CA1085533A

CA1085533A - Release coating silicone compositions

Info

Publication number: CA1085533A
Application number: CA272,979A
Authority: CA
Inventors: David K. Wald
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1976-03-05
Filing date: 1977-03-02
Publication date: 1980-09-09
Also published as: FR2343035B1; FR2343035A1; GB1575121A

Abstract

ABSTRACT OF THE DISCLOSURE A coating composition comprising an inert liquid carrier and solids including a binder containing a methyl-phenyl polysiloxane resin and an epoxy resin, and also a dimethyl polysiloxane fluid and a catalyst. The composition forms finishes that have thermal stability, release properties, hardness, glossy appearance, and abrasion resistance and are used as finishes for cooking vessels.

Description

BACKGROUND OF THE INVENTION
Field of the Invention This invention relates to a coating composition that is useful as a release finish for cooking vessels because of its thermal stability, release properties, hardness, glossy appearance and abrasion resistance.
Particularly, this invention relates to a coating composi-tion useful for coating tin-plated steel cooking vessels because it is curable at a temperature which is lower than the melting point of tin.
Prior Art Cookware and bakeware items have been coated with fluorocarbon polymers, such as polytetrafluoroethylene and copolymers thereof. Finishes of polytetrafluoroethylene have excellent thermal stability and good release properties and have been widely used and well accepted. ~owever, a primer is generally required for these coatings, along with special treatment of the metal substrate, to obtain excel-lent adhesion of the coating. Additionally, since the temperature rieeded to fuse the fluorocarbon polymer is higher than the melting point of tin, it cannot be used upon tin-plated metals. Therefore, it would be desirable to have a coating composition that could be applied to unprimed metal substrates, including tin, and would fo~m a finish that has release properties and also good abrasion and scratch resistance.
The novel coating composition of this invention can be applied directly to metal without the use of a primex, forms a finish that has release properties, out-standing adhesion to unprimed metal, thermal stability, good hardness, abrasion reJistance, and glossy appearance and i8 an ldeal coating composition ~or cooklng vessels, partlcularly tln-plated ones.
SUMMARY OF 'l'~E INVENIION
....
The coatlng compo81tlon has a solids contcnt of 25-80% ~r ~eight in an lnert liquid carrier and i~ co~-pri~ed of (a~ a binder co~prised o~
(1) 60-99% by ~eight, ba~ed on the ~eight o~ the binder, o~ a s$1icone resin ~h$ch i~ a heat curable 8tructured Eethylphenyl polysllo~ane resin ha~in8 the repeating structural ~ormNla ~ o}Esl-o}E~ ~sl o3 a ~ b ~ c ~ d ~hereln lc a runctlonal group ~hich allo~
cros~-linking at it~ slto, and a, b, c, and d aro ~ositlve lnteg~r~
~hich are ~urriclentl~ lar~e to provlde the res~n, ~hen ~ea~ur~d at 60~ re~ln ~olld~ ln ~yl~ne at 25-C, with a ~i~cosity o~ 30-~0 ¢entipolso~, specl~lc 6ra~ity o~
1.03-1.2, and re~ractlve lndo~ Or 1.~5-1.57, and ha~ng a ~llanol content abo~e 4% ba~ed on the ~i~t Or the r~

(2) 1-40% by ~elght, based on the ~eight Or th~

~B

~.~8~533 binder, of an epoxy resin having the rormula:

CH2_CllCH2~_~_R-~-ocH2cHcH2~;;o-~)-~-~-ocH2-cH-cH2 wherein n is an integer rrom 0-42, and R i~ an alkylene group Or 1-4 carbon-atoms;
(b) 0.2-10% by ~eight, based on the welght Or the blnder, Or sll~cone rluid ~hich i8 a dimethyl polysiloxane rluid ha~ing a ~lscosity Or 50-5000 centistoke~ measured at 25C;
(c) 0.2-10~ by ~eight, based on the weight Or the binder, Or catalyst ~hich 1~ a metal salt; and (d) inert liquid carrler.
A metal cooking ~essel coated ~lth a cured rllm producod by the above composition i8 al80 part Or the lnv~ntlon.

