CA1105167A

CA1105167A - Fluoropolymer coating compositions having improved adhesion

Info

Publication number: CA1105167A
Application number: CA257,232A
Authority: CA
Inventors: Thomas P. Concannon; Eustathios Vassiliou
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1975-07-21
Filing date: 1976-07-19
Publication date: 1981-07-14
Also published as: US4016125A; US4070525A; SE426500B; FR2318912B1; JPS6035379B2; NL181438B; JPS5213532A; BR7604697A; FR2318912A1; LU75418A1; BE844356A; NL181438C; GB1563815A; DE2632884A1; DE2632884C2; SE7608258L; IT1062136B; NL7608076A

Abstract

ABSTRACT OF THE DISCLOSURE
Compositions containing fluoropolymer, alkali metal silicate, the suite 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 INVENT.ION

- In recent yearæ, the use of ~luoropolymers as lubricious coa~ings has become widespread. Cookware coated with polytetra~luoroeth~lene (PTFE) has become especially popular because o~ its non-stick characterO

The physical nature o~ fluoropolymers makes it diffi-cult to bond them well enough to substrates to preven~ coat~ngs of æuch polymers ~rom blistering a:nd peeling away during u~e.
Th~s is cspecially true o~ PTFE.

10 ' ~3~

It has now been ~ound that the adhesion o~ a ~luoro-polymer coating to its substrate can be signi~icantly impro~ed ~f a composit~on o~
(a) ~luoropolymer~
(~) a binder comprising an-alkall meta~ silicate and ~he amine salt of a polyamide acid, (c) ~ coalescing agen~, and (d) a l~q~id carrier .
: 20. is used to prime the ~ ~trate before the ~luoropolymer topcoat ~ is applied~

:; DETAIIED .DESCRIPTION OF THE_INVENTION

~; The fluoropolymers used in the composltions of the inYention are homopolymers and copolymsrs: (meaning the polymer , g contains:~wo or more di~erént monomer units) ~f~mono-ethylenically unsaturated hydrocarbon monomer~ completely ~:~ 2 , -substituted with ~luorine atoms or completely 6ubskituted with a combination of ~luorine atoms and chlorine atoms. ~ncluded in this group are per~luorole~in polymers such as polytetra-~luoroethylene (PTFE~ and copolymers o~ tetrafluoroethylene and hexa~luoropropylene in all monomer unit weight ratios~
~luorochlorocarbon polymers such as polymonochlorotrifluoro-ethylene, and the normally solid copolymers o~ tetra~luoro-ethylene and per~luoroalkyl vinyl ethers (wherein the alkyl group ls of 1-5 car~on atoms). Mlxtures of these can also be used. PTFE is pre~erred ~or its thermal stability.

The ~luoropolymer used is pa~ticulate~ The particles are preferably small e~ough to pass through the nozzle o~ a : epray gun without clogglng it and are also small enough to give the resulting fllm integrity.

The ~luoropolymer should have a number a~erage mole-cular welght of at least about 20,000, for a polymer with a molecular weight of less than this tends to be waxy and unsuited ~or use. The number ave~age molscular weight is preferably at least 200~000. Number average molecular weight o~ PTFE ls measured by the method described b~ Suwa9 Takehisa and Machi in Journal o~ Applied_Polymer Science, Volume 17, pp. 3253-3257 . (1973~. The number average molecular weigh~ o~ a tetrafluoro-: ethylene/hexa M uoropropylene copolymer is measured by ~irst determining its melt ~low rate (MFR) accord~ng to ASTM D2116, using the ~FR to determine melt viscosity tMV) according to the~e~uation .
MV ~ pi x pressure used (mm) x ori~ice radius (mm) ; , ' , ~ .
.

and then determining molecular weight (MW)accordlng to the equation ., ~

MW = ~ 3-1~ ~ MV -~ -V 1062 x ~

~umber average molecul~r welght o~ a ~l~orochlorocarbon poly-mer is measured by ASTM D1430.
, Althotlgh a powder of ~luoropolymer can be used and a carrier provided separately, a polymer in the form of an aqueous dlsperslon is pre~erred ~or its stability and because it is most eas~ly obtained in that form.

.. . . .. . . _ . . . __ ~_, . _, .. _ _ _ .__ .. .--.. ._. ~ .. _ _ . .. ......... ..... .. . .. . . . .
.The fluoropolym~r is ordlnaril~ present ~n the com-position at ~ concentratlon o~ about 10-90%~ pre~erably about 60-80~, by weight of~the total of ~luoropolymer and binder (as will be de~ined later ) .

