How to solve the caking problem of powder coating?

March 21, 2023 Longchang Chemical

How to solve the caking problem of powder coating?

 

Powder coatings have the tendency to caking at a certain temperature, which is mainly due to the softening of the resin, leveling agent and other materials in the powder coating when they encounter heat.

The resins of thermosetting powder coatings as the main film-forming materials are organic polymers with low molecular weight.

 

These resins have a physical property, at a lower temperature, it is hard and brittle glass state, when the temperature rises to a certain degree, the resin begins to turn into a certain elasticity and produce a state of adhesion, below this temperature, the resin returns to the non-adhesive glass state, the glass state of the resin and the viscoelastic state of the mutual transformation temperature is called the resin glass transition temperature.

 

Different resins have different glass transition temperature, the glass transition temperature of epoxy resin and polyester resin is about 50 degrees Celsius, the glass transition temperature of ‍-liquid leveling agent is below zero degrees Celsius.

 

The greater the amount of low glass transition temperature material added to the powder coating formulation, the lower the glass transition temperature of the system. The glass transition temperature of the powder system is positioned at approximately 40 degrees Celsius at the time of production, and this temperature is set as a safe temperature for powder coating agglomeration. ‍

The increase in temperature will make it easier for powder coating products to agglomerate, so how do we prevent powder coating agglomeration in our work?

First of all, we have to establish a concept that

 

Powder coating caking at a certain temperature is a natural law. To prevent powder coating caking must be in the whole process of powder coating production such as grinding, packaging, storage and transportation, powder coating products are below their glass transition temperature. ‍

Based on this view, the following solutions are available.

 

1) In the production of polyester resin, choose some alcohols or acids that can increase its glass transition temperature, or reduce the amount of alcohols that can lower the glass transition temperature of resin to increase the glass transition temperature of polyester resin.

(2) Reduce the amount of low glass transition temperature polymer used in powder coating formulation design, such as leveling agent and lightening agent, to ensure that the glass transition temperature of powder coating system will not be reduced.

(3) In production, the broken fragments from the steel belt should be cooled sufficiently before entering the grinding process, and the feeding speed should be reduced, the induced air volume should be increased, and the inlet air should be equipped with cold air conditioner to control the grinding temperature. However, if the grinding before the crushed pieces of material cooling down, the latter means will not play a good role, consider the method of forced cooling of broken debris material for low temperature treatment, which is more effective than the addition of air conditioning. ‍

Summer is coming, and powder coatings often have powder clusters and even lumps when in use. This is because of the high ambient temperature and humidity during production, storage, and transportation, and the glass transition temperature of powder coatings is more than 40 degrees. In order to prevent powder coating from caking and agglomeration during the application process, the following aspects should be noted.

1. When choosing resin, use some resin with high glass transition temperature (TG). The glass transition temperature of conventional epoxy resin and polyester resin is about 50℃, which can meet the general demand. When the storage requirements are high, the synthetic resin can reduce the use of alcohols that will reduce the glass transition temperature of the resin, curing epoxy available alicyclic amine curing.

2. In the powder coating formulation design to reduce the use of low glass transition temperature polymer, such as leveling agent, glass transition temperature at about 30 ℃, you can add a little less. The addition of plasticizer will reduce the rigid group of resin and lower the TG of resin, and the addition of excessive TGIC will also lower the TG of resin.

3. Auxiliary additives include dry powder flowing agent and additional anti-caking agent, white carbon black. The main component of dry powder flow agent is some wax powder organic material for anti-adhesion. Anti-caking agent mainly belongs to the silicate class of inorganic substances, and the sheet is crushed together with the use of sieve, can play a role in preventing powder adhesion caking. Adding silica is mainly fumed, light specific gravity, easy to absorb moisture. Therefore, when using to disperse well, avoid moisture, plus the dark color will not have white spots.

4. The production process mainly controls the extrusion and grinding two links. When extruding, the long track and fan physical cooling can effectively reduce the temperature of the flakes, and when the flakes are cooled, then grinding. When grinding powder, the feeding speed can be reduced, and the amount of induced air can be increased appropriately, and a chiller can be added to the air inlet to reduce the temperature in the pipeline if necessary. Grinding powder, to be cooled to below room temperature before packing. Some powder plant will be a box of powder with two inner bags separated packaging, but also to prevent the accumulation of powder, to a certain extent to prevent powder clumping.

5. Powder should be stored in a light-proof, dry workshop, some high-gloss and floating flowers easily caked products need to be stored in air-conditioned warehouses, or wrapped in a layer of aluminum foil around the product insulation. Transport should use air-conditioned vehicles or trucks covered with shade cloth, not box trucks, and should avoid the accumulation of heavy objects.

Conclusion: powder raw materials encounter high temperature easy to clump is a natural process, not necessarily an indicator of the evaluation of raw materials good or bad. In addition to high temperature anti-caking, fire prevention and disaster prevention with to be important, similar to the workshop to stop smoking, prohibit electric car charging, equipment wire exposed, etc., are small negligence caused by big problems.

The way to deal with powder coating caking.
1. In the production of polyester resin, choose some alcohol or acid that can improve its glass transition temperature, or reduce the amount of alcohol that can reduce the glass transition temperature of resin to improve the glass transition temperature of polyester resin.
2. In the planning of powder coating formulations, reduce the amount of low glass transition temperature polymers, such as leveling agents and long-faced agents, to ensure that the glass transition temperature of the coating system does not drop.
3. In terms of production, the broken fragments should be cooled sufficiently before entering the grinding process, and the grinding speed should be reduced and the air volume should be increased to control the grinding temperature.

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