Totally-synthetic cutting fluid special for copper parts and preparation process of totally-synthetic cutting fluid

Abstract

The invention provides a fully synthetic cutting fluid special for a copper part and a preparation process thereof. The fully synthetic cutting fluid special for the copper part comprises the following raw materials in parts by weight: 0.5-2.0 parts of a defoaming agent, 1.0-3.0 parts of an antioxidant, 0.5-2.0 parts of a corrosion inhibitor, 2.0-5.0 parts of a surfactant, 0.5-1.5 parts of a metal ion complexing agent and 80.0-100.0 parts of deionized water. Wherein the antioxidant is prepared from carnosol, daidzin and butylated hydroxytoluene. The carnosol and the daidzin can be used for reducing the oxidation reaction by capturing free radicals, and the butylated hydroxytoluene can be used for further enhancing the anti-oxidation effect by interrupting the oxidation chain reaction. And through the combined action and mutual complementation of different anti-oxidation mechanisms, a synergistic effect is formed, and oxidation of the copper part in the machining process is effectively prevented. Wherein the metal ion complexing agent is composed of isothiocyanate, polyaspartic acid and amino trimethylene phosphonic acid, multiple protection mechanisms are formed, and the corrosion resistance of the copper part is remarkably improved. And the existence of the metal ion complexing agent is also beneficial to improving the stability of the cutting fluid.

Description

Technical Field

[0001] The invention relates to the technical field of cutting fluids, and in particular to a fully synthetic cutting fluid special for copper parts and a preparation process thereof. Background Art

[0002] Copper parts play a key role in the electrical industry, machinery manufacturing, construction industry and other fields because of their good electrical conductivity, thermal conductivity and corrosion resistance. For example, in the electrical industry, copper parts are used to manufacture wires, cables and electrical components to ensure the stable transmission of current and the safe operation of the power system. In the field of machinery manufacturing, copper parts are processed into parts of various complex shapes due to their excellent thermal conductivity and ductility, and are widely used in the manufacture of mechanical components such as radiators, bearings, gears, etc. In the processing of copper parts, cutting is a commonly used processing method. In the cutting process of copper parts, the use of cutting fluid is very important. Cutting fluid can not only effectively reduce cutting temperature and reduce tool wear, but also improve processing accuracy and surface quality. For copper parts, due to their material characteristics, a large amount of heat and chips are easily generated during the cutting process. Cutting fluid can cool and flush away chips in time to avoid scratches on workpieces and tools by chips, ensuring the smooth progress of the processing process. However, the easy oxidation of copper parts also brings challenges to the use of cutting fluid. Copper parts come into contact with air during processing and are easily oxidized to form oxides. These oxides mixed into the cutting fluid will cause the cutting fluid to change color, thereby causing the cutting fluid performance to deteriorate. Summary of the invention

[0003] In view of this, the present invention proposes a fully synthetic cutting fluid specially used for copper parts and a preparation process thereof to solve the above problems.

[0004] The technical solution of the present invention is achieved as follows: a fully synthetic cutting fluid specially used for copper parts comprises the following raw materials in parts by weight: 0.5-2.0 parts of a defoamer, 1.0-3.0 parts of an antioxidant, 0.5-2.0 parts of a corrosion inhibitor, 2.0-5.0 parts of a surfactant, 0.5-1.5 parts of a metal ion complexing agent, and 80.0-100.0 parts of deionized water, wherein the antioxidant consists of carnosin, daidzin, and butylated hydroxytoluene.

[0005] Furthermore, a fully synthetic cutting fluid specially used for copper parts comprises the following raw materials in parts by weight: 1.2 parts of a defoaming agent, 2.0 parts of an antioxidant, 1.2 parts of a corrosion inhibitor, 3.5 parts of a surfactant, 1.0 parts of a metal ion complexing agent, and 90.0 parts of deionized water.

[0006] Furthermore, the defoaming agent is one or a combination of polysiloxane, polyvinyl alcohol, polyimide, polyether-modified silicone oil, and alkylated silicone oil.

[0007] Furthermore, the mass ratio of carnosin, daidzin and butylated hydroxytoluene in the antioxidant is (0.4-0.8):(0.5-1.5):(1.5-2.5).

