Note: Descriptions are shown in the official language in which they were submitted.
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Easy-to-Clean Coating
Description:
The invention relates to a method for producing a product having
an easy-to-clean surface by coating the surface with a
hydrophobic substance, as well as the products obtained with the
method.
It is generally known to provide objects with dirt-repelling
substances. Thus, for example, it is also known to treat surface
of glasses, glass ceramics, glazes and also rocks by means of
silicones to make them dirt-repellent and water-repellent. This
is usually accomplished by rendering the surface of the objects
to be treated hydrophobic by applying a liquid composition. For
this purpose, a plurality of chemicals are usually used, in
particular, however, silicone oils and/or fluorinated silanes.
Surfaces which have been treated in this way have shown to be
difficult to wet, as a result of which water beads up and runs
off. Dirt only adheres slightly to the treated surface and can
thus be easily removed. This is especially advantageous for
outdoor use since, for example, in the case of skylights and/or
glass roofs such as on winter gardens, etc., the deposited dirt
is entrained and removed as a result of the rainwater beading up
and running off. In this way, it is possible to keep such
windows appearing permanently clean without additional cleaning.
However, this method has the disadvantage that the applied
chemicals only react and form permanent bonds with OH groups
available directly on the substrate material. As there are
insufficient reactive OH groups on the surface of objects, in
particular glass, without an appropriate pretreatment such as,
for example hydrogen/oxygen plasma, only a very thin, mostly
monomolecular hydrophobic layer can be produced by means of such
methods which is quickly rubbed off during use, in particular
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also under a mechanical stress such as e.g. during cleaning
and/or abrasion by wind and dust, as a result of which the
desired self-cleaning property is lost.
Therefore, attempts have already been made to increase the
durability of coatings of this type. For example, EP-A 0 658 825
describes the preparation of a water-repellent multilayer film.
Three different sol solutions are thereby produced, mixed,
applied to a glass substrate and a gel coating produced on the
glass surface. By heating, a metal oxide surface is then
produced. A fluoroalkyl silane layer is then applied to this
metal oxide layer, as previously described.
A method is described in JP-A 11 092 175 according to which
methoxy silane or an ethoxy silane compound containing a
fluorocarbon chain is fixed on the surface of small particles
having a diameter of 100 nm. The thus modified particles are
then dissolved in an aqueous medium and applied to a surface to
be coated, the solvent is then removed and the residue baked.
In this way, a surface coated with small hydrophobic particles
is obtained.
WO 99/64363 describes the preparation of a water-repellent
surface in which the surface of the glass is first roughened and
all metal ions present on the surface are removed. A water-
repellent film is then applied to the previously treated surface
in a known manner. By roughening the surface, the thus obtained
roughening valleys are filled with the water-repellent agents.
WO 99/02463 describes the preparation of a scratch-resistant
coating in which an organic substance with silicone-like networks
is applied to the surface. This is followed by a heat treatment
in which the temperature and duration are selected so that the
purely organic layer applied is substantially degraded and/or
removed, but a compound of inorganic molecules of the support
material and organic molecules of the applied substance can form
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in the uppermost molecular layers. In this way, an organic
substance, for example a methyl group, is directly attached to
the silicon atom of the glass surface with formation of a Si-C
bond.
DE 695 02671 T2 (WO 95/24053) describes an indicating device with
a screen which has a non-absorbing cover layer consisting of a
hybrid inorganic-organic material and an inorganic network
consisting of silicon dioxide and metal oxide. The polymer
chains are thereby intertwined with the inorganic network and
form a hybrid inorganic-organic network. However, it was shown
that organic components, in particular hydrophobic organic
components such as fluoroalkyls, do not set homogeneously in such
a layer, but accumulate essentially on the surface facing away
from the support layer. For this reason, this outer hydrophobic
layer can rub off relatively easily.
All hydrophobic as well as optionally dirt-repellent properties
produced with these methods are shown to be insufficiently
durable in use and are quickly lost, in particular under
mechanical stress.
Therefore, the object of the invention is to provide an easy care
object whose easy-care and dirt-repellent finish is durable and
which also remains abrasion-resistant under stress, as a result
of which the aforementioned easy-care properties on the object
or product are retained for a long time.
Moreover, the object of the invention is to provide such a finish
for optical elements which do not, or not noticeably, change the
optical properties of the element.
According to the invention, this object is attained by the method
defined in the claims as well as the product obtained therewith.
