Final Report for the Source-Efficient Paper and Board Making Research Programme Area

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Final Report for the

Source-Efficient Paper and Board Making

Research Programme Area

Innventia Research Programme 2015-2017

Caroline Ankerfors, Ida Östlund, Magnus Gimåker, Paul

Krochak, Catherine Östlund, Peter Hansen, Claes

Holmqvist, Klas Johansson

Innventia Report No.: 1049

May 2018

Innventia Research Programme 2015-2017

Source-Efficient Paper and Board Making, pre-competitive part

Distribution restricted to: Andritz, BillerudKorsnäs, Fibria, Hansol Paper, Holmen, ITC, Metsä Board, Metsä Fibre, Saica, Smurfit Kappa, Solenis, Stora Enso,

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Acknowledgements

The Swedish Energy Agency, RISE and the financing companies in Innventia’s

research program “Source-Efficient Paper & Board Making” (Andritz, BillerudKorsnäs, Fibria, Hansol Paper, Holmen, ITC, Metsä Board, Metsä Fibre, Saica, Smurfit Kappa, Solenis, Stora Enso, Tetra Pak and UPM) are acknowledged for their financial support. The teams at the paper chemistry laboratory, the FEX pilot paper machine, and the physical testing laboratories at Innventia/RISE Bioeconomy are acknowledged for their skilful work.

The authors would also like to acknowledge Andrew Horvath, formerly employed at Innventia and one of the founders of this research programme.

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Summary

This is the final report for the Innventia/RISE Bioeconomy research programme area “Source-Efficient Paper and Board Making”, which was executed 2015-2017. The overall aim of the Source Efficient Paper and Board Making was to improve the resource efficiency in paper and board production. This was achieved by combining paper chemistry, paper physics and process technology. A particular goal was to reduce raw material consumption through the use of stronger materials or creation of bulk, which are needed to maintain bending stiffness and mechanical properties if the grammage is reduced. The work in the project has been carried out in laboratory scale and in pilot scale using the FEX pilot paper machine and the dynamic flow loop.

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Executive summary

The Source-Efficient Paper and Board Making research programme area was executed from January 1st, 2015, to December 31st, 2017. The overall aim of the Source-Efficient Paper and Board Making was to improve the resource efficiency in paper and board production. The approach was to combine paper chemistry, paper physics and process technology. A particular goal was to reduce raw material consumption through the use of stronger materials or by achieving higher bulk, which are needed to maintain bending stiffness and mechanical properties if the grammage is reduced.

The structure of the programme area is shown in the figure below. The programme area consisted of one pre-competitive research (PCR) section and three application-oriented research (AOR) projects. All companies in the programme area were participating in the pre-competitive section, i.e. the results from this section were presented and reported to all participating companies. The AORs were, however, different as not all companies participated in all applications. Only the companies in the specific AOR project have access to the results and the reports from that application.

The companies participating in the programme area were Andritz, BillerudKorsnäs, Fibria, Hansol Paper, Holmen, ITC, Metsä Board, Metsä Fibre, Saica, Smurfit Kappa, Solenis, Stora Enso, Tetra Pak, and UPM.

In the PCR section, five themes (green, horizontal lines) have been studied. This final report summarizes the activities and main findings within each of the five themes within the PCR part of the programme area.

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Theme P1. Chemical additives and fibre modification

Chemical approaches are an additional option to control the properties of paper outside of the traditional papermaking unit operations, such as refining, forming, pressing, and drying. Various technologies can be introduced either before the paper mill or within the wet-end of the paper machine to expand the property space of existing products. This is necessary to reduce the fibre grammage, which can only be realized by increasing the bulk and/or strength properties. Both approaches have been studied within Theme 1. For increasing bulk, two principal chemical technologies have been developed at Innventia. The first one is based on the attachment of carboxymethyl cellulose (CMC) on the pulp fibres followed by ion exchanging the pulp to its aluminium form. Studies of the mechanisms behind the bulking effect of CMC attachment showed that increased fibre surface friction is a plausible mechanism for the wet bulking technology. The other technology for increasing bulk involves pulp drying in the presence of AlCl3. NMR-studies were performed to clarify the mechanisms behind the bulking effect. It was found that the presence of AlCl3 upon drying increased the average fibril aggregate size, which could be interpreted as a more pronounced hornification effect.

While increased bulk is needed to maintain bending stiffness at reduced basis weight, the reality is that mechanical properties will be negatively affected. It is therefore likely that strength additives will be needed. The potential of strength additives, such as cellulose microfibrils (CMF), starch and synthetic polyelectrolytes, were therefore studied.

When using CMF as a wet end strength additive, it is known that dispersion of CMF is a critical issue. In a pilot scale trial, a mechanical approach was used to enhance the dispersion of CMF into the pulp by altering the CMF dosage position. It was shown that the tensile strength index increased with the CMF addition, independently of the studied dosing positions and it was concluded that CMF is a very robust strength additive. In another study, the influence of various CMF pre-treatment steps was studied by

measuring the tensile strength of CMF films. This work showed that a CMC addition to enzymatically treated pulp improved the tensile strength of the films, but even higher tensile strength values were obtained by CMC addition when no enzymatic pre-treatment had been made.

Other means to increase the strength-giving potential further when using CMF is by pre-mixing with polymers and filler prior to the addition to the furnish. A method was developed to identify shear-stable flocs of CMF and filler, which showed very good correlation with the drainage flow rate, formation, and retention of the sheets produced. In another attempt to enhance strength, the polyelectrolyte multilayer (PEM) technique and other dosage strategies, using combinations of cationic and anionic strength additives, have been studied in laboratory and pilot scale and compared to

single-dosages of cationic strength additives. The results showed that the PEM method enabled higher adsorbed amounts of strength additives and that the paper strength increased significantly without impairing bulk. These positive effects were demonstrated in clean conditions as well as in real mill process waters.

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Theme P2. Retention, formation and dewatering

Good formation is a major quality requirement for all paper and board products. While chemical additives and filler play an essential role in papermaking, incorrect dosage and inadequate mixing can lead to poor retention and formation. This results in reduced efficiency on the paper machine and can introduce web structure variability that deteriorates mechanical and surface properties in the final product. Dewatering is coupled to retention and formation, but the balance between high retention, good formation and dewatering is not easy to control.

The focus of this work package was to optimize the usage of different chemical

additives, including cellulose microfibrils, CMF, to enable increased filler retention and improved formation without hurting dewatering.

Strategies for improving formation through turbulence dampening in the wet-end were investigated on a pilot scale flow loop. Here it was shown that re-flocculation rates could be reduced through the addition of APAM along with reductions in turbulence levels.

