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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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

Caroline Ankerfors, Magnus Gimåker,

Gunborg Glad Nordmark

Innventia Report No.: 1032

April 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.

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Summary

In this study, the addition of up to four layers of PEM was 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).

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. By comparing the effect from single-addition of each cationic component to the effect of 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. For CPAM/APAM, however, very little advantage of using a multilayering approach was seen.

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.

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 by repeating some of the trial points of the CS/APAM system in mill process water. Firstly, this part of the study showed that PEMs can be successfully built in mill 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 and their strengthening effects remained.

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1 Introduction

The strength of a paper sheet is determined by the strength of the fibres, the number and strength of the bonds between the fibres, and the sheet formation. In reality, the choice of fibre is more or less predefined, but the paper strength can be further increased through the proper selection and use of chemical additives and addition strategies. Due to the desire to for instance increase filler content or the increased use of recycled fibres, there is an interest in achieving larger strength increases than what is possible by using a single strength additive. The strive towards lower basis weights is also

associated with a strength loss, thus also giving rise to an increased interest in strength additive systems that can give large effects.

1.1 Polyelectrolyte multilayers, PEM

Instead of the addition of a single polymer, alternating layers of cationic and anionic polyelectrolytes can be adsorbed, thereby forming a polyelectrolyte multilayer, PEM (sometimes also referred to as Layer by Layer, LbL). With the PEM technique, multi-layered nanometer-thick films can be built onto a surface (Decher et al. 1992, Decher and Schmitt 1992, Decher 1997). The technique was first introduced to the paper area by Wågberg et al. (2002) and has been shown to lead to a significant increase in paper strength (Wågberg et al. 2002, Eriksson et al. 2006, Torgnysdotter and Wågberg 2006, Lingström et al. 2007, Pettersson et al. 2007, Marais et al. 2016).

As an example, cationic and anionic starches of various grades were adsorbed onto unbeaten, bleached softwood kraft fibres to form three layers (Eriksson et al. 2005). When the cationic starches were in the outermost layer of the fibre treatment, i.e. the first or third layer, improvement in all investigated strength properties was detected. PEMs built by cationic and anionic starches have also been investigated by Lundström et al (Lundström Hämälä et al. 2009, Lundström 2009), but added to a TMP pulp instead. In one of the studies, the addition of PEMs made of CS/AS was compared to the strength enhancing effects of beating and it was found that PEM gave good strength improvements, while giving much less densification than beating.

The strengthening effect of PEMs is ascribed to the addition of increasing amounts of adsorbed polymer thus increasing the number of contact points between the fibres, the area in molecular contact in each contact point and the specific strength of the contacts (Eriksson 2004, Torgnysdotter et al. 2007). The use of PEM in papermaking can be envisioned as a way to reduce the use of energy-consuming beating processes, as well as to tailor the properties of paper materials by controlling the structure and properties of the polyelectrolyte multilayers formed on the surface of the pulp fibres during the papermaking process.

Multilayering has the advantage that high amounts of additives can be adsorbed which otherwise can be difficult with single-component systems. The technique, thereby, avoids a high concentration of non-adsorbed polyelectrolytes in the water phase, which can be detrimental to paper machine runnability. In real full-scale systems, only a few layers may be practically relevant, but may still have good effects on properties such as paper strength.

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The effects of multilayering of microfibrillated cellulose (MFC) onto a CTMP pulp, from which the fines material had been removed, were investigated with regard to the mechanical properties of hand-sheets (Ankerfors et al. 2016). It was found that multilayering using PAE and an anionic MFC gave a higher strength gain than using either cationic and anionic MFC or cationic and anionic starch. Multilayering with cationic and anionic MFC gave lower strength gain at a low addition level compared to starch multilayering, but there was a continuous increase in strength using MFC

multilayering. These trends were the same for all studied properties (tensile index, strain at break, TEA and Z-strength) except for tensile stiffness index, in which no clear differences between the chemical approaches could be seen. Sheet density was slightly affected (<14%) by the multilayering techniques used in these experiments.

1.2 Polyelectrolyte complexes, PEC

An alternative way to adsorb the same amount of strength additives would be to add all the additives in fewer steps, using a dual-addition dosage strategy, which in some way would be comparable to the formation of polyelectrolyte complexes, PECs.

PECs can be formed in two fundamentally different ways; in situ or by pre-formation (i.e. premixing of the cationic and anionic polyelectrolyte before addition to the pulp fibres) (Ankerfors 2012, Ankerfors and Wågberg 2014). Both methods have shown good results on paper strength properties. With the in-situ PEC formation method, the two polyelectrolytes are added sequentially to the pulp suspension. With this method, PECs are either formed directly on the fibre surfaces or in the dispersed state and later deposited onto the fibres and the same effects as from PEMs can be achieved, but with fewer additions and adsorption steps.

