Opportunities in Wet-End Chemistry: Feature Essay, Posted Oct. 2001
Good Chemistry - Looking towards the Future of Papermaking Additives
Martin A. Hubbe
Dept. Wood & Paper Sci., N.C. State Univ., Box 8005, Raleigh, NC 27695-8005
Citation (public domain): http://www4.ncsu.edu/~hubbe/new/goodchem/
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. Our challenge is to educate our fellow citizens that chemicals, used appropriately, are absolutely essential in this effort and that they also can be safe to use.
What about the chemicals that one adds at the wet end of a paper machine? The public sometimes associates the word "chemical" with words like "pollutants," "emissions," "toxicity," or "hazard." As noted in an article by Reinbold (1994), "the public no longer views technology as something beneficial." Some advocates for the environment have described paper as a "chemical cocktail." The goal of this essay is to consider how we, as papermakers, can do more in the years ahead to minimize environmental impacts and also to achieve a more favorable impression in the eyes of the public.
Fig. 1. Full description given by Gottsching (1993)
Fig. 2. Full description given by Gottsching (1993)
Where do we look for answers? In my opinion there are basically three answers to our situation as suppliers and users of papermaking additives. I will spend the rest of this essay expanding on each one of them in turn. The first answer is for us in the industry to show that each additive to a paper machine has a clear and beneficial role. The second answer is to demonstrate progress in understanding and minimizing environmental impacts of specific papermaking additives. The third answer is to envision the types of chemical additives and their uses in a hypothetical future paper mill. Our ideal paper mill of the future should be both profitable and as nearly "invisible" as possible in terms of its impact on the environment.
Part 1 - A Purpose for Each Wet-End Chemical
Think about your reaction when you see a really long list of ingredients on the container of a processed food item. Do you ever read down through the list and wonder whether all of those odd-sounding chemical items are really needed? It's far worse for those who happen to be allergic to one or more of those additives. Unlike packaged food, paper products come with no list of ingredients. Except for some factors that I will discuss below, we are in a situation somewhat resembling the years before food labels. It is possible to list about 3000 different kinds of chemicals that have been proposed for use in papermaking (Reinbold 1994). Ingredient labels for paper products may or may not be a good idea; but it is also clear that there is an opportunity for the paper industry to tell the public what is used in paper and why.
Progress in explaining the environmental consequences of papermaking additives already has been achieved in a series of publications that appeared in the early 1990's. First, an article by Reinbold (1994) clarifies just how few chemicals papermakers actually use. If one ignores brand designations, differences in concentration, and minor variations in molecular mass or composition, then only about 200 individual chemicals are commonly added to paper machines, not 3000. The relatively low number of chemical additives used in papermaking is consistent with the fact that this industry mainly makes low-cost, high-volume products; we simply can't afford to use superfluous chemicals.
An article by Göttsching (1993) makes the further point that papermaking practices are generally compatible with the environment. If one were to omit all chemical additives from a papermaking process, then the consequences would include larger increases in emissions of solids, biological oxygen demand, and even of noxious gases - resulting from uncontrolled growth of slime in paper machine systems. This article, together with a publication by Webb (1993) give an excellent run-down of the main types of chemical additives and the status of each of these additive relative to various environmental impacts.
Saving Energy: Let's take a closer look at how wet-end additives can reduce the energy required used in papermaking. Removal of water uses by far the largest component of that energy. Most water is removed by gravity drainage, application of vacuum, inertial effects, and pressing. However, most of the energy is expended during a subsequent process, drying by evaporation (Hersh 1981; Specht 1992). Approximately 2 to 9 million BTU are required per ton of product, to evaporate water. Substantial savings in energy can be achieved by shifting a greater proportion of the water removal to the preceding unit operations of forming and pressing (Manson 1980; Nelson 1981; Manfield 1986; Marley 1990). One way to accomplish this goal is to accelerate dewatering with chemical additives. There has been much work in this area (Auhorn 1982; Allen, Yaraskavitch 1991; Litchfield 1994; Raisanen et al. 1995; McGregor, Knight 1996). I it generally agreed that each 1% increase in solids content of a paper web should yield about a reduction of 4 to 5% in the net drying load (Shirley 1980; Nelson 1981; Auhorn 1982; Strawinksi 1985; Marley 1990). Pulp mills are often net producers of energy in the form of steam or electricity, but savings in the energy of drying has the potential to either decrease the consumption of fossil fuels or decrease the production of greenhouse gases.
The goal sounds great, but what chemicals are we talking about in terms of additives? Three classes of chemicals stand out as the major drainage chemicals in current use (Allen, Yaraskavitch 1991; McGregor, Knight 1996; Scott 1996). These three classes are often called "coagulants," "flocculants," and "microparticles." Coagulants used in papermaking are generally multivalent or polymeric compounds of high positive charge density. Commonly used coagulants include aluminum sulfate ("papermakers' alum"), polyamines, and polyethyleneimine (PEI). The word "coagulate" implies that the negative surface charges of suspended solids, fibers, and colloidal material are neutralized, removing the electrical repulsion between these surfaces. Flocculants complete the process of bringing fine particles together; the most widely used type of flocculants in the paper industry are very high mass copolymers of acrylamide (Horn, Linhart 1991). Amounts typically less than 0.05% based on product mass are sufficient to increase the retention of fine particle in paper as it is being formed. Microparticles are tiny negatively charged particles such as colloidal silica, bentonite, or highly branched carboxyl compounds; they interact with cationic polyacrylamides or cationic starch to further promote dewatering (Langley, Litchfield 1986; Knudson 1993; Honig et al. 1993; Andersson, Lindgren 1996; Swerin et al. 1996). A common characteristic of all of these drainage-promoting chemicals is that, to perform their function, they adsorb onto the surface of solids in the papermaking furnish. That means that these chemicals tend to be retained well in the paper; relatively little of it remains in liquid effluent from paper machines, even before wastewater treatment.
