DESC:
FIELD OF THE INVENTION
[01] The present invention relates to a hydrophobic glaze for coating a shaped ceramic body of a porcelain insulator before firing. The hydrophobic glazed insulators are operable in overhead transmission lines to withstand salinity levels up to 224 kg/m3. The said glaze serves to improve pollution performance in very heavy 5 pollution environments. The invention also relates to a method for making a hydrophobic glaze for coating shaped ceramic body of a porcelain insulator.
BACKGROUND OF THE INVENTION
[02] The performance of outdoor high-voltage porcelain insulators is adversely 10 affected by environmental pollution in coastal and industrial regions. With rapid industrialization over the years, the atmospheric pollution level has risen sharply, requiring electrical insulators to perform well against pollutants to ensure uninterrupted power supply. Porcelain insulators coated with a ceramic glaze tend to accumulate dust and pollution on their surface. The pollution layer that settles on the 15 insulator surface becomes a conductive electrolyte when the insulator surface is wetted by light rain or fog. This occurs due to the inherent hydrophilic nature of ceramic glaze. The conductive electrolyte allows leakage currents to flow over the insulator surface. As the pollution layer thickness varies, it creates different amount of leakage current. Heating occurs when amount of leakage current is high. Due to 20 heat, a dry band forms which can lead to arcing. Frequent arcing with enough arcing length to reach another arcing leads to flashover in insulators. In recent years, this pollution induced flashover is the single largest reason for transmission/distribution line outages.
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[03] In recent years, polymer insulators have grown in popularity because of their superior pollution performance owing to their inherent hydrophobicity, light weight and low cost. However, the lifespan of polymer insulators is low due to polymer degradation with age or getting modified by sunlight and exposure to wind, moisture and dust. Also, polymer insulators often get pecked on by birds and are not durable 5 for use over a long time and so, their use is limited to frequent use lines. Thus, there is a need to continue using porcelain insulators, yet focusing attention on improving the pollution performance of porcelain insulators.
[04] In order to address the pollution induced flashover of transmission line outages, which is one of the most important high voltage energy transmission 10 problems, research and development is being done to introduce design modifications in the porcelain insulators, or to introduce surface texture modifications. Surface texture modifications can be done through introduction of Room Temperature Vulcanizing (RTV) coatings e.g. silicone coatings. These coatings get damaged during handling and installation causing disruption of the outer hydrophobic skin 15 layer. Design modifications often result in very heavy insulators which causes handling issues while texture modifications raise challenges for adherence between the porcelain surface and the standard glaze coating done to block porosity and give the insulator certain standard colours. Therefore, design modifications or surface texture modifications of the types discussed hereinabove raise doubts about 20 consistent pollution performance throughout the life span of the porcelain insulator.
[05] There is therefore a need to modify the existing porcelain insulator to improve its performance against environmental pollution and where such an insulator would be viable for long term use even in high salt environments like coastal areas. Ideally,
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such an insulator should withstand long term exposure to the elements and any modifications made to this end should not lead to an increase in weight of the insulator, nor require complicated additional processes requiring new manufacturing set-ups.
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BRIEF DESCRIPTION OF THE DRAWINGS
[01] Fig. 1 is a photographic representation of the hydrophobicity level achieved with five samples of hydrophobic glaze prepared according to an embodiment of the invention, i.e. H1 to H5 versus that of regular porcelain glaze (RPG). The hydrophobicity level was measured as per the procedure of IEC62073. 10
[02] Fig. 2 is a photographic representation of the surface microstructure features of RPG as compared to the hydrophobic glaze H5 prepared according to an embodiment of the invention.
SUMMARY OF THE INVENTION 15
[03] According to an embodiment of the invention there is provided a hydrophobic glaze for coating a shaped ceramic body of a porcelain insulator, wherein said glaze comprises the following constituents: 26 to 30% by weight quartz, 27 to 35% by weight feldspar, 5 to 8% by weight dolomite, 1.5 to 3.5% by weight zircon, 1.5 to 5% by weight barium carbonate, 10 to 17% by weight China clay, 2 to 5% by weight ball 20 clay, 0.5 to 2% by weight alumina, 5 to 14% by weight calcite and 0.25 to 2% zinc oxide. [04] According to another embodiment of the invention, there is provided a method of making a hydrophobic glaze-coated shaped ceramic body of a porcelain insulator, the method comprising: 25
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a. grinding and mixing the following constituents with water: -26 to 30% by weight quartz,
-27 to 35% by weight feldspar,
-5 to 8% by weight dolomite,
-1.5 to 3.5% by weight zircon, 5
-1.5 to 5% by weight barium carbonate,
-10 to 17% by weight China clay,
-2 to 5% by weight ball clay,
-0.5 to 2% by weight alumina,
-5 to 14% by weight calcite, and 10
-0.25 to 2% zinc oxide; b. spray coating the shaped ceramic body with or dipping the shaped ceramic body into the mixed and ground wet constituents prepared in step (a) to obtain a glaze-coated ceramic body; and c. firing the glaze-coated ceramic body of step (b) in a reduction atmosphere in 15 the temperature range of 1230 to 1280oC for 26 to 35 hrs.
