Production and characterization of natural fiber-polymer composites using ground tire rubber as impact modifier

Production and characterization of natural

fiber-polymer composites using ground tire rubber as

impact modifier

Mémoire

Navid Nikpour

Maîtrise en génie chimique

Maître ès sciences (M. Sc.)

Québec, Canada

© Navid Nikpour, 2016

Résumé

Ce travail porte sur la production et la caractérisation de matériaux composites hybrides basés sur un polymère thermoplastique (polyéthylène de haute densité, PEHD), une fibre naturelle (chanvre) et un caoutchouc recyclé provenant de pneus usés (GTR) comme modificateur d'impact. L'addition d'un agent de couplage (polyéthylène maléaté) est également étudiée. Les échantillons sont mélangés par extrusion à double-vis et fabriqués par un moulage en injection. À partir des échantillons obtenus, une caractérisation morphologique et mécanique complète est effectuée. Les résultats montrent que la bonne dispersion est obtenue en raison des bonnes conditions de mélanges sélectionnées et une bonne adhésion interfaciale entre toutes les phases est atteinte en raison de la présence d'anhydride maléique greffée au polyéthylène (MAPE). Enfin, pour des propriétés mécaniques choisies, des modèles de régression non-linéaire sont proposés pour prédire et contrôler les propriétés finales de ces composés par des comparaisons faites sur la base des propriétés de la matrice seule.

Abstract

This work aims at the production and characterization of hybrid composites based on a thermoplastic polymer (high density polyethylene, HDPE), a natural fiber (hemp) as reinforcement and ground tire rubber (GTR) as an impact modifier. The addition of a coupling agent (maleated polyethylene) is also investigated. The samples are compounded by twin-screw extrusion and produced by injection molding. From the samples obtained, a complete morphological and mechanical characterization is performed. The results show that good dispersion is obtained due to the selected processing conditions and good interfacial adhesion between all the phases is achieved due to the presence of maleic anhydride grafted polyethylene (MAPE). Finally, for selected mechanical properties, nonlinear regression models are proposed to predict and control the final properties of these compounds and comparisons are made based on the neat matrix properties.

Table of content

Résumé ... III Abstract ... V Table of content ... VII List of tables ... IX List of figures ... XI Abbreviations ... XIII Symbols ... XV Acknowledgements ... XVII Foreword ... XIX Chapter 1: ... 1 1. Introduction ... 1 1.1 Waste Tires ... 1

1.2 Ground tire rubber (GTR)... 5

1.3 GTR applications ... 6

1.4 Thermoplastic Elastomers (TPE)... 6

1.4.1 Advantages and Disadvantages of TPE ... 9

1.5 GTR use in thermoplastic elastomers ... 9

1.5.1 GTR particle size ... 10

1.5.2 Adhesion of GTR ... 11

1.6 Hybrid composites based on rubber and natural fiber ... 12

1.6.1 Natural fibers ... 12

1.6.2 Mechanical properties of natural fibers ... 13

1.6.3 Advantages and Disadvantages of natural fibers ... 13

1.6.4 Surface treatment methods ... 16

1.6.4.1 Physical methods ... 17

1.6.4.2 Chemical methods ... 17

1.6.4.3 Compatibilizing agents ... 20

1.7 Thesis objectives ... 23

2. Effect of coupling agent and ground tire rubber content on the properties of natural fiber polymer composite ... 25 2.1 Résumé ... 26 2.2 Abstract ... 27 2.3 Introduction ... 28 2.4 Materials ... 29 2.5 Compounding ... 29 2.6 Characterization ... 32 2.7 Empirical models ... 32

2.8 Results and discussion ... 33

2.8.1 Morphology ... 33

2.9 Mechanical properties... 35

2.10 Properties modeling ... 39

2.11 Goodness of fit and prediction of the model ... 41

2.12 Refinement and Analysis of the Model ... 42

2.13 Model application ... 44

2.14 Conclusion ... 46

Chapter 3: ... 49

3. General conclusion and recommendations ... 49

3.1 General conclusion ... 49

3.2 Recommendation for future work... 50

References ... 53

Appendix A: ... 63

List of tables

Table 1.1. Overview of the world natural rubber situation (thousands tons) [1]. ... 1

Table 1.2. Overview of the world natural rubber situation (thousands tons) [1]. ... 2

Table 1.3. European tire production (1000 tons) [10]. ... 4

Table 1.4. Common thermoplastic elastomer materials [17]. ... 8

Table 1.5. Mechanical properties of natural fibers compared to man-made fibers [34]. ... 15

Table 1.6. Chemical treatments used for modification of natural fibers [53]. ... 19

Table 2.1. Sample coding and composition of the compounds produced. ... 31

Table 2.2. Mechanical properties of HDPE/hemp/GTR composites. ... 36

List of figures

Figure 1.1. U.S worn tire trends for the period 2005-2013 [2]. ... 3

Figure 1.2. U.S. scrap tire disposition for 2013 (total amount generated annually) [2]. ... 4

Figure 1.3. European tire production (1000 tons) [10]. ... 5

Figure 1.4. U.S. ground rubber markets for 2013 (percent of total pounds of ground rubber consumed) [2]. ... 6

Figure 1.5. Classification of non-wood fibers [31]... 12

Figure 1.6. Structure of a natural fiber cell. ... 13

Figure 1.7. Typical structure of a fiber-matrix interface [36]. ... 16

Figure 1.8. Morphology of a natural fiber surface before (a) and after (b) plasma treatment [35]. ... 17

Figure 1.9. Schematic representation of the interface area of epoxy matrix/silane-modified fiber [55]. ... 20

Figure 1.10. Structure of MAPE (coupling agent) and cellulosic fibers at the interface [56]. ... 21

Figure 1.11. SEM micrographs of untreated HDPE/sisal samples at two different magnifications [60]. ... 22

Figure 1.12. SEM micrographs of treated HDPE/sisal samples with MAPE at two different magnifications [60]. ... 23

Figure 2.1. SEM micrographs of the fractured surface of: (a,b) PH(55/45) at two different magnifications, (c,d) PG(40/60) at two different magnifications, and (e) PHG(85/7.5.7.5). ... 35

Figure 2.2.Transition curves to determine the properties of neat HDPE as a function of hemp and GTR contents. A) tensile strength, B) tensile modulus, C) flexural modulus, and D) impact strength. ... 45

Abbreviations

ABS Acrylonitrile-butadiene styrene

ASP -aminopropyltriethoxysilane

EPDM Ethylene propylene diene monomer

EVA Ethylene-vinyl acetate

𝐸′ _{Elastic modulus}

FP Fluoro polymers

GF Glass fiber

GTR Ground tire rubber

HDPE High density polyethylene

LCP Liquid crystal polymers

LDPE Low density polyethylene

MA Maleic anhydride

MA-g-SEBS Maleated styrene-ethylene/butylene-styrene

MAPE Maleic anhydride grafted polyethylene

MAPP Maleic anhydride grafted polypropylene

MRPS -mercaptopropyltrimethoxysilane

NFC Natural fiber composite

NR Natural fiber

PA Polyamide

PA-4,6 Polyamide-4,6

PAI Polyamide imide

PAR Polyarylate

PC Polycarbonate

PE Polyethylene

PEEK Polyetheretherketone

PEI Polyether imide

PES Polyether sulfone

PMMA Polymethylmetacrylate

POM Polyoxymethylene

PP Polypropylene

PPA Polyphthalamide

PPC Polyphthalate carbonate

PPO Polyphenylene oxide

PPS Polyphenylene sulfide

PPSU Polyphenyl sulfone

PS Polystyrene

PTFE Polytetrafluoroethylene (Teflon)

