Gradual Impregnation during the
Production of Thermoplastic Composites
Veronika Anna Bühler
Vollständiger Abdruck der von der Fakultät für Maschinenwesen der Technischen Univer- sität München zur Erlangung des akademischen Grades eines
Doktor-Ingenieurs genehmigten Dissertation.
Vorsitzender: Prof. Dr.-Ing. Veit Senner Prüfer der Dissertation: Prof. Dr.-Ing. Klaus Drechsler
Prof. Paul Compston, Ph.D.
Die Dissertation wurde am 12.04.2017 bei der Technischen Universität München eingere- icht und durch die Fakultät für Maschinenwesen am 21.06.2017 angenommen.
Lehrstuhl für Carbon Composites Boltzmannstraße 15
D-85748 Garching bei München
Tel.:+ 49 (0) 89 / 289 - 15092 Fax: + 49 (0) 89 / 289 - 15097 Email: info@lcc.mw.tum.de Web: www.lcc.mw.tum.de
Steiner Oma & Opa
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Ich erkläre hiermit ehrenwörtlich, dass ich die vorliegende Arbeit selbstständig und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe; die aus fremden Quellen (einschließlich elektronischer Quellen) direkt oder indirekt über- nommenen Gedanken sind ausnahmslos als solche kenntlich gemacht.
Die Arbeit wurde in gleicher oder ähnlicher Form noch keiner anderen Prüfungs- behörde vorgelegt.
...
Ort, Datum
...
Unterschrift
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I would like to express my gratitude to my supervisor Prof. Klaus Drechsler for giving me the opportunity to write a PhD thesis at the Chair of Carbon Compos- ites (LCC). Being a part of the institute’s Material Behavior and Testing group I was guided by Dr. Hannes Körber. I want to thank him and the deputy head of the institute, Dr. mont. Elisabeth Ladstätter, for their constant as well as indis- pensable support and for giving me sufficient confidence to work independently.
I express my deepest thanks to my co-supervisor Prof. Paul Compston for the fruit- ful discussions we had, his valuable technical input to my thesis and for making this long journey from Australia to take part at my examination. He also enabled a two-month research period at the Australian National University in Canberra where I got to know Sherman Wang, an expert in nano-indentation, who greatly supported me and gave valuable input to my thesis. I also want to express my gratitude to Dr. Christopher Stokes-Griffin for valuable discussions, great support and especially for the fantastic time when we shared an office. Thank you!
My PhD project was made possible by the financial support from SGL Carbon GmbH. I am very thankful for their generosity and highly appreciate the close col- laboration, the many valuable discussions and the great support by Dr. Andreas Erber, Patrik-Vincent Brudzinski, Dr. Steffen Janetzko, Dr. Christian Stang, Dr.
Oswin Öttinger and Veronika Hirschinger.
In addition, my great time at the LCC owes to my colleagues. I specially want to thank Cigdem Filker for her perpetual support in administrative matters and for being not only a wonderful office colleague but also a valued friend. I also highly appreciate the technical discussions, support, friendship, chats and coffees with Jan Krollmann, Luciano Avila Gray, Rhena Helmus, Philipp Hörmann, Philipp Bruck- bauer, Alex Schwingenschlögel, Stefan Ehard, Andreas Kollmannsberger, Thorsten Hans, Philipp Picard, Marina Plöckl, Peter Kuhn, Ludwig Eberl, Philipp Fahr, Christoph Ebel and Swen Zaremba. I also would like to thank the workshop for enabling almost impossible things.
It was also a great pleasure to work with excellent students during my time at the LCC. Thank you very much Christian Heckel, Heiko Baumann, Patrick Consul, Miriam Ernst, Adrián García López and Stefan Ender.
Moreover, I’d like to thank my family and friends for their indispensable encour- agement and great support not only during my PhD but all my life. Thank you so much, Mama, Papa, Steffie and Sebastian for everything! Thank you, Joe, for your loving support, incredible encouragement and unbowed belief in me!
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Commonly, pre-formed or pre-impregnated intermediates are used to produce car- bon fiber reinforced thermoplastics (CFRTP). However, impregnation of the fiber with the polymer is a time-consuming process, due to the high melt viscosity of ther- moplastics leading to high manufacturing costs. Existing approaches to overcome the challenges during impregnation deal with advancements in the impregnation technology, improvements in process control or the use of low viscous prepolymers.
In this work, partially impregnated tapes are developed which can be manufactured with increased production rates to reduce the costs of intermediates. The partially impregnated tapes are intended to completely impregnate throughout subsequent heating and consolidation processes, required to produce a final component made from CFRTP.
To begin with, the fiber-matrix compatibility between various polyamide types and carbon fibers with different sizings was investigated to produce high-performance composites from powder-coated tows. Based on results from macro- and micro- mechanical tests, the most suitable material combinations were selected for subse- quent investigations.
To enable the comparability of production methods that are characterized by signif- icantly different cooling rates, the crystallization kinetics of the selected polyamides was studied. This study on neat polymers, as well as in presence of differently sized carbon fibers, was conducted by using the differential scanning calorimetry. Using a new method to determine the crystallized fraction in fiber reinforced polymers, mechanical properties were correlated to different cooling rates used for the pro- duction of test panels.
Based on literature review, the transverse impregnation was modeled using Darcy’s law. To verify the derived model experimentally, an impregnation study was con- ducted to produce test panels from powder-coated tows by varying the three main process parameters that drive impregnation: time, temperature and pressure. The experimental design followed the design of experiments to consider extreme process settings, suitable for model verification. By post-processing micrographs from the test panels, an experimental procedure was developed to determine the degree of impregnation. Comparing the experimental results obtained from the impregnation study with the values predicted by the model, a good correlation was found for the selected material combinations.
To enable gradual impregnation throughout component production, process-related effects on the polymer influencing the impregnation behavior were investigated.
Temperature profiles with different dwell times were derived from a typical CFRTP production process. The selected polyamides were subjected to these profiles and
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to the temperature profiles yielded significant increases in the melt viscosity due to thermo-oxidative degradation. A processing window of 5 minutes was identified for the various process steps where gradual impregnation is enabled. Beyond this processing window, the observed increase in viscosity can prevent complete impreg- nation during the actual process step or in the following steps.
By adding a suitable antioxidant, the extent of the degradation reactions was re- duced and substantial increases in viscosity were limited. Additional modification of the selected polyamides with a lubricant further decreased the viscosity leading to reduced impregnation times.
Eventually, partially impregnated tapes were produced from powder-coated tows in a double-belt press with increased production rates. With a completely im- pregnated tape as a reference, the partially impregnated tapes were processed by press forming and thermoforming with different dwell times to simulate a typical production of a CFRTP component. The impregnation of partially impregnated intermediates was found to be completed after the repeated heating processes dur- ing CFRTP production. Studying the flexural properties of test panels produced from the partially impregnated tapes, comparable values to completely impreg- nated tapes were achieved upon press forming for 5 to 10 minutes. The analysis of the manufacturing costs yielded cost savings between 40 to 60 % as the partially impregnated tapes can be produced with double or quadruple production rates, compared to completely impregnated tapes.
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Zur Herstellung von carbonfaserverstärkten Thermoplasten (CFTP) werden übli- cherweise thermoplastische Halbzeuge verwendet, wobei die Fasern meist bereits vollständig mit der thermoplastischen Kunststoffmatrix imprägniert sind. Die Im- prägnierung der Fasern mit der Matrix ist aufgrund der hohen Schmelzviskosität von Thermoplasten jedoch ein zeit- und somit kostenintensiver Prozess. Bisherige Ansätze zur Bewältigung dieser Herausforderung bestehen in der Weiterentwick- lung der Imprägnierungstechnologien, der Verbesserung der Prozessführung oder der Verwendung von niederviskosen Vorpolymeren.
