CA1341016C - Polymer-metal bonded composite and method of producing same - Google Patents

Abstract

A coating composition including a polyether resin and comprising a major amount of resin and a minor amount of a property-improving additive, said resin being:
(A) a fluorocarbon resin selected from the group consisting of (1) perfluoroalkoxy tetrafluoroethylene copolymer resin (PFA), (2) ethylenechlorotrifluoro-ethylene copolymer resin (E-CTFE), (3) ethylenetetra-fluoroethylene copolymer resin (E-TFE), (4) poly-(vinylidine fluoride) resin (PVDF), (5) tetrafluoro-ethylene-hexafluoropropylene copolymer resin (FEP), (6) poly(chlorotrifluoroethylene) resin (CTFE), or a mixture of two or more of said fluorocarbon resins;
and/or (B) a polyether resin selected from the group consisting of (7) polyethersulfone resin (PES), (8) polyether ketone resin (PEK) and (9) polyether ether ketone resin (PEEK) or a mixture or two or more of said polyether resins;
said additive being:
(C) a poly(phenylene sulfide) (PPS); or (D) an inorganic crystalline ceramic powder and/or fluorocarbon resin when said resin is a polyether of (B) above; or (E) an inorganic material selected from the group consisting of a nitride, an oxide, a diboride, and a carbide of silicon, of zirconium, of tungsten or of boron, and/or a polyether when said resin is a fluoro-carbon of (A) above;
Coatings and articles including a fused form of said composition and a method for forming said fused composition.

Description

POLYMER-METAL BONDED COMPOSITE
AND METHOD OF PRODUCING SAME
FIELLI OF THE INVENTION
This invention relates generally to the field of bonding polymeric materials to metal materials and particularly to bonding fluorinated polymers and polyether resin: to metals, including ferrous-based metals.
REPORTED DEVELOPMENTS
In the chemical processing industry, as well as many other industries, a variety of composite materials are used to fabricate apparatus used therein. In many instances, metals are used to provide the structural strength for such apparatus.
However, such apparatus, many times, is required to be exposed to highly corrosive materials which are being processed; some of th.i.s exposure is at elevated temperatures and/or elevated pressures which tend to exaggerate the corrosive properties of the materials being processed. It has been found necessary, in many applications, to protect the metals used in such apparatus from the effects of corrosion, under varying conditions of temperature and pressure, particularly at elevated temperatures, and at increased pressures.
The approach, generally, to this problem has been to shield the si~ructural metals from corrosive materi als. This is done by forming a composite by super imposing other materials onto those surfaces of the structural meta:Ls which would otherwise be exposed to corrosive attack, the concept being to only permit contact of corrosive materials to barrier materials which will resi:~t the effects of such corrosion which are formed on the surfaces of underlying or substrate structural meta:Ls. Of course, such overlayed or barrier materia:Ls are selected to have relatively little, or idea:Lly no, reaction to those materials which otherwise corrosively attack the substrate structural metals.
One concept of protecting structural metals from corrosive attack is to bond a glass coating to those surfaces of the substrate structural metal which are to be exposed to materials which would corrode those structural metals if contact were permitted. This concept has been used for many years and is quite satisfactory where substantially no flexing of the structural metal is possible. If flexing is a possibility, glass-metal composites may encounter problems as the glass overlay, or barrier coating, of the composite may tend to crack, thus providing an avenue for corrosive material to reach the base sub-strate structural material. Also, glass coated metal materials tend to be highly susceptible to mechanical damage. Finally, glass, being an amorphous material, is not resistant to corrosive attack by various common chemicals.

1341p16 Another concept has been to overlay, or form a barrier coating, of relatively thin sheets of non-corrosive metals, such as titanium, tantalum, hafnium, etc. onto structural_ base metals, for example, mild 0.25% carbon low carbon steel. This concept requires the bonding of very a}:pensive overlay, or barrier coat, metals onto othEar_ dissimilar structural base metals. Not only are the overlay, or barrier coat, metals expensive:, but the process of bonding, usually requiring extensive and complicated welding techni-ques, is very e~c:pensive. Nevertheless, this concept is commercially used where the apparatus is to be exposed to a combinat_Lon of extreme corrosion and extremely elevated temperatures and where extreme temperature differentials, which occur rapidly, are to be encountered.
A third concept has been to apply a polymer to the surface of t:he structural metal, the polymer being bonded to the surface of the structural metal. This concept has had some success where the corrosive effects of the c:orros:ive materials are relatively mild and where the e7_evated temperatures to be experienced are modest, being below the heat degradation points of the polymers usod. Also, this concept has been used where anti-stic~:ing properties are important, such as in roll coverings used in dryer rollers, carrier rollers, etc.

In an attf~mpt to overcome some of the limitations of most polymers, in rE=spect to corrosive resistance and:to limitations on rangE= of elevated temperature use, fluorinated polymers have been used as overlays, or barrier coats, on base structural material;. As is well known, fluorinated polymers exhibit relatively high corrosion-resistance in comparison to other polymers. Al;~o, fluorinated polymers have a relatively high operating temperature point of degradation, in comparison to other polymers. Finally, fluorinated polymers, as well as other polymers are :relatively much more flexible in comparison to glass, and are e:~sentially inert to most common chemicals up to the melting poinl~ of such fluorinated polymers.
In developing polymeric barrier coatings as applied onto metals, a true composite is formed only where the materials are bonded together with high integrity bonds. This is to say that the ==esin used, which is in contact with the metal substrate, should be as firmly bonded to that metal as possible.
It is also necessary that microvoids (porosity) of the coating be essentially eliminated if the polymer barrier coating of the composite is to be utilized to prevent corrosion of the underlying substrate metal.
All polymeric barrier coatings, to one extent or another, are subjeci~ to molecular permeation by gaseous chemicals. Permeani~ flow is accelerated by elevated temperatures and by increased pressure. This phenomenon exists because it is virtu<~lly impossible 'to remove all of the voids (porosity) in the coating. However, the fewer voids, the less the permeation. The permeation, of course, is not detrimental to the resin itself because, hopefully, the polymer selected for the barrier coai~ is chosen because of its inertness in respect to corrosive: attack by a particular chemical or set of r chemicals. However, the barrier coat is necessary, in the first place, becaus~s the chemical or chemicals in question do corrosively attack she underlying substrate metal, while the barrier coat should impede this process.
If the bonding of the resin to the substrate metal is not complete, that is, if a substantial percentage of the resin is not completely banded to the adjacent metal surface, the metal at those poini~s is open to attack by the permeants.
Generally, the subsl~rate metal, particularly in the form of vessel walls, is non-isothermal with respect to the thermal condition of the co=rrosive medium, which, in the case of vessels, comprises i~he contents thereof. Thus, the metal surface acts as a hf=at sink, and the permeants tend to condense and collect on thos<~ portions of the colder metal surface to which the resin is not bonded. When this occurs, the corrosive substance causes note only deterioration of the exposed metal, but also deterioration of the adjacent metal underneath the metal-resin particle bonds that do exist. The result may be that the existing bonds a:re destroyed and delamination occurs.
This phenomenon can show -upon the surface of the barrier coat as blisters. Such blisters may be caused by gas and/or liquid build up, beneath the barrier coat, where the barrier coat has begun to become del<~minat~~d. These blisters indicate that the metal substrate underneath the blisters is suffering corrosive attack. Of course, the corrosive attack to the substrate metal frequency create di;~coloration of that metal as salts and oxides are formed. This discoloration, or blanching, as well as blistering, are ~risual:ly detectable on the barrier coat surface as the underlying corrosion becomes pervasive.
On the other hand, the more resin bonded to the metal, the less metal substrate surface area there is exposed to the permeants. ~Cherefore, the less condensation there is which occurs on the metal. Also, the fewer voids there are X

_ X341016 within the barrier coating, the less opportunity for the permeants, gaseous ~~r otherwise, to get through the barrier coating to the substrate metal.
In additi~~n to increasing the integrity of bonding of the resin to the substrate metal, thus decreasing voids and decreasing metal surface available to attack, another approach is considered desir:~ble. This is basically to increase the thickness of the barrier coat itself, the theory being that this will hinder permeation because, simply, the permeants have a greater distance i~o travel, and the possibility of tortuous pathways, through interconnected voids, being blocked by resin is increased because there is more resin between the barrier coat surface and thc~ underlying metal substrate surface.
Fluoropol~,rmers are well known for their inert characteristics in :=espect to a wide variety of different chemicals. In addit=ion, fluropolymers are well known for their high temperature capabilities relative to other polymers.
Therefore, fluoropo=Lymers are primary candidate materials for chemical barrier co<~tings.
On the other hand, fluoropolymers characteristically are very long chain,, high molecular weight, high melt viscosity polymers with a narrow temperature .range, relative to most other polymers, between melt and degradation. Fluoropolymers are also very poor conductors of heat, complicating the approach to developung heat input to induce melting, thus producing void free barrier coatings. The combination of these factors makes proce:~sing of fluoropolymers difficult, if not impractical, under rnany circumstances. Therefore, although fluoropolymers may be primary candidate materials for chemical barrier coatings, they are=_ difficult to process and apply, which in many circurnstances substantially diminishes this candidacy.
x As implied above, it is known in the art that the chemical permeability of barrier coating varies inversely with the thickness of that barrier coating. However, it is quite difficult to form relatively thick coatings of fluoropolymers because of their inherent high viscosities which result in low melt flow and slow fusion characteristics. To bond the particles of fluoro_oolymer resin to each other and to underlying substrate materials, the particles must be brought to above melt temperature but kept below the practical degradation temperature. Inability to control this process may result in entrapment of air between particles, ultimately resulting in the formation of bubbles in the barrier coating.
It is very difficult to control this process when such coatings are applied to relatively large or complex metal shapes, as it is difficult to convrol the temperature of each discrete point of such pieces within a narrow range such that each discrete point is above the melt point of the fluorinated polymer being applied but not above the practical degradation point. Also, it must be ensured ~~hat the surrounding atmosphere, adjacent to the exposed face of the barrier coat being applied is, likewise, within thf~ narrow practical range of temperature.
And finally, all particles of the fluorinated polymer across the thickness of the=_ coating must, likewise, be within that narrow practical temperature range, notwithstanding the fact that fluorinated po:Lymers are notorious for poor heat transfer.
Attempts have been made to build up series of thin coats of fluorinated polymers, as barrier coatings, overlaying one on another, and using a heating cycle in between each thin layer to bond it to the previous layer. In commercial applications, as arc=_ well known, the powdered polymer resin particles are suspended in a carrier fluid, usually water, and sprayed in a thin layer, onto the substrate metal, followed by a heating step. This is followed by a repeat of the cycle, 134~0~
.. s _8_ many times, each time laying down a 0.001" to 0.010" thick layer. This method has encountered difficulties as small quantities of the c,~rrier fluid tend to remain trapped within the lattice formed :by the powdered resin particles. On heating, the carrier fluid vaporizes and expands, which can separate the layers from one another and may prevent bonding.
This can appear as .surface bubbles. However, even when each sprayed layer is carefully dried, acceptable bonding may not occur between the t:zin layers of the barrier coating; the reason for this is :zot clear.
Also, there are major problems in developing and maintaining a uniform suspension of fluorinated polymer resin particles in the carrier fluids. A variety of additives in the form of surfactants, antifoaming agents and other "wetting aids" and "processing aids" are used in an attempt to overcome 134101 fi ' _g_ these problems. It is bE~:lieved that these additives hinder the bonding of successfu:l:ly built up thin layers of the polymers to each other, Even in situations where no bubbling occurs between such layers.
More recE~ntly, attempts have been made to apply dry powdered fluorinatErd polymer resins to metal substrates using elect rostat is deposit io:n, f loccing and f luidized bed techniques. Although coatings in Excess of 0.012" thickness have been accomplished, attempts to form coatings up to 0.040"
thickness have failed due to the formation of bubbles and voids during the heating stage, regardless of whether a single thick layer is applied or multiple thin layers, interspersed with heating steps, are attempted. The cause of such failures is not clear.
The following publications disclose coatings comprising fluorocarbon polymers: U.S. Patent Nos. 4,064,963 and 4,605,695; U.K. Patent No. 2,051,091, and EPO Publication No. 10152.
The major problem in using fluorinated polymers as an overlay, or barrier coat, in composites with substrate metals, is that it is difficult to produce high integrity bonding of the fluorinated polymers to the base structural metals. One composite of fluorinated polymer, overlayed as a barrier coating onto steels, has been successfully marketed, under the trade name Fluoroshield*, by W.L. Gore and Associates, Inc. This composite is believed to be detailed in British Patent No. 2,05:1,091. Fluoroshield coated metals, * Trade-mark -9a-however, as will be' later specifically detailed, do not appear to exhibit the lone-term bonding integrity or chemical resistance which i~~ deemed necessary, by those with skill in the field, to ensure the extended higher temperature corrosion-resistance necessary for reliable use in chemical processing equipment.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there it; provided a substrate coated with a non-porous corrosion-resistant reslnou:> coating formed by fusing a coating composi_tior~ including a polyether resin and comprising a mayor amount (more than about 50 wt.~) of resin and a minor amount ( less than C~bout 50 wt . ~ ) of a property-improving additive, said resin being:
( A ) a f luoroca.rbon res in se leca ed f rom t he group consist ing of ( 1 ) ~~erf luoroa:lkoxy t: et raf luoroethylene copolymer resin (PfA), (a?) ethylene-chlorotrifluoro-ethylene copolymer resin (E-CTFE), (3) ethyJ.ene-tetra-fluoroethylene copolymer resin (E-TFE), (4) poly(vinylidine fluoride) resin (PVDF), (5) tetrafluoroet.hylene-hexafluoropropylene copolymer resin (FEP), (6) poly(chlorotrifluoroethylene) resin (CTFE), or a mixture of two or more of said fluorocarbon resins; and also containing, a;a an addit ive, a ceramic powder, the ceramic powder being a tne~t:al carbide, silicon nitride, boron nitride, titanium dibor de or aluminum diboride powder.
Another aspect: of i:he present invent ion is the provision of a coating composition which is capable of being fused, that is, melted at elevated temperature and then cooled to form the aforementioned corrosion-resistant resinous coating. The present invention encompasses within its scope coat ing composit ions in ~,rhich the resin const ituent is present in a mayor amount :.in the form of a mixture of resins (A) and/or (B) above combined with a mj.nor amount of the property-improving additive. In addition, t;he composition includes within its scope tr.e use of a mixture of additives, for example, a mixture of two or more of the additives of (C), (D) and (E) above, and, withj.n the group of additives of (E) above, a mixture of two or more of such additives.
Accordinl~~ to a further a~>pect of the invention there is provided a method of forming a <:oating on a metal surface, which method comprises :fusing a pol.yether-containing resin to said metal surface to form a base <:oat; fusing a top coat to said base-coated mEtal surface, saj.d top coat containing (A) a fluorocarbon resin selected from the group consisting of (1) perfluoroalkoxy tetrafluoroethylene copolymer resin (PFA), (2) ethylene-chlorot rif luoro--ethylene copolymer resin ( E-CTFE ) , (3) ethylene-tetra-fluoroE~thylene copolymer resin (E-TFE), (4) poly- ( vinyl idine f I uoridE~ j yes in ( F'VDF ) , ( 5 ) tet raf luoro-ethylene-hexafluorupropyl.E~ne copolymer resin (FEP), (6) poly(chlorotrifluoroethylene) resin (CTFE), or a mixture of two or more of said fluorocarbon resins; and, as an additive, (B) a ceramic powdery, t:he ceramic powder being a metal carbide, silicon nitride, boron nitride, titanium diboride or aluminum diboride ~~owder,.
Speaking generally, the property-improving additive can be selected to improve various properties of coatings formed from the co~T~positj_on of the present invention, for example, properties; such as corrosion-resistance, abrasion-resistance, and/or bonding characteristics.

