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Chapter 5 Two-Component Waterborne Epoxy Coatings 1
Frederick H. Walker and Michael I. Cook
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Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501
The chemistry and properties of two component, waterborne epoxy coatings are reviewed. The coatings are classified by the type of epoxy resin employed. Type I utilize low molecular weight, liquid epoxy resin, and type II use pre-formed dispersions of higher molecular weight, solid epoxy resins. The physical and coatings chemistry of waterborne epoxy coatings is discussed, with particular emphasis on the film formation process. After a description of the general structure-property requirements for the coatings components and the surfactant properties of amine hardeners, a detailed description of the chemistry of the binders for both type I and type II systems is provided, based mostly on the patent literature. General principles of coatings formulation are then provided.
Since their first viable commercial introduction at least some 25 years ago, (1) waterborne epoxy coatings have become a commercially important technology. Growth has been driven by consumer desire for the low odor and water cleanup of coatings based on thermoplastic latices, in uses that require ambient application conditions and the performance advantages typically associated with thermosetting polymers. Various environmental and worker safety regulations have also contributed to this growth. Despite this interest, to date there appears to be no systematic published review of the various technologies employed in this market. In this paper, after first classifying the technologies available, we will discuss important aspects of polymer and colloid chemistry pertaining to waterborne epoxy coatings. The principal chemistries now employed in this technology will then be reviewed. No attempt has been made to be encyclopedic in coverage. Instead, the discussion will emphasize those chemistries that the authors suspect to be of commercial importance, or which represent novel approaches to the technology. Much 1
Current address: Air Products Nederland bv, Kanaalweg 15, Box 3193, GD Utrecht, Netherlands
© 1997 American Chemical Society
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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of the information on the chemical technologies is drawn from the patent literature. It should be appreciated by the reader that commercial practice frequently differs to some degree from that described in the patent. Classification of Waterborne Epoxy Technologies. For the purposes of this review, we classify waterborne epoxy coatings into two categories, based on the physical state of the base epoxy resin employed. Type I systems, which were the first to achieve commercial success, generally utilize the diglycidyl ether of bisphenol-A (DGEBA, la), which is commonly referred to as liquid epoxy resin, as the principle epoxy resin. Commercial grades used in coatings are slightly oligomerized (n « 0.1) and have a viscosity at 25°C of about 12,000 m-pa-s. The diglycidyl ether of bisphenol-F (Π, DGEBF, which also contains the ο,ρ' and ο,ο' isomers in addition to the ρ,ρ' isomer shown) may be substituted for all or part of la. Various epoxy reactive diluents (generally glycidyl ethers of various phenols or aliphatic alcohols and diols) are used to modify the viscosity and crosslink density of the resin.
In type I systems, the amine hardener is usually designed to act as the emulsifier for the epoxy resin, although sometimes the epoxy resin is pre-emulsified in water with surfactants, primarily to adjust package ratios. Thus, hardeners for type I systems are amphiphilic molecules, possessing both hydrophilic and hydrophobic sections. Type Π waterborne epoxy systems are based on much higher molecular weight epoxy resin, lb. At room temperature, commercial solid epoxy resin with an epoxy equivalent weight of 450 - 550 (lb, n « 2) is a solid with a melting point of 75 80°C. Its viscosity is orders of magnitude higher than la: about 10,500 m-pa-s at 100°C. (2) To effect a small and reproducible particle size, dispersion of such viscous materials requires specialized processing equipment, and the application of heat or the use of solvents. Thus, such dispersions are always pre-dispersed either by the manufacturer or raw material supplier. Interestingly, the amine hardeners for type Π systems also tend to be amphiphilic in nature, though they no longer serve to emulsify the resin. It is difficult to point to performance differences between the two systems that are true without exception, but the following observations generally hold. Most type I a v g
