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

Importance of the Topic

Epoxy resins are essential thermosetting polymers widely used as adhesives, coatings and insulators. Reliable quality control of their chemical reactivity and flow behavior is critical for consistent performance. Traditional titration and rheology methods are time consuming and require chemicals. Visible-near infrared spectroscopy (Vis-NIRS) offers a rapid, non-destructive alternative for simultaneous assessment of multiple quality parameters.

Objectives and Overview of the Study

The aim of this study was to demonstrate that Vis-NIRS can accurately predict weight per epoxy equivalent (WPE) and viscosity at various temperatures in a single measurement. A total of 175 epoxy resin samples with reference values determined by standard methods were analyzed by Vis-NIRS in reflection mode over 400–2500 nm. Chemometric models were developed to quantify WPE and viscosities at 25, 150, 175 and 200 °C.

Methodology and Instrumentation

Samples were measured in large sample cups using a Metrohm NIRS DS2500 Analyzer. Each sample was scanned in moving mode to reduce inhomogeneity effects, and four spectra per sample were acquired. Spectral pre-treatment included second or first derivatives and scatter corrections. Partial least squares (PLS) regression with cross-validation was used to build prediction models.

Instrument setup

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

Main Results and Discussion

Chemometric models achieved high correlation with reference methods:

• WPE: R2 = 0.989, SEC = 3.2 g/eq, SECV = 3.4 g/eq (650–770 g/eq)
• Viscosity at 25 °C: R2 = 0.973, SEC = 12.5 mPa·s, SECV = 13.9 mPa·s (402–1044 mPa·s)
• Viscosity at 150 °C: R2 = 0.995, SEC = 51.8 mPa·s, SECV = 58.6 mPa·s (1950–4630 mPa·s)
• Viscosity at 175 °C: R2 = 0.991, SEC = 101.4 mPa·s, SECV = 143.5 mPa·s (2125–4733 mPa·s)
• Viscosity at 200 °C: R2 = 0.907, SEC = 182.6 mPa·s, SECV = 196.3 mPa·s (2210–4250 mPa·s)

High R2 values and low prediction errors confirm that Vis-NIRS can reliably replace traditional assays for rapid quality assessment.

Benefits and Practical Applications

Vis-NIRS reduces analysis time to under one minute and eliminates the need for reagents, enabling eco-friendly and cost-effective quality control. It supports real-time monitoring of raw materials, in-process control and final product inspection, accelerating decision-making in production environments.

Future Trends and Opportunities

Potential developments include:

• Integration into process analytical technology for online monitoring
• Extension to other thermosetting systems and composite materials
• Application of advanced machine learning algorithms for enhanced accuracy
• Development of portable Vis-NIR instruments for field and on-site analysis

Conclusion

This study confirms that Vis-NIR spectroscopy is a powerful tool for simultaneous quantification of epoxy equivalent and viscosities at multiple temperatures. Implementation of optimized chemometric models offers rapid, accurate and non-destructive quality control for epoxy resin production.

Reference

• Metrohm Application Note NIR-067: Simultaneous determination of multiple quality parameters in epoxy resins using Vis-NIR spectroscopy, Version 1, August 2018