GB2234857A - Microwave-absorbing materials - Google Patents

Abstract

Microwave reflection from a surface is reduced by covering the surface or part thereof with a fabric formed of filaments of a polymer filled with carbon particles or with a polymer sheet material filled with carbon particles. The filaments or the polymer sheet material preferably have a modified surface produced by treatment with an organic liquid to increase the reflectance of microwave radiation. A preferred fabric adapted to absorb microwave radiation is a three-dimensional woven or knitted fabric formed of filaments of a polymer filled with carbon particles.

Description

Microware-Absorbing Materials This invetion relates to textile materials and sheet materials adapted to reduce reflection of microware radiation in the range of frequencies 100 MHz to 300 GHz, particularly frequencies of 600 MHz to 94 GHz. By a textile material we mean a fabric, which can be woven, knitted or non-woven fabric, or yarn, tow, filaments or fibres.Fabrics adapted to absorb microwave radiation can be used to make garments or screens to reduce detection of i individuals, vehicles or weapons by microwave radiation, drapes to separate areas, one of which contains a source of microwave radiation or as a coating on some surfaces of a device generating microwave radiation Background to the invention Microware-absorbing filaments and fabrics filled with ferrite are described in British Patent 2,127,739. Films filled with iron or carbon powder for absorbing microwave radiation are described in fuzz British Patent 893,007.A microwave-absorbing film or nm sheet formed with protrusions in the surface is described in British Patent 1,074,898.

Summary of the invention According to one aspect of the present invention process for reducing microwave reflection from a surface comprises covering the surface or part thereof with a fabric formed of filaments of a polymer filled with carbon particles.

The carbon preferably has a particle size of 0.5 to 10 rjn (nanometres), most preferably 1.5 to 2 nm, and can for example be that so3d cormercidll- as carbon black. The proportion of carbon particle in the filaments is preferably 10 to 22 per cent by volume (about 20 to 35 per cent by weight).

The filaments are piefer-ibl formed by melt spinning a fibre-forming thermoplastic polymer. The polymer can for example be a polyolefin such as polypropylene or polyethylene, a polyester such as polyethylene terephtalate, a polyamide or a vinyl polymer sucn as polyvinyl chloride. Polypropylene filaments are preferred. Alternatively the filaments can be formed by wet spinning or by dry spinning, for example wet spun acrylic or viscose filaments.

The fabric 5 preferably a woven fabric but can be a non-woven or knitted fabric. A three-dimensional woven or knitted fabric is particulary preferred. The fabric is preferably formed from continuous multi filament yarns of the carbon-filled filaments but can alternatively be formed from staple fibre yarns formed from the carbon filled filaments.

The filaments can additionally contain ferrite and/or iron particles, for example polypropylene may be filled with 10 to 20 per cent by volume carbon and 1 to 5 per cent by volume ferrite or iron. The particle size of the ferrite or iron is generally in the ranee of 0.1 to 100 microns and preferably 0.5 to 10 microns.

According to another aspect of the invention a fabric adapted to absorb microwave radiation is a three-dimensional woven or knitted fabric formed or filaments of a polymer filled with carbon particles. The invention also includes a process for reducing microwave reflection from a surface bv covering the surface or part thereof with such a fabric.

By a three-dimensional fabric we mean a fabric having a surface roughness substantially greater than that of a plain weave fabric of the same yarn, and preferably greater than 0.5 mm. Examples of three-dimensional fabrics are honeycomb weave fabrics and Raschel knitted fabrics. Fabrics in which one face presents an array of substantially pyramidal or conical indentations or protrusions, as can be formed by honeycomb weave, are particularly prefferred. The surface roughness of the fabric, measured from the lowest to the highest point at the same face of the fabric is preferably 1 to 10 mm.

'l".e wavelength of the surface r roughness of the fabric, that is the distance along the fabric from the lowest point to the highest point and back to the lowest, is preferably 1 to 30 mm. For the higher frequencies of microwave radiation, particularly in the range 30 to 94 GHz, it may be advantageous that the depth and wavelength of the surface roughness are each similar to the wavelength of the radiation that the fabric is expected to encounter.

According to a further aspect of the invention a textile material or polymer sheet material adapted to absorb microwave radiation, which is a textile material formed from filaments filled with carbon particles, or a film, sheet or tape of a polymer filled with carbon particles is treated with an organic liquid to decrease its reflectance of microwave radiation. The invention also includes a process for reducing microwave reflectioll from a surface by covering the surface or part tlerecf with such a fabric, film, sheet or tape.

