Inorganic coating compositions

This invention provides an inorganic coating composition comprising (a) about 100 parts by weight (based on solids) of a binder component prepared by thermally dissolving about 10 to about 40 parts by weight of fine particles of silica in about 100 parts by weight (based on solids) of potassium silicate and (b) about 15 to about 100 parts by weight of pearl luster pigment.

FIELD OF THE INVENTION 
The present invention relates to inorganic coating compositions, and more 
particularly to iris-colored inorganic coating compositions which are 
useful in forming a decoratively colored coat on articles made of 
substantially any kind of material except those made of plastics and woods 
and which can produce a coat capable of inhibiting rust on metallic 
articles. The invention also concerns with coating methods using said 
coating compositions. 
BACKGROUND OF THE INVENTION 
Known alkali silicate-type inorganic coating compositions are so poor in 
curability as to form coats low in resistance to boiling water and like 
properties and exhibit insufficient rust inhibition and corrosion 
resistance when applied to metallic articles. A highly active curing agent 
has been used to produce a coating composition with improved curability, 
thus free from these drawbacks. However, the use of highly active curing 
agent significantly accelerates curing, thereby shortening the pot life of 
coating composition to a great extent. Accordingly the use thereof 
necessitates cumbersome procedures of separately storing a coating 
composition and a curing agent and mixing them immediately before 
application of the composition, hence undesirable. 
In preparation of gold- or silver-colored or like metallic inorganic 
coating compositions, metal powder is used to impart a particular metallic 
luster to a film to be formed. Metal powder, however, when subjected to a 
high temperature of more than 500.degree. C. is oxidized to become black 
and devoid of metallic luster. Further, iris-colored inorganic metallic 
coating compositions with fully satisfactory properties have not been 
developed heretofore. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an inorganic coating 
composition having a high curability. 
It is another object of the invention to provide an inorganic coating 
composition capable of forming coats outstanding in properties such as 
resistance to boiling water. 
It is a further object of the invention to provide an inorganic coating 
composition capable of forming on metallic articles coats having 
resistance to rust and corrosion. 
It is a still further object of the invention to provide an inorganic 
coating composition having a significantly long pot life. 
It is an additional object of the invention to provide an inorganic coating 
composition capable of producing oxidation-resistant coats which can 
retain metallic luster even under high temperature conditions. 
Other objects and features of the present invention wil become apparent 
from the following description. 
The present invention provides an inorganic coating composition comprising 
(a) about 100 parts by weight (based on solids) of a binder component 
prepared by thermally dissolving about 10 to about 40 parts by weight of 
fine particles of silica in about 100 parts by weight (based on solids) of 
potassium silicate and (b) about 15 to about 100 parts by weight of pearl 
luster pigment. 
The present invention further provides a coating method comprising the 
steps of applying an inorganic coating composition to the surface of an 
article to be coated which coating composition comprises (a) about 100 
parts by weight (based on solids) of a binder component prepared by 
thermally dissolved about 10 to about 40 parts by weight of fine particles 
of silica in about 100 parts by weight (based on solids) of potassium 
silicate and (b) about 15 to about 100 parts by weight of pearl luster 
pigment, and heating the coated article to a temperature of about 
200.degree. to about 400.degree. C. for curing. 
DETAILED DESCRIPTION OF THE INVENTION 
Usable as potassium silicate in the present invention is any of those 
commercially available in liquid form. 
The fine particles of silica to be used in the present invention are those 
which have silanol (Si-OH) groups on the surface of particle and 
optionally partly inside thereof, which are soluble in alkali silicate and 
which are about 40 .mu.m or less in mean particle size. Fine particles of 
silica useful in the invention can be any of those commercially available 
and include so-called white carbon (product prepared from sodium silicate 
by wet process) and fine particles of anhydrous silica (product prepared 
from halogenated silicon by dry process). 
Pearl luster pigments useful in the invention include widely commercially 
available non-toxic micatitanium oxide pearl luster pigments such as a 
pigment by EM Chemicals Co., Ltd. U.S.A. (available under a trade name 
"Iriodin"). Pearl luster pigments are various and include those of 
silver-, iris color- and colored-type and the like. Any of them is usable 
in the invention. 