In der vorliegenden Arbeit wird die Entwicklung von teilimprägnierten Tapes vor- gestellt. Derartige Tapes können mit höheren Prozessgeschwindigkeiten produziert werden, um so die Herstellungskosten zu senken. Diese teilimprägnierten Tapes sollen im Laufe der anschließenden, zur Bauteilherstellung nötigen Aufheiz- sowie Konsolidierungsprozesse vollständig imprägniert werden.
Zunächst wurde die Faser-Matrix-Kompatibilität zwischen verschiedenen Polyamid- typen und Carbonfasern mit unterschiedlichen Schlichten untersucht, um hochleis- tungsfähige Verbundwerkstoffe aus pulverbeschichteten Fasern herzustellen. Basie- rend auf den Ergebnissen aus makro- sowie mikromechanischen Materialprüfungen wurden die Materialkombinationen mit der höchsten Kompatibilität von Faser, Schlichte und Matrix abgeleitet.
Um Produktionsprozesse mit signifikant unterschiedlichen Kühlraten miteinander vergleichen zu können, wurde die Kristallisationskinetik der ausgewählten Polyami- de bestimmt. Die Untersuchung mittels der Differenzkalorimetrie erfolgte sowohl an reinen Polymeren als auch an den hergestellten Verbundwerkstoffen. Durch die Verwendung einer neuen Methode zur Bestimmung des kristallinen Anteils in fa- serverstärkten Kunststoffen konnten die mechanischen Kennwerte mit den Kris- tallisationsgraden, die sich während der Herstellung von Prüfplatten aufgrund der verschiedenen Kühlraten einstellen, korreliert werden.
Basierend auf einer Literaturübersicht wurde die Imprägnierung in Dickenrichtung modellhaft mit dem Gesetz von Darcy beschrieben und eine Imprägnierstudie zur experimentellen Verifikation durchgeführt. Dazu wurden in einer statischen Pres- se Prüfplatten aus pulverbeschichteten Fasern mit verschiedenen Kombinationen der Prozessparameter Zeit, Druck und Temperatur, die die Imprägnierung maß- geblich beeinflussen, hergestellt. Zur Versuchsplanung wurde Design of Experi- ments verwendet, um auch extreme Prozessparametersätze zu berücksichtigen. Die Entwicklung einer automatisierten Nachbearbeitung von Schliffbildern der in der Imprägnierstudie hergestellten Prüfplatten erlaubte die Bestimmung des sich ein- stellenden Imprägnierungsgrades. Der Abgleich der Ergebnisse für den Imprägnie-
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Um eine graduelle Imprägnierung während der Bauteilherstellung zu ermöglichen, wurden prozessinduzierte Effekte auf die Matrixeigenschaften untersucht, welche das Imprägnierungsverhalten beeinflussen. Dazu wurden zunächst Temperaturpro- file aus typischen Produktionsprozessen für thermoplastische Verbundwerkstoffe abgeleitet. Die Auswirkungen der Temperaturprofile auf die Polymereigenschaf- ten wurden mithilfe der Differenzkalorimetrie, der thermogravimetrischen Analyse, der Rheometrie sowie der Gelpermeationschromatographie untersucht. Die thermi- sche Belastung aufgrund der angelegten Temperaturprofile verursachte einen star- ken Anstieg der Viskosität der untersuchten Polyamidtypen. Für die verschiedenen Prozessschritte wurde ein Prozessfenster von 5 Minuten identifiziert, in welchem die graduelle Imprägnierung ungehindert stattfinden kann. Außerhalb dieses Zeit- fensters kommt es zum beobachteten Viskositätsanstieg, welcher eine vollständige Imprägnierung im vorliegenden oder nachfolgenden Prozessschritt erschweren kann.
Durch die Beigabe eines geeigneten Antioxidanten wurde das Ausmaß der Degrada- tionsreaktionen und der Viskositätsanstieg begrenzt. Eine zusätzliche Modifikation der ausgewählten Polyamidtypen mit einem Fließmittel führte zu einer weiteren Senkung der Viskosität und ermöglicht somit eine erhebliche Verkürzung der Im- prägnierungszeit.
Schlussendlich wurden teilimprägnierte Tapes in einer Doppelbandpresse mit er- höhten Produktionsgeschwindigkeiten hergestellt. Diese sowie vollkommen imprä- gnierte Tapes wurden in Press- und Thermoformverfahren weiterverarbeitet, um eine typische Bauteilherstellung zu simulieren. Nach den wiederholten Aufheiz- und Konsolidierungsprozessen zur Bauteilherstellung wurde eine vollständige Tränkung der zunächst teilimprägnierten Tapes erreicht. Verglichen mit vollständig imprä- gnierten Tapes wurden nach dem Pressen für 5 bis 10 Minuten vergleichbare Bie- geeigenschaften für die teilimprägnierten Tapes erzielt. Eine anschließende Kosten- analyse ergab ein potentielles Einsparpotential von 40 bis 60 % für die Herstellung von teilimprägnierten Tapes, da diese mit der doppelten beziehungsweise vierfachen Prozessgeschwindigkeit produziert werden können.
