CA1105166A

CA1105166A - Fluoropolymer coating compositions having improved adhesion

Info

Publication number: CA1105166A
Application number: CA254,993A
Authority: CA
Inventors: Eustathios Vassiliou; Thomas P. Concannon
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1975-06-18
Filing date: 1976-06-16
Publication date: 1981-07-14
Also published as: NL7606653A; DE2627415A1; SE424556B; NL183947C; FR2316301A1; FR2316301B1; BE843149A; LU75180A1; DE2627415C2; NL183947B; JPS5214630A; SE7606783L; GB1563814A; US4011361A; JPS6011756B2; BR7603880A; IT1063379B

Abstract

ABSTRACT OF THE DISCLOSURE
Compositions containing fluoropolymer, colloidal silica, the salt of a polyamide acid, a coalescing agent and a liquid carrier are useful for priming metal, especially cookware, before topcoating it with a fluoropolymer enamel.

Description

BACKGROUND OF THE rNVENTION

In recent years, the use of fluoropolymers as lubricious coatings has become widespread. Cookware coated with polytetrafluoroethylene (PTFE) has become especially popular because of its non-stick cha,racter.

The physical nature o~ fluoropolymers makes it diffi-cult to bond them well enough to substrates to prevent coatlngs of such polymers from blistering and peeling away durlng use.
This is especially true of PTFE.

SUMMARY OF THE INVENTION

It has now been ~ound that the adhesion of a fluoro-polymer coating to its substrate can be signif~cantly improved if a composition of (a) fluoropolymer, (b) a binder comprising colloidal sllica and the amine salt of a polyamide acld, (c) a coalescing agent, and (d) a liquid carrier is used to prime the substrate before the ~luoropolymer topcoat is applied.
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DETAILED DESCRIPTION OF ~

; The fluoropolymers used in the compositions o~ the invention are homopolymers and copolymers (meaning the polymer contains two or more di~erent monomer units) of mono-ethylenically unsaturated hydrocarbon monomers completely ,: :

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5~6~i substituted with fluorine atoms or completely substituted with a combination o~ fluorine atoms and chlorine atoms. ~ncluded in this group are per~luorole~in polymers such as polytetra-fluoroethylene (PTFE) and copolymers of tetrafluoroethylene and hexafluoropropylene in all monomer unit weight ratios, fluorochlorocarbon polymers such as polymonochlorotrifluoro-ethyleneg and the normally solid copolymers o~ tetra~luoro-ethylene and perfluoroalkyl vinyl ethers (wherein the alkyl group is o~ 1-5 carbon atoms). Mixtures of these can also be used. PTFE is preferred ~or its thermal stability.

The ~luoropolymer used is particulate~ The particles are pre~erably small enough to pass through the nozzle of a spray gun without clogging it and are also small enough to give the resulting ~ilm integrity.

The fluoropolymer should have a number average mole-cular weight of at least about 20,000, for a polymer with a molecular weight of les~ than this tends to be waxy and unsuited for use. The number average molecular weight is pre~erably at least 200,000. Number average molecular weight of PTFE is measured by the method described by Suwa~g Takehisa and Machi in ~ournal of Applied Polymer Science, Volume 17, pp. 3253-3257 ~ (1973). The number average molecular weight o~ a tetra~luoro- ' ,~ ethylene~hexafluoropropylene copolymer ~s measured by first determining its melt ~low rate (MFR) according to ASTM D2116 using the ~ to determine melt viscosity (MV) according to ~the equation ~
, ~ , .
MV , pi x pressure used (mm) x ori~ice radius (mm) x ~rK--x oriI`ice le-~g~h~~~(mmj------- ~ ' - 3 - ~,-' . . ~ , .
.. . . .-and then determining molecular weight (MW)according to the equation . . .
MW = ~ 3 4 ~
V l.62 x l() -13--Number average molecular weight o~ a ~luorochlorocarbon poly-mer is measured by ASTM Dl4300 Although a powder of ~luoropolymer can be us0d and a carrier provided separately, a polymer in the ~orm of an aqueous dispersion is pre~erred for its stability and because it is most easily obt~ined in that form. Dispersions of fluoro~
polymers in organic liquids such as a~cohols, ketones, ali-phatic or aromatic hydrocarbons, or mixtures of these, can also be used. In either case, the liquid generally serves as the carrier ~or the composition.

