What are the reasons for poor powder curing in powder coating?

In powder coating, poor film curing is mainly caused by two factors: first, an unreasonable powder coating formulation, resulting in a film performance that fails to meet requirements; second, the powder coating itself may be functional, but improper coating process control leads to the film's failure to meet technical specifications.

1. Considering Powder Coating Formulation Design

If the resin and curing agent are not properly matched in the formulation design—for example, if the resin's reactivity is not selected appropriately, or the type and amount of curing agent are not chosen correctly, or if the filler's mass percentage or volume concentration is too high—these issues will affect the physical and mechanical properties of the coating, thus leading to poor curing during application.

2. Considerations from the Powder Coating Process Perspective

The following are some reasons why the powder coating process might affect the curing of the coating film:

2.1 The baking temperature of the oven fails to reach the required baking temperature for the powder coating

Although the oven temperature control can easily reach the design temperature or the required baking temperature for the powder coating under no-load conditions, under full load, due to insufficient total heat supply, it may not always reach the design temperature or the required baking temperature for the powder coating. This is especially true in northern regions where workshop temperatures are low in winter. If the oven insulation is poor, the temperature of the material being coated is low before entering the oven, and the material has thick walls and a large heat capacity, this problem is more likely to occur.

When the oven temperature does not reach the powder coating's baking and curing temperature, the temperature problem can be solved by increasing the heating power; alternatively, the baking and curing time can be extended to achieve the desired curing.

2.2 The baking and curing time in the oven does not reach the required baking time for the powder coating

In powder coating baking conditions, the baking time refers to the baking time calculated after the material being coated reaches the baking temperature, not the time calculated from the moment the material is placed in the oven.

The thicker the coating material and the more components it supports, the greater its heat capacity, and the longer it takes to reach the controlled temperature. In a baking oven of a given length, with a constant conveyor chain speed, the effective baking time is shorter. If the effective baking time is insufficient, the coating will not cure completely, and its physical and mechanical properties will not meet requirements.

Therefore, in powder coating, it is best to use an oven temperature tracking meter to measure the temperature change of the coating material in the baking oven under full load conditions to determine a reasonable baking and curing temperature and time. This ensures complete curing of the coating and guarantees its physical and mechanical properties.

2.3 Poor Surface Preparation of the Coated Material

If the surface of the coated material is not properly cleaned of oil, scale, or rust, or if the phosphating film is of poor quality and contains residual phosphating solution, the adhesion between the powder coating and the substrate will be affected, and the coating's impact resistance will also be poor. This situation must be resolved by improving the surface preparation quality of the coated material.

2.4 Regarding coating performance testing, it should be conducted strictly according to the test temperature and coating thickness specified in the product standard

Generally, when the coating thickness is thicker than specified and the test temperature is lower than specified, the coating's impact strength, flexibility, cupping test, and adhesion properties will be poor and unlikely to pass.

When the coating thickness is thinner and the test temperature is higher than specified, these physical and mechanical properties are more likely to pass. From the perspective of ensuring product quality, testing coating performance indicators under relatively stringent conditions is essential to guarantee the quality of the coated product.

In addition, it is also necessary to consider that excessively high baking and curing temperatures and excessively long baking times can cause changes in coating color due to thermal aging, and simultaneously reduce the coating's flexibility, impact strength, and other physical and mechanical properties. Therefore, excessively high baking and curing temperatures and excessively long times are also detrimental to the physical and mechanical properties of the coating.

3. Conclusion

In powder coating spraying, the baking and curing temperature and time play a decisive role in the coating's curing performance. Therefore, it is essential to conduct multiple trials and properly classify the objects to be coated in order to determine the number and arrangement of hangers, as well as the corresponding baking temperature and time, in order to ensure the quality of the coated products.