ELECTRICAL INSULATING FLUID

Abstract: This invention relates to electrical insulating fluids especially for use in transformers and which have excellent low temperature performance. The electrical insulating fluid comprises esters with 17 to 20 carbon atoms formed by the reaction of a saturated monobasic carboxylic acid and a saturated monohydric alcohol. Suitable esters include n-decyl 2-ethyl hexanoate, 2-ethyl hexyl 3,5,5-trimethyl hexanoate, isodecyl 2-ethyl hexanoate, isodecyl isooctanoate, isodecyl 3,5,5-trimethyl hexanoate and n-octyl/n-decyl 2-ethyl hexanoate. In addition to the ester, the electrical insulating fluid may contain as an additive, a stabiliser against oxidation. Suitable additives include an antioxidant such as a hindered phenol and a metal passivator such as a benzotriazole or a derivative thereof.

Description: ELECTRICAL INSULATING FLUID This invention relates to electrical insulating fluids, more especially to such fluids for use in transformers (particularly railway traction transformers), and most especially to such fluids that have excellent very low temperature performance. The liquid filling in a transformer serves two important functions. It acts as a coolant and also as an insulant, and must possess the characteristics necessary to provide good performance in both these aspects, across the range of operational conditions the transformer will experience. It is also critical that the fluid should have good electrical properties. Due to the possibility of leaks, the environmental behaviour of such fluids is of increasing concern, and as a result, biodegradability and low toxicity are highly desirable in such fluids. Carboxylate esters are generally superior to hydrocarbons and silicones in this regard. Transformers used in railway locomotives which experience very cold weather conditions require excellent low temperature properties. In particular a pour point of below -50°C and a viscosity ideally less than 400cSt at -50°C. To achieve these characteristics a kerosene-base product has been used in the past. However, the flash point of kerosene is only 95°C, and the fire point is 105°C, and this presents a fire hazard since the windings in the transformer can reach 120°C. Carboxylate esters typically have much higher flash points than mineral oils. However, commercially available esters do not have sufficiently good low temperature properties. For example, di(2-ethylhexyl) adipate has a viscosity of 4920cSt at -54"C, di(2-ethylhexyl) phthalate has a viscosity of 1300cSt even at -10°C and a pour point of -30°C and dibutyl sebacate has a pour point at -15°C. Polyol esters (such as those based on pentaerythritol) commonly used in transformers have much too high viscosities at low temperatures (e.g. 24,000cSt at -40°C). It is clear that although these materials have greater resistance to flammability than mineral oils, their relatively high viscosities and pour points could result in solidification in the transformer, with the danger of formation of voids in the windings. This would result in dramatic loss of insulation with the resultant risk of flashover. Also, critical loss of heat transfer efficiency (due to high viscosity) could result when the ambient temperature drops to very low levels (such as in Arctic or Russian winter conditions). A number of patents (WO 95/15364, WO 93/23491, EP 386636, EP 374672, EP 374671 and EP 386638) describe a range of different monobasic carboxylate esters for use in oil- containing drilling emulsions where moderate low temperature performance is required (e.g. pour point <-15°C, and viscosity at 20°C <20cSt, in addition to having as high a flash point as possible >100°C and preferably >140°C - (WO 95/15364). These patents, however, do not consider any electrical applications, nor do they consider extreme low temperature performance. US 4519932 describes low temperature hydraulic fluids where carboxylate esters are mixed with a 2 cSt synthetic hydrocarbon (roughly 30:65 proportions) to give a fluid having a viscosity of <3000cSt at -54 β C. This patent does not consider electrical performance, and the viscosity of the resulting fluid mixture is well outside the set requirements for low temperature electrical insulating fluids, (ca. 400cSt at -50°C). FR 2282467 describes a dielectric liquid composition for condensers and transformers based on 2-ethyLhexyl 2-ethylhexanoate (all or in part). In particular a mixture of the ester with chlorobiphenyls is disclosed. The ester is claimed to be particularly superior (e.g. compared to dioctyl phthalate) in terms of thermal and chemical stability. Although pour point is measured at -70°C, low temperature viscosity is not given. Having a total of only 16 carbon atoms, this ester will have flammability characteristics that are only