DRYING OILS AND RESINS Polymeric Functionality with Relation to the Addition Polymerization of Drying Oils’ THEODORE F. BRADLEY American Cyanamid Company, Stamford, Conn.
eventually find complete experimental correlation with the polymerization phenomena. The observed molecular weight-iodine value relations of heat-thickened tung and linseed oils correspond most closely t o those which are typical of a hexafunctional system, particularly in the case of tung oil. Departure from polymerization theory is occasioned in those cases where intramolecular additions, partial hydrolysis, or other extraneous reactions have occurred. The latter are most prominent in the case of linseed oil but vary with reaction conditions, particularly with temperature. Solvent extraction of stand oils has enabled the partial separation of polymers of varied molecular weight and of certain of the extraneous reaction products. Continued investigation of such fractions seems desirable as a means of further progress.
The general conception of polymerization has failed t o provide a basis upon which a number of important phenomena relating t o the chemistry of drying oils may find rational explanation. Consequently, many of these phenomena have been frequently ascribed t o colloidal association. The sudden rise of viscosity with formation of gelled particles but without substantial decrease of iodine number, such as has been observed during the later stages of stand oil formation, is a typical case. The phenomena of degelation and of the varied oxygen requirement for gelation are other instances. The modern concepts of polymerization permit of a mathematical treatment which, when applied t o existing data, provides a new and probably more fundamental insight into these phenomena. This approach relegates the colloidal aspects t o a secondary role, but they may
I
N A REVIEW of the chemistry of drying oils Morrell(16)
for oxyn formation in solution which wm observed for different concentrations were additional factors that favored the association concepts. On the other hand, it appears to some that the alleged failure of the general principles of polymerization to explain the processes of film formation may be due more to a general lack of appreciation of these principles or of their application than to difficulties in elucidating changes of bond structure, It is believed that the proper use and application of these theories should prove helpful in elucidating these changes of bond structure and likewise in casting further light upon those very phenomena which now appear to others as decidedly opposed to the polymerization concepts. Previous papers (2, 3) showed that, in considering polymerization, it is necessary to take all of the functional groups into account and to distinguish between the functional groups and those which are merely potentially functional. The apparent necessity of repeating some ideas that have previously been published is regretted; yet the objection of Houwink (9)to certain of the Carothers’ concepts and the failure to make use of the latter and of those concepts more recently advanced by Kienle (la)and by Bradley (2) are believed to warrant such publication.
called attention to a number of unsettled matters and rendered opinions which merit detailed consideration. He stated that ‘Ithe general principles of polymerization have not met with great success in explaining the processes of film formation from heat-thickened oils, mainly because of the difficulties of elucidating changes in the bond structure owing to inadequate identification of the products.” He also held that the incomplete identification and characterization of heat-thickened drying oils and of their acid radicals in addition to the observation that “tung oil heated to 310” C. shows no tendency to gelate” cast doubt upon the idea that film formation resulted from the dimerization of these oils or of their acid radicals. I n common with several previous investigators and commentators, Morrell’s observation that the formation of aggregates or gelled portions of a drying oil may occur during heat or oxidation processes without appreciable disappearance of double bonds was held to favor the association theories rather than the polymerization theories of film formation. He also considered that the high diene value of thickened p-eleostearin and the variable oxygen requirement 1
For previous pspers i n this series, see literature citations B a n d 5.
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VOL. 30, NO. 6
FIGURE 1. THEORETICAL RELATION OF MOLECULAR GROWTHTO DEGREE O F REACTIONAS DETERMINED BY INITIALDEGREEOF FUNCTIONALITY
General Polymerization Concepts The essential points of the current theory may be considered to be as follows:
FIG I
0
10
10
20
PO
40 50 PERCENT REACTION
30
30
80
40 50 PERCENT REACTION
FUNCTION&LITY Of I8
70
80
90
I(
(P)
EO
70
80
80
18
(P)
SYSTEMS 12
1. Polymerization, a form of molecular growth, is concerned solely lwith intermolecular reaction. It occurs by diverse mechanisms and by reason of the polyreactivity of the compound or compounds involved. 2 . In considering the degree of polyreactivity from the polymerization viewpoint, it is necessary to distinguish between active and potentially active groups, 3. It is necessary to distinguish between bi- or bi-bifunctional polymerizations and the more polyfunctional systems, not merely because of the essential difference in the character of their polymerides but mainly for the reason that in the first case polymerization proceeds without the accumulation of unreacted or potentially functional groups whereas in the latter case there develops an accumulation of unreacted groups with every step of the polymerization ( 1 2 ) .
