JP2004352748A - Conductive crosslinked polyethylene foam and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conductive crosslinked polyethylene foam exhibiting excellent sound-absorbing characteristics and conductivity, and its manufacturing method. <P>SOLUTION: An expandable crosslinkable resin composition is obtained by admixing an active component for increasing a dipole moment, a blowing agent, an auxiliary blowing agent and a crosslinking agent with a mixed resin composition, as a base material, of a low density polyethylene and an ethylene/vinyl acetate copolymer having uniformly kneaded therewith an electroconductive carbon powder. The expandable crosslinkable resin composition is loaded in a closed mold, heated under pressure to crosslink, subsequently heated under normal pressure to expand and then mechanically pressed to cause cells to communicate each other, thereby to give the conductive crosslinked polyethylene foam. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to a conductive crosslinked polyethylene foam excellent in radio wave absorption characteristics and sound absorption characteristics, and a method for producing the same.
[0002]
[Prior art]
Conventionally, a radio wave absorber is often mounted on a radio wave reflector in order to suppress reflected radio waves from a radio wave reflector such as a ship, a building, or a steel tower. In addition, a radio wave absorber is also installed in an anechoic chamber for performance inspection of precision equipment.
[0003]
Various radio wave absorbers having different shapes and materials have been proposed. For example, there are (1) a material using a combination of foamed polyurethane and carbon having an open cell structure, and (2) a material using foamed polystyrene. However, the material (1) has extremely poor weather resistance, and the material (2) has poor radio wave absorption characteristics in a low frequency range of 1 to 2 GHz.
[0004]
After that, a radio wave absorber (Japanese Patent Publication No. Hei 7-101785) using a conductive cross-linked polyethylene as a base, which has solved the above-mentioned disadvantages (1) and (2), has been proposed. The radio wave absorber had excellent radio wave absorption characteristics and excellent weather resistance, but had almost no sound absorbing performance.
[0005]
As a material for a radio wave absorber, the applicant of the present invention has prepared a conductive crosslink made by adding and kneading (a) polyolefin with carbon, a foaming agent and a crosslinking agent, crosslinking in a mold under pressure, and then heating and foaming under normal pressure. A method for producing a polyolefin foam (Japanese Patent Publication No. 2-29095) was developed.
[0006]
Further, the present applicant kneaded and kneaded (b) 10 to 40 parts by weight of low-density polyethylene and 90 to 60 parts by weight of an ethylene-vinyl acetate copolymer with a conductive carbon, a foaming agent and a crosslinking agent to form a closed system. A method of cross-linking in a mold, foaming by heating under normal pressure, and then mechanically connecting the bubbles was proposed (Japanese Patent Publication No. 4-55619).
[0007]
However, although (a) is a closed cell and (b) is an open cell, the sound absorbing performance is low, and there is no material for a radio wave absorber having both the radio wave absorbing property and the sound absorbing property.
[0008]
[Problems to be solved by the invention]
Therefore, an object of the present invention is to develop a conductive crosslinked polyethylene-based foam excellent in radio wave absorption characteristics and sound absorption and a method for producing the same, and to provide a base material for a radio wave absorber.
[0009]
[Means for Solving the Problems]
The conductive crosslinked polyethylene foam of the present invention is based on a mixed resin composition of a low-density polyethylene and an ethylene-vinyl acetate copolymer, and has conductive carbon, an active component that increases the amount of dipole moment, a foaming agent, and foaming. An auxiliary agent and a crosslinking agent are added.
[0010]
The conductive crosslinked polyethylene foam of the present invention preferably has an open cell structure of open cells. This is because the higher the open cell ratio, the better the sound absorbing performance.
[0011]
The conductive crosslinked polyethylene foam of the present invention preferably contains the active ingredient in an amount of 2 to 9 parts by weight based on 100 parts by weight of the mixed resin of the low-density polyethylene and the ethylene-vinyl acetate copolymer. When the amount of the active ingredient is 2 parts by weight or more, the sound absorbing performance is excellent. When the amount of the active ingredient exceeds 9 parts by weight, elongation of the resin during foaming is poor, and foaming is inhibited.
