Measurement of composite surface contamination using the Agilent 4100 ExoScan FTIR with diffuse reflectance sampling interface

Importance of the Topic

Composite materials are widely used in aerospace and industrial applications, but surface contamination by hydrocarbons and silicones can compromise adhesive bonding and repair integrity. Rapid, accurate detection of these contaminants on carbon–epoxy surfaces is critical to ensure bond performance and structural longevity.

Objectives and Study Overview

This application note evaluates the capability of the handheld Agilent 4100 ExoScan FTIR with diffuse reflectance sampling interface to detect and quantify silicone and hydraulic-fluid contaminants on composite coupons. The study aims to demonstrate field-ready, nondestructive analysis and to assess the effectiveness of plasma cleaning.

Methodology and Instrumentation

Samples:

• Silicone contamination: Composite coupons spiked with 0, 40, 78, and 300 µg/cm² of silicone via solvent evaporation on ~10 cm² areas.
• Hydraulic-fluid contamination: Coupons saturated with hydrocarbon fluid, cleaned with acetone wipe, then half-surface treated with one of three plasma processes.

Instrument and Parameters:

• Agilent 4100 ExoScan FTIR spectrometer with diffuse reflectance sampling interface.
• Measurement conditions: 128 co-added scans at 8 cm⁻¹ resolution (~30 s per sample).

Key Results and Discussion

Silicone Detection:

• Characteristic IR bands at 1260, 1095, 1018, and 800 cm⁻¹ were observed as negative peaks after baseline correction, a feature of strong reflectance samples.
• Calibration of the 800 cm⁻¹ band area yielded a nonlinear response across the full concentration range but a linear fit at low levels, suggesting a limit of detection near 10 µg/cm².

Hydraulic-Fluid Analysis and Plasma Cleaning:

• Contaminated samples exhibited clear CH stretch at 2930 cm⁻¹ and a C=O shoulder at 1730 cm⁻¹.
• Sample 3’s plasma treatment fully removed the fluid without substrate damage.
• Sample 1 was under-cleaned, retaining residual fluid.
• Sample 2 was over-cleaned, erasing the epoxy signature and indicating surface damage.

Benefits and Practical Applications

• Portability: Field-deployable instrument enables on-site composite inspection without sample transport.
• Speed: Rapid analysis (~30 s per measurement) supports high throughput in production and repair environments.
• Nondestructive: Maintains part integrity while providing chemical specificity for both silicones and hydrocarbons.

Future Trends and Potential Applications

• Expanded calibration protocols for additional contaminants and improved low-level quantification.
• Integration with automated mapping or robotic inspection systems for large-area composites.
• Development of real-time data analytics and machine-learning models to enhance detection sensitivity and decision support.

Conclusion

The Agilent 4100 ExoScan FTIR with diffuse reflectance sampling interface provides a fast, reliable, and nondestructive approach for detecting silicone and hydraulic-fluid contamination on composite surfaces. It offers practical benefits for in-field quality control and repair verification, with the potential for further enhancements in quantification and automation.