Quantification of Release Agent on a Carbon-Fiber-Reinforced Polymer using a Hand-Held FTIR

Importance of the Topic

Release agents are critical in composite manufacturing to enable clean demolding of carbon-fiber-reinforced polymer (CFRP) parts. Residual release agent on CFRP surfaces can compromise paint adhesion and bonding strength, posing significant risks in aerospace, automotive, and industrial applications. Rapid, nondestructive assessment of release agent residues is therefore essential for quality assurance and reliable performance of bonded or coated composite components.

Objectives and Study Overview

This study evaluates the use of a handheld FTIR spectrometer to quantify residual release agent on a T700/M21 CFRP system. It forms part of the ENCOMB project, which investigates various contamination scenarios affecting CFRP bond strength. The primary goal is to develop and validate a portable, in situ method that correlates FTIR spectral data with the amount of polydimethylsiloxane-based release agent present on the composite surface.

Methodology and Instrumentation

• CFRP Samples: Six-layer unidirectional coupons of T700 carbon fiber with M21 epoxy, treated with Henkel Frekote 700NC release agent at four contamination levels (2.2 to 10.5% silicon by XPS).
• Mechanical Testing: Mode-I critical strain energy release rate (GIc) measured on an MTS Universal Testing Machine in accordance with ISO 15024 and 25217.
• Handheld FTIR: Agilent 4100 ExoScan with diffuse reflectance interface, 128 coadded interferograms at 8 cm–1 resolution, 16 sampling points per coupon (71 spectra total), 40 seconds per measurement.
• Reference Method: X-ray photoelectron spectroscopy (XPS) to determine surface silicon as a proxy for residual release agent.
• Data Analysis: Multivariate analysis using partial least squares (PLS) regression with cross validation to build a prediction model for percentage silicon from FTIR spectra.

Main Results and Discussion

Bond strength, as indicated by GIc, decreases sharply with increasing residual release agent, dropping from over 1,000 J/m2 at low contamination to around 40 J/m2 at the highest level. The PLS model achieved an R2 of 0.96, predicting surface silicon within ±1 to 2% compared to XPS. The handheld FTIR method reliably classified contamination severity levels, demonstrating strong correlation between spectral features and silicon content.

Benefits and Practical Applications of the Method

The handheld FTIR approach offers rapid (<1 minute), nondestructive, in situ quantification of residual release agent without sample preparation. An embedded pass/fail algorithm and color-coded severity classification enable operators with minimal training to verify surface cleanliness before bonding or coating, enhancing manufacturing quality control and maintenance efficiency.

Future Trends and Opportunities

Advancements in portable FTIR technology, such as the newer Agilent 4300 model, will further improve sensitivity and ergonomics. Integration of handheld spectrometers into digital quality management systems could enable real-time monitoring of multiple contamination factors, including moisture, acidic residues, and thermal damage. Expanded applications may include on-site validation of plasma treatments and broader use in field inspections across aerospace, automotive, and consumer composites.

Conclusion

The study demonstrates that handheld FTIR, combined with multivariate calibration, provides a fast and accurate method to quantify release agent residues on CFRP surfaces. Adoption of this technique can prevent bond failures and paint adhesion issues, supporting reliable composite manufacturing and maintenance practices.

References

• ENCOMB Consortium. ENCOMB Extended Nondestructive Testing of Composite Bonds.
• Markatos D. N., Tserpes K. I., Rau E., Ehrhart B., Pantelakis S. Compos. Part B: Eng. 2013, 45(1), 556–564.
• Markatos D. N., Tserpes K. I., Rau E., Pantelakis S. J. Adhesion 2014, 90(2), 156–173.
• Markatos D. N., Tserpes K. I., Chamos A., Pantelakis S. ICEAF III, 2013.
• Henkel Frekote 700NC Material Safety Data Sheet.
• Higgins F. Agilent Application Note 5991-4037EN, 2014.
• Rein A., Tang P. L. Agilent Application Note 5991-4033EN, 2014.