CA2214496C - Infrared reflective coverings - Google Patents

Abstract

A textile material is described which provides thermal image masking or suppression in the mid and far infrared region without co mpromising the effectiveness of visual and near IR camouflage or comfort level, or the effectiveness, and mobility of a person. This is achieved by incorporating a metallized microporous membrane into a typical article of clothing or covering, e.g. tents, which suppresses thermal imaging. Specifically the invention is directed to an air permeable, moisture vapor transmissive, waterproof, heat reflecting material consisting essentially of at least one metallized microporous membrane (20) laminated to at least one other layer or textile backing material (22) such as woven, nonwoven or knitted nylon, polyester, cotton, silk, etc. or additional microporous layers, in which the metal in the metallized membrane forms a discontinuous layer (21) on the surface and on the pore walls adjacent the surface of the microporous membrane.

Description

TITLE OF THE INVENTION

Infra-Red Reflective Coverings FIELD OF THE INVENTION

~ This invention relates to ele_~ùnnagnetic reflective and l,dns",issive materiais and to the use of the ",alerials as electromagnetic camoufbge particularly at infra-red wave'enyU,s.
BACKGROUND OF THE INVENTION

Instnuments which detect ll,e""al rddialion are well known. Ra~i~tion from the human body or other objects can easily be de~cted by infra-red detecting instruments.
These instruments operate in the al",ospheric lranspa,ency windows of 3 to 5 mic,u"~eters and 8 to 12 micrullleters. Infra-red imaging atwavelengths outside of these windows is not prd- Ucdl due to dl-"ospheric absorption. In images obtained with these devices objects with high emissivities and or objects having a higher temperature relative to the background appear as bright silhouettes. This is due to the emitted power of these objects. The emitted power is described by the equ~tion:
W = ~T~
where W = emitted power in BTU/hr.-ft.2 ~ = emissivity ~s = the Stephan-Bolk-"a" con~ldr,l and T = tei~Iperdl-Jre in degrees Rankine.
From this e~uation it can be seen that there are two ~os .ible approaches to subdue a U ,e""al image; use low emissivity i"ateri ~ s on the exterior surface or reduce the exterior surface temperature. The typical approach is to use low emissivity Illdtel idlS on the exterior surface and then cover the low emissivity surface with materials which are l.dnspa,t n~ in infrared (IR) wavelengths but optically opaque to provide visual camouflage. The second approach is to use ll,e"nal insulation to reduce the e~tenor surface temperature. A third approach is a CG"~ ~ . ,alion of these methods.
It has long been a desi, ~le goal to develop materials that protect persons or equipment from dete. Uon by elecl.c""agnetic and esperial~y infra-red detecting equipment without detracting from the mobility of the personnel or equipment.

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For example. U.S. Patent 5 281 460 provides a pattem of strips attached to a porous nylon mesh. The strips are coated with silver copper or pigment.
lJ.S. Patent 4 495 239 employs a base layer of textile fabric having a vapor deposited metallic reflecting layer on it followed by a camouflage paint.
U.S. Patent 4 659 602 employs a woven material that has a metal foil on it and a polyethylene sheet containing a conductive partic~ ~~ste.
In U.S. Patent 4 621 012 a textile is coated with a ll,e""opl~slic that has selected dipole material in it and which has a metallic layer to reflect infra-red.
U.S. Patent 4 467 005 employs a support netting that a carrier web on each side having an IR reflecting metal coating. The malerial is water vapor permeable.
U.S. Patent 4 533 591 provides a thermopl~slic resin having disc,t"e ele~l,o,naynetically pailicles cJispe,~ad in it.
U.S. Patent 4 064 305 provides a knit fomled of strands of noncontinuous polymeric fibers and nonconl;n.Jous metal fibers which reflect radar waves.
U.S. patent 4 529 633 teaches an GIE~ l,ur,~agnetic ~lle~ ~ing material made of a layer of polyethylene a layer of a metal coating an adhesive and a fabric.
Because of the presence of plastic layers the co",posilions of the patents do not allow water vapor to escape easily and when wom as garments are uncomrol i le or when draped over equipment cause sweating of the aquipment.
An sxceplion is U.S. Patent 4 467 005 which claims water-vapor permeability but not air permeability. To a person skilled in the art it would be readily apparent that the te-_l ,n ~ e described to acl ..9ve water vapor permeability and wdlel~ r~,or"ess would not result in a sufficiently high water vapor permeability to be of any practical value. Any improvements in water vapor permeability would result in a con~po,)ding reduotion in 30 walel~,roor.,ess. The ",alelials descnbed in the aro,e-"anlioned patent provide a salisraclùry surface for metallizatlon and are acceFta~lo for uses where a high degree of flexibility and mobility are not required such as a covering for slalionary lo~ ~ ~s but many disad\dnlagas surface when these materials are used to provide thermal imaging p,utecliûn for an individual 35 person. Chief among these disadvantages are the lack of drape low moisture vapor permeability and weight. In addilion to the drort,l"entioned disadvantages the metallized surface is on the exterior of the la", ~dleS where it is in a position to be damaged or scraped off while moving through brush. It ~ ' CA 02214496 1997-09-02 .
~ -3-is desirable from a physic'~gi~l standpoint to reduce the heat stress of the person wearing IR camouflage gamlents to the largest extent possible. This can be accomplished by increasing the evaporali~/e cooling of the body by allowing moisture vapor to easily permeate through the lami.)a~, and by 5 reducing weight and thickness of the total thermal camouflage package.

