Dianhydride-Cured Epoxy and High Glass-Transition Temperatures

Very high glass transition temperatures (Tg) in epoxy resin formulations are enabled by dianhydride curing agents. One question would be how high, particularly when compared with other more frequently utilized curing agents.

Table I compares the highest achievable Tg from a variety of curing agents when utilized with a standard epoxy resin.

It is crucial to note a few points to understand the significance of this data properly. Firstly, for any epoxy formulation, the highest achievable glass transition will never be more than 15-20 °C higher than the highest cure temperature used. This is a limitation that comes from the laws of polymer physics and chemistry.

The phenomenon “vitrifies” the material as the Tg builds up to the curing temperature. Effectively, this “freezes” the growing polymer network and limits the molecular-level movement of any leftover reactive species.

Further curing reactions are stopped, locking in the Tg. If the formulation allows, a further, higher temperature post-cure is needed to boost Tg.

Table 1. Highest achievable Tg from different curing agents in combination with standard liquid epoxy resin (DGEBA, EEW 182 g/eq; ambient viscosity 6-8 Pa.s). Appropriate post cures were utilized to ensure that Tg developed past vitrification stages. Source: CABB Group GmbH

Curing Agent Type Stoichiometry (curing agent / resin) Maximum Tg (°C)
Polyoxypropylene diamine D-400 Amine 1 56
Polyoxypropylene diamine D-230 Amine 1 90
IPDA (isophorone diamine) Amine 1 149
Dicyandiamide Catalytic 8 phr (g, per
100 g of resin)
120
2-Methyl imidazole Catalytic 4 phr (g, per
100 g of resin)
148
MTHPA (methyl tetrahydrophthalic anhydride) Monoanhydride 0.90-0.95 125
NMA (norbornene methyl anhydride) Monoanhydride 0.90-0.95 165
BTDA (benzophenonetetracarboxylic dianhydride) Dianhydride 0.45-0.55 238

 

The examples demonstrated eliminate this option by using sufficiently high cure temperatures as needed by each formulation. Here, the highest potential Tg from each example formulation mentioned will be compared.

Second, based on use levels for each curing agent, some variation is viable. The values shown are taken for optimum stoichiometry (usage) levels as recommended by key suppliers of the material classes outlined.

As shown, at a use level about half its stoichiometric theoretical amount, the dianhydride (BTDA) outperforms all other curing agents by a substantial margin.

It is worth noting that all examples in the table utilized the simplest, difunctional DGEBA epoxy resins, addressing both cost and security of supply issues for the formulator.  

This information has been sourced, reviewed and adapted from materials provided by JAYHAWK Thermoset Additives.

For more information on this source, please visit JAYHAWK.

 

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