WO2003071846A1

WO2003071846A1 - High polymer microcellular foam conductive gaskets and method for preparing thereof

Info

Publication number: WO2003071846A1
Application number: PCT/KR2002/001689
Authority: WO
Inventors: Jae-Moon Song; Won-Hyung Kang
Original assignee: S & K Poly Tec Co., Ltd.
Priority date: 2002-02-19
Filing date: 2002-09-17
Publication date: 2003-08-28
Also published as: CN1276698C; CN1620845A; AU2002328450A1

Abstract

The present invention relates to a gasket to shield electronic apparatus from electric and electromagnetic waves and to a method for preparing the gasket, wherein the gasket is made of a conductive polymer elastic body. More particularly, an elastic body itself or an elastic body laminated with conductive fabric or metal film is punched to make holes, then a conductive material is applied thereto by dipping (filling) and coating to reduce surface resistivity and volume resistivity, and as a result, a gasket having impact and vibration absorption properties and conductivity is prepared.

Description

High Polymer Microcellular Foam Conductive Gaskets and Method for Preparing Thereof

FIELD OF THE INVENTION

The present invention relates to a conductive gasket for shielding a communication apparatus from electromagnetic interference (EMI) and/or radio frequency interference (RFI), and to a method for preparing the gasket. The gasket of the present invention is prepared by endowing a polymer elastic body with conductivity, such that the gasket has easy applicability, high conductivity, mechanical and physical strength. In addition, while maintaining high impact absorption and vibration damping with low compression set, the gasket of the present invention maintains original hardness and uniform cell structure of the foam required in general use.

BACKGROUND OF THE INVENTION

In designing various electronic communication apparatuses, undesirable conducted or radiated electrical disturbances from electrical or electronic apparatus are main causes of confusing the function of electronic communication apparatus such as deteriorating the functionality thereof, making noise, damaging image, shortening the life cycle and causing the production of inferior goods. As shielding materials to shield electromagnetic interference an/or radio frequency interference, metal plate, metal plated fabrics, conductive paints, conductive tapes or the like are conventionally used.

As conventional conductive materials with a polymer elastic body or a polymer sponge elastic body, open cell low-density polyurethane foam or closed cell polyethylene foam are known. However, because they have a low density, inferior shock and impact absorption properties and low conductivity (volume resistivity: 10⁴ Ohm- cm or greater), they are mainly used for packaging and carrying the electronic communication apparatus.

As a conventional elastic body used in the preparation of gaskets for preventing impact or vibration, there is high-density polyurethane foam. This polyurethane foam is prepared by mixing a common poly-hydroxy compound, organic polyisocyanate compound, chain extender, catalyst, silicone foam stabilizer and air or inert gas such as nitrogen in a mixer to obtain a foam composition (specific gravity: 0.1 — 1.0 grs/cu.cm), then cross-linking the foam composition and casting in a uniform thickness by using common coating apparatus on an endless conveyor belt equipped with heat treatment. Fabrics or plastic film may be laminated on one side or both sides of the polyurethane foam, if necessary (US patent No. 3,755,212, US patent No. 3,862,879, US patent No. 4,216,177 and US patent No. 5,859,081).

To endow conductivity on the high-density polyurethane foam, the following method may be applied.

