M. R. Acocella*a,
C. Esposito Corcione*b,
A. Giurib,
M. Maggioa,
A. Maffezzolib and
G. Guerraa
aDepartment of Chemistry and Biology and INSTM Research Unit, Università di Salerno, Fisciano, SA, Italy. E-mail: macocella@unisa.it
bDipartimento di Ingegneria dell'Innovazione, Università del Salento, Lecce, Italy. E-mail: carola.corcione@unisalento.it
First published on 25th February 2016
The influence of different graphite-based nanofillers on epoxide ring opening reactions, as induced by amines for diglycidyl ether of bisphenol A (DGEBA), is studied. Direct kinetic studies, with full chemical characterization and quantitative evaluation of the low molecular mass products, for reactions of DGEBA with primary and secondary monoamines as well with alcohols, are conducted. Moreover, the kinetic behavior of a commercial epoxy resin based on DGEBA and a diamine, leading to crosslinked insoluble networks, is studied by indirect methods, such as differential scanning calorimetry (DSC) and rheometry. The reported results show a relevant catalytic activity of graphene oxide on epoxy resin crosslinking by amines. For instance, for a graphene oxide content of 3 wt%, the exothermic crosslinking DSC peak is shifted (upon heating at 10 °C min−1) from 113 °C down to 96 °C, while the gel time at 50 °C is reduced by a factor of 2.5. This behavior is due to the ability of graphene oxide to catalyze primary amine–epoxy, secondary amine–epoxy and mainly hydroxyl–epoxy additions.
Graphene is generally dispersed in various polymer matrices as reduced graphene oxide. Polymer matrices reinforced with graphene and nano-graphite platelets present higher electrical and thermal conductivity sharing with those reinforced with other layered nanofillers improved strength, modulus, heat distortion temperature and barrier properties.23–26
In previous works from some of us25,27,28 a significant effort was directed to preparation of epoxy nanocomposites reinforced with nanographite and to the analysis of their structural, rheological, mechanical, and thermal behaviour. In particular, in ref. 28 comparative thermal analysis of epoxy resins filled with different carbon nanofillers, for different thermal histories, were conducted. The higher Tg values observed at low curing temperatures, for epoxy resins with graphite-based nanofillers, were rationalized by suggesting a catalytic activity of graphene oxide layers on the reaction between the epoxy and amine groups of the resin, leading to higher crosslinking density in milder conditions. This hypothesis, formulated on the basis of the well established catalytic behavior of graphene and graphene oxide for many organic reactions,29–34 has been supported by experiments on the epoxide ring opening reaction, for few monofunctional epoxide and amine reactants.28 However, analogous data have been rationalized by different hypothesis: restricted molecular mobility of the epoxy matrix by the nanofiller35 or possible crosslinking of graphene oxide layers via epoxy chains.36,37
Moreover, although additional studies have shown some catalytic activity of graphene oxide layers on curing reactions of different epoxy resins,38–40 other studies have reported the opposite effect, i.e. the occurrence of retardation effects.41–43
In this paper, the possible catalytic activity of graphite-based nanofillers on epoxide ring opening reactions, for the bi-functional basic component of most epoxy resins (diglycidyl ether of bisphenol A, DGEBA, Fig. 1), is explored. This study mainly reports kinetics of epoxide ring opening reactions with monofunctional amines, which produce easy to characterize low-molecular-mass products. The possible catalytic behavior is also explored by studying kinetics of crosslinking (cure) of a commercially available DGEBA-based resin, by indirect phenomenological approches,44 based on differential scanning calorimetry (DSC) and on rheological evaluation of viscosity increases.
The epoxy resin is characterized by an epoxy equivalent weight (EEW) of 190 g eq.−1 corresponding to an average molecular weight (Mw Epikote 828) of 380 g mol−1. Chemical formula of DGEBA and of corresponding oligomers are reported, in the left side of Fig. 1. Since neat DGEBA is characterized by an EEW = 170 g eq.−1, that corresponds to an average molecular weight (Mw DGEBA) of 340 g mol−1, Epikote 828 is a mixture of neat DGEBA and of its oligomer with n = 1.
Epikote 828 crosslinking was obtained with isophorondiamine (IPDA) supplied by Aldrich, which is characterized by an Hydrogen Equivalent Weight, HEW = 43. It was selected since it is a typical hardener used for the fabrication of epoxy based composites. According to the stoichiometric ratio, 22 parts of hardener were added to hundred parts of DGEBA (22 phr).
The used primary monoamines (benzylamine and cyclohexylamine) were selected because they exhibit amine groups in chemical environments similar to IPDA. The di-amine IPDA and used primary mono-amines are shown in Fig. 1.
