CHAPTER IV

Varnishes prepared using tannins based resins and linseed oil

IV.l Introduction

Tannins, which are polyphenols compounds, can be used as a substitute

for phenol in the preparation of phenol - formaldehyde resins [1]. Due to the

sustained efforts by Pizzi et al [2], wattle tannins were used in the preparation of

adhesives and the product was commercialized.

Phenolic resins encircle a wide variety of polymeric substances; each

polymer is composed of a multitude of structures, and a wide variety of raw

materials and catalyst. Phenolic resins represent a mature and classical polymer

family with a complexity and range of capabilities that make them now, as in the

past, the engineering materials of choice to meet a tremendous variety of needs.

Phenolic resins are prepared by the reaction of a phenol or substituted phenol

with an aldehyde, especially formaldehyde, in the presence of an acid or basic

catalyst.

The base catalysed resins are known as resoles. Resole type phenolic

resins are normally produced with a molar ratio of formaldehyde to phenol of

1.2:1 to 3.0:1. For substituted phenols the ratio is normally 1.2:1 to 1.8:1.

Commonly used alkaline catalysts used for the preparation of resoles are NaOH,

Ca(OH)2 and Ba(OH)2. While acid catalysed phenol formaldehyde resins result

in a limited number of structures and properties, resoles cover a much wider

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spectrum. Resoles are usually solids or liquids, water soluble or insoluble,

alkaline or neutral, slowly curing or highly reactive.

When base is used as a catalyst in the preparation of phenolic resins, the

first step is the formation of phenolate ion. Since the ion formed is anionic in

nature, electron density of aromatic ring is dominant at the ortho and para

positions. The high electron density of the aromatic ring favours the

electrophilic condensation reaction at the ortho and para positions of the

benzenoid nuclei. The ratio of ortho to para substitution depends on the nature of

the carbon and the pH [3,4]. Para substitution is favoured at lower pH and by

divalent cations, such as Ba2,' Ca2+ and Mg +.

The polymerization rate is pH-dependent, with the highest rates occurring

at both high and low pH. When base is used as a catalyst in the preparation of

phenol-formaldehyde type resin, large number of methylol groups will be formed

[5,6]. These methylol groups are responsible for the oil reactivity of the resins

with the unsaturated vehicle in the production of oil modified air drying

varnishes. The molecular weight of the resins normally falls in the range of 500-

5000 with most being below 2000.

The tannins of tamarind seed testa, cashewnut seed testa and red onion

skin, as discussed in the chapter II, belongs to the condensed type which are

structurally related to flavonoids and more appropriately based on procyanidins.

Large quantities of tamarind seed testa are available as a residual by-product

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during the production of tamarind kernel powder. This material is used as

tanning cum dyeing agent and also as an adulterant in the tea industry. The

cashewnut seed testa accumulates as waste in the production of cashew kernel,

which has good food value. This raw material is also used as an adulterant in tea

and also used rarely in tanning industry. The onion skin is one of the kitchen

wastes.

In this chapter, the tannins of tamarind seed testa, cashewnut seed testa

and red onion skin are utilised in the preparation of 18 different acid catalysed

tannin based phenol-formaldehyde type resins and three different base catalysed

Phenol-formaldehyde resins (resoles). These resins are then utilised to prepare

different varnishes.

IV.2 Experimental

IV.2a Preparation of tannin based resins (acid catalysed)

The tannins of these biomaterial wastes are extracted by adopting the

procedure given in chapter 11. These tannins are then condensed, with acid

hydroxylates containing furfural of various raw materials obtained by adopting

the procedure given below.

0.01M of tannins isolated from biomaterial wastes is refluxed with acid

hydroxylates (whose furfural concentration is equivalent to 0.01M) and

formaldehyde (0.04M) at 363 K. for 2.5 hours during which a resin is developed.

The resin obtained is repeatedly washed with distilled water to remove any

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monomers present and dried subsequently to get pure resins, which are then

weighed.

Eighteen different resins (R-13 to R-30) are prepared using the tamarind

seed testa tannins, cashewnut seed testa tannins and red onion skin tannins with

acid hydroxylates of various biomaterial wastes and formaldehyde using acid as

catalyst. The raw material sources for the preparation of these resins following

the methodology given above is given in table 14.