One method is that, as conductive pigments, fine powders of common carbon black, graphite or metals such as silver, copper, nickel, aluminum and the like are uniformly dispersed in the poly-hydroxy compounds before foaming process of the polyurethane foam. However, in order to acquire proper conductivity by using these conductive pigments in high-density polyurethane foam, the conductive pigments should form a continuous pathway to connect the conductive pigments particles in the cross-linked polyurethane foam. That is, fine powdered particles of carbon black, graphite or metal, as conductive pigments, should be positioned very close to each other to be able to pass electrons through. For example, to obtain conductivity by compounding carbon black to the polyurethane foam, about 15~30 wt% of carbon black to the total weight of poly-hydroxy compounds should be added according to the particle size and conductivity of the carbon black. To obtain higher conductivity, more than 40wt% of carbon black should be added. However, when these large quantities of carbon black are added, dispersion of the particles is difficult and the melt viscosity of the foam compound becomes very high, and as a result, the carbon black particles conglomerate with each other; such a high viscosity of the foaming compound interferes with the proper feeding of the components, and the mixing and foaming processes become extremely difficult. In addition, the specific gravity of the resulting product is high and the physical properties reduced. As a result, it is impossible to use the above polyurethane foam as a gasket material that requires impact and vibration absorbing properties. On the other hand, when metal powders are used, more than 2~3 times the weight of the carbon black are required to acquire stable conductivity, which makes the dispersion much more difficult and increases the specific gravity. Therefore, the above conductive pigments are not applicable to the polyurethane foam with impact and vibration absorption properties in addition to volume conductivity. (Fig. 9) Conventionally, as EMI and RFI shielding materials that require surface conductivity only, metal plate, metal plated fabrics and conductive tapes are used, wherein they are prepared by conductive pigments which is coated on various textiles, non-wovens, papers, plastic films or the like. However, because these materials do not have volume conductivity, they are used for only where surface conductivity is required, and as a result, polymer sponge materials that have both surface conductivity and volume conductivity are not yet disclosed.

Whereas, polymer sponge materials such as soft elastic polyurethane foams having low specific gravity were used in packaging and transporting various electronic components or semiconductor chips and in shielding window frames or electromagnetic equipment. However, since these soft polyurethane foam materials are not conductive and have high electrical resistance, to acquire a required shape having conductivity specific processes are needed, that is, after additional cutting or casting of the polyurethane foam (polymer sponge) into a required shape, then adhering conductive textiles or non-wovens applied with adhesives on the surface thereof was performed; or a complicated molding process such as injecting polyurethane foaming compounds into casting mold wherein conductive textiles or non-wovens are already mounted on the surface of casting mold is performed. As the result, manufacturing costs are high, and so these are not widely used as the gasket material for electronic communication apparatus that requires sufficient impact and vibration damping properties as well as EMI shielding effect. (Fig. 9) To endow the polymer sponge with necessary volume conductivity and

EMI- and RFI- shielding properties, common conductive carbon black, graphite, gold, silver, copper, nickel, aluminum or the like should be compounded into the polymer-sponge-foaming compound. However in this case, considering the process difficulties and physical properties, there are limitations to the amount of conductive pigments compounded to endow conductivity, as mentioned above, and as a result, it is difficult to obtain volume resistivity less than 10⁴ Ohm - cm. Therefore, highly shock absorbing and impact damping polymer sponge gaskets having high surface and volume conductivity with low hardness and compression set values have not yet been prepared. Under this circumstance, the present inventors studied and researched to solve the above problems and to endow the polymer elastic foam with conductivity.

As the result, the present inventors accomplished the present invention and prepared a gasket for electronic communication apparatus by perforating an elastic body, then applying a conductive material onto the surface of the elastic body, and coating the walls of the perforated holes or filling the holes with the conductive material; or by laminating conductive fabric or metallic film on both sides of the elastic body or on one side of the elastic body, then perforating the elastic body, and coating the walls of the perforated holes or filling the holes with the conductive material. The gasket of the present invention is prepared by a very simple process compared with the above-mentioned conventional arts and does not change the original physical properties of the elastic body. Further, the gasket of the present invention can have surface and volume conductivity while simultaneously maintaining the shock absorbing and vibration or impact damping abilities at the same time.

Therefore the purpose of the present invention is to provide a gasket that is easily prepared and that can obtain the required surface conductivity and volume conductivity at the same time while maintaining the properties of shock absorbing and vibration damping ability without sacrificing its original physical properties.

A further purpose of the present invention is to provide a method for preparing the gasket.

SUMMARY OF THE INVENTION

The present invention provides a gasket for electronic apparatus prepared by cutting a conductive sheet, prepared by perforating an elastic body sheet then applying conductive paints containing conductive materials on the surface of the elastic body sheet and applying the conductive paints on the walls of the perforated holes or filling the holes with conductive paints.

