The relationship between defoamer dosage and paint shrinkage

In the production and use of paint, defoamer plays a vital role. It can effectively eliminate bubbles generated during the manufacture and construction of paint, and improve the quality and construction effect of paint. However, the dosage of defoamer needs to be precisely controlled, and adding too much may cause paint shrinkage.

1. Working principle of defoamer

The bubbles in the coating are mainly caused by the air being drawn into the coating during the production process, such as stirring and dispersing, or certain surfactants in the coating formula that reduce the surface tension, making it easy for air to wrap and form bubbles. Defoamers are usually composed of active ingredients, carriers and additives. The active ingredients are generally organic silicon, polyether and other substances with low surface tension. When the defoamer is added to the coating system, the active ingredients will quickly adsorb on the surface of the bubbles, and with its low surface tension, the surface tension of the local bubble film will be reduced, causing the bubble film to be unevenly stressed. Under the pull of the higher surface tension in other parts of the bubble film, the bubble film becomes thinner and ruptures, thereby achieving the purpose of defoaming.

2. The Origin of Shrinkage 

Pores Shrinkage pores, also known as pits, refer to the phenomenon of circular pits appearing locally on the surface of the paint during the drying process. Shrinkage pores are mainly caused by the uneven distribution of the surface tension of the paint. When the surface tension of a certain area on the paint surface is lower than that of the surrounding area, the surrounding paint with higher surface tension will flow to the area with lower surface tension, thus forming shrinkage pores.

3. Reasons for shrinkage caused by excessive addition of defoamer

1). Excessive reduction in surface tension: 

The active ingredients of the defoamer have low surface tension. Adding an appropriate amount can balance the surface tension of the coating and eliminate bubbles. However, if it is added in excess, the local surface tension of the coating will be excessively reduced. When these areas with extremely low surface tension coexist with the surrounding normal surface tension areas, a surface tension gradient will be generated. During the drying process of the coating, driven by the surface tension gradient, the surrounding coating flows to the low surface tension area, thereby forming shrinkage holes.

2). Increased incompatibility: 

Some defoamers are not completely compatible with the coating system. When added in an appropriate amount, the defoamer can be evenly dispersed in the coating through stirring and other processes, and play a normal defoaming function. However, after adding too much, the defoamer exceeds the dispersion limit that the coating system can accommodate, and its incompatibility with the coating is aggravated. These incompatible defoamer micro-regions will form low surface tension points on the coating surface, causing shrinkage holes.

3). Leveling damage:

 Excessive defoamer will affect the leveling of the coating. The leveling of paint refers to the process of automatically forming a flat and smooth surface by relying on its own surface tension after construction. Too much defoamer will interfere with the normal flow of the paint, making it impossible for the paint to effectively fill the tiny pits formed by the bursting of bubbles during the leveling process. It may also cause abnormal local surface tension, further promoting the formation of shrinkage holes.

4. Measures to avoid shrinkage caused by excessive defoamer

1). Accurately control the dosage: 

Before the production of the coating, the optimal dosage of the defoamer should be determined through experiments. Different amounts of defoamer can be added to the sample coating to observe the defoaming effect and whether there are shrinkage holes and other undesirable phenomena, so as to determine the appropriate amount of addition. In large-scale production, add it strictly according to the determined dosage.

2). Select the appropriate defoamer: 

According to the type of coating (such as water-based coatings, solvent-based coatings, etc.), formula composition and construction process, select a matching defoamer. Ensure that the defoamer has good compatibility and dispersibility in the coating system to reduce the risk of shrinkage holes caused by incompatibility.

3). Optimize the addition process:

 When adding the defoamer, use the appropriate addition method and stirring conditions. For example, the defoamer can be diluted and added slowly, and moderately stirred during the addition process to ensure that it is evenly dispersed in the coating to avoid excessive local defoamer concentration.

Defoamers are indispensable in coatings, but the dosage must be strictly controlled. Understanding the principle of shrinkage caused by adding too much defoaming agent and taking effective preventive measures are crucial to ensuring the quality of coatings and improving construction effects.