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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coatings contain little or no co-solvent, whereas type Π require a coalescing agent. Type Π coatings, like solvent-borne coatings based on solid epoxy resin, will reach a touch-dry state as soon as enough of the water and co-solvent evaporate to increase the viscosity to the required level. This so-called lacquer dry is due to the very high viscosity of the binder. (3) Type I coatings require a significant amount of chemical reaction before the requisite viscosity is achieved, and are generally slower drying. Typically, the useable pot life of type I systems is also shorter than for type Π. The differences in resin molecular weight also result in different mechanical properties. Comparison of Waterborne and Solvent-borne Epoxy Coatings. In clear solventborne epoxy formulations, the resin and amine curing agent exist in a true, isotropic solution. Epoxy resins, on the other hand, are highly water insoluble. To be used in an aqueous medium, the epoxy resin must be dispersed with the aid of a surfactant into colloidal particles, ranging in size from about 100 to several thousand nanometers. In an aqueous medium, the amphiphilic amine hardeners will partition between the aqueous and epoxy phases and the epoxy/water interface. They may also exist in selfassembling aggregates of their own. Unlike most other aqueous dispersions used in coatings, the constituents of an epoxy coating react at room temperature. Indeed, water is an excellent catalyst for the amine/epoxy reaction, (4) as shown in Figure 1. The net result is that the molecular weight and chemical nature of an epoxy formulation is constantly changing once the components are mixed. In solvent-borne epoxy coatings the primary result of this reaction is an increase in viscosity that eventually indicates the end of the useable pot life of the system. In waterborne systems, the combination of colloidal phases and an ongoing reaction affect the rheology and pot-life in different ways, and also add significant complexity to the film formation process. Variability in performance also results from particle size effects, and from changes in the humidity and temperature of application. Η
Η
Figure 1. Water Catalyzed Epoxy Ring Opening With Arnines Rheology. Epoxy resin dispersions and emulsions, and their corresponding clear coatings, display a thixotropic or pseudoplastic rheology, typical of aqueous dispersions. (5) The viscosity of waterborne epoxy formulations exhibit complex
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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trends during the pot life. Depending on specific compositions, formulations may decrease in viscosity soon after mixing, remain relatively constant throughout the pot life, or exhibit an increasing viscosity. Examples are shown in Figure 2.
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Figure 2. Viscosity Profile of Waterborne Epoxy Formulations Of the systems depicted in Figure 2, the formulation with the increasing viscosity is a type I system, and the two other systems are of type Π, made from the same epoxy dispersion, but different hardeners. A n explanation for the drop in viscosity has not been reported in the literature. It may be the result of transfer of some of the amine curing agent from the aqueous phase (where its presence would increase viscosity) to the disperse phase (where its presence would have little or no effect on viscosity. (6) In any event, the effect is generally undesirable, since it can lead to a decrease in the sag resistance, and other inferior application properties of the coating. Pot Life. Though sometimes the end of pot life for a waterborne epoxy system is signaled by an increase in viscosity, it is more commonly indicated by a change in some important property of the resulting film. Figure 3 shows the gloss vs. time after mixing for two representative top coat formulations. As Wegman (7) has shown, the ongoing reaction increases the molecular weight and T of the disperse phase resins. This causes an increase in the minimum film-fonriing temperature (MFT) of the composition. Pot life can be increased by raising the cure temperature, or in some cases by addition of co-solvent or plasticizer to a formulation. In low gloss coatings the end of pot life may be indicated by some other property, such as a change in the humidity resistance of the film. g
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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Two-Component Waterborne Epoxy Coating?
