PAPERMAKING
New and emerging papermaking technologies are allowing P&W paper mills to improve sheet properties and cut production
Paper Machine Developments Open Door to Higher Quality, Production Efficiency
By Ken L. Patrick, Senior Editorial Consultant
This second part of a three-part series on papermaking trends in North America and Europe looks specifically at the paper machine wet-end, pressing, and drying sections. It explores new and emerging headbox developments, wet-end fillers/chemistry, shoe pressing, and advanced drying technologies. It also includes separate sidebox reports on recent advances in paper machine clothing and cationic starch applications.
Part 1 in the May issue of Pulp & Paper examined historical practices and papermaking technologies in Europe and North America, focusing on how and why they differ and what can be expected as the industry moves into the twenty-first century. It examined changing global markets and production trends, coating trends and practices, developments in the coating pigment areas, and emerging specialized grades.
The final report in this series will explore current and future limitations on paper machine speeds and widths, the present and future of fourdrinier formers, soft nip and other calendering developments, the impact of fiber recycling and a changing global fiber stream on papermaking processes, and automated grade changing.
WET-END TRENDS, HEADBOX DEVELOPMENTS. A continuing trend in literally all board and, more recently, in many paper grades, is “lighter, thicker, and stronger,” according to Ron Hunt, technology manager–paper, at BE&K Engineering Co. Almost all grades are getting to be “weight sensitive,” he points out, i.e., “bulk with low basis weight.” Today, the operating principal of linerboard is “as thick, light, and strong as possible. We will see this in all grades eventually,” Hunt believes.
Larry Chance, vice president, research and development at Beloit Corp., points out that the continuing move toward lighter basis weights creates a “vicious circle” of events, especially as more recycled fibers enter the furnish stream. For example, as basis weight goes down, strength and opacity decline. To boost opacity back up, the tendency is to use more fillers, which further weaken strength. To increase strength, mills tend to add more long-fiber virgin furnish, which increases costs and can harm bulk and opacity if substituted for mechanical fibers and can reduce surface and printing quality if directly replacing hardwood content.
In addition, the trend toward ever-faster paper machines also complicates and compounds problems at the wet end. But Chance adds that certain developments in paper machine design, including the use of shoe presses in graphic paper production, are helping to offset some of these basis-weight and speed-related problems. Also, recent developments in headbox and paper machine former designs, together with advances in filler and retention/drainage technologies, are providing some flexibility in this regard.
On the subject of machine speeds, Markku Karlsson, executive vice president, research and development, Valmet in Finland, explains that with lower grammages, new furnish and machinery solutions must be developed. “Increasing speed and maintaining efficiency in the 90% range is a must,” he says.
Doubling the machine speed (as has happened in the past 30 years) has quadrupled the headbox internal pressure, Karlsson continues. “In the process, this has increased the demands on headbox structural integrity, dimensional tolerances, and flow stability. To preserve these at higher speeds could mean that curved surfaces may have to replace flat surfaces to sustain higher pressures.”
Out of necessity, this requires new hydraulic designs, according to Karlsson, which may well be required to handle higher turbulence levels and the resultant instabilities this may create at the slice, assuming stock consistencies remain at their present low levels. “Stratification, or layering, may gain higher prominence once difficulties with formation, layer purity, and contamination of whitewaters have been resolved and once a need for such grades, together with their quality and economic advantages, are clearly demonstrated,” he says.
Chance emphasizes that the trend today is definitely toward dilution-type headboxes. In older headbox designs, he explains, the slice lip had to be bent in various places to control basis weight. The newer dilution-type units offer a significant advantage over the slice-bending headboxes, providing a much wider range of operations. For example, with board grades the headbox had very wide slice openings, but the slice lip could be bent only a small amount, relatively. So it had very little effect on basis weight.
The dilution headbox, however, can have a 10%+ effect on basis weight, Chance says, particularly on the heavier weights. The basis-weight profiles are also much better with dilution headboxes, with more discrete profiling—down to 35 mm on certain headboxes. By comparison, with older headboxes, actuators are placed from 75 mm to 150 mm apart.
