Presented before the Division of Paint, J-arnish, and Plastics Chernistrv at the 113th Meeting of the A4mericanChemical Society, Chicago, Ili.
+********
ries a Kenlueth A. Earhart C . S. lndicstrial Chemicals, I n c . , Baltimore 3, M d .
Several theoretical oil modified alkyd formulations are discussed for the purpose of explaining the theoretical reactions and structures involved. Oil modification is defined as the triglyceride calculated from the fatty acids in a completely esterified or analyzed alkyd and is suggested as the best practical guide to the properties of oil modified alkyds since glycerol phthalate has no definite composition as oil length is varied. Solubility, viscosity, application solids, apparent economy, actual economy, drying time, hardness, brushing ease, flow, sagging, grinding ease, initial gloss, gloss retention, color retention, can stability, and water impermeability are discussed as functions of percentage oil modification. The difference betw-een theory and fact in interpreting the ratio of moles of fatty acids against moles of phthalic anhydride to glycerol is presented together with a similar discussion on cross linking, as these same reactive ingredients are varied. The major dibasic acids and polyhydric alcohols in use today are discussed briefly. A method of blending alkyds to cover wide viscosity and nonvolatile ranges is presented. Evidence is shown to indicate that the logarithm of t h e viscosity of alkyd resin solutions increases directly with time during esterification unless the formulations or ingredients are such as to promote cross linking, whereupon an increased slope or curving is observed. The similarity of the functions in the observed plots with kinetic considerations involving order of reactions is shown.
B
the huge demand for alkyds which was ci catcd during World War I1 has continued unabated. Table I gives the latest available data on alkyd resins to emphasize the importance of the oil modified types as compared t o the total volume of alkyds produced In 1947, oil modified alkyds constituted approximately 70°& of the alkyds produced, whereas the total alkyd production amounted to approximately 60% of the resins and plastics used for protective coating purpose"
T ~ H LI.E ENDUSE AND PRODUCTION DATAI-OR R E ~ I N S ~ L ~ Y C F A C T CDURIW R E D 1947" End Use Protective coatings Molding a n d cnatiiig -ill other iises Total
E X DUSE D a i v - . l r , ~ PLASTICS Thousands of l'ounds
4.i3,00B 312,737 485,956
1,251,699 (Benzenoid (\-onbenz:enoid
737,714) 513,985) 1.231.699
Iturin iimiiified 76,862 With phthalic anhydride With pther riolybasic acids Oil modified and polyester types 207,202 Pure phthalic anhydride types P.A.b-glycerol P..4.-uentaervthritol P.A.-glyccroi-pentaer~,~hritol P.A.-other alcohols eic P.X. plus other dibasic aoids Ot,lier dibasic acids types and polyesters
ECXUSE oil modified alkyds constitute the iuain volume ot
alkyds in production today, the main theme of this paper will be centered on these important types. While the reaction of glycerol and phthalic anhydride was reported in 1901 (ad),i t remained for Arsern ( 2 ) and Howell (13) to divulge modification with oleic acid and castor oil, respectively, via patents in 1914. Although these were not the only patents obtained in thew early days, the high coat of phthalic anhydride created no incentive for practical development. FThen the price was lowered in the late 1920's, there was a sudden flurry of research and patents, especially in the drying oil modified alkyd field. For all practical purposes, the manufacture of alkyds of t h a t type was thrown open t o free enterprise when Patent 1,893,873 ( I S ) was declared invalid on ,June 12, 1835 ( I ) . The rest is history and
Grand totiti a
b
29,145 46,717 170,(501
24,374
134,324 12.292 7;208 16,777
12,227
283,064
Based on U. S. Tariff Cuminisrion figiirrv (2~5). P.A. = pbthalio anhydride.
O i l ,Modificuzztion TerrninoLlogly I n the early days of drying oil modified alkyds, the problem o f nomenclature or terminology with respect to oil modification c~ontinuallyarose. It was natural of 'course for the practical varnish man'to try to use the nomenclature which is still used for oleoresinous varnishes in order to get his bearings for evaluation purposes.
116
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
April 1949
Figure 1shows that a curve can be constructed which will theoretically allow conversion from one language to the other. It will become apparent, however, t h a t oil modified alkyds must be treated a s new compositions of matter. Some of the early failures of the varnish nomenclature when applied t o alkyds resulted because alkyds had the following properties as compared to varnishes of equal oil length: Alkyds brushed much harder; had higher viscosities or lower solubilities ( ~ o m required e aromatic solvents, Figure 1); did not dry as hard; and short oil alkyds gave early indications of drying fast, by the solvent release, but remained thermoplastic for some time. These factors plus the inability of alkyds to stand the water and alkali resistance tests commonly employed for the evaluation of varnishes no doubt militated against the gallons-oil-length terminology becoming attached to oil modified alkyds. The complexity of standardizing nomenclature might be illustrated as follows: Assume t h a t a mole of glycerol (92 grams with three reactive groups) is to be reacted with a mole of phthalic anhydride (148 grams with two reactive groups) and further modified with a mole of drying oil fatty acids (280 grams with one reactive group for esterification purposes). The formula would then look as follows: Grams Glycerol 92 Phthalic anhydride 148 Fatty adds 280
520
Grams c 520 17.70 28.45 53.85 a 56.2 Triglyceride
__
(1)
100.00 Unesterified resin ingredients
(2)
However, the final (esterified) alkyd will assay over 60% oil modification or oil length by the definition to be proposed later. Therefore, a correction should be made for loss of esterification water by subtracting 36 and dividing Formula 1 through again by 484: Glycerol Phthalic anhydride F a t t v acids
Parts 19 00 30.60 2 57.84 =.=
7% 3 9 . 5 Glycerol phthalate
60.5 Triglyceride - 107.44
100 0 Resin
(3)
Here the esterification water is tlhe number of parts exceeding 100. Assuming, however, the reverse order: Parts
T o make 39.5 units of glycerol phthalate will require T o make 60 5 units of oil will require
12.66 30.60 67.84 6.34
__
Glycerol Phthalic anhydride F a t t y acids Glvcerol
107.44
(4)
T h e result is the same formula as 3. If, however, the alkyd is made using oil instead of fatty acids the formula would be simplified to: Glycerol Phthalic anhydride Oil
Parts 12.66 30.60 60.50
103.76
(5)
Assuming the alkyd had been prepared in a perfect reactort h a t is, there were no losses except complete removal of esterification, water-and 100 grams were analyzed for the individual components, which were tjhen totaled, the result, would be: Parts 19.00 Glycerol 30.60 Phthalic anhydride % 57.84 Fatty acids o 60.50 Triglyceride
107.44
This totals the same as Formula 3 and if the fatty acids are converted to triglyceride, the result again is 60.5% oil modification. On analysis, the amount exceeding 100 is equivalent t o the esterification water which has been reintroduced by hydrolysis.
