Study on the Effect of Surface Energy of Polypropylene/Polyamide12 Polymer Hybrid Matrix Reinforced with Virgin and Recycled Carbon Fiber

Study on the Effect of Surface Energy of Polypropylene/Polyamide12 polymer Hybrid Matrix Reinforced with Virgin and Recycled Carbon Fiber.

Bruno Sena Maia

A thesis submitted in conformity with the requirements for the degree of Master of Science in Forestry Faculty of Forestry University of Toronto

Study on the Effect of Surface Energy of Polypropylene/Polyamide12 polymer Hybrid Matrix Reinforced with Virgin and Recycled Carbon Fiber.

Bruno Sena Maia

Master of Science in Forestry

Faculty of Forestry University of Toronto

2017

Abstract

The presented work is focused on characterization of thermal treated recycled and virgin carbon fibers. Their thermal performances, chemical surface composition and its influence on interfacial adhesion phenomena on PP/PA12 hybrid matrix were compared using TGA, FTIR and XPS analysis. Additionally, differences between hybrid matrix structural performances of PP/PA12 using both surface modifiers PMPPIC and MAPP were investigated. Final mechanical properties improvements between 8% up to 17% were reached by addition of PMPPIC in PP/PA12 hybrid matrix. For PP/PA12 matrix reinforcement using virgin and recycled carbon fibers, impact energy was improved up to 98% compared with MAPP modified matrix leading to a novel composite with good energy absorption. Finally, wettability studies and surface free energy analysis of all materials studied support the effect of the addition of PMPPIC, MAPP and carbon fibers in final composite surface thermodynamics bringing important data correlation between interfacial adhesion mechanisms and final composite performance.

Acknowledgments

My special thanks to Professor Mohini Sain and Professor Jimi Tjong for accepting me as part of the Centre for Biocomposites and Biomaterials Processing group. It meant a lot that they believed in my skills and capacity. Their valuable technical insights, critical perspectives, and overall support made this research possible. In addition, I would like to thank Professor Chandra Veer Singh for his prompt and excellent contribution, feedback and support during this research. Finally, Suhara Panthapulakkal, Shiang Law, Muhammad Pervaiz, Samir K. Konar, Javad Sameni and Tony Ung deserve a special thank you for their research support jointly with all Faculty of Forestry Staff that work hard to achieve excellent results in all aspects.

My sincere thanks also goes to Cristiane Gonçalves at Ford Brazil, who through her trust and confidence in my work, opened up the door to the possibility of graduate studies at the University of Toronto. Thanks to my professional CBBP colleagues Antimo Graziano, Ahmed Sobh, Birat KC, Masoud Akhshik and Otavio Titton Dias. I hope we can meet in the future to remember all the challenges and professional experiences we went through together. I must not forget to thank my Canadian friends Emilia Melo, Fabio Rocha (and the little Enzo!), Claudio Lopes and his family for all the support they gave me during my first days here. To everyone that directly or indirectly supported me during my path, thank you very much.

I would like to acknowledge the collaborative institutions that honored all my research funding support: MITACS Accelerate Program, Automotive Partnership Canada Program promoted by Natural Sciences and Engineering Research Council of Canada (NSERC), Ford Motor Company Canada and all professors and staff from University of Toronto, Faculty of Forestry. Extra thanks for Prof. Peter M. Brodersen, Prof. Edgar A. Costa and PhD. candidate Aurelio Stammitti from Chemical Engineering Faculty of University of Toronto for their patience, skilled support and advice.

Finally, this achievement could not have been accomplished without the support of my family. My character and principles are largely based on their guidance and support. Maria Alice Leal Sena Maia, Divanildo Branco Maia, thank you! To my sister, Ravena Sena Maia, for her advice and guidance through stressful periods. Most importantly, a special thank you to my wife Daniela Gonzalez de Freitas, who

iii accepted the challenge to live by my side. The Canadian winters and transition to student life have been a challenge to her. I could not be here without you my love and partner, thank you!

Table of Contents

Acknowledgments ...... iii

Table of Contents ...... v

List of Tables ...... viii

List of Figures ...... xi

List of Appendices ...... xviii

List of Symbols ...... xix

List of Abbreviations ...... xxi

Chapter 1 ...... 1

1 Research Background and Literature Review ...... 1

1.1 Background ...... 1

1.2 Carbon Fibers (CF) and Recycled Carbon Fibers (RCF) Overview ...... 6

1.2.1 Carbon Fibers ...... 6

1.2.2 Recycled Carbon Fibers ...... 16

1.3 Recycled Carbon Fiber (RCF): Reclaiming Processes and RCFRP-Recycled Carbon Fiber Reinforced Plastic Manufacturing ...... 18

1.4 Material Characterization Techniques for Carbon Fibers ...... 20

1.4.1 Thermogravimetric Analysis (TGA) ...... 21

1.4.2 Fourier Transformed Infrared Spectroscopy (FTIR) ...... 24

1.4.3 X-Ray Photoelectron Spectroscopy ...... 28

1.5 Surface Energy and Wetting Studies in Composite Materials ...... 30

1.5.1 Contact Angle and Surface Free Energy ...... 32

1.5.2 Carbon Fiber Surface Energy Studies ...... 34

1.5.3 Sessile drop techniques on polymer blends and composite materials ...... 36

1.6 Surface modifiers for polymer blends and fiber reinforced polymers ...... 38

1.6.1 Maleic Anhydride Polypropylene – MAPP ...... 39

1.6.2 Poly[methylene(Polyphenyl) Isocyanate] – PMPPIC ...... 44

Chapter 2 ...... 47

2 Research Motivation, Scope and Objectives ...... 47

2.1 Research Motivation ...... 47

2.2 Research Scope ...... 48

2.2.1 Recycled Carbon Fibers Characterization ...... 48

2.2.2 Reacted PP/PA12 Polymer Blend and Recycled Carbon Fiber Polymer Hybrid Composite ...... 48

2.2.3 Wettability and Surface Energy Studies in Recycled Carbon Fibers Hybrid Polymers ...... 49

2.3 Research Objective ...... 50

Chapter 3 ...... 51

3 Research Methodology ...... 51

3.1 Materials Description ...... 51

3.2 Material Compounding and Processing ...... 53

3.2.1 Material preparation ...... 53

3.2.2 Extrusion Process ...... 55

3.2.3 Injection molding process ...... 56

3.3 Research Test Methods ...... 57

3.3.1 Virgin and Recycled Carbon Fiber Characterization Tests ...... 57

3.3.2 Mechanical Properties Tests ...... 60

3.3.3 Surface Free Energy, Wetting Studies Tests ...... 62

Chapter 4 ...... 64

4 Results and Conclusions ...... 64

4.1 Recycled and Virgin Carbon Fiber Material Characterization Results ...... 64

4.1.1 Thermogravimetric Analysis (TGA) Results ...... 64

4.1.2 Fourier Transformed Infrared Spectroscopy (FTIR) Results for Recycled Carbon fibers ...... 74

4.1.3 X-Ray Photoelectron Spectroscopy (XPS) Results for Virgin and Recycled Carbon Fibers...... 76

4.2 Mechanical Properties Results ...... 86

4.2.1 Polymethylene Polyphenyl Isocyanate (PMPPIC) Optimization Results ...... 87

4.2.2 Polypropylene and Polyamide 12 Optimization and Mechanical Properties Results with PMPPIC and MAPP Surface Modifiers ...... 91

4.2.3 PP/PA12 Virgin and Recycled Carbon Fiber Reinforced Polymers Mechanical Properties Results ...... 95

4.2.4 Melt Flow Index – Material Viscosity Results ...... 100

4.3 Surface Thermodynamics Results ...... 103

4.3.1 Contact Angle/ Wetting Studies Results ...... 103

4.3.2 Surface Free Energy Results ...... 106

4.4 Conclusions ...... 112

4.5 Key contributions and recommendations for future research ...... 115

4.5.1 Key Contributions ...... 115

4.5.2 Recommendations for Future Research ...... 116

5 References ...... 117

Appendices ...... 126

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List of Tables

Table 1:Comparative of recycled and virgin carbon fibers: manufacturing energy used and final price, pound per kilo [15]...... 4

Table 2: Chemical events identified at different temperatures for the oxidation and carbonization processes [21, 36]...... 14

Table 3: Surface chemical groups for several HM and HS carbon fibers, as revealed by as result from FTIR analysis [40]...... 27

Table 4: XPS results showing O/C ratio for different carbon fiber oxidation treatments [39]...... 30

Table 5: Surface group compositions from XPS analysis for virgin and recycled carbon fibers [44]. .... 30

Table 6: Mathematical representations for different theories of surface adhesion [45]...... 32

Table 8: Contact Angles for carbon fibers in water and diiodomethane liquids from literature [52-56]. 35

Table 9: Total surface energies and its polar and dispersive contributions for carbon fibers from the literature [54, 55, 57, 58]...... 35

Table 10: Contact angles (above where Ra is the roughness) and calculated surface free energies for Glass/epoxy and Carbon/epoxy composites [60] ...... 38

Table 11: Mechanical properties for PP/PA12 blends without MAPP. (the N index represents the PA12 %) [67]...... 42

Table 12: Mechanical properties for PP/PA12 blends with MAPP (N index represents PA12%) [67]. .. 42

Table 13: List of materials used and their commercial names...... 51

Table 14: Material composition in % weight for the polymer blends, virgin and recycled carbon fiber reinforced polymers used...... 54

Table 15: Surface Tension reference data for Water and Diiodomethane...... 63

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Table 16: TGA results for virgin carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in air flow...... 67

Table 17: TGA results for virgin carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in nitrogen flow...... 69

Table 18: TGA results for recycled carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in air flow...... 71

Table 19: TGA results for recycled carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in nitrogen flow...... 74

Table 20: Surface groups identified by FTIR analysis for recycled carbon fibers...... 75

Table 21: XPS atomic material % composition of virgin and recycled carbon fibers treated in air and nitrogen at 400oC...... 77

Table 22: XPS analysis: O/C ratio for recycled and virgin carbon fibers at different treatments...... 78

Table 23: Performance comparison between MAPP, PMPPIC hybrid modified blend and pure hybrid blends...... 95

Table 24: Melt Flow Index values for Total PP 3622 polymers and PA12 Arkema Rilsamid AMNO . 101

Table 25: Melt Flow Index values for PP/PA12 pure blends...... 101

Table 26: Melt Flow Index values for 50%/50% - PP/PA12 polymer blends with PMPPIC surface modifier at 1%, 2% and 3%...... 101

Table 27: Melt Flow Index results for different PP/PA12 polymer blends with MAPP surface modifier...... 102

Table 28: Melt Flow Index results for different PP/PA12 polymer blends with PMPPIC surface modifier...... 102

Table 29: Melt Flow Index values for untreated and 400oC treated recycled and virgin carbon fibers with 20%/80% - PP/PA12 blends with PMPPIC surface modifier...... 103

Table 30: Melt Flow Index values for untreated and 400oC treated recycled and virgin carbon fibers with 20%/80% - PP/PA12 blends with MAPP surface modifier...... 103

Table 31: Contact angles results for pure materials investigated...... 104

Table 32: Contact angles results for water and diiodomethane for all PP/PA12 polymer blends studied with MAPP and PMPPIC surface modifiers and their respective virgin and recycled carbon fiber reinforced composites...... 107

Table 33: Total, dispersive and polar surface energies for polymers and virgin carbon fibers studied. 107

List of Figures

Figure 1-1: Structural (Body in white, closure, chassis, bumper) light-weighting strategy and material range forecast [5]...... 2

Figure 1-2: Carbon fiber demand in four market sectors, measured in “kilo tons annual (kTa)” [13]...... 4

Figure 1-3: Recycled Carbon Fibers Life Cycle for Automotive sector...... 5

Figure 1-4: Basic turbostratic structure of carbon fiber (left) and basic structure of graphite crystal (right) [17]...... 7

Figure 1-5: Carbon Fiber’s Basic Structural Unit (BSU) and micro fibrils on left and ribbon structure model for carbon fibers on the right [17, 21]...... 7

Figure 1-6: Voids appearance mechanism by ribbon structure misalignment [17, 21]...... 8

Figure 1-7: Carbon Fibers cross sections: a. concentric orthotropic, b. radial orthotropic, c. transversal, isotropic, d. concentric isotropic, centre transversal isotropic, e. radial orthotropic, centre, transversal, isotropic [22,29]...... 9

Figure 1-8: Carbon fiber surfaces oxygen groups [25,29]...... 10

Figure 1-9: Cost contributions of all stages of carbon fiber manufacturing [19]...... 11

Figure 1-10: Cyclization phenomena during oxidation phase (on left) and dehydrogenation and denitrogenation phases during carbonization process (on right) [73]...... 13

Figure 1-11: Fiber gas concentration curves during the oxidation and carbonization processes [21,50]. 15

Figure 1-12: Interface between fiber surface and polymer matrix [21]...... 16

Figure 1-13: Carbon fiber performance comparison between virgin and recycled carbon fibers (a) Young’s modulus (b) Tensile Strength (c) interfacial shear strength [15]...... 41

Figure 1-14: RCF reclamation and process review chart [15]...... 18

Figure 1-15: Upper, clean fibers (A); fibers with residual matrix (B). Lower, comparison between virgin carbon fibers (left) and recycled carbon fibers (right). [30,31] ...... 19

Figure 1-16: Comparison of TGA thermograms of (a) unstabilized PAN fibers and stabilized PAN fibers at (b) 235oC, 30 min; (c) 235oC, 2h; (d) 235oC, 4h; (e) 235oC, 8h; (f) 235oC, 16h. [36] ...... 22

Figure 1-17: Curves in argon flow for pitch carbon fibers brominated for different time treatments [74]...... 23

Figure 1-18: TGA analysis on pyrolysis for reclaimed carbon fibers from epoxy resin composites. Left, temperature treatment vs weight loss. Right, time dependency on isothermal treatments [76]...... 24

Figure 1-19: FTIR spectra of cellulose carbon fibers under different treatment temperatures [37]...... 25

Figure 1-20: FTIR absorbance spectra of UV- stabilized lignin- based carbon fibers [38]...... 26

Figure 1-21: Normalized IR spectra for untreated carbon fibers (a) and fibers treated by chemical oxidation from 30min to 6 hours from (b) to (h) [39]...... 27

Figure 1-22: Stacked XPS chart for different time oxidation PAN carbon fibers sized with epoxy resin [41]...... 28

Figure 1-23: Typical XPS Spectrum of carbon kinetic energy region spectrum (1486.6 eV) showing several binding energies of C1s belonging to different functional groups [42]...... 29

Figure 1-24: Five types of adhesion: mechanical (a), dispersive (b), chemical (c), adhesion by diffusion (d) and electrostatic adhesion (e), available on http://www.specialchem4adhesives.com/resources/adhesionguide/index.aspx?id=theory4 [48]...... 31

Figure 1-25: Surface tension forces for determination of Young equation [49]...... 32

Figure 1-26: Left: Work of adhesion of unsized and surface treated carbon fibers as function of pH o values of aqueous test liquids (o) fiber oxidized with ozone (0.75% O3 / 100 C/60s), (D) commercially o oxidized fiber, (x) fiber oxidized in pure oxygen (O2 / 400 C / 30min) and () unoxidized fiber. Right: Work of adhesion of different types of commercially oxidized fibers as a function of the pH value of the aqueous test liquids: (x) Celion fibers, (O) Tenax and (D) AS4 [29]...... 36

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Figure 1-27: Representations of ideal and real surface systems for contact angle analysis [49]...... 37

Figure 1-28: Maleic Anhydride Polypropylene interaction with fiber surface and polymer matrix [64]. 39

Figure 1-30: Young’s modulus, yield strength and % ductility of PP/PA12 blends (o) without MAPP and (•) with MAPP [66]...... 41

Figure 1-31: SEM micrographies with MAPP G3003 at 2% and 5% in weight. At left the " single pull out effect". At right, the "bundle pull out effect" [63]...... 43

Figure 1-32: Mechanical properties results for PP with different MAPP surface modifiers at several concentrations [63]...... 43

Figure 1-33: PMPPIC formula and functional groups interaction with fiber surface OH groups...... 44

Figure 1-34: Comparison of coupling effectiveness between different isocyanate surface modifiers (0.5% in weight) in PVC/CTMP blends [61, 68]...... 45

Figure 3-1: Virgin and Recycled Carbon Fiber samples: virgin on left, recycled on right...... 52

Figure 3-2: Sybron Thermolyne Furnace for carbon fiber treatment...... 55

Figure 3-3: Extrusion machine model Onyx TEC -25/40...... 56

Figure 3-4: Injected samples. Pure and 1%, 2%, 3% PP/PA12 blends with PMPPIC arranged from left to right (left picture). Virgin Carbon Fiber samples: with PMPPIC, MAPP and Recycled Carbon Fiber with PMPPIC (right picture)...... 57

Figure 3-5: TGA Analysis Q500 model...... 58

Figure 3-6: FTIR Tensor 27 model...... 59

Figure 3-7: Instron Universal Tester 3367 model. Tensile and Flexural Strength/Modulus measurement tests...... 61

Figure 3-8: Surface energy and contact angle measurement equipment, Dataphysics OCA15 EC...... 63

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Figure 4-1: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC and 500oC virgin carbon fibers in air flow for 30 minutes...... 66

Figure 4-2: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC and 500oC treated virgin carbon fibers in air for 30 minutes...... 67

Figure 4-3: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC, 500oC, 600oC and 700oC virgin carbon fibers in nitrogen flow for 30 minutes...... 68

Figure 4-4: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC, 500oC, 600oC and 700oC treated virgin carbon fibers in nitrogen for 30 minutes...... 69

Figure 4-5: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC and 500oC recycled carbon fibers in air flow for 30 minutes...... 70

Figure 4-6: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC and 500oC treated recycled carbon fibers in air for 30 minutes...... 71

Figure 4-7: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC, 500oC, 600oC and 700oC recycled carbon fibers in nitrogen flow for 30 minutes...... 72

Figure 4-8: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC, 500oC, 600oC and 700oC treated recycled carbon fibers in nitrogen flow for 30 minutes...... 73

Figure 4-9: FTIR absorption spectra results for recycled carbon fibers treated at different temperatures...... 76

Figure 4-10: XPS Stacked chart results for recycled and virgin carbon fiber at 400oC treatment in air and nitrogen...... 77

Figure 4-11: Stacked XPS curves of C1s element for different carbon fiber types and treatments...... 79

Figure 4-12: Carbon C1S deconvolution XPS curves for recycled carbon fibers thermal treated in air and nitrogen...... 80

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Figure 4-13: Carbon C1S deconvolution XPS curves for virgin carbon fibers thermal treated in air and nitrogen...... 81

Figure 4-14: Stacked XPS curves of O1s element for different carbon fiber types and treatments...... 82

Figure 4-15: Oxygen O1S deconvolution XPS curves for recycled carbon fibers thermal treated in air and nitrogen...... 83

Figure 4-16: Oxygen O1S deconvolution XPS curves for virgin carbon fibers thermal treated in air and nitrogen...... 84

Figure 4-17: Nitrogen N1S convolution curves for virgin and recycled treated carbon fibers in air and nitrogen...... 85

Figure 4-18: Chlorine Cl2p convolution curves for virgin and recycled treated carbon fibers in air and nitrogen...... 85

Figure 4-19: Sodium Na1S convolution curves for virgin and recycled treated carbon fibers in air and nitrogen...... 85

Figure 4-20: PA12-PMPPIC reaction resulting in new carbonyl groups and a branched-crosslinked molecule aligned structure...... 87

Figure 4-21: Flexural and Tensile Strength properties of 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers...... 89

Figure 4-22: Flexural and Tensile Young’s modulus of 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers...... 89

Figure 4-23: Impact Energy values for 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers...... 90

Figure 4-24: Tensile and Flexural Strength comparison between pure PP/PA12 blends and blends using PMPPIC and MAPP surface modifiers...... 91

Figure 4-25: Flexural modulus comparison of PP/PA12 pure blends and with MAPP and PMPPIC surface modifiers...... 92

Figure 4-26: Tensile modulus results for PP/PA12 blends and blends with MAPP and PMPPIC surface modifiers...... 93

Figure 4-27: Impact energy test results for Pure PP/PA12 blends and with MAPP and PMPPIC surface modifiers...... 94

Figure 4-28: Tensile Strength results for untreated and 400oC treated Recycled and Virgin Carbon Fiber Reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers...... 96

Figure 4-29: Tensile Modulus results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers...... 98

Figure 4-30: Flexural Strength results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers...... 98

Figure 4-31: Flexural Modulus results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers...... 99

Figure 4-32: Impact Energy (Izod notched) results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers...... 99

Figure 4-33: Contact angle vs time for several solid-liquid systems...... 105

Figure 4-34: Surface energy values for PP/PA12, pure blends and with MAPP and PMPPIC surface modifiers...... 108

Figure 4-35: Polar and dispersive surface energies for pure PP/PA12 blends...... 110

Figure 4-36: Polar and dispersive surface energies for PP/PA12 blends with MAPP surface modifier. 110

Figure 4-37: Polar and dispersive surface energies for PP/PA12 blends with PMPPIC surface modifier...... 111

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Figure 4-38: Surface energy results for virgin and recycled carbon fiber reinforced polymers with MAPP surface modifier...... 111

Figure 4-39: Surface energy results for virgin and recycled carbon fiber reinforced polymers with PMPPIC surface modifier...... 112

xvii

List of Appendices

Appendix 1 -Virgin Carbon Fiber Data Sheet...... 111

Appendix 2 - Recycled Fiber Material Data Sheet...... 111

Appendix 3 - Polypropylene Material Data Sheet …...... 112

Appendix 4- MAPP Material Data Sheet…………………………………………………….. 112

Appendix 5 - Polyamide 12 Material Data Sheet……………………………………………. 113

Appendix 6 - Surface energy literature data for Polyamide 12……………………………… 114

Appendix 7 - Surface energy literature data for Polypropylene……………………………… 114

xviii

List of Symbols

Symbols Metric Unit

2 Af Frontal area m b Interfacial Constant -

Cd Drag Resistance Coefficient -

Crr Rolling Resistance Coefficient - d Dispersive Forces J.m6 g Gravity m.s-2

La Axial Carbon Fiber Direction -

Lc Perpendicular Carbon Fiber Direction - m Mass kg

ρ Density kg.m-3 p Polar Forces J.m6

Ptract Tractive Power W q Angular displacement ° (degrees) r Radius m

ɣd Dispersive Surface Free Energy mN/m

ɣp Polar Surface Free Energy mN/m

ɣt Total Surface Free Energy mN/m

-1 Vi Initial Velocity m.s

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-1 Vi+1 Final Velocity m.s

List of Abbreviations

AB Acid-Base Interactions

ASTM American Society for Testing and Materials

BSU Basic Structural Unit

CAD Computer Aided Design

CAE Computer Aided Engineering

CFRP Carbon Fiber Reinforced Plastic/Polymer

CMTP Chemithermomechanical Pulp

DMTA Dynamic Mechanical Thermal Analysis

FBP Fluidized Bed Process

FTIR Fourier Transformed Infrared Spectroscopy

GWP Global Warming Potential

HM High Modulus

HS High Strength

IR Infrared Light

LCA Life Cycle Assessment

LW London-van der Waals forces

MAPP Maleic Anhydride Polypropylene

MDI 4,4’ – Methylene Diphenyl Isocyanate

MFI Melt Flow Index

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OEM Original Equipment Manufacturer

OWRK Owens, Wendt Rabel and Kaelble equation/theory

PA 6 Polyamide 6

PA10 Polyamide 10

PA11 Polyamide 11

PA12 Polyamide 12

PA66 Polyamide 6’6

PAN Polyacrylonitrile

PMDI Polymeric 4,4’ – Methylene Diphenyl Isocyanate

PMPPIC Poly[methylene (Polyphenyl) Isocyanate]

PP Polypropylene

PVC Polyvinyl Chloride

RCF Recycled Carbon Fibers

RCFRP Recycled Carbon Fiber Reinforced Plastic/Polymer

RTM Resin Transfer Molding

SEM Scanning Electron Microscopy

SMC Sheet Molding Compound

SUVs Sport Utility Vehicles

TEM Transmission Electron Microscopy

TGA Thermogravimetric Analysis

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TMI 3-Isopropenyl-α,α-dimethylbenzene isocyanate

TPD Temperature Programmed Desorption

TPE Total Primary Energy

UV Ultraviolet Light

VCF Virgin Carbon Fibers

XPS X-Ray Photoelectron Spectroscop

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Chapter 1

1 Research Background and Literature Review 1.1 Background

Sustainable and efficient transportation is one of the environmental pillars for most countries committed to greenhouse gas emission targets. In the US, almost one third of total emissions come from road transport vehicles. Rising awareness of the consequences of pollution among the general population has led to a general effort to reduce the environmental impact of human activity, which is in turn reflected in stricter government regulations for vehicle emissions. For this reason, the National Highway Traffic Safety Administration (NHTSA) and the Environmental Protection Agency proposed a target in 2016 that calls for fuel economy standards to be reduced to the levels of 37.8MPG for passenger cars and 28.8MPG for light Trucks [1]. The automotive industry is working on parallel strategies to address emission targets in a timely manner in order to comply with environmental regulations around the globe. One such strategy is the development of new powertrain configurations, such as electric, hybrid and hydrogen, which use energy sources more efficiently. Another strategy for reducing the environmental footprint of road transport – one that directly pertains to the focus of this study – is the reduction of rolling resistance coefficients, aerodynamics drag and overall inertia. This can be better visualized with the tractive power demand equation below [2]:

Ptract = [m.g Crr + 1/8 ρ.Cd.Af.(Vi+Vi+1)² + m.r.(Vi+Vi+1) + 1/2 m.g.sin q (Vi+Vi+1)] ( I )

According to the equation above, mass (m) is a proportional variable in 3 of 4 terms associated with vehicle power and energy demands. With this assumption in view, vehicle mass reduction can significantly improve fuel saving and efficiency. As a quantitative illustration of this equation, consider that each 10% reduction in vehicle weight improves overall efficiency from 6%-8%, representing savings of approximately 0.3l/100km for petrol engines. For electric vehicles this means a 100km increase in range, assuming a reduction of 100kg in vehicle weight [3]. The use of low-density materials is necessary, and for this reason light-weighting materials used in the production plays an important role in the

automotive supply chain towards sustainability. Many lightweight strategies are available due to the wide range of materials used in vehicle production. However, in addition to the weight of a given material, it is also important to consider its composition, manufacturability and end-of-life characteristics to improve life-cycle-impact assessments. These factors are demonstrated in the FORD-DOE Multi Material Lightweight Vehicle study, which uses a life cycle assessment perspective to show a reduction of 16% in global warming potential (GWP) and total primary energy (TPE) over the course of a vehicle’s lifetime when lighter and sustainable materials are used. [4,5].

