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SURFACE AND INTERFACIAL ENGINEERING OF FAST CURING

EPOXY/CARBON FIBER COMPOSITES

M. J. Rich, P. Askeland, E. Drown and L. T. DrzalComposite Materials and Structures Center

Michigan State University, East Lansing, MI 48824

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Background– Manufacture of fiber reinforced composites for vehicles requires

production rates of around 1 part per minute– The use of mold release is critical to achieving the speeds of this

manufacturing operation– External releases require removal of mold release and preparation of

the composite surface for adhesive joining and painting– Recent developments have led to the incorporation of “internal mold

release (IMR)” as part of the matrix formulation– A critical but unanswered question is if and how an IMR affects the

properties of these IMR formulated composites at the internal and external composite surface and interfaces

– AND if so, what options are available for IMR mitigation

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interface

surface

surface

interface

surface

Surfaces and Interfaces in Manufactured Composite

surface

surface

surface

surface

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CF-Epoxy Adhesion - CFRP Tension Test

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CF-Epoxy Adhesion - CFRP Compression Test

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CF-Epoxy Adhesion - CFRP Shear Test

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CF-Epoxy Adhesion - CFRP Fracture Test

Drzal et al., Fibre-Matrix Adhesion and Its Relationship to Composite Mechanical Properties J. Material. Sci., 28 596-610 (1993)

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Adhesives and JoiningObjective: This Task will address issues associated with adhesive bonding and painting of composites formulated with an Internal Mold Release (+IMR). Surface and Interfacial investigation of the issues associated with adhesive bonding and painting of composites.

Rationale: The manufacturing of carbon fiber composite parts for vehicles using the new generation of Dow fast-curing epoxies requires the incorporation of an Internal Mold Release (+IMR) to achieve the fast cycle times required for vehicle adoption.

The IMR concentrates at surfaces and interfaces as part of its role in the formulation. This can include the carbon fiber surface; the interface with the epoxy, the prepreg surface, the interlaminar surfaces and the external surface of the cured CFRC.

This task is to determine the identification, location, concentration, and effect on the mechanical properties, adhesion, durability and potential remediation strategies of the IMR to insure that the CFRC properties are optimum.

Goal: Determine the effect on: CF-matrix adhesion with (+IMR) and without (w/o IMR); Surface of the cured CFRP with (+IMR) and without (w/o IMR): Effect on multimaterial joining and paint adhesionIdentify Surface Treatment methods to remediate surfaces if required.

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Adhesives and Joining - APPROACH(1) Manufacture 30cm x 30cm compression molded CF‐Epoxy plaques (2) Conduct ASTM standard tensile strength, tensile modulus, flexural strength, flexural modulus, 

short beam shear, transverse flexure and end‐notched flexure fracture tests(3) Conduct SEM examination of the fracture surface to identify differences between the +IMR 

epoxy system and the w/o IMR epoxy system. (4) Conduct X‐ray Photoelectron Spectroscopic analysis of the plaque surface and internal 

fracture surface to identify the presence of and location of the internal mold release (IMR). (5) Conduct contact angle measurements on the cured plaque surfaces of the epoxy +IMR and 

w/o IMR.(6) Evaluate CF‐Epoxy adhesion and failure mode.  (7) Evaluate lap shear adhesive joints‐using automotive grade aluminum and steel surfaces 

bonded to the epoxy adhesive +IMR and w/o IMR.  (8) Evaluate paint adhesion in cured CF‐epoxy +IMR and w/o IMR.(9) Evaluate adhesive bonded specimens and painted specimens to Ford specifications for Salt, 

Wet and elevated temp ‐8 week conditioning.  (10)Conduct stress durability testing of the adhesive joints according to the Ford Laboratory Test 

Method (MSU)(11)Submit final report on the Adhesive and Joining Task (MSU)

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interface

surface

surface

interface

surface

Surfaces and Interfaces in Manufactured Composite

surface

surface

surface

surface

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X-Ray Photoelectron Spectroscopy

• Spectroscopic technique to examine up to the top 10Å of a surface

• Provides elemental and molecular information

h

Ephoton = h

h W + ½ mv2

W work function

½ mv2 kinetic energy of emitted electron

XPS Cured CFRP Composite ResultsSample Carbon Oxygen Nitrogenw/o IMR 78.0 16.1 5.9+ IMR  80.3 13.6 6.1

w/o‐IMR+ IMR

Differentially chargedcarbon fibers?

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Voltage Contrast XPS• Technique takes advantage of difference in conductivity between carbon fiber (Cond) and

sizing or matrix (non-Cond).

• Applying a bias via an electron flood gun will shift insulating materials to lower binding energies due to charging.

• The conductive carbon fiber’s peak will remain unchanged.

