Elastomeric alloys in rubber rollers.

Elastomeric alloys in rubber rollers

Elastomeric alloys (EAs) have emerged as an important new class of thermoplastic elastomers (TPEs) for use in a wide cross-section of rubber applications. Their wide acceptance in the marketplace is being maximized by the development of both application technologies and key products for certain market niches.

EAs are a newer class of TPEs with moderate cost and mid-range performance. EA-TPEs have effectively replaced thermoset rubbers in numerous applications requiring resistance to heat, aqueous fluids and oil-based fluids.

TPEs currently enjoy a significant share of the industrial rubber products market. This share is projected to grow to 9-12% within the next 7 to 10 years. The current TPE growth rate is approximately 8 to 10% per year, about four times the growth of the rubber industry as a whole. EAs are the fastest growing segment of the TPE family, with consistent double digit growth. This high growth will continue because of the wide acceptance of EAs in high performance applications and the introduction of new products filling niches in the performance spectrum.

Thermoset rubber-covered rollers are used in the textile and paper industries. The thermoset materials are polychloroprene, EPDM, nitrile and polyurethane. The breakdown of material usage for these large rubber applications in North America is shown in table 1.

EA covered rollers are potential materials for replacing thermoset rubbers.

The process for covering rollers with thermoset material is very labor intensive. If after autoclaving the covered roller has a major flaw, the rubber must be removed and the process of covering the hub repeated. Scrap thermoset rubber cannot be recycled. For the paper industry, EA covered rollers provide superior release characteristics. Polyolefin materials are known for their excellent release characteristics and specific technology provides the elastomeric properties required through crosslinked, finely divided rubber in a polyolefin matrix.

Roll cover technology

The technology developed to cover rolls can be broken into two application areas based on the size of the rolls. Small rolls are readily covered with a thermoplastic elastomer, and conventional technology can be applied. Very large rolls have special handling problems that make them more difficult to cover.

Small rollers for business machines

Smaller rollers, from a few centimeters to nearly a meter in length, have been covered with elastomeric alloys using insert injection molding, followed by machining the roll cover to tightly toleranced dimensions. In some uses, interlock to the roll is adequate adhesion for roll performance, but an adhesive can be applied to the roll to achieve a higher level of bonding between the elastomeric alloy and the roll surface. Another technology, used to cover small rolls successfully, is to extrude a tube-shaped preform. The preform can be press fitted over a roll using a lubricant or an air assist technique. For rolls requiring less stringent dimensional tolerances, the latter technique is an attractively low-cost means for preparing rubber covered rolls using an elastomeric alloy.

Tolerances

The exacting requirements of feed roller systems to start, propel and stop individual pieces of paper demand tight tolerances in the dimensions and properties of these feed rollers. More than 95% of the rollers used in paper handling equipment must be ground to the final true indicated radius (TIR). The tight tolerances to which EAs can be extruded and molded fit nicely with the need for rapid fabrication of roller systems which must meet a rigid TIR.

An EA can easily be extruded to a tolerance of [+ or -] 0.006 inch to [+ or -] 0.008 inch in the diameter of a rubber roller on a hard spindle. The final grinding operation to obtain the required TIR and the XY plane diameter tolerances can be done more quickly and consistently with EAs than with thermoset rubbers, which cannot be extruded to the same high tolerance. This results in less effort to grind down a roller to produce a finished product with the required diameter tolerance. This decreased time translates into lower production costs for elastomeric alloy covered rollers meeting the required dimensional tolerances.

Processing for higher production rates

Truly outstanding production rates can be achieved with EAs which can be extruded at a rate of 175-350 lbs. per hour with a 2.5 inch to 3.5 inch diameter thermoplastic extruder. The lower EA specific gravity (0.95-1.00) and the use of in-line cutting equipment enable rapid cutting of EA and its use in the assembly of rubber rollers.

Hardness

Though the softest grade commercially available EA is 55 Shore A, this elastomeric alloy can be chemically foamed to give rollers with a solid rubber skin and a density reduction up to 20%. A macroscopic hardness of 40-45 Shore A can be achieved on the surface of a rubber roller through the use of a foamed EA cover. Again, excellent tolerance control can be achieved.

