In the not too distant past, roll covers were considered a commodity by many papermakers. Wet press covers were predominately bone-hard and softer rubbers, although granite rolls were largely preferred in the center press position on newsprint and publication machines. The goal at that time was simply to rely on the supplier to provide the correct cover material, thickness, and hardness for a specific application. As long as the covers had reasonable wear resistance properties and acceptable overall life, questions about the cover relationship to sheet properties and paper machine operation seemed to be of secondary interest.
In the 1980s, in my opinion, two major factors were instrumental in changing the focus on roll covers in the U.S. paper market. One of these was the increase in production rates made possible by new paper machine designs. Papermakers soon realized that some of the basic cover types which had been used so reliably in the past were not completely suitable in the more demanding operating conditions, due to shorter life cycles and increased failure frequencies. Since the covers began to require grinding more frequently, the effect on sheet properties became more apparent, compared to the gradual effects occurring at previously lower speeds. But perhaps the most significant factor was the introduction of the polyurethane roll cover. These covers, initially used mainly in the wet press section on pressure controlled grades, such as newsprint, fine, and publication, allowed the papermaker to regain an acceptable running time between cover changes, and their effect on sheet properties began to be better understood. The benefit of an improved moisture profile was quickly observed, and some papermakers even resisted removing the polyurethane covered rolls at all for regrinding. It was also found that grooved and blind-drilled covers with higher void volumes could be used due to the material strength and wear resistance properties. Another benefit which was largely overlooked initially was the reduction in press drive energy when the covers were installed. I believe this was related both to cover deformity under load and also to the rolling resistance in the nip (similar to automobile tires on highways). Several mills claimed up to 10% reduction in drive HP was achieved, but this was not fully verified at the time.
The rapid acceptance of polyurethane covers beginning in the 1980's was remarkable, considering that the initial cover cost was about 4.5 times that of a rubber cover. The use of press section analyses and economic evaluation techniques were required to convince mills to approve cover expenditures above what was allocated in their budgets. Fortunately, the U.S. paper industry was a close knit "fraternity", and when expectations were met or exceeded, the success stories spread rapidly. Of course, better rubber covers were also being developed at the time, with the goal of securing positions where polyurethane was felt not to be suitable or cost effective at that time (for example, softer cover positions or in slower paper machines). Softer hydrogenated rubber covers had good success in the market. A number of machines evaluated operating without covers in the hard press roll positions, with varying degrees of success due to problems with wet felt life and very high nip pressures.
Papermakers also began to learn more about other important cover properties when polyurethane cover usage grew. A few of these are summarized below:
• Cover material hysteresis- This is the energy loss within a polymeric material when it is subjected to varying stress. It is commonly associated with heat buildup, and was originally emphasized due to concerns over polyurethane cover melting. However, most cover failures involving melting actually happened due to bond failures and the related excessive flexing of the loose area of the cover. Later, hysteresis was studied more in depth in regard to vibration in specific high speed paper machines.
• Impact resistance- Press roll covers are subject to damage from paper wads or foreign objects passing through the nip. Polyurethane covers were found to be more resistant to damage, and if damage occurred, the covers were often reliably repaired.
• Material resilience and compression set properties- It is very important that a cover material return to its original shape after compression or bending (resilience). Similarly, the material's ability to return to its original thickness after press compressive stresses are applied can be used as an indicator of whether the cover will develop a barring effect. Polyurethane has excellent resilience and resistant to permanent compression set, and papermakers soon identified these benefits.
• Permeation- This is the penetration of liquid, gas, or vapor through a material. In the case of polyurethane it is related to the material structure. Permeation became very important due to its association with bonding failures, requiring the development of stronger bonding systems. Harder polyurethane covers were less prone to these type detrimental effects than softer covers. Some papermakers attempted to "dry out" the covers between operating runs, but this technique was of limited benefit. The main effect seemed to be on the water retained in the cover from absorption.
The original polyurethane covers were mostly manufactured by casting liquid polymer into vertical or inclined molds, followed by post curing. Manufacturing later evolved into rotational casting, with faster curing polymers applied on cores turning in a horizontal mode. The latter technique offered the benefits of shorter manufacturing cycles, improved cover properties, and the ability to supply multi-layers of different hardness for more uniform stress distribution in the nip. More recently these covers have been used in tissue applications and for blade coater backing rolls.
Of course, cover evolution has not been limited to newer technology polyurethane and rubber covers. Sprayed ceramic coatings made significant progress in replacing granite rolls in Newsprint machines. These "covers" had to overcome major technical challenges in terms of release properties and resistance to corrosion effects. Both cast and wound resin composite covers were evaluated, and the cast covers in particular operated well in fine paper machines. Numerous specialty covers were made available for specific applications. In addition, "smart roll covers" were developed which allow dynamic display of nip conditions as an aid in improving loading uniformity and in troubleshooting press problems.
Roll covers have evolved to meet the increasing demands of today's paper machine operating conditions. But the future for roll covers will depend mainly on the design of tomorrow's machines, resulting from advancements in paper manufacturing technology.
Robert Moore is a retired chemical engineer, and is an experienced technical and fictional writer. His past work experience spanned the chemical, paper and equipment manufacturing industries, including holding management positions at Voith Paper, Scapa plc, and The Mead Paper Corporation. |