borderline, and would not give a sufficient safety margin in the operation of transformers (literature flash point of only 132°C). The present invention relates to the use of as an electrical insulating fluid certain monobasic carboxylate esters that have excellent low temperature properties, but also have improved flammability when compared to similar viscosity mineral oil. In particular flash points of at least 135°C and a fire point of at least 145 D C. For a new insulant to be considered as a significant advance on existing materials, the industry expects a flash point of at least 15°C greater than the temperature reachable by the windings. The esters have a viscosity <1200cSt at -50°C, preferably <800cSt and most preferably <400cSt at -50°C. Additionally, the esters have a pour point of <-30°C, preferably <-40 C and most preferably <-50°C. A balance has to be achieved between improving the fire safety of a fluid by having as high a flash point and fire point as possible, whilst retaining an acceptable viscosity at very low temperatures. We have found that only a narrow range of esters are suitable to meet these requirements. The range is defined by the total number of carbon atoms in the ester molecule. Esters with up to 16 carbon atoms will not have sufficient fire resistance. Esters of more than 20 carbon atoms will have too high a viscosity at very low temperatures. Thus, the present invention relates to the use of as an electrical insulating fluid, esters containing a total number of carbon atoms of from 17 to 20. Within this constraint on total number of carbon atoms, the molecular structural design will also critically influence performance. The most important of these are described as follows. Firstly, there has to be some degree of branching within the molecule, particularly in order to achieve the desired excellent low temperature properties. Secondly, both the amount and position of the branching can affect the properties of the resultant esters. It is impossible to predict accurately the precise effect of branching on key properties, but some general guidelines can be established: - (I) Some branching is necessary (to obtain good low temperature performance). (II) Branching can be on the alcohol or acid moiety or both. (III) Empirical observations show that the amount of branching should be from 10 to 50%, wherein the percentage is calculated as number of branched carbon atoms/total number of carbon atoms x 100%. (IN) Too much branching on the α-position of either acid or alcohol can lead to difficulty of manufacture (reactants becoming sterically hindered). Monobasic carboxylate esters are prepared from the reaction of an alcohol and an acid. To retain good oxidative stability, a further limitation is that the esters must contain no unsaturation, i.e. both the acid and alcohol starting materials must be fully saturated. In order to achieve an ester with a total carbon number of 17 to 20, a number of different combinations of acid and alcohol are theoretically possible. For optimum performance (electrical and low temperature) it is preferred to have the ester functionality near the centre of the molecule. Moreover, for a given molecular weight ester, flammability tends to decrease with increasing number of carbons in the alcohol moiety. As such a further limitation on the range of esters suitable is the choice of alcohols containing from 6 to 13 carbon atoms, preferably 8 to 10 carbon atoms, and choice of acids containing from 6 to 13 carbon atoms, preferably 8 to 10 carbon atoms, but giving a total of 17 to 20 carbon atoms in the esters. Commercially available alcohols include synthetic materials (e.g. derived from the OXO process). Examples include 2-ethylhexanol, isooctanol, 3,5,5-trimethylhexanol, isodecanol and tridecanol and also mixtures eg mixed Cl 1/C13 alcohol. Alternatively, linear naturally- derived alcohols can also be used such as n-octanol, n-decanol and n-dodecanol and also mixtures eg Lorol™ C8-C 10 Special (from Henkel). Similarly commercially available acids may be synthetic in origin, for example C7, 8, 9, 10, 11, 12, 13 Cekanoic™ acids (from Exxon) and mixtures thereof, and also n-heptanoic acid and 2-ethylhexanoic acid, Linear naturally derived acids are also suitable, for example capiylic acid, pelargonic acid, capric acid and lauric acid; in addition, mixtures eg Edenor™ V85 (C8/C10 fatty acid) (from Henkel). The flash and fire points of a carboxylate ester may be dramatically affected by volatile impurities, such as residual alcohol raw material left over from manufacture (especially for lower molecular weight alcohols). It is important that such impurities are minimised to limit adverse impact on the flash and fire points. As such, an important aspect of the present invention is that the ester can have