Ionic Liquid Systems: The Influence of Imidazolium

Article pubs.acs.org/IECR

Epoxy Resin/Ionic Liquid Systems: The Influence of Imidazolium Cation Size and Anion Type on Reactivity and Thermomechanical Properties Honorata Maka, Tadeusz Spychaj,* and Ryszard Pilawka West Pomeranian University of Technology, Polymer Institute, ul. Pulaskiego 10, 70-322 Szczecin, Poland ABSTRACT: The epoxy compositions with ionic liquids (ILs) differing in the alkyl chain length of imidazolium cation (butyl or decyl), anion type [N(CN)2¯, BF4¯, Cl¯], as well as ILs concentration (1, 3, and 9 phr) have been prepared, and their curing characteristics at ambient and elevated temperatures have been investigated. Rheometric, differential scanning calorimetry, and Fourier transform infrared measurements revealed substantial differences in the curing characteristics dependent mainly on the IL anion type and its concentration. TGA results showed that thermal decomposition of IL influenced the onset temperature of epoxy resins cross-linking reactions during heat treatment. Dynamic mechanical thermal analysis measurements showed that the type of IL anion had an effect on Tg and tan δ values as well. For materials cross-linked with ILs containing dicyanamide anion the highest Tg (ca. 170 °C) and the lowest tan δ (0.26) values have been found. On the basis of the obtained results and literature data the mechanism of epoxy resin cross-linking in the presence of imidazolium ionic liquids has been proposed. was described by our research group in 2003.14 However, in the last years, only a few papers on ILs utilization for epoxy resin cross-linking have been published.15,16 The reasons are probably the high prices of ILs and insufficient knowledge on chemistry as well as on the material science of epoxy resin/IL systems. Palmese’s team15 used 1-ethyl-3-methylimidazolium dicyanamide [EMIM]N(CN)2 in a range of 1−9 weight parts per 100 parts of epoxy resin (phr) and found a lower cross-linking temperature than that reported by Kowalczyk and Spychaj14 for [BMIM] BF4. Soares et al.16 very recently applied N,N′-dioctadecylimidazolium iodide [DODIM] I in a range of 1−20 phr for bisphenol A epoxy resin curing. When considering the mechanism of epoxy resin cross-linking with the ILs at a high temperature cross-linking process, the influence of both reactive IL ions, that is, imidazolium cation14−16 and tetrafluoroborate14 or dicyanamide15 anions has been taken into account. In addition to the works describing curing reactions of epoxy resins with imidazolium ionic liquids some papers that report using ILs for epoxy resins hardened with conventional amine hardeners could be found.16−19 Sanes et al. used 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) as an inner lubricant of epoxy material cured with a mixture of cycloaliphatic and aliphatic polyamines.17 An introduction of ca. 3 wt % [EMIM]BF4 influenced a substantial decrease of wear damage and friction coefficient of the epoxy resin/stainless steel surface contact. Guo et al. applied 1-butyl-3-methylimidazolium hexafluorophosphate as an effective dispersant of expanded graphite for making

1. INTRODUCTION Epoxy resins belong to the most important industrial reactive oligomers. Their broad range of properties as well as various curing possibilities allow for a wide range of commercial applications (coatings, adhesives, composites, etc.).1 The cross-linking of epoxy resin may be the addition or ionic polymerization reaction. Particularly important cross-linking agents are those exhibiting latent properties enabling the long storage of one-component epoxy resin/hardener system at ambient temperature. Thermally initiated curing agents are based on tertiary or quaternary nitrogen compounds, BF3 adducts, anhydrides, phenolic derivatives or dicyandiamide (DCDA).1 However, some of latent hardeners, for example, complexes of imidazole with metal cations,2 BF3 adducts, and DCDA are solid at ambient or even elevated temperature,1 which implies problems with dispersion of the curing agent in epoxy resin and could result in an unhomogeneous cure. Ionic liquids (ILs) are composed of organic cations and anionsas a rule the former being larger than the latterthat are often liquid at room conditions. ILs show unique properties such as thermal, chemical, and electrochemical stability, low volatility, nonflammability, ionic conductance, catalytic activity, and recyclability.3 They are often determined as “green solvents” and candidates for very broad practical applications. For instance, ILs can be used as media for performing polymerization processes,4 solvents or plasticizers for synthetic polymers5,6 as well as for polysaccharides (cellulose or lignocellulose biomass and starch),3,7,8 in separation techniques,9 polymer gel electrolytes,10 catalysts11 or even as efficient nanocarbonaceous particle dispersants (carbon nanotubes, graphenes).12,13 The easily miscible liquid latent curing agents able to initiate thermal epoxy resin polymerization and giving materials with good thermomechanical properties could eliminate disadvantages of the aforementioned solid latent hardeners. The first trial to use ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM]BF4 for cross-linking of epoxy resin © 2012 American Chemical Society

Received: Revised: Accepted: Published: 5197

October 10, 2011 March 2, 2012 March 6, 2012 March 6, 2012 dx.doi.org/10.1021/ie202321j | Ind. Eng. Chem. Res. 2012, 51, 5197−5206

Industrial & Engineering Chemistry Research

Article

Table 1. Description of Imidazolium Ionic Liquids Used

a

Ionic liquids obtained from commercial products by anion exchange.

