Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Process of treating scrap or waste product containing solid...
Reexamination Certificate
2000-03-29
2003-04-15
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
Reexamination Certificate
active
06548560
ABSTRACT:
BACKGROUND OF THE INVENTION
Millions of used tires, hoses, belts and other rubber products are discarded annually after they have been worn-out during their limited service life. These used rubber products are typically 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 similar things where resistance to weathering is desirable. However, a far greater number of tires, hoses and belts are simply discarded.
The recycling of cured rubber products has proven to be an extremely challenging problem. This problem associated with recycling cured rubber products (such as, tires, hoses and belts) 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 new tires and other rubber products. 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 to 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 recured into new rubber articles if it can be carried out without degradation of the rubber. In other words, the rubber could again be used for its original intended purpose. However, none of the devulcanization techniques previously developed have proven to be commercially viable.
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. Toluene, naphtha, terpenes, benzene, cyclohexane, diethyl carbonate, ethyl acetate, ethylbenzene, isophorone, isopropyl acetate, methyl ethyl ketone and derivatives thereof are identified as solvents that can be used in the process disclosed by U.S. Pat. No. 5,602,186.
SUMMARY OF THE INVENTION
By utilizing the process of this invention, cured rubber can be devulcanized using a simple technique without the need for microwaves, ultrasonic waves or an alkali metal. In other words, the cured rubber can be devulcanized in the absence of microwaves, ultrasonic waves or an alkali metal, such as sodium, potassium, lithium, or cesium. 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.
This invention is based upon the unexpected discovery that cured rubber can be devulcanized by heating it to a temperature of at least about 150° C. under a pressure of at least about 3.4×10
6
Pascals in the presence of an organic solvent selected from the group consisting of alcohols and ketones having a critical temperature which is within the range of about 200° C. to about 350° C. The molecular weight of the rubber can be maintained at a relatively high level if the devulcanization is carried out in the presence of the organic solvent at a temperature of no more than about 300° C. This devulcanization technique does not significantly change the microstructure of the rubber and it can accordingly be used in the same types of applications as was the original rubber. In other words, the devulcanized rubber can be recompounded and recured into useful articles in substantially the same way as was the original rubber.
This invention more specifically discloses a process for devulcanizing cured rubber into devulcanized rubber that is capable of being recompounded and recured into useful rubber products, 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 the presence of a solvent selected from the group consisting of alcohols and ketones, wherein said solvent has a critical temperature which is within the range of about 200° C. to about 350° C., to devulcanize the cured rubber into the devulcanized rubber thereby producing a slurry of the devulcanized rubber in the solvent; and (2) separating the devulcanized rubber from the solvent.
This invention also reveals a process for devulcanizing cured rubber into devulcanized rubber that is capable of being recompounded and recured 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 a solvent selected from the group consisting of alcohols and ketones, wherein said solvent has a critical temperature which is within the range of about 200° C. to about 350° C., 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 solvent, (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 solvent to a temperature of less than about 100° C. and (4) separating the devulcanized rubber from the solvent.
DETAILED DESCRIPTION OF THE INVENTION
Virtually any type of sulfur-cured rubber can be devulcanized by utilizing the process of this invention. For instance, it can be used to devulcanize natural rubber, synthetic polyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber, isoprene-butadiene rubber, styrene-isoprene rubber, styrene-isoprene-butadiene rub
Hunt Lawrence Keith
Kovalak Ronald Roy
Cain Edward J.
Lee Katarzyna Wyrozebski
Rockhill Alvin T.
The Goodyear Tire & Rubber Company
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