Chemical resistance

EAs are resistant to most inks, oils and fluids typically used in printers, copiers and facsimile equipment. Certainly, they provide a wider range of resistance than most EPDM and silicone compounds and do this at lower cost than many cast polyurethane compounds.

Process of covering large rollers

Two processes have been developed for covering the hubs with EAs.

Molten plying

The metal hub for the roll is preheated to 175 [degrees] C ([+ or -] 5 [degrees] C) in a hot air oven for approximately one hour. The hub is then mounted on the shaft of the variable speed wind-up system. A powdered adhesive (Polybond 1016) is sprinkled on to the outside surface of the hot hub as it is slowly rotated. After the hub has been uniformly coated with adhesive (approximately 0.031 g/square cm), it is returned to the oven for another 15 minutes to melt the adhesive.

The adhesive-covered hub is then remounted on the wind-up system. The elastomeric alloy is extruded through a standard sheet extrusion die with lips set at 0.5-2.8 mm thickness. The hot (200-210 [degrees] C) EA sheet is wound onto the adhesive covered hub using a 1.0-1.25 draw ratio. A pressure roller is used to bond the first layer of the EA onto the roller. Successive layers are wound onto the hub until the desired thickness is achieved (approximately 28 to 32 mm total thickness). The covered roll is then dismounted and quenched in water to cool (23 [degrees] C [+ or -] 5 [degrees] C) for approximately one hour. The roll is then machined on a standard lathe to the finished dimensions.

Shore 73A, 87A and 50D hardness EAs were successfully used with this process and exhibited void-free construction without delamination.

Cold plying

A two-step process for covering large metal hubs with elastomeric alloys has been developed. In the first step, a trapezoidal strip (80 mm wide and 8 mm thick) is extruded and wound up. In the second step, the hub is mounted on a wind-up system and a thin layer of epoxy coating (Metallon 2108) is applied to the hub. The trapezoidal strip is primed with Reno Primer 360. The primed trapezoidal strip is spirally wound on the hub to achieve the desired thickness. The priming of the first layer is necessary to promote adhesion between the EA and the epoxy covered hub.

Fabric shrink tape is tightly applied to the EA wound hub and the covered hub placed in an autoclave at 150-160 [degrees] C for 2-4 hours, depending on the size of the roller. The pressure generated during dry autoclaving fuses the layers. Rollers that have been covered using this process show void-free construction.

After the autoclave, the roller is air cooled to ambient temperature and machined to finished dimensions.

Short term creep behavior of TPEs at 23 [degrees] C and 100 [degrees] C

Elastomeric alloys offer good resistance to creep at ambient and elevated temperatures. Creep measurements were made on cylindrical specimens 18 mm in diameter and 13 mm in height, with an Instron Model 1332 Hydraulic testing machine. A load of 45.4 kg was placed on the specimens. The rate used to apply the deformation was 22.7 kg/second. The results are shown in figures 1-4. The excellent creep and fluid resistance of TPEs make them good candidates for roller applications.

Summary

The fabrication technology development using elastomeric alloys for rubber covered rollers offers superior cost performance benefit when compared to thermoset rubber covered rollers. Large rollers covered with thermoset materials require a vulcanization time from several hours to a day or two because of the large size of these roll coverings and their relatively low heat transfer of rubber. However, rollers covered with elastomeric alloys do not require vulcanization.

For those thermoset rubber rolls with quality flaws, the rubber has to be stripped off and the process repeated. The thermoplastic elastomer cover can be repaired in some cases, and, at worst, can be removed, reground and reprocessed.

Table 1 - material usage for rubber covered rollers

Material                Application         Quantity
Nitrile               Industrial roller    1,500 k lb.
Nitrile               Paper & textile      3,000 k lb.
Neoprene, EPDM, SBR      Paper             3,000 k lb.
Neoprene, EPDM, SBR      Textile           1,200 k lb.

[Figures 1 to 4 Omitted]