CA1121930A

CA1121930A - Synergistic primer for fluoropolymer coatings

Info

Publication number: CA1121930A
Application number: CA000316417A
Authority: CA
Inventors: Alkis Christofas; Julius E. Dohany
Original assignee: Pennwalt Corp
Current assignee: Arkema Inc
Priority date: 1977-12-29
Filing date: 1978-11-17
Publication date: 1982-04-13
Also published as: GB2012793B; FR2413455A1; US4179542A; IT7852492A0; IT1158195B; GB2012793A; JPS6250510B2; NL7812597A; SE431554B; SE7813388L; DE2856676A1; JPS54100447A

Abstract

Abstract of the Disclosure A primer coating containing, as the resin binder, a thermoplastic halogenated (at least partly fluorinated) ethylene polymer and a curable epoxy resin, and, as a fil-ler pigment, finely-divided mica and metallic pigment pow-der, is disclosed herein. The primer coating gives fluoro-polymer top coatings which have been simultaneously cured therewith unexpectedly high resistance to blistering and delamination when exposed to extreme conditions such as live steam under high pressure.

Description

-~1 2~.~ 3~

Syne gi-stic Primer for Fluor~polymer Coatings This invention concerns a primer composition for fluoropolymer coatings which.comprises a thermoplastis halogenated (at least partly fluorinated? ethylene polymer, an epoxy resin, powdered metallic pigment and wet ground mica; a composite structure comprising a base, the primer coating and a fluoropolymer top coating; and a me~hod of coating employlng said primer composition and a fluoro-polymer top coating.

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~2~930 It is well known that vinylidene fluoride polymer based coatings on metal are resistant to weather and pro-tect the metallic substrate against corrosion. It is also known that coatings based on vinylidene fluoride polymers without primers will not adhere sufficiently to metallic substrates to resist humidity, to chemicals and high pres-sure live steam. To improve the adhesion of vinylidene fluoride polymer coatings to metal, a primer composition has been proposed in the U. S. Patent 3,111,426 which con-sists of an epoxy resin and from about 40% to about 60%
by weight of vinylidene fluoride polymer. Although this system performs remarkably well in many applications, it fails to maintain adhesion when coatings of vinylidene fluoride polymers are exposed for an extended period to such extreme conditions as exposure to hot chemicals and high pressure live steam~
Another excellent primer for vinylidene fluoride poly-mer coatings consists of vinylidene fluoride polymer and from about 15% to about 30% of 325 mesh water ground mica -by weight of solids dispersed in organic solvents. This primer is used for vinylidene fluoride polymer protective coatings on fluid handling equipment. ~owever, coatings with this primer also lose adhesion when exposed to ex-treme temperature conditions for an extended period.
It is known ~hat stainless steel ~stay/steel) pig-men~ powders, e.g~ a specially ground A.I.S.I. type 304 . . , f l~Z~93~11 stainless steel, protect coatings in which they are incor-porated against detérioration from fast or high tempera-ture rise. The metallic flakes of stay/steel provide good heat dissipation. The higher the concentration of pigment, the greater the heat dissipation.
It is also known that finely-divided mica is an ex-tremely functional pigment which increases the corrosion resistance of coatings when properly dispersed. The term mica has the greatest commercial value. Finally, as men-tioned above, it is known that epoxy type resins improve a &esion of vinylidene fluoride polymer coatings to metal-llc substrates.
Although the above additives each may improve some-properties of a vinylidene fluoride polymer coating, none of them as a single additive enhance the adhesion of viny-lidene fluoride polymer coatings suf~iciently to impart the necessary resistance from delamination when such coat-ings are exposed to live steam temperatures as high as 14~C (300~F). Accordingly, the principal object o~ this invention is to provide a primer composition that will se-curely bond fluorocarbon polymer coatings to metal surfaces when exposed to extreme temperatures.
The above and other objects are accomplished in ac-cordance with this invention which is a primer coating com-position comprising from about 34 to about 70 percent by weight of the solids in said composition of a resin binder ~ .