According to the present invention, the binder component in the inorganic 
coating composition is prepared by thermally dissolving fine particles of 
silica in potassium silicate. The amount of fine particles of silica used 
is about 10 to about 40 parts by weight, preferably about 12 to about 38 
parts by weight, per 100 parts by weight of the potassium silicate based 
on solids. Use of less than 10 parts by weight gives a coat unsatisfactory 
in resistance to boiling water, whereas use of more than 40 parts by 
weight accelerates curing of the coat and tends to develop cracks in the 
coat. 
The amount of pearl luster pigment used is about 15 to about 100 parts by 
weight, preferably about 20 to about 90 parts by weight, per 100 parts by 
weight of the binder component based on solids. Use of less than 15 parts 
by weight results in formation of coat insufficient in hiding power. In 
this case, if a larger amount of coating composition is applied to provide 
a coat with an improved hiding power, blisters are likely to occur on 
thermal cure. Use of 100 parts by weight impairs the cohesion of the 
binder, leading to formation of coat lower in resistance to boiling water 
and to saline solutions and like properties and also in adhesion between 
the coat and the substrate. Moreover, use of excess pigment is 
uneconomical. 
The coating composition of the present invention is prepared by the 
following process. Into a reactor are placed specific amounts of potassium 
silicate and fine particles of silica. The mixture is stirred at a 
temperature of about 80.degree. to about 100.degree. C. for about 1 to 2 
hours to completely dissolve the silica into the potassium silicate, 
whereby the contemplated binder component is produced. In the preparation 
of binder component, water may be used in an amount of up to about 400 
parts by weight per 100 parts by weight, based on solids, of mixture of 
potassium silicate and fine particles of silica. The presence of water 
facilitates the mixing and dissolution of silica in potassium silicate, 
thereby improving the efficiency of coating operation. 
Subsequently the specified amount of pearl luster pigment is added to about 
100 parts by weight of the binder component thus prepared, and the mixture 
is homogenized by a stirrer or a ball mill for about 2 to about 4 hours, 
giving a coating composition according to the present invention. 
In the coating composition of the invention, potassium silicate acts as a 
main active component for the binder while the fine particles of silica 
serves as a curing agent for the potassium silicate. To cure the coating 
composition of the invention, potassium silicate and fine particles of 
silica are heated to a temperature of about 220.degree. C. at which 
condensation reaction of silanol groups proceeds between the silicate and 
the silica and between the same species of the two components. 
In preparing the binder component according to the present invention as 
stated above, the mixture of potassium silicate and fine particles of 
silica is heated to a temperature of about 80.degree. to about 100.degree. 
C. to dissolve the latter in the former. However, the condensation 
reaction of silanol groups is not completed in the above-specified 
temperature range so that the binder component thus prepared is 
substantially not cured. Further the condensation reaction of silanol 
groups does not proceed at room temperature. Accordingly the coating 
composition of the present invention with the components as mixed can be 
stored for a prolonged period of time. 
The coating composition of the present invention thus obtained can be 
applied by conventional methods such as spraying, dipping and like methods 
to the surface of articles made of substantially any kind of material 
except those of plastics or woods which exhibit no durability when heated. 
It is desirable in the present invention to apply the coating composition 
to an article to be coated in an amount of about 60 to about 100 g/m.sup.2 
based on solids and to form a film having a thickness of about 10 to about 
15 .mu.m. The coating composition applied is heated to a temperature of 
about 200.degree. to about 400.degree. C., preferably about 220.degree. to 
about 300.degree. C., for about 10 to about 20 minutes at which the 
unreacted silanol groups in the potassium silicate and fine silica 
particles are subjected to condensation reaction to cure the coating 
composition on completion of reaction. 
The inorganic coating composition of the present invention has the 
following advantages. 
(1) The coating composition of the invention is stable at room temperature 
and remains free from curing or change of properties during storage over a 
long period of time. 
(2) Unlike conventional inorganic coating compositions, the coating 
composition of the invention does not require incorporation of a curing 
agent before use, thereby enhancing the operational efficiency to a 
considerable extent and eliminating the need to install a costly coating 
equipment. 
(3) The coating composition of the invention can form a decoratively 
colored coat, namely an iris-colored coat, on articles including those 
made of substantially all kinds of materials except those made of plastics 
or woods. The coat thus produced has a high adhesion to the substrate and 
an improved hardness and is outstanding in thermal resistance, abrasion 
resistance, resistance to boiling water, corrosion resistance and like 
properties. 
