GB2144138A

GB2144138A - Electrically conductive layer

Info

Publication number: GB2144138A
Application number: GB08320606A
Authority: GB
Inventors: Alan William Atkinson; David Rushworth Hurst; Melanie Jane Rickman
Original assignee: T&N Materials Research Ltd
Current assignee: T&N Materials Research Ltd
Priority date: 1983-07-30
Filing date: 1983-07-30
Publication date: 1985-02-27
Also published as: GB2144138B; GB8320606D0

Abstract

An electrically conductive layer is formed on a substrate of plastics material by applying to the substrate a fluid composition comprising fine particles of exfoliated graphite impregnated with and suspended in a solution of a binder, preferably a polymeric organic material, in an organic solvent and then evaporating the solvent to leave a solid exfoliated-graphite-containing film of surface resistivity not greater than 100 ohms per square. The procedure is especially useful in treating the walls of a housing for electrical or electronic equipment, to reduce passage of electromagnetic radiation through the housing.

Description

SPECIFICATION Method of treating a substrate of plastics material This invention relates to a method of treating a substrate of plastics material.

Plastics materials, for example polyamides such as nylon, polycarbonates, polyethylene, polypropylene, ABS polymers and polymethylmethacrylate, are in general electrically nonconductive. We are concerned with treating them so as to make their surfaces conductive, by which we mean: give them a surface resistivity not greater than 100 ohms per square. The present invention provides a method of treating a substrate of plastics material to form on it an electrically conductive layer, and the method makes use of a film-forming composition which contains exfoliated graphite as its electrically conductive component.

Exfoliated graphite is a well known material used in the manufacture of gaskets, packings and sealing materials, and is made from natural flake graphite or well ordered synthetic graphite by a process whose first step involves treatment of graphite with a strong oxidising agent. Thus when graphite is treated with sulphuric acid under strong oxidising conditions it swells slightly owing to the interposition, between the layers of hexagonally arranged carbon atoms constituting the graphite lattice, of bisulphate ions HSO4- and neutral (unionised) sulphuric acid molecules H2SO4.

If the slightly swollen material is washed with a large volume of water, the intercalated neutral sulphuric acid molecules become replaced by water molecules, but the HSO4- ions are largely unaffected. When this compound is freed from adherent wash water and heated for a few seconds at temperatures of the order of 1,000"C, the interstitial water suddenly vapourises and causes a great expansion of the compound in the direction (i.e. perpendicular to the plane of the carbon atom layers), so that the final dimension may be 100 times or even more its initiai value.Owing to the suddenness of expansion, particles of the expanded substantially sulphate-free product do not possess the highly ordered structure of particles of the original graphite; they are of an irregular form which is usually described as vermiform (worm-like), and are extremely porous. They are also distinct from particles of the oxidised graphite material known as graphitic oxide or 'graphitic acid'.

According to the invention there is applied to a substrate of plastics material a fluid composition comprising fine particles of exfoliated graphite impregnated with and suspended in a solution of a binder in an organic solvent, and the solvent is evaporated to leave a solid exfoliated-graphite-containing film of surface resistivity not greater than 100 ohms per square.

As will appear later, by suitable formulation of the fluid composition films can be formed of surface resistivity much lower than 100 ohms per square. For one application of particular importance, namely treating a substrate of plastics material which forms or is to form a housing for electrical or electronic equipment, in order to shield the equipment from interference by electromagnetic radiation, or to reduce the extent to which radiation emitted from the equipment escapes to cause interference outside, it will usuaily be desired to obtain a film of surface resistivity not more than 20 ohms per square.

To avoid altering the external appearance of the housing surface treatment will usually be performed on that surface which forms (or is to form) the interior of the housing.

Usually, in order to obtain a film of good mechanical properties, and particularly of good resistance to abrasion, it is preferred to employ a fluid composition in which the weight ratio graphite particles:binder is not more than 1:2, and particularly not more than 1:4. A preferred lower limit is 1:40, and a particularly preferred lower limit is 1:20.

The preferred method of applying the fluid composition is by spraying, although other methods such as brushing or dipping can be used if desired. For spraying, the viscosity can be kept suitably low by controlling the weight ratio of solvent: (binder plus exfoliated graphite particles) in the fluid composition.

The binder is preferably an organic polymer material of molecular weight low enough to give it adequate solubility in the common organic solvents, such as aliphatic alcohols, ketones and esters, and liquid aromatic hydrocarbons, or mixtures of these solvents. Suitable organic polymeric materials are polyurethanes, alkyd resins, acrylic ester polymers and chlorinated rubbers. Suitable solvents are those of boiling point below about 200"C, and preferably those of boiling point below 150"C, for example ethanol, propanol, acetone, methyl ethyl ketone, ethyl acetate and toluene.