PVC Polyvinyl chloride

PVDF Polyvinyl diene fluoride

R2 _{Coefficient of correlation}

RPE Recycled polyethylene

RR Recycled rubber

RRP Reclaimed rubber polymer

SAN Styrene-acrylonitrile

SD Screw decompression

SMA Styrene-maleic anhydride

TDF Tire-derived fuel

TPE Thermoplastic elastomer

TPI Thermoplastic polyimide

UHMWPE Ultra-high molecular weight polyethylene

WF Wood flour

WPC Wood polymer composite

Symbols

F-test Fisher’s statistic test (-)

F-ratio Fisher’s statistic probability density (-)

L/D Length/Diameter (-)

P Pressure (Pa)

PB Back pressure (MPa)

P-value Fisher’s statistic probability distribution (-)

Q Year quarters (-)

R2 _{and adjusted R}2 _{Correlation coefficients (-)}

SM Shot size (mm) T Temperature (°C) VS Velocity/speed (%) X Experimental variables (-) Y Estimated response (-)  Confidence level (-) ß Regression parameter (-)

n Model coefficients (-)

 Residual error (-)

εb Elongation at break (%)

 Density (g/cm3₎

Acknowledgements

First and foremost, I would like to express my deepest appreciation to my committed supervisor, Professor Denis Rodrigue, who has the attitude and the substance of a genius: he continually and convincingly conveyed a spirit of adventure in regard to research and scholarship, and an excitement in regard to teaching.

I would also like to thank my family, especially my mother, father, sister and brother in law. This would never have been possible without their support and motivations.

I would like to thank my friends, and collogues at Université Laval for their encouragement and moral support which made my stay and studies more enjoyable.

I also acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC). The technical help of Mr. Yann Giroux was also much appreciated.

Finally, I acknowledge the Centre de Recherche sur les Matériaux Avancés (CERMA) and Centre Québécois sur les Matériaux Fonctionnels (CQMF) for technical and financial help.

Foreword

This master thesis consists of three chapters, including one article. The first chapter contains a general introduction on natural fibers, tire recycling, ground tire rubber, wood polymer composites (WPC), thermoplastic elastomers (TPE) and a literature review of each subject related to this work.

In the second chapter, various compounds with different filler concentrations (0 to 60% wt.) were made using extrusion/injection processing. Then, the effect of maleic anhydride polyethylene (MAPE) on the mechanical properties (tensile, flexural, impact) of hemp/HDPE composites is discussed. Secondly, the effect of rubber addition in the form ground tire rubber (GTR) is investigated. Finally, modeling of the mechanical properties is presented via nonlinear regression correlations. This chapter was submitted as a journal paper:

Navid Nikpour and Denis Rodrigue, Effect of coupling agent and ground tire rubber content on the properties of natural fiber polymer composites, International Polymer Processing, submitted (2016).

In the last chapter, a general conclusion on the work performed and recommendations for future works is presented

Chapter 1:

1.

Introduction

1.1 Waste Tires

In recent decades, the number of rubber (natural and synthetic) products manufactured and consumed increased substantially and Tables 1.1 and 1.2 present this increasing trend of the rubber industry.

Table 1.1. Overview of the world natural rubber situation (thousands tons) [1]. NATURAL RUBBER PRODUCTION 2013 2014 2015 Year Q1 Q2 Q3 Q4 Year Q1 Asia-Pacific 11386 2718 2389 2917 3159 11183 2622 EMEA 539 147 130 139 144 560 141 Americas 326 88 95 68 76 327 90 TOTAL 12251 2953 2614 3124 3378 12070 2853 NATURAL RUBBER CONSUMPTION 2013 2014 2015 Year Q1 Q2 Q3 Q4 Year Q1 Asia-Pacific 8229 2081 2264 2295 2261 8901 2117 EMEA 1485 396 404 387 368 1555 392 Americas 1674 434 448 420 402 1704 428 TOTAL 11388 2912 3116 3101 3030 12159 2936

Table 1.2. Overview of the world natural rubber situation (thousands tons) [1]. SYNTHETIC RUBBER PRODUCTION 2013 2014 2015 Year Q1 Q2 Q3 Q4 Year Q1 Asia-Pacific 8357 2244 2491 2481 2549 9765 2364 EMEA 4156 1051 954 978 964 3948 1028 Americas 2960 740 730 742 760 2972 745 TOTAL 15473 4035 4175 4201 4273 16685 4137 SYNTHETIC RUBBER CONSUMPTION 2013 2014 2015 Year Q1 Q2 Q3 Q4 Year Q1 Asia-Pacific 8965 2392 2564 2616 2701 10274 2456 EMEA 3687 935 913 864 843 3556 909 Americas 2815 711 746 733 2931 732 TOTAL 15467 4038 4219 4226 4278 16761 4097

A worn (used) tire is any tire that has been removed from its initial use and contains the whole or pieces of worn tires that are easily identifiable as worn tire by visual detection. Used tires are also scrap tires because they have been discarded by the original owner who is no longer interested to use it. For the U.S., worn tire trends between 2005 and 2013 are illustrated in Figure 1.1 [2]. So today waste tires management is one of the many environmental and recycling issues in developed countries and the comprehensive use of waste tires is the key to overcome these issues. If scrap tires are not properly handled, they can be a risk for the

environment. For example, disposal of waste tires in landfills causes valuable consumption of space due to their large void space [3]. Landfill disposal is currently being banned in some countries because of the environmental problems it creates [4]. Stockpile and dumping of waste tires are other disadvantages creating important health and safety risks such as fires or providing a breeding area for mosquitos which might carry diseases. Waste tire management plays an important role regarding to environmental issues. Tire grinding and separation of tire cord and metal is an interesting technique of waste management [5]. Furthermore, a high number of worth rubber product is used worldwide. But after it becomes waste, this product is usually used as a source of thermal energy [6]. The most important scrap markets for post-consumer tires in U.S. are shown in Figure 1.2 [2]. An important part of waste tire market is allocated to tire-derived fuel (TDF). However, it cannot be considered as a recycling method because it can create new problem like air pollution. Therefore, it is generally preferred to use more efficient recycling methods to manufacture worthier products from discarded tires [7]. The most straightforward option, which attracted a great deal of attention, is to use grinded discarded tires in polymer industries [8][9].

Figure 1.2. U.S. scrap tire disposition for 2013 (total amount generated annually)

[2].

Generally, it is difficult to recycle rubber because of its structure [4]. Actually, tire rubber is a thermoset with a crosslinked structure which is not easy to break by simple methods. The most common and most important product among all the rubber products is tire. Nearly one vehicle tire per year per person is discarded in industrialized countries [3]. Table 1.3 and Figure 1.3 present the tire production in Europe between the years 2006 and 2014. It can be seen that a 32.8% growth occurred between 2009 and 2014 (from 3.568 to 4.800 million tires) and this kind of statistics give importance of dealing with waste tires [10].

Table 1.3. European tire production (1000 tons) [10].

Year 2006 2007 2008 2009 2010 2011 2012 2013 2014 Production 4900 5100 4740 3568 4500 4800 4580 4670 4800 Change to previous year (%) +4.1 -7.1 -24.7 +26.1 +6.7 -4.6 +2.0 +2.8

Figure 1.3.European tire production (1000 tons) [10].

