CA1070004A

CA1070004A - Camouflage material

Info

Publication number: CA1070004A
Application number: CA248,680A
Authority: CA
Inventors: Gunnar Lindqvist; Erik W. Wallin
Original assignee: Barracudaverken AB
Current assignee: Barracudaverken AB
Priority date: 1975-04-16
Filing date: 1976-03-24
Publication date: 1980-01-15
Also published as: FR2308079B1; FR2308079A1; DE2616730A1

Abstract

ABSTRACT OF THE DISCLOSURE
Laminated, radar-defeating camouflage material for use under snowy, low-temperature conditions without delamina-tion or undue decrease in flexibility at temperatures at least as low as -40°C.

Description

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BACICGROUND OF T~E INVENTI()N
Increased use of radar surveillance has required ~he provision of camouflage which, when draped over an object readily observable by radar, will defeat radar by providing ~ver the area occupied by the object an apparent impedance substantially matching ~hat of the surrounding terrain.
Prior-art wor~ers have been able to provide such camouflage which is effective under woodland and desert conditions, or example, such camouflage being made according to U.S.
patent 3,733,606, issued May 15, 1973, to Sven-Goran Johansson.
Since warfare requires activities in snowy conditions, there has also been a continuing need for camouflage adapted to prevent detection of objects surrounded by snow, and excellent radar transparent camouflage materials are available, following the teachings of U.S. patent 3,300,325, issued January 24, 1967, to G~ W. ~indquist. However, attempts to produce radar ; defeating snow camou1age in flexible form, suitable to be draped over objects such as tanks, trucks and weapons, have not been generall~ successful.
~ ollowing accepted practices, radar defeating camou1age in flexible form requires use of a 1exible web which has been made to present a predetermined impedance to incident electromsgnetic ratiation, this usually bein~

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~^ :' '' : ' accomplished by securing to a ~ihrous web a multiplicity ~f tiny mctal or graphite fibrils, and c~overing the fibril-bearing surface o~ the web wlth a film o~ a polymeric ma-terial, such as a polyvinyl chloride fllm cast ~rom a plastisol onto a release web and" after curing to solid form, transferred to and adhered to ~he ~ibril--beariny surface. The fibrous web is typically 0.1-0.25 millimeter thick and the aahered polymer film is typically .03-.07 millimeter thick, so that the optical and physical character-istics of the fibrous web significantly affect the character-istics of the relatively thin finished laminate. When prior-art workers have attempted -to provide radar defeating snow camouflage by applying directly to the fibrous web a film of material o~ a nature such as to match the reflectance characteristics of snow, the finished product had neither th~
desired physical properties nor the necessary reflectance c~aracteristics. ~nd attempts to apply a snow camou~lage film over the polymer film conventionally adhered to the ~ibrous web have also failed to yield the necessary reflectance characteristics and, further, have resulted in a product wl~ich delaminates under low temperature conditionsO There is thus a critical need for a satis~actory radar defeating cnow camouflage~
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- OBJECTS OF TEE: INVFINTION
. A general object oE the inventi.on aecordinyly is to provide a flexible, radar-defeating camou1age n~terial suitable for use in snowy terrain.
Another object is to devise camouflage material which is eharacteri~ed by not only having the ult~a-violet reflectance neeessary for use as snow camouflage and ra~ar defeating ~ eharaeteristies but also adequate flexibility to be draped iat temoeratures at least as low as ~40Co and freedom from delamination at such temperatures.
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SUMMAR~ OF THE INVENliION
:The invention stems from the discovery that, when a film o pol~meric film, advantageously a film of polyvinyl ~chloride plasticized with a low temperature plasticizer, is .
: adhered to the surface of a flexible web ha~ing the overall ; ~ ~ impedance charaeteristics required for defeating radax by absorbing and reflecting the radar signals, the polymeric ilm eontaining 5-50% of a white pisment, a sur~aee layer accoxd-: : : ing to the principles of Lindquist U.S.A. patent 3,300,325, when : ~ established on the surface o the polymeri~ film, will have ~optical reflectance characteri.sties appxox~ating those o~
snow, despite prssence of the radar defea~g material, and ~ the multi~layer material will be adequately fl~xibIe and free from tend~sncies to delamilla~e, at -40C.

