Introduction
The Production Manager of PM 1 parks his hybrid car, nods to the lady guard at the gate and dons his helmet with goggles before inspecting his machine. Yes, this is his paper machine, his domain. After all, he is the one responsible for fulfilling the 500.000 metric ton production target annually. He pauses to glance at the film press with a satisfied smile – it is running smoothly and very, very fast; a bit below to 2000 m/min. There is something reassuring about the hum of the newly installed steel of the edge of liner technology. However, the very basic principle of this technology is not that new. Were we to take a jump across time and space we would find a similar expert taking pride in his work, size pressing hand sheets 600yrs ago in Germany, 2000yrs ago in China or 4000years ago in Egypt.
The use of starch worldwide in ancient days, today and, very likely, also tomorrow by papermakers must have a well-founded reason. These experts in their fields know what level of performance has been sought from their products. Or being more specific: what properties are non-wanted in the final paper or board grade in question.
– Physical strength measured by various means and devices and presented in more or less descriptive parameters have been a central issue all through papermaking history.
How come then? Why would this be so important? From a consumer point of view, maybe from the likes of me or you in our daily living, it is important that the product displays strong enough fiber-to-fiber bonding. From the studies and peeks at sub-fiber level we can tell that we are dealing with hydrogen bonds. And being more precise the number per physical contact area. This translates to the smart phone or HD-Led TV you ordered last week arriving safe and sound in your living room because the packaging box did withstand the physical stresses during the transport without failing. Another example would be toilet paper that should last in its ”wet-end application” for a specific time before ending up as separate fibers in the sewer piping as not to block them. Today failure to meet the designed end-consumer criteria usually means extra problems and costs of some sort.
All through time the papermaker has drained the fiber suspension to make a continuous wet web to be pressed and finally dried into a 2D paper structure. To achieve a sufficient level of bonding between the fibers, i.e. to increase strength, various means can be applied. The circular, oblong fibers are normally beaten to become somewhat flatter as well as ‘hairy’ while releasing fines. To boost the hydrogen bonding levels we may add some 5-15 kg well-cooked wet-end starch per ton fiber (dry/dry) to the papermaking furnish. This wet-end starch – aka internal sizing – works as a “bio glue” between the cellulosic fibers providing enhanced physical strength. Another approach is adding a continuous thin layer of starch onto the solid paper web to provide improved surface strength, aka surface-sizing.
Why could the production manager simply not refine his fibers a bit more in his conical refiners then?
At some stage towards the 0.5 Mt goal reality would catch up to him and it would become clear that these devices are quite costly to engage in the long run: with inorganic fillers in the paper furnish these get worn down requiring new fillings every now and then and on top of that a fair amount of electricity. Both of which equal €s or $s lost. No, the mechanical modification route alone towards paper strength won’t work for long in a mill this size. The paper chemistry route towards the same goal needs to be involved. The reason for starch being a central component in papermaking resides in its global availability, technological simplicity and favorable economy. Starch and cellulose are the two most abundant biopolymers and can be found in many different forms. Wood fibers are released, separated and cleaned to be rejoined but to a lesser degree and in shape of a continuous mat or paper web. Starch granules again are freed and enriched in a similar way, however requiring that their compact, spherical microstructure is expanded on molecular level by heating to provide a viscous liquid. Both processes utilize water as medium. The natural compatibility between the two biopolymers are employed when the papermaker add starch into the wet pulp or onto the paper web.
The natural synergy arising between starch and cellulose are partly due to them being close relatives, from a molecular point of view. Both are strong hydrogen bonders and can comply/complement each other herein. Stronger paper arises from improvements in inter-fiber bonding and so-called fiber segment activations. Starch addition alone has been shown to provide 30% strength enhancement in model systems (Vainio 2007). It has been indicated that starch adsorbed onto fibers behave like a thin gelatinous layer adsorbing water enhancing the inter-fiber bonding levels (Sand et al 2017). Moreover, it was proven that the starch adsorption onto pulp fibers takes place within seconds. In accordance with Nature’s laws, this adsorption can be enhanced by combining cationic and anionic molecular species. The papermakers have known this for a long time – that is the reason behind the high demand for Chemigate’s cationic starch grades. As the paper web is dried, every established fiber-starch-fiber hydrogen bond may contribute to the overall web force carrying capacity. The many small rivers combine into a large flood. Here the integrated sum reads strength improvement – what every paper- or -boardmaker is seeking for. That’s the main reasons why starch is denoted the mother of additives for papermakers.
We need stronger fiber-fiber bonding. This can be made by applying cationic starches. This universal green bonding agent works both for providing physical, interfacial and chemical strength. We in the business talk about internal bonding, surface sizing and printing ink migration dynamics.
Master Chen pours out the rest of the cooked starch and walks over to inspect his work for the day. He runs his hand over the high quality paper and nods contently. The monks over at the local monastery will surely be pleased. What makes him so pleased is that in addition to making the paper durable but not brittle, today he and his apprentice have managed to produce almost a whole cart load. A fruitful working day, Master Chen thinks when he walks over to join his dear ones at the home dinner table.
[Tom Lundin D.Sc. (Tech.) Senior Expert (Sales and R&D)]
References
Starch: Chemistry and Technology Eds.: BeMiller J. and Whistler R. 2009 ISBN: 978-0-12-746275-2
Vainio A. (2007) Interfibre bonding and fibre segment activation in paper – observations on the phenomena and their influence on paper strength properties, PhD
Lamberg, H. (2016) Tärkkelys ja tärkkelysteollisuus, MSc-work
Sand, A. et al. (2017) The effect of chemical additives on the strength, stiffness and elongation potential of paper, NPPRJ (32) 3