New tung oil derivatives

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New tung oil derivatives

Agricultural Research Service
UNITED STATES DEPARTMENT OF AGRICULTURE

SUMMARY

To extend the utilization of tung oil and improve its economic status,
chemists at the Southern Regional Research Laboratory are engaged in re-
search on the development of new chemical derivatives of the oil which may find
application as specialty products.

The chief constituent of tung oil is the glyceride of a/pAa-eleostearic acid,
a straight chain 18-carbon fatty acid that contains a special arrangement of alter-
nating single and double bonds known as a conjugated triene system. Alpha-tung
oil or its component a/p^a-eleostearic acid may be readily converted to the beta-
isomer, which also contains a conjugated triene system.

These very active systems of double bonds react in a readily predictable
fashion with a great number of different organic compounds, referred to as di-
enophiles, by what is known as the Diels-Alder Reaction. This particular reac-
tion has been used to advantage in preparing new chemical derivatives of tung
oil for use in the manufacture of plastics.

This paper describes the isomerization of alpha to beta tung oil, prepara-
tion and determination of the chemical structures of highly pure eleostearic
acids, alcoholysis of tung oil for the production of various esters, the reactions
of these materials with maleic anhydride, ^eto-propiolactone, acrylonitrile, and
fumaronitrile as dienophiles, and the results obtained on testing the ester adducts
as plasticizers for vinyl resins.

UNITED STATES DEPARTMENT OF AGRICULTURE

Agricultural Research Service

Southern Utilization Research Branch

Southern Regional Research Laboratory

New Orleans, Louisiana

NEW TUNG OIL DERIVATIVES

By Joan S. Hoffmann and W. G. Bickford
Southern Utilization Research Branch

INTRODUCTION

Tung, or China wood, oil has been used for many centuries by the Chinese people in the
manufacture of lacquers and waterproofing compounds. However, the culture of tung trees in the
United States had its origin just 50 years ago. Since those first experimental plantings, the
annual production of tung oil in this country has risen to over 40,000,000 pounds per year.

More than 80 percent of the tung oil consumed in American industries is utilized by manu-
facturers of protective coatings. In order to extend the utilization of tung oil and improve its
economic status, this Laboratory^ is engaged in research on the development of new chemical
derivatives which may find application as specialty products.

COMPOSITION OF TUNG OIL

Domestic tung oil is produced exclusively from tung kernels of the fordii species of the
genus Aleurites. Practically all vegetable oils are glycerides, and better than 95 percent of the
oil is composed of glycerine derivatives. Of this glyceride content, only about 10 percent is
glycerine itself and the other 90 percent consists of fatty acid. Fordii oil normally contains some
78 percent of eleostearic acid in the glyceride form, together with small percentages of other
fatty acid glycerides, such as oleic, linoleic, and stearic.

Two forms of tung oil are known -- the liquid, golden yellow oil designated alpha tung oil,
and the semisolid, butterlike beta tung oil. The eleostearic acid portions of these oils are called
aZpAo-eleostearic acid and 6eia-eleostearic acid, respectively. The exact chemical structures of
these two acids have been determined, and both acids have been shown to be straight-chain Cl8
fatty acids with triene unsaturation. In other words, highly reactive double bonds are located in
the 9, 11, and 13 positions along the 18-carbon chains of these acids. Furthermore, such an alter-
nating system of double and single bonds constitutes what is known as a conjugated system,
which is extremely reactive in particular chemical reactions. The only difference between alpha-
eleostearic acid and feeto-eleostearic acid lies in the double bond located in position 9 of the
carbon chain. In the case of the alpha acid this double bond is cis, and in the beta acid it is
trans, while the double bonds at positions 11 and 13 are trans in both acids, cis and trans are
merely terms describing the relative positions of hydrogen atoms around an ethylenic bond.

ISOMERIZATION OF TUNG OIL

The tung tree elaborates only one of the so-called isomeric forms of eleostearic acid, the
alpha form. Consequently, the eleostearic acid in fresh tung oil expressed from normal, undam-
aged tung kernels has the alpha configuration exclusively. Befo-eleostearic acid must be pro-
duced artificially by inducing the cis 9, 10 double bond of the alpha form of the acid to shift to a
trans configuration.

