GB2144273A - Housing for electrical or electronic equipment - Google Patents

Abstract

A housing for electrical equipment is made by moulding under heat a composition comprising an organic thermosetting resin which includes exfoliated graphite in an amount which is enough to impart to the moulded article an electrical resistivity not greater than 10 ohm-cm. The housing has an excellent capacity to reduce the level of electromagnetic radiation emitted by equipment within it and to shield electronic equipment from external electromagnetic radiation. <IMAGE>

Description

SPECIFICATION Housing for electrical or electronic equipment This invention relates to a housing for electrical or electronic equipment.

Housings for electrical or electronic equipment are often made of metal, but manufacturers are now preferring to use plastics materials (eg polycarbon ates),forthese havetheadvantageoflowerfabrica- tion costs, reduced weight and more flexibility in design. They have, however, the disadvantage that they are virtually transparent to electromagnetic radiation. As the number of radiation sources (such as citizens band radios) grows, and as electronic equipment (microprocessors for example) becomes more complex and so more sensitive to external electromagnetic fields, so it becomes more important to cut down radiation emitted from the one and to shield the other from that electromagnetic radiation which does reach it.

The present invention provides a housing for electrical or electronic equipment which is suitable for the purpose just outlined. The housing is hotmoulded from an organicthermosetting resin moulding composition in which there is included an amount ofthe material known as exfoliated graphite (sometimes called expanded graphite) which is enough to imparttothe moulded article an electrical conductivityequivalentto a resistivity notgreaterthan 10 ohm-centimetre. Because of the structure of exfoliated graphite, as little as 0.5% by weight of it can provide a continuous, electrically conductive network in the moulded article. Preferably, however, the content of exfoliated graphite in the moulding composition is in the range 2.5 - 20% byweig ht and a content of 5-20% by weight is particularly preferred.

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 or 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 bywater 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 orderof 1 ,000 C, the interstitial water suddenly vapourises and causes a great expansion ofthe compound in the direction (ie perpendicular to the plane of the carbon atom layers), so that the final c dimension may be 100 times or even more its initial value. Owing to the suddenness of expansion, particles ofthe expanded substantially sulphate-free product do not possess the highly ordered structure of particles of the original graphite; they are of an irregularform which is usually described as vermiform (worm-like), and are extremely porous. They are also distinctfrom particles of the oxidised graphite material known as 'graphitic oxide' or 'graphitic acid'.

In spite of their high porosity, particles of exfoliated graphite are not particularly fragile; they are indeed much less fragile than the hollow carbon microspheres whose inclusion in electromagnetic radiation absorption material is described in British patent specification 1 411731. Such microspheres are expensiveto make, andtheirspherical structure requires them to be used in considerably higher proportion in a non-conductive matrix in orderto obtain a composite with an electrical conductivity equal tothat imparted bya given content or exfoliated graphite.Thermosetting resin composi tionswhich include hollow carbon micro-spheres in a proportion adequate to give electrical conductivity suitable for our purposes are in fact very difficultto mould because of their low plasticity.

Thethermosetting resins preferably used are phenolic resins, particularly phenol formaldehyde resins, and unsaturated polyester resins, but epoxy resins, urea formaldehyde resins and melamine formaldehyde resins can be used if desired. Ordinari ly, the organicthermosetting resin will form at least 20% by weight of the moulding composition, but not more than 95% byweightthereof. The balance of the moulding composition is essentially non-conductive moulding additive such as reinforcing fibre and non-fibrous filler. There may be up to 60% by weight of reinforcing fibre, and up to 60% by weight of filler.

Other moulding additives such as catalyst, inhibitor and mould release agent may be employed in an amountnotgreaterthan about 10% by weight.

The invention is further illustrated by Examples 1 to 3 later, which utilise as starting material an exfoliated graphite which is conveniently prepared asfollows: Preparation of Exfoliated Graphite Natural flake graphite (10 gms; 99% carbon; 80% retained on a sieve of aperture 300,am diameter) was gently stirred for 112 hourwith a mixture (90 gms; added at room temperature) of sulphuric acid, nitric acid and water, in weight proportion such thatH2SO4: HNO3: H2O=74:15:11.

The reaction mixture was filtered off and the graphite compound formed by reaction (probably C24+ . HSOJ. .nH2SO4), was washed until substantial- lyfree 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 conventionallyflame exfoliated (temperature 1200 C) to volatilise the bound water content. The exfoliated product had a density ofabout4kgim3 (0.004gm/ cm3).

To avoid the difficulties of handling very large volumes of such low-density material, it may be convenientto compactthe product, to a density not greaterthan75-100kglm3.

EXAMPLE 1 Exfoliated graphite (409) was gently compacted into a slab of density 18kg/m3 and the slab was laid in a pool (20409) of alcoholic solution of phenolic resole resin (viscosity 60 centistokes at ambient temperature) containing 25% by weightsolids. When the slab had absorbed all the liquid it was broken down in a mixerto form crumbs of 2-1 Omm maximum dimen sion,andchoppedglassfibre(6mm long; 240g) having a dressing compatible with the phenolic resin was blended in. After 15 minutes the mixture was transferred to an oven and held at 1050for 100 minutes to evaporate the solvent and part-cure the resin.

