Anticorrosive water-borne paints

Progress in Organic Coatings, 15 (1987 ) 3 3 - 5 6

ANTICORROSIVE BARBARA

WATER-BORNE

BIEGAfiSKA,

33

PAINTS

MALGORZATA

ZUBIELEWICZ and EDWARD SMIESZEK

Institute of Plastics and Paints Industry, 44-l 01 Gliwice, Chorzowska 50 (Poland)

Contents Introduction.

.....................................................

33

Classification of water-borne paints ....................................

34

Raw materials used for the production of anticorrosive water-borne paints. ..... 3.1Binders ....................................................... 3.2 Pigments. ..................................................... 3.3 Additives ......................................................

35 37 37 39

Anticorrosive water-borne paints. ...................................... 4.1 Paints based on true solutions of resins in water. ....................... 4.2 Dispersionpaints ................................................ 4.3 Emulsion paints. ................................................ 4.4 Other water-borne paints. .........................................

41 41 46 50

Properties of water-borne paints. ......................................

51

Range of application of water-borne paints. ..............................

52

Conclusions .......................................................

52

References..........................................................

41

52

1 Introduction

Solvents contained in paints and added to dilute the latter allow painting materials to be produced ready for use and adapted towards various coating techniques in the best possible manner. When a paint is applied to a substrate, especially by spraying, solvents evaporate from the liquid material. This occurs when a paint dries at ambient or elevated temperature and it also applies to single- or multi-component paints. The evaporation of solvents is a physical process which is accelerated by an increase in temperature. However, there are exclusions from the rule, particularly when solvents react with the binder and enter into the composition of the coating. Apart from causing atmospheric pollution (since organic solvents are flammable, explosive, of unpleasant odour and sometimes toxic) they are subject to appropriate fire and industrial safety regulations which have to be 0033-0655/87/$8.90

0 Elsevier Sequoia/Printed in The Netherlands

34

observed. Moreover, the price of organic solvents, many of which are derived from petroleum, has risen appreciably over the years. Developments in paint technology have been dictated for quite some time by mutually dependent factors such as coating properties (which arise from paint formulations), manufacturing methods, energy consumption, accessibility to raw materials, etc. Over the past 10 years, greater attention has been paid to environmental pollution caused by the evaporation of organic solvents or other volatile components during paint application and drying. Many technical possibilities exist independent of the paint fo~ulation which could lead to a decrease in the atmosphe~c concen~ation of organic solvents, Le. thermal or catalytic combustion of vapours leaving spraying booths and drying ovens, and absorption, condensation or washing of the evaporated solvents with water. However, as often used in practice, these methods are energy consuming and, for this reason, other techniques have been used to decrease air pollution with organic solvents. Paints with high-solids content or solvent-less products (including powder paints) and particularly water-borne paints require the application of indirect methods of air pollution control. The use of water as a solvent or diluent for paints is advantageous in view of its non-flammability, lack of atmosphere pollution and physiological interaction. Indeed, developments in the production and application of paints containing water began with watersoluble paints capable of drying at elevated temperatures and designed for the dip painting of car bodies to eliminate the fire hazard created by large tanks filled with paints containing organic solvents. Further stages in the development of water-borne paints involved their use for anaphoretic and cataphoretic electrodeposition, and the design of paints and enamels for spray painting which could then be dried at elevated or ambient temperatures. Resins used in water-borne paints are, as a rule, products of polycondensation and polymerization reactions involving monomers containing acidic carboxylic or basic nitrogen groups. For monomers containing carboxylic groups, the resin is solubilized in water through the formation of salts with an amine or ammonia and, for monomers containing nitrogen groups, by neutralization with organic acids. Another possibility for the synthesis of water-borne resins is the introduction of strongly hydrophilic phosphonium or sulphonium functional groups into the polymer chain. Water-borne paints for anaphoretic or cataphoretic electrodeposition are not discussed in detail in this paper as, in the authors’ opinion, these should be the subject of a separate report because of their specific properties. 2 Classification of water-borne paints It is not possible to define water-borne paints explicitly. Thus, in the literature, such paints have been variously interpreted. In general, however,

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the term ‘water-borne paints’ is taken to cover paints which contain at least 80% water as the volatile component [l]. Furthermore, coatings derived from water-borne paints are also subject to different interpretations. Thus, such coatings have been categorized [Z - 41 as (a) water-soluble paints and (b) aqueous dispersions/emulsions. Water-soluble paints are based on true solutions of binders in water. They are transparent mixtures of water and a polymer, which remain clear on further dilution by water. Their ability to mix uniformly with water arises from the presence of specific functional groups introduced along the polymer chain which are then neutralized. On addition of water to such polymers, the latter undergo micellarization generating micelles of a sufficiently small diameter to allow the formation of a transparent solution [2, 5, 61. Although these paints are true solutions, nevertheless they also contain a certain amount of organic solvent, which at its maximum amounts to 20% by weight of the total amount of solvent present. Additives are not necessary to obtain a stable system [‘7, 81. Aqueous dispersions are essentially particles of a solid polymer dispersed in water, whereas emulsions are formed from dispersions of a liquid polymer in water. Aqueous dispersions are often called ‘latices’ and are normally obtained either by emulsion polymerization and copolymerization or by dispersing polymers in water, the polymers having been obtained by other methods [ 21. Paints based on aqueous dispersions or emulsions contain up to 5% by weight of organic solvent relative to the total amount of solvent present [5]. Water-borne coatings may also be classified in terms of the size of polymer particles they contain [4, 9 - 151, i.e. as true solutions, where the size of the polymer particles is
3 Raw materials used for the production of anticorrosive water-borne paints The materials used for the production of anticorrosive water-borne paints, such as binders, pigments and additives, are discussed in this section.

