INDUSTRIAL A N D ENGINEERING CHEMISTRY
86
ing alkyd resins. These data show that there was a marked reduction in hardness at‘ratios of 1 :3 and 1: 4 nitrocellulose to resin. Experience has shown that lacquer films with hardness values of about 10 or less have poor scuff resistance and are prone to collect dirt badly. Also the high resin ratios had poor water resistance. This is illustrated by both blush and softening effect. Ratios of 1: 1 and 1: 2 show very slight or slight temporary blush; the 1:3 ratio ranges from a medium temporary to a heavy permanent blush, depending upon the resin; the 1:4 ratio in every case showed a hedvy permanent blush. The lower 1: 1 and 1:2 ratios showed good resistance to softening, whereas the 1:3 and 1:4 ratios exhibited either medium or poor resistance to softening, depending on the reain. These data indicate that high-quality performance was maintained a t ratios up to 1:2 nitrocellulose to resin, but there was some sacrifice a t a 1:3 ratio and considerable sacrifice in performance when higher proportions of resin were used. Experience has shown that hot-spray application of nitrocollulose lacquers is possible a t temperatures as high as 70” C. without deteriorating the nitrocellulose in any way. At higher temperatures of 80-90: C., however, a definite degradation of the nitrocellulose has been found which is reflected in brittle films with poor durability. CONCLUSIONS
The data have demonstrated that nonvolatile content can be increased in nitrocellulose lacquers by using lower-viscosity nitrocelluloses and larger proportions of nonoxidizing alkyd resins with a more active solvent, or by applying the lacquers at an
Vol. 3’2,No. 1
elevated temperature up to 70” C. The gain in nonvolatile content contributed by each factor is cumulative, so that quite high solids are possible. T o maintain high-quality performance, the nitrocellulose used should be the 30-35 centipoise type or higher, and the proportion of nonoxidizing alkyd resin should not exceed a 1:2 ratio of nitrocellulose to resin. Table I1 indicates that it should be possible t o increase nonvolatile content from 17% in the 1:l ratio, using RS ‘/Z-second type with the standard solvent, to 29.5% in the 1:2 ratio, using 30-35 centipoise type with the high-solvency formula. This is an increase of 73.5% in nonvolatile content. Similarly, there was an increase from 17 to 38.5% when hot-spray application was employed. This amounted to an increase of 114% in nonvolatile content at spraying viscosity. Pigmentation would increase these gains somewhat, depending upon the pigment employed. LITERATURE CITED
Am. SOC.for Testing Materials, D445-37T, Tenhtive Methods of Test for Kinematic Viscosity. Doolittle, A. K., IND.ENQ.CHEM.,30, 189-203 (1938). Ericsson, R. L . , Am. Paint J . , 27, No.32, 40-52 (May 10, 1943). Gardner, H.A., “Physical and Chemical Examination of Paints. Varnishes, Lacquers and Colors”, 9th ed., p. 117, Washington. Inst. of Paint and Varnish Research, 1939. Koch, William, IND.ENG.CEEM.,36, 756-8 (1944). Lowell, J. H.(to D u Pont Co.), U.S. Patent 2,291,284 (July 28. 1942). (7) Ware, V. W., and Teeters, W. O., IND.ENG.CX~EY., 31, 1118-21 (1939). PRESENTED before the Division of Paint, Varnish, and Plsstics Chemistry a t the 108th Meeting of the AYERICAN CHEXICAL SOCIETY, New York, N. Y.
Polypengaerythritol Drying Oils Harry Burrell HEYDEN CHEMICAL CORPORATION, GARFIELD, N. J.
