Film Formation and Morphology in Two-Component, Ambient-Cured

Chapter 24

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Film Formation and Morphology in Two-Component, Ambient-Cured, Waterborne Epoxy Coatings 1

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Frederick H. Walker and Olga Shaffer 1

Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, PA 18195-1501 Emulsion Polymers Institute and Polymer Interfaces Center, Lehigh University, Bethlehem, PA 18015-4732

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Film formation from two component, waterborne, ambient cured coatings is a complex process involving a number of steps that must occur in the correct order. Iodocarboxylic acids such as 3-iodopropionic acid were used to stain amine containing domains in polymeric films based on amine cured water-borne epoxy resins. A film cast 0.5 hr. after mixing a solid epoxy dispersion and a polyethyleneamine type hardener shows phase-separated amine containing domains in an epoxy continuous phase. Two hours after mixing the morphology changes to show individual particles with amine-rich areas at the interparticle boundaries. In contrast, films cast from a water-borne combination of liquid epoxy resin and a highly epoxy-compatible, self-emulsifying amine hardener are shown to be of a much more uniform nature with this technique.

The development of two component, waterborne epoxy coatings is driven by environmental and worker safety regulations requiring ever lower usage of solvent, and consumer preference for coatings with low odor and water cleanup. Generally, these products are employed in markets that require ambient cure and certain advantages, such as enhanced chemical resistance, of a crosslinked polymer over a thermoplastic polymer. When considering the film formation process in these systems, it is convenient to divide the technology into two categories, depending on the molecular weight and physical nature of the epoxy resin employed. Type I, which has been in use for over 30 years ( 1 j , utilizes liquid epoxy resin (epoxy equivalent weight, E E W ca. 190) and a modified poly (ethylene amine) hardener. Usually, the hardener serves as the emulsifier for the epoxy resin, although sometimes the epoxy resin is pre-emulsified in water with surfactants, primarily to adjust package ratios. Typical hardeners are made by reacting a poly(ethylene amine)

0097-6156/96/0648-0403$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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such as methylene tetramine or tetraethylene pentamine with: i) fatty acids or dimer acids; and/or ii) epoxy resin, and then modifying that product in some way to reduce the 1 ° amine content. The strong catalytic effect of water on the amine epoxy reaction (2) results in too short a time period over which the mixture can be successfully applied to a substrate (i.e. pot life) without such modification. The amines are partially neutralized with a volatile carboxylic acid such as acetic acid, which increases the water solubility of the hardener and the colloidal stability of the combined, emulsified system. Nevertheless, pot lives are (with some exceptions) usually less than three hours. The humidity and corrosion resistance are generally inadequate for application directly to steel substrates with these older products, though some newer refinements of this technology show significant improvement in that regard. Chemical resistance is acceptable for many uses. The volatile organic content (VOC) achievable with this approach is very low or even zero, and the major market is for the protection of concrete substrates. The advent of solid epoxy resin dispersion technology (Type Π) gave much improved humidity and corrosion resistance, and longer pot life. These products are based on much higher molecular weight resin (EEW about 500 to 650). Because the process for preparation of dispersions of these resins is complicated, they are always supplied in a pre-dispersed form in water. The surfactant technology used to create stable dispersions can be sophisticated, and frequently involves modification of the surfactant for chemical incorporation into the resin. (3) The principal hardeners used in this case are the partially neutralized epoxy/poly(ethylene amine) adduct variety described above, modified to optimize performance with the epoxy dispersion. Pot lives of about five to eight hours can be achieved, and with skilled formulation corrosion and humidity resistance is sufficient for light duty maintenance of steel substrates. The VOC of a clearcoat is typically in the 240 - 360 g/L ( 2 - 3 lb/gal) range. The major market is for use over masonry and related substrates, where the very fast tack-free times (2 weeks. Film Based on Solvent Based Solid Epoxy Resin and Polyamide Hardener. A mixture of 86.6 g of solid epoxy resin (D.E.R. 671-PM75, Dow Chemical Co.; 75% N V in propylene glycol monomethyl ether), 20.1 g of polyamide hardener (Ancamide 350A, Air Products and Chemicals, Inc.; A H E W = 100), 9.5 g of n-butanol, and 9.5 g of xylene were thoroughly hand mixed, and after a 15 min. induction period, films were cast as described above. Film Based on Solvent Based Solid Epoxy Resin and Amine Adduct Hardener. A mixture of 67.4 g of D.E.R. 671-PM75, 15.4 g of a 60% N V (in an n-butanol/aromatic solvent blend) isolated 2:1 molar adduct of ethylene diamine and liquid epoxy resin (AHEW = 85.5 based on solids), 9.5 g of n-butanol, and 9.5 g of xylene were thoroughly hand mixed, and after a 15 min. induction period, films were cast as described above. The isolated adduct was prepared using a large excess of ethylene diamine, and removing the excess under vacuum until the residual ethylene diamine was less than 1% by weight as measured by GC. Film Staining and Microscopy. After removing the substrate, the films were crosssectioned with a cryoultramicrotome to about a 100 nm film thickness. After mounting on a T E M grid which has a polymer support film, the sections were section-side floated in a 2% solution of the staining reagent for 10 minutes, then rinsed several