The fluoropolymer is ordinarily present in the com-position at a concentration of about lO-90~, preferably about 60-80~, by weight o~ the total of fluoropolymer and binder (as will be de~ined later).
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The binder in the compositions o~ the lnventlon comprises a cDlloidal silica and the amine salt o~ a polyamide acid. This binder is ordinarily present in the composition at a concentration o~ about lO-90~ preferably about 20-40%, by welght o~ the total o~ fluoropolymer and binder.

The colloldal silica component o~ the binder can be an~ o~ those~which air dries to ~orm a continuous ~ilm when laid down by~itself. Such silicas are generally provided in the ~orm o~ aqueous sols containing aboub 30-50% o~ colloidal silica.

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Illustrakive o~ the colloidal sL:Llca 901s which can be used are those sold by E.I. du Ponk de Nemours and C'ompany as LUDOX HS-40~ I,IJDOX SM-3 LUDOX HS~ LUDOX TM~
LUDOX LS~ and LUDOX AS~ LUDOX AM~

The preferred colloidal silica sol is LUDOX AM~.
This product is a sodium stabilized colloidal silica having a pH of about 9 at 25C. J and a silica content (calculated as SiO2) of 30~, whose particles are surface modified with alu-minum and have a particle diameter of 13-14 millimicrons and a specific surface area of 210-230 square meters ~er gram.

Mixtures o~ colloidal silicaa can also be used.

The colloidal silica is ordinarily present in the binder at a concentration Df about 5-95~9 pre~erably about 20-80~, by weight o~ the binder.

Generally speaking) the polyamide acid salts used in the compositions of the invention are those of polyamide acids with tertiary amlnes. Such saltsJ which are further disclosed hereinafter~ are the subaect of Applicant's copen~ing appli-cation Seria~ No. 244 959, filed on February 3~ 1976~ of Thomas P. Concannon~ which corresponds to U.S. Patent 4 014 834J
`~ issued March 29J 1977.
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The polyamide acids can be repre~enked by khe structure > R \
-- HNC ~ CNH-Rl -~ _ ,. .-_ _ n where G is hydrogen or a car~oxyl ~roup;
de~otes i~omerism, R is a tetravalent organic radlc~l containing at least two carbo~ atoms, no more than two carbonyl groups of each polyamide acid unit being attached to any one carbo~
.: atom of ~aid tetravale~t radical;
Rl iS 2 divalent radlcal containin~ at least two carbon at~ms, the amlde groups of ad~acent polyamide acid units each bei~g attached ~o separa~e atoms o.~ said d~-valent radical~
a~d n :L~ a ~umber ~arge enough to give the poly-amlde acid an inherent viscos1t~ of at lea~t 0.1~ measured as a 0.5% solution i~ N~N -dimethyl acetamide at 30C.

These polyamide acids are ~eutralized with stoi~hio-metric amounts of ter~iary amlnes to glve the amine salts.

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The amine salts of polyamide acids pr~ferred ~or use in the binder component are those prepared from at least one amine represented by the structure H2N - Rl- NH2 and at least one acid anhydride represented by the structure O O O
,. , "
/C /C ~C\
HOOC - R O or O / R O
\C \C C
r~
10 0 o o where, in both structures, R and Rl are as pre~iously defined, neutralized with a stoichiometric amoun~ o~ a tertiary amine.

