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Effect of carbon nanotubes on electrical and mechanical properties of multiwalled carbon nanotubes/epoxy coatings.

Abstract In the present study, attempts were made to enhance conductivity of electrocoats (based on epoxy amine adduct) containing N[H.sub.2]-multiwalled carbon nanotubes (MWCNTs). The weight percent of incorporated MWCNTS into the electrocoat matrix varied in the range of 0.6-3.6 wt% to obtain a series of electrocoatings. These were then applied on steel substrates by a cathodic electrodeposition technique. Electrocoated films were characterized utilizing scanning electron microscopy and optical microscopy. The results illustrated that electrical conductivity was enhanced by increasing of the MWCNT load. At the percolation threshold, throwing power was dropped while the recoating ability was enhanced. Mechanical behavior of nanocomposites containing MWCNTs in the range of 0-2.8 wt% was investigated by dynamic mechanical thermal analysis (DMTA) and nanoindentation test methods. DMTA analysis revealed that the width of tan [delta] was increased by the addition of nanotubes up to 2.8 (wt%). Also, the results obtained from the nanoindentation test showed that the elastic modulus and hardness of the nanocomposites were decreased by the addition of MWCNTs.

Keywords Carbon nanotubes, Epoxy matrix, Electrical conductivity, DMTA, SEM


Coatings filled with nanoparticles have attracted great interest because incorporation of nanofillers can significantly improve mechanical, electrical, optical, and thermal properties of such coatings. (1,2) Highly conductive coatings have become one of the major themes of the thin film technologies of recent years. (3-4) Ideally, there are three methods possible for designing conductive coatings: utilizing conductive polymers as the continuous matrix, incorporating conductive pigments at a sufficient pigment volume concentration, and combination of both methods. (5) Due to economic reasons, however, in most cases, a highly conductive pigment is added to the matrix to provide a three-dimensional network of pigment particles throughout the composite. The conductive particles will percolate and the percolation threshold is characterized by a sharp drop in the electrical resistivity of the composite by several orders of magnitude. (6) Percolation theories are frequently used to describe the insulator-to-conductor transitions in composites made of a conductive pigment and an insulating matrix. Graphite nanoplatelets have attracted significant interest as conductive particles in coatings because of their excellent electrical, mechanical, and thermal properties. (7) Afshar et al. investigated the electroplating characteristics of graphite-bronze composite coatings using the electrodeposition method. They found a decrease of graphite concentration in coating with increasing the current density. The limitation in deposition rate of graphite was attributed to the gravity force. (8) Furthermore, oxide films are used in a wide variety of industrial applications ranging from sensors and superconductors to catalysis. Electrodeposition is an interesting technique to apply these films because of the flexibility of this method for large industrial applications at low cost. However, there are some limitations in the growing process of these coatings due to an ionic diffusion step through the film. Also, the oxidation of metals possesses several oxidations which cause some defects on coating. (9) Incorporation of the nanosized particles can improve the micro hardness and corrosion resistance of the coating. Furthermore, a high concentration of nanoparticles with lower size results in high concentration of the dispersed particles. (10)

