December 1. 1920 



THE INDIA RUBBER WORLD 



175 



acid dissociates and leaves hcxaxak-iit siilpluir, and the liberated 

 cyanhydric acid again functions as a catalyzer, as indicated above. 

 Thus, nitroso compounds would seem to react in the same way 

 as the aminic accelerators. 



The theory therefore is general and the true accelerator is 

 cyanhydric acid. In the presence of divalent sulphur, it changes 

 into unstable sulfocyanic acid at vulcanization temperature. 

 Under these conditions, the valence of the sulphur having been 

 modified to the liexavalent condition, is in the course of vul- 

 canization, while the regenerated cyanhydric acid would then 

 continue the cvclc of the reaction. 



THE DETERMINATION OF TRUE FREE SULPHUR 

 AND THE TRUE COEFFICIENT OF VULCANI- 

 ZATION IN VULCANIZED RUBBER' 

 By W. J. Kelly-' 



IT IS the object of this paper to present two methods, one for 

 the true free sulphur determination and the other for a more 

 accurate figure for the coefficient of vulcanization than could be 

 obtained by the older methods. 



The sulpliur present in vulcanized rubber may be divided into 

 four parts : 



Sulphur soluble in acetone: (1) combined with resins and 

 proteins; (2) true free sulphur. 



Sulphur insoluble in acetone: (1) combined with rubber; (2) 

 combined with resins and proteins. 



JIETHODS FOR DETEHMINING TRUE FREE SULPHUR 



Considering first the sulphur which is soluble in acetone, it is 

 evident that a solvent which would dissolve either the sulphur 

 or the organic material (part of which may contain sulphur of 

 combination) without affecting the other, would solve the problem. 

 Unfortunately, all solvents for either will dissolve both to a 

 greater or less extent. The entire extract very readily dissolves in 

 95 per cent alcohol. The alcohol will also dissolve about 0.04 grams 

 of sulphur in 100 cc. the amount of sulphur usually obtained in 

 an ordinary analysis. If, however, the extract is treated with 

 alcohol already saturated with sulphur, it will dissolve all of the 

 organic material present but none of the elementary sulphur. The 

 sulphur remains in the flask after the solution is decanted, and 

 can be weighed as such or oxidized and weighed as barium 

 sulphate. 



Prepar.\tion of .Sati'r.-\ted Alcohol. — The saturated solution 

 of sulphur in ethyl alcohol, to be referred to as "saturated 

 alcohol," is made by heating an excess of sulphur with the alcohol 

 for about three hours at 70 degrees to 75 degrees C. and allowing 

 to cool slowly to room temperature. Before used it is allowed 

 to stand at least 24 hours in order to insure complete crystalliza- 

 tion of the dissolved sulphur. As all later crystallizations are 

 carried out at room temperature a variation of a few degrees in 

 the temperature at which equilibrium is reached does not ma- 

 terially afTect the results. As a precaution, however, the solution 

 is kept in contact with solid sulphur to insure saturation at all 

 times. 



Details of Method. — A one-gram sample of the ground rubber 

 is extracted with acetone for 16 hours, in an Underwriters' ap- 

 paratus. The acetone is distilled off, the residue dried at 60 de- 

 grees to 65 degrees for half an hour and treated with .50 cc. of 

 saturated 95 per cent alcohol, and the flask and contents weighed. 

 An accuracy of O.S-gratn is sufficient for this weighing. It is 

 then warmed to about 50 degrees C. for a few minutes to insure 

 complete solution of all the organic material and allowed to cool 

 slowly to room temperature. Before it has reached this tempera- 

 ture the flask is again weighed and any alcohol lost during the 

 heating is replaced with pure 95 per cent alcohol. The flask is 

 then well covered and allowed to stand for 24 hours to permit the 

 complete crystallization of any of the elementary sulphur of the 



extract whicli may have dissolved. The liquid is then care- 

 fully decanted, and the residue washed two or three times by 

 dccantation with 5 cc. of saturated alcohol and dried. It may then 

 be weighed directly, or oxidized and weighed as barium sulphate. 



It is unnecessary to wash the residue with pure alcohol inas- 

 much as the amount of sulphur left by the evaporation of the 

 saturated alcohol is very slight. One cc. of 95 per cent alcohol 

 will dissolve only 0.0004-gram of sulphur and it is easily possible 

 to decant the liquid so that not over 0.52-cc. will remain in the 

 flask. 



coefficient of vulcanization 



The combined sulphur in rubber stocks has been considered to 

 be the sulphur which was not cxtractable with acetone. In case 

 inorganic sulphides were present a correction was made. As 

 stated above, however, it is possible that some of this sulphur may 

 be combined with resins and proteins in acetone-insoluble com- 

 pounds. Hence, a method whereby the residual sulphur could be 

 determined after these compounds had been separated from the 

 rubber would give a much better value for the coefficient ot vul- 

 canization. 



It has been shown' that about 85 per cent of the resin of Hrzra 

 rubber is saponifiable. Such being the case, it is probable that 

 part at least of any resin sulphur compounds which are insoluble 

 in acetone would also be saponifiable and rendered soluble in 

 alcoholic potash. The protein matter will also be hydrolyzed and 

 some of these products will dissolve in the alcoholic solution. In 

 any case any sulphur extracted by this method will not be sulphur 

 which has been combined with the rubber. The many failures to 

 remove the combined sulphur by any means substantiate this view. 



The experiments of Spence* show that all of the free sulphur 

 is extracted by acetone in about 8 hours, and hence it is probable 

 that a 16-hour extraction will remove every trace of it. 



.\ few experiments showed that from the stock used to deter- 

 mine the true free sulphur, alcoholic potash would remove an 

 additional 0.30 to 0.35 per cent of sulphur. In order to show 

 that this was not due to the removal of sulphur combined with 

 the rubber or to the disintegration of the sample, successive ex- 

 tractions were run on the same piece. Two 4-hour extractions 

 removed all the sulphur, absolutely no precipitate of barium sul- 

 phate being obtained when the third and fourth extracts were 

 oxidized and treated with barium chloride. 



Details of Method. — A one-gram sample of ground rubber is 

 extracted 16 hours with acetone and the residue dried. It is then 

 boiled for 8 hours with 75 cc. of a 5 per cent alcoholic potash 

 solution and washed once or twice with hot alcohol. As the 

 removal of the last traces of alkali is very difficult the sample is 

 extracted 16 hours (over night) with alcohol in an Underwriters' 

 apparatus. The solutions are then mixed and the alcohol distilled 

 off. The residue is oxidized first with 15 cc. of a solution of 

 bromine in potassium bromide (120 grams potassium bromide and 

 160 grams bromine in one liter of water) in order to prevent the 

 violent action of fuming nitric acid on alkali, and finished with 

 nitric acid. The barium sulphate is precipitated in the usual 

 manner. 



As there is always considerable silica fornud from the glass 

 it is necessary to evaporate twice with hydrochloric acid, thor- 

 oughly dehydrate, and remove the silica before precipitating the 

 sidphate. 



APPLICATION OF METHODS 



These methods were applied to a series of cures on a stock 

 made up of 1(X) parts of pale crepe and five parts of sulphur. 



^Abstracted from the paper read before tbe Rubber Division of the 

 .American Chemical Society at St. bonis, Missouri, .'\pril lJ-16. lyJO. 



-K< search Labrratorics. The (loodyear Tire & Rubber Co,. Altron, Ohio. 

 ^Hinriehsen and Memmler, "Der Kantschuck und seine Priifung." 

 'Kolb.id-Zeitschrift. 9. 300. 



