clay nanocomposite

clay nanocomposite

Progress in Organic Coatings 106 (2017) 69–76 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier...

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Progress in Organic Coatings 106 (2017) 69–76

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage:

Preparation and characterization of a high performance powder coating based on epoxy/clay nanocomposite Mehrdad Sharifi, Morteza Ebrahimi ∗ , Samane Jafarifard Polymer Eng. and Color Tech. Dept., Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 4 September 2016 Received in revised form 23 January 2017 Accepted 14 February 2017 Keywords: Powder coatings Epoxy nanocomposite Mechanical properties Thermal properties Clay nanoparticles Corrosion resistance

a b s t r a c t In this work, epoxy/clay (Closite 30B) nanocomposite samples were prepared by using solution and melt dispersion methods The quality of clay dispersion in epoxy matrix was evaluated by using XRD and TEM techniques. The prepared nanocomposite samples were used in epoxy powder coating formulations (named as nano powder coatings). DSC analysis was used to study the curing behavior of samples. The physical, mechanical and thermal properties of prepared samples were evaluated by using tensile, DMTA, microhardness, TGA and LOI tests. Finally, the corrosion resistance of prepared epoxy nano powder coatings was determined using the salt spray test. The results illustrated that closite 30B nanoparticles could be dispersed properly in epoxy resin by using both solution and melt dispersion methods It was found that Closite 30B could accelerate the epoxy curing reaction. The results showed that the mechanical properties of nano powder coatings improved when various concentrations of clay nanoparticle (in the range of 1–5 phr) were used in formulation. It was observed that sample with 3 phr clay nanoparticles presented the best thermal and fire resistance properties. The salt spray measurements confirmed the superiority of corrosion resistance of epoxy nano powder coatings in comparison to that for neat epoxy powder coating formulation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, powder coatings have been used extensively in different industries because they have significant merits. These include environmental sustainability, VOC free and they produce minimal waste of applied materials. Epoxy resin is considered as one of the most suitable resins in powder coating formulations. Powder coatings based on epoxy resin have exhibited very good properties such as excellent adhesion to most substrates, good thermal and mechanical properties and strong resistance to moisture, solvent and corrosion [1–4]. Various materials such as rubber elastomers, thermoplastic resins, liquid crystalline epoxy and nanoparticles have been incorporated into epoxy matrix to improve its physical and mechanical properties such as toughness [5–8]. For example, Jia et al. [9] reported preparation of epoxy resin/SiO2 nanocomposite by solution dispersing method. They reported that an epoxy/SiO2 nanocomposite with 3 wt.% nano silica showed higher tensile strength (about 50%) and elongation at break (about 500%) compared to those for the neat epoxy resin.

∗ Corresponding author. E-mail address: [email protected] (M. Ebrahimi). 0300-9440/© 2017 Elsevier B.V. All rights reserved.

Various research groups have studied the effect of clay nanoparticle on properties of epoxy resin [10–14]. Nigam et al. [10] prepared epoxy/montmorillonite nanocomposite via in-situ polymerization. The modulus of an epoxy resin was improved up to 100% with incorporation of 6 wt.% clay nanoparticle into the formulation. Results showed that nanocomposite with 1 wt% montmorillonite had the best tensile strength and strain at break. In all the above researches, formulations of liquid epoxy resin were used (i.e. low molecular weight, mostly DGEBA). According to the best of knowledge, there are few publications on modification of solid epoxy resin for powder coating using clay nanoparticle. Therefore, in this study, the effect of organically modified clay nanoparticles was investigated on mechanical, thermal and corrosion protection of an epoxy resin (used for powder coating). Solution and melt dispersion methods were applied to prepare epoxy/clay nanocomposite. Morphology of the nanocomposite was studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Mechanical properties of nanocomposite powder coating (nano powder coating) were characterized by means of dynamic mechanical thermal analysis (DMTA), microhardness and tensile tests. Thermal properties of samples were determined by thermo gravimetry analysis (TGA) and limiting oxygen index (LOI). Differential scanning calorimetry (DSC) was used to study curing behavior of the nano powder coatings. Moreover, corrosion


M. Sharifi et al. / Progress in Organic Coatings 106 (2017) 69–76

Table 1 The properties of organically modified clay nanoparticles. Trade name

Organic modifier

Cloisite 30 B (NC) a



Modifier concentration

d-spacing (A0 )

Density (g/cm3 )

90 meq/100 g Clay



MT2ETOH: Methyl, tallow, bis-2hydroxyethyl, quarternary ammonium.

