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Fatty Acid Estolides: A Review

Authors:

Abstract

Estolides are bio‐based oils synthesized from fatty acids or from the reaction of fatty acids with vegetable oils. Estolides have many advantages as lubricant base oils, including excellent biodegradability and cold flow properties. Promising applications for estolides include bio‐lubricant base oils and in cosmetics. In this review, the synthesis of estolides from fatty acids using four different types of catalysts, namely, mineral acids, solid acids, lipases, and ionic liquids, is summarized. The summary includes the yield of estolide obtained from varying synthetic conditions (time, temperature, catalyst). Also reviewed are studies comparing the physical properties of estolides synthesized from refined fatty acids against those synthesized from fatty acid mixtures obtained from vegetable oils such as coconut, castor, Physaria, etc. By varying the structure of the fatty acids, estolides with a wide range of pour point, cloud point, and viscosity are synthesized to meet a wide range of application requirements. Currently, estolide products are being commercialized for personal care and lubricant base oils for automotive, industrial, and marine applications. The application areas and the demand for estolides is expected to grow as the drive for switching from petroleum to bio‐based products keeps growing.
REVIEW
Fatty Acid Estolides: A Review
Yunzhi Chen
1,2
· Girma Biresaw
2
· Steven C. Cermak
2
· Terry A. Isbell
2
· Helen L. Ngo
3
·
Li Chen
1
· Amber L. Durham
2
Received: 25 September 2019 / Revised: 4 December 2019 / Accepted: 11 December 2019
Published 2020. This article is a U.S. Government work and is in the public domain in the USA
Abstract Estolides are bio-based oils synthesized from
fatty acids or from the reaction of fatty acids with vegeta-
ble oils. Estolides have many advantages as lubricant base
oils, including excellent biodegradability and cold ow
properties. Promising applications for estolides include
bio-lubricant base oils and in cosmetics. In this review,
the synthesis of estolides from fatty acids using four dif-
ferent types of catalysts, namely, mineral acids, solid
acids, lipases, and ionic liquids, is summarized. The sum-
mary includes the yield of estolide obtained from varying
synthetic conditions (time, temperature, catalyst). Also
reviewed are studies comparing the physical properties of
estolides synthesized from rened fatty acids against those
synthesized from fatty acid mixtures obtained from vege-
table oils such as coconut, castor, Physaria, etc. By vary-
ing the structure of the fatty acids, estolides with a wide
range of pour point, cloud point, and viscosity are synthe-
sized to meet a wide range of application requirements.
Currently, estolide products are being commercialized for
personal care and lubricant base oils for automotive,
industrial, and marine applications. The application areas
and the demand for estolides is expected to grow as the
drive for switching from petroleum to bio-based products
keeps growing.
Keywords Bio-lubricant Estolide Estolide-free acids
Estolide 2-ethylhexyl esters Estolide number Estolide
synthesis
J Am Oil Chem Soc (2020).
Introduction
Estolides are oligomeric fatty acid esters that contain sec-
ondary ester linkages on the alkyl backbone of the mole-
cule (Bredsguard et al., 2016; Cermak et al., 2017;
Cermak and Isbell 2004a; Isbell and Kleiman, 1994).
Estolides have been detected in the natural seed oil of sev-
eral plant families such as Euphorbiaceae, Brasicaceae,
Limmanthaceae, and Asteraceae (Burg and Kleiman,
1991; Hayes and Kleiman, 1995; Kleiman et al., 1972).
Naturally occurring estolides have also been found in
secretions from the glandular hairs of a caterpillar Pieris
rapae (Smedley et al., 2002) and in human meibum lipids
as a wax ester (Butovich et al., 2009). Synthetic estolides
can be broadly classied into two categories: (1) glycer-
ide-based estolides and (2) fatty acid-based estolides
(Fig. 1). Glyceride estolides are obtained by the reaction
of vegetable oils (triacylglycerols) with fatty acids con-
taining hydroxyl groups somewhere in their chains (Isbell
and Cermak, 2002; Romsdahl et al., 2019). Fatty acid
estolides are obtained by the reaction of two fatty acids, at
least one of which contains a hydroxy, double-bond(s), or
**Girma Biresaw
girma.biresaw@usda.gov
1
School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou, 510641, China
2
Bio-Oils Research Unit, National Center for Agricultural
Utilization Research, Agricultural Research Service, United
States Department of Agriculture, Peoria, IL 61604, USA
3
Sustainable Biofuels and Co-products Research Unit, Eastern
Regional Research Center, Agricultural Research Service, United
States Department of Agriculture, Wyndmoor, PA 19038, USA
Mention of trade names or commercial products in this publication is
solely for the purpose of providing specic information and does not
imply recommendation or endorsement by the U.S. Department of
Agriculture. USDA is an equal opportunity provider and employer.
