Castor oil: A vital industrial raw material

D.S. Ogunniyi

Department of Chemistry, University of Ilorin, Ilorin, Nigeria

Abstract

Even though castor oil is inedible, it has long been an article of commerce. This is, in large measure, due to the versatility of the

oil. This article discusses the extraction of castor oil and its refining methods and reviews the industrial applications of the oil. Since

castor oil is not edible, it could be substituted in many industrial application areas where edible oils are used. An awareness of the

various uses of the oil can be used to make a strong case for an increase in its production as a vital raw material for the chemical

industries.

Keywords: Castor oil; Raw material

1. Introduction

The trade in castor oil as an item of commerce goes

back to antiquity. The oil is obtained from extractingor expressing the seed of a plant which has the botanical

name Ricinus communis of the family Eurphorbiacae

(Kirk-Othmer, 1979). The oil is not only a naturally-

occurring resource, it is inexpensive and environmen-

tally friendly. Castor oil is a viscous, pale yellow

non-volatile and non-drying oil with a bland taste and

is sometimes used as a purgative. It has a slight charac-

teristic odour while the crude oil tastes slightly acridwith a nauseating after-taste. Relative to other vegetable

oils, it has a good shelf life and it does not turn rancid

unless subjected to excessive heat. India is the world�slargest exporter of castor oil; other major producers

are China and Brazil as shown in Table 1.

There are different varieties of castor seeds but on the

average, they contain about 46–55% oil by weight. Cas-

tor seeds are poisonous to humans and animals becausethey contain ricin, ricinine and certain allergens that are

toxic. If castor seed is accidentally ingested, it will bring

about abdominal pain, vomiting and diarrhea. Indeed,

as little as 1 mg of ricin can kill an adult. The pure oil

however, if administered in recommended quantities,can be used as a laxative. It is noteworthy that the qual-

ity of seed oil is hardly affected by the variation in good

or poor seeds. There are numerous other medicinal uses

of castor oil but that is not the focus of this discussion.

The castor plant grows in the wild in large quantities

in most tropical and sub-tropical countries. It is avail-

able at low cost and the plant is known to tolerate

varying weather conditions. Specifically, castor plantrequires a temperature of between 20 and 26 �C with

low humidity throughout the growing season in order

to obtain maximum yields. The weather conditions for

its growth limit its cultivation to tropical areas of the

developing world. Also, the fear of accidental ingestion

of the poisonous seed by children does not encourage

the use of castor plant for ornamental purposes. Simi-

larly, the seed cake is poisonous and consequentlyunsuitable as animal feed. Indeed, some people who

work with the meal may develop allergic reactions such

as asthma. Generally, the toxicity of castor seed is a

reason why US farmers no longer grow the crop

Table 1

Production volume of castor oil by major producersa

Major producers 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

�000 t 000 t �000 t �000 t �000 t �000 t �000 t �000 t �000 t �000 t �000 t

India 192 239 232 242 271 333 344 278 304 294 324

China, PR 77 86 93 97 97 82 73 83 80 91 105

Brazil 77 73 54 28 28 22 21 43 21 19 52

Thailand 18 18 19 18 16 14 10 9 9 7 5

E.U.b 20 16 14 12 14 11 9 10 7 8 8

Others 54 52 49 41 20 21 22 19 20 23 23

Total 438 484 461 438 446 483 479 442 441 442 517

a Source—http://www.ciara.com.ar/estadize.htm.b E.U.—European Union.

extensively. The total world production of seeds is esti-

mated at one million tonnes and the oil extracted is

about 500,000 tonnes. Information about the agricul-

tural production of castor oil is available from past

work (Roetheli et al., 1991; Woodend, 1993), and other

publications (Weiss, 1971, 1983). The objective of this

review was to highlight the various uses of castor oil

in chemical production. Being a non-edible oil, its in-creased cultivation, especially in developing countries,

and its use can free up some edible oils used in industry

for human consumption.

