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Water soluble epoxy resins for cathodic electrodeposition coatings: A review

Abstract

Most of the resins used in the cathodic electrodeposition (CED) coatings are made from modified epoxy resin and blocked polyisocyanate in which the cross linking is achieved by polyurethane and polyurea formation. A resin system has been developed that is isocyanate free and avoids not only the toxicity aspect, but also, to a great extent, the weight loss of the coating upon baking. And research work has been carried out to explore new isocyanate-free curing chemistry for the CED coatings. Thus, the curing of epoxy resins via ring opening reaction between glycidyl groups (homopolymerization) can be readily done using tertiary amines. A resin system containing both epoxy groups and tertiary amine groups was prepared and upon neutralization of the tertiary amine groups with formic acid, it made the resin water-compatible as well as suitable for cathodic deposition. After deposition this tertiary amine groups catalyze the curing via homopolymerization.

Key word: Epoxy resin, Amines, Waterborne, Electrodeposition.

Introduction

THE method of applying waterborne coating called electrodeposition was introduced during 1960s. It fulfills the requirements of low pollution and high transfer efficiency[1]. Since its introduction, the process has been a growing coatings technology that provides excellent corrosion protection with more effective paint utilization, uniform thickness, good throwing power, operational efficiency and safety[2] Epoxy resins have been used in cathodic electrodeposition coatings since the 1980s. Their chemical properties include mechanical strength, adhesion to metals and heat resistance excellent corrosion resistance. These properties facilitate the protection of car bodywork from corrosion and other damages, in some instances doubling the lifetime of a car. The main application of epoxy resins in automotive – are usually applied by Cathodic Electro Deposition (CED). It is the most widely used technique. Number of advantages, including: minimizing the use of organic solvents; excellent adhesion to metals; coating of cavities and other complicated geometries; highly automated processes; depositing a uniform film thickness. Other epoxy coatings are applied using solvent-free powder coating. During the CED process, the epoxy resin layer is the first of five organic layers coating the metallic bodywork of a car. Usually approximately 20μ m thick, it has been estimated 2 to 8 kg of epoxy are needed per car to act as primer for further layers to be placed on top of the epoxy coating.

Fig 1: Mechanism of CED coating

The process involves immersing the car body for ± 4 minutes in an electrodeposition bath. An electrical charge applied on the car body deposits the positively charged epoxy resin to create a new layer. The bath consists of an epoxy-amine adduct and a blocked Isocyanate to crosslink adduct.

The electrodeposition coating was started with anodic electrodeposition (AED) but today the cathodic electrodeposition (CED) has almost completely surpassed it as it is devoid of major drawbacks associated with AED such as metal dissolution and film discoloration[3]. The currently used CED coatings are usually based on isocyanate curing systems using blocked isocyanates as curing agents[4]. The blocking agents released in the step of baking dissipate and form gum and soot, contaminating the environment inside and outside oven. In addition, the isocyanates formed upon deblocking are themselves toxic, posing an environment compatibility problem[5]. When an organo-tin compound is used as a curing catalyst it sometimes poisons an exhaust combustion catalyst of the baking furnace. Apart from this due to release of blocking agent, the weight loss and film shrinkage are produced which increases the total applied cost[6]. Electrolytic deposition really took off with the development of effective electrolytes for silver and gold deposition around 1840. These became the basis of an extraordinarily successful decorative plating industry, with the technology spreading from Great Britain and Russia to the rest of Europe. However, the electrolytes were extremely toxic, as they incorporated cyanide, and the search for better, safer substitutes continues. Finding environmentally friendly alternatives to established electrodeposition processes is still an important general challenge, and, recently, the Electrodeposition Division of ECS has initiated a symposium series entitled “Green Electrodeposition.”

In parallel with the development of new processes, considerable effort has been devoted to gaining a fundamental understanding of electrodeposition. Breakthroughs included the establishment of the linear relationship between over potential and the logarithm of the deposition current by Tafel in 1905, and its explanation on the basis of statistical thermodynamics. Some exceptionally elegant experiments on perfect silver crystals by Kaischew, Budevski and coworkers in Bulgaria in the 1960s led to a much deeper understanding of the role of defects, in particular screw dislocations, in the growth of electrodeposited metals; but experimental studies at an atomic level were hindered for a long time because the experimental techniques using electron scattering that were developed so successfully for studying surfaces under vacuum could not be used in an electrolyte.

