Simultaneous determination of multiple quality parameters in epoxy resins using Vis-NIR spectroscopy

Importance of the Topic

Epoxy resins are key thermosetting polymers used as adhesives, coatings, and insulators. Quality parameters such as the weight per epoxy equivalent (WPE) and viscosity at different temperatures critically influence curing behavior and final material performance. Conventional titration and rheological methods are time consuming and require chemicals. Vis-NIR spectroscopy offers a fast, reagent-free alternative for comprehensive quality control.

Objectives and Study Overview

This study evaluates visible–NIR spectroscopy for the simultaneous quantification of WPE and viscosities of epoxy resins at 25, 150, 175, and 200 °C. A set of 175 samples with known reference values serves to develop and validate multivariate calibration models.

Methodology

Spectral data were acquired in reflection mode over 400–2500 nm using moving samples to reduce inhomogeneity. Four replicate spectra per sample were collected. Pretreatments included second-derivative processing and standard normal variate correction for viscosity models. Partial least squares regression with optimized factor numbers and selected wavelength ranges was applied to build quantitative models. Reference values were obtained by titration for WPE and rheometry for viscosities according to ASTM and ISO standards.

Used Instrumentation

• Metrohm NIRS DS2500 Analyzer
• DS2500 large sample cup
• Vision Air 2.0 Complete software

Main Results and Discussion

The WPE model (PLS, 10 factors) achieved R2 = 0.989 with SEC = 3.2 g/Eq and SECV = 3.4 g/Eq over 650–770 g/Eq. Viscosity at 25 °C (PLS, 10 factors) showed R2 = 0.973, SEC = 12.5 mPa·s, SECV = 13.9 mPa·s. Models for 150 °C (PLS, 8 factors) and 175 °C (PLS, 7 factors) viscosities yielded R2 values of 0.995 and 0.991, respectively, with low prediction errors. The 200 °C model had reduced performance (R2 = 0.907). High correlations between NIR predictions and reference data confirm the feasibility of simultaneous multi-parameter analysis.

Benefits and Practical Applications of the Method

Vis-NIR spectroscopy eliminates sample prep and reagent costs, delivering results in under one minute. It enables accelerated raw material inspection, real-time process monitoring, and final product control. The technique supports high-throughput quality assurance in research and industrial environments.

Future Trends and Possibilities for Application

Advances in reference method accuracy are expected to further improve calibration quality. The approach may extend to other polymer systems and composite materials. Integration of inline NIR sensors can enable continuous monitoring and digital process control for smart manufacturing.

Conclusion

Visible–NIR spectroscopy has been demonstrated as a rapid, accurate, and cost-effective tool for simultaneous determination of chemical and rheological properties in epoxy resins. The developed PLS models provide robust predictions for WPE and viscosity at multiple temperatures, supporting efficient quality control.