[NaN(CN)2, Sigma Aldrich, 96%] were used for anion exchange reactions. 2.2. Ionic Liquids−Anion Exchange. The anion exchange reactions have been performed according to MacFarlane et al.:20 10 g of IL was dissolved in acetone and then NaN(CN)2 or NH4BF4 was added. The molar ratio of IL and sodium/ ammonium salt was 1:1; anion exchange reaction was performed for 24 h at room temperature. The reaction liquor was left for 1 day for sodium/ammonium salt sedimentation, and subsequently the supernatant liquid was decanted. The residual salt was washed with acetone, and the collected acetone solutions of IL were dried to obtain the ionic liquid. 2.3. Preparation and Investigation of Epoxy Resin/IL Compositions. IL was mixed with E6 resin at ambient temperature. The weight ratios of IL per epoxy resin were 1, 3, and 9 parts/100 parts (phr). The following tests were performed: (i) storage time determination, (ii) rheometric measurements, (iii) differential scanning calorimetry (DSC), (iv) infrared spectroscopy (FTIR), (v) dynamic mechanical thermal analysis (DMTA) of the hardened epoxy material, and (vi) thermogravimetric analysis (TGA) of IL. 2.4. Methods. The storage time of E6/IL compositions at ambient temperature was determined on a basis of viscosity measurements during storage at 23−25 °C for 1−70 days using an ARES rheometer (Rheometric Scientific), a plate−plate system, φ = 40 mm, and a gap of 1 mm. The curing process of the epoxy compositions was investigated using an ARES rheometer and DSC Q-100 (TA Instruments) at a heating rate of 5 °C/min at the temperature range of 30−350 °C. IR spectra was performed using a Nexus FTIR with Golden Gate (ATR) (Thermo Nicolet Corp.) equipped with Omnic software. The FTIR measurements for E6/IL mixtures with the highest IL amount (9 phr) were performed just after mixing and after 10 and 60 min reaction at 150 or 200 °C. The glass transition temperatures and tan δ values of E6/IL composite materials were determined using DMTA Instruments Q-800 (TA Instruments) with dual cantilever, at a heating rate of 2 °C/min from 30 to 220 °C, frequency of 1 Hz. The samples for DMTA measurements were cast into Teflon molds and hardened at 150 °C/2 h (compositions with ILs containing N(CN)2¯ anion), or at 230 or 240 °C/2 h, for E6/ [BMIM]BF4 as well as for E6/[DMIM]BF4, respectively. Thermogravimetric analysis (TGA) was conducted on TGA Q-5000 (TA Instruments) in nitrogen atmosphere in the

epoxy resin/graphite composites cross-linked with polyoxypropylenediamine (Jeffamine D-230).18 Very recently Donato et al.19 have used three imidazolium ILs as additives for formation of epoxy silica nanocomposites via the simultaneous sol−gel process and epoxy resin build-up. The application of different ILs allowed to control of the nanosilica structure and modification interphase interaction thus influencing the morphology and mechanical properties of the composites cured with Jeffamine D2000. The epoxy systems cured with ILs reported so far are limited to only a few types of imidazolium salts.14−16 Thermal curing characteristics of these systems differ substantially, which suggests that by changing one or both, the cation size and the anion type, it would be possible to control the kinetics of the epoxy hardening process and probably the thermomechanical properties of the final epoxy materials. A great number of possible ILs cation−anion type combinations creates a wide field for tailoring the physicochemical properties of the epoxy/ IL compositions as well as the hardened materials. In this paper for the first time the influence of imidazolium cation size and anion type as well as IL content on latency, thermal cure characteristics, and thermomechanical properties of the hardened bisphenol A-based epoxy resin is reported.