HEN the tung oil shortage developed as a result of the ?in+ Japanese war, the American paint and varnish industry converted available drying oils by a t least three processes to products which more closely resembled ‘‘wood oil”: (1) dehydration of castor oil; (2) increasing the effective unsaturation of the fatty acids by (a) fractionating by distillation or solvent extraction or (a) conjugating the double bonds by treatment with alkali; and (3).esterification of the unsaturated fatty acids by alcohols more polyhydric than glycerol. The present production of dehydrated castor oil is considerable, but the product requires careful preparation and formulation to avoid slow-drying soft films and aftertack. Increasing the unsaturation in the fatty acid components has been of limited helpfulness, and in many cases has been commercially successful only because such improved acids were esterified by higher polyhydric alcohols as described by Stingley (19) and Bradley and Richard-
son (6). The method of re-esterification has been a natural and logical development based on Carothers’ theory (IO) of functionality, especially as it was expanded and interpreted by Bradley (6). In practice, pentaerythritol has proved to be a useful polyalcohol. The ready acceptance by the trade of coating compositions containing pentaerythritol, augmented by other wartime demands, led to the production of this alcohol in tonnage quantities. The preparation of pentaerythritol (tetramethylolmethane) was reported inr1891 by Tollens and Wigand (60). It is a tetrahydric primary alcobol with a quaternary carbon atom. “Pentaerythritol” is an unfortunate misnomer since it is not accurately descriptive and is difficult to wonounce; nevertheless, the usage is firmly established and it seems best to retain it. The ether
polymers of pentaerythritol have been termed “polypentserythritols”. Drying-oil fatty acid esters of pentaerythritol were prepared over fourteen years ago by Krzikalla and Wolf (16) and later by I,.& Bruson (7) and Arvin (1). Drinberg and Blagonravova (?11, 19, 13) reported many data of theoretical value, including comparisons with esters of other alcohols, both more and less polyhydric; they also evaluated practical aspects. As tung oil replacements, the pentaerythritol esters of linseed and soybean acids are useful in many instances because drying time, water resistance, and weather resistance are satisfactory (8). A major difference between tung oil and the pentaerythritol drying oils lies in the slower bodying rates; on the other hand, one of the outstanding characteristics of the polypentaerythritol drying oils is the ability to approximate tung oil more closely in this respect (9). Pentaerythritol does not occur in nature but is manufactured by the alkaliie condensation of formaldehyde h l d acetaldehyde. The reaction occurs more or less stepwise with the preliminary aldol condensation to form pentaerythrose, followed by a Cannizarro-type reduction to the alcohol: 4CH20
+ CHlCHO
+
HOHzC
-
r
CHO
+ CHzO
-+
LH*OH CHzOH HOH2C--LC-H20H
I
CHpOH Pentaerythritol
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1945
&vera1 side reactions accompany the main reaction; they normally yield smaller amounts of related by-product alcohols. Among them are the ether polymers of pentaerythritol; two, dipentaerythritol (14)and tripentaerythritol (g), have been isolated and their structures proved to be:
YHnOH
YHsOH
YHsOH
CH~OH-&-CH~--Q--CH+~-CH+CH+~-CH~OH bH,OH
LH20H Tripentaerythritol
87
Polypentaerythritols are ether polymers of pentaerythritol. Dipentaerythritol and tripentaerythritol are of economic signiflcance. They are high-melting white solids, insoluble in water. The unsaturated fatty acid esters of the polypentaerythritols resemble tung oils in their ability to body rapidly, to form water- and alkaliresistant varnishes, and to withstand weathering. The esters are simply prepared by heating the fatty acids and alcohols above 230' C. with agitation. Esterification rates and bodying rates are given. The polypentaerythritol drying oils are compared with other synthetic and natural oils as to drying time, hardness, durability, and increase in yellowness.
bHIOH
At present these two are the polypentaerythritols of greatest economic significance. Technical dipentaerythritol of satisfactory purity has been manufactured, but tripentaerythritol occws in admixture with smaller amounts of dipentaerythritol and other related ether-polymers probably including a tetrapentaerythritol; such a mixture has been designated by the coined name "pleopentaerythritol". The functionality of pleopentaerythritol is not significantly different from that of tripentaerythritol. PREPARATlON O F ESTERS
