J. Coat. Technol. Res.

, 11 (2) 169183, 2014

DOI 10.1007/s11998-013-9521-3

REVIEW ARTICLE

CNSL: an environment friendly alternative for the modern

coating industry
Dinesh Balgude, Anagha S. Sabnis

 American Coatings Association & Oil and Colour Chemists Association 2013
Abstract Considering ecological and economical
issues in the new generation coating industries, the
maximum utilization of naturally occurring materials
for polymer synthesis can be an obvious option. In the
same line, one of the promising candidates for substituting partially, and to some extent totally, petroleumbased raw materials with an equivalent or even
enhanced performance properties, is the Cashew Nut
Shell Liquid (CNSL). This dark brown-colored viscous
liquid obtained from shells of the cashew nut can be
utilized for a number of polymerization reactions due
to its reactive phenolic structure and a meta-substituted unsaturated aliphatic chain. Therefore, a wide
variety of resins can be synthesized from CNSL, such
as polyesters, phenolic resins, epoxy resins, polyurethanes, acrylics, vinyl, alkyds, etc. The present article
discusses the potential of CNSL and its derivatives as
an environment friendly alternative for petroleumbased raw materials as far as polymer and coating
industries are concerned.
Keywords CNSL, Renewable resources, Resins,
Functional chemicals, Coatings

Introduction
Until now, a number of chemistries have been explored
in the coatings industry, such as epoxy, alkyd, polyurethane, phenolic, acrylic, polyester, silicates, etc.
Generally, these chemistries are derived from petroleum-based stocks. Though these petroleum-derived
polymers/resins have played a vital role in the coating
D. Balgude, A. S. Sabnis (&)
Department of Polymer & Surface Engineering, Institute
of Chemical Technology, Mumbai 400 019, India
e-mail: as.sabnis@ictmumbai.edu.in;
anaghasabnis@rediffmail.com

industry, their uses have been overshadowed by economical and ecological aspects of the modern coating
industry. These aspects involve exponentially rising
prices and high depletion rate, handling issues, toxicity
and health hazards of material derived from petroleum
stocks, and volatile organic compound (VOC) that are
emitted in the environment during synthesis and
application of petroleum-derived chemistries as they
are volatile in nature. So, considering these issues, it is
necessary to search and explore the new sustainable,
economical, nontoxic, and nonhazardous alternatives.
One of the possible solutions is the use of bio-based
materials for polymer/resin synthesis. The increasing
worldwide interest in the use of biomaterials is mainly
due to the fact that these materials are derived from
natural sources which are abundantly available and
their use would also contribute to global sustainability
without depletion of scarce resources. Also, biomaterials are comparatively easy to handle with no or less
toxicity and health-related issues. Unlike petroleumderived polymers, the polymers synthesized from such
bio-based materials can degrade in a controlled way
when they come in contact with the biological environment due to the enzymatic action of some microorganisms, thus promoting the conversions to biomass,
methane, carbon dioxide, water, and other natural
substances.1,2 Thus, less environmental impact, ease of
availability, more economical, and easy biodegradability makes bio-based material an attractive topic for
academic as well as for industrial research on synthesizing polymers and functional chemicals for the
coatings industry.3
The utilization of bio-based materials as such or by
chemical modifications for various applications like
resin synthesis, adhesives, paints, coatings, composites,
etc., has been well reported.410 Such materials include
cellulose, starch, sucrose, sugar, lignin, plant and
animal oils, etc. However, there exists a compound
such as Cashew Nut Shell Liquid (CNSL), which can

169

J. Coat. Technol. Res., 11 (2) 169183, 2014

production of cashew trees throughout the year. There

are a number of other challenges like land preparation,
spacing, fertilizer use, entomological/pathological
problems, etc., that need to be taken care of in order
to have maximum production. Also, during processing,
CNSL is difficult to remove from the shell with high
yields due to the hard outer shell, the intricate
honeycombed features of the pericarp and the thermally sensitive nature of the CNSL. Methods for
removing CNSL from the shell include roasting, hot-oil
bath, steam processing at 270C, quick roasting at
300C, cold methods, and the solvent extraction
method.1618 Though commercially being used, these
methods have some constraints like low yield, polymerization of CNSL at processing temperature employed, long extraction times (22336 h), large
amounts of solvent, harsh mechanical pretreatment,
etc.19 To overcome these issues, supercritical fluids

1,400,000
1,200,000

Production (tons)

be used as a possible substitute for petroleum-based

materials due to its availability, sustainability, cost
effectiveness, and reactive functionalities.
Physically CNSL appears as a reddish brown,
viscous fluid found in shells of cashew fruits of
Anacardium occidentale (as shown in Fig. 1) cultivated
in a large number of tropical and subtropical countries.
The tree is native to Brazil and the coastal areas of
Asia and Africa and is now being grown extensively in
India, Vietnam, Mozambique, the Malagasy Republic,
Tanzania, Philippines, and other tropical countries.11,12
In some of these regions, cashew is a popular
plantation product, while some others import cashew
nuts for processing. Figure 2 gives world production of
cashew nuts showing the increasing trend in the
production of CNSL from 1960 until now. Among
the various countries mentioned above, Vietnam,
India, Nigeria, Cote d\Ivoire, and Brazil have become
the top five cashew nut producing countries all over the
world in 2010 with the production of 1,159,600, 613,000,
594,000, 370,000, and 174,300 metric tons (MT),
respectively.14 Thus considering the increasing production of cashew nut from 1960 until now, across the
world, we can consider CNSL as a continuous source
available for industrial exploitation without depletion
of stocks which proves its sustainability.
Though having worldwide availability, there are
some challenges in CNSL. For example, the cashew
tree is usually grown from seeds placed directly in the
field. Seed nuts should be thoroughly dry, clean, and
free from insect or fungal attack. Cashew seeds should
be sown or planted during the rainy season. Once the
rainy season is over, seeds should be stored properly
until the next rainy season before they are planted in
the field. If not, they may lose their germination
capacity. As plantation of cashew seeds is seasonal,
harvesting is another challenge in ensuring the continuous

1,000,000
800,000
600,000
400,000
200,000
0
1961 1965 1970 1975 1980 1985 1990 1995 1998 2000

Years

Fig. 2: World production of cashew nuts (from 1961 to

2000)15

Cashew apple
(pseudo-fruit)