Further, the present invention provides a gasket for electronic apparatus prepared by laminating conductive material sheets such as conductive fabric or metallic film on the elastic body sheet before the perforation mentioned above. That is, after laminating conductive material sheets, for example, conductive fabric such as non-woven based conductive fabric, or metallic films of copper, aluminum, nickel, silver or gold, on both sides or on one side of the elastic body sheet, then perforating the laminated elastic body sheet, filling the holes made by the perforation with the conductive paints or coating on the walls of the holes with the conductive paints, and finally cutting the elastic body sheet (conductive sheet), the gasket of the present invention is provided.

DETAILED DESCRIPTION OF THE INVENTION

Any kind of elastic body may be used in the present invention without limitation on the condition that the elastic body is made of polymers having elasticity. For example, polymer resins such as polyurethane foam sheet, PVC, silicone, ethyl vinyl acetate copolymer or polyethylene sheet, rubbers such as NR, SBR, EPDM, NBR or Neoprene, synthetic rubber solid sheet or sponge sheet may be used.

Thickness of the sheet may be 0.5mm ~ 10.0mm. The thickness of the sheet may be selected according to the purpose of the application.

Conductive materials to endow electric conductivity are, for example, carbon black, graphite, gold, silver, copper, nickel or aluminum, but not limited thereto. These conductive materials are used in the form of fine powder for the compounding of coating paints to coat the surfaces of the sheet and to coat the walls of the holes or to fill the holes.

A person skilled in the art may easily control the dry film thickness of the layers of the conductive coating materials coated on the sheet according to the purpose of the application. The dry film thickness of the layers of the conductive coating is expressed on the basis of "mil" units, in which 1 mil is equal to 0.025mm. Preferably, the dry film thickness of the layers of the conductive coating is 0.1 mil to 3.0 mil, and more preferably 0.3 mil to 1.0 mil. However, the thickness may be different in the different applications. When laminating a conductive material sheet such as conductive fabric or metallic film on the elastic body sheet, the thickness of the conductive fabric or metallic film may be 1 — 10 mil, more preferably 2 ~8 mil. In particular, the thickness of the conductive fabric would be better in the range of 3 ~4 mil, and the thickness of the metallic film would be better in the range of 2 ~ 3 mil. The conductive fabric may be prepared by depositing or electrolessly plating copper, nickel, aluminum, silver, gold or mixed powders of copper/ nickel or copper/silver onto a flexible woven of polyester or nylon or onto a non-woven. Common chemical vapor deposition methods may be applied to deposit the metals onto the fabrics. Also common electroless plating methods may be applied to plate the metals onto the fabrics. Further, a conductive fabric may be prepared by weaving conductive yarn that is coated with conductive material.

As metallic films, gold film, silver film, copper film, nickel film, aluminum film or graphite films may be used. There are a lot of commercial conductive fabrics, for example, SR- W23B

(PET/Ni+Cu+Ni; Ripstop; manufactured by E-Song Corporation), ST-W29B (PET/Ni+Cu+Ni; Taffeta; manufactured by E-Song Corporation), SN-W05B (PET/Ni+Cu+Ni; Non-woven; manufactured by E-Song Corporation), SM-W13B (PET/Ni+Cu+Ni; manufactured by E-Song Corporation), SP-W22B (PET/Ni+Cu; manufactured by E-Song Corporation), SR-W23D (PET/Ni; manufactured by E-Song Corporation) or ST-W25E (PET/Cu; manufactured by E-Song Corporation).

As metallic films, conductive adhesive-tape-type metallic films such as T200 (Fabric/ Ripstop backing; manufactured by E-Song Corporation), T214 (Fabric/taffeta backing; manufactured by E-Song Corporation) or T254 (backing with Non-woven; manufactured by E-Song Corporation) may be used. And, as metallic foil adhesive-tape-types, there are T267, T288, T286 or T291 (manufactured by E-Song Corporation).

