Coating composition comprising a polysiloxane resin forming precondensate, an epoxy resin, a silicone fluid, and a catalyst

A coating composition comprising PA1 (a) a binder containing a polysiloxane resin-forming precondensate and an epoxy resin, PA1 (b) a dimethylpolysiloxane fluid, PA1 (c) a metal salt catalyst, and PA1 (d) an inert liquid carrier. The composition forms finishes that have thermal stability, release properties, hardness, glossy appearance, and abrasion resistance and are used as finishes for cooking vessels.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a coating composition having a binder of epoxy 
resin and polysiloxane precondensate 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 composition useful for coating tin-plated 
steel cooking vessels because it is curable at a temperature which is 
lower than the melting point of tin, about 232.degree. C. 
2. 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. However, a primer 
is generally required for these coatings, along with special treatment of 
the metal substrate, to obtain excellent adhesion of the coating. 
Additionally, since the temperature needed 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 form 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 primer, forms a finish that has release 
properties, outstanding adhesion to unprimed metal, thermal stability, 
good hardness, abrasion resistance, and glossy appearance and is an ideal 
coating composition for cooking vessels, particularly tin-plated ones. 
U.S. Pat. No. 3,170,962--Tyler discloses coating compositions of epoxy and 
polysiloxane resins. While the examples of Tyler indicate they use 
polysiloxanes having 3 or 4% silanol, this is before the copolymerization 
with epoxies. During the copolymerization, at least a substantial 
proportion of these silanol groups would be consumed, so that the 
copolymer would have far less than 4% silanol. If one attempted to make a 
coating composition of Tyler's copolymer, it would have a relatively low 
silanol content, substantially below 4%. U.S. Pat. No. 3,801,522--Vasta 
and U.S. Pat. No. 3,925,276--Merrill teach polysiloxane coatings which do 
not include epoxy resin. 
SUMMARY OF THE INVENTION 
The coating composition has a solids content of 25-80% by weight in an 
inert liquid carrier and is composed of 
(a) a binder comprised of 
(1) 60-99%, based on the weight of the binder, of a polysiloxane 
resin-forming precondensate which has one or more of the units 
##STR1## 
wherein X is a functional groups which allows cross-linking at its site, 
and 
a, b, c, d and e are of a magnitude and in proportions which provide the 
resin with a degree of substitution, measured as the ratio of phenyl plus 
methyl groups to silicon atoms, of at least about 1.0, and a ratio of 
phenyl groups to silicon atoms of at least about 0.3, 
and which has a silanol content above 4%, based on the weight of the 
precondensate; 
(2) 1-40%, based on the weight of the binder, of an epoxy resin having the 
formula: 
##STR2## 
wherein n is an integer from 0-42, and 
R is an alkylene group of 1-4 carbon atoms; 
(b) 0.2-10%, based on the weight of the binder, of silicone fluid which is 
a dimethyl polysiloxane fluid having a viscosity of 50-5000 centistokes 
measured at 25.degree. C.; 
(c) 0.2-10%, based on the weight of the binder, of catalyst which is a 
metal salt; and 
(d) inert liquid carrier. 
Preferably, the precondensate, when measured at 60% resin solids in xylene 
at 25.degree. C., has a viscosity of 30-50 centipoises, specific gravity 
of 1.03-1.2, and refractive index of 1.45-1.57. Precondensates having a 
random arrangement of the repeating units are, of course, included. 
(Percentages and proportions herein are by weight except where indicated 
otherwise.) 
A metal cooking vessel coated with a cured film produced by the above 
composition is also part of the invention. 
DESCRIPTION OF THE INVENTION 
The coating composition of this invention preferably has a relatively high 
solids content of about 30-70%, more preferably 38-50%, in a suitable 
liquid carrier. The film-forming binder constituents are dissolved in 
organic solvents such as toluene, xylene, tetrahydrofuran, butyl carbitol, 
ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and the 
like. 