The drying41 acid esters of the polypentaerythritols are rather simply prepared by heating together a mixture of the fatty acids and alcohol. However, because of the solid nature and the high melting point of these polyhydric alcohols (Table I), some difficulties are normally encountered in reacting them with those high-molecular-weight organic acids in which they are insoluble. In reacting pentaerythritol-type alcohols, it should be remembered that they are tetra-, hexa-, or even more polyhydric; as a consequence, some esters may cross link more easily than the corresponding glycerol esters and, therefore, convert more easily to gels. Pentaerythritol-type esters of highly unsaturated fatty acids are sometimes excessively polyfunctional, and should be prepared with due precautions t o guard against gelation. A detailed description of the methods for preparing pentaerythritol drying oils haa already been given (8). The same methods may be used with the polypentaerythritols if the above mentioned precautions are observed. ESTERIFICATION RATES. Figure 1 compares the esterification ratm of linseed fatty acids with three pentaerythritol-type alcohols and erythritol (straight-chain tetrahydric alcohol, CH,OHCHOHCHOHCH~OH). These data were determined by heating stoichiometric amounts of the acid and alcohols at 250' C. (482' F.) in an atmosphere of carbon dioxide with constant agitation. Samples were periodically removed, immediately cooled, and titrated with 0.1 N sodium hydroxide solution to determine the acid number; the latter values are plotted against time elapsed. There is no significant dwerence in reaca
HOURS Figure 1. Esterification of Linseed Oil Acids at 250" C. under CaGbon Dioxide 1. Erythritol
3. PentaerytMtol
2. Dipentaerythritol
4. Pleopentaerythritol
tion rate between pentaerythritol and dipentaerythritol, but pleopentaerythritol apparently reacts somewhat more rapidly. A considerable difference in reaction rate does exist between erythritol and the pentaerythritol-type alcohols as a group, the erythritol estefifying more slowly. This undoubtedly results from the secondary alcohol groups of erythritol. Esterification proceeds more rapidly if carried out in the presence of a hydrocarbon solvent as described in a previous publication (8). The rates for several polyhydric alcohols were determined by that method,ps Figures 2 and 3 show. The equation for esterification may be plabed in the form:
K
-
(U
- b)/tb
where K = reaction constant a = initial acid number b = acid number after t The reaction constants were calculated from several positions on the curves, and the average values are listed in Table 11. Table I1 also gives the temperature coefficients of reaction rates which may be expressed as:
TABLIO I. MELTINGPOINTS Alcohol Pentaerythritol urified Di entaerythrilo? purified
.
Trf)%entaerythritoii Tec pentaerythptol purified 46 Tech. pntaerythntol \43$ Tech. iipentaerythnto Pleopentaerythritol
gg]
Combining Weight
Cor. Melting Range, C.
34.0 42.4
261-264 219-222
46.6 86.7 39.6 43.6 60.0
242-248 166-226 180-200 215-221 230-240
Kt
where
Ko
reaction rate a t t o C. = reaction rate a t (t lo)' C. T1o = number of times reaction rate increases when temperature is raised 10' C. TIW number of times reaction rate increases when temperature is raised lOn' C,
+
5
-
+
88
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 37, No. 1
required to body from 2 to 15 poises at 310' C., referred to the natural glyceride as unity, are as follows: Pleopentaerythritol Dipentaerythritol Pentaer thritol GlyoeroP(natura1 oil) Trimethylolpropane
Figure 2. Esterification of Linseed Oil Acids at 200' C. 1. Pleopentaerythritol 2. Dipentaerythritol 3. Glycerol
4.
Pentaerythritol
5. Trimethylolpropane 6. Pentaerythritol Ca catalyst
+
The special form of esterification known as alcoholysis has been covered previously for pentaerythritol (8, 15, 18), and the conditions reported also pertain generally to the polypentaerythritols except that alcoholysis catalyzed by calcium naphthenate requires about three times as long as it does for.pentaerythritol-e.g., about 15 minutes at 230-250" C. (446-482" F.)which is still only one quarter of the time required for glycerol.
0.58
0.76 1.00 1.29
The type of fluidity exhibited by bodied polypentaerythritol drying oils is quite different from that of highly bodied linseed or tung oil. This may best be expressed by saying that much higher true-viscosity values may be obtained with the polypentaerythritol drying oils; that is, the numerical viscosities are higher. Bodied linseed or tung oils seem to be converted to the rubbery gel stage at lower measurable viscosities. Another manifestation of this behavior is that polypentaerythritol drying oils, bodied to the stage where they apparently are gels, will actually flow very slowly from an open container placed on its sjde, and will become quite fluid when heated. Apparently the same phenomenon was reported by Blagonravova and Laearev ( 4 ) in a paper unfamiliar to the paint and varnish industry because of its Russian origin. They studied the pentaerythritol linseed acids ester and state; "The strong heat reversibility of the synthetic esters, characteristic of highly polymeric liquids, is due to the fact that primary polymeric complexes in these substances are combined into secondary association complexes by physicaI (van der Waals) forces , .The processes of association play an important role in the formation of solid films (drying) of synthetic esters."
..
