ALKYD RESINS CORRELATION BETWEEN PROPERTIES OF ALKYDS AND COMPOSITION OF MODIFYING FATTY ACIDS DUDLEY T. AIOORE Fatty Acids and Esters Laboratory, Emery Industries, Inc., Cincinrmti, Ohio
.
I n order to assist the protective coatings industry to select raw materials for particular applications, an attempt is being made to correlate certain properties of alkyd resins with the composition of the modifying monobasic acids. Investigations dealing with fatty acids containing onlj substantially completely nonconjugated unsaturation show t h a t each parent oil used leads to a separate drying time curve (the curves are similar in shape); hardness is directly related to polyenoic acid content, without regard to the linoleic-linolenic ratio; and relative afteryellowving can be predicted quantitatively where the composition of t h e fatty acids is known. With formulations containing substantial proportions of conjugated unsaturation hyperbolic curves generally define the effect of poly enoic acid variation on drying time; increase in conjugated un-
saturation, up to about one half of the total unsaturation, hastens the drying process; the hardness of the finished alkyd is predictable on the basis of fatty acid composition; changes in the ratio between dienoic and trienoic acids, or between conjugated and nonconjugated unsaturation, do not affect hardness, if the total polyenoic acid content is unchanged; the relative degree of yellowing of white enamels, after a particular interval of time, is predictable on the basis of fatty acid composition; and the progressive development of yellowing is related to the duration of the test by a logarithmic relationship. These data, when extended to include alkyds of various oil-resin ratios, will place on a quantitative basis knowledge of the varying effects produced by the use of natural and processed oils in alkyd resins.
HOTECTIVE coating vehicles containing fatty oils of a high degree of unsaturation (tung, linseed, etc.) dry faster and produce harder film, which are subject t o more severe afteryellou ing, than those containing oils of a lower degree of unsaturation (solbean, cottonseed, etc.). In a qualitative way, it has long been recognized that these properties are associated r+ith the relative contents of linoleic and linolenic acids (3). However, little of a quantitative nature has been published (7-9). Within the past few years, there have become commercially available various oils and fatty acids from which, by one means or another, a part of the less highly unsaturated constituents has been removed. I n general, it is possible to vary the conditions under which any one of these processes is operated so that thc amount of "unsaturation enrichment" varies within rather A ide limits. Usually, the cost of operating the process is directly proportional t o the degree of unsaturation enrichment obtained. Accordingly, it is important to know Rhether or not any degree of enrichment exists, beyond xhich the improvement in pertinent properties becomes negligible. Furthermore, if sufficient data could be assembled, quantitatively relating the properties of some classes of coating vehicles to the composition of the fatty acids or oils from which they are prepared, the industry would be in a position more intelligently t o select the proper material for a particular application. This paper is a report on the effect of oil type and polyenoic acid content on three important properties of protective coatings, as they are manifested in enamels prepared from alkyds of mpdium oil length: drying time, hardness, and afteryello'lr.ing of the dry film.
Iri order to obtain the full range of vai iation in linoleic-liiiolciiic. ratio and total content of polyenoic acids desired for this woih, each raw material was subjected to fui the1 fractional crystalliztion, to yield still more highly unsaturated materials, and a l i o extended with oleic acid (9.2% linoleic, 1 . 5 7 linolenic) to yicjld less highly unsaturated mixtures. The full range of compositions used is shorn n in Table I. From each of these sixteen fatty acid mixtures there was prepared, oii n laboratory scale, an alkyd having the composition (as chargcd). 4 7 . 0 7 , f a t t y acid 1 0 . 0 % glycerol 11,870 pentaeythritol 3 1 . 2 % phthalic anhydride
The equipment used comprised borosilicate glass resin kettles,. nominal capacity 3 liters, electrically heat,ed, and provided vvith an electrically driven glass paddle stirrer, thermometer, and means for introducing carbon dioxide gas at, a predetermined rate. The fatty acids and glycerol were charged into the lrettlc and heated to 220" C. under a carbon dioxide blanket. The pcntaerythritol \vas then gradually added (15 minutes), followed by the phthalic anhydride added in the same manner. The charge was then heated to 245" C. and, with carbon dioxide gas blowing through the reaction mass, that temperature \Vas maintained. until the acid number and viscosity Ivere reasonably close to the
TABLE I.
C O M P O S I T I O S OF FATTY l C I D ?*IIXTURES
70
Source of Oil Acids Cottonseed
FATTY ACIDS O F NONCONJCGATED SATURATION EXPERIMENTAL
The fatty acids used in this program were Plastolein 9305 (derived from cottonseed acids by the Emersol process), Plastolein 9315 (similarly derived from soya acids), and distilled linseed acids. [Plastolein is the registered trade-mark for liquid fatty acids prepared by the Emersol process, OM ned and licensed by the Emery Industries, Inc., Cincinnati, Ohio (6).] Their compositions (determined spectrophotometrically, 4 ) were: Linoleic Acid, % Linolenic Acld, % Oleic 2.4 Plastolein 9305 51.1 5.7 Plastolein 9315 57.9 47.0 Linseed acids 15.9
Linseed
.j
6 7 8 9 10 11 12 13
Soybean
Soybean
+ Saturated, 70 46.5 36.4 97.1
(+lo%
S o ) bean-linseed
2348
No. 1 2 3 1
linseed)
14 15 16
c/o Linoleic 48 5 .jl.l
55.6 59.1 13.6 14.5 l5.j 15.9 22.0 40.0 50.0
57.9 63.9 40 0 90
o
40 0
Yo
Oleic Total Plus Polyonoiv Linolenic Saturated .4cids 49.9 50.1 I 6 465 J3.,5 2.4 42.1 57.9 2.3 38 3 61 7 2.6 54.5 46..5 31.9 52.3 38.0 47.5 4 0 . 5 5 9.5 44.0 62.9 17.0 37.1 68 9 46 9 31.1 5.0 55 0 45.0 5.0 45.0 55.0 5.7 36.4 63.6 6.2 29.9 70.1 10 0 50 0 50 0 IO o 40 a 60 o 20 0 40 0 60 0
72
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
Hardness. For hardness measurements, films of the white enamels were applied to plate glass panels with the doctor blade as above. After the films had aged for various periods of time, the hardness was determined with the Sward rocker, standardized to a value of 100 on plate glass. The results obtained after the films had dried for 72 hours, and after they had aged for 6 weeks, are given in Figure 2. Color Retention. For these tests, the white enamels were applied to steel panels by brushing. In order to ensure complete hiding, two coats were applied, with 6 hours between coats. After 24 hours’ air-drying, the tristimulus readings for each panel were determined on a Hunter multipurpose reflectometer, using the usual green, blue, and amber filters. One half of each panel was then covered with opaque black paper, and the panels were exposed in a south window for several months. At intervals, tristimulus values were again determined on each panel. For each observation, the degree of yellowing was calculated as the widely accepted expression: ( A - B ) / G , where A , B, and G refer to the values obtained with the amber, blue, and green filters, respectively. (As was to be expected, there were no large color changes of the exposed halves of the panels, and the readings for these are not given.)
