CaCO3 Pigments: Challenge for repulpable adhesive solutions in the paper industry

1. Calcium carbonate: Success formula in the paper industry

Calcium carbonate (CaCO3) is experiencing a genuine boom in the paper industry as a coating pigment and filler in papers. Paper producers use CaCO3 in two ways in order to gain ground in the marketplace: The whiteness level and opacity of the paper can be adjusted to meet customer requirements and technical printing properties such as smoothness and printing ink acceptance can also be influenced positively. Using CaCO3 as a filler is attractive because it allows the raw material costs of even uncoated papers to be reduced. Increased use of CaCO3 in paper manufacturing thus offers a formula for success that enables improved quality and competitiveness.

The numbers for Europe alone make this clear. According to the trade association CEPI, the use of CaCO3 as a filler and coating pigment within its member countries more than doubled between 1991 and 2010.

A precondition for success in the use of CaCO3 in paper manufacturing is a production process that is also continuously adjusted to high proportions of the pigment. From pulp to delivery to the customers: Every manufacturing stage in paper machines, calenders and roll cutting must satisfy consistent high quality and safety standards. Production steps in which adhesives or adhesive tape are used require particular attention, because they come directly into contact with the paper surface. And this means direct contact with potentially large mass percentages of CaCO3.

The more calcium carbonate is used, the more important it becomes to use adhesive solutions that are specifically adapted to the use of such pigments. If normal repulpable adhesive tapes are used, there is no guarantee that the adhesion will hold permanently. This can be problematic in permanent splices and roll-end adhesion that must remain loadable over long periods. For them, as with all other taping on high content paper during operations the following must also apply: Repulpability must be ensured at all times. But repulpable standard products can cause problems in precisely this area.

2. Repulpability of adhesive tapes: A basic prerequisite

All production waste that is internally produced during paper manufacturing must be completely repulpable, so that it can be returned to pulp as a valuable raw material. Repulpability of all raw and auxiliary materials is therefore standard in today’s paper production.

Leftover waste paper or residue from inferior, non-repulpable adhesives become a real problem here. They clump together, contaminate the fiber mix in the form of troublesome stickies and must be expensively filtered out. The risk that stickies remain in the pulp is high. Ultimately, this can become expensive: Even the smallest impurities block the screens and, in the worst case, the paper web tears – the machine is blocked. Costs are generated not only by the loss of production time but also because the machine must be cleaned. This can involve considerable expense, particularly for the wire section.

Complete repulpability is therefore a basic prerequisite for high quality adhesive tape products intended for use in the paper manufacturing industry. In addition, the manufacturer's challenge lies in appropriately balancing the three principal features of adhesive properties.

This can be illustrated by the example of a pressure-sensitive tape for permanent splices: high initial tack is very important for workability, which often takes place under time pressure. Strong adhesion is important for bonding to the paper surface. The adhesive cohesion determines the shear resistance of the splice.

Optimization of these three properties in the so-called "magic triangle" (Fig. 2) requires adhesive development with sensitivity and experience, because enhancement of one property always takes place at the expense of the other two. This is why adhesive tape manufacturers such as tesa offer an entire product range of well-balanced adhesive tape products for the paper industry, each of which is optimally configured for its particular task. One possibility in this regard is the precise setting of the adhesive cross-linking level. However, with increasing use of calcium carbonate, the developers must integrate additional know-how into their adhesive products. Otherwise, the magic triangle can easily get out of balance.

3. Adhesive properties: Repulpable adhesives are vulnerable

No technically-relevant, repulpable adhesive is immune to problems caused by CaCO3.

This is due to the particular chemistry that enables repulpability. The proven state of the art is water-soluble, pressure-sensitive adhesives using long chain polymer molecules on an acrylate base with high acrylic acid content. The acrylic acid content is important for repulpability and as a reactant in the customized cross-linking of the adhesive.

As a result, repulpable pressure-sensitive adhesives consist of long molecule chains that each carry numerous acid functions. These acid functions have a double benefit: Their large number – on average some 1000 functional acid groups per polymer molecule – are what make the adhesive repulpable.

Since excessive cohesion in the magic triangle inevitably reduces adhesive strength, the adhesive loses its original functionality: It is no longer pressure-sensitive.

Inconveniently, this can also happen during use of the finished, correctly-balanced adhesive tape – through external influences. The same CaCO3 that is so advantageous in many other respects undermines the repulpable adhesive.

