Moisture analysis in caprolactam

Importance of the Topic

Caprolactam is the key monomer for Nylon 6 production and serves in fibers, resins, and polymers. Its hygroscopic and water-soluble nature demands precise moisture control to ensure product quality, optimize processes, and minimize downtime in industrial operations.

Study Objectives and Overview

This application note demonstrates the development and validation of a near-infrared spectroscopy (NIRS) method for rapid, non-destructive moisture determination in caprolactam. The aim is to replace conventional Karl Fischer titration with a faster, waste-free alternative suitable for routine quality control.

Methodology and Instrumentation

A total of 44 caprolactam samples spanning different moisture levels were measured by diffuse reflectance NIRS. Spectra were collected using a robust Metrohm DS2500 Solid Analyzer over the 400–2500 nm range. To address sample heterogeneity, a rotating large sample cup was employed. Pre-processing of spectral data involved a second derivative transformation, followed by outlier detection using a maximum distance algorithm. Calibration was performed with partial least squares (PLS) regression against Karl Fischer titration reference values, and model performance was evaluated through cross-validation.

Used Instrumentation:

• DS2500 Solid Analyzer
• Vision Air Complete spectroscopy software
• DS2500 large sample cup for rotating sample measurement

Key Results and Discussion

The NIRS calibration model achieved a correlation coefficient (R²) of 0.98 compared to titration results, indicating excellent agreement. The standard error of calibration (SEC) was 53 mg/L and the standard error of cross-validation (SECV) was 58 mg/L, demonstrating high sensitivity and reproducibility. Spectral pre-treatment effectively minimized outliers and enhanced model robustness.

Benefits and Practical Applications

• Rapid moisture quantification in under one minute
• No chemical reagents or waste generation
• Minimal or no sample preparation
• Operational stability in harsh production environments

Future Trends and Opportunities

Emerging prospects include integration of NIRS into inline process monitoring, application of machine learning for adaptive calibration updates, and expansion of the approach to other hygroscopic monomers for real-time quality assurance.

Conclusion

Near-infrared spectroscopy using the DS2500 Analyzer provides a fast, accurate, and environmentally friendly solution for moisture analysis in caprolactam, supporting enhanced product quality and operational efficiency.