.1)

CH2CHSi(OH)3 + Fiber-OH → CH2CHSi(OH)2O- Fiber + H2O (2.2)

Abdelmouleh et al. [44] studied the reinforcement of low density polyethylene (LDPE) and natural rubber using different types of cellulosic fibers. Cellulose fibers were added to the matrices before and after chemical treatments using three silane coupling agents namely: γ-methacryloxypropyltrimethoxy (MPS), γ-mercaptoproyltrimethoxy (MRPS) and hexadecyltrimethoxy-silane (HDS). They proposed the following schematic illustration (Figure 2.5) to explain the interaction between LDPE and cellulosic fibers treated by MPS.

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Figure 0.5: Schematic representation of interaction between LDPE and MPS-modified

fiber [44].

Increase interaction between matrix and cellulosic fiber was confirmed by morphological and mechanical characterizations. Morphological observations revealed that surface treatment with silanes increase the compatibility between LDPE and cellulosic fibers (Figure 2.6). It is seen in Figure 2.6 that the surface of treated fiber is completely covered with the polymeric matrix.

Figure 0.6: Effect of cellulosic fibers surface treatment with MPS: a) untreated and b)

treated fiber [44].

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22

Enhancement in surface interaction between lignocellulosic fibers and polyolefins can also be achieved via introduction of an acetyl functional group (CH3COO-) to the surface of the

fibers. Acetylation of natural fibers with acetic anhydride (CH3-C(=O)-O-C(=O)-CH3), for

instance, substitutes hydroxyl groups on cellulose molecules with acetyl groups which leads to hydrophobic behavior of fibers [41]. The reaction is as follows:

Fiber – OH + CH3-C(=O)-O-C(=O)-CH3 → Fiber – OCOCH3 + CH3COOH (2.3)

Acetic acid (CH3COOH) is produced as a by-product of the reaction which must be

removed from the fiber before introduction into polyolefins.

Rong et al. [49] studied the effect of sisal fibers acetylation to reinforce epoxy resins. Acetylation was performed by a 50% acetic acid aqueous solution for 5 minutes (fiber/solution ratio: 1/25). The authors claimed that improved interfacial bonding is due to creation of hydrogen bonds between acetyl groups (on fiber surface) and hydroxyl or amine groups in the epoxy resin.

Maleated coupling agents can also be used for surface covering of natural fibers in order to compatibilize them with polymers. In this approach, natural fibers are soaked in a solution of maleic anhydride or maleated polymers. Maleic anhydride groups react with hydroxyl groups on the surface of cellulosic fibers which results in decreased hydrophilic behavior. Figure 2.7 represents the reaction of maleated polypropylene (MAPP) with cellulose molecules [50].

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Figure 0.7: Surface treatment of natural fibers with maleated polypropylene [50].

Modified surface of lignocellulosic fibers allows better interaction with thermoplastic matrices through decreased fiber hydrophilic behavior and also physical entanglement of PP chains with the matrix molecules.

Mohanty and Nayak [51] investigated the effects of natural fibers surface covering with MAPP in PP/jute composites. They immersed jute fibers in MAPP/toluene solution at 100°C. They reported that surface covering of jute fiber in MAPP solution (with 5% MAPP) led to increased tensile strength in the composites with 30% of jute. Tensile strength of composites increased from 24.4 MPa to over 31 MPa.

2.3.1.2 Physical surface treatment of natural fibers

Alkalization (or mercerization) is a common method for physical treatment of natural fiber surfaces. In this method, lignocellulosic fibers are immersed in aqueous NaOH solution for a period of time. NaOH solution dissolves lignin, wax and oils from the fiber surface and leaves a clean and porous cellulosic surface. This treatment leads to higher specific mechanical properties since cellulose has much higher mechanical properties compared to lignin and also increases specific surface area leading to better interaction with the matrix. The main drawback of this approach is increased hydrophilic behavior of natural fibers

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which happens due to higher concentration of hydroxyl groups on cellulose molecules compared to lignin. This is why this method is usually proposed along with other chemical modification methods such as addition of silanes or maleic anhydride [40].

Sgriccia et al. [48] studied the effect of alkali treatment on morphological and water absorption properties of several natural fibers. The fibers were submerged in 5% solution of sodium hydroxide for one hour at room temperature. They concluded, via SEM microscopy, that alkali treatment of hemp fiber led to hemicellulose and lignin removal from the fiber surface. As presented in Figure 2.8, surface of hemp fibers is much clearer after alkalization. The authors reported however that fiber alkalization increased their water uptake. After 700 hours of immersion in distilled water, epoxy composites based on untreated hemp absorbed around 16% water, while alkali treated hemp had a water uptake of around 22%.

