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Engineering the carbon fiber-vinyl ester interface for improved mechanical properties
F. Vautarda, S. Ozcanb, L. Xua, L. T. Drzala
aComposite Materials and Structures Center
Michigan State University
2100 Engineering Building
East Lansing, MI 48824
bOak Ridge National Laboratory, Materials Science and Technology Division
1 Bethel Valley Road
Oak Ridge, Tennessee 37831-6053
SPE Automotive Composites Conference & Exhibition Novi, MI, Sept. 9-11, 2014
New applications for carbon fiber composites
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Development of low cost carbon fibers from new cost-effective precursors: “textile”-grade PAN, polyolefin, lignin. →Low cost, high volume composite production →Need of cost-effective matrix + resistance to chemicals and corrosion for specific applications →Targeted organic matrices: vinyl ester, polyester, thermoplatics (PolyEthyene, Thermoplastic PolyUrethane, Nylon).
Pre-treatment Oxidation Carbonization Surface
treatment Sizing Spooling
Typical PAN-based carbon fiber production line
Oxidation Stage: Four identical furnaces operated at
different temperatures
POST-TREATMENT 3
Surface treatment:
•remove low cohesion pyrolytic carbon layers remaining from carbonization step
•generate oxygen-containing and nitrogen-containing functional groups to increase interactions with matrix (wetting, Van der Waals interactions, covalent bonding)
Sizing: polymer layer
•Process-ability for composite manufacturing
•Protect fiber during handling
•Conservative design approach → compatible with matrix (example: epoxy matrix →blend of epoxy monomers and additives)
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Carbon fiber production: post-treatment
Need of surface treatment and sizing specifically designed for each type of matrix.
InterLaminar Shear Strength Carbon fibers: IM7 with commercial epoxy sizing Unidirectional, 60 vol. %
Very low values
Interface/interphase in carbon fiber composites
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Nano-analysis on the structure and chemical composition of the interphase region in carbon fiber composite Q. Wu, M. Li, Y. Gu, Y. Li, Z. Zhang Composites Part A 56 (2014) 143-149
Origin: interactions carbon fiber surface -monomer/oligomer of the matrix ≠ monomer-monomer (oligomer/oligomer) interactions. → Interphase: region of the matrix with composition, molecular structure and properties (mechanical) different from the bulk.
Strategic region: stress is transferred from the matrix to the fibers.
Composite properties depend on the mechanical properties of its constituent but also on the mechanical properties of the interphase.
Example: Carbon fiber (commercial sizing)-epoxy matrix →Curing agent of matrix diffuses in epoxy sizing →Gradient of curing agent concentration in interphase region
Adhesion of Graphite Fibers to Epoxy Matrices: II. The Effect of Fiber Finish L. T. Drzal, M. J. Rich, M. F. Koenig, P. F. Lloyd The Journal of Adhesion 16 (1983) 133-152
Adhesion tests
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FIFSS
Single fiber push-in test
Unidirectional composites
• 90º flexural strength • InterLaminar Shear Strength (ILSS)-Short beam shear test
Tensile test on pure resin with bi-axial extensometer
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Influence of the preferential adsorption of some constituents of the initiating
system on the carbon fiber surface (depletion in interphase region)
Composite system: • Fibers: AS4 surface treated and non sized (Hexcel) • Matrix: Derakane 411-C50 • Initiating system:
• Initiator: cumene hydroperoxide (CHP) • Promoter: cobalt naphtenate (CoNap) • Catalyst: dimethylaniline (DMA)
Thermal program: 1h room T + 1h 90ºC + 1h 125ºC.
• No effect considering concentrations of CHP between 1.0 and 2.0 wt. %. • Most important parameter/mechanical properties of vinyl ester resins: thermal program. • Similar shear modulus in interphase region → same IFSS.
CHP
concentration
(wt. %)
CoNap
concentration
(wt. %)
DMA
concentration
(wt. %)
IFSS (MPa)
Push-in test
1.0 0.3 0.10 50 ± 15
1.5 0.3 0.10 54 ± 15
2.0 0.3 0.10 52 ± 15
2.0 0.0 0.10 42 ± 8
2.0 0.1 0.10 52 ± 4
2.0 0.2 0.10 46 ± 4
2.0 0.3 0.10 50 ± 14
2.0 0.3 0.00 56 ± 7
2.0 0.3 0.05 51 ± 5
2.0 0.3 0.08 55 ± 8
2.0 0.3 0.10 51 ± 12
Same values
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Optimization of surface treatment (1)
Atomic Force Microscopy (AFM)
Increase of mechanical interlocking
Fibers: Panex 35 (Zoltek) Continuous treatment in ozone-based gas reactive phase: Thermo-Chemical Treatment (TCT)
Comparison with commercial Electrochemical Treatment (ET)
X-ray Photoelectron Spectroscopy (XPS)
Surface relative densities
Preferential generation of COOH and -OH
% C % O % N
No surface treatment 98 2 < 1
ET 88 7 5
TCT 77 20 3
Better topography and better surface chemistry in order to improve interfacial adhesion.
