66 



THE INDIA RUBBER WORLD 



[November 1, 1916. 



Moiionitrobcnzene, however, gives completely satisfactory re- 

 sults.* 



The combining capacity of nitro-compounds increases with the 

 number of nitro-groups in the molecule, but we are convinced 

 that the vulcanizing power of nitro-compounds does not depend 

 on this cause. Ostromyslenski found that tetranitromethane unites 

 with ethylenic compounds of both the aromatic and aliphatic 

 series, but in no case has it been possible to vulcanize caoutchouc 

 with tetranitromelliane, although a large number of attempts have 

 been made. 



\arious other substances which, like nitro-ci impounds, arc able 

 to unite with ethylenic derivatives, have also been tried, among 

 them triphenylmethane and diami.notriphenylmethane. These 

 compounds, in perfect agreement with the above results, cause 

 no trace of vulcanization, the caoutchouc remaining sticky and 

 plastic, and retaining even its pale color.f These facts show that 

 the power of nitro-compounds to vulcanize caoutchouc is not 

 determined by their ability to combine with ethylenes. 



Is any role in the vulcanization played by the capacity of nitro- 

 compounds to o.xidize organic substances — by their property of 

 yielding active o.xygen with formation of nitroso-compounds? In 

 other words, does the vulcanizing action of nitro-compounds de- 

 pend on the combination of active o.xygen at the double linking 

 of the caoutchouc? This question must, as experiment shows, 

 be undoubtedly answered in the affirmative. First of all, nitroso- 

 and isonitroso-compounds do not vulcanize, as is shown by ex- 

 periments with nitrosobenzene and isonitrosocamphor under vari- 

 ous conditions. This result leads to the assumption that the vul- 

 canizing power of nitro-compounds belongs to one of the oxygen 

 atoms of the XO; radicle. It follows, therefore, that under suit- 

 able conditions caoutchouc should be vulcanized by ozone or 

 ozonides, or by various peroxides, per-acids, etc. 



This fundamental conclusion has been confirmed by direct ex- 

 periment, a second new method having been found for the hot 

 vulcanization of caoutchouc by compounds containing active oxy- 

 gen. Special attention has been paid to the vulcanization of natu- 

 ral and synthetic caoutchoucs with benzoyl peroxide and perben- 

 zoic acid. It is found that caoutchouc is vulcanized by benzoyl 

 peroxide incomparably more rapidly and easily than by sulphur or 

 even nitro-compounds. 



In order to confirm the deciding part played by the o.xygen 

 atom, attempts were made to vulcanize caoutchouc with barium 

 pero-xide. This substance yields its oxygen with moderate rap- 

 idity only at very high temperatures, and should not effect vul- 

 canization$ if the latter is determined by the combination of 

 oxygen at the double linking of the caoutchouc. Actual experi- 

 ment gives the results expected, since barium peroxide produces 

 no trace of vulcanization. 



These new methods of vulcanizing caoutchouc, and the favor- 

 able results obtained, are of undoubted scientific and practical in- 

 terest, and in the tirst place throw new light on the puzzling 

 mechanism of this process. 



We are convinced that the present day vulcanization of caout- 

 chouc begins -with a chemical process. Only certain classes of 

 substances — sulphur and some of its derivatives (Sj CU, Ca Sj. 

 etc.), nitro-compounds, pero.xides and per-acids — bring about vul- 

 canization. The physical constants and peculiarities of the vul- 

 canizing substance are without influence on the final effect. What 

 can there be common to the physical properties of gaseous oxy- 

 gen, sulphur, tetranitronaphthalene and perbenzoic acid? At the 

 same time it is sufficient to replace the oxygen of dinitrotriphenyl- 



* Sliglil r.dlu-sion between freshly cut surfaces, as is well known, does not 

 indicate that vulcanization is incomplete, especially with rubber which has 

 been only recently vulcanized. — H. P. S. 



t This again is not necessarily an indication that vulcanization has not 

 taken place. — H. P. S. 



