New developments in roll covers continue to improve runnability, efficiency and overall paper characteristics
August 2007
By Mike Radtke
Papermakers are experiencing increasing pressure to reduce costs, improve efficiency and maximize product quality. With energy and fiber costs on the rise, mills are forced to use lower quality fiber sources, more fillers and steam boxes to stay competitive. This places heavier demands on the paper machine.
Since roll covers have a major impact on machine efficiency and the quality characteristics of the end paper product, papermakers are finding recent developments in roll cover technology to be beneficial in alleviating the mounting demands. Advanced roll cover materials are helping to increase press dryness, felt life and life between grinds; prevent barring and vibration; improve doctor blade life; and increase the overall life of the roll -- all factors leading to more efficient paper production.
Voith Paper Rolls has introduced several such technologies in the past 12 months. This article provides a summary of three new technologies, guidelines for their application, and actual mill results.
Thermal sprayed ceramic coatings
Sophisticated rubber and resin coatings were developed in the mid 1980s as an alternative to granite. However, the new materials could not replace the release characteristics of a granite roll. This led to the development of thermal sprayed coatings that can be applied to a steel or cast iron shell and can match the release properties of granite. This type of coating continues to evolve.
Voith introduced TerraSpeed in 2006, with the goal of pushing the limits of the press section and achieving and maintaining sheet release without agents or on-machine roughening of the roll surface. TerraSpeed offers improved release characteristics and contaminant suspending micro turbulences. In addition, it is easier and more forgiving to doctor.
The TerraSpeed coating uses specially engineered ceramic powders and coating parameters that are combined and adjusted to meet a variety of papermaking demands. Compared with earlier generations of thermal coatings, TerraSpeed further improves release properties (Figure 1) and maintains the release over extended running periods, resulting in consistent paper production. It also provides cost savings by increasing doctor blade life and often eliminates the need for release agents. The coating can be continuously doctored with micro-abrasive or coated steel blades.
TerraSpeed is suitable for high-speed graphic machines and specialty machines with various furnishes, as well as some lower speed machines in abrasive environments.
Composite calender covers
Another recent cover development is the Vantis M, a fiber-reinforced, synthetic composite material introduced in September 2006. Voith's goal in creating the Vantis M was to improve cover surface properties, extending in-machine operating times and expanding the usable limits of the cover. Developed originally for use in multi-nip calenders, the attributes of this cover are also performing well in soft-nip and supercalender applications.
One of the major innovations applied to the cover is a nano-particle filler system with a very narrow particle size distribution. The difficulty in using nano-particle fillers is preventing their agglomeration into larger particles, which would negate the benefits seen from the uniform distribution of the particles.
By optimizing the filler type and blend, the direct benefits of the nano-particle technology are manifested in several cover properties. The fine size and blend allow for a smoother calender cover surface that will improve during operation due to a self-polishing effect. This typically results in roughness values of only 5-6 Ra (0.15 mm), Figure 2.
The Vantis M also incorporates improvements in mechanical and thermal properties that allow the cover to run safely in extreme conditions. The mechanical loss factor (tan delta) is reduced, meaning less of the applied energy is converted into heat in the cover. This enables the Vantis M to run at increased line loads and higher speeds.
Thermally, the Vantis M has improved heat conduction and a lower thermal expansion coefficient. This allows the cover to withstand higher surface temperatures and lowers the internal stress generated by repeated heating and cooling cycles during operation.
In addition to the property benefits of Vantis, Voith uses an exclusive bonding system to ensure optimal runnability of the paper machine. Results indicate that this adhesive system has significantly improved the resistance to mechanical damage and reduced the problem of delamination between the cover and the core. Testing has shown that even in cases of severe mechanical overload no delaminations were generated.
The combination of better impact resistance, a smoother and more wear-resistant surface, and improved thermal and dynamic properties all provide increased value to the papermaker. Fewer shutdowns for roll maintenance, reduced losses for paper quality, reduced grind losses and improved impact resistance all lead to lower total cost of ownership.
Polyurethane advancements
Polyurethane (PU) roll covers have a combination of abrasion resistance, toughness and durability, making them ideal press and suction press roll covers on demanding paper machines. With the evolution of polyurethane technology over the years, papermakers are experiencing benefits including better hydrolytic stability, high abrasion resistance and improved dynamic behavior. These cover improvements have allowed for more aggressive cover designs, providing enhanced machine operation.
Voith Paper Rolls has created the Solar series -- SolarPress for press rolls and SolarFlow for suction press rolls -- to meet papermakers' goals.
SolarPress has been designed for challenging press roll applications that require a medium hardness cover, including blind-drilled and grooved covers on graphic paper machines and blind-drilled rolls on long nip style presses for packaging grades. SolarFlow combines advanced polyurethane compounds with productive surface patterns to decrease water flow resistance in the nip and optimize water removal characteristics, Figure 3.
One of the properties where Solar shows improvements over previous covers is its hydrolytic stability. In the hot, wet environment of a paper machine press section or size press, polyurethane covers absorb water. Depending on the temperature of the application and chemicals being used, there can be softening of the roll cover. Laboratory testing of Solar has demonstrated a higher resistance to hydrolysis compared with previously available products.
For elastomeric covers on paper machine press sections, the behavior of the material under cyclic loading is critical. As the cover is compressed and relaxed going through the nip, part of this compression work is converted into heat inside the cover. The lower the hysteresis of the cover material, the lower the energy absorbed and converted to heat and the cooler the cover will run. Materials with too high hysteresis will fail on high-loaded, high-speed applications. The Solar series exhibits excellent dynamic performance at higher P&J values than other polyurethanes when running in demanding positions in both graphic and packaging machines.
In the field
In applications worldwide, the Solar series covers have provided a variety of measured benefits.
A mill utilizing SolarPress noted a speed increase of 10-12 m/min when replacing premium rubber covers with Voith PU blind-drilled covers. A weekly production record was set after the installation of SolarPress on a long-nip style press. These covers have also improved press nip dewatering and sheet quality on high-quality woodfree coated machine press sections. In addition, a performance improvement was realized in a highly loaded, long-nip style press section with a history of cover failures.
A mill in Canada installed a SolarFlow on a previously bare suction press shell on its newsprint machine. Running at speeds of 1,200 m/min and higher, the machine was monitored to compare performance over the first six months of running SolarFlow vs. the six-month pre-installation period. The mill's production manager reported improvements in machine efficiency and speed, as well as a reduction in steam usage. An increase in fabric life was also noted and was attributed to roll performance.
Other mill installation results using the SolarFlow cover include increased sheet dryness out of the press section of greater than 1%, a reduction in steam consumption of more than 4%, decreased press section to dryer section draw by more than 10% relative, a more than 50% decrease in press section breaks, and a lengthening of fabric life by one week or more. The technology has also helped eliminate suction roll shadow marking and improve the paper CD moisture profile.
Summary
New roll cover materials, coatings and surface configurations are constantly being evaluated and tested. Work is now being done in nanotechnology and thin film coatings for the electronics industry that shows applicability to the paper industry. Papermakers can expect continued improvements in roll cover performance, as these technologies are explored and successfully applied to improve efficiency and cut costs in our capital-intensive industry.
Mike Radtke is Product Manager for Voith Paper Rolls. He is based in Neenah, WI, USA
