MXPA98007633A - Composition resistant to tracking and eros - Google Patents

Abstract

A composition resistant to tracking and erosion includes (a) 100 parts by weight of organopolysiloxane, (b) between 1 and 15 parts by weight of magnesium oxide, (c) between 15 and 45 parts by weight of zinc oxide, and (d) between 5 and 40 parts by weight of iron oxide

Description

BACKGROUND AND EROSION RESISTANT COMPOSITION TECHNICAL FIELD OF THE INVENTION This invention provides scouring and erosion resistant compositions, especially organopolysiloxanes, which are suitable as high voltage insulation materials and which can be easily molded into complex shapes. BACKGROUND OF THE INVENTION Two factors that affect the performance of a high-voltage insulating material are its resistance to tracking and its resistance to erosion. Tracing refers to the formation of driving routes (trace) of the deteriorated material on the surface of the insulating material, caused by superficial electric discharges. A tracking fault occurs when a trace joins the gap between two or more conductors, leading to an electrical fault. Erosion refers to the progressive wear away from the insulating material by the electric discharges, with the failure eventually occurring because much of the insulating material has worn away. Organopolysiloxanes (also called silicones) have high-voltage insulating materials, due to their electrical properties, processability (including moldability), physical properties, chemical inertness and other convenient characteristics. The organopolysiloxanes used in high voltage equipment usually contain additives to improve their resistance to tracking and / or erosion (hereinafter referred to as anti-tracking additives).

A well-known anti-tracking additive is alumina hydrate (also referred to as aluminum hydroxide, alumina trihydrate, hydrated alumina or ALTH) alone in combination with other additives such as metal oxides. Illustrative descriptions that relate to alumina hydrate include Elliot, E.U.A. 3,965,065 (1976); Penneck, E.U.A. 3,969,308 (1976); Penneck, E.U.A. 4,001,128 (1977), Cammack, II et al., E.U.A. 4,100,089 (1978); Penneck et al., E.U.A. 4,189,392 (1980); Penneck, E.U.A. 4,399,064 (1983); Clabburn et al., E.U.A. 4,431,861 (1984); Penneck, E.U.A. 4,521,549 (1985); Adkins, E.U.A. 4,822,830, and Kunieda et al., E.U.A. 5,369,161 (1994). It has also been proposed to use alumina hydrate as an additive for other purposes, such as flaming enhancement or retardation: Bobear, E.U.A. 4,288,360 (1981). Alumina hydrate is sometimes combined with other additives, such as other metal oxides or phosphorus compounds. Another class of additives are the platinum compounds, which are normally used to improve flame retardancy: Laur et al., E.U.A. 3,635,874 (1972); Pfeifer et al., E.U.A. 3,711,520 (1973); Milbert, E.U.A. 3,821,140 (1974); Bargain, E.U.A. 3,839,266 (1974); Hatanaka et al., E.U.A. 3,862,082 (1975); Itoh et al., E.U.A. 3,936,476 (1976); Matusushita, E.U.A. 4,110,300 (1978); Bobear, E.U.A. 4,288,360 (1981); Ackermann et al., E.U.A. 4,419,474 (1983); and Derwent WPI Abstract No. 76-82267X / 44 (summary of JP-50-097644 (1975)). Again, other additives may be employed simultaneously, either for flame retardation or for other purposes such as heat stabilization or tracking / erosion resistance. Finally, several additives have been added to organopolysiloxane compounds, for various purposes ranging from those mentioned above, to thermal conductivity, reinforcement and stability at high temperatures. These additives include oxides of metals, silica and metal salts. Illustrative descriptions of these are: Koda et al., E.U.A. 3,884,950 (1975); Cole and others, E.U.A. 4,604,424 (1986); Szaplonczay et al., E.U.A. 4,897,027 (1990); Wolfer et al., E.U.A. 5,008,317 (1991); Bosch and others. E.U.A. 5,023,295 (1991); Mazeika et al., WO 95/06552 (1995): Rowe et al., EP 0,218,461 A2 (1987); and GB 1,538,432 (1979). In summary, an organopolysiloxane used as a high voltage insulating material may contain a package of complex additives. An organopolysiloxane can be formed into a complex form for a particular end use. For example, the WO'552 of Mazeika mentioned above describes a high voltage insulator having a covered organopolysiloxane housing which has been molded by a process leaving non-longitudinal molding lines (which are more susceptible to tracking failure). The combination of the critical placement of the molding lines and the complex shape of the molded part makes strict demands on the organopolysiloxane. It should flow easily enough to properly fill the mold cavity and, after molding, sufficiently docile to be demolded. However, many anti-tracking additives of the prior art interfere with the molding process for one reason or another. The alumina hydrate makes it difficult to demold the molded part, especially at load levels required to effectively improve the anti-tracking resistance, about 15 parts per 100 weight or more usually greater than 100 phr in commercial modes. In our experience, alumina hydrate levels greater than 75 phr make demolding more difficult. Many moldable organopolysiloxane compositions are cured (entangled) in the mold via a vinyl-hydride addition reaction.

