Non Wovens and Laminates Make Their Way Into Press Felts
By Pulp & Paper Canada
It's a pressing issue, one that has to be addressed. Changes in technology, evolutionary and revolutionary, have seen to that. The topic of course is the press and the changes to which press fabric ma...
By Pulp & Paper Canada
It’s a pressing issue, one that has to be addressed. Changes in technology, evolutionary and revolutionary, have seen to that. The topic of course is the press and the changes to which press fabric manufacturers have had to adapt to keep their products up to speed.
Traditionally, press fabrics were made of an inner base covered by batt on the machine and paper sides. While the majority of press fabrics are still comprised of an endless woven base fabric covered by batt fibres on both sides, there has been a significant increase in the use of laminated or multi-base structures. This helps provide bulk and stability.
New developments have been driven at ways to eliminate the woven cloth to some form of non-woven material to provide the bulk required by the base fabrics, according to Weavexx’ product development manager Rn Marchand. “Also, the biggest demand of the last few years has been to improve paper print quality, with the largest change being in board type grades. This has moved laminated felt designs to using very fine cloths to better distribute the pressure and reduce any possibility of mark or surface roughness being induced by the felts used.”
In their paper, Papermaking in the third millennium (PULP & PAPER CANADA, October 1999, p.23), I.I. Pikulik, N.A. Poirier and F. Lger, Paprican, wrote that press sections of the future will have accept sheets with lower solids contents than today. Recent surveys show solids contents of just 13% for webs entering the press of some machines. Pilot studies show that a sheet can be successfully pressed at a solids content as low as 10%. This, they wrote, could eventually lead to a merger of the former and press sections. “Fabrics used on first presses that will accept low-consistency webs will have to be very permeable and will require meticulous cleaning. A completely new generation of forming/pressing fabrics might be required for this application.”
New composite technologies are replacing woven technologies. The goal is uniformity (no knuckling) and a resilient product. That is, press fabrics are compressed in the nip. It is important that they recover their thickness. The new fabrics also have a greater void volume.
To improve product life, the trend is continuing to less staple fibre and more base (batt to base ratio is decreasing), Marchand added. “Even greater strides in this area have been enabled by the use of finer top bases on laminated constructions. Less fibre on the felt means a more stable, compaction-resistant structure that is easier to clean, due in part to the lower internal surface area of the felt.”
The most recent advances in base fabric design include woven and non-woven multi-axial technology as well as a completely non-woven engineered composite structures.
John McEwen, manager, sleeve/belts, Voith Fabrics, spoke about the company’s flow control technology, which addresses rewetting in the nip. This is especially important in shoe presses and longer dwell times. He described it as a layer in the felt that allows water to be pressed through but does not allow water to migrate back to the sheet. In a great analogy, he likened it to a “diaper liner”. “This is the direction the technology is going.”
The development of the shoe press has had and will continue to have the greatest effect on the evolution of press fabric design because it significantly alters the performance requirements of both the batt and base fabric.
There are almost 400 shoe presses operating worldwide on a range of paper grades. Yet, as Daniel Perron of Albany International pointed out, when the first one started in the early 1980s, it was designed to improve sheet solids content in linerboard. Shoe press technology now offers papermakers a new measure of control on paper properties such as bulk and finish as well as machine speed advantages.
The basic effect in the brown paper segment has been the evolution of new high void volume, compaction-resistant designs, three- and four-layer felts, added Marchand.
The first shoe presses for publication grade papers — SC, newsprint — were installed in the mid-1990s. There are different configurations, single- or double-felted, although most publication grade applications use a single felted design. The clothing manufacturers had to develop uniform base fabrics as dwell time in the nip is long, eight to 10 inches.
“In printing and writing grades, void volume has not been a major issue,” Marchand stated. “The biggest problem was more in the area of resistance to marking. This has been resolved by using finer laminated designs. Issues of rewet have also generated a lot of interest.” This has led to the use of various barriers in the felt.
As McEwen added, the claim to fame of the shoe press is its ability to dewater the web. “The issue regarding felts is how you handle the water you remove.” This is what led to the development of the flow control technology described earlier. In the future, some paper machine will feature two nips instead of as many as four.
Because the volume of water in the nip is significantly greater, felts are being asked to handle more water per unit area. So, McEwen noted, a shoe press does not necessarily mean pressure is higher because the pressure is spread out over a larger area. So, the unit pressure (lb/ft2) is less but the contact area is greater. There is a high linear load.
