Interfaces, boundaries between two phases, are important in coating and ink technology. The behavior of a system during production, storage, application and film formation is, to a large extend, governed by the composition of the interfaces that are present and by the total amount of interfacial area. A key property, associated with the composition of an interface, is interfacial tension. There are two categories of interfacial tension, represented by the Greek symbol γ (‘gamma’), that are most important in paints and inks.
Surface tension
Surface tension is a property of liquids that is governed by intermolecular interactions: it originates from the cohesive forces between molecules in a liquid. Thermodynamics tells us that systems, like paints and inks, strive to attain a state with a maximum amount of favorable interactions. This implies that liquids will shape in such a way that the number of bulk molecules is at a maximum and the amount of surface molecules is at a minimum.
Molecules at the surface of a liquid are not fully surrounded by their fellows; surface molecules are partly naked. This implies that molecules at the liquid-air interface experience less favorable interactions and are in a state of higher energy compared to molecules in the bulk of the liquid.Therefore, energy is needed to move molecules from the bulk of the liquid to the surface. The stronger the interactions between the molecules are, the more energy is required to increase the surface area of a liquid.
The surface tension of a liquid, γlg , is defined as the energy (in Joule) needed to create 1 m2 of new liquid-gas interfacial area. The dimension of γ is J/m2. Often the dimension N/m (Newton per meter) is used. Two subscripts are used to specify the interface to which an interfacial tension refers. An interface is a boundary between two phases. Therefore, the abbreviation of these phases should be included in the symbol for the interfacial tension. Surface tension is the interfacial tension of a liquid-gas interface. In paints and inks the ‘gas’ phase is most often the air above the system.
n-Hexane has a low surface tension because the mutual interactions between the molecules in the liquid are relatively weak. The surface tension of a liquid with high surface tension, like water, can be lowered by adding molecules that have a surfactant structure, like wetting agents.1,2 Such additives lower the surface tension of a liquid because the molecules adsorb and orient at the liquid-air interface in such a way that the hydrophobic tails point towards the air and the hydrophilic parts stick into the liquid. Wetting can be improved, and film defects can be prevented, by using suitable wetting agents.3
Surface energy
Surface energy is a key property of solid materials, like substrates that must be coated. Surface energy is the interfacial tension of a solid-gas interface. The property is represented by the symbol γsg, with the subscript ‘s’ standing for solid and ‘g’ for gas. The gas above the solid surface is most often air.
Strong interactions are possible with a solid surface that has a high surface energy. Unsurprisingly, a high surface energy is beneficial for the wetting of, and adhesion on, substrates. Solids with a low surface energy, like most plastics, are difficult to coat.
The surface energy of plastic objects can be raised by using pre-treatment techniques, such as corona treatment, plasma treatment or flame treatment, thus improving wetting and adhesion.4
References
- Surface Active Agents (Surfactants), Marc Hirsch, 25 September 2015. [https://www.ulprospector.com/en/eu/Coatings/Detail/30648/635175/Surface-Active-Agents-Surfactants]
- Breaking the Tension with Surfactants, Angie Pedersen, 27 July 2018. [https://www.ulprospector.com/knowledge/8516/pc-surfactant-infographic/]
- Overcoming Paint Film Defects: Causes and Remedies, Ron Lewarchik, 16 June 2017. [https://www.ulprospector.com/knowledge/6636/pc-overcoming-paint-film-defects-causes-remedies/]
- Plasma Processing of Plastic Surfaces, 30 December 2016, Andy Pye. [https://www.ulprospector.com/knowledge/5708/pe-plastic-surface-plasma-processing/]
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In powder coating, how present is the surface tensión and surface energy?
Surface tension and surface energy are important in powder coatings as well.
In the powdery state, the paint particles adhere to the substrate because of electrostatic forces. When the powder melts in the oven, the surface tension of the melt must be low enough to completely wet the substrate. In powder paints, often an additive is present to lower the surface tension of the melt. This is done to assure complete wetting of the substrate by the molten powder paint and to minimize film defects.
I invite you to take a look at article The Basics of Powder Coatings, that can be found on the UL Prospector website.
https://www.ulprospector.com/knowledge/8841/pc-powder-coatings-basics/
Hi Jochum,
In many sources I found the surface energy of steel and glass to be 700-1100 and 250-500 Dyne/cm (mJ/m2 or mN/m), respectively. Why these are so different from yours?
Thank you
Dear Ethan,
Indeed, many different surface energy values of substrates/materials can be found. It is important that the circumstances, under which the measurement was done, is reported. Pure steel, for example, has a very high surface energy in an inert atmosphere. Under normal circumstances, the surface of steel is hydrated because it is exposed to air that contains humidity. The consequence is that the surface is not iron (Fe), but iron hydroxide. This implies that the surface of steel, under ‘normal’ circumstances, is covered with hydroxyl (-OH) groups. The surface energy depends upon the type of steel and upon the pre-treatment.
Hi,
Can we make substrate such a way that Paint does not stick to it.
Is it possible ? It should be removed by simple rinsing with solvent.
In Paint industry, we use big trollies to make paint and it should be wash by solvent.
This make more harmful to person who is washing.
Can you please give some inputs ?
Dear Mr/Mrs Babu, I advise to apply a paint on the trollies that forms a smooth and hydrophobic coating during film formation. To assure that the coating is inert to solvents, I advise to use a 2-component system with high crosslink density. To obtain lang-lasting hydrophobicity, I advise to use a surface additive (either silicone- or fluorine-based) that crosslinks with the binder system during film formation.
Thank you for reading,
Jochum
What are the effects of increasing surface crystallization of semicrystalline polymer on surface tension and surface energy? Does the type of crystal formation and morphology also add to the surface tension and energy of semicrystalline plastics like polyolefins e.g. Polypropylene and polyethylene?
Dear Joe, thanks for reading the article!
A system, for example polyolefin in contact with air, will arrange itself in such a way that the interfacial tension (in this case surface energy) is as low as possible. Polyolefins, like PP and PE, do not contain groups that are able to form dipole-dipole interactions and/or hydrogen bonds. For these 2 reasons, I predict that the surface energy of amorphous polyolefin and crystalline polyolefin will be close to each other. But of course, the prove is in measuring the surface energy values of polymers via contact angle measurements with liquids of known surface tension.
Greetings, Jochum.