1.2 Ground tire rubber (GTR)

Grinding (size reduction) of waste tire is a recycling method which has been used recently due to recycle thermoplastics and thermosets as a blend with ground rubber. The whole tire needs to go through the following steps to be converted into GTR: cutting, separation of metal and textile, granulation and classification. Mechanical grinding of is done under wet condition at ambient, high or cryogenic temperature [5][11][12][13][14]. These techniques can be described as [15]:

1. Ambient grinding technique: generally done using two-roll cracker-type mill. The average particle size is approximately 200 µm and the temperature may rise up to 130ºC.

2. Under wet (ambient) grinding technique: the waste rubber is cooled down by water spraying, then water is removed from the GTR by drying.

3. High-temperature grinding technique: the average particle size obtained from this method is less than 100 µm and the temperature is about 130ºC.

4. Cryogenic temperature grinding technique: the rubber is cooled below its glass transition temperature which depends on rubber type (usually between -30 and -80ºC) and the required energy for grinding is significantly reduced. The frozen rubber pieces go through an impact-type mill and become shattered. Afterward, the GTR is dried, textiles and metals are separated, and then categorized into the desired mesh sizes.

1.3 GTR applications

GTR applications contain new rubber supplies, playground mulch, sport surfacing and rubber-modified asphalt. In 2013, the U.S. ground rubber market consumed 975 thousand tons (almost 60 million tires) of worn tires corresponding to 25% of the worn tire generated. U.S. ground rubber markets are reported in Figure 1.4 for 2013 [2].

Figure 1.4. U.S. ground rubber markets for 2013 (percent of total pounds of

ground rubber consumed) [2].

1.4 Thermoplastic Elastomers (TPE)

Thermoplastic elastomers are one of the most versatile plastic resins found in several different applications. These materials are actually a physical mixture of a rigid/thermoplastic phase with an elastic/rubber phase. These materials show the properties of both plastics as well as rubbers since no chemical or covalent bonding between both phases exists. This behavior created a new window in the polymer field and became an important part of polymer

science in general. TPE can find applications in adhesives, packaging, footwear, building and construction, medical devices, engineering devices, wires and cables, as well as automobile parts and others. The first generation of thermoplastic elastomers was based on polypropylene (PP) and ethylene propylene diene monomer (EPDM) [16]. Since TPE can be molded, extruded and reused, they have the potential to be recycled. Table 1.4 presents a list of common TPE used in modern markets [17].

Table 1.4. Common thermoplastic elastomer materials [17].

Amorphous Semi-Crystalline

Low cost

PVC Polyvinyl chloride HDPE High density polyethylene SAN Styrene-acrylonitrile LDPE Low density polyethylene

PS Polystyrene PP Polypropylene PMMA Polymethylmetacrylate ABS Acrylonitrile-butadiene styrene SMA Styrene-maleic-anhydride Medium Cost

PPO Polyphenylene oxide UHMWP

E Ultra-high molecular weight polyethylene PC Polycarbonate PPC Polyphthalate carbonate POM Polyoxymethylene PTFE Polytetrafluoroethylen e (Teflon) PA Polyamide PBT Polybutylene terephthalate High Cost

PAR Polyarylate PA-4,6 Polyamide-4,6

PES Polyether sulfone PPA Polyphthalamide

PEI Polyether imide PPS Polyphenylene sulfide

PPSU Polyphenyl sulfone LCP Liquid crystal polymers

TPI Thermoplastic

polyimide PVDF

Polyvinyl diene fluoride

PAI Polyamide imide FP Fluoro polymers

Thermoplastic elastomers have an advantage from the mechanical properties of the elastomer part (at room temperature), while having a thermoplastic part. The thermoplastic phase actually acts as the matrix making them possible to be recycled and reprocessed. Furthermore, by changing their formulation, it is possible to manufactured TPE based on desirable mechanical properties.

1.4.1 Advantages and Disadvantages of TPE

Main advantages of thermoplastic elastomeric materials compared to conventional rubber materials are listed as below [18]:

1. Simple processing with few steps.

2. Shorter times are need for processing leading to less expensive final parts. 3. Shorter molding cycles leading to lower energy consumption.

4. Possibility to reuse scrap having similar quality as a virgin material.

Despite of such good benefits, TPE have some disadvantages when compared to conventional rubber materials.

1. Melting at high temperature limits their selection because most thermoplastic elastomer materials have a melting point of less than 150C.

2. Lack of low-hardness TPE. Most of the thermoplastic elastomers are available with hardness higher than 80 Durometer A.

1.5 GTR use in thermoplastic elastomers

One of the best option to use waste rubber is to incorporate waste rubber into polymers to achieve thermoplastic elastomers or thermoplastics with high impact resistance. The properties of GTR-thermoplastic depend on GTR and plastic types, GTR loading and the adhesion between the matrix and GTR [19]. Over the last decades, some works have been done using GTR in blend formulation. For example, Scaffaro et al. investigated a method to prepare blends based on ground tire rubber and recycled polyethylene (RPE) under varying

processing conditions [20]. The results showed reasonably good properties at low GTR concentration and proper processing condition. They concluded that temperature, mixing speed and blending method have direct effects on the final blend properties.

Canavate et al. also reported that for blend of high density polyethylene (HDPE)/GTR, mechanical properties loss occurred when GTR content increases and incompatibility between both phases is responsible for this behavior [21].

In a work by Luo et al., it was reported that GTR can provide very good improvement in polypropylene impact strength: almost 191% improvement for blends of 40/60 PP/GTR compared to neat PP [22].

In another work, the mechanical properties (tensile strength, Young’s modulus and elongation at break) of various concentrations of polypropylene/natural rubber (PP/NR) and polypropylene/recycled rubber (PP/RR) were investigated by Ismail [23]. Their results show that blends with recycled rubber have better mechanical properties compared to others with natural rubber. Tensile strength and Young’s modulus decreased with increasing rubber content (from 20 to 60% wt.). For a PP/RR blend at 20% wt., the rubber phase remained as dispersed particles. Dispersion state and smaller size of the dispersed phase led to higher tensile strength and modulus of PP/RR blends. But tensile strength and Young’s modulus decreased with increasing NR content because of decreasing blend rigidity. However, tensile strength and Young’s modulus of the blends with recycled rubber were slightly higher than with natural rubber at the same rubber content.

In general, various thermoplastic resins such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS) and ethylene-vinyl acetate copolymer (EVA) can be blended with GTR as reported by Li et al. [11].

1.5.1 GTR particle size

Larger rubber particle size were investigated and showed poor mechanical properties [24]. If smaller rubber particle size is used, higher contact surface area is created to transfer the applied loads [25]. Larger surface area is also contributing to get better compatibility and elongational capability.

It was reported that a 20% improvement in impact strength can be obtained when the GTR particle size decreased from 350 m to 100 m [24]. It is important to mention that adding solid particles to a plastic increases melt viscosity, especially for smaller particle sizes. Da Costa et al. studied blends of PP/GTR (GTR concentration varied between 0 and 45% wt.) and PP/EPDM (EPDM content between 0 and 45%) produced with a single-screw extruder [26]. From their results, increasing EPDM content in PP/EPDM (55/45) blends resulted to noticeable impact strength improvement (over 500% compared to neat PP). However, for the sample with 45% wt. GTR, there was no significant change in impact strength because of weak adhesion between the matrix and the large GTR particles. It is believed that for PP/GTR/EPDM (50/30/20), EPDM acted as an emulsifier at the GTR particle surface when EPDM was used at lower content. Therefore, it is important to improve both particle size and distribution, as well as the uniformity of rubber particle in PP to obtain a good toughening effect.

