GB2158737A - Thermoplastic sheet shielded by means of an electrical conductive layer - Google Patents

Abstract

A sheet of plastic material suitable to constitute the casing of electronic equipment, coated with an electrical-noise shielding conductive layer, characterized in that it is constituted by an ABS resin and that the electrical conductive layer is constituted by a mixture comprising at least an ABS resin, polyvinyl chloride, a lubricant, a plastifier and graphite.

Description

SPECIFICATION Thermoplastic sheet shielded by means of an electrical conductive layer It is known the problem of the electronic equipment having to be protected against the electromagnetic interferences, and generally from electric noises.

These noises can be of different types, they can be caused e.g. by sudden changes of electrical potential in external artificial sources, such as light sources, electronic computers, electrical motors, radio transmitters or depending on natural phenomena.

The electrical disturbances of artificial origin are nowadays more and more frequent, due to the increasing diffusion of electronic and radio equipment, with an increase of the amount of electromagnetic signals in the atmosphere.

Due to the more and more sophisticated nature of the present electronic equipment, and hence to the higher sensitivity thereof of the electrical disturbances, also the need is increasing of being able to protect them at best.

Another aspect relating to the electromagnetic interferences is that of the protection of electronic equipment emitting signals whose interception is not authorized. In this case, the most effective shielding techniques are required.

In advanced-technology Countries, precise legislative regulations exist relatively to the protection of electronic equipment from electrical disturbances.

In the U.S.A. the Decree N 20780 of Federal Commission for Telecommunications fixes the highest intensities of the noise which a computer or a videogame may emit. In the Federal Republic of Germany, the VDE-Verlag (German Electrotechnic Engineers' Association) has published the standards (0871-0879) on the same subject.

In the U.K., the paragraph n0 10 of 1949 Law on aerial transmissions grants the Minister powers to formulate regulations on this subject.

The protection of electronic equipment can be achieved by externally coating the casings thereof with an electrical conductive shield, so that the electric noise can be absorbed, reflected or transmitted through such a shield.

The casings of electronic equipment are presently mostly constituted by plastic materials, whose shielding can be obtained e.g. by means of an outer coating of metal, or by painting. It is clear that the formation of a metal conductive layer or of a layer of shielding paint on the housings of the equipment represents an additional production cost which one would desirably avoid.

The present trend is hence that of resorting only to the plastic materials to the purpose of achieving the shielding: the plastic materials are however generally insulating materials, so that they difficultly absorb or reflect the electrical noise to a sufficient extent.

In this connection, shields of plastic materials such as polystyrene and polyethylene are known. With the use of these materials however further problems are associated, such as the insufficient mechanical, above all impact, strength, and the poor dimensional stability. It becomes therefore it necessary to provide these materials e.g. with reinforcing frames, which causes still further production costs.

Purpose of the present invention is to provide the solution of the technical problem of obtaining a material for the coating of electronic equipment, having ideal shielding characteristics against the electrical noise and the electromagnetic and radio interferences, whose mechanical strength and in particular the impact strength be high, which can be manufactured at low cost, which is suitable to be formed according to the desired design and geometrical structures, without requiring additional treatments of shielding by means of metal or paint coats, or of other protecting layers added after that the equipment housing has been formed.

These properties are achieved according to the present invention with a sheet of plastic material suitable to constitute the housing of electronic equipment, such sheet being coated with an electrical conductive layer shielding the electrical noise, characterized in that it is constituted by an ABS resin, and that said conductive layer is constituted by a mixture comprising at least an ABS resin, polyvinyl chloride, a lubricant, a plastifier, and graphite.

The sheet of the invention is produced e.g. by calendering or by extrusion in several thicknesses: the conductive layer, coupled on to one or on to both faces of the sheet is applied during the same manufacturing of the sheet. The sheet so manufactured can be thermo- formed, and can hence be moulded according to the desired shape to form the housing or the casing of the electronic equipment, without that it be thereafter necessary to apply on to the finished housing other shielding coatings.

The conductive layer according to the invention has been experimentally shown to be able to efficaciously shield the electrical noise within a very wide signal very close to 104 ohm. Moreover, the layer has shown to have high impact and tensile strength. It can also be stretched up to elongation ratios at least 3:1.

To purely exemplifying and certainly nonlimitative purposes, an Example of composition of a sheet according to the invention is shown.

Example ABS-resin sheet coated with a surface layer constituted by the following mixture (% by weight): ABS 50 % PVC 22 % Phtalic acid ester (plastifier) 15 % Fatty acid ester (lubricant 0,5 % Carbon black (graphite 9,5 % Stabilizer 3% The main properties of the sheet are the following: Surface resistivity ohm 1.104 IZOD impact strength J/m 350 Tensile strength MPa 35 Flexural modulus GPa 2 Density 1,1 From what has heen hereinabove disclosed and exemplified, it can be understood how in general a sheet according to the invention allows the whole of the previously listed characteristics to be achieved.

Such a sheet, by being produced as a single piece only, exclusively from plastic materials, without requiring any subsequent treatments or coupling, is a substantially cheap solution of the problem of obtaining shielding housings or casings of electronic equipment.

It is useful to stress once again the characteristics of thermo-formability of the plate to the desired shapes, a finished efficaciously shielding casing being thus obtained, not requiring further subsequent treatments.

This basic advantage is of course accompanied by the further advantages of a very high shielding power, and of optinum properties of mechanical strength.

Claims (7)

1. Sheet of plastic material suitable to constitute the housing of electronic equipment, such sheet being coated with an electrical conductive layer shielding the electrical noise, characterized in that it is constituted by an ABS resin, and that said conductive layer is constituted by a mixture comprising at least an ABS resin, polyvinyl chloride, a lubricant, a plastifier, and graphite.

2 Sheet according to claim 1, characterized in that said conductive layer has the following composition: ABS 50%, PVC 22%, plastifier 15%, lubricant 0,5%, graphite 9,5% stabiliser 3%.

3. Sheet according to claim 1, characterized in that said conductive layer is applied on to both faces of the sheet.

4. Protective casing for electronic equipment, constituted by a sheet according to claim 1.

5. A sheet of plastic material according to claim 1 and substantially as hereinbefore described.

6. A sheet of plastic material substantially as described in the Examples herein.

7. A protective casing for electronic equipment including a sheet according to claim 5 or 6.