news

The coating turns white, in simple terms: the paint film becomes milky white, hazy, and the transparency decreases. Essentially, there are a large number of tiny light scattering interfaces inside the coating, causing light to scatter and appear white to the naked eye. According to their causes, they can be mainly divided into three categories: white tide (whitening), precipitation whitening, and pore/aggregation whitening.

1、 Chao Bai

Core mechanism: Wet film cooling, where water vapor in the air condenses into the interior of the paint film, forming tiny water droplets. Resin and water have different refractive indices, causing a large number of tiny droplets to scatter light and resulting in a whitening of the paint film.

1. Solvent based paint: The solvent evaporates rapidly and absorbs heat, causing the wet film temperature to drop below the dew point of the air, and water vapor enters the paint film. The faster the solvent evaporates, the easier it is to turn white.

Countermeasure: Add anti whitening water (slowly evaporating alcohol ether solvent) to reduce the volatilization rate; Raise the substrate temperature and avoid construction in high humidity environments.

1. Waterborne paint: high humidity and low temperature, slow evaporation of moisture, and prolonged moisture content in the paint film; Or incomplete film formation, residual free water inside, and whitening after drying. Characteristics: Slight whitening, with the continued evaporation of water/solvent, the whitening may gradually recover; If water has already destroyed the resin, the whitening will be permanent and irreversible.

2、 Additive/substance precipitation whitening

The small molecular substances in the formula migrate to the surface of the paint film in the later stage, forming microcrystals or thin films, causing scattering and whitening.

1. Excessive wax, lubricant, and hydrophobic additives cause migration and precipitation, resulting in a layer of white frost on the surface;

2. Hydrophilic small molecules such as dispersants, emulsifiers, and residual neutralizing salts migrate to the surface upon contact with water;

3. Plasticizers and low molecular weight resins precipitate. Characteristics: The initial appearance of the dry film is normal, but it only turns white after being left for a period of time or soaked in water; Sometimes wiping the surface can result in white powder.

3、 Pores, aggregation of pigments and fillers, poor film-forming and whitening

1. Film forming defect: the minimum film forming temperature MFT of lotion is too high, and the resin particles cannot be fused when applied at low temperature. Micro voids are formed between particles, and air is filled in the pores. The difference in refractive index between resin and air is significant, causing light scattering and whitening. Low temperature construction of water-based paint is most likely to occur.

2. Poor powder dispersion: pigments and fillers agglomerate, and large particles cause scattering; Excessive addition of inorganic powders such as silica sol and gas silica results in numerous micropores in the paint film.

3. Pinholes and microbubbles: Tiny bubbles remain in the paint film, forming air scattering points. Characteristics: It immediately turns white after drying and will not disappear even after being placed; Raising the baking temperature and improving film-forming properties can alleviate the issue.

4、 Other types of whitening

Hydrolysis whitening: Epoxy, polyurethane and other paint films are soaked in water for a long time, causing resin hydrolysis, polymer chain breakage, structural damage, and internal micro pores; Whitening after immersion in water, partially reversible after removal, and permanent whitening after repeated immersion in water.

Salt precipitation whitening: Soluble salts in the system migrate to the surface after water infiltration, forming white salt frost.

Quick differentiation and judgment tips

✅  Just started construction, high humidity environment appears → high probability of whitening

✅  The sample will dry for a few days/soak in water before turning white → additives will precipitate, salt will precipitate, or resin will hydrolyze

✅  Low temperature construction, the dry film turns white directly and remains unchanged → film formation is poor, and the MFT of lotion is too high

Summary

The essence of coating whitening is the generation of a large number of microstructures with different refractive indices inside the paint film, which induce light scattering. It can be divided into whitening caused by water vapor, whitening caused by small molecule migration and precipitation, and structural whitening caused by poor film formation/powder agglomeration/micropores. Whitening is often related to construction temperature, humidity, and solvent evaporation rate; Analysis shows that the white is caused by improper compatibility of additives; The root cause of structural whitening lies in resin selection and film-forming conditions.