T h e s e c o n c e p t s a r e s u b j e c t t o mathematical treatments, two of which have been published (6, l a ) . Graphically they may be illustrated as follows in the case of systems possessing functionalities of 2, 3, 4, 6, 8, 9, 12, and 18, respectively (i. e., number of polymerizable groups or bonds per mole of initial reactant). Figure 1 shows how the number of monomeric units per average mole of polymer may be expected t o vary with respect to the degree of reaction (expressed as percentage) in each of these systems; this illustrates the principle that, except in the case of the bifunctional systems, the degree of reaction reaches a limiting value while the molecular weights may become infinite. Figure 2 illustrates how the potentially functional groups may be expected to accumulate in each of these systems as long as only polymerization or intermolecular reaction obtains. Figure 3 presents the relation of the growth of unreacted or potentially functional groups to the molecular growth, in terms of monomeric units. As shown by Carothers (6),the relations depicted in Figure 1 may be represented by the general equation : R E - 2- -
F
where P x
= =
F =
FIGURE2. THEORETICAL RELATION OF ACCUMULATION OF POTENFUNCTIONAL GROUPSWITH RESPECT TO DEGREE OF REACTION AS DETERMINED BY INITI.4L DEGREE O F FUNCTIONALITY
TIALLY
FIGURE3. THEORETICAL RELATXON OF ACCUMULATION OF POTENFUNCTIONAL GROUPSWITH RESPECT TO MOLECULAR GROWTH AS DETERMINED BY INITIAL DEGREE OF FUNCTIONALITY
TIALLY
2
xF
degree of reaction degree of polymerization, number of monomeric units per mole of polymer number of functional groups in original reactant or monomer.
However, as Kienle (19) stated, the functional groups per molecule increase in number with every step of the polymerization in the more polyfunctional systems and therefore appear to require a more involved mathematical treatment to account more fully for the polymerization. It is perhaps simpler to derive the equations for each system from the elementary statistical considerations and to employ these as such or to make direct use of their graphic forms.
JUNE, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
The equations for each of the systems in Figures 1 to 3 are listed in Table I.
TABLEI. POLYMERIZATION EQUATIONS Potnnt,ial. -
Functionality of Degree of Monomer Reaction, P 2
P=-
2 - 1
Deeree of Polv- Accumulated Gerization,Potentially s (Monomeric Functional Units) Groups, F 223-
F + N
F=2(con-
2
stant)
X
3
P=-
4
p
9
P=-
12
-2
22
32
=
22
2s
e
-2
92 x - I
P = -
6s
x - F - 2
F - 2 $ = -
x = x = -
2
F - 2
7 F - 2 10
Used Functional Groups, N N = Z x - 2
F = x + 2
N = 2 2 - 2
F = 2 s + 2
i V = Z x - 2
F = 7 x + 2
N = 2 ~ - 2
F = 1 0 r + 2
N = 2 ~ - 2
691
systems by the equations in Table 11, each of which has been derived from the elementary statistical considerations. The first case involves only intermolecular reaction or polymerization, the second case involves the loss of an additional function a t each step of the polymerization by intramolecular reaction, and in the third case the extent of this intramolecular reaction has been doubled. It is thus possible to develop the equations for any given system and to allow for any specified amount of intramolecular reaction. If the extraneous reactions involve oxidation with or without the elimination of by-products, this may also be considered. Similarly if, as in the case of drying oils, the iodine number is used as an index, one is compelled to take into consideration the chemistry of the system and to make allowances for the restricted iodine value in case of conjugate unsaturation. While it is generally advisable to take into account all of the established chemistry of any given system and t o deal only with homogeneous substances, i t is still possible to deal with mixed glycerides such as are represented by the natural drying oils purely on the basis of their average degree of unsaturation and of their average molecular weights and yet obtain significant results.
Addition Polymerization of Linseed Oil It is important to note that these equations represent the principles of polymerization for these particular systems, but that the experimentally determined values for any given system will vary in the following respects: 1. With the number of reactive groups of the monomer. 2. With the extent and manner in which these groups are
caused to react.
Any confusion or doubt which may have arisen concerning the principles of polymerization is believed to be due to incomplete consideration of the second point. Houwink's objection to the validity of Carothers' general equation (while entirely sound and of unquestioned importance with respect to the general chemistry of these more polyfunctional systems) is apparently concerned with the possibility that intramolecular reactions as well as intermolecular reactions may occur during the course of these polymerizations. (A second objection deals with the nonreactivity of certain groups, of which account has already been taken.) But the Carothers equation applies only to such intermolecular reaction as is involved in polymerization, and by definition any reactions involving the potentially functional groupls which do not bring about polymerization are excluded. Thus if, as in the case of drying oils, one of the double bonds reacts with oxygen but is not afterwards joined to another molecule of the drying oil, the functionality of that bond with respect to polymerization has been destroyed. Similarly, since polymerization is concerned only with molecular growth, a plurality of linkages between any two molecules is no more effective than one; therefore those in excess of one have lost their functionality with respect to polymerization (and may regain this only if the intramolecular structure be broken and rearranged t o an intermolecular linking). If Houwink's major objection were carried to its extreme, all of the potentially reactive groups of a monomer would react intramolecularly, and reaction would be completed without increase of molecular weight. It thus appears that the effect of any intramolecular or of extraneous reaction during the course of a polymerization must serve to reduce the potentid functionality of that system. It will also tend to reduce the average molecular weight of the reaction product for any given degree of reaction. This effect of intramolecular reaction may be illustrated in the case of hexafunctional
As an illustration of the application of these concepts to a specific case, one may consider a trilinoleic glyceride or linseed oil since the average degree of unsaturation of linseed oil approximates that of the trilinoleic glyceride-i. e., six carbon-to-carbon double bonds per mole, equivalent to an iodine number of 173.6 a t the molecular weight of 878. The problem selected is to deduce from these functionality concepts and the available experimental data the nature and extent of the double-bond reactions which occur during the heat thickening of linseed oil. The first experimental data to be used are those provided by Caldwell and Mattiello (6).
