CA2209821A1 - Lightweight hardened protective coating and method for making and using same - Google Patents

Abstract

An improved protective coating is disclosed that hardens to form a lightweight but extremely durable finish for a variety of substrates. The preferred coating comprises a mixture of lightweight comminuted porous expanded polytetrafluoroethylene (PTFE) particles suspended within a base paint mixture. When applied to and cured on a surface, the expanded PTFE particles form an interlocking grid network within the paint which is lightweight, durable, chemical and UV light resistant, and has a low coefficient of friction. The composition of the present invention is particularly suited for applications where a durable but lightweight coating is sought, such as in the aircraft industry.

Description

wo s6nl700 ~J/~ /0 .1 .

LIGHTWEIGHT HARDENED PROTECTIVE
COATING AND METHOD FOR MAKING AND USING SAME

eACKGROUND OF THE INVENTION

1. Field of the In~/entiGn The pl- s3nl i,)~ention relates paints and other ha,~ened coatings applied to surfaces requiring eA~ptiondl protective qualities and particularly to lightwei~ht prvte..ti~a coatin~s for vehides such as aircraft.

2. Description of Related Art The coating of aircraft and other vehicles for decorative andlor pl~tG- ti;c purposes is an area of conUnuing interest. Given the costs of such vehides and their i,lherènt maintenance requir- .~,er,ts there is a de",and for i,,,~,u~cd p~vt~5- ti~le coatings for these vehides to dec.l~se vfear, and hopefully i"",rula the laminar flow of air and water over the vehides.
Improved laminar nOw is a major coo~,., in aircraft and water craft applications, such as with racin~ vehides where high speeds are sought and in tightening competitive ",a,l~ets where escalating fuel costs must be constrained.
While there continues to be i~p~u1remanls in the pigments and solvents used for probctive coaan~s, the availability of a highly protective coating usually results in a weight gain for the paint product. This is a particular worry in weight sens;t;,/a envir~r""enb such as the w"""ereial aircraft industry where every i"~ 2se in weight tl--nslates into Jla",~t;c i"~ _ases in fuel costs over time.
Due to the reduced coemcient of drag Expe,ienced with totrafluoroethylene (TFE) monG..,ar or full density polytetrafluoroethylene (PTFE) poly.--er such as PTFE pol~."ar available from E. I. duPont de Nemours & Company. Wilmin~ton Delaware under the t~-J--",a,l~ TEFLON~
many investi~ t~,a have su~ested mixing this n~ate,ial with a variety of paints 30 and other coaffngs to reduce friction and improve protective properties. For example, in United States Patent 4,284,668 issued August 18, 1981 to Nixon a mixture of tetrafluorue;!~Jl~ne (TFE) ",ono",ar, a TFE dispe.~ion carrier an abla3;Je compound and an atllo!i;Je Ji_pelaiun carrier are applied to a surface and buffed in place to ~is ~ir~te the Ji~pe,aion carriers and poly",e.iLe the TFE
35 on the painted surface.

w o96nl700 ~ J~03261 A number of others have suggested suspending poly",e~ ed TFE in paint and then apply;ng this sul,alance to the surface to be painted. For inslance, in United States Patent 4,849,264 issued July 18, 1989, to Gira et al., PTFE particles are suspended in an oil phase and mixed with a resin film-5 forming paint composition. Upon arpl :~tion and curing of the resin film, theoil phase sepa,dt~s to form a protective overlayer of oil with PTFE suspended therein. In United States Patent 5,039,745 issued August 13, 1991, to Riddle a mixture of silicone~ resin, PTFE poly.,.er and polyun,l; ,ane polymer are combined to produce a chemical fesialanl paint with a non-stick top layer.
One conce", with this ap,uùach is that a full density PTFE fine powder, or dispe,~ion of the same, may fibrillate when added to a paint mixture. This can lead to an inconsial~nl mixture of PTFE material within the paint. In ,~sponse to this pr b'e -, is the appluacll p,uposed in United States Patent 5,081,171 issued January 14, 1992, to Nixon. In this case, negatively charge PTFE particles suspendad in a paint are fused to a positively cl,ar~ed substrate.
While the various previous dt~ n.pb to combine TFE or PTFE
particles into a paint may have had some limited suc~ss, there are a number of pre~le."s with this appruac,l,. First, as some of the above patents recogni~e, without further process steps or other measures, discrete PTFE
particles do not readily bond to a substrate or paint layer. The inher~nt lubricity of these particles can then result in poor adl ,esion and possible co,.,rJro",;se of durability Second, of even greater concem in the aircraft industry where every measure must be employed to avoid excess weight, full density PTFE (i.e., with a density of about 2.2 g/cc) pr~sent~ a possibly needless weight gain for an aircraft. As a result, even if more durable or slippery cGatings can be applied to aircraft by inclusion of a PTFE ",at~rial, it may still be more econG"~ic-' to forgo the improved p~pe,tics of this ,.,~erial in favor of a lighter coating.
Accordingly, it is a primary purpose of the present invention to provide a hard coating ~",poaition suitable for use on a variety of surfaces, and espe~ally for use on aircraft, that is liyl ,t~,ight, durable, chemical resistanl, and has a low cG~fri~.ient of friction.
It is a further purpose of the present invention to provide a coating cbmpo~iticn that forms a inte.~,onne~.led polymeric net~vork when applied, improving durability and aJhesio-, of the coating.

