The incorporation of carbon nanotubes influences the curing process of epoxy resin and an understanding of the cure reaction of CNT/epoxy composite is very important for the design, analysis and optimization of manufacturing materials. Therefore, for highly demanding manufactories, it is necessary to understand the nature of the curing process, once the final properties of the nanocomposite depend significantly on the curing conditions [1, 39, 40].
Most works in the literature related to CNT/epoxy composites are centered on the final properties of the material, and few of them are focused on processing. Nowadays, however, the trend is towards processing, making the products available at a lower cost. In order to achieve this goal it is necessary to study the curing of these composites.
The effect of different types of carbon nanotubes on the curing process of epoxy resin was shown in many articles, but the results are often conflicting due to the large number of variables that can influence the cure of this composite. For example, some authors found out different effects on the cure reaction using the same type of functionalizing reaction. However, besides the groups linked to the CNT walls, other parameters can influence the curing reaction. Some of these parameters are the differences in the aspect ratio and orientation of CNTs, as well as the amount and type of impurities present in CNTs. Thus, these differences can lead to changes in the homogeneity of the composites, changes in the CNT-epoxy interaction or in the cure kinetics of epoxy resin.
Different techniques can be used to monitor the cure of these composites. The most widely used technique is the differential scanning calorimetry (DSC). However, there are other techniques for monitoring the cure reaction that provide important and complementary data to those provided by DSC analysis, which have been less explored. Consequently, this article aims to bring together key studies already conducted on the effect of carbon nanotubes on the curing process of epoxy resin, separating them according to analysis technique. Thereafter, this article will seek a better understanding of how different types of carbon nanotubes may or may not interact with epoxy resin, how this may influence the cure of the epoxy resin and how each technique may help to characterize these studies.
Study of the Cure Reaction by DSC
Most articles in the literature study the cure reaction of CNT/epoxy resin composites by DSC. This technique starts with a freshly prepared sample and the cure reaction occurs in a crucible during the analysis, under different conditions of temperature and heating rate (isothermal and nonisothermal). DSC provides several pieces of information but the main ones are: cure enthalpy, temperature/time of each reaction step, and activation energy. If more than one cycle is performed, DSC also provides the glass transition temperature (Tg). Some DSC results of CNT/epoxy resin nanocomposites are reported in the following paragraphs.
Puglia et al. used DSC to investigate the incorporation effect of SWCNTs on the curing reaction of epoxy resin. They observed that SWCNT acts as a strong catalyst, where the shifting of the cure reaction peak was related to SWCNT contents. Both samples prepared with 5% (w/w) and 10% (w/w) of nanotubes shifted that peak to a lower temperature. The reaction temperature of the sample prepared with 10% was higher than that of the sample prepared with 5%, but as CNT content increased, the temperature difference was not proportional to CNT content. They concluded that this nonproportional effect was related to a saturation effect, which is typical of catalyst behavior. The thermal stability of composite and neat resin was examined by thermogravimetric analysis (TGA), showing a faster thermal degradation for composites. The authors attributed these effects to the high thermal conductivity of SWCNTs and high surface heat propagation .
The effect of different concentrations of carbon nanotubes in the epoxy resin cure was also examined by Tao et al. using the DSC technique. They prepared nanocomposites with only SWCNT and mixtures of SWCNT with CNT of double or triple walls. On this occasion, the authors showed that nanotubes can initiate the resin cure at lower temperatures. However, the overall curing process was slower for nanocomposites, which was shown by a lower total reaction heat and a lower Tg. They also showed that the addition of carbon nanotube in the epoxy resin may induce the composite thermal degradation at lower temperatures .
Yang et al.  showed that CNT/epoxy composites prepared with nonfunctionalized MWCNTs have a delayed effect on the curing reaction of epoxy resin as compared to neat resin. That is, the curing process of these composites occurred at higher temperatures than neat resin. However, composites prepared with epoxy resin and amino-functionalized MWCNTs have an effect of accelerating the cure, i.e., the curing process occurred at lower temperatures. They attributed this effect to the presence of amino groups on the CNT surface that act as a catalyst of the cure reaction. The presence of amino groups decreases the activation energy of the curing reaction. They also observed that the CNT presence increases the thermal stability of the nanocomposites.
