The difference and application of degassing agent and defoaming agent

Through the communication with customers in this Nanjing exhibition and the problems reported by salespersons recently, we learned that most customers have misunderstandings about degassing agents and defoaming agents, and they believe that they are the same type of additives. As the price of raw materials continues to rise, customers have gradually switched to polyester-HAA system products in consideration of cost issues. When recommending products and applying formulas to customers, it is particularly important to advise customers on appropriate methods for the problems they encounter. The following is a brief introduction to the difference between deaerators and defoamers and their application methods:

1. Degassing agent (degassing agent)

The degassing agent (degassing agent) is when the powder coating is melted and leveled to form a film, the air, moisture contained in the powder coating and the volatile substances such as small molecular compounds generated during the crosslinking and curing reaction of the powder coating escape out, and It is an auxiliary agent that can make up for the pinholes that escape the small molecule compounds in time, and avoid the disadvantages such as small pinholes or pores in the coating film (as shown in Figure 1). This additive is one of the common additives in powder coatings, and is generally added in powder coating formulations.

The degassing agent commonly used in powder coatings is benzoin, also known as benzophenone (commonly known as benzoin). Benzoin is a white or light yellow odorless crystal with a melting point of 133-137°C and a boiling point of 344°C. It is slightly soluble in water and ether, and soluble in hot acetone and ethanol; its disadvantage is that it easily causes yellowing of the coating film at high temperatures. In order to overcome the shortcomings of benzoin, modified benzoin and wax-type degassing agents that are not easy to yellow under baking and curing conditions have been developed.

In powder coatings with benzoin, the bubbles shrink very quickly when the powder coating melts and flow, and the viscosity of the coating has not changed, and almost all bubbles disappear; when benzoin is not added, the bubbles shrink very slowly. The degassing mechanism of benzoin is that benzoin consumes oxygen in the bubbles and reduces the diameter of the bubbles, which leads to an increase in the internal pressure of the bubbles, which speeds up the dissolution of nitrogen in the polymer, thereby reducing the diameter of the bubbles. Benzoin begins to vaporize at 120°C, which is another important factor for the degassing effect of benzoin.

The results of experiments and production practices show that in the cross-linking and curing process of powder coatings, all types of coatings that produce small molecular compounds need to add degassing agents. Generally, it is more reasonable to add degassing agent to epoxy, epoxy-polyester, polyester and polyurethane powder coatings. This is because the powder coatings are easy to absorb moisture during production and use. In epoxy matt and matt powder coatings, it is not easy to produce pores and other defects without adding benzoin and other degassing agents. The reason cannot be explained clearly. It may also be that the surface of the coating film is not as high-gloss and smooth, and some coating film defects The feeling that is not obvious.

At present, benzoin is still a commonly used degassing agent, and the amount used in powder coatings is about 0.5% of the total film-forming material, which can be appropriately adjusted within a certain range according to the variety and composition of the powder coating. In polyester-HAA (hydroxyalkylamide) powder coatings, taking into account the effect of benzoin on the yellowing of the coating film, its dosage is controlled at about 0.3% of the total film-forming material, and the dosage is minimized. In addition, synthetic wax-type smoothing degassing agent can also be used, and the dosage is about 1% of the total powder coating formulation.

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2. Defoamer

In powder coating formulations, the function of defoamer is to prevent coating film when powder coating cast iron, cast aluminum, hot-dip galvanized steel sheet and hot-rolled steel sheet with blisters or pinholes on the surface of the coated object (workpiece) Additives added to prevent defects such as particles (particles) or volcanic craters (as shown in Figure 2), that is, to prevent bubbles from forming in the coating.

When powder coating is applied to cast iron, cast aluminum, hot-dip galvanized parts or hot-rolled steel plates with blisters or pinholes, during the baking and curing process, the powder coating is melted and leveled while sealing the blisters on the surface of the coated object And pinholes. As the temperature of the coated object increases, the air in the blisters and pinholes of the coated object expands, and the internal pressure continues to rise. When the internal pressure is slightly greater than the strength of the molten coating film, the internal air will burst the coating film and form small bubbles to escape. Because the powder coating undergoes a curing reaction during the film formation process, the melt viscosity of the coating continues to increase, and finally becomes a solid coating film, so when the internal pressure in the small bubbles does not reach the energy to burst the coating film, these small bubbles are formed Particles or particles (sand particles) protruding from the surface of the coating film; when the internal pressure in the small bubbles is enough to burst the energy of the coating film, the small bubbles will burst, and the air inside will go to the atmosphere. If the coating film at this time has lost its leveling properties and cannot bridge the small pores that the air escapes, it will form typical particles or particles with volcanic craters (Figure 3), which becomes a serious problem of the coating film.

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If the defoamer is added to the powder coating, the defoamer can reduce the melt viscosity of the powder coating, and at the same time reduce the surface tension of the coating, so that the blisters and pinholes on the surface of the coated object will be affected by the baking temperature and the pressure will increase. , It is easy to rupture and level the uncured coating film and run into the atmosphere. At the same time, the gap of the coating film that escapes the air bubbles can also be easily bridged and leveled, so that the coating film avoids particles and particles, or particles with volcanic craters. Or particles.

Because the defoaming mechanism of water-based coatings and solvent-based coatings is completely different from that of powder coatings, the defoaming agents used in water-based coatings and solvent-based coatings cannot be directly used in powder coatings. Due to the particularity of powder coatings, there are not as many defoamers used in powder coatings as in water-based coatings and solvent-based coatings.

Reference formula:

References:

[1] Nan Renzhi. Powder Coating and Painting Technology [M]. Beijing: Chemical Industry Press, 2014.5