improving adhesive bonding of composites through surface...

The Joint Advanced Materials and Structures Center of Excellence

Improving Adhesive Bonding of Improving Adhesive Bonding of Composites Through Surface Composites Through Surface

CharacterizationCharacterization

Brian D. FlinnBrian D. Flinn

The Joint Advanced Materials and Structures Center of ExcellenceUniversity of WashingtonUniversity of Washington

2

Improving Adhesive Bonding of Composites

• Motivation and Key Issues – Adhesive bonding is being used for primary composite structure

in commercial transport aircraft manufacture and repair- surface preparation is a critical step

– Good bonds are produced but questions remain:What are appropriate techniques to inspect surfaces?What are key factors for making a good/poor bond?How to predict material and surface preparation compatibility?

• Objective– Further understand the requirements for surface preparation to

produce strong primary structural composite bonds with different substrates and adhesives


3

Improving Adhesive Bonding of Composites

• Approach– Investigate the effect of various surface preparation

procedures and material systems on the adherend surface chemistry/structure and relate to subsequent bond performance

– Materials & Methods:Carbon Fiber Epoxy 127º C (260º F) &176º C (350º F)Glass Fiber Epoxy 127º C (260º F) Surface Preparation: Sanding and Peel PliesAdhesives: paste & film (127ºC (260ºF) and 176ºC (350ºF))

– CharacterizationSurface Chemistry, SEM, Contact Angle, Mechanical Testing and Fractography


4

FAA Sponsored Project Information

• Principal Investigators & Researchers– Brian D. Flinn (PI)– Fumio Ohuchi (Co-PI)– Molly Phariss (Ph.D. Candidate, UW)– Jeff Satterwhite (Masters student, UW)– John “Jack” Aubin (Masters student, UW)– Conor Keenen (BSc 2008, UW)

• FAA Technical Monitor– Curtis Davies

• Other FAA Personnel Involved– Larry Ilcewicz

• Industry Participation– Boeing: Peter Van Voast, William Grace, Paul Shelly– Precision Fabrics Group, Cytec, Toray, 3M, Henkel, Yokahama

• JAMS Participation– Mark Tuttle: Technical Discussions, Wettability Envelopes – Lloyd Smith (WaSU): Parallel study on durability


Outline

Evaluation of Characterization Techniques• Surface Appearance/Topography• Surface Chemistry• Surface Energy/Wetting• Fracture EvaluationWetting and Fracture are most easily adapted to

factory/repair settings-What information can be obtained?

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Surface Appearance

• Visual Inspection– First step in evaluating surface preparation– Proper peel ply used (color, texture, marking) – Proper removal of peel ply

No visible remnants of peel ply fabricNo damage to substrate

– Proper abrasion (if used)Correct abrasiveRoughnessNo peel ply pattern visible

Inexpensive, Fast- should always be performed


7

Surface Appearance• Scanning Electron Microscope (SEM)

– Detailed view of surface (10-100,000x)Microscopic remnants of peel ply present? –BADAmount of “fresh” matrix exposed- Good

– Limited to lab/process development settingsNot portable, trained operator, not cheap

Cytec 970-dry polyester Cytec 970-dry nylon Cytec 970-wet polyester


8

Surface Chemistry

• X-Ray Photo Spectroscopy (XPS/ESCA)– Chemical composition & functional groups of surface– Sensitive to small amounts of contamination– Critical tool for research- NOT for manufacture/repair– Expensive, time consuming, small specimen size


9

Surface Energy

• Contact Angle Measurement/Wetting– Theory predicts surface energy of substrate

and adhesive related to bond quality– Contact Angle measurement adaptable to

factory and repair environmentsQuickPortable equipment available

– Does it WORK???e.g water break test


10

Surface Energy

• Young’s equation: θ

is contact angle

• Complete wetting means θ

approaches zero• Contaminants usually lower the solid’s surface energy (increase θ)• Surface preparations try to increase the solid’s surface energy and

clean off contaminants

Illustration of Young’s equation – drop of liquid on a solid surface

θγγγ coslvslsv +=

Can surface energy guide bonding?

γsl γsv

γlv

solidliquid

vapor

θ


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Surface Wetting

Weak boundary layer due to voids and contaminants

Figure from Pocius, A., Adhesion and Adhesives Technology: An Introduction, 2nd ed., 2002, Hanser Gardner, New York.

