Coating film having water repellency and low refractive index

A coating film having a refractive index of from 1.28 to 1.38 and a contact angle of water of from 90.degree. to 115.degree., adhered on a substrate surface, and prepared by reacting a particular tetraalkylsilicate, a particular fluorine-containing silicon compound, a particular hydroxy-containing compound, and oxalic acid in a particular range of ratios and in the absence of water, and under further specified reaction conditions.

The present invention relates to an improvement of a coating film formed on 
a substrate from a polymer solution of an alkoxy group-containing silicon 
compound. Particularly, the present invention relates to a coating film 
having a low refractive index and a large contact angle of water, which is 
formed as adhered on a substrate surface, by heat-curing on a substrate 
surface a coating composed of a polysiloxane solution prepared by 
co-condensing, in the absence of water, alkoxy group-containing silicon 
compounds having a specific composition. 
It is known that when a coating film showing a refractive index lower than 
the refractive index of a substrate is formed on the surface of the 
substrate, the reflectance of light reflected from the surface of the 
coating film decreases. Such a coating film showing a decreased light 
reflectance is utilized as an antireflection film and practically applied 
to various substrate surfaces. 
JP-A-5-105424 discloses a process for forming an antireflection film having 
a low refractive index, on a substrate, which comprises coating on a glass 
substrate such as a cathode ray tube an alcohol dispersion of fine 
particles of MgF.sub.2 formed by reacting a magnesium salt or an alkoxy 
magnesium compound as a Mg source with a fluoride salt as a F source, or a 
liquid having tetraalkoxysilane or the like added thereto for improving 
the film strength, as a coating fluid, followed by heat-treatment at a 
temperature of from 100.degree. to 500.degree. C. 
JP-A-6-157076 discloses a low reflection glass having formed on a glass 
substrate a thin film showing a refractive index of from 1.21 to 1.40 and 
having a thickness of from 60 to 160 nm with irregularities or micro-pits 
having a diameter of from 50 to 200 nm, by mixing a solvent such as an 
alcohol with at least two hydrolytic polycondensates different in the 
average molecular weight, such as tetraalkoxysilane, methytrialkoxysilane 
and ethyltrialkoxysilane, to obtain a coating fluid, forming a coating 
film from such a coating fluid by controlling the relative humidity and 
the mixing ratio at the time of the above mixing, and heating the coating 
film. 
JP-B-3-23493 discloses a low reflectance glass comprising a glass, a lower 
layer film having a high refractive index formed on its surface and an 
upper layer film having a low refractive index formed on the surface 
thereof. As a detailed description of the process for forming the upper 
layer film, this publication discloses a process which comprises 
hydrolyzing a fluorine-containing silicon compound having a 
polyfluorocarbon chain, such as CF.sub.3 (CF.sub.2)C.sub.2 H.sub.4 
Si(OCH.sub.3).sub.3, and a silane coupling agent such as 
Si(OCH.sub.3).sub.4 in an amount of from 5 to 90 wt % based thereon, in an 
alcohol solvent in the presence of a catalyst such as acetic acid, 
followed by filtration to obtain a liquid of a co-condensate, then coating 
this liquid on the lower layer film and heating it at a temperature of 
from 120.degree. to 250.degree. C. 
The process for forming a multilayer coating film on a substrate as 
disclosed in the above JP-B-3-23493, requires repetition of the coating 
and baking steps, and is not efficient. Besides, due to repetition of the 
baking step, cracks are likely to form in the coating film, the resulting 
coating film tends to be non-uniform, and deformation of the substrate is 
likely to occur. Further, in order to impart a low refractive index to the 
upper layer film formed from the coating fluid obtained by such a 
hydrolytic method, it is required to use a large amount of the 
fluorine-containing silicon compound at a level of at least 1.1 mol per 
mol of the silane coupling agent, and even in such a case, a coating film 
having a refractive index lower than 1.33 is hardly obtainable. Further, 
if the coating fluid obtained by such a hydrolytic method, is directly 
coated on the substrate, and the coating is heated, the resulting coating 
film, will not have sufficient hardness. 