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Optimization of surface treatment (2)
Composite systems: • Fibers: 50 vol. %
• Vinyl ester matrix: Derakane 782 + 1.5 wt. % tert-butylperoxybenzoate
Thermal program: 1h 150 ºC
• Epoxy matrix: Epon 828 + Epikure W (26.4 phr) Thermal program: 2 h 85 °C + 2 h 149 °C
90º flexural strength
ILSS: epoxy matrix ILSS: vinyl ester matrix
Sharp improvement of interfacial adhesion with epoxy matrix, moderate with vinyl ester matrix.
Fracture profile-90º flexural test
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* 30 *5 000 Untreated
TCT * 30
TCT
* 30
*5 000
*5 000
* 500
* 500
*500
Untreated
TCT
* 30
* 500
* 30 *5 000
*5 000
Epoxy
Vinyl ester
Interfacial rupture
Interfacial rupture
Cohesive rupture
Mixed interfacial-cohesive rupture
Epoxy
Vinyl ester
Adding of maleic anhydride in the vinyl ester matrix
Composite system 90º flexural
strength (MPa) ILSS (MPa)
AS4-Derakane 510A-40 (A) 30 ± 3 65 ± 3
O3 surf. treat. AS4-Derakane 510A-40
(B) 36 ± 3 71 ± 4
AS4-Derakane 510A-40 + 5wt. %
maleic anhydride (C) 39 ± 4 74 ± 2
O3 surf. treat. AS4-Derakane 510A-40
+ 5wt. % maleic anhydride (D) 54 ± 5 77 ± 3
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• Generation of covalent bonding at the interface • Synergy if carbon fiber surface rich in –OH groups
90º flexural test-fracture profiles
Supplemental improvement of interfacial adhesion with the adding of a compatible reactive monomer in the matrix.
Fibers: AS4 surface treated and non-sized (50 vol. %) Matrix: Derakane 510A-40 + 1.5 wt.% cumene hydroperoxide Thermal program: 2h 150ºC + 2h 170ºC
Carbon fiber surface-vinyl ester matrix interfacial interactions
Preferential adsorption of styrene at the surface of the fibers Molecular dynamics simulations of vinyl ester resin monomer interactions with a pristine vapour-grown nanofiber and their implications for composite interphase formation Nouranian S, Jang C, Lacy TE, Gwaltney SR, Toghiani H, Pittman Jr CU Carbon 2011;49:3219–32.
Still lower compared to carbon fiber-epoxy composites → other parameter affecting interfacial adhesion and specific to vinyl ester resins.
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Is it only about interactions at the interface ?
Cure volume shrinkage of the matrix during curing
• Epoxy resins: 3-4 %
• Vinyl ester resins: 7-9 % (poly ester resins: 11-13 %)
→ Influence of cure volume shrinkage on interfacial adhesion?
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• Equivalent chemistry, different values of cure volume shrinkage • Measurement of interfacial adhesion → push-in test (IFSS)
Fuchem 891 Derakane 411-350
Composition
(%)
CHP 1.4 -- -- 2.0 2.0 --
MEKP - 2.0 2.0 -- -- 2.85
CoNap 0.2 0.1 0.1 0.3 0.3 0.10
Thermal program 1 h room T + 1h
90 ºC + 1.5 h
125 ºC
1 h room T + 1h
90 ºC + 1.5 h
125 ºC
Room T
1 h room T + 1h
90 ºC + 1.5 h
125 ºC
Room T
1 h room T + 1h
90 ºC + 1.5 h
125 ºC
Tensile modulus (GPa) 3.28 ± 0.05 3.36 ± 0.03 3.13 ± 0.04 3.30 ± 0.05 2.88 ± 0.07 3.24 ± 0.04
Poisson ratio 0.388 ± 0.080 0.436 ± 0.022 0.441 ± 0.018 0.382 ± 0.040 0.434 ± 0.090 0.403 ± 0.010
Shear modulus (GPa) 1.181 ± 0.020 1.170 ± 0.015 1.086 ± 0.011 1.180 ± 0.010 1.005 ± 0.027 1.155 ± 0.008
Cure volume shrinkage S (%) 3.3 ± 0.2 2.9 ± 0.2 2.3 ± 0.2 8.2 ± 0.2 6.4 ± 0.2 8.4 ± 0.2
Properties Derakane
411-350 Fuchem 891
Vinyl ester monomer molecular weight (g.mol-1) 907 1500-2000
Styrene concentration (wt. %) 45 35
Tensile modulus (GPa) 3.20 3.12
Tensile strength (MPa) 86 75
Tensile elongation at break (%) 5-6 4
dc: density of cured resin (displacement method)
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• Negative influence of cure volume shrinkage if > 6 % (radial compression stress at the interface). • Effect counteracted by partially cured epoxy coating (diffusion of styrene in epoxy coating that makes it
swell)-generation of InterPenetrating Networks (IPN) epoxy coating/vinyl ester matrix.