According to the study, polymer composites are the most promising materials for vehicle lightweighting, reducing the use of iron and steel, and mitigating the use of traditional petrol-based engineering plastics, as shown in Figure 1-1. It is expected that soon approximately one third of the total structural vehicle mass will consist of composite materials [5]. To consolidate this trend, different research strategies are being carried out in natural fiber reinforced composites [6,9], carbon fibers and more recently, recycled carbon fibers [7]. Within these alternatives, areas of research focus are: the availability of material science technical data for CAD and CAE designs; material selection, manufacturing techniques and optimization to improve the economics of the application; and finally product design testing and feasibility of application in the final product [1,8].

Figure 1-1: Structural (Body in white, closure, chassis, bumper) light-weighting strategy and material range forecast [5].

In composite materials, fiber-reinforced plastics are gaining in importance due to their manufacturability and applicability, allowing the design of complex and integrated parts in many automotive systems and improving vehicle plant assembly time by light-weighting modularization [8]. The use of injection molding and other processes, such as resin transfer molding (RTM) and sheet molding compound (SMC), is well-known and widely used for many thermoplastics. Polypropylene (PP) is the most widely used thermoplastic in the composite industry due to its low density, good mechanical properties, good dimensional stability, good temperature resistance and impact strength [9]. Among the range of researched fibers that are available for use as polymer reinforcement, carbon fibers show the best weight-stress ratio despite the low density of natural fibers. This is due to the known fact that carbon fibers have the highest specific modulus and specific strength of all reinforcing fiber materials. Carbon fibers are chemically stable in several environments, not affected by moisture, solvents, acids and bases [10].

For the reasons mentioned above, carbon fiber reinforced thermoplastics are considered the most desirable composite materials for the automotive industry. The high performance of carbon fiber composites, coupled with known and inexpensive processes used to produce thermoplastics, are opening new possibilities for the lightweighting of parts, substituting traditional glass fiber composites and competing with aluminum and magnesium, which are non-renewable materials. Carbon fibers have many applications, such as airspace, defense, wind energy sports goods and high performance vehicles. Particularly in the automotive sector, several parts are already in production for many high-profile vehicles, such as Ford GT. Carbon fiber shafts, “A” pillars, underbody structures and body panels are already used in low-volume racing and sports cars and in expensive vehicles. This market provides the knowledge basis for design and supply chain maturity. Some success cases are also being implemented in medium scale vehicles (Luxury Sedans and SUVs) [3,11]. However, the market volume as proportion of the whole is still small, as there are well-known challenges and roadblocks to bringing carbon fiber products to mass scale production in the automotive sector. These challenges and roadblocks are: supply chain maturity and lower cost precursors, high confidence level of design and validation data from research, improve multi-material joining with other lightweight structures, develop repairing techniques to improve serviceability [12], lower manufacturing residence times and finally cost effective recycling and reclaiming processes aligned with most recent governmental regulations.

Carbon fiber marketing is increasing significantly in other industry segments. Sectors such as wind turbines, aerospace and defense, sports and costumer goods pull up the total annual production of carbon

fibers to a commercial scale where growth is projected to reach approximately 142 kilo tons annually in 2020 (see Figure 1-2[13]). Around 96% of the total carbon fiber available in the market is made from PAN precursors.

Figure 1-2: Carbon fiber demand in four market sectors, measured in “kilo tons annual (kTa)” [13].

Owing to this significant market expansion, the carbon fiber market requires higher supply and production capacities to meet future market demand expectations. In some segments, such as aerospace the carbon fiber material weight composition reaches up to 50%, for example, in the new Boeing 787 and Airbus A350 models. Wind turbines pull the demand for construction of wind farms and are the main segment for carbon fibers, responsible for 46% of total market demand [14]. However, the business case to implement these projects is limited by several factors, such as the price of the carbon fiber material, the lack of a well stablished supply chain, low product development maturity and unavailable manufacturing technology for mass application [3,11]. Table 1 below shows how recycled carbon fibers can help improve the economic parameters in the cases cited. The cost of recycled carbon fibers (RCF) is at most half that of virgin carbon fibers (VCF). This is financially appealing, and can improve the business case for virgin carbon fibers themselves, considering the possibility of recycled material commercialization after first usage. It is also important to consider the energy savings in both manufacturing processes. RCF uses only 5%-10% of the manufacturing energy needed to produce VCF and therefore has a proven sustainability appeal [15].

Table 1:Comparative of recycled and virgin carbon fibers: manufacturing energy used and final price, pound per kilo [15].

The use of recycled carbon fibers also addresses an emerging environmental concern about the waste generated by using carbon fibers products. Considering average lifetimes, carbon fiber products in vehicles on the market today will be landfilled in the next 10 years. The period rises to 30 years in the case of wind turbines and airplanes. The major challenge for recycled and virgin carbon fiber reinforced polymers is that inserting the fibers into the polymer matrix increases recycling complexity. This is mainly due to the processes traditionally used to separate the fibers from the matrix and to obtain pure polymers and fibers from these composites. These processes are very expensive, energy intensive and sometimes not fully sustainable, compared with traditional thermoplastic recycling processes. Consequently, the recycling total cost of the material is severely impacted, which in turn affects its overall life cycle assessment.

Figure 1-3: Recycled Carbon Fibers Life Cycle for Automotive sector.

In addition to this, many carbon fiber products use thermoset matrices, which are known for their non-recyclable character and are generally landfilled or incinerated, emitting harmful gases. Carbon fiber recycling technology is relatively recent, and much research remains to be performed in at least three areas: integrating this technology into the actual supply chain to meet global demand; decreasing the final price and energy spent on its recycling processes; and finally, bringing to market recycled carbon fiber materials that meet technical performance specifications. Considering all these points, Figure 1-3 shows a new perspective for the overall carbon fiber lifecycle in the automotive industry. The introduction of recycled carbon fibers into the overall chain brings a new pulse to the carbon fiber industry as a whole.

Some promising data about the end-of-life of carbon fibers are shown in the literature [15,16]. In the LCA study performed by FORD, the impact of waste and scrap generated using composites and the allocation impacts for recycling these materials, which include carbon fiber, represented the third and fourth main environmental issues. The results shown in this research represent scrap and disposal scenario for metal stamping process, plastic, tire and carbon fiber auto parts and transportation energy that affect Global Warming Potential and Third Pole Environment and less than 1% for scrap and 3% for transportation [4,5]. The use of recycled carbon fibers can not only reduce but also contribute to a vehicle recyclability level of 85% of weight content, complying with the end-of-life vehicle target in the European Union (EU) [3].

1.2 Carbon Fibers (CF) and Recycled Carbon Fibers (RCF) Overview

1.2.1 Carbon Fibers

Carbon fibers are defined as fibers made from a polymeric precursor or carbon allotrope building blocks with a minimum carbon content of 92% [17,19,20,21]. Their molecular structure is basically anisotropic, formed by hexagonal graphitic ribbons. Their layers are bonded by weak Van der Waals forces between the parallel planes. When these layers form a stack, a pattern built from hexagonal or rhombohedral unit cells, a graphite structure is formed. When the formed stacks are aligned in a random pattern, a structure called turbostratic is formed. Examples of graphitic and turbostratic structures are presented in Figure 1-4. In the graphitic crystal structure the measured distance between individual layers is 0.3354nm, while in the turbostratic structure it is 0.34nm. This difference is explained by the presence of aligned sp2 bonds in the graphitic structure, resulting in a more compacted cell arrangement as compared with the turbostratic structure. For commercial PAN carbon fibers, the interlayer distance is approximately 0.355nm.

To avoid differences between the graphitic and turbostratic structures in the carbon fiber, a basic structural unit (BSU) of carbon fibers was firstly defined by Fourdeaux [18]. The BSU can be considered

a primary representation of carbon fiber molecular arrangement in the bulk material. It is determined as a two-dimensional ribbon-shaped graphitic layer with a width from 5nm to 6nm and a length of several hundred nm. A carbon fiber ribbon is a columnar arrangement composed of multiple BSU alignments of turbostratic planes. The level of alignment is quantified by the angle q formed from its axial direction called La and the angle f formed from the perpendicular fiber axis direction Lc, as demonstrated in Figure 1-5 and Figure 1-6 below.

Figure 1-4: Basic turbostratic structure of carbon fiber (left) and basic structure of graphite crystal (right) [17].

Figure 1-5: Carbon Fiber’s Basic Structural Unit (BSU) and micro fibrils on left and ribbon structure model for carbon fibers on the right [17, 21].

Figure 1-6: Voids appearance mechanism by ribbon structure misalignment [17, 21].

When these ribbons are oriented in parallel to the fiber axis, the material possesses an exceptional resistance to traction due to the interlocked structure. However, in many cases, BSU micro vacancies (as shown on left side of Figure 1-4 and Figure 1-5), change the direction of the ribbon structure, and trapped voids are generated as a consequence. One reason for these micro vacancies to appear is the lack of temperature and gas flow control during the heating processes, resulting in an incomplete cyclization process that leads to misoriented graphitic formation. Another reason is the presence of oxygen or nitrogen functional groups. These groups react with the carbon end structure, modifying the graphitic formation pattern during the heating processes. Alternatively, they are converted to gases and trapped in the bulk of the material, acting as a void generator in the material [21]. The average length of these voids falls in the range of 20-30nm for micro fibrils of about 1-2nm in width [21,24]. These voids change the mechanical properties of the carbon fibers significantly and are generally controlled during the manufacturing process to produce high strength or high modulus carbon fibers. Overall, the turbostratic structure provides higher stress strength because its random position prevents interlayer displacement under a stress load. The graphitic structure provides carbon fibers with a higher modulus due to its organized bulk pattern and lower interlayer distances under stronger bonds, requiring higher energy to be broken or moved.

Another important structural description is the carbon fiber’s varying cross-sectional configurations. These differences in cross-sectional patterns can be shown to originate simultaneously in the type of precursors that influence the graphitic or turbostratic layer arrangement, the level of tension applied during the winding processes, the process temperature and the molecular weight [22,23]. The varying cross-sectional configurations that stem from these factors impact the anisotropy level of the fiber and its final properties. Christensen performed mathematical analysis to understand the differences in mechanical behavior between concentric and radial orthotropic configurations [22]. Herakovich made several experiments comparing different fiber cross-sectional configurations and concluded that different isotropic patters showed different mechanical properties, while orthotropic cross sections demonstrated identical performance when solicited axially [22]. The impact of the cross-sectional results on the final mechanical properties of composite materials was also shown. The patterns studied are shown in Figure 1-7.

As already indicated in the structural description above, carbon fibers are known for their high mechanical properties. Tensile strength is up to 7GPa for high strength types, and elasticity modulus is up to E≤900Gpa for high modulus carbon fibers, with good creep resistance and an associated density varying from 1.75-2.00 g.cm-3. Carbon Fibers are resistant to all chemical species; however, they lack resistance to oxidizing agents, such as hot air and flames, initiating self-combustion above certain temperatures [17,20].

Due to its random configuration, oxygen-containing groups can be found on the carbon fiber surface in varying levels. The presence of these groups is mainly due to processing and manufacturing factors. The resulting surface groups are divided into basic (pyrone-like structures), neutral (Quinone-like structures) and acidic (carboxylic and hydroxyl groups). These groups are shown as an example in Figure 1-8. In the acidic group, strongly acidic (group I) and weakly acidic (group II) levels can be found, in addition to hydroxyl and carboxylic groups (groups III and IV) [25].

Figure 1-8: Carbon fiber surfaces oxygen groups [25,29].

The origin of the precursor material is crucial for carbon fiber production and performance. Each precursor has an intrinsic way of transforming into final graphitic and/or turbostratic structure. Due to its unique character, the type of precursor used has an impact on all further processes, such as spinning, stabilization and carbonization of the precursor. Some of the most commonly-used precursors for carbon fiber production are: Pitch, Cellulose, Rayon, Poly(ethylene), Lignin and Poly(acrylonitrile). Poly(acrylonitrile) PAN has been used as precursor material for carbon fibers since 1961. Today, PAN is the most widely-used and important precursor for carbon fibers [17].

The automotive industry has already set a range of design targets regarding carbon fibers. The fibers should cost less than $11–$15.40/kg and have a tensile strength of 1.72 GPa (250 ksi; 0.17 kg-f.mm- 2) and a modulus of 172 GPa (25 Msi; 17.53 kg-f.mm-2) to be considered suitable for use in mass product development [11,19,26]. However, conventional PAN carbon fibers are estimated at $25.15/kg [11,19]. Carbon fibers are technically viable as an input into the automotive production process. They satisfy all mechanical property requirements and comply with the design sections of relevant engineering specifications. However, research is still required to make carbon fibers cost-effective. The various factors involved in their large-scale manufacturing process still need to be developed and optimized. This is the next frontier in the adoption of carbon fibers by the automotive industry.

Today, the costs associated with precursors amount to almost half of the total carbon fiber manufacturing costs [19]. In addition, the low yield and slow graphitization rate limits the widespread use of alternative precursors, which is further jeopardized by the low market demand for these alternative precursors compared with PAN carbon fibers. The price of PAN precursors is tied to the global crude oil price and is subject to the fluctuations in that market. Since petroleum is a non-renewable energy source, the marketing trend is pessimistic about petrol commodity prices. Some alternative oil-based precursors, such as textile PAN, pitch and polyolefin-based precursors, have a lower cost of production. However, petroleum dependence might represent a supplier source risk in the future due to prioritization of resources for most critical sectors.

Figure 1-9: Cost contributions of all stages of carbon fiber manufacturing [19].

The manufacturing process for carbon fibers is complex, detailed and sensitive, with several factors that impact final fiber properties. Each precursor material is submitted to a preliminary winding process that transforms the bulk material precursor to a fiber structure capable of being processed as carbon fiber. After the fiber precursor is subjected to the winding process, the manufacturing process proceeds to the oxidation phase of the precursor (also known as the stabilization phase) in temperatures between 200oC- 400oC. This is the longest manufacturing stage and accounts significantly for the overall low yield of the process, because most of the raw material weight is lost at this step by gas volatilization. Many studies have been done on this phase to understand and minimize these impacts [17,21,27,41]. This step is important to assure that the fiber will not melt or fuse in subsequent processes. The material is submitted to heat in air or another oxygen atmosphere. The temperatures should be controlled because the overall process is exothermic. The heat released in the reactions may bring the total temperature outside the range of 200oC-400oC, which impedes the complete cyclization to form aromatic carbon rings. The yield level of cyclization of its aromatic carbon rings is the main factor that defines the structure of the carbon fiber in the carbonization process, impacting the fiber’s level of alignment and consequently its crystallinity. Cyclization leads to the formation of a graphitic or a turbostratic structure during the carbonization process, which distinguishes the carbon fiber as a type HM or HS. Although the Young’s modulus of the fiber structure is not affected at lower temperatures (up to 200oC), a high gas evaporation is observed, as shown in Table 2 [36], significantly decreasing the overall process yield. Young’s modulus improvement initially appears at temperatures close to 400oC. An example of the chemical structure transformation during the stabilization process for PAN precursor carbon fibers is shown in Figure 1-10 [73].

There are three alternative heat-treatment process used in carbon fibers` stabilization. The isothermal process is very time consuming but can achieve better results by allowing enough time for complete formation of aromatic rings, resulting in a higher percentage of material graphitization during the carbonization phase. In the step-wise temperature increase approach, the fibers are placed in a stationary furnace where temperature is increased up to the maximum allowed at this process. The fibers are then removed from the furnace and proceed to next step. The third approach involves placing the carbon fiber material in a tubular furnace and subjecting it to increasing temperature under a flow of oxygen. The third process is the most widely-used in industry because it provides the best trade-off between efficiency and quality of final fibers. At the stabilization phase, material density increases in most cases. In PAN carbon fibers final stabilization material density values varies between 1.18 g.cm-3 and 1.38 g.cm-3 [21].

Figure 1-10: Cyclization phenomena during oxidation phase (on left) and dehydrogenation and denitrogenation phases during carbonization process (on right) [73].

The second phase is called carbonization. This is the phase where both graphitic and turbostratic structures are formed in the carbon fibers, providing the material’s known high strength and high modulus. The process is performed in an inert atmosphere (generally nitrogen or argon) and can be divided by temperature, depending on the fiber mechanical property output desired: low temperature carbonization (for temperatures of up to 1000oC), high temperature carbonization (for temperatures up to 2500oC) and ultrahigh temperatures (for temperatures above 2500oC) which is also called graphitization. At temperatures of up to 1500oC, the process results in fibers with good fiber strength. However, temperatures above this range are required if the final objective is fibers with higher Young’s modulus values. As preparation for the carbonization phase, it is recommended to dry the oxidized fibers to remove moisture that can create trapped voids in the final structure. The carbonization phase is characterized chemically by the elimination of nitrogen, oxygen and hydrogen and by several reactions between the fiber elements and the atmosphere. Dehydrogenation appears at temperatures between 400oC-600oC, where it is characterized

by the elimination of hydrogen mainly in the form of H2O and HCN. Denitrogenation occurs above these temperatures, with higher levels of N2 being released from the material jointly with other gases. The list of chemical events occurring at each temperature stage and the gas concentration curves in the low carbonization temperature range are shown in Table 2 and Figure 1-11, respectively [21,36,50].

Table 2: Chemical events identified at different temperatures for the oxidation and carbonization processes [21, 36].

Figure 1-11: Fiber gas concentration curves during the oxidation and carbonization processes [21,50].

Surface treatment is an important step for improving carbon fiber adhesion in the material’s final application. The first objective of carbon surface treatment is to modify the surface morphology. The surface area per weight of material (roughness) is increased, which results in better mechanical interaction between fibers and matrix. The second objective is to change the surface chemistry through the formation of surface groups (mainly carboxylic and hydroxylic groups). This results in better acid-base interactions, improving the dispersive and polar contributions of hydrogen/covalent bonds and Van der Waals forces in fiber adhesion [21,29]. There are several types of surface treatments available, and the type used depends on the final purpose of the fiber. Gas phase oxidation is performed with air or O2 diluted in an inert gas such as N2, CO, CO2, NO, NO2, O3, or steam. Liquid phase oxidation is performed by using liquids such as: HNO3, NaOCl, KmnO4, NaClO3, Na2Cr2O7 and NaIO4. Anodic oxidation is performed by running electrical current through the fibers and coating their resulting ionized surface with the polymer sizing [21].

Figure 1-12: Interface between fiber surface and polymer matrix [21].

The final step before obtaining the completed carbon fiber product is the sizing, which consists in the application of a finishing coating on the surface of the fiber. Sizing improves the inter-filamentary adhesion of the fiber, helping avoid surface damage during transport by maintaining the compacted shape of the tow. It further aids the wetting out of the fiber into a matrix by improving its dispersion. Finally, it prevents damage during processing by acting as a lubricant. Sizing can be performed by deposition onto the fiber surface from a solution of polymer or, alternatively, by deposition from a polymer onto the surface by electrodeposition, electropolymerization or plasma polymerization [21].

1.2.2 Recycled Carbon Fibers

Carbon fiber composites are made from a variety of carbon fiber types, such as woven, non-woven, chopped, long and short fibers and many others, and are based on a variety of precursors, including PAN, Lignin, pitch-based, cellulose–based and polyethylene-based [17,20]. Many of these products, when produced, have their composition and manufacturing processes closed and patented, making it more complex to track their life cycle. The form to which the carbon fiber product is aggregated—whether it is a thermoplastic or a thermoset composite—will determine the type of recycling process to which it will

be submitted. Generally, the type of residue defines the carbon fiber’s value in the market according to two designations. Post-industrial residues are more valuable because they generally have a size pattern and do not suffer much damage caused by environmental and loading cycling exposition during their life cycle. Also, these products can be easily tracked, making available historic knowledge of its mechanical data and its chemistry composition.

Post-costumer residues are more complex to process and track, adding reclaim process chain costs. However, they are cheaper due to availability and the growing need mitigate environmental impacts. From an environmental perspective, post-consumer residues should be the focus of attention due to their impact as waste and landfilling. A lack of product data means that there is less control over the recycling process, affecting the performance of these fibers and generating lower-value final materials and products.