Interesting candidates with potentially unique properties are carbon nanotubes (CNTs). CNTs exhibit an exceptionally high stiffness and strength, light weight, and a diameter-dependent specific surface area of up to 1300 [m.sup.2]/g, as well as being able to be available in a variety of aspect ratios of up to several thousand. (11,12) Several polymer/CNT nanocomposites have been obtained by incorporating CNTs into various polymer matrices, namely epoxies, (6,13) polyanilines, (14) polyimides, (15-17) polyurethanes, (18,19) and polypropylenes. (20-22) The chemical bonding in CNTs is composed entirely of [sp.sup.2] carboncarbon bonds, which is incidentally stronger than the [sp.sup.3] bonds found in diamond, which renders CNTs with extremely high mechanical properties. (23) CNTs are excellent candidates for multi-functionally nano-reinforcing a variety of polymer matrices because of their high strength (~100 times stronger than steel) and modulus (about 1 TPa), high thermal conductivity (about twice as high as diamond), excellent electrical capacity (1000 times higher than copper), and thermal stability (2800[degrees]C in vacuum). (24) CNTs have attracted much attention as electrical conducting particles for conducting polymer nanocomposites. (25-27) When dispersing conductive particles with diameters below 1 [micro]m in a medium of low viscosity, diffusion processes and particle-particle interaction forces play an important role in agglomeration and network formation. Electrostatic charging of particles can both aid dispersion and hinder aggregation required in order to achieve a network of touching particles. (6) Frankland et al. (28) confirmed the influence of functionalization on interfacial adhesion by simulations which predict that a functionalization of less than one percent would improve the interaction between nanotubes and the polymer without decreasing the strength significantly.

New approaches to metal finishing have arisen during the second half of the 20th century. In addition to the problems these approaches must solve, there is a desire to reduce or completely eliminate the employment of flammable and toxic organic solvents. One approach to solving this problem has been the development of aqueous paints. (29) Electrodeposition of composite coatings has been widely investigated for better wear resistance and dispersion stability. Electrodeposition has the advantage of cost effectiveness compared to spray and sputtering processes. (30,31) The development of the electrodeposition process is mainly attributed to such advantages as higher productivity together with provisions for better corrosion protection. The ability of extending paint film formation in such recessed areas as the inner surfaces of steel pipes, recessed parts of box-shaped structures, and structures with small entrances leading to relatively large inner areas, is called the "throwing power" and leads to higher productivity as well as to better corrosion protection. (32) Additionally, objects with complex surface geometry and more particularly, those having hollow spaces, are not covered effectively by spray application. For such objects, dipping paints were developed. (33) Generally, resins used for the matrix of electrocoatings include epoxy resins, acrylic resins, their copolymers, and polyurethane resins. Epoxy resins have good stiffness, specific strength, dimensional stability, and chemical resistance and show considerable adhesion to metallic substrates. (24)

Electrodeposition is a process for producing a dense, uniform, and adherent coating upon a conductive surface by the act of electric current. (34) The use of CNTs in coatings can provide a conductive coating which has the added advantage of provisions for recoating ability by a further electrodeposition step. The aim of this work is to characterize the electrical behavior of CNT/epoxy-based coatings utilizing cathodic electrodeposition technique, which is based on electrical conductivity of the deposited polymeric film on the metallic substrate. The recoating ability was analyzed as a criterion for electrical conductivity of the first CNT/epoxy-based electrocoated film. Furthermore, throwing power was used as a tool to characterize the electrical behavior of the CNT/epoxy-based electrodeposited film. The novelty of this research work is utilizing recoating ability and throwing power techniques to determine the electrical percolation threshold of MWCNT/epoxy coating applied to a metal surface by an electrodeposition process. Additionally, the optimal loading of the CNT in order to obtain a suitable coating with an appropriate throwing power as well as a good recoating ability was determined. Also, the effect of MWCNTs on mechanical behavior of nanocomposites was determined by dynamic mechanical thermal analysis (DMTA) and nanoindentation methods.


Materials and sample preparation

MWCNT having lengths of approximately 50 pm, an average diameter of 8-15 nm, and an amine content of 0.45 wt% was purchased from the Timesnano company, China. Figure 1 shows SEM and TEM micrographs of these MWCNTs. (35)

A commercial electrodeposition resin CED (ED B 3700) was obtained from KCC Company, Korea. The CED resin with a solid content of 20 [+ or -] 2%, a pH of 5.5, and a conductivity of 0.1116 S/m was used as an upper layer second coat too. Weight fractions of MWCNTs ranging from 0.6-3.6 wt% were dispersed in the resin by shear-intensive mechanical stirring (Heidolph RZR2020, Germany) at 2000 rpm for 20 h. In a second step, the temperature of the dispersion was reduced by the aid of an ice bath and the dispersion was sonicated for a further half an hour. A DC power supply was then used to deposit this mixture on steel panels for 2 min at adjusted voltages to obtain a film thickness of 20 [+ or -] 1 [micro]m.