[0008] Furthermore, the corrosion inhibitor is one of benzotriazole, sodium mercaptobenzothiazole, and nanographite.

[0009] Furthermore, the surfactant is one or a combination of polyethylene glycol, polypropylene glycol, nonylphenol polyoxyethylene ether-10, sodium dodecylbenzene sulfonate, fatty alcohol polyoxyethylene ether, and tetrapolyricinoleate.

[0010] Furthermore, the metal ion complexing agent is composed of isothiocyanate, polyaspartic acid, and aminotrimethylenephosphonic acid in a mass volume ratio of g / mL of (1-3):(2-4):(4-6).

[0011] Furthermore, the copper-specific fully synthetic cutting fluid is prepared by the following process, including the following steps:

[0012] S1. Heat deionized water to 50-70°C, add a metal ion complexing agent, and stir evenly to obtain a mixed solution I.

[0013] S2. Add corrosion inhibitor and antioxidant to mixed solution I, stir evenly to obtain mixed solution II.

[0014] S3. Add surfactant and defoamer to mixed solution II, stir evenly, and obtain mixed solution III.

[0015] S4. Place the mixed solution III into a magnetizing device for magnetization treatment to obtain a magnetized mixed solution.

[0016] S5. Filter and bottle the magnetized mixed liquid to obtain a fully synthetic cutting fluid specially used for copper parts.

[0017] Furthermore, the stirring speed of S1 is 200-400 rpm and the stirring time is 10-20 min, the stirring speed of S2 is 150-300 rpm and the stirring time is 15-25 min, and the stirring speed of S3 is 100-200 rpm and the stirring time is 10-15 min.

[0018] Further, S4 was magnetized at 0.4-0.8 T for 10-20 min.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. By adding an antioxidant composed of a specific ratio of carnosin, daidzin, and butylated hydroxytoluene, the carnosin and daidzin capture free radicals and combine with free radicals, thereby removing the free radicals generated during the cutting process and preventing the free radicals from attacking the surface of the copper parts, thereby reducing the occurrence of oxidation reactions. The hydrogen atoms on the phenolic hydroxyl groups of butylated hydroxytoluene are highly active and easily react with free radicals to form stable compounds. By donating hydrogen atoms, butylated hydroxytoluene can effectively neutralize free radicals and prevent them from continuing to induce oxidation reactions. The tert-butyl group in the butylated hydroxytoluene molecule has a large steric hindrance effect, which can prevent free radicals from approaching the surface of the copper parts, thereby further reducing the occurrence of oxidation reactions.

[0021] 2. By adding a metal ion complexing agent composed of a specific ratio of isothiocyanate, polyaspartic acid, and aminotrimethylene phosphonic acid, the sulfur atoms and nitrogen atoms in the isothiocyanate can form chelates with the metal ions in the cutting fluid, thereby reducing the catalytic activity of the metal ions. By chelating the metal ions, not only the corrosion tendency of the copper parts is reduced, but also the stability of the cutting fluid is improved. At the same time, the chelation of isothiocyanate enhances the adsorption capacity of nanographite on the surface of the copper parts, thereby forming a protective film on the surface of the copper parts, thereby preventing the harmful components in the cutting fluid from corroding the copper parts. The polyaspartic acid and aminotrimethylene can form stable complexes with metal ions such as copper ions to prevent metal ions from depositing on the surface of the copper parts to form corrosion products. In addition, aminotrimethylene can also be adsorbed on the surface of the copper parts to form a protective film to isolate oxygen and other corrosive substances in the cutting fluid, thereby further improving the corrosion resistance of the copper parts.

[0022] 3. By magnetizing the cutting fluid during the preparation process, the arrangement and orientation of the molecules in the cutting fluid are changed to make it more orderly, which helps to form a more stable and effective lubricating film during the cutting process, thereby reducing cutting resistance, reducing friction and wear between the tool and the workpiece, and thus improving the anti-oxidation and anti-corrosion properties of the workpiece. In addition, this orderly arrangement may help heat conduction, because heat can be transferred more efficiently along the molecular chain, helping to reduce the temperature of the tool and workpiece, reduce thermal stress and thermal deformation, and improve machining accuracy and surface quality. DETAILED DESCRIPTION

[0023] In order to better understand the technical content of the present invention, specific embodiments are provided below to further illustrate the present invention.

