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Progress in Organic Coatings 49 (2004) 244251

Studies on oleoresinous varnishes and their natural precursors

K.P. Vinod Kumar, M.G. Sethuraman

Department of Chemistry, Gandhigram Rural Institute, Deemed University, Gandhigram 624-302, Tamil Nadu, India

Received 18 August 2003; accepted 1 October 2003

Abstract

Condensed tannins extracted from the seed testa ofAnacardium occidentale is subjected to phytochemical and spectral studies such as

UV and 13C NMR.Shell ofCocus nucifera on treatment with acid yields acid hydroxylates containing furfural which when condensed with

tannins ofA. occidentale form phenolformaldehyde type resins. The spectral and physico-chemical properties of the resins are studied.

Varnishes are prepared from these resins using linseed oil/linseed oiltung oil. The service performances of the varnishes including their

impedance spectra are evaluated. The results show that varnishes with good gloss and chemical resistivity can be prepared from these waste

materials.

2003 Elsevier B.V. All rights reserved.

Keywords: Biomaterials; Condensed tannins; 13C NMR; Coatings; Impedance spectra

1. Introduction

Protection is the prime requisite of our invaluable assets

from natural decay caused by air and other corrosive envi-

ronment. Eventhough there are many ways of protecting thesurface of the materials, protective coatings are widely used.

The cost of the raw materials needed for the preparation of

surface coatings is increasing day by day and their availabil-

ity is also draining. It is known that utilisation of biomate-

rial wastes in the manufacture of useful end products like

varnishes not only solves the problem of waste disposal but

also can bring down the cost of end products [1].

Condensed tannins are polyphenolic compounds that

can be substituted for phenol in the preparation of phe-

nol formaldehyde (PF) type resins. Tannins occur widely

in plants and are widespread especially in woody species

[2]. Furfural, which can be obtained from the shell and

fibre of agrowaste materials, can be used as a substitute forformaldehyde in the preparation of PF type resins [3].

Phenolic tannins and products based on them have found

rewarding industrial outlets, notably in the technology of

polymeric resins for protective and decorative coatings and

adhesive applications [4]. There has been a report on the

production of varnishes from fibre of Borassus flabellifer

and dry skin of Allium cepa [5]. In the present study seed

testa ofAnacardium occidentale and shell ofCocus nucifera

Corresponding author.

E-mail address: [email protected] (M.G. Sethuraman).

are utilised as sources of tannins and furfural, respectively,

in the preparation of varnishes. The studies performed on

the raw materials and the end products are detailed below.

2. Experimental

A. occidentale (Anacardiaceae) commonly known as

cashew is widely cultivated in the tropics. C. nucifera (co-

conut) belonging to the family Arecaceae is found through-

out the tropics 20N20S [6]. The thin testa separated from

the edible portion of A. occidentale and the hard shell of

C. nucifera, which are available locally, are used as sources

of tannins and furfural, respectively. Double blown linseed

oil and tung oil are used as vehicles. Chemicals used are of

AR grade for the entire study.

2.1. Extraction of tannins/polyphenolics

The extraction of tannin is carried out by soaking 100 g

of cashewnut seed testa with 1000 ml of equi-volume mix-

ture of acetone and water at about 318 K for an hour. The

cooled extract is separated and concentrated in vacuo. The

residue obtained is tested for tannins by colour reactions and

estimation is carried out by spectrophotometric method [7].

Cryoscopic method is used to determine the molecular

weight [8], UV spectra are recorded using Perkin-Elmer

Lambda-35 to characterise the tannins with n-butanolic-HCl

and Ehrlichs reagent. [9,10]. Proton decoupled 13C NMR

0300-9440/$ see front matter 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.porgcoat.2003.10.004

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K.P. Vinod Kumar, M.G. Sethuraman/ Progress in Organic Coatings 49 (2004) 244251 245

spectra is recorded using JEOL model GSX 400 MHz in-

strument. The sample is prepared by diluting 40% (w/w)

aqueous solutions of tannin extract with D2O in 1:4 weight

proportion [11].

