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VIRGINIA M. LOEBLICH and RAY V. LAWRENCE Naval Stores Research Section, United States Department of Agriculture, Olustee, Fla.
Inhibition of Resin Acid Isomerization
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Partially neutralized resins from pine gum should find direct use as paper sizes and soaps, and as starting materials for making precipitated metal resinates and maleic-, fumaric-, or formaldehyde-modified resins
T m steam distillation of pine oleoresin into turpentine and rosin an irreversiis
ble process-admixing of rosin and turpentine does not give the original pine oleoresin. Certain reactions occur during the distillation which alter the composition of the acidic portion of the oleoresin. A complete understanding of these reactions and methods for controlling them have, for many years, been major problems in naval stores chemistry. As a result of studies on the composition of pine gum and rosin, in 1955 this laboratory reported isolation of a new resin acid-palustric acid-from pine oleoresin and rosin (4). The reactions of palustric acid, combined with certain anomalies in the properties of rosin previously observed, indicated that palustric acid could have been formed, to some extent, during the processing of the oleoresin into rosin. For some time it has been known that levopimaric acid (8) can be thermally isomerized to a mixture of resin acids containing predominant amounts of 1-abietic acid. However, the nature of the products formed by this isomerization, other than 1-abietic acid, had never been established; nor had the effect of time and temperature on the percentages of products formed been studied. As levopimaric acid is the most sensitive to heat and represents approximately one third of the resin acids in the oleoresin, a chromatographic analysis of the products of its thermal isomerization was made (5). Under conditions comparable to those used for processing pine oleoresin into rosin the levopimaric acid was changed into a mixture of 34% palustric acid, 52% 1-abietic acid, and 14% neoabietic acid. This reaction explains, to a large extent, the differences observed in the physical and chemical characteristics of the oleoresin and rosin. Isomerization of the levopimaric acid changes the ratio of the conjugated-diene acids in the oleoresin and rosin (Table I). As a result, rosin contains essentially no levopimaric acid and larger amounts of palustric, neoabietic, and 1-abietic acids than the oleoresin. This difference is almost
completely responsible for the irreversibility of the distillation process. The amounts of these acids obtained on distillation of the oleoresin into rosin vary, depending on the conditions to which the resin acids are subjected until, in cases of drastic or prolonged treatment with heat or strong acids, 1-abietic acid may account for 40 to 50% of total acids. The distribution of the conjugateddiene acids in rosin is important from the standpoint of reactivity and crystallinity. I n reactions involving the conjugated double bond system the reactivity of the resin acids varies considerably; hence the amount of each acid present in rosin affects the reactivity of the rosin. The crystalline characteristics of rosin also depend on the ratio of the acids present. A predominant Amount of any one acid will initiate crystallization and the rosin becomes a semicrystalline product containing a mixture of crystalline acids instead of a clear, homogeneous resin. The thermal isomerization studies of levopimaric (5) and neoabietic acids (6) showed that the isomerization was catalyzed by the carboxyl group present in the resin acid molecule. When the acid group was eliminated from the molecule by converting it to an ester or an alcohol, the rate of isomerization was greatly decreased-methyl levopimarate isomerized approximately as fast as the free acid at 155'; methyl
Table 1. Distribution of ConjugatedDiene Acids in Pine Oleoresin, Gum Rosin, and Partially Neutralized Resin
Acid Levopimaric Palustric I-Abietic Neoabietic
yo Acid Present in Partially neutralized Oleoresin Rosin resina 22 11
10
15
0
18
19 18 19
13 13 16
Enough sodium hydroxide added to pine gum before distillation to react with 43% of acids present.
neoabietate and neoabietenol showed marked stability at 200'. As isomeriza: tion of the pure resin acids could obviously be controlled or inhibited by conversion of the acid into a nonacidic derivative, this principle was applied to the processing of the oleoresin. I t was found that the extent of isomerization of the levopimaric acid in the oleoresin could be controlled by adding an alkali such as sodium hydroxide, sodium carbonate, or potassium hydroxide to the crude oleoresin before it was processed into rosin. The amount of levopimaric acid in the resin increased with an increase in the percentage of the acids neutralized until, in the case of complete neutralization, the resin acid composition of the resin was virtually the same as that of the original pine gum.