2. EXPERIMENTAL SECTION 2.1. Materials. Epoxy resin bisphenol A-based low molecular weight Epidian 6 (E6), epoxy equivalent 185, (viscosity 18 000 mPas at 23 °C), Organika Sarzyna, Poland, was used. The ionic liquids applied are collected in Table 1. The basic ILs 1-butyl3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and 1-decyl3-methylimidazolium chloride ([DMIM]Cl) have been modified by anion exchange (Figure 1) to obtain 1-butyl-3-methylimidazo-

Figure 1. Scheme of anion exchange reactions to obtain [BMIM]N(CN)2, [DMIM]N(CN)2 and [DMIM]BF4.

lium dicyanamide [BMIM]N(CN)2, 1-decyl-3-methylimidazolium tetrafluoroborate [DMIM]BF4, and 1-decyl-3-methylimidazolim dicyanamide [DMIM]N(CN)2, respectively. Ammonium tetrafluoroborate (NH4BF4, Fluka, 98%) and sodium dicyanamide 5198

dx.doi.org/10.1021/ie202321j | Ind. Eng. Chem. Res. 2012, 51, 5197−5206


Article

Figure 2. Influence of cation size and anion type of imidazolium IL (a,b) as well as content of IL in epoxy composition (c,d) on viscosity change during storage at ambient temperature.

temperature range of 40−900 °C, with a heating rate of 10 °C/min.

were used. The results of rheometric measurements are presented in Figure 3. Rheometric curves of epoxy compositions with ILs having dicyanamide anion showed a steep viscosity increase in a lower temperature range (125−150 °C) in comparison to the systems with tetrafluoroborate anion (200− 240 °C). The higher was the IL content in the epoxy composition, the lower was the onset temperature of the crosslining reaction. The range of the onset temperature as a function of IL content was rather narrow, for E6/[DMIM]N(CN)2 it was 130−145 °C and for E6/[BMIM]BF4 it was 205−228 °C, and it became broader for other systems: E6/[BMIM]N(CN)2, 115−150 °C, and E6/[DMIM]BF4, 210−240 °C. The influence of the IL cation alkyl chain length on the onset temperature for the highest IL concentration seemed to be equivocal: some shifts to higher temperature (120 → 130 °C Figure 3a,b) or to lower values (210 → 200 °C Figure 3c,d) were observed, respectively. The results of DSC measurements are shown in Figure 4 and Table 2. Characteristic features of the thermograms were bimodal exothermic maxima for cured epoxy/IL compositions. Similarly to rheometric results DSC data showed that epoxy compositions with a dicyanamide anion in the ILs started to react at lower temperature range than those with tetrafluoroborate. For epoxy materials cured with ILs with the N(CN)2 anion the onset temperature values of the first exothermic peaks were 130−140 °C, similar to compositions with ILs containing a Cl¯ anion (Table 2). The maximum values of the second exotherm for materials with dicyanamide were very similar: 166−173 °C

3. RESULTS AND DISCUSSION 3.1. Curing Process of Epoxy Resin/IL Compositions. Storage Time at Room Temperature. It is reported14,15 that the epoxy resin/IL systems exhibited prolonged storage time at ambient temperature. The results of comparative latency study for the investigated epoxy systems are presented in Figure 2. On the basis of the results, the following can be concluded: (i) compositions containing the N(CN)2¯ anion are substantially more reactive than those with BF4¯ (storage time up to ca. 20 days irrespectively of imidazolium cation size), (ii) the E6/ [DMIM]Cl systems (not shown in Figure 2) exhibited almost unchanged viscosities up to ca. 30 days, (iii) epoxy compositions with ILs containing tetrafluoroborate anion exhibited the highest latency (viscosities up to ca. 30 Pas for 50 days), (iv) cross-linking reaction rate increased with the IL content in epoxy compositions (storage time decreased from ca. 40 to 20 days with the IL content increase from 1.0 to 9.0 phr for E6/ [BMIM]N(CN)2, (v) no influence of imidazolium alkyl chain on storage time has been found. The results correlate with Palmese et al.15 and our group14 reports. No data on the influence of the imidazolium cation size on epoxy resin crosslinking reaction rate have been found in the literature. Curing Performed at Elevated Temperatures. For investigation of epoxy resin/IL compositions curing performed at elevated temperature rheometry, the DSC and FTIR techniques 5199



Article

Figure 3. Rheometric curves of curing process of the epoxy compositions: influence of cation size, anion type, and concentration of imidazolium ILs.