As adhesives to adhere the conductive fabrics or metallic films to the polymer elastic sheets, commercial conductive adhesives may be used. A person skilled in the art can choose a suitable adhesive. Preferably, elastic adhesive is better. The size of sheet and perforated hole and the distance between holes are different according to the application of the gasket. For example, in case of a notebook computer having a comparably large size, large and thick sheets are used, and the sizes of holes are large and the distances between holes are also large. However, in case of a cellular phone having a comparably small size, comparably thin sheets are used and the sizes of holes and the distances between holes are also small.

Preferably, the hole size of 0.1 ~ 3.0mm is better. The number of holes per 1 cm of sheet may be 1 ~ 100. The arrangement of holes is not restricted specifically, but considering efficiency of preparation and electric conductivity, the arrangement may change. For example, after perforating holes in a line, when perforating a parallel second line of holes, the angles between the nearest holes of the previous line may be arranged as 30° ~90° . The size, number and arrangement of the holes are different according to the applications of the gaskets. The reason for restricting the size of holes to 0.1mm~3.0mm is to allow the conductive coating paints to fill the hole easily. When the conductive pigments (for example, conductive carbon black, graphite, silver, copper, nickel and aluminum powder) are dispersed in a water-soluble or solvent mixture of acryl resin, epoxy resin or urethane resin to become conductive paints, due to high loading of conductive pigments, the viscosity of the conductive paints becomes high, therefore the sizes of the holes are adjusted to allow the viscous conductive paints to flow into the hole easily. The sizes of the holes may be controlled according to the viscosity and permeation properties of the conductive paints.

In a smaller electronic apparatus, when the size of hole is more than 1.0 mm, the ratio of the total area of the holes to that of the elastic sheet becomes more than 25%, and as a result, the strength of elastic sheet is weakened and shock and vibration absorption ability is damaged.

On the contrary, in case of a larger electronic apparatus, on the condition that the total area of the holes is limited to less than 25% of the elastic sheet area, the size of the holes can be increased more than 1.0 mm up to 3.0mm and the distance between the holes also can be increased more than 2.0mm up to 5.0mm, so that applying the conductive paints to fill the holes becomes easier and perforation cost decreases while the volume conductivity increases.

In case of a delicate small gasket for an extremely compact electronic apparatus, a hole size of 0.3mm is too big for application, and the size of the holes may decrease less than 0.3 mm to 0.1mm and the distance between the holes may decrease less than 0.5mm to 0.3 mm. In this case, in order to fill and coat the such small holes of 0.3mm ~ 0.1mm diameter effectively, the viscosity of the conductive paints may be reduced by water or solvent within a limit that does not deteriorate conductivity by dilution. As shown in Fig. 4, by a pressured vacuum absorbance process, the above diluted conductive paints are forced to fill the holes or to coat the walls of the holes having very small sizes of 0.1mm~0.3mm diameter.

To be able to do dies cutting the elastic sheet into desired sizes and shapes as a gasket and to keep the ratio of total areas of the holes constant, it is 02 01689

10

preferred to perforate holes at an angle of 45° (Fig. lc) or 60° (Fig. lb) to those in a parallel line.

Any methods of coating and filling the conductive paints on the elastic sheet and into the holes may be applied, unless the functions of the elastic sheets are not damaged. Preferably, the above conveyor belt coating system with pressure roller is more recommendable.

For example, in a conveyor belt coating system, after perforating holes to have a uniform size and distance between holes with constant angle, the high density foam sheet prepared by casting is coated and the holes is filled or the walls of holes is coated uniformly with proper conductive paints compounded with fine powders of carbon black, graphite, silver, copper, nickel or aluminum (Fig. 3 and 4). The same procedure may be applied for conductive fabrics or metallic film laminated sheets.

Because the gasket of the present invention is prepared by coating and/or filling conductive paints on the shock and vibration absorbing sheet and can be mounted in narrow and compact spaces of even an extremely small electronic apparatus, the gasket of the present invention can provide multiple functions such as shock absorbing and vibration damping properties to protect electronic apparatus and can shield EMI/RFI occurring from insides and outsides of the instruments, and as a result, it can maximize the function of the electronic apparatus.

