JP2992763B2 - Flame retardant radio wave absorber - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a radio wave absorber having flame retardancy.

[Conventional technology]

The use of a conductive cross-linked polyethylene foam as the material of the radio wave absorber is disclosed in Japanese Patent Application Laid-Open No. 63-192299, and the present applicant himself has produced a conductive cross-linked polyolefin foam useful as a radio wave absorber. A method has been developed and a patent application has already been filed (Japanese Patent Application No. 63-212388, etc.).

Such a conductive cross-linked polyolefin foam has extremely excellent weather resistance, and also has a uniform and low resistance value per unit length because carbon is uniformly dispersed, specifically, about 500 Ω / cm. Therefore, it has extremely excellent characteristics as a material of the radio wave absorber. Furthermore, since it is rich in elasticity and has higher mechanical strength than other foams, it is excellent in terms of workability, reliability, and durability when mounting the radio wave absorber.

[Problems to be solved by the invention]

However, plastic foams are generally easy to burn, and in particular, polyolefin resins, which are the bases of the conductive crosslinked polyolefin foams, are extremely easy to burn and have high combustion heat.

Therefore, in order to enhance the safety during use and handling as a radio wave absorber and to prevent the destruction of the radio wave absorber in the event of an accidental fire, the above conductive crosslinked polyolefin foam should be provided with flame retardancy. Is desired.

In this case, it is generally conceivable to add and disperse a flame retardant to the conductive crosslinked polyolefin foam (Japanese Patent Application No.
63-212388). However, even the radio wave absorber using the conductive cross-linked polyolefin foam thus obtained has a low level of flame retardancy and only passes a self-extinguishing test (for example, U.S. standard UL94HBF). Because, the conductive cross-linked polyolefin foam, the elongation of the polyolefin resin is reduced because a large amount of conductive carbon is added in the manufacturing process, and when a further large amount of a flame retardant is added, the expansion ratio is high. This is because a foam cannot be formed. In addition, the use of halogen-based flame retardants tends to be restricted due to the toxicity of the flame retardants and generated gas.

Accordingly, an object of the present invention is to provide a radio wave absorber which solves the above-mentioned drawbacks, retains the original excellent physical properties such as elasticity, flexibility and elongation and has flame retardancy. It is in.

[Means for solving the problem]

The flame-retardant radio wave absorber of the present invention is characterized in that the surface of a conductive cross-linked polyolefin foam is coated with an adhesive containing unexpanded thermally expandable graphite.

[Function and Mode of the Invention]

As in the present invention, when unexpanded thermally expandable graphite is dispersed in an adhesive and coated on the surface of a conductive crosslinked polyolefin foam, when a flame strikes the foam surface, it adheres to the foam surface. Even if the graphite is thermally expanded and has a pinhole in the coating layer, the surface of the foam can be completely covered and combustion of the foam can be prevented. In addition, since the adhesive serves as a binder between the unexpanded thermally expandable graphite and the foam, the unexpanded thermally expandable graphite does not fall off and impairs the flexibility of the foam itself. There is no.

As the material of the radio wave absorber, a conductive cross-linked polyolefin foam kneaded with a conductive substance such as carbon is used. The foam is obtained by an electron beam crosslinking method or an organic peroxide crosslinking method. For example, carbon to polyolefin,
Add a foaming agent, an organic peroxide and other additives as necessary, knead them, form them into a thick sheet or a long sheet with a mixing roll or extruder, press, jacket-type foamer or continuous foamer And foamed by heating. The present applicant has already applied for patents on a method for producing a conductive crosslinked polyolefin foam for an electromagnetic wave absorber (Japanese Patent Application Nos. 63-169079, 63-212388 and 1-93804). ). In the present invention, the conductive crosslinked polyolefin foam obtained by these methods can be cut into quadrangular pyramids to be used as a material for a radio wave absorber.
The present applicant has also developed a method for obtaining a radio wave absorber having a quadrangular pyramid shape with good material efficiency, and has filed a patent application (Japanese Patent Application No. 1-216825). A quadrangular pyramid conductive crosslinked polyolefin foam obtained by this method can also be used. The conductive cross-linked polyolefin foam used is
Particularly preferred is one having an average volume resistivity of about 500 Ωcm according to “3.2 Wightstone Bridge Method” of the Japan Rubber Association Standard SRIS2301.

The unexpanded thermally expandable graphite used in the present invention is produced by oxidizing or electrolytically oxidizing natural graphite powder, washing with water and drying (see Japanese Patent Publication No. 60-34492, Japanese Patent Application Laid-Open No. 63-195104). When heated with heat of around 1000 ° C, the layer spacing expands about 200 times and the bulk specific gravity becomes 0.0
It is the very light graphite of No. 1 and has excellent heat resistance.

The adhesive used in the present invention is not particularly limited, but an aqueous emulsion type adhesive is preferred from the viewpoints of odor, toxicity, flammability, influence on foams, and price.

