572 



THE INDIA RUBBER WORLD 



May 1, 1921 



REACTIONS OF ACCELERATORS DURING VUL- 

 CANIZATION 



Il-A THEORY OF ACCELERATORS BASED ON THE FORMATION 

 OF POLYSULPHIDES DURING VULCANIZATION 



By Winfield Scou and C. W. Bedford' 



THE INVESTIGATION of Organic accelerators, as shown by the 

 literature of the past live or six years, appears to be con- 

 fined largely to a search for new compounds or a combination 

 of compounds to catalyze the addition of sulphur to rubber. It 

 has been shown tliat these accelerators are almost entirely organic 

 nitrogen compounds, and as a result nearly all classes of nitrogen- 

 containing substances have been tried. Furthermore, it has been 

 shown that the nitrogen of such compounds is basic or becomes 

 basic during vulcanization by the action of heat, sulphur, or hydro- 

 gen sulphide. 



It has been previously proposed that a sulphur reaction of the 

 accelerator is necessary, and certain reaction products in some 

 way make sulphur available for vulcanization. In some cases a 

 sulphur reaction is doubtless necessary to form the true acceler- 

 ator, which is a polysulphide. 



The latest theory for the' action of accelerators during vulcani- 

 zation is that of Kratz, Flower and Coolidge'. These writers 

 attribute the accelerating action of amines, such as aniline, to the 

 formation of an unstable addition product of aniline and sulphur, 

 in which the sulphur is temporarily attached to the nitrogen, mak- 

 ing it pentavalent. 



The compound thus formed gives up its sulphur to the rubber 

 and is then regenerated by a further reaction with sulphur. 



The writer believes that the mechanism of the action of amines 

 is represented differently from that given by the above investi- 

 gators, and that hydrogen sulphide is one of the important factors 

 in acceleration. It is believed that, in general, amines catalyze the 

 addition of sulphur to rubber. As a specific example, dimethyl- 

 amine, with hydrogen sulphide and sulphur, forms a derivative 

 of ammonium polysulphide as follows : 



(CH,),NH + H,S >(CH3).NH=SH 



(CH,),NH,SH + xS >(CH3).NH,SH 



II 



Sx 

 Polysulphide compounds similar to the above are considered to 

 be the true accelerators that furnish the sulphur necessary for 

 vulcanization. That this type of sulphur is available for vulcani- 

 zation has been shown by Ignaz Block*, who states that hydrogen 

 poly sulphides (HjS, and HjS,) will cure rubber at ordinary tem- 

 peratures. C. O. Weber' quotes Gerard and his work showing that 

 alkali polysulphides in concentrated solution will also vulcanize 

 rubber. 



ORGANIC ACCELEEATOB.S 



.'MI organic accelerators do not function in the same manner 

 as the bases, and for this reason the writers choose to divide 

 accelerators into two classes. 



I. Hydrogen Sulphide Polysulphide Accelerators. In this 

 class belong those bases which form polysulphides similar to 

 yellow ammonium sulphide 



II. Carbo-sulphvdryl Polysulphide .Accelerators. This 

 includes all accelerators that contain the grouping = C-SH. such 

 as the thioureas, dithiocarbamates, thiurams. mcrcaptans or the 

 disulphides which may be formed from them by oxidation or by 

 reaction with sulphur." 



^Presented before the Rubber Division at the mcetinR of the Ameriran 

 Chemical Society. Chicago. Illinois. September 610. 1920: Part I, The 

 India Rubber World, lanuary 1, 1920. pages 206-207 



'The Goodyear Tire & Rubber Co., Akron, Ohio, and Quaker City Rubber 

 Co.. Philadelphia, Pennsylvania. 



•Journal of Industrial and EngineerinR Chemistry, 12, 1920, 317. 



'German patent No. 219.525. 



•"Chemistrv of India Rubber," page 47. 



•AlthouKh the term pf lysiilphidc is applied to each class of accelerators. 

 it should be noted that they are distinct types. In Class I. the polysulphide 

 sulphvir is related to a sulphydryl group attached to nitrogen, while in Class 

 II the polysulphide sulphur is held by a sulphydryd group attached to 

 carbon. In the sc-called disulphides and their higher sulphides, the hydrogen 

 of the sulphydryl group is considered as having been eliminated in hydrogen 

 stilphide. 



