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Cathodic electrodeposition of self-curable polyepoxide resins based on cardanol.
Electrochemical reactions
Acetic acid
Organic acids
Epoxy resins
Gums and resins industry
Kumar, Pramod
Pub Date:
Name: JCT Research Publisher: American Coatings Association, Inc. Audience: Trade Format: Magazine/Journal Subject: Business; Chemicals, plastics and rubber industries Copyright: COPYRIGHT 2011 American Coatings Association, Inc. ISSN: 1547-0091
Date: Oct, 2011 Source Volume: 8 Source Issue: 5
Product Code: 2850000 Paints & Allied Products; 2852330 Wire & Insulating Coatings; 2868402 Acetic Acid; 2821820 Epoxy Resins; 0173000 Tree Nuts NAICS Code: 32551 Paint and Coating Manufacturing; 325199 All Other Basic Organic Chemical Manufacturing; 325211 Plastics Material and Resin Manufacturing; 111335 Tree Nut Farming SIC Code: 2869 Industrial organic chemicals, not elsewhere classified; 2821 Plastics materials and resins; 2861 Gum and wood chemicals; 0173 Tree nuts

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Abstract Cathodic electrodeposition (CED) behavior and film properties of coating binders as modified polyepoxide resins based on cardanol, a constituent of Cashew Nut Shell Liquid (CNSL), have been studied. These coating binders were synthesized by cpoxidation of cardanol-formaldehyde novolacs (CN.sub.s) with cpi-chlorohydrin and subsequently modified with secondary amine to make them water-thinnable by neutralization with acetic acid, and suiTable for CED. Laboratory synthesized coating binders, modified, epoxidized cardanol-formaldehyde novolacs (MECNs), were cathodically electrodeposited on pretrealed MS panels to investigate their electrodeposition behavior and film properties of deposited coating binders. Among the nine MECNs prepared by varying molar ratios of cardanol to formaldehyde as well as ECN to DEtOA, only seven MECNs([MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9]) were water-soluble and electrodcpositable, whereas [MECN.sub.4] and [MECN.sub.5] were not water soluble, and hence not considered for further studies. These seven MECNs were used for the study of their electrodeposition characteristics, such as electrodeposition yield (ED yield), coulombic yield (CY), dry film thickness (DFT), and properties of their deposited films. The most suiTable molar ratio of ECN:DEtOA for the preparation of MECNs was found to be 1:1. The CN prepared by using cardanol and formaldehyde in the molar ratio of 1:0.7 was used for the preparation of [ECN.sub.2] and [MECN.sub.2] prepared from this was found to be the most suiTable in terms of deposition behavior and overall film properties.The film of [MECN.sub.2] electrodeposited at constant voltage (100 V) was found to be the best in terms of film properties along with corrosion resistance, as it passed 800-h exposure to salt spray atmosphere. [MECN.sub.2] was optimized for its electrodeposition characteristics like ED yield as 3.62 mg/[cm.sup.2], CY as 35.87 mg/C, and DFT as 25.26 [micro]m. Through a wide cure window, the films of [MECN.sub.2] were found to be self-curable at an optimum cure schedule of 160[degrees]C/30 min. The eleclrodeposited films of MECNs had good physical, chemical, and corrosion resistance properties, but demonstrated low resistance to xylene, in particular. The study emphasized the electrodeposition behavior and film properties of the prepared MECN resins as binders for CED coating formulations, which were self-curable without using any external crosslinkcr. The self-curing of the deposited films was achieved via a novel self-curing mechanism, i.e., one molecule chemistry through anionic polymerization. The prepared epoxide resins as MECNs could be cathodically eleclrodeposited as primer coat for the protection of metallic substrates against corrosion.

Keywords Cathodic electrodeposition, Electrodeposition yield, Coulombic yield, Dry film thickness, Modified epoxidized cardanol-formaldehyde novolacs


Cathodic electrodeposition (CED) is the most important technique used presently in the automotive industry for priming vehicle bodies. It is also applied for domestic appliances, agricultural equipments/implements, metal toys, jewelry, nuts, bolts, etc. (1) and has had tremendous economic impact, especially in the area of corrosion protection. CED permits the application of protective coatings on such areas of a workpiece, which are not accessible to spray application. (2) CED also offers high resistance of films to alkali, excellent adhesion of electrodeposited films to metal substrates and, as a consequence of these, enhanced corrosion resistance. CED is free of the major drawbacks of anodic electrodeposition ((AED).sub.1) such as excessive substrate dissolution and film discoloration.(3-5)

The main resin compositions (2), (6-9) used today in CED coatings are modified epoxy resins and blocked polyisocyanates like TDI, HMDI, IPDI, etc. However, the use of polyisocyanate inevitably generates blocking agent at elevated temperatures. The weight loss caused by deblocking and subsequent evaporation of the blocking agent from the coating systems usually amounts to 10-15 wt% based on the cured coatings, which is not negligible either from the point of view of material cost or the air pollution. (10) The use of polyisocyanates as crosslinkers, in a co-reacted form in existing systems also poses a risk of gelation during the CED resin synthesis. Now, with increasing concern for environmental protection and due to stringent regulations to ensure the same, it has become a must for industry today to find eco-friendly substitutes for all objectionable materials like phenols and volatile organic compounds (VOC) in organic coatings.

