Formation and Deterioration of Paint Films Polymerization of the Vinyl Type in the Drying of Oils P. 0. POWERS Armstrong Cork Company, Lancaster, Pa.
J. L. OVERHOLT AND A. C. ELM The New Jersey Zinc Company (of Pa.), Palmerton, Pa. Our data from the three preceding papers in this series and the work of Miller and Claxton (1928) are examined. The changes in iodine value, specific refraction, and density of drying oils during exposure are correlated with the amount of oxygen absorbed. The results obtained in this analysis suggest a polymerization of the vinyl type as the best explanation of the reactions occurring during the final stages of drying. Five steps in the drying and aging processes are discernible. The first step is the period of induction before oxidation is appreciable; the second step is the period where oxidation occurs most rapidly; this is followed by a third step where the oxygen apparently rearranges, forming conjugated systems if they are not already present; the fourth step is polymerization accompanied by a rapid increase in density, a rapid decrease in
specific refraction, and a rapid drop in iodine value; and the fifth step, the aging step, occurs more slowly and is characterized by depolymerization or oxidative decomposition of the polymers previously formed. Most of these points have been previously recognized, but it is believed that the occurrence of the vinyl type of polymerization in the case of drying oils has not been sufficiently emphasized and supported by experimental evidence. Examination of the possible mechanism of this stage of the drying process makes it clear that some type of vinyl polymerization must occur. The only question remaining is whether the reaction is a straight chain or a ring type of polymerization. Further work will be necessary to determine whether ring structures are formed in the drying process.
HIS investigation was undertaken to gather experimental data needed for a hypothesis of the process or processes involved in the formationand deterioration of complexdrying oil film systems. The three previous papers described the changes in chemical and properties of methyl (io), glycol (ii), and glyceryl(18)esters of unsaturated fatty acids on prolonged exposure to ultraviolet light, Considerable evidence was uncovered which demonstrated that the different esters, regardless of the nature of the alcohol radical and the degree of unsaturation of the acid radical, varied remarkably little in degree and rate of oxid%of the tion. The main differences in the drying various esters included in this investigation must therefore be sought in the polymerization step. The present paper reported in the of the attemptsan analysis of earlier papers in an effort to formulate a mechanism for the polyme%ation stage of the drying process. I n the three preceding papers the changes in the chemical and physical properties were plotted against time of exposure. It was found that a much better picture of the changes in these properties could be obtained by correlating them with the changes in the oxygen content. Of the properties that had been determined, iodine value, density, and specific refraction seemed to offer the best chance for establishing a relation. The changes in these properties to be expected from the addition of oxygen to an unsaturated ester can be calculated with a fair degree of accuracy if the nature of the oxygen bond is known.
T
Iodine Number
It has been fairly well established that the iodine value may be used as a measure of oxidation as far as oleate esters are concerned. This relation also seems to hold within certain limits for other unsaturated esters. I n our experiments the drop in iodine value as oxidation proceeded was normal for the oleate esters (Figure 1). About 10 per cent of oxygen was added to the glyceryl oleate without an appreciable drop in iodine value, but the other esters showed a fairly steady decline with increasing oxygen content. The linoleate esters exhibited a sharp drop in iodine value with little increase in oxygen content after an oxygen content of about l5-18 Per cent had been r ~ ~ ~ h(figure e d 2). Glycol and glycerol holenates showed a rapid decrease in iodine number a t 24 and 19 Per cent of oxygen, respectively (Figure 3). Density The density of the oxidized esters may be calculated from their ultimate analyses with the aid of the covolume relation developed by Traube (7). From this a straight-line connection between oxygen content and density would be expected. This was verified as far as the oleate esters are concerned (Figure 4) where each rise in oxygen content resulted in a proportionate rise in density. The linolenate esters (Figure 6) showed a more rapid rise, beginning with an oxygen content of about 20 per cent. The linoleate esters (Figure 5 )
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1258
exhibited the most abrupt change in density. The methyl ester cume did not turn sharply upward until the oxygen content reached about 27 per cent, while the glycol and glyceryl ester curves broke in the neighborhood of 17-18 per cent of total oxygen.