: ~ me bi~der ~ the composit ons o~ the invention comprises an alkali m~tal silicate and th~ amiLne ~alt o:E a polyamide acid. Thls b~nder 1~ ord~narily pres~t ~n a ¢G~o-- ition at a ~o~centr~tio~ o~ ~bout 10-9~, pre~er~bly abo~t . ~ .
20-40%, by ~lgh~ o~:th~ t~tal Or ~l~sr~pol~er and binder.

20~ :The al~ali me~al ~ilic~te ccmpo~e~t of the bi~der can b~ a~y alk~lt metal silicat~ h~v~n~ an al~ali met~l oxide ~1 co~te~ h ~ thl~ r~age of ~r~ a trace ~m~u~t to about 20%
o~ the tot~l ~u~r:o~ o~ alkali mRtal oxi~e a~d ~illcon :
~ dlo~ide. ; ~ ;
.

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"Trace amount" is u~,ed in the conventional sense of meaning a quantity so small that it i~ not detect,able at all or is barely detectable.

"Alkali metal" silicate means a silicate of sodium, potassium) lithiumJ rubidium, cesium or frankium. O:f theseJ
the silicates o~ lithium, sodium and potassium having alkall metal. oxide mol contents or 1~-12~ of the total num~er of mols of alkali metal oxide and silicon dioxide are pre~erred fo:r use because o~ their stabi.lity.

Mixture~ of alkali metal silicates can also be used.

The alkali metal silicate component can be made by blending a commercially available alkali mekal silicate con-taining more than 20 mol percent o~ alkali metal oxide with a commercially available colloidal silica to obtain an alkali metal silicate having the desired alkali metal oxide conten-t.

The alkali metal silicate is ordinarlly present in the binder at a concentration o~ about 5-95%~ preferably about 10-80~, by weight of the binder.

Generally speaking, the polyamide acid salts used in the compositions o~ the invention are those of polyamide acids with tertiary amines. Such salts, which are further disclosed hereinaf`ter~ are the sub~ect o~ Applicant's copendin~

application Serial No. 244 959J filed on February 3~ 1976~ of Thomas P. Concannon~ which corresponds to U.S. Patent 4 014 834, issued March 29, ].977.

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T.he polyamide acids can ~?e re~re~.ented by the structure G COOH
\
/~
_ _~C ~ CNH_Rl __ _.
" ..
O O

_ _ n where G is hydrogen or a carboxyl group;
denotes isomerism;
is a tetravalent organlc radical containlng at least two carbon atoms~ no more than two carbonyl groups o~ each polyamide acid unit being at~ached to any one carbon atom o~ said tetravalent radical3 Rl is a divalent radical containing at least two carbon atoms, the amide groups of ad~acent polyamide acld units each being attached to separate atoms of said di-valant radical, and : n is a number large enough to give the poly-amide acid a~ inherent viscosity of at least 0~1~ measured as a 0.5~ olutio~
in ~N -dimethyl aeetamide at 30C.

m ese polya~ide acids are ~eutr~lized with s~olchio-metric amou~ts o~ tertiar~ amines to glve the amine ~alts~

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~ ~5:~6 ~

The amlne salts o~ polyamide acids pre~erred ~or u~e in the binder component are tho~e prepared from at least one amine represented by the structure H2N ~ NH~

and at least one acld anhydride represented by the structure O, ' O O

~OOC - R / 0 or 0\ / R\ / 0 0 'O O O
;.. .
where~ in b~th structures~ R and Rl axe a~
pre~lously defined, ~' neutralized with a stoichiometrlc amount o~ a tertiary amine.
:: .
. The amine salts Or polyamide acids especially pre-ferred for use ln the binder component are those prepared ~ro~
-(13 meta- or para-ph~nylenedlamines~ bis~4~amino-. phenyl)ether, benzidine, 2,2-bis(4'-amino-~`~ pheny~) propane 3 bi~(4-aminophen~l)methane, bis~-aminophenyl)sulfone, bls(4~aminophenyl) ~ sul~ide or 1~6-bexamethylenediamine3 and . . - .

(2) p~romellltlc dianhydride,`trlmellitic anhydride 23 2-bis (3 '~, 4 ' -dicarboxyphen~l )propane ~ dianhydride, or ble(3~4-dicarboxyphenyl)~tber dia~hydrlde, , - : .. . , ., - .
. .

.