Spongy
Shell
Cashew Nutshell Liquid
(CNSL)
Almond
OH
OH
OH

Cashew Nut
(fruit)

Fig. 1: Cashew nut shell liquid origin13

170

OH

Cardanol

J. Coat. Technol. Res., 11 (2) 169183, 2014

have been suggested as an attractive alternative

method by Saito.20 So, with proper agricultural practices and optimization of various process parameters
like extraction pressure, extraction temperature, and
mass flow rate of supercritical fluid in the advanced
extraction techniques, one can overcome all the challenges related to the production of cashew tree and
CNSL extraction.
Besides having processing constraints, CNSL and
their derivatives possess a number of technical benefits
over other renewable oils. Unlike oils, the extracted
CNSL contains a number of useful phenolic derivatives
with meta-substituted long chain saturated/unsaturated
hydrocarbons which makes them suitable for a number
of polymerization reactions through addition as well as
condensation mechanisms. Also, the combination of
aromatic ring and long chain hydrocarbon helps to
maintain the good balance between flexibility and
hardness properties of the coatings. On this basis they
were used in a number of industrial applications,
including brake linings materials, laminating resins,
adhesives, ion-exchange resins, paint and coating
resins, foundry chemicals, lacquers, fine chemicals,
hybrid materials, water proofing agents, surface active
agents, synthetic rubber, wax compounding, etc., as
shown in Fig. 3.
Due to reactive phenolic hydroxyl, one-step synthesis of epoxy resin with 100% conversion can be
possible, unlike epoxies derived from oil. Also,
CNSL-based epoxy synthesis does not involve the use
of hazardous chemicals like peroxides which are used
in epoxidation of oils. Thus there are no handling or
health-related issues in CNSL-based resin synthesis.

Also, chemically unmodified CNSL has been reported

to reduce the corrosion rate of carbon steel by over
90% due to phenolic hydroxyl which gets adsorbed on
metal surface.24 This inhibitive property cannot be
achieved with oils. Due to structural similarity, CNSL
and their derivatives can easily replace toxic phenolic
compounds used in resin synthesis, like phenols in
phenolic resin synthesis, bisphenol-A in epoxy resin
synthesis with improved properties. In some cases,
epoxy resin derived from CNSL can replace conventional epoxy resin with equivalent or slightly higher
performance properties. Also, chemically modified
CNSL can replace hydroxyl functional resins derived
from petroleum-based stocks which can be used in a
number of applications like polyurethane synthesis,
crosslinkers, etc., with better performance properties.
With esterification, CNSL can replace conventional
toxic plasticizers like di-octylphthalate (DOP)/di-butylphalate (DBP)/di-ethylhexylphthalate (DEHP) which
are used in polyvinyl chloride (PVC) processing.
Commercially used hindered phenols for antioxidant
purpose can be replaced by chemically modified CNSL.
These are all chemical modifications and their performance against conventional ones are covered in the
subsequent sections.
Chemistry of CNSL and composition
of its extraction
As an agricultural by-product of the cashew nut
production, CNSL is one of the major economic
sources of naturally occurring phenols and is regarded

Lacquers & Varnishes

Specialty Polymers & Coatings

Decorative applications
Protective applications
Insulation Coatings
Buildings, Furnitures, &
Automobiles

Anti-biofilm Coating
Crosslinked Polymers
Molecularly Imprinted Polymers
Fire Retardant Polymers
Liquid Crystalline Polymers, etc.

Applications of
CNSL

Other Applications
Foundary Chemicals
Brake Lining & Clutch Facing
Laminates
Cement Hardeners
Diesel Oil
Adhesives
Hybrid Materials
Rubber Compounding
Age Resistors (Prevent degradation)
Vulcanizing Agent

Resin Synthesis
Alkyd
Polyesters
Epoxy
Polyurethanes
Acrylics
Phenolics
Ion Exchange Resins, etc.

Paints & Primers

Additives

Anticorrosive
Heat Insulating
Flame Resistant
Black Enamels

Antioxidants
Corrosion Inhibitors
Colorants & Dyes
Coupling Agents
Dispersants
Bactericides
Fungicides
Emulsifying Agents
Stabilizers
Accelerators
Plasticizers

Fig. 3: Potential applications of CNSL and its derivatives2123

171

J. Coat. Technol. Res., 11 (2) 169183, 2014

Table 1: Physicochemical characteristics of Cashew

Nut Shell Liquid (CNSL)26
Parameter
Appearance and nature
Refractive index
Specific gravity
Viscosity (30C) (centripore)
Moisture (%)
Ash (%)
Saponification value
(mgKOH/g)
Iodine value (mg/100 g)
Acid value (mgKOH/g)
Free fatty acid (mgKOH/g)

OH

OH
COOH

Observation
Reddish brown viscous liquid
1.6931.686
0.9410.924
4156
3.96
1.2
4758

C15H31

C15H31

(a)

(b)

OH

OH
H3C

(d)

(c)
215235
12.115.4
6.17.8

C15H31 =

C15H31

HO

C15H31

HO

(1)
8'
(2)
8'

as a versatile and valuable raw material for polymer

production. CNSL obtained from unroasted shells was
first found by Stadeler to consist chiefly of anacardic
acid which on heating decarboxylated to cardanol and
cardol. Since then, a number of authors have reported
on the chemistry, method of extraction, refining, and
compositions of the extracted CNSL.25 The physicochemical properties of CNSL are stated in Table 1.
On thermal distillation, CNSL yields a number of
phenolic derivatives like anacardic acid (6-pentadecenyl salicylic acid), cardol (5-pentadecenyl resorcinol),
and 2-methylcardol (2-methyl 5-pentadecenyl resorcinol) whose main component is cardanol (3-pentadecenyl phenol); a meta-substituted unsaturated
hydrocarbon chain having a chain length of C-15,27 as
shown in Fig. 4.
Possible reactions of CNSL and their derivatives
CNSL and their derivatives can undergo number of
chemical reactions, some of them being sulfonation,
nitration, esterification, halogenation, etherification,
epoxidation, etc.
CNSL was sulfonated28 to yield alkyl aryl sulfonic
acid or their metal salts. The reaction was carried out
at 108C using concentrated H2SO4. To prevent
polymerization during sulfonation, an aryl or alkali
group was substituted for hydrogen and the double
bonds of the aliphatic side chain were saturated by
hydrogenation before treatment with the acid.
Direct nitration of cardanol leads to simultaneous
oxidation and polymerization reactions. By nitration of
hydrogenated cardanol, 4-nitro and 6-nitro compounds
were obtained.29 Nitro-derivatives of cardanol are very
efficient antioxidants for gasoline, mineral hydrocarbons, petroleum products, and lubricating oils.
Cardanol esters can be synthesized by reacting
cardanol with acid chlorides in the presence of alkalis.
Thus benzoyl chloride gives benzoyl cardanol. Various
other esters of industrial importance have also been
reported.30

