Pulp and paper QC and product screening with NIR spectroscopy

Jan 15, 2024

Article

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.

Global paper, tissue, and board production share by country in 2019. Source: Statista

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.

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.

Left: Measurement of solid samples is typically done in sample cups. Right: The measurement mode is known as diffuse reflection, where the sample is exposed to light and the diffuse reflected light gets absorbed.

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.

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.

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.

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.