Rotatable surface conditioning rolls ("mill rolls") are commonly used in the manufacturing and processing of metal sheets and coils in continuous production lines. During the processing of metal sheets, mill rolls are generally utilized in so-called "wringer applications" as well as "oiler applications".
In wringer applications, mill rolls and like articles are used to prevent the cross contamination of treating solutions. For example, during manufacture of metal sheets many process operations use a variety of treating solutions designed to variously clean, degrease, remove scale, preserve, rinse, surface etch, lubricate, and/or protect a metal surface. During these operations, it is important that mill rolls prevent the contamination of these often incompatible treating solutions. Thus, mill rolls provide wringing ("damming") operations at surfaces of metal sheets to remove excess treating solution from metal surfaces as metal strips exit each treatment tank. The wringing operation prevents the inadvertent mixing of treating solutions. Wringer arrangements may include mill rolls made of conventional rotatable rubber or made of non-woven materials as well as wiper bars made of felt or cork.
During a surface conditioning process, metal sheets move rapidly over rotatable mill rolls at a rate of approximately 200-1200 feet/minute. The relatively non-compressible nature of rubber mill rolls, unlike the web construction of non-woven mill rolls, for example, makes them susceptible to cut propagation and other structural damage at the nip of a roll. Additionally, the surfaces of rolls are easily torn, gouged, or cut by edges of splices in the metal sheets. Once damaged, the initial cut, tear, or gouge in the roll will usually enlarge due to compressive forces at the nip and tension forces on either side of the nip. As a roll becomes increasingly damaged, a roll's ability to act as a wringer decreases and treating solutions may pass through the roll. Chemicals of one treatment tank are then inadvertently mixed with chemicals of another chemical treatment tank and may eliminate a surface modification activity of an initially pure chemical. If a treatment tank is contaminated by another chemical, the contaminated chemical mixture will have to be replaced with new chemical or chemicals. Discarding the contaminated chemicals may be difficult, raising environmental concerns. Also, frequent repair (e.g., surface dressing) or replacement of these articles has been required, thereby creating unscheduled production line downtimes with associated losses and costs.
When production lines are stopped, the rolls on the lines become dried as the sheets of metal are no longer moving and carrying liquid to the rolls. Non-woven rolls absorb liquid because they are porous and have a large void volume. The liquid from the surface modification process fills the void volume when the production line is in operation. When the production line operation stops, the liquid in the roll may precipitate and change the physical characteristics of the roll. Typically, the roll becomes stiff and may have to be replaced before production begins.
In some processes of modifying the surface of metal sheets, the last steps may include a passivating treatment comprising oxidizing acids. The oxidizing acids react with the surface of the metal to make the metal surface less reactive with its environment. Unfortunately, the oxidizing acids also react with the rolls and the rolls may loose physical integrity. A roll used in the passivating treatment step usually performs an application function of applying a thin layer of oxidizing acid to the surface of the metal sheets being treated. The application of a uniform layer of oxidizing acid is necessary to avoid problems. Excess oxidizing acid results in spots appearing on the surface of the metal sheets. Not enough oxidizing acid results in paints or other coatings not adhering well to the metal surface.