3 ; . , .. ~
It is still another purpose of the present invention to provide a coating composition which includes an additive with a porous structure of PTFE, presenting an improved surface for binding between paint pigments and the additive.
It is yet another purpose of the present invention to provide a method for 5 producing and using a coating material with the above properties.
These and other purposes of the present invention will become evident from review of the following specification.

SUMMARY OF THE INVENTION
The present invention is an improved coating material for use on a variety of suRaces and particularly for use on vehicles such as aircraft where there is a need for a highly protective but lightweight hardened coating.
The coating material of thQpresent invention comprises a blend of csmminuted porous expanded PTFE particles within a base paint mixture. The expanded PTFE
15 particles are evenly suspended within the base paint for application and then cured into a cohesive protective rletworked- grid of expanded PTFE.
With a typical density of expanded PTFE of 1.8 g/cc or less, the composition of the present invention forms a lightweight coating on the substrate, suit~le for weight-sensitive applications such as aircraft painting. Moreover, the coating of the 20 present invention is believed to have significantly improved properties in many other respects, including adhesion, durability, chemical resistance, ultraviolet (UV) light resistance, and low coeffficient of friction. These properties are also sought to improve paint quality. The cumulative effect of combining these properties creates a paint that has enhanced wear resistance, which will thus make it suitabLe for many 25 demanding applications like use on water craft, roads or metals in corrosive environments.
Due to the availability of expanded PTFE material and the properties of comminuted expanded PTFE particles which allow for ease in producing and applying the composition of the present invention, the present invention should be 30 suitable for a wide variety of applications. However, the lightweight properties of . .
present invention makes it particularly useful for providing protective coatings on vehicles, and especiaiiy aircraft. ~ - ~ ~
Other improved properties of coating of the present invention include: greater resistance to heat and flames (in fact, the addition of expanded PTFE

AM~NDED SHEET

to a material increases the number of fluorine groups to the matrix thus inhibiting combustion of the material); increased liquid repellence, both hydrophobic and oleophobic (i.e., expanded PTFE mixed with paint tends to gravitate to the surface of the mixture, providing a blanket layer of ePTFE, which will assist in repelling water or other fluids having surface tension energies greater than 31 dynes/crn at 23~C as per the MTCC 118-1983 Oil Repellency Test); improved resistance to steam and other gaseous mixtures; increased insulating properties; enhanced chip resistance capability over paint doped with standard PTFE since the ePTFE particles bond better in the paint due to their inherent morphology; improved abrasion resistance;
10 when loaded with large expanded PTFE particles, improved durable non-slip surface properties; resistance to flex and thermal expansion avoiding cracking and fractures; and enhanced ability to bond to other desirable materials oF active agents, such as anti-fungal, anti-bacterial, or anti-fouling agents, pigments (e.g., fluorescent pigments), radar absorbing materials, radar reflecting materials, etc.
DESCRIPTION OF THE DRAWINGS
The operation of the present invention should become apparent from the following description when considered in conjunction with the accompanying drawings, in which:
Figure 1 is a graph plotting the average particle sizes of comminuted expanded PTFE particles generated for use with the present invention, the graph comprising the cumulative volume and differential volume;
Figure 2 (plate 82) is a scanning electron micrograph (SEM) of a surface of a coating of the present invention, enlarged 100 times;
Figure 3 (plate 80) is a SEM of the surface shown in Figure 2, enlarged 500 times;
Figure 4 (plate 88) is a SEM of filings scraped off the surface of a cured coating of the present invention to show its cross-section, enlarged 100 times;
Figure 5 (plate 86) is a SEM of filings scraped off the surface of a cured 30 coating of the presen~ invention, enlarged 500 times;
Figure 6 (plate 91 ) is a SEM of filings scraped off the surface of a cured coating of the present invention, enlarged 5000 times.

