Oleophobic coatings are a critical technology that directly affects product performance and durability. To accurately evaluate their effectiveness, it is essential to select appropriate indicators that match the intended purpose.
Typical indicators include physical measurements such as contact angle, sliding angle, and friction coefficient. Each indicator reflects different surface properties, and combining multiple methods enables more precise evaluation. Since these factors are also linked to process conditions such as dip coating parameters, it is important to consider material characteristics and equipment settings.
This article introduces the main evaluation indicators and practical methods for selecting them effectively.
Main Evaluation Indicators for Oleophobic Coatings
Measuring Surface Energy by Contact Angle
The contact angle method evaluates surface energy by measuring the angle formed between a liquid droplet and the coating surface. A larger angle indicates that the liquid spreads less easily, showing higher oleophobicity.
The measurement is performed using a contact angle meter, which calculates the angle from the droplet shape. This method is simple, highly reproducible, and suitable for initial evaluation and material screening.
Assessing Droplet Mobility by Sliding Angle
The sliding angle indicates the tilt angle at which a droplet begins to move on the surface. While the contact angle measures static wettability, the sliding angle captures dynamic oleophobic behavior. A smaller sliding angle means that the droplet slides off more easily, indicating better oil repellency.
The measurement is performed using a tilting stage with adjustable angles and an observation camera.
Evaluating Dynamic Oleophobicity by Friction Coefficient
The friction coefficient quantifies the frictional force between a liquid droplet and the surface. A lower coefficient means smoother droplet movement and higher oleophobicity.
Measurements can be made using droplet movement on an inclined surface or with specialized test instruments. Since the results are affected by droplet type, viscosity, and surface microstructure, it is important to keep all conditions consistent during testing.
Criteria for Selecting Evaluation Indicators Based on Purpose
Relationship Between Material Selection, Liquid Concentration, and Contact Angle
The contact angle changes depending on the substrate material and the concentration of the coating liquid. When the liquid concentration is high, film thickness increases and the contact angle tends to become larger. In contrast, if the surface energy of the material is high, the droplet spreads more easily and the contact angle becomes smaller.
During the material design stage, measuring the contact angle helps identify the optimal material and coating combination.
Effect of Withdrawal Speed and Curing Conditions on Sliding Angle
In dip coating, withdrawal speed and curing conditions have a strong influence on film uniformity and adhesion. If the withdrawal speed is too high, film thickness variation and coating runs may occur, which can also affect the sliding angle. Improper curing conditions reduce surface smoothness and make droplet movement more difficult.
By adjusting process conditions, the sliding angle can be improved and the oleophobic performance enhanced.
Relationship Between Surface Cleanliness, Equipment Settings, and Friction Coefficient
The friction coefficient is highly affected by surface contamination and micro-roughness. Insufficient cleaning or foreign particles increase friction and prevent smooth droplet movement. Improper transport speed or drying conditions in the equipment can also increase the friction coefficient.
To obtain stable and reliable evaluation results, strict control of pretreatment and equipment settings is essential.
Summary
In evaluating oleophobic coatings, it is important to combine multiple indicators such as contact angle, sliding angle, and friction coefficient. The contact angle assesses static oleophobicity, the sliding angle captures droplet motion, and the friction coefficient quantifies surface smoothness. By using these indicators together, performance can be evaluated more accurately and objectively.
In addition, considering process factors such as material selection, liquid concentration, withdrawal speed, curing conditions, surface cleanliness, and equipment settings helps achieve evaluation results that better reflect target performance.
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