• The matrix and carbon fiber can then be separated and analyzed independently.

270275280285290295300

cps

Binding Energy (eV)

V offVE + CF

V=10 eV

CF

VE

E.g. AS4/Vinyl Ester Fracture surface

• Examination of unbiased fracture surface, shows one C1s peak---the carbon fiber and vinyl peaks overlap.

• Unable to differentiate the carbon fiber from the matrix.

• After a bias (10 eV) is applied, the vinyl ester peak shifts to a lower binding energy.

• Both components can be quantified.

0 Volts ‐8 Volts

matrix carbon

carbon fiber carbon

26.9% Carbon from matrix

Voltage Contrast XPS  + IMR

0 Volts

270275280285290295

200

400

600

800

1000

1200

1400

1600

1800

2000

17XPS1800.spe

Binding Energy (eV)

c/s

Pos. Sep. %Area279.24 0.00 6.23280.74 1.50 16.94282.24 3.00 14.79283.74 4.50 13.52284.87 5.63 41.96286.37 7.13 6.56287.87 8.63 0.00

‐8 Volts

carbon fiber carbon

matrix carbon

51.5% Carbon from matrix

VCXPS Cured CFRP Composite Results

Voltage Contrast XPS  w/o IMR

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Solid (S)

Vapor (V)

Equilibrium Contact Angle

0

0

0

( ) ( cos )

lim 0

( ) cos

SSL LVSV

S

A

SL LVSV

G A A

GA

SV SL

LV

cosLVSLSV

Young’s Equation

Most common application is to measure (SV), the solid surface energy.Contact angles can be measured experimentally, but interfacial tension (SL) is an unknown quantity and difficult to measure directly!

Note: subscript SV => solid in equilibrium with vapor. This implies an adsorbed film of vapor on the solid. A ‘film pressure’ term (o) is ideally subtracted from the LHS of Young’s equation above, but is small enough to be ignored for many applications.

Liquid (L)

Wettability of IMR CFRP and non-IMR CFRP Surfaces - Contact angle

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting (wettability) was measured by the solid-liquid contact angle made at the edge of a sessile drop of water resting at equilibrium on the solid epoxy surface with and without the IMR.

Solids with high surface energy (i.e. contact angles less than 900) e.g. clean metals, metal oxides, oxidized polymers

Solids with low surface energy (i.e. contact angles greater than 900) e.g. Hydrocarbon polymers, fluorinated materials, waxes, mold release

contact angle < 90o(wettable)

contact angle > 90o(non‐wettable)

P6300 (w/o IMR)63.0o +/‐ 8.3

CFRP (w/o IMR)97.0o +/‐ 2.1

M6400 (+IMR)102o +/‐ 1.0CFRP (+IMR)104o +/‐ 2.5

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ASTM D30300o & 90o Tensile Properties

, , Ey

ASTM D2344Short-Beam Shear

ASTM D790/D72640o & 90o Flexural Properties

, , Ey

ASTM D7905 Mode II Interlaminar Fracture

Toughness

CF-Epoxy Adhesion Dependent CFRP Properties

ScrapSpecimen0°

0° Tension

SBS

0°Fl

ex

Lap Shear

90° Tension

90° Flex

0°

Lap Shear

LapShear

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CF-Epoxy Composite Mechanical Properties w/o IMR + IMR

0° Tensile Modulus (GPa) 116 ± 4.9 140 ± 2.8

0° Tensile Strength (GPa) 1.69 ± 0.07 2.32 ± 0.08

0° Flexural Modulus (GPa) 113 ± 2.7 139 ± 2.4

0° Flexural Strength (GPa) 1.75 ± 0.08 2.18 ± 0.09

90° Tensile Modulus (GPa) 8.8 ± 1.5 8.9 ± 0.8

90o Tensile Strength (MPa) 43.8 ± 5.1 40.5 ± 1.2

90° Flexural Modulus (GPa) 8.3 ± 0.08 9.6 ± 0.2

90o Flexural Strength (MPa) 110 ± 2.2 85.6 ± 5.3

Short Beam Shear (MPa) 90.5 ± 1.5 84.7 ± 4.9

GIC Mode I Fracture Tough (KJ/m2) 0.69 ± 0.2 0.64 ± 0.2

Sample Test Density +/-LMG-061617-1 No IMR 0o Flex & 0o Tensile 1.5290 0.0004LMG-061617-2 No IMR 90o Flex & 90o Tensile 1.5192 0.0004LMG-061617-08 with IMR 0o Flex & 0o Tensile 1.5748 0.0002LMG-011617-09 with IMR 90o Flex & 90o Tensile 1.5724 0.0002

The differences in the composite properties between the samples might also be the result of differences in the volume fraction of the fibers. Those measurements are underway. However, density measurements (which can be affected by voids) show differences between the samples with and without the IMR:

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Composite Failure Modes – 0o Tension

w/o IMR +IMR

Composite Failure Modes – 90° Tension

w/o IMRFailure occurred at various locations,

some did intersect foil inclusions.