Specific Refraction For the purpose of calculating the specific refraction of the original and oxidized esters, it was assumed that they are composed of one, two, or three units containing 19 carbon atoms each-that is, 18 carbons for the fatty acid radical and 1 carbon for the portion of the alcohol radical attached to it. Two hydrogen atoms were allowed for the latter. This introduces a slight error in the value for the methyl and glyceryl esters, which, however, is so small as to exert no significant effect upon the final results. The values of Auwers and Eisenlohr (8) were used for all atoms except peroxide oxygen
l,ooi II L
225
25
PERCENT
12
for which a value proposed by Rieche (IS) was entered. These values are given below : 2.418 1.100 1.525 1.645 2.211 2.02 1.733
Conjugation of double linkages is known to increase specific refraction, a phenomenon called “exaltation”. No particular value can be assigned to this effect of conjugation, but any appreciable increase in specific refraction over expected values may be ascribed to conjugation. I n calculating theoretical values for the oxidized films, assumptions must be made as to the nature of the oxygen bond and the number of double linkages present. The oxygen in the ester linkage was assigned the usual ketonic and ether oxygen values. Any additional oxygen present might be given any of the oxygen values shown in the above table. I For the purpose of this discussion the rluuKL 1 ketonic oxygen value was used since it IODINE NUMBER OF O L E A T E E S T E R yields the highest specific refraction values. METHYL Yet many of the calculated figures were GLYCOL GLY CERY L still appreciably below the observed values. Thus any excess in the observed over the calculated values could not be blamed on an error in the choice of the oxygen value or the asscmption of the nature of the oxygen bond. It is reasonable to assume that they indicate an exaltation effect due to the presence of conjugated systems. The iodine values were used to calculate P E R C E N T OXYGEN the numbers of double bonds remaining in 20 25 the oxidized esters. The results of such calculations are in error to the same degree as the determination of the iodine number. The specific refraction of the various structures which may be formed upon oxidation of an unsaturated fatty acid ester Q i were calculated by means of the above values and are shown in Table I. The effect of changes in such structures during the oxidation of drying oil esters with relation to oxygen content is shown in Figure 7 . It is readily seen that oxidation, regardless of the nature of the oxygen bond, will result in a straight-line decrease of the specific refraction. Conjugation tends to FIGURE 3 counteract this decrease. The extent of IODINE NUMBER the effect of conjugation depends largely OF LINOLENATE ESTERS upon the value assigned t o exaltation due to this conjugation. Although no definite values can be assigned to exaltation, it is believed that values of E = 1 and E = 6 represent the extremes for the types of systems involved in film formation. This effect would be equally valid, whatever the form of conjugation. It has not been definitely established whether the conjugation that appears t o be present in oxidized or drying oil films is of the C=C-(3-0 the C=C-C=C type. The specific refraction of each ester was calculated from its ultimate analysis, with b assumptions made as discussed before. The OXYGEN 16 20 ,-\o 2p 28 calculated values are compared with the ob1
.
15
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
October. 1941
served in Table 11. When these values were plotted against oxygen content (Figures 8 and 9), the oleate esters showed a steady decline in specific refraction with increasing oxygen content. The declinein specific refraction of the linolenate esters was not quite so great until the oxygen content amounted to more than 20 per cent. The specific refraction then dropped sharply with only a slight change in oxygen content, approaching the oleate values a t higher oxygen concentrations. The linoleate esters (Figure 9) exhibited the most remarkable behavior, the specific refraction dropping off sharply at about 18 per cent for the glycol and glyceryl esters and a t 30 per cent for the methyl ester.
I
-1.02
I
b
I
-
FIGURE 4 D E N S I T Y OF O L E A T E ESTERS @ 0 METHYL
-
-0.98 t
-
P E R C E N T OXYGEN
Blown Oil A paper by Claxton on the air oxidation of methyl eleostearate (9) was oxamined to determine if iodine number, density, and specific refraction of a blown oil showed the same behavior with increasing oxygen content. The results are given in Table I11 and Figures 10, 11, and 12. For comparison, the theoretical curve for the specific refraction of oxidized methyl eleostearate was included in Figure 12. In calculating these theoretical values no allowance was made for conjugation. The differences between the calculated and the observed specific refractions for the ester up to about 18 per cent oxygen are due to the presence of conjugated systems. The sharp decrease in specific refraction above 18 per cent oxygen indicates that the conjugated systems present in methyl eleostearate were rapidly destroyed as oxidation progressed. I n general, these results are in good agreement with those obtained by Overholt and Elm with the esters of other unsaturated fatty acids.
