|More Publications: Online, By Others, and Critical Reviews|
ASA droplet size;
Starting a distance education program;
Enzyme use in papermaking;
Pitch and stickies - a chemist's view;
Does pitch have your attention?;
Recycling of additives;
Retention and drainage chemicals;
A cure for stickies?;
- Looking towards the Future of Papermaking Additives" (posted Oct.
There's an old story about a public hearing in which paper company executives were describing their plans for a green-field mill. A spokesperson ended her presentation with a listing of the maximum levels of various substances in the liquid effluent from the proposed plant. "Our effluent water will have a biological oxygen demand of less than 10 parts per million, and it will have a pH of 7." At this point someone near the back of the room stood up and said, "I am a citizen of this town, and I will insist that the pH value be reduced to zero before the water is discharged!"
Part of our challenge as papermakers is to maximize the efficiency of our operations and make them increasingly eco-friendly. But, as illustrated by the story above, we also need to be proactive in explaining the steps we are taking as an industry. (Continued - The full article has 8424 words and 158 literature references.)
|Suggested Readings in Papermaking Chemistry, By Topic Area|
Here's a summary of recent findings and trends in the field of papermaking chemistry. You will find topic areas arranged alphabetically in the list below. If you are more interested in wet-end chemistry research work at NC State University, then you should look in the Research page or in the Abstracts of Published Work page.
Italian hand-made sculpure of Louis Robert's first continuous paper machine
Starch for wet-end applications
Those who are interested in many more (mostly 1-2 page) reviews of recent articles of importance in papermaking chemistry ought to visit the "Our Literature Reviews" section of the website for the North Carolina State University graduate-level wet-end chemistry course, WPS 527.
Those who are interested in a wide range of background information about papermaking chemicals and related equipment are encouraged to visit the "Mini-Encyclopedia" section of this website, and there is also additional related information at the graduate-level courses website (see previous item).
Here's an article for those who thought either that (a) everything is already known about "papermakers' friend," or (b) alum is "dead" in the age of alkaline papermaking. Farley shows clearly that there's much more to add to the story (Tappi J. 75, 11: 193, 1992). It turns out that the cationic charge of alum is strongly dependent on the presence of calcium ions, i.e. "water hardness." In certain circumstances, especially if there is a lot of sulfate ion present, the performance of poly-aluminum chloride (PAC) may become indistinguishable from that of alum.
Colloidal mixtures related to alum with potential uses as microparticles can be formed by rapid neutralization in the presence of controlled amounts of strongly complexing ions such as tartrate and silicate, according to work reported by Ohman and Wagberg (J. Pulp Paper Sci. 23, 10: J475, 1997). At sufficiently high levels of the silicate ion, the colloidal aluminum hydroxide floc appeared to have a negative surface charge. Small, stable colloidal dispersions were achieved under conditions where the zeta potential was far enough away from the neutral point. A retention and drainage aid program based on these principles was described by Lindstrom, Hallgren, and Hedborg (Nordic Pulp Paper Res. J. 4, 2: 99, 1989).
CHARGE MONITORING & CONTROL
On-line titration of streaming current is emerging as a key element in strategies to monitor and control chemical properties of white water, leading to more stable production. Further evidence of this progress was documented by Bley and Winter (TAPPI 1997 Papermakers Conf., 297). The level of anionic colloidal charge of white water was correlated with frequencies of paper machine web breaks and retention of fine materials during the forming process. A report by Phipps (Tappi J. 82 (8): 157, 1999) gives insights on how to overcome practical difficulties in achieving reproducible results with streaming current tests.
On-line control of zeta potential has been considered by many papermakers as a strategy, but very few have pursued their dream to the point of publishing their results. Miyanishi is an exception. His pioneering work involved on-line trials with the fiber-pad streaming potential method (Tappi J. 82, 1: 220, 1999). This is the first time that we have seen real-time data clearly showing how the surfaces of a fiber furnish became more anionic as a system was converted from acidic to neutral papermaking conditions. Also, within the conditions of testing, there appeared to be an optimum zeta potential corresponding to maximum first-pass retention.