However, the development of low-cost, self-curable coatings and coatings curable at reduced temperatures still remains an important challenge in the field of organic CED coatings. To this end, the development of eco-friendly coating compositions for CED, cardanol, obtained from Cashew Nut Shell Liquid (CNSL) as an eco-friendly substitute for phenols, was used for the preparation of novolac resins. (11-14) Cardanol is a natural alkyl phenol present in CNSL which has a meta-substituted (C.sub.15) unsaturated chain containing 1, 2, or 3 double bonds at the 8, 11, or 14 positions, (15) and it is a potential source for biomonomers.

CNSL having cardanol as a natural phenol is considered to be less toxic and more eco-friendly as compared with ordinary phenol. (16), (17) The lower cost of cardanol (US$ 333/MT) (18) when compared with that of ordinary phenol (19) (US$ 2,154-2,177/MT) turns out to be an added advantage for the commercial use of CNSL.

In this research study, an attempt has been made to evaluate CED behavior of the prepared coating binders (modified epoxidized cardanol-formaldehyde novolacs, MECNs) in terms of electrodeposition characteristics, such as electrodeposition yield (ED yield), coulombic yield (CY), and dry film thickness (DFT). Finally, the electrodeposited films, after curing, were evaluated for their physical properties, chemical resistance and corrosion protection performance.



The materials used in the experimental study include laboratory synthesized cationic binders as MECNs from cardanol. Reagents such as acetic acid, sodium hydroxide, sulfuric acid, methyl ethyl ketone, xylene, sodium chloride, methanol (Qualigens Fine Chemicals, Mumbai India) of L. R. grade and mineral turpentine oil (MTO) of commercial grade were used for evaluation of chemical resistance properties of cured films. Laboratory prepared double-distilled water was used throughout the experimental study.

Evaluation of film properties

The following instruments/equipments were used for the evaluation of properties of the cured films of the electrodeposited resins:

Scratch hardness tester (Sheen, UK)

Pencil hardness tester (Mitsubishi Uni., Japan)

Flexibility tester (Conical Mandrel) (Sheen, UK)

DuPont impact tester (Ueshima Scisakusho Co. Ltd., Japan)

Salt spray cabinet (Sheen, UK)

DFT meter (Elcometer, England)

Triglossometer (Elcometer, England)

General mechanisms of cathodic electrodeposition

The fundamental aspects of the electrodeposition process and resin compositions have been reported by many authors in the past. (20-23)

As CED is mainly employed where corrosion protection is the main concern, and as the epoxies, by virtue of their chemical constitution, exhibit excellent corrosion resistance properties, it was, therefore, planned to combine both epoxide resins and CED technique to have a synergistic performance.

Thus, in this study, it was decided to develop a true self-curable, waterborne epoxy based system, which could be used for developing coating formulations suiTable for CED of films.

Chemical reactions involved in the preparation of water-soluble CED binders based on prepared epoxy resin are shown in Scheme 1.

Experimental design: cathodic electrodeposition of binders

The synthesis part of CED coating binders used for this study can be accessed through our forthcoming publication. However, a brief outline of the synthesis is described here.

CED binders were synthesized in the laboratory in a sequence of steps, as, first of all, cardanol-formaldehyde novolac resins (CNs) were prepared by reacting cardanol with formaldehyde in different molar ratios. The prepared CNs were epoxidized by reaction with epichlo-rohydrin to produce epoxidized cardanol-formaldehyde novolacs (ECNs). Further, ECNs were modified by reaction with secondary amine as diethanolamine (DEtOA) to produce ECN-DEtO A adducts having tertiary amino group(s) into their molecules to make them water-soluble and suiTable for CED, on neutralization with acetic acid. True self-curability of the coaling films has been achieved via anionic polymerization. Characteristics of the prepared CED coating binders as MECNs arc presented in Table 1.


Preparation of test panels

Pretreated mild steel panels (150 mm x 50 mm x 0.25 mm) were used as test panels. They were prepared as per the standard procedure described in ASTM D 609. (24) They were prepared by following a sequence of steps, viz. mechanical cleaning (by 400# abrasive paper), degreasing, rinsing, pickling with a hydrochloric acid solution at 1:1 dilution (30 s), rinsing with distilled water, phosphating as zinc phosphate based conversion coating at 60[degrees]C for 3 min, followed by chromate passivation and finally rinsing.

A phosphating chemical composition (having zinc phosphate, phosphoric acid, nickel nitrate, etc.) with the brand name of Nipaphos 3020M, a proprietary composition of M/S Nipa Chemicals Ltd., Chennai, India, was dissolved in DI water to make an aqueous solution of 20 g/L of zinc phosphate concentration, and was used for the purpose of phosphate treatment of mild steel panels.

Preparation of electrolytic composition

The required amount of adduct was weighed in a 500 mL glass beaker with a glass rod and Teflon coated magnetic bar. Approximately 100 mL of double distilled water was added to the paste. The adduct present in the paste was neutralized by adding drop wise from a burette the aqueous solution of acetic acid (0.5N) under thorough agitation. The pH of the solution was monitored continuously during neutralization. The concentration of this solution was adjusted by the addition of double distilled water.

Cathodic electrodeposition of coating binders derived from cardanol as MECNs

A glass beaker (500 mL capacity) was filled with 350 mL of the electrolytic composition (CED coating liquid). A weighed metallic panel (the cathode) and a stainless steel anode of the same size were immersed up to a depth of 50 mm into the solution. After giving electrical connections to the electrodes, the required time of deposition was set in the timer of ED rectifier (Allied Industries, Mumbai). The voltage was then increased to the required value in 5-10 s (the ramp time) with the help of the voltage regulator of the rectifier. The "peak current" and the "drop in current with time" were noted with the progress of film deposition at constant voltage. An ampere meter connected in series was used for this purpose. The voltage was applied for a preset time for deposition. The coulombs were read on coulometer of the rectifier, and recorded. The electrodeposition was performed at five constant voltages as 50, 100, 150, 200, and 250 V.