The binder of the novel coating composition of this invention is a blend of 
polysiloxane resin-forming precondensate and an epoxy resin. The 
precondensate and the epoxy resin are capable of reacting together to cure 
into a coating after the composition has been applied to a substrate. The 
binder contains about 60-99% of silicone resin and 1-40% epoxy resin in 
the amounts necessary to equal 100%. Preferably, the binder contains 
90-99% of silicone resin and 1-10% of epoxy resin. 
Also present in the composition is a silicone fluid which is a dimethyl 
polysiloxane. The amount of dimethyl polysiloxane fluid utilized is based 
on the total weight of binder. The amount of fluid will range from 
0.2-10%, based on the weight of the binder; preferably, the range will be 
1-3%. 
A catalyst is added to the composition to ensure good film formation and 
rapid cure. The amount of catalyst utilized is also based on the total 
weight of binder. It will range from 0.2-10%, based on the weight of the 
binder; preferably, the amount will be 0.4-1.2%. 
The combination of these components provides a coating composition that has 
excellent adhesion to unprimed metal substrates in combination with good 
release properties, excellent thermal stability, and low temperature cure. 
A particularly preferred methyl phenyl siloxane resin, when measured at 60% 
solids in xylene at 25.degree. C., 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.degree.-85.degree. C. 
Polysiloxane resins and resin-forming precondensates of the invention can 
be prepared by techniques known in the art. Generally, desired proportions 
of dimethyldichlorosilane, methylphenyldichlorosilane, and sometimes 
diphenyldichlorosilane are hydrolized to form cyclic structures, and then 
the cyclic structures are polymerized with acid or base to form the 
polysiloxane resin-forming precondensate. It is evident that appropriate 
proportions of the units selected for the structural formula must be 
provided in order to achieve the ratios of the preferred compositions and 
in order to obtain the desired resin-forming precondensates. For instance, 
those skilled in the art would know that if a, b, c, and e are each zero 
and the structure is made entirely of 
##STR3## 
the resulting composition would be hydrated silicon dioxide which is not a 
resin-forming precondensate. Also, if b, c and d were all zero and the 
resin were made entirely of 
##STR4## 
the result would be an elastomer rather than a resin. 
The high silanol content of the polysiloxane precondensates of the present 
invention makes a significant contribution to the bonding of the 
presently-claimed coatings to the substrate, although the precise 
mechanism is not known. In any event, the high silanol content permits 
versatility in formulating with commercially-available polysiloxane 
precondensates and epoxy resins. There is no need to synthesize 
epoxy-silicone copolymers; the expense of such a procedure is avoided and 
the resulting coating composition is more versatile in that the 
polysiloxane precondensate and the epoxy resin can be selected from a 
variety of materials and their ratios can be varied. This flexibility 
facilitates tailoring the adhesion and nonstick properties to a specific 
need. 
The in situ crosslinking of the presently-claimed coating compositions can 
be expected to give coatings with properties superior to those of 
prereacted copolymers. This is largely because of the high silanol content 
available in the claimed compositions which permits in situ crosslinking 
and contributes to adhesion to the substrate. It is well known in the art 
of nonstick coating compositions that superior adhesion to the substrate 
produces superior coatings. In addition to whatever epoxy adhesion 
mechanism other materials might have, the presently-claimed material has 
the further advantage that crosslinking in situ can create a coating which 
more closely follows the microdiscontinuities of the substrate surface. 
The presently-claimed materials can be expected to penetrate into such 
discontinuities more deeply than an analogous preformed copolymer. When 
the epoxy and polysiloxane precondensate are crosslinked in situ, a tough 
material is formed which locks the coating to the substrate more 
effectively. 
While such a mechanistic explanation of applicant's coating is hypothetical 
and not readily subject to direct proof, it is also logical and presents a 
fair representation of what is most likely happening. 