SUMMARY OF THE INVENTION

The object of this invention is to provide thermal image masking or 10 suppression in the mid and far infra-red region without coi"pr~.",;sing the effectiveness of visual and near IR camouflage or co",fc,l level, or the effectiveness, and mobility of a person. This objective is ach--eved by inco".ordli"g a met~ ed microporous membrane into a typical article of clothing or covering, such as tenbng, which suppresses themmal imaging of 15 objects underneath or behind the metallized membrdne.
Specifically the invention is directed to an air peu n ~ ~ t l ~, moisture vaportransmissive, waterproof, drapable heat reflecting material consisting essentially of at least one metallized microporous me",b,dne laminated to at least one other layer or textile backing material such as woven, nonwoven or 20 kniKed polyamides, polyolefins, polyester, cotton, silk, etc. or additional microporous layers, in which the metal in the metallized menJbrane forms a discontinuous layer on the surface and on the pore walls adjacent the surface of the ~"ic,~porous me"~brdne.

BRIEF DESCRIPTION OF THE DRl~WINGS

Figures 1a and 1b are cutaway side views of "~.- oporous membrane used in the invention which depicts how the metal layer can be discontinuous but sti!l provide an effective barrier to heat l,ana",ission.
Figure 2 is a side view of the metallized ",ai"brd,)e laminated to a backing material.

DET~ILED DESCRIPTION OF THE INVENTION

Referring to Figure 1a, an enlaryed side cutaway view of a "~.c opor~us membrane 10 is shown with polymer sheet 11 having irregularly shaped pores 12 running through it from side to side. In Figure 1 b a vapor deposi~ed metal 13 is shown in which the metal is depo~ited from above the membrane so that ' CA 02214496 1997-09-02 .

the metal coats the surface and the "open" pore walls, i.e., that portion of thepore walls that are open as viewed from above the membrane. Thus looking vertically down from the top in the direction of the arrow it appears that the metal has fommed continuous line-of-sight coverage. This is dep. 'ed by the dotted lines in Figure 1 b. But from the side, it is seen that the metal coating is discontinuous, leaving the pores open for passage of water vapor.
The use of metallized i" _ uporûus films and membranes, such as microporous polyethylene, polypropylene, polyurethane, expanded polytetrafluoroethylene, etc. in lamination with standard bxtile fabrics circumvents the disadvantages of the prior art for several reasons. First, the three dimensional nature of the microporous materials provides for 100% line of sight metal coverage on the surface as viewed from above, providing the IR
reflection required for adequate thermal image suppression. Second, the porosity in three dimensions required to allow large quantities of moisture vapor to permeate through the composite is preserved, thus reducing heat stress on the wearer. Third, the air entrained in the "~ pores of the membrane reduces the membranes themmal conductivity by providing an insulating air space. This forces more of the heat exchange between a human body and the environment to be through evapordli./e cooling. A large portion of the heat radiated through the microporous me",brdrle from the body is reflected back towards the body, in tum reducing the temperature of the exterior surface, thereby reducing the thermal image. The reflected heat is removed through the body's natural cooling me.,l,anis"" evaporation.
Microporous materials are also lighter, more flexible and drapeable than the materials cited in the prior art, which makes them more suit~ for clothing.
The metal used in the metallized n,i 'OpGIuUS films and membranes can be any metal that can be vapor deposited or sputtered on the film or membrane and produce the desired reflective effect, such as aluminum, silver, copper, zinc, or the like.
The mat1t'i~ation is on one side only and can be acco""~' ~hed through the use of physical vapor deposilion, e.g. sputtering, or chemical vapor deposition. The metal coating can range from 40 to 1200 angs(,ùms in thickness, and the mat " :~ membrane will have an optical density between 1 to 6 density units.
The met;~ ed ",-:~upo,ous film or ",e",brd.~e thickness measured as A
in Figure 1b can range from 0.001 to 0.125 inch and will vary depending on the desired air and moisture vapor permeability. The ll ,i._l~"ess of the metal coating is not so great as to close the pores of the "~: opor~us film or membrane but rather deposition takes place to the extent that the surface and part of the pore walls are covered to form a line-of-sight coating as explained further above with reference to Figure 1.
The textile employed should have the desired specific properties e.g. IR
5 transparency visible opacity etc.
The textile is adhered to the metallized membrane on the metal coating side by laminating with a discontinuously applied adhesive with heat and pressure or by direct heat fusion.
In one e,.,bo~ ent shown in Figure 2 a .n;croporuus expanded 10 polytetrafluoroethylene (ePTFE) membrane 20 is shown with metal 21 e.g.
aluminum deposited on the membrane. A textile shell material 22 such as woven silk or nylon is laminated to the coated me---bra"e using a .Jiscor,~i"uous polyurethane adhesive so that the metallized surface of the ' microporous membrane is facing the shell material. A liner fabric (not shown) 15 such as knitted polypropylene can be attached in the same manner as the shell. Altemate embodiments could include multiple layers of meP~ ed microporous membranes of the same or different chemistry addilional textile Iayers and fusion bonding instead of adhesive bonding. In addition the textile shell material may be coated with a topical coating of a material such as 20 barium titanate to modify its radiant ll,e"nal characteristics.

