Mono N-Alkylated DABCO-Based Ionic Liquids and Their Application as Latent Curing Agents for Epoxy Resins

Abstract

Research on ionic liquids is constantly providing information on their new and more advanced applications. Undoubtedly, the use of latent curing agents for epoxy resins is one of the most interesting applications of this group of compounds. In this work, a detailed description of the synthesis and physicochemical studies of DABCO-based (1,4-diazabicylo[2.2.2]­octane) ionic liquids is presented. Their most valuable feature is that they can be latent curing agents for polymerization of epoxy resins, which in turn can be used, when combined with fibers, as composites with special, new properties. The research includes detailed discussion of synthesis of new DABCO-ILs, focusing on obtaining pure compounds and expanding current knowledge on the subject. Thermal (DSC) and chromatographic (IC) properties have been discussed for all presented salts. Furthermore, ionic liquids with dicyanamide and triflate anion have demonstrated the ability to initiate polymerization of epoxy resins. Subsequently, all mixtures were cured into cubical shape and were subjected to the Shore hardness test (SHT). The main advantages of the obtained ILs are (i) the ability to store ionic liquids/epoxy resin mixture for prolonged periods of time without deterioration of curing properties and (ii) a significantly lower quantity of ionic liquids used in the process of curing epoxy resins.

Synthesis and Behavior of Hexamethylenetetramine-Based Ionic Liquids as an Active Ingredient in Latent Curing Formulations with Ethylene Glycol for DGEBA

“…At room temperature, they remain weakly active against the DGEBA molecule. In particular, this is a property observed with many ionic liquids used as curing initiators for epoxy resins and is fairly well explained [ 18 , 19 , 20 , 21 ].…”

“…The bromide salts obtained earlier were precursors on the way to their preparation and required further processing. For this purpose, two different pathways of the metathesis reaction were used between the bromide anion in the HMTA quaternary salt and (i) the dicyandiamide anion derived from the freshly prepared (according previously published work [ 21 ]) silver salt, or the (ii) bis(trifluoromethanesulfonyl)imide anion derived from lithium salt. Each pathway was analyzed at room temperature, however, using different process conditions, as shown in Figure 2 , and synthetic techniques, as described in detail in the synthesis section.…”

“…The wide area of application of ionic liquids is due to their properties, among which the following should be mentioned: high thermal stability [ 14 ], high electrochemical window [ 15 ], low volatility [ 16 ] and flammability [ 17 ]. At the same time, it turns out that these compounds can successfully replace, among others, hardeners (as a latent curing agents) of epoxy resins [ 18 , 19 , 20 , 21 ] as well as a large group of property modifiers: (i) flame retardants [ 22 , 23 , 24 ], (ii) hydrophobizers [ 25 , 26 ] and (iii) compounds that increase the mechanical parameters of the composite [ 21 , 27 , 28 , 29 , 30 ].…”

“…The most important substances in this group include: (i) 1,4-diazabicyclo[2.2.2]octane (DABCO), (ii) 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), (iii) 1-azabicyclo[2.2.2]octane (ABCO), (iv) 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and hexamethylenetetramine (HMTA). The first two, DABCO and DBU, remain in the interest of many researchers, especially as new ionic liquids with catalytic properties [ 21 ]; DBN is not very popular due to its high similarity to DBU, while ABCO, because of its very high price, is practically not studied at all. In contrast, the simplest, best known and most easily available, as well as the cheapest polycyclic amine—HMTA, remains to this day the most unexplored source of ionic liquids, which may have a few unique properties, from catalytic, electrochemical and anti-corrosion to fuel applications.…”

Optical Fiber Grating-Prism Fabrication by Imprint Patterning of Ionic-Liquid-Based Resist

“…There have been many advanced researches, discoveries and patents in the field of ILs in recent years due to their significant features and great applicability, but their commercial exploitation is still limited. Ionic liquids have many unique properties, including low vapor pressure, high thermal stability, low toxicity, high ionic conductivity and almost unlimited tunability, which have already allowed them to find many applications in chemistry, electrochemistry, extraction and separation processes [ 8 , 9 , 10 , 11 , 12 , 13 , 14 ]; however, the photonic application of these materials have so far been very limited, and their optical parameters have not been sufficiently explored. Their properties can be tuned either by anion and/or cation modification, making them photodefinable, transparent, colored or flexible.…”