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
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
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