Surface tension - Thermoplastic polymers, elastomers and additives

By surface tension, property of a liquid that makes it behave as if its surface were encased in an elastic sheet

The attractive forces between the molecules of a liquid can be considered as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals adhesion.

It is a thermodynamic quantity defined as work necessary to isothermally increase the surface area of ​​a liquid. The surface tension is a direct measure of the intermolecular forces at the level of the surface. Due to surface tension the tendency of liquids is to reduce their energy as much as possible, then to form drops. The surface tension varies with the temperature, the composition of the flow and the interactions between the alloy of the substrates. For these reasons, the surface tension of brazing alloys is not known precisely. In general, the surface tension tends to be lower in the air than in an inert atmosphere. The action of the environment is generally determined by the absorption of vapor on the polymeric surface, and it is what reduces the surface tension. If the surface tension of a liquid, placed on a surface, is greater than that of the solid, it forms a contact angle between the solid phase and the liquid phase. When the same liquid is placed in super? Ci With the increase in surface tension, the contact angle decreases.

The force that is required (in dynes) to break a 1 cm film. of length. The surface energy in ergs per square centimeter can be established in an equivalent way. The surface tension depends on the nature of each liquid. Surface tension is defined in physics as the amount of energy needed to increase the surface of a liquid and counteract that force of attraction inward. Water at 20 ° C has a surface tension of 72.8 dynes / cm compared with 22.3 for ethyl alcohol and 465 for mercury.

The measure is necessary:

- before painting: water-based paints require a wettability of at least 45 mN / m (dyne)

- Before printing: the adhesion of the printing inks requires a wettability of approx. 38 mN / m

- Before gluing: at least 42 mN / m

- After the treatment to verify that the above parameters have been reached.

It must be remembered that a good wettability of the surfaces to be glued, printed and painted is a necessary condition, but not always sufficient, and other factors that may condition adhesion sometimes intervene.

We indicate the intrinsic surface tensions of some untreated plastics:

Polypropylene 29 mN / m (dyne)

Polyethylene 31

Polystyrene 35

Polyamide <37

PBT 34

Silicone 23

PVC 38

PMMA <37

ABS 36

Mexpolimeros supplies inks for the wettability test in packages of 8 bottles of 30 ml, in the standard version from 30,34,38,40,44,46,52,58 mN / m. On request also different gradations.