Process technology: powder coating particle size and dry powder flow

China Coatings Online News Information: Laser particle size analyzer to determine particle size distribution, D10 (particle size less than its particle size accounted for 10% of the total number of particles), D50 (median diameter, particle size greater than it and particles smaller than it accounted for 50%) and D90 (particle size less than its particle size accounted for 90% of the total number of particles) and other related parameters.The analysis ofpowder particle size distribution test chart should focus on the following aspects:
-average particle size: generally expressed as Dv50, refers to powder particles larger than and less than that particle size each accounted for 50%, the general powder coating DV50 is about 30 to 35 m.
-fine powder content: Generally refers to less than 10 to 15 m of powder particles as a percentage of the total volume of powder particles, ordinary flat powder coating subdivision content should be controlled at less than 10%.
-coarse powder content: refers to the powder particles larger than the particle size of 90 m as a percentage of the total volume of powder particles. Too coarse powder particles, such > 115 m, should be avoided.
-graphical distribution: the ideal particle size distribution should be a standard normal distribution, our powder products often appear single or twin peak distribution, some peaks towering, some peak flat.
actual experience, the particle size distribution parameters of conventional powders are controlled as follows:
- high-voltage electrostectrical spray gun powder: Dv50 to 35 to 38 m, less than 10 m of fine powder <8%, greater than 90 m of coarse powder <3%.
-friction gun powder: Dv50 x 40 to 45 m, <10 m of fine powder content <6%, crude powder <70 m total powder particles >90%.
-fluidized bed powder: Dv50 is generally in 50 to 60 m. < the fine powder content of 10 m < 4%, > 3% of the crude powder content of < 170 m. the sprayability of
powder coatings has a lot to do with the fine powder content, and the charge performance of fine powders (especially ultra-fine powders of <5 m) is so poor that they cannot even be charged, which usually goes into the recovery system and becomes a direct loss. We have tested powder products in many powder factories, the vast majority of powder coatings have a fine particle size, the average particle size is more than 25 to 32 m, <10 m of fine powder is mostly about 15%, and >75 m of powder particles are less.The particle size control of the powder used in friction guns is more stringent than that of high-pressure electrostectrecturic powder coatings, because powder particles must rub violently against the polytetrafluoroethylene material on the inner wall of the gun tube at a certain speed to carry sufficient positive charge. If the powder particles are too fine, the area of friction with the inner wall of the tube is small enough to generate and maintain sufficient positive charge. There is no electrostation field between the spray gun and the work piece in the frictional charge spray system, the powder is deposited on the work piece, mainly relying on the amount of charge brought by the powder friction, if the powder belt charge is small, the static attraction is low, the powder is not easy to adsorption to the work piece. Experiments have found < particles with a value of 5 m are usually not available in friction gun spray systems. In addition, the powder used for friction guns must be as dry as possible, the formula with flow aids should be selected by gas phase alumina, to increase the frictional charge of powder coatings.test of the coagulation and laser particle size of the sample, it is found that the more uniform the powder particles, the smaller the condensation. The better the sphericality of the powder particles, the better their mobility. Powder particles are needles and flakes with poor mobility.