J Am Oil Chem Soc (2020)
DOI 10.1002/aocs.12323
J Am Oil Chem Soc (2020)
an epoxide functional group. A wide range of estolides
structures can be obtained by changing the structure of the
fatty acids (e.g., chain length, branching, etc.). The molec-
ular weights of estolides can also be varied by changing
the degree of oligomerization, which provides a capability
for synthesizing molecules with targeted physical proper-
ties (Cermak and Isbell, 2009; Isbell, 2011). The structure
of fatty acid estolides enables them to have outstanding
properties such as reducing unsaturation, which improves
oxidation stability, and the molecular branching, which
improves cold ow temperature properties. The excellent
hydrolytic stability, biodegradability, natural detergency,
low volatility, and high viscosity index of fatty acid
estolides make it applicable in many areas, including in
cosmetics, rubber, plastics, paints, coating, food, and bio-
lubricants. This review focuses primarily on the synthesis,
physical properties, and applications of fatty acid
estolides.
Synthesis of Fatty Acid Estolides
Estolides are obtained by oligomerization of fatty acids.
There are two types of oligomerization reactions in the syn-
thesis of fatty acid estolides, namely, condensation and
addition. In the condensation reaction, a hydroxy group in
one fatty acid reacts with the carboxylic acid of the second
fatty acid, a molecule of water is lost, and the hydroxy fatty
acid estolides is formed (Fig. 2). In the addition reaction,
the carboxylic acid group of one fatty acid is added to the
double bond of another fatty acid (Fig. 3) (Isbell et al.,
1992). In this review, we discuss four types of catalysts
used for the esterication reactions, namely, mineral acids,
solid acids, lipases, and ionic liquids.
Mineral Acids
In this section, we discuss the application of mineral acids
for estolide synthesis. Mineral acids when mixed with fatty
(d)
(c)
(b)
(a)
Fig. 1 Estolides from triglyceride (a,b) and from fatty acids (c,d)
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acids generate an emulsion that enables greater contact
between catalyst and fatty acids and can potentially result
in a higher product yield and reaction rate. Burton and
Byrne (1953) and Showell et al. (1968) reported that small
amounts of estolide byproducts were detected when they
treated oleic acid with concentrated sulfuric acid and 70%
perchloric acid. Isbell et al. (1994) studied several mineral
acids for the production of oleic acid estolides, and the
results are summarized in Table 1. Hydrochloric acid, phos-
phoric acid, and nitric acid gave a lower yield of estolides
due to the hydrolysis of estolides caused by a large
proportion of water in these catalysts. Due to its excellent
performance, perchloric acid was widely applied with other
fatty acids from new crop oils such as pennycress (Cermak
et al., 2015b), coriander (Cermak et al., 2011), cuphea
(Cermak and Isbell 2004b), and meadowfoam, from which
71% yield was obtained (Isbell and Kleiman, 1996). It was
observed that temperature plays a critical role in these ester-
ication reactions. The reaction rate is low at lower temper-
atures, while the yield decreased above an optimum
temperature due to the production of lactone byproducts
(Isbell et al., 1997; Isbell and Kleiman, 1996). The
Fig. 2 Example of estolide-free acid synthesized from hydroxy fatty acid
Fig. 3 Example of estolide-free acid synthesized from saturated and unsaturated fatty acids
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concentration of catalyst also inuenced the reaction signif-
icantly. Generally, high acid concentrations gave faster
reaction rates and higher yield but also increased the
amount of side products (Isbell et al., 1994, 1997).
