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 hexamethylene diamine possess greater chemical resistance properties because (HMD) and trimethyl hexamethylene diamine (TMD). their very high functionality results in a very tight These chemicals share the high reactivity of ethylene crosslink density. Second, the larger number of aromatic amines, but they offer moderate flexibility because of ring structures increases heat resistance in the final sys- their greater linearity (compared to ring structures) and tem. These properties tend to make the pure novolac the presence of two terminal primary amines. These ter- resin systems more brittle than bisphenol A or bisphenol minal amine groups are unhindered because they are at F resin systems, but the problem of brittleness is general- opposite ends of the molecule. This accounts for their fast ly addressed by formulation techniques and hardener se- reaction time. This also allows for greater flexibility be- lection. An overview of relative performance properties is cause the distance between two reaction sites is maxi- outlined in Table 2. mized by their location. The closer and greater the num-

JPCL May 2000 51 Copyright ©2000, Technology Publishing Company ing solvent resistance generally re- Table 3: quires blending with other more sol- Relative Performance Differences among Amine Curing Agents vent-resistant hardeners. Color Low Temp Water Film Heat Solvent Acid Metaxylylene diamine (MXDA) pro- Stability Viscosity Cure Sensitivity Flexibility Resistance Resistance Resistance vides excellent compatibility with Excellent Low Good Low Good Excellent Excellent Excellent epoxy because of an aromatic ring structure, but its relatively low molec- IPDA DETA MXDA MDA PAR MDA DETA MDA ular weight allows for some blushing DCH TMD or carbonation to occur. Its heat resis- TMD MXDA TMD tance is better than other aliphatic PEA PEA amines discussed because of the aro- DETA AA DCH TMD PACM PACM IPDA IPDA MXDA matic backbone. But this same aro- IPDA DCH matic group limits flexibility and re- DCH DETA PACM PACM DCH DCH silience. The most distinct feature is DCH MXDA IPDA its ability to cure at lower tempera- PEA tures than other aliphatics because of PAR TMD its unhindered primary amines. AA TMD PACM IPDA MDA PACM PACM Polyamides and Amidoamines PACM IPDA MXDA Polyamides and amidoamines share AA MDA several significant advantages over MXDA PEA MXDA TMD aliphatic (ethylene) amines. These ad- IPDA DETA PAR vantages result from the introduction AA DCH AA AA of a fatty acid into the backbone of PAR PAR the epoxy hardener. Amidoamines are AA MXDA PAR AA TMD a reaction product of tall oil fatty acid DETA DETA (TOFA) and ethylene amines. PAR DETA PAR Polyamides are based on dimerized TOFA and ethylene amines. Ami- PEA doamines have a lower viscosity. PEA PEA Unique properties include improved MDA MDA MDA PEA flexibility and wet-out, improved ad- Poor High Poor High Poor Poor Poor Poor hesion, and outstanding water resis- Key: tance. Additionally, corrosivity and AA: Amidoamines MXDA: Metaxylylene diamine (aliphatic) health hazards are reduced due to DCH: Diaminocyclohexane (cycloaliphatic) PACM: bis-(p-aminocyclohexyl) methane (cycloaliphatic) lower functionality. (Corrosivity is a DETA: Diethylene triamine (aliphatic) PAR: Polyamide resin measure of how rapidly a substance IPDA: Isophorone diamine (cycloaliphatic) PEA: Polyetheramines (aliphatic) will corrode or degrade a surface. MDA: Methylene dianiline (aromatic) TMD: Trimethyl hexamethylene diamine (aliphatic) Generally, this is a measure of impact on contact with skin and eyes.) This lower functionality also leads to ber of reactive amine groups are, the harder and more in- longer usable pot life, less sensitive mix ratios, and signif- flexible the cure system will become. Their compatibility icantly less risk for carbonation and surface defects than with epoxy is also better than that of ethylene amines, is the case with aliphatic amines. However, solvent and but they too require modification to overcome carbona- acid resistance suffer as a result. tion. TMD is often used as a flow and leveling agent, but Polyamide resins (PAR) and amidoamines (AA) are cost prohibits