Defoamer chemicals affect many aspects of papermaking, in addition to drainage, but it is the drainage benefits that have the clearest connection with environmental impact. A study by Brecht and Kirchner (1959) was among the first to clearly show that air bubbles in a stock suspension can have an effect very similar to that of fiber fines in slowing the rate of drainage from a paper web. Especially in the case of heavier weights of paper or paperboard, higher levels of fines or bubbles can be expected to clog the drainage channels in a wet sheet of paper (Gess 1989, 1991). Defoamers are added to the wet end in the form of emulsions; little droplets of oily material spread rapidly on bubble surfaces and cause the bubbles to coalesce. The result is less entrained air coming out of the headbox. In principle, improvements in drainage can be converted into dryer paper going into the wet-press section. In turn, a dryer sheet coming into the press section makes it possible to load the presses more without squashing the sheet. The happiest situation is when increased pressing results in a stronger, better-consolidated sheet, with less water remaining to be evaporated. The wild card in this situation is whether the resulting sheet still has enough caliper so that it can be calendared to meet a specified smoothness.
Decreasing Effluent Loads: A remarkable aspect of the "art" of papermaking is that paper is formed on a relatively coarse, continuous screen fabric; typically the openings in the fabric are large enough so that between about 5 and 50% of the solids delivered to the forming section are capable of passing through those openings. The small particulate material in paper, the "fines," may consist mostly of wood byproducts (Brecht, Klemm 1953; Scott 1986; Gess 1991; Luukko, Paulapuro 1999; Rundlöf et al. 2000). Even before it is refined, a typical kraft pulp contains about 5 to 10% by weight of such things as tiny parenchyma cells, used for food storage or conduction. The process of refining pulp - passing the pulp slurry between counter-rotating metal plates or cones having raised bars - is necessary to develop the bonding ability of fibers for most grades of paper, but refining also increases the level of fines in the slurry. But all of these wood-derived fines can be overwhelmed by fine material of a different type, the mineral fillers (Bown 1998). Calcium carbonate and clay are the major types of fillers used, and they make it possible to achieve opacity targets with less total materials.
To understand how retention aid chemicals can impact the environment it is worthwhile to view papermaking operations as the first step in a multi-step water clarification process (Leitz 1993). Though there is a great deal of overlap between "retention chemicals" and the fore-mentioned "drainage chemicals," the emphasis of a retention program is to increase the relative proportion of fine materials that stay with the wet paper web as it is being formed (Jaycock, Swales 1994; Gess 1998). The very-high-mass acrylamide copolymers, polyethylene oxide in combination with phenolic cofactors, and also high-mass acrylamides, in combination with microparticles, can be very effective retention systems, even in some cases where the surfaces of the suspended matter are far from being neutral in charge. Higher retention efficiency implies that less solid material is present in the water that drains from the paper. The traditional name that papermakers used to describe the filtrate water from papermaking is "white water." A generation ago it used to be more common for white water to contain so much clay, titanium dioxide, and air bubbles that it looked like milk. Now, largely thanks to chemical additives, together with screen devices called save-alls, solids levels of white water are kept under control and nearly all of the fine material eventually ends up as paper.
Avoiding Waste of Fibers: You may not think of strength aids as fiber-saving chemicals, but you should. Consider the case of recycled office waste fibers. Such fibers tend to loose a significant fraction of their bonding ability each time they are dried and reslurried (Lindström, Carlsson 1982; Klungness, Caulfield 1982; Howard, Bichard 1992; Nazhad, Paszner 1994; Zhang et al. 2001). The loss in bonding ability has been attributed to essentially irreversible closure of pores in the cell wall (Stone, Scallan 1966), resulting in a loss of flexibility of the fiber surfaces (Paavlilainen, Luner 1986). Strength specifications become more difficult to achieve. One approach is to try to make up for the strength loss by increased refining. However, the furnish is likely to already have a relatively low freeness, so there comes a point where more refining is not the answer. Rather, papermakers tend to use increased levels of strength additives, such as cationic starch or acrylamides (Marton 1980; Strazdins 1984; Howard, Jowsey 1989; Smith 1992; Iwasa 1993; Glittenberg et al. 1994).
Another situation in which strength additives can "save fiber" arises in the case of paper grades that are specified by strength rather than basis weight. Such is the case for containerboard grades made in accordance with the Rule 41 criteria (Gutmann et al. 1993). Briefly stated, the rule allows a producer to decrease the basis weight of a product as long as the combined board still meets various strength goals, such as crush resistance. In practice, papermakers use a combination of refining practices, dry-strength additives, and sometimes size-press addition to make the premium-strength board and take advantage of Rule 41 (Smith 1992).
Can Chemicals Added Initially Benefit Recycling? It has been shown that strength-enhancing chemicals added to never-dried kraft fiber can also have a beneficial effect after the same fibers are recycled (Higgins, McKenzie 1963; Grau et al. 1996; Laivins, Scallan 1996; Zhang, Hubbe 2000). Treatments found to be effective included cationic starch and combinations of cationic and anionic polymers. Results were consistent with the ability of such chemicals to act as inter-fiber bonding agents - both in the initial paper and also in the recycled paper, even when no additional polymeric material was added during the second generation of papermaking.