no more than 500ppm alcohol impurities, preferably no more than 200ppm and most preferably no more than lOOppm. The resulting fluid may be made up of predominantly a single ester, mixtures of esters, or a mixed product made up by using isomeric raw materials (acids and/or alcohols). In addition to these properties it is also extremely important that an electrical fluid should have sufficiently high electric strength, low dielectric dissipation factor, high specific heat and thermal conductivity, low coefficient of thermal expansion, excellent chemical stability, low density; good lubricity, low solvent power; good arc-quenching properties and also be non- toxic and preferably also biodegradable. In particular electric strength should be at lest 40kV, preferably at least 50kV and most preferably at least 60k V. Dissipation factor (tanδ at 20°C) should be less than 0.1 and preferably less than 0.05. Impurities can also adversely affect electrical performance, in particular small amounts of moisture can have a dramatic effect. It is thus another important feature of the present invention is that the esters must be thoroughly dried before use such that the water content is less than 250ppm, and most preferably less than lOOppm. Similarly residual alcohol can have a deleterious effect on electrical performance, and thus the residual alcohol content should be <500ppm, preferably <200ppm and most preferably < lOOppm. Similarly residual acid can also have adverse affects on electrical performance and thus the acid value of the ester should be less than 0.05mg gKOH and preferably less than 0.03mg gKOH. It is also critical that the ester be stabilised against oxidation. This requires the use of a suitable additive package that can be optimised by those skilled in the art. Typically this might include an antioxidant (eg a hindered phenol such as bisphenol F (Ethyl 702). 2,6-di-t-butyl-p-cresol (DBC) and Benzenepropanoic acid, 3,5-bis (1,1- dimethylethyl)-4-hydroxy-,l,6-hexanediyl ester (Irganox 259)) and a metal passivator (eg a benzotriazole or a derivative thereof). It is very important that the additive package be selected with due regard to its impact on the electrical properties of the resulting fluid. The resulting fluid (ester + additive package) can also be used in related applications, for example, in traction transformers and especially in distribution transformers e.g. pole- mounted transformers. The esters of the invention may be made any of the standard methods that are well-known in the chemical industry, most easily by the direct reaction of the acid with the alcohol (though an acid chloride may be used as a more reactive alternative to the acid) with an appropriate catalyst. For more detailed information refer to: Kirk-Othmer's Encyclopaedia of Chemical Technology 4th Edition, Volume 9, pp 755-780 or in Advanced Organic Chemistry by J March, pp 348-351 and references contained therein. Catalysts for the direct reaction can be inorganic acids, for example sulphuric acid, hydrochloric acid, phosphoric acid, phosphorous acid, hypophosphorous acid, phosphotungstic acid, boron trichloride etc; or organic acids eg p-tolueτ) e sulphonic acid, methane sulphonic acid, trifltc acid; or organometallic compounds, eg dibutyl tin oxide or tetrabutyl titanate; or heterogeneous acid catalysts such as cationic ion exchange resins, Nafion™ resin, or supported heteropolyacids. Examples Examples 1 to 9 (with no stabiliser) were prepared by a standard method (analysis given in Table 1) and evaluated for extreme low temperature performance and flammability. The results are described in Table 2. Table I Table 2 (1) Precipitation at -40°C, not as heavy as (2) (2) Precipitation at -40°C, approx 50% at -50°C (3) Light precipitation at -26°C, heavy at -50°C. All the examples above (except for 6) show good performance in low temperature fluid behaviour. Examples 3, 4, 5, 8, 9 (with no stabiliser) were further evaluated for electrical performance, and the results are given in Table 3. Table 3 P = Permittivity Tan Δ - Dielectric dissipation factor V.R. - Volume resistivity BDN = Breakdown voltage (IEC 156) Examples 3, 4, 8 and 9 (with no stabiliser) display good electrical properties. Example 5 is less good and demonstrates the importance of removing residual alcohol impurities. Although not shown in the Tables above, n-octyl n-decyl 2-ethyl hexanoate also gave good results. The ester of Example 8 containing 0.5% by wt. of the stabiliser 2,6,2' _6'-tetra-t-butyl bisphenol F and 0.01% by wt. of the metal passivator benzotriazole was tested for electrical performance under IEC 247 conditions at 90°C and exhibited a Permittivity of 3 and Volume Resistivity of 14 which shows that the V.R. of the ester is not adversely affected by the inclusion of the stabilisers.