90
BODYING
w C 70
Von Mikusch (17) studied the bodying rates of linseed and dehydrated castor oils. By his procedures, viscosity-time curves were determined at 288' and 310' C. (550" and 590' F.) for the linseed acids esters of several pentaerythritol-type alcohols in Figure 4. It is apparent that the polypentaerythritol esters of linseed acids tend to approach tung oil in bodying characteristics. Since the slopes of the curves are indicative of the bodying rates, data were taken from Figure 4 to construct Figure 5 in which the time required to increase the viscosity from' 2 to 16 poises is plotted against temperature. Von Mikusch's data for linseed oil are also included for comparison. At any given temperature, the relative bodying rate is given by the intersection with the temperature ordinate, so that a comparison of bodying rates of the different esters may be made by noting the relative heights at which the curves cross a chosen temperature ordinate. The temperature coefficient of bodying is given by the slope of the curves. A convenient way to express the temperature coefficient is to note the increase in temperature necessary to halve the time required to body. This temperature increase is known as "the doubling interval' '(17). Expressed mathematically: Tg TI= doubling interval when M o = 0.5 M I , where M I = minutes required to body at temperature TI, and Mz = minutes required to body at temperature 2'. The doubling intervals calculated from Figure 5 follow:
52 50
ro
-
Pleopentaerythritol Dipentaer thritol Pe?taerytikitol Trimethylolpropane Glycerol (natural oil)
0.48
17.0a C. 14.2 13.6
16.1 13.0
Figure 5 shows that, in the temperature range studied, the relative order of increasing bodying rates is: trimethylolpropane, glycerol (natural glyceride), pentaerythritol, dipentaerythritol, pleopentaerythritol. Expressed another way, the relative times
0 u c 30 20
0
fo
80
bo
JOO
MINUTES
Figure 3.
Esterification of Linseed Oil Acids at 230' C.
1. Glycerol 3. Dipentaerythritol 2. Trimethylolpropane 4. Pentaerythritol 5. Pleopentasrythritol
TABLE 11. ESTERIFICATION CONSTANTS Km. c. Kz10. c.
Reacting Aloohol Pentaery thritol Dipentaerythritol Pleopentaerythritol Trimet hylolpropane Pentaerythritol Ca naphthenate
+
0.0099
0.0136
0.0044 0.0124 0.0226
0,0160 0.0165
0.0118
.. . ,
0,0195
T*o 1.15
1.08
1.39
1.16
..
FILM PROPERTIES
Table I11 compares polypentaerythritol drying-oil films with other polyhydric alcohol esters of the same fatty acids. All oils were bodied to 13 * 0.2 poises before testing. The outstanding fact presented in Table I11 is that the pleopentaerythritol linseed acid ester approaches tung oil closely, the drying time and durability b.eing nearly identical. It has the advantage over tung oil of being extremely rapid in bodyhg but not nearly so prone to gel. It is very color stable and is a satisfactory paint vehicle. The same characteristics are shared to a slightly lesser
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Idnparp, 1945
89
oil. This last condition arises from the rapid bodying rate which allows high viscosities to be obtained in a short time, thus reducing oxidative and pyrogenoua decomposition of the oil t o a minimum. Such highviscosity low-acid-number oils are especially needed in wartime paint formulation where current trends are to cut the oil content of paints. Another point in favor of these synthetic oils is that the rapid bodying results in appreciable savings in processing costs. As already noted, the present tung oil shortage can be somewhat mitigated by the use of pleopentaerythritol linseed acids ester; although government restrictions at present prevent the use of soybean oil in coatings, there is every indication that soybean oil will be cheap when peace returns, a t which time a superior, competitive drying oil could be prepared.
v)
w
2
B
MINUTES
Figurd 4. Bodying of Linseed Oil Acids Esters a t 288’ and 310’ C. 1. Pleopentasr thritol 2. Dipentasrytgritol
3. P e n t a a y t M t o l
4. Trimethylolpropans
TABU 111. PROPERTIES OF POLYPENTAERYTHRITOL DRYINGOIL FILMS Tack-Free Drying Timeo
So bean acid esterp hopentaer thntol Dipentaar txritol Pentaerytiritol Natural ycerides Linseef oil Tung oil
% Yellowness Inoreages Light Dark
3 hr. 5 hr. 15 hr. 32 hr. 16 days 7 days
2.9 14.2 4.7
.. .. ..
4.1
6 hr.
7days 18daya
3.7 7.2 5.3
3.3 6.2
7daye 3 hr.
83.4
..
5.8 5.8
.. .. *.
Sward Hard-
nem@ 10 8 4
S&
Soft
Dwabihtya Month: 19
9
8
6. 13
2.9
6 2
Soft
13
16.8
...4
14 18
I .
