Contact angle measuring instrument measures the surface energy of resin after curing.

The contact angle goniometer evaluates surface energy by analyzing the contact angle formed by a liquid droplet on a solid surface, based on the principle of Young's equation. Measuring the surface energy of resin after curing is crucial for properties such as coating adhesion and bonding. During measurement, the sample surface must be clean and flat. A liquid with a known surface tension (such as deionized water) is dropped onto the sample, and the instrument automatically calculates the contact angle. The polar and dispersive components of the surface energy are then determined using the Owens-Wendt method. Results can be influenced by factors such as surface roughness and environmental conditions, so multiple measurements under controlled conditions are necessary to ensure accuracy. For example, the total surface energy of an epoxy resin was measured at 38.2 mJ/m², with the dispersive component being dominant, which can guide material optimization and application.

Rationale

A contact angle meter is an instrument that evaluates the wettability and surface energy of solid surfaces by analyzing the contact angle formed by liquid droplets on a solid surface. Its core principle is based on Young's equation, which describes the three-phase interfacial tension equilibrium relationship between solid-liquid-gas. When a drop of test liquid is placed on a solid surface, the instrument captures an image of the droplet profile through an optical system and automatically calculates the contact angle θ using a software algorithm. The size of the contact angle directly reflects the wetting characteristics of the solid surface: the smaller the contact angle, the better the liquid spreads on the solid surface, and the surface energy is usually higher; On the contrary, the larger the contact angle, the worse the wettability, and the surface energy is usually lower.

After curing, the surface chemical composition and micromorphology of resin materials will change, which directly affects their surface energy. Surface energy is a key parameter that characterizes the surface properties of resins, and has a decisive impact on the subsequent application properties such as coating adhesion, adhesion performance, printability and anti-fouling ability. By quantifying the surface energy of the resin after curing through the contact angle measuring instrument, it can provide objective data support for material formulation optimization, process adjustment, and quality control, ensuring that the product meets the design requirements in practical applications.

Measurement method and steps

Measuring surface energy after resin curing is usually calculated using a contact angle meter supplemented by a variety of test liquids with known surface tension. The standard measurement process includes sample preparation, instrument calibration, droplet deposition, image acquisition, and data analysis. First, make sure the resin sample surface is clean, flat, and free of contamination. The sample is placed horizontally on the instrument sample stage and a small drop of test liquid (e.g., deionized water, diiomethane, etc.) is deposited on the surface of the sample using a microsyringe. The instrument camera captures the droplet side view image, and the software automatically fits the droplet profile and calculates the static contact angle. In order to obtain surface energy, the Owens-Wendt-Rabel-Kaelble method is often used, which decomposes the surface energy into polar components and dispersion components, and solves it by the contact angle values of at least two liquids with different properties.

The formula for calculating surface energy is as follows:

γ_L(1+cosθ) = 2(√(γ_S^dγ_L^d) + √(γ_S^pγ_L^p))

Among them, γ_Lis the surface tension of the liquid, θ is the measured contact angle, γ_S^dand γ_S^pThe dispersion component and the polar component of the solid surface are γ_L^dand γ_L^pis the corresponding amount of liquid.

Influencing factors

The accuracy and repeatability of measurement results are affected by a variety of factors. Attention should be paid to the control of sample surface roughness, chemical uniformity, ambient temperature and humidity, and droplet volume. Surface migration or oxidation that may occur during resin curing can also alter surface properties. It is recommended to measure in a controlled environment and perform multi-point measurements on each sample to obtain statistically reliable data. Instruments are regularly calibrated using standard templates to ensure the accuracy of optical system and software analysis.

Application examples

The following is an example of an epoxy resin that uses two test fluids to measure the contact angle and calculate the surface energy component after curing. This data helps to understand the surface properties of the resin.

Test the liquid	Contact angle measurement (degrees)
Deionized water	78
Diiodomethane	42
Total surface energy (mJ/m²)	38.2
Surface energy polarity component (mJ/m²)	5.1
Surface Energy Dispersion Component (mJ/m²)	33.1

According to the data in the above table, the total surface energy of the epoxy resin after curing is 38.2 mJ/m², of which the dispersion component is dominant. The relatively low polarity component indicates that its surface chemistry is dominated by non-polar groups. This information can be used to predict the resin's compatibility with polar adhesives or coatings, and if adhesion is to be improved, it may be possible to improve its surface polarity, for example, through surface treatment.

Conclusion

Contact angle meters provide an efficient and quantitative analytical method for evaluating surface energy after resin curing. Through standardized measurement processes and reasonable data processing, surface energy parameters that reflect the wettability and chemical properties of the resin surface can be obtained. These parameters have reference value for material development, process optimization, and performance prediction. In practical applications, interpreting the measurement results in combination with specific process requirements can provide directions for improving the comprehensive performance of resin products.

References

Owens D K., Wendt R C. Estimation of the surface free energy of polymers. Journal of Applied Polymer Science, 1969.

International Standard ISO 19403-2:2017, Paints and varnishes — Wettability — Part 2: Determination of the surface free energy of solid surfaces by measuring the contact angle.

ZHANG Hua. Surface and interface analysis technology of polymer materials. Beijing: Chemical Industry Press, 2018.