AMENDED SHEET

o l~ r.
5 ~ n ~ ~ t ~ -- -- ~ -- ~ -- --DETAILED DESCRIPTION OF THE INVENTION
The present invention comprises a coating material which is applied to a substrate and hardened in place to form a relatively lightweight and durable protective finish on the substrate. The present invention provides numerous 5 improved properties, including serving as a vehicle for other agents.
Many improved properties can be imparted to polytetrafluoroethylene (PTFE) by heating and rapidly expanding the PTFE in one or more directions. By forming a network of polymeric nodes interconnected by fibrils, the PTFE experiences little decrease in its overall volume, but undergoes a dramatic decrease in its density in 10 this process. As a result, a microporous structure is created that has numerous unique properties, including low density, porosity, increased tensile strength, and increased resistance to creep. Such a product can be produced in a known manner,- ~ such as in accordance with the teachings of United States Patent 3,953,566 issued April 27, 1976, to Gore. ~enerally, this material has a density of less than 2.0 g/cc, 15 and preferably a density of 0.9 to 1.8 g/cc.
The coating compositiori of the present invention comprises a blend of a conventional paint mixture and comminuted particles of expanded polytetrafluoroethylene (PTFE). The expanded PTFE comprises a fine powder grind of expanded PTFE with an average particle size of about 5 to 100 micrometers, a 20 particle size of less than about 40 micrometers being preferred. The expandedPTFE particles and the paint mixture are agitated together to form an even suspension of the expanded PTFE particles within the paint mixture.
While the expanded PTFE particles can be generated in any known manner, it is preferred to form these particles by the following procedure:
1. Initially reducing the size of the expanded-PTFE in the form of sheet goods, - fiber etc., to a particle size of approximately 940 micrometers (20 mesh) using a shredding type mill; ~
2. Further reducing the size of the 940 micrometers (20 mesh) material using a modified Morehouse Super 800 series colloid mill. The Morehouse mill is modified by securing the mill stones as is taught in U.S. Patent Number 4,841,623 James C.
Rine, incorporated by reference. -The mill stones-are ,e~tdined circumferentially as opposed to the common axia~ mo~nting conf~uration. The circumferentially ~-~
mounting of the stones permits the stones to will,stand higher rotatior~al velocities.
- When the grinding stones rotate at the increased ~otational velocities in the colloid mill, it is found that expanded PTFE can be sized reduced t~ a mean particle size of 40 micrometers and smaller without severe degradation to the nodal-fibril structure inherent to the expanded PTFE material. Common size reduction techniques for PTFE and plastics use radiation to render the material fractal to allow the material to .~ AMENDED SHEET

6 ~ . . ' 7 O
be size reduced to particles sizes below 100 micrometers. Unfortunately, the irradiation process destroys the nodal-fibril structure of the expanded PTFE material.
The use of the modified Morehouse Colloid mill provides an alternative to the use of irradiation to yield particles of sized reduced PTFE and size reduced expanded 5 PTFE below 100 micrometers.
3. Adding tap water to the 940 micrometers (20 mesh) initially sized reduce material in the hopper which feeds the colloid mill. A water and ePTFE slurry isproduced in the hopper with a concentration of water to ePTFE of 50:50. Note that the higher the concentration of ePTFE to water is made, the better the efficiency of 10 the mill. Although there does exists a peak concentration ratio since too much ePTFE to water will result in excessive heat build-up on the stones. The stone must be kept cool during the milling operation. Excessive heat build-up in the stones will - render the stone useless. The preferred water and PTFE concentration is 35~0%
PTFE to 65-60% water to allow for adequate cooling of the stones. The size 15 reduced material exits the mill as a slurry.

4. Placing the slurry material on flat aluminum pans in a forced air convection oven at a temperature of 105 to 150~ C until the water evaporates. This produces a cake-like material.

5. Removing the pans from the oven and fracturing the cake-like material by 20 blending the material using a standard household food blender or simply by shaking it in a closed container.

