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
III
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.
V
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.
VII
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
Chapter 2: ............................................................................................................................. 25
VIII
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
Appendix B: .......................................................................................................................... 65
IX
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
Table 2.3. Summary of the regression coefficients for the mechanical properties. ............. 40
XI
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
XIII
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
PBT Polybutylene terephthalate
XIV
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
SEM Scanning electron microscopy
XV
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 R2 Correlation coefficients (-)
SM Shot size (mm)
T Temperature (°C)
VS Velocity/speed (%)
X Experimental variables (-)
Y Estimated response (-)
Confidence level (-)
ß Regression parameter (-)
XVI
n Model coefficients (-)
Residual error (-)
εb Elongation at break (%)
Density (g/cm3)
Tensile strength (MPa)
XVII
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.
XIX
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
1
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
2
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
3
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 1.1. U.S worn tire trends for the period 2005-2013 [2].
4
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
5
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.
6
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
7
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].
8
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
PEEK Polyetheretherketone
9
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 150C.
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
10
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.
11
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)
12
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
13
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
14
weight, low abrasiveness and less required equipment. Some of the natural and man-made
fibers with their mechanical properties (density (), elongation at break (εb), tensile strength
() and elastic modulus (E)) are listed in Table 1.5 [34].
15
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
16
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.
Figure 1.7. Typical structure of a fiber-matrix interface [36].
17
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
18
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.
19
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]
20
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
21
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).
22
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
different magnifications [60].
23
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
The main objectives of this work include:
24
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.
25
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
submitted to International Polymer Processing, January 2016.
26
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.
Mots-clés: HDPE, MAPE, GTR, propriétés mécaniques, optimisation.
27
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.
Keywords: HDPE, MAPE, GTR, mechanical properties, optimization.
28
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
29
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
30
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.
31
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
* The amounts are % based on total filler (hemp + GTR) weight.
32
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]:
y = f (x1, x2, x3, …) (1)
33
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
34
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.
35
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.
36
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)
Numbers in parentheses represent standard deviations.
37
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%,
38
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
39
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/cm3) have higher densities than neat HDPE (0.95 g/cm3). 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
(𝛽𝑛) was performed with Sigmaplot (v.10) via nonlinear regression analysis for each
mechanical property. The selected factors are not influenced by the errors.
40
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
41
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 𝑅2, 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
42
observed sample result) [63]. The correlation coefficient describes how well a statistical
model can fit the data and make it feasible to identify the model ability to predict the response
(Y), although the p-value or F-test helps to determine the significance of the results. R2 and
adjusted R2 should not be not less than 0.8 for the model to be acceptable [63]. In fact, 𝑅2
does not consider the degrees of freedom which can overestimate the describing power of the
regression results. This is why for small data sets, it is more common to report and compare
the adjusted 𝑅2 [93]. Therefore, the analysis is carried out using the adjusted R2 as the
correlation coefficient of the model. It is important to mention that the F-test (p-value) needs
to be less than the confidence level (α = 5%) for the model to be significant.
2.12 Refinement and Analysis of the Model
At this point, the aim of model refinement is to decide which variables should not be
investigated by exclusion of non-significant factors in the model. One by one, non-significant
factors were removed from the model. This process started with the one having the highest
p-value [64]. Consequently, the most significant factors from the model are analyzed.
Equations (5-9) represent the optimized regression models relative to the corresponding
factors for each mechanical properties studied (refined models). For each model, the 𝑅2 and
adjusted 𝑅2 as the correlation coefficients, as well as p-value as F-test related to the model
significance are used to quantify the model quality of fit. Nevertheless, our results indicate
that for the range of MAPE studied, the results were not significantly different; i.e. for all the
MAPE contents used, the mechanical properties were not statistically different. This is why
this parameter (X3) does not appear in the remaining equations.
Tensile strength
Eq. (5) presents the refined model for the tensile strength which contains five factors:
𝑌𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ = 18.09 + 14𝑋1 – 20.6𝑋2 – 46.6 𝑋1𝑋2 – 26 𝑋12+ 8.9 𝑋2
2 (5)
𝑅2 = 0.972 𝑅2𝑎𝑑𝑗 = 0.952 p-value < 1.00× 10−4
From the equation, it is clear that increasing hemp concentration (X1) leads to higher tensile
strength (positive coefficient) and the quadratic effect of GTR has a small enhancing effect
on matrix as well. However, the main effect of GTR, interaction between hemp and GTR,
43
and quadratic effect of hemp have negative effect on tensile strength which confirms the
experimental results.
