Rubber material with ultrahigh insulation stability and preparation method thereof

Abstract

The invention discloses a rubber material with ultrahigh insulation stability. The rubber material is prepared from the following raw materials in parts by weight: 40-80 parts of chloroprene rubber, 20-60 parts of brominated butyl rubber, 10-40 parts of ethylene propylene diene monomer, 50-120 parts of inorganic reinforcing filler, 4-15 parts of an active agent, 1-8 parts of an anti-aging agent, 1-15 parts of carbon black, 1-10 parts of alkylphenol formaldehyde resin, 1-10 parts of an anti-hydrolysis agent, 1-8 parts of an accelerant and 2-15 parts of zinc oxide. The rubber material has good flame retardance, high and low temperature resistance and ozone resistance at the same time. The rubber material with the ultrahigh insulation stability has ultrahigh insulation performance, and the volume resistivity reaches the ultrahigh 1015 omega.cm level. The rubber material with ultrahigh insulation stability still keeps high insulativity after resisting 35 MPa high-pressure seawater, the volume resistivity still reaches the 1014 omega.cm level, and long-time normal and stable transmission of underwater signals is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of new materials, and relates to a super-high insulating rubber material and a preparation method thereof. Background Art

[0002] Chloroprene rubber is an emulsion polymer of 2-chloro-1,3-butadiene (CH 2 =CCl-CH=CH 2 ), and has good adhesion properties, mechanical properties, and flame retardant properties. Although its resistance to thermal-oxidative aging, ozone aging, and weather aging is inferior to that of ethylene propylene diene monomer rubber and butyl rubber, it is superior to diene rubbers such as natural rubber, styrene-butadiene rubber, and cis-1,4-polybutadiene rubber. It is widely used as an electrical protection layer material, especially underwater electrical materials, to ensure the transmission stability of signals. However, chloroprene rubber belongs to polar rubber, has poor insulation, and is easy to absorb water. Therefore, it is particularly important to develop a chloroprene rubber material with super-high insulation and high insulation underwater.

[0003] At present, there are many defects in the performance of materials used on the market. First of all, the insulation is insufficient and the volume resistivity is unstable. Especially, the volume resistivity of the product drops significantly after being resistant to high-pressure water, and the probability of signal transmission loss is large. Moreover, some materials use strongly polluting lead substances, which are seriously harmful. Summary of the Invention

[0004] The purpose of the present invention is to provide a rubber material with super-high insulation performance and high resistivity stability in high-pressure water, and a preparation method thereof, so as to completely solve the defects of the materials currently used on the market.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A super-high insulation stability rubber material is made of raw materials including the following parts by weight: 40-80 parts of chloroprene rubber, 20-60 parts of bromobutyl rubber, 10-40 parts of ethylene propylene diene monomer rubber, 50-120 parts of inorganic reinforcing filler, 4-15 parts of activator, 1-8 parts of antioxidant, 1-15 parts of carbon black, 1-10 parts of alkylphenol formaldehyde resin, 1-10 parts of anti-hydrolysis agent, 1-8 parts of accelerator, and 2-15 parts of zinc oxide.

[0007] Preferably, the super-high insulation stability rubber material is made of raw materials including the following parts by weight: 50-60 parts of chloroprene rubber, 20-40 parts of bromobutyl rubber, 10-40 parts of ethylene propylene diene monomer rubber, 50-80 parts of inorganic reinforcing filler, 4-8 parts of activator, 1-8 parts of antioxidant, 1-15 parts of carbon black, 1-10 parts of alkylphenol formaldehyde resin, 1-10 parts of anti-hydrolysis agent, 1-8 parts of accelerator, and 2-15 parts of zinc oxide.

[0008] More preferably, the ultra-high insulation stability rubber material is made from the following raw materials in parts by weight: 50 parts of chloroprene rubber, 20 - 40 parts of bromobutyl rubber, 20 - 40 parts of ethylene propylene diene monomer rubber, 50 - 80 parts of inorganic reinforcing filler, 4 - 8 parts of activator, 1 - 8 parts of anti-aging agent, 1 - 15 parts of carbon black, 2 - 5 parts of alkylphenol formaldehyde resin, 1 - 5 parts of anti-hydrolysis agent, 1 - 4 parts of accelerator, and 3 - 10 parts of zinc oxide.

