Note: Descriptions are shown in the official language in which they were submitted.
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STRUCTURED SURFACES HAVING HYDROPHOBIC PROPERTIES
Field of the Invention
The present invention relates to articles having
structured surfaces made of materials having low surface
energy.
The present invention furthermore relates to
processes for the production of these articles having
structured surfaces.
Background of the Invention
Articles having surfaces which are difficult to wet
have a number of interesting and economically important
features. For example, they are easy to clean and do not
easily retain residues. These properties relate in particular
to transparent and aesthetically appealing articles.
Surfaces from which water readily runs off must be
either very hydrophilic or hydrophobic. Hydrophilic surfaces
have a small contact angle with water, resulting in rapid
distribution of the water over the surface and finally rapid
running off of the resulting water film from the surface.
Hydrophobic surfaces, on the other hand, ensure drop
formation by means of a large contact angle with water. These
drops can rapidly roll off inclined surfaces.
The use of hydrophobic material, such as
perfluorinated polymers, for the production of hydrophobic
surfaces is known; a further development of these surfaces
consists in structuring the surface in the m to nm range.
U.S. Patent No. 5,599,489 discloses a process in
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which a surface can be rendered particularly water-repellent
by bombardment with particles of an appropriate and subsequent
perfluorination.
Another process is described by H. Saito et al. in
Surface Coating International 4, 1997, page 168 et seq. Here,
particles of fluorine polymers were applied to metal surfaces
resulting in greatly reduced wettability of the surfaces with
respect to water. A considerably reduced tendency to icing
was also found.
U.S. Patent No. 3,354,022 and WO 96/04123 described
further processes for reducing the wettability of articles by
topological changes to the surfaces. Described therein are
artificial protuberances or indentations, having a height of
about 5 to 1000 m and a spacing of about 5 to 500 m, which
were applied to hydrophobic materials, or materials rendered
hydrophobic after the structuring. Surfaces of this type lead
to rapid drop formation, the drops which roll off picking up
dirt particles and thus cleaning the surface. No information
is given regarding an aspect ratio of the protuberances.
Surfaces of this type have a large contact angle
with water but are completely wetted by liquids such as oil.
Once wet, the effect of the large contact angle with water,
which results from the structure, is also lost. The use of
such material is thus limited to areas where no liquids
forming an oil film occur.
It is therefore a major object of the present
invention to develop industrially useful surfaces which have a
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very large contact angle with water and from which dirt, in
particular oily dirt, can be removed, for example, by washing
off with water, or which promote the running off of water.
It was surprisingly found that structured surfaces
having protuberances with a height of 50 nm to 10 m and an
average spacing of 50 nm to 10 m and a surface energy of the
unstructured material of 10 to 20 mN/m are virtually not
wetted by water and can also be readily cleaned to remove oily
dirt.
Summary of the Invention
The present invention therefore relates to an
article having a structured surface, wherein the structured
surface has protuberances having an average height of 50 nm to
10 m and an average spacing of 50 nm to 10 m and the
structured surface is made of a material which is hydrophobic
to such an extent that the material has, when structured, a
surface energy of 10 to 20 mN/m.
The present invention furthermore relates to a
process for the production of the article wherein
protuberances are formed by mechanical impression or are
etched by lithographic methods or are applied by shaping.
Materials for the purposes of the present invention
are products which already have their final form for use,
semifinished products or intermediate products, such as, for
example, granules or powders, which must pass through another
shaping process, such as, for example, melting, casting or
extrusion.
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Brief Description of the Drawing
Figure 1 is a schematic cross-section of a surface
superstructure having protuberances according to the
invention.
Detailed Description of the Preferred Embodiments
The structured surfaces of the articles according to
the invention have particularly large contact angles. This
substantially prevents the wetting of the surface and leads to
rapid drop formation of water. With an appropriate
inclination of the surface, the drops of water can roll off
the protuberances, picking up dirt particles and thus
simultaneously cleaning the surface.
Surfaces for the purpose of the present invention
may not only be hydrophobic but may also be oleophobic. This
property extends the ranges of use of the structured surfaces
also to include areas where oil-containing liquids or dirt are
to be expected, for example road, rail and air traffic, and in
industrial production plants.
Articles having surfaces structured according to the
invention can be very easily cleaned. Where rolling-off drops
of, for example, rainwater, dew or other water occurring in
the field of use of the article are not sufficient for
cleaning, the articles can be cleaned by simply washing off
with water.
