Formulating Tips for Epoxy-Anhydride Cure Systems

In this interview, experts from Jayhawk Fine Chemicals talk to AZoM about epoxy-anhydride cure systems.

Tell us about epoxy anhydride systems, and what value they bring to applications.

Epoxies, especially the DGEBA class of resins, respond beautifully to anhydride curing and have been regularly used this way since the 1960s. The value of these systems is their ability to retain physical, mechanical and electrical properties under high-temperature conditions for extended periods of time.

They have worked best in a variety of applications, including fusion bonded epoxies for anti-corrosive coatings in infrastructure; electric and autonomous vehicles, which consist not only of electric motors, but wire and cable, display systems and telematics; industrial lighting, specifically LED systems; and energy exploration where syntactic foams find use as an effective thermal insulator for rigid pipe in deepwater applications.

What are some of the challenges and requirements facing the development of these systems for new applications?

Challenges arise not only from processing but also from properties.

Processing questions now being asked are: "Can curing temperatures be reduced to as low as 140° C?” “By how much can we reduce the curing cycle?”

Heat resistance, a traditional propertyadvantage for epoxy-anhydride chemistry, implies that there is a high degree of cross-linking density, and in some cases, a high degree of rigidity. The question now asked is: “How can I have a reasonable amount of heat resistance with less rigidity?”

In the case of color, which may have been tolerated in the past, and not an issue for pigmented formulations,  water white appearance is now being requested.

A traditional mass cast type of formulation, for example in electrical insulators,  incorporates inorganic fillers, but the modern question being asked is: “What are the effects of carbon and glass fibers in composite formulations cured with anhydrides?”

Epoxy coating powders are prevalent in applications such as electric and autonomous vehicles. Image Credit:Shutterstock/nrqemi

Could you please give us a short background on epoxy-anhydride curing mechanisms?

There are three reactions in classic epoxy-anhydride chemistry. What's important to consider is that anhydrides do not react directly with epoxide groups.

The anhydride first seeks out a hydroxyl group, present either on the resin backbone, introduced as alcohol, as added water, or a contaminant within the system.  These two constituents react and form a monoester.

The monoester reacts directly with the epoxide and forms a diester, the essential component required for propagation and formation of the three-dimensional network, the epoxy polymer.

In some cases, there is a homopolymerization reaction, involving the combination of an epoxide with a hydroxyl group on the resin backbone.

Are there difficulties in mixing the different components of an epoxy formulation?

There are a variety of different components: some are solids, some are liquids, and some are pastes. For example, gentle heating of ingredients at 35 °C facilitates the mixing of solid dianhydrides in viscous epoxy resins. It's important to understand that simpler is quite often better.

What anhydride:epoxy ratios do you recommend?

The best technique is to calculate theoretical amounts based on stoichiometry, and then dial back considerably. This is recommended due to the high reactive nature of anhydrides, especially dianhydrides. Maintaining stoichiometry creates a system that may "run away" during cure and not crosslink correctly.  Empirical ratios actually respond better.

Epoxy coated slot insulation on motor armature of an electric motor.

Is it possible to cure the systems at room temperature?

Epoxy-anhydride systems require elevated cure temperatures. There are options available for curing at lower temperatures; however, these materials cannot be cured at room temperature.

There are certain types of anhydride systems that will cure at a moderate temperature, say 80 °C/100 °C, but then high Tgs are sacrificed.

What system would you recommend for a syntactic foam core with a prepreg outer layer?

Our previous work in the area of syntactic foams focused on the BTDA-NMA blend as curative.  There are actually a variety of monoanhydride combinations that can work with BTDA, but we achieved very favorable results with BTDA-MNA, and this speaks to the mass-cast, gentler exotherm processing discussed previously.

Can prepreg be formulated with epoxy-anhydride systems?

There was some concern initially that these formulations would not work well in a prepreg application, but the limited work we've done has shown it to be possible.  The two phase system is fully integrated with the fiber allowing for a good cure. We haven’t yet looked at shelf life stability on these systems, but in terms of preparing and using the prepreg in a relatively short period of time, they can work well.

Mass-cast epoxy insulators for power lines.

How does reheating, warming above Tg, affect strength and longevity?

Once cure or post-cure is established, the composite is basically a thermoset, and reheating above Tg will simply soften it. However, as long as it's not under undue stress, it can retain its dimensional stability, and regain most of the original properties upon cooling. So, barring degradation, the materials should be fairly innocuous to a change in temperature.

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Drawing from your work, do you have any tips for formulating epoxy-anhydride systems?

We came up the following key formulating tips based on our recent study of epoxy resins, anhydrides and accelerators:

1. Monoanhydrides greatly benefit from accelerators. They rely upon them to drive cure. Tertiary amines and imidazoles work well here.
2. Dianhydrides, particularly BTDA, don't require accelerators if a longer curing cycle can be tolerated. To expedite that, the best response can be obtained either from tertiary amines or ureas.
3. For the ultimate in Tg development and cross link density, BTDA is a very favorable choice.
4. When seeking a lower curing temperature, a faster cycle time, and an overall more rapid system, tertiary amines and BTDA-MNA blends work well together.
5. For latent systems, BTDA and ureas provide a good level of performance.
6. Homopolymerization, previously considered as a lesser curing mechanism, occurs in all systems. The best results for managing homopolymerization were obtained with tertiary amines.
7. For mass casting, prepregs, anything that may build heat during the cure cycle, consider the lower enthalpy afforded by BTDA and tertiary amines.
8. Perhaps less of a tip and more of an observation, one-component systems continue to be a challenge, due to the presence of contaminants, for example, the free acid content of anhydrides.  

How do I know I've achieved the right balance between processing and properties?

This is a multi-step exercise. Our view is that it's best to just start with the raw materials suppliers' recommendations. If the results and performance are somewhat lower than anticipated, then look further into tuning the formulation and process variables. Every change adds cost and resources, so it’s important to be practical with testing and expectations.

What types of areas are you looking to be working on in the future?

We've focused on Tg development, cross link density and cure efficiency, but we'd like to delve further into system rigidity. A study of different types of tougheners would be very helpful to understand how we can manage rigidity. A certain level of stiffness is good, and it can be offset by tougheners to reduce the rigidity of the system.  DSC has proven to be an excellent tool to investigate behavior, establish certain trends and make recommendations, and we intend to do more.

Where can our readers go to find out more?

To find out more please visit our website https://www.jayhawkchem.com/

About CABB Group GmbH

CABB Group GmbH is a leading custom manufacturer of starting materials, active ingredients and advanced intermediates; a major producer of high-purity monochloroacetic acid; and a supplier of premium fine chemicals. We are small enough to focus on attentive customer partnerships, yet large enough to master complex chemical synthesis. Customers benefit from CABB’s manufacturing excellence, product quality, security of supply, and collaborative approach to sourcing solutions.  Custom manufacturing services are offered from three complementary multi-purpose production sites: Kokkola, Finland; Pratteln,Switzerland; and Galena, Kansas USA ("Jayhawk")."