172

8'

11'

(3)
14'
(4)

11'

Fig. 4: Chemical composition of CNSL: (a) anacardic acid,

(b) cardanol, (c) cardol, (d) 2-methylcardol

Epoxidation of the phenolic group can be accomplished by the reaction of CNSL with epichlorohydrin.31 The chemical changes during this reaction are
similar to those of conventional epoxy synthesis. In this
regard, Unnikrishnan et al. have studied the use of
cardanol in place of a phenol or diphenol, in the
synthesis of epoxy system and compared with the
conventional epoxy. Further, the combination of cardanol and bisphenol-A have also been studied and it was
observed that introduction of 20 mol% cardanol into
bisphenol-A resulted in a resin having reduced tensile,
impact, and compressive strengths upon curing by a
polyamine hardener but considerable improvement in
elongation-at-break without much decrease in energy
absorption. All possible reactions are shown in Fig. 5.
CNSL and its derivative-based polymers, resins,
and functional chemicals
CNSL, a naturally available material, undergoes similar
kinds of reactions as those of phenols due to its phenolic
structures. In addition, the presence of long chain
unsaturated hydrocarbon chain provide additional
reacting site. Therefore, a diverse range of resins/
polymers can be synthesized using CNSL. It includes
epoxy, alkyd, polyurethane, phenolic resin, vinyl,
acrylic, etc. Further, these synthesized resins can be
formulated for different types of coatings like modified
alkyd-based coatings, epoxy coatings, waterborne coatings, UV-curable coatings, modified polyurethane coatings, phenolic coatings, etc. (as shown in Fig. 6).
Numerous authors have reported on the different
reaction conditions, number of reaction catalyst, and
various process parameters to polymerized CNSL.3235

J. Coat. Technol. Res., 11 (2) 169183, 2014

Sulphonation

OR
SO3

+
M

C15H31

H2SO4, Zn, Ca

H2O
Es

OR

rif
ica

tio

io

OCOC6H5

C15H31

O
+

NaCl

RCl

HCl

EC

OH

li

ka

Na

6H
5C

C15H31
OH

HCI

C15H31

OC
I

id
at
Ep
ox

te

Al

C15H31

HN

OR

C15H31

he

rif
ica

OH

OH
O2N

Et

2O

2S

2S

+
C15H31

C15H31
NO2

io

tio

itr

at

Fig. 5: Possible reactions of CNSL and their derivatives

Epoxy resins
Aggarwal et al.36 have developed an epoxy-cardanol
resin with better properties as compared to BPA-based
epoxy resin in terms of an increase in tensile strength
(31%), elongation (129%), and bond with steel (28%)
and reduced water vapor transmission of the film.
Further the synthesized resin was formulated for an
anticorrosive paint and cured with an aromatic polyamine adduct hardener. The formulated paint was
tested for physico-mechanical properties, chemical
resistance and corrosion protection efficiency and
compared with conventional epoxy resin-based anticorrosive paint. It was found that the modified resinbased paints exhibit about 25% higher tensile strength
and 15% more elongation than the paints made with
unmodified resin. Finally, authors have concluded that
the developed resin performed better as binder media
for the formulation of anticorrosion paints than the
unmodified epoxy resin.
Huang et al.37 have synthesized a light color cardanol-based epoxy curing agent from cardanol butyl
ether, formaldehyde, and diethylenetriamine and

compared to phenalkamine with a similar structure.

It was observed that etherification of phenolic hydroxyl
of cardanol improved the color stability and lowered
the viscosity. However, etherified cardanol was found
to be less reactive due to absences of phenolic hydroxyl
group as compared to phenalkamine.
Kim et al.38 have successfully synthesized an epoxidecontaining polycardanol by enzymatic route using two
different enzymes, viz. lipase and peroxidase. Lipase
catalysis was used for the epoxidation of the unsaturated alkyl chains of both cardanol and polycardanol,
and peroxidase catalysis was used for the polymerization of both cardanol and epoxide-containing cardanol.
The product was synthesized by two different routes
including synthesis of epoxide-containing cardanol in
presence of lipase, followed by the polymerization of
the phenolic functional groups of cardanol using
peroxidase. Another route involved a synthesis of
polymerized cardanol from cardanol in the presence
of peroxidase and, subsequently, the epoxide-containing polycardanol from polycardanol in the presence of
lipase. The curing of the resulting polymers proceeded
thermally at 150C, and yielded a transparent polymeric