A series of mixing trials were performed on FEX where the effect of early vs late addition of starch, CPAM and silica were compared for filler containing grades. Here it was shown that without the use of starch, improvements to the retention formation relationship could be realized with late addition of CPAM and Si, i.e. immediately prior the headbox, but under good mixing conditions. If starch was included in the stock, the late addition of all three components indicated possibilities to decrease the C-PAM consumption without hurting retention or product properties. The effect of starch dosage position, i.e. in the thick stock, short circulation, or prior the headbox, on product

properties was also investigated for a middle-ply board application, and comparison made with twin-wire and Fourdrinier forming. Here it was shown that late addition of starch reduced the density but also the tensile properties in the final product. However, in twin-wire forming, late addition was found to improve the Z-strength at a reduced density.

Dewatering studies in this work package were largely connected with the use of CMF in papermaking, where CMF is known to enable strength improvements, but with impaired dewatering. A laboratory investigation of two commercial micro-particle systems (Si and bentonite) and an aluminum hydroxide (Al(OH)3) system showed that both silica and bentonite were able to compensate for the impaired dewatering from the CMF. Al(OH)3 was shown to improve filler retention, and also improved dewatering, but not to the same extent as silica and bentonite. However, these dewatering improvements were not observed in a pilot scale (FEX) trial. A FEX trial did however show that with an increased headbox concentration the dewatering time could be reduced, and the formation could be improved simultaneously. It was further shown at pilot scale that a CMF dosage of 4-5% could optimize strength properties with minimal dewatering impairment.

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Theme P3. Web structure

The cross-thickness structure of paper and board products affects key performance characteristics, such as bending stiffness and convertability. This is exploited in layered sheets, where the composition and structure of the different layers are used to optimize properties. However, cross-thickness gradients in both material distributions (e.g. the local fines content) and properties (e.g. the local shear strength) are present also in single-ply grades and within each layer in a multi-ply grade.

The objective of this theme was i) to benchmark layered sheets produced by couching together separately formed plies (‘multi-ply sheets’) with sheets produced with different implementations of stratified forming with a single headbox (‘stratified sheets’), ii) to develop a method to determine the distribution of fine material across the thickness of the sheet, and iii) to make pilot machine trials in which the method was employed to determine how operational parameters affects the ZD fine material distributions and the corresponding effect on sheet properties.

In the benchmarking activities, two-layered stratified kraft sheets with a bleached top layer and an unbleached base layer were produced with pure roll forming using different types of separating vanes in the headbox, including various implementations of the Aqua-vane concept. Corresponding multi-ply sheets were produced by couching together a roll-formed base ply with a fourdrinier-formed top ply. The sheets were compared in terms of mechanical properties, layer purity and fibre orientation

anisotropy. The main observed difference was somewhat better CD-properties for the multi-ply sheet due to a less oriented sheet, a somewhat lower sheet density for stratification with a conventional lamella, best top ply coverage with a version of the Aqua-vane concept, and differences in the ZD-position of the rupture zone in ply bond tests. The Scott-Bond value was also higher for the stratified than for the multi-ply sheets, whereas the difference was less apparent for the Z-strength test.

Pilot trials were also performed to evaluate whether stratified starch dosage through an Aqua-vane yields any performance benefits. No significant effects compared to a homogeneous addition was observed.

A method was developed to detect cross-thickness fine material distributions. It is based on labelling the fraction targeted for detection with a cationic blue dye. By splitting the ready sheet into a number of different layers, which are then scanned and subjected to a colour analysis, it appears that the variation of dyed material across the sheet thickness can be determined (at least qualitatively, but possibly also quantitatively). This method will be a very valuable tool in future work to determine how machine operational parameters influence fines or CMF distributions, and how these profiles relate to other sheet properties (e.g. internal bond and ply bond). Some preliminary pilot machine trials to evaluate the feasibility of such investigations were undertaken with encouraging results.

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Theme P4. Pressing

Water removal is perhaps the most critical papermaking operation for reducing energy costs. A general rule of thumb is that a 1% increase in the solids content after pressing can result in a 4-6% reduction of dryer demand. Pressing strategies would ideally be designed to maximizing water removal, but overly aggressive pressing destroys bulk, deteriorates surface properties and printability, can create moisture variability that persists after drying and in extreme cases lead to web crushing. Despite several decades of research on the subject of wet pressing, there still remains many unanswered

questions concerning optimal press strategy for a given product quality.

The focus of this work package has been on understanding fundamental relationships between basic parameters of the press profile, i.e. the rate of compression, nip residence time and peak load, on dryness, density and bending stiffness. The response of different levels of fibre beating and ingoing dryness to different press profiles was also

investigated.

A literature study was performed first, where it was shown that non-uniform density profiles can be created in the web depending on the press profile, fibre type and level of beating. This was shown to be a complex interaction between the press profile, press felt and wet web. An extensive series of laboratory trials were performed investigating the effect four different press profiles on web dryness at different ingoing dryness levels and levels of beating. The roll press profile resulted in the lowest press dryness, while a shoe press profiles with low rate of compression resulted in the highest press dryness. These behaviours were independent of ingoing dryness. For low and moderate ingoing dryness, the high rate of compression shoe press resulted in low dryness and a higher rate of densification with respect to dryness. It was also shown that press dryness was reduced, and density increased with highly beaten pulps.

Results from a series of FEX trials (performed outside the Research Programme Area), comparing roll and shoe press nips, were analysed to elucidate their effect on web dryness, densification and paper properties. Here, it was shown that shoe press nips result in a high dryness, lower density web, however with reduced tensile properties. Roll press nips were shown to result in lower dryness but improved tensile properties. Both press strategies were shown to produce approximately the same bending stiffness. It was shown that roll nips created a thin but highly densified region on the web surface which likely accounted for the increase in tensile properties. The degree of densification was estimated using tomographic imaging.

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Theme P5. Sustainability

In this study the economic and environmental potential for production of fine paper with CMF has been evaluated for three cases based on raw material use, electricity used for refining, and steam used for drying. In Case 1, a CMF addition of 3.2% was used to increase the filler content while grammage and tensile strength was maintained. In Case 2 and 3, a CMF addition of 4.5% was used to decrease the grammage. In Case 2 the evaluation was made without filler and in Case 3 filler was used.

The addition of CMF had a positive effect on the cost for all evaluated cases when only considering the operating costs. In case the production must be decreased due to

limitations in evaporation capacity of the dryer, the impact of the decreased dry solids content to dryer with CMF was significant. When it came to the effect of decreased production due to decreased dryness to dryer, Case 2 and 3 were correlated to a need in decreased production. For Case 2, the concept was still economically feasible while Case 3 was just below the limit.

