INDUSTRIAL A N D ENGINEERISG CHEMISTRY
April, 1926
t h a t spray type equipment is more adapted for the absorption of very soluble gases and bubble type is more suitable for slightly soluble gases. This conclusion is of practical importance in another way, since it permits the mathematical treatment of absorption in equipment like bubble plate columns, where reaction occurs both with gas bubbles and with liquid drops. An over-all coefficient could be employed for such equipment without the necessity of separating the two types of absorption surface.
Absorption during Period of Drop Formation Incidentally, rate coefficients may be estimated for absorption while the drop is forming on the tip if the average surface area during this period is assumed. In lieu of any direct determination of this area it will be arbitrarily assumed to be 0.97 sq. cm. (the same as that of the spherical drop formed in 0.6 second). From Run 5 on carbon dioxide (Table 11) the total absorption is 0.000432 gram per cc. The zero intercept on Figure 3 shows that 0.000337 gram per cc. was picked up while the drop was falling. The difference, 0.000095 gram per cc., was therefore absorbed during the 4.94 seconds while the drop was being formed. Applying Equation 1, the absorption rate per unit area is W(A8),,.
o'oooo95
o'0836
= 4.94 sec. X 0.970 sq. cm.
X 3600 = 0.00597 gram/
hour/sq. cm.
The concentration difference, CQ- CL, O.ooOo3 = 0.00147. Therefore,
=
0.00150-
367
This figure is 20 per cent higher than that reported for a flat liquid surface stirred a t 60 r. p. m.' Run 7, on the absorption of ammonia, gives a calculated rate Coefficient, k ~ of, 45.6, although the true coefficient a t zero time is only 21.8 (from Figure 4). The difference in absorption per drop represented by these two figures is (45.6
-
-
0 326 sec. = 3600 0.000975 gram NHs
21.8) X 0.970 sq. cm. X 0.466 atmos. X
This absorption occurs during a period of 5.56 seconds. Theref ore,
(s)at.0.970 X 5.56 X 3600 0'000975
=
Therefore ,
= 0.652 gram/hour/sq. cm.
Po - PL = 0.476 - 0.005 = 0.471 k o = 652 L = 1.4 0.471
This value is 40 per cent lower than the gas film coefficient obtained by Whitman and Davis' above a stirred liquid surface. The ratio k ~ / isk 4.1/1.4 ~ = 2.9 for this quiescent period while the drop is forming on the tip. Although the actual values of the individual coefficients are doubtful owing to the necessity for assuming a surface area, the ratio of the two is far more accurate and is only twice as great as the ratio determined for a stirred liquid surface.
Oxidation of Rubber Exposed to Light' By Ira Williams FIRESTONE
TIRE& RUBBERCO.,AKRON,OHIO
which contains the oxygen. T IS generally recognized Oxidation of rubber may take place in three ways: To all appearances a sample that vulcanized rubber (I) deterioration throughout the rubber, (2) formation of rubber which has been deteriorates because of of a film on the surface of the rubber article, and (3) oxidized u n t i l t h e t e n s i l e o x i d a t i o n . Thompson* as cracking or checking. Experiments to demonstrate strength has been reduced to long ago as 1885 noticed that the catalytic effect of light were made both in the sunone-half of its original value rubber upon deterioration light and under artificial light, and by varying the conhas not changed, although an took up large quantities of ditions the different types of oxidation were produced. a p p r e c i a b l e a m o u n t of oxygen and a t the same time Light of short wave length is most effective in producacetone-soluble material has gave off a volatile yellow oil. ing surface oxidation, and in general the amount of Recently Jones3 has conbeen formed. oxidation varies with the intensity of the light. Ozone firmed both these findings by The oxidation of rubber is is concluded to be the active cause of cracking, but it carrying out the oxidation a t catalyzed to a great extent by has no effect on unstrained rubber. Cracking can be an elevated temperature, dethe presence of ordinary light, prevented by using copper salts to catalyze surface termining the increase in especially when the rubber oxidation weight, and collecting the oil is in a strained condition. t h a t was carried away by the current of oxygen in which the To demonstrate this a piece of pure-gum inner tube was buffed rubber was