1.5.2 Adhesion of GTR

Poor interface, resulting in weak stress transfer, normally leads to limited effect on the overall mechanical properties. Poor adhesion is usually considered to be the main issue (combined with large particle size) resulting in significant mechanical properties decreases observed upon the addition of GTR into polymers [24]. This poor adhesion is associated to the high crosslinking degree inside GTR particles. This high degree of crosslinking prevents molecular diffusion (chain mobility) across the interface limiting molecular interpenetration between the phases. For the moment, almost no effective methods have been proposed to produce highly filled GTR thermoplastics (higher than 50%).

One of the main factor to have good blend quality is the presence of a high degree of compatibility which depends on the dispersed phase particle size and the degree of phase separation. Addition of compatibilizing agents into thermoplastics is commonly done to improve the mechanical properties as well as quality of the blends [27]. An alternative way to improve thermoplastic elastomer properties is to mix them with materials having the ability to enhance the mechanical properties of polymers, and then the overall behavior.

Hassan et al. reported the compatibilizing effect of maleic anhydride [28]. They produced composites using reclaimed rubber powder (RRP) as the matrix and maleic anhydride (MA)

as the coupling agent, then glass fiber (GF) was added at different ratios. From the results obtained, tensile strength increased slowly with increasing GF content. A similar trend was found for hardness.

In a work by Kakroodi et al., MAPE was proposed as the matrix to produce TPE at intermediate and high concentration (up to 90%) [29]. Also, they made a comparison between their results with blends based on HDPE as the matrix. MAPE/GTR composites showed very good tensile properties, but adding more GTR led to lower elongation at break. They also reported that compounds with 70% GTR had optimum properties. Replacement of MAPE by HDPE gave rise to noticeable decrease in blend homogeneity which was confirmed by scanning electron microscopy (SEM). MAPE/GTR composites showed better elastic recovery in comparison with HDPE/GTR compounds.

1.6 Hybrid composites based on rubber and natural fiber

1.6.1 Natural fibers

In order to extend polymer applications and to manage their limitations, reinforcement (natural and synthetic materials) are often added. Natural fibers can be obtained from different sources and origins categorizing them into plants, minerals and animals fibers [30]. Plant based fibers can be subdivided into various groups. Figure 1.5 presents a classification of the different non-wood natural fibers.

Figure 1.5. Classification of non-wood fibers [31].

In general, the chemical composition of natural fibers depends on the kind of fiber. Firstly, fibers consist of cellulose, hemicellulose, lignin and pectin. The properties of each component can explain the overall properties of the fiber. But the most important component

in the fiber is hemicellulose since it is the one mostly responsible for moisture absorption, biodegradation and thermal degradation. The exact composition of natural fibers can change substantially from one to another. However, the highest amount is cellulose (60-80%), followed by lignin (5-20%), and moisture (up to 20%) [32]. Several parts of cellulose-lignin with hemicellulose layers create a complex multi-layer structure of fibers (Figure 1.5) [33]. Every single vegetal fiber has several cells built from crystalline cellulose microfibrils. Amorphous lignin and hemicellulose components connect the walls to the complete layer.

1.6.2 Mechanical properties of natural fibers

The high mechanical properties of natural fibers can be associated to the spiral angle of cellulose the fibrils in a fiber. Synthetic reinforcements have generally higher mechanical properties, but composites based on natural fibers are getting most of the attention over the last decades, mostly because of their lower costs and lower density.

Figure 1.6. Structure of a natural fiber cell.

1.6.3 Advantages and Disadvantages of natural fibers

Natural fibers have interesting advantages making them more interesting than manmade reinforcements. These advantages include: low cost, environmental friendliness, light

weight, low abrasiveness and less required equipment. Some of the natural and man-made fibers with their mechanical properties (density (), elongation at break (ε_b), tensile strength () and elastic modulus (E)) are listed in Table 1.5 [34].

Table 1.5. Mechanical properties of natural fibers compared to man-made fibers

[34].

Despite of the advantages of using natural fibers, there are also some disadvantages to be taken into the account. The large amount of hydroxyl group (-OH) on the cellulose and hemicellulose surface leads to negative effect on composite properties. Blending of hydrophilic natural fibers with hydrophobic polymers results in low interfacial bonding making natural fibers less able to transfer stresses at the polymer-fiber interface when mechanical loadings are applied. Furthermore, natural fibers have the ability to absorb water. Water absorption in these fibers causes some defects like lower mechanical properties, higher degradation level and swelling. It is also worth mentioning that hydrogen bonding can be formed between cellulose chains (because of existing hydrogen groups of cellulose) limiting

their dispersion [34]. Other drawbacks including low density (compared to thermoplastics), low thermal stability, low impact strength and low elongation also exist [35].

To overcome such problems, several alternatives have been proposed and applied to increase the bonding level at the polymer-fiber interface.

1.6.4 Surface treatment methods

Generally, the fiber-matrix interface plays an important role in composites due to load transfer control between the fibers and the polymer [36]. There is also an interphase region between the matrix and the bulk fiber leading to various layers of materials (Figure 1.7). The interphase depends on the composite composition leading different mechanical performances [37]. Composite with the low level of stiffness have soft interface, while stiff composites (low fracture resistance) have stiff interfacial region. This is why various modification methods have been considered to enhance the interface quality as well as compatibility between different fibers and matrices.

1.6.4.1

Physical methods

One of the way to improve the interface of lignocellulosic fibers is by physical methods. In this case, mechanical bonding between the matrix and the fibers can be improved by changing the surface properties and structure of the fibers. This method does not change the chemical composition of the fibers [34]. Physical methods such as thermal treatment [38], stretching [39], calendaring [40][41] and hybrid yarns [42] are the current ways to reinforce fibers [43]. Corona and cold plasma (electric discharge) are newer ways of physical treatment. The mechanism of Corona treatment is to change the surface energy of cellulose fibers [44]. For wood fibers, the aldehyde group content can be increased by surface activation [45]. Cold plasma treatment was found to have the same effect. Different gases can be used resulting in a diversity of surface modifications. In fact, plasma treatment remove impurities on the fiber surface and increase fiber porosity (Figure 1.8).

Figure 1.8. Morphology of a natural fiber surface before (a) and after (b) plasma

treatment [35].

1.6.4.2

Chemical methods

When two materials are not compatible (hydrophilic fibers [46] and hydrophobic polymers [47][48]), there is the possibility to add a third material having properties intermediate between the other two. Actually, this method can improve the properties of the fiber such as strength, surface, amount of impurities and matrix-fiber interaction [49]. Chemical modification methods can help to improve the interfacial adhesion between each phase

resulting in better overall mechanical properties and reduced water absorption [50]. In order to overcome the problems related to fiber water absorption, using hydrophobic aliphatic and cyclic structure for treating fiber have been developed. These cyclic structures consist of reactive groups having the possibility to make new bonds with reactive groups on the polymer. Therefore, the chemical treatment of natural fiber aims at making the fibers more hydrophobic leading to have stronger interfacial adhesion between the matrix and the fiber in composites [43] [51] [52]. Several chemical modifications such as dewaxing, bleaching, acetylation, delignification and chemical grafting are commercially used to enhance the mechanical performance via surface properties modifications [32]. Rowell et al. (1992) reviewed the different chemical treatments applied on natural fibers [53], and Table 1.6 presents a summary of these different chemical modifications.

Table 1.6. Chemical treatments used for modification of natural fibers [53].

Dewaxing is usually carried out by using benzene or alcohol followed by treatment with NaOH, then drying at ambient temperature [53]. Numerous bleaching agents like hydrogen peroxide, sodium hypochlorite and alkaline calcium are frequently used. Nevertheless, bleaching usually leads to a loss of tensile strength and weight [52]. This loss is mainly associated to the action of alkali or alkaline (bleaching agent reagent) on hemicellulose or lignin.