[0009] Most preferably, the ultra-high insulation stability rubber material is made from the following raw materials in parts by weight: 50 parts of chloroprene rubber, 30 - 35 parts of bromobutyl rubber, 30 - 40 parts of ethylene propylene diene monomer rubber, 50 - 80 parts of inorganic reinforcing filler, 4 - 8 parts of activator, 5 - 8 parts of anti-aging agent, 1 - 15 parts of carbon black, 2 - 5 parts of alkylphenol formaldehyde resin, 1 - 5 parts of anti-hydrolysis agent, 1 - 4 parts of accelerator, and 3 - 7 parts of zinc oxide.

[0010] The chloroprene rubber has a Mooney viscosity of 40 - 80 at 100 °C under ML(1+4).

[0011] The bromobutyl rubber has a Mooney viscosity of 40 - 70 at 125 °C under ML(1+8).

[0012] The ethylene propylene diene monomer rubber is 100% oil-extended ethylene propylene diene monomer rubber, and has a Mooney viscosity of 40 - 70 at 125 °C under ML(1+4). By using oil-extended ethylene propylene diene monomer rubber in the present invention, the risk that additional plasticizer oil is likely to be unevenly mixed and separated in different polar blended rubbers, thereby reducing the performance of the rubber material, can be avoided.

[0013] The total amount of raw rubber of the chloroprene rubber, bromobutyl rubber, and ethylene propylene diene monomer rubber is 100 parts.

[0014] The inorganic reinforcing filler is one or a combination of several of silica powder, white carbon black, silicate powder, and diatomite.

[0015] Specifically, the organic filler can be a combination of silicate powder and white carbon black with a weight ratio of 1:1.

[0016] Specifically, the silicate powder can be selected from silicate powder VM-56.

[0017] The activator is one or a combination of several of stearic acid, magnesium oxide, Si69, and calcium stearate. Si69 is used as a surface treatment agent for the inorganic reinforcing filler to improve the interaction force between the inorganic filler and rubber molecules, and can also release sulfur atoms as a vulcanizing agent. Stearic acid can further enhance the active combination between chloroprene rubber, bromobutyl rubber, and ethylene propylene diene monomer rubber.

[0018] Preferably, the active agent is a combination of stearic acid, magnesium oxide, and Si69 in a weight ratio of (1-2):(1-5):(2-5).

[0019] More preferably, the active agent is a combination of stearic acid, magnesium oxide, and Si69 in a weight ratio of 1:4:3.

[0020] The anti-aging agent is a combination of one or more of anti-aging agent DNP, anti-aging agent MB, anti-aging agent RD, and microcrystalline wax.

[0021] Preferably, the anti-aging agent is a combination of anti-aging agent DNP, anti-aging agent MB, and microcrystalline wax in a weight ratio of 1:1:1.

[0022] Anti-aging agent DNP and anti-aging agent MB have a passivating effect on metal ions such as copper, reducing the corrosion of rubber on copper wires. At the same time, they can improve the aging resistance of rubber and extend its service life; microcrystalline wax can migrate to the rubber surface to form a flexible film, blocking the infiltration of moisture, reducing the water absorption rate of rubber materials, and being beneficial to maintaining the resistivity.

[0023] The carbon black is a combination of one or more of N550, N774, N990, and N660.

[0024] Preferably, the carbon black is N550 carbon black.

[0025] The alkylphenol formaldehyde resin can not only increase the mutual adhesion between the three rubbers but also act as a vulcanizing agent to vulcanize the rubber. Specifically, the alkylphenol formaldehyde resin can be selected from the vulcanizing resin Tackirol 201.

[0026] The anti-hydrolysis agent is a combination of one or more of RhengranP-50, PCD, and UN-150.

[0027] Preferably, the anti-hydrolysis agent is RhengranP-50.

[0028] The accelerator is a combination of one or more of accelerator ETU, accelerator DPTT, accelerator TMTD, and accelerator M. Among them, accelerator TMTD can also release sulfur atoms as a vulcanizing agent as a sulfur carrier.

[0029] Preferably, the accelerator is a combination of accelerator ETU, accelerator TMTD, and accelerator M in a weight ratio of (0.4-1.5):(0.4-1.5):(0.3-1.5).

