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Sonam Tamanga,b, Anu Surendranc, Kamal P. Sharmad, Jyoti Girie, Sabu Thomasc, Takahiro Maruyamad, Sabita Shresthaa*, Rameshwar Adhikari a,b *(2023), Structural, thermal, and surface wetting properties of epoxy resin/multiwalled carbon nanotubes composites, 3 (1) 2023, 9-16, 10.55878/SES2023-3-1-2

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INTRODUCTION

Nanostructured materials have a special place in contemporary materials science and engineering due to their promising properties. These materials have been found suitable for several applications such as in structural and electrical applications owing to their desirable mechanical [1], [2] [3] [4], optical [5], electrical and thermal properties [6]. In this respect, materials with tailored mechanical and electrical properties have received a lot of attention in academia and industries due to their potential uses in sensors [7], capacitors, and structural components [8]. The nano-filled polymers comprising conducting fillers in different thermoplastics [9], rubbers [10], and thermosetting epoxy resins  [10][11][12][13][14] are of special interest.

Epoxy resins (EP) are organic compounds with a low molecular weight having more than one epoxide group. When properly cross-linked, they have a wide range of uses due to their superior thermal and mechanical characteristics [11]. Several curing agents, such as amines [12], are used to harden them resulting in solid, cross-linked structures. They have high mechanical strength and good adhesion to other materials [13] and thus are the most widely used thermosetting resins because of their excellent mechanical and bonding capabilities, outstanding chemical stability, and ease of processing [14] [15] [16]. The majority of the cured products, however, have a high cross-link density, making them susceptible to undergo brittle fracture when subjected to external forces. Thus, epoxy products generally exhibit low impact and environmental stress-cracking resistance demanding extensive research works aimed at reinforcing the resins with nanoparticles, fibres, graphene, and nanotubes. [17] [18] [19] [20] [21].

It is known that multiwalled carbon nanotubes (MWCNTs) are used in biomedical fields [22] [23], as coatings and films, in microelectronics, energy storage devices, for environmental remediation, and in biotechnology [24]. In addition, MWCNT finds applications in designing nanocomposite membranes [25], transparent electrodes [26], and catalysis [27] and transistor [28]. Because of the extraordinary mechanical properties of MWCNTs, they are also used as reinforcing filler in the EP matrix. However, it has been reported that there is no noticeable increase in the strength of epoxy nanocomposites, which has been related to the poor adhesion between the MWCNTs and the EP matrix. The surface functionalization of the MWCNTs via fluorination [29], silanization [30], carboxylation, and amidation [31] [32] has been employed as a strategy  to improve the filler matrix adhesion. It has been demonstrated that polymer nanocomposites with functionalized MWCNT have better thermal, electrical, and mechanical properties than the corresponding nanocomposites with pristine nanotubes [33]. Furthermore, the nanocomposites containing different functional groups [36] are applicable in the construction of scaffolds for tissue engineering [37] [38]. It has been reported that EP/MWCNT nanocomposites are very useful for coating applications [34] whereby the surface wettability is especially important to determine the quality of adhesive bonding of the coatings to the substrate [35].

In the context of the above discussions, it is desirable to develop MWCNT-based nanocomposite materials with optimized electrical, mechanical, thermal, and surface adhesion properties. This paper aims at investigating the effect of the addition of a different form of MWCNT on the thermal and surface physical properties of epoxy thermoset.

 

MATERIALS AND METHODS

Materials

Pristine multiwalled carbon nanotubes (MWCNTs, Nanocyl 7000) from Nanocyl SA were used in this study. Epoxy Lapox L-12 (the diglycidyl ether of bisphenol A, DGEBA) and Hardener K-6 (the aliphatic polyamines) supplied by Atul Ltd. Polymer Division, were used for the preparation of nanocomposites. The laboratory grade Silicon oil with a viscosity of 370-390 m Pa was used for releasing the thermosetting resin from the mould after the solidification. Acetone and nitric acid (Fischer Scientific, India) were used as a solvent for epoxy material and to oxidize the MWCNT, respectively. The pristine MWCNTs used in this study have an average diameter of 44 nm and a length of several hundred nanometres, as shown by FESEM images given in Figure 1.

 

 

 

Figure 1: FESEM images showing the bundles of pristine MWCNT (a) and highly magnified nanotubes (b)) used in this work

METHODS

Chemical modification of MWCNTs

Pristine MWCNT is inert and hydrophobic. It is difficult to attach metal, polymer and other kinds of particles to their surface in a neat form. Therefore, the functionalization with different oxygen-containing groups on the MWCNTs surface through an oxidation process is necessary to improve their hydrophilicity and reactivity. There exist several methods for the oxidation of carbon nanotubes such as air oxidation, sonication, annealing, acid oxidation etc. [7][9][23][35].

In this work, the acid oxidation method has been employed for chemical functionalization of the nanotubes. In a typical experiment, 0.5 g of MWCNTs were first sonicated in 40 mL of concentrated nitric acid for 15 min followed by reflux for 5 h. The mixture was filtered under suction using membrane filter paper with a pore size of 0.2 µm. The residue was washed with distilled water until pH value of 7 and dried in an oven maintained at a temperature of 100 ºC for until it dried [39]. The samples prepared in this work are listed in Table 1.