One of the laboratories of the Southern Utilization Research Branch, Agricultural Research Service, U. S. Department
of Agriculture.

- 2-

lodine, sulfur, selenium, and sunlight are variously reported to effect this alpha to beta
transformation. However, it has been observed in this Laboratory that the isomerization of
alpha to 6efa-eleostearic acid is most readily accomplished by treatment of the tung oil with a
small quantity of saturated potassium iodide solution, followed by exposure to diffused daylight.
In any event, the transformation from alpha to beta is easily accomplished whether the eleostearic
acid is in the form of the glyceride or the free fatty acid.

TUNG OIL ACIDS AND ESTERS

Numerous procedures have been reported in the literature for the preparation of pure alpha-
and 6e£a-eleostearic acids. Varying in their degrees of complexity, some require special appa-
ratus for excluding oxygen, while others employ numerous washings and recrystallizations of the
intermediate and final products from various solvents. Such multiple operations are not only time
consuming, but by their very nature afford ample opportunity for these highly unstable acids to
undergo deterioration. We have developed a simplified method involving only one recrystallization
for the preparation of the pure eleostearic acids in good yields. The tung oil is saponified under
mild conditions, then acidulated with dilute hydrochloric acid. The liberated acids are immediately
dissolved without further treatment in 95 percent ethyl alcohol and crystallized at -20° C. The
acids are recrystallized from ethanolic solution at +5° C. After vacuum drying, the white, fluffy,
crystals are stored in evacuated ampoules at low temperatures. Ultraviolet absorption data obtained
on these highly pure acids serve as a basis for the more accurate determination of the alphor and
fceia-eleostearic acid contents of tung oil.

In preparing new chemical derivatives of tung oil, it is often more expedient to utilize various
esters of the eleostearic acids instead of the fatty acids or glycerides as such. A convenient
method of preparing the esters without appreciable isomerization occurring during the process is by
alcoholysis. This .consists of heating the glyceride with an excess of the appropriate alcohol in
the presence of the corresponding sodium alcoholate. For example, in the preparation ofethyl
oZpAa-tungoate, aZpAa tung oil is gently refluxed with a mixture of absolute ethyl alcohol and
sodium ethylate. The sodiimi ethylate is prepared simply by dissolving freshly shaved metallic
sodium in absolute ethyl alcohol.

DIELS.ALDER REACTIONS

As was pointed out earlier, the conjugated system present in tung oil, its fatty acids, and
its fatty esters is unique, with a chemistry all its own. This highly reactive system of double
bonds reacts in a readily predictable fashion with a great number of different organic compounds by
what is known as the Diels-Alder Reaction.

In order for a Diels-Alder Reaction to readily occur, there must be present in the reaction
mixture a trans, trans diene, and a dienophile (a "diene-loving" compound). /1/pAa-eleostearic
acid has a irons, trans conjugated system present in carbons 11-14, while 6efa-eleostearic acid has
a trans, trans diene not only at position 11-14, but also at position 9-12. Therefore, tung oil acids
and esters admirably fulfill the diene requirements for a Diels-Alder Reaction. Organic chemicals,
which are suitable dienophiles, are far too numerous to list, but almost all of them have a particu-
lar type of structure, consisting of an ethylenic bond conjugated with another type of double bond,
such as a carbonyl bond. A true Diels-Alder Reaction always yields a particular type of adduct,
formed by cis addition of the dienophile to the first and last carbon atoms of the conjugated diene
system (1, 4 addition). For example:

- 3-

CH2 = CHCOOH CH3(CH2)3CH = CH - CH = CH - CH = CH(CH2)7COOH

Acrylic acid /4/pAa-eleostearic acid

HC=CH

CH3(CH2)3HC CH - CH = CH(CH2)7COOH

He CH

H COOH
Diels-Alder Adduct

Aa adduct produced in this way contains a cyclohexene nucleus and one ethylenic bond out^
side the nucleus referred to as an exocyclic double bond. This exocyclic double bond may be
cis or trans, depending on whether alpha- or beta- eleostearic acid was employed as the reactant.

TUNG OIL DERIVATIVES

Selection of dienophiles for reaction with theeleostearates was based principally on the
probable utility of the predicted reaction products as plasticizers, biologically active agents,
emulsifiers, and sticking agents, as well as their facile addition to the eleostearates without
complicated side reactions such as polymerization.

