Epoxy Resins and Hardeners Used in Paints and Coatings – A Guide to Hardener Selection

Epoxy resins are a general term for reactive prepolymers and polymer materials containing epoxide groups. Also known as polyepoxides and synthesized by Paul Schlank (1) in Germany in 1934, epoxy resins were first produced in their modern form in 1936 by Dr. Pierre Castan (2) and Dr. Sylwan Owen Greenlee (3). Since the early 1950s, they have found widespread applications across many industrial sectors. Although epoxy resins have many different application areas, as of 2020, approximately 23% of the epoxy resin market valued at USD 10.5 billion (4) comprises the paints and coatings industries. (5)

• Adhesives, • Composite materials, • Marine, • Wind turbines/energy, • Electronics, • Art, • Furniture, • Biomedical systems, • Aerospace and space technology

 

Some epoxy resin applications are shown in Figure 1 and technical properties are presented in Table 1.

[caption id="attachment_126717" align="aligncenter"] Figure 1. Different Epoxy Applications[/caption]   The most common epoxy resins are systems produced from a two-stage reaction between epichlorohydrin (ECH) and bisphenol-A (BPA). Different formulations have also been developed to produce specialty resins. The ECH and BPA reaction scheme is shown in Figure 2. In general, epoxies are known for excellent adhesion, chemical and heat resistance, strong mechanical properties and electrical insulation characteristics. The ability of epoxy resin systems to provide strong mechanical properties depends on the linear epoxy resins' ability to form three-dimensional cross-linked thermoset structures. This process is made possible through a curing reaction performed using materials called curing agents or hardeners (6). Common hardeners used for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols among their basic functional groups. Amine-based hardeners are particularly preferred in the paints and coatings sector due to their ability to cure at room temperature and to improve the physical properties of the product.

1. Hardener Selection According to Different Applications

Epoxy applications encompass considerable variety. In floor coating applications, alongside primer and topcoat systems, some examples of different work determined by application area and requirements are listed as follows: • Self-leveling, • Orange peel appearance, • Epoxy mortar, • Epoxy paint, • Anti-corrosion epoxy primer, • Epoxy ceramic adhesive, • Epoxy adhesive, • Epoxy joint filler. The epoxy resin and hardener selections to be used according to applications will differ, as will the additives and fillers that must be used, which should be determined based on system requirements. Hardeners used in primer applications are suitable for use in situations without color sensitivity, in industrial coatings and primers, in epoxy mortars, in high-filled systems and in applications requiring high chemical resistance. Before proceeding with application, a soil survey of the area to be coated should be performed and the surface cleanliness should be verified. If there are pores in the surface, they should be filled with repair mortar. After all floor preparations are complete, the surface is painted with an epoxy primer coat. For the second operation, the primer coat is allowed to fully cure. In some cases, an intermediate coat can be applied. After the epoxy primer coat application has fully cured, the final coat can be applied. In this operation, coloring is often preferred. If the mixture is transparent and a different color is desired, pigment paste is added to the prepared system for coloring. After the final coat operation is also completed, the application is allowed to fully cure. Information on curing time is provided in Table 2, where we share the technical properties of our products. Epoxy curing time can vary depending on the floor type and system to be applied.

2. Effect of Hardener Base Group

The use of different hardener groups affects the properties of the final product. The chemical resistance, gloss and color stability of the product vary depending on the effectiveness of the base groups to be used. In amine-based hardener groups, the effect of the selected base amine on the product properties has been schematically determined and presented in Figure 3. As can be seen in Figure 3, resins cured using aliphatic amine-based hardeners have good resistance to alkalis, certain inorganic acids and solvents. Curing at low temperature is provided and pot life is quite low. Aliphatic amine-based hardeners can be used particularly in applications requiring solvent resistance. On the other hand, cycloaliphatic amine-based hardeners are necessary in situations where color stability and gloss of the product are important. Having lower viscosity compared to aliphatic amine-based products provides ease of work while also contributing to the creation of a wide range of products in terms of chemical and mechanical resistance. Polyamide-based hardeners are preferred in primer applications where water resistance becomes important and in moisture barrier products. In addition, excellent adhesion can also be achieved. However, the long pot life and the transition process from film formation to full cure should be considered. With amidoamine-based hardeners, products with strong adhesive properties can be obtained. They should be used in situations where chemical resistance and water resistance are expected. The application areas of hardeners with different base amine groups are given in Figure 4.

