Fabrication and high radiation-resistant properties of functionalized carbon nanotube reinforced novolac epoxy resin nanocomposite coatings

Abstract

Carboxyl-functionalized multi-walled carbon nanotube (c-MWCNT) reinforced novolac epoxy resin composite coatings with high radiation-resistant performance were successfully fabricated and investigated. Well-dispersed c-MWCNTs were prepared via a γ radiation method. The microstructure, abrasion resistance and impact resistance of the composite coatings with varying content of c-MWCNTs (0 wt%, 0.1 wt%, 0.26 wt%, 0.5 wt%, and 0.75 wt%, 1 wt%) were studied in detail via high-dose Co-60 γ radiation (6000 kGy). All of the results reveal that the addition of c-MWCNTs obviously enhanced the radiation-resistance and comprehensive performance of the composite coatings. 0.75 wt% c-MWCNTs was the optimum content. The process and mechanism were generally discussed. The resulting composite coatings can be widely applied in nuclear power plants and other radiation environments.

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1. Introduction

Materials used in nuclear power plants, aerospace and other nuclear technology facilities have to tolerate a radiation environment, which is crucial to their safety and service periods.^1–3 Polymer-based coatings are playing important roles in protecting equipment and facilities against corrosion under complex conditions.⁴ Compared with metal and inorganic materials, polymers are usually more easily damaged by ionizing radiation.⁵ Therefore, many efforts have been made to improve the radiation resistance of polymer-based materials via fabricating radiation-resistant molecular structures or adding radiation protective agents.^6–9

Epoxy resins are widely used in the nuclear industry due to their good comprehensive performance and radiation resistance.^10,11 Epoxy resin based composites are rapidly developing and attracting more and more attention.^12,13 Their radiation stability and behavior are also investigated in detail.^14–17 High energy radiation can directly or indirectly cause degradation or crosslinking of organic molecules, which mostly take place in sensitive sites such as C–C, C–O, and C–H in aliphatic structures.^18,19 Benzene structures in polymers are much more stable than other structures under radiation.^20,21 The radiation resistance of epoxy resin is largely dependent on the existence of benzene structures. However, aliphatic structures existing in the main chains of many epoxy resins have become the bottleneck of radiation stability of entire composites, especially under high-dose irradiation.

Carbon nanotubes (CNTs) have been widely applied in many areas including in the reinforcement of composites.²² The special chemical structures of CNTs and graphene give them a certain structural radiation stability and capability of free radical scavenging.^23,24 Najafi and Shin fabricated poly(methyl methacrylate)–CNT nanocomposite films and found that CNT fillers dramatically improved their stability under UV-ozone and e-beam radiation.²⁵ Recently, CNTs have been employed to reinforce low density polyethylene (LDPE) and ultrahigh molecular weight polyethylene (UHMWPE), whose performances under γ irradiation (50–500 kGy) were investigated.^26–28 Graphene reinforced DGEBA epoxy and LDPE/UHMWPE composites have been separately prepared and studied under 365 nm UV radiation or γ radiation (25 kGy, 50 kGy, and 280 kGy).^29–32 All of the results reveal that CNTs and graphene can obviously enhance the radiation stability of polymers. However, their dispersibility and bonding in the cross-linking network can not only affect the mechanical performance but are also critical to the radiation resistance of the composites. Their performances under high-dose γ irradiation (e.g. >5000 kGy) are seldom studied, even though it is important to evaluate their availability and safety in a strong radiation environment.

This study was carried out to explore high radiation-resistant nanocomposite coatings and evaluate their performances under high-dose γ irradiation, and was based on the following considerations: (1) increase the content of benzene structures in the composites, (2) reduce the aliphatic structures in the main chains of the polymers, (3) improve the dispersibility and bonding of MWCNTs in the cross-linking network of the polymers.

In this paper, a kind of high radiation-resistant novolac epoxy resin composite coating with carboxyl-functionalized multi-walled carbon nanotubes (c-MWCNTs) was designed and successfully fabricated. In order to obtain well-dispersed and functionalized MWCNTs, raw MWCNTs were treated by γ radiation firstly. The radiation-resistant properties of the as prepared nano-composite coatings with varying content of c-MWCNTs (0 wt%, 0.1 wt%, 0.26 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%) were investigated via high-dose ⁶⁰Co γ radiation (6 × 10³ kGy). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) were employed to examine the microstructure of the composites. The abrasion resistance and impact resistance of the composite coatings were also tested in detail. The process and mechanism were generally discussed.