In the present invention the method for preparing sheet is not restricted. The sheet can be prepared by a common method of preparing polymer foam resin. The method of perforating is not restricted. A person skilled in the art may prepare a sheet and perforate the sheet for the use of gasket.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. la is a plan view of the perforated high-density polyurethane foam sheet of the present invention having shock and vibration absorption properties.

Fig. lb shows a perforation with an angle of 60° between holes in parallel lines. Fig. lc shows a perforation with an angle of 45° between holes in parallel lines.

Fig. 2 is a side view of the perforated high-density polyurethane foam sheet of the present invention having shock and vibration absorption properties.

Fig. 3 is a processing schematic view of applying, filling holes with, drying and cross-linking conductive paints on the perforated high-density polyurethane foam.

Fig. 4 is a processing schematic view of applying and filling holes with conductive paints by using a pressured roller and a vacuum absorbing device, and drying and cross-linking conductive paints on a shock and vibration absorbing high density polyurethane foam sheet that is finely perforated with a hole size of 0.3mm or less.

Fig. 5 is a high-density polyurethane foam sheet of the present invention having shock and vibration absorption properties, prepared by perforating, then by applying, filling holes with, drying and cross-linking conductive paints. Fig. 6 is a gasket for electronic communication apparatus obtained by dies cutting the high density polyurethane foam sheet having shock and vibration absorption properties, prepared by perforating, then by applying, filling holes with, drying and cross-linking conductive paints.

Fig. 7a is a gasket obtained by die cutting the high density polyurethane foam sheet prepared by laminating a polyester fabric deposited with a copper/ nickel mixture onto both sides of a polyurethane foam sheet, perforating, then by applying, filling holes with, drying and cross-linking conductive paints.

Fig 7b is a cross-sectional view of the gasket of Fig. 7a. Fig. 8a is a gasket obtained by die cutting the high density polyurethane foam sheet prepared by laminating a polyester fabric deposited with a copper/ nickel mixture onto one side of a polyurethane foam sheet, perforating, then by applying, filling holes with, drying and cross-linking conductive paints. Fig 8b is a cross-sectional view of the gasket of Fig. 8a.

Fig. 9 is a conventional EMI/RFI shielding foam wherein conductive fabrics are adhered on the whole surface of the polyurethane foam to endow conductivity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is described more specifically with reference to the examples.

As a polymer elastic body sheet, various common polymer sponge elastic bodies conventionally used may be adopted. However, in the present invention, mechanically frothed high-density polyurethane foam that is foamed in a speed mixer under the presence of air or an inert gas such as nitrogen then molded and formed on an endless conveyor belt to form an endless sheet shape was adopted. Particularly, high-density polyurethane foam prepared by mixing polyhydroxy compound, organic polyisocyanate compound, chain extender, catalyst, silicone foam stabilizer and air or inert gas such as nitrogen in an intensive speed mixer, then casting to a desired thickness on an endless conveyor belt and finally heat treated to form cross-linking is preferable.

[Methods for preparing gasket]

Method 1

Commonly-made polymer elastic body sheet having shock and vibration absorbing properties is perforated uniformly, and conductive paints are applied on the whole upper and bottom surfaces of the elastic body sheet and in the holes to coat the surface of the sheet together with to coat or fill (or dip) the holes in order to obtain required surface and volume conductivity depending on the applied conductive paints, and finally the above coated elastic body sheet is dried and cross-linked to be used as conductive gasket for electric communication apparatus having shock and vibration properties.

