Analysis of the reasons for poor curing of powder coating

In powder coating, poor film curing stems from two main causes: first, an unreasonable powder coating formulation, resulting in substandard film performance; second, while the powder coating itself may be functional, improper coating process control leads to the film failing to meet technical specifications.

From the perspective of powder coatings, improper matching of resin and curing agent in the formulation design—for example, incorrect selection of resin reactivity or the type and dosage of curing agent—

excessive filler mass percentage or volume concentration can also affect the physical and mechanical properties of the coating.

From the perspective of the powder coating process, the following factors contribute to poor film curing:

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

Although the oven temperature can easily reach the design temperature or the required baking temperature under no-load conditions, under full load, insufficient total heat supply may prevent reaching the design temperature or the required baking temperature for the powder coating.

This problem is particularly prevalent in northern regions where workshop temperatures are low during winter. If the oven's insulation is poor, the material being coated is cold before entering the oven, and the material itself is thick-walled and has a large heat capacity, this issue is more likely to occur.

This problem is also more likely to occur when the oven temperature does not reach the powder coating's baking and curing temperature.

When the oven temperature does not reach the powder coating's baking and curing temperature, this can be addressed by increasing the heating power or extending the baking and curing time.

2. In the oven, the baking and curing time does not meet the required baking time for the powder coating.

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

The thicker the material being coated and the more components being mounted, the greater its heat capacity, and the longer it takes to heat up to the controlled temperature.

In an oven of a certain 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 the requirements.

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

3. Poor surface preparation of the substrate.

If oil stains, oxide scale, or rust are not thoroughly cleaned from the surface of the substrate, or if the phosphating film is of poor quality and contains residual phosphating solution, etc.,

In such cases, the adhesion between the powder coating and the substrate will be poor, resulting in poor impact resistance. This problem can be solved by improving the surface preparation quality of the substrate.

4. For coating performance testing, it should be strictly carried out 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 results, and adhesion are poorer and less likely to pass.

However, when the coating thickness is thinner and the test temperature is higher than specified, these physical and mechanical properties are more likely to pass. To accurately determine coating performance, testing must be conducted under conditions specified in the standard or close to those conditions.

From the perspective of ensuring product quality, testing coating performance indicators under relatively stringent conditions is crucial to guaranteeing the quality of the coated product.

5. Excessive baking and curing temperatures must also be considered.

Prolonged baking times can cause color changes due to thermal aging of the coating, and simultaneously reduce its flexibility, impact strength, and other physical and mechanical properties. Therefore, excessively high baking and curing temperatures and times are also detrimental to the physical and mechanical properties of the coating.

In powder coating, the baking and curing temperature and time of the powder coating play a decisive role in the coating's curing performance. Therefore, multiple trials are essential, and the objects to be coated must be properly categorized to determine the quantity and arrangement of the coating attachments, as well as the corresponding baking temperature and time, in order to guarantee the quality of the coated product.

Source: Coatings Alliance Network