1. The activity of powder coating and its detection
The mobility of the powder refers to the level of the powder coating being fluidized by air in the fluidized bed, and it is an important basis for evaluating the construction performance of the powder coating. The mobility of powder coatings can usually be weighed by the angle of repose or fluidity coefficient. The angle of repose is a direct detection method for powder activity. The smaller the angle of repose, the better the activity. There are currently two commonly used detection methods for activity:
1.1 Normative funnel method
This method is mainly used to detect the mobility of metal powder. It measures the time required for 50g of metal powder to flow through a funnel hole of a standard size, in s/50g. The longer it takes, the worse the mobility, and vice versa.
1.2 Natural accumulation angle
Let the powder flow down naturally through a set of screens and pile up on a flat circular plate with a diameter of 1 inch. When the powder is piled up on the circular plate, the bottom angle of the powder cone is called the angle of repose or pile powder angle. The larger the pile powder angle, the worse the mobility of the powder, and vice versa, the better the mobility.
2. Factors affecting activity performance
Powder mobility is the fundamental characteristic of powder, which refers to the difficulty level of powder movement. Powder activity performance is related to many factors, such as powder particle size, shape, roughness, dry humidity and so on. Generally speaking, increasing the coefficient of friction between particles will make it difficult for the powder to move. Generally, spherical particles have the best mobility, while powders with irregular shape, small size and rough appearance have poor mobility.
In addition, the mobility of the powder is affected by the adhesion between the particles. If the surface of the particles absorbs moisture, gas or participates in the forming agent, the mobility of the powder will be reduced. The mobility of the powder directly affects the average mixing performance. The mobility is too poor and it is easy to mix. Adhesion and clumping cannot be mixed evenly.
The glass transition temperature of the powder coating resin determines the activity and storage stability of the powder coating. The glass transition temperature of T polymer is improved, the stability of the powder is improved, the agglomeration is reduced, and the mobility of the powder coating is improved. The glass transition temperature increases with the increase of molecular weight. When the molecular weight increases to a certain level, the glass transition temperature of the polymer does not change significantly. The glass transition temperature is too high, the number average molecular weight of the resin is large, the melt viscosity is large, the mobility is poor, and the mechanical properties of the powder coating film decrease. The molecular weight of the commonly selected resin in consumption is 2000-5000, and it is more suitable to control the glass transition temperature in the range of 50-65°C.
3. The influence of activity performance on powder coatings
3.1 Impact on painting construction
In the process of electrostatic spraying of powder coatings, the mobile powder, like water boiling, appears very loose and has the effect of flowing water. From the powder supply barrel to the spray gun, the powder is transported and heavy, and the powder is atomized well from the nozzle, and the powder is restrained. Agglomeration of paint results in abnormal consumption such as spraying gun blocking or "powder spitting". The opposite is true for the powder with poor mobility in the powder tank, which leads to poor powder coating efficiency.
3.2 The impact on the consumption of stored procedures
The mobility of powder coatings directly affects the storage stability of the product. During the product packaging process, the powder begins to agglomerate. As time progresses and squeezed during storage, the powder begins to harden and agglomerate. The texture powder loses its texture effect after spraying, and the flat powder also shows a metamorphosis phenomenon.
4. Improvement of powder coating activity performance
The mobility of powder coatings is closely related to storage stability. The storage stability of powders is mainly determined by the glass transition temperature of the resin and curing agent. They occupy a large proportion in the formulation design, so they play a more important role. Although the amount of curing agent is small, it has a significant impact on the storage properties of powder coatings. Under the condition of ensuring the appearance of the coating film, try to choose a resin and curing agent with a high glass transition temperature.
In powder coating formulations, the mass fraction of pigments and fillers has a significant impact on mobility. The pigments and fillers of powder coatings are mainly used for coloring and improving rigidity. If the mass fraction of pigments and fillers is too high, the resin is relatively less, powder coatings The performance of the activity is worse. At the same time, in the formulation design process, try to add as little or no additives as possible that affect the glass transition temperature and storage stability, such as adding less leveling agents, wax-type matting agents and accelerators, etc.
Adding a loosening agent to improve the looseness of powder coatings by internal addition can reduce the angle of repose of powder coatings and make the powder less likely to agglomerate. After addition, it can adjust the affinity and cohesion between the film-forming substances in the powder coating and improve the anti-caking of the powder coating. Temperature, improve the storage stability of powder coatings, and improve the performance of dry powder; in addition, when electrostatic powder spraying, the powder coating is not easy to block the spray gun, preventing agglomeration and sticking to the wall. Hubei Laisi Chemical New Materials Co., Ltd. dry powder active agent 445 belongs to this category of additives.
In addition, additives such as fumed white carbon black and alumina, which are larger than the surface area and have strong moisture absorption ability, are added in an external way to form a barrier between the powder coating particles, reduce the collision and agglomeration of the powder coating particles, and reduce the space between the powder coating particles. Absorb and rub each other to avoid contact between particles, so that the powder coating is not easy to agglomerate. Hubei Laisi anti-caking agent 466 is made of inorganic minerals as the main body and auxiliary organic surface active agent. It is processed by air jet pulverization. The particle and density of the anti-caking agent are similar to powder coatings. The addition amount is 0.4%, which can be directly added to the waste powder, and can also be crushed and sieved together with the flakes.
The mobility of powder coatings is the result of the comprehensive use of multiple elements. In addition to the main elements discussed above, it is also related to other elements, such as commonly used pigments, filler properties and amount of participation, spraying operation process conditions, angle of repose detection methods, etc., which require scientific and technical personnel to explore from time to time during consumption. Discussing the mobility of powder coatings, improving powder coating formulations and spraying consumption process control parameters are of great significance to guiding consumption theory.