Losses in fiber bonding ability due to drying, aging, and recycling of paper made from kraft pulp may be minimized by alkaline papermaking conditions. Some benefit of alkaline conditions may result from reduced hydrolysis of cellulose macromolecules (Wilson, Parks 1983; Nazhad, Paszner 1994). Further benefits may be associated with reduced closure of pores in the cell walls (Lindström, Carlsson 1982), and reduced stiffening of fibers. Though papermakers adjust pH values in various different ways, one type of additive stands out in terms of adjusting the pH to minimize damage to fibers. Give up? That additive is calcium carbonate filler. Recent recommendations for archival papers require at least two percent calcium carbonate to make sure that the paper remains buffered in a weakly alkaline pH range to make it resistant to gradual embrittlement (McComb, Williams 1981; Kelly, Weberg 1981; Anon. 1993). Work by Pycraft and Howarth (1980) shows further that over-drying of virgin paper is likely to harm the properties of the fibers, if they are to be used later for recycled paper.
The Sludge Dewatering Press is Like a Little Paper Machine: Recycling of paper requires more fossil fuels or electrical energy, compared to new pulp and paper from wood or sawmill waste. The recycling of paper also can produce a lot of waste sludge. Nevertheless, recycling usually is regarded as having a favorable net impact on the environment (Pajula, Kärnä 1995; Jorling 2000). A key goal of increased recycling helps keep the rate of tree harvesting below the growth rate of new trees.
Saving land-fill space is another motivation to recycle paper: it turns out that chemicals can play a beneficial role in helping to achieve this goal. The reason is that sludge from wastepaper recycling can contain a lot of water (Dorica, Allen 1997; Kantardjieff 2000). The water content adds to the weight of sludge to be discarded, and it also makes it more fluid-like, not the ideal characteristic for building a stable landfill. Chemicals coagulants such as poly-aluminum chloride (PAC), essentially the same coagulants used in paper formation, can be used to assist pressing more water from sludge (Ghosh et al. 1985; Leitz 1993; Pawlowska, Proverb 1996). Side benefits of sludge dewatering may include a) more stable, solid-like sludge, b) the colloidal materials in the sludge will tend to be insolubilized in polyelectrolyte complexes and precipitates, and c) the sludge will be more valuable as a fuel source, if that option is considered (Harila, Kivilinna 1999). In principle well coagulated waste sludge is expected to have reduced rates of leaching.
Part II - Minimizing the Environmental Impacts of Each Type of Additive
"You work for the paper industry? Then maybe you can explain that smell when I drive into [you fill in the place name]." To put the present discussion into context it is worth noting that most recent public concern has been directed at issues other than papermaking additives. Rather, greater attention has been directed towards issues of pulping, tree harvesting practices, paper recycling, and, yes, air emissions (Vasara 2001). Another, possibly more authoritative measure of environmental concern comes in the form of legislation. Pulping and bleaching have been center-stage in the so-called "cluster rule" regulations (Vice, Carroll 2001). While keeping this context in mind, we still have to seriously consider the potential impacts of papermaking additives, if and when they enter the environment. The good news is that substantial progress has been accomplished in the area of papermaking additives with respect to their toxicity, their biodegradability, and their ability to be removed from the water phase during wastewater treatment (Jorling 2000; Hamm, Göttsching 1994; Swann 2000). Later in Part II we will consider various papermaking additives, focusing on their potential hazards.
Fig. 3. See article by Goettsching (1993)
Fig. 4. Full description given by Vasara (2001)
A subtle, and often overlooked influence on chemical additives for papermaking comes in the form of Material Safety Data Sheets. "MSDS" information often is kept in orange or yellow loose-leaf notebooks, adjacent to places where industrial chemicals are being used. As noted by Allen (1991), these documents have encouraged a trend towards greater awareness of what it being added to paper machines. Toxicity and safety information in MSDS has provided a starting point for making improvements, and making substitutions toward less toxic materials.
After toxicity, perhaps the second most serious issue is biodegradability of chemical additives for papermaking. Essentially all excess water from US paper mills undergoes wastewater treatment before it is discharged. Bacterial action during the secondary wastewater treatment converts many organic chemicals into benign forms, and most of the biological oxygen demand (BOD) is consumed. Some approximate rules to predict biodegradability have been proposed. For instance, compounds that contain chlorine, nitrogen, sulfonic acid, or azo-groups are more likely to resist breakdown during water treatment (Hamm, Göttsching 1994). Other factors that appear to hurt biodegradability include toxicity, long chain length of polymers, branching, and chemical substituents along polymer chains. Unsubstituted alkyl chains also resist biological degradation (Swann 2000). The problem with persistent chemicals is that they might have the potential to accumulate in the environment or in particular organisms.
Wet-strength agents: Environmental concerns about wet-strength chemicals are often associated with their monomer composition, possible residual monomers, and the possibility of regenerating these monomers and releasing them into the environment. The traditional wet-strength resins most often used for acidic papermaking conditions are based on formaldehyde (Dulany 1989; Espy 1995; Spence 1999). Possibly in response to these concerns, the usage of phenol-formaldehyde and melamine-formaldehyde resins has decreased dramatically in the US paper industry. Poly-amidoamine-epichlorohydrin (PAAE) resins have been replacing the formaldehyde resins in most paper applications requiring durable wet-strength (Espy 1995; Fischer 1996; Spence 1999).
