A Sticky Situation: Epoxies and Curatives (2024)

Original article date: Nov. 1, 2019
Updated May 11, 2022

Epoxy resins have many applications, including metal coatings, electronics/electrical components, electrical insulators, fiber-reinforced plastic materials and adhesives.

Uncured epoxy resins have only poor mechanical, chemical and heat resistance properties. Better properties are obtained by reacting them with curatives to form three-dimensional cross-linked thermoset structures. Curing of epoxies is an established mature technology. Here, Andy Pye covers the established technologies and emphasizes recent developments.

Increasingly, there is an emphasis on polymers being biodegradable. Most biodegradable resins are produced from natural, water-based, non-toxic materials, such as starch, organic sugar or oils. However, tests show that two bacteria, Rhodococcus rhodochrous and Ochrobactrum anthropic, will cause biodegradation of epoxy resin. The decomposition rate increases rapidly with reaction temperature. Some sources claim it takes between 5 to 7 days for compostable resins to decompose, provided that conditions are favorable.

Note also that partially cured resins can be poisonous if inhaled, ingested, or absorbed into the body. Inhaling the vapors of epoxy resin, especially during sanding, can result in asthma and other respiratory problems.

Common classes of curatives for epoxy resins include amines, acids, acid anhydrides, phenols, alcohols and thiols. While some epoxy resin/hardener combinations will cure at ambient temperature, many require heat, with temperatures up to 150°C being common. Temperature is sometimes increased in a step-wise fashion to control the rate of curing and prevent excessive heat build-up from the exothermic reaction. Other additives such as accelerators, tougheners, fillers and flame retardants may also be needed.

Anhydrides

Anhydrides have been used as epoxy thermal curatives for decades, as they often provide ease of handling and excellent pot life. The low viscosity and high latency of anhydride hardeners make them suitable for processing systems which require the addition of mineral fillers prior to curing, such as for high voltage electrical insulators. There are two classes of anhydrides used as epoxy curatives: monoanhydrides and dianhydrides.

Generally, they impart a superior mix of performance properties, including high tensile and flexural strength, thermal stability, dielectric behavior, resistance to a wide range of chemicals and minimal shrinkage during cure. The effects of accelerator selection, stoichiometry and processing are also important.

Anhydrides do not react readily with epoxies at ambient temperatures and typically require an accelerator to speed curing to meet cycle time requirements in processes such as pultrusion or filament winding. Liquid monoanhydrides are frequently favored in epoxy formulations for ease of mixing and metering. Alicyclic anhydrides or liquid anhydride blends based on methyltetrahydrophthalic anhydride (MTPHA) are among the most commonly used curing agents. They are highly reactive and favor high cross-link density, which in turn provides for enhanced physical properties of the cured systems.

Polyphenols

Polyphenols, such as bisphenol A or Novolacs, can react with epoxy resins at elevated temperatures (130–180°C), normally in the presence of a catalyst. The resulting material has ether linkages and displays higher chemical and oxidation resistance than typically obtained by curing with amines or anhydrides. Since many Novolacs are solids, this class of hardeners is often employed for powder coatings.

Halogenated epoxy resins, in particular brominated and fluorinated epoxy resins, are also used for special purposes. Brominated bisphenol A is used when flame retardant properties are required, such as in printed circuit boards. Fluorinated epoxy resins have been investigated for their high chemical resistance and low water absorption. However, commercial use of fluorinated epoxy resins is limited by high cost and low glass transition temperature (Tg).

Amines (see Table 1)

Amines are classified into primary, secondary, and tertiary amines, in which one, two, and three hydrogen molecule(s) respectively of ammonia (NH3) have been substituted for hydrocarbon. Amines are also classified into aliphatic, alicyclic and aromatic, according to the types of hydrocarbons involved, and they are all important curing agents for epoxy resin.

Aliphatic amines cure at room temperature, giving excellent properties and heat resistance around 100°C. Resins that have been cured using aliphatic amines are strong and have excellent bonding properties. They resist alkalis and some inorganic acids and have good resistance to water and solvents, but they are not so good when exposed to many organic solvents. Aliphatic amine irritates the skin and is toxic, although those that have high molecular weight and low vapor pressure are less toxic.

Aromatic amine has been developed to achieve greater heat and chemical resistance than aliphatic amine but only slowly cures at room temperature. In addition, it has good electrical properties and excellent chemical resistance, particularly against alkalis, and thus it is a curing agent that is highly resistant to solvents.

Ketimine is attracting attention as a curing agent for high solid paints and is formed by the reaction between aliphatic polyamine such as diethylenetriamine (DTA), p-tolylamine (TTA), dipropylenediamine (DPDA), and m-xylylenediamine (m-XDA), and ketones such as methylethyl ketone (MEK) and isobutyl ketone (MIBK). Ketimine cures very slowly when mixed with epoxy resin, but it works as a kind of latent curing agent by absorbing moisture in the air and regenerating amines to cure at room temperature.

Polyamide resin has been widely used as a curing agent for epoxy resin. It will cure at or below normal temperature with moderate heat generation. It cures so slowly that it has a long pot life. The cured resin features high tensile, compression, and bending strengths, while it is stiff, strong, and excellent in shock resistance.

Imidazoles

These anionic polymerizing curing agents are characterized by a relatively long pot life, the ability to form cured resin with a high heat deformation temperature by thermally treating at a medium temperature (80 to 120°C) for a short time, and the availability of various derivatives having moderate reactivity that improved workability. They are used in compound resin compositions, such as one-part thermosetting coating adhesives, casting materials, and filling materials.

In addition, imidazoles can be used as an accelerator for organic-acid anhydrides, dicyandiamide, polyhydric phenol and aromatic amine.

Thiols (mercaptans)

Also known as mercaptans, thiols contain sulfur, which reacts very readily with the epoxide group, even at ambient or sub-ambient temperatures. Curing at 0°C to -20°C, they require a tertiary amine as an accelerator. At normal temperature, polymercaptan has a pot life of 2 to 10 minutes and rapidly cures and reaches practical strength in 10 to 30 minutes. While the resulting network does not typically display high temperature or chemical resistance, the high reactivity makes it useful for domestic DIY adhesives and chemical rock bolt anchors.

Polysulfide

Polysulfide resin does not have low-temperature and fast-curing properties and is used as a curing agent which doubles as a flexibility. Due to its good water resistance, polysulfide resin has been used in adhesives, sealing agents, and casting materials.

References:

1. Epoxy resins cured with Dytek amines have excellent properties for use in coatings, civil engineering, adhesives, marine, and composite applications.
   https://dytek.invista.com/markets-and-applications/differential-reactivity/
2. D.J. Weinmann, K. Dangayach, and C. Smith—Shell Chemical Co.*. Amine-Functional Curatives for Low Temperature Cure Epoxy Coatings. Journal of Coatings Technology, Vol. 68, No. 863, December 1996
3. Properties and performance of amine based curing agents. Osamu Hara Custom Group, Technical Department Research Laboratory Three Bond Co Ltd. Curing Agents for Epoxy Resin [PDF], Three Bond Technical News, December 20, 1990. See table 1 on page 10 – note that this is a different Table 1 to that included in this article!
4. Epoxy Design Challenges with Advanced Thermal Curatives; Sponsored by Jayhawk Fine Chemicals Corporation Sep 5 2016.
5. Is Resin Biodegradable?

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