Analysis of plasma treated carbon fiber reinforced polymer (CFRP) composites by portable Fourier Transform Infrared Spectroscopy (FTIR)

Importance of the Topic

Carbon fiber reinforced polymers (CFRP) are extensively used in aerospace applications for their exceptional strength-to-weight ratio. Following peel-ply removal, residual polydimethylsiloxane (PDMS) or wax-based release agents hinder adhesive bonding. Plasma treatment is widely adopted to clean and activate CFRP surfaces, but precise, non-destructive monitoring of treatment efficacy is essential to ensure optimal bond strength while avoiding substrate damage.

Objectives and Study Overview

This work assesses the capability of portable Fourier Transform Infrared (FTIR) spectroscopy to detect and quantify chemical changes induced by plasma treatment on CFRP surfaces. The aim is to develop a multivariate predictive model linking spectral features to treatment parameters and adhesive peel strength, thereby defining under-treated, optimal, and over-treated regimes.

Methodology and Instrumentation

• Samples: CFRP coupons retaining either PDMS or hydrocarbon-based release agents.
• Plasma treatment: Radio-frequency plasma nozzle scanned across the coupon at 6 cm/s with gap varying from 20.5 mm (under-treatment, < 100 °C) to 5 mm (over-treatment, > 260 °C).
• Spectral acquisition: Agilent 4100 ExoScan and 4300 Handheld FTIR spectrometers with high-efficiency diffuse reflectance interfaces; 128 co-added interferograms at 8 cm⁻¹ resolution (~1 min per spectrum).
• Temperature mapping: Thermal camera recordings synchronized with nozzle position.
• Adhesive peel testing: G1c measurements using an Instron 5566 Universal Testing Machine.
• Data analysis: Partial least squares (PLS) regression applied to spectral regions 780–1850 cm⁻¹ and 2715–3700 cm⁻¹.

Main Results and Discussion

Infrared spectra showed systematic variations in O–H stretching (~3400 cm⁻¹), alkyl C–H stretches (~2900 cm⁻¹), and carbonyl absorption (~1720 cm⁻¹) as a function of treatment severity. Over-treatment increased carbonyl intensity and depleted hydroxyl and alkyl signals, indicating oxidative modification of both release agent and CFRP matrix. The optimized PLS model predicted nozzle position within ± 1 cm, effectively mapping spectral changes to thermal exposure. Correlating model predictions with G1c peel strength and XPS-derived silicon content delineated three zones: under-treated (insufficient cleaning, low strength), optimal (maximum cohesive failure strength), and over-treated (thermal damage, mixed failure).

Benefits and Practical Applications

• Non-destructive, on-site verification of plasma treatment quality.
• Immediate feedback for process control, reducing rework and scrap.
• Integration into quality assurance workflows in aerospace, automotive, and general composite manufacturing.

Future Trends and Opportunities

Advances in detector sensitivity, miniaturization, and real-time multivariate algorithms will further improve handheld FTIR capabilities. Coupling FTIR monitoring with automated plasma systems could enable closed-loop surface treatment control. Expanding this approach to diverse composite chemistries and industrial processes offers broader application potential.

Conclusion

Portable FTIR spectroscopy, exemplified by the Agilent 4100 ExoScan and 4300 Handheld FTIR systems, provides an effective, non-destructive method to evaluate plasma treatment of CFRP surfaces. The developed PLS model correlates spectral signatures with treatment intensity and adhesive performance, enabling precise identification of optimal processing conditions.

References

• Rein A., Tang P.L., Agilent Technologies, Inc. (2015). Analysis of plasma treated carbon fiber reinforced polymer composites by portable FTIR. Application Note 5991-4033EN.