INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1931
881
T a b l e 11--Thermal Decomposition of P r o p a n e ~~
MOLEFRACTIONS RUN TEMP.
PREHEATER
Hs
C ~ H E CHI
REACTION COIL
CaH4
CzHs
0.008
0.000 0.001 0.001 0.001 0.009
Hz
CsHs
CHI
C2H4
0.020 0.049 0 064 0.042 0.078
0.024 0.048 0.075 0.066
0.021 0.043 0.063 0.057 0.072
CnHs
587 611 635 648 679
0.009 0.016 0.032 0 026 0.043
0.012 0.017 0.035 0.011 0.045
0.013 0.020 0.037 0.039 0.044
0.013 0.027 0.028 ...
0.021 0.047 0.062 0.055 0.075
Conclusions 1-The values of the velocity constant for the thermal dissociation of ethane and of propane and the hydrogenation of ethylene in the temDerature ranae of 600" to 700" C. are satisfictorily represented by the Fespective equations:
-T
log k = 13.44
-13,500
log k' = 8.79
Bb
1
- Ea OF C O N - Bb TACT
0.084
0.001 0.003 0.003 0.002 0.012
Sec. 11.70 7.30 4.25 2.26 1.17
0,0209 0,0614 0.0734 0.0608 0.0793
k
0,00412 0.01936 0,0398 0.0619 0,1560
propane are dehydrogenation and demethanization, the rates of the two reactions being approximately the same. Acknowledgment The authors wish to acknowledge their indebtedness to
H. 0. Forrest and P. K. Frolich, director and assistant director
15,970
log k = 15.12
TIME
lor,
Za
I
c. 7 5 6 8 9
1
of this laboratory, respectively, at the inception and during the early part of the work. They are also indebted to K. H. Butler and D. Quiggle for securing a portion of the experimental data.
T
9140
-T
Literature Cited
2-With short times of contact and at temperatures below 700" C., ethane decomposes predominately to ethylene and hydrogen, and secondary reactions, polymerization and d e methanization of ethylene, are negligible. &The principal reactions in the thermal decomposition of
(1) Davis and Quiggle. IND. ENG.CHEM.,Anal. Ed., 2, 39 (1930) (2) Frey and smith, IND. END. CHEM.,ao, 948 (1928). (3) Hurd and Meinert, J . A m . a m . SOL Sa, 4978 (1930).
il;
~ ~ ~ s ~ , a ~ ~ ~(lgZ9). ~ ~ ~ (6) Pease and Durgan, I b i d . , 60, 2715 (1928). (7) Pease and Durgan, Ibid., sa, 1262 (1930).
~
,
e
~
~
~
The Drying and Yellowing of Trilinolenic Glyceride' A. C. Elm THE
NEW J E R S E Y
Z I N C COMPANY, PALMERTON,
A previous investigation into the yellowing of white paints and enamels had led to the conclusion t h a t a solution of the yellowing problem might be expected to result from a study of the chemistry of simple glycerides when exposed to normal and abnormal conditions. The rate of oxidation of trilinolenic glyceride exposed under partly controlled conditions was measured by determining, from time to time, its gain in weight, iodine addition and substitution, peroxide, acid, and ester values. In the 6rst stage of the drying process the formation of peroxide groups accounts almost quantitatively for the decrease in ethenoid linkages.
PA.
The experimental evidence presented is insufficient to permit definite conclusions as to the chemical changes undergone by the peroxides in the second stage of the drying process. However, it proves fairly conclusively that the oxidized glyceride polymerizes to association colloids during the solidification of the oil film'. Subsequent changes in the oil film, including yellowing, are probably caused, not by further oxidation, but by variations in the degree of association. Morrell's theory that the yellowing of oils is due to the formation of ketoxy groups could not be verified.
..............
I
N T H E course of an investigation of the yellowing of white paints and enamels (59), it became apparent that the final solution of the yellowing problem might be expected to come from a thorough study of the chemistry of the drying process of drying oils. Although a hypothesis had been advanced to explain the drying of oils on a purely colloidal basis ( 2 , 3, 4, 9, 60, 61, 62, 63), the opinion now prevails that the drying of fatty oils, with the probable exception of tung oil, must be preceded by a chemical reaction between oxygen and the olefinic compounds of the oils (16, 17, 37, 40, gs, 56, 63,58). 1 Received April 9, 1931. Presented before the Division of Paint and Varnish Chemistry st the 81st Meeting the Ameaican Chemical Society, Indianapolis, Ind., March 30 t o April 3, 1931.
Natural fatty oils, being very complicated mixtures, make an intelligent interpretation of the results almost impossible. Eibner, recognizing the necessity of studying the drying of oils on simple glycerides, suggests the use of p-eleostearin (17, 21). Since, however, the drying process of wood oil is essentially different from that of linseed $1, as would be expected from their respective chemical constitutions, it was thought unsuitable for a study of the autoxidation process. Synthetic trilinolenic glyceride was therefore selected, because (1) it is one of the most important drying principles of the members of the linseed oil group; - . (2) . . its chemical composition and constitution are kniwn; and (3) it dries without the aid of m&llic driers in a reasonable length of time, thus simplifying considerably the experiments as well as the
~
.
