INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1930
(4) Harries, "Untersuchungen iiber die natiirlichen und kiinstlichen Kautschukarten," p. 260 (1919). (5) Harries, Ibid., p. 170. ( 6 ) Harries, Ibid., pp. 193, 170, 207, 199; and unpublished work of the authors. (7) Kaufmann with Adams, J . Am. Chem. Soc., 46, 3029 (1923). (8) Kyriakides, Ibid., 56, 532 (1914).
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(9) Kyriakides, Ibid., S6, 987 (1914). (10) Macallum and Whitby, Trans. Roy. SOL. Con.,111, 22, 39 (1928); Rubber C h e m . Tech., 1, 341 (1928). (11) Salkind, J . Russ. Phys. Chem. SOL.,37, 486 (1905); Chem. Z e n f r . , 76, (11) 752 (1905).used potassium cyanide as the condensing agent. (12) Schotz, "Synthetic Rubber," p. 69 (1926). (13) Voorhees with Adams, J. Am. Chem. SOC.,44, 1397 (1922).
Effect of Pigmentation on the Work of Retraction of Rubber Compounds' F. S. Conover THE NEW JERSEY ZINC COMPANY, PALMERTON, PA.
HE reenforcement of rubber by pigments has been
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studied by a number of investigators (4, 6) by considering the effect of pigmentation on the extension stress-strain curves and on the areas enclosed between these curves and the elongation axes, which areas are a measure of the work of extension. (Figure 1) These investigations, in conjunction with other data, have developed two opinions concerning the mechanism of reenforcement; one (2), that reenforcement is caused by a bond between the rubber and the pigment particles; the other (3), that reenforcement is primarily a function of the particle size of the pigment and that the bond is of secondary importance.
mately the same work of retraction from any elongation which is common to all the compounds, although the work of extension to the same elongation for these compounds is quite widely different. It follows that the retraction curves for two compounds differing only in the kind and amount of pigment used must cross in returning from the same elongation, as is illustrated in Figure 2. The stiffer the stress-strain curve on extension, the lower is the retraction curve when returning to zero stress. The data further show that work of retraction does depend, in some measure a t least, upon the quality and condition (state of cure) of the rubber used. The interpretation of this behavior involves many considerations, and though it undoubtedly has considerable bearing on the theory of reenforcement, any discussion of this aspect of the work will be reserved for a future paper. Conditions of Experimental Work
I n carrying out this work it was necessary to take several important factors into consideration: (1) The pigments chosen should be representative of a wide range of particle size and behavior in rubber compounds. These pigments were Red Label Kadox, carbon black, XX red zinc oxide, clay, and ground barytes. Three of these are high-grade reenforcing pigments and the other two are fillers. They were used in concentrations of 5-10-20-3040 and 50 volumes of pigment to 100 volumes of rubber. Strain Figure I-ALTypical Rubber Stress-Strain Extension a n d Retraction Curve The 1entire area between the extension curve (heavy line) and the elongation axis is a measure of the work of extension. The cross-hatched portion between the retraction curve (dotted line) and the elongation axk is a measure of the work of retraction.
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The work cited above shows that pigments exert different effects on work of extension, and it has consequently been assumed that they would have somewhat similar effects on work of retraction as measured by the areas enclosed b e tween the retraction curves and the elongation axes. (Figure 1) The data assembled in this report, however, show that work of retraction from a given elongation is much more independent of the kind or amount of the pigment used. For example, if several pigments and several volume loadings are chosen a t random, it will be found that a &volume carbon black, a IO-volume Kadox, a 20-volume clay, and a 30-volume XX red zinc oxide stock mill all have approxi1 Received April 16, 1930. Presented before the Division of Rubber Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April ?to 11, 1930.
Figure 2-Crossing of Retraction Curves of Two Corn o u n d s One C o n t a i n i n g 20 Volumes of Carbon B l a c f (1). a i d the Other Containing 20 Volumes of XX Red Zinc Oxide ( 2 )
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treatment for every compound. To this end the above base compound with the sulfur omitted was master-batched on a 16 X 42 inch mill and the pigments were later added on a 6 X 12 inch laboratory mill. The sulfur was added last. All milling was carried out with the rubber a t 70" C. (4) The vulcanization periods should be such as to have every compound a t a comparable cure. With pigments of such widely different characteristics it was impractical to choose any one time which would be satisfactory for all stocks. It was decided to choose that cure which had the highest work of extension after 14 days' aging in a Geer oven a t 70" C. Figure 3 illustrates the method. Work of extension was plotted against time of cure for all the Red Label Kadox stocks after they had been aged for 14 days a t 70' C. The dotted line traces the shift of time to the comparable cure as the pigment concentration increases. The time to the comparzble cures for all compounds is shown in Table I.