Some authors presented recycled carbon fibers with good performance in comparison with virgin carbon fibers, as shown in the Figure 1-13 [15]. Their performance level can reach up to 95% compared with virgin ones. Recycled carbon fibers show competitive performance especially for low-cost, semi- structural applications in the automotive industry as an alternative for traditional glass fibers or expensive polymer blends. Several challenges for the development of recycled carbon fiber materials still remain: a cheaper fiber reclaiming processes, a reliable and mature RCF supply chain, scalability of recycled carbon fibers and development of a portfolio of recycled carbon fiber reinforced polymers that complies with design requirements of automotive manufacturers by further understanding fiber behavior in the matrix and its final properties [15,16]. The type of reclaiming process directly impacts the fiber’s final properties and generally decreases the fiber diameter.

Figure 1-13: Carbon fiber performance comparison between virgin and recycled carbon fibers (a) Young’s modulus (b) Tensile Strength (c) interfacial shear strength [15].

1.3 Recycled Carbon Fiber (RCF): Reclaiming Processes and RCFRP-Recycled Carbon Fiber Reinforced Plastic Manufacturing

To be used as thermoplastic reinforcement, the material can be either mechanically shredded or crushed, and then grinded to be reinserted through injection molding in the new matrix. Alternatively, the waste material passes through a previous preparation by being oxidized at high temperatures to eliminate all matrix and contaminants, thus assuming the “fluffy” pure form, and is then reinserted in the new matrix. The mechanical process has the advantage of simplicity and recovers the fiber resin inside the polymer matrix. However, the trade-off is that it results in lower mechanical performance properties due to its heterogeneity.

Figure 1-14: RCF reclamation and process review chart [15].

To solve this issue, fiber preparation is necessary by oxidation, allowing cleaning the fiber surface from resin to improve its adhesion [15,29]. A summary of all process available today is presented in Figure 1-14.

With pyrolysis, processed carbon fibers can recover their original fiber surface shape with some loss of diameter and severe defects if the process is very aggressive [30]. However, diameter can be improved by the remaining deposition of resin if residues could not be fully eliminated [31]. Setting the correct temperature, atmosphere flow and time during the pyrolysis step are crucial factors in improving interface quality between the recycled fiber and the polymer matrix. The top pictures in Figure 1-15 show a comparison in surface quality between virgin clean fibers and recycled fibers with some residual matrix. Although these residues can improve mechanical adhesion between fibers and matrix, this improvement is not significant compared to the loss of chemical bond adhesion. The bottom pictures show a comparison between pyrolysis-reclaimed RCF and VCF: the surface was completely recovered, which demonstrates the importance of the pyrolysis process in improving the quality of the RCF.

Figure 1-15: Upper, clean fibers (A); fibers with residual matrix (B). Lower, comparison between virgin carbon fibers (left) and recycled carbon fibers (right). [30,31]

In the fluidized bed process (FBP), the chambers are fed with the fiber scrap. Inside these chambers a steady flow of hot air passes over the material and decomposes the old polymer matrix once deposited

on fiber surface. By decantation, the heavier removed contaminants and old polymer particles are settled down in a silica bed and the fluffy fibers are carried by the stream flow [16]. This process has the advantage of having a well-established process in the industry for several other applications. However, due to fiber exposition to an oxidized environment at high temperature, degradation occurs and a fiber length patterned size is not easy to achieve.

When RCF processing is complete, the final fibers recover their fiber mechanical properties to a good degree, as shown in Figure 1-13. Some studies show a recovery level of at least 80% or more for tensile strength and Young`s modulus [15,30]. When compounded as composite material, a good interface adhesion between recycled fibers and the matrix can improve mechanical properties to around the same levels registered by the same composite using virgin carbon fibers.

1.4 Material Characterization Techniques for Carbon Fibers

Several surface characterization techniques have been used to identify the surface composition of carbon fibers. The most common are SEM and TEM microscopies, which are useful to investigate surface morphology and fiber diameters and to verify the interface adhesion behavior between the fibers and matrix [29-32]. However, to understand the fundamental chemical mechanisms involved in carbon fiber behavior, additional characterization techniques must be used, such as Thermogravimetric Analysis (TGA), Fourier Transformed Infrared Spectroscopy (FTIR), X-Ray Photoelectron Spectroscopy (XPS), and Inverse Gas Chromatography. These four techniques are the most common and are used in many high- impact carbon fiber studies [21].

Characterization methods on recycled carbon fibers are not new. However, the availability of data is low [31,33,34] due to the lack of market appeal for this material type. Nowadays, this data is crucial, since the supply of recycled carbon fibers on the market is growing, whereas a lack of confidence in recycled carbon fiber properties [35] and in the final composites of recycled carbon fibers keeps market demand low. The importance of this type of data is crucial to reach a higher design engineering confidence level, because in many supplier sources of recycled carbon fibers its performance in composites are not well known. There are multiple unknowns that contribute to the lack of a complete tracking identification of recycled carbon fibers, including whether the material suffered a contamination attack or any thermal

or mechanical fiber damage during the manufacturing processes (mainly in post-industrial residue carbon fibers), as well as the kind of finishing, load and environmental conditions the product was subjected to during its life-time usage (mainly in post-costumer residue carbon fibers). Nonetheless, it is very important for research and academic institutions to establish engineering data links and exchanges between manufacturers of recycled carbon fibers, the automotive supply chain and OEMs. Full material characterization will help identify and fill all outstanding gaps in data collection and material quality, in order to improve these materials to a level that meets all standards and specification requirements.

1.4.1 Thermogravimetric Analysis (TGA)

The Thermogravimetric analysis technique is important to understand the thermal behavior of carbon fibers. Samples are placed in an atmosphere- and temperature-controlled chamber. Their mass loss is evaluated in relation to the temperature. Generally, small weight samples are used, from 5mg to 10mg, and temperatures can vary from 0oC up to 1000oC. With this test, it is possible to evaluate the degradation rate of the material in relation to temperature and detect the peaks of material degradation under a determined temperature. TGA is a very important technique for carbon fibers, because its wide treatment temperature range of up to 1000oC can accommodate the high temperatures used in the recycled carbon fiber manufacturing process, as previously described. In addition, TGA allows stabilization process behavior to be analyzed under air flow conditions (Figure 1-16), while pyrolysis phenomena can be further understood under a nitrogen atmosphere with all carbon fiber precursors available.

Figure 1-16: Comparison of TGA thermograms of (a) unstabilized PAN fibers and stabilized PAN fibers at (b) 235oC, 30 min; (c) 235oC, 2h; (d) 235oC, 4h; (e) 235oC, 8h; (f) 235oC, 16h. [36]

The temperature point of many chemical carbon fiber phenomena can be detected through localized weight-loss percentage in TGA and further analyzed by the following two characterization techniques described below (FTIR and XPS). Stabilization or oxidation singular events, such as loss of mass, can be further understood when TGA is performed in the temperature range from 100oC to 400oC, whereas the maximum degradation temperature can be detected with air flow in the temperature range from 500oC to 800oC. Pyrolysis or carbonization phenomena can be further analysed in nitrogen flow from 400oC to 1000oC. There is much TGA research performed that places special focus on the carbon fiber reclaiming process. Another interesting application was using TGA to detect the carbon fiber content in CFRP [28]. It was shown that the degradation temperature improves with higher content of carbon fiber in the composite. TGA studies performed on commercial available recycled carbon fibers to understand their thermal performance were not found in literature, although they are available for virgin carbon fibers, as shown in Figure 1-16 for the stabilization stage of PAN fibers [36] and in Figure 1-17 for brominated pitch based carbon fibers [74] .

Figure 1-17: Curves in argon flow for pitch carbon fibers brominated for different time treatments [74].

The recycling process of carbon fibers was further investigated by performing TGA on recycled carbon fiber composites under reduced oxygen levels [31]. This improved understanding of their combustion kinetics and the degree to which the fibers to be used as new material reinforcement avoided degradation. Pyrolysis phenomena at different temperatures were also fully investigated using TGA, with a view to reclaiming carbon fibers from epoxy resin composites. Results showed that partial oxidation does not promote significant fiber damage. With the pyrolysis optimization performed, it would be possible to indicate the parameters for an industrial-scale carbon fiber recycling process by pyrolysis. The results, presented in Figure 1-18 below, show a maximum weight loss limit of 31.8% for the resin material in a nitrogen atmosphere and total combustion at 900oC in air. Time is another important factor evaluated by TGA in pyrolysis and is also presented below. For higher temperatures, more time spent means more weight loss degradation when air is used. Finally, the recycled carbon fibers presented good mechanical performance after reclamation [76].

Figure 1-18: TGA analysis on pyrolysis for reclaimed carbon fibers from epoxy resin composites. Left, temperature treatment vs weight loss. Right, time dependency on isothermal treatments [76].

1.4.2 Fourier Transformed Infrared Spectroscopy (FTIR)

FTIR is another well-known and widely used technique to characterize carbon fiber materials. The technique proceeds through the emission of Infrared radiation through the sample. Some of the radiation is absorbed by the material, while the rest is transmitted. When it is absorbed and/or transmitted, the radiation changes the energy level of the atoms and molecules of the material, resulting in a wavelength spectrum that is known as the fingerprint of the sample.

Since this fingerprint reveals most of the molecular bonds present in the material, FTIR is very useful in characterizing the qualitative aspect of carbon fibers. Understanding the chemistry groups contained in the precursors of carbon fiber allows for the presence of these groups to be confirmed in the carbon fiber and for new groups to be detected. Many of these new chemical groups originate in the stabilization and pyrolysis stages of the RCF and VCF manufacturing process, when the material is subjected to high temperatures and gas flow. In addition, gas/liquid surface treatments and sizing processes introduce new carbon fiber surface chemistry groups and characterize the behavior of these surfaces in fiber interfacial adhesion with the polymer matrix, which in turn influence the performance of the final material composite.

The example in Figure 1-19 shows the value of FTIR analysis in understanding precursor contribution to final carbon structure and chemical constitution. FTIR spectra vs transmittance of cellulose-based carbon fibers under different carbonization temperatures were performed by Ma, Xiaojun [37]. Hydroxyl groups (3427-3429 cm-1) decreased as temperature increased. The analysis also showed small methylene absorption peaks (2925-2850cm-1). A C=C benzene ring structure appears at 1610- 1504cm-1 and some absorption peaks at 1631-1639 cm-1. C-O-C was also detected but did not change absorption values after 500oC. In conclusion, FTIR also indicated that the carbonization process decreased the –OH, -CH2, -O-C and phenyl groups. Finally, the researcher found that the analyzed cellulose-based carbon fiber sample was difficult to graphitize to create higher modulus carbon fibers [37].

Figure 1-19: FTIR spectra of cellulose carbon fibers under different treatment temperatures [37].

Figure 1-20 shows a different analysis, FTIR spectra vs absorbance from UV-stabilized Lignin- based carbon fibers. The figure shows the spectra range from 900-1800cm-1. After UV stabilization, some small changes were verified on C=O and C=C groups [38].

Figure 1-20: FTIR absorbance spectra of UV- stabilized lignin- based carbon fibers [38].

A weak point of FTIR analysis for carbon fibers has to do with the material’s high infrared absorption properties, leading to a scattering of the infrared radiation used in FTIR. Therefore, characterization is incomplete and further investigation is required to quantify the material’s composition fully. The most common additional technique is X-Ray Photoelectron Spectroscopy (XPS). A comparison between XPS and FTIR characterization of the carbon fiber surface was performed by T. Ohwaki and H. Ishida on PAN-based carbon fibers treated by chemical and electrochemical oxidation. FTIR sensitivity was comparable to that of XPS. The study also revealed that oxidation time is relevant for fiber surface functionality. As evidence for this finding, consider the more pronounced appearance of C=O groups at 1720 cm-1 wavelength as treatment time increased, as shown in Figure 1-21. Overall the study demonstrates that, in addition to mapping the infrared absorption properties of carbon fibers, FTIR is useful for determining changes in their surface chemistry and adhesion behavior caused by different treatments, such as oxidation [39].

Figure 1-21: Normalized IR spectra for untreated carbon fibers (a) and fibers treated by chemical oxidation from 30min to 6 hours from (b) to (h) [39].

The surface group chemistry of various surface-oxidized HT and HM carbon fibers was studied by U. Zielke et al. The results are shown in Table 3 below, which also includes surface group intensity [40]. Complementary XPS analysis was performed and will be discussed in section 1.4.3 below. Many of the surface groups presented in the table are also confirmed by Hopfgarten to be present on the fiber surface and at 50nm below the surface. The groups found on the surface in order of intensity were: C-O, C-OH, C-S, COOH, C=O and C-Cl. At 50nm below the surface the most common groups were: C-O, C- S, C=O and COO [21].

Table 3: Surface chemical groups for several HM and HS carbon fibers, as revealed by as result from FTIR analysis [40].

1.4.3 X-Ray Photoelectron Spectroscopy

XPS is a versatile technique used not only to determine the surface composition of the carbon fiber but also to analyse the differences in surface chemistry and bulk structure between different carbon fiber precursors. One of the drawbacks of XPS is that it is not sensitive to H or He elements. For this reason, other characterization techniques, such as FTIR, should be used to qualify the chemical fundamentals when adopting new manufacturing optimization techniques, surface treatments and sizing. Nevertheless, XPS remains crucial for detecting and quantifying differences on bulk structure.

Figure 1-22: Stacked XPS chart for different time oxidation PAN carbon fibers sized with epoxy resin [41].

Figure 1-22 above is a typical example of an XPS application for carbon fiber from PAN precursors oxidized at different time periods and finished with an epoxy size. The values demonstrate a increase in Nitrogen from 6% to 9% composition, which is characteristic of unsized fibers. Oxygen composition increased from 9% to 13% with treatment. The levels of the various elements can be thoroughly interpreted with an atomic composition chart showing each % composition [41].

Figure 1-23: Typical XPS Spectrum of carbon kinetic energy region spectrum (1486.6 eV) showing several binding energies of C1s belonging to different functional groups [42].

XPS and temperature programmed desorption (TPD) analysis of the carbon fiber surface characterization and its adhesion with thermoplastics was performed by G. Krekel [ 42]. This study showed that oxidized fibers presented higher surface concentrations of hydroxyl and ether groups and smaller carboxylic and lactone groups compared with non-oxidized fibers. To improve carboxylic oxidation groups, the Ozone treatment was recommended. The XPS analysis also showed that at 400oC in oxygen all carboxylic groups were decomposed in hydroxyl and ether. Results differed with ozone treatment, where stronger carboxylic groups were detected. The detailed carbon region spectra from this study are shown in Figure 1-23.

An important comparison between XPS and FTIR techniques used with carbon fibers was performed by T. Ohwaki and H. Ishida. Results from the FTIR analysis were discussed in the previous topic. Considering the XPS analysis, the fiber surface of studied samples consisted of 79.8% carbon, 17.6% oxygen and 2.6% nitrogen. The oxygen/carbon ratios are presented in Table 4. Within the C1s spectrum, the presence of two different carbon species were detected, and they were not associated with the turbostratic or graphitic form when the samples were treated by chemical or electrochemical oxidation. The graphitic carbon peak is shown at 284.5 eV, while two other peaks were shown at 286 eV and were identified as C-O (ether/hydroxyl groups) and C=O (ester/carboxyl groups) at 288.4 eV. One of the findings of this work is that oxidative treatment on carbon fibers has a slightly negative impact on tensile

strength. In addition, when a cyanate polymer is used as a matrix, some functional groups reduce the thermal stability of the matrix in the fiber-matrix interphase [39].

Table 4: XPS results showing O/C ratio for different carbon fiber oxidation treatments [39].

A similar analysis was performed by Connor, M. in 2008 for recycled carbon fibers. In this study, unsized and sized (CF-A and CF-B) virgin carbon fibers from Hexcel (AS4 and IM7 types) were compared with two different recycled carbon fibers (CF1 and CF4), namely Hexcel AS4 fibers reclaimed by Adherent Technologies and Milled Carbon. Recycled carbon fibers presented a higher level of hydroxyl and carbonyl groups compared to virgin fibers and a smaller portion of graphitic carbon, as shown in Table 5 [44].

Table 5: Surface group compositions from XPS analysis for virgin and recycled carbon fibers [44].

1.5 Surface Energy and Wetting Studies in Composite Materials

Adhesion mechanisms have been investigated by the automotive and aerospace industries for more than 50 years. Initially the interest was focused on the polymer surface and paint substrate layer and adhesives [45]. Recently, with the urgent appeal for lightweighting and sustainable materials, the existing

characterization analyses were complemented by an investigation into the mechanical properties of the materials, blends and composites, since they were not sufficient to understand fully the scientific fundamentals of engineering polymers and composites. Techniques to investigate the interfacial adhesion between polymer blends and the polymer matrix with fiber reinforcements have been developed to fill the scientific gaps and to complement traditional material science characterization techniques.

The scientific fundamentals on adhesion and surface tension are available from several textbooks [46, 47, 51]. There are five types of adhesion, as shown in the Figure 1-24. Mechanical adhesion is defined by roughness friction between the material surfaces. Chemical adhesion is defined by chemical reactivity of the interface surface groups, resulting in covalent and hydrogen bonds. Dispersive adhesion is based on molecular dispersive forces defined as Van der Waals bonds. Adhesion by diffusion is characterized by interfacial adsorption phenomena causing entanglement of molecules supported by dispersive forces. Finally, electrostatic adhesion takes place through attraction between different electrical potentials from charged or ionized surfaces [48].

(a)

(b) (c)

(d) (e)

1.5.1 Contact Angle and Surface Free Energy

The level of wettability is defined by the hydrophilicity or hydrophobicity of the liquid with the material surface, most commonly attributed to water. Contact angles higher than 90o characterize the material as hydrophobic, and angles lower than 90o as hydrophilic. Young considered these parameters using a known surface tension liquid to determine the strength of adhesion for a system, as displayed in Figure 1-25 below.

Figure 1-25: Surface tension forces for determination of Young equation [49].

Table 6: Mathematical representations for different theories of surface adhesion [45].

From this system, the use of cosine force relationships at the equilibrium force state leads to the first Young equation (see Table 6) [45], which defines the surface tension between a liquid and a solid surface. With this equation, and knowing the surface tension of several liquids in advance, the surface tension of a solid can be calculated with confidence. One limitation of the Young equation is that it presupposes ideally planar surfaces. It is known that in real systems material chemical heterogeneity,

roughness and environmental conditions increases the range of contact angle hysteresis, which is defined as the difference between the advancing (maximum) contact angle and receding (minimal) contact angle.

Two main theories define the surface tension equations that are recently used. The first approach considers dispersive forces and polar forces (also known as Van der Waals and Hydrogen bond forces, respectively). The second uses only one contact angle measurement and considers the equation of state. The Fowkes equation is a variation of the Young equation and is listed as the second equation in Table 6. It was derived by considering that surface free energy is divided into two parts: dispersive forces, which are variously described as London, Keesom, Debye and Lifshitz and Van der Walls forces; and hydrogen bonding forces between acid-basic components. These forces are shown in the Table 7 below [51].

Table 7: Dispersive and Polar Interaction forces considered for determination of surface energies' equations [51].

The Fowkes equation (the second equation in Table 6) calculates surface energy by taking only dispersive interactions into consideration. The geometric mean theory was further developed to incorporate polar forces (hydrogen bonds). The geometric mean theory is also known as Owens, Wendt, Rabel and Kaelble (OWRK) theory and is the third equation in Table 6. This theory extends the Fowkes equation by calculating the surface tensions of solids and liquids as the geometric mean of both dispersive and polar forces. Another variation considering both dispersive and polar forces was introduced by Wu and is presented as the fourth equation in Table 6. Wu’s equation uses the harmonic mean instead of the geometric mean. Since these methods take both dispersive and polar forces into account, they require the use of a minimum of two liquids to calculate the surface free energy: one polar and other non-polar. Water is extensively used as the polar liquid, while several organic liquids such as hexane and diiodomethane are used as non-polar liquids.

Acid-base theory introduces some changes in the way the polar and dispersive components of surface energy are represented. The polar term in the Fowkes and Geometrical mean equations is represented by the superscripted letter “p”. This now changes to “AB”, referring to the hydrogen bonds as acid-base interactions. The dispersive forces, previously represented by the superscripted letter “d”, are now labeled as “LW”, indicating all London-van der Waals forces. The polar term AB is further divided by superscripted positive and negative terms, which indicate contributions from electron donors (-) and electron acceptors (+). Given these considerations, the surface energy acid-base equation is combined with Young equation on the left side to form the fifth equation in Table 6.

The last equation in Table 6 is the equation of state developed by Kwok and Neumann. It defines interfacial tension by a formation of two equations and two unknowns (interfacial tensions of solid, liquid and vapor states) and the use of an interfacial constant b.

1.5.2 Carbon Fiber Surface Energy Studies

There are several recent efforts to determine the contact angle, and consequently the surface energy, of carbon fibers. As demonstrated earlier in Chapter 1, carbon fibers are subjected to surface treatments as part of their manufacturing process. It is therefore important to understand the impact of surface treatments on the final adhesion between carbon fibers and their interface materials. Of the many

methods used to determine surface energy, the most common is the Wilhelmy’s method [46, 47]. Several studies were performed using an adapted version of this method [52-56]. Some values of contact angles and surface energies found in the literature are shown in Table 8 and Table 9.

Table 8: Contact Angles for carbon fibers in water and diiodomethane liquids from literature [52-56].

Table 9: Total surface energies and its polar and dispersive contributions for carbon fibers from the literature [54, 55, 57, 58].

Previous studies found contact angles for Diiodomethane to range between 30.3o to 52.6o and for water between 65.8o to 82.5o [52-55]. In a study of the variation of contact angles for thermal treated carbon fibers, A. Bismarck found that thermal treatment changed the wetting of the fibers in water, improved its hydrophilicity and increased its angle with diiodomethane [53, 57]. For recycled carbon fibers, Jiang, G. and Pickering, S. found values of 73.5o [46]. A detailed and accurate study of the wettability of carbon fibers in water was performed by Si Qiu [52]. It yielded a contact angle of 65.8o for the sample analysed.

The importance of acid-base theory emerges in the results by G. Krekel in a series of papers [29, 42, 43], which show how the adhesion mechanisms between treated carbon fibers and thermoplastics vary with the composite materials’ mechanical properties. Measurements performed on different oxidized

treatment samples show that work of adhesion changes with the pH of the liquid used. Accordingly, it is shown that the work of adhesion of the carbon fibers is lower for acid liquids. Unoxidized fibers also presented lower work of adhesion values compared with treated carbon fibers. It was demonstrated that the work of adhesion in these cases is formed by the carboxylic acid groups in the carbon fiber surfaces [29].

Figure 1-26: Left: Work of adhesion of unsized and surface treated carbon fibers as function of pH values of aqueous test o liquids (o) fiber oxidized with ozone (0.75% O3 / 100 C/60s), (D) commercially oxidized fiber, (x) fiber oxidized in pure o oxygen (O2 / 400 C / 30min) and () unoxidized fiber. Right: Work of adhesion of different types of commercially oxidized fibers as a function of the pH value of the aqueous test liquids: (x) Celion fibers, (O) Tenax and (D) AS4 [29].