By the process disclosed in the above JP-A-5-105422, the bond strength 
among the fine particles of MgF.sub.2 is weak, so that the formed coating 
film is poor in the mechanical strength, and the adhesive strength to the 
substrate is inadequate. Besides, this coating film made of MgF.sub.2 does 
not essentially show a refractive index lower than 1.38, and depending 
upon the type of the substrate, no adequate antireflection property can be 
obtained. 
The process disclosed in the above JP-A-6-157076 is cumbersome in the 
preparation and incorporation of the polycondensates having different 
molecular weights and further requires control of the relative humidity 
during the film-forming and the surface irregularities of the coating 
film. Thus, this process is not practically useful. 
Each of the coating films disclosed in the above JP-A-5-105422 and 
JP-A-6-157076 is susceptible to staining of its surface during practical 
use, and to prevent such staining, it has been common to coat a treating 
agent having higher water repellency on its surface, such as a 
stain-proofing agent made of a fluorine-containing compound. 
It is an object of the present invention to provide a process for forming 
an improved coating film on a substrate simply and efficiently. 
Particularly, the present invention is intended to provide a coating film 
formed on a substrate as adhered to the surface of the substrate and 
having a refractive index of from 1.28 to 1.38 and a contact angle of 
water of from 90.degree. to 115.degree.. 
Namely, the present invention provides a process for forming a coating film 
on a substrate surface, which comprises preparing a reaction mixture 
comprising a silicon compound (A) of the following formula (1): 
EQU Si(OR).sub.4 ( 1) 
wherein R is a C.sub.1-5 alkyl group, a silicon compound (B) of the 
following formula (2): 
EQU CF.sub.3 (CF.sub.2)nCH.sub.2 CH.sub.2 Si(OR.sup.1).sub.3 ( 2) 
wherein R.sup.1 is a C.sub.1-5 alkyl group, and n is an integer of from 0 
to 12, an alcohol (C) of the following formula (3): 
EQU R.sup.2 CH.sub.2 OH (3) 
wherein R.sup.2 is a hydrogen atom, or an unsubstituted or substituted 
C.sub.1-12 alkyl group, and oxalic acid (D), in a ratio of from 0.05 to 
0.43 mol of the silicon compound (B) per mol of the silicon compound (A), 
in a ratio of from 0.5 to 100 mol of the alcohol (C) per mol of the total 
alkoxy groups contained in the silicon compounds (A) and (B), and in a 
ratio of from 0.2 to 2 mol of the oxalic acid (D) per mol of the total 
alkoxy groups contained in the silicon compounds (A) and (B); heating this 
reaction mixture at a temperature of from 50.degree. to 180.degree. C. 
until the total amount of the silicon compounds (A) and (B) remaining in 
the reaction mixture becomes at most 5 mol%, while maintaining a SiO.sub.2 
concentration of from 0.5 to 10 wt % as calculated from silicon atoms in 
the reaction mixture and maintaining absence of water, to form a 
polysiloxane solution; then coating a coating fluid comprising the 
polysiloxane solution on a substrate surface to form a coating; and 
heat-curing the coating at a temperature of from 80.degree. to 450.degree. 
C. to form a coating film having a refractive index of from 1.28 to 1.38 
and a contact angle of water of from 90.degree. to 115.degree., as adhered 
on the substrate surface; and such a coating film formed by the process. 
The above polysiloxane solution is transparent and contains no polysiloxane 
of gel form. Although a large amount of the alcohol (C) and a relatively 
large amount of oxalic acid (D) are coexistent, since the silicon 
compounds (A) and (B) are heated in a reaction mixture in the absence of 
water, this polysiloxane is not the one formed by condensation of 
hydrolyzates of the silicon compounds (A) and (B). When a polysiloxane is 
formed from an alkoxysilane by a method of hydrolysis in an alcohol 
solvent, the liquid tends to be turbid, or a non-uniform polysiloxane is 
likely to form, as the hydrolysis proceeds. With the above reaction 
mixture of the present invention, no such phenomenon will take place. 