Table 2 Formulation of epoxy nano powder coating.

Table 3 Samples code based on nanoparticles amount and dispersion method.


Epoxy or epoxy nanocomposite

Aradur 2844




Sample code






Content (%)






NC loading (phr) Preparation method

0 –

3 Solution

1 Melting

3 Melting

5 Melting

2. Experiment

to obtain an average particle size of about 40 ␮m. Final samples were prepared by electro spraying the powder coatings on to aluminum panels and then baking at 150◦ C for 20 min. Thickness of samples was adjusted to about 70 ␮m.

2.1. Materials

2.4. Measurement and characterization

A bisphenol-A based epoxy resin (Epon resin 3003, Hexion Co.) with an epoxy equivalent weight of 750 eq/g and melting point of 80 ◦ C was used as resin for a powder coating formulation. A dicyandiamide derivative (i.e. Aradur 2844, Huntsman Inc.) was used as a hardener. Both chemicals used as received. An organically modified clay (Closite 30B) was purchased from Southern Clay Co. The properties of this type of clay are presented in Table 1. The structure of Closite 30B is schematically shown in Fig. 1. In addition, different additives such as leveling agent (PV-88, Huangshan Jinfeng Industrial, China), degassing agent (benzoin, Huangshan Jinfeng Industrial, China) and surface and processing modifier (P520, Shin Woun Chemical Co. South Korea) were also used in powder coating formulations.

Dispersion of NC nanoparticles in nanocomposite was evaluated by using XRD analysis (Philips Xpert Co, Holland). In addition, a transmission electron microscopy (ZEISS Co., 30EM902A) was used to visualize the extent of nanoclay particle interclation/exfoliation in epoxy resin. Curing behavior was characterized in the samples using a DSC (Netzch, 200F3) instrument. The experiments were carried out under nitrogen atmosphere. The samples were heated from 25 to 270 ◦ C at the heating rate of 10 ◦ C/min. Tensile properties of cured samples were measured using tensile test specimens of dimensions 2*10 cm2 and thickness of 600 ␮m. The gauge length of test specimens was 50 mm as per ASTM D638 [17]. The samples were tested by universal tensile testing machine (UTM, Galdabini, 1890, Italy) with a speed of 1 mm/min. DMTA was conducted on powder coating samples using a DMA-Triton, Tritec 2000 instrument. Microhardness was measured in samples using a Vikers hardness tester (Linkam, VMHTMOT) according to ASTM E384 [18]. Thermo gravimetric analysis (TGA) was carried out over a temperature range of 30–400 ◦ C at the heating rate of 10 ◦ C/min and under a nitrogen atmosphere (Dupont Instruments, 951). Limiting oxygen index (LOI) values were measured on a flame meter (Stanton Redcraft) using a modified method. In this test, the minimum percentage of oxygen in an oxygen/nitrogen mixture required to sustain the flame for a specific time was taken as LOI (ASTM D2863). In this method, the sample was free film of samples in dimension of 5*10 cm2 and located in a vertical direction [19]. The corrosion resistance evaluations were made for samples using a salt spray test based on ASTM B117-03 [20]. Temperature of the salt spray cabinet was set at 40 ± 2 ◦ C and a brine solution with a concentration of 3.5 wt.% and pH = 6.5–7.2 was sprayed on the samples for 15 min every 45 min. Sample codes and their specifications are shown in Table 3.

resistance of the best sample was evaluated using the salt spray test.

