Vis-NIR spectroscopic analysis of wood pulps for multiparameter prediction

Importance of the Topic

The pulp and paper industry produces over 400 million tons of material each year, making rapid and reliable quality control essential. Conventional laboratory assays for kappa number, freeness, strength, and density are time-consuming and generate chemical waste. Visible-near-infrared spectroscopy (Vis-NIRS) offers a fast, non-destructive alternative that can streamline process control and reduce environmental impact.

Objectives and Study Overview

• Develop quantitative Vis-NIR models to predict six pulp quality parameters: kappa number, pulp freeness (CSF), breaking strength (SCT), buckling strength (RCT), tensile strength, and applied density.
• Compare Vis-NIR predictions with standard laboratory reference methods.
• Assess the feasibility of simultaneous multi-parameter analysis in under one minute.

Methodology

Two hundred thirty-six standard hand-sheet samples were measured in triplicate in reflection mode across 400–2500 nm. Spectral data were pre-treated using a second derivative and Standard Normal Variate to minimize baseline and scatter effects. Partial Least Squares regression with internal cross-validation and defined wavelength regions (excluding water bands around 1400 and 1900 nm) was employed to develop individual prediction models.

Used Instrumentation

• NIRS DS2500 Analyzer (Metrohm 2.922.0010)
• Vision 4.03 chemometric software (Metrohm 6.6069.102)

Main Results and Discussion

• Kappa number: R² = 0.986, SECV = 3.0 mg, showing excellent agreement with titration.
• Applied density: R = 0.903, SECV = 0.0308 g/cm³, indicating precise mass-per-volume prediction.
• Pulp freeness (CSF): R² = 0.785, SECV = 71 mL, moderate correlation reflecting structural variability.
• Breaking strength (SCT): R² = 0.803, SECV = 2.6 MPa, suitable for routine tensile testing.
• Buckling strength (RCT): R² = 0.768, SECV = 7.4 MPa, consistent with ring crush standards.
• Tensile strength: R = 0.874, SECV = 5 MPa, strong performance across 8–68 MPa range.

Benefits and Practical Applications

Vis-NIR reduces analysis time to approximately 30 seconds per sample and eliminates the need for reagents. It enables rapid, on-site quality checks, real-time process monitoring, and integration into quality assurance workflows without hazardous waste generation.

Future Trends and Opportunities

Advances in portable NIR devices and enhanced chemometric algorithms will further enable online process control. Expanding the method to diverse pulp grades, integrating machine-learning models, and combining spectral data with process sensors can lead to comprehensive digital monitoring systems in pulp and paper mills.

Conclusion

This study demonstrates that Vis-NIR spectroscopy can simultaneously predict six critical pulp parameters with high accuracy and speed. The approach offers a sustainable and efficient alternative to traditional laboratory assays, supporting improved productivity and environmental stewardship in pulp production.

Reference

No external literature references were provided in the original application note.