Epoxy resin technology goes back to the early 1940s when Dr. Greenlee synthesized the first glycidated bisphenol A derivatives.5 These early epoxy resins gave significantly better performance than alkyds in industrial primers. Shell Chemical Co. took an immediate interest in this new chemistry because it was based on several of its base feedstocks — propylene leading to epichlorohydrin; and cumene leading to phenol, acetone and, ultimately, to bisphenol A. Shell, the original U.S. licensee of epoxy technology, concentrated on developing new applications for this chemistry,6 as well as developing commercial-scale manufacturing facilities to supply these new epoxy resins to the coatings industry.
Today, epoxies are the metal protection primer of choice due to their superior adhesion and outstanding overcoatability. Epoxies are the predominant binder used in the marine, industrial-maintenance and steel pipe coatings sectors. They are the preferred primers for use under polyurethane topcoats and the base polymer for cathodic electrodeposition primers on automobiles.
The first two-component waterborne epoxy system7 came to market in the late 1970s to early 1980s. It was characterized by slow dry, slow cure and short potlife. The second binder system came into being only two years later.8 The improved binder system extended the useable potlife from two hours to between four and six hours, but it still exhibited slow dry times and limited hardness development properties. Even with these limitations, customers liked these products because they were nonflammable and low odor, which made them well-suited for interior tile-like coatings for masonry substrates. Both systems are based on liquid epoxies that become water-miscible when combined with a “salted” amine curing agent. These waterborne binder systems are generally not suitable for metal applications because of inadequate corrosion resistance.
The first solid epoxy resin dispersion and modified amine adduct suitable for metal protection came into use around 1982.9 It was characterized by good potlife (6–8 hours), shorter dry times (4–6 hours) and corrosion resistance (500–1,000 hours salt fog, depending on film thickness).10 This second-generation system gives reasonable performance, but is limited by lower film hardness and poorer solvent resistance than conventional epoxy/polyamide solventborne coatings. In the mid-1980s, the third generation waterborne epoxy binder was developed.11 It is characterized by faster dry, better corrosion protection and improved water resistance. It can be formulated for improved solvent resistance, abrasion resistance and flexibility by changing the binder’s stoichiometry;12 however, it does not provide the total range of properties in one formulation. This binder system has become the industry’s performance standard for waterborne technology over the last 10 years.
Now, after more than two decades of waterborne epoxy usage, a totally “new generation” system has been developed that brings waterborne epoxy technology into a position to squarely challenge the performance of epoxy/polyamide systems on a much broader basis. This new waterborne epoxy technology has been trademarked as NEW GENTM, to distinguish it from previous binder systems. This new binder system13 uses a bisphenol-A/ECH, 1-type solid epoxy resin and a hydrophobic amine adduct hardener that are predispersed in water.13 Both dispersions use a proprietary nonionic surfactant that has been pre-reacted into the epoxy and amine resin oligomers. These dispersions have been designed for enhanced resin/curing agent compatibility and optimized reaction kinetics. Tables 1–2 show the typical physical properties for these two dispersions.
Results and DiscussionTraditional waterborne epoxy systems use curing agents that are either water-soluble or are rendered water-soluble by dissolving them in a co-solvent. The epoxy resin particles are inherently hydrophobic in nature. For the systems to cure, the water-soluble curing agent must diffuse into the epoxy resin particles. Since the curing agent prefers the water phase, relatively high concentrations remain at the epoxy resin particle’s surface throughout the film formation process. This high concentration leads to some crosslinking and surface hardening of the resin particles. Ultimately, the hardened shells of the resin particle retard complete coalescence, which results in film morphology having areas of higher resin or curing agent concentration.
In contrast, this new waterborne system has hydrophobic resin and curing agent particles that remain separate until coalescence, which, in this case, can simply be considered de-emulsification. This process occurs very rapidly and, if the dispersed materials have similar solubilities, will result in turbulent mixing at a particulate level. The result is excellent coalescence with resulting smooth film formation.14 The photomicrographs in Figure 1 show this effect quite dramatically.
