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

Importance of the Topic

Two-component polyurethane clearcoats are widely used in industrial applications for their superior durability and aesthetics. Ensuring the correct A:B mix ratio prior to thermal activation is critical to achieve full cure, avoid surface defects, and minimize costly rework or scrappage.

Objectives and Study Overview

This study demonstrates the use of a handheld Agilent 4300 FTIR with an external reflectance interface to quantify the wet mix ratio of 2K PU paint applied to aluminum coupons. The goal is to build and validate a rapid multivariate calibration model for on-site QA/QC of spray-application ratio control.

Methodology and Instrumentation

• Paint system: Industrial-grade 2K PU OEM clearcoat with aliphatic polyol (Component A) and blocked isocyanate curative (Component B).
• FTIR measurement: Handheld Agilent 4300 FTIR with 45° external specular reflectance interface, collecting 64 scans at 4 cm⁻¹ resolution (~40 s per spectrum).
• Sample preparation: Aluminum coupons sprayed with A:B ratios of 3.99:1, 3.06:1 and 2.49:1 (gravimetric mixing), with ten sampling points per coupon.
• Data analysis: Partial least squares (PLS1) regression using Microlab Expert software to develop calibration and validation models capturing key spectral features before and after thermal stoving at 140 °C.

Key Results and Discussion

• Distinct reflectance spectra were obtained for components A and B and for uncured vs. cured paint, revealing characteristic absorbance changes associated with curing chemistry.
• The PLS1 model built from 24 calibration spectra and validated with six independent spectra achieved R and R² > 0.99 and a standard error of prediction (SEP) of 0.036, enabling mix ratio predictions within ±0.04 ratio units over a 2.5–4.0 range.
• Model robustness prevents misuse with other paint systems and accounts for intra-sample variability, outperforming simple univariate approaches.
• The FTIR instrument interface provides real-time, color-coded (green/red) alerts based on user-adjustable ±5% tolerance thresholds around the ideal 3:1 A:B ratio.

Benefits and Practical Applications

• Enables sub-40 s in-situ verification of wet paint mix ratios directly at the spray application point, reducing the risk of improper curing and coating defects.
• Supports real-time QA/QC and equipment calibration for automated and manual spray processes, minimizing rework and warranty claims.
• Adaptable to other multicomponent coatings and chemical formulations using the same FTIR protocol.

Future Trends and Applications

• Integration with robotic spray systems for closed-loop control of component feed rates.
• Real-time monitoring of cure kinetics and solvent evaporation during application.
• Cloud-based chemometric models for multi-site deployment and remote QA/QC support.

Conclusion

The handheld FTIR method combined with a PLS1 multivariate model provides a rapid, accurate, and robust solution for on-site quantification of 2K PU paint mix ratios. Implementing this approach enhances process control, ensures coating performance, and reduces operational costs associated with coating defects.

References

• Leung Tang. Rapid Quantification of the A:B mix-ratio of a 2K Industrial OEM PU paint prior to autoclave thermal activation. Agilent Technologies, Publication number: 5991-8323EN, 2017.