1259
1.02
I
FIGURE 5 DENSITY OF L I N O L E A T E E S T E R S
0.98
>
I-
z w a 0.94
-
0.90 PERCENT
I5
20
I
I
OXYGEN
I
25 I
I
Mechanism of Polymerization I t is not necessary to review here the various mechanisms which have been proposed from time to time in an effort to explain the drying of oils. Several such reviews are available (3, 4,6, 6). It is generally accepted that film formation consists in principle of three overlapping stages-namely, the induction period, the oxidation step which is characterized by the formation of peroxides, and the polymerization step which is characterized by a rapid increase in density, viscosity, and decrease in iodine value without appreciable change in oxygen content. This discussion deals primarily with the reactions which may serve to explain this third period. The marked change in iodine values, density, and specific refraction of the linoleate and linolenate esters with increasing degree of oxidation is believed to be significant and
FIGURE 6 DENSITY OF LINOLENATE E S T E R S
PERCENT 15 I
OXYGEN 20 1
25
I I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
to be the key to a satisfactory explanation of the polymerization phase of the drying process. Such an explanation will therefore be attempted in full realization of the highly speculative nature of this undertaking. The peroxides formed in the first step-that is, the oxidation step of the drying process-could undergo polymerization by combining either with unoxidized radicals or molecules by way of the double linkage or with other peroxides. When peroxides combine with unoxidized double linkages, two structures could result. One would be a dioxane derivative (I) while the other would be a chain containing alternate C-0-0-C and new C-C linkages (11) or alternate C-0-C and old C-C linkages (111): H
H
H -c-0-C-
H
CH
I
I
H
H
+ r + -c-0-c-
-c-0 H
FIGURE 7 C A L C U L A T ED SPEC IFI C REFRACTION OF OXIDIZED
a -0.28
b (L
U
a W
v)
I
PERCENT 15
OXYGEN
20
I
I
\I A I
HC-0-
\I A I
HC-0-
H
O-CH-
-Hc-O-O-CH-
Reaction IV, proposed by Goldschmidt and Freund ( 4 ) , does not account for the rapid decrease in iodine value and specific refraction during the second (polymerization) stage. Further oxidation of unsaturated compounds cannot account for the drop in iodine value and specific refraction, either because there is no reason why the rate of oxygen absorption during this stage should be greater than during the first stage, or because ultimate analyses and gain in weight measurements do not indicate an abnormally rapid oxygen absorption. The same arguments seem t o rule out the possibility of the peroxide polymer V as suggested by Staudinger (14). On the basis of these deductions it seems improbable, then, that the polymer is formed through bridges involving C-0-C or C-0-0-C linkages. This deduction receives considerable support from the observation that there are only insignificant differences between the amount of oxygen found in oxidized drying oil esters by ultimate analysis and the sum of oxygen present in ester, peroxide, aldehyde, and hydroxyl groups (IO, 11, 1%'). It appears reasonable to or C-0-0-C linkages assume that if any C-0-C
FIGURE 8 EClFlC REFRACTION OF O L E A T E A N D LINOLENATE ESTERS NOLENATE ESTERS
FIGURE 9 SPECIFIC REF RACTION OF LINOLEATE ESTERS OMETHYL @ *GLYCOL @ QGLYCERYL
0
I
H
The formation of a dioxane ring system is questionable on several grounds: (a) The presence of such a system in a drying oil film has never been proved analytically; ( b ) the peroxide content should decrease at the same rate as the iodine value; (c) if such a reaction could take place, glycol and glyceryl oleates should be capable of forming films. The last two arguments also make it highly improbable that reactions I1 and I11 could be accepted as satisfactory explanations of the drying process. Peroxides could combine with each other according to the following schemes : --HC-0
1
p 3 2
H(i-o--CH /
Vol. 33, No. 10
P E R C E N T OXYGEN
20
30
October, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY
existed in the final film, they would not have been detected or measured quantitatively by the analytical methods for the determination of the oxygen-containing groups enumerated above. The conclusion therefore appears unavoidable that the oxygen does not link together the various oxidized molecules but serves merely to activate neighboring double linkages so that they may enter into polymerization reactions. The exact mechanism of this activation is not known. For the purpose of this discussion such knowledge does not appear to be necessary. The activated double linkages may then enter into polymerization reactions according to the following scheme which is known to be operative in the vinyl type of polymerization: 2 R-CH=CH-R'