: The amine salts o~ polyamide acids especially pre-~erred ~or use in the binder component are those prepared from ~: :
(1) meta- or para-phenylenediamines~ bis(4-am~no-phenyl)ether, benzidine, 2,2-bis(4'-amino-phenyl) propane, bis(4-aminophenyl)methane, : bis(4-aminophenyl)sul~one, bis(4-aminophenyl) sulfide or 1,6-hexamethylenediamine;
and

(2) pyromellitic dianhydride,: trimellitic anhydride 2,2-bis(3',4'-dicarboxyphenyl)propane dianh~dride~
or bis(3~4-dicarboxyphenyI)ether dianhydride, . ~ . ~ . ~ , . . . .

neutralized with a stoichiometric amount of tri-methylamine, triethylamine, dimethylethanolamine, diethyl 2-hydroxyethylami~e~ tributylamine, tris(2-hydroxyethyl~amine, ethyl bis(2-hydroxyethyl) amine, N,N-dimethylaniline, morpholine, pyridine, N-methylpyrrole, or mlxtures of these.

The polyamide acid salt most preferred ~or use in the binder component is one prepared from (bis(4~aminophenyl) methane and trimellitic anhydride, neutralized with a stoichio-me~ric amount o~ a 2/1 weight rat~o mixture o~ triethylamineand diethyl 2-hydroxyethylam~ne.

The amine salt is ordinarily present in the binder at a concentration of about 5-95~, preferably about 20-80%~
by weight of the binder.

The coalescing agent used in the composition o~ the invention is, generally speaking, an organic liquid which can dissolve the polyamide acid amine salt used. Organic liquids o~ this type are highly polar and have boiling points above 100C. Illustrative ar~
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N-methylpyrrolidone dimethyl~ormamide dimethylacetamide dimethylsulfoxide cresylic acid sulfolane and ~ormamide.

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The coalesc~ng agent preferred ~or use because o~
its availability and low toxicity is N-methylpyrrolidoneO

The coalescing agent is ordinarlly present i~ the compositions o~ the invention at a concentration of about 10-90~o by weight of the polyamide acid amine salt used, pre-ferably about ~0-50%.

The compositions o~ the in~ention will have more workable viscosities at higher solids content i~ they contain about 15-20G~, pre~erably about 80-150~, by weight of the polyamide acid amine salt used, o~ a viscosity reducing agent which is an organic liquid whose solubility parameter~ ST~
falls within the range of 10-21.6 and which ls miscible with water. A de~inition o~ solubility parameter appears in Kirk-Othmer, "Encyclopedia o~ Chemical Technology"~ second edition (196~), pages 889-896. Total solubility parameter, T, has khe following component parameters:

Dispersion component, gD 7.0-10.0 ~olar component~ ~p 2.0-11.0 Hydrogen bonding component, ~ H 7.0-14.0 where the equ~tlon ~T2- ~DZ ~p2 ~ 2 describes the realtionship o~ the components to the total solubility parameter.
--Illustrative of such vlscosity reducing agents are furfuryl alcohol, l-butanol, 2-propanol, ethylene glycol ~-monoethyl ether, triethylene glycol~ formic ac~d, and mixtures o~ these. Furfuryl alcohol is pre~erred ~or its ef~ectiveness.
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The composltions of the invention can also contaln such c~nventional additives as pigments, flow control agents~
surfactants and plasticizers as are necessary or seem desirable.
These additives are added for the usual reasons, in the usual ways and in the usual amounts.

The amount of total solids in the coating composi-tions will be governed by the substrate to ~hich the composi-tions are to be applied~ ~he method of application, ~he curing procedures, and like ~actors. In the usual case, the composi-tion will contain about 10-80~, by weight, o~ solids.

The compositions can be made by simply mixing together appropriate amounts o~ the proper components. Pig-ments can be added, if this is desired, by ~irst convention-ally preparing a suitable pigment dispersion and then adding thi~ dispersion to the composition.

The resulting compositions can be applied by spray-ing~ brushing, roller-coating, dipping, or by electrodeposi-tlon. If the substrate is metal~ this is preferably pre-treated by grit blasting, by the ~lame spraying of metals or metal oxides, or by frit-coating, although the compositions can also be applied to phosphated and chromated metals. If the substrate is glass, it is pre~erably ~irst grit blasted or frit-coated.

A composition of the invention is ordinarily applied to a thickness o~ about 2-15 microns (dry) and is then air-dried. This primer coat is then topcoa~ed with a conventional ~ -clear or pigmented ~luoropolymer enamel, and baked in the usual way to give an adheren~ ~luoropolymer coating.