173

J. Coat. Technol. Res., 11 (2) 169183, 2014

OH
OCH2CHCH2OH

P
Ad olya
du mi
ct ne

H
yd
ro
ly
sis

MDI

Epoxy Coating

O
O

C15H31

Polyurethane
Coatings

DBTDL

Modified
Polyol
Water Soluble
Binder

C15H31

on

HO

OH

ECH

High Mol.Wt
Epoxy, 180C

C15H31

OH

Epoxy-Phenolic
Coating

OH

HCHO

Malenization

H+

180-200C

Water Borne
Coatings

ti
i
iza lam
no
a
h
iet

r
ut

Ne

OH HO
O

C15H31

C15H31

NaOh, Reflux

Modified Phenolic
Resin

Pd, Co
Driers

al

NaOH
OH

C15H31

ne

Chlorohydrin

OH HO

OH
OCH2CHCH2OH

P
Li htha
ns li
Gl eed c an
Oi hy
yc
l dr
er
ol ,
id
e

IP

Pd, Co

DI

Hy
,
d
et rox
ha
y
cr eth
yl
yl
at
e

Modified Alkyd
Resin

C15H31 m

UV-Curable
Photoinitiator
Coating Material
Diluent

Driers

Modified Alkyd
based Coatings

Modified Acrylic
Coating

Fig. 6: Coatings based on CNSL and its derivatives

film. The pencil scratch hardness of the films was

improved compared with that of polycardanol. Owing
to the epoxide content in the polymerized cardanol, the
film cured with phenalkamine showed a higher hardness
value after a relatively short curing time.
Longo and co-workers39 have synthesized two different novolac resins, named Nov-I and Nov-II, containing an amount of unreacted cardanol of 35 and 20
wt%, respectively, by the condensation reaction of
cardanol and para formaldehyde using oxalic acid as
catalyst. The cardanol-based novolacs were tested as
curing agents for diglycidyl ether of bisphenol-A epoxy
resin employing 2-ethyl-4-methyl-imidazole as catalyst.
Differential scanning calorimeter (DSC) and thermogravimetric studies were performed to identify the
thermal properties of the cured resins. In addition, the
epoxy resins cured with the synthesized novolacs were
evaluated for tensile tests and synthesized novolacs
and were shown to be worthy of consideration as
effective epoxy curing agents.
A new class of phenalkamine (Mannich reaction
product) from cardanol, formaldehyde, and polyamines
was successfully synthesized by Pathak and Rao.40 The
product was characterized by high-pressure liquid chromatography (HPLC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance
spectroscopy (1H-NMR). The presence of characteristic
methylene linkages of Mannich bases at d 3.54.0 ppm
was observed by 1H-NMR. Further, the synthesized
curing agent was used to cure diglycidyl ether of bisphenol-A at room temperature and the curing times were
optimized. The cured samples showed good adhesion
with different metal surfaces, in particular, higher values
were observed with copper due to its high surface energy.
Further, the coatings were analyzed for viscoelastic
properties and thermal stability properties by dynamic

174

mechanical thermal analysis (DMTA) and thermogravimetry analysis (TGA). The storage modulus (E) was
found to be on the order of 109 Pa and tan d values were
around 90C. A reduction in storage modulus (E) and an
increase in tan d values on postcuring were observed.
TGA showed two-stage degradation above 250C; the
first stage being the decomposition of the aliphatic chain
of the Mannich base and the second stage due to cured
epoxy polymer degradation.
Tan and Nieu41 have investigated the thermal,
dielectrical, chemical, and mechanical properties of a
newly synthesized carbon fiber composite based on
tetrafunctional epoxy resin namely N,N,N,N-tetraglycidyl-2,2-bis[4-(4-aminophenoxy)phenyl]propane modified with cardanol. It was observed that the use of
cardanol in epoxy resins at cardanol/epoxy molar ratios
less than 0.3/1 improved the chemical resistance as well
as the mechanical properties of the composites, such as
flexural strength and modulus, tensile strength and
modulus, and interlaminar shear strength. Higher
cardanol contents decreased such properties. The
maximum values of all properties of the composites
were observed with the epoxy-cardanol resin having a
cardanol/epoxy molar ratio of 0.3/1.
Alkyd resins
Madhusudhan and Murthy42 have synthesized a polyfunctional compound from cardanol by reacting with
maleic anhydride under various experimental conditions to yield up to 70% conversion. Further, the
products were evaluated as intermediates for preparing
water-soluble binders and as alkyd resin modifiers. It
was concluded that the modification improved resistance to water and chemicals and showed high scratch

J. Coat. Technol. Res., 11 (2) 169183, 2014

hardness values (1900 g for water-soluble binder and

1100 g for modified alkyd compared to 800 g of neat
alkyd).
Polyurethanes
Tan et al.43 have investigated the synthesis of cardanolglycols (CGs) and polyurethane (CGPU) films
thereof. The films were characterized for FTIR and 1HNMR spectroscopy, swelling test and DSC studies. The
content of cardanol in CGPUs was found to be
inversely proportional to the molecular weight of
glycols and affected the crosslinking density of the
films. The reduced crosslinking density strongly
affected the swelling property and glass transition
temperature. Further, the crosslinking of CGPUs was
improved
by
autooxidationautopolymerization
through the double bonds of the cardanol side chain,
catalyzed by cobalt salt.
A class of tough and crosslinked polyurethanes was
successfully synthesized from a derivative of CNSL by
Gopalakrishnan and co-workers.44 A three-stage synthesis of hydroxyl functional resins for polyurethane
involved a synthesis of novolac resin using cardanol and
formaldehyde in three different molar ratios followed by
epoxidation and subsequent hydrolysis to obtain hydroxyalkylated cardanolformaldehyde resin. The synthesized hydroxyl functional resin along with a
commercial polyol (PPG-2000) was used to cure diphenylmethane diisocyanate (MDI). Polyurethane prepared using a higher mole ratio of cardanol/
formaldehyde of hydroxyalkylated cardanolformaldehyde resin was found to possess better thermal and
mechanical properties than the polyurethane prepared
from a lower molar ratio.
Asha and co-workers45 have established a one-pot
synthetic step to prepare UV-curable urethanemethacrylate crosslinkers from cardanol. The methodology
involved an end capping of isophorone diisocyanate with
one equivalent of hydroxyethyl methacrylate followed
by condensation with cardanol. The structures of the
resins were characterized by FTIR, 1H-NMR, 13CNMR, matrix-assisted laser desorption/ionization time
of flight (MALDI-TOF) spectroscopies, and size exclusion chromatography (SEC). Further, the synthesized
acrylate oligomer was formulated for UV-curable coatings. The experimental results revealed that the hydrogen bonded crosslinkers based on cardanol and its
derivatives had higher double bond conversion when
compared to a nonhydrogen bonding standard such as
hexanediol diacrylate (HDDA) under identical conditions. The temperature effects on the hydrogen bonding
and thereof on the curing process were also investigated.