717
PERCENT OIL
Figure 1. Oil Length us. Per Cent Oil as Applied to Varnishes and Alkyd Resins
It is suggested that the fatty acids contained in 100 parts of theoretically completely esterified alkyd be calculated t o triglyceride and used t o define in per cent the oil length or oil modification of a n alkyd; this will eliminate the tendency t o define oil modification in terms of fatty acids. If oil is the starting ingredient, no calculations are necessary by the method indicated, and it is apparent t h a t analytical data on the fatty acid content of alkyds is easily convertible t o oil length.
Theoretical Oil Modified Alkyd Structures While the above definition of oil length for alkyds is advocated, they really contain little free oil unless added mechanically after resin formation, or unless the reaction is controlled in such a way as t o leave free oil molecules. With the theoretical resin previously discussed which contained a mole each of glycerol, phthalic anhydride, and fatty acids, if the phthalic anhydride is assumed to react only on alpha hydroxyl groups, while the fatty acids react with the beta hydroxyl groups, a linear type of polymer like that shown in Formula A of Figure 2 results. The size of n is unknown and most likely varies for the polymer units formed so t h a t actually there is a molecular weight distribution in the form of a probability curve. Of course the assumption made makes a pretty picture when drawn on paper but some of the fatty acids may be attached t o alpha hydroxyls and some phthalic anhydride to beta hydroxyls, not to mention the fact t h a t phthalic anhydride could also cause branching and cross linking. Formula B of Figure 2 is shorthand for the reaction product of 1 mole each of glycerol, phthalic anhydride, and fatty acids and is not t o be interpreted as a structural formula. Another formulation which can be drawn on paper and discussed with a reasonable amount of assurance is t h a t shown in Figure 3. I n this case 2 moles of glycerol are theoretically reacted with 1 mole of phthalic anhydride and 4 moles of drying oil fatty acids. A picture of what is believed t o be the reaction product is shown in Formula A, and another probability is shown in Formula B. The latter is a mixture of the resin shown in A of Figure 2 plus a triglyceride and is not actually as drawn using the shorthand formula shown in 2-B. While the mixture shown in Figure 3-B is a distinct possibility, various experiments, involving the evaluation of blends of oils with shorter oil alkyds against the cooking of long oil alkyds equivalent t o the blends, leads the writer t o believe t h a t the configuration shown in Figure 3-A is closer to the true state of affairs. When mole ratios other than those shown are employed, a suitable guide, such as oil length, becomes necessary t o correlate modification and it will be helpful t o convert the components t o triglyceride and glycerol phthalate as previously described.
718
INDUSTRIAL AND ENGINEERING CHEMISTRY R C 0
HO-C€I~-
A.
n c
R
c
E
0 0 I
H-CH200C
0 0
COO-
H-CHB-OOC
c
0 0
c
0 0
0 0
It Moles 1 1 1
HzO of esterification
Parts 19.00 30.60 57 84
520 -36
107.44 -7.44
484
100.00
-
Yield Figure 2.
Molecular Grams 92 148 280
-
COOH
c
B. Materia Glycerol Phthalic anhydride Fatty acids
Vol. 41, No. 4
= Fatty acid radical
39.5% Glycerol phthalate 60 5% Oil (triglyceride)
Theoretical Structural and Compositional Kelation for an Alkyd Formulated with Mole Ratio of 1:l:l Glycerol t o Phthalic Anhydride to Fatty Acids
.issume that 6 nioles of glycerol are to be reacted with 8 moles of phthalic anhydride and 2 moles of fatty acids. I n terms of reactive positions for esterification there are 18 from the glycerol, 16 from the phthalic anhydride, and 2 from the fatty acids: this gives a mixture which theoretically could be esterified to a zero acid number. However, if this nere cooked i t would gel long before a low acid number could be obtained and then there would be the problem of whether to leave it at the high acid number or to formulate it to a lower acid number. Suppose the esterification of the alkyd shown in Figure 4-I3 was stopped in order to retain the phthalic anhydride half ester groups, since their esterification would cause cross linking and gelation if the reaction were continued to completion. This resin would have a high acid number. Next, suppose that 2 moles of phthalic anhydride are eliminated and, therefore, the alkyd is formulated with 6 moles of glycerol, A moles of phthalic an-
hydride, and 2 mo1t.s of fatty acida. This is essentially the resin depicted in Figure 4 - 1 and when cooked it would be found that the acid number would come down to around 6 to 12 and analysis would show that the oil length was somewhere around 31.59f0. If the glycerol used in calculating the fatty acids to triglyceride were subtracted from the total glycerol and the ratio of phthalic anhydride to the remaining glycerol Rere calculated, the result would be a ratio of about 1.1 to 1. This figure closely approximates that found when analyzing low acid number commercial alkyds of this oil length. In a recent and comprehensive paper on alkyds by Wiclts (267, the subject of functionality and mole ratios of fatty and dibasic acids to polyhydric alcohols (including pentaerythritols) waa treated. Two of the curves (minus pentaerythritol data) are included here (Figures 5 and 6) for the sake of discussion. In Figure 5 the moles of dibasic acid per mole of glycerol are plotted against the moles of fatty acids necessary to completely esterify the hydroxyl groups of the glycerol when less than 1.5 R K K H moles of dibasic acid are used. In Figure 6, the moles of fatty acids are expressed as percentages of fatty acids and it should be C C c c 0 0 0 0 emphasized that the method of treatment involves the ingredients before reaction. While this method is useful and is recommended for visualizing the theoretical proportions of H2H-CHI-00C COO-CHsH- HI ingredients as the ratio of dibasic acid to total polyhydric alcohol is changing, it is only of academic interest since the acid value of 12 R R the resulting resins will increase from left to right due to the t l a c c c c c c inability to completely esterify before gelation. I n fact, the 0 0 0 resin from 1.5 moles dibasic acid prr mole of glycerol will gel at acid values above 100. CHz- H-CHg Tn England, Lynas-Gray (17, 18) recently m o t e two articles k z - L J H 2 on oil modified alkyd resins. Molecular In the first article he went Parts to considerable length, from a Material Moles Grams Glycerol 2 184 13 50 mathematical viewpoint, to ex1 148 10 86 14% Glycerol phthalate plain oil modified alkyd resins Phthalic anhydride Fatty acids 4 __ 1120 82 24 86% Oil (triglyceride) and then reduced the calculations to a graphical method 1452 106.60 HSO of esterification - 90 -6 60 of formulating which handled . ___ Lhe variables glycerol, phthalic Yield 1362 loo. 00 anhydride, and fatty acid by R = F a t t y acid radical plottingon tri-coordinate paper. In the second the strutFigure 3. Theoretical Structural and Compositional Relation for an Alkyd Formulated ture and oil length of alkyds with Mole Ratio of 2:1:4 Glycerol to Phthalic Anhydride to Fatty Acids
