Epoxy Design Challenges with Advanced Thermal Curatives

Epoxy resins are used globally in a number of industry sectors, such as energy, automotive and infrastructure. With a rise in the number of performance requirements and the number of end-use applications, the demands for curing agents that improve the durability and strength of these resins have also increased.

Curatives such as dianhydrides, monoanhydrides or blends of both form crosslinked thermoset networks and can improve the chemical, electrical, thermal, and mechanical behavior of epoxies. The polymers created can be used in applications ranging from adhesives to wire enamels and advanced composites.

Thermal curatives play an active role in electronic componentry for wind power.

Figure 1. Thermal curatives play an active role in electronic componentry for wind power.

Beyond curative choice, many variables can influence end-use performance.  Formulators have a wide variety of epoxy resins to choose from. They must consider the ratio of curative to epoxy, understand the epoxy and anhydride equivalents of these components and how to best balance them. Other additives such as accelerators, tougheners, fillers and flame retardants may also be needed. Of equal importance to formulating are mixing procedures, processing, and cure schedules.  Failure to carefully calibrate these variables can lead to systems that are difficult to work with and which may yield a finished product that does not meet the end customer’s specifications.

Anhydrides as Thermal Curatives

Anhydrides have been used as epoxy thermal curatives for decades, as they often provide ease of handling and excellent pot life. They also 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.

There are two classes of anhydrides used as epoxy curatives: monoanhydrides and dianhydrides.

Anhydrides do not react readily with epoxies at ambient temperatures and thus have the advantage of latency in formulated systems. In fact, anhydrides typically require the use of 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, such as NADIC® Methyl anhydride, or liquid anhydride blends based on methyltetrahydrophthalic anhydride (MTPHA) are among the most commonly used curing agents. They are highly reactive and favor high crosslink density, which in turn provides for enhanced physical properties of the cured systems.

NADIC Methyl anhydride.

Figure 2. NADIC Methyl anhydride.

Of those dianhydrides that are best suited to serve as an epoxy thermal curative, the highest mechanical, physical, and chemical property enhancements have been provided by 3, 3’,4,4’- Benzophenone-tetracarboxylic dianhydride (BTDA®) (Figure 3).

BTDA.

Figure 3. BTDA.

Glass Transition Temperature

Tg is the temperature range where a cured polymer transforms from a hard, glassy material to a soft, rubbery material. Materials that have high Tg retain their properties at high temperatures, for instance in chips, capacitors, resistors, and transformers which are exposed to soldering and aggressive in-service environments.

BTDA has been recognized for Tg improvements as high as 30°C in different epoxy resins. Figure 4 shows the Tg performance of different anhydrides.

Tg performance with anhydride curatives.

Figure 4. Tg performance with anhydride curatives.

Blends

The incorporation of BTDA in epoxy resin formulations can be a difficult task as BTDA is a high melting solid. While dry mixing is a commonly employed technique, liquid mixing has to be closely monitored.

However, blending BTDA in low viscosity monoanhydrides can enable the formulation of pastes and liquid dispersions that can make it easy to incorporate into liquid resins. This greatly simplifies processing for both new and experienced customers and increases the performance levels that can be achieved with monoanhydrides alone. Some of the various possibilities for blending systems are listed below:

Electricals

Blends can be employed to produce adhesives that attach integrated circuits to electronic components (Figure 5). They can be used in powder coatings that offer chemical, corrosion, and insulation resistance. Blends can be useful in molding compounds that provide dielectric and thermal performance. They can also be applied in manufacturing motor varnishes and wire enamels.

Powder coated electronic components.

Figure 5. Powder coated electronic components.

Syntactic Foams

Syntactic foams (Figure 6) that are employed in offshore drilling applications can use blends for various purposes. Currently cured with monoanhydrides, these foams increasingly require the crush resistance and temperature stability available from BTDA to compensate for the greater depths, higher pressures, and hotter extraction processes of today's drilling operations.

Syntactic foam insulation.

Figure 6. Syntactic foam insulation.

Fusion-Bonded Epoxies

Interior and exterior steel pipe coating applications are another area where blends can be used. Epoxies cured using blends can be employed as exterior coatings or primers for shale gas transmission and other onshore piping, or as interior coatings for petroleum and crude oil derivatives.

Epoxies improve the abrasion and corrosion resistance of underground pipe coatings that are exposed to bacteria, soil acids and alkalis, fungus and soil stress; and underwater pipe coatings that are exposed to corrosive gases, solvents, wastewater, petrochemicals, and salt water.

Conclusion

Thermal curing of epoxies with anhydrides can offer higher sustainable electrical and mechanical properties under extreme conditions. However, a suitable understanding of the effects of accelerator selection, stoichiometry, and processing is essential. In order to ensure that customers receive both the product and value-added support, using blended curatives from quality suppliers is essential. This helps customers to bring numerous advanced downstream technologies to the market.

NADIC is a registered trademark of Dixie Chemical Company

BTDA is a registered trademark of Jayhawk Industries AG

Partners in Your Success

Formulators and end users alike have a wide range of choices to make with regard to epoxy thermal curatives - opting for monoanhydrides, dianhydrides or alternatives.  The increasing design complexity of electronics and related materials, for example, regularly challenges formulators to meet the demands of in-service thermal and dielectric properties of epoxy systems.

For those well versed in epoxy formulating, the use of fine or micronized powder dianhydrides in dry mixes - for example, in electrostatic spray or fluidized bed processes - is well known.  However, in liquid preparations, blends of solid dianhydrides with liquid monoanhydrides have been used to improve ease of incorporation while enhancing performance.

Jayhawk Fine Chemical Corporation - a leading dianhydride manufacturer – and their partnership with Dixie Chemical Company - a well-known manufacturer of monoanhydrides and sundry products for epoxy chemistry – serves customers with more in-depth technical support and blended curative systems combining the best attributes of dianhydride and monoanhydride chemistry. These will not only simplify customers' work, but also enhance the characteristics of epoxy coatings, molding compounds, and composite materials - especially those relating to sustained mechanical, thermal, and dielectric properties in adverse conditions over longer periods of time

This information has been sourced, reviewed and adapted from materials provided by Jayhawk.

For more information on this source, please visit Jayhawk

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