The order of addition of the polycation and the polyanion has been investigated by for example Wågberg et al (1987) by analysing the formation and size of the formed fibre flocs by the addition of APAM/PAE to a bleached softwood. The largest effect on flocculation was found when the cationic PAE was added first. The same influence of the order of polyelectrolyte addition was seen on the tensile strength (expressed as breaking length) using sequential addition of PDADMAC and CMC to pulp made from recycled copy paper; the highest paper strength being measured after addition of

PDADMAC followed by CMC (Lofton et al. 2005). At a high level of addition of PDADMAC (supposedly enough to oversaturate the fibre surfaces), an increase in the amount of CMC added resulted in higher strength. It was hypothesized that this procedure led to the formation of a PEC, which was deposited on the fibres.

1.3 Purpose and goal of the present study

To conclude, there are many promising results to be found for both PEM and PEC in the literature. However, all of these were achieved using clean and well controlled

conditions. The high concentrations of electrolytes, dissolved anionic substances as well as colloidal materials, and especially the variation in these concentrations over time that are a reality in most paper mills, make the implementation of polyelectrolyte

multilayering difficult and challenging. This study will address the effect of presence of dissolved and colloidal substances (DSC) to investigate the possibilities of

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The goal of the present study was to find a suitable polyelectrolyte system for a future FEX pilot trial, by testing a few industrially relevant systems in laboratory scale, both in tap water and in mill process water. The work was conducted as pre-competitive

research activity P1.2d The effect of different chemical additives on strength and bulk within the programme area Source-Efficient Paper and Board Making.

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2 Experimental

2.1 Materials

The pulp used was a bleached softwood kraft pulp supplied by Stora Enso. The pulp was beaten with 50 kWh/tonne using a Voith lab refiner (“LR40”, bar code 3-1.0-60, cutting edge length = 0.5 km/s and specific edge load = 2.0 J/m) before polyelectrolyte addition. No extra washing of the pulp was made, meaning that fines were present. The SR number of the refined kraft pulp was approximately 16.

Three pairs of a cationic and anionic components were studied, see Table 1 (all data according to manufacturer, unless else stated).

Table 1. Properties of the cationic and anionic polyelectrolytes for the three PEM combinations, PVAm/CMC, CPAM/APAM, and CS/APAM.

Cationic component Anionic component

PVAm/CMC Polyvinylamine, Hercobond 6950, Mw ~350 kDa, charge density 7.5 meq/g at pH 7, Solenis Carboxymethylated cellulose, Cat no. 419281, DS 1.2, charge density 7.4 meq/g, ~250kDa, Sigma Aldrich CPAM/APAM Cationic polyacrylamide,

Hercobond L1220, charge density 1.4 meq/g (calc.), medium-high Mw, Solenis

Anionic polyacrylamide, Hercobond 2800, 3.5 meq/g, Mw ~550 kDa, Solenis

CS/APAM Cationic starch, Solbond PC80, potato starch, DS 0.08, 0.49 meq/g1_{, Solam}

See above.

1_{The molecular size and structure is determined by the fact that the source is conventional potato. The} ratio amylose:amylopectin is roughly 80:20. The cationization of the starch does not influence the molecular structure to any significant extent. The interested reader is referred to Pérez et al. (2009) and Jane (2009), which give a very thorough review about what is known about the molecular structure of different starches.

The cationic starch was gelatinized prior to use by dispersing the starch in water at a concentration of 3 g/l, after which it was gelatinized at 96-98°C and heavy agitation for 20 minutes. The gelatinized starch was then cooled to room temperature before use. All other chemicals were dissolved in deionized water before use.

For the first part of the study, tap water with an addition of NaHCO3 up to a conductivity of 1100 µS/cm was used. In the second part of the study, mill process water was received from the BillerudKorsnäs mill in Frövi, Sweden. The process water was taken from the production of unbleached liquid packaging board (where CTMP is a major component in the middle layer pulp mix, meaning that the white water contains a fair amount of dissolved and colloidal substances).

For all trials including mill process water, a fixative was used (DiMAPA, Praestafix DC1250, from Solenis) to neutralise the detrimental substances in the process water. The fixative dosage was chosen so that the cationic demand was reduced but without reducing the zeta potential to any large extent.

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2.2 Methods

Suitable polymer dosages for each layers of the PEMs were determined by adding varying dosages of each polymer and measuring the net fibre charge using Mütek SZP and cationic demand value in a fibre suspension filtrate using Mütek PCD.

The adsorption of cationic starch was measured by determining the sugar content in the white water using a standard colorimetric method (Dubois et al. 1956).

All adsorptions were performed in a Britt Dynamic Drainage Jar (BDDJ) with 1000 rpm stirring to ensure well controlled and constant mixing conditions. For the trials using tap water the pulp consistency was 20 g/L. For the trial using mill process water the pulp consistency was 5 g/L.