~341p16 - 11 a --The preferred property-improving additive is a metal carbide, most prefE~rably silicon carbide or zirconium carbide or a mixture of such carbides, and,. for use with fluorocarbon resins, also PEEK.
It is ex~~ected that the _Lnvention will be used most widely in connection with forming coatings on metallis surfaces, part iculo.rly anon substrates . However, non-metallic surfaces can be co~~ted ,also with a composition of the present invention.
The present invention encompasses also a method for forming a coating from the composition of the present invention, including process means for the application of such a composition to an underlying substrate.
DETAILED DESCRIPTION OF THE INVENTION
The present invention includes a composite of a polymer barrier coaxing which is highly integrally bonded to a substrate metal sur:Eace, the coating exhibiting substantially fewer voids and improved resistance to corrosion and abrasion.
Thus, given a certain thickness of the barrier coating of the present invention, in comparison with an equal thickness of barrier coatings found in the prior art, the present invention is responsible for affecting substantially decreased corrosive attack to the under:Lying metal substrate at elevated temperatures and/or pressures over a prolonged period of time.
The present invention also provides a method of producing such a composite as well as a formulation, the use of which will produce the barrier coatings, sheets and shaped articles of the present invention.
As will bc~ described in greater detail below, the additive-containing polymer compositions of the present invention may be di~,rided for convenience into three groups based upon the uses to which they are put: primer coatings, barrier coatings, and abr,asion/wear resistant functional coatings.
Primer coatings bond very strongly to the underlying metal substrate and themselves provide a substrate to which coatings having other properties may be strongly bonded. In applications where umprov~ed bonding of a protective resin overlay is sought, t=he coating composition may be applied as a "primer coating" to the underlying metal substrate.
Barrier coatings provide a barrier between the substrate to which t=hey are bonded and a corrosive environment.
In applications where improved corrosion-resistance of a x protective resin overlay is sought, the coating composition may be applied directly to the metal substrate or over a previously applied resin coat, for example, over a primer undercoat.
In certain instances, there may be an overlap in the use to which a given composition may be put, that is, certain of the compositions bond strongly to a metal substrate and at the same time provide a superior barrier to chemical attack.
In addition, as a gf=_neral rule, polymer compositions of the present invention which bond most strongly to the substrate are useful also as abra;~ion-resistam coatings applied directly to the substrate or over other polymer coats.
According:Ly, i.n applications where improved abrasion-resistance of the coating is sought, the coating composition may be applied as an outermost layer over previously applied resin layers or in direct contact with the metal substrate.
Compositions useful particularly for forming primer coatings include (l~ a fluorocarbon resin of (A) above in admixture with an aciditiv~e of (C) and/or (E) above and (2) a polyether resin of (B) above in admixture with fluorocarbon resin, preferably a resin of (A) above, and/or a ceramic powder of (D) above.
Compositions useful parti~~ularly for forming a corrosion-resistant barrier coating include a fluorocarbon resin of (A) above =Ln admixture with an additive of (E) above.
Compositions useful particularly for forming abrasion- and wear-resistant surfaces or top coatings include a fluorocarbon and/or polyether resin of (A) and (B) above respectively in adm=~xture with a non-resin additive as identified in (D) and (E) above.

The fluor~~carbon resins and polyether resins of (A) and of (B) above ar~= known classes of resins. Species of such resins are available commercially.
The fluorocarbon and polyether resins of (A) and (B) above are used in the practice of the present invention in fine particle size form. It is recommended that the particle size of the resins be about 1 to about 200 microns, preferably about 20 to about 120 microns.
As mentioned above, PFA is a preferred fluorocarbon resin for use in thc: practice of the present invention.
Examples of commercially available PFA resins are TEFLON*-P
532-5012 PFA powder resin which is manufactured by E. I. DuPont de Nemours & Company, Inc. of Wilmington, Delaware and which is described in DuPont Fact Sheet TI-:14-84, and Neoflon* AC-5539 and Neoflon AC-5500 PFA powder resin, both manufactured by Daikin Industries, htd. of Osaka, Japan.
DuPont recommends said 532-5012 resin for use as an intermediate resin t=o be overlayed onto other primer systems, followed by the application of other top coat resins overlayed onto a coating formed from the 532-5012 resin. Daikin, likewise, recommend: said AC-5500 o:r AC-5539 resins for use as a top coat resin to be ov~~rlayed onto an intermediate resin which, in turn, is overlayed onto a primer resin. (DuPont's 850-300 Teflon primer system, believed to consist of a chromium oxide-containing po=Lytetrafluroethy:lene, is recommended by both Daikin and DuPont a:~ a primer resin.) However, the aforementioned 532-5012 resin and the AC-5500 and 5539 resins have been found to be *Trade-mark i Tr 1341 01 ~

a quite acceptable resins for forming primer coatings when used in accordance with t:he present invention.
Examples of other commercially available fluoro-carbon resins for vse in the practice of the present invention include AUSIMONT'S HALARF~ 6014 E-CTFE copolymer resin;
DuPont's TEFZELR 5~~2-600() E-TFE copolymer resin; and Kreha Corporation of America's KF polymer. poly(vinylidine fluoride) resin (PVDF).
With respect to polyether resins, they have outstanding mechanical ;properties (flexural and tensile, resistance to abra~,ion, wear and creep under load) and have the best radiat ion res ist:ant propert ies of any plast is material. PolyethE~retherketone (PEEK) is a particularly useful polyether, 1-iaving a lower oxygen permeability and water vapor transmission level than the most resistant fluoropolymers. Pc~lyethE~rketone (PEK) and polyethersulfone ( PES) are also part icularly useful. .
Polyether~ resin:3 also have excellent adhesion to metals as well as good compatibility with fluorocarbon resins making polyether resins Excellent primers for top coatings formed from fluorocarbon resins.
Turning now to a description of the property-improving additive of thE~ present invention, they are also known materials . F~referably, the part isle size of the additive does not exceed the particle size of the resin (A) or B ) const ituent . I:t appE~ars that the addit ive is present in the fused coat ing in discrete part icle form. In the case of an organic addit iv~~, the shape of polymeric part icles are ~34~~~6 -15a-changed by the heat of 'the fusing process.
With res~~ect to additive (C) above, that is poly(phenylene sulfide), examples of commercially available PPS resins which cam be used are Ryton* type * Trade-mark ~341~1 V-1, P-4 or P-6 as manufactured by the Phillips Chemical Company or Bartlesv:ille, Oklahoma. The Ryton type V-1 PPS
resin is most preferred. The PPS should be used in fine particle size form. It is recommended that the particle size thereof be about 1 1~o about 200 microns, preferably about 10 to about 100 microns.
With respect to the ceramic powder of additive (D) above, this include; fine particle size, inorganic crystalline materials. A cerami~~ powder is characterized typically by its ability to be converted by sintering into a chemically inert material. Examples of ceramic powders that can be used as additive (D) above <~re: refractory carbides such as silicon carbide, tungsten carbide, molybdenum disilicide and boron nitride; metal oxidE~s such as alumina, chromic oxide, powdered quartz, cerium oxidf~, silicon oxide, beryllia and zirconium oxide; silicon nitride, titanium diboride and aluminium diboride.
The ceramic powder can be in various forms, for example, in the form of regularly or irregularly shaped crystals, whisker fibers, long fibers, and platelets.
Metal carbide powders are a preferred additive for use in the present .invention. The preferred carbides include silicon carbide, zirconium carbide, tungsten carbide and boron carbide, silicon carbide being most preferred.
A consideration in selecting the type of ceramic powder to be used i;s its resistance to the corrosive effects of the chemical materi,~l with which the resin composite material is to be used. It is believed that alpha silicon carbide is the most corrosive :resistant type of ceramic powder available in respect to corrosive attack by a very broad range of chemical materials. Thus, it is highly preferred. In addition, silicon carbide is a low-cost material. However, for x 1341p16 a variety of reason:, such as cost factors, etc., another type of ceramic powder m<~y be selected.
Examples of commercially available silicon carbide powders are 39 Crysi~olon green silicon carbide flour as marketed by the Nori~on Company of Worcester, Massachusetts and Arendahl SIKA SiC powder, marketed through Standard Oil Electrominerals Co. of Niagara Falls, New York. These are recommended for use in the practice of the present invention.
The ceram:ic powder to be used is preferably no larger than the particle size of the resins with which it is to be mixed and should preferably be within the size range of about 1 micron to about 40 microns, most preferably up to about 5 microns in size.
With respect to the fluorocarbon resin of additive (D) above, such resin additive can be any one or more of the fluorocarbon resins of (A) above. Other fluorocarbon resins can be used also as the property-improving additive for use with a polyether re:~in of (B) above. Such other resins are those which include polymers that do not have hydrogen atoms and in which there are at least three fluorine atoms for each other halogen atom (for example chlorine) that may be in the polymer. Polytetra:Eluoroethylene (PTFE) is an example of such a fluorocarbon resin.
The particle size of a fluorocarbon additive for use in admixture with a polyether resin of (B) above should preferably be about 1 to about 200 microns, preferably about 20 to about 120 micron;.
With respc=ct to the non-resinous, property-improving additives of (E) abcwe, such materials are selected ceramic powders within the class of ceramic powders of (D) above. The particle sizes of s,.~ch selected ceramic powders can be liked __ X341016 those of the genera:Lly described ceramic powders mentioned above. Similarly, and as also described above, the preferred additive of (E) abo~,re is a metal carbide most preferably silicon carbide.
With respE~ct t.o the polyether additive of (E) above, the particle size thereof can be like those of the generally described polyether resins mentioned above. The preferred polyether resin additive of (E) above for use is admixture with the fluorocarbon re:~in of (A) above is PEEK.
There fol:Low hereafter general descriptions of the effects that the property-improving additives have on coatings formed from composii~ions of the present invention and also general observation: respecting the characteristics of coatings of the present invention.
In genera:L, it :has been observed, most notably in the use of ceramic powders, particularly with fluorocarbon resins, that bond strength between the coating and an underlying metal substrate increases with increased quantities of ceramic powder in the composition. On t:he other hand, resistance to corrosion by chemical attack .LS observed to be highest where relatively small amounts of ceramic :powder are added to the resin, corrosion resistance being observed to decrease as amounts of ceramic powder in the resin are further increased.
As mentioned above, polyether resins have excellent adhesion to metals as well as good compatibility with fluorocarbons, making polyethers excellent primers for fluorocarbon resin-based top coatings. Where the specific need for the special properties of a polyether resin-based coating is required in a mei~al protecting coating, polyether resins in admixture with eithf=_r ceramic powders or fluorocarbon polymers or both may be used to advantage. Silicon carbide is a preferred ceramic powder and PFA is a preferred fluoropolymer.

