Designed Monomers and Polymers
ISSN: (Print) 1568-5551 (Online) Journal homepage: www.tandfonline.com/journals/tdmp20
Cashew nut shell liquid (CNSL) - a versatile
monomer for polymer synthesis
Mary C. Lubi & Eby Thomas Thachil
To cite this article: Mary C. Lubi & Eby Thomas Thachil (2000) Cashew nut shell liquid (CNSL) a versatile monomer for polymer synthesis, Designed Monomers and Polymers, 3:2, 123-153,
DOI: 10.1163/156855500300142834
To link to this article: https://doi.org/10.1163/156855500300142834
Published online: 02 Apr 2012.
Submit your article to this journal
Article views: 11771
View related articles
Citing articles: 47 View citing articles
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=tdmp20
Designed Monomers and Polymers, Vol. 3, No. 2, pp. 123 – 153 (2000)
 VSP 2000.
Review article
Cashew nut shell liquid (CNSL) — a versatile monomer for
polymer synthesis
MARY C. LUBI and EBY THOMAS THACHIL ∗
Department of Polymer Science and Rubber Technology, Cochin University of Science and
Technology, Kochi 682022, India
Abstract—Cashew nut shell liquid (CNSL) is a by-product of the cashew industry. It is a naturally
occurring substituted phenol which can take part in a variety of reactions. It is a cheap and
renewable substance and can be employed for the manufacture of a multitude of useful products.
It can replace phenol in many applications with equivalent or better results. CNSL by itself is
useful for insecticidal, fungicidal, anti-termite, and medicinal applications, and as an additive, in
many plastic formulations. Resins derived from CNSL are employed widely in the fields of friction
materials, automobiles, surface coatings, adhesives, laminates, rubber compounding, and have several
miscellaneous applications. Greater utilization of CNSL as a monomer for indusrial polymer products
can be an attractive proposal in view of its low cost, abundant availability, and chemically reactive
nature, amongst other attributes. This review gives an accounts of the composition, extraction,
reactions, and applications of CNSL based on the available literature.
Keywords: CNSL; cashew nut shell liquid; phenolics; monomer; polymer synthesis; renewable
sources.
1. INTRODUCTION
Cashew nut shell liquid (CNSL) occurs as a greenish-yellow viscous liquid in the
soft honeycomb of the shell of the cashew nut. Cashew nut is a product of the
cashew tree, Anacardium occidentale L. [1], which is native to Brazil. The plant
may grow to 12 m in height. Cashew nuts are the cash crop yielded by the tree, but
it also produces cashew apple, wood suitable for boats, charcoal, etc., and a gum
similar to gum arabic. The cashew nut is greenish-gray in color and is attached to
the cashew apple, which when ripe is either bright yellow or red. The cashew apple
is used in beverages, jams, and jellies. The nut has two walls or shells, the outer one
being smooth and somewhat elastic and olive green in color before maturity. When
∗ To whom correspondence should be addressed.
124
M. C. Lubi and E. T. Thachil
the nut matures, the outside shell becomes grayish-brown in color. The inner shell is
harder and must be cracked like the shells of other nuts. The honeycomb is housed
between the inner and outer shells. The kernel is inside the shell and is covered with
a brown skin known as testa. The nut has a kidney shape and may vary in size from
2.5 to 4 cm. The oil has a reddish-brown color, due to charring as well as chemical
changes occurring during frying. Industrial grade CNSL is reddish-brown in color.
CNSL constitutes about 20– 25% of the weight of the cashew nut.
The main constituents of CNSL are phenolic compounds with long-chain substitution at the meta position. These compounds give most of the reactions of phenol
and can be the basis for producing a number of polymeric substances of wide utility
in industrial applications. CNSL, which enjoys a considerable cost benefit, can be
a substitute for phenol in many applications. The use of CNSL in place of phenol
is an excellent example of conservation of a synthetically derived substance and the
utilization of a cheap agro by-product. In the modern context, CNSL occupies a position of great importance because of its renewable nature [2]. CNSL is abundantly
available in many parts of the world, such as India, Brazil, Bangladesh, Tanzania,
Kenya, Mozambique, tropical regions of Africa, and South-East and Far-East Asia.
In some of these regions, cashew is a popular plantation product, while some others
import cashew nuts for processing.
2. COMPOSITION
The earliest work published concerning the composition of cashew nut oil was by
Staedeler [3]. Since then, many researchers have investigated the constitution of the
oil [4– 6]. Naturally occurring CNSL contains mainly four components: cardanol,
cardol, anacardic acid, and 6-methyl cardol. Figure 1 shows the chemical structures
of these compounds.
Commercial-grade CNSL contains hardly any anacardic acid because of decarboxylation during the roasting process, which converts anacardic acid to cardanol
or 2-pentadeca-diethyl phenol [7]. The roasting process also leads to polymerization, which accounts for 20– 25% of polymeric substances in the oil. Crude CNSL
is corrosive but becomes less so by decarboxylation and removal of H2 S during the
refining process. The effect of processing parameters on the refining of CNSL has
been studied [8].
The components of CNSL are themselves mixtures of four constituents differing
in side-chain unsaturation, namely saturated, monoene, diene, and triene. The
structures of the side-chains of varying degrees of unsaturation occurring in the four
major components of CNSL are identical. Figure 2 shows these linear structures.
Paul and Yeddanapally [9] identified the components of anacardic acid as
1-hydroxy-2-carboxy-3-pentadecyl benzene, 1-hydroxy-2-carboxy-3-(80 -pentadecenyl)benzene, 1-hydroxy-2-carboxy-3-(80 ,110 -pentadecadienyl)benzene, and
1-hydroxy-2-carboxy-3-(80 ,110 ,140 -pentadecatrienyl)benzene. Table 1 lists the constituents of anacardic acid as determined by various techniques.
Cashew nut shell liquid
Figure 1. Components of CNSL.
Figure 2. Components of the side-chain.
125
126
M. C. Lubi and E. T. Thachil
Table 1.
Composition of the constituents of anacardic acid as determined by various techniques (values are in
wt%) [5]
Constituents
of anacardic
acid
Technique employed
Low temp.
TLC–UV
crystallization
TLC–
grav.
TLC–
GLC
HPLC
TLC–mass spectrometry
Saturated
Monoene
Diene
Triene
4.00
15.00
44.00
37.00
—
38.40
17.30
44.10
4.00
38.3
17.30
40.40
4.30
33.4
19.90
42.4
3.65
38.19
16.50
41.62
—
38.70
16.30
45.10
2.2–3.0
25.0– 33.3
17.8– 32.1
36.3– 50.4
Table 2.
Composition of the constituents of cardanol as determined by various techniques (values are in wt%)
[5]
Constituents
of cardanol
Saturated
Monoene
Diene
Triene
Technique employed
Molecular
distillation
Argentated
column
TLC–GLC
—
60.00
10.00
30.60
5.40
48.50
16.80
29.30
2.68
29.50
16.60
51.2
TLC–mass spectrometry
3.11
36.10
20.10
40.60
1.98
31.31
15.23
51.47
3.94– 4.35
21.64– 32.2
15.36– 18.22
45.23– 58.99
Table 3.
Composition of the constituents of cardol as determined by various techniques (values are in wt%) [5]
Constituents
of cardol
Technique employed
Saturated
Monoene
Diene
Triene
0.30
8.10
21.90
69.70
TLC– GLC
TLC–mass spectrometry
—
9.60
25.20
62.20
0.24
10.74
20.64
68.39
0.19– 2.70
8.43– 15.15
24.2– 28.92
36.46– 67.18
Symes and Dawson [10] and Cornelius [11] identified the components of
cardanol as 3-pentadeca-anisole, 3-(80 -pentadecenyl)anisole, 1-methoxy-3-(80 ,110 pentadecadienyl)benzene, and 1-methoxy-3-(80 ,110 ,140 -pentadecatrienyl)benzene.
Table 2 gives the composition of the constituents of cardanol. The same group
of workers also identified the components of cardol as 5-pentadecyl resorcinol,
5-(80 -pentadecyl)resorcinol, 5-(80 ,110 -pentadecadienyl)resorcinol, and 5-(80 ,110 ,140 pentadecatrienyl)resorcinol. The composition of the constituents of cardol is given
in Table 3.
Cashew nut shell liquid
127
Table 4.
Composition of 2-methyl cardol as determined by various techniques [5]
Constituents
of 2-methyl
cardol
Technique employed
TLC–GLC [34]
Saturated
Monoene
Diene
Triene
1.66
13.90
18.50
65.90
TLC–mass spectrometry [117]
2.43
15.30
20.40
61.50
3.92
18.43
20.15
57.50
0.96– 1.34
16.27– 25.28
20.56– 24.36
49.82– 62.20
Table 5.
Phenolic composition of natural and technical CNSL (values are in wt%) [6]
Component
Natural CNSL
Technical
CNSL
Cardanol
Cardol
2-Methyl cardol
Polymer
Anacardic acid
1.2
11.31
2.04
20.3
64.93
62.86
11.25
2.08
23.8
—
Tyman [6] established the components of 2-methyl cardol as 2-methyl-5-pentadecyl resorcinol, 2-methyl-5-(80 -pentadecenyl)resorcinol, 2-methyl-5-(80 ,110 -pentadecadienyl)resorcinol, and 2-methyl-5-(80 ,110 ,140 -pentadecatrienyl)resorcinol. The
relative proportion of the saturated and unsaturated constituents of 2-methyl cardol
is shown in Table 4. It has been shown that cardanol, the main constituent, consists
of species of varying degrees of unsaturation in the side-chain. Table 5 gives typical
compositions of natural and technical CNSL.
