Chemical Technology of Papermaking

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22 Hochwirksame Mais-St?rke im Wet-End als kostengünstige Alternative zur

Kartoffelst?rke Highly effective cornstarch in the wet-end as a low-cost alternative to

potato starch

Detlev Glittenberg Ph.D, Krefeld, Deutschland Richard J. Tippett B.Sc., Krefeld, Deutschland Peter Leonhardt Dipl.-Chem (FH), Krefeld, Deutschland Maurice Timmermans M.Sc., Bergen Op Zoom, Niederlande

Zusammenfassung

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20 Abstract

Cationic wet-end starches are primarily added as a dry strength additive. In addition to this role in the dried paper sheet, however, they also heavily influence dewatering as well as re-tention and formation in the wet part of the paper machine. That means that the type, amount and addition point of a wet-end starch is important to establish the best possible synergies with synthetic retention, dewatering and sizing chemicals.

Typically potato-based cationic wet-end starches are used in heavy duty applications, mean-ing highly filled or hard-sized papers or papers being produced on high speed machines, be-cause in most cases they outperform traditional cationic corn starches of the same cationic-ity.

With the planned reduction of the EU potato starch quota and the foreseeable effects on the pricing, research was performed with the objective to optimise the performance of corn based wet-end starches in order to close the performance gap to potato starch and make possible the application of products based on this readily available raw material.

When comparing potato and corn starch, one obvious difference is the viscosity level that is achieved upon cooking both starches at identical concentration: potato starch pastes yield a significantly higher viscosity level. There are ways to increase the viscosity of starches well known to starch chemists, however, these methods have to be applied with care.

In the first chapter of the paper it is described how by careful choice of cationic reagent, reac-tion conditions and additional chemical modification? ‘viscosity optimised‘ cationic wet-end starches have been developed. A special test method has been applied to be able to monitor small but significant differences in the retention performance of these products to support this development process. As a result corn-based wet-end starches were invented that showed far higher fines, filler and self retention levels than their non-optimised predecessors.

The second chapter focuses on case studies where the newly developed products have been benchmarked against potato based products from different sources in an industrial en-vironment. These studies comprise highly filled wood-free papers as well as wood-containing papers and waste based packaging papers. In all cases the new corn-based wet-end starches performed at least as good as their potato counterparts in a variety of retention sys-tems. In most cases the formation is improved when using the corn-based products which is good for strength. Furthermore, the zeta-potential of the pulp is less affected, even if the nominal degree of substitution of the corn based product is higher than that of the potato based product. An explanation for this behaviour is that these new starches occupy less space on the fibres but stretch more into the water phase. Logically this allows more bridging between particles and also explains the improved retention. As a consequence this means that more starch can be added before the pulp is charge neutralised or that there is more room for other cationic additives.

The third chapter describes the use of these new starches as ASA emulsifying starch. After the results of the previous chapter with respect to retention and dewatering performances, it was no surprise that these products are also very useful for ASA sizing purposes. Independ-ent test results of a leading ASA producer are discussed that confirm the comparability of the results for the classical potato-based products and the new generation corn starches. Due to the high viscosity of the corn starches and different setback properties of its paste, certain precautions have to be taken for a successful application.

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This new generation of corn based cationic wet-end starches opens new opportunities for cost-efficient high performance.

Dr. Detlev Glittenberg Cerestar ACIS Cerestarstrasse 2 D-47809 Krefeld Deutschland

detlef_glittenberg@https://www.360docs.net/doc/014646847.html,

Richard J. Tippett Cerestar ACIS Cerestarstrasse 2 D-47809 Krefeld Deutschland

richard_tippett@https://www.360docs.net/doc/014646847.html,

Peter Leonhardt Cerestar ACIS Cerestarstrasse 2 D-47809 Krefeld Deutschland

peter_leonhardt@https://www.360docs.net/doc/014646847.html,

Maurice Timmermans Cerestar ACIS Lelyweg 31

NL-4612 PS Bergen op Zoom Niederlande

maurice_timmermans@https://www.360docs.net/doc/014646847.html,

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This is of course only a model of an ideal world. In reality the ‘relation’ between wet-end starch and fibre is distracted by a lot of factors which have to be counteracted through the design of the starch and addition of ‘supportive’ chemicals if necessary (Fig. 2).

Fig. 2: Influence Parameters of Starch Adsorption Kinetics

System Parameters

? Pulp Type ? Fines Content ? pH

? Anionic Trash Level

? Conductivity (Ca 2+-level) ?

