Research On Particle Size Distribution Control And Fluidization Properties Of Ultrafine Powder Coatings

Ultrafine powder coatings are one of the development directions in powder coatings area,as they can achieve thin coatings with good leveling and high surface smoothness comparable to liquid coatings,without the emission of Volatile organic compounds（VOCs）during production and application.According to Geldart’s Powder Classification,the ultrafine powder coatings can be categorized in Group C particles(D₅₀ under 25μm～40μm),which are difficult to fluidize due to their relatively large interparticle attractive forces.While the high flow and fluidization properties are one of the most important prerequisites for powder coating spraying processes,and it is of great importance to study the overall characters of fluidization mechanism of Group C particles to improve the flow and fluidization performances of ultrafine powder coatings.The fluidization performances of Group B and D particles with different size distributions were studied previously.However,little systematic research is so far reported on the effect of particle size distribution on flow and fluidization abilities of Group C particles.From this point of view,the effects of span and median particle size of ultrafine powder coatings with Gaussian-distribution on the pressure fluctuation,bed expansion rate（BER）and the minimum fluidization velocity(U_mf)et al.were carefully investigated.This thesis also presents the performances of Group C particles in dense phase conveying systems.Based on the mechanism of the influence of span on the flowability of Group C particles,a modified grinder design is proposed to decrease the excessive impact of grinding pins and therefore reduce span of particles produced.The main work of this thesis summarized as below:Firstly,the flow and fluidization properties of Group C particles with similar median particle size and different span were studied,and a comparison between the flowabilities of A and C particles was made.Analysis on the Angle of repose（AOR）and Avalanche angle（AVA）was conducted to characterize the flowabilities of Group C and A particles with different span.Fundamental studies including normalized pressure drop,BER,U_mf and dense phase BER were carried out.Compared with Group A particles,the effect of span on fluidization abilities of Group C particles is much more significant.Meanwhile,the flow and fluidization abilities of particles are mainly affected by the median particle size in contrast to span.Secondly,the effects of fluidization additives’concentrations on the flow and fluidization performances of Group C and A particles were investigated.With the increase of span,the specific surface areas of Group C particles increase significantly due to the increasing contents of fine particles,which leads to the different fluidization behaviors of particles even at the same additive concentrations.The surface coverage ratio of C2.6A0.8 is 19%±5%which is calculated by image processing with threshold segmentation.The additive concentration of 0.8 wt.%is saturated for C2.6A0.8 while insufficient for C3.5A0.8.Thirdly,the performances of Group C particles in dense phase conveying systems has been investigated in comparison with Group A particles.The absolute pressure of conveying system and solid-gas ratio was characterized.The enhancement of flow ability of Group C particles is observed with the help of the fluidization additives.At last,the theoretical analysis and experimental studies on the particle size control were conducted in the production of ultrafine powder coatings.The“cushioning”effect produced by the modified pins decreases the excessive impact between smaller particles and grinding pins and therefore reduces the over-grinding of ACM.The D₁₀ of ultrafine powder coatings(D₅₀=21μm)produced by P4increases by about 34%,while the corresponding span decreases by about 7%.To study and evaluate the surface optical quality of coating films,the ultrafine powder coatings with different span were sprayed on panels.The specular gloss and surface toughness of coatings with smaller span are significantly higher than those with larger ones.