Water-soluble epoxy resins for surface coatings

Water-soluble epoxy resins for surface coatings

Progress in Organic Coatings, 11 (1983) 167 - 197 WATER-SOLUBLE 167 EPOXY RESINS FOR SURFACE COATINGS K. KRISHNAMURTI Re~ona~ Research Labo~to~, ...

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Progress in Organic Coatings, 11 (1983) 167 - 197

WATER-SOLUBLE

167

EPOXY RESINS FOR SURFACE COATINGS

K. KRISHNAMURTI

Re~ona~ Research Labo~to~,

~yde~bad

500 009 tIndia~

Contents 1 Introduction. ............................................. 2 General requirements for water-soluble epoxy resins. ................... 3 Binders for anodic deposition. .................................. 3.1 General considerations ................................... 3.2 Maleinized epoxy resin fatty acid esters ........................ 3.3 Maieinized fatty acid epoxy resin esters ........................ 3.4 Partial esters of dicarboxylic short oil epoxy esters. ................ 3.5 Epoxy esters modified with vinyl monomers ..................... 3.6 Epoxy ether resins. ..................................... 3.7 Epoxy resins modified with graft copolymers .................... 3.8 Epoxy resins modified with urethanes ......................... 3.9 Epoxy resins modified with other organic compounds. .............. 3.10 Epoxy resins modified with natural resin ....................... 3.11 Two-component epoxy systems ............................. 3.12 Curing with crossfinking agents. ............................. 3.13 Electrodeposition of binders. ............................... 4 Binders for cathodic deposition ................................. 4.1 Generai considerations ................................... 4.2 Epoxy resins containing amino groups ......................... 4.3 Epoxy resins containing ammonium groups. ..................... 4.4 Epoxy resins containing sulphonium or phasphonium groups. ......... 5 Binders for anodic or cathodic deposition (amphoteric type) .............. 6 Conclusions. ............................................. Acknowledgements ........................................... References. ................................................

167 168 169 169 169

171 173

17’7 179 180 181

181 183

183 183 184 186 186 187 190 191 191 193 193 193

1. production The use of water-soluble resins has increased significantly in recent years in the surface coating industry. Motivation for the growth of such resins has arisen from their ease and cleanness of application and their low odour levels, toxicity and ~~~rnab~i~. The air pollution caused by solvents and their bad effects on human health led to the advent of Rule 66 in the mid 1960s and increasing regulations on pollution control. Consequently, the surface coating industry is being forced to adopt low-solvent and water-borne coating technology. In addition, the ever-increasing prices of coating solvents is another reason to accelerate the changeover to water base/soluble coatings. 0033-0655/831$9.80

@ Elsevier Sequoia/Printed in The Nethertands

168

The rapid development in electrodeposition (ED) (both anodic and cathodic) techniques as commercial methods of paint application has further promoted the use of various types of water-soluble binders. Electrodeposition of water-soluble organic coatings on metal substrates has been known since the early 1930s [l, 21. It was only in the 1960s that the ED method of painting on electroconductive substrates made an important breakthrough in industrial painting. The rapid development of ED is due to the following features. - deposition of protective film in highly recessed areas, - uniform coating thickness, good coating edges and no run-off of film, - maximum utilization of coating materials, - use of water as medium which virtually eliminates fire and pollution hazards, - easily automated, application process, - over-all saving in materials, labour, facilities, investment, electric power, etc. is about 20 - 30 per cent compared to spray, electrostatic spray or dip coating, painting methods [3], and - electroconductive substrates such as iron, steel, aluminium, copper, zinc, brass, tin, nickel and chromium as well as pretreated metals may be coated [4]. Earlier, water-based industrial maintenance coatings were more highly priced than solvent-based counterparts. The situation is now reversed due to the rapid rise in the price of oil feed stocks. The excellent record of two-component epoxy solvent-based systems in industrial maintenance and marine coatings has led to the use of water-soluble epoxy coatings. Thereafter, the progress of these coatings is rapid due to their good chemical and corrosion resistance when applied to non-phosphated steel substrates by the ED process. In earlier review articles, the mechanism and fundamental physical aspects of electrodeposition of paint [ 51, the chemistry of binders for ED [ 61, water-soluble thermosetting organic polymers [ 71, water-soluble alkyd coatings [ 81 and electro coating processes [ 9, lo] are described. The chemistry of water-soluble epoxy resins, their synthesis, constitution, behaviour in ED, crosslinking reactions and properties of unpigmented and pigmented systems are covered in this review.

2. General requirements for water-soluble epoxy resins They are obtained by introducing hydrophylic groups into the epoxy polymer backbone and by subsequent neutralization with bases in the presence of. water-miscible cosolvents. Clear or pigmented aqueous solutions are converted to water-insoluble coatings by oxidative or condensation polymerization. In general, amino, phenolic or isocyanate resins are used as crosslinking agents.

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Poor hydrolytic and pH stability, reduction in viscosity and precipitation of the resin fractions during storage are possible difficulties assodamd with water-soluble epoxy resins, The main cause for this instability is the hydrolysis of the ester groups catalysed by the neutralizing amine in the presence of water. Hence, tertiary amines are preferred to primary or secondary amines, as they are unlikely to attack the polymer system through ammonolysis and/or depolym~~a~on [ 11 f . The use of water-soluble epoxy resins introduces film defects such as ‘crawling’ and ‘cratering’ due to improper wetting of the substrate. Therefore, small amounts of organic watir-miscible alcohols and cellosolve solvents md surface-active agents 1121 are often used to control the flow and wetting properties of the polymers in use, One of the most spout factors in designing these resins is hence the careful selection of raw materials and proper formulation. A number of chemical approaches to water-soluble epoxy resins that meet the desired requirements were explored and will be discussed in the following sections. 3. Binders for anodic deposition 3. I General considerations In an anodic ED process, the article to be coated is made the anode of an electrical circuit and is immersed partially or wholly in an aqueous resin system. Resins employed in this process have carboxyl groups, which on ~~utr~ization with ~rnoni~ an organic amine or an alkanof amine become ionic. When electric current is applied across the electrodes, the dispersed resin is discharged on to the anode to which it adheres, As the coating becomes sufficiently thick, it behaves as an electrical insulator and the current ceases to flow. The anode is then withdrawn from the solution, washed and baked to provide a durable coating. Several types of binders are suitable for anodic ED; no one system is better than the others in every respect. A class of binders of great importance for ED is the epoxies, i.e., epoxy esters, epoxy ethers, epoxy phenolic resins and other epoxy derivatives. Modified epoxy resins have considerable applications in conventional coatings technology. They exhibit good hardness, flexibility and corrosion resistance. Therefore much work has been done to utilize the properties of this class of resins for ED. 3.2 Maleinked epoxy resin fatty acid esters Drying oil fatty acids readily react with the epoxy goup of an epoxy resin to form a fatty acid epoxy polyester. The remaining hydroxyl groups in the resin and the newly formed hydroxyl groups can be further esterified with additional fatty acids [ 13,141. The esterification is fast in the presence of a tertiary amine or a sulfonium salt at temperatures above 100 “C [ 15,X6]. This fatty acid epoxy ester, having no free hydroxyl groups, is then maleinized in the presence or absence of an eladinization catalyst, Xt becomes water soluble on neutralization with a tertiary amine (see Fig. I ).

