Patented Nov. 9, 1943 ‘ ‘2,333,810 UNITED STATES PATENT OFFICE \' DEVULCANIZIN G RUBBER Arthur Morrill Neal and James Ralph Scha?er, Wilmington, Del., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a cor _ poration of Delaware No Drawing. Application May 28, 1940, Serial No. 337,630 I3 Claims. (Cl. 260-712) This invention relates to rubber and more par factorily by use of the thiophenols even in the ticularly to new and improved methods of devul presence of substantial amounts of alkali. canizing rubber. We have found that the treatment of whole tire Reclaimed rubber is more and more ?nding a scrap presents diii‘erent problems. Particularly, place in the rubber art not only because the‘ re we have found that the thiophenols are ine?ec claimed rubber is in general cheaper than natu tive as assistants for reclaiming whole tire scrap ral rubber but also because of certain properties in the presence of substantial amounts of alkali, which are obtainable through the use of _ re as in‘ the normal alkali process. While we are claimed rubber as a compounding ingredient not certain as to the exact reason for this and which are obtainable in no other manner. l0 phenomena, it appears most probable that it is There are two general methods used in the pro due to the fact that whole tire scrap contains duction of reclaimed rubber; these are: the open relatively large amounts of compounding in-_ steam or pan process and the alkali process. The gredients and ?llers, particularly carbon black, open steam or pan process is generally used in whereas inner tube scrap contains relatively reclaiming rubber stock which contains little or 15 small amounts of such compounding ingredients no fabric, e. g., the reclaiming of old inner tubes. and ?llers and generally no carbon black. The alkali digestion process is used in reclaim It is an object of the present invention to pro-' ing rubber which contains large amounts of vide an improved method for devulcanizing rub fabric, e. g., the reclaiming of old rubber tires. ber and particularly whole tire scrap. Another In this process the fabric is destroyed by the use 20 object is to provide a method for devulcanizing of sodium hydroxide. In carrying out this alkali ‘whole tire scrap, whereby devulcanized rubber, digestion process, the rubber scrap is ?rst ?nely having a greater degree of devulcanization and ground and then loaded into an autoclave in the any increased tendency to sheet out to a smooth presence of an aqueous solution of sodium hy sheet on a rubber mill, is obtained. A further droxide. In general, concentrations of 4 to 6% 25 objectris to provide a method of rendering thio of sodium hydroxide are used. The autoclave is phenols effective in the devulcanization of whole then closed and heated to 178° to 198° C. The tire scrap. A still further object is to provide time of digestion‘ varies from 8 hours for'the a method of combining the thiophenol devulcan higher temperature to 20 hours for the lower izing process with the alkali process to obtain temperature. This treatment causes almost 30 an improved devulcanized rubber from whole tire complete destruction and removal of the fabric. scrap. Other objects are to provide new com At‘the conclusion of this treatment; the auto positions of matter and to advance the art. Still clave is cooled down and the reclaimed rubber is other objects will appear hereinafter. washed well with water to remove the alkali and The above and other objects of our invention is then converted into a homogeneous mass either 35 may be accomplished by subjecting whole tire on a mill roll or in an internal mixer. scrap separately to the action of a small propor It has been proposed in the past to use many‘ tion of a thiophenol at temperatures of from di?erent softening agents in connection with the about 150° C. to about 200° C. for a su?icient reclaiming process. These have been products length of time to materially devulcanize the rub such as the various oils, tars and pitches derived 40 ber and to the action of a dilute aqueous solution from the distillation of wood, petroleum or coal, of sodium hydroxide at from about 160° C. to also fatty acids and asphalt‘ic base softeners. ' about 200° C. for a su?icient length of time to has been found, however, that the use of such substantially destroy fabric in the rubber. The materials as softening agents for the reclaiming r'treatment with the thiophenol may follow the is relatively ine?icient. Large quantities are re treatment with the sodium hydroxide solution, quired to e?ect any unusual softening of the re _ but, in such case, it is essential to wash most of claim and these large quantities of added sub the alkali from the-rubber prior to treatment stances produce reclaim stocks