Determination of amine number and solid content of dipping paint

Importance of the Topic

The electrophoretic deposition of dipping paints provides an efficient way to apply uniform, corrosion-resistant coatings to metal substrates. Accurate determination of amine number and solid content is critical for ensuring coating performance, consistency in layer thickness, and optimal curing behavior. Traditional wet-chemical methods are time-consuming and require skilled personnel, creating a demand for rapid, user-friendly analytical alternatives.

Goals and Overview of the Study

This application note demonstrates how visible-near infrared (Vis-NIR) spectroscopy can be applied to quantify both amine number and solid content in water-based electrophoretic dipping paint. The study aims to replace conventional laboratory assays with a single, non-destructive spectroscopic measurement that can be performed by non-experts, thereby streamlining quality control workflows.

Methodology and Instrumentation

The spectral data were acquired using a NIRS XDS SmartProbe Analyzer in transflection mode. White dipping paint samples with known ranges of solid content (11.5–20.4 %) and amine number (37.5–48.3 mmol/kg) were continuously stirred in a beaker to prevent sedimentation during measurement. Chemometric modeling was performed in Vision Software 4.0.3 using Partial Least Squares (PLS) regression. Pre-processing included second derivative transformation (segment size 10 nm) and Standard Normal Variate (SNV) correction to mitigate baseline shifts and scattering effects. Internal cross-validation assessed model robustness.

Main Results and Discussion

Amine Number Model:

• PLS factors: 5
• Wavelength range: 1120–1920 nm
• Coefficient of determination (R2): 0.9678
• Standard error of calibration (SEC): 0.7345 mmol/kg
• Standard error of validation (SEV): 2.9284 mmol/kg
• F-value: 156.3

Solid Content Model:

• PLS factors: 6
• Wavelength range: 1120–1920 nm
• R2: 0.9239
• SEC: 0.7484 %
• SEV: 0.8606 %
• F-value: 331.1

Both models demonstrate strong correlations between Vis-NIR predictions and reference methods, indicating reliable quantification across the tested concentration ranges.

Benefits and Practical Applications

Vis-NIR spectroscopy offers:

• Rapid, simultaneous measurement of multiple quality parameters
• Minimal sample preparation and consumables
• Operation by non-specialists with minimal training
• Potential for at-line or inline process monitoring

These advantages support tighter process control in coating production and improved consistency in end-product performance.

Future Trends and Potential Uses

Advances in spectrometer miniaturization and real-time data processing may enable fully automated, inline monitoring of electrophoretic coating baths. Expansion of calibration libraries could extend the method to additional additives, binder types, and paint chemistries. Integration with manufacturing execution systems (MES) could further enhance quality assurance and regulatory compliance.

Conclusion

This study confirms that Vis-NIR spectroscopy, combined with robust chemometric models, provides an efficient, accurate alternative to conventional methods for determining amine number and solid content in dipping paints. Implementation of this approach can accelerate quality control, reduce labor, and enable proactive process adjustments.

Used Instrumentation

• NIRS XDS SmartProbe Analyzer
• Vision Software 4.0.3