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Valley Roller Roll Coverings for Industry


Roll Coverings for Industry

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Silicone/EPDM is as the name implies a blend of silicone and EPDM elastomers. Most polymers do not lend themselves to blends, but silicone rubber and EPDM work well together and produce some interesting properties. As one might expect, this material has properties intermediate to the two individual elastomers. Its release properties are better than EPDM, but not as good as straight silicone rubber. Also, it will withstand higher temperatures than EPDM alone, but not as high as silicone alone. In addition, the material is tougher than silicone rubber, but not as tough as EPDM. Silicone/EPDM may be well suited for applications where silicone is just not quite tough enough, and there is room for compromise in release characteristics and temperature requirements, or when EPDM does not have quite enough release characteristics.

Fluoroelastomers are commonly known by the trade names of Viton or Fluorel. It is a fluorocarbon elastomer based primarily on vinylidene fluoride and hexafluoropropylene. This material is unique chemically in that it withstands a broad range of solvents and chemicals, and is capable of operation at 500° F and above. It is the best elastomer available for exposure to most chlorinated solvents. Because it is a very expensive material it is only used by the roller industry for applications that can justify its cost.

Thiokol, or polysulfide rubber, was the first synthetic rubber to be developed. It is noted for its resistance to a broad range of solvents, and it works well in certain solvent blends where other elastomers are unsatisfactory. Coating rollers have been the primary application for thiokol for this reason. The material lacks in physical properties, and it has an objectionable odor. Also it is limited in service temperatures up to 212° F. As a general rule, thiokol is used only where nothing else will work. Its availability is limited.

Butyl Rubber is a copolymer of isobutylene and isoprene. It is a stable elastomer due to its saturated polymer backbone, and has chemical and solvent resistant characteristics similar to EPDM. Butyl rubber has very low air permeability; thus its principal utilization has been in the inner tube market. Butyl also has high shock absorption characteristics. Because of its low air permeability it is more difficult to make blister and void free rollers with butyl rubber, and its shock absorption properties tend to produce low resilience coverings. Typically EPDM coverings are used rather than butyl rubber.

Epichlorohydrin Rubber, or polyepichlorohydrin has found limited applications within the roller industry. It may best be characterized by comparing it to nitrile rubber. Its oil resistance is similar to nitrile rubber, but its chemical backbone is somewhat more stable as evidenced by better ozone resistance and higher temperature resistance. Epichlorohydrin rubber has excellent dynamic properties, but otherwise its physical properties are very similar to NBR. Also, it can be readily formulated to yield anti-static and electrically conductive coverings. Epichlorohydrin’s major drawback, when compared top nitrile, is cost.

Polyurethane is produced from either a liquid cast technology, as thermoplastic resins which are typically molded, or in a millable form, which can be processed on typical rubber equipment. The primary reason for using polyurethanes in roll applications is its extreme toughness over a broad range of hardnesses. A second reason for using ester type urethanes is their resistance to aliphatic and aromatic type solvents.

Polyurethanes are available in two basic chemical types: polyester and polyether. Polyesters have better solvent and fuel resistance than polyether types. Polyethers work better in applications that come in contact with water, especially at high temperatures, because they exhibit superior hydrolytic stability over polyesters.

Natural Rubber chemically is polyisoprene. Natural rubber is noted for its good strength characteristics, however it has limited chemical resistance and has virtually no oil resistance. An easy to work with elastomer, natural rubber is used extensively to make very hard roller coverings, particularly for the paper making industry. It has also been found to work well in laser engraved rollers for printing continuous patterns.

SBR, a copolymer of styrene and polybutadiene, has been the general elastomer of the rubber industry, and particularly the tire industry for years. While it is similar to natural rubber in chemical resistance and poor resistance to oils, it lacks the good toughness and rebound properties of NR. A considerable amount of SBR is used to produce economical low quality coverings for pulley laggings and other non-critical conveyor type rollers. Generally however, SBR is limited by its narrow range of properties, and is supplemented by elastomers for critical roller applications.

Silicone chemically it is polymethylsiloxane and differs from all other elastomers in that its polymer backbone is inorganic; i.e. it is not based on carbon atoms. Two primary properties of silicone rubber as it applies to rollers are its high temperature resistance and its release characteristics.

Silicone rubber withstands up to 500° F continuous exposure, and can operate at higher temperatures intermittently. Release is another important property of silicone rubber. Applications such as the processing of hot plastic films, e.g. laminating machines, or glue applicators are perfect for silicone. Silicone also exhibits good resiliency.

Silicone has other unique properties. It is very inert chemically, and thus resists attack by many chemicals. Electrically, silicone has a very high dielectric constant. One application where this property has been exploited is in the corona treatment of polyolifin films. Silicone has poor physical properties at room temperature when compared to other elastomers. Consequently it is used only where other elastomers will not work, and the unique properties.

 
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