Non-destructive Evaluation of Urethane-Epoxy Coating Systems Using - - PowerPoint PPT Presentation

Non-destructive Evaluation of Urethane-Epoxy Coating Systems Using the Scanning Kelvin Probe Technique

By: David Borth

Advisor: Douglas C. Hansen Ph.D.

October 2018

Outline

• Introduction
• Background Information

– Scanning Kelvin probe (SKP) – Rain Erosion Coat (REC)

• Materials and Methods
• Results and Discussion
• Conclusion
• Future Work

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Introduction

• Problem

– Standard lifetime predictions fail to estimate the functional lifetime of polyurethane coatings1 – Recoating procedures are expensive and time consuming – The Air Force Spends $5.4M/year (2009) on corrosion related issues

• Goals

– Develop a non-destructive evaluation technique

• Use the SKP to differentiate between degraded and

non-degraded polyurethane coatings

– Verify these results with Spectroscopy and Thermomechanical Analysis

Borth 3 1Tiong, U. H., & Clark, G. (2010). “The structural environment as a factor affecting

coating failure in aircraft joints”. Procedia Engineering, 2 (2010).

Background Information

4

SKP Operating Principle

• A) Two materials with different work functions
• B) When in electrical contact potential will be balanced by

movement of electrons

• C) A bias voltage VB = -VCPD negates potential1

Borth 5 1 Lu, J., E. Delamarche, L. Eng, R. Bennewitz, E. Meyer and H.-J. Guntherodt. “Kelvin Probe Force Microscopy on

Surfaces: Investigation of the Surface Potential of Self-Assembled Monolayers on Gold” Langmuir 15, (1999)

SKP Operating Principle

• The probe is vibrated at 90Hz and distance (d)
• The capacitance (C) is dependent on distance1

– ! = ##$

% & '(∆'*+,(./)

– + = 123'

'! '/

• PID controller balances the current generated with a

bias voltage U – + = (123' − 5)'!

'/

• When i = 0 then Vcpd = U
• U is the difference in work function
• The Work Function of the probe is known

– The work function of the material being scanned is also known2

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1 G. Grundmeier et al. “Novel Electrochemical Measurement Techniques in Corrosion”.

• Vol. I & II, (2000).

2I.R Peterson. “Kelvin Probe Liquid-Surface Potential Sensor” Rev. Sci. Instr. 70, (1999).

What the SKP Detects

• Conductive substrate (aluminum)
• Galvanic activity
• Interfaces (β)

– Metal-Oxide (β1) – Primer-Oxide (β2)

• Electric double layer formation1

– Primer-REC (β3) – REC-Air (β4) – Probe (W)-Air (β5)

• Changes capacitance

– ! = ##$

% & '(∆'*+,(./)

• Probe height (d)
• Dielectric permitivity2 (ε)

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1 Nazarov, A., and Thierry, D. ”Scanning Kelvin probe study of metal/polymer interfaces”.

Electrochimica Acta, 49 (2004).

2Hansen, et al. “Scanning Kelvin probe measurements for the detection of corrosion processes

beneath applied paint coatings on aluminum alloy and steel substrates”. Polymer Preprints, 4 (2004).

Polyurethane REC Epoxy Primer

Aluminum

Aluminum Oxide

β3 β2 β1 β4 Air

Probe

β5

+

• ε

Polyurethane Rain Erosion Coat (REC)

• Hydrolysis occurs on the carbonyl carbon of ester bonds of the

soft segments1

H2O Δ

+

R R

• 1M. Malíková, et al. "Assessing the Progress Of Degradation In Polyurethanes By

Chemiluminescence" Polymer Degradation and Stability 95. (2010).

Hard Segment Soft Segment Chain Extender Hard Segment

Caprolactone MethyleneBis(4-Cyclohexyl Isocyanate) 2,2-oxybis(ethanol)

Materials and Methods

9

Sample Preparation

1) Coat 3x6 inch 2024-T3 aluminum panel

▫

1 mil Deft 02Y40 primer (MIL-PRF-23377)

▫

13 mil Caapcoat B-274 rain erosion coat (MIL-PRF- 85285 )

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Primer Rain Erosion Coating Sample with Primer- Rain Erosion Stack-up

Cross section

Degradation Process

1) Cut sample to make two 3”x 3” sections 2) Tape edges of one half to prevent delamination during exposure 3) Expose taped half in an autoclave at 121˚C and 100% RH 4) Exposure times 4, 6, 8, and 12 hours (then dried)

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1) 3) 4) 2)

SKP Test Procedure

• Optical Surface Profile (OSP) scan

▫ Topographical data

• SKP with height tracking

▫ Work function measurement

• Repeat 6 times (2 day period)
• 2 panels per exposure time

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Brass insulator 500µm diameter tungsten tip

Sample Analysis

• Scan before and

after exposure

– Scan area (yellow Box)

• Compare the scans

– Use Excel™ to determine average values – Exclude large features to determine bulk value

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0hr Exposure 8hr Exposure SKP Scan Optical Image

Spectroscopic Test Procedure

• FT-IR

–Thermos Scientific Nicolet IS50

• 1064nm wavelength
• Aperture 80%
• Optical velocity 0.4747cm/s
• 12 samples/point

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TMA Test Procedure

• TA Instruments Q400
• Penetration Probe: 0.05N load
• Penetration tip: 1mm diameter
• Temperature range: 25-250°C at 5°C/min
• Nitrogen atmosphere: Purge gas 50mL/min

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Sample Setup for TMA testing.