IV.2b Preparation of resoles

Tannins (0.07 M) obtained from various biomaterial wastes listed already

are added with resorcinol (0.03 M). To this mixture, one ml of NaOH (0.25 M)

solution, formaldehyde (0.06 M) and 250ml of distilled water are added and

wanned. The unreacted monomers and water are distilled off to get pure resins

(R-31 to R-33), which are then dried and weighed.

IV.2c Preparation of varnishes using acid catalysed tannins based resins

Oil modified air drying varnishes are prepared by making use of these

tannin based resins and linseed oil.

Varnishes (V-13 to V-30) are prepared using 2g of resin along with 75g of

linseed oil and refluxing it at 520 K for about 3 hours with constant stirring in the

presence of hexamine. The hexamine is added to improve the oil bodying of resin

[7]. To the homogeneous solution of varnish obtained after filtration, 0.3% of

cobalt naphthenate to the total weight of the vehicle is added along with 0.5ml of

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antiskinning agent. Ethyl methyl ketoxime is the antiskinning agent used for the

present study.

To improve the oil bodying of these highly cross-linked and polar resins

in linseed oil, additives such as hexamine, soylecithin and sodium lauryl sulphate

are added in various proportions. The effects of these additives are studied.

Varnishes, V-1.3 to V-30 are prepared using the tannin based resins

R-13 to R-30 respectively (obtained from tamarind seed testa tannin, cashewnut

seed testa tannin and red onion skin ) along with linseed oil by adopting the

methodology given above.

IV.2d Preparation of resole resins based varnishes

5g of R-31 to R-33 are ground well separately and heated around 413 K

and 75g of hot linseed oil are added and refluxed at 510 K for 2.5 hours. The

varnishes obtained are filtered off to get homogenous solutions of varnishes,

V-31 to V-33 respectively.

I V.3 Evaluation of resins and varnishes

The evaluation of resins and varnishes are done following by the

methodologies given in chapter HI.

IV.4 Results and Discussion

The electrophilic substitution reactions of tannins with different

electrophiles are established through the halogenation, alleviation and

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The condensed tannins have many reaction sites at A and B rings of a

monomelic unit [9]. Electrophilic substitution reactions can take place at any of

these positions. However it has been reported that many different electrophiles

react preferentially with A ring of condensed tannins unit [10]. Moreover the

attack is dominant at the C-6 followed by the C-8 of the condensed tannin unit

[10]. The vicinal hydroxyl groups activate the B ring without any localized

effects such as those found in A ring [11]. The accessibility of nucleophilic sites

at positions 6 and 8 on A ring during reaction with formaldehyde is not inhibited

by steric effects due to adjoining 7 or 5 hydroxyl functions of the monomer unit

of condensed tannins [10].

The presence of the hydroxy groups on the A ring results in structures that

are activated toward electrophilic aromatic substitution. The phenolic hydroxyl is

a powerful electron donor which increases the electron density in the ring and

thereby making it a better nucleophile as well as stabilizing carbocation

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intermediates and it forms the basis for electrophilic attack on the positions ortho

and para to the hydroxy groups.

As discussed in chapter II, the tannins of tamarind seed testa, cashewnut

seed testa and red onion skin belong to the procyanidin type. The A ring of these

tannin molecules are phloroglucinolic type. Hence electrophilic condensation

reactions will take place preferentially at the C-8 of the condensed tannin unit.

The C-6 position also takes part in the condensation reaction type, as tins

position is also not occupied by any substituent group [12-14],

It is reported that for the formation of methylol groups besides the

formation of bridging methylene groups, the amount of aldehyde added should

be more than the amount of phenol added [15]. hi the preparation of tannin based

acid catalyzed phenol - formaldehyde type resins, a ratio of 1:5 mole of

phenolics to aldehyde is used. Since the tannin molecules have many reaction

sites these aldehydes are mainly consumed for electrophilic condensation

reactions forming bridging methylene groups and a small proportion of the same

for the formation of methylol groups.

In the base catalysed reaction, a ratio of nearly 1:1 mole of phenolics to

aldehyde is added. Hence aldehydes are consumed for the formation of

methylene bridges between phenolic moieties. However, it has been reported that

in base catalysed reactions methylol groups are formed during the resin

formation [2,3],

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IV.4a Physical properties of resins

The resins (R-13 to R-33) have been tested for their melting points,

molecular weight and solubilities in various solvents such as acetone, xylene and

toluene and the results are given in table 14.