In addition, with today’s dilution headboxes, the basis weight and fiber orientation are basically “decoupled,” according to Chance. “Before, when you bent the slice lip, you not only affected basis weight, but you also affected fiber angles. The dilution headbox keeps the slice lip very straight. Also, response times are significantly faster with dilution headboxes. This is very important when changing grades on a machine. You can change over much faster and get on-grade with an excellent basis-weight profile very rapidly,” he points out.
Gregory Wedel, director, product management at Beloit, adds that headbox stability and not just response time is considerably better with dilution units. The cross-direction differences between zones has been significantly reduced, and the stability of control has increased accordingly. “We believe that the days of slice-bending headboxes are about over,” he concludes.
Bill Paxman, of Sandwell’s engineering and construction services group in Atlanta, Ga., agrees that the most significant headbox developments in recent years have been the move from aid-padded to hydraulic and the use of consistency dilution. “These are fairly significant advances, and the rest are more or less variations of these. To warp the lip means getting all of the mechanisms in a very close gap. If you don’t have to do that, the lip can be made much stronger.
“We really haven’t had a lot of feed-back about how consistency control is working,” he reports. “In theory, certainly it should work. There may be some control problems in the injection of dilution water—where and how it’s put in. Controlling turbulence and getting proper mixing might still be lingering problems. Perhaps more attenuation will be required as we go to higher speeds, because we’re now talking about much higher pressures. With hydraulic units, any air entrainment or variations in the circuit will show up more dramatically. But once these potential problems are overcome, consistency control has to be a better method than warping the lip—it just has to be.”
Don Lee, also of Sandwell’s engineering and construction services group, adds that “variation” is a key word, because with the twin-wire and gap former units being used today, what comes out of the slice is “more or less what you’ve got from there on.” To a large degree, the sheet is virtually frozen at that point, so “it’s vital that what comes out of the slice is exactly what the papermaker wants.”
The major driving force in the forming area, Hunt concludes, is “high speed.” He also agrees that the most significant paper machine developments “in the past 50 years” have been dilution consistency headboxes and shoe presses. “And these have been very positive developments for the industry,” he says.
In general, Karlsson states, quality and economic considerations aside, future paper machine wet ends that operate at very high speeds will have to contend with a number of new issues—huge volumes of moisture-laden air pumped around by machine clothing and rotating rolls, generation of unacceptable noise, mist, and dust, and higher use of water, chemicals, and energy—all of which will have some environmental impact.
“Solutions to these problems will require new technologies and higher levels of process automation. The most immediate issue is how to reduce the use of freshwater without upsetting the operation,” he states.
STRATIFIED FORMING. Stratified or layered headboxes, as Karlsson mentioned, may play an increased role in the future of papermaking—after some key operating problems are worked out.
Herrin Cabe of Sandwell in Atlanta explains that, initially, stratified headboxes were intended to augment the effectiveness of secondary headboxes on board machines by allowing inferior fibers to be placed in inner layers and the secondary headbox to be used for maximizing better fibers on the outside printing surface. The technology then found its way into tissue, he explains.
Paxman adds that on heavier weights “you can use the stratified headbox as you would an I-beam. You put strength and quality on the two outer layers and the middle layer is used mainly for bulk—to keep the two outer layers apart.”
Wedel of Beloit explains that stratified headbox technologies date back to the 1960s. “It quickly grabbed the attention of the board and tissue industry, but not much between. During the past 20 years, we have been trying to close that gap between board and tissue, and I think the entire industry is expecting that to happen. But, as yet, only a very small number of printing and writing paper mills have even tried it,” he says.
Lee of Sandwell adds that “there are advantages to stratified forming, but the control situation is always questionable. Migration between layers can be controlled fairly well if a mill’s control system is stable enough to handle it, and many of the newer DCS’s can,” he believes.
Frank Ludovina of Sandwell explains that stratified headboxes may be particularly applicable with twin-wire/gap former machines. “If you’re going to set a sheet very quickly and you want to have any layering capability, the stratified headbox seems to be the only viable option. If you’re not going to set the sheet quickly, the stratified headbox loses a lot of its potential, because the plies are going to intermingle and you’re pretty much forced into having a secondary headbox,” he says.