[0024] Unless otherwise specified, the experimental methods used in the embodiments of the present invention are all conventional methods.

[0025] Unless otherwise specified, the materials, reagents, etc. used in the embodiments of the present invention can be obtained from commercial sources.

[0026] Example 1

[0027] A fully synthetic cutting fluid for copper parts, comprising the following raw materials by weight: 0.5 parts of defoamer, 1.0 parts of antioxidant, 0.5 parts of corrosion inhibitor, 2.0 parts of surfactant, 0.5 parts of metal ion complexing agent, 80.0 parts of deionized water. Among them, the defoamer is polysiloxane, the antioxidant is composed of carnosin, daidzin, and butylated hydroxytoluene in a mass ratio of 0.4:0.5:1.5, the corrosion inhibitor is benzotriazole, the surfactant is polyethylene glycol, and the metal ion complexing agent is composed of isothiocyanate, polyaspartic acid, and aminotrimethylenephosphonic acid in a mass volume ratio of g / mL of 1:2:4.

[0028] Example 2

[0029] A fully synthetic cutting fluid for copper parts, comprising the following raw materials in parts by weight: 2.0 parts of defoamer, 3.0 parts of antioxidant, 2.0 parts of corrosion inhibitor, 5.0 parts of surfactant, 1.5 parts of metal ion complexing agent, and 100.0 parts of deionized water. The defoamer is composed of polyvinyl alcohol and polyether modified silicone oil in a mass volume ratio of 1:1 g / mL, the antioxidant is composed of carnosin, daidzin, and butylated hydroxytoluene in a mass ratio of 0.8:1.5:2.5, the corrosion inhibitor is sodium mercaptobenzothiazole, the surfactant is composed of sodium dodecylbenzene sulfonate and polypropylene glycol in a mass volume ratio of 1:1 g / mL, and the metal ion complexing agent is composed of isothiocyanate, polyaspartic acid, and aminotrimethylenephosphonic acid in a mass volume ratio of 3:4:6 g / mL.

[0030] Example 3

[0031] A fully synthetic cutting fluid for copper parts includes the following raw materials in parts by weight: 1.2 parts of defoamer, 2.0 parts of antioxidant, 1.2 parts of corrosion inhibitor, 3.5 parts of surfactant, 1.0 parts of metal ion complexing agent, and 90.0 parts of deionized water. The defoamer is composed of polyimide, polysiloxane, and alkylated silicone oil in a mass volume ratio of 1:1:1 g / mL, the antioxidant is composed of carnosin, daidzin, and butylated hydroxytoluene in a mass ratio of 0.6:1.0:2.0, the corrosion inhibitor is nanographite, the surfactant is composed of polyethylene glycol, nonylphenol polyoxyethylene ether-10, and tetrapolyricinoleate in a volume ratio of 1:1:1, and the metal ion complexing agent is composed of isothiocyanate, polyaspartic acid, and aminotrimethylenephosphonic acid in a mass volume ratio of 2:3:5 g / mL.

[0032] The fully synthetic cutting fluid for copper parts described in the above embodiments 1-3 is prepared by the following process, including the following steps:

[0033] S1. Heat deionized water to 60° C., add a metal ion complexing agent, and stir at 300 rpm for 15 min to obtain a mixed solution I.

[0034] S2. Add corrosion inhibitor and antioxidant to mixed solution I, stir at 230 rpm for 20 min to obtain mixed solution II.

[0035] S3. Add surfactant and defoamer to mixed solution II, stir at 150 rpm for 12.5 min to obtain mixed solution III.

[0036] S4. Place the mixed solution III into a magnetizing device and magnetize it at 0.6 T for 15 min to obtain a magnetized mixed solution.

[0037] S5. Filter and bottle the magnetized mixed liquid to obtain a fully synthetic cutting fluid specially used for copper parts.

[0038] Example 4

[0039] Compared with Example 3, the present embodiment is different in that the special fully synthetic cutting fluid for copper parts is prepared by the following process, including the following steps:

[0040] S1. Heat deionized water to 50° C., add a metal ion complexing agent, and stir at 200 rpm for 20 min to obtain a mixed solution I.

