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)
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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
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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
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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.
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