Infrared Spectroscopy: A Potential Quality Assurance Method for Composite Bonding 2012 Technical Review Ashley C. Tracey & Brian D. Flinn University of Washington Materials Science and Engineering

Infrared Spectroscopy: A Potential Quality Assurance Method for Composite Bonding

•  Motivation and Key Issues –  Most important step for bonding is SURFACE

PREPARATION!! –  Inspect surface prior to bonding to ensure proper

surface preparation •  Objective

–  Develop QA technique for surface preparation •  Approach

–  Use FTIR to characterize different surface preparations and levels of contamination §  CFRP prepared with peel ply for secondary bonding §  Peel ply contamination §  Manual abrasion to remove peel ply texture from CFRP

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Quality Control Methodologies for Surface Preparation Processes for Composite Bonding

•  Principal Investigators & Researchers –  Brian D. Flinn (PI) –  Ashley Tracey (PhD student, UW-MSE) –  Elise Santa Maria (undergraduate, UW-MSE)

•  FAA Technical Monitor –  David Westlund

•  Other FAA Personnel Involved –  Larry Ilcewicz –  Curtis Davies

•  Industry Participation –  Toray Composites –  Precision Fabrics & Richmond Aerospace & Airtech International –  The Boeing Company (Kay Blohowiak, Peter Van Voast, William

Grace, Tony Belcher, Paul Vahey, Paul Shelley, Greg Werner and Marc Piehl)

Composite Bonding and Surface Preparation

•  Peel ply is a desirable surface preparation because it produces repeatable and consistent surfaces and can prevent surface contamination

•  Paste adhesives not compatible with peel ply surface à

further surface treatment required •  Composite bonds are materials system specific (prepreg/

surface prep/adhesive) à potential problems that could degrade bond quality: –  Incorrect peel ply? –  Contamination? –  Improper abrasion?

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Peel ply

Composite

FTIR to Examine Surfaces

•  Why might FTIR be used for QA? –  Requirement of adhesion:

§  formation of primary chemical bonds between the adherend and adhesive

–  FTIR is used to measure surface chemistry §  surface prep and contaminants influence surface chemistry

–  Portable units available §  Suitable for factory and field applications

•  Need to assess ability of FTIR as a QA technique

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Experimental Overview

•  Can FTIR detect different peel or release ply materials? –  Nylon, polyester, SRB

•  What level of siloxane detectable with FTIR? –  Various levels of contamination (0.1%-2% and SRB)

•  Can FTIR detect proper abrasion? –  Different levels of manual abrasion to remove peel ply

texture/residue

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Assess ability of FTIR to identify improperly prepared surfaces

FTIR Methodology

Bruker Vertex 70 FTIR

•  Bench-top instrument •  Diamond attenuated total

reflectance (DATR)

Agilent Technologies Exoscan FTIR

•  Handheld device •  Diffuse reflectance

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http://www.aoc.kit.edu/english/612.php www.chem.agilent.com

DATR Methodology

•  MIR data region: 4000 cm-1 to 400 cm-1

•  Data collection: 16 scans with 4 cm-1 resolution for background and specimen

•  Pressure clamp used for intimate contact between crystal and sample à max pressure: 30 ksi

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An infrared beam path for a single bounce DATR

Diffuse Reflectance Methodology

•  MIR data region: 4000 cm-1 to 650 cm-1

•  Data collection: 90 scans with 16 cm-1 resolution for background and specimen

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An infrared beam path for diffuse reflectance

Spectra Analysis

•  FTIR spectra of CFRP surfaces complex –  Multiple constituents à many spectral peaks

•  How to analyze spectra with confidence? –  Multivariate analysis!

