JPH0322075B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a radio wave absorbing molded article used for preventing or blocking the reflection of radio waves, and in particular provides a radio wave absorbing molded article that satisfactorily absorbs a wide range of high frequencies in the high frequency range of 1 to 20 GHz. It is. More specifically, in a radio wave absorbing molded product made of ferrite, a short fiber conductive filler, and a high molecular polymer, by using a modified conductive filler, a radio wave absorbing molded product that has stable high frequency absorption performance can be obtained. It provides the body. Radio wave absorbing molded bodies have long been used, for example, to prevent radar interference caused by phantom images from structures such as bridges and buildings. Furthermore, in recent years, with the progress and diversification of electronic devices, opportunities for using high frequencies have increased, and the need for blocking high frequencies is rapidly increasing. Conventionally, such high-frequency radio wave absorbers include those that use carbon black as a filler, those that use ferrite, and those that use a mixture of both. Furthermore, in recent years, a thin radio wave absorber having a broadband high-frequency absorption band has been developed in which ferrite and a conductive filler with a relatively high aspect ratio are used in combination, dispersed in a high molecular weight polymer, and molded. This type of radio wave absorber can utilize the mutual effect of radio wave absorption performance due to high conductivity due to the inclusion of a conductive filler and high magnetic loss due to the inclusion of ferrite.
Shows excellent radio wave absorption performance. Such a radio wave-absorbing molded article is produced by dispersing and blending an appropriate amount of ferrite and a short fibrous conductive filler into a powder, pellet, or solution of a high molecular weight polymer.
Then, it is molded into a sheet or a housing by extrusion sheeting, injection molding, compression molding, or the like. These molded bodies are generally used alone or in combination with a separately obtained sheet having a 1/4λ deformation function. By the way, although this radio wave absorbing molded product made of ferrite, short fibrous conductive filler, and high molecular weight polymer is recognized to exhibit excellent absorption performance, there are large variations in performance between molded products. It has been pointed out that a major problem is that product management using predetermined standards results in extremely poor yield rates. For example, in order to control the performance standards of a radio wave absorber based on two factors: reflection absorption loss (dB) and frequency band (Hz), it is possible to control the performance standards of a radio wave absorber by molding the same polymer, the same compound composition, and the same molding method. Some of the molded products have extremely narrow effective frequency bands, and in some cases, the effective frequency band does not reach the predetermined level of reflection absorption loss. In addition, it is recognized that the occurrence of directionality in radio wave absorption performance is one of the manifestations of dispersion. It is slightly speculated that the cause of these variations in radio wave absorption performance is that the conductive filler in the form of short fibers is oriented along the flow of the molten polymer during the molding process, resulting in directional properties. However, no clear cause has been identified. In view of these circumstances, the present inventors conducted intensive studies to obtain a radio wave absorber with stable high-frequency radio wave absorption properties, and as a result, pre-treated the short fibrous conductive filler used in the present invention with a specific modifier. The present inventors have discovered that a molded article having dramatically stable radio wave absorbing properties can be obtained by using the same method, and have completed the present invention. That is, the present invention provides a radio wave absorbing molded article comprising a ferrite, a short fibrous conductive filler, and a high molecular weight polymer, wherein the conductive filler is selected from mono- or bis-organic phosphate esters or salts thereof. It is a radio wave absorbing molded article that has been modified with at least one or more modifiers. The ferrite used in the present invention is a ferrite powder represented by the general formula MFl ₂ O ₄ (M is a divalent metal such as Fl, Mn, Ni, Co, etc.), and is generally iron ferrite (Fl ₃ O ₄ ) is used as the main component. From a practical point of view, ferrites of the type that incorporate heavy metals after they have been used to treat wastewater containing heavy metals, so-called by-product ferrites, can be advantageously used. The short fibrous conductive filler used in the present invention is a metal fiber such as brass, aluminum, or iron, or a carbon fiber, and has a relatively large aspect ratio, which is the ratio of the length of the fiber to the diameter. is used. Examples of preferred ranges include diameters of 5 to 200 μm and lengths of 0.1 to 20 mm.