2. Extraction

The extraction of oil from castor seed is by one or acombination of mechanical pressing and solvent extrac-

tion. In mechanical pressing, the seeds are crushed and

then adjusted to low moisture content by warming in a

steam-jacketed vessel. Thereafter, the crushed seeds are

loaded into hydraulic presses and they are pressed by

mechanical means to extract oil. The oil from mechani-

cal pressing has light colour and low free fatty acids

(Kirk-Othmer, 1979). However, mechanical pressing willonly remove about 45% of the oil present and the

remaining oil in the cake can be recovered only by sol-

vent extraction. In the solvent extraction method, the

crushed seeds are extracted with a solvent in a Soxhlet

extractor or commercial extractor. Solvents used for

extraction include heptane, hexane and petroleum

ethers.

Table 2

Characteristics of castor oil grades

Properties Cold-pressed

oil

Solvent-extracted

oil

Dehydrated

oil

Specific gravity 0.961–0.963 0.957–0.963 0.926–0.937

Acid value 3 10 6

Iodine value (Wij) 82–88 80–88 125–145

Saponification value 179–185 177–182 185–188

3. Refining

As in other vegetable oils, it is usual to refine the

crude oil obtained from either mechanical pressing or

solvent extraction. The main aim of refining is to remove

impurities (e.g., colloidal matter, free fatty acid, colour-

ing matter) and other undesirable constituents, thusmaking the oil more resistant to deterioration during

storage. Refining includes: (a) removing solid and colloi-

dal matter by settling and filtration, (b) neutralizing the

free fatty acid by alkali, (c) removing coloured matter by

bleaching, and (d) deodorizing by treatment with steam

at high temperature and low pressure. The general

method of refining used for edible oils is applicable to

castor oil.

4. Properties

Castor oil, like all other vegetable oils, has different

physical and chemical properties that vary with the

method of extraction. Cold-pressed castor oil has low

acid value, low iodine value and a slightly higher sapon-

ification value than solvent-extracted oil, and it is lighter

in colour. Typical properties are given in Table 2 whilerepresentative composition of the oil is given in Fig. 1.

The chemistry of castor oil is centered on its high

content of ricinoleic acid and the three points of func-

tionality existing in the molecule. These are: (1) the

carboxyl group which can provide a wide range of este-

rifications; (2) the single point of unsaturation which can

be altered by hydrogenation or epoxidation or vulcani-

zation; and (3) the hydroxyl group which can be acety-lated or alkoxylated, may be removed by dehydration

to increase the unsaturation of the compound to give a

semi-drying oil. The hydroxyl position is so reactive

the molecule can be split at that point by high-tempera-

ture pyrolysis and by caustic fusion to yield useful prod-

ucts of shorter chain length. The presence of hydroxyl

group on castor oil adds extra stability to the oil

and its derivatives by preventing the formation of

Fig. 1. Equation showing the constitution of castor oil.

hydroperoxides. The ricinoleic acid comprises over 89%of the fatty acid of the oil. Other fatty acids present are

linoleic (4.2%), oleic (3.0%), stearic (1%), palmitic (1%),

dihy- droxystearic acid (0.7%), linolenic acid (0.3%), and

eicosanoic acid (0.3%). Castor oil consists mainly of es-

ters of 12-hydroxy-9-octadecenoic acid (ricinoleic acid);

thus the presence of hydroxyl groups and double bonds

makes the oil suitable for many chemical reactions and

modifications (see Table 3). The oil is characterized byhigh viscosity although this is unusual for a natural veg-

etable oil. This behaviour is due largely to hydrogen

bonding of its hydroxyl groups. It is soluble in alcohols

in any proportion but it has only limited solubility in ali-

phatic petroleum solvents.

Although castor oil is a unique naturally-occurring

polyhydroxy compound, a limitation of the oil is the

slight reduction of its hydroxyl value and acid valueon storage; both values may change by about 10% if

Table 3

Various reactions of castor oil

Nature of reaction Added reactants

Ester linkage Hydrolysis Acid, enzyme, or T

Esterification Monohydric alcoh

Alcoholysis Glycerol, glycols,

compounds

Saponification Alkalies, alkalies p

Reduction Na reduction

Amidation Alkyl amines, alka

compounds

Double bond Oxidation, polymerization Heat, oxygen, cros

Hydrogenation Hydrogen (modera

Epoxidation Hydrogen peroxid

Halogenation Cl2, Br2, I2

Addition reactions S, maleic acid

Sulphonation H2SO4

Hydroxyl group Dehydration, hydrolysis,

distillation

Catalyst (plus heat

Caustic fusion NaOH

Pyrolysis High heat

Halogenation PCl5, POCl3Alkoxylation Ethylene and/or pr

Esterification Acetic-, phosphori

anhydrides

Urethane reactions Isocyanates

Sulphation H2SO4

Source: http://www.groshea.com.

stored for about 90 days. The reduction of these values

is due to the reaction between hydroxyl and carboxyl

groups in the oil molecule to form estolides.