Acetylation aims at making the fiber more hydrophobic via fiber surface modification [54]. Therefore, acetyl groups (CH3CO) should react with the hydroxyl groups (OH) of the fiber. Acetylation of OH groups is shown as:

From the literature, this method was found to be beneficial to reduce water absorption in natural fibers. For example, a decrease of moisture uptake by about 50%-60% for acetylated jute fibers and pine fibers was reported [34].

Several investigations reported on the mechanisms and influences of silane modification on the mechanical properties of various composites. For instance, Abdelmouleh et al. [55]

investigated that fiber treatment with silane led to improved mechanical performance of bleached soda pulp/PU and bleached soda pulp/epoxy. For composites based on epoxy, the elastic modulus (E') increased from 2.55 GPa to 2.93 and 3.2 GPa (for the composites having untreated fibers), respectively, for the composites with treated fibers (-mercaptopropyltrimethoxysilane (MRPS) and -aminopropyltriethoxysilane (APS)). Figure 1.9 illustrates the fiber treatment with silane as a coupling agent showing the ability of the fibers to react with epoxy and unsaturated polyester resin due to covalent bonding at the matrix-fiber interface.

Figure 1.9. Schematic representation of the interface area of epoxy

matrix/silane-modified fiber [55].

1.6.4.3

Compatibilizing agents

Another modification method to increase matrix-fiber interactions in composite is by the addition of a coupling agent. Coupling agents are used in small quantities to modify the interface by making new bonds between each component. They can act as compatibilizers in composites containing hydrophobic polymers and hydrophilic fibers. Fiber dispersion in the polymer can be also be improved by using coupling agents. The main mechanisms to improve the matrix-fiber interface is to form new interfaces and to reduce interfacial energy level resulting in lower fiber agglomeration.

According to earlier works, some coupling agents are used in wood polymer composites making the phases more compatible by reducing interfacial tension [35]. Addition of maleic

anhydride grafted polyolefins has been mostly done because of their ability to improve stress transfer and eventually the mechanical properties of natural fiber composites [34].

Several coupling agents such as maleic anhydride grafted polyethylene (MAPE), maleic anhydride grafted polypropylene (MAPP), maleated styrene-ethylene/butylene-styrene (MA-g-SEBS) and styrene-maleic anhydride (SMA) were shown to form new chemical bonds between MA and fiber hydroxyl groups (OH). Entanglement of polymer chain and hydrogen bonding (secondary interaction) are the other mechanisms of the coupling agents when dealing with the polymer matrix.

Today, MAPE is the most common coupling agent used when HDPE is the matrix. Figure 1.10 shows that interfacial interactions between HDPE and natural fibers are composed of both physical (hydrogen bonding) and chemical (ester bonding) interactions between maleic anhydride groups in MAPE and hydroxyl groups (OH) on the fibers.

Figure 1.10. Structure of MAPE (coupling agent) and cellulosic fibers at the

interface [56].

Compatibilizing agents are various in type and specification which have different but important effects on the interfacial adhesion between the different phases (matrix and fiber).

For a composite, improved interfacial bonding results in better mechanical and physical properties relying on acid number (maleic anhydride groups), molecular weight and concentration [57]. Low coupling agent concentration, moderate acid number and high molecular weight are the best combination leading to improve interfacial bonding then improve mechanical properties of composites. Since a high amount of compatibilizer can have a negative influence on interfacial adhesion, an optimum value must be searched. Nevertheless, direct effects on the matrix-fiber bonding strength was found with increasing molecular weight of the coupling agent [58].

As an example, two typical micrographs (fractured surface) of untreated and treated composites are shown in Figure 1.11 and 1.12. From Figure 1.11, it can be seen that spaces between the matrix and the fiber (because of fiber pullout) are present. This behavior is related to weak interfacial adhesion and insufficient wetting of the untreated fibers inside HDPE. Conversely, as seen in Figure 1.12 the fibers were finely dispersed in the matrix leading to a reduction in the number of gaps/voids between the matrix and the fibers. It is also clear that HDPE layers (coating) on the fiber particles were pulled out together at the fracture, which further support cohesive coupling between HDPE and treated fibers [59].

Figure 1.11. SEM micrographs of untreated HDPE/sisal samples at two

Figure 1.12. SEM micrographs of treated HDPE/sisal samples with MAPE at

two different magnifications [60].

Incorporation of natural fiber into polymers often leads to lower ductility because of stress concentration. In order to improve the impact strength of such composites, the addition of rubber particles in the composite through melt-mixing is usually proposed. Elastomer phases such as copolymers of polypropylene or polyethylene like styrene ethylene butylene styrene tri-block (SEBS) and ethylene propylene diene monomer (EPDM) are the most common materials being used [61].

The effect of rubber addition into polypropylene(PP)/polyethylene (PE)/wood flour (WF) was investigated by Clemons [62]. Using two impact modifiers (ethylene propylene diene monomer (EPDM) and maleated EPDM), a noticeable increase (63%) in impact strength after the incorporation of 10% MA-EPDM for the sample PP/PE: 25/75 with 30% WF was observed, although the same EPDM content had a lower effect (46% increase). It was also reported that tensile modulus decreased by 32% for samples with MA-EPDM compared to 26% for samples with EPDM.

1.7 Thesis objectives

1. To use recycled tire as a worldwide environmental concern and use them as an impact modifier in polymer composites.

2. To use natural fibers which have several advantages based on cost and availability due to their reinforcing effect on thermoplastic resins.

3. To produce and characterize hemp/ground tire rubber hybrid composites using industrial processes (extrusion/injection).

4. To investigate the effect of a coupling agent (MAPE) on the mechanical properties and determine its effect of morphological properties determined via scanning electron microscope (SEM) micrographs.

5. To produce thermoplastic elastomer/natural fiber compounds over a wide range of concentration (up to 60% wt.) to have a wide range of mechanical properties.

6. To study the effect of recycled rubber particles on impact strength.

7. To model the mechanical properties (correlations) via nonlinear regression.

Based on the main objectives of this work, this thesis is composed of three main chapters: The first chapter presented a general introduction based on ground tire rubber and natural fibers (hemp) as the fillers used in this project. Their main use in the production of thermoplastic elastomer/natural fiber composites was also discussed. Then, a literature review is presented mainly focusing on the mechanical properties of natural fibers polymer composites as well as thermoplastic elastomers.

The second chapter presents the effect of MAPE and GTR on the mechanical properties of NFC. The effect of coupling agent addition is investigated via morphological analysis of the fractured surfaces of the produced samples. Then, hybrid composites with different compositions are produced and characterized. Finally, an attempt is made to model the experimental results.

The third chapter consists of the general conclusions based on the results of the mechanical characterizations, as well as regression models for prediction and optimization purposes.

Chapter 2:

2.

Effect of coupling agent and ground tire rubber content on

the properties of natural fiber polymer composite

Navid Nikpour and Denis Rodrigue

2.1 Résumé

Dans ce travail, des composites à base de polyéthylène de haute densité (PEHD) et de fibres de chanvre sont produits par extrusion suivie d'un moulage par injection. En particulier, l'effet de caoutchouc recyclé sous forme de pneus usés (GTR) et d’un agent de couplage (polyéthylène maléaté, MAPE) a été étudié pour modifier les propriétés mécaniques de ces matériaux composites à base de fibres naturelles (NFC). À partir des échantillons produits, une caractérisation complète a été réalisée, y compris la morphologie, la densité, la dureté, ainsi que les propriétés mécaniques en traction, flexion et impact. Les résultats indiquent qu’une amélioration substantielle de la résistance au choc des NFC est produite après l’addition de GTR, tandis que les propriétés en tension et en flexion sont réduites. D'autre part, l'addition d'un agent de couplage a pu améliorer l'adhésion entre chacune des phases menant à de meilleures propriétés des composites. Dans l'ensemble, les propriétés finales des matériaux composites représentent un équilibre entre l'élasticité/ténacité du GTR et la rigidité/résistance du chanvre et du MAPE. À partir des données obtenues, des modèles de régression pour différentes propriétés sont présentés afin d’aider à la conception/contrôle des propriétés finales de ces composites.