“Theoretically, a mill should be able to engineer a sheet using a layered headbox—to put ash wherever it’s needed for maximum printability, bulk fiber in the center for better stiffness, etc.,” Chance of Beloit states. “All of that is possible today to some degree. But the stratified systems are much more complex, with three separate fan pumps, stock approach systems, etc. It’s especially difficult to retrofit this kind of technology, and to justify putting in such complex systems on an existing fine paper machine. Companies struggle with the costs involved.
“With board machines, these separate systems are already there. It’s just a matter of replacing secondary or tertiary headboxes with a stratified unit. With tissue, it’s a quality consideration—to get perceived softness and ‘drapability’ by putting different fibers on the surfaces. For fine papers, the key will be in the stock supply systems, and some promising work is currently going on in this area.
“Stratification technology is not as good today as we would like it to be. Even though we are seeing some differences between the top and middle layers, we would prefer a 90% to 95% ply purity. Also, if fibers in the ply furnishes are of different colors, a lot of basis weight is needed for cover up, to prevent streaking. So mills really need to run furnishes of similar color, especially with fine papers,” Chance explains.
Both Chance and Wedel feel that an engineered fine paper sheet is viable in the future. “It’s possible to preferentially place various minerals, chemicals, and fibers throughout the thickness of a sheet to obtain desired characteristics. And I believe we will see more of this in the future. Layer purity will definitely be a driver, but I don’t think it will be a technological limitation. Economics has been the primary impediment so far. The value in terms of sheet quality exists, but the up-front costs have not been worked out yet,” Chance concludes.
WET-END FILLERS. The use of alkaline fillers in fine paper grades, both precipitated (PCC) and natural ground (GCC) versions, has increased dramatically in recent years—especially in Europe, but also in North America as the “alkaline revolution” continues. While North American mills are typically in the 10% to 15% filler range, some free-sheet mills in Europe, where bleached fiber costs are still considerably higher than in the U.S., are using fillers in the upper 20% range, or even higher.
Hunt of BE&K says that some mills in the U.S. are running near 20%, but doesn’t think usage will increase much beyond current levels. “It was done here primarily for quality reasons. If it wasn’t for the quality gains, I doubt that North American mills would have bothered with it.”
Chance reports that while 10% filler levels might be common in the U.S., “I know of at least a few that are running around 16%.” One North American machine, he says, is running an ash content of 16% and reportedly does not go higher because of the loss in stiffness.”
As far as calcium carbonate use goes, “it’s still basically a GCC coating and PCC filler scenario, but we do see the wet end changing over the short haul,” Harry Bigelow, vice president of technology for Omya, points out. “Today, we see more blends of PCC and GCC in the wet end, and we expect that to continue.
“What we have found is that the papermaker can improve drainage with GCC in the blend, which can translate into increased machine speeds. At the same time, bulk and stiffness are maintained with the addition of GCC, and formation and tensile strength can be improved as well, compared with a 100% PCC filler,” Bigelow explains.
According to Bernd Balzereit, market development manager–paper for Omya, mills have found that no one particular filler is optimal for their entire grade line. Filler blends of PCC and GCC have become commonplace in uncoated free-sheet. “Today, many mills are looking at various filler blends and are also investigating higher filler levels, which favors the use of mineral combinations at the wet end,” he says.
Warren McPhillips, vice president of sales and marketing for Omya, explains that in the wet-end area, there are really two mineral filler scenarios—uncoated free-sheet and coating base stock. “GCC is being used extensively in coating base stocks, especially in the 60-lb to 70-lb weights and above. In uncoated free-sheet grades, mills are tending to use blends of PCC and GCC, the proportions again depending on specific grades.
“GCC is allowing mills to increase ash levels with improved drainage characteristics. Typically, larger mills have been limited to 12% to 16% ash levels, but with GCC some of these have been able to go up to 24%. It’s basically a filler-for-fiber replacement situation with uncoated free-sheet,” McPhillips emphasizes.
In the LWC free-sheet areas, very little virgin filler (2% to 3%) is being used in the wet end, according to Bigelow, especially as weights go from 40 lb to 45 lb. Almost all of the filler in these grades comes from the coated broke, so it really goes back to what pigments are being used in the coating formulation. But, 10%-filler content is “bumping the ceiling” in these grades, he says.