desired values. The resin was then allowed to cool somewhatoand reduced to 50% nonvolatile in mineral spirits. All the alkyds were a t top heat for between 2.25 and 2.75 hours, and had a Gardner color of 9 1 and an acid number of 5 + 1. The viscosities were between Y and Z1 with one exception, the alkyd prepared from the fractionally crystallized linseed acids (No. 9 in Table I) having a viscosity of 25. A white enamel was prepared from each of the above alkyds b grinding 245 grams of titanium dioxide and 190 grams of a g y d on a three-roller mill to a fineness of 7 to 7 1 / 2 (North Standard) and adding 400 grams of alkyd, 3 grams of 24% lead naphthenate, 1 gram of 6% cobalt naphthenate, and sufficient mineral spirits t o give a viscosity of 74 to 76 Krebs units.
*
I
I
I
2349
iO0)
VI
h
I G
Y
I
I
I
ao
3
:
~~
50 55 60 65 PERCENT TOTAL POLYUNSATURATES
45
a 40
70
r t 30
Figure 1. Drying Time of Unpigmented Alkyds
In addition, a series of five alkyds (and enamels) was prepared in which the composition was constant and only the vehicle viscosities were varied. The linseed acids (No. 8, Table I) and the formulas given above were used. The alkyds were at 246” C. for 90, 105, 120, 135, and 150 minutes, and the final acid numbers were 9.2, 8.0, 6.7, 5.4, and 4.8, respectively. The viscosities obtained, to two significant figures, were 10, 17, 23, 37, and 100 poises. These five enamels were used only in color retention tests, reported below. Drying Time. Drying time tests were performed on the unpigmented alkyds: 60 ml. of alkyd 20 ml. of xylene 0 . 6 ml. of 24% lead naphthenate 0 . 3 ml. of 6% cobalt naphthenate
Films were applied to plate glass panels with a doctor blade having a clearance of 0.003 inch. The time required to reach the stage “dried hard” was then determined. (Dried hard is that stage a t which the pressure that can be exerted between the thumb and finger does not move the film or leave a mark which is noticeable after the spot is lightly polished.) 4
1
45
50 55 60 65 PERCENT TOTAL POLYUNSATURATES
Figure 2.
60
I Figure 3.
\ I
\
I
\
I\
Yellowing us. Vehicle Viscosity
The degree of afteryellowing observed in the series of five enamels in which the vehicle composition was constant and only the vehicle vimosity was varied is shown in Figure 3. The degree of afteryellowing is inversely proportional to the logarithm of the viscosity. The results given in Figures 4 and 5 have been corrected accordingly, by multiplying the observed values by the 11. factor: [(logloviscosity - 1)/5 Figure 4 shows the degree of yellowing, a t 1and a t 3 months, of the sixteen white enamels (corrected for differences in vehicle viscosity as above) plotted against a function of the composition of the modifying fatty acids. The function used (linoleic acid content plus five times the linolenic acid content) was chosen on a trial-and-error basis. In Figure 5, the yellowing data on three of the enamels, chosen at random, have been replotted to bring out the relationship between the degree of yellowing and the duration of the exposure. During the course of the project, qualitative observations were made as to certain other physical characteristics of the enamels, including initial gloss, gloss retention, package stability, and ease of brushing. No significant differences were found except for the difficult brushing of the enamel made from the very viscous alkyd prepared from the fractionally crystallized linseed acids.
+
70
Hardness of Enamel Films
The results are shown in Figure 1. All the points except one may be deemed to lie on smooth sigmoid curves, a separate curve for each parent oil. The one exception is the alkyd based on a soybean-linseed mixture, containing about 3,5% linseed acids (No. 16 in Table I).
DISCUSSION
The nature of the curves shown in Figure 1 indicates that little improvement in the drying speed of an alkyd can be expected as the polyenoic acid content is increased beyond about 70%. The following somewhat oversimplified view may explain this. A polymer segment consists of alternate glyceryl and phthalyl radicals, with fatty acid radicals projecting a t intervals from the chain. A certain proportion of these fatty acid radicals will have
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
2350
two or more double bonds each. There is not universal agreement on the mechanism of drying, but it is generally accepted that the solid film is built up through cross linking by these polyenoic fatty acids. When the percentage of polyenoic acids is relatively low, they must, on the average, be scattered relatively infrequently along the polymer chain. Hence, the chance that two such radicals, attached to the same polymer segment, will react together is very small, Under such circumstances, double-bond reactions must be almost entirely intersegmental; each such reaction increases the average polymer size, and the drying speed is directly governed by the polyenoic acid content.
.160L 50 PERCENT
I
I
I
IO0 I50 200 LINOLEIC PLUS FIVE TIMES
Figure 4.