170

l

HZ!-

1 SUCCINIC ADDUCT H2F yH_ ? O=:. y?),

9

fH -FHum

1 DELS-AIDE&

fso

‘p&7” -CH-c f” CH

E” CM //O

CH2- c
ADDUCT

Y- FH? P

oq

F-O R

+n -FH-CC

RCOOH (UNSATURATED FATTY ACID)

//O \

IH /O :n -cH-c,o 4H2)5 in3

Fig. 1. Maleinized epoxy ester of unsaturated fatty acids.

Failure to esterify all the hydroxyl groups in the epoxy resin results in gelation at the maleinization stage [ 171. If the epoxy resins are esterified with a molar excess of unsaturated fatty acids before maleinization, nongelled products are obtained, which give clear solutions in water on neutralization [ 181. This type of maleinized epoxy resin fatty acid ester dries by oxidation and has greater hydrolytic stability in the aqueous phase because of the polymer back-bone having essentially ether, rather than ester linkages

WI.

Such resins when compounded with fumed silica (so that the silicon content is 5 per cent by weight of the binder content) yield water-soluble binders which can be electro-deposited on phosphate-coated steel substrates

ia.

Epoxy resins can also be esterified with a combination of linseed oil fatty acids [21] or ricinoleic acids and an aromatic monocarboxylic acid. This ester is then maleinized, followed by reaction with ammonia, to yield water-soluble products, suitable for coating by ED over aluminium, nickel and phosphated steel substrates [ 221.

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Another way of synthesizing water-soluble malein’lzed epoxy resin fatty acid esters is to partially esterify the epoxy resin with dehydrated castor oil (DCO) or tall oil fatty acids and to subsequently esterify with a hydrolysed adduct of maleic anhydride and unsaturated fatty acid [ 23 - 251. Addition of 0.15 per cent triphenyl phosphite improves the colour of the product. The primer paints based on this resin system and iron oxide could be electrodeposited at a continuous voltage of 100 volts. The films, baked at 170 “C for 30 min, exhibit good corrosion resistant properties [26 1. The electrodeposited films when baked have excellent properties [ 271. These resins may be blended with melamine and resols based on phenolic acids. The bath stability of these resins is limited by the occurrence of two reactions. First, the solubilizing carboxyl groups are eliminated by the hydrolysis of ester linkages. Secondly, an increase of the molecular weight takes place due to autooxidative crosslinking reactions. The bath stability can be improved by preparing linseed fatty acid esters using tin octoate as catalyst and then maleinizing with maleic anhydride (5 per cent of the weight of epoxy ester) using butyl oxitol as cosolvent. The product, after neutralizing with triethylamine, is soluble in water and has good bath and pH stability [28]. The ED primers prepared using red iron oxide, titanium dioxide, clay and strontium chromate pigments stoved at 135 “C for 30 min have good anticorrosive properties with good adhesion, hardness, impact strength and appearance [28]. Bath stability can also be improved by partially esterifying the epoxy resins with o-branched monocarboxylic acids such as versatic acid [ 291. In another modification, epoxy resin with a molecular weight of 900 could be esterified with a mixture of castor and tall oil fatty acids by a solvent cooking process till the acid value reached 14 - 15. This mixture is then treated with butanol and fumaric acid to complete the Diels-Alder adduct and is then reacted further with styrene in the presence of cumene hydroperoxide. On neutralization with ammonia this product is water soluble and suitable for coatings by ED [30]. Epoxy resins having better compatibility with aminoplast precondensates are obtained by esterification of maleinized epoxy fatty acid esters with diols or propylene oxides [ 30, 311. Incorporation of water-soluble phenolic or aminoplast resins in the water-soluble epoxy resins greatly improves the film durability [32 - 361. When these coatings are applied electrophoretically on zinc phosphated steel sheets and baked at 170 “C for 30 min, they can resist 160 hours in salt spray as per ASTM [37]. The results show that phenoplast combinations performed better than aminoplast combinations in corrosion tests. 3.3 Maleinized fatty acid epoxy resin esters Drying oil or its fatty acids can be maleinized first, followed by the partial esterification of an epoxy resin. The resulting maleinized fatty acid epoxy ether esters have different properties when compared with the maieinized epoxy fatty acid esters [ 38 - 411. These type of esters are reported to be hydrotically stable [42] (see Fig. 2).

172 UNSATURATED ,

FATTV ACIDS

I+ MALEIC AN~~V;DE

SUCCINIC ADDUCT

DIELS ALDER ADDlXIT

I

I

I

I . EPOXY RESIN CH2- CH v I P

CH M b 1

on I

(;H2)6 t”

Fig. 2. Epoxy ester of maleinized unsaturated fatty acids.

Maleinized fatty acids are prepared by the reaction of maleic anhydride with unsaturated fatty acids on heating the mixture at 200 “C for four hours. The product is believed to be a mixture of a bicyclic anhydride derived from 1-4 addition and a substituted succinic anhydride by 1-2 addition. These anhydrides have a high acid value and are economical to use in preparing water-soluble epoxy binders. The fatty acids used in this type of binder are generally of linseed, DC0 and tall oils and/or their mixtures [43]. The epoxy resins used are of bisphenoLAdiglycidy1 ether type, having molecular weights in the range 380 - 2700 [ 441. Maleinized (8 per cent) DC0 and linseed fatty acid esters of bisphenolA-diglycidyl ethers with molecular weights of 900 - 1400 are suitable for coating by ED and other conventional methods. The coatings baked at 130 “C for 30 min have excellent mechanical properties [45]. The primers formulated using inert pigments such as synthetic iron oxide, titanium dioxide and carbon black, and active pigments such as lead chromate, basic lead silicochromate and zinc phosphate at 20 pigments volume concentration (PVC) have good storage stability baking characteristics and their films showed excellent mechanical and corrosion resistance. The paints based on synthetic iron oxide alone performed better than the conventional solvent-borne anticorrosive primers [ 451. The effect of modification of maleinized DC0 fatty acid epoxy resin esters with polyethylene glycols, acrylic acid, ally1 alcohol and urethanes on

