November 1, 1916.] 



THE INDIA RUBBER WORLD 



65 



Mechanism of the Process of Vulcanization of Caoutchoucs. 



By I. I. Ostromyslenski. 



[From the "Journal of the Russian Physico-Chemical Society," 1915, pases 1,453-1,461. Translated from the original Russian by 

 Thomas H. Pof'e. B.Sc. Translation revised by Dr. H. P. Stevens, published in the "India Rubber Journal," September 30, 1916.] 



THE hot vulcanization of caoutchouc discovered by Goodyear 

 (1839) proceeds, as is well known, under simple conditions; 

 a homogeneous mixture of the caoutchouc and sulphur 

 is heated at 130 to 145 degrees. That is all. As a result, the ini- 

 tial caoutchouc loses its plasticity, and separate pieces of fresh 

 fractures no longer exhibit tlie power of adhesion. The solubil- 

 ity is lowered, and the "interval of elasticity" increased ; the fatal 

 temperature of well vulcanized natural caoutchouc lies at about 



— 35 degrees, that of the chemically pure product being about 



— 18 degrees. What takes place during the heating of the caout- 

 chouc? Attempts to explain this peculiar process have exhausted 

 all the theoretical possibilities. Some investigators regard it as 

 an exclusively physical process, and others as solely a chemical 

 reaction, whilst many authors consider vulcanization to be deter- 

 mined by both physical and chemical changes. 



Since all phenomena, at any rate, of unorganized nature, are 

 divided into only two groups — the physical and the chemical — 

 there can be no essentially new theory of vulcanization. Never- 

 theless, the nature of the mechanism of the process even yet re- 

 mains unexplained. 



The supporters of Weber's chemical theory regard vulcanized 

 solid caoutchouc (ebonite) as a polynieride of the compound, 

 C,„ H,j Si- (16 per cent of sulphur), whilst others, for instance, 

 Erdmann, consider it to be the thiozoide, C,„ H,j Sj, or even a 

 ditliiozonide. On the other hand, many identify the vulcaniza- 

 tion of caoutchouc with the process of "swelling" (Quellung) of 

 colloids or that of gelatinization or adsorption, that is, with the 

 processes of formation of solid or "semi-solid" solutions, etc. 



Some of the supporters of the "mixed" theory consider that the 

 sulphur itself swells or is adsorbed or dissolved in the free caout- 

 chouc, whereas other authors assume the preliminary formation 

 of a compound of the caoutchouc with the sulphur — although onlv 

 in insignificant amount — this compound being then adsorbed in 

 the still unchanged caoutchouc. 



1 shall not devote time to the extensive literature of this ques- 

 tion, but shall proceed immediately to the conclusions which re- 

 sult from my observations and my new methods for vulcanizing 

 caoutchouc. 



Until now no method of vulcanizing caoutchouc has been 

 known ni which any organic or mineral compound not contain- 

 ing sulphur is used as vulcanizing agent.* But the chemical and 

 especially the physical theories of vulcanization anticipate the 

 possible existence of a whole series of such compounds. I de- 

 cided to attempt to find substances which may replace sulphur 

 in the vulcanization of caoutchouc. 



It was thought that the investigation of the action of homo- 

 logues and analogues of such substances on caoutchouc and that 

 of the external conditions of the new process — the influence of 

 different admixtures, accelerators, etc. — might elucidate the mech- 

 anism of vulcanization itself. 



This task has now been completed, and two new methods for 

 the hot vulcanization of caoutchouc have been discovered. 



When heated with unsaturated hydrocarbons, sulphur produce* 

 a twofold effect : it combines at the double bond with forma- 

 tion of thiozonides (Erdmann), or it oxidizes the ethylene group- 

 ing, removing hydrogen in the form of hydrogen sulphide, a new 



'The process of vulcanization is often termed the sulphuring of caoutchouc. 