after 10 min of heating at 200 °C (Figure 5a) were very similar, and for the 60 min heated sample only a slight decrease of epoxy group absorption at 908 cm−1 has been found (Table 3). In the case of E6/[DMIM]Cl heated at 200 °C more evident changes in FTIR spectra were registered (Figure 5b): the epoxy group absorption band at 918 cm−1 decreased by almost 2 times with thermal treatment (Table 3) and a new absorption band at 1740 cm−1 appeared as a result of the carbonyl group formed during epoxy resin polymerization. Some changes of the FTIR spectra, that is, more than 60% intensity increase (A2950/A1183) from 0.62 up to 1.05 can be observed in a range of 2930− 2950 cm−1 with the epoxy cross-linking progress, caused by the imidazolium cation or products of its thermal decomposition (see also Figure 6). The most significant changes in the FTIR spectrum for the E6/[DMIM]N(CN)2 system after it was heated at 150 °C for 10 or 60 min has been noted (Figure 5c). These changes are as follows: 918 cm−1 (epoxy group absorption), 1740−1750 cm−1 (carbonyl absorption), and 2930 cm−1 (stretching band of methylene group). Similarly to the FTIR of E6/[DMIM]Cl, the composition absorbance band at 918 cm−1 decreased twice (A918/A1183 from 0.35 to ca. 0.17) and some intensification of the 2930−2950 cm−1 band is observed (from 0.34 to ca. 0.58 or 0.48 after 10 and 60 min reaction, respectively). A very specific feature has been found for the carbonyl group band at 1740− 1750 cm−1: an increase from A1750/A1183 ≈ 0 up to 0.66 after 10 min reaction and a drop to 0.30. It could mean that carbonyl groups in the reaction conditions (150 °C, 60 min) underwent some secondary decomposition processes.

for E6/[BMIM]N(CN)2 and 164−172 °C for E6/[DMIM]N(CN)2. The enthalpy values of cross-linking reactions referring to exotherms for E6/IL N(CN)2 systems were comparable to the main (i.e., the second) exotherm: 483 and 532 J/g for E6/ [BMIM]N(CN)2, 462 and 510 J/g for E6/[DMIM]N(CN)2, and were significantly lower, 25−59 J/g, 20−39 J/g, for the first maximum. For epoxy compositions containing [DMIM]Cl the second exotherms were noted in a higher temperature range 225−240 °C than for those with dicyanamide salts (Table 2). The enthalpy values for both cross-linking transitions of E6/ [DMIM]Cl system and for material based on dicyanamide ILs were similar. Another tendency has been found for epoxy compositions with tetrafluoroborate imidazolium ILs. In the case of material based on an IL with a shorter alkyl chain on the imidazolium ring, practically only one exotherm in the high temperature region 281−308 °C could be observed (Table 2, Figure 4c). The composition of the IL with a longer aliphatic chain on the imidazolium ring exhibited an isotherm with two maxima placed in the high temperature region: at 268−291 °C and at 322−329 °C, respectively. The DSC results correlate with the reports on epoxy resin/IL BF414 and epoxy resin/IL N(CN)215 systems. The FTIR analysis could be a powerful tool for evaluation of the epoxy oligomers curing reactions21,22 including systems with ionic liquids.14−16 Figure 5 shows the spectra of E6/IL compositions with 1-decyl-3-methylimidazolium salts containing various anions: N(CN)2¯, BF4¯, and Cl¯ directly after the components are mixed, and after 10 or 60 min of heating at 150 or 200 °C. The FTIR spectra of the E6/[DMIM]BF4 mixture and 5200



Article

Figure 4. DSC thermograms of epoxy compositions during heating: influence of cation size, anion type, and concentration of imidazolium ILs.

and maximum decomposition temperatures (T0 and Tm, respectively) have been found for ILs with Cl¯ and N(CN)2¯ anions. Substantially higher characteristic decomposition temperatures have been found for ILs containing tetrafluoroborate anion. The order of decreasing decomposition temperatures has been established as follows (Figure 6): [BMIM]BF4− (T0 = 365 °C, Tm = 470 °C) ≥ [DMIM]BF4− (T0 = 350 °C, Tm = 450 °C) ≥ [DMIM]N(CN)2− (T0 = 250 °C, Tm = 290 °C) ≥ [DMIM]Cl− (T0 = 215 °C, Tm = 290 °C). The reaction activity of ionic liquids toward epoxy resin was connected with its thermal decomposition characteristics. Easy decomposing ILs with dicyanamide and chloride anions were able to cross-link epoxy resin at ca. 150 °C, whereas the reactions of temperature resistant ILs containing tetrafluoroborate anion were above 200 °C. 3.2. Thermomechanical Properties of Epoxy Resin/IL Composites. The DMTA curves presenting tan δ dependences for epoxy materials cross-linked with 1-butyl-3-methyl and 1-decyl-3-methylimidazolium salts containing dicyanamide and tetrafluoroborate anions are given in Figure 7. The curves maxima, determining Tg values of particular samples, were shifted to higher values with increasing IL content in the starting epoxy composition. However, the highest Tg values have been determined for the composition-containing medium amount of IL, that is, 3 phr (Figure 7). Tg values for

Table 2. DSC Characteristic Parameters: Maximum Temperatures and Enthalpies of Exothermic Reactions for Epoxy Resin/IL Compositions maximum temperature composition acronym

Tp1 [J/g]

Tp2 [J/g]