~
(
~
Vol. 23, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
882
correlation and interpretation of the results. A careful, systematic study of the chemical changes taking place in this oil during normal and abnormal exposures was expected to throw some light on the yellowing of oils. Method
The exposure conditions greatly affect the drying rate of oils, and in order to get comparable results the trilinolenic glyceride was exposed under partly controlled conditions. It was brushed on opalite glass panels a t the rate of approximately 2 grams per square foot (0.093 square meters) and allowed to dry in a cabinet equipped with a 75-watt Mazda daylight bulb and a fan. The panels were a t an average distance of 10 inches (25.4 cm.) from the light source. The temperature of the cabinet averaged 42" C. No attempt was made to control the humidity. Samples of the oil or film were taken a t regular intervals and analyzed for gain in weight, halogen addition and substitution (35), peroxide (N),acid and ester values, and increase in molecular weight. The data are presented in Tables I to IV. Note-Previous determinations of the iodine number of oxidized oils are probably seriously in error owing t o the presence of peroxides in these oils. I? acid solution these peroxides liberate iodine from potassium iodide and thus make the apparent quantity of iodine absorbed too low. For this reason a 0.1 N solution of iodine monochloride in water-free carbon tetrachloride (24, 34) was used and thus a truer measure of the num4er of double bonds remaining in the oil or film was obtained. T a b l e I-Gain EXPOSURE Hours
in Weight of Trilinolenic Glyceride d u r i n g Drying
T a b l e 11-Changes
EXPOSURE Hoirrs
GAIN
EXPOSURE Hours
%
GAIN o/,
in Iodine Addftion a n d S u b s t i t u t i o n VaIues a n d Peroxide N u m b e r " IODINE IODINE ADDITIONSUBSTITUTION PEROXIDE VALUE VALUE TOTAL VALUE
0
Grams 258 R 247.0 238.5 235.5 226.5 214.0 193.5
Grams
Grams
0
0
Grams 258 6
1 18 0 26S.i 1.5 2 20 5 1.3 260.3 27 5 264.8 1.8 3 266.3 2.9 3 5 36 9 256.6 4 38 6 4.0 249.8 48 7 7.6 5 245 6 7.8 52 3 6 185..5 242.2 57 2 6 5 175 0 10 0 234.3 64 0 7 161.0 9 3 227.6 67 8 147.8 8 13.0 65 0 9 71.5 64 3 175:8 72.5 14 39'0 63 6 174.7 73.8 17 37.3 62 0 171.0 71.5 20 37 5 48 67.8 55.7 120 38.5 34: 1 33.9 106:5 2200 20.0 29.0 11.7 60.7 a The iodine addition values were calculated just like iodine numbers; that is they express grams of iodine added t o 100 grams of oil To bring the sudstitution and peroxide numbers on the same basis with the iodine addition numbers, they were calculated as grams of iodine substituted and grams of iodine liberated by 100 grams of oxidized oil, respectively. Values of 29 are then approximately equivalent to one double linkage, two hydroxy groups, or one peroxide group, respectively. T a b l e 111-Increase
EXPOSURE
in Molecular Weight d u r i n g t h e Drying of Trilinolenic Glyceride MOLECULAR WEIGHT Benzene freezing-point Rast's method camphor method
H"*Y.i . ..
0 1 2 2.5 4 5.5 6 7 S 9
870 982 ioi6
868
made to obtain an insight into their colloidal structure. The results of these tests are tabulated in Tables V to VIII. The data are, with a few exceptions, averages of several representative tests. This minimizes the influence of slight variations due to exposure conditions and similar factors. T a b l e IV-Changes
in Acid a n d Ester Values of Trilinolenic Glyceride
EXPOSCRE Hours 0 4 6,5 18 24
4igo
.. ..