its widespread use. used in primers or tie coats for steel and concrete. For Polyetheramines provide good color retention, good outstanding chemical resistance, however, other amino flexibility, and reduced carbonation tendencies but react hardeners are selected for topcoats. Modifications such as more slowly than other aliphatic amines. In addition, oxy- adduction are common to improve compatibility with genated solvents attack the polyether backbone. Enhanc- epoxy. Solvent resistance can also be improved by formu- 52 JPCL May 2000 Copyright ©2000, Technology Publishing Company lating with other amine hardeners Table 4: such as ethylene amines or other Epoxy Systems for Components of a Wastewater Treatment Facility short, highly functional curing agents. WWT Structures Epoxy Resin Type Curing Agent Type Cycloaliphatic Amines Administration building floors Bis A Aliphatic, cycloaliphatic Isophorone diamine (IPDA) and di- Aeration tanks Bis F Aliphatic, cycloaliphatic aminocyclohexane (DCH) are the Aerobic digesters Bis F Aliphatic, cycloaliphatic most widely used cycloaliphatic Anaerobic digesters Bis F Aliphatic, cycloaliphatic amines. This class of amine is charac- Air pollution control equipment Bis F Aliphatic, cycloaliphatic terized by an amino group on the six- Bar screen chamber Bis F Aliphatic, cycloaliphatic carbon ring structure. Through the Chemical feed room walls All All presence of a non-chromophoric ring All structure, IPDA and DCH provide the Chemical feed room floors All most light-stable systems of all amine Chlorine contact chambers EPN*, Bis F — structures. This structure also pro- Clarifier tanks, steel Bis F Polyamide primer, cycloaliphatic vides greater heat resistance than lin- topcoat ear aliphatic amines. Cycloaliphatics Clarifier tanks, concrete Bis A/Bis F Aliphatic, cycloaliphatic Control room floors Bis A/Bis F Aliphatic, cycloaliphatic Control room walls Bis A/Bis F Aliphatic, cycloaliphatic Demineralization units Bis A/Bis F Aliphatic, cycloaliphatic Distribution chambers Bis A/Bis F Aliphatic, cycloaliphatic Grit chambers Bis A/Bis F Aliphatic, cycloaliphatic Influent collection channel Bis A/Bis F Aliphatic, cycloaliphatic Lift stations Bis A Polyamide, aliphatic Manholes Bis A Polyamide, aliphatic Nitrification reactor tanks EPN Cycloaliphatic Operations building floors Bis F/Bis A Aliphatic, cycloaliphatic Process building floors Bis F/Bis A Aliphatic, cycloaliphatic Pump house floors Bis F/Bis A Aliphatic, cycloaliphatic Secondary containment EPN, Bis F Cycloaliphatic Sedimentation tanks Bis F/Bis A Aliphatic Slope/fill Bis A All Sludge thickeners EPN Aliphatic, cycloaliphatic Storage tanks, exterior Bis F/Bis A All, not topcoats Structural steel Bis F/Bis A All, not topcoats Hydrochloric acid tank in secondary containment at a desalination water treatment facility. Secondary con- Sumps All All tainment lining requirements here are similar to those Trenches All All for secondary containment at wastewater plants. *Epoxy Novolac

increase rigidity, which improves mechanical strength clohexyl) methane (PACM). This amine combines the low over both aliphatic and fatty acid-based amines. They are temperature cure characteristics of MXDA with the heat slower reacting than aliphatic amines but faster than resistance and mechanical strength of the other cy- polyamides. Chemical resistance (non-solvent) is superior cloaliphatics. It is characterized by two ring structures to aliphatic amines and polyamides. Carbonation is still a with ring-bound amines connected by a methylene potential problem, but adduction with epoxy has become bridge. It yields significantly tougher coatings than do standard in the industry to reduce this effect. In general, other cycloaliphatics because of its low functionality, accelerators are necessary to complete the reaction be- which results in a low crosslink density. It gives better tween the epoxy and the hindered ring-bound amine. solvent resistance than that given by PAR but poorer re- A relatively new cycloaliphatic amine is bis-(p-aminocy- sistance than that given by aliphatics. Its compatibility