It is evident from the table 14 that the acid catalyzed tannin based resins

(R-13 to R-30) had the melting point above 633 K. Their solubility in organic

solvents such as acetone, xylene and toluene is very low. The reaction of tannins

with aldehydes is at the C-6 and C-8. The reactivity of the tannins are increased

in acidic medium and hence they enter into cross-linking condensation reactions

[10]. The properties such as low solubility and high melting point of the resins

are attributed to the high level of cross-linking by furfural and formaldehyde

through the formation of methylene groups between the tannin molecules. It is

reported by previous researchers that highly cross-linked phenolic resins are very

much insoluble in common organic solvents except for its solubility in phenol

[16].

When base is used as a catalyst in the preparation of phenol-formaldehyde

type resins, large number of methylol groups are formed usually [17] at the ortho

and para positions of the phenolic ring. These tannin molecules when used in the

preparation of resoles form good number of methylol groups. It is corroborated

by the solubility of these (R-31 to R-33) resins in polar solvents such as acetone

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[17,18]. The raethylol groups formed during resin formation, being polar in

nature, are responsible for the dissolution of these resins in polar solvents.

IV.4b IR spectra of resins

The important characteristic IR bands of the prepared resins are given in

table 15.

The IR spectra for acid catalyzed tannin based resins have bands around

2925 cm"1 which correspond to the methylene bridges formed by furfural and

formaldehyde between the tannin molecules in the resins. The bands around

760 cm"1 correspond to the aromatic nuclei of tannins. Bands corresponding to

C=C stretching vibrations of aromatic rings are around 1600 cm"1, 1520 cm"1 and

1450 cm"1. Bands at 3350 cm"1 are also seen for all the resins, which is attributed

to the presence of phenolic hydroxyl group.

The IR spectra of all the resoles revealed the presence of a methylene

bands around 2932 cm"1 (R-31), 2942 cm"1 (R-32) and 2954 cm'1 (R-33)

respectively. It indicates that formaldehyde has reacted with phenolic

compounds to form methylene linkages in between them. The formation of

methylene groups between the phenolic compounds is responsible for the chain

growth. Band around 3300 cm"1 is seen for all the resins which indicates the

presence of phenolic hydroxyl groups. The bands around 753 cm"1 highlight the

presence of aromatic ring of the resins. There are also bands corresponding to the

C=C stretching vibrations of aromatic ring.

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IV.4c Oil bodying of acid catalysed tannins based resins

The tannin molecules have many reactive positions prone for electrophilic

condensation reactions. So the tannin molecules during the condensation

reactions may enter into cross-linking reactions through methylene linkages.

These cross-linkages lead to a fewer number of methylol groups. Hence the oil

bodying of the resins in linseed oil is less. To increase the level of oil bodying of

these resins, additives such as hexamine, soylecithin and sodium lauryl sulphate

are added separately and the effects of these additives are investigated.

Hexamine is a reaction product of 6 moles of formaldehyde with 4 moles

of ammonia. It is generally used as an antiseptic and has been given the name

urotropine. It has four tertiary nitrogen atoms and hence it is highly basic [7],

A lecithin is a glyceride containing two usually different fatty acid ester

groups (eg. stearic and oleic acid) and a phosphocholine group which on

saponification gives inorganic phosphate and quaternary base choline.

Lecithin extracted from egg yolk or soybean oil is a waxy hygroscopic

white substance, which rapidly becomes yellow or brown in ah". The acid

components of soybean lecithin are palmitic, stearic, palmitoleic, oleic, linoleic

and arachidonic acids [19].

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IV.4g Oil bodying of tamarind seed testa tannin based resins

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 'A' ring of tannin molecule is more prone to form the

methylol groups, the formation of chrornane ring or the reactivity of the tannins

based resins with the oil is mainly through 'A' ring.

The increase in viscosity from 19 sees for varnishes prepared without

hexamine to 27 sees in the presence of 1.33% by weight of hexamine to the total

weight of vehicle indicate the high level of reaction of these resins with the oil.

The results of oil bodying of R -13 with linseed oil in the presence of hexamine

are given in table 16.

Previous researchers studied the reaction of unsaturated compounds such

as styrene and maleic esters with methylol phenol and a mechanism for the

formation of ether like compound is given [20]. It is then proved through the

reaction of o-methylol derivative with p-t-butyl-o-cresol and oleic acid [21].

