ASME 1990 Citrus Engineering Conference CEC1990 March 29, 1990, Lakeland, Florida, USA 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 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.