The fluid composition employed can be made by impregnating exfoliated graphite with a solution of the binder in organic solvent, and subjecting the impregnated material to high shear (as in a colloid mill or a ball mill). It is important that the exfoliated graphite should be impregnated before being subjected to high shear:otherwise the operation of shearing results in undue compression of the exfoliated material, and this makes absorption of binder solution very slow.

For the impregnation step, it is convenient to employ exfoliated graphite which has been lightly compacted into a body of density in the range 15-60 kg/m3.

The invention if further illustrated by Examples 1 and 2 later, which utilise as starting material an exfoliated graphite which is conveniently prepared as follows: Preparation of Exfoliated Graphite Natural flake graphite (10 gms; 99% carbon; 80% retained on a sieve of aperture 300 tm diameter) was gently stirred for + hour with a mixture (90 gms; added at room temperature) of sulphuric acid, nitric acid and water, in weight proportion such that H2SO4: HN03: H2O = 74:15:11.

The reaction mixture was filtered off and the graphite compound formed by reaction (probably C24. + HSO4- .nH2SO4) was washed until substantially free from the molecular species H2SO4 by running water at room temperature (20'C) for one hour. The fully washed material was drained from adherent water and dried at 80"C for 3 hours, and then conventionally flame exfoliated (temperature 1,200he) to volatilise the bound water content. The exfoliated product has a density of about 4 kg/m3 (0.004 gm/cm3).

Example 1 Exfoliated graphite was gently compacted into a slab of density 40 kg/m3 and the slab was laid in a pool of liquid obtained by diluting with toluene the acrylic ester resin/solvent composition (solvent, toluene and xylene mixture) commercially available under the designation UCB 902 (Union Chimique Belge). The liquid contained about 25% solids, and was used in a quantity such that its solids content amounted to 10 times the weight of the slab. All the liquid was absorbed into the slab.

The impregnated slab was then ball-milled for 8 hours. Conditions were: milling pot capacity, 5 litres; pot diameter, 23 cm; ceramic balls diameter, 2.5 cm; pot rotation speed, 67 rpm.

The fluid composition which resulted from ball-milling the impregnated slab could be sprayed by means of a conventional paint sprayer. On application to a rigid panel of plastics material (polymethylmethacrylate, thickness 5 mm) it lost solvent by evaporation overnight at ambient temperature (25'C) to form a tough film (thickness about 0.2 mm) of surface resistivity 7-10 ohms per square.

To measure its effectiveness as a shield against electromagnetic radiation, the coated panel was mounted over a square aperture of size 25 cm cut in the side of a wall of a room otherwise well shielded. A range of signal genrators with appropriate transmitter antennae was placed outside the room, facing the aperture, and matched receivers were mounted inside the room, facing the aperture.

Transmitter, receiver and antennae combinations were selected to permit measurements over frequencies which included the range QFnO-? MHz.

By measuring received signal levels at selected frequencies, with and without the test panel in place over the aperture, the attenuation characteristics of the coated panel were determined over the frequency range 960-450 MHz. The measured attenuation was 26-36 dB. (Attenuation of 20-40 dB is regarded as satisfactory for most domestic and commercial equipment.) Example 2 The preparative procedure of Example 1 was repeated with the difference that ballmilling was carried out for a shorter time (3 hours instead of 8).

The fluid composition thus formed was sprayable, but owing to the larger size of the exfoliated graphite particles it was less easy to spray than the composition of Example 1. The dried film formed on evaporation of the solvent had a desirably low resistivity (7-8 ohms per square) and good attenuation(30-40 dB over 960-450 MHz frequency range).

Claims

1. A method of treating a substrate of plastics material to form on it an electrically conductive layer, in which there is applied to the substrate a fluid composition comprising fine particles of exfoliated graphite impregnated with and suspended in a solution of a binder in an organic solvent, and the solvent is evaporated to leave a solid exfoliated-graphite-containing film of surface resistivity not greater than 100 ohms per square.

2. A method according to claim 1 in which said flm has a surface resistivity not greater than 20 ohms per square.

3. A method according to claim 1 or 2, in which the weight ratio of graphite particles: binder in the fluid composition is less than 1:2.

4. A method according to claim 3, in which said weight ratio is less than 1:4.

5. A method according to any one of claims 1 to 4 in which the binder is an organic polymeric material.

6. A method according to claim 5, in which the binder is an acrylic ester polymer.

7. A method according to any one of claims 1 to 6 in which the organic solvent is one having a boiling point less than 150"C.

8. A method according to any preceding claim in which the substrate of plastics material forms a housing for electrical or electronic equipment.

9. A method of treating a substrate of plastics material to form on it an electrically conductive layer substantially as herein described with reference to the Examples.

10. A substrate of plastics material having on it an electrically conductive layer formed by the method of any preceding claim.