xx
8 :
+
7 ," 7
719
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1949
R C
R C
H
H
R = Fatty acid radical Figure 4. Theoretical Structural Relation for Alkyds of Low Oil Content with Varying Phthalic Anhydride to Glycerol Mole Ratios
was treated from a functionality and cross-linking viewpoint and a graph was developed to depict degree of cross linking against oil length (Figure 7 ) . Once again it must be pointed out that, while graphs of this type do an excellent job of depicting the theoretical ideal conditibns, the degree of esterification, as indicated by acidity before gelation when processing such alkyds, points towards much lower degrees of cross linking-for example, since the zero oil lengt,h alkyd will gel a t an acid number of over 100, the degree of cross linking will fall far short of one in practice. It should be evident, therefore, that practical alkyds cannot be formulated based on concepts fashioned around theoretical proportions. The method of calculating fatty acids to triglyceride, and then concentrating attention on the mole ratios of phthalic anhydride to glycerol for the "remainder" of the resin as oil length changed, produced more practical results.
Therefore, since glycerol phthalate has an indefinite and varying composition from a mole ratio standpoint as oil modification changes, an arbitrary oil-length definition, such as the one previously suggested, must be adopted.
Fullacw of Phthalic Anh dride Content and Durubil& During World War I1 great emphasis was put on the phthalic anhydride content of alkyds, especially those appearing in government specifications. However, phthalic anhydride content does not guarantee a resin which is infallible with respect to durability and certain other important characteristics. The preparation of alkyd resins is much more complex than has been pointed out here and a few examples will illustrate what could happen a t equal phthalic anhydride content. Even though an analyst might indicate that two resins were identical in so far as their composition of phthalic anhydride, polyhydric. alcohol, and fatty acid content is concerned, the properties, which would be evident on using the resins, could be far apart. For instance, suppose an alkyd of around 55% oil modification was blended with sufficient oil to make a total of 6570 when analyzed. Then assume that an alkyd which contained 65y0 oil modification w m cooked. If these two alkyds were put through preliminary drying tests, little if any difference would be found with the possible exception of a slightly tackier film for the alkyd which
MOLES OF DlE4siC ACID PER MOLE OF GLYCEROL
Figure 5. Theoretical Variation of Moles of Phthalic Anhydride and Fatty Acids Necessary t o Satisfy 1 Mole of Glycerol
Analysis of all the alkyds on the market would show that there is no simple relation between phthalic anhydride content and glycerol phthalate. This is another way of saying that the mole ratio of phthalic anhydride to glycerol (excluding that used for triglyceride calculation) changes according to oil length. Other ingredients, which might be added to modify and give the particular alkyd properties other than that found in straight oil modified glycerol-phthalate resins, further complicate the picture and do not allow generalization on a factor with which to multiply phthalic anhydride to convert it to glycerol phthalate.
I
, 1v2
I I
I I/>
MOLES OF PHTHALIC ANHYRIDE PER M7LE OF RYCEROL
Figure 6. Percentage of Fatty Acids i n Theoretical Mixtures of Fatty Acids and Phthalic Anhydride per Mole of Glycerol as Shown i n Figure 5
720
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
DEGREE OF CROSS LINKING Figure 7. Effect of Oil Modification on Theoretical Degree of Cross Linking Occurring in Glycerol and Pentner: .$+olPhthalate iilkyds
had oil blended into it. However, Lhis alkyd-oil blend, 011brushing, would be found to slip easier under the brush and the formulator would no doubt wish to use it in preference to the alkyd which was cooked with all the oil present. Now suppose these two alkyds were pigmented and put out on an exposure fence. I n a short period of time it would be evident that the first alkyd was inferior in gloss retention to the second; yet, both had the same phthalic anhydride content. Phthalic anhydride content can be misleading also because the ratio of phthalic anhydride to the polyhydric alcohols used is a n important consideration. If, a t any particular oil length, acid or hydroxyl groups are left unreacted in higher than optimum quantities, the alkyd will suffer in one or more important respects. Therefore, only alkyds developed by reliable companies and thoroughly tested as to durability and other important properties of the alkyd type should be used. Oils All other factors being equal, the oils used in preparing modified alkyd resins are important and may be deciding components. For example, if an alkyd with moderate drying speed but good color retention is desired, the standards of the industry today are soybean oil modified types, although other oils such as sunflower and walnut seed may be used practically interchangeably. However, where fairly fast drying is wanted and color retention is secondary, linseed oil modified alkyds are the standard. Other oils which are used for special properties are castor, dehydrated castor, tung, fish, and coconut. To understand oil modified alkyds, i t is necessary t o understand the oils and the reactions of drying or, if they do not dry, what polar properties in the oil can be useful in a particular coating composition. There is not as much difference in drying rates between soybean modified alkyds and linseed modified alkyds as would be expected based on behavior of the oils alone. The reason for this phenomenon is that when oxidation of an oil modified alkyd starts, the fatty acids attach themselves to one another by a mechanism still not clearly understood; but the fact remains that the fatty acid modified alkyd resin molecules are of sufficiently high degree of polymerization that a small amount of cross linking by way of oxidation of the fatty acid groups causes a film to set to a gellike structure and makes i t appear dry long before the final oxidation reactions have been completed. Thus only a small amount of cross linking by soybean or linseed fatty acids is necessary to cause incipient gellation of a film. I n general, however, it has been found that the higher iodine number oils or fatty acids dry faster, and give harder films, higher gloss, freedom from bloom, and high compatibility.
