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November 2008

By Norbert Schall, Ellen Krüger, Rainer Blum, Martin Rübenacker (BASF)

European manufacturers of packaging papers normally apply surface starch or, in isolated cases, internal starch, in compensation for a lack of dry strength. In Germany, annual consumption of surface starch for corrugating stock is at approx. 150,000 t. However, since internal starch additions to kraft liner substitute and test liner 1 qualities are limited to a maximum of approx. 1.5 % per tonne of paper, this approach frequently fails to meet the specified strength levels.

In recent years, it has become ever more difficult for papermakers to achieve the strength specifications of corrugating stock and especially so in the case of premium qualities – despite the use of high-grade raw materials and surface starch. This trend is attributable to the inferior quality of recovered papers in Europe and abroad.

Since the 1980s, the share of Kraft liner products in the raw material for recycled furnishes has drastically decreased whilst filler contents of corrugating stock underwent a steady rise during that same time period. Moreover, the number of fibre recycles has multiplied over the past years.

A viable alternative to surface starch are cationic polymers on a polyacrylamide and polyvinylamine (PVAm) basis. They are applied internally to give very high strength levels. When performing additional functions in a PM circuit, PVAm favourably impacts not only the fixation of disturbing substances but also dewatering and retention. The PVAm polymer was first successfully applied as a fixative for internal starch.

Dry strength is optimally enhanced by adding a combination of PVAm and an anionic copolymer on a polyvinylformamide basis. The strength-enhancing effects of this combination coincide with the strengths attained in a one-sided conventional size press. Combined application of the two chemicals offers the additional benefits of improved dewatering in the wire and press sections, reduced steam consumption levels and higher PM speeds.

Over decades it had been assumed that dry strength was achieved via hydrogen bonding between single fibres. Meanwhile SEM microscopy has demonstrated that this theory is presumably wrong (Linhart et al.). Instead, dry strength seems to result from fibril entanglement of two fibres, which are thus bonded to each other.

These SEM pictures illustrate the improbability of the hydrogen bond theory. It lacks plausibility in view of the fact that a single fibril is approx. 1000x larger in diameter than is the maximum bridgeable length of a hydrogen bond. Hydrogen bonds are below the visibility limit of even SEM microscopes. Considering the distance between fibrils, there arises the question of whether the number of hydrogen bonds that can possibly be set up in the process of dry strength evolution is sufficient to essentially impact paper strength (Fig. 1).

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Figure 1. SEM pictures of a paper surface showing a fibre network with fibre crossings. The photograph on the right shows the marked zone 200x magnified with clearly visible fibrils at the fibre-fibre interfaces.

Today, technology advocates the theory that the bond between two fibres is fortified as the fibrils entangle while the dry strengthening effect comes on. If the dry fibrils are elastic, they tend to bend and open so that they are more easily drawn apart under force effects – in other words, the stiffness of the hooks substantially contributes toward dry strength.

Having excellent filming properties, all known dry strength agents form strong films when drying. Synthetic polymers typically produce particularly strong and tough films, whereas starch films are strong but at the same time very brittle. This might explain the superior performance of synthetic polymers as against starch – solid against solid.

Within the fibre batt, the retained molecules of dry strength agents are prevented from forming a continuous film because the amount of polymer present is insufficient for the available surface. However, if the molecules attach to fibres and to fibrils in particular, the stiffness and strength of their films adds to those of the fibres and fibrils. The introduction of dry strength agents fosters the entanglement process and thus ultimately boosts dry strength.

Another finding supports this hypothesis: dry strength increases at higher refining energy, which creates a larger number of fibrils. Accordingly, more fibrils will entangle in the fibre batt with the result of added dry strength. This theory is underlined by Brecht's remarks on the strength increase achieved by refining: "The combined effects of swelling and compression make the fibres more pliable and more prone to entangle by fibrillation … (Fig. 2)"

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Figure 2. Strength agents for packaging board grades.

In corrugating stock production, a variety of dry strength agents are applied. The dominant chemical is starch, which is used both internally and as surface starch. Application is primarily in size presses. In corrugating stock production in Germany, over 150,000 t/a enzymatically, thermochemically or oxidatively degraded native starch find application as surface starch.

To meet the specified dry strengths, papermakers switch between the one-side and two-side mode of the size press. Internal sizing with starch plays a minor role with less than 10 % compared to the amount of size press starch. This is explained by the fact that the sizing effect of internal starch diminishes as water circuits are more highly closed. Basic disturbing influences in this context are the high levels of salt and interfering substances that are typically experienced in closed loop systems.

Synthetic dry strength agents by contrast perform excellently also in severely contaminated systems on account of their higher cationic charge and lower molecular mass. Specifically, a differentiation is made between polyacrylamides, glyoxilated polyacrylamides and polyvinylamines (Fig. 3).

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Figure 3. Increase in recycled pulp strength achieved by 3 synthetic dry strength agents. Being a measure for strength enhancement, the SCT factor (= SCT value divided by basis weight) indicates the performance of the various polymers. 1% additions of polyacrylamide and glyoxilated polyacrylamide produce a nearly linear rise of SCT and so does glyoxilated PAM, but on a slightly higher level. Increasing dosages of polyvinylamine by contrast cause strength values to rise more steeply up to a maximum that cannot, however, be exceeded if additions are further increased.

To characterize the performance of PVAm in recycled pulps, two different methods were applied. The first study looked into the adsorption behaviour of PVAm on recycled fibre. The polymer was added to the fibres under ideal conditions using de-ionized water. After centrifuging, the residual polymer volume was determined in the aqueous phase.

The studies showed a 100% adsorption of the polymer up to a dosage of 0.5%. Higher dosages gave rise to a saturation of the fibres so that adsorption decreased. The curve derived from zeta potential measurements was virtually identical to the adsorption curve (Fig. 4).