For the Carbon Footprint, improvements could be seen for Case 2 and 3 but not for Case 1. The reason is that PCC has a higher CO2-eq factor than the pulp in this study, so the total Carbon Footprint was increased since the PCC partly replaced the pulp in both Case 1a and 1b. GCC has a lower CO2-eq factor than the pulp, so if GCC would have been used instead of PCC, the Carbon Footprint would have been lower for the papers with CMF also in Case 1.

From an economic point of view, an increase of filler is favorable for the production cost, thanks to the low price of filler. However, this study shows that it is not

necessarily favorable from an environmental point of view. The relationship between the CO2-eq factor for the pulp and the filler decides if the outcome is positive or not.

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Theme 1. Chemical additives and fibre modification

P1.1 Chemical bulking technologies P1.1a Acid grafting of CMC

During the project, the nomenclature for the attachment of CMC onto the fibre surfaces was changed. The method which previously was called grafting is now referred to as CMC attachment or CMC adsorption.

Project activities

There is a renewed interest in “bulking fibers”, primarily from board manufacturers to enhance the bulk in the middle layer(s) of paperboard, which is needed to decrease the basis weight while maintaining the bending stiffness. Recent research efforts at

Innventia have resulted in several novel approaches to increase the bulk of paper. Two chemical methods for producing bulking fibers can be distinguished:

• Dry bulking method: Treatment of fibers with aluminum or other multivalent salts before drying the fibers, resulting in less swollen fibers when re-slushed that give higher bulk.

• Wet bulking method: Attaching carboxymethyl cellulose (CMC) to cellulosic fibers and then transferring the fibers to their aluminum form, which leads to high bulk.

The existing method for attaching CMC onto fibres requires high temperature and long reaction times in high electrolyte concentration using divalent ions. For an industrial application, the removal of the divalent ions must be addressed. In order to achieve a more industrially realistic process and avoiding the divalent ions, adsorption of CMC onto pulp fibres under acidic conditions may be a simpler method. The aim with this study was therefore to investigate the possibilities of adsorbing CMC onto pulp fibers under acidic conditions, investigating pH, reaction time and temperature. The study was conducted in laboratory scale.

Key findings

The efficiency in attaching 2% CMC increased with decreasing pH and increasing temperature, see Figure 1. For 100% attachment of CMC, a pH of 2.5 and a treatment time of 120 minutes were required. However, the study also showed that when lower added amount of CMC (1%) was used, the attached amount increased and a high attachment rate was achieved at higher pH and shorter treatment times.

It was found that adsorption of CMC under acidic conditions was a simpler method than using divalent metal salts and also allowed reasonable residence times. However, it required low pH-values (~2.5), which may be cumbersome in commercial practice. Higher pH and lower CMC addition combined with charge multiplication using layer by layer technology may be an alternative route to alleviate the use of low pH-values. Alternatively, adsorption of CMC onto fibres may possibly be performed in acidic bleaching stages, which has been patented by others.

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Figure 1. Attached amount of CMC for 2% CMC addition at 30 minutes treatment time in different pH as a function of temperature.

Publication reports

Horvath, A. E., Gimåker, M. and Lindström, T. (2017)

Adsorption of CMC to cellulosic fibres under acidic conditions

Innventia Report No. 1031 0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 Attached amount of CMC (%) Temperature (C) pH 2,5 pH 3,0 pH 3,5 pH 4,0

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P1.1b Mechanisms behind hornification from multivalent salt treatment

Recent research efforts at Innventia have resulted in several novel approaches to increase the bulk of paper. Two major groups of bulking fibers can be distinguished:

• Dry bulking method: Treatment of fibers with aluminum or other multivalent salts before drying the fibers, resulting in less swollen fibers when re-slushed that give higher bulk.

• Wet bulking method: Attaching carboxymethyl cellulose (CMC) to cellulosic fibers and then transferring the fibers to their aluminum form, which leads to high bulk.

This activity focused on finding the mechanism behind the increased hornification for the dry bulking method where the fibres are treated with aluminium salt before drying and re-slushing. The impact on the cellulose supramolecular structure due to the presence of aluminium salts was investigated using the CP/MAS 13C-NMR technique. In this study a cellulose-rich never-dried acetate grade eucalyptus dissolving pulp was used. Four different samples from this pulp were investigated; 1. never-dried pulp, 2. dried and re-slushed pulp, 3. Al-treated, dried and re-re-slushed pulp, and 4. Al-treated, dried, re-re-slushed and washed pulp.

Key findings

The results are presented in Table 1. The presence of AlCl3 increased the average lateral fibril aggregate dimensions some 25% above that reached by the dried pulp. The

observed changes in cellulose supramolecular structure due to the addition of AlCl3 are large considering the low AlCl3 concentration that was used (5 mM). No change in degree of crystallinity was observed as the result of drying, either with or without AlCl3 addition. Although the mechanism of action for AlCl3, causing the observed effects on the cellulose supramolecular structure, is currently not known, the interpretation made was that the presence of AlCl3 during drying increased the degree of cross-linking.

Table 1. Results from fitting of the C4 spectral region of CP/MAS 13_{C-NMR spectra. LFD:} average lateral fibril dimension, LFAD: average lateral fibril aggregate dimension, Cr: degree of crystallinity, LFAD SSA: the specific surface area in the water swollen state, computed from LFAD dimension. Values in parenthesis are one standard error and is based on the quality of the fit.

LFD (nm) LFAD (nm) Cr (%) LFAD SSA (m2_/g)

Ref ND 4.2 (0.1) 20 (1) 53% (1%) 136 (4) Ref D 4.3 (0.1) 24 (1) 54% (1%) 113 (3) Al 4.3 (0.1) 30 (1) 54% (1%) 89 (4) Al washed 4.3 (0.1) 30 (1) 54% (1%) 89 (4)

Larsson, P.T., Lindström, T. and Glad Nordmark, G. (2017)

The impact of AlCl3 on the cellulose supramolecular structure during drying. A CP/MAS 13C-NMR study.

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P1.1c Effect of grafting technology on surface friction and consolidation

During the project, the nomenclature for the attachment of CMC onto the fibre surfaces was changed. The method which is called grafting in this activity is referred to as CMC attachment in activity P1.1a.

The traditional approaches to produce bulk in board and paper is through fiber selection, but paper and board makers could be limited in their fiber sources. Therefore, an

alternative route could be to use chemical bulking technologies to reduce grammage but maintain the bulk. Innventia have carried out research on developing new bulking methods and one of them is to adsorb carboxymethyl cellulose (CMC) onto the fibers and then exchange the counter ions to aluminum. The exact bulking mechanism for this method is not known but one hypothesis is that bulk is created due to an increase in the surface friction of the fibers, due to that the electrostatic double layer is shielded together with electrostatic cross-linking from aluminum ions. The higher friction between fibers prohibits the fibers to slide and reduces the sheet consolidation during drying. The aim of this study was therefore to investigate the effect of counter ion on the frictional forces, expressed as the fiber network strength.