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Contents xviii
List of Figures xxv
List of Tables xxxi
1 Introduction 1
1.1 Motivation . . . 1
1.2 Thermoplastic composites . . . 2
1.2.1 Intermediate materials . . . 3
1.2.2 Production methods . . . 6
1.3 State-of-the-art . . . 8
1.4 Objectives and outline of the thesis . . . 9
2 Fiber-matrix compatibility 13 2.1 Investigated materials . . . 13
2.1.1 Carbon fibers . . . 13
2.1.2 Polyamides . . . 14
2.2 Experimental methods . . . 16
2.2.1 Four-point bend test . . . 16
2.2.2 Double-cantilever beam test . . . 17
2.2.3 Statistics . . . 18
2.2.4 Nano-indentation . . . 18
2.2.5 Scanning electron microscopy . . . 20
2.3 Sample preparation . . . 20
2.3.1 Intermediate production . . . 20
2.3.2 Test panel production . . . 21
2.3.3 Test specimen preparation . . . 22
2.3.4 Micrographs . . . 23
2.4 Results . . . 23
2.4.1 Influence of the sizing . . . 23
2.4.2 Influence of matrix ductility . . . 28
2.4.3 Development of an interphase . . . 31
2.5 Selection of compatible material combinations . . . 33
3 Crystallization of polyamides 35 3.1 Theory . . . 35
3.1.1 Crystallization and nucleation . . . 35
3.1.2 Crystallization kinetics . . . 37
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3.2.2 Crystallinity ratio . . . 40
3.2.3 Four-point bend test . . . 41
3.2.4 Visualization of crystals . . . 41
3.3 Results . . . 42
3.3.1 Preliminary tests . . . 42
3.3.2 Neat polymers . . . 44
3.3.3 Influence of carbon fibers on crystallization . . . 51
3.3.4 Relation between mechanical properties and crystallinity ratio 53 3.4 Conclusion and implications . . . 55
4 Impregnation model 57 4.1 Transverse resin flow . . . 57
4.2 Processing phenomena of individual constituents . . . 59
4.2.1 Fiber bed properties . . . 59
4.2.2 Matrix . . . 62
4.3 1D through thickness model . . . 64
4.3.1 Assumptions . . . 64
4.3.2 Model derivation . . . 64
4.4 Experimental work . . . 66
4.4.1 Rheology . . . 66
4.4.2 Experimental determination of degree of impregnation . . . 68
4.4.3 Design of experiments . . . 69
4.4.4 Interlaminar shear test . . . 71
4.5 Results . . . 71
4.5.1 Viscosity data . . . 72
4.5.2 Influence of processing on impregnation progress . . . 74
4.5.3 Model calibration . . . 78
4.5.4 Influence of degree of impregnation on interlaminar shear strength . . . 80
4.6 Conclusion and implications . . . 80
5 Degradation of polyamides 83 5.1 Literature review . . . 83
5.1.1 Thermal degradation . . . 83
5.1.2 Thermo-oxidative degradation . . . 83
5.1.3 Post-condensation . . . 85
5.1.4 Influence of degradation on processing . . . 86
5.2 Temperature profiles . . . 87
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5.3.2 Differential scanning calorimetry . . . 88
5.3.3 Thermogravimetric analysis . . . 89
5.3.4 Rheometry . . . 90
5.3.5 Gel permeation chromatography . . . 90
5.4 Results . . . 92
5.4.1 Influence on melting temperature . . . 92
5.4.2 Effect on mass loss . . . 93
5.4.3 Impact on complex viscosity . . . 94
5.4.4 Effect on molecular composition . . . 96
5.4.5 Processing window . . . 97
5.4.6 Conclusion and implications . . . 98
6 Thermal stabilization and flow promotion of polyamides 101 6.1 Thermal stabilization . . . 101
6.1.1 Chain breaking antioxidants . . . 101
6.1.2 Radical scavengers . . . 103
6.1.3 Preventive antioxidants . . . 103
6.1.4 Investigated antioxidants . . . 104
6.2 Flow promotion . . . 106
6.2.1 Internal and external lubricants . . . 106
6.2.2 Investigated lubricants . . . 108
6.3 Experimental methods . . . 109
6.3.1 Polymer samples . . . 109
6.3.2 Composite samples . . . 110
6.4 Results . . . 112
6.4.1 Effectiveness of additives . . . 112
6.4.2 Effects of combined use of additives . . . 116
6.4.3 Influence of matrix modification on impregnation . . . 121
6.5 Conclusion and implications . . . 122
7 Gradual impregnation during production 125 7.1 Manufacture of differently impregnated tapes . . . 125
7.2 Manufacture of thermoplastic composites . . . 128
7.2.1 Prediction of dwell times . . . 128
7.2.2 Test panel production . . . 129
7.2.3 Four-point bend test . . . 129
7.3 Influence of initial degree of impregnation on mechanical properties 130 7.3.1 Final degree of impregnation after production . . . 130
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7.4.1 Procedure . . . 135 7.4.2 Data collection . . . 137 7.4.3 Monetary effect of partially impregnated tapes . . . 139 7.5 Correlation of mechanical properties and manufacturing costs . . . 140 7.6 Conclusion and implications . . . 141
8 Summary and outlook 143
8.1 Summary and conclusion . . . 143 8.2 Future work . . . 148
Bibliography 151
A Appendix 169
A.1 to Section 4.4.3 . . . 169 A.2 to Section 6.4 . . . 171 A.3 to Section 7.3.1 . . . 171
B Publications 173
C Supervised student theses 175
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Symbol Unit Meaning
a [mm] Delamination length
a0 [mm] Initial delamination length
A [nm2] Project area of Berkovich indenter
A1 [-] Constant for modified compliance calibration
As [Pa] Empirical constant
aT [-] Temperature shift factor
C [mm/N] Compliance or
C [°C] Cooling rate
C1 [-] Fiber packing constant
CR [-] Crystallinity ratio
DOI [%] Degree of impregnation
DOIf [%] Final degree of impregnation DOIi [%] Initial degree of impregnation
E [GPa] Elastic modulus
Ea [kJ/mol] Activation energy
Ef1 [GPa] Longitudinal flexural modulus Ef2 [GPa] Transverse flexural modulus
G [MPa] Storage modulus
G [MPa] Loss modulus
GI [J/m2] Mode I interlaminar fracture toughness
GIc [J/m2] Opening mode I interlaminar fracture toughness
H [GPa] Hardness
ΔHc,∞ [J/g] Enthalpy of crystallization at the end of the crys- tallization process
ΔHc [J/g] Enthalpy of crystallization ΔHcc [J/g] Enthalpy of cold crystallization
ΔHf [J/g] Enthalpy of fusion
ΔHf0 [J/g] Enthalpy of fusion of a 100% crystalline polymer hp [nm] Plastic depth of penetration
ILS [MPa] Interlaminar shear strength
k [10−2min−n] Crystallization rate constant according to Avrami
K [m2] Permeability
K(T) [-] Cooling function
Kzz [m2] Permeability in z-direction
kzz [-] Kozeny constant
kzz [-] Kozeny constant, modified by Gutowski
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l [mm] Specimen length
m [-] Ozawa exponent or
m [-] Population mean
Mn [g/mol] Molar mass
Mw [-] Molar weight
M W D [-] Molecular weight distribution
n [-] Number of measurements or
n [-] Avrami exponent
nd [-] Avrami exponent for crystal dimension nn [-] Avrami exponent for crystallization type
OIT [min] Oxidation induction time
p,P [bar] Pressure
Pmax [N] Maximum force
R [J/molK3] Universal gas constant
rf [m] Fiber radius
s [mm] Deflection or
s [-] Standard deviation
T [°C] Temperature
t [s] or [min] Time or
t [mm] Specimen thickness
t0.925 [-] Confidence level of 95%
Tg [°C] Glass transition temperature
Tm [°C] Melting temperature
Tp [°C] Peak crystallization temperature uf [mm/s] Fiber bed velocity vector
um [mm/s] Matrix velocity vector
V0 [%] Initial fiber volume content
Va [%] Maximum possible fiber volume content Va [%] Maximum available fiber volume content Vc [Vol.-%] Volume fraction of crystalline phase
Vf [%] Fiber volume fraction
Vf,max [%] Maximum fiber volume content
w [mm] Specimen width
Wc [wt%] Crystallized mass fraction
¯
x [-] Arithmetic mean of n measurements
X [%] Relative degree of crystallinity
α, λ, n [-] Fitting parameters Carreau-Yasuda model
δ [mm] Load point deflection or
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η [Pa s] Viscosity
η [Pa s] In-phase component of complex viscosity η [Pa s] Out-of-phase component of complex viscosity
η∗ [Pa s] Complex viscosity
η0 [Pa s] Zero-shear viscosity
˙
γ [1/s] Shear rate
γ0 [-] Strain amplitude
φ [wt%] Fiber weight fraction
ρ [g/cm3] Density
ρa [g/cm3] Density of amorphous polymer fraction ρc [g/cm3] Density of crystalline polymer fraction
σ [MPa] Stress
σf [MPa] Flexural stress
σf1 [MPa] Longitudinal flexural strength σf2 [MPa] Transverse flexural strength
σyield [MPa] Yield stress
τ,τ0 [Pa] Shear stress, shear stress amplitude
ξ [°] Angle of twist
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Abbreviation Meaning
AFP Automated fiber placement
AMMRF Australian Microscopy and Microanalysis Research Fa- cility
ANU Australian National University, Canberra ASTM American Society for Testing and Materials
ATL Automated tape laying
B3L Toughened polyamide 6 grade from BASF B3S Low-flow polyamide 6 grade from BASF
B40 Polyamide 6 grade with high molecular weight from BASF
C2000 Semi-aromatic co-polyamide PA10T/X from Evonik CAM Centre for Advanced Microscopy
CB Chain breaking; group of antioxidants
CB-A Chain breaking acceptor
CB-D Chain breaking donor
CF-EPY SIGRAFIL C T50-4.0/240-E100 carbon fibers from SGL Group
CFRP Carbon fiber reinforced plastics
CFRTP Carbon fiber reinforced thermoplastics
CF-TP SIGRAFIL C T50-4.0/240-T140 carbon fibers from SGL Group
DCB Double-cantilever beam
DIN Deutsches Institut für Normung e.V.