The graph of Figure 1 illustrates a typical range of particle sizes of expanded PTFE generated through the above procedure. Curve 10 represents the cumulative 25 volume; the bar graphs 12 represent the differential volume. The graph indicates that the greatest quantity of pa,li.'os (i.e., 50%) produced in this manner are between 17 to 50 ",icro",eters. To acquire particular particle sizes for use with the present invention, the ground expanded PTFE material can be separated in any known manner, such as through screening, thermal air separation, floatation, 30 cyclone rill,ation and cyclone separation, also possibly fluid bed separation.
Preferably the pa, licles are separated through the process of a thermal air separator.
Comminuted expanded PTFE particles- are the preferred matèrial for use in the present invention for a couple of reàsons. First,- in addilion to ~ow density, with a typical density of less than 1.8 g/cc, an ek,uanded PTFE fiber can have a very high 35 matrix tensile strength when compared to standard PTFE. For example, a conventional PTFE particle may have a matrix tensile strength of 211 Kg/sq cm (3000 psi). By contrast, a comminuted particle of expanded PTFE made from a fiber stretched with a ratio of 80:1 will have a matrix tensile strength of about 7030 Kg/sq AMEN~~D SI~IEET

cm (100,000 psi). In this context, tensile strength is determined in accordance with ASTM D-882 (Tensile Properties of Thin Plastic Sheeting) using an INSTRON
Tensile Tester Model 1130 outfitted with clamping jaws suitable fomest,aining sheet goods. This machine is available from INSTRON Machine of Canton, MA. The 5 tensile strength was determined using sheet goods of expanded PTFE. It is thought that the material is isotropic.
Depending upon the paint mixtures used and the properties desired, the particles can be mixed in virtually any proportion, with the understanding that excessive loading of expanded PTFE particles can lead to an uneven painted 10 surface and, in extreme, possible compromise of adhesion. A proportion of expanded PTFE in the final composition of 1/8% to 25% by weight is considered appropriate for most ~pplications. For use as an aircraft or water craft paint, where a smooth, gloss finish is often desired, the expanded PTFE preferably comprises , .
'10% by weight of the final blended composition.
The blending of the paint and expanded PTFE is preferably performed using mechanical agitation, such as~through use of a magnetic stirrer or for more viscous matter, an electrical lab propeller. This step is performed by slowly adding theground ePTFE particles into the agitated beaker containing the paint. A vibrating sifter or other powder dispensing device should be used to dispense the co"~",inuted 20 ePTFE material into the container holding the paint. Other suit~'e mixing processes may include wetting out of the comminuted expanded PTFE with a solvent, such as isopropyl alcohol (IPA), and suLjecli"g the mixture to ultrasonic agitation, such as through the use of an ultrasonic horn submerged in a beaker containing the IPA and then slowly adding the comminuted ePTFE into the beaker using any suitable 25 powder dispensing device or appardtus. Other potential mixing apparatus may include a mechanical vibrdting table, or a shaker table.
For use with oil-based paint mixtures which are "filmogen" (i.e., film-forming material or binder), such as vegetable oil, linseed oil, and oleoresinous varnish, the expanded PTFE may become suspended through mere agitation. However, with 30 most paints, such as water based latex paints (e.g., acrylic, butadiene-styrene, polyvinylacetate), the hydrophobic nature of expanded PTFE may require the introduction of a wetting agent, such as a hydrophilic surface coating of SPECTRACOTE (polyuretl,ane coating) from Flexible Products Company of Marietta, Georgia, or a surfactant of TRITON X100 from Rohm & Haas or OP10 from 35 ICI, Inc. These materials should be mixed such that the surfactants are less than 3% of the total liquid volume.
Once the final composition is formed in the above manner, it is readily applied to a substrate through any conventional manner, including but not limited to use of . .
AMENDED SHEET

CA 0220982l l997-07-08 8 , ' ~ , ~' ' ' '- --.
brushes or other mechanical applicators, spraying equipment, and pad printing.
Once applied, the composition will dry and cure in accordance with the properties of the paint mixture used. Unlike previous attempts to mix a full density PTFE or similar material with a paint, the expanded PTFE particles, which are pre-fibrillated, are believed to form a stronger interlocking grid network within the hardened paint surface without requiring any further treatment. It is anticipated that this network will serve both to improve the interface between the expanded PTFE
and the paint pigment and to provide a stronger and more durable finish to the coating.
Another important property of the expanded PTFE particles is that their microporous structure and randomized surfaces provide an excellent surface to which paint pigment can bond. As a result, a far stronger and more cohesive coating can be provided than is possible with existing fully densified fluorinated paints. In light of these advantages, it is believed that the preferred size for the ePTFE particle 15 is greater than 5 microns to assure sufficient porosity of the particles.
Without intending to limit the scope of the present invention, the following examples serve to illustrate how the present invention can be made and used.

