ComparisonsComparisons ofof EpoxyEpoxy TechnologyTechnology forfor ProtectiveProtective CoatingsCoatings andand LiningsLinings inin WastewaterWastewater FacilitiesFacilities By John D. Durig, General Polymers, Cincinnati, Ohio, USA Aeration tank at a wastewater plant. Bis F epoxy resin with an aliphatic or a cycloaliphatic amine curing agent is appropriate. Editor’s Note: This article was first presented at SSPC (Photos courtesy of the author) 99, The Industrial Protective Coatings Conference and Exhibit, November 14-18, 1999, in Houston, TX, USA, and published in The Proceedings of the Seminars, SSPC 99-14, pp. 31-37. poxy technology and methods of curing and Epoxy Resin Technology E reacting with amine-based hardeners have There are three types of epoxy resins that find application continued to evolve since the first epoxy patents were is- in wastewater treatment facilities: bisphenol A, bisphenol sued in the 1930s. The possible reactions combined with F, and novolac resins. These resins all result from reac- wide-ranging formulation additives have resulted in a myri- tions of epichlorohydrin with phenolic compounds. The ad of products that can easily confuse decision makers type and number of phenolic groups determine both when it comes to product selection. Adding to the confu- physical and performance properties of the cured resin. sion is the wide range of environmental factors that must be considered when choosing a protective coating system. Bisphenol A Resin Structure This article will identify the primary differences be- Bisphenol A is a reaction product of phenol and acetone. tween three types of epoxies and four types of amine- Bisphenol A is reacted with epichlorohydrin to form based hardeners typically used in coatings for wastewater diglycidylether bisphenol A resin or DGEBA. The resul- treatment facilities. A brief description of chemical struc- tant epoxy resin is a liquid with a honey-like consistency. ture will assist those with some chemistry background, DGEBA is most often used in solvent-free coatings and but the important issue is the performance derived from flooring systems. each specific chemistry. After the discussion of perfor- The molecular weight of the formulation is increased mance, combinations of epoxy resins and curing agents by adding more bisphenol A to liquid DGEBA to form suitable for specific structures and areas in wastewater fa- semi-solid or solid resins. These resins are cut in solvent cilities will be identified. Finally, to assist decision makers to allow their use as maintenance primers for steel or as in selecting the right products, a simple method utilizing corrosion-resistant films. material safety data sheets (MSDS) will be presented. The higher the molecular weight is, the higher the vis- JPCL May 2000 49 Copyright ©2000, Technology Publishing Company Table 1: centipoises [cP]) that of bisphenol A Typical Epoxy Properties resins. Additionally, there is a higher proportion of trifunctional epoxy mol- Property DGEBA DGEBF Novolac ecules, which increases the function- Molecular weight 370 370 504 ality available for crosslinking from 1.9 to 2.1. Viscosity @ 25 C (77 F) 11,000-15,000 cps 2,500-5,000 cps 20,000-50,000 cps Epoxide equivalent weight 177-192 159-172 185-200 Novolac Epoxy Resins Functionality 1.9 2.1 2.6-3.5 Novolacs are modifications of bisphe- nol F resins formed using excess phe- nol. Bisphenol F is the simplest novolac resin, but should not to be confused with its related higher functionality analogs. Its functionality and performance properties are quite different than true novolacs. Bisphenol F epoxy resin performance in wastewater treatment facilities is between bisphenol A epoxy resin and true novolac. For the purpose of this article, bisphenol F epoxy resin will be considered its own class. This will be an important is- sue for decision makers; some manufacturers do not dis- tinguish between bisphenol F and other novolac epoxy resins. Not distinguishing between the two resins can re- sult in using a product that does not perform adequately or buying a more expensive and more durable system than the exposure environment requires. The viscosity of novolac resins is significantly higher than that of bisphenol F resins. As important, the func- Secondary containment at a wastewater treatment facility. An epoxy tionality is considerably greater. The higher viscosity and novolac resin or a Bis F epoxy resin is appropriate along with a cycloaliphatic amine curing agent. greater functionality of the novolacs make their heat and chemical resistance properties superior to those of bisphenol F. Table 1 summarizes the key chemical prop- erties of the epoxy resins described. cosity and functionality of the resin are. Therefore, in- creasing molecular weight