Besides the issues with biodegradability, users of wet-strength agents face two additional concerns. First, difficulties in repulping wet-strength paper increase the likelihood that the fiber will be sent to landfills after its first use. Second, if papermakers decide to repulp the wet-strength paper, one needs to be concerned about the chemicals used as repulping aids. Hypochlorite bleach is sometimes used to repulp wet-strength broke (Espy 1992; Fischer 1997). Elevated pH or temperature also may be required to redisperse the fibers. At a minimum, recycling of wet-strength paper is likely to require higher energy input in the repulping operation. That means that there is an environmental price to wet-strength treatment; sometimes the price is paid in terms of increased landfilling, sometimes in increased water treatment requirements, and sometimes in increased energy expenditures. The ideal, in terms of wet-strength treatments, would be to find a non-toxic, biodegradable material that provided efficient, durable wet-strength under conditions of use, but which also repulped easily under slightly higher temperatures and hydrodynamic shear conditions in a repulping operation.
Dyes: Papermaking colorants tend to have relatively poor biodegradability (Webb 1993; Wahaab 2000). Fortunately there has been a trend towards dyes with relatively high affinity for solid surfaces. That means that the dyes tend to leave the paper machine as part of the product, not in the water to be treated. In addition, dyes entering the wastewater plant tend to be removed with biological sludge (Webb 1993). High affinity onto solid surfaces is generally achieved by development and use of relatively large, planar molecules - the so-called "direct" dyes. Affinity for fibers is further promoted by the trend for more use of cationic direct dyes, in cases where these are appropriate. Jackson (1993) noted that dye suppliers can minimize adverse environmental impacts by careful selection of adjunct materials used to stabilize liquid dyes.
Acrylamide copolymers: Considering their benefits in reducing the waste of unretained fines, it is easy to love retention aids. Copolymers of acrylamide are the most widely used very-high-mass flocculants to promote fine-particle retention. On the one hand, acrylamide products are expected to contribute much less to biological demand (BOD), compared to the amounts of starch products needed to render equivalent benefits in terms of either retention or dry strength (Iwasa 1993). On the other hand, they are not easily biodegradable (Webb 1993), as is to be expected, based on their molecular mass (Hamm, Göttsching 1994). The maximum permissible level of monomers present in acrylamide copolymers is 750 ppm, compared to 100 ppm in the case of other polymers (Swann 2000). Acrylamide products have received the more lenient limits due to their history of 40 years of use in the paper industry without evidence of harm.
Another issue to consider is the use of mineral oil as the continuous phase of common retention aid emulsion products (Swann 2000). Oil introduced with retention aids probably is mostly adsorbed by fibers, with no adverse effects. However, it is possible to imagine a bad effect resulting from the following sequence: a) a low-grade mineral oil, having a significant aromatic content, is used in the formulation; b) some of the same paper is recycled in a batch that includes colored papers; and c) the paper is bleached with elemental chlorine (Fleming 1995; Lancaster et al. 1992). Fortunately, this combination of circumstances is probably rare these days due to the use of purified, alkyl mineral oils and the elimination of elemental chlorine from most pulp bleaching operations in the US (Deardorff 1997).
Another way to address concerns about oils in retention aid products is to eliminate them from the formulation. One of the side-benefits of oil-free formulation can be a substantial reduction in shipping weight and transportation costs for a given amount of active materials. Many water-in-oil retention aid emulsions have active solids contents in the range of 25 to 50% (Horn, Linhart 1991). Dry granular or "bead" acrylamide-type flocculants, which have been available for many years, have nearly 100% active content. If it weren't for the perceived convenience of pumpable liquid formulations it is likely that dry products would enjoy more widespread use. An especially elegant solution to this dilemma involves a dispersion of acrylamide-copolymer particles in aqueous solutions (Feng et al. 2001). Normally such copolymers would dissolve in water, but the ion concentrations can be adjusted to prevent this from happening.
Highly cationic copolymers: Efforts by papermakers to conserve fiber resources and water have led to increased usage of highly charged cationic polymers. One of the ways to conserve fiber resources is to use high-yield pulps, such as thermo-mechanical pulp (TMP). Wood pitch from TMP can be a source of tacky deposits on papermaking equipment, forcing the mill to shut down often for cleanups (Back, Allen 2000). Another way to conserve fiber is through recycling. Wood pitch is less of a problem with recycled fibers, but the problem is replaced by stickies from pressure-sensitive adhesives and coating latex (Hsu 1997; Douek et. al. 1997; Venditti et al. 1999; Wilhelm et al. 1999). Some of the tackiness problems can be minimized by use of talc (Braitberg 1966; Allen et al. 1993). Also the furnish usually can be treated with highly charged cationic materials such as polyethylene-imine (PEI), polyamines, or poly-diallyldimethyl-ammonium chloride (poly-DADMAC). Such cationic treatments can help to bind the tacky materials to fibers so that they can be purged from the system (Gill, 1993; Fogarty 1993; Shetty et al. 1994; Magee, Taylor 1994; Moormann-Schmitz et al. 1994). Highly cationic polymers or soluble aluminum compounds are used for the neutralization of excess anionic colloidal charge in papermaking furnish - often the first key step in optimization of drainage and retention systems (see references cited in Part I). Yet another use of highly cationic polymers is in the spraying or forming fabrics or press felts to inhibit deposition of tacky substances from the paper (Allen 1991; Sawada 1997); here again, the use of these agents is helping in the effort to use wastepaper and high-yield pulps, both of which are worthy environmental goals.
