The structure and properties of synthetic resins are the main factors that determine the performance of powder coatings and coating films. Therefore, we must take care of the choice of resin. When choosing a synthetic resin, the following factors should be considered.
① Melting temperature and decomposition temperature
The powder coating is sprayed on the object to be coated in a powder state, and is heated and melted to form a film, that is, the resin needs to be heated above the melting temperature. If the decomposition temperature of the resin is very close to the melting temperature at this time, it is very difficult to control the heating temperature. During the manufacture of powder coatings and during construction applications, it is desirable to have a large temperature difference between the melting temperature and the decomposition temperature of the resin. The resins with large temperature difference include epoxy, polyester and acrylic resins, and the relatively small ones are polyvinyl chloride.
② Melt viscosity
Because the powder coating is applied to the object to be coated by electrostatic spraying, fluidized bed dipping and other methods, and is melted and leveled to form a film. At this time, the air existing between the powder coating particles has a great influence on the surface tension. When the melt viscosity is high, it is difficult to form a smooth and uniform coating film, and when the melt viscosity is low, it is easy to level. Therefore, low melt viscosity is required. In addition, it is also desirable that the melt viscosity range of the resin is narrow, and when the temperature reaches a temperature above the melting point, the resin viscosity will drop rapidly. Epoxy and polyester resins have this property.
③Electrical properties
During electrostatic powder coating, powder coating particles are charged and adsorbed to the object to be coated with the help of Coulomb force. The greater the Coulomb force, the better the electrostatic adsorption effect. The Coulomb force F can be expressed by the following formula:
F=q/16πεh 2
In the formula, q——the charge carried by the powder coating particles;
ε——vacuum dielectric constant;
h——the distance between the powder coating particles and the object to be coated.
The greater the value of q, the greater the Coulomb force. Generally, the resin used for powder coating has relatively large resistance and is easy to discharge. The potential of polyvinyl chloride resin drops rapidly, and it is easy to cause the attached powder coating particles to fall off.
④ Resin stability
Generally, the paint spilled in powder coating should be recycled and reused. However, some powder coating particles have been in contact with the air for a long time and deteriorate due to the influence of moisture and temperature in the air. Therefore, it is hoped that the physical and chemical stability of the resin is better.
⑤ Adhesion
The adhesion of powder coatings to the object to be coated mainly depends on the resin. If the adhesion between the resin and the object to be coated is good, no primer is required, and the performance and durability of the coating film are also good; generally, the adhesion of thermoplastic resin is not good, but it can be improved after modification, while thermosetting resin The adhesion is better.
⑥Mechanical pulverization performance
The particle size distribution and particle shape of powder coatings have a great influence on the appearance and construction performance of the coating film. Crushing is required to obtain a suitable powder coating particle size. The manufacture of powder coatings generally adopts mechanical pulverization or chemical pulverization, among which mechanical pulverization is mainly used. Therefore, the mechanical pulverization performance of the resin is required to be good. Generally, the mechanical pulverization of thermoplastic resins is not good, and some resins need to be frozen and pulverized.
In addition to the above conditions, from the perspective of industrial production, the synthesis process, source of raw materials, price, toxicity, etc. of the resin are all important conditions to be considered.