Figure 0.8: SEM micrographs of (a) untreated and (b) alkali treated hemp fibers [48].

2.3.2 Modification of polymeric matrix

Modification of the matrix is also a common method to enhance surface interaction between lignocellulosic fibers and polyolefins. This type of compatibilization is usually

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25 achieved via two different approaches: (1) inclusion of a maleated polymer to the matrix and (2) chemical modification of the matrix by grafting active groups (especially maleic anhydride). In both methods the active group (maleic anhydride) reacts with the hydroxyl groups on the natural fiber surface, while the polymeric chains entangle with the matrix [40,52].

Efficiency of maleic anhydride grafted polymers (as compatibilizer) depends on acid number and molecular weight. Macromolecules with longer chains have better ability to entangle with the matrix. Acid number, on the other hand, represents the concentration of maleic anhydride groups in maleated polymers. Needless to say that molecules with higher acid number provide higher interaction with natural fibers.

Keener et al. [52] studied the effects of different grades of MAPP as compatibilizer in PP/agro-fiber (jute and flax) composites. They reported that addition of different types of MAPP increased the mechanical properties of both PP/jute and PP/flax composites, while the increase was more significant for specific grades of MAPP. For instance, adding 3% of Epolene G-3003 and Epolene E-43 to PP/flax (70/30) increased tensile strength by 50% and 38%, respectively. It is reported that the acid number of Epolene E-43 is 5 times higher and its molecular weight is 80% lower compared to Epolene G-3003. They also supported their findings with morphological observations. It is clear in Figure 2.9 that adding MAPP resulted in increased surface interaction between natural fibers and PP as less fiber pull-out is seen and the natural fiber surfaces are covered with matrix molecules (Figure 2.9-b).

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Figure 0.9: SEM micrographs of PP/agro-fiber composites (a) before and (b) after

compatibilization with MAPP [52].

2.4 Matrices for natural fiber composites

Selection of a suitable matrix for natural fibers is a vital step in the fabrication of composites. Shape, environmental tolerance, surface appearance and total durability of natural fiber composites are controlled by the matrix phase. Currently, 80% of the composite matrices are based on non-renewable petroleum resources which have caused concerns due to environmental issues [53]. Generally, two different approaches have attracted attention to decrease these environmental issues caused by the production of natural fiber composites.

2.4.1 Bio-based polymers

Bio-based plastics are suggested as an important substitute to petroleum based plastics to reduce the dependence on petroleum and also decrease the environmental impacts coupled with use of petroleum resources. Several new polymers have been developed from renewable resources. Figure 2.10 presents current and emerging plastics as matrices for composites regarding their biodegradability [53]. It is clear that bio-based plastics are environmentally degradable in comparison with common petroleum based plastics.

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Figure 0.10: Current and emerging matrices for natural fiber composites and their

biodegradability [53].

Poly(lactic acid) (PLA) has been investigated as a matrix for fabrication of kenaf fiber reinforced composites by a number of researchers. PLA is a bio-based material that can be produced from lactic acid (a fermentable sugar) and polyhydroxyalkanoate (PHA). Huda et al. [54] studied the mechanical properties of kenaf fiber reinforced poly(lactic acid) laminated composites. They also investigated the effects of compatibilizing kenaf fibers with PLA through alkalization and silane-treatments. They reported that inclusion of both silane-treated and alkali-treated kenaf fibers led to increased PLA mechanical properties. Alkali treatment resulted in 50% improvement in impact strength of surface-treated

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composites, while silane treatment resulted in more improvement in flexural performance. The flexural modulus and strength of surface-treated composites increased up to 69% and 50%, respectively.

Starch is also a naturally occurring polymer which has attracted attention as a plastic matrix for natural fiber composites. Natural fiber composites based on biodegradable starch matrix and sisal fibers were made by Alvarez et al. [55-57]. The thermal, rheological and creep properties of the composites were studied extensively.

2.4.2 Use of waste plastics

As mentioned in Chapter 1, recycling of plastic materials is a promising approach to deal with environmental impacts caused by waste plastics. Among all waste plastic materials, polyethylene and polypropylene have attracted a great deal of attention due to ease of separation from other polymers and processing in the form of PE/PP blends. Many researchers have also investigated the possibility of such blends to serve as the matrix phase for natural fiber composites.