~lZ193~1 consisting of a thermoplastic resin selPcted from the group consisting of vinylidene fluoride homopolymers and copolymers of at least 40 mole percent of at least one fluorinated aliphatic monomer having a terminal ethylenic bond with at least one different aliphatic monomer having a terminal ethylenic bond, and from about 18 to about 45 percent, based on the weight of said thermoplastic resin, of a heat curable epoxy resin; and, as the remainder of said composition, a fil]er pigment consisting of finely-divided mica and from about 20 to about 40 percent, based on the weight of said mica, of metallic pigment powder.
The metallic pigment powder is preferably stainless steel pigment powder although other metallic powders and flakes, for example, aluminum, copper and copper alloys, e.g., bronze may be used. The primer coating is preferably dis-persed in a liquid carrier which may be water, an organic sol~ent or mixture of both.
The primer coating described above is effectively utilized in a heat cured composite structure comprising a) a solid base member, b) a primer coating of a thick-ness ranging from about 1 to about 10 mils adhering to at least one surface of said base member comprising a mixture of from about 34 to about 70 percent by weight of said mixture of a resin binder consisting of a thermoplastic resin selectPd from the group consisting of vinylidene fluoride homopolymer and copolymers of at least 40 mole percent of at least one fluorinated aliphatic monomer hav--~lZ~

ing a terminal ethylenic bond with at least one different aliphatic monomer having a terminal ethylenic bond, and from about 18 to about 45 percent, based on the weight of said thermoplastic resin, of an epoxy resin, and, as the remainder of said mixture, a filler pigment consisting of finely-divided mica and from about 20 to about 40 percent, based on the weight of said mica, of metallic pigment pow-der, preferably stainless steel pigment powder, and c) a top coating having a thickness of at least about 2 mils adhering to said primer coating, said top coating compris-ing a thermoplastic resin containing at least 50 mole per-cent of polymerized monomeric units which are identical to those of the thermoplastic resin of said resin binder.
Additionally, this invention concerns the method of making a co~posite structure wherein the primer coating is uniformly applied to a substrate or base having a suitably prepared surface, the primer is dried at low temperature, if applied from a liquid carrier, the proper fluoropoly-mer top coating is applied, and the primer coating and top coating are simultaneously heat cured at a temperature above the melting point of the fluoropolymer of the top coat whereby a pin-hole free surface having high resis-tance to blistering and delamination is established. More specially, the method of ma~ing a composite structure com-prises a) providing a clean, grease free solid substrate, b) coating the substrate with a liquid primer composition comprising from about 34 to about 70 percent by weight of ~ 9 3~

the solids in said composition of a resin binder consist-ing of a thermoplastic resin selected from the group con-sisting of vinylidene fluoride homopolymers and copoly-mers of at leas~ 40 mole percent of at least one fluori-nated aliphatic monomer having a terminal ethylenic bond with at least one different aliphatic monomer having a terminal ethylenic bond, and from about 18 to about 45 percent, based on the weight of said thermoplastic resin, of a heat curable epoxy resin; and, as the remainder of said composition, a filler pigment consisting of finely-divided mica and from about 20 to about 40 percent based on the weight of said mica, of metallic pigment powder, preferably stainless pigment powder, c~ drying the primer at temperature no greater than about 90Cs d) top coating the primer with a coating composition consisting essen-tially of a thermoplastic r~sin containing at least 50 mole percent of polymerized monomer units of the resin binder o~ said primer, and e) subjecting ~he top coating to a fusion temperature above the melting point but below the decomposition temperature of the top coat resin where-by a pin-hole free surface is produced.
The substrate or base for this invention preferably comprises a planar metallic surface but any solid clean, grease-free material is included, for example, sheet metal, cast metal, metal pipe and fittings, stone, slate, resin wood, ceramic glass and similar solid materials.