The phenolic nature of the constituents of CNSL, along with varying degrees of
unsaturation in the side-chain, makes it a highly polymerizable substance amenable
to a variety of polymerization reactions. Of overriding importance, however, is the
condensation reaction with active methylenes that formaldehyde exemplifies. This
leads to a phenol– formaldehyde type condensation.
3. EXTRACTION
Traditionally, a number of methods are employed to extract the oil from the
nuts. Often a particular region follows a technique by convention rather than by
deliberate choice. The following are the popular methods currently employed for
the extraction of the oil [5].
128
M. C. Lubi and E. T. Thachil
3.1. Hot oil bath method
The raw nuts are passed through a bath of hot CNSL at 180– 200 ◦ C. The outer part
of the shell bursts open and releases CNSL. About 50% of the oil is thus recovered.
This process makes the decortication of nuts easy, without adversely affecting the
quality of the kernels. Improvements over this basic technique include initial surface
wetting and dipping in water at 20– 25 ◦ C and subsequent steam treatment prior to
exposure to the hot CNSL bath. The excess moisture content of 7– 10% of the
weight of the nuts causes the cells to burst, with the result that the oil oozes into
the bath. Another 20% could be extracted by passing the spent shells through an
expeller and the rest by a solvent extraction technique. The expeller oil can be
upgraded by acid washing followed by centrifugation and heating.
3.2. Roasting method
Sudden exposure to high temperature from ambient is the basis of this method. The
shells are charred during this process, producing an explosive pressure in the cellular
structure which forces the liquid out of the shell. One variation of this method
is abrading at 100– 300 ◦ C for about 1 h and subsequent roasting at 400– 700 ◦ C
in an inert atmosphere. In yet another patent, the nuts are first abraded and then
treated with moisture and heated in an infrared oven. Finally they pass through a
high-frequency electric field, where the liquid flows out. Often this technique is
employed in conjunction with an expeller, where the oil is expelled from the shells
to an extent of 90%.
3.3. Miscellaneous methods
Heating by superheated steam to force out the oil and subsequent expulsion using
an expeller form the basis of a recently reported method. Extraction of the oil
by solvents such as benzene,toluene, petroleum hydrocarbon solvents or alcohols
[12] or supercritical extraction using a mixture of CO2 and isopropyl alcohol are
other reported techniques. Reports of a power-operated cashew nut sheller have
also appeared [13].
4. REFINING
Sulfides, nitrogenous materials, and minerals contained in the crude CNSL directly
affects the quality of the oil. Hence CNSL is often subjected to chemical treatment
with materials such as hydrocarbon sulfates and sulfuric acid before industrial
use. Several acid treatment processes have been found to reduce the poisoning or
vesicant action [1]. Aqueous solutions of acids such as hydrochloric acid, sulfuric
acid, acetic acid, chloroacetic acid or phosphoric acid, or acid sulfates such as
sodium hydrogen sulfate are employed for this purpose.This performs two important
functions: mineral salts of ammonium, calcium, and potassium are precipitated and
Cashew nut shell liquid
129
Table 6.
Physical constants of the oil before and after chemical treatment [1]
Iodine No.
Refractive index
Specific gravity
Saponification value
Viscosity at 25 ◦ C
Raw oil
Treated oil
269
1.5158– 1.5162
958
19.6
400
254
1.5212– 1.5218
96
29.7
435
there is a reduction in the content of skin vesicant present in CNSL. The chemical
treatment is accompanied by the evolution of hydrogen sulfide. The reduction in
sulfur content has been found to correspond roughly to the removal of the agent
responsible for the skin irritation. Treatment with amines or with hydroxides of the
metals of Group IA or IIA has been found to reduce the cardol content.
The refined CNSL can be readily distilled under reduced pressure or hydrogenated, both processes leading to products with good color stability. Steam treatment of H2 SO4 -treated CNSL followed by distillation has been found to deodorize
this oil to a substantial extent.
Physical constants of CNSL before and after chemical treatment are summarized
in Table 6.
5. PHYSICAL PROPERTIES
CNSL is a dark viscous oil with a characteristic smell, quite unlike other vegetable
oils. It is opaque and when applied as a thin film, it is reddish-brown in color. It
is immiscible with water but is miscible with most organic solvents. CNSL has
germicidal and fungicidal properties. It is used traditionally as a cure for fungal
attack of the feet in Kerala.
6. REACTIONS OF CNSL
The chemical structure of CNSL is such that it can take part in a large number
of reactions. Reactions of CNSL are of commercial importance because several
useful industrial products can be produced starting from this substance. A few
reviews have appeared summarizing the known reactions of CNSL [6, 14]. Many
reactions of CNSL have been reported in the form of patents issued all over the
world in the last three-quarters of this century. Often the mechanism underlying
these reactions and the intermediate steps are left uninvestigated or unreported. The
reaction schemes given in the following sections are based on cardanol, the major
constituent of CNSL.
130
M. C. Lubi and E. T. Thachil
Figure 3. Decarboxylation of CNSL.
Figure 4. Hydrogenation of cardanol.
6.1. Decarboxylation
As mentioned earlier, anacardic acid decarboxylates on heating to cardanol. This
reaction can occur during the extraction of CNSL or the refining process. Distillation of the crude CNSL also leads to decarboxylation as the distillate is found
to contain mainly cardanol. Figure 3 shows the chemical changes involved during
decarboxylation.
6.2. Hydrogenation [15]
Hydrogenation is generally carried out using hydrogen and catalysts such as copper,
nickel, palladium, and platinum (Fig. 4). Hydrogenated cardanol can be separated
by distillation, as is done with crude CNSL. The saturation in the aliphatic sidechain can be carried out to completion, but evidence of hydrogenation of the
aromatic ring has not been forthcoming.
6.3. Polymerization
CNSL can be polymerized by a variety of techniques. The presence of the aliphatic
side-chain gives these resins pronounced hydrophobicity, which is a valuable
property for many applications.
The unsaturation in the side-chain can be the basis for addition polymerization
[15– 17] using free radicals or ionic initiators. Acids such as H2 SO4 , HCl, paratoluene sulfonic acid (PTSA), etc. can give polymeric products. The polymerization time in the presence of H2 SO4 is one of the Indian Standard specification for
CNSL (Table 7). The acid polymer of CNSL is rubber-like and has a low susceptibility to oxidation. The kinetics and mechanism of the polymerization of cardanol
using catalysts such as H2 SO4 and H3 PO4 and Lewis acids such as boron trifluoride etherate have been investigated. These studies show that cardanol undergoes
Cashew nut shell liquid
131
Table 7.
Indian standard specifications for CNSL
Characteristic
Requirement
Specific gravity
0.95– 0.97
Viscosity at 30 ◦ C, cp (max)
550
Moisture, % by wt (max)
1.0
Matter insoluble in toluene, % by wt (max)
1.0
Loss in weight on heating, % by wt (max)
1.0
Ash, % by wt (max)
1.0
Iodine value (max)
(a) Wijs method
(b) Catalytic method
250
375
Polymerization
(a) Time, min (max)
(b) Viscosity at 30 ◦ C, cp (min)
(c) Viscosity after acid washing at 30 ◦ C, cp (min)
4
30
200
oligomerization under acidic conditions through the side-chain unsaturation and
that the oligomerization is initiated by protonation of the side-chain followed by
cationic chain growth. CNSL can be polymerized using salts such as zinc chloride,
stannic chloride, ferrous sulfate, aluminum sulfate, boron trifluoride, and salts of
iron, cobalt, nickel, boron, chromium, lead, silver, mercury, manganese, etc. These
salts are generally used to the extent of 1– 6%.
The phenolic properties of CNSL are utilized to make condensation polymers
by reaction with formaldehyde, furfural, HMTA, etc. [18– 20]. These polymers
are similar to the product of phenol and formaldehyde condensation but show less
reactivity during synthesis and later during cross-linking. Both resole and novolaktype resins can be made.
In the presence of alkaline catalysts and with more formaldehyde, the methylol
phenols condense either through methylene linkages or through ether linkages. In
the latter case, subsequent loss of formaldehyde may occur with methylene link
formation. In the case of the resole-type polymers, the structure of the final product
has been studied by Tyman (Fig. 5). Working back from this structure and in
analogy with similar reactions for phenol the reaction scheme leading to this final
cross-linked network can be given as shown in Fig. 6.
In the presence of acid catalyst and with less formaldehyde, the resin has no
reactive methylol groups and therefore is incapable of condensing with other
132
M. C. Lubi and E. T. Thachil
Figure 5. Resole resin made from cardanol [53].
molecules on heating in the absence of hardening agents. To complete resinification,
more formaldehyde is added, resulting in cross-linking. The final structure of the
novolac product has also been given by Tyman (Fig. 7).
It is observed that condensation of CNSL takes place at one of the ortho positions
and the para position of the phenolic ring. Condensation does not occur at the
ortho position between the side-chain and the OH group. This can be attributed to
a stoichiometric deficit of formaldehyde and steric hindrance caused by the lengthy
side-chain. The expected reaction scheme for the novolac product is given in Fig. 8.
This has been obtained by considering the final polymer structure and the reaction
of phenol under similar conditions.
The condensation reaction in this case is more difficult because of the immiscibility of CNSL and HCHO layers and the steric hindrance caused by the lengthy sidechain. Longer processing times are needed and the reaction product is a mushy, high
viscosity substance, which tends to solidify at comparatively low degrees of polymerization. This reddish-brown substance is the most well-known product derived
from CNSL. In certain applications, a preliminary step of polymerization through
the side-chain is done before condensation polymerization.