Maleic anhydride reacts rapidly with alpha- and 6efa-eleostearic acids and esters, either in
the melt or in solution. The aZpAa-eleostearic acid forms only one adduct, while feefa-eleostearic
acid produces two adducts with maleic anhydride. These adducts and various derivatives were
used in establishing definitely the structures of the two eleostearic acids. Esterifi cation of the
maleic anhydride adducts with alcohols leads to the formation of compounds which contain three
ester groups per molecule (tricarboxylic acid esters). Peracid oxidation of the adducts results in
saturation of the exocyclic double bonds of the adducts with oxygen, forming oxirane derivatives.
Oxygen does not attack the cyclohexene nucleus in any of the maleic anhydride adducts. Catalytic
hydrogenation of the adducts results in the formation of completely saturated compounds.

Seta-propiolactone, in the presence of small amounts of potassium carbonate, reacts with the
eleostearates to produce acrylic acid derivatives. Since acrylic acid is an unsymmetrical dieno-
phile, it would be expected that a greater number of isomeric products would result from this reac-
tion than from the maleic anhydride addition. Two isomeric dicarboxylic acids were isolated from
the reaction of fcefa-propiolactone and a/pAa-eleostearic acid esters. Esterification of the adducts
with alcohols results in the formation of diesters. Peracid oxidation of these adducts did not
proceed selectively as in the case of the maleic anhydride adducts, but rather, resulted in satura-
tion of both the cyclic and exocyclic double bonds. Reaction with peracids under hydroxylation
conditions similarly resulted in attack at both centers of unsaturation. The adducts were catalyt-
ically hydrogenated to form completely saturated compounds.

Acrylonitrile and fumaronitrile react with the eleostearates for the production of compounds
which contain cyano (C-N) groups on the cyclohexene nuclei of the adducts. The acrylonitrile
adducts contain only one cyano grouping, while the fumaronitrile adducts contain two such groups
per molecule. The unsymmetrical character of acrylonitrile makes possible the formation of more

isomeric compounds in the Diels-Alder Reaction than can be expected from the addition of the
symmetrical fumaronitrile molecule. Also, reaction of any given dienophile with 6efa-eleostearic
acid results in the production of more isomeric compounds than does an analogous reaction with
a/pAoeleostearic acid, since as was previously noted, the beta-acid has two reactive centers for
dienophilic attack compared to the single reactive center of the alpha-acid. Acrylonitrile also
reacts with raw tung oil via the Diels-Alder Reaction, producing nitrile derivatives of the oil
itself.

The structures of the derivatives prepared in the reactions described above, together with
certain of their properties, are presented in the APPENDIX.

PLASTICIZERS

The rapid expansion of the plastics industry has greatly increased the demand for effective
plasticizing materials, especially since one pound of plasticizer is required for every two pounds
of certain vinyl resins.

These resins are very stiff, brittle rraterials; therefore, in order to prepare suitable plastics
from them, chemical compounds referred to as plasticizers must be incorporated into the resins to
impart flexibility and other desirable characteristics. The prime requisite for a plasticizer is that
it must be compatible with the resin and not sweat or bleed out of the finished plastic.

Among the most important characteristics employed to evaluate the performance of a plasti-
cizer are tensile strength, elongation, modulus, and brittle point of the plasticized composition.
Tensile strength is the load in pounds per square inch supported by the sample at the moment of
rupture. Percentage elongation is the ratio of the length of the sample at the moment of rupture to
the initial length. Modulus is a measure of the ease or difficulty with which a plastic can be
elongated. Modulus is reported in pounds per square inch; the lower the modulus, the greater the
ease of elongation. Brittle point is a measure of the low temperature characteristics of the
plasticizer -- it is the temperature at which the plastic fractures on impact.

To achieve desirable properties in the plastic, it is frequently expedient to employ a mixture
of plasticizers, each of which has some particularly advantageous property. When a plasticizer is
used in such a manner, it is referred to as a secondary plasticizer.

In evaluating plasticizers, it is customary to compare their properties with a reference
material such as dioctyl phthalate (DOP) or tricresyl phosphate (TCP). Plasticizer data for
esters of the maleic anhydride — 6efa-eleostearic acid adducts and their derivatives are presented
in Table I.