The panel with freshly deposited film was removed from the electrodeposition bath (the electrolytic com-position) and rinsed under a mild current of running tap water, followed by a distilled water rinse.

The panel having the electrodeposited film (wet and uncured) was then hung inside the oven maintained at the predetermined temperature for the desired time, to effect film cure. The panel with the dried and cured film was taken from the oven, cooled to room temperature in a desiccators, and reweighed to determine the difference in the weight of the cathodically electrode-posited film (dried/cured). The area of the deposited film was measured on both sides of the panel. Using values of the weight of deposited film, area of the deposited film and the coulombs passed; the values of electrodeposited yield (in mg/[cm.sup.2]) and CY (in mg/C) were calculated.

Results and discussion

Cure characteristics of deposited coating binders as MECNs

To determine the suiTable molar ratio of DEtOA to ECN for the preparation of ECN-DEtOA adducts (MECNs), the effect of molar ratio of each ECN to DEtOA on its cure characteristics was studied. Each ECN, viz., [ECN.sub.1], [ECN.sub.2], [ECN.sub.3], and [ECN.sub.4] was reacted (modified) with nine different molar ratios of DEtOA as shown in the fifth column of Table 1. The molar ratio of DEtOA per mole of ECN was varied from 1 to 2.5 mol in view of the maximum number of epoxy groups available in the ECN molecule (epoxy functionality of ECN), i.e., 2.04, 2.20, 2.92 and 2.12 for [ECN.sub.1], [ECN.sub.2], [ECN.sub.3], and [ECN.sub.4], respectively. The curability was checked by baking the film at a fixed temperature (range 100-180[degrees]C) and fixed time (range 30 or 60 min). Resins, not curing at the highest temperature during 60 min, were considered incurable. Cure schedules of 100-180[degrees]C by 20[degrees]C increments and 30-min duration were used as shown in Table 2. No cure for any film was observed at or below 120[degrees]C while [MECN.sub.2] and [MECN.sub.8] cured slightly at 140[degrees]C and fully cured at 160[degrees]C. MECN, and [MECN.sub.6] required further higher temperature (180[degrees]C) for their complete curing. The curing temperature of MECNs seems to be dependent on their epoxy functionality with at least one cationic amino group present in their molecule. MECNs having higher epoxy functionality have been observed to cure at lower temperatures.

Optimization of the electrodeposition parameters

The prepared modified epoxidized cardanol-formaldehyde novolacs (epoxy-amine adducts), viz., [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], were neutralized with acetic acid and dissolved in distilled water to result in the solution to be used for electrodeposition experiments. The electrodeposition, using the solutions having optimized bath parameters such as concentration (10%, w/w) and pH (5.5), was carried out at room temperature at applied voltages of 50, 100, 150, 200, and 250 V for 60 s as deposition time. In fact, deposition time was varied up to 90 s, and it was found that almost all adducts attained the highest DFT in 60 s. The electrodeposition characteristics such as ED yield, CY, and DFT of the deposited and cured film of adducts were determined/ measured, and the values are given in Table 3.

Current density

The values of current densities (i, expressed in mA/[cm.sup.2]), at the optimized voltages of 50, 100, 200, 250, 250, 100, and 200 V for adducts, such as [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], respectively, vs times of deposition have been plotted in Fig. 1.


The graph shows that the current density, having a high initial (peak) value, dropped very rapidly during the initial period of deposition, as is evident by a steep curve corresponding to the deposition time of less than 30-40 s, and it attained its lowest (constant) value in 60 s, as shown in Fig. 1.

Electrodeposition yield

The voltage applied across the electrolytic cell (electrodeposition bath) was varied from 50 to 250 V. The values of ED yield, CY and DFT for chosen adducts at different voltages are tabulated in Table 3. It can be seen from the Table that the ED yield of adduct [MECN.sub.1]at 50 V was the lowest (1.34 mg/[cm.sup.2]) which increased to 4.06 mg/[cm.sup.2] at 200 V. On further increasing the applied voltage, the ED yield did not increase. Similarly, there was no significant increase in DFT with an increase in applied voltage after 200 V. The maximum ED yield and DFT that could be obtained by varying the applied voltage for [MECN.sub.1] were 4.06 mg/[cm.sup.2] and 25.26 [micro]m, respectively at 200 V. At an applied voltage of 250 V, the deposited film was found to be rough due to film rupture, as was observed by sudden upsurge of current. Thus, the voltage of 50 V was considered to be a suiTable applied voltage for the CED of adduct [MECN.sub.1] to produce smooth films with suiTable ED yield. On the same lines, the suiTable applied voltages for the optimized deposition of adducts as [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9] were found to be 100, 200, 250, 250, 100, and 200 V, respectively, on the basis of outstanding film properties and smoothness.

The number of cationic amino groups and the hydrophobic content of the molecule are the key factors and go hand-in-hand to govern the micelle size. It has been mentioned by several researchers (25), (26) that the molecule having net higher hydrophobicity would aggregate to a larger extent to form larger micelles. In this context, net hydrophobicity refers to the degree of the overall dominance of hydrophobicity over hydrophilicity.

