Current chemistry takes over

Chemistry is an integral part of every papermaker's operations, whether they like it, or not. While some companies prefer to pass on the responsibility of papermaking chemicals to someone else, others are keen to invest in research that will benefit the production process in the long run

Eight years ago, there was a PPI article on papermaking and its raw materials looked at from a chemical perspective, entitled "Papermaking chemistry: exact science or black art?" In the introduction, it was noted that the type of chemistry employed at a paper mill is not the same subject that sometimes grabs the imagination at school - frothing test-tubes, big bangs or colorful crystals. Paper mill chemistry is more to do with subtle interactions between rather unexciting white or brown particles (fibers) and invisible (water-soluble) polymers. These interactions are so subtle that it may be easy to miss where chemistry comes into the process at all.

Some mills recognize its significance and invest a lot time and money in having well-qualified, well-trained chemists (not engineers sent on a week-long course), while others cannot wait to off-load chemical management to someone else. Of course, outsourcing functions like energy supply has been a popular topic in recent years, but chemicals directly affect the paper that is being made.

Coming back to the title of the 1994 article, is papermaking chemistry a black art or has it become a bit more scientific? It is certainly not an exact science, but it is apparent that universities, institutes, suppliers and occasionally papermakers continue to carry out some excellent fundamental research. Applying the research can still be a struggle against the conservatism of the industry, though. This attitude is understandable given the enormous capital investment required to build a state-of-the art paper machine. But the colossal quantities of machine broke are often, although certainly not always, a testament to the industry's chemical uncertainties.

Looking at the main elements of the production process, water and fibers are the two constants on all paper machines. While the quality of the fresh water could be said to form the baseline chemistry of the papermaking system, the myriad of substances in pulps (including mill broke) play the most important role in the overall chemical picture. Just as fresh water is not only H₂O, pulps are not only cellulose. On top of that, most of these non-cellulosic substances have the ability to cause problems of one sort or another. Apart from the substances that cause pitch or sticky deposits, most of these problems (slime, foam, interferences) arise from substances that dissolve in water. Quite a bit of research has been carried out over the years on the factors that influence dissolution, but most of this was on mechanical pulps due to their high content of non-cellulosic materials. However, some recent research by the Swedish Pulp and Paper Research Institute (STFI) as part of a European Union (EU) funded project has shed new light on dissolution from both elemental chlorine free (ECF) and totally chlorine free (TCF) bleached kraft pulps.

For these pulps, there is not much left to dissolve other than hemi-cellulose carbohydrates, but the research has indicated big differences in the physical nature of the material between softwood and hardwood. Whereas most of the carbohydrate from hardwood is of relatively high molecular weight (>20,000 Daltons), in softwood it is composed of lower molecular weight materials (10,000-20,000 Daltons). The actual amount dissolved depends on several variables, including the extent of mechanical action (slushing time and energy input during refining) and the concentration of electrolytes, some of which may also originate from the pulp. Under all conditions, the highest levels (up to about 7 kg/tonne of pulp) came from hardwood. One of the important pulp variables influencing the dissolution of carbohydrates is the total content of anionically-charged substances (a subset of the total carbohydrates). The higher the value is, the greater is the dissolution of carbohydrate, some of which is itself anionic (Figure 1). The dissolution of these anionic hemi-celluloses (such as the glucuronoxylans) also increases substantially on refining, particularly for eucalyptus pulp.

The dissolution of anionic substances from pulp is undesirable on two counts. Firstly, it contributes to the anionic trash fraction. Secondly, it removes of material from the fiber surface that helps to attract and anchor cationic additives such as cationic starch and wet strength resins.

In some cases, it has been found necessary to add extra anionic material to enhance the adsorption of cationic additives, the best example being the addition of carboxymethylcellulose (CMC) before polyamide-based wet strength resins. As the name suggests, CMC is a derivative of cellulose in which the molecule has been given a negative charge at various points along the polymer chain and this also makes it water-soluble.

The TrumpJet dosing concept is installed on a large SC machine in Finland

Some recent work, again at STFI, has used this chemical to try to boost the anionic charge on the fiber surface, albeit at conditions not usually found at the wet end, for example, a high temperature of 80°C and high electrolyte levels. Provided that the chain length of the CMC is long enough, the polymer only adsorbs on the external fiber surfaces as it is too big to penetrate the fiber's fine pore structure. From work elsewhere, it is known that peroxide-bleached TCF pulps retain more anionic substances than ECF or other types of TCF pulp. As tissue producers, for example, tend not to refine their pulps to preserve bulk and softness, the anionic substances should remain on the fiber surface and boost the retention of cationic wet and dry strength chemicals.