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

24. WALKER & SHAFFER

Ambient-Cured Waterborne Epoxy Coatings

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times with DI water, dried, and examined with a transmission electron microscope (TEM) operated at 100 k V accelerating voltage. The stain most commonly employed was 3-iodopropionic acid, but 2-iodoacetic acid, 3-iodobenzoic acid, and phosphotungstic acid were also used as noted. Electrochemical Impedance Spectra. The impedance spectra were measured on 2 mil films cast on grit blasted steel using a roll bar, and cured under the same conditions as the free films (above). The EIS coating cell exposed a 1 cm diameter circle of the coating to 1.0 M NaCl in water. A platinum gauge counter electrode was used, and potentials were referenced to the aqueous Ag/AgCl electrode. A Princeton Applied Research (PAR) Model 273A potentiostat and model 5210 lock-in amplifier was used, and PAR model 398 impedance software (version 1.11) was used for data acquisition. Experiments were run in 2 parts: single sine waves were used from 100 down to 5 kHz, and a composite waveform was used from 10 Hz to 0.05 Hz. Data were combined into one spectrum for analysis. Sine wave excitation voltages of 10 and 15 mV were used for the single and multi-sine experiments, respectively. Spectra took 13 min. to acquire. Testing was done after 1 and 24 hr. of immersion in NaCl solution. Results and Discussion The use of various stains such as osmium tetroxide, (10) ruthenium tetroxide, (11) and phosphotungstic acid (12) to examine the morphology of polymeric films is a common technique. To our knowledge the use of iodocarboxylic acids to stain amine containing domains within a polymeric matrix is new. (13) The stain was designed to work by the reaction of the carboxylic acid with the amine groups to form the corresponding salts. The iodine atom, which has a very high nuclear density, scatters the incident electron beam, with the result that amine containing areas within the film appear dark in a photograph. A cross-section of a solvent borne clearcoat prepared from a solid epoxy resin with an E E W of about 475 crosslinked with a typical medium solids polyamide hardener is shown in Figure 1. The cross-section was stained for 10 minutes with a 2% solution of iodopropionic acid. This is a typical epoxy resin and curing agent combination that has seen widespread use in coatings for the protection of metal substrates in highly corrosive environments for decades. There are some diagonal stripes visible in the photograph which are an artifact caused by the microtome's diamond knife as it sliced the cross-section, and a few specks of dust are also visible. Otherwise, the film is homogeneous at this level of resolution. Type II Film Late In Pot Life. The morphology of films based on a Type Π solid epoxy dispersion was studied using a published clearcoat formulation, employing an epoxy dispersion with an EEW of 625 (based on solids) and the recommended amine/epoxy adduct hardener with an AHEW of 163. A noteworthy feature of this formulation is that it utilizes a large stoichiometric excess of 1.5 epoxy groups per amine hydrogen. This is about the minimum excess necessary to improve the water resistance and corrosion resistance of the coating for direct-to-metal applications, ( 14) and excesses as large as 2:1 are sometimes recommended. This formulating practice is



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Figure 1. Cross-section of a film from a solvent borne solid epoxy resin cured with a polyamide hardener, stained for 10 min. with iodopropionic acid.

Figure 2. Cross-section of a film prepared from the solid epoxy dispersion with no curing agent, stained 2 hr. with iodoacetic acid.