Claims: CLAIMS 1. The use as an electrical insulating fluid of an ester obtainable by the reaction of a saturated monobasic carboxylic acid and a saturated monohydric alcohol wherein the resulting ester contains from 17 to 20 carbon atoms. 2. The use as claimed in claim 1 wherein the ester has a degree of branching from 10 to 50% where the branching is calculated as the number of branched chain carbon atoms/the total number of carbon atoms x 100%. 3. The use as claimed in claim 2 wherein the branching is on the alcohol moiety or the acid moiety or both. 4. The use as claimed in any of the preceding claims wherein the ester is a mixture of at least two components. 5. The use as claimed in any of the proceeding claims wherein the acid component of the ester contains from 6 to 13 carbon atoms. 6. The use as claimed in claim 5 wherein the acid component of the ester contains from 8 to 10 carbon atoms. 7. The use as claimed in any of the preceding claims wherein the alcohol component of the ester contains from 6 to 13 carbon atoms. The use as claimed in claim 7 wherein the alcohol component of the ester contains 8 to 10 carbon atoms. 9. The use as claimed in any of the preceding claims wherein the acids and alcohols are selected from synthetic acids and alcohols obtained from the OXO process and linear acids and alcohols derived from naturally occurring sources and also 2-ethyl hexanol and 2-ethyl hexanoic acid. 10. The use as claimed in any of the preceding claims wherein the ester has a moisture content of less than 250 ppm. 11. The use as claimed in claim 10 wherein the ester has a moisture content of less than lOO ppm. 12. The use as claimed in any of the proceeding claims wherein the ester has a residual alcohol content of less than 500 ppm. 13. The use as claimed in claim 12 wherein the ester has a residual alcohol content of less than 200 ppm. 14. The use as claimed in claim 12 or 13 wherein the ester has a residual alcohol content of less than 100 ppm. 15. The use as claimed in any of the preceding claims wherein the ester has an acid value of less than 0.05 mg/g KOH, 16. The use as claimed in claim 15 wherein the ester has an acid value of less than 0.03 mg/g KOH. 17. The use as claimed in any of the preceding claims wherein the ester has a flash point ofat least l35°C 18. The use as claimed in any of the preceding claims wherein the ester has a fire point of at least 145°C. 19. The use as claimed in any of the preceding claims wherein the ester has a viscosity of up to 1200cSt at -50°C. 20. The use as claimed in claim 19 wherein the ester has a viscosity of up to 800cSt at -50°C. 21. The use as claimed in claim 19 or 20 wherein the ester has a viscosity of up to 400cSt at -50°C. 22. The use as claimed in any of the preceding claims wherein the ester has a pour point below -30°C. 23. The use as claimed in claim 22 wherein the ester has a pour point below -40*C. 24. The use as claimed in claim 22 or 23 wherein the ester has a pour point below -50°C. 25. The use as claimed in any of the preceding claims wherein the ester has a breakdown voltage of at least 40kV. 26. The use as claimed in claim 25 wherein the ester has a breakdown voltage of at least 60kV. 27. The use as an electrical insulating fluid of n-decyl 2-ethyl hexanoate, 2-ethyl hexyl 3,5,5-trimethyl hexanoate, isodecyl 2-ethyl hexanoate, isodecyl isooctanoate, isodecyl 3,5,5-trimethyl hexanoate or n-octyl/n-decyl 2-ethyl hexanoate. 28. An electrical insulating fluid comprising an ester as specified in any of claims 1 to 27, an antioxidant and/or a metal passivator. 29. An electrical insulating fluid as claimed in claim 28 wherein the antioxidant is a hindered phenol. 30. An electrical insulating fluid as claimed in claim 29 wherein the hindered phenol is 2,6,2',6 l -tetra-t-butyl bisphenol F, 2,6-di-t-bυtyl-p-cresol or Benizenepropanoic acid, 3 ,5-bis (1,1 -dimethylethyl)-4-hydroxy-, 1 ,6-hexanediyl ester. 31. An electrical insulating fluid as claimed in claim 28 to 30 wherein the metal passivator is benzotriazole or a derivative thereof. 32. The use of the electrical insulating fluid of any of claims 28 to 31 in transformers. 33. The use as claimed in claim 32 in traction transformers and distribution transformers. 34. The use as claimed in claim 32 or 33 down to a temperature of -50 P C.

Inventors: HILL JONATHAN SIMON, PHILLIPS WILLIAM DAVID, HILL, JONATHAN, SIMON, PHILLIPS, WILLIAM, DAVID
Applicants: GREAT LAKES CHEMICAL CORP, HILL JONATHAN SIMON, PHILLIPS WILLIAM DAVID, GREAT LAKES CHEMICAL CORPORATION, HILL, JONATHAN, SIMON, PHILLIPS, WILLIAM, DAVID
Application year: 1999
Main classification: C09K7
IPC codes: C10M105