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20 40 60 80 100 Time of cure in minutes at 30 uounds Figure 3-Determination of t h e Comparable Cures for a R a n g e of Volume Loadings of Red Label Kadox The smooth dotted line connects the approximate pojnts of highest work of extension after accelerated aging. The intersection of this line with the solid curves determines the time to the comparable cures more accurately than could be done by inspection of the solid curves alone.
(2) The pigments should be used in a base compound which would be fairly representative of commercial practice, and which would contain sufficient sulfur to overcome any tendency towards reversion which might be manifest if a smaller amount were used, due to the chemical combinations of the sulfur with some of the pigments. The base compound selected was: Pale crepe Sulfur Diphenylguanidine Mineral Rubber Stearic acid Red Label Kadox
920 55 9 2 4.6 2.3 168
100 volumes 6% on rubber by wt. 1% on rubber by wt. 0 . 5 % on rubber by wt. 0 . 2 5 % on rubber by wt. 3 volumes
(3) . The milling practice should be such as to preclude any possibility of pre-vulcanization set-up and to insure the same
Table I-Time Necessary for C o m p o u n d s to Reach Their Comparable Cures When Vulcanized at 30 Pounds (2 atm.) S t e a m Pressures (134.5O C.) VOLUMESXX RED RED OF ZINC CARBON LABEL GROUND PIGMENT OXIDE BLACK KADOX CLAY BARYTES Minutes Minutes Minutes Minules Minutes 0 60 (60) 60 (60) 60 (60) 70 (65) 65 (60) 5 42 (60) 70 (70) 10 57 (50) 43 (60) 70 (70) 20 38 (35) 51 (50) 55 (70) 30 27 (30) 53 (50) 42 (60) 40 16 (20) 59 (60) 40 (40) 50 60 (60) 5 ( 7) (The figures in parentheses are time to the tensile peak before aging.)
There are various ways of determining time to the comparable cure, each of which has its advantages and disadvantages. One method, ivhich is independent of aging, is to choose that time a t which the compound reaches its highest initial tensile strength. If that had been done in this case, the comparable cures would not have differed appreciably from those found, as is shown in Table I. The times to comparable cure are in line with general experience; increasing amounts of carbon black retard the cure through the ordinary compounding range (up to 20 volumes of pigment), and Kadox causes a considerable acceleration.
The retraction curves were taken on the first cycle so that they would be comparable to the considerable amount of data already published on work of extension. Unpublished data by W. W. Vogt, of the Goodyear Tire & Rubber Company, have shbwn that the results are decidedly different if different conditions of test are used. Vogt measured the rebound of a weighed pendulum from the surface
0 200 400 600 800 1000 Per cent elongation Per cent elongation Per cent elongation Figure 6 Figure 4 Figure 5 Work of Retraction for a R a n g e of Volume Loading8 of Various P i g m e n t s (Numbers on curves represent volumes of pigment)
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axes a t elongations starting a t 300 per cent and increasing by 100 per cent to as near the breaking point of the piece as possible. The measurements were then calculated as foot-pounds per cubic inch or kilogram cen189.9 *g timeters per cubic centimeter. Ultimate work of retraction a t break was determined by extrapolating the work of retraction$ elongation curves to the breaking elongation of the compound. For every different elonga1 2 6 6 5 tion it was necessary, of course, to employ 2 unused test pieces. The extension and retraction stress-strain E curves were made according to the standard $ A. S. T. M. testing methods ( 1 ) 63 3 The test pieces were stretched on a regular rubber-testing machine. The jaws separated a t a speed of 20 inches (51 em.) per minute until the required elongation was obtained. A throw of a switch immediately reversed the jaws and they returned with the same speed a t which they had separated.
