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Photoelastic study of rubber
June, I933.]
U.
S.
BUREAU
745
OF S T A N D A R D S N O T E S .
P H O T O E L A S T I C STUDY OF RUBBER.
When transparent rubber is stretched, it exhibits the wellknown p h e n o m e n o n of double refraction. A s t u d y is being made of the double refraction of rubber as a function of the stress and the composition. T h e measurements cover the range of stresses from I to IOO bars (the bar is a unit of pressure, equal to 14.5o lbs./in."), which correspond, for the samples employed, to elongations from about 2o to 6oo per cent. One rubber compound which was studied in some detail was made according to the following formula: Pale crepe r u b b e r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc oxide (Kadox) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stearic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tetramethylthiuramdisulphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parts by Weight IOO.O I.O I .o 2.o o.3 lO4.3
In order to ascertain the effect of the sulphur content on the photoelastic behavior of rubber, similar compounds were made with 4, 5, and 6 per cent. of sulphur, respectively, b u t w i t h o u t the tetramethylthiuramdisulphide. T h e measurements are expressed in terms of the stressoptical coefficient in brewsters. A coefficient of one brewster signifies t h a t when a beam of light is passed through the substance rendered doubly refracting by the stress, the extraordinary ray is retarded I Angstrom unit behind the ordinary ray when the thickness of the sample is I millimeter and the stress is I bar. T h u s the brewster is lO-13 cm. 2 dyne -1. T h e double refraction at any value of the stress is related to the stress-optical coefficient, C, by the equation, n o - - n e -~- I o - 7 C S ,
where no and ne are the refractive indices for the ordinary and the extraordinary rays, respectively, and S is the stress in bars. Since the refractive indices, and hence the stress-optical coefficient, m a y vary with the wave-length of the light, all measurements were made with monochromatic light of wavelength 5,461 A. U. obtained from a mercury arc. The stress-optical coefficient for the accelerated rubber compound of the formula given above is about 2,ooo brewsters
746
U.S.
BUREAU OF STANDARDS NOTES.
[J. F. I.
at a stress of 2.5 bars (elongation, 2o per cent.), and increases to 2,I00 brewsters at 13 bars (elongation, IiO per cent.) and remains constant within the probable experimental error of 4- IO brewsters up to a stress of 55 bars (elongation, 330 per cent.). At higher stresses rubber of this composition exhibits optical creep. T h e compounds which were vulcanized with sulphur alone, without the organic accelerator, do not show a similar constancy of the stress-optical coefficient with varying elongation. The values for the coefficient range from 2,000 to over 3,300 brewsters and appear to be a function of both the stress and the sulphur content. Inasmuch as stress-optical data m a y not be familiar to investigators in other fields, it m a y be of interest to note t h a t the coefficient for rubber is high in comparison with coefficients for most other transparent materials. Inorganic crystalline substances and glasses have coefficients in the range I to 5 brewsters, celluloid has a coefficient of about IO brewsters, and bakelite, a coefficient of about 50 brewsters. Gelatines, however, have coefficients of 5,000 to 20,000 brewsters and are the only c o m m o n materials higher than rubber in this respect. EFFECT OF TEMPERATURE
O N LITHOGRAPHIC PAPERS.
Further information has been obtained at the bureau to assist lithographers in overcoming misregister of successive color prints, which is the most prolific source of waste in the industry. Five samples of lithographic papers, representative of the different types of papers most commonly used in offset p r i n t i n g , have been studied at eight different temperatures, ranging from 68 ° to IiO ° F. Dimension changes, the most common cause of misregister, were determined by measuring specimens approximately 24 by 24 inches to o.ooI inch, and moisture content changes were determined by weighing somewhat smaller samples on a chemical balance to o.oooi gram. The relative h u m i d i t y was maintained constant at 45 per cent. t h r o u g h o u t so t h a t the temperature was the only factor varied. The temperature changes resulted in regular moisture content changes, the average change being 0.2 per cent. per
U.