Figure 5. Bodying Rates of Linseed Oil Acids Esters
10
1 Trimsthylol mpans
8
2: t i p ~ e d o i ~ p I o n
Mikuioh data)
6
0.03% Co, 0.03% Mn, and 0.6% Ca, BB naphthenatea, wing the 5ngertip method films on glass b Dsternhned by m e a h n g light transmimion with amber blue, and sen filters of a Hunter reflectometer, and calculating the fractidn A-E)/C’. &terminations were made immediately after drying and again alter storage for one year in diffwe dayli ht and in the dark. 0 Deterrmned with the Swardrocker on 512 weeks old, wing glssa 100 BLI standard. “Soft” meane too soft to measure. d Two coats of clear oil plw the above drierso, on maple aneh ex osed at Garfield, N. J., 46” to horizontal, facing south; the mont%8 recor&d were those to failure to the point where the woad waa no longer protected. Q
-
3. PsntaerytMtol 4. MpentaerytMtol 5. Pleopentaerythritol
In view of the characteristics of the polypentaerythritol drying oils, it is a definite possibility that the present commercial success of technical pentaerythritol has resulted from the dipentaerythritol “impurity”. The characteristics reported hew for drying oils carry over to alkyd resins, ester gums, and modified phenolic resins. Studies on these will be reported in future papers. LITERATURE CITED
extent by the dipentaerythritol ester. I n both the soybean and linseed series, as the functionality of the alcohol increases, the drying time decreases and the hardness increases. The effect of alcohol functionality on durability is not so consistent (probably because of the difficulty of controlling and evaluating outdoor weathering); but it shows the definite trend in both series of increased durability with increase in number of hydroxyl groups. It is also evident that the yellowing of the films on aging, both in the dark and in light, is much less with the pentaerythritol-type esters than with natural linseed oil. The value of the polypentaerythritol drying oils is evident from the data reported. They are compatible with the same resins aa linseed oil, and satisfactory varnishes have been prepared with phenolics and with copals, but this field has not been thoroughly investigated. Preparation of paints is another aspect on which insufficient data are available to permit reliable statements, but indications are that a11 the usual pigments may be used; the hiding power with a given pigment is equal to or exceeds that of linseed oil, and the stability may be better with reactive pigments became of the lower acid number of the bodied
(1) Arvin, J. A., U. 5. Patent 2,029,851(Feb. 4,1936). (2) Berth, R. H., unpublished data. (3) Blagonravova, A. A., and Drinberg, A. Y., J. Applied Chem. (U.S.S.R.), 11, 1642-7 (1938). (4) Blagonravova, A. A., and Lazarev, A. M., Ibid., 12, 1718-22 (1939). (5) Bradley, T. F.,IND.ENO.CHEM.,29, 440-5, 579-84 (1937); 30,689-96 (1938). (6) Bradley, T.F., and Richardson, D.. Zbid., 34,237-42 (1942). (7) Bruson, H.A., U. S. Patent 1,835,203(Dec. 8,1931). (8)Burrell, H.,Oil & Socqp, 21,206-11 (1944). (9) Burrell, H.,and Bowman, P. I., U. S. Patent Application pendmg.
(10) Carothers, W.H., J. Am. Chem. SOC.,51,2548-70 (1929). (11) Drinberg, A. Y.,Byull. Malyarnoi Tekh., No. 4,24-8 (1939). (12) Drinberg, A. Y., Org. Chem. Znd. (U.S.S.R.), 4, 114-17 (1937), (13) Drinberg, A. Y.,and Blagonravovs, A. A,, Natl. Paint, Varnish Lacquer Assoc., Sci. Sed. Circ. 501 (Feb., 1936). (14) Friederich, W., and Brun, W., Ber., 63B,2681-90 (1930). (16) Gauerke, C. G.,U.8.Patent 1,979,260(Nov. 6,1934). (16) Krzikda, H.,arid Wolf, W., German Patent 629,483 (1930); French Patent 703,792 (1931). (17) Mikusch, 3. D.,von, IND.ENQ.CHIW., 32,1061-9 (1940). (18) Robinson, P., U. 8. Patent 2,123,206(July 12, 1938). (19) Stingley, D.V.,IND.ENQ.CHEM.,32, 1217-20 (1940). (20)Tollens. B.,and Wigand, P., Ann., 265, 316-40(1891).
