Powder coatings have a special position in the coatings industry. Compared with traditional organic coatings, there are many differences. There are many similarities in the final use performance of the cured coating film. From the perspective of the production process, the manufacture of powder coatings should be attributed to plastics. Although powder coatings seem to be simple from the elements of coating formulations, powder coatings are actually very complicated due to a comprehensive process involving manufacturing process, storage, film formation and final application characteristics.
1 The molecular weight of the base material
Like all polymers, the resin used in powder coatings is a mixture of molecules with different molecular weights. Therefore, the average molecular weight of the resin must be known. Among several different ways of expressing average molecular weight, number average molecular weight (Mn) and weight average molecular weight (Mw) are the most important for the characteristics of powder coatings. The mechanical properties of powder coatings, such as tensile strength and impact resistance, mainly depend on the number average molecular weight, and the weight average molecular weight mainly determines the melt viscosity of the resin. To ensure that a commercial polymer has good tensile strength and impact resistance, its average molecular weight should be between 20,000 and 200,000. We must take this fact into consideration and use it in powder coatings.
2 Functionality of powder coating components
Powder coating formulations are more sensitive to changes in the correct ratio of functional groups. This problem can be solved by increasing the functionality of the curing agent or resin. In this way, a very large network structure must be formed through stoichiometry to reduce the sensitivity of the system. According to Gordon's branching process theory, a powder coating system is classified and substituted for calculation. The composition of the coating system is a carboxyl polyester with an average molecular weight of 3800, a functionality between 2 and 3.25, and a number average molecular weight of 1500, with functionality It is 2 bisphenol A epoxy resin.
3 Glass transition temperature (Tg)
Amorphous polymers are mainly used in thermosetting powder coatings. The glass transition temperature of the coating components is a parameter that must be paid attention to for resin and coating chemists. It will directly or indirectly affect the physical and chemical stability of the coating components during storage, and the rheological behavior during production and film formation, and ultimately lead to internal stress in the cured coating during use. Assuming that the particles on the powder coating are affected by the gravity of the upper powder particles, if the Tg of the powder is higher than the storage temperature, due to the lack of mobility of the chain links, there is no segment-level or molecular-level material diffusion between different particles; at low Tg In the case of storage temperature, the molecular chains between different powder particles are always highly interpenetrated, and the mobility of the chain links is high enough to make the powder agglomerate. This phenomenon is considered to be the poor physical stability of powder coatings, so a high Tg value of powder coatings is a prerequisite for good physical stability, but it is difficult to determine the highest Tg value that can ensure good powder stability.