TABLE11. EFFECT OF INTRAMOLECULAR REACTION ON A POLYMERIZABLE SYSTEM HAVING A POTENTIAL FUNCTIONALITY OF 6 Functionality of Degree of System Reaction, P 6
6-1 6-2
x - 1 3s 2s 2 P = 3s 2 - 1
Degree of, Polymerization, x (Monomeric Units)
p=-
-
P = 2
x = -
x=x=-
F - 2 F - 4
F
+N 6
.
Accumulated Functional Groups, F
Used Functional Groups, N
F = 4 ~ + 2
N = 2 x - 2
F = 2 ~ + 4
N = 4 x - 4
F = 6
N =6x - 6
These data were obtained on linseed oil which was thickened during 22 hours a t 287.8" C. The observed molecular weights in common with those of many other investigators are low for the monomer and, in view of the work of Gay (8) and of Brocklesby (4), have been multiplied by a correction facttor of 1.116. First it is desired to determine the potential functionality of the system. The reactivity is centered in the six carbon-tocarbon double bonds. The functionality may be 12 or 6, depending upon whether these linkages may be caused to undergo the Staudinger type of chain polymerization or whether polymerization proceeds as by ring formation between the unsaturation of the acid radicals of adjacent molecules of the glyceride. As shown in Figures 1 to 3, a system with an initial potential functionality of 6 will have developed a t the dimeric stage a potential functionality of 10 and have exhausted two functions, the reaction being one-sixth complete. If the initial
INDUSTRIAL AND ENGINEERING CHEMISTRY
692
TABLE 111.
FRACTIONATION OF HEAT-THICKENED LINSEEDOIL (7)
VOL. 30, NO. 6
Application to Isolated Fractions
Elod and Mach (7) recently reported on the chemistry of the heat thickening and of the oxidation processes of linseed oil in which the evidence was considered by them to favor the colloid association mechanism of gelation. I n this work linseed oil was heat-thickened both when exposed to air and when processed in vacuo. The thickened oils in each case were fractionated by means of acetone into one insoluble . and two soluble fractions] and one soluble porVarnish 0, Exposed to Air tion was again fractionated by methanol and by 8 Acetone-601.1 54.0 38.67 131.0 1398 ethanol. The various fractions and significant 1776 32 '45 when combined 9 Acetone-sol. I1 10.5 39.62 123.0 (17 .58for Original mixt') 10 Acetone-insol. 35.5 53.55 105.0 3588 analytical data are listed in Table 111. 11 Methanol-sol. 13.5b 97.83 140.8 844 Figure 5 shows the theoretical iodine numberfrom 8 molecular weight relations of a hexafunctional 12 Ethanol-sol. 5.5) 45.18 133.2 1155 1.08 10.39whencombined 1.541 (5.25 for original from 8 system. Curve 6 represents purely intermo13 Ethanol-insol. 35.0b 9.27 127.7 1561 from 8 lecular reaction or polymerization. Curve 6-1 14 Entirevarnish . ... 45.70 123.8 1947 17.58 represents the same system in which it has been 5 Per cent of sample 1. assumed that for each step of the polymerization b Per cent of sample 8. an additional loss of unsaturation has also occurred, corresponding to one double bond or potential function per monomer molecule. Curve TABLEIV. THEORETICAL MOLECTJLAR WEIQHT-IODINE NUMBER 6-6 has also been included to illustrate 100 per RELATIONS OF POLYMERIZED LINSEEDOILAND LINSEEDACIDS cent intramolecular reaction of the double bonds. -Linseed OilLinseed Acids Mol. Iodine Mol. Iodine Mol. Iodine Each fraction of Table I11 has been located on z F weight No. x F weight No. z F weight No. Figure 5. Here again in both preparations all of 1 2 280 8 2 2240 22.7 878 173.6 180.1 1 6 the fractions are somewhat less than hexafunc20.2 1756 144.6 2 2 560 9 2 2520 90.7 2 10 18.1 2634 135.0 3 2 10 2 2800 840 60.5 3 14 tional with respect to polymerization and, on 4 2 1120 45.3 11 2 3080 16.5 3512 130.1 4 18 12 2 3360 15.1 4390 127.3 5 2 1400 36.3 5 22 the basis of their average compositions (7 and 2 3640 14.0 30 2 13 6 2 1680 6604 125.3 6 26 14), fall somewhere between the first two curves. 7 2 1960 25.9 14 2 3920 13.0 Upon fractionation, however, it is apparent that the major acetone-soluble portions (i. e., 1and 8) represent mixtures in which the extent of intrapotential functionality is 12, the dimer should possess a potenmolecular reaction is greater and the extent of polymerization tial functionality of 22 and have exhausted two functions, the less than the average, that the methanol- and ethanol-soluble reaction being one-twelfth complete. This corresponds to fractions (4,5, and 11) have not polymerized and may represent only monomers which have undergone partial intraiodine numbers of 144.6 and 159.5, respectively, whereas the experimental value is 128. The system is thus indicated to molecular reaction, and that the acetone-insoluble fractions represent that monomer which has undergone the maximum be even less than hexafunctional. degree of polymerization. At this point it becomes advisable to plot the iodine numbers against the molecular weights of a hexafunctional and of At this point one may inquire whether the apparent intramolecular addition reactions are a reality. Thermal decoma tetrafunctional system (Figure 4). The experimental values position of a portion of the polymerizing glyceride with are next plotted. These