Of course, the big story on retention aids over the last 20 years has been micro-particle systems using a cationic polymer such as starch or polyacrylamide with an anionic inorganic solid phase, for example silica or bentonite. The other essential component in all these systems is to make use of the natural variations in process hydrodynamics to apply some shear to the papermaking stock at just the right time. The dimensions of the small particles in these systems can be just a few to a few hundred nanometers (billionth of a meter), so nano-particles would be a better name for them. A few years ago, these systems were joined by an organic micro-particle in the form of a branched polyacrylamide, which was developed by Cytec. Ciba now markets the product under the Polyflex banner. A recent paper reported that about 5 million tonnes/yr of paper and board are made with Polyflex. The polymer is made with the same two chemicals that are used to produce conventional anionic polyacrylamides including acrylic acid and acrylamide, plus methylenbisacrylamide as a cross-linking agent. The micro-emulsion that is formed works best when the droplet size is in the sub-micron range. In contrast with the rigid structure of the inorganic micro-particles, the flexibility of the polymer is believed to be an important factor in its ability to knit together the cationized fiber and filler surfaces to lift retention and drainage.

A new retention chemistry that is not yet in commercial use is based on dendrimer or cloudburst molecules that extend outward from a central hub. The commercial materials are manufactured by DSM, a Dutch chemical company, under the Astromal name. However, they differ from the above micro-particles in being strongly cationic and also being much smaller in molecular size even than colloidal silica. Investigations by Paprican, the Canadian pulp and paper research institute, on a difficult mechanical pulp furnish show that they can achieve superior or at least comparable performance to normal retention polymers at similar doses.

Focusing on retention chemicals, polyethyleneoxide (PEO) is unique among working retention aids in being nonionic (uncharged). One of the interesting questions about PEO is just how it works? Its potential use as a retention aid came to light in the 1950s, but serious development work only started around 20 years ago. Research focused on finding a retention chemical that worked with mechanical pulps, as normal cationic polymers had little effect at economic doses due to the high level of interfering substances. Early on, it was discovered that PEO worked with some pulps, but not with others where it required the addition of a second component. Research to find the best second component has trawled a range of chemistries, but has centered on modified lignin compounds and various resins, the most common of which are based on phenol-formaldehyde. In the future, PEO systems may be used more as one of their characteristics seems to be a relatively high tolerance of anionic interfering substances.

Before moving on from retention chemicals, a new development on the application side merits a few words. Conventional polymers require a small amount of water for preparing the concentrated solution, but much larger quantities of water for pre-dilution just ahead of dosing into the stock to give uniform mixing. Normally, this uses fresh water. But the practice has a negative effect on machine closure and overall retentions. As mills increasingly close machine loops, the water used for chemical dilution accounts for a larger proportion of total fresh water use. Wetend Technologies in Finland has developed a new dosing concept called TrumpJet, which gets around the problem by allowing the use of recycled whitewater or even thin stock. The system uses a specially designed injector nozzle. One to four nozzles may be used, depending on the pipe diameter. The benefits of TrumpJet are said to include a 10-15% decrease in retention aid consumption, an 8% increase in paper machine speed, a 25-40% decrease in consumption of sizing agent and 15-25% higher wire retention level of fines and filler.

Turning to the development of new methods to control microbiological problems in papermaking, this has been a constant issue over the years. A continuing trend has been the move away from toxic biocides to more subtle ways of stopping bugs forming their small communities better known as slime. Researchers' current interest is mainly in surfactant-based biofilm inhibitors. The majority of microbiological problems stem from this type of surface-attached activity, particularly from the anaerobic slime layer close to the supporting surface. Problems can include accelerated corrosion from sulfate reduction, safety problems from some gases, notably hydrogen and hydrogen sulfide, and generation of odors - organics acids and H2S.

However, microbiological activity from non-attached micro-organisms will continue to degrade the functionality of biodegradable raw materials and their conversion into unwanted by-products.

As bacteria are individually in the colloidal size range (but as big as or even bigger than fibers when they are clumped together), they are inevitably retained in the product along with fiber fines and fillers through the action of normal retention aids. In fact, recent work in Portugal has shown that some biocides, in this case dithiocarbamates, can give the bacteria a positive charge, promoting their deposition on negatively-charged fibers. Although the contamination of the finished paper or board with micro-organisms is minimized by the disinfecting action of the high sheet temperatures achieved during drying, some manufacturers of food-contact products have recently begun to use a technique well-known in the food industry to further minimize any consumer risks. This technique introduces yet another acronym to add to the papermaking lexicon - Hazard Analysis and Critical Control Points or HACCP (pronounced hassip). As the name suggests, it provides a structured framework for identifying where hazards are most likely to arise in any manufacturing process and then for managing the process to minimize these hazards and associated risks.