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Figure 3. Gloss Profile of Waterborne Epoxy Formulations Film Formation. Though a typical solid epoxy dispersion particle is only 500 nm in diameter, on a molecular level this is a large distance. Taking the density of epoxy resin to be 1.16 g/mL, assuming a molecular weight of 1000, and ignoring the effect of any swelling of the particle by water, it is easily calculated that there are about 4.6 χ 10 molecules per 500 nm diameter particle. To achieve uniform film formation comparable to a solvent-borne epoxy formulation, it is necessary for the amine to diffuse uniformly throughout this viscous resin phase. For this reason, film formation in some waterborne epoxy systems is incomplete, and complex, heterogeneous morphologies result. Meanwhile, the ongoing chemical reactions continually raise the viscosity even higher, increasing the barrier to diffusion. At a certain degree of reaction, T exceeds room temperature, at which point diffusion becomes very slow. (8) Consequently, the morphology of films may also change as the pot life progresses. 7
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El-Aasser and coworkers (9) used transmission electron microscopy to look at the structure of films formed from a solid epoxy resin dispersion and a conventional polyamide curing agent dispersion. Both dispersions were stabilized with a hexadecyltrimethylammonium bromide surfactant plus an unspecified co-emulsifier and a water-immiscible solvent, then homogenized with a Manton-Gaulin disperser, and the solvent removed by steam distillation. They had extremely small particle sizes ranging from about 30 to 200 nm, and thus are not typical of commercially available dispersions. Nevertheless, the results were quite interesting. The surface of the films prepared from freshly mixed dispersions and examined by a replication technique was shown to gradually coalesce over a period of 15 days. When a catalyst for the amine/epoxy reaction was included in the epoxy dispersion, no coalescence occurred. The films were also cross-sectioned and stained with osmium tetroxide. The osmium stains the double bonds present in the dimer acid moieties of the polyamide curing agent. The cross sections of both the catalyzed and uncatalyzed films showed
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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heterogeneous morphologies, with epoxy domains dispersed in a polyamide matrix, although there was clearly much more coalescence in the absence of catalyst. Walker and Schaffer (10) developed the use of 3-iodopropionic acid as a stain to look for the presence of amine-rich domains in microtomed cross-sections of cured waterborne epoxy films. Figures 4 and 5 show the transmission electron photomicrographs obtained from a type Π system consisting of a solid epoxy dispersion with an equivalent weight of 625 and epoxy-amine-adduct curing agent. The films were prepared 0.5 hours and 2 hours after mixing, respectively, then cured for >2 weeks at ambient temperature. The dark domains at 0.5 hours are attributed to phase separation of some of the amine from the epoxy continuous phase. Two hours after mixing, more of the amine has reacted with the epoxy particles. This increases the compatibility of the amine with the epoxy, removing the thermodynamic driving force for phase separation. However, the individual particles are still present, indicating incomplete coalescence. The particles also stain darker at the particle boundaries than in the cores, suggesting a higher amine concentration at the interparticle boundaries. It was speculated that these morphologies may explain some of the difference in water resistance properties generally associated with the use of waterborne epoxy coatings when compared to their solvent-borne counterparts.
Figure 4. Cross-section of a type Π Figure 5. Cross-section of a type Π waterborne epoxy film, cast 0.5 hours waterborne epoxy film, cast 2 hours after after mixing. Stained with 3mixing. Stained with 3-iodopropionic iodopropionic acid. acid. (Figures 4 and 5 reproduced with permission from reference 37. Copyright 1995 University of Southern Mississippi.) The same technique was applied to a type I system that was developed to specifically improve the film coalescence. Liquid epoxy resin was employed because its lower viscosity would be expected to lead to higher diffusion rates, and the resin was emulsified and cured with a modified, highly epoxy-compatible cycloaliphatic amine. The film formation was much more uniform at this level of resolution. Electrochemical impedance spectroscopy showed that these films also gave a very high pore resistance, which is a measure of the resistance of the film to the penetration of ions, when immersed in a sodium chloride solution. This points to an interesting dilemma as chemists work to improve the water resistance properties of these coatings. By employing type Π resins with their higher