The chemical structure of the polysiloxane of the present invention is 
complex and can not be specifically defined. However, it is considered 
that a co-condensate polysiloxane of the silicon compounds (A) and (B) 
having a degree of polymerization suitable for forming a solution and 
having a relatively uniform structure, will form, even though it may have 
a branched structure, as polymerization proceeds, probably as the alcohol 
(C) acts on an intermediate formed by the reaction of the silicon 
compounds (A) and (B) with oxalic acid (D). 
By heating the coating containing the above polysiloxane solution, coated 
on the substrate, removal of volatile components from the coating and a 
curing reaction of polysiloxane in the coating will proceed, whereby an 
insoluble coating film adhered to the substrate surface and having a low 
refractive index and water repellency, will form. The larger the molar 
ratio of the silicon compound (B) to the silicon compound (A), the lower 
the refractive index of this coating film, and the larger the contact 
angle with water. However, as is different from the upper layer film 
disclosed in the above JP-B-3-23493, the coating film of the present 
invention has a refractive index lower than the refractive index of the 
above upper layer film, in spite of the fact that it is formed from a 
coating fluid having a low content of the silicon compound (B). 
Now, the present invention will be described in detail with reference to 
the preferred embodiments. 
Examples of the alkyl group R in the above formula (1) include methyl, 
ethyl, propyl, butyl and pentyl. Preferred examples of the silicon 
compound (A) include tetramethoxysilane, tetraethoxysilane, 
tetrapropoxysilane and tetrabutoxysilane. Among them, particularly 
preferred are tetramethoxysilane and tetraethoxysilane. These compounds 
may be used alone or in combination as a mixture of two or more of them. 
Examples of the alkyl group R.sup.1 in the above formula (2) include 
methyl, ethyl, propyl, butyl and pentyl. Preferred examples of the silicon 
compound (B) include trifluoropropyltrimethoxysilane, 
trifluoropropyltriethoxysilane, tridecafluorooctyltrimethoxysilane, 
tridecafluorooctyltriethoxysilane, heptadecafluorodecyltrimethoxysilane 
and heptadecafluorodecyltriethoxysilane. These compounds may be used alone 
or in combination as a mixture of two or more of them. 
Examples of the unsubstituted alkyl group R.sup.2 in the above formula (3) 
include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl. 
Examples of the substituted alkyl group R.sup.2 includes hydroxymethyl, 
methoxymethyl, ethoxymethyl, hydroxyethyl, methoxyethyl and ethoxyethyl. 
Preferred examples of the alcohol (C) include methanol, ethanol, propanol, 
n-butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl 
ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl 
ether, propylene glycol monomethyl ether and propylene glycol monoethyl 
ether. Among them, particularly preferred is ethanol. 
A uniform polysiloxane solution is hardly obtainable from a reaction 
mixture in which the silicon compound (B) is used in an amount exceeding 
0.43 mol per mol of the silicon compound (A). From a reaction mixture 
wherein the silicon compound (B) is used in an amount of less than 0.05 
mol per mol of the silicon compound (A), a coating film having a 
refractive index of 1.38 or less will hardly be formed, and the coating 
film thereby formed will not exhibit water repellency showing a contact 
angle of water of at least 90.degree.. It is particularly preferred that 
the silicon compound (B) is used in an amount of from 0.05 to 0.25 mol per 
mol of the silicon compound (A). 
If the alcohol (C) is used in an amount smaller than 0.5 mol per mol of the 
total alkoxy groups contained in the silicon compounds (A) and (B), it 
takes a long time to form the polysiloxane, and it tends to be difficult 
to form a coating film having high hardness from the liquid containing the 
polysiloxane thereby obtained. On the other hand, if the alcohol (C) is 
used in an amount larger than 100 mol per mol of the total alkoxy groups 
contained in the silicon compounds (A) and (B), the SiO.sub.2 
concentration in the obtained polysiloxane-containing liquid tends to be 
inadequate, and concentration will be required prior to coating, such 
being inefficient. It is particularly preferred to use the alcohol (C) in 
an amount of from 1 to 50 mol per mol of the total alkoxy groups contained 
in the silicon compounds (A) and (B). 
