Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Process of treating scrap or waste product containing solid...
Reexamination Certificate
2002-01-17
2002-10-08
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Process of treating scrap or waste product containing solid...
C521S044000, C264S912000
Reexamination Certificate
active
06462099
ABSTRACT:
BACKGROUND OF THE INVENTION
After they have been worn-out during their limited service life, millions of used tires, hoses, belts and other rubber products are discarded annually. These used rubber products are typically discarded and hauled to a dump because there is very little use for them after they have served their original intended purpose. A limited number of used tires are utilized in building retaining walls as guards for protecting boats and in other similar applications. However, the number of worn-out tires that need to be disposed of annually far exceeds the demand for them in these types of applications.
The recycling of cured rubber products has proven to be an extremely challenging problem. This problem associated with recycling cured rubber products arises because, in the vulcanization process, the rubber becomes crosslinked with sulfur. After vulcanization, the crosslinked rubber becomes thermoset and cannot be reformed into other products. In other words, the cured rubber cannot be melted and reformed into other products like metals or thermoplastic materials. Thus, cured rubber products cannot be simply melted and recycled into new products.
Since the discovery of the rubber vulcanization process by Charles Goodyear in the nineteenth century, there has been interest in the recycling of cured rubber. A certain amount of cured rubber from tires and other rubber products is shredded or ground to a small particle size and incorporated into various products as a type of filler. For instance, ground rubber can be incorporated in small amounts into asphalt for surfacing roads or parking lots. Small particles of cured rubber can also be included in rubber formulations for various rubber products that do not have high performance requirements. For instance, reclaimed rubber can be ground and compounded into formulations for floor mats or tire-turf for playgrounds. However, it should be understood that the recycled rubber serves only in the capacity of a filler because it was previously cured and does not co-cure to an appreciable extent with the virgin rubber in the rubber formulation.
Various techniques for devulcanizing cured rubber have been developed. Devulcanization offers the advantage of rendering the rubber suitable for being reformulated and recurred into new rubber articles if it can be carried out without degradation of the rubber. The recycled rubber could again be used for its original intended purpose rather than simply as a filler. In other words, the devulcanized reclaimed rubber could again be used at higher levels in applications where there are high performance requirements; such as, in manufacturing tires, hoses and belts. The large scale commercial implementation of such a devulcanization technique could potentially be used to recycle vast quantities of worn-out tires and other rubber products that are currently being discarded to landfills. However, to the present time, no devulcanization technique has proven to be commercially viable on a large scale.
U.S. Pat. No. 4,104,205 discloses a technique for devulcanizing sulfur-vulcanized elastomer containing polar groups which comprises applying a controlled dose of microwave energy of between 915 MHz and 2450 MHz and between 41 and 177 watt-hours per pound in an amount sufficient to sever substantially all carbon-sulfur and sulfur-sulfur bonds and insufficient to sever significant amounts of carbon-carbon bonds.
U.S. Pat. No. 5,284,625 discloses a continuous ultrasonic method for breaking the carbon-sulfur, sulfur-sulfur and, if desired, the carbon-carbon bonds in a vulcanized elastomer. Through the application of certain levels of ultrasonic amplitudes in the presence of pressure and optionally heat, it is reported that cured rubber can be broken down. Using this process, the rubber becomes soft, thereby enabling it to be reprocessed and reshaped in a manner similar to that employed with previously uncured elastomers.
U.S. Pat. No. 5,602,186 discloses a process for devulcanizing cured rubber by desulfurization, comprising the steps of: contacting rubber vulcanizate crumb with a solvent and an alkali metal to form a reaction mixture, heating the reaction mixture in the absence of oxygen and with mixing to a temperature sufficient to cause the alkali metal to react with sulfur in the rubber vulcanizate and maintaining the temperature below that at which thermal cracking of the rubber occurs, thereby devulcanizing the rubber vulcanizate. U.S. Pat. No. 5,602,186 indicates that it is preferred to control the temperature below about 300° C., or where thermal cracking of the rubber is initiated.
U.S. Pat. No. 5,891,926 discloses a process for devulcanizing cured rubber into devulcanized rubber that is capable of being recompounded and recurred into useful rubber products, and for extracting the devulcanized rubber from the cured rubber, said process comprising (1) heating the cured rubber to a temperature which is within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×10
6
Pascals in 2-butanol to devulcanize the cured rubber into the devulcanized rubber thereby producing a mixture of solid cured rubber, solid devulcanized rubber and a solution of the devulcanized rubber in the 2-butanol, (2) removing the solution of the devulcanized rubber from the solid cured rubber and the solid devulcanized rubber, (3) cooling the solution of the devulcanized rubber in the 2-butanol to a temperature of less than about 100° C. and (4) separating the devulcanized rubber from the 2-butanol.
SUMMARY OF THE INVENTION
This invention is directed to a commercially viable technique for recycling large quantities cured rubber from reclaimed rubber articles. The technique of this invention involves grinding the reclaimed rubber to a particle size which is within the range of about 325 mesh to about 20 mesh and then devulcanizing the surface of reclaimed rubber crumb. It has been unexpectedly found that, by doing so, the surface devulcanized reclaimed rubber can be blended into and cocured with virgin rubber. This offers a tremendous commercial advantage in that it is only necessary to devulcanize the rubber on the surface of the reclaimed rubber crumb. Thus, the cost of the devulcanization procedure is only a fraction of the cost associated with devulcanizing the total quantity of reclaimed rubber being recycled.
The surface devulcanized reclaimed rubber of this invention can be used in manufacturing rubber articles that demand high performance characteristics (such as, tires, hoses and belts) when blended with virgin rubbers in quantities of up to about 40 phr. In fact, such blends of the surface devulcanized reclaimed rubber with virgin elastomers have cure properties and tensile properties that are comparable to blends made with totally virgin materials.
By utilizing the process of this invention, the surface of cured rubber crumb can be devulcanized using a simple technique without the need for microwaves, ultrasonic waves or an alkali metal. In other words, the surface of the cured rubber crumb can be devulcanized in the absence of microwaves, ultrasonic waves or an alkali metal. The employment of the process of this invention also preserves the original microstructure of the rubber and allows for it to maintain a relatively high molecular weight. Thus, the process of this invention primarily breaks sulfur-sulfur bonds and/or carbon-sulfur bonds rather than carbon-carbon bonds. The surface devulcanized reclaimed rubber can accordingly be used in the same types of applications as was the original rubber.
This invention more specifically discloses a process for devulcanizing the surface of reclaimed rubber crumb into surface devulcanized reclaimed rubber crumb that is suitable for being recompounded and recurred into high performance rubber products, said process comprising the steps of (1) heating the reclaimed rubber crumb to a temperature which is within the range of about 150° C. to about 300° C. under a pressure of at least about 3.4×10° Pascals in the presence of 2-butan
Beers Roger Neil
Benko David Andrew
Lee Katarzyna W.
Rockhill Alvin T.
The Goodyear Tire & Rubber Company
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