2.2. Hydrolysis of coconut shell

The coconut shell is reduced into powder form and sieved

over a 60 mesh screen. A known quantity of the powdered

shell is reacted with 40 times its weight of 2.5 N HCl and

cooked for 3 h. Thereafter the mixture is cooled and fil-

tered. The residue is washed with 2.5 N HCl and the filtrate

is subjected to solvent extraction using toluene to separate

furfural. The physical constant of the separated furfural is

determined. Estimation of pentosans and furfural is done by

iodometric method [8].

2.3. Preparation of resins (R-1, R-2 and R-3)

The acid hydroxylate equivalent to 0.1 M furfural is re-

fluxed with 0.2 M phenol at 353 K until a coloured product

is developed. This is followed by the addition of 0.12 M

formaldehyde (36%, w/v) and it is refluxed continuously

for 3 h. Solid resin (R-1) formed is separated, dried and

weighed.

Tannins (0.01 M) of cashewnut seed testa are refluxed with

acid hydroxylate (whose furfural concentration is equivalent

to 0.01M) and formaldehyde (0.04 M) at 363 K for 2.5 h to

get PF type resins. The resin (R-2) obtained is repeatedly

washed with distilled water to remove any monomers present

and the dried resin is weighed.Tannins (0.07 M) obtained from cashewnut seed testa are

added to resorcinol (0.03 M). To this mixture, 1 ml of NaOH

(0.25 M), formaldehyde (0.06 M) and 250 ml of distilled wa-

ter are added and warmed. The unreacted monomers and

water are distilled off to get pure resin (R-3).

2.4. Preparation of varnishes (V-1, V-2, V-3 and V-4)

R-1 and R-3 of 5 g are ground well separately and heated

around 413 K and 75 g of hot linseed oil are added to the

resins and refluxed at 510 K for 2.5 h. The resulting solutions

obtained are filtered off to get V-1 and V-3, respectively.

Varnish (V-2) is prepared using 2 g of resin along with

75 g of linseed oil and refluxing it at 520 K for about 3 h with

constant stirring in the presence of hexamine. The hexamine

is added to improve the oil bodying of resin [12].

To improve the oil bodying of this highly cross-linked

and polar resin in linseed oil, additives such as hexamine,

soylecithin and sodium lauryl sulphate are added in various

proportions. The effects of these additives are studied.

Following the same methodology of preparation of V-2,

V-4 is prepared, but without making use of hexamine. The

ingredients used in the preparation of V-4 are 1:1 ratio of

linseed oil and tung oil along with resin R-2.

To identify a suitable drier system for the prepared var-

nishes, naphthenates of cobalt, manganese and lead are

added separately to the varnishes as also various com-

binations of them. The drying time before and after the

addition of driers are found out. Ethyl methyl ketoxime

(0.5 ml) is also added as antiskinning agent to the varnishes

prepared.

2.5. Evaluation of resins

The melting point of the resins is measured using Gal-

lenkamp melting point apparatus. The solubility of these

resins in solvents such as acetone, xylene and toluene is also

found out. The molecular weight of the resins is found out

by cryoscopic method [8] and the IR spectra of resins are

recorded by KBr pellet method using Perkin-Elmer FTIR

model 1600.

2.6. Evaluation of varnishes

Viscosity [13], drying time [13], scratch resistance

[14], skinning test [15], film thickness [13], gloss [16],

non-volatile content [17], flexibility [18], impact resistance

[13], salt spray chamber test [19], acid, alkali and water re-

sistances [13], and insulation measurements [20] are carried

out for the prepared varnishes.

Impedance measurements are made by using a M398

AC impedance system with a three-electrode configuration,

over a frequency range of 10 kHz10 mHz using a 10 mV

peak-to-peak sinusoidal voltage. A glass tube is attached on

the coating surface using an adhesive (M seal) and the tubeis filled with NaCl solution (3%, w/v). The surface area of

the coated panel exposed to the electrolyte is kept as 1 cm2.