I It has been already found that the melting point of the vulcanizing 

 substance does not affect the process. Thus, caoutchouc is readily vulcanized 

 by nitrobenzene, which is a liquid, and by tetranitronajihthalene, which 

 melts at 218 degrees, whereas the vulcanization proceeds at 116-145 degrees. 



methane by liydrogen or to remove from the nitro-group of nitro- 

 benzene one atom of oxygen, to obtain a compound — diamino- 

 iriphenylniethane, nitrosobenzene — absolutely devoid of the power 

 to vulcanize caoutchouc. 



In the process of vulcanization, chemical reactions are allotted, 

 therefore, a definite but still quite modest place. Chemical ac- 

 tion with the vulcanizing compound occurs with only a negligible 

 fraction of the initial caoutchouc. Thus, it is found that the 

 complete vulcanization of 100 parts of natural Para caoutchouc 

 requires only 0.5 parts of nitrobenzene or 1:3: 5-trinitroben- 

 zene. 



There can be no question here of molecular proportions, since 

 100 parts of Cm H,c would correspond with a minimum of 156 

 parts of C» Hs (NOOa- Even if it is assumed that Q„ H,„ re- 

 quires only one atom of active oxygen — which is not true — and 

 that the molecule of trinitrobenzene contains three atoms and 

 that of nitrobenzene one atom of active oxygen, 100 parts of 

 caoutchouc would require 52 parts of trinitrobenzene or 90 of 

 nitrobenzene. Even the corresponding solid ebonite is, however, 

 obtained by vulcanizing rublier in presence of 10-15 per cent, of 

 trinitrobenzene. 



Thus, W'ith the actual methods for vulcanizing caoutchouc only 

 a vanishing part of the latter enters into chemical reaction, but 

 this reaction is actually indispensable. The further course of this 

 interesting process is conditioned by physical interaction between 

 the vanishing quantity of caoutchouc which has reacted and that 

 which has remained unchanged. 



Thus, we arrive at the conclusion that the vulcanization of 

 caoutchouc is .divided sharply into two fundamental phases: (1) 

 A chemical reaction affecting only an insignificant part of the 

 caoutchouc, and (2) adsorption or swelling of the unchanged 

 caoutchouc into the product of this chemical reaction. 



Vulcanization may, however, be imagined as an exclusively 

 physical process, since theoretically it may begin with the second 

 phase of the proce.ss. Thus, instead of bringing nitro-compound, 

 sulphur, or peroxide into contact with caoutchouc, we may iso- 

 late and make use of the minute pjoportion of substance formed 

 in our first phase ; by heating this mi.xtnre we should undoubt- 

 edly obtain vulcanized caoutchouc. In such case vulcaniza- 

 tion takes place in a single phase — adsorption or swelling of the 

 initial caoutchouc into the mixed product, and represents an ex- 

 clusively physical process typical of caoutchouc. In vulcanization 

 by means of sulphur the existence of the latter in the free state 

 is of no importance, as it is necessary only for the preliminary 

 formation of its compound with caoutchouc, and then only in 

 negligible amount.* 



The elastic and other properties of caoutchouc vulcanized, for 

 instance, by trinitrobenzene, are qualitatively and quantitatively 

 identical with those of caoutchouc vulcanized with sulphur. Both 

 substances are devoid of plasticity and stickiness and exhibit 

 similar difficult solubility, etc. 



Only by chemical analysis might these two vulcanizates be dis- 

 tinguished, although they are obtained by treatment of caoutchouc 

 by absolutely different compounds. The nature of the vulcanizing 

 substance, is, therefore, almost without influence on the physical 

 properties, solubility and all the elastic properties of the resulting 

 caoutchouc ; it has, further, no effect on the chemical properties 

 of the vulcanizate, since the latter contains only a negligible pro- 

 portion of foreign substance. 



It may again be emphasized that the characteristic changes in 

 the properties of caoutchouc produced i)y vulcanization are de- 

 termined exclusively by a physical process — the adsorption or 

 "swelling" of the caoutchouc. 



These new methods of vulcanization of caoutchouc open up a 

 wide perspective, and it may be that the nitro-compounds, per- 

 oxides, and per-acids represent only the "first swallow" and that 



'It may be that this compound vulcanizes caoutchouc only when in "statu 

 Ttasccudi." 