Wetting required for good bond- intimate contact


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Surface Wetting

• Surface energy can be used to develop wettability envelops to predict wetting of substrate by adhesive

Highest energy surface…Good wetting-small θ

…lower…

…even lower… …lowest.Poor wetting-large θ

Figure from Pocius, A., Adhesion and Adhesives Technology: An Introduction, 2nd ed., 2002, Hanser Gardner, New York.

A liquid epoxy adhesive is on each surface.


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Wettability envelopes showed the difference in the prepared surfaces.

• Fluids inside the envelope will wet spontaneously– Critical condition for

bonding? • Wettability envelopes a

potential method to determine suitability of a surface for bonding

• Epoxy adhesives* on boundary for nylon prepared BMS8-276 surfaces

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0 5 10 15 20 25 30 35

Polar Component

Dis

pers

ive C

om

ponent

60001

52006

SRB

Epoxy Adhesives*

* Literature values for aerospace epoxies-

Curves generated using WET program (M. Tuttle)

BMS 8-276


• Example to show contamination of adherend by nitrile gloves• Materials & Surface Preparation

– Hexcel T300/F155– Orbital sanded with 100 grit Al2 O3 sand paper – Roughness, 1.194 µm (47µin) to 1.549 µm (61µin)– Cleaned with compressed air – Contact with external glove surface– Adhesive Henkel EA9394

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Surface Energy-Detecting Contamination

Profilometer

measuring surface roughnessOrbital sanding Contacting surface with glove

The Joint Advanced Materials and Structures Center of ExcellenceUniversity of WashingtonUniversity of Washington 15

Video


No Glove Kleen

Guard Titan NDEX

Glove surface NA 29.1º 92.2º 104.2º

Composite surface 78.9º 69.8º 109.0º 115.3º

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Contact Angle and Wettability Envelopes

Average contact angle of DI water on glove surfaces compared to glove contaminated composite surface

Contact angle of DI water on glove surfaces.

TitanKleen

Guard NDEX


Sample Polar component Dispersive Component

Total surface energy

Kleen

Guard 9.6 (mN/m) 41.0 (mN/m) 50.6 (mN/m)

NDEX .01 (mN/m) 38.8 (mN/m) 38.81 (mN/m)

Titan .6 (mN/m) 35.3 (mN/m) 35.9 (mN/m)

No Glove .2 (mN/m) 73.3 (mN/m) 73.5 (mN/m)

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Surface energies of composite surfaces.

Surface energy decreased after contact with gloves



Wettability envelopes for composite surfaces with various glove exposures


X-Ray Photoelectron Spectroscopy

BINDING ENERGY - EV1107.3 1007.3 907.3 807.3 707.3 607.3 507.3 407.3 307.3 207.3 107.3

CO

UN

TS

3.8K

3.6K3.4K

3.2K3K

2.8K

2.6K2.4K

2.2K2K

1.8K

1.6K1.4K1.2K

1K800

600400200 B

r 3p

Br 3

s

Br 3

d

Sb 4

d

C 1

s

Mg

a

O 1

s

K 2s

N 1

s

Sb 3

d3

Sb 3

p3

Sb 3

p1O a

BINDING ENERGY - EV1107.3 1007.3 907.3 807.3 707.3 607.3 507.3 407.3 307.3 207.3 107.3

CO

UN

TS

6K

5.5K

5K

4.5K

4K

3.5K

3K

2.5K

2K

1.5K

1K

500

Si 2

s

Si 2

p

N 1

s

Ca

2s Ca

2p

O 1

s

O a

C 1

s

XPS spectrum for the as-

abraded (No Glove) composite

surface.

XPS spectrum for composite surface exposed to the NDEX

glove.

C C

N

O

ON


• Reduced oxygen* and nitrogen on contaminated surfaces.

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XPS continued…

Elemental composition (atomic %) as determined by XPS.

* Kleen

Guard has higher O, but also Ca, possibly CaCO3

C O N Ca S ClNo Glove 78.6 15.35 4.7 0.9

Kleen

Guard 60.65 26.9 4.3 4.05 1.45 1.9

Titan 90.2 7.05 0.65 0.95 0.55

NDEX 93.9 4.9 0.9 0.5


• NDEX glove contaminated surface shows reduced fracture toughness

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Fracture Toughness

The average strain energy release rate from five DCB tests/sample.