Then a platinum foil (counter electrode) and a saturated

calomel electrode (reference electrode) are put in the solu-

tion and the AC impedance measurement is carried out [21].

3. Results and discussion

3.1. Characterisation of tannin and furfural

The quantitative estimation of tannin extract of cashewnut

seed testa, revealed the presence of 82.5% of total polyphe-

nolics out of which 80% are tannins while the remaining

is phenolic constituents. The molecular weight of the tan-

nins of cashewnut seed testa as calculated by the cryoscopic

method is 1815.

It was reported earlier that the max values for prodel-

phinidin (I) and procyanidin (II) (in n-BuOH-HCl medium)

are 558 and 547 nm, respectively. Bate-Smith has outlined

a method to determine the relative proportions of procyani-

din and prodelphinidin based on accurate determination of

max [9] and this method has been adopted to characterise

the nature of tannins present in cashewnut seed testa.

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246 K.P. Vinod Kumar, M.G. Sethuraman/ Progress in Organic Coatings 49 (2004) 244251

The max of cashewnut seed testa (in BuOH-HCl medium)

is at 545 nm, which indicates that these tannins contain pro-

cyanidin as the major constituent [9].

The Ehrlich reagent (4-dimethyl amino benzaldehyde)

can distinguish between the resorcinolic and phlorogluci-

nolic A rings of the condensed tannin molecules. This

reagent produces immediate pink colour with phloroglu-

cinolic A rings while it does not produce any colour

with resorcinolic A rings. The tannins of cashewnut seed

testa produced colour reaction with this particular reagentindicating the presence of phloroglucinolic A rings,

i.e. procyanidin-based tannin molecules. The max is at

551 nm, which is an indication of procyanidin-based tannin

molecules [9,22].

The 13C NMR spectra (400 MHz, D2O) of cashewnut seed

testa tannins have signals at 97 and 95 ppm corresponding

to free C-6 and C-8. It also has signals at 154 ppm corre-

sponding to the C-5 and C-7 atoms. The downfield shift of

these atoms indicates the presence of OH groups at these

positions. Carbons at 3 and 4 resonate at 143 ppm, indi-

cating the presence of catechol type nucleus in the ring B.

Thus the 13C NMR spectra indicates the ploroglucinolic A

ring and catechol type B ring in the tannins of cashewnutseed testa. The interflavonoid linkage positions have been

determined based on the interpretation given by Pizzi et al.

[23,24]. The intensity of a signal at 108 ppm is similar to

that of pine bark tannins indicating C-4 to C-8 interflavonoid

linkage.

The structure of cashewnut seed testa tannins as inter-

preted from the spectral data is given below:

The percentage of pentosans in the shell of C. nucifera

as estimated by iodometric method is 40. In industries, sug-

arcane bagasse with 33% of pentosan content is used as a

raw material for the isolation of furfural. Thus the estima-

tion of pentosans in the shell of C. nucifera shows it to be a

Table 1

Yield, melting point, molecular weight and solubility of phenolic resins

Resin Yield

(g)

Melting

point (K)

Molecular

weight

Solubility

Acetone Xylene Toluene

R-1 18 413 968 HSa SSb SS

R-2 2.1 >633 2192 SS SS SS

R-3 4.8 397 2096 HS SS SS

a Highly soluble.b Sparingly soluble.

better raw material for furfural. The furfural content in acid

hydroxylate is found out to be 1.12%.

3.2. Characterisation of resins

The preparation of R-1 (acid catalysed reaction) involved

2 mol of aldehyde to 1 mol of phenol. The high ratio of alde-

hyde to phenol permits the formation of free methylol groups

in the resin structure, which are the sources of reactivity of

the resin with the oil [13]. It is reported that methylol groups

are formed rather more in the base catalysed reaction [25].

R-1 and R-3 have good number of methylol groups, which

assist in blending of resins with the vehicle easily, and hence

termed as heat reactive resins. This is substantiated by their

solubility in polar solvents (Table 1). Due to the low solu-

bility of R-2 in polar solvents and lesser oil reactivity it is

termed as heat non-reactive resin.

