Enhancing Powder Coatings with Graphene Nanotubes

By Troy Newport

Nanotubes are defined as “tiny cylinders of atoms with a diameter in the nanometer range.” Sounds like a typical grade school science class definition, doesn’t it? Hold on to your chemistry hats, because we’re about to embark on a microscopic trip to explain these fascinating molecules that have increasingly revolutionized materials science since they were brought to the attention of the scientific community in 1991 by Sumio Iijima and Toshinari Ichihashi.

To assist with the explanation of this technology, we enlisted the expertise of Philippe Bichot, CASE (coatings, adhesives, sealants, and elastomers) applications manager for the U.S. market for OCSiAl. Established in Luxembourg in 2010 and named after four elements, oxygen (O), carbon (C), silicon (Si), and aluminum (Al), OCSiAl is one of the world’s leading manufacturers of graphene nanotubes. Produced at industrial scale, its graphene nanotubes are used in a wide variety of applications including fuel cells and batteries, elastomers/rubbers, paints and coatings, composites, and plastics.

So, what are nanotubes and why are they so special?

The Basics
When you think of Earth’s strongest materials you may think of diamonds. They are formed naturally deep in the Earth when carbon atoms are placed under immense pressure and heat until the atoms are squeezed so tightly together that they begin touching. Over time, the carbon atoms bond together and crystallize in a tetrahedral structure, forming diamonds. That means that diamonds are simply a different form, or allotrope, of carbon.

Graphite is another allotrope of carbon and is typically formed near the Earth’s surface through the same means as diamonds, except the carbon atoms aren’t subjected to the extreme temperatures that atoms deeper in the Earth experience during diamond formation. Another major difference between diamonds and graphite is the way in which the carbon atoms bond during formation. During diamond formation, each carbon atom is attached to four other carbon atoms, creating four strong covalent bonds directed toward the corners of a regular tetrahedron. For graphite formation, each carbon atom is bonded to three other carbon atoms and arranged at the corners of a hexagon, which forms a flat lattice or honeycomb pattern and rolls into a tubelike structure. With this configuration, each carbon atom is left with a spare electron, which forms a sea of delocalized electrons within the tube, meaning nanotubes can conduct electricity. The arrays are layered on top of one another, creating interactions between layers which strengthen the material and form graphite. Graphene, discovered in 2004, is a single sheet of graphite and is considered up to 100 times stronger than steel and 40 times stronger than diamonds, while being five times lighter than aluminum. The distinct honeycomb pattern created by the carbon bonds can inherently bend and twist, which lends itself to holding a sturdy, tube-shaped structure.

The Nanotube
A nanometer is one billionth of a meter. For context, a human hair is between 80,000 and 100,000 nanometers wide. A carbon nanotube is a tubular molecule, on a nanoscale, composed of a large number of carbon atoms. Therefore, a single-walled carbon nanotube (SWCNT) is a thin, single layer of graphite “rolled up” in the form of a hollow cylinder, while multi-walled carbon nanotubes (MWCNT) consist of multiple concentric layers of tubes arranged inside one another (Figure 1).

SWCNT are longer in length and have a single-layer thickness, while MWCNT are much shorter and thicker. Due to the longer, thinner, more flexible structure of SWCNT, fewer of them are required to enhance the properties of the materials to which they are added. Philippe simplifies the difference between the two nanotube types using a pasta comparison: the difference between spaghetti and macaroni noodles. “For example,” Philippe says, “you would only need five spaghetti noodles to cover the same area as roughly 50 macaroni noodles.” This is one reason OCSiAl strictly focuses on the production of SWCNT, while their properties tell the rest of the story.

SWCNT are shown to have immense advantages as a multifunctional additive to create new products with properties that were previously unachievable. Depending on the application, adding concentrations as low as 0.01% to 0.05% weight (wt.) can provide the intended benefits. Just like traditional composites, where two or more different materials are combined to form a material stronger than the individual materials by themselves, SWCNT have the capability to revolutionize many of the materials used today. Since only a small amount of SWCNT need to be incorporated into other materials, we can substantially reduce the quantity of materials needed to accomplish the same task.

SWCNT Properties

• Stronger than steel and diamond.
• Thermal stability up to 2,912 degrees Fahrenheit in a vacuum.
• Low thermal expansion coefficient.
• Length to diameter (L/D) ratio of over 3,000.
• High conductivity, yet five times lighter than copper.
• Inert and chemically compatible with most materials.