Today, most groundwood mills in Europe are using carbonate in their coating mix—in the range of 40 to 70 parts—because of the quality advantages it affords, according to Balzereit. Thus, they are running their wet ends under neutral or alkaline conditions, he says, adding that the same quality reasons are also driving conversions in the U.S.
“Some mills producing very lightweight coated groundwood grades, 32-lb and below, are also beginning to look at GCC on the coating side. At least from our perspective, the coating pigment is definitely driving a lot of the wet-end alkaline conversions,” he explains.
The use of carbonate in groundwood grades actually began in North America in the early 1990s, according to Bigelow, but conversions in Europe date back much earlier. “Typically, there’s a 5- to 10-year span between successful technology changes in Europe and their application in North America,” he says.
To compensate for strength loss in these LWC base stocks, as minerals and recycled fiber content increases, most mills in North America “sweeten their furnish with long virgin fibers, as they more or less have always done,” Bigelow explains. But, generally, this happens at the expense of mechanical fiber, with corresponding decreases in opacity.
“Because of our wood basket in North America, sweetening has been the common practice here. Northern mills, for example, sweeten primarily with spruce and fir, but as they begin to deplete these species in some areas, they will have to use alternative species. So, because of losses in bulk and opacity and because the fiber base is changing in North America, sweetening may not be the long-term answer,” Bigelow emphasizes.
Mills in Europe either don’t sweeten, or do so to a much lesser degree, because their processes are designed differently, he continues. European paper machines, for example, are designed to run at higher filler levels. They tend to be more “closed,” i.e., there are no open draws on the paper machine. From the headbox through the former, press, and dryer sections, the design is specifically for these levels. The coating operations are also much gentler on the sheet than in North America. “We put much more stress and strain on the sheet than European mills do,” he explains.
“But it’s more than just the paper machine design and coating operations,” he adds. “Mills in Europe pulp and bleach specifically for higher fiber strength. By comparison, their mills are able to use up to 80% mechanical fiber in LWC grades, on the average, whereas North American mills are averaging only about 50% to 60%,” he concludes.
GAP FORMERS/MULTI-WIRE UNITS. Also cited among the industry’s more significant papermaking technology developments in recent years, along with dilution headboxes and shoe presses, gap formers have opened the door to new production standards worldwide.
Karlsson of Valmet strongly believes that gap formers will become the dominant formers for most grades in the future. Further development of these units, he says, will continue to revolve around increased dewatering capacity, solids retention, and improved web properties and profiles in its three principal directions.
This control, he maintains, must be effective over an increasingly smaller area of the web, approaching that of an individual fiber. To achieve such small scale control will require further developments in fabric uniformity and in the former’s dewatering elements, in combination with improvements in headbox jet stability and control. Ultimately, the headbox and former will be integrated into one composite unit to achieve this, Karlsson believes.
With the wide range of multi-wire units and gap formers in operation today, Wedel of Beloit believes that “once again, we’re pushing the open wire to its limits.” Top wire units have moved closer and closer to the headbox, he says, and now gap formers more or less abut the headbox directly. The roll-blade-type formers, the counter-blade units, etc., are all extending the range of application today.
“In some respects, with blade forming—the Bel Baie type of formers—we still have the entire forming zone to tweak and turn and fine-tune the formation. And with counter blades, we’ve added yet another tool for controlling formation—various suction/vacuum boxes, etc. Unlike a single-roll former where you just take what you get, we have retained the ability to do some tailoring.
“But I would classify these developments as evolutionary in nature—incremental steps from the groundwork laid many years ago. But currently some new revolutionary-type developments are being explored that can bring dramatic change to the industry,” Wedel states.
But certainly compared with a fourdrinier table, a gap former “does tend to freeze the sheet,” Chance agrees. “However, on the other hand, the gap former’s single profile is going to be better than that for a fourdrinier.” A major concern with gap formers is going to be forming fabric stability as machines are pushed to higher speeds (2,000+ m/min), he maintains. “These wires will have to accept all of the momentum of the jet at these speeds, which tends to lift the outside wire off.”
