INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Crude Rubber Testing-Rate
Vol. 16, No. 9
of Cure’
By C. W.Sanderson THEFISKRUBBER Co.,CHICOPEE FALLS,MASS.
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T IS highly desirable to adopt a method for the determination
of the rate of cure of crude rubber which will permit of this property being determined by one cure in contrast to that where a series of cures is made. It is possible to do this, but in order that the limitations of the method may be understood the principles will be explained in some detail. As soon as it was recognized that samples of rubber did vary in time of cure, i t was realized that the determination of the proper cure was essential, not only as a guide to adjustment necessary when putting the rubber into use, but also as a basis for comparing the other properties of the rubber. The term “optimum cure” has by common usage come to mean that cure to which the rubber should be vulcanized to compare its properties with other samples. The optimum cure as determined by the different methods has usually been selected as that cure which will show the properties-particularly tensile-of the sample a t their best. One method largely used, especially in this country on account of our method of testing by the straight test piece, consists of curing over a range of cures and of comparing samples on the basis of the cure which gives the highest tensile. Another modification consists of selecting the highest product of tensile times elongation. In either case the cure giving the highest figure determines the rate for the sample.
Another method, which also is subject to modifications, depends for its determination on the stress-strain curve rather than on the end point. The fundamental properties of the stressstrain curves for a rubber-sulfur mix were expounded by Schidrowitz.? He later3amplified this and gave a method of determining the “correct cure.” De Vries has developed this principle into a method which is essentially that recommended here, except that the standard point has been changed slightly. On referring to Fig. I, which shows a typical set of curves for a mix of 100 rubber-10 sulfur cured a t 141O C. (40 pounds steam), it will be noted that the change in position of the curve is proportional to the change in time of cure. This does not hold true for the markedly undercured or the overcured regions of the curves, and it does not hold true for mixes of low sulfur content, but it does hold for the mix used to the extent that the state of cure is shown by the position of the curve. Assuming for the moment that the curves for different rubbers will coincide for the same state of cure, it is only necessary to choose a standard curve, make one cure, plot it, and by its distance from the standard estimate the time of cure necessary to make i t give a cure falling on the standard. The standard may be chosen more or less arbitrarily, but is usually designed t o represent a cure quite close to that giving the maximum tensile, provided a suf-
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This method has the disadvantage of requiring several cures, and also the greater objection due t o the fact that the determination depends on the breaking figures, the uncertainty of which is well recognized. 1 Part of report of the Crude Rubber Committee presented before the Division of Rubber Chemistry at the 67th Meeting of the American Chemical Society, Washington, D. C., April 21 to 26, 1924.
ficiently large number of tests are made to determine this tensile accurately. The problem is not quite so simple as outlined, however. Schidrowitz showed that different grades of rubber did not give Rubber Ind., 1Q14, 212. a J. SOC.Chem. I n d . , S8, 347 (1919). 2
September, 1924
INDUSTRIAL AND ENGINEERING CHEMISTRY
parallel curves (in the upper portions) at equivalent cures, and that a t equivalent cures the elongation at a given load for one sample might be lower than for another. This is illustrated in
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minus the elongation a t 0.60 kg. and dividing by 2.5. This, of course, is purely conventional, but is much simpler than the actual determination of the slope. The figure should be determined on all samples, for reasons which will be shown, even though one is not working to the accuracy of allowing for different standard curves or points for different slopes. While it is true that the slope does vary slightly for different cures, it is sufficiently accurate to determine the figure on any test within, say, 1 hour of the standard. The figures for slope found by De Vries vary from 35 to 38 on the first grades and run up to as high as 50 in rare cases on lower grades. As stated by De V r i e ~ , ~ “high figures for slope, such as 42 and more, generally indicate a rubber which on keeping becomes tackey.” Therefore, to determine if the sample falls within the grade represented by the standard, the slope should always be obtainedSs RECOMMENDED METHOD
A single cure of 3 hours a t 141O C. (Fig. IV) is run on the sample mixed in the proportion 100 rubber to 10 sulfur. The pieces are tested b y any of the usual methods to get the full stress-strain data, so that the full curve may he drawn. Usually, four pieces are sufficient, and the results are averaged and one curve is plotted (Fig. 111). The slope is then determined in this curve by subtracting the per cent elongation a t 386 kg. (850 pounds), which equals 785, from the per cent elongation a t 672 kg. (1480 pounds), which equals 870, and dividing by 2.5, th,e result being 34. (These figures are chosen to compare with those of Schid-
Fig. 11. Here we have a series of cures for a very low grade of rubber. It is obvious that the cure representing the optimum for the low-grade rubber will go through a point of very much lower elongation for a given load than will the optimum cure for a high-grade rubber, and it would be necessary, therefore, to have a different standard point for such a rubber. Theoretically, it would be necessary to have a different standard point for each different slope. I n actual practice, however, De Vries, for example, considers it unnecessary to work t o such accuracy, particularly because vaiiation of slope large enough to be considered in this connection occurs quite rarely and only in very low-grade rubber. The slope is determined conventionally by Schidrowitz by taking the elongation a t a load of 1.04 kg. per sq. mm.