Wear and .Load-bearing properties of polyether resin-based coatings are _Lmprov~~d by the addition of ceramic powders, particularly silicon carbide.
Substitut=Lon of PES for PEEK or PEK is useful where some temperature and chemical resistance can be sacrificed.
The major advantage of PES is its exceptionally low cost as compared to both PEK and :PEEK.
Applications of composite mixtures of SiC-containing polyether resins as coatings to metal roll surfaces which are subject to high abrasion <~nd wear as well as high nip roll loading at elevated temperature pro~;ride exceptional life performance in resistance to damage in applications such as papermaking, calendering and extrusion lamination, for example, of plastics employed in packaging and similar industries.
Where release characteristics are desired, a fluoropolymer, for example=_ PFA, may be added to the formulation to impart release properties. Other applications for release, corrosion-barrier, wear- and load-resistant coatings will be evident to those with expc=_rience in end use application materials.
In genera7_, the smaller the particle size of the resin constituent, t:he bel~ter the properties of the coatings.
The major constituent of the composition of the present invention i~~ a fluorocarbon resin and/or a polyether resin of (A) and (B) abovf=_, the property-improving additive being present in a minor amount. Although the additive can be used in an amount approaclZing 50 wt. % of the composition, it is preferred that the amount of additive comprise a lesser amount. The property-imp=roving additive can be used in a bond-improving amount, preferably about :L to about 40 wt. %. Such amounts improve also the abrasion-resistance of the coating.

-2°- ~34~0~6 Additives providing an improved resin barrier coating which inhibits corrosion of an underlying metal substrate should be used in amounts of about 1 to about 25 wt. %, preferably about 1 to about 20 wt. %; and most preferably about 2 to about 5 wt.
o.
There fol=Low descriptions of preferred embodiments within the scope of the present invention.
In a preferred embodiment of the present invention, a perfluoroalkoxy (PF~3) resin is used to form a primer coating.
The primer coating, that which is directly in contact with the underlying substrate, most typically a metal surface, is a PFA
resin, predominantl~r in a powder sire range of about 1 micron to about 200 micron:, preferably predominantly in a range of about 20 microns to about 120 microns, preferably modified with the addition of a polyeth~=r.
A preferrE=d PFA-based primer coating may be formed from polyether resin, preferably in an amount of about 2 to about 40 wt. %, most: preferably about 5 to about 20 wt. %. A
very acceptable prirner coating can be prepared by mixing 15 wt.
% PEEK type 150 PF (Batch No. SPG9-:191p), as manufactured by ICI America, Inc., with 85 wt. % of Neoflon AC-5500 PFA resin.
An alternative :PFA-based primer-coating may be formed from PPS which is preferably present in an amount of about 2 to about 20 wt. %, most. preferably about 5 to about 10 wt. %.
A very acceptable primer coating can be prepared from a composition comprusing '7 wt. % of Ryton type V-1 PPS resin and 93 wt. % of Neoi=lon AC-5500 PFA resin.
In another embodiment of the present invention, ethylene-chlorotrif=Luoroethylene (E-CTFE) copolymer resins, ethylenetetrafluoroethyle:ne (E-TFE) copolymer resins, or .~ 134116 poly(vinylidine fluoride) (PVDF) resins are used to form primer coatings, modified with the addition of a polyether and also a selected ceramic powder, preferably a metal carbide, and most preferably silicon carbide or zirconium carbide, in an amount less than 50 wt. %, preferably in an amount of about 1 to about 25 wt. %, most preferably about 2 to about 20 wt. %.
In anothe:= embodiment of the present invention, a composite coating i:~ formed from the aforementioned primer compositions and an overlay or barrier top coating is formed from a composition comprising PFA and ceramic powder, using as the PFA TEFLON-P 53:?-5010 PFA powder resin which is marketed by DuPont (Fact Sheet SCI-13-84.) The ceramic powder is used in an amount preferably w_Lthin the range of about 0.5 to about 5 wt.
%, most preferably about 1 to about 3 wt. %.
Where the fluorocarbon resins are used to form a corrosion-resistant barrier coating, they may be modified to advantage with a se=Lected ceramic powder in an amount of about 0.5 to about 5 wt. '-'s, preferably about 1 to about 3 wt. %, and most preferably about 2.5 wt. %.
In applications subject to abrasion and wear, an outer top coating oj= any of PFA, E-CTFE, E-TFE and PVDF in admixture with less than 50 wt. % o:f ceramic powder, preferably silicon carbide or s;ircon.ium carbide, may be used to advantage.
A very acc:eptab:le polyether-containing primer coating of E-CTFE can be prepared from about 10 wt. % 39 CRYSTOLON
green silicon carbide flo,ar (up to 5~ in particle size) and AUSIMONT'S HALARR 6014 E-CTFE resin.
A very acceptable corrosion-resistant barrier top coating of E-CTFE can be formed from about 2.5 wt. % of 39 CRYSTOLON green sil_LCOn carbide flour (up to 5~ in particle 'x .. 1341016' size) and about 97.5 wt. % of AUSIMONT'S HALARR 6014 E-CTFE
resin.
Additiona:L polyether-containing primer coatings may be formed f rom about. 2 5 wt . % 3 9 CRYSTOLON s i l icon carbide and DuPont TEFZEL 532-6000 E-'TFE copolymer. A 5 wt. % SiC-containing coating of TEFZEL may be used to excellent advantage as a corrosion-resi:~ting :barrier coating.
A composii~ion comprising 5 wt. % 39 CRYSTOLON silicon carbide and 95 wt. '-'s Kreha Corporation of America KF polymer PVDF resin may be u:~ed to advantage in forming a barrier coating having exce=Llent corrosion-resistant properties.
In a most preferred embodiment of the invention, there is provided a composite of a build up of PFA mixed with the addition of about 1 to about 20 wt. %, preferably about 1 to about 5 wt. %, and most preferably about 2 wt. % of a selected ceramic powder dispersed within the PFA resin, as a top barrier coating,, overlayed onto and integrally bonded to a primer coating of PFA/polyether which in turn is overlayed onto and integrally bonded to a metal substrate, in particular a metal substrate.
A primer or barrier coating may be formed also from a composition comprisung PE:K, PEEK, o:r PES admixed with about 1 to less than 50 wt. %, preferably about 2 to about 25 wt. % of a ceramic powder. ~Che composition can be applied to a metal substrate in dry powder form, for example, by electrostatic means used for fluor_opolymer-based coating compositions or by other known methods such <~s, for ex<~mple, fluidized bed methods, floccing methods, etc.
The addituon of SiC in the range of about 20 to about 25 wt. % to about 80 to about 75 wt. % of either PEK or PEEK
produces a polyether resin coating composition which can be 'x formed into a coating which exhibits significant reduction in abrasion, wear and ~~reep under load relative to neat polyether resin-based coating; which do not contain SiC. The addition of chemically resistani~ SiC with its exceptional hardness enhances the already superior mechanical properties of the polyethers.
Polyether resin-based compositions containing either or both fluoropolymE=_rs and ceramic powders, for example, PFA
and silicon carbide, may be used to advantage when applied as a coating to the chem:LCal seal and drive portion of agitators employed in chemica:L vessels for mixing corrosive chemicals, as such coatings have excellent chemical-resistance and very desirable wear-, abrasion- and creep-resistance under load, particularly at ele~rated temperatures.
The same preferentially applied polyether resin-based composite is also u:~eful when applied to the tips of agitator blades subject to h_Lgh abrasion and wear, particularly when exposed to mixing liquids containing abrasives. In such applications, prefe~~red coatings include a SiC-containing polyether coating applied directly to the metal substrate or over SiC-containing fluorocarbon resin-based primer coating.
The present invention encompasses applying an undercoat of resin/additiwe composition to a substrate and integrally heat bonding it to the substrate followed by the application of successively built-up top coat layers and integrally bonding each, :respective:Ly, to both the undercoat and each preceding =_ayer of the top coat. The coating composition may be applied in a dry powder form, electrostatically, or by a wet spray system, or by other known methods such as, for example, fluidized bed methods, floccing methods, rotomoldinc~, and rotolining etc.
The present inv~=ration encompasses also a method of applying coating cornposition by wet spraying to form both the top coating and the primer coating, as well as the process of forming the aforementioned as a barrier coating and also a formulation for wet-spraying the coating composition.
PRE-APPLI(~ATION PREPARATION OF TOP COAT RESIN
In prepar:Lng the resin for application to the substrate, a prefer:=ed procedure is described below for a fluorocarbon resin <~nd ceramic dry powder mixture of PFA and silicon carbide.
1. Five micron particle size SiC powder is weighed out and sieved through <~ series of Tyler mesh screens to break up agglomerations. The powder is first sieved through a 42 mesh screen, second through a 100 mesh screen, third through a 325 mesh screen and finally through a 400 mesh screen. The sieving operation is accomp:Lished by shaking the screens either manually or preferably mechanically using an automatic sieve shaker apparatus as is well known to those with skill in the field. Two wt. o o:E the sieved silicon carbide powder is then placed in a suitabl<~ container for roller mixing.
2. Ninety-ei<~ht wt. % PFA resin is then added to the sieved silicon carb:Lde powder in the roller mixing container and that container :LS sealed. A suitable container for mixing is a polypropylene ;jar or bottle which can be obtained from a variety of differeni~ sources as is well known to those with skill in the field. It does not matter whether the resin is added to the silicon carbide powder in the jar or the silicon carbide powder is added to the resin therein. Whatever is most convenient will suf:Eice.
3. After the two components are placed in the container ~~ ,r X

-w 1341016 and the container i;~ sealed, the container should be shaken thoroughly to dispe=rse and separate the particles of silicon carbide powder to ensure that agglomeration does not occur.
4. The sealed container is then placed on a roller mill and rolled for aboui~ 0.5 hour to fully and evenly mix the two components together to produce a homogenous mixture of the resin and the silicon carbide powder. An acceptable roller mill for this purpo:~e is a Norton 735 RM Jar Mill, marketed by the Norton Company, although many competitive devices are also on the market and readily obtainable as is well known to those with skill in the f:field.
5. The mixture, now ready fo:r application, is emptied from the container into t:he fluidized bed hopper of an electrostatic spraying apparatus. An acceptable electrostatic spraying apparatus, including a fluidized bed powder container (hopper), is the Ransburg GEMA 701 unit, marketed by Ransburg Corporation of Indianapolis, Indiana, although a variety of competitive models are available from other sources as is well known to those with skill in the field. This unit is used to apply the primer re:~in, a;s well as the overlay or top coat resin mixture, the preparation of which has been described above.
PREPARATION OF 1'~ETAL SUBSTRATE FOR COATING
Before an~r of the resins can be applied, the substrate metal must. be prepared. 'Typically a mild, low carbon steel substrate metal is most commonly used; however, various other ferrous and non-ferrous metals may be used as the substrate metal. It. is preferred to employ carbon steel as a metal substrate because of its low cost, although the surfaces of other metals may be coated if prepared to accept the primer -26- 1341~~~
resins of the present invention. The preparation of the substrate metal sur:Eace is substantially the most important consideration, as distinguished from the species of metal to be used as a substrate. Specifically, the substrate metal surface must be cleaned such that it is free of oils, greases, blasting grit, water and othE~r contaminants to the degree reasonably practicable in gene=rally accepted shop conditions in the coating industry. 'L'his can be done, for example, using standard solvent cl<~aning techniques as are well known to those with skill in the a..t. After the surface of the substrate metal has been cleaned of surface contaminants, the following procedure may be used for the preparation of a mild low carbon steel substrate:
6. The steel piece is grit blasted with 3/0 (000) size silica (sand) which is both fresh (unused) and dry.
Alternatively, a reusable blasting medium such as 24 grit aluminum oxide can be used. After blasting, the blasted surface should not be touched with bare hand; it is recommended that clean gloves be worn. Care should be taken to avoid contamination of the blasted surface from water, oil, grease, dirt, etc. The bla:~ted s~~urface should also be inspected at this point to find <~ny su:rface defects in the metal. If there are any deep groove;, sharp edges, pinholes or weld defects, such should be repaured at this point and the surface re-cleaned and reblasted as above. The grit blasting roughens the metal surface of the metal and, thus, enhances the bonding of the primer resin thereto.

7. Within twelve (12) hours of the above described blasting step, preferably sooner, the metal pieces should be placed into an oven for heating. An example of an electric oven which can be u;~ed and which was used in the examples described hereinafter, is the Ramco Model RT-215 (Serial No.

813054) as manufact~.~red by Ramco Equipment Corporation of Hillside, New Jersey.

8. Optionall~~r, the pieces may be blasted a second time, this time preferabl:~ with an 80 grit size aluminum oxide/titanium oxide grit, within about 0.5 to 1.0 hour before they are placed into the oven. This second blasting is recommended in humid weather to eliminate any rust which may have formed on the ;surface since the first blasting step.

9. Whether o:r not one or two blasting steps are utilized, just before the pieces are to be placed into the oven, the pieces should be vacuum cleaned, using a suction type vacuum cleaner, to :remove any residue of blasting grit.
Following the vacuum cleaning, the pieces should be thoroughly brushed with a non-metal :bristle, non-shedding brush. The blasting steps accomplish two things: firstly, the surface is cleaned, and, secondly a surface texture is developed which is most advantageous for and facilitates the bonding of the primer resin thereto.

10. As the piE=ces are being loaded into the oven, a thermocouple should be attached to each on a surface of the metal which is not t=o be ~eoated with a barrier coating. These thermocouples should be connected to standard apparatus to enable the monitoring of the temperature of each piece within the oven.