5. Dehydration

Castor oil has only one double bond in each fatty

acid chain and, so, is classified as a non-drying oil. How-

ever, it can be dehydrated to give semi-drying or drying

oil which is used extensively in paints and varnishes. It

must be noted that coatings that incorporate castor oil

alone will never achieve complete cure through oxidative

crosslinking as do coatings that contain oil with multipledouble bonds in their fatty acid components.

As the name implies, dehydration involves the re-

moval of water from the fatty acid portion of the oil.

Being a polyhydroxy compound, its hydroxyl function-

ality can be reduced through dehydration or increased

by interesterification with a polyhydric alcohol. The

dehydration process is carried out at about 250 �C and

in the presence of catalysts (e.g., concentrated sulphuricacid, activated earth) and under an inert atmosphere or

vacuum. Under this condition of dehydration, the hy-

droxyl group and an adjacent hydrogen atom from the

C-11 or C-13 position of the ricinoleic acid portion of

the molecule is removed as water (see Fig. 2). This yields

a mixture of two acids, each containing two double

bonds but in one case, they are conjugated. The presence

Type of products

witchell reagent catalyst Fatty acids, glycerol

ols Esters

pentaerythritol, and other Mono- and diglycerides,

monoglycols, etc.

lus metallic salts Soluble soaps, insoluble soaps

Alcohols

nolamines, and other Amine salts, amides

slink agent Polymerized oils

te pressure) Hydroxystearates

e Epoxidized oils

Halogenated oils

Polymerized oils, factice

Sulphonated oils

) Dehydrated castor oil,

octadecadienoic acid

Sebacic acid, capryl alcohol

Undecylenic acid, heptaldehyde

Halogenated castor oils

opylene oxide Alkoxylated castor oils

c-, maleic-, phthalic Alkyl and alkylaryl esters,

phosphate esters

Urethane polymers

Sulphated castor oil (Turkey red oil)

Fig. 2. Equation showing the dehydration of ricinoleic acid.

Fig. 3. Production of sebacic acid and capryl alcohol from castor oil.

of an acid containing conjugated double bonds results in

an oil resembling tung oil in some of its properties.

Thus, castor oil, which is non-drying, can be treated

and converted into a semi-drying or drying oil known

as dehydrated castor oil.

Fig. 4. Production of x-aminoundecanoic acid from castor oil.

6. Industrial uses

Although castor oil is not edible, it is more versatile

than other vegetable oils as it is widely used as a starting

material for many industrial chemical products because

of its unique structure. It is one of those vegetable oils

that have found usage in many chemical industries. It

is a raw material for paints, coatings, inks, lubricantsand a wide variety of other products.

Because of its hydroxyl functionality, the oil is

suitable for use in isocyanate reactions to make poly-

urethane elastomers (Quipeng et al., 1990), polyurethane

millable (Kirk-Othmer, 1979; Yeganeh and Mehdi-

zadeh, 2004), castables (Heiss, 1960; Lyon and Garret,

1973), adhesives and coatings (Yeadon et al., 1959;

Trevino and Trumbo, 2002; Somani et al., 2003), inter-penetrating polymer network from castor oil-based

polyurethane (Patel and Suthar, 1988; Xie and Guo,

2002) and polyurethane foam (Ehrlich et al., 1959;

Ogunniyi et al., 1996). Some semi-rigid foams that have

potential uses in thermal insulation were produced when

castor oil/polyether mixture was reacted with toluene

diisocyanate (Ogunniyi et al., 1996).

