AU3138193A

AU3138193A - Materials for electromagnetic radiation attenuation

Info

Publication number: AU3138193A
Application number: AU31381/93A
Authority: AU
Inventors: Beth A. Dispenza
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-11-15
Filing date: 1992-11-13
Publication date: 1993-06-15
Also published as: WO1993009944A1; CA2123598A1

Description

MATERIALS

FOR

ELECTROMAGNETIC RADIATION ATTENUATION

TECHNICAL FIELD OF THE INVENTION

This invention relates to a novel coating product for attenuation of electromagnetic radiation, and to a method for production of same.

BACKGROUND OF THE INVENTION

A continuing demand exists for a simple, inexpensive coating which can be used to reduce the level of electromagnetic radiation which reaches personnel and equipment. This is particularly true with respect to a variety of "low level" radiation sources, such as electromagnetic radiation from electronic, video, or computing equipment, and from electrical power transmission lines. Also, as the earth's ozone layer decreases in effectiveness with respect to blockage of ultraviolet ("UV") radiation, a need arises to provide better UV protection to those exposed to natural sunlight. More specifically, a need exists to produce lightweight personnel protective clothing which provides a proven measure of protection against unwanted electromagnetic radiation.

Coatings or other protective devices of the character described above which provide the general capabilities desired have heretofore been proposed. Those of which we are aware are disclosed in U.S. patents Nos. : 4,831,210, issued May 16, 1989, to Larson et al. for SHIELDS FOR ELECTROMAGNETIC RADIATION; 4,843,641 issued July 4, 1989, to Cusick et al. for RADIATION SHIELD GARMENT; and 4,869,970, issued September 26, 1989 to Gulla et al. for RADIATION ATTENUATION SHIELDING. More generally, an article entitled "RF Shielding Effectiveness and Light Transmission of Copper or Silver Film Coating on Plastic Substrate," by Samuel Y. Liao, which appeared in the IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-18, No.4, November, 1976, provides background on the theoretical basis for effective shielding against radio frequency electromagnetic radiation, and describes use of a thin copper or silver film to block or attenuate unwanted electromagnetic radiation. For the most part, the documents identified in the preceding paragraph disclose coatings and materials which include the presence of some type of metal either contained within a support matrix, or deposition (such as by electroplating) of metal over a substrate.

Unfortunately, the type of support matrix or substrate materials used heretofore have been unsuitable for use in lightweight articles, such as cotton cloth or other textiles that might be utilized in clothing or protective equipment.

Another common deficiency of the heretofore available electromagnetic radiation attenuation devices of which we are aware is the lack of portability, often due to the sheer weight of devices, and the lack of flexibility, due the the rigid type of structure most commonly found. Thus, the dual advantages of flexibility and lightweight construction of electromagnetic radiation attenuation articles fabricated using our novel radiation attenuation coating, is important and self-evident.

SUMMARY OF THE INVENTION

We have now invented, and disclose herein, a novel, improved electromagnetic radiation attenuation coating product which does not have the above-discussed drawbacks common to those somewhat similar products heretofore used of which we are aware. Unlike the electromagnetic radiation attenuation coatings or devices heretofore available, our product is simple, lightweight, relatively inexpensive, easy to manufacture, and otherwise superior to those heretofore used or proposed. In addition, it provides a significant, demonstrated measure of attenuation against unwanted electromagnetic radiation.

Another important feature is the fact that our novel coating is not electrically conductive. This provides a unique safety feature when compared to may previously known electromagnetic attenuation methods and devices.

Our novel electromagnetic radiation attenuation coating, and articles made according to our novel method of applying the coating, differ from those products mentioned above in one respect in that a thin, lightweight, electrically non-conductive coating is provided. When used on grey goods, such as on cotton or other textiles for flexible articles such as clothing, the thin coating is comparable or lighter in weight and handling characteristics than conventional flexible plastic coated clothing, such as vinyl plastic coated raincoats.

OBJECTS, ADVANTAGES, AND FEATURES OF THE INVENTION

From the foregoing, it will be apparent to the reader that one important and primary object of the present invention resides in the provision of a novel, improved coating to provide a means for electromagnetic radiation attenuation, thereby preventing or reducing the amount of radiation reaching equipment or personnel so protected.

Other important but more specific objects of the invention reside in the provision of an electromagnetic radiation attenuation coating as described in the preceding paragraph which: can be manufactured in a simple, straightforward manner; results in comparatively light articles which include the coating;

in conjunction with the preceding object, have the advantage that they can be embodied in selected personnel protective clothing; and,

which provides articles including the coating which are easy to use, install and remove.