JPCL May 2000 53 Copyright ©2000, Technology Publishing Company with epoxy is excellent due to the double ring structure, and carbonation is less likely than with aliphatic amines. Table 5: Acid resistance suffers as a result of the long distance be- Index of Epoxy Resins and Curing Agents tween reactive amines. The unhindered primary amines Chemical Name Abbreviation C.A.S.# are easily accessible for reaction, which makes this a fast- Resins reacting system. Adduction is a typical modification to re- Diglycidyl ether bisphenol A Bis A resin 25068-38-6 duce corrosivity and further improve compatibility with epoxy resin liquid (DGEBA) epoxy. Diglycidyl ether bisphenol A Bis A resin 25036-25-3 epoxy resin solid 9003-36-5 Aromatic Amines Diglycidyl ether bisphenol F Bis F resin (DGEBF) and their Modification Epoxy phenol novolac Novolac resin 028064-14-4 For years, the most widely used aromatic amine was (EPN) methylene dianiline (MDA). It offered a long pot life, the best acid and heat resistance, and excellent mechan- Curing Agents ical properties. It was also non-corrosive and non-irritat- Aminoethyl piperazine (aliphatic) AEP 140-31-8 ing, moisture insensitive, and flexible. Color stability Ethylene diamine (aliphatic) EDA 107-15-3 111-40-0 was its major drawback. Unfortunately, it has been Diethylene triamine (aliphatic) DETA Triethylene tetramine (aliphatic) TETA 112-24-3 found to be systemically toxic. In addition, restrictions Hexamethylene diamine (aliphatic) HMD 124-09-4 imposed by the U.S. Occupational Safety and Health Ad- Trimethyl hexamethylene diamine TMD 025620-58-0 ministration (OSHA) and health concerns in handling (aliphatic) MDA have forced most formulators to stop using MDA. Polyetheramines (aliphatic) PEA 9046-10-0 Fortunately, formulators have found alternative Metaxylylene diamine (aliphatic) MXDA 1477-55-0 chemistries to satisfy most of the demands for perfor- Polyamide resins PAR 68410-23-1 mance properties of MDA. Amidoamine resins AA 68953-36-6 2855-13-2 Table 3 summarizes the performance differences that Isophorone diamine (cycloaliphatic) IPDA Diaminocyclohexane (cycloaliphatic) DCH 694-83-7 can be expected by most of the types of amine-cured bis-(p-aminocyclohexyl) methane PACM 1761-71-3 epoxies described above. These differences are relative (cycloaliphatic) to each other within each class of epoxy. Ultimately, Methylene dianiline (aromatic) MDA 101-77-9 coating performance will depend on the formulator’s skill in designing a product for a specific niche or appli- cation. Nevertheless, this information on relative perfor- mance can assist decision makers in understanding the products specified and purchased. MSDSs helps the specifier locate information on the coatings’ ingredients and compare generically similar Practical Application coatings to learn of any significant differences between of the Chemistry Described them. To bring this chemistry into the world of wastewater treatment facilities, Table 4 identifies the appropriate Conclusion choice of a cured epoxy system for typical structures in This article has identified several variations within an a wastewater treatment facility. Table 5 gives names of epoxy formulation. Resin options include bisphenol A, the resins and curing agents discussed, their abbrevia- bisphenol F, and novolac to vary base functionality and tions, and their Chemical Abstracts Service (C.A.S.) chemical resistance. Modification of the hardener allows numbers. (The C.A.S. is an organization that provides for enhanced performance, cure properties, chemical re- an index to information in Chemical Abstracts, which is sistance, flexibility, and light stability. When selecting published by the American Chemical Society. The C.A.S. epoxies for wastewater facility application, it is important numbers help readers locate information about particu- to select the best and most appropriate formulation to lar substances in the Chemical Abstracts.) The material meet the demands of the environment without incurring safety data sheet (MSDS) of a coating will list the spe- excess cost. cific curing agents and resins in the coating composi- tion. The MSDS must list the C.A.S. number of each in- gredient. Thus, knowing the C.A.S. numbers from 54 JPCL May 2000 Copyright ©2000, Technology Publishing Company