-Si- H + H2C = CH2-S¡- -Si- CH2- CH2- Si- The cure can be effected by a platinum catalyst, such as hexachloroplatinic acid. The amounts of platinum in excess of those used for curing can use an additive; however, in cases where the platinum additive can also catalyze the healing reaction, leading to premature or roasting. Another disadvantage of platinum as an additive is its high cost. In addition, there are increasing demands for performance for high-voltage insulating materials, for example, where power generation or distribution plants are built in pollutant coastal areas, places where tracking and erosion are particularly severe problems. Consequently, there is a need for more effective anti-tracking packages that do not exhibit the limitations of the prior art packages. SUMMARY OF THE INVENTION The present invention provides an organopolysiloxane composition resistant to screening and erosion that is easily molded into articles of complex shape. Many of these compositions also have tracking and / or erosion resistance properties that are notoriously superior to those of the prior art. Said composition comprises (a) 100 parts by weight of organopolysiloxane; (b) between 1 and 15 parts by weight of magnesium oxide; (c) between 15 and 45 parts by weight of zinc oxide; and (d) between 5 and 40 parts by weight of iron oxide. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a suitable molding for molding an insulator of complex shape made from a composition of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The organopolysiloxanes used in this invention have a repeating unit predominantly of the structure -O- Si- I 2 wherein Ri and R2, which may be the same or different, are monovalent hydrocarbon or halogenated hydrocarbon radicals having from 1 to 30 carbon atoms, such as methyl, ethyl, propyl, 3,3, 3-trifluoropropyl and phenyl, with methyl being preferred. A preferred organopolysiloxane is polydimethylsiloxane, that is to say that R ^ and R2 are each a methyl. For curing, the polymer contains reactive functional groups, such as vinyl and hydride, placed in terminal positions in the polymer chain or at the branch point within the polymer chain. The healing chemistry is usually catolized by platinum or peroxide. Said organopolysiloxanes are well known and are available in a large selection of grades from many suppliers such as gums Q44758 and Q44758 available from Dow Corning STI, preferably mixed with a Shore A hardness tester (ASTM D2240) of between 45 and 60. The present invention is especially suitable for organopolysiloxanes which are cured by a hydride addition chemistry with platinum catalysts. After curing (3 minutes at 332.2 ° C, 4 hours after curing at 382.2 ° C) the organopolysiloxane preferably has an elongation greater than 400%, when tested in accordance with ASTM D412 at 50.8 cm / min. The breaking strength of Die B is preferably greater than 1785.0 kg / m, preferably more than 2320.5 kg / m, more preferably greater than 2377.5 kg / m. We have discovered that magnesium oxide works synergistically and inexplicably with zinc and iron oxides to provide exceptional tracking and erosion resistance, without adversely affecting demolderability, as might be the case with ALTH. The amount of magnesium oxide should be between 1 and 15, preferably between 2 and 7, parts by weight per 100 parts by weight of organopolysiloxane (phr). A preferred magnesium oxide has the form of a powder at least 94.5% pure, with at least 99% passing through a sieve of 325 mesh size and an average surface area greater than 32m2 / g. The amount of zinc oxide present preferably is between 15 and 45, preferably between 15 and 25 phr. A preferred zinc oxide has the form of a powder at least 99.63% pure, with at least 99.99% passing through the mesh size sieve of 325 and an average surface area of at least 7.5 m2 / g. Among iron oxides, red iron oxide (Fe203) is preferred. The amount of the iron oxide is preferably between 5 and 40, preferably between 5 and 20 phr, and more preferably between 5 and 10 phr. The iron oxide is conveniently added from a master batch in an organopolysiloxane gum. Preferably, the screening and erosion resistant compositions of this invention consist essentially of the organopolysiloxane (including any catalyst required for the curing process), the first oxide material, and the second oxide material. However, as long as they do not materially affect the basic and novel features of this invention, other additives may be present, for example, fillers (reinforcement or without reinforcement), stabilizers, thermally conductive fillers, flame retardants and pigments. Other exemplary specific additives include titanium dioxide, cerium oxide, aluminum hydrate, fumed silica and carbon black. Preferably, the composition is free of platinum, except that it is needed as a catalyst for the curing reaction. Generally, the amount of platinum required for the catalysts is between 15 and 30 parts by weight per parts per million by weight of organopolysiloxane (parts per million or ppm). As previously noted, alumina trihydrate has a tendency to adversely affect the moldability of the organopolysiloxane, especially at the high loads required for the maximum anti-tracking effect. While some alumina trihydrate may optionally be added to the composition of the present invention, no particular benefit accompanies said addition, and in addition, its addition in amounts greater than 75 phr is undesirable from a moldability point of view and should be avoided. Preferably, a composition of this invention is essentially bound to anti-tracking additives or other than the aforementioned magnesium oxide, zinc oxide, and iron oxide. A filler such as Minusil ™ silica can be present in an amount of about 5 to 20 phr (parts by weight per 100 parts by weight of organopolysiloxane). The resistance to tracing and erosion of an insulating material can be evaluated quantitatively by the procedure published in ASTM D2303-90. Basically, this method measures the resistance of an insulating material to voltage voltages along its surface when it is moistened with an electrically conductive ionizable liquid pollutant, conditions that stimulate exposure to dirt and atmospheric moisture condensed during service. The test method is of the inclined plane type, in which a contaminating solution is dropped onto a test specimen held at a 45 ° angle while a voltage is applied simultaneously. A voltage is applied in increments of 250 V, with a holding period of 1 hour in each increment (unless a failure is indicated). The time and voltage in which the fault occurs is observed. The following procedure is representative: the sample is scraped with 