According to the invention, it was found that a uniform,
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resistant coating or a coating on a product can be obtained
having properties which are homogeneous in cross section by
providing its surface with a layer which comprises a thin metal
oxide network or a metal oxide matrix, a hydrophobic substance
being uniformly distributed in the network. The layer is usually
a uniform layer consisting of a coherent metal oxide network
spread out in a sheet-like manner. The metal oxide networks of
the invention can have open or closed pores.
The metal oxide layers according to the invention axe formed by
thermal treatment of an applied gel layer and remain as a firm
finish or coating on the product. In the coating according to
the invention, the hydrophobic substance is uniformly
distributed, i.e. it is present on the layer side adhering to the
support material to the outer layer surface over the cross
section in a homogeneous concentration and does not accumulate,
exclusively or primarily, on the outer coating surface. In this
way, the surface layer also retains the desired properties
according to the invention when there is surface abrasion.
The gels used according to the invention are, in particular,
metal oxide gels which are produced by a sol-gel process. The
gels are thereby formed in situ during the application to the
object or product to be coated, as a result of which a uniform
continuous gel network is produced on the surface of the object
to be coated. Preferred metal oxides are Si02, A1203, Fe203, In203,
Sn02, Zr02, BZO4 (sicJ and/or TiOz. Hydrogels, alkogels, xerogels
and/or aerogels are preferred gels. The result of the addition
of the hydrophobic, optionally also oleophobic substance
according to the invention to the sol mixture prior to or during
formation of the gel is that the hydrophobic substance is
uniformly distributed throughout the forming gel network and is
chemically bound by polycondensation, for example of its silanol
groups. In this way, it is possible to provide the surface thus
treated with especially wear-resistant and durable dirt-repellent
properties.
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The general production of gel layers by means of a sol-gel
process is known in itself and has been frequently described.
Usually, with this process, a polymer reaction is produced by
means of hydrolysis in a solution, preferably an aqueous and/or
alcoholic solution, with inorganic metal salts or metal organic
compounds such as metal alkoxides, whereby a colloidal
suspension, i.e. a sol, is produced. By further hydrolysis, a
coherent gel network is formed from the sol. Preferably, the
formation of the gel is produced directly during coating. The
final formation of the entire gel network is preferably
accelerated by heating. Typical temperatures for this are
between 0°C and 200°C, preferably between 20°C and
200°C, in
particular between room temperature and 170°C, whereby a
temperature of 150°C is especially preferred. By selecting the
hydrolysis conditions, it is possible to produce very dense, i. e.
more or less pore-free gel networks or networks having only the
tiniest pores. Metal alkoxides are preferably C~ - C4 metal
alkoxides, metal methylates and metal ethylates being especially
preferred. Among the metal salts, metal nitrates are preferred.
The hydrolysis with sol formation is usually started with an
excess of distilled water and the sol formation is completed by
allowing it to stand at ambient temperature and optionally also
at elevated temperature for an extended period of time, for
example two to four days.
Generally, suitable hydrophobic substances are all hydrophobic
substances which can be incorporated into the forming gel. For
the method according to the invention, it is preferable to use
those hydrophobic sustances which are capable of distributing
themselves as uniformly as possible in the gel-forming sol
solution. Therefore, the hydrophobic substances used according
to the method of the invention are preferably slightly water-
soluble in themselves or can be made water-soluble by means of
solubilizers or by hydrolysis. In a further preferred
embodiment, the oleophobic substances used according to the
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invention have a chemical modification which impart water
solubility. Modifications of this type are water-soluble groups,
e.g. amino residues or acid groups. Examples of this are natural
and synthetic oils and/or long-chain fatty acids, in particular
fatty acids having a chian with at least six carbon atoms,
preferably at least 10 carbon atoms. However, especially
preferred are hydrophobic oleophobic substances, in particular
silicones sand silanes, siloxanes, silicone oils and silicone
greases. The silicone compounds used according to the invention
can be linear or branched or possibly also contain cyclic silane
groups. In a preferred embodiment, they contain a water-
solubility imparting function, e.g. an amino group, whose
hydrogen atoms can also be optionally substituted.
The hydrophobic substances used according to the invention
preferably contain fluorine and have, in particular, at least 5%,
preferably at least 10~ fluorine atoms (relative to the total
number of atoms of the hydrophobic substance finally incorporated
after sintering). Preferably, however, they have at least 20%
fluorine atoms, at least 30% being especially preferred.
Although it was found that the incorporation of the hydrophobic
substances according to the invention by the in-situ process
results in lasting dirt repellency, it is preferred to chemically
link the hydrophobic substances with the gel network by means of
reactive groups, in particular by means of reactive silanol
groups. Hydrophobic substances with methoxy, ethoxy, propoxy,
butoxy or isocyanate groups and clorosilanes are especially well
suited.