[0030] More preferably, the accelerator is a combination of accelerator ETU, accelerator TMTD, and accelerator M in a weight ratio of 0.5:1:0.5.

[0031] Another object of the present invention is to provide a method for preparing the ultra-high insulation stability rubber material, comprising: first putting neoprene and bromobutyl rubber into a mixer for mixing, then adding ethylene propylene diene monomer rubber for blending, then adding an antioxidant, a hydrolysis inhibitor, an activator, carbon black and an inorganic reinforcing filler, and after the above raw materials are fully mixed evenly, adding a promoter, an alkylphenol formaldehyde resin and zinc oxide, discharging the rubber, and vulcanizing to obtain the ultra-high insulation stability rubber material.

[0032] Specifically, it includes the following steps:

[0033] Step (1): Set the temperature of the mixer to 45±5°C, put neoprene and bromobutyl rubber into the mixer, and carry out mixing for 0.5 to 2 minutes; then put ethylene propylene diene monomer rubber into the mixer for mixing for 0.5 to 2 minutes;

[0034] Step (2): Add an antioxidant, a hydrolysis inhibitor, and an activator other than Si69, mix for 1 to 2 minutes until the temperature reaches 55°C, then add Si69, carbon black and an inorganic reinforcing filler, and mix for 1 to 2 minutes until the temperature reaches 60 to 70°C; lift the upper ram, clean, and sweep the raw materials that have not been mixed into the corners such as the upper ram and the feed port back into the mixed rubber, press the upper ram, and continue to mix for 0.5 to 3 minutes until the temperature reaches 80 to 90°C; then add a promoter, an alkylphenol formaldehyde resin, and zinc oxide, mix for 1 to 2 minutes (the temperature is lower than 110°C), and discharge the material; pass through a two-roll mill to produce a sheet of mixed rubber;

[0035] Step (3): Let the mixed rubber stand in an environment with a temperature below 30°C for 12 hours, and then vulcanize at a temperature of 160±2°C for 30±5 minutes to obtain the ultra-high insulation stability rubber material.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] Neoprene has excellent flame retardancy and high strength, but poor water resistance and strong polarity, resulting in poor insulation; bromobutyl rubber has excellent airtightness due to the dense methyl groups on the main chain side, excellent barrier property to water vapor penetration, and good insulation at the same time; ethylene propylene diene monomer rubber has excellent water resistance and insulation; both bromobutyl rubber and ethylene propylene diene monomer rubber have excellent heat and oxygen aging resistance, ozone aging resistance, and high and low temperature performance, which can greatly improve the heat and oxygen, ozone and high and low temperature performance of neoprene; the combination of the three rubbers achieves excellent synergistic effects.

[0038] Chloroprene rubber is a polar rubber, with a too large polarity difference from common non-polar rubbers (natural rubber, styrene-butadiene rubber, cis-butadiene rubber, ethylene propylene diene monomer rubber), making it difficult to be fully and evenly mixed; and rubbers such as natural rubber, styrene-butadiene rubber, and cis-butadiene rubber have a relatively larger water absorption. Bromobutyl rubber has a certain polarity and can be well mixed with chloroprene rubber; ethylene propylene diene monomer rubber and bromobutyl rubber have the characteristic of being mixed in any ratio. In the present invention, chloroprene rubber and bromobutyl rubber are first kneaded and then co-blended with ethylene propylene diene monomer rubber, which can ensure a good co-blending effect of the three rubbers and achieve the coexistence of key properties such as insulation, water resistance, flame retardancy, adhesiveness, high strength, resistance to high and low temperatures, and ozone resistance.

[0039] In the present invention, zinc oxide, alkylphenol formaldehyde resin, and sulfur carrier (accelerator TMTD) are used in combination to achieve the co-vulcanization of chloroprene rubber, bromobutyl rubber, and ethylene propylene diene monomer rubber.

[0040] The rubber material of the present invention simultaneously has good insulation, flame retardancy, high and low temperature resistance, and ozone resistance. The specific manifestations are as follows:

[0041] The ultra-high insulation stability rubber material of the present invention has ultra-high insulation performance, and the volume resistivity reaches an ultra-high level of 10 15 Ω·cm level. After the ultra-high insulation stability rubber material of the present invention is exposed to high-pressure seawater of 35 MPa, it still maintains high insulation, and the volume resistivity still reaches 10 14 Ω·cm level, ensuring the long-term normal and stable transmission of underwater signals.