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In one particular aspect the present invention provides in a camouflage material having both radar-defeating capabilities and reflectance characteristics relative to visible and near visible electromagnetic radiations suitable for camouflaging under snow conditions, the comblnation of a flexible web having impedance characteristics which render the same radar~defeating;
and a covering overlying and adhered to one surface of said web, said covering comprising at least an outer film comprising a cured flexible binder heavily loaded with at least one white pigment which is highly reflective to light in the wavelength range of 3000 - 4000 Angstrom units, individual particles of said at least one white pigment protruding from the exposed surface of said ` ~uter film but being-anchored therein, the difference between the refractive index between said at least one pigment and said binder and the refractive index between said at least one pigment and air being such that said film reflects electromagnetic radiations within substantially the same range of wavelengths and to substantially the same extent as would said at least one pigment alone, said camouflage material being ade~uately flexible at -~0C
to be draped in camouflaging fashion and being capable of ~eing rolled and unrolled at -40C without delamination, In order that the manner in which the foregoing and other objects are achieved accordlng to the invention can be understood in de~ail, particularly advantageous embodiments thereof will be ~escribed with reference to the accompanying drawing, which ~ :
forms part of the original disclosure of this application, and ln which the single figure is a cross~sectional view of camouflage material according to the invention.
DETl~ILED DESCRIPTION OF THE INVENTION
The drawing illustra~es a typical flexi~le camouflage material made according to the invention and comprislng an intermediate web 1, polymeric fllms 2 and 3 whlch cover and are .
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adhered to the respective major surfaces of web 1, and films 4 and 5 which cover and are aclherecl to the surfaces of films 2 and 3, respectively. The drawlng i.s greatly enlarged, the actual thickness oE web 1 typically being 0.1-0.25 millimeter, and fi.lms

2-5 each having a ~ypical actual thickness of .0bl-.08 millimeter.
In accordance with conventional U.S.A. pract:ice for raclar defeating camouElage web 1 is a non-woven web of fibers of a thermoplastic polymeric material, the fibers .

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being fusion bonded, fiber-to-fiber, t~ estab:Lish a stable fahric. Typically, web 1 can be a "spun bonded n~lon" non-woven web oE the type marketcd by Monsan-to Chemical Co., 5t. Louis, ~o., under the trademark C~REX. To provide the web with an overall impedance such as to render the finished material radar defeating, metal or graphite fibrils 6 are - distributed on one surface of web 1 in a random isotropic orientation, the fibrils being secured to ~he non-woven web by a suitable polymeric bonding material, typically an aqueous latex of a polyvinyl chloride-methyl acrylate copolymer highly plasticized with a phosphate ester type plasticizer.
Typically, the fibrils are laid on the non-woven web by slurrying the fibrils in an aqueous liquld medium and flowing the liquid medium through and over the web while the web is being passed continuously ~hrough a substantlal volume of the liquid medium, the web then being passed over a section box, the bonding material then being applied over the fibril-garni.shed surface .of the web, and the web then passed through an oven for curing and drying. According to UoS~A~ practice, the fibrils 6 are o stainless steel, and are applied in an amount and di-sposition to provide a radax re~lectance power of about 40%, ~ based on a metal plate o~ the same area, and a one-way : transmission attenuation of at least 6 decibels. The ibrils .

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axe small, having a typical diametcr in the rangc of ~ ~o 20 microns and leng-ths on the order of, e.g., 3-30 millimeters.
Graphi-te fibrils are also employed and, while longer lengths appear to be preferred for graphite fibrils, present practice does not contemplate fibrils of any type lonyer than, e.g~, S0 millimetersO i Films 2 and 3 are advantageously polyvinyl chloride films cast from a plastisol onto a release web and thermally fused. The films are laminated each onto a different major surface of web 1 by thermal bonding. This is accomplished by running the polyvinyl chloride film, still carried by the release web, into flush engagement with web 1 and applying sufficient heat to bring the polyvinyl chloride to the fusion point and sufficient pressure to assure a uniform bond. The laminate is then cooled and t-he release webs are stripped from the polyvinyl chloride films.
Films 4 and 5 are then applied, as by reverse roller coatiny, and cured in accordance wit~l the xequirements of the particular binder chosen in the formulations for these films, protuberance of the pigment particles resultiny because of the high proportion of pigment and the film forminy character ~f the binder.

