362 



THE INDIA RUBBER WORLD 



[April 1, 1919. 



What the Rubber Chemists Are Doing. 



NATURE OF THE VULCANIZATION PROCESS.' 



THE following observations are condensed from the remarks 

 of Dr. G. Van Iterson on the nature of the vulcanization 

 process. 

 The investigations of the Institute were inade primarily for 

 technical purposes, but offer an opportunity for some theoretical 

 considerations. In what follows, the results of others are taken 

 into account but the object is not completeness in every direction. 



1. A theory on the nature of the hot vulcanization process — 

 and it is that with which I intend to deal exclusively— will have 

 to reckon with the following established facts : 



a. The process proceeds additively, and formation of sul- 

 phureted hydrogen is subordinate. 



b. Sulphur is transferred into the so-called "combined" state. 

 It is then practically insoluble in the usual solvents. 



t-. The "binding" may practically be deemed "complete," the 

 opinion that part of the sulphur remains uncombined as "free" 

 subhur having been refuted." 



d. Very small quantities of combined sulphur make an im- 

 portant change in the properties of considerable quantities of 

 rubber. Rubber with one per cent combined rubber is entirely 

 different from ^ non-vulcanized rubber-sulphur with one per cent 

 Sulphur. The usual solvents do not dissolve non-vulcanized rub- 

 ber from the vulcanized. 



I 1'. The change in the properties of rubber during vulcaniza- 

 tion is perfectly continuous. Change of mechanical properties 

 iaries with the am-Qivnt of sulphur being taken up. Xo boundaries 

 can be delineated between prevulcanization, undervulcanization 

 |nd overvulcanization.' 



: /. The velocity of combining has a coefficient of temperature 

 between two and three.' The binding of sulphur begins -much 

 below the ordinary vulcanization temperatures.' 



g. The velocity of combining of sulphur varies considerably 

 for different varieties of rubber under equal conditions of tem- 

 perature, mixing, molding, etc." 



It. The velocity of vulcanization is strongly influenced by 

 some inorganic and organic compotinds. This influence, espe- 

 cially that of some organic compounds, is entirely catalytic' 

 Acids act retardingly, alkalies in weak concentrations, acceler- 

 atingly.' 



i. First latex plantation rubbers on being vulcanized under 

 constant conditions produce mechanical properties which strongly 

 correlate with the vulcanization coefficients." With accelerated 

 vulcanization, by catalyst or by excess' of sulphur, or under 

 greatly deviating conditions, this correlation ceases.'" 



/. Powerful mechanical treatment of the raw rubber changes 

 the mechanical properties of the vulcanized product, but not the 

 velocity of vulcanization." 



2. There is no agreement as to what is the maximum quantity 

 of sulphur which can be combined during vulcanization. A re- 

 view of the results of various investigators leads the author to 



'"Communications of the Netlierland Government Institute for Advising 

 the Rubber Trade and the Rubber Industry." Part VII, page 239. 



=Spcnce and Young, "Kolloid-Zeitschrift," 1912, page 28, and 1913, page 

 265. Skellon, "The Rubber Industry," London, 1914, page 172. 



Tart VI, page 216. 



•Dourn, "The India-Rubber Journal," 1913, page 120; Si ence and Young, 

 /. c: and Fol and Van Heurn, Part VI, page 187. 



'^riinricbsen and Kindscher, "Kolloid-Zeitschrift," 1911, page 245. 



'Stevens, "Kolloif'-Zeitschrift." 1914, p.ige 91; Gotllob, "Gummi-Zeitung." 

 1916, page 307; and Part VI, chapter 40. 



"Martin, "The Eiibbcr Industry," 1914. page 205. 



•Part V, chapter 40. 



'"Part VI,. chapter 60. 



"Spence and Ward, "Kolloid-Zeitschrift," 1912, page 274; and Part VI, 

 page 181. 



conclude that 32 per cent is probably not the maximum of sul- 

 phur which can be "combined" with caoutchouc, meaning such 

 sulphur as cannot be extracted with the ordinary extraction 

 liquids and under circumstances whereby all sulphur is removed 

 from a non-vulcanized rubber-sulphur mixture. 



3. An unsettled problem is that of the reversibility of the vul- 

 canization process. The fact ascertained by Spence and Young, 

 /. c, that no free sulphur is extracted from a mixture with ten 

 per cent of sulphur having previously been vulcanized for a long 

 time, even when extracting for 48 hours, is a strong indication 

 that there is no question of a reversibility by ordinary extrac- 

 tion. Bary and Weyndert" think themselves justified in con- 

 cluding the reversibility from the fact ascertained by them, that 

 sulphur can be extracted from vulcanized rubber, previously 

 deprived by extraction of free sulphur and then heated in a car- 

 bon dioxide atmosphere. Unpublished experiments carried out 

 at the Institute make it evident that on heating vulcanized rubber 

 free from uncombined sulphur, a very important regeneration 

 occurs, characterized by the product becoming plastic without 

 ■iulphur being transformed from the combined into the free state. 

 On further continued heating a part of the rubber is trans- 

 formed into the soluble form and through this, part of the sul- 

 phur becomes "soluble." 



4. It has not been certainly determined whether the com- 

 bined sulphur takes the place of the bromine during bromination 

 of vulcanized rubber. 



5. Opinion is not settled as to the course of the combining 

 of sulphur with the hot vulcanization. 



Fol and Van Heurn found the following vulcanization coeffi- 

 cients after vulcanizing at SO pounds with varying quantities of 

 sulphur. 



too , 



Hbber.) 



14.30 6.85 



17.70 7.68 



21.20 8.32 



25.00 9.66 



Considering that about half the added sulphur was combined 

 .qt the close of the vulcanization in the above table, it appears 

 I hat for mixtures of 20 parts sulphur and less there is approxi- 

 mate proportionality between the vulcanization coefficient and the 

 quantity of added sulphur. 



6. Judging about the course of the combining of the sulphur, 

 one has to take into consideration that at the temperatures of 

 vulcanization in use in factories, only a limited quantity of free 

 sulphur is being taken up by the rubber. Skellon has shown 

 that at about 130 degrees C. ten per cent free sulphur can be 

 taken up very rapidly in non-vulcanized rubber, by diffusion or 

 adsorption. 



7. For the explanation of a rectihnear course of the curve 

 representmg the binding of sulphur with the duration of the 

 vulcanizing process one encounters difficulties when a rectilinear 

 course is assumed for all sulphur binding lines. In that case a 

 different rectilinear course will have to be explained for dif- 

 ferent additions of sulphur. Now the straight line points to an 

 independence of the velocity of binding of the concentration of 

 the free sulphur, while the different direction of straight lines 

 would show precisely the dependence on increasing additions of 

 sulphur. It is very probable that the rectilinear course with ten 

 parts of sulphur and the doubly bent course with higher sul- 

 phur concentrations, must be attributed to the same cause, namely, 

 an auto-catalytical acceleration of the process. In order to ex- 

 "^■Comptes Ecndues," 1911, page 676. 