[0042] The ultra-high insulation stability rubber material of the present invention improves the ozone resistance of the rubber material, and it still does not crack under the ozone condition of "200 pphm * 40 °C * 168 h * 25% elongation".

[0043] The ultra-high insulation stability rubber material of the present invention improves the high and low temperature resistance range of the chloroprene rubber material, and it can be used for a long time between -50 °C and 130 °C.

[0044] The present invention eliminates the serious harm of the existing lead materials to the environment. Specific embodiments

[0045] The present invention will be described in detail below with reference to specific embodiments.

[0046] The components and weight parts of the ultra-high insulation stability rubber materials in Examples 1 - 3 are shown in Table 1.

[0047] Table 1. Raw materials and weight parts of the rubber materials in Examples 1 - 3 and Comparative Examples 1 - 4

[0048]

[0049]

[0050] Note: The Mooney viscosity of chloroprene rubber M-40ML(1+4) is 48 at 100 °C; the Mooney viscosity of bromobutyl rubber BB X2ML(1+8) is 46 at 125 °C; the Mooney viscosity of ethylene propylene diene monomer rubber 4869C ML(1+4) is 48 at 125 °C with an oil filling of 100%; the Mooney viscosity of natural rubber CV60 ML(1+4) is 60 at 100 °C; the Mooney viscosity of styrene butadiene rubber 1712ML(1+4) is 49 at 100 °C with an oil filling of 25-30%.

[0051] Example 1

[0052] A rubber material with ultra-high insulation stability, the raw materials and their weight parts are shown in Table 1, and it is prepared according to the following steps:

[0053] Step (1): Set the temperature of the internal mixer to 45±5 °C, put chloroprene rubber M-40 and bromobutyl rubber BB X2 into the internal mixer, and carry out mixing for 1 min; then put ethylene propylene diene monomer rubber 4869C into the internal mixer and mix for 1 min;

[0054] Step (2): Add antioxidant DNP, antioxidant MB, microcrystalline wax 9108, stearic acid, magnesium oxide, Rhengran P-50 (anti-hydrolysis agent), and mix for 1 min until the temperature reaches 55 °C; then add Si69, carbon black N550, silica powder VM-56, and white carbon black (inorganic reinforcing filler), and mix for about 2 min until the temperature reaches 70 °C; lift the upper ram, clean, press down the upper ram, and continue to mix for 2 min until the temperature reaches 85 °C; then add accelerator ETU, accelerator DPTT, accelerator M (accelerator), curing resin Tackirol201, and zinc oxide (vulcanizing agent), and mix for 1.5 min until the temperature reaches 105 °C, discharge the material, and obtain the mixed rubber through the two-roll mill.

[0055] Step (3): Let the mixed rubber stand in an environment with a temperature below 30 °C for 12 h, and then vulcanize at 160 °C for 30 min to obtain the rubber material with ultra-high insulation stability.

[0056] Example 2

[0057] A rubber material with ultra-high insulation stability, the raw materials and their weight parts are shown in Table 1, and it is prepared by referring to the preparation method of Example 1.

[0058] Example 3

[0059] A rubber material with ultra-high insulation stability, the raw materials and their weight parts are shown in Table 1, and it is prepared by referring to the preparation method of Example 1.

[0060] Comparative Example 1

[0061] A rubber material, the raw materials and parts by weight thereof are shown in Table 1, and it is prepared by referring to the preparation method of Example 1.

[0062] Comparative Example 2

[0063] A rubber material, the raw materials and parts by weight thereof are shown in Table 1. Referring to the preparation method of Example 1, except that the feeding order of the raw rubber materials is: first add neoprene M-40 and styrene-butadiene rubber, and then add natural rubber, and the others are the same as in Example 1.

[0064] Comparative Example 3

[0065] A rubber material, the raw materials and parts by weight thereof are shown in Table 1. Referring to the preparation method of Example 1, except that the feeding order of the raw rubber materials is: add neoprene rubber and bromobutyl rubber, and the others are the same as in Example 1.

