Talking about the principle of wetting agent: surface tension

Surface and interface

In an inhomogeneous system, there are at least two phases with different properties. The coexistence of two phases must have an interface. It can be seen that the interface is the result of the inhomogeneity of the system. Generally refers to the transition region of about a few molecules thickness at the junction of two phases, if one of the phases is a gas, this interface is usually called the surface. It is customary to call the interface between liquid or solid and air the surface of liquid or solid. Common interfaces are: gas-liquid interface, gas-solid interface, liquid-liquid interface, liquid-solid interface, solid-solid interface.

Definition and principle of surface tension

At the boundary between the liquid and the gas, that is, at the surface of the liquid and the interface between the two immiscible liquids, a very small pull is generated due to the attraction between the molecules. Imagine that there is a thin film layer at the surface, it is subjected to the tensile force of the surface, this tension of the liquid is called the surface tension. It's called gamma in N/m.

Factors affecting surface tension

(1) The effect of intermolecular interaction: the stronger the chemical bond, the greater the surface tension. γ (metallic bond) > γ (ionic bond) > γ (polar covalent bond) > γ (non-polar covalent bond)

(2) The effect of temperature: the temperature increases, the surface tension decreases

(3) The effect of pressure: the general pressure increases, and the surface tension decreases

In general:

The surface tension of water is 72.8mN/m(20℃).

The surface tension of organic liquids is less than that of water.

The surface tension of organic liquid containing nitrogen, oxygen and other elements is larger.

The surface tension of liquid containing F and Si is the smallest.

High molecular weight high surface tension;

Aqueous solution: if it contains inorganic salts, the surface tension is greater than that of water; Contains organic matter, surface tension is smaller than water;

The weakest surface tension of liquid (above 0 ° C) is alcohol.

Definition of substrate wetting

Substrate wetting is defined as the replacement of air on the surface of the substrate by a liquid coating, a process that is critical to the successful coating of the coating.

The spreading of the coating on the substrate can be microcosmic as the boundary surfaces of the three media intersect at a point, and the three boundary surfaces intersect at a certain Angle, which is called the contact Angle θ.

The size of the contact Angle θ depends on the surface energy γs of the substrate, the surface tension γl of the liquid coating and the interfacial tension γls between the liquid coating and the substrate, the smaller the contact Angle θ, the better the wetting effect of the substrate. Therefore, the most convenient way to get a good substrate wetting is to reduce the surface tension of the liquid coating.

Selection of substrate wetting agent

The substrate wetting agent is mainly used to reduce the surface tension of water-based coatings. The main solvent in this system is water with high surface tension, and the surface tension of water is 72.8mN/m(20℃). Therefore, an aqueous solution of 0.2% substrate wetting agent is used to test its dynamic and static surface tension.

Dynamic surface tension test method: bubble pressure method

Aqueous solution of 0.2% substrate wetting agent

Note: The pressure required to create bubbles in a liquid is directly proportional to the dynamic surface tension. The different bubble production rates can reflect the ability of different surfactants to quickly and directionally arrange on the newly created surface.

As the bubble life becomes longer, the measured surface tension tends to be stable, and the surface tension is the static surface tension of the liquid.

peroration

Reducing the surface tension of water-based coatings can make it a good wetting spread on different substrates, and different substrates can be adapted by selecting different substrate wetting agents with different surface tension reduction capabilities.