+IMRAll failures occurred inside the tabs,

within 2mm of the gage section

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Composite Failure Modes - Flexure

0° Flexure

Note: Axial and transverse failure, both sets.

90° Flexure

w/o IMRPanel: LMG-061617-01

Material: XPR-4607 roll 1.6

+ IMRPanel: LMG-061617-08

Material: XPR-8324-01 roll 1.9

w/o IMRPanel: LMG-061617-02

Material: XPR-4607 roll 1.6

+ IMRPanel: LMG-061617-09

Material: XPR-8324-01 roll 1.9

Note: Transverse failure only, both sets.

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SEM Results: w/o IMR

Cra

ck d

irect

ion

SEM Results: +IMR

Cra

ck d

irect

ion

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CF-Epoxy Composite Mechanical Properties w/o IMR + IMR

0° Tensile Modulus (GPa) 116 ± 4.9 140 ± 2.8

0° Tensile Strength (GPa) 1.69 ± 0.07 2.32 ± 0.08

0° Flexural Modulus (GPa) 113 ± 2.7 139 ± 2.4

0° Flexural Strength (GPa) 1.75 ± 0.08 2.18 ± 0.09

90° Tensile Modulus (GPa) 8.8 ± 1.5 8.9 ± 0.8

90o Tensile Strength (MPa) 43.8 ± 5.1 40.5 ± 1.2

90° Flexural Modulus (GPa) 8.3 ± 0.08 9.6 ± 0.2

90o Flexural Strength (MPa) 110 ± 2.2 85.6 ± 5.3

Short Beam Shear (MPa) 90.5 ± 1.5 84.7 ± 4.9

GIC Mode I Fracture Tough (KJ/m2) 0.69 ± 0.2 0.64 ± 0.2

Sample Test Density +/-LMG-061617-1 No IMR 0o Flex & 0o Tensile 1.5290 0.0004LMG-061617-2 No IMR 90o Flex & 90o Tensile 1.5192 0.0004LMG-061617-08 with IMR 0o Flex & 0o Tensile 1.5748 0.0002LMG-011617-09 with IMR 90o Flex & 90o Tensile 1.5724 0.0002

The differences in the composite properties between the samples might also be the result of differences in the volume fraction of the fibers. Those measurements are underway. However, density measurements (which can be affected by voids) show differences between the samples with and without the IMR:

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w/o IMR: 0° Flexural & Tensile w/o IMR: 90° Flexural & Tensile

+ IMR: 0° Flexural & Tensile + IMR: 90° Flexural & Tensile

Composite Cross Section and Fracture Surface

Crack direction

Crack direction

w/o IMR: Mode I

+ IMR: Mode I

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PATTI Test for Adhesion for Adhesives, Paints and Films• The P.A.T.T.I.® system applies a true axial (relative to

stub axis) tensile pull test. The tensile values obtained quantitatively measure the bond between a paint, film, coating or adhesive, and the substance substrate. The P.A.T.T.I.® conforms to ASTM D4541, “Pull Off Strength of Coatings Using Portable Adhesion Testers,” is the only self-aligning, pneumatic instrument.

Pull-off stub with cut-off ring to eliminate the meniscus formed by the adhesive.

• Substrates: Epoxy/CF Composites with and with-out IMR in an 8 ply, [±45]2s layup.

• Surface Preparation: Swabbed using a Wipe-All® saturated with isopropyl alcohol, let stand 10 sec wet, wiped dry.

• Adhesive: 2K-Flex at a 10:0.8 ratio of A:B, 375 µm glass beads added (1 % w/w) as bond line spacers. Cure cycles:

– 8 hr @ 60 °C– 8 hr @ 60 °C followed by a high-heat cycle of 10 min @ 190 °C.

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Bonding of 2K-Flex to CFRP Surfaces

0100200300400500600700800900

1000

8 hr @ 60 °C 8 hr @ 60 °C & 10 min @190 ° C

Pull-

off T

ensi

le S

tren

gth

(psi

)

PATTI Testing of 2K-Flex on a CFRP + IMR and w/o IMR

No IMR IMR

w/o IMR: 8 hr @ 60 °C, 10 min @ 190 °Cw/o IMR: 8 hr @ 60 °C

+ IMR: 8 hr @ 60 °C+ IMR: 8 hr @ 60 °C, 10 min @ 190 °C

PATTI Tests on Composite Surfaces Fully Cured 2K Flex Bonded to Composite 

w/o IMR + IMR

Higher Pull‐off Strength  Lower Pull‐off Strength

Cohesive in 2K Flex

Adhesive Failure in 2K‐Flex Composite and Cohesive in 

Composite 

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II. PATTI Testing – Bonding of 2K-Flex to Aluminum

• Substrate: Aluminum sheet, supplied by Ford (May 2018).