[0012]
In the conductive crosslinked polyethylene foam of the present invention, it is preferable that the active component is N, N-dicyclohexylbenzothiazyl-2-sulfenamide in terms of compatibility with a resin and sound absorption.
[0013]
The method for producing a conductive cross-linked polyethylene foam of the present invention is based on a mixed resin composition of low-density polyethylene and an ethylene-vinyl acetate copolymer, and has a conductive carbon, an active ingredient for increasing the amount of dipole moment, and foaming. The foamable crosslinkable resin composition obtained by adding the mixing agent, the foaming aid and the crosslinking agent is filled in a closed mold, heated under pressure, crosslinked, and then foamed by heating under normal pressure, and then mechanically. This is a manufacturing method in which bubbles are communicated by pressing.
[0014]
In the production method of the present invention, it is preferable that the active ingredient is contained in a ratio of 2 to 9 parts by weight based on 100 parts by weight of the mixed resin of the low density polyethylene and the ethylene-vinyl acetate copolymer. When the amount of the active ingredient is 2 parts by weight or more, the sound absorbing performance is excellent, and when the amount exceeds 9 parts by weight, elongation of the resin during foaming is inhibited, and foaming is inhibited.
[0015]
In the production method of the present invention, the active ingredient is preferably N, N-dicyclohexylbenzothiazyl-2-sulfenamide.
[0016]
[Embodiment of the present invention]
The conductive crosslinked polyethylene foam according to the present invention is composed of a mixture of a low-density polyethylene and an ethylene-vinyl acetate copolymer, 8 to 15 parts by weight of conductive carbon, and 2 to 9 parts by weight of an active ingredient for increasing a dipole moment. A foamable crosslinkable composition obtained by adding and kneading a foaming agent, a foaming aid and a crosslinking agent is heated, the composition is filled in a closed mold, and heated under pressure to form a foam having an expansion ratio of 1 to 3 times. This is a method in which a crosslinkable composition is obtained, then the composition is foamed by heating under normal pressure, and then mechanically pressed to make the bubbles open.
[0017]
In the present invention, it is preferable that the cell structure of the conductive crosslinked polyethylene-based foam is an open cell in terms of sound absorption characteristics.
[0018]
The low-density polyethylene of the present invention refers to a density of 0.910 to 0.935 g / cm3, preferably 0.915 to 0.930 g / cm3, and a melt flow rate of 0.5 to 5.0 g / 10 min, preferably 2.5 It is a low-density polyethylene of 4.0 g / 10 min.
[0019]
The ethylene-vinyl acetate copolymer of the present invention has a density of 0.920 to 0.945 g / cm3, preferably 0.925 to 0.940 g / cm3, and a melt flow rate of 0.5 to 4.0 g / 10 min, preferably It is an ethylene-vinyl acetate copolymer having a vinyl acetate content of 1.0 to 3.5 g / 10 min and a vinyl acetate content of 10 to 20% by weight, preferably 12 to 16% by weight.
[0020]
The composition ratio of the above two resins is preferably 95 to 20 parts by weight of low density polyethylene, particularly preferably 90 to 30 parts by weight, 5 to 80 parts by weight of ethylene-vinyl acetate copolymer, and particularly preferably 10 to 70 parts by weight. Parts by weight.
[0021]
The density and the melt flow rate are measured by the methods specified in JIS K6922.
[0022]
Examples of the conductive carbon black used in the present invention include furnace-based carbon black, acetylene-based carbon black, and channel-based carbon black. These may be used alone or in combination of two or more. Particularly, among carbon blacks, furnace black having a surface area (nitrogen adsorption method) of 900 m ² / g or more is a preferred conductive carbon black.