Vd. 41, No. 4
In the case of alkyds which are modified with nondrying oils the key to the situation seems to be polarity, exemplified by castoi. oil, or the length of the fatty acid chains, exemplified by the acids in coconut oil: Figure 8 shows that, in general, alkyds above 50% oil, modification are soluble in mineral spirits while those below 50% oil modification need aromatic solvents, such as xylene. Stated. in other words, the solubility of alkyds in any particular solvent increases with the oil length and the related property, viscosity, decreases with oil length. Three other important variables which depend on oil modiiication are application solids, apparent economy, and actual economy. At any constant viscosity in the aromatic or minerd spirit soluble range of alkyd resins the application solids wili increase with oil length while the appment economy will be proportional t o the decrease in oil length because the percentage solids a t application will be lower. IIowever, actual economy is proportional to oil length since less solvent is necessary to produce a film of constant thickness (of course these statements are predicated on alkyd constituents cost,ing approximately the same per pound). With respect to drying time, i t has been said earlier that the drying of oil modified alkyds is dependent on the chemistry of drying oils-that is, the chemistry involved when the fatty acids of these oils are oxidized. Those with experience hsve observed that drying time increases with oil length when starting a t a.bout 50% oil modification--that is, in the mineral spirits soluble range. However, below this oil modification, it may be surprising b find indications of increase in drying time as the oil length is decreased. As the oil length of an alkyd decreases, the fatty acid moleculm become less in number and spatially farther apart. This means that their chances of causing cross linking through oxidation become less and less. At the same time, however, solvent release becomes more rapid as the oil length is decreased and tende to give the impression of higher speed drying. This leads to some confusion as to the meaning of dry in connection with short oil modified alkyds. Possibly this point can best be clarified by pointing out that while shorter oil length alkyds give th? impression of being dry, they are actually thermoplastic, in t,erms of print tests, until
yoOil Modification 30
40 Aromatic (e.g., xylol)
50
60 70 Aliphatic (e.g., mineral spirit)
Solvent
-6-----------3.
Propertiest1
z
Viscosity Solubility Application solids* Apparent economyb Actual economyb Drying time Hardness Brushing ease Flow Sagging Grinding ease Initial gloss Gloss retention Color retention Exterior durability Can stability Water impermeability
Properties increase in directions shown by arrows; broken arrows are used when solvent has bearing on property involved. I, -4t constant viscosity. Figure 8. Effect of Varying Oil iModification with Respect to Various Physical Properties of Drying Oil Modified Alkyds
‘April 1949
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
721
will fail by cracking, checking, peeling, or other means which indicate a loss of flexibility or adhesion. Above 50% oil modification, the durability will tend t o fall off because the glycerol phthalate content, which is known t o be the vital part of the film with respect t o durability, decreases in direct proportion t o the oil length. However, if a specific surface like bonderized steel is involved, and the proper primer is used, optimum durability can be shifted below 50% oil length. Can stability of course depends on a lot of factors such as type of oil, how far the resin has been polymerized, what type of functional ingredients are present, etc. However, to generalize, and with all other variables being equal, can stability is proportional to the oil length and, although hard to 45 50 55 60 B5 show graphically, solvent can play a n important part. PERCENT OIL MODIFICATION Although there are other marginal propcrties Figure 9. Oil Modification us. Two Stages i n the Drying of Soybean which might be discussed in terms of oil length, the and Linseed Oil Modified Alkyd Resins last to be considered here is the subject of water impermeability. Arrows in Figure 8 indicate oxidation of the oil modification causes sufficient cross linking to that the water impermeability reaches a maximum a t 50% oil effect a rigid gel structure. It has been found t h a t a convenient modification. D a t a on this particular subject was found in a means of identifying the thermoplastic state is by scratching a patent recently issued to Sloane and Patterson ($3) and is given film with a knife or spatula: a clean, nonjagged, edge of the in Table 11. scratched film (sometimes called cheesiness) indicates insufficient Polgfunctionat AcMs oxidation to cause gelation. When the threshold of gelation is reached, it will then be observed that the sides of the grooves in Table I11 (column 1) gives some difunctional, dibasic acids the scratched film become slightly jagged. This observation and anhydrides. Of course phthalic anhydride reigns supreme has led the author to coin the phrase polymerization time to in the field of oil modified alkyd resins which are used alone for cover t h e time elapsed until the appearance of the jagged edges. film forming purposes by air drying or baking. The next four, Therefore, to graphically depict this state of affairs the setsuccinic, adipic, azelaic, and sebacic acids, are useful in the to-touch-time and polymerization time for a series of oil modified formulation of plasticizers, especially when used in conjunction alkyds employing linseed and soybean oil has been plotted (Figure with glycols. The next three anhydrides which, respectively, are the maleic adducts of butadiene, isoprene, and piperylene are 9). Returning to Figure 8, it may be said that, in general, the not so well known but were recently discussed by Cosgrove hardness of a n alkyd resin film, after i t has dried at constant film and the author ( 7 ) as modifiers for linseed oil in comparison with thickness] is proportional t o the amount of glycerol phthalate maleic anhydride and fumaric acid. and therefore inversely proportional to the amount of oil modiTable I11 (column 2) shows three