Partially Neutralized Resins from Pine Oleoresin
Preparation. Partially neutralized resins (7 to 98%) were prepared with only minor changes in the present process for distilling the oleoresin (9). I n the laboratory preparations, the alkali was added to the oleoresin in an aqueous solution prior to removal of the turpentine by steam distillation. When commercially cleaned pine gum was used as the starting material, the aqueous alkali solution was added to the cleaned pine gum prior to distillation. If not more than half of the resin acids were neutralized, the only change in processing was the use of a higher distillation temperature, as the partially neutralized resins have a higher melting point than a similar rosin not treated with alkali. The resins were clear, homogeneous materials resembling rosin in physical appearance. If over half of the resin acids were neutralized, the last traces of turpentine and water were removed by spraying the hot solution into an evacuated chamber to obtain a dry powder. Properties and Composition. A series of partially neutralized resins was prepared ranging from 7 to 98% neutrality (Table 11). The grade, acid VOL. 50, NO. 4
APRlL 1958
61 9
number, softening point ( I ) , specific rotation (2,% solution in 95% ethanol), and per cent levopimaric acid (gravimetric method) ( 2 ) were determined o n each sample. I n determination of the levopimaric acid content on the sodium salt of the resins, the samples were dissolved in pentane and washed with 2 N acetic acid to remove sodium ions before adding maleic anhydride. The resins listed in Table I1 were prepared from commercial pine oleoresin having an [ a ]= ~ -27.4", acid No. = 125.7, and levopimaric acid content = 21.8y0 of the total acids. The rosin prepared from this oleoresin by standard distillation methods had an [CY]I, = 14.9 ', acid No. = 168, grade WW, and levopimaric acid content = 0. Compared with rosin prepared by standard distillation methods, with increase in the percentage of acids neutralized the specific rotation became more negative, the softening point increased, the acid number decreased, and the levopimaric acid content increased. Addition of relatively small amounts of alkali markedly decreases the rate of isomerization of levopimaric acid, as shown by lack of change in specific rotation and the levopimaric acid content of the finished resin. For example, a clear, homogeneous, 34% neutralized resin contained 73y0 of the levopimaric acid present in the original pine gum. Beyond this point, the effect of increasing the amount of acids neutralized is not so marked; however, it is possible to prepare a dry turpentine-free, neutral resin which has essentially the same resin acid composition as the nonvolatile portion of the untreated oleoresin. Chromatographic and ultraviolet investigation of the amount of conjugateddiene resin acids present in a rosin prepared from a 43% neutralized oleoresin gave: levopimaric acid, 18%; palustric acid, 13%; 1-abietic acid, 13%; and neoabietic acid, 167,. When these values are compared with acids in the oleoresin and rosin (Table I), the increase in the palustric, neoabietic, and I-abietic acid content is small, 18% levopimaric acid is still present in the rosin,
+
Table II.