.~ ~s~ LYf~ ~1 neultr~ d w~th ~ ~tolchlome~,3rlc ambunt o~ tri~
~ethylam:lne., trlelkhyïamir~ lmekhyl~th~olamln09 di~thy~ 2-hydroxyethylamiLne3 trlbutylamln~
trl~ ( 2-hydr~xyethyl )a~ln~3 e1;hyl bi~ (2-hydroxye thyl ) amin~ N3 N-dim~thylanlline~ morpholîrle, pyrldlrle, ~methylpyrrole" or mix~ure~ o~ the~.

q~he pol~mide acld ~alt mos~ pr~erred for 7188 in th~ b~nder component ~ olle prepar~d rrom (bl~(4~amlnophenyl) methane and trirne31i~1c ~hydrldeg neutr~lized ~ stoichlo~
10 m~tric ~mc~unt o~ a 2/1 welght ratlo mix~ure o* trie~hyl~mlt e ~d dle~21yl 2-hydroxy~thylamlneO

The amlne ~alt i~ ordinarily pre~en* lrl the binder al; a con~ntratîon o~ a:baut 5~95%3 pre~rably a~out 20090,~, by welght o~ th~ blnder.

me coale~clng Qgents u~ed in the compositlorls o~ th~
ntion are3 generally ~pea~3 organic ll~uidg whlGh c~
di~olYe th~ polyamide at~ld amine ~alt uæed. Org~ic llquld~
of thi~ type ar~ hlghly polar a~d have bolll~3 polnts ab~r~e 100C. Illu~tr~tlYe ~re ~methylpyrrolid~
~ methyl~or~mi~e di methyl ac etamld~
~im~thyl~ulf oxlde cre~y~îc ac ~ul~
and iorn~nideO

: .

The coale~cing agent pre~rred ~or u~e becau~e o~
its ~ ilabllity ~nd lo~ toxicity i~ ~-m~l;hylpgrrolldone~

The coale~clrlg agent i~ ordin~ily present ln a compo~ltion of the i~entiorl at ~ concentr~tion ~ abaut 10-9û% by ~elght o~ the polya~d~ acld amlne ~Llt used, pre~erably about 20 50%.

The carrlers u~ed in the com3?o~1tions oP the in~ention can be any o~ ~ho~e Ilorma~ly u~ed in compo~ltion~
oî this type~, The carr~r i~ ~ lnert, not a reacltlve or 10 ~unctional (iXl th~ ~e~e o~ contributlng to the worklng o~
the invention ) compon~nt and its n~ture i~ ther~ore o~
secondary im~?ortance, ~Ct serv~ only to keep th~ compo~it~on homogeneous and acts ~l8 a ~n~ch~nlsm ~or c~ eylng the ~olld compone~ts to the sub~trate. ~fter a coat~g h~s be~n depo~lted, the carrier evapora~s,, It i8 ne~essary" th~refore" o2lly th~t lt be ~omp~tible wlth the other components of the cor~posltion and that it h~ve no ~d~rer~e e~ect~ on th~ coaking lt~elf, Co~monly9 the fluor~poly~r used i~ the co~o3ition i~
proYid~d in the form o~ an aqueo~ls d~p~r~l~n, and the w~ter 20 introdu~ed irlto th~ compo~ition with the pvl~ r a flO s~r~e~
a~ all or ~ part Or the c~rrler. Or~anic llquld~ ~uch a~
alcohols, ke Iton~, allph~tlc and aroma~ic hy~rocarbons3 or mixture~ oP th~, c~n Ql~o bs u~edO
... .

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The compositions o~ the :in~ention will ha~e more workable viscosities at higher sol:lds content if they contain ab~ut 15-200~ preferably about 80-150%, by weight o~ the polyamide acid amine Ralt used~ o~ a viscosity reducing agent hich is an organic liquid whose solubility parameter3 ST~
falls withln the range o~ 10-21.6 and which is miscible with water. A definition o~ solubllity parameker appears in Klrk-Othmer, "Encyclopedia o~ Chemical Technology"~ eecond edition (1963), pages 889-8960 To-tal solubilit~ parameter, T~ has the . io following component parameters: .

Dispersion component~ ~D 7~0-10.0 Polar component, ~p 2.0-11.0 Hydrogen bonding componenk, ~H 700-14.0 where the equ~tion :- describes the realtionship of ~he components to the total solubility parameter.

~llustrativ~ o~ such ~i~cosity reducing agents are ~ ~ur~uryl alcohol, l-butanol, 2-propanol, ethylene glycol 20~ monoethyl ether, t:riethylene gl~c~ ormic acid3 and mixtures : ~ o~ these. Fur~ur~l alcohol ls preferred ~or ~t~ ef~ec~iveness.