All of the adduct esters impart higher tensile strength to the vinyl resin than does DOP, and
about the same elongation, although somewhat inferior moduli and brittle points.

Plastics made with vinyl chloride decompose slowly with the evolution of HCl, which
induces deterioration of the plastic. The epoxy derivatives are not as good plasticizers as the other
materials tested, but they would have the advantage of acting as HCl scavengers, thereby stabili-
zing vinyl chloride-containing plastics.

Plasticizing characteristics of diesters of the acrylic acid — eleostearic acid adducts are
summarized in Table II.

It is apparent that the diesters of the acrylic acid adducts of both the alpha- and beta acids
impart substantially identical characteristics to the resin with the exception of the brittle point.
The butyl esters prepared from the beta-acid adduct are decidedly superior from the standpoint of
low-temperature plasticizing characteristics.

The diethyl esters of the acrylic acid adducts of both alpha- and 6efa-eleostearic acid are
comparable plasticizers to DOP with respect to modulus, tensile strength, and elongation. The
dibutyl esters, on the other hand, are somewhat inferior in each' respect. Brittle points imparted
by the a/pAo- acid derivatives are about the same as for the control DOP, while those for the beta-
acid derivatives are somewhat better, about midway between those of DOP and di-2-ethylhexyl
adipate in the case of the dibutyl ester. Volatilities of the beta adduct stocks run from about the
same to one-half that of DOP, while those for the alpha adduct stocks run from about the same to
twice as much, llydrogenation results in reduced volatility for both alpha and beta derivatives,
and can be expected to result in improved thennal stability, although it shows little or no consist-
ent influence on the other plasticizing characteristics of these materials.

Primary and secondary plasticizer data on the acrylonitrile and fumaronitrile adducts of the
Ai-butyl esters of alpha- and 6efa-eleostearic acids appear in Table III.

All of the adducts were compatible at the time of milling and molding, although at 60 days
the stock plasticized with the adduct of butyl feefa-eleostearate was bleeding, showing definite
incompatibility. Both the butyl alpha- and butyl 6e^a-eleostearate — acrylonitrile adducts were
satisfactory secondary plasticizers when incorporated with either DOP or TCP.

The screening tests show that the stocks plasticized by the fumaronitrile adducts are some-
what better in tensile strength, compatibility, and volatility than those plasticized by the acrylon-
itrile adducts, although the latter excell the former in modulus and brittle point. It is apparent
from these results that the presence of two cyano groups on the cyclohexene nucleus of the adduct
leads to greater permanence and an enhanced degree of compatibility between the plasticizer and
the resin.

»

By incorporating the acrylonitrile adducts with DOP or TCP, it is possible to achieve either
a reduction in volatility of DOP plasticized stocks or an improvement in the modulus and low-
temperature performance of TCP plasticized stocks. This does not entail any sacrifice in the
desirable plasticizing characteristics of DOP or TCP.

-6-

LISTOF PUBLICATIONS

(1) Bickford, W. G., DuPre , E. F., Mack, C. H., and O'Connor, R. T., The Infrared Spectra and
the Structural Relationships Between alpha- and 6efa-Eleostearic Acids and Their Mai ei c
Anhydride Adducts, Jour. Amer. Oil Chemists Soc, 30, 376 (1953).

(2) Hoffmann, J. S., O'Connor, R. T., Magne, F. C, and Bickford, W. C, The Reaction o{ beta-
Propiolactone with alpha- and 6eia-Eleostearates and Plasticizer Properties of Derived
Esters, Jour. Amer. Oil Chemists' Soc, ^, 533 (1955).

(3) Hoffmann, J.S., O'Connor, R. T., Magne, F. C, and Bickford, W. G., The Reaction of Acrylo-
nitrile and Fumaronitrile with alpha- and 6eto-Eleostearates. Plasticizer Properties of the
re-Butyl Esters of the Adducts, Jour. Amer. Oil Chemists' Soc, 33, 410 (1956).

(4) Hoffmann, J. S., O'Connor, R. T., Heinzelman, D. C, and Bickford, W. G., A Simplified
Method for the Preparation of alpha- and feeta-Eleostearic Acids, and a Revised Spectrophoto-
metric Procedure for their Determination, in press (Jour. Amer. Oil Chemists' Soc.)

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