The textile material treated can advantageously be a three-dimensional fabric as described above. A microwaveabsorbing polymer sheet material can advantageously be formed with protrusions at the surface, for example as described in British Patent 1074898.

The organic liquid used to treat the textile material or sheet material Is preferably a hydrocarbon, a halogenated hydrocarbon, an ether, a ketone or an alcohol. For textile or sheet materials formed from polyolefin a hydrocarbon or halogenated hydrocarbon is preferred such as xylene, toluene, petroleum ether, trichloroethylene cr pcrchloroethylene or carbon tetrachlcyide. The tetilc mateilal which is treated with the organic liquid is preferably a fabric.The fabric is preferably immersed in the organic liquid for a period of 0.1 to 720 minutes, for example 1 to inn minutes. Treatment at ambient temperature is generally sufficient although higher temperature can Do used, for example treatment can be carried out at up to 100 C or treatment at ambient temperature can be followed by heating at up to 100 C.The treatment can be carried out in apparatus conventinally used for dry cleaning fabrics and garments. Alternatively a continuous length of fabric can be passed through a treatment bath, particularly if such immersion is followed by heating in an oven. The textile material can alternatively be a yarn or tow, which can be treated using apparatus designed for dyeing yarn or tow, but fabric treatment is flore convenient.

A film, sheet or tape can for example be immersed for 0.1 to 20 minutes by passing through a treatment bath, preferably followed by heating.

In carrying out the process of the invention more than one layer of fabric may be applied to the surface which is to be masked. The layers of fabric can The the same or different.

It may be advantageous to use a fabric having a higher carbon loading next to the said surface, with a facing layer having a lower carbon loading. One or more of the fabric layers can be solvent treated as described above, lor example the layer in contact with the surface may be. so treated with the facing layer being untreated. Either or both of the fabrics may be three-dimensional fabrics.

The invention is illustrated by the following Examples.

Example 1 Polypropylene containing 30 per cent by weight (17 per cent by volume) carbon black (Cabelec 3140 sold by Cabot) and 0.7 per cent lubricant was melt-spun to foil. a 1200 decitex/30 filament conductIve yarn. The yarn as woven into a plain weave fabric having a weight per unit area of 319 grams per square metre.

The fabric was tested fc.r its ability to reduce reflection of microwave radiation. In this test the sample, cf one or wore fabric ayers, is ilaccA in contact with a metal plate and is illuminated by microwave radiation and the returned signal Is measured and related to the incident signal in amplitude and phase by means of vector network analyser.

The reflectivity is expressed as the reflection loss, (in dB) of the sample compared to a metal reference plate of the same size and shape, and is plotted as a function of frequency.

Measurements are conducted in one of two small anechoic chambers. in the first the same hori is used both to illuminate the sample and to collect the reflected signal, giving true monostatic operation with normal incidence. In the second, separate horns are used for the transmit and receive functions, and each is mounted at 50 from the normal to the sample. The bistatic angle thus obtained, however, is sufficiently small for the measurements to be indistinguishable from the monostatic case.

When tested in this manner a three-layer thickness of the fabric of Example 1 gave at least 10 decibels reflection loss over a range of 4.3 GHz.

Example 2 The conductive yarn of Example 1 was woven into a honeycomb weave fabric having a repeat pattern of 6 x 6 mm squares and a thickness of 1.8 iron. The fabric weight was 368 grams per square metre When tested as described in Example 1 a three-layer thickness of this fabric gave at least 10 decibels reflection loss over a range of 4.9 GHz.

Fainple 3 The conductive yarn ol example 1 was woven into a honeycomb weave fabric having a repeat pattern of 2 x 2 mm squares with a thickness ci 3.1 nun. The fabric had a weight per unit area of 652 grams per square metre.

When tested as described in Example 1 a three-layer thickness of this fabiic gave at least 10 decibels reflection loss over a range of 3.0 GHz.

Example 4 The honeycomb weave fabric of Example 2 was treated with perobloroethyleno in a dry-cleaning machine at ambient temperature for 2 hours. Some shrinkage occurred, increasing the weight per unit area of the fabric to 404 grams per square metre.

When tested as described in Example 1 a single layer of the solvent-treated fabric gave a reflection loss of at least 10 decibels over a range of 4.9 GHz.