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the film properties of both unpigmented and pigmented systems are reported elsewhere [ 461. The primers based on acrylic- and allyl-modified epoxy esters withstand 1000 hours in a corrosion cabinet and 750 hours in salt spray. Polyethylene glycol-modified epoxy resins have inferior anticorrosive properties. Water-soluble binders are also obtained by reacting maleinized (30 per cent) linoleic acid with polyglycidyl ethers of 2,2 bis(4-hydroxy phenyl) propane and tert-butyl phenol [47]. Water-soluble epoxy resins can be produced by condensing the itaconic or fumaric adduct of Ci,,-Cl4 unsaturated fatty acids with epoxy resin [38, 481. These esters are reported to possess good alkali and detergent resistance, pH stability, hardness, impact resistance and salt-spray resistance [ 28, 491. A fatty acid mixture of oleic and linoleic acids can be reacted with refined linseed oil and fumaric acid and then with epoxy resins till the acid value of 94 is reached. This resin blended with 10 per cent methylated methyl01 melanine resin gives flexible films by spraying on steel panels bonderized with zinc phosphate [ 381. In another synthesis of water-soluble epoxy resins, phenolic resols are reacted with an epoxy resin and then with a mixture of linseed oil, DC0 and maleic anhydride [ 50, 511. These products are solubilized in butyl cellosolve and neutralized with triethyl amine prior to dilution in water [52]. During ED of these resins, it is necessary to use baths with screened electrodes (dialysis cells) since the electrical resistance of electrodeposited epoxy films is fairly high [ 531. 3.4 Partial esters of dicarboxylic short oil epoxy esters The short oil epoxy resin esters can be made water dilutable by reacting them with polybasic anhydrides such as maleic, phthalic or succinic anhydrides, followed by neutralization with amines. They form half esters with monoanhydrides provided oil length, method of esterification, anhydride addition and reaction temperatures are properly adjusted (see Fig. 3).

“2:

P f*O R

7” OH

My

Cd=0 L

L-0 coon

s cn-

coon

OH

7”’

edI d

Fig. 3. Half esters of dicarboxylic short oil epoxy esters.

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During the partial esterification stage of epoxy resins with drying oil fatty acids, the following reactions take place: - carboxyl/hydroxyl esterification - carboxyl/epoxide esterification - epoxide/hydroxide etherification -addition reactions between the olefinic centres of these fatty acids 1541. Etherification reactions lead to higher molecular-weight branched structures and higher viscosities. By the addition of alkaline materials, sodium carbonate or sodium salts of fatty acids, the dissociation of fatty acids is promoted and the carboxyl ions formed react more readily with epoxide groups and promote esterification at the expense of etherification [ 551. Naser and Ramadan 1561 claimed that the type and concentration of catalyst were responsible for the esterification of epoxy resins with various monobasic fatty acids. According to them, lithium hydroxide and zinc oxide catalysts are most effective at a concentration of 0.375 element equivalent per 100 g epoxy resin. Increasing the unsaturation of monobasic acid promotes the rate of esterification. The temperature of the reaction has a significant effect on the acid value-viscosity relationship of epoxy ester. At temperatures over 200 “C in the presence of base catalysts, the epoxide/hydroxyl reaction takes place and results in branched chain formation, leading to high viscosity [ 571, The use of strong esterification catalysts such as zirconium octoate and higher reaction temperatures (230 “C - 280 “C) is recommended to promote epoxide/carboxyl reaction during the early stages of the reaction, thus reducing the chain branching. Water-soluble epoxy esters of this type are prepared in two stages. In the first, an epoxy resin is esterified with drying oil fatty acids so that the resulting ester possesses mainly secondary hydroxyl groups, which’ are further reacted with cyclic mono-anhydride in the second stage [55, 56,581. The following variables have considerable effect on the quality of the final product [55]: - type of anhydride, - reaction conditions, - concentration of the anhydride, - acid value of the epoxy ester, - oil length of the epoxy ester, - type of fatty acid used. The optimum recommended temperatures for addition of phthalic anhydride are 130 “C - 140 “C. At lower temperatures the formation of phthalic anhydride diester is greater and thus the water solubility of the resin is reduced. A marked difference exists in the reaction speeds of various anhydrides used in the second stage of the reaction. The hexahydrophthalic anhydride reacts faster than succinic anhydride and its derivatives, while phthalic anhydride is the slowest of all such anhydrides. Excessive amounts of free fatty acids in the epoxy semi esters have detrimental effects on the

175

behaviour of the water-soluble binders on their application by ED. Several amines such as diisopropanolamine [ 551, triethylamine or a-methyl piperazine [58] and dimethylethanolamine [45] are recommended for neutralizing these resins. Water-soluble driers shorten the drying time. Manganese acetate is found to be the most effective one when used in the proportion 0.03 per cent metal/resin since higher concentrations result in its precipitation [ 561. In one of the methods, an epoxy resin is first esterified with 0.4 mole equivalent of lauric acid, then reacted with succinic anhydride and reduced to 80 per cent non volatiles with buthyl cellosolve. This material is combined with hexamethoxy melamine to provide a water-borne baking composition [ 591. In some formulations azelaic acid and trimellitic anhydride can also be used instead of succinic anhydride [60]. A similar epoxy binder can also be obtained by replacing lauric acid with safflower fatty acids and/or hydroxy stearic acid [61]. Short oil epoxy esters of drying oil fatty acids have acid values 50 - 100 when reacted with phthalic or any other unsaturated poly carboxylic acids or anhydrides and are water soluble on neutralization with a volatile base [62,63]. Such resins are suitable for coatings inside beverage cans [62,64]. A British patent claims the production of a water-soluble epoxy modified resin having acid value 58, by heating the epoxy resin with linseed fatty acids at 220 “C followed by the addition of phthalic anhydride [65]. The resin, when pigmented with micronized iron oxide, titanium dioxide and lead silicochromate (1:3:3 by weight), has good bath stability in electrodeposition at medium and high turnover rates. However, the stability may not be so satisfactory under conditions of low turnover rates [ 551. This paint has good adhesion, hardness, flexibility and corrosion resistance on zinc phosphated mild steel when deposited at the anode from a 10 per cent bath (110 V, 2 min) and cured for 30 min at 150 “C.