[0066] Comparative Example 4

[0067] A rubber material, the raw materials and parts by weight thereof are shown in Table 1, and it is prepared by referring to the preparation method of Example 1.

[0068] The physical and mechanical properties of the rubber materials prepared in Examples 1 - 3 and Comparative Examples 1 - 4 are shown in Table 2.

[0069] Table 2 Physical and mechanical properties of the rubber materials prepared in Examples 1 - 3 and Comparative Examples 1 - 4

[0070]

[0071] It can be seen from Table 2 that in the present invention, neoprene rubber is used as the main rubber, and bromobutyl rubber and ethylene propylene diene monomer rubber with excellent water vapor barrier property, high and low temperature resistance, and ozone resistance are used in combination to make up for the disadvantages of neoprene rubber such as easy water absorption, poor resistivity, and not good enough high and low temperature performance; at the same time, microcrystalline wax, metal ion inhibitors, hydrolysis inhibitors, etc. are used to resist the water absorption of the rubber material and extend the service life. The volume resistivity of the rubber material of the present invention can reach an ultra-high insulation level of 10 15 Ω·cm. Especially after withstanding a high water pressure of 35 MPa for a long period of 35 MPa, the volume resistivity still remains at the high insulation level of 10 14 Ω·cm; after undergoing hot air aging at 100 °C for a long period of 1008 h, the resistivity of the rubber material is still 10 15 Ω·cm level. It shows that bromobutyl rubber and ethylene propylene diene monomer rubber have obvious improvement in the insulation and thermal aging properties of neoprene rubber. The white carbon black and silica powder ensure the high strength of the rubber material and at the same time maintain the high insulation of the rubber material.

[0072] Comparing Comparative Example 1, Example 3 and Comparative Example 3, it can be seen that when the proportion of chloroprene rubber increases, the resistivity shows a downward trend, especially the resistivity decreases more significantly after high water pressure. Comparing Example 1 and Example 2, after the amount of bromobutyl rubber increases, the decrease in resistivity after high-pressure water resistance decreases. Comparing Example 1 and Comparative Example 2, it can be seen that in Comparative Example 2, when natural rubber and styrene-butadiene rubber are used instead, the resistivity decreases significantly after high-pressure water resistance, and cracks appear in the rubber compound during ozone resistance. Comparing Example 1 and Comparative Example 4, it can be seen that after the amount of carbon black N550 in Comparative Example 4 increases to 20 parts, the resistivity decreases significantly.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting them. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention should be included within the protection scope of the present invention.

Claims

1. A rubber material with ultra-high insulation stability, characterized in that: The invention is prepared from the following raw materials in parts by weight: 40-80 parts of chloroprene rubber, 20-60 parts of brominated butyl rubber, 10-40 parts of EPDM rubber, 50-120 parts of inorganic reinforcing filler, 4-15 parts of activator, 1-8 parts of antioxidant, 1-15 parts of carbon black, 1-10 parts of alkylphenol formaldehyde resin, 1-10 parts of anti-hydrolysis agent, 1-8 parts of accelerator and 2-15 parts of zinc oxide.

2. The ultra-high insulation stability rubber material according to claim 1, characterized in that: The invention is prepared from the following raw materials in parts by weight: 50-60 parts of chloroprene rubber, 20-40 parts of brominated butyl rubber, 10-40 parts of ethylene propylene rubber, 50-80 parts of inorganic reinforcing filler, 4-8 parts of activator, 1-8 parts of antioxidant, 1-15 parts of carbon black, 1-10 parts of alkylphenol formaldehyde resin, 1-10 parts of anti-hydrolysis agent, 1-8 parts of accelerator and 2-15 parts of zinc oxide.

3. The ultra-high insulation stability rubber material according to claim 2, characterized in that: The ultra-high insulation stability rubber material is made of the following raw materials in parts by weight: 50 parts of chloroprene rubber, 20-40 parts of brominated butyl rubber, 20-40 parts of EPDM rubber, 50-80 parts of inorganic reinforcing filler, 4-8 parts of activator, 1-8 parts of antioxidant, 1-15 parts of carbon black, 2-5 parts of alkylphenol formaldehyde resin, 1-5 parts of anti-hydrolysis agent, 1-4 parts of accelerator and 3-10 parts of zinc oxide.