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', ~ ' ' . ' Although the best coating i6 obtained with the two-coat system just described, a strongly adherent fluoropolymer coati.ng can be obtained with a one-coat operation. To do this, a coat of a composition of the invention ls applied to a thick-ness of 5-50 microns (dry). This coat is then air-dried and baked at a temperature high enough to fuse the fluoropolymer used.

The compositions o~ the invention are most use~ul ~or priming metal cookware3 especially ~rypans~ before coat-ing them with PTFE~ but the compositions can also be used to prime other articles requiring tightly bonded fluoropolymer coatings. These other articles can be made of glass or any other material capable of withstanding the baking temperatures used. For example, the compositions can be used to prime or coat bearings~ valves, wire9 me~al foil, boilers, pipes, ship ;~ bottoms, oven liners, iron soleplates, wa~fle irons, ice cu~e trays, snow ~hovels and plows, chutes, conveyors, dies, tools such as saws, ~iles and drills, hoppers and other industrial containers and molds.

The ~ollowing examples illustrate the invention. In these examples, all parts and percentages are by weight unless indicated otherwise.

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The following were added to a ve~sel in the order stated and mixed:

1. The polyamide acid am.ine salt solution of~ Example 1 of appli-cation S.N. 244 959, filed - ~ebruary 3, 1976 (cor~taining 18 o~ fur~uryl alcohol and lG% ~:~
N-methylpyrolidone) 151.8 parts 2. Fur~uryl alcohol 15.2 parts

3. Deionized water 520 parks

4 . White pigment dispers:l on 52. 6 parts (Made by ball-milling a mixture of TiO2 46 parts Water 46 parts Triethanolamine/oleic acid mixture 2/18 parts)

5. PTFE dlspersion in water 222.5 parts (60% sol~d~) : 6. LUDOX AM colloidal silica sol29.8 parts The resulti~g composltion ~as sprayed on a grlt-blasted aluminum pan to a thickness o~ 5 microns (dry) a~d then air-dried. The pa~ ~as then topcoated by spraying it with TEFLON ~ enamel-Clas~ic White 455-102 (sold by E. I. du Po~t de Nemours and Co~pany) to a thickn~ss o~ about 100 micrvns (dry)3 baking lt ~or 15 ~ tes a~ 95C~ raising the baklng temperatur@ to 425C., and then remo~i~g the pan fr~m the oven.
The result$ng coating i st~ongly adherent to th~ -pan.

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Exa~e~

~he following were added to a vessel in the order stated and mlxed:

1, The polyamide acid amine salt solution of Example 156r 15 parts 2. Furfuryl alcohol 5.62 ~artY
3. Deioni~ed water192.33 parts 4. PTFE dispersion of Ex,a~ple 1 358.77 parts 5. LUDOX AM colloidal ~illca sol 201.28 parts

6. White pigment di~persion 121.44 parts (Made by ball-m~lling a mlxture of TiO2 44 parts Water 44 parts Triethanolamine/oleic acid mixture 2/1 12 parts) me resulting composition was applled to a *rit-coated aluminum pan3 topcoated and then cur~d as in Example 1, with substantially the saa~e results.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows

1. A coating composition comprising (a) about 10%-90%, by weight of the total of (a) and (b) solids, of a polymer of mono-ethylenically unsaturated hydrocarbon mono-mers completely substituted with fluorine atoms or a combination of fluorine atoms and chlorine atoms, the polymer having a number average molecular weight of at least 20,000;
(b) about 10%-90%, by weight of the total of (a) and (b) solids, of a binder which comprises (1) about 5-95%, by weight of the total of (1) and (2) solids, of a colloidal silica which air-dries to form a continuous film when laid down by itself, and (2) about 5-95%, by weight of the total of (1) and (2) solids, of a salt of a poly-amide acid represented by the structure where G is hydrogen or a carboxyl group;
denotes isomerism, R is a tetravalent organic raidcal con-taining at least two carbon atoms, no more than two carbonyl groups of each polyamide acid unit being attached to any one carbon atom of said tetravalent radical;

