Journal of Photopolymer Science and Technology Volume 28, Number 2 (2015) 151 155 2015SPST Enameled Wire Having Polyimide-silica Hybrid Insulation Layer Prepared by Sol-gel Process Atsushi Morikawa 1, Kazunori Suzuki 2, and Kenji Asano 2. 1 Department of Biomolecular Functional Engineering, Ibaraki University, Hitachi, Ibaraki 316-8511, Japan 2 Power System Laboratory, Hitachi Cable, Ltd., Hitach, Ibaraki 319-1411, Japan Heat resistant enameled wire that possessed a polyimide-silica hybrid insulation layer prepared using the sol-gel process was prepared. Tetraethoxysilane (TEOS) was hydrolyzed, followed by polycondensation in polyamic acid varnish, and the varnish was then used as an enamel. Conductor wire was coated with the enamel through a die to control the coating thickness. The wire was then passed through an oven for solvent evaporation and imidation. Interconnected uniform silica globules with a diameter of 50 nm were observed in the insulation layer by scanning electron microscopy (SEM). The enameled wire showed higher breakdown voltage than that without silica after thermal deterioration at 300 C or 400 C. Keyword: sol-gel process, polyimide-silica hybrid, enameled wire, breakdown voltage 1. Introduction Polyimides are known as reliable high temperature polymers, especially in the aerospace and electrics industry. Polyimide, which is usually insoluble in organic solvents, is prepared from a soluble precursor, polyamic acid. Heat resistant enameled wire, which has a polyimide insulation layer, is prepared by coating a conductor with a varnish of polyamic acid, then heating for solvent evaporation and imidation. Enamel wire with a ceramic layer is used as a higher heat resistant wire. However, ceramic wire is not suitable for uses requiring windability due to its low bending strength. The heat resistance was improved while maintaining windability by dispersing the inorganic particles homogeneously in a matrix organic insulation layer. Silica nanoparticles were surface modified with silanes using a sonochemical method, and dispersed in polyester varnish [1]. The varnish was used as an insulation layer for the enamel wire. Dispersing the silica particles in the polyester enhanced the thermal resistance as well as the dielectric breakdown strength of the enamel wire. We examined the preparation of the insulation layer for enameled wire using the sol-gel reaction of TEOS [2]. TEOS was hydrolyzed and then polycondensed in a Kapton-type polyamic acid varnish, and the conductor was coated with the silica-containing polyamic acid varnish and then passed through an oven for solvent evaporation and imidation. The sol-gel process is a method for preparation of inorganic metal oxides starting from organic metal alkoxides. The reaction consists of hydrolysis of the metal alkoxides, followed by polycondensation of the hydrolyzed intermediates [3]. The sol-gel reaction has been applied to the reaction in a polymer matrix to prepare inorganic and organic hybrid materials. Polyimide-silica hybrid film has been successfully prepared by the sol-gel reaction of TEOS in polyamic acid solution, and by subsequently heating the resulting film at 270 C. The hybrid film showed higher modulus, lower thermal expansion coefficiency, and higher 10 wt% loss temperature [4]. Received March 17, 2015 Accepted April 30, 2015 151
2. Experimental 2.1. Materials Kapton-type polyamic acid varnish purchased from Du Pont Corporation and TORAY Corporation was used to prepare the enameled wire. TEOS was purchased from Wako Pure Chemical Industry. Copper conductor or nickel plating copper was used as the wire with a diameter of 0.5 mm or 1.1mm. 2.2. Preparation of polyimide-silica hybrid film (Entry 3) 0.52 g (2.5 mmol) of TEOS and 0.38 g (21 mmol) of water were added to 10 g of 10 wt% polyamic acid varnish in N-methyl-2-pyrrolidone (NMP), and stirred until the varnish became homogeneous. The film was prepared by casting the homogeneous varnish on a glass plate. After the film was dried at 80 C for 2 h, the film was heated at 270 C for 3 h. 2.3. Preparation of polyimide-silica hybrid enameled wire 305 g (1.46 mol) of TEOS and 225 g (12.5 mol) of water were added to 1500 g of 15 wt% polyamic acid varnish in NMP, and stirred until homogeneous. The homogeneous varnish was used as the enamel. The conductor was coated with the enamel through a die to control the coating thickness. The wire was then passed through an oven at 260-360 C for several seconds for solvent evaporation and imidation. The operation was repeated several times to control the thickness of the insulation layer. The silica content calculated by assuming that the sol-gel reaction proceeded completely was 31 wt %. The breakdown voltage of the enameled wire was measured by twisting two enameled wires according to JIS C 3003. 