ASME 1990 Citrus Engineering Conference CEC1990 March 29, 1990, Lakeland, Florida, USA CEC1990-3605 Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021

LIMONENE - A VERSATILE CHEMICAL

BY Joseph J. McBride Consultant Ariiona Chemical 807 Wood Avenue Panama Ciy, Florida 32401

ABSTRACT

Following a brief review of the chemistry and properties of limonene as a chemical entity, d-limonene recovered as a by-product in the processing of citrus fruit is discussed in more detail. The largest use of d-limonene, the production of tackifying resins for the adhesive industry, is given special attention, although other important uses, such as in the synthesis of I-cawone, and in specialty solvents and cleaners are also discussed. d-Limonene's price history is compared with those of its competitors in the resin market, piperylene and sulfate turpentine. Its current relative position in the large and growing market is explained and estimates of its future availability and potential use in the market are proposed.

LIMONENE -A VERSATILE CHEMICAL

d-Limonene is the major component of the oils of citrus fruits. Chemically it is classified as a "terpene hydrocarbon." Terpenes are compounds of hydrogen and carbon and sometimes oxygen, and can be considered as made up of two or more units of isoprene, which is the building block of natural rubber. Monoterpenes, like limonene, consist of two isoprene units, sesquiterpenes of three isoprene units, diterpenes of four isoprene units, etc. Menthol and camphor are monoterpenes, Vitamin A is

Published with permission. CRUDE LIMONENE FROM FLORIDA--- CITRUS

Low Boilers, % 0.1 Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021 a-Pinene, % 0.6

Sabinene, % 0.4

Myrcene, % 1.9

Limonene, % 94.6

Octanal, % 0.2

High Boilers, % 2.2

Specific Gravity, 25~/25'~ 0.84

Boiling Point, OF 310

Flash Point, T.O.C., 'E' 115

Refractive Index, ZO'C 1.471 1

Color Colorless to Pale Yellow

FIGURE 1 a diterpene and steroids, which are much in the sports pages today, are tri-terpenes. Many flavors, spices and perfumes owe their distinctive taste or odor to terpene components. Although terpenes are among the most abundant types of natural products, the most important commercial source is the pine tree from which turpentine, a complex mixture of mainly monoterpenes and rosin, a diterpene, are obtained. While rosin and turpentine are still obtained by "gumming" trees or extracting stumps, the major source today is the Draft pulping process in which they are obtained as a by-product of the paper industry.

Limonene comprises over 90% of orange, grapefruit and mandarin oils and over 80% of lemon and Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021 lime oils. The composition and some physical properties of a typical sample of Florida "stripper oil," or crude limonene, are shown in the first figure (Fig. 1). The " high boilers" include aldehydes and esters present in the parent oil.

The chemical structure of limonene shown in the next figure (Fig. 2) is a planar representation of a three-dimensional molecule. This brings us to the significance of the "d" in d-limonene. Since limonene contains what chemists call an "asymmetric" carbon atom, i.e., one which has four different groups attached to it, it is optically active and will rotate the plane of a polarized light shone through it. These optical isomers, i.e., compounds which have identical compositions and properties except for their effect on polarized light, are related as are our left and right hands. We can place our two hands together palm-to-palm and the fingers and thumbs will correspond, but if we place one hand on top of the other we see that the fingers do not correspond, i.e., the hands are not superimposable. Since limonene from citrus causes this light to turn to the right, it is designated as "d" limonene from the Latin "dexter" meaning right. There is also a left-turning or "I" limonene found in Douglas and silver fir and in Russian peppermint oil and a d, I-limonene which is optically inactive since it consists of equal amounts of the d - and I- forms whose opposite rotations cancel each other. The major source of d, I-limonene is sulfate turpentine from the paper industry. This d, I-limonene from turpentine is commonly known as "dipentene."

Since most physical properties of optical isomers, such as boiling point, vapor pressure, specific gravity, etc., are identical, the presence of optical activity is of no significance for most commercial uses. The major exception is in biological activity, i.e., activity in living systems. Many natural products synthesized in plant tissues and in animal bodies are optically active and the surfaces of tissues and organs have optically active sites. In order for such a site to be specifically and exclusively affected, the agent used must have a complementary structure. For example, with many drugs, only one form, the d- or the I- will be active since only it will precisely fit the receptor site. Since most chemical reactions are not stereo-specific, i.e., equal amounts of d- and I- forms are usually produced if an asymmetric center is present, the drug will frequently be a racemic mixture, the inactive form "just going along for the ride." Monosodium glutamate, or MSG, the flavor enhancer exists in d- and I- forms; but only the I- form is effective.