•  Multivariate analysis: Principal Component Analysis (PCA) –  Identification of peak locations and intensities not

obvious to the observer –  Can be used to develop test interfaces to remove

operator interpretation à go/no go output

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Multivariate Analysis: Unscrambler X

•  Principal Component Analysis (PCA) –  Exploratory data analysis – identify trends –  2 principal component analysis –  Some spectra required preprocessing: Savitsky-

Golay 1st derivative with 5 smoothing points fit to a 2nd order polynomial §  Derivative to amplify changes in slope of spectral peaks while

reducing effects of baseline offsets §  Smoothing points to reduce influence of noise

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Materials

•  Toray 3900/T800 unidirectional laminates –  Autoclave cure (177°C, 0.6MPa)

•  Peel ply surface prep –  Precision Fabric Group (PFG) 60001 polyester peel ply –  PFG 60001 polyester peel ply with controlled siloxane

amounts §  0.1% to 2% siloxane

–  PFG Super Release Blue (SRB) siloxane release ply –  PFG 52006 nylon peel ply

•  Manual abrasion surface prep –  Random orbital sander with Diablo 120 grit ceramic

abrasive sanding discs

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Peel Ply Type

•  Peel ply surface preparation for adhesive bonding is materials system specific –  Previous research showed CFRP prepared with

different peel ply materials and bonded with MB 1515-3 film adhesive affect bond quality

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Ø  Need to ensure correct peel ply material used

DATR Sensitive to Peel Ply Material

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ü  DATR can differentiate between peel plies Ø  Peel ply prep detectable on CFRP surface?

Polyester peaks: C=O

Polyester SRB

Nylon Silicone peak

Silicone peak

Amide II Band: C-N stretch + CO-N-H

bend Amide V

Band: N-H out-of-plane

bend

Amide IV Band: C-CO

stretch

Silicone peaks

Trans configuration of ethylene glycol unit

CH2 wagging mode of trans conformer of glycol moiety

(indicative of crystalline

domains in PET)

Gauche configuration of -OCH2CH2O- group (largley associated with amorphous region of PET)

Benzene ring

DATR of CFRP Prepared with Different Peel Ply

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ü  Small differences between CFRP peel ply prep observed Ø  Multivariate analysis?

Silicone band at 1260

cm-1 ?

Polyester prepared SRB prepared Nylon prepared

DATR of CFRP Prepared with Different Peel Ply

•  Multivariate Analysis: PCA of two PCs –  Preprocessing: Savitsky-Golay 1st derivative with 5

smoothing points fit to a 2nd order polynomial

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•  PCA identifies no significant difference between CFRP peel ply prep Ø  Peel ply prep detectable with diffuse reflectance?

1=Nylon prep 2=SRB prep

3=Polyester prep

Diffuse Reflectance Sensitive to CFRP Prepared with Different Peel Plies

•  Multivariate Analysis: PCA of two PCs –  Preprocessing: Savitsky-Golay 1st derivative with 5

smoothing points fit to a 2nd order polynomial

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1=Nylon prep 2=SRB prep

3=Polyester prep

ü  PCA identifies CFRP peel ply surface prep Ø  What level of peel ply contamination is detectable?

Si Contamination of Peel Ply

•  Surface contamination detrimental to bonding –  contamination cannot

be completely eliminated from all composite fabrication environments – siloxane

Ø Need to inspect CFRP surfaces for contaminates

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Solid Green: Cohesive Capability

Dashed Blue: Carbon Delamination

Red Arrow: Contaminated Bond

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B-RAT results showing bond quality is degraded of CFRP substrates with peel ply contamination levels at 1% siloxane and

greater1

1VanVoast, P.J., P.H. Shelley, R.L. Blakely, C.B. Smith, M.P. Jones, A.C. Tracey, B.D. Flinn, G. Dillingham, B. Oakley. “Effect of Varying Levels of Peel Ply Contamination on Adhesion Threshold.” SAMPE 2010. Seattle, WA, May 17-20, 2010.

Si contamination on CFRP difficult to quantify with Diffuse Reflectance FTIR

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•  PCA identifies some differences between siloxane contaminated samples •  PCA does not show difference between acceptable and unacceptable

levels of contamination Ø  Different technique to identify level of contamination?

How to quantify amount of Contamination Transferred to CFRP Substrate

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•  All spectra show characteristic silicone peaks at 1260 cm-1, 1100 cm-1, 1022 cm-1 and 800 cm-1

•  Peak at 1260 cm-1 used for analysis of amount of siloxane transferred to surface

Hexane Extraction Results using FTIR with Vertical Integrating Sphere (sensitive to small amounts of material): 2” x 2” samples to quantify how much siloxane transfers to surface

Hexane Extraction 1260 cm-1 Peak Areas as a Function of Peel Ply Contamination

•  CFRP surface contamination increases with increased peel ply contamination – maybe use a wipe and analyze residue to determine contamination level?