R1 is a divalent radical containing at least two carbon atoms, the amide groups of adjacent polyamide acid units each being attached to separate atoms of said divalent radical;

and n is a number large enough to give the polyamide acid an inherent viscosity of at least 0.1, measured as a 0.5% solu-tion in N,N -dimethyl acetamide at 30°C., neutralized with a stoichiometric amount of a tertiary amine;

(c) about 10-90%, by weight of the polyamide acid amine salt, of N-methyl pyrrolidone, dimethyl-formamide, dimethylacetamide, dimethylsulfoxide, cresylic acid, sulfolane or formamide;
and (d) a liquid carrier.

2. The composition of claim 1 additionally con-taining about 15-200% of an organic liquid, miscible with water, having a solubility parameter of about 10-21.6, the parameter having a dispersion component of about 7-10, a polar component of about 2-11 and a hydrogen bonding component of about 7-14.

3. The composition of claim 2 wherein the organic liquid is furfuryl alcohol, l-butanol, 2- propanol, ethylene glycol monomethyl ether, triethylene glycol, formic acid, or a mixture of these.

4. The composition of claim 1 wherein the colloi-dal silica is a sodium stabilized colloidal silica.

5. The composition of claim 1 wherein the salt of the polyamide acid is one prepared from at least one amine represented by the structure H2N - Rl- NH2 and at least one acid anhydride represented by the structure or where, in both structures, R and R1 are as defined in claim 1, neutralized with a stoichiometric amount of tertiary amine.

6. The composition of claim 5 wherein the salt of a polyamide acid is one prepared from (a) meta- or para-phenylene diamine, bis(4-aminophenyl) ether, benzidine, 2,2-bis(4'-aminophenyl)propane, bis(4-aminophenyl)methane, bis(4-aminophenyl)sul-fone, bis(4-aminophenyl)sulfide or 1,6-hexamethylene-diamine, and (b) pyromellitic dianhydride, trimellitic anhydride 2,2-bis(3',4'-dicarboxyphenyl)propane dianhydride or bis(3,4-dicarboxyphenyl) ether dianhydride, neutralized with a stoichiometric amount of a tertiary amine.

7. The composition of claim 1 wherein the fluoro-carbon polymer is PTFE.

8. The composition of claim 1 wherein the carrier is water.

9. The composition of claim 1 wherein (a) the fluorocarbon polymer is PTFE;
(b) the colloidal silica is a sodium stabilized colloidal silica;
(c) the salt of a polyamide acid is one prepared from trimellitic anhydride and bis(4-aminophenyl) methane, neutralized with a 2/1 weight ratio combination of triethylamine and diethyl 2-hydroxy ethylamine;
and (d) the liquid carrier is water.

10. The composition of claim 8 additionally con-taining about 15-200%, of furfuryl alcohol.

11. An article bearing a primer coat of the composition of claim 1, topcoated with a fused fluoropolymer enamel.

12. An article bearing a primer coat of the composition of claim 2, topcoated with a fused fluoropolymer enamel.

13. An article bearing a primer coat of the composition of claim 3, topcoated with a fused fluoropolymer enamel.

14. An article bearing a primer coat of the composition of claim 4, topcoated with a fused fluoropolymer enamel.

15. An article bearing a primer coat of the composition of claim 5, topcoated with a fused fluoropolymer enamel.

16. An article bearing a primer coat of the composition of claim 6, topcoated with a fused fluoropolymer enamel.

17. An article bearing a primer coat of the composition of claim 7, topcoated with a fused fluoropolymer enamel.

18. An article bearing a primer coat of the composition of claim 8, topcoated with a fused fluoropolymer enamel.

19. An article bearing a primer coat of the composition of claim 9, topcoated with a fused fluoropolymer enamel.

20. An article bearing a primer coat of the composition of claim 10, topcoated with a fused fluoropolymer enamel.

21. In a method for improving the adhesion of a fluoropolymer coating to its substrate, the improvement com-prising priming the substrate, before the fluoropolymer coat-ing is applied, with a composition according to claim 1.