2.2 Abstract

In this work, high density polyethylene (HDPE)/hemp fiber composites were produced by extrusion compounding followed by injection molding. In particular, the effect of ground tire rubber (GTR) and coupling agent (maleated polyethylene, MAPE) content was studied to modify the mechanical properties of these natural fiber composites (NFC). From the samples produced, a complete characterization was performed including morphology, density, hardness, as well as mechanical properties in tension, flexion, and impact. The results indicate that substantial improvement in NFC impact strength occurred after GTR addition, while tensile and flexural moduli/strengths decreased. On the other hand, the addition of a coupling agent was able to improve adhesion between each phase resulting in better composite properties. Overall, the final properties of the composites represent a balance between elasticity/toughness from GTR and rigidity/strength from hemp and MAPE. From the data obtained, a regression model for different properties is presented to design/control the final properties of these composites.

2.3 Introduction

Natural fibers (mostly lignocellulosics) are still being increasingly used as reinforcements in polymer composites instead of synthetic materials which are often non-biodegradable. Several investigations were devoted at using natural fibers like hemp, jute, flax, etc. to reinforce thermoplastics and thermosets [65][66][67][68]. The main driving force to introduce natural fibers in polymer matrices is mostly economics (low raw material cost) with acceptable mechanical properties (good specific strength). Other advantages are local availability, light weight, renewability, and lower equipment abrasion making them interesting materials, especially in building/construction and automotive markets [69][70][71][72].

However, natural fibers have some disadvantages like low thermal stability limiting processing temperature below 200°C. This is why only a few thermoplastics like polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC) have been commercially developed [73]. Also, natural fibers have usually weak interaction with most resins because of differences in surface energies between hydrophobic polymers (thermosets and thermoplastics) and hydrophilic fibers [74][75]. Poor interfacial fiber-matrix interactions usually lead to low mechanical properties of the resulting composites. To solve this problem, coupling agents are often added for interfacial adhesion improvement [70][76]. Several investigations showed that maleic anhydride-grafted polymers, like maleated polyethylene (MAPE), are very efficient for polyethylene/natural fiber composites [77][78][79]. As a result, significant improvements in mechanical properties of wood composites were obtained after maleated polyolefin addition [80][81].

But a major disadvantage of adding natural fibers to thermoplastic matrices is substantial loss of impact strength [62]. Nevertheless, several solutions have been proposed to solve this problem, especially for wood flour reinforced thermoplastics. One way to recover impact strength is to introduce a rubber phase [82][83]. In this case, several studies added different elastomers including ethylene-propylene-diene monomer (EPDM), styrene-butadiene rubber (SBR), and natural rubber (NR). For example, Ruksakulpiwat et al. [84] reported that adding more than 20 wt.% of rubber powder (natural and EPDM) to vetiver grass-polypropylene composite led to better impact resistance, but lower tensile strength and modulus were

obtained. Biagiotti et al. [82] also reported a modulus reduction of PP/flax fiber/EPDM composite with increasing rubber content. For instance, a 250% increase in impact strength was observed when 30% EPDM was added to the matrix (PP). Nevertheless, one main drawback is that elastomers are generally more expensive than natural fibers and most thermoplastics. To reduce raw material costs, recycled rubber is an interesting way and ground tire rubber (GTR) should be a good choice to achieve competitive properties. Since ground rubber tire creates environmental issues, several attempts have been made to reuse tires under different forms as a filler to produce thermoplastic elastomers. Recently, these works focused on blending GTR with thermoplastics as fillers or active components [29][85][86].

In this work, high density polyethylene (HDPE)/hemp composites are produced by extrusion/injection molding. To improve the strength/rigidity of these NFC, a coupling agent is added at different concentrations. Then, to control their toughness/impact strength, the effect of GTR content is studied. Finally, the samples produced are characterized and from the results obtained, a nonlinear regression model is proposed to optimize the balance between rigidity and strength depending on the final application.

2.4 Materials

High density polyethylene (HDPE) HD 6719 (Exxon Mobil chemicals) was used as the matrix. The MFI for this HDPE is 19 g/10 min (190°C/2.16 kg) with a density of 0.952 g/cm3_. Hemp fibers, from the Hemp Trade Alliance (Quebec, Canada), were sieved to keep only particles less than 250 microns. The ground tire rubber (EPDM) used (density of 1.29 g/cm3 and particle sizes less than 250 microns) was provided by Royal Mat Inc. (Canada). Maleic anhydride grafted polyethylene (MAPE) was supplied by Westlake Chemical Corporation (USA) with the tradename Epolene C-26. Its melting temperature and density are 121°C and 919 kg/m3_{, respectively.}

2.5 Compounding

The fibers were dried overnight in an oven (80°C) before being processed. All the samples were melt blended in a co-rotating twin-screw extruder Leistritz ZSE-27 with a L/D ratio of 40 (total of 10 heating zones). The temperature profile was set at 170°C for all the zones and

the screw speed was constant at 120 rpm. Then, the extruded compounds were cooled down in a water bath (room temperature) at the die (5.9 mm in diameter) exit and subsequently pelletized. GTR (when used) was fed alone via the main hopper (zone 1) to have the possibility of having smaller particle sizes (shearing force) [85] or to produce partial EPDM regeneration [87][88], while HDPE (which was dry-blended with MAPE) and hemp were fed together via a side-feeder placed at zone 3 to avoid thermal degradation and process overload. Sample codes are presented in Table 1 where P, H, and G represents HDPE, hemp, and GTR, respectively. Finally, the pellets were dried overnight at 75°C before being injection molded using a Nissei PS60E9ASE machine with a temperature profile between 180 and 185°C producing samples with dimensions of 110 × 25 × 3 mm3_{. The samples for characterization} were cut in these rectangular bars.

Table 2.1. Sample coding and composition of the compounds produced.

Sample code HDPE

(wt. %) Hemp (wt. %) GTR (wt. %) MAPE (-)* HDPE 100 0 0 0 PH(85/15) 85 15 0 1.5 PH(70/30) 70 30 0 3 PH(55/45) 55 45 0 4.5 PH(40/60) 40 60 0 6 PG(85/15) 85 0 15 1.5 PG(70/30) 70 0 30 3 PG(55/45) 55 0 45 4.5 PG(40/60) 40 0 60 6 PHG(85/7.5/7.5) 85 7.5 7.5 1.5 PHG(70/15/15) 70 15 15 3 PHG(55/22.5/22.5) 55 22.5 22.5 4.5 PHG(40/30/30) 40 30 30 6

2.6 Characterization

Sample morphology was analyzed with a scanning electron microscope (SEM) JEOL model JSM-840A at 15 kV. The molded samples were soaked in liquid nitrogen and cryogenically fractured. The exposed surface was coated with a layer of gold-palladium alloy before images were taken at different magnifications.

Tensile properties were evaluated according to ASTM D638 (type V) on an Instron model 5565 with a 500 N load cell at a strain rate of 10 mm/min. At least five specimens were tested at room temperature (23°C).