 

Table 1: The list of samples used in the present work with their codes and description

 

Preparation of EP/MWCNT nanocomposites

The pristine and acid-treated MWCNTs were incorporated into the EP matrix via physical mixing assisted by ultrasonication. For this purpose, 2.5 g of Lapox – L12 was dissolved in 2.5 mL of acetone taken in a 50 mL beaker, followed by 15 min of ultrasonication. The mixture was agitated for 1 h at 60 ºC with a magnetic stirrer to evaporate the residual solvent. The resulting mixture was allowed to cool down to room temperature, and the amine hardener was added (volume ratio of epoxy to hardener being 10:1). Finally, the mixture was cast into a plastic mould and pre-cured at 25 ºC for 24 h followed by post-curing at 70 ºC for 5 h.

 

 

Characterization Techniques

Fourier Transform Infrared (FTIR) spectra of the materials were recorded with an IR Prestige 21 spectrophotometer equipped with a ZnSe ATR crystal over a wavenumber range between 4000 cm-1 and 600 cm-1 with a resolution of 4 cm-1.

X-ray Diffraction (XRD) of the samples was carried out using a diffractometer equipped with the radiation source of Cu Kα with a wavelength of 0.15406 nm. The specimens were scanned over the diffraction angle 2θ values between 5° and 80°.

Thermogravimetric Analysis (TGA) of the specimens was performed by SDT 0600, TA instruments over a temperature range of 30 °C to 700 °C at the heating rate of 10 °C /min.

Contact Angle Measurements of the nanocomposites were conducted in the FTA 100 series (First Ten Angstrom, Portsmouth, Virginia, 23704, USA) using distilled water. The measurements were carried out on samples of dimensions 1 cm x 1 cm x 1 mm which were subjected to the tests at 25 °C in closed chambers, maintaining the volume of the sessile drop at 5 µl in all cases using a microsyringe.

 

RESULTS AND DISCUSSION

Structural Characterization of Nanocomposites

The FTIR spectra of neat EP, EP/pMWCNT(0.05), and EP/mMWCNT(0.05) are shown in Figure 2. For simplifying the discussion, we present only the FTIR results obtained from the nanocomposite comprising a low amount of MWCNTs.

The FTIR spectra reveal information regarding the functional groups as well as the variations in mechanical and thermal properties of various samples with their molecular structures [40]. The spectrum in Figure 2 shows that the addition of 0.05 wt.-% of nanotubes does not significantly alter the chemical structure of the EP matrix. The peaks centred at 2960 cm-1, 2830 cm-1, and 1521 cm-1 represent the benzene ring [41]. The EP groups present in all the samples are further denoted by the bands located at 824 cm-1, 915 cm-1, and 1220 cm-1. It can be noticed that the peak centred at 2960 cm-1 is eliminated in the case of both types of nanocomposites. In addition, the peak centred at 915 cm-1 has a slight position shift towards a higher wavenumber in the case of the EP/MWCNT nanocomposites which may be due to the uniform dispersion of MWCNT on the EP matrix [42].

Figure 2: FTIR spectra of neat EP, EP/pMWCNT(0.05) and EP/mMWCNT( 0.05) nanocomposites

The broad band was observed in the range of 3132 cm-1- 3783 cm-1 indicating the presence of hydroxyl groups attached due to acid treatment on the surface of the samples which is responsible for creating hydrogen bonds with other nucleophilic groups. The intensity of these bands is found to increase in the nanocomposites comprising mMWCNT, which may be due to the interaction of the epoxy groups of the resin with the -COOH groups introduced onto the surface of the nanotubes through acid treatment, thereby enabling the formation of hydrogen bonds between the –COOH group and EP resin resulting in cross-linked structures [43] as well as hydrogen bonds between the unreacted carboxylic groups and –OH groups within the mixture [44].

XRD was performed to extract an overview of the microstructure evolution of nanocomposites with various nanotube loadings. The XRD patterns of neat EP, EP/pMWCNT(0.5), and EP/mMWCNT(0.5) nanocomposites are displayed in Figure 3.

Figure 3: XRD patterns of neat EP, EP/pMWCNT(0.5) and Ep/mMWCNT(0.5) nanocomposites

XRD patterns of the materials present some broad peaks centred on 2θ values of around 19º and 43º, which can be attributed primarily to the amorphous structure of the epoxy resin [42]. The nanocomposites comprising both pristine and acid-treated filler also show similarly positioned broad bands which imply a quite uniform dispersion of the nanotubes in the epoxy network [43]. The XRD patterns of the nanocomposites further attest that the incorporation of MWCNT  in epoxy has not exercised any influence to modify the inherent epoxy resin matrix morphology on the microscopic scale [45]. The decreased peak intensity observed in both the nanocomposites may be correlated to the exfoliation of nanotubes bundles resulting from the diffusion of epoxy chains between each bundle of nanotubes space of carbon nanotubes filled nanocomposites [46]. In both nanocomposites, the characteristic peaks of the nanotubes were not observed, indicating the homogeneous dispersion of the nanotubes in the polymer matrix [47].