400A sand silicon carbide paper and rinsed with distilled water. The initial tracking voltage method of ASTM D2303 was followed, starting at 2.5kV and a contaminant flow rate of 1.5 mL / min. The contaminant was 0.1% ammonium chloride. The voltage increased 0.25kV per hour, reaching 4.0 kV in the sixth hour. The voltage was kept constant at 4.0kV after both the tracking voltage and the failure time were recorded. The flow rate of the contaminant was changed according to Table I in ASTM D2303. The compositions of this invention have utility as insulating materials in high voltage electrical equipment, for example insulators, surge suppressors, elbows, unions, terminations, transformer bushings, fuse shorts and disconnect switches. As noted above, the compositions of this invention have excellent demolding ability, making them especially suitable for molded parts with complex designs, such as coated insulators, particularly those that are free of longitudinal mold lines. Mazeika, WO 95/06552 (1995) describes a molding process for forming said insulators, the description of which is incorporated herein by reference. In this process, the elastomeric properties of the material allow the use of molding plates which are joined substantially perpendicular to the longitudinal axis of the part, allowing the curing of parts where the flammable material of the mold line is along the periphery of the mold. the ends of the covers reducing the need for damping and cycle time when it leads to increased tracking resistance. In addition, compositional variations of the in-line composition of the mold on the limbs have little or no effect on the performance of the cover. The elastomeric molding material allows the radially walled, multiplied, molded part to jump through the mold plates especially upon application of a vacuum to the internal structure of the tubular molded part during mold extraction. The molding process can further be understood by referring to Figure 1, which shows a mold comprising plates 1, 2, 3, 4, 5 and 6 and a tubular forming insert 50 exiting plate 6. The covers or at least the provisions of the cover of the molded part are illustrated in the elements 10, 20, 30 640 of Figure 1, while the tubular nature is derived from the insert 50. The mold lines or pulse points for the material instead of being along the longitudinal axis of the part occurs between 11 and, 21, 23 and 31, 33 and 41, 43 and 51, respectively, when closing the mold and injecting the elastomeric composition. During the molding operation, plates 1, 2, 3, 4, 5 and 6 are joined with sufficient heat, time and temperature pressure for the injection and curing of the elastomeric material. The molding process can be carried out with a 165 ton Engel injection molding machine with vertical rollers or similar machines. The Engel 165 machine was modified to inject the part from one side of the mold as well as the need to move the plates. The reason for this modification is that the segment / free space of the machine opening between the rollers when fully opened must provide space for the operator or robot between at least two plates such as plate 2 and plate 3 as well as the plate 1 and plate 2 so that they can remove the plate part 2. Other equipment similar to a shuttle press or a rotary press can be used and is preferred for manufacturing higher volumes mainly due to the ease of adding more plates to the mold or more radial cover elements or other details and reduce wear on the mold pins. The support for the mold could be very simple on a rotating shuttle table while providing greater capacity to open the plates and remove the part. The barrel of the molding machine is heated to approximately 49 ° C and the barrel temperature can be adjusted up or down depending on the temperature of the mold as well as the size of the part and the gate / vent of the mold of 15 ° C. A suitable molding time is a 2-minute cycle from clamping to release until re-clamping, ie from closing the mold by injecting the material, molding the material and opening the plate, removing the finished part and reclosing to start a new cycle. The cycle time depends more specifically on the temperature of the barrel, the position used in the mold and the temperature of the mold. During the actual molding operation, a suitable molding temperature is from about 149 ° C to about 204 ° C, preferably from 160 ° C to 193 ° C and more preferably from about 182 ° C. Having described the overall sequence, the following description follows the molding cycle once the mold plates are closed and pressed. With a closed mold, the preferred elastomeric material of the invention is injected and has an average cure time of about 1 min uto. Then, the mold opens between the plates 5 and 6 and the core attached to the plate 6 is pulled from the part. The rest of the mold is held together using bolts and an injector system on one side of the press as well as springs. Subsequently, the mold plates 4 and 5 are opened via a spacer pin to release these sections. After this operation, the molding plates 3 and 4 are opened, which releases the section of the part and collapses in a space where the core is removed and pulled through plate 4. After the part it is pulled through plate 4, the mold is opened apart between plates 3 and 2 and the part is pulled through plate 3 as in the previous step. The mold opening plates continue for plates 1 and 2 with the mold in a fully open position, the part is on the side of the plate facing plate 3 at which point the operator or robot of the mold is able to remove the part between the open plates 2 and 3. The present invention can also be understood by reference to the following examples, which are provided by way of illustration and not limitation. Table I provides illustrative formulations according to the invention, together with comparative examples that are not in accordance with this invention. Samples were prepared by mixing in a 3-liter sigma spatula mixer at room temperature. The gums were pre-mixed until they became homogeneous. The fillers and additives were then added and mixed until well dispersed. Formulas 1 to 4 are according to this invention, Formulas 1 to 3 being examples of platinum cured systems and Formula 4 being an example of a peroxide cured system. Formulas 5 to 9 are comparative examples that do not agree with the example. In Formula 5, the amount of magnesium oxide is very low. In Formula 6, the amount of magnesium oxide is greater than that prescribed. In Formula 7, there is no iron oxide. In Formula 8, there is no magnesium oxide. In Formula 9, there is no zinc oxide. Tabia I I provides the tracking and erosion data for the formulas in Table I.

