Use of esters as dielectric fluids for electrical discharge machines

Abstract

The present invention relates to the use as a dielectric fluid for electrical discharge machines of an ester of formula (I) or formula (II) or a mixture of esters of formula (I) and formula (II).

Description

[0001] "USE OF ESTERS AS DIELECTRIC FLUIDS FOR ELECTRICAL

[0002] DISCHARGE MACHINES"

[0003] Cross-Reference to Related Applications

[0004] This Patent Application claims priority from Italian Patent Application No. 102024000000219 filed on January 8, 2024, the entire disclosure of which is incorporated herein by reference. Technical Field

[0005] The present invention relates to the use of an ester of formula (I) or an ester of formula (II) or a mixture of esters of formula (I) and formula (II) as a dielectric fluid for electrical discharge machines.

[0006] State of the art

[0007] The present invention is applied in the field of the electrical discharge machining (EDM) process, commonly used in metalworking.

[0008] This process is defined as an unconventional metalworking process, because it is based on a physical process that does not involve contact between the tool and the workpiece. The removal of excess metal is in fact the result of a series of controlled, non-stationary, high- frequency electrical discharges between a tool and a workpiece, which cause local micro-melting of metal. The space between the tool and the workpiece must be invaded by a special dielectric fluid, which performs several functions, such as electrical insulation, cooling, precise, repetitive and constant definition of the discharge area, and flushing of electro-ferrous metal slag.

[0009] The EDM process is divided into four basic operating technologies .

[0010] "Wire": used to generate profiles and / or actual cuts on metals. In this case, water can be used as dielectric fluid if in the presence of metals or hydrocarbon fluids if in the presence of hard metal.

[0011] "Die-sinking": used to generate complex volumetric shapes, without any constraints of a geometric or volumetric nature. In this case, petroleum-based dielectric fluids are currently used, never water.

[0012] "Micro-drilling by EDM": with water to generate micro holes for which a high degree of dimensional accuracy is not required or with a hydrocarbon dielectric to generate holes with high dimensional accuracy. "Grinding by EDM": for machining PCD, industrial diamond.

[0013] With the exception of wire EDM and micro-drilling where mainly deionised water is used (except when high precision cutting needs to be achieved or hard metal needs to be machined for which so-called "oil" technology must be used), the dielectric fluid has always consisted of light hydrocarbon fractions and the evolution has mostly been in the area of purity improved over the years by increasingly refined distillation and separation processes and consequently in the ever-widening range of viscosimetric grades.

[0014] Therefore, over the years, there has been a shift from the use as a dielectric fluid of dark, foul-smelling kerosene with indefinite and inconsistent viscosity, rich in aromatic compounds, to light petroleum bases consisting of colourless, odourless, practically aromatic-free N-Paraffins or GTL (Gas to Liquid) with very narrow distillation intervals and thus stable viscosities over time.

[0015] However, these fluids are still hydrocarbons, which are therefore polluting and do not always perform satisfactorily.

[0016] Summary It is thus an object of the present invention to provide the use of a dielectric fluid in EDM that effectively performs the functions of electrical isolation, cooling, defining the discharge area, and washing out electro-ferrous metal slag, while at the same time having the lowest possible environmental impact.

[0017] This object is achieved by the present invention as it relates to the use as defined in claim 1.

[0018] Brief description of the Figures

[0019] Figure 1A illustrates a dielectric strength comparison graph (IEC 60156 method) between a hydrocarbon fluid and a dielectric fluid according to the present invention (Synthetic Fluid 1).

[0020] Figure IB illustrates a dielectric strength graph (IEC method 60156) of a second embodiment of the dielectric fluid according to the present invention.

[0021] Figure 1C illustrates a graph of the dielectric strength of a third embodiment of the dielectric fluid according to the present invention.

[0022] Figure 2 illustrates the results of metallographic tests conducted in comparison between hydrocarbon-based EDM fluid and fluid according to the first embodiment of the invention.