3. Results and Discussion 3.1. Preparation of polyimide-silica hybrid film Before preparing the enameled wire, polyimide-silica hybrid films were prepared by the sol-gel reaction of TEOS in the polyamic acid varnish, which was a solution of 10 wt% polyamic acid in NMP. First, TEOS was reacted with water in the varnish, then the resulting homogeneous mixture was cast onto a glass plate to prepare the polyamic acid-silica hybrid film. Finally, the polyimide-silica hybrid film was obtained by heating the precursor film at 270 C. Table 1 summarizes the results of the preparation of polyimide-silica hybrid films. The silica (SiO 2 ) content in the table denotes the values calculated by assuming that the sol-gel reaction proceeded completely. All the varnish was homogeneous before casting on a glass plate. The cast films were dried at 80 C on the glass plate to evaporate the solvents as well as to develop the siloxane network of silica. Polyimide-silica hybrid films having silica content up to 62 wt% were obtained. The films of entries 1, 2, and 3 were transparent and yellow even after the films were heated at 270 C, whereas the films containing more than 34 wt% silica were opaque and yellow. Table 1.Preparation of polyimide-silica hybrid films Entry PAA TEOS H 2 O Silica Remark b varnish (g) (g) (g) (wt%) a 1 10.0 0 0 0 T 2 10.0 0.1 0.07 3 T 3 10.0 0.52 0.38 14 T 4 10.0 1.63 1.1 34 O 5 10.0 5.17 3.6 62 O a Calculated of silica content in the hybrid films. b T, transparent; O, opaque. The chemical structure of polyimide-silica hybrid film was confirmed by means of IR spectroscopy. Figure 1 shows the IR spectra of entry 1 (0 wt% SiO 2 ) film and entry 4 (34 wt% SiO 2 ) film. The formation of polyimides was confirmed by the appearance of absorption bands at 1780 and 1720 cm -1 (C=O) and 1380 cm -1 (C-N) characteristic of imide groups. In addition, the appearance of absorption at about 1100 cm -1 indicated the formation of silicon-oxygen bonds in the IR spectrum of the entry 4 film. Figure 1. IR spectra of polyimide-silica hybrid film and polyimide film. 152
The silica phase in the hybrid films was observed by SEM. Since the silica phase could not be clearly seen in the film, the film was heated at 800 C in air to thermally decompose the polyimide phase, and the residual silica was observed by SEM. The morphology of the silica in the hybrid film was found to be retained after oxidation at 800 C. [5] Figure 2 shows SEM photographs of entry 3 (14 wt% SiO 2 ) film and entry 4 (34 wt% SiO 2 ) film. Interconnected uniform globules were seen as observed in hybrid films prepared previously by the sol-gel reaction in a methanol solution of polyamic acid triethylamine salt [5]. The sizes of silica globules in entry 3 film and entry 4 film were 0.1 and 1.0 m, respectively, smaller than those in the hybrid films prepared previously by the sol-gel reaction in a N,N-dimethylacetamide solution of polyamic acid [4]. Figure 3. Variation in viscosity of polyamic acid-silica varnish. Polyimide-silica hybrid enameled wire was prepared as shown in Figure 4. The conductor was coated with the homogeneous polyamic acid-silica varnish and then passed through an oven for solvent evaporation and imidation. The temperature in the oven was 260-360 C, and the wire was passed through the oven in several seconds. An insulation layer of 20-30 μm was formed by repeating the operation ten times. Figure 2. SEM photographs of polyimide-silica hybrid films containing 14 and 34 wt% silica. 3.2. Preparation of polyimide-silica hybrid enameled wire TEOS was reacted with water in varnish, which was a solution of 15 wt% polyamic acid in NMP. TEOS was insoluble in the polyamic acid varnish, but silica, which was generated by the sol-gel reaction of TEOS, was soluble due to its polar silanol groups. The polyamic acid-silica varnish was used to prepare polyimide-silica enameled wire, and the silica content calculated by assuming that the sol-gel reaction proceeded completely was 31 wt%. Figure 3 shows variation in viscosity of the homogeneous varnish during standing at room temperature or 4 C. The values barely changed over a period of one month, showing that the varnish was very stable. Figure 4. Procedure for preparation of polyimide-silica hybrid enamel wire. 153