If oxalic acid (D) is used in an amount smaller than 0.2 mol per mol of the 
total alkoxy groups contained in the silicon compounds (A) and (B), it 
tends to be difficult to form a coating film having high hardness from the 
resulting polysiloxane-containing liquid. On the other hand, if the oxalic 
acid (D) is used in an amount larger than 2 mol per mol of the total 
alkoxy groups contained in the silicon compounds (A) and (B), the 
resulting polysiloxane-containing liquid contains a relatively large 
amount of the oxalic acid (D), and from such a liquid, it tends to be 
difficult to obtain a coating film having the desired properties. It is 
particularly preferred to use the oxalic acid (D) in an amount of from 
0.25 to 1 mol per mol of the total alkoxy groups contained in the silicon 
compounds (A) and (B). 
To form the reaction mixture, an alkylalkoxysilane may be incorporated as a 
modifier (E), for example, in an amount of 0.02 to 0.2 mol per mol of the 
silicon compound (A), as the case requires, in addition to the silicon 
compounds (A) and (B), the alcohol (C) and the oxalic acid (D). 
Preferred examples of the modifier (E) include trialkoxysilanes such as 
methyltrimethoxysilane, methytriethoxysilane, ethyltrimethoxysilane, 
ethyltriethoxysilane, propyltrimethoxysilane, propyltriethoxysilane, 
butyltrimethoxysilane, butyltriethoxysilane, pentyltrimethoxysilane, 
pentyltriethoxysilane, heptyltrimethoxysilane, heptyltriethoxysilane, 
octyltrimethoxysilane, octyltriethoxysilane, dodecyltrimethoxysilane, 
dodecyltriethoxysilane, hexadecyltrimethoxysilane, 
hexadecyltriethoxysilane, octadecyltrimethoxysilane, 
octadecyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, 
vinyltrimethoxysilane, vinyltriethoxysilane, 
.gamma.-aminopropyltrimethoxysilane, .gamma.-aminopropyltriethoxysilane, 
.gamma.-glycidoxypropyltrimethoxysilane, 
.gamma.-glycidoxypropyltriethoxysilane, 
.gamma.-methacryloxypropyltrimethoxysilane and 
.gamma.-methacryloxypropyltriethoxysilane, and dialkoxysilanes such as 
dimethyldimethoxysilane and dimethyldiethoxysilane. Such a modifier (E) is 
capable of lowering the temperature for curing the coating on the 
substrate and improves the adhesion of the coating film to the substrate. 
The reaction mixture comprising the silicon compounds (A) and (B), the 
alcohol (C) and the oxalic acid (D), may be formed by mixing such 
components, or by further incorporating the above modifier (E) thereto. To 
such a reaction mixture, no water may be added. This reaction mixture is 
preferably heated in the form of a solution. For example, it is preferably 
heated as a reaction mixture in the form of a solution obtained by 
preliminarily adding the oxalic acid (D) to the alcohol (C) to form an 
alcohol solution of oxalic acid and then mixing the silicon compounds (A) 
and (B) and the above modifier (E) to the solution. The reaction mixture 
comprising the silicon compound (A), the silicon compound (B), the alcohol 
(C) and the oxalic acid (D) in the above-mentioned ratios, usually has a 
SiO.sub.2 concentration of from 0.5 to 10 wt % when silicon atoms 
contained therein are calculated as SiO.sub.2. Also in the case of the 
reaction mixture containing the above modifier (E), such a modifier (E) is 
incorporated so that the mixture will have a SiO.sub.2 concentration of 
from 0.5 to 10 wt % when silicon atoms contained therein are calculated as 
SiO.sub.2. Such a reaction mixture is maintained at the above SiO.sub.2 
concentration and in the absence of water during the heating of the 
reaction mixture. This heating can be carried out in a usual reactor at a 
liquid temperature of from 50.degree. to 180.degree. C. Preferably, it is 
carried out, for example, in a closed container or under reflux, so that 
no evaporation or volatilization of the liquid from the reactor occurs. 
