Introduction
In industries such as coatings, inks, adhesives, and composites, the wetting and spreading properties of resin liquids on substrates directly affect the adhesion, uniformity, and appearance quality of the final product. The wetting and spreading process is essentially the result of the three-phase interface interaction between solid-liquid-gas, and its driving force is closely related to the surface tension of the liquid, the interfacial tension of the solid-liquid and the free energy of the solid surface. As an instrument for accurately measuring the surface and interfacial tension of liquids, the surface tensiometer provides key data support for the quantitative evaluation of the wetting and spreading properties of resin liquids. The purpose of this paper is to explore how to systematically evaluate the wetting and spreading behavior of resin liquids by using surface tensiometer and related theories.
Wet spread
The wetting process is usually described by Young's equation, which defines the relationship between the contact angle θ and the three-phase interfacial tension:
γSV = γSL + γLV cosθ
Among them, γSVIt is the free energy of the surface of a solid surface in a vapor environment, γSLIt is the tension of the solid-liquid interface, γLVis the surface tension of the liquid (that is, the interfacial tension between the liquid and the vapor). The spreading coefficient S can be used to directly measure the tendency of a liquid to spread spontaneously on a solid surface, which is defined as:
S = γSV - γSL - γLV
When S ≥ 0, the liquid spreads spontaneously on a solid surface. Therefore, the γ of resin liquids is determined by a surface tensiometerLVand the interfacial tension with specific liquids (e.g., diiodomethane, water), combined with known solid surface energy data, can indirectly calculate or evaluate its spreading capacity.
Measurement principle
Modern surface tension meters mostly use the hanging ring method, hanging plate method or hanging drop method. For resin liquids, due to their non-Newtonian fluid properties or surface tension over time, it is recommended to use methods suitable for dynamic measurement, such as the droplet method or the maximum bubble pressure method. Measurements are subject to controlling temperature, humidity, and liquid volatility, usually in accordance with relevant standards (e.g., ASTM D1331, ISO 304). Key measurement parameters include static surface tension, dynamic surface tension (surface life effect), and interfacial tension with the reference liquid.
Evaluation process
The evaluation process can be divided into three steps: First, the surface tension γ of the resin liquid is measured directlyLV； second, the contact angle between the resin liquid and two reference liquids with known surface energy components (commonly polar and non-polar liquids) is measured, or its contact angle on a known solid is measured directly using a surface tensiometer accessory; Finally, combined with the solid surface energy data (if unknown, it can be estimated by Owens-Wendt and other models), the spreading coefficient S is calculated or the contact angle change trend is analyzed. Measurement of dynamic surface tension helps to understand the wetting behavior of resins during construction.
Influencing factors
The wetting and spreading properties of resin liquids are affected by their composition, additives, temperature and substrate properties. The addition of surfactants or wetting agents significantly reduces surface tension but may affect interfacial tension. The measurement data can be organized as follows to aid in analysis:
| Measurement parameters | Influence trend on wetting spread |
| Static surface tension is reduced | It is usually conducive to spreading |
| Fast dynamic surface tension decay | It is conducive to quick wetting |
| Low resin-substrate interfacial tension | Directly promote the spread |
| The contact angle is reduced | Indicates improved wettability |
It should be noted that a single surface tension value is not sufficient to fully evaluate wettability. For example, too low surface tension can lead to insufficient cohesion strength in the resin. Therefore, it should be combined with the interfacial tension and spreading coefficient for comprehensive judgment.
Application examples
In industrial R&D, the type and dosage of wetting agents can be optimized by comparing the surface tension and spread coefficient of different formulations of resins. For example, in coating applications, ensuring that the resin has the right spreadability on metal or plastic substrates can avoid defects such as shrinkage holes and fisheyes. When measuring, attention should be paid to the uniformity and cleanliness of the resin, and the real-time impact of solvent volatilization on the measurement results should be considered. For high-temperature curing resins, measurements close to process temperatures are recommended.
Conclusion
Surface tensiometer provides a reliable means to quantify the wetting and spreading properties of resin liquids. By systematically measuring surface tension and interfacial tension and calculating the spreading coefficient in combination with wetting theory, the development of resin formulation and the optimization of substrate treatment process can be scientifically guided, so as to improve the interface quality and performance consistency of coatings and composites. In the future, surface tension measurement methods combined with automation and real-time monitoring technology are expected to further meet the evaluation needs of complex process conditions.
References
ASTM D1331-14, Standard Test Methods for Surface and Interfacial Tension of Solutions of Surface-Active Agents.
ISO 304-1985, Surface active agents — Determination of surface tension by drawing up liquid films.
Owens, D. K., & Wendt, R. C. Estimation of the surface free energy of polymers. Journal of Applied Polymer Science, 1969.
Adamson, A. W., & Gast, A. P. Physical chemistry of surfaces. John Wiley & Sons, 1997.