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
iza-
Over
cl=
L o -
3
'
40-
\
A L K R L I ~ RRECLI)IL E
S H O E RE'III~M
I
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.
I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1930
Genthe, Coffey, and Rogers and Taylor also considered the effect of temperature, but carried their experiments only up to 100' C. All three investigators agreed that as the temperature was increased any autocatalytic effect that might be present was minimized. Genthe introduced the concept of
1371
investigation, however, a slightly modified baking oven was used, as shown in Figure 1. I n this oven the test films were inserted from the top, a slit in a tightly fitting chrome-steel cover providing both an entree for the test films and an exit for vapors. A suitably balanced counterweight device allowed for rapid insertion and removal of the test specimens. Experimental Procedure
+
To study the baking process at any given temperature and oven conditions, the baking oven was adjusted to the desired conditions. A set of films was then made, all baked a t successive intervals of time, to give films from the initial to the deteriorated state. Each film was examined with a spectrophotometer, 90 determine the percentage of light of wave length 6600 A. reflected, the change of which was used as a relative measure, at any instant of time, of the degree of bake. The reason for the adoption of the wave length 6600 A. for the spectrophotometer measurements was that the examination of films covering the entire baking range, on aluminum by reflected light and on glass by transmitted light, showed that the predominating wave length, although shifting slight1 , was in the red of the visible spectrum, the wave length 6600 . being a good average value.
w
-=INTI Figure 1-Modified
Baking Oven
a secondary reaction, which he postulated was a combustion process whose rate was directly proportional to the oxygen concentration. He then gave as an equation that better expressed his high temperature results: dx d - = k(a
- X)
+ ki
X(U
- X)
where k = velocity constant of the secondary reaction k l = velocity constant of the ordinary low-ternperature reaction The studies of Genthe, Coffey, and Rogers and Taylor constitute the only investigations that were found, in which baking, as a means of setting varnish films, was treated in any degree. As true baking is usually carried out above 100' C., it is apparent that our knowledge of the kinetics of the baking process is indeed meager. Genthe's equation for his results a t higher temperatures is interesting and it is quite evident that as we approach a higher and higher temperature, if we follow Genthe's argument, kl gets smaller and smaller compared with IC. The second term in the equation will then become negligible and we will have
z
=
k(a
- x)
the equation for a first-order reaction. This is the equation found best to express the experimental data that were obtained in the present investigation. However, there was no evidence whatever that the cause was due to the increasing prominence of a combustion reaction as Genthe had postulated. Materials a n d Apparatus The raw materials, varnish, and the preparation of the various films have already been described ( 6 ) . I n the present
Figure 2
To be assured that all films were as near alike as possible, every film was prepared in an identical manner. The aluminum test strips were submerged three-quarters of their length in the varnish, removed with the dip machine at a rate of 2 feet (61 cm.) per minute, and allowed to drain 5 minutes before using. I n the experiments made in an oxygen atmosphere, all films were given in addition a 5-minute prebake a t 125' C., in order to eliminate thb spontaneous combustion which sometimes occurred when working in this atmosphere if most of the solvent was not removed. I n order to obtain constant conditions, the gas flow through the baking oven was maintained at 2 liters per minute. This rate was controlled with a flowmeter separately calibrated for each gas used. This rate was chosen following a study with air of the effect of rate of flow on the baking velocity which study showed that this rate gave the optimum results. When the rate was either greater or less than that specified, the baking was retarded: in the former case probably because a cooling effect occurred; in the latter case either because an insufficient supply of oxygen was present or more likely because the vapors evolved were ineffectively removed.
IXDCSTRIAL AND ENGINEERISG CHEMISTRY
1372
Vol. 22,
KO.
12
Reaction Velocity of Baking Process
Temperature Coefficient
The baking process was studied in air, oxygen, and nitrogen atmospheres. I n each case isotherms of the baking process were obtained for the temperatures 275", 295', 315", ar$ 335" C. by plotting the percentage reflected light a t 6600 A. as a function of time. The experimental results are shown in Figures 2, 3, and 4 .
Plotting the logarithm of the reaction-velocity constant against the reciprocal of the absolute temperature gave a series of straight lines as shown in Figure 5 . The temperature coefficients of the baking process for the several gas atmospheres investigated were obtained directly from these curves. Exlsressea, as is customarv. in terms of a 10-deuee temperature-difference, the temperature coefficients (IC 3&"/ IC 295") as read from Figure 5 were: oxygen 1.40, air 1.35, nitrogen 1.29.