Clemons [19] studied the characteristics of composites based on PE/PP blends containing wood flour. Although he did not use recycled materials, the fact that the matrix was a blend of PE and PP simulates the recycling conditions. He studied the effects of maleated compatibilizers on surface interactions between wood flour and matrix and also between different phases in the matrix (PE and PP). He reported that inclusion of maleated ethylene-propylene-diene monomer (MA-EPDM) as compatibilizer between all three phases (PE, PP and wood flour) leads to increase in strength and deformability of wood plastic composites. Tensile strength of composite with PE/PP (75/25) as matrix and 30% wood flour increased from 23 to 25 MPa after inclusion of 10% MA-EPDM. However, tensile modulus of the composite decreased around 32% due to inclusion of the rubber phase.

Najafi et al. [58] also fabricated composites based on virgin and recycled plastics (PE and PP) and 50% wood sawdust. They concluded that tensile modulus of composites containing recycled PE/PP (50/50) as matrix (6.5 GPa) was comparable with composites based on

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29 virgin PE/PP (50/50) matrix (7.1 GPa). Tensile strength of composites with recycled PE/PP blend as matrix was less than 10 MPa compared to around 11 MPa for composites with the virgin matrix.

Gao et al. [59] fabricated wood plastic composites based on PP/PE (80/20) blends containing 60% wood flour. In order to increase the compatibility between the thermoplastic matrix and natural fibers, they grafted maleic anhydride groups on PE and PP through twin-screw extrusion in presence of dicumyl peroxide (DCP) as initiator. The authors reported that increasing maleic anhydride concentration up to 1% resulted in higher mechanical properties such as flexural and impact strength. Flexural modulus of the composite with 0.5% maleic anhydride (5.1 MPa) was lower than the unmodified composite (5.8 MPa), while inclusion of higher concentrations of maleic anhydride resulted in increased modulus. Incorporation of 1.5% of maleic anhydride resulted in flexural modulus of 6.2 MPa.

Dintcheva and La Mantia [60] performed one of few researches dealing with separation and recycling of true light fraction of plastic waste stream and studied the effects of inclusion of wood flour to such blends. They reported that inclusion of wood flour led to increased tensile modulus of PE/PP blend, while tensile strength, elongation at break and impact strength decreased. For instance, tensile modulus increased from around 600 MPa to around 900 MPa when wood content increased from 20 to 40%, while tensile modulus decreased from 12.5 MPa to 11.5 MPa.

Dintcheva et al. [61] also studied the effects of different filler types (namely wood fiber, glass fiber and calcium carbonate) and processing equipment (discontinuous mixer, single and twin-screw extruder) on the mechanical properties of light fraction based composites. They reported that inclusion of 20% of all types of fillers led to similar effects on the mechanical properties of the composites as presented in Figure 2.11. They also reported that although mechanical properties of light fraction (with no filler) was not affected by the processing method used, wood fiber based composites were shown to have higher mechanical performance after injection molding in comparison with compression molding.

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This might be due to increased fiber alignment caused by higher levels of shear stress during injection molding.

Figure 0.11: Elastic modulus (E), tensile strength (TS), elongation at break (EB) and

impact strength (IS) of light fraction (LF) based composites with different fillers (WF: wood fiber, GF: glass fiber) [61].

2.5 Structural design of composite materials

Composite materials are extensively used in many modern constructional applications to provide certain characteristics. Structural modification of composites, in terms of multilayered structures results in fabrication of more efficient designs which provide great potential in comprehensive functions. For multilayered structures, the flexural properties (such as stiffness) are dependent on layer configuration and are no longer determined via the simple rule of mixture. This behavior can be used in optimization of layer configuration, thickness and stacking sequence [62].

Today, multilayered structures are attracting increasing attention in several applications due to their efficiency and advantages regarding load distribution. These structures can have layers for special purposes such as damping, decrease density or protection from environmental effects. Key feature of such structures include inhomogeneous distribution of mechanical properties though the thickness. Such structures can be produced using a

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31 wide range of materials such as metal alloys, ceramics, polymers, foam and wood (plywood). Tables 2.3 and 2.4 present some applications with positive/negative effects of such structures, respectively [63].

Table 0.3: Applications of multi-layered structural composites [63].

Branch of industry Application

Rocket construction Aircraft construction Machine building Automotive industry Medical equipment

Load-carrying structural elements, fuel tanks, aerial elements Tail assembly, stabilizers, inner lining of cabins

Transmission cases, gear wheels, machine elements

Wheel rims, trunkcovers, hoods, steering columns, inner lining of cabins Implants, artificial joints

Sports industry Surfing, skis, clubs, canoes Telecommunication Parabolic aerials

Oil production Civil engineering

Elements of frames for offshore drilling rigs Facing materials

Energetics

Industrial engineering

Rotor blades of wind power stations Reservoirs, pipelines

Table 0.4: Advantages and disadvantages associated with application of multilayered

structures. [63].