For the build-up of PEMs, the solutions of the cationic and anionic components were added sequentially to the BDDJ, without washing between each consecutive addition. For the experiments made in tap water, two different adsorption times (i.e., time between additions) were used; 30 seconds respectively 15 minutes. Since no positive effect of a longer adsorption time was seen, only 30 seconds adsorption time was used for the experiments using actual mill process white water.

The experiments made in tap water used an extra addition of NaHCO3 to get a

concentration of 0.01 M, which gives a conductivity of approximately 1100 µS/cm and a pH at around 8.2-8.3.

The industrial advisory committee considered the CS/APAM system to be the most interesting, and selected trial points for this system were repeated in mill process water. In these cases, the pulp was re-slushed in the process water. Conductivity and pH was not further adjusted.

In all cases using mill process water, a fixative (DiMAPA, Praestafix DC1250, from Solenis) was added as a first step and a mixing time of 2 minutes was used to assure proper mixing of the fixative before any other additives were added.

In the end, pulp suspensions with adsorbed polyelectrolytes were transferred to a Finnish sheet former and sheets with a basis weight of 100 g/m2 were formed in accordance to ISO 5269-1 using tap water in the sheet former. The sheets were then wet-pressed at 400 kPa for 5 min, the blotting paper was changed, and the wet sheet was again pressed at 400 kPa for 2 min. Finally, the sheets were dried under constraint at 23°C and 50% RH. Four sheets per trial point were made.

Grammage, density and in-plane mechanical (tensile) properties were measured on the formed sheets. The methods used are presented in Table 2.

Table 2. Properties tested and the methods used for the laboratory hand sheets.

Property Method

Standard atmosphere for conditioning and testing ISO 187:1990, 23C, 50% RH

Grammage ISO 536

Structural density SCAN-P 88:01

Apparent density ISO 534

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3 Results

In this study, the addition of up to four layers of PEM was 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.

3.1 Adsorption of a single strength additive onto pulp fibres

The adsorption behaviour of single additions of PVAm, CPAM and CS was studied by adding increasing amounts of the polyelectrolytes to a pulp fibre suspension. The effect on the net surface charge of the fibres was measured as zeta potential value (Figure 1) and the adsorbed amounts were calculated from the excess of polyelectrolyte in the filtrated measured by titration with PESNa solution using Mütek PCD03 (Figure 2).

Figure 1. Zeta potential against the total added amount of PVAm, CPAM and CS in tap water, using an adsorption time of 15 minutes.

Figure 2. Adsorbed amounts against the total added amount of PVAm, CPAM and CS in tap water, using an adsorption time of 15 minutes.

As can be seen from Figure 1 and for all three strength additives, the zeta potential value increased almost linearly with the polyelectrolyte addition at first, and levelled off as higher amounts were added. At various addition levels, the zeta potential value changed sign from negative to positive values. A lower amount of PVAm was needed to recharge the pulp fibres to net cationic zeta potential values, compared to when CPAM or CS was used (recharge after the addition of approx. 4, 12, and 10 mg/g fibre for PVAm, CPAM and CS, respectively). Also, with PVAm, higher positive zeta potential values could be achieved than for either CS or CPAM.

As can be seen from Figure 2, the retention of CPAM was the highest (least difference between added and adsorbed amounts) whereas this difference increased with higher added amounts of CPAM or CS, which can be interpreted as a saturation of the fibres for the two latter cases leading to a lower retention of the added CS or PVAm. This deviation occurred roughly at the same addition level as the charge reversal. While the retention of CPAM seemed to be almost unaffected even at the highest dosage in this study, 30 mg/g fibre, the zeta potential value levels off and does not increase further at the higher dosages. For PVAm, the opposite effect could be seen; the retention decreased at dosages above approx. 10 mg/g fibre, but the zeta potential value continued to increase even above this dosage. The difference in adsorption behaviour

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may indicate that CPAM and PVAm have different conformations on the fibre surfaces, but this would need to be further investigated.

Figure 3 shows the tensile strength index, strain at break, TEA index, and tensile stiffness index values of hand sheets prepared from pulp after the addition of various amounts of PVAm, CPAM or CS.

The graphs show that there was a strengthening effect from the addition of all three additives used as single component strength additives, although the effects were quite small. For tensile strength index (TI) and TEA index, the strength increased with increased addition levels of either CPAM or CS at first, and the effect levelled off after addition of approx. 15 mg/g. For PVAm, no such trend could be seen. The figures also show that the effect on strength properties of adding CPAM was slightly higher than adding PVAm or CS. This is in line with the result from the measurement of adsorbed amount, which showed that the retention of CPAM was higher than for PVAm or CS. However, this difference was seen at the higher dosages only and a continued strength increase at higher addition levels of CPAM (above approx. 15 mg/g) would have been expected but could not be shown here.