Yellowing
I
I
250 3 00 PERCENT LINOLENIC
z’s. Polyunsaturates
At the other end of the scale, when the polyenoic acid content is high, these polyenoic radicals appear on the polymer segment at much more frequent intervals. The chance of intrasegmental reaction thus becomes greater, and such reactions, which do not increase the polymer size, account for a considerable proportion of the total linkages formed. Thus, in the higher ranges, added increments of polyenoic acids would be expected to have a continuously decreasing effect on the drying speed of the alkyd and the slope of the curve would gradually change, the curve becoming more and more nearly horizontal. That a separate drying curve was obtained for each of the three parent oils should be no cause for surprise. On the one hand, it is known that the vegetable oils contain various antioxidants, and the oils may be expected to differ among themselves as to the amounts of these substances present. On the other hand, it is also rather generally supposed that linolenic acid is more potent than linoleic acid in contributing to drying speed. This latter point would lead to results in the proper order, but analysis of the data shows that this cannot alone account for the results observed. For, if the linolenic acid contents are weighted to allow for the third double bond, it is seen that of two fatty acid mixtures, the one containing the more unpaturation does not necessarily produce the faster drying alkyd. Thus, for example, compositions 4, 12, 15, and 16 (Table I ) all contain fewer double bonds than composition 6, yet all four dry faster. The hardness data in Figure 2 present two pointfi of interest. First, the hardness is directly proportional to the polyenoic acid content. This means not only that higher total polyenoic acid contents, with the attendant greater degree of cross linking, produce harder films (as would be expected), but also that the linolenic acid contributes no greater hardness to the films than does linoleic acid-for example, in composition 2 the ratio between linoleic and linolenic acids is more than 20 to 1, in composition I1 the same ratio has a value of 10 to 1, and in composition 6 the ratio is less than 0.4 to 1; yet all three yield alkyds of the same hardness, within the accuracy of the measuring instrument. Secondly, the 72-hour values show the need for sufficient aging of films before making evaluations. It is not unusual for a technician to
Vol. 43, No. 10
compare coatings when they have been applied for only 1 to 3 days. This may lead to erroneous conclusions. A correlation between afteryellowing and vehicle viscosity was to have been expected. Both are influenced by the polyenoic acid content of the fatty acids, and, in addition t o other factors which may affect color retention, the more double bonds used up in forming viscosity-promoting cross linkages, the fewer remain for formation of colored bodies in the film. As viscosity is related to time of processing, and hence to the proportion of double bonds reacting, in a logarithmic manner, and to degree of afteryellowing in the same manner, it appears that yellowing i s inversely proportional to some function of the percentage of double bonds destroyed in the polymerization process. The work summarized in Figure 4 places on a quantitative basis what has long been qualitatively known-that linolenic acid is much more potent than linoleic acid in producing yellowing. The factor of 5, by which the linolenic acid content was multiplied, was arrived a t by trial and error in the process of attempting t o fit the data to some simple curve. It is probably no more than a coincidence that this is almost exactly the ratio between the fourth powers of the number of double bonds in the two acids. Figure 5 suggests the possibility of shortening the time usually devoted to color retention tests, a t least with alkyd enamels. It is not unusual to conduct such tests over a period of 6 months or even a year. If this can be shortened to 4 or 6 weeks, much time will be saved. These data apply directly, of course, only to the particular formulation from which they were derived. However, it is bclieved that an “extrapolation” may be made to cover alkyds of oil lengths different from that used.
.400 300
.IO0
.080 I 0
,o
I I
2
,
I
I
I
I
I
I
I
1
4
6
8
IO
12
I
I 14
WEEKS
Figure 5.
Yellowing vs. Time of Exposure
That a longer alkyd will drj. more slowly, be less hard, and yellow more severely than those discussed herein is an expected concomitant of the higher percentage of fatty acids present. The properties of shorter alkyds, within the usual range of airdrying compositions, should similarly move in the opposite dirrction. But it is reasonable to suppose that variations in polyenoic acid content within the framework of any particular formulation would still give drying curves of the same general shape as those in Figure 1, that the factors discussed above would still operate t o cause flattening of the curve a t the higher polyenoic acid percentages, that the hardness of the dry film would still be proportional to the amount of unsaturation present, and that yellowing would still be determined by time, by viscosity, and by linoleic and (especially) linolenic contents. CONCLUSIONS
The following generalizations are suggested, subject to revision as further data become available: The rate of drying is a function of polyenoic acid content. This rate increases rather rapidly up to a polyenoic acid content
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
of about 50 %. Above that figure a limiting value is gradually approached. (It would seem that relatively pure linoleic acid if commercially available, would offer little advantage over the 65 to 70% material now on the market.) At constant polyenoic acid content, linseed-based vehicles dry somewhat more rapidly, and cottonseed-based vehicles somewhat more slowly, than their soybean-based counterparts. The limiting rate for cottonseed-based vehicles is as high as, or higher than, the rate for a vehicle prepared from soybean oil of average composition; and the limiting value for soybean-based vehicles is as high as, or higher than, that of a vehicle made from linseed oil of average composition. (Thus, the various commercial processes by means of which %nsaturation enrichment” is achieved may yield products which dry as well, or almost as well, as linseed-based vehicles without linseed’s notoriously poor color retention.) The hardness of the dry film is proportional to the polyenoic acid content, At constant polyenoic acid content, a change in the linoleic-linolenic ratio produces no appreciable change in hardness. Color development in white alkyd enamels is a logarithmic function of the viscosity of the vehicle. Other things being equal, the B log V , afteryellowing fits a formula of the form: Y = A where Y = degree of yellowing, V = viscosity, and A , B = constants. Color development in white alkyd enamels is for a period (the duration of which has not yet been determined), an exponential function of time. After an initial period of 2 or 3 weeks, during which the yellowing seems to be primarily governed by a secondorder reaction, the yellowing then fits a formula: log Y = A B T , where Y = degree of yellowing, T = time, and A , B = constants. Color development in white alkyd enamels is proportional to the polyenoic acid content, with the linolenic acid weighted several (about 5) times as heavily as the linoleic-that is, the afteryellowing fits a formula: Y = A(L1 5Ls) B, where Y = degree of yellowing,’4= % linoleic acid, Lz = % linolenic acid, and A , B = constants. Composition of the fatty acids has no apparent effect on initial gloss, gloss retention, package stability, or ease of application, except in so far as the more highly unsaturated mixtures are allowed to produce alkyds of higher viscosities.
