WU Jian, HAN Wen＊, ZHAO Yalin, YANG Bin, JING Yan, GENG Mingxin,BAI Xiaochun, TAN Haiying, GE Qizhong, LENG Pei, SUN Jiuxiao＊

(1. Shaanxi Electric Power Research Institute, Xi’an 710100, China; 2. College of Materials Science and Engineering, Key Laboratory of Textile Fiber and Products (Ministry of Education), Wuhan Textile University, Wuhan 430200, China)

Abstract: Because of its merits, acrylic resin was chosen to improve the mechanical, conductive and hydrophobic properties. Carbon fiber powders (CF), carbon nanotubes (MWCNT), and nano-TiO2 were incorporated into the acrylic resin to prepare the corona-proof conductive composite coatings. The incorporation of CF and MWCNT may improve the conductivity and mechanical strength of the coatings. However, the addition of nano-TiO2 may increase the hydrophobicity of the coatings. Thus, the effects of different additives on the mechanical properties, conductivity, hydrophobicity and heat resistance of the conductive film were studied. The experimental results show that the incorporation of carbon fiber powders and multi walled carbon nanotubes can significantly improve both the conductivity and mechanical properties of the conductive coatings,and the addition of nano titanium dioxide can improve the hydrophobicity of the conductive film.

Key words: carbon fiber powders; carbon nano tube; acrylic resin; coating; mechanical performance;self-cleaning

1 Introduction

Corona performance high voltage transmission has attracted great attention on the problems of radio interference, corona loss, and audible noise. The main reason for corona loss is that the conductors are damaged in the process of transportation and installation, or their diameter is relatively small due to imperfect design considerations. Conductor corona loss are widely distributed, and it is difficult to control them by adopting sound insulation barriers and other measures. Therefore, it is necessary to control the corona loss[1-3]. The fundamental method to control corona loss is to reduce local field intensity. Adding voltage equalizing equipment is the commonly used method to control the corona loss. However, when there are many corona points on a single section of wire the mass of the wires will be increased. Moreover,this method is complex and not suitable for large area use[4-6]. Thus, it is urgently needing a simple and convenient method to reduce the corona loss.

Acrylic resin (AA) coating it is a new kind of coating with excellent adhesion, good weatherability,light color, paint film colorful plump, durable, resistant to gasoline, good corrosion resistance and mechanical properties of high performance properties, therefore they are widely used in aviation, military industry products, light industrial products and instruments and meters, vehicles,etc[7-9]. However, the poor toughness,poor electrical conductivity and strong hydrophilicity of acrylic resin have largely limited the application of acrylic coating in high voltage transmission. There are three main reasons for this: (1) The high resistivity of acrylic resin makes it difficult for its surface charge transmission, and the corona loss prevention effect is poor; (2) The mechanical properties of acrylic resin is not satisfied. Thus, it may be easily damaged or even fall off by external forces (eg, strong wind)when it is used in the transmission wires; (3) The strong hydrophilicity makes the coatings can easily be damages (eg, falling off) when it is used outdoors.Therefore, it is urgent to prepare a conductive acrylic coating with good mechanical properties(good strength and toughness) with hydrophobic properties. To solve the afore-mentioned problems, carbon fibers has been added to the acrylic resin to improve its mechanical properties, and multi-wall carbon nanotubes has been incorporated to improve its electrical conductivity,and nano titanium dioxide was added to maintain its surface wettability. Improving the conductivity of acrylic coating will benefit to the charge transfer and reduction of corona. On the other hand, increasing the hydrophobicity of the coating will avoid its erosion damage when outdoor used.

Thus, acrylic acid based conductive composite coatings for high-voltage transmission was prepared with acrylic acid resin as the matrix material, carbon fiber powders(CF), multi-wall carbon nanotubes(MWCNT) as conductive filler and nano TiO2as selfcleaning agent in this paper. The acrylic composite coating can be used to repair the burr scratches on the surface of wires and to fill the seam of strands. Due to the strong plasticity of composite coating, it can realize the effective repair of wire surface damage.On the other hand, the good fluidity of the composite coating made it suitable for filling the gap of the conductor strand, change the local morphology of the conductor surface, and make the surface of the conductor smooth and round. To a certain extent, it can enlarge the diameter of the wires, which can reduce the local electric field and decreased the corona loss of the conductor. Furthermore, the acrylic composite coating has good hydrophobic and self-cleaning propertymakes it suitable for outdoor used in high voltage transmission.

2 Experimental

2.1 Materials

Acrylic resin (AA, industrial grade) was purchased from Guangzhou Yongyi Chemical Co. Carbon fiber powders (CF) were purchased from Yancheng Carbon fiber Technology Co., Ltd.MWCNT was purchased from Nanjing Xianfeng Nanomaterials Technology Co., Ltd, whose diameter ranges from 10 nm to 20 nm. Silane coupling agent 3-aminopropyltriethoxysilane (KH550, chemically pure) was purchased from Macklin Co. Nano titanium dioxide (Nano TiO2) with the particle size 8000 mesh was purchased from Shaoxing Lijie Chemical Co., Ltd.Toluene (AR) and ethyl acetate (AR), were purchased from Sinopac Chemical Reagent Co., Ltd. Fluidic agent BYK-4511was purchased from Beecker Company(Germany).