Advantages _{Disadvantages}

High rigidity characteristics relative to mass Thermo-insulation

Sound-proofing

High fatigue characteristics High corrosion resistance

Loss of strength due to aging of adhesive joints High technological requirements to the accuracy of production

Necessity of modifying the methods of non-destructive testing of structures

High sensitivity to impact loads Brittleness

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Decrease in the number of assembling operations due to development of more complex components

-

In case of reinforced plastics, fabrication of multilayered structures suffers from a lack of information about the effects of design parameters on their mechanical performance. The most important design parameters for multilayer composite structures include: fiber orientation, concentration of reinforcement phase and cross-sectional arrangement of each layer.

Dyer et al. [64] studied the effects of cross-sectional design of different fiber reinforced composites for prosthodontic applications. Figure 2.12 presents the changes in elastic modulus and toughness with unidirectional R-glass fiber and different cross-sectional designs.

Figure 0.12: Effect of different cross-sectional designs on properties of composites. Lines

depict corresponding fiber placement. Superscript groups are not statistically different for each test [64].

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33 It is presented in Figure 2.12 that alteration in structure design (at equal fiber content) led to noticeable change in mechanical properties of the composites. The authors reported that flexural modulus increased when one group of glass fiber reinforcements was located in the compression side of the specimen (intrados) during flexion tests. They also reported that toughness increased when the fiber group was placed in the tension side of the specimens and also when fiber content increased.

Salomi and coworkers [65] studied the flexural performance of double-layered structural composites under different load directions. The structure contained a layer of PP/E-glass fiber (40/60) coupled with a layer of linear low density polyethylene. Figure 2.13 shows load-deflection curves of the structures under flexural load from different directions: i.e. the composite layer is in the intrados or extrados. It is clear that flexural strength and deformability were higher when the composite layer is in intrados. Beam stiffness, on the other hand, was shown to be independent of load direction.

Figure 0.13: Three-point flexion test when the composite layer is in extrados or intrados

sides (the intrados represents the position of layer placed under the load nose, while the extrados represent the position of other layer in a double-layer laminate) [65].

In addition, the authors studied the failure modes of specimens in bending tests. Figure 2.14 shows that the failure started in composite layer for both load directions. In cases where the

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composite was in extrados, tension failure occurred (Figure 2.14-a), while buckling was responsible for composite failure in intrados layer (Figure 2.14-b).

Figure 0.14: Fractured areas of specimens from flexion test with composite layer placed in

the (a) extrados side and (b) intrados side [65].

Smith and Partridge [62] studied the flexural properties of planar multilayered structures based on two dissimilar materials, namely high strength titanium alloy (Ti-834) and titanium metal matrix alloy (Ti-MMC). They studied the development and evolution of several types of two-material multilayer systems and Figure 2.15 presents the effects of changing layer configuration and thickness on the flexural modulus of such beams. As shown in Figure 2.15, the layer position strongly influenced the stiffness of symmetric and asymmetric systems.

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Figure 0.15: Flexural modulus of two and three layered systems with dissimilar materials

(different colors in these structures represent the different materials: 834 and MMC for white and black layers, respectively) [62].

For symmetric and asymmetric multilayered systems with a given volume fraction of each phase (50/50), the flexural modulus is shown to vary significantly with altering layer configuration (Figure 2.16). Maximum flexural modulus was observed when the stiffer layers (Ti-MMC) were placed as the top and bottom skins. On the other hand, the minimum stiffness was achieved when Ti-834 (less stiff) were in the top and bottom skins. For a given volume fraction of layers, all other proposed configurations where shown to have intermediate stiffness values. This proves that the flexural moduli of multilayered structures are controlled by the skin layers. They also concluded that layer position strongly changes other mechanical properties such as strength, impact toughness and damping capacity.

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Figure 0.16: Flexural modulus of symmetric and asymmetric systems with similar volume

fraction of phases (MMC in black and 834 in white) [62].

2.6 Thesis objectives and organization

Despite the extensive efforts towards improvement of PE/PP blends for plastic recycling purposes, only a limited number of research works were found in the literature dealing with true plastic waste materials. Therefore, the main objective of this research work is to adopt the current enhancing techniques i.e. polymer compatibilization, inclusion of wood fiber residue and structural design, on municipal plastic waste streams to improve their mechanical and morphological performance.

Chapter 1 presented a general overview of plastic recycling with the aim of introducing the mechanical techniques as the most appropriate method in recycling municipal plastic waste streams. In addition, the light fraction (PE/PP blend) was introduced as the major component of municipal plastic wastes. Then, the technical challenges in mechanical