Solid Acids
Unlike homogeneous catalysts, the application of heterog-
enous catalysts for estolide synthesis has several advan-
tages such as recyclability and easy separation, which is
highly desirable in industrial products. Erhan and Isbell
(1997) reported the application of modied clay catalysts
to produce estolides from meadowfoam fatty acid and
oleic acid. The esterication reaction was catalyzed by
acidic sites on the clay. Yields catalyzed by montmoril-
lonite clay, montmorillonite clay treated by Fe
3+
,and
montmorillonite clay treated with cationic surfactants
were as follows: 21.0%, 27.0%, and 30.0%, respectively.
Borugadda and Dalai (2018) reported the synthesis of
estolides from epoxidized canola biodiesel and oleic acid
using mesoporous aluminosilicates with the Modernite
Framework Inverted-type pentasil structure. These hetero-
geneous acidic catalysts, at 15.0 wt% of loading, 6 hours
of reaction time, and 110 C of reaction temperature,
result in >95.0% conversion of reactant. Nordin et al.
(2011) used silica containing 5.045.0% perchloric acid to
synthesize oleicoleic estolides for 10 hours at 70Cand
observed 99.0% conversion of the oleic acid and 64.0%
oleicoleic monoestolide acid selectivity. Nordin et al.
(2012) screened several heterogeneous catalysts, includ-
ing HClO
4
/silica, HClO
4
/silica gel, and HClO
4
/alumina,
and found that silica gel treated with 10.0 wt% perchloric
acid was best for the oleicoleic estolides synthesis. It
gave a nal conversion of 97.5% with 79.8% selectivity to
oleicoleic monoestolide acid and 17.7% selectivity to
oleicoleic diestolide acid.
Lipases
The use of enzymes as biocatalysts allows for milder reac-
tion conditions (e.g., lower temperatures in the range
2040C). Enzymes also produce better selectivity than
inorganic catalysts. Yamaguchi et al. (1989) were the rst
to report the esterication of castor oil in a two-step
enzymatic reaction to obtain estolides with high yield.
Since then, numerous reports have conrmed that lipases
are effective catalysts for the synthesis of hydroxy fatty
acid estolides. Generally, they provide good stereo-
selectivity and fewer side reactions compared to conven-
tional catalysts. Hayes (1996) summarized the effect of
various lipase types on the hydroxyl group of different
hydroxy fatty acids. It is concluded that the selectivity of
lipases was strongly dependent on the position of the
hydroxyl group in the hydroxy fatty acid chain, slightly on
the hydroxy acid chain length, and on lipase concentration.
Hayes and Kleiman (1995) ran a series of esterication
reactions using a mixture of lesquerolic acid and
octadecenoic acid. They observed a yield of 62.85% in the
presence of Pseudomonas sp. at 22 C after 23 days.
Bódalo-Santoyo et al. (2005) reported the synthesis of
ricinoleic acid estolides with Candida rugosa. They
obtained ricinoleic acid estolides with an acid number of
65. Aguieiras et al. (2011) used commercial lipases such as
Novozym 435, Lipozyme RM-IM, and Lipozyme TL-IM
to synthesize estolides from oleic acid and methyl
ricinoleate and observed up to 15% of conversion. Martin-
Arjol et al. (2013) synthesized 10(S)-hydroxy-8(E)-
octadecenoic acid mono-estolides with Novozym 435 cata-
lyst and obtained a maximum of 30% yield. Todea et al.
(2015) synthesized estolides from 16 and 18 hydroxy fatty
acids using several native immobilized lipases and obtained
estolides with a degree of polymerization of up to 10 and a
conversion of greater than 80%.