Synthetic Graphite 8427® as trademark with a high surface area (HSAG, of about 308 m2 g−1) and a high shape anisotropy of the crystallites,45 was purchased from Asbury Graphite Mills Inc. All other standard reagents were bought from Aldrich.
Exfoliated graphite oxide (eGO) samples were prepared by GO powders introduced in 125 mL ceramic jars (inner diameter of 75 mm) together with stainless steel balls (10 mm in diameter) and dry-milled in a planetary ball mill (Retsch GmbH 5657 Haan) for 2 h with a milling speed of 500 rpm and a ball-to-powder mass ratio of 10 to 1.
All yields reported refer to the isolated products obtained by column chromatography on silica gel in gradient elution with petroleum ether–ethyl acetate. All 1H NMR and 13C NMR spectra were recorded with a DRX 400 MHz Bruker instrument, by using CDCl3 (d = 7.26 ppm in 1H NMR spectra and d = 77.0 ppm in 13C NMR spectra) as solvent (400.135 MHz for 1H and 100.03 MHz for 13C NMR). Chemical shifts are reported in ppm, multiplicities are indicated by s (singlet), dd (double doublet), bs (broad singlet). Mass spectrometry analysis was carried out using an electrospray spectrometer Waters 4 micro quadrupole. The elemental analyses were performed with FLASH EA 1112 Thermo equipment.
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Rheological properties of filled and unfilled mixtures were evaluated by monitoring the evolution of the storage modulus G′ and the loss modulus G′′ as function of time. Measurements were made using a strain-controlled rheometer (Ares, Rheometric Scientific) with a parallel plate geometry (12.5 mm radius) at a temperature of 50 °C, with a frequency of 1 Hz and a deformation of 10%. Three runs were made for each sample. The gel time is observed for constant value of the degree of reaction, according to Flory47 and hence it represents a fast way to compare the rate of polymerization reactions of thermosetting resins with the same reagents and under different conditions, such as temperature or catalysis.
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Fig. 2 X-ray diffraction patterns (Cu Kα) of the high-surface-area graphite (HSAG, A), of the derived GO (B) and eGO (C). |
Additional characterizations of the used nanofillers (elemental analysis, BET surface areas and FTIR spectra) are added as ESI.†
Analogous studies are presently extended to a much more reactive di-epoxide, i.e. DGEBA that is the most common epoxide used in commercial epoxy resins. These studies are conducted in the presence of two different mono-amines (benzylamine and cyclohexylamine), which exhibit amine groups in chemical environments similar to those experienced by the two amine groups of the crosslinking agent used in Epikote 828 (isophorondiamine, IPDA, see Fig. 1). The use of mono-amines rather than di-amines avoids cross-linking, thus making easy characterization of the obtained products. The epoxy ring opening reactions were conducted at room temperature in presence of 3 wt% of carbon nanofiller, in solvent free condition and by using different epoxide/amine ratios. The study of reactions at room temperature rather than in the temperature range 50–180 °C (usual for epoxy resins), is useful to put in evidence the catalytic activity of the used nanofillers.
The considered reactions, both in the absence and presence of nanofillers, are highly regioselective (Fig. 3), due to a SN2 mechanism with the nucleophile attack to the less substituted end. For molar ratios between epoxide and amine groups close to one, both reactions lead to a single product (3 and 5 in Fig. 3) and the conversion of the di-epoxide is complete already at room temperature, if sufficiently long reaction times are allowed.
Reaction rate is higher with benzylamine and for both amines it is faster in the presence of eGO. In particular, after 7 hours at room temperature, from benzylamine 2, product 3 was obtained with yields of 70% and 30%, for eGO catalyzed and uncatalyzed reactions, respectively. Moreover, after 24 hours at room temperature, from cyclohexylamine 4, product 5 was obtained with yields of 60% and 15%, for eGO catalyzed and uncatalyzed reactions, respectively.
The influence of all the considered graphitic nanomaterials (HSAG, GO and eGO) on reaction rate from 2 to 3, for epoxide/amine molar ratio of 1.0/1.2 and at room temperature, is shown in Fig. 4, as yield of product 3, versus reaction time. All nanofillers increase reaction rates, with maximum increases for graphene oxide (eGO) due to the further activation exerted by its acidic functionalities. In fact, in presence of HSAG the epoxide activation derives only from π-stacking interactions between DGEBA and graphitic surface, as already suggested, also on the basis of DFT calculations, for other reactions.33,34 In the presence of GO, instead, due to its OH and COOH groups, both mechanism, π-stacking activation and acidic activation could be involved, increasing the rate of the reaction. Moreover, an induction time longer than 1 hour occurs only in the absence of nanofillers. In fact, the typical autocatalytic behavior of epoxy–amine reactions, characterized by a maximum yield rate at time > 0,48 is clearly observed only with neat resin. This indicates a catalytic activity of all nanofillers on the epoxide ring opening reaction, which eliminates the induction time and makes irrelevant the formation of OH groups for the reaction beginning.