Highly substituted, synthetic polymers of the type used for precipitation of tacky materials and the neutralization of excess colloidal charge are not expected to be highly biodegradable (Hamm, Göttsching 1994). For example Wahaab (2000) observed very poor biodegradability in the case of a commercial, highly cationic polymer used for treatment of forming fabrics.
One step towards addressing concerns about possible environmental impacts of highly cationic polymers is to avoid using more than is needed. For instance in the spraying of forming fabrics it is possible to minimize the chemical use by proper dilution and by use of a well-designed spray boom (Sawada 1997). When used to neutralize excess colloidal charge, it is possible to avoid overdose of highly cationic polymers by carrying out online or laboratory charge titrations with streaming current instruments (Bley 1992; Stitt 1998; Phipps 1999; Gill 2000; Rantala, Koskela 2000; Chen et al. 2001). Charge control to the neutral range has the advantage of tending to maximize precipitation of most polymers and fines onto fiber surfaces, reducing the amounts of polymeric and colloidal substances that are sent to the wastewater treatment system.
Recently there is yet another option to consider, the use of highly cationic polymers based on starch or other natural products. Already a highly cationic polymer based on starch has been used for charge neutralization and optimization of wet-end operations (Vihervaara, Paakkanen 1992). Presumably such materials might be more easily biodegraded, compared to their synthetic counterparts. "Not necessarily so," says Reinbold (1994). Rather, there is a wide range of variability in the biodegradation of both natural and synthetic polymers.
Biocides: Conventional slimacides are highly toxic. They have to be to perform their function. Many do not break down readily during treatment of wastewater (Webb 1993). Concerns over these types of biocides have resulted in pressure against biocide use for papermaking in Sweden (Swann 2000). One of the goals, then, is to develop biocides that do their job and then self-destruct (Allen 1991).
Enzymes are very good at self-destruction. The fragile nature of enzymes is due to the fact that they consist of complex proteins with many loops and coils that have to fit together in an exact way to perform some kind of function. Even moderate changes in pH or temperature can temporarily or permanently destroy the enzyme's activity. Enzymes such as amylases are already used for cleaning up deposits on starch-preparation equipment and paper machine wet-ends (Swann 2000).
Another way to minimize the need for toxic agents to control slime involves biodispersants (Crill 1993). Biodispersants make sense because bacteria attached to surfaces, the so-called "sessile" bacteria, tend to cause more problems than freely floating bacteria in paper mill systems. Although it is premature to expect that biodispersants can eliminate the need for toxic biocides, or of oxidizers such as chlorine dioxide, it is reasonable to expect the dependency on such materials to be reduced.
Starch: Starch products probably wouldn't even be included in the present discussion, but for the fact that the paper industry uses so much of them. The largest proportion of starch is added to the surface of paper at the size press or in coating formulations. Additional starch is commonly added at the wet end in levels up to about 1% on paper mass. Native, underivatized starch is close to ideal in terms of its biodegradability (Hijiya 1999). In addition to providing strength and helping certain retention aid programs, starch products also are based on a renewable resource. The most common grade of starch used in the US is a byproduct of processing corn sweetener for soft drinks and other processed foods. The trouble is, size-press starch often makes up 1 to 5% of the mass of various paper products. This is certainly true of printing papers. Since the kinds of starch most often used at the size press are poorly attached to fibers, large amounts of starch can become solubilized through the repulping of dry-end broke. Such starch is likely to be a major contributor to BOD of liquid effluent from the mill. In other words, the problem is in the large amount of starch products in the effluent water, not in their rate of degradation in a well-run biological wastewater treatment system.
Work carried out by Roberts et al. (1987) showed a very effective way to minimize BOD contribution of starch in effluent from paper machine systems. The answer is to use cationic starch (Webb 1994). Roberts showed a case in which about 85% of cationic starch was retained at neutral pH, whereas only about 10% was retained when the experiments were repeated with unmodified starch. It should come as little surprise that most starch now added at the wet-end of paper machines is either cationic or amphoteric (i.e. having both positive and negative charged groups attached to the chain). The down side is that cationization of starch appears to make it less biodegradable (Hamm, Göttsching 1994). In summary, the higher retention of cationic starch and its good, though not perfect biodegradability make it highly beneficial in terms of overall environmental impact of paper mills.
Sizing Agents: Internal sizing agents are truly remarkable in their ability to transform the nature of paper, even when the added dosages are typically well below 1% of the dry mass of product. The chemical composition of wood-derived fibers makes them highly water-loving. Paper uses for cups, bags, cartons, and various printing applications can require that it resist water absorption and penetration.
Rosin size has been criticized for its toxicity and for the fact that rosin sizing usually requires the use of aluminum compounds (Webb 1993). But rosin products can claim a positive attribute not shared by the common alternative sizing agents; rosin is a byproduct of wood pulping. Rosin is a renewable, biodegradable material (Webb 1993). There is an interesting balance between rosin's efficiency and its biodegradability; most rosin is reacted with maleic or fumaric anhydride to produce "fortified" rosin size. The fortified size is more storage-stable and more efficient in use. However, it also is less biodegradable than natural rosin (Webb 1993).
