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THE INDIA RUBBER WORLD 



Vulcanization b> the Use of the Sulphur Reaction Product 

 of a Nitrogen Accelerator. 



BRITISH Patent No. 130,857, recently granted, covers vulcan- 

 ization of rubber by the use of the sulphur reaction product 

 of a nitrogen accelerator. 



The specification states that in the curing of rubber no prac- 

 tical or commercial product has ever been produced from the re- 

 action of sulphur with caoutchouc without the aid of a nitrogen- 

 containing body present during vulcanization. This nitrogenous 

 body may be found in natural rubber or may be added in 

 the process of manufacture. The absence of all nitrogen, how- 

 ever, gives a vulcanization product that is of no value com- 

 mercially. It has been the practice to add to the rubber before 

 vulcanization certain nitrogen bodies (accelerators'), which ap- 

 pear to have the effect of shortening the time required to cure 

 the product and to improve its properties. 



It has been found that these nitrogen accelerators are not 

 usually the ultimate bodies which assist in the vulcanization of 

 caoutchouc, but that they must first react with sulphur and that 

 the sulphur reaction product thus formed is the agent which 

 either aids or is entirely responsible for the satisfactory vulcan- 

 ization of the rubber with sulphur. 



Although this principle is believed not to have been recog- 

 nized heretofore, a specific example has been proposed, namely 

 the use separately as accelerating substances, of liquid and solid 

 products of the interaction beween paranitroso-dimethyl-aniline 

 or any of its homologs, and sulphur. The use of these partic- 

 ular accelerating substances is excluded from the present inven- 

 tion. 



Vulcanization of rubber under this invention is divided into 

 two distinct steps : first, the production of a sulphur reaction 

 product of a nitrogen accelerator, and second, vulcanization 

 proper, the reaction product being incorporated in the mix. A 

 number of advantages result. For example, the temperatures 

 used in vulcanization of caoutchouc are only occasionally and 

 by merest coincidence, those most suitable for a reaction be- 

 tween sulphur and a nitrogen accelerator. In some cases, as 

 in the use of hexamethylene-tetraminc, the reaction with sulphur 

 is violent, and produces large volumes of gas including hydrogen 

 sulphide and other malodorous compounds, resulting in the 

 formation of a vulcanized product which is often porous and 

 bad smelling. In other instances the temperature of vulcaniza- 

 tion is not sufficiently high to cause satisfactory reaction be- 

 tween the nitrogen accelerator and the sulphur and the accelerat- 

 ing effect is partly or wholly unattained. For example, carbanilide 

 is almost inactive at the temperature corresponding to 40 pounds 

 of steam pressure (141.4 degrees C.) while at 60 to 80 pounds 

 pressure (153 to 162 degrees C.) it shows very valuable qualities. 



By carrying out the reaction between the nitrogen accelerator 

 and the sulphur before compounding them with rubber, the ex- 

 act temperature best suited to this particular reaction may be em- 

 ployed and all undesirable by-product be removed before the fin- 

 ished reaction product is introduced into the rubber mixing. In 

 many cases the curing value referred to the nitrogen content is 

 more than doubled by using the sulphur reaction product of a 

 nitrogen accelerator instead of the nitrogen accelerator itself. 



A mixture by parts of sulphur 1, zinc oxide 16, and plantation 

 pale crepe rubber 16, cures to the best product in from 2^ to 

 254 hours at 141 degrees C. The addition of two parts of 

 anhydro-formaldehyde-aniline (methylene-aniline) shortens the 

 cure to about 45 minutes. 



This accelerator when caused to react with aniline and sul- 

 phur forms a sulphur-nitrogen compound 1% parts of which used 

 in place of the anhydro-formaldehyde-aniline in the above mix, 



shortens the time of cure to 20 minutes. This shortened cure 

 benefits the product as evidenced by higher tensile strength and 

 higher modulus of elasticity. 



By anhydro-formaldehyde-aniline (methylene-aniline) is meant 

 the reaction product of two molecules of formaldehyde on two 

 molecules of aniline, giving two molecules of methylene-aniline 

 which polymerizes and may be conveniently considered a di- 

 polymer. 



EXAMPLE I. 



The preferred method for producing this sulphur-nitrogen ac- 

 celerator is as follows : 210 parts (one molecule') methylene- 

 aniline and aniline are boiled under a reflux condenser for five 

 hours at 195 to 198 degrees C. (thermometer in the liquid). The 

 mixture is then cooled to 150 degrees C. at which point the 

 sulphur is added. The temperature is next raised slowly to 175 

 degrees C. and held until 56 to 60 parts by weight (about two 

 molecules) of hydrogen sulphide are lost. The free aniline is 

 distilled ofT by steam distillation, care being taken to remove 

 the aniline from the reaction mixture to such an extent that 

 the product will cool to a hard brittle mass. 



Thiocarbanilide or methylene-diphenyldiamine may be substi- 

 tuted in the foregoing example with the proper precautions and 

 a similar product may be produced having a higher curing power 

 than the original base. The reaction products thus obtained when 

 dried are suitable for use in rubber mixings and show many 

 advantageous properties as accelerators. 

 EXAMPLE n. 



Methylene-diphenyldiamine 198 parts (one molecule) ; sulphur 

 64 parts. These ingredients are slowly melted and heated to 120 

 to 150 degrees C. until 34 to 36 parts (one molecule) of hydrogen 

 sulphide has been removed. The reaction product is then steam- 

 distilled, removing about 93 parts (one molecule) of aniline 

 which has been produced during the reaction. 

 EXAMPLE III. 



Methylene-aniline, 210 parts (one molecule of di-polymer) ; 

 sulphur, 64 parts. The ingredients are slowly melted and then 

 heated to 150 to 170 degrees C. until 34 parts (one molecule) of 

 hydrogen sulphide are removed. 



There is also produced some carbon di-su!phidc, which may 

 be estimated by absorption in aniline and allowed for, although 

 this is not necessary. The product is then steam-distilled until 

 no more aniline will come over. 



EXAMPLE IV. 



Straight sulphur reaction on tri-phenyl-guanidine ; 287 parts tri- 

 phenyl-guanidine ; 64 parts sulphur. The mixture is heated to 

 225 degrees C. under reflux until 34 parts (one molecule) hydro- 

 gen sulphide is lost. The compound on cooling is ground and 

 compounded with rubber and has the same curing power, nitro- 

 gen for nitrogen, as tri-phenyl-guanidine itself. 

 EXAMPLE V. 



Sulphur on tri-phenyl-guanidine with 287 parts tri-phenyl- 

 guanidine ; 186 parts aniline ; 64 parts sulphur. The mixture is 

 heated gradually to 236 degrees C. until 34 parts of hydrogen 

 sulphide has been removed. In order to raise the temperature 

 the aniline must be partially distilled off with a simultaneous 

 evolution of gas. 'When the aniline removed equals 186 parts, or 

 nearly so, and the hydrogen sulphide has reached or passed 

 34 parts, the reaction is complete, and on cooling is found to be 

 a hard reddish-brown resin having a concoidal fracture. It is 

 equal in curing power to tri-phenyl-guanidine. 



The term "nitrogen accelerators" is intended to include, with 

 the exceptions cited above, all of those nitrogen-containing bodies 