Storage and hydrolytic stability Storage and hydrolytic stabilities are problems associated with these resins. Their shelf life is improved by defunctionalizing the oxirane groups with anhydrides [ 54,66,67]. Sometimes such polymers do not retain their water solubility for acommercially adequate length of time. Turpin [68] was able to identify the causes of this instability and indicated techniques for the synthesis of extremely hydrolysis-resistant polymer species. According to him, the steric factor and the anchiometric effect have direct bearing on the resistance to hydrolysis. Epoxy resins of the bisphenol A-epichlorohydrin type contain potentially two primary hydroxyl and a number of secondary hydroxyl groups, depending on the molecular weight of the resin. If the epoxy resin is completely esterified to zero hydroxyl content and carboxyl groups are introduced at sites far from ester groups, then it would have the most stable structure. Any weakness in the structure can be attributed to the two primary

176

ester groups whose concentration decreases with increasing molecular weight of the epoxy resin. Fully maleinized fatty acid esterified epoxy resin is an example of this type of resin. Partial epoxy esters are less resistant to hydrolysis due to the presence of weak hydroxy esters, ff carboxyls are produced using anhydrides, the unstable acid-ester group is formed and hydrolytic resistance suffers further. Thus the combination of primary ester, hydroxyester, and acid ester in an epoxy resin produces very poor hydrolytic resistance [68]. The hydrolytic resistance of glutaric half esters is 1.5 times greater than that of succinic half esters [42,69, 701, whereas phthahc half esters hydrolyse readily. Salt-sprayresistance Salt-spray resistance is still an important part of paint testing specifications [71], not withstanding the doubtful practical significance of this test. development work on wa~r-soluble epoxy binders frequency includes the tasting of coated panels for their resistance to salt spray. The main reason is that paints based on such binders are being increasingly used as primers in automotive industry. Van Westrenen [ 723 has studied the behaviour of water-soluble epoxy coatings under salt-spray exposure and highlighted the difficulties in relying on the knowledge of the nature of a resin binder to predict the degree of corrosion inhibition that is likely to be given by a paint based on it. The general design of epoxy ED binders having excellent corrosion resistance for steel is based on the following considerations: - preparation of binders with hydroxyl-rich backbones on epoxy resins, -reaction of this backbone with cyclic anhydrides for the desired wa~r-solubility characteristics, - combination of these binders with small amounts of curing resin, such as melamine resins, for a sufficient degree of crosslinking and a high level of hydroxy groups (200 - 400 mole equivalents OH/100 g cured film). Based on the above considerations, terminal hydroxy groups can be introduced into the solid epoxy resin (molecular weight 900 or 1400) by reacting the epoxy groups with stoichiomet~c qu~tities of hydroxy acids (lactic or dimethylol propionic acid). The resulting hydroxyl-rich ‘backbones’ (8 - 12 hydroxyl groups/molecule) are then reacted with cyclic anhydrides to introduce the carboxyl groups necessary for solubilizing in water. They are suitable for highly corrosion resistance primers, but are unsuitable as outdoor finishing coats because of their tendency to chalking [ 741. Short oil epoxy esters prepared from rosin acids in a ratio of 0.5 - 1.0 acid equivalents of rosin acids per epoxy equivalent can be reacted with unsaturated fatty acids and cyclic anhydrides. Primers containing red iron oxide and titanium dioxide pigments electrodeposited on bonderized steel panels show very good resistance towards salt spray, even in the absence of chromate pigments, and also resistance to alkali [75]. Primers in the presence of an alcoholic moderator can also be applied by roller, spraying and dipping [ 761.

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3.5 Epoxy esters modified with vinyl monomers Water-soluble binders with special properties can be obtained by reacting the epoxy esters with polyvinyl resins. Epoxy resin first esterified with methacrylic acid-styrene copolymer and then with maleinized linseed oil fatty acids represents a styrenated type of binder. Such a styrenation method offers the following advantages over the classical method of post styrenation or the use of styrenated fatty acids [77] : - easily controlled esterification reaction, - better conversion of styrene-methacrylic acid copolymerization than of the ester-styrene polymerization, - stymnation via acidic polystyrene scarcely restricts or influences further reactions (via) double bonds, and -permits a much wider choice of fatty acid, including the saturated ones. The styrene-methacrylic acid copolymers recommended in these systems are of narrow molecular weight distribution and fairly low molecular weight. Epoxy resins modified with styrene-methacrylic acid copolymers and maleinized drying oil fatty acids become water soluble on neutralization with triethanolamine. This system is claimed to give enhanced throwing power of 9.75 inches in an ED bath at 290 V for 2 min [78]. When styreneally1 alcohol copolymers are reacted with the maleinized fatty acid epoxy esters, they yield resins suitable for ED. If maleic adducts are replaced with fumaric adducts, the products gel [ 791. In order to prepare a hydrolytically stable backbone having pendant ally1 ether groups, a copolymer of methyl methacrylate and ally1 glycidyl ether is made and reacted in a mole ratio with trimethylol propane diallyl ether. The carboxyl groups are introduced by alkaline hydrolysis of methyl ester groups in the copolymers [80]. Air dried films have less tendency to yellow and excellent stability to salt solutions when the resin is modified with drying oil epoxy esters. Pure partial ally1 ethers of polyhydric alcohols are required for this work. Partial ether alcohols are prepared by reacting the polyols with ally1 bromide in the presence of aqueous sodium hydroxide

[811. Water-soluble epoxy ester may be prepared by the reaction of an epoxy resin with soya fatty acid diethanol amides in the presence of boron trifluoride, followed by further reaction with styrene and acrylic acid. Claims have been made for water-soluble epoxy esters prepared by etherifying epoxy resin with ethanol and further esterifying with linseed fatty acid and styreneacrylic acid copolymer [ 82, 831. They have better stability in alkaline conditions. Styrenated epoxy resin combined with chromic acid anhydride or phosphoric acid gives baking primers for the corrosion protection of steel [84, 851. The copolymerization products of epoxy ester of unsaturated carboxylic acid with methyl methacrylate, butyl acrylate or styrene become water

178

soluble when esterified with long chain unsatura~d fatty acids and subsequently maleinized [ 36 - 391. The addition of cu-isobutyl acrylic acid during condensation of epoxy resins with fatty acid leads to the formation of watersoluble bright weather-resistant epoxy oligomers [ 901. Copolymerization of epoxy drying oil fatty ester with styrene and acrylic acid and other vinyl monomers in the presence of an initiator such as di-tert-butyl peroxide and a chain terminating agent (dodecyl mercaptan) gives water-soluble vehicles having superior ah-round properties [91,92]. These products show good bath stability, good adhesion, high resistance to corrosion and low loss of weight on baking. Epoxide esters reacted with one or more vinyl or vinylidene compounds in the presence of vitiator, preferably cobalt salts or the steel walls of the reaction vessels, and reacted further with ammonia produce water-soluble epoxy resins. These can be applied mechanically on wood or by electrophoretic means on non-ferrous metals [ 931. Epoxy resins modified with (a) DC0 fatty acids and copolymers of styrene-methacrylic acids (Fig. 4) and (b) Diels-Alder adducts of DC0 fatty acids and methacrylic acid copolymers (Fig. 5) are suitable for preparing paints which can serve as a single coat system functioning both as primer as well as top coat [ 941. These paints withstand 1500 hours in a corrosion cabinet and 750 hours in a salt spray. The creep of rust under the paint film OH -a-

2-o-7=0

cn2 - y4

TCH3

OH HC--cc ,C!+C6tfn

HOOC-(CH&-HC’ k-Or

F 1,1:,3 c+o bH

Fig. 4. Epoxy ester of maleinized DC0 fatty acid and copolymer crylic acid.

