Analysis on the formation mechanism and prevention of shrinkage cavity in powder coating

Shrinkage, as a common coating defect, has been widely concerned by the coating and coating industry, mainly due to the change of surface tension during the coating curing process

Shrinkage

There are generally two cases of shrinkage on the coating surface, one is circular shrinkage, and the other is hexagonal polygonal shrinkage

Figure 1 The formation of Bernard's vortex

At the same time, some overseas experts believe that the shrinkage of the powder coating comes from insufficient wetting of the substrate after the coating is melted

Figure 2 The effect of acrylic additives% on the contact angle of pigment-free epoxy and polyester powder coatings

When there is a low surface tension substance on the surface of the coating, there is a difference in surface tension between it and the surrounding coating.

Analysis of foreign matter caused by shrinkage cavity

For the shrinkage caused by the gradient of the surface tension of the coating, as mentioned above, the generation of shrinkage can be reduced by increasing the wettability of the system

The small molecules inside the coating are mostly caused by insufficient dispersion of the formula or the inability of the organic components in the system to fully wet the inorganic components

The introduction of foreign matter from the outside will cause penetration and core shrinkage

How to prevent shrinkage

First of all, for shrinkage cavities in the system, some lower surface tension additives should be added to improve the wetting of the coating system and the wetting of the coating to the substrate, so as to reduce the uneven surface tension of the coating

Secondly, avoid heat, moisture and introduction of impurities during the coating storage, transportation and coating process
.
In the coating process, compressed air must be ensured and the introduction of water and oil must be controlled
.
The coating line body, curing furnace and flue are cleaned regularly to reduce the residue of impurities such as coking particles and residues generated by the high temperature in the curing furnace on the coating line body
.
Pay attention to the control of the humidity of the coating environment to reduce the dust, fibers and other impurities flying on the spot, and contaminate the uncured coating through electrostatic adsorption
.

In short, the difference in local surface tension during the coating curing process is the main cause of shrinkage
.
Regardless of the impact of the coating itself or the coating process, how to reduce the introduction of uneven substances is the key to avoiding shrinkage
.