~2193~9 The fluoropolymer resins of the primer and top coat-ings described here includes at least two different fluo-rine-containing polymers each being normally solid resins preferably of high molecular weight. One such fluoropoly-mer contains 90 up to 100 mole percent polymerized vinyli-dene fluoride. Preferred examples of this fluoropolymer are vinylidene fluoride homopolymer, and copolymers of vinylidene fluoride with one or more of the following monomers: tetrafluoroethylene, trifluoroethylene and hexa-fluoropropene. The other fluoropolymer is a copolymer of at least 40 mole percent, and preferably no greater than ~0 mole percent of at least one fluorinated aliphatic monomer having a ~erminal ethylenic bond. Preferably,-the aliphatic monomer will have from 2 to 4 carbon atoms. Pre-ferred examples of this fluoropolymer are vinylidene fluoride copolymers mentioned above having from about 40 to 90 mole percent vinylidene fluoride, copolymers of te-trafluoroethylene with ethylene or propylene, copolymers of chlorotrifluoroethylene with ethylene and copolymers of tetrafluoroethylene with vinylidene fluoride. Some examples of other monomers which may be used as copoly-merizable components of these fluoropolymers include vinyl fluoride, vinyl chloride, vinylidene chloride, pentafluoro-propene, acrylic and methacrylic acid and lower alkyl esters the eof. Other copolymeriza~le aliphatic monomers having terminal ethylenic bonds which produce fluoropoly-mer resins having good melt flow properties for use in coatings can also be employed as minor copolymerized com-ponents.
The epoxy resin used in this invention is based on glycidyl polyether compounds derived from the reaction of epichlorohydrin with polyhydroxy compounds and mixtures of these reaction products with acrylic and/or methacrylic acid ester polymers. These products are commercially available under various trademarks and are very well kno~m to the art. Particularly useful polyether resins for this invention are reaction products of 2, 2 bis (4-hydroxy-phenyl) propane (known as bisphenol A) having an epoxide value from 0.5 to about 0.02 equivalents per 100 grams.
~ To effect bonding of the glycidyl polyether it is usually necessary to incorporate with the epoxy compounds certain functional compounds often mentioned as the cur-ing agents, which will react with the eopxide to form re-action products which will be mechanically strong, resis-tant to chemical attack and adherent to the substrate.
Examples of appropriate curing agents include: Hydroxy-pyridines, metal cyandiamide, and melamine; the preferred curing agent is dicyandiamide. The amount of curing agent employed may vary over a considerable range, such as from 1 to 100% by weight of ~he epoxide with the exact range depending on the particular type of curing agent selected.
With dicyandiamide as curing ag~nt concentration varies between 1% and about 25%, the preferred range being from about 4% to about 10% by weight of epoxide present.

:llZ193~

Mixtures of epoxide resins and methacrylic acid poly-mer as described in U. S. Patent 3,008,848 are included herein. These mixtures yield strongly adherent primer coatings to which thermoplastic and thermo~etting resins will firmly adhere. Such compositions are thermally stable at temperatures in excess of 300C and are there-fore highly desirable for use for the primer composition of the present invention. These mixtures contain from about 1% to about 98% by weight preferably from 10% to 50% by weight of an acrylic and/or methacrylic acid ester polymer. In general, epoxy resin composition which are useful to prepare the primers of this invention are dis-persions with long shelf life containing from about 10%
to about 50% solids preferably from about 20% to about 25% solids by weight.
The ingredients of the primer composition are prefer-ably dispersed in an organic liquid. Optionally water may also be used as the dispersing medium. The primer dispersions may contain from about 5% to about 60% by weight solids and are sprayable, brushable, or rollable at room temperature. Solvents suitable for the formula-tion of these primers are generally high boiling liquids.
The organic solvent which may be added to wet all ingre-dients of the primer and control the viscosity of the dis-persion are: toluene, xylene, dioxane, methyl ethyl ke-tone, methylisobutyl ketone, and methyl cellosolve acetate.