Influence of matrix cure volume shrinkage on interfacial adhesion
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Interpenetrating
networks
Carbon fiber
Vinyl ester matrix
Epoxy coating
Covalent bond
Increase of the surface density of covalent bonds with the carbon fiber surface (better compatibility between the chemistries of epoxy resins and the carbon fiber surface)
Creation of interpenetrated networks to convey the improvement of adhesion with the carbon fiber surface to the matrix + counteracting the influence of the cure volume shrinkage of the matrix + hindering the preferential adsorption of some constituents of the matrix on the carbon fiber surface
Improvement of interfacial adhesion by generation of InterPenetrating Networks (IPN)
Obtaining sizing optimized conditions
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Effect of coating solution concentration on quality of coating SEM observations
Coating thickness → TGA analysis Fraction conversion of epoxide groups → kinetics study by FTIR spectroscopy
Bridges between single fibers, concentration too high.
Homogeneous sizing
Coating solution: Epon 828 + Jeffamine T-403 30 wt.% (stoichiometric amount) dissolved in acetone. • Thickness of coating function of concentration of the coating solution. • Fraction conversion of epoxide groups function of time.
Optimization of reactive coating thickness and conversion fraction 90º flexural strength
InterLaminar Shear Strength Conversion fraction
Conversion fraction 17
90º flexural test-fracture profiles
Surface treated, no coating
Surface treated, optimized coating
Using concentrations of curing agent below the stoichiometric amount
Issue with using stoichiometric amounts of curing agent
→ coating keeps curing until full cure → not scalable for industrial production (tow hard
and not flexible)
Concentration of Jeffamine T-403 in
Epon 828 (wt. %) 7.5 10 12 13.5 15
Fraction conversion of epoxy coating
(determined by FTIR) 0.21 0.25 0.38 0.41 0.50
Thickness (nm) 112 136 142 152 176
ILSS (MPa) 90 ± 3 92 ±3 93 ±2 97 ± 2 83 ± 3
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Concentration of Jeffamine T-403 in
Neoxil 5716 (wt. %) 10.9 17.2 13.5 13.5
Ozone based surface treatment
applied to the carbon fibers No No Yes Yes
ILSS (MPa) 84 ± 4 88 ± 2 91 ± 2 88 ± 2
Use of a water-based coating solution for industrial applications
Approach with partially cured epoxy coating affective and totally scalable.
High values of ILSS obtained with water-based coating solution.
Selection of epoxy curing agent to generate extra covalent bonding in
the inter-diffusion zone
Curing agent 90º flexural strength
(MPa)
90º flexural modulus
(GPa) ILSS (MPa)
Jeffamine T-403 52 ± 6 8.0 ± 0.2 75 ± 4
Maleic anhydride 76 ± 5 8.7± 0.1 80 ± 1
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Partially cured epoxy coating with maleic anhydride and triethylamine as catalyst
90º flexural test-fracture profiles
Curing agent: Jeffamine T-403 Curing agent: Maleic anhydride
Further improvement of interfacial adhesion by selecting an epoxy curing agent also reactive during the polymerization of the vinyl ester matrix Conditions not optimized → potential for further improvement.
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Carbon fiber surface-vinyl ester matrix interfacial interactions
when using a reactive epoxy coating.
High values of interfacial adhesion achieved (cohesive rupture). Mechanical properties similar to the ones obtained with an epoxy matrix.
Conclusions
• High level of interfacial adhesion → reactive coating
• Generation of covalent bonding with the fiber surface
• Generation of IPN or covalent bonding with the matrix
• Flexibility in order to counteract stress generated at the interface by
the cure volume shrinkage of the matrix
• Strategy that can be applied to other types of matrices.
• Partially cured epoxy coating simple and effective
approach, potentially scalable for carbon fiber industrial
production.
• New potential applications for carbon fiber-vinyl ester
composites for which mechanical properties are important.
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Acknowledgements
• US Office of Naval Research (Y. Rajapakse)
• Florida Atlantic University (R. Granata)
• U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies
• Mr. Truman Bond and RMX Technologies Co.
• Zoltek
• Hexcel
• Ashland
• Hexion
• Huntsman
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