brings the resin to a consisten- Performance Differences cy that requires solvent to allow for application to the among Bisphenol A, Bisphenol F, substrate. and Novolac Resins Functionality is fundamentally the number of sites Bisphenol A epoxy resin is the workhorse resin for the available for reaction with curing agents. More reactive majority of chemically curing epoxy coatings for concrete sites per molecule result in a tighter and more three-di- and steel. It is used extensively because of its excellent mensional crosslink density. Increasing functionality thus adhesion, toughness, wear resistance, and chemical resis- increases strength and chemical resistance, thus allowing tance. the resins to be used in maintenance primers. Bisphenol F resins have been steadily gaining ground in civil engineering applications because of their resis- Bisphenol F Resin Structure tance to a wider range of chemicals. There are two Bisphenol F is similar to bisphenol A except phenol is re- main reasons for their chemical resistance properties. acted with formaldehyde rather than acetone. The resul- First, bisphenol F systems have slightly higher function- tant phenolic chemical does not have the two methyl ality than bisphenol A. The higher functionality pro- groups that are present between the ring structures in vides more reaction sites, leading to a tighter and more bisphenol A resins. Bisphenol F is reacted with epichloro- three-dimensional crosslink density. Crosslink density hydrin to form diglycidylether bisphenol F (DGEBF) determines chemical resistance. Second, relative to resins. Because of the missing methyl groups, the viscosi- bisphenol A resins, bisphenol F resins have lower vis- ty of bisphenol F resins are typically 1/3 (2,500-5,000 cosity. Lower viscosity means fewer additives and dilu- 50 JPCL May 2000 Copyright ©2000, Technology Publishing Company Amine-Based Table 2: Epoxy Curing Agents Relative Performance Properties of Epoxy Resins For most coatings, the ambient tem- Bis A Bis F Novolac perature curing requirements demand Performance Property* Epoxy Epoxy Epoxy the use of amine-based curing agents. While the epoxy resin selection sets Adhesion 3 3 3 some limits on performance, the type 1 1 1 UV protection of curing agent provides significant Abrasion resistance 3 3 3 performance enhancements. Under- VOCs 2 3 3 standing the chemistry of basic curing Crystallization 1 3 3 agents will assist in recognizing the performance differences they impart. Moisture tolerance 3 3 3 The classes of amine hardeners in- 1 2 3 Heat resistance clude Chemical resistance 1 2 3 • aliphatic amines, –Sulfuric acid 1 2 3 • polyamides and amidoamines, –Acetone 1 2 3 • cycloaliphatic amines, and • aromatic amines. –Methanol 1 2 3 3 3 3 –Sodium hydroxide Aliphatic Amines –Organic acids 1 2 3 and their Modifications Aliphatic ethylene amines were the *Key: Scale is from 1 to 3, with 3 reflecting the highest performance among the epoxies described. first amine hardeners used in epoxy coatings. They are simple commodity chemicals that were available to react ents are needed to enhance application properties. Addi- with epoxies. They include aminoethyl piperazine (AEP), tives and diluents diminish the crosslink density, which diethylene triamine (DETA), ethylene diamine (EDA), and in turn lowers chemical resistance of any epoxy system. triethylene tetramine (TETA). The benefits of using these (Keep in mind, however, that resin content must be bal- amines include high reactivity (fast cure) at ambient tem- anced by formulators who impart other desirable prop- perature and excellent solvent resistance because of their erties by this means. Therefore, even with a pure high functionality. Disadvantages such as limited flexibili- bisphenol F resin system, additives are necessary to pro- ty and inefficient chemical reaction with the epoxy resin, vide the right surface appearance and application prop- which leads to surface carbonation or blushing, have rele- erties.) gated these amines to an additive status in their pure Compared to bisphenol A resins, bisphenol F resins state. Modifications such as adduction or pre-reaction also have less of a tendency to crystallize at low tempera- with a small amount of epoxy overcome flexibility prob- tures. Heating the resin will re-liquefy the crystals, but lems and improve compatibility with epoxy resins to re- heating is difficult to do on a job site. If crystallization is duce surface defects. Formulators generally use these severe, the crystals may appear in the final film. modified ethylene amines with other hardeners to obtain Novolac resins provide two important performance ad- desired performance properties. vantages over bisphenol F resins. First, novolac resins Other aliphatic amines include