Flexural tests were conducted on rectangular samples (75 × 12.8 × 3 mm3_{) according to} ASTM D790 using an Instron model 5565 at room temperature. The tests were performed at a crosshead speed of 10 mm/min with a 50 N load cell and a span of 60 mm. The values reported are the average of at least five samples.

Notched Charpy impact tests were carried out according to ASTM D256 at room temperature. The tests were done with a minimum of 10 samples for each composition on a Tinius Olsen model 104. Notches were prepared with an automatic sample notcher from Dynisco (model ASN 120m).

Density was measured with a gas pycnometer Ultrapyc 1200e (Quantachrome, USA) using nitrogen.

Finally, hardness was determined by a PTC Instruments Model 307L (ASTM D2240). The values reported represent the average of at least 10 repetitions.

2.7 Empirical models

Based on experimental results, it is possible to assume that the results rely on the experimental conditions. From this set of data, it is possible to describe these results as a function (f) based on the experimental variables (xi) as [63]:

Based on the three selected parameters (GTR, hemp, and MAPE content), the simplest model for any property may contain only first order terms describing only linear relationships between the experimental variables and the mechanical properties as:

y = ß₀+ ß₁X₁ + ß₂X₂ + ß₃X₃ + ɛ (2)

where  is the error. As a second step, interactions between the different experimental variables can be included as:

y = ß0 + ß1X1 + ß2X2 + ß3X3 + ß12X1 X2 + ß13X1 X3 + ß23X2 X3 + ɛ (3) Finally, second order terms can be introduced to be able to determine optima (minimum or maximum). Introduction all of these terms help to determine the nonlinear relationships between the experimental variables and their responses via:

y = ß0 + ß1X1 + ß2X2 + ß3X3 + ß12X1 X2 + ß13X1 X3 + ß23X2 X3 + ß11X1 2 +

ß22X2 2 + ß33X3 2+ ɛ (4)

These models will be used and discussed based on the experimental results obtained.

2.8 Results and discussion

2.8.1 Morphology

To get good thermoplastic composites mechanical properties, not only a large interfacial area is required, but also good compatibility (adhesion) between all the phases present. To this end, hemp and GTR particles dispersion in the polymer composite and filler-matrix interface state were characterize using scanning electron microscopy (SEM). Figure 1a,b show typical SEM micrographs of HDPE/hemp fiber composites in presence of MAPE as a coupling

agent. Figure 1a presents the composite with 55% hemp showing a good dispersion of the fibers in the matrix. The image has very few holes and gaps which implies good compatibility between the matrix and the fibers. Adding MAPE led to better interfacial adhesion and interfacial tension reduction due to better stress transfer at the interface [89]. For sample PH(55/45), it can be seen that HDPE is covering the hemp fibers which are difficult to detect (Figure 1b). Furthermore, the fracture was able to break hemp particles and no fiber pull-out is observed, an indication that load transfer from the polymer to the fibers was successful. SEM images for HDPE/GTR composites with MAPE at two different magnifications are shown in Figure 1c,d. Once again, good dispersion of GTR particles in the composite is observed. From Figure 1c, the SEM image shows a typical morphology of a thermoplastic elastomer (TPE) which is a fine dispersion of a rubber phase (darker zones) in the polymer matrix (light grey). Figure 1d presents the SEM image of a HDPE/GTR composite (PHG(40/30/30)) at higher magnification showing again good dispersion of rubber particles inside the matrix. Figure 1e shows an example of a typical SEM image for a sample containing all the components: PHG(85/7.5/7.5). Again, good dispersion and strong interfacial interaction between all the fillers and the polymer matrix can be observed. In all cases no void and hole can be observed; i.e. the matrix totally covers GTR and hemp particles which is an indication of good interfacial contact without any particle pulled-out.

Figure 2.1. SEM micrographs of the fractured surface of: (a,b) PH(55/45) at two different

magnifications, (c,d) PG(40/60) at two different magnifications, and (e) PHG(85/7.5.7.5).

2.9 Mechanical properties

Table 2 presents the results of the mechanical characterizations with respect to the different compositions studied. The results for tension, flexion, density, hardness, and Charpy impact strength are discussed and compared next.

Table 2.2. Mechanical properties of HDPE/hemp/GTR composites.

Sample code Tensile modulus (MPa) Tensile strength (MPa) Elongation at break (%) Hardness (Shore D) Density (g/cm3₎ Flexural modulus (MPa) Impact strength (J/m) HDPE 425 (61) 18.5 (0.6) 1324 (228) 64.2 (2.0) 0.95 (0.01) 1001 (40) 42.8 (0.7) PH(85/15) 429 (17) 18.8 (0.2) 23 (6) 64.8 (2.0) 1.00 (0.01) 1166 (34) 35.5 (4.7) PH(70/30) 581 (3) 19.7 (0.1) 13 (1) 69.2 (1.5) 1.11 (0.01) 1550 (45) 30.6 (1.8) PH(55/45) 748 (16) 20.7 (0.2) 6 (1) 70.4 (3.0) 1.16 (0.10) 1875 (57) 29.5 (1.7) PH(40/60) 819 (90) 16.4 (0.7) 4 (1) 72.5 (4.0) 1.31 (0.10) 2862 (119) 28.3 (0.5) PG(85/15) 320 (14) 15.1 (0.2) 733 (42) 62.4 (4.0) 0.98 (0.02) 728 (29) 50.2 (3.7) PG(70/30) 254 (16) 13.0 (0.1) 405 (50) 57.6 (3.0) 1.01 (0.05) 582 (17) 105 (22) PG(55/45) 175 (9) 10.7 (0.1) 367 (22) 56.2 (3.3) 1.07 (0.03) 401 (10) 198 (9.0) PG(40/60) 111 (7) 8.8 (0.1) 136 (9) 49.0 (4.0) 1.14 (0.05) 264 (10) 529 (42) PHG(85/7.7/7.5 ) 412 (17) 17.6 (0.2) 182 (20) 64.6 (1.7) 1.02 (0.06) 955 (28) 43.8 (3.3) PHG(70/15/15) 371 (20) 14.9 (0.1) 57 (2) 63.8 (2.7) 1.07 (0.10) 877 (27) 46.0 (2.7) PHG(55/22.5/2 2.5) 363 (11) 13.3 (0.1) 34 (2) 62.0 (2.5) 1.13 (0.10) 816 (43) 70.6 (5.0) PHG(40/30/30) 295 (47) 10.6 (0.1) 19 (2) 58.0 (3.1) 1.22 (0.10) 780 (31) 75.6 (3.5)

According to Table 2, tensile modulus increased continuously with hemp content showing a value 93% higher than the neat matrix at 60% (from 425 to 819 MPa). This behavior shows the reinforcing effect of hemp on HDPE for the range studied. It can also be seen that a decrease in tensile modulus was produced by adding even small amounts of GTR because of its low modulus (around 2 MPa) [90]. The results show a modulus as low as 111 MPa for PG (40/60) which is 74% lower than neat HDPE (425 MPa). For samples having both fillers (hemp + GTR), tensile modulus values are in between the values of each component used alone. Nevertheless, the effect of the elastomer is more important than hemp because the tensile moduli of all samples with both fillers (for example PHG(70/15/15) with a tensile modulus of 371 MPa) are always lower than neat HDPE (425 MPa).