ESTERVALUES
189 195 205 180 183
0.58 14.5 28.7 71.8 78.5
T a b l e V-Iodine Addition a n d S u b s t i t u t i o n Values of Soluble a n d Insoluble Portions of Trilinolenic Glyceride Films a f t e r 24 H o u r s in C a b i n e t IODINE ADDITION IODINE SUBSTITUTION VALUE VALUE SOLVENT Soluble Insoluble Soluble Insoluble Acetone Benzene Carbon tetrachloride
%
570
7%
70
38 2 41 3 35 4
39 5 37 5 33 5
29 9 30 8 30 8
29 7 29 6 26 8
of Trilinolenic Glyceride F i l m s Exposure i n C a b i n e t SOLVENT Acetone Benzene Carbon tetrachloride
T a b l e VI-Solubility
a f t e r 24 Hours'
% 67.6 47.3 23.5
T a b l e VlI-Molecular Weights of Soluble Porrions of Trilinolenic Glyceride Films a f t e r 24 Hours' Exposure in C a b i n e t (Rast's C a m p h o r Method) Acetone 1020 Benzene 1132 Carbon tetrachloride 996 Table VIII-Com s r i s o n of Covered ' Y e l l o w ) a n d Uncovered (Colorless) Portions of T h n o l e n i c Glyceride F i l m s a f t e r 120 Hours' Cabinet Exposure COVERED UXCOVERED /\'ELLOW) (COLORLESS) Iodine addition number 59.0 38.5 Iodine substitution number 19 9 34.1 Peroxide number 26.3 33.9
Weight Changes during Drying
As indicated by Table I and Figure 1, the drying of trilinolenic glyceride is much less complicated than that of linseed oil. The drying curve is much steadier and does not show sudden breaks which are so characteristic of linseed oil. It is quite reasonable to conclude that a negligible amount of volatile decomposition products is formed and that the trilinolenic glyceride film under ordinary conditions is a much more stable substance than has generally been assumed. The curve illustrates clearly that the trilinolenic glyceride loses weight immediately after application and after drying dust-free. The same phenomena have been observed with linseed oil, but cannot be explained as yet. Chemical Changes during Drying
A good insight into the chemical changes during the first stage of the drying process of trilinolenic glyceride may be obtained from Table I1 and Figure 2. The halogen addition value decreases a t almost the same rate as the peroxide value increases. This would lead to the conclusion that all the oxygen absorbed by the oil is added a t the double linkages in the form of peroxides as expressed by the following equation:
b6a 0 2
864 1875
d u r i n g Drying ACIDVALUES
i62 925 1065 1050
Since the results of these tests led to the conclusion that most of the characteristic properties of solid oil films cannot be explained by their chemical composition, attempts were
R.CH=CH.R
A + +R.CH-CH.R 0 2
The amount of hydroxy groups formed during this stage as measured by the iodine substitution value is negligible. The explanation of the reactions taking place during the f i s t 7 hours of exposure-that is, up t o the time when the oil becomes very viscous and stringy-seems, therefore,
I S D U S T R I A L AND E,VGINEERIiVG CHEMISTRY
August, 1931
comparatively simple. However, “The difficulties,’’ write Norre11 and Wood (/to), “in a satisfactory representation of the drying of an oil begin at this stage. What is the fate of the peroxide? Is it polymerized as such, or does it act with A = more oil according to the following scheme: A02 2-40?’’ As shown in Figure 2 , from this stage on the iodine addition value decreases very rapidly, indicating a fast saturation of do u b 1ti linkages , while t h e peroxide 2 a v a l u e remains almost constant. This might be taken as evidence in favor of the above reaction which would result in t h e formation of FIGURE N0.I Some investigators (22, 43, ,@have j) proposed linDURING DRYING. oxyn formulas based on t h i s assumption. If t h e y were correct, a treatment of the films with slightly acidified boiling water should result in the formation of alcoholic compounds according to the following equation (26):
+
-
R CH,
R CH
/3
+ H?O+
0
/\
-CH-CH-
+-CHz-CO-
Quantitative ultimate analyses of trilinolenic glyceride h e have shown that their oxygen content approaches the equivalent of eighteen atoms per molecule (33.1 per cent); that is, two atoms of oxygen per double linkage originally present in the glyceride (SO). These few tests make it appear improbable that the oil films contain monoxides, but it seems that they are peroxide derivatives, the constitution of which is unknown as yet. Colloidal Structure and Polymerization Reactions
R CH OH
Certain physical and chemical properties of the oils during and after drying, such as the rapidly increasing viscosity, the swelling of films when exposed to water or organic liquids, etc., prove that the originally non-colloidal oils are transformed into colloidal systems upon exposure to oxidizing conditions. Oxidation alone, however, cannot account for the formation of a colloidal system from oils, and polymerization of the oxidized glycerides has been assumed for this reason.
I
0
+ R”=C=C=R”
1.043 at 25” C., was soluble in alcohol (95 per cent), had an iodine number of 72.8, a molecular weight of 975, and a refractive index of 1.4883 at 25’ C. Apparently, three of the nine double linkages in the trilinolenic glyceride had remained intact. This oil, when spread out in a thin film and exposed under conditions under which trilinolenic glyceride dries in less than 10 hours, did not dry at all. A few tests made in an attempt to verify the chemical constitution assumed for this compound indicated that, not the oxido, but the corresponding keto compound, had been obtained. It seemed that the alpha monoxide is so unstable that it undergoes a tautomeric change to the keto form immediately upon its formation (see also 10):
R CH OH
Since filnis turn yellow upon treatment with boiling water, and this color, according to some authors, is caused by the presence of hydroxy groups, an investigation of this reaction became of vital importance. Trilinolenic glyceride was allowed to dry under standard conditions for 48 hours. X portion of the film scrapings was treated for several hours with boiling water slightly acidified with sulfuric acid and then dried in a vacuum desiccator over barium oxide. The untreated film, which was almost colorless, had a substitution value of 43.3, while that of the treated film, which had turned a deep yellowish brown, was 39.5. Some of the hydroxy compounds had apparently been dissolved out hy the water, which Fas slightly colored, causing a decrease in the substitution value. New alcoholic compounds had not been formed, however, which means that either the alpha monoxides are stable toward boiling water or the films do not contain monoxides of this type. The first conclusion is very improbable, because alpha monoxides are generally highly unstable compounds and are easily acted upon by chemical agents. I n order to obtain evidence for or against the second conclusion, an attempt was made to prepare an alpha monoxide from trilinolenic glyceride. Alpha monoxides or oxido compounds may be prepared by oxidation of olefinic compounds with perbenzoic acid (7,11, 25, 65). The reaction takes place according to the following equation : CBH, CO,H
883
= C6H,COlH
/\
+ R”=C-C=R”
The benzoylhydroperoxide was prepared following a method suggested by Hibbert and Burk (23). A solution of slightly more than the calculated amount of perbenzoic acid (1 per cent escess) in chloroform was mixed with an ice-cold chloroform solution of trilinolenic glyceride and allowed t o stand at room temperature for 2 weeks. The benzoic acid formed was removed by mashing with dilute sodium carbonate solution. The cliloroform solution was dried oyer anhydrous sodium sulfate and the solvent evaporated in vacuo. h lightcolored, T-iscous oil was obtained, which had ‘3, density of
’
