A hydrophobic definition means “tending to repel or fail to mix with water.” Coatings that offer a hydrophobic (EU) or superhydrophobic surface can impart multiple advantages to the coating surface and substrate they are applied to. Advantages may include decreased dirt retention, self-cleanability, improved moisture and corrosion resistance, as well as extended life expectancy of the coating and substrate. To fully explain and quantify hydrophobicity, it is necessary to define the relationship between contact angle and the hydrophobic/hydrophilic (EU) character of a surface.
Figure 1 – Contact Angle for Hydrophobic Coating Surface and a Hydrophilic Coating Surface
Figure 2 – Contact Angle and Superhydrophobicity
Accordingly, the surface characteristics can create different coatings, ranging from hydrophilic (water-loving) coatings to superhydrophobic coatings, which are highly water-repellent. Several factors impact the contact angle of a water drop on the surface of a coating. These include the macro, micro, nano-surface profile, and the surface tension of the coating on which the water droplet is resting. Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible.
As Table 1 below illustrates, water has a higher surface tension than common solvents used in the paint industry. This is attributed to the high attraction of water molecules for each other as a result of hydrogen bonding. Another important factor in determining the hydrophobicity of coatings is the microscopic geometry of the surface.
Table 1 – Surface Tension of Paint
Nature has multiple examples of superhydrophobic and hydrophobic definition. One of the most notable surfaces is that of the Lotus leaf. The contact angle of water on the surface of a Lotus leaf is greater than 150°. The cause of self-cleaning properties of the Lotus leaf is the hydrophobic water-repellent double structure of the surface. This enables the contact area and the adhesion force between surface and droplet to be significantly reduced and results in a self-cleaning process allowing water to readily roll off the leaf and collect dust deposits on the way. This micron size double structure is formed at the surface of the plant and it’s comprised of needle-like projections from the surface that are covered by wax.
The wax-covered projections are 10 to 20 µm in height and 10 to 15 µm in width. These waxes are hydrophobic and form the top layer of the double structure. Some plants show contact angles up to 160° and are called super-hydrophobic, meaning that only 2–3% of the surface of a water droplet is in contact with the surface. Since the surface contact area is less than 0.6%, this leads to the self cleaning effect.
Thus far, we’ve defined the factors that contribute to the hydrophobicity or the lack thereof including contact angle, surface structure, and why most organic solvents tend to wet a surface better than water as a consequence of their lower surface tension. The next segment will concentrate on how to impart greater hydrophobicity to a coating system, especially from a surface perspective.
To maximize the surface hydrophobicity of a coating, the surface energy (EU) should be as low as possible. A low surface energy, coupled with an appropriately structured surface, maximizes hydrophobicity. Surface energy has the same units as surface tension (force per unit length or dynes/cm). A high surface tension liquid such as water will have maximum hydrophobicity and thus have poor wetting (high contact angle) over a coating surface that has a low surface energy. As Table II illustrates, surface energy can vary greatly depending on the nature of the surface that comes in contact with water.
Table II – Surface Energy of Materials[1]
For example, a coating comprised of polyhexafluoropropylene (12.0 Dynes/cm) on the surface will provide a more hydrophobic surface than that of polymethylmethacrylate (EU) (40.2 Dynes/cm). In general terms, to provide the greatest hydrophobicity, the material’s most hydrophobic moiety should be positioned on the surface. For example, if an organofunctional trimethoxysilane (EU) is used for surface modification, the methoxysilane (EU) groups should be engineered to be positioned at the surface. Perfluoro and aliphatic (EU) groups at the coating surface offer greater hydrophobicity than that of ester or alcohol groups. For example, from lowest to highest surface tension:
Providing increased hydrophobicity throughout a properly engineered coating can also provide additional attributes such as improved corrosion and moisture resistance.