As Daniel Hedou, press product business leader AstenJohnson, said, “Peak nip mechanical pressures will not rise and long dewatering times in the shoe press will greatly reduce sheet and fabric hydraulic loads. Uniformity of pressure application will become a critical factor.”
McEwen said speed has been the driver behind most fabric developments. As speeds increase, dwell times in the nip decreased. Therefore, felts had to be redesigned to compensate for shorter dwell times. Besides dwell times, speed relates to other issues such as nip cycles and sheet handling characteristics. All these drive the development and design of new press fabrics.
When asked about belt technology, McEwen said there are three aspects to consider: sleeves, transfer belts and process belts. The latter two are used in place of felts for different purposes. There are two purposes. The first (belt on bottom, felt on top) transfers the sheet without rewetting, e.g., a rubber belt through a press nip. In the nip, the sheet transfers to the bottom belt and goes through the press thus giving it the name transfer belts. The second application is when, as McEwen said, “You have a condition where you are trying to impart a property to the sheet through the use of a process belt called a PressFlex belt.” Used in brown papers grades where the bottom felt in a double felted press configuration is replaced by the PressFlex belt to impart specific surface properties. McEwen said it is very new and is where “the market is going.”
The former, what McEwen referred to as sleeves, may also be called belts. Albany’s Perron said the combination of belts/sleeves and felts is important. This type of belt can be plain, blind-drilled or grooved. For plain ones, fabrics must be engineered to handle all the water in the nip. For grooved belts, the fabrics can be engineered to have a lower void volume in the nip so the belts will get some water. He said that in most publication grades, grooved belts/sleeves have increased sheet dryness by 1 to 2%.
No more marking
Although seamed felts have been used in the press section, as recently as 1997 there were still concerns about marking. Now, the problem has been largely overcome, although AstenJohnson noted that improvements are still needed, particularly with laminated fabrics.
Perron noted that mills are running high-speed fine paper machines (including coated freesheet) with seamed felt with excellent results. Even tissue machines are using seamed felts in the press section.
Marchand called seamed felts “the fastest growing product segment at this time”. They are in use in practically all applications. Recent developments in terms of the seam construction, with the use of bicomponent fibr
es and various types of chemical treatment has allowed the making of felts that run virtually mark-free. (Weavexx has patented the use of bicomponent nylon fibre.)
R&D is a constant
No supplier in this highly competitive field can afford to let up in its pursuit of new fabric design. This includes constant R&D into materials. This has led partly to the trend of consolidation that has touched all areas of the industry. Asten and JWI became AstenJohnson. Scapa became part of the Voith group. Albany took over Geschmay. Geschmay was particularly known for its press fabrics for high-speed machines, particularly for third and fourth press positions. Perron said a task force has been established to study what Geschmay did — design, process, materials, “everything” — to discover the Geschmay “recipe”, to see what it did well. “There will be a lot of technical exchanges,” Perron added. “The goal is to have two product portfolios that complement each other.”
All suppliers agree that ongoing R&D is a must. “You have to know the paper machine, furnish, sheet characteristics before each press, the hardness of the press, roll diameter; all influence felt design,” Perron said.
Furnish is constantly changing as more mills try to save on virgin fibre. This means more filler and recycled fibre. “The more fillers you have, the more problems you’ll have,” Perron added. The type of filler is important. Calcium carbonate is more abrasive than clay. Therefore, felt design/construction must take this into account. The higher the recycled content, the more likely plugging problems will occur.
Perron said Albany works closely with Valmet. There are technical exchanges. The company also has its own shoe press in its Swedish R&D facility where it can conduct its own experiments.
Weavexx recently opened a multi-million R&D facility in Glottnitz, Austria, site of its European operations. Marchand said the industry has “never been as competitive as it is today.”
Over the last 15 years, Marchand added, the direction of R&D has been more oriented toward practical applications rather than “pure” research. He called this a good thing, resulting in more products that have shown real practical value to papermakers. He cited seamed felts as one example. “What started out with limited application has now found sue in the broader market due to constant product refinements.”
In the area of materials, the industry in general is still using mainly polyamide fibres. Perron noted that Albany is also working with polymeric screens. Instead of a woven fibre, it is a combination of a textile fabric with a polymeric layer. This layer helps provide better pressure uniformity than a textile fabric alone. It is very smooth and uniform. “We are getting very good results,” Perron said. “This is completely new technology that did not exist five years ago.”