In order to investigate the recoating ability of MWCNT/epoxy films, a light gray upper layer second coat was applied on the prepared primer films. The recoating ability of MWCNT/epoxy films was determined by measuring the deposition yield and the thickness of the second layer.

The recoating time interval was kept constant at 120 s, the voltage was set at a predetermined desired value of 100 V, and the temperature of the bath was maintained at 28 [+ or -] 1[degrees]C.

Electrodeposition procedures

The electrodeposition cell consisted of a cubic 132 mL glass vessel containing 120 mL of the prepared depositable paint dispersion. The anode was a stainless steel panel of 12 cm x 2 cm dimension. The cathode was a steel panel with a depositable area of 10 cm x 2 cm. The anode and the cathode were connected to the respective terminals of a 0^150 V DC power supply (supplied by Eamen Tablo Co., Iran). The distance between anode and cathode was about 4 cm. The electrodeposition cell was held in a water-bath, the temperature of which was controlled at 28 [+ or -] 1[degrees]C. Paint films of approximately 20 [+ or -] 1 pm thickness were deposited on the cathodic steel panels at various voltages ranging from 20 to 250 V.


Electrical resistivity of the electrodeposited films was measured using an ohmmeter (Fluke 1550B MegOhmMeter, USA). The MWCNTs were dispersed in the electrodeposition bath using an ultrasonic processor (Hielscher UP400S, Germany). A rectifier was used to deposit various dispersions onto the test panels. Field emission scanning electron microscopy (FESEM) (Philips XL30, Japan) and optical microscopy (Leica DMR, Germany) were utilized for characterizing the morphology of the deposited films. DMTA was carried out by means of a Tritec2000 on nanocomposites at 1 Hz. The heating range was 0 to 200[degrees]C with a heating rate of 1[degrees]C/min. This experiment was carried out on only a single specimen for each nanoparticle loading. The MWCNTs were dispersed in the electrodeposition bath using an ultrasonic processor (Hielscher UP400S, Germany). The tube penetration test was used to measure the throwing power of the MWCNT/epoxy resin dispersions.

In the measuring procedure, the MWCNT/epoxy dispersion was poured into a stainless steel cylindrical vessel up to a height of 26 cm from the bottom, and the bath was maintained at 28 [+ or -] 1[degrees]C. The test strips (8 mm x 220 mm x 0.8 mm) and the hollow glass tube were immersed to a depth of 23 cm. The coating time was kept constant at 120 s and the voltage was set to a predetermined desired value of 100 V. The distance measured in centimeters from the bottom of the strip upwards was taken to be the throwing power of each MWCNT/epoxy dispersion.

Results and discussion

Electrodeposition of samples

One of the important factors which affects the deposition yield of electrocoats is electrodeposition time. In order to investigate the effect of time, the variation of electrical current was measured as a function of deposition time. The results obtained are shown in Table 1.

Table 1 shows that at initial stages of applied voltage, the deposition current density decreases significantly. However, it reaches a constant value upon further deposition. The sharp current drop is due to a sudden increase in concentration of a resistive film on the cathode. After an initial surge of nanotubes deposition on the cathode, the deposition on the cathode becomes stable, and therefore, the deposition current maintains a virtually constant value. Figure 2 shows the decrease of the current density vs time for the resin without MWCNT and MWCNT-containing epoxy resin.


As can be seen in this figure, the rate of electrodeposition is affected by the conductivity of the electrodeposition bath. By increasing the percent loadings of MWCNT in the epoxy resin, the conductivity of the resultant films is increased, so the residual current is also raised. The current density decreases significantly at initial stages and becomes constant after some time. As the thickness of the film increases, there is a tendency for the rate of deposition to decrease as a consequence of the increasing resistance of the deposited film. (36) The increase in the resistive nature of the deposited film results in a constant current density after some time.

