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Coated surfaces can impart  a wide range of affinity related to water, from hydrophilic (water loving), to hydrophobic (water repelling) to superhydrophobic (super water repellency). These surface characteristics are obtained by the proper combination of surface morphology at the micro and/or the nanoscale level, combined with a low surface energy material.

Superhydrophobicity and the lotus leaf

A prime example of superhydrophobicity in nature is the lotus leaf. The lotus leaf has a microstructure  comprising small protuberances or spiked papillae 10 – 20 microns in height and 10 – 15 microns in width which have a second hydrophobic wax layer. The combination of a structured surface combined with a low energy wax provides superhydrophobicity to the surface. To fully explain and quantify hydrophobicity, it is necessary to define the relationship between contact angle and the hydrophobic/hydrophilic character of a surface.

Contact angles of 150° or more and are called superhydrophobic – meaning that only two to three perfect of the surface of a water droplet is in contact with the surface. Since the surface contact area is less than 0.6 percent, this provides a self-cleaning effect. The ramifications of imparting lotus leaf water repellency characteristics to a coating surface has profound performance implications which can include the following:

• Self-Cleaning – Contaminants that fall on a superhydrophobic/hydrophobic surface are removed as water droplets will roll off.
• Improved moisture resistance – Improved blister resistance and gloss retention
• Improved corrosion resistance – Lowering moisture penetration reduces or even eliminates water and soluble salt penetration to the metal substrate which greatly slows the onset of corrosion.
• Extended life cycle for coating and substrate – Increased coating weatherability and resistance to the penetration of soluble salts and moisture positively impacts the longevity of the coated article.

The role of surface tension

We have discussed the role that surface morphology plays in imparting hydrophobicity; the other  critical component for hydrophobicity is surface energy.

• Surface tension is the elastic tendency of liquids that make them acquire the least surface area possible.
• Surface tension is measured along a line, whereas surface energy is measured along an area.

Components of surface tension mainly include dispersive and polar, hydrogen bonding and acid-base contributions. In general lower surface energy materials provide higher hydrophobicity. Table 1 and 3 lists the Surface Free Energy of several polymer types and modifiers, respectively, used in coatings, whereas Table 2 provides surface tensions of commonly used solvents in coatings.

Table 1 – Surface Free Energy of Polymers

Table 2 – Surface Tension of Solvents

Table 3 – Surface Free Energy of Potential Surface Modifying Agents

When two different liquid materials are applied to a solid surface, the liquid with the lower surface tension will flow or wet out on the solid surface, for example polyethylene, more so than the liquid with the higher surface tension. For example, water (surface tension 72.8 Dynes/cm) will form a higher contact angle than will Toluene (surface tension 28.4 Dynes/cm).

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.

Maximizing surface hydrophobicity

To maximize the surface hydrophobicity of a coating, the surface energy 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 lowsurface energy.  As Table II illustrates, surface energy can vary greatly depending on the nature of the surface that comes in contact with water.

For instance, a coating surface that is rich in polydimethylsiloxane (Surface Energy 19.8 mN/m) at the surface will provide a more hydrophobic surface than that of polystyrene (40.6 mN/m). In general terms, to provide the greatest hydrophobicity, the material’s most hydrophobic moiety should be positioned on the surface.

As another example, if an organofunctional trimethoxysilane is used for surface modification, the methoxysilane groups should be engineered to be positioned at the surface. Perfluoro and aliphatic groups at the coating surface offer greater hydrophobicity than that of ester or alcohol groups. Ester and alcohol groups are more polar in nature and thus more receptive to water deposited on the surface. For example, from lowest to highest surface tension:

Providing increased hydrophobicity throughout a properly engineered coating can provide additional attributes such as self-cleaning, improved corrosion and moisture resistance and an extended life cycle for the coating and substrate.

Recent advances in silane technology have enabled the availability of silanes for use in waterborne systems for improved hydrophobicity. Accordingly, resin selection, flattener, extender pigments and opacifier pigments can also be selected to maximize hydrophobicity.

Secondly, formulations utilizing nanoparticles must be tailored to provide proper acceptance rather than as a drop-in to achieve a desired property.

Search UL Prospector^® for hydrophobic raw materials.

You might also be interested in…

• Surface Tension & Surface Energy
• Breaking the tension with surfactants [INFOGRAPHIC]
• Hydrophobic Coatings Explained
• Dispersing & Wetting Hydrophobic Pigments & Fillers in Water-Based Paints to Avoid Pigment Flooding & Floating

Sources

• Prospector Knowledge Center: Hydrophobic Coatings Explained
• Gelest 2016 product literature

Silanes were first discovered and identified in 1857 by German chemists Heinrich Buff and Friedrich Woehler among the products formed by the action of hydrochloric acid on aluminum silicide.¹ Since that time silane chemistry has proven to be a versatile means to enhance performance of organic-based coatings, or to provide siloxane-modified coating systems with a variety of performance characteristics not readily achievable with other technologies.