The effect of carboxyl and fluorine-functionalized MWCNTs on epoxy curing reaction was studied by Abdalla et al. using the DSC technique . They showed that the presence of these groups on the SWCNT surface does not change the final cure degree and the glass transition temperature of the neat epoxy. The reaction with carboxyl groups showed the highest value of activation energy, while the reactions with fluorine groups and neat epoxy had the same values of activation energy. The cure enthalpy was lower for nanocomposites than for neat resin, but composites prepared with fluorine-CNT showed values more similar to neat resin. The cure mechanism of nanocomposites was explained in terms of homogeneity, since the authors observed a good dispersion of fluorine-CNTs in the epoxy matrix, whilst the carboxyl modified CNTs were not well dispersed.
Zhengping et al. studied the concentration effect of amino-functionalized MWCNTs on the curing behavior of epoxy resin using DSC. They observed that the nanocomposite activation energy was lower than the neat epoxy one. The cure enthalpy of the system epoxy/1% (w/w) of amino-MWCNT was higher than for neat resin, while the enthalpy of the system epoxy/nonfunctionalized MWCNTs was lower than that of neat resin. However, when the concentration was 2 wt% of MWCNTs, the cure enthalpy of the composite prepared with functionalized and nonfunctionalized MWCNTs was lower than neat resin . The difference was explained by the increase in viscosity caused by the addition of CNTs, especially the nonfunctionalized ones and due to the homogeneity degree of the nanocomposite.
Qiu and Wang  studied the effect of CNT size and various functionalization types (with amine and epoxide groups) in the curing of epoxy/CNT composite. They showed, by DSC, that the total cure enthalpy is lower for all nanocomposites in relation to neat resin, which was explained by the decrease in the epoxy monomer mobility. Shorter CNTs provoked remarkable effects, decreasing both the reaction enthalpy and the extent of conversion, because they elevated the size effect-induced interference. This size effect is related to the change of diffusivity of epoxy and curing agent molecules because shorter CNTs have the size in the same scale as epoxy resin molecules. The lowest effect, that is, the higher enthalpy, was observed for the CNT functionalized with the epoxide group, since this group participated in the curing reaction and was present in great concentration. All CNTs (functionalized or not) reduced the onset curing temperature, accelerating the epoxy cure reaction at the initial stage. The largest acceleration occurred when they used epoxide-functionalized CNTs. However, amino-CNTs increased the activation energy of the overall cure reaction, while epoxide-CNTs slightly decreased the activation energy (compared with neat epoxy).
Xie et al.  studied the effect of MWCNT (1 and 5 wt%) on cure of epoxy resin by DSC. They observed that CNT behaves as an autocatalyst and that MWCNT/epoxy nanocomposites showed the highest initial reaction rates, the lowest time to reach the maximum rate and the lowest activation energy values in the initial reaction stage (in relation to neat epoxy). However, the CNT presence no longer influenced the last stage of the cure reaction. The catalytic effect of CNTs was explained by the presence of OH groups on the MWCNT surface, which exerts catalytic effects for epoxide ring opening. Table 2 summarizes the main information of each work cited above, and some additional information.