I
I
FIGURE NQ2 CHANGES IN IOWNE ADDITION AND SUBSTITUTION VALUES AND PEROXIDE NUMBER. I
200
150
100
50
IO
20
30
40
50
60 HW.
Polymerization is the combination of two or more molecules to forni one. There are various types of polymerizing reactions (60): (1) the combination of molecules, with coincident liberation of water, hydrochloric acid, etc., condensation; ( 2 ) the combination of unsaturated compounds by primary chemical linkages to form larger molecules of a less unsaturated or saturated type, polymerization; and (3) the combination of unsaturated organic compounds by means of residual valencies to form less unsaturated larger molecules, association.
It is very unlikely that condensations take place during the natural drying of trilinolenic glyceride or similar oils, and the question remains as to which of the last two reactions
,
INDUSTRIAL AND ENGINEERING CHEiWIXTRY
884
Vol. 23, No. 8
causes the formation of the colloidal system resulting finally in gelation and drying. Polymerization without the active participation of the absorbed oxygen is highly improbable, because it would lead to the formation of new C C linkages and rings which have never been detected in linseed oil films. The formation of polymerides with the aid of oxygen has been suggested by Marcusson (39) and others. Marcusson’s latest idea on this reaction may be illustrated as follows:
have been oxidized to the same state as the insoluble, solid portions. The results in Table VI show also that the film consists, not of only two distinct phases, but of an indefinite number of phases varying in their solubilities in various solvents. It seems that carbon tetrachloride dissolves the lower, benzene the medium, acetone the higher association products, while the highest remain undissolved in any of these solvents. This statement must be slightly modified, however. When some of the acetone-insoluble residue is treated with acetone a t 100” C. in a closed tube, it dissolves almost com-CH=CH-CH-CH-CH-CHpletely. This would indicate that the solvents do not merely / \ I I remove from the film certain polymerides, but actually exert 202 +o=o o=o+o 0 + 0 2 a depolymerizing effect. The degree of depolymerization \ / I 1 -CH=CH-CH-CH-CH-CHdepends on such conditions as type of solvent, concentration, and temperature (13). The final product of this reaction contains a 1,4-dioxane An attempt to find the average molecular weight of the ring, which, however, has never been found in linseed oil soluble portions by the freezing-point method in benzene films. This and other evidence brought forth make this failed because the acetone- and carbon tetrachloride-soluble type of polymerization highly improbable. portions did not dissolve clearly in benzene. The benzeneThe main question is whether the peroxides polymerize soluble portion had an apparent molecular weight of 1335. as such or, if not, what changes they undergo preceding or Using Rast’s camphor method, the results contained in during polymerization. Although the experimental data Table VI1 were obtained. It seems that the three solvents collected thus far make the formation of monoxides from the caused depolymerization of different proportions of the film peroxides under normal exposure conditions highly improb- substance to the monomeric state or that the camphor comable, they are insufficient to permit a final decision in favor of pleted the depolymerization begun by the solvents. The one or the other idea. The following considerations, how- latter appears to be the more plausible explanation, as the ever, are principally the same for both and will be carried results in Table I11 indicate. out using the peroxide as an example. All this evidence points toward the association type of The more important possible polymerization reactions are polymerization during the drying and aging of trilinolenic as follows: glyceride. Association during the drying of oils can easily explain the conflicting data on molecular-weight changes (a, 8, 15, 4.6) R R R R R found in the literature. The apparent molecular weight I 1 I I I CH-02-CH CH-02-CH CH obtained in each case depends to a large degree on the type of solvent used, the concentration of the solution, the condi-CH tions of solution and the method, ebulboscopic methods CH-02-CH CH-02CH tending to give lower results than cryoscopic methods. I I I I I R R R R R Marcusson (33), unable to detect an increased apparent L J L _I molecular weight of the glyceride, but finding dimeric fatty x-2 (1) acids after saponification (this phenomenon will be discussed in detail later), claimed that not inter- but intraR molecular polymerization had taken place. Intramolecular polymerization, with or without the aid of oxygen according to the following scheme, would result in a denser, more compact molecule:
+
x1)o-
I
[q
L
i
x
Reaction 1 leads to the formation of true polymerides, while 2 yields associated compounds. The principal difference between these two types of colloids is that the association colloids, being formed by neutralization of free secondary valencies, are greatly affected by solvents, while the colloidal state of the others is affected by chemical reagents and reactions only. Furthermore, association polymers do not differ chemically from the monomers because the distribution of main chemical valencies has remained unchanged. The differences are of a physical, and especially a colloidal, nature. The acetone, benzene, and carbon tetrachloride soluble and insoluble portions of the trilinolenic glyceride films differ chemically only within the experimental limits (Table V). The solid-insoluble phase of the films differs from the liquid-soluble phase only in the degree of polymerization (see also 31). It has been said that the liquid phase of the a m is unchanged oil and that the film is a two-phase system. However, the above results show clearly that the soluble portions