For strain at break and tensile stiffness index, no clear effects could be detected and any conclusions on trends are more difficult to draw. Possibly, CPAM showed slightly higher values than PVAm or CS.

Figure 3. Tensile strength index, strain at break, TEA index, and tensile stiffness index using various single-addition levels of the strength additives PVAm, CPAM, and CS.

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The structural sheet density for the hand sheets prepared with various single-addition levels of the three strength additives PVAm, CPAM, and CS is shown in Figure 4.

Although with some variability at the lower dosage levels, especially in the PVAm case, the results show that there was no significant increase in density in this study, keeping in mind that these are ISO hand sheets. According to our experience ISO hand sheets tend to show less densification effect with addition of strength additives as compared to e.g. Rapid-Köthen sheets.

Figure 4. Structural density with various single-addition levels of the three strength additives PVAm, CPAM, and CS.

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3.2 Adsorption of multilayers onto fibres and effects of paper properties

PEMs were built onto the fibre surfaces by alternating addition of cationic and anionic polyelectrolytes to the pulp suspension. The reversal of the net surface charge of the fibres (change of sign of the zeta potential) from negative before any polyelectrolyte addition to positive after addition of the first, cationic polyelectrolyte layer (PVAm, CPAM or CS) enables the adsorption of the second, anionic polyelectrolyte layer (CMC or APAM). The charge reversal back to negative value of the zeta potential by addition of CMC or APAM enables the adsorption of the third, cationic layer, and so on.

The suitable dosage for each of the layers in the PEMs were determined by measuring the zeta potential value of the pulp during the step-wise addition of small amounts of each polyelectrolyte and monitoring the charge reversal. The dosage levels were chosen so that the net zeta potential was well over on the other side, preferably around

± 5 – 10 mV. For CS this was not possible on the cationic side, and the net zeta

potential values for layers 1 and 3 of the CS/APAM PEM were therefore slightly lower than the others (comparing the absolute values). Figure 5 shows how the zeta potential value alternates between positive and negative values with the addition of each new layer in the PEM.

Figure 5. Zeta potential development for the addition of 1-4 layers of PEM built by consecutive additions of PVAm/CMC, CPAM/APAM, or CS/APAM.

The dosage used for the third layer in the PVAm/CMC PEM (i.e., PVAm) was later slightly adjusted and 5 mg/g was used both for layer number one and three in the continued studies, instead of 2 mg/g for the third layer as shown in Figure 5. A

summary of the selected dosages for building the three types of PEMs in the continued work presented in this study is shown in Table 3.

Table 3. Added amounts for each layer in the PEMs (values in mg/g).

1st_layer ₂nd_layer ₃rd_layer ₄th_layer

PVAm/CMC 5 1 5 1

CPAM/APAM 15 1 20 1

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3.2.1 CS/APAM PEM and dual-additions

Mechanical properties of laboratory hand sheets prepared after the addition of up to four PEM layers of CS and APAM are shown in Figure 6 through Figure 9, and compared to those with various dosages of CS only.

Figure 6. Tensile strength index against total added amount for single-additions of CS and PEM of CS/APAM.

Figure 7. Strain at break against total added amount for single-additions of CS and PEM of CS/APAM.

Figure 8. TEA index against total added amount for single-additions of CS and PEM of CS/APAM.

Figure 9. Tensile stiffness index against total added amount for single-additions of CS and PEM of CS/APAM.

As can be seen from the graphs, both addition strategies (CS as a single additive or together with APAM in a PEM) increased the tensile strength index and TEA index values (Figure 6 and Figure 8) for the produces hand sheets. Increases were also seen in the strain at break and tensile stiffness index data (Figure 7 and Figure 9) although these positive effects were not as pronounced as for TI or TEA index.

The highest strength values in this comparison were achieved with the addition of 2–4 PEM layers. Whereas the single addition of CS initially increased the TI gradually as the dosage increased, the addition of the PEM layers seemed to increase the TI in a step-wise fashion with the largest increase with the addition of the second layer (APAM). At the higher CS addition levels, the strengthening effect levels off, whereas the same total added amount using the PEM technique resulted in the highest strengths.

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As can be seen in Figure 6 through Figure 9 as well as in the following sections, the mechanical properties of the two sets of reference sheets (i.e. the trial points at 0 mg/g added chemical amount) differed although they should have been the same. The “true” value is probably somewhere between the two values shown here and the trends that can be seen when adding strength chemistry should be considered as valid despite this. Tensile strength index of laboratory hand sheets prepared using two different dual-addition strategies is shown in Figure 10 (left: CS added first, and right: APAM added first). Additional graphs (strain at break, TEA Index and tensile stiffness index) can be found in Appendix, section 6.

Figure 10. Tensile strength index against total added amount for dual-additions of CS/APAM with either CS added first (left) or with APAM added first (right). The inserted numbers indicate the charge ratio between the two components, in the order of addition.