DESCRIPTION OF THE INVE~TIO~
The coatlng co~posltlon Or thls in~ention prererably has a relatively hlgh solids content Or about 20-70 perc~nt by ~eight in a sulta~le liqutd carrler. The rllm rorming binder constltuents are dlssolYed in or&anic sol~ents such as toluene, ~ylene, tetrahrdr0~uran, ~utyl CARBITO~*, ethylene glycol monoethyl ether, ethylene glycol ~onobutyl ether, and the llke.
The blnder Or the novel co~ting compo~ltlon Or thl~ inventlon i8 a blend of ~ilicone resin ~hich i~ a highly reactive meth~lph~l polyslloxane and an epox~

denotes trade mar}~

B

resin. The bind~r contains about 60-99 percont by ~elght of silicone resin and 1-40 percent by ~eight epoxy resin in the a~ounts necossary to equal 100 percent. Prererably, the blnder contain~ 90-99 percent by ~eight o~ silicone resln and 1-10 percent by ~eight Or epoxy resin.
Also present ln the composition i~ a sillcone rluid ~hich is a di~ethyl polysilo~ane. The amount of dlmethyl polysiloxane fluid utili~ed iB based on the total ; ~eight Or binder. me amount o~ fluld ~ill range rrom 0.2-10% by ~elght, based on the ~eight Or the binder;
prererably, the range ~111 be 1-3% by weight.
A catalyst i6 added to the compositlon to ensure good rllm rormation and rapld cure. m e amount o~
cata}yst utlllzed 18 also based on the total ~elght Or b1nder. It ~111 range rrom 0.2-10% by welght, based on tho ~eight Or the binder; prererably, the ~elght wlll be 0.4-1.2% by ~eight.
The comblnatlon Or the~e components provide~
a coatlng co~po~ltlon that adheres ~ell to unprl~ed ~etal 2Q substrat~s ln comblnatlon ~lth good release properties, de~lreable thermal stablllty, and low tomperature cure.
The hlghly reactlv- m thyl phenyl polyslloxane resln 18 heat curable, ha8 a ~ilanol contsnt abo~e 4%, prererably 5%, based on the ~elght o~ the re~in, and has the repeating structural ~ormula:

3~ ~
3 a X b X c d ~herein B

. ~ .
x is a functional group which allows cross-linking at its site, and a, b, c, and d are positive integers which are sufficiently large to provide the resin, when measured at 60~ resin solids in xylene at 25C, with a viscosity of 30-50 centipoises, specific gravity of 1.03-1.2, and refractive index of 1.45-1.57.

A particularly preferred methyl phenyl siloxane resin, when measured at 60~ solids in xylene at 25C, has a viscosity of about 35-45 centipoises, specific gravity of about 1.05-1.12, and a refractive index of about 1.50-1.53;
it also has a softening point of about 75-85C.
The epoxy resins utilized in the present inven-tion are commonly known in the art. One class of such resins has the generalized formula CH2~HCH2{ --~)--R ~ OCI~2C!~C~Iz ~ R--~)--OCH2--cH--c1/2 where n is an integer from 0-42 and R is an alkylene group of 1-4 carbon atoms. The epoxy resins utilized in the present invention contain at least two epoxy groups per molecule and therefore, upon curing of the composition, introduce no uncross-linkable extractable portions into the coating.
Preferably, to obtain a coating with high gloss, a liquid epoxy resin is used. An undiluted liquid epoxy resin where the average value of n is about 0-3, ~ is .
T 7 _ 10~S533 isopropylidene, the viscosity i8 1.2-225 poises at 25C
as measured by ASTM-D-445, and the epoxy equivalent about 150-470 is preferred. m e epoxy equivalent is defined as the gram~ of resin containing one g~am-equivalent of epoxide functionality as measured by A~TM-D-1652. A
coating composition containing EPON 828* is particularly preferred because finishes produced by such compositions have high gloss while maintaining high adhesion. EPON 828 is a liquid epoxy re~in where the average value of n ls about 0, R is isopropylidene, the vicosity of the undiluted resin is 100-160 poises at 25C as measured by ASTM-D~445, and the epoxy equlYalent i8 185-192.
Modifications of epoxy reslns can also be utilized in the coating composition of the present invention. For example, it is known to those skilled in the art that when an epoxy compound containing a hydroxy group i8 brought ln contact with an acid, there results an ester or mixture of esters. Thus, when phosphoric acid i~ added to an epoxy resin, a reactlon occurs at one or more of the epoxy groups of the molecule and the resulting mixture contains both the mono- and diesters of phosphoric acld. A product of this reaction i8 exemplified by the formula 2 2 ~ ~ R ~ OCN2CNCN2 ~ l O