The silanes preferred in the method according to the invention
have the general formula
( CFxHy ) - ( CF.Hb ) "- ( CF,.Hb. ) ~-S i. - ( OR ) 3
wherein x and y independently of each other stand for 0, 1, 2 or
3 and x + y = 3, and a, a' and b, b' independently of each stand
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for 0, 1 or 2, and a + b as well as a' + b' - 2 and n and m
independently of each other denote an integer from 0 to 20 and
together add up to a maximum of 30 and R is a straight-chain,
branched, saturated or unsaturated (optionally containing
heteroatoms) C1 - Cg alkyl group. Preferred alkyl groups are
methyl, ethyl and propyl groups as well as their amino
derivatives. According to the invention, silanes which have
functional groups comprising heteroatoms or a heteratom that
increase or impart water solubility of the silane, are preferred.
The heteroatoms and/or functional groups are incorporated in the
backbone of the alkyl carbon chain and/or the fluoroalkyl carbon
chain and/or adhere thereto as a substituent. According to the
invention, amino alkyl groups and/or amino fluoroalkyl groups are
especially preferred.
In a preferred embodiment, x = 3 and y = 0, so that the above
general formula has an end CF3 group. In a further preferred
embodiment of the invention, a = 2 and a' = 0, so that CF2 and CH2
blocks are formed. Of course, there can also be more than 2
blocks in the chain and the CFZ blocks and CHZ blocks can be
interchanged. However, it is preferred to have the fluorinated
blocks ending at the Si atom. Preferred values for n are 1 - 10,
in particular 1 - 8, and for m 0 - 10, in particular 0 - 8. In
the gel solution to be applied, the weight ratio of the
hydrophobic substance to the gel network is preferably 0.01:1 to
1:1, ratios between 0.05:1 and 0.2:1 being preferred.
The mixture of gel and hydrophobic substance according to the
invention is applied by means of conventional coating methods,
spraying and dip-coating being preferred. The thickness of the
coating can thereby be controlled by adjusting the viscosity and
the rate at which the object to be coated is withdrawn from the
dipping solution. Thus, in a special embodiment, the coating
mixture also contains viscosity modifiers such as PVP, PVA and
PEO. Layer thicknesses preferably produced according to the
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invention are between 0.5 nm - 1 ~cm, layer thicknesses of < 200
nm being preferred. After application, the layer is preferably
dried at room temperature for at least 1 minute, preferably at
least 3 minutes, and then hardened at an elevated temperature at
which optionally added substances, such as viscosity modifiers
are pyrolyzed or burned. The drying time depends on the layer
thickness produced, on the actual temperature and on the vapour
pressure of the solvent and is preferably at least 1 minute and
is, in particular, at least 3 minutes. The drying times are
usually 4 - 6 minutes. The sintering or hardening fo the applied
layer preferably occurs at temperatures of 150°C - 400°C,
preferably at 250°C - 380°C. The duration of the hardening is
usually maximum of 1 hour, a maximum of 45 minutes and in
particular a maximum of 30 minutes being preferred.
By means of the degree of hydrolysis, it is possible to set the
viscosity of the coating solution, in particular the dipping
solution, accurately to a drawable value. In this way, with a
known viscosity and a known drawing rate, the layer thickness
produced in each case can be exactly reproduced. A change in the
viscosity when using the coating or dipping solution can be
easily adapted to the respectively desired value by diluting it
with solvents, e.g. ethanol, or by adding a further hydrolyzable
sol-gel solution.
In the method according to the invention, it is also possible to
adapt the refractive index of the coating to the support
material. This is possible, for example, by mixing various metal
oxides. The refractive index of Si02 is n = 1.45, Ti02 is n =
2.3. In a Si02/Ti02 system, any refractive values desired within
these extreme values can be set, depending on the composition.
By setting the refractive index and the layer thickness, the
method according to the invention is also especially suitable for
producing interference coatings, e.g. to reduce reflectivity.
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In a special embodiment of the invention, the durable hydrophobic
layer is provided with a surface microstructure by means of
appropriate measures taken before, during or after the thermal
hardening, as a result of which the hydrophobic properties of the
coating are enhanced and its cleaning facilitated or the layer
is provided with an antireflective effect or this effect is
enhanced. Such effects can be achieved by incorporating
particles or by embossing. In this way, surface microstructures
can be obtained, which have, for example, knobs which limit a
contact of dirt particles and the surface coated according to the
invention to a few contact points as is the case, for example,
with the so-called lotus effect. In this way, the desired
cleaning effect is further enhanced.