Ionic Liquids
Ionic liquids are salts of organic cations and inorganic and/or
organic anions. Ionic liquids are environmentally friendly and
have attracted much attention because of their low volatility,
thermal stability, and easy separation properties. They have
been used as green solvents and catalysts for many reactions
(Wang and Sun, 2017). Adnan et al. (2010) reported a series of
Lewis acid ionic liquid catalysts, including choline chloride-
zinc chloride (ChCl-ZnCl
2
), choline chloride-iron(III) chloride
(ChCl-FeCl
3
), choline chloride-tin(II) chloride (ChCl-SnCl
2
),
and choline chloride-copper(II) chloride (ChCl-CuCl
2
), for the
synthesis of the monoestolides from oleic acid. ChCl-ZnCl
2
exhibited the highest activity of 98% oleic acid conversion and
Table 1 Synthetic conditions for oleic acid estolides using different mineral acid catalysts
Mineral acid catalyst Oleic:catalyst equivalent Temperature (C) Reaction time (h) Estolide yield (%)
Perchloric acid 1.0:1.0 50 2 76
Sulfuric acid 1.0:1.0 50 6 65
Boron triuoride etherate 1.0:0.2 50 7 71
p-Toluenesulfonic 1.0:1.0 100 24 45
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80% selectivity of monoestolides. Wang and Sun (2017)
compared different ionic liquid catalysts, including
1-butylsulfonic-3-methylimidazolium tosylate [BSO
3
HMIM]
TS, 1-butylsulfonic-3-methylimidazolium hydrogen sulfate
[BSO
3
HMIM]HSO
4
, 1-butylsulfonic-3-methylimidazolium
triuoromethanesulfonate [BSO
3
HMIM]OTF, and
1-butylsulfonic-3-methylimidazolium hydrophosphate
[BSO
3
HMIM]HPO
4
for the synthesis of ricinoleic acid
estolides. Among the tested ionic liquids, [BSO
3
HMIM]TS
showed the best performance and gave a nal acid number
of 48 under the optimized synthesis conditions.
Physical Properties of Fatty Acid Estolides
Fatty acid estolides are becoming more popular for indus-
trial applications and scientic research because they can
be produced in a wide range of molecular structures and
molecular weights. In the past several decades, many fatty
acid estolides with varying properties have been synthe-
sized by using different combinations of fatty acids and dif-
ferent reaction conditions. There are several factors that
inuence the physical properties of estolides, including
estolide number (EN = n+ 1, where n is the degree of olig-
omerization of estolides), degree of unsaturation, hydrocar-
bon chain length, and position of the estolide ester groups.
For bio-lubricant applications, low-temperature ow prop-
erties, viscosity, and oxidation stability are among the most
critical properties that need to be evaluated.
Temperature affects the ow properties of lubricants.
The viscosity of oil increases below its pour point, and its
ow to bearings and other machine elements is restricted.
High viscosity requires high startup torque for engines, or
it might even prevent startup, cause excessive friction, and
a complete failure. The low-temperature limit for starting
an oil-lubricated engine depends on the cloud point and
pour point of the oil. Cloud point (ASTM D-2500) refers
to the temperature below which waxes and clouding
appears. The presence of solidied waxes thickens the oil
and clogs fuel lters and injectors in engines. Pour point
is the lowest temperature at which oil will ow when chil-
led under prescribed laboratory conditions (ASTM D97).
For most mineral-based industrial lubricants (designated
as turbine, hydraulic, industrial, and machine oils), the
pour point is the temperature at which the parafnmole-
cules of the oil solidify into white crystalline wax that
eventually immobilize the oil. Thus, lower pour point and
cloud point are very desirable for bio-lubricants. Estolides
usually have lower cold ow temperatures than
triacylglycerols due to the presence of branching in their
structures. Thus, estolides are promising materials for bio-
lubricant applications.
Viscosity is a measure of the internal friction in a uid.
It is one of the most important physical properties of a uid
for lubricant applications. Viscosity decreases with increas-
ing temperature. Viscosity index (VI) is used to evaluate
the temperature dependent on uid viscosity. The higher
the VI of an oil, the less its viscosity changes with tempera-
ture. The VI can be calculated using the method described
in Stachowiak and Batchelors publication (Stachowiak and
Batchelor, 2013). Oils with higher VI have benets,
e.g., higher viscosity gives thicker lubricant lm and lower
wear. Estolides with different viscosity grades can be pro-
duced to meet the different application requirements of
lubricants.
Oxidation stability is an important property of lubricants.