11. The bare ;~ubsta:nce metal pieces are then "baked" in the oven. The oven temperature, being set at 760°F, the pieces must be soaked in the oven for a sufficient time to bring the temperature of the pieces up to 740°F, as the pieces will never reach the set temperature of the oven, due to convection, conductance, etc. a;~sociated with the design characteristics of ovens of such type. In addition to this temperature equalization step, preferably when the temperature of the pieces has reached '740°F, a timer should be set so that the pieces are "baked" :in an .air atmosphere for a period of at least one (1) hour but not more than eighteen (18) hours (to avoid unnecessary surface oxidation.) The purpose of this extended "bake-out" time is to drive out essentially all of the gases, organics and other contaminants which may be trapped within the interstii~ial metal structure thereof. Higher "bake-out" temperatures and/or "bake-out" periods may be used. Once the "bake-out" is finished, the pieces are now ready to be removed from the oven and sprayed with the primer resin.
APPLICATIN OF FLUOROCARBO:L~
RESIN AND ADDITIVE MIXTURE
The spraying of the resin powders, both the primer resin and the top coat resin mixture, requires particular care as control of the substrate temperature ranges are important.
Also control of the ranges of thickness of the coats is important. Finally, it is important that the thicknesses of each coating be conl~rolled within defined ranges, from one section to another <~cross the surfaces of the substrate metal pieces; that is to ;gay that the spray application of any given layer of coating should be controlled such that it is not too thick at any given ~~oint and/or not too thin at any given point. A procedure used for the application of a PFA/PPS
primer coat to the metal substrate, following "bake-out" of the pieces is preferable as follows.
A. PRIMER LAYER APPLICATION

12. The piece~~ should be removed from the oven with the 1341n16 temperature of the ~~ieces being at least about 700°F, with the thermocouples still attached to each piece. The first spraying of the primer resin should be commenced preferably within thirty (30) seconds from the time the pieces are removed from the oven with the temperature of the pieces preferably not being lower than about 680°F, although it is quite acceptable that the finishing t=ouches of the spraying may be added as long as the discrete section of each piece, which is then being sprayed, is above the resin melting point. Spraying may be done, simply, but not preferably, until no more of the powder primer resin melts onto t:he pieces. The melt range of the PFA/PPS resin mixture is 575°F to 600°F, but the powder resin will not normally melt as it hits the metal unless the metal is at about 600°F or above. The thermocouples attached to each piece only show the temperature of that portion of each piece which is immediatel~r adja~~ent to the thermocouple attachment point, while other ;~ectio:ns of each piece may be higher or lower in temperature, depending on the rate of cooling of each discrete section of each piece. Thicker sections will cool more slowly while thin se~~tions wil:1 cool relatively rapidly.
Preferably, the thickness of the primer resin will be within the range of about 0.002" to about 0.005", although primer coat thicknesses in the range of about 0.001" to about 0.025" have been found acceptable. The setting on the Ransburg GEMA 701 electrostatic spray apparatus will vary according to the size of the pieces, the thickness of each discrete section of the substrate metal, and the geometry of the pieces. An appropriate setting for a 1/4" x 8" x 8" mild steel plate is 40 Kv. The primer rerun is preferably applied in a single spraying, forming a single layer, although in some circumstances an additional layer or more may need to be applied. If such i:~ nece;ssary, additional coats of primer resin may be applied as follows.

X341 X16 ' B. APPLICATIC7N OF .ADDITIONAL "PRIMER" LAYERS

13. After the first layer of primer resin has been sprayed onto the pic=ces, additional layers may be applied but are not necessary to improve bond strength. Such additional layers may be used i.o provide a transition between the metal substrate which characteristically has a low coefficient of expansion and top coat material which has a higher coefficient of expansion. If ail additional layer of primer resin is to be applied, the pieces are then returned to the oven. The oven should be set at about 700°F. When the temperature of the pieces has reached '700°F and all portions of the first coating have reached the me_Lt phase, as is determined by visual inspection through a window in the oven (usually requiring a soaking of about twenty (:20) minutes), and if it is determined that one or more additional layers of primer resin are necessary, the pieces are again removed from the oven and a second layer of prirner resin is sprayed on, overylaying the first layer. Becau:~e of notably poor heat transfer in fluorinated polymer;, the first layer will hold the temperature of the pieces allowung ample time for the second layer to be sprayed on at this temperature. The actual surface temperature of the first coat may drop to about 650°F or less, but should not drop below about. 600°F. The objective is to obtain a primer resin build up of about 0.002" to about 0.02". In applying the second layer (and any necessary additional layers) of the primer resin, the Ransburg GEMA 701 electrostatic spraying apparatus may be set. at about 30 to 40 Kv. for a 1/4" x 8" x 8"
piece of mild steel..
x 1 3 41 0 1 6 "~

14. Whether o=r not additional layers of primer resin are applied over the fi=rst layer of primer resin, following the last layer of prime=r resin sprayed on, the pieces are returned to the oven which i:~ set at 700°F until inspection determines that the last layer of primer resin has reached the melt phase.
A procedu:=a used for the application of PFA/SiC top coat to a "primed" metal substrate is preferably as follows.
C. APPLICATION OF 'TOP COAT LAYERS

15. The pieces are :removed from the oven and sprayed with the first layer of topcoat resin mixture, using the Ransburg GEMA 701 electrostatic spraying apparatus, which may be set at about 30 Kv. for a .L/4" x 8" x 8" piece of mild steel. Care should preferably be taken to ensure that the temperature of the pieces being sprayed ahould always remain at or above the 575°F to 600°F melt range of PFA. The thickness of each layer of top coat resin which ins sprayed should preferably be within the range of about 0.006" to about 0.01", although layer thicknesses within t:he range of about 0.001" to about 0.015"
have been found to be acceptable.

16. After the first layer of top coat resin mixture has been sprayed onto tree pieces, they are placed back into the oven and heat soaked unti:L the just-sprayed resin coat has fully melted.

17. SuccessivE: layers of top coat resin mixture are applied in the same manner, following the specifications set forth in Step. Nos. 15 and 16 above. The objective is to form an overall barrier <:oating on the pieces which is at least 0.040" thick but wh__ch may be of a greater thickness. Thus, as many layers of top coast resin mixture are applied as are x -32- 134101fi necessary to achiev<~ such a thickness. After the last layer of top coat resin is a~~plied, the pieces are first reduced in temperature to 550°F' in the oven, by shutting the oven off but continuing to circu:Late air in the oven with the oven blower.
When the pieces reach 400'°F, they are removed from the oven and cooled to room tempE=rature, thus being ready to place into service.
WET SPRAY APPLICATION
Where the=re is a need to form a barrier coat type composite, in accordance with the present invention, in relation to the sub;~trate metal of relatively large metal apparatus, such as banks and pressure vessels, there is a problem in applying the fluorocarbon resins to the substrate metal. Normally, in such apparatus, the barrier coating is needed on the inside walls of tanks, vessels and the like. To follow the procedures described above would necessitate manual spray of the interior of a tank or vessel which is at a temperature in exce;~s of 600°F. It would be impossible to place a person inside a tank or vessel at such a temperature to effect the required spraying operations.
It is envisioned that such spraying of the interior of hot vessels migh~~ be effected by the application of robotics technology; however, until such is developed, an alternate approach has been followed for spraying the fluorocarbon resins, and mixture; thereof with fine crystalline ceramics, onto substrate metals at ambient temperatures, while still achieving the high integrity bonding of those resins and resin mixtures to the substrate metal, and to each other, substantially free ~~f voids (porosity) .

To develop the :barrier coating of the composite of the present invention, each layer of both the barrier resin and the top coat resin rnay alternatively be applied to a piece when that piece is at ambient (room) temperature. However, each of those layers is spr<~yed o:n wet, rather than as a dry powder, and electrostatic spraying apparatus is not used. A preferred method of developing the composite of the present invention, using wet spraying, is as follows.
A. PREPARATION OF .AQUEOUS DISPERSION
Preliminary, a top coat resin mixture in a water suspension is prepa=ed most preferably comprising: 96.04 wt.
of PFA resin; 0.04 ~Nt. % of Dow Corning Anti-Foam A antifoaming additive, marketed by Dow Corning Corporation of Midland, Michigan; 1.96 wt. 'o of 5,~ silicon carbide; and water (in the laboratory, de-ioni:~ed water may be used but in production, standard "softened" water, with the minerals removed, is quite satisfactory.) To prepare a sample of the mixture in a water suspension, 100 ml of refrigerated, de-ionized water is deposited into the mixing container of a blaring model 34BL21 high speed blender. The :blender is then turned on and the speed is adjusted to the :highest speed which will still maintain a smooth vortex without splashing. Two grams of Triton X-100 are added to i=he blending water with a standard eye dropper, a drop at <~ time. Then add 0.05 gram of Anti-Foam A
x 134~~16 to the blending watf~r and reset the Variac control on the blender to 20-30V. Mix t:he solution at this speed for one (1) minute. Then reset the blender Variac to 60V and slowly add 2 grams of silicon carbide to the solution. Then set the blender Variac to 60V-70V and add 98 grams of PFA resin slowly to the center vortex of the blending solution. If the PFA resin does not disperse, add ac3ditio:nal water to the blending solution in 5 ml increments until the PFA resin disperses. The dispersion of the PFA resin is aided by using refrigerated water, although this is not a neces:~ity. It has been found that this formulation, in modified form, produces acceptable coatings where as much as 99.9 grams of PFA resin are added and as little as 0.1 grams of silicon carbide powder is mixed in.
B. PRE-APPLICATION PREPARATION OF METAL SUBSTRATE
Secondly, the piece to which a barrier coating is to be bonded is prepared in .exactly the same way as specified above in No. 6 through 11, with the prior cleaning step, prior to sand blasting, a:~ specified above, included. Once the "bake-out" procedure has :been completed, the piece is removed from the oven and air cooled to below 100°F.

C. WET SPRAY APPLICATION OF PRIMER COAT
Thirdly, <~ prim~sr resin coat should be applied. A
preferred primer re;~in coat may be the PFA/PPS resin mix described above or .Lt may be the DuPont 532-5012 resin. Either of these may be app=Lied electrostatically using the Ransburg GEMA 701 electrostat=ic spray apparatus by spraying a preferred thickness of 0.002" to 0.005" per layer of resin to the cold (room temperature) piece. Then the piece, with the thermocouple attached thereto as described above, is inserted back into the oven, set at 700°F. The piece is brought up to temperature and held there until all portions of the primer resin layer have reached the melt phase, as determined by visual inspection through a window in the oven following this, the oven is shut ofi_ with the oven blower still running, and the piece is slow cooled to 550°F, as described above. Finally, the piece is removed from the oven to rapidly air cool to below 100°F for application of t:he next layer of primer resin, if necessary. Succeeding layers of electrostatically sprayed primer resin layers may b~~ applied in exactly the same way.
Applying the primer 'x res,~n , n this mar.n~ar .r,ay require multiple layers as the sprayed on czry powder, to some extent, tends to fall off of the piece, decreasing the thickness of the layer.
A1'~heugh the electrostatic dry spray method is preferred, alte.~nati.vely, the primer resin may be applied wit. L:~>ual_ly, a 0.00?." to 0.005" thi.ckriess of primer resv~n cant can be applied in a single layer.
mhe top coat re:~in misaure in a water suspension, described a:Jove, may, in a slightly modified form, be used as a primer re~_,in. For example, in the case of a PFA/PPS pr=L"'.er res,~n described above, the only change necessary _is to reduce. the 96.04 wt. o of PFA resin to 90.00 opt. -'~ anci to add G.04 wt. o of PPS resin. In preparing a sample mixture, as a preliminary step, 6.2 grams of PPS resin are thoroughly mixed and blended ~~;ith 91.3 grams of :~?I~'~, resin using a roller mill as described above. T.nen the sample mixture procedure for the water :~uspensi.on, descr,~bed above, is used 2C except that the PFA/PPS resin mixture is added instead of the straight PFA resin. The silicon carbide may be, aptian.ally; taker, out in the primer resin mixture water suspension.
Prior to spraying on the primer resin mixture water suspe.~,sion (pri.:~er wet spray) , the "baked-out"
piece, prepared as described above, is removed from the furnace anal air cooled to 1_ess than 100°F. The primer wet spray is loaded into a one (1) quart pot for a Bin'Ls model 1'~ ;pray gun which is used to apply the primer ;aet ~~pray to the piece, as marketed by Bin;cs manufacturing Company of l~rarklin Park, Illi-nois. Prefera._ly, the Binks model spray gun is equipped with ~ Bin:K~> No. 66SS fluid nozzle, a Binks ~'o. 66SF air nozzle and a Binks No. 65 needle. The primer wet spry in the one (1) quart pot should, preferably, be shaken 'intermittently, but frequently, to keep ti:e solids ~._ the primer wet spray in suspen-sion. mhe air pre~;sure used to apply the primer wet spray should, preferably, be within the range of about 40 psi to aboL:t 50 p"i.
Tn spray~_ng the piece, new below 100°F in temperature, preferably at room temperature for ease of handl i ng, t=he sprG.y should be applied evenly, first to the critical areas w::ere complex relatively sharp curvatures, corners, etc. ex it, then to the relati-vely more ;month, 1e~>s curved, flat, etc. areas. Care should be tal~:on to avoid running and overspray of the primer wet spray on the piece. Also, as mentioned 1:~ before, care must beg taken to see that the solids suspended ,~n the wager, in the primer wet spray, do not separate during the spraying operation. All spraying of the primer wet spray should preferably be done without a break in the operation. Stopping the operation. will alloH; the primer wet spray on the piece to dry. When this occurs and the spraying is recom-menced, the dried material can easily be blown off by the atomizing air of_ the spray gun. Thus, it is recommended that the spraying always be done against a trailing wet edge, as is well known to those with skid in the vfield. An even ~_ayer of primer wet spray should be app?ied by spraying by a steady, even movement or t:~e spray gun. The primer wet spray shcuid be applied until a single layer in a range of about 0.00?_" ~o O.Ot?5" thickness is built up, prefer-ably in a range of about 0.00" to 0.005" as measured in the wet co~dition. A Nord~~on Wet Film Thickness Gauge, as mar'~:eted by Nordson Corporation of Amherst, Ohio, may be used t:o determine the thickness of the wet sprayed layer.
Once the primer wet spray coating has been applied, a~ a sing:Lc layer, the piece is preferably air dried for abo~,~t fifteen (1.5) minutes. Then the piece shoo-d ire pl aced :in a preheated oven set at 350 ° F, wit'_: a -thermocouple at.t:ached as explained above. After_ the ~z_ece is so placed in the oven, without delay, the oven should be reset to 720°F.
When the temperature of the p~_ece reaches 700°F and the primer wed spr:ay coating, fully dried, and all surfaces thereof have reached the melt phase, as determined visually, the oven is turned off, with the oven blower still r~.nnir~g, and the piece is cooled in the oven until it reaaches 550"F. At this point, the piece is removed from the oven and air cooled to below 100°F, preferably to room temperature, for application of the top coat wet spray.
D. WET SP_T2AY APPLICATIO:v' OF TOP COAT SUSPENSION
The top coat wet spray may be the top coat resin mixture in a water suspension described above, without FPS resin and defin_~'~ely with silicon carbide in-eluded. The v.op coat wet spray is sprayed onto the piece, prerer~.bly at= room temperature, in layers, exactly follor'ing the spraying techniques described above for the prime~~ wet spray resin, except that the thickness of each layer of top coat wet spray is greater, preferably :in the range of about 0.010" to about 0.01", with due care being taken to avoid running and o-~~erspray. The objective is to build up a:~ overall ba~~rier coating of at least about 0.040" in thickness.