It is in the area of organoleptic, or taste and odor, properties that the optical activity of limonene is significant. Approximately 3 million pounds of d-limonene are used annually in the production of I-cawone, the flavoring agent of spearmint oil. In this three step synthesis, d-limonene is reacted with nitrosyl chloride to give limonene nitroso chloride. Heating this compound causes elimination of hydrogen chloride and the formation of the oxime, which on hydrolysis gives the desired I-cawone. If one would carry out the same syntheses with I-limonene, d-cawone would be obtained. d-Cawone is not at all like spearmint; it taste like dill - a quite different flavor!

For the other major uses of limonene, its optical activity is of no consequence. These uses are in the manufacture of synthetic resins, as a solvent replacing petroleum distillates and chlorofluorocar- bons, and as an odorant for petroleum-derived solvents.

Strictly in terms of solvent power, limonene has little to recommend it over the much more abundant and much less costly petroleum distillates. However, it pleasant odor and perceived safety (it is a "natural product") have secured it a place in certain specialty products such as bathroom cleaners, waterless hand cleaners and aerosols. The implication of chlorofluorocarbons in destruction of the ozone layer has resulted in the partial replacement of these solvents by limonene in the electronics LIMONENE Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021

H-C-H C-H

H-C-H

FIGURE 2 industry. These "specialty" uses probably account for 25-30% of the limonene produced today. The potential size and strength of this market remains to be seen, but in view of the recent large increase in limonene availability, we may soon have some idea. DuPont and General Chemical Co. have announced the successful development of less harmful solvents for the electronics industry, and mineral spirits is an excellent solvent and degreaser at a fraction of the current price of limonene.

The largest use for limonene is in the manufacture of tackifying resins. The chemical industry consumes probable 65% of the world production for this purpose. Tackifying resins are essential and major ingredients of formulated adhesives. I say "formulated" adhesives to distinguish these from Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021 natural adhesives such as animal glues, vegetables, gums and other natural polymers. More than one billion pounds of formulated adhesives were produced in the U.S. alone in 1989 and annual growth of 5% is projected at least through 1993.

With the advent of synthetic polymers, the raw material base for adhesives was potentially greatly expanded. However, while the new polymers had good cohesive strength, they generally lacked adhesive strength. It is the tackifying resin which makes the adhesive "sticky." The raw materials for these resins are rosin, also from the pine tree, petroleum hydrocarbons, turpentine and lirnonene. From these are produced rosin resins, hydrocarbon resins, terpene resins and hybrid hydrocarbon- terpene resins. Despite the large proportion of the world's limonene which goes into the manufacture of these resins, limonene is a minor factor since the tackifying industry in the U.S. alone amounted to 550 million pounds and is growing at about 6% a year. A recent estimate shows the distribution of tackiiing resins as 70% hydrocarbons, 22% rosinderived and 8% terpene of which perhaps half can be classified as limonene resins.

Hydrocarbon and terpene resins are low molecular weight polymers prepared by the cationic polymerization of monomer feedstocks. The terpene monomers used are alpha and beta-pinenes and dipentene from turpentine, and d-limonene from citrus fruits. The petroleum hydrocarbon feedstocks are mainly co-products produced in the steam-cracking of petroleum fractions in ethylene production. The most important of these co-products for the production of tackifying resins is piperylene, a C-5 hydrocarbon. U.S. 1989 production of C-5 derived resins is estimated at 259 million pounds while terpene and terpene-hydrocarbon hybrids amounted to less than 57 million pounds.

These resins are produced as follows. In either a batch or continuous process, the monomer (limonene, other terpenes, hydrocarbon or mixtures of these) in a hydrocarbon solvent are caused to polymerize by a Friedel-Crafts catalyst, usually aluminum chloride. The reaction is exothermic and the temperature must be controlled by cooling. When the polymerization reaction has essentially stopped, the mixture is quenched with water or dilute hydrochloric acid. The polymer solution is washed to remove any residual aluminum salts and the solvent is removed by distillation. The residue is the crude tackifying resin and consists of a mixture of polymer molecules made up of two to about twelve molecules in the case of terpene resins. The desired softening point of the final product is attained by removing, usually under vacuum, the more volatile compounds. A tackifying resin plant is a sophisticated collection of equipment. The monomers and solvents are highly flammable and the catalyst and its hydrolysis products are extremely corrosive to most metals including stainless steel alloys. I have samples of the majgr types of limonene resins if anyone cares to see them. is a straight limonene resin, ZONAREZ 7100 and the other is a limonene - styrene resin, ZONATACOn8 105. The numbers "100" and "105" refer to the softening points in degrees Centegrade.

