Analysis of pulp and paper using near-infrared spectroscopy

Importance of the topic

Near-infrared spectroscopy (NIR) offers rapid, non-destructive, and accurate analysis of critical parameters in pulp and paper manufacturing. This technology significantly reduces turnaround time compared to conventional wet chemistries, improving process efficiency and product quality while lowering operational costs.

Objectives and overview

This bulletin demonstrates multiple NIR applications across pulp and paper quality control. Seventeen case studies illustrate the determination of softwood ratio, kappa number, lignin, resin, wax, coatings, moisture, polymers, and specialty additives in pulps, papers, and related materials.

Used instrumentation

• NIRS XDS RapidContent Analyzer Solids (model 2.921.1120): core instrument for reflectance and transmission studies.
• NIRS XDS RapidLiquid Analyzer (model 2.921.1410): used for moisture analysis in liquid coatings.
• NIRS XDS Process Analyzer (model 2.928.0310) with DirectLight non-contact sampling: applied in inline/process monitoring of silicone on tissue paper.

Methodology

Most studies employed reflectance mode over the 1100–2500 nm range, with occasional transmission measurements in thin cuvettes. Calibration strategies included single-wavelength linear regression on characteristic absorption bands and multivariate partial least squares (PLS) to deconvolute overlapping signals and account for path length variations. Sample presentation ranged from standard cups and coarse cells to remote reflectance heads and spinning modules, ensuring representative averaging of paper and pulp matrices.

Main results and discussion

• Softwood content and ratio of hardwood/softwood in pulps were quantified with moisture SECs below ±0.2% and tree-species classification achieved via discriminant analyses.
• Kappa number and its logarithmic estimate (K No.) in various pulp blends and blowline samples were predicted with SECs around 0.5–1.2 kappa units by targeting lignin-related bands near 1670–1680 nm.
• Lignin in pressed pulps, resin and wax in wood fibers, and phenolic/formaldehyde resins were monitored with SECs typically <1% by focusing on absorption bands at 1556–2172 nm for lignin and 1725–2310 nm for wax/resins.
• Moisture in papers, coatings, and lacquer layers on foil-backed papers was detected via water combination bands near 1960 nm and 2346 nm, enabling rapid moisture and lacquer-weight assessments.
• Coating levels on diverse papers and Mylar, nitrocellulose in casings, triacetin in acetate fibers, super absorbent polymers in fluff pulps, asphalt in joint mats, and silicone on tissue papers were successfully quantified using tailored calibration wavelengths and PLS models with SECs ranging from 0.1% to approximately 1%.

Benefits and practical applications

• Non-destructive, real-time analysis supports inline quality control and reduces batch rejection.
• Elimination of hazardous reagents and waste associated with traditional wet methods.
• Broad multi-parameter capability from a single instrument accelerates laboratory throughput.
• Data driven decisions enhance consistency in product performance and meet stringent industry standards.

Future trends and potential applications

Advances in chemometric algorithms, miniaturized and portable NIR devices, and integration with process automation will expand inline monitoring of pulp and paper processes. Coupling NIR with machine learning and digital twins can enable predictive maintenance, real-time control of fibre blends, and optimization of energy-intensive bleaching and coating stages.

Conclusion

NIR spectroscopy has proven to be a versatile, efficient, and accurate tool for comprehensive quality assessment in pulp and paper industries. The broad range of case studies validates its capability to replace traditional laboratory methods, offering significant gains in speed, sustainability, and cost-effectiveness.