4. The ultra-high insulation stability rubber material according to claim 3, characterized in that: The invention is prepared from the following raw materials in parts by weight: 50 parts of chloroprene rubber, 30-35 parts of brominated butyl rubber, 30-40 parts of ethylene propylene rubber, 50-80 parts of inorganic reinforcing filler, 4-8 parts of activator, 5-8 parts of antioxidant, 1-15 parts of carbon black, 2-5 parts of alkylphenol formaldehyde resin, 1-5 parts of anti-hydrolysis agent, 1-4 parts of accelerator and 3-7 parts of zinc oxide.

5. The ultra-high insulation stability rubber material according to any one of claims 1 to 4, characterized in that: The Mooney viscosity of the chloroprene rubber ML (1+4) at 100°C is 40-80; the Mooney viscosity of the brominated butyl rubber ML (1+8) at 125°C is 40-70; the EPDM rubber is 100% oil-extended EPDM rubber, and the Mooney viscosity of ML (1+4) at 125°C is 40-70.

6. The ultra-high insulation stability rubber material according to any one of claims 1 to 4, characterized in that: The inorganic reinforcing filler is one or a combination of silicon micropowder, white carbon black, silica powder and diatomaceous earth; The active agent is one or a combination of stearic acid, magnesium oxide, silicon 69, and calcium stearate; preferably, the weight ratio of stearic acid, magnesium oxide, and silicon 69 is (1-2): (1-5): (2-5); The carbon black is one or a combination of N550, N774, N990, and N660, preferably N550 carbon black; The anti-hydrolysis agent is one of Rhengran P-50, PCD, and UN-150 or a combination of several thereof, preferably Rhengran P-50.

7. The ultra-high insulation stability rubber material according to any one of claims 1 to 4, characterized in that: The antioxidant is one or a combination of antioxidant DNP, antioxidant MB, antioxidant RD, and microcrystalline wax, preferably a combination in which the weight ratio of antioxidant DNP, antioxidant MB and microcrystalline wax is 1:1:

1.

8. The ultra-high insulation stability rubber material according to any one of claims 1 to 4, characterized in that: The accelerator is one or a combination of accelerator ETU, accelerator DPTT, accelerator TMTD, and accelerator M, preferably a combination of accelerator ETU, accelerator TMTD, and accelerator M in a weight ratio of (0.4-1.5):(0.4-1.5):(0.3-1.5).

9. A method for preparing the ultra-high insulation stability rubber material according to claim 1, characterized in that: First, chloroprene rubber and brominated butyl rubber are put into an internal mixer for mixing, and then EPDM rubber is added for blending, followed by adding antioxidant, anti-hydrolysis agent, activator, carbon black and inorganic reinforcing filler. After the above raw materials are fully mixed, accelerator, alkylphenol formaldehyde resin and zinc oxide are added, and the rubber is discharged and vulcanized to obtain a rubber material with ultra-high insulation stability.

10. The method for preparing the ultra-high insulation stability rubber material according to claim 9, characterized in that: The following steps are involved: Step (1), setting the temperature of the internal mixer to 45±5° C., adding chloroprene rubber and bromobutyl rubber into the internal mixer, mixing for 0.5 to 2 minutes; then adding ethylene propylene diene monomer rubber and mixing for 0.5 to 2 minutes; Step (2), adding antioxidant, anti-hydrolysis agent, and active agent other than silicon 69, mixing for 1-2 minutes to 55° C., then adding silicon 69, carbon black and inorganic reinforcing filler, mixing for 1-2 minutes to 60-70° C.; lifting the top bolt, cleaning, and re-sweeping the raw materials that have not been mixed into the corners such as the top bolt and the feed port into the mixed rubber, pressing the top bolt, and continuing to mix for 0.5-3 minutes to 80-90° C.; then adding accelerator, alkylphenol formaldehyde resin, and zinc oxide, mixing for 1-2 minutes, and discharging; discharging through an open mill to obtain a mixed rubber; Step (3), the mixed rubber is placed in an environment with a temperature below 30° C. for 12 hours, and then vulcanized at a temperature of 160±2° C. for 30±5 minutes to obtain the ultra-high insulation stability rubber material.

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