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Per cent elongation Per cent elongation Figure 7 Figure 8 Work of Retraction for a R a n g e of V o l u m e Loadings of Various P i g m e n t s (Numbers on curves represent volumes of pigment)
of rubber compounds. The length of the swing was adjusted so that equal penetration of the weight into the sample was secured. It was found that the energy of rebound, which might be comparable to work of retraction, was dependent upon the kind and amount of pigment used. This should not be unexpected, however, because the conditions of test in the two cases were radically different. Energy of rebound is essentially a very rapid test under very nearly adiabatic conditions. Work of retraction is a test which
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The data are given in Figures 4 to 8. These curves show that work of retraction from any given elongation is practically independent of the kind or amount of pigment used with the exception of the barytes compound. As illustrations of these facts, Figures 9 and 10 are presented. All points on the curve in Figure 9 are very close, in spite of the fact that they represent four different pigments and four different volume loadings. I n Figure 10 the barytes compound is the only one which does not fall on the general curve. This may be explained by the fact that vacuoles 189 9
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400 600 800 1000 Per cent elongation Figure 9-Work of Retraction a t 300, 400, a n d 500 Per C e n t for a 5-Volume Carbon Black, a 10-Volume Kadox, a 20-Volume Clay, a n d a 30V o l u m e XX Red Zinc Oxide Stock 0
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is, comparatively, very slow and the conditions are much more nearly isothermal. Vogt further found that the state of cure of the compound had very little effect on energy of rebound; this being in line with published work by Williams ( 6 ) . The complexity of the problem is further shown by the fact that state of cure has a decided effect when a large block of rubber is tested with the Whitney rebound tester. Experimental Procedure
Work of retraction for these compounds a t the cures mentioned above was determined by measuring the areas enclosed between the retraction curves and the elongation
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600 800 1000 Per cent elongation Figure 10-Work of Retraction for All 20-Volume C o m p o u n d s 200
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were formed during extension, and that consequently the rubber was under less strain than it was in the other compounds. When ultimate work of retraction, calculated to the breaking elongation, is plotted against volumes of pigment as in Figure 11, it is seen that it also is virtually independent of the kind of pigment used. It is, however, dependent upon the ultimate elongation of the compounds which are greatly influenced by the amount of pigment used. That work of retraction and ultimate work of retraction are influenced by the condition, or state of cure, of the rubber in the compound, can be seen by an examination of Figure 12. For obtaining these data the 20-volume XX red zinc oxide compound was used, cured for 30, 45, 75, 90, and 105 minutes a t 30 pounds. Work of retraction a t elongations
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Figure 11-Effect of P i g m e n t a t i o n on Work of Retraction Calculated to Breaking Elongation
short of break has increased for every increase in time of cure. Ultimate work of retraction has also increased to a point beyond the optimum, which occurred in 70 minutes, and has then started to decrease. Literature Cited (1) American Society for Testing Materials, Specifications for Testing Rubber Products, D15-24 (1927).
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400 600 800 1000 Per cent elongation Figure 12-Effect of Cure on Work of Retraction (The numbers on the curves re resent time ot cure iu minutes.? (2) Blake, I n . ENG. C H E M . , 10, 1084 (1928). (3) Depew, Rubber Age ( N . Y.),14, 378 (1929). (4) Lunn, Trans. Insl. Rubber I n d . , 4, 396 (1929). (5) Wiegand, Can. Chem. J . , 4, 160 (June, 1920). (6) Williams, IND. ENG. C H E M . , 81, 872 (1929).
Solubility of Oxygen in Rubber and Its Effect on Rate of Oxidation' Ira Williams and Arthur M. Neal E. I.
DU FONT DE
Nemouas &
COMPANY, WILMINGTON,
DEL.
A study has been made of the effect of the concenrapidly at 70' C. if the oxytration of oxygen in rubber upon its aging properties. gen pressure is increased. If tests applied to rubber Such factors as temperature and pressure on the the concentration of the rubis intended t o d e solubility of oxygen in rubber have been studied. ber remains constant, so that termine its probable deterioraThe solubility of oxygen in rubber follows Henry's the oxidation reaction could tion with age. It has long law and decreases with increasing temperature. The be considered to revert to s been recognized that the deoxidation of rubber proceeds at a uniform rate as long type a8 simple as the first terioration is caused princias the oxygen concentration remains above a certain order, the increase in oxygen pally by oxidhtion and two minimum. The concentration of oxygen in rubber pressure from that of atmostests have been devised for exposed to air at 70' C. is less than that required to pheric oxygen to 21.1 kg. per increasing the speed of oxidaproduce uniform oxidation at a maximum rate. High sq. cm. (300 lbs. per sq. in.) tion. The Geer test increases temperature appears to produce a type of oxidation will increase the deteriorathe rate of the reaction by which is affected by high pressure. The simultaneous tion rate 100 times, or 31/2 placing the ruhber in air at use of high temperature and high pressure should be days in the b o m b w o u l d a n elevated t e m per a t u r e. avoided. equal 1 year of aging in the The Bierer-Davis test emoven. This condition is not ploys both an elevated temperature and an atmosphere of oxygen under pressure. It found in practice. If the oxidation reaction is of a higher is well known that at elevated temperature the rubber is order, the increase in pressure should make the rate in the affected by factors other than oxidation, such as continuation bomb a t 70" C. extremely rapid. This emphasizes the imof vulcanization, volatilization of materials from the rubber, portance of the knowledge of the concentration of oxygen and solubility of various ingredients contained in the rubber. under different conditions. Most of the previous work on the solubility of gases in No consideration seems to have been given to the effect of change in the concentration of oxygen under various con- rubber has been related to their permeability through rubber, Venable and Fuwa (6) investigated the effect of pres ditions of testing. The concentration of oxygen would be expected t o affect sure and temperature on the solubility of carbon dioxide in the rate of oxidation of rubber. This supposition would seem rubber and correlated the data so obtained with the permeato be justified by the fact that rubber deteriorates more bility of this gas. They also found values for the solubility of various other gases a t one temperature and pressure. 1 Received April 18, 1930. Presented before the Division of Rubber No detailed study of the effect of temperature and pressure Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, on the solubility of oxygen in rubber was made. Ga., April 7 to 11, 1930.
NE of the important
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