These modified epoxy resins can still function as epoxy resins in the coating compositions o~ the invention.
i The dimethyl polysiloxane fluid utillzed in the coating composition of this inventlon can have viscosity o~ o.65 to over a million centistokes measured at 25C, but ;-* denotes trademark , -. .~, "~ _ O
~4' preferably has a vlscoslty of 100-5000 centistokes. To form partlcularly high quallty composltions, a vlscoslty Or 500-1500 cent~stokes i8 preferred. ~he dlmethyl poly-~lloxane fluld 18 used to lncrease the release character-lstic o~ the rllm produced.
Cataly~ts useful in the coating composltlon of this lnventlon are metal salts, partlcularly metal salts Or fatty aclds such as zlnc octoate and cobalt naphthenate.
A prererred zinc octoate is Nuodex~ (a zinc octoate 801u-tlon hav~ng an 8~ zinc metal content sold by Tenneco). Acoupling solvent, such as V.M. and P. Naphtha, is sometlmes used to en~ure solublllty Or the catalyst wlthin the coating composltlon.
Suitable liquid carriers i~clude such organic solvents ~8 those used ~or dissolving the rllm-rormlng blnder constltuent~, ~or example, toluene, xylene, tetrahydrofuran, butyl GARBITOL, ethylene gl~col oonoethyl ether, ethylene glycol monobutyl ether, and the li~e.
Option~lly, plgments can be u~ed in the coating co~posltion Or the invention. A ratlo of pigment to binder Or about 1/100-400/100 can be utlllzed, prererably, about 1/100-200/100, more pre~erably, in compo~ltlona having excellent propertles, about 1/100-50/100. Typical pi~ments that can be used are, for example, carbon black, titanium dlox~de, brown, blac~, and yellow iron oxides, alu~tinum ~llicate, m~ca, talc, china clay, metal powder~, carbon~tes, ; and the like.
Optlonally, to provlde the composition ~ith in-creased ~cratch resistance, abrasion resi~tance, hardness, blister resist~nce, and decreasing poro~ity, finely div~ded t B

inorganic hardening agent may be added in a ratio of harden-ing agent to binder of about 5/100 to about 400/100. The hardening agents that can be used include zirconium silicate, zirconium oxide, silicon dioxide, reactive and unreactive aluminas, and filler pigments such as aluminum flake, potassium titanate fibrils, titanium dioxidP fibrils, silicone dioxide fibrils, and alumina monohydrate fibrils.
Examples of aluminas which can be used are calcined aluminas, low-soda aluminas, reactive aluminas, high-purity aluminas, tabular aluminas, calcium aluminate cement, and hydrated alumina. The preferred hardening agents are reactive alumina.
A preferred reactive alumina which is useful as a coating composition having excellent properties is "Reactive Alumina A-15SG" which is sold by Alcoa. This alumina con-6ists of 23 99-5+~
Na20 .08 sio2 , 07 :
Fe2 3 .01 It, with compaction and sintering aids, can pro-vide an all-alumina composition with a green body density of 2.86 gm/cc at 5000 psi, and fired density of 3.93 gm/cc, with only 10.1 percent linear shrinkage after one hour at 1665C.
When a reactive alumina is used as the hardening agent, its ratio to binder is about 10/100 to about 70~100;
preferably 20/100 to 40/100.
The coating composition of this invention can be prepared by many methods known to the artisan. One method 30 is to first prepare a mill base by mixing pigment, silicone -- 10 -- .