2.2. Preparation of epoxy/clay nanocomposites Two methods, namely melting and solution methods were applied for the preparation of nanocomposite samples. 2.2.1. Melting method In this method, epoxy resin was charged into an internal mixer (PL2200, Brabender Co. Germany) with a capacity of 60 cm3 and then NC nanoparticle was added to melted epoxy. The temperature, rotating speed and processing time of the internal mixer were adjusted to be 90 ◦ C, 60 rpm and 10 min, respectively. Finally, the mixture was taken out of the internal mixer and was cooled at ambient temperature [15]. 2.2.2. Solution method Epoxy resin was dissolved in acetone and then NC nanoparticle was added to the solution. The mixture was mixed rigorously (2000 rpm) by using a mechanical mixer (IKA Labortechnik, Jankekunel, GmbH) for 10 min. Then, the nanoparticle was well dispersed in the solution by using an ultrasonic instrument (Hielscher Co., Model: UP 400S, 400w, 24 kHz) for 10 min. Finally, the solvent was evaporated under vacuum condition [16]. 2.3. Preparation of the nano powder coating The formulation of the epoxy powder coatings is shown in Table 2. A mixture of prepared nanocomposite, curing agent and additives was extruded by using a co-rotating twin-screw lab extruder (OMC Saronno Co. Model: EBVP 20/24, temperature 95 ◦ C, rotation momentum 65%). Extruded materials were crushed by a grinder (Retsch Co. Model: 2M100), followed by sifting (mesh 325)

3. Results and discussion 3.1. Dispersion of clay nanoparticles in nanocomposite 3.1.1. Effect of dispersion method Fig. 2 shows XRD patterns of 3 M and 3S nanocomposite samples prepared by using melt and solution dispersion methods, respectively. The d-spacing of clay nanoparticles was calculated based on Bragg law from XRD patterns [21,22]. As can be seen, d-spacing increased considerably in both samples compared to those for NC (i.e. 18.5 Ao , Table 1). This result proposes that clay nanopartticles could be intercalated/exfoliated properly in epoxy matrix. In addition, results showed that the sam-

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Fig. 1. Schematic structure of Closite 30B.

Fig. 2. Comparison of XRD patterns for nanocomposites with different dispersion methods.

ple prepared via the solution method had a greater layer space. It seems that epoxy solution could diffuse between the layers more easily compared to melt epoxy resin and cause better intercalation/exfoliation of clay particles [23]. TEM was also used for visualizing dispersion quality of the clay nanoparticle and a closer look at the dispersion of clay in epoxy matrix. TEM images of 3 M and 3S samples are presented in Fig. 3. As can be seen, the clay nanoparticles were clearly intercalated/exfoliated in both samples. It can also be recognized that the clay particles were more exfoliated in the 3S sample compared to that of 3 M. It shows once again that the method of solution dispersion was more effective than the melt dispersion method. However, it should be noted that the melting dispersion method is more practical, economical and environmentally friendly for application in the powder coating industry. Moreover, residual solvent in samples prepared by solution dispersion might cause some deterioration in the final cured powder coating product [24]. Therefore, the melt dispersion method was selected for preparation of samples in the rest of this work.

3.1.2. Effect of clay nanoparticles loading Fig. 4 shows XRD patterns of epoxy/nanocomposite samples with various NC loadings prepared by the melt dispersion method (i.e. 1M, 3M and 5M; Table 3). As expected, the values of d-spacing for all the nanocomposite samples with different NC concentrations were higher than those for NC nanoparticles. Moreover, it was found that d-spacing was more significantly increased in samples with lower amounts of clay nanoparticle. It seems that agglomeration might form when at higher concentrations of NC incorporated into an epoxy matrix.

Table 4 Characteristic temperatures of curing behavior of nanocomposite powder coatings. Sample

Blank 1M 3M 5M

Characteristic Temperatures Tonset (◦ C)

Tmidpoint (◦ C)

Tend (◦ C)

100 98 96 91

179 173 165 159

231 227 223 219

TEM images of 3M and 5M samples are also shown in Fig. 5. It, once again, confirmed that d-spacing for the 3M sample was larger than that of the 5M sample. 3.2. Curing behavior of nano powder coatings The characteristic temperatures for curing of all samples were determined using the DSC technique (Table 4). Tonset shows the starting temperature of curing reactions, Tmax illustrates the temperature at which maximum exothermic curing reactions occurred and Tend shows the temperature at the end of the curing reaction. As can be seen, the curing reaction started at a lower temperature when clay nanoparticles were incorporated into the formulations. In addition, Tmax and Tend values were found to be lower for the nano powder coating samples compared to those for neat epoxy. These results confirmed that Closite 30B had a catalytic effect on the curing reactions. It is reported that an alkyl ammonium component in Closite 30B can catalyze epoxy ring opening reactions [10,25]. It is obvious that the concentration of alkyl ammonium would be higher for 5M samples compared to other samples. Therefore the catalytic effect of the sample was found to be more significant.


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Fig. 3. TEM images of 3M and 3S nanocomposites, 250000 magnification.