Although Pira in the UK did a lot of work in the 1970s on the microbiological quality of papermaking raw materials and products, not much appears to have been done recently. However, an SCA packaging mill in Sweden has reported some research on linerboard. The results showed relatively low levels of aerobic bacteria and fungi in samples taken early in the year, but some much higher levels later in the year (Figure 2). The difference was attributed to microbial growth within the recovered paper fraction, which was present at varying levels in all the samples. One of the concerns in food packaging is the possibility of increased microbial hazards in papers containing recycled fiber. The study confirmed that there was a significant correlation between recycled content and microbial levels in packaging. As expected, the vast majority of the bacteria in the products were spore-formers, notably Bacillus cereus, which can withstand high temperatures. However, research and more detailed studies on this aspect by the German association for manufacturers of, among other things, paper auxiliaries (TEGEWA) showed that there was no significant transfer of microbes from paper or board products to dry, moist or greasy food.

An interesting new research project on improved microbiological control in papermaking is now approaching its halfway point. The EU-funded Biotech Control project has been looking at the application of new techniques to monitor slime build-up and to characterize slime-forming bacteria, which will then be used to assess various slime control techniques. The new microbiological monitor uses a quartz crystal microbalance (QCM) from the Swedish company, Q-Sense. The microbalance consists of a thin quartz disc sandwiched between a pair of electrodes. The quartz crystal can be made to oscillate by applying an AC voltage between its electrodes. When a thin film is attached to the sensor crystal, the frequency decreases. If the film is thin and rigid, the decrease in frequency is proportional to the mass of the film. Laboratory testing has shown that the system is very sensitive to the early stages of biofilm attachment, when visual assessment is often inconclusive. Housing for an online QCM sensor is being developed, which would allow real-time assessment of slime-forming ability on the paper machine.

In the last year or so, a radically new method of slime control has begun to be implemented. The process involves the application of an electrical potential through the placement of an electrode or array of electrodes within a metal pipe or tank. The approach has been investigated independently by two companies - US-based Zeta and Savcor Process in Finland - although the two concepts are quite different.

The process from Zeta is described as an electrostatic approach in which a capacitor is set up from an inserted electrode or ceramic dielectric, the Zeta Rod, and the grounded body of the pipe or tank. One mechanism at work here is for the high applied voltage (30-35 kV DC) to increase the surface charge on particles, making them less likely to aggregate together. A Canadian mill has used the system on two paper machines to stop slime growth within a press shower system using inline filtration. Continued running with a 50% reduction in biocide addition has yielded an 80-85% drop in plugging of shower orifices combined with a payback time of two months.

The Finnish approach is different, not least in that the applied voltage is much lower and that it works by changing the surface conditions at the metal exterior. Depending on the chemistry of the water surrounding the metal surface (M), a number of reactions may occur:

• Anode reactions:
  M => Mⁿ+ + ne^-
  2H₂O => O₂ + 4H⁺ + 4e^-
• Cathode reactions:
  O₂ + 4H⁺ + 4e^- => 2H₂O
  O₂ + 2H₂O + 4e^- => 4OH^-

It is evident that several of these reactions affect the pH in the vicinity of the metal surface. The cycling of surface pH in response to managed changes in the applied voltage is what is believed to limit microbial attachment. As the application of an inappropriate voltage could exacerbate corrosion, it is critical to optimize the electrochemical conditions for each application.

Cloudy filtrate tank fitted with Savcor electrochemical system (left) and untreated tank 20 days after cleanig (right)

Looking to the future of papermaking chemicals, it is still somewhat surprising that, some 50 years after they were first introduced, research continues on how alkylketene dimer (AKD) sizes work. It has been known for some time that even if they can react with cellulose under papermaking conditions, only a minor fraction of the added AKD does so and this occurs during the drying of the paper. Recent research has shown that AKD does not distribute itself over the fiber by a spreading mechanism, as it cannot wet the surface. Instead, it may be distributed by vaporization followed by adsorption. This mechanism could also explain the observed loss of sizing in some papers during storage.

Rosin has been used even longer for sizing paper, but there is no sign of any end to its research possibilities. Using new analytical techniques to probe the distribution in paper, Japanese researchers have shown that fibers are not uniformly covered with rosin, at least at neutral pH. As with most problems with rosin sizing at neutral pH, this is likely to be related to the chemistry of the auxiliary aluminum salt, particularly the difficulty in achieving good distribution in the stock when the aluminum ion is in the process of being hydrolyzed and precipitated.

With similar research projects on papermaking chemicals being carried out around the world, perhaps it will not be too long before paper mill chemists can through away their witches' hats and cauldrons for good.