Table 2. Thermodynamic and kinetic parameters of epoxy cure reaction.
|Ref.||CNT type||wt% CNT||Dispersion||Enthalpy (ΔH)||Activation energy (E)||Explanation|
|||A||5, 10||Regular||ΔHA < ΔHJ||—||Thermal conductivity|
|||C||0.5||—||—||EC> EJ > ED||Steric hindrance of MWNTs|
|||D||0.5||High||—||Amine: act as curing agent|
|||E||1||Low||ΔHE < ΔHF < ΔHJ||EE > EJ ∼ EF||Heterogeneous dispersion: hinder mobility|
|||D||1||High||ΔHD > ΔHJ||ED < EJ||Good dispersion|
|||C||1||Low||ΔHC < ΔHJ||EC ∼ EJ||Heterogeneous dispersion|
|||D||2||High||ΔHD < ΔHJ||ED < EJ||High viscosity|
|||C||2||Low||ΔHC < ΔHJ||EC ∼ EJ||High viscosity|
|||A, G, H, I||—||—||ΔHH < ΔHG < ΔHA<ΔHI <ΔHJ||EI < EJ < EG< EA < EH||Hinder mobility, CNT groups: participate in cure|
The works cited above and Table 2 shows some controversy in the results of CNT influence on the cure process of epoxy resin. Each author attributes their own explanation about this controversial behavior, which can be: an increase or decrease in the steric hindrance effect; an increase in thermal conductivity due to excellent CNT thermal conductivity; an increase or decrease in viscosity; a catalytic effect due to the chemical groups of the CNT walls etc. However, despite this controversy, CNT dispersion homogeneity into epoxy resin and viscosity of the medium seem to be the most responsible for the anomalous behavior of this class of nanocomposites. For example, the effect of non-functionalized CNTs can lead to a faster cure, but sometimes to a slower one as well. This contradictory effect is due to a great number of variables that can influence the curing reaction, e.g. length and diameter of the CNT, purity, type and quantity of chemical groups attached to their walls, CNT amount, dispersion method etc. Thus, composites prepared with the same CNT type can lead to different dispersion degrees of CNTs in epoxy resin and different viscosity values, which in their turn determine the kinetics and thermodynamics of the cure reaction. For example, amine-functionalized CNTs can catalyze the curing reaction, but if they are added in large amounts they will greatly increase the viscosity of the noncured resin, hindering the cure reaction. Moreover, dispersion homogeneity seems to be a decisive factor which will determine the curing reaction of the epoxy/CNT composites. One can say that heterogeneous CNT dispersion slows down the rate of epoxy resin cure and reduces the heat curing reaction [42, 43]. Therefore, in order to clarify the effect of CNTs on the curing reaction of epoxy resin, information related to CNT dispersion homogeneity must be supplied, such as the aspect ratio of CNTs, the amount added, the purification and dispersion method, as well as fracture surface micrographs and viscometry.
Furthermore, CNTs are often cited as acting as catalysts for the epoxy resin cure at early steps and for their minimal influence at the final stage. The catalyst action of CNT is often attributed to CNT thermal conductivity (especially when they are added in large amounts), and to the presence of functional chemical groups, such as amine, hydroxyl or fluorine. These amine groups act as curing agent since they have a similar chemical structure to hardener, and the hydroxyl group can catalyze the ring opening of epoxy. Fluorinated CNT may react with the amine curing agents, so they can essentially act as a curing agent and react with other epoxy groups . Some authors also suggested that the metallic residue from the manufacturing process that is still in the CNT interior could also act as a hydrogen donor and catalyze the cure reaction, especially at the early stages . However, the extension degree of total cure is often similar to or lesser than that of neat resin. The lowest extension degree of cure reaction is described by the increase in the viscosity caused by the CNT addition, which hinders the mobility of epoxy chains. Furthermore, the CNT addition into epoxy resin changes the optimized curing ratio between epoxy/hardener.
Figure 4 summarizes the possible effects of CNTs on the curing reaction of epoxy resin and the factors that may interfere on the reaction.
Study of the Cure Reaction by DMA
The dynamic mechanical analysis (DMA) is mainly used for obtaining the glass transition (Tg) of polymer samples. The glass transition value of a composite sample is often related to the cure extension of epoxy resin, as well as to the interfacial interaction of epoxy/CNT. Both the interface strength and the cure extent can reduce the mobility of epoxy resin chains in cured samples, increasing the glass transition temperature.