2. Experimental

2.1. Materials

Multi-walled carbon nanotubes (MWCNTs, 95 wt% purity, Chengdu Institute of Organic Chemistry, China), polyfunctional phenol novolac glycidyl ether (EPN1179, Hunstman Company, China), benzene modified amine hardener (Aradur 830, Hunstman Company, China), N,N-dimethylacetamide (DMAC, Sinopharm Chemical Reagent Company, China), defoaming agents (BYK-A530 and BYK-A141, BYK Company, Germany), sulfuric acid (Nanjing Chemical Reagent Company, China), and deionized water.

2.2. Preparation of c-MWCNT reinforced novolac epoxy resin coatings

Raw MWCNTs were dispersed in 1 mM sulfuric acid to form a 1 mg mL⁻¹ solution. Then, the above solution was put in a sealed bottle and irradiated by γ-rays (3 × 10⁵ Ci Co-60 γ source, 9 kGy h⁻¹) for 90 kGy. After that, the solution was centrifuged (4000 rpm) for 10 minutes. The sample was washed three times using deionized water to remove sulfuric acid and other matter. Then, the carboxylic and short multi-walled carbon nanotubes (c-MWCNTs) were obtained.

A certain amount of the as prepared c-MWCNTs was well dispersed in DMAC by ultrasonication (40 kHz) for 30 minutes. The above solution was gradually added into an EPN 1179 resin under stirring, and then dispersed under ultrasonication for another 30 minutes. DMAC solvent was completely eliminated from the above mixture by decompression distillation. Afterwards, the Aradur 830 hardener (H/OH = 1.0) and defoaming agents (0.1 wt% BYK A-530 and 0.2 wt% BYK A-141) were successively added into the above solution under strong stirring and formed a c-MWCNT/novolac epoxy resin composite paint.

The as prepared composite paint was rolled uniformly on the surface of iron sheets using a coating machine (XT-200CA). Before coating, the iron sheets were pre-treated by sand paper. Then, the iron sheets with coatings were dried at 80 °C for 30 minutes and formed c-MWCNT/epoxy resin composite coatings. Samples with varying content of c-MWCNTs (0 wt%, 0.1 wt%, 0.26 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%) were prepared separately.

2.3. Characterization

In order to evaluate their radiation-resistant performances, the samples were irradiated by γ-rays (3 × 10⁵ Ci Co-60 γ source, 9 kGy h⁻¹) for 6 × 10³ kGy, at room temperature in air. After γ irradiation, the samples were tested. Transmission electron microscope (TEM) and scanning electron microscope (SEM) images were taken on a Philips CM100 transmission electron microscope and a Hitachi S-4800 scanning electron microscope. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker OPUS 80V FT-IR spectrometer. The abrasion resistance of the coating was tested on an ABER SN-5135 machine (750g/1000r) according to the ASTM D4060-10 standard. The impact resistance of the coating was determined on a BGD-500 impact resistance tester according to the ASTM D-2794 standard and represented as the maximum non-destructive height (cm).

3. Results and discussion

3.1. Fabrication of c-MWCNT/novolac epoxy composite coatings

The preparation process of the c-MWCNT reinforced novolac epoxy composite coatings is shown in Fig. 1. In order to obtain functionalized and well-dispersed MWCNTs, raw MWCNTs are treated in sulfuric acid solution using high energy γ radiation, which leads to the formation of short nanotubes and the production of carboxyl groups on the surface of the nanotubes. The significant decrease of their length and the massive existence of carboxyl groups on their surface can obviously improve the dispersibility of the MWCNTs and make them well dispersed in the solution containing polymers. When they are heated, the carboxyl groups on the surface of the c-MWCNTs can react with the amino and hydroxyl groups in the epoxy resin and hardener. The c-MWCNTs take part in the formation of the crosslinking network, which can increase their stability in the polymer matrix. Novolac epoxy resin and benzene modified amine hardener are optimized as matrix polymers because they both have many aromatic structures and only one carbon atom existing between two benzene rings in their structures, which is good for the radiation stability of the composite coatings.

Fig. 1 Fabrication schematic of the c-MWCNT/novolac epoxy composite coatings.