Especially in the field of polymers, CNSL has primarily

been studied as a modifier of phenolformaldehyde
(PF) resins due to its structural similarity with phenol.
CNSL reacts with formaldehyde under a variety of
conditions, yielding both resole and novolac resins
depending on the catalyst used. Figure 7 shows the
possible structure of crosslinked CNSLformaldehyde
resin where R represents the side chain.
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.
The most obvious and common method of obtaining
polymeric materials from CNSL is the condensation
reaction with formaldehyde.
Mahanwar and Kale46 experimentally investigated
the effect of process condition during replacement of
phenol with CNSL on the properties of novolac and
resole resins. The addition of CNSL into phenol seems
to increase reaction times for the preparation of
phenolic resins. This increase in reaction time can be
due to the low reactivity of the CNSL, arising from the
stearic hindrance caused by the side chain. Experimental results revealed that an acid value of CNSL
played an important role in resin synthesis. When
CNSL with acid value more than 10 was used, only a
viscous fluid with very low resin content was obtained.
Finally, the authors concluded that only CNSL with an
acid value less than 10 was suitable for resin preparation; the addition of CNSL leads to a decrease in
tensile strength but an improvement in the impact
strength and electrical properties of the resole resins.
Similarly, the effect of partial replacement of phenol
by CNSL in PF resin has studied by Papadopoulou and
Chrissafis.47 Further, the synthesized resin was compared with a conventional petroleum-based PF resin.
The resins were characterized for physicochemical and
thermal properties. Wood panels impregnated with
these resins were also evaluated for thermal properties
by DSC from an end application point of view. The
DSC measurements revealed that the wood reduces
the curing temperature of both resins, but it has greater
OH

OH

H2
C

R
H

HO

Phenolic resins
CNSL has potential industrial applications such as for
resins, friction lining materials, and surface coatings.

H2
C

H2C

Fig. 7: Crosslinked structure of CNSL-formaldehyde resin

175

J. Coat. Technol. Res., 11 (2) 169183, 2014

effect on the CNSL-modified PF resin (PCF) where it

brings a reduction of 7C. In the case of the PF
standard resin this reduction corresponds to only 3.6C.
It was proven that, although the neat PCF cured at
longer time and higher temperature than a conventional PF resin, wood affects it more significantly,
resulting in the equalizing of their curing performance.
Further, the adhesion strength of synthesized resins
was investigated by their application in plywood
production. The plywood panels were tested for their
shear strength and wood failure performance while
their free formaldehyde emissions were determined
with the desiccator method. This was a novel finding
that manifests the possibility of replacing a conventional PF resin with a CNSL-modified one in the
plywood production, without changing any of their
production conditions and with improvement to their
overall properties.
The various reaction parameters like reaction kinetics, reaction mechanism, composition of resin for acid
as well as alkali catalyzed cardanol-based phenolic
resins have been extensively studied by a number of
authors.4852
Eswaran and co-workers53 have demonstrated a
novel methodology for the synthesis of a new series of
CNSL/cardanol-based high ortho novolac copolymers, used as photoresists for microlithography. The
authors have used gel permeation chromatography
(GPC) and both 1-D and 2-D NMR spectroscopic
techniques to elucidate the exact microstructure of
synthesized copolymer and to calculate the percentage
incorporation of different monomers in the polymer
microstructure. The lithographic performance of photoresists using novolac resins based on cardanol (fractionated CNSL) and diazonaphthoquinone ester was
also evaluated.
A novel phenolic type of thermoset resin with
improved mechanical and toughness properties was
successfully synthesized by Cardona et al.54 The
modification involved a copolymerization of phenol
with cardanol at different weight ratios. The modified
phenolic resins (CPF) were prepared at various molar
ratios of total phenol (phenol with cardanol) to
formaldehyde. CPF resins with maximum content of
40 wt% of cardanol were synthesized and used. Both
resins (CPF/PF) were mixed in different proportions,
and their thermal and mechanical properties were then
established. An increase in the content of cardanol
resulted in a proportional increase of the flexural
strength and fracture toughness together. The results
obtained by the DMA analysis of the post cured resin
CPF/PF blends revealed a decrease in the crosslink
density and Tg values with increasing cardanol content
and also with the decreasing total phenol/formaldehyde molar ratio. This could be due to the flexibility
enhancement by introduction of cardanol inside the
phenolic molecular network.
Souza et al.55 have developed cardanol-based phenolic resins and blended them in situ with polyaniline
(PANi) for pressure-sensitive applications. The final

176

polymer blend was found to be composed of a soft solid

material and insoluble in ordinary solvents. Samples
were characterized through X-ray scattering, FTIR,
electrical conductivity, and pressure sensitivity measurements. FTIR results indicated that the insertion of
PANi into the blends did not change the chemical nature
of the resin. According to wide-angle X-ray scattering
results, PANi was dispersed homogeneously in the final
polymer samples which improved the sensitivity of the
electrical conductivity to pressure variations. Pressure
sensitivity and electromechanical analysis indicated that
the produced blends could be used as pressure-sensing
materials. Among the tested materials, the blend containing 5 wt% PANi presented a larger variation of
conductivity (340%). The increase of the PANi concentration led to a decrease in the conductivity variation.
This could be related to the increasing number of contact
points among the PANi chains.
A number of researchers have reported on the
synthesis of epoxidized cardanol-based novolac type
phenolic resins.5658 Though having several outstanding characteristics, epoxy resins exhibit a low impact
resistance in their cured state which limits the applications of epoxy resins. To alleviate this deficiency,
epoxy resins were modified by the incorporation of a
carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN).5962 In this regard, Yadav et al.63
have tried to produce the modified epoxy matrices
based on cardanol and improved its impact resistance
by physical blending with CTBN. Further, CTBN
blended epoxidized novolac resin was cured with a
stoichiometric amount of polyamine curing agent. The
formation of various products during the synthesis of
cardanol-based novolac resin, epoxidized novolac
resin, and blending of epoxidized novolac resin with
CTBN has been studied by FTIR analysis. The number
average molecular weight was determined by GPC
analysis. The blend sample, having 15 wt% CTBN
concentrations, showed minimum cure time and the
most thermally stable system.

Miscellaneous coating materials

ANTIBIOFILM COATINGS: Some species of natural
unsaturated hydrocarbon phenols such as cardanol and
some component of lacquer tree sap (sap extracted from
lacquer tree) have been reported to have an antibiofouling effect. Kim et al. reported polymerization of
cardanol by enzymatic reaction and its potential
application as antibiofilm coating material.64
Choi et al.65 have studied an antifouling property of
newly synthesized polydimethylsiloxane (PDMS) matrices impregnated with natural unsaturated hydrocarbon
phenols, i.e., urushiol from the sap of natural lacquer
tree and a mixture of cardol and cardanol from refined
CNSL. Incorporation of naturally available unsaturated
phenols showed excellent antimicrobial property to
both Escherichia coli and Saccharomyces cerevisiae.