DETAILED DESCRIPTION OF THE INVENTION
[05] In the following description, the embodiments of the invention are described in sufficient detail to enable those skilled in the art to practice the invention and it is 20 understood that other embodiments may be utilized and that logical processual changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known
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to those skilled in the art. The following description is, therefore, not to be taken in a limiting sense and the scope of the illustrative embodiments are defined only by the claims appended to this specification.
[06] In an embodiment of the invention, there is provided a hydrophobic glaze for coating a shaped ceramic body of a porcelain insulator, wherein said glaze comprises 5 the following constituents: 26 to 30% by weight quartz, 27 to 35% by weight feldspar, 5 to 8% by weight dolomite, 1.5 to 3.5% by weight zircon, 1.5 to 5% by weight barium carbonate, 10 to 17% by weight China clay, 2 to 5% by weight ball clay, 0.5 to 2% by weight alumina, 5 to 14% by weight calcite and 0.25 to 2% zinc oxide. [07] In another embodiment of the invention, there is provided a method of making 10 a hydrophobic glaze-coated shaped ceramic body of a porcelain insulator, the method comprising: a. grinding and mixing the following constituents in water: -26 to 30% by weight quartz,
-27 to 35% by weight feldspar, 15
-5 to 8% by weight dolomite,
-1.5 to 3.5% by weight zircon,
-1.5 to 5% by weight barium carbonate,
-10 to 17% by weight China clay,
-2 to 5% by weight ball clay, 20
-0.5 to 2% by weight alumina,
-5 to 14% by weight calcite, and
-0.25 to 2% zinc oxide;
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b. spray coating the shaped ceramic body with or dipping the shaped ceramic body into the mixed and ground wet constituents prepared in step (a) to obtain a glaze-coated ceramic body; and c. firing the glaze-coated ceramic body of step (b) in a reduction atmosphere in the temperature range of 1230 to 1280oC for 26 to 35 hrs. 5
[08] Preferably, 56 to 67% of the ground constituents should have a particle size of =10µm. The glaze coated on the ceramic body should ideally have a thickness in the range of 0.20 to 0.50mm. Preferably, carboxymethyl cellulose is added to the mixed and ground wet constituents of step (a) prior to step (b) to obtain a glazing slurry Preferably, the mixed and ground wet constituents or the glazing slurry is 10 passed through a sieve having 0.25 – 0.075 mm size openings prior to step (c).
[09] Hydrophobicity of materials can be obtained through one of two ways: (1) through inherent electronic property of materials (i.e. by non-polar and polar interaction) and (2) through tailoring the surface microstructure of the material. As ceramics are inherently hydrophilic, the present invention successfully tailors the 15 microstructure of glaze materials.
[10] Since ceramics by nature are hydrophilic, the hydrophobic glaze of the present invention provides hydrophobicity to the surface of porcelain insulators. Rare earth oxides are super-hydrophobic in their pure state. Addition of rare earth oxides materials to existing ceramic glaze renders hydrophobicity; but it raises two 20 challenges, namely cost and multicolour in the product due to reduction process in the kiln. Therefore, rare earth oxides were excluded from the glaze in the product of the present invention. Other transition metal oxides like copper oxide can also be useful to introduce hydrophobicity to a glaze coating, but this also raised colour issues
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in addition to the issue of weakening the mechanical strength of the product. Thus, it was found to be very difficult to introduce hydrophobicity by adding specific materials and relying upon the said materials’ inherent character. Thus, tailoring the microstructure of the glaze was attempted in the present invention. In order to tailor the materials’ microstructure, conventional regular porcelain glaze (RPG) was altered 5 through compositional tailoring to control melting of the glaze and particle size distribution.