Fig. 4. Comparison of XRD patterns for nanocomposites with different amounts of NC nanoparticles.

Fig. 5. TEM images of 3M and 5M samples, 25,0000 magnification.

3.3. Physical and mechanical properties of nano powder coatings 3.3.1. Tensile properties The tensile properties of cured nano powder coatings are presented in Fig. 6. As can be seen in Fig. 6a, the tensile strength of samples increased with an increase of NC concentration in nanocomposite. The results showed tensile strength increases of 15%, 34% and 37% with additions of 1, 3 and 5 phr of NC in the formulation, respectively. Fig. 6b illustrates the elongation at break results of nanocomposite with various loadings of NC nanoparticle. Elongation values at the break of samples improved by 10%, 24% and 26% when NC amounts of 1, 3 and 5 phr were used in the nanocom-

posite, respectively. Fig. 6c shows the tensile modulus evaluations of nano powder coating samples. This parameter also increased up to 19%, 44% and 49% for 1M, 3M and 5M samples, respectively. It seems that a good interaction between NC particles and epoxy matrix was responsible for improved tensile properties of the nano powder coating samples. The same trend was reported by other researchers [9,13]. It is proposed that the hydrophobic nature of alkyl ammonium provides this good interaction between the nano filler and epoxy matrix. A closer look at Fig. 6 demonstrates that tensile properties were considerably more improved up to NC concentration of 3 phr. It

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Fig. 6. Mechanical properties a) tensile strength, b) elongation at break of, and (c) tensile modulus of the nano powder coatings at different NC concentrations.

seems that some agglomeration might form at a higher concentration of NC (i.e. 5 phr) in formulation. 3.3.2. Microhardness A Vickers microhardness tester was applied to evaluate hardness of the nano powder coating samples and the results are shown in Fig. 7. As can be seen, hardness of coatings increased by incorporation of clay nanoparticles in powder coating formulation. It was found that hardness increased by 6%, 25% and 31% for samples with NC amounts of 1, 3 and 5 phr respectively, compared to that for the Blank formulation. These results confirmed the load bearing improvement of nano powder coatings. As results show, the extent of the increase in microhardness was more significant at the range of 1–3 phr NC loading. The same trend was observed for the tensile properties improvement. Therefore, optimum formulation was determined at NC of 3 phr NC. 3.3.3. Glass transition temperature The Tg of samples was measured by using DMTA analysis. Fig. 8 shows the Tg amounts of nano powder coatings with different NC loadings. As can be seen, the Tg of cured nano powder coatings decreased slightly by increasing the amount of NC loading in formulation. It should be noted that presence of NC in epoxy composite has several contradictory effects. The incorporation of a hard filler such as clay in thermoplastic polymer matrix usually increases the Tg if there is a good filler/polymer interaction. On the other hand, the situation would be more complex for thermoset polymer matrix. In the latter case, the curing reaction would also be affected by the presence of nanoparticles. For example, Ferdosian et al. [21] studied the curing behavior of an epoxy/clay nanocomposite and found that the system became diffusion-control at lower

conversions because of an increase of viscosity. Therefore, the final conversion of these systems would be lower and consequently their Tg would also be lower. On the other hand, they claimed that Closite 30B had some catalytic effect on the curing reaction of epoxy resin and it might improve the extent of reaction and consequently the Tg of system. In another research, Akbari et al. [11] claimed that the homo-polymerization of epoxy oligomers between the clay layers were responsible for the decrease of final nanocomposite glass transition temperature. In summary, it seems that the inferior effect of clay nanoparticle on the Tg of nano powder coatings were more pronounced [15,26,27]. 3.4. Thermal properties of nano powder coatings 3.4.1. TGA analysis Thermo gravimetric analysis (TGA) was performed to determine thermal stability of the blank and nano powder coatings in a nitrogen atmosphere. Table 4 shows the results of TGA analysis of nano powder coatings with different NC loadings. The results show that initial degradation temperature (IDT) did not change when 1 phr NC was included in the formulation. However, it was found to be higher for 3M and 5M samples compared to the blank one. The higher amounts determined for IDT for 3M and 5M samples could be because of molecular mobility inhibition of polymer chains at the initial steps of degradation reactions [28,29]. It is worth mentioning that the IDT of 5M was lower than that for the 3M sample. The possible agglomeration of NC in 5M sample might be the reason for this observation. The maximum weight loss temperature (Tmax ) results are shown in Table 5. As can be seen, all the nano powder coatings had lower Tmax compared to the Blank one. In addition, the nano powder


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Fig. 7. Microhardness test results of nano powder coatings.