(4) The coating composition of the invention displays rust resistance and 
corrosion resistance when deposited on a metallic substrate.

The present invention will be described below in greater detail with 
reference to the following examples. 
EXAMPLE 1 
Using components in the amounts specified below in Table 1 (part by 
weight), inorganic coating compositions were prepared by the following 
procedure. 
To 100 parts by weight (based on solids) of alkali silicate were added fine 
silica particles, and then water was added in the amount specified per 100 
parts by weight of mixture of alkali silicate (based on solids) and fine 
silica particles. The mixture was heated with stirring in a reactor for 
dissolution to a temperature of 90.degree. to 100.degree. C. for 2 hours, 
giving a binder component. 
To 100 parts by weight (based on solids) of the binder component thus 
obtained was added pearl luster pigment and the mixture was agitated by a 
ball mill for 3 hours to give a homogeneous coating composition. In this 
way, there were obtained specimens Nos. 1 and 2 (coating compositions of 
the present invention) and specimens Nos. 3 to 6 (comparison coating 
compositions). Each of the coating compositions thus obtained was applied 
by spraying to a commercially available cold-rolled steel sheet (as 
specified in JIS G 3141 SPCC) in an amount of 60 to 90 g/m.sup.2. The 
coated sheet was heated to a temperature of 220.degree. C. for 20 minutes 
for curing to obtain a test panel. Table 1 below shows the results. 
TABLE 1 
__________________________________________________________________________ 
Alkali silicate 
Potassium silicate (*1) 
Sodium silicate (*2) 
Lithium silicate (*3) 
Specimen No. 1 2 3 4 5 6 
__________________________________________________________________________ 
Fine silica particles (*4) 
15 25 35 56 5 20 
(part by weight) 
Water (*5) (part by weight) 
260 280 227 165 328 330 
Pearl luster pigment (*6) 
50 50 50 50 50 50 
(part by weight) (*7) 
Test (*8) 
Coating thickness (cured) 
13 15 12 14 -- -- 
(.mu.m) 
Appearance of coating 
A A A A C C 
surface 
Resistance to boiling 
A A C C -- -- 
water 
Spraying test for resistance 
A A B B -- -- 
to saline solutions 
Outdoor exposure test 
A A C C -- -- 
Thermal resistance 
A A C A -- -- 
Adhesion 100/100 
100/100 
100/100 
100/100 
-- -- 
Pencil hardness 
Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
-- -- 
Abrasion resistance (mg) 
0.38 0.31 0.34 0.26 -- -- 
__________________________________________________________________________ 
Note: 
(*1) Commercially available product, potassium silicate (product of The 
Nippon Chemical Industrial Co., Ltd., Japan) 
(*2) Commercially available product, Water Glass No. 1 (trade name, 
product of Osaka Soda Co., Ltd., Japan) 
(*3) Commercially available product, lithium silicate (1:4 mixture, 
product of Honjo Chemical Co., Ltd., Japan) 
(*4) Commercially available product, Nip Seal E220 (trade name, product o 
Nippon Silica Kogyo Kabushiki Kaisha, Japan) 
(*5) Amount of water used per 100 parts by weight of mixture of alkali 
silicate (based on solids) and fine silica particles 
(*6) Commercially available product, 4670 Iriodin 100 Silver Pearl (trade 
name, product of EM Chemicals Co., Ltd., U.S.A.) 
(*7) Amount used per 100 parts by weight (based on solids) of the binder 
component 
(*8) Test 
(i) Appearance of coating surface: 
The test panel was heated to 220.degree. C. for 20 minutes to evaluate the 
change in appearance of coating surface according to the following 
ratings. 
A: Uniform coat was formed without crack or peeling. 
B: Cracks and peeling occurred. 
C: Cracks and peeling took place during heating and curing. 
(ii) Resistance to boiling water: 
The test panel was immersed in boiling water for 100 hours to evaluate the 
change in appearance of coating surface according to the following 
ratings. 
A: No change. 
B: Discolored (partially whitened). 
C: Peeled off when rubbed with a finger. 