heated. Bierer4 has studied the oxidation of on an emery wheel until a sufficient quantity of finely divided rubber at an elevated temperature and in an atmosphere of rubber was secured. Equal quantities of the buffings were oxygen a t a pressure of several hundred pounds, and has placed in two glass bulbs of about 50 cc. capacity. The bulbs found that the extent to which oxidation takes place depends were then filled with oxygen and were sealed together by a glass upon the temperature and the concentration of oxygen. capillary about 18 inches long, the two bulbs being separated This oxidation apparently takes place throughout the rubber by a drop of mercury a t the center of the capillary tube. and only in extreme cases can any evidence of a surface lIThenone of the bulbs was covered with tinfoil and the appaeffect be noticed. This general oxidation is characterized by ratus exposed to the sun the drop of mercury traveled toward the change of the rubber into an acetone-soluble material the exposed rubber. In a second experiment two similar bulbs containing 5 grams of rubber buffings were connected 1 Presented before the joint meeting of the Division of Rubber Chemistry and the Akron Section of the American Chemical Society, Akron, to a measuring tube. One of these bulbs was covered with Ohio, February 22 and 23, 1926. tinfoil and the two were exposed side by side to a July sun. 2 J . SOC.Clrem. Ind., 4, 710 (1885). The absorbed oxygen was measured by the amount of 8 THIS JOURNAL, 17,571 (1925). mercury entering the measuring tube. The exposed rubber ' I b i d . , 16, i l l (1924).
I
I
~ I 2 V l ~ E i r ! 1 CIIh!.WIS1'& .V~~ Y
365
Yol. 18. so. 4
nl~sorbcdm y ~ r i :iti t l w n*io uf 0.N cc. per hour wliili! oiily defitiitc I,niid of color, was I> ed t,lirouglr a miter filter 0.13 rl.. / ~ ! Iwiir r u:cs absorbed by the covered bulb. 'I'IIBFI? t.o renwvc heat radiation and was focused on an area of tile differences are g r a t e r than would be expeet,ed from t,lic riililm about 5 mm. square. Four different bands of the
tcmpcnitore diffrrenccs whir% must have existed duo t o sunlight falling on tlie unprot,ret,ed rubber and inisst. bc dnc to the longer wave Icngt~hradiation since the glass of the bulb removed light of short wave length. The success of this experiment depended upon the fact that the fresh surfaces of rubber rlung together and t.he small particles of rubber were obtained in a strained condition. When utistrained strips of rubber are used in similar experilmiits the effect. of light is very small. Surface oxidation
'Tu study the effect of light and oxygen on the surfacc cd the rubber three strips were cut. froin the same slali of vulcanized gray sidewall stock and were stretched on a rvoodcn frame to an elongation of 5 per cent.. Strip 1 was sealeil i n an air-filled glass tube, strip 2 in a tube containing oxygen, and strip 3 in a tube which was esacuated. These werc exposed to a duly sun continuously during ench day for 2 weeks. At the eiirl of this time the siirface of t,lic adrips in air and in oxygen p:i(~liappeared slightly wriirklcd. lipon releasing the strain, tllc wrinkled nppearanee beCkkKnl! very pronounced, showing that 11 none1 e skin had formed on t,he surface of tlic riilht.r. Iliith s i had increased slightly i i i weight. Tlie strip in tlic evacuated tutie apprired t.o IIC itncliangril. 7'lic thrw r q x i s i ~ saniplss J :ire shown in P1:itc 1 . TIE oxidatioii irotliicts fiirined dining surI face oxidation :ire prnhrrlilg t,he same as tiiosv frmieil [hiring iinturd or artificial ngiiig. .\n rx:ciii iriat ion of s t r i p which h:id hccn siilijccted t,o this type of oxidation showed t / i : L i o n l y t h e s u r f a c e had been affected. The skin which had formed on the s u r f a c e p r e s e n t e d ic glazed appearance, \ almost inelastic hut not, Ptnte I-Surface Oxidation of Rubber Fittle, and wax soh1hk in a l c o h o l . After reinoval of the suriilce tlie rubber ui1derncat.h appeared to he unchanged a i d no ilecrease in tho tensile strength of tho rubber eouhl be noticed. In coritrast t o the ordinary oxidation, surface oxidation takes place rapidly at rooni temperature, the actual speed of skin formation depending upon the strain in the rubber, the conditions of light, and the conipositiun of the rubher. .hi elongation of 25 per
