December, 1930
INDUSTRIAL AND ENGINEERISG CHEMISTRY
1367
Water Absorption of Rubber Compounds' H. A. Winkelmann and E. G. Croakman PHILADELPHIA RYBBERWORKSCOHPAZY, A K R O N , OHIO
The water absorption of a vulcanized rubber comtherefore are not shown. UBBER products pound is of considerable importance in evaluating a The water-absorption data which are exposed t o rubber compound. In this study it has been shown of the compounds containing m o i s t u r e and water that fillers such as whiting, barytes, clays, zinc oxide, 5 , 10, and 15 volumes have under varying conditions thermatomic carbon, and carbon black have different been averaged to give a comshould have as low water effects on the water absorption. Thermatomic carbon posite curve for each pigabsorption as possible. A gives the lowest water absorption. Volume loading ment. The control comstudy of the variables which apparently has very little effect. Water absorption of pound (Figure 3) containing might have an effect on the vulcanized rubber decreases with increased time of no pigment shows a minimum water absorption, such as the cure, reaching a minimum at the optimum cure, after water absorption at about the effect of various compoundwhich it increases again. 80-minute cure for all periods ing ingredients, volume loadRubber under stretch absorbs water faster than at of immersion. Whiting (Figing, time of cure, the effect of rest. A tread compound which has been immersed ure 4) and barytes (Figure 5 ) tension, and the effect of the in water and subsequently dried gives higher physical show a minimum water abpresence of salts in the rubtests than the original compound. The presence of sorption a t the 60-minute ber compound, has been metallic acetates in a vulcanized rubber compound cure. The compound conmade. The results indicate causes an increase in the water absorption. A hardtaining clay (Figure 6) shows certain facts which may be of rubber compound absorbs water in mush the same higher water absorption on value in the preparation of manner as a soft-rubber compound, but decreases the overcures than on the water-resisting compounds. rapidly as the optimum cure is reached. Reclaimed undercures. After 30 days' The water absorption was immersion clay shows the rubbers vary considerably in water absorption, those determined on compounds having the greatest amount of residual alkali present lowest water absorption for containing whiting, barytes, giving the highest water absorption. Shoe reclaim the optimum cure. The comclay, zinc oxide, thermatomic pounds containing zinc oxide has the lowest water absorption. carbon, and carbon black, (Figure 7), thermatomic carrespectively. The effect of voiume loading and of time of cure was determined for each bon (Figure 8), and carbon black (Figure 9) show greater individual ingredient, Samples representing a range of cure water absorption on the undercures than on the overcures. for each compound were immersed in water a t room t'empera- Thermatomic carbon after 1 day's immersion is an exception ture and the percentage increase was recorded a t intervals and shows higher water absorption on the overcures. The optimum cure based on the time required to reach over a period of 30 days. The effect of the presence of various metallic acetates in vulcanized rubber on water absorption maximum tensile strength is 40 minutes on all compounds, is shown. I n the tread compound the effect of water absorp- with the exception of 10 and 15 volumes of zinc oxide and 5 tion on the physical properties was determined. The in- volumes of carbon black, in which the optimum is 60 minutes. fluence of time of vulcanization on the water absorption of The optimum cure with 10 and 15 volumes of carbon black is a tread compound is shown. The importance of the choice 80 and 120 minutes, respectively. The minimum water of the proper reclaim is shown in the comparison of the rate absorption of the control without pigments is observed a t the 80-minute cures. Zinc oxide after 7 and 30 days' immersion of water absorption of various reclaimed rubbers. and carbon black after 1 and 7 days' also show a minimum a t Effect of Fillers 80 minutes. The minimum water absorption on the other The following pure-gum compound was used as a control in compounds is on the 60-minute cure, or a little past the optimum cure. Clay shows a minimum water absorption a t the making the water-absorption studies: 20- to 40-minute cures. Smoked sheets.. .......................... 93 Zinc oxide.. .............................. 3 A comparison of whiting, barytes, and the control (Figure 5 Sulfur., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diphenylguanidine. ....................... 0.75 10) after 30 days' immersion shows but little difference in the To this compound 5, 10, and 15 volumes of whiting, barytes, undercures. Barytes a t the optimum cure decreases the clay, zinc oxide, thermatomic carbon, and carbon black were water absorption slightly, whereas whiting appears to exert no added. Each compound was cured 20, 40, 60, 80, 100, and influence whatsoever on the water absorption of the optimum 120 minutes a t 141.7" C. The test pieces 2 X 2 X 0.1 cure. I n the overcures whiting gives the highest and the inch ( 5 >! 5 X 0.25 cm.) were cut from the cured slabs and control the lowest mater absorption. Both whiting and barytes increase the mater absorption of the overcures comimmersed in water a t room temperature (28" C.). The percentage increase in weight of the test pieces was pared with the control. The undercures show higher water determined a t various intervals for 30 days. The test was absorption than the overcures. Clay and zinc oxide (Figure 11) increase the water absorpthen discontinued because it was felt that this was sufficiently tion of vulcanized rubber and show higher results than whiting long enough to show the trend of water absorption. There is pract'ically no effect of increased volume loading of or barytes. Zinc oxide shows the maximum water absorption any given pigment on water absorption. This is illustrated at the undercures, which decreases to a minimum a t the optiby the water absorption of the compounds containing 5, 10, mum cure after which it increases but slightly for the overand 15 volumes of barytes (Figure 1)and thermatomic carbon cures. Clay shows the opposite behavior of zinc oxide, since (Figure 2 ) . The other pigments show similar results and it shows the greatest water absorption a t the overcures. The water absorption of the undercures is not much higher than Presented before the Division of Rub1 Received September 20, 1930. that of the optimum cure, after which the water absorption ber Chemistry a t the SOth Meeting of the American Chemical Society Cinincreases rather sharply. cinnati, Ohio, September 8 to 12,1930.
