Technology Spotlight: Episode 3
Why Ethynilic-based reactive end-cappers outperform alternatives for polyamic acid/polyimide molecular weight control and latent thermal curing of insulative coatings
Reactive, functional end-cappers have found their rightful place. They control the degree of polymerization of aromatic polyimides for improved processing, and enable heat-activated addition curing to deliver in-service properties similar to those of high molecular weight analogs. This chemistry has been proven in low melt viscosity, resin transfer molding and resin infusion of structural composites for aerospace, and more recently, in coating formulations for the insulation of microelectronics. Episode 1 of this series serves as a quick refresher.
But why C≡C reactive groups? Aren’t there better end-capper designs? Different ones, yes, but better, no. Let’s explore them and answer these nagging questions.
Early thermocurable polyimides from the 1970s incorporated norbornene derivatives, such as Nadic® anhydride, resulting in the Nadimide group shown in the adjacent table. This modification enabled addition polymerization to occur upon heating to 350-370 °C, suitable for matrix resins but certainly not for solution processing of coatings.
In the 1990s, Phenylethynyl pendant groups were discovered to be ideal for low melt viscosity resins, with a large processing window, similar thermal curing profile, ability to form several polyene and cyclic structures, and, wait for it…excellent thermo-oxidative stability rivaling traditional high-temperature polyimides.
Nexam Chemical AB offers Phenylethynyl phthalic anhydride as NEXIMID 100A. You may know it as PEPA from its many literature citations and casual jargon.
In order to shift the curing temperature of reactive end-cappers closer to the processing requirements for coatings, other compounds were introduced. Both Cyanate and Maleimide groups, while allowing for curing at 200-300 °C, were inadequate for long-term thermal stability. A simpler analog to Phenylethynyl functionality, the Ethynyl group was found to lower the curing temperature to 250 °C, with a smaller processing window as the compromise, so not in the ballpark quite yet.
Further variants were developed in subsequent decades, including Methylethynyl phthalic anhydride (NEXIMID 500), Ethynyl phthalic anhydride (NEXIMID 200)*, and most recently, Phenylethynyl trimellitic anhydride (NEXIMID 300, shown at left). NEXIMID 300 enables thermal curing at 190-230 °C, which can be reduced to 70 °C (yes, you read that correctly!) with selected catalysts. The proverbial jackpot can now be hit with this specific chemistry, ideal for solution processing of coatings.
So what then are the key takeaways for ethynilic-based reactive end-cappers?
Volatile-free thermal curing of polyamic acids and polyimides
Variable processing conditions to meet the broad requirements of advanced composites and insulative coatings
Low curing temperatures suitable for solution processing
Retention of mechanical and thermal properties typical of high molecular weight polymers
Commercially available from Nexam Chemical
Curious to learn more? Reach out for a free initial consultation today.
*Note: Ethynyl or acetylene terminated polyimides (ATI) were produced in the late 1970s via Ethynyl-substituted aromatic amines e.g. Thermid 600, but these were intended for aerospace resins and adhesives. Similar short processing windows were encountered then as well due to the reactivity of the terminal groups.