-“-cn2- on :”

fi

A LO ,CH,-

MXK-

ccy17

-c<

CH.P;CH

Fig. 5. Epoxy esters of Dids-Alder

I CH

2, ,CH-(CH~,~-CHJ

of styrene and metha-

-o-c*0 F”cnz OH H3C-_(H2C)5-C* g

1

y-W3 i”’ :” (CH2), I cow

adducts of DC0 fatty acid and methacrylic acid.

179

for this duration of test is insignificant. The gloss of the paint film based on lead chromate at 10 PVC is good and is retained for six months. 3.6 Epoxy ether resins The amionic ED binders based on epoxy ester resins have a limited bath stability because of the hydrolysable ester groups present in the system. Therefore binders based on epoxy resins have been developed by etherification of epoxy resins with unsaturated alcohols which do not show this disadvantage [ 21, 951. Bisphenol A-epoxy resin can be etherified with an unsaturated fatty alcohol or ally1 alcohol using Lewis acids as catalyst. Crosslinking reactions due to self etherification of the epoxy resin can be avoided if alcohols are used in excess. The partial ally1 ethers of epoxy resins can be further modified with maleinized adducts of linseed or DC0 fatty acids [ 94,961.

Similarly the safflower fatty acid alcohols can be etherified partially and subsequently esterified with maleinized adducts of drying oil fatty acids. ~O-CH~-FHHCH~-Q-CHZ_R OH

1

0-F

a0

Lo

OH

F :” F”-COO” jH--CH=c+-(m),-

WOC-(ocL)t 043

-a,

+

-

~c”-cH2,.-C”3

cn-cn

ii” CH I (5”2)7

The unsaturated fatty acid epoxy ethers can be grafted by hard and soft carboxyl groups containing vinyl monomers (Fig. 6).

180 H2C \J

CH

M

l

R-o”

-R-o-

CH2-:n

I R-

v OH

OH

STVRENEACRYLIC CO- POLYMER

ACID

O-CH2-~FH~ OH

J

STYRENE. ACRYLIC CO - POLYMER

Fig. 6. Vinyl copolymers of unsaturated epoxy ethers.

The neutralized resins are water diluted and stable to hydrolysis. These can be crosslinked with phenoplasts or aminoplast resins [97]. Soft monomers improve flexibility and decrease throwing power, while hard monomers improve throwing power, but give brittle films. Long chain unsaturated alcohols improve throwing power and corrosion resistance when compared with short chain alcohols. Epoxy groups etherified with alkanolamides and/or oxalolines of unsaturated fatty acids treated with maleic anhydride give water-soluble products on neutralization with amines [ 981. 3.7 Epoxy resins modified with graft copolymers One technique by which a carboxyl group can be attached to a polymer backbone via non-hydrolysable linkages leads to the use of the well-known grafting reactions. When 80 parts of high molecular weight bisphenol-A epoxy resin are used as solvent for copolymerization with 20 parts of carboxylcontaining ethylenically unsaturated monomers, interesting graft copolymers are produced [ 991. They have a comb-like structure, with the grafted components being, of short length compared to the length of the main backbone. The composition and the structure of these graft copolymers can be determined using gel permeation chromatography, 13C!-NMRspectroscopy, multiple solvent fractionation analysis and thermal analysis. These products are suitable as food and beverage can lacquers. When graft copolymers, comprising methylmethacrylate, slyrene, ethyl acrylate, acrylonitrile or mixtures of these are introduced in the backbone of a long chain epoxy maleinized drying oil fatty acid ester, water-soluble epoxy compositions are obtained [ 28,100,101]. These are useful as aqueous sealers. Acrylic/epoxy graft copolymers are solubilized in aqueous media by diethanolamine added in the proportion 0.7 - 1.5 moles per epoxy equivalent of the resin [ 1021. New raw materials for water-borne coatings which combine the weathering advantages of acrylics with cured properties of epoxies are reported elsewhere [ 1031. They are made from water-soluble amine functional acrylics. This backbone is formed through a free radical polymerization of common vinyl esters and acids and unsaturated aromatics. This new hybrid is suitable

181

for air-drying industrial maintenance coatings and in low-energy curing operations over a wide variety of substrates. 3.8 Epoxy resins modified with urethanes Urethane-modified anionic epoxy resins are obtained by the reaction of polyisocyanate with an ester of a monohydroxycarboxylic acid and a liquid epoxy resin. The esters primarily resulting on hydrolysis to the free acids are claimed as good ED binders [ 1041. These can also be prepared by reacting epoxy esters of a drying oil fatty acid with toluene diisocyanate (TDI) to give a hydroxyl-containing polyurethane. This intermediate is partially esterified with trimellitic anhydride [ 1051. Similar types of products can also be obtained by the reaction of two moles of a diisocyanate with one mole of a C&s diol and subsequent esterification with maleinized drying oil fatty acid ester of epoxy resin [46]. Epoxy-modified polyurethanes based on the reaction product of drying oils, polyalkylene oxides and isocyanates were reported to have high gloss [106]. According to a German patent [ 1071, water-thinnable air-drying paints giving high gloss films are prepared in three stages. In the first stage an epoxy resin based on the reaction between bisphenol and propylene oxide is prepared and esterified with maleinized linseed oil fatty acids. In the second stage, a hydroxy-terminated polymer is prepared by reacting TDI and trimethyl01 propane, which is then reacted further with the epoxy ester in the third stage. This product is soluble in water when neutralized with triethyl amine. The use of pentaerythritol, polypropylene glycol (molecular weight 3000) and phthalic anhydride in the compositions is also reported [ 106,107]. Incorporation of polyethylene glycol in the polymer system yields better drying, gloss and mechanical properties of the films [108]. Tung oil fatty acids can also be employed in the place of linseed oil fatty acids [ 1091. 3.9 Epoxy resins modified with other organic compounds Many interesting techniques are available for converting hydrophobic polymers into water-soluble forms. In preparing water-soluble epoxy resin, bisphenol-A can be replaced by methyl ester of diphenolic acid, followed by the hydrolysis of methyl ester and neutralization [ 1101 (Fig. 7).

~,_,,-0-([email protected]~~

~O-CHZ-CM &I

:H2 cooe

Fig. 7. Epoxy polymer containing

pendant carboxyl

groups [99].

