"The Selection and Application of Waterborne Coating Additives"

Water-based coating additives are an important component of coating products.

The editor will talk to you about the aspects that need to be paid attention to in the selection of additives for water-based coatings:

Wetting and dispersing agent

Wetting and dispersing agent has both wetting and dispersing effects, and is often used to solve the problem of difficult pigment dispersion.

01 Wetting phenomenon

When solid and liquid are in contact, the original solid-gas interface disappears and a new solid-liquid interface is formed.

02 Wetting and wetting agents

When wetting the pigment filler with liquid and making it penetrate into the agglomerates and aggregates of the pigment filler, if a small amount of surfactant is added to the liquid, the wetting and penetration are easier, which is the wetting effect

The surface properties of pigments and fillers, the type and amount of wetting and dispersing agents, and the performance of dispersing equipment are the main factors affecting dispersion

defoamer

The foam is generated.

1) When the paint is produced, the air will be entrained into the paint due to mechanical stirring;

2) Air brought in during the coating process, such as brush coating, roller coating, high pressure airless spraying,
etc.

3) For two-component paint, when mixing before construction, stir the mixed air
.

4) There are many pores in the coated object, and air bubbles are formed due to the infiltration of the paint, such as when painting on porous wood and cement walls
.

5) The bubbles generated by chemical reactions, such as the reaction of polyisocyanates in two-component PU coatings with trace amounts of water, will produce carbon dioxide
.

Foam is inherently unstable, and its breaking has to go through three processes, namely bubble redistribution, film thickness reduction and film rupture
.
However, for a relatively stable foam system, it takes a long time to achieve natural defoaming through these three processes, and defoamer is mostly used in all production
.

It is generally believed that the defoaming agent reduces the local surface tension of the foam liquid film and defoaming
.
Because of the low surface tension of the defoamer itself, it always penetrates into the foam system in the form of microparticles to locally reduce the surface tension of the foam film
.
Since the liquid with low surface tension always flows to the liquid with high surface tension, the membrane wall gradually becomes thinner, and is strongly pulled by the surrounding membrane layer with high surface tension, resulting in an imbalance of stress, which eventually leads to bubble bursting
.

At present, there are three general types of defoamer: mineral oil defoamer, silicone defoamer, and silicone-free polymer defoamer
.

Choice of defoamer

The choice of defoamer, there are many factors in the coating formulation that affect foaming and foam stabilization
.

NO.
1 Surface tension

The surface tension of the coating has a great influence on the defoamer.
The surface tension of the defoamer must be lower than the surface tension of the coating, otherwise it will not be able to have defoaming and anti-foaming effects
.
The surface tension of the coating is a variable factor, so when choosing a defoamer, the surface tension should be constant, and then the surface tension factor should be taken into account
.

NO.
2 Influence of other additives

Most of the surfactants used in coatings tend to be functionally incompatible with defoamers
.
In particular, emulsifiers, wetting and dispersing agents, leveling agents, thickeners, etc.
will affect the effect of defoaming agents
.
Therefore, when using various additives together, we must pay attention to the relationship between different additives and choose the best balance point
.

No.
3 Baking temperature

When the coating enters high temperature baking at room temperature, the viscosity will drop instantly at the beginning, and the bubbles can move to the surface.
However, due to the volatilization of the solvent, the curing of the coating, and the increase of the surface viscosity, the bubble film tends to be more stable, and it will shrink when left on the surface.
holes and pinholes, so the baking temperature, curing speed, and solvent evaporation rate also affect the effect of the defoamer
.

No.
4 Solid content, viscosity, elasticity of coatings

Coatings with high solid content and thick film, high viscosity and high elasticity are very difficult to defoamer.
In these coatings, the diffusion of defoamer is difficult, the speed of microbubble growth is slow, the ability of foam to migrate to the surface is reduced, and the viscoelasticity of the foam film is large.
There are many unfavorable defoaming factors
.
The foam in these coatings is quite difficult to eliminate, and it is best to use a defoaming agent and a defoaming agent
.

No.
5 Coating method and application temperature

There are many coating methods for coating construction, such as brush coating, roller coating, curtain coating, blade coating, high pressure airless spraying, screen printing,
etc.
The degree of blistering of different coatings is also different depending on the coating method used
.
Brushing, roller coating foam more than spraying and scraping, the most foam is the screen printing of ink, and it is not easy to eliminate
.
When the temperature is high, there is more foam than when the temperature is low, but when the temperature is high, the foam is easier to eliminate than when the temperature is low
.

Use of defoamer

1) It must be fully stirred before use to mix well
.

2) Add defoamer while the paint is stirring
.
3) Before use, generally do not need to be diluted with water, it can be added directly
.
If some varieties need to be diluted, they need to be used with the preparation
.
4) The dosage should be appropriate
.
Too much dosage will cause problems such as shrinkage, shrinkage, poor brushability and poor recoatability.
If the dosage is too small, the defoaming effect will be poor
.
The optimal point should be determined
.
5) It is best to divide into two additions, namely before grinding the pigment paste and after adding the emulsion paint
.
Generally, the amount added each time is half of the total amount, and it can also be adjusted according to the foam situation
.
In the stage of grinding the pigment slurry, a defoamer with good anti-foaming effect can be selected, and a defoamer with good foam-breaking effect can be selected in the stage of paint mixing
.
Experiments show that it is best to add silicone defoamer in the stage of grinding the pigment slurry, and add as little water as possible, so that it can be fully dispersed and the dosage can be reduced to a minimum
.
It is advisable to add a non-silicon type defoamer in the paint mixing stage
.
6) After the defoamer is added, it takes at least 24 hours to achieve the balance between the defoaming performance, edge shrinkage and shrinkage cavities
.
Therefore, the test of coating performance must be carried out after 24 hours
.