TABLE 1 Formula Formula Formula Formula Formula Formula Formula Formula Formula Component 1 2 3 4IX 5 6 7 8X 9X Silicone Base 1 (pep) at 100 100 100 100 100 100 100 100 100 100 Silicone Rubber 11 (pep) at 3 3 3 3 3 3 3 3 3 Inhibitor "1 (pep) to 0.3 0.3 0.3 0 0.3 0.3 0.3 0.3 0.3 Hydride interleaver (pep) to 4.75 2.2 2.2 0 2.2 2.2 4.75 4.75 2.2 Total silicone (pep) b 115.55 113 115.55 110.5 113 110.5 115.55 113.05 110.5 Platinum catalyst (phr) c 0.39 0.31 0.18 0. 0.40 0.41 0.41 0.13 0.14 Oí Silica (pep) at 10 10 10 10 10 10 10 10 10 Iron oxide Lot Maestrovl (pep) at 15 15 15 15 15 10 10 10 10 Iron oxide real (phr) d 6.49 6.64 6.49 6.79 6.64 4.52 4.52 4.42 4.52 Zincv oxide "(pep) a 25 25 25 25 25 25 25 15 0 (phr) c 21.64 22.12 21.64 22.62 22.12 22.12 22.62 13.27 0 Magnesiovl oxide" (pep) b 3 7 3 3 1 1 20 0 2.5 (phr) a 2.60 6.19 2.60 2.71 0.88 0.88 18.10 0 2.26 'Q44758 and Q44768, from Dow Corning STI' SQM35, from Dow Corning STI II "I IV ETCH, Dow Corning STI No.63570, Dow Corning STI vMinusil ™ SM VI 50% Silicone Rubber Mapico 567A / 50% Oxide red iron vllNo.20553-2, Aldrich Chemical VIII, lagite K, Calgon lxAdditionally contains peroxide Varox xAdditionally contains 10 pbw titanium oxide aParts in weight "Total silicone base plus silicone gum plus inhibitor plus hydride interlayer plus half of the master batch of iron oxide. cParts in weight per hundred parts by weight of total silicone dPartes by weight of red iron oxide in master batch per hundred parts by weight of total silicone.