Fig. 8. Tg of nano powder coatings with different NC loadings.

Table 5 TGA results of nano powder coatings with different amounts of NC nanoparticles.

Table 6 LOI of nano powder coatings.


Initial Degradation Temperature (IDT, ◦ C)

Maximum Weight Loss Temperature (Tmax ,◦ C)


Limiting Oxygen Index (LOI, %)

Blank 1M 3M 5M

225 225 250 240

380 375 366 355

Blank 1M 3M 5M

20 20 27 24

coatings with a higher amount of NC nanoparticle showed a lower Tmax . This result was expected due to the presence of alkyl ammonium groups between layer structures of clay nanoparticle (NC). As reported in the literature, after initiation of degradation, amines had an accelerating effect on degradation reactions and caused a lower Tmax . The downward trend of Tmax as NC content increased in nano powder coating showed that a higher concentration of amine groups had an accelerating effect on the thermal degradation reaction [10,30]. 3.4.2. LOI measurements Limiting oxygen index (LOI) is defined as the minimum fraction of oxygen in a mixture of O2 /N2 atmosphere needed for combustion of a material and is a quantitative criteria in determination of flame

retardant property of a material [15,31]. Table 6 shows LOI results of nano powder coatings with different amounts of NC nanoparticles. As can be seen, the LOI values of 1M and Blank samples were the same. However, LOI values for 3M and 5M samples were found to be bigger than 21 (i.e. 27 and 24) this means that they were more fire resistant in comparison to the Blank sample. It seems that the barrier effect of NC nanoparticle was responsible for this increase of LOI. This observation suggests that clay nanoparticles with a layered structure can promote the flame retardant property of nano powder coatings. In addition, results showed that the LOI of 5M sample was lower than that for 3M sample. As previously mentioned, the agglomeration of layered NC structure could deteriorate the barrier effect of clay in this sample.

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Fig. 9. Pictures of exposed samples to salt spray conditions after 500 h, 3M sample (left) and Blank sample (right).

Fig. 10. Schematic diagram of corrosion agents penetration path through Blank sample (left) and 3 M sample (right).

3.5. Corrosion resistance of nano powder coatings The results of mechanical properties showed no significant difference in mechanical properties between samples 3M and 5M. In addition, the 3M sample presented superior thermal properties compared to other samples. Therefore, it was decided to investigate corrosion resistance of the 3M sample. The corrosion performance of 3M sample was evaluated by using a salt spray test for 500 h. Fig. 9 shows the pictures of exposed Blank and 3M samples. As can be seen, both samples were affected and corroded in salt spray conditions. However, the numbers of blisters in the Blank sample were clearly more than that in the 3M sample. It is obvious that the blisters spread all over the surface of the Blank sample but they existed mainly around the cross lines in the 3M sample. It seems, as other researchers have also stated [1,12], that the layered structure of clay nanoparticle served as a barrier against corrosive ions. As shown schematically in Fig. 10 the precense of NC layerd nanoparticles increased tortuosity of the coatings against penetration of corrosive ions. In this condition, corrosive agents must travel a longer path and this phenomenon can delay the process of corrosion. 4. Conclusions Organo modified montmorillonite clay nanoparticle (NC) was incorporated in an epoxy powder coating formulation. This work demostrates the feasibility of preparing epoxy nano powder coatings by using both melt and solution dispersion methods. Although solution method provided a better nanoclay intercalation, however, the melt dispersion method was found to be more practical in powder coating industry and were used for sample preparation in the rest of research. It was also found that the presence of Closite 30B could catalyze the curing reaction. In addition, the mechanical properties of nano powder coating samples were found to be better

than that for Blank sample. The glass temperatures of nano powder coatings were slightly lower than that for Blank one. The presence of clay nanoparticle caused an improvement in the initial degradation of nano powder coating samples and their fire resistance. Based on mechanical, thermal and fire resistance properties, it was concluded that the incorporation of 3 phr clay nanoparticles in to the formulation could provid the optimum performance. Finally, it was found that the corrosion resistance of sample with 3 phr clay naoparticle was supperior compared to Blank sample.

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