July, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited
Acknowledgment The author is indebted to Phillips Petroleum Company for granting permission to publish this paper. He is also deeply indebted to K. H. Hachmuth of Phillips Petroleum Company for helpful guidance and suggestions in developing this study.
Nomenclature %H
weight per cent of less volatile component yo L weight per cent of more volatile component PCH = critical pressure of less volatile component, Ib./sq. in. abs. POD = deviation of critical pressure of mixture from additive law value, Ib./sq. in.; defined by Equations 2 and 3 PcL = critical pressure of more volatile component, lb./sq. in. abs. P C M = critical ressure of mixture of less volatile and more volatife components, lb./sq. in. abs. TCH = critical temperature of less volatile component, TcL = critical temperature of more volatile component,’*F. T O M = critical temperature of Fixture of less volatile and more volatile components, F. No. of C in H = number of carbon atoms in less volatile constituent No. of C in L = number of carbon atoms in more volatile constituent No. of C’s = sum of number of carbon atoms in less volatile and more volatile components = =
849
(1) Cummings, Stones, and Volante, IND.ENQ.CHEM., 25, 728 (1933). (2) Gibbs, “Collected Works of J. Willard Gibbs”, Vol. 1, New York, Longmans, Green and Co.,1928. (3) Gilliland and Scheeline, IND. ENO.CHEM.,32, 48 (1940). (4) Guter, Newitt, and Ruhemann, Proc. Roy. SOC.(London), A176, 140 (1940). (5) Kat5 and Kurata, IND. ENQ.CHEM.,32, 817 (1940). (6) Kay, Ibid., 28, 1014 (1936). (7) Ibid., 30, 459 (1938). (8)Ibid., 32, 353 (1940). (9) Ibid., 33, 690 (1941). (10) Lu, Newitt, and Ruhemann, Proc. Boy. SOC. (London), A178, 506 (1941). (11) Nysewander, Sage, and Lacey, IND. ENG.CHEM.,32, 118 (1940). (12) Pawlewski, Be?., 15,460 (1882). (13) Roesr, J. Inst, Petroleum Tech., 22, 665 (1936). Proc. Rov. SOC.(London). A171. 121 (1939). (14) Rlihemann. - (16) Sage, Hicks, and Lacey, IND. ENQ.CHEM., 32, 1085 (1940). (16) Sage and Lacey, Ibid., 32, 992 (1940). (17) Sage and Laoey, “Volumetric and Phase Behavior of Hydrocarbons”, Stanford Univ. Press, 1939. (18) Sage, Laoey and Schasfsma, IND.ENQ.CHEM.,26, 214 (1934). (19) Sohoch, Hoffmann, and Mayfield, Ibid., 33, 688 (1941). (20) Smith and Watson, Ibid., 29, 1408 (1937). (21) Taylor, Wald, Sage, and Lacey, Oil Gas J., 38, 46 (1939). \--I
PRFBBNTBD before t h e Division of Petroleum Chemistry a t the 103rd Maeting of t h e A M ~ R I C ACNE ~ M I C A SOCIETY. L Memphii. Tenn,
Ester Gums from Rosin and Modified Rosins W. D. POHLE AND W. C. SMITH Naval Stores Research Division, Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Washington, D. C.
HE ester formed by the reaction of rosin with glycerol is known commercially as ester gum and was first made by (11) in 1886. Ester gum is prepared by heating T8chaal the rosin and glycerol together in such a manner that the water formed by the esterification process is removed. Three important properties of ester gum are color, softening point, and acid number. Ellis and Rabinovitz (6) called attention to the discoloration of ester gum when exposed to air or oxygen during esterification and the desirability of maintaining an inert atmosphere over the esterifying materials during processing. At that time ester gum with an acid number as low as 6 or 7 was generally darker in color than ester gum having an acid number of 13 (6). The softening point of ester gum is raised by oxidation (2, 12); however, such treatment darkens the product. Ellis and Rabinovits (6),Murray (8), and Beegle (1)investigated the effect of certain conditions on the rate and extent of esterification. They found the temperature between 270’ and 290” C. most effective for rapid and almost complete esterification. As none of these investigations have compared ester gums made from various commercial rosins and modified rosins, the Naval Stores Research Division has undertaken such a study.