Similar reactions can be conducted with hydrogenated cardanol and the product
converted to methyl ether and then oxidized with KMnO4 , giving methoxyterephthalic acid (Fig. 9).
Mishra and Pandey investigated the kinetics of alkali-catalyzed formaldehyde
condensation after thoroughly reviewing the results previously reported by various
workers [21]. For the reaction catalyzed by NaOH, second-order kinetics has been
reported when the concentration of the active position of the reactants is used instead
of their molar concentration. The specific reaction rate constant for various reaction
conditions and the energy of activation, E, have been calculated. A quick method
for the determination of the composition of the resin at any stage of the reaction has
also been suggested. O’Conner and Blum have studied the thermal stability of some
formaldehyde condensate resins [22].
The structure and properties of CNSL novolac resin prepared using succinic acid
as a catalyst have been investigated [23]. The synthesis and characterization of a
large number of resins using different hydroxy aromatic compounds, fomaldehyde,
furfural, and substituted aromatic compounds like CNSL in the presence of acid
Cashew nut shell liquid
133
Figure 6. Polymerization of CNSL using alkali catalysts.
and base as catalysts have been studied [24]. The synthesis and properties of resins
from CNSL and different aldehydes have been the subject of a number of other
investigations [25].
Oxidation polymerization using acid oxidizers such as HNO3 can also give
polymers, which have proved to be excellent for surface coatings and paper and
fabric impregnation [26]. CNSL on treatment with copper, lead, aluminum, etc.
undergoes polymerization. The air-drying qualities of CNSL were found to improve
134
M. C. Lubi and E. T. Thachil
Figure 7. Novolac resin made from cardanol.
Figure 8. Polymerization of CNSL using acid catalysts.
by this. Enzymatic polymerization of alkyl phenols derived from CNSL has been
carried out in a dioxan– water mixture [27].
Heating is the simplest method of polymerizing CNSL. When CNSL is heated
to 200 ◦ C or above, it undergoes polymerization. Cationic, anionic or oxidizing
agents can accelerate the process. When the polymerization is proceeding with heat
alone, salts present in CNSL catalyze it. Alkalis can also be used as catalysts for the
polymerization of CNSL by heat. The first step is usually the reaction of phenolic
hydroxyl with the alkali, and the compound thus obtained acts as a catalyst for the
polymerization.
Cashew nut shell liquid
135
Figure 9. Condensation of hydrogenated cardanol followed by etherification and oxidation.
Figure 10. Sulfonation of cardanol.
6.4. Sulfonation
Sulphonation CNSL derivatives can be sulphonated to yield alkyl aryl sulphonic
acid or their metal salts. The reaction is carried out using concentrated H2 SO4 at
108 ◦ C. The reaction scheme is shown in Fig. 10.
To prevent polymerization during sulfonation, an aryl or alkali group is substituted
for hydrogen and the double bonds of the aliphatic side-chain are saturated by
hydrogenation before treatment with the acid.
6.5. Nitration
Direct nitration of cardanol or CNSL leads to simultaneous oxidation and polymerization. But nitration of hydrogenated cardanol gives the trinitro-derivative, which
136
M. C. Lubi and E. T. Thachil
Figure 11. Nitration of cardanol.
Figure 12. Etherification of cardanol.
is a very useful product. The yields are better when sulfonated compounds are used
as the starting material (Fig. 11).
6.6. Halogenation
Halogenation of CNSL can be accomplished with comparative ease. For example,
chlorine gas can be passed through CNSL dissolved in kerosene to obtain 15%
(w/w) chlorination. Chlorinated CNSL reacts with active methylenes at a faster rate
and are more flame-resistant. The halogenation is accomplished by polymerization
and thickening and a large exotherm.
6.7. Etherification [28]
Cardanol can be reacted with dialkyl sulfates in presence of alkalis to give dialkyl
cardanols. Similar reactions can be carried out on hydrogenated cardanol. Another
route to cardanol ethers is by the reaction of halogen compounds with CNSL
in the presence of alkalis. Acid ethers of cardanol can be obtained by reaction
with 1-chloropropionic acid. Allyl ethers can be obtained by treating CNSL
with allyl chloride, bromide, iodide or sulfates. Glyceryl ethers of hydrogenated
cardanol have been prepared by the reaction of hydrogenated cardanol with glycerin
monochlorohydrin or glycol monochlorohydrin. Glyceryl ethers of cardanol have
also been obtained by the reaction of epichlorohydrin. Figure 12 shows the chemical
changes involved during etherification.
6.8. Esterification
Cardanol can be reacted with acid chlorides in the presence of alkalis to give
cardanol esters. Thus, benzoyl chloride gives benzoyl cardanol (Fig. 13). The same
reaction can be carried out on anacardic acid, present in raw CNSL, to give mixed
ether ester. By treating an epoxy compound derived from CNSL with unsaturated
Cashew nut shell liquid
137
Figure 13. Esterification of CNSL.
Figure 14. Epoxidation of cardanol.
dicarboxylic acids or their anhydrides polyester can be obtained. This, in turn, can
be mixed with a vinyl monomer to obtain an unsaturated resin composition [29].
Various other esters of industrial importance have also been reported.
6.9. Epoxidation
Epoxidation of the side-chain can be accomplished by the reaction of CNSL with
epichlorohydrin [30]. This is the basis for numerous international patents. The
chemical changes during this reaction are shown in Fig. 14. CNSL polymers
were reacted with glycerin monochlorohydrin or glycol monochlorohydrin in the
presence of alkali to obtain polyhydroxy-alkyl halides. Epoxy resin can also be
prepared from glycidyl ethers by reaction with CNSL.
6.10. Miscellaneous reactions
Pillai and co-workers have investigated the phosphorylation of cardanol and its condensate with phosphoric acid and the prospective applications of the products [31].
Urethanes have been synthesized from CNSL, as in the case of other hydroxy
compounds, by the reaction with isocyanates.
Acetals of CNSL have been prepared by the reaction of dimethyl acetal or
polymeric acetals such as poly-vinyl acetal or poly-vinyl butyral. Acetals of CNSL
resin have advantages such as better electrical properties due to the total absence
of water, ease of processing because of the non-volatile nature of the reactants, less
dermatic effect on the skin, and faster reactions compared with phenolic resins.
Isocyanates have been prepared from hydrogenated cardanol by reaction with
SO2 Cl2 followed by nitration, methylation, and reduction [32]. Azo dyes are made
by the reaction of phenols with primary aromatic amines after diazotization of
hydrogenated cardanol [33]. By reacting CNSL with triethylene-tetramine, phenolic
138
M. C. Lubi and E. T. Thachil
polyamines can be prepared [34]. Benzotriazole derivatives are prepared from
hydrogenated cardanol by diazotizing with aromatic amines followed by reduction.
Styrenation of CNSL resin can be done by reacting styrene with CNSL hexamine
condensate [35]. Acrylic resin and a variety of other polymers derived from CNSL
have also been reported.
7. APPLICATIONS
Polymers derived from CNSL have certain outstanding properties which make them
unique for many applications. The most attractive aspect of CNSL as a starting
material is its low cost. The polymer shows flexibility, due to internal plasticization
resulting from the presence of a long side-chain. This long aliphatic side-chain also
gives water resistance. It has low fade characteristics, which make it a desirable
component of brake lining formulations. CNSL has inherent anti-microbial and
anti-termite properties. It is compatible with a wide variety of polymers such as
alkyds, melamines, polyesters, etc. Its ready solubility in a number of common
solvents makes it a natural choice for a large number of surface-coating applications.
A serious problem of CNSL-based polymers is the color imparted by the oil.
Although there are several suggested methods [1] for obtaining a transparent, lightly
colored substance, it is doubtful whether economical methods exist for making
brilliantly colored products from CNSL.
There have been a number of reviews on the applications of CNSL-based
materials [36– 45]. Most of these draw upon information available from the patent
literature. In the present context, we have attempted to include all available reports
of scientific and academic research in the area, in addition to information collected
from several patents.
7.1. Brake linings and clutch facings [46, 47]
This industry is the largest consumer of CNSL. CNSL polymers are a natural
choice for this application because of an excellent blend of properties such as
absorption of heat generated due to friction, longer retention of braking efficiency,
high impact strength and modulus, low fade characteristics, and low cold wear.
These resins gave good miscibility with rubber and other natural and synthetic
materials. CNSL friction materials are ideal for low-speed automobiles where the
temperature reached does not exceed 250 ◦ C . CNSL furfural reaction products have
very even frictional characteristics over wide temperature ranges.
Numerous patents exist on the technique of producing friction materials starting
from CNSL. The preparation of resins for brake linings reportedly involves an
acid catalyst side-chain polymerization followed by conventional formaldehyde
condensation. CNSL aldehyde or CNSL, phenol and aldehyde friction composition
can be obtained with desired properties when synthesized in the presence of aryl
borates. Drying oil can be used to modify CNSL aldehyde products to be used
Cashew nut shell liquid
139
as friction elements. CNSL-modified phenolic resin with carbon fiber, powdered
cast iron, and BaSO4 was found to have excellent fade and wear characteristics.
CNSL-based resins and cashew dust have been used for preparing corrosion and
skid-resistant friction materials. A friction material having low brake noise was
obtained from blends of CNSL-modified phenolic resins with rubber and certain
additives. Friction materials with enhanced thermal stability were obtained from
formulations having CNSL-based particles along with other ingredients.