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0

0.5

1

1.5

2

2.5

Pea

k A

rea

(m

g/f

t2)

Sample

Increasing Peel Ply Contamination

•  Diffuse reflectance or DATR to analyze residue? Ø  Are different levels of abrasion detectable with FTIR?

Manual Abrasion to Remove Peel Ply Texture

•  Previous research shows polyester peel ply prep of 177°C cure composites fail in adhesion when bonded with paste adhesives2 –  Surface abrasion can be used to remove peel ply texture/residue

from composite à cohesive failure when bonded with paste adhesives2

–  Want to detect levels of abrasion to remove peel ply surface layer

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Peel Ply Texture, No Abrasion

> 75% Peel Ply Texture Evident

< 50% Peel Ply Texture Evident

No Peel Ply Texture

Nylon Prepared

Polyester

Prepared

2Bossi, R., R. Carlsen, F.J. Boerio and G. Dillingham. “Composite Surface Preparation QA for Bonding.” SAMPE 2005. Long

Beach, CA, May 1-5, 2005.

DATR and Diffuse Reflectance Sensitive to Manual Abrasion to Remove Polyester Peel Ply Texture

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ü PCA of diffuse reflectance spectra shows difference between acceptable and unacceptable abrasion levels to remove polyester peel

ply texture/residue à more sensitive than DATR

PCA of DATR Spectra PCA of Diffuse Reflectance Spectra

0=polyester peel ply surface 1= >75% peel ply surface evident 2= <50% peel ply surface evident

3=no peel ply surface evident

Unacceptable abrasion

Acceptable abrasion

DATR and Diffuse Reflectance Sensitive to Manual Abrasion to Remove Nylon Peel Ply Texture

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PCA of DATR Spectra PCA of Diffuse Reflectance Spectra

0=nylon peel ply surface 1= >75% peel ply surface evident 2= <50% peel ply surface evident

3=no peel ply surface evident

Unacceptable abrasion

Acceptable abrasion

ü PCA of diffuse reflectance spectra shows difference between acceptable and unacceptable abrasion levels to remove nylon peel ply

texture/residue à more sensitive than DATR

Summary of FTIR Results

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FTIR Sampling Interface DATR Diffuse

Reflectance Peel Ply Prep ? ✔ Si Contaminants N/A ?

Abraded Surfaces to Remove Peel Ply Residue

✔- ✔

Scarfed Surfaces/Repair TBD TBD

DATR vs. Diffuse Reflectance

•  Diffuse reflectance shows greater sensitivity to surface preparation than DATR –  Due to sampling volume?

§  DATR spectra from surface in contact with DATR crystal – ideal for smooth surfaces, less contact with rough surfaces

§  Diffuse reflectance spectra better for rough surfaces – can get information from peel ply channels

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Conclusions

Diffuse reflectance more sensitive to rough surfaces than DATR

•  Peel Ply Prep:

–  Diffuse reflectance can identify polyester vs. nylon vs. SRB prep •  Peel Ply Contamination:

–  Diffuse reflectance can identify some differences between siloxane contaminated samples

–  Hexane wipe alternative? Diffuse to analyze wipe? •  Abrasion to Remove Peel Ply Texture:

–  Diffuse reflectance can identify removal of nylon and polyester peel ply texture

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FTIR has potential as a quality assurance technique for adhesive bonding process

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Looking 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 characterization vs. bond quality model

§  Bond quality vs. amount of peel ply texture on abraded surface –  QA methods 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

•  FAA COE: JAMS, AMTAS

•  The Boeing Company

–  Kay Blohowiak, Pete Van Voast, Will Grace, Marc Piehl, Paul Shelley, Paul Vahey, Greg Werner

•  Precision Fabric Group

•  Richmond Aircraft Products

•  Airtech International

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End of Presentation.

Thank you.

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