1.5.3 Sessile drop techniques on polymer blends and composite materials

The sessile drop technique is one of the first techniques employed to measure wettability on homogeneous surfaces. This technique originated the development of the Young equation for surface tension on solid surfaces discussed in Section 1.5.1 above. Given the adoption of more precise, automated systems and sophisticated software image analysis for this purpose, one of the next challenges of this technique is to improve the precision of correlate wettability with adhesion of polymer blends and composite materials. With the support of additional material characterization techniques, quantitative wettability data can achieve a direct correlation with adhesion phenomena and the mechanical properties of the material. To provide an accurate measurement, contact angles should be performed on an ideal smooth solid surface that is homogeneous, chemically and physically inert and has a certain level of hydrophobicity. It is known that real surfaces do not offer these conditions, as can be seen in Figure 1-27.

Composite and polymer blend materials offer several challenges due to their heterogeneity and extra roughness. There are theories that were developed specifically to address these issues. For example, the Wenzel theory uses roughness constants to approximate the disparities between real and ideal surfaces [59].

Figure 1-27: Representations of ideal and real surface systems for contact angle analysis [49].

Very little research using the sessile drop technique was made on carbon fiber composites. One such study was performed on carbon/epoxy and glass epoxy composites by Q. Bénard et al. using several liquids (water, glycerol, formamide, ethylene glycol and diiodomethane) and employing the Good-Van Oss (Acid-Base) approach. Samples were washed and the peel ply treatment technique was applied on the same surface to compare its performance with untreated samples. The results of the contact angle are demonstrated in Table 10 below [60].

Table 10: Contact angles (above where Ra is the roughness) and calculated surface free energies for Glass/epoxy and Carbon/epoxy composites [60]

One of the conclusions of the cited work is that the composite surface cannot be considered a polymeric resin alone. The nature of the composite reinforcement influences the chemical composition of the surface and its surface energy. The peel ply technique improved the surface energy mainly by increasing the effective surface available. [60]

1.6 Surface modifiers for polymer blends and fiber reinforced polymers

The use of surface modifiers, commonly referred as coupling agents or polymer reagents, is required when it is necessary to improve one or more properties of the polymers, such as viscosity (using plasticizers), strength and toughness. In addition, surface modifiers improve the interphase compatibility between polymers blends, as well as adhesion in the interface of fiber surface and polymer matrix.

Most thermoplastic matrices have non-polar surfaces and are hydrophobic. On other hand, their reinforcement fibers are always polar and hydrophilic. This incompatibility results in poor adhesion and consequently lower energy transfer from matrix to the fibers, which affects the final performance. Generally, surface modifiers promote covalent, hydrogen bonds and polymer entanglement simultaneously. However, one effect is always dominant depending on the material boundary conditions, such as surface morphology, chemistry and processing. The surface modifier, which generally accounts for 1% to 3% of total material weight [61], also helps promote compatibility between immiscible polymers.

There are several coupling agents for polymers available on the market, and they are suitable for a variety of application purposes. A general classification can divide these agents into organic, inorganic and organic-inorganic. The most popular agents available are anhydrides, isocyanates, silanes and anhydride-modified copolymers. Several techniques are also employed to coat the polymers or fibers surface and are discussed in literature [61-65].

The focus of this research will be on two surface modifiers: Maleic Anhydride Polypropylene (MAPP) and Poly[methylene(Polyphenyl) Isocyanate] (PMPPIC) and their interaction in PP/PA12 hybrid polymer matrix. In the next chapters, some polypropylene – polyamide 12 interface mechanism studies will be presented revealing what is known about the addition of surface modifiers on its structure.

1.6.1 Maleic Anhydride Polypropylene – MAPP

Maleic Anhydride, which belongs to the anhydride surface modifier group, was initially used as a surface modifier in 1972 by Gaylord to combine cellulose and polyethylene. There are several formulations of Maleic Anhydride grafting with polymers. Two examples are MAPP (polypropylene grafted) and MAPE (polyethylene grafted) co-polymers. All of them have functional carboxylic groups [-

(CO)2O-] that make the polymer reactive and enable it to interact with the fiber. The graft co- polymerization in the polymer matrix backbone occurs through carbon-carbon double bonding (C=C) with the user of a catalyst. The adhesion of the co-polymer surface modifier with the reinforced fibers is performed by hydrogen bonds at OH surface functional groups, promoting covalent/hydrogen bonds as shown in Figure 1-28 [62-64].

Figure 1-28: Maleic Anhydride Polypropylene interaction with fiber surface and polymer matrix [64].

MAPP is one example of this co-polymerization performed on Maleic Anhydride to create a larger molecular structure comparable to fiber and polymer matrix structures. It allows better alignment and promotes Van der Walls bonds, while changing the electrical dispersion of the structure and turning it into a reactive polymer. The MAPP co-polymerization process should be started by using a peroxide reagent to initiate free radicals on the PP structure [65]. After co-polymerization, the surface modifier can be blended with the polymer matrix or the reinforced fiber can be coated before compounded in final composite.

One important study related to this topic was performed by Aranburu, N. and Eguiazabal, J. I. They investigated the effect of MAPP on PP/PA12 blends by graphitization based on the known fact that PP is non-polar and PA12 has a polar structure. The polymers studied were PP070 from Repsol YPF and PA12 Rilsan AMNO TLD by Arkema, whereas the MAPP used was from Fusabond PMZ203D by DuPont with 0.74% content. The blend composition range studied was from 0%-100% to 100%-0% PP/PA12. Results of this study demonstrated a biphasic composition revealed in the DMTA results, where the temperature glass transition of PP was clearly distinct from PA12. Also, higher PP crystallization percentages were found for higher PA12 blend fractions. The impact of MAPP on the dispersion of biphasic structure can be seen below in Figure 1-29. MAPP acts by dispersing the PP phase in the material blend. This process is enhanced by injection molding due to the presence of mechanical forces and high temperature, which promote adhesion of the MAPP groups with the terminal amine groups of PA12 and lead to improved interfacial adhesion [66].

Figure 1-29: SEM micrographics of 40/60 PP/PA12 blends. On left, biphasic structure without MAPP. On right, with MAPP [66].

Figure 1-30: Young’s modulus, yield strength and % ductility of PP/PA12 blends (o) without MAPP and (•) with MAPP [66].

Mechanical properties are shown in Figure 1-30 above. Young`s modulus did not improve significantly by the addition of MAPP and tends to decrease with higher PA12 fractions. Its lack of linearity is justified by the change in crystallinity morphology of the PP phase (increasing from 45% to 65%) under richer PA12 compositions. The crystallinity morphology is the main toughness contributor. Strength values improved, approximating a linear pattern for higher PA12 fractions with MAPP. An improvement in ductility was also observed when applying MAPP to PP/PA12 blends [66].

The mechanical properties values presented above can be compared to another study of PP/PA12 with and without MAPP performed by Jose S. [67], as shown in Table 11 and Table 12 below.

Table 11: Mechanical properties for PP/PA12 blends without MAPP. (the N index represents the PA12 %) [67].

Table 12: Mechanical properties for PP/PA12 blends with MAPP (N index represents PA12%) [67].

The use of MAPP with recycled carbon fiber reinforced polymers was studied by K.H. Wong et al. This study used recycled carbon fiber at 30% weight composition with PP and compared three different coupling agents: E43, G3015 and G3003 from Eastman UK. The results, shown in Figure 1-32, demonstrate that the RCF composite with PP-MAPP presented higher tensile and flexural properties, whereas impact fracture and toughness were reduced. It was demonstrated that better compatibility was directly associated with the MAPP molecular weight and the quantity of anhydride groups. A greater number of anhydride groups provide better tensile and flexural strengths, while maximum strength is determined by molecular weight. Toughness improvement is only associated with the MAPP Young`s modulus. SEM pictures, presented in Figure 1-31, show RCF30%-PP with MAPP at different

concentrations. The increase of interfacial adhesion of MAPP with the fibers results in a better tension transfer along the material, since the treated fibers now act as a bundle when under stress. This “bundle effect” resulted in a more efficient load transfer from the matrix to the fibers and enables support for higher loads. When the amount of MAPP surface modifier is low or zero, single fibers are pulled out from the matrix, exhibiting a “single pull out effect” due to lack of adhesion and consequently a low load transfer from the matrix to the fibers [63].

Figure 1-31: SEM micrographies with MAPP G3003 at 2% and 5% in weight. At left the " single pull out effect". At right, the "bundle pull out effect" [63].

Figure 1-32: Mechanical properties results for PP with different MAPP surface modifiers at several concentrations [63].

1.6.2 Poly[methylene(Polyphenyl) Isocyanate] – PMPPIC

Poly[methylene(Polyphenyl) Isocyanate] also known as PMPPIC is an organic isocyanate formed from 4,4’-Methylene diphenyl isocyanate, also known as MDI. MDI forms polymeric MDI (PMDI) by a condensation reaction in a commercial process using aniline and formaldehyde, with hydrochloric acid as the catalyst. PMDI forms as a dark brown liquid at room temperature, with an MDI monomeric concentration that varies from 25% to 80%. The PMPPIC monomer is [-C6H3(NCO)CH2-]n and has isocyanate groups (R-N=C=O) that generally react with primary and secondary amines and with substances with carboxylic groups (-COOH), hydroxyl groups (OH) or –SH groups. The reaction attacks the carbon of the isocyanate in a similar manner to carboxylic acid derivates such as esters or anhydrides. The reaction generally takes seconds or minutes and is exothermic. When exposed to humid air or to water, PMPPIC reacts with water and releases CO2, forming a solid surface of polyurea [65,69].

The use of PMPPIC as a surface modifier is initially dated to 1988, when it was used to improve adhesion between polyethylene and cellulose fiber [61]. Methyl isocyanates are known as monofunctional reactants lowering the surface energy of the fiber by making it non-polar [62]. The PMPPIC interaction with carbon fiber surfaces is based on the reaction of alcohols with the isocyanates, catalyzed by tertiary amines or salts of metals, resulting in urethanes. The reaction is formulated in Figure 1-33. PMPPIC is one of the most reactive surface modifiers in the isocyanates group [68] as can be seen in Figure 1-34 below

R-N=C=O + R'-OH --> R-NH-(C=O)-O-R' .

Figure 1-33: PMPPIC formula and functional groups interaction with fiber surface OH groups.

Figure 1-34: Comparison of coupling effectiveness between different isocyanate surface modifiers (0.5% in weight) in PVC/CTMP blends [61, 68].

The use of PMPPIC as a wood fiber surface modifier is one of its several demonstrated applications. In a study of the effect of PMPPIC on wood fibers, Pickering found improvement of 11.5% in strength compared with unmodified PP/Wood Fiber composites and 4% compared with the PP matrix. Young’s modulus has improved 77% compared to the PP matrix. The same study also measured mechanical properties values generated by the combined use of MAPP and PMPPIC and results were better than the choice of use one surface modifier but still, needs further analysis [64]. Studies by Maldas, D. and Kokta B.V. found that both surface modifiers can work together on reinforced PP. The –NCO functional group additionally reacts with acid groups of MAPP, bringing even better properties than the individual use of these surface modifiers [64,68].

In addition to surface fiber modification, several recent efforts have focused on finding alternatives to blend polypropylene and polyamides. Since one polymer is reactive (Polyamide) while the other is inert (Polypropylene), the non-reactive polymer should be functionalized to obtain a reactive compatibilization. The use of surface modifiers, or coupling agents, to blend two immiscible polymers such as Polypropylene and Polyamide was successfully performed for Maleic Anhydride reagents by graft polymerization, as discussed previously. But MAPP has presented some constraints, such as limited toughness. When reinforced fibers are added to this PP/PA12 blend system, MAPP has low influence as a surface modifier in improving adhesion between the fiber and the matrix, because most of the reagent is already used up

for blending compatibilization. Therefore, to be effectively used in the composite, MAPP should be selected either as a coupling agent or as a surface modifier, but not both.

For this reason, there are very few studies examining PMPPIC application on composites. The same is true for PP/PA blends. In one of the main studies on this topic, H. Cartier and G.-H. Hu produced a final PP/PA6 blend by activating Polypropylene using e-Caprolactam polymerized anionically as catalyst to promote the copolymerization of 3-Isopropenyl-a,a-dimethylbenzene isocyanate, known as TMI. The researchers used a batch mixer to promote the in-situ reactivation of PP with TMI and to increase the capacity of the functionalized material for compatibilization in PP/PA6 blends. The advantage of this process is that PA6 is formed in-situ with the e-Caprolactam and need not be added as a final polymer. FTIR showed that the isocyanate group peak of 2255cm-1 disappeared, while three new peaks appeared between 1500-2800cm-1 wavelength, more specifically at 1708cm-1, 1668cm-1 (the highest peak) and 1533cm-1. These peaks corresponded to C=O from the tertiary amide, C=O from urea and NH from the urea of acyl caprolactam, respectively. Solubility tests were performed with PP and PA6 solvents and showed up to 16.1% of insoluble material, which represented the maximum amount of compatibilization in this study [71,72].

Chapter 2

2 Research Motivation, Scope and Objectives

2.1 Research Motivation

Given the literature review previously presented, this section shows the research motivation of the project, with a focus on literature gaps and opportunities found in the areas previously discussed, some of which will be shown in the next paragraphs.

Despite the previously discussed environmental and economic challenges, there are well known recycled carbon fiber reclaiming technologies, as presented in Figure 1-14. Some of them, such as pyrolysis and fluidized bed processes, are already in mass scale production. The next problem is how to improve the use of these materials as reinforcements in composites, given that many recycled carbon fiber properties are not well controlled, such as length variation and length distribution in the matrix. The material’s origin is also a big issue because recycled fibers are often from different suppliers and are produced by different manufacturing processes. Finally, surface quality of the fibers must be improved in order to produce better adhesion between fiber and matrix [80]. In light of these problems, the use of recycled carbon fibers from a single supplier source mitigates the material origin issue because the fibers would come from the same industrial process.

Another important motivation is the need to collect engineering data for recycled carbon fiber composite materials to improve confidence level and enhance fiber’s competitiveness among material composites’ field. The goal is recycled carbon fiber be considered during the material selection phase of future automotive projects. This can be achieved by design and validation of these material composites in automotive components. The material behavior, characterization and primary properties data under known standards, are crucial information considered by design engineers in furthering component development.

In transportation lightweighting design processes, materials with multiple properties can substitute or mitigate the use of structural materials. Composite polymers have a very promising prospect in this regard

47 48 due that many components are made of composites that boast strength, yet are lacking in toughness and energy absorption. Parts made of composites that combine higher toughness and energy absorption with satisfactory levels of strength would result in substantial secondary mass savings, allowing lighter structural materials to be used efficiently for vehicle safety design purposes [81].

2.2 Research Scope

The research scope of the project can be divided into three areas of knowledge based on the discussion in the literature review: recycled carbon fibers characterization (section 1.2.1); polymer hybrid blend matrix and the effect of fiber reinforcement on final composite properties (section 1.2.2); and finally wettability and surface energy investigation in all materials studied (section 1.2.3). There are many research gaps and opportunities in this area that will be demonstrated in the items below.

2.2.1 Recycled Carbon Fibers Characterization

There is well-established fundamental knowledge of PAN virgin carbon fibers through which its performance in many final composite materials is understood. However, when these materials are exposed to different production processes and usage, and are then submitted to recovery and recycling processes, the final fiber properties and performance change. There are few studies done on the characterization of these fibers that investigate their behavior when reinserted as a new composite. Thermal and surface parameters, such as chemical groups, morphology and surface free energy, are important to understand the interfacial adhesion phenomena between these fibers with their respective matrices and the final effect on material composites’ properties. Since limited data is available for recycled carbon fibers, it is very important to expand database knowledge of different recycled carbon fiber sources/suppliers regarding to fiber’s performance, in order to take advantage of these materials in their recycled form.

2.2.2 Reacted PP/PA12 Polymer Blend and Recycled Carbon Fiber Polymer Hybrid Composite

48 49

The use of Polyamides in automotive industry is not new and well-known. However, the addition of Polypropylene is an interesting alternative, with a view to keeping prices competitive without impairing material performance. Polyamide 12, in particular, is an especially expensive material compared with other engineering polymers, and its sources in the industrial chain are limited. The challenge, as already presented in the literature review, is to improve the interfacial adhesion of both phases of the material when mixed, resulting in better overall properties. The previously discussed use of MAPP for PP/PA12 Blends in general improves tensile and flexural strength and modulus. However, impact energy is not improved. Molecular weight of grafted PP and Maleic Anhydride concentration are important surface modifier factors when mixed into polymer matrix. PP/PA12 with MAPP yield a novel complex when recycled carbon fibers are used, and the interaction between PP, PA12 reacted with MAPP and fiber surface requires further investigation.

The uses of PMPPIC as a surface modifier and compatibilizer are new, since some isocyanate group reagents were used in single polymers with many different fibers. For this reason, a novel hybrid matrix with PP/PA12 and PMPPIC is presented in this work. The use of recycled carbon fibers in this hybrid composite is novel. Therefore, the basic overall properties and fundamental scientific phenomena in the interface between the PP/PA12/PMPPIC and recycled carbon fibers must be further investigated.

2.2.3 Wettability and Surface Energy Studies in Recycled Carbon Fibers Hybrid Polymers

The fundamental science behind wettability and surface energy in solid materials is not new. However, due to its nanometer scale and high sensitivity, the available experimental methods are not well- established and cannot provide accurate and fast results. Carbon fibers are a good example: methods are sensitive and time consuming and are in still at a rudimentary level. For this reason, different test results yield significant differences that are mainly due to material surface chemistry and morphology, but test methods still impact significantly on final accuracy of the contact angle measurements. Polymers are in a better position. Non-polar polymers, such as polypropylene, are better understood and have more results available in the literature than polar polymers such like Polyamide 12.

The contact angle and surface energy relationship between pure polymers and their composites are not well understood. It is not well-established how the general measurement parameters involved in interface

49 50 microstructure influence on final surface energy and how they impact the final blend mechanical properties. Although many materials have already been studied, further test data and analytical procedures are needed to establish a mathematical relationship. For this reason, novel measurements are presented in this work for reacted PP/PA12 blends with MAPP and PMPPIC.

The problem becomes more complex when fibers are added, resulting in a final composite. Length, distribution, fiber type and many other parameters are added as new parameters into the system. Its relationships, therefore, need to be investigated, mainly to understand adhesion behavior between fibers and matrix. For this reason, contact angle measurements in virgin carbon fibers are presented here as a novel procedure, contributing valuable research data to the study of this composite.

2.3 Research Objective

The objective of this research is to address all those points mentioned in the research scope. They are summarized in the list below:

I. Characterize recycled and virgin carbon fibers and understand the impact of thermal treatment on carbon fiber surface properties;

II. Present a novel optimization of PP/PA12 blend with PMPPIC surface modifier;

III. Use virgin and recycled carbon fiber as polymer reinforcement to evaluate the performance of the novel PMPPIC on mechanical properties;

IV. Investigate contact angles and correlate virgin and recycled carbon fiber surface free energy with interface adhesion mechanisms in PP/PA12 blends with surface modifiers;

V. Present an alternative recycled carbon fiber composite material with higher toughness and impact energy properties to increase the lightweight portfolio of automotive applications focused on design safety for powertrain engine components.

50 51

Chapter 3

3 Research Methodology

The methodology of this research work is divided into four parts. The first part is dedicated to the characterization of recycled and virgin carbon fibers. The second part focuses on polymer blend optimization with PMPPIC and MAPP surface modifiers. The third part concerns reinforced fiber materials with untreated and thermal treated carbon fibers. Finally, the fourth part is dedicated to surface energy and wetting studies and results. This chapter will describe the entire research methodology, including materials used, material preparation, blend and composites compounding and processing. Finally, all test specifications and standards will be covered in detail.

3.1 Materials Description

All raw materials used in this research are demonstrated in Table 13 below, and their individual material data sheet properties can be found in the appendix. The main properties and material characteristics will be discussed in the next paragraphs under several points.

Table 13: List of materials used and their commercial names.

51 52

Figure 3-1: Virgin and Recycled Carbon Fiber samples: virgin on left, recycled on right.

The specific polypropylene used is PP3622 from Total® S.A. Company. It is an homopolymer for general injection molding and extrusion purpose with good processability at temperatures between 200oC- 232oC. Its melt flow index is 12g/10min and its density is 0.905g/cm³. Tensile strength is 33.1 MPa and its modulus is 1.38GPa. Flexural modulus is 1.17GPa and impact energy is at 1.4J/m (Izod notched).

Polyamide 12 used is Rilsamid® AMNO from Arkema®. Its density is 1.02 g/cm³ and it has a Tensile Strength Yield of 38.0 MPa. Its tensile and flexural modulus are 1.17 GPa and 1.20 GPa respectively. Its Charpy impact energy is 0.900 J/cm3. The melting point is 180oC, higher than that of the polypropylene used, which is 165oC.

The Maleic Anhydride Polypropylene used is Orevak® CA 100 from Arkema®. Its density is 0.905 g/cm3 and its melting point is 167oC. The flexural modulus is 880 MPa and its tensile strength is 22 MPa.

Poly[methylene(polyphenyl) isocyanate] used is from Polysciences, it has a NCO content of 30%, molecular weight of 360 and density of 1.24g/cm3. It is a viscous liquid in environmental temperature and it needs to be stored in a moisture free environment to avoid drying and consequent solidification. It is not recommended to be used in it solidified form.

The Recycled carbon fiber used in this research is commercial branded Recafil® from SGL Company. Its aspect can be described as fluffy, as shown in Figure 3-1. Its average length is 40 mm, with tensile strength as high as 4400 MPa. Its elastic modulus is 255 GPa, density is 1.80 g/cm3 and mass per unit of length is 3.45 g/m. The sizing amount on these recycled carbon fibers is 1% (epoxy resin compatible). Other fiber types are found mixed with the material, such as glass fiber (4%) and polymer fiber (3%).

52 53

The virgin fiber studied is from Zoltek Company, named PX35, with density of 1.81g/cm3 and chipped form as shown in Figure 3-1. Its tensile strength and modulus are 4137 MPa and 242 GPa, respectively. The fiber diameter is 7.2 microns with carbon content as high as 95% and mass per unit of length of 3.74 g/m.

3.2 Material Compounding and Processing

3.2.1 Material preparation

Before processing, all materials were previously prepared according to Table 14 below. For pure material blends 4 packs of 500 g with different compositions of PP/PA12 were used. The same process was used to prepare the material with MAPP 3% in weight, with 4 packs of 500g used here as well.

To process the PP/PA12 with PMPPIC blend, the grafting method was used. In this method the surface agents are mixed initially in the polymer matrix. The PP and PA12 pellets were weighted according the table, and a solution of 2 (two) parts of acetone for 1 (one) part in volume of the PMPPIC were diluted, added and mixed to the material in a metallic tray. First it was made with fractions of 1%, 2% and 3% of PMPPIC on 50%/50% PP-PA12, and after all the results were obtained, the other fractions of PP/PA12 with higher PA12 content were made. The material was exposed for 24 hours in environmental conditions to ensure total evaporation of the acetone volatile content. It was manually mixed 3 times during the drying process to ensure a good dispersion of the diluted PMPPIC in the material.

The virgin and carbon fibers were weighted per the percentage material table below and treated in a high temperature furnace, the Sybron Thermolyne Model shown in Figure 3-2 below. The furnace was pre-heated until the treatment temperature of 400oC was reached. Each material was treated individually for 30 min and then removed to be added to the matrix.

To process the virgin and recycled carbon fiber reinforced polymers, the same process described previously was used to prepare the polymer matrix composition for MAPP and PMPPIC surface modifiers. In an additional step, chips of untreated and 400oC virgin carbon fiber were added to their respective

53 54 packages with the other pellets. They were mechanically mixed and then submitted to the extrusion process. The recycled carbon fibers were kept separated from the matrix pellets up to the extrusion process.