It is a measure of the chemical stability of the lubricating
oil when subjected to high temperature under oxygen in the
presence of water and catalyst. Poor oxidation stability
leads to oil degradation and changes in oil viscosity, acid
number, generation of varnish, and sludge deposits. Several
methods are used to measure the oxidation stability. One of
the methods is the rotating pressure vessel oxidation test
(RPVOT)-ASTM D2272, formerly called the rotating
bomb oxidation test (RBOT). In this method, a longer
RPVOT time (in minutes) corresponds to better oxidation
stability of the oil.
Estolides can be synthesized from puried fatty acids or
vegetable oil fatty acids. Rened fatty acids are highly puri-
ed fatty acids (>90%) obtained commercially, such as
oleic acid, lauric acid, stearic acid, etc. Vegetable oil fatty
acids, on the other hand, are fatty acids obtained by split-
ting (hydrolyzing) vegetable oils such as coconut, soybean,
and canola oils. Vegetable oil fatty acids are mixtures of
fatty acids with compositions corresponding to that of the
vegetable oil. Table 4 gives the fatty acid compositions of
some vegetable oils. The physical properties of estolides
from rened fatty acids and vegetable oils fatty acids are
summarized below.
Physical Properties of Estolides from Rened Fatty
Acids
Estolides synthesized from rened fatty acid are usually
purer and easier to analyze than those from fatty acid mix-
tures obtained from vegetable oils. The properties of
estolides are inuenced by the properties of the fatty acids,
such as chain length, molecular weight, and degree of
unsaturation. Estolides synthesized from fatty acids can be
categorized into two groups: (1) estolide-free acids, where
the estolide fatty acid is not esteried (Fig. 4a), and
(2) estolide esters, where the estolide fatty acids are esteri-
ed (Fig. 4b). As illustrated in Fig. 4b, most estolides are
esteried with 2-ethyl-hexanol (2-EH).
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Estolide-free acids and estolide 2-EH esters have differ-
ent physical properties. The physical properties of
estolide-free acids and 2-EH esters of oleic acid with
homologous series of rened fatty acids are shown in
Tables 2 and 3, respectively. The data show that, as the
carbon number of the fatty acid increases, the EN decrease
because the shorter-chain-length fatty acids are less reac-
tive. A capped rate reects the degree of unsaturation of
estolides. Addition of saturated fatty acids to the reaction
mixture reduces the reactive double bonds and results in
shorter oligomer estolides. Consequently, the estolide is
stopped at this point from further rapid growth; thus, we
refer to the estolide as being capped(Cermak and Isbell,
2001) (Fig. 5). Thus, higher capping corresponds to higher
saturation, higher pour point, and higher cloud point.
Estolide 2-EH esters generally have a lower pour point
and lower cloud point than estolide-free acids because the
branched chain structure of the 2-EH group. Estolide
2-EH esters also have lower viscosity than estolide-free
acids. VI does not vary with the degree of capping.
Estolide 2-EH esters generally have higher VI than
estolide-free acids.
(a)
(b)
Fig. 4 Example of estolide-free acid (a) and estolides 2-ethylhexyl ester (b)
Table 2 Physical properties of estolide-free acids synthesized from oleic acid and a homologous series of pure saturated fatty acids
Estolides EN Capped (%) Pour point (C) Cloud point (C) Viscosity 40 C (cSt) Viscosity 100 C (cSt) Viscosity index
OL-4:0 3.3 33 27 26 410.0 39.9 146
OL-6:0 3.3 34 24 27 515.5 39.7 122
OL-8:0 2.9 42 24 24 389.1 37.7 143
OL-10:0 2.7 53 21 342.0 34.0 142
OL-12:0 2.2 58 25 27 262.6 28.7 145
OL-14:0 1.8 65 18 6 282.3 30.4 146
OL-16:0 1.9 68 10 12 267.1 28.7 143
OL-18:0 1.4 43 32 296.5 31.0 143
Source: Cermak and Isbell (2002).
4:0, butyric; 6:0, caproic; 8:0, octanoic; 10:0, decanoic; 12:0, lauric; 14:0, myristic; 16:0, palmitic; 18:0, stearic; EN, estolide number; OL,
oleic acid.