Method 2

A gasket for electronic apparatus may also be prepared by laminating a conductive material sheet onto both sides of the elastic body sheet, then perforating the laminated elastic body sheet to make uniform holes, applying conductive paints on the surfaces and the holes made by the above perforation (dipping and coating), and finally drying and cross-linking. In the above process, the conductive material sheet comprises conductive fabrics prepared by depositing or electrolessly-plating copper, aluminum, nickel, silver, gold, copper/nickel mixture or copper/silver mixture on non- woven or the like. In addition, conductive fabrics prepared by weaving a conductive yam coated with conductive coating materials may be used, and metallic films of copper, aluminum, nickel, silver or gold can be used as conductive material sheet. In this gasket, the degree of surface conductivity and volume conductivity can be easily controlled by the selection of the conductive material sheet or the conductive paints.

Method 3 This method is the same as Method 2 except that the conductive material sheet such as conductive fabric or metallic films of copper, aluminum, nickel, silver or gold is laminated only on one side of the elastic body sheet.

In these cases, the degree of the surface conductivity and volume conductivity can be easily controlled according to the selection of the conductive material sheet and conductive paints applied.

Conventionally, conductive pigments are directly compounded with the foaming composition, and therefore, the physical properties of conductive elastic polymer foam deteriorate and the quantities of conductive pigments to be used are restricted, and as a result, it is impossible to obtain less than 10⁴ Ohm- cm of volume resistivity. However, in the present invention, surface conductivity and volume conductivity can be easily controlled according to the conductive material sheet and conductive coating paints applied. For example, when carbon black or graphite is used as conductive pigments of the conductive coating paint, volume resistivity of 10³ Ohm- cm is obtained, and when silver or nickel powder is used, volume resistivity of 10^"5 Ohm- cm is available. Therefore, the gasket of the present invention satisfies the need for shielding the electronic communication apparatus from EMI/RFI.

Hereinafter, the examples of the present invention are described in detail with reference to the drawings.

<Examρle 1>

Fig. 1 shows perforated holes with 0.5mm diameter on a high-density polyurethane foam sheet (hereinafter called "foam sheet") that was used as a polymer elastic body of the present example. By using a hank punching press conventionally used in perforating metal plate and a punch with a 0.5mm hole diameter, and by hitting the foam sheet on a die with an angle of 60° between holes in parallel lines and with a pitch of 1.5mm, the perforated holes were made as shown in Fig. lb.

Resulting perforated foam sheet has the plan view of Fig. la. Thickness of the foam sheet was 1.0mm.

The above perforated foam sheet is die cut to be a gasket for electronic apparatus having the properties of shock and vibration absorption and shielding EMI/RFI (electromagnetic interference/radio frequency interference) and to have a shape necessary for every requirements. To maintain a uniform volume resistivity, the angle between holes in parallel lines was set at 60° as seen in Fig. 1 (Fig. lb).

Fig. 2 is a cross-sectional side view of the perforated foam. Surface conductivity and volume conductivity can be easily controlled by selecting the conductive coating material, thickness of the dried film layer coated on the foam sheet, hole size, and perforating density of holes and pitches.

Fig. 3 is a processing schematic view showing the procedure of applying, filling holes with, drying and cross-linking the conductive paints. The above perforated foam sheet was rolled in a feeding device (100), and Black

Conduction 103™ made by Kemco International Associates, Inc., (West Lake,

Ohio) wherein conductive pigments are conductive carbon black was used as a conductive coating paint. As seen in Fig. 3, the conductive coating paint was applied on the foam sheet by a double sided (upper and lower) dipping device

(102) placed in a container (103), and then the coated foam sheet was compressed by compressing rollers (104, 105) that is designed to coat the conductive paint in a uniform thickness by controlling the space between the two rollers to have the thickness of the foam sheet reduced to half of the original thickness, which means that the pressure between the compressing rollers was controlled to make the original 1.0 mm thickness of the foam sheet become 0.5 mm after the pass. The foam sheet coated on one side with conductive paints passed a cross-linking tunnel (106) equipped with a heated blowing device at a temperature of 100 ^°C ~ 150 ^°C , and was finally wound in a winding device ( 107) . The foam sheet coated on one side and cross-linked was inverted and transferred to the feeding device (100) again, and went through the above steps again to coat the opposite side of the sheet by turning the side of coating. Thus, conductive coating layers were uniformly formed on both sides of the foam sheet and the perforated holes were also uniformly filled. As a result, a polyurethane foam sheet having uniform volume and surface conductivity without sacrificing the original physical properties of the foam was obtained.

