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All kinds of materials can be used to make paper, e.g., reeds, flax, cotton, and even bagasse (sugarcane pulp). However, most paper is made from treated wood fibers. Processing wood for this purpose is done with pulp mills and paper mills which involve the use of chemicals and a lot of energy. Screening and quality control (QC) are important measures to take along the entire production chain from the timber to the finished paper product. This blog article shows how near-infrared spectroscopy (NIRS) is able to monitor several key QC parameters simultaneously in the pulp and paper manufacturing process.

Introduction to the pulp and paper industry

The pulp and paper industry produces and sells cellulose-based products derived from different types of wood. Pulp and paper mills process timber using mechanical and chemical methods to produce various paper products for all kinds of uses.

In Figure 1, the global paper, tissue, and board production share by country in 2019 is shown [1]. China accounts for the largest share, followed by the United States, Brazil, and Japan. These five countries account for more than half of the global production.

Figure 1. Global paper, tissue, and board production share by country in 2019 [1].

Wood, the precursor to paper, is primarily composed of cellulose, hemicellulose, and lignin. Differences in its microstructure allow wood to be classified as either softwood or hardwood. Coniferous trees (with needles and cones, e.g., pines, firs, and spruces) are composed of softwood. Deciduous trees (with leaves and seeds, e.g., oak, beech, walnut) are hardwood species.

Hardwoods have shorter fibers and produce weaker paper. However, the final product is smoother, more opaque, and better suited for printing. Softwoods yield long and strong fibers that impart strength to paper. This wood type is more commonly used for making boxes and other packaging.

The pulp and paper manufacturing process

The pulp and paper manufacturing process consists of four main stages, namely: raw material preparation, pulping, bleaching, and papermaking. Each of these steps is described in the sections below. A general overview of the complete process is shown in Figure 2.

Preparation of raw materials (timber) typically involves size reduction, debarking, chipping, and screening. 

As the name implies, debarking removes the outer layer of bark from logs prior to chipping since bark is a contaminant in the pulping process. After debarking, the logs are run through a chipping machine. This produces wood chips of a consistent size and shape in order to maximize the efficiency of the pulping process. 

The final stage of raw material preparation includes screening of the wood chips. Wood chips that are too small or large are separated from the bulk and either used for energy recovery or sent for further processing.

During the process of chemical pulping, lignin and cellulose linkages are broken down. This occurs in a high-pressure digester using various chemicals. The most common method used for wood pulping is the Kraft process. 

An aqueous solution of sodium hydroxide and sodium sulfide, known as «white liquor», selectively dissolve the lignin. After two to four hours, the mixture is discharged from the digester. The pulp is washed to separate it from the byproducts («black liquor», the pulping chemicals, and wood waste). 

This treated pulp, called «brown stock» at this point in the process, is ready to be bleached.

Bleaching involves adding chemicals to the treated pulp in varying combinations, depending on the end use of the product. 

The most common bleaching chemicals are chlorine, chlorine dioxide, hydrogen peroxide, oxygen, caustic soda, and sodium hypochlorite. Concerns over forming chlorinated byproducts such as dioxins, furans, and chloroform have resulted in a shift away from the use of chlorinated chemicals in the bleaching process. 

Effluent from this step is collected in tanks and is either reused in other stages as wash water or sent for wastewater treatment.

Pulp preparation includes slushing, mechanical pretreatments, blending of pulps with different qualities, and introducing fillers and other additives to make paper. The type of process differs for each kind of paper quality.

Figure 2. Illustration of the general pulp and paper manufacturing process.

Screening and monitoring various quality control parameters before, during, and after the manufacturing process is critical to make high-quality paper products that consumers can rely on. Near-infrared (NIR) spectroscopy is a method that can be used for quality control of all stages during the production of pulp and paper. This article explains how NIRS works in general and shows how it is superior to other methods for QC and screening in the pulp and paper industry using the example of wood pulp.

Near-infrared spectroscopy technology overview

Light and matter interact in many ways (e.g., absorption, reflection, scattering, emission, and transmission). When discussing the light used in spectroscopic methods, it is typically described in wavelength or wavenumbers. NIR spectrometers, such as the Metrohm DS2500 Solid Analyzer, generate spectra full of information by measuring the light-matter interaction (Figure 3). 

Figure 3. Near-infrared spectra resulting from the interaction of NIR light with paper samples. Note the peaks attributed to moisture, cellulose, and hydroxyl content.

The sensitivity of NIRS to the presence of certain functional groups makes it an excellent method to quantify chemical parameters like water content (moisture), kappa number, lignin content, and resin content. As this interaction is also dependent upon the sample matrix, the detection of physical parameters (e.g., density and strength) is also possible with NIRS. 

The sheer amount of information contained about a sample in the resulting spectrum makes NIRS ideal for quick multi-parameter analysis.

NIRS measuring mode for wood pulp and paper products

Deciding which NIRS measuring mode to use depends on the sample type. When analyzing solids like wood pulp and paper products, the diffuse reflection mode should be used (Figure 4). In this measuring mode, the sample is exposed to NIR light, absorbing some of it. The unabsorbed NIR light reflects back and is measured by the detector. 

Figure 4. a) Measurement of solid samples is typically done in sample cups. b) The measurement mode is known as diffuse reflection, where the sample is exposed to light and the diffuse reflected light gets absorbed.