In our system, which is unique in itself, hydrophilicily and hydrophobicity would be caused by number of cationic amino groups and the number of long C (15) (hydrocarbon) chain, respectively, that are present on the molecules. The overall dominance of hydrophobicity would result in larger micelle size. The number of hydrophobic C (15) chains per molecule has been calculated for all MECNs, as tabulated in Table 4.

Based on the above data, it is clear that the increasing micelle size will observe the following trend:


Our experimental results also confirm that the MECN in group (I), having smallest micelles, would deposit suitably at the lowest voltage (50 V). Accordingly, MECNs in group (II) would deposit at 100 V, group (III) at 200 V and group (IV) at 250 V.

The ED yield and DFT for MECNs were determined/measured, and they were found to behave in the same manner, as shown in Table 3. Identical trends of these two ED characteristics confirm that the two terms, viz., ED yield and DFT, used to indicate the film-build, manifest the same feature. However, measurement of both the characteristics is useful, as ED yield can be conveniently measured quickly and very accurately in the laboratory, by weighing the panels before deposition and after curing the deposited film on the panels. On the other hand, the film-build on large articles, as encountered in commercial productions, is measured by DFT only.

Figure 2 shows that, for all MECN resins, the average ED yield is dependent on molecular weight. The lowest molecular weight resin, [MECN.sub.1], had an ED yield of only 2.32 mg/[cm.sup.2], whereas the higher molecular weight resins [MECN.sub.6] and [MECN.sub.7] had ED yields of 4.92 and 4.84 mg/[cm.sup.2], respectively. Molecular weight and the number of amino groups are key factor in controlling film-build.


Coulombic yield

Coulombic yield (CY) of all adducts was found to be independent of the time of deposition and remained constant for all adducts, whereas the ED yield increased steadily with time of deposition up to 60 s. From Table 3, it can be observed that the value of CY is almost constant in the range of 25.85-26.70 mg/C for [MECN.sub.1], 35.85-36.65 mg/C for [MECN.sub.2], 34.10-34.75 mg/C for [MECN.sub.3], 37.56-38.43 mg/C for [MECN.sub.6],39.50-40.85 mg/C for [MECN.sub.7],35.31-36.73 mg/C for [MECN.sub.8] and 33.31-34.87 mg/C for [MECN.sub.9] with the increase of applied voltage from 50 to 250 V. Independence of columbic yield on deposition time can be explained well by Faraday's laws of electrolysis, based on the fact that the mathematical form of laws have built-in time term in the expression, as the coulomb is current multiplied by time. As a matter of fact, CY of a material (adduct) is nothing but its electrochemical equivalent as per Faraday's laws of electrolysis.

However, the higher values of CY for all adducts, obtained experimentally, as compared to that of the theoretical values, i.e., calculated by Faraday's laws, can be attributed to a number of facts such as the self-polymerization of adduct during preparation, micelle formation of neutralized adduct molecules in the electrolytic solution and partial neutralization of the adducts by acetic acid. Acetic acid, being a weak acid ([pK.sub.a] = 4.76) might have not been able to react fully with tertiary amino groups of the adduct due to steric hindrance. As a consequence, some of the tertiary amino groups would have remained shielded and un-neutralized, to increase the value of electrochemical equivalent of the neutralized adducts, i.e., micelle formation in the electrodeposition bath. Secondly, as a consequence of micelle formation following physical and/or chemical aggregation, the adductcations as individual ions might not have migrated to deposit, rather aggregates/clusters of adductcations could have been dragged as a single cation and deposited on the cathode.

Dry film thickness

The thickness of the dried/cured films (called Dry Film Thickness, DFT) was measured at a number of equidistant points on the deposited film using a digital electromagnetic induction film thickness gauge. The readings (4 or 5 in number) were used to calculate the average film thickness. DFTs of the films have been given in Table 3.

The average rates of film growth of adducts [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], were calculated and found to have fallen in the range of 0.33-0.44, 0.41-0.46, 0.40-0.48, 0.36-0.50, 0.41-0.65, 0.40-0.48, and 0.54-0.56 um/s, respectively. In fact, the film deposition was rapid in the beginning and slowed down with the progress of deposition. The film growth almost stopped beyond 60 s of deposition time because of the near-complete insulation of the substrate by the deposited film. Thus, the optimum time for deposition was considered to be 60 s.

Kinetics of film growth during cathodic electrodeposition

To investigate the kinetics of growth of film during CED, resin adducts as MECNs prepared from ECN and DEtOA at 90[degrees]C for 3 h of reaction time were used. Pierce (27) proposed a model for the kinetics of film growth which relates DFT (denoted by letter ([delta]) with time of deposition (t)). The mathematical expression for the relation of ([delta]) with (t) can be given as

[delta] = [[2c[[sigma].sub.F]V].sup.1/2][t.sup.1/2]

where c = coulombic efficiency,[[sigma].sup.F] = specific conductivity of the deposited film, and V = applied voltage.

From the equation, it can be appreciated that the slope of the straight line portion of the curve obtained by plotting [sigma] vs [t.sup.1/2] (Figs. 3-9) should be equal to [(2c[[sigma].sub.F]V).sup.1/2] which can give the value of coulombic efficiency, c (in [cm.sup.3]/C), if the specific conductivity, [[sigma].sub.F] (in [mu]S/cm) is known. From Figs. 3-9, slopes of the straight line plots of [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9] have been calculated to be 2.6, 3.19, 3.83, 3.96, 4.78, 3.01 and 4.47 [micro]m/s1/2 at 50, 100, 200, 250, 250, 100, and 200 V, respectively.