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The composltions o~ the invention can also contain such convent~onal additives as pigments~ ~low control agents, sur~actants and plasticizers as are necessary or seem desir~ble.
These additi~es are added ~or tha usual reasons, in the usual ways and in the u~ual amounts.

The amount o~ total soli~s in the coatlng composi-tions will be ~overned by the subs-trate to which the composi-tions are to be applied, the method of applicationJ the curing procedures, and like ~actors. In the usual cas~, ~ composi-tion will contain about 10-80~, by weight, of solids.

The compositions can be made by si~ply mixing together ~ppropriate amounts of the proper componentsO Pig-ments can be added, i~ this is desired~ by first convention ally preparing a su~table pigment dispersion and then adding this dispersion ~o the com~osition.

The resulting compositlons.can be applied by spray-ing, brushingg roller-coating, dlpping, or by electrodeposi-tion~ I~ the substrate is meta}, thi~ is preferably pre~
~- treated by grit blasting, b~ the ~lame spraying o~ metals or metal oxides, or by ~rit-coating, although the compos~tions ~an also be applied to phosphated and chromated metals. If the substrate is glass2 $t i~ pre~erably ~irst grit-blasted o~ ~rit-coated.

: ~ A compo~lt~on of the lnvention is ordinarily app~ied to a thickness o~ about 2-15 microns (dry) and is then air-dried~ Thls prlmer coat 1s ~hen topcoated with a conve~tional clear or~pigmented fluoropolymer enamel, and baked in the usual way to give an adherent ~luoropolymer coating.

.:

Although the best coating is obtained with the two coat system ~ust descrlbed, a strongly adherent fluoropolymer coating can be obta~ned with a one-coat operation. To do this, a coat of a composition o~ the invention is applied to a thick-ness of 5-50 microns (dry). Thls coat is then alr-dried and baked at a temperature high enough ~o ~use the M uoropolymer used.

The composltions of the :~nvention are most use~ul for priming metal cookware, especially ~rypans, be~ore coat-ing them with PTFE, but the composltions can a~so be used topr~me other articles requiring tightly bonded fluoropolymer coatings. These other articles can be made o~ glass or any other material capable of wlthstanding the baking t2mperatures used. For example, the compositio~s can be used to prime or coat bearings, valvesj wire, metal ~oil3 boilers, pipes~ ship bottoms, oven liners3 ~ ron soleplates, wa~le irons, lce cube trays, snow ~hovels and plows, chutes, conveyors, dies, tools such as saws, f~les and drills, hoppers and other industrial con~ainer~ a~d molds..
~ 20 me rOllO~ng ex~mple illu~trate~ the inventio~
In the ~x~mplef all parts and percentage~ are b~ wei6h~.

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EXAMPLE

The following were added to a vessel in the order stated and mixed:

I. me polyamlde acid amine salt solut~on o~ Example 1 o~ appli-cation S.~. 244 959~ flled February 3, 1976 (con~aining 18 o~ ~urfuryl alcohol and 10~ of N-methylpyrrQlidone)303.6 parts 2. Furfuryl alcohol 30~ part6

3. Deionized water1040.0 parts

4. Pigme~t dis.persio~105.2 parts (Made by ball-milling a mixture o~
TiO244 partg Water48 parts Triethanol-amine oleic acid mlxture 2/18 parts)

5. PTFE dispersion (60~ solids in waterJ
sold by Eo I. du Pont de Nemours and Com~any as "Te~lon'~ Dispersion T-30) 445 0 parts ~ o 200 parts of the resulting mixture were then added, with: mixi~g3 8.86 parts 9* llthium silicate solution in water (21~ soli.ds, sold by E. Io du Pont de ~emours and Compan~ as LITHI~M POLYSILICATE 85*.
~ "
The resu].tin~ ~omposition was sprayed on a gr:it-blasted aluminum panel to a thicknes~ of 5 microns (dry) ~nd * denotes trade mar~

~ 13 -i~, :
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then air-dried~ The pan was then topcoated by spraylng it wlth "Teflon" Enamel-Classic White 455-102 (sold by E. I. du Pont de N~mours and Company) to a thickne~ of about 100 microns (dry~ baking it ~or 20 minutes a~ 95~C., ralslng the baking temper~ture to 425~C. and the~ removing the panel ~rom the oven~

The resulting coating was stronely adherent to the panel.

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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 an alkali metal silicate having an alkali metal oxide mol content within the range of from a trace amount to about 20% of the total number of mols of alkali metal oxide and silicon dioxide, 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, dimethylsufoxide, 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, 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 alkali metal silicate of sodium, potassium or lithium.

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-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 alkali metal silicate is lithium silicate;
(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 l, 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.