fluid, solvent, and silicone resin and then grinding the mixture by conventional techniques such as pebble mill grinding, ball mill grinding, sand mill grinding and the like. To the mill ~ase is added more silicone resin and solvent, epoxy, catalysts and other constituents; this is blended to form the coating composition.
The composition of this invention is applied to the interior of a vessel by first roughening the interior surface of the vessel, preferably by grit blasting the surface with grit, or the surface can be roughened by other techniques. The surface is then cleaned.
The coating composition of this invention is then applied by conventional techniques, such as, spraying, electrostatic spraying, roller coating, and the like to the surface of the vessel to a thickness of 0~2-5 mil (dry), preferably 0.5-1.5 mil (dry) and is then baked for about 5-45 minutes at about 175-400C.
~ he coating composition of this invention forms excellent finishes, not only on cooking vessels, but also on ice cube trays, dough cutters, paper cutters, and can be used as a lubricant coating on bearing and curtain rods and can be used as a coating on coin machine slots, fan vents, shovels, and discardable aluminum utensils.
The following example illustrates the invention.
All ~uantities are on a weight ~asis unless otherwise indicated.

A coating composition was prepared as follows: -(a) (1) Preparing a resin solution by mixing until dissolved:

- Sillcone re~in rlake whlch ls a heat Parts By Wei~ht curable structured methyl phenyl poly-slloxane resin having the repeating structural formula C~3 a X b c d whereln X is a ~unctional group which allows cross-llnkin~ at lts ~ite, and a, b, c, and d are positive integers whlch are ~ur nciently large to provide the re~in, : when measured at 60% resin sollds in -~
xylene at 25C, with a vlscoslty Or 40 centipoises, ~peclfic gravity Or 1.08, and re~ractl~e index Or 1.518, and havln~
a silanol content above 5% based on the welght Or the re~in. 20.00 ~7i''' xylene 20.00 and (2) addlng to the product Or (a) (l) and then grindlng ~or 16 hours:
xylene 56 -tltanlum dioxlde plgment lO0 ~lllcone fluld (D.C.~ 200, a polydlmethyl siloxane having a vi~coslty at 25-C o~ lO00 centi~to~es, sold by now Cornlng) B

(b) Adding to the ~111 of (a), the ~ollowing and mixing untll dissolved:
Partæ By ~ ei~ht silicon res~n flake (descr~bed ln (1)) 455 xylene 455 (c) Adding the ~ollowing to the produce of (b) and then blending together:
EPON 828 (a liquld epoxy resin sold by Shell Chemical Company) 25 cobalt naphthen~te 7.05 -Nuodex~ (a zinc octoate solutlon, having 8% zinc ~etal content sold by Tenneco 9.25 V.M. and P. ~apht~a 50 1201.30 A tln plat~ steel murrin pan ~as wlped ~ith xylene to remove ~ny resldue grease and then was grit blasted with 200 ~esh grlt at 80 pounas per square inch.
The pan wa6 then cleaned Or grit partlcles wlth a blast co~pressed alr and ~a~ wlped wlth xylene.
, m e coating co~position was then sprayed onto I the pan to a fllm thickness Or 0.7 mll (dry) and alr drled or 10 min~tes and then ba~ed for 15 minutes at 215C.
m e re~ltiag ~lnlsh on the pan had a good glos~y appear~nce, excellent adhe~on to the metal substrate, a pencil hardness of F, and wa~ fully cured a~ measurea by a ~0 double wlpe test ~ith toluene~
m e coated pan was sub~ected to a 10 ba~e te~t st 220C ~ith blueberry muf~in mix and meat loa~ mix. me release properties, ease of cleaning, and appearance of the finish remained excellent.