A parallel plate rheometer was used where oscillatory shear was applied with controlled strain in order to measure the fiber network strength. The fiber network strength can be related to fiber-fiber friction. CMC-grafted pulp in different ionic forms were used and compared to an un-grafted pulp.

Key findings

When comparing CMC-grafted pulp in different ionic forms it can be concluded that when aluminum is the counter ion, the fiber network strength (i.e. the apparent yield stress, 𝝉) is higher compared to hydrogen, calcium and sodium, see Figure 2. However, un-grafted pulp that has only been rinsed in tap water has similar network strength as grafted pulp with aluminum as counter ion. It might be that the fibers of CMC-grafted pulp in aluminum form and the un-CMC-grafted pulp are stiffer, an indication for that is the lower WRV i.e. less swollen fibers, which causes higher friction and stronger fiber network. As the fiber swelling increases, the fiber network strength decreases. Based on the results in this study, it seems like the fiber surface friction is a mechanism for the wet bulking technology. However, a comparison with un-grafted pulp in the different ionic forms should be made to fully understand the development of bulk using the wet bulking technology.

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Figure 2. Apparent yield stress (𝝉) as a function of WRV measured in deionized water (0 µS/cm). The yield stress was calculated for 2 and 4% pulp consistency.

Horvath, A. E. (2016)

Effect of grafting technology on surface friction and consolidation

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P1.2 CMF and chemical additives

P1.2a-1 Dispersion and dilution of CMF, mechanical means

Previous papermaking lab- and pilot-trials at Innventia with cellulose microfibrils (CMF) as strength additive, have shown different strength increase of the final sheet, where the lab studies showed a higher strength increase from CMF than the pilot trials. This discrepancy is hypothesised to be due to poor dispersion of the CMF in the paper produced in pilot scale. Therefore, the objective of this pilot study was to test different mechanical means to dilute and disperse the CMF in the pulp and study if the strength-giving effect of the CMF could be further increased. The contact time for CMF was varied by introducing CMF at three different locations; i) to the thick stock before the refiner, ii) to the thick stock after the refiner, which is the standard position and iii) to the short circulation. The effect from extra dilution of the CMF was also investigated as well as throttling a valve close to the headbox as this has been proven to increase the mixing in other studies, see activities P2.1c and P2.1g.

Paper sheets were produced on FEX using bleached softwood kraft pulp without CMF and with the addition of 4.5% CMF. Tensile properties, z-strength, dewatering time, dry solids content, retention, formation, density and air permeance were evaluated.

Key findings

The tensile strength index increased with 35% with the CMF addition, see Figure 3. Furthermore, it was shown that the strength increase was independent of dosing

position, where the CMF was added either to the thick stock before or after the refiner, or into the short circulation. Moreover, increasing the mechanical mixing by throttling a valve had no significant influence on the mechanical properties. The strength increase showed no significant difference when the two addition concentrations of 10 g/l and 3 g/l were used for CMF.

The dry solids content after press was impaired by the addition of CMF. It was however shown, that the low dry solids content after press in the presence of CMF could be compensated by increasing the press load.

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Figure 3. Left: Tensile strength index for the different CMF dosing positions. Right: Tensile strength index for open and throttled valve close to the headbox (circles and squares), low concentration of the injected CMF (3 g/L compared to 10 g/L) as well as throttling on the CMF injection hose (+). In all trial points with CMF, the dosed amount was 4.5%. The error bars represent ±1 standard deviation.

To conclude, this study shows that the strength-giving effect from CMF could not be improved by the mechanical means investigated, and the reason for the discrepancy between the lab and pilot trials is still unknown. On the other hand, the results show that using CMF as a strength additive in papermaking is very robust and gives the same strength increase independent of where the CMF is added.

Håkansson, K. M. O. and Östlund, I. (2016)

Mechanical means for dilution and dispersion of CMF to use its full strength giving potential.

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P1.2a-2 Dispersion and dilution of CMF, chemical means

Innventia has developed enzymatically pre-treated CNF (cellulose nanofibrils, also referred to as CMF, cellulose microfibrils) and CNF, which has been CMC grafted as a pre-treatment method. It is also known that dispersion of CNF materials is a critical issue, so it was of interest to explore possible synergistic effects when treating fibres both with an enzyme and CMC. It is also known that the original inventors of CNF materials discovered that the addition of polysaccharides and other hydrophilic polymers offered some benefits to the delamination process. The upscaling of the pre-treatment with CMC, has been difficult as low pH-values or high electrolyte

concentrations in combination with high temperatures are required for quantitative grafting yields. Therefore, both plain addition of CMC or partial grafting (mild

conditions) of CMC was investigated. The effects were investigated by evaluation of the tensile strength properties of vacuum filtrated CNF-films. The study was performed in laboratory scale.

Key findings

In Figure 4, the film strength of the different CNFs produced are shown at different energy levels. The results show that a CMC addition to an enzymatically treated pulp improves the tensile strength of CNF films, but even higher tensile strength values were obtained by CMC addition when no enzymatic pre-treatment was made. The effect is related to the fact that enzymatic hydrolysis slightly decreases the degree of

polymerization and, hence, the strength properties. Partial grafting of fibres with 8% CMC gives films with strength values in parity with the theoretical strength (around 170 Nm/g). With partially pre-grafted fibres there is no need for a refining stage in order to reach a high tensile strength, but refining is, however, of benefit for the strain at break/tensile energy absorption. The latter effect is not understood from papermaking considerations.

Investigations regarding the processing order of CMC, enzymatic treatment and secondary refining did not point to an order that offered a clear benefit.

In conclusion, CMC offers a benefit to film properties, with or without an enzymatical treatment, but no synergistic effects could be traced during these investigations.

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Figure 4. The evolution of the tensile index (CNF film strength) with the delamination energy (secondary refining and homogenization). Different pre-treatment methods were used before homogenization; mechanical and enzymatic pre-treatment (enzyme), addition of CMC to the mechanical and enzymatic pre-treatment (enzyme + CMC) and combination of CMC and mechanical pre-treatment (CMC).