20 40 60 PERCENT TOTAL POLYENOIC ACIDS
0
-
+
+
+
2351
Figure 6. Drying Times of Alkyds
saturation is based on the total polyenoic acid content, and not on the total fatty acids. The “dry hard” time of each of the alkyds was determined in the usual manner. From the previous results (Figure 1) the curve, to a different scale and extended to lower polyenoic acid contents, for the soybean-based alkyds (Table 11, 1L to 1T) has been included in Figure 6, to afford a direct comparison with the results obtained with each of the conjugated-unsaturation groups. Curves I1 to VI refer to the fatty acid compositions grouped in Table 111. Separate curves corresponding to groups VI1 and VI11 are not shown, because, within experimental error, the drying times for the alkyds in group VI1 were the same as for the corresponding items in group IV, and for group VI11 the same as for the corresponding ones in group VI.
FATTY ACIDS O F CONJUGATED UNSATURATION EXPERIMENTAL
The formulations and testing procedures used were as given above, The compositions of the various fatty acid mixtures used are given in Tables I1 and 111. Table 11,which lists the compositions containing substantially no conjugated unsaturation, includes the mixtures reported above, with some others in addition, Table I11 shows a stepwise increase in conjugated unsaturation, arising from the inclusion of increasing amounts of tung and dehydrated castor fatty acids. The compositions of these two materials were:
T u n g acids ’ Dehydrated castor oil acids
Conjugated Trienoio 71.7 0.1
Conjugated Dienoic 4.5
29.6
0
20
40 PERCENT POLYENOIC
60
80
ACIDS
Nonconjugated Trienoic 0.0
Nonconjugated Dienoic 11.6
Figure 7. Hardness of White Enamels
0.8
62.1
Hardness readings were taken on pigmented films with the Sward rocker (Table IV). The results represent “ultimate” hardness, further aging of the panels producing no consistent changes in the results. Except in the cases of the safflower acid alkyds (Table 11, 1U to lX), the results given are after 6 weeks’
The compositions of the soybean, cottonseed, and linseed acids used in the mixtures of Table I11 may be found in Table 11, a8 lT, lB, and lH, respectively. The amount of conjugated un-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2352
TABLE 11. COVPOSITIOIV OF FATTY ACIDMIXTURES (Essentially nonconjugated)
%
Alkyd Xo.
19 1B 1c 1D 1E 1F 10 1H 1J 1K 1L 1V i1 1N 10 1P 1R 18 1T
1u
111TT
1x
%
%
Soybean Acids
... ... ...
...
..
67 80 93.3 100 100 35.3
...
...
..
,,. ,
.., I . .
... ...
58.4 17.0 33.3 48.9 630 61 4 83.7 82.3 100
... , . . , . .
...
... ...
, . .
... ,..
... ...
, . .
...
...
... ...
,.,
... ... ...
... ... ... ... ... .. .. ,.
...
11.0
...
...
...
., .. ..
..
33 206 7
..
,..
...
1.8 13.0
... .. ., ..
...
...
% ConOleic jugated Acid Trienoic
.. ..
.. .. ..
., .. ,.
...
...
Safflower Acids
, . .
... ...
%
%
Cottonseed Acids 100 100 100 100
Linseed Acids
37.7 71.8 85.8 100
6’3 83.0 66.7 51.1 35.2 25.6 16.3 6 7 4i:3 28.2 14. . 2
0.6 0.1 0 3 0.2 0.1 0 0 .. 1 1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0 1 0.0 0.0 0 .0 0.0
5%
%
%
%
Conjugated Dienoic
Nonconjugated Trienoic
Nanconjugated Dienoic
Total Nondrying
1.2 1.7 1.7 2.0 1.6 1 1 .. 6 6 1 6 2.5 2.1 0 2
1.1 2.4 2.0 2.4 31.9 3 4 37 .. 98 46.9 46.8 19.9 1.: 2 5 3.2 4,9 9.9 4,9 9 9 5.6 0.4 0.3 0 .1 0.0
47.3 49.4 53.8 57.1 12.1 1 3.0 13.9 14.3 19.5 38.0 12.8 22.1 31.0 39.1 38.1 47.7
49.8 46.4 42.2 38.3 54.3 47.5 40.5 37.1 31.1 39.9 85,l 74.8 65.0 55.0 49.8 45,l
43.6 53.5 6 73 3 .. 4 3
44.9 3 25 4 .. 0 9
0.5
0 7 0.9 2.1 2.2 2.2 2.4 1.0
1.3 1 1 .. 85
;;;; 5g.O
OF FATTY ACIDMIXTURES TABLE 111. COXPOSITION
Alkyd NO.
%
Tung Acids
%
DCO Acids
7 0 %
Linseed Acids
2h 2B 2C 2D 2E 2F
1.4 1.4 2 1 2 1 2.8 2 8
12.1 16.3 17.0 19.9 19.9 24.8
9.9 9.2 14.2 12.8 18.4 17.7
3A 3B 3c 3D 3E 3F
2.1 2.1 2.8 2.8 3.5 3.5
22.7 30.5 32.6 36.9 39.0 46.8
7.8 7.1 11.6 11.6 13.5 13.5
Soybean Acids
,.
13.5 22.0 24.1 27.0 29.1 36.2
.. .. . ,
..
..
..
5A 5B 5C 5D 5E 5F 5G 5H 55
2.8 5.0 7.1 2.4 4.7 2 4 4.7 9.4 13.5
..
.
5:O 24.5 34.8
6A 6B 6C 6D 6E 6F 6G 6H 6J
2.1 4.3 5.0 2.3 4.7 2.3 4.7 9.2 12.8
8.5 15.6 24.1 44.4 44.0 56.8 55.4 41.1 42.5
4.3 11.3 13.5 6.1 13.0 6.7 12.8 27.6 37.6
..
14.2 27.0 39.7 45.3 43.6 58.1 36.5 40.2 75.9
.. , ,
.. ,
3 7 8.3 17.7
7:0
TI.
7F 7G 7H
7.8 9.9 12.8 15.6 17.0 18.4 19.8 22.7
8.4 8B 8C 8D 8E 8F 8G
13.5 19.7 26.2 32.6 36.2 39.0 42.6
7D 7E
5.7 16.3 25.5 35.4 40.4 46.4 50.4 50.3
8.5 15.6 23.4 56.7 60.3 35.4 77.6 4,s
51.7
..
.. .. ..
.. .,
.. .. .. ..
VIII.
5:o
11.3 17.7 19.7 22.7 25.5
.. .. , . ., .. ., ..
