Film Dictionary

Surface tension

The surface tension of films is a decisive characteristic

The tension of the respective surface is an important factor in certain applications of films. Only if the surface tension – sometimes also referred to as surface energy – of films is high enough will adhesives or printing inks adhere securely to the surface of the films.

How does surface tension occur?

In liquids, molecular forces occur that keep the respective surfaces small. The tension of the surface results from this process. This varies depending on the liquid. For example, the surface of water has relatively high tension, while oil has comparatively lower surface energy or surface tension. The extent of surface tension can be summed up in a rule of thumb: the more strongly the particles attract each other, the greater the surface tension.

The temperature of liquids influences surface tension

Surface tension also depends on the temperature of the liquid. As a rule, surface tension decreases as temperature increases. In this case, heat causes expansion, so that the particles in the liquid increase their distance from one another. At the same time, this also reduces the attractive force between the particles.

In principle, the surface of liquids behaves similarly to a stretched, elastic film. In nature, this effect is responsible, for example, for the formation of water droplets. The phenomenon of some insect species being able to walk on water is also directly related to molecular forces and the surfaces kept small as a result.

Differences in surface energy between liquids and solids

The tension of the surface in a solid is influenced either by repulsive or attractive forces. This is due to the immobile particles, which are fixed in position. In a liquid, by contrast, all molecules are mobile. This means that the distances between the individual particles can change. As a result, attractive and repulsive forces can act on the liquid molecules. This situation can also be illustrated using a Lennard-Jones potential, which describes binding energy.

High tension of film surfaces ensures good adhesion

Surface energy or surface tension is of essential importance for films, especially when applying inks, varnishes, adhesives and coatings. This is because the strength of the tension influences the adhesion of additional materials to the film. This also applies to the wettability of film or plastic surfaces. A typical example is polyethylene films used as packaging material for water-containing foods.

Typical example of the importance of surface energy

With films of this type, water released from the food can collect on the inside of the film in the form of small water droplets. This process can be prevented by specifically modifying the surface energy, thereby maintaining the transparency of the films.

In this context, Bleher Folientechnik has focused on industrially compostable, food-safe PLA films made from renewable raw materials. These not only have a very good moisture transmission rate and good transparency for UV light, but also a high level of surface tension.

This specific pretreatment can increase surface tension

If required, surface tension can be additionally increased by specific pretreatment of the films. The so-called corona treatment specifically increases the natural tension of the film surface and of other plastic, paper and polymer objects. By increasing the tension of surfaces, both the wettability of the materials and the adhesive strength of adhesives, printing inks and coatings can sometimes be significantly increased.

In addition, corona treatment improves print quality, enables faster press speeds and reduces waste. Overall, this means an optimization of quality and productivity.

How polymer films benefit from improved surface energy

Untreated polymer film surfaces are pore-free, chemically inert and have comparatively low surface tension. As a result, the respective film can only bond with difficulty to printing inks, substrates, adhesives and coatings. Corona treatment provides a remedy precisely here by increasing the tension of the film surface and at the same time improving adhesion.