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rowitz.) The distance from the standard point is then determined. For the standard point an elongation of 850 per cent a t a load of 840 kg. (1850 pounds) has been taken. (This compares with De Vries’ figures of 890 per cent a t 1.30 kg., or 1850 pounds per square inch.) The distance of the 3-hour cure from this point is found to be 4.5 units on the per cent elongation “Estate Rubber,” p. 470. For further detailsin regard to the methods indicated above see Schidrowitz, footnote 3; Whitby, “Plantation Rubber,” p. 340; De Vries, “Estate Rubber,” chapters 17 and 18. 4
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scale. By taking results from a large number of tests, it has been found that a difierence of cure of 60 minutes produces a movement of 13.5 units. Therefore, by proportion, since the 3-hour curve is 4.5 units from the standard, the difference of cure necessary to bring this rubber to the standard is 20 minutes. For important tests and for a check i t is well to cure another sample a t the estimated cure and see how closely it agrees with the standard. I n applying this method it should be borne in mind that the test is suggested simply as a means of determining the time of cure. It is not intended to discuss other qualities in this paper. It should also be recognized that in using this method it is necessary to choose certain arbitrary points, and in these there may be some difference of opinion. For example, curing a rubber to an elongation of 850 per cent a t a load of 840 kg. (1850 pounds) represents a technically overcured rubber. The argument for a lower cure is that it will better correspond with technical practice, while the argument for the cure as used in this paper is that the rubber is cured to a state where the full properties are shown to best advantage. Looking a t it in this way we are only interested in its properties for test purposes, and its deterioration with age may be determined in a separate test if desired.6 When this work was started by the Crude Rubber Committee, it was thought advisable to cure the tests a t 141O C. (40 pounds steam pressure), and the foregoing explanation is carried out on that basis. After the first preliminary work, however, it was 6
Comments on this point are given by De Vries, I n d z a Rubber J.,61, 87
(1921).
Vol. 16,No. 9
recognized that there was no essential difference in the character of the curve, whether cured a t 141" C, (40 pounds) or 148" C. (50 pounds), and, therefore, in order to save time it was decided to use the 148" C. (50 pounds steam pressure) as a standard for testing practice on =this formula. The comparison between 141" C. (40 pounds) and 148" C. (50 pounds) cures is shown in Figs. IV and V. On Fig. V, Curves A , C, and E represent t h e cures a t 141" C. (40 pounds), while B, D,and F, the cures a t 148" C. (50 pounds). Although the corresponding cures do not quite coincide, they were considered to be close enough to illustrate that the rubber did not behave essentially different a t the high steam pressure. Fig. IV shows the behavior of these cures on accelerated aging test, and this also shows that the 148' C. (50 pounds) cure behaves essentially the same as the 141' C. (40 pounds) cure. Another point to be recognized is t h a t the method as outlined is really a short cut and should not be applied unless the principle back of it is understood. No one should start out to use t h e method without first running test samples at several cures, i n order to determine the extent to which the curve is advancedi by increased cure. I t is well also to secure a sample a t the estimated standard to check the determination. It may be found that for the grades other than first quality rubbers it will be necessary to choose different standards depending on the slopes of the samples. This point is not gone into in detail in this paper, as it can be worked out by any one familiarwith the method, and might confuse the explanation of the principles given here.
A Pioneer in P u r e Foods and Drugs' Lewis C. Beck,'A.B., M.D. By L. F. Kebler BUREAUOF CHEMISTRY, WASHINGTON, D. C.
H E Committee on Agriculture, of the House of Representatives, on'January 13, 1830, reported the following resolution:
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Rerolued, That the Secretary of the Treasury cause to be prepared a welldigested manual, containing the best practical information on the cultivation of sugar cane, and the fabrication apd refinement of sugar, including the most modern improvements; and t o report the same to the next session of Congress." (House Jouvnal, 21st Cong., 1st Sess., p. 157.)
The resolution was laid on the table, called up on January 25, and, after some debate, agreed to without division. [Debates in Congress, 6, 554 (1830).] I n August, 1832, Dr. Benjamin Silliman was engaged to do the work. Several other chemists collaborated in the investigation; among them may be mentioned C. U. Shepard and J. B. Avequin. The report entitled "Manual on the Cultivation of the Sugar Cane and the Fabrication and Refinement of Sugar" was submitted in May the following year. A thousand copies of 122 pages were printed. These workers appear to be the first chemists employed by the United States Government to study some of its agricultural production problems. The first chemist to be engaged under a specific appropriation by Congress to make chemical analyses and study the adulteration and deterioration of food products was Dr. Lewis C. Beck, a man of unusual attainments. Energy, enthusiasm, integrity, thoroughness, and love of science dominated his life. He was a 1 Presented under the title "Lewis C. Beck, M.D., a Pioneer in the Food and Drug Adulteration Movement in America" before the Section of History of Chemistry a t the 67th Meeting of the American Chemical Society, Washington, D. C., April 21 to 26, 1924.
physician by education and engaged in the practice of medicine a t the age of nineteen. I n this profession Dr. Beck and two of his brothers became eminent. Biographies of these three brothers appear in Dr. Samuel D. Gross's "Lives of Eminent American Physicians and Surgeons of the 19th Century," 1861. Another biography of Lewis C. Beck appears in the Annals of the Medical Society of Albany, New York, 1864. I n these biographies his work in connection with pure foods, drugs, and other commodities is only briefly mentioned. Dr. Beck received his A.B. degree from Union College in 1817, and the College of Physicians and Surgeons of New York City conferred its degree on him later. I n February, 1818, he was. admitted to practice. ATotwithstanding the fact that Dr. Beck engaged in the practice of his profession early in life and gained eminence therein, it is evident t h a t his energies were directed largely along educational and investigational lines. He began , his public life in 1824 as teacher in botany in Berkshire Medical College and the same year became professor of botany in Rensselaer Polytechnic Institute at Troy, N. Y. In 1826 he accepted the chair of botany and chemistry in the Vermont Medical Academy. Four years later we find him professor of chemistry and natural history in Rutgers College of New Jersey, the successor of Queen's College, organized in 1766. He was professor of chemistry in the University of New York City in 1836, and accepted the professorate of chemistry and pharmacy in the Albany Medical College in 1840. He held his Albany and Rutgers College positions simultaneously for many years. He continued to lecture to his class a t Albany until shortly before : his death in 1853.