182

OH

8

n

OOC WO-CH2

-

9H FH ~CH-CH2-O~CoO”

-

OH

Fig. 8. Epoxy acid [ 991.

polymer

h

OH

containing

terminal carboxyl

+ 2 H 2N

Ct3H2 ““:-ICH 0

-u

0

groups through p-hydroxy

benzoic

COOH

I

O

+

0

&H-CH2-NH++COO’

OOC

AM

Fig. 9. Epoxy

polymer

containing

terminal carboxyl

groups through p-aminobenzoic

acid

1991. IV.++ CH- CHz . ‘0’ wvv

HS- Cl42- COOCH3

I

CH-CCH2 I OH

-

S -

1 -

CH-‘“2

-

CH2

-

CH2

-

COOCH3

HOH S -

COO

8

OH

Fig. 10. Epoxy

polymer

containing

terminal carboxyl

groups using methyl

thioglycolate

[991. They display very good storage stability. Methyl esters of para-hydroxy benzoic acid can be reacted with an epoxy resin, followed by hydrolysis and neutralization [ill] (see Fig. 8). Another technique, which makes an epoxy resin water soluble, is to react with pam-aminobenzoic acid at the oxirane group (Fig. 9). Instead of paru-aminobenzoic acid, anthranilic acid can also be used. The resins stable to hydrolysis and their coatings baked for 5 - 10 min at 185 205 “C are resistant to methyl ethyl ketone and have good adhesion, flexibility and blush resistance in steam processing [ 112 - 1141. When methyl ester of thioglycolic acid is reacted with an oxirane ring containing polymer then hydrolysed and neutralized, a water-soluble product is formed [115] (see Fig. 10). A water-soluble epoxy resin can also be prepared by reacting an epoxy resin having an epoxy equivalent of at least 500 with an organic sulphide and a protic acid [116].

An electrophoretic paint binder is produced when dimethylol propionic acid is reacted with a liquid aliphatic epoxy resin [117]. Fast drying compositions are reported when the water-soluble epoxy resin is mixed with silicon tetraacetate [ 118 J. Epoxy resins modified with carboxy terminated polybutadienes are water soluble and suitable for ED [llQ]. Electra-deposited coatings of these vehicles have excellent hardness. Conventional water-soluble epoxy esters and alkyd resins have been compared with liquid polymers made of diene monomers such as butadienes and 1,3-pen~dienes as vehicles for ED coatings [ 120]. A wa~r-soluble paint composition conning epoxy resin, an amine and isobutene-maleic anhydride copolymer is a good primer on baking [ 1211, when mixed with urea or melamine resin. 3.10 Epoxy resins modified with natural resin (shellac) Epoxy resins can be modified with shellac by refluxing in butanolxylene mixtures and can be dissolved in water in the presence of triethanolamine. This vehicle is suitable for water-thinned red oxide primers [ 122, 1231. Primers baked at 150 - 160 “C for 30 min are very hard, flexible and resistant to water, mild alkalis, mineral acids and a few solvents and can also withstand salt spray for 10 days [ 1231. 3.11 ho-comp~ne~ t epoxy systems Two-component air drying systems based on pigmented epoxy resins mixed with a polyamide-epoxy adduct and then dispersed in water have special interest for coating large metal structures [ 1241. The process has the limitations of a two-component system, together with paint loss due to dragout. The loss can be reduced to an acceptable level by working at low solids content. Curing of the paint films takes place even under water. The films withstand 250 hours in a salt spray and a humidity cabinet and their chemical and solvent resistance is of a high order. A three-stage process is reported for converting epoxy resins into ecologically acceptable war-soiubie systems 11251. The first of these involves modifying the epoxy resin with hydrophylic moieties to make the polymer water soluble. The second involves the use of a resin surfactant combination. In the third, the resin-surfactant combination is mixed with a solvent to achieve emulsification. 3.12 Curing with crosslinking agents Neutralized water-soluble epoxy resins esterified with versatic acid and succinic anhydride can be crosslinked with butylated melamine resin of low reactivity [ 1261. In electrocoating of this system, the coulombic yield rises linearly with increasing amount of melamine resin. This shows that unreacted aminoplast is carried into the diffusion layer at the anode by the ionic epoxy ester. The adducts of monoepoxy compounds and poly~~o amide condensation products of an alkylene polyamine and unsaturated fatty acid polymers

184

can be used as water-thinnable crosslinking agents [127 - 1281. When these are combined with water-dilutable epoxy resins, the coatings have the following properties [ 1291: - free from flash rusting, - acceptable salt-spray resistance, - acceptable acid, alkali and water resistance, - ability to coat on damp masonery surfaces, - mild chemical resistance. They can be used as industrial maintenance primers, enamels and glaze tile coatings. Combinations of epoxy polyether with an alkoxy melamine and methyl ricinoleate give ED binders after saponification of the ester functions- [ 1301. Anionic aminoplast resins are used for curing water-soluble coating compositions formed by the reaction of a polyepoxide, a primary amine and a polycarboxylic acid [ 1311. Anionic phenoplast precondensates containing carboxylic groups are useful ED binders. They are obtained by transetherification of conventional phenol resol ethers with hydroxycarboxylic acid esters and subsequent hydrolysis of the ester functions [ 132,133]. When an epoxy resin is reacted with this phenoplast or with resol based on &cyclic acid ester f134, 1351, self cross linking binders are formed. They can also be prepared by reacting pheno plast precondensates with bisphenol-A-resol etherified with butanol and chemically bound to polyacrylic ED binders [136,137]. When epoxy resins are cured with phenolic or amino resins, the hydrolytic and storage stability and corrosion resistance of the resins are improved [ 1381. 3.13 Electrodeposition of binders In anodic electrodeposition, four electrochemical processes are involved: electrolysis, electrophoresis, electrocoagulation and electroosmosis [ 139, 1401. The following reactions induce the electrocoagulation of the resin: - precipitation of the resin in the vicinity of the anode: RCOO-

+ H+ + RCOOH.

- coagulation

by the formation

of metallic complexes

Me + Me”+ + n en RCOO-

+ Me”+ + Me(RCOO),.

- coagulation dation [ 1411:

by crosslinking

RCOO- + R’ + e- + CO2 2R’ + R-R.

of the polymer

due to Kolbe-type

degra-

185

The significance of the formation of metallic complexes based on the above reactions was studied by Giboz and Lahaye [142]. They electrodeposited epoxy resins simultaneously on iron and platinum anodes at constant current density of 1.5 mA/cm’ and found: - The reactions of coagulation by acid precipitation and by degradation are responsible for the deposition on platinum electrodes. - There are two well-defined periods in the case of iron anodes. During the first (deposition time 2 min), the mechanism is identical to that observed in the case of platinum. During the second, the formation of metallic complexes plays a significant role in the deposition of the resin (37 per cent for a deposition of 10 min). A systematic study of decarboxylation of modified epoxy resin during an electrodeposition reveals that the contribution of decarboxylation of the resin to the mechanism of deposition is 3.25 per cent when the current density is 0.75 mA/cm’ and 13 per cent when the current density is increased to 1.50 or 3.00 mA/cm’ for a 10 min deposition [143]. During the ED of a modified epoxy resin at constant current density on an iron anode, it is shown that the coagulation of resin by metallic ions occurs by flocculation and that during the first period of the deposition, an insoluble iron compound is formed by anodic oxidation. The curves representing the potential difference between the electrodes and the deposition time show two maxima. While the first of these appears to correspond to the beginning of depolarization of the anode (evolution of oxygen), the second corresponds to the beginning of the flocculation of the resin by the metallic ions [ 1441. Electrodeposition of acrylic and epoxy coatings on iron or platinum are independent of current density, but dependent on the type of polymer and the anode material. In acrylic polymers Fe++ formed at the anode does not affect the coating thickness, whereas in epoxy resins, Fe++ contributes to polymer precipitation and increases the coating thickness. The carboxyl group content of the undeposited coating system increases during ED, indicating that macromolecules with carboxyl content below average are preferentially deposited [ 1451. The weight average molecular weight of the acrylic coatings is higher than that of the polymer in the aqueous phase in which the weight average molecular weight remains constant during the ED, indicating the molecular association in the coatings. On the basis of the chemistry of the ED process, galvanic cell potentials and analysis of various phosphated surfaces, three causes for a reduced saltspray resistance of electrodeposited maleinized epoxy drying oil esters coatings are apparent. These are damage to the interface between the sub strate and the coating by the presence of water-soluble salts, the presence of electrodissolved inorganic ions in the coating and the reduction in the thickness of the phosphate coating. The most probable cause of poor salt-spray results is the electrodissolved ions occluded in the coating during the ED process. Better ED coatings can be obtained by minimizing the electrodissolution reactions during the ED process by increased coulombic efficiency [ 1461.