• Surface Preparation:– As Received – No cleaning performed, visible oily substance coating the

surface– IPA Cleaned – Swabbed using a Wipe-All® saturated with isopropyl

alcohol, let stand 10 sec wet, wiped dry.

• Adhesive: 2K-Flex at a 10:0.8 ratio of A:B, 375 µm glass beads added (1 % w/w) as bond line spacers. Compounded in a Speed-Mixer® using two cycles of 30 s at 3000 rpm. Materials were kept at 60 °C during handling to allow dispensing & mixing. Four specimens for each condition were prepared.

• Cure cycles: – 8 hr @ 60 °C– 8 hr @ 60 °C followed by a high-heat cycle of 10 min @ 190 °C.

• The aluminum was found to have deformed as a result of the testing.

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II. Bonding of 2K-Flex to Aluminum

MaterialPull-off Tensile Strength (psi) Failure Mode(s)

As Received8 hr @ 60 °C

685 ± 44 Adhesive failure at the aluminumCohesive failure in the 2K-Flex (minor)

As Received8 hr @ 60 °C /10/min@190°C

795 ± 25 Cohesive failure in the 2K-FlexAdhesive failure at the aluminum (minor)

IPA Cleaned8 hr @ 60 °C

792 ± 47 Adhesive failure at the aluminumCohesive failure in the 2K-Flex (minor)

IPA Cleaned8 hr @ 60 °C / 10 min@190°C

826 ± 40 Cohesive failure in the 2K-Flex

0100200300400500600700800900

1000

8 hr @ 60 °C 8 hr @ 60 °C & 10 min @ 190 ° C

Pull-

off T

ensi

le S

tren

gth

(psi

) As-ReceivedIPA Cleaned

As Received: 8 hr @ 60 °C As Received: +10 min @ 190 °C

IPA Cleaned: 8 hr @ 60 °C IPA Cleaned: +10 min @ 190 °C

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Adhesion to CF/Epoxy Composite Surface• Plasma Treatment of Composite Surfaces for Adhesion Promotion to Paint and Bonded

Joints– PlasmaTreat unit commissioned at MSU and at SURF– Conducting trials to investigate the effects of distance from nozzle and speed – XPS, Contact Angle and PATTI tests underway on composites +IMR and w/o IMR

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

IPA Wipe 2.0 1.0 0.5 0.25

Change in XPS Atomic Ratio with Plasma Treatment SpeedN:C O:C

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Interim Summary - Adhesives and Joining(1) Manufacture 30cm x 30cm compression molded CF‐Epoxy plaques (2) Conduct ASTM standard tensile strength, tensile modulus, flexural strength, flexural modulus, 

short beam shear, transverse flexure and end‐notched flexure fracture tests(3) Conduct SEM examination of the fracture surface to identify differences between the +IMR 

epoxy system and the w/o IMR epoxy system. (4) Conduct X‐ray Photoelectron Spectroscopic analysis of the plaque surface and internal 

fracture surface to identify the presence of and location of the internal mold release (IMR). (5) Conduct contact angle measurements on the cured plaque surfaces of the epoxy +IMR and 

w/o IMR.(6) Evaluate CF‐Epoxy adhesion and failure mode.  (7) Evaluate lap shear adhesive joints‐using automotive grade aluminum and steel surfaces 

bonded to the epoxy adhesive +IMR and w/o IMR.  (8) Evaluate paint adhesion in cured CF‐epoxy +IMR and w/o IMR.(9) Evaluate adhesive bonded specimens and painted specimens to Ford specifications for Salt, 

Wet and elevated temp ‐8 week conditioning.  (10)Conduct stress durability testing of the adhesive joints according to the Ford Laboratory Test 

Method (MSU)(11)Submit final report on the Adhesive and Joining Task 

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Interim Summary of IMR Investigation• IMR concentrates at:

– surface of CF, – interface between CF and cured epoxy, – and cured epoxy surfaces

• IMR can be detected via surface analysis:– XPS, VCXPS, contact angle

• IMR reduces adhesion between:– CF and epoxy– Cured epoxy and adhesive

• IMR affects composite properties:– Improves Interlaminar Consolidation– Enhances 0o CFRP Strength and Modulus– Lowers 90o CFRP Strength, Short Beam Shear and Mode I Fracture toughness

• Surface Treatments (Plasma, OV-O3) have potential to remediate excess IMR