If the heating to form the polysiloxane is carried out at a temperature 
lower than 50.degree. C., the liquid tends to have turbidity or tends to 
contain insoluble substances. Therefore, this heating is carried out at a 
temperature higher than 50.degree. C. If the temperature is high, the 
operation can be completed in a short period of time. However, heating at 
a temperature higher than 180.degree. C. is inefficient, as no additional 
merits will be thereby obtained. The heating time is not particularly 
limited. For example, it is usually about 8 hours at 50.degree. C and 
about 3 hours under reflux at 78.degree. C. Usually, the heating is 
terminated when the amount of the remaining silicon compounds (A) and (B) 
becomes at most 5 mol %, based on the total charged amount of the silicon 
compounds (A) and (B). If a polysiloxane-containing liquid in which these 
silicon compounds remain more than 5% based on the total amount of the 
silicon compounds (A) and (B) charged, is coated on a substrate surface 
and then the coating is heat-cured at a temperature of from 80.degree. to 
450.degree. C., the resulting coating film tends to have pinholes, or it 
tends to be difficult to obtain a coating film having adequate hardness. 
The polysiloxane solution obtained by the above heating, may be used 
directly as a coating fluid for the next coating step. However, if 
desired, it may be concentrated or diluted to obtain a solution useful as 
a coating fluid, or the solvent may be substituted by other solvent to 
obtain a solution useful as a coating fluid. Otherwise, an optional 
additive (F) may be added thereto to obtain a coating fluid. Examples of 
such an additive (F) include a silica sol, an alumina sol, a titania sol, 
a zirconia sol, a magnesium fluoride sol and a ceria sol, which are in the 
form of sols of colloidal inorganic fine particles. These sols may be used 
alone or in combination as a mixture of two or more of them. Such sols are 
preferably organo sols. Particularly preferred are organo sols using the 
alcohol (C) as the dispersing medium. The amount of the sol to be added, 
may be selected optionally, so long as the amount of colloidal inorganic 
fine particles is at most 70 wt %, based on the total weight of the heat 
cured solid content in the coating fluid. As other additives (F), metal 
salts or metal compounds may, for example, be mentioned. These additives 
are suitable for controlling the water repellency of the coating film. 
The coating fluid to be used in the coating step, is preferably a fluid 
which contains from 0.5 to 10 wt %, as calculated as SiO.sub.2, of silicon 
atoms derived from the above polysiloxane transparent solution. If this 
SiO.sub.2 concentration is less than 0.5%, the thickness of the coating 
film formed by one coating operation tends to be thin. If the 
concentration exceeds 10 wt %, the storage stability of such a coating 
fluid tends to be inadequate. It is particularly preferred that the 
SiO.sub.2 concentration of this coating fluid is from 2 to 8 wt %. 
The substrate is not particularly limited so long as it permits formation 
of an adhesive coating film thereon. In order to form an antireflection 
coating film thereon, it is preferred to use a substrate having refractive 
index higher the refractive index of the coating film, such as usual glass 
or plastics. 
The above polysiloxane solution or a coating fluid comprising such a 
solution, can be coated on the substrate by a conventional method such as 
a dipping method, a spin coating method, a brush coating method, a roll 
coating method or a flexo printing method. 
The coating formed on the substrate may directly be heat-cured. However, 
prior to such heat-curing, it may be dried at a temperature of from room 
temperature to 80.degree. C., preferably from 50.degree. to 80.degree. C., 
and then heated at a temperature of from 80.degree. to 450.degree. C., 
preferably from 100.degree. to 450.degree. C. The time for this heating 
may be from 5 to 60 minutes for adequate heat-curing. If this heating 
temperature is lower than 80.degree. C., the hardness, chemical resistance 
or the like of the resulting coating film tends to be inadequate. In the 
case of a heat resistant substrate such as glass, heating may usually be 
carried out at a temperature of at least 300.degree. C. However, at a 
temperature higher than 450.degree. C., no adequate water repellency tends 
to be imparted to the resulting coating film. Such heating can be carried 
out by a conventional method, for example, by using a hot plate, an oven 
or a belt furnace.

Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted to such specific Examples. 
EXAMPLE 1 
70.8 g of ethanol was charged into a four-necked flask equipped with a 
reflux condenser, and 12.0 g of oxalic acid was gradually added to this 
ethanol with stirring, to prepare an ethanol solution of oxalic acid. 