Colloidal solution slightly opalescent small 200 000 moderate, slight influence of molecular weight moderate good to very good dependent upon pigments moderate dependent upon cosolvent type and amount moderate moderate moderate good at lower amounts of solvents good to very good good to very good good to very good good to very good

True solution

transparent none 20000 high, strongly dependent on molecular weight low very good dependent upon pigment

ready ready

moderate to high ready ready very good

good good good good

Property

appearance Tyndall effect molecular weight viscosity

solid content pigment dispersibility stability of pigment dispersion foaming control viscosity control

organic solvent content paint formulation application film formation

gloss mechanical properties chemical properties weather resistance

Comparison of true solutions, colloidal solutions and dispersions

TABLE 1

moderate to good very good very good very good

low complicated difficult coalescent required

difficult requires thickener

opalescent distinct 200 000 low, independent of molecular weight high dispersing agent required good

Dispersion

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3.1 Binders [5, 16 - 181 Most conventional resins used for the production of solvent-based paints may be obtained in a water-soluble form through the incorporation of specific functional groups into the polymer backbone. Such groups are hydrophilic or can be made hydrophilic after appropriate modification. They have been introduced into alkyd, polyester, acrylic and epoxy resins, amongst others. Alkyd resins possess a composition which is similar to that of the conventional material. One important difference, however, is the presence of carboxyl groups in the molecule, leading to acid values ranging from 40 to 65. Alkyd resins are generally modified by means of acrylic and silicone resins. High quality paints obtained from acrylic-modified alkyds are superior to conventional paints, whereas the silicone-modified materials also show improved heat resistance. Polyester resins do not contain fatty acids and, generally, they are based on long carbon-chain dibasic acids, e.g. adipic acid and suitable combinations of polyols. They need crosslinking at elevated temperatures. Three procedures have been developed for introducing epoxy resins into ecologically acceptable water-borne systems: (i) modification with hydrophilic moieties which make the polymer soluble in water; (ii) combination of the resin with surfactants which allow dispersion in water; and (iii) formation of resin/surfactant/solvent systems that can subsequently be emulsified. Each one of these procedures possesses certain specific advantages and disadvantages. Recently, formulations consisting of a water-dilutable epoxy resin in the aqueous phase and either an emulsified or dispersed resin in the particulate phase have been prepared. Both water-soluble thermoplastic and thermosetting acrylic resins have been used for paint production. Anionic water-soluble acrylics can be produced by the copolymerization of maleic anhydride and methacrylic, acrylic or itaconic acids. Cationic resins are produced by using dialkylaminomethacrylates. Amino resins are mainly used as crosslinking agents. There are available as water-soluble urea-, melamine- and guanamine-formaldehyde resins. Many other binders have also been employed, for example butadienestyrene emulsions, carboxylated emulsions of a butadiene-styrene copolymer which had been further modified by the addition of water-borne oligomers, vinyl acetate copolymers with added ethylene or vinyl chloride, vinyl acetate epoxide copolymers with added alkenes, and polyurethane latices, the latter being characterized by special properties such as considerable hardness and flexibility at low curing temperatures.

3.2 Pigments A great problem in the preparation of anticorrosive water-borne paints is the choice of appropriate pigments to simultaneously maintain paint stability and good protective properties. The use of inhibiting pigments with

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too low a water solubility results in a decrease in the protective properties of the coating, whereas the application of more soluble pigments can cause poor paint stability. To obtain paints of good stability the following factors need to be taken into account: pH; precipitation of the dispersing agent by cations of the pigment; and chemical structure which can minimize or eliminate interaction between the cation and the dispersant [ 191. Bierwagen and Rich [ZO] have discussed a method for determining the critical pigment volume concentration (CPVC) as a function of the pigment volume concentration (PVC) and means of testing for anticorrosive properties which depend on the pigment volume concentration of dispersion paints. Pigments commonly used in traditional solvent-borne paints can be employed for water-borne paints, i.e. zinc chromate, zinc phosphate, lead silicone chromate, calcium borosilicate, calcium, zinc, lead, strontium and barium molybdate and barium metaborate [ 16,19, 21 - 251. Tests have been carried out on the application of zinc chromate in a dispersion paint based on an acrylic-styrene resin [22, 261. The effectiveness of zinc chromate depends on the specific solubility of chromate in water. It has been recommended that a zinc chromate solubility of 0.09 g CrO, per 100 ml HZ0 should be used in order to achieve 7% concentration in the formulation. Chromate pigment effectiveness may be improved by the addition of 20% zinc phosphate, yielding a pigment volume concentration in the paint of 24 - 35%. According to Repin [27] such a mixture of zinc phosphate and chromate exhibits a synergistic effect, and is recommended for paints designed for the protection of steel structures in aggressive environments. In contrast, an anticorrosive water-borne paint containing chromium and cupric oxides, and a mixture of iron and chromium oxides, where the ratio of chromium oxide to other metallic oxides is 1:(0.3 - 15), has been patented in Czechoslovakia [ 281. Whereas zinc chromate was previously the most popular pigment used in water-borne paints, because of the attendant health hazard it (together with pigments containing lead compounds) is now being replaced in many countries by pigments of lower toxicity, such as barium metaborate, calcium borosilicate, calcium phosphosilicate, calcium-zinc molybdate, a zinc phosphate/oxide complex, zinc borate and zinc phosphate [21, 26, 291. These pigments exhibit very good properties and give a better protection than chromate or lead pigments. For example, the zinc phosphate/oxide complex pigment shows better salt spray resistance and durability in a marine environment than zinc chromate [ 291. Furthermore, this pigment is stable in any type of binder and exhibits little tendency to ‘flash rust’. The fact that it is white allows a paint of any desired colour to be obtained. Comparative studies have been carried out [30 - 321 of the corrosion resistance of both acrylic-styrene emulsions and acrylic dispersions, pigmented with borosilicates and phosphosilicates. It was found that paints based on phosphosilicates show a diminished tendency to foam and exhibit better corrosion resistance.