Figure 5 shows the typical thermogravimetric curves of the insulation layer of the polyimide-silica hybrid enameled wire and the enamel wire without silica. Remarkable weight loss was not seen below 500 C, and solvent evaporation and imidation proceeded completely even though the wires passed through the oven for only several seconds. The weight of the residues at 800 C was in agreement with the value calculated by assuming that the sol-gel reaction proceeded completely. hybrid film. The size of the globules was 50 nm, smaller than that in the hybrid film. Silica is insoluble in polyamic acid or polyimide, thus phase separation occurred in the drying process. The enamel wire was heated at 260-300 C, and the solvents were evaporated very fast. The varnish was thought to solidify before the silica grew larger due to the interfacial tension. Figure 5. TG curves of insulation layer in polyimide-silica hybrid enamel wire and polyimide enamel wire. Figure 6. SEM photographs of polyimide-silica hybrid enameled wire after aging at 800 C in air. The silica phase in the insulation layer was observed by SEM (Figure 6). The sample was also heated at 800 C in air to thermally decompose the polyimide phase. Interconnected uniform globules were seen as was observed in the Figure 7. Variation in breakdown voltage of polyimide-silica hybrid enamel wire and polyimide enamel wire after aging at 300 C or 400 C. The breakdown voltage of the enamel wire was measured after aging at 300 C or 400 C to examine the thermal resistance. The enamel wires were deteriorated by aging at high temperatures, and the breakdown voltages decreased with the aging period. Figure 7 shows the variations in the values compared with those of the polyimide enamel wire, which has a polyimide insulation layer without silica. The polyimide-silica hybrid enameled wire did not show a large decrease of the breakdown voltage as did the polyimide enamel wire without silica, and the polyimide-silica hybrid enameled wire aged at 300 C for 1000 h and at 400 C for 10 h maintained high values of more than 6 times and 4 times that of the polyimide enamel wire without silica, respectively. Figure 8 shows SEM photographs of the enamel wire, which was aged at 400 C for 16 h. Voids generated by thermal deterioration were observed, and the voids in the polyimide-silica hybrid 154
enameled wire were smaller and fewer in number than those in the enamel wire without silica. The mobility of the polyimide molecules was thought to be low due to hydrogen bonds between the silanol groups in the silica and the carbonyl groups in the polyimide, and the thermal deterioration was slow 4. Conclusion TEOS was hydrolyzed then polycondensed in polyamic acid varnish, and enameled wire, which had a polyimide-silica insulation layer, was prepared by coating conductor wire with the polyamic acid-silica varnish and heating. The silica phase had an interconnected globular structure with a diameter of 50 nm. The enameled wire showed higher breakdown voltage than that without silica after thermal deterioration at 300 C or 400 C, and higher thermal resistance. Figure 8. SEM photographs of polyimide-silica hybrid enameled wire and polyimide enamel wire after aging at 400 C for 16 h. References 1. Y. Kim, S. Y. Park, S. Y. Kwon, S. J. Kim, J. Kim, and Y. S. Seo, Thermochimica Acta, 542, (2012) 62-68. 2. A. Morikawa, K. Suzuki, and K. Asano, JP-A-Hei-10-289622 (1998). 3. L. L. Hench and J. K. West, Chem. Rev., 90. (1990) 33. 4. A. Morikawa, Y. Iyoku, M. Kakimoto, and Y. Imai, Polym. J., 24 (1992) 107-113. 5. A. Morikawa, H. Yamagichi, M. Kakimoto, and Y. Imai, Chem. Mater., 6 (1994) 913-917. 155