Specimen specimen 1 specimen 2 specimen 3 specimen 4 specimen 5Kleen

Guard

Interlaminar/ Cohesive





NDEX Adhesion Interlaminar/ Cohesive

Adhesion Adhesion Interlaminar/ Cohesive

Titan Interlaminar/ Cohesive





No Glove Cohesive Cohesive Cohesive Cohesive Interlaminar/ Cohesive

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Mode of Failure

• Glove contamination changed failure mode of DCB specimens• NDEX gloves –

3 failures in adhesion


Modes of Failure

Example of interlaminar/cohesion failure (left) and adhesion failure (Right).

No Glove NDEX Glove


Objectives◦

To investigate nitrile gloves as a potential source of contamination on composites surfaces prior to bonding.

◦

To further understand the relationship between surface energy and bond strength.

Results◦

Certain brands of gloves did reduce the surface energy of a composite surfaces after contact

◦

In some cases the lower surface energy was reflected in reduced average GIC and a change in the mode of failure.

◦

This research further supports the concept that spontaneous wetting is necessary but not adequate for a quality bond

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Summary- Glove Study


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Surface Energy

• Important Considerations– Theory considers a uniform surface– Are composite surfaces uniform?

Peel Ply TextureFiber orientationVariations in surface roughness

– Do these factors influence contact angle?


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Surface Energy

• Peel Ply Texture- effects contact angle

Uniform surfacepeel ply surface


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Surface Energy

• Fiber Orientation effects contact angle, θ– surface energy of fiber is less than matrix– fluid spreads along matrix

easier than fiber– θfiber

>θmatrix

γm

> γf


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Surface Energy

• Our research has shown wetting is a necessary but not always sufficient condition for a strong bond.

• Contact angle has been shown to detect changes in surfaces due to contamination

• Proper choice of fluids critical• May have surface energy anisotropy

– Peel Ply Texture– Fiber orientation– How to account for that?


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Surface Energy

• Need to evaluate other surface energy theories– Lewis Acid Base vs. Polar-Dispersive model

• Need to evaluate surface energy at cure temperature

• Potential Applications– Compare with previously characterized surfaces– Monitoring of surfaces for bonding– Verification of surface preparation

RepairManufacturing


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Fracture Evaluation

• The only sure way to measure bond quality is to break it– Not very good business plan– Use process control to ensure reproducible bond quality– Standard Specimens: Lap Shear and DCB

Time & material consuming, requires test frame

– Need for a “quick” testRapid Adhesion Test (RAT) methodInstrumented RAT (i-RAT)


Fracture Evaluation

• Failure mode and relation to bond strength: – Adhesion

(weak bond)х

Considered unacceptable in aerospace– Cohesion

(bond as strong as adhesive itself)Acceptable

– Interlaminar

(bond as strong as laminate itself)Acceptable

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Frac

ture

Ene

rgy

Failure Mode

Adhesion

Cohesive

Interlaminar↑

or↓

Composite

Adhesiveinterfacial/adhesion

cohesive in adhesive

Composite

cohesive in adherend/interlaminar


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Rapid Adhesion Test (RAT) Method

– A quick, low cost test to assess the adhesion– A modification of metal-to-metal peel test– The backing adherend clamped to while the

peeling adherend is removed– Failure mode representative of bond

Adhesion Failure-Poor BondCohesive Failure-Strong Bond

– Failure modes correlate with DCB test with ~90% less cost and flow time

Adhesive filmFEP crack starterBacking adherend (0.063” Al- PAA)

Peeling adherend (0.020” Al PAA+ single ply of composite- peel ply surface)

RAT Specimen Geometry


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RAT Method Assessment

Cohesive failure

(left) vs. Adhesion failure

(right)

Peel ply patternGlass Fabric pattern

FEP starter crack FEP starter crack


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RAT Method Assessment

Cohesive failure here (OK)

Failure in adhesion here (BAD)

Mixed failure mode of RAT specimen (looking at substrate surface after peeling off adhesive); Cytec Cycom MXB 7701/7781 – P 60001 – Cytec FMx209

Failure in adhesion (Peel ply prepared surface still visible)

Resin material

Glass fiber tows

Cohesive failure


Instrumented RAT (i-RAT)

• Need for quantitative comparison of bond “strength”• Produce a quicker, less expensive method of generating

a quantitative measure of bond “strength” through the development of the i-RAT test method

• Use i-RAT data to evaluate different composite/adhesive/peel ply combinations in an array of composite bonds

• Quickly and easily identify mode of failure and relative strength in bonded samples to screen samples for further testing (DCB, Lap Shear, etc.)