A n.' OpGlOUS ePTFE me~"t"a"e 0.001 inch thick of nominal .2 ~m pore 25 size obtained from W. L. Gore & Asso~ s Inc. was metallized by vapor depositing aluminum by evapG,dtion and condensalion to an optical density of 3.0 density units (as dete""ined on a Densito-.-~ter of Tobias Assoc. Inc.
(Model TRX-N). Specifically aluminum wire was heated in an oxide crucible at a high vacuum (2 X 10~ Torr at about 1220 ~C. The aluminum vaporized.
30 The eP'TFE me,.,brdne with a hlm backing to block entry of vapor on one side was passed over the crucible with the bac~dng on the side away from the crucible. Vapor from the cruicibl~a rose to form the .~iscontinuous coating on the adjacent side of the men,brane. The coated "~e"~brdna was then wound on a roll. After the backing was removed the aluminized ,n-- uporous 35 membrane was laminated to a 2.7 ounce/yard woven nylon taslite shell material so that the aluminized surface is closest to the shell'material using adiscontinuously applied polyurethane adhesive. A third layer of knitted nylon was then laminated to the non-metallized side of the ePTFE la"~;ndle.

To test infrared image suppression, a Hughes/Texas Instruments night vision system (dielectric volometer - Part #6245935) was used. The dielectric volometer recorded heat emission from a human object. When the la"~inale was placed over the object, the image of the object was sub~Le~l)t;e-"y reduced.

A metallized microporous ePTFE l"e",~re~ne was prepar~d as in Example 1.
10A piece of one ounce per square yard China silk was placed on a 6 X 9 inch rubber pad. A 6 X 9 inch piece of fusible open, nonwoven adhesive such as Spunfab #EV3014 was placed over the silk. A piece of the metallized film was placed over the adhesive layer with the metal side fadng the adhesive.
This rubber pad/silk/adhesive/metallized me",~rdne comhine~lion was placed in 15a press heated to 123~C. The press was closed and pressurized to 2000 PSI
for 10 seconds. The laminated sa",~'ss were then removed. IR image suppression properties were determined as in Example 1. The image was substantially reduced.

Claims (7)

I CLAIM:

1. An air permeable, moisture vapor transmissive, waterproof, drapable, heat reflecting material comprised of at least one metallized microporous membrane having pores from one side to the other which is laminated to at least one other layer of textile material, in which the metal in the metallized membrane forms a discontinuous layer on the surface and the adjacent pore walls of the microporous membrane such that when viewed from above the surface of the membrane the metal appears to be continuous.

2. The material of Claim 1 wherein the membrane is microporous expanded polytetrafluoroethylene and the metal is aluminum.

3. The material of Claim 2 wherein the textile material is silk.

4. The material of Claim 1 in the form of a garment.

5. The material of Claim 1 forming a part of a garment.

6. The material of Claim 1 in the form of a tenting material.

7. The material of Claim 1 forming a part of a tenting material.