The following article examines the reasons underpinning the high performance of anhydride curing agents in epoxy resin applications.
The Types of Curing Agents for Epoxy Resins
Epoxy resin formulations can be cured using a variety of curing agent (hardener) chemistries, each offering distinct advantages. Common options include amines, anhydrides, dicyandiamides, dihydrazides, imidazoles, organic acids, and boron trifluoride complexes.
Some, such as tertiary amines and substituted imidazoles, are often used in smaller amounts and can also act as accelerators when combined with a primary curing agent from this broader list.
Selecting the right curing agent requires a methodical approach, considering factors such as performance targets, curing time and temperature, formulation cost, stability, handling challenges, and—most importantly—health and safety concerns.
Amines represent the most widely used category of epoxy curing agents, dominating technical literature more than any other class. One key reason for their prevalence is that many amine-based epoxy systems can cure at ambient temperature, whereas other curing agents typically require external heat to be effective.
Anhydrides
After amines, anhydrides are the second most widely used class of epoxy-curing agents. What sets them apart is their ability to react with both oxirane (epoxide) and secondary hydroxyl groups present in commercial epoxy resins.
Additionally, when an anhydride reacts with an oxirane group, it generates a secondary hydroxyl group along the epoxy resin molecule.
As the curing reaction progresses, this newly formed hydroxyl group further reacts with another anhydride group within the mixture. This reaction pathway leads to a higher degree of crosslinking in cured epoxy systems than one might initially expect.
Figure 1. A simplified two-step cure mechanism for the esterification reaction between epoxides and anhydride groups which leads to crosslinking. Image Credit: CABB Jayhawk Fine Chemicals
The widely used industry-standard liquid epoxy resins are derived from diglycidyl ethers of bisphenol A (DGEBA), such as those with epoxide equivalent weights (EEW) ranging from 176 to 190 g/eq.
These resins are recognized as di-functional in terms of their epoxide groups, and when used with amine curing agents, they function as difunctional reactants. However, with anhydrides, such an epoxy resin behaves as a tetra-functional reactant, enabling significantly higher levels of crosslinking.
Figure 2. Chemical structure for standard difunctional epoxy resins (diglycidyl ethers of bisphenol A). Image Credit: CABB Group GmbH
A key advantage of high crosslink density in cured epoxy systems is enhanced performance, including exceptional heat resistance, electrical insulation, solvent resistance, and overall durability.
To achieve these specialized properties, epoxy formulators often turn to higher-functionality epoxy resins, such as epoxy novolacs and multifunctional epoxies (glycidyl amines).
However, it’s important to note that similar performance can often be achieved with standard epoxy resins simply by using anhydride curing agents—particularly when dianhydrides are incorporated into the formulation.
There are three common anhydride types used with epoxy resins:
1. Monoanhydrides
Monoanhydrides can be either alicyclic or aromatic. Common alicyclic monoanhydrides used in epoxy systems include methyltetrahydrophthalic anhydride (MTHPA), methylhexahydrophthalic anhydride (MHHPA), and NADIC methyl anhydride (NMA). Phthalic anhydride (PA), an aromatic monoanhydride, was widely used in the past but is now being phased out due to its volatility. At ambient temperatures, PA sublimates into the air, posing fire and inhalation hazards. As a result, less volatile alternatives like MTHPA and MHHPA are now preferred.
A monoanhydride is essentially a dicarboxylic acid that has undergone dehydration to form an anhydride ring. When the ring opens, it becomes difunctional in terms of carboxyl groups, enabling its reactivity in epoxy curing. Commercial monoanhydrides are typically supplied as eutectic mixtures to maintain a liquid state at ambient conditions, making them easier to handle in various epoxy applications.
Figure 3. Examples of monoanhydrides (MTHPA & NMA) used to cure epoxies. Image Credit: CABB Group GmbH
2. Monoanhydride with a Free Carboxyl Group
Trimellitic anhydride (TMA) is a well-known commercial example of this category. As an aromatic monoanhydride, TMA contains a single anhydride ring along with a free carboxyl group. Chemically, it is derived from trimellitic acid, a tricarboxylic acid in which two of the acid groups have been dehydrated to form the anhydride ring, leaving the third carboxyl group intact.
When the anhydride ring opens during curing, TMA becomes trifunctional in terms of carboxyl groups. This added functionality results in higher crosslink densities compared to standard monoanhydrides, enhancing the performance of epoxy systems. However, due to its classification as a Substance of Very High Concern (SVHC) by the European Chemicals Agency (ECHA), TMA is being phased out of many epoxy applications.
Figure 4. Chemical structure of trimellitic anhydride (TMA). Image Credit: CABB Group GmbH
3. Dianhydrides
Dianhydrides can be aliphatic (like butanetetracarboxylic dianhydride), alicyclic (such as cyclopentanetetracarboxylic dianhydride), or aromatic, including compounds like BTDA (3,3’,4,4’-benzophenone tetracarboxylic dianhydride) and PMDA (pyromellitic dianhydride).
In epoxy applications, aromatic dianhydrides are typically favored due to their ability to enhance high-temperature performance. While dianhydrides are difunctional in terms of anhydrides, they exhibit tetrafunctionality regarding their equivalent carboxyl groups, effectively making them tetrafunctional for epoxy curing.
Since a simple difunctional epoxy resin already acts as a tetrafunctional reactant, this combination can lead to extremely high crosslink densities, resulting in outstanding dielectric properties, solvent resistance, and elevated glass transition temperatures.
Although PMDA contributes many high-performance characteristics, its rigid backbone structure can render the cured epoxy excessively brittle. In contrast, BTDA enables a good balance of high crosslinking and sufficient flexibility. This flexibility stems from the central carbonyl group bridge within the BTDA molecule, which permits some rotational movements within the structure.
Figure 5. Chemical structures for 3,3’,4,4’-benzophenone tetracarboxylic dianhydride (BTDA) and pyromellitic dianhydride (PMDA). Image Credit: CABB Group GmbH
In anhydride-cured epoxies, the highest levels of crosslinking are attained using dianhydrides. However, it can be advantageous to optimize formulations based on other criteria, as mentioned earlier in this article.
Utilizing stoichiometrically excess epoxy resin (i.e., reducing the amount of dianhydride) may help balance other properties, such as mechanical performance.
For instance, a glass transition temperature (Tg) of 239 °C was achieved with Olin D.E.R.® 383 epoxy resin cured with only half the BTDA compared to full stoichiometry. Alternatively, formulations can be made easier to handle in the customer's curing process by incorporating liquid monoanhydrides alongside a dianhydride.1
Figure 6. Dynamic mechanical analysis (DMA) of Olin D.E.R. 383 Epoxy resin cured with BTDA. The anhydride/epoxide equivalent ratio (A/E) for the formulation was 0.5, meaning only half the BTDA was used vs. that required per stoichiometry. The excess resin helps manage crosslink density and mechanical performance. Image Credit: CABB Group GmbH
To summarize, anhydrides offer unique benefits to epoxy formulations due to the unique chemical reaction mechanisms with which they cure epoxy resins.
Reference:
- V. Mishra, J. Dimmit, N. Bilic, Dianhydrides: powerful curatives for the epoxy formulator’s toolbox, Proceedings from 2022 annual meeting of Thermoset Resin Formulators Assoc., Dallas, TX, USA.
This information has been sourced, reviewed and adapted from materials provided by CABB Group GmbH.
For more information on this source, please visit CABB Group GmbH.