422 



THE INDIA RUBBER WORLD 



increase in breaking strain takes place, ulien analyzed and aver- 

 aged gives the following figures : 



Average Period (Weeks) 

 During Which 

 Increase in Breaking 

 Average Coefficients Between Strain Takes Place. 



2 and 3 = 2.52 54 



3 and 4 = 3.43 33 



4 and 5 = 4.55 10 



5 and 6 = 5.69 •'' 



over 6 = 6.56 O 



The position of the breaking-strength curves is dependent on 

 the coefficient and not necessarily on the time of cure, showing 

 that the coefficient is the essential index of the degree of vul- 

 canization. 



The load required to produce a given elongation in sheet rub- 

 ber is greater than in crepe rubber when both types are cured 

 to give the same coefficient. The curves for elongation all slope 

 downward, showing a gradual reduction in the final length with 

 the period of aging. With low coefficients the reduction is very 

 gradual; with coefficients over five the vulcanized specimens are 

 perished before the full period of aging is completed. 

 The average figures are as follows: 



Percentage Decrease 

 Average Coefficients Between in Final Length. 



2 and 3 = 2.52 7.7 



3 and 4 = 3.43 13.9 



4 and 5 = 4.55 39.9 



over 5 = 6.02 nearly 100 



Conclusions. 

 These experiments illustrate the general instability of vulcan- 

 ized rubber in contrast to the raw material. Plantation rubber 

 may be stored for ten years and, when vulcanized, give results 

 comparable with those yielded by the freshly gathered raw mate- 

 rial. Vulcanized rubber, however carefully vulcanized, com- 

 mences to change in physical properties the momei.t the vul- 

 canization process is completed. When the original coefficient 

 exceeds three, appreciable deterioration takes place in two years. 

 This may resuh in a loss of 50 per cent of the tensile strength 

 when the coefficient reaches four, and complete "perishing" with 

 a coefficient of five or more. A reasonably permanent product 

 of primary vulcanization can be obtained only when the coeffi- 

 cient does not exceed three, which figure is regarded by the 

 author as a suitable standard for vulcanization when compar- 

 ing the physical properties of different specimens of vulcanized 

 rubber. 



RESULTS BY DE. 0, DE VRIES AND W. SPOON. 



The changes in vulcanived rubber during the first days after 

 vulcanization have been reported by O. de Vries and W. Spoon 

 in "Archief Voor De Rubbercultuur," November, 1918, page 814. 



In order to determine if a rest of only 24 hours is ample 

 for stabilization of the physical properties of vulcanized mixtures 

 of rubber and sulphur, de Vries and Spoon tested a large number 

 of samples of first-quality crepe, smoked sheet, and lower grades 

 of plantation rubbers. The tests w^ere made on mixtures of 

 92Y2 parts of rubber and 75^ parts of sulphur vulcanized as slabs 

 in an autoclave in live steam at 148 degrees C. It was found 

 that during a rest period of five days the tensile strength showed 

 no change. 



When tested 24 hours after vulcanization the stress-strain 

 curve is found somewhat higher than when tested after intervals 

 of 72 or 96 hours, but the difference amounts to only five or 

 ten per cent in length at 1.30 kilograms, corresponding to an 

 increase of two to three minutes in time of cure. The change 

 for crepe rubber seems to be somewhat smaller than for smoked 

 sheet or for lower grades. 



Experiments to determine whether testing 24 hours after vul- 

 canization gives results as regular and as reliable as testing after 

 a longer period of rest showed that the figures for the position 

 of the stress-strain curve are at least as regular and as reliable 

 when testing 24 hours after vulcanization as compared with a 

 rest of 72 hours. 



RESULTS BY DR. 0. DE VRIES. 



Remarking on the aging of vulcanized rubber, Dr. O. de Vries, 

 in "The India- Rubber Journal," January 11, 1919, summarizes 

 his results on vulcanized mixtures of 92J^ parts of rubber and 

 7K> parts of sulphur, as follows: 



Tlie evidence seems sufficient to prove that for the above 

 rubber-sulphur mixture there is no specially stable state of cure 

 with a coefficient of vulcanization of nearly three per cent. 

 The changes on aging are in a general sense the same as for 

 nii.\tures cured to a coefficient of vulcanization of two or four 

 per cent. The coefficient of vulcanization alone does not con- 

 stitute a sufficient guide to judge of the aging properties. In 

 cases where comparison is to be made of vulcanized rubbers 

 which are not prepared according to the same procedure, both 

 the stress strain and the coefficient of vulcanization, and perhaps 

 other properties, will have to be taken into account to gain an 

 insight into the state of the sample and its probable life or aging. 



Aging of mixtures of 92J/2 rubber and 7j^ sulphur at elevated 

 temperatures (65-72 degrees C), had the following effects: 



(1.) The stress-strain curve shifted position in the same way 

 as in ordinary vulcanization. The change during the first 24 

 hours was greater the shorter the time during which the sample 

 was cured before aging. The change on further heating is prac- 

 tically the same for cures with a coefficient of vulcanization of 

 two to four per cent. The change on prolonged heating becomes 

 progressively less until brittleness is reached. 



(2.) The tensile strength for a given stress-strain curve, 

 though increasing in prolonged vulcanization, does not reach 

 the value obtained by ordinary vulcanization. Samples with 

 a coefficient of two,, three, and four per cent first show an in- 

 crease of tensile strength and later on the rubber becomes brittle. 



(3.) At temperatures below 80 degrees C. the coefficient of 

 vulcanization shows only a small increase. The physical 

 changes in aging which in ordinary vulcanization accompany 

 an increase in the combination of sulphur and rubber are not 

 coupled with this chemical reaction. 



For practical testing purposes the changes during the first 

 days after vulcanization are of special interest. As the vul- 

 canized product is not stable, a fixed period of rest must be ob- 

 served before the sample is tested. 



An extended study to determine the proper duration of this 

 period of rest was made on 27 different grades of rubber, sample 

 mixings being tested, respectively, 24, 48, 72, 96 and 120 hours 

 after vulcanization. In the first 24 hours after vulcanization the 

 change was great but during a further period of six days the 

 figures did not show greater oscillations than the error of de- 

 termination. A shifting in the position of the stress-strain 

 curve is to be expected by aging during the days after vul- 

 canization, but the change is only small. 



In conclusion, the method adopted at the Central Rubber 

 Station, Buitenzorg, Java, based on these studies, is to test 24 

 hours after vulcanization. This procedure is found to give 

 regular and reliable results. 



CHANGES IN VULCANIZED RUBBER AT ELEVATED TEMPERATURE. 



Dr. O. de \'ries summarizes on this topic as follows :° 



The following changes in vulcanized slabs (92^4 rubber and 



7'/$ sulphur) were obtained by aging at 65 to 72 degrees C. 



(equivalent to 149 to 161.6 degrees F.). 



1. The stress-strain curve shifted its position in a way 

 analogous to that in ordinary vulcanization. The change during 

 the first day was smaller the longer the specimen subjected to 

 the aging test was cured, but during following days the changes 

 for differently cured specimens were parallel. 



2. The tensile strength changed in the same sense as in or- 

 dinary vulcanization, but the values obtained remained more and 

 more behind those obtained in ordinary vulcanization. Rubber 



de Ruhlx 



1918, page 805. 