that aredi?icult with the thiophenol. to handle in practice. The tendency of the re When the treatment with ' claimers has been to reduce materially or elimi 50 the thiophenol precedes the treatment with the nate entirely the amount of these materials sodium hydroxide-no intermediate washing step ' which are added to the reclaim. , is necessary but the sodium hydroxide treatment B. S. Garvey in Patent 2,193,624 has proposed may follow the thiophenol-treatment without in to improve the processes of devulcanizing rubber terruption or intermediate handling of the prod and particularly inner tube scrap by carrying out 55 uct. We have found that, by such combination . the reclaiming processes in the presence thio of steps, the thiophenol is rendered e?ective to phenols. Such process operates quite satisfac substantially devulcanize the whole tire scrap torlly under the conditions disclosed by Garvey . and the resulting devulcanized rubber has a particularly when inner tube scrap is treated. greater degree of devulcanizatlon and an in Inner tube scrap may even be reclaimed satis 60 creased tendency to sheet out as a smooth sheet 2 2,338,810 on a rubber mill than could be obtained by either increase in devulcanizatlon was calculated by step alone and than would be expected from the using the following formula: results obtained from the individual steps. By the term “whole tire scrap,” we mean rubber (A—_I%£iq9=% increase in devulcanization tires which have been ground or broken up into relatively small pieces. Usually, the rubber tires where A=the weight of the chloroform extract will be ground, as is usual in reclaiming processes. of the product obtained through the aid of the The term “a thiophenol,” as employed herein thiophenols and B represents the weight of the and in the claims, will be understood to mean an chloroform extract of the product obtained in the aromatic compound containing a sulfhydryl absence of the thiophenols. (—SH) radical in which the sulfur is bonded di In carrying out the chloroform analysis, 10 rectly to a carbon atom in in the aromatic ring. grams of the material was ?rst extracted with In other words, they are phenols in which a acetone in a Soxhlet apparatus for 24 hours. The phenolic oxygen has been replaced by a sulfur purpose of the acetone extraction was to remove atom. The aromatic ring may contain sub any rubber resins or softening agents which might stituents such as alkyl, hydroxyl, halogen, addi be present in the compound and which might af tional sulfhydryl groups and the like. The thio fect the chloroform extract through their solu phenols may contain ‘one, two or more benzene bility in chloroform. The rubber, which had rings, but preferably will contain from one to two been acetone extracted, was then extracted with benzene rings and particularly a single benzene 20 chloroform for 48 hours in the same type of appa ring. Representative aromatic groups are phenyl, ratus. The chloroform was then evaporated of! tolyl, xylyl, biphenyl, naphthyl, methyl, naphthyl, and the weight of the chloroform extract deter anthracyl, diphenyl-methane and like groups. mined by evaporating to constant weight in a 50° Preferably, the thiophenol will be one which, ex C. oven. - cept for the sulfur of the mercaptan group, con 20 In order to illustrate our invention more clear sists of carbon and hydrogen, and, of these, the ly, the preferred modes of carrying the same alkylated thiophenols, containing only one ben into effect and the advantageous results to be ob zene ring and particularly those in which the tained thereby, the following examples are given: alkyl groups are lower alkyl groups, will be pre EXAMPLE I ferred. By “lower alkyl groups,” as used herein, 80 we mean saturated aliphatic hydrocarbon radi Pan-alkali process cals containing from 1 to 6 carbon atoms. Suit 300 grams of ground whole tire scrap were able alkyl groups are methyl, ethyl, propyl and placed in an enameled pan and mixed with 1.5 butyl. Amongst the compounds which we have grams of xylyl mercaptan introduced in the form found to be particularly satisfactory for our pur 35 of a 36% solution in kerosene. The enameled pose are xylyl mercaptan, para-thio-cresol, ortho pan was covered with a lid and heated in open thio-cresol, thio-alpha-naphthol and thio-beta steam at 80 pounds square inch pressure for 18 naphthol. Of these xylyl mercaptan is preferred. hours. At the end of this period, the steam pres By the term “xylyl mercaptan,” as employed sure was released, the pan and charge was cooled, herein and in ,the claims, we mean the technical 40 and the mass was removed from the pan and mixture of isomers prepared from the technical ' charged into an autoclave equipped with agita mixture of isomeric xylenes.