Estimation of amine value in epoxies with Raman spectroscopy

Significance of the topic

Accurate quantification of reactive amine groups governs the stoichiometry in epoxy formulations, ensuring complete curing and optimal mechanical and chemical properties. Traditional titration methods, while precise, are time-intensive and generate hazardous waste, highlighting the need for faster, greener alternatives.

Objectives and study overview

This study evaluates Raman spectroscopy as a rapid, nondestructive secondary method to estimate the amine value (AV) of epoxy hardeners. By correlating Raman data with standard potentiometric titration results, the work aims to establish a reliable calibration model and assess performance on calibration, validation, and blind sample sets.

Methodology and instrumentation

Hardeners were dissolved in glacial acetic acid to prepare calibration standards spanning 0 to 554 mg per 25 mL, validation samples at 308 and 514 mg, and five blind samples. AV was determined by potentiometric titration following ASTM D2073 and by Raman spectral analysis using contactless measurements through the sample beaker.

Instrumentation used

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

Main results and discussion

Titration of calibration standards yielded a perfect linear fit (R2=1.0000, RMSEC=0.018) and validation deviations below 0.2%. Raman spectra revealed a distinct hardener peak at 1002 cm−1 and minimal overlap with acetic acid in selected regions. Simple linear regression of the 1003 cm−1 band achieved R2=0.9965, while a multiband chemometric model reached R2=0.9999 and RMSEC=0.79, predicting validation AV within ±0.5%. Blind sample predictions matched titration within 0.1–4.4% (RMSE=2.53), acknowledging titration’s intrinsic ±2% error margin.

Benefits and practical applications of the method

Raman spectroscopy enables rapid (seconds), nondestructive, contactless AV estimation without reagents or extensive sample preparation. It supports in-process and quality control workflows, reducing analysis time and chemical waste in industrial epoxy production.

Future trends and opportunities

Expanding the calibration dataset, integrating advanced chemometric algorithms, and deploying inline or through-container measurements can enhance robustness and enable real-time process monitoring. Application to other reactive functional groups and automated QC platforms offers further potential.

Conclusion

Raman spectroscopy demonstrates strong agreement with potentiometric titration for AV determination in epoxy hardeners, providing a fast, reliable secondary method that complements conventional analysis in quality control and process development.

References

1. Sukanto H; Raharjo WW; Ariawan D; et al. Epoxy Resins Thermosetting for Mechanical Engineering. Open Engineering. 2021;11(1):797–814. doi:10.1515/eng-2021-0078.
2. ASTM D2073-07R19. Standard Test Methods for Total, Primary, Secondary, and Tertiary Amine Values of Fatty Amines by Alternative Indicator Method. ASTM International; 2019.
3. Izumi A; Shudo Y; Shibayama M. Network Structure Evolution of a Hexamethylenetetramine-Cured Phenolic Resin. Polym J. 2019;51(2):155–160. doi:10.1038/s41428-018-0133-8.