Then, this solution was heated to its reflux temperature, and a mixture 
comprising 11.0 g of tetraethoxysilane and 6.2 g of 
tridecafluorooctyltrimethoxysilane, was dropwise added to this solution 
under reflux. After completion of the dropwise addition, heating was 
continued for 5 hours under reflux, followed by cooling to obtain a 
polysiloxane solution (L.sub.1). 
This solution (L.sub.1) was analyzed by gas chromatography, whereby no 
alkoxide monomer was detected. This solution (L.sub.1) was coated on the 
surface of a calcium fluoride substrate, and then, the coating was heated 
at 300.degree. C. for 30 minutes to form a coating film adhered to the 
surface of this calcium fluoride substrate. Then, with respect to this 
coating film, the spectrum of transmitted light was measured by means of 
an infrared spectroscope, whereby absorption by a silanol group was 
observed in the vicinity of 3,200 cm.sup.-1 and 980 cm.sup.-1, absorption 
by a methylene group was observed in the vicinity of 2,800 cm.sup.-1, 
absorption by Si--O--Si was observed in the vicinity of 1,100 cm.sup.-1, 
and absorption by C--F was observed in the vicinity of 1,200 cm.sup.-1. 
EXAMPLE 2 
72.4 g of ethanol was charged into a four-necked flask equipped with a 
reflux condenser, and 12.0 g of oxalic acid was gradually added to this 
ethanol with stirring, to prepare an ethanol solution of oxalic acid. 
Then, this solution was heated to its reflux temperature, and a mixture 
comprising 12.5 g of tetraethoxysilane and 3.1 g of 
tridecafluorooctyltrimethoxysilane, was dropwise added to this solution 
under reflux. After completion of the dropwise addition, heating was 
continued for 5 hours under reflux, followed by cooling to obtain a 
polysiloxane solution (L.sub.2). This solution (L.sub.2) was analyzed by 
gas chromatography, whereby no alkoxide monomer was detected. 
EXAMPLE 3 
70.6 g of ethanol was charged into a four-necked flask equipped with a 
reflux condenser, and 12.0 g of oxalic acid was gradually added to this 
ethanol with stirring, to prepare an ethanol solution of oxalic acid. 
Then, this solution was heated to its reflux temperature, and a mixture 
comprising 9.4 g of tetraethoxysilane, 6.2 g of 
tridecafluorooctyltrimethoxysilane, 1.2 g of 
.gamma.-glycidoxypropyltrimethoxysilane and 0.6 g of 
.gamma.-aminopropyltrimethoxysilane, was dropwise added to this solution 
under reflux. After completion of the dropwise addition, heating was 
continued for 5 hours under reflux, followed by cooling to obtain a 
polysiloxane solution (L.sub.3). This solution (L.sub.3) was analyzed by 
gas chromatography, whereby no alkoxide monomer was detected. 
EXAMPLE 4 
149 g of ethanol and 51.0 g of a methanol-dispersed silica sol containing 
colloidal silica having a particle size of 8 nm in an amount of 15.7 wt % 
as SiO.sub.2, were added to 100 g of the solution (L.sub.3) obtained in 
Example 3, followed by mixing thoroughly, to obtain a polysiloxane 
solution (L.sub.4). 
EXAMPLE 5 
223.6 g of ethanol and 76.4 g of a methanol-dispersed silica sol containing 
colloidal silica having a particle size of 8 nm in an amount of 15.7 wt % 
as SiO.sub.2, were added to 100 g of the solution (L.sub.3) obtained in 
Example 3, followed by mixing thoroughly, to obtain a polysiloxane 
solution (L.sub.5). 
Comparative Example 1 
Into a four-necked flask equipped with a reflux condenser, 43.7 g of 
ethanol, 16.6 g of tetraethoxysilane and 9.3 g of 
tridecafluorooctyltrimethoxysilane were charged and mixed to obtain an 
ethanol solution. Then, this solution was heated to its reflux 
temperature, and a mixture comprising 24.9 g of ethanol, 5.4 g of water 
and 0.1 g of nitric acid as a catalyst, was dropwise added to this 
solution under reflux. After completion of the dropwise addition, heating 
was continued for 5 hours under reflux, followed by cooling to obtain a 
liquid (L.sub.6) comprising hydrolyzates of the alkoxysilanes. 
