Need for Conductivity Agents
Need for Conductivity Agents
Electrostatic spray coatings have the advantage of less paint consumption compared to conventional non-electrostatic spray-coating methods.
Some special products are required to enhance the electrostatic paint performances and avoid problems from batch to batch. These are better known as Conductivity Agents or Conductivity Improvers.
The electrostatic spray application consists of creating an electrostatic field with the help of high voltage source. It enables wrapping up complex objects with a paint layer. Thanks to this field, the paint is forced-orientated following the electrostatic lines and is able to cover even the shadow parts.
Due to this forced orientation, the electrostatic spray application offers better yields (around 30% more) than common air spray techniques.
Combined with an airless application, it can also significantly reduce the solvent mist and as a consequence the VOC emission.
Let's first understand the role of electrical conductivity & resistivity in electrostatic spray coatings in detail.
Role of Resistivity and Conductivity
Role of Resistivity and Conductivity
In order to be charged and electrostatically attracted to the substrate, the paint must have a defined conductivity / resistivity. These two parameters can be defined as follow:
Electrical Resistivity (ρ)
Resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm meter (Ω.m).
where,
- ρ is the electrical resistivity
- R is the electrical resistance of a uniform specimen of the material.
- l is the length of the specimen.
- A is the cross-sectional area of the specimen.
Electrical Conductivity (σ)
Conductivity is the reciprocal quantity, and measures a material ability to conduct an electric current.
The SI unit is Siemens per meter (S.m−1). The relation between the electrical Resistivity and Conductivity is:
An electrostatic liquid paint has generally a resistivity between 1 and 50 MΩ.cm.
- 1 MΩ.cm could cause electrical short circuit
- 50 MΩ.cm reduces the charge of the particles and the paint ability to cover properly the pieces
Conductive Paint by Bare Conductive®
Now since we know about electrostatic spray coatings, let's turn our attention towards improving coating systems with the right choice of conductive agent.
Conductivity Agents for Coating Systems
Conductivity Agents for Coating Systems
Waterborne and Solventborne Coatings
Due to the water dipole characteristics,
water-based paints are too conductive. They require specifically designed electrostatic spray units. Specific conductivity agents can be used to bring some antistatic properties.
For solventborne coatings, one of the best conductivity agent is
indium tin oxide. Nevertheless, this transparent conducting material is really expensive for most of coatings technologies.
More cost effective alternatives, based on halogen-free cationic compounds, such as amine of fatty acid or quaternary ammonium salt derivative also exist.
- Coatings containing non polar solvents (such as alkyds, melamine, cyclized rubber systems…), need a conductivity agent as the solvents cannot conduct electricity.
- When containing polar solvents (such as epoxy, polyester, acrylic systems…) the conductivity agent provides the fine tuning and enhances the paint properties. In this case, choosing the polar solvents carefully may considerably help in improving the conductivity, and the conductivity agents will work in synergy with these solvents.
Conductivity Agents for Powder Coatings
In powder coatings, there is less need of conductivity agents, as the electrostatic potential is provoked by the specific designed parts of the spray gun (tribo or corona). The powder particles are charged and cover the substrate.
Nevertheless, addition of a conductivity agent to the powder reduces the powder resistivity, resulting in improved charging efficiency. Spraying time can be reduced.
- At the same voltage, the powder layer builds more rapidly
- At a lower gun voltage; it improves penetration in Faraday cage areas
Many of the antistatic agents are quaternary ammonium salts (cationic) or alkyl sulfonates (anionic) based on fatty-acid derivatives. They should be added prior to the extrusion.
Besides these different chemistries, some metallic powders or specific pigments, such as
conductive carbon black or carbon fibers can be used for specific applications (such as the formulation of a long term conductive paint, or a conductive primer for instance). Such products can be used either in solventborne or waterborne coatings. In powder coatings, using these products will require more attention, especially regarding the application process, when a too conductive powder may provoke some power shock.
» Also, get started with our exclusive guide on tests and standards to characterize powder coatings.
Let's explore the points to consider while selecting the right conductivity agent for your coating formulation.
Selecting the Right Conductivity Agent
Selecting the Right Conductivity Agent
As the electrical conductivity is a sensitive parameter, it is important to check the performances of different agents and evaluate their tendency to provoke some unwanted side effects. Indeed, due to their chemistry, conductivity agents may reduce the pot life in case of 2K-PU systems, or increase the drying time of air drying alkyds. Besides, the efficiency of quaternary ammonium derivative may vary with the relative humidity, so, the application parameters must be once defined and stay constant.
Points to Consider
Following charts will give some indications regarding the conductive agent efficiency and negative side effects that the formulator must take in consideration when selecting the right product.
In a melamine-based paint formulated with non-polar solvents, different conductivity agents were added at different amounts and the conductivity was measured. In this experiment, the quaternary ammonium salt based product shows a better efficiency at low dosage.
In a 2K-PU paint, different conductivity agents were added to reach the same conductivity level. The pot-life was tested by measuring the liquid paint viscosity. In this experiment, the quaternary ammonium salt product shows a lower viscosity increase in time.
Further recommendation:
-
In epoxy resin coatings, conductivity agents based on quaternary ammonium compounds should be used in combination with other chemistry or types to reach the desired efficiency.
- Due to their special chemistry, conductivity agents may also have an influence on the haze and gloss.
- When possible, modifying the polar solvent types / ratio may help a lot to decrease the resistivity when it is not possible to increase the conductivity agent dosage for side effect or cost considerations.
To conclude, it would be important to mention that using a conductivity agent or not largely depends on the selected application method. In case of conductivity improvement required for an electrostatic spray application, most of the products in the market will bring you the required conductivity level.
Nevertheless, the negative sides effect may strongly vary. That is why it is further more important in this application to check not only the product performances, but also all the side parameters. Besides, when possible, combining the right conductivity agent with a polar solvent to obtain a perfect synergy is a great way to improve the conductivity at a moderate cost.
So, let's start formulating with a complete range of conductivity agents available today for all types of coating and ink formulations (aqueous, solvent-based, high-solid or 100% solids systems and for pigment concentrates). You can check tech profile for each product, ask for samples or discuss your case with producer’s tech staff.