It is a short fiber with an aspect ratio of 5~
4000 range. In the present invention, what is satisfactorily used as a modifier for the short fibrous conductive filler is a substance selected from mono- or bis-organic phosphate esters or salts thereof (hereinafter collectively referred to as acidic organo-phosphate esters). It is. In the present invention, the reason why the performance of the radio wave absorbing molded article obtained by pre-treating the short fibrous conductive filler with an acidic organic phosphate ester is stabilized is not clear, but in any case, It is a surprising fact that a radio wave absorbing molded article with stable performance can be obtained by such a relatively simple pretreatment. The acidic organic phosphoric acid ester referred to in the present invention is

[Formula] or

It is represented by the structural formula [Formula] (where R, R ₁ and R ₂ are aliphatic or aromatic hydrocarbons or derivatives thereof), and is one of the three acidic groups of phosphoric acid or Two are in esterified form. These salts are those in which the remaining acidic groups that have not yet been esterified are neutralized with alkali, and include sodium salts, potassium salts, ammonium salts, calcium salts, magnesium salts, and the like.
In the present invention, one or more of monoorganic phosphates, bisorganic phosphates, and salts of these acidic organic phosphates are selected and used as the modifier. Further, each acidic organic phosphate ester does not need to be pure. For example, when acidic organic phosphates are produced industrially, a mixture of monoorganic phosphates and bisorganic phosphates is obtained. This mixture is used as a modifier as is. The acidic organic phosphate esters used as modifiers are typified by the substances listed below. Monomethyl phosphate, bismethyl phosphate,
Monoethyl phosphate, bisethyl phosphate, monopropyl phosphate, bispropyl phosphate, monoisopropyl phosphate, bisisopropyl phosphate, monobutyl phosphate, bisbutyl phosphate, monooctyl phosphate, bisoctyl phosphate ester, mono-2-ethyl-hexyl phosphate, bis-2-ethylhexyl phosphate, monoisodecyl phosphate, bisisodecyl phosphate, monodecanol phosphate, bisdecanol phosphate, mono Butoxyethyl phosphate, bisbutoxyethyl phosphate, monochloroethyl phosphate, bischloroethyl phosphate, mono-2,3-dichloropropyl phosphate,
Bis-2,3-dichloropropyl phosphate, mono-2,3-dibromopropyl phosphate, bis-2,3-dibromopropyl phosphate, monophenyl phosphate, bisphenyl phosphate, hydroxymethyl phosphate monoacrylate acid esters, hydroxyethyl bisacrylate phosphate, hydroxymethyl monomethacrylate, hydroxyethyl bismethacrylate phosphate, and the like. Further, there are sodium salts, potassium salts, ammonium salts, calcium salts, magnesium salts, etc. of these esters. Methods for modifying the short fibrous conductive filler with these acidic phosphate esters include (1) a method of bringing the modifier into direct contact with the conductive filler without using a medium, and (2) a method of modifying the conductive filler in a medium. This method is one in which the medium is removed after contacting the medium. In addition, in the method (1) above, several seconds is sufficient for the contact time between the modifier and the conductive filler. In the method (2) above, 1 minute to 30 minutes at most is usually sufficient. Regarding the modification method (1) above, it should be noted that most of the modifiers used in the present invention are liquid at room temperature, so the modifier and conductive filler are mixed in a mixer. Modification can be carried out by supplying and mixing.The amount of modifier used in this case is 0.5 to 10% of the amount of conductive filler.
The amount is preferably on the order of % by weight. When using these modifiers, the viscosity decreases when the modifier is heated to 50 to 100°C, so it is also possible to employ the following modification method. That is, the modification is carried out by immersing the conductive filler in the heated modifier or by bringing the modifier and the conductive filler into contact in a sprayed state. Further, if necessary, excess modifier can be removed by washing with a lower alcohol such as methanol or ethanol. To explain the modification method (2) in more detail, water, alcohols, or a mixture thereof is used as a medium, and the modifier of the present invention is dissolved or partially suspended in these media. Since the modification proceeds effectively in any of these conditions, the medium can be appropriately selected depending on the process. Alcohols here include methanol, ethanol, propanol,
It refers to lower alcohols such as isopropanol. When water is used as the medium, a surfactant can also be used to improve dispersion of the modifier in water. In a modification method using a medium, drying is the easiest unit operation for removing the medium. An example of an embodiment using such a medium is shown below. The modifier is dissolved or suspended in water, alcohol, or a mixture thereof.
