Welcome to the fourth 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.
You have heard this before. Dianhydride curing agents allow for very high glass transition temperatures (Tg) in epoxy resin formulations. A fair question would be how high, especially in comparison with other more commonly used curing agents.
So here we go. Table I compares the highest achievable Tg from an assortment of curing agents when used with a standard epoxy resin.
Table I. 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.
(curing agent / resin)
|Polyoxypropylene diamine D-400||Amine||1||56|
|Polyoxypropylene diamine D-230||Amine||1||90|
|IPDA (isophorone diamine)||Amine||1||149|
(g, per 100g of resin)
|2-Methyl imidazole||Catalytic||4 phr
(g, per 100g of resin)
(methyl tetrahydrophthalic anhydride)
(norbornene methyl anhydride)
To properly understand the significance of these data, it is important to note a few points:
- First, for any epoxy formulation, the highest achievable glass transition will never be more than 15-20°C higher than the highest cure temperature employed. This is a limitation imposed by the laws of polymer chemistry and physics. The phenomenon is termed vitrification, which “vitrifies” the curing material as the Tg builds up to the curing temperature. This effectively “freezes” the growing polymer network, limiting the molecular-level movement of any leftover reactive species. Additional curing reactions are halted, locking in the Tg. A subsequent, higher temperature post-cure is required to boost Tg if the formulation permits. The examples shown eliminate this option by utilizing sufficiently high cure temperatures as required by each formulation. So, we are comparing the highest potential Tg from each example formulation mentioned.
- Second, some variation is possible based on use levels for each curing agent. The values reported are taken for optimum stoichiometry (usage) levels as recommended by major suppliers of the material classes mentioned.
As can be seen, the dianhydride (BTDA) outperforms all other curing agents by a substantial margin, at a use level about half its stoichiometric theoretical amount. (For a discussion on why dianhydrides are recommended at below stoichiometry, see Entry 1 in this series).
Note that all examples in the table used the simplest, difunctional DGEBA epoxy resins. Using higher functionality resins will increase Tg for all curing agents, but dianhydride is truly in a league of its own.