trifunctional dibasic acids or fication. anhydrides-name!y, maleic anhydride, fumaric acid, and the Four other properties which can be treated together are brushmaleic adduct of cyclopentadiene. I n view of their higher ing ease, flow, sagging, and grinding ease. All of these properties functionality these particular acids and anhydrides can be used are dependent to a high degree on the rheological condition of the only in limited quantities in conventional oil modified alkyd alkyd resin. Brushing becomes easier as the oil modification is resins but maleic anhydride and fumaric acid were used in rather increased and of course as the degree of polymerization or the sizes large volume during the war to improve the properties of drying of the alkyd resin molecules decrease. Flow and sagging are in oils (4, 6, $2). I n a recent paper by Cosgrove and the author the same category-that is, in general they increase with per( 7 ) data were presented to indicate t h a t the maleic adduct of centage oil modification. There is no doubt t h a t the rheological cyclopentadiene was more than difunctional although the mechaproperties can be altered at a given oil length but the author nism of how this particular adduct reacts with drying oils is not knows of no systematic study on this subject which has been clear, published even though such a study would be of fundamental Table IV depicts the relation between citric acid, aconitic acid, importance. While grinding ease is proportional to the oil itaconic anhydride, citraconic anhydride, and mesaconic acid. modification, due to the fact t h a t the polymers get smaller and Since none of these acids are used commercially in large quantismaller as the oil modification increases and the fatty acid groups act as wetting agents on the pigments involved, there are many other contributing factors. TABLE JI. IMPERMEABILITY OF AIXYDS RESINFILMS TO WATER Both initial gloss and gloss retention arc, in general, propor($8) tional t o the glycerol phthalate content (for the same type oil) Impermeability5 to Water and are indicated by the arrows pointing t o the left in Figure 8. (0,005-Inch Film a t 3 5 O C. Composition after 6-Mo. Outdoor Exposure) When using the same oil for modification] the color retention 35 Linseed oil modified alkyd resin 4.2 increases with the glycerol phthalate content. 4 0 7 Linseed ’ oil modified alkyd resin 8.4 50% Ljnseed oil mod/fied alkyd resin 17.0 Exterior durability is a subject on which it is hard to generalize 5 0 % Linseed oil modified alkyd resin 13.4 from a n oil modification viewpoint but if application of all 62% Linseed oil modified alkyd resin 6.8 80% Linseed oil modified alkyd resin 1 .Q types of enamels t o all types of surfaces is considered simul1.2 Linseed oil, heat bodied Linseed oil, air blown 0.8 taneously, it can be said that the exterior durability reaches a Chinawood oil, heat bodied 2.8 maximum at around 5001, oil modification; this statement can be a Impermeability values were expressed in terms of the number of hours further qualified by saying that below this oil content, if an alkyd required f o r 1 square om. of film to transmit 1 mg. of water. is put on wood or over old oil or varnish type finishes, the film
INDUSTRIAL A N D ENGINEERING CHEMISTRY
722
TABLE
111.
SONE
Vol. 41, No. 4
DIBASIC ACIDSAND ANHYDRIDES
Difunctional
Trifunctional
CHCO
Maleic anhvdride
Phthalic anhydride
CHCO
Succinic acid Adipic Azelaic Sebacic
HOOC-(CH,),COOH HOOC-( CH2)rCOOH
Fumaric acid
HOOCCH
I/
HOOC-( CHz) GOOH EIOOC-(CHz) qCOOH
Maleic adduct of butadiene (1 2 3 6tetrahydrophthalic anhydrid;) ' '
CH-COOH Maleic adduct of cyclopentadiene (endo-czs-1,2,3,6-tetrahydro-3,6endomethylene-phthalic anhydride) (6)
co
Maleic adduct of isoprene (1,2,3,6tetrahydro 4-methylphthalic anhydride) Xlaleic adduct of piperylene (1,2,3,6tetrahydro 3-methylphthalic anhydride) CHI
Citric acid
+
CH2COOH 1
- HzO
TABLE Iv. CITRIC&ID Aconitic acid CEI-COO13 I' ( - C02) C-COOH ( - Hz0)
DERIVaTIVES
Citraconic anhydride CHB
I
AH,COOH
RIesaconica acid
CHI I
KOH -f C-coo1-i H,O II HOOC-CH
6-CO
C-COOH I
c--2
6HzCOOH Itaconic acid Ties for alkyds, the relation between them is currently of academic interest but eventually may become of practical interest provided these compounds bccomc products of commerce.
PoilglLydric AlcohoC~ .I polyhydric alcohol is defined ( 1 1 ) as one containing inorr than two hydroxyl groups. The glycols have been included by many people in their discussions of alkyd resins because ethylene glycol with dibasic acids forms linear polymers. Ethylene glycol and diethylene glycol have been most available unt,il recently. Propylene and dipropylene glycol are now articles 01 commerce and the rapidly moving chemical scene no doubt will feature other dihydric alcohols in quantity in t,he near future. Glycerol is the workhorse in the alkyd indust,ry of today and is used in the largest amount. Much research using penherythritol and sorbitol has been carried out and some alkyds of commerce now cont,ain these polyfunctional alcohols. A t this time it would seem that pent,aerythritol will be used rriosbly in long oil modified alkyds and that its four primary alcohol groups will stand it in good stead in making more impermeable type films which dry more rapidly than alkyds formulated from glycerol. On the other hand, its high functionality will no doubt lead to problems concerning viscosity, gelation, and skinning unless the alkyds are properly formulated; there is
also the quest,ion of durability at equal viscosity since the oil length must be increased and the phthalic anhydride must, be decreased when pentaerythritol is used in place of glycerol. Sorbitol theoretically has a functionality of 6, but practically, it has been found to have little more functionality than 4, due to inner ether formation. While there is considerable interest in the use of sorbitol in alkyd resins, much still remains to be done with respect to a final evaluation of it in comparison with glyoerol and pentaerythritol. Due to the German economy, trimethylol ethane and trimethyl01 propane were used extensively in Europe prior to and during the war. These polyhydric alcohols no doubt will receive attention in this country aud, after evaluation, will most likely find some place in the alkyd industry of the future.
