Calenders, rolls and covers: new materials for improved performance: new materials create new opportunities for papermakers.

The improved performance of modern calenders, especially multi-nip calenders, would not be possible without advanced materials. New materials are being used in calender components, including rolls, coatings and covers. Some of the most significant developments have been in cover materials.

Dr. Paul McCarten, vice president, technical applications for Voith Paper Service Division, Charlotte, North Carolina, USA, noted that the soft covers used are generally a composite material comprised of a thermoset resin mixture with reinforcing fillers and fibers.

"There have been material advances in all three areas," he said. "The fibers impart crack resistance and toughness to the composite at the expense of slight non-uniformity. New fiber materials--such as para-aramids with higher toughness and reduced diameters--are promising for even better toughness but with improved cover uniformity."

Developments in new filler materials include more uniform particle distribution and increased hardness, according to McCarten. He noted that the use of nano-particles to enhance wear resistance is especially promising. "Improvements in these areas have led to enhanced abrasion resistance for such covers as Safir S," he said.

While advances in fibers and fillers have kept pace with the requirements of load and abrasion resistance for calender covers, soft nip calenders and multinip calenders are also operating under higher temperatures. "Resin mixture properties set the upper temperature limits for the calender. Earlier attempts to raise the operating limit of calender covers led to brittleness," noted McCarten. "New resin technologies from the aerospace industry may offer increased temperature capability with high toughness."

New materials have also been developed for hard coatings on metal calender rolls. These coatings have hard, carbide-type particles in a metal matrix. Smaller hard phase particles, including new types of carbides, have been developed. "Nano-particles, combined with metals with improved corrosion resistance, have led to hard coatings with lower surface Ra that can be maintained even when doctored," he said.

UN-NIPPED ROLL COVER TECHNOLOGY

Another promising area is "unnipped" roll covers, according to Cody Merrion, general manager for Coldwater Resins Inc. "Un-nipped roll covers have often been considered 'non technical' due to the misconception that they do not have a direct affect on papermaking," he said. "Yet they typically represent 80% or more of the covered rolls in a machine. They perform a critical task: carrying the wire, pulp, felt and sheet from the head box to the reel. Whether the roll is driving the wire/felt, or the wire/felt is driving the roll, traction is paramount."

Most un-nipped, rubber and epoxy roll covers in the paper industry operate at a hardness of 0-1P&J "bone hard" in order to extend the life of the roll cover, but putting a hard cover on a roller built for traction is counterproductive, Merrion noted. The hard cover will invariably slip against the wire/felt, causing friction, heat and wear in wires and felts, leading to costly damage of expensive machine clothing.

Materials used to produce roll covers in the wet end of a paper machine have traditionally been rubber and epoxy, very hydrophobic materials that do not adsorb moisture. "A harder material is much more durable than a softer one," said Merrion. "The industry has adopted a technology that promotes longer cover life but sacrifices traction to the wire and causes premature wear."

Companies have replaced rubber and epoxy with polyurethane roll covering materials that provide better traction and durability, but they have been limited to press rolls and soft nip calendar rolls. Since polyurethane tends to adsorb moisture, press rolls are routinely taken out of the machine and "dried out." As a result, polyurethane roll covers were not used in un-nipped positions since those rolls typically ran continuously for years and could not be removed.

New technology now provides polyurethane roll covers for unnipped roll covers that do not adsorb water or suffer from hydrolytic degradation.

"These materials incorporate composite materials into the polyurethane matrix to impart better traction, durability and hydrolytic resistance into the roll cover. For example, papermakers can now use a 5-20 P&J polyurethane roll cover where they are currently using a bone hard rubber or epoxy cover," said Merrion. "Softer covers provide better traction without sacrificing cover durability. This provides a significant advantage, especially on faster machines where traction is at a premium." S!

ALAN ROOKS, Editorial Director