Example 5 Polypropylene containing 30 per cent by weight carbon black and 0.7 per cent lubricant was mixed with further polypropylene and (Carbonyl Iron' iron particles of particles size 4 to 6 microns which had been pre-treated with a titanate coupling agent to form a composition containing 14 per cent by volume carbon arid 4 per cent by volume iron. This composition was melt spun to form a 1200 decitex/30 filament yarn. The yarn was woven into a plain weave fabric of weight 371 grams per square metre.

When tested as described in Example 1 a three layer thickness of this fabric gave a reflection loss of at least 10 decibels over a range of 4.5 GHz.

Example 6 Polypropylene containing 25 per cent by weight (15 per cent by volume) carbon black and 0.7 per cent lubricant was melt spun to form a 1200 decitex/30 filament yarn. The yarn was woven into a honeycomb weave fabric having a repeat pattern of 4 x 4 nun squares and a thickness of 2.5 mm and a weight of 467 grams per square metre.

Example 7 A layer of the perchloroethylene-treated fabric of Example 4 was applied to a metal backing and was covered by a layer of the fabric of Example 6. The assembly was tested as described in Example 1 and gave at least 10 decibels reflection loss over the whole range 8-18 GHz, with at least 15 decibels reflection loss over the range 10-17 GHz.

Example 8 The conductive yarn of Example 1 was woven into a honeycomb weave fabric having a repeat pattern of 2 x 2 mm squares and a thickness of 3.9 mm and a weight of 465 grams per square metre. The fabric was treated with perchloroethylene as described in Example 4.

Example 9 A layer of the treated fabric of Example 8 was applied to a metal backing and was covered by a layer of the fabric of Example 6. The assembly was treated as described in Example 1 and gave at least 10 decibels reflection loss over the whole range, with at least 15 decibels reflection loss over the range 8-17 GHz and about 20 decibels reflections loss over most of the range.

Claims (21)

1. A process for reducing microwave reflection from a surface, said process comprising covering the surface or part thereof with a fabric formed of filaments of a polymer filled with carbon particles.

2. A process according to claim 1, wherein the particle size of the carbon particles is 0.5 to 10 nm.

3. A process according to claim 1 or 2, wherein the proportion of the carbon in the filaments is 10 to 22 per cent by volume.

4. A process according to any of claims 1 to 3, wherein the filaments are melt-spun filaments of a thermoplastic polymer.

5. A process according to any of claims 1 to 3, wherein the filaments are polyolefin filaments.

6. A process according to claim 5, wherein the polyolefin is polypropylene.

7. A process according to any of claims 1 to 6, wherein the fabric is a three-dimensional woven or knitted fabric.

8. A process according to any of claims 1 to 7, wherein the filaments of the fabric have a modified surface produced by treatment with an organic liquid to decrease the reflectance of microwave radiation from the fabric.

9. A process according to claim 8, wherein the polymer is a polyolefin and the organic liquid is a hydrocarbon or halogenated hydrocarbon.

10. A process for reducing microwave reflection from a surface, carried out substantially as hereinbefore described with reference to any of the foregoing Examples.

11. A fabric adapted to absorb microwave radiation, said fabric being a three-dimensional woven or knitted fabric formed of filaments of a polymer filled with carbon particles.

12. A fabric according to claim 11, which is a honeycomb weave fabric.

13. A fabric according to claim 11, which is a Raschel knitted fabric.

14. A fabric according to any of claims 11 to 13, wherein the surface roughness of the fabric, measured from the lowest to the highest point at the same face of the fabric, is 1 to 10 millimetres.

15. A fabric according to any of claims 11 to 14, wherein the filaments of the fabric have a modified surface produced by treatment with an organic liquid to decrease the reflectance of microwave radiation from the fabric.

16. A fabric according to claim 15,wherein the polymer is a polyolefin and the organic liquid is a hydrocarbon or halogenated hydrocarbon.

17. A fabric adapted to absorb microwave radiation and substantially as hereinbefore described.

18. A process for reducing microwave reflection from a surface, said process comprising covering the surface or part thereof with a polymer sheet material filled with carbon particles, said polymer sheet material having a modified surface produced by treating the polymer sheet material with an organic liquid to decrease its reflectance of microwave radiation.

19. A process according to claim 18, wherein the proportion of carbon particles in the filaments is 10 to 22 per cent by volume.

20. A process according to claim 18 or 19, wherein the polymer sheet material is formed of a polyolefin and the organic liquid is a hydrocarbon or halogenated hydrocarbon.

21. A process according to claim 20 wherein said polyolefin is polypropylene.