Coating forming process
Coating-forming process can be divided into melt coalescence to form a coating film leveling three stages.
At a given temperature, the control molten coalescence rate the most important factor is the melting point of the resin, the viscosity of the molten state of the powder particles and the size of the powder particles. In order to best coalescence of molten shall as soon as possible, in order to have a longer time to complete the leveling phase flow effects. The use of a curing agent shortened available to flow and leveling of the time required, and thus the coating film formed of those extremely active powder often presents orange peel.
The key factors that affect the coating flow and leveling is a melt viscosity of the resin, the surface tension of the system and the film thickness. In turn, the melt viscosity, particularly depends on the curing temperature, the curing rate and the heating rate.
The various factors mentioned above, together with the particle size distribution and the film thickness, is usually determined by the required film properties to be painted objects and powder construction conditions decided. Powder coating flow and leveling of the power from the surface tension of the system, this front has also been mentioned. The force with applied to the attraction between the molecules in the coating film on the contrary, the result, such as the melt viscosity is higher, the greater the resistance against flow and leveling. Thus, surface tension, and molecular size of the difference between the gravitational determine the extent of the coating film leveling.
For a coating with good flowability, it is clear that the surface tension of the system should be as high as possible, and the melt viscosity is as low as possible. These can be achieved by adding the additives to the surface tension of the system can be improved and the use of a low melting point of the resin of low molecular weight.
Coatings can be prepared according to the above conditions having excellent flow properties, but because of its high surface tension causes the shrinkage, due to the lower melt viscosity will generate sagging, and the corners Poor coatability. In practical work, the surface tension and melt viscosity of the system are controlled within a specific range, so qualified coating surface appearance can be obtained.
The impact of the surface tension and melt viscosity of the flow of the coating film is shown in Figure 2. As can be seen in the figure, the surface tension of the too low or too high a melt viscosity will prevent coating flow, resulting in a coating film poor flowability, and the surface tension is too high the film forming process will appear craters. The physical storage stability of the melt viscosity is too low will make the powder deteriorates Poor coatability of a corner construction, and the construction in the facade sagging.
In summary, it is clear, the final surface condition of the obtained powder coating film, defects and deficiencies (such as orange peel, poor flowability, craters, pinholes, etc.) are closely interrelated, and also in the deposition process involved in the phase change in the rheological force control. The powder particle size distribution also affects the surface appearance of the coating film. The smaller the particles, the larger particles of low due to its thermal capacity, so its melting time is shorter than that of the large particles, coalescents also faster, and better surface appearance of the coating film is formed. Large powder particles melting time than the length of small particles, the coating film formed on may be generated the orange peel effect. Electrostatic powder construction methods (corona discharge or friction discharge), but also leads to the formation of a factor in the orange peel.
How to reduce or avoid the orange peel effect to promote flow and leveling can reduce or avoid orange peel. The system uses a low melt viscosity, leveling extended time and a higher surface tension in the curing process can be improved flow and leveling. The important parameters to control the surface tension gradient is reduced orange peel, while also controlling the surface tension of the coating film surface is uniform, in order to obtain the smallest surface area.
Flow-promoting agent or a leveling agent is often used in the actual work to improve the appearance of the coating, in order to eliminate surface defects such as orange peel, craters, pinholes. The good performance of a flow-promoting agent can reduce the melt viscosity, thus contributing to the melt mixing and the pigment dispersion, to improve the wettability of the substrate, the flow and leveling of the coating, helps to eliminate the surface defects as well as to facilitate the release of air.
The flow modifier dosage and effect relationship should be investigated. Insufficient amount will cause shrinkage and orange peel, excessive consumption will lead to loss of gloss, haze, and produce recoat adhesion problems on the upper. Typically, the flow modifier in the premix added. Or made of a resin master batch (resin and the additive ratio of 9/1 to 8/2), or is adsorbed on the inorganic carrier in powder form. The amount of the additives in the powder paint is 0.5 to 1.5% (in Binders calculated effective polymer), but at low concentrations may also good.
Polyacrylate flow modifier resins most widely used, such as polyacrylic acid butyl ester (“Acronal 4F”), acrylic acid ethyl – ethyl hexyl acrylate copolymer and butyl acrylate – acrylic acid-hexyl acrylate copolymer, etc.. They can be used in a very wide concentration range. Usually polyacrylate little effect on surface tension, they can contribute to the coating forming a relatively constant uniform surface. Compared with those of the surface tension reducing additives (such as silicone or the like), they do not reduce the surface tension, and therefore can be used to accelerate the leveling. Reduce the surface tension of the additives include surface active agents, fluorinated alkyl esters, and silicone. They join the amount is very sensitive. Benzoin is a degassing agent, also has the effect of reducing the surface tension, is widely used to improve the surface appearance of the coating film of the powder coating.
Coating forming process