Comparative Example 2 
Into a four-necked flask equipped with a reflux condenser, 72.0 g of 
ethanol was charged, and 11.4 g of oxalic acid was gradually added to this 
ethanol with stirring, to prepare an ethanol solution of oxalic acid. 
Then, this solution was heated to its reflux temperature, and a mixture 
comprising 11.0 g of tetraethoxysilane and 5.6 g of 
octadecyltrimethoxysilane, was dropwise added to this solution under 
reflux. After completion of the dropwise addition, heating was continued 
for 5 hours, followed by cooling to obtain a polysiloxane-containing 
liquid (L.sub.7). 
Comparative Example 3 
Into a four-necked flask equipped with a reflux condenser, 53.7 g of 
ethanol and 20.8 g of tetraethoxysilane were charged and mixed to prepare 
an ethanol solution of tetraethoxysilane. Then, this solution was heated 
to its reflux temperature, and a mixture comprising 20.0 g of ethanol, 5.4 
g of water and 0.1 g of nitric acid as a catalyst, was dropwise added to 
this solution under reflux. After completion of the dropwise addition, 
heating was continued for 5 hours at a reflux temperature, followed by 
cooling to obtain a liquid comprising a hydrolyzate of the alkoxysilane. 
Then, to the entire amount of this liquid, 700 g of ethanol and 100 g of a 
methanol-dispersed silica sol containing colloidal silica having a 
particle size of 12 nm in an amount of 30 wt % as SiO.sub.2, were added 
and thoroughly mixed, to obtain a mixed liquid (L.sub.8). 
EXAMPLE 6 
Each of the above liquids (L.sub.1) to (L.sub.8) was used as a coating 
fluid and spin-coated on a substrate to form a coating, and then this 
coating was dried on a hot plate at 80.degree. C. for 5 minutes, followed 
by heating at a temperature identified in Table 1 in a baking furnace, to 
form a coating film on the substrate surface. Then, with respect to each 
coating film thus obtained, the pencil hardness, the refractive index, the 
reflectance, the contact angle of water and the film thickness were 
measured by the following methods. 
For the measurements of the pencil hardness and the reflectance, the 
coating film was formed on the surface of a soda lime glass substrate 
having a refractive index of 1.52 and a reflectance of from 4 to 5%, and 
for the measurement of the refractive index, the coating film was formed 
on the surface of a silicon substrate. 
Method for measuring pencil hardness: In accordance with the method 
prescribed in JIS K5400. 
Method for measuring refractive index: Using Ellipsometer DVA-36L, 
manufactured by Mizojiri Kogaku K.K., the refractive index of light with a 
wavelength of 633 nm was measured. 
Method for measuring reflectance: Using spectrophotometer UV 3,100 PC, 
manufactured by Shimadzu Corporation, the reflectance of light with an 
wavelength of 550 nm was measured at an angle of incidence of 5.degree.. 
Method for measuring contact angle of water: Using an automatic contact 
angle meter CA-Z model, manufactured by Kyowa Kaimen Kagaku K.K., the 
contact angle when 3 .mu.l of pure water was dropped, was measured. 
Method for measuring film thickness: The coating after drying, was cut by a 
cutter and then heat-cured to obtain a coating film, and with respect to 
the coating film, the film thickness was measured by measuring the 
difference in level by means of a Talystep, manufactured by Rank Taylor 
Hobson Company. 
The results of these measurements are shown in Table 1. 
TABLE 1 
______________________________________ 
Contact 
Coat- 
Curing Film Pencil 
Refrac- 
Reflec- 
angle of 
ing temperature 
thickness 
hard- tive tance water 
fluid 
(.degree.C.) 
(nm) ness index (%) (.degree.) 