[11] In order to tailor the microstructure of a material, pigments were also excluded from the composition. Pigments cause melting and pigments with variable valence might lead to electrolytic corrosion over insulators in corrosive/polluted 10 environments. Also, pigments like iron oxide, chromium oxide, manganese oxide weaken the network of Si-Al bonding as they work as glass network modifier or intermediate; they break strong tetrahedron network of Si-O sigma bond and create pi bonds. This loose structure has such a high flow that it does not create required microstructure over the glaze to impart hydrophobicity. Therefore, a hydrophobic 15 glaze for porcelain insulators was developed using quartz powder, feldspar, dolomite, zircon, barium carbonate, china clay, ball clay, alumina, calcite and zinc oxide, but without pigments.
[12] The conventional porcelain glaze contains pigments like iron oxide, chromium oxide, manganese oxide and other pigments. However, the method of 20 preparation of the two glazes differs mainly in terms of certain process parameters, for example, grinding duration for the hydrophobic glaze is much less than the grinding duration for the conventional porcelain glaze one, thereby requiring less power consumption. This is because the amount of particles having =10µm particle
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size in the conventional glaze is higher than in the hydrophobic glaze of the present invention. It is assumed that the presence of a substantial proportion of large-sized particles along with less melting property of glaze helps in creating required microstructure for hydrophobicity.
[13] Table 1 provides details of constituents (by weight percentage) of the 5 hydrophobic glaze of the present invention versus the regular porcelain glaze.
[14] Table 1: Compositions of glaze
Raw Material
Hydrophobic glaze,
weight range, %
Regular porcelain glaze, weight range, %
Quartz
26 -30
27 – 32
Feldspar
27-35
29.5 – 35.5
Dolomite
5 – 8
4.5 – 5.7
Zircon
1.5 – 3.5
2.15 – 2.9
Barium Carbonate
1.5 – 5
1.40 – 1.52
China Clay
10 – 17
9 – 11
Ball Clay
2 – 5
2.5 – 4.5
Alumina
0.5 – 2
0.89 – 1.21
Calcite
5 – 14
6.78 – 7.25
Manganese di oxide
0
2.9 – 3.5
Iron oxide
0
1.78 – 2.08
Chromium oxide
0
0.88 – 0.93
Zinc Oxide
0.25 – 2
0
Total
100.00
100.00
[15] The base green porcelain insulator body for glaze is made of clay, quartz and feldspar. All the raw materials were mixed in water, followed by screening, ferro-10 filtering magnetic iron separation, filter pressing, pugging and shaping. After shaping was done, the greenware porcelain insulators were sent to a dryer for moisture removal. Normally after dryer, the moisture of the green body remains in around 1%. The green body remains in leather hard condition. This body is either dipped in or sprayed with glaze slurry for glazing purpose. The present hydrophobic glaze of the 15
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invention is also applied with the same way. No alteration of process was required. The firing zone temperatures along with duration are described in Table 2.
Table 2: Zone wise kiln temperature and duration
Zone
Temperature range, ?C
Duration, min
Start temperature, ?C
End temperature, ?C
Preheating zone
20 – 42
780 - 820
570-620
Oxidation
780 – 820
920 - 980
210 – 235
Reduction
920 – 980
1230 - 1270
195 – 225
Soaking
1230 – 1270
1230 - 1270
78 – 83
Cooling
1230 – 1270
170 - 130
650 – 690
[16] The following experimental examples are illustrative of the invention but not 5 limitative of the scope thereof:
Example 1:
[17] A green porcelain insulator was prepared and its shaped ceramic body was coated with 5 samples of hydrophobic glaze (H1 to H5) having respective 10 compositions as shown in Table 1. The profile / design of the green porcelain insulator was in accordance with the requirements of IEC 60815-2 and IS 731. The compositions of the hydrophobic glaze samples are compared with the composition of a conventional regular porcelain glaze (RPG) in Table 3. While H1, H2 and H3 reflect tailoring of constituent raw materials, the modification in process parameters 15 based on H3 is reflected in H4 and H5, since H3 showed better properties than H1, H2 and RPG. Particle size distribution (=10µm) above and below the percentage used in H3 were used for H4 and H5 respectively. The particle size distribution for H4 was maintained to 76 – 80% of =10µm, while the particle size distribution for H5 was maintained to 56 – 67% of =10µm. 20
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[18] For H1, the amount of pigments were reduced, as compared to RPG, in order to reduce melting. For H2, H3, H4 and H5 no pigment was used in order to reduce melting. H3, H4 and H5 has 2% by weight zinc oxide unlike RPG, H1 and H2.