Though it still is worth considering environmental implications of rosin size products, there has been a strong trend over the past 20 years towards alkaline papermaking conditions and the use of calcium carbonate filler (Gill, Scott 1987; Laufmann et al. 2000). Values of pH higher than about 7 make it increasingly harder to size paper with conventional rosin products (Liu 1993; Schultz, Franke 1996; Wang et al. 2000). Fortunately, two widely used "alkaline sizing agents" are available. Alkenylsuccinic anhydride (ASA), which is very popular for production of printing papers and gypsum board liner, is a byproduct of petroleum (Webb 1993). By contrast, alkylketene dimer (AKD) is made from fatty acids, a renewable resource. In either case, alkaline sizing agents tend to be much more efficient than rosin in terms of the amounts needed to reach equivalent levels of resistance to fluids.
Surfactants: Some surface-active materials are added to paper intentionally, whereas others come along for the ride as stabilizers for other chemicals or as residuals from de-inking. If we use a broad definition, then the list of intentionally added surfactants would include sizing agents (e.g. rosin soap size), components of certain defoamers (i.e. water-insoluble surfactants), certain deposit-control additives, and debonding agents used in certain tissue products. Various nonionic and fatty-acid-based surfactants are used in flotation de-inking (Johansson, Ström 1998; Rao, Stenius 1998) and for the agglomeration of xerographic toners (Darlington 1989; Heitmann 1994; Bast-Kammerer, Salzburger 1995). Nonionic surfactants also are used to stabilize such additives as retention aid emulsions, dyes, and certain sizing agents.
Probably the most obvious adverse environmental impact of surfactants would be cases of visible foam. But the more serious impacts should be evident to anyone who has opened their eyes in soapy water, or when shampooing. Has anyone interviewed a fish on this subject?
Issues of toxicity and persistence have been raised in the case of non-ionic surfactants (Hamm, Göttsching 1994). Nonylphenol-ethyoxylate products have been replaced, especially in Europe, due to concerns about their toxicity (Swann 2000). Linear alkyl (or alcohol) ethoxylates have taken their place in many applications. Though the latter are not regarded as toxic, the saturated alkyl chains tends to make them poorly biodegradable (Swann 2000). Perhaps the next logical extension is to use unsaturated aliphatic (alkenyl) poly-ethers. Alternatively, perhaps the most economical solution is to do a better job at removing surfactants before effluent water is discharged.
Chelating Agents: The most common function of chelating agents such as diethylenetriaminepentaacetate (DTPA) in papermaking is to keep certain metal ions from interfering with peroxide bleaching of mechanically defibered pulps. Strictly speaking this is not an issue of wet-end chemistry; usually the pulping and bleaching operations are regarded as separate from papermaking. That matter aside, the problem with chelating agents is that they resist biodegradation (Göttsching 1993; Hamm, Göttsching 1994; Reinbold 1994). The potential adverse effect of persistent chelating agents follows from their likely tendency to interfere with natural uses of calcium and other metals in aquatic organisms. Since peroxide bleaching is often used for recycled pulp, especially when it contains mechanical fibers, there is active interest in finding biodegradable alternatives to chelating agents. One approach is to use sequesterants such as silicates. In layman's terms, a sequesterant is something that binds objectionable metal ions less efficiently than a chelating agent, but enough to permit peroxide bleaching. Since the byproducts of peroxide bleaching tend to be non-toxic, it would be highly beneficial to find other ways of increasing its efficient use in pulp mixtures that are likely to contain manganese, iron, and other divalent transition metal ions.
While on the subject of metals, it is worth considering the environmental consequences of heavy metals in effluent from paper mills. In the past there were concerns about heavy metals in various printing inks. As noted by Göttsching (1993), papermakers have to work with their associates in publishing and converting companies to avoid contaminating the waste fiber supply with persistent hazardous materials. D'Souza et al. (1998) observed that between 75% and 100% of various metals entering a paper mill system by way of waste paper were removed as a component of sludge. However, the levels of metal in the sludge, and also in the product, were both below the level of concern.
Part 3 - A Vision for the Future
"I don't know what they do in those buildings next door. They seem to do a lot of business and process a lot of waste materials. They seem to ship a lot of product. They always keep their lawn mowed and the people are always polite." My vision for the future paper mill is that it should be "invisible" in terms of its effects on the environment. Neighbors, from urban people to rural fish, ought to hardly notice its presence. The goal of Part 3 is to consider what kinds of wet-end additives and related processes are likely to take place in that paper mill.
Fig. 5. Paper technologist thinking of word "chemistry."
Fig. 6. Using the other dictionary definition of "chemistry"
A lot of effort and capital goes into the production of fibers from wood, as well as from alternative fiber sources such as sugar cane residues (bagasse), straw, and cotton. These are renewable resources. When managed properly, every tree that gets converted into pulp for paper products gets replaced by new planting and new growth of trees or other fibrous materials. Actually, the situation is even a bit more complicated than that. Rather than using all trees cut from the forest, the paper industry gets much of its wood fiber in the form of used paper an waste from lumber mills and related operations (Smith 1984; Kramer, Jurgen 1998).
Even before one considers the effect of papermaking additives, plant fibers already have the following highly desirable attributes: a) they easily bond to each other without needing any glue; b) they can easily be redispersed in water and formed into recycled paper; c) they do not originally contain toxic materials; and d) after they have become too degraded or contaminated to be worth recycling, they still can be used for energy generation (Göttsching 1993; Delefosse 1993; Norris 1998; Weigard 2001). By using fibrous waste products to fuel power boilers at the paper mill it is possible to displace some of the need for fossil fuels and also reduce landfill requirements. Landfilling of paper products can result in production of greenhouse gases such as methane (Wiegard 2001), so it makes more sense to use waste wood products for fuel and leave more petroleum, natural gas, and coal reserves in the ground.