Advantages for paper producers when using NIRS

NIR spectroscopy offers users many advantages for analytical applications—particularly when it comes to quality control and screening. Time and cost savings are among the top benefits for multiple reasons. Sample preparation is not required, and results are delivered in seconds. NIRS is a multi-parameter analysis technique, eliminating the need to perform several separate time-consuming analyses on different instruments for the same results. The technique is also non-destructive and does not require the use of any chemical reagents.

Aside from these points, NIRS is also approved for quality control purposes by standard organizations such as ASTM [2]. Furthermore, NIRS is user-friendly and can be operated by non-technical personnel, distinguishing it from more complex analytical techniques.

Product screening and QC parameters for pulp and paper production

Research, development, and quality control all rely on laboratory testing. Pulp and paper mills use several standardized test methods for the determination of physical and chemical properties of the raw materials, intermediates, and final products. The most relevant test parameters for screening and QC of pulp and paper products are found in Table 1.

Table 1. Various screening and quality control parameters for pulp and paper products along with the typical method used for analysis.

Part of process Parameter Conventional analysis method
Raw material analysis Bulk density Densimetry
Moisture content Karl Fischer titration
Research for genetic improvement:
Pulp yield Gravimetric analysis
Cellulose content Gravimetric analysis
Lignin content Hydrolysis / Gravimetric analysis / Photometry
Chemical pulping and bleaching Lignin content Hydrolysis / Gravimetric analysis / Photometry
Cellulose content Gravimetric analysis
Resin content  HPLC
Kappa number Titration
Breaking strength Short span Compression Test (SCT)
Buckling strength Ring Crush Test (RCT)
Tensile strength Tensometry
Pulp freeness Canadian Standard Freeness Test (CSF)
Liquor and crude tall oil Titration
Finished product QC Moisture content Karl Fischer titration
Paper grammage Gravimetric analysis
Ash content Oven
Silicone levels GC
Hardwood / Softwood ratio Microscopy
Coating layers Photometry

A turnkey solution for pulp and paper analysis

The Metrohm NIRS solution for pulp analysis comes with a ready-to-use pre-calibration model for the determination of the kappa number, density, buckling strength, breaking strength, tensile strength, and pulp freeness (Table 2). Because of this pre-calibration set, this solution can be used as a turnkey starter model without any prior method development.

Table 2. Available NIRS pre-calibrations for analysis of key quality parameters in pulp products.

Parameter Range Standard Error of Cross Validation (SECV) R2
Applied density (g/cm3) 0.2–0.65 0.039 0.855
Buckling strength (MPa) 20–94 7.4 0.821
Breaking strength (MPa) 6–32 2.6 0.828
Tensile strength (MPa) 6–68 5 0.905
Kappa number 0–175 3.8 0.996
Pulp freeness (mL) 130–800 72 0.766

Table 3 shows the results of the quality control of processed wood pulp. This example shows that visible near-infrared spectroscopy (Vis-NIRS) can simultaneously determine six different QC parameters (kappa number, applied density, pulp freeness, breaking strength, buckling strength, and tensile strength) in wood pulp. 

Table 3. Example of multi-component analysis in wood pulp using Vis-NIRS.


Parameter		 Figures of merit
R2 Standard Error of Calibration (SEC) SECV
Kappa number 0.986 2.9 mg 3.0 mg
Applied density 0.903 0.0292 g/cm3 0.0308 g/cm3
Pulp freeness 0.785 68 mL 71 mL
Breaking strength 0.803 2.5 MPa 2.6 MPa
Buckling strength 0.768 7.2 MPa 7.4 MPa
Tensile strength 0.875 5 MPa 5 MPa

Summary

Using NIRS for quality control and screening of pulp and paper products is preferable to other conventional reference laboratory methods. Not only does this spectroscopic method save producers time and ensure a high-quality product, it is also environmentally friendly and easy to use. Simultaneous analysis of multiple key quality parameters is possible with a single scan. 

Results for the kappa number, applied density, pulp freeness (CSF), breaking strength (SCT), buckling strength (RCT), and tensile strength in wood pulp can be delivered in 30 seconds, even by non-technical personnel. Tighter control can be taken over production as data is generated more frequently than with other laboratory techniques.

References

[1] Statista Research Department. Paper production global capacity share by country. Statista. https://www.statista.com/statistics/664968/global-paper-production-capacity-share-by-country/ (accessed 2023-11-23).

[2] ASTM International. Standard Practices for Infrared Multivariate Quantitative Analysis; ASTM E1655-17; ASTM International, 2018.

Your knowledge take-aways

Application Note: Multi-parameter analysis of wood pulp using Vis-NIR spectroscopy

Application Note: Determination of hardwood/softwood content in wood products by near-infrared spectroscopy

Application Note: Determination of lignin in wood pulp

Application Note: Determination of the cotton linter and pulp content in cellulose

Application Note: Purity, degree of substitution and water content of carboxymethyl celluloses

Application Note: Moisture content and pH value in crude tall oil (CTO)

Boost efficiency in the QC laboratory: How NIRS helps reduce costs up to 90%

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Underestimating QC processes is one of the major factors leading to internal and external product failure, which have been reported to cause a loss of turnover between 10–30%. As a result, many different norms are put in place to support manufacturers with this. However, time to result and the associated costs for chemicals can be quite excessive, leading many companies to implement near-infrared spectroscopy in their QC process. The following white paper illustrates the potential of NIRS and displays cost saving potentials up to 90%.

Authors
Guns

Wim Guns

International Sales Support Spectroscopy
Metrohm International Headquarters, Herisau, Switzerland

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Lanciki

Dr. Alyson Lanciki

Scientific Editor
Metrohm International Headquarters, Herisau, Switzerland

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