Influencing factors and improving methods of flow performance of plastic powder coatings

Influencing factors and improving methods of flow performance of plastic powder coatings

1, plastic powder coating fluidity and detection

The fluidity of powder refers to the degree of the powder coating being fluidized by air in the fluidized bed, which is an important basis for evaluating the construction performance of powder coating. The fluidity of powder coatings can usually be measured by the Angle of repose or the fluidity coefficient. Repose Angle is a direct detection method of powder fluidity, the smaller the repose Angle, the better the fluidity. At present, there are two common detection methods for liquidity:

1.1 Standard funnel method

(Hall current meter)

The method is mainly used to detect the fluidity of metal powder, measuring the time required for 50g metal powder to flow through the standard size funnel hole, the unit is s/50g, the longer the time, the worse the fluidity, and the other way around, the better the fluidity.

1.2 Natural accumulation Angle

(rest Angle, powder pile Angle)

Let the powder flow naturally through a set of screens and pile up on a flat round plate with a diameter of 1 inch. When the powder is piled up on the round plate, the bottom corner of the powder cone is called the resting Angle or pile powder Angle. The larger the pile powder Angle, the worse the fluidity of the powder, and the better the fluidity.

2. Influencing factors of flow performance

Powder fluidity is the basic characteristic of powder, which refers to the difficulty of powder flow. The flow performance of powder is related to many factors, such as particle size, shape, roughness, dry humidity and so on. Generally speaking, increasing the coefficient of friction between particles will make the powder flow difficult. Generally, the powder fluidity of spherical particles is good, while the powder with irregular particle shape, small size and rough surface has poor fluidity.

In addition, the powder fluidity is affected by the adhesion between particles, if the particle surface adsorbed water, gas or the addition of forming agents will reduce the fluidity of the powder, the powder fluidity directly affects the mixing uniformity, the fluidity is too poor, it is easy to adhere when mixing, and the agglomeration can not be mixed evenly.

The glass transition temperature of powder coating resin determines the flow performance and storage stability of powder coating. The glass transition temperature of T polymer is increased, the stability of powder is increased, the caking property is decreased, and the fluidity of powder coating is increased. The glass transition temperature increases with the increase of molecular weight, and the change of glass transition temperature is not obvious when the molecular weight increases to a certain degree. The glass transition temperature is too high, the number average molecular weight of the resin is large, the melting viscosity is large, the fluidity is poor, and the mechanical properties of the powder coating film are reduced. In the production, the molecular weight of the resin is generally selected in 2000 ~ 5000, and the glass transition temperature is controlled in the range of 50 ~ 65℃.

3, the impact of flow performance on powder coating

3.1 Influence on painting construction

In the electrostatic spraying process of powder coating, the powder with good fluidity, like water boiling, appears very fluffy, and has the effect of flowing water, from the powder supply barrel to the spray gun, the powder transfer is light, the powder is atomized well from the nozzle, and the production anomaly such as the spray blocking gun or "spitting powder" caused by the powder coating agglomeration is overcome. The powder with poor fluidity is the opposite in the powder barrel, resulting in low powder difference efficiency on the coating.

3.2 Impact on the production stored process

The fluidity of powder coating directly affects the storage stability of the product, and the powder begins to aggregate during the production and packaging process of the product. Over time and squeezed during storage, the powder begins to harden into clumps. Texture powder lost texture effect after spraying, flat powder also appeared metamorphism.

4, powder coating flow performance improvement

The fluidity of powder coating is closely related to storage stability, and the storage stability of powder is mainly determined by the glass transition temperature of resin and curing agent, which occupies a large proportion in the formulation design, so it plays a more important role. Although the amount of curing agent is small, it has obvious influence on the storage of powder coating. In the case of ensuring the appearance of the film, try to choose the resin and curing agent with high glass transition temperature.

In the powder coating formulation, the mass fraction of pigment and filler has a significant effect on the fluidity, the powder coating filler is mainly used to color and improve the stiffness, the mass fraction of the filler is too high, the less resin is relatively less, the flow performance of the powder coating is worse. At the same time, in the formulation design process, as little as possible to add or not add additives that affect the glass transition temperature and storage stability, such as less leveling agent, wax type matting agent and accelerator.

In addition, the loose agent is added to improve the loose property of powder coating, reduce the repose Angle of powder coating, so that the powder is not easy to aggregate, and adjust the affinity and cohesion between film forming materials in powder coating after adding, which can improve the anti-caking temperature of powder coating, improve the storage stability of powder coating, and improve the dry powder flow performance. In addition, when electrostatic powder spraying, the powder coating is not easy to plug the spray gun, to avoid agglomeration and wall adhesion phenomenon. Hubei Laisi Chemical New Materials Co., Ltd. dry powder flow agent 445 belongs to this kind of additives.

In addition, additives such as vapor phase silica and alumina with large specific surface area and strong moisture absorption ability are added in an external way to form a separation layer between powder coating particles, reduce the chance of collision and condensation of powder coating particles, reduce the mutual attraction and friction between powder coating particles, prevent mutual contact between particles, so that powder coating is not easy to aggregate. The anti-caking agent 466 is composed of inorganic minerals as the main body and auxiliary organic surfactants. The particle and density of the anti-caking agent are similar to that of powder coatings. Add 0.4%, can be directly added to the finished powder, can also be crushed together with the sheet sieve.

5. Conclusion

The fluidity of powder coating is the result of many factors. In addition to the main factors discussed above, it is also related to other factors, such as common color, filler properties and addition amount, spray operation process conditions, repose Angle detection methods, etc., which require scientific and technological personnel to continue to explore in production. It is of great significance to study the fluidity of powder coating, improve the formula of powder coating and control parameters of spraying production process.