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

026496.0ln01862.0875696.0

2

f

n

f

m

f d

d

E

G

d

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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Thank you for your attention!

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