Then, the conductive filler is added under stirring and stirring is continued for 1 minute to 30 minutes. The slurry is strained to obtain a wet cake. By drying this, a modified conductive filler is obtained. Incidentally, an important requirement of the present invention is to modify the short fibrous conductive filler.
Just to be sure, it has been confirmed that there is no problem even if ferrite, which is another component, is modified in the same way. Therefore, for convenience of the manufacturing process, it is acceptable to modify the ferrite and the conductive filler at the same time. The polymers used in the present invention include, for example, phenolic resins, urea resins, epoxy resins, unsaturated polyester resins, silicone resins,
Thermosetting resins such as polyurethane resins, polyethylene, polystyrene, polypropylene, polyvinyl chloride, ABS, nylon, polycarbonate,
Examples include thermoplastic resins such as polyacetal, polybutyl terephthalate, polyphenylene oxide, and polyether sulfone. These polymers contain various commonly used improving additives, such as weather resistance improvers, heat stabilizers, facial dyes,
It does not matter at all that it contains lubricants for molding processing. Further, the above-described high molecular weight polymers may be used alone or as a mixture of two or more. In the radio wave-absorbing molded article of the present invention, the blending ratio of the ferrite, the conductive filler modified with acidic phosphate ester, and the high molecular weight polymer can be selected in light of the performance standards desired by the user. However, a preferred blending range is a blend of 5-80 parts by weight of ferrite, 1-40 parts by weight of modified short fibrous conductive filler, and 10-50 parts by weight of high molecular weight polymer. Can show quantity. These blends are carried out using a common drum blender, ribbon blender, or the like. These blended compositions are molded into a processed product such as a sheet or a housing by an appropriate method depending on the polymer used. For example, for thermosetting resins such as phenolic resins, urea resins, epoxy resins, unsaturated polyester resins, silicone resins, and polyurethane resins, press sheet methods, compression molding methods, transfer molding methods, etc. are commonly used. In the case of unsaturated polyester resins, etc., a B-stage sheet is obtained by the SMC method (sheet molding compound method) and then press molded, or by the BMC method (bulk molding compound method), A method of obtaining a block body and then press-molding it into a molded body may also be used. In addition, an extrusion sheet molding method or an injection molding method may be used. For thermoplastic resins such as polyethylene, polystyrene, polypropylene, polyvinyl chloride, ABS, nylon, polycarbonate, polyacetal, polybutylene terephthalate, polyphenylene oxide, polyether sulfone, etc.
Rather, extrusion molding methods, injection molding methods, etc. are generally used. However, in the production of small lots, it may be more economical to use the press method.
The thus obtained radio wave-absorbing molded product can be made into a sheet and pasted on building walls, bridges, etc. to prevent radio wave illusions, or can be molded into casings for electronic devices to prevent signal noise in devices. It can be used for. In addition, as one mode of use, a further radio wave absorption effect can be obtained by laminating and using the molded article according to the present invention and a molded article having 1/4λ metamorphic ability. Hereinafter, examples of the present invention will be shown and further explained. Example 1 As a short fibrous conductive filler, aluminum fibers having an average thickness of 60 μm and a length of 3 mm were selected, and 1.5 kg of the fibers were modified using monoisodecyl phosphate ester. The modification was carried out at room temperature in a reaction vessel equipped with a 15 stirrer. In other words, in the reaction pot
10 g of water was charged, and then 30 g of monoisodecyl phosphate was added and dispersed in the water. The above aluminum fiber was added to this and stirred for 30 minutes. Next, the obtained slurry is passed through a centrifuge,
The resulting cake was dried at 60° C. for 5 hours in a hot air circulation dryer to complete the modification. This modified aluminum fiber, by-product ferrite, and polypropylene powder (Mitsui Noblen JHN, manufactured by Mitsui Toatsu Chemical, (trade name)) were mixed at a weight ratio of 18:52:30, and a 40 mm diameter uniaxial It was extruded using a screw extruder to obtain pellets weighing about 8 kg. Using this pellet as raw material, the thickness is 1.2mm and the external size is 15mm.