._ After each layE=r of top coat wet spray is sprayed onto the piece, the piece is air dried for fifteen (15) minutes and placed into a ~>reheat~ed oven set at 350°F, with the thermocouple attached as described above. Then, without delay, the oven is reset for 650°F and the temperature of the part is brought up to the ~~oint where visual inspection assures that the now dried top coat wet spray layer is in the melt phase, or until the piece temperature reaches 620°F, whichever comes first. If the melt phasE: is reached before the piece temperature reaches 620°F, preferably, continue to heat the piece for about ten (10) minutes or until the piece has reached 620°F, whichever comes first. Then the oven is shut off with the blower still running and the piece is cooled to 550°F, followed by removal of th.e piece from the oven for air cooling to below 100°F. Th=~s heating-cooling cycle is repeated for each layer of top coat wet spray applied.
Additional advantages of the present invention are that the use of a property improving additive, as described herein, is effective in increasing the rate at which the coating composition can be fused. By way of background and as exemplary of the aforementioned, it is known to those with skill in the field, as exemplified by the two (2) DuPont Fact Sheets mentioned abcwe, that it is difficult to get good fusion or sintering of pari~icles of PFA resin to each other, or to primer systems over metal substrates, at temperatures below about 700°F within time periods that: can be used practically in a commercial setting. Thus, it is ~~onventionally recommended that PFA resins should be fused at about 725°F for about 20 minutes. However, .Lt is also known that PFA, as a fluorinated polymer, deteriorates (degrades) relatively quickly at 1341016' temperatures of aboL:+. 700°F and above, as fairly rapid oxidaticn occurs. 'Thus, the objective is to effect a complete fusion of the resin particles before the resin. itself degrades. It can be appreciated that it is relatively diffic::lt to get good bonding of PFA
resin. to metal substrates and to get one layer of PFA
resin bonded t.~ another.
By way of example, neat powdered PFA resin, app lied directly to a 8" x 8" x 1/4" mild steel plate brought to a temperature of about 725°F, fuses com-pletely in abo,~t 20 minutes; at 675°F, fusion is completed in about 30 minutes; and at 620°F, fusion is completed in about 40 to about 50 minutes. The addition of pc;adere.d ceramic material to the resin 1F significantly reduces the time necessary to complete the fusion of i:he resin particles. When powdered PFA
resin and cermaic material is applied to a mild steel plate heated to abcut 725°F, fusion of the resin is completed in about 10 minutes; at 675°F, fusion is completed in about 15 to about 20 minutes; and at 620°F, fusion '_s completed in about 30 to about 40 minutes.
Thus, a preferred method for forming a fused coating from t_ue composition of the present invention includes heating said composition to a temperature for a period of time no =longer than a predetermined period of time, said temperature being at least 25 F° below the temperature at which the resin of said composition in neat form can be f.'usec~ completely by heating for no longer than said precLetermined period of time without substantially degrading said resin.
It has also been observed that where neat powdered resins are applied to an already resin-coated metal substrate, the time to complete fusion is 1341A~6 increased to even longer periods than those just recited above. I:t is suspected that this longer fusion. time ~~s due to the heat insulating property of the previous_Ly applied neat resin layer. Thus, to effect compic~te fusion. of the top-most resin layers, it is necessary to prolong the period over which the plate is heaved, they-eby degrading the bottom-most resin layers in closest proximity to the substrate surface.
The addition of ceramic powders to the resin speeds t:he fusion process, perhaps due to the improved heat conductivity of ceramic powder-containing resins.
Thus, a'~1 of the resin layers may be brought to fusion temperature :r.ore quickly, reducing the exposure time and concomit:~nt degradation of the bottom-most layers.
It is also n.nown that it is relatively difficult to achieve b~~ndirg of straight PFA resin to metal substrates, regardless of the temperature of fusion or sintering us?d. It has now been determined that the addition of the above specif=ied quantity ranges of PPS
resin to the PFA resin, surprisingly results in a very high quality integral bonding of the PFA resin to substrate metals at temperatures in the range of about 675°F to about 720°F without significant deterioration (degradation) to the PFA. It is also now known that a very similar phenomenon occurs in dry sprayed PFA
resin without the addition of PPS resin, but with the addition of the specified quantity ranges of ceramic powder, in those same temperature ranges. No explana-tion of why these phenomena occur is known and no speculation. thereof is offered herein.

134~~~6 EXAMPhES
EFFECT OF ADD=CNG SiC TO PFA
TO ELIMINATE RESIN BUBBLING
In the e:fampl-es which follow, Sample Plates (A-G) were fabryca'<~d to determine the effects of adding silicon carbide to PFA resin in respect to the occurrence of bubbling of top coat layers during the build up of the barrier coat. In all cases, DuPont 1.0 TEFLON-P 532-~>0~2 PFA resin was used as the primer resin and Dur~~ort TEFLON-P 532-5010 PFA resin mixed with silicon c~arb~.de powder (as indicated) was used as the top coat resin. The sample plates were formed using 1/~'r" x a3" x. 8" size mild steel plates and 1.5 composites were formed thereon. in accordance with the foregoing procedures. The results are as follows in Table 1.

a:0 Sample Plate A B C D E F G
Pr imer Thickness .021 .02_2 .021 .018 .024 .017 .001 25 (inch) No. of Layers 2. 2 2 2 2 2 1 -s0 =----____-_--:--_-___-----__--_--_-____________________ Top Coat 100 99.75 99.5 99 98 98 99 PFA wt.o .5 SiC wt. % 0 0. 2.5 0. 50 1 2 2 2 (Particle Size) - 5~.c 5h 5~C 5~t 5~ 5u ~.0 Thickness .008 .007 .007 .007 .008 .040 .018 (inch) 1341p~6 No. of Layers 1. 1 1 1 1 7 3 Steel Temp.
(°F) 675 675 675 675 675 675 675 Barrier Coat .029 .029 .028 .025 .032 .057 .019 Total Thick--;inches) Bubblinc_~ Yes Slight Very No No No No Slight It can }~e seen from the results reported in Table 1 above, tha': when sample plate A was sprayed with a top coat of :L00~ PFA, it exhibited bubbling after a sing a 7_ayer of top coat was applied. With the addition of 0.25 wt. % of silicon carbide to the PFA
top coat, tre bubbling was significantly reduced with the applecat:ion of a single layer of top coat as shown in Sample Plate i_s. The single layer of top coat applied to Sample Plate C had 0.5 wt. o of silicon carbide added, and the bubbling was reduced to a point where it waC just barely noticeable upon visual inspection. In ~>ample Plate D, 1.0 wt. o of silicon carbide was ;~ddecl to the PFA top coat, and in the single layer app~.ied, no bubbling was detected. In Sample Plate E, 2.0 wt. o of silicon carbide was added to the PFA t~Jp coat, and in the single layer applied, no bubb'_ing was detected. Sample Plate F was prepared identically vo Sampie Plate E, except that 6 addition-al layers of top coat were applied for a total of 7;
again, no bubbling was detected.

In the preparation of Sample Plates A-F, a relatively thwick primer coat was applied, using two layers in each case. To determine whether or not a reduction of the thi.c:kness of the primer coat, and a single layer applicat=ion thereof, would generate bubbling in the PFA top coat, with 2 wt. % silicon carbide added, Samp1_e Plate G was prepared, with a single layer cf primer coat, only .001" in thickness, being appl;.~ed and then ove.rlayed with 3 layers of 10 PFA/2 wt. ~ S_C mixture top coat resin. Again, no bubbling was c.etectod. It should also be noted in regard to Samr~le Flutes D and E that thicknesses of 0.007" - 0.00." of )?FA/SiC mi:ftures, as a top coat, were applied in single layers without bubbling, 15 whereas, it i_=> known to those with skill in the field that straight PFA cannot be applied in coating layers greater than 0.003" thickness without the occurrence of bubbling. This :is pointed out in DuPont Fact Sheet TI-13-84 referenced above.

Several <zdditional Sample Plates (H-L) were fabricated wherein composites were formed on under-lying mild steel substrates in accordance with the 25 foregoing procedures. The object of developing these samples was t« determine the effect of lowering the metal substrate temperature in respect to the occur-rence of bubbling of top coat layers during the build up of the barrier coat. Again, in all cases, DuPont 30 TEFLON-P 532-5012 PFA resin was used as the primer resin and DuP~~nt ~'EFLON-P 532-5010 PFA resin mixed with silicon carbide powder (as indicated) was used as the top coat resin. The Sample Plates were formed 134~p~ fi using 1/4" x ~" 8" steel The x size plates.
mild results are a_, in Table2.
follows TABLE
?.

Sample Plate H I J K L

Primer Thickness .0:L7 .024 .010 .022 .011 (inch) 2v'o. of Layers 2 3 2 2 2 Toy Coat :L00 99.5 99 98 99 PFA wt.

SiC wt.% 0 0.5 1 2 1 (Particle size) - 5u 5u 5u 5u Thickness .015 .031 .006 .008 .034 (inch) No. of Layers 3 5 1 1 7 Steel Temp. ("F) 625 615 625 625 625 Barrier Coat .032 .055 .016 .030 .045 Total Thickr.c~s "_-5 (Inch) Bubbling Yes No No No No 4.0 In Sa:npl~~ Plates I-L, where silicon carbide was mixed with the PFA resin of the top coat, no bubbling was detecaed where the steel substrate metal tempera-ture was only raised to a range of 615°F to 625°F
while significant bubbling was noted in Sample Plate H
~.5 which used straight PFA resin, without silicon 1341o~s carbide, as ;she top coat where the steel substrate metal temperaa~ure was only raised to 625°F.
EFFECT OF' SiC PAR~'ICLE
SIZE ADDED TO TOP COAT
Several additional Sample Plates (M-R) were fabricated wherein composites were formed on underly-ing mild stee:L substrates in accordance with the foregoing pro~:edures. The object of developing these 7_0 samples was to determine the effect of adding larger sized pa~-ticl~~s of: silicon carbide powder to the PFA
top coat resi.:~ in respect to the occurrence of bubbling of t~~p coat layers during the build up of the barrier coat. Again., in all cases, DuPont TEFLON-P
._5 532-5012 PFA :resin was used as the primer resin and DuPont TEFLON-P 532-5010 PFA resin mixed with silicon carbide powder (as indicated) was used as the top coat resin. The sample plates were formed using 1/4" x 8"
x 8" size mii_~3 steel plates. The results are as a?0 follo~,as .in Ta:bl a ~3 .

Sample Plate M N O P Q R
Primer Thickness .010 .021 .020 .020 .020 .020 (inch) No. of Layers 1 3 2 2 3 2 Top Coat 90 99.5 90 80 99 98 PFA wt . o SiC wt.% 10 0.5 10 20 1 2 ~~0 (Particle Size) 7u 7~ 7u 7~C 14~C 14~t ~3'~~~16 Thickness .035 .026 .040 .040 .021 .021 (inch) NO. Of Layers 6 5 6 10 4 4 Steel Temp.
(°F) 680 615 600 650 620 620 Barrier Coat .045 .047 .060 .060 .041 .041 Total Thick ness (Inch) Bubbling No No No No No No In regard to Sample Plates M-P, silicon carbide powder of 7~~, size was mixed with PrFA resin and applied to form the t:op coat with no bubbles detected therein.
The top coat, of S<~mple Plates Q and R had 14~. sized silicon carb~_de powder added to the PFA resins before application. Also, it should be noted in regard to Table 2 that the temperature of the substrate steel metal was varied i:~ a range from 600°F to 680°F.
Finally, it :>hould be noted i.n regard to Table 2 that the wt. ~ of silicon carbide which was mixed with the PFA resin. to form 'the top co:~t resin mixture was varied i.n the range of 0.5 wt. % to 20 wt. ~. In none of these cases, reported in Table 5, did any visibly detectable bubbling occur.