DOE Design of experiment
DSC Differential scanning calorimetry
EBS Bis-stearyl ethylenediamine / ethylenbisstearamide
EC European Community
FIT Fibre impregnée thermoplastique GMT Glass-mat reinforced thermoplastics
GPC/SEC Gel-permeation chromatography/ Size-exclusion chro- matography
LFT Long-fiber reinforced thermoplastics
LPL Laboratory prepreg line
MCC Modified compliance calibration
NCF Non-crimped fabric
OoA Out-of-Autoclave
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PEEK Polyetheretherketone
PEI Polyetherimide
PES Polyether sulfone
PPA Polyphthalamide
PPS Polyphenylene sulfide
PVC Polyvinyl chloride
RFI Resin film infusion
RIM Reaction injection molding
RTM Resin transfer molding
SCB Side-clamped beam
SEM Scanning electron microscopy
TGA Thermogravimetric analysis
T-RTM Thermoplastic-resin transfer molding
UD Unidirectional
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1-1 Classification of thermoplastics [8]. . . 3 1-2 Overview of potential manufacturing routes originating from the pro-
duction of intermediates for thermoplastic composites, based on [7, 10, 11]. . . 4 1-3 Principle process steps during thermoplastic composite production
along with the governing process parameters time, pressure and tem- perature; based on [29]. . . 6 1-4 Principle production steps to manufacture thermoplastic composites;
pictures provided as courtesy by SGL Group and from Celanese [44]
as well as from the Institut für Verbundwerkstoffe (IVW) Kaiser- slautern [45], as indicated. . . 10 1-5 Schematic structure of the present thesis. . . 11 2-1 Chemical structure of aliphatic PA6. . . 15 2-2 Chemical structure of semi-aromatic co-polyamide (PA10T). . . 15 2-3 Four-point bend test setup with support span L and a load span
L’=L/3. . . 16 2-4 a) DCB test specimen with initial delamination lengtha0 from load
line to end of insert and b) test specimen clamped to the SCB test fixture mounted to a Hegewald & Peschke 100 kN universal testing machine. . . 17 2-5 a) Principle of nano-indentation on carbon fibers surrounded by ma-
trix; b) SEM image of Berkovich indenter tip [72]. . . 19 2-6 Schematic of the prepreg line used to produce powder-coated tows
on a laboratory scale. . . 21 2-7 a) Stacking, b) processing in a static press and c) produced test panel. 22 2-8 a) Stress-strain curves of CF-EPY/B3S and b) CF-TP/B3S. . . 24 2-9 R curves for a) CF-EPY/B3S and b) CF-TP/B3S. . . 24 2-10 a) Mean transverse flexural strengthσf2 and b) mean mode I inter-
laminar fracture toughness GIc of CF-EPY/B3S in comparison to CF-TP/B3S. . . 25 2-11 Fracture surface analysis of tested DCB specimens made of a) CF-
EPY/B3S in comparison to b) CF-TP/B3S. . . 25 2-12 Stress-strain curves for a) CF-EPY/C2000 and b) CF-TP/C2000. . 26 2-13 a) R curves of CF-EPY/C2000 and b) CF-TP/C2000. . . 26 2-14 a) Mean transverse flexural strengthσf2 and b) mean mode I inter-
laminar fracture toughness GIc of CF-EPY/C2000 in comparison to CF-TP/C2000. . . 27
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2-16 Micrographs of four-point bend test panels made of a) CF-EPY/C2000 and to b) CF-TP/C2000. . . 28 2-17 Fracture surface analysis of tested DCB specimens made of a) CF-
EPY/C2000 compared to b) CF-TP/C2000. . . 28 2-18 Stress-strain curves of a) CF-TP/B3L in comparison to b) CF-TP/B40. 29 2-19 R curves for a) CF-TP/B3L and b) CF-TP/B40. . . 29 2-20 a) Mean transverse flexural strengthσf2 and b) mean mode I inter-
laminar fracture toughness GIc of CF-TP/B3L and CF-TP/B40 in comparison to CF-TP/B3S. . . 30 2-21 Fracture surface analysis of tested DCB specimens made of a) CF-
TP/B3S, b) CF-TP/B3L and c) CF-TP/B40. . . 30 2-22 Nano-indentation on a) CF-TP only and b) B3S only. . . 31 2-23 Nano-indentation on carbon fibers coated with a) epoxy-based and
b) thermoplastic-based sizing, surrounded by B3S; blue arrow indi- cates indenting direction and covered area. . . 32 2-24 Close-up view of a) CF-EPY fibers with highlighted gap between
fiber and matrix, b) CF-TP fibers, surrounded by B3S, under the SEM. . . 32 3-1 Time-delayed development of nucleation and growth rate during
crystallization as a function of temperature, redrawn from [75]. . . . 36 3-2 Rucks thermoforming unit and used aluminum tool to perform ther-
moforming of flat test panels. . . 42 3-3 Influence of pre-drying process on crystallization behavior of a) un-
dried and b) pre-dried B3S. . . 43 3-4 Subjection of C2000 to ten heating and cooling cycles. . . 44 3-5 Development of the relative degree of crystallinity X(t) for a) neat
B3S and b) C2000. . . 45 3-6 Plot of log−ln[1−X(t)]versus logtfor the isothermal crystalliza-
tion of a) B3S withρa = 1.08 g/cm3 and ρc = 1.24 g/cm3 b) C2000, based on Wc according to [98]. . . 46 3-7 Development of the relative degree of crystallinity X(T) for a) neat
B3S and b) C2000. . . 48 3-8 Plots of log−ln[1−X(T)]versus ln|(dT /dt)−1|for a) neat B3S at
205, 200, 195, 190 and 185 °C; b) neat C2000 at 235, 230, 225, 215 and 205 °C. . . 48 3-9 Plots of cooling rate versus enthalpy of crystallization ΔHc for B3S
and C2000. . . 50
xxvi
3-11 Close-up view of B3S specimens with a) epoxy-sized fibers and b) polyamide-sized fibers that were etched by oxygen plasma to visual- ize crystalline structures . . . 52 3-12 Influence of CR on σf2 and Ef2 of a), c) CF-TP/B3S and b), d)
CF-TP/C2000. . . 54 4-1 Idealized arrangements of fiber packings in a composite yielding
maximum fiber volume contents Vf,max of 78.5 % (quadratic) and 90.7 % (hexagonal). . . 60 4-2 Comparison of production processes for thermoplastics with regard
to shear rates; redrawn from [133]. . . 63 4-3 Schematic of the flow front progression according to the derived 1D
through thickness model. . . 65 4-4 a) Parallel-plate and b) cone-plate fixtures for use in rotational
rheometers. . . 66 4-5 a) Oscillatory measurement and b) time-delayed shift of stress re-
sponse compared to applied strain rate; redrawn from [137]. . . 66 4-6 Schematic of the degree of impregnation of thermoplastic interme-
diates. . . 68 4-7 Micrographs of cross-sections of tapes with a) highlighted entire fiber
bundle area, b) highlighted non-impregnated area and binary pic- tures of c) the entire fiber bundle area and d) the non-impregnated area within the fiber bundles. . . 69 4-8 Graphical representation of a Box-Behnken Design with three factors
having two extreme factor levels (−1 (minimum) and 1 (maximum)) and a center point (0); redrawn from [142]. . . 70 4-9 Viscosity curves at different temperatures for a) B3S, B3L, B40 and
b) C2000. . . 72 4-10 Plots of temperature shift factor aT versus 1/RT for a) B3S, B3L,
B40 and b) C2000; the slope of the lines represents the activation energy Ea. . . 73 4-11 Viscosity curve of B3S calculated according to Arrhenius for a typical
temperature profile during thermoforming. . . 73 4-12 Micrographs of CF-TP/B3S test panels after a press time of a) 1
minute revealing non-impregnated fiber bundles and b) 10 minutes showing complete impregnation. . . 74
xxvii
vanced impregnation progress. . . 75 4-14 Micrographs of CF-TP/B3S test panels pressed with a) 5 bar re-
vealing non-impregnated fiber bundles and b) 30 bar with increased DOI. . . 75 4-15 Main effect plot for DOI of CF-TP/B3S. . . 76 4-16 Interaction plot for DOI of CF-TP/B3S. . . 76 4-17 Main effect plot for DOI of CF-TP/C2000. . . 77 4-18 Interaction plot for DOI of CF-TP/C2000. . . 77 4-19 Development of the experimentally determinedVf (◦) as a function