Bjärestrand, A. and Lindström, T. (2017)

Effects of delamination on CNF film strength after enzymatic and/or carboxymethyl cellulose (CMC) treatment

Innventia Report No. 832

0 20 40 60 80 100 120 140 160 180 200 0 1000 2000 3000 4000 T e n s il e i n d e x , k N m /k g Delamination energy, kWh/t Enzyme Enzyme+CMC CMC

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P1.2b Labelling of CMF to study solution behaviour and retention

Characterizing CMF is generally complicated by the length scale and chemical

similarities to fine material. This makes direct measurements difficult and dictates that researchers rely on indirect methods such as turbidity measurements, carbohydrate analysis and rheological characterization.

A method for labelling CMF with blue dye (Pergasol Blue 67L, 8kg merchandise product /ton dry material) for direct characterization was studied, with a focus on dispersion, dosing and mixing and quantification of CMF in a sheet structure.

This activity was done jointly with activity P3.1d where the method was developed and with activity P2.2c where the labelling was test in comparison with unlabelled CMF at a FEX trial.

Key findings

The methodology of splitting paper in plies using hot lamination and plastic pouches was used to access different regions in the thickness direction. The amount of dyed material in a ply was monitored by the parameter -a*L/opac where the opacity and the CIE colour parameters a* and L was measured using L&W Elrepho.

Studies on laboratory sheets showed that dyed CMF could be chemically retained to a high degree in a pulp prior to the forming of a conventional hand sheet, such that a homogeneous thickness-directional distribution of dyed CMF could be achieved. A series of sheets with increasing amounts of CMF gave a proportional increase in -a*L/opac, see Figure 5.

Figure 5. CTMP with different dosages of dyed CMF. The position of the middle of each ply is associated with the value of -a*L/opac acquired for the ply. The ZD distribution of CMF was flat and corresponded to the CMF dosage.

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Dyed CMF was compared with undyed CMF in a FEX trial. The dosage of CMF was in both cases 4% and the pulp used was a CTMP/bleached softwood kraft 50/50 mixture. The retention system used was starch, CPAM and silica 5 kg/tonne, 0.35 kg/tonne, 0.35 kg/tonne, respectively.

The measured result from the trial point using dyed CMF was compared to the corresponding trial point using undyed CMF.

The conclusion is that the dye had no effect on the papermaking process, neither chemically nor mechanically. For example, the roll dewatering time was the same and the dry solid contents were similar for dyed and undyed CMF. The density and tensile strength index was the same. Slight shift in other properties measured, seemed as natural consequence of the paper with dyed CMF having somewhat higher grammage. Paper samples were splitted and monitored by the -a*L/opac parameter in the cross-thickness direction. Knowing that a difference of 1 unit of -a*L/opac corresponds to approx. 1% dyed dry material, it was concluded that the middle of the paper contained about 2% CMF while up to 5-6% CMF are retained close to the surfaces, due to mechanical retention of flocculated materials. In total the paper contains approx. 4% CMF, Figure 6.

Figure 6. Analysis of cross-thickness distribution of dyed CMF in paper made at FEX.

Hansen, P. (2017)

Development of a method to determine CMF or fine materials distributions in the thickness direction

Östlund, I., Hansen, P. and Gimåker, M. (2017)

Changes in strength and dewatering when using CMF as strength additive Part 2. Altering the paper grammage

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P1.2c Sizing of bulky materials – not started

Increasing bulk and strength is needed to facilitate grammage reductions, but the overall performance of paper and board must also be maintained. While strength properties are often in focus, sizing is a critical parameter that will be influenced at higher bulk. An important aspect of sizing is edge wicking, i.e. the imbibition of liquid into unprotected edges on liquid packaging board.

The two major properties influencing the rate of edge wicking is pore size distribution and contact angle. The large pores that will exist at high bulk simply give too little resistance towards water transport and thus a high rate of edge wicking. It is very difficult to decrease the contact angle between cellulose fibres and water any further with the internal sizing concepts available. The best would thus be if the pore sizes could be reduced without influencing the density to any greater extent.

By using porous microparticles, the larger voids in the fibre network structure could be filled, thereby shifting the pore size distribution to smaller sizes without increasing the sheet density too much. The plan for the activity was to identify suitable highly porous microparticles and make some laboratory trials with these to establish if it was a feasible way forward or not.

The activity was down-prioritized before it started.

Key findings

None.

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P1.2d The effect of chemical additives on strength and bulk

Cationic starch (CS) has traditionally been a cost-effective way to increase the

mechanical properties of paper and board, but is limited as a wet-end additive in terms of efficiency at higher adsorbed amounts. Furthermore, polysaccharides such as cationic starch and cellulose microfibrils (CMF) generally improve strength, at least partly, by increased consolidation, which leads to increased density and thereby decreased bulk. All of this may be problematic especially when aiming for higher bulk, when further strength will be needed to maintain mechanical performance.

Instead of the addition of a single polymer, alternating layers of cationic and anionic polyelectrolytes can be adsorbed, thereby forming a polyelectrolyte multilayer, PEM. When applied in paper making, this method has been shown to lead to a significant increase in paper strength with only little or no negative impact on sheet density. In this activity, the addition of up to four layers of PEM has been studied and compared with the use of single-additions or dual-additions of the same chemicals with respect to their effect on strength and bulk properties of paper sheets produced in the laboratory. First, this was made under clean conditions, i.e. in tap water, to set a baseline for the performance. The systems studied were cationic/anionic polyacrylamide

(CPAM/APAM), polyvinylamine/carboxymethyl cellulose (PVAm/CMC) and cationic starch/anionic polyacrylamide (CS/APAM).

The later part of this activity also addressed the influence from dissolved and colloidal substances (DCS) to investigate the possibilities of implementing the polyelectrolyte multilayering technique in practice. The goal of this activity was also to find a suitable polyelectrolyte system for the FEX pilot trial within activity P1.2e, by testing a few industrially relevant systems in laboratory scale, both in tap water and in real mill process water.

Key findings

One of the main findings of the study was that with single-additions with increasing dosage levels of PVAm, CPAM or CS, the tensile strength index of the produced sheets increased at first, but the effect seemed to level off at higher dosages, see Figure 7 (top left). By comparing the single-addition of each cationic component to a polyelectrolyte multilayer (1-4 layers) of the same component together with an anionic component, it was found that significantly higher tensile strength could be reached with the PEM strategy for the combinations PVAm/CMC and CS/APAM (Figure 7, top right and top left). For CPAM/APAM, however, very little advantage of using a multilayering approach was seen (Figure 7, bottom right).

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Figure 7. Tensile strength index of hand sheets made with single-additions of PVAm, CPAM or CS (top left), single-addition or PEM of CS/APAM (top right), single-addition or PEM of

PVAm/CMC (bottom left) and single-addition or PEM of CPAM/APAM (two dosage levels, bottom right).