Conductivity of coated films containing MWCNT

In order to investigate the effect of MWCNTs on electrical conductivity of nanocomposites, the electrical conductivity of nanocomposites was measured. This experiment was carried out on three specimens for each MWCNT loading. The variation in electrical conductivity of electrodeposited films with percent loading of MWCNT is plotted in Fig. 3.

Nonconducting pure epoxy coating becomes conductive as MWCNT is incorporated into this matrix. As shown in Fig. 3a, the electrical conductivity of the final paint film increases as the loading of MWCNT is increased. A sharp increase in conductivity is observed at loadings of 2-2.8 wt% MWCNTs. This indicates the formation of a percolated network of MWCNTs in the epoxy matrix. An electrical percolation threshold is apparent approximately at 2.5 wt% of MWCNTs. In the classical percolation theory, the dependence of specific conductivity ([[delta].sub.DC]) of electropaints on concentration of conductive pigment (p) above the percolation concentration ([p.sub.c]) can be described by a scaling law of the form shown in equation (l). (37)

[[sigma].sub.dc] = [[sigma].sub.0] [(p - [p.sub.c]).sup.t], (1)

where [[sigma].sub.0] is a fitted constant and t is the critical exponent. The conductivity exponent, t, generally reflects the dimensionality of the system with values typically around 1.3 and 2.0 for two and three dimensions, respectively. (6)

The straight line in Fig. 3b represents the best fitted line to the experimentally measured conductivity data as a function of p-pc expressed as a weight percent. The value of exponent t was determined from the slope of the least-square line on a log-log scale as seen in the straight line. This analysis reveals a percolation threshold of about 2.5 wt% and a scaling exponent, t, of 1.78 for these MWCNTs. The obtained critical exponent is higher than the critical exponent (t = 1.44 [+ or -] 0.30) of the MWCNT/epoxy resin mixture reported by Barrau et al. (38) It was found that the percolation threshold can vary by as much as a factor of 10 for exactly the same source of CNTs and matrix combination, when prepared under different mixing conditions. (25,39,40) Since Barrau et al. did not study functionalized MWCNTs and water-based epoxy resin, it is logical for the degree of dispersion and the interfacial characteristics to differ from those of functionalized MWCNTs evaluated in the present study. Also, the percolation threshold occurs in a significantly higher percent of CNTs in comparison to Li et al. Since Li et al. (25) did not study multiwalled CNTs and water-based epoxy resin, these factors can account for variations in [[sigma].sub.0], [p.sub.c]. and t values obtained from the percolation equation for two similar electropaint systems. In addition to the abovementioned factors, they have applied their films by casting methods which is different from the electrodeposition method. With the introduction above, we believe that in the percolation theory, the value of [[sigma].sub.0] in equation (1) is the intercept of the least-square line on a log-log scale as seen in the insert of Fig. 3 which calculates to be 0.77 S/m. It should really approach the conductivity of the MWCNT itself (i.e., 100 S/m). However, there exists a contact resistance between the MWCNT or clusters of MWCNT and polymeric matrix in the system, which decreases the effective conductivity of the MWCNTs themselves. (15)


In order to investigate the recoating ability of the coatings, the second layer was applied. So, by the use of CNTs as conductive pigments, it is possible to produce electrocoats with recoating ability by a further electrodeposition step. As can be seen in Figs. 4 and 5, the deposition yield and thickness of the second coat increased with increased loadings of MWCNT.

However, at certain specific ranges of loadings, the deposition yield and the coating thickness increase dramatically; after which, they both level off to their respective constant values.


Morphological properties of the films containing MWCNT

In order to investigate the surface morphology of electrocoats before and after the percolation threshold, optical microscopy was used. At the percolation threshold, conductive paths are formed which cause the conversion of insulator to conductor behavior in coatings. The surface morphology of deposited films can be seen in Fig. 6.