(iii) Spraying test for resistance to saline solutions: 
The test was performed according to JIS-Z-2371 by observing the appearance 
of coating surface 720 hours after spraying a saline solution to evaluate 
the change in appearance of coating surface according to the following 
ratings. 
A: No change. 
B: Discolored (partially whitened), but free of formation of red rust. 
C: Red rust formed. 
(iv) Outdoor exposure test: 
The test panel was exposed outdoor in an industrial district for 1 year to 
evaluate the change in appearance of coating surface according to the 
following ratings. 
A: No change. 
B: Sparsely covered with white powder but to an extent of the powder being 
readily rubbed out by lightly wiping with a piece of cloth. 
C: Densely covered with white powder to an extent of the powder being 
unremovable by wiping with a piece of cloth. 
(v) Thermal resistance: 
The test panel was left to stand in an electric furnace at a temperature of 
500.degree. C. for 100 hours to evaluate the change in appearance of 
coating surface according to the following ratings. 
A: No change. 
B: Discolored. 
C: Blistered. 
(vi) Adhesion: 
The test panel was cut crosswise with a knife to the substrate to form 100 
squares, each 10 mm.times.10 mm and an adhesive cellophane tape was 
adhered to the surface thus crosswise cut and peeled off to count the 
number of squares to which the tape remained adhered. 
(vii) Pencil hardness: 
Using a Mitsu-Bishi pencil, UNI (trade name, product of Mitsubishi Co., 
Ltd.), the hardness of the film was evaluated. 
(viii) Abrasion resistance: 
The amount (mg) of abraded portion was measured by a Taber's abrasion 
resistance tester, CB-10 (1000 g.times.1000 revolutions). 
Table 1 shows that the inorganic coating compositions prepared using alkali 
silicate other than potassium silicate gave the unsatisfactory films, and 
that more specifically, the coating compositions containing sodium 
silicate produced the films sparsely covered with white powder and lower 
in resistance to boiling water while the coating compositions containing 
lithium silicate were incapable of forming a film. 
EXAMPLE 2 
Coating compositions (specimens Nos. 8 to 12) of the present invention and 
comparison coating compositions (specimens Nos. 7 and 13) were prepared by 
repeating the procedure of Example 1 with the exception of using silica 
and water in amounts (part by weight) listed below in Table 2 per 100 
parts by weight of potassium silicate to prepare a binder component and 
mixing the binder component thus obtained with pearl luster pigment by a 
ball mill for 2 hours. Test panels were produced in the same manner as in 
Example 1 with the exception of applying each coating composition in an 
amount of 90 to 100 g/m.sup.2, and tests were performed using the test 
panels by the foregoing methods. Table 2 below shows the results. 
TABLE 2 
__________________________________________________________________________ 
Specimen No. 7 8 9 10 11 12 13 
__________________________________________________________________________ 
Fine silica particles (*1) 
8 10 20 25 30 40 45 
(part by weight) 
Water (part by weight) 
200 200 200 250 270 300 350 
Pearl luster pigment (*2) 
45 45 45 45 45 45 45 
(part by weight) (*3) 
Test 
Coating thickness (cured) 
12 12 13 15 15 14 15 
(.mu.m) 
Appearance of coating 
A A A A A A B 
surface 
Resistance to boiling 
C B A A A A -- 
water 
Spraying test for resistance 
B A A A A A -- 
to saline solutions 
Outdoor exposure test 
B A A A A A -- 
Thermal resistance 
B A A A A A -- 
Adhesion 100/100 
100/100 
100/100 
100/100 
100/100 
100/100 
-- 
Pencil hardness 
Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
-- 
Abrasion resistance (mg) 
0.41 0.38 0.37 0.36 0.32 0.31 -- 
__________________________________________________________________________ 
Note: 
(*1) Commercially available product, Nip Seal E200 (trade name, product o 
Nippon Silica Kogyo Kabushiki Kaisha, Japan) 
(*2) Commercially available product, 4285 Iriodin 205 Rutile Platinum Gol 
(trade name, product of EM Chemicals Co., Ltd.) 
(*3) Amount used per 100 parts by weight (based on solids) of the binder 
component 
Table 2 shows that the coating composition containing fine silica particles 
in less than the specified amount gave the film inferior in resistance to 
boiling water (specimen No. 7) while the coating composition containing 
the same in more than the specified amount exhibited a lower film-forming 
property (specimen No. 13). 