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Ellinger [33] has discussed the application of aluminium pigments in aqueous systems. Use of these pigments creates two important problems, viz. the wetting of aluminium particles in an aqueous system and the stability of aluminium in the paint or paste. These problems can be solved in several ways depending on whether the addition of organic solvents is permissible or not for ecological reasons, i.e. the application of dry pigments (although this sometimes gives rise to characteristic problems), the application of conventional pastes in hydrocarbon solvents which requires emulsification of the organic binder, the application of pastes containing hydrocarbons and emulsifier, or the application of solvent pastes which are mixable with water. When the addition of solvent is prohibited, the recently prepared ‘aqueous’ pastes may be used. They contain water and inhibitors which ensure the stability of the paste itself during storage as well as the stability of the paint thus obtained for a predetermined period of time. Due to the chemical composition of the aqueous paste, it is recommended that tests should be undertaken on the complete paint system.

3.3 Additives With water-borne paints, additives play a more important part in comparison to other solvent-borne paints. The choice of additive should be limited to those which have no adverse effect on the water resistance of the binder. Such additives should only be added in amounts sufficient to obtain the required paint properties, i.e. viscosity, stability, antifoaming, etc., since these substances which partially activate the corrosion process are hydrophilic and, as a result, can cause higher susceptibility of the binder to water. They can be washed out from coatings by rainwater making such coatings porous and, at the same time, increasing their tendency to blister and underrrust. The effect of thickening agents on paint properties should also be carefully tested. Polyvalent metal ions derived from active pigments (e.g. zinc ions from zinc chromate and zinc phosphate) can also adversely affect paint stability as a result of their reaction with thickening agent. For example, it has been found that the simultaneous application of an acrylated thickening agent and zinc chromate causes an increase in the viscosity of the product [ 221. The use of thickening agents in water-borne paints has been commented upon adversely recently, and it has been suggested that viscosity control should be achieved by varying the pH of the system. Paint additives also include agents for improving the storage stability of the paint. Hydroxylated polymers, poly(viny1 alcohol), starch and cellulose methylhydroxypropylcellulose derivatives such as hydroxyethylcellulose, and methylcellulose can be used for this purpose [19, 23, 241. Such agents form a protective film on the surface of the pigment and polymer particles to protect them against mutual contact. Copolymers of ethylene and propylene oxides are very good paint stabilizers. However, their amount should not exceed 3% relative to the amount

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of resin solids. Primers containing such additions have been shown to be stable for three months at a temperature of 60 “C [23, 351. These polymers can be also used as dispersing agents in paints, in a similar fashion to agents such as sodium hexametaphosphate. The recommended amount of such polymers in primers is 0.8 wt.% of the total weight of pigment. Emulsifiers are used as primary additives for the production of emulsion paints. The effect of various surface-active agents (anionic, cationic, non-ionic) on the properties of paint coatings has been investigated [ 36 - 391. Lein et al. [40] have found that of many surface-active agents investigated only a non-ionic/anionic agent (a mixture of ethoxylated and sulphonated salts of fatty acids) showed an adverse effect on the properties of air-drying water-borne paints. Coalescents lower the minimum temperature for film formation [23, 411. Examples are n-butyl glycol and some diisobutyl esters. Antifoaming agents play a very important part particularly in the case of paints used for airless spraying. Examples of such agents include longchain alcohols, polyglycols, silicones and higher hydrocarbons [ 23,26,42]. Other paint additives (based on water-borne binders) are those which eliminate coating defects such as ‘flash rusting’ and ‘early rusting’. These defects are caused by improper drying conditions (high humidity, low temperature), the presence of readily soluble ferric salts on the surface to be painted and a reduced film thickness [ 21, 411. As demonstrated by Humphries [ 231, ‘flash rusting’ depends on the pH of the paint. This defect does not occur when the thickness of the coating is greater than 60 pm, provided that the pH of the paint is 5 or less. In the case of white primers, the addition of ammonia is not recommended. Grourke [21] tried to eliminate ‘flash rusting’ by changing the type of latex, and by a discrete choice of coalesce&s and oil modification in the paint. Unfortunately, such changes yielded negative results except that replacement of the anticorrosive pigment with barium metaborate gave a considerable improvement in coating resistance to ‘flash rusting’. Another method for eliminating ‘flash rusting’ consists of adding compounds such as sodium, potassium or calcium nitrites, sodium salicylate, sodium or ammonium benzoate, phosphoric acid and high-boiling amines (for example, morpholine) [ 16,21, 24, 431. Good results are obtained by the application of so-called complexones as corrosion inhibitors. These contain acidic and basic centres in the molecule and form durable complexes with metal cations [ 161. A typical example is ethylenediaminetetraacetic acid, although it is also possible to use phosphoorganic complexones. According to Parmentier [443, the application of amines such as triethanolamine, diethylamine or morpholine improves the resistance of coatings towards salt spray. Better anticorrosive properties can also be obtained by replacing volatile amines with non-volatile amines [ 451 and by the application of corrosion inhibitors such as calcium or magnesium alkylbenzenesulfonate [ 46 J.

