Simple determination of surface tension of plastic diaphragm

The printability of the plastic film is fundamentally determined by its surface tension. In order for the ink to adhere, the surface tension of the diaphragm must be higher than the surface tension of the ink. You can check if the surface tension of a plastic diaphragm is high enough by some simple methods.

Surface tension characterizes the behavior of the interface between a liquid (such as water) or a solid (such as a diaphragm) and a gas (such as air), so it is also called interfacial tension. For example, the nature of water naturally seeking to minimize its surface area can prove this phenomenon. Therefore, a drop of liquid that does not withstand external forces will always assume an approximately spherical shape during, for example, free fall.

Molecular interaction

Adjacent molecules are subject to attraction and repulsion, the so-called cohesion. In a large volume of liquid, these forces can interact equally in all directions. However, this is not the case for molecules on the surface, because they have fewer molecules adjacent to them than those inside. Inside the liquid, the movement of a single molecule exists in a state of energy balance, however, the movement on the surface consumes energy to break molecular bonds.

Therefore, if the surface of a liquid is to be increased, a certain amount of work must be done. The work required to increase the surface depends on the surface tension of the liquid. The ratio of the work done to the surface increase formed is the surface tension (represented by the symbol "Sigma")

σ = ΔW / ΔA

It is usually expressed in units of mN / m (millinewton / m), which is equivalent to 0.001 kg m / s2 or mJ / m2 (mJ / m2) of SI. Water at 20 ° C showed a surface tension of 72.8 millinewtons / meter, compared with 484 millinewtons / meter for mercury and 21.7 millinewtons / meter for isopropyl alcohol.

The surface tension is related to temperature and usually decreases with increasing temperature. Surface active substances, such as surfactants in fountain solution additives, will reduce the liquid surface tension and improve the wetting of offset printing plates.

Liquid measurement method

In most cases, the surface tension of a liquid will be measured in a way that increases the surface area of ​​the liquid and then determines the amount of work done to achieve this. Examples include the ring method developed by Pierre Lecomtedu Noüy, the Wilhelmy plate method and the frame method named after Philipp Lenard.

In all three methods, a solid (ring, plate or frame) is immersed in the liquid and then pulled out to keep a layer of liquid film attached to the solid. The pulling force is gradually increased until the liquid film breaks. The surface tension can then be calculated from the maximum pulling force, the external dimensions of the solid immersed in the liquid, and the density of the liquid.

Measurement methods for solids

The surface tension of a solid can only be measured indirectly by a similar method. When using the contact angle method, a solid is wetted with two different liquids whose surface tension is known. The selected liquids are often water and diiodomethane. In the Young's equation shown in the side view, the signs S and L represent "solid" and "liquid"; the symbols σS and σL describe the components of the surface tension of the two stages; γSL represents the interfacial tension between the two stages And θ represents the contact angle with the angle between the vectors σL and γSL. In order to determine the surface energy, various initial equations of γSL are combined with Young's equations, and cosθ represents a function of the stage surface tension. Using this equation system, the surface tension of the solid can be calculated.

A series of instruments are available on the market for determining the contact angle quickly and simply. This type of goniometer consists of a system that injects test liquid and a camera that measures the cross-section of the water drop. Then use this measurement to calculate the corresponding contact angle value. If the liquid spreads, ie if the solid is completely wetted, it can be counted as a contact angle of 0 °. Contact angles between 0 ° and 90 ° are considered to represent good wetting, while contact angles between 90 ° and 180 ° are treated as poor wetting. A contact angle of 180 ° means that the liquid is repelled in a bead shape. This phenomenon is often referred to as the "lotus leaf effect" and is named after the corresponding characteristics exhibited by the leaves of the lotus plant.

Another typical method for determining the surface tension of solids is to use test inks. Apply an ink of known surface tension to the material to be tested with a brush. If the test ink wets the surface, the surface tension of this material is equal to or higher than the surface tension of the ink. Conversely, if the test ink retracts into a bead within 3 seconds, the surface tension of the solid is lower than that of the test ink.

Ideal test ink for plastics

The method for testing inks is based on the knowledge that if the surface tension of the liquid is lower than the surface tension of the solid in question, the liquid can achieve good wetting. This is why printing on substrates with low surface tension is problematic. In order to ensure that the printing ink can fully wet the substrate, it must be properly selected or pre-treated to ensure that its surface tension is higher than the surface tension of the ink. Therefore, a set of test inks will be a very convenient tool for checking the properties of a specific plastic substrate.

Surface tension of different plastic substrates

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