Hydrophobic material

Details Hydrophobic material Hydrophobic material

2002-05-01

2004-06-01

Cameron, Erma (Department: 1762)

Coating processes

Solid particles or fibers applied

C427S387000

Reexamination Certificate

active

06743467

ABSTRACT:

TECHNICAL FIELD
The invention relates to the technology of coatings. In particular, the invention relates to coating materials which are hydrophobic, and to methods for making coatings from those materials.
BACKGROUND ART
Hydrophobic coatings are water-proof coatings which have immediate uses in reducing icing and fouling of other surface. Such coatings can also render protected surfaces resistant to attachment by water soluble electrolytes such as acids and alkalies, and by microorganisms.
In the past, surfaces have been protected against encrustation, corrosion, icing and fouling by means of coatings containing polymer films, hydrophobic solid fillers and hydrophobic liquids. One disadvantage of the use of such coatings is that they do not achieve multi-purpose protection since they are not generally versatile enough to protect against damage from a variety of causes.
It is well understood that the wettability of various materials is dependent on both the physical and chemical heterogeneity of the material. The notion of using the contact angle &thgr; made by a droplet of liquid on a surface of a solid substrate as a quantitative measure of the wetting ability of the particular solid has also long been well understood. If the liquid spreads completely across the surface and forms a film, the contact angle &thgr; is 0°. If there is any degree of beading of the liquid on the surface of the substrate, the surface is considered to be non-wetting. For water, the substrate surface is usually considered to be hydrophobic if the contact angle is greater than 90°.
Examples of materials on which liquid droplets have high contact angles include water on paraffin, in which there is a contact angle of about 107°. Many applications require a hydrophobic coating with a high contact angle of at least 150°, and preferably at least 165°.
A “gel” is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. The liquid prevents the solid from collapsing, and the solid prevents the liquid from escaping. The solid skeleton can be formed by linking colloidal particles together.
The present inventors have now developed methods for producing materials which, when coated on a surface, render that surface hydrophobic.
SUMMARY OF THE INVENTION
In a first aspect, the present invention provides a method of forming a material capable of being applied to a surface, the method including the steps of:
(a) providing precursors capable of reacting to form a gel;
(b) reacting the precursors together to form the gel;
(c) adding a particulate material to the gel to form a mixture, the particulate material being capable of chemically bonding with the gel; and
(d) treating the mixture such that a modified gel is formed in which the particulate material is bound to the gel, and the modified gel is capable of forming a surface which is chemically hydrophobic and has a surface roughness which physically enhances the surface hydrophobicity, such that water has a contact angle on the surface of at least 150°.
In a second aspect, the present invention provides a method of forming a coating on a substrate, the method including steps (a) to (d) of the first aspect of the present invention, and further including the steps of:
(e) applying the modified gel to the substrate; and
(f) treating the applied modified gel such that a coating is formed on the substrate, the coating having a surface which is chemically hydrophobic and has a surface roughness which physically enhances the surface hydrophobicity, such that water forms a contact angle of at least 150°.
Preferably, the hydrophobic surface defined in either the first or the second aspect of the present invention is such that water forms a contact angle of at least 155° on it. More preferably, the contact angle is at least 160°. Even more preferably, the contact angle is at least 165°.
The hydrophobicity of the hydrophobic material when applied to a surface is preferably due to both the chemical properties of the modified gel and physical roughness of the material. It is envisaged that the modified gel of the first aspect of the present invention could be used to make solid materials in a range of possible forms, including bulk materials, thick coatings, and thin films.
The gel functions as a cross-linking agent which binds the particulate material, and attaches the modified gel to the substrate if required. Any known process for forming a gel may be used. Typically, the precursors defined in step (a) of the first and second aspects of the present invention at least include water, a solvent, and a metal alkoxide such as one of the following:
tetramethoxysilane (abbreviated TMOS), Si(OCH
3
)
4
tetraethoxysilane (abbreviated TEOS), Si(OCH
2
CH
3
)
4
;
titanium tetraisopropoxide, Ti(O-iso-C
3
H7)
4
;
titanium tetramethoxide, Ti(OCH
3
)
4
;
titanium tetraethoxide, Ti(OC
2
H
5
)
4
;
titanium tetrabutoxide, Ti[O(CH
2
)
3
CH
3
]
4
;
zirconium n-butoxide, Zr(O-n-C
4
H
9
)
4
.
The solvent may comprise an alcohol such as methanol, ethanol, isopropanol, or butanol. Alternatively, the solvent may comprise hexane or diethyl either.
For example, silicate gels may be synthesised by hydrolysing an alkoxide dissolved in an alcohol with a mineral acid or base, or organic acid or base. The end product is a silicon dioxide network.
Step (b) of reacting the precursors together in the first and second aspects of the invention may be implemented by refluxing the precursors for an extended period, such as a period in the range from 4 hours to 24 hours.
The particulate material defined in step (c) of the first and second aspects of the invention may consist of particles having substantially equal diameters, or alternatively having a spectrum of diameters. Preferably, at least some of the particles have diameters within a range from 1 nm to 500 &mgr;m. More preferably, the range is from 1 nm to 100 &mgr;m. Still more preferably, the range is from 1 nm and 1 &mgr;m. Still more preferably, the range is from 1 nm and 100 nm, and even more preferably the range is from 5 nm and 50 nm. In one embodiment, the particulate material consists of particles with diameters in a range from 1 nm to 500 &mgr;m. In a further embodiment, the primary particle diameter of the particulate component is the range from 5 nm to 50 nm. In yet a further embodiment, the average particle size is in the range from 5 nm to 20 nm. In yet a further embodiment, the average particle size is about 15 nm.
Step (d) of the first and second aspects of the invention may be implemented by firstly thoroughly mixing the mixture, such as in an ultrasonic bath. Optionally, an alcohol such as isopropanol may be added to the mixture during this step to aid in dispersing the particulate material. Secondly, the mixture may be refluxed to cause chemical bonding between the particulate material and the gel.
Step (e) of applying the modified gel to a substrate may be carried out by any known technique of forming a coating from a liquid, such as spin coating, dip coating or spray coating.
Step (f) may involve drying the applied modified gel until a solid coating is formed. There may be solvents which need to be removed from the modified gel, and in such a case, the drying may include heating the applied modified gel to a temperature which is at least high enough to evaporate the solvents. It will be appreciated that the drying temperature will depend on the melting point of the substrate and the type of gel. The drying time for a particular application will usually depend on the temperature used, and to some extent on the thickness of the coating. In the case of silica coatings, it has been found that a heating temperature of in the range from 120° C. to 400° C. over a period of 10-30 minutes is suitable when the substrate is capable of withstanding such a temperature. Vacuum drying, or a combination of vacuum drying and heating, may be preferable when the substrate has a low melting point.
Elasticity and flexibility of the coating may be enhanced by adding a polymer component to the gel during step (c). Alte

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