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 Borth 2

Introduction • Problem – Standard lifetime predictions fail to estimate the functional lifetime of polyurethane coatings 1 – 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 o Use the SKP to differentiate between degraded and non-degraded polyurethane coatings – Verify these results with Spectroscopy and Thermomechanical Analysis 1 Tiong, U. H., & Clark, G. (2010). “The structural environment as a factor affecting coating failure in aircraft joints”. Procedia Engineering , 2 (2010) . Borth 3

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 V B = -V CPD negates potential 1 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) Borth 5

SKP Operating Principle • The probe is vibrated at 90Hz and distance (d) • The capacitance (C) is dependent on distance 1 % – ! = ## $ & '(∆'*+,(./) '! – + = 1 23' '/ • PID controller balances the current generated with a bias voltage U – + = (1 23' − 5) '! '/ • When i = 0 then V cpd = 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 known 2 1 G. Grundmeier et al. “Novel Electrochemical Measurement Techniques in Corrosion” . Vol. I & II , (2000). 2 I.R Peterson. “Kelvin Probe Liquid-Surface Potential Sensor” Rev. Sci. Instr . 70 , (1999). Borth 6

What the SKP Detects • Conductive substrate (aluminum) • Galvanic activity • Interfaces (β) Probe – Metal-Oxide (β 1 ) – Primer-Oxide (β 2 ) β 5 o Electric double layer formation 1 Air β 4 – Primer-REC (β 3 ) ε Polyurethane REC – REC-Air (β 4 ) β 3 + Epoxy Primer – Probe (W)-Air (β 5 ) β 2 - Aluminum Oxide • Changes capacitance β 1 Aluminum % – ! = ## $ & '(∆'*+,(./) o Probe height (d) o Dielectric permitivity 2 ( ε ) 1 Nazarov, A., and Thierry, D. ”Scanning Kelvin probe study of metal/polymer interfaces”. Electrochimica Acta , 49 (2004). 2 Hansen, 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). Borth 7

Polyurethane Rain Erosion Coat (REC) MethyleneBis(4-Cyclohexyl Caprolactone 2,2-oxybis(ethanol) Isocyanate) Hard Segment Soft Segment Hard Segment Chain Extender • Hydrolysis occurs on the carbonyl carbon of ester bonds of the soft segments 1 H 2 O + Δ R R 1 M. Malíková, et al. "Assessing the Progress Of Degradation In Polyurethanes By Chemiluminescence" Polymer Degradation and Stability 95 . (2010).

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 ) Primer Cross section Sample with Primer- Rain Erosion Stack-up Rain Erosion Coating Borth 10

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) 2) 1) 3) 4) Borth 11

SKP Test Procedure • Optical Surface Profile (OSP) scan Brass ▫ Topographical data insulator • SKP with height tracking 500µm ▫ Work function measurement diameter • Repeat 6 times (2 day period) tungsten tip • 2 panels per exposure time Borth 12

Sample Analysis Optical SKP Scan Image • Scan before and after exposure 0hr Exposure – Scan area (yellow Box) • Compare the scans – Use Excel™ to determine average values – Exclude large features to determine bulk 8hr value Exposure Borth 13

Spectroscopic Test Procedure • FT-IR –Thermos Scientific Nicolet IS50 o 1064nm wavelength o Aperture 80% o Optical velocity 0.4747cm/s o 12 samples/point Borth 14

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 Penetration Probe Erosion Coating Sample Setup Erosion Coat for TMA testing . DSC Sample Pan Lid Borth 15

Results and Discussion Borth 16

TMA Results • Drop in T g as exposure time increases • Indicative of bond breakage Borth 17

SKP Results 0 hr 4 hr Average Surface WF Difference Between Exposed and Unexposed Surfaces 400 Change in Work Function (mV) 350 300 6 hr 250 200 150 8 hr 100 4hr 6hr 8hr 12hr Exposure Time 12 hr Borth 18

Comparison of FT-IR Spectra 1160 cm -1 1720 cm -1 (C=O)-O C=O asym. 1227 cm -1 (C=O)-O sym. C-O-C • 0hr (pink) • 4hr (orange) • 6hr (green) (C=O)-NH- • 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 ) Clear REC FTIR 0.37 (C=O)-O asym. C=O 0.35 0.33 0hr Absorbance 4hr 0.31 (C=O)-O sym. 6hr 0.29 8hr 0.27 12hr 0.25 1720 1227 1160 Wavenumber (cm -1 ) Borth 20

Cause of Work Function Change • Ester hydrolysis • Increased work function with increased exposure • Not dependent on: Air –Coating thickness β 4 ε d Polyurethane REC –Water β 3 + Epoxy Primer β 2 - o Interface Aluminum Oxide β 1 o Bulk polymer Aluminum –Aluminum substrate or oxide layer • Leaving the dielectric of the REC as the most likely contributor Borth 21

Cause of Work Function Change • Hydrolysis creates discontinuities –With free hydroxyls • These act as electron traps 1 + R –Increase in dielectric permittivity ( ε) 1 –Increase in dielectric hysteresis 1 o Speed and completeness of capacitor discharge • Proposed mechanism: Increase in electron trapping sites causes work function to increase o Work function = work to remove electrons 1 Lee, 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). Borth 22

Conclusion 1. TMA showed a decrease in T g 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 • 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) 1 Caap Co. “Caapcoat B-274 Rain Erosion Coating MSDS” (1996). 2 Heinkel. “Prepolymer MSDS” Revision number: 002.1 (2009). Borth 26

Polymerization 27

Spectroscopic Peak Assignments 1,2,3,4 7 Wave- Band number Assignment Origin (cm-1) Hydrogen Bonded N-H and 1 N-H O-H 3300 O-H Vibration Band 3 2900- Asymmetric and Symmetric 2 R-CH2-R 2800 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 Methylene Asymmetric 6 R-CH2-R 1440 Bend 3 7 (C=O)-O-C 1250 Ester Symmetric Stretching 7 1160 Ester Asymmetric Stretching 8 C-O-C 1090 Ether Stretching (crystalline) Ether Stretching 1040 (amorphous) 1 Vandenabeele, Peter. “Practical Raman Spectroscopy: An Introduction”. West Sussex, United Kingdom: Wiley, 2013. 2 PerkinElmer, Inc. “FT-IR Spectroscopy Attenuated Total Reflectance (ATR)”. Shelton, CT (2005). 3 Socrates, G. “Infrared and Raman Characteristic Group Frequencies: Tables and Charts”. New York : Wiley, 2001. 4 Bruckmoser, K., and K. Resch. "Investigation of Ageing Mechanisms In Thermoplastic Polyurethanes By Means Of IR And Raman Spectroscopy." Macromolecular Symposia 339:1 (2014). Borth 28