4. Risk due to pigments: Calcium ions weaken adhesive strength

Investigations in the Hamburg tesa laboratory confirm that pigments can cause the same damage as an overdose of cross-linkers. To test this, repulpable adhesives were spread on paper containing CaCO3 and changes in the properties of the adhesive were tracked over time. In addition, the distribution of chemical elements by type and quantity in a similar sample with altered adhesive properties was measured.

The interpretation is clear: Over time, Ca2+ ions that have freed themselves from the calcium carbonate in the coating or raw paper, migrate into the adhesive (Fig. 4). There they meet the wide-meshed network of polymer molecules with their numerous free functional acid groups. The calcium ions now become cross-linkers – precisely like the small amounts of metal ions that are intentionally used in the production of the adhesive to set the cohesion.

But this time the process is not controlled. Fig. 3 shows a snapshot of the process. The negative Ca2+ ion gradient, i.e. the declining concentration in the adhesive with distance from the paper surface, means that the migration of ions is continuing because the concentrations of calcium ions in the paper and adhesive are not yet in equilibrium. Since there is a large surplus of Ca2+ ions in the paper, they migrate practically continuously into the adhesive. They find a number of free acid groups there, and cross-link the polymer molecules far beyond a healthy level.

The magic triangle becomes unbalanced and cohesion excessively increases, while initial tack and particularly adhesive strength significantly decline. Naturally, the concentration of Ca2+ ions increases most rapidly at the adhesive boundary surface to the paper (Fig. 4). Just where the adhesive should adhere to the substrate, adhesion capability is reduced most substantially.

The increasing cohesion can lead to complete loss of adhesive properties and also greatly reduce repulpability.

5. Damage analysis in the laboratory: Varnishing is a matter of time

In order to find out how quickly the varnishing effect occurs, papers with different CaCO3 content were coated with repulpable adhesive. Critical storage conditions were selected which greatly accelerated the slow process that exists in reality. This allowed observation of how the adhesive properties changed at short time intervals. Fig. 5 shows an example of the progress of adhesive strength.

The Ca2+ effect is most clearly seen in coatings with 100% calcium carbonate content in the pigment portion of the coating: After three days, the adhesive strength has already sunk below one tenth. For 50% CaCO3, it already amounts to one quarter of the original adhesive strength after the same time period. And on the 10% CaCO3 coatings as well, the adhesives have still lost one-seventh of their adhesive strength after three days and after a full two weeks it has sunk to one half (Fig.5, dashed curves).

Such high speeds are only realistic in hot and damp areas such as Asian production locations, where climatic conditions match those in the laboratory. In more temperate climate zones the damaging chemical processes occur more slowly – but nevertheless continuously. There is thus always migration of Ca2+ ions into the adhesives. And thereby a creeping loss of loading capability and adhesion reliability at minimum.

Even with the most careful process planning, the results can be economically significant. By the time an apparently correctly taped paper roll is printed, the permanent splices may already be many days old. Even if the adhesive film is not completely varnished yet, it may, nevertheless, have already become fragile. If it is stressed while running over deflection rollers in the machine, it can come loose or even come off. With the high web speeds that have become normal in the printing industry, the danger that the paper web will tear is considerable. That stops the entire print run. The printer loses capacity, time and thus money. The paper supplier has to reckon with a complaint in cases of doubt.

6. The correct formula: The CaCO3 problem can be solved

That repulpability and adhesive strength of adhesive tapes can be retained despite high CaCO3 content in the paper to be taped is another result from tesa research at its Hamburg headquarters. Through special modification of the adhesive, the chemical cross-linking effect of Ca2 ions can be slowed down so effectively that varnishing of the adhesive within the relevant time frame essentially no longer represents a problem (Fig. 5, solid curves).

While the adhesive strength of normal adhesives on papers with 50% CaCO3 content in the pigment portion of the coating sinks below one quarter of the original value within three days under laboratory conditions, the value for adhesives with the new modifications remain at 98.2% after the same period. Even one month later, adhesive strength has declined by less than 10%. Together with adhesion, the other properties in the magic triangle remain practically optimal. The adhesive in the adhesive tapes remains durably elastic, strong and loadable. Gelling can no longer significantly contribute to damage.

Meanwhile, after completion of comprehensive tests, these modified adhesives could be taken from the research stage to market readiness. The current products for roll taping and permanent splices are available in the tesa range as CaCO3-insensitive further developments. The compatibility of modern production processes in the paper industry with adhesive solutions for smooth operation remains the enduring, primary concern of tesa research.

In this instance a new technological solution has been achieved and the “magic triangle” has been preserved.