Choosing the Right Epoxy Hardener for Fast, Low-temperature Curing

TAGS:  Epoxy Adhesives    

A cured epoxy resin should have a tightly cross-linked molecular structure for optimal physical and chemical properties. During the curing process, an epoxy resin is combined with a compatible catalyzing hardener to start the polymerization reaction. Curing characteristics such as pot life and cure speed, depend heavily on the choice of a hardener.

Thermoset properties such as tensile strength, glass transition temperature, adhesion, transparency and hardness can also be controlled to a certain extent by choosing an appropriate epoxy hardener. The desired balance of thermoset properties can vary for adhesives, coatings, composites and other applications.

Let's explore the key features offered by some commonly used classes of epoxy hardeners (phenols, anhydrides, amines, thiols...) and learn to select the right hardener for your applications.

Types of Epoxy Hardeners

Epoxies cure by forming a copolymer with polyfunctional curatives or hardeners. A hardener generally has a reactive hydrogen that reacts with the epoxide groups in the resin. Some commonly used classes of hardeners, in increasing order of reactivity are:

• Phenols
• Anhydrides
• Aromatic amines
• Cycloaliphatic amines
• Aliphatic amines
• Thiols

Most classes of hardeners require high temperatures of around 150°C for complete curing. Insufficient heat during cure results in incomplete polymerization and leads to sub-optimal mechanical, chemical and heat resistance properties.

To maximize cured epoxy performance, cure temperature should typically reach the glass transition temperature (Tg) of the fully cured resin. Some epoxy resin/hardener combinations, however, can be cured at ambient temperature.

Phenol Hardeners

Polyphenols, such as bisphenol A and novolacs react with epoxy resins at temperatures in the range of 130–180°C in the presence of a catalyst to form ether linkages. The resulting material displays better oxidation and chemical resistance than epoxies cured with amines or anhydrides. This class of hardeners is typically used for powder coatings as many novolacs are solids.

Anhydride Hardeners

Epoxy resins, thermally cured with anhydrides, show prolonged property retention at elevated temperatures. Reaction and subsequent crosslinking occur after the opening of the anhydride ring. Anhydride hardeners have high latency which makes them suitable for applications with mineral fillers, such as high voltage electrical insulators.

Amine Hardeners

Primary amines react with the epoxide group to form a hydroxyl group and a secondary amine. The secondary amine in turn reacts with another epoxide to form a tertiary amine and an additional hydroxyl group. The reactivity of a primary amine is approximately double that of the secondary amine.

Polyfunctional amines form a 3D cross-linked network. Aliphatic, cycloaliphatic and aromatic amines are all employed as epoxy hardeners with aliphatic amines being the most reactive and aromatic amines being the least reactive. Aromatic amines offer excellent end properties. However, in recent years concern about the health effects of aromatic amines has led to increased use of cycloaliphatic and aliphatic amines.

Thiol Hardeners

The sulfur in thiols readily reacts with the epoxide group, even at ambient or sub-ambient temperatures. During curing, thiols are ionized by a base catalyst. The high nucleophilicity of ionized thiol makes it highly reactive to the epoxy group. The thiol breaks the epoxy ring to produce a thiolate anion. The thiolate anion undergoes a proton exchange reaction to regenerate the base catalyst.


The high reactivity of thiols makes them ideal for applications that require a fast cure or curing at room temperature such as domestic DIY adhesives and chemical rock bolt anchors.

Primary and secondary thiols can both be used as hardeners for epoxy resins.

• In primary thiols, the carbon bonded to the thiol group is directly attached to only one other carbon atom.
• In secondary thiols, the carbon with the thiol group is attached to two carbon atoms. The extra carbon (methyl group) in secondary thiols, shields the ester structure.

This structural difference lowers the reactivity of secondary thiols (the affective of steric hindrance) but gives them several performance advantages over primary thiols.

Primary vs. Secondary Thiols as Epoxy Hardeners

Secondary thiols offer several benefits over primary thiols when used as a hardener for epoxies. Some of these benefits are discussed below:

Good Stability

Secondary thiols show better stability than primary thiols. Although their low-temperature curing rate is lower as compared to primary thiols, they still offer a remarkable improvement over amine hardeners in terms of curing rate. For applications that require good stability in addition to fast low-temperature curing, secondary thiols are a good choice.

High Water Resistance

Epoxy resins cured with primary thiols and modified amines can lose transparency when exposed to water. Epoxy resins cured with secondary thiols show improved water resistance. The extra methyl group in secondary thiols, protects the ester structure giving it higher water resistance. Epoxy resins cured with a secondary thiol resin remain stable after immersion in hot water, while primary thiol resin and modified amine resin cured epoxies can lose transparency and turn opaque.

Resistance to Acids and Bases

Epoxies cured with secondary thiols show good resistance to both acids and bases. Primary thiol cured epoxies can completely dissolve in a 5% NaOH solution within 7 days. Epoxies cured with amine hardeners show swelling under acidic conditions.

Strong Adhesion

Epoxy resins cured with primary thiols show good adhesion. For enhanced bond strength, primary thiols can be replaced by secondary thiols. Secondary thiols can increase adhesion strength by 50-80% as compared to primary thiols.

Transparency

Some applications require high transparency in the cured epoxy resin. Secondary thiols can be used to produce highly transparent cured epoxies. Use of a suitable catalyst with a secondary thiol can also help avoid yellowing. A phosphorus-based catalyst helps remove the yellowness observed with an amine-based catalyst producing a highly transparent solid.

Low Odor

Residual impurities are the major cause of unpleasant odor in primary thiols. Removal of these impurities, after the production process, can significantly reduce the odor. The odor of secondary thiols can be reduced to less than 5% of the odor that is typical of primary thiols such as PETMP.

Key Applications of Epoxy Hardeners

Formulation of One-component Epoxy Resins Fancryl 500 Series Monomers

Secondary thiols can be used to formulate one-component (1K) epoxy resins when combined with a latent catalyst. These 1K resins have excellent storage stability and rapid, low-temperature curability. The latent catalyst is activated by the dissolution of the powder upon heating. 1K formulations with latent curing catalysts help achieve a long shelf life as well as a short gel time, balancing stability and curability.

Ingredient	Quality (phr)
Bisphenol A type epoxy resin	100
Secondary Thiol	72
Curing Catalyst (imidazole or aliphatic tertiary amine)	0.5 - 5

One-Component Epoxy Resin Composition

Epoxy/Acid Anhydride Hardening Accelerator for High Heat Resistant Applications

For applications that require high heat resistance, Resonac also offers an acid anhydride epoxy resin curing agent “MHAC-P”. The curing agent has a unique Norbornane structure which enhances heat resistance, raises the glass transition temperature (Tg), and improves the insulation function of the cured resin. The extended pot life after compounding provides improved handling.

DISCLAIMER: All images, tables, and graphs in this article are used with the consent of Resonac.