Monitoring the epoxy resin cure reaction using DMA is very difficult since the sample is liquid before the curing process. Thus, DMA is often used to study cured samples, providing Tg values. Tg can also be determined by DSC analysis, but the DMA technique provides more accurate values than DSC.
Allaoui et al.  showed that composites prepared with nonfunctionalized SWCNT had lower Tg than neat epoxy due to the bundling tendency of SWCNT. However, the Tg was not affected by CNT addition when they were well dispersed in the matrix . Therefore, the glass transition temperature is greatly influenced by the homogeneity of the composite, as is the curing reaction.
Luo et al.  determined the Tg and storage modulus (G′) of composites prepared with amino-functionalized CNT by DMA. They showed that composites with higher homogeneity and good adhesion between the resin and nanotubes had higher Tg and G′ than neat epoxy. In particular, CNTs functionalized with phenylbiguanide had the highest values of Tg and storage modulus (G′) . Grikas et al.  studied how the time interval and amplitude pulse of the sonication step during CNT dispersion in epoxy resin have an influence on Tg and G′ and their results were similar to Luo et al. Abdalla et al.  determined the following order of glass transition temperature for some functionalized CNT/epoxy composites: fluorinated CNT > carboxylated CNT > neat epoxy, which agrees with the order of chemical strength bonding between the functional group and epoxy resin.
Gude et al.  prepared neat epoxy and CNT/epoxy samples with different concentrations of curing agent and submitted them to DMA and flexural tests. The highest glass transition temperature was obtained by samples with stoichiometric composition of curing agent, while the epoxy-rich samples showed the highest glassy storage and flexural modulus. In addition, they concluded that the curing agent concentration is much more relevant than the CNT addition for the epoxy curing process, while CNT addition was more relevant in the epoxy-rich resin, increasing the elastic modulus and Tg. Therefore, the authors suggested that the amount of the curing agent is by far the most relevant parameter to be controlled in the epoxy resin synthesis. Nevertheless, considering synthesis with the same curing agent/epoxy resin ratio, the type, concentration, and mainly the dispersion degree of CNT into epoxy resin became very important parameters.
Study of the Cure Reaction by Rheology
The manufacturing process of polymer composites requires information such as viscosity and resin cure. The viscosity of composites depends not only on the cure degree, but also on the shear rate. The control of the rheological properties is very important for obtaining reproducible CNT/epoxy resin samples. However, despite the importance of this information, few works can be found in the literature related to the rheology of these composites. Unlike DMA testing, the rheometer provides insights into the initial step of cure reactions. For many authors, the beginning of the cure reaction is the step where the most important differences between the neat epoxy and the composites happen. Therefore, the initial steps of the cure of epoxy/CNT composites are very important to be analyzed and some results are summarized in the next paragraphs.
Fan and Advani  prepared samples with several dispersion degrees of nanotubes in epoxy resin using oscillatory experiments, and observed significant changes in the CNT/epoxy rheological properties with the dispersion degree. Figure 5 shows two types of CNT dispersion which showed different rheology behaviors . The authors showed that good CNT dispersion, high CNT aspect ratio and high CNT concentration in the matrix caused strong CNT networking, which was shown by the high values of storage modulus (G′) and complex viscosity (|η*|).
Figure 5. Structure of two types of nanotubes: (I) with agglomerates, (II) without agglomerates . Reprinted with permission from Fan, Z., Advani, SG, v. 51, p. 585–604. Copyright 2007, The Society of Rheology, DOI: 10.1122/1.2736424.
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Johnson and Pitchumani  studied the curing process of epoxy/CNT composite using CNTs in different concentrations and aspect ratios by DSC and rheology techniques. Their results showed that the CNT presence did not change the cure mechanism and cure heat. However, the CNT addition decreased the activation energy of cure resin. In addition, they showed that the CNT presence increased the viscosity and introduced a distinct shear-thinning rheology, with the shear thinning effect being greater with the higher amount of CNT.