It has been reported that many different electrophiles re-

act preferentially with A ring of procyanidin tannin unit.

Moreover, the attack is dominant at C-6 followed by C-8

of the condensed tannin unit [2628]. The vicinal hydroxyl

groups activate the B ring without any localised effects suchas those found in A ring [29]. The accessibility of nucle-

ophilic sites at positions C-6 and C-8 on A ring during reac-

tion with formaldehyde is not inhibited by steric effects due

to adjoining 7 or 5 hydroxyl functions of the monomer unit

of condensed tannins [3032]. The yield, melting point and

molecular weight of R-1, R-2 and R-3 are given in Table 1.

The IR spectra of resins R-1, R-2 and R-3 revealed

the presence of a methylene band around 2920, 2925 and

2942 cm1, respectively. This band clearly indicates that

formaldehyde and furfural are embedded between phenolic

nuclei by forming the methylene bridges. This shows that

the chain growth is by the formation of methylene linkages

between phenolic nuclei. Bands around 3300, 3342 and

3274 cm1 are seen for R-1, R-2 and R-3, respectively,

which indicate the presence of phenolic hydroxyl groups.

Bands corresponding to aromatic C=C stretching frequen-

cies are also seen for these resins. The aromatic bands

are seen at 753, 766 and 773 cm1 for R-1, R-2 and R-3,

respectively.

3.3. Blending of the resins with the vehicle

The blending of acid catalysed tannin-based resin, R-2

is very low due to lesser number of methylol groups as is

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Table 2

Effect of oil bodying of R-2 (2 g) (coconut shell hydroxylate/cashewnut seed testa tannin/formaldehyde) in linseed oil (75 g) with additives

Amount of additives (wt.%) Flow time of V-2 (s)a

Without additives With hexamine With soylecithin With sodium lauryl sulphate

0 19

0.26 21 23 14

0.53 22 24 140.80 25 26 16

1.06 27 27 18

1.33 30 29 19

a Flow time measured using Zhan cup No. 5.

evident from the solubility studies. To increase the level of

oil bodying of R-2, additives such as hexamine, soylecithin

[33] and sodium lauryl sulphate are added separately and

the effects of these additives are investigated.

Hexamine being a tertiary amine is basic and can abstract

the phenolic proton of the resin to form phenolate ion, which

reacts with the double bond of the linseed oil. Since the Aring of tannin molecule is more prone to form the methylol

groups, the formation of chroman ring or the reactivity of

the tannin-based resins with the oil is mainly through A

ring [26].

It is found that addition of 1.33% (w/w) of hexamine to

vehicle has increased the viscosity of V-2 from 19 to 30 s.

The results of oil bodying of R-2 with linseed oil in the

presence of various concentrations of hexamine are given in

Table 2.

Previous researchers studied the reaction of unsaturated

compounds such as styrene and maleic esters with methy-

lol phenol and a mechanism for the formation of ether-likecompound is given [34]. It is then proved through the re-

action of o-methylol derivative with p-t-butyl-o-cresol and

oleic acid [35]. Based on this reaction the mechanism of

p-t-butyl phenol novolac with unsaturated oil in the pres-

ence of hexamine [12] has been studied. Similar mechanism

is proposed for the formation of chroman ring in V-2. The

probable mechanism is indicated below. Additional IR band

at 1240 cm1 in V-2 supports this mechanism [5]:

The emulsifying activity of soylecithin is also encourag-

ing as the viscosity of the varnish prepared by adding 1.33%

(w/w) of soylecithin to vehicle increased to 29 from 19 s.

Soylecithin with its quaternary ammonium groups anchors

the phenolic resin by forming an ion pair [36]. It has an oil

soluble end and an ionic end, which is responsible for the

emulsification process. An ion pair may be generated be-

tween the resin molecule and the anchor group, which is the

quaternary ammonium group of the lecithin. Soylecithin can

also form hydrogen bonds with the resin and the vehicle to-

gether, which further increases oil bodying [36]. Hence thedispersion of the resin with the vehicle is more, resulting in

the increased viscosity of the resultant phenolic varnish.