In the present invention, as a method of dispersing the unexpanded thermally expandable graphite in the adhesive, if the adhesive is of an aqueous emulsion type, the viscosity is reduced by diluting with water, and the unexpanded thermally expandable graphite is added. And stir to disperse uniformly. Subsequently, the adhesive is applied to the surface of the conductive cross-linked polyolefin foam by spraying, brushing, or the like, thereby covering the foam surface,
dry.

The coating amount of the unexpanded thermally expandable graphite on the foam surface is preferably 3.0 mg / cm ² or more, particularly preferably 7.0 mg / cm ²
That is all.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the examples below.

Example 1 50 parts by weight of water was added to 100 parts by weight of an acrylic water-based emulsion-type adhesive (trade name: Bond SP220, nonvolatile content: 54%, manufactured by Konishi Co., Ltd.) to reduce the viscosity, and then unexpanded thermal expansion 100 parts by weight of graphite (trade name: CA-60, manufactured by Nippon Kasei Co., Ltd.) and 5 parts by weight of a surfactant were added to the adhesive, stirred and uniformly dispersed.

Next, a conductive cross-linked polyolefin foam (each product: New Perka LCX-200T, apparent density 0.055 g / cm ³ , volume resistivity 500 Ωcm, manufactured by Sanwa Kako Co., Ltd.) is square pyramid (height: 25
(0 mm, the bottom is a square having a side of 100 mm), and the adhesive in which the unexpanded thermally expandable graphite was dispersed was applied twice with a brush to the entire surface of the square pyramid.

The weight change of the quadrangular pyramid and the insoluble content of the unexpanded thermally expandable graphite and the adhesive were calculated by the following formula, and as a result, the amount of the unexpanded thermally expandable graphite per unit area was 10.0 mg / cm ^2. Was.

Here, the weight of the foam before treatment (g): A The weight of the foam after applying the adhesive to which the unexpanded thermally expandable graphite is added (g): B The unexpanded thermally expandable graphite and water Weight of adhesive added (g): C Weight of water added to adhesive (g): D Non-volatile content of adhesive used (percentage) / 100: E Surface area of foam coated with adhesive (cm ²⁾ ): F Adhesion amount of unexpanded heat-expandable graphite (mg / cm ² ): G
Gas burner (Product name: Prince gas burner GB2001,
(Prince Development Co., Ltd.) was radiated from a position 40 mm away for 2 minutes so that the flame reached the highest temperature (1130 ° C.). As a result, the surface of the square pyramid was depressed by about 1 cm,
The conductive crosslinked polyolefin foam did not ignite.

Example 2 A conductive crosslinked polyolefin foam was prepared in exactly the same manner and under the same conditions as in Example 1, except that an adhesive containing unexpanded thermally expandable graphite was applied in a single application. Processed.

The adhesion amount per unit area of the unexpanded thermally expandable graphite was calculated by the same calculation formula as in Example 1, and as a result, it was 5.5 mg / cm ² . In addition, as a result of performing a combustion test in the same manner as in Example 1, a depression having a depth of 2 cm was generated, but the conductive crosslinked polyolefin foam did not ignite.

Comparative Example 1 An adhesive prepared by adding the unexpanded thermally expandable graphite prepared by the same formulation and the same method as in Example 1 was thinly applied once on a quadrangular pyramid made of the same conductive crosslinked polyolefin foam as in Example 1. Was applied.

2.2 m of unexpanded, heat-expandable graphite per unit area
g / cm ² , and the same combustion test as in Example 1 was carried out. As a result, the conductive crosslinked polyolefin foam was ignited.

From these results, in order to impart sufficient flame retardancy to the radio wave absorber, the amount of the unexpanded thermally expandable graphite should be 3.0 mg / cm ²
It is necessary.

〔The invention's effect〕

As described above, the radio wave absorber of the present invention is obtained by dispersing unexpanded thermally expandable graphite in an adhesive, and coating this on the surface of the conductive crosslinked polyolefin foam. Due to the excellent heat resistance of expanded graphite and expansion when heated, it is extremely flame-retardant.
Title No.14 of CFR (Code of Federal Regulations)
(Aeronautics and Space), and pass the combustion test applied to flooring materials, seat cushioning materials, etc. as specified in Part 25.853 (b).

Furthermore, since the unexpanded thermally expandable graphite is coated on the surface of the conductive cross-linked polyolefin foam with an adhesive, it is different from the conventional one in which a large amount of flame retardant is mixed in the foam. On the contrary, the excellent properties of the conductive crosslinked polyolefin foam as a radio wave absorber are maintained without impairing the flexibility and elongation of the foam.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-20536 (JP, A) JP-A-1-235399 (JP, A) JP-A-1-194398 (JP, A) JP-A-62-162 81799 (JP, A) JP-A-2-39933 (JP, A) JP-A-3-99496 (JP, A) JP-A-3-99491 (JP, U) (58) Fields investigated (Int. ⁶ , DB name) H05K 9/00

Claims (1)

(57) [Claims] 1. A flame-retardant radio wave absorber characterized in that the surface of a conductive cross-linked polyolefin foam is coated with an adhesive containing unexpanded thermally expandable graphite.