Abdalla et al.  studied uncured epoxy resin/CNT samples and observed that the rheological properties are affected by the covalent bonds of CNTs. Systems without CNT showed a Newtonian behavior, while the CNT presence led to a non-Newtonian behavior, mainly if the composite had been prepared with fluorinated nanotubes. Another study  showed that the presence of CNTs functionalized with carboxyl groups decreased considerably the gel time of epoxy resin in relation to neat resin and to fluorine-functionalized CNT . Thus, hydroxyl groups act as catalysts for the early stages of cure reaction.
Terenzi et al.  showed by the rheometry technique that the cure reaction of epoxy resin composites and amino-functionalized CNTs was accelerated by the presence of amino group on CNTs. They showed the composite reached higher cure degree in shorter times, as well as the gel point occurring in less time when compared with neat epoxy.
Rheology is a simple technique that allows for studying the influence of carbon nanotubes in the cure of epoxy resin, but it also provides information concerning the homogeneity, viscosity, and chemical-physical interactions of a composite. Thus, it is a technique that has great potential to be applied in studying the effect of different kinds of variables on the cure process of composites prepared with epoxy resin and carbon nanotubes. Moreover, this technique is important to the manufacturing process of these composites, since it allows the obtaining of viscosity values at different shear rates.
The previous results showed that composite homogeneity is a critical parameter in determining the rheological behavior of the samples. Besides that, the CNT catalytic effect on the initial stages of curing reaction was observed by rheology studies as well as by the DSC technique.
Study of Cure Reaction by Raman, Luminescence Spectroscopy, and FT-IR
Luminescence spectroscopy is a very sensitive technique which has been used to monitor the cure of epoxy resin and other composites, such as epoxy/carbon fiber and epoxy/glass fiber. This technique had not been applied to study the curing of epoxy/CNT, until the work performed by Cividanes et al. . In this study , two unconventional techniques were applied to study the influence of CNTs, functionalized or not, in the cure of epoxy resin: Raman and luminescence spectroscopy. The cure process was monitored by Raman spectroscopy using the intensity of two bands of the Raman spectrum: 1610 cm−1, characteristic of the stretching of aromatic ring C=C, and 3065 cm−1, characteristic of epoxy ring stretching. On the other hand, luminescence spectroscopy was applied to monitor the cure by calculating the cure degree, using the Equation:
where Ii means the intensity of the emission peak obtained from the studied sample after pre-cure at 80°C for 1 h; If means the intensity of the emission peak obtained using the studied sample after cure at 120°C for 1080 min; It is the intensity of the emission peak obtained using the studied sample after thermal treatment at 120°C for a given period of time (t).
The presence of CNTs, functionalized or not, increased the resin cure degree. The composites prepared with annealed CNT (purification to 1800°C, vacuum) and acid-treated CNT had considerably higher cure rates at the reaction beginning. Composites prepared with amino-functionalized nanotubes and neat epoxy resin had a lower cure rate. The difference in the cure rate was explained by means of the sample's homogeneity (greater in the composite prepared with amino-CNT) and the presence of chemical groups.
A problem observed when using Raman spectroscopy to measure the cure intensity of epoxy resin/CNT is the influence of background fluorescence in the Raman spectra. Depending on the type of epoxy resin and hardener used, this effect can change the intensity of the peaks exactly in the region of interest, as in the aforementioned work .
An advantage of luminescence spectroscopy in relation to other techniques is its sensitivity to detect the curing reaction, particularly at its final stages. Therefore, this technique can be used to monitor the cure reaction after the gel point .
Vega et al.  used in situ Raman spectroscopy and electrical conductivity measurements to monitor the internal stresses arising from the curing process of composites epoxy/SWCNT. They did preliminary investigations to analyze how SWCNT Raman spectra changes with temperature. They showed that the G band position was dependent on temperature and on cure degree. However, the effect of adding 0.2% of SWCNT in epoxy was found to be negligible for the curing process.