[0023]
As an active ingredient added to the polyolefin resin used in the present invention, for example, N, N-dicyclohexylbenzothiazyl-2-sulfenamide (DCHBSA), 2-mercaptobenzothiazole (MBT), dibenzothiazyl sulfide ( MBTS), N-cyclohexylbenzothiazyl-2-sulfenamide (CBS), N-tert-butylbenzothiazyl-2-sulfenamide (BBS), N-oxydiethylenebenzothiazyl-2-sulfenamide (OBS), a compound containing a mercaptobenzothiazyl group such as N, N-diisopropylbenzothiazyl-2-sulfenamide (DPBS), and a benzotriazole having an azole group bonded to a benzene ring as a mother nucleus and a phenyl group 2- {2′-hydroxy-3 ′ having a group bonded thereto (3 ", 4", 5 ", 6" tetrahydrophthalimidomethyl) -5'-methylphenyl} -benzotriazole (2HPMBB), 2- {2'-hydroxy-5'-methylphenyl} -benzotriazole (2HMPB), 2- {2'-hydroxy-3'-t-butyl-5'-methylphenyl} -5-chlorobenzotriazole (2HBMPCB), 2- {2'-hydroxy-3'-5'-di Among compounds having a benzotriazole group such as -t-butylphenyl} -5-chlorobenzotriazole (2HDBPCB) or compounds having a diphenylacrylate group such as ethyl-2-cyano-3,3-di-phenylacrylate Or two or more selected from the group consisting of:
[0024]
The cross-linking agent used in the present invention has a decomposition temperature of at least the flow start temperature of the polyethylene resin in the polyethylene resin, and is decomposed by heating to generate free radicals to generate intermolecular or intramolecular molecules. Organic peroxides which are radical generators that cause cross-linking to be formed, such as dicumyl peroxide, 1,1-di-tert-butyl peroxide, 1,1-di-tert-butyl peroxy-3,3,5- Trimethylcyclohexane, 2,5-dimethyl-2,5-di-tert-butylperoxyhexane, 2,5-dimethyl-2,5-di-tert-butylperoxyhexyne, α, α-di-tert-butylperoxyisopropylbenzene, Tertiary butyl peroxyketone, tertiary butyl par -Oxybenzoate and the like, but the most appropriate organic peroxide must be selected depending on the resin used at that time.
[0025]
Examples of the blowing agent used in the present invention include azodicarbonamide, benzenesulfonyl hydrazide, dinitrosopentamethylenetetramine, toluenesulfonyl hydrazide, and 4,4-oxybis (benzenesulfonyl hydrazide).
[0026]
In the present invention, a foaming aid can be added according to the type of the foaming agent. Examples of the foaming aid include compounds mainly containing urea, metal oxides such as zinc oxide and zinc, compounds mainly containing salicylic acid, stearic acid, and the like, that is, higher fatty acids and metal compounds of higher fatty acids.
[0027]
In the present invention, for the purpose of improving the physical properties of the composition to be used or lowering the price, a compounding agent (filler) which does not significantly affect the cross-linking, for example, zinc oxide, titanium oxide, calcium oxide, magnesium oxide, oxide Metal oxides such as silicon, carbonates such as magnesium carbonate and calcium carbonate, or fiber materials such as pulp, or various dyes, pigments and fluorescent materials, and other conventional rubber compounding agents can be added as necessary. it can.
[0028]
Next, a method for producing the conductive crosslinked polyethylene foam of the present invention will be described.
[0029]
The above-mentioned low-density polyethylene, ethylene-vinyl acetate copolymer, and a known foaming agent, a foaming aid and a crosslinking agent were added to a mixture of conductive carbon black and an active ingredient, and the mixture was kneaded to obtain a foamable crosslinkable resin. The composition is pressurized and filled into a closed mold, and is heated and molded under pressure at a heating temperature of 120 to 160 ° C. for a heating time of 40 to 90 minutes.
