Powder coating fluidity and its test

Powder fluidity refers to the degree to which powder coating is fluidized by air in a fluidized bed.The fluidity of powder coatings can be measured by rest Angle or fluidity coefficient.Resting Angle is a direct method to detect powder fluidity. The smaller resting Angle is, the better fluidity is.At present, there are two commonly used liquidity detection methods:

1.1 standard funnel method(hall tachometer)GB/T1482-2010

This method is mainly used to detect the fluidity of metal powder, and measure the time required for 50g metal powder to flow through the funnel hole of standard size, unit s/50g, the longer it takes, the worse the fluidity is, and vice versa.

1.2 natural accumulation Angle(rest corner, dusting corner)

Let the powder flow naturally through a set of screens and accumulate on a flat circular plate with a diameter of 1 inch. When the powder is filled with the circular plate, the bottom Angle of the powder cone is called rest Angle or powder pile Angle.The larger the powder Angle, the worse the powder fluidity, and vice versa.

2. Influencing factors of flow performance

Powder fluidity is the basic characteristic of powder, which refers to the difficulty degree of powder flow.Powder flow performance is related to many factors, such as powder particle size, shape, roughness, dry humidity, etc.Generally, increasing the coefficient of friction between particles makes it difficult for the powder to flow.Generally, powder with spherical particles has the best fluidity, while powder with irregular shape, small size and rough surface has poor fluidity.

In addition, the powder fluidity is affected by the adhesion between particles. If water and gas are adsorbed on the surface of particles or forming agent is added, the powder fluidity will reduce the powder fluidity. The powder fluidity directly affects the mixing uniformity.The vitrification temperature of powder coating resin determines the flow performance and storage stability of powder coating.The vitrification temperature of T - polymer increases, the stability of the powder increases, the caking property decreases, and the fluidity of the powder coating increases.The glass transition temperature increases with the increase of molecular weight, but the change of glass transition temperature is not obvious when the molecular weight increases to a certain extent.The vitrification temperature is too high, the average molecular weight of the resin is large, the melt viscosity is large, the fluidity is poor, the mechanical property of the powder coating film is decreased.Generally, the molecular weight of the selected resin is in 2000 ~ 5000, and the glass transition temperature is controlled in 50 ~ 65℃.

3. Influence of flow performance on powder coating

3.1 influence on coating constructionIn

the powder coating electrostatic spraying process, the flow of good powder, like water boiling, appears very fluffy, has the effect like flowing water, from the powder barrel to the spray gun, powder transmission light, powder from the nozzle atomization, overcome the powder coating group and cause spray gun or "spit powder" and other abnormal production phenomenon.The powder with poor fluidity is reversed in the powder barrel, resulting in low powder efficiency on coating.

3.2 impact on production stored procedures

The fluidity of the powder coating directly affects the storage stability of the product.As time passes and the powder is squeezed during storage, it begins to harden.The texture powder lost the texture effect after spraying, and the smooth light powder also deteriorated.

4, powder coating flow performance improvement

The fluidity of powder coating is closely related to the storage stability, which is mainly determined by the glass transition temperature of resin and curing agent. They occupy a large proportion in the formula design, so they play a more important role.Although the dosage of curing agent is small, it has obvious effect on the storage of powder coating.Resin and curing agent with high vitrification temperature should be selected to ensure the appearance of the film.In powder coating formula, the mass fraction of pigment and filler has a significant impact on the fluidity, powder coating yan filler is mainly to play coloring and improve the stiffness, yan filler mass fraction is too high resin is relatively less, powder coating flow performance is worse.At the same time, in the formulation design process, as little as possible or not to affect the glass temperature and storage stability of the additives, such as less leveling agent, wax extinction agent and acceleratorIn addition to improve the looseness of powder coating, reducing the rest Angle of powder coating, so that the powder is not easy to agglomerate, after adding to adjust the affinity and cohesion between film materials in powder coating, can improve the anti-caking temperature of powder coating, improve the storage stability of powder coating, improve the dry powder flow performance;In addition, when electrostatic powder spraying, so that the powder coating is not easy to block the spray gun, to avoid the phenomenon of agglomerating and sticking to the wall.Hubei laisi chemical new material co., LTD. Dry powder flow agent 445 belongs to this kind of auxiliary agent.Add specific surface area is big, in the form of plus moisture absorption ability of the gas phase silica and alumina additives, make insulation layer is formed between powder particles, decreases the chance of powder particle collision and coagulation and reduced the attraction and friction between the powder particles, prevent intergranular contact each other, make the powder coating is not easy.Hubei leishi anti-caking agent 466 is composed of inorganic minerals as the main body and auxiliary organic surfactant, which is processed by airflow crushing. The particles and density of anti-caking agent are similar to that of powder coating.Add 0.4%, can be added directly to the finished powder, can also be crushed with the sheet material sieving.