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common with solid epoxy dispersions, and is in contrast with solvent borne epoxy formulations, where stoichiometries closer to 1:1 are normally employed. In Figure 2, a cross-section of film made from a solid epoxy dispersion and stained for 2 hours with 2% iodoacetic acid is shown. Note that this is a control film that contains no amine hardener. The film was very soft and difficult to cut. We ascribe the vague shadowy pattern present to nonuniformity in the cross-sectioning caused by the softness of the film. Figure 3 shows the unstained cross-section of the cured, formulated clear coat obtained from a film that was cast 2 hours after mixing the amine and epoxy dispersion, and Figure 4 shows a cross-section of the same film that was stained for 10 min. with 2% iodopropionic acid. In the absence of stain, a cell pattern is barely visible, but with the stain, a core-shell type structure becomes clearly visible. Individual particles are clearly present. The particles are about the same size as the starting dispersion, which was measured by T E M to have a particle size of about 500 nm. The particles appear to have been somewhat elongated by the cutting process. They stain darker on the outside of the particles than in the interior regions, indicating that amine concentrations are higher at the particle boundaries than in the interiors. Unfortunately this is only a qualitative technique, and we have no way to measure what the difference in concentration is. A stained cross-section obtained 4 hours after mixing is virtually indistinguishable from Figure 4. In Figure 5, the stained film obtained 8 hours after mixing is shown. The structure is similar to the film obtained 2 hours after mixing, except that now some of the largest particles appear to be deformed into dumbbell-like shapes. The core-shell type structure seen in Figures 4 and 5 probably arises as follows (see Figure 6). The hardener is a step-growth oligomer formed by the reaction of multifunctional epoxides with a polyethylene amine, followed by further adduction with aromatic monoepoxides, which is then partially neutralized with acetic acid. (15) Thus it contains both hydrophobic and hydrophilic regions in the same molecule, and the number average molecular weight would be anticipated to be on the order of 1000. When mixed into an aqueous medium containing some glycol ether, it would be anticipated that such an amine adduct would partition into the particle cores, into the aqueous phase, and onto the particle surface. This situation would tend to cause the amines to react preferentially with epoxy groups at or near the surface of the particle, based simply on where the amines are located, as well as the catalytic effect of water on the reaction. Reaction near the surface will result in the formation of a crosslinked shell. This shell would in turn raise the barrier to diffusion of amine into the particle interiors. If this model is correct, one would expect that small particles, with a larger surface-to-volume ratio than larger particles, would achieve a higher degree of cure, at least during early stages of the curing process when film formation occurs. Therefore the Tg and hardness of small particles would also be greater. Small particles could then act as a mandrel around which larger particles could be deformed by capillary and other forces (16) present during the film formation process. This explains the formation of the deformed particles seen 8 hours after mixing. The need for large stoichiometric excesses of epoxy to reduce water sensitivity may be related. With more amine present in a formulation, more will be partitioned into the aqueous phase, and thus more could be trapped at the particle boundaries in



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Figure 3. Cross-section of a film from a solid epoxy dispersion and hardener, film cast 2 hour after mixing, no stain.

Figure 4. Cross-section of a film from a solid epoxy dispersion and hardener, film cast 2 hour after mixing, stained 10 min. with iodopropionic acid.




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Figure 5. Cross-section of a film from a solid epoxy dispersion and hardener, film cast 8 hour after mixing, stained 10 min. with iodopropionic acid.

A

A

A

A

Figure 6. Process for the formation of core-shell type morphology from a waterborne epoxy coating. Ά ' represent unreacted amines, and the circle represents an epoxy resin particle. The crosslinked shell is represented by the hash pattern.