The formulated adhesive consists mainly of the high molecular weight polymer such as natural or synthetic rubber, ethylene-vinyl acetate or a block elastomer and the tackifying resin. the ratio of tackifier to elastomer varies widely depending on the properties desired but typically ranges between 1:2 to 2:l. The largest single use of limonene-containingresins today is in the adhesive systems used to build "disposables" such as diapers and sanitary napkins. Other important uses are in label and book-binding adhesives and in chewing-gum base.

The next figure (Fig. 4) shows the price history over a 15-year period of crude d-limonene, i.e., stripper oil, vs. the other terpene monomer source, crude sulfate turpentine and the major hydrocar- bon monomer, piperylene, used in these resins. It is obvious that (1) limonene has been much more Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021

FIGURE 3 costly on the average, than the two competing feedstocks and (2) perhaps as important from the points of view of the resin producers and the adhesive manufacturer, much more volatile in price. It is these two factors which have kept limonene's share of the terpene and terpene-hydrocarbon market at a low 4-5% despite significant advantages of limonene resins in certain applications. During the past several years, the resin manufacturers have developed turpentinederived extenders and substitutes for limonene with the result that, while terpene-based tackiiing resin production has grown, limonene usage has remained level, cr in some cases, actually decreased. At the same time, the adhesive manufacturers are decreasing their dependence on limonene resins by developing new

adhesive formulations. These efforts continue today while the adhesive industry grows at 6-7% per Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021 year and world production of limonene appears to be on the verge of considerable growth.

A source which I would consider well-informed has estimated, based on projections of the Florida Department of Citrus, end-of-the-century Florida plus Brazil production of orange oil plus limonene to be 160 million pounds as compared with about 100 million pounds today.

It should be apparent that the tackifying resin industry has the potential to consume much larger quantities of citrus limonene.

What does this industry need in terms of limonene qualitp We said earlier that tackifying resins, the largest use for limonene, are made by polymerization. Polymerization has been defined as the combination of several molecules to form a more complex molecule having the same percentage elemental composition. For example, polyethylene is made by connecting, end-to-end, as many as 100,000 ethylene molecules in a single chain. Ethylene contains 86% carbon and 14% hydrogen; polyethylene has the same percentage composition. Typically, the reactants for a polymerization reaction must be of a very high degree of purity since impurities might react with the growing chain and stop the reaction. While tackifying resins have very short chains compared with polyethylene, it is essential to good yields that certain types of impurities be removed from the monomers. In the case of crude limonene, the major undesirable components are oxygenated compounds, commonly called "oxyterpenes". The surprising consequence of this fact is that the resin manufacturer is rather indifferent to limonene quality since even high quality "orange terpenes" produced in the folding of cold-pressed oil is not pure enough for his use. The "assay" or limonene content of the product is his only concern. Since he must refine whatever he receives from the citrus processor, it really makes little difference if the color is light or dark, the odor pleasant or unpleasant. He will probable buy de-waxed off-grade cold-pressed oil if offered at the same price as stripper oil. The "specialty" market users, on the other hand, frequently require a water-white product or a product meeting strict odor criieria. Additionally, unlike the specialty users, the resin maker buys in bulk. He uses such large quantities that handling of drums is both uneconomical and bothersome.

Having said that the resin producer is relatively indifferent to limonene quality, I must point out that lemon and lime oils contain large amounts of terpenes harmful to the polymerization reaction. These terpenes are impossible to remove economically and limonene from these fruits is not suitable for resin production.

While I, a chemist, would not presume to tell and audience of engineers how best to recover limonene, I have been convinced that more total limonene plus oil will be recovered by always recovering oil, rather than by-passing the centrifuges as is often done with early and mid-season fruit and that this incremental recoverywill more than compensate forthe cost of operating the centrifuges.

Finally, I'd like to present some data on oil and limonene recovery from two large plants. These data include only oranges and mandarins. You may like to compare these recoveries to your experience in your plant (Fig.5).

In case there may be some in the audience who are not closely associated with the processing of citrus fruit in the concentrate plant, the next figure (Fig. 4) is a schematic which I believe shows the sources of d-limonene and cold-pressed oil. CITRUS FRUIT Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021

(ma) (OIL k VATBE Mw

I (PBRL k PULP) I 1 WAPOUTOR mCB J - mw -

CON-TED WATBR & OIL VAm COLD-PRESSW NICB VAPORS OIL & I

gSsKNCB OIIS

mpmon. (CRUDE LWOMNE)

Dmurn m WATER-PEASE

FIGURE 4 Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1990/99779/72/2370597/cec1990-3605.pdf by guest on 23 September 2021

COLD PRESSED OIL AND LIMONENE RECOVERY ------

TOTAL OIL, US LIMONENE, LBS & LIMONENE /I000 BOXES /I000 BOXES /I000 BOXES ------.. ------Plant # 1

------. Plant #2

FIGURE 5