0.1 0.1 0.1 0.1 0.1 0.1 0.1
Dienoic
.. ,.
..
%
13.5 13.3 17.3 16.9 12.0 22.1
%
Total Nondrying
5.9 5 8 8.7 8.7 11.4 11.4
23.5 32.2 33.4 38.1 39.6 48.1
65.3 55.2 50.4 44.9 40.1 29.9
4.7 4.5 6.1 6.2 7.5 7.6
21.6 29.3 31.8 35.2 37.6 44.8
65.1 55.1 49.7 44.9 40.0 30.2
0.9 0 9 0.0 4.9 9.9 4.9 9.9 19 8 1.3
9.9 16.2 22.5 26.5 26.8 32.4 32.8 27.4 45.9
84 9 74.8 64 8 55.0 49.9 45.3 40.3 40.1 30.1
4.0 7.2 10.2 3.4 6.7 3.2 6.7 13.3 19.6
8.3 13.8 18.9 38.6 38.4 47.8 47.6 48.1 37.9
84 9 74.8 64.9 54.7 49.8 45.2 39.9 40.1 31.0
2.9 6.1 7.1 3.3 6.5 3.3 6.6 13.3 18.1
7.8 10.9 16.8 26.6 26.4 33.3 33.3 27.1 29.2
85,1 75.0 64.9 55.1 50.2 44 8 39.9 40.0 29.9
0.9 0.9 0.8 0.8 0.8 0.8 0.8 0.7
6.5 11.8 16.7 21.7 24.3 26.8 29.4 34.4
85.0 74.9 65.2 55.1 50.0 45.0 39.7 29.8
0.8 0.: 0.I 0.7
4.2 7.2 10.8 14.5 15.9 17.5 19.2
84.7 76.4 65.2 54,7 50.1 45.4 40.3
7.2 9.6 10.4 11.7 12.4 14.9
11.1
%
Nonconjugated Dienoic
Sonconjugated Trienoic
TRIEKOIC .$CIDr C O K T U G A T h D 0.3 83.0 2.0 3.6 0.6 .. 67.1 0.9 51.8 5.1 6:s 34.1 1.7 1.6 28.0 3.4 1.7 j2:4 9.8 1.7 2.1 12.7 3.4 2.4 61:6 ,. 6.7 1.8 .. .. 9 7 1.8 Or
..
Borii DIEXOIC A N D TnIENorc 1.5 .. 85.1 68.8 3 0 .. .. 57.4 3.6 .. 1.6 218 44.4 .. , , 38.3 3.4 1.6 218 32.4 .. .. 2.8 24.3 3.4 .. ,. 22.1 6.6 , . ,, 7.1 9.1
..
..
%
Conjiigated
DIENOIC .$CIDS C O N J C G . 4 T E D 0.1 4.2 85.8 0.1 73.0 8.0
..
, .
..
..
1 5 1.5 2.0 2.0 2.5 2.5
l/8
.. ,.
53.9 38.3 29.9 22.7 14.9
44:9 3.5.8 32.4 23.2 17.9 14.2
VII. 7A 7B 7c
OF
6613 9.8 20.6 9,s 20.3 41.9
v. .. .. .. ..
l/8
..
4A 4B 4C 4D 4E 4F 4G 4H 45
%
11. 15% POLYEKOIC ACIDS COSJUGATED 4.3 .. 28 4 48.2 1.0 42 6 30.5 5,s 1.0 16 3 28.4 1.5 22’0 6.0 24.1 1.5 6.8 41.1 16.3 2.0 6.9 42.6 8.6 47.5 7 2 .. 2.0
IT’.
..
%
Cotton% Conseed Oleic jugated Acids Acid Trienoic
$CIDS
COsJuGaTED
2.7 5.0 7.6 13.4 13.5 17.0 16.8 13.0 13.7
POLSENOIC Acrns CONJUGATED 5.6 2.0 .. 86.5 73.8 7.1 5,3 .. 9.2 8.1 61.7 ,. 11.2 49.0 11.2 , . 1 2.7 42.6 1 2 . 2 , . 13.2 14.2 .. 36.2 14.2 15.8 , . 29.8 18.9 17.0 16.2 .,
l / ~
%/a POLSENOIC ACIDS COXJUGATED
..
.. .. . , .
..
..
86.5 75.3 62.5 49.7 44.1 38.3 31.9
9.7 14.2 18.8 23.4 25.9 28.0 30.5
0.6
0.6 0.5
Vol. 43, No. 10
a g i n g . T h e safflow!r-baec:d alkyds reached ultimate hardness more slowly, and these results are after 9 weeks’ aging. The “calculated” v a l u e s given in Table IV arc derived from the equation: H = 4000/(340 - SP), where P = total polyenoic acids and H = calculated hardness. As Sward hardness valurs are not usually expressed in terms of fract,ional or decimal values, the figures calculated from the aboveequation have been rounded to the nearest whole number. 9 grouped representation of the hardness data is shown in Figure 7. The band extending diagonally upward across the center of the graph is 2 Sward units wide-that is, it repreeents the values drrived from equation above, +l unit. Tlie numbers within, and to either side of, this band are the nurnbers of alkyds, having approsimately the polgenoic acid contents indicated, giving hardness readings within, and without, that band, as the case mav be. Of the eighteen a l k ~ d s falling outside the band, only one departed from the calriilated value by more than 2 units. hfterj-elloaing values have been multiplied by 1000 for convenience. In Table V are showi t ’ l r c b results obtained on all thc enamels after a 3 months’tcst,ing period. The results indicate the amount of yelloaiiig developed by each enamel during the exposure period; thc initial color reading has hccn subtracted from the observed value in each case. In addition, a correction factor, baaed on vehicle viscosity, has been applied. The calculated values included in Table Y are derived from the equation: Y = 0.55(D 5T 3C) 242, where Y’ = degree of yellowing, D = % dienoic acids, T = % trienoic acids, and C = % conjugated acids. The data from Table V arc repeated graphically in Figure 8, the line drawn thereon being the locus of the equation above. I n addition, twenty of thc enamels-five from each of Table I1 and Table 111, groups IV, V, and VI-remained on trst for one year. The readings
+
+
+
.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
October 1951
TABLE IV. SWARD HARDNESS OF ALKYDS (In order of increasing polyenoic acid content) Alkyd
Hardness Obsd. Calcd. 14 14 14 14 12 14 14 14 14 12 14 13 15 15 15 15 I5 15 15 14 15 16 15 14 17 18 17 19 17 18 17 I6 17 17 17 18 17 17
No.