Table 14: Material composition in % weight for the polymer blends, virgin and recycled carbon fiber reinforced polymers used.

54 55

Figure 3-2: Sybron Thermolyne Furnace for carbon fiber treatment.

3.2.2 Extrusion Process

Before the material is processed by the extrusion process, all PP/PA12 pellets were dried using an aluminum tray in an oven at temperatures between 90oC-100oC for 2 hours to eliminate moisture and ensure good viscosity. This avoids intermittent flow in the screws and consequently trapped gas in the bulk of the material by water evaporation.

All pure blends, MAPP and PMPPIC surface modified blends, virgin and recycled carbon fiber reinforced polymers were submitted to an extrusion process in a Lab co-rotation twin screw extruder: model Onyx TEC -25/40, shown in Figure 3-3, with capacity from 2kg to 15kg per hour, screw diameter of 25mm and 40:1 L/D ratio, three volatile venting ports and 10 heating zones. All polymer blends and virgin carbon fiber reinforced polymers were processed in temperatures between 180oC and 210oC under the following sequence of temperature zones: 180/190/190/200/210/210/210/210/200/200. The feed rotating speed was set to 9 rpm and the rotor screw speed was 110 rpm. The material was cooled down in a water bath. An exception was observed for PP/PA12 with PMPPIC at 3%, in that the processing temperature range had to be increased from 190oC to 230oC because of the material’s high viscosity. The temperature zones were modified accordingly: 190/200/210/220/230/220/210/210/200/200 degrees each. The rotor screw speed was increased to 130 rpm.

55 56

Figure 3-3: Extrusion machine model Onyx TEC -25/40.

For the recycled carbon fibers, the material had to be added manually along with the matrix pellets in the screw feeder port to assure that the chopped fibers were pulled in the screw channel. The material showed good aspect and dispersion, like the automatically fed virgin carbon fiber composites. After all materials were extruded at this stage, they were subjected to the next step, the injection molding process.

3.2.3 Injection molding process

As preparation for the injection molding process, all extruded pellets were also submitted to an oven drying process at temperatures between 90oC-100oC for 2 hours to avoid moisture, as described above. The material was injected at a temperature of 210oC, its cooling time was set to 8 seconds and the molding time was 2 seconds. All materials presented good surface quality and finishing. An exception was found when PP/PA12 at 50%/50% were processed with PMPPIC 3%. In this case, the samples presented some flow issues and the molded part was not totally filled due to their high viscosity, suggesting that a higher processing temperature in the injection machine was needed. Figure 3-4 shows some injected samples that were submitted to mechanical properties tests. The left picture describes samples with 1%, 2% and 3% PMPPIC concentration in hybrid PA12/PP matrix, the right side shows parts processed with MAPP and PMPPIC modifiers and reinforced with virgin and recycled carbon fibers.

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3.3 Research Test Methods

3.3.1 Virgin and Recycled Carbon Fiber Characterization Tests

To optimize characterization, TGA was performed initially to identify the best temperature at which to treat the carbon fibers without significant loss and degradation. FTIR was performed on the recycled carbon fiber material to understand the change in chemical surface groups composition behavior between the untreated state and one temperature studied. XPS was performed on virgin and recycled carbon fibers at untreated and 400oC treated temperatures. Each characterization process is detailed below.

3.3.1.1 Thermogravimetric Analysis (TGA)

Thermogravimetric Analysis was performed on all virgin and recycled carbon fiber samples. The equipment used was model Q500, produced by TA Instruments, USA. All samples were weighed to ensure a total mass below 10 milligrams before insertion into the equipment. Untreated fibers were subjected to a temperature increment of 10oC/min in Air and Nitrogen, at 60ml/min each, until the maximum

57 58 temperature of 800oC was reached, at which point the process concluded. To understand the behavior of the material under different treatment temperatures, isothermal periods with the duration of 30min were set in the TGA software for temperatures of 400oC, 500oC, 600oC and 700oC in both Air and Nitrogen chamber environments.

The same 10 (ten) analyses (Air and Nitrogen at untreated, 400oC, 500oC, 600oC and 700oC) were performed for recycled carbon fibers and for virgin carbon fibers.

Figure 3-5: TGA Analysis Q500 model.

3.3.1.2 Fourier Transformed Infrared Spectroscopy (FTIR)

FTIR analysis was performed in FT-IR Tensor 27 Spectrometer model (Figure 3-6) on recycled carbon fiber samples. The untreated RCF sample was directly subjected to FTIR analysis, whereas the thermally treated samples were initially exposed to a controlled air environment at 60ml/min in a temperature increment of 10oC/min and isothermally maintained at temperatures of 400oC, 500oC and 600oC for 30 minutes.

The sample was prepared by mixing 5mg of the material with 200mg of Potassium Bromide KBr powder. The powder was smashed with a stone pestle, compressed in a hand press up to 10000 psi to form the pellets and then inserted into the sample compartment to be analyzed.

58 59

Figure 3-6: FTIR Tensor 27 model.

3.3.1.3 X-Ray Photoelectron Spectroscopy (XPS)

The XPS instrument model is a K-Alpha XPS system from Thermo Fisher Scientific (East Grinstead, UK). Monochromatic Al K-Alpha x-rays were used in this analysis. The system used a micro-focused x- ray spot, ranging in size from 30um to 400um (major axis of a 2:1 ellipse). The vacuum pressure is ~5x10- 8 mbar under normal conditions (when the charge compensation source is used), with the clear majority of residual pressure being Argon-associated with the operation of a charge compensation source.

The survey spectrum is typically acquired in a high pass energy (200 eV), low point-density (1 point/eV) scan mode. Regional spectra used to determine relative atomic composition and to obtain chemical information is usually acquired in a low pass energy (50 eV), high point-density (0.1 eV spacing) scan mode. These are 'high-resolution' scans of smaller regions of the spectra, centered on individual spectral features of interest (e.g., C1s region).

In analyzing the data, a smart background function was used to approximate the experimental backgrounds. Surface elemental compositions were calculated from background-subtracted peak areas derived from transmission-function-corrected regional spectra. Sensitivity factors provided by the instrument manufacturer were used to calculate the relative atomic percentages. Lorentzian-Gaussian peaks were used to fit the spectra.

59 60

Untreated and 400oC treated Recycled and virgin carbon fibers were selected based on TGA results. Samples were previously exposed to controlled air and Nitrogen environments at 60ml/min in a temperature increment of 10oC/min, and isothermally maintained at temperatures of 400oC for 30 minutes to perform the thermal treatment. Six samples were analyzed.

3.3.2 Mechanical Properties Tests

Four main mechanical tests were performed to understand how material behavior changes by the addition of the surface modifier. Its performance was then compared to different types of carbon fiber reinforcement polymers. All tests were performed under the indicated ASTM standards. Their procedures are described below.

3.3.2.1 Tensile Strength and Tensile Modulus Tests

Tensile Strength tests were performed on Instron Universal Tester, Model 3367 (Figure 3-7) as per ASTM D638 (Standard Test Method for Tensile Properties of Plastics). 6 (six) samples were used to obtain the tensile properties. Type III specimens were used in this test. They were conditioned for 48 hours at a temperature of 25oC and relative humidity of 55% and were tested using crosshead speed of 12.5mm/min with 30 kN load cell. Calculations for the average and standard deviation are presented in chapter 4, Results and Conclusions.

60 61

Figure 3-7: Instron Universal Tester 3367 model. Tensile and Flexural Strength/Modulus measurement tests.

3.3.2.2 Flexural Strength and Flexural Modulus Tests

Flexural strength and modulus tests were performed as per ASTM D790 (Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials). Tests were performed with the Instron Universal Tester, Model 3367 shown in Figure 3-7. 6 (six) specimens with dimensions of 12.7mm in width, 3.2mm in thickness and 127mm in length were used to obtain the flexural properties. Specimens were conditioned at a temperature of 25oC and relative humidity of 55% for 48 hours, and then tested at the same temperature and relative humidity using 12.5mm/min as crosshead speed with 2kN load cell. Calculations for the average and standard deviation are presented in Chapter 4, Results and Conclusions.

3.3.2.3 Impact Tests

Impact energy properties tests were performed using the Izod Notched method as per the ASTM D256 (Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics) Method A. The Machine used is Tinius Olsen 892 model. 7 (seven) Izod Type specimens were conditioned at a temperature of 25oC and relative humidity of 55% for 48 hours and then tested at the same temperature

61 62 and relative humidity. Calculations for the average and standard deviation are presented in Chapter 4, Results and Conclusions.

3.3.2.4 Melt Flow Index Tests

Melt Flow Index Tests were performed as per ASTM D1238 (Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer) Procedure A. Tests were performed on a Custom Scientific Instruments Inc. equipment at 25oC environment temperature and 55% relative humidity. The temperature in the test was set to 230oC and the weight used was 1.2kg for all samples, with extraction times between 0.5-3 minutes, varying according to the MFI registered in the first test turn. From each test turn, 2 to 5 samples (depending on viscosity and time intervals used) could be removed and weighted and 5 values were registered to complete a sample average. A total of 5 measured samples were obtained. The final average and standard deviation calculations are presented in the Chapter 4, Results and Conclusions.

3.3.3 Surface Free Energy, Wetting Studies Tests

To perform the surface free energy tests, the sessile drop method was chosen and performed as per ASTM 7334 (Standard Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement) and ASTM D7490 (Standard Test Method for Measurement of the Surface Tension of Solid Coatings, Substrates and Pigments using Contact Angle Measurements). The test used a contact angle measuring equipment, Dataphysics OCA 15EC, with a contour analysis system and a Single-Direct dosing system (OCA 15EC package 1), as shown in Figure 3-8. Tests were performed under laboratory environmental conditions (25oC with 50% relative humidity). The shape and finishing of all specimens were obtained from injected molded samples (impact test samples) to reach testing compatible surface roughness. Sample preparation followed the ASTM D7490 recommendations: all samples were washed with water and laboratory soap to remove dust and hand contamination and were exposed to dry in environmental conditions for 20-30 minutes. The liquids selected were Distilled water (Type II reagent according with ASTM D1193) with surface tension values obtained from reference data supplied by Rabel. Diiodomethane liquid from Sigma Aldrich (density 3.33g/mL at 25oC and 98.5%

62 63

Purity) was used with surface tension reference data from Owens et al. The surface tensions of the liquids used are shown in Table 15 below.

Table 15: Surface Tension reference data for Water and Diiodomethane.

Figure 3-8: Surface energy and contact angle measurement equipment, Dataphysics OCA15 EC.

For both liquids, three drop measurements were performed on each sample specimen. The left and right contact angles were registered from each drop, and its total average was recorded as a sample measurement. A total of 5 (five) sample measurements were obtained. The total average and standard deviation are presented on Chapter 4, Results and Conclusions.

Surface Free Energy values were calculated from the contact angles measured for Water and Diiodomethane liquids as per ASTM D7490 using the Geometric mean, also known by Owens-Wendt- Kaelble equation (OWK) previously described in the literature review. According to ASTM D7490, dispersive, polar and total surface energies for each sample were calculated based on contact angle values measured for both liquids, as described previously. Each respective total value was considered as the average of the five samples measured. The total average and standard deviation are presented in Chapter 4.

63 64

Chapter 4

4 Results and Conclusions

Results are presented in the same order as the research methodology discussion in the previous chapter. Characterization results are presented in section 4.1, PMPPIC optimization results and mechanical properties comparison between MAPP and PMPPIC Blends are presented in section 4.2, mechanical properties results for the virgin and recycled carbon fiber reinforced composites are presented in section 4.3. Section 4.4 will be dedicated to wetting studies and surface free energies analysis of all materials used in this research. The last item will be dedicated to the conclusions of this work.

4.1 Recycled and Virgin Carbon Fiber Material Characterization Results

4.1.1 Thermogravimetric Analysis (TGA) Results

Thermogravimetric Analysis results are presented in the following order: virgin carbon fiber treated with air and nitrogen respectively, then recycled carbon fiber results for air and nitrogen are shown.

4.1.1.1 Virgin Carbon Fiber TGA Results

When treated with air, virgin carbon fibers demonstrated very good thermal stability at temperatures up to 500oC with no significant variation in curve shape, as shown in Figure 4-1. For treatment temperatures of 600oC and 700oC the level of weight loss degradation caused by the combustion

64 65 with air resulted in fibers being turned to ash before the test finish. For this reason, the curves at these temperatures are not shown. However, the total weight loss of these tests is shown in Table 16. A small weight loss curve slope was detected in Figure 4-1 at temperatures around 300oC. The same phenomenon was detected by IM Alarifi, 2015 in his TGA analysis for PAN fibers with prior carbonization that it was explained as pyrolytic reactions occurred at 302oC [27]. Because the stabilization phase occurs at this temperature range, the weight loss is associated with remaining PAN fiber thermal cyclization of graphite structural carbon rings and consequently gas emission such as carbon dioxide (CO2), carbon monoxide

(CO), NH3, HCN and water vapor [21,73]. Significant increase of weight loss at temperatures between 550oC-600oC indicates the maximum temperature range of the fibers without significant degradation by combustion.

Figure 4-2 shows the derivative weight loss per temperature that indicates the weight loss gradient at each temperature registered. At the remaining carbon fiber stabilization step temperature (303.73oC), values of 0.037%/oC were registered. At treatment temperatures of 400oC and 500oC the registered gradient losses were 0.019%/oC and 0.050%/oC, respectively. After 600oC the weight loss gradient curve is exponentially shaped until the complete material burn. The maximum recorded weight loss gradient values were 1.132%/oC at 791.35oC for untreated samples, 1.023%/oC at 792.87oC for 400oC treated samples and 1.079%/oC at 795.40oC for 500 oC treated samples. It can be observed that lower total weight loss comes at higher temperatures for treated carbon fibers. This phenomenon can be explained through the initial isothermal treatment, which caused the formation of aligned ribbon carbon strips through the energy applied during the process. This alignment transformed some turbostratic structures to graphitic structure that are typical for High Modulus carbon fibers, which increases Young’s modulus for its respective reinforced polymer, as will be shown in the mechanical properties results below.

65 66

TGA Analysis for Virgin Carbon Fibers in Air flow

100%

80%

60% VCF Untreated

VCF 400°C Weight % 40% VCF 500°C

20%

0% 0 100 200 300 400 500 600 700 800 900 Temperature °C

Figure 4-1: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC and 500oC virgin carbon fibers in air flow for 30 minutes.

Table 16 shows the total weight loss at the final temperature and during the 30 minutes of treatment exposure of the material. The total loss for untreated carbon fibers in air was 79.50%. For 400oC and 500oC treatments, the values were 77.79% and 71.66%, respectively. This decrease is due to the graphitization phenomena and the higher temperature degradation resistance of the material under these conditions. Total weight loss percentage values were very low for both treatments: 0.41% and 0.26% for 400oC and 500oC respectively, indicating a low level of degradation at these treatment temperatures.

66 67

TGA Analysis for Virgin Carbon Fibers in Air flow

1,20 791.35°C, 1.132%/°C 795.40°C, 1.079%/°C 792.87°C, 1.023%/°C 1,00 C) ° 0,80

0,60 VCF Untreated VCF 400°C 0,40 400.24°C, 0.019%/°C VCF 500°C Deriv. Weight(%/ 303.73°C, 0.037%/°C 500.29°C, 0.050%/°C 0,20

0,00 0 100 200 300 400 500 600 700 800 900 Temperature (°C)

Figure 4-2: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC and 500oC treated virgin carbon fibers in air for 30 minutes.

Table 16: TGA results for virgin carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in air flow.

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TGA Analysis for Virgin Carbon Fibers in Nitrogen flow 102%

100%

98% VCF Untreated

VCF 500°C 96% VCF 400°C Weight % VCF 600°C 94% VCF 700°C

92%

90% 0 100 200 300 400 500 600 700 800 900 Temperature °C

Figure 4-3: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC, 500oC, 600oC and 700oC virgin carbon fibers in nitrogen flow for 30 minutes.

Thermogravimetric analysis results for Nitrogen flow treatments are shown in Figure 4-3 and Figure 4-4. The same remaining material stabilization phenomena occur at a just over different temperature range than the results for air, from 303.25oC to 319.42oC, with a total weight-loss average of 3% in the slope range. Figure 4-3, which has an enlarged y-axis scale range compared with the figures in the previous air analysis, shows in higher detail the similar small percentage of weight loss at these treatment temperatures. An exception in the shape format was found for 700oC, showing a significant degradation at isothermal phase compared with other treatments. The total weight loss measurements during each 30-minute treatment are shown in Table 17. The values registered were 0.28%, 0.29% and 0.07% of remaining material for 400oC, 500oC and 600oC, respectively. These results represent a better thermal behavior compared to air due to the lack of oxygen and combustion phenomena. The 700oC treatment registered considerably higher weight loss, at 3.14%, possibly caused by denitrogenation combined with peaks of H2O and CO gas formation [74].

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TGA Analysis for Virgin Carbon Fibers in Nitrogen flow 0,50

0,45 700.93°C, 0.46%/°C

0,40 C)

° 0,35 VCF Untreated 0,30 315.88°C, 0.035%/°C 303.25°C, 0.045%/°C VCF 400°C 0,25 500.82°C, 0.029%/°C

307.79°C, 0.040%/°C VCF 500°C 0,20 319.42°C, 0.034%/°C

306.28°C, 0.039%/ 600.44°C, 0.005%/°C VCF 600°C 0,15 Deriv. Weight(%/ 396.83°C, 0.074%/°C VCF 700°C 0,10

0,05

0,00 0 100 200 300 400 500 600 700 800 Temperature °C

Figure 4-4: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC, 500oC, 600oC and 700oC treated virgin carbon fibers in nitrogen for 30 minutes.

Table 17: TGA results for virgin carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in nitrogen flow.

As shown in Table 17, total weight loss percentages in the Nitrogen TGA analysis were: 5.46% for untreated fibers, 3.49% for 400oC, 4.28% for 500oC, 3.21% for 600oC and 8.86% for 700oC. Weight loss gradient per temperature was registered as 0.040%/oC for 400oC, 0.020%/oC for 500oC, 0.005%/oC for 600oC and 0.306%/oC for 700oC. The last value evidences the denitrogenation phenomena at this temperature level.

69 70

4.1.1.2 Recycled Carbon Fiber TGA Results

As expected, recycled carbon fiber TGA results showed, in general, a higher level of degradation compared to virgin fibers. Thermal maximum stability temperatures for recycled carbon fibres reached the 500oC–550oC range, 50oC lower in average than the virgin carbon fibers studied. The stabilization phenomena registered previously for virgin carbon fibers were much less detectable for recycled carbon fibers treated in air flow. However, it becomes more observable for nitrogen atmosphere at temperatures o o between 411 C and 413 C. At this level, emissions of carbon dioxide (CO2), carbon monoxide (CO), NH3, HCN and water vapor were again responsible for most of the weight loss like previous virgin carbon fiber related results.

During the air flow experiment for recycled carbon fibers a degradation peak appeared at 596.67oC for untreated carbon fibers and at 593.17oC for 400oC treated carbon fibers. The same did not occur for the 500oC treated sample, because the material was severely burned and degraded during its isothermal phase resulting in significant curve variation (Figure 4-6). The peaks of derivative weight loss presented at these temperatures evidence the uncontrolled exothermal process that took place during combustion at maximum degradation temperature.

TGA Analysis for Recycled Carbon Fibers in Air flow 100%

80%

60% RCF Untreated

Weight % 40% RCF 400°C RCF 500°C 20%

0% 0 100 200 300 400 500 600 700 800 900 1000 Temperature °C

Figure 4-5: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC and 500oC recycled carbon fibers in air flow for 30 minutes.

70 71

TGA Analysis for Recycled Carbon Fibers in Air flow

4,00 500.76°C, 3.46%/°C 3,50

3,00 527.01°C, 2.43%/°C C) °

2,50 553.46°C, 0.65%/°C

2,00 RCF Untreated 596.67°C, 0.90%/°C 1,50 RCF 400°C 593.17°C, 0.83%/°C RCF 500°C

Deriv. Weight(%/ 1,00 399.58°C, 0.14%/°C 0,50

0,00 0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C)

Figure 4-6: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC and 500oC treated recycled carbon fibers in air for 30 minutes. At 400oC and 30-minute treatment the registered temperature weight loss rate for recycled carbon fibers was 0.14%/oC. It was higher than values registered for virgin carbon fibers under the same test conditions. At this temperature, total weight loss registered for recycled carbon fibers was 1.66%, as shown in Table 18. Recycled carbon fibers treated at 500oC lost 25.95% in weight, confirming a high degradation level (Figure 4-5). Total loss in all samples was greater than 96%, meaning that only ashes remained.

Table 18: TGA results for recycled carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in air flow.

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The final TGA discussion is related to recycled carbon fiber samples treated with a nitrogen atmosphere. Each temperature treatment weight loss step can be observed in Figure 4-7. It can be seen in the Figure 4-8 that the remaining-material stabilization phenomena can also be observed. However, the weight loss gradient peaks are found at higher temperatures compared with virgin carbon fibers results, 411.23oC for untreated samples, 411.89oC for 500oC treated samples and 413.05oC for 600oC treated samples. Surprisingly, for 700oC treated samples this phenomenon was not observed. Instead, a denitrogenation peak was observed at 700oC, with a higher weight loss rate compared to virgin carbon fibers, reaching values of 0.74%/oC. In the isothermal phases, at 400oC the gradient weight loss rate was 0.072%/oC, at 500oC a value of 0.031%/oC was registered, and at 600oC the rate was 0.049%/oC.

TGA Analysis for Recycled Carbon Fibers in Nitrogen flow

102%

100%

98%

96% RCF Untreated 94% RCF 400°C 92%

Weight % RCF 500°C

90% RCF 600°C

88% RCF 700°C

86%

84% 0 100 200 300 400 500 600 700 800 900 Temperature °C

Figure 4-7: Thermogravimetric analysis results, weight % vs temperature for untreated, 400oC, 500oC, 600oC and 700oC recycled carbon fibers in nitrogen flow for 30 minutes.

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TGA Analysis for Recycled Carbon Fibers in Nitrogen flow 0,80

0,70 699.92°C, 0.74%/°C

0,60 C) ° 0,50 RCF 700°C

0,40 RCF Untreated 411.89°C, 0.024%/°C RCF 400°C 0,30 413.05°C, 0.042%/°C RCF 500°C

Deriv. Weight(%/ 411.23°C, 0.052%/°C 0,20 RCF 600°C 500.82°C, 0.031%/°C 600.87°C, 0.049%/°C 0,10 400.77°C, 0.072%/°C

0,00 0 100 200 300 400 500 600 700 800 Temperature °C

Figure 4-8: Thermogravimetric analysis results, derivative weight (%/oC) vs temperature for untreated, 400oC, 500oC, 600oC and 700oC treated recycled carbon fibers in nitrogen flow for 30 minutes.

Table 19 shows each treatment’s total weight loss at isothermal treatment phase and total weight loss at the end of the experiment. At isothermal treatment phase, values of 0.86%, 0.38% 0.32% were found for 400oC, 500oC and 600oC, respectively. For 700oC weight loss at treatment temperature was 4.88% due to denitrogenation. Total sample loss at the end of the experiment had values of 5.29% for untreated samples, 5.47% for 400oC treated samples, 5.22% for 500oC treated samples, 5.26% for 600oC treated samples and finally 13.71% for 700oC treated samples.