E6/[BMIM]N(CN)2 1.0 E6/[BMIM]N(CN)2 3.0 E6/[BMIM]N(CN)2 9.0 E6/[BMIM]BF4 1.0 E6/[BMIM]BF4 3.0 E6/[BMIM]BF4 9.0 E6/[DMIM]N(CN)2 1.0 E6/[DMIM]N(CN)2 3.0 E6/[DMIM]N(CN)2 9.0 E6/[DMIM]BF4 1.0 E6/[DMIM]BF4 3.0 E6/[DMIM]BF4 9.0 E6/[DMIM]Cl 1.0 E6/[DMIM]Cl 3.0 E6/[DMIM]Cl 9.0

140.0 135.0 133.5

173.2 172.0 166.0 291.9 308.2 280.9 172.3 171.5 163.5 328.6 322.3 291.4 233.4 240.5 225.0

138.5 131.3 131.9 267.9

136.1 135.4 129.5

enthalpy ΔH1 [J/g] ΔH2 [J/g] 6.7 25.0 58.5

4.2 20.0 38.7 81.6

3.9 16.6 51.5

171.7 483.4 531.6 401.7 469.3 318.6 145.4 462.4 509.8 407.9 453.8 444.7 92.6 529.2 535.6

The ILs rheometry, DSC and FTIR results can be correlated with thermogravimetric data. TG and DTG curves of the ILs are illustrated in Figure 6. The lowest values of the beginning 5201



Article

Figure 5. A-E6/[DMIM]BF4, A1-E6/[DMIM]BF4 10 min/200 °C, A2-E6/[DMIM]BF4 60 min/200 °C, B- E6/[DMIM]Cl, B1-E6/[DMIM]Cl 10 min/200 °C, B2-E6/[DMIM]Cl 60 min/200 °C, C-E6/[DMIM]N(CN)2, C1- E6/[DMIM]N(CN)2 10 min/200 °C, C2- E6/[DMIM]N(CN)2 60 min/200 °C. 5202



Article

Table 3. Changes of FTIR Spectra for the E6/ILa Compositions before and after Heating for 0, 10, or 60 min at 200 or 150°C absorbance ratio Ax/A1183 E6/[DMIM]BF4 200 °C absorbance cm 918 1740−1750 2930−2950 a

−1

E6/[DMIM]Cl 200 °C

E6/[DMIM]N(CN)2 150 °C

0

10

60

0

10

60

0

10

60

0.37 0.05 0.48

0.42 0.03 0.57

0.34 0.03 0.52

0.39 0.04 0.62

0.26 0.35 0.90

0.20 0.33 1.05

0.35 0 0.34

0.16 0.66 0.58

0.18 0.39 0.48

Weight ratio of ionic liquid/epoxy resin = 9/100.

Figure 6. Thermal gravimetric analysis of imidazolium ILs applied for epoxy resin curing.

epoxy materials cross-linked with ILs containing dicyanamide anion were higher (177.8 and 167.7 °C for E6/[BMIM]N(CN)2 and E6/[DMIM]N(CN)2, respectively) than for ILs containing tetrafluoroborate anion (150.6 and 146.6 °C for E6/ [BMIM]BF4 and E6/[DMIM]BF4, respectively). A longer alkyl chain of imidazolium cation caused this decrease of glass transition temperature of the cross-linked epoxy resin. The analysis of Tg data from Figure 7 and Table 4 allowed us to conclude that 1 phr of IL per epoxy resin was not enough for efficient epoxy resin curing. However, 9 phr seemed to be too much to reach high Tg values. The glass transition temperature values of E6/[DMIM]Cl materials were substantially lower (55.9−104.3 °C) than for other obtained epoxy materials. These findings evidenced the influence

of both anion type and cation size on the thermomechanical properties of epoxy materials. Moreover, the tan δ value at the curve maximum gives information on the ratio of energy E′ stored and dissipated as heat E″ (tan δ = E″/E′). Usually, for polymer composite materials E″ < E′; E = 0 for ideal elastic material and E′ = 0 for ideal viscous material. The lowest tan δ values were found for epoxy materials cross-linked with dicyanamide anions, that is, 0.27 and 0.26 for E6/[BMIM]N(CN)2, 3.0 phr, and E6/[DMIM]N(CN)2 3.0 phr, respectively, showing high cross-linking efficiency and low loss of modulus values. On the other side, rather high tan δ values (0.97−1.69) have been registered for materials cured with [DMIM]Cl, which could prove to be an unacceptable degree of samples cross-linking (Table 4). 5203



Article

Figure 7. Influence of cation size and anion type of imidazolium IL on tan δ values and their maximum temperatures (i.e., Tg values) of the crosslinked epoxy materials.

