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

Significance of the Topic

Rapid and reliable determination of component mix ratios in two-part polyurethane industrial coatings is critical to ensure coating performance and longevity. Incorrect mixing can lead to surface defects, compromised abrasion and weather resistance, and costly remedial actions or scrappage of painted parts. Hand-held Fourier-transform infrared spectroscopy (FTIR) offers a non-destructive, on-site approach for quality control of wet spray coatings before curing.

Objectives and Study Overview

This study aimed to develop and validate a rapid method for quantifying the A:B mix ratio of an industrial two-component (2K) polyurethane (PU) clearcoat using a hand-held Agilent 4300 FTIR equipped with an external reflectance interface. Experiments involved collecting spectra of individual paint components, correctly mixed coatings, and deliberately varied mix ratios applied to aluminum coupons. A multivariate calibration model was then built to predict mix ratios in under 40 seconds per measurement.

Methodology and Instrumentation

• Coating system: Industrial-grade OEM two-component PU paint with blocked isocyanate curative (Component B) and aliphatic polyol resin (Component A).
• Instrumentation: Agilent 4300 FTIR with 45° specular reflectance interface; sacrificial foil used to protect the sampling window.
• Spectral acquisition: 64 scans at 4 cm⁻¹ resolution, acquisition time <40 s per spectrum.
• Mix-ratio calibration: Gravimetric preparation of three ratio sets (resin-poor 2.49:1, near correct 3.06:1, resin-rich 3.99:1 A:B), 10 sampling points each.
• Chemometric modeling: Partial least squares (PLS1) regression using Microlab Expert software; eight spectra per coupon for calibration and two for independent validation.

Key Results and Discussion

• Distinct spectral features of Components A and B enabled construction of a reference library and a robust PLS1 model.
• Cure-related spectral changes (before vs. after stoving at 140 °C) were clearly identified, supporting cure monitoring capabilities.
• The PLS1 model delivered excellent linearity (R and R² >0.99) with a standard error of prediction of 0.036, allowing mix-ratio determination with ±0.04 accuracy over the 2.5–4.0 A:B range.
• Multivariate over univariate models proved necessary due to complex overlapping absorption bands and intra-sample variance.

Benefits and Practical Applications

• Non-destructive, on-site verification of spray gun mix ratios prior to curing reduces risk of defective coatings and downstream rework.
• Rapid analysis (<40 s) facilitates real-time QA/QC in automated and manual paint application processes.
• Color-coded instrument interface provides immediate pass/fail feedback relative to user-defined mix ratio thresholds.
• Method adaptable to other 2K formulations and chemical systems by replicating the experimental protocol.

Future Trends and Potential Applications

• Extending FTIR-based QA tools to additional industrial coatings, adhesives, and sealants with complex curing chemistries.
• Integration with automated spray booths and robotics for closed-loop control of mix-ratio adjustments.
• Development of cloud-based chemometric libraries to support cross-manufacturer coating systems.
• Coupling with other handheld spectroscopic techniques (e.g. Raman) for enhanced multi-modal analysis.

Conclusion

The presented method demonstrates that a hand-held Agilent 4300 FTIR combined with PLS1 chemometrics can rapidly and accurately quantify the component mix ratio of a 2K PU industrial paint in wet form. Implementation of this approach at the point of spray application minimizes defective coatings and associated costs, ensuring adherence to manufacturer specifications and reliable long-term coating performance.

Reference

1. Agilent Technologies, Inc. Publication number: 5991-8323EN, 2017