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A1 ~70~0 4k Films 2 and 3 can be of any thermoplastic polymeric material which can be converted into a self-suppor-ting film o a thickness in the range of .04-.07 millimeter, with the film being adequately flexible to retain its integrity under conditions of camouflage use at low temperatures. Polyvinyl ehloride is particularly advantageous because it can be cast ~rom a plastisol into a film of precisely controllecl thickness and can be eompounded with plasticizers suitable for low temperature conditions. Other suitable polymeric materials include polyvinyl aceta~e, dispersion grade acrylates, including polyethyl acrylate and polymethyl methacrylate, .
and polyurethane.
Films 2 and 3 contain major proportions of at least one white pigment, typically titanium dioxide ran~ing from about 5% to about 50% of the total weight of the cured film, depending upon the specific nature of web 1 and fibrils 6, and the formulation of ilms 2 and 3. The main purpose of this white pigment content is to optically mask the interm~diate web 1, and the fibrils 6 carried by web 1, so as to provide a background whi~h is more nearly neu~ral from the standpoint o~ visible and nesx visible electromagnet~F radiation, :' ' ,` '` ' ':

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With polyvinyl chlo~ide employed as the polymeric material, ilms 2 and 3 can compris~ 25~56% by weight polyvinyl chloride, 15-40% by weight tol:al plasticizer and 5-50% by weight white pigmen-t, the ~alance being made up by stabilizers, flame retardants and other conventional consti-tuents. Since flame resistance is a required characteristic for the camouflage material, it is advantageous to include an~imony trioxide, which is an excellent fl~me retardant, as well as a white pigment in the white pigment content of the layers 2 and 3, with the antimony trioxide amounting to 3~5%
of the total weight of fused ~ilm, any balance of the total white pigment being made up by a less expensive pi~ment such as titanium dioxide. Since the cast polyvinyl chloride film must be thermally fused, a stabilizer is required to inhibit autocatalytic decomposition of the polyvinyl chloride~ Any suitable conventional stabilizer or combi~tion of stabilizers can be employed, and conventional barium-c~mium-zinc stabilizers axe typical. Use of low temperature plasticizers or the polyvinyl chloride is highly advan~ageous. Typical low temperature plasticizers which can be ~sed include dioctyl aæelate, dioctyl phthalate, trioct~l phosphate, .