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i-RAT Methodology

• Climbing Drum Peel (CDP) test– Primarily used to measure peel torque values for the removal of material from a surface

Composite facesheet from honeycombPeel ply from composite substrate

– Test drum is “rolled” up compositesurface, removing peel ply that hasbeen clamped to the drum

• i-RAT Test MethodCDP fixtureLoad vs. displacement recordedPeel torque calculatedRelation to bond strength or GIC

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• Mechanical Testing -

Data

Common Plot of load vs. displacement for a CDP test specimen used to calculate peel torque values. [6]

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Review of CDP

• Data analysis– The P2 value is a measure of the applied load necessary to:

• A: Overcome the drum rolling up the peel material– Equal to P1 and determined from calibration run

• B: Induce failure in the bonded joint• C: Bend the peel material

– Zero if no load required to bend peel material (ex: peel ply)– The value for P = (P2 -P1 ), in units of lbf, can be converted into a

peel torque value, in units of J/m2, by a simple equation relating drum diameter to sample geometry


i-RAT Methodology

• Instrumented Rapid Adhesion Test (i-RAT)– Intermediary test between RAT and DCP or Lap Shear testing– Combination of RAT test with Climbing Drum Peel (CDP) test

method and fixture (ASTM D 1781-98)– Attain quantitative values for bond strength– Same quick sample prep and cost benefits as with RAT method

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1 student evaluated 9 combinations in 1 day (including lay up, & 2 autoclave cure cycles)


Fractography

Peel ply patternFabric pattern

Cohesive failure, good (left) vs. Adhesion failure, bad (right)

Hexcel BMS 8-79 with 3M AF163-2 Adhesive

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Substrate -

cohesion failure Substrate -

adhesion failure

Adhesive-adhesion failure


i-RAT Results

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i-RAT Bond Evaluation Method

• Advantages– Inexpensive, quick test method for ranking bond strength– Yields fracture surface for characterization

Adhesion vs. Cohesive– Quick screening process for down-selecting bonded

samples for further testing

• Limitations– Bending/Plastic deformation of Al backing

MeasurableAccountable in determining “toughness” value?

– Failure mode influence by specimen geometry? Differing compliance of adherends

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Summary

• No Single method to characterize surfaces to ensure a good bond

• XPS and SEM are valuable tools • Contact angle/wetting can NOT predict bond quality • Contact angle/wetting can detect contamination• Strict process control is required (even type of gloves)• Simple peel tests can be used to quickly (and at much

lower cost) evaluate bond quality• Of course… more research is needed


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A Look Forward

• Benefit to Aviation– Better understanding of peel ply surface prep.– Guide development of QA methods for surface prep.– Greater confidence in adhesive bonds

• Future needs– Surface energy (wetting) vs. bond quality– Surface energy at cure temperature– QA method to ensure proper surface for bonding– Applicability to other composite and adhesive systems– Model to guide bonding based on characterization,

surface prep. and material properties


Acknowledgements

• Funding FAA JAMS-AMTAS• Peter Van Voast & Will Grace at The Boeing

Company• Mark Tuttle for his advice and “WET” software

utility• Material donations from Cytec-Cycom, Toray

Composites America, Airtech International, Henkel, Richmond Aerospace and Precision Fabrics


QUESTIONS ?

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Improving Adhesive Bonding of Composites Through Surface CharacterizationImproving Adhesive Bonding of CompositesImproving Adhesive Bonding of CompositesFAA Sponsored Project InformationOutline Surface AppearanceSurface AppearanceSurface ChemistrySurface EnergySurface EnergySurface WettingSurface WettingWettability envelopes showed the difference in the prepared surfaces.Surface Energy-Detecting ContaminationVideoContact Angle and Wettability EnvelopesContact Angle and Wettability Envelopes Contact Angle and Wettability EnvelopesX-Ray Photoelectron Spectroscopy XPS continued…Fracture Toughness Mode of FailureModes of FailureSummary- Glove StudySurface EnergySurface EnergySurface EnergySurface EnergySurface EnergyFracture EvaluationFracture EvaluationRapid Adhesion Test (RAT) MethodRAT Method AssessmentRAT Method AssessmentInstrumented RAT (i-RAT)i-RAT MethodologySlide Number 37i-RAT MethodologyFractographyi-RAT Resultsi-RAT Bond Evaluation MethodSummaryA Look ForwardAcknowledgementsQUESTIONS ?

improving adhesive bonding of composites through surface...

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