68 



THE INDIA RUBBER WORLD 



(November 1, 1916. 



fliience on the vulcanization of caoutchoucs by means of sulphur, 

 nitro-compouiuls or peroxides; they increase the extensibility 

 and tlie constant K', i.e., the tensile strength of the vulcanizate. 



On normal vulcanization by means of benzoyl peroxide the 

 physical structure of caoutchouc is not destroyed. It is, how- 

 ever, necessary to avoid excess of the peroxide and, for every 

 given benzoyl peroxide mixture, to establish exactly the necessary 

 temperature and time for the vulcanization. If not, the vul- 

 canizate will exhibit, like "abnormal" and also like chemically 

 pure taoutchoucs, negligible extensibility and tensile strength* : 

 the protein compounds may be oxidized by the benzoyl pero.xidc, 

 and their destruction may be accompanied by that of the pliysical 

 structure of the given caoutchouc. 



Caoutchoucs normally vulcanized by benzoyl pero.xide exhibit 

 both qualitatively and quantitatively all the typical properties of 

 caoutchoucs vulcanized by either sulphur or nitro-compounds ; 

 when kept, they do not change.f Caoutchoucs vulcanized with 

 a slight excess of benzoyl peroxide soon (1-S days) develop on 

 their surface soft, colorless, crystalline leaflets, which are as 

 transparent as glass, and possess pronounced lustre ; after the 

 lapse of a longer time (1, 3 or 5 months) the vulcanizate begins 

 to oxidize and gradually becomes sticky ; finally it runs, becom- 

 ing converted into a sticky, more or less viscous, plastic masst. 

 The vulcanizate decomposes especially rapidly when in contact 

 with the original, non-vulcanized mixture, which evidently acts 

 as a "detonator." 



Conse(|uently, when different mixtures of caoutchouc and 

 benzoyl peroxide are either heated or stored, two processes take 

 place simultaneously: (1) Vulcanization of the original caout- 

 chouc, this being connected with partial or complete union of the 

 oxygen of the peroxide w-ith the caoutchouc, and (2) oxidation 

 of the caoutchouc by the benzoyl pero.xide with formation of the 

 highly sticky mass mentioned above. 



The relative rates of these two processes determine the effect 

 of the vulcanization, and experiment shows that these rates de- 

 pend on the proportion of benzoyl peroxide, on the temperature, 

 on the prolongation of the vulcanization, and on the nature and 

 quantities of the foreign matters in the initial mixture. 



X'ulcanization of caoutchouc with benzoyl pero.xide requires, 

 therefore, increased attention or skill in the operator. 



When once started at a high temperature, the process of vul- 



*PresiiniabIy corresponding with over-vulcanization in the case of ordi- 

 nary rvibber and sulphur compounds. — H. P. S. 



tl have samples of vulcanized caoutchouc which have been kept for six 

 months without change. 



tSome caoutchoucs, for instance, normal erythrene caoutchouc, vulcanized 

 with a large amount of benzoyl peroxide, gradually solidify when kept, 

 yielding a dense, brittle mass, easily powdered but absolutely without 

 stickiness. 



T.XELE B.— VtJrX.\NIZ.\TION 



canization continues comparatively rapidly, even at the ordinary 

 temperature. Thus, it was found that a mixture of normal 

 erythrene caoutchouc and a small excess of benzoyl peroxide con- 

 verted after 27 days into a very sticky, viscous mass, which later 

 gradually runs or assumes the form of the containing vessel. 

 When previously heated, without access of air, for two minutes 

 at 85 degrees, the same mixture does not run when kept (at the 

 ordinary temperature) ; on the other hand, the stickiness already 

 present disappears spontaneously ; the plasticity of fresh sections 

 and their proneness to become sticky are lost, and the mixture 

 .gradually vulcanizes at the ordinary temperature, and finally even 

 over-vulcanizes owing to the excess of benzoyl peroxide present. 



It is seen that the relative velocity of oxidation, on the one 

 hand, and of vulcanization on the other, depends on the charac- 

 ter of the preliminary treatment, in the given case on the two- 

 minutes' heating at 85 degrees. 