The TI values in Figure 10 show that the addition of CS and APAM using a dual-addition strategy resulted in strength increases in the same order as did the PEM

additions. The dual-addition strategy may therefore be a good alternative to the build-up of PEMs. The trial points with the highest total added amount presented in Figure 10 had the same total added amount as the four PEM layers in Figure 6 and with the same cation to anion ratio. By comparing the results from these addition strategies (PEM, dual-addition with either cation or anion added first), it can be seen that the strength increase is in the same order of magnitude for all strategies.

By comparing the three trial points having similar total amount added (namely 10+1, 10+1.5 and 10+2 mg/g of CS+APAM, respectively, in Figure 10, left, or the same dosages but in the reverse addition order in Figure 10, right), but with the charge ratio varied, the ones with the component added last in excess with respect to charges resulted in slightly higher TI values. This would however need further investigations.

1:1 3:2 2:3 3:2 2:3 3:2 _2:3 1:1

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3.2.2 PVAm/CMC PEM and dual-additions

Mechanical properties of laboratory hand sheets prepared after the addition of up to four PEM layers of PVAm and CMC are shown in Figure 11 through Figure 14 (open

squares), and compared to those with various dosages of PVAm only (filled squares).

Figure 11. Tensile strength index against total added amount for single-additions of PVAm and PEM of PVAm/CMC.

Figure 12. Strain at break against total added amount for single-additions of PVAm and PEM of PVAm/CMC.

Figure 13. TEA Index against total added amount for single-additions of PVAm and PEM of PVAm/CMC.

Figure 14. Tensile stiffness index against total added amount for single-additions of PVAm and PEM of PVAm/CMC.

As can be observed in all of the properties shown in Figure 11 - Figure 14, the PEM addition clearly improved the properties of the formed sheets, especially with the addition of the second and third layers. The trends are similar as for CS/APAM, but the advantage with using the PEM technique over the single-additions of PVAm is much clearer in this case, due to the poor strength development achieved when adding various dosages of PVAm only.

It could also be observed that the combination of PVAm/CMC in a PEM increased the tensile stiffness value (Figure 14), an effect that could not be seen with the other PEM combinations in this study, nor with the single additions tested in this study.

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Using the dual-addition dosage strategy, two different PVAm dosages (4 and 8 mg/g) were tested, and these were also combined with various CMC dosages (from 2 to 8 mg/g), resulting in various charge ratios between the added cationic and anionic charges (as a result of the relative added amounts of PVAm and CMC, which have approximately the same charge density). Tensile strength index values of laboratory hand sheets prepared using the dual-addition strategy with various ratios are shown in Figure 15 (PVAm added first), Figure 16 (CMC added first), and Figure 17 (both components added simultaneously). Additional graphs showing strain at break, TEA Index, and tensile stiffness index can be found in Appendix, section 6.

Figure 15. Tensile strength index against total added amount for dual-additions

of PVAm/CMC with PVAm added first. The inserted numbers indicate the charge ratio of the two added components, in the order of addition (cation:anion).

Figure 16. Tensile strength index against total added amount for dual-additions of PVAm/CMC with CMC added first. The inserted numbers indicate the charge ratio of the two added components, in the order of addition (anion:cation).

Figure 17. Tensile strength index against total added amount for dual-additions of PVAm/CMC with both components added simultaneously. The inserted numbers indicate the charge ratio of the two added components (cation:anion).

3:2 2:3 4:1 1:8 1:1 2:3 1:1 1:1 3:2 4:1 1:1 1:1 1:4 1:4 1:4

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Compared to when PVAm was used as a single-component additive (filled squares in Figure 11), adding an anionic polymer to the system could clearly help boost the effect of the PVAm and higher tensile index values were achieved, which is shown in Figure 15. No clear differences between the varied charge ratio could be seen (the inserted numbers in Figure 15), but all values seem to show similar relation between added amount of chemicals and their effect on tensile strength index.

CMC was also added first followed by the addition of PVAm, as well as at the same time as the PVAm addition. Tensile strength index values for hand sheets produced using these two dosage strategies are shown in Figure 16 and Figure 17. In Figure 16, where the anionic CMC was added first, the trial point with the highest tensile strength index had the charge ratio 1:4 (CMC:PVAm), but a repeated experiment with the same dosages resulted in significantly lower tensile strength index value, which clearly

indicates the poor repeatability in this case. This result could point at efficient mixing as one very important parameter, among other.

In Figure 17, where the two additives where dosed simultaneously, the trial point with a 4:1 charge ratio (here: 8 mg/g PVAm followed by 2 mg/g CMC), resulted in the highest tensile strength index value, and a much higher tensile strength index value compared to a single-addition of the same amount of PVAm, but not quite at the same level as with four PEM layers of the same components (see Figure 11 for comparison). This high PVAm dose (8 mg/g) is close to the maximal dose that can be retained by the fibres, and the fibre surfaces are maximally recharged (as seen in Figure 1 and Figure 2). The following CMC addition can now be adsorbed by the fibres, and possibly also helps anchoring the previously adsorbed PVAm, which may be favourable for the

development of sheet strength.