NITUGGEN
Heat of Activation The determination of the reaction-velocity constants and the temperature coefficient allowed the calculation of the heat of activation in calories per gram molecule by use of the Arrhenius equation (9). The heats of activation as calculated are as follows: oxygen 22,000, air 19,500, nitrogen 16,000 calories per gram-mol. Discussion
Figure 3
For each gaseous atmosphere the percentages of reflected light for the initial and the deteriorated states, ROand R,, respectively, were also determined. Using these values and analyzing the data of the various isotherms, it was found that a satisfactory reaction-velocity constant could be calculated when the unimolecular reaction velocity equation was used, 1. e.: ax
=
k
or wherein a
=
k(a
- x)
l / t In -?!-
a - x
total change in reflected light for complete baking,
=
R Q - Rm x = change in reflected light up t o time t,
and
RQ - Rt
The complete analysis of the data for one isotherm is included in Table I. This is the 275' C. isotherm for the baking process in air. Table I-Reaction-Velocity I s o t h e r m for Baking Process in Air T = 275' C. (548" A,) Gas flow = 2 liters per minute
R (reflected light at 6600
I
Sec. 0
30 90 120 180 240 300 360 480 600 1200 m 5
A.)
a-xa
&
86 81 72 65 53 42 38 35 32 22 20
1:&1 1.194 1.322 1.621 2.045 2,260 2.458 2.684 3.910 4,300
Per cent 94 89 80 73 61
50 46 43 40 30 28 8
0
Calculated from Rt
...
k
log
....
....
0.0267 0.0770 0.1212 0,2008 0.3107 0,3543 0.3906 0 4288 0.5927 0.6335
0.0020 0.0020 0,0023 0.0026 0.0029 0.0027 0.0026 0.0021 0.0023 0.0014
....
....
0,0022
Av.
- R ,.
I n Table I1 is given a summary of the various reactionvelocity constants as determined from the data plotted in Figures 2, 3, and 4. Table 11-Reaction-Velocity
$ X 108
TEXPBRATURE *e. OA.
275 295 315 335
548 568 588 608
182 176 170 164
Although the baking process of a typical oil varnish has been shown to follow the mathematical expression for the unimolecular equation, it does not necessarily mean that the baking process is actually monomolecular. Since we are dealing with a heterogeneous system, all we can say with certainty is that the baking process, involving as it does both
Constants of Baking
AIR k 0,0022 0.0046 0.0090 0.0158
NITROGEN k 0,00065 0.00097 0.00171 0.00213
OXYGEN
k 0.010
0.019 0.035 0.060
-
T I M E O F &?KING
l3EC
Figure 4
oxygen or intermolecular linkage between molecules, proceeds in accordance with the unimolecular law. I n other words, what has been shown is only that the rate of change of the fraction of oil varnish oxidized or polymerized a t any instant of time is directly proportional to the fraction of oil varnish unoxidized or unpolymerized. As baking proceeds, undoubtedly a series of reactions occurs and the rate of the slowest, which apparently follows the unimolecular equation, governs the rate of the process. Another possibility is that diffusion of the oxygen to, and of the vapors from, the varnish film is the controlling factor. This follows because diffusion satisfies the unimolecular law. The importance of the proper removal of the volatile products has already been amply demonstrated (6). The foregoing suggestion makes somewhat doubtful the significance of the heats of activation. However, their order of magnitude is the same as that of other polymeric processes ( 5 ) . Also a difference is obtained for the several gas atmospheres leading to the belief that we are actually measuring a truly chemical process. The fact that the baking process follows the unimolecular equation furthermore supports the equation previously derived (6) empirically interrelating the time required to reach a given end point and the absolute temperature, namely: A
logt = T
-B
INDUSTRIAL AND EN GIhTEERINGCHEMISTRY
December, 1930
This is as required by the unimolecular law. The value of this equation from an engineering viewpoint is interesting. It allows the calculation of the time that would be required to REACTION WELOC/TI CONSTANT
-
TEMPERATURE CURVES
1373
gained by increasing the baking temperature can thus be estimated. Establishment of the manner in which the removal of vapors from the varnish film affects the baking process is of interest in baking-oven design. Proper consideration of this factor has not only definite theoretical significance, but also decided practical importance, particularly in the design of ovens for baking varnish where nearly all the heat is by radiation-as, for example, in electric ovens. It is hoped that this investigation will lead to others and thus improve our understanding of this widely used industrial process. Literature Cited
Figure 5
accomp1ish"any given industrial varnish baking process, a t least as a first approximation, provided that time a t two other temljeratures,-is known. The saving of time that can be
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Coffey, J . Chem. Soc., 119, 1152, 1408 (1921); 121, 17 (1922). Fahrion, 2. angew. Chem., 22, 11, 1451 (1909). Fokin, J . Russ. Phys. Chem. Soc., 39, 607 (1907); 40, 276 (1908). Genthe, Z. angew. Chem., 19, 2087 (1906). Kienle, ILD. ELG. CHEM.,22, 590 (1930). Kienle and Adams, I b i d . , 21, 1279 (1929). Rhodes and Van Wirt, I b i d . , 16, 1135 (1923). Rogers and Taylor, J . Phys. Chem., 30, 1335 (1925). Taylor,"Treatise on Physical Chemistry." Vol. 11, p. 902, Van Noatrand
The Adsorption of Sulfuric Acid by Leather'" John Beek, Jr. BUREAU OF STANDARDS, WASHIXGTON, D. C.