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An improvement in the protective properties of water-borne paints has been obtained by the addition of tannin [47], while the addition of benzotriazol to poly(viny1 acetate) also improves paint properties [ 481.

4 Anticorrosive water-borne paints In the following discussion of anticorrosive water-borne paints, it has been assumed that the size of the polymer particles contained accords with the classification presented above in Section 2. 4.1 Paints based on true solutions of resins in water For such coatings, saturated polyesters and modified alkyd resins are generally used [ 1, 49 - 541. The coatings thus generated show good impact strength and resistance towards chemicals and water. Polyester coatings possess a diminished resistance towards weathering relative to alkyd coatings. A recently developed silicone-modified alkyd resin which forms a true solution in water has been used for the production of anticorrosive paints. Such paints dry completely in 12 h whereas the drying time of solvent silicone-alkyd paints is 24 h or greater [55]. Another new water-borne alkyd paint recently described also has a very short drying time [ 531. In this case, it showed excellent resistance to water only 3 h after application. This paint has been recommended for the protection of transport vehicles, farm machines and equipment which operates outdoors. A new alkyd paint has been developed [56] which dries at high air humidity (up to 90%). 4.2 Dispersion paints Despite the good properties of coatings based on true solutions of binders in water, there is an increasing tendency for the wider use of colloidal systems and aqueous dispersions or emulsions, since the organic solvent content can be substantially reduced only in these systems. Acrylic and vinyl homo-, co- or ter-polymers are generally used as binders for dispersion paints. Aqueous dispersions of traditional alkyd, epoxy and polyester resins are also of great importance. Research work is being undertaken in the Soviet Union aimed at replacing fatty acids and vegetable oils by synthetic materials, In this case, lowmolecular polymers and diolefin copolymers or liquid rubber are often used. Thus, Babkina [57] has modified water-borne alkyd resins with a rubber based on cis-1,4-butadiene. Primers obtained in this way show good mechanical and anticorrosive properties relative to those prepared more conventionally. Modification of water-borne alkyd resins can be undertaken by incorporating multifunctional monomers into the alkyd resin chains. Thus, by adding 5 wt.% of monomer (for example, trimethylpropane a&ate) product parameters such as drying time, and water and salt fog resistance may be improved [ 581.

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Advantageous properties of paints may be achieved by combining various types of resin, for example the combination of an alkyd resin and acrylic latices [59]. The properties of this type of binder are substantially better than those produced by using the resins seperately. Thus, in comparison to alkyd resins, this resin exhibits a shorter drying time, better stability, greater hardness and adhesion to galvanized steel. In comparison to latex, this combination shows better gloss, improved corrosion resistance (even without the addition of anticorrosive pigment) and ease of application. It has been used in maintenance paints for the automotive industry. A method of modifying alkyd resins with epoxy resins has been presented in a Japanese patent [60]. The water-borne paint thus obtained can be used for painting reactors, canisters, machine bodies, etc., and is characterized by good water, chemical and corrosion resistance. A corrosion-resistant paint has been used in East Germany for the protection of car chassies. The patent describing this paint indicates that it is comprised of water-borne alkyd resins, a synthetic butadiene-styrene latex, non-organic pigments and a crosslinking agent [ 611. Saratovskij and Simajev [62] have prepared a priming enamel based on water-borne urethane-modified polyester. This enamel which dries at ambient temperature can by applied to rusty surfaces. The coatings so obtained exhibit very good anticorrosive properties which are better than those obtainable with presently employed paints, and an application of these paints to boilers has brought substantial savings. The introduction of trimethylpropane acrylate increases the corrosion resistance of water-borne polyester paints. By adding this monomer to airdrying and baking products, their water permeability is decreased [58]. Appropriately modified epoxy resins are also suitable as binders for anticorrosive dispersion paints [63 - 661. Indeed, epoxy resins have been designed for dispersion, emulsion or true solution type paints [63]. Paints based on these resins are characterized by good adhesion to the substrate, high durability and resistance to chemicals, temperature and corrosion. Epoxide oligomers, dispersed in water, find application as anticorrosive and chemically resistant paints. In general, modified epoxy acrylic products are used for this purpose. Tests have been carried out to examine the effect of the amount and type of curing agent, anticorrosive pigments and extenders on the protective properties of two-component epoxy aqueous dispersions [64, 651. It has been found that the type of the curing agent has no effect on the corrosion resistance. In fact, in this case the type of epoxy dispersion plays a more important part. Through the use of active anticorrosive pigments the protective properties of paints are significantly improved. However, the use of natural barium sulphate as an extender leads to worse results than with calcite. No differences in the properties of coatings based on epoxy dispersions, or of solvent-thinned and pigmented coatings have been found after two years of natural weathering. Water-borne epoxy resins have been formulated by combining waterdilutable, hydrophilic-modified epoxy resins and dispersed high-molecular