[0023] Detailed description of the invention According to the present invention, an ester of formula

[0024] (I)

[0025] ((I) wherein R is a linear or branched C4-C12 alkyl of C4- C12 monocarboxylic acid or is a linear or branched C4-C6 alkyl of C4-C6 polycarboxylic acid and R' is 2-ethylhexyl of a 2-ethylhexanol esterified with C4-C12 monocarboxylic acid or C4-C6 polycarboxylic acid, or 1-octyl of a 1-octanol esterified with C4-C12 monocarboxylic acid or C4-C6 polycarboxylic acid; is used as a dielectric fluid for electrical discharge machines.

[0026] R is preferably selected from the group consisting of butyl (nC4Hg), isobutyl (iC4Hg), tert-butyl (tC4Hg), pentyl (nCsHu) hexyl (nCeHis), heptyl (nCgHis), isoheptyl (iCgHis) octyl (nCsHi?), isooctyl (nCsHi?) and nonyl (nCgHig).

[0027] R' is 2-ethylhexyl (CH3CH2CH2CH2CH (CH2CH3)CH2) or octyl (nC8Hi7).

[0028] According to the present invention, the ester, used as a dielectric fluid for electrical discharge machines, can also be an ester of f wherein

[0029] R2is a linear or branched C2-C12 alkyl,

[0030] R3, R4, R5, R6and R7, the same or different from one other, are selected from the group consisting of hydrogen; hydroxide or hydroxide esterified with linear or branched C2-C12 monocarboxylic acid, or with linear or branched C2- C12 polycarboxylic acid; C1-C4 hydroxy alkyl or C1-C4 hydroxy alkyl esterified with linear or branched C2-C12 monocarboxylic acid, or with linear or branched C2-C12 polycarboxylic acid; linear or branched C1-C15 alkyl, and n ranges between 0 and 8.

[0031] In detail, R2represents the residue of linear or branched C2-C12 monocarboxylic acid, or linear or branched C2-C12 polycarboxylic acid, which participates together with the polyol in the esterification reaction, described later.

[0032] For example, R2is preferentially selected from the group consisting of butyl (nCiHg), isobutyl (iC4H9), tertbutyl (tC4Hg), pentyl (nCsHn) hexyl (nCeHis), heptyl (nCvHis), isoheptyl (iCvHis) octyl (nCsHi?), isooctyl (nCsHi?) and nonyl (nC9Hi9).

[0033] For example, R3, R4, R5, R6e R7, which are the same or different, are selected from the group consisting of 2- ethylhexyl (CH3CH2CH2CH2CH (CH2CH3)CH2), octyl (nC8Hi7), nonyl (nC9Hi9), isononyl (iC9Hi9), isodecyl (iCioHgi) and isotridecyl (1C13H25)• Also according to the present invention, the ester, used as a dielectric fluid for electrical discharge machines, can alternatively be a mixture of esters of formula (I) and formula (II). The method for producing these esters is known in the art.

[0034] A specific example of a production method of the ester of formula (I) is as follows.

[0035] Alternatively, a specific example of the synthesis of the ester of formula (II) and the mixture of esters of formula (I) and (II) is as follows:

[0036]

[0037] According to the specific example illustrated above, the ester of formula (II) is pentaerythrityl tetravalerate wherein R3and R4are hydrogen; R5, R6and R7are Cl alkyl hydroxyl esterified with valeric acid and n equals 1.

[0038] According to the specific example above, the mixture of esters consists of pentaerythrityl tetravalerate and 2- ethylhexyl valerate.

[0039] For example, the mixture of esters consists of 20% by mass of pentaerythrityl tetravalerate and 80% of 2- ethylhexyl valerate by mass of the total mass of this mixture.

[0040] Depending on the type of EDM, it is preferable to use a different ester.

[0041] In particular, for die-sinking EDM, the ester-based fluid that will be referred to as Synthetic Fluid 1 or the fluid that will be referred to as Synthetic Fluid 2 or the fluid that will be referred to as Synthetic Fluid 4 is preferable. For wire EDM, Synthetic Fluid 1 is preferable.

[0042] For micro-drilling by EDM, Synthetic Fluid 1 is preferable. For grinding by EDM, ester-based fluid is preferable, which will be referred to as Synthetic Fluid 3.

[0043] The dielectric fluid, called Synthetic Fluid 1, consists of ester formulated with 2-ethylhexanol or 1- octanol having the following formula.