of applied pressure for a) CF-TP/B3S and b) CF-TP/C2000. . . 78 4-20 Comparison of experimentally determined (◦) and calculated DOI
for various temperatures at 17.5 bar for a-c) CF-TP/B3S and d-f) CF-TP/C2000. . . 79 4-21 ILSS and the DOI for a) B3S at constant time, b) C2000 at constant
time, c) B3S at constant temperature, d) C2000 at constant temper- ature, e) B3S at constant pressure, f) C2000 at constant pressure. . 81 5-1 Principle scheme of thermal decomposition of PA6 [145]. . . 84 5-2 Basic mechanism for chain scission during oxidation of aliphatic
polyamides [156]. . . 85 5-3 Considered CFRTP production process to derive temperature profiles. 87 5-4 Temperature profiles P1 and P2 for a) B3S and ) C2000 derived from
a CFRTP production process. . . 88 5-5 Determination of Tm for bimodal melt peaks as present for C2000. . 89 5-6 Procedure to determine the mass loss that has occurred during every
process step of P1 and P2 for B3S as an example. . . 90 5-7 Size separation and detection of dissolved molecules by GPC; re-
drawn from [173]. . . 91 5-8 DSC thermograms for a) B3S and b) C2000 subjected to temperature
profile P2 in air. . . 92 5-9 Development of Tm of a) B3S and b) C2000 under air and nitrogen
atmosphere when subjected to temperature profiles P1 and P2. . . . 92 5-10 Mass loss of a) B3S and b) C2000 samples subjected to temperature
profile P1 and P2 under air and nitrogen gas atmosphere in TGA. . 94 5-11 Development of the complex viscosity η∗ of a) B3S and b) C2000
during subjection to temperature profile P2 in air and nitrogen gas atmosphere. . . 94
xxviii
5-13 MWD of a) B3S and b) C2000 as-received and after exposure to temperature profile P2 under air and nitrogen gas atmosphere. . . . 96 5-14 Development of the DOI of CF-TP/B3S as a function of a) constant
viscosity and b) viscosity development as measured for temperature profile P2 during the laminate production step. . . 97 5-15 Development of the DOI of CF-TP/C2000 as a function of a) con-
stant viscosity and b) viscosity development as measured for tem- perature profile P2 during the laminate production step. . . 98 6-1 Stabilization reaction using sterically hindered phenols [176]; R1,R2,
and R3 denote moiety. . . 102 6-2 Stabilization reaction of aromatic amines [176]. . . 102 6-3 Effect of lubricants as a function of solubility in the host polymer;
redrawn from [192]. . . 107 6-4 Temperature profiles used for compounding additives to B3S and
C2000 in a twin-screw extruder. . . 109 6-5 The OIT of B3S samples at 320 °C and C2000 samples at 340 °C,
both neat and modified with antioxidants. . . 112 6-6 Mass loss of neat and single-modified a) B3S samples and b) C2000
samples during temperature profile P1. . . 113 6-7 Complex viscosity of neat and modified a) B3S and b) C2000 sub-
jected to temperature profile P2 under oxidative and inert atmosphere.114 6-8 Complex viscosity of neat and modified C2000 subjected to temper-
ature profile P2 under oxidative and inert atmosphere. . . 116 6-9 Complex viscosity of neat and multi-functionalized a) B3S and b)
C2000 subjected to temperature profile P2 under oxidative and inert atmosphere. . . 118 6-10 Results from four-point bend testing a) in fiber direction and b)
perpendicular to fiber direction of test panels produced from non- modified and multi-functionalized polymers at different dwell times in a press. . . 119 6-11 MWD of as-received without processing, single-modified and multi-
functionalized polymers after subjection to temperature profile P2 in air of a) B3S and b) C2000; MWD of non-modified and multi- functionalized samples extracted from four-point bend test panels of c) B3S and d) C2000. . . 120
xxix
profile P2 during the laminate production step of b) non-modified B3S and c) multi-functionalized B3S. . . 121 6-13 Development of the DOI of CF-TP/C2000 as a function of a) con-
stant viscosity and viscosity development as measured for temper- ature profile P2 during the laminate production step of b) non- modified C2000 and c) multi-functionalized C2000. . . 122 7-1 Double-belt press with seven heated sections used to produce CF-
TP/B3S tapes with different DOIi; modified from [201]. . . 126 7-2 Differently impregnated CF-TP/B3S tapes with highlighted non-
impregnated areas produced in a double-belt press with a) 2 m/min at 40 bar b) 4 m/min at 5 bar and c) 8 m/min at 5 bar. . . 127 7-3 Predicted final degree of impregnation (DOIf) after press forming
tapes with a DOIi of a),c) 80 % and b),d) 90 % at a dwell time of 90 s and 300 s. . . 129 7-4 Micrographs of laminates produced with varying dwell times based
on differently impregnated tapes with highlighted non-impregnated areas if applicable. . . 130 7-5 a) Longitudinal flexural strength σf1 and b) longitudinal flexural
modulus Ef1 of test panels made from CF-TP/B3S tapes with dif- ferent DOIi that were processed with varying dwell times in a static press or thermoformed (+TF). . . 132 7-6 a) Transverse flexural strength σf2 and b) transverse flexural mod-
ulus Ef2 of laminates made of tapes with different DOIi that were processed with varying dwell times in a static press or thermoformed (+TF). . . 133 7-7 Correlation of costs to mechanical performance for a) σf1, b) σf2,
c) Ef1 and d) Ef2 compared to reference values obtained from com- pletely impregnated tapes pressed for 1200 s. . . 140
xxx
1-1 Comparison of intermediate materials; based on [7]. . . 6 2-1 Comparison of tensile strengthσ, tensile modulusE and sizing type
of investigated carbon fibers. . . 14 2-2 Properties of investigated polyamides including densityρ, yield stress
σyield (dried), tensile modulus E and Tm [54–58]. . . 15 2-3 Investigated material combinations produced by the powder-coating
technique along with required lay-up to obtain desired test panel thickness for four-point bend and DCB testing. . . 22 2-4 Summary of the mechanical properties for the investigated material
combinations of differently sized carbon fibers and polyamides. . . . 33 3-1 Avrami exponent n, nucleation mode, crystal growth shape accord-
ing to [76, 80]. . . 38 3-2 Kinetic parameters for the isothermal crystallization of neat B3S. . 46 3-3 Kinetic parameters for the isothermal crystallization of neat C2000. 47 3-4 Effect of cooling rate on crystallization of neat B3S. . . 49 3-5 Effect of cooling rate on crystallization of neat C2000. . . 50 3-6 Overview of produced test panels along with matrix mass fraction
determined by acid digestion. . . 53 4-1 Comparison of boundary conditions as present in Darcy’s Law and
thermoplastic matrix flow through carbon fiber bed [121]. . . 58 4-2 Experimental design with three factors - time, temperature and pres-
sure - for CF-TP/B3S and CF-TP/C2000. . . 70 4-3 Zero-shear viscosity data for B3S used as input parameter. . . 74 4-4 Zero-shear viscosity data for C2000 used as input parameter. . . 74 4-5 Input parameters for model calibration. . . 78 6-1 Selected additives for thermal stabilization of B3S and C2000. . . . 105 6-2 Selected additives to increase flowability. . . 108 6-3 Effectiveness of different lubricants on B3S and C2000, compared
