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. tion. 780 grams of a 5% solution of sodium hy The thiophenols may be used in the proportion droxide were then added. The autoclave was of from about 0.05% to about 5% by weight, closed and the mass heated with agitation to based on the rubber scrap. In most instances, 45 180° C. for 12 hours. The autoclave was then we prefer to employ from about 0.1 to about 1% cooled and the reclaim removed. The rubber of thiophenol, based on the rubber scrap. mass was separated from the solution and washed Generally, the. treatment with the thiophenols with water. It was then placed on an ordinary will be effected at temperatures .of from about rubber wa'sh mill and washed with water until 150° C. to about 200° C. and preferably from about 50 the wash water was no longer alkaline. The 150° C. to about _175° C. However, when the reclaimed rubber was then dried on an ordinary treatment with the thiophenol is caused to take smooth roll rubber mill and drawn out into a place in the presence of large amounts of water thin sheet. The chloroform extract was deter and sodium hydroxide is to be added to the re mined on this sample in the manner hereinbe ' action mass immediately upon completion of the 55 fore described. desired action of the thiophenol, the action of the ' EXAMPLE II thiophenol will preferably be caused to take place This example was exactly the same as Exam at temperatures of from about 175° C. to about ple I, except that no xylyl mercaptan was used 190° C. The action of the sodium hydroxide will in the process. In order to avoid variations in generally be caused to take place at temperatures 60 results due to any effect of the kerosene, a weight of from about 160° C. to about 200° C. and prefer of kerosene, equal to that introduced in Example ably from about 175° C. to about 190° C. I, was added to this control example. The in There has been considerable discussion in the crease in devulcanization of Example I over Ex literature as to what is the best method for deter ample II, as calculated by the formula given here mining the e?iciency of any reclaiming process. inbefore, was 27.7%. The consensus of opinion seems to be that the Another test was run, repeating Example I degree of devulcanization is intimately connected and Example II but using 1% of xylyl mercap with the percent of chloroform extract in the re tan instead of the 1/z% previously used. The in claimed rubber and this method has been used as 70 crease in devulcanization, brought about by the a criterion of the increase in devulcanization use of xylyl mercaptan in this case, was 46%. brought about through the use of our invention. EXAMPLE III Chloroform extractions were made on samples of products, prepared both in the presence and inv Alkali-pan process the absence of the thiophenols. The percent of 76 600 grams of ground whole tire rap and 1560 2,833,810 grams of a 5% sodium hydroxide solution were Table I I charged into an autoclave and heated with agita- ' tion to 180° C. for 12 hours. The, reclaimed Per cent rubber was ?ltered, washed and dried on the Pcptizing agent. bgseéghotn. creasePer cent in inde ordinary rubber mill, as described in Example I. _ . rubber ’ vulcanization 150 grams of this product were‘ torn into small. pieces and mixed with 1.5 grams of xylyl mer xylyl mercaptain. 0. 5 49. 6 Do ...... _- 0. 108 32. 8 'captan, introduced in the form are. 36% solu 0.05 12. 7 tion in kerosene. This mixture was then heated , - incovered enameled pans at 80 pounds/square 10 inch steam pressure for 18 hours. The reclaim These results show quite conclusively that the was then milled into a thin sheet on a 70° C. use of as small ‘an amount of xylyl mercaptan as mill. The chloroform extract of this reclaim 0.05% still results in a very marked increase in was determined in the standard manner.’ the amount of devulcanization obtained by the 15 “modi?ed alkali process.” EXAMPLE IVv The modi?ed alkali process of Example V and This was exactly' the same as Example III, further illustrated in Tables I and II constitutes except that no xylyl mercaptan was used. As in - the preferred embodiment of our invention. the case of the other control examples, a weight . In order to illustrate more clearly the differ of kerosene, equal to that added along with the 20 ences between inner tube scrap and whole tire xylyl mercaptan in Example III, was added to scrap, the different problems involved and the this control. The increase in devulcanization advantages of our- invention, the following ex of Example III over Example IV, as calculated amples are given: by the formula hereinbefore given, was 83.2%. EXAMPLE VII EXAMPLE V 25 150 grams of ground red innertube scrap, 390 Modi?ed alkali process g. of. 5% sodium hydroxide solution and 1.5 g. of A mixture of 300 grams of ground whole tire xylyl-mercaptan, introduced in the form of a 36% solution in kerosene, were charged into an scrap, 740 grams of water'and 1.5 grams of xylyl 30 mercaptan, introduced as a 36% solution in kero autoclave equipped with agitation and "heated to sene, 'was charged into an autoclave equipped 180° C. for 12 hours. The autoclave was then - with agitation and heated to 180° C. for 12 hours. cooled, the reclaim removed, washed free of The autoclave was then cooled to 30 :1; 5° C. and b caustic on a rubber wash mill and ?nally dried and sheeted out on' an ordinary rubber mill. The 134 grams of a 30% solution of sodium hydroxide 35 were added. The mixture was then heated to chloroform extract of this sample was determined 180° C. and held for 12 hours. The autoclave by the standard procedure. ‘ was then cooled. The reclaim was removed from EXAMPLE VIII the autoclave, washed free of caustic on arubber wash mill and ?nally dried and sheeted out on This example was identical with Example VII, an ordinary rubber mill. The chloroform ex 40 except that no xylyl-mercaptan was'added. As tract of this sample was determined by the stand in the case of the other control samples, a weight ard procedure. of kerosene, equal to that added in the kerosene EXAMPLE VI solution of xylyl-mercaptan in Example VII, was' ’ 45 added to this charge. The increase in devulcani This example was identical with Example V, zation of Example‘ VII over Example VIII, calcu- ' except that no xylyl mercaptan was added. As lated by the formula given above, was 40%. in the case of the other control examples, a weight of kerosene, equal to that added in the EXAMPLE IX kerosene solution of xylyl mercaptan in Example V, was added to- this charge. The increase in 50 \ 150 grams of ground whole tire scrap, 390 g. devulcanization of Example V over Example VI, of 5% sodium hydroxide solution and 1.5 g. of as calculated by the formula hereinbeforegiven, xylyl-mercaptan,‘ introduced as a 36% solution in kerosene, were charged into an autoclave was 49.6%. 4‘ Other experiments, similar to those of Exam equipped with agitation and heated to 180° C. ple V and Example VI using the “modi?ed a1 55 for 12 hours. The autoclave was then cooled, kali reclaiming process,” were carried out in the reclaim removed and washed free of caustic which other aryl mercaptans were substituted on a rubber wash mill. It was ?nally dried and . for the xylyl-mercaptan of Example V. The re sheeted out on an ordinary rubber mill and the sults of these tests are tabulated in Table I. chloroform extract of this reclaim determined by 60 the standard procedure. Table I EXAMPLE X - Per cent ' Per cent in This example was identical with Example IX, Pe tizin a cut we ight crease in de p g g bfusggezn vulcanization except that no xylyl-mercaptan was added. As 65 in the case of the other control samples, a weight ' of kerosene, equal to that added in the kerosene Para-thiocresol ______.. 0. 5 41. 4 Ortho-thio-cresol ______0.5 25 _ solution of xylyl-mercaptan in Example IX, was Crude thio-alpha-naphthol_-_. -. 0. 58- 56. 3 Thio-beta-naphthol ______._ 0. 5 28 added to this charge, The increase in devulcani 70 zation of Example IX over Example X,‘ calcu~ A series of experiments were carried out as in ’ lated by the formula given above, was 0%. - Example V in which the amount of xylyl mer ' EXAMPLE XI captan, used in the “modi?ed alkali process,” was varied. The results of this seriesof tests are The process of Example IX was repeated, vary given in Table II. ing the amount of xylyl-mercaptan and the times 4 2,333,810 and temperatures of treatment. The results are group, consists of carbon and hydrogen at tem 'shown in the following Table III: peratures of fromabout 150° C. to about 200° C. Table III for a. su?icient length of time to materially de ' vulcanize the rubber and to the action of a di lute aqueous solution of sodium hydroxide, at Per cent of Temperature Per cent in ' xylyl- mer of digestion, crease in de ‘ from about 175° C. to about 190° C. for a su?l captan used 0. vulcanization cient length of time to substantially destroy fab ric in the rubber. 