Specific conductivity of the deposited film Ideally, the conductivity of the electrodeposited film of adduct should be zero, i.e., the film should be totally nonconductive, as it is composed of electrically non-conductive organic material (cardanol-derived epoxy-amine adduct). In practical situations, however, a freshly deposited film has pores filled with the same electrolyte from which the film was being deposited. As a result, current conduction can take place through these pores. In general, the conductivity of the film is regarded as the conductivity of electrolyte filled in pores of the film. (26) If we consider these pores as conductors of electricity, then Ohm's law should hold true with respect to resistance or conductance.

In equation (2), [delta] is the pore depth which would obviously be equal to the wet film thickness. According to equation (2), the current density (i) and the field strength (V/[delta]) across the deposited films should be linearly proportional:

The values of specific conductivities ([sigma].sub.F) of the films have been calculated with the help of equation (2) using the values of current density (i) and field strength (V/[delta]), and these have been found to be 0.0313, 0.0179, 0.0185, 0.0219, 0.0252, 0.0257, and 0.0193 [micro]S/cm for [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], respectively. By using these values of [[signa].sub.F] and the values of slope of ([delta] vs [t.sub1/2] curves, in accordance with equation (1) and from Figs. 3-9, the values of coulombic efficiency have been calculated and found to be 0.0215, 0.0284, 0.0198, 0.0142, 0.0181, 0.0175, and 0.0258 [cm.sup.3]/C, respectively, for the above-mentioned MECNs.

Measurement of gloss

Gloss of cured films of all the MECNs was measured at a number of equidistant points on the deposited film using a digital triglossometer (Elcometer) at 60[degree] as per standard test method ASTM D [523-99.sub.28]The reported values are the averages of four or five readings to determine the accurate value of gloss. The values of gloss of the deposited films of all the MECNs are given in Table 5.

The gloss generally increased with the increase in voltage from 50 to 250 V because thicker films were deposited on the substrate at higher voltages. It is also observed that lower molecular weight resin has higher gloss value as compared to the resins having higher molecular weight.

Evaluation of film properties

Based on the curability of MECNs, the applied films of MECNs, viz., [MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8] were selected to be used in this part of the study. These films were evaluated for their physical and chemical resistance properties after proper curing as per their selected cure schedules as mentioned in Table 2, and maturing the cured films for 4 days at room temperature. The films were applied on pretreated mild steel panels for evaluation of physical properties, while glass panels were used for determining the chemical resistance properties.

Physical properties

The films were evaluated for physical properties such as adhesion, (29) flexibility, (30) hardness, (31), (32) and impact resistance. (33) The results on physical properties of the films of different MECNs have been tabulated in Table 6.

Adhesion of films to mild steel panel was measured by cross-hatch tape test in which all the MECNs showed 100% adhesion, i.e., no square was lifted. It probably is due to the presence of a large number of hydroxy1 groups per mole of MECN.

In the measurement of flexibility, films of all MECNs passed 1/4 in. mandrel bend test, but films of [MECN.sub.6] and [MECN.sub.8] failed on 1/8 in. Based on this qualitative measurement, it can be said that the films had reasonably good flexibility due to the presence of [C.sub.15] side chain in cardanol. Films of [MECN.sub.6] and [MECN.sub.8] were found to be slightly inferior, probably, due to their higher initial molecular weight than that of [MECN.sub.1] and [MECN.sub.2].

Hardness of the films of MECNs can also be said to be good, as indicated by scratch hardness values of 1200, 1500, 1400, and 1800 g for [MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8], respectively, and pencil hardness as 2H-3H. This probably can be attributed to the higher degree of polymerization effected by the presence of a number of epoxy groups, acting synergistically to the higher degree of hydrogen bonding resulting from the considerable amount of hydroxy1 groups present in the MECNs molecules.

Impact resistance of a film is generally governed jointly by two factors, viz., adhesion to the substrate and the film flexibility. The failure of a film against impact (rapid deformation) is determined by the net effect of the two, i.e., the predominance of one factor over the other. The impact resistances of the films of [MECN.sub.1] and [MECN.sub.2]were found to be excellent, as the films of these resins passed impact tests of 50 and 45 cm, respectively, by 1000 g of weight. The rest of the MECNs have lower impact resistances of 25-35 cm, probably due to the higher film hardness. Thus, some of the prepared MECNs produced films with optimum combinations of adhesion and flexibility.

Chemical resistance properties

The films of MECNs were evaluated for their chemical resistance properties (34) by immersing the coated films in a variety of chemicals such as acid, alkali, salt, organic solvents, and distilled water. Conditions of the films were observed after every hour in the beginning, and then after 4, 8, 12, and 24 h, and subsequently after 2 days, 4 days, 1 week, 2 weeks, 3 weeks, 4 weeks, and finally 1 month, 2 months, 3 months, etc. Chemical resistances of the films in terms of their conditions after 1-week and 3-month exposures are given in Tables 7 and 8, respectively.

The alpha-numerical codes have been assigned to the condition of the exposed films, for brevity in presentation, as given below:

NE = No Effect; FD - Fully Dissolved; SG = Slight Loss in Gloss; S = Swelling of film, ranging from 1S to 10S, where 1S is minimum swelling, i.e., maximum resistance of films to the chemical, and 10S is maximum swelling, i.e., minimum resistance to the chemical; L = Degree of Film Lift Off, ranging from 1L to 10L, where 1L is minimum film lifting, i.e., maximum resistance to the chemical, and 10L is maximum film lifting, i.e., the lowest resistance to the chemical; D = Degree of Dissolution of Film, ranging from 1D to 10D, where 1D is minimum film dissolution, i.e., maximum resistance to the chemical, and 10D is maximum film dissolution, i.e., the lowest resistance to the chemical.