Based on this reaction the mechanism of p-t-butyl phenol novolac with

unsaturated oil in the presence of hexamine [7] has been studied. Similar

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The emulsifying activity of soylecithin is also encouraging as the viscosity

of the varnish prepared using 1.33% by weight of soylecithin to the total weight

of vehicle increased to 27 secs'from 19 sees, the viscosity of varnishes prepared

without it. Soylecithin with its quaternary ammonium groups anchor the phenolic

resin by forming an ion pan [23]. It has an oil soluble end and an ionic end,

which is responsible for the emulsification process. An ion pair may be generated

between 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 together, which further increases oil bodying [23].

Hence the dispersion of the resin with the vehicle is more, resulting in the

increased viscosity of the resultant phenolic varnish.

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mechanism is proposed for the formation of chromane ring in V-13. The

probable mechanism is indicated below. Additional IR band at 1240 cm"' in

V-13 supports this mechanism [22].

Sodium lauryl sulphate is expected to give emulsification effect similar to

that of soylecithin. Sodium lauryl sulphate has both aliphatic chain and an ionic

group. But this additive does not have any effect on the oil bodying of the resin.

The inactive nature of sodium lauryl sulphate can be explained as follows. The

sulphonic acid group present in sodium lauryl sulphate has lesser ability than

quaternary ammonium group present in soylecithin to form an ion pair which is

responsible for the oil bodying of the resins [23], The hydrogen bonding

capacity which is almost veiy much less when compared to soylecithin hence the

oil bodying in the presence of sodium lauryl sulphate is low [23].

IV.4f Oil bodying of cashewnut seed testa tannin based resin

The oil bodying of resins made out of cashewnut seed testa tannins are

very low. It may be due to the high level of cross-linking between the tannin

molecules through formaldehyde and fuifural and the low number of methylol

groups and other factors such as steric strain. All these factors reduce the oil

bodying of these resins. Hence for the oil bodying of these resins, additives such

as hexamine, soylecithin and sodium lauryl sulphate are used. The results are

given in table 17.

The effect of oil bodying of these resins by additives given above is

similar to that of tamarind seed testa tannin based resins. The oil bodying of the

resins has been enhanced when hexamine and soylecithin are used while sodium

lauryl sulphate produced no effect. The formation of chromane ring is also

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observed as evident from IR absorption bands for varnishes prepared using these

additives.

IV.4i Oil bodying of red onion skin tannin based resins

The red onion skin tannin based varnishes do not have good oil bodying

characteristics. The oil bodying of these resins is very low and similar to

tamarind seed testa tannin based resins. From the table 18, it is evident that

hexamine and soylecithin enhance the reactivity of the resin with the oil as is

evident from the increased viscosity. Chromane ring formation is also

observed for these varnishes, which is substantiated by IR absorption value

around 1240 cm'1. However sodium lauryl sulphate does not have any impact

on the oil bodying of these resins.

IV.4j Identification of suitable drier

The varnishes prepared using tannin based resins along with linseed oil

have diying time above 200 minutes. To lower the drying time to around 60

minutes, various driers are added and studies are carried out. Catalytic driers

such as cobalt naphthenate and manganese naphthenate and cross-linking drier

viz., lead naphthenate are used to minimize the drying time of these tannin resins

based varnishes. These driers are added separately in various concentrations

ranging from 0.1 to 0.3% weight to the total weight of the vehicle, linseed oil.

Varnishes V-13, V-19 and V-25 prepared using tannins of tamarind seed testa,

cashewnut seed testa and red onion skin respectively are taken for the study and

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the results are given in table 19. The drying times with these driers for resole

based varnishes are given in table 20.

It is evident from the table 19 that the drying times of the three different

acid catalysed tannin resins based varnishes are almost the same with various

concentrations of these different driers. The drying time of these varnishes

without any drier is above 200 minutes. But the addition of driers markedly

decreases the drying time. Cobalt naphthenate performs well in all the three

varnishes and it is in good agreement with the previous research work [24], The

manganese naphthenate, though it is a catalytic, surface drier, does not have any

significant impact on the drying time of these varnishes, when compared to lead

naphthenate. The mechanism of action of these driers may be the same as given

in chapter III.

It is evident from the table 20 that cobalt naphthenate of 0.3%

concentration reduces the drying time to around 60 minutes for all the three

resoles based varnishes. The lead and manganese naphthenate has very little

effect on the drying time of the varnishes. The result is in good agreement with

the previous reports that cobalt naphthenate acts as a catalyst for drying and

increases the rate of formation of dry film [24].