The addition of PVAm and CMC to a charge ratio of 1:1, would theoretically result in the formation of in-situ-formed, uncharged complexes when added simultaneously or with the anion CMC added first, with low possibilities to interact with the fibre surfaces and a low effect on paper strength would thereby be expected. Tendencies of support to this reasoning can be seen in the low effect of the 1:1 dosages in Figure 16 and Figure 17, even at high total added amounts. In Figure 15, however, where the cation PVAm was added first, the 1:1 addition resulted in the highest TI value, just slightly lower than that of four PEM layers, see Figure 11. With this addition order, it can be imagined that the high amount of PVAm overcharges the fibre surface, and the following CMC

addition contributes by anchoring the adsorbed layer. The same amount of PVAm added without the following CMC dosage resulted in much lower strength values (PVAm single-additions in Figure 11 - Figure 14).

A general observation for all trials presented in Figure 15 - Figure 17 is that all

measured TI values for sheets made with a low PVAm dose (4 mg/g, circles) are lower than for the high PVAm dose (8 mg/g, triangles). This could be due to that at the lower PVAm dosage, the zeta potential value of the fibre suspension (Figure 1) is around zero, and the fibres should thereby be less prone to interact with any added CMC, which thereby may be poorly retained. At the higher PVAm dosage, the fibre surfaces are recharged and an interaction with the CMC can be expected.

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3.2.3 CPAM/APAM PEM

Mechanical properties of laboratory hand sheets prepared after the addition of up to four PEM layers of CPAM and APAM are shown in Figure 18 - Figure 21, and compared to those with various dosages of CPAM only. The figures below also include the

mechanical properties for a series of 1-4 PEM layers with lower dosage level (squares), i.e. where the fibres were not recharged by the addition of each new layer, but always stayed net anionic.

Figure 18. Tensile strength index against total added amount for single-additions of CPAM and PEM CPAM/APAM with lower dosages (open squares) or higher dosages (open circles).

Figure 19. Strain at break against total added amount for single-additions of CPAM and PEM CPAM/APAM with lower dosages (open squares) or higher dosages (open circles).

Figure 20. TEA Index against total added amount for single-additions of CPAM and PEM CPAM/APAM with lower dosages (open squares) or higher dosages (open circles).

Figure 21. Tensile stiffness index against total added amount for single-additions of CPAM and PEM CPAM/APAM with lower dosages (open squares) or higher dosages (open circles).

As can be seen in all graphs above, using CPAM in combination with APAM in a PEM did not result in any increase as compared to using CPAM alone as a single additive for the mechanical properties presented here.

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Further, the increases in mechanical properties with the lower PEM dosages (unfilled squares) were smaller than for the higher PEM dosages (unfilled circles), which could be merely a consequence of the low total added amount in the low PEM series. Due to the poor effect of combining CPAM and APAM using the PEM method over using the CPAM alone as a single-additive, no trials using CPAM and APAM in dual-addition strategies were performed.

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3.3 Influence of dosage strategy on sheet density

Figure 22 and Figure 23 show tensile strength index against structural and apparent density, for all dosages and strategies tested in this study.

Figure 22. Tensile strength index against the structural density of hand sheets made with a single-addition (open symbols), PEM (black symbols) or dual-addition (grey symbols) of PVAm/CMC (squares), CPAM/APAM (circles) or CS/APAM (triangles).

Figure 23. Tensile strength index against the apparent density of hand sheets made with a single-addition (open symbols), PEM (black symbols) or dual-addition (grey symbols) of PVAm/CMC (squares), CPAM/APAM (circles) or CS/APAM (triangles).

Although all measured sheet density variations were small, there were indications that the density was lower for PEMs, medium for single-dosages and higher using the dual-addition strategies. These tendencies were slightly more pronounced in the structural than in the apparent density measurement values.

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3.4 Influence of mixing time

Initially, two adsorption times, 15 minutes and 30 seconds, and their influence on tensile strength index of the formed hand sheets was tested (see Figure 24 and Figure 25). To large extent, the results were similar, albeit with some indications that a shorter adsorption time would be favourable for developing strength of the produced paper sheets. This may be explained by polyelectrolyte reconformation processes taking place on the fibre surfaces and in the PEM structures, resulting in a flatter and less effective PEM conformation. Given that the mixing of the added polyelectrolyte solution to the pulp suspension is sufficient, the adsorption process in itself should be fast enough to take place during the 30 seconds.

Figure 24. Tensile strength index against total added amount for PEM of CS/APAM, with 30 s (unfilled symbols) or 15 min (grey symbols) adsorption time for each added layer.