Finding the root causes of deposits on papermaking equipment (or spots in the product) can involve a lot of detective work. As shown by Potter, modern analytical chemistry can help (TAPPI 1996 Papermakers Conf., 315). In the cited work it was found that different kinds of aluminum-containing deposits formed, depending on the pH range of papermaking. Under "acidic" conditions of pH 5-7 the deposits were a complex mixture. At higher pH, the deposits consisted of relatively pure aluminum hydroxide. In general it was concluded that alum deposits can be minimized by avoiding high pH, long dwell time, heat, and the build-up of high levels of aluminum ions in the wet end.
DRAINAGE - MICRO-PARTICLE DRAINAGE-AID PROGRAMS
Wall and coworkers at Chalmers University in Sweden provided some intriguing insights into the mechanism of a microparticle drainage-aid program based on cationic starch and colloidal silica (J. Colloid Interface Sci. 151, 1: 178, 1992). The interaction between the starch and the silica was strongly charge-dependent and appeared to result in polymeric bridge structures. Certain flocs appeared to contract after the addition of the colloidal silica, but only if the particles of colloidal silica were very small. The observations are consistent with a mechanism in which the anionic microparticles penetrate into the loops of adsorbed starch, causing the starch to contract and release water.
Pierre and Carre showed that alum can act a lot like a micro-particle. In other words, it can play a role in promoting drainage under alkaline conditions (TAPPI 1993 Papermakers Conf., 163). The effect was observed when alum was added to a system that contained cationic potato starch. It is likely that anionic phosphate groups that are natural in potato starch were responsible for at least part of the reaction with colloidal alum byproducts. The effect was found to be synergistic with the use of colloidal silica in the same system.
Those who are interested in bentonite as part of a drainage program are urged to read a delightful article by Kundson (TAPPI 1993 Papermakers Conf., 141). This is a very readable, but authoritative description of what bentonite is and how it is likely to behave in papermaking applications. An article by Wagberg and coworkers (Tappi J. 79, 6: 157, 1996) explores the mechanism. The strongest interaction between the furnish and the bentonite was achieved under conditions where cationic polyacrylamide retention aid would be expected to have extended loops and tails of polymer extending outward from the fiber surface. This affect can be achieved by either (a) adding a higher dosage of the cationic PAM before the bentonite, of (b) pre-treating the pulp with a low-mass cationic polymer to block many of the surface sites where the PAM otherwise would be able to lie down.
Microparticle drainage-aid programs have achieved widespread use for improving drainage rates, especially under alkaline papermaking conditions. But Honig and coworkers showed that it was possible to stretch the definition of "microparticle" and replace the colloidal silica or bentonite with highly branched anionic polymers (Tappi J. 76, 6: 135 (1993). Because of the "open" structure of a branched polyelectrolyte, compared to solid mineral particles, it doesn't require as much "micro-polymer" to achieve a substantial boost in drainage rates. The authors show promising results for paper produced at a high loading of calcium carbonate filler.
Believe it or not, certain lignin byproducts actually can be used to promote drainage. the system has been shown to work as long as a sufficient amount of cationic polymer has been added first (TAPPI 1996 Papermakers Conf., p. 439). After demonstrating this fact in the lab and pilot plant, Vaughan has taken on a more daunting task. The challenging step is to persuade papermakers that there can be merit in adding what is essentially black liquor to their wet-end systems. The approach is especially suited to products formed from old-corrugated-container (OCC) fiber.
DRAINAGE RATE MONITORING AND CONTROL
On-line monitoring of the drainage characteristics of papermaking furnish is becoming a reality. For instance, see a report by Renaud and Olsson (TAPPI '99, Vol. 1, p. 271). These authors used their Dewatering Rate Analyzer to demonstrate the drainage-promoting effects of cationic starch and retention aid.
