Epoxy/imidazolium‐based ionic liquid systems: The effect of the hardener on the curing behavior, thermal stability, and microwave absorbing properties

Abstract

A new transparent microwave absorbing coating was developed by compounding 1-butyl-3-methyl imidazolium tetrafluoroborate (bmim.BF 4 ) ionic liquid (IL) with diglycidyl ether of bisphenol A-type epoxy resin. The systems were crosslinked with the IL alone or combined with conventional hardeners, as anhydride or aromatic amine. The curing behavior was investigated by thermal and spectroscopic analysis performed at high temperatures. Neat bmim.BF 4 was able to cure epoxy resin, giving rise to networks with outstanding thermal stability compared with the systems cured with anhydride or aromatic amine. bmim.BF 4 accelerated the curing process in the presence of aromatic amine but retarded this event when anhydride was used as an external curing agent. The glass-transition temperature evaluated by dynamic mechanical analysis decreased when the amount of IL increased, which can be attributed to side reactions during the curing process, as well as the plasticizing effect of IL. The epoxy networks cured with bmim.BF 4 alone or in combination with anhydride or aromatic amine were transparent and presented considerable microwave absorbing properties in the X-band frequency range (8-12 GHz), being the best performance observed for the systems cured with bmim.BF4/anhydride curing system, with reflection loss value around −16 dB at 11.3 GHz.

New Imidazolium Ionic Liquids from Recycled Polyethylene Terephthalate Waste for Curing Epoxy Resins as Organic Coatings of Steel

“…The data summarized in Table 2 confirm that the crosslinking densities of DGEB/PAIIL 4:1 and 2:1 are the most suitable weight ratios to obtain highly crosslinked epoxy networks. These data agree with that determined from DMA (Figure 10) to confirm that DGEB/PAIIL 4:1 and 2:1 have been cured for the shortest time at the lowest temperature and indicating a relatively uniform networks [35][36][37]. It is also noticed that DGEB/PAIIL weight ratios 3:1 and 1:1 undergo more pronounced lower temperature beta transitions before going through the glass transition temperature at similar temperatures to indicate that their networks containing different structural domains (heterogeneous).…”

“…Moreover, the disappearance of the DGEB epoxy ring (Figure 7a) at 910.64 cm −1 from the spectrum of DGEB/PAIIL (Figure 7a) indicating that all epoxy ring was consumed in the reaction, which is in support of Schemes 2 and 3. The disappearance of carbonyl bands from 1700 to 1750 cm −1 attributed to de-alkylation mechanism of IIL [35][36][37] confirms that the de-protonation mechanism (Scheme 3) was preferred to cure DGEB resin with PAIIL. Thermal stability of the cured DGEB epoxy resin with different weight ratios of PAIIL (DGEB/PAIIL; 4:1, 3:1, 2:1, and 1:1) was investigated from TGA thermograms represented in Figure 8.…”

“…The concentration of the hydroxyl groups is increased with the second proposed mechanism for oxirane ring opening with imidazolium ILs (Scheme 3) that influenced by anions of imidazolium [35]. The reaction of imidazolium (IILs) with epoxy group was completed either by decomposition of IILs (de-alkylation) in basic condition or deprotonation (proton exchange) that affected by the basicity of their ions [35,36]. The dealkylation mechanism usually was occurred with using dialkylimmidazolium acetate IILs to form cured epoxy without imidazolium cations via formation of highly crosslinked networks.…”

“…The DMA analysis determined in terms of variation tanδ versus temperature of all cured DGEB/PAIIL epoxy resins (Figure 9b and Table 1) shows the same results of T g that determined from the maximum of the tanδ peaks. These mean that the increasing of PAIIL (wt.%) produces rigid networks with increasing T g which may be referred to decomposition of PAIIL due to large amounts of acetic acids produced the curing of DGEB/PAIIL weight ratio (1:1) to decrease the plasticizing effect of imidazolium cations [35][36][37]. The T g data of the DGEB/PAIIL determined from DMA were increased than that determined from DSC experiment were attributed to the higher temperatures used to DSC under dynamic mode.…”

Time‐dependent properties of epoxy resin with imidazolium ionic liquid

“…1,2 Despite the excellent thermo-mechanical properties, the high crosslink density of epoxy resins makes them brittle. In recent literature, three ways to improve fracture toughness of epoxy are discussed: (1) adding hard or rubbery particles 3,4 ; (2) manipulating the chemical structure of the resin with additives and modifiers 5,6 ; or (3) by changing the curing conditions, such as temperature. 7,8 Amine-based hardeners are widely used with epoxy, where epoxy resins cured with imidazoles may have comparatively superior physicochemical properties.…”