3n The amounts of solvents required for the preparation of the dispersion will depend upon the viscosity required for the intended application. In general, the viscosity of the dispersion is regulated with the level of solids.
The dispersions are prepared by methods well known in the art, that is, by first placing the epoxy resin so-lution in a high shear blending apparatus, often called a dissolver, and then other ingredients are added in the following order: wet ground mica, metallic pigment pow-der, fluoropolymer powder, and finally a liquid diluent is added to adjust viscosity and solids to the desired level.
The primer dispersion is then applied to a cleaned and degreased metal surface by spraying or any other me-thod conventional in the art. A wet lay-up of the primer should be so adjusted that the dry film obtained after the primer is dried will be between 1 and 10 mils, pre-ferably between l.S and 5 mils. The wet primer layer is ambient or warm air dried. The fluoropolymer top coat is then applied directly over the non-~used primer coat by any of the well known methods in the arts such as elec-trostatic spraying of powder and the entire primer-top-coat system is baked in one operation at temperatures above the melting point of the topcoat resin.
In the examples given below, to illustrate this in-vention in a non-limiting mannerJ the same general mode ~1~2~.~33~) of operation has been used throughout while exchangeing substrates, primer and exposure tests in order to empha-size the importance of the primer composition. The in-vention is applicable to other known methods of coatings applications from both liquid and powder based coatlngs of fluoropolymers.
Surface Preparation Metal surfaces were cleaned and degreased in usual manner. Two types of substrates were used in the follow-ing experiments:
1. 24 gauge, 4 inch x 12 inch, cold rolled steel panels were purchased from the Parker Company, De-. troit, Mich. 48220, which have Bonderite 37 finish (zinc phosphate conversion coating produced by Oxy Metal Finishing Corporation, a Division of Oxidental Petroleum Corporation).

2. The second type of samples were 8 inch x 8 inch x 1/4 inch hot rolled steel panels heated to 343C
(650~) before sandblasting to burn off all contami-nants from the metal's surface. Ths sandblasting was carried out using 100 mesh alumina with 100 psig air pressure until a satin gray finish was obtained.
The primer formulations were prepared as follows:
25% solids containing solution of epoxy resin was placed in a laboratory Waring blender, wet ground mica was added while the mixer was running at low speed, thereafter the desired weight of stay/steel flakes were added and the mixture was mixed for five minutes at high speed. Then KYNAR 301F powder (vinylidene fluoride homopolymer resin) was added slowly to the above blend with the mixer at low speed. When KYNAR powder addition was completed, the mixer speed was increased to high speed for about 3 to 10 minutes keeping the temperature of the dispersion be-low 100C(212F). The mixer speed was then reduced from high to low and methylcellosolve acetate was added to re-duce solids level to 40 wt. % non-volatile material. A
portion of the primer dispersion was applied by spraying on prepared metal surfaces. The primer coated panel was dried at room temperature over night to yield a primer film between 1.5 and 5 mils thickness.
The primed panels were topcoated with the electro-static spraying method using ~YNAR 960ES powder [cryo-genically ground vinylidene fluoride homopolymer resin, a product of Pennwalt CorporationJ. The electrostatic spray application was carried out at room temperature and the single layer topcoat was fused at 249C (480F) metal temperature for five to ten minutes. As soon as fusion was achieved, the part was removed from the oven, resprayed with powder while hot, and returned to the oven for anGther bake cycle. These cycles were repeated until the desired film thickness was achieved as shown in the following examples. ~pon fusion of the final layer, the part was removed from the oven and allowed to air cool.