across the first three process steps (powder-coating until laminate production) and the final step (thermoforming) of temperature pro- file P2 in an oxidative and inert atmosphere. . . 116 6-4 Mass loss of neat and multi-functionalized (MF) B3S and C2000
recorded during subjection to temperature profile P1 under an ox- idative atmosphere. . . 117
xxxi
7-2 Input parameters for the prediction of dwell times to completely impregnate partially impregnated UD tapes during press forming. . 128 7-3 Dwell times used to process differently impregnated tapes in a static
press and via thermoforming along with the final DOIf of all test panels. . . 131 7-4 Assumptions made for cost analysis based on forecast for 2020. . . . 135 7-5 Collected data used for the cost analysis. . . 138 7-6 Calculation of the machine hour rate for varying operating speeds
of a double-belt press. . . 139 A-1 Three-factor Box-Behnken Design for CF-TP/B3S . . . 169 A-2 Three-factor Box-Behnken Design for CF-TP/C2000 . . . 170 A-3 FVC averaged over three samples of test panels produced from non-
modified and multi-functionalized B3S and C2000 reinforced by CF- TP carbon fibers . . . 171 A-4 FVC averaged over three samples of test panels produced with var-
ious dwell times from differently impregnated tapes . . . 171
xxxii
1.1 Motivation
Fiber reinforced composites have established in the aerospace industry over decades due to their specific properties. More than 50 % of the primary structures of the 787 Dreamliner launched by Boeing are made of composites enabling fuel savings up to 20 % [1]. Every reduction in weight and corresponding fuel savings are crucial during the design due to the long service life of aircraft. The automotive industry has a strong interest in weight reduction of cars as alternative propulsion concepts such as electric vehicles with heavy battery packs gain in importance, too. By introducing the i model series, BMW undertook a major step towards the mass production of carbon fiber reinforced plastics (CFRP). There is a high demand for cost-efficient production of CFRP since these costs can comprise 50 % [2] of the total CFRP component cost.
The main driver in reduction of process costs is automation. Automated fiber place- ment (AFP), automated tape laying (ATL) or press forming techniques (e.g. di- aphragm forming, thermoforming) are examples for highly advanced techniques that can reduce processing costs by their high level of automation involving repro- ducibility and accuracy.
Another possibility for cost reduction lies in the use of suitable materials. By intro- ducing intermediate materials such as carbon fibers pre-impregnated with matrix (prepregs) ready for the use in automated processes, the time-consuming infiltra- tion or impregnation step of carbon fibers with matrix is mostly finished before the actual component production starts.
In case of CFRP, two polymer groups are commercially used: Thermosets and ther- moplastics. In 2014, thermosets represented the most common (49 %) group used for CFRP amongst other matrix materials such as metals, ceramics, hybrids, car- bon or thermoplastics [3]. Established production processes and the initial chemical constitution of thermosets are responsible for their wide use in industry. Thermosets initially consist of two or three low-molecular constituents (resin, hardener and cat- alyst) with very low viscosities. During curing, these components start crosslinking and form non-fusible polymers. The uncured thermosets facilitate wetting and in- filtration of thin reinforcing fibers due to the low viscosity.
However, the eminent problems in processing epoxy-based thermosets were already identified in 1980: high brittleness, absorption of water/moisture and long manu- facturing times due to crosslinking [4]. In contrast, thermoplastic matrix systems are already completely polymerized making them fusible, shapeable and more suit- able for repair and recycling processes. In addition, they are typically more ductile
1
and tougher than thermosets. With having the impregnation process completed, thermoplastics can be processed with short forming and consolidation times of sev- eral minutes as no crosslinking reactions of up to several hours are required. The short processing times of thermoplastic composites can meet the low cycle times that govern the automotive industry.
Besides economic advantages, thermoplastics meet also ecologic demands due to their recycling potential. The last two decades were dominated by the increasing importance of environmental impact. The European Community (EC) guideline [5]
for reuse or disposal of 95% of the automobile weight came into effect in January 2015. Other investments by the EC such as CleanSky worth several billions em- phasize the strong need for more environmentally-friendly air transportation.
However, the long and branched molecular chains of polymerized thermoplastics are the cause of melt viscosities that are 100-1000 times higher than for thermosets even at processing temperatures well above the melting point [6]. The high melt vis- cosity of thermoplastics complicates wetting during processing. Especially carbon fibers with diameters of about 7 μm are difficult to impregnate. Thus, the produc- tion of intermediate products is costly due the time-consuming impregnation that requires high temperatures as well as pressures explaining the modest industrial use of thermoplastics as matrix materials in continuously reinforced composites [3].
In order to expand the use of thermoplastics as matrix materials, either the costs of the intermediate materials need to be reduced or the production of carbon fiber reinforced thermoplastics (CFRTP) needs to become more efficient. A combined approach may be most successful and will lead to an increased acceptance of ther- moplastic composites.
1.2 Thermoplastic composites
The component production of continuously carbon fiber reinforced thermoplastics (CFRTP) follows three essential steps: impregnation, consolidation and solidifica- tion. The impregnation step is usually carried out in a separate intermediate step before the actual component production starts. In contrast to thermoset-based CFRP, fibers and matrix are combined to obtain partially or completely impreg- nated intermediate materials that form the raw materials for CFRTP production.
Thus, the time-consuming impregnation step is separated from the actual compo- nent production to make use of the potentially short forming and consolidation times of thermoplastics.
The quantitatively most used intermediate materials with thermoplastic matrix are glass-mat reinforced thermoplastics (GMT) and long-fiber reinforced thermoplas- tics (LFT) [7] suitable for mass production of automotive thermoplastic compos-
ite components with medium strength. For high-performance components made of thermoplastic composites, intermediate materials with continuous carbon fibers are required. These intermediate materials are the focus of this thesis.
1.2.1 Intermediate materials
The production of intermediate materials to combine fibers and matrix follows the same principal steps from impregnation, consolidation to solidification as known from the CFRTP component manufacture. The carbon fiber reinforcement for inter- mediate materials covers the whole range of available fiber architectures including spread unidirectional (UD) tows, woven fabrics, non-crimped fabrics (NCF), knit- ted or braided preforms.