Preparation of Ester Gums The first step was the adoption of a laboratory method for preparing the ester gums t o be compared. The temperature used for the esterification and the amount of glycerol to be reacted with the rosin were selected after the effects of temperature and amount of glycerol on the rate and extent of esterification were determined. For each test about 500 grams of rosin or its equivalent were chosen arbitrarily as a convenient amount for the laboratory experiment. The amount of glycerol to be used with a given weight of rosin was determined by the acid number of the rosin, since a constant ratio between rosin acids and glycerol was to be maintained in all tests. To facilitate the calculation of the amount of glycerol t o be used with a given 500 grams of rosin, Figure 1 was prepared. In the laboratory preparation of ester gum, 500 grams of rosin and the amount of U. S. P. glycerol desired were placed in a 1-liter three-necked flask equipped with a constant-speed stirrer, thermometer, inlet for carbon dioxide (used to maintain an inert atmosphere in flask), and a distillation connection to remove the mater as it was formed. All connections were made with standard-taper ground-glass joints. The stirrer was also glass. The flow of carbon dioxide was regulated a t
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 34, No. 7
TABLE I. PROPERTIES OF ROSIXSAND ESTERGUXS,AND CONDITIOSS UNDER WHICHESTERGEMWAS PREPARED Grade Test NO. 1 2 3 4
5 6 7 8
9 10 11
12 13 14 15
16
17
18 19 20 21
22
Oxidized Softening Point Acid No. Ester ester Ester Ester Description of Sample Rosin gum guma Rosin gum Rosin gum Slash rosin M G 77 93 165 9 Slash rosin M G 77 93 165 8 Slash rosin hl G 77 92 165 24 Slash rosin N G 77 96 165 16 Longleaf rosin >I G 89 171 8 Slash rosin 31 G 77 89 171 8 Rosin from crystalline fraction of slash gum WG G 79 85 178 8 Rosin from iiauid fraction of slash N H 73 89 8 gum 164 Slash rosin oxidizedb 2 days I< F 91 7 171 H Slash rosin oxidizedb 4 days 92 F 79 171 8 Abietic acid WG 89 / 85 G 184 7 I< Abietic acid 85 F SO/ 11 184 Abietic acid0 H B 84 7 180 Slash rosin vacuum-distd. 3A 82 .. 175 8 Hydrogenited slash rosin IC 76 75 9 165 Stabilized (commercial) rosin I WG 72 72 9 170 Crystalline acids from stabilized rosin I 3A .* 81/ 82 184 9 Stabilized (cornrnerrial) rosin I1 IC 68 55 165 21 Crystallized acid from stabilized rosin I1 X 84f 64 184 12 Slash-rosin oxalic acidd M F 85 171 9 sulfuric acids Slash rosin Green 70 17; 22 Wood rosin (commercial) WG .. ?i 82 160 7 Powdered and exposed in a thin layer t o air for 14 days. b Rosiq powdered and exposed t o t h e air in a thin layer. c Iron-contaminated; iron is known t o increase t h e r a t e of esterification. d 15 grams of oxallc acid added and heated t o 260' C. in 1 hour. glycerol then added. e 2 mi. concentrated sulfuric acid added before flask containing rosin and glycerol was placed in oil bath. / Softening point of fused acids. K O glycerol present, esterification was complete. ~
..
..
+ +
.. ..
..
Charged
~
..
500
Temperature, C . Ester Oil Hours gum bath Processed
500 500
60 60 60 50 63 62
261 290 321 290 290 290
440
55
290
305
300 500 500 500 500 500 500 445 500
37 62 63 70 67 67 64 55 62
290 290 290 290 290 200 290 290 290
305 305 305 305 305 305 305 305 305
500
500
67 62
290 290
305 305
500 500 XI0 500
67 64 63 60
290 290 290 290
305 305 305 305
500 000 500
276 303 336 303 305 305
8
4
2.58
4
4 4
4 4
4 4 6
5 4 6.5 4 5
6.5
4
5.5 5
5 5
the rate of about 25 ml. per minute. When the thermostatically controlled oil bath used for heating the flask had reached the desired temperature, the flask was lowered into the bath to a given depth. The receiver for the water formed in the esterification was connected and the stirrer started as soon as the melting rosin permitted, The temperature of the oil bath mas maintained a t the chosen temperature throughout the test. Samples were removed from time to time for acid number determinations by which the progress of the esterification was followed. When the acid number had been reduced to the desired point and any excess glycerol distilled off, the flask was removed from the bath and the product cooled to about 210" C. before it was poured into the containers for testing and storage.