3. Temperature

In epoxy curing reactions, curing rate is directly proportional to temperature. In systems using amine-based hardeners, room temperature is generally sufficient for curing; however, it should be noted that the curing reaction is an exothermic reaction and heat will be released during the period until gelation is complete. The amount of heat released in the stated exothermic reaction increases depending on the amount of epoxy used and the application thickness. More heat is retained in a large mass, which causes a faster reaction and more heat. The shape of the mixing container and the amount being mixed have a major effect on this exothermic reaction. A curing epoxy mass in a plastic mixing container can generate enough heat to melt the container, while the same amount spread in a thin layer allows the exothermic heat to dissipate. The thinner the applied epoxy layer, the less it is affected by exothermic heat and the more slowly it cures. If temperature decreases, the curing completion process will be prolonged. For faster curing, the ambient temperature must be increased. Figure 5 shows the effect of epoxy temperature on curing stages.

4. Resin/Hardener Ratio

The properties and quantities of epoxy resin and hardener, along with the resin/hardener ratio, form the most important factor group in determining quality and product functions. Besides this, the effectiveness of parameters such as fillers and other additives used, ambient temperature and thickness of the applied product also become important. 4. Resin/Hardener Ratio The properties and quantities of epoxy resin and hardener, along with the resin/hardener ratio, form the most important factor group in determining quality and product functions. Besides this, the effectiveness of parameters such as fillers and other additives used, ambient temperature and thickness of the applied product also become important.

Example Calculation:

Let us assume that the epoxy resin to be used is KER 828, which is well known in the market and has high viscosity. As component A, the epoxy equivalent weight (EEW value) of KER 828 which we selected is determined by the manufacturer and is given in TDS documents. For KER 828, this value (EEW) is defined as 188 (g/eq). Let us use Recure 503 as the hardener in this example. (See Table 2.) Recure 503 AHEW: 115 (g/eq) The mixing ratio varies depending on the resin and hardener brand. For this reason, it must be recalculated for each system and the mixture must be prepared in accordance with the calculated ratio for proper curing. Contrary to what might be thought, increasing the hardener amount for faster curing can disrupt the reaction balance of the system, causing it to remain more flexible. [caption id="attachment_126757" align="aligncenter"] Figure 7. Appearances of some hardener groups from left to right respectively; Recure 340, Recure 350, Recure 503, Recure 502, Recure 503 and Recure 507.[/caption]   Reaksiyon Kimya A.Ş. has been conducting raw material supply and production activities for the needs of the construction materials market for nearly 20 years. Epoxy resin, epoxy hardener, cellulose ether, powder polymer, pigment paste, wax and acrylic emulsion are among the main product groups that the company manufactures. Reaksiyon Kimya Anonim Şirketi operates production in a modern technology-equipped factory built on a 4,500 m² area in Kocaeli Dilovası Organized Industrial Zone. The company's entire operational processes are supported by a fully equipped research and development and quality control department to ensure customer satisfaction. With the importance it attaches to R&D work, the company's sole objective is to increase production volume and reduce Turkey's dependence on imports in this sector.

References 1. P. Schlack, I. G. Farbenindustrie, German Patent 676 117 (1938), US Patent 2 136 928 (1938). 2. US 2444333, Pierre Castan, "Process for the manufacture of thermosetting synthetic resins by the polymerization of alkylene oxide derivatives", issued 1948-06-29, assigned to DeVoe & Raynolds 3. ^ US 2456408, Sylvan Owen Greenlee, "Synthetic drying compositions", issued 1948-12-14, assigned to DeVoe & Raynolds 4. TMR, (2015). SpecialChem Coatings and Inks Formulation, (Website: http://www.specialchem4coatings.com) Copyright TMR 5. Epoxy Resins Committee > Socioeconomic Analysis 2013, https://epoxy-europe.eu/wp-content/uploads/2018/11/Epoxy_Socioeconomic_Study_Main_Findings_August-2017.pdf 6. Hakiki, Farizal et al. (2015). Is Epoxy-Based Polymer Suitable for Water Shut-Off Application? SPE-176457-MS. SPE/IATMI Asia Pacific Oil & Gas Conference and Exhibition, 20–22 October, Nusa Dua, Bali, Indonesia. doi: 10.2118/176457-MS