The performance of coatings were evaluated in the following four tests:
l. Resistance to elevated temperature and 100% rela-tive humidity.
Test panels, with coated side down, formed a cover of a pan containing deionized water at constant level and at 82~C (180F). At this temperature the relative humidity in the environment in contact with the coated surface is at 100%. The panels were crosshatched to the metal and checked for blistering, rust propagation at the crosshatched area, or any other changes indicat-ing failure of the coating as a protective cover.
2. Resistance to boiling water and water vapors while the non-coated surface of the panel is exposed to am-bient temperature.
In this test the coated panels were clamped to the opposite ends of a standard 3 inch diameter. Pyrex "Tee" with the coated surfaces exposed to water and its vapor at the boiling point 100C (212F). The liquid level was at l/2 of the "Tee" diameter as to expose l/2 of the coating to liquid and l/2 of the coat to vapor. The third opening of the "Tee" was provided with a reflux condenser, temperature indica-tor, etc. The exposed panels were examined periodi-cally for loss of adhesion, blistering, or any sign of failure to protect the metal surface from corro-sion. It has been established that a total of about ~lZ~93 300 hours exposure to boiling water under the above conditions provides pertinent information on the effi-ciency of the coating to resist permeation and pro-tect adequately the metallic surfaces.

3. Resistance to boiling water, water vapors, and galvanic effect.
This test is carried out in the same equipment as described for test 2 except for the addition of a mag-- nesium rod which was inserted as an anode into the boiling water to provide cathodic protection. The anode is connected with an electrical conductor with the panels under test.

4. Exposure to live steam under pressure.
The coated panels are clamped to opposite sides of - a 6 inch diameter steel pipe spool fitted with appro-priate piping and controls to expose the coating to steam at 149C (300F) and 3.5 bars (50 psig) for 300 to 350 hours. The panels were examined for loss of adhesion, blistering or signs of substrate corrosion due to the penetrating potential of high temperatures and pressure steam.
Examples 1-16 Table 1 shows the examples, type of substrate, pri~er composition and topcoats of vinylidene fluoride polymers all sf which were prepared as described above to illu-strate this invention. Table 2 shows the results of the -~

3~3 tests which were conducted as described above.
The epoxy resin "252P" in all examples was a commer-cial product produced by the M & T Chemical Company, Rah-way, N. J. This is a mixture consisting of a polyepoxide compound based on bisphenol A, poly (methylmethacrylate), dicyandiamide, TiO2 and china clay pigment mixed in methyl ethyl ketone to obtain a dispersion containing about 25%
solids by weight.
Examples 17-24 ' These examples show that if one or more constituents of the synergistic primer composition is left out the coating will fail in at least one of the four tests des-cribed above. Table 1 shows composition of these examples while Table 2 shows the test results.
Exam~e 25 For comparison with the compositions of this invention, example #l of the U. S. Patent 3,111,426 has been repeated.
50 grams of KYNAR 301F powder (vinylidene fluoride homopolymer resin) was dispersed in 50 grams of methyl-ethyl ketone in a laboratory Waring blender. Then a mix-ture of 37.5 g of an epoxy resin (made by condensation of bisphenol with epichlorohydrin and having an epoxy equiva-lent of 191 - as Araldite 1010 - Ciba Corp.~ and 12.5 g of a polyamide resin having an amine value of 290 320 and a Brookfield viscosity at 75~C of 7-9 ~Versamid 125 -General Mills) was added and blended into the dispersion.

llZ~30 The resulting dispersion was sprayed 1) onto 4" x 8" cold rolled steel panels from Parker Company with a Bonderite 37 conversion coating to a thickness of .001"; 2) onto a sandblasted 8" x 8" x l/4" hot rolled steel panels to a thickness of .001". The coating was cured by first allow-ing the solvent to evaporate at ambient temperature and then holding the panels at 100C for five minutes after which the coating was still tacky.
Then the top coating of KYNAR 960ES powder (vinylidene fluoride homopolymer resin) was applied electrostatically and heat fused as previously described to a total thick-ness of .010". The coated panels were subjected to tests 1,.2 and 3. The results are shown in Table 2.

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~11 ~i O ~1 ~ ,~ ,~ ~ N ~ C~ C`l 1~2~3 Notes: 1. All primer application air dried.
2. Each topcoat application fused for 5 min.
at 249C (480F) (metal temperature).