Carbon nanotubes are very easy to aggregate in polymer and other solutions. As is shown in Fig. 2(a), the raw MWCNTs are very long and aggregate together, which makes them difficult to disperse well and exist stably in the solution. The aggregation of carbon nanotubes in polymers can seriously affect the properties of the as prepared composites. Fig. 2(b) shows that the irradiated MWCNTs (c-MWCNTs) are very short and have excellent dispersibility in the solution. There is no deposit after standing for 12 h. Fig. 2(c) shows that the as prepared c-MWCNTs can be well-dispersed in the novolac epoxy matrix and have no obvious aggregation, which is very critical to their performance.

Fig. 2 TEM images of raw MWCNTs (a), c-MWCNTs (b) and the as prepared composite coating with 0.75 wt% c-MWCNTs (c). The insets in (a) and (b) are the photographs of them in solution before (left) and after (right) standing for 12 h.

3.2. Morphology of composite coatings before and after irradiation

Fig. 3 shows the surface morphology of the as prepared nano-composite coatings. As is shown in Fig. 3(b)–(f), the c-MWCNTs are well-dispersed in the resin coatings and there is no obvious aggregation even when the content of c-MWCNTs in the composite coatings reaches 1 wt%. High-dose γ radiation causes varying destruction of the composite coatings with different amounts of c-MWCNTs (Fig. 3(g)–(l)). The surface of the coatings without c-MWCNTs is seriously damaged. Damages induced by γ radiation decrease with an increase of the amount of c-MWCNTs in the coatings. There are no obvious radiation damages in the samples with 0.75 wt% and 1 wt% c-MWCNTs, which exhibit high radiation resistance.

Fig. 3 SEM images of the composite coatings with varying content of c-MWCNTs (0 wt%, 0.1 wt%, 0.26 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%) before (a–f) and after (g–l) irradiation.

3.3. Abrasion resistance of composite coatings before and after irradiation

The abrasion resistance of the samples is shown in Fig. 4. The samples with a smaller weight loss have higher abrasion resistance. The weight losses of the samples with 0.5 wt% and 0.75 wt% c-MWCNTs are only 64.2% and 59.2% of that of the samples without c-MWCNTs, respectively. The weight losses of the samples with 1 wt% and 0.75 wt% c-MWCNTs are almost the same. The results reveal that the abrasion resistance of the samples is gradually enhanced as the content of c-MWCNTs increases. However, there are no notable changes of abrasion resistance when the content of c-MWCNTs increases to a certain value. Compared to the samples without irradiation, the weight loss of all of the samples is increased in varying degrees after γ irradiation. The changes of weight loss between the samples before and after irradiation are reduced with an increase of c-MWCNTs, which indicates that c-MWCNTs can enhance the abrasion resistance of coatings after irradiation and elevate the radiation-resistance of composite coatings.

Fig. 4 The abrasion resistance of the composite coatings with varying content of c-MWCNTs before and after irradiation.

3.4. Impact resistance of composite coatings before and after irradiation

Fig. 5 shows that the impact resistance of the samples is gradually enhanced with an increase of the amount of c-MWCNTs in the composite coatings. However, there are no obvious changes of impact resistance when the content of c-MWCNTs increases to a certain value. The impact resistance of the samples with 1 wt% and 0.75 wt% of c-MWCNTs is almost the same. After γ irradiation, the impact resistance of all of the samples decreases by varying degrees compared to the samples without irradiation. The impact resistance of all of the samples after irradiation can be enhanced by the addition of c-MWCNTs. The maximum non-destructive height of the composite coating with 0.75 wt% c-MWCNTs remains more than 70 cm after 6 × 10³ kGy γ irradiation, which illustrates that it has a good radiation resistance. These results indicate that c-MWCNTs play an important role in improving the abrasion resistance and impact resistance of coatings.

Fig. 5 The impact resistance of the composite coatings with a different content of c-MWCNTs before and after irradiation.