Resistance to acid ([H.sub.2][SO.sub.4]) solution

The films of all the four resins, viz., [MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8] immersed into acid solution (IN sulfuric acid), remained unaffected for 1 week, but after 3 month's immersion of the films of these MECNs, films became swollen to varying degrees, and lost gloss. This behavior of the films can be because the films were composed of alkaline resin (a tertiary amine), and hence the films were attacked by the acid solution significantly.

Resistance to alkali (NaOH) solution

The films, due to their alkaline nature, were not affected (attacked) by the alkali solution (1N sodium hydroxide) in a week of film immersion. However, varying degrees of swelling (in the range of 2S-4S) of the film were observed for films of resin, viz., [MECN.sub.1], [MECN.sub.2], [MECN.sub.6], and [MECN.sub.8] after 3 months of film immersion. The films of these MECNs were lifted slightly from the dipped edges of the panel after 3 months of film immersion. When the films were removed from the panels (as free films) after 3 months of immersion, the removed (free) films had good integrity and enough strength. This indicates that the alkali could not affect the film-forming material but, of course, it attacked at the film-substrate interface to detach the film from the panel. The film of resin [MECN.sub.2] had the least effect due to its good adhesion.

Resistance to salt (NaCl) solution

The salt solution (5% w/w, sodium chloride) did not cause any damage to the films of all the resins in 1 week of immersion. However, after 3 months of exposure, the films of MECNs observed loss in gloss, swelling and film lift-off.

Resistance to solvents

Solvent resistance of the cured films of MECNs was checked in mineral turpentine oil (MTO), xylene, methyl ethyl ketone (MEK), and methanol as solvents. The effects of these solvents in 1 week and 3 months of immersion of the films are shown in Tables 6 and 7, respectively.

From the results, it can be inferred that the films had dominating nonpolarity due to the presence of C15 (hydrocarbon, nonpolar) chains in the resin molecule. Accordingly, xylene as an aromatic hydrocarbon had the most significant effect on the film and methanol (highly polar) exhibited the least effect. The effect of MEK, modatory polar, had an intermediate degree of effect.

Apart from the nonpolarity introduced by the C15 chain, the degree of polymerization resulted in during film curing, which could be the result of the number of cationic amino groups as well as the number of unreacted epoxide groups in the resin molecule before curing.

Resistance to water

Films of all the resins designated as MECNs were totally unaffected even after 3 months of immersion period in distilled water. This indicates that all MECNs have excellent water (immersion) resistance at ambient temperature, due to much higher polarity of water as compared to that of the resin films, leading to a high order of noncompatibility of the two.

Corrosion resistance: salt spray test

The electrodeposited films were exposed for 800 h in the standard salt spray atmosphere; exposed films were assessed by the corrosion creepage (in mm) across the scribe, number (density), and size of blisters developed. The blisters were assessed as per ASTM B 117-94. (35)

Table 9 show that, overall, the cathodically electrodeposited films of all adducts exhibited excellent corrosion resistance properties, as the corrosion creep-age across the scribe was very low, between 1.0 and 2.2 mm, along with small blister size (2-6 only) for all the adduct films even after 800 h of continuous exposure to saline (salt spray) atmosphere.


Based on this study, the following conclusions can be drawn:

(1.) Cardanol has been successfully incorporated within the resin molecule synthesized for CED, and hence these resin systems are cost effective, nontoxic and eco-friendly.

(2.) Cathodically electrodeposited films of the developed polyepoxide resins (MECNs) are self-curable without use of any external crosslinker. Nonisocyanate chemistry of curing of such polyepoxide resins has been investigated via anionic polymerization mechanism.

(3.) Films of developed MECNs are cathodically electrodeposiTable even at a lower voltage of 100 V, achieving good ED characteristics in terms of ED yield, DFT, and CY, which were found to be 3.62 mg/[cm.sup.2], 25.26 pm, and 35.87 mg/C, respectively.

(4.) ED yield increased with the increase in the molecular weight and number of tertiary amino group(s), but decreased with increase in epoxy functionality of MECNs. The maximum ED yield of lowest molecular weight resin ([MECN.sub.1]) was found as 4.06 mg/[cm.sup.2] and the highest molecular weight resin ([MECN.sub.7]) had the ED yield as 9.75 mg/[cm.sup.2].

(5.) It was found through the studies on cure schedule that in general higher molecular weight MECNs with higher epoxy functionality and lower number of tertiary amino group(s) were cured at lower temperature, and vice versa.

(6.) Based on the kinetic studies, specific conductivities ([[sigma].sub.F]) of the deposited films of MECNs have been calculated and found to be 0.0313, 0.0179, 0.0185, 0.0219, 0.0252, 0.0257, and 0.0193 [micro]S/cm for [MECN.sub.1], [MECN.sub.2], [MECN.sub.3], [MECN.sub.6], [MECN.sub.7], [MECN.sub.8], and [MECN.sub.9], respectively.

(7.) With the increase in molecular weight of resins, the physical properties of the films varied significantly in terms of hardness, flexibility, and impact resistance. In fact, hardness increased with the increase in molecular weight and epoxy functionality (decreased in tertiary amino groups), whereas flexibility and impact resistance decreased.

(8.) In general, higher molecular weight resins with higher crosslink density due to more epoxy functionality demonstrated belter resistivity against chemicals. Conceptually, e.g., [MECN.sub.6] having higher molecular weight and less epoxy functionality (0.93) was affected more than [MECN.sub.2] having higher epoxy functionality (1.21).