Thermal Behavior of Nanocomposites

Thermogravimetric analysis is a highly valuable tool for determining the overall quality of the material. The neat EP, EP/pMWCNT, and EP/mMWCNT nanocomposite samples were subjected to thermogravimetric analysis (TGA) to study the impact of adding pristine and functionalized MWCNTs on their thermal degradation behaviour.

Figure 4a shows the TGA thermogram, and Figure 4b shows the DTG plot of EP, EP/pMWCNT, and EP/mMWCNT. In all the cases, there is no weight loss below 315  ºC, and drastic weight loss (about 90 %) occurs at a temperature ranging from 315 ºC to 450 ºC with a single DTG curve at 361 ºC (EP), 370 ºC  (EP/pMWCNT(0.2) and 374 ºC  (EP/mMWCNT(0.2)), which showed one-step degradation mechanism. The nature of the TGA and DTG curves indicates that the presence of MWCNT does not affect the breakdown of the EP matrix. The residual masses of the EP, EP/pMWCNT(0.2), and EP/mMWCNT(0.2) composites are around 10, 9, and 7 wt.-%, respectively [48].

On careful examination, it is found that, as compared to neat EP, the EP/mMWCNT(0.2) nanocomposites showed greater thermal stability. The well-dispersed nanotubes and strong interfacial adhesion between the nanotubes and the EP matrix contribute to the nanocomposite's increased thermal stability [49].

Figure 4: TGA (a) and DTA (b) plots of neat EP along with that of EP/pMWCNT(0.2) and EP/mMWCNT(0.2) nanocomposites

Surface Wetting Behavior of Nanocomposites

The water contact angle indicates the wettability of the solid substrate surface correlates with the wettability of the solid. The contact angle is important wherever the intensity of the phase contact between liquid and solid substances needs to be assisted, such as in coating, painting, cleaning, printing, bonding, dispersing etc. The wettability of the nanocomposites was studied by measuring the contact angle with the help of the sessile drop technique using a built-in camera system operated through software [49]. The values of contact angles (θ) made by water droplets on various sample surfaces are indexed in Table 2. Figure 5 shows further the photographs illustrating the surface wetting properties of the nanocomposites using the sessile drop technique are presented in Figure 5. It can be observed that the contact angles of the EP/pMWCNT(0.2) and EP/mMWCNT(0.2) nanocomposite are greater than that of the neat EP, indicating that these nanocomposites are more hydrophobic than the hydrophilic neat EP matrix.

Figure 5: The sessile drop geometric shapes on (a) neat EP, (b) EP/pMWCNT(0.05), (c) EP/pMWCNT(0.2), (d) EP/mMWCNT(0.05) and (e) EP/mMWCNT(0.2) nanocomposites

Table 2. Contact angles of EP and that of EP/pMWCNT(0.2) and EP/mMWCNT(0.2) nanocomposites measured using sessile drop technique

 

 

 

 

 

 

 

The arrangement of the MWCNT nanostructure in the epoxy matrix may have an impact on the surface behaviour, and the heterogeneity effects may further be responsible for an increase in contact angle values [50]. The increase in contact angle values may be attributed to the surface roughness of the samples caused by the incorporation of the nanotubes. The further increase in the contact angles in the materials comprising the mMWCNT may be interpreted as an effect of the presence of oxygen-containing functional groups in the nanofiller [51].

 

CONCLUSION

On adding the MWCNTs to the EP resin, the constituents are found to be well cross-linked to each other, the phenomenon being more pronounced in the cases with acid-treated nanotubes, implying the latter's potential prospect of enhancing the composites' physical properties for structural applications. From the XRD analysis, it can be concluded that the incorporation of MWCNT in epoxy resin has no impact on the primary structure of the resin.

The thermal analysis of the materials indicate that the addition of the nanofiller does not affect the degradation behaviour of the matrix polymer over the investigated composition range while a slight improvement in the thermal stability is visible for the EP/mMWCNT nanocomposite. The hydrophilicity has been successfully introduced to the epoxy by addition of the mMWCNT.

The details on the correlation of the nanocomposites' detailed morphological features (as observed by electron microscopy) with their structural and physical properties shall be discussed in a separate publication.

CONFLICT OF INTEREST

All authors declare that there is not any kind of conflict of interest.

ACKNOWLEDGEMENT

ST gratefully acknowledges the Nepal Academy of Science and Technology (NAST), Lalitpur, Nepal, the Polymer Service GmbH Merseburg (PSM), Germany, and Mahatma Gandhi University, Kerala, India for providing scholarships for carrying out the research works. We gratefully acknowledge St. Thomas College in Pala, India for providing the XRD results and cordially thank the Japanese Ministry of Science and Technology for supporting the research stay of ST and JG at Meijo University, Nagoya, Japan in the frame of "Sakura Science Program 2020".



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