J. Coat. Technol. Res., 11 (2) 169183, 2014

FIRE RETARDANT MATERIAL: The recognition of

toxicity and environmental persistence of halogenated
flame retardant (FR) materials has prompted the
reduction in their usage across the globe. There is an
immediate need for new types of nontoxic and
effective FR produced preferably through sustainable
routes. In this regard, Ravichandran et al.67 have
reported the synthesis and characterization of a new
polyphenolic FR material-based cardanol. Cardanol
was polymerized in aqueous media using various types
of oxidants. The thermal properties of the resulting
polymers were investigated. Polycardanol synthesized
using a specific type of oxidant exhibited good thermal
stability and low heat release capacity. Finally, authors
concluded that the preliminary results obtained from
the study were quite promising and indicated the
possibility of synthesizing new types of FR materials
from bio-based phenols.
CROSSLINKED POLYMER: A novel crosslinked polymer
from cardanol was synthesized easily by Bai et al.68
through solvent-free polymerization with FeCl3. The
methodology involved grinding of cardanol and
anhydrous FeCl3 powder using a glass pestle in a
mortar at ambient temperature and solvent-free
condition, yielded up to 80% in 5 min. It was
concluded that the rigid structure of conjugated
condense rings improved the thermal stability of the
polymer which was in good agreement with
thermogravimetric graphs as shown in Fig. 8.

100
90
80
70
60

Wt%

PROCESSABLE AROMATIC DIAMINE: More and coworkers66 have synthesized a processable aromatic
diamine monomer, viz., 4-(4-aminophenoxy)-2pentadecylbenzenamine containing pendant pentadecyl
chain from CNSL for electronic applications. A series of
new polyazomethines containing flexibilizing ether
linkages was synthesized by polycondensation of
synthesized diamine monomer with commercially
available aromatic dialdehydes viz., terephthaldehyde
(TPA), isophthaldehyde (IPA), and varying mixtures of
TPA and IPA. Inherent viscosities and number average
molecular weights of (co) polyazomethines were in the
range 0.500.70 dL/g and 10,49040,800 (GPC,
polystyrene standard), respectively, indicating the
formation of medium to reasonably high molecular
weight polymers. Polyazomethines containing pendant
pentadecyl chains were found to be soluble in common
organic solvents such as chloroform, dichloromethane,
tetrahydrofuran, pyridine, m-cresol and could be cast into
transparent and stretchable films from their solution in
chloroform. Polyazomethines exhibited glass transition
temperatures (Tg) in the range 2148C. The observed
depression of Tg could be ascribed to the internal
plasticization effect of pentadecyl chains. The
temperature at 10 wt% loss (T10), determined from
TGA in nitrogen atmosphere of polyazomethines was in
the range 434441C indicating their good thermal
stability.

50
40
30

20

10
100

200

300

400

500

Temperature (C)

Fig. 8: TG curves of the polycardanol by solvent-free

polymerization (a) and by liquid-phase polymerization (b)

MICROBIAL CATALYZED POLYMER: Kim et al.69 have

successfully carried out an oxidative polymerization of
cardanol in waterorganic solvent mixtures using a
fungal peroxidase from Coprinus cinereus (CiP). So far,
only uneconomic plant peroxidases, such as soybean
peroxidase (SBP), have been used to polymerize
cardanol. The fungal peroxidase used was easily
produced by cultivating C. cinereus, and was purified
by ultrafiltration and size exclusion chromatography.
Microbial CiP-catalyzed the cardanol polymerization
as efficiently as SBP. The effects of reaction
temperature and peroxide concentration on the CiPcatalyzed polymerization of cardanol were investigated
in aqueous 2-propanol. It was found that a low reaction
temperature of 10 and 15C increased the polycardanol
yield (to 91%) and the hydrogen peroxide feed rate
was found to affect the initial reaction rate and the final
conversion. Finally it was concluded that the microbial
CiP could be more useful for the synthesis of a range of
polyphenols from renewable resources than plant
peroxidases.
COPOLYMER CURING SYSTEM: Rao et al.70 have
developed a new copolymer curing system based on
newly synthesized monofunctional benzoxazine (CBO)
and hydroxyl functionalized benzoxazoline monomer
(HBO), 2-(4-hydroxy phenyl)-2-oxazoline from
cardanol derivatives. Further, the curing system was
evaluated for thermal and mechanical properties by
varying proportions of CBO and HBO in copolymer
system. A significant reduction in curing temperature
was observed from thermal studies and an increase in
heat of polymerization value with incorporation of
HBO was also noted. The activation energy of
cardanol benzoxazine was found to reduce with
incorporation of 25 mol% of hydroxy benzoxazoline
due to the catalytic effect of OH group lowering the

177

J. Coat. Technol. Res., 11 (2) 169183, 2014

activation energy. The flexible cardanol benzoxazine

polymer displayed a lower storage modulus of 2.7 9
108 (Pa) and a tan d of 104C, with incorporation of
rigid hydroxy benzoxazoline monomer in the
copolymers exhibiting significant enhancement of
these values. The width of tan d peak of copolymers
was found to increase, suggesting an enhancement of
toughness value.
WATER-SOLUBLE MANNICH BASES: Ramasri et al.71
have synthesized water-soluble Mannich bases from
cardbisphenol, a reaction product of distilled cardanol
and phenol by Mannich reaction, as shown in Fig. 9.
The effect of electrodeposition parameters on the
film formation from synthesized binders and from the
pigmented composition was studied. It was found that
the polymers gave uniform coatings with good mechanical properties and the pigmented systems exhibited
high resistance to organic solvents and excellent
corrosion resistance properties.
POLY(AMIDEIMIDE): A novel class of aromatic diacylhydrazide monomer was successfully synthesized from
cardanol by More et al.72 The synthesized monomer was
used to developed a new series of poly(amideimide)s
containing
flexibilizing
ether
linkages
and
pendant pentadecyl chains by a two-step solution
OH

(CH2)7

H
C

(CH2)6CH3
OH

2-ethylaminoethanol
0 to 5C

HCHO

HO
OH
N
H
C

(CH2)6CH3
OH

HO

Mannich base

Fig. 9: Mannich Bases from cardanol for cathodically

electrodepositable system

178

MOLECULARLY IMPRINTED POLYMER: Recently, Philip

et al.73 have used monomers from CNSL to develop
molecularly imprinted polymers. The extracted CNSL
was used to synthesize anacardanyl acrylate (AnAcr)
and anacardanyl methacrylate (AnMcr) monomers and
were characterized by FTIR and 1H-NMR. Different
imprinted bulk polymers based on AnAc, AnAcr, and
AnMcr functional monomers were separately
copolymerized in toluene with ethylene glycol
dimethacrylate and divinylbenzene as crosslinkers,
using racemic propranolol as a model template. The
experimental results showed that the AnAc-based
polymer revealed a meager rebinding ability, the
imprinted polymers made from AnAcr and AnMcr
displayed highly specific propranolol binding. At a
polymer concentration of 2 mg/mL, AnAcr- and
AnMcr-based imprinted polymers were able to bind
over 50% of trace propranolol. Under the same
condition propranolol uptake by the two nonimprinted
control polymers was less than 20%.