(9.) The corrosion resistance performance of electrodeposited films of MECNs was found to be satisfactory, as all films demonstrated good salt spray resistance (800 h).

In view of the self-curability, low-cost, and nontoxicity of MECNs, as well as their good ED characteristics and film properties, it can be expected that the developed resin systems through this investigation would gain wide commercial acceptance.

Acknowledgments The authors wish to express their gratitude to the All India Council for Technical Education (AICTE), New Delhi (India) for the financial assistance provided for running the project entitled "Development of eco-friendly and energy-efficient polymeric coalings for cathodic electrodeposition" under its TAPTEC scheme (AICTE File No. 8021/RID/NPROJ/TAP-12/2002-03). The authors also wish to thank Satya Cashew Chemicals Ltd., Chennai (India) for providing free samples of cardanol and CNSL for this study.


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Raju, P. Kumar (El)

Department of Oil & Paint Technology, Harcourl Butler

Technological Institute, Kanpur 208 002, India




DOI 10.1007/s 11998 -011-9337-y
Table1:Characteristics of MECN resins: mode ratio,
epoxy functionality, and amine value

S.       MECN resin       Molar (a)         Epoxy      Molar ratio
No.                       ratio (C:F)   functionality  (ECN:DEtOA)
                                            of ECN

1    [MECN.sub.1]           1:0.6          2.04          1:1.0
2    [MECN.sub.2]           1:0.7          2.20          1:1.0
3    [MECN.sub.3]           1:0.7          2.20          1:1.5
4    [MECN.sub.4] (c)       1:0.8          2.92          1:1.0
5    [MECN.sub.5] (c)       1:0.8          2.92          1:1.5
6    [MECN.sub.6] (c)       1:0.8          2.92          1:2.0
7    [MECN.sub.7] (c)       1:0.8          2.92          1:2.5
8    [MECN.sub.8] (c)       1:0.9          2.12          1:1.0
9    [MECN.sub.9] (c)       1:0.9          2.12          1:1.5

S.       Epoxy      Amine value of             [Conversion.sup.c]
No.  functionality    MECN resin                       (%)
        of MECN       (mgKOH / g)
                    (Theoretical)   (detd.)

1        1.04           59.49        58.53         98.38
2        1.20           50.17        49.65         98.96
3        0.70           71.89        61.72         85.85
4        1.92           47.18        45.74         96.94
5        1.42           67.78        68.54         98.17
6        0.92           86.70        85.77         98.92
7        0.42          104.15        95.83         92.01
8        1.12           50.44        40.40         80.09
9        0.62           72.26        58.33         80.72

S.       Epoxy        Watersolubility       Selfcurability
No.  functionality
        of MECN

1        1.041        Soluble               Curable
2        1.21         Soluble               Curable
3        0.85         Soluble               Uncurable
4        1.98         Insoluble               -
5        1.45         Insolubie               -
6        0.93         Soluble               Curable
7        0.46         Soluble               Uncurable
8        1.40         Soluble               Curable
9        0.77         Soluble               Uncurable

(a) C:F is molar ratio of cardanol to formaldehyde used for
preparation of corresponding CN resin

(b) ECN to MECN, calculated by using theoretical and determined amine

(c) Resins had inadequate pot life, as they gelled in a short period
of time, and hence they were not used for application as films and
for evaluation of film properties

Table 2: Cure temperature of modified epoxidized
cardanol-formaldehyde novolac (MECN) resins

S.       MECN                                      Temperature
No.      resin                                     (cure time: 30

                   100[degrees]C  120[degrees]C  140[degrees]C

1.   [MECN.sub.1]        NC               NC               NC
2.   [MECN.sub.2]        NC               NC               SC
3.   [MECN.sub.3]        NC               NC               NC
4.   [MECN.sub.6]        NC               NC               NC
5.   [MECN.sub.7]        NC               NC               NC
6.   [MECN.sub.8]        NC               NC               SC
7.   [MECN.sub.9]        NC               NC               NC

No.  160[degrees]C  180[degrees]C

1.         SC               c
2.         c                c
3.         NC               SC
4.         SC               c
5.         NC               SC
6.         C                c
7.         SC               SC

NC, not cured; SC, slightly cured; C, cured

Table 3: Electrodeposition characteristics of MECN resins
electrodeposited at different applied voltages

S.       MECN      Applied      ED        Columbic         DFT
No.      resin       voltage    yield     yield (mg /  ([mU]m)
                     (V)      (mg /         C)

1.   [MECN.sub.1]     50        1.34        26.31        20.08
                     100        2.32        25.85        22.86
                     150        2.92        26.11        24.40
                     200        4.06        25.88        25.26
                     250        4.06        26.70        25.56
2.   [MECN.sub.2]     50        2.05        35.95        24.60
                     100        3.62        35.87        25.26
                     150        4.69        36.00        26.36
                     200        6.54        36.39        28.10
                     250        6.59        36.65        28.00
3.   [MECN.sub.3]     50        2.19        34.54        24.26
                     100        4.04        34.75        26.65
                     150        5.40        34.10        27.46
                     200        6.42        33.92        28.96
                     250        6.67        34.49        28.88
4.   [MECN.sub.6]     50        3.33        37.79        21.83
                     100        4.92        37.56        22.33
                     150        7.84        38.22        28.08
                     200        8.60        38.43        30.46
                     250        9.31        37.26        30.34
5.   [MECN.sub.7]     50        2.94        40.05        24.76
                     100        4.84        40.72        28.84
                     150        8.34        39.73        35.67
                     200        9.46        40.85        36.34
                     250        9.75        39.50        36.87
6.   [MECN.sub.8]     50        3.18        35.31        24.50
                     100        4.39        36.09        25.79
                     150        7.11        36.75        28.55
                     200        7.62        36.15        28.76
                     250        7.46        35.71        28.86
7.   [MECN.sub.9]     50          -            -            -
                     100        4.27        33.59        32.80
                     150        6.61        33.31        33.10
                     200        7.84        34.67        33.30
                     250        7.86        34.87        33.68