1L 6A 7A 4A 5A 8A 8R 6B 7B 4B 5B 1M 2A 7c BC 3A
1N
5c BC
Alkyd No. 4c 2B 3B 6D 7D 10
1u
4D 5D 8D 1E 2C 6E 8E 7E 4E 5E 1A 1P
Hardness Obsd. Calcd. 17 17 20 19 20 19 19 19 20 19 20 18 20 20 20 20 20 20 20 20 20 21 21 23 21 22 21 21 21 22 21 20 21 21 21 20 21 23
Alkyd
No. 3c 1F 1B 8F 4F 5F 1R 7F
1v
2D 3D 6F 1c 1G 4G 8G
1s
2E 4H
Hardness Obsd. Calcd. 22 21 24 22 23 22 23 23 26 23 24 23 24 23 23 23 23 23 23 23 24 23 22 23 23 24 26 25 25 25 27 25 26 25 25 25 27 25
r
-- -0
50
150
IO0
D
Figure 8.
+ 5T
200
250
300
do not increase the polymer size. Accordingly, it appears that the equation should be modified by a term calculated to Alkyd Hardness account for such behavior. No. Obsd. Calcd. The idea of this “intramolecular reaction term” may be developed as follows. 3E 25 25 Usually, double-bond reactions between 1K 27 25 5G 25 25 two fatty acid radicals will occur only if each contains two or more double bonds. 1D 26 26 Thus, if kinking, coiling, or other move1H 28 26 IT 27 27 ment of a polymer segment brings two fatty acid radicals attached thereto into 5J 32 30 sufficiently cloRe proximity so that a reac3F 31 31 45 30 31 tion might take place between them, such reaction will normally occur only if both 7H 29 31 radicals are polyenoic. For any such pair 1x 34 35 of fatty acids, the probability that either will be polyenoic is proportional to P, the percentage of polyenoic acids present. The probability that both will be polyenoic is therefore equal to the product, of the separate probabilities involved-that is, it is proportional to P2. Hence the modified equation must be of the form: T ( P - P 2 / K ) = K‘ where K is of such magnitude that ( 100)2/Kis equal to the percentage of wasted polyenoic acids in a pure polyenoic acid modified alkyd. Some further refinements could probably be achieved by introducing a factor, or factors, related to the ratios existing between dienoic and trienoic acids, and between conjugated and nonconjugated forms. However, except in the region mentioned above, the observed data agree very well with the values calculated from the equation as given, after substitution of suitable, empirically determined, values for K and K’-for example, the equation T ( P - P2/200) = 2600 gives the following results relative to the observed drying times from curve VI (Figure 6):
ii
i;
;g
ig
!Iv
if
gi
:!
:;
:’
t 3C 15 190 187
Yellowing of White Enamels
obtained a t 1, 2, 4, 8, and 12 months are given in Table VI. A s in Table V, the initial color value has been subtracted from the observed value in each case. However, the vehicle viscosity correction factor has not been used, as it would affect all the values (for a given enamel) to the same extent. The calculated values included in Table VI are derived from a family of equations of the form: log Y = K - K’/T, where Y = degree of yellowing, T = time (months), and K , K’ = constants empirically derived for each separate enamel. DISCUSSION
2353
Minutes, obsd. Minutes, calcd.
25 126 119
Polyenoic Acid Content 35 45 50 55 85 70 65 65 90 74 69 65 Standard deviation = 3 7
60
60 62
70 60 57
The hardness data on fatty acids of nonconjugated unsaturation having covered only a limited range of polyenoic acid contents, a reasonably good straight-line correlation between hardness and polyenoic acid content appeared to exist. Extension of the range to higher and lower polyenoic acid percentages has shown that, although such a direct relationship is a useful approximation within a limited range, the actual correlation is hyperbolic. In Table IV, the “Rtandard deviation” from tbe calculated values of the whole group of observations is 1.2 units, and of the
The authors are still without any explanation for the anomalous behavior of the alkyds containing only small amounts of conjugated unsaturation, in the range of 45 to 60% polyenoic acid TABLE V. YELLOWING OF WHITEEKAMELS content (Figure 6, curves Soya and 11). (In order of increasing predicted severity) Aside from this region, the various dryYellYello __ wing Yellolwing Alkyd 0wing Alkyd Alkyd Alkyd ing time-composition curves are roughly No. KO. Exptl. Calcd. NO. Exptl. Calcd. NO. E X Calcd. hyperbolic and suggest that, as a first ap1L 5G 305 307 5H 259 255 1x 286 279 proximation, the data might fit a family 307 4A 3c 256 260 308 7E 5B 284 287 2D 308 1M 308 4H 265 262 1R 287 289 of curves of the form: PT = K , where P 318 312 1E 6A 6B 7 c 273 267 289 289 314 4F 267 312 8C 3A 5A 269 288 289 = yopolyenoic acid, T = drying time, and 320 3 D 313 7 F 5F 270 274 289 1u 289 K = constant. However. closer examina311 4E 1N 313 6H 270 29 1 266 8A 290 311 6 F 4B 314 2B 1F 272 265 293 297 tion indicates that such a curve needs 321 8B 316 1T 1v 7G 275 274 294 291 320 5J 1A BE 276 319 7B 275 294 298 modification to produce more rapid flatten2E 319 8D 319 7A 1P 277 272 295 294 ing a t high polyenoic acid contents. 324 45 320 3B 10 279 275 298 294 1G 321 7H 3E 322 4D 1B 280 283 300 303 This discrepancy is in accord with the 1K 323 323 1w 284 65 5E 300 300 281 325 . 325 1H 4G 4c 282 281 301 295 1s supposition that, at higher polyenoic acid 2F 1c 321 8E 282 303 328 6D 284 306 7D 1J 330 329 concentrations, an increasing amount of 5D 6C 284 283 304 302 331 330 2A 2C 6G 8F 285 304 310 289 the cross-linking potential is “wasted” 328 1D 3F 331 315 5c 8G 285 305 294 through intramolecular reactions, which ~
Yellc,wing __ ~Exptl. Calcd. 334 338 341 339 344 340 346 340 345 342 343 348 351 351 359 357 358 353 365 364 370 371 375 378 374 375 378 372 383 387 381 383 387 389 395 391 402 409
2354
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
Vol. 43, No. 10
and if the number of cross links formed were to vary with the number of double bonds originally (As a function of duration of exposure) present, it would seem that the terminal struc8 Months 2 Months4 Months 12 months tures produced from the various polyenoic species Alkyd B hNo. Exptl. Calcd. Exptl. Calcd. Exptl. Calcd. Exptl. Calcd. Exptl. Calod. be so nearly that the observed differ1K 119 119 196 196 255 251 280 284 298 296 ences would not exist. 10 108 108 182 182 238 238 269 270 283 283 1P 82 84 159 158 214 217 256 255 278 269 The data in Table VI show the progressive de1R 108 102 183 184 243 246 283 286 302 300 274 289 286 velopment of color (after a preliminary period of 1s 105 110 190 186 236 240 269 4D 81 83 151 151 209 205 238 239 251 251 4 weeks, during which the yellowing is rapid and 4E 93 102 167 166 214 217 242 241 249 251 4F 77 84 160 159 222 219 255 256 268 270 not readily predictable) as the result of a particu4G 78 81 168 160 223 223 267 280 280 267 275 287 287 iar testing procedure. It is recognized that 4H 121 123 202 196 247 247 270 172 237 5D 90 90 172 237 276 280 294 296 different curves might be obtained if the condi5E 103 105 190 184 244 244 279 280 293 293 5F 94 94 168 168 222 225 262 261 274 274 tions of exposure were changed ( d ) , but in view of 104 5G 104 172 179 237 237 270 270 283 283 285 297 297 the widespread use of this procedure, it is felt that 5H 112 114 192 192 249 249 287 6D 92 94 167 167 230 226 262 262 269 274 these results have special significancefor the protec6E 97 97 183 174 236 233 269 269 281 283 6F 90 92 171 171 236 234 270 273 289 289 tive coatings industry. 