186

Robinson and Tear [ 1471 used Laponite (a synthetic clay) in primers pigmented with iron oxide, titanium dioxide and carbon black and found that its addition prevented preferential pigment deposition in anodic ED at 170 V for 2 min. They also referred to zeta potential measurements for the special role of TiOz surface treatment components, aluminium hydroxide and hydrated silica on the pigmentation of ED paints [ 148 - 1501. Zeta potentials for several TiOz pigments with air drying linseed/wood oil epoxy esters are reported [ 1511.

4. Binders for cathodic

deposition

4.1 General considerations The electrodeposition process and adequate binding media concentrated on anodic deposition, whilst cathodic deposition found little industrial application till 1974. The main disadvantage of cathodic deposition is the limited range and high cost of raw materials. In addition, there are not so many possibilities for the introduction of basic groups into the macromolecules. The cathodic ED process has the following advantages over the anodic one [ 971: - Electrochemical oxidation of the resin at the substrate is not possible because of the absence of nascent oxygen. -Neither contamination nor discolouration of the paint film occurs due to the absence of metal ions from the substrate. - Passivating layers on the substrate are not attacked in the alkaline medium at the cathode region and so the coating has better corrosion resistance. - Cathodically-deposited paint films are expected to protect the substrate against the attack of alkaline agents. Deposition mechanism, the chemistry of binders and properties of anodic and cathodic ED materials were recently compared and reviewed [97, 1521. Cathodic ED resins are either poly cations or neutral polymers. The main reaction involved in cathodic deposition is the electrochemical decomposition of water [6] : H,O+&+

4 H; +OH-.

Amine resins neutralized with acids are discharged and lose their solubility [ 61:

by an acid-base

reaction

+ OHX-

soluble

insoluble

Deposition is caused by the reduced solubility of the amine base compared with the amine salt. In the cathodic deposition process the formation OH- ions is responsible for deposition.

of

187

4.2 Epoxy resins containing amino groups Epoxy resins play1an important role in the synthesis of binders suitable for cathodic ED. Introduction of a sufficient number of amino groups into the epoxy polymers and successive neutralization with acids results in the formation of water-soluble resins (see Fig. 11). H-

N

(

RI-OH Rz-OH

1

;;I+)-Cn,-

;H

NWI

OH 0 -LO-R,

I

m

coon

>N-CH~-CH -$-O-R2 0

Fig. 11. Epoxy-amine

AME, bw

adduct and subsequent esterification [ 1521.

These resins cannot be hardened and they must be esterified with drying oil fatty acid or with carboxylic/acrylic polymers and then blended with phenolic or melamine resins [ 153 - 1551. Epoxy-amine adducts prepared by using primary monoamines, secondary monoamines or polyamines can be made water soluble by neutralizing them with formic, acetic or oxalic acids [156,157]. Acrylic polymers containing epoxy groups such as glycidyl acrylate can also be used as the starting material [ 1581. Amine-modified epoxy resin esters prepared from drying oil fatty acids can give excellent salt-spray ratings on phosphated steel, but poor performance on bare steel. This is due to the fact that cathodic ED prevents the degradation of the phosphate layer and then leads to superior corrosion resistance on pretreated steel [ 731. The reaction product of an epoxy resin and diethanol amine reacted partially with ricinene acid and polyacrylic acid and crosslinked with melamine-formaldehyde resin or a resol type of phenolic resin results in coatings having good corrosion resistance [ 1521. Curing agents such as polyamino amides or polyimidazolines can be added to binders suitable for cathodic ED [ 1591. The presence of free epoxy groups reduces the bath stability, however they cure at very low temperatures [ 1241. It is not easy to produce white or pale coloured films of binders based. on amino functional epoxy polymers, but Reeves [ 1601 reported the synthesis of an epoxy-polyamide system giving white and non-yellowing films deposited cathodically. The paint based on this binder is not recommended for electropriming of automotive bodies because of its poor throwing power. Epoxy resins modified with tetraethylene pentamine neutralized with acetic acid can be combined with etherified hexamethylol melamine. This

188

could be electrodeposited on steel [153]. Amine functional copolymers electrodeposited cathodically and cured at lower than usual baking temperatures are useful as corrosion-resistant coatings [ 1611. Epoxy resins containing high levels of residual hydroxy groups are excellent oil-free cationic binders with good corrosion resistance [ 73 J. The hydrophilic character of resins having oxyalkylene structures can be enhanced by reacting them with boric acid esters containing basic nitrogen atoms [156,162] (Fig. 12). -

o C:-:H2

+ [ ~~,:“‘)s_,,+

a-q

/ EPOXV RESIN MODIFIED WITH -0 OXVALKVLEN GROUPS

-

,C”Z

CHi-

0, B-0-CH2-CH2-N<

-CH2-0’

C”3 C”3

I ‘CH2 STRUCTURE NOT ESTABLISHED

Fig. 12. Reaction of epoxy resin with boric acid esters containing nitrogen atoms [ 1521. EPOXY RESIN MODIFIED WITH OXYALKYLENE GROUPS

N-

R- COOH

Fig. 13. Modification of epoxy resins with ethoxylated acid.

ethanol and tert-aminocarboxylic

Another method of modification is reacting epoxy resin and ethoxylated ethanol with secondary or tertiary amine having hydroxyl or carboxyl groups [156] (Fig. 13). The resin can also be modified by reacting its hydroxyl groups with the amino group containing isocyanates [ 1631: NC0 -