______________________________________ 
L.sub.1 
300 100 7H 1.36 1.2 105 
L.sub.1 
350 98 8H 1.35 0.9 104 
L.sub.1 
450 98 8H 1.32 0.8 105 
L.sub.1 
550 95 8H 1.39 1.5 10 or less 
L.sub.2 
300 105 8H 1.38 1.5 100 
L.sub.3 
100 97 7H 1.38 1.5 105 
L.sub.4 
100 110 6H 1.35 1.1 103 
L.sub.5 
300 90 7H 1.29 0.6 100 
L.sub.6 
300 100 7H 1.42 2.3 95 
L.sub.7 
300 100 7H 1.43 2.3 80 
L.sub.7 
350 95 8H 1.42 2.3 30 
L.sub.7 
450 93 8H 1.42 2.3 10 or less 
L.sub.8 
300 110 7H 1.33 0.9 10 or less 
______________________________________ 
As shown in Table 1, when the coating of the coating fluid (L.sub.1) was 
heated at a temperature of 300.degree. C., 350.degree. C. or 450.degree. 
C., the coating film of the present invention was obtained. Whereas, when 
the coating of the coating fluid (L.sub.1) was heated at 550.degree. C., a 
coating film of a Comparative Example was formed which had a contact angle 
of water of 10.degree. or less and a refractive index of 1.39. 
Each of the coating films obtained by heating the coating fluid (L.sub.2) 
at 300.degree. C., the coating fluid (L.sub.3) at 100.degree. C., the 
coating fluid (L.sub.4) at 100.degree. C. and the coating fluid (L.sub.5) 
at 300.degree. C., respectively, was excellent. 
Each of the coating films obtained by heating at 300.degree. C., the 
coatings of the comparative coating fluid (L.sub.6) obtained by hydrolysis 
and the comparative coating fluid (L.sub.7) prepared without using the 
silicon compound (B), respectively, failed to show a refractive index of 
1.38 or less. 
The coating film formed by heating at 300.degree. C. the comparative 
coating fluid (L.sub.8) containing the hydrolyzate of tetraalkoxysilane 
and the colloidal silica, showed a refractive index of 1.33, but had a 
contact angle of water of 10.degree. or less. 
The polysiloxane solution used for forming the coating film of the present 
invention has a stability durable for storage of about 6 months at normal 
temperature and thus can be presented as an industrial product. The 
coating film of the present invention can readily be obtained by a step of 
coating on a substrate surface a coating fluid comprising this solution of 
industrial product and a step of heat-curing the coating thereby formed. 
By forming the coating film of the present invention on a substrate having 
a refractive index higher than the refractive index of the coating film of 
the present invention, such as on the surface of usual glass, this 
substrate can readily be converted to an antireflecting substrate. The 
thickness of the coating film of the present invention can be controlled 
by the thickness of the coating, but it may readily be controlled by 
adjusting the SiO.sub.2 concentration in the coating fluid. The coating 
film of the present invention may be effectively used as a single coating 
film on the substrate surface, but it may be used also as an upper layer 
coating film on a lower layer coating film having a high refractive index. 
It is known that between the thickness d (nm) of the coating film having a 
refractive index a and the wavelength .lambda. (nm) of light, of which 
reduction in reflectance by this coating film, is desired, there is a 
relation represented by the formula d=(2b-1).lambda./4a, wherein b is an 
integer of at least 1. Accordingly, by determining the thickness of the 
coating film using this formula, it is readily possible to prevent 
reflection of a desired light. For example, prevention of reflection from 
a glass surface of a light having a center wavelength of visible light of 
550 nm by a coating film having a refractive index of 1.32, can readily be 
accomplished by employing a coating film thickness of 104 nm which is 
obtainable by substituting these numerical values for .lambda. and a in 
the above formula and 1 for b, or a coating film thickness of 312 nm which 
is likewise obtainable by substituting 2 for b. 
The coating film of the present invention may be applied to the surface of 
various products for which antireflection of light is desired, including 
cathode ray tubes made of glass, displays for computers, mirrors having 
glass surface and show cases made of glass. The coating film of the 
present invention also has excellent water repellency, and by forming this 
coating film on a hydrophilic substrate surface, the hydrophilic substrate 
surface susceptible to staining can be converted to a stainproof surface.