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the cured film. Since the amines are hydrophilic, the interparticle regions may represent a channel through which water can diffuse more rapidly than it can through other parts of the film. Type II Film Early In Pot Life. Figure 7 shows the stained cross-section of a film obtained much earlier in the pot life of this formulation, only 1/2 hour after mixing. The morphology is completely different. There are amine containing domains embedded in an epoxy continuous phase. Some of the amine domains are quite large, exceeding 1 μπι in diameter. Numerous modifications to the staining technique were run to check for the possibility that these results were due to some artifact of the experiment. Staining in ethanol and ethyl acetate, instead of the usual aqueous medium, gave the same result. To eliminate the possibility that an SN2 reaction with amine was eliminating iodide which might migrate to some other area of the film, staining was conducted with 3iodobenzoic acid in ethanol, also with the same result. Finally, staining was reduced from 10 min. to only 10 sec. Since we believe the stain functions through acid-base reactions which are generally considered to occur at diffusion controlled rates, (17) this change should make no difference, as was the case. These photomicrographs indicate that films formed shortly after mixing the amine and epoxy components have phase separated. It is well known in solvent borne coatings that amine curing agents based on polyethylene amines are not highly compatible with epoxy resins. This is why it is generally recommended that coatings based these curing agents be mixed with the epoxy for about 30 min. to 1 hr. before application, in order to avoid the exudation of amines to the surface of the coating, which is known in the industry as 'blush'. (18) In Figure 8, the stained cross-section of a solvent borne formulation based on a solid epoxy resin and an isolated amine adduct hardener is shown. This film was cast only 15 min. after mixing. Similar amine-rich domains appear to be present, although they are not as prevalent as was the case in the waterborne system. The amine domains in the waterborne system also arise from the initial incompatibility of the amine and epoxy resin. The fact that more of these domains are present probably results from the phase separated state of the waterborne system when the films are cast. In the solvent borne systems, incompatibility develops as the solvent evaporates, since the formulations are initially clear or at most slightly hazy. We believe that the change in morphology of the waterborne system results from the reaction of the curing agent with the surface of the solid epoxy dispersion. Once the surface has been modified sufficiently, the dispersion surface and any remaining amine are no longer incompatible, and the thermodynamic driving force for phase separation is eliminated. Novel Type I Film. Operating under the premise that the morphologies discovered above may be responsible, at least in part, for the generally poorer performance of waterborne epoxy coatings relative to their solvent borne counterparts, we tried to develop a waterborne epoxy system that would have a more uniform film morphology. In doing so we employed three principles.





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Figure 7. Cross-section of a film from a solid epoxy dispersion and hardener, film cast 0.5 hours after mixing, stained 10 min. with iodopropionic acid.

Figure 8. Cross-section of a film from a solvent borne solid epoxy resin and amine adduct hardener, film cast 0.25 hours after mixing, stained 10 min. with iodopropionic acid.



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The first was to base the system on liquid epoxy. Its much lower molecular weight and viscosity should reduce the barriers to coalescence. The second was to utilize a hydrophobic, epoxy-compatible amine as the basic building block for the curing agent. For this purpose PCPA appeared to be ideal. PCPA is a complex amine mixture made up of numerous components. Representative constituents are I and II. In contrast to the polyethylene amines, considerable experience with the formulation of PCPA has shown it to be highly compatible with epoxy resin, and its water solubility is very low. This type of amine structure also contains primary amines on a secondary carbon. The rate of reaction of such amines with a glycidyl ether is slower than that for the primary amine on a polyethylene amine, which would then extend the time after mixing the amine and epoxy during which the viscosity and molecular weight would stay low enough to afford good application and film formation.

The third principle was to modify a small portion of the PCPA stream by attaching water soluble polyethylene oxide chains in a proprietary process, resulting in a mixture of products typified by III. In this way, III serves as the emulsifier for the unmodified PCPA and the liquid epoxy resin. In addition, the emulsifying chains eventually become part of the crosslinked network. This is generally thought to be a way to reduce water sensitivity. (19) The formulation given above was developed using experimental design techniques. It contains a plasticizer (nonylphenol), as is usually required in the formulation of an epoxy resin and cycloaliphatic amine hardener. In the absence of a plasticizer, the Tg of the network exceeds room temperature at an early stage in the cure process, at which point the amine-epoxy reaction essentially stops. The formulation also contains a small amount of glycol ether coalescing solvent, and has a calculated V O C of 144 g/L (1.2 lb/gal). The stained cross-sections of films cast 0.5 hr. and 4 hr. after mixing are shown in Figures 9 and 10, respectively. A few light regions are visible in some areas of the film, and may indicate the presence of some undispersed epoxy resin. However, the vast majority of the films appear to have a uniform morphology within the limitations of the resolution of this technique. Also, the morphology does not change through the useful pot life of the system, which is about 4 hours as measured by the 60° gloss of the clearcoat.





Figure 9. Cross-section of a film from liquid epoxy resin and self-emulsifying curing agent, film cast 0.5 hours after mixing, stained 10 min. with iodopropionic acid.

Figure 10. Cross-section of a film from liquid epoxy resin and self-emulsifying curing agent, film cast 4 hours after mixing, stained 10 min. with iodopropionic acid.