To establish if the used nanocarbons can be considered real catalysts, the reusability of both eGO and GO was investigated by repeated tests for the reaction leading to product 3. The nanofillers, recovered after extraction from the aqueous solution and dried at 60 °C overnight, were used without any further treatment. The reaction conditions, room temperature and 3 h, were kept constant for all cycles. Yields, for four consecutive reaction cycles, in the presence of GO and eGO are shown in Fig. 5.
In the presence of eGO, the yield is maximum for the first run and markedly decreases to a nearly constant value, starting from the second run. In the presence of GO, on the contrary, the yield is maximum for the second run and starting from the 4th run reaches values close to those observed for eGO.
These data can be easily rationalized on the basis of the structural reorganization of the GO and eGO, after the first reaction cycle and removal from the reaction system. In fact, both for the ordered graphite oxide (Fig. 6A) and the disordered graphene oxide (Fig. 6B), relevant changes of the WAXD patterns are observed, which lead to rather similar patterns for both nanofillers (Fig. 6A′ and B′).
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Fig. 6 X-ray diffraction patterns (Cu Kα) of GO (A and A′) and eGO (B and B′), before (A and B) and after (A′ and B′) the first reaction cycle and removal from the reaction system. |
In particular, the pattern of Fig. 6A′ shows that the 001 graphite oxide peak at d = 0.835 nm is completely lost and replaced by narrower peaks at d = 3.4 nm and d = 1.7 nm. These peaks can be rationalized as 001 and 002 reflections of an intercalate compound of graphite oxide,49–52 with the used amine (benzylamine). The pattern of Fig. 6B′ shows that the eGO sample, although initially fully exfoliated, after removal from the reaction system partially reorganizes leading to a similar intercalate compound with slightly increased interlayer spacing d = 4.2 nm (only the d002 = 2.1 nm is clearly apparent).
The yield data for consecutive reaction runs of Fig. 5 clearly confirm that the influence of carbon nanofillers on epoxide ring opening reaction corresponds to a real catalytic action. However, the yield reduction (after the first run for eGO and after the second run for GO), can be attributed to a partial loss of functional groups, mainly due to amine reaction with epoxy groups on the GO layers.53 This conclusion is mainly supported by the formation of GO/benzylamine intercalate structures, as shown by the WAXD patterns of Fig. 6A′ and B′.
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Fig. 7 DSC scans (heating rate = 10 °C min−1, exotherms downward) for the pure epoxy resin (black) and for composite resins wit 3 wt% of: HSAG (purple); GO (blue) and eGO (green). |
Sample | Delta H [J g−1] | T Peak [°C] |
---|---|---|
Epoxy | 313 ± 35 | 113 |
Epoxy + HSAG | 339 ± 35 | 112 |
Epoxy + GO | 373 ± 35 | 104 |
Epoxy + eGO | 375 ± 35 | 96 |
The cure reaction was also studied by isothermal rheological measurements, at 50 °C. The ratio between the modulus of complex viscosity |η*| and the complex viscosity measured at t = 0 of the neat resin and of the resin filled with 3 wt% with the three considered nanofillers, are reported in Fig. 8 as a function of time.
The dynamic-mechanical properties of a curing system depend on the degree of reaction, dramatically changing when the gel point is approached. The time to gelation or gel time (tg) can be determined according to one of the different criteria, as reviewed by Laza et al.54 In this paper we have chosen as gel times those corresponding to |η*|/|η*0| = 1000. The measured values of gel time and the corresponding complex viscosity measured at t = 0 (|η*0|), for the neat resin and the resin filled with 3 wt% with the three considered nanofillers, have been collected in Table 2. The gel time of the neat resin (tgel = 2776 s) is only slightly reduced in the presence of HSAG (tgel = 2560 s) while it is strongly reduced in the presence of graphite oxide (tg = 1650 s) and graphene oxide (tgel = 1550 s). A similar trend is obtained adopting any of the others criteria described in ref. 55. This result can be again rationalized by a relevant catalytic activity of graphite oxide (mainly of eGO) and a nearly negligible catalytic activity of HSAG toward the whole process of epoxy resin crosslinking.
Sample | tgel (|η*|/|η*0| = 1000) [s] | |η*0| [Pas] |
---|---|---|
Epoxy | 2776 | 0.136 |
Epoxy_HGSA | 2560 | 0.239 |
Epoxy_GO | 1650 | 0.596 |
Epoxy_eGO | 1550 | 0.564 |
These apparently conflicting results can be rationalized by considering beside primary amine–epoxy additions also the other two major reactions involved in amine curing of epoxy resins: secondary amine–epoxy and hydroxyl–epoxy additions (as schematically shown in Fig. 9).43,55 The latter reaction, of course, becomes significant only after an adequate increase of OH group concentration, as a result the first two reactions.