Wedel adds that the challenge will be not only mechanical stability, but “profile stability” as well. “Permeability/ stability—the ability to maintain cleanliness at high speeds—will also be critical. The number of times you have to shut down, take the sheet off of the machine, and wash up can have a very detrimental effect on machine efficiency. A lot of fabric work will be needed in the future,” he emphasizes. Clothing considerations with gap formers and shoe presses are discussed in more detail in the sidebox on page 72.
Wedel further explains that, basically, the turbulence of the gap former jet is increasing at velocity squared. This high degree of turbulence can lead to sheet defects, and can literally blow the sheet apart. So, most likely, the length of the free jet will have to be minimized in the future. Dilution headbox technology will possibly help in this regard, because there won’t be a need for “slice benders” extending into the forming zone, he says.
As an aside, Paxman of Sandwell points out that machine vibrations caused problems with some of the early gap formers. When Black-Clawson first introduced the Vertiformer, he explains, the headbox was on top of the structure and squirted downward. It was soon discovered, however, that even small vibrations in the wet-end structure impacted the jet. They then put the headbox on the floor so that it squirted the jet upward. “But I wonder how soon it might be before vibrations in the headbox itself, at high speeds, will begin causing disturbances in the jet,” he asks.
SHOE PRESS TECHNOLOGIES. Today, shoe presses are standard for most board grades and are rapidly becoming standard for many paper grades, according to Karlsson. Currently, they operate on newsprint, SC, LWC, and fine paper machines. The reason for their wide acceptance, he says, is their superior water removal capability without crushing and their ability to improve paper properties (notably two-sidedness), permeability, and bulk.
Shoe presses, in combination with the new anti-rewet felts specially developed for dewatering and for web transfer applications, are already designed to convey the web from former to dryer sections with no open draws, Karlsson explains. “This has eliminated the last press-related bottleneck, namely further increases in speeds.”
Wedel reports that Beloit has installed some 20 Extended Nip Presses (ENPs) on graphic paper machines, and estimates that the total number of fine paper installations, including shoe presses supplied by other builders, is between 40 and 50. “It has taken from the 1970s, when the first ENP was introduced, until the early 1990s for this technology to finally make it into the fine paper arena.”
Wedel adds that, putting all paper machine drivers into perspective, “there really are only three: quality, production, and cost per ton.” The main one, of course, is quality. “If you don’t have quality, there’s not much reason to even consider the other two.” But once a mill reaches a certain quality benchmark, he continues, it can sell what it makes. Then and only then does production (efficiency x width x speed—discussed in more detail in Part 3 of this report in July’s Pulp & Paper) and cost per ton become more critical.”
The capital cost of shoe presses can be quite high but the cost per ton can drop significantly, because much more water is being removed, Wedel stresses. “If a machine does not have sufficient dryer capacity, then production can be improved. But most publication paper machines do have sufficient drying capacity. So the only remaining criteria is cost per ton, which has not been a large enough driver in North America, in terms of the capital cost required, until fairly recently,” he explains.
Chance points out that, in the past, it was generally thought that shoe presses would not have much application on bulk-sensitive grades, because they tend to densify the product. “With bulk-sensitive grades, mills want as much caliper as possible to maintain stiffness, and fine papers generally fall into this category.
“But in recent years, studies have shown this not to be the case. In fact, mills have been surprised to find that, compared with roll presses at the same dryness, they could actually get more bulk or caliper with a shoe press—just the opposite of what was anticipated. Once this was discovered, the use of shoe presses on woodfree grades began escalating.”
Wedel notes that in regard to rebuild opportunities, for example, a mill may have a machine with sufficient drying capacity but it wants to remove some of the dryers and install a film coater. This mill may find that a shoe press is a very advantageous option. It can make up for the one-third (or so) of drying capacity lost to the online film-press installation, he explains.
PAPER MACHINE DRYER SECTION. Generally—other than single-tier drying—not many new developments have occurred in the dryer section in recent years, at least until recently. Impulse or impact drying does offer some potential, but the technology has not progressed significantly during the past decade.