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weight epoxy resins with a new water-borne phenolic resin [ 661. The resulting compositions can be used as binders for paints which exhibit excellent film properties. Recently, patents have been published for a great number of anticorrosive epoxy dispersions [67 - 721. The greatest number of such dispersions are those based on acrylic and modified acrylic resins. Very good protective properties are obtained from film-forming substances such as poly(alkylacrylates), poly(alkylmethacrylates), acrylic and methacrylic acid derivatives containing, amongst others, amide groups. In addition, copolymers of acrylic and styrene monomers, vinyl acetate, vinyl chloride or vinylidene chloride have often been used in practice [21, 73 - 841. The effect of the type of acrylic monomer on the rate of crosslinking of dispersions at elevated temperatures has been studied by Mondt and Wirth [85]. They found that the addition of 10 wt.% melamine resin, based on the weight of polymer present, significantly improved the crosslinking process in acrylic resins. The most suitable resin for this purpose was an etherified melamine resin [l, 9, 10, 86, 871. Acrylic dispersions, modified with a melamine resin, are used in anticorrosive primers which have excellent technological and application properties. The addition of appropriate dispersing agents to acrylic paints provides long-term anticorrosion protection of steel, the properties of such paints being improved in comparison to alkyd paints. It has been found, however, that some components of anticorrosive paints based on aqueous dispersions have a deleterious effect on coating properties and, hence, the number of such paint components should be limited [41]. Seven components provide the essential minimum for the production of anticorrosive water-based paints, i.e. base pigment, extender, reactive pigment, pigment dispersant, defoamer, thickener and can preservative, and fungicide. The authors have reported that zinc oxide is a very useful reactive pigment even when only present in small amounts in a paint. Apart from the seven paint additives mentioned above, the authors recommend the addition of five further components, i.e. surfactant, coalescent, glycol, modifier and a second reactive pigment. Brushwell [34, 881 has described some new water-borne acrylic resins suitable for air-dry or baking type primers on steel, galvanized steel, zinc and

aluminium .

An acrylic-styrene copolymer has also been recommended for waterborne paints [22, 26, 891. Melan [22] has described the principles of primer formulations based on this binder which can be pigmented with basic zinc chromate, red lead or modified barium metaborate. The effectiveness of chromate pigments can be improved by the addition of 20% zinc phosphate based on the pigment content. The choice of appropriate additive for improving the paint and coating properties is also discussed. New water-borne acrylic copolymers [90 - 941 used for anticorrosive paints show good adhesion to iron and non-ferrous metals, and provide effective protection to substrates covered with residual rust against further

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corrosion. Such coatings are water, oil and petrol resistant. The addition of small amounts of anticorrosive pigments which decrease the PVC from 20% to ca. 15% provides an adequate means for improving corrosion resistance. However, for protecting steel, it is necessary to apply two or more coats of paint to provide a total thickness of 150 - 200 pm, whereas to protect nonferrous metals one coat of ca. 100 pm thickness should be sufficient. An acrylic primer pigmented with zinc phosphate and iron oxide red, with added corrosion inhibitors, has been shown to be non-toxic on welding [64, 651. The primer contained 2% organic solvent, and the coating of 15 - 25 pm thickness produced protected the substrate for a period of at least three months. This dispersion paint has been used for the temporary protection of steel structures subject to welding. Initially, water-borne coatings were not accepted by the European Coil Coating Association. For this reason, a new acrylic dispersion designed for coil-coating paints was developed. Paints containing this binder were comparable to traditional products as regards quality and price. When mixed with water-borne amino resins, this binder exhibits good adhesion towards steel and aluminium, and excellent hardness, flexibility and impact resistance. Paints based on vinyl copolymers provide the next group of aqueous dispersion paints. Thus, an improvement in the protective and physicochemical properties of poly(viny1 acetate) dispersions can be achieved by modifying them with oxidizing agents, e.g. hexavalent chromium compounds [95] or KMn04 [96]. Vinyl acetate dispersions, modified with KMn04, can be used for the production of primers for metals. The poly(viny1 acetate) dispersion, after modification with oligomers and with added reactive pigments and extenders, has been used for the production of a surface paint for steel and non-ferrous metals, and for concrete. This is resistant to oils and petrol, and can be used under high humidity conditions [ 971. An aqueous dispersion of ethylene and vinyl acetate copolymer, with the addition of chlorinated polypropylene and waxes, has been designed for the production of paints and adhesives and patented in Japan [ 981. Aqueous dispersions of vinyl and vinylidene chloride with the addition of an inhibitor (an aqueous solution of tannin [ 991) have been prepared in the Soviet Union. Anticorrosive paints based on polyurethane latices exhibit very good properties, i.e. high hardness, flexibility, water resistance and weatherability. Several polyurethane water-borne resins [ 1001 have been designed for primers and enamels. These polymers are colloidal dispersions of polyurethanes of high molecular weight and are recommended for metal protection. Some patents have been filed describing various polyurethane dispersions useful for paints [ 101,102]. Primers for cabs, motor-car bodies and farm machines, based on waterborne urethane resins, have been developed in the Soviet Union. These paints which dry at 130 - 190 “C are characterized by high corrosion resistance at thicknesses of cu. 15 - 17 pm [ 1031. A primer containing alkyd resins and an aqueous polyisocyanate dispersion with blocked isocyanate groups has been patented in Japan [ 104 3. After