Previous studies have shown that gloss development is a good predictor of film performance. The most likely explanation of this observation is that gloss development parallels film coalescence. Figure 2 shows the gloss development of each system — evaluated with and without an “induction” time. Although the third-generation system performs better with significant induction, it is more limited at the extremes of relative humidity. Customer feedback suggests that it is possible for the third-generation system to be effectively applied at high humidity, provided there is adequate ventilation or air movement over the substrate; but this restricts its overall field applicability. The test results in Figure 2 indicate that the NEW GEN waterborne system can generally be effectively applied at humidity levels as high as 92% R.H. but at 95% relative humidity, the system does not adequately coalesce.
Results of the salt fog, water immersion and condensing humidity results after 5,000 hours of exposure are shown in Figures 3–5. The electronic photos not only show the results of these tests but also demonstrate the capabilities of this new aqueous epoxy system. Please note that the rust above the scribed area is due to rust staining from an uncoated welded hanger attachment at the top edge of the panel. This welded attachment also caused the rust stain at the water/air interface on the immersion panels.
ConclusionThis article began by listing four key concerns regarding whether aqueous epoxy systems are capable of performing like a conventional solventborne, epoxy/polyamide system. Conventional epoxy/polyamide coatings based on 1-type solid resin and high-viscosity polyamides4have been the accepted performance standard for more than half a century but use of such coatings is increasingly limited by VOC regulations. A waterborne epoxy alternative that performs like this solventborne standard would find immediate application in many end uses.
Regarding film formation and physical properties, testing (described in the footnoted reference articles) indicates that the NEW GEN system can form smooth, glossy films like solventborne coatings. Furthermore, the final test film properties were comparable to those of an epoxy/polyamide coating. The results also indicate that the NEW GEN system not only can dry faster (at standard ambient cure conditions) but can also achieve better early hardness than the solventborne standard.
With respect to humidity extremes, tests indicate that the NEW GEN waterborne system may have a wider application window than current commercial waterborne systems. Independent feedback from customers evaluating this new technology has been consistent with these conclusions.
Concerning long-term film performance, the 5,000 hours of salt spray, hot-water immersion and condensing humidity resistance indicate that this new waterborne system can be a viable alternative to solventborne epoxy/polyamide coatings. Additionally, based on their experiences with this technology, several inhibitive pigment and additive suppliers have found that this new system surpasses previous levels of waterborne coating performance. With the expertise of coating formulators to optimize the starting point formulations tested in this salt spray study (by use of flash rust additives, optimized defoamers and flow agents), one might expect that this new system could become the industry’s performance standard for the next century.
For more information on waterborne epoxies, contact George Roy, PO Box 2463, Houston, TX 77252; phone 800/TEC.EPON; visit www.resins-versatics.com; e-mail firstname.lastname@example.org.
Sidebar: Epoxy Resins Demand to Reach 690 Million Pounds in 2004Demand for epoxy resins in North America is forecast to increase nearly 3% per year to 690 million pounds, valued at $1.4 billion, in the year 2004.
The industry is currently in the midst of a major restructuring period, with two of the three U.S. producers of liquid epoxy resins announcing the divestiture of their operations. In January, Ciba Specialty Chemicals entered into an agreement to sell its epoxy resin business to investment firm Morgan Grenfell.
As such, the next five years will be marked by integration and restructuring activities on the part of new owners. These and other trends are discussed in Epoxy Resins in North America, a study from the Freedonia Group Inc., Cleveland. Solid demand in the Canadian and Mexican markets, as well as ongoing shifts to higher solids, low-solvent formulations in both coatings and adhesives, will partially offset negative factors of the restructuring. The United States is predicted to continue its strong domination of epoxy production in the region, with output benefiting from stable domestic demand and solid export markets to Canada, Mexico, and Asia. With usage rates already high, North American suppliers will need to boost capacity to meet rising demands.
Coatings will remain the largest outlet for epoxy resins, accounting for over half of total demand in 2004. Applications include industrial coatings for container, appliance, furniture, marine, aerospace and other markets. As reformulation continues, epoxies are finding the bulk of their new applications in powder coatings and other environmentally friendly technologies, including waterborne, electrodeposition, and radiation-cured coatings.
Copies of the study are available from the Freedonia Group for $3,600. For more information, call Corinne Gangloff, 440/684.9600; fax 440/646.0484; or e-mail email@example.com.