and dioetyl sebacate. Combinatlons of Low temperakure plastieizers with other plasticizers, such as dioetyl phthalate, ean he employed. Typieally, the initiaL plastisol can inelude 60-80 parts by weight total plastieizer per 100 parts by wei~ht polyvinyl chloride, with the low temperature plasticizer present in an amount equal to at least 10 parts per :L00 parts polyvinyl chloride. ~dditional additives, sueh as eonventional fungicides, can be employed.
Outer films 4 and 5 are formulated aeeording to Lindquist pat~nt 3,300,325 and eharaeterized by being so heavily loaded with at least one pigment whieh is highly reflective to light in the wavelength range of 3000-4000 Angstrom units that individual partieles 7 of that pigment protrude from the exposed surface of the film. Thus, as will be clear from the drawing, the protruding pigment particles are present throughout the exposed surface of the ~i:lmO
~hrouyh protruding, ~he particles at the film surface are firmlr anchored by the binder. Suitable pigments having such eharaeteristie reflectivity include the sulfates, earbonates, silieates and oxides o~ barium, ealeium, magnesium and anti~
mony. Suitable film ~orming binders for the pi~ments inelude polyvinyl ehloricle eopoly~ers sueh as eopo]~rmers of vinyl ehl~ride and vinyl aeetate, linseed oil alkyd, polyvinyl aeetate, polyvin~l isob~1tyl ether, etllyl eellulose and ., .
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nitrocellulose, such binders being dissolved in a solvent such as butyl acetate, xylene, toluene, ethyl acetate or alcohol, or dispersed or emulsified ln a suitable aqueous medium, the use of a volatile organic solvent being preferable and advantageous. The pigment or pigmentsO
binder and solvent are formulated in accordance with conventional paint practices so that, after evaporation of the solvent or liquid medium/ particles 7 of the pigment or pigments protrude from the surface of the filmO as shown, so that as a result the difference between the refractive index ~etween pigment and binder and the refractive index between pi~ment and air is such that the film reflects light within the same ran~e of wavelengths and to the same extent as the pigment contained in the layer. The proportion of the special picrment employed which will result in such protuberance varies with the choice of film orming binder. For most hinders, an amount of pigment in the range of 50 75% of the to-tal weight of the cuxed film will sufice. ~he pigment or pigments employed advantageously have an average maximum particle dimension in the range of .001-.010 millimeter.
~ he drawing illustrates a camouflage material both sides of which exhibit reflectivity characteristics approximately .
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those of snowO I-Iowevex, film S can be omitte~, and film 3 can then be other ~han white. For e~ample, wi-th Eilm 5 omitted, film 3 can be of a color suitable Eor camouflage under conditions other than snow.
When b~th sides of the camouflage material are for snow camouflage purposes, both films 4 and 5 being employed, and we~ 1 carries impedance elements such as fibrils 6 on ~ne surface only, the film which covers the fibril-carrying surface of the web can be more heavily loaded with white pigment than is the Eilm covering the other surface of the web. Thus, in the emhodiment illustrated in the drawin~, film 2 can contain substantially more white pigment than does film 3, so that better masking of fibrils 6 is achieved.
For example, film 2 can include 23% titanium dioxide and 2%
~antimony trioxide, with film 3 containing 3% titanium dioxide and 2% antimony trioxide~ the percentages being by weight based on the total weight of the cured film.
Altèrnatively, when fibrils 6 or other i~pedance elements are present on one surface only of the intermediate web 1, the ~ilm which covers that surface can be rela~ively thicker and ~h film covering the other surf~ce can be thinner, Thus~ for examplel film 2 can be .07 millimeter thic~ and the thickIless o film 3 can be .05 millimeterO

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~07~)004 The following example is illustrative:
Example 1 A conventionally produced "spun b~nded nylon" non-woven fabric web of approximately .08 millimeters Qn thickness, garnished with stainless steel fibrils of a diameter of 8 microns and a length of about 45 millimeters is employed as the intermediate web 1. The'fibrils are laid on the nylon 'web in a random isotropic orientation and secured to the web by use of an aqueous polyvinyl chloride-methyl acrylate copolymer plasticized with a phosphate ester plasticizer. By standard U.S.A. Army test procedures, the web has a radar reflectance of 40% based on a metal plate of the same area, and a one-way transmission attenuation of 6-7 decibels.
Films 2 and 3 are established on a rayon release web by conventional film casting from a plastisol of the following composition:
Ingredient Parts by weight Dispersion grade polyvinyl chloride 100 Dioctyl phthalate, plasticizer' 50 Dioctyl aæelate, plasticizer 20 Titanium dioxide, pigment 20 ~ntimony trioxide, pigment " 5 Barium-ca~nium-zinc stabilizer 2 ' :

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The plastisol is spread as a uniEorm coating on the running release web and cured by passing through an ovcn at 180C., with a residence time of ~.5 minu-tes in the oven, the -thickness of the plastisol coating being controlled, with use of a monitoring beta gage, to yield a cured polyvinyl chloride ~ilm having a thickness oE .065 millimeter. q~o such films are prepared, each on its own release web. Using a conventional laminating machine, the two polyvinyl chloride films are run each onto a different surface of the intermediate web and secured thereto by heat and pressure. The resulting lamination is run into a stripping machine and the two release webs are stripped away, leaving a laminate consisting of web 1 and films 2 and 3, with the stainless steel fibrils 6 carried by we~ 1 and embedded in and adhered to the inner surface of fllm 2.
Films 4 and 5 are applied to the exposed surfaces of films 2 and 3~ respectively, by reverse roller coating with a : composition of the following formulation:
Ingredient Calcium carbonate, pigment 40 Vinyl chloride-vinyl acetate copolymer 16 ~ Butyl acetate as solvent 44 ., . :.