CHz.OOC.(CH~)~~~H-C,H.CH~~CH=CH.CHZ.CH-CH.CHZ~ I ( i i 1 bH.OOC. (CH2):-CH-CH.CH2.CH-CH.CH2Ck-CH.CH2.CH3 ; I CHZ.OOC.(CH2)j.CH=CH.CH2.Ck-C~.CH2.CH=CH.CH*.CH3
I
Such a structure could hardly account for the increased viscosity and could not explain the colloidal properties of the film, Furthermore, it is highly improbable that the glyceride molecule has a plane structure. It is more reasonable to assume a three-dimensional structure for this molecule according to the following scheme, the fatty acid radicals extending into space at angles of 120 degrees (Figure 3). Intramolecular polymerization would result in the formation of a highly strained and unstable molecule, which is not in agreement with fundamental conceptions of organic chemistry. The possibilities of intermolecular polymerization or association are much greater (40). When determined by Rast’s camphor method, the molecular weight of the trilinolenic glyceride increased during drying from 868 to 1050, approximately-just enough to account for the oxygen absorbed. After saponification of the films the molecular
August, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
weight of the oxidized fatty acids by the same method was 690, which indicates dimerization. Apparently the free fatty acids, on account of the polarity of the carboxyl group, are capable of forming associated molecules in camphor while their esters are in monomolecular solution in this solvent (see also 6 and 12). The oxidized trilinolenic glyceride associates a t least to the tetrameric state, as indicated by the molecular weights determined in benzene (Table 111). This does not mean that all the molecules are tetrameric. Their stages of association vary from the monomer to polymers of unknown molecular size, so that the average apparent molecular weight is equal to that of the tetramer. The final film, in other words, is a solid solution of chemically similar or identical organic compounds. This, however, is the main characteristic of a resin, which has been defined, not as a definite chemical compound, but as a characteristic physical and colloidal state (50). I n fact, the analogies between dry oil films and resins are so striking that the drying of oils has been called a “resinification” (18, 19, 20, 46, 47, 48, 49). Eibner, in studying the drying of oils, found that the free fatty acids, their methyl, ethyl, and other lower esters, do not dry (see also 18,36), and claimed that the glycerol radical takes an active part in the drying process. This is true when intended to mean that, although these compounds oxidize and associate, their molecules do not reach a sufficient size and degree of complexity to acquire colloidal oharacteristics and to set to a gel. Higher esters of linolenic acid should then dry better and faster than the glyceride, as has been found to be true with the mannitol ester (SO). Nagel and Gruss (41) have arrived a t the same conclusion in their study of the p-eleostearic acid and its esters. Summary of Mechanism of Drying Process
Summarizing the results and conclusions, it can be said that the drying process consists of three distinct phases: (1) oxidation of the olefinic compounds through the peroxide stage to associable compounds; (2) polymerization of these products to association colloids; and (3) gelation and aging of the colloidal system thus formed. It has been customary t o explain film properties and failures by oxidation, neglecting entirely the influence of polymerization or association of the oxidized oil molecules. An attempt to correlate paint experience with a hypothesis based on the proper balance of oxidation and polymerization reactions in drying oils is likely to yield some interesting results. Yellowing
Numerous investigations into the yellowing problem have resulted in a better understanding of this most annoying phenomenon, although its complete solution has not been accomplished. I n a recent investigation Morrell and Marks (37) came to the conclusion that the yellowing of oils is due to the formation of ketoxy compounds. If this hypothesis were correct, yellow films would show a higher hydroxy value than films that had not yellowed. Hydroxy groups in organic compounds are usually measured by the amount of acetic acid required for their esterification. For their quantitative determination the compound is boiled with acetic anhydride until the esterification is complete. Under these conditions acetic anhydride may react with other groups thought to be present in oxidized drying oils and give an apparent hydroxyl value not due to hydroxy groups. The acetyl value of oxidized oils cannot, therefore, be considered a true measure of hydroxy groups present (see also 6 and 42). The halogen substitution, a much less violent reaction, was therefore used in an attempt experimentally to test the hypothesis postulated by Morrell and
885
Marks. As shown in Table VIII, this hypothesis was not verified. The covered and yellow films had apparently stopped a t a lower degree of oxidation and association than the uncovered films. Neither had a treatment of the films with boiling water resulted in a higher substitution value, although the films had turned a deep yellowish brown. The yellowing of oils can therefore hardly be due to the formation of hydroxy compounds.
