CA1105168A

CA1105168A - Fluorocarbon primer having improved scratch resistance

Info

Publication number: CA1105168A
Application number: CA257,996A
Authority: CA
Inventors: Eustathios Vassiliou
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1975-07-31
Filing date: 1976-07-28
Publication date: 1981-07-14
Also published as: BE844759A; IT1064694B; NL182001B; JPS5218740A; FR2319695A1; BR7604973A; LU75497A1; FR2319695B1; JPS6035380B2; DE2634422A1; NL7608510A; DE2634422C2; SE7608573L; GB1565842A; SE426502B; NL182001C

Abstract

ABSTRACT OF THE DISCLOSURE
Compositions containing (a) fluoropolymer, (b) colloidal silica, (c) the salt of a polyamide acid, (d) a coalescing agent, (e) mica particles, mica particles coated with pigment, or metal flake, and (f) a liquid carrier are useful for priming metal, especially cookware, before topcoating it with a fluoropolymer enamel.

Description

BACKGROUND OF rrHE I~ENTION
Cookware coated with ~luoropolymers" especially polytetrafluoroeth~lene (PTFE), has come into wldespread use in recen-t years. ~ny cooks pre~er to use such cook-ware because of its non-stlck prope:r t:Les and because it is so easy to clean.
It is w~ll known that the adheslon of fluoropolymer~
to metal is of a rather low order3 so in the fabricatlon of such cookware it is customary to improve adhesion by applying a primer composit~on to the metal before applying the ~luoro-polymer topcoat. Such a primer composition is described ln Canadian application Serial No. 254 993 o~ E. Vassiliou and T.P. Concannon~ filed June 16, 1976. m e composition described there comprises (a) a ~luoropolymer, (b) a binder comprising colloidal silica and the amine salt of a polyatnide acid, ( c ) a coalescing agen~, and (d) a llquid carrier.
Whi le cookware prlmed with such a composition is quite durable3 its entire ~luoropolymer coati~ tends to become scratched after extended use~
SUMMAR~ O~' THE INVENTION
It ha~ ~ow been found that the scratch resistance of such a coating is improved i~ the primer composition contains mica particles~ mi.ca particles coated with pigment, or metal ~lake .

-, :-9~y~LL~ .5 ~IF~ION OF ~IE INVENTION

The mica particles, coated mica particles and metal flake used in the compositions o~ the invention can be any of those available commercially. These particles and flakes have an average longest dimension of 10 to 200 microns, preferably 15-50 microns~, with no more than 50%
of the particles oP ~lakes having longest dimensions of more than about 500 microns. Particle and ~lake size ls ~easured optically against a standard.

The mica particles coated with pigment pre~erred for use are those described in U.S. Patent 3,087,827 to Klenke and Stratton3 and U.S. Patents 3Jo87J828 and 3,087,829 to Linton.

The micas described in these patents are coated with oxides or hydrous oxides o~ titanium, 3i rconium, aluminum, zinc, antimony, tin~ iro~ copper, nickel3 cobaltJ chromium or vanadium. Titanium dloxide coated mica is preferred because o~ its availabilitg. Mixtures o~ coated micas can also be used.

Representative of the metal ~lake which can be used are aluminum PlakeJ stalnle~ steel Plake, nickel ~lake and bronze flake. Mixtures of flake can also be used.

The ~ica, coated mica or metal ~lake is ordinarlly present in a composition of the invention at a concentration ~ ' .

of about 0.2-20%, by weight of the to-tal solids, preferably about 2~15~, even more preferably about 5-12%. "Total solids" means -the sum of fluoropolymer, mica, coated mica or metal flake, polyamide acid salt, pigment and colloidal silica content.
The fluoropolymers used in the composition of -the invention are homopolymers and copolymers (meaniny the polymer contains two or more different monomer units) of monoethy-lenically unsaturated hydrocarbon monomers completely substi-tuted with fluorine atoms or completely substituted with a combination of fluorine atoms and chlorine atoms. Included in this group are perfluorolefin polymers such as polytetrafluoro-ethylene and copolymers of tetrafluoroethylene and hexafluoro-propylene in all monomer unit weight ratios, fluorochlorocarbon polymers such as polymonochlorotrifluoroethylene, and the normally solid copolymers of te-trafluoroethylene and perfluoro-alkyl vinyl ethers (wherein the alkyl group is of 1-5 carbon atoms). Mixtures of these can also be used. PTFE is preferred for its thermal stability.
The fluoropolymer used is particulate. The particles are preferably small enough to pass through the nozzle of a spray gun without clogging it and are also small enough to give the resulting film integrity.
The fluoropolymer preferably has a number average molecular weight of at least about 20,000, even more preferably at least about 200,000, because such fluoropolymers give harder finishes. Number average molecular weight of PTFE is measured ~y the method desc:ribed by Suwa, Takehisa and Machi in Journal of Applied Polymer Science, Volume 17, pp. 3253-3257 (1973).
The number average molecular weight of a tetrafluoroethylene/
hexafluoropropylene copolymer is measured by first determining its melt flow rate (MFR) according to ASTM D2116, using the MFR