+R-CH2-CH-R' I
R~=CH-R/
1261
characteristic curves because of the presence of the third double linkage which complicates both the oxidation and polymerization reactions. The methyl esters of the more highly unsaturated fatty acids oxidize and polymerize but do not yield dry films because they lack the necessary functionality of the alcohol radical. The glycol and glycerol esters dry and yield fusible or infusible films, depending upon the functionality of the alcohol radical. The drying of the eleostearate esters appears to be slightly more complicated than that of the linolenate esters but follows the same general principles. During the oxidation stage (Figure 12) the specific refraction decreases slowly since only one of the three conjugated double linkages is oxidized. Because of this observation it is reasonable to assume that one of the two outside double linkages of the triple system and not the center linkage is oxidized. During the second and polymerization stage one of the two remaining double linkages is consumed, and conjugation is thus completely destroyed. For this reason the specific refraction drops sharply.
This mechanism readily explains the increase in density and the decline in specific refraction with little or no change in oxygen content during the second or polymerization step of the drying process. The density increase in the TABLEI. THEORETICAL VALUES FOR SPECIFIC REFRACTION case of the methyl eleostearate and glycol Mol. % RefraoType Formula Wt. Oxygen F Struoture Mr tion linoleate is sufficiently large (0.92-0.99) to Triene CieHsiOz 291.44 10.98 3 (C=C-C)a 89.096 0.3064 suggest the formation of polymers higher than dimers, I n some cases the decrease in speci0-0 fic refraction, especially in the case of the Peroxide C~eHslOa 323.44 19.8 2 91.402 0.283 methyl eleostearate, is appreciably larger than would be expected from the disappearance of double linkages. I n these cases it is Diperoxide CleHaOe 355.44 27.1 1 -C-C2 92.705 0.261 believed that conjugated systems are also Hydrate CieH3303 309.46 15.52 2 HO H 91.09 0.255 destroyed as the double bonds are used up I 1 c-c in polymerization. The possibility of ring I H closure according t o the Diels-Alder reaction is not excluded from these considerations. Dihydrate CleHs6Oc 327.47 19.53 1 93.08 0.284 The rapid drop in iodine value in a few cases is considerably greater than the expected loss of one half double bond per fatty acid for the formation of a dimer or for the one Glycol CisIIaaOn 325.46 19.67 2 HO OH 92.61 0.285 double bond lost in the Diels-Alder reaction. L A It is possible that in these cases the accurate determination of the degree of unsaturation Diglycol ClQH36Os 359.47 26.7 1 96.63 0 269 by the iodine number method is difficult. The limitations of this method have been discussed repeatedly. They are again menH H B tioned to suggest that the decrease in double O H linkages may not be so great as indicated by b-c I 89.77 0 252 Ketone CloHslOa 307.44 15.63 2 the decrease in iodine number. I H At first it might seem unusual that the linoleate esters show a particularly character90.25 0.279 Diketone CiaHa104 323.44 19.8 1 istic behavior and that the linolenate esters show less remarkable curves in spite of their higher degree of unsaturation and their greater drying power. Upon more careful consideraHydrate C ~ ~ H B B618.92 OD 15.52 3 OH 180.45 0.291 polymer tion, however, this does not seem so strange. A-c-c As stated before, during the first or oxidaOC-t: tion stage of the drying process one of the I OH two double linkages in the linoleate esters is oxidized. This oxidized linkage then actiDihydrate CasHioOrr 654.94 19.53 1 polymer vates, through some mechanism as yet unOH known, the other double linkage which can 90.17a 0.2955Q Ketone oonCiaHzsOs 305.42 1 5 . 7 2 3 0 then enter into polymerization reactions of jugation 95.176 0.312b e-c=c the vinyl type. An ester containing only one double linkage per acid radical cannot dry Diketoneoon- CieHz04 319.41 20.1 3 0 0 91.125 0.2860 jugation 101.126 0 . 3 0 % because the oxidized double linkage does not b-c=c-Lc =c itself enter into the polymerizing reaction and there is no second double linkage which it could activate. The linolenate esters show less
-&-A-
[ "-0 ]
[3-] 2
LA
[_"gq
INDUSTRIAL AND ENGINEERING CHEMISTRY
1262
150
FIGURE 10 IODINE NUMBER OF METHYL ELEOSTEARATE
TABLE 11.