The model reactions of DGEBA with monoamines (Fig. 3–5) have demonstrated catalytic activity of all the considered nanofillers on reaction 1 of Fig. 9. Additional model reactions are conducted to evaluate the possible influence of the considered graphitic nanofillers on secondary amine–epoxy addition (reaction 2 in Fig. 9) and on hydroxyl–epoxy additions (reaction 3 in Fig. 9). In particular, we have compared reactions of DGEBA with a secondary monoamine (dibenzylamine, Fig. 10) and with an alcohol (benzyl alcohol, Fig. 11), in the presence of 3 wt% of carbon nanofillers in solvent free condition and by using molar ratios between reacting groups close to one.
The obtained results for the secondary amine–epoxy addition of Fig. 10, are rather similar to those obtained for primary amine–epoxy addition of Fig. 3, showing a similar catalytic activity of graphene oxide and graphite on the ring opening reaction. For instance, after 19 h, in the presence of eGO as well as of HSAG the product 7 is obtained in a 43% yield while in absence of nanofillers the reaction only proceeds up to 20% yield.
Completely different are the results obtained for the model reaction for the hydroxyl–epoxy addition (Fig. 11), showing a relevant influence of graphene oxide but a negligible influence of graphite on the ring opening reaction. In particular, reactions conducted in the presence of graphene oxide after 24 h lead to 20% and 50% of overall yield of 9 and 10, when conducted at 90 °C and 110 °C, respectively. For the same reactions, if conducted without nanofillers or in the presence of HSAG, no traces of products are detected. This result can be explained based on the reduced nucleophilicity of alcohol with respect to the amine group, that needs a more activated electrophile to provide the desired product. In fact, only in the presence of acidic activation the etherification reaction is catalyzed.
In summary, according to the results of ring-opening model reactions of present section, graphene oxide is able to catalyze all three major reactions that are involved in amine curing of epoxy resins (Fig. 9), while graphite-based nanofillers are only able to catalyze amine–epoxy additions (reactions 1 and 2, in Fig. 9). Because only nanofillers based on graphene oxide (but not those based on graphite) are able to catalyze crosslinking of epoxy resins (Section 3.3), the present results indicate, in agreement with a literature suggestion,55 that the hydroxyl–epoxy addition (etherification reaction) is the rate-determining step of epoxy resin crosslinking.
Direct kinetic studies of epoxide ring opening reaction for DGEBA in the presence of different mono-amines (benzylamine, cyclohexylamine and dibenzylamine), with full chemical characterization and quantitative evaluation of low molecular mass products, have been conducted. The reported results indicate a strong influence of carbon nanofillers, mainly of graphene oxide but also of graphite oxide and high-surface-area graphite, on epoxide ring opening reactions by amines. For instance, for the reaction with benzylamine for the epoxy/amine group molar ratio of 1.0/1.2, at room temperature, the addition of 3 wt% of nanofillers leads to elimination of the reaction induction time, which is instead longer than 1 hour in the absence of the nanofillers.
The occurrence of a real catalytic activity of these carbon nanofillers on amine–epoxy additions has been clearly proved by repeated tests for the reaction between the di-epoxide and a mono-amine (benzylamine), as conducted in the presence of recovered nanofillers. After few runs the yields become similar for graphite oxide and graphene oxide. This phenomenon is mainly due to the exfoliation of graphite oxide in the reaction conditions, already occurring during the first reaction run.
Kinetics of crosslinking of a commercial epoxide resin with a diamine have confirmed a relevant catalytic activity of graphite oxide (mainly of graphene oxide) while have shown a negligible catalytic activity of graphite. In particular, for a nanofillers content of 3 wt%, the exothermic DSC peak corresponding to the crosslinking reaction (at a heating rate of 10 °C min−1) is shifted of 17 °C and 1 °C for eGO and HSAG, respectively. Analogously, gel time, as evaluated by rheological data at 50 °C in the presence of 3 wt% of nanofillers, is reduced of nearly 45% in the presence of eGO while it is reduced of less than 10% for high-surface-area graphite.
These results have been rationalized by showing, for model reactions of DGEBA with an alcohol, that only graphene oxide (but not graphite) is able to catalyze hydroxyl–epoxy additions (etherification reactions). The latter results also suggest that etherification reaction is the rate-determining step in amine curing of epoxy resins.
In summary, nanofillers based on graphene oxide reduce temperature and/or time of crosslinking of epoxy resins by amines, by catalyzing primary amine–epoxy, secondary amine–epoxy and mainly hydroxyl–epoxy additions.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00485g |
This journal is © The Royal Society of Chemistry 2016 |