Adhesives

Cardbisphenol

(CH2)7

polycondensation in N,N-dimethylacetamide. The

solubility of poly(amideimide)s in N,N-dimethylacetamide, 1-methyl-2-pyrrolidinone, pyridine, and m-cresol
at room temperature was found to be significantly
improved by incorporation of pendant pentadecyl chains.
In addition, the synthesized polymer was characterized for
wide-angle X-ray diffraction, while thermal stability was
determined by TGA in nitrogen atmosphere. From the
experimental results, the authors concluded that the
thermal stability of poly(amideimide)s was excellent.
Also, a glass transition temperature of poly(amideimide)
was in the range 162198C. It was observed that the
plasticization effect of attached pentadecyl side chains
induced the depression of Tg.

Kim74 developed CNSLformaldehyde (CF)-based

resin and its alloy with polyvinyl acetate (PVAc) resin
for the maple face of the veneer bonding on plywood.
The CF resin was used to replace ureaformaldehyde
(UF) resin in the formaldehyde-based resin system in
order to reduce formaldehyde and VOCs emissions
from the adhesives used between plywoods and fancy
veneers. The use of PVAc was found to introduce
reactive sites in the CF resin. The green adhesives with
varying percentages of PVAc resins such as 5, 10, 20, and
30% were evaluated for surface bonding strength by
Universal Testing Machine (UTM) in the tensile mode,
light microscopy, scanning electron microscopy, formaldehyde emission test, and a VOC analyzer test. The
CF/PVAc resins showed better bonding than the commercial natural tannin adhesive with a higher level of
wood penetration, as shown in Fig. 10.
The bonding strengths of the nontreated (before
boiling), engineered flooring samples made using CF/
PVAc hybrid adhesives were considerably higher than

J. Coat. Technol. Res., 11 (2) 169183, 2014

Bonding strength (kgf/cm2)

Non-treated
After bonding

20

15

10

10

15

20

25

30

PVAc content in CNSLformaldehyde/PVAc green adhesive

Fig. 10: Bonding strength between the face of the fancy

veneer and plywood substrate in engineered flooring:
CNSLformaldehyde (CF) resin and CF/PVAc green adhesives

those of the CF resin. With increasing PVAc content,

the bonding strength was increased up to 20% of PVAc
content.
Lee et al.75 have studied the properties of green
adhesives based on tannin (a naturally occurring
phenolic compound) and CNSL for the replacement
of conventional formaldehyde-based toxic and hazardous adhesives in indoor environments.
Laminating resins
Sridhar et al.76 have synthesized a series of resole
resins from distilled multivalent phenol obtained during the carbonization process of a lignite source, along
with other phenolic derivatives like cardanol. They
established a low cost method for making electricalgrade laminate from the synthesized resin on a
laboratory scale. The properties of the resoles were
evaluated and found to be similar to that of pure
phenol (C6H5OH) resins. The resole varnishes prepared were used for making cotton paper phenolic
laminates by hand impregnation and the compression
molding technique. The paper laminates were evaluated for physical, chemical, mechanical, and electrical
properties. The experimental investigations indicated
that the distillate of multivalent phenol can be a useful
inexpensive substitute for conventionally used phenols
in the manufacture of P3 grade laminates.
Modifying agents for resins and plastics
Although providing excellent application properties,
some of the resins like PVC, low molecular weight
epoxies, and unsaturated polyesters when cured with

styrene and methyl ethyl ketone peroxide (MEKP)

have shown low impact resistance and flexibility in the
final cured state. In this regard, some of the researchers
have reported the use of cardanol derivatives as
renewable plasticizers/flexibilizers to achieve the required properties.
Greco et al.77 have studied two different plasticizers
obtained by esterification of the cardanol hydroxyl
group (cardanol acetate) and further epoxidation of
the side chain double bonds (epoxidated cardanol
acetate). The synthesized renewable plasticizer was
characterized for DSC to study the miscibility with
PVC. The miscibility was correlated to the chemical
structure of plasticizer by means of the Hansen
solubility parameter analysis. Results obtained indicated that esterification of cardanol yields a partial
miscibility with PVC, whereas esterification and subsequent epoxidation yield a complete miscibility with
PVC. Therefore cardanol acetate, obtained by solventfree esterification of cardanol, was used as a secondary
plasticizer of PVC. Mechanical and rheological analysis showed that the cardanol acetate can partially
replace commercially used di-ethyl-hexyl-phthalate
(DEHP) plasticizer in PVC formulation.
Coating additives
ANTIOXIDANT AGENT: Dantas et al.78 have synthesized
novel tert-butyl substituted phenolic compounds from
CNSL at ortho and para position, through simple and
low cost methodology. The electron donor character of
the substituent increased the electronic density of the
phenolic oxygen atom, and hence, yielded a good
proportion between the captive and the captor radicals
which helped to retard/inhibit the oxidative
degradation as compared to commercially available
additives.
Additionally, an antioxidant property of newly
synthesized phosphorated cardanol on the mineral oils
NH10 and NH20 was investigated by Facanha et al.79
using TGA. It was found that the addition of 1.2
2.0 wt% of phosphorated cardanol compound to the
mineral oils improved their thermal oxidative stability
on 1418C, respectively. The occurrence of major
thermal degradation events at higher temperatures
(Tmax) on additivated oils confirmed an antioxidant
property of phosphorated cardanol compound.
Lomonaco et al.80 reported on the synthesis of
phosphorylated compounds derived from cardanol and
its application as antioxidants for biodiesel. These
compounds were added in biodiesel samples in three
different concentrations (500, 1000, and 2000 ppm) and
their antioxidants activities were tested by TGA by
evaluating their integral procedure degradation temperatures (IPDTs). The results showed that the addition of new antioxidants increased the thermal stability
of biodiesel, making this biofuel more resistant to the
thermo-oxidative process.