Table 3: Compositions of sample glazes H1 to H5 versus RPG 5
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[19] The raw materials for the hydrophobic glaze were ground in a ball mill or pot mill in water media. It was found that 56 to 67% of the constituents should have a particle size of =10µm to maintain preferred particle size distribution, as shown in H5. Using percentages lower than 56% could compromise the strength of the glaze. Using very high percentages of constituents having particle size of =10µm results in 15 smooth finish with fewer air gaps. Using percentages of 56% to 67% gives a slightly
Raw materials
Existing glaze (RPG)
Sample glaze
H1
H2
H3
H4
H5
Quartz, %
29.75
29.75
29.75
29.75
29.75
29.75
Feldspar, %
35
35
35
35.00
35.00
35.00
Dolomite, %
4.8
6.87
6.87
6.87
6.87
6.87
Zircon, %
2.4
2.4
2.40
2.40
2.40
2.40
Barium Carbonate, %
1.5
1.5
1.50
1.50
1.50
1.50
China Clay, %
9
10
10
10.00
10.00
10.00
Ball Clay, %
3.6
3.6
3.6
3.60
3.60
3.60
Alumina, %
1
1
1.00
1.00
1.00
1.00
Calcite, %
6.81
6.81
9.88
7.88
7.88
7.88
Manganese di oxide, %
3.34
1.67
0.00
0.00
0.00
0.00
Iron oxide, %
1.9
0.95
0.00
0.00
0.00
0.00
Chromium oxide, %
0.9
0.45
0.00
0.00
0.00
0.00
Zinc Oxide, %
0
0
0.00
2.00
2.00
2.00
Total
100.00
100.00
100.00
100.00
100.00
100.00
Particle size (=10µm), %
74.89
71.09
68.12
68.05
78.09
63.78
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rough finish to the glaze with more air gaps on the surface of the glaze which prevents water droplets from wetting the surface of the glaze.
[20] The hydrophobic glaze of the invention does not settle down when it is discharged above a density of 1.65 gm/cc. At this stage fluidity is quite low, at around 100 – 150 ?/swing. To make it workable, binder (in present case, carboxymethyl 5 cellulose, CMC) was added. However, addition of binder is optional. Once binder is homogeneously mixed with the hydrophobic glaze slurry, it is sieved through 60 - 200# sieve (having 0.25 – 0.75 mm openings). It is necessary to ensure that there should not be any iron particles in the composition. Generally permanent magnetic bar is used to remove iron particles. Porcelain samples are coated with this glaze via 10 any process like dipping or spraying. Glaze thickness should be kept in the range of 0.20 - 0.50mm. This glaze is fired in a reduction atmosphere in the temperature range of 1230 – 1280oC for 26 – 35 hrs. The fired insulator of the invention develops hydrophobicity of HC2 level as per IEC 62073. Water contact angle of 60-70o was obtained. 15
[21] The properties of the 5 sample glazes versus the RPG and standard properties are documented in Table 4:
Table 4: Properties
Properties
Standard
RPG
H1
H2
H3
H4 H5
Colour variation in all samples
No
No
Yes
No
No
No No
Flow of glaze at top (1246?C), mm
50 – 60
55.55
56.05
50.67
50.50
51.55 50.50
Flow of glaze at bottom (1242?C), mm
51 – 60
57.78
57.92
51.98
51.84
51.96 51.84
MOR (silica body), kg/cm2
=1200
1335
1356
1237.00
1296.00
1316 1266.00
MOR (Alumina body), Kg/cm2
=1600
1775
1795
1538.00
1542.00
1439 1439.00
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Compression of glaze on silica body, %
=25%
38.7
40.88
28.52
34.65
36.72 31.53
Compression of glaze on alumina body, %
=20%
34.43
35.95
16.48
16.69
9.01 9.01
Thermal expansion at 650?C
0.25 -0.36
0.34
0.34
0.37
0.37
0.3658 0.37
Hydrophobicity as IEC62073
HC 1- 3
HC 5
HC 4
HC 3
HC3
HC4 HC 2
Water mark over glaze surface
No
Yes
Yes
Little
Little
Little Little
Hydrophobicity on water mark
Yes
No
No
No
Yes
Yes Yes
Example 2: Hydrophobicity visual assessment
[22] Fig. 1 is a photographic representation of the hydrophobicity achieved with the 5 samples i.e. H1 to H5 versus that of RPG. The level of hydrophobicity of the glaze on the porcelain insulators’ surface as per the procedure of IEC62073 has been 5 shown for visual assessment in Fig. 1. No distinct droplets appear on the surface of RPG, but instead wetting of the whole surface occurs which is undesirable and indicates no hydrophobicity. In contrast, the droplets of water are seen on H2 to H5, with H5 being most preferable.