The dry film thickness of the conductive coating layer (paint layer) on the foam sheet was 0.3 mil (1 mil is 0.025mm.), and the volume resistivity and surface resistivity were both 10 Ohm of degrees. For reference, when the film thickness of the conductive coating layer after drying was increased to 2 mil, volume resistivity of the foam sheet for gasket reduced to 35 Ohm - cm and surface resistivity reduced to 40 Ohm/sq. The above resitivity was measured by using a "VOAC86A Multimeter" from Iwatsu Electric Co., Ltd, Japan.

Fig. 5 shows a high density polyurethane foam gasket obtained by dies cutting the high density polyurethane foam sheet of the present invention having shock and vibration absorption properties, that is prepared by perforating, then by applying, filling holes with, drying and cross-linking conductive paints. As seen in Fig. 5, conductive coating film layers were formed on upper and lower sides (301, 302) of the gasket and side walls (303) of the perforated holes, and in particular, the holes were filled with conductive coating paints. This gasket has a low permanent compression set, excellent shock and vibration absorbing properties, and excellent volume conductivity and surface conductivity at the same time. The gasket prepared above may be preferably applied as an LCD protecting gasket in an electronic or communication apparatus requiring high accuracy and functionality such as cellular phone, PDA, note book computer or the like, to enhance their functionality and durability.

The same procedure as of Example 1 was adopted except that the thickness of sheet was 0.5mm, hole diameter was 0.3mm, hole pitch was 1.5mm and the conductive paints were applied as shown in Fig. 4.

The method of Fig. 4 is applied to make a gasket for a minute electronic or communication apparatus. When the diameter of perforated hole is as small as 0. lmm ~ 0.3mm, it is difficult to fill the perforated holes with the conductive coating paints by the method described in Fig. 3 of Example 1, therefore, a method as shown in Fig. 4 is recommendable instead. As shown in Fig. 4, while foam sheet rolled in a feeding device (200) was passing the coating rollers (201), conductive coating paints (CP) were supplied onto the foam sheet by a knife coater (202). The above-coated foam sheet was compressed by press rollers (203, 205) while passing vacuum absorbing devices (204, 206) equipped with beneath the press rollers to coat and fill the holes with conductive paints by force, and passed through a drying and cross-linking tunnel (207) equipped with a heated air blowing device at a temperature of 100^°C~ 150^°C , then wound in a winding device (208).

The next process was the same as that of Example 1. In this example, "C-100™" (conductive carbon basis) manufactured by Conductive Compounds, Inc Londonderry, NH 03053 was used as the conductive coating paints. The dry film thickness of the conductive coating paints layer on the foam sheet was 1 mil after drying. The volume resistivity and the surface resistivity were both less than 75 Ohm of degree.

<Example 3> The same procedure as of Example 1 was adopted except that the angle between holes in parallel lines was 45° (Fig. lc).

The dry film thickness of the conductive coating paints layer was 0.5mil, and the volume resistivity and the surface resistivity of the gasket foam sheet were both less than 10² Ohm of degree.

The same procedure as of Example 1 was adopted except that a kind of water base urethane coating "CO- 120™" containing conductive carbon black and manufactured by Conductive Compound, Inc. was used as conductive paints.

The dry film thickness of the conductive coating paints layer was 0.3mil, and the volume resistivity and the surface resistivity of the gasket foam sheet were both less than 10⁴ Ohm of degree.

The same procedure as of Example 2 was adopted except that the angle between holes in parallel lines was 45° (Fig. lc).

The dry film thickness of the conductive coating paints layer was 0.5mil, and the volume resistivity and the surface resistivity of the gasket foam sheet were both less than 700 Ohm of degree.

The same procedure as of Example 2 was adopted except that "G972A™" using Conductive Iron Oxide/Graphite CO- 120 manufactured by Kemco International associates, Inc. was used as conductive material.