[0030]
Next, the foamable crosslinkable composition thus formed is placed in a mold having a desired shape such as a rectangular parallelepiped which is not hermetically sealed at normal pressure, and a metal bath using a rose alloy, a wood alloy or the like, an oil bath, sodium nitrate, A heating medium conduit (heating medium: steam, etc.) is provided on the outer wall of a salt bath using a molten salt of one or more kinds of salts such as potassium nitrate and potassium nitrite, in a nitrogen stream, or in a rectangular parallelepiped type. The foam is obtained by heating for a predetermined time in this state or in a state covered with an extensible iron plate or the like, followed by cooling. The heating temperature is 145 to 210 ° C, preferably 160 to 190 ° C, depending on the type of polyethylene used, and the heating time is 30 to 180 minutes, preferably 50 to 150 minutes.
[0031]
Next, the obtained foam is mechanically compressed and deformed by a constant-speed two-roll mill or the like, whereby the cell membrane is broken and the cells are made to communicate.
[0032]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples.
[0033]
Example 1
90 parts of low density polyethylene (trade name: Novatec LC-520, density 0.9226 g / cm ³ , melt flow rate 3.8 g / 10 min, manufactured by Mitsubishi Chemical Corporation) and ethylene-vinyl acetate copolymer (trade name: Evatate) P-1403, density 0.93 g / cm ³ , melt flow rate 1.3 g / 10 min, 10 parts by Mitsui / Dupont Polychemical Co., Ltd.), furnace type carbon black (trade name: Ketjen Black EC, Mitsubishi Chemical Corporation) 15 parts, N, N-dicyclohexylbenzothiazyl-2-sulfenamide (DCHBSA) 2 parts, azodicarbonamide (trade name: Vinylol AC # 3, manufactured by Eiwa Chemical Co., Ltd.), 7 parts, activated zinc white 0.1 parts, α, α'-bis (t-butylperoxy) diisopropylbenzene (trade name: Dox 14/40) A composition consisting of 0.7 parts was kneaded in a kneader at 110 ° C, and the kneaded product was filled in a mold (31 × 200 × 400 mm) in a press heated to 140 ° C. The mixture was heated under pressure for 1 minute to form a foamable crosslinkable composition.
[0034]
The foaming ratio of the molded product was 1.1 times.
[0035]
Next, the molded product is placed substantially at the center of a non-airtight openable metal mold (90 × 500 × 1000 mm) provided with a flow path of heated steam, and 7.0 kg / cm ² of heated steam is applied to the flowed steam. The foam was passed through a path and heated for 240 minutes to decompose the remaining foaming agent and cross-linking agent. After cooling, the foam was taken out.
[0036]
The obtained foam was passed between two constant-velocity two rolls set at a roll interval of 20 mm five times to break the bubble film and to communicate the bubbles.
[0037]
The obtained foam had an apparent density of 48 kg / m ³ , a surface resistance of 0.5 × 10 ⁶ Ω, and an open cell ratio of 95%.
[0038]
The surface resistance is a numerical value obtained by measuring the insulation resistance in the longitudinal direction of 1 cm × 5 cm long using an insulation resistance meter 8060A manufactured by Fluke Corporation.
[0039]
When the sound absorbing properties of the obtained foam were measured by a reverberation chamber method sound absorbing rate according to JIS A1409, as shown in FIG. 1, it showed a better sound absorbing effect than the conventional conductive crosslinked polyethylene foam.
[0040]
Example 2
A foam was obtained by foaming in the same manner as in Example 1 except that the ratio of the low-density polyethylene to the ethylene-vinyl acetate copolymer in Example 1 was changed to 25:75.
[0041]
The obtained foam had an apparent density of 49 kg / m ³ , a surface resistance of 0.5 × 10 ⁶ Ω, and an open cell ratio of 95%. The sound absorption characteristics were the same as in Example 1.
[0042]
Comparative Example 1
Except that the ratio of the low-density polyethylene to the ethylene-vinyl acetate copolymer in Example 1 was changed to 100: 0, foaming was carried out by the same formulation and the same method as in Example 1 to obtain a foam.
[0043]
Although the obtained foam had an apparent density of 48 kg / m ³ and a surface resistance of 0.5 × 10 ⁶ Ω, it was hard, had a low open cell ratio of 60%, and had a sound absorbing property as shown in FIG. Was inferior.