*
ACID RADICAL
a
I N PLANE OF PAPER
w
Figure 3-Structure
of Glyceride Molecule
A fairly satisfactory explanation of yellowing is possible on the basis of oxidation in conjunction with polymerization of the glycerides (see also 14, a?‘). It is well known that the drying of linseed oil is greatly accelerated by light. According to Grothus’ law a photoreaction can take place only when the light is partially or wholly absorbed. The deciding factor for the light absorption of a compound is its degree of unsaturation. The stronger the fields of residual valency the more pronounced and the more selective is the light absorption which, when sufficiently strong in the range of the visible rays, will result in color (29). A reduction in the strength of the fields of residual valence causes a displacement of the absorption toward shorter wave lengths and usually a decrease in selectivity. In any case, polymerization must result in a decrease of the depth of the color of the monomer (51). These thoughts applied to the yellowing of oils lead to the following picture. When drying under normal conditionsthat is, in the presence of light-the oxidized glycerides polymerize to colorless compounds of fairly high molecular weight. As a matter of fact, it has often been observed that oils bleach on drying. I n the absence of light the rate, as well as the ultimate degree, of polymerization is lower, resulting in colored compounds of low molecular weight. This would also explain why yellow b s are tacky and soft. When this yellow film is then exposed to light, the colored compounds are polymerized to the colorless polymers. Upon reexposure in the absence of light more of the monomers or low polymerides are formed from the peroxides and oxidized glyceride and the film yellows again. This time, however, the concentration being lower, it yellows less. This process may be repeated until all molecules have been polymerized to a sufficiently high degree. To test this hypothesis a dry film of a white enamel was half covered with paper and then exposed to ultra-violet light for 366 hours. The paper mask was then turned by an angle of 90 degrees and the panel was exposed to conditions accelerating yellowing. The results are clearly shown in
Figiin: 4. 'Tlie Jiigliiy ~ ~ ~ d y r n e r i sqiiartcrs ed ( I aid 2) d m \ v 1i:trdly any yeIl&ng, rvhile the quarters dried atid exposi:,l ul&r ordinav cmditions (3 and 4) yellrivred considerably. If the yellon coiripounds were oxidative decornpositiw
the most highly oxidised film should yellow ilie most. LLnvernr, a film thoroughly dried in the presence of direct light, pri:ferably sunlight or ultra-violet light, yellows less than anothcr m e of tlic same age dried in &Rune liglit or a,Inenre of IiKht.
Pi*"re 4--liffoct Of Polymerization on Yellowin$ (1) , Expored aucces*iveiy to ultra-violet liehi and light in yellowi~ie~ ca"i"er 12) Expored LO ultra-violet light and darkness i n ytllowing cabinet. (3 and 4) No ultra-violet light but capored to light and darkner. i n yelluwinp cabinet.
atid soon x point was rcaclied wlwre the pink color faded a\t-ay rather slowly. This is obviously the cud point of tlie free fatty acid present. The titration was continued until the pink color persisted for a long time without any appreciable fading. Thus a point was reached wiiicli has been used in the calculation of the acid niiinber reported in Table IV. It is apparent that some decomposition had taken place during the titration causing the high acid numbers. Apparently, as indicated by the slowly fading elid points, the acidic dc(!omposition products are not present in the free state, but, are formed under the influence of a very slight excess of potassium liydrosicle. An excess of aicoiiolic potassiurri hydroxide solution wlls ailded to this reaction mixture, w h i c l i TIW heated on the hot phte lintil complctcly saponified. After titration of the B X C ~ S Sof potash, tlie ester value was calculated in tho usual way. Considering the difficulty of obbaining a slisrp and definite end point in this titration, the results correspond well enough wit.!: the theoretical cster value to indicate that no hydrolysis of the glyceride had taken place. If there is any increhse in the acid number of dving oils during the drying process, it is due to the formation of acidic decomposition products of low inolerular weight.
..
Tile hypothesis in the above f m n , however, does not explain t.he inflimice on yellowimg of moisture, which a(:cording to most investigators of this problern is absolutely esential. A modification of this hypothesis to meet the above requirement is not difficult if we apply Trchirsch's ideas on the reactions causiiig rancidity of oils and fats (67). According to this author, these reactions may be expressed hy the following equation: i
CIi t
I
~
CH
CH
1
Vol. 23, so, x
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The morioxide or tire tautorricric keto compinmd fonned from the peroxide under tlro influence of water can then enter into the polymerization reactions outlined above and lead to a ycllou~or colorless film. Such a reaction would, of course, require that hydrogen peroxide be fornred just prior tci yellowing and in amounts proportional to the depoe of yellowing. Tho evolution of hydrogen peroxide by oils has been inwho did not, vestigated by Stilts, Nelson, and Schmuto (K), however, point out its possible connection with the yelloning of oils. Such an investigation might yield some interesting and valuable data. Hydrolysis of Drying Oils
It lias been reported that the acidity of oil films increases rapidly on drying aiid aging. Eibncr found a,cid nnmhers OS 45 after 4 days, 73 after 60 days, and lY2 after 2 years, and claims that partial hydrolysis of the glycerides is resp,msible for this plienomenon. Aithoirgh this statement iins bcon cont.mdictcil am1 corrected by d'dns and .\Iersl,aciics ( 1 ) and by Slansky (is),acid a.nd csber values of trilinolenic glyeeridc i l n r i n ~drying were determined i n order to kit their possible comec:tiiin iritli ycllowing. Tlicse are Sound in Talile IF'. 'The oil and film sanijiles w r e taker1 up witii ether arid carefiilly titrated with cold aicoiidic potash (0.1 h-), using phenolphttiaiein as au indicntor. A t the beginning the pink color of the indicator dis:lppo:ircd rather rapidly
Acknowledgment
The author wishes to acknowledge the assistance and criticisms of J. S.Long, Lehigh University, and S. Werthao and the other members of the Remarch t)ivisiori of The Sew Jersey Zinc Company. literature Cited (11 d'Anr and Mrirlvacher. ('hem. Umrcholi 34, :io0 (1027); 8 % 174 (192s). I21 Auer, / b i d . , 86, 11, 27 119281.