Hunt of BE&K says that one of the major drawbacks of these technologies is energy efficiency. “It looks good on some special applications, especially on older machines with production problems. But, eventually, this could be one of the industry’s biggest developments. I believe that in the future the worldwide paper industry will have to run more like the tissue business—using methods and technologies that don’t damage the sheet,” he says.
According to Karlsson of Valmet, the dryer section is the longest, heaviest, and until recently, “the most ignored part of the paper machine.” Today, he adds, a number of dramatically different drying technologies are being developed, ranging from conventional steam heated systems to high-intensity systems such as Condebelt, impulse drying, and impingement drying. Some of these have already been commercialized.
“The push for new drying concepts stems from both quality and economic reasons. Only recently has it been fully appreciated that many of the web’s important properties are developed during drying. Therefore, the dryer should be designed not only to dry the sheet, but to impart desired properties.
“The key economic consideration is how to dry the web in the shortest possible time (reduced machine length) at the lowest possible energy cost. Since the latter differs among regions, this will be reflected in the designs of future dryer configurations,” Karlsson believes.
There is a wide range of “things” between current single-tier and the potential for the future, according to Wedel—“things that run the gamut from pressing within the dryer section to breaker stacks on dryers, to what you might call impact drying, all the way up to extreme impulse drying.” Today, none of these have achieved much commercial success, he adds, mainly because they are revolutionary in nature and require an enormous amount of design and development work.
“The Beloit group is focusing heavily on impulse drying, which is on the upper end of this family of new drying concepts. We’ve been developing this concept for 8 to 10 years, working with various other companies and institutions. Some of the problems that have to be solved include high-temperature fabric manufacturing, roll covers, systems for generating the high surface temperatures needed, and development of the paper product itself, because the product, at least initially, will be different than conventional paper products,” Wedel reports.
Chance adds that “one of the biggest problems we still face” is that of water boiling in the sheet at the extreme temperatures of impulse drying. When the sheet goes into the nip, he explains, it is heated to a temperature above the boiling point of water. Under elevated pressure within the nip, the water does not boil, but when it exits the nip into atmospheric pressure, it can quickly boil and cause sheet delamination problems. “We have some ideas that hopefully can solve the problem within the next five years,” he states.
Cabe of Sandwell points to air cap technology as a possible key drying development in the near future. Using a gas-heated aircap and then reusing the waste heat from that gas for pocket ventilation air has considerable potential, he explains, especially with single-tier dryer systems. “It’s maybe five years away from commercial application, but I believe it will happen,” he says.
Cationic starch update
European papermakers have been using cationic starches in the wet end and the size press for many years, and the practice is now spreading rapidly throughout North America. The switch to cationic starches in Europe was environmentally motivated to some degree, at least initially. But mills there quickly discovered other production and quality benefits.
AT THE SIZE PRESS. Advancements in size press technology and the continued expansion of uncoated paper capacity in recent years has had a major impact on the consumption of starch at the size press. Introduction of the metering size press, in particular, has given papermakers a reliable tool to optimize starch pickup and penetration, depending on end-product demands.
The choice of starch at the size press is dependent on a myriad of factors, including economics. Both unmodified and modified starches have reportedly provided similar performance in conventional as well as metering size presses at equal levels of addition. But, generally, the trend in recent years has been toward modified starches.
As Mike Ducey, vice president of sales and marketing at Cerestar International, explains, the use of cationic corn starch in European size presses “began as an answer to concerns about BOD (biological oxygen demand) discharge from paper mills.” He points out that non-ionic starch had been a major source of BOD in the effluent discharges from these predominantly non-integrated operations, and that environmental pressures forced a change toward cationic starches beginning in the 1980s.
Operating experience at European mills has “clearly shown” a 50% or better reduction in BOD with cationic starches at the size press, Ducey continues. It adheres with considerable tenacity to the surface of anionic fibers and fillers, which are typically much higher in European sheets than in counterpart products made by North American mills.
The tenacity of cationic starches also has economic benefits. Significantly less starch is lost to the sewer with reprocessed machine broke, compared with non-ionic starches. Performance-wise, sheets surface-sized with cationic starch are typically stronger and more stable, helping to reduce press picking and dusting in the converting and printing processes, Ducey reports.