EXAMPLE 1:
A comminuted expanded PTFE was produced from unsintered expanded PTFE material by grinding it using a Morehouse Super 800 series colloid mill. The Morehouse mill was modified by securing the mill stones as is taught in U.S. Patent Number 4,841,623 James C. Rine. The 940 micrometers (20 mesh) material was further size reduced using a modified Morehouse Super 800 series colloid mill. The 25 milling apparatus was equipped with 90 grit stones. The mean particle size of the comminuted material was about 45 micrometers.
The expanded PTFE material used in this context comprised of 1/2 inch (12.7 mm) diameter PTFE cord material which was expanded 8:1 times longitudinally in accordance to U.S. Patent No. 3,953,566. This material is commercial expanded 30 PTFE joint sealant material without the adhesive strip and is available from W. L.
Gore & Associates, Inc., Elkton, MD, under the trademark GORE-TEX~). The expanded beading was then shredded to a mean particle size of approximately 500-700 micrometer using a shredder from Cumberland, Inc. The shredded flake material was further size reduced using a Morehouse colloid mill which was modified 35 as described above in accordance to U.S. Patent No. 4,841,623. The modification to the mill involved restraining the stones around the circumference of the mill stones as opposed to restraining the stones axially, as is commonly done. This novel A~llENDED SHEEt ., , . ~, 9 , ~
... .. .. ..
restraining configuration permits the stones to rotate at a greater velocity without destroying the stones due to centrifugal forces.
One g of comminuted expanded PTFE was added to 10 g of AWL-CAT #2 G3010 (92-C-39) thinner available from U.S. Paint Corporation, Inc. of St. Louis, 5 Missouri. This thinner comprises xylene, toluene, ethyl acetate, cellulose acetate and aliphatic polyisocyanate. The expanded PTFE particles and thinner were agitated togéther by hand with a glass stirring rod. The expanded PTFE material easily wetted-out to produce a homogenous suspension.
Next, 10 g of pigmented polymer were added to the suspension while stirring 10 continued with the glass rod. The pigmented polymer comprised a glossy black G-2001 Alumi-Grip Linear Polyurethane topcoat paint available from U.S. Paint Corporation, Inc. of St. Louis, Missouri. This is a common high gloss paint comprising a base urethane polymer, which is frequently employed for coating aircraft and race cars. An even suspension of expanded PTFE in the paint material 15 was achieved.
The mixture was then applied by natural bristle paint brush to a piece of aluminum sheet metal of the kind used in aircraft construction. The sheet metal was degreased with a toluene based solvent and wiped with lint-free towels. The coating composition was allowed to cure under the following conditions: Ambient temperature, under laboratory hood with a face velocity of 0.5 m/sec (100 ft/min) for 24 hours.
Assuming all the thinner volatized during the curing operation, the comminuted expanded PTFE constitutes about 9.1% of the final mixture.