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EFFECT GF VOLUUELGPUI,?IC~ ON WATER ABSORPTION.
EFFECT OF Voiunr LOADINL ON
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1368
WATER f/BSOR?TIOjV
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VOl. 22, No 12
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Carbon black (Figure 12) does not appreciably affect the water absorption of the undercures, whereas thermatomic carbon decreases it. I n the overcure carbon black shows slightly higher water absorption and thermatomic carbon slightly lower than the control. I n the undercures whiting, barytes, carbon black, and control show about the same water absorption, whereas clay and zinc oxide cause it to increase. I n the overcures clay shows the highest water absorption, followed by zinc oxide and whiting. Carbon black and the control are very nearly the same and thermatomic carbon is lowest in both the underand overcures. All the pigments, regardless of volume loading or time of immersion. decrease in water absorDtion to a m k n u m up to the optimum cure, after which i h e water absorption increases again with increasing time of cure.
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Figure 12
molecular equivalents were added to the following control recipe : Pale crepe.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zinc oxide.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfur.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diphenylguanidine. ......................
100 5 5 0.75
The acetates were chosen in preference to the soluble salts of other acids, because acetic acid is used in the coagulation of rubber latex. The effect of water absorption on small amounts of acetates formed by the reaction of the traces of acetic acid with various compounding ingredients may be appreciated from these results. The acetates with the amount of each added to the above compound are as follows:
Effect of Metallic Acetates
Aluminum acetate.. . . . . . . . . . . . . . . . . . . . . . . . 1.62 Ferric acetate,, . . . . . . . . . . . . . . . . . . . . 4 . 0 Zinc acetate,, . . . . . . . . . . . . . . . . . . . . . . . . 2 . 3 7 Potassium acetate.. . . . . . . . . . . . . . . . . . . . 1 . 0 Lithium acetate,. . . . . . . . . . . . . . . . . . . . . . . . . 1.02 Magnesium acetate.. . . . . . . . . . . . . . . . . . . . . . . 2,14
To determine the effect of the presence in a rubber compound of various metallic acetates on water absorption,
The percentage of water absorption for the 30-, 60-, and 90minute cures is shown on Figure 13. The presence of the salt
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A study was made of the effect of water on the physical properties of a tread compound. For this purpose the following recipe was used: Rubber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10; Zinc o x i d e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Stearic a c i d . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Pine tar.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Carbon b l a c k . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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Cures were obtained for 30, 60, 75, 90, and 120 minutes at 126.6' C. Standard I/r-inch (6-mm.) test pieces were cut from each cure. Tests were obtained on the original compound and on test pieces which had been immersed in water for 30 days and in one case tested immediately, and in the other after drying to constant weight in a desiccator over calcium chloride. These results are shown on Figure 14. The original tests are somewhat lower on the 3 0 - a n d 60minute cures, while on the 90- and 120-minute cures the tensile strength is somewhat higher. I n the case of the samples dried after 30 days' immersion higher tensiles were obtained over the full range of cures. The modulus results are almost within experimental error, but again indicate higher results for the samples which were dried after 30 days' immersion. The highest elongation is obtained on the original compound. The effect on the resistance to tear was also studied using the Winkelmann tear test. These results are shown on Figure 15. The original compound, with the exception of an inconsistency on the 75-minute cure, appears to give the highest resistance to tear. The samples which were immersed in water check very closely and, with the exception of the points a t the 75-minute cure, are somewhat lower than the original. Observations made on the resistance to abrasion as determined on the Grasselli abrader indicate that the poorest
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Figure 18
Water Absorption of a Hard-Rubber Compound
Water-absorption tests were made on a hard-rubber compound cured from 30 to 240 minutes a t 147.6' C. at-30minute intervals. The following recipe mas used: Smoked s h e e t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sulfur.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100 40