Basic Reactions Of the basic reactions involved in the preparation of oil niodified alkyds,thc major ones a,re alcoholysis (19) and esterification. Alcoholysis is basically a reaction involving an alcohol and an ester containing a different alcohol. If a monohydric alcohol of one variet'y is present in excess, and the alcohol in the ester is more volatile than the first, a complete change from one type of an ester to another is theoretically possible; this is commonly called enter interchange. However, in alkyd resins t'he reaction
April 1949
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 A. 100
8.
40 30
0
723
Comporition o l Blends 80 66 40
PO
0
PO
80
100
40
60
n ~
A
I\
Figure 10. Deviation of Log Viscosity from Straight-Line Relation with Time when Cross Linking Occurs or Extra Functionality Becomes Effective while Cooking an Alkyd Resin usually involves an alcohol and an ester of the same alcohol. Since the alcohol is polyfunctional, the reaction is more a matter of obtaining equilibrium between various possible reaction products than true alcoholysis, as earlier defined. For example, when glycerol is reacted with a drying oil, under conditions of heat and catalyst, the fatty acids from the triglyceride tend to shift to the glycerol which has no fatty acids. However, the reaction is more complex than first appears as glycerol plus oil can be converted to diglyceride or monoglyceride, and equilibrium concentrations depend on many factors, including the concentration of the first reactants, temperature, and time. With respect to esterification and the preparation of oil modified alkyds, two general methods are usually employed: the esterification of a mixture consisting of polyhydric alcohol, dibasic acid, and fatty acids; or, if an oil has been previously alcoholized with a polyhydric alcohol, the esterification of the alcoholysis products with a dibasic acid. Many fundamental studies have been carried out and papers published on the reaction of glycerol with phthalic anhydride, phthalic acid, and other simple dibasic acids, but little has been published on the preparation of alkyds of the oil modified types except in the patent literature. One of the earliest investigations of the glycerol phthalate reaction was carried out by Kienle and Hovey (14). Later Savard and Diner ( 2 1 ) investigated the reaction when the mole ratios of glycerol and phthalic anhydride were varied over wide limits. Kienle, van der Muelen, and Petke studied the reaction of glycerol with phthalic anhydride in one case (16), and phthalic acid in another (26). By carefully controlling the reaction in terms of water evolved, curves were plotted showing water formed against time, acid value against time, and other important variables. Their results indicate some acid formation in the early stages of esterification with some anhydride formation in the later stages. Brasseur and Champetier (3) wrote an article on the reaction of phthalic anhydride with glycerol and used the results of Savard and Diner (df) to expound a theory that, in addition to the phthalic anhydrideglycerol reaction which forms half esters, a secondary reaction occurs, in the presence of an excess of phthalic anhydride, whereby the newly formed half esters link through their hydroxyl groups. Water evolved as a by-product of this linkage in turn hydrolyzes the excess phthalic anhydride to phthalic acid. Proceeding to oil modified alkyds still another variable comes into the picture-namely, fatty acids. Therefore, i t is almost
impossible to study this reaction in terms of the proportions of half esters, phthalic acid, phthalic anhydride, or fatty acids present a t any particular moment. Recently, Goldsmith (IO) discussed the relative reactivity of phthalic anhydride, phthalic half esters, and fatty acids with the alpha and beta hydroxyl groups of glycerol while preparing oil modified alkyds. It has been found, after plotting the various physical characteristics of drying oil modified alkyds, that the reciprocal of the viscosity varies directly with the acid value. This is just another way of saying that the acid value comes down slowly as the viscosity approache6 infinity, although the acid value a t infinite viscosity cannot be obtained directly. An extrapolation of the curve indicates the acid value which a particular alkyd would have had, had the reaction continued t o gelation or to infinite viscosity. Wiederhorn (27) took these facts and evolved a mechanistic interpretation of how oil modified alkyd resin formation might occur. The data which he presented, plus the discussion on how to use the equations which he derived, are helpful in understanding the reaction and also in formulating alkyd resins which are much closer to perfection. Under certain conditions during preparation, the log viscosity of a resin solution may be plotted against time to give a straight line curve, assuming that esterification variables are held constant (Figure 10). An equation which would fit these facts is Log viscosity = mT
+b
where m is the slope of the line, T is time, and b is the intercept on the log axis. Under certain other conditions, the plot of the line is found to curve upwards from the straight-line relation as the viscosity of the resin being formed approaches gelation. In trying t o develop a theory as to what caused the upward curve in the plot of log-viscosity against time, the basic mathematics (9) in connection with order of reactions was examined. If the viscosity is assumed to be directly proportional to the algebraic expression for the initial concentrations a n d decrease of ingredients during regular chemical reactions, the similarity between the two types of equations is apparent (Table V) and explains
INDUSTRIAL AND ENGINEERING CHEMISTRY
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1'. T H E O R E T I C A L RELATIOSS I. Log-viscosity againbt time for reacting alkyd resills 11. Equations expressing the kinetic= involved in rear.t,ion o r d c ~ rrARLE
I. Log v i a c o s i t y = n f T + b
r
I i t n r d c r r(>nctioii
I
2nd order reaction
I
:3rd order r e a c t h i .