Anacardic acid can be reacted with tris(2-hydroxyl ethyl)isocyanurate to obtain
products which can be used as improved binders for friction materials. Several
methods to modify the CNSL resin chemically for increased thermal stability and
the effect of these modifications on the fade property have been reported. Fatty acids
from oils can be reacted with CNSL and then converted to hard infusible particles
useful as friction dust. Potassium acetate, pyridine, etc. are used as catalysts for
the fatty acid– CNSL reactions at 200– 300 ◦ C and the products are found to be
suitable for friction dust formulations. A number of CNSL-based friction materials
with specific performance properties have been reported. Clutch facings with
low hygroscopy have been developed from CNSL. Friction compositions without
any volatile products, and possessing improved fading resistance, good resistance
to noise, wear resistance, improved reliability, heat resistance, high resistance
to hygroscopy, corrosion resistance, and rust resistance have also been reported.
PCNSL could be an excellent matrix resin for brake linings. Phenol-cardanolaldehyde resins have been used as binders for extrudable brake linings. Molding
compositions have also been made from CNSL resins to obtain filled compositions
useful for brake and clutch linings as well as floor coverings.
7.2. Surface coatings
7.2.1. Paints and primers. The chemically stable nature of CNSL, the solubility
of the resin in various solvents, the inherent hydrophobicity and chemical resistance,
the film-forming nature, and a high degree of unsaturation make CNSL polymers
highly suitable for surface coating applications. Cured CNSL formaldehyde films
soften at elevated temperatures and remain in a rubbery condition. The formulation
and manufacture of various classes of paints based on CNSL have been described
by Ramanujam [48].
CNSL condensates can be reacted with alkyl ortho ester to give films of high
heat resistance and excellent elecrical properties. Phenolic resin using 5% cardanol is used for making coatings for cans used in food packaging. Lac– CNSL
combinations have also been found useful for surface coatings which are weatherresistant and as metal primers which are anti-corrosive and water and solventresistant [49, 50].
The condensation product of CNSL, phenol, and HCHO with an acid was treated
with chloroacetic or bromoacetic acid under alkaline conditions and neutralized
with ammonium hydroxide or an amine to give a water-based coating material.
Water-based CNSL –HCHO paint was also prepared by treating CNSL with HCHO
140
M. C. Lubi and E. T. Thachil
and cross-linking with maleic anhydride and neutralization with ammonia [51].
Possibilities of developing a water-thinnable coating composition from CNSL have
been indicated. Paints can be formulated as water-based stable emulsions of
oil-in-water type using CNSL –HCHO resins. Water-based coatings with good
hot water and chemical resistance have been prepared by reacting CNSL –HCHO
condensation products in the presence of an acid with bromoacetic or chloroacetic
acid.
Anti-corrosive paint formulations designed for ship bottoms have been widely
reported [52, 53]. Coatings of various colors can be made from CNSL oxidized
with HNO3. CNSL oxidized with HNO3 was found to give coatings that air-dry very
quickly. The products are suitable for making paints, varnishes, electrical insulation,
impregnating paper, woven fabrics, etc. Coated papers for bottle caps have been
developed from CNSL [54]. The most satisfactory coating appears to be CNSL
cured at 150 ◦ C for 10 min. Primers based on PCNSL dry within 2 h. The addition
of lead and cobalt naphthenates or barium and potassium chromates and lead and
silico-chromates is found to give improved corrosion resistance [55]. Materials
such as kaolin, bauxite residue, barium and zinc chromates, lead silico-chromates,
manganese, and mica have been added to CNSL formulations as fillers. Some
of these formulations showed enhanced corrosion resistance [56]. CNSL –HCHO
reaction products can be reacted with waste walnut kernel oil to obtain stoving
enamels. Co-ordinatively bound anti-corrosion paint useful for clean metal surfaces
has been prepared from CNSL –salicylic acid formaldehyde resin.
Black slate enamels were prepared from CNSL or CNSL distillate residue using
silica and emery powder as pigments. Baking enamels were prepared by condensing
CNSL with HCHO in the presence of linseed oil or turpentine.
CNSL polymerizes with metal or metal oxides and forms a good medium for
paints for further cooking with drying oils. The addition of unsaturated oils to
CNSL improves their flexibility and drying characteristics. The simplest method of
incorporating a drying oil is by adding CNSL and heating the mixture [57]. CNSL
distillation residue can be heated with castor oil to obtain surface coatings. Coating
compositions have also been made from CNSL –aldehyde products in the presence
of rosin, furfural, drying oils, fatty acids or their combinations. CNSL –HCHO
bound emulsion paint can be employed on freshly plastered walls [58]. CNSL
primers containing barium potassium chromate, linseed oil, zirconium chromate
or red oxide have been studied [59]. CNSL –aldehyde anti-fouling paints have been
prepared by incorporating copper aceto-arsenite or linseed oil rosin condensate [60].
Coating made from castor oil by reaction with maleic anhydride can be improved
by incorporating a small amount of CNSL.
Styrenation of CNSL-based phenolic resin improved the paint performance.
Factors which affect the copolymerization of CNSL resin with styrene have been
studied. Polymerized CNSL can be mixed with oils and resins such as linseed,
china-wood, and coumaron to give coatings for cloth, fabrics, papers, and metal
sheets, and wires dipped in these liquids carry a film of the liquid which, on drying,
Cashew nut shell liquid
141
is flexible but hard and tough. The products are suitable for preparing electrical
insulating materials such as impregnated cloth fabrics or paper for coating conductor
bars, sheets or wires and also for producing water-proof cloth fabrics, lining for food
containers,bottle caps, etc. CNSL is made non-vesicant by reaction with vegetable
oils, terpentine, etc. in the presence of nitric acid. The products give an air-drying
film for wood, paper, and metals. The CNSL distillation residue can also be reacted
with phosphoric acid to give baking primers with improved adhesion and corrosion
protection ability [61]. A modified solvent process for the manufacture of CNSLbased resin for coating formulations has been described [62]. The process solves the
problem of pollution and reduces the processing time. The condensate of CNSL and
aldehyde, polyols and polybasic acids, and modifiers are polycondensed and mixed
with cross-linking agents to obtain coatings with good hardness and a fast drying
rate. Anacardic acid is reacted with unsaturated fatty acids to reduce the hydroxyl
content to 80– 90% of the original to obtain products that can be dried in air [63].
Epoxy resins based on CNSL are used as protective coating materials; autoprimers; linings for cans, drums, and pipes; potting and encapsulation of electrical
and electronic components; and in electrical laminates. Being soluble in common
solvents such as naphtha and benzene, the coatings are air-drying and do not require
any expensive hardener, as in the case of conventional epoxy resins.
Polyester resin based on CNSL and its derivatives are reported to be good coating
materials [64].
7.2.2. Specialty coatings. Vinyl resin based on cardanol gives films with excellent adhesion and flexibility and resistance to oils and greases, water, and chemicals. The products can be used in paints and varnishes and the modified composition has high chemical resistance. Phosphated CNSL –HCHO reaction products
give coatings with improved heat resistance, flexibility, and adhesion. CNSL oil
modified alkyd resin finds use in the surface coatings industry and printing inks.
Chlorinated resinous coating compositions suitable for fire-retardant and insecticidal coatings have been prepared from CNSL. Fire-resisting and heat-insulating
paints based on sodium silicates, CNSL, chlorinated CNSL or their condensation
products have been developed. A molten mixture of CNSL, asphalt, rubber, and
petroleum resin could be used as a rust-proof tacky composition. CNSL modified by heating at 160– 300 ◦ C in the presence of certain accelerators gives stoving
enamels resistant to alkali and acid solution, mineral and fatty oils, and various
organic solvents. CNSL distillate residue is also used as a coating for increasing
the durability of light roofing corrugated sheets [65]. A coating based on CNSL
distillate residue was used for the interior of ferroconcrete domes used for the collection of gobar gas. Coating compositions possessing insecticidal properties were
obtained by adding DDT, gamexane, etc. to CNSL or chlorinated CNSL after treatment with HCHO. A coating of CNSL containing copper sulfate extends the life
of coconut leaf thatch from 1 to 4 years [66]. Specialty coatings based on CNSL
for wooden surfaces of fishing boats and modified with polybutadiene and epoxy-
142
M. C. Lubi and E. T. Thachil
modified CNSL have been reported. CNSL –HCHO condensation product in an
alkaline medium on styrenation gives a resin which can be used as a varnish or pigmented to yield an enamel or paint. The film has good resistance to saline and alkaline waters. Rapid-drying coatings containing epoxides were prepared by reacting
cardanol with ethylene oxide, hexamine, and toluene diisocyanate. The copolymer
so obtained is mixed with dibutyl tin dilaurate, lead, and cobalt naphthenate before
use. Surface coatings with epoxy groups have been prepared by the reaction of
epichlorohydrin with CNSL –phenol– formaldehyde resins [67]. A coating based
on CNSL resins modified with p-butyl phenol has improved weatherability.
Heating CNSL with copper at 200 ◦ C or by heating with 10% nitric acid and
dissolving it in a solvent, a coating material for containers can be made. Containers
made from paperboard, wood, gelatin, cellulose acetate, and metals can be coated
and made waterproof by its application. The products are useful for food containers
as the coatings are not affected by fruit juices, alkalis, acetic acid, brine, pickling
solution, or hydrocarbons.
Mixing of CNSL –HCHO resin dissolved in solvents with Portland cement gives
a cement that sets in 6 h at room temperature and has good resistance to water, oil,
and heat. This can also be used as paint. CNSL reacts with alkaline materials such
as cement, alumina, barium hydroxide, sodium carbonate, etc. to give coatings for
walls of cement, wood, fabric, etc. which are impervious to water.
Electropainting compositions [68] have also been prepared from CNSL. CNSL
was treated with HCHO using alkali catalysts to give a water-soluble resin for use
in electrodeposition on mild steel [69].