Figure 6a shows several agglomerated MWCNTs at the surface of the deposited film. However, Fig. 6b shows that at 2.8 wt% loadings of MWCNT, conductive MWCNT pathways are formed, indicating that the entanglement of MWCNTs became much looser, which may allow easier formation of conducting networks. (38) According to the optical microscopy test and the results of conductivity, it has been shown that by increasing the percent of multiwalled carbon nantotubes to 2.8 (wt%), the conductivity of multiwalled CNTs/epoxy resin increased significantly.






Dispersion of multiwalled CNTs in electrocoats is shown by FESEM in Fig. 7.

Figure 7 depicts FESEM micrographs of a loading of 2.8% of MWCNT in the epoxy matrix (a) in the bulk and (b) at the surface of the film. Although the weight percent of CNT in which the percolation threshold occurs is high, the high aspect ratio of CNTs was preferred because of the higher rate of percolated network formation for higher aspect ratio. The long period of mixing and sonication caused a good dispersion of high aspect ratio of CNT, which is shown in the FESEM images. Both images show that the MWCNTs are fairly well dispersed throughout the surface and bulk of the matrix and very large agglomerates are not visible. However, some localized regions of low and high content of MWCNT are evident.

Throwing power measurement

The ability to coat recessed areas of complex metal shapes is called the "throwing power" and measured by throwing power cell. Variation of throwing power with increased loadings of MWCNT is depicted in Fig. 8.

As can be seen before percolation, an electrical field is available for film growth up the metal strip and the sequence is repeated until the resistance of the bath becomes so great, or so much current leaks through the previously deposited areas that sufficient electrical field is not available for further deposition. In other words, as the deposited film precipitates, the resistance increases. The increase in resistance is proportional to the thickness of the deposited film and at a given voltage, the current density decreases as the film becomes thicker until it ultimately reaches a point where deposition has stopped occurring. Throwing power is highly dependent on deposition time and conductivity of the film. Therefore, it is expected that as loadings of MWCNT are increased, the conductivity of the film will also increase and consequently the throwing power must decrease. Again, at a specific range of loadings near the percolation threshold, the throwing power decreases dramatically as expected and levels off eventually.

Mechanical properties of MWCNT/epoxy coatings

DMT A analysis

It has been shown that by using certain values of MWCNT, the conductivity was significantly enhanced. However, the mechanical properties of the nanocomposites and also the curing behavior are important factors which could be considered. In fact, it is not acceptable to produce coating films with high electrical conductivity with low mechanical properties. Therefore, studying the mechanical property is as important as studying the electrical conductivity of the electrocoats containing MWCNT. DMT A was used in order to evaluate the effect of MWCNT on tan [delta] and storage modulus. Variation of tan [delta] and storage modulus as a function of temperature is shown in Fig. 9.


Figure 9a shows that by increasing the percent of nanotubes up to 2 (wt%), the width of tan S is decreased and in 2.8 (wt%) it has been increased, since the amount of aggregates are increased by increasing the percent of CNTs and it releases more energy against viscoelastic deformations. In other words, the decrease of the width of tan [delta] for higher nanotube contents can be interpreted as an increasing susceptibility to agglomeration that causes less energy dissipating in the system under viscoelastic deformation.

Figure 9b shows that the addition of nanotubes to the epoxy resin has a strong influence on the storage modulus in the glassy region. However, a strong decrease could be observed in the rubbery region and in the vicinity of the glass transition temperature. There is a significant increase in the storage modulus for 2 wt% of MWCNTs. This behavior can be explained in terms of an interaction between the CNTs and the epoxy due to the enormous surface area. This interfacial interaction reduces the mobility of the epoxy matrix around the nanotubes and leads to the observed increase in thermal stability. At higher contents of MWCNTs (2.8 wt%), the MWCNTs aggregation occurs which acts as mechanical failure concentrators. Furthermore, a low decrease of the storage modulus for 2 wt% MWCNTs above [T.sub.g] is mainly attributed to the low wettability between MWCNTs and epoxy resin. With further addition of MWCNTs, the formation of agglomerates leads to less wettability between MWCNTs and epoxy resin due to the difficulty of homogenously dispersing MWCNTs. Air entering may cause the decrease of modulus at higher MWCNTs contents.