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curing at a temperature of 160 “C, a coating 25 pm in thickness showed high resistance towards corrosive agents. Acrylic, alkyd and vinyl latices can be modified with urethane dispersions to obtain coatings with increased hardness and flexibility [ 111. Styrene-butadiene latices [105 - 1071 comprise a new group of binders for anticorrosive water-borne paints. To obtain anticorrosive air-drying primers with properties as good as those of alkyd materials modified with phenolic oligomers, aqueous dispersions of paints based on oxidized styrenebutadiene latices have been used. These have been used for the protection of electrotechnical apparatus, and for the temporary and permanent protection of motor-car body elements exposed to striking stones. Aqueous dispersions of primers based on butadiene-styrene latex are oxidized with chromium(V1) compounds [ 1061. Chemical modification of the above dispersions, especially oxidation with hexavalent chromium compounds in the presence of phosphoric acid, improves the properties of the resulting coatings, simplifies the surface preparation technology prior to painting (phosphatizing and passivation operations can be omitted) and lowers the coating crosslinking temperature. Paints based on modified butadiene-styrene latices are used for priming electrotechnical apparatus and tram cars. Furthermore, these paints can be used for the temporary protection (of up to 1 year) of steel structures [108]. A new rust-converting primer based on butadiene-styrene latex has been prepared in the Soviet Union. This paint contains a modifier which acts as an inhibitor on a clean steel surface and converts rust when the paint is applied to a rusty surface through the formation of iron complexes. Tests have shown that the protective properties of this paint are better than those of paints presently available. Coatings derived from this paint can be covered with alkyd, vinyl and acrylic enamels [log]. Anticorrosive paints based on butadiene-styrene latices find application in the automotive industry [llO, 1111, in building engineering [ 1121 and in the machine industry [ 113). Water-borne silicone-alkyd coatings [114] are resistant to the environment and their properties are similar to solvent-type silicone paints. Siliconealkyd paints are used for painting household goods and steel structures in the building industry. In comparison with other water-soluble coatings, these paints show a better resistance towards temperature and weathering, being characterized by a greater hardness and increased water resistance. A number of new anticorrosive binder-based compositions have been patented, for example [115] a composition resistant to ‘flash rust’ comprising an aminoethylated vinyl polymer in a water-miscible solvent, pigmented with aluminium or stainless-steel flakes with epoxy resins as additives [ 1151. A patent [ 1161 on the anticorrosive composition of nitrile latex claims very good chemical resistance and low oxygen permeability, while a new composition consisting of polyphenylene, sulphide, furan, polysulphone resin and inorganic extenders dispersed.in water has been developed [ 1171. The addition of substituted phenol and dimethylurea has been recommended [ 1181 as a means of improving the adhesion of hexafluoropropylene and vinylidene fluoride copolymer latex.

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A water-borne intercoat has been developed [63] from the combination of a dispersion and a true binder solution in water. This intercoat contains no organic solvents although it contains ca. 60% solids. Its mechanical properties and resistance to salt fog are said to be comparable to the properties of solvent paints. Water-based coatings also find application in the painting of non-ferrous metals [ 119 - 1213. Such products are suitable for applying over chromate coated zinc, aluminium, brass and copper. They can also be used for painting zinc or aluminium in the absence of chromate layers. 4.3 Emulsion paints Many factors account for emulsion paints not finding wider application in corrosion protection in the past. The properties of emulsion paints which are currently available are much better than those existing in the 196Os, i.e. their surface tension is less, the wettability of the pigments and the stability of the paints are better, and their viscosity is controlled by means of pH rather than by the addition of a thickener. Present investigations are directed towards improving the corrosion resistance of aqueous emulsions. Emulsifiers are invariably necessary for the production of emulsion coatings. However, these have a disadvantageous influence on the resistance of the coating. This effect can be eliminated by the use of emulsifiers which take part in the crosslinking reaction and become chemically bonded during the drying of the coating [ 121. For anticorrosive coatings, the alkyd resins which are supplied in a neutralized form contain 80% solids. Prior to painting, water is added to the alkyd resins until the required viscosity is achieved. Such resins confer properties such as good pigment wettability, short drying time and very good corrosion resistance. Initially, aqueous epoxy emulsion compositions were obtained by emulsifying epoxy resins in water in the presence of a surfactant, protective colloids and other traditional additives. The advantages of these products were their very good technological properties, whilst their disadvantages arose from unsatisfactory protective properties linked with the presence of surfactants in the coating. Further studies have concentrated on the modernization of the emulsification of epoxy resins. Better protective properties have been obtained through the direct introduction of an aqueous solution of the curing agent prior to applying the coating to the substrate. In a similar manner to their solvent-type counterparts, water-borne epoxy coatings can be used to protect steel structures, industrial equipment, ships, etc. Water-based epoxy coatings suitable for spraying were developed in 1979. These could be used for painting the internal surfaces of cans for containing beer and fruit juice [I]. The binder in such coatings is an epoxy emulsion into which carboxyl groups have been introduced (through the polymerization of high-molecular epoxy resin with acrylic acid or other monomers). The following components of epoxy emulsions have been

41

produced [ 1221: (i) a non-crystallizing epoxy resin containing a reactive solvent, and (ii) a modified polyamine hardener (80% solution in water). An epoxy undercoat pigmented with borate or zinc phosphate shows very good protective properties [123]. A coating system consisting of this undercoat and a topcoat based on a similar binder shows very good resistance towards brine and sea water (2000 h). No changes were observed in such a system after 4.5 years exposure in an industrial environment. A patent filed in Japan describes an anticorrosive composition of low toxicity which forms coatings at high air humidity [ 1241. The composition which consists of an epoxy emulsion and a polyamide curing agent with iron oxide as the pigment is resistant towards sea water. A thermally resistant emulsion paint used for the protection of metal surfaces and containing an aqueous dispersion of an epoxy resin modified with a fatty acid (Cl,, _ 22) or polyester resin, non-ionic emulsifier and chromic acid or its ammonium salt has been patented in Czechoslovakia [ 1251. Aqueous acrylic emulsions may be used as binders in anticorrosive undercoats or topcoats for steel, zinc and aluminium. Amongst other applications, they have been used for painting car and machine elements, household goods and for the coil coating of steel sheets. Early acrylic emulsions were based on monomers such as ethyl acrylate or vinyl acetate. Nowadays they are obtained from monomers such as the esters of acrylic or methacrylic acid, acrylonitrile, styrene, etc. On replacing a portion of the methylmethacrylate in an acrylic copolymer by styrene, the hardness, water and alkali resistance of the coating thus obtained is increased [ 1261. The binders now available exhibit an increase in the minimum filmforming temperature, although they require additives, for example volatile coalescents of the glycol ether type. The small sizes of the emulsion particles and the use of plasticizers which reduce the internal viscosity of the polymer particles provide emulsions with better properties relative to those available earlier [ 1271. A comparison of the properties of earlier and newly-developed acrylic emulsions is presented in Table 2. An example of an emulsion paint with particle sizes
48 TABLE 2 Comparison of the properties of earlier and newly-developed acrylic emulsions Properties