Rate of Esterification As a guide in selecting the temperature best suited for our study, tests were made a t three temperatures, using about a 33 per cent excess of glycerol, to determine the effect of temperature on the rate of esterification. The data for these tests are given in Table I, tests I, 2, and 3, and are plotted in Figure 2. It is evident that temperature has a marked effect on the rate of esterification. Because the decarboxylation of rosin is appreciable a t 320" C., and no data are available on the breakdown of ester gum a t this temperature, it was thought best not to use 320" C. for preparing ester gums for comparative purposes. However, the acid number of the ester gum reached a constant value in about 2 hours when the esterification was carried out a t 320" C., due no doubt t o complete removal of the excess glycerol. As there appears to be no difference between the ester gums prepared a t 260" and 290" C., and the rate of esterification a t 290" C. was much greater (Table I, tests 1 and 2), the latter temperature was selected for preparing the ester gums. To determine the effect of the amount of glycerol on the rate of esterification, two tests, one using 50 and the other 60 grams of glycerol, were compared (Table I, tests 2 and 4). The data are expressed graphically in Figure 3. At the end of 4 hours the runs were terminated, as no glycerol was present in the ester gums. The data show that under the conditions of these tests a t 290" C. it is necespary to use about a
FIQURE1. AMOUNTOF U. S. P. GLYCEROL TO BE USEDWITH 500 GRAMS
OF
ROSINBASEDUPON THE ACID NUMBER ROSIN
OF THE
33 per cent excess of glycerol if the acid number of the ester gum is to be reduced to 10 or less in 4 hours. The advantages in preparing ester gums for comparison from different rosins by this laboratory procedure are: (a) Thermostatic control of the oil bath used for heating the reaction flask ensures that the products will not be exposed to temperatures higher than that for which the oil bath is set; (b) agitation keeps the contents of the reaction flask thoroughly mixed and the temperature throughout the mass uniform; (c) all-glass equipment eliminates the contamination by (foreign) metals during the esterifying process; (d) maintaining an atmosphere of carbon dioxide in the flask eliminates the possible action of oxygen on the product during formation, which in some cases might be appreciable, in others slight.
July, 1942
INDUSTRIAL AND ENGINEERING -CHEMISTRY
851
line resin acids, while the other is completely liquid. Ester gums were prepared from rosins made from the crystalline and from the liquid portion of the gum (tests 7 and 8). The rosin made from the crystalline portion of the gum is lighter in color and has a higher softening point than that made from the liquid portion. However, the ester gum made with rosin produced from the crystalline fraction of the gum is lighter in color, less stable toward oxygen as tested, and softer than the ester gum made with rosin produced from the liquid fraction of the gum. The greater stability toward oxygen of ester gum made with the liquid fraction indicates that the rosin acids are, on the whole, more stable toward oxygen than those present in the rosin made from the crystalline fraction of the gum. This is in agreement with tests made on these fractions when studying the color that rosin contributes to soap (10). Although the rosin made from the crystalline fraction of the gum had a higher acid MINUTES PROCESSED number and was harder than the rosin produced OF TEMPERATURE ON RATEOF ESTERIFICATION FIGURE 2. EFFECT from the liquid portion of the gum, the ester gum made with this rosin was softer. The high grade of the rosin made from the liquid fraction Properties of Products Studied indicates that the rosin contains a minimum of oxidized rosin The following determinations were made on the ester gums: acid. Thus the oxidized rosin acid will have little effect on color grade on the rosin standard scale (S), acid number, the properties of the ester gum of test 8. The lower softening softening point by the A. S. T. M. ball and ring method, point of the ester gum made with the crystalline fraction of the crystallization from ethyl acetate, and in some cases the effect gum may be due to its higher abietic acid content (10). As on color of exposing powdered ester gum to air. the softening point of ester gum made with abietic acid (tests The color grades are United States Department of Agricul11, 12, and 13) is lower than that made with normal gum ture