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The reverse roller coating operation is carried out se~uentially to establish films 4 and 5 with each Eilm, when cured by evaporation of the solvent,being approxi.mately .055 millimeter in thickness. The cured films 4 and 5 are characterized by the fact that, throughout the entire extent of the ilm, individual particles 7 of the calcium carbonate pigment protrude from the exposed surface of the film, with the protruding particles being closely adjacent to each other and securely anchored in the film.
The finished product thus obtained is adequately flexible to be draped over objects to be protected against surveillance at temperatures as low as -40C. and to be rolled, unrolled, folded, unfolded and crinkled at that temperature without delamination. As to electromagnetlc radiation in the visible and near visible range, particularly in the wavelength range of 2000-4000 Angstrom units, the refleetanee eharaeteristies of ~he camouflage material are so elose to those of snow as to prevent detection under snow eonditions, and pre;senee of films 2-5 has no delsterious ~ffeet on the ra~ar~defeating charaeteristics of the int0r-mediate web and fibrils 7D

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Claims

1. In a camouflage material having both radar-defeating capabilities and reflectance characteristics relative to visible and near visible electromagnetic radiations suitable for camouflaging under snow conditions, the combina-tion of a flexible web having impedance characteristics which render the same radar-defeating; and a covering overlying and adhered to one surface of said web, said covering comprising at least an outer film comprising a cured flexible binder heavily loaded with at least one white pigment which is highly reflective to light in the wavelength range of 3000-4000 Angstrom units, individual particles of said at least one white pigment protruding from the exposed surface of said outer film but being anchored therein, the difference between the refractive index between said at least one pigment and said binder and the refractive index between said at least one pigment and air being such that said film reflects electromagnetic radiations within substantially the same range of wavelengths and to substantially the same extent as would said at least one pigment alone, said camouflage material being adequately flexible at -40°C. to be draped in camouflaging fashion and being capable of being rolled and unrolled at -40C. without delamination.

2. The combination as defined in claim 1, wherein said covering also comprises an inner film of polymeric material having at least one white pigment distributed therethrough in an amount equal to 5-50% of the total cured weight of said inner film.

3. The combination as defined in claim 1 and further comprising a second covering overlying and adhered to the other surface of said web, said second covering comprising at least an outer film comprising a cured flexible binder heavily loaded with at least one white pigment which is highly reflective to light in the wavelength range of 3000-4000 Angstrom units, individual particles of said exposed surface of said second covering but being anchored therein the difference between the refractive index between said at least one pigment and said binder and the refractive index between said at least one pigment and air being such that said film reflects electromagnetic radiations within substantially the same range of wavelengths and to substantially the same extent as would said at least one pigment alone.

4. The combination as defined in claim 3, wherein said second covering also comprises an inner film of polymeric material having at least one white pigment distributed there-through in an amount equal to 5-50% of the total cured weight of the inner film.

5. The combination as defined in claim 1, wherein said flexible web is a non woven thermally bonded web of thermoplastic polymeric fibers carrying conductive elements on one surface of the web.

6. The combination as defined in claim 2, wherein the white pigment content of said inner film comprises a flame-retarding amount of antimony trioxide.

7. The combination as defined in claim 6, wherein the polymeric material of said inner film is polyvinyl chloride.

8. The combination as defined in claim 4, wherein the white pigment contents of both of said inner films include a significant amount of antimony trioxide, and the white pigment content of the inner film of said first-mentioned covering includes a white pigment other than antimony trioxide in an amount greater than the antimony trioxide of that film.

9. The combination defined in claim 4, wherein said inner film of said first-mentioned covering contains significantly more white pigment than does the inner film of said second covering.

10. The combination as defined in claim 4, wherein aid inner films are of polyvinyl chloride.