CH -

CH2

c

NH-$-0-CH2-CH2-N<

OH

CH3

0 I

0

0-CH2

-

CH2 c N<

CH3

CH3 CH3

189

These binders can be externally crosslinked with phenoplasts, but difficulties may arise in electrodeposition due to the strong basic nature of the binder [ 1531 which could be reduced by the addition of tricresyl phosphate [164]. Reaction products of epoxy resins with partially capped polyisocyanates reacted with compounds containing primary or secondary amino groups are able to crosslink at elevated temperatures by the reaction of the uncapped isocyanate groups with hydroxy or amino functions of the resin [156,165]. One of the methods of building up self crosslinking systems is to combine a reaction product of one mole of TDI with one mole of capping agent, e.g., 2-ethyl hexanol with epoxy resin having a hydroxy group [ 156,166, 1671. NC0 maw CH’-;H

-

CH2 m + H3C

OH 1 -

CH2-CH

- CH2 cr C,H3

b

I

0

= C-NH

4 0

NH-:

-OR 0

The blocking agent is released on curing and the free isocyanate reacts with hydroxyl and/or amino functions of the epoxy resin [ 1681. The introduction of hydroxyl-containing acrylic monomer into the system increases crosslinkable double bonds [ 163,268] : MIENCHz-

CH- CH2 M OH

+

NH-;-

0-CH2-CH2-O-

6 0

0

NH-;-O0

CH2-CH2-O-

F-C”

-CH sCH2

rCH2

0

The products with terminal amino groups may be capped by the formation of ketimines. Combinations of similar cationic epoxy resins with cationic polyamides and partially capped polyisocyanates give binders that can be deposited aathodically. These can be cured with amino- or phenoplast resins [169, i70].

190

Diprimary amines like CY,o diamino polyalkylene oxide reacted with the epoxy resins and diisocyanates can be cured with precondensed waterdispersible phenolic resins [ 171. TDI half capped with 2-ethyl hexanol can be treated with dimethyl ethanolamine and aqueous lactic acid. The product is thinned with propanol and reacted with epoxy resin and dibutyl tin laurate. It can be deposited cathodically on zinc-phosphated steel panels [ 1721. If lactic acid is replaced by acetic acid in the presence of cellosolve solvents, the coatings have better surface properties and impact strength [ 1731. Further modifications of these products give glossy, hard and flexible coatings. This combination of hardness, gloss and solvent and corrosion resistance is unusual in the case of aqueous coatings deposited by cathodic ED [ 1741. Another group of binders for cathodic ED are epoxide-modified Mannich bases of polyphenols. These were the first cathodically-depositable resins used in Europe. A polyphenol is reacted with formaldehyde and secondary amine (Mannich reaction) and subsequently reacted with a polyepoxide to form a resin [ 1571. This resin has self crosslinking properties due to methyl01 and the Mannich-base groups: iH3 -

0-CH2-

;H -CH2

[email protected]

OH

CH3

7H2 NR2

During curing a crosslinking mechanism similar to that of phenol-formaldehyde resin takes place. With these products pH values above seven in the ED bath can be achieved. They can even be dispersed in water without being neutralized completely [ 1751. 4.3 Epoxy resins containing ammonium g:oups Ammonium groups can be introduced into epoxy resins by reacting them with tertiary amines in the presence of acids having a pK value less than 5. In order to get sufficient water solubility, more than 20 per cent of the nitrogen should be present as +NR4 groups [ 1621: 0-CH2-SH-CH2-N& OH

Bosso et al. [ 1761 have found that water-soluble synthetic resins with epoxy groups and chemically bound quarternary ammonium base groups when solubilized with lactic acid having pK > 5 can be applied cathodically. Water-soluble thermosetting compositions based on epoxy resin reacted with tertiary amine such as N-methyl morpholine, N-methyl pyrrole and alkylated products of amino resins have been developed [ 1771. These coatings can be applied by all conventional methods of application. They have a high

191

degree of ~exibili~ during mach~i~g and s~p~g of the coated articles, corrosion resistance and gloss. The food and beverage can coating industry utilizes this type of thermosetting composition [ 1781. These coatings, having good adhesion, protect the metal walls of the cans from attack by food and beverage and their low extractables and sorption prevent taste adulteration, Completely capped polyisocy~a~s can be used in ~ornb~a~on with epoxy resins containing ~monium and hydroxy groups to give externally crosslinking systems [179], Two anionic ED systems (an acrylic and a maleinized polybutadiene) and two cationic ED systems (an epoxy amine derivative capped with isocyanate and a Munich base) are compared elsewhere f97] (see Table 1). A polyepoxy adduct containing at least two epoxy groups can be reacted with mercaptoethanol. The resulting tbioethers can be alkylated with alkylene oxides in the presence of acids to form sulphonium groups [MO]. These resins can be crosslinked with aminoplasts. They can also be made self-crosslinking binders by reacting free hydroxyl groups with partially capped diisucyanates. In another method, sulfanilic acid is incorporated into amine-modified epoxy resin esters in the presence of other amines, leaving the sulphonic group intact. The binder of this type crosslinks well and leads to coatings with excellent salt-spray resistance both on phosphated and bare steel [ 731. Epoxy resins chemically hound with ternary sulphonium base groups give clear aqueous sohrtiuns when solubilixed with an acid [Ml]. Polyepoxides can be reacted with mixtures of sulphides (diethyl sulphide or thiodiethanol) and acids or phosphines (trimethyl phosphine) and acids. These resins are water dispersible and lose their onium structure at baking ~rnpera~~s* becoming non-polar and water resistant [182]. They have good mechanical properties and corrosion resistance and can also be pigmented in light colours for light coloured paints [ 183 J. 5. Binders for anodic or cathodic deposition (amphoterie type) Technical advantages can be, achieved by changing the charge of the resin from positive to negative and converting anodic systems to cathodic systems. Chemical mo~~cations of the epoxy backbone are now being researched to accomplish this objective. A few advantages are: - insensitivity to pH, - better corrosion resistance, - improved throwing power, - sharp current cut-off and - good pot life. Epoxy binders prepared by Van ~es~e~en 1731 can be modified by reacting the substituted succinic anhydride rings present in the epoxy resin w$th Z-dimethyl aminoethanol into the amino acid functions, i.e.,

193

R-YH--CooH CH2-COO~H2~H,-N’

CH3 \ CH3

Such modified epoxy systems could be electrodeposited cathodically, depending upon the pH.

anodically or

6. Conclusions In this review several types of water-soluble epoxy resins and their modifications described in the literature are discussed, with special reference to their use in electrodeposition and other methods of application. Most of the literature on this subject is cited under patents and the information available therein is insufficient for drawing specific conclusions. It is hoped that this article may help in designing and developing new types of epoxy resins having the desired properties and will also serve as a source of references in this field. Acknowledgements My thanks are due to Dr. M. A. Sivasamban and Dr. M. M. Shirsalkar for their encouragement in writing this review., I also express my appreciation for the useful criticisms of Dr. M. Yaseen and Dr. P. S. Sampathkumaran.

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