m = slope'
T = time b = a conetant
A . B , C = initial concentrations of reactants S = decrrase of each reactant after time, 7' K = velocity constant
the reason for a straight-line plot of log-viscosity against time. However, when the number of functional esterifying groups reacting increases, or if unsaturated bonds should increase functionality through polymerization, it is also easy to see t h a t logviscosity and the slope would change with respect to time while cooking a n alkyd. I n the preparation of oil modified alkyd resins, i t is the author's belief t h a t essentially linear molecules are being formed while the curve remains linear and of relatively low slope but t h a t a n increa,se in the slope indicates cross linking due to a higher order of functionality. Upward curving in the cases of resins which contain tung oil, maleic anhydride, and other materials capable of higher functionality has been observed. It has been determined also t h a t when extra functionalit'y is involved, the theoretical equations worked out by Wiederhorn (27) do not apply, as evidenced by the fact t h a t gelation will occur at higher acid numbers than theoretical. This is another way of saying t h a t the mechanistic interpretation was based on disappearance of acid groups to increase viscosity t'hrough simple linear polymers and t,hat,, if extra functionality through double bonds is involved, the theoretica.1relation will not hold. If an upward curve on a normally straight-line relation of log-viscosity against time occurs or if higher acid numbers are obtained a t gelation than called for by the Wiederhorn equation, one has qualitative evidence t h a t funct'iona.lities of higher order are coming into play.
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Chlorinated rubber has a rathcr linear type of molecule and alkyds with considerable proportions of fatty acid radjcals have been found to be the most compatible. I n ot,her wmds, alkyds from around 45% oil length and longer have proved, LT-hen dissolved in aromatic thinners, to be compatible. T i t h respect to the compatibility of allq-d resins with oleoresinous varnishes in general, it map be said that the longer the oil. length of the alkyd, the better tho compatibility, and that 50% oil modification is roughly the minimum. The extent of compatibility depends on the degree of polynierization of the alkyd as well &s the degree of polymerization of the varnish. Asphalt compositions are compatible with certain alkyd resins but the author has found that the shorter oil length alkyds are the most compatible in view of the ryclic groups obtained by the high phthalic anhydride content. Castor oil modified alkyds of the shorter oil lengths, however, are not compatible.
Blending of Alkgds For all practical application purposes, the viscosity of an alkyd resin solution varies logarithmically with the percentage solids (by weight) when thinned with LI given solvent. This fact may be employed in a graphical method for determining the amount of any two alkyds of different oil lengths t o blend to obtain viscosities equivalent t o those of intermediate oil length. Figure 11 shows a typical blend and reduction-viscosity chart. Reduction viscosity is used t o describe the viscosity of any alkyd resin solution at any solids selected below that of its initial concentration-for example, alkyd resin A in Figure 11 has a reduction viscosity of 2.3 poises a t 37.5Yc solids whereas resin H has a reduction viscosity of 2 poise3 at! 7,570 solids. This method
Composition of Blends
A.
I00 0
75 25
-_-
+
50 50 -
25
a
75
100
65
70
-
Coinpat ibilit g I n general, conipatibility of alkyd resins is proportional to the polarity of the molecules and inversely proportional t'o the degree of polymerization. I n nitrocellulose lacquers the compatibility of alkyds varies over a wide range because the ester groups are polar, and possibly the cyclic nature of the phthalic anhydride units is polar to the cyclic units of the nitrocellulose. The fatty acid modification acts as a diluent, except in special cases, but it is remarkable t,hat compatibility of alkyds with nitrocellulose extends up to rather high oil lengths, in fact, as high as 60% oil modification. Short chain modified alkyds-for example, those modified with coconut fatty acids-are found to be exceptionally well suited for use with nitrocellulose. Castor oil modified alkyds have hydroxyl groups which are polar in nature and help compatibility. While i t is not practical, the compatibility of oil modified alkyds could be increased by blowing with air at rather low temperatures t o build up keto and hydroxyl groups which, in view of their polarity, would practically put the alkyd resins in the class of solvents for nitrocellulose.
40
45
50
% Solids in
SS 60 Mineral Spirits
Figure 112. Regularity of Reduction Viscosity Curves of Two Alkyds and Their Blands A = 52 70 oil modified5 B = 58 70 oil modified
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
is best used when the blending alkyds contain the same solvent as the intermediate alkyd which i t is desired to duplicate. Figure 11 shows also the reduction-viscosity curves of four intermediate alkyds, C, D, E, and F. Now, having alkyd resin solutions A and B, whose reduction-viscosity curves fall on opposite sides of C, D, E, and F, a suitable number of A and B blends are made to obtain a smooth curve; the composition of the blends is depicted by the upper abscissa whereas per cent solids is indicated by the lower abscissa. If the blend curve is found not to cross one of the intermediate reduction-viscosity curves, the latter may be extrapolated as shown by the dotted line for C. All that needs to be done now is find the point where a n intermediate alkyd crosses the blend curve and go up vertically to the composition abscissa. I n this case A and B are approximately 50% and 70’% oil modified, respectively. Figure 12 shows a blend curve for two alkyds which are approximately 5201, and 58y0oil modified, respectively. I n this case the reduction-viscosity curve was included for each of the blends to emphasize the regularity of the straight lines obtained. C, D, and E happen to be three alkyds from three different manufacturers; the last of these falls outside the blend curve and therefore its viscosity characteristics cannot be duplicated exactly by blending. Some of the resin solutions-for example, E and F in Figure 11-have different slopes. I n this case, E has a cross-linking ingredient which inhibits the dispersion of the resinous molecules in the solvent; thus the slope of the reduction-viscosity curve is kept below t h a t of F as the per cent solids is decreased. Alkyd blend charts can be of great help in reducing the number of alkyds carried in inventory by the user. However, blends should not be expected to solve every problem involving duplication of alkyds having intermediate viscosities since successful use depends on the basic compositions of the alkyds t o be blended being similar to those to be duplicated. I n addition, blends of alkyds varying more than 10% in oil length are not recommended because of rapidly changing characteristics in even such a short span. It is suggested that, wherever possible, alkyds with oil lengths differing by 10% be blended-for example, blend 40 with 50, 50 with 60, 60 with 70, etc.