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Figure 4. Mechanism of polyvinylamine applied as a single component.

It may safely be assumed that polymer adsorption is primarily charge-driven. This theory has been substantiated by adsorption tests of polymers carrying different cationic charges.

In actual mill practice, the degree of polymer adsorption is additionally subject to influences of deleterious matter. Depending on the PM circuit loading, dosages of highly cationic polyvinylamines are restricted in amount. In the case of highly anionically loaded systems, highly cationic PVAm reduces the levels of anionic trash so that a strength increase will be achieved (Fig. 5).

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Figure 5. For corrugating stock, the effects of cationic PVAm are reflected by the CMT variable. To characterize the PM circuit, the zeta potential and the cationic demand were measured during the individual test stages. The zeta potential remained comparatively constant for all dosage amounts whereas the cationic demand declined as polymer additions were increased. Shortly before the isoelectric point is reached, polymer adsorption becomes incomplete so that part of the polymer resists fixation on the fibres – a phenomenon that may lead to increased foaming.

Is it possible to improve the adsorption of a cationic polymer by adding an oppositely charged anionic polymer on a PVAm basis? To clarify this question, 0.5% cationic PVAm was introduced first before an anionic PVAm was added. Up to a polymer dosage of 1.2%, polymer adsorption was of a quantitative nature, which means that 0.7% anionic in addition to 0.5% cationic dry strength agent had adsorbed at the fibres. In other words, the anionic component, too, contributes toward a strength increase. Rising dosages of the anionic polymer are accompanied by a drop of the zeta potential into the negative range (Fig. 6).

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Figure 6. Addition of oppositely charged anionic PVAm improves polymer adsorption onto the fibres.

This dual system relies on a purely ionic mechanism. Since retention of an anionic dry strength agent is only possible in the presence of a cationic carrier, the dual system with synthetic polymers on a PVAm basis is called "carrier system".

In the BASF Technical Centre, test liner/corrugating stock was produced from 100% recycled fibre. To achieve a strength increase, the carrier system was introduced internally. The dosages of up to 0.8% solid polymer were made up of 50% cationic and 50% anionic polymer. In mill practice, dosages are at 30 – 70% of the maximum additions applied in the pilot tests, depending on the basis weight of the product.

The relevant strength levels for corrugating stock, i.e. burst, SCT, CMT, were found to be enhanced by 25 – 30% (Fig. 7).

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Figure 7. During a pilot test conducted in the BASF Technical Centre Paper, the carrier system achieved a 25 – 30% strength increase for recycled fibre based corrugating stock.

In a second trial, internal dosage of the carrier system was compared with size press application of oxidatively degraded starch. Low additions of the dual system actually boosted the strength level to a mark attained by surface starch only with substantially higher dosages. With the brittle starch product, the fibril entanglement tends to be broken apart at an early stage – a phenomenon that can only be controlled by increased additions.

The pilot tests revealed that the carrier system achieved a strength increase identical to that of a one-sided size press. This is due to the fact that, in the case of high carrier system additions and subsequent treatment with size press starch, the strength increase adds up from both the carrier system and the size press. The carrier system when combined with 2% surface size applied in a one-sided size press raises the SCT value to an identical level to that achieved with 4% starch applied in a two-sided size press without carrier system (Fig. 8).

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Figure 8. Starch has to be applied in substantially higher dosages if an identical strength level to that of the carrier system is to be obtained.

The additional cost of the carrier system is offset by savings of other process chemicals, such as retention aids or size press starch – and above all by the potential for a production increase. This finding has been confirmed for corrugated stock in a mill environment. As against a standard chemical system, the carrier system with 0.4% total polymer increases the CMT (0) by approx. 20%.

This strength gain enables one-side operation of the size press – an approach that causes the CMT to drop to a mark still above the initial level. The specific steam consumption is reduced by some 20 – 25%. Together with the significantly improved pulp dewatering, this allows a 15% PM speed increase as becomes evident from the vPM index and from the absolute machine speed. It is true, the strength levels undergo a further decrease when the PM speed is increased, but the target values are met easily and reliably.

The extra chemical cost incurred for the carrier system is more than compensated for by a higher output (Fig. 9).

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Figure 9. The carrier system enables higher PM speeds without jeopardizing paper strength.

It goes without saying that there exist paper machines where cationic internal starch is used for premium-quality papers. However, raw material variation and circuit fluctuations may give rise to quality inconsistencies. In the pilot trials, the target values were no longer met in spite of size press application plus internal sizing. An analysis revealed raised starch concentrations in the whitewater.

After introduction of cationic PVAm, the starch content in the whitewater was reduced by 30%. The ultimate strength increase results from the added effects of improved starch retention plus a strength increase caused by PVAm; target values were, however, reliably achieved. In actual practice, 5 – 10% PVAm in relation to internal size is needed to improve starch retention (Fig. 10).

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Figure 10. Cationic PVAm improves starch efficiency with premium quality papers.

Meeting the strength specifications for recycled fibre based corrugating stock has become a major challenge to papermakers and particularly so for high-grade qualities. Polyvinylamines are synthetic dry strength agents with a very broad performance spectrum – a prerequisite for customer specifications being safely met. When applied in combination with internal starch, they enhance both starch retention and dry strength.

If, however, internal starch totally loses performance in the presence of high salt levels, PVAm may be applied as a single component to compensate for the lack of strength, thus enabling a 2 – 3% productivity increase. The carrier system achieves the highest increase in strength – on condition that production conditions enable one-sided operation of the size press. This will provide for production increases in the range of 14 – 18% (Fig. 11).

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Figure 11. Polyvinylamine – a dry strength agent with a broad performance spectrum

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