10 



THE INDIA RUBBER WORLD 



lOtTODEK 1, 1910. 



What the Ruhher Chemists Are Doln^. 



THK STAltll.ITV ( il- VULCANIZED RUBBEK AND THK Ol'TIMlM 



CURE. 



UNDER the aliove title Dr. Henry P. Stevens, in the "Journal 

 of the Society of Chemical Industry" (August 31, 1916), 

 presents the results of investigations made for the Rubber 

 Growers' Association. The author remarks that in the manu- 

 facture of vulcanized goods the greatest care is taken to adjust 

 the conditions of vulcanization to suit the particular compound 

 to be vulcanized. K.xcess temperature or time will over-cure the 

 product, and it will not age satisfactorily. Pronounced over-curing 

 may exhibit inferior mechanical properties within a very short 

 period of vulcanization, but in most cases the freshly vulcanized 

 compound at fir.st exhibits good mechanical properties, and it is 

 only after a period of months or years that the over-curing be- 

 comes apparent by the gradual perishing of the rubber. 



Pronounced under-curing also causes gradual deterioration. 

 but the effect is not so marked as with over-cured rubber, and is 

 more easily detected, as the mechanical properties of under-cured 

 rubber are not fully developed. 



It is obvious that for each particular rubber compound there 

 must exist certain conditions of vulcanization, principally time 

 and temperature, which will produce the optimum results. This 

 cure has been termed the "optimum" or "perfect cure." 



Schidrowitz and Eaton and Grantham have published their 

 work on this subject. Each of these works was undertaken to 

 compare the vulcanizing qualities of raw rubbers and to evolve 

 a standard method of testing. These methods are based on the 

 figures for tensile tests made on simple compounds of rubber with 

 sulphur only, vulcanized for varying periods under constant con- 

 ditions of temperature. 



Eaton and Grantham say "we have always taken as the opti- 

 mum, that point at which the product of the breaking load per 

 unit cross section and the elongation is a maximum." Strictly 

 speaking, they have taken the final length at rupture : that is, the 

 elongation plus the original length, and multiplied this by the 

 breaking load to give the product. This latter calculation ap- 

 pears to be the more correct. 



The corresponding figure obtained liy Dr. Stevens, termed 

 "Tensile product," represents the breaking load per unit cross- 

 sectional area of test piece, when the latter is fully stretched, as 

 the volume of a piece of vulcanized rubber does not show ap- 

 preciable alteration on deformation. The specimen giving the 

 maximum product is held by Eaton and Grantham to be that 

 which has been subjected to the optimum cure. It is obvious 

 that if a numl)er of vulcanizations be carried through with one 

 compound for progressively increasing periods, some specimens 

 may be under-cured, some over-cured and one will lie in the 

 neighborhood of the correct cure, but it would be a mere chance 

 if this last specimen were vulcanized for exactly the period re- 

 quired to produce the optimum effect. 



These methods fail to take into consideration the fact that 

 tensile properties of a vulcanized rubber are not constant, but 

 vary with the age of the specimen. 



Research work of recent years has shown that vulcanization 

 proceeds after the actual vulcanization process is completed, and 

 that vulcanization takes place, although very slowdy, at the ordi- 

 nary temperature of the air. 



Dr. Stevens regards the vulcanization process as a sort of im- 

 pulse, and the momentum induced carries on the vulcanizing 

 effect after the vulcanization process is complete. 



A specimen of vulcanized rubber, if tested at intervals of time 

 over a given period, shows progressive changes during that period. 

 Thus the product of the breaking strain and elongation of a 

 specimen tested the day after vulcanization will usually give 



ligures which are lower than those ohtaintd if llie test be made 

 a week or a month later. The term "optimum cure" if it is to 

 have any practical meaning, must connote not only the physical 

 properties of the freshly vulcanized rubber but also its aging. 



There is another guide to correct curing, namely, the figure for 

 the coefficient of vulcanization. With many types of compound 

 this figure is difficult to ascertain, even approximately, but in a 

 simple type, consisting entirely of rubber and sulphur, it is only 

 necessary to extract exhaustively with acetone and to determine 

 the sulphur in the residual rubber, expressing the results as a 

 percentage of the raw rubber originally taken. 



Dr. Stevens has carried through a comprehensive series of 

 tests with the compound employed by Eaton and Grantham, com- 

 posed of rubber with 10 per cent of sulphur. In each case the 

 tests were carried out on the same lines as those of Eaton and 

 Grantham, except that the vulcanized specimens were tested at 

 intervals, the last test being put through approximately ten 

 months after vulcanization. The rubbers employed were pale 

 crepe and smoked sheet rubbers of ordinary commercial quality. 



The results of the investigation prove conclusively that a rub- 

 ber compound subjected to Eaton and Grantham's "optimum 

 cure" is undoubtedly over-vulcanized, or over-cured, and that 

 the word "optimum" is consequently incorrectly applied. If, 

 however, Eaton and Grantham's definition be modified so as to 

 include the aging factor, we have what appears to be a very 

 good definition of the optimum or correct cure. It may be pro- 

 visionally defined as that cure which gives the maximum figure 

 for tensile product after full period of aging. It is not at pres- 

 ent possible to define more exactly the period of aging. With 

 rubber compounded with 10 per cent of sulphur only, the re- 

 sults show that the period should exceed the ten months allowed 

 in the present experiments. Probably two or three years will 

 be required. The vulcanized specimens must be preserved under 

 suitable conditions and should not be exposed to light. It must 

 not be forgotten that the temperature and possibly also hygro- 

 metric and other conditions of the atmosphere affect both the 

 aging qualities and the physical tests. 



The coefficient of vulcanization can only be taken as an ap- 

 proximate guide to the condition or degree of vulcanization. A 

 figure for the coefficient exceeding 3.50 per cent for a vulcan- 

 ized rublier and sulnliur compound is an almost certain indica- 

 tion of over-curing. 



ANILINE OIL POISONING. 



In a communication to "The Journal of Industrial and Engi- 

 neering Chemistry," W. W. Sanders relates the experience 

 of the Goodyear Tire & Rubber Co., Akron. Ohio, with regard 

 to its manufacture and use of aniline oil. 



Since November, 1914. the company has manufactured about 

 1,500 tons of aniline, without a single fatality due to poisoning. 

 This is the result of a persistent campaign to eliminate every 

 jiossible source of danger and to care promptly for such cases 

 of poisoning as have actually resulted. Men who are working 

 constantly in the aniline plant and who have, in a measure, be- 

 come immune to the effect of aniline vapor, show bluish lips 

 and skin ; the poisoning does not seem to go beyond that stage, 

 and the men are not inconvenienced in any way. The severest 

 cases have resulted from the spilling of aniline, nitrobenzol. or 

 aniline hydrochloride directly upon the skin. 



Realizing that poisoning may be caused either by inhalation 

 of vapor or direct contact with liquid, the manufacturing pre- 

 cautions have been based on these two points. The building is 

 provided w-ith suction ventilating fans in the roof. Several 24- 

 incli fans, located in ihe exterior side wall of the building, play 