fftll between the two systems, and liberation of free fatty acids would also disturb the iodine the form of the curve departs considerably from either. Is number-molecular weight relations. The experimental data this due to the influence of intramolecular reactions substantially in accordance with Houwink's contentions? show that the isolated fractions are decidedly acidic. If the partial hydrolysis has not been selective and has liberated Two additional curves are then plotted for the polymerizaundecomposed fatty acids characteristic of the average comtion of a hexafunctional system in which it is assumed that a position, then these may possess a potential functionality with second and a third double bond, respectively, have been respect to addition polymerization equivalent to one-third caused to react intramolecularly a t each step of the polyof that of the glyceride, or 2. The hexafunctionality of the merization. At once it appears that the experimental curve oil will then be reduced proportionately to the extent of the closely approximates the first case and departs from it to hydrolysis by the bifunctional adulterant. some extent only during the latter stages. The calculated iodine number-molecular weight relations The addition reactions occurring during the heat thickening of the bifunctional linseed acids are shown graphically in of linseed oil thus seem to be typical of the polymerization of Figure 6 . These relations and those of the neutral glyceride a hexafunctional system in which further reaction occurs inare listed in Table IV. tramolecularly. Also the extent of this intramolecular reFrom these data or their graphs one may ascertain the action appears to be approximately equal to that of the intertheoretical iodine values of each of these systems a t the molecular reaction or polymerization during the earlier stages molecular weights which were observed for each fraction by of the heat treatment but later seems to become smaller than Elod and Mach. From the observed acid values of each fracthat which results in polymerization. In any case the fall of tion one may then calculate the percentage of fatty acid and of iodine number is greater, and the increase of molecular weight neutral glyceride represented by each fraction. Although less, than that to be expected from polymerization alone. It the acidic and neutral portions of each fraction have preis significant, however, that Long, Knauss, and Smull (IS@ sumably interpolymerized, one may nevertheless calculate previously detected intramolecular reaction during the polythe theoretical iodine value for each mixed fraction on the merization of linseed oil by means of the hexabromide and basis of their average composition a t the observed molecular iodine numbers. Varnish V, Vacuum-Processed Sample % of Acid Iodine Mol. -% to Adsorb Oxygen in DryingNo. Fraction Total No. No. wt. Tests 1 Acetone-sol. I 49.50 22.46 136.1 1338 20.38 when 'OmbinEd 2 Acetone-sol. I1 8.25 22.87 131.4 1832 (14.06for Original mixt.) 3 Acetone-insol. 42.25 45.25 109.6 3986 4 Methanol-sol. 9.255 87.78 147.8 848 0.97 1 from 1 5 Ethanol-sol. 4.75O 20.24 146.7 904 3.46 10.70 when combined from 1 6 Ethanol-insol. 35.505 2.93 135.3 1390 6.27 (7.45for Originsl mixt.) frpm 1 7 Entirevarnish .. . 32.00 128.0 1900 14.06
I
JUNE, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
weights. Although this procedure is invalid from the standpoint of the probable chemical mechanism (in which interpolymerization of the fatty acids and glycerides is believed to occur), it may be justified on the ground that the calculated molecular weight-iodine number relations are identical with those calculated on the assumption that mixed polymers are formed and permits of more simplified statistical treatment. I n Table V and Figure 7 are presented the calculated and observed iodine values of each fraction.
693
IW
140 6
I20
100 6-1
TABLEV. Fraction KO.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
CALCULATED AND OBSERVED IODINE VALUES OF FRACTIONS ISOLATED BY ELOD AND MACH % Fatty
Acid Equivalent 11.23 11.44 27 63 43 89 10 12 1 47 16 00 19.34 19.81 26.78 48,96 22.59 4.63 22.85
Obsvd. Iodine No. 136 1 131 4 109 6 147 8 146 7 135 3 128 0 131.0 123.0 105.0 140.8 133.2 127.7 123.8
Calcd. Iodine NO.
140.1 129 8 96.8 123.8 159.4 149,8 123.6 128.9 121.2 100.3 143.4 132.6 142.5 115.1 I
80 4
Obsvd. Mol. Weight 1338 1832 3986 848 904 1390 1900 1398 1776 3688 844 1155 1561 1947
60
mo (11
2080
1280
2480
6-2 2880
(3)
6
03
.IO 100
The agreement with the polymerization theory is for some reason better in the case of varnish 0 (fractions 8 to 14) than for varnish V (fractions 1 to 7 ) but, with few exceptions, is sufficiently close to appear significant. Figure 8 presents data of Caldwell and Mattiello (corrected for molecular weight) for an oil processed a t 287.8' and for another processed a t 320.8"C. The latter developed more acidity than the former under the influence of the higher reaction temperature, but the acidity in all cases was much less than for the oils that were reported to have been processed a t 280' C. by Elod and Mach.