Epoxy-Amine waterborne adduct

Conventional electrocoating processes tend to use low molecular weight resins to enhance flow and coalescence of the film as coated and as cured. However, such resins deposit and cure relatively slowly. The present invention permits the use of relatively high molecular weight resins with low amine content for efficient electrodeposition. Coalescing aids can improve the integrity of the film as initially deposited and during cure. Crosslinking agents can act to some extent as coalescing aids and do help further to provide proper baked film properties. The water- borne coating composition used in the present invention is a solution or dispersion of the reaction products of an epoxy resin, a tertiary amine, and a carboxyl-functional polymer. By mixing these components in a random order and utilizing aqueous solutions of highly specific tertiary amines such as dimethyl ethanol amine, a stable, water soluble or dispersible salt of a polymeric quaternary ammonium hydroxide and a carboxyl functional polymer results which can be cross linked without the addition of external crosslinking agents. The optional addition of an external crosslinking agent, such as a nitrogen resin, also affords a crosslink able solution or dispersion which is stable at room temperature. Both compositions, the salt and the solution or dispersion containing an external crosslinking agent, are infinitely dilutable with water.[7] The water compatible polyamine-epoxy resin is formulated by reacting a polyamine with a mixture of epoxide resins consisting essentially of monoepoxide and polyepoxide. The polyamines suited for preparing the water compatible polyamine-epoxy adduct are ones that typically contain from about 2 to 20 carbon atoms per molecule and have from 2 to 10 amine nitrogen atoms. Preferably, the amine has from about 3 to 6 nitrogen atoms with two of the nitrogen atoms being primary amine nitrogen atoms. Particularly suited for producing the water compatible polyamine-epoxy adduct are the polyalkylene polyamines and preferably the polyethylene polyamines represented by the formula:

H2NR-[NHR] n-NH2

where n is an integer from 0 to about 6 and R is an alkylene group, preferably C2-3 alkylene. Examples of alkylene polyamines include ethylene diamine, diethyl enetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, dipropylenetriamine, tripropylenetetramine and tributylenetriamine. Other polyamines include the aminopropylated polyamines which can be formed by the reaction of a polyamine, polyol or alkanolamine with acrylonitrile and the subsequent reduction of the cyano group to the primary amine. The polyamines suited for amino propylation include the polyalkylene polyamines such as those described above. Examples of polyols suited for amino propylation include polyols having from 2 to 6 carbon atoms and from 2 to 4 hydroxyl groups such as ethylene glycol, propylene glycol, trimethylolpropane, and pentaerythritol. Alkanolamines are those of similar carbon content to the polyols and have both amine and hydroxy functionality. Examples include ethanolamine and diethanolamine. Conversion of the cyano group to the amine group can be carried out in conventional manner by contacting the cyanoethylated polyamine or polyol with hydrogen in the presence of a hydrogenation catalyst such as a Raney nickel or palladium catalyst metal carried on a support such as silica, diatonaceous earth, carbon, alumina and the like. Examples include bis(3-methoxypropyl)amine and bis(aminoproxy ethoxypropyl)amine. The polyamine used in forming the water compatible polyamine-epoxy adduct is reacted with a monoepoxide in an amount to endcap from 10 to 50%, preferably 20 to 40% of the primary amino hydrogen atoms in the polyamine. The monoepoxides are compounds having from 7 to 21 carbon atoms and they can be aliphatic or aromatic. [8]

The kinetics and mechanism of organic film growth during the cathodic electrodeposition process

The primary process in the case of cathodic electrodeposition of a waterborne organic coating on a metal electrode is hydrogen evolution by H2O discharge:

2H2O + 2e– H2 + 2OH– …..(1)

Followed by electrocoagulation of the resin micelles at the cathode surface by neutralization of positively charged groups in the resin with electrochemically generated OH– ions. The deposition of a coating will occur when the hydroxyl ion concentration (pH of the system) achieves a critical value:

R–NH3+ + OH–gR–NH2 + H2O ....(2)

Constant voltage experiments carried out at lower voltages enabled the observation of current density changes more accurately than is possible at higher voltages and a mathematical model of organic film growth has been proposed.[9] An increase in the applied voltage decreases the time necessary to achieve the maximum value of the current density and gives a larger value for that maximum. At voltages higher than 150 V, a maximum was not observed. If the film formation and the film growth kinetics are the same for the process at both lower and higher voltages, the current density, j–time, t, relationship can be explained by dividing the process into three steps. The first step is given by Eq. (1) and is represented by an initial plateau of the current density- time curve. The second step is given by Eq. (2) and is represented by a rapid decay of the current density after the initial plateau. In the third step, the electrodeposition process takes place mainly through the porous film, as the surface coverage approaches a value of unity. This step is represented by a maximum of the current density-time curve followed by decrease in the current density. In the third step, reaction (1) takes place in pores. The generated hydroxyl ions react with resin particles outside the pores where transport of resin particles from the bulk ensures a constant resin concentration and a new layer of the polymer film forms. If both reactions (1) and (2) are pseudo-first order reactions, then:

Where, A is water, B is hydroxyl ion, k1 and k2 are the rate constants of the first and the second step of the electrodeposition process, respectively.