affording N-heterocyclic carbenes, high acidity at C2 position, and easier deuterium exchange have significant implications in the chemistry of this type of ILs.23,24 This is why the imidazolium ILs are likely to be chemically unstable under basic conditions. Handy et al. demonstrated that anions in imidazolium ILs had strong influence on the conditions required for proton exchange.25 For instance more basic anions such as N(CN)2¯ resulted in ILs that would undergo deuterium exchange in the absence of any added base while weakly coordinating, and nonbasic anions such as BF4¯ resulted in salts which required an external base for deuterium exchange to occur. So in the case of such a base as N(CN)2¯ (or Cl¯) (as well as some basic impurities in ILs) thermal decomposition of 1,3-dialkyl imidazolium liquids at relatively lower temperature could follow via a highly stabilized N-heterocyclic carbon structure (Figure 8a). As a result imidazole or its 1-alkyl derivatives are formed. The reaction mechanism of epoxy resin with imidazoles was examined in several reports,26−28 and most of the authors have agreed upon this mechanism. In the first step a pyridine-type of nitrogen in an imidazole derivative reacts with the epoxy group to form a 1:1 adduct (Figure 8b). In the case of imidazole as the decomposition product, H+ is transferred at this 1:1 adduct transforming the pyrrole-type nitrogen into pyrridine-type nitrogen in imidazoles. The newly formed pyridine-type nitrogen can react with an epoxy group to form a 1:2 adduct.26−28 Very recent investigations on thermal decomposition of 1ethyl-3-methylimidazolium bromide ([EMIM]Br) using tunable vacuum UV photoionization time-of-flight mass spectrometry

Table 4. Thermomechanical Properties of Crosslinked E6/ IL Materials composition acronym

tan δ

Tg [°C]

E6/[BMIM]N(CN)2 1.0 E6/[BMIM]N(CN)2 3.0 E6/[BMIM]N(CN)2 9.0 E6/[BMIM]BF4 1.0 E6/[BMIM]BF4 3.0 E6/[BMIM]BF4 9.0 E6/[DMIM]N(CN)2 1.0 E6/[DMIM]N(CN)2 3.0 E6/[DMIM]N(CN)2 9.0 E6/[DMIM]BF4 1.0 E6/[DMIM]BF4 3.0 E6/[DMIM]BF4 9.0 E6/[DMIM]Cl 1.0 E6/[DMIM]Cl 3.0 E6/[DMIM]Cl 9.0

0.83 0.27 0.51 0.64 0.51 0.78

75.0 177.8 144.0 112.4 150.6 125.5

0.26 0.42 0.71 0.51 0.58 1.69 0.97 1.04

167.7 145.7 111.4 146.6 121.9 55.9 96.3 104.3

3.3. Discussion of Epoxy Resin/Imidazolium IL Curing Mechanism. In the contributions referring to cross-linking of epoxy resin with ILs only some suggestions on a possible reaction mechanism14,15 or simplified reaction mechanism have been presented.16 Considering the mechanism of the epoxy curing with 1,3dialkylimidazolium liquids the noninnocent nature of imidazolium ionic liquids should be taken into account.23 Deprotonation 5204



Article

Figure 8. Proposed mechanism of epoxy resin cross-linking with 1,3-dialkylimidazolium ILs: (A) decomposition of IL into 1-alkylimidazoles, (B) 1alkylimidazole/epoxy resin 1:1 adduct formation and epoxy resin anionic polymerization, (C) 1-alkylimidazole regeneration and final cross-linked epoxy resin structurization.

(Figure 5, Table 3, bands at 1740−1750 cm−1). Soares et al.16 investigating IR spectra of the cured system epoxy resin/ [DODIM]I system reported also absorbancy at 1720− 1730 cm−1. However, the authors interpreted that fact as an effect of epoxy material thermal degradation. Considering the bimodal character of DSC thermograms in a case of epoxy systems cured with ILs with N(CN)2¯ and Cl¯ anions (Figure 4) we propose an explanation of this phenomenon by 1:1 adduct formation and chain addition polymerization (the first peak at lower temperature) while the higher temperature peak is assumed to be caused by further anionic polymerization being a result of imidazole regeneration.30 Another situation is in a case of ILs with BF4¯ because of the reasons mentioned earlier. The proposed mechanism of curing epoxy resin with 1,3-dialkylimidazolium liquids seems to bind the result of this work and literature data.

and thermographic analysis mass spectrometry have shown that the decomposition products were 1-alkylimidazoles (i.e., 1-methyl and 1-ethylimidazole) and alkyl bromides (methyl and ethylbromide).29 Taking this into account adduct 1:1 could be formed only as an effect of 1-alkylimidazole(s) interaction with epoxy resin (Figure 8b), without formation of the mentioned earlier adduct 1:2 type. Subsequently, the anionic polymerization of epoxy molecules based on the 1:1 adduct is performed (Figure 8b). Next there are two possible options of the following reactions in the cross-linked epoxy system, both leading to regeneration of 1-alkylimidazoles (Figure 8c). The evidence of imidazole (or its derivatives) regeneration is well documented in the literature.27,30 In the first option a cyclic epoxy polymer structure is formed as a result of the N-dealkylation reaction and in the other an unsaturated structure of epoxy polymer is finally transformed by tautomerization into the structure with a carbonyl group (Figure 8c). IR bands in a region of 1650−1770 cm−1 (i.e., double bond and/or carbonyl vibrations) have been found by some authors studying the epoxy resin curing mechanism with imidazoles31 (see also some references cited in there) similarly to this study

