Silicone-covered rolls

Epoxy coated rolls (450-500 inVkW) offer a more durable covering than silicone or Hypalon and are also a logical choice for basic corona treating applications. Epoxy coatings have harder surfaces than silicones or rubbers and offer added insurance against unexpected roll failure from exposure to excessive corona discharges, heat build-up, or accidental operator-inflicted damage from an operator s knife. A pinhole in an epoxy roll can usually be repaired on the production site. Life expectancy of an epoxy covered roll is approximately one year. 

Although highly insulative roll coverings, such as ceramic, cost more when compared to silicone sleeve roll coverings, they offer a more precise roll covering... 

Comparing universal roll system performance to systems utilizing Hypalon , silicone, and epoxy covered rolls, the higher efficiency ceramic coating requires less power to achieve the same surface tension results, therefore reducing the cost of the power supply required. 

Finally, Rong and coworkers discuss the roll of surface oxygen on the MCS process75. Rong employed a lab-scale stirred bed reactor and then applied XPS to analyze the silicon samples before and after the reaction. The reactivity of silicon depended on the initial thickness of the native oxide on the silicon. After the reaction the surfaces of all of the samples were mostly covered with Si02. There was no observed correlation between the surface and bulk O content. XPS analysis showed the presence of Al, Ca and Ti impurities in some samples. Titanium on the surface appeared to increase the reactivity, whereas Ca decreased the selectivity of Di formation. Addition of ZnO to the silicon before CuCl improved reactivity and also decreased the induction period of the reaction. XPS studies of samples prepared in this manner exhibited a lower Zn surface concentration compared to the samples where CuCl, Si and ZnO were mixed together. 

Latir. [Laur Silicone Rubber Congxling.] Silicone elastomers used fix o-rings, roll covering, hot stanq>ing dispersions used as adh ives, size coating, spUc-ii agem. 

Silopren. Payer] Silicone rubber used in seals and ofoer technical moldings and extrodates, roll covers, conveyor belting, cable and wire insulations, pharmaceutical goods. 

The equipment used for extrusion coating is similar to that used for the extrusion of flat film. Figure 2.30 shows a typical extrusion coating setup. The thin molten film from the extruder is pulled down into the nip between a chill roll and a pressure roll situated directly below the die. The pressure between these two rolls forces the film on to the substrate while the substrate, moving at a speed faster than the extruded film, draws the film to the required thickness. The molten film is cooled by the water-cooled, chromium-plated chill roll. The pressure roll is also metallic but is covered with a rubber sleeve, usually neoprene or silicone rubber. After trimming, the coated material is wound up on conventional windup equipment. 

The most common roll covering is a silicone sleeve. These sleeves have good dielectric strength at 350-400 inVkW, which allows them to handle all basic corona treating applications (a lower rating indicates that more power can be directed at the roll). For roll diameters greater than 8 in, vulcanized silicone must be used. 

Silicone sleeves are very economical and replaceable, since they can be easily slipped off the roll and a replacement sleeve slipped on. The downside is that they can be damaged from over-exposure to corona or accidentally by the slip of an operator s knife. While the cost of a new silicone roll covering is not high, costs from unexpected downtime and comprised product can quickly add up. The life expectancy of a silicone sleeve is, on average, six months. 

Table 4.1 Performance Comparison - Silicone vs. Ceramic Roll Coverings...

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