'34~~16 CORROSION RE:~ISTANCE DETERMINATION
FOR FLUOROCA:~BON POLYMER BARRIER
COATING CF THE PRIOR ART
British Patent No. 2,051,091, believed to be the detailed specification for developing Fluoroshield coatings, te:?ches using a composition comprising a mixture of P'rFE (polytetrafluoroethylene) resins, and PFA resin to form overlays or_ barrier coats. These dry powder resin mixtures are mixed with a carrier liquid and, it appears in commercial applications, glass powder, for wet spray applications. British Patent No. 2_,051,091 teaches that "To obtain non-porous coatings it is necessary to densify the applied coating. This may be accomplished by rolling the coating prior to heating the coating to coalesce it."
It also teac'zes that '°...a pure PFA coating is unsuitable a:nd failed to provide a uniform, non-porous coating."
Althoug":~ British patent No. 2,051,091 discloses several exam:~les, of what are now known commercially as Fluoroshield coatings, which were spark tested at 10,000 volts, none of the examples in that patent discloses actual corrosion tests.
One apparatus which is widely accepted by those with skill in the field for testing corrosion is the Atlas Cell, as marketed by Custom Scientific Glass, Inc. of Elkton, Maryland, U.S.A. Basically, the Atlas Cell tests materials or exposed material surfaces to the effects of corros~_on at either ambient or elevated temperatures, as desired, and for extended periods of time as dess_red. Various tests have been made on Fluoroshield coated mild steel samples, with one side of each being Fluoroshield coated, exposing only that Fluoroshield coated surface of each of those pieces to the tests. All test samples were commercially fi~~~Ot6 _ acquired specimens of Fluoroshield coated mild steel apparatus. .?~11 samples of the tested Fluoroshield coated contained finely ground glass powder. The samples and the test results are as follows in Table 5 4.

Fluoroshield Corrosive 10 Sample No. _ Material Temperature Time 1. 70 wt. % 252°F 600 Hours Nitric Acid 15 2. 20 wt. % 220°F 600 Hours Hvd.rochloric Acid 20 3. 70 wt. % 252°F 1,000 Hours Nitric Acid 4. 20 wt. % 220°F 1,000 Hours 25 Hyc!rochloric Acid 30 The evaluation of each of these samples, follow-ing the abo~~ a to st.~>, was by subj ective observation, in each case relative to a sample in accordance with the present ,~rvention, as described hereinafter, and comprise the opinions of the inventors of the present 35 invention. That s_<_> to say that each Fluoroshield coated test sample, recited above, was tested under equivalent conditions used t:o test a corresponding non-Fluoroshield c:oat.ing test sample, in accordance with the present invention, and compared thereto. The 40 results of such tests of non-Fluoroshield coated test samples are deta.il_ed hereinafter in Tables 6-8. The subjective o~~servations in regard to each Fluoroshield coating sample are as follows in Table 5.

Fluoroshield Sample No. Evaluation 1. Coating severely blanched and fully blistered.
2. Caating blanched and blistered, slight degradation of underlying steel evident, slight delamination evident.
3. Coating severely blanched and fully blistered, almost total delamination evident, coating substantially pealed away from substrate metal, substantial degradation of underlying substrate metal evident.
4. Coating severely blanched and fully blistered, degradation of underlying steel evident, substantial delamination evident.
In each of the foregoing Fluoroshield samples, the blistering ~_s an indication that the overlayed barrier coating may not be firmly bonded to the underlying substrate steel and that the coating is beginning to separate. The blanching or discoloration is an indication that the corrosive medium has penetrated into and even through the pores of the overlayed barrier coating, attacking both additives which have been mired into the coatings as well as, possibly, the underlying Cubstrate metal. Delamination indicates that the bonding of the Fluoroshield coating to the substrate metal has failed.

'341D16 As is demonstrated by the Fluoroshield coating tests, above, there is substantial room for improve-ment in both the integrity of the bond between the overlayed barrier_ fluorinated polymer coating and the underlying substrate metal, in particular, steel.
Also there is substantial room for improvement in diminishing porosity in the overlayed barrier fluori-nated polymer coat.i.ng, irrespective of spark test evaluations.
The use of At.l.as cell testing for corrosion testing has been briefly described above in regard to the testing of Flu.oroshield coated mild steel samples.
An Atlas ce,~l is arranged in the form of a hollow cylindrical section with both ends being open. Ports extend through the sides of the cylindrical section through which instrument sensors and heating elements are inserted into the hollow of the cylindrical section. The open ends of the cylindrical section are capped with the test samples which are to be exposed to corrosion. Such test samples are normally in the form of fiat plates, one surface each of which is abutted against an open end of the cylindrical secticn. The open ends of the cylindrical section are capped T,aith the test samples which are to be exposed to corrosion. Such test samples are normally in the form of flat plates, one surface each of which is abutted against an open end of the cylindrical section and clamped or otherwise mounted thereto so that the joint between is ~;ealed. Because there are two (2) open ends to the hollow cylindrical section, two (2) test samples are used to cap those respective open ends, thus two (2) test samples are simultaneously and concurrently subjected to testing by each Atlas cell test program.

Atlas cell test programs are normally set up to test the effects of a corrosive liquid medium, for example, an acid, which is introduced into the hollow cylindrical section after the open ends thereof have been capped .end sealed with the test pieces. The corrosive liquid is introduced through one of the open ports e:ltending through the wall of the cylindrical section, after which the ports are sealed. The corrosive li.~uid, either heated or at ambient tempera-ture, is left within the Atlas cell for extended periods of time amounting to several hundreds of hours or more.
In each of the above reported 1,000 hour Atlas cell tests of Fluoroshield coated mild steel samples, Fluoroshield Sample Nos. 3 and 4, the opposite end of the Atlas Cell was capped with a sample plate which was a composite according to the present invention.
These sample plates, in accordance with the present invention are described hereinafter as Sample Nos. 13 and 14 which correspond, in their Atlas cell testing, to Fluoroshield Sample Nos. 3 and 4 respectively. The sample numbering system is used for convenience in making cross comparisons; thus Fluoroshield Sample No.
1 corresponds to Sample No. 11 and Fluoroshield Sample No. 2 corresponds to Sample No. 12, etc.
The preparation of Sample No. 11 through 14 was in accordance with. the procedures described above in accordance with th.e present invention. In all cases, the primer coat resin used was DuPont TEFLON-P 532-5012 PFA rein and. the top coat resin mixture was 98 wt. % of DuPont TEFLON-P 532-5010 PFA resin mixed with 2 wt. °s of 5~, sized silicon carbide powder. In all cases the barrier coat overall thickness of Sample Nos. 11 through 14 exceeded .040", but did not exceed .060". The details of the preparation of Sample Nos.
11 through 1~= are as follows in Table 6.

Sample No. Primer Coat Top Coat 11 Heated Heated Dry (Electro- Dry (Electro-static) static) 12 Heated Heated Dry (Electro- Dry (Electro-static) static) 13 Heated Heated Dry (Electro- Dry (Electro-static) static) 14 Heated Heated Dry (Electro- Dry (Electro-static) static) To furtzer explain the nomenclature used in Table 6, above, th~~ term "Heated" in regard to the "Primer Coat" indicates that the procedure used is that described ab~we, from the point just following step No. 5 throug:~ Step No. 11 are followed. The term "Dry (Electrostatic)" in regard to the "Primer Coat"
indicates that the procedure used is that described above, from 'the point just following Step No. 11 through Step No. 14. The term "Heated" in regard to "Top Co<it" indicates that the procedure used is that described ab~we, in Step No. 14. The term "Dry (Electrostatic)" in regard to the "Top Coat" indicates that the procedure used is that described above, in Step No. 15 through Step No. 17 Table 7, following, shows the Atlas cell test conditions which Sample No. 11-14 were subjected to, each corresponding to the individual test program to which Fluoro=_:hield Sample Nos. 1-4 were subjected, 5 respectively.

Corrosive 10 Sample No. Material Temperature Time 11 70 wt. 0 252F 600 Hours Nitric Acid 15 1?. 20 wt. 0 220F 600 Hours Hydrochloric Acid 20 13 70 wt. 0 252F 1000 Hours Nitric Acid 14 20 wt. % 220F 1000 Hours 25 Hydrochloric Acid 30 The subjective observations in regard to the evalua-tion of Samp=Le Nos. 11-14 are as follows in Table 8.

35 Sample h'o. -_ Evaluation 11. Very slight blanching detected. No blistering detected.
40 12. No blanching detected. No blistering detected.
13. Slight blanching detected. No blister-45 ing detected.

13410' 14. No blanching detected. No blistering detected.
In comparing the evaluations of Table 5 and Table 8, it is clear that all of the Fluoroshield samples tested were significantly deteriorated and degraded by the Atlas cell test while none of the samples in accord with the present invention suffered any significant deterioration or degradation. Sample Nos.
13 and 14 were further tested, under the same cor-responding conditions stipulated above in Table 8 for an additional 300 hours each. In all cases, the evaluation of the=.e samples, after the additional 300 hour exposures, remained unchanged.
BOND STRENGTH DETERMINATIONS FOR
SiC-CONTAINING E-C'TFE COATINGS
The next group of examples illustrates the effect on bond strength between the coating and the underly-ing metal substrate by increasing the concentration of SiC in the primer coat layer..
Several additional sample plates (S-X) were fabricated wherein composites were formed on underly-ing mild steel substrates in accordance with the procedures described above with the following depar-tures from the enumerated protocol. The workpieces to be coated were cleaned by a single grit blasting with 80 grit aluminum oxide (Step 6) and "baked" in an oven at 600°F (St.ep 11;. A primer coat was applied to workpieces brought to a temperature of about 500°F
(Step 12). Workpieces were returned to the oven and brought back up to 500°F before application of each succeeding coat (Step 16). Coatings were applied before the workpiece cooled below 465°F (Step 15).

All bond ~~trengt~h determinations were made according to A~~TM D3167-76 (Reapproved 1981), entitled "Standard Test Method for Floating Roller Peel Resistance of Adhesives", with the exception that an equivalent to t:he fi:rcture for supporting the test specimen, described :in paragraph 4.2 of ASTM D3167-76, was used to the same end result.
In sampler S-W, Ausimont's HALARR 6014 ethylene-chlorotrifluoroethylene copolymer (100 E-CTFE) was used as a primer coat resin with Norton Company's 39 Crystolon gree:z silicon carbide flour 4647 (1000 grit) in admixture t:zerewith in the amounts indicated in Table 9 below. The primer coat layer applied to each plate was fol7_owed by five ten-mil thick coats of 1~> neat, 1000 E-CTFE, giving a total coating thickness of 53 to 55 mils.
In sample plate X, Ausimont's HALARR 6614 E-CTFE
primer system, believed to contain, as its major ingredient, E-CTFE, with chromium oxide as a minor additive, was first applied followed by five applica-tions of AUSIN:ONT'S HALARR 6014 neat E-CTFE top coat resin.
Sample plates were formed using 1/4" x 8" x 8"
size mild steal planes. The results of the bond strength testing are as follows in Table 9.

