Improved cure kinetics of phthalonitrile resins using dicyanamide‐based ionic liquids

• Published in: Journal of applied polymer science Vol. 140; no. 8
• Main Authors: Wolfgang, Josh D., Dysart, Jennifer L., Laskoski, Matthew
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 20-02-2023; Wiley Subscription Services, Inc
• Subjects: Additives; Amines; composite; crosslinking; Crossovers; Curing; ionic liquid; Ionic liquids; Ions; Materials science; Mechanical properties; Organic salts; phthalonitrile; Polymers; Resins; Rheological properties; Temperature; Thermal stability; Thermosetting resins
• ISSN: 0021-8995; 1097-4628
• DOI: 10.1002/app.53534

Phthalonitrile (PN) polymers are high‐temperature thermosetting resins with excellent thermal and mechanical performance, but they exhibit notably long cure‐times hindering their use in certain applications. Ionic liquids (ILs) and aromatic amines provide an opportunity to cure at lower temperatures and shorter times. Dicyanamide‐based ILs are an interesting class of high‐temperature organic salts due to their high thermal stability and –CN functionality. The addition of ionic liquid in concentrations of 10 and 20 wt% enabled a decrease of the crossover modulus by 30 and 45°C, respectively. When the samples were mixed with both 3 wt% of an amine curing additive and the ionic liquid, the temperature of the crossover modulus was decreased even more dramatically with cure temperatures up to 96°C below that of the PN resin without additives. The differential scanning calorimetry results support the rheological findings that the ILs decrease the temperature and/or time required to cure PN resins. Herein, a dicyanamide‐based ionic liquid catalyzed the curing of PN resins by lowering the gelation temperature by up to 40°C, decreased the processing viscosity by five‐ to eight‐fold, and increased the density of the resin thermosets by nearly 3% on average.
Phthalonitrile (PN) resins thermally crosslink to form numerous nitrogen‐containing heterocycles, including triazine rings. The steric hindrance associated with these rings produces lower density thermosets than what is achieved upon adding dicyanamide‐based ionic liquids. The ionic liquid used herein also enabled the tunability of the curing profile for the PN resins.