2.2 Acrylic conductive film preparation

The pre-treated carbon fiber powders were dispersed in acetone solution by ultrasonic for 1h,followed by filtration and water washing to obtain the purified carbon fiber powders. Then the carbon fiber powders was dispersed in toluene solution of the preprepared silane coupling agent KH550 toluene solution,stirred at 60 ℃ for 4 h, and then the carbon fiber powders modified by the coupling agent was extracted,filtered, washed and dried. Then, a certain amount of carbon nanotubes were dispersed in ethyl acetate under ultrasonic for 1 h to obtain 5wt% carbon nanotubes ethyl acetate dispersion solution(dispersion solution A).After that, the acrylic resin was dissolved in toluene to obtain 50wt% the acrylic acid solution(solution B). Nano-titanium dioxide was dispersed in toluene with ultrasonic to obtain dispersion liquid C. Finally,a certain amount of dispersion solution A, solution B,and dispersion solution C were mixed and stirred to achieve homogeneous dispersion. Furthermore, a trace amount of leveling agent BYK-4511 was added and stirred for 30 min. Finally, the mixture was coated on the polytetrafluoroethylene flat plate to obtain acrylic conductive film with the thickness of films at 120 μm.Four kinds of acrylic coatings were prepared: pure acrylic resin (AA); composite coating of acrylic resin(50wt%) and carbon fiber (50wt%) was denoted as(AA+CF); Composite coatings of acrylic resin (45wt%),carbon fiber powders (50wt%) and multi-wall carbon nanotubes (5wt%) was denoted as (AA+CF+MWCNT)composite coatings of acrylic resin (40wt%), carbon fiber powders (50wt%), multi-wall carbon nanotubes(5wt%) and Nano titanium dioxide (5wt%) was denoted as AA+ CF + MWCNT + Nano TiO2.

2.3 Characterization

The tensile properties of acrylic conductive films were tested by a universal testing machine(Instron 5967). The test standard was GB/T 1040-2006, and the tensile speed of the samples was 5 mm/min.The conductivity of acrylic conductive film was measured by a RTS-9 double electric four-probe tester.Each sample was tested for 10 times, and the final conductivity was averaged. The surface contact angle of acrylic conductive film was measured by a JY-PHA type contact angle meter and the water drop was 3 μL. The thermal stability of acrylic conductive film was tested by a Q50 thermogravimetric analyzer. The temperature was increased from room temperature to 600 ℃ with the heating rate at 10 ℃/min. The dispersion state of nanoparticles in acrylic films was observed by scanning electron microscope(JSM-6700F). Before the test, the samples were frozen in liquid nitrogen and brittleed.The microscopic morphology of the quenched section of acrylic conductive films was observed.

3 Results and discussion

3.1 Tensile properties of acrylic conductive films

Firstly, the tensile properties of acrylic conductive composite films were tested, and the experimental results are shown in Fig.1. As shown in Fig.1, the elastic modulus, tensile strength, and elongation at break for AA-CF composite coating are 3.0 GPa, 12.9 MPa, and 0.5%, respectively. Compared with the pure AA resin coating, the tensile strength increases by 268.6%, and the elastic modulus and elongation at break decrease by 30.2% and 57.1%, respectively.After the addition of carbon nanotubes, the toughness of the material(AA+ CF + MWCNT) is improved and the strength is maintained well. After the addition of nano-titanium dioxide, the strength of the material remained good, but the toughness decreases to a certain extent. Because the titanium dioxide(nano-TiO2) is round nanoparticle, the incorporation of nano-TiO2will produce defect in the sample, resulting in the decrement of the strength. However, in order to obtain an acrylic composite coating with excellent comprehensive properties, the other properties of the obtained samples should be further analyzed and comprehensively investigated.

Fig.1 Stress-strain curves(a) and tensile properties(b) of the four type of acrylic based conductive films (AA, AA+CF,AA+CF+MWCNT, and AA+CF+MWCNT+Nano TiO2)

3.2 The conductive properties and selfcleaning ability of acrylic conductive films

The conductivity of the films was further tested,and the results are shown in Fig.2(a). As shown in the figure, the incorporation of both CF and MWCNT,the conductivity of the films increases, and AA+ CF+MWCNT shows the best conductivity. This is mainly because the carbon nanotubes formed a bridging structure directly in the carbon fiber, which will benefit for its charge transmission. However, the addition of TiO2(AA+ CF+ MWCNT+Nano TiO2) decreases the conductivity slightly. Volume resistance and surface resistance of AA + CF + MWCNT+Nano TiO2are 0.8Ω·cm and 83.2 Ω, respectively.