Table 9 E-CT:E'E COATING BOND STRENGTH TESTING
Sample Plate S T U V W X
Primer Coat E-CTFE' wt. % 100 95 90 85 75 N. D.3 SiC2 wt. % 0 5 10 15 25 0 Particle Sia - 5~C 5~, 5u 5~, 1.5 Cr203 wt. % 0 0 0 0 0 N.D.' Thickness approx. 3-5 mil (all) Top Coat ~:0 E-CTFE wt. % 100% (all) Thickness 50 mil (all) No. of layers 5 (all) a~5 Peel Strength 60 >180° >134' >1104 >1044 75 (pli) 'Ausimont; HALAI~'z 6014 E-CTFE resin.
30 ZNorton Company 39 Crystolon green silicon carbide flour 4647, Worcester, Mass.
3Ausimont: HALA:2'~ 6614 E-CTFE primer system, believed to consist. of a major volume of E-CTFE and a 3 5 minor amount o f Cr_ X03 .
' Value :>hown is actually a measurement of the cohesive strength of the coating. Adhesive (bond) strength of the coating to the substrate believed to ~i0 be considerably higher than cohesive strength value shown.
In sample plate S, coated with neat (0 wt. ~ SiC) .~5 E-CTFE in both the "primer" layer and top coat layers, bond strength between the "primer" coat layer and metal substrate wa=. relatively low, around 60 pounds/linear inch (pl_i). Sample plate X, prepared with the man'ufacturer's recommended primer system, (HALARR 6614 E-CTF~~~ primer) believed to contain a minor amount of chromium oxide, fared slightly better than sample plate S, giving a bond strength between the primer coat layer and metal substrate of 75 pli, an improvement of about 25%.
In sample plate T, the addition of only 5 wt. %
SiC to the primer coat resin resulted in a bond strength value in excess of 180 pli, better than a 300% improvement in bond strength over plate S; 240%
over plate X coated with the manufacturer's recom-mended primer system. The actual bond strength of the sample plate T coating to the substrate could not be precisely determined because the strips of coating being peeled away from the substrate during testing ripped apart shortly after peeling was initiated.
Thus, the value shown is actually a measure of the cohesive strength of the coating being peeled away from the substrata during the test.
In sample plates U, V, and W, having 10, 15 and wt. % SiC, respectively, in admixture with the E-CTFE resin as the primer coat, the bond strength between the coating and metal substrate was, like 25 sample plate T, so great that it exceeded the cohesive strength of the strip of coating being peeled away from the substrate. Thus the peel strength values expressed in Table 9 for plates T, U, V, and W reflect the cohesive strength of the resin coating itself; the adhesive strength of the coating to the substrate is believed to be substantially in excess of the cohesive strength.
The apparent decrease in bond strength in coatings having increased amounts of SiC, actually a 1341p16 decrease in 'the coating cohesive strength, not adhesive strength, is believed to be due to the increased brittleness of the primer coat layer brought about by the elevated SiC content. Thus, as the strip 5 was peeled away f-.rom the substrate during the test, the more brittle, higher SiC-containing primer coat layer caused the strip of coating being peeled to tear more easily, the tear being initiated by a fracture in the brittle :primer coat.
10 In an effort to determine an actual bond strength value for Si.C-containing E-CTFE coatings, a sample plate having a coating corresponding to that applied to sample plate U was prepared, this time with a piece of metal screen embedded in the top coat layers. The 15 screen, 6" x. 9" ir,, dimension, was positioned over the 10 wt. o SiC-containing primer coat layer and the top coat layers applied over the screen in the same manner as described for the preparation of plate U. The screen was intended to provide a substantial reinfor-20 cement of the coating as it was being peeled away from the substrate. Despite the addition of the reinforc-ing screen in the strip of coating being peeled away from the substrate, the strip fractured, stretching the screen embedded in it, just after the point at 25 which a value of 1.50 pli had been measured.
CORROSION-RESISTArfCE DETERriIINATION
FOR SiC-CONTAININCT E-CTFE COATINGS
In the examples which follow, the corrosion 30 resistance of SiC--containing E-CTFE coatings is illustrated. Sample plate Y, corresponding to sample plate U described above, was prepared by applying a 3-5 mil thick primer coat layer of E-CTFE (HALARR 6014) having 10 wt. % SiC admixed therewith, followed by 35 five successive.coats of E-CTFE (HALARR 6014) having 2.5 wt. % SiC admixed therewith. A 10 wt. % SiC-containing E-CTFE primer coat layer was selected on the basis of the bond strength test results reported in Table 9 above and was deemed to represent a primer coat having a preferred bond strength. A 2.5 wt. %
SiC-containing E-fTFE top coat was selected on the basis of the superior corrosion test results observed for 2 wt. % SiC-containing PFA top coat resins.
For purposes of r_omparison, sample plate Z was prepared in the same manner as sample plate X in Table 9 above, (one 3-.5 mil primer layer coat of Ausimont HALARR 6614 :E-CTFE, believed to contain chromium oxide, followed by five ~>uccessive 10 mil coats of neat E-CTFE (Ausimont HAhARR 6014)).
Both sample plates Y and Z were subjected to Atlas Cell testing as described above (20% HC1 C
220°F) and observations of the respective coatings made at 300, 600 and 1000 hours. Prior to the Atlas cell testing, each plate was spark tested for pinholes in the coating by testing with a Wegener WEG 20 High Frequency Spark Tester set to 20 KV (AC). The power level at whv.ch spark testing was conducted was considerably more demanding of the coatings being tested than is recommended by the Society of Plastics Industry (SPI) Test Method for Detecting Faults in Corrosion RE~sistant Fluoropolymer Coating Systems, No.
FD-128. SPI: recommended test voltages do not exceed 6,000 volts (DC). No pinholes were detected in either plate spark tested. The results appear in Table 10 below.

1341~1~

Table 10 E-CTFE CORROSION RESISTANCE TESTING
Sample Plate Hours Y Z
300 no change no change 600 :mall blister no change (3mm) forming 1000 :mall blister in- no change creasing in size (l4mm) and beginning to crack; second small blister (3mm) forming As Table :LO shows, SiC-containing E-CTFE barrier coatings were unaffected by exposure to hot (220°F) 20 HC1 even after exposure for 1000 hours.
In comparison, the AUSIMONT barrier coat system began to blister after 600 hours of exposure under the 2~~ same acid conditions. Blistering suggests two modes of failure for the system: (1) permeation of the top coat layer_ by 13C1 and (2) insufficient bonding of the primer coat la:~er to the metal substrate (75 pli), permitting a direct chemical attack on the underlying metal substrat~n, lifting the coating, exacerbating coating failure. No failure was evident after 1000 hours of hot acid exposure in the SiC-containing E-CTFE system.
BOND STRENGTH DETERMINATION FOR
ZrC-CONTAINING E-CTFE COATING
In this example, the bond strength of a 10 wt. ~
zirconium carbide-containing E-CTFE coating was deter-mined. The zirconium carbide (Z-1034, a product of Zerac/Pure, Milwaukee, Wisconsin 53233) was less than 44 a in particle size. Coating application and bond strength testing were carried out as described for SiC-containing E-CTFE. Bond strength was determined to be in excess of 190 pli, the value measured just prior to the cohesive failure of the strip of coating being peeled fro:~i the substrate.
BOND STRENGTH DETERMINATIONS
FOR SiC-CONTAINING E-TI?E COATINGS
The object of developing these samples was to determine the effect on bond strength between an ethylene-tetrafluoroetylene copolymer (E-TFE) coating and underlying metal substrate of adding SiC in increasing concentrations to the primer coat layer.
Sample plates were prepared wherein E-TFE composite barrier coatings were :formed on underlying mild steel substrates i.n acc:ordance with the procedures described immediately above for :E-CTFE with the exception that after grit blast~.ng, workpieces were "baked" at 530°F
(Step 11) and the primer coat and all subsequent top coats applied with the workpiece at a temperature of 525°F (Step 12).
In samples ~~A-EE, Dupont's TEFZELR ethylene-tetra-fluoroethylene copolymer (E-TFE) was used as a primer coat layer with 2Jorton Company's 39 Crystolon green silicon carbide :Flour 4647 (1000 grit) in admixture therewith in the amounts indicated in Table 11 below.
Table 11 E-TFE COATING BOND STRENGTH TESTING
Sample Plate AA BB CC DD EE
Primer Coat E-TFE1 wt. % 100 95 90 85 75 SiC2 wt. 0 0 5 10 15 25 Particle Size - 5~, 5~ 5~C 5~t Thickness 3-5 mil (all) Top Coat E-TFE wt. % 100%
Thickness 27 N.D. 30 30 35 (mils) No. of layers 10 N.D. 10 10 10 Peel Strenctth 29 N.D. 28 31 37.5 (pli) 1TEFZELR 532-6000 ethylene-tetrafluoroethylene copolymer sold by Duport.
239 CRYSTOLON green silicon carbide flour 4647 of Norton Co.
From Table 11, bond strength between E-TFE and the metal substrate i~> observed to improve measurably with increased amount~> of SiC. A 23% improvement over neat E-TFE is o~~taine~i by the addition of 25 wt. %
SiC.
CORROSION-RESISTANCE DETERMINATION
FOR SiC-CONTAINi:NG E-TFE COATINGS
In the exa~~ples which follow, the corrosion resistance of Si.C-ceni~aining E-TFE coatings is illustrated. Sample plate GG was prepared corresponding to samp:Le plate EE above, that is, having a "primer" coals layer of 75 wt. % E-TFE/25 wt.
SiC and "top" coat :Layers of 95 wt. % E-TFE/5 wt. %
SiC. For comparative purposes, sample plate FF, corresponding to samp:Le plate AA above, was prepared having a neat E--TFE coating applied thereto.
E-TFE coatings applied to sample plates FF and GG
and corrosion test re:~ults are summarized in Table 12 below.

Table 12 E-TFE CORROSION-RESISTANCE TESTING

Primer Coat E-TFE wt. % 100 75 10 SiC wt. 0 0 25 Thickness (mil.s) 3-5 3-5 Top Coat 15 E-TFE wt. s 100 95 SiC wt. 0 0 5 Thickness (mil_s) 37 40 20 No. of Coats 10 6 Corrosion Test 300 hours no change no change 25 600 hours no change no change 1000 hours test area has a single pin-developed hun- hole has devel-d:reds of pin oped in the 30 holes through- coating oat due to the e:Ktensive cracking o:f the exposed coating surface 35 After 1000 hours of hot acid (20% HC1 at 220°F) exposure, the neat E-'rFE coating of sample plate FF
was literally r~_ddled with pinholes (2-3 pinholes/cmz) with extensive ~iiscre~te micro-cracks throughout, each crack about 3mm or so in length. In comparison, the 40 SiC-containing ~;-TFE coating of sample plate GG
developed a single pinhole and no evidence of any cracking after x_000 hours of hot acid exposure.

Pinholes were detected using a WEG 20 Wegener High Frequency Spark Tester set to 20 KV (AC).
In addition to the superior corrosion-resistance of the SiC-containing E-TFE coating, the admixture of SiC was seen to reduce shrinkage of the coating applied. On a scale of 1 to 5 (1 being no visible coating shrinkage and. the coating is observed to flow smoothly around the sample plate edge without thinning; 5 being severe coating shrinkage and the coating is observed t:o form shrinkage ridges pulled in over 1/4" from the edge of the sample plate), neat E-TFE coatings experienced severe shrinkage, for a rating of 5, while shrinkage of 95 wt. % E-TFE/5 wt.
SiC coatings (a.pplied over a 3-5 mil primer coat of 75 wt. % E-TFE/25 wt. % SiC) was very low, for a rating of 2 (slight shrinkage - just starting to thin or pull in at the corners).
Further, t:he addition of SiC was seen to improve surface uniformity o:E the applied coating. Neat E-TFE
coatings tended to be uneven with large bumps and waves unevenly distributed across the coating surface, giving a mottled effect, whereas the addition of only 5 wt. % SiC rendered E-TFE coat smooth and uniform.
High surface g:'_oss of E-TFE coatings remained unaffected by i:he addition of SiC.
Still_ furt=her, the addition of SiC to E-TFE was seen to improve the "buildability" of the electrostatica:Lly applied coating. Ten coats of neat E-TFE were required to achieve a 37 mil thick top coat whereas only 6 coats of 5 wt. % SiC containing E-TFE
were required ~~o achieve a 40 mil thick top coat, an improvement over nearly 180% (6.6 mils/coat vs. 3.7 mils/coat). The phenomenon observed appears to reside in the amount ~~f dry E-TFE powder that will adhere to 134101 fi ' the sample plate during electrostatic deposition.
SiC-containing E-TFE dry powder was observed to build to a greater depth than neat E-TFE dry powders. A
possible explanation for the observed phenomenon may be that the negatively charged resin powder insulates the relatively positively (i.e. grounded) charged piece being electrostatically coated. Once insulated, the charged workpiece cannot attract additional powder and, in fact, additional powder sprayed on the piece is repelled or simply falls away. The addition of SiC
to the resin powder may i.mpr_ove the powder conduc-tivity, thereby permitting a thicker layer of dry resin powder to be attracted to the substrate before reaching a thickness great enough to insulate the underlying substrate.
Overall, the addition of SiC to E-TFE coatings was seen to improve coating bond strength, dramatically improve corrosion resistance, markedly reduce coating shrinkage, significantly improve surface uniformity, and provide nearly a two-fold improvement in "buildability".
CORROSION-RESISTANCE DETERMINATION
FOR SiC-CONTAINING PVDF COATINGS
In the following examples, the corrosion resistance of SiC-containing poly(vinylidene fluoride) (PVDF) top coats applied over a Cr203-containing PVDF
"primer" coat is illustrated. Sample plates HH and II
were coated accorc.ing to the procedures already described for E-CTFE with the exception that after grit blasting, the plates were "baked" at 550°F (Step 12). Between sub~;equent: coating applications, the plates were returned to 500°F. Top coatings were applied before thE~ plates cooled below 350°F (Step 15).

?341p16 A 3-5 mil thick primer coat of KF Polymer poly-(vinylidene fluoride) (PVDF), a Kreha Corporation of America produci:, admixed with 5 wt. % chromium oxide (Cerac, Inc., Milwaukee, Wisonsin), was applied to both sample plates tested. To sample plates HH and II, were applied top coats of PVDF resin having in admixture therewith 0 and 5 wt. ~ SiC respectively.
Plates HH and II were concurrently subjected to Atlas cell- tesi~ing (20% HCl at 220°F) and observations of the coating:> made at 300, 600 and 1000 hour intervals. Each plate was spark tested at 20 KV (AC) and found free of pinholes. Results of Atlas cell tests are summ<~rized in Table 13 below.
1F~ Table 13 Sample Plate HH II
Primer Coat PVDF1 wt. % 95 95 Cr203 wt . 0 5 5 Particle Siz~a <_10u 510 Thickness 3-5 mils 3-5 mils Top Coat PVDF wt. % 100 95 SiC 0 5 Particle S i z a -- 5~C

No. of layers 3 4 Thickness (mil) 45 50 Atlas Cell Test 300 large (llmm) very small b:Lister (4mm) blister formed beginning to form 600 b:Lister enlarged blister to 15 mm causing enlarged d:isbonding in to 6mm area 1000 blister continues pinhole to enlarge developed in (23mm); pinhole 6mm blister in blister formed.
'PVDF KF Polymer poly(vinylidine)fluoride, Kreha Corporation of ~~meri.ca.
ZCr203, Cerac, Inc., P.O. Box 1178, Milwaukee, Wisconsin 53201 As Table 1:3 shows, sample plate II, having a 5 wt. % SiC/95 wt. % PVDF top coat, was much less susceptible to :glistering than sample plate HH
having a neat P'JDF top coat. The addition of SiC
to the PVDF top coat significantly reduced permeation by hst HC1 (20% HC1 C 220°F) as evidenced by the greatly reduced extent of blister-ing, notwithstanding the use of a Cr203-containing "primer" coat layer of PVDF.
BOND STRENGTH DETERMINATION FOR SiC AND/OR HIGH
PERFORMANCE THERMOPLASTIC-CONTAINING PFA COATING
In the examples which follow, sample plates MM, NN, 00, PP, QQ, FR and SS were prepared in order to compare bonf, strengths for PFA resin coatings having 0, 10 and 20 wt. % SiC; 20 wt.
polyphenylene sulfide (PPS); 20 and 15 wt.
polyetheretherketone (PEEK); and 10 wt. % SiC in admixture with 20 wt. ~ PEEK. All samples were t34t~t6 prepared in the same manner described earlier for PFA coatings. r,ond strength data for each are presented in Table 14 below.
5 Table 14 r' PFA COATING BOND STRENGTH TESTING
10 Primer Coat NCI NN 00 PP QQ RR' SS

PFA' 1000 90 80 80 80 85 70 SiC2 0 10 20 0 0 0 10 Thickness 4/19 4/21 4/19 4/33 4/31 3/23 4/26 20 (~ coats/mil) Bond Strength <5 10-15 <5 <5 >405 N. D.6 N. D.6 25 1PFA (perfluoroal_koxy resin) , NEOFLON AC-5500 PFA

resin, Daikin Industries, Osaka, Japan.

