Compositions for producing difficult-to-wet surface

Details Compositions for producing difficult-to-wet surface Compositions for producing difficult-to-wet surface

2001-04-25

2004-01-27

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S493000, C524S543000, C524S544000, C524S546000, C524S556000, C524S560000, C524S562000, C524S570000, C524S571000, C524S577000, C524S578000, C524S579000, C524S581000, C524S582000, C524S588000, C524S601000, C106S013000, C252S008620, C526S939000, C516S001000

Reexamination Certificate

active

06683126

ABSTRACT:

The present invention relates to a composition, especially a coating composition, for producing difficult-to-wet surfaces and to a process for producing such surfaces. The present invention relates in particular to the use of the coating compositions of the invention to produce a surface with a self-cleaning effect.
Usual surfaces are generally wetted by liquids. The degree of wetting is the result of an interplay between the forces of cohesion in the liquid and the forces of adhesion between liquid and surface.
In many cases, the wetting of a surface by a liquid is unwanted. For example, the wetting of surfaces with water results in the retention of water droplets on the surface and their evaporation, with the solids suspended or dissolved in the water remaining as unsightly residues on the surface. This problem exists in particular with surfaces exposed to rainwater.
The wetting of a surface with water is frequently also a trigger for its corrosion or for infestation with microorganisms and with growths such as algae, lichen, mosses, bivalves, etc.
In the context of packaging and storage vessels for liquids, low wettability of the interior surfaces is desired, so that only small amounts of liquid remain when the packaging or storage vessel is emptied.
In the field of apparatus and plant construction, as well, low wettability of components which come into contact with liquids is desired. If, indeed, the wettability of the components is high, there is a risk of increased formation of coverings and deposits. Furthermore, increased wettability generally has the consequence of increased flow resistance of liquids in pipelines.
It is known that the wettability of a surface by hydrophilic liquids may be reduced by a hydrophobic coating of the surface. Suitable coating materials in this context include polysiloxanes and perfluorinated polymers, especially the extremely hydrophobic polytetrafluoroethylene (Teflon). The coating reduces the forces of adhesion between liquid and wetted surface.
Furthermore, it has proven favorable to structure hydrophobic surfaces. Surface structures of this kind generally have regular or irregular elevations or depressions in the range from 0.1 to 1000 &mgr;m. The structuring firstly reduces still further the adhesion of the surface for polar liquids such as water. Furthermore, the structuring leads to reduced adhesion of solid deposits such as dirt particles on the surface. It has been found, moreover, that, given appropriate structuring, the dirt particles are flushed from the surface by moving water. This effect is also referred to as the self-cleaning effect or lotus effect (see Barthlott et al., Biologie in unserer Zeit, 28, No. 5, 314-322).
Accordingly, for example, WO 96/04123 describes self-cleaning surfaces of articles possessing an artificial surface structure which has elevations and impressions, the structure being characterized in particular by the distance between the elevations and the height of the elevations. The surfaces are produced, for example, by applying Teflon powder to an adhesive-treated surface or by embossing a structure onto a thermoplastically deformable hydrophobic material. U.S. Pat. No. 3,354,022 discloses similar surfaces. Here again, the surface is produced either by embossing a structure or by applying hydrophobic particles—wax particles, for example—to a hydrophobic surface. Also described is a surface comprising glass dust in a wax matrix.
EP 933 388 discloses a process for producing structured surfaces having hydrophobic properties, in which first of all a negative mold is produced by photolithography, this mold is used to emboss a polymer film, and the polymer film is subsequently hydrophobicized using fluoroalkyl silanes.
EP-A 909 747 describes a process for generating self-cleaning properties on ceramic structures such as roofing tiles, in which a dispersion of clay particles in an organic silicone resin solution is applied to the ceramic structures and the coating is cured.
JP 7328532-A discloses a coating process in which fine particles having a hydrophobic surface are applied to a wet coating, which is cured. In this case, water-repelling surfaces are obtained.
The methods the prior art describes for producing difficult-to-wet surfaces are either highly complex or do not lead to satisfactory results. The production of a structured surface by embossing techniques is complex and can only be employed economically with planar surfaces. Surfaces where structuring is brought about by subsequent application of hydrophobic particles are frequently difficult to reproduce or possess poor mechanical stability. Moreover, this process is also highly complex. Additionally, there is often a need for organofluorine compounds or fluoropolymers which not only are very expensive but also are ecologically objectionable.
It is an object of the present invention to provide a coating composition for producing surfaces with low wettability that can be used to produce, simply and reproducibly, stable coatings of low wettability.
We have found that this object is achieved in a coating composition comprising at least one porous powder having a high specific surface area which, additionally, is hydrophobic and at least one hydrophobic binder for the powder particles, the weight ratio of hydropobic powder to binder being at least 1:4.
The present invention accordingly provides a composition, especially in the form of a coating composition, for producing difficult-to-wet surfaces, comprising
i) at least one finely divided powder whose particles have a hydrophobic surface and a porous structure characterized by a BET surface area (determined in accordance with DIN 66131) of at least 1 m
2
/g, and
ii) at least one film-forming binder characterized by a surface tension <50 mN/m, the weight ratio of powder binder being at least 1:4.
The compositions are easy to apply to surfaces. They reduce almost completely the wettability of the surface and produce a self-cleaning effect on these surfaces. It has also been found that the compositions of the invention can be processed to form shaped articles which have a difficult-to-wet surface. The present invention accordingly also provides a process for producing difficult-to-wet surfaces by applying a composition of the invention to a surface that is to be coated, and also provides for the use of the compositions to produce surfaces having self-cleaning effects.
To characterize surfaces with regard to their wettability it is possible to employ the static contact angle of a drop of liquid on a surface. The static contact angle is defined as the angle enclosed by said surface and a tangent along the surface of the liquid drop in the region of the contact point of the liquid drop with the surface, the contact angle being measured through the liquid drop. A contact angle of 0, accordingly, denotes complete wettability and no drop formation, whereas a contact angle of 180° denotes complete unwettability. The contact angle may be determined in accordance with known methods, with the aid, for example, of a microscope equipped with a goniometer (see also C. D. Bain et al., Angew. Chem. 101 (1989) 522-528 and also A. Born et al., Farbe & Lack 105 (1999) 96-104).
The surfaces treated with the compositions of the invention generally have static contact angles ≧120° and in particular ≧140° (determined at room temperature) for a variety of liquids, particularly water. The contact angle for water and aqueous solutions is generally at least 140°. Frequently, indeed, contact angles >160° are achieved, especially in the case of water. Contact angles above 160° cannot generally be determined with sufficient accuracy. A contact angle above 160° corresponds, however, in general to complete unwettability of the surface.
Another measure of the wettability of the surface is the repellent power F
R
, which is defined as the reciprocal of the weight force F
H
required to cause a drop of liquid to run from an inclined surface. The repellent power is calculated using the following formula:
F
R
=
1
F
H
=
1
sin
&it

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