Analysis of pulp and paper using near-infrared spectroscopy

Importance of Topic

Near-infrared (NIR) spectroscopy has emerged as a powerful analytical tool in the pulp and paper industry, offering rapid, non-destructive, and cost-effective measurement of critical quality attributes. By replacing or supplementing traditional wet chemistry methods, NIR enables faster decision-making, reduced chemical usage, and improved process control across raw material intake, processing, and finished goods inspection.

Objectives and Study Overview

This bulletin compiles 17 application studies demonstrating how NIR spectroscopy quantifies parameters such as softwood/ hardwood ratio, kappa number (lignin content), moisture, resin and wax levels, polymer additives, coating weights, and specialty components in pulp, paper, and related materials. Each case evaluates feasibility, calibrates models, and validates performance under both laboratory and production conditions.

Methodology and Instrumentation

All applications employ NIR scanning in the 1100–2500 nm range using reflectance or transmission sampling modes. Samples are presented in cuvettes, coarse sample cups, transport modules, or remote probe heads, often rotated or spun to average inhomogeneities. Calibration strategies include single-wavelength least-squares regression for simple constituents and multivariate (partial least-squares) modeling to resolve overlapping bands and correct pathlength variations. Derivative pretreatments are applied in some studies to enhance spectral features and reduce baseline effects.

Used Instrumentation

• NIRS XDS RapidContent Analyzer Solids (2.921.1120)
• NIRS XDS RapidLiquid Analyzer (2.921.1410)
• NIRS XDS Process Analyzer with DirectLight/NonContact probe (2.928.0310)

Key Results and Discussion

• Softwood proportion in pulp determined with moisture calibration at 1960 nm (SEC ~0.1%).
• Kappa number in blends and blowline samples measured near 1670–1680 nm (SEC ~0.5–1.2 units).
• Lignin content in pressed pulp tracked using dual-wavelength models (SEC ~1%).
• Wax and phenolic resin in wood fibers quantified at ~1728–2172 nm (SEC 0.1–0.4%).
• Superabsorbent polymers in fluff pulp mapped by remote reflectance at 1752 nm (SEC ~6%).
• Resin in paper, moisture and lacquer on foil-backed paper, and calcium propionate sprayed levels all show high sensitivity in targeted regions (SEC in low ppm or percent levels).
• Coating weights on various paper substrates and triacetin or moisture in fiber rods demonstrate the versatility of NIR for surface and bulk analyses.
• Nitrocellulose in casings and asphalt in expansion joint mats quantified via reflectance and PLS modeling (SEC <1%).
• Silicone on tissue paper monitored successfully with non-contact reflectance probes.

Benefits and Practical Applications

By integrating NIR spectroscopy into quality control and process monitoring, pulp and paper producers gain real-time insights, reduce sample preparation time, minimize solvent and reagent consumption, and enhance product consistency. Inline and at-line installations support continuous production adjustments, while simple laboratory setups accelerate R&D and troubleshooting.

Future Trends and Opportunities

Advances in chemometric algorithms, machine learning, and miniaturized sensor designs will expand NIR capabilities for on-machine and handheld monitoring. Coupled with digital process control and Industry 4.0 frameworks, these developments promise predictive quality assurance, remote diagnostics, and seamless data integration across supply chains.

Conclusion

The collected case studies confirm that NIR spectroscopy offers robust, accurate, and versatile analysis of a wide range of pulp and paper parameters. Its adoption can significantly enhance productivity, product quality, and environmental sustainability in the industry.