For adhesion to a surface or for multiplication on a
surface, bacteria and other microorganisms require water,
which is not available on the hydrophobic surfaces of the
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present invention. Surfaces structured according to the
invention inhibit or prevent the growth of bacteria and other
microorganisms and are thus bacteriophobic and/or
antimicrobial.
The characterization of surfaces with regard to
their wettability can be effected by measuring the surface
energy. This quantity is obtainable, for example, through the
measurement of the contact angle of various liquids on the
smooth material (K.D. Owens, R.C. Wendt, J. Appi. Polym. Sci.
13, 1741 (1969)) and is stated in mN/m (milli-Newton per
meter). Determined according to Owens et al., smooth
polytetrafluoroethylene surfaces have surface energy of 19.1
mN/m, the contact angle with water being 1100. In general,
hydrophobic materials have a contact angle of over 90 with
water.
The contact angle or the surface energy is
expediently determined on smooth surfaces in order to ensure
better comparability. The material property "hydrophobicity"
is determined by the chemical composition of the uppermost
molecular layer of the surface. A large contact angle or
lower surface energy of a material can therefore also be
achieved by coating methods.
Surfaces according to the invention have larger
contact angles than the corresponding smooth materials. The
macroscopically observed contact angle is thus a surface
property which reflects the material properties plus surface
structure.
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A particularly low surface energy is necessary when
not only hydrophobic but also oleophobic behaviour is
required. This is the case in particular with nonsolid, oily
dirt. This in fact leads, in the case of nonoleophobic
surfaces, to wetting with oil, which has a permanent adverse
effect on the stated properties. For such applications, the
surface energy of the smooth, unstructured surfaces should be
below 20 mN/m, preferably 10 to 20 mN/m.
The surface properties of the surfaces according to
the invention are dependent on the height, the shape and the
spacing of the protuberances or protrusions. Protuberances
having an average height of 50 nm to 10 m and an average
spacing of 50 nm to 10 m have proven useful.
For specific fields of use or materials to which the
articles are applied, other dimensions of the structure
according to the invention may also be used.
The average height of the protuberances is therefore
preferably 50 nm to 4 m with an average spacing of 50 nm to
10 m. Alternatively, the average height of the protuberances
may be 50 nm to 10 m with an average spacing of 50 nm to 4
m. Particularly preferably, the protuberances have a height
of 50 nm to 4 m with an average spacing of 50 nm to 4 m.
The ratio of height to width of the protuberances,
the aspect ratio, is likewise important. The protuberances
preferably have an aspect ratio of from 0.5 to 20, more
preferably from 1 to 10, and particularly preferably from 1 to
3Ø
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In addition to the structural properties of the
material the chemical properties thereof are also important
for achieving the required small contact angles. Here, the
chemical composition of the uppermost monolayer of the
material is particularly decisive.
Surfaces according to the invention can therefor be
produced from materials which already exhibit hydrophobic
behavior before the structuring of their surface. These
materials contain a particular bulk polymers with
polytetrafluoroethylene, polyvinylidene fluoride or polymers
of perfluoroalkoxy compounds, either as homo- or copolymers or
as a component of a polymer blend.
Also possible are mixtures of polymers with
additives, which align in the shaping process in such a way
that hydrophobic groups predominate on the surface. Suitable
additives are fluorinated waxes, for example the Hostaflons*
from Hoechst AG.
The structuring of the surface can be carried out
after the hydrophobic coating of a material.
The chemical modifications can also be carried out
after shaping so that the protuberances can be subsequently
provided with a material having a surface energy of 10 to 20
mN/m.
Since the chemical properties of the uppermost
monopolymer of the material are particularly important with
respect to the contact angle, surface modification with
compounds which contain hydrophobic groups may be sufficient
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to achieve the desired surface properties.
Processes of this type include covalent bonding of
monomers or oligomers to the surface by a chemical reaction,
for example treatments with alkylfluorosilanes, such as
Dynasilan* F 8261 from Sivento Chemie Rhienfelden GmbH, or
with fluorinated ormoceren.
Processes in which radical sites on the surface are
initially produced and are reacted, in the presence or absence
of oxygen, with monomers capable of radical polymerization may
furthermore be mentioned. The activation of the surface can
be effected by means of plasma, UV, or radiation and special
photoinitiators. After the activation of the surface, i.e.
production of free radicals, the monomers may be polymerized
onto the surface. Such a process generates a mechanically
resistant coating.
The coating of material or a structured surface by
plasma polymerization of fluoroalkenes or perfluorinated or
partially fluorinated vinyl compounds has proven particularly
useful.
