32

1.0 INTRODUCTION

2.0 OBJECTIVE

Fluorescent Surface Coatings for Aerospace Composite Damage Detection Ryan Toivola, University of Washington MSE

1

4.0 RESULTS AND DISCUSSION

3.0 MATERIALS AND METHODS

5.0 CONCLUSIONS & FUTURE WORK

6.0 REFERENCES

ACKNOWLEDGMENTS

Develop a system of aggregate sensitive fluorescent dye molecules compatible with currently used coating systems (primer and topcoat) (Jen Research Group, UW MSE)Utilize custom dyes designed to crosslink into coating network systemsWhen coatings become stressed, dyes aggregate or deaggregate, causing emission change

Damage in aerospace composites can be extremely difficult to detect using surface inspection techniques – Especially Barely Visible Impact Damage (BVID)1

Damage from BVID can cause subsurface damage that affects composite part performance while being undetectable at the part surface2

Current inspection techniques that can detect BVID (ultrasonic C-scan) are costly and require significant aircraft downtime

A technique with the ability to quickly and inexpensively identify impact events and stressed areas in composite parts is very desirable

Fluorescent molecules with deformation-sensitive fluorescent emission behavior have been identified in solid polymersMethodology: Aggregation induced quenching,

emission, or color change

Aggregation sensitivity of dyes (Jen Research Group, UW MSE)Established by changing monomer to dimer

absorbance ratio with changing concentrationTesting done in liquid solution Mechanism varies, but any detectable

change is acceptable

Fluorescent coatings for use in damage detection were developed using customized dyes and existing aerospace coating systems.

Cure Behavior and Thermal properties of coatings were not affected by dye incorporation

One dye species showed observable color and intensity changes when strained; changes were consistent with design

After coating relaxation, strained coatings did not maintain the changesreversible transition

1. Cantwell, W.J., Curtis, P.T., Morton, J., Comp. Sci. & Tech. 25 (1986), 133-48.

2. Poon, C. et.al., Theor. & Appl. Fract. Mech. 13 (1990) 81-97.

3. Yang et.al., J. Appl. Polym. Sci. 82 (2001) 2041-8.

4. DesoPrime 7501 Product Data Sheet, PPG Aerospace, Pittsburgh, PA. www.ppg.com

Project Principal Investigators: Dr. Brian Flinn, Dr. Alex Jen (UW MSE)Project Collaborators: Dr. Sei-Hum Jang, Dr. Zhengwei Shi (UW MSE), Dr. Gary Georgerson (The Boeing Company)Research was funded by The Boeing Company Project Code B8LDLMaterials were donated by PPG Aerospace, Woodinville, WA

Materials: Aerospace Coating systems (PPG Aerospace)Epoxy-Based Primer (Epoxy)

A 2-part liquid cures to a stiff, opaque primer

Polyurethane Topcoat (PU)A 3 part liquid cures to a flexible, clear

topcoat

BVID – subsurface damage not apparent at surface of impacted

composite part

Left: Aggregated (orange) and separated (green) states of dye molecule emission

Right: dyes in solid PMMA, before and after tensile deformation3

Determine if coatings are changed materially by incorporation of the dyesCure Behavior, Thermal-Mechanical Properties

Characterize Fluorescent Response of dyes in coating systems with and without stress Fluorescent Absorbance in liquid pre-polymer coating componentsFluorescent Absorbance in solid polymer coatingFluorescent response of coatings under tensile stress

Materials: Customized Fluorescent Dyes (Jen Research Group, UW MSE)Based on known and newly developed

aggregation-sensitive dye systems Functionalized end groups to allow dyes to

participate in cure of both epoxy and PU systems

Aggregation State

Separated State

Aggregation State

Aggregation State

Separated State

Methods: Coating synthesis Dyes customized for solubility in liquid componentsMixed and spread cast onto various substrates for testing

Methods: Cure and Thermal AnalysisCure behavior analyzed with Differential Scanning Calorimetry (DSC)Glass transition temperature measured via Dynamic Mechanical Analysis (DMA)

Methods: Fluorescence Response testingAbsorbance of dyes in solid PU coating (epoxy too opaque)Observed emission of dyes on PET substrates as tensile stress applied

Coating schematic4

Various custom dyes

Fluorescent coatings with incorporated custom dye moleculesEstablished by measuring absorbance of dye

in solid PU filmsSimilar absorbance behavior to dye in liquid

solvent

-40 -30 -20 -10 0 10 20-2

-1.5

-1

-0.5

0

PU w/P. 6, 3.3x10^-4 mol/L

Epoxy w/p. 7, 1.3x10^-3 mol/L

Temp. (deg.C)