Amorphous or semi-crystalline thermoplastic matrix systems for intermediate ma- terials are selected from all application areas that are depicted in Figure 1-1.
PEEK PEKKPEK
PMMA PC PET
PSU
PA6
PPS
PPA
PBT POM
PE
HD-PE PP
Standard thermoplastics Engineering thermoplastics
High-performance thermoplastics
Price, performance
PS ABS
PVC PEI
SAN PA66
Semi-crystalline Amorphous
LD-PE
Figure 1-1 Classification of thermoplastics [8].
In general, thermoplastics are used in form of pellets, ground powder, suspension (with water) or solution (dissolved polymer) to produce intermediate materials.
Amorphous thermoplastics such as polyetherimide (PEI) or polyether sulfone (PES) with high viscosities and no melting point are often processed as powder, solu- tion or suspension [7]. Semi-crystalline thermoplastics such as polyetheretherketone (PEEK), polyphenylene sulfide (PPS) or polyamides (PA6, PA66, PA10T) cannot be dissolved properly due to their high chemical resistance against most solvents.
They are usually processed via melt impregnation or powder-coating [9].
Over the last decades, new types of thermoplastic intermediates appeared simulta- neously to the invention of new manufacturing techniques for thermoplastic com- posites [10]. Considering various forms of intermediates, the production may be divided in the following principal process steps with regard to the thermoplastic matrix [7]:
• Matrix application (suspension, solution, melt, film, powder)
• Heating (oven, infra-red source, calender, nozzle, double-belt press)
• Cooling/calibration (calender, double-belt press)
Due to this large variety, potential manufacturing routes for thermoplastic com- posites are presented in Figure 1-2, starting from reinforcing fibers and matrix to a final part.
Pre-forming
UD tapes, tows, textile prepregs Powder-coated
tows, commingled yarns, FlT* bundles
Pre-consolidated sheets Woven fabrics, braids
Manufacturing technique for shaping and forming
Autoclave, vacuum consolidation, AFP, ATL, filament winding, pultrusion, press forming, thermoforming
PART *FIT: Fibres Imprégnées de
Thermoplastique
Reinforcement (e.g. unidirectional (UD) spread tows,
woven fabrics, NCF, braids)
Thermoplastic matrix (e.g. powder, fiber, film, suspension)
Pre-impregnation
Techniques:
• Film-stacking
• Powder-coating
• Fiber hybridization Techniques:
• Melt impregnation
• Solution impregnation
Figure 1-2 Overview of potential manufacturing routes originating from the production of in- termediates for thermoplastic composites, based on [7, 10, 11].
The production of intermediates can be divided into two principal techniques:
pre-impregnation and pre-forming [10]. Pre-impregnation is commonly reached by melt [12–18] or solution impregnation [19–21], and other exotic techniques such as impregnation by using aqueous suspensions [22, 23].
The pre-forming technique brings reinforcement and matrix together in a defined way without impregnation. Here, the impregnation takes place during part manu-
facture by forming or shaping. Using such intermediates, reinforcement and matrix reveal a weak link due to the non-impregnated state leading to a high degree of drapeability that is maintained during lay-up. At the same time, a large flow dis- tance has to be covered in pre-formed intermediates. This makes the wetting as well as the impregnation to the critical phase during component production [10].
In the case of commingled yarns, the matrix is mixed in the form of thermoplastic fibers to reinforcing fibers producing a hybrid yarn that becomes rigid after consol- idation [24]. Another example for pre-formed intermediates is represented by the film-stacking method where thin polymer films and reinforcement layers (fabrics, NCF, spread tows etc.) [10] are consecutively stacked and consolidated in a double- belt press [25].
Using the powder-coating method, polymer powder with particles in the range of 5 to 200 μm is deposited on the reinforcement [10]. The powder deposition can take place in an impregnation bath by using a fluidized bed or a fine suspension of powder particles in a liquid. The powder may also be directly applied by a needle roller or electrostatic deposition. To avoid loss of powder, the fabric, NCF or spread tows coated with powder subsequently pass a heat system. By means of an oven, calender or heater, the powder is surface-fused ensuring sufficient adhesion to the reinforcement without impregnation [7].
In 1983, Ganga [26] patented a special type of powder-coated intermediates: Fibre Impregnée Thermoplastique (FIT). Here, the tows are powder-coated and enclosed by a flexible sheath made of preferably the same thermoplastic as the powder parti- cles. Thus, the powder maintains its position while these intermediates are further processed [27]. In general, FIT and commingled yarns are usually further processed to more complex preforms such as braids, knits, fabrics or three-dimensional pre- forms and consolidated afterwards. Film-stacked and powder-coated intermediates can transform into pre-impregnated intermediates after passing a heat system un- der pressure e.g. in a double-belt press as indicated in Figure 1-2 [7, 28].
The introduced intermediates vary with regard to the degree of impregnation (DOI) and the remaining flow path. Both characteristics determine the type of production technology that can be used for the subsequent processing to a CFRTP compo- nent. Table 1-1 compares the introduced intermediate materials with regard to the initial degree of impregnation (DOIi) before component production along with the remaining flow path, production rate, flexibility in relation to the material avail- ability and equipment costs. Here, the expression “hybrid” designates commingled yarns or FIT bundles. As not every thermoplastic polymer can be spun into a fiber or dissolved the flexibility of hybrid and solvent-impregnated intermediates is con- sidered to be low. Intermediates produced via melt impregnation show the highest DOIi with the lowest remaining flow path.
Table 1-1Comparison of intermediate materials; based on [7].
Intermediate Pre-formed Pre-impregnated
types Powder Film Hybrid Melt Solution
DOIi low medium medium high medium
Remaining flow path high medium medium low medium
Production rate high medium medium medium medium
Flexibility medium medium low high low
Equipment cost medium high low-high medium high
1.2.2 Production methods
According to the classification of the previously introduced intermediates into pre- formed and pre-impregnated forms, different production methods are required. Pre- impregnated materials undergo three principle process steps: heating/melting above glass transition temperatureTg or melting temperatureTm, consolidation and cool- ing/solidification below Tg [10, 29] as depicted in Figure 1-3.
Pressure [MPa]
ܶ
Time [s]
Heating/Melting Consolidation Cooling/
Solidification
Temperature [°C]
ܶ
Figure 1-3 Principle process steps during thermoplastic composite production along with the governing process parameters time, pressure and temperature; based on [29].
Pre-impregnated intermediates are typically completely impregnated and enable the use of production techniques such as laser-assisted AFP or filament winding that are aimed at in-situ consolidation. ATL and other automated placement tech- nologies such as FiberForge [30] were designed for quick and automated lay-up of laminates without in-situ consolidation between plies. AFP, filament winding or the FiberForge process technology are also applicable to pre-impregnated interme-
diates with low to medium DOI along with subsequent consolidation in a press or in an autoclave. In addition, pre-impregnated intermediates are suitable for pul- trusion to profiles.
Pre-formed intermediates such as commingled yarns or FIT generally involve an- other textile processing step to form a complex structure before the actual compo- nent production starts. Film-stacked or powder-coated reinforcements may be fur- ther processed in a calender, double-belt press or pultrusion equipment to obtain completely impregnated prepregs for subsequent use in automated production tech- nologies. In addition, they may be directly processed to pre-consolidated sheets in a static press or in an autoclave under vacuum. By feeding alternately reinforcement layers and polymer films into a double-belt press according to the film-stacking method, pre-consolidated sheets with multiple plies are produced [7, 10]. The di- rect production of multi-ply laminates in a double-belt press can also be achieved by using powder-coated tows. In this way, a previous manufacture of pre-impregnated materials can be eliminated and shorten the overall process chain.