Stabilitg, Skinning, and Wetting Power I n general, i t may be said t h a t any alkyd which drys (polymerizes) fast tends t o skin, wet poorly, and be unstable. For example, if a n alkyd, made from a drying oil subject to easy cross linking, is p u t into solution prior to gelation, it is subject, on evaporation of the solvent (and even in solution), to gelation by the action of oxygen through a cross-linking mechanism. Stability of alkyds may be defined as their ability to be ground with pigments and to retain reasonably stable viscosity characteristics with age. Alkyds which have been brought close to the gelation point before being p u t into solution are easily affected by pigments which are surface active, such as carbon black (and even titanium dioxide) and, despite the fact t h a t the formulator may want exceptionally fast drying, it is sometimes necessary t o use some material in the grinding medium which will be adsorbed on the pigment much faster than the alkyd if stability is t o be maintained. Skinning is a result of oxidation, and/or polymerization. As was mentioned previously, a drying oil modified alkyd can become oxidized in solution and a small amount ofoxygen will sometimes cause skinning, especially if the alkyd was near the point of gelation when p u t in solution. I n general, the degree to which a n alkyd resin wets pigments is proportional to the oil modification. An additional factor is whether or not this oil niodification has been allowed to cross link while making the alkyd. From still another viewpoint, wetting ability is inversely proportionpl t o the degree of polymerization of the alkyd resin.
72%
Future of Oil Modtfied Alklyds While the development of new varieties of alkyds during t h e late twenties and the early thirties was rather rapid, the rate of progress now has slowed down awaiting developments in related fields. Although phthalic anhydride continues to be the most promising dibasic acid, other acids with functionality of 2 or more continue to be investigated in a more or less routine fashion. However, it would seem for many reasons (one being cost) t h a t n o other dibasic acids excepting maleic and fumaric have yet been found to be of real value. Glycerol continues as the favorite polyhydric alcohol, although pentaerythritol commands interest in some applications; sorbitol is still subject t o much research in order t o take advantage of i t s high functionality and minimize the dehydration of some of i t s hydroxyl groups. While the conventional vegetable oils will continue t o be used as modifiers for alkyds, the fatty acids obtained from the FischerTropsch synthesis will no doubt be a subject of investigation and, in view of their success in Germany, presumably will find their place in American alkyds of the future. Although oil modified alkyds no doubt will continue to be improved as knowledge of fatty acids continues t o expand, other approaches by way of employing unused double bonds, carboxyl groups, and hydroxyl groups for cross-linking purposes, after application of alkyd films, will get their share of attention. The use of diisocyanates with alkyds has been practiced abroad (8) and also has been patented in this country (90). Combinations of isocyanates with alkyds may open up new specialized fields of uses, especially in adhesives. I n conclusion i t is predicted t h a t oil modified alkyd resins will continue to be used in increasing volume for many years to come. The endless permutations and combinations possible with the chemicals of today and those of t h e future leave many variations still to be explored and evaluated.
Literature Cited Anon., District Court, E.D., New York, Equity 7192 (June 12, 1935): IND. ENG.CHEM.NEWSED., 13, 24, 314 (1935). Arsem, W. C., U. S. Patent 1,098,777 (June2, 1914). Brasseur, P., and Champetier, G., Bull. SOC.Chem., 57, 265 (1946). Butler, 1%’. H., U. S. Patent 2,397,240 (March 26, 1946). Chem. Rev., 31, 498 (1942). Clocker, E. T., U. 5 . Patent 2,188,882 to 2,188,890 (Jan. 30, 1940): 2,262,923 (Nov. 18, 1941); 2,286,486 (June 16, 1942). Cosgrove, C., and Earhart, K. A . , presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 112th Meeting of the A.C.S., New York, N. Y. DeBell, J. M., Goggin, T I T . C., and Gloor, W. E., “German Plastics Practice,” Springfield, Mass., DeBell and Richardson (1946). Glasstone, S., “Physical Chemistry,” 5th printing, New York, D. Van Nostrand (1943). Goldsmith, H. A., IND. ENG.CHEM.,40, 1205 (1948). Hackh’s Chemical Dictionarv. 3rd ed.. Philadelohia. . . Pa.. Blakiston Co. (1946). Howell, K. B., U. S. Patent 1,098,728 (June 2, 1914). Kienle, R. H., Ibid., 1,893,873 (Jan. 10, 1933). Kienlc, R. H., and Hovey, A . G.. J. A m . L’hem. Soc., 51, 509 (1929). Kienle, R. H., van der Meulen, P. A., and Petke, F. E., I b i d . , 61, 2258 (1939). Ibid., p. 2268. Lynas-Gray, J. I., P a i n t Technol., XI,No. 124, 129 (1946). Ibid., XII,No. 133, 7 (1947)Ott, Bernard, and Frick, U. S. Patent 2,044,747 (June 16, 1936) Rothrock, H. S., Ibid., 2,282,827 (May 12, 1942). Savard, J., and Diner, S., Bull. SOC.Chem., 51, 597 (I 932). Schwarcman, A., U. 8. Patents 2,412,176 and 177 (Dec. 3, 19461. Sloane, G. K., and Patterson, G . B., Ibid., 2,410,187 (Oct. 29, 1946). Smith, W., J. SOC.Chem. I n d . ( L o n d o n ) , 20, 1075 (1901). U. S. Tariff Commission, “Synthetic Organic Chemicals,” U S. Production and Sales, preliminary (1947). Wicks, Z . W., Interchem. Rev., 6 , No. 3, 63 (1947). Wiederhorn, N. A , , presented before the Division of Paint, Varnish, and Plastics Chemistry, at the 110th and 111th Meeting5 of the A.C.S., Chicago, Ill., and Atlantic City, N. J . , respectively. RECEIVED March 25, 1948. Presented (in part) as a p a r t of the Symposiuiii on Alkyd Resins before the Division of Paint, Varnish, and Plastics Chemistry a t the 113th Meeting of t h e AMERICAN CHEnr1c.u SOCIETY, Chicago, 111.
