Résumé : |
The author explores the developments in fluidisation of additives for powder coatings.
All powder coating systems that use either tribo-charging or corona-ion equipment require a flow additive to optimise performance and prolong the stability of the coating material in storage during spraying and handling.
Powder cohesion is the result of structural forces within the material that compete in various ways, depending on particle size, humidity and particle composition, with the gravitational forces experienced by the particles.
When dry particle are larger than 75 microns, gravitational forces are greater that the structural or interparticle forces, and particle-particle attractions are negligible. As a result, powder can fluidise and flow with much less energy, since the particle can move independently of one another.
When particles are less than 75 microns, structural forces tend to dominate and the resultant particle-particle interaction lead to a powder that is usually cohesive and will fluidise and flow poorly. This is particularly the case with particles 25 microns or less. Flow additives such as fumed silicate or fumed alumina, overcome this problem of they have the appropriate size distribution and an affinity with host particle's surface, a random mixture results. In this case the flow additive and host particle tend to self-associate and produce pockets of additive within a matrix of host particles. |