[0041] S2. Add corrosion inhibitor and antioxidant to mixed solution I, stir at 150 rpm for 25 min to obtain mixed solution II.

[0042] S3. Add a surfactant and a defoaming agent to the mixed solution II, stir at 100 rpm for 15 min, and obtain a mixed solution III.

[0043] S4. Place the mixed solution III into a magnetizing device and magnetize it at 0.4 T for 20 min to obtain a magnetized mixed solution.

[0044] S5. Filter and bottle the magnetized mixed liquid to obtain a fully synthetic cutting fluid specially used for copper parts.

[0045] Example 5

[0046] Compared with Example 3, the present embodiment is different in that the special fully synthetic cutting fluid for copper parts is prepared by the following process, including the following steps:

[0047] S1. Heat deionized water to 70° C., add a metal ion complexing agent, and stir at 400 rpm for 10 min to obtain a mixed solution I.

[0048] S2. Add corrosion inhibitor and antioxidant to mixed solution I, stir at 300 rpm for 15 min to obtain mixed solution II.

[0049] S3. Add surfactant and defoamer to mixed solution II, stir at 200 rpm for 10 min, and obtain mixed solution III.

[0050] S4. Place the mixed solution III into a magnetizing device and magnetize it at 0.8 T for 10 min to obtain a magnetized mixed solution.

[0051] S5. Filter and bottle the magnetized mixed liquid to obtain a fully synthetic cutting fluid specially used for copper parts.

[0052] Comparative Example 1

[0053] The difference between this comparative example and Example 3 is that the raw material of the special synthetic cutting fluid for copper parts does not contain an antioxidant.

[0054] Comparative Example 2

[0055] The difference between this comparative example and Example 3 is that the antioxidant consists of daidzin and butylated hydroxytoluene in a mass ratio of 1.0:2.0.

[0056] Comparative Example 3

[0057] The difference between this comparative example and Example 3 is that the antioxidant consists of carnosin and butylated hydroxytoluene in a mass ratio of 0.6:2.0.

[0058] Comparative Example 4

[0059] The difference between this comparative example and Example 3 is that the raw material of the fully synthetic cutting fluid specially used for copper parts does not contain a metal ion complexing agent.

[0060] Comparative Example 5

[0061] The difference between this comparative example and Example 3 is that the metal ion complexing agent consists of polyaspartic acid and aminotrimethylenephosphonic acid in a mass volume ratio of g / mL of 3:5.

[0062] Comparative Example 6

[0063] The difference between this comparative example and Example 3 is that the metal ion complexing agent is composed of isothiocyanate and aminotrimethylenephosphonic acid in a mass volume ratio of g / mL of 2:5.

[0064] Comparative Example 7

[0065] The difference between this comparative example and Example 3 is that no magnetization treatment is performed in the process of preparing the special synthetic cutting fluid for copper parts, that is, after the step S3 is completed, the mixed liquid III is filtered and filled to obtain the special synthetic cutting fluid for copper parts.

[0066] 1. Performance test

[0067] 1.1. Antioxidant performance test

[0068] Five cylindrical brass pieces with a diameter of 50 mm, a thickness of 10 mm, and a roughness of Ra 0.8 μm were selected and placed in a cutting machine. Examples 1-5, Comparative Examples 1-7, and commercially available fully synthetic cutting fluids were added for processing. A carbide tool was selected and the spindle speed was set to 2000 rpm, the cutting depth was set to 2 mm, and the cutting time of each copper piece sample was set to 15 min. After processing, the number of oxidation spots in each group was immediately observed and recorded, and the average number of oxidation spots was calculated and recorded in Table 1. At the same time, the cutting fluid was observed to see if there was any color change, and the color change was recorded in Table 1.

[0069] 1.2. Anti-corrosion performance test

[0070] Take a copper piece with a size of 25mm×50mm×3mm as a test piece, and soak it in Examples 1-5, Comparative Examples 1-7 and commercially available full synthetic cutting fluids, respectively. Use 50mL of full synthetic cutting fluid to soak it at 55°C for 12h. After soaking, observe the surface of the test piece. Rust-free and shiny as new is grade A, slight discoloration is grade B, moderate discoloration is grade C, and severe discoloration is grade D. The results are shown in Table 1.