Studies are conducted to find the combined effect of these driers in

various proportions and combinations. The results of the study are given in table

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21 and 22 for acid catalysed tannin resins based varnishes and resoles based

varnishes respectively.

It is evident from the table 21 that a mixture of 0.1% concentration of each

of these driers decreases the drying time. Moreover any combination having

cobalt naphthenate as one of the constituents has significant effect on the drying

time. However it is found that the manganese naphthenate does not have any

telling impact on the drying time of these varnishes.

From the table 22, it is evident that a mixture of 0.1% of these driers

decreases the drying time of V-31, V-32 and V-33 to 70, 68 and 75 minutes

respectively. The results of table 20 and table 22 suggest that cobalt naphthenate

of 0.3 % concentration is the best drier followed by 0.1 % combination of each

drier. The drying effect of manganese naphthenate is not good, as it could not

reduce the drying time to around 60 minutes, and it is the same in the case of lead

naphthenate.

IV.4k IR spectra of varnishes

Bands corresponding to C-H stretching vibrations of CH3 groups of fatty

oil are seen around 2926 cm"1 and 2854 cm"1 for all the acid catalysed tannin

resin based varnishes. The bands corresponding to phenolic hydroxyl groups of

resin are seen for all the varnishes. Bands corresponding to aromatic nuclei of

resins are also retained for all the varnishes. The presence of ketonic group is

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shown by a strong band at 1740 cm"1 for all the varnishes. The formation of

chromane ring envisaged is also seen around 1240 cm"1 for all the varnishes.

IR spectra of resole based varnishes have bands around 2925 and 2855

cm"1 for -CH3 and -CH2 groups of linseed oil respectively. The bands around

3600 cm"1 indicate the presence of phenolic hydroxyl group. There are also bands

corresponding to the carbonyl groups of the linseed oil.

IV.41 Physical properties of tannins based varnishes

The viscosity, non-volatile content, diying time and anti skinning property

of the prepared varnishes are given in table 23.

Varnishes are termed as Newtonian liquids due to their flow behaviour

having constant plasticity, thixotrophy etc. The viscosity of the prepared tannin

based varnishes (V-13 to V-30) increased to 475 cps from 230 cps, which is the

viscosity of the vehicle, linseed oil. The increased viscosity indicates the level

of oil bodying of these resins with the oil.

The viscosity of all the varnishes prepared using the resoles and linseed

oil are around 600 cps, which is around 3 times higher than that of the linseed oil.

It indicates the high level of oil bodying of these resins with the vehicle. Since

resoles have large number of methylol groups, the blending of resins with the

vehicle is very high resulting in the high viscosity of the varnishes.

The non-volatile content of these tannin based varnishes (V-13 to V-33)

are around 99.5 which indicates low amount of volatile substances. This may be

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due to the fact that solvents are not used for dilution of these varnishes. The

value of volatile matter may be related to volatile solvent, present in the drier

system and also due to the antiskinning agent present in the varnishes.

The diying time of these varnishes are studied with different

concentrations of naphthenates of various metals such as cobalt, manganese and

lead. The results indicate that cobalt naphthenate of 0.3% weight of driers to the

total weight of vehicle is found as the suitable drier. For this concentration of

cobalt naphthenate these varnishes have the minimum drying time around 60

minutes.

Coating materials packed in containers for a longer period should not

form skin on the surface of the container. Some coatings have the tendency to

form skin because of oxidation due to the presence of enclosed air. The skin may

vary from soft skin to tight skin depending on the coating formulations. A spatula

may remove the tight skin whereas the soft skin may pose problems while

coating. If they remain in the coating material they appear as irregularities on the

surface of the finish, and constitute one of the forms of seeding. The varnishes

prepared pass the skinning test for 48 hours. No skin or gel formation is observed

for these varnishes that are subjected to skin test.

The scratch hardness, flexibility, impact, insulation and gloss values for

the prepared varnishes are given in table 24.