The epoxy resins utilized in the present invention are commonly known in 
the art. One class of such resins has the generalized formula 
##STR5## 
wherein 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, R is isopropylidene, the viscosity is 1.2-225 poises at 
25.degree. C. as measured by ASTM-D-445, and the epoxy equivalent about 
140-470 is preferred. The epoxy equivalent is defined as the grams of 
resin containing one gram-equivalent of epoxide functionality as measured 
by ASTM-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 
resin where the average value of n is about 0, R is isopropylidene, the 
viscosity of the undiluted resin is 100-160 poises at 25.degree. C. as 
measured by ASTM-D-445, and the epoxy equivalent is 185-192. 
Modifications of epoxy resin 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 
is brought in contact with an acid, there results an ester or mixture of 
esters. Thus, when phosphoric acid is added to an epoxy resin, a reaction 
occurs at one or more of the epoxy groups of the molecule and the 
resulting mixture contains both the mono- and diesters of phosphoric acid. 
A product of this reaction is exemplified by the formula 
##STR6## 
These modified epoxy resins can still function as epoxy resins in the 
coating compositions of the invention. 
The dimethyl polysiloxane fluid utilized in the coating composition of this 
invention can have a viscosity of 0.65 to over a million centistokes 
measured at 25.degree. C., but preferably has a viscosity of 100-5000 
centistokes. To form particularly high quality compositions, a viscosity 
of 500-1500 centistokes is preferred. The dimethyl polysiloxane fluid is 
used to increase the release characteristic of the film produced. 
Catalysts useful in the coating composition of this invention are metal 
salts, particularly metal salts of fatty acids such as zinc octoate and 
cobalt naphthenate. A preferred zinc octoate is Nuodex.RTM. (a zinc 
octoate solution having an 8% zinc metal content sold by Tenneco). A 
coupling solvent, such as V.M. and P. Naptha, is sometimes used to ensure 
solubility of the catalyst within the coating composition. 
Suitable liquid carriers include such organic solvents as those used for 
dissolving the film-forming binder constituents, for example, toluene, 
xylene, tetrahydrofuran, butyl carbitol, ethylene glycol monoethyl ether, 
ethylene glycol monobutyl ether, and the like. 
Optionally, pigments can be used in the coating composition of the 
invention. A ratio of pigment to binder of about 1/100-400/100 can be 
utilized, preferably, about 1/100-200/100, more preferably, in 
compositions having excellent properties, about 1/100-50/100. Typical 
pigments that can be used are, for example, carbon black, titanium 
dioxide, brown, black, and yellow iron oxides, aluminum silicate, mica, 
talc, china clay, metal powders, carbonates, and the like. 
Optionally, to provide the composition with increased scratch resistance, 
abrasion resistance, hardness, blister resistance, and decreasing 
porosity, finely divided inorganic hardening agent may be added in a ratio 
of hardening 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 
dioxide 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 consists of 
______________________________________ 
Al.sub.2 O.sub.3 
99.5+% 
Na.sub.2 O .08 
SiO.sub.2 .07 
Fe.sub.2 O.sub.3 
.01 
______________________________________ 
It, with compaction and sintering aids, can provide 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 1665.degree. C. 
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 is to first prepare a mill base by mixing 
pigment, silicone 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 base 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 (5-125 
.mu.m) (dry), preferably 0.5-1.5 mil (12.5-37.5 .mu.m) (dry) and is then 
baked for about 5-45 minutes at about 175.degree.-400.degree. C. 
The 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 quantities are on a 
weight basis unless otherwise indicated. 