6-1
80
6-6
FIG. 3
60 1)
6)
(2) 1240
840
2040
1640
2440
2840
(4) 3640
3240
(5) 4440
4040
4840
W
f 0
80
40
0
FIG. 6 20
2
FIGURE 4. THEORETICAL IODINE KUMBER-MOLECULAR WEIGHTRELATIONS
880
480
1280
1680
2080
2480
2880
3880
3280
4080
I
Polymerization of a hexafunctional and] a tetrafunctional monomer of molecular weight 880 (curves 6 and 4, res ectively); variations t o be expected in t h e event of intramolecular aaditions in a hexafunctional system (curves 6-1 and 6-2); a n d experimental values for linseedgtand oil (curve E) according to Caldwell and Mattiello (6).
FIGURE5 . THEORETICAL IODINESUMBER-MOLECULAR WEIGHTRELATIONS Addition polymerization of linseed oil as a hexafunctional syatem (curve 61, with a n equal loss of unsaturation by intramolecular addition (curve 6-1) a n d with complete intramolecular addition (6-6). These a r e employed with reference t o t h e values observed by Elod a n d Mach (7), on isolated fractions of linseed s t a n d oils.
FIGURE6. THEORETICAL IODINENUMBER-MOLECULAR WEIGH'f RELATIONS I N THE ADDITIONPOLYMERIZATION OF MIXED FATTY ACIDS OF LINSEED OIL FIG.8
FIGURE7 . ATTEMPTEDCORRELATION OF OBSERVED AND THEORETICAL VALUES FOR THE STAXD O I L FRACTIONS OF ELOD AXD MACH AFTER ALLOWINGFOR OBSERVED HYDROLYSIS Points on t h e broken line represent t h e theoretical values after allowance for hydrolysis. FIQURE 8. ATTEMPTEDCORRELATION OF OBSERVED AND THEORETICAL VALUES FOR STANDOILS OF CALDWELL AND MATTIELLO AFTER ALLOWINGFOR OBSERVED HYDROLYSIS Curve Curve Curve Curve Curve
6. 6-C-1. E-1. 647-2. E-2.
Theoretical for the pure lyceride Theoretical for the 287.8' C. oil. Observed for t h e 287.8O C. oil (6) Theoretical for t h e 320.8' C,, oil Observed for the 320.8' C. oil (6)
130
120
BW
980
IO80
1180
1280 1380 1480 MOLECULAR W E I G H T
1580
I880
1780
1880
INDUSTRIAL AND ENGINEERING CHEMISTRY
694
Figure 8, curve 6, represents the theory for a hexafunctional system of molecular weight 878 in which polymerization alone is assumed; curve 6-1 represents the same system in which an equal loss of unsaturation occurs by intramolecular addition. The points on broken line E-l show the data observed for the oil processed a t 287.8" C.; those on line E-2 represent the data observed for the oil processed at 320.8' C. Since these oils developed acidity, curve 6 is no longer applicable and becomes 6-C-1 for the 287.8' C. oil and 6 4 - 2 for the 320.8" C. oil in accordance with the previously described means of calculation. 180
I b 2 -PETROLEUM-ETHER-SOLUBLE F R I C T I O N S FROM 2 2 O 0 C . 0 I L
I60
140
w
J 2
g
120
la
0 IO0
80
3
-
-PETROLEUM-ETHER-SOLUBLE F R A C T I O N S F R O M 360.C OIL
8-0 80 840
1240
1640 2040 2440 MOLECULAR WEIGHT
2840
3240
3640
FIGURE9. ATTEMPTEDCORRELATIONS OF OBSERVED AND THEORETICAL VALUES FOR TUNG STAND OILSAND THEIR FRACTIONS AS OBTAINED BY RHODES AND WELZ( 1 7 )
Collectively these data are believed to show that the heat thickening of linseed oil is primarily an addition polymerization characteristic of a hexafunctional system but is modified by extraneous reactions. The experimental data are insufficient to characterize the latter fully. The observed acid numbers, however, seem to indicate that, under the thermn! processing, partial hydrolysis has occurred with liberation of fatty acid; the extent of this reaction is conditioned by the processing treatment employed. It is further indicated that the addition reaction of the double bonds has proceeded not only intermolecularly with resulting polymerization but also to some extent intramolecularly with consequent additional loss of unsaturation and without a corresponding increase of molecular weight. Interpretation of this sort is conditioned by the reliability of the physical and chemical analyses employed. The molecular weight and iodine values, however, are here presumed to be as accurate as present methods afford. The molecular weights are corrected in accordance with the work of Gay (S), of Brocklesby (4, and of Elod and Mach ( 7 ) , who believed that their results definitely excluded association effects. Consideration of the data of Elod and Mach suggests that their nonpolymerized nondrying alcohol-soluble fractions may consist of free fatty acids and of other products of the thermal decomposition and of the intramolecular reaction of the oil rather than the oleic and linoleic glycerides which they assumed. The iodine value is considered somewhat too high to support their assumption; also other investigators (13) showed that similar fractions can be obtained from synthetic trilinolenic glyceride, while Kino (13A) has found the methanol soluble portion of heat bodied methyl linolenate to comprise an unpolymerized, intramolecular reaction product. Detailed chemical analyses therefore appear desirable to establish the facts.