S. No.  MECN resin           Film appearance (after curing)

1.      [MECN.sub.1]                Smooth
2.      [MECN.sub.2]                Smooth
3.      [MECN.sub.3]                Rough
4.      [MECN.sub.6]                Rough
5.      [MECN.sub.7]                Rough
6.      [MECN.sub.8]                Rough
7.      [MECN.sub.9]                FND

FR, films ruptured drastically, hence not considered in study;
FND, film not deposited; deposition time: 60 s; bath
concentration: 10% (w/w)

Table 4: Theoretical dominance of hydrophobicity over hydrophilicity

MECNs         Molecular    No. of       No. of     Hydrophilic
                weight     cationic   [C.sub.15]    cationic
               of MECN     group      chain          group
                         per mole   per mol         contente
                          (charge   (2 + DP        (equiv./
                            per     n value           kg)

1.               943         1        2.0804       1.0604

2.              1118         1        2.4608       0.8944

3.              1170        1.5       2.5739       1.2820

4.              1294         2        2.8434       1.5455

5.              1346        2.5       2.9565       1.8573

6.              1112         1        2.4478       0.8992

7.              1164        1.5       2.5608       1.2886

MECNs         Hydrophobic
               (equiv. /

1.              2.2061

2.              2.2010

3.              2.1999

4.              2.1973

5.              2.1965

6.              2.2012

7.              2.2000

DP, degree of polymerization; n, number of repeat units in the molecule

Table 5: Gloss of electrodeposited films of MECN resins after curing

Applied             Gloss measured at 60[degrees]

         [MECN.sub.1]  [MECN.sub.2]  [MECN.sub.3]  [MECN.sub.6]

50       88.90         67.43         63.16         48.90
100      93.96         68.30         64.56         52.64
150      94.47         68.06         66.37         55.60
200      95.73         67.93         67.76         58.93
250      85.4          68.30         69.40         66.13

Applied       Gloss measured at 60[degrees]

         [MECN.sub.7]  [MECN.sub.8]  [MECN.sub.9]

50       30.40         55.86         -
100      34.36         70.36         82.93
150      35.68         71.34         87.94
200      37.56         72.33         99.00
250      39.47         74.83         79.63

Table 6: Physical properties of MECN resins

S. No.  Property/test

1.      Adhesion (%)
        Cross hatch (1 X 1 mm) tape test    100

2.      Flexibility (Mandrel bend test)
        (a) (1/4 in. mandrel)               Pass
        (b) (1/8 in. mandrel)               Pass

3.      Hardness
        (a) Scratch hardness (g)            1200
        (b) Pencil hardness (pencil grade)  2H

4.      Impact resistance (cm)              50)

S. No.  Result/value

        (MECN.sub.2)  (MECN.sub.6)  (MECN.sub.8)

        100           100           100

        Pass          Pass          Pass
        Pass          Fail          Fail

        1500          1400          1800
        2H            3H            3H

4.      45            35            25

Table 7: Conditions of the films of the prepared MECN resins after
 1 week immersion in different chemicals

S.      Resin     Condition of the film after immersion for 1 week

                  [H.sub.2]S[O.sub.4]  NaOH  NaCI  Xylene  Methanol
No                        (1N)         (1N)   (5%)

1.  [MECN.sub.1]          NE            NE    NE     3D       NE
2.  [MECN.sub.2]          NE            NE    NE     2D       NE
3.  [MECN.sub.6]          NE            NE    NE     3D       NE
4.  [MECN.sub.8]          NE            NE    NE     3D       NE

S.    MEK  MTO     Water
No                (distilled)

1.  NE   NE       NE
2.  NE   NE       NE
3.  NE   NE       NE
4.  NE   NE       NE

Table 8: Conditions of the films of the prepared MECN resins
 after 3 months immersion in different chemicals

S.       Resin       [H.sub.2]S[O.sub.4]  NaOH  NaCI  Xylene  Methanol
No.                        (1N)           (1N)  (5%)

1.   [MECN.sub.1]          4S            3S    SG     7D       2S
2.   [MECN.sub.2]          3S            2S    NE     5D      1.S
3.   [MECN.sub.6]          4S            3S    3S     6L       2S
4.   [MECN.sub.8]          5S            4S    6S     7L       4S

S.   MEK  MTO     Water
No.                (distilled)

1.   4D   3D       NE
2.   2D   2D       NE
3.   5D   3D       NE
4.   4D   4D       NE

Table 9: Salt spray test results of electrodeposited MECN resins

S.   Resin                   Salt spray
No.                          resistance

                   Creepage  Blister     Blister
                   (mm)      (grade)     density

1.   [MECN.sub.1]  2.2       6           Medium

2.   [MECN.sub.2]  1.5       4           Medium

3.   [MECN.sub.6]  1.0       2           Medium

4.   [MECN.sub.8]  1.5       4           Medium
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