EXAMPLE 1 
A coating composition was prepared as follows: 
______________________________________ 
Parts by 
Weight 
______________________________________ 
(a) (1) Preparing a resin solution by mixing until 
dissolved: 
Silicone resin flake which is a heat 
curable structured methyl phenyl poly- 
siloxane resin having the repeating 
structural formula 
##STR7## 
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.degree. C., with a viscosity of 40 
centipoises, specific gravity of 1.08, 
and refractive index of 1.518, and having 
a silanol content above 5% based on 
the weight of the resin. 20.00 
xylene 20.00 
and 
(2) adding to the product of (a) (1) and then 
grinding for 16 hours: 
xylene 56 
titanium dioxide pigment 100 
silicone fluid (D.C. .RTM. 200, a 
polydimethyl siloxane having 
a viscosity at 25.degree. C. of 1000 
centistokes, sold by Dow 
Corning) 
(b) Adding to the mill base of (a), the following 
and mixing until dissolved: 
silicon resin flake (described 
in (1) ) 455 
xylene 455 
(c) Adding the following to the produce 
of (b) and then blending together: 
"Epon 828" (a liquid epoxy resin 
sold by Shell Chemical Company) 
25 
cobalt naphthenate 7.05 
"Nuodex" (a zinc octoate solution, 
having 8% zinc metal content 
sold by Tenneco 9.25 
V.M. and P. Naptha 50 
1201.30 
______________________________________ 
A tin plated steel muffin pan as wiped with xylene to remove any residue 
grease and then was grit blasted with 200 mesh grit at 80 pounds per 
square inch. The pan was then cleaned of grit particles with a blast of 
compressed air and was wiped with xylene. 
The coating composition was then sprayed onto the pan to a film thickness 
of 0.7 mil (dry) and air dried for 10 minutes and then baked for 15 
minutes at 215.degree. C. 
The resulting finish on the pan had a good glossy appearance, excellent 
adhesion to the metal substrate, a pencil hardness of F, and was fully 
cured as measured by a 50 double wipe test with toluene. 
The coated pan was subjected to a 10 bake test at 220.degree. C. with 
blueberry muffin mix and meat loaf mix. The release properties, ease of 
cleaning, and appearance of the finish remained excellent. 
COMATIVE TEST 
A series of experiments, described below, has been conducted to show that 
the use of reactive polysiloxanes with a silanol content above 4%, based 
on the weight of the precondensate, gives coatings which are superior to 
those made with nonreactive polysiloxanes having silanol contents of about 
1%. Except for the silanol content, the polysiloxanes in both experiments 
are within claim 1 of this application. Other than using different 
polysiloxanes, the coating compositions for each experiment were 
formulated, applied and baked under identical conditions. 
The table below shows the formulations of coating composition A, with the 
reactive silicone, and B, with the less reactive silicone. 
TABLE 
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FORMULATIONS 
A (g) B (g) 
______________________________________ 
Reactive Silicone (4.7-7.8% silanol) 
255.20 -- 
Less Reactive Silicone (1% silanol) 
-- 255.20 
Zn Octoate (8% Zn in mineral spirits) 
14.70 14.70 
Silicone Releasing Fluid 
6.00 6.00 
Carbon Black 43.70 43.70 
Calcined Alumina 109.40 109.40 
Liquid Epoxy Resin 54.70 54.70 
Butyl Acetate 194.80 194.80 
Methyl Isobutyl Ketone 66.30 66.30 
TOTALS 1000.00 1000.00 
______________________________________ 
Coatings of each composition were applied to identical aluminum panels 
under identical conditions to produce coatings with a dry thickness of 
10-12.5 .mu.m. Both panels were baked at 218.degree. C. for 15 minutes. 
The panel with composition A has a much glossier appearance than the panel 
with composition B. 
After baking the coating, a cloth saturated with toluene was rubbed with 
light finger pressure back and forth 50 times on each panel. This resulted 
in exposing bare metal on the panel with composition B and little 
noticeable effect on the panel with composition A. This demonstrates that 
composition B did not give a satisfactorily-cured coating. 
These experiments demonstrate that, in order to obtain optimum coating 
results, especially when curing at relatively low temperature, it is 
important to provide a polysiloxane precondensate of the invention with 
high reactivity. A less reactive polysiloxane with a smaller proportion of 
silanol groups does not cure in situ to give the advantages of the 
invention.