3.5. FT-IR analysis of composite coatings before and after radiation

The FT-IR spectra of samples ranging from 400 cm⁻¹ and 4000 cm⁻¹ are given in Fig. 6. The insets are the magnification of several characteristic peaks. Different changes induced by radiation are observed in the samples with different amounts of c-MWCNTs. The peaks at 1512 cm⁻¹ and 1610 cm⁻¹ correspond to the stretching vibrations of C=C and C–C in the benzene ring, which are typical signals of an aromatic group.^33,34 Benzene rings are very stable under radiation. So, all FT-IR spectra in Fig. 6 have been normalized according to the peak at 1512 cm⁻¹. The peaks at 815 cm⁻¹ and 938 cm⁻¹ correspond to the stretching vibrations of C–O–C and C–O in the epoxide ring. The peak at 1241 cm⁻¹ corresponds to the stretching vibration of the C–O–benzene ring. As is shown in Fig. 6, the peaks at 815 cm⁻¹ and 1241 cm⁻¹ decrease by varying degrees after irradiation, which indicates that radiation can damage the C–O–C structures in the coatings. The degradation of the C–O–C structures decreases with the increase of the amount of c-MWCNTs in the composite coatings. The peak at 1719 cm⁻¹ is ascribed to the stretching vibration of C=O, which is associated with the formation of oxidation products such as saturated aldehyde, ketone or acid.^13,17 The intensity of this peak is enhanced under irradiation and the rising decreases with the increase of c-MWCNTs in the composite coatings. These results indicate that the addition of c-MWCNTs reduces the structural damages of the composite coatings under radiation. As the amount of c-MWCNTs is increased in the composite coatings, the structural changes of the samples before and after irradiation become less obvious. Samples containing 0.75 wt% and 1.0 wt% c-MWCNTs exhibit good structural stability under radiation, which is consistent with the results of the SEM and mechanical properties.

Fig. 6 FT-IR spectra of the composite coatings with varying content of c-MWCNTs before and after irradiation.

3.6. Mechanism of as prepared high radiation resistant coatings

Fig. 7 illustrates the mechanism of high radiation resistance of the c-MWCNT/novolac epoxy composite coatings. Ionizing radiation can directly act on the polymer molecules and induce cleavage or crosslinking. Radiation can also produce many kinds of active radicals which can cause damages to the molecular chains in polymers. The radiation stability of organic molecules is closely related to their chemical structures. A benzene ring is a special chemical structure and has been proven to have high structural stability under irradiation.^20,21 Novolac epoxy resin and benzene modified amine hardener both have many aromatic structures in their molecules, which elevates the content of benzene rings in as prepared composite coatings and enhances their radiation resistance. Compared with benzene structures, the aliphatic structures are more sensitive to radiation. Most cleavage and crosslinking take place in the sites containing aliphatic structures.^18,19 In the main chains of as prepared composite coatings, only one carbon atom exists between two benzenes, which guarantees the high radiation resistance of body structures. Crosslinking and bonding among aliphatic structures are only formed during curing. Even though these chains are lightly damaged by radiation, the entire polymers can also keep their performance. Carbon nanotubes have special chemical structures and exhibit high radiation stability, which allows them to keep a high effectiveness in composites under irradiation.²³ Besides their high radiation stability, carbon nanotubes can also transfer energy and are capable of free radical scavenging.²⁶ c-MWCNTs bonded in the polymer matrix can reduce the radiation damages to other polymer structures. The dispersibility and stability of nano-fillers in the polymer are very important for the performance of the composites. In the as prepared composite coatings, short and carboxylic MWCNTs were well dispersed in the matrix polymer and bonded in the crosslinking network. The content of c-MWCNTs in the composite coatings is critical to their radiation-resistant properties. Experiment results indicate that the as prepared nanocomposite coatings with 0.75 wt% and 1.0 wt% c-MWCNTs have high radiation resistant properties and comprehensive performance. 0.75 wt% c-MWCNTs is the optimum content.

Fig. 7 Radiation resistant mechanism of the as prepared nanocomposite coatings.

4. Conclusion

c-MWCNTs prepared via a γ radiation method were employed to reinforce novolac epoxy resin and fabricate high radiation-resistant nanocomposite coatings. Samples with different amounts of c-MWCNTs (0 wt%, 0.1 wt%, 0.26 wt%, 0.5 wt%, 0.75 wt%, and 1 wt%) were prepared and studied in detail using high-dose γ radiation (6 × 10³ kGy). All of the results reveal that the radiation-resistant properties of the samples were enhanced with an increase of c-MWCNT content. 0.75 wt% c-MWCNTs is the optimum content, which allows the nanocomposite coating to exhibit excellent mechanical behavior and radiation-resistant performance. The preparation process and mechanism were generally discussed. The resulting composite coatings can be widely applied in nuclear power plants and other radiation environments.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (NS2014061, NJ20150022), the National Natural Science Foundation of China (11105073, 11575086), the Cooperative Innovation Fund of Jiangsu Province (BY2013003-09), the Natural Science Foundation of Jiangsu Province (BK2011739) and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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