In principle, by means of the method according to the invention,
it is possible to coat any materials capable of withstanding the
sintering temperatures described above.' These include, in
particular, metals, plastics, inorganic minerals and rocks, such
as e.g. marmor, granite, burned clay. However, it is especially
preferred to coat glass and glass ceramics with the method
according to the invention. Preferred glasses for this purpose
are borosilicate, soda-lime and optical glasses. The method of
the invention is especially suitable for producing easy-to-clean
flat glasses and, in particular, float glasses, curved glasses,
optical lenses, glass tubes, TV and PC screens and front glasses
therefor, furthermore, glass ceramic products, motor vehicle
glass enclosures, enamelled and/or ceramic products. Preferred
flat glasses are, for example, window glass, mirror glasses,
shower enclosure glasses, glass shelves, cover glasses for solar
collectors, sight glasses, instrument glasses, glass keyboards,
touch screen panels, display cover glasses, for example, for
mobile telephones and laptops, glasses for stoves, for example
baking oven panels, glass baking trays and/or glass baking
containers, lamp cover glasses and glasses for refrigerators and
furniture. Curved glasses are, for example, spotlight glasses,
lamp cover glasses, watch glasses and/or sanitary glasses. Glass
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lenses are, for example, spectacle lenses, ocular and objective
glasses in optical devices. Glass tubes are, for example, solar
collector tubes and wastewater pipes. Vehicle glass enclosures
are, for example, windows and instrument covering glasses for
automotive vehicles, for rail-borne vehicles such as trains,
etc., for ships and airplanes. Enamelled products are, for
example, baking trays, sauce pans and sanitary objects such as
wash basins, urinals, bathtubs and toilet bowls. Ceramic
products are, for example, tiles, roofing tiles and the
aforementioned sanitary objects.
The method of the invention is also suitable for coating
household items such as drinking glasses, glass cooking utensils
and cooking areas made of glass-ceramic as are available, for
example, under the trade name CERAN. The method according to the
invention was also found to be suitable for coating enamelled
cooking utensils, e.g. pots and pans.
However, it is also possible to produce multiple interference-
optical layers, for example reflectivity-reducing layers, by
means of the method of the invention. Such reflectivity-reducing
coatings according to the invention are preferably produced as
the outermost layer exposed the surroundings or air.
The invention will be described in greater detail by the
following examples.
Example 1 - Preparation of Hydrophobically Modified SiO,, Dipping
Solutions
a) A mixture is produced from 13.6 g tetramethylorthosilicate
(CAS:681-84-5 available under the name Dynasil"' M from Degussa,
Frankfurt/Germany) and 13.6 g 96% ethanol (mixture A), as well
as a mixture of 3.75 g distilled water and 0.15 g 36 HC1 (mixture
B) . Mixture A and B are mixed and stirred for 10 minutes at room
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temperature. A mixture of 1.4 g of a water-soluble modified
fluoroalkylsiloxane (to CAS 64-17-5 available under the name
Dynasylan'''" F8800 from Degussa, Frankfurt/Germany) and 175 g 96%
ethanol where added while stirring. This mixture is used as a
dipping solution.
b) Similar to a), a mixture A consisting of 13.6 g
ethylpolysilicate (from tetraethylsilicate, available under the
name Dynasyl'"' 40 from Degussa AG, Frankfurt/Germany) and 13.6 g
96% ethanol and a mixture B consisting of 3.8 g water and 0.15
g 36% hydrochloric acid is prepared, then the two mixtures are
added together and stirred for 10 minutes. A mixture of 1.4 g
of a water-soluble modified fluoroalkylsiloxane which contains
aminoalkyl-functional substituents (CAS No. 64-17-5, available
from Degussa AG, Frankfurt/Germany under the name Dynasylan"'
F8800) and 175 g 99.5% ethanol is then added while stirring.
c) A mixture of 254.2 g 99.5% ethanol, 77.6 g water, 7.2 g
glacial acidic acid and 90.8 g tetramethylorthosilicate (Dynasi h"
M, see above) are stirred together and allowed to stand for 24
hours. 25 g of the concentrate thus obtained are then mixed
while stirring with 75 g 99.5% ethanol and then stirred together
with a mixture of 100 g 99 . 5 % ethanol and 1.4 g of a fluoroalkyl-
functional water-soluble polysiloxane which is made water-soluble
by means of aminoalkyl-functional substitution (CAS No. 64-17-5,
Dynasylan"' F8800), as a result of which the finished dipping
solution is produced.
d) 88.6 ml silicic acid methylester, 80 ml of distilled water
and 10 ml glacial acidic acid are stirred into 240 ml ethanol.