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Table 19: TGA results for recycled carbon fibers. Weight loss at treatment temperature and total material weight loss at selected temperatures in nitrogen flow.

4.1.2 Fourier Transformed Infrared Spectroscopy (FTIR) Results for Recycled Carbon fibers

Table 20 shows FTIR absorption vs spectra results for recycled carbon fiber surface groups at different oxidation treatment temperatures. Figure 4-9 shows the stacked FTIR curves for better visualization. OH hydroxyl groups were found in the 3570-3200cm-1 range in stretching mode. OH bending vibration spectra were identified in the 1480-1400cm-1 range. The absorption peak increases with higher oxygen treatment temperature. Also detected were Alkane Methyl groups (-CH3) in the 2880- -1 -1 2860cm range and Methylene groups (-CH2) in the 2935-2915cm range, their absorption decreased from untreated to treated recycled carbon fibers. This is indicative of remaining H2 formations during the stabilization of carbon fibers necessary to build new turbostratic structures with pure carbon, as will be shown further in the XPS analysis. As consequence, the hydrogen formed reaches the fiber surface and reacts with remaining carbon opened rings. Aldehyde, Ketone and Esters -C=O groups were registered in the 1750-1720 cm-1 range, becoming less pronounced at higher treatment temperatures. Other C=O structures, carboxylic acid groups (-COOH) and the unsaturated structure of benzene overlapped in the 1700-1520 cm-1 range and had a small absorption increment. Bending modes from methyl, methylene and OH were revealed overlapped in the range of 1480-1400cm-1, reinforcing their presence. Absorption peaks at 1383cm-1 for 500oC and 600oC temperatures appeared, representing bending vibration modes of Alkanes and Methyl. This change from stretching to bending vibration mode could be explained by the

74 75 evolution from turbostratic to graphite structures, necessary for high modulus carbon fibers. The remaining dehydrogenation of the bulk structure also reinforces this hypothesis. Two peaks of secondary alcohols (CH-OH) were also detected, the last one in lower absorptions levels for 400oC-treated RCF. The first peak appeared in the 1124-1087cm-1 range, overlapping with amine groups from the nitrogen groups remaining on the material surface due to denitrogenation. The second peak of secondary alcohols was detected in the 1100-840cm-1 range.

Wavelength range Wavelength Peak detected (cm-1) Functional Group Itensity / Vibration Type region (cm-1) 400 °C 500 °C 600 °C Untreated treated treated treated 3570-3200 3431 3438 3445 3438 Hydroxyl [-OH] (Strong) Strong / Stretching

2935-2915 2925 Methylene (=CH2) (Medium to Strong) Medium to Strong / Stretching

2880-2860 2870 Methyl (-CH3) (Medium to Strong) Medium to Strong / Stretching 1750-1720 1738 Aldehyde and Ketone / Esters (-C=O) Strong / Stretching C=C Benzene Ring / -C=O / Carboxylic 1700-1520 1624 1630 1630 1630 Strong / Stretching Groups (-COOH) 1480-1400 1458 1458 1413 1417 CH2, CH3 and OH Medium / Bending 1388-1382 - - 1383 1383 Alkane / Methyl deformation Medium / Bending Strong (-CH-OH), Medium (- 1124-1087 1114 1113 1115 1116 Secondary Alcohols (-CH-OH) / Amine (-CN) CN)/Streching 1100-840 1039 1035 1025 1025 Secondary Alcohols (-CH-OH) Strong/Strecthing 600-500 - - - - Alkyne (-C-H) and Alkene -(C=H) Groups Medium/Bending

Table 20:Surface groups identified by FTIR analysis for recycled carbon fibers.

FTIR Spectra for air treated Recycled Carbon Fibers 0,12

0,1

0,08

0,06 Absorption

0,04

0,02

0 4000 3600 3200 2800 2400 2000 1600 1200 800 400 Wavelength (cm-1) Untreated RCF 400 °C treated 500 °C treated 600 °C treated

75 76

Figure 4-9: FTIR absorption spectra results for recycled carbon fibers treated at different temperatures.

Finally, the fingerprint region did not show clear peaks. However, higher absorption levels were detected in the 600-500 cm-1 range, representing alkyne and alkene groups from the bulk structure.

From this analysis, it can be concluded that the surface groups of these recycled carbon fiber samples are mainly acidic, and that with oxidation the level of surface acid groups is increased. The FTIR results of this work can be related to results from G. Krekel et al. and others, as previously demonstrated in Figure 1-8, Figure 1-20, Figure 1-26 and Table 3 in the literature review. The results obtained here matched with previous literature studies and indicate that surface adhesion is also affected by the oxidation treatment—a phenomenon that leads to a lower pH level of the material surface in contact with liquid used [42].

4.1.3 X-Ray Photoelectron Spectroscopy (XPS) Results for Virgin and Recycled Carbon Fibers

The results from XPS analysis performed on recycled and virgin carbon fibers are shown in stacked chart format in Figure 4-10 below. The spectra analysis allows identification of several elements, including carbon (C1s and Ckl 1), oxygen (O1s) and nitrogen (N1s), sodium (Na1s), chlorine (Cl2p) and Silica (Si2s, Si 2p3, Si 2p1). Table 21 shows the carbon content comparison between treatments performed. Virgin carbon fibers treated at 400oC in air had a slightly decreased carbon content, from 81.11% to 78.44%. The opposite was found when the sample was treated with nitrogen (an increase from 81.11% to 86.68%). In case of virgin carbon fibers treated at 400oC the lower carbon values are justified by the remaining sizing volatilization in high oxygen temperatures and the CO, CO2 formation. For recycled carbon fibers, C1s content improved in both treatments, suggesting that stabilization phenomena were taking place. Oxygen values in all samples decreased in the case of air, resulting in an impact on the O1s/C1s ratio. The abundant oxygen atmosphere contributes to formation of some surface groups

(hydroxyl and carboxyl) and causes CO and CO2 gases to be formed from the combustion process. In the

Nitrogen atmosphere NO2 gas formation takes place.

76 77

Table 21: XPS atomic material % composition of virgin and recycled carbon fibers treated in air and nitrogen at 400oC.

O 1s C 1s

C kl1 Na 1s N 1s Cl 2p RCF 400°C - N2

RCF 400°C - Air

RCF Untreated

VCF 400°C - N2

VCF 400°C - Air

VCF Untreated

1400 1200 1000 800 600 400 200 0 Binding Energy (ev)

Figure 4-10: XPS Stacked chart results for recycled and virgin carbon fiber at 400oC treatment in air and nitrogen.

Table 22 shows that O/C ratios are 0.2653 for RCF untreated, 0.2126 for VCF untreated, 0.2182 for VCF 400oC air treated, 0.1275 for VCF 400oC nitrogen treated, 0.1285 for RCF 400oC air treated and

77 78

0.1426 for RCF nitrogen treated. Nitrogen percentage has increased in all samples, which is predicted by the reviewed literature as being characteristic of unsized carbon fibers. The presence of Na and Cl suggests the type of treatment to which the fibers were subjected—gas or liquid: NaOCl or NaClO3 surface treatments. Traces of silica can be explained as presence of dust.

Treatment O/C Ratio RCF Untreated 0.2653 RCF 400°C Air 0.1285

RCF 400°C N2 0.1426 VCF Untreated 0.2126 VCF 400°C Air 0.2182

VCF 400°C N2 0.1275

Table 22: XPS analysis: O/C ratio for recycled and virgin carbon fibers at different treatments.

The carbon C1s deconvolution analysis, presented in Figure 4-11, Figure 4-12 and Figure 4-13, verifies the result from the earlier FTIR analysis that C=O, C-O and C-O-C groups representing esters, secondary alcohols and carboxylic acid and/or ethers are present in untreated virgin and recycled carbon fibers. Their presence impact is shown as a second and third peaks in the convolution. Secondary alcohols of C-O bond type are more present in untreated recycled carbon fibers and carboxylic acids and/or ethers C-O-C bond types are more present in untreated virgin carbon fibers. The presence of other carbon groups and sizing on VCF untreated sample increase the spread of the C1s peak out of the C1s binding energy of 284.8 eV and same is observed for recycled carbon fiber as observed in Figure 4-11. For treated samples, higher peaks of C1s were observed. The explanation for these peaks is that the remaining stabilization phenomena inside the material resulted in more carbon rings layers, as was explained previously, and this layered orientation facilitate transitioning from turbostratic to graphitic structures. With less oxygen, as shown in Table 21, the curves for treated carbon fibers have higher peaks at the C-C, C-H binding energy levels, meaning that all remaining oxidized functional groups once available as structural defects of the turbostratic structure were transformed to gases and some of them reached the surface forming oxygen surface groups as shown in FTIR results at Table 20.

Lower oxygen levels are clearly shown on the convolution analysis for O1s in Figure 4-14. Figure 4-15 and Figure 4-16 shows the deconvolution of all O1s carbon fiber curves and showed the presence of esters and secondary alcohols. Esters (C=O) groups were predominant for both untreated recycled and

78 79 virgin carbon fibers. The effect of air and nitrogen treatment resulted in a more stable surface condition with the presence of more secondary alcohols (C-O) groups. Higher peaks for untreated VCF were observed according with Figure 4-14. However, higher percentage amount for RCF in XPS percentage chart implies that oxygen in RCF represents more functional groups than VCF. This result was also obtained by Connor M. [38], as shown in Table 5 in the literature review. In fact XPS deconvolutions for O1s in Figure 4-15 and Figure 4-16 shows that for recycled carbon fibers the presence of secondary alcohols were much higher than in virgin carbon fibers and this can be an explanation for the higher percentage of oxygen in XPS table. Figure 4-17 presents the nitrogen convolution analysis and its smoothed curve. As can be seen from the figure, a level of disturbance impacted a correct correlation with the atomic percentage of each sample. However, the curves show a good level of representation of the atomic composition. Significant levels of chlorine were found only in VCF 400oC air treated and RCF 400oC air and nitrogen treated samples, as shown in Figure 4-18. However, the disturbance level of the analysis makes the correct correlation with XPS percentage table difficult to perform. Peaks of sodium at 1071.8 eV showed a good correlation with the XPS percentage table and caused some interference peaks in the 500eV region in the samples in which they were detected (RCF treated in air and nitrogen and VCF treated in air). These peaks are shown in the convolution curves in Figure 4-19.

C1S Scan

o RCF 400 C N2

RCF 400oC Air

RCF 400oC Untreated Signal (Count/s)

o VCF 400 C N2

VCF 400oC Air VCF 400oC Untreated

290 285 280 Binding Energy (eV)

Figure 4-11: Stacked XPS curves of C1s element for different carbon fiber types and treatments.

79 80 C 1s Scan

100000 80000 RCF Untreated CC 1s 1s Scan Scan VCF Untreated 80000 100000 C-C 60000 C-C 100000 RCF Untreated 80000 VCF Untreated RCF Untreated VCF Untreated C-O-C C-C 4000060000 C-C 50000 C-C 60000 C-C C-O C-O-CC-O 2000040000 50000 C-O-C

C-O-C Signal(Count/s) 40000 C=O Signal(Count/s) 50000 C-O C=O C-O C-O 200000 0 C-O C-O-C Signal(Count/s) C=O

Signal(Count/s) 20000 C=O

C-O-C Signal(Count/s) C=O

Signal(Count/s) 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 C=O288 286 284 282 280 0 0 Binding energy (eV) Binding energy (eV) 0 0 150000 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284VCF 400°C282 Air280 150000 300 298 296 294 Binding292 energy290 (eV)288 286 284 282 280 300 298 296 294 Binding292 energy290 (eV)288 286 284RCF 400°C282 Air280 150000 Binding energy (eV) C-C Binding energy (eV) 100000 VCF 400°C Air 150000 150000 100000 C-CRCF 400°C Air VCF 400°C Air 150000 C-C RCF 400°C Air 100000 50000 C-C 100000 C-C 100000

50000 Signal(Count/s) C-O-C 100000 C-C C-O

Signal(Count/s) 50000 C-O-C C-O 0

50000 Signal(Count/s) 50000 0 C-O-C C-O Signal(Count/s)

50000 Signal(Count/s) C-O-C 0300 298 296 294 292 290 288 286 284C-O-C282 280 C-O C-O Signal(Count/s) 300 298 296 294 292 290 288 286 C-O-C284 282 280 Binding energy (eV) 0 C-O 0 Binding energy (eV) VCF 400°C N 0 300 298 296 294 292 290 288 286 284 282 2280 300 298 296 294 292 290 288 286 284 282 280 Binding energy (eV) RCF 400°C N2 300 298 296 294 292 290 288 286 284 282 280 100000 100000 C-C Binding energy (eV) VCF 400°C N 300 298 296 294 292 290 288 286 C-C284 282 280 Binding energy (eV) 2 Binding energy (eV) RCF 400°C N 2 VCF 400°C N2 100000 100000 C-C C-C 50000 RCF 400°C N2 50000 100000 100000 C-C C-O-C C-C C-O-C

50000 C-O Signal(Count/s) 50000 C-O Signal(Count/s) 0 0 50000 C-O-C 50000 C-O-C

C-O Signal(Count/s) C-O

Signal(Count/s) C-O-C 300 298 296 294 292 290 288 286 284C-O-C282 280 0 300 298 296 294 292 290 288 286 284 282 280 0

C-O Signal(Count/s) Binding energy (eV) C-O

Signal(Count/s) Binding energy (eV) 0 0 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 Binding energy (eV) Binding energy (eV) 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 Binding energy (eV) Binding energy (eV)

Figure 4-12: Carbon C1S deconvolution XPS curves for recycled carbon fibers thermal treated in air and nitrogen.

80 81 C 1s Scan C 1s Scan 100000 80000 RCF Untreated VCF Untreated 100000 80000 RCF Untreated C 1s Scan VCF Untreated C-C 60000 C-C 100000 80000 C-C RCF Untreated 60000 C-C VCF Untreated C-O-C 40000 50000 C-C 60000 C-O-C C-C 40000 50000 C-O C-O 20000 C-O-C C-O 40000 50000 C-O-C Signal(Count/s) C=O C-O Signal(Count/s) C=O 20000

C-OC-O-C Signal(Count/s) C=O Signal(Count/s) 0 C-O 0 C=O 20000

C-O-C Signal(Count/s) 0 C=O Signal(Count/s) 0 300 298 296 294 292 290 C=O288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 0 Binding energy (eV) 3000 298 296 294 292Binding290 energy288 (eV)286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 150000 Binding energy (eV) VCF 400°C Air Binding energy (eV) 150000300 298 296 294 292 290 288 286 284 282 280 150000 300 298 296 294 292 290 288 286 284 282 280 RCF 400°C Air Binding energy (eV) VCF 400°C Air 150000 Binding energy (eV) C-C 150000 RCF 400°C Air 100000 VCF 400°C Air 100000150000 C-C C-C RCF 400°C Air 100000 100000 C-C C-C 10000050000 100000 C-C 50000 Signal(Count/s) 50000 C-O-C C-O Signal(Count/s)

50000 C-O-C Signal(Count/s) 50000 C-O 0 C-O-C C-O Signal(Count/s) 500000 Signal(Count/s) C-O-C 0 C-O-C C-O C-O

Signal(Count/s) 300 298 296 294 292 290 288 286 284 282 280 C-O-C 0 C-O 0 300 298 296 294 292 290 288 286 284 282 280 Binding energy (eV) 0 300 298 296 294 292 290 288 286 284 282 280 Binding energy (eV) VCF 400°C N 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292Binding290 energy288 (eV)286 284 282 2802 RCF 400°C N 300 298 296 294 Binding292 energy290 288(eV) 286 284 282 2802 Binding energy (eV) VCF 400°C N 100000 100000 C-C 2 Binding energy (eV) C-C VCF 400°C N RCF 400°C N2 2 100000 100000 C-C C-CRCF 400°C N2 100000 100000 C-C 50000 C-C 50000

50000 C-O-C 50000 C-O-C

C-O Signal(Count/s) C-O 50000 50000 Signal(Count/s) C-O-C C-O-C 0 0

C-O C-O-C Signal(Count/s) C-O C-O-C Signal(Count/s)

0 C-O Signal(Count/s) 0 C-O Signal(Count/s) 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 0 0 Binding energy (eV) Binding energy (eV) 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294 292 290 288 286 284 282 280 300 298 296 294Binding292 energy290 (eV)288 286 284 282 280 Binding energy (eV) Binding energy (eV) Binding energy (eV)

Figure 4-13: Carbon C1S deconvolution XPS curves for virgin carbon fibers thermal treated in air and nitrogen.

81 82

O 1s Scan

o RCF 400 C N2

RCF 400oC Air

RCF 400oC Untreated

o VCF 400 C N2

Signal (Count/s) VCF 400oC Air

VCF 400oC Untreated

544 542 540 538 536 534 532 530 528 526 524 Binding energy (eV)

Figure 4-14: Stacked XPS curves of O1s element for different carbon fiber types and treatments. O 1s Scan

RCF Untreated 80000 VCF Untreated 60000 C=O C=O 60000

40000 40000

20000 C-O Signal(Count/s) 20000 Signal(Count/s) C-O

0 0 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV)

60000 VCF 400°C Air 40000 RCF 400°C Air C=O

30000 C-O 40000 C=O

C-O

20000 Signal(Count/s)

Signal(Count/s) 20000 82

10000 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) 40000 40000 VCF 400°C N2

RCF 400°C N2 C-O C=O

30000 30000

C-O

20000 20000 Signal(Count/s)

C=O Signal(Count/s)

10000 10000 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) O 1s Scan

RCF Untreated O 1s Scan 80000 VCF Untreated 60000 C=O RCF Untreated 80000 C=O VCF Untreated 60000 C=O 60000 C=O 40000 60000

40000 40000 40000 20000 C-O Signal(Count/s) 20000 Signal(Count/s) C-O 20000 C-O Signal(Count/s) 20000 Signal(Count/s) C-O 0 0 545 540 535 530 525 545 540 535 530 525 830 0 Binding energy (eV) 545 540 Binding energy535 (eV) 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) 60000 VCF 400°C Air 40000 RCF 400°C Air 60000 VCF 400°C Air 40000 C=O RCF 400°C Air C=O 30000 C-O 40000 30000 C=O C-O 40000 C=O C-O

20000 Signal(Count/s)

Signal(Count/s) 20000 C-O

20000 Signal(Count/s)

Signal(Count/s) 20000

10000 545 540 535 530 525 545 540 535 530 525 10000 545 540 Binding535 energy (eV) 530 525 Binding energy (eV) 545 540 535 530 525 Binding energy (eV) 40000 Binding energy (eV) 40000 VCF 400°C N2 40000 RCF 400°C N 40000 2 VCF 400°C N2 C-O C=O RCF 400°C N2 C=O 30000 C-O 30000 30000 30000 C-O

20000 20000 C-O Signal(Count/s)

20000 C=O Signal(Count/s) 20000 Signal(Count/s)

C=O Signal(Count/s) 10000 10000 545 540 535 530 525 545 540 535 530 525 10000 10000 545 540Binding energy535 (eV) 530 525 545 540 Binding 535energy (eV) 530 525 Binding energy (eV) Binding energy (eV)

Figure 4-O15: Oxygen 1s O 1SScan deconvolution XPS curves for recycled carbon fibers thermal treated in air and nitrogen.

RCF Untreated 80000 VCF Untreated 60000 C=O C=O 60000

40000 40000

20000 C-O Signal(Count/s) 20000 Signal(Count/s) C-O

0 0 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV)

60000 VCF 400°C Air 40000 RCF 400°C Air C=O

30000 C-O 40000 83 C=O

C-O

20000 Signal(Count/s)

Signal(Count/s) 20000

10000 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) 40000 40000 VCF 400°C N2

RCF 400°C N2 C-O C=O

30000 30000

C-O

20000 20000 Signal(Count/s)

C=O Signal(Count/s)

10000 10000 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) O 1s Scan RCF Untreated RCF Untreated 80000 VCF Untreated 60000 80000 VCF Untreated 60000 C=O C=O C=O C=O 60000 60000 40000 40000 40000 40000 Signal(Count/s)

20000 C-O Signal(Count/s) 20000 C-O 20000 Signal(Count/s) 20000 C-O Signal(Count/s) C-O

0 0 0 0 545 540 535 530 525 545 540 535 530 525 84 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) Binding energy (eV) Binding energy (eV) 60000 60000 VCF 400°C Air 40000 VCF 400°C Air 40000 RCF 400°C Air RCF 400°C Air C=O C=O

30000 C-O 40000 30000 C-O 40000 C=O C=O

C-O 20000 Signal(Count/s) C-O

20000 Signal(Count/s) Signal(Count/s) 20000

Signal(Count/s) 20000

10000 545 540 535 530 525 545 540 535 530 525 10000 545 540 535 530 525 545 540 535 530 525 Binding energy (eV) Binding energy (eV) Binding energy (eV) Binding energy (eV) 40000 40000 VCF 400°C N 40000 40000 VCF 400°C N2 RCF 400°C N 2 RCF 400°C N2 C-O 2 C=O C-O C=O 30000 30000 30000 30000

C-O C-O 20000 20000 20000 20000 Signal(Count/s)

C=O Signal(Count/s) Signal(Count/s)

C=O Signal(Count/s)

10000 10000 10000 545 540 535 530 525 10000 545 540 535 530 525 545 540 535 530 525 545 540 Binding energy535 (eV) 530 525 Binding energy (eV) Binding energy (eV) Binding energy (eV)

Figure 4-16: Oxygen O1S deconvolution XPS curves for virgin carbon fibers thermal treated in air and nitrogen.

N 1s Scan N 1s Scan

RCF 400°C N2

RCF 400°C N2 RCF 400°C Air RCF 400°C Air RCF Untreated RCF Untreated

VCF 400°C N2 VCF 400°C N2 Signal (Count/s) Signal (Count/s) VCF 400°C Air VCF 400°C Air

VCF Untreated VCF Untreated

410 408 406 404 402 400 398 396 394 392 390 410 408 406 404 402 400 398 396 394 392 390 Binding Energy (eV) Binding Energy (eV)

84 85

Figure 4-17: Nitrogen N1S convolution curves for virgin and recycled treated carbon fibers in air and nitrogen.

Cl 2p Scan Cl 2p Scan

RCF 400°C N2 RCF 400°C N2

RCF 400°C Air RCF 400°C Air RCF Untreated RCF Untreated Signal ( Count/s) Signal ( VCF 400°C N2 Signal (Count/s) VCF 400°C N2 VCF 400°C Air VCF 400°C Air VCF Untreated VCF Untreated 210 208 206 204 202 200 198 196 194 192 190 210 208 206 204 202 200 198 196 194 192 190 Binding energy (eV) Binding energy (eV)

Figure 4-18: Chlorine Cl2p convolution curves for virgin and recycled treated carbon fibers in air and nitrogen.

Na 1S Scan

RCF 400°C N2

RCF 400°C Air

RCF Untreated

VCF 400°C N2 Signal (Count/s)

VCF 400°C Air

VCF Untreated

1080 1078 1076 1074 1072 1070 1068 1066 1064 1062 1060 Binding Energy (eV)

Figure 4-19: Sodium Na1S convolution curves for virgin and recycled treated carbon fibers in air and nitrogen.