Fedlr, Oclr, Warhir l f o r z r .
(3) I41 (51 (6) 17) (8)
Auer. I'orbcn-Zlu., 81, 1240 (10'B). Auer. I b i d . , 84, 1729. 1790 (10271. Unuer, "Die trocknciicien Oelc," p. 255, Stuttgarf, 1928. Hauer, I b i l . . P. 219. Baucr and 1+6hr. I. Prnkl. Chem., 141, 201 11929). Bauei end Kutrcher. Chem. Lrmiihox P d l r , Ode, IVoihre N @ n c . 84, 57 11925) (9) Blom, Z. ongew. Chem., 4D, 146 11927). (10) llodendorl. Arch. Phorm.. 468, 491 (1930). sei. rrdnm, ~ 84.377 ,(1'3~9). (11) m s c k o l . s n i t , and ~ ~proc. ~ i .A m i i~ (12) Biiedeb, 2. 9hwik. Chem., 10, 205 119301. 1131 Cutter, I. Oil Colmrr Chem. Ariocn., 18, 66 (1930). (14) Editor's Note, Pnrbc h c k , 19S8. 606. 11s) Eibner, "Ueber Fettlr Ode;' Munich, 1922. I161 Eihnei. Porbcn-Zla.. 88. 2165 (19281. 117) Eibner, I b i d . , 85, 585 (i9291. (18) Eihner. Chrm. Umirhur Felie. Oelc, Woihic I f o r r c , Sa. 36 (19201 (191 Eibibner, Z. nngmu. Chcm., 86, 34 (1923). I201 Rihner, Pur6e Lo&, 1996. 378, 403. (21) Ribner and Munrert, Chem. Um.%hou Pelis, O a k . Waihzc I f 1 7 r i C . a&. 105 (1927). (22) Fahrion, Z. G ~ Z L W . Chem.. 48, 722 (1910). (23) ftibbrrt and Hurk, J. Am. Chem. Sor., 11, 2240 (1925). (241 Nildt, Reo. prod, chim., 41, 254 (1918). (25) Rouben. "Die Mcthudeiz der arsanischen Chemie," Vol. I I . p. 134,
Leipzip. 1925.
(26) Rouben, I b i d . , p. 135. (27) Kienle end Admmr. I w a . ENO.Cirsw., 41, 1379 (1929). (28) Kiiehino Kino, J. Soc. Chon. l n d . Jopon Supnl,, 88, 153: C h c n .
. (31) (32) (33) (34)
. i o n s , Zinmerman, and Nevins. INO. E m . Cneu.. PO, 806 (1928) hlarcurion, Z. ungcil;. Chrm., 88, 148 110261. Marcusson. /bid.. 39, 476 (1836).
Marrl~ali,I. .So'. C~icin.I n d . , 19, 213 (1900). (35) Mcllhincy, .I. ilm. Chcm. SOC.. al, 1084 (18991 (SC,) Moirell, J. 011 Coluirr Chem. A ~ i O c n . ,I , 153 (19241. (37) Moriril and Mrrks, /bid.. la. 183 (1929). (381 Morrril and Marks. I b i d . , 10, 186 11927). (391 Morrrll and Marks, Anolyri, 64, 503 (1'3a9). (40) Morrcll and Wood. '"Chernisto01 Drying Oils," i'. 93, 4 e w York. 1'325. (41) Srgel aod Cruris. Z. iingero. Chem.. 39. 10 (1928). (4%) xormnnu, Cicm. Umsdhai I r i l o , O d e . W e r h i e Horse, 86, 81 (19291.
I S D USTRIAL A S D E-VGIIVEERING CHEMISTRY
August, 1931
(43) Orlov, J . Russ. Phrs. Chem. Soc., 42, 658 (1910). (44) Rhodes and van Wirt, IND. ENG. CHEM., 16, 1135 (1923); 16, 960 (1924). (45) Salway, J. Soc. Chem. I n d . , 39, 324 (1920). (46) Scheiber, Farbe Lack, 1927, 75. (47) Scheiber, Zbid., 1926, 295. (48) Scheiber, 2. angew. Chem., 40, 1279 (1927). (49) Scheiber. Chem. Umschau Feffe, O d e , Wachse Harze, 34, 1 (1927). (50) Scheiber and Sandig, “Die kunstlichen Harze,” Leipzig, 1930. (51) Scheiber and Sindig, Zbid., p. 60. (52) Slansky. Chem. Umschau Felte, Oele, rt’achse Harze, 34, 148 (1927).