To the first class belong all basic organic accelerators or such 

 compounds as produce basic accelerators under curing conditions. 

 Certain inorganic accelerators may also be included. These will 

 be discussed later in the paper. 



The second class includes certain of the Schitf bases' which 

 form thiourea derivatives by a sulphur reaction during the cure. 

 Further discussion of this class will be reserved for a later paper*. 



The phenylated guanidines belong to both classes, since at 

 curing temperatures they easily react with hydrogen sulphide to 

 form thioureas and free amines. Diphenylguanidine, for example, 

 gives thiocarbanilide and ammonia. 



The present paper deals with the first-mentioned class of ac- 

 celerators, i. e., with those which, in the presence of hydrogen 

 sulphide under curing conditions, form polysulphides analogous 

 to those of sodium and ammonium. 



The structural relationships of the polysulphides of the nitrogen 

 bases and the more positive metals are not clearly understood 

 at present, although it is known that some of the sulphur is held 

 in a more or less loose form of chemical combination. This is 

 evidenced by the precipitation of sulphur from concentrated 

 solutions on dilution, and the generation of heat when sulphur 

 dissolves in sulphide or hydrosulphide solutions. It is certain that 

 the sulphur of polysulphides is quite different from rhombic or 

 a-sulphur, and that the aggregate S, is changed to the sulphur of 

 polysulphides by the combined action of hydrogen sulphide and 

 basic accelerators. 



It is a well-known fact that sulphur will react with rubber 

 resins and proteins at temperatures near 140 degrees with the 

 formation of hydrogen sulphide. This hydrogen sulphide in the 

 presence of basic accelerators forms hydrosulphides which in 

 turn take up sulphur to form polysulphides These polysulphides 

 pass on part of their sulphur to the rubber and constitute the true 

 curing agents. Such a mechanism applies also to the curing 

 action of alkali and alkaline-earth hydroxides. The fact that 

 basic magnesium carbonate will react with hydrogen sulphide and 

 sulphur in water suspension to form polysulphide solutions no 

 doubt accounts for its mild accelerating power. Lime and 

 magnesia do not function well in deresinated rubbers where much 

 of the hydrogen sulphide producing materials have been removed. 

 The sulphides and polysulphides of the alkali and alkaline-earth 

 metals should function in deresinated or synthetic rubbers. 



The relative accelerating power of the organic bases is de- 

 pendent upon the facility with which they form polysulphides 

 and the extent to which they are able to activate sulphur and 

 make it available for the rubber. This will, in some measure, 

 be dependent upon the basicity. In a previous paper by the 

 writers' it was stated that at least a part of the accelerating action 

 of hexamethylene tctraniine is due to the fact that during the 

 cure there are produced, among other products, ammonia and 

 carbon disulphide which, alone or with basic products present in 

 the rubber, form dithiocarbamates. It may be added that "Hexa" 

 also forms hydrogen sulphide by sulphur reaction, which with the 

 ammonia undoubtedly iorms ammonium polysulphides. This 

 accelerator may, therefore, be classed under both types since it is 

 both a hydrogen sulphide and a carbo-sulphydryl polysulphide ac- 

 celerator. 



.Aldehyde ammonia, by the action of heat alone, forms ammonia, 

 while with sulphur it also gives hydrogen sulphide. Heat also 

 produces other bases such as the alkyl pyridines or collidines. 

 This material appears to be solely a hydrogen sulphide polysul- 



'Bedford and Scott, Journal of Industrial and Fncineering Chemistry, 12, 

 1920, 31. 



•'The reaction cf carbon disulphide ^ on amine; to form thioureas and 

 hydrogen sulphide is reversible, and it is entirely possible that by the action 

 of hydrogen sulphide during vulcanization rhe thioureas are changed to the 

 more pov.'erful dithiocarb.'^matPs which are intermediate to the complete 

 transformation to amine and carbon disulphide. It is also prissiblc that the 

 thioureas may form polythio compounds direct, through the carbo-sulphydryl 

 group. 



"Journal of Industrial and Engineering Chemistry, 12. 1920. 3L 