Other important objects, features, and additional advantages of our invention will become apparent to the reader from the foregoing and the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

Figure 1 is a perspective view of a flexible article including a cotton fabric substrate which has been coated with the novel coating of the present invention.

Figure 2 is an enlarged view of an end section of the article first illustrated in Fig. 1, taken thru article 2-2 of Fig. 1, now revealing a bi-directional weave with a top coating according to the present invention.

Figure 3 is a schematic of a method of application of the coating of the present invention to grey goods such as the cotton fabric substrate first illustrated in Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION

As was previously discussed, the present coating of the present invention provides attenuation of electromagnetic radiation. Specific examples of various types of electromagnetic radiation of concern include low frequency radiations from sources such as power lines, slightly higher frequencies such as radio broadcast radiation, still higher frequencies such as microwave radiation, followed by infrared, visible, and ultraviolet light. At the high end of the electromagnetic spectrum, x-rays, gamma rays, and cosmic rays are found. While the present invention may be useful in attenuating, by either reflectance or absorbance, electromagnetic radiation throughout the spectrum, it is particularly useful with respect to protection against both electrical and magnetic fields generated by electrical equipment such as computers, and with respect to ionizing radiation such as ultraviolet light.

Turning now to Figure 1, there is shown an artiςle 10 of manufacture having a flexible substrate 12 having a coating 14 according to the present invention. The flexible substrate 12 may be any suitable material, however, we prefer to utilize textiles, known as "grey goods" in the industry, such as cotton or polyester fabric. More preferably, grey goods such as one hundred percent (100%) cotton fabric are used, such as a 100 % cotton fabric having a 78/54 thread count. Most preferably, a mixed fabric, having 75% cotton and 25% polyester, is utilized. We prefer lightweight grey goods so as to minimize the weight of the finished product, but any convenient substrate may be utilized without varying from the teachings of our coating material.

Adhering to one side of the flexible substrate 12 is a flexible coating 14. This coating adheres to the grey goods by way of a method of manufacture such as the one indicated below, but preferably will consist of a cured or dried carrier 15. The carriers that are easiest to use are those which exist in a liquid state in a raw or uncured form, and then when dried or cured, form a solid, flexible membrane layer 14. A preferred carrier is polyvinylchloride plastic. A urethane or polyurethane coating also provides a suitable membrane.

This plastic, urethane, or like substance, acts as a carrier for finely dispersed particles of metal such as copper 16, which are embedded within the cured coating layer. Turning now to Figure 2, further details of the construction of the flexible electromagnetic radiation attenuation article 10 are visible. The flexible coating 14 has a top surface 18, formed as hereinbelow described. The coating 14 extends downward and fills in the interstitial voids such as location 19 surrounding the cross directional threads 20 and the longitudinal threads 22 of the grey goods flexible substrate 12.

Dispersed within the coating or flexible membrane layer 14 are particles of finely divided copper dust 16. The molecular state of the copper utilized is important, since elemental copper is needed to attain the maximum possible electromagnetic attenuation. In this regard, oxidation of the copper will result in loss of effectiveness. Therefore, we have found that when obtaining copper, one particular type of source is preferred. Copper flakes, formed from finely ground copper wire, and preserved with the anti-oxidant stearic acid, have provided the best attenuation results. In this regard, we normally μse a commercially available copper dust mixture that is greater than ninety eight percent (98%) copper and less than two percent (2%) stearic acid. Other electrically conductive metals, such as silver or gold, or even nickel, cobalt, or lead, may be utilized. The percentages of the just mentioned and other like EMF attenuating metals may be varied to any desired degree to attain the required electromagnetic attenuation at an acceptable level of raw material cost. However, from a cost effectiveness and raw materials supply standpoint, we have found that copper provides the best all-round results.

We prefer to utilize copper dust 16 having a size ranging from 5 microns in diameter to 50 microns in diameter. More preferably, the diameter of the copper dust 16 utilized will have an average diameter of about 20 microns. The coating 14 thickness may vary as necessary to suit the particular type of carrier utilized, and is of not particular consequence except that minimizing the weight is always of primary importance. However, we have found that it is necessary to disperse the copper dust 16 to a density of between 200,000 and 350,000 particles per square centimeter of upper surface area 18 of the article 10. More preferably, the copper dust 16 is dispersed to an average density of approximately 250,000 particles per square centimeter of surface area 18. In providing this type of coverage, we have found the copper dust 16 application rate to be about 0.0024 grams of copper dust per square cm (0.00492 pounds per square foot).