The structure of a nanotube influences its properties. Length, diameter, symmetry, and alignment all affect how a nanotube will behave when added to another material, so mastering the production process is critical. While nanotubes can be formed naturally as a byproduct of soot, synthesizing them in a production setting is ideal to control quality. Plasma arc discharge, laser evaporation, and chemical vapor deposition (CVD) are the primary methods of creating nanotubes, with CVD currently the most utilized method for commercial production. However, Philippe reveals that OCSiAi utilizes a patented, proprietary process to produce their SWCNT nanotubes at industrial scale.

Enhancing Powder Coatings
OCSiAl estimates that SWCNT can be used in up to 50% of all materials market applications, and Philippe estimates that more than 1,500 industrial companies worldwide already utilize them in their products. He also notes SWCNT were first used in U.S. coatings around 2014. They are mixed into powder coating formulations using standard production technologies with no adaptations required, making them easy toincorporate by powder manufacturers. Philippe says that a concentrated form of their product (TUBALL™ MATRIX) is often processed with a speed mixer during the premixing phase of powder production, before being extruded. He explains that working with a concentrate during manufacturing is much easier than handling raw materials. In addition, powder coatings containing SWCNT can be electrostatically sprayed and cured using traditional technologies, so no new equipment or processes are required for end users.

Electrically conductive powder coatings with graphene nanotubes are compatible with most engineering plastics and metal substrates. Utilizing conductive coatings helps to conduct or insulate humans from static electricity buildups that tend to discharge by friction between two or more bodies, leading to a spark which could ignite flammable materials or gases. They also prevent static electricity buildups on surfaces that might cause electromagnetic interference (EMI), damaging or disabling electronic devices. SWCNT are currently used for electrostatic- sensitive applications in hazardous explosive atmospheres (ATEX in the EU; HAZLOC in the U.S.), as well as to protect the instrumentation and surfaces in the medical, marine, aerospace, defense, and electronics (processor, chips) industries.

Philippe reveals that a much smaller dosage of SWCNT is required than conventional techniques for producing antistatic powder coatings. “SWCNT provide permanent, stable, homogeneous surface resistivity ranging from 103 Ω/sq to 109 Ω/sq without insulative spots or any dependence on humidity,” he advises. (Ω/sq, or Ohms per square, is the unit of an electrical measurement of surface resistivity across any given square area of a material. It is the measurement of the opposition to the movement of electrons across an area of a material's surface and normalized to a unit square.) “Traditionally formulated high conductivity powder systems often rely on conductive carbon black or conductive mica, which limits pigmentation options. By switching to a graphene nanotube system requiring drastically lower dosages, a significantly wider range of color options are available.” Because you are using 0.01-0.05% wt. SWCNT versus 5-20% wt. of a traditional additive, you are maintaining significantly more of the color and mechanical properties of the formulation (Figure 2). “In the powder coating world, we have for the first time, the possibility to get almost any kind of color or shade and achieve the three levels of electrical conductivity; anti-static, dissipative, and conductive,” Philippe says.

Last but not least, using much less conductive material enables improved production time and energy savings, making the final powder coating product highly competitive and much more sustainable when compared to other conductive additives.

Philippe expects warehousing and distribution center operations to be a tremendous growth area for powder coatings containing SWCNT. The U.S. has one of the largest networks of warehouses in the world, with Amazon alone averaging a total of 319 million square feet of space. This network supports our complex supply chain that moves goods efficiently from manufacturers to consumers. As this sector continues to grow and rapidly automate, replacing humans with robotics that can operate 24/7, SWCNT coatings will be necessary to protect equipment from EMI. Robots can take between five and 15 minutes to reboot, so racks and floors at minimum must utilize antistatic coatings to prevent damage to equipment and costly delays in sorting and moving goods.

The incorporation of SWCNT into powder coatings offers a multitude of benefits. The use of SWCNT can enhance safety by providing anti-static properties, improving coating durability, and reducing reliance on hazardous additives. These combined benefits make SWCNT a valuable additive for powder coatings, offering opportunities to create coatings with improved performance, longevity, and safety across various industrial and commercial applications.

Troy Newport is publisher of Powder Coated Tough.