DSC Sample Pan Lid Erosion Coat Penetration Probe

Erosion Coating

Results and Discussion

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TMA Results

• Drop in Tg as exposure time increases
• Indicative of bond breakage

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SKP Results

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100 150 200 250 300 350 400 4hr 6hr 8hr 12hr Change in Work Function (mV) Exposure Time

Average Surface WF Difference Between Exposed and Unexposed Surfaces

0 hr 4 hr 6 hr 8 hr 12 hr

Comparison of FT-IR Spectra

1720 cm-1 1227 cm-1 1160 cm-1

C=O (C=O)-O sym. (C=O)-O asym. (C=O)-NH- C-O-C

• 0hr (pink)
• 4hr (orange)
• 6hr (green)
• 8hr (light blue)
• 12hr (red)

FTIR Vibration Band Intensity Comparison

–Decreased Ester Bands

(1350 cm-1 to 1160 cm-1 ) – Decrease in Carbonyl C=O (1730 cm-1 )

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0.25 0.27 0.29 0.31 0.33 0.35 0.37 1720 1227 1160

Absorbance Wavenumber (cm-1)

Clear REC FTIR

0hr 4hr 6hr 8hr 12hr

C=O (C=O)-O asym. (C=O)-O sym.

Cause of Work Function Change

• Ester hydrolysis
• Increased work function with increased exposure
• Not dependent on:

–Coating thickness –Water

• Interface
• Bulk polymer

–Aluminum substrate or oxide layer

• Leaving the dielectric of the REC as the most likely

contributor

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Polyurethane REC Epoxy Primer

Aluminum

Aluminum Oxide

β3 β2 β1 β4 Air

+

• ε

d

Cause of Work Function Change

• Hydrolysis creates discontinuities

–With free hydroxyls

• These act as electron traps1

–Increase in dielectric permittivity (ε) 1 –Increase in dielectric hysteresis1

• Speed and completeness of capacitor discharge
• Proposed mechanism: Increase in electron trapping sites

causes work function to increase

• Work function = work to remove electrons

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1Lee, S., Koo, B., Shin, J., Lee, E., Park, H., & Kim, H. “Effects of hydroxyl groups in polymeric

dielectrics on organic transistor performance”. Applied Physics Letters, 88:16 (2006).

+

R

Conclusion

1. TMA showed a decrease in Tg with increasing exposure time 2. The breaking of bonds through hydrolysis caused a change in work function of the PU coating as demonstrated through TMA, Spectroscopy and SKP 3. The change in work function was detected by the SKP allowing it can distinguish between degraded and pristine PU rain erosion coats. 4. Spectroscopy showed peak intensity changes in vibration bands related to hydrolysis (specifically ester bands) with increasing exposure time Overall: The scanning Kelvin probe is sensitive to changes in coating

• properties. The non-destructive nature of this technique makes it a good

candidate for a tool to assess the integrity of coatings currently in service.

Future Work

• Collect more data to verify/improve correlation
• Develop: Portable SKP for field testing

– Faster scanning of large areas – 5 axis motor to scan curved parts

• Look into other applications

– Paint bridge failure – Stress corrosion cracking

Any Questions?

25

CAAPCOAT B-274 [MIL-C-83231]1

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• Solvents:

▫ Xylene (39.6 wt%) ▫ Methyl isobutyl ketone (13.8 wt%)

• Monomers: (46.4 wt%) (Proprietary)2

▫ Caprolactone ▫ MethyleneBis(4-Cyclohexyl Isocyanate) ▫ 2,2-oxybis(ethanol)

• Addadive:0.1%

▫ Carbon Black (for color)

1Caap Co. “Caapcoat B-274 Rain Erosion Coating MSDS” (1996).

• 2Heinkel. “Prepolymer MSDS” Revision number: 002.1 (2009).

Polymerization

27

Spectroscopic Peak Assignments1,2,3,4

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Band Origin Wave- number (cm-1) Assignment

1

N-H O-H 3300 Hydrogen Bonded N-H and O-H Vibration Band

2

R-CH2-R 2900- 2800 Asymmetric and Symmetric Methylene Stretch

3

C=O 1730 Ester C=O Stretching 1630 Urethane C=O Stretching

4

C=C 1600 Carbon Black?

5

(C=O)-NH- 1520 Amide II N-H Bend

6

R-CH2-R 1440 Methylene Asymmetric Bend

7

(C=O)-O-C 1250 Ester Symmetric Stretching 1160 Ester Asymmetric Stretching

8

C-O-C 1090 Ether Stretching (crystalline) 1040 Ether Stretching (amorphous)

1Vandenabeele, Peter. “Practical Raman Spectroscopy: An

Introduction”. West Sussex, United Kingdom: Wiley, 2013.

2PerkinElmer, Inc. “FT-IR Spectroscopy Attenuated Total Reflectance

(ATR)”. Shelton, CT (2005).

3Socrates, G. “Infrared and Raman Characteristic Group Frequencies:

Tables and Charts”. New York : Wiley, 2001.

4Bruckmoser, K., and K. Resch. "Investigation of Ageing Mechanisms

In Thermoplastic Polyurethanes By Means Of IR And Raman Spectroscopy." Macromolecular Symposia 339:1 (2014).

3 7 3 7

1600cm-1 Peak

• Blue line contains carbon black
• Red line is an identical formulation with the exception of carbon black
• Raman settings are the same
• Carbon black should be causing the changes

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200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 Int 800 1000 1200 1400 1600 1800 Raman shift (cm-1)

5x

Surface Features 6hr Exposure

50x

Feature Comparison

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