Accordingly, resin selection, flattener, extender pigments and opacifier (EU) pigments can also be selected to maximize hydrophobicity. Secondly, formulations utilizing nanoparticles (EU) must be tailored to provide proper acceptance rather than as a drop to achieve a desired property. In summary, properly formulated coatings utilizing nanoparticle technology can achieve performance attributes heretofore not obtainable by other means. Some suppliers of materials to enhance surface hydrophobicity listed in Prospector include BYK, Evonik Industries Ag Functional Silanes, ICM, Momentive, Phibro, Shin-Etsu Silicones of America, Tianjin Boyuan New Materials Co., Ltd., Evonik Industries AG Silica and Wacker.
Suppliers available in Europe: BYK, Evonik Industries Ag Functional Silanes | Momentive | Tianjin Boyuan New Materials Co., Ltd. | Evonik Industries AG Silica | Wacker
For additional information concerning the selection of materials to enhance hydrophobicity, please navigate to www.ulprospector.com (EU).
[1] Gelest 2006 Product Brochure
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.
VERY INFORMATIVE .JUSTICE HAS BEEN DONE WITH THE SUBJECT.
Excellent Mr.Ronald.The points are so crisp and deep in subject.Good article.Gives good clarity and idea.
N.SRIDHAR
Hydrophobicity/superhydrophobicity is well explained an I liked it much.
As a formulator of epoxies/ polyurethanes for potting/encapsulating of medium/high voltage transformer I am aware of several hydrophobic coatings ,for example offered by Dow Cornings. Lasting adhesion to epoxy// polyurethanes is a tricky issue …
Are your hydrophobic coatings based on Rovene 6106?
I need a novel starting formulation(s) based on siloxanes.
FROM ABOVE – ….. In general terms, to provide the greatest hydrophobicity, the material’s most hydrophobic moiety should be positioned on the surface. … HOW ??
Hello Yucel,
There are multiple mechanisms to position the desired hydrophobic moiety on the surface. The mechanism to achieve the desired results is dependent on the paint system (water vs. solvent, powder, application technique and ambient cure vs. thermal etc.). The same is true for all surface active ingredients.
Small note on “superhydrophobicity / Lotus effect”. This effect is achieved by a sort of air cushion. The air is stuck between the nanoparticles prevents liquid from touching the substrate. This is not really a superficial tension modifying effect.
So it’s just a imitation of reduced surface tension?
In other words, prerequisite for super-hydrophobicity (lotus effect) consists in a hydrophobic coating having micro- and nano-roughness. So, liquid drop rests on the surface as yogi on nails.
Ron, Great website!!
Very nice article. I have two questions.
1. How do we achieve water-repellent double structure of the Lotus Leaf in a paint or coating? Seems very tricky!!
2. What would be the tangible advantages of water repellent paint that one can monetize? Do you see improved long term durability or any other benefits in exterior paints?
Thanks,
The right additive will provide the necessary hydrophobicity, this coupled with a surface structure that provides a spaced nanostructure (like a Lotus leaf) will provide superhydrophobicity. There are multiple patents on means to achieve superhydrophobicity, some even involve using materials that enable superhydrophobicity throughout the coating as well as on the surface. In our evaluations we have seen coatings that provide improved corrosion and humidity resistance as a result of improved hydrophobicity. Since one of the principal mechanisms of how coatings degrade is a result of UV light coupled with moisture, coatings that have long long term hydrophobic properties should provide improved weathering properties.
Ron, this is very nice article and thank you for published this in common forum.
I want to have your opinion on ” Is it possible to develop a durable and transparent super hydrophobic coating for glass surface?”. Durability means sustaining of super hydrophobic Coating against abrasion.
Hello Mr. Sunder,
Multiple environmental conditions will contribute to the degradation of a Superhydrophobic glass coating including UV light, moisture, chemicals, coating erosion (1 micron or less) and lastly abrasion. The challenge is to maintain excellent clarity of the applied glass coating and at the same time sufficient film thickness to ensure longevity. One can extend the durability of a superhydrophobic composition, but no coating offers indefinite life expectancy.
Amazing effect! Nice article dear Ron, thank you for publishing.
Which p0lymer do you suggest for high hardness and highgl0ss hydroph0bic coating that cures via m0isture ?