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

Importance of the topic

CFRP composites combine low weight, high strength and impact resistance, making them indispensable in aerospace applications. However, residual release agents such as PDMS or wax from peel ply layers hinder adhesive bonding. Plasma treatment is widely used to remove these agents and activate the surface, but under- or over-treatment can lead to poor bond strength or substrate damage. Portable FTIR offers a non-destructive, in-situ means to monitor chemical changes and ensure optimal surface preparation.

Objectives and study overview

This work evaluates the capability of portable FTIR instruments (Agilent 4100 ExoScan and 4300 Handheld FTIR) to detect and quantify plasma-induced chemical changes on CFRP surfaces with residual release agents. A secondary aim is to correlate FTIR spectral changes with adhesive peel strength (G1c) and develop a predictive multivariate model for real-time quality control.

Methodology

• CFRP coupons with PDMS- or wax-based release agents were prepared.
• RF plasma nozzle scanned across coupons at 6 cm/s with gap varying from 20.5 mm (under-treatment) to 5 mm (over-treatment).
• Surface temperature recorded by thermal camera; ranged from <100 °C to >260 °C depending on gap.
• Adhesive peel tests (Instron 5566) measured G1c to classify under-, optimal, and over-treated zones.
• Multivariate PLS regression correlated FTIR spectral regions (780–1850 cm⁻¹ and 2715–3700 cm⁻¹) with treatment position.

Used instrumentation

• Agilent 4100 ExoScan FTIR with high-efficiency diffuse reflectance interface
• Agilent 4300 Handheld FTIR with MCT detector option
• Infrared thermal camera for surface temperature monitoring
• Instron 5566 Universal Testing Machine for peel strength measurements

Main results and discussion

FTIR spectra revealed decreasing O–H stretching (3300 cm⁻¹) and alkyl bands (2900 cm⁻¹) and increasing carbonyl signal (1720 cm⁻¹) at high temperature (over-treated) conditions, indicating bond degradation and oxidation. The cross-validated PLS model predicted nozzle position within ±1 cm and delineated three zones: under-treated (low chemical change, poor adhesion), optimal (cohesive failure, highest G1c), and over-treated (thermal damage, mixed failure). Model predictions agreed closely with peel strength and XPS silicon concentration data.

Benefits and practical applications

• Enables non-destructive, at-site evaluation of surface treatment efficacy.
• Provides real-time feedback to ensure consistent bond quality.
• Reduces need for destructive testing and laboratory delays.
• Can be integrated into QC workflows in aerospace, automotive and composite manufacturing.

Future trends and potential applications

• Expansion of multivariate models to different composite materials and treatment chemistries.
• Integration with automated robotics for in-line surface monitoring.
• Development of cloud-based analytics for data aggregation and predictive maintenance.
• Broader adoption of handheld FTIR in field service and repair operations.

Conclusion

Portable FTIR systems, combined with multivariate analysis, offer a robust, non-destructive approach to monitor plasma treatment of CFRP surfaces. The predictive model correlates spectral changes with bond strength, enabling rapid, on-site quality control and reducing the risk of adhesion failures or substrate damage.