Having said all these nice things about plant fibers, especially those from wood, one of our high priorities as an industry ought to be aimed at preserving their quality and in continuing to use renewable plant fibers as the main component in our products. Calcium carbonate is known to inhibit aging of paper by buffering the pH in the alkaline range. By contrast, acidic paper tends to become embrittled during drying and storage, and the cellulose molecules gradually suffer hydrolysis (McComb, Williams 1981). In addition to its beneficial buffering ability, calcium carbonate may be preferred over clay products due to a relatively high purity of its deposits, so that mining of CaCO3 generates less volume of "pits" in the ground and "piles" of tailings (Webb 1993). In cases where sludge from treatment of paper mill wastes is used for compost, the calcium carbonate provides useful pH buffering.
Mineral fillers, though abundant, are non-renewable, so there seems little point in trying to load up paper with high percentages of calcium carbonate, beyond what is needed to achieve opacity and smoothness specifications; rather it has been suggested that papermakers ought to concentrate on achieving high smoothness and covering the paper with relatively thin layers of mineral-based coatings (Lindström 1994; Swann 2000). In that way any printing inks are likely to adhere to the coating materials and the fibers can be more readily recovered "clean" when the resulting wastepaper is de-inked and recycled.
A recent project at North Carolina State University has involved efforts to minimize or compensate for loss of bonding ability of kraft fibers when they are dried (Zhang et al. 2000). The vision that comes out of this type of work is that fibers ought to be treated gently during each cycle of papermaking. One of the key strategies in this regard is to avoid excessive drying temperatures or very low moisture contents, i.e. "over-drying" (Pycraft 1980). It also is recommended to avoid excessive energy or intensity of pulp refining (Baker 1995). In this regard, dry-strength chemicals such as cationic starch can help to achieve strength objectives with moderate savings in refining energy. Our recent work indicates that the proportional effect of dry-strength additives added to never-dried pulp may be greater when the fibers are recycled, compared to their effect on the initial paper.
In a paper mill of the future I envision that not only fiber, but also water, is handled as a precious resource. Future mills are likely to be choosing between the following two alternatives: a) continue the gradual trend of many years towards operation with less and less fresh water per unit of product (Springer 1978; Swann 1999); and b) operate with zero or very little discharge of liquid effluent - in a so-called "closed water cycle" mode (Pietschker 1996; Wiseman, Ogden 1996). In either case I envision that paper mill operations will increasingly turn to their own wastewater treatment systems as a source of "fresh" water. The logic is as follows: Some level of treatment is required even of "clean" water from rivers or springs to remove sand, humic acids, and to control microbes. On the other hand, a paper mill will already have expended considerable effort in purifying the wastewater; it may be cleaner in some respects than untreated "fresh" water. In fact, some system already in place to condition white-water for internal re-use are very similar to conventional primary clarification of wastewater (Sugi 1997).
Not only are future paper mills likely to reuse some of their wastewater, but it appears likely that some of them will essentially "bring the wastewater treatment plant into the paper mill." The motivation for this trend is a need to control the build-up of biological oxygen demand and colloidal materials - a probable consequence of increased recycling of both fibers and water (Pietschker 1996; Zhang 1999). Successful applications of this type of technology have been reported (Delefosse 1993; Norris 1998). In some cases it is possible to justify the cost of such processes as ultrafiltration (Norris 1998) and ozonization to purify water to be reused in papermaking, whereas the same treatments would be considered too expensive if the wastewater were to be discharged (Demel, Kappen 1999). Combinations of aerobic and anaerobic treatment have been recommended to minimize the volume of sludge (Göttsching 1993; Demel, Kappen 1999). Compact reactors for biological treatment may make sense in terms of minimizing the volume of water as paper mills begin to incorporate these operations as part of their system (Tenno, Paulapuro 1999; Gubelt et al. 2000). The most important attribute of paper chemicals, in order to be compatible with the biological treatment systems just described, is biodegradability. Some progress has been made in this area (Hamm, Göttsching 1994; Wahaab 2000), but much more work is needed.
Another essential part of efforts to reduce water usage is to select combinations of additives that tend to "self-purge" themselves from the system by becoming retained on fibers. In principle that implies avoiding substances like simple salts, sugars, and oils that have little affinity for fibers, even in the presence of coagulants or retention aids. In isolated cases it may make sense to remove excess salts by evaporation or reverse osmosis (Wigsten 1995; Norris 1998; Tenno, Paulapuro 1999). In principle it is possible to maximize the retention of both colloidal and fibrous materials in paper by control of highly cationic additives to achieve near neutral zeta potential (Bley 1992; Moormann-Schmitz 1994), followed by very-high-mass flocculants to collect primary particles into a particles large enough to be mechanically retained (Horn, Linhart 1991). It is remarkable the extent to which these principles parallel those used in treatment of fresh water and wastewater (Leitz 1993).
"Frugal of Energy and Raw Materials"
I envision the ideal paper mill of the future as being frugal in terms of energy and raw materials. Losses of fine materials can be reduced to very low percentages in the paper forming process by use of an effective retention aid program on a paper machine, plus the use of a saveall to recover fine material from white water. Closing up the water system it is possible to conserve heat (Springer 1978; Wigsten 1995). Hot water promotes more rapid drainage, and extra heat energy has to be supplied to the extent that fresh water is used. In these respects the paper industry already seems to be doing a very good job.