Table 2 also shows that tensile strength increased with hemp content and was the highest for sample PH (55/45). A value of 20.7 MPa (11% improvement) is reported for this sample compared to 18.5 MPa for the neat matrix. As reported several times in the literature, increasing tensile strength with fiber content is usually an indication of good interfacial stress transfer [91]. This is also confirmed by the micrographs reported in Fig. 1. For the effect of a rubber phase addition, Biagiotti et al. [82] reported that the tensile strength of PP/flax composites decreased with rubber (EPDM) content. For example, tensile strength decreased from 32.1 to 20.1 MPa after the addition of 30% EPDM. Also, Mahallati and Rodrigue [84] reported a reduction of tensile strength by up to 51%, while up to 67% decrease in tensile modulus was observed after incorporation of recycled EPDM to neat PP. In all cases, high hemp content (60%) led to lower values probably due to fiber-fiber contact (aggregation) at high filler content[92]. This shows that PH(55/45) has a hemp content close to the optimum under the experimental conditions studied. Presence of rubber particles also has the same effect as modulus on tensile strength leading to lower values. For instance, tensile strength decreased from 18.5 to 8.8 MPa (52% decrease) when 60% of GTR was added to neat HDPE. It is clear again that the tensile strength values of hybrid composites are in between the values of compounds based on GTR or hemp alone. For example, PHG(70/15/15) has a tensile strength of 14.9 MPa compared to 19.7 and 13.0 MPa for PH(70/30) and PG(70/30), respectively.

As presented in Table 2, the elongation at break has its lowest values for the samples with hemp fiber alone: PH(85/15), PH(70/30), PH(55/45), and PH(40/60)) with 48, 13, 6, and 4%,

respectively. This behavior is related to the low elasticity of rigid fibers since the elongation at break of hemp is around 2-4% [32]. This behavior is also related to the production of more interfacial area with increasing hemp content. Nevertheless, elongation at break decreases with GTR content, but to a lower extent when compared to hemp. This difference can be associated to the more elastic nature of the recycled rubber than rigid hemp. Although the material is more elastic, it is still highly crosslink to start with. There is also the possibility of lower interaction level between HDPE/GTR limiting chain mobility (possible entanglement). Nevertheless, good contact between both phases was observed in Figure 1. For example, the addition of 15% GTR led to a 45% decrease in elongation at break of the neat HDPE (from 1324 to 733%). Once again, hybrid composites have intermediate values. For example, the elongation at break of PHG(70/15/15) is 57% compared to 34% for sample PHG (55/22.5/22.5), as well as 13% and 405% for PH(70/30) and PG(70/30), respectively. The flexural modulus data follow the same trends as for the tensile modulus. It is clear that increasing hemp content increases the values of the neat matrix (1001 MPa) by approximately 16, 55, 87, and 186% with 15, 30, 45, and 60% hemp, respectively. Nevertheless, a noticeable decrease in flexural moduli can be seen by adding GTR alone for all compositions. For instance, the flexural modulus decreased by about 60% when 45% of GTR was added to HDPE. Compounds with both hemp and GTR have lower flexural modulus than neat HDPE showing again that the rubber phase has more effect than hemp on mechanical properties. For instance, sample PHG(40/30/30) has a flexural modulus of 780 MPa which is around 22% lower than the modulus of neat HDPE (1001 MPa), but in between the values of 1550 and 582 MPa for PH(70/30) and PG(70/30), respectively.

Table 2 presents the Charpy impact strength results for the samples produced. Addition of hemp fibers led to lower impact strength with fiber addition. At the maximum hemp content (60%) the lowest impact strength was observed at 28.3 J/m which represents a 34% decrease compared to neat HDPE (42.8 J/m). On the other hand, as expected, incorporation of GTR as an impact modifier was highly successful at improving the impact strength of neat HDPE. For example, adding 30% GTR increased the value by over 145%, while for sample PG(40/60) the improvement is about 1136%. In case of hybrid composites, the values were again in between the neat hemp and GTR results for all compositions. Once again, the effect

of the rubber phase is more important compared to hemp as all the impact strengths were above neat HDPE.

Hardness results are also reported in Table 2. As hemp fiber loading increases from 0 to 60%, a small hardness increase (from 64.2 to 72.5 shore D) can be seen which is due to increased stiffness of the composites. For example, incorporation of 45% hemp to HDPE increased hardness by around 10% (from 64.2 to 70.4). But adding GTR to HDPE produced the opposite effect because of the more “soft” and “elastic” nature of GTR. Again, the effect of GTR is more important since all the hybrid composites have hardness values lower than neat HDPE. For example, PHG(40/30/30) has a hardness value of 58 shore D which is lower than the value of neat HDPE (64.2 shore D).

Finally, composite density increased with hemp and GTR content because hemp fiber (1.35 g/cm3_{) and GTR (1.29 g/cm}3_{) have higher densities than neat HDPE (0.95 g/cm}3_{). So density} increases with filler content, but very small effects are detected depending on filler composition: hemp, GTR of a mixture of both.

2.10 Properties modeling

The results of Table 2 were then used to determine the relations between compositions and mechanical properties. Since filler concentrations were varied independently, the independent variables (factors) were selected as weight fraction for hemp (X1), GTR (X2), and MAPE (X3). To limit the calculations, only five properties were selected: tensile strength, tensile modulus, elongation at break, flexural modulus, and impact strength. Table 3 presents the results of the regression calculations performed. Determination of the model coefficients (𝛽_𝑛) was performed with Sigmaplot (v.10) via nonlinear regression analysis for each mechanical property. The selected factors are not influenced by the errors.

Table 2.3. Summary of the regression coefficients for the mechanical properties. Tensile strength (MPa) Tensile modulus (MPa) Elongation at break (-) Flexural modulus (MPa) Impact strength (J/m) Coeff (MPa) p value (t-test) Coeff (MPa) p value (t-test) Coeff (MPa) p value (t-test) Coeff (MPa) p value (t-test) Coeff (MPa) p value (t-test) Constants 18.1 5.00 × 10-4 401 2.80 × 10-3 10.8 3.15 × 10-2 1020 2.10 × 10 -3₁₀-3 54.7 2.97 × 10-1 Main effects X1 15.9 1.00 488 1.00 -43.6 1.00 467 1.00 72.4 8.42 × 10-1 X2 -18.7 1.00 -490 1.00 -7.59 1.00 -1362 1.00 -337 2.95 × 10-1 X3 -19.0 1.00 -15.8 1.00 -220 1.00 -3736 1.00 -1111 7.53 × 10-1 Interaction effects X1 X2 -20.3 1.00 331 1.00 61.0 1.00 3617 5.8 × 10-1 -251 9.80 × 10-1

2.11 Goodness of fit and prediction of the model

The goodness of fit of a model explains how good the model proposed fits a series of data. It is also described by the correlation coefficient 𝑅2_{, adjusted 𝑅}2_{, and p-value (function of the}

X1 X3 8.85 1.00 -18.6 1.00 12.5× 10−1 1.00 -147 9.88 × 10 -1 ₉₆₅ 9.92 × 10-1 X2 X3 -557 1.00 7718 1.00 2228 1.00 115457 8.56 × 10-1 22806 9.80 × 10-1 Quadratic effects X12 -20.5 1.00 -786 1.00 23.6 1.00 -1788 9.09 × 10-1 -578 9.26 × 10-1 X22 -20.5 1.00 -786 1.00 23.6 9.58 × 10-1 -1788 9.09 × 10 -1 _-578 9.26 × 10-1 X32 57.0 1.00 -559 1.00 14.3 9.92 × 10-1 -2155 9.69 × 10 -1 ₇₉₀₈ 7.23 × 10-1 Model goodness of fit

R2 _{0.972 0.986 0.821 0.993 0.972} R2 adj 0.889 0.946 0.287 0.972 0.890 p value (F-test) 3.38 × 10-2 1.18 × 10-22 3.97 × 10-1 4.44 × 10-33 3.3 × 10-2