Sodium lauryl sulphate has both aliphatic chain and an

ionic group. The sulphonic acid group present in sodium

lauryl sulphate has lesser ability than quaternary ammonium

group present in soylecithin to form an ion pair. The hydro-

gen bonding capacity is less when compared to soylecithin

and hence the oil bodying in the presence of sodium lauryl

sulphate is low [36].

3.4. Identification of suitable drier

Driers are added separately in various concentrations

ranging from 0.1 to 0.3% (w/w) to the vehicle, linseed oil.

The results are given in Table 3 for varnishes V-1, V-2,

V-3 and V-4. Cobalt naphthenate performs well in all the

varnishes and it is in good agreement with the previous

report [37]. The manganese naphthenate, though it is a cat-

alytic, surface drier, does not have any significant impact on

the drying time of these varnishes, when compared to lead

naphthenate. The lead and manganese naphthenates have

very little effect on the drying time of the varnishes. The re-

sult is in good agreement with the earlier reports that cobalt

naphthenate acts as a catalyst for drying and increases the

rate of formation of dry film [37].

The results of the study to find the combined effect of

these driers are given in Table 4. It is evident from Table 4

that a mixture of 0.1% concentration of each of these driers

decreases the drying time. Moreover, any combination hav-

ing cobalt naphthenate as one of the constituents has signif-

icant effect on the drying time.

The drying activity of metal is attributed to the fact that

they have more than one valence states. These metals readily

undergo oxidationreduction processes. Co and Mn fall in

the above category of driers. It is quite likely that better

performance by cobalt as drier is due to the repeated rapid

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Table 3

Effect of various driers on air drying of varnishes (V-1, V-2 and V-3)

Percentage weight

of drier to the

total weight of oil

Drying time (min)

Cobalt naphthenate Lead naphthenate Manganese naphthenate

V-1 V-2 V-3 V-1 V-2 V-3 V-1 V-2

0.1 119 0.82 140.1 0.76 124.6 0.47 >200 >200 >200 >200 >200

0.2 89 0.82 99.6 1.24 95.0 0.0 >200 200.5 1.3 >200 200.5 1.32 >200

0.3 49.6 0.47 60.3 1.69 60 1.6 >200 164.5 1.25 >200 161.6 1.25 >200

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Table 4

Effect of mixed driers on drying time of acid catalysed tannin resins based varnishes (V-1, V-2 and V-3)

Percentage weight of drier to the weight of oil used Total % of driers Drying time (min)

Lead naphthenate Cobalt naphthenate Manganese naphthenate V-1 V-2 V-3

>200 >200 >200

0.1 0.1 0.1 0.3 64.3 0.47 70.2 0.76 68.3 0.47

0.1 0.1 0.2 97.6 0.82 110.0 0.28 100.0 0.00.1 0.1 0.2. 76.6 0.94 80.0 0.50 77.6 1.6

0.1 0.1 0.2 >200 >200 >200

Table 5

Properties of phenolic varnishes

Varnish Viscosity (cps) Non-volatile content (%) Drying time (min) Skinning test

V-1 551 99.96 0.0004 49.6 0.47 Passes skinning test for 48 h




transitions from Co(II) Co(III) Co(II) during drying

as suggested by Muller in his mechanistic study of cobalt

driers [38]. It is reported that cobalt forms a weak complex

with unsaturated moieties in the ester [39]. This is accom-

panied by an increase in conjugation, which is identified as

rate-determining step for oxygen uptake in the presence of

cobalt. It is reported that the energy of activation for drying

is found to be 1.3 kcal/mol whereas it is 10.3 kcal/mol in the

absence of cobalt. These results illustrate the catalytic effect

of cobalt on oxygen absorption and drying [39].