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Physical Properties of Estolides from Vegetable Oil
Fatty Acids
Even though highly rened fatty acids give highly pure
estolides with improved properties, they are also expensive
and very uncompetitive in cost. On the other hand, lower-
cost fatty acids that are typically less rened or are mixtures
of fatty acids of varying chain lengths and unsaturation pro-
vide a cheaper alternative for estolide synthesis (Isbell and
Cermak, 2014). Table 4 gives the fatty acid compositions
of some vegetable oils. The physical properties of estolide-
free acids and 2-EH esters synthesized from different vege-
table oil fatty acids are summarized in Table 5. The data
show the lowest pour point of 54 C and a viscosity range
of 29679 cSt at 40 C.
Cermak et al. (2015a) also synthesized oleiccoconut
estolides esteried with different branched chain alcohols
such as 2-butyloctanol, 2-hexyldecanol, 2-octyldodecanol,
Table 3 Physical properties of estolide 2-EH esters synthesized from oleic acid and a homologous series of saturated fatty acids
Estolides EN Pour Point (C) Cloud point (C) Viscosity 40 C (cSt) Viscosity 100 C (cSt) Viscosity index
OL-4:0-2EH 2.8 30 36 125.5 19.3 175
OL-6:0-2EH 3.5 30 34 114.5 17.9 174
OL-8:0-2EH 3.0 36 41 104.4 16.8 175
OL-10:0-2EH 2.7 39 93.8 15.5 176
OL-12:0-2EH 2.2 36 32 73.9 13.0 179
OL-14:0-2EH 2.0 25 22 80.5 13.9 179
OL-16:0-2EH 1.4 12 13 81.6 13.5 174
OL-18:0-2EH 1.1 15 4 81.8 14.0 177
Source: Cermak and Isbell (2002).
OL, oleic acid; 4:0, butyric; 6:0, caproic; 8:0, octanoic; 10:0, decanoic; 12:0, lauric; 14:0, myristic; 16:0, palmitic; 18:0, stearic; 2-EH, 2-ethyl
hexyl; EN, estolide number.
(a)
(b)
Fig. 5 Example of estolide-free acid without double bond (capped) (a) and with double bond (uncapped) (b)
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and iso-stearyl alcohol. The coco-oleic dimer estolide esters
(EN = 1) had the lowest pour point of 45 C when esteri-
ed with 2-hexyldecanol. The viscosity of the coco-oleic
dimer estolide esters varied from 27.5 to 51.7 cSt at 40 C
and 3.0 to 9.5 cSt at 100 C. The viscosity of coco-oleic tri-
mer plus estolide esters (EN 2) ranged from 120.8 to
227.7 cSt. at 40 C and 17.9 to 29.4 cSt. at 100C.
Overall, most fatty acid estolides have excellent cold
ow properties and VI. The data indicate promising proper-
ties for estolides in the bio-lubricant area.
Application of Estolides
Bio-lubricant is one of the major applications of fatty acid
estolides. As early as 2000, Isbell et al. (2000a) evaluated
oleic estolide ester as a potential bio-based base oil for
lubricant applications. Other applications of estolides
include motor oils, truck engine oils, truck piston oils,
scooter/motorcycle oils, transmission uids, gear oil, pro-
cess oils, hydraulic oils, compressor uids, grease, metal-
working uids, and cylinder oils (Bredsguard, 2016;
Table 4 Fatty acid composition of vegetable oils (%, w/w)
FA Phy.
a
Cas.
a
Coco.
b
Mdf.
c
Cuph.
d
HOS
e
Cori
b
Penn.
f
Cram.