[0071] Table 1

[0072] Number of oxidation spots Cutting fluid color Corrosion protection grade Example 1 0 No change Level A Example 2 0 No change Level A Example 3 0 No change Level A Example 4 0 No change Level A Example 5 0 No change Level A Comparative Example 1 3 Slight discoloration C-Class Comparative Example 2 1 No change Class B Comparative Example 3 1 No change Class B Comparative Example 4 3 Slight discoloration C-Class Comparative Example 5 1 No change Class B Comparative Example 6 1 No change Class B Comparative Example 7 2 Slight discoloration Class B Commercially available 4 Slight discoloration C-Class

[0073] It can be seen from Table 1 that the antioxidant and anti-corrosion properties of the fully synthetic cutting fluids of Examples 1-5 are better than those of Comparative Examples 1-7 and commercially available fully synthetic cutting fluids.

[0074] During the copper cutting process, due to friction and high temperature, a large number of free radicals will be generated on the copper surface. These free radicals can trigger a series of free radical chain reactions, thereby causing copper oxidation. The copper-specific fully synthetic cutting fluid of embodiment 1-5 is added with an antioxidant composed of carnosin, daidzin and butylated hydroxytoluene in a specific ratio, wherein the carnosin and daidzin capture free radicals and combine with free radicals to remove the free radicals generated during the cutting process, prevent the free radicals from attacking the copper surface, thereby reducing the occurrence of oxidation reactions. The phenolic hydroxyl functional group in the carnosin molecule reacts with the copper ion to form a stable complex, which is closely adsorbed on the copper surface to form a protective film, effectively isolating the contact between oxygen and the copper surface, thereby slowing down the oxidation rate of the copper. The hydrogen atoms on the phenolic hydroxyl group of butylated hydroxytoluene have high activity and are easy to react with free radicals to generate stable compounds. By donating hydrogen atoms, butylated hydroxytoluene can effectively neutralize free radicals to prevent them from continuing to trigger oxidation reactions. The tert-butyl group in the butylated hydroxytoluene molecule has a large steric hindrance effect, which can prevent free radicals from approaching the surface of copper parts, thereby further reducing the occurrence of oxidation reactions. Therefore, carnosin and daidzin can reduce the occurrence of oxidation reactions by capturing free radicals, while butylated hydroxytoluene further enhances the antioxidant effect by interrupting the oxidation chain reaction. Through the combined action of different antioxidant mechanisms, they complement each other and form a synergistic effect, thereby significantly improving the antioxidant performance of the cutting fluid and effectively preventing the oxidation of copper parts during processing.

[0075] The copper-specific fully synthetic cutting fluid of embodiment 1-5 is added with a metal ion complexing agent composed of isothiocyanate, polyaspartic acid, and aminotrimethylene phosphonic acid in a specific ratio, wherein the sulfur atoms and nitrogen atoms in the isothiocyanate can form chelates with the metal ions in the cutting fluid, thereby reducing the catalytic activity of the metal ions. By chelating the metal ions, not only the corrosion tendency of the copper is reduced, but also the stability of the cutting fluid is improved. At the same time, the chelation of the isothiocyanate enhances the adsorption capacity of nanographite on the surface of the copper, thereby forming a protective film on the surface of the copper, thereby preventing the harmful components in the cutting fluid from corroding the copper. This chelation not only reduces the corrosion tendency of the copper, but also improves the stability of the cutting fluid. The polyaspartic acid and aminotrimethylene can form a stable complex with metal ions such as copper ions to prevent metal ions from depositing on the surface of the copper to form corrosion products. In addition, aminotrimethylene can also be adsorbed on the surface of the copper to form a protective film, isolating oxygen and other corrosive substances in the cutting fluid, thereby further improving the anti-corrosion performance of the copper. Isothiocyanate, polyaspartic acid and aminotrimethylenephosphonic acid work together in the cutting fluid to form a multiple protection mechanism. They can not only form stable complexes with metal ions such as copper ions to reduce the formation and accumulation of corrosion products, but also adsorb on the surface of copper parts to form a protective film to isolate corrosive substances. This multiple protection mechanism significantly improves the corrosion resistance of copper parts. The presence of metal ion complexing agents also helps to improve the stability of the cutting fluid. They can form stable complexes with metal ions in the cutting fluid to prevent metal ions from precipitating or crystallizing in the cutting fluid, thereby extending the service life of the cutting fluid.