EXAMPLE 3 
Coating compositions of the present invention (specimen No. 15 to 18) and 
comparison coating compositions (specimens no. 14 and 19) were prepared by 
repeating the procedure of Example 1 except that a binder component was 
prepared from silica and water in amounts specified below in Table 3 per 
100 parts by weight of potassium silicate and that the binder component 
thus obtained and pearl luster pigment were mixed together by a ball mill 
for 4 hours. Test panels were produced in the same manner as in Example 1 
using the coating compositions thus prepared and were tested with the 
results shown below in Table 3. 
TABLE 3 
__________________________________________________________________________ 
Specimen No. 14 15 16 17 18 19 
__________________________________________________________________________ 
Fine silica particles (*1) 
24 24 24 24 24 24 
(part by weight) 
Water (part by weight) 
200 
250 270 280 290 290 
Pearl luster pigment (*2) 
10 30 50 70 100 120 
(part by weight) (*3) 
Test 
Coating thickness (cured) 
-- 12 13 14 14 16 
(.mu.m) 
Appearance of coating 
B A A A A A 
surface 
Resistance to boiling 
-- A A A A C 
water 
Spraying test for resistance 
-- A A A A C 
to saline solutions 
Outdoor exposure test 
-- A A A A A 
Thermal resistance 
-- A A A A A 
Adhesion -- 100/100 
100/100 
100/100 
100/100 
100/100 
Pencil hardness 
-- Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
Over 9 H 
Abrasion resistance (mg) 
-- 0.38 0.41 0.39 0.42 0.48 
__________________________________________________________________________ 
Note: 
(*1) Commercially available product, Nip Seal AQ (trade name, product of 
Nippon Silica Kogyo Kabushiki Kaisha) 
(*2) Commercially available product, Cloisonne Green (trade name, product 
of The Mearl Corporation) 
(*3) Amount used per 100 parts by weight (based on solids) of the binder 
component 
Table 3 reveals that the coating composition containing pearl luster 
pigment in less than the specified amount produced the film inferior in 
all properties (specimen No. 14) while the coating composition containing 
the same in more than the specified amount gave the film lower in 
resistance to boiling water and resistance to saline solutions (specimen 
No. 19). 
EXAMPLE 4 
A 45 part by weight portion of pearl luster pigment, "Cloisonne Copper" 
(trade name, product of The Mearl Corporation) was added to 100 parts by 
weight, based on solids, of the binder component obtained in Example 3 for 
specimen No. 17. The mixture was homogenized by a ball mill for 2 hours, 
giving a coating composition according to the present invention. The 
coating composition thus obtained was applied by spraying to each of a 
commercially available plate made of asbestos cement and calcium silicate 
and a commercially available concrete block in an amount of 90 g/m.sup.2. 
The coated substrate was heated to 220.degree. C. for 20 minutes. The film 
thus formed had an attractive reddish brown pearl luster and was excellent 
in abrasion resistance and high in adhesion and hardness. 
EXAMPLE 5 
To 100 parts by weight, based on solids, of potassium silicate was added 20 
parts by weight of fine silica particles (commercially available product, 
trade name "Aerozil-200", product of Nippon Aerozil Kabushiki Kaisha, 
Japan). Water was added to the mixture in an amount of 260 parts by weight 
per 100 parts by weight of mixture of potassium silicate (based on solids) 
and fine silica particles. The mixture was heated in a reactor with 
stirring at 90.degree. to 100.degree. C. for 2 hours for dissolution to 
obtain a binder component. A 30 part by weight portion of pearl luster 
pigment (a commercially available product, trade name "4708 Iriodin 220 
Perlblau/Blue Pearl," product of EM Chemicals Co., Ltd.) was added to 100 
parts by weight, based on solids, of the binder component thus obtained. 
The mixture was homogenized by a stirrer for 30 minutes, giving a coating 
composition according to the present invention. The coating composition 
thus prepared was applied by spraying to a commercially available aluminum 
plate (specified in JIS H-4000-A5052P) in an amount of 70 g/m.sup.2. The 
coated plate was heated to 220.degree. C. for 20 minutes for curing. The 
film thus formed had an attractive blue pearl luster and was excellent in 
corrosion resistance and high in adhesion and hardness.