AMENDED SHEET

wo s6nl700 r~ s~

The fe" / :.19 results were observed: the mixture easily ~leased from the paint brush onto the sheet mebl surface. I lo~ cr, brush strokes were eYident in the final coated product since the paint failed to level sumciently during curing. The cured surface was somewhat rough and dull in appea~nca, a suitable finish for water craft deck use, aircraft step areas, and similar env;.o"",~nts where traction may be desired.
To test the durability of this surface, after the coating had completely cured for 24 hours, app,uxi,,,alely 1.0 ml of a test solvent was applied to the painted surface. The solvent cG",p,ised an aircraft hydraulic fluid of tributyl~JhGsphate, dibutyl phenyl phosphate marketed by MDnsanto Company of St. Louis, Missouri under the llaJelllalh SI~YDROL. After the fluid was left in place for 96 hours, no appar~nl effect was observed on the painted surface.
By way of co",parison, a test area was p~pa,e:d on the same sheet metal a~ply;ng the glossy black G-2001 Alumi-Grip Linear Poly~-,ell,ane lopcGat paint alone. The paint applied to the surface easily and cured for 24 hours to a glossy black finish. When the hydraulic fluid was applied to the paint for the same 96 hour period, the painted surface was severely drre.,led, with the paint ~issolv;, .9 in the solvent, blistering from the surface, and easily wiping off.
EXAMPLE 2:
Using the same materials, anotl.er test sample was pr~pa,ed employing 2 9 of comminuted eA~,anded PTFE added to 10 9 of G3010 thinner. The ex~,anded PTFE particles and thinner were gently agih~ed by hand using a glass stirring rod. Again, the ePTFE material easily wetted-out to produce a ho",ogeneous suspension. 10 9 of G-2001 pig,.. ented polymer were added to the suspension while sUrring the mixture with a glass stirring rod.
The mixture was then applied to the aircraft sheet metal using a natural bristle paint brush. Assuming 100YO of the thinner is vol t;~ed during 30 the curing operation, the comminuted ex~,anded PTFE constitutes about 16.6% of the final coating.
In this i"slance the mixture easily l~ilcased from the paint brush onto the sheet metal surface. Similar to Example 1, bNsh strokes were pr~senl in the fnal coating bec~use the paint did not fully level before curing. The cured 35 surface was more rough and dull in appea,~nce than the surface of produced in ac~rJance with Example 1.

WO g6m700 PCI/US95/03261 When the hydraulic fluid was placed on the surface of the paint for 96 hours, no appaf~nt effect was observed.
Both of these examples de.-,on~t~dted a number of illluG~nt ~r~.pe.ties of the ~r~Gn~ invention. First, that e~pan~ed PTFE pa~ticl2s can S be readily mixed with solvents and paint to acl..eve an even suspension without agglo..,or~tion of the eA~anded PTFE particles. Second, that this suspension can be roadily applied to a sheet mebl surface and fomm a stron ~dl .esion thereto. Third, that the combination of the eApdnded PTFE particles and the paint produce a finish which is far more chemical resistant than paint 1 0 alone.
Scanning electron mi~u~ hs (SEMs) were then taken of the product of Example 2. These are shown in Figures 2 and 3. Figure 2, a 1 OOx magnification of the surface of the coatin~, shows the noda~porous nature of the coating's surface. Figure 3, a 500 magnification of the surface of the coating, shows how the ex~anded PTFE particles are enc~ s~ ted and/or bonded to the epoxy paint. It is in.po, ~n~ to note little shade or co. ~t,~sl di~fcr~nce between the eA~.andad PTFE partides and the black pigment. This is believed to indicate that the e~andad PTFE particles have been wetted out by the pigmont.
Surface filings were s~,3ad off the test sample of Example 2 to study the cross-section of the cured paint. SEMs of these sc~ed filings are shown in Figures 4 through 6. Figures 5 and 6 show a defined nodal-fibril structure in the eApanded PTFE. This structure may have resulted from the scraping action of the test sample during sample prt,pa~aliûn. Figures 4 and 5 show pookstc of epoxy pigment, but there a~ ."s to be a di_~rupGl tionate amount of eApandad PTFE on the surface. The in~-,ased amount of eApanded PTFE on the surface may indicate that the eA~,anded PTF
migrates to the surface of the paint during curing rather than being a het~n~g2neous mixture across the cross s~ction of the cured paint. This may explain why the final coating has an e~,~.ely durable and chemically resistant finish.
The coating of the present i,.l/ention is believed to have numerous advantages over previous paints. First, low density e,~uandad PTFE when blended with regular paint will reduce the total weight per gallon of the combined mixture. Given the substantial cost of fuel, this form of weight reduction can be a ve~y si4gnificant factor in the aircraft industry.