~f~

~ ' ' :

$~1 to determine melt viscosity (M~) according to the equation MV - pi x pressure used (mm) x orifice radius (mm) 8 x MFR x orifice length (mm) and then determining molecular weight (MW) according to the equation r ~, MW = ~3.4/ MV
1.62 x 10-13 Number average molecular weight of a fluorochlorocarbon poly~
mer is measured by ASTM Dl430.
Although a powder of fluoropolymer can be used and a carrier provided separately, a polymer in the form of an aqueous dispersion is preferred for its stability and because it is most easily obtained in that form.
The fluoropolymer is ordinarily present in the composition at a concentration of about 10-90%, preferably about 60-80%, by weight of the total of fluoropolymer and binder (as will be defined later.) The binder in the compositions of the invention comprises a colloidal silica and the amine salt of a polyamide acid. This binder is ordinarily present in the composition at a concentration of about 10-90%, preferably about 20-40%, by weight of the total of fluoropolymer and binder.
The colloidal silica component of the binder can be any of those which air-dries to form a continuous film when laid down by itself. Such silicas are generally provided in the form of aqueous sols containing about 30-50%~ by weight, of colloidal silica.

~ ' ~ 1~4~

Illustrakive of the colloidal silica sols which can be used are those sold by E.l. du P~nt de Nemours and Company as LUDOX* HS-40 I,UDOX SM-30 LUDOX HS LUDOX TM
LUDOX LS and LUDOX AS LUDOX AM

The preferred colloidal silica sol is LUDOX AM.
This product is a sodium stabiliæed colloidal silica havin~ a pH of about 9 at 25C., 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 milllmicrons and a specific surface area of 210-230 square meters per gram.

Mixtures of colloidal silicas can also be used.

The colloidal silica ls ordinarily present in the binder at a concentration of about 5-95%~ preferably about 10-80%, by weight of the binder.

Generally speaking, the polyamide acld salts used in the compositions of the invention are those of polyamide acids with t,ertiary amines. Such salts, which are further disclosed hereinafter, are the subJect of Applicant's copending appli-cation Serial No. 244 9599 filed on February 3, 19769 of Thomas P. Concannon, which corresponds to U.S. Patent 4 01~ 83~, issued March 29~ :L977.

* denotes trade mark : .

The polyamlde acids can be represented by the struc kure G COOH
\ R
~INC ~ CNH-Rl--~.
". "
O O

where G ls hyd~ogen or a carboxyl group;
~ denotes isoDIerism;
R is a tetrav~lent organic radical containing at least t~o carbon atoms, no more than two carbonyl groups of each polyamide acid tmit being attached to any one carbon atom o~ said tetravale~t radical;
R is a divaler~t radical contairling at leas~
two carbon atoms, gh~ amide groups o~
ad~acent polyamide acid units each being attached to separate atoms of s~id di valent radical;
and n is a nu~iber large enough to give the poly-amide acid an ir~herent viscosiky of at least 0.1, m~a~ured a~ a 0. 5~ solution Lll N,i~ -dimethy1 acetamide at 30C.
These polyamide ac:i ds are ~eutr~liæed with ~toichio metric amounts of tertiary amir~es to give th~ amine salts.

.
. ~ .. . ;

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The amine salts of polyamide acicls preferred for use in the binder component are those prepared from at least one amine represented by the structure H2N ~ R1 N 2 and at least one acid anhydride represented by the structure O O o .. .. ..
/c \ /c\ /c \
HOOC - R \ O orO\~ R /O

ll ll ll O O O
where, in both structures, R and R1 are as previously defined, neutralized with a stoichiometric amount of a tertiary amine.
The amine salts of polyamide acids especially pre-ferred for use in the binder component are those prepared from (1) meta- or para-phenylenediamines, bis(4-amino-phenyl3ether, benzidine, 2,2-bis(4'-amino-phenyl) propane, bis(4-aminophenyl)methane, bis(4-aminophenyl~sulfone, bis(4-aminophenyl) sulfide or 1,6-hexamethylenediamine;
and

