Accurate vessel cell measurement improves runnability of high-end papers
Figure 1:
Vessel cell
Figure 2:
Picking of a vessel cell and its re-deposition on the sheet by the rubber blanket
Vessel cell defects are an annoying side-effect of hardwood pulp that spoil paper quality. What if there were an easy way to improve output by predicting – and countering – vessel cell problems?
In certain paper-making applications, there is an annoying tendency for white spots to appear on the paper, especially after printing. Until now, papermakers have had little choice but to resign themselves to the problem. But new analytical techniques pioneered at mills like M-real Biberist in northern Switzerland suggest effective new strategies for anticipating and resolving these vessel-cell-related problems, for more consistent quality output of high-end printing papers.
Vessel cells (water conducting cells from the wood’s water-transporting tissue) are present in a variety of fibrous raw materials, of which eucalyptus is one of the most widely used hardwoods. Since eucalyptus is cultivated in plantations in many parts of the world, it’s a common raw material for the paper industry. Compared with other hardwoods, the vessel cells in eucalyptus wood are smaller in number, but larger in size.
BTG’s Pulptec™ MFA-5000 Morphology Fiber & Shive Analyzer can help mills combat vessel cell picking through accurate online analysis of pulp characteristics
The photograph above was taken with a Keyence VHX-100 microscope. The shape of the white spot in the centre suggests that it is a vessel cell. Evidently, vessel cells tend to ‘swallow’ colored inks.
Vessel cells located at the sheet surface and not firmly bonded to surrounding fibers may be picked off by the rubber blanket and be re-deposited on the paper sheet in a different place. Figure 2 below shows a classic example of this phenomenon, known as ‘vessel picking’, which is the underlying cause of uneven printing problems.
Initial analysis
Located near Bern in north-western Switzerland, M-real Biberist produces around 450,000 tons of coated fine papers for the graphic arts industry and offset printing applications, and uncoated paper for office, preprint and offset printing.
Switzerland’s M-real Biberist mill favors the use of eucalyptus wood, taking routine pulp measurements with a BTG Pulptec™ MFA-5000 Morphology Fiber & Shive Analyzer.
Since the MFA-5000 – or MorfiTrac, as it is commonly known – has been developed to automatically and continuously sample and analyze pulp fibers, fines, vessels and shives, it was deemed a good choice for measuring fiber properties and vessel cell characteristics in short-fiber pulps.
Figure 3:
Variations in vessel cell numbers
Figure 4:
Vessel cells vs print quality results
M-real Biberist’s pulp supplier was adamant about the consistent quality of its pulp deliveries, attributing the mill’s print problems to paper machine issues. Since PM 6 is being run with around 80% short-fiber material, pulp deliveries were regularly tested over a period of several months.
These investigations soon revealed quality fluctuations in the eucalyptus raw material, in terms of both the percentage and size of vessel cells.
MFA-5000 test results showed widely varying numbers of vessel cells in the pulp (Figure 3).
As far as vessel pick was concerned, however, the impact of vessel cell numbers was less pronounced than expected, with no evidence that picking in printing occurred whenever the number of vessel cells exceeded a certain mark.
Figure 5 right :
Picked vessel cells
Figure 5 left:
Picked vessel cells
On the other hand, pick tendency decreased or increased substantially in the presence of vessel cells that fell within certain size ranges, in terms of cell lengths and width (Figure 4).
Since microphotographs (Figure 5) showed the presence of vessel cells of a size range defined as critical, paper samples of ‘good’ and ‘off-spec’ output have been sent to the laboratory for breakdown and measurement with the MorfiTrac, with the focus on the size distribution of vessel cells.
Analysis quickly revealed that the majority of good print results coincided with a certain vessel cell size range (Figure 4).
Figure 6:
Print test results vs vessel cell dimensions
Disregarding the distribution of the scatter plot, the vessel cell length on the Y-axis was replaced by the Shape factor. To calculate this factor, the vessel cell width is divided by its length, which transforms the scatter plot into a line. This helps determine the range in which vessel cells do not cause any problems in print tests.
Figure 6 illustrates good and off-spec print results. A black line has been added to the graphic to facilitate identification of the current range and enable accurate predictions of good or off-spec results.
In-depth investigation
In light of the above findings, random tests of all M-real Biberist’s incoming pulp deliveries were taken to measure the properties of fibers and vessel cells. Tests showed that some pulp samples perfectly matched the pattern indicating ‘good’ production, i.e. they fell within the range below the curve. In view of this, PM 6 was preferably run with these pulps whenever vessel picking problems were experienced in the in-house print shop.
Initially, the correlation was excellent, and reliable forecasts were made regarding the picking behavior of the paper sheet in the printing unit. Surprisingly, however, off-spec results were occasionally obtained for productions with acceptable size distributions. This suggested that the size distribution of vessel cells was not the only factor influencing picking problems.
In a more thorough vessel cell analysis, the number of vessel cells was intentionally decreased by separating them in cleaners, and modifying their size in refiners.
Studying the various fiber morphologies established relationships between certain fiber parameters and paper properties. Since the MorfiTrac does not perform freeness measurements, the Schopper-Riegler (SR) value was measured in the lab. Previous investigations by the Centre Technique du Papier (CTP) in Grenoble had already revealed a correlation between the macrofibrils and Schopper-Riegler.
Figure 7:
Fibrillation indices of different refiner plates
Using the MorfiTrac, the percentage share of macrofibrils was measured. Provided the above relationship between the fines and the degree of fibrillation is confirmed in industrial practice, the SR value – which does not always fit 100% – might be replaced by a new value.
The figure to the right compares the inflow and outflow of a long-fiber refiner. In all trials, M-real Biberist noted that the plates on the left showed a higher SR value and a higher fibrillation index (Figure 7).
At the conclusion of the texts, data was exported to the statistics program StatMorf, which performed an evaluation of the data and created a formula enabling a forecast of SR values from the raw fiber analysis data.
The chart below illustrates the relationship between the macrofibrillation index and the SR value. This diagram only relates to one specific refiner plate (Figure 8).
Statistical evaluation results:
- Correlation coefficient r = 0.983
- Coefficient of determination r² = 0.966
- Degrees of regression: y = B*x + A = 15.03*x - 4.67*10-3
Figure 8:
SR value vs macrofibrillation index
The macrofibrillation index thus enables a prediction of SR values.
The above examples demonstrate the manifold benefits of MFA-5000 analysis of fiber morphology. To determine the percentage and size of vessel cells, an analysis on reception of pulp deliveries can be made, and the separation of vessel cells in cleaners, and their reduction, can also be monitored in an online process.
While the Schopper-Riegler value can be calculated, online fiber measurements provide more in-depth information about fiber properties. M-real Biberist’s tests show that it’s not always sufficient to simply be aware of a change in the SR value – it’s also important to know whether this change was caused by a changed fines share, or by the beating degree of the pulp.