Non-Destructive Analysis of Substrates and Contaminants by FTIR with Specular Reflectance Interface

Importance of the Topic

This application note addresses the critical need for non-destructive, in-situ cleanliness testing of substrates prior to bonding or coating. Contaminants such as oils or release agents can impair adhesion, reduce bond strength, and lead to premature component failure. A portable, field-deployable technique with rapid measurement capability is essential for quality assurance in manufacturing, aerospace, automotive, and other industries relying on strong adhesive bonds.

Objectives and Study Overview

The study demonstrates the use of the Agilent 4300 handheld FTIR spectrometer equipped with a 45° specular reflectance interface to:

• Qualitatively identify common contaminants on reflective and non-reflective surfaces.
• Quantify trace levels of a PTFE-based aerosol release agent on aluminum coupons.
• Integrate quantitative models into a portable workflow for onsite verification of surface cleanliness.

Methodology

Two sets of samples were prepared:

• Glass-lined vessel surfaces smeared with six different lubricants or release agents to evaluate qualitative detection.
• Aluminum coupons (40.85 × 50.80 mm) contaminated with accurately weighed PTFE aerosol ranging from 0.08 µg/cm2 to 495 µg/cm2 for quantitative modeling.

All spectra were collected over 16 seconds (64 scans at 8 cm–1 resolution). The specular reflectance (IRRAS) mode allowed a double pass of IR light through surface films without physical contact, eliminating sample damage or contact pressure variability.

Instrumentation

• Agilent 4300 handheld FTIR spectrometer with 45° specular reflectance interface featuring an FEP-coated silicone O-ring.
• MicroLab PC and MicroLab Expert software for data acquisition, library search, and multivariate modeling.

Main Results and Discussion

Qualitative analysis produced distinct spectra for the clean glass vessel and each lubricant in the CH stretching (2,800–3,000 cm–1) and fingerprint regions, supporting quick library searches for contaminant identification.

Quantitative analysis used partial least squares (PLS1) multivariate models:

• Global model spanning 0–495 µg/cm2 (R2 = 0.9911).
• Low-range model spanning 0–33 µg/cm2 (R2 = 0.9868).

Both models delivered accurate PTFE predictions with minimal spectral preprocessing and were implemented in the handheld workflow. Predicted values closely matched actual loadings, with reproducibility confirmed by boxplot analysis.

Benefits and Practical Applications

• Rapid, non-destructive (<30 s) and force-free analysis of large or awkward substrates in situ.
• Portable (~2 kg) instrument with simplified user interface for non-expert operation.
• Customizable software methods supporting pass/fail traffic-light displays and automatic model selection.
• Enhanced QA/QC for adhesive bonding, coating processes, and reaction vessel maintenance.

Future Trends and Applications

Potential developments include:

• Expansion to multi-analyte quantification of diverse contaminants or surface treatments.
• Integration with Industry 4.0 systems for automated data logging and remote monitoring.
• Advanced chemometric interfaces for real-time process feedback and adaptive cleaning protocols.
• Extension to composites, ceramics, and coated assemblies in aerospace and automotive sectors.

Conclusion

The Agilent 4300 handheld FTIR with specular reflectance interface offers a versatile, sensitive solution for verifying substrate cleanliness. Its rapid, non-destructive measurements and integrated multivariate models enable reliable detection and quantification of contaminants, ensuring optimal adhesive bonding and coating adhesion in demanding industrial applications.

References

1. Extended Non-Destructive Testing of Composite Bonds (ENCOMB), FP7, CORDIS, European Commission, 2013.
2. Tang, P. L.; Rein, A. Quantification of Release Agent on a Carbon-Fiber-Reinforced Polymer using a Handheld FTIR; Agilent Technologies Application Note; Publication 5991-5595EN, 2018.
3. Tang, P. L. Comparison of Portable FTIR Interface Technologies for the Analysis of Paints, Minerals & Concrete; Agilent Technologies Application Note; Publication 5991-8359EN, 2017.