FAQ

Powder coatings hold a special place in the coatings industry, differing significantly from traditional organic coatings. However, they share many similarities in terms of the final performance of the cured film. From a production process perspective, powder coating manufacturing should be categorized under plastics. Although powder coatings may seem simple in terms of formulation elements, they are actually quite complex due to the integrated process involving manufacturing, storage, film formation, and final application characteristics.

While the chemical reactions during curing are the same for powder coatings and solvent-based coatings, the physical properties of the resulting coatings are drastically different. Solvent-based coatings can have their application characteristics adjusted by selecting solvents or solvent mixtures that have a smaller impact on the cured film. Powder coatings, lacking solvents, cannot do this. Most properties of powder coatings are determined solely by the binder. Therefore, to produce powder coatings with good overall properties, it is essential to understand the influence of various resin factors on coating characteristics. A basic understanding of the relationships between different resins and coating parameters, and their impact on the final film properties, is more useful than extensive mastery of powder coating formulations.

1. Molecular Weight of the Binder

Like all polymers, the resins used in powder coatings are mixtures of molecules with different molecular weights. Therefore, the average molecular weight of the resin must be known. Among the various ways of expressing average molecular weight, number-average molecular weight (Mn) and weight-average molecular weight (Mw) are the most important for the properties of powder coatings. The mechanical properties of powder coatings, such as tensile strength and impact resistance, depend primarily on the number-average molecular weight, while the weight-average molecular weight mainly determines the melt viscosity of the resin. To ensure that a commercially viable 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 account and apply it to powder coatings. Imagine a linear carboxyl-terminated polyester resin with a number-average molecular weight Mnp undergoing a curing reaction with a linear bisphenol A epoxy resin with a number-average molecular weight Mne. During the curing process, if the carboxyl groups of the polyester resin are slightly in excess and all epoxy groups react sufficiently, the number-average molecular weight Mn of the cured coating can be simply calculated using the following formula:

Mn=(x+1)Mnp+xMne

In the formula, x refers to the degree of polymerization of the block copolymer composed of block polyester resin (P) and epoxy resin (E).

2. Functionality of Powder Coating Components

Powder coating formulations are quite sensitive to changes in the correct ratio between functional groups. This problem can be solved by increasing the functionality of the curing agent or resin. This requires forming a large network structure through stoichiometry to reduce the system’s sensitivity. Based on Gordon’s branching process theory, a certain powder coating system was calculated using a hierarchical substitution method. The composition of the coating system is a carboxyl polyester with an average molecular weight of 3800 and a functionality between 2 and 3.25. and a bisphenol A type epoxy resin with a number-average molecular weight of 1500 and a functionality of 2. During the curing process, the relationship between the conversion rate of epoxy groups and the number-average molecular weight of the system shows that the average molecular weight increases more rapidly with increasing functionality. To obtain a coating with a number-average molecular weight of 20.000. the conversion rate is 86% when the functionality is 2. and 62%, 24%, and 8% when the functionality of the polyester resin is 2.5. 3. and 3.25. respectively.

High functionality leads to a rapid increase in viscosity, thus shortening the flow time of the coating during film formation. This results in poor leveling properties and severe orange peel effect. Thermo-viscoelastic spectroscopy (TVA) was used to determine the effects of changes in molecular weight and polyester resin functionality on the rheological behavior of a pigment-free polyester/epoxy hybrid powder coating melt at 120°C, and the effect on the dynamic modulus of the system during dynamic temperature changes from 120°C to 200°C.

The functionality (Fn), number-average molecular weight (Mn), and acid value (AV) or hydroxyl value of the polyester resin are not independent variables. This means that, to maintain a constant polyester/epoxy resin weight ratio under the same acid value, increasing the molecular weight is necessary to increase functionality.

Another method is to keep the molecular weight constant while increasing the acid value, but this directly affects the required polyester/epoxy resin ratio. Increasing functionality also increases the molar fraction of branched components in the resin formulation. During random esterification, with a constant acid value, the number-average molecular weight increases linearly with increasing functionality, while the weight-average molecular weight increases rapidly.

Therefore, although a resin with a functionality of 4 can be obtained, it is impractical due to its excessive viscosity. Furthermore, a too-short gel point interval affects the safety of the production process. The gel point interval refers to the ratio of the degree of reaction at the gel point to the degree of reaction required to obtain a resin with specific properties.

3. Glass Transition Temperature (Tg)

In thermosetting powder coatings, amorphous polymers are mainly used. The glass transition temperature of the coating components is a parameter that resin and coating chemists must pay close attention to. It directly or indirectly affects the physical and chemical stability of coating components during storage, as well as their rheological behavior during production and film formation, ultimately leading to internal stress in the cured coating during use. Assuming the particles in the powder coating are subjected to gravity from the upper powder particles, if the powder’s glass transition temperature (Tg) is higher than the storage temperature, there is no segmental or molecular-level material diffusion between different particles due to the lack of chain segment mobility. When the Tg is lower than the storage temperature, the molecular segments between different powder particles remain highly interpenetrating, and the chain segment mobility is high enough to cause powder agglomeration. This phenomenon is considered to indicate poor physical stability of the powder coating; therefore, a high Tg value is a prerequisite for good physical stability, but it is difficult to determine the optimal Tg value that ensures good powder stability. Based on practical experience, the generally accepted Tg value for powder coatings is no lower than 40°C. For thermosetting powder coatings, the glass transition temperature also affects the chemical stability of the system. Because a pair of reactants—resin and curing agent—coexist in the coating, it can be assumed that certain reactions will occur during storage. From a thermodynamic point of view, when reactants meet each other, a reaction can occur as long as there is sufficient energy (at least equal to the activation energy). When the Tg of the powder coating is higher than the storage temperature, the probability of the two functional groups on the resin and crosslinking agent meeting each other is very low because the mobility of the polymer chain segments is restricted.