(2) pyromellitic dianhydride, trimellitic anhydride - -2,2-bis(3',4'-dicarboxyphenyl)propane dianhydride, :~ 20 or bis(3r4-dicarboxyphenyl)ether dianhydride, - 8 - ~ :

~ .

neutralized with a stoichiometric amount of trimethylamine, triethylamine, dimethyl-ethanolamine, dlethyl 2-hydroxyethylamine, tributylamine, tris(2-hydroxyethyl)amine, ethyl bis(2-hydroxyethyl)amine, N,N-dimethyl-aniline, morpholine, pyridine, N-methylpyrrole, or mixtures of these.
The polyamide acid salt most preferred ~or use in the binder component is one prepared from (bis(~-aminophenyl) methane and trimellitic anhydride, neutralized with a stoichio-metric amount of a 2/1 weight ratio mixture of trie-thylamine and diethyl 2-hydroxyethylamine.
The amine salt is ordinarily present in the binder at a concentration of about 5-95%, preferably about 20-90%, by weight of the binder.
The coalescing agent used in the composition of the invention is, generally speaking, an organic liquid which can dissolve the polyamide acid amine salt used. Organic liquids of this type are highly polar and have boiling points above 0 100C. Illustrative are N-methylpyrrolidone dimethylformamide dimethylacetamide dimethylsulfoxide cresylic acid sulEolane and formamide.

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- .' , The coalescing agent preferred for use because of its availability and low toxicity is N-methylpyrrolidone.
The coalescing agent is ordinarily present in the compositions of the invention at a concentration of about 10-90% by weight of the polyamide acid amine salt used, pre-ferably about 20-50~.
The compositions of the invention will have more workable viscosities at higher solids content if they contain about 15-200%, preferably about 80-150%, by weight of the polyamide acid amine salt used, of a viscosity reducing agent which is an organic liquid whose solubility parameter, ~T~
falls within the range of 10-21.6 and which is miscible with water. A definition of solubility parameter appears in Kirk-Othmer, "Encyclopedia of Chemical Technology", second edition (1963), pages 889-896. Total solubility parameter, T, has the following component parameters:
Dispersion component, ~D 7.0-10.0 Polar component, ~p 2.0-11.0 Hydrogen bonding component, ~H 7.0-14.0 where the equation T2 = ~D2 ~p2 ~H2 describes the relationship of the components to the total solubility parameter.
Illustrative of such viscosity reducing agents are furfuryl alcohol, 1-butanol, 2-propanol, ethylene glycol monoethyl ether, triethylene glycol, formic acid, and mixtures of these. Furfuryl alcohol is preferred for its effectiveness.

.

.,,~.

- : .
- . ~ :
- - - .

The compositions of the ~lvention can also contain such conventional additives as pi~ments, flow control agents, surfac-tants and plas-ticizers as are necessary or seem desirable. These additives are added for the usual reasons, in the usual ways and in the usual amounts.

The carrier used in the compositions of the invention can be any o~ those no~mally used in composi-tions of this type. It is an inert~ not a reactive or functional (in the sense of contributing to the working of the invention) component and its nature is therefore o~ secondar~ importance. The carrier serves only to keep the composition homogeneous and acts as a mechanism for conveying the solid components to the substrate. After a coating has been deposited, the carrier evaporates. It is necessary, therefore, only that it be compatible with the other components of the composition and that it have no adverse effects on the coating itsel~.

` Commonly, the fluoropolYmer used in the comPo-sition is provided in the ~orm of an aqueous dispersion, and the water introduced into the composition with the polymer also serves as all or as part of the carrier for the composition. Organic liquids such as alcohols~ ketones, aliphatic and aromatic hydrocarbons, or mixtures o~ these, can aleo be used :

- ~' , ~ 5 ~

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~ the mekhod o~ application~ the curing procedures, and like ~actors. ~n the usual case, the composi-tion will contain about 10-80~, by weight, of solids.

The compositions can be made by simply mixing together appropriate amo~mts of the proper components. Pig-ments can be added, i~ this is desired, by flrst convention-ally preparing a suitable pigment dispersion and then adding this dispersion to the composition.

The resulting compositions can be applied by spray-ing~ brushing, roller-coating, dipping, or byUelectrodeposi-tion. The substrate is pre~erably pretreated by grit blasting, b~ the flame spraying o~ metals or metal oxides~ or by frit-coating, although the compositions can also be applied to phosphated and chromated metals.