WO96n1700 P~-JI~ 9~103261 Second, as has been noted, the fibrous nature of e,~panded PTFE
creates an inlerlGcking grid network which should enhance the final codting's tensile stlell~tll. Further, this material is far more f~.,y;~;,lg to stresses from thermal cyding or flexing, significantly redudng ~ ~ in~ or fractures in the S paint underthese conditions. Additionally, when co",pa~d to the sl-~cked pattern that can occur when full density PTFE spheres are added to paint, the grid network of expanded PTFE also creates a barrier which greatly enhances the coat;ny's chemical ,esistance. While convenlional PTFE is UV light resislant, it is believed that the i"terlo.,ki"g grid fommed with the present 10 invention may present an even more protective coating in this regard. Finally, the inlerlo~i"g grid may also improve the heat nesisla.lce of the final coating.Third, the density and chemical prupc, lies of the eA~,anded PTFE
particles should cause them to gravibte toward the surface of the coating during the curing p,ocess. When cured in this position, the ~"~t nalion of 15 eA~anded PTFE and paint binder is believed to offer a far superior barrier to wear, light dey~da~ion, and chemical attack.
Fourth, the inte,lGcking nature of the e~ande.J PTFE partides may also allow a paint manufacturer to formulab a paint that is SIIIG~JU ,er than one containing full densitv PTFE. This should lead to i"~pro~ed laminar flow over 20 the coated surface, and improved performance and reduced fuel consumption in vehides.
Fifth, the eA~,anded PTFE should in~ease the flame retar~an~ of the mixture since by the addition of PTFE to the matrix, the number of fluorine compounds presenl are i"ueased thus lende,i"g the mixture less fla,~""able.
25 Fluorine compounds help reduce the degree of flammability of mal~,ials.
Thus, the final product is more flame r~tar~ant.
Sixth, the voids and pores of the expanded PTFE should improve the illteilGcki"g of pigments with the eA~,andad PTFE. This is a sig"ifieanl improvement over conventi~aal PTFE sphert,s, where only the minimal surface 30 aJl,esion of the pigments to the sphe,es can be pro~ided. This allows the eA~.anded PTFE particles to seNe as a skeletal sponge for the paint pigments, improving the wetting and suspension prupe,~;es of the eA~,anded PTFE
particles while producing an improved final product.
Seventh, the voids and spheres of the eApanded PTFE offer a region 35 for other materials to occupy and be placed into the paint vfh~ lg added enhance" ,en~s to the paint. Active agents that can be in~i ~,Gra~ed into the expanded PTFE pa.licles include: anti-fungal agents (e.g., magnesium W O gGnl700 1~ S~3261 -1~
borate), anti-fouling agents (e.g., metallic naphU.-.nate, mercury compounds), i-.~e ti~id~l agents (e.g., dica"U.on, dieldrin), radar rt~ne~.ti..g medium (e.g., aluminum flakes), radar absorbing medium (e.g., soft ferrites), etc.
It should be evident from the above description that the p.esent S invention has numerous properties particularly suited for use on a variety of land, water, air, and space vehicles. In fact, the lightwei~ht nature of the p,esent invention should be especially useful when used in all forrns of aircraft applicaffons (e.g., conventiondl planes and hel :~pters, space vehides, etc.).
In addition to employment on vehides, it should be und6.:.tood that many of 10 the pn~p."~ies of the pr~sent invention may also be used in other areas, including: certain machinery coatin~, such as heavy construction equipment which is subjected to harsh en~l;,ufi,..ent~; building, and especially roof p~te tion and roadway paint. Additionally since ePTFE is both hyd~phob.e and o'eoFheb:~, and since the ePTFE particles Send to migrate to the surface 15 of the coating, a coating of the present im/entiûn is particularly suibble as an improved wabr sealant or chemically ,~:sis~nt finish.
.'JlDreo~ar, as has been nobd, the e~and.,d PTFE p .ticles may serve as a matrix or carrier for other materials. In this ".anner, various chemical compounds may be incGI,uGrdted or ena~ps.llated in the comminuted 20 ePTFE material prior to mixing into the paint. One advantage of this ~ncept is that materials that are resistant to mixing uniformly into a paint alone can be suc~ssfully and unifommly in~l~GIdted throughout the paint through di~pe,aion via the ePTFE particles.
The paint of the present im~enl;on has virtually endless possible 25 arF' ~tions. Suitable paints mixtures (e.g., oil based paints) of the pr~senl invention could be used to coat machinery, cement, asphalt, bridges, buildings, tanks and other surfaces .,~l~osed to harsh env;.~r".,ental conditions. Fu.llle----Gr~, the comminuted ex~,andad PTFE may mix even more readily with oil based paints, becoming susp~nd~d through mere 30 agitation. This may make paint production easier and less e)t,uensivc.
Applications for the latex based paints, either in a standard form or which havebeen modified as to be suitable for cl~ct~ùal~tic or ele_~ ophw~:tic applications, may be used to coat a wide variety of products, including buildings, autu...at.'es, and other vehicles, and certain aquatic appa-d~us, etc. The 35 matenal of the pl~Sel It invention can also reduce the chance for galvanic corrosion by minimizing the penetration through the paint of CCill05i'~ liquids,like salt water, thus stopping the electro-chemical process from ever wos6n~700 ~ JS9~0326 beginning.
vVhile pa~ '~ embodiments of the present invention have been illustrabd and des~.,il,ed herein, the present invention should not be limited to such illuall~tions and des~i,~tions. It should be appare"t that changes and 5 modifications may be i, IcouJordbd and embodied as part of the present invention within the scope of the f~IILJ~j~I9 claims.