An article by Stratton shows that how and where dry-strength resins are added to the system can make a big difference (Nordic Pulp Paper Res. J. 4, 2: 104, 1989). Tests were carried out with PAE resin (see notes in the "Wet-Strength section farther down this page) and anionic carboxymethyl cellulose (CMC). The strength aids were added separately to the fines and long-fiber fractions of refined unbleached kraft softwood pulp. The highest strength was achieved when the resins were added only to the long-fiber portion. Also, wet-end addition of the resins had a much higher impact on tensile and compressive strength, compared to surface addition (as in a size press).
Spraying cationic polymer on the forming fabric can be a very effective way to combat the build-up of pitch, as explained by Nguyen and Dreisbach (TAPPI 1996 Papermakers Conf., 511). The problem is mainly due to hydrophobic, tacky substances in the white water. Adsorption of cationic polymers onto plastic test surfaces rendered them hydrophilic, reducing the likelihood of pitch deposition. Related methods are widely used in paper mills that face issues with pitch and other sticky materials.
Work reported by Shetty, Greer and Laubach (Tappi J. 77, 10: 91, 1994) showed how related benefits in pitch control can be achieved by adding similar cationic polymers to the fiber furnish. Poly-DADMAC promoted coalescence of pitch particles, allowing them to be retained in the paper. The treatment also rendered surfaces more hydrophilic, as shown by the previous item (Nguyen and Dreisbach).
RECYCLING - IT'S EFFECT ON WET-END CHEMISTRY
Sjostrom and Odberg showed that charged chemicals and a sizing agent added during papermaking continue to have an impact after the paper is recycled to make a secondary fiber slurry. The secondary furnish was exposed to typical conditions of de-inking and washing. Samples of pulp that had been treated with cationic poly-acrylamide (cPAM) or cationic starch during the first cycle had a decreased ability to adsorb the same materials during the second cycle, relative to previously untreated pulp. Likewise, secondary pulp that had been treated with alkylketene dimer (AKD) size during the first cycle required less AKD to reduce water absorption to the same degree in secondary sheets, compared to reference sheets with no AKD added in the first cycle. Only a fraction of the "activity" of the original AKD survived the processes of repulping, de-inking, and washing.
Wagberg and Lindstrom have shed light on the mechanism of dual-polymer retention aid programs (Nordic Pulp Paper Res. J. 2, 2: 49, 1987). This type of system involves treating the furnish first with a moderate-mass, high-charged cationic polymer such as a polyamine (e.g. dimethylamine-epichlorohydrin condensation product), and then with a very high mass anionic acrylamide copolymer (aPAM). The rate of flocculation was rapid under all conditions of polymer concentration, charge density, salt concentration, and shear. The size and strength of flocs were affected by these variables. Results were consistent with a bridging mechanism of flocculation.
"Site-blocking" is a mechanism described by Swerin, Gland-nordmark, and Odberg to explain why addition of a low-mass, high charge cationic polymer can help the flocculating effect of a high-charge cationic polymer (Proc. 1996 International paper and Coating Chemistry Symposium, 61). It's worth noting that the low-mass additives had no flocculating ability when added alone to stirred suspensions of microcrystalline cellulose. These authors used highly washed pulps, so the effect was not due to scavenging of anionic colloidal material. Rather, they proposed that the low-mass material on the surface of fibers kept the high-mass flocculants from lying down flat on the fiber surfaces. The more extended loops and tails of adsorbed polymer are expected to be more effective for flocculation. Recently Kuhn publicized a practical system involving simultaneous addition of a low-mass, high-charge catioinc polymer (a PEI product) and a cationic PAM flocculant (PaperAge, July 1998).
High conductivity of white water can hurt the performance of the most popular types of retention aids. Buontempo, Sherman, and St. John compared the effects of different cationic poly-acrylamide (cPAM) products over a range of salt concentrations (TAPPI 1996 Papermakers Conf., 49). At the lowest levels of salt tested there was no significant difference in retention performance of cPAM products having different cationic charge density. At a conductivity corresponding to a typical paper mill with a moderately low level of fresh water use the efficiencies of all the cPAM retention aids were reduced; however, there appeared to be an optimum charge density.
