May 1, 1919.] 



THE INDIA RUBBER WORLD 



421 



What the Rubber Chemists Are Doing. 



THE AGING OF VULCANIZED PLANTATION RUBBER, 



THE following abstracts of five important papers dealing with 

 the deterioration of vulcanized rubber by aging, present 

 the latest results of elaborate research on the part of three 

 highly esteemed authorities, Dr. H. P. Stevens,' Dr. O. de 

 Vries, °-'"' and W. Spoon." 



RESULTS BY DR. H. P. STEVENS. 



Lack of space compels omission of the curves illustrating Dr. 

 Stevens' papers. The results in his first paper are reviewed by 

 Dr. Stevens as follows : 



Experiments were conducted on crepe and sheet rubbers of 

 which the vulcanized specimens were retested physically at in- 

 tervals of a few months. It was shown that in those cases in 

 which the coefficient of vulcanization exceeded 3.2 per cent, de- 

 terioriation set in a short time after vulcanization, and that the 

 higher the coefficient the more rapid the deterioriation. 



Tliese experiments were extended to carry out again tests over 

 a longer period, and to comprise a wider range of types, includ- 

 ing rubber from matured coagulum (slabs). The mixings of 90 

 parts of rubber 10 parts of sulphur were cured at 35 pounds 

 steam pressure for 2, 2^, 3, 35^2, 4 and 4J4 hours, and the speci- 

 mens tested one week after vulcanizing and subsequently at in- 

 tervals extending over 120 weeks. The specimens were pre- 

 served in the dark, or in semi-darkness, but otherwise no par- 

 ticular care was taken with them and they were exposed to the 

 ordinary fluctuations of room temperature. 



When comparing the results it must not be forgotten that 

 determinations of breaking strain are subject to appreciable ex- 

 perimental error W'hen the rubber is overcured and brittle or 

 "perished." 



The effect of aging on the physical properties is more evident 

 at summer temperatures than at those of winter. 



The coefficient of vulcanization was determined shortly after 

 vulcanization and again at the conclusion of the aging period. 

 The results were as follows : 



Rolled Sheet. Unrolled Sheet (Sl.\b). 



Beginning 

 of Aging 

 Period. 



jf Aging 



Period. 



3.16 



4.17 



End of Aging 



Period. 



3.44 



These figures indicate that with coefficients of over three, there 

 is a tendency for the coefficient to increase with the aging of the 

 specimen, the efifect being greater the higher the coefficient. It 

 is probable that this increase is accompanied by decomposition 

 of the rubber. It is well known that sulphuric acid is formed 

 when rubber perishes. It appears, however, that where the co- 

 efficient is low appreciable deterioration does not set in during 

 the period under test, and little or no increase takes place in the 

 coefficient. 



The higher the coefficient the shorter the period of increase 

 in breaking strain. The figures are approximately as follows : 



Period Ouring 

 A'hich Increase 



Breaking Strai 

 Takes Place. 

 . . . 35 weeks 

 ...22 " 

 ...18 " 



Period During 



Which Increase o 



Breaking Strain 



Takes Place. 



21 weeks 



15 " 



These results confirm the conclusion drawn from earlier ex- 

 periments that a rubber vulcanized to give a coefficient of 3.5 is 

 certainly overcured and cannot be expected to retain its physical 

 properties for a reasonable length of time. To secure approxi- 

 mate permanency the coefficient should not exceed three per 

 cent. Finally, it is clearly seen that the maximum breaking 

 strain cannot be obtained without overcuring the rubber to such 

 a stage that it commences to deteriorate within a few days of 

 vulcanization and loses tensile strength at an average rate of 

 about one per cent per week. 



In his subsequent communication. Dr. Stevens gives the re- 

 sults of a series of experiments exactly similar to those above 

 reported, on smoked sheet, smoked "slab," ordinary pale crepe, 

 and smoked crepe. 



The figures for the coefficients of vulcanization determined at 

 the commencement and conclusion of the aging experiments were 

 as follows : 



Aging Period. 

 (1) Smoked (2) Smoked 



Start. 

 2.82 

 3.37 



End. 

 2.92 

 3.53 

 4.42 

 5.63 

 6.16 

 6.80 



These figures confirrr 

 the increase with age in 

 overvulcanized samples. 



and those 

 suits : 



the previous conclusion, namely, 

 the coefficient is more noticeable 

 Thus, an analysis of the above figures 



in the previous paper gives the following 



crage Coefficients Betw 



Average Increase in 



Coefficient During 



Aging Period. 



Tabulating the maximum breaking strains corresponding to 

 coefficients of four to five, obtained from tests made shortly 

 after vulcanization, we have the following: 



rage Coefficients Betwe 



5.69 



6.56 



Maximum 

 Breaking Strain. 

 1,560 



.790 



Tliese figures show clearly that the maximum breaking strain 

 is not attained without considerably overcuring the rubber. All 

 types of rubber, when cured to give a coefficient of 4-5, attain 

 the maximum breaking strain within 10 or 15 weeks of vulcani- 

 zation, after which they rapidly lose strength. In a year or so 

 they become hardened and perished. On the other hand, if the 

 vulcanization be carried only so far as corresponds to a co- 

 efficient of 2-3, there is obtained with all types of rubber a 

 product which, although only of medium strength when freshly 

 cured, improves on an average for about a year and then only 

 very slowly loses strength, so that at the end of two years the 

 rubber is still stronger than when first vulcanized. 



The correlation of the coefficient and the period during which 



'11 V si->.,, Il,e Agine of Plant.ition Rubber." "Journal of the 



Sod. u .1 I :: r ,; • InJustry," November 30, 1918. and December 31. 1918. 



•n - . ' W. Spoon. "Changes in Vulcanized Rubber Shortly 



.Afi. r \ ., ■ ^'.Archief voor de Rubbercultuur in Nederlandsch- 



'O. de Vries, "Some Remarks on the Aging of Vulcanized Rubber." 

 "The India-Rubber Journal." January 11. 1919. 



*0. de Vries, "Changes in Vulcanized Rubber at Elevated Temperature," 

 ".\rchicf voor de Rubbercultuur in Nederlandsch-Indie," November, 1918. 



