March 1, 1917.] 



THE INDIA RUBBER WORLD 



325 



What the Rubber Chemists Are Doing. 



COEFFICIEKT or VULCANIZATION AS A CHECK ON THE STATE OF 



CURE. 



DR. O. de Vries, Director of the Central Rubber Station at 

 Buitenzorg, Java, publishes in "The India Rubber Journal" 

 (January 20, 1917) a paper in which he presents proof 

 "That the chemical process of the combination of rubber and 

 sulphur is in itself quite independent of the physical one de- 

 termining the position of the stress-strain curve." 



Following is his presentation in condensed form : 

 The large differences in coefficient of vulcanization, noted by 

 Schidrowitz and Goldsborough, are not found by Dr. de Vries 

 in actual vulcanization practice, who points out that the per- 

 centage of combined sulphur does not increase at temperatures 

 below 80 degrees C. On the other hand, the curve shifts its 

 position even at ordinary temperatures, as shown by plotting 

 the figures for the heating points after different lengths of 

 time. These points are demonstrated by several series of ex- 

 periments. The first, in which a number of samples of the 

 same crepe rubber were vulcanized together but tested at dif- 

 ferent times, shows only small changes in the coefficient of vul- 

 canization, the percentage of combined sulphur not increasing 

 perceptibly. The position of the curve (length at load of 1.30 

 kilograms) changes a total of 42 per cent in length and in 

 this case carries with it practically no difference in coefficient 

 of vulcanization. The same effect is reached by keeping a vul- 

 canized sample for a longer time. It is not necessary to make 

 experiments for many months at the ordinary temperatures, a 

 few days at 70 to 75 degrees C. have the same effect. The curve 

 proceeds lower down on the paper, just as it does when the 

 time of cure is lengthened, but the percentage of combined 

 sulphur remains the same. From these experiments it is clear 

 th^t the reaction between sulphur and rubber, and the changes 

 in the rubber-sulphur mixture which determine the mechanical 

 properties as expressed by the stress-strain curve, are two 

 different processes, which may proceed independently, and that, 

 in general it is not feasible to make deductions from one re- 

 garding the other. It may be assumed that the mechanical 

 properties are more important, and that the curve is a better 

 gage to judge of the properties of the cured rubber than the 

 chemical condition as expressed by the coefficient of vulcani- 

 zation. 



METHODS OF ANALYSIS. 



NITROGEN IN HUBBEH. 



D J- EATON and F. W. F. Day, in the "Agricultural Bulletin," 

 *-*• Federated Malay States, August, 1916, describe their investi- 

 gations on the distribution of nitrogen in coagulum and serum of 

 Hevea latex. One hundred parts by weight of latex yielded 67 

 parts of wet coagulum and 33 parts of serum, the coagulum 

 being lifted from the pan and allowed to drain for a short time 

 before weighing. 



Following are the weight percentages of nitrogen contents of 

 the various portions: latex, 0.11; wet coagulum, 0.15; serum, 

 0.06. 



The nitrogen content of the serum, which was contained in a 

 tall, covered cylinder, fell after 14 days to 0.04 per cent, and 

 after 60 days to an average of 0.03 per cent, after which no fur- 

 ther loss appeared to take place. 



One hundred parts, by weight of latex, yielded 67 parts by 

 weight of unpressed coagulum or slab, and this on hand rolling 

 was reduced to 50 parts by weight containing 0.26 per cent of 

 nitrogen, equivalent to 0.78 per cent of nitrogen calculated on 

 dry material. 



Samples of similar coagulum were converted to sheet and crepe 

 on the day following coagulation with the following results : 



One hundred parts, by weight of latex, yielded 19.7 parts of 

 wet sheet containing 0.30 per cent nitrogen, equivalent to 0.35 

 per cent calculated on dry weight, which on drying was reduced 

 to 16.9 parts of sheet containing 0.38 per cent nitrogen. Also, 

 100 parts, by weight of latex, yielded 21.3 parts of wet crepe con- 

 taining 0.30 per cent nitrogen, equivalent to 0.38 per cent of nitro- 

 gen calculated on dry material. This on drying was reduced to 

 16.9 parts of dry crepe containing 0.40 per cent nitrogen. 



Thus the nitrogen content of the dry sheet is only slightly lower 

 than that of the dry crepe. It was observed that the nitrogen 

 content of the sheet and crepe is about twice as great as that of 

 coagulum left unpressed for a period of six weeks and then 

 washed and creped, and nearly twice as great as that calculated 

 for the dry coagulum after keeping, showing that little or no 

 alteration of nitrogenous constituents of the rubber has been 

 effected in the sheet and crepe, causing the formation of nitroge- 

 nous substances soluble in water or gaseous loss of nitrogen in 

 any form. 



METHODS OF TEST. 



PHYSICAL TESTING OF EUBBEE MAIEEIAtS. 



"T^HE following methods are standard for the physical ex- 

 amination of vulcanized rubber goods as specified by the 

 Board of Estimate and Apportionment of New York City. 



S.\MPLiNG. The contracting department shall select and take 

 all samples for testing. The number of samples and the quantity 

 to be taken from the deliveries will depend upon the size of the 

 articles and the quantity delivered. 



Samples shall fairly represent the delivery, and pieces shall be 

 taken from not less than one per cent of the number of units 

 delivered. 



AvER.'^GES. The results of tensile strength, elongation and set 

 tests as reported, shall be the average obtained from the samples 

 received by the laboratory. Not less than three test pieces from 

 each sample shall be tested and their results taken in calculating 

 the average unless some individual result is apparently in error, 

 in which case a retest shall be made. 



Temperature of Testing Room. Physical tests of rubber shall 

 be made with the temperature of the air not lower than 65 or 

 higher than 90 degrees F. 



Time. All measurements of time shall be taken with an ac- 

 curate stop watch. 



PREPARATION OF TEST PIECES. 



Test pieces of rubber shall be stamped out with a die, when- 

 ever practicable to do so. 



Tensile Strength, Elongation and Set Test Pieces. Test 

 pieces of rubber for tensile strength, elongation and set tests 

 shall be cut out with a die, either of the constricted bar or ring 

 type. The same test piece shall be used for making all three 

 tests. When the bar test piece is prepared a die should be used 

 that will make the constricted part of such a width that the 

 cross section will be approximately one thirty-second of a square 

 inch. 



All pieces for these tests shall have the backing entirely re- 

 moved, and any corrugations or irregularities of any kind shall 

 be accurately buffed off to make a uniform smooth surface. 



Test pieces which have become burnt in buffing shall be dis- 

 carded. 



Test pieces shall be kept constantly wet during the buffing. 



Test pieces of rubber valves and odd-shaped rubber articles 

 shall whenever possible be cut down on a lathe to an even thick- 

 ness of not more than one-eighth inch and then cut out to shape 

 for testing with a die. 