Shear

Additives Parameters ? Chemistry of Trash Catchers ? Type of Retention System

Starch Parameters ? Starch Source

? Molecular weight ? Cationicity (Cat. D of S) ? Additional Chemical Modification(s)

2. Design of Wet-End Starches with improved Retention

With the planned reduction of the EU potato starch quota and the foreseeable effects on the pricing, research was performed with the objective to optimise the performance of corn-based wet-end starches in order to close the performance gap to potato starch and make possible the application of products based on this readily available raw material.

When comparing potato and corn starch, one obvious difference is the viscosity level that is achieved upon cooking both starches at identical concentration: potato starch pastes yield a significantly higher viscosity level. There are ways to increase the viscosity of starches well known to starch chemists; however, these methods have to be applied with care.

The first step to optimise the viscosity of a cationic starch was to free the cationisation re-agent from all impurities that influence the starch viscosity by unwanted cross-linking reac-tions (Fig. 3). As a result the viscosity fluctuations that we normally were faced with during production were reduced to a minimum. This stable process then allowed to optimise the re-action conditions in order to avoid any breakdown of the molecular structure during this chemical modification. This measure led to an average viscosity increase of 40%.

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Fig. 3: Quality Improvement of Cationic Wet-End Starches

After this was successfully established we went one step further to improve the retention properties of our starches by an additional chemical modification. The result of this is shown in Fig. 4 where, depending of the addition of the second reagent, the filler level in the lab sheet was able to be more than doubled.

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The chosen approach works with all kinds of base starch. This allows to optimise the reten-tion properties of e.g. cationic corn starch to close the performance gap to conventional cati-onic potato starches.

A side-effect of this double modification is that the influence on the system’s zeta potential is reduced. This means that more cationic starch or other cationic chemicals can be added to the system before it is overcharged, thus giving more freedom to the papermaker to optimise his furnish. We believe that both the improved retention and the reduced ‘blocking’ of anionic sites on the fibre are due to the effect that the rearrangement of the starch molecule onto the fibre is hindered (Fig. 6).

Fig.6: Rearrangement of Cationic Starches onto Fibres

Conventional Starch

Long Contact Time

Optimised starch Long Contact Time

2.1 Replacement of cationic potato by optimised cationic corn starch

These findings encouraged us to try to replace conventional cationic potato starches on a range of paper machines producing a variety of paper grades. In most cases we were suc-cessful in matching or even outperforming the incumbent starches. Sometimes this was only possible by applying a corn starch with a higher cationic Degree of Substitution compared to their counterparts. However, this did not impair the economics. In this context one has to take into account that, due to the natural protein content the effective D of S of a cationic corn starch is 0.006 lower than the nominal one measured by Kjeldahl Nitrogen determina-tion.

2.1.1 Case Study Newsprint

As an example we would like to present the results obtained on a newsprint machine that was producing newsprint under the conditions outlined in Fig. 7:

Fig. 7: Set-up of a trial on a newsprint machine

? Newsprint machine: 200 000 t/year ? Typical grammage 45 g/m2 ? Machine speed 1200 m/min

? 65% DIP / 35% TMP dithionite bleached ?

Starch:

? Reference Cat. Potato, D of S 0.044

? C Bond HR 05947, effective D of S 0.047 ? Addition rates: 0.5 - 0.7%

? Retention system: Cat. PAM 0.035% ?

System chemicals: ATC, Biocides

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this machine. Moreover this result was obtained at 0.5 % starch dosage already compared to 0.7% of the incumbent starch (Fig. 10).

Fig. 10: Effect on IGT-dry

P o tato 0,7%

0.5%

0.7%

This better efficiency of the new corn starch can on one hand be attributed to the better self

retention, and obviously has links to the improved formation on the other. Formation analysis

showed a small but significant advantage for the trial with 0.5 % cationic corn starch.

Whether this improved formation is due to the different character of the starches or to the re-duction of the synthetic retention agent remains a matter of speculation (Fig. 11).

Fig. 11: Formation Analysis

0,7% potato starch Formation value: 9,0

0,5% corn starch

Formation value: 9,2

0,7% corn starch

Formation value: 9,1

3. Use of new cationic corn starches for ASA sizing

3.1 Advantages of ASA

In order to create a sufficiently hydrophobic paper surface for offset printing, improve dusting and control dimensional stability, all uncoated printing and writing papers are produced with addition of a hydrophobation agent or sizing agent in the wet-end 1,2). With the era of neutral papermaking, the so-called synthetic sizing agents AKD (Alkyl Ketene Dimer) and ASA (Al-kenyl Succinic Acid Anhydride) replaced to a large extent the natural resin size / alum sizing systems.

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fig 12:

In comparison with AKD, ASA has some technical advantages like: faster curing / lower cur-ing temperature, less steep addition rate response curve, reliable sizing also in the pseudo-neutral environment 3), besides being often the more economical solution.