6G 97 97 171 177 249 240 279 279 289 293 The standard deviation of the differences be268 272 129 217 213 309 309 6H 129 320 322 tween the experimental and calculated values is 3.26 units or L5%, which again is only slightly larger than the degree of uncertainty of the mpasalkyds fmm each separate group, is respectively, 1.2, 1.1, 0.8, 1.3, urements assumed above. The form of the equation is that of a firsborder reaction, which 1.1, 1.2, 1.4, and 0.9 unit. is in accord with either the view that color is formed as a result of Assuming that the average of a considerable number of obsersimple double-bond shifts, or that it results from an oxidative revations is the “true value,” it has been shown ( 1 , 6) that the precision obtainable with the Sward rocker (as used in this study) is action, the oxygen concentration being virtually constant. The first of these alternatives might be favored in view of the such that about 70% of the observations will be within 1 unit of the true value and about 7% of the observations may differ from fact that oxidation apparently comes virtually to a standstill after a limited time (IO); that the panels are masked from the the true value by more than 2 units, The standard deviation of light and the circulation of air beneath the mask is probably the determinations is about 1.4 units. somewhat restricted (the supposition that the film is in an enAs the agreement between the experimental and calculated vironment of constant oxygen content therefore probably being values is within the limit of precision of the measuring instruerroneous); and that the yellowing that develops during the first ment, it seems probable that the correlation suggested above few weeks, when oxidation is known to be taking place, definitely actually exists. does not follow a &st-order course. Perhaps the most striking fact implicit in the hardness data reLeaving aside any further consideration of the mechanism that sides in the constancy of the hardness resulting from a given may be involved, it appears that, in any case, the rapidity of the polyenoic acid content. Regardless of the type of fatty acids afteryellowing having once been established, the further course used, the relative contents of dienoic and trienoic acids, and the of the color development may be predicted with considerable acrelative contents of conjugated and nonconjugated unsaturation, R hen the total content of all polyenoic components is the same, curacy. the films produced are of equal hardness, within the limits of error CONCLUSIONS of the measurement. This fact may be taken as indicative of the constancy of the Certain potentiallv uReful relationships exist between the drynumber (probably of CrOs9 links formed by each fatty acid ing time, hardness, and color retention of an alkyd of medium oil For if, for example, linolenic acid formed two cross length and the composition of the fatty acids with which it is links and linoleic acid only one, the linolenic acid would be exmodified, pected to produce the harder film. This is not in accord with the experimental results. Hence, the suggestion is that the number For any ratio between conjugated and nonconjugated unof cross links formed is the same, regardless of the number of saturation, the drying time is related to the content of polydouble bonds in the reactants. As is shown below, the afterenoic acids by an equation of the form: T P ( 1 - P / K ) = K’. The presence of increasing amounts of conjugated unsaturation yellowing data are consistent with this view. has a beneficial effect on drying time up to a limit in the general In Table V, the devintisns between the experimental and the neighborhood of one half of the total unsaturation present. calculated afteryellowing values range from 0 to 10 units, apThe hardness of the alkyd may be closely predicted from an proximately 80% being 4 units or less. Tee standard deviation equation of the form: H ( K - P ) = K’. is 3.76 units or 1.2%. This is approximately equal to the degree The hardness of an alkyd is not appreciably affected by the of uncertainty involved in making the reasonable assumption ratio between dienoic and trienoic acids, nor the ratio between conjugated and nonconjugated unsaturation. that each reading taken on the Hunter reflectometer is in doubt Degree of afteryellowing, after a particular interval of time, ie by & I unit. related to the composition of the fatty acids through an equation The standard deviations for the alkyds from each of the sepaK’. of the form: Y = K ( D UT bC) rate groups are, respectively, 1.3, 1.4, 1.2, 1.2, 1.3, 1.1, 1.4, Degree of afteryellowing of an alkyd is related to the duration and 1.1%. Considering the relatively small number of alkyds in of the exposure (after a preliminary period of 2 to 4 weeks) K’/l’. through an equation of the form: log Y = K each group, it is evident that the deviations have an extremely low level of statistical significance. Inasmuch as there are wide differences in the contributions to ACKNOWLEDGMEYT afteryellowing from the various polyenoic forms present in the The constructive criticisms and the suggestions received from fatty acid mixtures, it is believed that these are supplementary R. G. Kadesch and V. J. hluckerheide have been very helpful. evidence in favor of the view that only one cross link is formed Thanks are also due B. R. Krabacher who assisted with the exby each fatty acid radical reacting. For it is generally believed perimental work and the preparation of the graphs, and t o W. C. that the residual double bond structure arising from the polyClark who made the spectrophotometric analyses. enoic components is a potent factor in the development of color,
TABLE VI. YELLOWING O F WHITE ENdMELS
+
-+-
+
-
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
October 1951
LITERATURE CITED
v. J., and-Myers, L.D., I b g . , 23, 146-50 (1946). (6) Moore, D.T., Paint, Oi2 & Chem. Rsv., 113,41 (1950). i 5 j Kistler, R. E., Muckerheide,
2355
(7) Morrell, R. 9.. J . Oil & Colour Chemists' Assoc., 10, 186-201
RECEIVED February 27, 1950. Presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 117th and 119th Meetings of the AMNRICAN CHEMICAL SOCIETY, Detroit, Mich., and Boston, Mass.