Valentini et al.  used Raman spectroscopy not to monitor the cure, but to understand the interaction between CNTs and epoxy resin during the cure. They studied three types of composites: (a) fluorinated SWCNT (F-SWCNT); (b) F-SWCNT grafted with butylamine (BAM-SWCNT); (c) unfunctionalized SWCNT (u-SWCNT). They observed that BAM-SWCNT acts as a strong catalyst during the cure reaction because of cross-linking interactions between epoxy resin and BAM-SWCNTs, since BAM is the hardener of epoxy resin. Raman spectra of the epoxy/u-SWNT composite did not show frequency shifts of typical bands of SWNTs, while F-SWNTs showed an important chemical affinity with epoxy, resulting in a shifting of its Raman peaks. This peak shifting toward higher frequencies is due to the opening of CNT bundles which was produced by the cross-linked resin intercalation.
FT-IR technique is often used to study epoxy/CNT composite curing for comparing the area of the epoxy ring peak before and after the curing process. Yang et al.  compared the peak area of an epoxy ring (peak at ∼913 cm−1) with a reference peak (at 1610 cm−1) which is characteristic of a benzene ring and does not vary with the cure degree. They found that the final cure degree of the composites prepared with nonfunctionalized CNT and amino-CNT was basically the same (∼98%). In another study , a very similar cure degree between neat resin (95%) and composites prepared from carboxylated and fluorinated MWCNTs (96%) were obtained.
Table 3 summarizes some information about the curing reaction of epoxy resin in presence of carbon nanotubes, describing the advantages and disadvantages of each technique.
Table 3. Characteristics of different techniques used to study the curing of epoxy resin/CNT composites.
|DSC||Enthalpy, time/temperature of peak, E, Tg, cure degree.||Study all stages of the reaction.||Destructive|
|DMA||Tg, viscoelastic study after gel point.||Tg with greater accuracy.||Usually used only after gel point.|
|Rheometry||η* under various shear rates, gel time, viscoelastic study before gel point.||Study of the initial reaction.||Usually used only before gel point, destructive test.|
|Luminescence||Cure degree, photophysical and photochemical study.||Greater sensitivity, especially in the final times, non destructive.||Usually used only after gel point|
|Raman||Cure degree.||Simplicity of the sampling requirements, non destructive.||Interference of fluorescence.|
|FT-IR||Cure degree.||Study of the initial and intermediate stages.||Low sensitivity for final cure stages.|
The DSC technique is the most used because of its practicality and the large amount of information that can be obtained through it. Even so, a lot of information can be obtained from other techniques with advantages over DSC. For example, luminescence spectroscopy is a very sensitive technique for monitoring the cure reaction, making it possible to obtain data which are impossible from others. Moreover, luminescence spectroscopy is used together with the FT-IR technique, since the latter is able to easily detect the initial cure reaction, while the former is highly sensitive to the final cure reaction.
Raman spectroscopy also allows monitoring of the cure reaction. However, this technique can be influenced by the fluorescence of epoxy resin and of hardener, which is a disadvantage of this technique.
These three techniques (Luminescence, FT-IR, and Raman) are capable of detecting the cure degree of the composite during the reaction. For this, it is necessary to cure the sample at different times/temperatures (t/T), and thus perform the analysis of each point t/T. Therefore, they are not as practical as DSC, which is a continuous analysis of the curing reaction. However, it is possible to couple an oven to the equipment to increase the practicality of the cure analysis with these techniques. That is, the samples are cured and tested at the same time in the same equipment. These three techniques also provide information about the chemical groups belonging to nanocomposites, thus further studies can be performed while studying the cure process. For example, study of the degradation of epoxy resin by FT-IR in tandem with studying the cure.
The rheometer and DMA techniques are also often used together, since the first one evaluates the viscoelastic properties of composite until the gel point and the second, after the gel point. The rheometer allows for obtaining information that is not obtained with any other technique considered, such as the variation of viscosity with shear rate and gel time, which is valuable information for processing and fabrication of composites. On the other hand, DMA is a much more sensitive technique than DSC for determining Tg of polymers.