CA1132941A

CA1132941A - Microwave product flow additive

Info

Publication number: CA1132941A
Application number: CA332,901A
Authority: CA
Inventors: Melvin W. King; Keith A. Tyler
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 1978-08-25
Filing date: 1979-07-31
Publication date: 1982-10-05
Also published as: IT7950011A0; DE2934381A1; AU525500B2; JPS6150972B2; MX153126A; FR2434177B1; AU4969379A; GB2028835A; JPS5531886A; BR7905423A; FR2434177A1; GB2028835B; DE2934381C2; ZA794017B; AR222833A1; IT1120498B; PH14369A

Abstract

MICROWAVE PRODUCT FLOW ADDITIVE
Abstract of the Disclosure Sulfur-vulcanized elastomer containing polar groups can be readily and effectively devulcanized by grinding the sulfur-vulcanized elastomer to pass about a 3/8" screen to give particulates of elastomer and then pass the particles containing a coating of silicous partition agent such as fine silica or talc through a microwave field having a microwave frequency of 915 to 2450 mHz at a dose rate of 50 to 150 watt hours per pound and a temperature of between 450 and 800° F.

Description

ll~Z941 BACKGROUND OF THE INVENTION
.
This invention relates to an improved method for the devulcanization of rubber by the use of microwave energy which results in the production of a material which can be recompounded and revulcanized into a new vulcanized rubber article.
PRIOR ART
It is known to employ microwave energy for many applications relating to rubber products. Examples of such known applications are vulcanization, bonding, preheating, drying, foaming, and foam curing. It is also known to employ microwave energy as well as irradiation to depolymerize rubber to form carbon black and liquid and gaseous hydrocarbons. U. S. patent 4,104,205 describes a basic method of devulcanizing cured elastomers with microwave energy.
Applicants' invention is an improved method of devuloanizing sulfur vulcanized elastomer containing polar groups which comprises applying a partition agent along with controlled dosesof microwave energy in an amount sufficient to sever substantially all carbon-sulfur and sulfur-sulfur bonds and insufficient to sever significant amounts of carbon-carbon bonds. The partition agents are silica, clay, talc and commercial silicates available under the trade designation Hi-Sil 215 or 233. Although various frequencies in the microwave spectrum could be employed by varying the dose and doee rate, as a practical matter only 915 and 2450 mHz are regulated for use in industrial processes of this type. The process is ecologically sound by recovering more than 95 percent of the total material needed in the process and no harsh chemicals are discharged into the environment. In 9~

, ~

~13Z941 addition, unvulcanized waste or unvulcanized waste contain~n~ some contamination from vulcanized pieces of similar materlal can be vulcanized and subsequently reclaimed in the same single stage process.
The present invention is directed to an improved method of devulcanizing sulfur vulcanized elastomer containing polar molecules which comprises applying a controlled dose of microwave energy of between 915 and 2450 mHz and between 50 and 150 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 the improvement comprising mixing particulates of said elastomer capable of passing a 3/8" screen with a coating of Silicous partition agent preferably powdery in form to form a mixture and subjecting said mixture to microwave energy.
In a preferred embodiment the method is performed where the elastomer is maintained in continuous motion by means of a transport frequency being between 915 and 240 mHz; the dosage being between 50 and 150 watt hours per pound and the elastomer temperature being between 450 and 800F and the vulcanized elastomer is ~ixed with from about 0.5 to 3% by weight of a partition agent selected from fine powdery silica.
Articles produced from the material resulting from the process of the invention have good physical properties ; and such material can be employed in the production of articles requiring higher physical properties than could be accomplished with the prior art reclaim. When the material produced by the process of applicants' invention B

ll~Z941 is mixed with ne~ rubber compound in a ratio which reflects the total reuse of the commercial waste produced (in ranges ~hich ma~ run up to 20 or 25%~ in a hose manufactur-ing process, the physical properties of the hose being produced are not noticeably degraded. Applicants have employed the process of the inv~tion on vulcanized compounded EPDM and butyl scrap and have been able to return the product of the inventive process to the original manufacturing process and indluded in the regular manufacturing process to make hose of the same kind without loss of physical properties on the hose employing the devulcanized and revulcanized stock. Prior art reclaiming processes produce a material which is uneconomical due to the poor physical properties of the reclaim. This invention will enable an economic waste to be reused. Beyond this, as will be seen in the examples, the material resulting from the process-o~ applicants' invention can be devulcanized and revulcanized a second , time through the waste recycling process without signifi-cant loss of physical properties in the ultimate vulcani-zate.

.

