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Orange peel is a common surface leveling defect that appears on the surface of the coating after coating application, with a concave convex ripple like appearance similar to orange peel. The core essence is that during the curing process of the coating, the leveling speed cannot keep up with the curing speed of the film, and the difference in surface tension causes liquid flow, which is ultimately “frozen” and solidified to form undulating textures. Below are the main categories for analyzing the mechanism:

1、 Internal factors in the formula system

1. Resin and viscosity system coatings have high viscosity and strong thixotropy, making it difficult for wet films to rely on gravity to level and smooth out small undulations after coating spraying/rolling. Improper selection of thickeners in aqueous systems (such as HEUR and alkaline swelling thickeners with imbalanced combinations) can cause a rapid decrease in viscosity under high shear, but a rapid rebound in viscosity under low shear, resulting in a shorter wet film leveling window period. The glass transition temperature Tg of the resin is too high. After the moisture/solvent in the wet film evaporates, the film layer quickly hardens without time for leveling, and the orange peel becomes heavier.

2. Imbalance of solvent/volatilization rate: The solvent evaporates too quickly: the solvent on the surface of the wet film quickly dissipates, and the surface viscosity sharply increases to form a “hard skin”, while the interior is still in liquid state; The internal solvent continues to evaporate and expand, lifting the surface layer and forming ripples. Slow solvent evaporation: The wet film is excessively flattened and prone to sagging, requiring a matching gradient evaporation solvent.

Water based coatings: The rate of water evaporation is crucial, as environmental temperature and humidity directly affect the evaporation of water.

1. Pigments, fillers, and additives affect the addition of powders (titanium dioxide, barium sulfate, talc powder). Excessive addition and poor dispersion can cause powder agglomeration, increase frictional resistance within the wet film, and hinder flow; Insufficient or incorrect selection of leveling agents: Lack of suitable polyether and acrylic leveling agents, unable to eliminate surface tension gradients; Excessive leveling agent can cause shrinkage and loss of gloss. Improper combination of defoamers can leave tiny pits after the rupture of tiny bubbles, further exacerbating the appearance of orange peel.

2. Surface tension gradient (Marangoni effect) results in inconsistent solvent evaporation rates in different regions of the wet film, leading to differences in local surface tension levels. The liquid flows from the low surface tension region to the high surface tension region, spontaneously forming vortex convection and generating ripples on the surface. Once it solidifies before the ripples disappear, the ripples are permanently retained, which is orange peel.

2、 Construction process factors

1. Spray parameters

Insufficient spray gun pressure, poor atomization, large droplet size of paint, and uneven surface when it falls onto the substrate itself;

The paint output is too large, the spraying distance is too close, and the wet film is applied too thick at once; The distance is too far, and the paint droplets dry prematurely (dry spray), forming rough orange peel;

Unstable gun speed and unreasonable overlap rate.

1. Base material conditions: The surface of the base material is rough and has large pores, and the coating is quickly absorbed by the base material. The wet film thickness is uneven, and local areas quickly lose adhesion and flow is obstructed; The substrate temperature is too high, which accelerates solvent evaporation upon contact with the coating.

3、 Environmental conditions

The high ambient temperature and high wind speed accelerate the evaporation of surface solvents/water; In a humid air environment, the evaporation of moisture on the surface of water-based paint slows down, which can also change the film-forming rhythm. Fluctuations in temperature and humidity can amplify Marangoni convection and exacerbate orange peel.

4、 The opposition between orange peel and sagging

There are trade-offs in formula design:

Improve leveling and reduce orange peel → prone to sagging

Improve thixotropic anti sagging → Easy orange peel formula engineer through layered rheological design: low viscosity under high shear for easy spraying and atomization; After spraying, the low shear viscosity slowly increases, retaining a short leveling time while suppressing sagging.

5、 Improvement measures

1. Rheology: Adjust the thickening system, optimize the thixotropic curve, and extend the leveling window;

2. Volatility: Mix solvents to control gradient volatilization and reduce premature crust formation on the surface;

3. Additives: Add suitable leveling agents to improve surface tension and weaken Marangoni effect; Ensure sufficient dispersion of pigments;

4. Construction: Optimize atomization pressure, spraying distance, and wet film thickness; Control the temperature and humidity of the construction environment and avoid strong winds blowing directly.