A non-ionic retention aid, polyethylene oxide (PEO) holds great promise as the paper industry faces the challenges of further reductions in the amounts of fresh water that we use. In principle PEO is not sensitive to high levels of salts in the process water. Pelton and Gato did some intriguing experiments in which PEO was mixed with a co-factor, i.e. the second component usually used in such retention programs (see TAPPI '99, vol. 1, p. 281). Then they measured the strengths of little gel-balls formed from the mixture. These experiments led them to conclude that (a) there was an ideal ratio of PEO to co-factor, and (b) the degree of stretch of the gel-balls implied that many PEO chains are entangled and partly linked by the co-factor.
A lot of questions have been raised as to how these non-ionic retention aid systems actually work. Van de Ven found evidence that PEO did not adsorb onto fibers in the absence of the phenolic resin co-factor (J. Pulp Paper Sci. 23, 9: J447, 1997). Association of PEO with the co-factor appeared to change its behavior.
SIZING - ACIDIC
Ever wonder why liquid rosin products, i.e. "soap sizes" can behave differently from rosin emulsion products? An article by Marton tells all (Nordic Pulp Paper Res. J. 4, 2: 77, 1989). The soap can be precipitated onto fibers in the wet end when alum is added. Alum can help to retain the emulsion particles, but there's not much orientation or true anchoring of the size molecules until the sheet is dried. The article gives some rules of thumb for optimizing your system.
SIZING - ALKALINE
Alkyl ketene dimer (AKD) is firmly established as a preferred product for increasing the water resistance of products formed at neutral or alkaline conditions. In particular, it is valued for meeting the requirements of beverage containers, cups, containers, and color ink-jet printing. But first you have to retain it in the sheet. A report by Isogai, Kitaoka, and Onabe (J. Pulp Paper Sci. 23, 5: J215, 1997) provides a lesson on how basic wet-end chemical principles can be used to understand conditions that favor retention of AKD.
Droplets of melted AKD do not spread sufficiently to account for the chemical's ability to make paper water-resistant. This is the conclusion that I draw from a wonderfully understated article by Garneir and Lu (J. Pulp Paper Sci. 25, 7: 235, 1999). The authors used atomic force microscopy (AFM) to obtain images of glass to which commercial AKD dispersions had been placed. The presence of starch and other materials associated with commercial AKD did not have a significant effect on spreading. The calculated fractional area covered by AKD in a typical application was estimated to be between 1-5%. Even if one were to use different assumptions about addition levels and surface area, this still does not seem to be a high enough number to explain the high efficiency of AKD size under ideal conditions. It will be interesting to see whether this report inspires further work involving gas-phase transport of AKD molecules to adsorption sites on fibers.
Surface sizing depends on wet-end sizing. Often the best practical solution is to add hydrophobic materials both in the wet end and at the size press. That's what Tsai, Colasurdo, and Inuoe reported (TAPPI '99, Vol. 1, p. 111). The combination of alkylketene dimer (AKD) in the furnish and styrene acrylate emulsion (SAE) at the size press gave the best density of ink-jet images.
A cationic styrene emulsion can be used as a wet-end size, at least in principle, as shown by Yang and Deng (TAPPI '99, Vol. 1, 111). The sizing response was highest at a moderately low (but non-zero) level of cationicity. Water hold-out increased with increasing doage and temperature of cure, presumably due to a melting and spreading mechanism.
Alkaline rosin sizing is an old idea that refuses to go away. Hedborg and Lindstrom showed that rosin sizing was an excellent fit with their retention and drainage strategy based on alum under alkaline conditions (Nordic Pulp Paper Res. J. 8, 3: 331, 1993). In that system a finely divided Al(OH)3 floc is added to a furnish that contains cationic potato starch. The drainage benefits appear to be related to the mechanisms of micro-particles such as colloidal silica. Best results were achieved at an optimum degree of hydroxylation of the aluminum.