A cm square flat plate was injection molded. The injection molding machine used was a screw in-line injection molding machine (Meiki Seisakusho SJ-35) with a capacity of 4 ounces. Separately, a 3.5 mm thick sheet made of polypropylene (Mitsui Noblen JHH) and by-product ferrite in a 60:40 weight ratio was injection molded and used as the λ modified layer. That is, this λ modified layer sheet and the radio wave absorbing sheet containing the modified aluminum fiber were laminated, and the reflection absorption loss (dB) curve between 8 GHz and 13 GHz was measured. As an evaluation standard, 7G
Hz to 10.5GHz (3500M as frequency band)
Hz) with a reflection absorption loss coefficient exceeding 20 dB, and the pass rate was measured. The result is
Of the 47 samples, 44 passed, resulting in a pass rate of 93.6%, an extremely high value that had never been achieved before. Comparative Example 1 A radio wave absorbing sheet was obtained in the same manner as in Example 1 except that the aluminum fiber was not modified, and the pass rate of the reflection absorption loss coefficient in the same frequency band was measured. As a result, 27 out of 43 samples passed, giving a pass rate of 62.8%. Example 2 Brass fibers with an average thickness of 80 μm and a length of 3 mm were used as short fibrous conductive fillers.
1.5 Kg thereof was modified with a mixture of mono-2-ethylhexyl phosphate and bis-2-ethylhexyl phosphate (mixture molar ratio 7:3). The modification method was the same as in Example 1. The modified brass fiber is mixed with by-product ferrite and polystyrene powder in a weight ratio of 20:56:24, and extruded using a single screw extruder to produce approximately 7 kg.
of pellets were obtained. This pellet is heated to a thickness of 1.0 mm and the inner size is
Sprayed in a 15cm square spacer, the temperature was 220℃,
A sheet was obtained by hot pressing at a pressure of 30 kg/cm ² . This sheet was evaluated in the same manner as in Example 1,
A λ-modified sheet made of polypropylene and ferrite is used to measure the reflection absorption loss curve.
As a result, 29 out of 30 samples passed, giving a pass rate of 96.7%, demonstrating extremely stable performance. Comparative Example 2 A press sheet was obtained in the same manner as in Example 2, except that the brass fiber was not modified, and the pass rate of the reflection absorption loss coefficient was measured. As a result, out of 30 samples, 22 passed the test.
The pass rate was 73.3%. Example 3 A brass fiber modified with a mixture of mono-2-ethylhexyl phosphate and bis-2-ethylhexyl phosphate (mixture molar ratio 7:3) was prepared in the same manner as in Example 2. did. Unsaturated polyester resin suitable for bulk molding compound (manufactured by Mitsui Toatsu Chemical Co., Ltd., Ester-M-2101 (trade name)), by-product ferrite, and the above-mentioned modified brass fiber were mixed in a weight ratio of 24:56:20. 0.72 parts of magnesium oxide and tertiary butyl perbenzoate
Add 0.24 parts and knead for 10 minutes in a kneader.
A block-shaped bulk molding compound was obtained. Wrap this in vinylon film and age for 3 hours, then use a 1mm thick spacer to
A cm square sheet was produced using a hot press. The temperature of the press was 140° C., the press time was 7 minutes, and the pressure was 40 kg/cm ² . The reflection absorption coefficient pass rate of this sheet was measured in the same manner as in Example 1. The λ modified layer is a 60:40 mixture of unsaturated polyester (Mitsui Toatsu Chemical Co., Ltd., Estar M2101 (trade name)) and by-product ferrite.
Magnesium oxide in a mixture consisting of (weight ratio) 2.4
Part, tertiary butyl perbenzoate 0.6
A 3.5 mm press sheet obtained by hot-pressing the bulk molded compound by adding 50% of the total amount was used. As a result of the evaluation, 48 out of 50 samples passed the test, for a pass rate of 98%. Comparative Example 3 A bulk molding compound sheet of unsaturated polyester resin was obtained in exactly the same manner as in Example 3, except that the brass fiber was not modified, and the pass rate of the reflection absorption loss coefficient was measured. As a result, 11 out of 20 samples passed, for a pass rate of 55%.

Claims (1)

[Claims] 1. In a radio wave-absorbing molded article consisting of ferrite, a short fibrous conductive filler, and a high molecular polymer, the conductive filler is at least one selected from mono- or bis-organic phosphate esters or salts thereof, or A radio wave absorbing molded article characterized in that it has been modified with two or more modifiers.