The imparting of hydrophobic properties to a
structure surface by means of an HF hollow cathode plasma
source with argon as carrier gas and C4F8 as monomer at a
pressure of about 0.2 mbar is a technically simple and elegant
variant for subsequent coating.
In addition, an already produced article can be
coated with a thin layer of a hydrophobic polymer. This can
* Trade-mark
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be effected in the form of a finish or by polymerization of
corresponding monomers on the surface of the article.
Solutions or dispersions of polymers, such as, for example,
polyvinylidene fluoride (PVDF), or reactive finishes can be
used as the polymeric finish.
Suitable monomers for polymerization on the
materials or their structured surfaces are in particular
alkylfluorosilanes, such as Dynasilan* F 8261 (Sivento Chemie
Rheinfelden GmbH, Rheinfelden).
The shaping or structuring of the surfaces can be
effected by impression/rolling or simultaneously during
macroscopic shaping of the article, such as, for example,
casting, injection molding or other shaping methods.
Corresponding negative shapes of the desired structure are
required for this purpose.
Negative shapes can be produced industrially, for
example by means of the Liga technique (R. Wechsung in
Mikroelektronik, 9, (1995), page 34 et seq.). Here one or
more masks are first produced by electron beam lithography
according to the dimensions of the desired protuberances.
These masks serve for exposure of a photoresist layer by deep
X-ray lithography, with the result that a positive shape is
obtained. The intermediate spaces in the photoresists are
then filled by electrodeposition of a metal. The metal
structure thus obtained is a negative shape for the desired
structure.
* Trade-mark
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In another embodiment of the present invention, the
protuberances are arranged on a somewhat coarser
superstructure (Fig.1).
The protuberances have the above-mentioned
dimensions and can be applied to a superstructure having an
average height of 10 m to 1 mm and an average spacing of 10
m to 1 mm.
The protuberances and the superstructure can be
formed by mechanical impression simultaneously or in
succession, applied by lithographic methods or by shaping.
Such "double" structuring has proven useful
particularly in the case of course dirt and has a higher
mechanical load capacity compared with the "single"
microstructuring.
The protuberances and the superstructure may have a
periodic arrangement. However, stochastic distributions of
the dimensions of the superstructure and of the protuberances
are also permissible, simultaneously or independently of one
another.
In the case of surfaces having a superstructure, as
in the case of surfaces only having a microstructure, the
shaping or structure of the surfaces is expediently effected
in one operation. Subsequent imparting of hydrophobic
properties or chemical modification of an already produced
"doubly" structured surface is of course also possible.
Surfaces produced according to the invention are
transparent from structuring smaller than 400 nm and are
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therefore suitable for all applications in which high
transmittance or good optical properties are important. In
particular, the production or coating of headlamps,
windscreens, advertising surfaces or coverings of solar cells
(photovoltaic and thermal) may be mentioned here.
A further field of use of the surfaces according to
the invention is in containers to be emptied without leaving a
residue or holders to be rapidly cleaned, such as, for example
wafer holders in semiconductor production. Within their
production process, wafers are transported with special
holders (cassettes) into various baths. To avoid transfer of
the various bath liquids, cleaning steps, in particular for
the holders, are required. The cleaning or drying steps are
dispensed with if the respective bath liquid drips off
completely from the holder on removal of the wafer from the
bath.
Surfaces according to the invention are therefore
extremely suitable for the manufacture of products whose
surface promotes the running off of liquids. Surfaces
according to the invention are preferably used for
manufacturing objects which are self-cleaning as a result of
water running off. Preferred products are containers,
transparent bodies or holders.
The example below is intended to describe the
present invention in more detail without restricting its
scope.
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Example
A negative shape was produced by UV lithography of a
photosensitive plastic and subsequent electroforming with
nickel. This shape was used to cast a polycarbonate film
having a microstructure with protuberances of about 2 m wide
(measured at half height) and about 4 m high with a spacing
of 4 m. These protuberances are arranged on a superstructure
having a height of about 23 m and a spacing of about 48 m.
The structure of the shape has the same dimensions with
opposite sign. Fig. 1 shows a schematic cross-section
(abscissa and ordinate in [ m]).
The polycarbonate film structured in this manner was
then rendered hydrophobic with Dynasilan* F 8261 (Sivento
Chemie Rheinfelden GmbH, Rheinfelden).
A film rendered hydrophobic in the same manner but
unstructured had a contact angle with water of 109.8 + 0.4
and a surface energy of less than 20 mN/m (determined
according to Owens et al.) and the structured film a contact
angle of 150 . Even stubborn contamination with oily
substances could be removed by simply washing off with water.
*Trade-mark
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