All measured variations in sheet density were small, although with some indications that the density was lower for sheets with PEM, medium for sheets made with a single-dosage strategy and highest for sheets made with the dual-addition strategies. From the results of this comparison, it was decided that the system most suitable for a

continuation in the form of a FEX trial would be CS/APAM.

By repeating some of the trials above in process waters achieved from BillerudKorsnäs Frövi (unbleached CTMP), the influence from DCS was investigated. Firstly, the study showed that PEMs can be successfully built in real process waters. Further, it was found that although the adsorbed amounts might differ compared to in the cleaner system, the trends for the dosage strategies were the same.

Ankerfors, C., Gimåker, G and Glad Nordmark, G (2018)

A comparative study of polyelectrolyte multilayers and other chemical dosage strategies - Effect on properties of paper sheets produced in laboratory scale using tap and mill process waters

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P1.2e FEX trial with strength additives

With the strive towards a maximally efficient use of fibre sources, driven by the desire to for instance increase the filler content or increase the use of recycled fibres, there is an interest in achieving higher strength increases than what is possible by using a single strength additive. Another application where efficient strength additives are of great importance is when aiming for higher bulk. In such applications, avoiding densification as a side-effect from the strength additives is crucial.

In this pilot scale study, polyelectrolyte multilayers (PEM) with one to four layers of cationic starch (CS) and anionic polyacrylamide (APAM) were built by consecutive addition of the two components to the thick stock. The results were compared to the addition of cationic starch alone (two dosage levels) and to the addition of the same amount of strength additives as in the four layers, but added in a dual-addition strategy with either the cation or the anion added first. The pulp used was a slightly refined bleached softwood kraft pulp and the process water was taken from a mill in order to have dissolved and colloidal substances present during the trial. A constant amount of fixative was used in all trial points.

The goal of the study was to test if PEMs can be built in mill process waters and to study and compare the effect of PEMs on strength and bulk to the use of a single strength additive alone.

Key findings

The results showed that polyelectrolyte multilayers could be successfully built in mill process waters. With the PEM addition, large increases were seen for almost all the mechanical properties of the produced paper (see Figure 8), with the largest increase after the addition of four layers and with especially high strength effects in tensile strength index and z-strength. Possibly, this was an effect of the higher adsorbed amount of strength additives as compared to the addition of the highest amount of cationic starch alone, which in almost all cases resulted in lower strength values than with a lower dosage, possibly due to poor retention of the starch at the higher dosage level. It was hypothesized that the APAM had an important role in retaining the cationic starch in order to gain its full potential as a strength additive.

For tensile strength index, the improvement was larger with PEM than when adding the same amount of strength additives using the dual-addition strategy. For z-strength and all other mechanical properties both dual-addition dosage strategies gave comparable results as four layers (Figure 8).

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Figure 8. Tensile strength index, geometrical mean (left) and z-strength (right) plotted against the total chemical dosage. The circles represent the dual-addition strategy where either the cation (filled circles) or the anion (unfilled circles) was added first.

Other important findings were that the dewatering on the wire was unaffected during the whole trial and that the sheet density remained basically unaffected for all dosage

strategies (Figure 9). The dry solids content before press increased with increasing number of PEM layers, whereas the dry solids content after press was basically unaffected for all trial points.

Figure 9. Structural density of sheets of all studied dosage strategies plotted against total chemical dosage (left) and tensile strength index plotted against structural density (right).

Ankerfors, C., Gimåker, G and Östlund, I. (2018)

A comparative study of polyelectrolyte multilayers and other chemical dosage strategies – Effect on properties of paper sheets produced in pilot scale using mill process waters

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P1.2f Pre-mixing CMF and filler

It is well known that pre-mixing cellulosic fines and fillers in the presence of a flocculant have clear advantages regarding strength and retention over adding the components separately to the fibre furnish. There are granted patents for the concept of pre-mixing CMF and filler (EP2425057, US8945345). RISE Innventia’s assessment is that these patents are too general to be strong.

It was shown earlier that shear resistant flocs are a necessity for pre-flocculation

concepts (polymer added to the filler suspension) to work well. Studies need to be made using relevant turbulence levels. Previous work has shown the importance of the right particle size distribution that is needed to get both good retention and formation when the filler flocs are used in papermaking.

The goal of this activity was to find suitable chemistry for creating shear stable flocs with filler and CMF (pre-mixing). Different chemicals and combinations of chemicals were investigated in the laboratory, and later in a dynamic flow loop, regarding their ability to create shear stable flocs of a certain floc size. The study was made with CMF (pre-mixing) and without CMF (pre-flocculation).

- Stability and resistance of these flocs when exposed to higher shear in a beaker. - Impact of the pre-mixing or pre-flocculation strategy on the retention, formation,

drainage and strength properties on papers produced in a modified Finnish sheet former.

- Evaluation of the fibre and filler flocs’ shear resistance in a Dynamic Flow Loop.

Key findings

The shear strength resistance of the flocs was evaluated for both mixing and pre-flocculation strategies with different chemical systems, whose optimal dose was identified in pre-trials using the FBRM methodology.

The following results were obtained from the re-flocculation studies (see Figure 10): - CPAM flocs were easily broken,

- Cationic starch flocs were more stable, during high-speed shear and the obtained flocs were larger with CMF than without CMF

- Addition of bentonite gave smaller flocs with CMF than without CMF, but was still the system giving the larger flocs at the end of the test

- Silica addition showed no effect at all

With those results, it was decided to run the next step comparing the behaviour of the different systems on drainage, formation, retention and mechanical parameters.

There was a high correlation between the particle size of the filler flocs and the drainage flow, formation and retention of the sheets, i.e. there was a good correlation between the results obtained in the FBRM tests and the final sheet properties, see Figure 11. The formation improved with increased filler floc size, which also resulted in a higher filler retention. Even if the drainage flow speed was reduced with the addition of CMF to the filler (pre-mixing strategy), there were significant improvements in formation and mechanical properties showing potential benefits.

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Figure 10. Results for the pre-flocculation (left, no CMF) and pre-mixing trials (right, with CMF) with the different chemicals. 1st_{component addition at 00:01 = CPAM, 1}st_{component addition at} 00:03 = CPAM (red curve) or CS (green curve), 2nd_{component addition at 00:03 = bentonite} (orange curve) or silica (yellow curve).

Figure 11. Correlation between the particle size of the filler flocs and the sheet formation (left) or drainage flow (right).

Comparison of floc size measurements obtained with the FBRM and image analysis on the dynamic flow loop showed the better shear stability of the CPAM/Bentonite system. However, the image analysis method is not able to capture the high filler flocculation obtained with the CPAM/Bentonite system as seen with the FBRM. Similarly, the FBRM is not able to differentiate the fibre flocculating effect between the cationic starch and the CPAM/Bentonite system.