85 86

4.2 Mechanical Properties Results

The second part of the results discussion is focused on the mechanical properties of all materials studied. The mechanical properties analysed in this section were Tensile Strength and Modulus, Flexural Strength and Modulus, Impact Energy and Melt Flow Index. The results of the Melt Flow Index analysis can be found in this section for the sake of convenience, based on the fact the viscosity is a physical property of the material. The analysis begins with all properties of single materials mentioned in the literature review and the subsequent blends/composites comparisons. Some insights about the impact of MAPP surface modifiers on PP/PA12 blends can also be found in the literature review. Mechanical properties of pure blends ranging from 50%-50%, 40%-60%, 30%-70% to 20%-80% PP/PA12 compositions were analysed. These values can be considered as “base line” values to understand the impact of MAPP and PMPPIC surface modifiers on the mechanical properties of the materials studied. The optimization of PMPPIC was performed using 50%-50% PP/PA12 samples, as described in the research methodology. The best-performing material was chosen to be analysed with higher PA12 compositions. The PMPPIC results in all compositions were compared with MAPP results. Finally, this section will discuss the mechanical properties of virgin vs recycled reinforced carbon fiber polymer composites, both untreated and treated at 400oC, and blended with both surface modifiers.

After the initial discussion of the PP/PA12 blends and their biphasic behavior, and before presenting the analysis results, it is important to mention the main chemical events that occurred between PP-MAPP-PA12 and PP-PMPPIC-PA12 blends and how the addition of virgin and recycled carbon fibers impact these chemical events improving the final results.

As discussed in the literature review, once the PP reacts with MAPP, it becomes polarized and its anhydride groups reacts with the amine PA12 groups by imidation. This increases the PP phase dispersion in the PA12 phase material, lowers the interfacial tensions and improves adhesion area, which in turn promotes material strength. Esterification is present between the PP-MAPP-PA12 and the surface groups of carbon fibers as a chemical adhesion reaction due to an abundance of surface carbon fibers hydroxyl groups reacting with the anhydride groups of the functionalized polypropylene. Carbon fiber hydroxyl groups also find amide groups to react with, which improves fiber matrix adhesion.

86 87

For PP-PMPPIC-PA12 blends the polymeric shape of the reagent allows some molecule alignment by the reaction of the amide groups of PA12 and the isocyanate groups from the PMPPIC, resulting in a branched molecule with a high level of agglomeration. The isocyanate-amide reaction affects the dispersion polarizabilities of both molecules, improving their reactivity with new carbonyl-imide like structures. Additionally, some localized urethane formation can be found within the isocyanate group structure and atmosphere water moisture during process if environment is not well controlled. Urethane reactions can also be found between carbon fiber hydroxyl surface groups and the remaining isocyanate groups not reacted with polyamide matrix. The final blend bulk structure becomes more biphasic compared with the pure PP-PA12 blends due to the higher polarity and agglomerated structure of PA12- PMPPIC and for this reason has its internal energy increased. The copolymeric branched alignment hinder PP penetration in the PA12 phase, leading to a higher phase separation and higher interfacial tensions. The PA12-PMPPIC structure can be seen in Figure 4-20 below.

CH2(CH2)9CH2 O NCO N n = O C CH2(CH2)9CH2 + NH N n n Polyamide 12 Poly[methylene(polyphenyl) isocyanate]

Figure 4-20: PA12-PMPPIC reaction resulting in new carbonyl groups and a branched-crosslinked molecule aligned structure.

4.2.1 Polymethylene Polyphenyl Isocyanate (PMPPIC) Optimization Results

Flexural and tensile strength results for PMPPIC optimization are shown in Figure 4-21. The base values for 50%-50% PP/PA12 pure blends are 47.33 MPa for flexural strength and 29.24 MPa for tensile strength. Among the PMPPIC blends studied, PMPPIC at 1% showed the best results at flexural strength

87 88 of 50.56 MPa without significant change in tensile strength, as shown, with its values falling below the standard deviation. Higher compositions of PMPPIC (2% and 3%) presented lower values in proportion to the increment in PMPPIC content: 39.78 MPa and 37.86 MPa for flexural strength and 19.47 MPa and 18.06 MPa for tensile strength for 2% and 3% respectively. Material strength in this biphasic composition is intricately related to the interfacial adhesion between the PP and PA12 phases. Interfacial stress occurs mainly because of the biphasic structure and increases with each modulus material phase increment. Each phase toughening results in more local interfacial tensions, leading to premature failure due to microstructural defects expansion. The result is a lower tension load capability. At 1% PMPPIC concentration, several effects on the PA12 phase are observed: bonding between PA12 and PMPPIC is mainly occasioned by the reaction of isocyanate group NCO with PA12 amide groups, which improves the PA12-PMPPIC branching under high temperature and mechanical mixing processing conditions. Stiffness improvement for the overall PA12 material results from these conditions. PP crystallinity was also affected at 60% PA12 concentration, as previously proved by Aranburu, N. and Eguiazabal, J [66], improving the overall modulus of the material.

At higher PMPPIC compositions, saturation of isocyanate groups in the bulk of the material leads to the formation of localized polyurea due to natural degradation caused by atmospheric water moisture exposition. The dispersion of polyurea in the PA12 phase drastically increases the modulus which leads to a compromised biphasic adhesion at the PP/PA12 interface due to increase of interfacial stress. Also, at 2% and 3% PMPPIC concentrations, saturation did not allow a significant portion of the surface modifier to reach the interface region to promote effective branching. Thus, the outer material surface region starts to degrade during the extrusion mixing process, when exposed to high temperature and air with moisture, resulting in very high surface brittleness which impact significantly the overall material flow as can be seen in the MFI results. Another complementary factor can be found in the anisotropic behavior of PP crystallinity phase due to injection molding process as described by Varlet, J. et al [77].

Analysis of the flexural and tensile modulus is shown in Figure 4-22. The flexural modulus of the PP/PA12 improved from 1.17 GPa for the pure PP/PA12 blend to 1.28GPa, 1.29 GPa and 1.47 GPa for 1%, 2% and 3%, respectively. However, the tensile modulus was only improved for PMPPIC at 3%, 1.63GPa. Other PMPPIC concentrations showed no significant change: for 1% the value was 1.45GPa, and a still lower value of 1.35GPa was found for PMPPIC 2%.

88 89

Flexural and tensile strength values for PP/PA12 with PMPPIC surface modifier at different concentrations

30 Flexural Strength 50,56 47,33 Tensile Strength 20 39,78 37,86 29,24 28,97 10 19,47 18,06

Flexural and tensile strength (MPa) 0 50% PP / 50% PA12 50% PP / 50% PA12 + 50% PP / 50% PA12 + 50% PP / 50% PA12 + PMPPIC 1% PMPPIC 2% PMPPIC 3%

Figure 4-21: Flexural and Tensile Strength properties of 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers.

Flexural and tensile modulus for PP/PA12 with PMPPIC surface modifier at different concentrations

1,8 1,6 1,4 1,2 1 Flexural Modulus 0,8 1,63 1,48 1,45 1,47 1,28 1,29 1,35 0,6 1,17 Tensile Modulus 0,4 0,2

Flexural and tensile modulus (GPa) 0 50% PP / 50% PA12 50% PP / 50% PA12 + 50% PP / 50% PA12 + 50% PP / 50% PA12 + PMPPIC 1% PMPPIC 2% PMPPIC 3%

Figure 4-22: Flexural and Tensile Young’s modulus of 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers.

89 90

The impact energy analysis (Figure 4-23) showed that the addition of PMPPIC affected the material’s performance against fracture. Among all PMPPIC percentages used best values were found at 1% PMPPIC composition. Pure blends presented energies of 48.55 J/m, whereas 1% PMPPIC reached 37.87J/m. The PA12 branched structure improved phase energy absorption due to agglomeration and becomes more evident for higher concentrations of PMPPIC. The increase in tension stress between the PP and PA12 phases enhances the formation and propagation of micro fractures through the material during impact, resulting in fragile mode fractures.

In the previous analysis of the behavior of different PMPPIC compositions, 1% PMPPIC concentrations demonstrated the best average values. Furthermore, 1% PMPPIC concentrations displayed better Melt Flow Index values. This implies better manufacturing capability due to its better mixing and carbon fiber dispersion during all processes. The next comparison is between PMPPIC 1% and MAPP 3% at different PP/PA12 blend fractions and will be shown in the following chapter.

Impact Energy values for PP/PA12 with PMPPIC surface modifier at different concentrations

60,00

50,00

40,00

30,00 48,55 20,00 37,87 Impact Energy (J/m) 36,76

10,00 22,66

0,00 50/50 PP-PA12 / PMPPIC 50/50 PP-PA12 / PMPPIC 50/50 PP-PA12 / PMPPIC 50/50 PP-PA12 (J/m) 1% (J/m) 2% (J/m) 3% (J/m)

Figure 4-23: Impact Energy values for 50%-50% PP/PA12 polymer blends and with 1%, 2% and 3% PMPPIC surface modifiers.

90 91

4.2.2 Polypropylene and Polyamide 12 Optimization and Mechanical Properties Results with PMPPIC and MAPP Surface Modifiers

The mechanical comparison between the surface modifiers used in this work, MAPP and PMPPIC, is presented in this sub-section. Figure 4-24 compares flexural and tensile strength results among pure PP/PA12 blends and blends with MAPP 3% and PMPPIC 1% surface modifiers.

Tensile strength for PMPPIC showed only a small improvement compared to its respective pure blends for all PP/PA12 compositions. Consequently, the addition of PMPPIC 1% as surface modifier maintains the tensile properties of the original polymer blend. MAPP tensile strength improvement was in the order of 17.7% relative to its respective pure blends. This is comparable to the predictions of other studies using this type of material [63, 64, 66, 67].

Flexural and Tensile Strength for PP-PA12 blends with MAPP and PMPPIC 65

60 58,28 58,51 56,47 55,18 55,14 57,52 55 51,88 50,56 53,89 Flexural Strength PP/PA12 50 52,70 Pure Blend 50,52 Flexural Strength 3% MAPP 45 47,33 Flexural Strength 1% PMPPIC Tensile Strength PP/PA12 40 Pure Blend 36,24 36,23 36,07 35,58 Tensile Strength 3% MAPP 35 Flexural and Tensile Strength (MPa) 31,78 Tensile Strength 1% PMPPIC 29,84 30,09 28,97 30 30,39 30,78 29,24 29,31 25 50/50 PP-PA12 40/60 PP-PA12 30/70 PP-PA12 20/80 PP-PA12

Figure 4-24: Tensile and Flexural Strength comparison between pure PP/PA12 blends and blends using PMPPIC and MAPP surface modifiers.

91 92

Flexural strength values for PMPPIC represented an improvement of up to 8.5% compared to pure blends. At 80%/20% PA12/PP composition, the PMPPIC values were superior than MAPP and pure blends. The PMPPIC curve trend along the PA12 increment was like that of pure blends. This is important evidence to show that PMPPIC improves the mechanical properties at PA12 bulk phase and does not affect or promote interfacial adhesion mechanisms between PP and PA12 phases. The average low concentration of the amide end groups, which reacted mostly with the PMPPIC groups, coupled with the slow chain mobility, reduces somewhat the probability of finding a PP phase and disperse. This is further hampered by the immiscible nature of the blend and becomes more explicit in higher concentrations of isocyanate groups, as shown previously for 2% and 3% PMPPIC concentrations.

Flexural modulus for PP-PA12 blends with MAPP and PMPPIC

1,50

1,45 1,42 1,40 1,39 1,34 1,35 1,37 1,35 1,34 1,34

1,30 1,31 1,31 1,28 1,25 1,27 Flexural Modulus PP/PA12

Flexural Modulus (GPa) Pure Blends 1,20 Flexural Modulus 3%MAPP

1,18 1,15 Flexural Modulus 1% PMPPIC

1,10 50/50 PP-PA12 40/60 PP-PA12 30/70 PP-PA12 20/80 PP-PA12

Figure 4-25: Flexural modulus comparison of PP/PA12 pure blends and with MAPP and PMPPIC surface modifiers.

The flexural modulus comparison between PP/PA12 pure blends and blends with MAPP and with PMPPIC is shown in Figure 4-25. Both materials with surface modifiers presented better performance compared to the pure blend. However, the MAPP surface modifier performed better for 50% and 70% PA12 compositions, while PMPPIC presented better results for 60% and 80% compositions of PA12.

92 93

MAPP acted directly on PP chains, promoting grafting and polymer functionalized chains and causing the polymer to bond with PA12 amides. These molecular-level reactions improved phase miscibility and compatibility decreasing bulk energy of the material. The resulting increase in interfacial adhesion explains the better strength performance of MAPP in comparison with PMPPIC at higher percentages of PP. However, as previously explained, when the PA12 percentage increases, the PMPPIC blocking in PA12 phase significantly improves the flexural modulus of PP/PA12 blends with higher PA12 content (70-80%). At 70% though, the MAPP-PMPPIC difference does look like it’s within the margin of error.

Tensile modulus for PP-PA12 blends with MAPP and PMPPIC

1,60

1,53 1,55 1,51

1,50 1,47

1,45 1,45 1,48 1,46 1,41 1,40 1,40

1,35 1,36 1,34 1,30 Tensile Modulus PP/PA12 Pure Tensile Modulus (GPa) 1,28 Blends 1,25 Tensile Modulus 3% MAPP

1,20 Tensile Modulus 1% PMPPIC

1,15 /50 PP-PA12 /60 PP-PA12 /70 PP-PA12 /80 PP-PA12 50 40 30 20

Figure 4-26: Tensile modulus results for PP/PA12 blends and blends with MAPP and PMPPIC surface modifiers.

Finally, Figure 4-26 shows surface modifier compatibility and performance as measured by the tensile modulus. For pure blends, the modulus tends to decrease as the content of PA12 in the blend increases. Since PA12 is not known for its toughness, this shows that the brittle behavior of PP contributes to the stiffness of the pure blend. The same curve trend is observed for MAPP-modified blends. However, the results show a higher tensile modulus for all MAPP compositions, an improvement of up to 12% compared to pure PP/PA12 blends for 40%/60% grades. PMPPIC presented a solid modulus performance increment with the addition of PA12 blend content. This result supports the hypothesis of the surface modifier’s high compatibility with this type of material and its higher performance structure. However,

93 94 for lower values of PA12 the biphasic lack of adhesion phenomenon significantly impacted the tensile modulus due to high stress in the interfacial area. PMPPIC 1% with 20%-80% PP/PA12 blends presented one of the best tensile modulus values, 8.2% better than their respective pure blends and 3% better than MAPP modified blends.

Impact Energy for PP-PA12 blends with MAPP and PMPPIC 70,00

60,71 60,00 56,15 51,70 48,55 50,00 50,02 52,00 47,86 40,00 37,87 3% MAPP 40,08 1% PMPPIC 30,00 33,54 32,56 30,63 Pure Blend

20,00

10,00 Izod (notched) Impact Energy (J/m)

0,00 50/50 PP-PA12 (J/m) 40/60 PP-PA12 (J/m) 30/70 PP-PA12 (J/m) 20/80 PP-PA12 (J/m)

Figure 4-27: Impact energy test results for Pure PP/PA12 blends and with MAPP and PMPPIC surface modifiers.

Impact test energy results are shown in Figure 4-27. The curve for pure blends and MAPP modified material followed the same shape, showing good reproducibility in test procedures and material composition behavior. MAPP is known as a reagent that promotes ductility and tensile stress material improvements with lack of toughness and the results reinforced this point. PMPPIC results evidenced toughness improvement behavior relative to MAPP, with the best overall results obtained in PMPPIC 1% with 80% PA12 blend composition. PMPPIC at 1% improves toughness by 16.8% compared to pure blends and is 98% better than the MAPP surface modifier. The impact improvement can be related to the newly-formed PA12-PMPPIC structure, as explained in the beginning of this chapter, and in particular its high-energy absorption behavior due to its molecular agglomerated form. In the end based on percentage performance comparison summary in Table 23 below, the 20%-80% PP/PA12 hybrid matrix performed better in four of five mechanical properties compared with MAPP and based on these criteria, this material

94 95 was selected to be reinforced with virgin and recycled carbon fiber. Results of these composite materials are presented and discussed in the next section.

% performance comparison between pure and modified hybrid matrices

80%/20% PA12/PP 1% 3% Pure blends B (W) PMPPIC MAPP

Tensile Strength 30.78 3.2% 17.7% (Mpa)

Tensile Modulus 1.34 8.2% 5.2% (Gpa)

Flexural Strength 53.89 8.5% 6.7% (Mpa)

Flexural Modulus 1.27 11.8% 5.5% (Gpa)

Impact Energy 52.00 16.8% (30.3%) (J/m)

Table 23: Performance comparison between MAPP, PMPPIC hybrid modified blend and pure hybrid blends.

4.2.3 PP/PA12 Virgin and Recycled Carbon Fiber Reinforced Polymers Mechanical Properties Results

Mechanical properties results of recycled and virgin carbon fiber reinforced polymers with 25% content in weight are presented in Figure 4-28 4-29, 4-30, 4-31 and 4-32. Each figure shows differences between untreated virgin and recycled carbon fibers and their respective treated samples at 400oC for 30 minutes. In general, material strength behavior in composites with the MAPP surface modifier was better than PMPPIC. On the other hand, better toughness capabilities were achieved with PMPPIC, as shown by the higher values of impact energies registered for composites with that surface modifier.

Figure 4-28 below shows the tensile strength for the materials studied. MAPP untreated recycled and virgin carbon fibers showed better tensile strength results relative to all other samples. PMPPIC showed expected lower results, mainly influenced by the presence of the isocyanate surface modifier. This finding was obtained in the previous matrix tensile strength analysis involving the same surface modifier and was discussed there. Another significant observation is that all fibers treated at 400oC presented lower

95 96 tensile strength than untreated ones. The fiber oxidation treatment possibly eroded the fiber surface, creating micro voids that drastically impact mechanical adhesion in the fiber-matrix interface area. Added to this point, the increase in OH groups, as demonstrated in the FTIR analysis, was not significant due to overall oxygen loss shown by XPS. Consequently, chemical bonding probabilities between fiber surface and matrix were also decreased. In the case of MAPP, the OH groups available in the carbon fiber surface can possibly form hydrogen bonds with dispersed available maleic anhydride end chain of the reactive polypropylene or amide groups in polyamide 12. This reactive polymer behavior was not found in PP/PA12 blends with PMPPIC. A stronger hypothesis is that most of the isocyanate groups of PMPPIC end chains reacted with PA12 amide end chains and formed a more agglomerated and crystallized structure and then left no significant amount of isocyanate reactive groups available to bond with the OH remaining on the surface of the carbon fibers.

Tensile Strength Values for modified PP/PA12 blends reinforced with virgin and recycled carbon fibers 120

100

60 106,39 Tensile Strength 3% MAPP

40 84,38 83,83 Tensile Strength 1% PMPPIC

Tensile Strength (MPa) 64,35 62,57 59,79 55,91 20 45,05

0 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% RCFU RCF 400 ∘C VCFU VCF 400 ∘C

Figure 4-28: Tensile Strength results for untreated and 400oC treated Recycled and Virgin Carbon Fiber Reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers.

Tensile modulus results for all reinforced composites are shown in Figure 4-29. Although differences between MAPP and PMPPIC in this case were lower, MAPP samples still performed better. The observations made for tensile strength in the paragraph above are also valid here. Even though the

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PMPPIC blend material possess a higher matrix modulus, the lack of adhesion between the carbon fibers and the matrix leads to poorer tensile modulus performance relative to MAPP. Based on the considerations made in the TGA analysis, 400oC-treated virgin carbon fibers composites presented a higher modulus than its untreated ones. However, this can be observed only in MAPP composites and is a strong evidence of better final hybrid MAPP blend interfacial adhesion with the carbon fibers` surface promoting a better load transfer between fiber and hybrid matrix.

Flexural Strength and Modulus values are presented in Figure 4-30 and Figure 4-31. The same behaviour found in the tensile strength and modulus comparisons between MAPP and PMPPIC can be observed here. Untreated fiber composites showed better results than treated ones, except for thermal treated virgin carbon fibers with MAPP. The carbon fiber bulk graphitization and good hybrid matrix interfacial adhesion are the main reasons for this behavior. This finding confirms the previous tensile modulus discussion and is supported by the TGA analysis results. The higher values for composites with MAPP have already been explained based on better adhesion phenomena between MAPP anhydride end chains and fiber surface OH groups, which are available in a better dispersed biphasic material. It is important to mention that fiber size was also an important factor in relation to the final mechanical properties discussed. Higher recycled carbon fiber length resulted in higher effective load transfer area and then better final flexural and tensile strength and modulus properties. However, this behavior was strongly affected by the thermal treatment as well as by lack of adhesion already commented upon. Hence, treated recycled carbon fiber composites showed lower values compared with their respective virgin carbon fiber composites.

Finally, Impact Energy results show better performance values for PMPPIC surface modifier blends relative to their MAPP correlates, as seen in Figure 4-32 PMPPIC performed 43% and 55% better than MAPP for untreated and 400oC-treated recycled carbon fibers and 55.9% and 40.4% better for untreated and 400oC treated virgin carbon fibers. This is associated not only with the carbon fiber reinforcement and its high modulus but also with the higher toughness capability of the PMPPIC polymer hybrid matrix relative to the respective MAPP blend, a difference that is due to the high energetic material bulk agglomerate and crystallized arrangement between PMPPIC and the PA12 matrix.

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Tensile Modulus Values for modified PP/PA12 blends reinforced with virgin and recycled carbon fibers 9

4 7,64 6,82 6,32 6,45 3 5,94 5,86 5,64 Tensile Modulus 3% MAPP Tensile Modulus (GPa) 2 3,88 Tensile Modulus 1% PMPPIC

0 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% RCFU RCF 400 ∘C VCFU VCF 400 ∘C

Figure 4-29: Tensile Modulus results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers.

Flexural Strength Values for modified PP/PA12 blends reinforced with virgin and recycled 180 carbon fibers 160 140 120 100

80 154,38 Flexural Strength 3% MAPP 135,12 137,22 60 115,90 Flexural Strength 1% PMPPIC 105,6 101,48 96,88 40 80,62 Flexural Strength (MPa) 20 0 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% RCFU RCF 400 ∘C VCFU VCF 400 ∘C

Figure 4-30: Flexural Strength results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers.

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Flexural Modulus Values for modified PP/PA12 blends reinforced with virgin and recycled carbon fibers 9

4 Flexural Modulus 3% MAPP 7,54 6,82 Flexural Modulus 1% PMPPIC 3 5,81 6,06 5,29 5,17 4,52

Flexural Modulus (GPa) 2 3,78

0 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% 20/80 PP-PA12 + 25% RCFU RCF 400 ∘C VCFU VCF 400 ∘C

Figure 4-31: Flexural Modulus results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers.

Impact Energy Values for modified PP/PA12 blends reinforced with virgin and 140,00 recycled carbon fibers

120,00

100,00

80,00

60,00 117,81 100,57 103,50

40,00 75,59 70,27 73,93 73,70

47,58 20,00 Izod (notched) Impact Energy (J/m)

0,00 RCF Untreated (J/m) RCF 400°C (J/m) VCF Untreated (J/m) VCF 400°C (J/m) 20/80 PP-PA12 + MAPP 3% 20/80 PP-PA12 + PMPPIC 1%

Figure 4-32: Impact Energy (Izod notched) results for untreated and 400oC treated recycled and virgin carbon fiber reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers.

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4.2.4 Melt Flow Index – Material Viscosity Results

The following tables present Melt Flow Index results for all materials studied in the sequence of optimization. Temperature condition and weight used are noted in each table. Values of pure polymers are presented in Table 24. The respective pure blends viscosity values are shown in Table 25. Table 26 presents values for 1%, 2% and 3% PMPPIC PP-PA12 blends. Viscosity comparison between PMPPIC and MAPP is presented in Table 27 and Table 28. Finally, the comparison of carbon fiber reinforced polymer values is presented in Table 29 and Table 30.