standards and are designated by X, WW, WG, N, M, K, rosin, one would expect that the rosins which contain more I, H, G, F, E, and D. They range in color from a very light than the usual amount of abietic acid would make softer ester yellow for X grade to a very dark red for D grade when a gums. Previous tests in this laboratory indicated that rosin '/pinch cube is viewed against a clear sky. made from the crystalline fraction of the gum will have more The products studied were: slash pine rosin, longleaf pine abietic type acids than one made from the liquid fraction (10). rosin, rosin from crystalline fraction of slash pine gum, rosin from liquid fraction of slash pine gum, oxidized slash pine rosin (powdered and exposed in a thin layer to air at room temperature for 2 and 4 days), 1-abietic acid, 1-abietic acid contaminated with iron, vacuum-distilled slash pine rosin, laboratory-hydrogenated slash pine rosin, commercial stabilized rosins, crystalline acid from commercial stabilized rosins, and commercial wood rosin. Ester gums were prepared from various rosins and rosin acids. The properties of the rosins and ester gums and the conditions under which each ester gum was prepared are summarized in Table I. Gum rosin is made from the pine oleoresin that exudes from the longleaf and slash pine when the tree is wounded by cutting through the bark and into the wood of the tree. Although commercially no distinction is made between rosins from these gums individually or from various mixtures of them, it was desirable in this study to determine what differences, if any, exist between ester gums made with rosins produced from straight longleaf and straight slash pine gums. The ester gums made with longleaf and slash pine rosins did not differ in color, softening point, change of color by oxidation, or UWUTES PROCESSED time required for processing (tests 5 and 6). This does not FIGURE 3. EFFECTOF AMOUNTOF mean that gum rosins made from straight longleaf and straight GLYCEROLON ESTERIFICATION AT slash pine gums are identical in the type of rosin acids and the 290" C. quantity of these rosin acids they contain, but rather that the over-all effect of the rosin acids yields ester gums with the Rosins may vary in the amount of oxidized resin and rosin same properties. acids present. To check the effect of oxidized rosin acids on an ester gum, the rosin used in test 6 was oxidized 2 and 4 Crystalline and Liquid Fractions days and then made into ester gum. This degree of oxidaticp Shortly after clear pine oleoresin exudes from the tree, darkens the rosin and ester gum and slightly raises the softenportions of the resin acids crystallize. Thus the gum can be ing point of the ester gum (tests 9 and 10 compared with divided by filtration into two fractions; one contains crystaltest 6).
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
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Test 21 was made to determine the effect of a stronger acid on the ester gum. In this case the color of the ester gum was not characteristic of normal ester gum, and the softening point was much lower. These tests indicate that if an ester gum with a high softening point is desired, the stronger acids should not be added to gum rosin. Commercial ester gums are made from both gum and wood rosin. The ester gum made from wood rosin is consistently lighter in color than the rosin from which it is made and the softening point is lower than that of gum rosin ester ghm. To determine how laboratory results check with those obtained commercially, tests 6 and 22 were compared. The laboratory results with regard to color and softening point are in agreement with commercial practice. The laboratory data indicate that gum rosin esterifies more rapidly than wood rosin. These findings could not be checked with commercial practice, as no commercial data were found on this point. The difference in rates of esterification of gum rosin, abietic acid, and crystalline acids from commercial stabilized rosin is shown graphically in Figure 4 for the results from tests 6, 11, and 19. The type of rosin acids is apparently a factor in determining the rate of esterification. The only ester gums to show crystallizing tendencies when 10 grams were dissolved in 10 grams of ethyl acetate and kept for 48 hours a t 0' F. mere those of tests 16, 18, and 19. At present we have no explanation as to why test 17 was negative. The answer to this question may show how to eliminate the crystallizing tendencies of ester gums from stabilized rosins or rosin acids.