He points out that unconverted cationic starches provided the answer to viscosity questions at some European mills. “These mills could have switched to pre-converted starch (e.g., hydroxyethylated), but they wanted to keep their option of cooking in-house to control viscosity to their process specifications.
“Today, some U.S. papermakers are using a pre-converted cationic corn starch at the size press for easy cooking purposes. They are finding that it can be very expensive, and unfortunately, they must also choose from a narrow list of available viscosities. More recently, however, an unconverted cationic corn starch has become available on the North American market, which can be converted in-house to the mill's desired viscosities, as in Europe. And, typically, it is at least 30% less costly,” Ducey says.
WET-END APPLICATIONS. Accelerated growth of the cationic starch market in the U.S. in recent years is partly due to capacity expansion and the conversion to alkaline in the uncoated free-sheet sector, and also the increased use of recycled fibers. Again, the selection of cationic starch for the wet end, as with the size press, depends on many factors, including economics.
Michael Linscott, director-marketing and sales, papermaking chemicals at National Starch & Chemical, believes that in the U.S. more is happening with surface sizing starches than in the wet end. But he points to some important recent advances in wet-end cationic starch technology.
In Europe, he explains, most papermakers are now moving toward the elimination of synthetic polymer retention and drainage systems. These are being replaced with more environmentally friendly and easier to control silica-based micro-particle systems, with a moderately-charged cationic starch added to the thick stock for strength, and a highly-charged cationic starch added to the thin stock.
These highly-charged cationic starches have treatment levels higher than the U.S. FDA currently allows, Linscott adds, but points out that “we expect U.S. mills not requiring FDA approval to be moving in this direction soon. We also expect that the FDA will allow higher cationic treatment on starches within the next year.”
New developments in clothing for gap formers and shoe presses
In recent years, industrywide implementation of gap forming and shoe press and/or wide-nip technologies in both brown, and more recently, fine paper grades has placed increasing demands on fabrics used in these critical sections of the paper machine. Paper machine clothing manufacturers have quickly responded to many of these demands.
GAP FORMER CONSIDERATIONS. As Tom Gulya, vice president of technology, Weavexx, explains, “with brown grades, the setup of the headbox is critical in gap forming.” The papermaker, he continues, has to be very accurate with headbox individual ply rush/drag ratios because the fiber orientation of the outer layers is set immediately.
“The rush/drag ratios appear to be significantly higher than on a conventional fourdrinier,” he says. “But on the positive side, the papermaker has an easier time maintaining good ply bond and stratification. On a typical gap former for brown paper operating at 2,400 fpm, within 0.3 seconds the sheet has reached the same consistency that would require 1.2 seconds to achieve on a conventional linerboard machine. This reduced drainage time means that any flow marks in the fabric will be more evident, and the choice of fabric pattern becomes critical.”
Gap former fabrics must be stable and wear-resistant, according to Gulya. For instance, he says, a typical gap former conveying length is 100 ft, while the base-liner wire of the fourdrinier is 180 ft. The gap former wire makes 1.8 times as many revolutions per minute. Drag loads appear to be similar to those generated by a standard fourdrinier.
He further points out that a comparison of horsepower used by a group of brown paper machines showed a requirement of 0.22 to 0.3 hp/in./100 fpm. “This means that, although the gap former conveying position will not create any worse fabric stretch problems, fabrics will tend to wear faster. There is also evidence that the gap former puts significantly more pressure on the center of the forming fabric, thereby requiring a more stable fabric.”
In addition, Gulya continues, the typical wear pattern on returned fabrics has changed. On a typical fourdrinier, the edges are significantly more worn unless some type of edge protection is used. With the gap former, the edges are hardly worn and the wear profile of the body of the fabric is very uniform.
The gap former, according to Ken McCumsey, product technology team leader at Asten, allowed process hydraulic and vacuum forces to be used to enhance the basesheet. Increased dewatering rates allowed formation optimization with lower consistencies. With gap formers, limitations in adjusting the headbox lip when changing stock or grade became unnecessary.