For improving adhesion and flexibility, drying oils, bisphenols, and epoxy resins
were incorporated into CNSL –aldehyde reaction products. CNSL, in combination
with linseed oil resin as a matrix, is used in anti-fouling marine paint formulations.
Polymerized CNSL can be mixed with oils and resin such as linseed, china wood,
and coumaron to give coatings for fabrics, paper, and metal sheets. CNSL –epoxy
systems are useful as coatings on rusted surfaces. Resins prepared by the reaction
of bisphenol epoxy resins with CNSL and unsaturated fatty acids are used for
the preparation of air-drying corrosion-resistant coatings [70]. Surface coatings
for the protection of concrete against attack by chemical fertilizers have been
developed from CNSL. CNSL –hexamine resins are used with lead and cobalt
naphthenates after mixing with calcined mud, dehydrated castor oil, and solvents to
give corrosion-resistant primers. Coatings giving tough elastic films were prepared
from CNSL –glycerin reaction products [71].
7.2.3. Varnishes. Esters of acrylic acid with cardanol give products useful for the
manufacture of varnishes which are resistant to water and dilute Na2 CO3 solutions
[72]. Copolymers of CNSL and bhilawan nut shell liquid (BNSL) have been used
in the manufacture of varnishes and stoving enamels. CNSL –aldehyde products
can be mixed with raw or cooked oils and the mixture can be used to prepare
varnishes. Varnishes resistant to water and gasoline were prepared by incorporating
Cashew nut shell liquid
143
sulfur in CNSL. Corrosion-resistant varnishes were prepared by heating CNSL in
the presence of litharge, dryers, and thinners [73].
Electrical insulating varnishes were prepared by treating CNSL with HCHO
and compounding the resulting material with pure phenolic resin or alkyd resin
in suitable proportions [74]. Films of these materials are water- and chemicalresistant and can be used as insulating varnishes with high electrical resistance and
as bobbin enamels and laboratory table tops. Pale-colored lacquers and varnishes
were obtained by copolymerizing cardanol formaldehyde condensate with drying
oil, alkyd resin, styrene, other resins, and monomers. These are highly resistant to
water and have a superior electrical insulating property. These resins can be used
as insulation varnish, food can lacquers, and wood finishes. Electrical windings
impregnated with CNSL have improved mechanical properties. The rate of heat
dissipation is also high. Films of CNSL are insoluble in petroleum spirit and oils
and impervious to water and can be repeatedly heated to 250 ◦ C and above and
cooked without impairing their electrical properties.
Water and gasoline-resistant varnishes can be prepared from sulfur-containing
CNSL having 13% sulfur [75]. The addition of drying oils to CNSL –aldehyde
or CNSL –phenol– aldehyde resins also results in improved varnishes.
Polymerized CNSL can be mixed with linseed, china wood or fish oils to obtain
varnishes. Mixtures of polymerized CNSL and bodied linseed oil have been used
to make quick-drying varnishes [76]. Varnishes prepared from cadnanol, styrene,
and dehydrated castor oil have reduced water vapor permeability and improved
mechanical properties [77]. Condensation of CNSL with diphenyl oxide and
subsequent treatment with formaldehyde can give heat-curable varnishes. For
making varnishes from natural gums, it is necessary to heat gums with solvents for
long times to render them miscible with each other. When CNSL is used together
with solvents, the gums become soluble in a shorter time and the products are
completely soluble in conventional solvents. Reaction products from CNSL and
natural resins or gums prepared by reacting them together at 300 ◦ C until brittle
and then diluting them with solvents are used as varnishes. CNSL can be used to
modify shellac and the products are used to insulate electrical coils. Varnish from
cardanol– HCHO–epoxy resin can be baked at a low temperature. Water-thinnable
varnishes have also been prepared from CNSL.
7.2.4. Lacquers. Lacquers made from CNSL could be used for insulation and
protective or decorative purposes for furniture, buildings, automobiles, etc. [78].
Their films have optimum toughness and elasticity, excellent gloss, and superfine
adhesive qualities. The dried films are superior to those of ordinary oil paints with
respect to resistance to oils, greases, moisture, and chemicals. Cashew lacquers
can be easily applied to the substrates by brushing, spraying, and dipping. In terms
of total cost per unit area, they are cheaper than ordinary varnishes. Lacquers for
special applications such as screen lacquering are prepared by condensing CNSL
and formaline in the presence of alkalis, and for gold lacquering CNSL –HCHO
144
M. C. Lubi and E. T. Thachil
condensate is treated with Ti(OBu)4 . Water-thinnable thermosetting lacquers have
also been made. Lacquers suitable for painting porcelain or glassware have been
developed by thermal polymerization of rosin ester with a drying or non-drying oil
and CNSL. Lacquers for inlaying were prepared from CNSL –HCHO condensate
under alkaline conditions in ketones or cellulose-containing Ti(OBu)4 catalyst.
7.3. Adhesives
The high polarity, inherent tackiness of phenolic materials, and the ease with
which liquid to solid conversion can be accomplished make CNSL a desirable
starting material for adhesive formulations. The rheology of CNSL adhesives
has been studied [79]. Relaxation in a Couette viscometer and penetrometry of
CNSL –HCHO resin adhesives revealed shear thickening, yet the system possessed
a yield value. Heat-sensitive adhesives used on the back of reflectors have
been synthesized from CNSL. The preparation of adhesives by condensation of
xylol, HCHO, and CNSL using various catalysts and hardeners has been reported
[80]. Pressure-sensitive adhesives and adhesives for bonding rubbers to fibers
have also been developed from CNSL. PCNSL shows a five-fold increase in
adhesive properties from those of conventional CNSL –HCHO resin [81]. CNSL
adhesives with superior water resistance and shear strength have been prepared.
The hardening rate of CNSL-based adhesives was noticeably accelerated by a small
amount of colloidal polymers with a considerable amount of hydroxyl groups.
CNSL –aldehyde resins mixed with tannin aldehyde resin have been used with
tamarind seed powder as a filler to control the flow properties [82].
Cold setting wood adhesives are made from CNSL –PF resin prepared with NaOH
as a catalyst by adding 10– 12% PTSA as a hardener. CNSL-based resins have been
used as adhesives for plywood [83]. Polymerized CNSL can also be used with
aldehydes to make plywood adhesives by treating with phenol and formaldehyde
in the presence of acids. Plywood adhesives prepared from CNSL –HCHO can
be made fireproof by incorporation of ammonium phosphate, and ammonium
dihydrogen phosphate or diammonium hydrogen phosphate [84]. CNSL oxidized
with KMnO4 or MnO2 at 100 ◦ C and reacted with HCHO and compounded with
CuCl2 gives adhesives suitable for plywood. CNSL modified with furfural, aniline,
xylol, etc. also gives good plywood adhesives [85]. An aqueous emulsion of
cardanol is used for coating plywood for laminating purposes. Low-grade wood
species impregnated with the resin show upgrading of quality. CNSL polymerized
with H2 SO4 and mixed with low condensate PhOH–HCHO resin and 5% CuCl2 can
be used as an adhesive for plywood [86]. Exterior-grade plywood adhesives have
also been developed from CNSL. Two to 3% KMnO4 as an accelerator decreases the
pressing time [87]. Red onion skin extract and HCHO resins modified with CNSL
have been used in the formulation of wood adhesives with better water resistance
and shear strength [88].
A CNSL –xylene– HCHO-based composition has been prepared as a binder resin
for particle boards [89]. Borax can be added to increase the fire retardancy of
Cashew nut shell liquid
145
particle boards [90]. CNSL can be used as a binder for making wood substitute from
agricultural waste. A medium-density fiberboard comprising CNSL and coconut
fibers as the major constituents has been developed for use in place of wood [91].
CNSL adhesive formulations based on CNSL and HCHO are suitable for replacing PhOH–HCHO adhesives in the manufacture of moisture-proof adhesives. Adhesives based on CNSL with improved bonding to other materials, such as metals, have been reported. Polymerized CNSL –furfural– furfural alcohol mixtures
have been used as binders for heat exchangers made from carbon plates. Adhesives can also be prepared by using CNSL with gelatinous materials such as animal
glue, blood and egg, albumin, etc. Adhesives are also prepared from casein and
CNSL [92].
Epoxy resins have been modified to obtain solventless adhesives which could
cure at room temperature and give greater impact strength and shear resistance by
preparing liquid resin from bisphenol A, epichlorohydrin, and a diglycidyl ether of
CNSL as plasticizer [93].
7.4. Foundry
The phenolic nature of CNSL facilitates the use of resins derived from it in place of
conventional phenolic materials used in the foundry industry. The fire-retardant and
ablative properties associated with the phenolic structure are responsible for this.
CNSL is being used as a substitute for linseed oil in the manufacture of foundry core
oil, which is used as a binder in the foundry [94]. CNSL-modified phenolic resins
are mixed with amines and ammonium salts of inorganic acids and water-soluble
surfactants to give low-viscosity caking agents for sand molding. CNSL, phenol,
and cyclohexanone condensate prepared in the presence of formalin and acids has
also been used as binders for sand cores, along with hexamine and potassium
stearate. Resin prepared from phenol and CNSL ethers by reaction with aldehydes
has also been used in molding powders or as a binder. CNSL paraform condensates
are reacted with diacetoacrylamine and polyamines to obtain adhesives for sand
castings. Polymerized CNSL –formaldehyde resin can be modified with phthalic
anhydride and glycerin and cured at 125 ◦ C with 3% litharge to obtain binders for
the foundry industry [95]. No-bake core binders, used by the metal casting industry,
have been prepared by reacting isocyanates with cardanol– paraformaldehyde and
cardanol– alkyd resins [96].