[0044] Synthetic Fluid 1: C4, C5 and C6 monocarboxylic acids esterified with 2-ethylhexanol or 1-octanol

[0045] As illustrated in the following examples, this molecule - which is totally different from the hydrocarbons used in the prior art - performed equally well in the EDM process. In some cases, even better. The viscosity of this dielectric fluid is 2.7 cSt at 20 °C.

[0046] To meet market demands, a second molecule was also used. Internationally, the most widely used hydrocarbon fluid has the highest viscosity. Therefore, it is necessary to provide a dielectric fluid of equal viscosity to top up the installed fluid until complete replacement. In this way, erosion times will also remain unchanged as the viscosity will be the same as the hydrocarbon product in operation. The dielectric fluid, called Synthetic Fluid 2, has the following formula.

[0047] Synthetic Fluid 2: C4, C5, C6, C7 , C8 and C9 monocarboxylic acids esterified with 2-ethylhexanol or 1-

[0048]

[0049] The viscosity of this dielectric fluid is 3.6 - 4 cSt at 20 °C.

[0050] The dielectric fluid, called Synthetic Fluid 3, has the following formula:

[0051] Synthetic Fluid 3: CIO, Cll and C12 monocarboxylic acids or C4, C5 and C6 polycarboxylic acids esterified with 2- ethylhexanol or 1-octanol

[0052]

[0053] The viscosity of this dielectric fluid is 6.8 - 9 cSt at 40 °C.

[0054] The dielectric fluid on which the mixture of esters of formula (I) and formula (IT) is based, called Synthetic Fluid 4, has a viscosity of 3.6 - 4.4 cSt at 20 °C.

[0055] For example, the mixture of 20% by mass pentaerythrityl tetravalerate and 80% 2-ethylhexyl valerate has a viscosity of 3.84 cSt at 20 °C. Examples

[0056] Experimental tests were performed to evaluate the performance of different dielectric fluids with respect to a hydrocarbon-based dielectric fluid according to the prior art . The chemical-physical characteristics of the fluids subjected to laboratory tests and subsequent processing tests are summarised in the table below:

[0057] * 20% by mass of pentaerythrityl tetravalerate and 80% of 2- ethylhexyl valerate.

[0058] • according to the ASTM D92-90 method

[0059] With regard to Synthetic Fluid 4, further chemical-physical properties are indicated:

[0060] - acidity number of 0.05 mgKon / g according to the AOCS 3d-63 method

[0061] - density at 20 °C of 0.895 g / ml according to the ASTM method D1298-99

[0062] - combustion point of 131 °C according to the ASTM D92- 90 method

[0063] - electrical permittivity of 3.32 according to the IEC 60247:2004s method

[0064] - distillation characteristics shown in the table below according to the ASTM D86-23 method

[0065] Example 1 - Dielectric Strength

[0066] Dielectric strength tests (method IEC 60156), designed to measure the "DISCHARGE VOLTAGE STRENGTH" of the products according to the invention, were conducted in parallel with a hydrogenated hydrocarbon fluid of equal viscosity, using them separately in their pure state and adding known and increasing amounts of water.

[0067] The fluids according to the invention showed excellent insulating capacity with at least 10 times as much water as those of the hydrocarbon, as can be seen from the following tables and Figures 1A-1C.

[0068] Example 2 - Specific heat Specific heat measurements carried out in parallel with a hydrogenated hydrocarbon fluid of the same viscosity showed more consistent values in favour of the dielectric fluid according to the invention (Synthetic Fluid 1). Specific heat was measured with Setaram's C80 heat flow calorimeter; there is no standardised method for this determination. The data are shown in the table below.

[0069] The functionality aspect is highlighted by tests conducted by GFMS as described below in comparison with a competing fluid of approx. 3.8 cSt viscosity at 20 °C. It was also expected that, being a profoundly different chemical compound with respect to an N-paraffin, more polar, more complex, containing oxygen in the molecule, erosion times would be higher or however different.

[0070] Instead, erosion times were similar, in some cases better; the GFMS technicians realised how a variation in the machine's PLC setting could greatly improve this condition to the point of decreasing erosion times. The test was performed under the following conditions:

[0071] Machine: GFMS FORM 200 • Dielectric fluid tank capacity: 600 litres

[0072] • Copper electrode with 2 open cavities

[0073] • Objective Ra 20 VDI

[0074] • Modified pulse sequences (not reported by the manufacturer)

[0075] The results are shown in the following table.