Sebacic acid, a 10-carbon dicarboxylic acid, is manu-factured by heating castor oil to high temperatures

(about 250 �C) with alkali. This treatment results in

saponification of the castor oil to ricinoleic acid that is

then cleaved to give capryl alcohol (2-octanol) and seba-

cic acid (see Fig. 3). The preparation of sebacic acid and

2-octanol from castor oil has been reported (Vasishtha

et al., 1990). Although the sebacic acid yields are low,

this route has been found to be cost competitive. Sebacicacid and hexamethylene diisocyanate react through con-

densation polymerization to produce nylon-6,10. Fur-

thermore, the esters of sebacic acid also are used as

plasticizers for vinyl resins and in the manufacture ofdioctyl sebacate—a jet lubricant and lubricant in air-

cooled combustion motors. Capryl alcohol is used in

plasticizers in the form of dicapryl esters of various

dibasic acids.

The pyrolysis of castor oil at 700 �C under reduced

pressure has been used to obtain heptaldeyde and

undecylenic acid (Das et al., 1989) (Fig. 4). Undecylenic

acid and heptaldehyde are important intermediates inthe preparation of perfume formulations. When undecy-

lenic acid is mixed with isobutylamine, an insecticidal

synergist is obtained. Heptaldehyde can be further

hydrogenated to produce alcohol for use as a plasticizer.

In addition, undecylenic acid is used in preparing ath-

lete�s foot remedy. Also, the synthesis of E-2-Nonenal,

an ingredient of natural flavours and fragrances has

been reported (Kula et al., 1994).In order to obtain x-aminoundecanoic acid (Bryd-

son, 1975; Saunders, 1988), castor oil is subjected to

methanolysis to yield methyl ester of ricinoleic acid by

the route shown in Fig. 4. In the first step, the pyrolysis

of methyl ricinoleate is carried out at about 500 �C to

give n-heptaldehyde and methyl undecylenate. The

methyl undecylenate is hydrolysed to give undecylenic

acid, which is treated with hydrogen bromide in a

non-polar solvent in the presence of peroxide. Under

these conditions, reverse Markownikoff addition occurs

and the main product is x-bromoundecanoic acid. Theproduct is then treated with ammonia to give x-ami-

noundecanoic acid, which is a crystalline solid.

Aminoundecanoic acid is the starting material for ny-

lon-11. It is claimed that a French company produces

nylon-11 by this route (Kovaly, 1982).

Another characteristic of castor oil is the hydrogen

bonding of its hydroxyl group; this confers a high vis-

cosity on the oil. This property makes the oil useful asa component in blending lubricants (Kirk-Othmer,

1979).

The castor cake is mainly used as a fertilizer. It is

unsuitable as an animal feed because of the presence

of toxic protein called ricin and toxic allergen often re-

ferred to as CBA (castor bean allergen). However, it is

noteworthy that none of the toxic components is carried

into the oil. Some methods reported for the detoxifica-tion of the cake include treatment with ammonia, caus-

tic soda, lime and heat (Kirk-Othmer, 1979; Weiss, 1971;

Gardener, 1960; Horton and Williams, 1989). When the

cake is steamed, the ricin is detoxified and the allergen is

inactivated. Although the use of detoxified cake as cattle

feed has been reported (Woodend, 1993), extreme cau-

tion and experimentation are desirable before the cake

is fed to farm animals. Another method of detoxifyingcastor seed meal involved the wet mixing with sal (Sho-

rea robusta) seed meal so that the toxic constituents of

castor seed were neutralized by tannins. (Ghandi et al.,

1994).

In addition, some people in parts of South-Eastern

Nigeria have long developed a method for treating and

detoxifying the unextracted seed that is subsequently

used as food seasoning known as ogiri-igbo (Weiss,1971; Okagbue, 1993). In this case, the method used to

detoxify castor seed involves fermentation. The seeds

are first dehulled and boiled in water for about 18 h.