7.5. Laminates
CNSL-based resins improve the resistance to heat, the bonding of the reinforcement
materials, and the flexibility of laminates, while reducing brittleness and age
hardening. Resins for laminated products are made from CNSL and phenol by
reaction with aldehydes. Cardanol can also be used in place of CNSL. CNSLmodified phenolic resins are used for making laminates with good dimensional
stability and warp resistance. Punchable heat-resistant laminates of paper with
146
M. C. Lubi and E. T. Thachil
copper foil can be made using CNSL –HCHO resin. Coated paper for bottling
chemicals, pharmaceuticals, and other preparations has been made from CNSL after
curing at 150 ◦ C for 10 min [97]. Epoxidized CNSL increases the tensile strength
and flexural strength of laminates. Thermally stable and fire-resistant laminates
based on CNSL have been reported. Metal– polymer laminates requiring high
separation forces have been developed from CNSL [98].
7.6. Rubber compounding
The incorporation of CNSL has been found to improve the properties of rubber
products. It aids processing and enhances the vulcanizate properties [99]. It
also helps the incorporation of ingredients into rubber and increases its resistance
to moisture. The effect of cardanol on the processing, mechanical, and aging
properties of elastomers has been studied [100]. CNSL can be added to rubber
compounds to improve the processability and resistance to crack and cut and
thermal stability [101]. CNSL acts as a plasticizer for sulfur and helps the
incorporation of sulfur in rubber and other products. Methyl, ethyl, propyl, and
butyl ethers of anacardic materials and thickened CNSL products act as excellent
plasticizers for synthetic rubber and also lower the Shore hardness and improve
the aging and elongation characteristics of cured rubber. Styrene– CNSL reaction
products have been used as plasticizers for rubbers to improve the intake of
fillers. Vibration-absorbing rubber compositions have been developed starting from
CNSL. Tyre rubber composition having good processability and hardness have been
developed using CNSL.
CNSL enhances the insolubility of NR vulcanizates in petroleum solvents and
acts as an antioxidant in NR vulcanizates [102]. The effectiveness of various
antioxidants prepared from cardanol have been studied. 1,4-Hydroxy-2-pentadecylphenyl-3-phenyl thiourea synthesized from CNSL improves the heat stability of
NR vulcanizates [103]. If NR is treated with CNSL either by milling or by
heating with organic solvents, no decrease in unsaturation is observed [104].
Compounding of less than 10% cardanol with NR reduces viscosity, increases
tackiness, and improves tensile strength, abrasion resistance, and aging properties
[105]. If less than 10% cardanol is added to filled NR products, the modulus and
abrasion resistance are improved [106]. Cutting pads for leather are made from NR
and CNSL –HCHO resin. Products from acid polymerized CNSL and hexamine
are millable materials which serve as a plasticizer for NR. The processability
characteristics of the NR –halogenated CNSL polymer system have been studied.
The rheology of blends of NR and PCNSL has been studied [107]. The increase
in pseudoplasticity index and the decrease in consistency index, viscosity, shear
modulus, and activation energy of the melt flow with increasing concentration of
PCNSL, temperature, and shear rate indicate the plasticizing effect of PCNSL.
PCNSL can be used to increase the tensile properties of NR and helps to give a lower
degree of stress relaxation at higher strain rates and strain levels [108]. The selfadhesion strength of NR compounds can be modified by PCNSL. PCNSL-modified
Cashew nut shell liquid
147
NR vulcanizates show improved thermal stability and resistance to fatigue failure.
Bromo-derivatives of PCNSL improve the flame retardancy of NR vulcanizates
[109].
Polymerized CNSL acts as a biological plasticizer for nitrile rubber and its blends
[110]. Cellular rubber compositions have been developed from CNSL –phenol–
aldehyde– nitrile rubber mixes. Cardanol can be used for the vulcanization of
butyl rubber [111]. Pentadecylbenzoquinone-dioxime synthesized from CNSL gave
heat-resistant vulcanizates for SBR and IIR [112]. Resoles based on CNSL were
modified with butanol, phenyl isocyanate, and rosin, and used for the vulcanization
of butyl rubber. CNSL resin blends with SBR have been used to produce railroad
brake shoes. The low-temperature properties of SBR plasticized with Cardolite625, the ethyl ester of the monophenolic component of CNSL, are superior to
those of similar compounds containing naphthene-based plasticizers. CNSL –amine
condensation products are used as an effective antioxidant for SBR [113]. Cardanolbased resoles used for the vulcanization of SBR were found to impart superior
physical properties to the vulcanizates. Latexes of SBR or NBR can be mixed
with CNSL-modified phenolic resins and hexamine and then mixed with fillers to
obtain compositions suitable for molding or extrusion. Chloroprene can be reacted
with CNSL to obtain products ranging from resilient rubber-like materials to hard
rigid ones. Hydrocarbon ethers of anacardic materials are useful for plasticizing
polychloroprene. Difficulties observed in processing cis-1,4-polybutadiene can be
overcome by blending it with liquid ethers obtained by the reaction of CNSL with
polyhydric phenols such as resorcinol.
7.7. Plastic materials and additives
A study by Pillai et al. has shown that high-performance and specialty polymers can
be produced from cardanol by a variety of methods [114]. Flame-retardant prepolymers were obtained from cardanol by simultaneous phosphorylation and oligomerization. About 10– 15% CNSL increases the thermal stability of substituted phenolic resin. The effect of replacement of phenol by CNSL on the properties of
novolac and resole resins has been experimentally investigated [115]. Lightweight,
sandwich-type plastic, composite panels suitable for partitions, cladding, flush door,
etc. [116]. can be made using CNSL –HCHO resins. Foam plastics developed from
CNSL are found to become lighter upon aging for 12 days at 100 ◦ C [117]. The effect of the addition of cardanol on peroxide curing of polyester resin has also been
studied [118]. PCNSL enhances the thermal stability of LDPE [119]. A waterproofing resin composition was developed from CNSL by heating CNSL with shellac at 100 ◦ C. A heat-resistant resin can be developed from CNSL when it is subjected to phase transfer-catalysed KMnO4 oxidation or when it is polymerized with
TiCl4 or ZnCl4 in the presence of a Grignard reagent in an inert atmosphere. CNSL
can be used for making an ablative fiber-reinforced polymer by the introduction of
bromine into the phosphorylated prepolymer [120]. The reaction products of CNSL
and fatty acids have been reacted with glycerin to obtain resins suitable for electri-
148
M. C. Lubi and E. T. Thachil
cal insulations. The epoxy derivative of CNSL can be used as a stabilizer for PVC
[121]. CNSL reduces the rate of dehydrochlorination of PVC [122]. Sulfurated
CNSL is used as a plasticizer [123] and cardanol condensed with epichlorohydrin
is used as a stabilizer [124] for PVC.
CNSL can be used as a chemical modifier for asphalt to improve low- and hightemperature performance [125]. Polymerized CNSL can be reacted with furfural
to give resins suitable for insulation parts for storage batteries, because of their
resistance to acids and alkalies. Methylation with dimethyl sulfate and acetylation
with anhydride and acetic acid can give epoxide products with a pale color obtained
from CNSL for use as a plasticizer [126]. Curing agents for epoxy resins are
prepared by the reaction of CNSL with diethylene tetramine. Phenolic polyamines
for curing epoxy resins are also prepared by using mixtures of CNSL with phenol,
cresol or other alkylated phenols.
Linseed oil-modified CNSL –HCHO resin has outstanding flexibility and chemical resistance [127]. CNSL can be condensed with cyanuric chloride to improve
the thermal properties of the products [128]. CNSL dimethylol urea isogel resin has
been prepared by treating urea with formaline and CNSL. Rigid polyurethane foam
has been prepared by treating CNSL with HCHO and ethanolamine at 70– 90 ◦ C,
blending the reaction mixture with polyols, and treating the mixture with polyisocyanate and blowing agent at less than 25 ◦ C.
The addition of CNSL can increase the heat resistance of a phenolic molding
composition. A phenolic resole molding composition useful in the manufacture
of commutators has been reported. A phenolic molding composition for dynamic
transmission has been developed using a mixture of CNSL-modified phenolic resin,
chopped vinyl fibers, and nitrile rubber. Moldings having a wood-like feel and
good antistatic and electromagnetic properties and coatability useful for building
materials have been prepared from CNSL resins. Molding compounds without
blisters based on CNSL have been reported.
7.8. Miscellaneous applications
CNSL is used for the treatment of acne vulgaris. It is also used as an antibacterial
agent [129]. CNSL finds use as a rubefacient and vesicant in treating skin diseases.
A range of drug analogues of warfarin, phenol phthalein, saligenin, salicylamide,
and salicylanilide have been prepared from 3-pentadecyl phenol and 5-pentadecyl
resorcinol [130].
CNSL is used as an antioxidant for food and cosmetics. Insecticidal compositions
have been made from chlorinated products of CNSL by blending with rotenone,
DDT, etc. CNSL has been used for the stabilization of pyrethrine. CNSL is treated
with AS2 O3 or boric acid to improve its toxicity towards termites [131].
CNSL can be used as a superplasticizer for cement paste and mortar [132].