Vol. 33, No. 10
cOMP.4RISOX OF CALCULATED AND FOR SPECIFIC REFRACTION
Time Hour;
C
H
0 29 70 144 312 936
76.5 71.8 72.3 71.6 05.9 56.7
13. 4 13.1 12. 1 12.0 11.4 10.7
0
5 14.5 143 503
76.1 69.2 62.9 58.6 48.1
0 9.8 16 48 408
77.0 67.2 65.2 59.6 54.4
0 24 200 710
76.5 73.3 67.2 59.6
0 5 14 24
76.4 72.4 73.0 70.3
0
Formula
FOUND VALUE8 Sp. Refraction Calod. Found
Methyl Oleate 10. 1 15.1 1 5 ,6 16.4 22. 7 32, 6
CisH39.701.81 ClsH4lOa CloHsaO3.06 CisHss0s.a Cl9H39.ZOa.s Cl9H42.7OS.K
0.317 0.312 0.298 0.286 0.287 0.270
0.308 0.302 0.296 0.2015 0.2816 0.265
0.310 0.294 0.284 0.253 0.237
0.3081 0.302 0.295 0,2645 0.253
0.308 0.279 0.274 0.260 0.242
0.3097 0.296 0.286 0.270 0.250
0.305 0.296 0.282 0.268
0.3038 0.2965 0.2766 0.2594
0.302 0.295 0.278
0.300 0.30648
Methyl Linoleate
Methyl Linolenate
Glycol Oleate
Glycol Linoleate
11.1
10.9 10.0 10.3
12.5 16.7 17.0 19.4
CssHa604.7 C38H6800.6 Cs3HszOa.6 C38HssOs
. . L
0.28
.....
Glycol Linolenate
5 24
77.8 74.1 67.9 65.3 61.7
0 10 48 144
76.6 75.8 73.1 67.8
0 5 10 24
76.8 75.6 73.6 71.3
11.1 11.0 10.6 10.3
0 1 3 5
77.1 77.2 72.5 67.1
10.6 10.6 10.0 9.6
0 1 3
0.307 0.297 0.283 0,280
0.30772 0.3046 0.292 0.277
0.302 0.299 0.291 0.279
0.308 0.3039 0.2953 0.282
0.301 0.301 0.293 0.283
0.3062 0.3063 0.3073 0.2748
0.304
0,3068 0.3074 0.301 0.2849
...
.....
Glyceryl Oleate
Glyceryl Linoleate 12.1 13.4 15.8 18.4
C67HlooO7 CmH0~07.r CnH~7.aOi0.5 C67H99Oll
Glyceryl Linolenate 12.3 12.2 17.5 23.3
CarHeaOi CsrHsaO7 C5rHssOm.a CarH97016
:
0 263 0,280
TABLE111. SPECIFICREFRACTION OF BLOWN METHYLELEOSTEARATE
Time, Hours 0 1 5 7 63 a
Per Cent
Molecular Refraction Calculated Formula 12.52 CigHaa.rOi.aa Fa 0.314' E 3 0.324 16.98 CigHae0s.r Fz:0.2991 E = 4:2, 0.314 20.03 CiQH3804.1 FI, 0.290 23.8 Cl9H82.6O6.1 Ft, 0.275 23.9 CnHan.aOa.1 Fo, 0.270
0
Found 0.324 0.314 0.277a 0.268
M,ol. Weight 301.17 318.88 330.08 342.64 342.64
A t 6 hours.