Turning now to Figure 3, one advantageous method for manufacturing electromagnetic radiation attenuating articles 10 is illustrated. A container 30 having carrier 32 therein, in liquid form, is provided. A commercially available liquid polyvinyl chloride resin preparation is one suitable carrier 32 that we prefer. Copper dust 16 is provided from a shipping or other container 34 and the dust is mixed by way of mixer 36 into the liquid carrier 32. The container 30 is then set up so that carrier 32 containing dust 16 is emptied into a liquid pool 38 whereby the upper surface 40 of grey goods 42 is wetted by the copper dust containing carrier 32. The grey goods 42 may be on a winder 44, and the operation may be accomplished by way of a calender stack 46, or by other suitable means as commonly practiced in the vinyl coating industry.

Once the uniform application of liquid carrier 32 is accomplished on to grey goods 42, the wetted grey goods is taken through a curing or drying process, such oven 50, again as commonly practiced for the type of carrier chosen; here, as normally conducted in the polyvinylchloride plastic coating industry. Once the carrier 32 has cured and into the flexible membrane 14 identified above, the finished article 10 may be wound on a finish roll 52 or cut as desired. As mentioned above, we desire to produce a relatively lightweight finished article 10, and have found that preparing a single thickness article of about 0.0103 grams per square cm (0.021 pounds per square foot) is easily accomplished. This contrasts greatly with the weight of many articles known heretofore, and is a substantial improvement in the art.

Testing of articles 10 prepared as described and of construction as described above has been extensively conducted. Electromagnetic radiation attenuation characteristics have been characterized.

Referring now to Example I, in the "ELF" or extra low frequency and electrical power frequencies, the electrical field reduction from a single thickness of article 10, at a 0.3 meter range, is about 0.5 to 0.6 volts per meter. In the 2.0 meter range, the electrical field reduction is between 0.05 to 0.07 volts per meter. In the ELF and electrical power frequencies, the magnetic field reduction at a 30.48 cm distance from a flyback transformer ranges from 40 to 48 mA/meter.

Attention is now directed to Example II, where results for attenuation of ultraviolet radiation are shown. An unshielded UV source of 73.0 uW/cm² intensity was provided. Shielding the source with a single layer of our novel attenuation coating on a cotton substrate reduced the exposure to 1.1 uW/cm². A single layer of our coating reduced the exposure to 0.6 uW/cm² at thirty-eight and 1/10 cm (38.1 cm) distance, and to 0.1 uW/cm² at seventy-six and 2/10 cm (76.2 cm) distance.

Turning now to Example III a surprising and exemplary result is shown. With respect to attenuation of X-rays, at 25kV, 5 mAs wavelength, four (4) layers of our electromagnetic radiation attenuation article 10 provided better reduction of electromagnetic strength than a 0.1 mm Aluminum foil sheet.

In Example IV, the superb electrical insulating properties of our electromagnetic attenuating coating are illustrated. When dry, the material exhibits an infinite resistance, and when wet, a ten (10) M Ohm impedance is maintained when the test specimen was subjected to 500 Volts, thus minimizing or eliminating any concerns of electrical shock hazard when using garments, for instance having our novel electromagnetic attenuating coating.

EXAMPLE I

Extra Low Frequency/Power Frequency Tests

I. E- Field Test

A. 0.3 Meter Range-

Unshielded Field Intensity: 8.4 to 8.9 volts/meter

Single-Thickness Shielding: 7.9 to 8.3 volts/meter

B. 2.0 Meter Range-

Unshielded Field Intensity: 1.58 to 1.67 volts/meter

Single-Thickness Shielding: 1.51 to 1.62 volts/meter

II. H- Field Test

A. 0.3 Meter Range-

Unshielded Magnetic Fields: 74 to 86 mA/meter

Single-Thickness Shielding: 74 to 86 mA/meter

III. H- Field Test- 12 inches from Flyback Transformer

Unshielded Magnetic Field: 508 to 530 mA/meter

Wrap of H - Field Sensor: 460 to 490 mA/meter

EXAMPLE II

LOW LEVEL ULTRAVIOLET RADIATION TEST

Distance Exposure

Unshielded Probe:

76.2 cm 11.5 uW/cm²

38.1 cm 26.2 uW/cm²

Single Layer Coating:

76.2 cm o.l uW/cm²

38.1 cm 0.6 uW/cm²

Cotton Fabric:

76.2 cm 3.7 uW/cm²

38.1 cm 7.3 uW/cm²

Unshielded Probe at source: 73.0 uW/cm²

Single Layer coating at source: l.l uW/cm² EXAMPLE III

X-RAY TESTS

X-Ray Exposures: 25 kV, 5 mAε

Exposure Shielding

25.1 mR/h Probe Only

23.2 mR/h 1 - Layer

21.4 mR/h 2 - Layers

19.0 mR/h 4 - Layers

Comparison with Aluminum Filtration:

19.2 mR/hr 0.1 mm Aluminum

EXAMPLE IV

ELECTRICAL CONDUCTIVITY TEST

A 500 Volt electrical charge was induced into various lengths of dry cotton fabric having theron a coating as taught herein. A Megohm meter was used to determine the resistance and electrically insulating characteristics of the article, and breakdown voltages were examined. Results were as follows:

60.96 cm infinite Ohms

30.48 cm infinite Ohms

15.24 cm infinite Ohms

5.08 cm infinite Ohms

2.54 cm infinite Ohms

2.54 cm (WET) 10 M Ohms Thus, it can be seen that we have developed and have set forth herein an exemplary electromagnetic attenuation material, and a method for producing the same. The material is lightweight, flexible, and extremely useful for the manufacture of personnel protective clothing. The coating method itself may be used on non-textile substrates without departing from the scope and spirit of our invention.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalences of the claims are therefore intended to be embraced therein.

Claims

CLAIMSWe claim:

1. A coating for attenuation of electromagnetic radiation, comprising:

a) a carrier, said carrier having an uncured state wherein said carrier is in liquid form, and having a cured state wherein said carrier is in solid form,

b) particles, said particles disposed throughout said carrier, said particles further comprised of a conductive metal.

2. The coating of claim 1 wherein said particles are further comprised of one or more metals selected from the group of copper, silver, gold, nickel cobalt, or lead.

3. The coating of claim 2 wherein said said particles are copper.

4. The coating of claim 3 wherein said particles are comprised of copper dust.

5. The coating of claim 4 wherein said particles further include an anti-oxidant covering.

6. The coating of claim 5 wherein said anti-oxidant covering comprises stearic acid.

7. The coating of claim 4 wherein said copper dust particles are substantially in the range of 5 microns to 50 microns in diameter.

8. The coating of claim 7 wherein said particles are have an average diameter of approximately 20 microns.

9. The coating of claim 1, wherein said particles are dispersed to a density substantially between 200,000 particles per square centimeter of surface area and 350,000 particles per square centimeter of surface area.

10. The coating of claim 9 where said particles are dispersed to a density of approximately 250 , 000 particles per square centimeter of surface area.

11. The coating of claim 4 wherein said particles are dispersed to a density of approximately 0.00492 pounds of copper dust per square foot of surface area.

12. The coating of claim 1, further including a textile substrate.

13. The coating of claim 12, wherein said textile substrate further comprises a cotton cloth.

14. The coating of claim 13, wherein said finished textile weight is approximately 0.021 pounds per square foot.

15. The coating of claim 12, wherein said cotton cloth has a 78/54 thread count.

16. The coating of claim 12, wherein said textile substrate further comprises a cloth comprised of cotton and polyester.

17. The coating of claim 16, wherein said textile substrate consists essentially of 75 percent cotton and 25 percent polyester.

18. A flexible membrane for attenuation of electromagnetic radiation, comprising:

a) a textile substrate; and

b) a coating, said coating including

(i) a cured carrier, and

(ϋ) particles, said particles disposed throughout said carrier, said particles further comprised of a conductive metal.

19. The flexible membrane of claim 18 wherein said particles are further comprised of one or more metals selected from the group of copper, silver, gold, nickel cobalt, or lead.

20. The flexible membrane of claim 19 wherein said said particles are copper.

21. The flexible membrane of claim 20 wherein said particles are comprised of copper dust.

22. The flexible membrane of claim 21 wherein said particles further include an anti-oxidant covering.

23. The flexible membrane of claim 22 wherein said anti-oxidant covering comprises stearic acid.

24. The flexible membrane of claim 21 wherein said copper dust particles are substantially in the range of 5 microns to 50 microns in diameter.

25. A method of providing an electromagnetic attenuating membrane suitable for covering a substrate, comprising:

a) providing a carrier, said carrier in liquid form and of composition suitable for drying or curing into a flexible membrane,

b) providing a metal dust,

c) adding said dust to said carrier,

d) mixing said carrier to disperse said dust uniformly therein, e) covering said substrate with a thickness of said dust carrying carrier, said thickness being effective to attenuate said electromagnetic radiation,

f) curing or drying said carrier to form a finished, flexible membrane over said substrate.

26. The method of claim 25, wherein said metal dust further comprises copper.

27. The method of claim 26, wherein said copper dust comprises particles having a nominal particle size ranging from 5 to 50 microns.

28. The method of claim 26, wherein said copper dust further comprises particles having an average size of 20 microns.

29. The method of claim 25, wherein said metal dust density varies from approximately 200,000 particles per square centimeter to approximately 350,000 particles per square centimeter.

30. The method of claim 26, wherein said copper dust particles are dispersed to a density of approximately

0.00492 pounds of copper dust per square foot of surface area.