Cashew polymers react with HCHO to give a rubbery gel, which can be used as
a cement-hardening agent over a period. The set mass adheres well to porous
brickwork, steel, and carbon blocks, and is not affected by acids or alkalis. It can
Cashew nut shell liquid
149
be used for the cementing of floors subject to chemical attack. Chemically resistant
covering sheets useful as table tops and floor coverings are made by treating CNSL
with aldehydes and compounding the hard gel with NR and curing systems. Acidpolymerized CNSL reacted in the cold with paraformaldehyde or furfural gives a
cold-setting composition with excellent acid and alkali resistance. These products
find extensive use as stopping and filling compounds in the electrical industry in
industrial flooring and chemical plants. CNSL –HCHO reaction products have been
used for floor coverings and insulated air plane ignition conductors in combination
with cellulose ethers, PVA, etc. Polymerized CNSL –HCHO resins with carbon or
graphite fillers are used to make chemically resistant linings and joints in chemical
plants.
Cation exchange resins can be prepared by the reaction of CNSL with H2 SO4 ,
and anionic exchange resins by the reaction with tetraethylene pentamine. Cation
exchange resins inert to alkali have also been developed. Rubber-based cation
exchange resin membranes are made from CNSL and aldehyde by milling with
crepe rubber [133]. Various types of lubricants have been prepared from CNSL
[134].
Activated carbon can be prepared from CNSL using CO2 and KOH as activating
agents. The rate of moisture absorption can be reduced by using CNSL in kerosene
and methylated spirit [135]. Organometallic preservatives can be prepared from
CNSL [136]. CNSL –HCHO resin suitably compounded has been used for coating
coarse cloth to give linoleum. Fish nets can be coated with CNSL to prolong
their lives. Heat-processed reaction products from CNSL were found to confer
water-proofing properties to cardboard and cotton fabrics. High boiling solvents
and heat exchange media have also been developed from CNSL. Phenolic resins
made from CNSL and aldehyde can be modified by reaction with amide and can be
used as a substitute for vegetable wax. Slow-acting fertilizers have been developed
from CNSL. CNSL can be reacted with glycerin and other polyhydric alcohols
to give substances which are resistant to flame, and acids. CNSL is also used as
a hypergolic rocket fuel [137]. Various types of surfactants and additives have
been derived from CNSL. Ionic conducting films have been prepared from CNSL
[138]. New dyes and inks have also been developed from CNSL. The spontaneous
combustion characteristics of CNSL –HCHO products can be considerably reduced
by the incorporation of a small amount of aniline. When china-wood oil or linseed
oil is heated with CNSL, there is no danger of overheating in contrast to when
they are heated alone [139]. The moisture absorption of superphosphate can be
reduced by coating with a solution of CNSL in kerosene or methylated spirit
[140]. A number of products based on CNSL have been reported to be extremely
useful additives, such as antioxidants, stabilizers and de-emulsifiers for petroleum
products.
150
M. C. Lubi and E. T. Thachil
8. OUTLOOK FOR THE FUTURE
In spite of the massive literature accumulated on the applications of CNSL, many
areas remain which have yet to utilize this attractive raw material. Disadvantages
such as non-uniformity in the quality of the oil and its composition when it comes
from different locations and the unattractive color have been stumbling blocks
in its large-scale utilization. It is hard to believe that organic chemists have
exhausted all possibilities of synthesizing more valuable and useful derivatives from
CNSL, which has a most unusual structure for a starting material. The cracking
of this high-molecular-weight product is one area which merits more detailed
information. The use of CNSL in the field of engineering polymers has not been
widely reported. While the possibility of making useful thermoset polymers such
as polyurethanes, epoxies, and polyester has been demonstrated, there has not been
sizable consumption of the oil for this purpose. This can be an attractive proposition
in view of the dwindling global petroleum resources. While forest resources being
depleted at a fast rate, the manufacture and use of wood substitutes based on CNSL,
utilizing wastewood, sawdust, woodchips, etc., can be an attractive proposal. CNSL
has outstanding anti-microbial properties which can help mankind fight parasites
and harmful micro-organisms in the agricultural and medicinal fields. It can be a
valuable alternative to many of the troublesome chemical agents in use today.
9. CONCLUSIONS
The present survey has demonstated the immense potential of CNSL to satisfy many
of our material requirements in diverse fields. Certain problems such as variation
in the composition and quality of the oil have to be solved by proper blending
and quality control. Considering the large number of different polymer materials
that can be synthesized from CNSL, it is surprising that very little global effort
is directed towards greater and fuller utilization of this versatile starting material.
The technological world will do well to concentrate on the development and use
of value-added products from CNSL in the interests of economy, natural resource
conservation, ecology, integrated agricultural development, and the overall progress
of mankind.
REFERENCES
1. M. T. Harvey and C. Caplan, Ind. Eng. Chem. 32 (10), 1306 (1940).
2. S. Manjula, J. D. Sudha, S. C. Bera and C. K. S. Pillai, J. Appl. Polym. Sci. 30, 1767 (1985).
3. Stadeler, Ann. Chem. Pharm. 63, 137– 164 (1847).
4. A. R. R. Menon, C. K. S. Pillai, J. D. Sudha and A. G. Mathew, J. Sci. Ind. Res. 44, 324 (1985).
5. P. H. Gedam and P. S. Sampathkumaran, Prog. Org. Coat. 14, 115 (1986).
6. J. H. P. Tyman, Chem. Soc. Rev. 8, 499– 537 (1973).
7. J. H. P. Tyman, D. Wielezynski and M. A. Kashani, J. Am. Chem. Soc. 55, 663 (1978).
8. P. A. Mahanwar and D. Kale, Indian J. Technol. 3, 191– 193 (1996).
Cashew nut shell liquid
151
9. V. J. Paul and L. M. Yeddanapally, Nature 174, 604 (1954).
10. W. F. Symes and C. R. Dawson, Nature 171, 841 (1953).
11. J. A. Cornelius, Trop. Sci. 8 (2), 79 (1966).
12. J. H. P. Tyman, J. Am. Oil. Chem. Soc. 66 (4), 533– 537 (1989).
13. R. K. Jain and K. Sivala, J. Food Engg. 32 (2), 339– 345 (1997).
14. L. C. Anand, J. Petro Chem. 12 (5), 3 (1981).
15. V. Madhusudhan, M. Sundararamaiah and N. B. Naidu, Indian J. Tech. 11, 347 (1973).
16. H. Barron, Use of CNSL resins in modern plastics, 2nd edn, p. 84. Chapman and Hall, London
(1949).
17. R. Antony, C. K. S. Pillai and K. J. Scariah, J. Appl. Polym. Sci. 41, 1765 (1990).
18. N. R. Krishnaswamy and K. P. Govindan, Chem. Ind. 1892 (1958).
19. I. N. Hemalatha, H. Dakshinamurthy and J. S. Aggarwal, Paint Manuf. 39 (11), 41 (1969).
20. D. S. Basu and A. K. Kar, Chem. Age India 24 (6), 341 (1973).
21. M. K. Mishra and G. N. Pandey, J. Appl. Polym. Sci. 30, 969– 977 (1985).
22. D. O’ Conner and F. D. Blum, J. Appl. Polym. Sci. 33, 1933– 1941 (1987).
23. K. Sathialekshmi, Bull. Mater. Sci. 16, 137 (1993).
24. S. Lenka, A. P. Das and P. L. Nayak, J. Polym. Sci. Polym. Chem. 30, 4619 (1985).
25. J. S. Aggarwal, Paint Manuf. 36, 29 (1966).
26. M. T. Mortimer, Cashew News 11, 16 (1977).
27. K. K. Alva, Shridara, J. Macromol. Sci. A 34 (4), 665– 674 (1997).
28. S. C. Sethi and B. C. S. Rao, Indian J. Tech. 2, 206 (1964).
29. M. S. Bhatnagar, Pop. Plastics, 79 (1982).
30. D. Narayanamurthy, Res. Ind. 13 (3), 134– 136 (1968).
31. A. R. R. Menon, C. K. S. Pillai and G. B. Nando, J. Adhes. Sci. Tech. 9, 443– 451 (1995).
32. N. D. Ghatge and U. S. Patil, Angew. Makromol. Chem. 19, 83 (1971).
33. N. D. Ghatge and R. G. Gokhale, Indian J. Tech. 9 (10), 391 (1971).
34. N. D. Ghatge and R. G. Gokhak, Rubber India 13, 2406 (1972).
35. D. A. Rahul, Paintindia 40 (8), 59– 60 (1990).
36. L. C. Anand, Paintindia 28 (6), 13– 33 (1978).
37. A. Orazo, Chem. Ind. 61 (10), 718– 726 (1979).
38. A. Orazo, Chem. Ind. (Melan) 78 (6), 693– 696 (1996).
39. J. S. Aggarwal, J. Colour Soc. 14 (3), 1–9 (1975); 15 (1), 14– 25 (1976).
40. S. Ramanujam, Paintindia 10, 91– 106 (1960).
41. J. S. Aggarwal, Paintindia 17 (1), 103– 112 (1967).
42. A. E. Oats, Ind. Chem. 323 (1956).
43. M. Sunsaramaiah, Fette Seifen Anstriokn 78, 472 (1976).
44. S. P. Potnis and J. S. Aggarwal, J. Colour Soc. 13 (1), 10– 12 (1974).
45. J. S. Aggarwal, Paint Manuf. 43 (3), 29– 31 (1973).
46. (a) F. D. Blum, J. Am. Chem. Soc. 37, A1 (1996);
(b) F. D. Blum, Polym. Prepr. 37, 733– 734 (1996).