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FILM FORMATION IN WATERBORNE COATINGS 1E+11

I

SME+IO-J
10 ohms/cm after 1 nr. immersion. This is a very high pore resistance for a waterborne epoxy coating, and is comparable to the initial pore resistance of high quality traditional solvent based maintenance coatings. (20) Only a slight drop in pore resistance is observed after 24 hr. of immersion. 2

10

2

Conclusions Iodocarboxylic acids have been utilized to stain amine containing areas in crosssections of an epoxy network. The technique shows that films prepared from an aqueous solution of a solid epoxy dispersion and amine adduct curing agent have a heterogeneous morphology that changes as a function of the time between mixing of the reactive components and the preparation of the film. At short time intervals, the amine phase separates within an epoxy continuous phase, whereas at later times the film is composed of particles that have amine rich shells. The change in morphology was attributed to compatibilization of the amine with the epoxy dispersion surface due to reactions between them. By grafting emulsifying polymer chains onto a hydrophobic and highly epoxy resin-compatible mixture of amines, a curing agent capable of emulsifying liquid epoxy resin was prepared. Films made from this system exhibited a much more uniform morphology, similar to that obtained from traditional solvent borne epoxy resin formulations. These films also exhibited a very high initial pore resistance as measured by electrochemical impedance spectroscopy.




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Acknowledgments. The authors thank Dr. A. Gilicinski for obtaining and assisting in the interpretation of the electrochemical impedance spectra. The assistance of Mrs. K.E. Everett, who prepared most of the coatings for this work is gratefully acknowledged.


Literature Cited 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Bolgar, L. Br. Patent 1 108 558. Rozenberg, B.A. Adv. Polym. Sci., 1986, 75, 113. (a) Elmore, J.D.; Cecil, J.D. U.S. Patent 4 315 044 , 1982. (b) Williams, P.R.; Burt, R.V.; Golden, R. U.S. Patent 4 608 406, 1986. (c) Becker, W.; Godau, C.; U.S. Patent 4 886 845, 1989. (d) Dreischoff, H.; Geisler, J.; Godau, C.; Hoenel, M . U.S. Patent 5 236 974, 1993. Wicks, Z.W.; Jones, F.N.; Pappas, S.P. Organic Coatings: Science and Technology, John Wiley and Sons, Inc.: New York, NY, 1992, Vol. 1, pp 35 48. Wisanrakkit, G.; Gillham, J.K. J. Coatings Tech. 1990, 62, (783), 35 - 40. (a) Winnik, M.A.; Wang, Y.J. J. Coatings Tech., 1992, 64, (811), 51 - 61. (b) Linne, M.A.; Klein, Α.; Miller, G.A.; Sperling, L.H.; Wignall, G.D. J. Macronol. Sci.-Phys., 1988, B27, 217 - 231. Wicks, Z.W.; Jones, F.N.; Pappas, S.P. op. cit., Vol. 1, ρ 47. Wegman, A. J. Coatings Tech., 1993, 66, (827), 27. Clear Enamel Formulation 24-294, Hi-Tek Polymers, Inc., 1988. (a) Kato, K. Polymer Letters, 1966, 4, 35. (b) El-Aasser, M.S.; Vanderhoff, J.W.; Misra, S.C.; Manson, J.A. J. CoatingsTech.;1977, 49 (635), 71. Trent, J.S.; Scheinbeim, J.I.; Couchman, P.R. Macromolecules, 1983, 16, 589. Hess, K.; Gutter, E.; Mahl, H. Naturwissenschaften, 1959, 46, 70. Walker, F.H.; Everett, K.E.; Kamat, S. Proc XXII High Solids, Waterborne, and Powder Coatings Symposium, 1995, 88. Galgoci, E. Proc. XXII High Solids, Waterborne and Powder Coatings Symposium, 1995, 119. Degooyer, W.J. U.S. Patent 4 539 347, 1985. Eckersley, S.T.; Rudin, A. J. Coatings Tech., 1990, 62, (780), 89 - 100. Lowrey, T.H.; Richardson, K.S. Mechanism and Theory in Organic Chemistry, Harper & Row: New York, NY, 1976, ρ 406. Hare, C. Protective Coatings; Technology Publishing Co.: Pittsburgh, PA, 1994; ρ 210. Wicks, Z.W.; Jones, F.N.; Pappas, S.P. op. cit., Vol. 1, ρ 70. (a) Monetta, T.; Bellucci, F.; Nicodemo, L.; Nicolais, L. Prog. Org. Coatings, 1993, 21, 353; (b) Rammett, V.; Reinhard, G. ibid., 1992, 21, 205.


Film Formation and Morphology in Two-Component, Ambient-Cured

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