Earlier emulsions

Newly-developed emulsions

particle size molecular weight viscosity at 25 “C (cps) surface tension (dyn cm-‘) minimum film forming temperature (“C) pigment dispersibility viscosity control flow solids content gloss hardness resistance durability


20

poor thickener poor high low low excellent excellent

good pH control excellent moderate to high high high excellent excellent

resins have been investigated. The physical and mechanical properties of these products were very good; however, their corrosion resistance was less than satisfactory. Sullivan and Vukasowich [24] have discussed the properties of anticorrosive undercoatings based on molybdate-pigmented acrylic emulsions. Such undercoats are resistant to ‘flash rusting’ and ‘early rusting’. They can be applied to a cold substrate and the coating is found to be resistant to a humid atmosphere soon after application. Acrylic emulsion undercoats pigmented with zinc chromate or iron oxide also have very good functional properties [ 1331. TABLE 3 Comparison of the properties of an emulsion acrylic paint with an alkyd paint Properties

Acrylic paint

Alkyd paint

flow drying time gloss gloss stability fastness to light heat resistance alkali resistance freeze-thaw resistance flammability steam permeability flexibility

6a lh up to 60% 10 10 5-6 10 4 non-flammable higher 10

10 5-6h high 8 6-8 6-8 4 8 flammable lower 8

aProperties evaluated = very good.

as: 4 = poor;

5 = moderate;

6 = satisfactory;

8 = good;

and

10

49

Acrylic emulsions have also found application in the production of rustconverting agents. An example here is the agent composed of an acrylic emulsion, an alkyd resin and a water-soluble organic acid, the latter being capable of chelating with iron to form insoluble compounds, e.g. tannic acid, methylene disalicylic acid, etc. [134]. A similar product [135] exhibiting anticorrosive properties for iron surfaces contains an aqueous emulsion or dispersion of an acrylic or alkyd resin plus iron complexing agents, e.g. those obtained from gallic acid or phthalic acid anhydride, Single-or two-pack acrylic emulsions have also found application in the production of paints resistant to elevated temperatures and various chemicals [136]. Paints based on acrylic resins [ 137 - 1421 exhibit very good adhesion to various substrates. The resulting coating is dust-free after 15 min (at a relative humidity of 50% this time is increased to 30 min). Such coatings are characterized by their hardness and resistance towards water and the weather. A paint based on another acrylic binder pigmented with zinc or strontium chromate showed improved resistance towards chemicals, solvents and petrol in comparison to the unpigmented variety. A reduced drying time was obtained by modifying an acrylic resin with a water-soluble alkyd resin. The system comprising a primer, interlayer and enamel based on acrylic resin exhibits very good protective properties. Such a composition has been used for maintenance painting of an old lighthouse in South Carolina. Young [ 1431 has discussed new air-drying acrylic-epoxy water-borne coatings which combine the advantages of acrylic coatings (weather resistance and colour stability) with those of epoxy coatings (ready crosslinking). The corrosion resistance of the system, as tested in a salt spray chamber, was >lOOO h for the case of cold-rolled steel. Air-drying acrylic-styrene emulsions have very good mechanical and protective properties (1000 h in a humidity cabinet and 500 h in a salt spray chamber). Such coatings could be used to replace conventional alkyd and cellulose nitrate paints. Binders and paints based on a vinyl-acrylic copolymer belong to the next group of emulsion paints. Coatings derived from these paints have the advantage of hardening at ambient temperature. New air-drying vinyl-acrylic copolymers designed for anticorrosive paints and enamels have been developed [ 8, 23, 35,144 - 1471. Acrylate-modified vinyl and vinylidene chloride copolymers of this type yield coatings resistant to aggressive environments which dry very quickly and show no tendency to ‘flash rust’. Such coatings are hard and flexible, show good adhesion to metal and resistance to salt fog over a period of 1000 h. These results are better than those for acrylic or alkyd emulsions, being comparable to or better than those for solvent-based chlorinated rubber and vinyl paints [ 1441. Brandon [ 1471, Burgess and collaborators [35], Humphries [ 231 and Caine [ 1481 have presented in detail methods for obtaining paints based on such systems and discussed their properties. In contrast to other paints for steel, paints based on the above binders are characterized by a very low steam permeability (18 g mW2in 24 h for

