Entry 1 – Considerations for Properly Formulating Epoxies with Dianhydrides

Welcome to the first entry of The Dianhydrides Diaries, a bi-weekly series that will explore all the benefits of dianhydrides, along with the chemistry, applications, tips, and advice on how to gain a competitive edge with your next project.

Welcome to the first entry of The Dianhydrides Diaries, a bi-weekly series that will explore all the benefits of dianhydrides, along with the chemistry, applications, tips, and advice on how to gain a competitive edge with your next project.

Dianhydrides cure epoxy resins to extremely high degrees of crosslinking via esterification reactions. This leads to superior performance, such as resistance to high temperature, chemicals, and electrical voltage. But to derive the benefits from such resin systems, the formulator must be careful of certain points.

During cure of dianhydride-epoxy formulations, the buildup of a densely crosslinked polymer network easily leads to premature vitrification well before full cure. Vitrification leads to residual reactive groups, which can cause potential problems especially during long term service. E.g., unreacted anhydrides groups, likely transformed to acid, will cause sensitivity to water and chemicals. Even if complete cure is somehow achieved, say via post-cure at elevated temperature, the finally realized crosslink density may be too high. This can lead to brittleness and generally non-optimum physical characteristics.

Therefore, dianhydrides are best used at far below stoichiometric levels vs the epoxy resin. (See a comparison with mono-anhydrides in the table below.) This approach brings the target crosslink density a bit lower and helps optimize the resulting physical properties. And it helps avoid residual anhydride (or acid) groups in the cured product. This also leads to many excess epoxide groups in the formulation. However, they will get reacted within the cured network, thanks to the wonderful side-reaction of epoxy homopolymerization. This etherification reaction lowers the crosslink density in the network and reduces brittleness, when compared with a resin system which is fully crosslinked via esterification alone with a dianhydride.

Interestingly, this epoxide side reaction is generally undesired by many epoxy formulators. And yet, users of dianhydrides benefit from it routinely, deliberately designing it into their formulations. Like they say – a knife can either harm or serve – it just depends on how you use it!

Table 1 – Suggested Anhydride/Epoxide ratios (A/E) for different anhydride curatives, with standard liquid epoxy resin (DGEBA with EEW 190g/eq). Fully stoichiometry would mean A/E =1.00.