VOL. 30, NO. 6
Gelation, Degelation, and Polymerization of Tung Oil The ['failure of tung oil to gelate when heated to 310° C." is believed to require amplification, particularly as to the conditions which obtained. Tung oil, like phthalic glyceride or other convertible esters, will gel1 if the temperature is raised gradually to 300' C., but a t higher temperatures or under certain other hydrolytic conditions it may degel. If the rate of heating exceeds a critical velocity, the gel may be destroyed as fast or faster than it can be formed and hence never be recognized to exist. In the case of phthalic glyceride, Kienle ( l a ) and Kienle and Barnes (IS) definitely related this behavior to decomposition or thermally induced hydrolytic reactions. As early as 1916 Schumann (18) investigated "superheated" tung oils and showed that such products were characterized by abnormally high acidity, low iodine, and saponification numbers, and from 8 to 12 per cent unsaponifiable matter. Moreover, while the "superheated" product was found to be nonheat convertible, extraction of the 25 per cent which was soluble in petroleum ether and in alcohol left a residue which was found to gel readily when reheated. Rhodes and Welz (17) (Figure 9) compared tung oil processed a t 220" and a t 360" C., the fractions which were obtained from this oil by solvent extraction, and the fatty acids recovered after saponification of these fractions. Their analytical results, in general, confirmed those of Schumann's and showed that the acids recovered from the 360' C. oil were lower in molecular weight but also possessed greatly reduced iodine value compared to the acids recovered from the 220" C. oil. Dimers isolated from the latter possessed iodine values which are now seen to agree well with the theory for a hexafunctional system. (Eleostearin, while more unsaturated than a hexafunctional system, may be expected to appear hexafunctional because of the well-known influence of conjugate unsaturation upon iodine number determinations.) The work of these and of subsequent investigators, as interpreted by the polymerization concepts, appears to show that the normal heat thickening of tung oil adheres most closely to polymerization theory since hydrolytic, intramolecular, or other extraneous reactions occur to almost negligible extent and thus present an interesting contrast with the behavior of linseed oil. It appears, however, that when tung oil is subjected to excessive temperatures, it may depolymerize or undergo hydrolytic or other extraneous reactions. The resulting destruction or loss of functional groups apparently coincides with the observed loss or impairment of the gelation characteristics. Jordan (11) discussed these phenomena. R. H. Kienle advised the author that, in accordance with his experience, linseed oil will degel even more readily than tung oil: "In fact, if care is not taken, it is possible to pass through the gel state rapidly a t lower temperatures even than in the case of tungoiland not know that one has passed through this stage. . . Also, we were able to show rather conclusively that while these degelled vegetable oils do not air convert, they will heat convert if sufficient volatile products are removed. They can also be made to air convert provided a small amount of glycerol is added and esterification is effected, presumably by the acids that were developed during the degelation."
Polymerization and Heat Thickening The concepts of polymerization are believed to provide an adequate explanation of the relatively high iodine number of the polymerized drying oils and of the high diene value of the eleostea,rins during their heat thickening, because both may be expected to attain a limiting value and the accuracy of their determination becomes highly significant. The molecular weights may increase enormousIy with but inappreciable
JUNE. 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
change of iodine or of diene numbers as the gel point is approached. Various investigators have, from time to time, noted in the heat thickening of drying oils a sharp drop of iodine number accompanied by relatively little increase of viscosity during the early stages of the reaction and a reverse effect during the later stages. This seems to have puzzled many investigators and caused some to conclude that the earlier stages represent chemical polymerization and condensation whereas during the later stages colloidal association must be chiefly responsible for the observed phenomena. The polymerization concepts, however, show that it may be unnecessary to postulate colloid mechanisms or to assume entirely different re action mechanisms a t different stages of the thickening processes. It has been shown ( I O , It9) that association becomes greater or more pronounced as the molecular weight is increased through polymerization, and that the structural or shape factors which distinguish the soluble forms of threedimensional polymerides from the linear polymeridea likewise contribute to the powers of association, have also been shown. Association, like polymerization, may then be related to the molecular architecture, particularly to the functionality and to the nature of the functional groups. The suspicion thus remains that, since the forces of association are thus conditioned by the degree of polymerization and by the functionality of the polymeride, they are consequently more incidental and less fundamental than the latter. Consequently, if the powers of association represent merely an inherent property of a polymeride, one is compelled to search more deeply for the fundamental factors. The functionality and polymerization concepts should therefore find much application in this direction. One group of investigators (1) recently stated that “the iodine value is the best measure of the progress of polymerization (of drying oils) during kettling” and “that this is true is quite compatible with the theory of polymerization.” Plotting observed iodine number against viscosity, they drew the conclusion that, since the iodine number reached a limiting value a t which point the viscosity rose enormously, then a t or (‘beyond this point the viscosity has no relation to polymerization but is influenced by association.” Objection to these conclusions must be raised. The iodine value is reduced by addition reactions whether these result in increased molecular weight (intermolecular reaction or polymerization) ar whether no increase of molecular weight is involved (intramolecular reaction). It may even fail to detect certain forms of unsaturation. For these reasons the iodine value may be used only cautiously and then merely with reference to the observed and theoretical molecular weights as has been illustrated. Granting that association forces contribute to the exaggerated rise of viscosity, it nevertheless appears that fundamentally this may originate from an increased degree of polymerization. Solvent extraction and analyses of the various fractions for their iodine value, molecular weight, and acid, saponification, and hydroxyl numbers, and the determination of the viscosity-temperature and viscosity-concentration relation of these fractions should help to decide this matter. (R. H. Kienle advised the author that the sudden rise of viscosity with relatively little change of iodine value is due not so much to the increased molecular weight per se as to the formation of tetramers and octamers of the hexafunctional monomer which, because of their complex structure, tend to immobilize the unpolymerized monomer and lower polymers. Slight variations of the concentration of the higher polymeric fractions may produce no appreciable change of iodine value but a considerable change of viscosity. This viewpoint is entirely in accord with polymerization theory and is considered to be supported by much of the experimental data now on record.)