(53) (54) (55) (56) (57) (58) (59) (60) (61) (62) (63)
887
Slansky, Z . angew. Chem., 36, 389 (1922). Slansky, I b i d . , 32, 533 (1921). Smit, Rec. trau. chim., 49, 675 (1930). Stutz, Nelson, and Schmutz, IND. ENG. CHBM.,17, 1138 (1925). Tschirsch, Chem. Umschau Felle, O d e , Wachse Hame, 32, 29 (1925). Vollmann, Farben-Ztg., 33, 1531, 1599 (1928). Werthan, Elm. and Wien, IND.ENG.CREM.,22, 772 (1930). Wolff, Farben-Ztg., 31, 1239, 1457 (1926). Wolff, Chem.-Zfg., 48, 897 (1924). Wolff, F a r b e Lack, 1928, 262. Wolff, Chem. Umschau Felte, Ode. Wachse Harse, 36, 313 (1928).
Generalized Thermodynamic Properties of Higher Hydrocarbon Vapors’ J. Q. Cope, W. K. Lewis, and H. C. Weber DEPARTMENT OF CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
All available, direct, experimental determinations of deviations from the gas laws for the saturated vapors of hydrocarbons having more t h a n two carbon atoms per molecule, when plotted against the ratio of reduced pressure to reduced temperature, fall on a single curve within a maximum deviation of about 3 per cent. Analysis of the data available leads to the conclusion that, up to moderate pressures, the deviations from the gas laws of hydrocarbons having more than three carbon atoms per molecule are approximately the same for equal values of reduced temperature and pressure whether the vapors are saturated or not. These relations, shown graphically, are valid up to a value of RT/P,V = about 4.0, but not beyond. They make it possible to estimate the vapor volume and vapor density of a hydrocarbon under any given conditions of temperature and pressure
from a knowledge of molecular weight and critical pressure and temperature alone. These graphical relationships may be expressed by approximate algebraic forms corrected by graphical functions obtained from the original data, from which, by graphical integration, the internal energy and total heat of hydrocarbon vapors as functions of temperature and pressure are obtained. With these it is possible to compute the heat consumption and energy changes of various processes. Granting additivity of volumes and total heats when, mixing vapors a t constant pressure and temperature, it is possible to estimate the volumetric and thermal relations of vapor mixtures. The material presented may be considered as a graphical method of estimating the volume and the thermal properties of the vapor of any higher hydrocarbon up to and somewhat beyond its critical conditions.
N THE processing of hydrocarbons, the trend of industrial
three carbon atoms per molecule, the ratio is remarkably constant at approximately 3.85, as is shown by Table I based on Young’s data. Furthermore, for these compounds the critical density is practically constant, indicating a critical volume proportional to the molecular weight. Ring compounds have a somewhat lower value of RT,/P,T’,, but even in the case of the aromatics this value falls only to about 3.7. If one wishes to use an average value of 3.78, it seems safe to conclude that a deviation of the ratio RT,,’P,T‘, of more than about 2 per cent from this value need not be anticipated.
I
practice is toward higher pressures and temperatures. From the point of view of engineering design, this introduces serious difficulty, because little is known regarding the properties of hydrocarbons under these conditions. Indeed, the limited data available are restricted to hydrocarbons of relatively low molecular weight, so that, in the case of higher boiling compounds with which the engineer deals predominantly, one is a t present restricted to methods of estimation which may be far in error. It is the purpose of this article to present certain generalizations regarding the P-T’-T relationships and the thermal properties of vapors of higher hydrocarbons, which, it is believed, offer a sounder basis for estimation in the solution of the problems of design involving these materials at high temperatures and pressures than any hitherto available. P-V-T Similarities a t Critical Point
It has long been appreciated that, in certain respects, the various hydrocarbons are extraordinarily similar in their P-V-T relationships. Thus Young’s data (12) on the hydrocarbons emphasize the fact that, at the critical point, the ratio RT,/P,T7, varies but little, particularly if compounds with less than three carbon atoms per molecule are not considered. This ratio indicates, of course, the extent the compound in question deviates from the gas law at the critical point. In the case of normal paraffins containing more than 1
Received April 9, 1931.
T a b l e I-Critical HYDROCARBON
TC
c. n-Pentane n-Hexane %Heptane n-Octane Isopentane Diisobutyl Hexamet hylene Diphenyl Benzene
197 234 266 296 187 276 280 281 526 288
2 8
85 20 80 8 7 5
PC Atm. 33 00 29 60 26 80 24 65 32 95 24 55 39 8 40 4 41.3 47 9
Relationships RT, VC
PC
Cc./g. mol
311.0 367,O 429.0 491 308 484 306.5 311.0 490 256.7
~
PCV,
0.232 0.234 0.234 0.2327 0.234 0.236 0.270 0.2735
3 762 3.83 3 845 3 860 3 73 3 795 3 71 3 72
0.3045
3.74
...,
For a series such as the normal paraffins above propane, for which the critical density is constant, the constancy of RT,IP,V, is equivalent to the constancy of the ratio MP,/T,. Data on this ratio, from International Critical Tables and Landolt-Bornstein-Meyerhofer Tabellen, are given in Table 11. It will be noted that for normal paraffins above propane the maximum deviation from 5.01 is * 0 . 1 1 4 . e.,