3.5. Characterisation of varnishes

The results of viscosity, non-volatile content, drying time

and skinning tests are given in Table 5. The viscosity of the

varnishes, V-1 to V-4 are found to be higher than that of

the vehicle, linseed oil, indicating the extent of the body-

ing of the resins with the oil. The resins, R-1 and R-3, as

discussed previously, have good number of methylol groups

and hence are more viscous than R-2. As a matter of fact,

the oil bodying of these resins are high and hence the vis-

cosities are high. All the varnishes prepared have very high

non-volatile content. The prepared varnishes (V-1 to V-4)

pass the skinning test for 48 h.

The results of scratch hardness, flexibility, impact, insu-

lation and gloss values for the prepared phenolic varnishes

are given in Table 6. Varnishes, V-2 to V-4 have very good

Table 6

Film properties of phenolic varnishes

Varnish Scratch hardness (g) Flexibility property Impact resistance Insulation property (V/m) Gloss at 60 head

V-1 1250 1.33 Passes 1/8in. conical mandrel test Passes 0.65kgC m Passes 220V for 105 5m 67.8 1.08

V-2 955 4.08 Passes 1/8 i n. conical mandrel test Passes 0.65 k g C m 4.43 0.16 63.1 0.89

V-3 875 4.1 Passes 1/8in. conical mandrel test Passes 0.65kgC m Passes 220V for 110m 30.2 1.43

V-4 1050 8.5 Passes 1/8 i n. conical mandrel test Passes 0.65 k g C m 3.55 0.01 58.0 1.25

scratch hardness values. Since the varnishes are prepared

with tannin-based resins and linseed oil as the vehicle, the

adhesion may be due to the valence forces and by interlock-

ing action of the varnishes on the metal surfaces [40,41]. The

scratch hardness value is around 900 g for acid catalysed tan-

nin resins based varnishes (for 30 5m) and around 875 g

for resoles based varnishes (for 40m) which are very good.

The values are also good for V-1. Previous research works

also indicate that oleoresinous varnishes based on phenolic

resins and linseed oil have good adhesion strength [42,43].

All the varnishes pass the 1/8 in. conical mandrel test,

which is an indication of the high level of flexibility. Pre-vious researchers report that oleoresinous varnishes based

on phenolic resins and linseed oil have good flexibility

[42].

All the varnishes prepared pass the 0.65 kg cm impact test,

which indicates the high level of toughness and adhesion of

the coating film. It is supported by the flexibility and scratch

hardness values. It was also reported earlier that oleoresinous

varnishes based on phenolic resins and linseed oil have good

impact resistance value [42].

All the varnishes pass 220 V for the film thickness of

about 110 5m which indicates good insulation property

of the varnish coated film.

It is obvious from Table 6 that the gloss values for these

varnishes are very good, which indicate the homogeneity of

the resin in the vehicle and the smooth surface of the varnish

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250 K.P. Vinod Kumar, M.G. Sethuraman/ Progress in Organic Coatings 49 (2004) 244251

Table 7

Reactivities of phenolic varnishes towards various media and salt spray testa

Varnish After 24 h of contact with Salt spray chamber test (5% NaCl)

Acid (10%) Alkali (5%) Water 60 h 72 h

V-1 ++ + ++ NA FRV

V-2 ++ + ++ NA for 36 h FRV after 48 h

V-3 +++ + ++ NA for 48 h FRV after 60 hV-4 +++ ++ ++ NA FRV

a +++: film stable, gloss retained; ++: film stable, gloss diminished slightly; +: film slightly washed away, gloss diminished; NA: not affected;

FRV: few rusts visible.

coat on the substrate. The gloss value is especially good in

the case of V-2.

It is clear from Table 7 that the varnish coated films are

very stable and the gloss property is retained when these

films are subjected to acid resistance test. The varnish coated

films are slightly washed away and these films underwent

slight loss in gloss when subjected to alkali and water resis-

tance test. The effect is more adverse in the case of alkalithan water. The vehicle being the non-conjugated linseed oil

in V-1 to V-3, produce more peroxy linkages than the much

more stable carboncarbon double bonds or ether bonds in

the dry film. The low resistivities of the varnish films when

exposed to these media are attributed to the cleavage of per-

oxy linkages of the dried film. The alkali and water resistiv-

ity of V-4 is relatively good because of conjugated tung oil

being used as the vehicle. It produces the more stable CC

bonds. The relatively lower resistance of the film towards

alkali is due to the fact that linseed oil being a saponifi-

able vegetable oil, forms soaps or salts with basic substances

even in the polymerised state. The vegetable oils have thetendency to absorb moisture and transmitting ability. This is

the cause for the low level of alkali resistivity [44,45].