Advantages and Uses of Partially and Completely Neutralized Resins
These neutralized resins possess properties which should be of distinct advantage to certain rosin consumers. The lower l-abietic acid content should impart a stability toward autoxidation and yellowing. The resins should show less tendency toward crystallization. Retention of levopimaric acid in the resins should increase their reactivity in reactions involving the conjugated double bond system. The ability to inhibit the isomerization of the resin acids by reduction of the total acidity of the rosin can be used to control isomerization when rosin is stored in heated tanks for prolonged periods of time and when hot rosin is shipped in insulated tank cars. Partially neutralized wood rosin has
Properties of a Series of Partially Neutralized Resins
% Neutralization
Grade
0
ww
7%b 12 24 34 41 43 48" 74 98
and isomerization of the levopimaric acid has been greatly inhibited. The ultraviolet absorption spectrum of pine oleoresin and rosin can also be used to measure changes in composition. The conjugated-diene resin acids exhibit characteristic ultraviolet absorption spectra at the following wave lengths: I-abietic acid, 241 mp, a = 77; neoabietic acid, 251 mp, a = 80; palustric acid, 266 mp, a = 30; and levopimaric acid, 272 mp, a = 19. A typical rosin or oleoresin will exhibit its maximum absorption at 241 mp, and the specific extinction coefficient at this wave length is a measure of the amount of I-abietic acid in the sample. I n comparing the ultraviolet spectra of pine oleoresin and rosin, it is seen that the maximum a t 241 mp increases from a specific extinction coefficient, a, of 24.8 for the acid portion of the oleoresin to 37.2 for the acid portion of the rosin, showing the formation of I-abietic acid during processing (Figure 1). The spectra of a series of partially neutralized resins show that as the amount of neutralization is increased, the extent of formation of I-abietic acid is decreased; until, in a completely neutral resin, the ultraviolet absorption spectrum becomes almost identical to the spectrum of the original oleoresin.
Soft. Pt., M M
M M M M M
... ...
( 4 D
-I-14.9 +7.1 $5.4 -12.9 -17.0 -16.7 -17.0 -21.0 -23.0 -27.2
O
c.
78.5 81.5 83.5 87.5 92.5 95.0 99.0 99.5
... ...
Commercial cleaned pine gum used as starting material. Alkali was sodium carbonate.
620
INDUSTRIAL AND ENGINEERING CHEMISTRY
Acid No. 168.0 156.7 148.7 127.8 110.2 99.1 94.2 88.0 44.0 3.5
7p
Leropimaric Acid 0 5.8 8.1 13.9 16.6 16.0 16.4 18.0 19.0 21.0
LL 2 2 0 240 260 2 WAVE
8
G
LENGTH
- rnw
Figure 1 . Ultraviolet absorption spectra of partially neutralized resins
been described by Palmer (7) and Georgi (3). Although it is different in composition from the products discussed here, their general fields of utility would be similar. These partially neutralized resins should find direct uses as paper sizes and soaps, and as starting materials for the preparation of precipitated metal resinates and maleic-, fumaric-, or formaldehyde-modified resins. Such reactions could be carried out at lower temperatures than are necessary when rosin is used, because of the presence of the highly reactive levopimaric acid. I n general, the partially neutralized resins should be equal or superior to rosin in most reactions involving the conjugated-diene system of resin acids. literature Cited
(1) .4m. SOC. Testing Materials, Philadelphia, Pa., Designation P 465-51, 1955 Book of ASTM Standards, Pt. 4,p. 692, 1956. (2) Fleck, E. E., Palkin, S., IND.EXG. CHEM.14, 146 (1942). (3) Georgi, E. A. (to Hercules Powder Co.), U. S. Patent 2,085,151 (June 29,1937).
(4) Loeblich, V. M., Baldwin, D. E., Lawrence, R. V., J . Am. Chem. Sac. 77, 2823 (1955). (5) Loeblich, V. M., Baldwin, D. E., O'Connor. R. T.. Lawrence. R. V.. Zbid.,77,6311 (1955). (6) Loeblich, V. M., Lawrence, R. V., Zbid.,79, 1497 (1957). (7) Palmer, R. C. (to Newport Co.), U. S. Patent 1,787,281 (Dec. 30, . , 1930). (8) Ruzicka, L., Balas, F., Vilim, F., Helv. Chim. Acta 7, 458 (1924). (9) Smith, W. C., Reed, J. O., Veitch, F. P., Shingler, G. P. (to United States of America), U. S. Patent 2,254,785 (Sept. 2, 1941). RECEIVED for review May 17, 1957 ACCEPTED January 9, 1958 Division of Paint, Plastics, and Printing Ink Chemistry, Naval Stores Symposium, 131st Meeting ACS, Miami, Fla., April 1957.
