Factors Affecting Shrinkage of Waterborne Coating Films

2019-06-25

Factors Affecting Shrinkage of Waterborne Coating Films
In practice, the main causes of shrinkage are as follows:
1. The substrate is contaminated by oil or impurities
2. Apply a paint to the old paint or finish without sanding
3. The interval between constructions of the two paints is too long
4. Too much defoamer in the paint
The following focuses on two factors: defoamer and impurity contamination.
The effect of defoaming agent
Emulsions, wetting agents, dispersants, defoamers, thickeners, etc. are used in latex paints. The main principles of using defoamers are as follows:
  • The raw material contains a surface active material or a surfactant, and bubbles are easily formed. The foam hinders the dispersion of the pigment and filler, reduces the utilization rate of the equipment, prolongs the production cycle, and seriously brings about coating defects, pinholes and shrinkage cavities.
  • After the second coating is used, the construction tool is brought into the air, and the coating wets the substrate to displace the air in the gap of the substrate. The air becomes a bubble under the action of the surface active material, so the antifoaming agent is used to eliminate these bubble.
The surface tension of the defoaming agent should be less than the surface tension of the defoaming liquid. The solubility in the defoaming system is as small as possible and has good dispersibility. Can not react with other substances in the system, does not affect the stability of pigments and emulsions, has a strong diffusion capacity, and can not affect the performance of the coating. The most important choice for defoamer selection is the performance of the dry film.

To protect the normal production and construction of the coating, defoamers are often used in coating formulations. From the analysis of the relationship between coating composition and properties, although the defoaming agent has little effect on the properties of coatings and coatings, it is a surfactant. The surface tension of the defoamer is lower than that of the coating system, and its HLB value is usually around 3. The defoamer is not dissolved in the coating system, but can be dispersed in the coating system in the form of tiny droplets. The optimal droplet size is the wall thickness of the bubble wall. If the droplets of the defoamer are too large or too small, the performance of the coating will be affected. The droplets of the defoamer will be too small (the excessive dispersion of the defoaming agent) will weaken the defoaming performance; if it is too large, the surface tension in the coating system will be lower than that. The presence of droplets of the bulk coating results in shrinkage of the coating film. Therefore, the shrinkage cavities caused by the defoaming agent are mainly due to the poor compatibility of the defoaming agent itself, or the poor dispersibility, the particles of the defoaming agent dispersed in the coating system are too large, or the defoaming active substances are easily aggregated into Larger defoamer droplets.


The effect of contamination

The second reason for shrinkage cavity in coating system is the existence of micro-phase separation. There are two sources of contamination:
  • Water-based latex paint is contaminated with raw materials - inorganic pigment filler powder, using contaminated powder to produce paint, while using some mineral oil defoamers, powder and defoamer may adsorb and accumulate with each other, exacerbating The degree of shrinkage. In the case of serious pollution, large and small flocculent oils are formed on the surface of the paint when the paint slurry is ground, which seriously damages the quality of the paint.
  • The shrinkage film of the coating film caused by the construction problem is usually contaminated by the coated substrate. The substrate is not thoroughly cleaned before painting. There are oil stains and stains on the substrate, which causes the surface tension of the contaminated surface to be lower than the surface tension of the surrounding paint.The coating cannot spread in the contaminated area and causes shrinkage.

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