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Oxygen- Induced Polymerization of Oils The variable oxygen requirement for the production of oxyn in solutions of 6-eleostearin, such as was mentioned by Morrell, is believed to require amplification. The low oxygen requirement a t high concentrations may have resulted in oxyn containing much ungelled phase; the latter phase might have been isolated by suitable extractions whereas the oxyn produced in more dilute solution may have contained relatively less of the ungelled phase. During the oxidation of trilinolenic glyceride it was recorded that the oxyn from the pure ester contained 27.8 per cent of liquid phase after 175 days of exposure of the gel; the same material upon dilution with 68 per cent of olein formed an oxyn from which could be extracted an amount of liquid only slightly exceeding the olein content (14). The oxidation studies of Elod and Mach ( 7 ) , Table 111, showed that the separated fractions of stand oil exhibited far greater oxygen absorption when separately tested and then totaled than did the original mixtures. They observed that in the mixtures of higher viscosity where early gelatinization develops, oxygen absorption is frequently hindered, presumably because of checks on diffusion. In the case of the oxygen conversion of drying oils it appears that many more intramolecular or extraneous reactions are involved than in the case of heat conversions. Fairly extensive saturation of the double bonds in the case of the nonconjugated oils has been especially noticeable. However, beyond the small minimum threshold value of oxygen required to effect gelation, the amount of oxygen absorbed may vary considerably under the environmental conditions which obtain. In the case of the eleostearins and of their maleic anhydride adducts, the work of Morrell and associates (16) is considered especially significant. This work demonstrated the inequivalency of the various double bonds and indicated that only the remote double bonds ordinarily become involved in the oxygen-induced polymerization, whereas the remaining double bonds may react with oxygen to form ketohydroxy groupings or may even fail to function. This is entirely in accord with the general conclusions derived with respect to the oxygen conversion of the drying oils on the basis of the polymerization concepts ( 2 ) . Moreover, it appears probable that the tendency of the remote double bonds to form peroxides which may alone become functional over a period of time may help to explain why freshly formed oxyns still contain much liquid phase and only gradually approach complete conversion. It is considered not improbable, however, that in the case of stoving varnishes and enamels, additional double bonds may be caused to become functional by the combined effect of heat and of oxygen.
Conclusions The mechanism of the inter- and intramolecular addition reactions occurring during the thermal treatment of linseed and of other drying oils which appears to agree best with the polymerization concepts, with the gelation characteristics of the various esters of the drying oil acids and of other acids of known constitution, and with the bulk of the analytical evidence to date, is that which the writer previously described ( 2 ) . The splitting of glycerides and addition of the unsaturated acids to the mono- or polymeric glycerides during the heat bodying of the oils requires no change of the proposed mechanism, merely serving to affect the rate and extent of the polymerization. It has not been found possible to differentiate between the formation of four- or of six-membered ring systems in the polymerization of the drying oils since these are statistically identical. From the standpoint of polymerization and of the
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gelation phenomena it appears immaterial as to which type of ring is formed. The proposed mechanism seemingly requires that, regardless of the molecular weights attained during the polymerization of the drying oils, saponification and hydrolysis will result in the recovery of mono- and of dibasic acids, whereas a polystyrene type of chain polymerization is expected to result in the recovery of appreciable amounts of more polybasic acids of higher molecular weight. The advisability of continued and more exacting analytical studies is indicated. While cognizant of the influence of association forces in connection with the formation of colloidal aggregates, the polymerization-minded investigator is inclined to regard these colloidal aspects as more incidental than fundamental. He finds it difficult to conceive of the formation of films or gels of insoluble and infusible form by any process which does not involve primary valence bonds. When viewed in the light of the polymerization concepts, the various reactions which occur during the heat thickening or oxidation of the natural and synthetic drying oils and resins assume increased significance and interest. The influence of extraneous reactions upon the course of polymerizations and upon the physical properties of polymers is believed to warrant particular consideration. In the case of the heatthickened drying oils it appears that variations not merely of degree of polymerization