Though the paper machine tends to be frugal, relatively large amounts of fiber fines, fillers, and fibers can be lost when paper is recycled (Paula, Kärnä 1995; Dorica, Allen 1997; Kantardjieff 2000). It is likely that much of such waste consists of ink, colloidal materials, and fiber fines too small to be of much value in papermaking. However there may be opportunities to recycle the mineral content of waste paper or of sediment in the clarifiers at paper mills. Studies have shown that it is possible to "burn off" various organic materials and recover gray filler particles that are useful for paper products with intermediate brightness targets (Sohara, Westwood 1997; Johnston et al. 2000; Moilanen et al. 2000; Wiseman et al. 2000).
Further savings in energy, per ton of paper, are likely to come in two areas. The use of chemicals to promote dewatering and reduce the need to evaporate water was discussed already in an earlier section. It is possible that further savings will be achieved by reducing the amount of water that needs to be pumped. Said another way, it will be possible to save electrical energy expended at the fan pump by increasing the typical consistency of headbox furnish. Higher-consistency forming has been considered in various publications (Case 1990; Waris 1990). Already, modern headbox designs have been helpful in being able to still form uniform paper with slightly less water (Kiviranta, Paulapuro 1990). But it seems that chemical additives will be needed to that minimize fiber flocculation at the higher solids levels. Conventional "formation aid" strategies can have a devastating effect on drainage (Wasser 1978; Lee, Lindström 1989). This is an area of wet-end chemistry that may become important in the future.
It seems that no vision of the future ought to be complete without the words "high tech." In terms of papermaking chemicals, the key "high tech" trends to look out for will include automation, new sensors, bio-engineered processes or additives, and nano-technology. Recently it seems that nano-technology is a growth area for research. In fact, papermakers have been involved in nano-technology for many years. How big are the colloidal silica "microparticles" used in drainage-enhancing programs? The answer is "usually about 1 to 5 nm" (Moffett 1994; Andersson, Lindgren 1996; Swerin et al. 1996). So, in fact, we already use nano-technology.
Bio-tech solutions are recently becoming important in the use of enzymes for deposit control and slime control (Webb 1994). Enzymes also can be used to reduce the cationic demand of process water, especially in cases involving thermomechanical fiber (Buchert et al. 1996). In the future we can expect to see more progress in the use of enzymes to assist with strength development and to promote more rapid drainage (Eriksson et al. 1997).
The large-scale, continuous, capital-intensive nature of papermaking operations make them attractive subjects for improved process control strategies. The last couple of decades have brought substantial progress in the development and implementation of tray-water solids sensors (Bernier, Begin 1994; Artama, Nokelainen 1997). These have made it possible to even out swings in first-pass retention by varying the addition rates of retention aids. However, in at least one case it was shown that the demand for retention aid was strongly correlated with variations in cationic demand of the furnish (Tomney et al. 1997). Therefore it makes sense to control cationic demand as well, with a goal of getting closer to the root cause of the variations. Significant progress has been achieved in online charge control, especially with automated streaming current titrating devices (Tomney et al. 1997; Gill 2000; Rantala, Koskela 2000). In principle the same type of data can be obtained more accurately and reliably by a new streaming potential titration method (Hubbe 1999). Other devices that are likely to become more common, especially in large papermaking facilities, include automated freeness testers (Lehtikoski 1991), online evaluation of fiber flocculation (Wågberg 1985; Alfano et al. 1998; Hubbe 2000), and automated monitoring of bio-films related to the growth of slime deposits (Robertson, Rice 1998; Dickinson 1999; Flemming et al. 2000).
Earning the "Good Chemistry" Label
The goal of "good chemistry" with the public, with our clients, and with our investors will require a long-term approach. We have to face the fact that papermaking is a highly capital intensive enterprise with long lead times for new construction and replacement of existing facilities. We cannot expect to keep up with every new change in focus of environmental issues (Vasara 2001). The challenge will be continue to make meaningful, practical improvements in our practices affecting the environmental even through changes of issues and economic cycles.
So what about the "chemistry" between the paper industry and the public? We need to encourage an atmosphere of "working together" on environmental issues for the sake of long term progress is exemplified by a Wisconsin initiative to create a private-public partnership (Schmidt 1998). A focus on headlines sometimes can lead to a view that our society is highly polarized around issues of environmentalism versus profitability (Reinbold 1994). However, a more cautious analysis of public opinions reveals that the bulk of the American public tends to see issues in a much more balanced light, compared to their politicians (Wolfe 1998). Technical people in the paper industry have a responsibility to be environmental advocates. Some worthy environmental goals for papermakers include a) continuing to rely mainly on a renewable, recyclable resource - wood fibers, b) taking steps to avoid deforestation - by replanting, recycling, and avoiding waste, c) minimizing water pollution by careful development, selection, and use of wet-end chemicals, and d) minimizing energy use. As noted by Siekman (1998), some environmentally sound practices can be profitable, in addition to their intrinsic benefits.
No, it probably wouldn't do much good to place an ingredients label on each sheet of paper, or even on each ream wrap, carton, or jumbo roll. But already there have been proposals to label certain products as "eco-friendly" (Rogers 1993). For instance, such labels could be awarded by an independent agency based on a point system, with part of the score coming from such issues as wet-end chemical practices, bleaching practices, or the amount of energy used in the life-cycle of a product. Ideally this ought to be a voluntary system, something like the ISO certifications of paper mill practices. In that way, paper companies will have the incentive to get their products certified so that they have the right to label their products as "eco-friendly."
Achieving "good chemistry" on the paper machine and with the public will require more than good intentions. It will require significant technological input and a long-term commitment on the part of us in the industry to continue to make the needed progress.
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