Determination of amine number and solid content of dipping paint

Significance of the topic

Electrophoretic dipping paints are widely used to protect and decorate metal substrates. Key quality parameters such as amine number and solid content directly influence coating performance, film thickness, and curing behavior. Traditional laboratory methods for these measurements are laborious and time-consuming, motivating the adoption of rapid Vis-NIR spectroscopic approaches.

Study objectives and overview

This application note demonstrates the development of quantitative Vis-NIR calibration models to determine amine number (DIN 53176) and solid content (DIN 53219) in water-based electrophoretic coating baths. The aim was to replace conventional titration and gravimetric procedures with a single non-destructive spectral measurement accessible to non-specialist operators.

Methodology and instrumentation used

The study employed a Metrohm NIRS XDS SmartProbe Analyzer with a transflection probe and Vision Software 4.0.3 for data acquisition and chemometric modeling. White dipping paint samples covering the range 11.5–20.4 % solid content and 37.5–48.3 mmol/kg amine number were stirred to prevent sedimentation during transflection measurement. Spectra were recorded from 1120 nm to 1920 nm. Pretreatment involved a second derivative (segment size 10 nm, gap 0 nm) and Standard Normal Variate correction to mitigate scattering effects. Partial Least Squares regression with internal cross-validation was used to build the predictive models.

Main results and discussion

Amine number calibration achieved a coefficient of determination R² = 0.9678, SEC = 0.7345, cross-validation error SEV = 2.9284 mmol/kg, F-value = 156.3, and PRESS = 274.4, indicating excellent predictive accuracy. Solid content modeling yielded R² = 0.9239, SEC = 0.7484 %, SEV = 0.8606 %, F-value = 331.1, and PRESS = 19.81, demonstrating robust quantification of non-volatile solids.

Benefits and practical applications

• Rapid, non-destructive analysis enabling multiple parameter determination in a single measurement
• Minimal sample preparation and operator training
• Improved throughput for in-process quality control in paint production
• Real-time monitoring potential for bath stability and consistency

Future trends and potential applications

Advances in near-infrared detector technology and machine learning algorithms are expected to further enhance model robustness and expand the range of measurable properties. Integration of Vis-NIR probes into production lines or flow-through cells could enable continuous monitoring of electrophoretic baths. Additionally, extension to other coating chemistries, pigment systems, and multilayer formulations presents promising opportunities.

Conclusion

The Vis-NIR spectroscopic method provides a fast, reliable alternative to conventional laboratory assays for amine number and solid content in dipping paints. The developed PLS models deliver high accuracy and can be operated by non-specialists, supporting efficient quality control and process optimization in the paint industry.

Reference

Metrohm. NIR Application Note NIR-30: Determination of amine number and solid content of dipping paint.