[0044]
Comparative Example 2
Except that the ratio of the low-density polyethylene to the ethylene-vinyl acetate copolymer in Example 1 was changed to 0: 100, foaming was carried out by the same formulation and the same method as in Example 1 to obtain a foam.
[0045]
The resulting foam had an apparent density of 49 kg / m ³ and an open cell ratio of 90%, but had a large surface resistance of 1.2 × 10 ⁷ Ω and was inferior in conductivity.
[0046]
Comparative Example 3
A foam was obtained by the same blending and the same method as in Example 1 except that the amount of the active ingredient N, N-dicyclohexylbenzothiazyl-2-sulfenamide in Example 1 was changed to 1 part. .
[0047]
The resulting foam had an apparent density of 48 kg / m ³ , a surface resistance of 0.5 × 10 ⁶ Ω, and an open cell ratio of 95%, but was inferior in sound absorbing properties as shown in FIG.
[0048]
Comparative Example 4
Except that the amount of N, N-dicyclohexylbenzothiazyl-2-sulfenamide, which is the active ingredient in Example 1, was changed to 10 parts, foaming was carried out by the same formulation and the same method as in Example 1, but satisfactory. No foam was obtained.
[0049]
FIG. 1 shows the measurement results of the reverberation chamber sound absorption coefficient of the conductive crosslinked polyethylene foam of the present invention.
[0050]
【The invention's effect】
As described above, the conductive cross-linked polyethylene-based foam according to the present invention can obtain excellent sound absorption by adding an active component that increases the amount of dipole moment to form open cells. In addition, by adopting a structure in which carbon powder is uniformly kneaded, excellent dispersibility can be obtained and the resistance value can be uniform, so that excellent radio wave absorption characteristics can be obtained. That is, it has both excellent sound absorbing properties and excellent radio wave absorption characteristics, and is suitable as a material for a radio wave absorber used in an anechoic chamber for measuring the performance of precision equipment.
[Brief description of the drawings]
FIG. 1 shows a measurement result of a sound absorption coefficient of a conductive crosslinked polyethylene foam of the present invention in a reverberation chamber method.

Claims (7)

Conductive cross-linked polyethylene based on a mixed resin composition of low-density polyethylene and ethylene-vinyl acetate copolymer, and added with conductive carbon, an active ingredient that increases the amount of dipole moment, a foaming agent, a foaming aid and a cross-linking agent Based foam. The conductive crosslinked polyethylene foam according to claim 1, wherein the cell structure of the conductive crosslinked polyethylene foam is open cells. 3. The conductive crosslinked polyethylene according to claim 1, wherein the active ingredient is contained in a proportion of 2 to 9 parts by weight based on 100 parts by weight of the mixed resin of the low density polyethylene and the ethylene-vinyl acetate copolymer. Based foam. The conductive crosslinked polyethylene foam according to claim 1, 2, or 3, wherein the active ingredient is N, N-dicyclohexylbenzothiazyl-2-sulfenamide. Foamable cross-linking based on a mixed resin composition of low-density polyethylene and ethylene-vinyl acetate copolymer, and adding and mixing conductive carbon, an active ingredient that increases the amount of dipole moment, a foaming agent, a foaming aid and a cross-linking agent A conductive crosslinked polyethylene foam which is filled with a conductive resin composition in a closed mold, heated under pressure and crosslinked, heated under normal pressure to foam, and then mechanically pressed to open the bubbles. Manufacturing method. 6. The conductive crosslinked polyethylene foam according to claim 5, wherein the active ingredient is contained in a proportion of 2 to 9 parts by weight based on 100 parts by weight of the mixed resin of the low density polyethylene and the ethylene-vinyl acetate copolymer. How to make the body. The method for producing a conductive crosslinked polyethylene foam according to claim 5 or 6, wherein the active ingredient is N, N-dicyclohexylbenzothiazyl-2-sulfenamide.

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