The dry film thickness of the conductive coating paints layer was 1 mil (0.025mm), and the volume resistivity and the surface resistivity of the gasket foam sheet were both less than 350 Ohm of degree.

Conductive fabrics having surface resistivity of 0.06 Ohm/sq were laminated on both sides of polyurethane foam sheet having shock and vibration absorbing properties and thickness of 0.5mm, and holes were perforated with a diameter of 0.3mm, a pitch of 1.5mm and an angle between holes in parallel lines of 60° .

In this example, as a conductive fabric, a conductive polyester fabric

"ST-W29B" (PET/Ni+Cu+Ni; manufactured by E-Song Corporation) that was woven by a yarn deposited with copper/nickel/copper was applied and laminated on the polyurethane foam. The thickness of the conductive fabric was 3 mil and weight was 80 g/m². As a conductive coating paints, "Series 599-Y1325" mainly containing copper and manufactured by Spraylat Corporation, Mt. Vernon, NY was applied on the above perforated polyurethane foam laminated with a conductive fabric as shown in Fig. 4 for coating and filling. The methods for applying, coating and filling the holes with the conductive paints were the same as those of Example 2.

The surface resistivity of this example was 0.06 Ohm/sq and volume resistivity was 0.05 Ohm- cm.

<Example 8> The same procedure as of Example 7 was adopted except that the conductive fabric was laminated on one side of the high-density polyurethane foam.

The film thickness of the conductive coating layer was 1 mil, and the volume resistivity of the gasket foam sheet was 0.04 Ohm- cm and surface resistivity was 0.05 Ohm/sq.

<Example 9> The same procedure as of the Example 7 was adopted except that "Series

599-B3730M" using silver coated copper from Spraylot Corporation was used as a conductive coating paint.

The dry film thicknesses of the conductive coating layers of the sheets and perforated holes were 1 mil, the volume resistivity of the gasket foam sheet was 0.03 Ohm ^• cm and the surface resistivity was 0.04 Ohm/sq.

INDUSTRIAL APPLICABILITY

The gasket of the present invention can be easily prepared with simple equipment and has excellent physical and mechanical properties such as excellent shock and vibration absorption properties, and therefore is useful for producing high quality electronic or communication apparatus.

Claims

1. A method for preparing a gasket for electronic apparatus, having both surface conductivity and volume conductivity, comprising the steps of: preparing a polymer elastic body sheet; perforating the sheet to make holes; applying conductive coating paints on the sheet to coat the surface of the sheet and to fill the holes or to coat the walls of the holes, in order to make a conductive layer; and cutting the sheet having a conductive layer thereon.

2. The method for preparing a gasket for electronic apparatus according to claim 1, wherein the polymer elastic sheet is polyurethane foam, PVC, Silicone, Ethylene vinyl acetate copolymer, Polyethylene, NR, SBR, EPDM, NBR, Neoprene sheet or sponge sheet.

3. The method for preparing a gasket for electric apparatus according to claim 1, wherein the conductive coating paints contain at least one selected from the group consisting of powders of carbon black, graphite, gold, silver, copper, nickel and aluminum.

4. The method for preparing a gasket for electronic apparatus according to claim 1, wherein the angle between holes in parallel lines is 45° or 60° .

5. The method for preparing a gasket for electronic apparatus according to any one of claims 1 to 4, further comprising a step of laminating a conductive material sheet such as conductive fabric or metal film on one side or both sides of the elastic body sheet before the step of perforating the sheet to make holes.

6. The method for preparing a gasket for electronic apparatus according to claim 5, wherein the conductive fabric is prepared by depositing or electrolessly plating copper, nickel, aluminum, silver, gold, copper/nickel mixture or copper /silver mixture on polyester, nylon or non- woven to give conductivity.

7. The method for preparing a gasket for electronic apparatus according to claim 5, wherein the conductive fabrics are such fabrics as those being woven by using yarns having conductive material coated thereon.

8. A gasket prepared by a method disclosed in any one of claims 1 to 7.