Pre-consolidated sheets, also referred to as multi-ply laminates or “organo-sheets”, are commonly shaped into complex structures by using different techniques of press forming or thermoforming. The actual forming and consolidation of the pre- consolidated laminates can occur in a matched-metal mold (matched-die molding), in a rigid female mold with flexible male mold (rubber forming, rubber-pad form- ing, hydroforming) or in between two membranes (double-diaphragm forming) [10].
To heat or melt the pre-consolidated laminates, the used molds or press plates of the press equipment are heated and cooled during the press forming process. In the thermoforming process, pre-consolidated sheets are pre-heated externally by an infra-red source or heater and then transferred to the not-heated mold or press plates. As mold and/or press plates are not heated and cooled, thermoforming al- lows short processing times involving high cooling rates.
In more recent approaches such as SpriForm [31], injection molding and thermo- forming are combined into a single process. Here, multi-ply laminates are shaped in the cavity of an injection molding machine while more complex structures such as ribs are injection-molded by using neat, short-fiber or long-fiber reinforced ther- moplastics. This process is also referred to as back-injection molding.
Direct production techniques such as vacuum consolidation on a heated plate, a static press or in an autoclave are independent of the used intermediate materials and their DOIi. However, partially impregnated prepregs are not suitable for pro- cessing in AFP or filament winding as long as in-situ consolidation and completion of the impregnation progress is required. The pressure applied during AFP and filament winding is insufficient and the processing time too short to completely impregnate and consolidate prepregs with remaining flow path [32].
1.3 State-of-the-art
Within the last decades, several approaches were developed to overcome the dif- ficulty of impregnating fibrous reinforcements with high viscous thermoplastics.
There are several paths to achieve a more efficient production of intermediates or thermoplastic composites which are summarized in the following:
• Optimization of the impregnation technology
• Integration of an online-impregnation device into processing technologies
• Reduction of the impregnation time by matrix modification and/or suitable process settings
By using pre-formed materials such as commingled yarns or powder-coated tows the initial flow path is reduced and the actual impregnation occurs during part processing. By feeding commingled yarns into a pultrusion device, a good impreg- nation level is reported for high pultrusion speeds up to 10 m/min [33]. However, commingled yarns are constrained in width enabling the production of rods or pro- files but are less suitable for sheet production.
Considering typically completely impregnated prepregs, the throughput can be in- creased by improving the impregnation technology. Marissen et al. [34] used specif- ically designed bars/pins which reduce the viscous drag and hence the relative speed between spreader bars and fiber bundles to enhance the throughput of the thermoplastic pultrusion process. Weustink [35] refined the impregnation device developed by Marissen et al. by identifying some important key aspects for fixed and driven pins. The device itself including the pins shall be heated to achieve the lowest possible viscosity of the thermoplastic. Additionally, fiber bundles which were produced by using the impregnation device were directly fed into a filament winding machine.
This combination of placement or winding technologies with online-impregnation was also investigated by several other researchers. The operating efficiency of on- line melt-impregnated fiber bundles with direct processing in a filament winding device was found to be capable of competing to the use of pre-impregnated in- termediates [36]. In other approaches, powder-coated fiber bundles [37] or FIT bundles [38] are fed into filament winding equipment.
On a more comprehensive level, the overall process time to produce thermoplastic components can be reduced by integrating different material forms into one pro- cess. Instead of consecutively proceeding processes, commingled yarns and powder- coated tows may be placed locally onto an injection-molded polymer part in an integrated manufacturing cell [39]. This enables the production of complex parts with short cycle times.
Another path to produce sufficiently impregnated intermediates or components at economically attractive production rates is the use of low viscous thermoplastics.
This can be achieved by impregnation with thermoplastics previously dissolved in a suitable solvent [40] or by using prepolymers. The polymeric precursors enable the use of production techniques generally designed for thermosets such as resin transfer molding (RTM), resin film infusion (RFI) or reaction injection molding (RIM) [41]. During thermoplastic RTM (T-RTM), the low molecular prepolymers facilitate impregnation of fibrous reinforcement and form the thermoplastic matrix after impregnation by in-situ polymerization [41, 42].
The previous studies showed that focus is put on efficient production of either in- termediates or components. A more comprehensive approach lies in the production of intermediate materials at enhanced production rates that are intended to fur- ther impregnate during the processing steps to obtain a final CFRTP component.
Hayashi et al. [43] investigated the effects of different pressures and processing tem- peratures during thermoforming of completely and semi-impregnated (semi-preg) materials on their mechanical properties. The semi-pregs could be produced two or four times faster than the fully impregnated prepreg. Mechanical properties of semi- pregs were found to be decreased compared to completely impregnated prepregs as it was not aimed at complete impregnation when processing intermediates to com- ponents. However, the DOIi of these semi-pregs was not evaluated and thus could not be correlated to the obtained mechanical properties. In addition, a relation be- tween mechanical properties to the monetary effect due to increased productivity during semi-preg manufacture was not investigated.
1.4 Objectives and outline of the thesis
Currently, mostly completely impregnated intermediate materials are used for the production of thermoplastic composites. Considering a principal production process for thermoplastic composites, the manufacturing steps require repeated heating of the polymer above Tg or Tm from intermediate production to tape or laminate consolidation until forming to a component as presented in Figure 1-4.
The overall objective of the present work is the production of cost-efficient interme- diates by increasing the operational throughput. Therefore, partially impregnated tapes are developed that can be produced with enhanced production rates inde- pendent of the used manufacturing technology. The idea behind the use of partially impregnated tapes lies in utilizing the repeated heating cycles during the produc- tion of thermoplastic components to complete impregnation. However, the remain- ing flow path must be adjusted such that gradual and complete impregnation is enabled throughout the subsequent process steps.
Intermediate Products
[SGL Group]
Raw Materials
[SGL Group] [IVW]
Final component Preforms, multi-ply
laminates
[Celanese]
Heating Heating Heating
[SGL Group]
[SGL Group]
Figure 1-4 Principle production steps to manufacture thermoplastic composites; pictures pro- vided as courtesy by SGL Group and from Celanese [44] as well as from the Institut für Verbundwerkstoffe (IVW) Kaiserslautern [45], as indicated.
The major research objectives of this work can be summarized as follows:
1. Characterization of suitable fiber-matrix combinations on microscopic and macroscopic level as a function of the fiber sizing.
2. Study of the crystallization behavior to compare production processes that involve different cooling rates.
3. Modeling the thermoplastic impregnation of fiber bundles to determine the influence of process parameters on the DOIi of intermediates as well as com- ponents.
4. Evaluation of viscosity changes induced by degradation reactions which de- velop during repetitive heating processes.
5. Prevention of viscosity changes due to degradation by thermal stabilization and viscosity reduction by adding suitable lubricants.
6. Investigation of the gradual impregnation of partially impregnated tapes throughout component production and evaluation of intermediate cost.
Based on these objectives, the structure of the present thesis was developed and is schematically shown in Figure 1-5.
In Chapter 2, the compatibility of various carbon fiber sizings to several polyamides is studied. The transverse four-point bend and the double-cantilever beam test are selected to characterize the adhesion between fiber and matrix. By using scanning electron microscopy, the adhesion behavior is analyzed qualitatively. The nano- indentation technique serves the investigation of a three-dimensional phase that