This type of polymerization is in general agreement with the mechanism suggested by Bradley (1) for the formation of polymers in drying oil systems. It is not certain that each acid radical enters into polymerization reaction with only one other fatty acid radical. The formation of films from glycol esters points to the formation of higher order polymers and three-dimensional structures.
October, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
Since some evidence has been presented that drying oils polymerize through reactions of the Diels-Alder type during heat bodying (8), it would be interesting to determine whether similar reactions occur during air drying. Unfortunately the experimental evidence available is insufficient to decide this question definitely.
Literature Cited (1) Bradley, T. F., IND. ENQ.CHEM.,29, 440,579 (1937). (2) Brod. J. S., France, W. G., and Evans, W. L., Ibid., 31, 114 (1939). (3) Bull, H.B., “Biochemistry of the Lipids”, p. 108, New York, John Wiley & Sons (1937).
1263
(4) Dean, H. K., “Utilization of Fats”, p. 218,New York, Chemical
Publishing Co., 1938. (5) Elm, A. C., IND. ENQ.CIIEM.,23, 881 (1931). (6) Gee, G.,Trans. Faaraday SOC.,32, 187 (1936). (7) Getman, F. H.,“Outlines of Theoretical Chemistry”, p. 104, Now York, John Wiley & Sons, 1922. (8) Gilman, “Organic Chemistry”, p. 1739, New York, John Wiley & Sons, 1938. (9) Miller, A. B.,and Claxton, E., IND. ENQ.CHEM..20, 43 (1928). (10) Overholt. J. L.. and Elm, A. C., Ibid.,32. 378 (1940). (11) Ibid., 32, 1348 (1940). (12) Ibid., 33, 658 (1941). (13) Rieche, A., “Alkyl Peroxides and Ozonides”, p. 93 (1931). (14) Staudinger, H.et al., Ber., 58, 1075 (1925). PRESENTED before the Division of Paint, Varnish, and Plastics Chemistry at the lOlst Meeting of the Amerioan Chemiosl Sooiety, St. Louis, M o
Viscosity-Molecular Weight of Rubber Cryoscopic Deviation of Rubber Solutions from Raoult’s Law A. R. KEMP AND H. PETERS Bell Telephone Laboratories, New York, N. Y .
The S taudinger viscosity-molecular wcight relation is critically examined. The following equation is recommended : M = log
~r
X K,,/C
based on the Arrhenius (1) relation log qr/ C = K,, which holds true within narrow limits, provided the viscosity range is not too great. Using both benzene and cyclohexane in the cryoscopic measurements, the viscositymolecular weight constant IC,, has been detcrrnined on fractions of depolymerized rubber, the average molecular weights of which vary from about 1200 to 8000. S o h -
‘
ONSIDERABLE attention has recently been given to the Staudinger viscosity-molecular weight method because of its simplicity and possible application to a wide variety of high polymers. Numerous questions, however, have been raised regarding the influence of several variables on the results by the method, and there appears to be little agreement in respect to the most suitable equation for calculating the average molecular weight from viscosity data. The purpose of the present paper is to help clear up some of these questions and to provide a more satisfactory basis than now exists for the estimation of the average molecular weight of rubber and related substances. In the present work the
tions of fractions with molecular weights in excess of about 1400 have shown increased deviation from Raoult’s law as their molecular weight increased whereas the lower polymers behave ideally. A K,, value of 0.75 x 104 was found in contrast with the value of lo4obtained by Staudinger who had not considered the effect of polymer size or concentration in his cryoscopic measurements. The effect of various solvents on the viscosity of polyprene sols has been investigated, and i t has been found that benzene, toluene, and chloroform are the best for general use.
Staudinger (14) viscosity-molecular weight relation is critically examined, and various solvents and procedures for determining the molecular weight of rubber by the viscosity method have been studied. In this, as in a previous study (7), the solutions were prepared by diffusion methods and shaking was avoided in order to obtain true molecular dispersion and avoid inclusion of gel particles or molecular aggregates which may occur upon violent agitation. An Ostwald viscometer was employed and the temperature held at 25” C. * 0.10”. This control of temperature is satisfactory since the difference in the relative viscosity of dilute rubber sols at 20” and 25” C. is practically nil. The length of the capillary of the viscometer used was