MuCumsey adds that while gap formers have allowed major operating and quality improvements, they have had a dramatic impact on forming fabric design. For example, surfaces on both sides of the sheet became very critical in balancing drainage and support at higher speeds. Surfaces moved to much finer designs with special weave structures imparting new levels of sheet smoothness.
Fabric rigidity and stability became more critical in the forming zone due to the high level of water removal in a short distance, he continues. “Today, thinner, very high-fiber support fabrics have been optimized for gap former requirements. The combination of suction rolls, ceramic support surfaces, minimum vacuum requirements, and forming zone configurations have added up to reduced drag load, longer fabric life, and a significantly higher level of productivity.”
As gap former speeds continue to increase in the future and furnishes include even more recycled fiber, forming fabric designs will have to be further modified, according to John Craft, technical director, Scapa Forming Fabrics. Major factors that will force improvements are MD and CD fabric stability, paper quality in regard to smoothness and two-sidedness, and fiber and filler retention, he says.
“The basic weave types of the double-layer extra support designs and the triple-layer designs available today will probably fill the role from a design standpoint. However, these designs require new materials that will allow yarn sizes to decrease while improving characteristics needed for stability.
“These smaller sizes will allow more yarns to be incorporated in the structure to provide improved fiber support, which will boost retention of short fibers, fines, and additives. Materials being evaluated today appear promising in filling these needs,” Craft surmises.
SHOE PRESS FABRICS. According to Bob Crook, vice president–product market development, Scapa Press Fabrics, shoe presses are generally characterized by lower peak pressure, which results in a slower rate of felt compaction and belt fiber damage. But increased nip residence time and sheet dewatering can aggravate two-sidedness and other quality concerns.
While the demands of the particular press and paper grade being produced, along with choice of sleeve venting, will be critical to specific press fabric design parameters, some rule-of-thumb criteria Cook lists for felts include the need to: apply a high level of mechanical pressure uniformly to the sheet; keep flow resistance low through the nip; keep recovering after compression in the nip so that excessive hydraulic pressure does not build as the felt ages; and maintain stability.
Gulya of Weavexx reports that typical shoe press loadings range from 450 to 900 kN/m over a nip width of 200 to 250 mm vs 80 to 140 kN/m with a nip width of 10 mm to 40 mm for a roll press. The blankets have been blind drilled, grooved, or plain.
The key issues in traditional pressing, he continues, are nip dewatering, marking, and rewetting. “It should have been no surprise that these are also critical issues for the shoe press.” However, he explains, an additional set of requirements comes into play. The initial felts trialed on shoe presses were designed biased toward experiences learned in pressing linerboard.
These felts were designed with high void volumes and heavy felt weights to carry large volumes of water from the press to be removed at the uhle box. These felts ran wet and made it difficult to achieve the desired speeds. While typical felt weights for brown paper shoe presses start at 1,700 g/m2, laminated felts developed for shoe pressing lightweight sheets are much lighter in weight (1,200 to 1,700 g/m2 range) and are thin to facilitate nip dewatering, according to Gulya.
“The new felt structures incorporate lightweight, thin base fabrics with a smoother top cloth, low flow resistance, and significant load hysteresis to facilitate fast nip dewatering, nip impulse resistance, and minimized rewetting. After development of an optimized felt design, we have seen felt lives ranging from 30 to 60 days at press speeds from 900 to 1,200 m/min,” he explains.
Billy Summer, product and applications manager at Asten, points out that, initially, sheet quality was one of the major concerns with shoe press fabrics. “Press fabric development was taken to a new level for shoe press positions on machines making heavyweight grades. The challenge was to provide structures that would maintain low flow resistance, consistent water removal, and uniform pressing (no marks). New laminated structures were developed to meet these requirements.
“The trend in the industry is the elimination of surface defects in the sheet and the improvement of sheet properties. The introduction of the shoe press to the lightweight paper market has increased the focus of press fabric development on improving sheet properties as well as water removal. New laminated structures, designed with top base fabrics that have a very uniform pressing surface, are placed over structures that are engineered to maintain water handling capacity and consistency of performance,” Summers points out.
Ken L. Patrick, senior editorial consultant for Pulp & Paper, is president of Paper Industry Communications, Inc., Atlanta, Ga.
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