Pro

be

Po

sit

ion

(m

m)

375 425 475 525 575 6250

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

no dye

3.31x10^-4 mol/L

6.62x10^-5 mol/L

3.31x10^-5 mol/L

1.65x10^-5 mol/L

Wavelength (nm)

No

rma

lize

d a

bs

orb

an

ce

Cure Behavior of coatings with incorporated dye moleculesEstablished by measuring residual

exothermic or endothermic events in cured coatings

No measureable activity in coatings with dye molecules after curing, indicating complete cure reached and all solvents fully evaporated

50 70 90 110 130 150 170 190-0.3

-0.2

-0.1

-2.77555756156289E-16

0.0999999999999997

0.2

0.3

0.4

0.5

Temp. (deg.C)

DS

C, (

mW

/mg)

50 70 90 110 130 150 170 190-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

Temp. (deg.C)

DS

C (

mw

/mg

)

Glass Transition Temperature of coatings with incorporated dye moleculesEstablished by measuring residual

exothermic or endothermic events in cured coatings

No statisticallly significant effect is caused by dye incorporation

2 dye species in liquid solvent, showing relative monomer and dimer absorbance activity

Dye in solid PU film, showing absorbance activity consistent with dye in liquid solvent

Dyes (colored) in cured epoxy (left) and PU (right). Black curves show uncured exo- and endothermic

activity, not evident in cured filjms

Ep

ox

y

Ep

ox

y ...

Ep

ox

y w

...

Ep

ox

y w

...

Ep

ox

y w

...

Ep

ox

y ...

PU

PU

w/P

. 6

PU

w/P

. 7

PU

w P

. 8

PU

w P

. 9

PU

w P

...

-10

0

10

20

30

40

50

60

45.239.4

48.6 49.8

40.9

49.6

4.6 5.2 5.81.8 3.7 1.7

Gla

ss T

rans

ition

(deg

.C)

Left: representative plot of DMA data for epoxy and PU film. Right: Tg data showing no significant

change due to dye incorporation

Fluorescent Response of Dyes in coating systemsEstablished by measuring emission

spectra caused by excitation at peak absorbance wavelength

Dyes are fluorescent active in coatingsDifferent dye species emit different

spectra

400 450 500 550 600 650 700 750 8001

10

100

1000

10000

Wavelength (nm)

Lo

g I

nte

nsi

ty

Incident photons

Emitted photons

Peak emission λ

Left: representative fluorescent emission due to incident UV light of 2 dye species. Right:

representative emission spectra of dyes in solid films due to mono-wavelength excitation

Fluorescent coatings mechanical testing

Dye-incorporated coatings spread cast onto flexible PET substrates

Dye-substrate bilayer strained in tensile test frame

Testing stopped at various strain levelsDye in solid coating at various strain levels during

tensile testin g. Images taken while sample illuminated with UV light.

Fluorescent behavior of coatings while being strained Evident color change as strain increases for one dye species Possible intensity change as strain increases

1( 0 strain) 2 (25% strain) 3 (50% strain)

Fluorescent behavior of coatings after strainCoatings were elastically deformed and allowed to relax before

quantitative fluorescent emission testingConcentration dependence of peak wavelength and quantum

yield change is consistent with design strategy of dye moleculeNo observed strain dependence of fluorescent behavior

indicates the observed changes are reversible with strain

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.40.600000000000001

0.650000000000001

0.700000000000001

0.750000000000001

0.800000000000001

7.38x10^-4 mol/L 1.48x10^-4 mol/L

7.38x10^-5 mol/L

Tensile Strain (mm/mm)

PL

Qu

antu

m Y

ield

Decreasing concentration

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4550

560

570

580

590

600

7.38x10^-4 mol/L 1.48x10^-4 mol/L

7.38x10^-5 mol/L

Tensile Strain (mm/mm)

Pea

k λ

(nm

)

Decreasing concentration

Left: Fluorescent Quantum yield measurements show increasing intensity with decreasing concentration. Right: Peak emission wavelength shows

shift from red to red-orange emission with decreasing concentration. Both plots show no strain dependence.

Future work will test the coatings’ in-situ strain dependent fluorescent properties in tension and compression

4.0 RESULTS AND DISCUSSION

1