Fig.2 (a)The conductive properties and (b)The hydrophobicity results of the four type of acrylic based conductive films (AA, AA+CF, AA+CF+MWCNT, and AA+CF+MWCNT+Nano TiO2); (c) Schematic shows the possible structure of the nanofillers in the acrylic resin

Moreover, the hydrophobic properties is also important for the acrylic based coatings. Therefore the hydrophobic ability of the films was further detected and the results are shown in Fig.2(b), after the incorporation of CF and MWCNT, the hydrophobic of the film increased. After the addition of nano-TiO2,contact angle of AA+ CF+ MWCNT + nano TiO2films reached to 103.7° higher than that of AA+ CF+MWCNT composite film(10.8%), which shows the best hydrophobic properties. because the incorporation of Nano TiO2may form micro-nano structure on the surface of the coating material, making the material have good hydrophobicity.

In summary, the mechanical properties,conductive properties and hydrophobicity of AA+CF+ MWCNT+Nano TiO2composite film all meet the requirements of anti-corona coating on the condition that the material needs to discharge corona. For AA+CF composite system, the 3D conductive network in is only formed through the contact between carbon fibers,while CF and MWCNT formed three-dimensional(3D) conductive network in the coating(Fig.2(c)) and were alternately connected to each other. In addition,there may be a certain degree of riveting between CF and MWCNT in the films, which plays a key role in improving its tensile and conductive properties[10]. In this case, the conductivity and mechanical properties of AA+ CF+ MWCNT+ Nano-TiO2are higher than that of AA+ CF[11].

3.3 Heat resistance of the conductive acrylic films

The thermal stability of acrylic based composites is also a key index to evaluate its application as coatings. The thermal stability of the composite film was further studied through thermogravimetric analysis and is shown in Fig.4. It can be seen from the figure that AA+CF, AA+CF+MWCNT, and AA+CF+MWCNT+Nano TiO2composite conductive films all have good heat resistance. The maximum decomposition temperature increased from 385.0 ℃(pure AA) to 390.1, 396.1, and 400.2 ℃, respectively.The increment of the films reached to 1.3%, 2.9%, and 3.9%, accordingly. This because CF, MWCNT, and Nano-TiO2have better thermal stability than that of AA and they can capture free radicals, which greatly delays the degradation of AA. In addition, nanotitanium dioxide can be used as flame retardant and heat stabilizer in polymer composite system[12]. Thus,the addition of nano-TiO2will further improve the thermal stability of the films. Moreover, the surface of carbon fiber(CF) modified by silane coupling agent has more amino groups, which can react with hydroxyl and carboxyl groups in acrylic resin to form a stable chemical bond[13]and enhance the interface between the nano fillers and AA. In summary, AA+ CF+ MWCNT+Nano TiO2composite conductive film has the best thermal stability.

Fig.3 TGA(a) and DTG(b) curves of the four type of acrylic based conductive films(AA, AA+CF, AA+CF+MWCNT,and AA+CF+MWCNT+Nano TiO2)

3.4 Microstructure of acrylic conductive films

According to the previous experimental results,we believe that the electrical properties, mechanical properties and hydrophobicity of the composite are closely related to the distribution of nano-packing in acrylic acid. Therefore, the distribution of nanopacking in acrylic acid conductive composite film is further studied by SEM(Fig.4). The SEM images of the quenching section of the film shows that the pure AA was brittle fracture. While, CF surface coated acrylic resin in AA+CF composite film(Figs.4(b) and 4(e)) demonstrated the strong interfacial interaction between the carbon fibers and acrylic resin. For AA+CF+ MWCNT + Nano TiO2films (Figs.4(d), 4(e),and 4(f)), all the nano filler dispersed well in the acrylic resin, CF and MWCNT formed a 3D network structure, and present a layered fracture quenching break, and a phenomenon of drawing can be observed in Fig.4(f). All the results demonstrated that the fillers in the acrylic resin formed 3D network as is shown in Fig.2(c), and resulting in the increment of conductivity and mechanical properties of the coatings.

Fig.4 SEM images of (a) pure acrylic resin coating, (b) AA+CF coatings, (c-f) SEM images of AA+CF+MWCNT+Nano TiO2 at different detection places, (b1) enlarge of Fig.4(b), and (c1)enlarge of Fig.4(c)

4 Conclusions

In this paper, anti-corona acrylic conductive coating for high voltage wire was prepared by the incorporation of carbon fiber powders, carbon nanotubes as conductive fillers and nano titanium dioxide as self-cleaning agent to acrylic resin. The acrylic based composite coatings showed good mechanical properties, conductivity and thermal stability, which meets the needs of the high voltage transmission application. Volume resistance and surface resistance of conductive coatings, reached to 0.8 Ω/cm and 83.2 Ω, respectively. Elastic modulus, tensile strength and elongation at break of the composites films reached of 5.5 GPa, 12.4 GPa, and 1.1%. And the contact angle is 103.7°, which meets the requirements high voltage corona application.