g
8:0
h
—— 5.1 0.6 ——— —
10:0 —— 4.9 65.6 0.5 ——
12:0 ——48.3 3.2 ——— —
14:0 ——21.7 6.5 ——0.1 0.1
14:1 —— — —
15:0 —— — —
16:0 1.1 1.0 10.6 0.5 7.2 5.3 3.4 2.5 1.6
16:1 0.7 —— — — 0.2 0.3
17:0 —— — —
17:1 —— — —
18:0 1.8 7.0 0.8 3.4 0.8 0.4 0.8
18:1 15.4 3.7 2.4 1.4 9.1 81.5 81.3 11.4 17.9
18:2 6.9 4.4 0.5 5.9 6.9 14.0 20.7 6.9
18:3 12.2 —— — — —17.1 6.7
20:0 0.2 ——0.5 0.6 0.1 0.7
20:1 1.0 ——64.0 0.3 5.9 2.5
20:2 0.2 —— — — 1.8 0.3
22:0 —— — 1.0 0.1 1.4
22:1 —— —10.0 ——35.6 58.6
22:2 —— —19.0 ——0.8 0.3
22:3 —— — — 0.1 0.3
24:0 —— — 0.5 —— 0.3
24:1 —— — — 3.2 1.2
18:1-OH 0.6 89.0 ——— —— —
20:1-OH 55.4 1.1 ————— —
20:2-OH 3.8 —— — — ——
Cas., castor oil; Coco., coconut oil; Cori., coriander oil; Cram., crambe oil; Cuph., cuphea oil; FA, fatty acid; HOS, high oleic sunower oil;
Mdf., meadowfoam oil; Penn., pennycress oil; Phy., physaria oil; Tal., tallow oil.
a
Cermak et al. (2006).
b
Cermak et al. (2011).
c
Erhan et al. (1993).
d
Cermak and Isbell (2004b).
e
García-Zapateiro et al. (2010).
f
Cermak et al. (2015b).
g
Earle et al. (1966).
h
Numbers x:y, represent total carbons in the fatty acid, x; and total number of double bonds in fatty acid, y;OHhydroxyl group somewhere
on the chain.
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Bredsguard et al., 2012a, b; Forest and Bredsguard, 2016;
Forest and Parson, 2015). Fatty acid estolide products are
also being developed for application in personal care such
as cosmetics and emollients (Isbell et al., 2000b; Parson
et al., 2015). Stolp et al. (2019) reported that fatty acid
estolide esters made from soybean oil fatty acids have
potential applications as plasticizers. In summary, fatty acid
estolides are promising bio-based products with excellent
physical properties and industrial applications.
Conict of Interest The authors declare that they have no conict
of interest.
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Table 5 Estolide-free acids and estolide 2-EH esters synthesized using mixture of fatty acids from different vegetable oils
a
Estolides EN Capped
(%)
PP
(C)
CP
(C)
MP
(C)
Visc.
40 C (cSt)
Visc.
100 C (cSt)
VI RPVOT
(min)
Oleic-Castor
b
—— 54 <54 34.5 7.6 196 159
Stearic-Castor
b
—— 32341.7 8.6 191
Coconut-Castor
b
—— 36 30 29.0 6.5 186
Oleic-Physaria
b
—— 48 35 35.4 7.8 200
Stearic-Physaria
b
—— 31238.6 8.2 195
Coconut-Physaria
b
—— 24 <24 40.4 8.4 192
Crambe
c
—— — — 0 679.0 58.6 151
Crambe-2EH
c
—— — —12 184.4 26.1 177
Meadowfoam
c
—— — — 6 229.8 27.4 154
Meadowfoam-2EH
c
—— — — 1 104.2 16.5 172
Oleic-Cuphea
d
1.8 27 23 213.4 24.0 140
Oleic-Cuphea-2EH
d
1.3 48 33 34 65.3 11.7 177
High Oleic Sunower
e
—— — — 430.8 50.1 ——
Coconut-Coriander-2EH
f
1.4 45.6 24 25 55.9 9.8 162 273
Oleic-Pennycress-2EH
g
2.2 7.4 24 17 191.7 27.1 178
Coconut-Oleic
h
1.8 27 29 317.7 33.0 145
CP, cloud point; EN, estolide number; MP, melting point; PP, pour point; RPVOT, rotating pressurized vessel oxidation test; VI, viscosity index;
Visc., viscosity.
a
Fatty acid mixtures were obtained from hydrolysis (splitting) of triacylglycerols of castor, Physaria, crambe, meadowfoam, cuphea, high oleic
sunower, coriander, and pennycress. The fatty acid compositions of the vegetable oils are given in Table 4.
b
Cermak et al. (2006).
c
Isbell et al. (2001).
d
Cermak and Isbell (2004).
e
García-Zapateiro et al. (2010).
f
Cermak et al. (2011).
g
Cermak et al. (2015b).
h
Cermak et al. (2015a).
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