50

coatings of 25 pm thickness), which is achieved because of the high chlorine content in the copolymer. One disadvantageous property of water-based polymers containing chlorine, however, is their tendency to dehydrochlorinate at pH values above 7. This leads to an increase in the concentration of chloride ions in the aqueous phase. Although the rate of dehydrochlorination can be limited by decreasing the chlorine content in the polymer, this modification leads to a decrease in the protective properties of the polymer. Latex paints are normally obtained at basic pH, generally over the range 7 to 9. A paint with a pH value of 4 to 5 ensures high stability which, with a low steam permeability through the coating, enables paints with excellent protective properties to be obtained. Addition of a block copolymer of ethylene or propylene oxide stabilizes the paint at acidic pH values. The zinc phosphate-pigmented primer based on a vinyl-acrylic copolymer shows good adhesion to steel and other primer coatings, and possesses a short drying time (a coating of 100 pm thickness is dust free in 10 min and is thoroughly dry after 30 min at a relative humidity of 50%). The stability of the product is greater than 12 months while the coating is characterized by good chemical and mechanical resistance. An anticorrosive emulsion [ 1491 based on the product of the reaction between a non-saturated fatty acid and glycidal ester with a vinyl monomer in the presence of a surfactant and a protective colloid, e.g. a water-based alkyd resin and a maleate-modified polybutadiene, has been patented in Japan. Another new emulsion [150] composed of vinyl acetate, dicyandiamide and plasticizers provides the possibility of coatings at low temperatures (2 “C or lower).

4.4 Other water-borne paints Publications relating to a new type of coating, i.e. aqueous suspensions of powder paints (so-called ‘slurry paints’ [50, 151 - 1571 are encountered more and more frequently in the literature. Paints of this type can be applied by methods used for traditional solvent-based paints, while the properties of the resulting coatings correspond to those of powder paints. A major problem to be solved in the development of these paints was the dispersion of solid binders and their stabilization in the aqueous phase. In most cases, surfactants have been employed for this purpose, although to date an exact explanation has not been provided for the influence of surfactants on the stabilization of the components in a suspension. Research work aimed at using these products as interlayers and topcoat paints is proceeding, with the problem of paint stability at high shear stresses requiring to be solved in the near future [ 501. The advantages of these paints may be summarized as follows: low coating thickness (ranging from 15 to 30 pm), ability to apply coatings wet upon wet, various methods of paint application (for example, by air, airless or electrostatic spraying), easy maintenance, ready change of paint colour by the introduction of aqueous tinting pastes into the paint.

51

Electra Powder Coatings (EPC) [50, 53, 158, 1591 comprising of aqueous suspensions of powder paints in the binder for electrodeposition purposes constitute the next group of aqueous suspensions. Use of the EPC method leads to an improvement in the physical and mechanical properties of the coating in comparison to that achieved with powder paint applied by electrostatic spraying, a method which was first developed in Japan. The EPC process has been used for the protection of car bodies with the painting material consisting of an epoxy powder paint dispersed with a hardener in a cataphoretic binder based on epoxy resin. The thickness of the coating obtained over 15 - 60 s was 40 - 80 pm. The disadvantage of the method is its poor throwing power, whereas its advantage is the greater flexibility of the coating and its enhanced resistance towards striking stones in comparison to coatings applied by traditional cathodic electrodeposition.

5 Properties of water-borne paints As mentioned above, anticorrosive water-borne coatings possess both advantages and disadvantages in comparison to traditional solvent-based paints. In relation to the water content and binder type, the following problems can arise when applying water-based paints: 1. A longer drying time is necessary in comparison to organic solvent-type paints, particularly in an environment with a high humidity. 2. If the amount of water in the paint is increased, the tolerance towards the the fatty substrate is decreased and there is an increased tendency towards cratering. 3. Interlayer adhesion is unsatisfactory due to presence of surfactants. 4. When coatings are dried at high temperature, a ‘boiling’ effect takes place. To avoid this effect, the time necessary for preliminary evaporation should be increased. 5. It is difficult to obtain thicker coatings. 6. The air in the spraying apparatus must be free from oil. 7. Water-borne paints must be stored at temperatures above 0 “C. The main advantages which compensate the above disadvantages are: 1. Emissions of organic substances are decreased so that expensive installations for burning the vapour evolved from baking ovens can be eliminated. 2. The explosion and fire hazard are reduced. 3. Despite the higher energy requirement for water evaporation (2260 J g-’ in comparison to 393 J g-’ in the case of xylene), the total energy saving arising from water evaporation is estimated to be 25% relative to conventional paints. 4. The spraying apparatus can be washed with water. 5. The savings in organic solvents amount to ca. 200 - 400 kg per tonne of paint.

52

6 Range of application of water-borne paints Water-borne paints are used in all areas of industry. Examples are as follows: (i) the priming of car bodies by anodic or cathodic electrodeposition; (ii) the painting of metal packages (barrels) [160 - 1621; (iii) painting of engine blocks [ 162 - 1641; paint systems for cars [165 - 1701; paints for coilcoating of steel sheets [ 131, 1711; corrosion protection of steel plates, steel and galvanized steel structures [129,171] ; and the coating of small packages, for example tins for beer and other drinks [ 231. Other applications have been described above.

7 Conclusions Restrictive environmental and safety legislation, and necessary savings in raw materials and energy, have given rise to the development of new anticorrosive coatings including water-borne paints. Although the properties of early developed paints were not satisfactory in comparison to solvent-borne paints, during recent years there has been a rapid increase in the development of new binders, additives and pigments. As a result, a great number of anticorrosive water-borne paints are now available on the market, with properties comparable and even superior to the properties of traditional solvent paints. If the demands of customers require the use of water-based paints, a wide variety of such paints is now available. To date, however, these paints have only been used on a large scale in the automotive industry despite their unquestionable advantages.

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