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* Severe discoloration of the primer at the tempera-ture (250C) and residence time (7 min.) necessary for fusion and flow of the powder coating.
** Starting decomposition and outgassin~ of the primer at the temperature (250C-280 C) and the residence time (10-15 min.) necessary for fusion and flow of the powder on the 3/16" thick steel plate.
*** This test was only applicable to panels of type 2 substrate. The steam pressure in this test is 3.5 bars (50 psig) which requires the heavier steel panels of 1/4 inch thickness.

llZ~3~

SUPPLEMENTARY DISCLOSURE
. Additionally, it has been found that the incorpora-tion of small amounts of a fluxing agent consisting of a high boiling latent solvent for poly(vinylidene fluoride) in amounts of from about 3 to about 25 percent, preferably - about lO to about 15 percent based on the combined weight of the vinylidene fluoride polymer and epoxy resin (resin binder) increases the cohesive strength of the primer, enhances the bond of the resin-pigment, primer-metal and primer-topcoat interfaces.
Still further, it has been found that vinylidene fluoride polymer powders as disclosed herein for the top-coat can advantageously contain from about 5 to about 25 percent by weight one or more ~luxing agents as mentioned above based on the weight of the vinylidene fluoride poly-mer. The addition of the fluxing agent to the topcoat does not interfere with the free-flowing properties of the poly-mer powder and improves the melt flow, coalescing proper-ties of the powder when fused to provide topcoats of im-proved appearance and properties and, additionally permits the ùse of powder coating polymers of higher molecular weight.
The fluxing agents as employed herein are latent sol-vents which will lower the observed crystalline melting point (Tm) of poly(vinylidene fluoride) as measured by the ASTM Method D3418 at temperatures above 63C while having no significant effect on the polymer at temperatures below . ., _ _ .

112~93 this temperature. Examples of these fluxing agents in-clude dimethyl phthalate, diethyl phthalate, triethyl phosphate, dimethyl succinate, diethyl oxalate, tetraethyl urea, dimethyl adipate, diethyl adipate, isophorone, pro-pylene carbonate, Kodaflex triacetin, butyl cellosolve acetate, and diisobutyl ketone. The preferred fluxing agents will lower the Tm to less than 130C and boil from 160 to 300Co In preparing the primer coating composition contain-ing the fluxing agent, the coating ingredients were mixed as previously set forth herein for Examples 1-16~ Then the speed of the laboratory mixer was reduced from high to slow and the fluxing agent, for example, a blend of methyl cellosolve acetate and dimethyl phthalate, was added to reduce the solids level, e.g., to 45% of the nonvolatile materials in the mix. Typically the above methyl cello-solve acetate -dimethyl phthalate blend contained 10-15%
by weight dimethyl phthalate based on the combined weight of the poly(vinylidene fluoride) -epoxy resin solids in the primer.
Example 26 A primer coating composition containing a fluxing agent, prepared as described above but containing as the fluxing agent, 10% by weight of dimethyl phthalate based on the combined polymer-epoxy weight, was applied by spraying on the surface of carbon steel plates prepared as follows:
*Trade Mark ~ r-~1~12~93 An 8" x 8" x 3/16" hot rolled carbon steel panel was heated at 343C for 3 hours to burn any organic matter absorbed on the surface, sandblasted and coated by spray-ing,as described for Examples 1-16,with the primer con-taining the fluxing agent and~ after drying overnight had a coating thickness of .004 - .005 inches. The primed panel was topcoated with KYNAR 960ES powder as described for Examples 1-16 to provide a total topcoat thickness of .'~25 inch. This coated assembly was subjected to the lp steam test 4 as described herein-before except that the stea~ was at a temperature of 121C. After 360 hours ex-posure in the test no blistering or loss of ~dhesion was notedO After terminating this steam test the panel was subjected to a temperature recycling of 10 cycles, each cycle consisting of 24 hours at -18C and 5 hours at am-bient temperature. After this recycling procedure no blistering was noted and a force of 20 pol1nds per linear inch was necessary to remove the coating from the panel.
In comparison, considerable loss of a &esion was noted for a coated panel prepared in the same manner but wherein the primer lacked the addition of the fluxing agent, when this panel was subjected to the same 10 cycle test procedure after being subjected ~o ~he same steam test. The coating of this panel could be peeled off with a force of less than five pounds per linear inch.
*Trade Mark , ';~ - ` .