Hedou explained that new opportunities have opened for the use of highly specialized materials in engineered structures that were not possible with woven bases. AstenJohnson is working closely with its partner, Heimbach, in conducting fundamental press fabric research. This includes experimental and theoretical modeling of fabric and sheet nip dewatering, extensive dynamic characterization of press fabric compressive properties and fundamental investigation of fabric pressure uniformity and rewet.
Discussing bicomponent nylon fibre, Marchand said it features a nylon core with a low melt polymer sheath, resulting in “dramatic” improvements to a felt’s wear resistance. The trend in nylon staple is moving to higher viscosity materials. This gives better wear and compaction resistance.
“The use of molded components or polymer sheets in press felts is driven by the desire to reduce the amount of woven materials used in press felts for the most part,” Marchand explained. “The reason for this move is related to the pricing pressures affecting the clothing industry with the advent of large corporate deals and high discount levels. This is all part of a very competitive environment that has pushed the major players to have to draw on all available means to provide good quality products at prices the market can bear.”
McEwen noted that Voith Fabrics is part of an organization that includes a roll and paper machine manufacturer. He explained that the company has evolved from one in which dewatering technology was the focus to one where expertise exists in all three areas: felts, rolls and press. “Before, the customer had to go to three places; now, he can go to one. We can take a customer’s conditions and study all three parts of the process and use all our expertise to develop a single solution.”
For example, he cited one case in which the company’s Qualiflex sleeves had wear problems. The P&J of the mating roll was changed. “The solution came about because of the union of the three technologies.”
McEwen sees felt design as an evolutionary process. Belts are the revolutionary area now. “This will change the process in the future. This is where the big R&D push is now.”
Changes in operating conditions, furnish changes, machine and process advancements must somehow all combine to make a better end product more economically and efficiently. Mills won’t accept anything else today. As McEwen said, “Suppliers can no longer say: ‘It’s someone else’s fault.’ It won’t be accepted.”
The following people contributed to this article: Daniel Hedou, Richard Delage, Graham Jackson, AstenJohnson; John McEwen, Tony Relich, Voith Fabrics; Daniel Perron, Albany International, Rn Marchand, Weavexx.P&PC
What’s out there
DYNATEX MULTI-AXIAL PRESS FABRICS. The yarn systems are arranged in four directions. The yarns are woven at an angle to the two- or three-layer laminated base. There are more crossover points than traditional base fabrics, leading to higher lateral flow. The felts also feature high compaction resistance resulting in a more even sheet moisture profile.
APERTECH: It contains up to 50% non-textile polymeric materials by weight. It provides a high level of compression recovery and its construction allows precision design of the water removal paths under compression to predict and quantify flow resistance.
PRESSPRINT: A series developed for publication grades running on shoe presses. Most are laminated with a very fine, very uniform top base to optimize Z-direction pressure distribution in the nip.
PRIZM: It features a multi-axial technology that addresses modern press section needs: mass and pressure uniformity and steady state operation which translates into faster start-ups and longer useful life.
EQUATOR: This is a new batt material that is designed to improve sheet finish and increase water removal in many applications.
QUADRASEAM: Seamed fabrics are now an option for shoe press machines. This has removed product application hurdles allowing the design of better performing products for these applications.
The SPECTRA series uses a combination of new materials (polyurethanes) and is non-woven. It is claimed to have unique properties in resiliency. It will compress in the nip but totally recover its membrane thickness.
FLOW CONTROL: A patented, chemically-treated hydrophobic material that can be inserted into the batt structure of any felt.
SMOOTHPRINT: Designed to improve paper smoothness, it was introduced along with new needling technology, Huyperpunch, which enables the needling of a felt with virtually no “needle tracks” or patterns on the surface of the felt. It has shown excellent results in Europe.
FLOMAXX: Designed for brown paper grades, these felts have a high void volume and compaction resistance. One of the designs is FloMaxx S2, a four-layer construction for high-load shoe presses.
EXTENDER: This uses the latest bicomponent fibre and treatment technology as well as seam construction modifications to enhance the wear resistance of the felt and seam area.
DRYSTAR: A new rewet barrier concept that uses a polymeric matrix in the felt structure. It can be used for
all applications, from tissue to brown paper.