The solvent-free part-cured mixture was then transferred to a heated mould and press-cured at 170 Cforone hourtoform a housing with walls 3.5mm thick, of density 7 500kg/m3 and resistivity 0.18 ohmcm The capacity of the material of the housing to reduce transmission of electromagnetic radiation at various frequencies was investigated as follows ~~ A panel of the material was mounted over a square aperture of size 25cm cut in the side of a wall of a room otherwise well shielded. A range of signal generators with appropriate transmitter antennae was placed outside the room, facing the aperture, and matched receivers were mounted insidethe room, facing the aperture.Transmitter, receiver and antennae combinations were selected to permit measurements over frequencies in the range of 50-1000 MHz.

By measuring received signal levels at selected frequencies, with and without the test panel in place overthe aperture, the attenuation characteristics of the panel were determined overthe stated frequency range. The measured attenuation was 32-64dB, as set out below. (Attenuationof20AOdBisregardedas satisfactoryfor most domestic and commercial equipment.) Frequency (MHz) Attenuation IdBJ 960 64 700 64 450 64 200 56 100 52 50 32 Mechanical properties ofthe moulded material forming the housing were:~ Tensile strength (Mpa) 13 Tensile modulus (Gpa) 8 Flexural Strength (MPa) 46 Flexural Modulus (GPa) 7 Izod impact strength (Notched edgewise) (Jim) 80 EXAMPLE2 The following ingredients were tumble mixed for 10 minutes::~ A B Exfoliated graphite (compacted to bulkdensity 11 kg/m3) 509 1009 Chopped Glass fibre (3mm long) 5509 5009 * Powdered phenolic resin 400g 4009 Calcium stearate (mould release agenl) 159 15g * Novolac resin admixed with 12% by weight of hexamine; meltviscosity = 82 kpoise min. at 136 C Each of mixtures A and B was press-cured (moulding pressure 4-16 MN/m2) for onehour at 1700C in a heated mould to give a housing of wall thickness 3.5mm and density about 1750 kgim3.Other properties of the press-cured materials were:~ Electromagnetic radiation shield capacities: Attenuation (dB) Frequency(MHz) A B 10000 120 140 960 72 100 700 70 100 450 64 110 200 58 90 100 44 85 50 32 55 Tensile strength (MPa) 45 39 Tensile modulus (GPa) 14.3 12 Flexural strength (MPa) 108 73 Resistivity (ohm-cm) 0.35 0.07 EXAMPLE3 The following ingredients were mixed in a Z-blade mixer: Exfoliated graphite (compacted to bulk density 11 keg/ m3) 100g Polyester resin solution in styrene (2 parts by weight of polyester derived from propylene glycol, maleic acid and phthalic anhydride; 1 part by weight of styrene) 700g followed by Polyvinyiacetate solution in styrene (1:1) (low profile additive) 200g Conventional catalyst and inhibitor mixture 10g Followed by Chalk (filler; average particle size 3,am) 770g Calcium stearate (mould release agent) 30g followed by Glass fibre (6mm long; with polyester-compatible dressing 200g After a total of 20 minutes mixing, the material was transferred to a heated mould and pressured at 1 300Cfor 1.1/2 minutes to form a housing of wall thickness 4mm and density 1590 kg/m3. Its resistivity was 0.5 ohm-cm.

Its capacity to reduce transmission of electromagnetic radiation at various frequencies was:~ Frequency (MHz) Attenuation (dB) 10000 106 960 50 700 50 450 54 200 48 100 32 50 26 Mechanical properties of the material of the hous ing were:~ Tensile strength (MPa) 18 Tensile Modulus (GPa) 8 Flexuralstrength (MPa) 50 Flexural modulus (GPa) 8 Izod impact strength 166 (notched) (Jim)

Claims (11)

1. A housing for electrical or electronic equipment made by moulding under heat a composition com prising an organicthermosetting resin which con tails an amount of exfoliated graphite which is enough to impart to the moulded article an electrical resistivity not greaterthan 10 ohm-cm.

2. A housing according to claim 1, in which the amount of exfoliated graphite forms 0.5to 20% by weight of the composition.

3. A housing according to claim 2, in which the amount of exfoliated graphite forms 2.5to 20% by weight ofthe composition.

4. A housing according to claim 3, in which the amount of exfoliated graphite forms 5 to 20% by weightofthe composition.

5. A housing according to any one of claims 1 to4, in which the organicthermosetting resin forms 20-95% byweight of the composition.

6. A housing according to claim 5, in which the composition contains upto 60% byweightof reinforcing fibre.

7. A housing according to claim 5 or 6, in which the composition contains upto 60% byweightof non-fibrousfiller.

8. A housing according to any one of claims 1 to 7, in which thethermosetting resin is a phenolic resin.

9. A housing according to any one of claims 1 to 7 in which thethermosetting resin is an unsaturated polyester resin.

10. A housing for electrical or elctronic equipment made substantially as described herein with refer enceto Example 1 or2.

11. Ahousingforelectrical orelectronicequipment made substantially as described herein with reference to Example 3.