Interfaces are important in coating and ink technology. The behaviour of a system during production, storage, application and film formation is, to a large extent, governed by the composition of the interfaces that are present. A key property, associated with an interface, is interfacial tension. In this article, two categories of interfacial tension, represented by the Greek symbol γ ('gamma'), are discussed.
Surface tension is a property of liquids 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 maximum amount of favourable interactions. This implies that liquids will shape in such a way that the amount of bulk molecules is maximum and the amount of surface molecules is minimal.
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 feel less favourable 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 surface area. The dimension of γ is J/m². Often the dimension N/m (Newton per metre) is used. Note that 2 subscripts are used to indicate the interface γ refers to. An interface is a boundary between 2 phases and therefore the abbreviation of the 2 phases should be included in the symbol for the interfacial tension. In this case, γlg is the interfacial tension of the liquid-gas interface. In coatings and inks the 'gas' phase is most often the air above the system.
The surface tension of a liquid of high surface tension, like water, can be lowered by adding molecules that have a surfactant structure, like wetting agents. These additives lower the surface tension of a liquid because the molecules adsorb and orient at the liquid-air interface in such a way that hydrophobic tails point towards the air.
Surface energy is a key property of solids, like substrates that must be coated. It is the interfacial tension of a solid-gas interface and it is represented by the symbol γsg, the subscript 's' stands for solid and 'g' for gas. The gas above the solid is most often air.
When a solid has a high surface energy, it implies that strong interactions are possible with that surface. It will come as no surprise that a high surface energy is beneficial for, for example, adhesion. Solids with a low surface energy, like most plastics, are difficult to coat. The surface energy of plastics can be raised by treatment techniques, such as corona treatment or flame treatment, thus improving wetting and adhesion.
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.