Self-sizing is a process by which paper can sometimes become more hydrophobic when it is heated, even if no known sizing agent was used. Work by Ness and Hodgson showed that self-sizing can occur even under alkaline papermaking conditions if the paper is formed from mechanical fiber that has a high enough resin content (Nordic Pulp Paper Res. J. 14, 2: 111, 1999). Treatment of the resinous fiber with peroxide rendered the same pulp incapable of self sizing. No information was provided regarding behavior of the same materials in the presence of aluminum ions.
There's a bit of an art to staying ahead of slime-forming organisms in the wet end. The combination of starch, wetness, and warmth makes a wonderful environment for bacteria and fungal growth. Stitt recently published some practical strategies to keep ahead of the bugs (PIMA's North Amer. Papermaker 79, 9: 54, 1997). For instance, it pays to schedule boilouts of the wet-end system at optimum intervals.
High white-water conductivity, i.e. high salt levels, can pose a challenge for users of wet-end starches. This fact was confirmed in a recent study by Beaudoin, Gatton, and Turcotte (J. Pulp Paper Sci. 21, 7: J238, 1995). Best results were achieved by using a more highly cationic starch. Also, a waxy maize product outperformed dent corn and wheat starches in terms of the dry strength achieved under the test conditions.
A curtain coater can be used to apply starch directly into the wet-web as it travels down the Fourdrinier table, according to technology described by Fulger and Parisien (TAPPI '99, Vol. 1, p. 141). In principle one can use inexpensive, uncooked starch. The procedure has no adverse effect on either drainage or drying. Effects are of the same type that can be achieved by spraying uncooked starch onto the surface of the wet web (or between plies in the case of cylinder board).
Reaction with carboxyl groups at fiber surfaces appears to play a key role in the mechanism of poly-amidoamine-epichlorohydrin (PAE) resins. That's what Wagberg and Bjorklund showed based on (a) tests with artificially carboxylated pulps, and (b) IR spectrophotometric evidence (Nordic Pulp Paper Res. J 8, 1: 53 (1993).
Polyamido-amine epichlorohydrin (PAE) resins have market leadership, especially in the case of paper that is formed at neutral or alkaline pH. But now there is a new competitor, polyisocyanates. You can learn about them in a report by Roick and Matthews (TAPPI '99, Vol. 1, p. 51).
|Meetings and Conferences:|
TAPPI, the Technical Association of the Pulp and Paper Industry, is a great place to associate with others who are interested in papermaking additives. The Additives Committee of TAPPI sponsors an annual conference called the Papermakers Conference. Typically it is held in the Spring, though in recent years the times have been moved around in order to join with some of the other TAPPI divisions in presenting "megaconferences." The 2000 conference will be in April in Vancouver, BC. For the latest updates, you can "visit" TAPPI at http://www.tappi.org . Papermakers Conference are generally held in the spring, sometimes in combination with other TAPPI events.
SHORT COURSE: TAPPI also produces a very popular 3-day short-course entitled "Introduction to Wet-End Chemistry." Recently I have been assisting TAPPI as Chair of this short-course offering. If you are interested in attending or helping out with these short courses, please look at information listed ELSEWHERE ON THIS WEBSITE.
An "International Paper and Coating Chemistry Symposium" takes place every couple of years, usually in Stockholm, Sweden or Canada. The focus of discussion will be the physical and chemical principles for the performance of chemical additives in papermaking.
PIRA is the paper technical organization that has extensive laboratory and pilot plant facilities in the UK. Many of their conferences and collaboraborative research products are of particular interest to wet-end chemists. A conference entitled Scientific and Technical Advances in the Internal and Surface Sizing of Paper and Board is scheduled for Dec. 2-3, 199 in Milan, Italy. For details and registration go to http://www.pira.co.uk .
|Research Needs: Areas for Future Research in Wet-End Chemistry|
The critical needs for research in the area of paper machine wet-end chemistry go way beyond the capabilities of any existing research center. The need is great, and there are hardly enough researchers working in this area. Here's a preliminary list of some of the areas that we anticipate needs for graduate research and industrial research over the coming decade:
If you can think of other items that ought to be added to this list, please send an E-Mail message.