.~ :

Claims

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

1. A coating composition having a solids con-tent of 25-80% by weight in an inert liquid carrier and comprised of (a) a binder comprised of (1) 60-99% by weight, based on the weight of the binder, of a silicone resin which is a heat curable structured methylphenyl polysiloxane resin having the repeating structural formula wherein X is a functional group which allows cross-linking at its site, and a, b, c, and d are positive integers which are sufficiently large to provide the resin, when measured at 60% resin solids in xylene at 25°C, with a viscosity of 30-50 centipoises, specific gravity of 1.03-1.2, and refractive index of 1.45-1.57, and having a silanol content above 4% based pm the weight of the resin;
(2) 1-40% by weight, based on the weight of the binder, of an expoxy resin having the formula:

wherein n is an integer from 0-42, and R is an alkylene group of 1-4 carbon-atoms;
(b) 0.2-10% by weight, based on the weight of the binder, of a silicone fluid which is dimethyl polysiloxane fluid having a viscosity of 50-5000 centistokes measured at 25°C;
(c) 0.2-10% by weight, based on the weight of the binder, of catalyst which is a metal salt; and (d) inert liquid carrier.

2. The coating composition of Claim 1 wherein the binder comprises (a) 90-99% by weight, based on the weight of the binder, of silicone resin, and (b) 1-10% by weight, based on the weight of the binder, of epoxy resin.

3. The coating composition of Claim 2 containing (a) 1-3% by weight, based on the weight of the binder, of said silicone fluid, and (b) 0.4-1.2% by weight, based on the weight of the binder, of catalyst.

4. The coating composition of Claim 1 wherein the silicone resin has a silanol content above 5%, based on the weight of the resin, a softening point of about 75-85°C, and, when measured at 60% resin solids in xylene at 25°C, a viscosity of about 35-45 centipoises, specific gravity of about 1.05-1.12, and refractive index of about 1.50-1.53.

5. The coating composition of Claim 1 wherein the epoxy resin is a liquid having a viscosity of 1.2-225 poises at 25°C, an epoxy equivalent of 150-470; and wherein n is an integer from 0-3, and R is isopropylidene.

6. The coating composition of Claim 1 wherein the silicone fluid has a viscosity at 25°C of about 500-1500 centistokes.

7. The coating composition of Claim 1 wherein the catalyst is a metal salt of a fatty acid.

8. The coating composition of Claim 1 containing pigment in a pigment to binder ratio of about 1/100 to about 400/100, and inorganic hardening agent in a hardening agent to binder ratio of about 5/100 to about 400/100.

9. The coating composition of Claim 1 wherein (a) the binder is comprised of (1) 90-99% by weight, based on the weight of the binder, of a silicone resin having (a) silanol content above 5% based on the weight of the resin, (b) softening point of about 75-85°C, and (c) when measured at 60% resin solids in xylene, a viscosity of about 35-45 centi-poises, a specific gravity of about 1.05-1.12, and refractive index of about 1.50-1.53; and (2) 1-10% by weight, based on the weight of the binder, of liquid epoxy resin having a viscosity 1.2-225 poises at 25°C, and an epoxy equivalent of 150-470;
and wherein n is an integer from 0-3, and R is isopropylidene;
(b) 1-3% by weight, based on the weight of the binder, of said silicone fluid having a viscosity of 500-1500 at 25°C;
(c) 0.4-1.2%, by weight, based on the weight of the binder, of a catalyst which is a metal salt of a fatty acid;
(d) pigment in a pigment to binder ratio of about 1/100 to about 400/100;
(e) inorganic hardening agent as a hardening agent to binder about 5/100 to about 400/100;
and (f) inert liquid carrier.

10. A cooking vessel having its food contacting surface coated with a film produced by curing the coating composition of Claim 1.