0. 5 12 180 -—5 3. The method of devulcanizing rubber which l. 0 12 180 —3 ~ 1. 0 7% 180 —9 10 comprises subjecting whole tire scrapseparately l. 0 12 210 —-i to the action of a small proportion of xylyl mer captan at temperatures of from about 150° C. to From Examples 9‘ to 11 inclusive, it will be ap about 200° C. for a su?icient length of time to parent that the thiophenols are substantially in materially devulcanize the rubber and to the ac effective to devulcanize whole tire scrap in the 15 tion of a dilute aqueous solution of sodium hy presence of substantial amounts of alkali. By droxide at from about 160° C. to about 200° C. a comparison of such examples with Examples for a su?icient length of time to substantially de 1 to 6 inclusive, it will be apparent that by our stroy fabric in the rubber. . invention we have solved the problem and made 4. The method of devulcanizing rubber which it possible to obtain the advantages of both the 20 comprises subjecting whole tire scrap consecu thiophenol process and the alkali process in the tively to the action of a small proportion of a treatment of whole tire scrap. thiophenol at temperatures of from about 150° C. While we have disclosed the preferred embodi to about 200° C. for a suf?cient length of time ments of our invention and. the preferred modes to materially devulcanize the rubber and then to of carrying the same into effect, it will be readily 25 the action of a dilute aqueous solutionof sodium apparent to those skilled in the art that many hydroxide at from about 160° C. to about 200° C. variations and modi?cations may be madethere for a sufficient length of time to substantially in without departing from the spirit of our in destroy fabric in the rubber. vention. The above examples are given for il 5. The method of devulcanizing rubber which lustrative purposes only and we intend to claim 30 comprises subjecting Whole tire scrap consecu our invention broadly as in the appended claims. tively to the action of a small proportion of a Other thiophenols may be substituted for those thiophenol containing from one to two benzene illustrated in the examples. Some of such other‘ ‘rings and which, except for the sulfur of the thiophenols are: mercaptan group, consists of carbon and hydro Thiophenol 35 gen at temperatures of from about 150° C. to about 200° C. for a su?‘icient length of time to Mercapto-ortho-cresol materially devulcanize the rubber and then to 1-mercapto-2-naphthol - the action of a dilute aqueous solution of sodium Dithio-resorcinol hydroxide at from about 175° C. to about 190° C. Mono -t;1io-catechol ' 40 for a su?lcient length of time to substantially de Meta--chloro-thiophenol stroy fabric in the rubber. Para-phenyl-thiophenol 6. The method of devulcanizing rubber which 4-mercapto-diphenyl-methane ' 2 ,B-dimercapto-naphthalene comprises subjecting whole tire scrap to the ac 1,5-dimercapto-naphthalene tion of a small proportion of a thiophenol in the Z-mercapto-anthracene 45 presence of water and at temperatures of from Ethyl-thiophenols about 150° C. to about 200° C.‘ for a su?icient Propyl-thiophenols length of time to materially devulcanize the rub Butyl-thiophenols her, then adding an aqueous solution of sodium - Stearyl-thiophenols hydroxide to form with the water a dilute solu Thio—salicyllc acid 50 tion of sodium hydroxide and then heating the mixture at from about 160° C. to about 200° C. Para-hydroxy-thiophenol for a sufficient length of time to substantially Still other thiophenols will occur to those destroy fabric in the rubber. skilled in the art. Also mixtures of two or more 7. The method of devulcanizing rubber which thiophenols may be employed. Usually, it will 55 comprises subjecting whole tire scrap to the ac be preferred to employ mixtures such as techni tion of a small proportion of a thiophenol con cal mixtures of xylyl mercaptans and technical taining from one to two benzene rings and which, mixtures of thio-cresols as ordinarily produced except for the sulfur of the mercaptan group, in syntheses of such compounds. consists of carbon and hydrogen in the presence We claim: ' of water and at temperatures of from about 150° 1. The method of devulcanizing rubber which C. to about 200° C. for a su?icient length of time comprises subjecting whole tire scrap separately to materially devulcanize the, rubber, then adding to the action of a small proportion of a thiophe