llZ193 Ex~nple 27 ~~
To demonstrate the additional advantageous effect of incorporating a fluxing agent in both the primer and top-coat 9 the following procedure was followed:
A panel was coated with a primer coating cont~ining fluxing agent as described in Example 26. A topcoat was prepared by blending KYNAR*961 powder [ground poly(vinyli-dene fluoride)resin having an initial melt viscosity of lp,OOO poise~ for three hours in a P-K blender with 15%
by weight of dimethyl phthal~te based on the weight of the KYNAR powder. The resulting free-flowing blend was densified by extruding at approximately 204C through a melt extruder apparatus and pelletized to provide peliets of about 1/16 inch diameter and length. The pellets were frozen in liquid nitrogen ~and ground by using a hammer -mill with a stream of liquid nitrogen to maintain the low - temperature during grinding. The resultant powder was classified by screening and the portion passing through a 170 mesh screen was used for electrostatic deposition on the primed panel as described for Examples 1-16 herein.
The topcoating was extremely smooth and no surface fissures or particle interface outlines could be observed even under lO~X magnification after exposure ~o steamO
Observation of such surface imperfections is usually the case with topcoatings not containing the fluxing agents after similar exposure to steam. The panel sample passed the temperature cycling procedure of Example 26 with imr *Trade Mark _ . . _ . .

: - 25 -.

proved maintenance of adhesion and, when the coating was . pried loose and removed, it left the surface of the steel panel clean without any sign of attack.

, .
~`` ~'

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A primer coating composition comprising from about 34 to about 70 percent by weight of the solids in said com-position of a resin binder consisting of a) a thermoplas-tic resin selected from the group consisting of vinylidene fluoride homopolymers and copolymers of at least 40 mole percent of at least one fluorinated aliphatic monomer hav-ing a terminal ethylenic bond with at least one different aliphatic monomer having a terminal ethylenic bond, and b) from about 18 to about 45 percent, based on the weight of said thermoplastic resin, of a heat curable epoxy re-sin; and, as the remainder of said composition, a filler pigment consisting of a) finely-divided mica and b) from about 20 to about 40 percent, based on the weight of said mica, of metallic pigment powder.

2. The primer coating composition of claim 1 wherein the metallic pigment powder is stainless steel pigment powder.

3. The primer coating composition of claim 1 wherein the thermoplastic resin is a vinylidene fluoride homopolymer or a copolymer of at least 40 mole percent vinylidene fluoride and a copolymerizable fluorinated aliphatic mono-mer having a terminal ethylenic bond.

4. The primer coating composition of claim 3 wherein the copolymerizable monomer is selected from the group consist-ing of tetrafluoroethylene, chlorotrifluoroethylene, tri-fluoroethylene, hexafluoropropene, vinyl fluoride and pentafluoropropene.

5. The primer composition of claim 3 wherein said curable epoxy resin is a glycidal polyether prepared from the re-action of epichlorohydrin and bisphenol A.

6. The primer composition of claim S wherein the compo-sition contains a curing catalyst for said epoxy resin in an amount sufficient to promote curing of said epoxy re-sin when said composition is heated to its curing tempera-ture.

7. The primer coating composition of claim 6 wherein the metallic pigment powder is stainless steel pigment powder.

8. The primer coating composition of claim 1 mixed with a liquid carrier.

9. The primer coating composition of claim 8 wherein said liquid carrier comprises an organic liquid.

10. The primer coating composition of claim 8 wherein said liquid carrier comprises water.

11. The primer coating composition of claim 9 wherein said organic liquid is a solvent in which said composi-tion is dispersed.

