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The shrinkage of water-based metal coating is related to the cleanliness of the coating and the substrate. Coating or substrate is polluted for a variety of reasons, mainly the following points:

1:Metal base material degreasing is not clean, the surface is greasy, resulting in shrinkage

2: Coating environment cleanliness is poor, dust or grease (silicone) pollution sources occasions.

3: Some raw materials in the coating formulation hydrolyze or react to produce insoluble low surface energy substances.

4: Leakage of industrial lubricating oil in coating and drying equipment contaminates the paint film.

5: Operator's non-standard operation, the introduction of grease, resulting in shrinkage cavity.

Pollutants generally than the surface tension of the oil or silicone coating tension is low, made base material or coating surface with low surface tension and surface tension caused by uneven, paint film on the surface under the action of tension difference, from low to high surface tension in surface tension, the results form center concave hole - namely the formation of shrinkage cavity. The uneven surface tension of paint film is the internal cause of the formation of shrinkage cavity. Other properties of paint, such as viscosity, thixotropy, paint drying speed and film thickness, can intensify or weaken the flow ability of paint fluid, which will intensify or weaken the degree of shrinkage cavity, these factors are attributed to the external cause of shrinkage cavity.

There are two main methods to eliminate the influence of pollutants on the shrinkage of the paint film. One is to control the pollution source head, skim clean, strictly control the foreign oil pollution into the system, and solve the problem from the internal cause. The second is to add appropriate surfactants, weaken or eliminate the shrinkage pore brought by pollutants, solve the problem from the external cause.

• Causes of shrinkage in waterborne metal coatings

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