MFI values for pure polypropylene were 4.79g/10min and 25.42 g/10min for PA12. The literature lists the polypropylene value for tests performed under the same standard as 12g/10min. However, weight and temperature were not shown in the previous studies and were probably higher than the ones used in the test performed in this work. Therefore, the test was performed again to ensure that all materials were tested under the same parameters. Values for pure blends are shown in Table 25. As expected, MFI values increase as the proportion of PA12 in the total material composition grows, due to the naturally lower viscosity of PA12. The minimum value was 6.87g/10min for 50%/50% PP/PA12. The maximum was 16.23g/10min, registered for 20%/80% PP/PA12 blends. Lower viscosity values (i.e., higher MFI values) are generally associated with less machine power during the extrusion and injection manufacturing processes. Blends with higher PA12 are thus more suitable for that purpose, due to its more amorphous and less molecular crystallized structure compared with PP.

Viscosity tests were performed on 50%/50% PP/PA12 blends with three PMPPIC compositions, as part of the PMPPIC optimization investigation. The results of the are shown in Table 26. Values of the MFI decreased significantly for each 1% increment in PMPPIC surface modifier concentration. Compared to pure blends, the MFI values for 50%/50% PP/PA12 blends decreased from 6.87g/10min without reagent to 3.91g/10min at 1% and 1.04g/10min for 2%. At 3% PMPPIC concentration the viscosity increased so significantly that it was necessary to increase the temperature from 230oC to 270oC to obtain flow results under the same 1.2kg weight parameter. MFI values for 3% PMPPIC were registered as 2.43g/10min at 270oC/1.2kg.

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Table 24: Melt Flow Index values for Total PP 3622 polymers and PA12 Arkema Rilsamid AMNO

Table 25: Melt Flow Index values for PP/PA12 pure blends.

Table 26: Melt Flow Index values for 50%/50% - PP/PA12 polymer blends with PMPPIC surface modifier at 1%, 2% and 3%.

The viscosity comparison between PMPPIC and MAPP reagents with different PP/PA12 compositions are shown in Table 27 and Table 28 below. Compared to pure blends, MAPP presented higher MFI values for all blends, with the exception of the 40%/60% blends due to PP crystallization in the 45-65% range of PA12 proportion in the PP/PA12 blend composition [66]. This overall behavior is mainly explained by the PP dispersion in the PA12 phase due to MAPP polarization on the PP backbone lowering the bulk material energy. It is demonstrated in the SEM analysis results obtained by Aranburu and Eguiazábal and presented in Figure 1-29 of their study [66]. PMPPIC values showed that the material has a higher viscosity with lower MFI values compared to MAPP and pure blends. Nonetheless, PMPPIC still improves, yielding higher MFI values at higher PA12 percentages. The main cause of this drastic reduction is the formation of PMPPIC-PA12 agglomerated phase with the reaction between amide and isocyanate groups leading to a higher energy of this bulk material phase. This higher energy PMPPIC- PA12 phase difficult the phase displacement and phase dispersion during material flow. Additionally, degradation during processing leads to the formation of polyuria, which drastically impacts material flow

101 102 due to its high viscosity. The remaining portion of Polymethylene polyphenyl is less reactive and its molecule branch on the backbone structure of PA12 blocks deformation and movement inside PA12 molecular chains.

Table 27: Melt Flow Index results for different PP/PA12 polymer blends with MAPP surface modifier.

Table 28: Melt Flow Index results for different PP/PA12 polymer blends with PMPPIC surface modifier.

Similar surface modifier behavior comparisons regarding viscosity can be applied when the blends are reinforced with untreated and 400oC treated recycled and virgin carbon fibers. The addition of carbon fibers increased viscosity values in all cases, when compared with the corresponding pure and surface- modified blends. Overall, MAPP presented lower viscosity values (higher MFI) than PMPPIC. The 400oC treatment performed on carbon fibers also increased MFI values (lowered viscosity) in all respective reinforced composite materials, as can be seen in Table 29 and Table 30. Recycled carbon fibers in general presented higher MFI values compared to virgin carbon fibers. An exception was found between VFC and RCF with MAPP surface modifiers.

The obtained values show an intrinsic relationship between adhesion of the fibers and their dispersion in the material during processing. As explained earlier, the recycled and virgin carbon fiber thermal treated surface had its chemical and mechanical adhesion compromised by lower oxygen levels

102 103 and micro erosion voids in the surface. Consequently, this lack of adhesion allowed the fibers to spread more easily in the matrix, causing a decrease in viscosity.

Table 29: Melt Flow Index values for untreated and 400oC treated recycled and virgin carbon fibers with 20%/80% - PP/PA12 blends with PMPPIC surface modifier.

Table 30: Melt Flow Index values for untreated and 400oC treated recycled and virgin carbon fibers with 20%/80% - PP/PA12 blends with MAPP surface modifier.

4.3 Surface Thermodynamics Results

4.3.1 Contact Angle/ Wetting Studies Results

The contact angles of pure materials were measured by the process described in chapter 3. Polypropylene was the first material measured, and its results were very close to those in the literature, as shown in appendix 7. Contact angles for all other materials are shown in Table 31. Initial wettability data was the basis for evaluation of other phenomena, such as evaporation and liquid-surface stabilization, in which contact angle variation over time was measured. These values are shown in Figure 4-33 below, with the curves showing the trend values in degree/min. The relationship is very close to linear, except for

103 104 polypropylene and polyamide 12 in diiodomethane at 10 min, where a slope change was detected. This is mainly because the high volatility of diiodomethane causes a non-linear relationship between spread area and surface tension. The conclusion of this analysis is that the linear trend is mainly due to evaporation, with some deviations caused by the force equilibrium changes between the smaller drop area and the solid wetted surface.

Contact angles for all polymer blends/composites were also measured and are available in the Table 32. Contact angle values for MAPP are close to polypropylene, which seems reasonable since MAPP is a polypropylene-based grafted co-polymer. Values for polyamide 12 were 91.52o for water and 41.22o for diiodomethane. These contact angles were higher compared with literature values cited in appendix 6, where water had its maximum value at 77o. The difference might be associated with material structure and low amide content. Another hypothesis is that surface amide groups were reoriented towards the inner bulk of the material, lowering its surface polarity [78,79]. Contact angles for untreated virgin carbon fibers were measured at 80.67 degrees for water and 29.74 degrees for diiodomethane. The thermal treatment changed these values drastically: to 48.05 and 9.85 degrees respectively. The hydrophilicity improvement result is compatible with the literature results shown in Table 8 for 385oC treated fibers. However, the behavior in diiodomethane showed lower values relative to the untreated fibers studied and to the literature values.

Table 31: Contact angles results for pure materials investigated.

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Figure 4-33: Contact angle vs time for several solid-liquid systems.

The contact angles measured for all the blends and composites in this study are available in Table 32. All standard deviations presented values of less than 2o variation, showing a good reproducibility and are in accordance with the ASTM D7334 and D7490 standards. For pure blends, the contact angle decreased for both liquids when higher fractions of PA12 were present. With diiodomethane, values decreased from 54.45o to 40.98o and with water from 105.7o to 98.03o. The decrease in values for water indicates higher material polarity due to amide groups [78,79], whereas the decrease in values for diiodomethane is probably from surface roughness morphology change by higher compositions of PA12 with smoother surface properties under the same processing conditions. The addition of MAPP improved the average contact angle values for diiodomethane and kept the trend curve as PA12 concentration increased, with the values ranging from 55.59 degrees to 49.74 degrees. The same trend is not observed for water, which showed higher contact angles at 60%/40% and 70%/30% fractions. This can be explained by the change of crystallization rate of polypropylene at high PA12 fractions, as was previously shown in

105 106 the literature review. Another hypothesis to consider is the higher dispersion level between reactivated polypropylene in the polyamide 12 matrix, making available an excess of amides which affect the surface polarity. In general, the addition of PMPPIC improves the hydrophobicity for water, while the contact angles for diiodomethane increase with higher PA12 fractions. Reinforced carbon fiber composites with the MAPP surface modifier in general presented lower contact angles with diiodomethane and water than the composites with PMPPIC. The thermal treatment also raised the contact angle for all samples in diiodomethane.

4.3.2 Surface Free Energy Results

Surface energies with their polar and dispersive values are presented in Table 33 below. All values showed good agreement with the literature. As expected, polypropylene and MAPP showed very close values for surface energy and dispersive contributions. The polar contribution for MAPP was nearly double relative to polypropylene, this can be attributed to the Maleic Anhydride concentrations. PA12 showed a higher surface energy value, with most of its contribution from dispersive energy. Isothermal treatment resulted in a significant improvement in surface energy for virgin carbon fibers, from 45.66mN/m for untreated fibers to 61.58mN/m for fibers treated at 400oC. This change is mostly attributed to the increase in polar surface energy contribution. The dispersion energy values did not change with the treatment. This demonstrates that the sizing decrease in surface energy closer to the levels of the polymer matrix improved adhesion, as was shown in the discussion of the mechanical properties results.

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Table 33: Total, dispersive and polar surface energies for polymers and virgin carbon fibers studied.

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Total Surface Energy for PP/PA12 blends with MAPP and PMPPIC Surface modifiers

39,98 39,15 39,36 40

35,39 34,28 34,16 34,49 35

32,19 33,54 33,86 32,63 30 31,15 Surface Energy mN/m)

25 Total Surface Energy (Pure Blends) Total Surface Energy (MAPP 3%) Total Surface Energy (PMPPIC 1%) 20 PP/PA12 50-50 PP/PA12 40-60 PP/PA12 30-70 PP/PA12 20-80

Figure 4-34: Surface energy values for PP/PA12, pure blends and with MAPP and PMPPIC surface modifiers.

The surface energy values for all polymer blends are shown in Figure 4-34 above. For 50%/50% pure blends the values fall between the individual surface energies values of PP and PA12. However, when the PA12 composition increases the surface energies reach the same level as those of pure PA12. The biphasic rearrangement due to the presence of polypropylene might have an impact on the amide orientation of the PA12 located near the surface. An interesting behavior was observed in blends with MAPP and PMPPIC, in that the surface energies follow a trend that increases for MAPP and decreases for PMPPIC. For both MAPP and PMPPIC blends, the level of surface energy for all compositions of 60% of PA12 or higher have decreased compared with their respective pure blends. For MAPP-modified blends, values start from 31.15 mN/m and increase gradually to 34.49 mN/m. The known dispersion behavior of the functionalized polypropylene in the polyamide matrix, together with the lower contribution of the less polar PP phase, are the main contributors to the surface energy increase in PP-

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MAPP-PA12, with its trend towards PA12 pure values of 39.02mN/m. Further investigation is necessary to increase understanding of the polar and dispersive contributions in PP-MAPP-PA12 blends.

For PP-PMPPIC-PA12 blends, surface energy values were the highest for 50%-50% and 60%- 40% blends, recorded at 34.28 mN/m and 35.39 mN/m respectively, and showed a decreasing trend thereafter, reaching values of 32.63 mN/m. This was the lowest value found among all 80%-20% material blends studied. The main hypothesis is that PA12-PMPPIC has a non-miscible structure with low molecular mobility, with the consequence that its polar groups are incapable of reaching the surface to interact with the liquid affecting the surface free energy. Also significant is the low polar behavior of the majority methylene composition in the backbone chain in both polyamide and PMPPIC affecting overall polarizability.

The polar contributions in pure blends are mainly associated with amide contributions from the PA12 phase, as shown in Figure 4-35. Overall surface energies decreased with the addition of PMPPIC and MAPP surface modifiers. However, the polar contribution increased, and it was not possible to predict a trend. The values and details are shown in Figure 4-36 and Figure 4-37. The surface energy values in MAPP blends increased due to lower PP-MAPP and higher PA12 surface availability. Higher dispersive increments appeared due to higher contribution from PA12 methylene groups when PA12 composition is raised. With PMPPIC the situation is reversed. The agglomerated structure affects the molecular arrangement, altering the dispersive behavior due to higher intermolecular distances on PA12 functionalized material. This increases the polar contribution due to higher reactivity and significantly lowers the dispersive contribution on the PA12 phase. The total surface energy decreases as a result.

For virgin and recycled reinforced carbon fibers, results showed some impact on surface energy depending on the type of fibers used. However, there was no noticeable impact of surface modifiers in this case. These results are shown in Figure 4-38 and Figure 4-39. Lower surface energy values were observed for recycled carbon fibers. This result reinforces the lower mechanical properties values for these materials, shown in Section 4.2 above, and indicates a lower adhesion interface between fibers and matrix. Thermal treatment also impacted the surface energies for all materials studied. Lower surface energy values were observed for treated fibers, again indicating a lack of adhesion between fibers and matrix. This is likewise in accordance with the mechanical properties results previously shown.

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Surface Free Energy Analysis for PP/PA12 Pure Blends 45 0,01 0,27 0,31 40 35 0,07 30 25 Surface Energy Polar Pure PP/PA12 Blend 20 39,97 38,85 39 Surface Energy Dispersive Pure PP/PA12 15 32,13 Blend

Surface Energy (mN/m) 10 5 0 PP/PA12 50-50 PP/PA12 40-60 PP/PA12 30-70 PP/PA12 20-80

Figure 4-35: Polar and dispersive surface energies for pure PP/PA12 blends.

Surface Free Energy Analysis for PP/PA12 with MAPP 3% Surface Modifier

35 0,28 34 0,21 1,4 33

31 Surface Energy Polar 1,08 33,88 Surface Energy Dispersive 30 33,34 33,09

Surface Energy (mN/m) 29 30,06 28

27 PP/PA12 50-50 + MAPP PP/PA12 40-60 + MAPP PP/PA12 30-70 + MAPP PP/PA12 20-80 + MAPP 3% 3% 3% 3%

Figure 4-36: Polar and dispersive surface energies for PP/PA12 blends with MAPP surface modifier.

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Surface Free Energy Analysis for PP/PA12 with PMPPIC 1% Surface Modifier 36

35 0,72 0,11 34

33 1,32 Surface Energy Polar Surface Energy Dispersive 34,67 0,76 32 34,17 Surface Energy (mN/m)

32,54 31 31,87

30 PP/PA12 50-50 + PP/PA12 40-60 +PMPPIC PP/PA12 30-70 + PP/PA12 20-80 + PMPPIC 1% 1% PMPPIC 1% PMPPIC 1%

Figure 4-37: Polar and dispersive surface energies for PP/PA12 blends with PMPPIC surface modifier.

Surface Free Energy Analysis for PP/PA12 with MAPP 3% with RCF and VCF Reinforcement 39

37 1,22 1,56 36 0,78 35 Surface Energy Polar 0,76 34 36,57 Surface Energy Dispersive Surface Energy (mN/m) 35,85 33 35,44 33,81 32

31 PP/PA12 20-80 + MAPP PP/PA12 20-80 + MAPP PP/PA12 20-80 + MAPP PP/PA12 20-80 + MAPP 3% + RCFU 3% + RCF400 3% + VCFU 3% + VCF400

Figure 4-38: Surface energy results for virgin and recycled carbon fiber reinforced polymers with MAPP surface modifier.

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Surface Free Energy Analysis for PP/PA12 with PMPPIC 1% matrix with RCF and VCF Reinforced

38 1,38

37 0,08 0,8 36

35 Surface Energy Polar 0,64 34 37,17 Surface Energy Dispersive 36,37 35,94 33 Surface Energy (mN/m) 33,85 32

31 PP/PA12 20-80 + PMPPIC PP/PA12 20-80 + PMPPIC PP/PA12 20-80 + PMPPIC PP/PA12 20-80 + PMPPIC 1% + RCFU 1% + RCF400 1% + VCFU 1% + VCF400

Figure 4-39: Surface energy results for virgin and recycled carbon fiber reinforced polymers with PMPPIC surface modifier.

4.4 Conclusions

The main findings and conclusions of this research are presented here related with each point mentioned in research objective section.

I. Characterize recycled and virgin carbon fibers and understand the impact of thermal treatment on carbon fiber surface properties;

• In the characterization studies performed on virgin and recycled carbon fibers, RCF presented good thermal stability in air atmosphere, with a maximum degradation temperature that was 50oC lower (500oC to 550oC range) than that of virgin carbon fibers (550oC to 600oC). Thermal treatments in air flow up to 400oC can be performed on recycled carbon fibers without any significant weight loss.

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• Some degree of stabilization occurred at temperatures of 303oC for VCF in air and in the range 303oC to 315oC for VCF in nitrogen. For RCF, the same process was detected only in nitrogen atmosphere in the range 400oC to 415oC. As evident from the TGA analysis, the thermal treatment process stimulated, by stabilization phenomenon, the formation of more aromatic carbon rings. The latter effect led to a more graphitic bulk structure and a higher modulus carbon fiber.

• The FTIR study showed that acidic groups are formed on the recycled carbon fiber surface. Thermal treatment in air increased the appearance of OH groups. These acidic groups decreased the work of adhesion of the fibers however, it improves the chemical adhesion with an polar matrix, based on the correlation with final mechanical properties of the composite.

• The XPS analysis showed that levels of carbon C1s found in RCF were lower compared to those in VCF. Air and nitrogen thermal treatments improved the carbon content for RCF. The XPS study revealed that oxygen levels decreased, as demonstrated by lower O/C ratios. A reduction in absolute oxygen % content, sodium and chlorine suggests that some surface treatment occurred and some sizing was removed in the RCF based on the XPS analysis.

II. Present a novel optimization of PP/PA12 blend with PMPPIC surface modifier;

• In the optimization of the isocyanate surface modifier in a 50%/50% - PP/PA12 polymer blend, the surface modifier showed the best overall performance at 1% concentration. Higher isocyanate concentrations (2% and 3%) affected viscosity drastically and improved toughness. Isocyanate groups react with amide groups in Polyamide 12, building a branched agglomerated structure with high internal bulk material energy that increases the modulus of the PA12-PMPPIC and consequently raises the interfacial stress between PP/PA12 phases, creating a more accentuated phase separation between the two polymers.

• In the comparison of 4 (four) different compositions of non-reinforced PP/PA12 blends with MAPP and PMPPIC surface modifiers, PMPPIC presented better impact energy absorption. PMPPIC performed 17% better than same pure PP/PA12 blend and 98% better than MAPP, without a compromise in strength compared to pure blends. Better tensile modulus and flexural strength and modulus for 20%-80% PP/PA12 blends were also observed.

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III. Use virgin and recycled carbon fiber as polymer reinforcement to evaluate the performance of the novel PMPPIC on mechanical properties;

• The results of impact energy tests for reinforced carbon fiber composites show that PMPPIC performed 43% and 55% better than MAPP for untreated and 400oC treated recycled carbon fibers and 55.9% and 40.4% better for untreated and 400oC treated virgin carbon fibers. This can be associated not only with carbon fiber presence but also with an improvement obtained in the matrix energy absorption capability. Overall impact energy improvement in RCF and VCF treated with PMPPIC ranged from 42% to 126% relative to its respective pure material.

IV. Investigate contact angles and correlate virgin and recycled carbon fiber surface free energy with interface adhesion mechanisms in PP/PA12 blends with surface modifiers;

• Thermal treatment decreases the hydrophobicity of VCF, as evident from the wettability data, where an increase in VCF surface free energy is observed. MAPP surface energy values are slightly lower than polypropylene and its polar contribution is twice than Polypropylene.

• Surface energy values of the blends with surface modifiers fall between individual values of the PP and PA12 materials. However, they are lower than the respective pure PP/PA12 blends. At higher PA12 fractions, surface energies increase for PP/PA12 with MAPP and decrease for PP/PA12 with PMPPIC, indicating that PMPPIC has low surface energy. These results are mainly related with the polarity of the blend formed.

• The addition of carbon fiber increases the surface free energy of all final composite materials compared with their respective hybrid matrices. Lower surface energies were observed for both RCF composites as compared to VCF composites. Also, all untreated fibers composites presented lower surface free energy values as compared to thermal treated fibers. These values are in good agreement with the obtained mechanical properties results, indicating the level of adhesion of the carbon fibers in the matrix.

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• All fundamental material properties of the new recycled carbon fiber reinforced polymer presented here are the first step inside of the material selection for automotive design process. With the technical information presented here, potential composite benchmarking research done for vehicle parts materials already in use can reveal real opportunities for effective recycled carbon fibers reinforced polymers application.

4.5 Key contributions and recommendations for future research

4.5.1 Key Contributions

The key scientific contributions of this work are summarized in the list below:

• This work presented a lightweight alternative material for use in the automotive industry that potentially meets the industry’s high impact energy requirements for safety without compromising strength.

• It presents novel engineering data for PP-MAPP-PA12 materials reinforced with virgin and recycled carbon fibers.;

• It demonstrates and explains the scientific fundamentals behind the performance of an innovative PMPPIC surface modifier and presents mechanical properties data as per accredited standards. Mechanical properties data for a PP-PMPPIC-PA12 polymer blend with recycled and virgin carbon fibers are shown for the first time.

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• This work presents characterization data of recycled and virgin carbon fibers to evaluate their potential innovative application as reinforcement in a PP/PA12 hybrid matrix, with their final composite mechanical properties and adhesion performance results shown;

• Significant contribution to the scientific database for contact angle and surface energy values for virgin carbon fibers, Polypropylene and Polyamide 12;

• It makes a significant contribution to the scientific database of contact angle and surface energy values for the following materials:

o Virgin carbon fibers; o Polypropylene; o Polyamide 12; o Maleic Anhydride Polypropylene; o Pure PP/PA12 blends; o PP/PA12 blends with MAPP and PMPPIC surface modifiers; o PP/PA12 recycled and virgin carbon fiber reinforced polymers with PMPPIC and MAPP.

4.5.2 Recommendations for Future Research

Several recommendations for future research are presented in the list below:

• Optimization of PMPPIC as a PP/PA12 blend should be performed at concentrations between 0.5% and 1.5%, on PP/PA12 polymer blends with PA12 composition of more than 50%.

• It is highly recommended to perform PMPPIC optimization not only with Polyamide 12 but also with other Polyamide polymers, such as PA10, PA11, PA6 and PA66, in combination with the researcher’s blends.

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• It is also strongly recommended to use MAPP and PMPPIC together as surface modifies in PP/PA12 blends, based on the known facts that PMPPIC interacts better with the PA12 phase and that MAPP functionalize the polypropylene phase.

• While this work used graft co-polymerization, PMPPIC processing with carbon fibers and a blend matrix should be investigated. One suggestion is to perform the PMPPIC coating process on carbon fibers and then compound the fibers in the polymer blend matrix.

• Contact angle and surface energy values for recycled carbon fibers should be investigated and compared with the virgin carbon fiber values in this study. This will further investigate the composite adhesion comparison results obtained for recycled and virgin carbon fibers.

• The contact angles and surface free energy of PMPPIC should be measured and compared to the surface energies and wetting studies data obtained in this study, with a view to further understanding PMPPIC’s adhesion properties.

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Appendices

Appendix 1: Virgin Carbon Fiber Data Sheet.

Appendix 2: Recycled Fiber Material Data Sheet.

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Appendix 3: Polypropylene Material Data Sheet.

Appendix 4: MAPP Material Data Sheet.

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Appendix 5: Polyamide 12 Material Data Sheet

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Appendix 6: Surface energy literature data for Polyamide 12.

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Appendix 7: Surface energy literature data for Polypropylene.

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