No matter what kind of coating method is used for water-based coatings, after construction, there is a process of leveling and drying to form a film, and then gradually form a flat, smooth and uniform coating film. Whether the coating film can be smooth and smooth is called leveling. In the paint industry, obtaining a flat paint film is often the goal pursued by paint formulation engineers. The surface of the roll film is smooth and free of defects, and good appearance is an important indicator to measure the performance of the paint.

Poor leveling will:

• Reduce the hiding efficiency of the paint and increase the cost;
• Reduce the smoothness of the paint film, affect the appearance, cause paint film defects, and reduce the protective effect of the paint film on the substrate.

Generally, the additives that can promote the flow of the paint film and help to obtain a smooth surface are called surface control additives, also known as leveling agents. Most surface control additives affect the surface tension and surface tension gradient distribution of the paint film and improve the flow. sex to achieve the corresponding effect.

The coating film formation and leveling process can be described and theoretically deduced by the following model.

(1) Initial solid-liquid interface, wetting distribution of the coating film on the substrate - contact angle model (Young's wetting square)

γ _sv = surface tension of the substrate, the main driving force for the spreading of liquid flow.
γ _lv = the surface tension of the liquid, always tends to shrink the liquid, is the resistance to the flow of the wetting spread and the driving force for the contraction of the liquid.
γ _ls = The surface tension of the liquid-solid interface is always the resistance of the liquid movement, no matter the liquid tends to spread or shrink, it is always a force that must be overcome. Even when γ _s > γ _l , the contact angle of the droplet is <90°, the liquid can wet the solid surface but not necessarily fully self-spread; only when γ _sv > γ _lv + γ _ls cosθ, the liquid can fully spread cloth. The surfactant migrates to the liquid-solid interface, and the non-polar hydrophobic group of the surfactant faces the non-polar substrate, and the hydrophilic group faces the water-soluble system, so that γ _ls is greatly reduced.

(2) Bernard vortices in the vertical direction

The early flow of the wet film is due to the wetting of the substrate and the low viscosity of the initial system, so the initial surface is quickly leveled. However, with the solvent volatilization, resin system (amine neutralization system and two-component crosslinking system), defoamer and other reasons, the gradient of surface tension is inevitable. The process of violent movement in the middle is shown in the figure, that is Bénard Cell (Bénard vortex), which is different from horizontal flow, but vertical movement.

(3) Sine wave model flow equation of surface leveling (Newtonian liquid)

△ t=kλ ⁴ h/γd ³

Flow equation (Newtonian liquid) where: k-constant; t-flow time; λ-wavelength of sine wave; η-viscosity of liquid; γ-surface tension of liquid; d-thickness of coating film.
The flow equation formula is an important model to explain the flow process of the wet film from the corrugated state to the flat state. This flow model has preset assumptions, including the assumption that the liquid is a Newtonian fluid
without thixotropy: the surface tension of the liquid is assumed to be uniform everywhere; and the dynamic factors of solvent volatilization are not considered. The trough part is driven by the surface tension of the liquid and tends to shrink the interface as a leveling force. The higher the surface tension, the greater the actuation ability.
The above three models can correspond to the three main stages of wet film wetting and leveling, in which factors such as surface tension, shear force, thixotropic behavior, viscosity change, and solvent volatilization play an important role in each stage. They influence each other and play a comprehensive role. Substrate wetting agent and leveling agent are additives added to affect the intricate balance in the above-mentioned different stages, in order to obtain the desired ideal surface control effect.

From the classification of chemical structure, leveling agents mainly include polysiloxanes, non-silicon polymers and high boiling point solvents.
Polysiloxanes can be divided into polyether-modified siloxanes, polyester-modified siloxanes, and aralkyl-modified siloxanes. Non-silicon polymers can be divided into polyol type, polyacrylate type, fluorine-modified polyacrylate type, etc. Due to the low surface tension of the siloxane segment and the limited compatibility with the resin system, polysiloxane leveling agents can often migrate to the coating surface at a faster rate than non-silicon leveling agents, thus It shows a faster leveling speed, and its effect is more reflected in the early leveling, which helps to eliminate the Bernard vortex caused by the volatilization of the cosolvent in the early stage of coating film formation. The polymer non-silicon leveling agent is more reflected in the later leveling aspect. Because of its relatively high surface tension and great leveling power, silicone and non-silicon polymers are often used together in the formula. use. Polysiloxane leveling agents have the following performances: leveling speed, wetting ability, recoatability, compatibility, slipperiness, foam stability, anti-shrinkage and anti-blocking properties. These are closely related to its chemical structure, and performance optimization can be achieved by adjusting the siloxane segment, hydrophilic segment and end-capping group to avoid some side effects.