Estimation of amine value in epoxies with Raman spectroscopy

Significance of the Topic

The amine value (AV) of epoxy hardeners is a pivotal parameter for ensuring correct stoichiometry and full cure of resin formulations. Traditional titration-based AV determination is accurate but slow, laborious, and generates chemical waste. Raman spectroscopy presents a rapid, nondestructive, and contactless technique that can streamline quality control and in-process monitoring of epoxy systems.

Objectives and Study Overview

This study evaluates the feasibility of using Raman spectroscopy as a secondary method for AV estimation by correlating spectral data with results from standard potentiometric titration (ASTM D2073). Calibration, validation, and blind sample sets were measured to assess method performance.

Methodology and Instrumentation

Samples of a commercial epoxy hardener were dissolved in glacial acetic acid to match titration conditions. Calibration standards covered 0–554 mg of hardener per 25 mL solvent; validation and blind sets tested intermediate and unknown concentrations. Raman spectra were collected through the beaker wall without sample preparation, and titrations were run in parallel to provide reference AV values.

Instrumentation Used

• Raman system: 785 nm laser excitation; BAC102 fiber probe; SpecSuite software
• Titration system: Metrohm 907 Titrando; 50 mL Dosino burette; Solvotrode electrode; OMNIS software

Main Results and Discussion

• Titration calibration yielded AVs from 30.8 to 258.7 mg KOH/g with R²=1.0000 and RMSEC=0.018.
• Raman spectra featured a strong amine-associated peak at 1002 cm⁻¹; low overlap regions (650–850, 930–1270, 1550–1630 cm⁻¹) supported quantification.
• Simple linear regression of the 1003 cm⁻¹ peak gave R²=0.9965; an optimized model using multiple bands improved to R²=0.9999, RMSEC=0.79.
• Blind sample predictions deviated by 0.1–4.4% (RMSE=2.53), aligning closely with titration (±2% inherent error).
• The total uncertainty reflects both Raman performance and propagated titration variability; expanding the calibration set is expected to enhance robustness.

Benefits and Practical Applications

Raman spectroscopy enables rapid AV screening without reagents or cleanup, making it well suited for high-throughput quality control, process monitoring, and intermediate evaluation in industrial epoxy production.

Future Trends and Potential Applications

Integration of portable Raman probes for inline process control, development of advanced chemometric models, and extension to other functional group analyses will further broaden the technique’s utility in real-time manufacturing environments.

Conclusion

Raman spectroscopy has been demonstrated as a reliable secondary method for AV determination in epoxy hardeners. The strong correlation with potentiometric titration, combined with speed and nondestructive operation, makes it an attractive tool for supplementary quality assurance and process optimization.

References

1. Sukanto H., Raharjo W. W., Ariawan D. et al. Epoxy Resins Thermosetting for Mechanical Engineering. Open Engineering 2021;11(1):797–814.
2. ASTM D2073-07r19. Standard Test Methods for Total, Primary, Secondary, and Tertiary Amine Values of Fatty Amines by Alternative Indicator Method.
3. Izumi A., Shudo Y., Shibayama M. Network Structure Evolution of a Hexamethylenetetramine-Cured Phenolic Resin. Polymer Journal 2019;51(2):155–160.