[0076] The fully synthetic cutting fluid for copper parts of embodiment 1-5 can change the arrangement and orientation of molecules in the cutting fluid by magnetizing treatment, making it more orderly. This ordered molecular structure helps to form a more stable and effective lubricating film during the cutting process, thereby reducing cutting resistance, reducing friction and wear between the tool and the workpiece, thereby improving oxidation resistance and corrosion resistance. In addition, under the action of the magnetic field, the molecules in the cutting fluid are affected by the Lorentz force and tend to be oriented. This ordered arrangement may help heat conduction, because heat can be more efficiently transferred along the molecular chain, which helps to reduce the temperature of the tool and the workpiece, reduce thermal stress and thermal deformation, and improve machining accuracy and surface quality.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A fully synthetic cutting fluid for copper parts, characterized in that: The invention comprises the following raw materials in parts by weight: 0.5-2.0 parts of a defoamer, 1.0-3.0 parts of an antioxidant, 0.5-2.0 parts of a corrosion inhibitor, 2.0-5.0 parts of a surfactant, 0.5-1.5 parts of a metal ion complexing agent, and 80.0-100.0 parts of deionized water. The antioxidant consists of caryophyllene, daidzin, and butylated hydroxytoluene.

2. A fully synthetic cutting fluid for copper parts as claimed in claim 1, characterized in that: The invention comprises the following raw materials in parts by weight: 1.2 parts of a defoaming agent, 2.0 parts of an antioxidant, 1.2 parts of a corrosion inhibitor, 3.5 parts of a surfactant, 1.0 parts of a metal ion complexing agent and 90.0 parts of deionized water.

3. A fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The defoaming agent is one or a combination of polysiloxane, polyvinyl alcohol, polyimide, polyether modified silicone oil and alkylated silicone oil.

4. A fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The mass ratio of carnosin, daidzin and butylated hydroxytoluene in the antioxidant is (0.4-0.8):(0.5-1.5):(1.5-2.5).

5. A fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The corrosion inhibitor is one of benzotriazole, sodium mercaptobenzothiazole and nanographite.

6. A fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The surfactant is one or a combination of polyethylene glycol, polypropylene glycol, nonylphenol polyoxyethylene ether-10, sodium dodecylbenzene sulfonate, fatty alcohol polyoxyethylene ether, and tetrapolyricinoleate.

7. A fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The metal ion complexing agent is composed of isothiocyanate, polyaspartic acid and aminotrimethylenephosphonic acid in a mass volume ratio of g / mL of (1-3):(2-4):(4-6).

8. The process for preparing a fully synthetic cutting fluid for copper parts as claimed in claim 1 or 2, characterized in that: The following steps are involved: S1. Heat deionized water to 50-70°C, add a metal ion complexing agent, and stir evenly to obtain a mixed solution I; S2, adding corrosion inhibitor and antioxidant to the mixed solution I, stirring evenly to obtain a mixed solution II; S3, adding a surfactant and a defoaming agent to the mixed solution II, stirring evenly, to obtain a mixed solution III; S4, placing the mixed solution III into a magnetizing device for magnetization treatment to obtain a magnetized mixed solution; S5. Filter and bottle the magnetized mixed liquid to obtain a fully synthetic cutting fluid specially used for copper parts.

9. The process for preparing a fully synthetic cutting fluid for copper parts as claimed in claim 8, characterized in that: The stirring speed of S1 is 200-400 rpm and the stirring time is 10-20 min, the stirring speed of S2 is 150-300 rpm and the stirring time is 15-25 min, and the stirring speed of S3 is 100-200 rpm and the stirring time is 10-15 min.

10. The process for preparing a fully synthetic cutting fluid for copper parts as claimed in claim 8, characterized in that: The S4 is magnetized at 0.4-0.8T for 10-20 min.