Say Goodbye to "Orange Peel": Optimization Strategies and Practices for Poor Leveling in Release Film Coating (Part 1)

Causes of Orange Peel Formation

1. Coating Solution Formulation and Properties

• Excessively High Viscosity: Overly high viscosity of the coating solution leads to poor fluidity. Unable to level effectively under the action of gravity and surface tension, it tends to form an uneven surface.
• Too Fast Solvent Evaporation Rate: Surface solvents evaporate preferentially. If the coating solution contains a high proportion of low-boiling, volatile solvents, or if the inlet temperature and air velocity of the oven are excessively high, a large amount of surface solvents will evaporate instantly, causing a sharp increase in viscosity and rapid skin formation on the surface.
• Formation of Bénard Cells: After the surface dries rapidly and forms a skin, the underlying solvents continue to evaporate and diffuse toward the surface. If a high-viscosity "skin" has already formed on the surface, the vapor of the underlying solvents will break through the surface to form small holes, while driving the movement of surrounding high-viscosity materials, resulting in a vortex-shaped concave-convex structure, namely orange peel.
• Solvent Gradient and Surface Tension Difference: Solvent evaporation creates a concentration gradient inside the coating, which in turn induces a surface tension gradient. This drives the material to flow from areas of low surface tension (high solvent concentration) to areas of high surface tension (low solvent concentration), forming convection (the Marangoni effect) and exacerbating orange peel formation.
• Mismatched Surface Tension: When the surface tension of the coating solution is higher than the critical surface tension of the substrate (release film), wetting properties are poor. The coating solution shrinks on the substrate, leading to local thickness unevenness, which manifests as orange peel after drying.
• Excessively High Solid Content: High solid content means relatively less solvent in the coating solution, resulting in higher viscosity and poorer fluidity, which makes leveling difficult.

2. Coating Process Parameters

• Excessively Fast Coating Speed: An overly fast coating speed means the coating solution does not have sufficient time to level by its own surface tension before drying after spreading on the substrate.
• Improper Setting of Coating Gap/Thickness: A too-thick wet film requires a longer time for solvent evaporation, increasing the risk of underlying solvent vapor breaking through the already-skinned surface and thus the likelihood of orange peel formation.
• Uneven Coating Thickness: Improper setting or wear of the coating head (e.g., comma coater, microgravure roll, die head) leads to inherently uneven coating thickness, which becomes more pronounced after drying and shrinkage, showing as local orange peel.

3. Drying Conditions

• Unreasonable Oven Temperature Setting: Excessively high inlet temperature is one of the most common causes. An over-high temperature at the inlet section of the oven causes violent evaporation of surface solvents as soon as the coating solution enters the oven, leading to rapid skin formation and subsequent orange peel issues.
• Unreasonable Temperature Gradient: The temperature inside the oven should rise gently from low to high (gradient heating). Abrupt temperature jumps can also trigger localized violent solvent evaporation.
• Excessively High or Uneven Air Velocity: Excessively high air velocity accelerates solvent evaporation on the coating surface, causing rapid surface drying and skinning. Uneven distribution of hot air also leads to localized over-fast drying of the coating.
• Insufficient Drying Time: Inadequate overall drying time prevents the coating from achieving full leveling and solvent release, which also impairs the surface condition.

4. Substrate (Release Film) Factors

• Low Surface Tension/Poor Wettability: If the release film itself has low surface tension (e.g., certain silicone release films), the coating solution struggles to wet and spread on it, easily forming craters or dot-like accumulations, which in severe cases manifest as orange peel.
• Poor Surface Flatness: Substrate surface defects, contamination, static electricity, or inherent poor flatness affect coating uniformity, leading to localized coating defects that show as orange peel after drying.

5. Environmental Factors

• Excessively High Environmental Humidity: For water-based systems or certain hygroscopic solvent-based systems, a high-humidity environment affects solvent evaporation rate and may even cause surface water absorption of the coating, damaging leveling properties. High humidity can also reduce oven efficiency.
• Excessively Low Environmental Temperature: Low temperatures significantly increase the viscosity of the coating solution, reduce fluidity, and are unfavorable for leveling.