This fact explains immediately why incomplete vulcanization 

 protects caoutchouc from oxidation or decomposition in the air. 



The benzoyl peroxide may be replaced by perbenzoic acid, and 

 probalily by ozone, ozoides of caoutchouc or terpenes, oxides of 

 nitrogen, certain metallic peroxides, hydrogen peroxide, etc. 



Further, my observations show that barium peroxide produces 

 no trace of vulcanization in caoutchouc. Into natural Para 

 caoutchouc were introduced 1 per cent, 5 per cent, 10 per cent, 

 15 per cent and 50 per cent BaO^., the mixtures being vulcanized 

 for 5 minutes, 10 minutes, 30 minutes, and 2 hours with steam 

 at 2, 3 and 4 atmospheres' pressure in a press ; under these con- 

 ditions the mixture underwent no change, its plasticity and even 

 its light color remaining qiiite unaltered. This interesting ob- 

 servation lends further confirmation to the fact that vulcanization 

 of caoutchouc by the above method takes place at the expense, 

 not of the peroxides themselves, but of their active oxygen. 



The accompanying tal?le contains recipes for the vulcanization 

 of different caoutchoucs with benzoyl peroxide. It must be 

 pointed out, however, that the external conditions indicated in 

 tliis table are by no means ideal.* 



To conclude, in presence of 0.5-3 per cent of benzoyl peroxide, 

 normal synthetic caoutchouc obtained on coagulation of its solu- 

 tion, undergoes at about 80 to 120 degrees C. incomplete 'vul- 

 canization ; the external appearance, and all the new properties 

 of the product obtained compel the assumption that some forms 

 of natural rubber represent products of incomplete (incipient) 

 vulcanization caused by active oxygen.f 



*The detailed recipes for the vulcanization of caoutchouc by means of 

 benzoyl peroxide, together with other documents kept in my pocket-book, 

 were unfortunately stolen from me. 



tOr by compounds containing 

 BY ORG.\NIC PEROXIDES. 



active oxygen, etc. 



° " - 3 •- =; 



>~ E c-= c -5 



.S'n £ - = 'i'>''x 



3 ^ Caoutchouc Employed. ^ °'« £ ^ H 



1 Natural Para 20 6 of 30% 



2 Natural Para 20 .4 of 20% 



i Mixture of experiment 1 6 



A Mixture of experiment 1 



5 Natural Para 5 1 of 20% 



6 Natural Para 5 I 



7 Natural Para 5 1 



Dimelhylerythrene "photopolymeride" 15.6 1.6 of 10% 



9 Normal erythrene 

 10 Natural Para .... 



l.J of 20% 



100 10 of 109 



1.5 



1:3 y. 





1.5 





5 ;:-c = t 



1 almos. (119") 



2 atmos. 

 1 atmos. 



15 

 5 



0.5 atmos. 

 2 atmos. 



15 

 15 



5 Observations. 



Vulcanization complete; prod- 

 uct quite transparent; pale 

 cinnamon color. 



X'ulcanization complete. 



Vulcanization complete; prod- 

 uct differs from No. 2 only 

 by its darkish color and its 

 n on- transparency. 



'Products of acid odor are formed, evidently identical with 

 fWhen kept, this product does not change in six months. 



Vulcanization complete; prod- 

 uct opaque. 

 15 \'ulcanization complete; prod- 

 uct opaque. 

 1.5 . . . . 2 atmos. 15 Vulcanization complete; prod- 



uct opaque and tougher and 

 more leathery than Nos. 5 

 and 6. 

 1 5 Almost complete vulcaniza- 

 tion ; product transparent; 

 at higher temperatures the 

 caoutchouc undergoes pro- 

 found oxidation.* 

 . . 6 of colophony 1 atmos. 6 Vulcanization complete; over- 



vulcanized even ; product 

 trnnsiTicent. 

 3 atmos. 30 Vulcanization complete: prod- 



uct opaque and possessing 

 those appearing when this caoutchouc is kept in the air. sufficient tensile strength.! 



2 atmos. 

 2 atmos. 



Vulcanized at 80° 