ZSiC (green silicon carbide flour) 39 CRYSTOLON

4647 (1000 grit; Norton Company, Worcester, Mass.

30 'PPS (polyphenylene sulfide resin) Ryton type V-1;

Philips Chemica_L Co., Bartlesville, Oklahoma.

4PEEK (polyetheretherketone) VictrexR 150 PF, Batch No. SP69-:L91P, ICI Americas, Inc., Wilmington, 35 Delaware 19897.

Sexceeded c:ohesi~re strength of coating.

6bond strength too great to initiate peeling.

40 'Plate RR was At7_ac cell tested for corrosion resistance (70 pat. . nitric acid at 225F) and, after 300 hours of te:~ting, shows no evidence of pinholes or blistering.

1341 p' 6 In comparing bond strength of PFA with the various additives above indicated, it is clear that 20 wt. % PEEK provides air least an 800 % increase in bond strength over noat coatings, 20 wt. % PPS and 20 wt.
SiC-containing fFA "primer" coatings, and a 3 to 4 fold increase over a :LO wt. % SiC-containing PFA
"primer" coating.
In the above described examples, microscopic examination of ~'PS and PEEK-containing PFA coatings shows that PPS remaina, for the most part, in its , particulate stage as discrete spheres in the PFA
resin. Where P3?S particles are in direct contact with the metal substrate, there is some evidence of PPS
flow at the point of contact.
PEEK particles, on the other hand, in the involved examples appear to flow to a greater extent at their point of contact with the metal substrate, appearing as rounded mounds rising from the substrate.
PEEK particles not in contact with the substrate form interconnected :strings, anchored to the mounds, and form a matrix through which the PFA resin flows.
Coating composites of about 10 to about 40 wt. %
PEEK/about 90 to about 60 wt. % PFA "primer" coat and about 2 wt. % SiC/98 wt. % PFA top coat provide a superior barrier resistant coating resulting from the vastly superior bond strength of the PEEK-containing PFA "primer" system and the corrosion resistance pre-viously demonstrated for a 2 wt. %-containing PFA "top coat".
In Table 14A below, bond strengths and Atlas Cell test results ar~~ presented for such composite coatings. The sample plates TT, UU, W, WW, XX, and YY were prepare~3 by applying 5 coats of a "primer"
coating composition of PFA resin and 5, 10, 20 and 40 1341 t11 6 wt% PEEK or 8 wt:% PPS, and 5 coats of a "top coat"
composition inc7_uding PFA resin and 2 wt% SiC. All samples were prepared in the same manner described earlier for PFA coatings.
Table 14A
COMPOSITE COATINGS
BOND STRENGTH AND ATLAS CELL TEST RESULTS
_ Sample uT U1:J W WW XX YY

Primer Coat PEEK' 0 ~0 20 0 40 0 ThlCkneSS 10/36 10/x.210/41 10/3910/36 10/43 COatS~ml_1 ) Bond Strength 24 38 >40 >40 >40 336 Corrosion Resistance?

(500 hrs) 0K$ OK OK OK OK OK

(1000 hrs) 0K OK OK OK NO9 NO

1PFA (perfluoroa7_koxy NEOFLON AC-5500 PFA
resin), resin, Daikin Industries,Osaka, apan.
J

2PFA (perfluoroaT_kox y resin), NEOFLON AC-5600 PFA

resin, Daikin Industries,Osaka, apan.
J

'PEEK (polyetherethe rketone) VictrexR
150 PF, Batch No. SP69-191P, :LCI Americas, Wilmington, Delaware Inc., 19897.

~ 34~ o t ~

4PEEK (polyethE:retherketone) VictrexR 450 PF, ICI
Americas, Inc., Wilmington, Delaware 19897.
SPPS (polyphenylene sulfide resin) Ryton type V-1;
Philips Chemical Co., Bartlesville, Oklahoma.
6Lower and higher concentrations of PPS result in lower bond strength.
7Atlas cell te~;ting conditions for corrosion resistance (70 wt. % nitric acid at 252°F).
BIndicates slight blanching but no blistering was detected.
9Indicates that slight blanching and either blistering or disbonding was detected.
PREPARATION OF SiC-CONTAINING PFA SHEETS
The object of the example which follows was to demonstrate the preparation of a sheet of SiC-containing PFA
resin.
The sheet was formed on a mild steel plate which had been cleaned according to the procedures described above, including grit blasting with 80 grit aluminum oxide grit, vacuum cleaning and baking in an oven for eight hours to drive contaminants from t:he steel. The surface of the cleaned plate was first sprayed with a heat stable release agent (Frekote*
33, a fluoropolymer product manufactured by Frekote, Inc. of 170 W. Spanish River Blvd., Boca Raton, Florida 33431) to permit a subsequently applied resin coating to be stripped cleanly away from the underlying substrate, then heated in an oven to between 680-700°F. The heated, release agent-treated plate was then sprayed with six 8 to 10 mil thick coats of a 2 wt.

*Trade-mark x - 73a -SiC/98 wt. % PFA dry powder mixture to give a total coating thickness of 60 mils. The plate was repeated to 680-700°F
between each coat. After the final coat was applied, the coated x metal plate was allowed to cool to ambient temperature and the coating stripped cleanly away from the substrate in thc~ form of a sheet.
Continuous sheet production is contemplated through the use of endless thin steel belts, treated with a suitable relea:~e agent, heated between resin coating applications by passing the belts through a series of ovens heated, for example, by both convection and infrared radiation, the resin compositions being applied through spray nozzles spaced between ovens.
Stainless steel belts, 18 to 24 gauge thick, manufactured by Sandv.ik Co. of 1702 Nevins Road, Fair Lawn, New Jersey 07410 would be suitable for this purpose.
PREPARATION OF ;>HAPED ARTICLES
ARTICLES OF SiC--CONTAINING PFA
The object of the example which follows was to demonstrate the preparation of a shaped article comprised of a :resin/ceramic powder mixture of the present invention.
In this example, a 3" pipe elbow having a 50 mil wall thickness ;end comprised of 98 wt. % PFA/2 wt.
SiC was prepared from a two-part mold assembled from a 3" carbon steel pipe elbow, cut in half along its length. The in;~ide surface of each pipe half was grit blasted with 80 grit aluminum oxide, the residue removed and the pipe halves baked in an oven for eight hours at 7G0°F to remove contaminants. After cooling the pipe halves to room temperature, they were re-as-sembled and the interior thereof coated with a release agent (Frekote 33) and the liquid excess removed.
The release agent-treated mold was returned to the oven and brought to a temperature of between 680 and 700°F after which the mold was removed from the oven and the interior thereof electrostatically sprayed with the 98 wt. o PF1~/2 wt. % SiC dry powder to form a coating thereon 8 to 10 mils thick. The thus-coated mold was return<~d to 'the oven and again heated to 680-5 700°F until the resin,/SiC powder had fused to a smooth and glossy film. The coating process was repeated until the coating was 50 mils thick. After cooling, the molded piece was tested for pinholes using a WEG 20 spark tester se-t to 50 KV (AC). No pinholes were 10 detected. The mold was disassembled and the PFA/SiC 3"
elbow removed from the mold.
It is cont~Amplated that the fluorocarbon polymer/additive coatings can be used in a variety of applications including those for which wear and load 15 resistance, corrosion resistance, and/or release characteristics is desired.
For example, composite coatings of the present invention may be applied to the chemical seal and drive portion of agitators commonly employed in chemical 20 vessels for mixing corrosive chemicals. Particularly useful in this regard are polyether/fluorocarbon polymer composites described above. Such composite coatings may also be applied to the tips of blades of such agitators which are subject to high abrasion and 25 wear.
Still further, composites of the present invention may be applied as coatings to metal roll surfaces of the type found on rol:Lers used in paper making, calendaring, and extrusion lamination, which rollers 30 are usually subjected to abrasion, wear, and high load.
Many of the conventional primer systems used with the application of fluorinated polymer coatings to metal substrates, include chemicals, such as for example chromic: oxide, which are considered detrimental to the environment and definitely are not approved for use with food stuffs :for human consumption. On the other hand PFA, PPS, and PVDF, PES and silicon carbide have been approved by the U.S. Food and Drug Administration as coating materials which can be used in the processing of food stuffs for human consumption.
Such approval h<~s likewise been given to many of the species of crysi:alline ceramics. Thus, the use of the barrier coating system of the present invention and the composites formed therewith exhibit an additional advantage wherein applied to process equipment used in the preparation of such food stuffs.

Claims (15)

1. A coating which fuses to form an adherent solid cohesive and non-porous, corrosion-resistant material at ambient pressure, the coating including a base coat and at least one top coat, said coating characterized by:
said base coat containing a mayor amount of a polyether resin;
said top coat containing a major amount of a fluorocarbon resin selected from the group consisting of (1) perfluoroalkoxy tetrafluaoroethylene copolymer resin (PFA), (2) ethylene-chlorotrifluoro-ethylene copolymer resin (E-CTFE), (3) ethylene-tetra-fluoroethylene copolymer resin (E-TFE), (4) poly-(vinylidine fluoride) resin (PVDF), (5) tetrafluoro-ethylene-hexafluorcpropylene copolymer resin (FEP), (6) poly(chlorotrifluoroethylene) resin (CTFE), or a mixture of two or more of said fluorocarbon resins; and also containing, as an additive, a ceramic powder, the ceramic powder being a metal carbide, silicon nitride, boron nitride, titanium diboride or aluminum diboride powder.

2. A coating according to Claim 1, in which the coating has a thickness of at least about 1 mm (40 mils).

3. A coating according to Claim 1 or 2, in which the base coat has a thickness of about 1 mil (0.03 mm) to about 25 mils (0.64 mm).

4. A coating according to any one of claims 1 to 3, in which the polyether and fluorocarbon resins have a particle size of about 20 to about 120 microns.

5. A coating according to any one of claims 1 to 4, in which the additive of the top coat further comprises poly(phenylene sulfide) (PPS).

6. The coating of any one of claims 1 to 5, wherein said base coat consists essentially of a polyether resin.

7. The coating of any one of claims 1 to 6, wherein said fluorocarbon resin consists essentially of PFA.

8. The coating of any one of claims 1 to 7, wherein said polyether resin contains PEEK.

9. The coating of any one of claims 1 to 8, wherein said top coat comprises about 1 to about 25 wt.% of said additive.

10. The coating of any one of claims 1 to 9, wherein said additive comprises about 0.5 to about 5 wt.% SiC.

11. The coating of any one of claims 1 to 10, further containing an additional top coat comprising a fluorocarbon resin and silicon carbide.

12. The coating of any one of claims 1 to 11, wherein said base coat consists essentially of a polyether resin, and said top coat comprises PFA, PVDF, E-CTFE or E-TFE.

13. The coating of any one of claims 1 to 12 fused to a metal substrate.

14. A method of using a coating according to any one of Claims 1 to 12 comprising fusing said base coat and then heating said top coat to fuse it to said base coat at a temperature for a period of time no longer than a predetermined period of time, said temperature being at least about 25°F
(13.8°C) below the temperature at which said fluorocarbon resin of said top coat in neat form can be fused completely by heating for no longer than said predetermined period of time without substantially degrading said fluorocarbon resin.

15. A method of forming a coating on a metal surface, which method comprises fusing a polyether-containing resin to said metal surface to form a base coat; fusing a top coat to said base-coated metal surface, said top coat containing (A) a fluorocarbon resin selected from the group consisting of (1) perfluoroalkoxy tetrafluoroethylene copolymer resin (PFA), (2) ethylene-chlorotrifluoro-ethylene copolymer resin (E-CTFE), (3) ethylene-tetra-fluoroethylene copolymer resin (E-TFE), (4) poly-(vinylidine fluoride) resin (PVDF), (5) tetrafluoro-ethylene-hexafluoropropylene copolymer resin (FEP), (6) poly(chlorotrifluoroethylene) resin (CTFE), or a mixture of two or more of said fluorocarbon resins; and, as an additive, (B) a ceramic powder, the ceramic powder being a metal carbide, silicon nitride, boron nitride, titanium diboride or aluminum diboride powder.