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The scratch hardness, otherwise called as mar resistance of an organic

coating is its ability to withstand scratching and scuffing actions, which tend to

disfigure or mar the surface appearance of the coating. Mai1 resistance, as defined

above, is a resistance of the surface of the coating to permanent deformation, as a

consequence of the application of dynamic mechanical forces. The mar

resistance is actually a measure of adhesion. Adhesion is defined as the state in

which two surfaces are held together by interfacial forces which may consist of

valence forces or interlocking action, or both [25]. The varnishes in the present

study are prepared using phenolic tannin based resins and linseed oil as the

vehicle. There are reports about the chemical adsorption/complexation of tamiin

molecules on the metal surface to prevent corrosion [26,27]. Since the varnishes

are prepared with tamiin based resins and linseed oil as the vehicle, the adhesion

may be due to the valence forces and by interlocking action of the varnishes on

the metal surfaces. The scratch hardness value is around 900g for acid catalysed

tamiin resins based varnishes (for 30+5 |im) and around 875 g for resoles based

varnishes (for 40 jam) which are very good. The resistance towards scratch is

provided by the veiy strong adhesion of the varnishes on the metal surface by

valence and interlocking forces. It is reported that oleoresinous varnishes have

much more adhesion strength than is required for good service and it is much

superior to nitrocellulose coatings. It is also reported that phenolic varnishes

have good adhesion strength [28,29].

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The flexibility requirements for coatings vary with the substrate on which

the coating is applied. Coatings on metal sheets, automobile bodies and tin

containers, is subjected to considerable flexing during normal use. Coatings on

paper vary considerably with the use; most of them require a good degree of

flexibility. The prepared tannins based varnishes pass the 1/8 inch mandrel test

which indicates the high level of flexibility. This is in good agreement with the

previous report that oleoresinous varnishes based on phenolic resins and linseed

oil possess good flexibility [30].

The impact resistance test gives a measure of the combination of

toughness and adhesion of coatings. All the varnishes pass the 0.65 kg.cm

impact test. Since the varnishes prepared pass this test it has good toughness and

adhesion which is confirmed by scratch hardness test and flexibility

measurements. It is again supported by the fact that oleoresinous3 phenolic

varnishes have good adhesion, flexibility and impact resistance [31].

The insulation property of these varnishes (V-13 to V-30) measured

following the Indian standard 350 (IS: 350) is good and the varnishes have the

breakup voltage of around 4.5 V for a micrometer thickness of the varnish film.

The insulation properties of all the three resoles based varnishes (V-31 to

V-33) measured by ISO 350 standard are fairly good and these varnishes pass

220 V for the film thickness of 100±5 jim.

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The specular gloss is a factor, which depends on the refractive index of

the coated film and indicates the smoothness of the surface. If the surface is

smooth then the incident beam of light will not spread out but reflect at an angle.

It follows that for smooth surface the gloss values are very high. On the other

hand for wrinkled or rough surface the gloss value, when measured at 60 °, is

low due to diffused reflection. The varnishes prepared have good gloss value,

which indicates the smoothness of the surface and the homogeneity of resin in

vehicle [25,26].

The results of chemical resistivities and salt spray test of the tannin based

varnishes are given in table 25.

The acid resistivities of all these tannin resins based varnishes are very

good. There is no loss in gloss and the films are very stable. The water resistant

character of all the varnishes is. also good but with slight loss in gloss. Further it

is found that these varnishes, (V-13 to V-33) did not possess alkali resistivity

with the films getting slightly washed resulting in the loss of gloss also. The low-

water and alkali resistance of films derived from non conjugated linseed oil may

be due to the formation of more peroxy linkages man the much more stable

carbon-carbon or ether bonds in the dry film. When such films are exposed to

water or alkali, the peroxy linkages cleave which results in the low resistivity of

the film [31, 32]. Linseed oil, a vegetable oil being saponifiable, forms soaps or

salts with basic substances even when in the polymerized state and vegetable oils

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have a certain measure of moisture absorbing and transmitting ability. Many of

these soaps are water-soluble and hence the alkali resistance is actually the

moisture resistance. This is the cause for the low level of alkali resistance and

water resistance of the varnishes prepared. The linseed oil coated film when

exposed to alkali is almost washed away resulting in the poor resistivity towards

this medium.

The corrosion resistance property for the varnishes V-13 to V-33 has been

evaluated by salt spray test. The films after 36 hours of continuous exposure to

5% NaCl does not show any degeneration of the surface, while V-31 to V-33

remains intact even after 48 hours. For varnishes V-13 to V-3, the 48 hour

exposure leads to visible appearance of a few rust spots on the panels. After 60

hours as can seen in table 25, blisters appear for V-13 to V-30. The increased

protective value of V-31 to V-33 can be attributed to the high oil bodying of the

resins [33].

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