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Example 3: Pollution testing
[23] As sample H5 showed the most desirable properties, pollution testing was carried out for samples made with H5 glaze only. The hydrophobicity of H5 is enough for a porcelain insulator to pass through salinity level of 224kg/m3 in salt fog test conducted as per IEC60507. This is the required salinity level for porcelain insulators 15 to pass through for application in very heavily polluted areas according to IEC 60815. The Artificial pollution test was carried out at 420kV voltage level at the Central
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Power Research Institute, Hyderabad. It must be noted that the present invention will, nevertheless, work for all voltage levels. It must also be noted that, while the product tested in the present working example was a cap and pin type porcelain insulator, nevertheless the present invention will work for all types of porcelain insulators such as long rods, hollow and solid core insulators etc. The porcelain insulators with the 5 hydrophobic glaze of the invention passed the maximum pollution level of 224kg/m3 in salt fog test, whereas the wares with existing RPG glaze showed performance up to 160kg/m3 salt fog test.
Example 4: 10
[24] In order to differentiate between surface textures of RPG and H5 glaze, microstructural analysis has been carried out and the microstructural surface features are shown in Fig. 2. From microstructural observations, the difference seems to be crack formation on H5 glaze while RPG does not have any such feature. It must be noted that these cracks did not adversely affect properties or influence any of the test 15 results which are conducted for evaluating porcelain insulators as per the standards of IEC, ANSI and PGCI. It is possible that these cracks do not propagate in such a depth so as to degrade the insulators body.
[25] Porcelain insulators coated with both the glazes i.e. RPG and H5 passed under various standard electrical insulators testing like electro-mechanical strength (EMS), 20 thermos-mechanical performance test (TMPT), temperature cycle test, power frequency test as per IEC60383, impact test as per ANCI 29.2, steep wave front test as per PGCI, etc. Microstructural analysis till 100k could not reveal any crystal formation in any of the glaze.
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[26] The above examples are non-limiting. The invention is defined by the claims that follow: ,CLAIMS:1. A hydrophobic glaze for coating a shaped ceramic body of a porcelain insulator, wherein said glaze comprises the following constituents: 26 to 30% by weight quartz, 27 to 35% by weight feldspar, 5 to 8% by weight dolomite, 1.5 to 3.5% by weight 5 zircon, 1.5 to 5% by weight barium carbonate, 10 to 17% by weight China clay, 2 to 5% by weight ball clay, 0.5 to 2% by weight alumina, 5 to 14% by weight calcite and 0.25 to 2% zinc oxide.
2. The hydrophobic glaze as claimed in claim 1, wherein 56 to 67% of the constituents should have a particle size of =10µm. 10 3. A method of making a hydrophobic glaze-coated shaped ceramic body of porcelain insulator, the method comprising: a. grinding and mixing the following constituents with water: -26 to 30% by weight quartz,
-27 to 35% by weight feldspar, 15
-5 to 8% by weight dolomite,
-1.5 to 3.5% by weight zircon,
-1.5 to 5% by weight barium carbonate,
-10 to 17% by weight China clay,
-2 to 5% by weight ball clay, 20
-0.5 to 2% by weight alumina,
-5 to 14% by weight calcite, and
-0.25 to 2% zinc oxide;
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b. spray coating the shaped ceramic body with or dipping the shaped ceramic body into the mixed and ground wet constituents prepared in step (a) to obtain a glaze-coated ceramic body; and c. firing the glaze-coated ceramic body of step (b) in a reduction atmosphere in the temperature range of 1230 to 1280oC for 26 to 35 hrs. 5
4. The method as claimed in claim 2, wherein 56 to 67% of the ground constituents should have a particle size of =10µm.
5. The method as claimed in claim 2, wherein the glaze coated on the ceramic body has a thickness in the range of 0.20 to 0.50mm 6. The method as claimed in claim 2, wherein carboxymethyl cellulose is added to the 10 mixed and ground wet constituents of step (a) prior to step (b) to obtain a glazing slurry.
7. The method as claimed in claim 2 or 6, wherein the mixed and ground wet constituents or the glazing slurry is passed through a sieve having 0.250 – 0.075 mm size openings prior to step (b).