The corrosion resistance property evaluated by salt spray

test indicates few rust spots after 72, 48, 60 and 72 h, re-

spectively, for V-1, V-2, V-3 and V-4.

The results of the impedance studies for different var-

nishes are given in Table 8. It is obvious from Table 8 that

of these different varnishes, V-1 and V-3 give the highest

protection from NaCl induced corrosion. The deviation of

open circuit potential (OCP) value from 630 V (OCP value

Table 8

Impedance parameters of varnishes

Varnish OCP (mV) Rp (cm2) Cdl (10

10 F/cm2)

V-1 +121 185.2 M 54.25

141 102.3 M 60.04

V-2 395 374.5 k 0.027

396 36.49 k 0.047

V-3 372 1.281 M 0.687

423 956.3 k 0.728

V-4 402 273.9 k 0.309

427 252.0 k 0.313

of mild steel) to +121 V for V-1 indicates the level of shift

in the potential, which is indicative of the protection from

corrosion [21]. Deviation from OCP values are also good

in the case of resole based and tung oil incorporated var-

nishes. V-2 also induces the deviation from OCP value of

mild steel, but only to a lesser extent.

The paint resistance values for V-1 and V-3 are very good.

The values of phenolic varnishes are above 108 cm2 whilefor V-3, the value is in the order of 106 cm2. Even after

24 h contact, the paint resistance values are in the range of

108 cm2 in the case of V-1, which indicates the sustain-

ability of the varnish films towards NaCl. It also suggests

that the coatings are not porous and they do not have any

other defects. Hence they prevent the ingression of the elec-

trolyte/water into the substrate. Pores, pinholes and coating

deficient areas in the system, which have high ionic conduc-

tivity, act to short circuit the coating dielectric [46]. These

surface defects aid the rapid transport of electrolyte to the

coating/metal interface [21]. The resistance value of V-3 is

sustained even after the exposure of the film to electrolytefor 24 h. It indicates the sustainability of the varnish film

against the NaCl induced corrosion. By the exposure of the

film to NaCl solution, one will be able to understand the di-

electric strength of the film and also the porosity of the film.

Better the permeation resistance, better is the film property.

The double layer capacitance (Cdl) values increase in the

case of all the varnishes as the exposure time is increased

which indicates that solution ingression has taken place and

corrosion has been initiated. However, the values are better

in the case of V-1 and V-3 than the other two varnishes as

is evident from Table 8.

The impedance studies indicate the superiority of V-1

and V-3. It may be due to the fact that V-1 and V-3 have

higher resin content than V-2 and V-4. It also indicates the

homogeneity of varnishes which, results in smooth, even

thickness varnish films without pores or any other defects

associated with the ingression of the solution to the substrate.

4. Conclusion

It is obvious from the results of the present study that

preparation of good quality varnishes can be achieved by

using agro-wastes such as A. occidentale and C. nucifera.

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8/8


Varnishes with good gloss, flexibility, scratch resistance and

corrosion resistance properties can be prepared from these

agro-wastes. Large-scale preparation of these varnishes can

be taken up in an effort to further minimise the cost and

also to improve the quality of the varnishes. Through proper

network, large quantities of raw materials can be collected.

Acknowledgements

The authors express their sincere gratitude to CSIR, New

Delhi, India, for financial assistance. They also record their

profound thanks to Dr. P. Jeyakrishnan and Dr. G. Venkat-

achari of CECRI, Karaikudi, India, for their technical assis-

tance in recording of impedance spectra. The authors also

register their thanks to Dr. Madhulatha of CLRI, Chennai,

for helping them in UV studies and the authorities of Gand-

higram Rural Institute for all the help.

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