As would be expected, the percentage of water absorbed decreases with the time of cure. It is interesting to note, however, that after 210 minutes' curing there is a slight increase on all three curves (Figure 18). Water Absorption of Reclaimed Rubber
I n order to study the effect of water absorption on various t,ypes of reclaimed rubber, four representative grades were mixed with sulfur and cured for 15,30,45, 60, and 75 minutes a t 141.7" C. Five parts of sulfur were added to 100 parts of alkali tube reclaim, neutral tube reclaim, and alkali whole-tire reclaim. Two parts of sulfur were added t o 100 parts of shoe
1370
INDUSTRIAL ALVDENGINEERING CHEMISTRY
=z 2
the entire range Of cures. The lowest results are obtained with a shoe reclaim. It is quite evident from these that the presence of residual alkali in a reclaim
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VOl. 22, No. 12
i-Water absorption of a hard-rubber compound decreases mith increasing time of cure, &Reclaimed rubbers show considerable variation in water absorption, depending on the type of reclaim, process of devulcanization, and materials which are present.
-
Summary of Results
1-Increased volume loading of any given pigment has very little effect on the water absorption of vulcanized rubber. >The water absorption of vulcanized rubber decreases with increasing time of cure reaching a minimum just after the optimum cure after which it increases again. The tread compound (Figure 18) is an exception because it shows the highest absorption a t the optimum cures. The significant fact is that the water absorption of a compound varies over a range of cures. 3-Whiting and barytes increase the water absorption of overcured vulcanized rubber. Clay and zinc oxide show the greatest increase in water absorption. Carbon black has a very little effect whereas thermatomic carbon decreases water absorption. &The presence of salts such as metallic acetates increase water absorption, The presence of salts in compounding
before attempting a theoretical explanation of the results obtained. Further work should be done on the effect of softeners, accelerators, immersion a t elevated temperatures, and source of pigment. Water absorption of the uncured compound is often important. Additional work will show whether the observations made here hold for different types of compounds containing combinations of pigments, reclaims, and accelerators. The fact that the control without pigment shows water absorption of about the same range as some of the pigments would indicate that water absorption when pigments are present is more or less independent of the tendency for these pigments to absorb moisture. Higher water absorption for the undercures may be due to porosity, the compound becoming denser with increasing time of cure; the increase in water absorption after the optimum cure may be due to a chemical change of the protein during vulcanization. A consideration of these problems will be helpful in the preparation of reclaims as well as rubber products subject to exposure to water and steam.
Kinetics of the Baking Process of Oil Varnishes' R. H. Kienle GENERAL ELECTRIC COMPANY, SCHENECTADY, ?i. Y.
The baking process of a typical oil varnish has been Historical N A previous article studied kinetically, using a colorimetric method to Genthe (c) to have (6) it was shown that the baking process of an measure the degree of bake. been the first to study the Measurements of the reaction velocity at four temkinetics of the setting of a oil varnish could be repreperatures were made in air, oxygen, and nitrogen varnish film. He studied the sented by a chemical equation atmospheres. The baking Process was found to follow process a t ordinary temperawhich in its general form the expression for a unimolecular reaction. tures ifi air, measuring the contained two factors--one The temperature coefficients and heats Of activation reaction quantitatively by the involving oxidation, the other po~werization, ~ ~ for the ~ several ~ atmospheres h were ~ calculated. ~ rate a t which oxygen was abSeveral Practical applications of the data obtained sorbed, giving full consideramore, it was shown that the were discussed* tion to such volatile products degree of bake could be measured quantitatively by a as were formed. He showed colorimetric method. By use of this method it has been pos- the process to be essentially autocatalytic. sible to determine the kinetics of the baking reactions and Fahrion (W), Fokin (S), Coffey ( I ) , Rhodes and Van Wirt thus gain further insight into the baking process. (Y), and Rogers and Taylor (8) also have investigated the kinetics of the reaction a t ordinary temperatures and agree 1 Received September 19, 1930. Presented before the Division of an exin genera' with Genthe* Rogers and Paint and Varnish Chemistry a t the 80th Meeting of the American Chemical cellent review of the literature in this field. Society, Cincinnati, Ohio, September 8 to 12, 1930.
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