The solution thus obtained is allowed to stand for 72 hours. It
is then diluted with 1.580 ml ethanol and the hydrolysis stopped
with 2 ml of a 37% hydrochloric acid. 8.6 ml
tridecafluorooctyltriethoxysilane (available under the name
Dynasylan F8261 from DEGUSSA-HLTLS, Frankfurt, Germany) are then
added while stirring.
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The coating was applied according to the invention by means of
a single dipping step. It was subsequently dried for five
minutes at room temperature and baked at 250°C for a maximum of
30 minutes, as a result of which the silica gel hardened.
Example 2 - Preparation and Testing of a Coatinct according, to the
Invention
A clean 2-mm thick, 10 x 20 cm panel of borosilicate glass was
immersed at room temperature in the Si02 dipping solution
described above in Example 1 and was then withdrawn from the
solution at a rate of 20 cm/minute. The coating film thus
applied was allowed to dry for 5 minutes at room temperature and
was then baked for 20 minutes in an oven at 250°C (Table 1,
Coating 1) or at 300°C (Table 1, Coating 2). After the baking,
the coating of the invention was about 120 nm thick. The
hydrophobing process was evaluated by determining the contact
angle with water. This was done with a model G 10 contact angle
meter of the firm KRUSS, Hamburg [Germany]. With this method,
freshly cleaned glass surfaces show a contact angle of < 20°,
coated glass surfaces, an angle of about 60° and surfaces freshly
rendered hydrophobic, an angle of >_ 100°.
Immediately after the preparation according to the invention, a
value of 110° was measured at room temperature in this manner.
Thereafter, a Schrubb test was performed as follows: a piece of
felt having a contact surface of about 3 cm2 and moistened with
water was subjected to a total load of m = 1 kg and moved back
and forth on the test specimen. In this case, a load cycle
corresponds to a back and forth movement.
After 500 load cycles by the Schrubb test, the contact angle was
still 102°, after 1000 cycles it was 1033 and after 2000 cycles
it was still 100°, within in accuracy of ~ 3°.
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Example 3 - (Comparative Example) - Hydrophobing by Use of a
Fluoroalkylsilane
A hydrophobic glass surface was produced in accordance with the
prior art by applying tridecafluorooctyltriethoxysilane (F 8262,
available from DEGUSSA-HI1LS) : The f luoroalkylsilane was applied
to the entire surface with a textile cloth and fixed for 20
minutes at 200° or 250°. Measurement of the contact angle with
water showed a value of 108° immediately after preparation.
After 500 load cycles by the Schrubb test (see above), the
contact angle was 81°, after 1000 cycles it was 68° and after
2000
cycles it was still 67°. Similar values were also obtained for
identically tested hydrophobic glass surfaces from different
manufacturers.
Example 4 - (Comparative Example) - HydrophobinQ by Use of a
Silicone Oil
By applying hydromethylpolysiloxane (Fluid 1107, available from
DOW CORNING), a hydrophobic glass surface according to the prior
art was produced: The silicone oil was applied to the entire
surface with a textile cloth and fixed at 180° for 20 minutes.
Measurement of the contact angle with water showed a value of
102° immediately after preparation. After 500 load cycles by the
Schrubb test (see above), the contact angle was 87°, after 1000
cycles it was 71° and after 2000 cycles it was still 51°.
Similar
values were also obtained for identically tested hydrophobic
glass surfaces from different manufacturers.
Example 5 - (Comparative Example L - Performance of Commercially
Available Hydrophobic Glass Surfaces
Four commercially available hydrophobic glasses from different
manufacturers were subjected to a loading or Schrubb test as
described in Example 2. The test results are summarized in Table
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1.
Table 1: Contact Angle with Water in Degrees on Different
Hydrophobic Glass Surfaces After n Load Cycles
Pre aration Ori in n = 0 n = 500 n = 1000 n = 2000
Coating 1, according 114 106 102 101
to
the invention, Example
2
(250C)
Coating 2, according 110 102 103 100
to
the invention, Example
2
( 300C)
Example 3 (Comp. Example108 81 68 67
according to prior art,
coated with
fluoroalk lsilane
Example 4 (Comp. Example102 87 71 51
according to prior art,
coated with silicone
oil)
commercially available 90 - 99 54 - 89 50 - 71 -
hydrophobic glass
surfaces as per Example