Figure 25. Tensile strength index against total added amount for PEM of CPAM/APAM, with a higher (circles) or lower dose (squares) and with 30 s (unfilled symbols) or 15 min (grey symbols) adsorption time for each added layer.

Another observation can be done by comparing the high and low dosage PEM levels of CPAM/APAM in Figure 25: for the lower dose, the shorter time resulted in higher TI values, but for the higher dose, the longer time was slightly more effective, but the relative differences were smaller. One hypothesis based on this observation may be that at the lower dose, the conformation of the polyelectrolytes on the fibre surface is much more important, and the longer time enables the polyelectrolyte to adapt a flatter structure, less effective in contributing to strength. At higher dosage levels, the polyelectrolytes are abundant and any reconformation on the fibre surface or in the PEM structure are less critical.

Based on these findings, an adsorption time of 30 seconds was used for all PEMs and dual-additions. The shorter adsorption times would be more feasible for implementation and scale-up (although this will be highly case and machine dependent).

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3.5 Build-up of CS/APAM PEMs in mill process water

In all cases using mill process water, a fixative (DiMAPA) was added as a first step. The purpose of the fixative is to work as trash collector before the other more expensive polymers are added. The fixative dose was chosen to decrease the cationic demand as much as possible without decreasing zeta potential value too much and this dosage was the same for all trial points shown in Figure 26 through Figure 28.

Figure 26 shows the zeta potential values of a suspension of fibres of the same type as before but reslushed in mill process water, after the addition of the fixed dose of fixative and after the addition of various dosages of cationic starch.

Figure 26. Zeta potential against the total added amount of CS in mill process water.

As Figure 26 shows, a recharging of the fibres surfaces by the addition of cationic starch was possible in the mill process waters. Although the dosage of cationic starch needed to do so was somewhat higher compared to in the lab trials in tap water (see Figure 1), the trend was the same. Also, the highest zeta potential value achieved was slightly lower in mill process water compared to in tap water.

Based on zeta potential measurements as in Figure 26, the dosages needed for the recharging with each added layer in the PEMs were slightly adjusted.

Figure 27 shows tensile strength index values of laboratory hand sheets from fibres reslushed in mill process water and with the addition of up to four PEM layers of CS and APAM (open triangles) or with the addition of various amounts of CS only (filled triangles).

Figure 28 shows tensile strength index values for the same type of sheets but with the chemical additions made using the dual addition strategy, adding either the cation first (filled circles) or the anion first (open circles). The dosage levels for the dual additions in Figure 28 corresponds to the dosage in all four layers of the PEMs.

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Figure 27. Tensile strength index against total added amount for single-additions of CS and PEM of CS/APAM in mill water, with fixative added.

Figure 28. Tensile strength index against total added amount for dual-additions of CS/APAM with either CS added first (filled symbols) or with APAM added first (open symbols), in mill process water, with fixative added.

Figure 27 shows similar trends as the previous trials with addition of CS and APAM made in tap water shown in Figure 6, with higher tensile strength index values obtained with the combination of CS and APAM, than with increased CS dosage. However, the difference between the two dosage strategies seem somewhat less pronounced in this latter comparison, made in mill process water.

Looking at the effect of PEM addition in more detail, it can be seen that the largest increase in TI was achieved when adding the second layer (APAM), whereas the 3rd and 4th layers did not increase the TI further. Possibly, the effects of adding the 3rd and 4th layers could be improved by optimization of the dosage levels for these layers. Figure 28 shows an increase in TI of about the same magnitude as the addition of the same amount of additives, added as a PEM. In this comparison, however, the best effect in relation to added amount was achieved by adding the two PEM layers as shown in Figure 27 (i.e. dual addition with CS added first, but with a lower dosage compared to the dosages in Figure 28).

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4 Conclusions

In the present study, the influence on mechanical properties of paper sheets after fibre treatment using the polyelectrolyte multilayering (PEM) technique was studied and compared to the effects of adding the cationic components of the PEM only, as a single-component additive with various dosages. One of the main findings 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. 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 indices could be reached with the PEM strategy for the combinations PVAm/CMC and CS/APAM with especially good strength improvement with the addition of the second layer. For CPAM/APAM, however, very little advantage of using a multilayering approach was seen.

The results from using a dual-addition strategy (with either the polycation or polyanion first) showed similar potential as the PEM technique for the two tested polymer pairs. However, the results are more difficult to interpret, and it is believed that mixing is very important for these additions.

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 and knowledge in the industrial advisory committee, it was decided that the system most suitable to test in mill process water as well is in future continuation in the form of a FEX pilot machine trial would be CS/APAM. The results in this study showed that the strength gain was very similar in the clean system as in the white water system.

