Rapid Quantification of the A:B mix- ratio of a 2K Industrial OEM PU paint prior to autoclave thermal activation

Significance of the Topic

Modern industrial clearcoat layers must deliver superior resistance to weathering, chemicals, abrasion and UV radiation while providing high gloss. Two-component polyurethane (2K PU) systems are widely used for these applications, but incorrect mixing ratios prior to curing can lead to surface defects, compromised long-term performance, and costly remedial actions or part scrappage. A rapid, on-site method for verifying the liquid A:B ratio in wet paint would enable manufacturers and applicators to ensure quality control at the point of application and avoid downstream failures.

Objectives and Study Overview

This study aims to develop and validate a quick, reliable approach for quantifying the mix ratio of a 2K industrial OEM PU paint in its wet state, prior to thermal activation. A handheld Agilent 4300 FTIR spectrometer, coupled with a 45° external reflectance interface, was used to collect spectra from sprayed aluminum coupons. A multivariate Partial Least Squares (PLS1) model was built to predict component ratios and integrated into the instrument’s software for real-time QA/QC feedback.

Methodology

The experimental protocol comprised:

• Collection of FTIR spectra for pure components A (aliphatic polyol) and B (blocked isocyanate curative) at 64 scans, 4 cm⁻¹ resolution.
• Preparation of three aluminum coupons sprayed with gravimetrically determined mix ratios: resin-rich (3.99:1), near-target (3.06:1) and resin-poor (2.49:1) A:B.
• Acquisition of 10 spectra per coupon using a sacrificial pierced foil to protect the interface, with under 40 s per spectrum.
• Development of a PLS1 calibration model using eight spectra per coupon (24 spectra total) and validation using the remaining six spectra.

Used Instrumentation

The analytical setup included:

• Handheld Agilent 4300 FTIR spectrometer.
• 45° specular external reflectance sample interface.
• Pierced sacrificial foil to prevent paint adhesion on the interface.
• Microlab Expert chemometric software for PLS1 model creation and deployment.

Main Results and Discussion

Key findings of the study are:

• Spectral libraries for components A and B exhibited rich, distinctive reflectance features suitable for multivariate analysis.
• Thermal stoving induced pronounced spectral changes correlating with curing chemistry, suggesting FTIR can also monitor cure progress.
• The six-factor PLS1 model delivered excellent calibration (R and R² > 0.99) and low standard error of prediction (SEP = 0.036), accurately resolving mix ratios across the 2.5–4.0 range.
• Model predictions were visualized in real time on the FTIR display with color-coded in-spec (green) or out-of-spec (red) indicators and adjustable tolerance thresholds (±5%).

Benefits and Practical Applications

This approach offers significant advantages:

• Rapid, non-destructive verification of wet paint mix ratios in under 40 s per measurement.
• On-site QA/QC capability allows immediate corrective action, reducing defective coatings and warranty claims.
• Integration into automated or robotic spray systems to continuously monitor mix ratio delivery.
• Extensible to different 2K chemistries and manual mixing protocols using the same workflow.

Future Trends and Potential Applications

Emerging opportunities include:

• Cloud-based databases for centralized spectral libraries and model updates across multiple production sites.
• Real-time feedback loops in robotic painting lines for closed-loop process control.
• Expansion of chemometric algorithms (e.g., machine learning) for enhanced robustness across diverse formulations.
• Integration with other spectroscopic methods or sensing modalities for comprehensive coating quality assessments.

Conclusion

Handheld FTIR combined with a multivariate PLS1 model provides a fast, accurate, and user-friendly solution for quantifying the A:B mix ratio in two-component PU paints before curing. This method supports in-line QA/QC, helps prevent coating failures, and can be adapted to various industrial coating systems, delivering measurable cost savings and improved product reliability.