Dianhydrides are able to cure epoxy resins to extremely high degrees of crosslinking. This results in enhanced performance, for example, resistance to chemicals, high temperature and electrical voltage.
However, the formulator must be careful of certain points to derive the advantages from such resin systems.
The rapid buildup of a densely crosslinked polymer network easily results in vitrification, trapping unreacted anhydride groups. Likely transformed to acid, these sites may create sensitivity to chemicals and water exposure.
When a complete cure is achieved, for example, via post-cure at elevated temperature, the finally realized crosslink density could be too high. This can result in brittleness and generally non-optimum physical characteristics.
So, dianhydrides are best utilized at far below stoichiometric levels vs the epoxy resin. The table below shows a comparison with mono-anhydrides. This method brings the target crosslink density slightly lower and helps optimize the resulting physical properties.
This also helps to avoid residual anhydride (or acid) groups in the cured product, which results in many excess epoxide groups in the formulation. Via the side-reaction of epoxy homopolymerization, they will react within the cured network.
This etherification reaction decreases brittleness and lowers the crosslink density in the network when compared with a resin system which is fully crosslinked via just esterification with a dianhydride.
Many epoxy formulators do not usually want this epoxide side reaction, yet, users of dianhydrides frequently benefit from it, deliberately designing it into their formulations.
Table 1. Suggested Anhydride/Epoxide ratios (A/E) for different anhydride curatives, with standard liquid epoxy resin (DGEBA with EEW 190 g/eq). Fully stoichiometry would mean A/E =1.00. Source: CABB Group GmbH
||Suggested A/E ratios for
|BTDA (Benzophenone tetracarboxylic dianhydride), a dianhydride
||0.50 - 0.60
|NMA (Nadic methyl anhydride), a mono-anhydride
||0.90 - 0.95
|MTHPA (Methyl tetrahydrophthalic anhydride), a mono-anhydride
||0.93 - 0.97
This information has been sourced, reviewed and adapted from materials provided by JAYHAWK Thermoset Additives.
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