The Application of Contact Angle Measurement Instruments in the Study of the Relationship Between Ink Printing Wettability and Substrate Surface Tension

Application principle:

During ink printing, the surface wettability of the substrate is one of the key factors that determine the printing quality. The contact angle meter can quantitatively characterize the degree of wetting of the liquid to the solid by analyzing the contact angle of the droplet on the solid surface. According to Young's equation, there is a direct correlation between contact angle and interfacial tension: γS = γSL + γL cosθ (where γS is the solid-gas interfacial tension, γL is the liquid-gas interfacial tension, γSL is the solid-liquid interfacial tension, and θ is the contact angle). When the contact angle is less than 90°, it indicates that the liquid can wett the substrate well. If it is greater than 90°, the wettability is poor. The wettability of ink to the substrate directly affects the ink transfer efficiency, graphic clarity and adhesion.

Measurement method

The standard test solution (such as deionized water, diiodine, etc.) was dropped on the surface of the substrate using a precision syringe, and the droplet volume was controlled at 2-5μL to avoid the influence of gravity. The droplet profile is captured by a high-speed camera, and the static contact angle is calculated using gonometry or fitting algorithm. The surface tension of the substrate is indirectly calculated by a variety of test solutions with known surface tension, combined with the Oss-Good equation or Wu harmonization method.

Table 1 lists the data of commonly used test fluids and their surface tension components:

Table 1 Reference value of surface tension of test liquid

Relationship analysis

The relationship between the contact angle and the surface tension of the substrate follows the Zisman law: when the surface tension is higher than the surface tension of the ink, the contact angle is small and the wetting is sufficient. Experimental data show that for untreated polyolefin substrates (surface tension is about 30 mN/m), the contact angle of water-based inks (surface tension is about 40 mN/m) is often greater than 90°, and the surface tension of the substrate needs to be increased to more than 38 mN/m by corona or plasma treatment to obtain the wettability to meet the printing requirements.

Table 2 shows the correspondence between contact angle and surface tension under different treatment methods:

Table 2 Wetting characteristics of different treatment substrates

Influencing factors

The measurement results are affected by the ambient temperature and humidity, the cleanliness and roughness of the substrate. It is recommended to operate in a constant temperature and humidity environment. The addition of additives to ink formulations can change its surface tension, such as adding surfactants to reduce ink surface tension and improve wetting of low-energy surfaces. In addition, the microstructure of the substrate surface can be enhanced by the Wenzel model: cosθ actual = r cosθ intrinsic (r is the roughness factor).

Conclusion

Contact angle measurement provides a direct basis for the optimization of ink printing processes. By accurately measuring the contact angle and calculating the surface tension of the substrate, the ink spreading effect can be predicted, and the surface modification of the substrate or the adjustment of the ink formula can be guided. This method is suitable for the printing suitability evaluation of various substrates such as plastic film, metal foil, and paper.

References

[1] Zisman W A. Relation of the equilibrium contact angle to liquid and solid constitution. Contact Angle, Wettability, and Adhesion, 1964.
[2] Owens D K, Wendt R C. Estimation of the surface free energy of polymers. Journal of Applied Polymer Science, 1969.
[3] Relevant technical standards: GB/T 30693-2014 Determination method of contact angle between plastic film and water.