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5 References

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KTH Royal Institute of Technology, Sweden

Ankerfors C and Wågberg L (2014)

Polyelectrolyte Complexes for Tailoring of Wood Fibre Surfaces

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Ankerfors M, Lindström T and Glad Nordmark G (2016)

Multilayer assembly onto pulp fibres using oppositely charged microfibrillated celluloses, starches, and wet-strength resins − Effect on mechanical properties of CTMP-sheets

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Fuzzy nanoassemblies: toward layered polymeric multicomposites

Science 277(5330): 1232-1237

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Buildup of ultrathin multilayer films by a self-assembly process:III. Consecutively alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces

Thin Solid Films 210-211(Part 2): 831-835

Decher G and Schmitt J (1992)

Fine-tuning of the film thickness of ultrathin multilayer films composed of consecutively alternating layers of anionic and cationic polyelectrolytes

Prog. Colloid Polym. Sci. 89(Trends Colloid Interface Sci. VI): 160-164

Dubois M, Gilles K A, Hamilton J K, Rebers P A and Smith F (1956)

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Eriksson M (2004)

Interaction Between Thin Interfacial Layers: The Influence of Polyelectrolyte Multilayers on Paper Strength

Eriksson M, Pettersson G and Wågberg L (2005)

Application of polymeric multilayers of starch onto wood fibres to enhance strength properties of paper

Eriksson M, Torgnysdotter A and Wågberg L (2006)

Surface modification of wood and fibers using the polyelectrolyte multilayer technique: Effects on fiber joint strength and paper strength properties

Ind. Eng. Chem. Res. 45: 5279-5286

Jane J-l (2009)

Structural features of starch granules II

Starch (Third Edition), Elsevier: 193-236.

Lingström R, Notley S H and Wågberg L (2007)

Wettability changes in the formation of polymeric multilayers on cellulose fibers and their influence on wet adhesion

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Lofton M C, Moore S M, Hubbe M A and Lee S (2005)

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The adsorption of polyelectrolyte multilayers (PEM) of starch on mechanical pulps for improved mechanical paper properties

Lundström L (2009)

Polyelectrolyte multilayers of cationic and anionic starch and their use for improving the strength of paper made from mechanical pulps

Marais A, Enarsson L-E, Pettersson G, Lindström T and Wågberg L (2016)

Pilot-scale papermaking using Layer-by-Layer treated fibres; comparison between the effects of beating and of sequential addition of polymeric additives

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6 Appendix

The graphs in Figure 29 show additional mechanical properties of laboratory hand sheets prepared using the two CS/APAM dual-addition strategies as described in Section 3.2.1 (left: CS added first, and right: APAM added first).

Figure 29. Strain at break, TEA Index and tensile stiffness index against total added amount for dual-additions of CS/APAM with either CS added first (left) or with APAM added first (right).

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The graphs in Figure 30 to Figure 32 show additional mechanical properties of

laboratory hand sheets prepared using the three PVAm/CMC dual-addition strategies as described in Section 3.2.2, adding either PVAm or CMC first or adding both

components simultaneously.

Figure 30. Strain at break against total added amount for dual-additions of PVAm/CMC with either PVAm added first, with CMC added first or with both components added simultaneously.

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Figure 31. TEA Index against total added amount for dual-additions of PVAm/CMC with either PVAm added first, with CMC added first or with both components added simultaneously.

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Figure 32. Tensile stiffness index against total added amount for dual-additions of PVAm/CMC with either PVAm added first, with CMC added first or with both components added

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Innventia Database information

Title

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

Author

Caroline Ankerfors, Magnus Gimåker and Gunborg Glad Nordmark

Abstract

In this study, the addition of up to four layers of PEM was studied and compared with the use of single or dual-additions of the same chemicals with respect to their effect on strength properties of the produced hand sheets. The systems studied were cationic/anionic polyacrylamide

(CPAM/APAM), polyvinylamine/carboxymethyl cellulose (PVAm/CMC) and cationic starch/anionic polyacrylamide (CS/APAM). It was found that with increasing single-dosage levels of PVAm, CPAM or CS, the TI of the produced sheets increased at first, but the effect levelled off at higher dosages.

By comparing the effect from single-addition of each cationic component to the effect of 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. For CPAM/APAM, however, very little advantage of using a multilayering approach was seen. 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 using dual-addition strategies.

This study also addressed the influence from dissolved and colloidal substances (DCS) by repeating some trial points in mill process water to investigate the possibilities of implementing the polyelectrolyte multilayering technique in practice. It was shown that PEMs can be built in process waters and that the adsorbed amounts might differ compared to in the cleaner system but the trends in strengthening effects for the dosage strategies remained.

Keywords

Paper additive, paper properties, strength properties, dry strength, dry strength agent, wet end additive, wet end chemistry, polyelectrolyte

Classification

1180, 1240

Type of publication

PCR report Source-Efficient Paper and Board Making, InnRP 2015-2017

Report number

Innventia Report No. 1032

Publication year

2018

Language

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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