[0076] Example 3 - Metallographic tests

[0077] Metallographic tests conducted in comparison between hydrocarbon-based EDM fluid and according to the invention (Synthetic Fluid 1) have shown that the latter significantly reduces the surface compound layer of steels subjected to the EDM process, thus improving the metallographic characteristics and consequently the quality of the manufactured product obtained. See Figure 2 and the following table.

[0078]

[0079] The values given in the table refer to the thickness in μm of the compound layer (white layer) measured with an electron microscope at magnifications of 500X to 1000X. The traces were made with 15 x 15 mm copper electrodes

[0080] 2 mm deep, according to frequency and power ramps set by the machine's CNC to achieve the desired surface roughness

[0081] (expressed in VDI).

[0082] Below is the conversion from VDI to pm:

[0083] VDI12 = 0.4 μm / VDI18 = 0.8μm / VDI24 = 1.6μm / VDI28 = approx. 2.5 μm

[0084] Example 4 Environmental Tests

[0085] In-use environmental tests were conducted on Synthetic

[0086] Fluid 1 to verify the impact on operators and the working environment compared to a hydrocarbon fluid of the same viscosity.

[0087] The machine used for this purpose is the same one used for the erosion tests on the various materials shown in examples 1 to 3.

[0088] In this case, however, the operating conditions were varied, as the power output of the generator is 80 A with an electrolytic copper electrode, in order to create severe operating conditions.

[0089] The smoke / vapour sensors were placed as follows:

[0090] • Pl placed at the edge of the machine near the protective grille;

[0091] • P2 placed near the machine at the control station;

[0092] • P3 placed about 2 metres from the machine. For all stations, two tests of 30 minutes each were carried out for the C.O.T. parameters. (total organic carbon), V.O.C. (volatile organic compounds), aldehydes and ketones parameters and 1 test of 240 minutes for the "oil mist" parameter in order to have better sensitivity and reproducibility.

[0093] Below is an extract of the most significant measurements.

[0094] Formaldehyde

[0095] Formaldehyde was listed by IARC in 2004 as a Group I compound (definite carcinogen): as an agent with probable carcinogenic action, the lowest possible concentration level is recommended.

[0096] For the Formaldehyde parameter, the Association of Industrial Hygienists ACGIH proposes a TLV-TWA threshold limit value of 0.12 mg / m3and a TLV-STEL value of 0.37 mg / m3, a concentration to which workers may only be continuously exposed for a short period of time.

[0097] The WHO has set a guideline value of 0.1 mg / m3(average over 30 minutes): the values measured in the working environments (Pl - P2 - P3) during the first measurement campaign were always less than 1 / 10 of the recommended values; according to EN689 / 1997, the environmental value is considered to comply with the occupational exposure limits if 3 out of 3 measurements are less than 10% of the OELV.

[0098] The table above shows how the use of ester in EDM significantly reduces the presence of formaldehyde in the working environment to the benefit of operators.

[0099] V.O.C. (Volatile Organic Compounds)

[0100] This parameter was sampled on a vial with suitable support to retain the analytes and subsequently analysed in the laboratory using a GCMS detector (capillary gas chromatography with quadrupole detector) after separation of the sample in the GC column with the possibility, therefore, of identifying and quantifying the various compounds captured.

[0101] T.O.C. (Total Organic Carbon) This parameter was analysed using an FID, Flame Ionisation Detector.

[0102] Despite being an organic molecule, the ester shows a V.O.C. and T.O.C. presence in the atmosphere in line with hydrocarbons (in this case of high purity).

[0103] Oil mists

[0104] The oil mist values, whose TLV-TWA is 5 mg / m3for refined oils, were all below the detection limit.

[0105] *T.n.d. = undetectable traces - when the result is below the limit of quantification of the analysis, the analytical sensitivity value referring to the sampled volume is given in the table. The above results showed the following:

[0106] • Reduced oil mist formation

[0107] • Reduced Total Organic Carbon in line with current regulations

[0108] • Lower formaldehyde emissions compared to hydrocarbon

[0109] This results in an improvement of the working environment and the resulting health and safety of the operators.