Interested in using an organosilane in your next formulation?

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Depending on the proper selection of reactive silane, a variety of improved performance attributes can result, including:

• Weathering
• Adhesion
• Hardness
• Flexibility
• Moisture resistance
• Lubricity
• Cross-link density
• Corrosion resistance

Silane and Siloxane Structures:

In the presence of water, a trialkoxysilane can hydrolyze as a first step in the reaction to liberate methanol (for a trimethoxysilane) or ethanol (for a triethoxysilane) and self-condense to form a siloxane or react with available alcohol groups on a pigment, polymer or substrate to provide a siloxane linkage.

Silanes are used in a number of applications to:

• Improve adhesion to inorganic or organic surfaces – Silanes, when added to paints, can enhance adhesion to inorganic surfaces including metals and glass
• Coupling Agents – Silanes are used for coupling organic polymers to inorganic materials including pigments and fillers
• Crosslinking Agent – Selective organofunctional alkoxysilanes can react with organic polymers to provide a trialkoxysilyl group into the polymer backbone. In turn, the silane can then react with moisture to crosslink and form a three-dimensional siloxane cross-linked structure.
• Dispersing Agent – Used to increase the hydrophobicity of inorganic pigments and improve flow characteristics and the ability to be dispersed in organic polymers and solvents.
• Improved hydrophobicity – Selective reactive silanes can be modified to provide superb hydrophobicity (to be discussed more in the sequel to this article)
• Moisture Scavenger – In moisture sensitive formulations, the three alkoxysilane groups can scavenge water by reacting with moisture to form alcohol molecules.
• Pretreatment for metal surfaces – Specialized waterborne silanes for pretreatment of various metal surfaces (e.g. Evonik’s Dynasylan SIVO product group)

A silane that contains at least one carbon silicon bond (CH₃ – Si -) is called an organosilane. Reactive silane is the term used to define compounds that have a trialkoxysilyl group and an alkyl group (R) containing a reactive constituent.

Trialkoxysilyl groups can react directly, or indirectly in the presence of water with hydroxyl groups. As illustrated in Table 1, the other organofunctional group (R) can participate via a crosslinking reaction with another reactive site in a coating.

In regard to the reactions and interactions with a surface, there are many complexities and dependent variables. For example, the rate of hydrolysis of the trialkoxysilyl groups with moisture to form silanol groups (R – Si- OH), which in turn self-condense or crosslink compete with the reaction of the silanol groups with the substrate hydroxyl groups. These competing reactions can vary depending on moisture level, pH, and rates of reverse reactions. as hydrolysis is reversible. Hydrolysis of trialkoxysilyl groups to silanols and the subsequent self-condensation to form a siloxy crosslink (- Si – O – Si -) can be accelerated by the use of a suitable tin catalyst such as dibutyltin dilaurate.

On the other hand, the best catalyst for promoting co-condensation between a resin and -the silicone intermediate are titanate-based catalysts such as tetraisopropyl titanate.

Except for those applications requiring polymerization of a reactive silane into a resin backbone, most of the reactions illustrated in Table I can occur under ambient conditions.

Table I: Reactions of Trialkyloxy Organofunctionalsilanes and Their Applications

Reactive silanes provide utility to improve coating performance in a number of applications, including:

• Pigment wetting
• Improving hydrophobicity and increasing contact angle
• Enhancing adhesion over a number of metallic and inorganic surfaces
• Coupling agent between differential materials
• Scavenging moisture to provide improved stability
• Crosslinking to improve physical and environmental properties

A variety of siloxane-based reactive trimethoxy silane prepolymers are also available with functional groups including acylate, isocyanate, amino, hydroxyl, epoxy and vinyl. These enable a variety of opportunities to improve cross-link density, adhesion, weather resistance, moisture resistance, hydrophobicity and chemical resistance.

Resources

1. Wikipedia: Silane
2. UL Prospector
3. Evonik, ACS Product presentation
4. Organic Coatings, Science and Technology, 3^rd Edition

Electrically-conductive particles for use in coatings have taken a quantum leap forward in recent years. Revolutionary materials like carbon nanotubes (CNTs) and graphene possess 200 times greater strength than steel, with conductivity better than copper. This unique combination of strength, conductivity and high temperature resistance have the promise to impart exceptional properties in coatings that challenge the imagination. This article will discuss and contrast conductive materials with an emphasis on conductive nanomaterials and their potential advantage in coatings applications.