.

ll~Z941 In reusing the material made by the practice of applicants' invention, it is necessary to compound it with conventional amounts of zinc oxide and the customary vul-canizing agents prior to subjecting it to the normal vu~-canizing procedure. mis contrasts with the prior art reclaimed rubber which requires vulcanizing agents but no zinc oxide to revulcanize. The vulcanizing agents and zinc oxide may be added to the devulcanized rubber either before or after it is blended with the virgin rubber. Alterna-tively, the devulcanized rubber, virgin rubber, curatives and zinc oxide may all be mixed simultaneously.
Subsequent to the devulcanization process, applicants found it desirable to pass the devulcanized material through a refining step and then to form the resulting smooth sheet into the conventional slab form for convenience in process-ing it through the conventional equipment present in rubber processing plants.
Detailed Description of_Invention Rubber Startin~ Material In order to be usable in the process of the invention, the waste material must be polar in order that the micro-wave energy will generate the heat necessary to devulcanize.
This polarity may be an inherent characteristic of the rub-ber compound itself, as for example polychloroprene, nitrile rubber, or chlorinated polyethylene. In addition, the polarity may be achieved as a result of some other material compounded into the rubber, for example carbon black. The size of the starting product must be reduced to a size where the material is reasonably compact when il3Z94~

subjected to the ml rowave heating. Preferably it should be reduced to ~articles of a size able to pass through a 3/8 inch screen.
Microwave Source The microw~ave energy may be produced by any of the numerous commercially available industrial microwave generators which are known to the art. Either single or multiple applicators may be employed in practicing the invention. The variables in the process are the dose, dose rate, frequency, and temperature. These variables, while demonstrated by the examples disclosed hereinafter, will vary depending upon the particular vulcanizate being pro-; cessed but may also be varied with respect to each other on a given stock. In devulcanizing the elastomeric compounds generally employed today in manufacturing industrial pro-ducts, such as belts and hose, the dose will generally be 7 in the range from 50 to 150 watt-hours per pound and pre-ferably from 85 to 95 watt-hours per pound; and the fre- -quency which theoretically covers the microwave band of the electromagnetic spectrum will of necessity be restricted to 915 or 2450 Megahertz (mHz).
The most efficient relationship of dose~ dose rate and frequency must be determined empirically for each specific elastomeric compound being devulcanized. Different types of carbon black are known to affect the devulcanization rate.
In addition, if fillers such as silica are employed, the par-ticle sizes of the silica will affect the devulcanizing rate.
Processin~ Equi~ment The equipment employed in the invention may be either , ll;~Z941 batch or con-tinuous. The structure which contains -the material to be devulcanized must be constructed of glass, ceramic or other material which is transparent to microwave.
The best mode of practicing the invention known to applicarts is the continuous method of operation. This latter method utilizes a glass or ceramic tube inclined downward at an angle of about 15 and rotating at a rate to feed the stock through the tube at the desired microenergy exposure rate.
The rate of transport can be controlled by controlling the speed of rotation of the tube, which may be used to adjust the flow rate for different types of rubber. Also, the ground scrap rubber can be transported by an auger. A batch process is more subject to generation of temperature gradients and hot spots in the rubber being subject to the 5 microwave treatment than the continuous method.

~ EPDM hose end trim and butyl tire bladders were devulcanized by the process of the invention employing a rotating 4~' inside diameter glass tube feed inclined at 15 to move the rubber stock through the tube. By operating the microwave generator at maximum capacity and varying the microwave dose by means of controlling the speed of transport the effect of microwave dose on physical properties such as oscillating disk rheometer torque (L) and tensile strength may be determined. Such data enables an operator to establish operating conditions which result in a devulcanized product equivalent to a virgin control elastomer sample.
me rubber scrap ground to pass a 3/8" screen was charged at the rate of 100 parts by weight per minute to the 11;~2941 above transport tube alone with 0.5 to 3, and preferably 1, parts by weight of a finely divided powdery silica, available as Hi-Sil 215 or 233, a trade name of Pittsburgh Plate Glass Company. This gives a rubber scrap containing 0.5 to 3% by weight of partition agent with about 1~ being preferred.
me use of Hi-Sil 215 or 255 at the above rate permitted transport of rubber scrap through the transport system at faster speeds and thus required less energy to reclaim the rubber. Also, the powdered silica prevented the reclaimed rubber particles from adhering together in large chunks, thus preventing handling problems.
It is preferred to mix the ground scrap rubber and partition agent as they are fed into the transport tube and rotation of the tube will cause the partition agent to coat the particles of ground scrap rubber. This will reduce the tendency for the rubber particles to adhere together.
While certain representative embodiments and details have been shown for the purpose of illustrating the invention it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved method of devulcanizing sulfur vulcanized elastomer containing polar molecules which comprises applying a controlled dose of microwave energy of between 915 and 2450 mHz and between 50 and 150 watt hours per pound in an amount sufficient to sever carbon-sulfur and sulfur-sulfur bonds and insufficient to sever significant amounts of carbon-carbon bonds the improvement comprising mixing particulates of said elastomer capable of passing a 3/8" screen with a coating of silicous partition agent to form a mixture and subject-ing said mixture to microwave energy.

2. The method of Claim 1 wherein the elastomer is maintained in continuous motion by means of a transport system while present in the microwave field; the microwave frequency is between 915 and 2450 mHz; the dose is between 50 and 150 watt hours per pound; and the elastomer temperature is between 450 and 800°F.

3. The method of Claim 1 wherein the vulcanized elastomer is mixed with from about 0.5 to 3% by weight of a partition agent selected from fine powdery silica.