Extreme wear resistance is required in paper manufacturing
Made with Therban®
Rolls play an important part in many areas of industry. They convey, press, finish, emboss, guide and brake, apply liquid media, heat and cool. The materials used for covering these rolls must be tailor-made for each of these applications, however. Rolls used in paper manufacturing for squeezing water out of "raw" paper pulp, for example, must meet particularly high specifications. They must have an extremely smooth, hydrophilic surface and good damping characteristics (reduced heat build-up) as well as being highly resistant to wear and (most importantly) hard. Roll covers based on Therban®, the HNBR rubber marketed by LANXESS, meet these requirements, which is why they
have been able to assert their superiority over other materials in many paper factories.
"Being hard" is not something which can be taken for granted in the paper industry. The rolls used in large-scale paper production must be capable of withstanding peripheral velocities of up to 2,000 meters per minute (= 120 km/h) and high contact pressures. Coating materials which are too soft become very warm under these extreme conditions owing to the repeated pressing cycles. Materials which are too hard, such as graphite or ceramics, are heavy and too brittle therefore susceptible to failure. For a long time, the gap between soft and glass-hard was bridged by highly crosslinked, hard rubber coatings based on natural rubber (NR). The hardness of these rubbers was not sufficiently consistent over the range of in-service temperatures, however, and their resistance to aging left a lot to be desired, partly because of the sulfur crosslinking.
Hydrogenated nitrile rubber (HNBR), marketed by LANXESS under the name of Therban®, is an elastomer which rises to the extraordinary challenges of the paper factory with a balanced mix of physical and mechanical properties. Almost completely lacking in sensitive double bonds, peroxide-crosslinked Therban® is characterized by excellent aging and ozone resistance and a low compression set even at high temperatures. It is suitable for manufacturing extremely hard roll covers, with Shore D values of over 70 and Pusay & Jones values of less than ten. The heat build up under stress is correspondingly low. Despite the hardness of the vulcanizate, the green compounds are easy to process.
Acrylate-filled, light-colored and non-discoloring grades of Therban® are also characterized by a highly abrasion-resistant and particularly tight and smooth surface which (unlike with rolls made of NR hard rubber) lends itself to regrinding. This is important because the accurate finishing of press rolls, which can often measure five meters in width and weigh several tons, is ultimately precision work. Rolls covered with Therban® also retain their original dimensions and hardness over a wide range of temperatures, even in continuous operation. Peroxide crosslinking also results in good stretch properties, which give the covers the necessary robustness for long-term use. Reduced machine down-time ensures that Therban covered rolls have improved economics when compared with the old fashioned NR rolls.
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