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  1. 1. D-Limonene 1. Introduction With the production phaseout of chlorofluorocarbons (CFCs) and other ozone-depleting chemicals and increased awareness of workplace safety, many different cleaning solvents have been introduced. d-Limonene has shown great effectiveness in the cleaning market and is experiencing a growing acceptance as the solvent of choice in a number of different applications. d-Limonene is a non-water-soluble solvent. It can be used straight, thickened to a gel or blended with an emulsification system to produce a water dilutable/rinsable product. It is capable of effectively removing organic dirt loads ranging from light cutting oils and lubricants to heavy greases. It had been customary to use petroleum-derived or halogenated hydrocarbon solvents or high levels of caustics to clean soils such as greases, dirt, grime, asphalt deposits and burned or oxidized films. But all of those cleaning materials are either hazardous when used or provide residues that are increasingly troublesome environmental pollutants. Solvents of the type mentioned above have several disadvantages. Some of the halogenated hydrocarbon solvents have been shown to adversely affect worker health, so their use either has been eliminated or drastically curtailed by regulations. Also, many of these solvents and the compositions derived from them are flammable. This limits their use in situations in which fire and explosion are potential hazards. Among the chemical alternatives turned to by manufacturers seeking to avoid these environmental problems is d-limonene. In addition to being a powerful solvent, d-limonene leaves a fresh fragrant, natural odor that dissipates over time. Limonene (C10H16) (without the chemical prefix "d") is a major component of orange and lemon oils and belongs to a group of hydrocarbon compounds known as monoterpenes. Monoterpenes, a subclass of terpenes, can have an acyclic or cyclic C10 hydrocarbon structure or can be their oxygenated derivatives. Some molecules possess “handedness.” They call these left and right handed pairs enantiomers, which are mirror images of each other. The favorite example of enantiomers which everyone can relate to is the chemical which is responsible for the smell of oranges and lemons. The name of the chemical, limonene, comes from the lemon but is present in many citrus fruit rinds. D-Limonene (Orange) & L-Limonene (Lemon) 1
  2. 2. By the look of the way that molecular diagram is drawn, there is no difference between the two molecules. The picture shows the mirror plane but does not illustrate the difference. The filled in wedge (D-Limonene) compared to the dashed-line wedge (L-Limonene) indicate whether that bond in the chemical comes out of or into the plane (i.e. out of your monitor or into your monitor). D-Limonene (Orange) L-Limonene (Lemon) In many cleaning applications, the soils to be removed are organic oils and greases. Because of the inherent chemical differences between organic and inorganic materials, such as polarity and ionic effects, organic solvents tend to perform much better for cleaning these types of soils than water-based solutions. To compare organic solvent strengths, the Kauri-butanol (KB) value, an ASTM method (D1133-97), has been established. The more toxic chlorinated solvents and benzene and its related compounds are all extremely effective cleaning solvents and have high KB values. The KB value of d-limonene is a bit lower, but higher than that of petroleum-derived products. It is not possible to perform the KB test on oxygenated compounds, so there is no listed value for methyl ethyl ketone (MEK) or acetone. KB Values, Comparative Strength (Solvency) of Industrial Solvents (Higher Values = Higher Dissolving Power) Solvent KB Value Methylene Chloride 136 Trichlorethylene 129 Benzene 107 Toluene 105 Xylene 98 Perchloroethylene 92 d-limonene 67 Mineral Spirits 37 Naphtha 34 Kerosene 34 Stoddard Solvent 33 MEK N/A Acetone N/A 2
  3. 3. 2. Production d-Limonene is the major component of the oil extracted from the citrus rind during the citrus juicing process. When the fruit is juiced, the oil is pressed out of the rind, then separated from the juice and distilled to recover certain flavor and fragrance compounds. The bulk of the oil is left behind and collected. This is food grade d- Limonene. After the juicing process, the peels are conveyed to a steam extractor. When the steam is condensed, a layer of oil floats on the surface of the condensed water. This removes the bulk of the oil from the peel. This is technical grade d-Limonene. The chart below shows the citrus oil manufacturing process and the specific oils that come from each part of the process. d-Limonene is a thin, relatively colorless liquid. The typical physical and chemical properties for technical grade are: TYPICAL PROPERTIES Slight yellow to water Color white Odor Orange aroma Specific gravity (25°C) 0.83800.843 Refractive index (20°C) 1.4710 to 1.4740 Optical rotation (25°C) +96° to +104° Flash point 115°F (46,1°C) Boiling point 178°C (310°F) Freezing point -96°C (-140°F) Evaporation rate 0.05 vs. butyl acetate Water solubility Insoluble Vapor pressure (20°C) 1.4 mmHg 3
  4. 4. 3. Safety and Environmental Concerns From a personal-safety standpoint, d-limonene is a much safer product for use than most other solvents. The oral LD50 (lethal dose required to kill 50% of test animals within a specified time) of d-limonene is greater than 5000 mg/kg body weight. For comparison, the typical mineral spirit LD50 is around 2000 mg/kg body weight. d-Limonene is also noncaustic and nonreactive to metal surfaces. It does not leave any significant residue and it is non-corrosive to metals. It does not cause rusting or oxidation of any materials. But it will attack some plastics, rubber, paint and polymers at high concentration. This is important because seals in high-pressure pumps can be adversely affected by d-Limonene. It has been classified as a slight skin irritant, because it can remove the naturally occurring oils from skin, but has not been shown to cause lasting damage. It is not carcinogenic or mutagenic. d-Limonene is not itself and does not contain any ozone-depleting chemicals. It is currently regulated as a volatile organic compound (VOC). The evaporation rate of d-limonene is relatively low, so the actual VOC emissions are small. d-Limonene is not considered an air toxic or hazardous air pollutant (HAP), and is not regulated under the Clean Air Act. The issue of global warming as it pertains to the recovery and use of d-limonene is difficult, and no reliable estimate has been completed. When plants create d-limonene, or any terpene, they use carbon dioxide and water. When a terpene is destroyed or degrades, carbon dioxide and water are produced. So the creation and destruction of d-limonene would result in a net zero global-warming effect. When d-limonene/surfactant/water systems are made, the closed cup flash point will generally rise to about 130°F (54,4°C). These solutions will have a very high open cup flash point, and will not support a flame at any temperature below boiling. Pure d-limonene is lighter than water. The commercial product has a boiling range of 310-352°F (154-177°C). It has a flash point of 119 °F (48°C) and is a combustible liquid. It has negligible solubility in water. d-Limonene is an extremely effective and relatively safe cleaner and solvent for use in many industries. It can be used in a wide variety of applications and in most cases will perform better and longer than the classic solvents. Although it is not perfect, it is a good option to be considered when choosing a cleaning system or looking for an effective solvent replacement. 4