179

J. Coat. Technol. Res., 11 (2) 169183, 2014

COLORANTS AND DYES: CNSL and its derivatives have

been found to be excellent raw materials for the
preparation of colorants and dyes. A number of azo
compound-modified cardanol-based dyes have been
well reported as a colorant for polymer/plastic and
coatings.81,82
Similarly, Thamyongkit and co-workers83 successfully synthesized a novel bis (azo) dye from the
coupling of cardanol with a series of diazotized
aromatic amines and diamines. The dyes were highly
soluble in a variety of common organic solvents and
gasoline as a consequence of the cardanol unit. Based
on the colorimetric analysis, the practical concentration of the synthesized dye that gives the most similar
gasoline color compared to that of the commercial one
was 6 or 18 ppm. Further from experimental results of
stability and solubility of synthesized dye in gasoline
91, authors concluded that the synthesized dye can be
successfully used as a coloring agent in gasoline 91.
CORROSION INHIBITOR: Philip et al.84 have reported a
mechanism of interaction of CNSL with metallic
substrate and their effect on the dissolution rate of
SAE 1008 carbon steel in CO2 saturated NaCl
solution. It was observed that CNSL acts as an
anionic inhibitor at higher solution pH (i.e., a basic
type inhibitor). The phenoxide ions, RC6H4O , of
the CNSL inhibitor was adsorbed on SAE 1008 carbon
steel surface in aqueous CO2 saturated 3% NaCl
solution by electrostatic interaction as shown in
Fig. 11.
DISPERSANT: The principal reasons for applying
pigmented coating to paper and paperboard are to
improve printability and appearance. The simplest
form of coatings contains a pigment and a binder to
bind the pigment particles both to one another and to
the base sheet of paper. It is very important that the
pigment be fully dispersed to ensure satisfactory
performance and to fully contribute to the properties
of the coated paper. A number of systems are used for
pigment dispersion, all of which involve the addition of
chemical dispersant and the use of mixing equipment.

0
+

Metal
Fig. 11: Electrostatic adsorption of the anionic corrosion
inhibiting CNSL to positively charged SAE 1008 carbon
steel surface

180

Chemical dispersant serves to aid in the wetting of the

pigment particles, adjust the surface charges of the
pigment particle to prevent flocculation, and reduce
the viscosity. In this regard, Suryanarayan et al.85 have
studied the sodium salt of sulfonated CNSL as a
dispersant for china clay and calcium carbonate and
compared it with the conventional dispersant
(polyacrylate) used in paper mills. Finally, the
properties of coated papers were studied and it was
found that the bio-based dispersant gave better results
compared to conventional one at an optimum dose of
0.8%.
COUPLING AGENT: The term coupling agent generally
applies to silicon-containing species capable of forming
chemical linkages between dissimilar materials. The
materials to be linked are often organic polymers and
inorganic fillers, as in pigmented coatings, although
silane coupling agents can also be useful with other
kinds of fillers and polymers. Small amounts of silane
coupling agents, used at an interface, can greatly
improve the mechanical properties of the coating.
Silane coupling agents are found in a broad range of
applications as varied as metal coatings, dental
materials, and contact lenses.86
In this regards, Tanaka et al.87 have developed a
cardanol-modified silane coupling agent, by reacting
cardanol or a derivative thereof with an epoxy silane
coupling agent (3-glycidoxypropyl trimethoxy silane)
or an isocyanate silane coupling agent (c-isocyanatopropyl trimethoxy silane/c-isocyanatopropyl triethoxy
silane/c-isocyanatopropyl methyl diethoxy silane/c-isocyanatopropyl methyl dimethoxy silane) which can
improve strength and toughness by improving adhesion
at an interface between a filler and a cellulose resin
when being used as an surface treatment agent

Conclusion
CNSL, one of the major sustainable resources, mainly
extracted by hot-oil and roasting process, contains
number of useful phenolic derivatives like cardol,
cardanol, 2-methyl cardol, and anacardic acid with
meta-substituted unsaturated hydrocarbon chain
(chain length of C15). The combination of reactive
phenolic structure and unsaturated hydrocarbon chain
makes CNSL a suitable starting material to synthesize
various resins like epoxy, alkyd, polyurethanes, acrylics, phenolic resins, etc. In addition, a number of other
useful products, such as modifiers like flexibilizer and
reactive diluents, adhesives, laminating resins, antioxidants, colorants and dyes, etc., have also been
developed from CNSL and its derivatives. So, considering the high depletion rate of petroleum-based
stocks and the range of possible applications, CNSL
can be accepted as a greener and sustainable
approach for future expansion in the modern coating
industry.

J. Coat. Technol. Res., 11 (2) 169183, 2014

Future trends
Due to availability of unsaturation in the long chain
and reactive phenolic hydroxyl group of CNSL and
their derivatives, a number of functional groups like
carboxyl, hydroxyl, epoxy, amines, isocyanate, etc., can
be incorporated via Dielsalder reaction mechanism
and addition mechanism, respectively, to yield chemically modified CNSL (CMCn). This modified CNSL
can be further modified or utilized as such for coating
applications. This functionality can be utilized for
synthesizing hyperbranch polymers followed by their
application in coatings as wetting and dispersing agent
or crosslinker, etc. Also, CMCn can be used in high
solid coating formulations as it can act as a reactive
diluent maintaining the viscosity of the formulation
and later on becoming part of the film. CMCn can be
utilized in UV-curable coating formulation as such or
with little modifications. A number of water-based
coatings can be synthesized with CMCn. Today, sol
gel-derived organic inorganic hybrid coatings are
widely used in the coating industry due to a number
of advantages they possess, including eco-friendly
technology, room temperature synthesis, chemical
inertness, high oxidation and abrasion resistance,
excellent thermal stability, very low health hazard,
etc. In this regards, CMCn can be further modified with
organofunctional silane like 3-glycidoxypropyltrimethoxy silane (GPTMS) or c-isocyanatopropyl trimethoxy silane or aminopropyltrimethoxy silane
(APTMS), etc., to get CNSL-based hybrid precursor
which can further be hydrolyzed to yield CNSL-based
hybrid coatings. Similarly, CNSL or CMCn can be
utilized in a number of coating applications like
hyperbranch polymers, water-based coatings, UV-curable coatings, and hybrid materials, etc. The commercialization of all these technologies, however, will
require further research and development for costeffective solutions.

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