Vis-NIR spectroscopic analysis of wood pulps for multiparameter prediction

Significance of the topic

Efficient quality control in pulp production is crucial to maintain product consistency, reduce processing costs and limit environmental impact. Traditional laboratory assays for key parameters such as kappa number, freeness and mechanical strength are time-consuming and generate chemical waste. Implementing rapid, non-destructive spectroscopic techniques can streamline operations and support real-time monitoring in the pulp and paper industry.

Objectives and overview

This study aimed to evaluate visible-near-infrared (Vis-NIR) spectroscopy as a single-step method to quantify six critical wood pulp attributes:

• Kappa number
• Applied density
• Pulp freeness
• Breaking strength (SCT)
• Buckling strength (RCT)
• Tensile strength

Instrumentation used

The following equipment was employed for spectral acquisition and data analysis:

• NIRS DS2500 Analyzer (Metrohm part no. 2.922.0010) operating in reflection mode over 400–2500 nm
• Vision 4.03 software (Metrohm part no. 6.6069.102) for chemometric modeling

Methodology

A set of 236 standard paper hand sheets with reference values for the target parameters was measured in triplicate. Spectral regions affected by water absorption around 1400 nm and 1900 nm were excluded. Data preprocessing included a second derivative transformation and standard normal variate correction to minimize baseline and scatter variations. Partial least squares (PLS) regression models were developed with internal cross-validation to predict each parameter simultaneously.

Main results and discussion

Strong correlations between Vis-NIR predictions and reference analyses were observed:

• Kappa number: R² = 0.986; SEC = 2.9 mg; SECV = 3.0 mg
• Pulp freeness: R ≈ 0.785; SEC = 68 mL; SECV = 71 mL
• Breaking strength: R² = 0.803; SEC = 2.5 MPa; SECV = 2.6 MPa
• Buckling strength: R² = 0.768; SEC = 7.2 MPa; SECV = 7.4 MPa
• Tensile strength: R = 0.874; SEC = 5 MPa; SECV = 5 MPa
• Applied density: R = 0.903; SEC = 0.0292 g/cm³; SECV = 0.0308 g/cm³

These results demonstrate the feasibility of capturing multiple pulp quality metrics within a 30 s measurement, offering significant time savings over conventional chemical and mechanical tests.

Benefits and practical applications

Vis-NIR spectroscopy delivers:

• Rapid, simultaneous quantification of multiple parameters
• Elimination of chemical reagents and associated waste
• Non-destructive analysis suitable for high-throughput quality control
• Cost and time efficiency for laboratory and process environments

Future trends and potential applications

Emerging developments may further enhance Vis-NIR utility:

• Integration of online and inline process monitoring systems
• Advanced chemometric methods and machine learning algorithms
• Portable spectrometers for on-site measurements
• Combination with hyperspectral imaging for spatially resolved analysis

Conclusion

Vis-NIR spectroscopy using a DS2500 analyzer and PLS modeling provides a robust, fast and environmentally friendly alternative to traditional pulp testing. Its ability to predict key quality attributes in a single measurement supports efficient QA/QC workflows and aligns with sustainable manufacturing goals.

References

• Metrohm, NIR Application Note NIR-39: Vis-NIR spectroscopic analysis of wood pulps for multiparameter prediction