On the other hand ASA is more demanding in the application, due to the fact than an in-situ emulsion has to be prepared.

fig 13:

Due to the higher reactivity care has to be taken to avoid hydrolysis leading to sticky depos-its.

3.2 Comparative lab benchmarking trials

Despite the general belief that cationic potato starch is the ultimate type for ASA sizing Cer-estar has had already some good experience with corn based products.

When running the first industrial trials with our new, improved retention starches on ASA siz-ing, however, we were facing some hurdles. So we started a systematic study in order to de-fine the conditions to apply these starches for ASA emulsification. This was done together with a well known supplier of ASA.

The basic benchmarking was done in a model system (shown in fig. 14)

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fig 14:

consisting of a hard/soft wood blend filled with GCC and strengthened with 0.5 % cationic po-tato starch. ASA/starch emulsions were prepared at different ASA : starch ratios and tem-peratures. It was found that the starch temperature before emulsification has a small but sig-nificant effect on the particle size.

fig 15:

As evident from fig. 15 a lower starch temperature leads to a finer emulsion especially with respect to the amount of particles below 1 micron (56 % at 40°C vs. 48 % at 60°C).

Of course a lower temperature is also of advantage with respect of ASA hydrolysis in the wet-end. Also the effect that the speed of ASA hydrolysis increases proportionally to the par-ticle size (fig. 16) agrees with what is to be expected from thermodynamics.

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fig 16:

The sizing results of the benchmark study for different starch types are show in fig. 17.

fig 17

The ASA-starch emulsions were added to the thin model stock just before sheet forming. The sheets were dried on the dryers of the Rapid K?then sheet former and stored at 25°C / 53 % rel. humidity for 24 hours before testing. Obviously there are only insignificant differences be-tween the different starches with respect to the sizing efficiency.

3.3 Best practice of industrial ASA emulsification

Corn has proven to be a suitable raw material for this application and lab trials with the new generation of HR starches based on corn have shown some excellent results. One additional hurdle to overcome when applying HR starches deals with the settings of cooking conditions at the customer’s site.

As described in the previous chapter, the starch paste should be as cold as possible before the emulsification of ASA takes place (fig. 18).

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fig 18:

This cooling is usually performed with a relatively small and simple plate heat exchanger. The relatively low viscosity potato starch is obviously cooled down more efficiently with a given plate cooler configuration than the new generation HR corn starches that exhibit 10 – 30 times higher Brookfield viscosities (fig. 19) after jet cooking.

fig 19:

Systematic jet-cooking and plate-cooling trials led to the following results (fig. 20): With a very high paste viscosity the temperature differential between the paste entering and leaving the cooler is quite low. Furthermore it decreases very fast over time being indicative that a layer of gelled starch is formed on the cooled plate surfaces. This layer prevents a good heat transfer and in the worst case leads to a blockage of the cooler.

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fig 20:

By raising the cooking temperature from 125°C to 130°C the starch paste viscosity was dras-tically reduced. By doing so , not only the cooling efficiency—expressed by the temperature differential—could be improved, but there was also no drop in the efficiency noticed for an hour. As an acid test we then almost doubled the cooling water flow to provoke gel formation inside the cooler. As the graph shows the cooling efficiency was improved and the starch paste (??) remained stable.

Thus we became aware of the fact that the starch paste cooling is a crucial step in ASA emulsification when using highly viscous starches. The operating window (fig. 21) is smaller than expected. In order to move away from the danger zone of cooler fouling there are differ-ent options:

- use a flash vessel to get rid of steam surplus and dilute paste after that

- increase cooking temperature to reduce the paste viscosity

- use a sufficiently dimensioned and designed heat exchanger

These measures alone or in combination with each other will enhance performance of HR cornstarches in the application of ASA emulsification. It will allow the papermaker to use these new wet-end starches and experience the positive effects they generate on retention and strength also for ASA sizing.

fig 21:

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4. Conclusions:

The new generation of HR corn starches for wet end applications has proven to be a viable alternative to potato-based products. - A new modification chemistry has boosted the retention properties - A lower effect on Zeta-potential allows higher addition rates and thus strength effects - Often, formation is improved, helping strength development - Synthetic retention additives can be reduced due to increased synergies - ASA emulsification, if performed well, leads to fully satisfying degrees of sizing.

If you believe facts more than myths, please join the club and try it.

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20 Bibliography:

1 GLIESE,

ASA als Leimungsmittel

Internationale Papierwirtschaft, 2003; 9; S. 42 – 46

2. SCHULTZ,

W.S.

Leimungsmittel am Beispiel Feinpapier

Das Papier 51, V110-113 (1997), Nr. 6A

3. BAEHR,

Leimung mit System – ASA

Wochenblatt für Papierfabrikation 129, 1112-1116 (2001), Nr. 17

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