Study On Flowability Mechanism And Improvement Of Ultrafine Powder Coatings

This study aims at solving the issue of the poor flowability of ultrafine powder coatings(Group C powder,D₅₀<25μm),due to the high cohesion between particles,which makes it difficult to be applied in coating process.The factors that affect the flowability of powder coatings,including the flow additive type,flow additive dosage,powder particle size and particle size distribution,were systematically investigated,and new parameters to predict the powder flowability was proposed.Finally,the structure of the classifier impeller of the commonly used powder equipment air classifier mill（ACM）was modified to control the particle size distribution.The traditional commercial nano flow additive,nano-silica,was modified in this study.The surface of the nano-silica flow additive was encapsulated with a layer of polyester resin.The results show that the flowability of powder is significantly improved by the surface-modified nano additives compared with commercial additives.The addition dosage of flow additives has an optimal value,beyond which the flowability will not be further improved,or even be reduced.The reason is that when exceeding the threshold,there will be too many flow additive aggregates covering the surface of the host coating particles and the adjacent host particles will be connected together by flow additive aggregates,due to the bridging effect,and hence leading to poor powder flowability.At the same D₅₀of powders,the narrower the powder particle size distribution（the smaller the span）,the better the powder flowability,because the smaller particles in powders have a greater negative impact on powder flowability.As the particle size distribution narrows,the content of small particles decreases and the powder flowability improves.When the D₅₀ of powders are not the same,span can’t predict the powder flowability precisely.Therefore,the novel parameters of DA₅₀（particle size corresponding to the cumulative area distribution of 50%in particles）,volume moment and area moment were proposed in this study to characterize the particle size and distribution of powders.The results show that the novel parameters can predict powder flowability more accurately.The results of ACM impeller structure modification show that the dentate impeller can narrow the particle size distribution.When the required particle size D₅₀ is small,a coarse-sawtooth impeller can be used to prepare the powders with smaller span,while when D₅₀ is large,it is better to use a fine-sawtooth impeller.