Nano indentation test

In order to investigate the effect of MWCNT on elastic modulus and hardness of MWCNT/epoxy composites, a nanoindentation test was performed. Figure 10 illustrates variations of elastic modulus and hardness of MWCNT/ epoxy nanocomposite with various nanotube contents.

It is clearly seen that additions of nanotube to epoxy compositions greatly decreases hardness at various MWCNT contents. The sample containing 2.8% MWCNT shows the lowest hardness, which could be explained by less migration of MWCNT to the surface. It seems that MWCNTs tend to remain more in the bulk of coating through an agglomeration process. This behavior is clearly observed for the highest MWCNT concentration, which indicates less migration of MWCNTs due to the agglomeration process. Moreover, the addition of MWCNT to the epoxy decreases the elastic modulus. Variations in elastic modulus may be related to the influencing effect of nanotube on the curing reactions of an epoxy resin. Adding the nanotubes leads to decrease of crosslinking density of resin and incomplete curing.


MWCNT was used in order to improve the electrical conductivity of electrocoatings. It was found that increased MWCNTs loadings led to increased conductivity of the formed films together with enhanced recoating ability at the expense of decreased throwing power. Also, it is shown that the addition of nanotubes to the epoxy resin shows a strong increase of the nanotube content on the storage modulus in the glassy region. However, a decrease could be observed in the rubbery region and in the vicinity of the glass transition temperature. The width of tan[delta] has been increased since the amount of aggregates are increased, which caused the incomplete curing process of the MWCNT/ epoxy composites. The incomplete curing process of MWCNT-containing epoxy composites can be attributed to CNT surface areas containing oxidized groups such as phenolic hydroxyl groups or carboxylic acid. Incorporating a large amount of CNT might prevent the curing of epoxy-based resin by changing the optimum ratio of the epoxy-based functional group with the curing agent. The results of the nanoindentation test show the decrease in elastic modulus and hardness of MWCNT/epoxy composites with the addition of MWCNTs. The decrease of the hardness was attributed to less migration of MWCNTs to the surface due to the agglomeration process. The decrease of elastic modulus may be related to the effect of MWCNTs on the curing process of MWCNT/epoxy composites.

DOI 10.1007/s11998-015-9723-y

M. Zabet ([mail]), S. Moradian

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran, Iran


S. Moradian, Z. Ranjbar

The Center of Excellence for Color Science and Technology, Institute of Color Science and Technology, Tehran, Iran


Z. Ranjbar

Department of Surface Coatings and Corrosion, Institute of Color Science and Technology, Tehran, Iran

N. Zanganeh

School of Chemical Engineering, Mississippi State University, Oktibbeha, MS 39762, USA


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Table 1: The change of the current density as a function
of time in carbon nanotube-containing epoxy resin

Percent of      Initial           Drop in          Residual
MWCNT           current       current density    current after
(wt%)           density         after 10 s           120 s
            (mA/[cm.sup.2])   (mA/[cm.sup.2])   (mA/[cm.sup.2])

0                32.66             1.9               0.9375
0.6              31.13             2.86              1.19
1.2              31.87             2.93              1.3
2                34.98             2.95              1.6
2.8              38.87             3.2               2.77
3                36.54             3.32              2.81
3.4              39.66             3.43              2.86
3.6              45.3              4.47              3.1
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Author:Zabet, M.; Moradian, S.; Ranjbar, Z.; Zanganeh, N.
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jan 1, 2016
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