S.
BUREAU
745
OF S T A N D A R D S N O T E S .
P H O T O E L A S T I C STUDY OF RUBBER.
When transparent rubber is stretched, it exhibits the wellknown p h e n o m e n o n of double refraction. A s t u d y is being made of the double refraction of rubber as a function of the stress and the composition. T h e measurements cover the range of stresses from I to IOO bars (the bar is a unit of pressure, equal to 14.5o lbs./in."), which correspond, for the samples employed, to elongations from about 2o to 6oo per cent. One rubber compound which was studied in some detail was made according to the following formula: Pale crepe r u b b e r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc oxide (Kadox) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stearic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulphur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tetramethylthiuramdisulphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Parts by Weight IOO.O I.O I .o 2.o o.3 lO4.3
In order to ascertain the effect of the sulphur content on the photoelastic behavior of rubber, similar compounds were made with 4, 5, and 6 per cent. of sulphur, respectively, b u t w i t h o u t the tetramethylthiuramdisulphide. T h e measurements are expressed in terms of the stressoptical coefficient in brewsters. A coefficient of one brewster signifies t h a t when a beam of light is passed through the substance rendered doubly refracting by the stress, the extraordinary ray is retarded I Angstrom unit behind the ordinary ray when the thickness of the sample is I millimeter and the stress is I bar. T h u s the brewster is lO-13 cm. 2 dyne -1. T h e double refraction at any value of the stress is related to the stress-optical coefficient, C, by the equation, n o - - n e -~- I o - 7 C S ,
where no and ne are the refractive indices for the ordinary and the extraordinary rays, respectively, and S is the stress in bars. Since the refractive indices, and hence the stress-optical coefficient, m a y vary with the wave-length of the light, all measurements were made with monochromatic light of wavelength 5,461 A. U. obtained from a mercury arc. The stress-optical coefficient for the accelerated rubber compound of the formula given above is about 2,ooo brewsters
746
U.S.
BUREAU OF STANDARDS NOTES.
[J. F. I.
at a stress of 2.5 bars (elongation, 2o per cent.), and increases to 2,I00 brewsters at 13 bars (elongation, IiO per cent.) and remains constant within the probable experimental error of 4- IO brewsters up to a stress of 55 bars (elongation, 330 per cent.). At higher stresses rubber of this composition exhibits optical creep. T h e compounds which were vulcanized with sulphur alone, without the organic accelerator, do not show a similar constancy of the stress-optical coefficient with varying elongation. The values for the coefficient range from 2,000 to over 3,300 brewsters and appear to be a function of both the stress and the sulphur content. Inasmuch as stress-optical data m a y not be familiar to investigators in other fields, it m a y be of interest to note t h a t the coefficient for rubber is high in comparison with coefficients for most other transparent materials. Inorganic crystalline substances and glasses have coefficients in the range I to 5 brewsters, celluloid has a coefficient of about IO brewsters, and bakelite, a coefficient of about 50 brewsters. Gelatines, however, have coefficients of 5,000 to 20,000 brewsters and are the only c o m m o n materials higher than rubber in this respect. EFFECT OF TEMPERATURE
O N LITHOGRAPHIC PAPERS.
Further information has been obtained at the bureau to assist lithographers in overcoming misregister of successive color prints, which is the most prolific source of waste in the industry. Five samples of lithographic papers, representative of the different types of papers most commonly used in offset p r i n t i n g , have been studied at eight different temperatures, ranging from 68 ° to IiO ° F. Dimension changes, the most common cause of misregister, were determined by measuring specimens approximately 24 by 24 inches to o.ooI inch, and moisture content changes were determined by weighing somewhat smaller samples on a chemical balance to o.oooi gram. The relative h u m i d i t y was maintained constant at 45 per cent. t h r o u g h o u t so t h a t the temperature was the only factor varied. The temperature changes resulted in regular moisture content changes, the average change being 0.2 per cent. per