L h' 1 I*-GUM
175
ROSIN
ACID(*^^- 87.) X-CRYSTALLINE ACIDS FROM A COMMERCIAL STAQIUZEO ROSIN
0-ABIETIC
I 0
I 100
I
I
I
I
200 300 MINUTES PROCESSED
I
FIQURE 4. ESTERIFICATION OF ROSIN PRODUCTS The term "abietic acid" refers to the rosin acid, 1-abietic acid, which has a high negative rotation and not to the rosin acids as a whole or a mixture of rosin acids. The abietic acid used in tests 11, 12, and 13 had a levorotation of more than -80" and was prepared by the method of Palkin and Harris (9). I n contrast to gum rosin ester gum, the abietic acid ester gum is lighter in color than the fused abietic acid from which it is made. Although the softening point of the fused abietic acid is appreciably higher than that of gum rosin, the softening point of the abietic acid ester gum is lower than that of gum rosin ester gum. The rate of esterification of abietic acid and glycerol, tests 11 and 12, is less than that of gum rosin and glycerol.
Effect of Various Treatments Vacuum distillation has been suggested as a means of making high-grade rosins from lower grades ( 4 , 6 , 7). The results obtained with a vacuum-distilled rosin are given in test 14. The ester gum is lighter in color and softer than the gum rosin ester gum. The vacuum-distilled rosin did not react so rapidly with glycerol as the gum rosin. The higher abietictype acid content of this product (10) may explain the properties of the resulting ester gum and its slower rate of esterification. Ester gum was made from slash pine rosin hydrogenated in the laboratory a t 190" C. and 800 pounds pressure using a nickel catalyst (test 15). The hydrogenated rosin ester gum was darker in color and softer than the ester gum made from the original gum rosin. The dark color of the ester gum could no doubt be improved with a little experimenting, but the lower softening point of the ester gum appears to be characteristic of ester gum made with hydrogenated rosin. Two commercially shbiliaed rosins and the crystalline acids obtained from them by crystallization from acetone were used to make ester gums (tests 16 to 19). The softening point of these ester gums is lower than that of one made with gum rosin, particularly in tests 18 and 19. As oxalic acid is sometimes used t o improve the grade of rosin when the rosin is contaminated with iron, an ester gum was prepared from a slash pine rosin which had been heated w?th oxalic acid (test 20). The only effect of this treatment was a lowering of the softening point of the ester gum, which is no doubt due to the increase in abietic-type acids (IO).
Summary A laboratory method has been described for preparing ester gums under controlled conditions so that they can be compared with respect to color, softening point, and rate of esterification. Using this method, ester gums were prepared from various kinds of special rosins, modified rosins, and rosin acids. The following observations were made: Above 260" C. temperature had a marked effect on the rate of esterification. Ester gums made from longleaf and slash pine rosins had the same properties. There was no correlation between the softening point of the rosin or fused acid and the softening point of the ester gum. Oxidation of the unstable rosin acids darkened the ester gum and raised the softening point of the ester gum. Abietic acid ester gum was lighter in color than the fused abietic acid, and the softening _ point _ was lower than that of gum rosin ester gum. Ester gum made with vacuum-distilled gum rosin was lighter in color and softer than the gum rosin ester gum. The vacuumdistilled rosin did not esterify so rapidly as gum rosin. Hydrogenated rosin ester gum and commercial stabilized rosin ester gum was softer than gum rosin ester gum. Ester gum made from stabilized rosin or stabilized rosin acids had a tendency to crystallize from a cold ethyl acetate solution, while ester gums made from unstabiliaed rosins did not show this tendency. Gum rosin esterified more rapidly than wood rosin. Crystalline acids from commercial stabilized rosin and abietic acid did not react so rapidly with glycerol as gum rosin.
Literature Cited (1) Beegle, IND. ENG.CHEM.,16, 953, 1075 (1924). (2) Bradley, U. S. Patent 1,893,982(1933). (3) Brice, J . Optical SOC.Am., 30, N o 4,152-8 (1940). (4) Donk, U. S. Patent 1,219,413(1917). (5) Ellis and Rabinovitz, J. IND. ENG.CHEM.,8,406-11 (1916). (6) Hitch, U. S.Patent 1,904,464(1933). (7) Kressman, Ibid., 1,992,754(1935). (8) Murray, Chem. & Met. Eng., 25,475 (1921). (9) Palkin and Harris, J . A m . Chem. Soc., 56, 1935 (1934). (IO) Pohle and Speh, Oil & Soup, 17, 100-6 (1940). (11) Schaal, U. S. Patent 335,485(1886). (12) Ibid., 1,583,014(1926).