12. A heat cured composite structure comprising A) a solid base member, B) a primer coating of a thickness ranging from about 1 to about 10 mils adhering to at least one surface of said base member comprising a mixture of from about 34 to about 70 percent by weight of said mix-ture of a resin binder consisting of a) a thermoplastic resin selected from the group consisting of vinylidene fluoride homopolymer and copolymers of at least 40 mole percent of at least one fluorinated aliphatic monomer hav-ing a terminal ethylenic bond with at least one different aliphatic monomer having a terminal ethylenic bond, and b) from about 18 to about 45 percent, based on the weight of said thermoplastic resin, of an epoxy resin, and, as the remainder of said mixture, a filler pigment consisting of a) finely-divided mica and b) from about 20 to about 40 percent, based on the weight of said mica, of metallic pigment powder, and C) a top coating having a thickness of at least about 2 mils adhering to said primer coating, said top coating comprising a thermoplastic resin contain-ing at least 50 mole percent of polymerized monomeric units which are identical to those of the thermoplastic resin of said resin binder.

13. The composite structure of claim 12 wherein said solid base member is metallic.

14. The composite structure of claim 12 wherein said metallic pig-ment powder is stainless steel pigment powder.

15. The composite structure of claim 12 wherein the thermoplastic resin of the primer coating is a vinylidene fluoride homopolymer or a copolymer of at least 40 mole percent vinylidene fluoride and a copolymerizable fluorinated aliphatic monomer having a terminal ethylenic bond.

16. The composite of claim 12 wherein the copolymerizable monomer is selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, hexafluoropropene, vinyl fluoride, pentafluoropropene.

17. The composite of claim 12 wherein said epoxy resin is a glycidal polyether resulting from the reaction of epichlorohydrin and bisphenol A.

18. The composite of claim 12 wherein said base member is a planar metallic base, said thermoplastic resin of the primer coating is vinylidene fluoride homopolymer and said epoxy resin is glycidal polyether resulting from the reaction of epichlorohydrin and bisphenol .

19. A method of making a composite structure comprising a) providing a clean, grease free solid substrate, b) coating the substrate with a liquid dispersion of a primer composition as described in claim 1, c) drying the primer dispersion at a temperature no greater than about 90°C, d) topcoating the primer with a coating composition com-prising a thermoplastic resin containing at least 50 mole percent of polymerized monomer units identical to those of the resin binder of said primer, and e) subjecting the top coating to a fusion temperature above the melting point, but below the decomposition temperature of the top coat resin whereby a pin-hole free surface is produced.

20. The method of claim 19 wherein the primer composition is that described in claim 6.

21. The method of claim 19 wherein the primer composition is that described in claim 3.

22. The method of claim 19 wherein the primer composition is that described in claim 7.

23. The method of claim 22 wherein the top coat is applied as a dry fine powder.

24. The method of claim 23 wherein the substrate is a planar metal.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE:

25. The primer coating composition of claim 1 additionally containing from about 3 to about 25% of a fluxing agent based on the combined weight of said resin binder.

26. The primer coating composition of claim 7 additionally containing from about 10 to about 15% of a fluxing agent based on the weight of said resin binder.

27. The composite structure of claim 12 wherein the primer coating additionally contains from about 3 to about 25% of a fluxing agent based on the weight of said resin binder.

28. The composite structure of claim 12 wherein said top-coating contains from about 5 to about 25% of a fluxing agent based on the weight of said thermoplastic resin.

29. The composite structure of claim 27 wherein said top-coating contains from about 5 to about 25% of a fluxing agent based on the weight of said thermoplastic resin.

30. The method of claim 19 wherein the primer composition is that described in claim 26.

31. The method of claim 30 wherein the top coat is applied as a dry fine powder.

32. The method of claim 31 wherein the substrate is a planar metal.