TABLE II Formula Formula Formula Formula Formula Formula Formula Formula Formula Formula Property Measure 1 2 3 4 5 6 7 8 9 Initial tracking voltage (kV) 4.00 4.00 4.00 3.00 4.00 3.00 3.50 4.00 4.00 Tracking erosion time (min) Low value3 840 840 840 156 462 143 288 442 477 High value 840 840 840 719 840 840 840 840 840 Average of 10 samples 840 840 840 432 802 359 417 781 687 Erosion (% lost weight) Low value 0.41 0.24 0.50 0.91 1.19 0.50 0.60 0.51 0.69 High value 0.67 0.76 3.10 4.33 1.99 3.10 1.52 3.60 5.95 Average of 10 samples 0.55 0.40 1.56 1.85 1.53 1.59 1.04 1.32 2.67 'Test discontinued at 840 min when the samples still did not fail.

It is noted that the formulations according to this invention consistently exhibited high tracking erosion times, greater than 840 minutes. Conversely, the comparative formulas achieved a tracking time of 840 minutes occasionally, but did not do so consistently. Since the consistent performance of high voltage materials is important, the superiority of the compositions of the present invention was demonstrated. The above detailed description of the invention includes all appropriate combinations of information found in different passages. Similarly, although different descriptions thereof refer to specific embodiments of the invention, it should be understood that when a specific aspect is described in the context of a particular embodiment, that aspect may also be used, to the appropriate degree, in the context of another embodiment, in combination with another aspect, or in the invention in general.

Claims (7)

1. CLAIMS 1. A composition resistant to tracing and erosion, characterized in that it comprises: (a) 100 parts by weight of organopolysiloxane; (b) between 1 and 15 parts by weight of magnesium oxide; (c) between 15 and 45 parts by weight of zinc oxide; and (d) between 5 and 40 parts by weight of iron oxide.
2. 2. A material resistant to tracking and erosion according to claim 1, wherein the amount of magnesium oxide is between 2 and 7 parts by weight.
3. 3. A material resistant to tracing and erosion according to claim 1, wherein the amount of zinc oxide is between 15 and 25 parts by weight.
4. 4. A material resistant to tracing and erosion according to claim 1, wherein the amount of iron oxide is between 5 and 20 parts by weight.
5. 5. A material resistant to tracing and erosion according to claim 1, which is essentially free of antirastreo additives other than magnesium oxide, ie zinc oxide and iron oxide.
6. 6. A material resistant to tracing and erosion according to claim 1, which is cured by a catalyzed addition reaction with platinum and vinyl hydride.
7. 7. A material resistant to tracing and erosion according to claim 1, which is cured with peroxide cure. RESU MEN A composition resistant to tracing and erosion includes (a) 100 parts by weight of organopolysiloxane; (a) 100 parts by weight of organopolysiloxane; (b) between 1 and 15 parts by weight of magnesium oxide; (c) between 15 and 45 parts by weight of zinc oxide; and (d) between 5 and 40 parts by weight of iron oxide.