an aqueous solution of sodium hydroxide to form nol at temperatures of from about 150° Cfto with the water a dilute solution of sodium hy about 200° C. for a su?lcient length of time to 65 droxide and then heating the mixture at from materially devulcanize the rubber and to the ac about 160° C. to about 200° C. for a suf?cient tion of a dilute aqueous solution of sodium hy length of time to substantially destroy fabric in droxide at from about 160° C. to about 200° C. for the rubber. a su?icient length of time to substantially de 8. The method of devulcanizing rubber which stroy fabric in the rubber. > 70 comprises subjecting whole tire scrap to the ac 2. The method of devulcanizing rubber which tion of a small proportion of a thiophenol con comprises subjecting whole tire scrap separately taining only one benzene ring and which, except to the action of a small proportion of a thiophe for the sulfur of the mercaptan group, consists of nol containing from one to two benzene rings and carbon and hydrogen in the presence of water Ihich, except for the sulphur of the mercaptan 75 and at temperatures of from about 150° C. to 5 ‘ 2,333,810 ' about. 200°- C.~ior a suii‘lcient length‘of time to length of time to substantially destroy fabric in materially" devulcanize the rubber, then adding the rubber. \- . ‘ an aqueous solution of sodium hydroxide to form - 11.j'I‘he method otdevulcanizing rubber which with the water a dilute solution of sodium 'hy- ’ comprises subjecting whole tire scrap to the ac droxide and then heating the mixture at from tion or a small proportion of zylyl mercaptan in about ‘160°. C. to about 200° C. for a su?icient . the presence of water and at temperatures of from length of time to substantially destroy fabric in about 150° C. to about 200° C. for a su?iclent length oi! time to materially devulcanize the rub. > the rubber. . 9. The method of devulcanizing rubber which I ber, then adding an aqueous solution of sodium comprises subjecting whole tire scrap to'the ac ,10 hydroxide to form with the water a dilute solu tion of a small proportion of an alkylated thie tion of sodium hydroxide and then heating the phenol containing only one benzene ring and mixture at from about 160° C. to about 200° C. which, except for the sulfur of the mercaptan for a su?icient length of time to substantially group, consists of carbon and hydrogen and in j destroy fabric in the rubber. ' which the alkyl groups are lower alkyl groups in 12. The method of devi'lcanizing rubber which the presence of water and at temperatures of comprises subjecting whole tire scrap separately ‘from about 150° C. to about 200°C. for a suffi to the action of a small proportion of thio-alpha cient length of time to materially devulcanize the naphthol at temperatures of from about 150° C. rubber, then adding an aqueous solution of so to about 200° C. for a suf?cient length of time dium hydroxide to form with the water a dilute 20 to materially devulcanize the rubber and to the solution of sodium hydroxide 'and then heating action of a dilute aqueous solution of sodium the mixture at from about 160° C. to about 200° hydroxide at from about 160° C. to about 200° C. C. for a suf?cient length of time to substantially for. a suf?cient length of time to. substantially destroy fabric in the rubber. _ destroy fabric in the rubber. 10. The ‘method of devulcanizing rubber which 13. The method of devulcanizing rubber which ' comprises subjecting whole tire scrap to the ac comprises subjecting whole tire scrap separately tion of a small proportion of a thiophenol con- . to the action of a small proportion of thio-beta taining only one benzenering having substituted naphthol at temperatures of from about 150° C. thereon only one mercaptan group and one to to about 200° C. for a su?icient length of time. to materially devulcanize the rubber and to the ac two methyl groups in the presence of water and 80 at. temperatures» of from about 150°C. to about tion of a-dilute aqueous solution of sodium hy 200° C. for a su?lcient length of time to mate droxide at from about 160° C. to about 200° C. rially devulcanize' the rubber, then adding an for a su?lcient length of time to substantially aqueous solution of sodium hydroxide to form destroy fabric in the rubber. ~ 7 - with the water a dilute solution “of sodium hy dromde and then heating the mixture at from ARTHUR MORRILL = about 160° C. to about 200° C. for a sufficient‘ JAMES RALPH scnAFFER.