Digital Technologies in Assessing Quality of Paper and Cardboard Forming by Cross-Sectional Microstructure

A very instructive, educational, though challenging text (don’t complicate your day if you don’t really need it)

The world has changed in recent years and the pandemic contributed to this. Today, the trend of digitalization of research is observed. In my opinion, this direction is also correct for our industry, especially for educational and scientific activities related to pulp and paper industry.

I was isolated and had no access to the science lab of my university as well as many of my colleagues. It was time to pay more attention to the part of the work that was associated with the digitalization of scientific research. And I would like to share some of the results, using the example of evaluating the uniformity of paper forming.

We looked at the uniformity of forming paper and cardboard in a different way. The main difference from the existing methods is that the uniformity of forming is estimated by the microstructure of the cross section. I do not criticize existing methods, which quickly and relatively simply can estimate the molding index. The proposed method to date can be seen as a supplement or alternative in some cases.

In general, the study of the microstructure of paper and cardboard originates from the end of the 17th century. The first picture of the microstructure of the beech tree was published by Robert Hooke at the Royal Society of London in 1665.

After then, microscopy did not develop as actively. The breakthrough in the study of the microstructure of paper and cardboard is associated with the development of scanning electron microscopy (SEM). The resolution increased to 3-10 nm. Electron microscopy confirmed the hypothesis about the physicochemical nature of the formation of bonds between fibers.

Combination of microscopy and digital technology are successfully used in the analysis and processing of data from studies of the microstructure of paper and cardboard fibers. In this way Fiber Tester, MorfiCompact were invented. Also, the methods of microstructure of paper and cardboard were developed.

One of the directions of research development in the field of paper microstructure was the production of a high-quality cross section. Obtaining a high-quality cross-sectional image is difficult because, the fibers are deformed during preparation. You can see the difference between the quality of the slice, which is obtained by a knife tool and ion cutting (Slides 5, 7).

We used ion beam cut technology to obtain high-quality cross section. In order to apply it for the preparation of paper samples, indicators were selected: cutting time; ionic beam energy; material thickness (Slide 5). We have established indicators that allow you to obtain a cross-section with high quality (Slides 7–10).

Interesting results were obtained by graphical and analytical processing of the cross-sectional microstructure. The developed method allows to determine more than 12 characteristics (Slide 11).

We tested the method on samples different in their microstructure, obtained from the same material. In this case, three technologies for the production of double-layer cardboard have been studied (Slide 12):

A — cover layer of paperboard obtained by aerodynamic molding;

B — cover layer of paperboard obtained from fibers prepared by a dry defibration and added to the stock;

C — cover layer of cardboard is obtained from fibres prepared by dry defibration followed by grinding in stock.

According to the patent, the lengths of the lines along which the fibers contact and their uniformity of distribution are estimated (Slides 13, 14).

By their length, mechanical characteristics are evaluated, uniformity of fibre sections distribution and porosity of cardboard are determined (Slides 15–18).

Also, uniformity of forming the layer along cross-sectional profiles is determined, for which purpose it is divided into equal sections of 5 μm and then the thickness of each layer in cross-section is set. This allows you to construct histograms of the distribution of thicknesses in the layer, which then evaluate the uniformity of forming (Slides 19, 20).

The obtained cross section profiles allow to determine roughness according to the method presented in GOST 25142-82 (Slide 21).

Interesting results are obtained by estimating the chalk distribution when an electron microscope is equipped with BSE module (Slide 22). According to foreign companies, manufacturers of microscopes, today there are modules that can map the cross-section not only on chemical elements, but also on lignin and pulp.

One new study is to assess the uniformity of paper and paperboard forming, which is characterized by a formation index and considers samples at the macro level.

Interesting results in this field were carried out in the Northern (Arctic) Federal University (Arkhangelsk, Russia). The sample is located between the light source and the detector. The light intensity is then adjusted to the distribution function by which the forming index is estimated (Slide 23).

Аn area of fibrous material cut along a vertical plane was highlighted. It was a one of the results of the graphical processing of the cross section. In order to assess the uniformity of forming, it is necessary to consider the regions of fiber clots (floccules) as the existing methods.

In order to quantify these methods, it is necessary to establish the relationship between the proposed and existing methods. To this goal, consider one of the results of the forming index evaluation shown in Slide 23. It can be seen that the liner board has a uniformity on the lumen, there are regions where the fiber concentration is higher (floccules) or lower.

The larger the fibrous material in the thickness of the cross-section of the paperboard, the lower the intensity of the transmitted light in this section. That is, the intensity of the transmitted light is an amount inversely proportional to the thickness of the cut fibers in the cross section. Using the cross-sectional images shown in Slide 24, areas of fiber clots in each layer were isolated separately (Slides 25–27).

For the coating layer Imax according to the proposed method is 47 μm, for the lower — 42 μm, for the double-layer cardboard — 68 μm. The minimum value in all three cases is zero. In addition to the above calculation results, diagrams of the distribution of the sizes of floccules in the vertical plane were built, which are in reverse dependence on the brightness of the tones passing through the fibrous material of light.

Slide 28 shows graphs of the vertical distribution of the floccules, which characterize the uniformity of the distribution of the floccules in the cross section of the double-layer cardboard.

The distribution diagrams of the flocules in the vertical plane shown in Slides 28, 29 make it possible to estimate the uniformity of the forming of each layer and the two-layer board.

Similar to determining the forming index, a ratio can be established between the frequency of repeating sizes and the difference between the maximum and minimum floccule size.

However, an important and fundamental difference is that the clearance indices are inversely proportional to those obtained from the diagrams presented on Slide 29.

Thus, an estimate of the quality of paperboard forming is developed, in addition to the existing forming index, which, unlike the existing one, allows assessing the uniformity of the distribution of floccules, both in each layer individually and in the board as a whole. This index can be used either alone or in conjunction with the forming index.

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