4. CONCLUSIONS Imidazolium ionic liquids containing shorter (butyl) or longer (decyl) alkyl chains and dicyanamide, tetrafluoroborate, or chloride anions have been used as latent hardeners of bisphenol 5205



Article

(8) Zhu, S.; Wu, Y.; Chen, Q.; Yu, Z.; Wang, C.; Jin, S.; Ding, Y.; Wu, G. Dissolution of cellulose with ionic liquids and its application: A mini-review. Green Chem. 2006, 8, 325. (9) Liu, H.; Wang, H. Z.; Tao, G. H.; Kou, Y. Novel imidazolium-based ionic liquids with a crown-ether moiety. Chem. Lett. 2005, 34 (8), 1184. (10) Reiter, J.; Vondrak, J.; Michalek, J.; Micka, Z. Ternary polymer electrolytes with 1-methylimidazole based ionic liquids and aprotic solvents. Electrochim. Acta 2006, 52 (3), 1398. (11) Olivier-Bourbigou, H.; Magna, L.; Morvan, D. Ionic liquids and catalysis: Recent progress from knowledge to applications. Appl. Catal. A: General 2010, 373 (1/2), 1. (12) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Molecular ordering of organic molten salts triggered by single-walled carbon nanotubes. Science 2003, 300 (5628), 2072. (13) Ji, Q.; Honma, I.; Pack, S. M.; Akada, M.; Hill, J. P.; Vinu, A.; Ariga, K. Layer-by-layer films of graphene and ionic liquids for highly selective gas sensing. Angew. Chem. 2010, 122, 9931. (14) Kowalczyk, K.; Spychaj, T. Ionic liquids as convenient latent hardeners of epoxy resins. Polimery, Warsaw 2003, 48 (11/12), 833. (15) Rahmathullah, A. M.; Jeyarajasingam, A.; Merritt, B.; VanLandingham, M.; McKnight, S. H.; Palmese, G. R. Room temperature ionic liquids as thermally latent initiators for polymerization of epoxy resins. Macromolecules 2009, 42 (9), 3219. (16) Soares, B. G.; Livi, S.; Duchert-Rumeau, J.; Gerard, J. F. Synthesis and characterization of Epoxy/MCDEA networks modified with imidazolium-based ionic liquids. Macromol. Mater. Eng. 2011, 296, 826. (17) Sanes, J.; Carrión-Vilches, F. J.; Bermúdez, M. D. New epoxyionic liquid dispersions. Room temperature ionic liquid as lubricant of epoxy resin-stainless steel contacts. e-Polym. 2007, 005, 1. (18) Guo, B.; Wan, J.; Lei, Y.; Jia, D. Curing behaviour of epoxy resin/graphite composites containing ionic liquid. J. Phys. D. Appl. Phys. 2009, 42 (14), 145307. (19) Donato, R. K.; Matejka, L.; Schrekker, H. S; Plestil, J.; Jigounov, A.; Brus, J.; Slouf, M. The multifunctional role of ionic liquids in the formation of epoxy-silica nanocomposites. J. Mater. Chem. 2011, 21, 13801. (20) MacFarlane, D. R.; Forsyth, S. A.; Golding, J.; Deacon, G. B. Ionic liquids based on imidazolium, ammonium and pyrrolidinium salts of the dicyanamide anion. Green Chem. 2002, 4, 444. (21) Fraga, F.; Burgo, S.; Rodriguez Nunez, E. Curing kinetic of the epoxy system BADGE n = 0/1, 2 DCH by Fourier transform infrared spectroscopy (FTIR). J. Appl. Polym. Sci. 2001, 82 (13), 3366. (22) Wu, L.; Hoa, V.; Tan, M.; That, T. Effects of composition of hardener on the curing and aging for an epoxy resin system. J. Appl. Polym. Sci. 2006, 99 (2), 580. (23) Sowmiah, S.; Srinivasadesikan, V.; Tseng, M. C.; Chu, Y. H. On the chemical stabilities of ionic liquids. Molecules 2009, 14 (9), 3780. (24) Canal, J. P.; Ramnial, T.; Dickie, D. A.; Clyburne, J. A. C. From the reactivity of N-heterocyclic carbenes to new chemistry in ionic liquids. Chem. Commun. 2006, 1809. (25) Handy, S. T.; Okello, M The 2-position of imidazolium ionic liquids: Substitution and exchange. J. Org. Chem. 2005, 70 (5), 1915. (26) Ham, Y. R.; Kim, S. H.; Shin, Y. J.; Lee, D. H.; Yang, M.; Min, J. H.; Shin, J. S. A comparison of some imidazoles in the curing of epoxy resin. J. Ind. Eng. Chem. 2010, 16 (4), 556. (27) Jíšová, V. Curing mechanism of epoxides by imidazoles. J. Appl. Polym. Sci. 1987, 34 (7), 2547. (28) Cho, Y. S.; Lee, H. K.; Shim, M. J.; Kim, S. W. Effects of additive on electrical tree behavior in epoxy resin. J. Ind. Eng. Chem. 1997, 3 (3), 171. (29) Chambreau, S. D.; Boatz, J. A.; Vaghjiani, G. L.; Koh, C.; Kostko O.; Golan, A.; Leone, S. R. Thermal decomposition mechanism of 1ethyl-3-methylimidazolium bromide ionic liquid. J. Phys. Chem., published online November 18, 2011, doi.org/10.1021/jp209389d. (30) Ooi, S. K.; Cook, W. D.; Simon, G. P.; Such, C. H. DSC studies of the curing mechanisms and kinetics of DGEBA using imidazole curing agents. Polymer 2000, 41, 3639. (31) Omrani, A.; Simon, L. C.; Rostami, A. A.; Ghaemy, M. Study on curing mechanism of DGEBA/nickel−imidazole system. Thermochim. Acta 2008, 468 (1/2), 39.