Tensile modulus
The refined tensile modulus model is presented by the following equation:
𝑌𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 = 401 + 486 X1 – 491 𝑋2 – 1474 𝑋1𝑋2 + 408 𝑋12+ 2.68 𝑋2
2 (6)
𝑅2 = 0.986 𝑅2𝑎𝑑𝑗 = 0.976 p-value < 1.00× 10−4
The positive coefficient of factor X1 (main effect) shows the reinforcing effect of hemp
content on tensile modulus and the inverse effect (negative coefficient) of GTR content (X2).
However, the quadratic effect of the fiber and the rubber phase have a small and medium
positive effect, respectively. In this case, the interaction between X1 and X2 is highly
important leading to lower moduli due to its large negative value.
Elongation at break
The refined model for the elongation at break (consists of five factors) is shown as follows:
𝑌𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 𝑎𝑡 𝑏𝑟𝑒𝑎𝑘 = 1081 - 6572 𝑋1 - 2967 𝑋2 + 9323 𝑋1𝑋2 + 8327 𝑋12+ 2497𝑋2
2 (7)
𝑅2 = 0.822 𝑅2𝑎𝑑𝑗 = 0.695 p-value = 0.0148
Only the main effect of hemp and GTR decrease the elongation at break. The other factors
(interaction between hemp and GTR and quadratic effect of hemp and GTR) have positive
effect due to their positive coefficients. Nevertheless, the model is not very good as the
regression coefficients are low.
Flexural modulus
Eq. (8) represents the refined flexural strength model which includes five factors:
𝑌𝐹𝑙𝑒𝑥𝑢𝑟𝑎𝑙 𝑚𝑜𝑑𝑢𝑙𝑢𝑠 = 1021 + 121 𝑋1 - 1708 𝑋2 - 2665 𝑋1𝑋2 + 4719 𝑋12+ 739 𝑋2
2 (8)
𝑅2 = 0.993 𝑅2𝑎𝑑𝑗 = 0.988 p-value < 1.00× 10−4
From the equation, the main effect of hemp and the quadratic effects (hemp and GTR) have
positive effect on flexural modulus. On the other hand, the main effect of GTR and interaction
44
between hemp and GTR have substantial negative effect due to their high negative
coefficients.
Impact strength
The impact strength refined model of Eq. (9) has five factors which is shown as follows:
𝑌𝐼𝑚𝑝𝑎𝑐𝑡 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ = 40.4 – 23.2 𝑋1 – 26.7 𝑋2 – 211.5 𝑋1𝑋2 + 1.5 𝑋12+ 830 𝑋2
2 (9)
𝑅2 = 0.992 𝑅2𝑎𝑑𝑗 = 0.986 p-value = 2.00× 10−4
From the equation obtained, the main effect of hemp and interaction between hemp and GTR
have a decreasing effect on impact strength (negative coefficients). On the other hand,
although the main GTR effect is negative, the quadratic effect is important leading to
improved impact strength.
2.13 Model application
For example, if one wants to improve on the properties of neat HDPE, there is a set of
hemp/GTR content leading to satisfy this criterion. In our case, for each property (tensile
strength, tensile modulus, flexural modulus, and impact strength), the transition plots are
provided in Figure 2 to illustrate the properties of neat HDPE.
45
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.
Tensile strength
From the experimental data, it was confirmed that adding rubber particles led to lower tensile
strength. Therefore, in this case, the area below the curve in Figure 2A corresponds to the
region giving higher properties than neat HDPE. For example, adding 40% of hemp leads to
a maximum of 4% GTR to get at least the tensile strength of the base matrix. The results also
show that it is not possible to put more than 6% GTR for the conditions studied.
Tensile modulus
Once again, the region below the curve in Figure 2B represents the conditions giving better
tensile modulus than the neat matrix. For instance, if 25% of hemp is added, the maximum
46
amount of GTR is 11%. In all cases, the maximum amount of GTR is about 12% for the
conditions studied.
Flexural modulus
The flexural modulus behavior a different than tensile modulus. The region below the curve
in Figure 2C represents the conditions leading to higher flexural modulus than neat HDPE,
but the trends (curvature) are different as higher GTR content can be used. For example, at
30% hemp up to about 20% GTR can be added. This behavior can be associated to the
different ways the stresses are distributed in the sample while tested. In a tensile test, the
stresses are applied directly on the particles resulting is high shear stresses mostly controlled
by interfacial adhesion and compatibility. On the other hand, in a flexural test, some of the
particles are under compression and have better resistance to deformation, especially at small
strains where the moduli are measured. The limit is related to the maximum packing fraction
of the particles as enough matrix is needed to recover all the particles.
Impact strength
In this case, the region above the transition plot in Figure 2D produces better impact strength
than the neat polymer. For instance, a minimum of 13% GTR must be added for a sample
having 20% hemp fiber. This shows again that GTR has a higher effect than hemp as a lower
amount is needed to modify the properties of the neat matrix.
2.14 Conclusion
In this study, hybrid composites based on hemp fiber (0 to 60 wt.%) and recycled gro
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