Mononitration of Cumene J. W. HAW AND KENNETH A. KOBE Department of Chemical Engineering, University of Texas, Austin, Tex. Cumene was produced in large amounts during World War I1 as a blending agent for aviation gasoline. This production ceased at the end of the war, but some fields of chemical utilization should be available. Nitration is the first step in making many chemical products. Cumene was nitrated with a yield of 94.5% mononitrocumene using a 2 to 1 ratio of sulfuric acid to hydrocarbon, acid concentration SI%, and 20% excess nitric acid at 15" C. A new theory of aromatic nitration which considers the nitryl ion, NOI+, as the nitrating agent has been used to develop a new series of process variables for aromatic nitration to replace the concept of dehydratingvalue of sulfuric acid. The ratio of acid to hydrocarbonand the initial concentration of the sulfuric acid (on a nitric acid-free basis) replace the dehydrating value of sulfuric acid as a process variable. The advantage of this new concept is shown. The orientation of the entering nitro group i8 shown to be 24% in the ortho position and '76% in the para position, contrary to a report that the orientation changes from essentially ortho to completely para when the temperature of nitration changes from 0" to 45" C.
B
EFORE World War 11, cumene (isopropylbenzene) was a
relatively rare aromatic hydrocarbon, produced in limited quantities for special uses. However, its use a8 a high-octane blending agent in aviation gasoline during the war brought about a spectacular increase in its production-from 400 barrels per day in May 1942 to 15,000 barrels per day in December 1944 (19). The end of the wartime emergency resulted in a sharp decrease in demand, and it was felt that some investigation of the utilization of cumene as a chemical raw material was needed. As nitration is one of the important reactions used to introduce functional groups into the aromatic nucleus, a study of this unit process applied to cumene would give important information concerning an initial step in chemical utilization. The work of Sterling and Bogert (86) had indicated that the temperature of nitration has an unusually strong effect on the position taken by the entering nitro group, and a thorough study of this particular process should make some contribution to the theory of orientation in the benzene nucleus. After some exploratory work, it was decided to attempt a new approach to the study of the nitration process, using the fundamental data now available on the kinetics and mechanism of the nitration reaction. In the past few years a considerable amount of fundamental data has been published on nitration, and it is believed that the present work presents the unit process of nitration in a new light rand will lead to generalizations not heretofore recognized.
THEORY OF AROMATIC NITRATION
The basic mechanism of the process of aromatic nitration has been the subject of intermittent investigation and controversy for many years. The earliest proposal as to the mechanism was made by Kekule (18) and supported by others (1, SO), on the basis of the addition of nitric acid to ethylenic linkages; it held that the nitric acid added to the "double bond" of the benzene structure to yield a nitrohydrin which was then dehydrated by strong acids to yield the nitro compound. More recently, on the basis of a reinvestigation of the addition of nitric acid to alkenes, Michael (3.8)and Michael and Carlson (IS) proposed a slightly different mechanism for the nitration reaction. This mechanism involves the addition of nitric acid to the hydrocarbon in an aldolization reaction, followed by a loss of water to form the nitro compound. The essential common feature of the two mechanisms outlined is the concept of the function of the sulfuric acid as a dehydrating agent. This has led to the usual method of presenting process data on nitration reactions, in which the dehydrating value of sulfuric acid (D.V.S.) is used to indicate the composition of the mixed nitration acid (14). Dehydrating value is defined as the ratio of the weight of sulfuric acid present to the weight of water present a t the end of the nitration reaction, assuming a theoretical yield of nitro compound. These mechanisms lead to an implicit belief that the sulfuric acid serves only to combine with one of the products of reaction, thus driving the equilibrium toward completion of the reaction. However, Gilman (12) presents data indicating that the nitration reaction is irreversible and there is no equilibrium to be so affected; and it has been shown (11) that nitration will not take place in the presence of phosphoric acid or phosphorus pentoxide, although these materials have a higher affiity for water than does sulfuric acid. These data, and many other lines of evidence, indicate that a more thorough study of the fundamental characteristics of the nitration reaction is necessary. Recently, Gillespie and Millen (11) published an extensive review of the literature on nitration reactions, and summarized the results of a long-term investigation of the reaction by a group of British workers. The mechanism proposed involves direct electrophilic displacement of a hydrogen atom on the benzene nucleus, in the form of a hydrogen ion, by the nitryl ion, NOz+, a suggestion originally made by Euler ( 8 )in 1901. Gillespie and Millen consider the sulfuric acid (or other strong acid) only as an ionizing medium in which the reaction takes place, and which is a strong proton donor. The presence of the nitryl ion is supported by the spectroscopic data of Chedin ( 6 ) , whose data on the mixed acid system are presented by Gillespie and Millen (11) in the form of curves of constant amount of nitryl ion in gram-moles per 1000 grams of mixed acid on a
'