A composition of the invention is ordinarily applied to a thickness of about 2-15 microns (dry) and is then air-dried. This primer coat is then topcoated with a conventional clear or pigmented fluoropol~mer enamel and baked in the usual way to give an adherent fluoropolymer coatingD

Although the best coating is obtained with the two-co&t system just described, a strongly a~herent fluoropolymer co~ting can be obtained with a one~coat operation. To do this~
a coat of a composition of the invention is applied to ~ 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.
: :

- ~2 --. .. . ~
. ~ . . .
....... : . . . .

5~

The compositions of the invention are most use~ul for priming metal cookware, 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 o~ glass or any other material capable of withstanding the baking temperatures used. For example, the compositions can he used to prime or coat bearings, valves, wire, metal foil, boilers, pipes, ship bottoms, oven liners, iron soleplate, waffle irons, ice cube trays, snow shovels and plows, chutes, conveyors, dies, tools such as saws, files and drills, hoppers and other industrial containers and molds.
The following example illustrates the invention.
In this example, All parts and percentages are by weight unless indicated otherwise.

'.~;

', EXAMPLE
_____ The ~ollowing were added to ~ vessel in the order given and mixed:
(A) (1) m e polyamide acid amine sal~
solution of Example 1 of appli-cation S.N. 244 959, filed February 3, 1976 (containing 18~ of ~urfu~yl alcohol a,nd 10~ o~ N-methyl pyrrolidone)31.98 parts (2) Furfuryl alcohol 3.21 parts

(3) Deionized water 109~47 parts (L~) Pigment dlspersion 27.39 parts - Made by ball-milling a mlxture of Cobalt oxide 45 parts Water 55 parts (5) TiO2 coaked mica 7.6 p~rts ("Af~lalr" ~ NF-152-DJ
sold by E.I. du Pont de Nemours and Company) Component (5) ~s added with vigorous mixing. A~ter ~he addition was complete, the product was stirred ~or 10 minutes. To it were then added3 with mixing (6) PTFE dispersion 91.17 parts (60~ solid~ in water3 "Te~lon" ~ TÆ-Fluorocarbon Re~in, Aqueous Disperslon, T-30~ sold by E.I. du Pont de Necnours a~d Compa~y) 3 (7) "Ludox" ~ AM 36.78 parts .

. .

The resulting primer composi tlon was ~prayed on a degreased aluminum parlel to a thickness o:~ a~out 8 microns (dry) and air-dried.
A topcoat composltion was prepared by adding the following to a vessel in the order stated, w~th mixing:
(B) (8) The PTFE dispersion of179.7 parts ( 6 ~ above (9) Deionized water 2.7 parts (10) T102 coated mlca of (5) above 9.5 p~rts ( 11) Pigment dispersion of 3 . 5 parts

(4) above ( 12 ) Mixture OI
Toluene11. 24 parts Txiethanolamine 14.78 parts Butyl carbitol 4.27 parts Olelc acid4.72 parts Cerium octoate 2.88 parts solutlon ~12~ in 2-ethylhexanoic acid) ~RITON* X-100 1.61 parts (non-ionic sur~actant sold by Rohm ~ Haas Co.) (13) Methyl methacrylate/
eth~l Qcrylate/meth acrylic acid 39/57/
4 terpolymer dispersion (40% in water) 33.4 part6 The resulting topcoat composition was sprayed to a thickness o~ about 20 micro~s on the panel prlmed in (A). The panel was then baked ~or flve m~nutes at.425C.
The resultlng ~inlsh showed signL~icantl~ better scratch resistance than the same o~e lackln~ the coated mica compon~nt.
~ enotes trade mark - 15 -

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 radical 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 group 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-dimethylacetamide 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;
(d) about 0.2-20%, by weight of the total of (a) and (d), of mica particles, mica particles coated with pigment, or metal flake;
and (e) 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, 1-butanol, 2-propanol, ethylene glycol monomethyl ether, triethylene glycol, formic acid, or a mixture of these.

4. The composition of Claim 1 wherein the colloidal 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 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-hexa-methylenediamine, and (b) pyromellitic dianhydride, trimellitic anhydride 2,2-bis(3',4'-dicarboxyphenyl) ether 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 the (d) component is mica particles coated with TiO2.

10. The composition of Claim 1 comprising (a) PTFE;
(b) a sodium stabilized colloidal silica;
(c) the salt of a polyamide acid prepared from trimellitic anhydride and bis(4-aminophenyl) methane, neutralized with a 2/1 weight ratio combination of triethylamine and diethyl 2-hydroxy ethylamine;
(d) mica coated with TiO2;
and (e) water as a carrier.

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

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

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

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

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

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

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

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

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

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

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

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

23. 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.