Claims (19)

The invention claimed is:

1. A process for applying a lightweight coating to a substrate which comprises:
providing a base paint;
providing fine ground particles of expanded polytetrafluoroethylene (PTFE), said expanded PTFE being obtainable by heating and rapidly expanding PTFE in one or more directions thus forming a network of polymeric nodes interconnected by fibrils, having an average particle size of between about 5 and 100 micrometers and with a density no greater than 1.8 g/cc;
mixing the particles of expanded PTFE with the base paint to produce a mixture comprised of a base paint and a suspension of expanded PTFE particles;
applying the mixture to the substrate to provide a hardened coating thereon.

2. The process of claim 1 which further comprises comminuting the expanded PTFE particles to an average particle size of less than about 40 micrometers.

3. The process of claim 1 which further comprises mixing the base paint and the expanded PTFE particles in a ratio of 0.125 to 25% by weight expanded PTFE particles.

4. The process of claim 3 which further comprises mixing the base paint and the expanded PTFE particles in a ratio of less than 10% by weight expanded PTFE particles.

5. The process of claim 1 which further comprises mixing the paint and expanded PTFE particles through agitation.

6. The process of claim 5 which further comprises wetting out the expanded PTFE with a solvent prior to applying the agitation.

7. The process of claim 1 which further comprises providing expanded PTFE particles with a matrix tensile strength of ~ 7030 Kg/sq cm (100,000 psi) and an average particle size of about 5 to 100 micrometers.

8. The process of claim 1 which further comprises forming a coating on the substrate with the expanded PTFE comprising an interlocking grid network therein.

9. A hardening coating material which comprises:
a base paint; and comminuted particles of expanded polytetrafluoroethylene (PTFE), with a density of ~ 1.8 g/cc and an average particle size of about 5 to 100 micrometers, suspended within the base paint;
wherein the expanded PTFE particles comprise 0.125 to 25% by weight of the coating material.

10. The coating material of claim 9 wherein the average particle size of the expanded PTFE is ~ 40 micrometers.

11. The coating material of claim 9 wherein the expanded PTFE
particles comprises ~ 10% by weight of the coating material.

12. The coating material of claim 9 wherein the base paint includes an organic solvent in which the expanded PTFE particles are wetted out.

13. The coating material of claim 9 wherein the base paint includes an aqueous solvent and the expanded PTFE particles are wetted out therein through use of a wetting agent to place the particles into suspension.

14. The coating material of claim 13 which further includes a wetting agent selected from the group consisting of hydrophilic surface coatings and surfactants.

15. The coating material of claim 9 wherein the expanded PTFE within the coating material forms a protective interlocking grid network within the hardened coating.

16. The coating material of claim 9 which further comprises an active agent incorporated within the expanded PTFE particles.

17. The coating material of claim 9 comprises the incorporation of particles greater than 5 micrometers to roughen the texture of the paint.

18. A method for providing a lightweight, protective coating to an aircraft which comprises:
providing a base paint comprising pigment and solvent;
providing comminuted particles of expanded polytetrafluoroethylene (PTFE), said expanded PTFE being obtainable by heating and rapidly expanding PTFE in one or more directions thus forming a network of polymeric nodes interconnected by fibrils, the particles of expanded PTFE having a density of ~ 1.8 g/cc and an average particle size of 5 to 100 micrometers;
mixing the base paint and particles of expanded PTFE to form a mixture with the particles suspended within the base paint, the particles comprising 125 to 25% by weight of the mixture;
applying the mixture to the aircraft to form the protective coating, with the expanded PTFE forming an interlocking grid network therein.

19. The method of claim 18 which further comprises providing particles of expanded PTFE with an average size of ~ 40 micrometers and mixing the particles with the base paint to form a mixture comprising ~ 10% by weight expanded PTFE
particles.