47. S. S. Bera and C. K. S. Pillai, Indian J. Tech. 27 (8), 343– 347 (1989).
48. S. Ramanujam, Paintindia 11 (1), 47– 112 (1961).
49. S. S. Chopra, Paintindia 15 (1), 136– 138 (1965); 16 (3), 25– 30 (1966).
50. Vo Dhien and Tap Chi, Hoa Hoc. 20 (2), 29– 32 (1982).
51. W. Sailan, Tuliao Gongye 6, 14– 16 (1995).
52. M. Shirsalkar and M. A. Sivasamban, J. Colour Soc. 16 (2), 13– 18 (1976).
53. A. Ramamurthy and M. D. Das, Indian Cashew J. 11, 5 (1977).
54. N. C. Jain and R. S. Shah, Research and Ind. 5 (4), 94– 95 (1960).
55. M. Selvaraj, Paintindia 37 (7), 19– 22 (1987).
56. S. B. Roy and H. D. Sircar, Paintindia 12, 18 (1963).
152
M. C. Lubi and E. T. Thachil
57. T. Ramalingam, B. G. K. Murthy, M. A. Sivasamban and J. S. Aggarwal, Paintindia 20 (10),
29 (1970).
58. Mannare and D. A. Rahul, Paintindia 40 (8), 59– 60 (1990).
59. S. Guruviah and K. S. Rajagopalan, Paintindia 17 (4), 15 (1967).
60. K. Ravindran and R. Balasubramanian, Paintindia 18 (7), 28 (1968).
61. T. Ramalingam, B. G. K. Murthy and M. A. Sivasamban, Paintindia 21 (8), 22 (1971).
62. S. Malik and S. N. Shimtre, Paintindia 39 (4), 55– 58 (1989).
63. N. K. H. Purandare and J. S. Aggarwal, Paintindia 15 (6), 25 (1965).
64. K. G. Srivastava, S. P. Potnis and J. S. Aggarwal, Paint Manuf. 45 (3), 48 (1973).
65. B. G. K. Murthy and M. A. Sivasanban, Cashew Couseric 1 (3), 8– 10 (1979).
66. C. K. S. Pillai, Indian Coconut J. 14 (1983).
67. P. Potnis and M. Venugopal, Paintindia 14 (4), 29 (1964).
68. M. Selvaraj, Paintindia 37 (7), 19– 22 (1987).
69. M. Seivarajm and S. Guruvia, Bull. Electro Chem. 5 (7), 528– 532 (1989).
70. S. Vargiu and G. Guliani, Bull. Assoc. Ital. Tech. Ind. Vermi Affine 65, 3– 8 (1978).
71. M. T. Harvey, Cashew News Bull. 11, 5– 8 (1978).
72. T. B. Desai, Paintindia 27 (5), 13– 19 (1977).
73. S. P. Potnis and M. Venugopal, Paintindia 14 (2), 12 (1964).
74. Da Rocha and Silva Rev, Quim. Ind. 53 (515), 22– 25 (1975).
75. S. K. Ragunathan and K. G. Tandon, Paint Tech. 11, 94 (1946).
76. S. I. Patil, M. C. Menon and J. S. Aggarwal, Paintindia 11 (4), 17 (1961).
77. V. Madusudhan, B. G. K. Murthy and M. A. Sivasamban, Paint Manuf. 42 (6), 368 (1972).
78. V. Madusudhan, B. G. K. Murthy and M. A. Sivasamban, Cashew Couserie 6 (2), 12– 16
(1984).
79. D. Narayanamurthy and R. C. Gupta, Rheo Acta 1, 514 (1961).
80. C. P. Dhamany, Paintindia 26, 20 (1976); 33 (3), 7– 10 (1983).
81. J. D. Sudha, C. Pavithran and C. K. S. Pillai, Res. Ind. 34 (2), 139– 141 (1989).
82. D. Narayanamurthy and C. P. Dhamany, Adhesion 9 (10), 398 (1965).
83. C. P. Dhamany, Paintindia 18 (8), 17 (1968).
84. D. Narayanamurthy and R. Ramprasad, Holz. Roh. Werkst 27 (2), 45 (1969).
85. C. P. Dhamany, Paintindia 28 (9), 27 (1978).
86. N. C. Jain and C. P. Dhamany, Paintindia 14 (1), 8– 9 (1969).
87. C. P. Dhamany, Paintindia 20 (11), 21 (1970).
88. K. Anon, Bioresour Tech. 56 (223), 279– 280 (1996).
89. D. Narayanamurthy, Composite Wood 4 (6), 97– 103 (1957).
90. C. P. Dhamany, Paintindia 30 (4) (1980); 29 (6), 40 (1979).
91. S. P. Aggarwal, Res. Ind. 40 (1), 1–4 (1995).
92. C. P. Dhamany, J. Colour Soc. 9 (4), 1–5 (1970).
93. S. S. Stivala and W. J. Powers, Ind. Eng. Chem. 50, 935 (1958).
94. D. N. Batia, N. J. Kazi and A. Asak, Foundry J. 21 (6), 10– 15 (1975).
95. R. P. Sinha and A. K. Kar, Res. Ind. 17 (4), 134 (1972).
96. S. S. Mahajan and R. S. Khisti, Res. Ind. 29, 26– 28 (1984).
97. N. C. Jain and Shah, Res. Ind. 5 (4), 94 (1960).
98. B. Banerjee, N. P. Suryanarayana and C. S. Inamadar, Indian Patent 142572 (1977).
99. N. D. Ghatge and R. G. Gokhale, Rubber Age 101 (2), 52 (1969).
100. L. K. Sangi, J. Inst. Rubber Ind. 8 (5) 188– 191 (1974).
101. C. G. Mothe, J. Thermal Analysis Polymer Testing 15 (1), 91– 97 (1996).
102. R. A. Rajapakse, Polymer 19 (2), 205– 211 (1978).
103. N. D. Ghatge and R. G. Gokhala, Rubber India 24 (6), (1972).
104. V. Krishnan and B. K. Shah, Pop. Plastics 11 (2), 20 (1966).
105. S. K. Banerjee, Indian J. Tech. 9 (11), 424 (1971).
Cashew nut shell liquid
153
106. S. K. Banerjee, Indian J. Tech. 9 (12), 467 (1971).
107. C. K. S. Pillai, Polym. Sci. 2, 657– 661 (1994).
108. A. R. R. Menon, J. Appl. Polym. Sci. 65 (11), 2183– 2189 (1997).
109. A. R. R. Menon, J. Fire Sciences 15 (1), 3–13 (1997).
110. S. K. Banerjee, Rubber India 30 (4), 13– 17 (1978).
111. N. D. Ghatge, Elastomeric 11 (12), 48– 50 (1979).
112. N. D. Ghatge and M. N. Malda, Rubber Chem. Technol. 52 (2), 353– 360 (1979).
113. R. A. Rajapakse, Programme Pop-Rubber Conf., IRMR Thana, India 10th 1978, 116– 26.
114. C. K. S. Pillai, V. C. Prasad, J. P. Sudha and A. R. R. Menon, J. Appl. Polym. Sci. 41 (9– 10),
2487– 2501 (1990).
115. A. Mahanwar and P. D. Kale, J. Appl. Polym. Sci. 61 (12), 2107– 2111 (1996).
116. R. K. Jain, K. Kasthana and S. Rawats, Res. Ind. 24, 15– 18 (1979).
117. C. P. Dhamany, Paintindia 28 (11), 29 (1978).
118. R. K. Jain and K. Kasthana, Pop. Plast. 16 (11), 269 (1971).
119. V. S. Prasad and C. K. S. Pillai, Polym. Sci. 2, 823– 828 (1994).
120. C. K. S. Pillai, Pop. Plastics 33 (9), 53– 55 (1988).
121. S. D. Vernekar, Indian J. Tech. 18 (4), 170– 172 (1980).
122. I. K. Varma and S. K. Dharak, J. Polymer. Mat. 6 (3), 167– 173 (1989).
123. N. D. Fregade and N. D. Millans, Rubber India 33 (7), 17– 22 (1981).
124. E. Neuse and J. D. Van Schalkwyk, Afr. J. Sci. 72 (8), 223– 227 (1976).
125. M. Vikas, Ind. Eng. Chem. 24 (3), 478– 484 (1985).
126. Y. K. Kammath and B. D. Vidvans, Curr. Sci. 32, 72 (1963).
127. K. Kchakrawarthy, Indian J. Tech. 25 (3), 109– 113 (1987).
128. D. M. Kulkarni and N. D. Ghate, J. Shivaji Univ. 36 (1,2) (1968).
129. H. Masaki, J. Agri Food Chem. 39 (2), 418– 421 (1991).
130. A. S. Gulate and B. C. Subhrow, Indian J. Chem. 2 (8), 3378 (1964).
131. J. K. Jain, Narayan, J. Timber Div. Associ India 37 (2), 25– 27 (1991).
132. I. Masood, J. S. Aggarwal, Cem. Conc. Res. 24 (2), 291– 302 (1994).
133. N. P. Suryanarayana and K. M. Joshy, J. Appl. Polym. Sci. 8 (3), 1491 (1964).
134. K. D. Pathak, Indian J. Tech. 1 (11), 433– 34 (1963).
135. G. S. Khanvilkar, Indian Agr. Sci. 33, 44– 101 (1963).
136. E. H. G. Seargent, British Patent 678019 (1952).
137. G. G. Pant, V. Y. Oka, S. K. Chathopadhya, P. K. Dutta and U. C. Durgapal, J. Armament Stud.
14 (1) 47– 51 (1978).
138. S. Guruviah and J. G. Mayue, Proc. Congr. Mat. Corros. 5th, pp. 656– 659 (1972).
139. G. S. Khanvilkar and A. Sen, Indian J. Agric. Sci. 33, 94 (1963).
140. K. D. Pathak and B. C. Sudha, Indian J. Tech. 1, 433 (1963).