More than 200 conductive materials

That’s what you’ll find in Prospector®. Conductive additives, resins, pigments…Find materials faster with Prospector.

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Other more commonly used electrically-conductive particles include conductive carbon black, graphite, quaternary ammonium salts, copper, aluminum, silver and combinations thereof. In addition to conductive particles, there are a number of conductive polymers that can provide conductive coatings that may be discussed in a future article. Surface resistivity of coatings is nearly independent of relative humidity. Figure 1 illustrates the structure of graphene, graphene is a two-dimensional structure and can be thought of as a single walled carbon nanotube in sheet form.

Table 1 – Common Terminology of Conductive Materials

For electric current flow across a surface, surface resistivity (ohms/square) can be defined as the ratio of voltage drop per unit length, to the surface current per unit width.

Antistatic refers to the property of a material that inhibits triboelectric charging. Antistatic coatings have a surface resistivity of at least 1 X 10⁹, but less than 1 X 10¹².

Static dissipative coatings have:

• a surface resistivity of 1 X 10⁵ ohms/sq. or
• a volume resistivity of 1 X 10⁴ ohm-cm, but less than 1 X 10¹² ohm/sq. surface resistivity or
• 1 X 10¹¹ ohm-cm volume resistivity.

Conductive coatings have a surface resistivity of less than 10⁵ ohm/sq., or a volume resistivity less than 10⁴ ohm-cm.

Table 2 – Films with conductive additives

An important aspect of obtaining optimum conductivity from both graphene and CNTs for water or solvent-borne applications is to ensure that the dispersion is optimal and stabilized. If graphene and/or CNTs are not effectively dispersed, the particles will not be adequately separated and form bundles of particles. These materials tend to self-associate and form clusters, which in turn do not provide adequate connectivity to support optimum conduction of an electric current.

To overcome this issue, in many cases nonconventional means of dispersion are employed. Sonication is one such method that untangles nanoparticles, and thus provides the connectivity to support a current (percolation).

The agglomeration of graphene and CNTs is known to have a major impact on the percolation threshold and thus electrical conductivity of their nanocompositions. The interface between the conductive materials also has a profound effect on conductivity. The percolation threshold is the threshold at which the concentration of conductive nanomaterial of the composition displays an abrupt transition from insulator to a conductor. The aspect ratio of a conductive material (width:height) also plays an important contribution to the percolation threshold (lower aspect ratios favor conductivity).

The reason for the conductivity of graphene and more so CNTs is their extensive network of sp² bonds and stacking of their pi bonds.

Graphene can be described as a single layer thickness of graphite. Graphite is also electrically conductive and available in different purities as well as amorphous or crystalline forms.

One of the other reasons why nanoparticles such as graphene and CNTs are so conductive is that they are ultrasmall. As the dimension of a particle decreases, the ratio of surface area to volume increases quite dramatically. Larger surface area produces greater interaction of particles and higher attractive forces. For example, a surface area over 100nm in size normally defeats the advantage that nanoparticles provide to enhance performance.

The proper use of conductive nanoparticles in coatings can impart multiple beneficial properties. Stabilization of dispersed nanoparticles is essential to optimize the full benefits of these materials. Secondly, formulations utilizing nanoparticles must be tailored to provide proper acceptance, rather than as a drop-in to achieve a desired property.

In addition to their electrical properties, graphene, graphite and CNTs provide good stability at high temperatures. Furthermore, due to their unique molecular structure, when properly dispersed, they can also enhance mechanical properties. The temperature stability of CNTs in air is reported to be 750° C, graphene and graphite in excess of 600° C.

Applications for future coatings using the next generation of conductive materials will include:

• coatings to prevent electronic discharge for electrical applications
• communication equipment
• consumer electronics
• computer equipment
• flexible electrical equipment
• electronic applications requiring high temperature resistance and enhanced mechanical properties.

In summary, properly formulated coatings utilizing conductive particles and especially conductive nanoparticles can achieve performance attributes heretofore not obtainable by other means.

References and further reading:

• Journal of Applied Physics, August 7, 2015, Wang et.al.
• Prospector: Nanoparticles – When Smaller is Better
• ResearchGate: Nano-Bio-Technology and Sensing Chips: New Systems for Detection in Personalized Therapies and Cell Biology
• ResearchGate: ChemInform Abstract: Carbon Nanotubes: An Example of Multiscale Development – A Mechanistic View from the Subnanometer to the Meter Scale