De San Pio, I., Johansson, K. and Setterwall, H. (2018)

Pre-mixing CMF with filler

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Theme 2. Retention, formation and dewatering

P2.1 Retention and formation

P2.1a Adapting the flow loop for mixing and flocculation studies

The flow loop at Innventia was originally developed to perform basic investigations relating to mixing and flocculation. However, it needed to be further adapted to give more flexibility in terms of complex chemical dosage strategies and to allow for a wider range of measurement techniques, such as FBRM for filler floc size, longer fibre

flocculation measurement sections, and more accurate estimates of turbulence levels. Additionally, the transparent headbox developed by Yan needed to be connected so that flocculation processes could be measured for at different points in a headbox nozzle. This activity was completed during 2015 and the new setup has been used within the activities “P1.2f Pre-mixing CMF and filler” and “P2.1e Turbulence dampening for improved formation.”

Key findings

See activities “P1.2f Pre-mixing CMF and filler” and “P2.1e Turbulence dampening for improved formation.”

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P2.1b Turbulence dampening and the effect on flocculation and dewatering

The use of CMF as a turbulence dampener for improving formation was studied during 2015. Turbulence dampening, expressed in terms of turbulent drag reduction, was studied in the flow loop to investigate synergistic effects of CMF, cationic polyacrylamide (C-PAM), and anionic polyacrylamide (A-PAM) based systems. The influence of C-PAM and A-PAM dosage on mean floc size and turbulence dampening in pipe flows was evaluated based on high-speed filming and automated image analysis at various retention levels. Dewatering was not investigated in these systems.

The activity was continued as “P2.1e Turbulence dampening for improved formation” and the findings from these two activities were reported together.

Key findings

See activity “P2.1e Turbulence dampening for improved formation.”

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P2.1c FEX trials to show how dosing and mixing affect retention and formation, Part 1. Fine paper case

Mixing of chemical additives into the paper stock represents a fundamental unit operation in papermaking. A well-mixed stock can lead to improved additives performance and reduce their consumption during production. Good mixing is also crucial to improving retention and formation. On the other hand, poorly mixed additives can lead to severe runnability and product quality issues. The importance of mixing retention aids has been made clear by the wide range of specialized dosage equipment introduced to the market within the past few years alone.

Previous pilot studies on FEX at Innventia have shown significant improvements to both retention and formation when retention aids were added close to the head box under optimal turbulent conditions. In these studies, turbulence was manipulated by placing a gate valve just upstream the headbox and closing it partially, i.e. throttling. In some cases, improvements to both filler retention and formation were found to be more than 50% simultaneously. Those studies were, however, carried out without any wet end starch and at low conductivity levels. The aim of the present study was to investigate the effect of wet end starch and realistic conductivity levels in conjunction with the addition of retention aids close to the headbox on improving the retention-formation relationship. A throttling valve was used just upstream retention aids addition to improve the retention aids mixing quality.

The different cases for dosing and mixing investigated in this study were i) the standard case without throttling a gate valve close to the head box and where the retention aids were added at standard positions, and ii) the novel case where the retention aids were added very close to the head box and immediately after a throttled gate valve. Both the standard and the novel case were investigated with and without wet end starch and at different dosage levels of retention aid.

The pulp used was a mixture of 70% refined bleached hardwood kraft pulp and 30% refined bleached softwood kraft pulp. Target filler content was 20% in the paper. Retention, formation, dewatering, strength properties and optical properties were determined on the produced sheets.

Key findings

The effect from the novel approach for controlled dosing and mixing was different depending on if wet end starch was used or not, see Figure 12. In the absence of C-starch, the retention-formation relationship was broken when applying the novel case compared to the standard case; the formation was improved at maintained retention. In the presence of C-starch, the changes followed the retention-formation relationship with improved formation at the expense of impaired retention. Moreover, the effect from increasing the C-PAM dosage on the retention-formation relationship was almost negligible for the novel case in the presence of starch, i.e. an increased level of C-PAM had no effect. The reason for the strong influence of C-starch is not fully

understood but may involve different retention levels or different dewatering times. The dosage strategies here had only very small effects on strength and optical properties.

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Figure 12. Total formation number against first pass retention of solid material when varying the C-PAM dosage level (constant silica dosage). A low formation number means good formation. CS = cationic starch. Novel case = throttling the gate vale close to the headbox and late addition of retention aids.

Östlund, I. and Krochak, P. (2017)

Effect from wet end starch on strategies for dosing and mixing of retention aids. Fine paper case.

10 11 12 13 14 15 50 60 70 80 90 100 F o rm a ti o n F to t 0 .3 -3 0 m m , %

Retention of solid material, %

0% CS, standard case 0% CS, novel case 0.5% CS, standard case 0.5% CS, novel case

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P2.1c FEX trials to show how dosing and mixing affect retention and formation, Part 2. Starch dosage positions for twin-wire or Fourdrinier forming

Within this activity, two pilot production trials were performed on FEX aimed at investigating starch dosage strategies for improving the bulk-strength relationship for the middle ply in packaging grades. The products specifications were for a 100 g/m2 sheet consisting of 50/50 CTMP/bleached softwood kraft. Trials were repeated with twin-wire and Fourdrinier forming for comparison of the two production methods. The starch dosage position was varied throughout the trials, including thick stock addition, short circulation addition, and addition immediately prior to the headbox. The mixing conditions prior to the headbox were manipulated by throttling a gate valve, positioned just upstream the headbox. Trials were repeated with and without the throttled gate valve for comparison. Mechanical properties of the dried sheets were evaluated for samples collected at the jet-wire speed closest to the minimum fibre orientation anisotropy.

Key findings

It was shown that as the starch was added closer to the headbox, the bending stiffness and bulk were improved while the in-plane tensile properties were reduced. With twin-wire forming, the z-strength improved as starch dosage was made closer to the headbox. Figure 13 shows a comparison of the tensile stiffness index at the different dosage positions, where the dosage is closest the headbox at DKA/DKB.

Figure 13. Tensile stiffness index (geometric mean) for the different dosage positions, where DK8 represents the short circulation addition and DKA immediately prior the headbox. The open triangles represent no valve throttling while the solid squares represent with valve throttling.

Comparing the two forming methods it was found that twin-wire forming was more sensitive to the starch addition location than with Fourdrinier forming. Although the z-strength was found to be significantly higher in Fourdrinier forming, the z-z-strength improved as starch was added closer to the headbox with twin-wire forming, see Figure 14.

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Figure 14. Z-strength measurements for the different dosage positions, where DK8 represents short circulation addition and DKA immediately prior the headbox. The open triangles represent no valve throttling while the solid squares represent with valve throttling.