Recent developments

Work has been carried out to explore new isocyanate-free curing chemistry for the CED coatings. Thus, the curing of epoxy resins via ring opening reaction between glycidyl groups (homopolymerisation) can be readily done using tertiary amines [10]. A resin system containing both epoxy groups and tertiary amine groups was prepared and upon neutralization of the tertiary amine groups with formic acid, it made the resin water-compatible as well as suitable for cathodic deposition. After deposition this tertiary amine groups catalyze the curing via homopolymerisation. The objective of the research work is to explore this reaction as newer isocyanate free curing chemistry for the CED coatings. The glycidyl groups readily react with secondary amines forming tertiary amines[11] which can catalyze the homopolymerization of epoxy resin. Therefore, the research work is going to develop isocyanate-free self-curable epoxy coatings for CED.

New types of cathodic electro coatings are continually being developed to improve corrosion Performance, coating appearance and mechanical properties and to reduce the environmental Impact of the processes. Self-curable cathodically depositable coatings were developed from glycidyl functional epoxy ester-acrylic graft co-polymer (EEAG) without using any external cross linking agents. The EEAG-amine adducts (EEAGAs) were prepared by reacting EEAG with varying amount of diethanolamine (DEoA) which are neutralized with acid and dispersed in deionised water to give stable dispersion for cathodic electrodeposition (CED) coatings[12]. The dispersions were cathodically electrodeposited on phosphated steel panels and thermally cured to give uniform coating. The coatings were evaluated for different mechanical, chemical and corrosion resistance properties. The coatings were evaluated for their thermal properties using thermo gravimetric analysis (TGA). The final properties of the coatings were found to be affected by the amount of amine reacted with epoxy. The coating films showed good overall performance properties for their use in coating industry. The typical properties of the resin are given in Table 1.

Table.1: Properties of epoxy resin used in the study

The prepared adduct is cooled to 30°C and neutralized with 20% formic acid solution to 90% degree of neutralization. The neutralized resin was transferred to separate vessel equipped with high speed stirrer and dispersed in deionised water under high speed stirring (2000 rpm) to give stable dispersion with 12% solid content.

Curing of the test panels is an important step, because the properties of the final film will be governed by the extent of cross linking reaction which occurs. The novel curing mechanism was analyzed in detail using sophisticated instrumentation. The rinsed electrodeposited test panels were cured thermally in a constant temperature oven at different predefined stoving schedules of temperature from 130°C to 260°C for 30 minutes. The curing was measured according to ASTM D 5402 by the MEK double rub test.

The prepared dispersions were cathodically deposited on to the test panels having an electrodeposition (ED) bath – the chemical and solvent resistant rectangular plastic tank having dimensions 6 in.×6 in.×3 in. (length×height×width) DC power supply, magnetic stirrer and temperature controller system. The anode (stainless steel plate of dimension 150mm×750mm×0.3 mm) and the test panel were connected to the anode and cathode terminal of the DC power supply respectively. The dispersions were filled in the bath and the panels were deposited at potential of 75V for 120 s under constant stirring at 25°C. The panels were then rinsed with deionised water to remove cream coat and were given flash off for 10 min at 80°C. The curing of the coatings was done at 220°C for 30 min to give coherently cured films on the test panels.

Properties of coatings

The cured films on the test panels were evaluated for different mechanical properties such as adhesion (ASTM D 3359), flexibility (ASTM D 522), impact resistance (ASTM D 2794) and Scratch hardness (IS 101: 1964). The films were also analyzed for chemical resistance (ASTM D 1308) by dipping the panels in 5% NaOH solution and 5% H2SO4 solution for 96 h while In hot water and lubricating oil (2T engine oil) for 7 days. The solvent resistance was analyzed by double rub test with methyl ethyl ketone (MEK) (ASTM D 5402). To evaluate corrosion resistance of the films the salt spray test (ASTM B 117) and humidity resistance (using controlled condensation) tests were carried out on the test panels for 200 h then the performance was rated according to the standard method (ASTM D 1654) for evaluation of panels subjected to corrosive environment.

Conclusion

The results of film properties show that cathodic electrodeposited films of coating compositions made from water soluble epoxy-amine adducts had better adhesion, flexibility, impact resistance and water resistance than the cathodic electrodeposited films of coating compositions made from epoxy-amine adducts. The characteristic medium band at 905.08 cm−1 of C–O stretching of epoxy group is observed in the spectra of film. This reveals that the copolymer can be cured through ring opening polymerization of epoxy group. In a futuristic look ahead, we might imagine the significant contribution of next generation cathodic electrodeposition coatings to smart coatings capable of diagnostic or time dependent and responsive.

References

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Author Details

B. S. Suryawanshi

Kansai Nerolac Paints Ltd.

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