,
Values are given for the adsorption of sulfuric acid of the system if there were of the effect of sulfuric on a vegetable-tanned leather. The measurements no adsorption was calculated acid on the life and propwere made in aqueous solutions, at concentrations from the weight of solvent from 0 to about 1.5 molal. Considering the observed erties of leather, it was found present there and the equithat a knowledge of the limitadsorption as the sum of two effects-combination librium concentration of the ing combining ratios betmeen of sulfuric acid with the leather, and adsorption due to solution; and the difference sulfuric acid and the leathers the surface tension effect-an expression is developed between these quantities was employed would be of use in giving the adsorption as a function of the equilibrium taken as the amount ade x p l a i n i n g the results obconcentration. A similar expression, involving a consorbed. tained. No data could be sideration of the effect of swelling, is derived. This found in the literature giving expression is tested in its application to data, given by Experimental Procedure the quantity of acid adsorbed Kubelka and Wagner, for the adsorption of hydrochloric by any v e g e t , a b l e - t a n n e d acid on collagen. Commercial v e g e t a b l e tanned steer-hide leather was leather a t c o n c e n t r a t i o n s great enough to give the equivalent weight, of the leather ( 7 ) . selected for these experiments. From each of two bends of This research was carried out in order to determine whether the same tannage, a strip approximately 12 inches (30 cm.) the results of an adsorption experiment would give a definite long and 4 inches (10 cm.) wide was cut lengthwise of the value for the equivalent weight of a leather. bend, beginning a t a point approximately 36 inches (91 em.) The usual method of measuring adsorption, which consists from the root of the tail and 16 inches (41 cm.) from the in measuring the change in the concentration of a solution backbone. These pieces were analyzed for moisture and put in contact with the adsorbent, involves tthedetermination mater-soluble matter by the official methods of the American of a small difference between two large quantities; that is, Leather Chemists' Association, and for total sulfur by oxidizthe total quantities of solute present in the solution before ing a sample with nitric acid and then determining total and after adding the adsorbent are determined, and the sulfate gravimetrically by the barium sulfate method. The difference between these quantities is taken as the quantity results of these analyses are given in Table I. adsorbed. As the equilibrium concent'ration is usually T a b l e I-Analyses of L e a t h e r Used determined in a sample pipetted from the solution, the error is WATER-SOLUBLE TOTALSULFUR multiplied by the factor used to give the total quantity of PIECE M O I S T U R E ~ MATTERb ( A S H ~ S O Ib) solute in the solution. Per cent Per cent Per cent A 10.0 18.9 0.84 A more direct method of measuring adsorption was used in B 10.0 17.2 0.76 this work. The quantity of solute in the part of the system Per cent of air.dry leather, b Per cent of dry leather. immediately associated with the adsorbent was determined; the quantity of solute which mould be present in that part Twenty-four samples were cut from the two strips and 1 Received September 27, 1930. Presented before the Division of used for determining the adsorption of sulfuric acid from Leather and Gelatin Chemistry a t the 80th Meeting of the American Chetniso~utionsvarying in concentration from 0 to about 12 per cal Society, Cincinnati. Ohio, September 8 to 12, 1930. cent H2S04' The were prepared by pipetting from a 2 Publication approved by t h e Director of the Bureau of Standards of gravimetrically analyzed solution of sulfuric acid. the U. S. Department of Commerce.
I
N T H E course of a study