The effect of valve throttling (mixing) resulted, in most cases, in improved formation and slight improvements in tensile properties, however decreased the product bulk (increased its density). Formation was however found to be significantly different with the two forming methods, where small scale formation was significantly better with Fourdrinier forming and large scale formation was significantly better with twin-wire forming.

Krochak, P., Östlund, I. and Hermansson, L. (2016)

The influence of starch addition location and mixing conditions on product properties.

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P2.1d Dosing and mixing strategies for the retention of CMF – not started

To reach the full potential of CMF as a strength additive, it is imperative that it is retained to the fibres by adsorption. An undesired retention mechanism is that the retention aid only binds the CMF, such that the resulting agglomerate is retained by filtration into the web on the wire and leads to poor dewatering. Lab studies were planned to screen various single component retention aids and dosing strategies in relevant ionic conditions.

The objective of this activity has been investigated in other activities: “P1.2a Dispersion and dilution of CMF” and “P2.2a Comparison of microparticle retention aids on

dewatering”.

Key findings

See activities “P1.2a Dispersion and dilution of CMF” and “P2.2a Comparison of microparticle retention aids on dewatering”.

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P2.1e Turbulence dampening for improved formation

This work aimed to demonstrate the hypothesis that turbulence in the approach flow and headbox is detrimental to formation and that APAM can be used as a means to

minimize this turbulence, thereby improving formation. An investigation was made on the synergistic effects of the combined use of anionic polyacrylamide (APAM), cationic polyacrylamide (CPAM), together with either silica micro-particle (Si) or cellulose micro-fibrils (CMF) on the flocculation and re-flocculation rates in a semi-pilot scale flow loop and headbox nozzle. It was assumed that by reducing the average floc size in the wet-end, formation can be improved. Flocculation processes were characterized under dynamic conditions through flow visualization using high speed filming with automated image analysis. Filming was made in transparent piping and again in a transparent headbox.

In the first part of this work, synergies between CMF, CPAM and APAM on turbulence dampening were investigated at different flow rates. In the second part of this work, synergies between CPAM, silica and APAM were investigated and the effects of local turbulence levels and hydrodynamic shear applied to the fibre phase prior to dosage were studied. The addition order of APAM and silica were also varied in order to optimize the dosage strategy.

Key findings

APAM was shown to reduce the mean floc size in flowing suspensions along with turbulence levels in the approach flow (pipe flow), under industrially relevant

conditions. Moreover, these reductions could be observed with a relatively low APAM dosage level, i.e. 300 g/ton was found to result in a significant reduction in the mean floc size. CMF was shown to have no effect on the mean floc size nor on turbulence levels. Figure 15 shows the mean floc size, measured downstream the addition of CMF, CPAM and APAM at different flow rates.

Figure 15. Mean floc size of a fibre suspension and the influence from CMF, APAM and CPAM measured in a flowing suspension using high speed filming and automated image analysis. The dosage level of both APAM and CPAM were 300 g/tonne.

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Similar results were shown with systems consisting of CPAM, silica and APAM, where APAM was shown to reduce the floc size in the approach flow, however to less of an extent when the silica addition levels were high. However, flocculation in the headbox was found to be higher when APAM was present in the system, see Figure 16, while the rate of re-flocculation in the headbox nozzle was slower. It is believed that the APAM dampened the turbulence created by the headbox tube packets, minimizing floc rupture. At the same time, the reduced turbulence levels resulted in slower re-flocculation in the headbox nozzle. An example of the mean floc size at different points in the headbox is shown in Figure 16.

Figure 16. Shown on the left is the mean floc size as measured at different positions in a transparent headbox. Shown on the right are the different filming positions in the transparent headbox.

Krochak, P., Johansson, K., Lindström, T., Hillergren, M. (2017)

Turbulence dampening for improved formation: The effectiveness of CMF and APAM as wet-end formation aids.

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P2.1g FEX trial to show if adequate mixing can enable late addition of starch for strength

This activity was a continuation of the activities performed within P2.1c, with the objective to further investigate the effect of cationic starch (C-starch) on the retention-formation relationship in combination with cationic polyacrylamide (C-PAM), and silica for a fine paper grade (62.5 g/m2, 70/30 hardwood/softwood with 20% filler). Two addition strategies were compared, (1) a reference case, where C-starch was added in the thick stock, C-PAM was added prior the headbox pump and silica added directly after the pump, and (2) a novel case where all three components (C-starch, C-PAM and silica) were added directly before the headbox. For the novel case, mixing conditions were optimized by use of a throttled gate valve positioned immediately upstream the addition locations. Additionally, the addition levels of C-starch and C-PAM were 0, 5 or 10 kg/tonne and 900, 500 or 300 g/tonne, respectively.

Key findings

When all components (C-starch, C-PAM, and silica) were added just prior to the headbox, the formation worsened, despite the high mixing energy created by the throttled valve, see Figure 17. However, the C-PAM dosage needed to obtain a certain retention level was about half of that needed when the components were added in their normal positions. This would indicate possibilities to significantly decrease the C-PAM consumption while still maintaining a given level of retention.

Figure 17. First pass retention vs C-PAM addition level (left) and formation vs retention relationship (right) for the two different addition strategies. The C-starch addition level was 5 kg/tonne.

Strength properties increased with increasing dosage level of C-starch. However, despite a very short contact time and the starch addition level of 5 kg/ton, late addition of starch gave almost the same tensile properties as the standard addition point in the thick stock, see Figure 18. This would indicate potential optimization of the C-starch/C-PAM dosage for improving filler retention without deteriorating product properties.

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Figure 18. Tensile strength index as a function of CPAM dosage (left) and tensile stiffness index vs C-PAM dosage (right). The C-starch addition level is 5 kg/tonne.

Krochak, P. et al (2017)

Possible benefits of adding retention aids as well as cationic starch for dry strength very close to the headbox.

PaperCon 2017.

Östlund, I., Gimåker, M. and Krochak, P. (2018)

Possible benefits of adding retention aids as well as cationic starch for dry strength very close to the headbox.

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P2.1h Use of electrical fields to improve efficiency of retention systems – not started

The adsorption of polyelectrolytes (which all conventionally used retention polymers are) is driven by increased entropy by release of counter-ions upon adsorption. The rate of the adsorption process is dependent on mass transport of the polyelectrolyte to the charged surface, i.e. cellulose fibres. By application of an oscillating electrical field around the polyelectrolytes and cellulose fibre surfaces, momentary changes in the distribution of counter-ions could be induced. The hypothesis was that these momentary changes could influence the adsorption kinetics positively.

The activity was down-prioritized before it started.

Key findings

None.

Final Report for the Source-Efficient Paper and Board Making Research Programme Area