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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epoxy equivalent weights, the required amount of amine curing agent is substantially reduced. Since the curing agent is generally more hydrophilic than the epoxy resin, this results in a composition that has an overall more hydrophobic balance. On the other hand, the higher molecular weight results in higher barriers to film coalescence. When type Π technologies were first developed, they offered improved water resistance properties over the earlier type I systems, and were the first technology to see significant use in direct-to-metal applications. It would now appear that water resistance properties of certain type I systems is at least comparable, based on reported salt spray and EIS data. It will be interesting to see how these competing approaches compare as waterborne epoxy technology continues to advance. Mechanical Properties of Films. The stress-strain and dynamic mechanical properties of a proprietary solid epoxy resin dispersion and curing agent combination were reported by Arora and coworkers. (11) Interestingly, the tensile strength, elongation, storage modulus, calculated molecular weight between crosslinks (Mc) and T were fairly close to those obtained for a solvent-based solid epoxy resin of about the same equivalent weight cured with the same hardener. Surprisingly, however, the values of Mc and T showed little change when the amine to epoxy stoichiometry was varied by 20% to either side of unity. When these results are compared to the morphology data discussed under film formation it would be interesting to discover if the system studied by Arora has a uniform morphology, in which case mechanical properties similar to a solvent borne film would be expected. Also of interest are how the mechanical properties of heterogeneous waterborne systems compare to those formed from materials of similar molecular weight and functionality, but cast from solvent. We have conducted some informal experiments of this nature in our laboratories, and determined that systems cast from solvent had, for example, much greater impact resistance than corresponding formulations cast from water. (12) g
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Waterborne Amine Hardeners: General Structural Requirements. In many type I systems, the curing agent serves a dual role: i) as emulsifying agent for the liquid epoxy resin; and ii) as crosslinking agent. There are three generic types of curing agents, all based on polyethyleneamines, which have been in common use in traditional solvent borne epoxy coatings and which have the basic amphiphilic structure required. The first of these are amides prepared by reaction of the amine with a fatty acid, ΙΠ, which are commonly referred to as arnidoamines. The second are similar condensates prepared from dimer fatty acids, IV, known as polyamides. The final type, V, are prepared by reaction of the amine with an epoxy resin, and have come to be called amine adducts. It should be noted that ΠΙ - V are highly idealized structures. The polyethyleneamines themselves are mixtures of linear, branched, and cyclic structures. Since the polyethyleneamines are multifunctional, unless special techniques are employed, reactions with dimeric reagents such as dimer acid or epoxy resin will result in some degree of polymerization. Even in reaction with a monofunctional fatty acid, there will be significant numbers of species formed with more than one fatty acid moiety attached to the rx)lyemyleneamine. The fatty acids employed in both the arnidoamines and polyamides are complex blends of mostly Cie
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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fatty acids with differing amounts of unsaturation. The dimerization of tall oil fatty acid yields numerous isomers besides that depicted. Finally, for both amidoamines and polyamides there is at tendency for the β-aminoamides to lose a second mole of water to form the corresponding imidazoline, V I . (CH ) CONH(CH2CH NH)nCH2CH2NH2 2
H
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Though the traditional curing agents H I - V frequently can be seen to be the basis of waterborne curing agents, they must be modified before they will be acceptable both as surfactants and film crosslinkers. The specific chemistries employed are discussed in the section on type I curing agents, below. However, there are some common themes to these modifications. To extend pot life of waterborne epoxy systems, it is usually necessary to decrease the amine reactivity. Primary amines are more reactive with epoxy functional groups than secondary amines. (13) By treating the hardener with a reagent that reacts preferentially with primary amines, the overall reactivity is reduced. Examples of commonly employed reagents for this purpose include monofunctional epoxides, formaldehyde, and unsaturated reagents capable of undergoing Michael addition reactions such as acrylonitrile. The general incompatibility of epoxy resins and amines can also lead to difficulties. This incompatibility has always been a problem with traditional epoxy coatings, which have a tendency to exude amine on their surface. This can result in the formation of carbonate salts known as 'blush , (14) or a greasy surface layer. In waterborne formulations incompatibility may lead to other surface appearance defects such as cratering, and may also decrease the stability of the epoxy emulsion. Compatibility of a hardener is frequently enhanced by reaction with monofunctional epoxy diluents, particularly aromatic diluents, or by reaction with epoxy resin. Because polymer compatibility normally decreases with increasing molecular weight, these modifications are even more important in type Π technologies due to the higher molecular weight of the epoxy dispersions. These adduction procedures have the desirable effect of also reducing reactivity. 1
In Technology for Waterborne Coatings; Glass, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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