 Vulcanization hy calcium or sodium hypochlorite or free hypochlorous acid, 

 like vulcanization by hologens (bromine, iodine, or iodine bromide), leads, 

 as is known, only to "horny" rubber, i.e., to ebonite-like substances. Com- 

 pare Marckwald and Frank, "Uber Herkommen und Chemie des Kautschuks," 

 Dresden, page 62. 



ethylemc derivative, or a new compound containing sulphur being 

 thus formed.* 



On the physical side, sulphur is characterized, besides by the 

 ordinary constants (specific gravity, melting point, etc.), and by 

 its ability to exist in different polymorphic modifications (rhom- 

 bic, hexagonal, amorphous, etc.). 



In searching for organic substances which vulcanize caout- 

 chouc like sulphur, the first to be investigated are those which 

 resemble sulphur in oxidizing ethylenes, and at the same time 

 are able to unite at the double linking. Of the physical constants 

 of such substances the essential ones are the melting point and 

 the vapor pressure at the temperature of vulcanization; after 

 these, the solubility in caoutchouc, specific gravity, etc. Besides 

 possessing physical constants near to those of sulphur, the sought 

 for compounds should exist in polymorphic modifications. 



This explains why, in this investigation, I first of all made a 

 halt at compounds containing the nitro-group. These oxidize or- 

 ganic substances (c. g., in Skarup's synthesis of quinoline), and 

 at the same time readily combine with various ethylenes (atten- 

 tion may be called to the compounds of Ar (NO..) with poly- 

 cyclic hydrocarbons and to the author's use of tetranitromethane 

 as a reagent for double bonds). 



1 :3 : 5-Trinitrobenzene has a melting point, 118 degrees, very 

 near to that of sulphur, i.e., below the temperature of vulcaniza- 

 tion, and in specific gravity it also resembles sulphur. Furtlier. 

 most polynitro-conipounds exist in polymorphic modifications. 



1:3: 5-Trinilroben2ene was the first instance which I hoped 

 would serve as a substitute for sulphur in vulcanization. Ex- 

 periment completely confirmed my expectation. It was found 

 that both synthetic and natural caoutchoucs are vulcanized more 

 rapidly and easily by various nitro-compounds than by sulphur 

 itself under the same conditions. The result was a product pos- 

 sessing all the associated physical properties of caoutchouc vul- 

 canized by rneans of sulphur. Experiments were made with both 

 fatty and aromatic nitro-compounds, and vulcanization took place 

 with nitrobenzene, dinitrobenzenes, trinitrobenzenes, tri- and 

 tetra-nitronaphthalenes, picric acid, picramic acid, picryl chloride, 

 "artificial musk." nitro-cyclohexane, and many other compounds. 



Further investigation showed that the vulcanizing properties of 

 nitro-compounds do not depend on their capacity for combining 

 at the dculile linking. As is well known, picric acid combines 

 with ethylenic compounds considerably more readily than most 

 other nitro-compounds of the aromatic series, and yields more 

 stable products. Next in order come picryl chloride, picramic 

 acid, trinitrobenzene, etc.; dinitro- and mononitro-benzenes do 

 not unite at all with ethylenic derivatives. 



On the other hand, according to their vulcanizing power, nitro- 

 compounds are arranged in the reverse order, or, more accur- 

 ately, in an order which reveals no analogy between the proc- 

 esses of vulcanization and of combination at the double linking. 



Caoutchouc is vulcanized most rapidly and easily by 1 : 3 : 5- 

 Irinitrobcnzene, after which come dinitrobenzene, mononitroben- 

 zene, tetranitronaphthalene. Picric acid and picryl chloride do not 

 yield satisfactory products; vulcanization undoubtedly begins, but, 

 in spite of many scries of experiirients, 1 have never succeeded in 

 liringing it to completion ; the caoutchouc partially retains its 

 plasticity, and sticks when fresh fractures arc pressed together. 



*When acenaphthere is heated with sulphur, the hydrocarbon CigHis 

 (decacyclenc) is formed. 