[0110] Advantages

[0111] The use of the ester or mixture of esters according to the present invention enables the four functions described for an ideal dielectric fluid to be fully satisfied. In addition, the environmental impact of using said ester is radically reduced in terms of both raw material procurement and the working environment.

[0112] In fact, to produce the dielectric fluid used in the present invention, the raw materials come largely from renewable sources and not from oil extraction and refining.

[0113] In addition, in use, at the point where the spark strikes where temperatures can reach up to 20,000 °C, molecule cracking phenomena occur, but the use of the dielectric fluid according to the present invention, at the same viscosity as the hydrocarbon fluid, offers the following advantages: i. higher flash point; ii. reduced evaporation, thus reducing fumes and mists in the working environment; iii. reduction of emissions of harmful compounds (formaldehyde) during use; iv. reduced formation of aromatic compounds absent in any case at origin; v. less aggression on the skin; vi. reduced consumption; vii. extremely narrow distillation interval; viii. greater viscosity stability.

Claims

CLAIMS1. Use of an ester of formula (I) or an ester of formula(II) or a mixture of esters of formula (I) and formula (II)whereinR is a linear or branched C4-C12 alkyl of a C4-C12 monocarboxylic acid or is a linear or branched C4-C6 alkyl of a C4-C6 polycarboxylic acid;R' is 2-ethylhexyl of a 2-ethylhexanol, esterified with C4-C12 monocarboxylic acid or C4-C6 polycarboxylic acid, or 1-octyl of a 1-octanol esterified with C4-C12 monocarboxylic acid or C4-C6 polycarboxylic acid;R2is a linear or branched C2-C12 alkyl, of a C2-C12 monocarboxylic acid or is a linear or branched C2-C12 alkyl, of a C2-C12 polycarboxylic acid;R3, R4, R5, R6and R7, the same or different from one other, are selected from the group consisting of hydrogen; hydroxide or hydroxide esterified with linear or branched C2-C12 monocarboxylic acid, or with linear or branched C2-C12 polycarboxylic acid; C1-C4 hydroxy alkyl or C1-C4 hydroxy alkyl esterified with linear or branched C2-C12 monocarboxylic acid, or with linear or branched C2-C12 polycarboxylic acid; C1-C4 alkyl ester; ester; linear or branched C1-C15 alkyl; and n ranges between 0 and 8 as a dielectric fluid for electrical discharge machines.

2. Use according to claim 1, wherein R is selected from the group consisting of butyl (nCiHg), isobutyl (iC4H9), tertbutyl (tC4Hg), pentyl (nCsHn) hexyl (nCeHis), heptyl (nCvHis), isoheptyl (iCvHis), octyl (nCsHi?), isooctyl (nCsHi?) and nonyl (nC9Hi9).

3. Use according to claim 1 or 2, wherein the ester, obtained from C4-C6 carboxylic acid esterified with 2- ethylhexanol or 1-octanol, is used for die-sinking or wire EDM or micro-drilling by EDM.

4. Use according to claim 1 or 2, wherein the ester, obtained from C7-C9 monocarboxylic acid esterified with 2- ethylhexanol or 1-octanol, is used for die-sinking EDM.

5. Use according to claim 1 or 2, wherein the mixture of esters is used for die-sinking EDM.

6. Use according to claim 1 or 2, wherein the ester, obtained from C10-C12 monocarboxylic acid or C4-C6polycarboxylic acid esterified with 2-ethylhexanol or 1- octanol, is used for grinding by EDM.

7. Use according to claim 1, wherein the ester of formula (II) is obtained by an esterification reaction of linear or branched C2-C12 monocarboxylic acid, or linear or branched C2-C12 polycarboxylic acid, with a polyol.

8. Use according to claim 7, where the polyol is pentaerythritol .

9. Use according to claim 1 or 2, wherein the mixture of esters consists of pentaerythrityl tetravalerate and 2- ethylhexyl valerate.

10. Use according to claim 9, wherein the mixture of esters comprises 20% by mass of pentaerythrityl tetravalerate and 80% by mass of 2-ethylhexyl valerate with respect to the total mass of this mixture.

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