A-based epoxy resin. The storage time of the epoxy resin/IL compositions at room temperature significantly depended on the anion type and ionic liquid concentration in the epoxy compositions and was in the range of 20−70 days. The rheometric and DSC measurements exhibited the following differences in the curing processes of epoxy resin/ IL systems: (i) with IL content increasing in the curing system a shift to lower temperatures was observed; (ii) the curing process started at lower temperature (120−150 °C) when ILs with N(CN)2¯ anion were applied, in comparison with those containg BF4¯ anions (200−240 °C); (iii) the alkyl chain length of imidazolium cation influenced slightly the onset temperature curing range: for decyl substituent, 200−240 °C, and butyl, 210−230 °C; (iv) as a rule bimodal exotherms appeared on DSC thermograms, the first was placed at a lower temperature range above 110 °C [compositions with ILs and basic N(CN)2¯ anion] and the second was placed above 250 °C (compositions with ILs bearing BF4¯ anion). The cross-linked epoxy materials exhibited Tg values dependent on the ionic liquid type and concentration in the system with epoxy resin. The highest Tg (150−180 °C) and the lowest tan δ values determining efficiency of curing reactions have been found for medium IL concentration (i.e., 3 phr). It means that higher IL amounts could plastify the cross-linked epoxy material. The epoxy resin/IL systems with tetrafluoroborate anion exhibited lower Tg values (ca. 150 °C) than systems containing dicyanamide anion (165− 180 °C). A longer alkyl chain of imidazolium cation caused a lower glass temperature of hardened epoxy material. The observed bimodal exotherms and FTIR absorption bands at 1740−1750 cm−1 were explained by the proposed mechanism of epoxy resin anionic polymerization initiated by thermal decomposition products of 1,3-dialkylimidazolium liquids.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail:[email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from Ministry of Science and Higher Education (Grant No. 508 44 13 36) is gratefully acknowledged.

■

REFERENCES

(1) Czub, P.; Bończa-Tomaszewski, Z.; Penczek, P.; Pielichowski, J. Chemistry and Technology of Epoxy Resins. WNT: Warsaw, Poland, 2002. (2) Pilawka, R.; Spychaj, T.; Leistner, A. Epoxy compositions and materials cured with complexes of imidazoles and Cu(II) cations. Polimery, Warsaw 2008, 53 (7/8), 526. (3) Wilpiszewska, K.; Spychaj, T. Ionic liquids: Media for starch dissolution, plasticization and modification. Carbohydr. Polym. 2011, 86, 424. (4) Kubisa, P. Ionic liquids as solvents for polymerization processesProgress and challenges. Prog. Polym. Sci. 2009, 34, 1333. (5) Weyershausen, B.; Lehmann, K. Industrial application of ionic liquids as performance additives. Green Chem. 2005, 7, 15. (6) Wan, S.; Zhang, X.; Wang, H. Acrylic fibers processing with ionic liquid as solvent. Polym. Adv. Technol. 2009, 20, 857. (7) Tan, S. S. Y.; MacFarlane, D. R. Ionic liquids in biomass processing. Top. Curr. Chem. 2010, 290, 311. 5206


Ionic Liquid Systems: The Influence of Imidazolium

Recommend Documents