but also of the degree of the intramolecular and thermally induced “hydrolytic” reactions may account for many physical and chemical differences which have been frequently dismissed as colloidal phenomena. Variations of the acetone-insoluble portions of linseed stand oils are stated by Elod and Mach to range from 30 to 70 per cent, depending upon the conditions of formation. The work of the New York Production Club (16)confirms these data. Their vacuum-processed stand oils were shown by analyses to be relatively free from acidic or hydroxylated “hydrolytic” products. But they exhibit iodine value-molecular weight relations which show evidence of intramolecular addition as well as of the more prominent hexafunctional addition polymerization, It is unfortunate that this otherwise comprehensive investigation did not include the fractionation and analysis of the acetone-insoluble and -soluble phases. The observed data, however, when replotted on a new basis 4.e., per cent of acetone-insoluble phase us. average molecular weight-illustrates the definite relation of the acetoneinsoluble phase to the degree of polymerization. Further analyses of the materials and application of the methods here described may be expected to provide additional information. Application of the polymerization concepts to the chemistry of the heat-thickening processes of tung and of linseed oils showed that in both cases the loss of unsaturation and increase of molecular weight conform most closely to the theory for a hexafunctional system. The agreement of observed and theoretical values in the case of tung oil is attributed to the absence of intramolecular or of other extraneous reactions. I n the case of linseed oil, as well as (‘superheated” tung oil, the evidence is such as to demon s t r a t e that intramolecular additions (which may in some cases originate concurrently with the polymerizatipn or, more usually, as a
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result of depolymerization) and thermally induced partial hydrolysis are prominently featured and proceed concurrently with the polymerization in varying degree in accordance with the conditions of the processing treatment. It is hoped that the present article may help to dispel doubt as to the value and significance of iodine number and molecular weight determinations when applied to heat-thickened drying oils, and will serve to demonstrate the advisability of including both determinations in future studies. It is also hoped that the work of recent foreign investigators as well as the present considerations of applied polymerization theory will discourage the current trend in this country to refer the observed analytical values of processed drying oils to such a complex function as the over-all viscosity of these concentrated polymeric species rather than to their average molecular weights or to the viscosity constant a t extreme dilution (19). While the industrial importance of these viscosity variations and their measurement is unquestioned, their use as a fundamental index appears unwise because of the numerous factors which are known to influence and determine them.
Acknowledgment The author gratefully acknowledges the helpful advice and suggestions of his associates in connection with the preparation of this paper, especially those which were rendered by G . M. J. Mackay, D. L. Fuller, and R. H. Kienle. Appreciation is likewise expressed to the American Cyanamid Company for its support of this work and permission to publish.
Literature Cited (1) Am. Fed.$aint &Varnish Clubs, Sci. Sect., Los Angeles Group, Natl. Paint, V a r n i s h Lacquer Assoc. Circ. 546, 263-71 (1937) ; Am. P a i n t J., 22 (3E), 23 (1937). ’ (2) Bradley, T. F., IND. ENG.C H ~ M 29, . , 440-5, 579-84 (1937). (3) Bradley, T. F., Kropa, E. L., and Johnston, W. B., Ibid., 29, 1270-6 (1937). (4) Brocklesby, H. N., Can. J . Research, 14, 231 (1936). (5) Caldwell, B. P., and Mattiello, J., IND. ENG.CHEM.,24, 159-60 (1932). (6) Carothers, W. H., Trans. Faraday SOC., 32, 44 (1936). (7) Elod, E., and Mach, U., KoEZoid-Z., 75, 338-48 (1936). (8) Gay, P. J., J . SOC. Chem. I n d . , 52, 703-5 (1933). 19’1 Houwifik. R.. Trans. Faradau SOC..32. 49 (1936). (iOj Houwink; R., and Klaasens,-K. H., Kolloi‘d-Z.,‘70, 329 (1935) ; 76, 217 (1936). (11) Jordan, L. A,, J . Oil Colour Chem. Assoc., 17, 47-66 (1934). Chem. I n d . , 55, 229-37T (1936). (12) Kienle, R. H., J . SOC. (13) Kienle, R. H., and Barnes, R. B., paper presented before Div. of Paint and Varnish Chemistry a t 93rd Meeting of Am. Chem. Soc., Chapel Hill, N. C., April 13, 1937. (13A) Kino, K., Sci. Papers I n s t . Phys. Chem. Research ( T o k i o ) , 26, 91-7 (1935). (13B) Long, J. S., Knauss, C. A., Smull, J. G., IND.ENG.CHEM., 19, 62-5 (1927). (14) Long, J. S., Rheineck, A. E., and Ball, G. L., Ibid., 25, 1089 (1933). (15) Morrell, R. S., J . SOC.Chem. I n d . , 56, 795-8 (1937). (16) New York Production Club, Natl. Paint, Varnish Lacquer Assoc. Circ. 523, 435-6 (1936). (17) Rhodes, P. H., and Welz, C., IND. ENQ.CHEW,19, 68-73 (1927). (18) Schumann, C. L., Ibid., 8 , 10 (1916). (19) Tachimori, M., J . SOC. Chem. I n d . J a p a n , 39, Suppl. Binding, 473-5 (1936); 40, Suppl. Binding, 19-21 (1937). RECEIVED March 12. 1938.
