Ionic Liquid-Cured Epoxy/PCL Blends with Improved Toughness and Adhesive Properties

Abstract

In this work, ionic liquid (IL)-cured epoxy resins were modified by adding poly(ε-caprolactone) (PCL). Three different ILs were used in order to study how (a) the chemical structure of the ILs and (b) the PCL content affect the phase behaviour, microstructure, mechanical and adhesive properties. Regardless of the IL used or the PCL content, the obtained materials showed a single phase. The addition of PCL to the epoxy resin resulted in plasticizing of the network blends, lower glass transition temperatures (Tg), and crosslinking densities (νe). Low PCL contents did not have a significant impact on the mechanical properties. However, the adhesive properties improved significantly at low PCL contents. Higher PCL contents led to a significant increase in toughness, especially in the case of the imidazolium-based IL. The balance achieved between the mechanical and adhesive properties of these IL-cured epoxy/PCL blends constitutes an important step towards sustainability. This is because a biodegradable polymer (PCL) was used to substitute part of the epoxy resin, and the ILs—which are non-volatile and cure effectively at much lower contents—were used instead of conventional curing agents. Given the wide use of this kind of materials in the adhesive industry, the practical significance of these results must be emphasised.

Ionic Liquids as Alternative Curing Agents for Conductive Epoxy/CNT Nanocomposites with Improved Adhesive Properties

“…Regarding the role of ILs as curing agents in epoxy-based systems (as we reported in previous works [ 45 , 46 ]), imidazolium- [ 38 , 39 , 40 , 41 ] and phosphonium- [ 36 , 37 , 42 , 43 , 44 , 45 , 46 ] based ILs are effective substitutes for traditional volatile and toxic curing agents. In addition, as they act as initiators rather than as comonomers [ 42 ], less IL is required to cure the epoxy resin effectively.…”

Effect of the Simultaneous Addition of Polycaprolactone and Carbon Nanotubes on the Mechanical, Electrical, and Adhesive Properties of Epoxy Resins Cured with Ionic Liquids

“…Moreover, a lower crosslinking density was observed in the epoxy/PCL/IL systems. These results point to the miscibility of the epoxy resin with the PCL [ 10 , 18 , 45 ]. Regarding the effect of adding CNTs to the epoxy/IL system, neither the T g nor the ν e changed significantly in the epoxy/CNT/IL NCs compared to the unfilled epoxy resin.…”

“…Based on our previous results, the PCL content selected was 20 wt.% [ 45 ] with 0.2 wt.% CNTs [ 62 ]. Before preparing the samples, the DGEBA was degassed in a vacuum oven at 80 °C for 1 h. The epoxy/PCL/CNT/IL NCs and the reference epoxy/IL, epoxy/PCL/IL, and epoxy/CNT/IL systems were prepared as follows (see flowcharts in Figure S1 ):…”

“…Finally, one of the main environmental concerns associated with epoxy resins is the use of toxic, volatile hardeners. This is why imidazolium- [ 41 , 42 , 43 , 44 , 45 , 46 ] and phosphonium- [ 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ] based ionic liquids (ILs) have been proposed as more sustainable epoxy curing agents. ILs are salts that melt at temperatures below 100 °C and have excellent properties: they are comparatively stable, are ionically conductive, and have a low vapour pressure.…”

“…In addition to being non-volatile, smaller amounts of ILs are needed to cure epoxy resins because they act as initiators rather than as co-monomers [ 47 ]. They have also been reported as effective curing agents in several epoxy/thermoplastic systems, such as epoxy/poly(2,6-dimethyl-1,4-phenylen ether) (PPE) [ 53 ], epoxy/polyetherimide (PEI) [ 54 ], and epoxy/poly(ε-caprolactone) [ 45 ], which we included in our previous work. Furthermore, ILs have been used as dispersing agents in CNT-filled thermoplastic [ 55 , 56 , 57 ] and thermoset [ 58 , 59 , 60 , 61 ] NCs.…”