The boiled seeds are cooled and wrapped together with

banana leaves and allowed to ferment in the fire place

for about five days. The fermented seeds are then

mashed by pounding using a mortar and pestle. This is

followed by addition of ash from burnt palm kernelhusk which gives it a dark colour. The dark, mashed

product is allowed to mature for a further period of five

days after which it is packaged for sale. It is believed

that most of the detoxification takes place during fer-

mentation and it leads to the elimination of the toxic

factors. Microbiological studies have shown that the

bacteria involved are spore-forming bacteria, especially

members of the genus Bacillus (Odunfa, 1985).Castor oil has been used as a plasticizer for celluloid

and in lacquers but the blown oil has been discovered to

perform better. Blown or oxidized castor oil is prepared

by blowing air or oxygen into it at temperatures of 80–

130 �C, with or without catalyst to obtain oils of varying

viscosity. The blown oil is used widely as a plasticizer in

lacquers, artificial leathers, hydraulic fluids and adhes-

ives (Kirk-Othmer, 1979; Weiss, 1971).Castor oil also can be modified by reduction with

hydrogen to produce hydrogenated castor oil (HCO),

which is a wax-like material with melting point of

86 �C. Hydrogenated castor oil is used in cosmetics, hair

dressing, ointments, preparation of hydrostearic acid

and its derivatives; and in certain cases as wax substi-

tutes and for polishes. Sometimes HCO is used as a

paint additive, solid lubricant, pressure mould releaseagent in the manufacture of formed plastics and rubber

goods (Kirk-Othmer, 1979; Weiss, 1971).

Another product formed from the modification of

castor oil is sulphated castor oil (also known as ‘‘Turkey

red oil’’). Sulphated castor oil is prepared by adding

concentrated sulphuric acid to castor oil at 25–30 �C

for several hours, followed by washing and neutralizing

with sodium hydroxide solution. It is an active wettingagent. As such, it is used extensively in dyeing and in fin-

ishing of cotton and linen. Generally, the ability of cas-

tor oil and some of its derivatives to wet surfaces make

them useful as excellent carriers of pigments and dyes.

Also, the action of sulphuric acid on castor oil produces

a useful emulsifier for certain insecticidal oils (Kirk-

Othmer, 1979; Weiss, 1971).

Even though a small amount is involved, about 0.7parts per hundred of castor oil is added to latex or wet

rubber to promote crumbling and thus produce the

crumb rubber grade (Rubber Research Institute of

Malaysia, 1966).

Dehydrated castor oil (DCO) is used in the prepara-

tion of alkyd resins (Ogunniyi and Njikang, 2000) that

are in turn used for paints, enamels, lacquers and

varnishes with high gloss, good adhesion and wettingqualities. It has advantages over tung oil because it is

non-yellowing (Weiss, 1971). The vulcanization of

DCO with sulphur has been reported (Botros and Mei-

necke, 1987): factice, the resulting product, has been

found to be a rubber additive with antiozonant and

good flow properties.

Castor oil is used in the preparation of brake fluids;

as an ingredient for soaps, lubricants, inks, paint andvarnishes; and as the main ingredient in motor oils for

high speed automobile engines. Other miscellaneous

applications of castor oil are in the formulation of

cathartic, the formulation of contraceptive creams, as

a component of synthetic flower scents, the preparation

of bland emollient to treat skin diseases, and for induc-

ing labour in pregnancy at term. Castor oil-based syn-

thetic detergents are less prone to foaming and thedisposal of the detergent is hastened since microbiolog-

ical breakdown is simplified. Since DCO contains unsat-

urated acids, it is felt that epoxidation of DCO should

be feasible (Kirk-Othmer, 1979; Weiss, 1971). If DCO

is epoxidized, the product can be evaluated in poly

(vinyl) compounds as a plasticizer/stabilizer with the

possibility that epoxidized castor oil may be capable of

replacing epoxidized soybean oil. It must be noted thatedible oils such as soybean oil are useful in industrial

production, but they are in short supply in Africa. The

possibility of using non-edible oil such as DCO for

industrial production will make available nutritious oil

such as soybean oil for human consumption.

7. Conclusions

There is no doubt that castor oil is an important

renewable resource. This is evident from the fact that

much has been written about the oil and it is certain that

more literature will still be written. In this short review,

the various uses of castor oil have been outlined. How-

ever, it must be realized that there are other chemicals

that can be produced from castor oil that have not beenmentioned. This is because such chemicals are not yet

produced in commercial quantities. In many countries

with little or no petrochemical feedstock, castor oil will

come in handy as a versatile renewable resource. Such

oil, with a variety of uses, will be a suitable substitute

in industrial production where many edible oils are

being used. Generally, it is considered that non-edible

oils should be exploited as far as it is possible so that edi-ble oils can be freed for human consumption. This is

especially important in developing countries where food

security poses a challenge.

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