Particle Coating and Surface Modification
Surface modification of particles is one of the most important and least understood processes for particle processing. It can be used to mask flavors or odors, add color, as well as improve the reactivity, solubility, flowability and wetting characteristics of the particles. Part of the difficulty of this field of particle processing is the diversity of applications and the variety of the methods that can be used. AVEKA makes many of these applications available for its customers.
Magnetically Assisted Impact Coating (MAIC, pronounced “mace”), is a patented process used to coat smaller particles onto larger particles or to apply small amounts of liquids onto particles. The system relies on a peening process in which small magnets in an oscillating magnetic field are fluidized to rapidly move in both a translational and rotational manner to peen or force the coating material onto the larger particles. By adding a smaller coating particle and a large core particle into the assembly of the oscillating magnets, the small particles are readily and strongly coated onto the core particles. Likewise, the magnets will distribute a small amount of liquid, such as an organosilane, evenly throughout the core particles and robustly apply them to the surface. The core and coating materials flow continuously through the magnet beds at rates of 8-600 pounds per hour. Materials that have been used in this process include glasses, pigments, metals, metal oxides, polymers, and organic and inorganic powders. A unique aspect of the particle on particle product is that, in many cases, the dry coated composite particle will not dissociate in the dry state or when the composite is put into a dispersion.
The MAIC process also has a large and permanent effect on flowability of the treated materials. When fumed silica is added to particle assemblies using traditional methods, the mixture flows better than the core material, but the fumed silica is not attached to the surface of the particles and will be removed from the surface with relatively minimal forces. After a MAIC treatment, two effects are seen for silica flow agent addition: first, the amount of silica needed to improve the flow is reduced significantly, up to an order of magnitude as compared to the traditional mixing/application methods; second, the resultant mixture is stable to shaking and transportation. Essentially, the mixture of small and large particles does not separate, even with the vibration seen with handling.
When the MAIC process is used to apply liquids to the surface of a particle, the large number of rapidly moving magnets acts like an assembly of mixers to uniformly distribute a very small amount of liquid over the surface of the particles without agglomerating the particles. In other liquid applications, it is not uncommon to see small, but significant, increases in particle size as the liquid being applied to the surface acts as a liquid bridge between particles before the liquid can be distributed. The MAIC process, with its continuous flowthrough characteristics and the high speed movement of the magnets (3600 rpm), minimizes this tendency as seen in the particle size distribution graphs and data below of titania before and after organosilane treatment:
Titania
Titania with organosilane surface treatment
| Titania | Coated Titania | |
| Mean | 0.757 µm | 0.746 µm |
| d5 | 0.470 | 0.474 |
| d10 | 0.512 | 0.516 |
| d50 | 0.716 | 0.712 |
| d90 | 1.074 | 1.035 |
| d95 | 1.449 | 1.338 |
Finally, MAIC has been used in combination with other surface treatments to expand the functionality and utility of the surface modification process. For instance, titania described above has been treated with an acrylate functionalized organosilane and then an acrylate polymer was graft polymerized onto the surface through the acrylate functionalities. Alternatively, we have also used the MAIC to coat powders onto a thermoplastic core particle and then melt-bonded the core particles to the coating particles by passing the coated assemblies through a hot air zone. By rapidly cooling the melt bonded particles we find that no agglomeration occurs (see the Spheroidization Description below).
Spray drying is one of the oldest methods for coating particles and, in large volumes, it is relatively inexpensive. The particle to be coated is added to an aqueous solution or slurry of another material. The combined slurry is atomized into a warm chamber where the water is dried off, ideally leaving a dry particle that has the second material coated onto it. The resultant particle may also be a matrix with some of the “core” particles sticking out of the matrix. Please see the Spray Drying section of our web site for additional information.
Prilling, also known as spray congealing, spray chilling, or melt atomization, is the process of atomizing molten liquids or mixtures and cooling the resultant droplets to form prills or beads that are the final product. When particles are added to the molten material, they will become embedded and be encapsulated in the matrix of the final bead. In general, the matrix material must be a material that is solid at room temperature, has a discreet melting and freezing point and a low melt viscosity. The Prilling section of our web site has additional information.
Fluid bed coating or Wurster coating is a process that was developed at the University of Wisconsin in the 1950’s. The particles to be coated are fluidized in a bed of moving air, and a solution of dissolved polymer, sugar, inorganic salts, sol gels, or other materials is sprayed onto the fluidized particles. As the fluidized particles dry, the coating dries on them, creating a core/shell encapsulation. Due to the nature of fluidized beds, particle coating using this method is generally limited to particles larger than 100 microns.
V-blending can be used to apply smaller particles or a small amount of a liquid coating onto other particles. The liquid is added to the mix via an intensifier bar, and the mixture is rotated in the blender for some period of time. Frequently this is used to apply silane onto non-reactive particles or to apply a flow aid or other coating particles to the material.
Hot Blending is a basic method for coating non-melting particles with a small amount of a thermoplastic material. Initially, the core particles are blended with the thermoplastic material at room temperature, then the temperature is increased to the melting point of the thermoplastic. Blending continues as the temperature is returned to ambient. The resultant particles have a light coating of the material, such as a wax, that may improve the flow and reduce bridging of the powder.
Spheroidization allows one to round the edges of a meltable particle. This is used to improve the flow characteristics of powders or to make an imperfect coating more continuous. The particles are rapidly passed through an intensely heated area, the outside material melts, flows, and, as it cools, forms a smooth surface. If there are small meltable particles on the outside of the core material, these will melt and flow to provide a better coating. If the core is thermoplastic, it softens and allows the coating particles to become more strongly adhered.
Tablet Coating is a process that tumbles particles in a rotating bed where hot air or cool air is used to dry or freeze atomized coating solutions or melts onto particle surfaces. This is a process where thin coatings can be applied to the surface of particles or heavy coatings up to 100% of the core particle can be applied.
| Method | Core | Coating | Final Size | Load wt % | Present Production Capability |
| MAIC | 0.25 µ to 2 mm | Particles ten times smaller than the core or liquids | Same as original core particle | Coating from 0.25 to 4 % | Up to 600 pounds per hour |
| Spray Drying | Insoluble in water, under 50 microns in size | Water soluble material or small particle slurry | 10-125 microns | Depends on fill material | From one pound to thousands of pounds |
| Prilling | Stable in a wax, under 50 microns in size | Low melt viscosity, solid at room temperature | 15 microns to 3 mm | Particulate up to 40% | One pound to thousands of pounds |
| Fluid Bed Coating | Particles >100 µm that can be fluidized | Water soluble materials, limited solvent capability | Similar to starting size, plus the coating material | Coating up to 50% | One Pound to hundreds of pounds |
| V-Blending | Liquids, such as silane |
Same as starting particle |
Coating from 0.25 to 5 % |
One pound to tons |
|
| Hot Blending | Non-melting particle | Thermoplastic | Same as starting particle | Coating from 0.25 to 5 % | One pound to tons |
| Spheroidization | Up to 0.5 mm | Meltable material | Same as starting particle | One pound to tons | |
| Tablet Coating | 0.5 mm to 10 mm | Aqueous solutions, emulsions, or melts | Similar to starting size, plus the coating material | Up to 100% | One pound to tons/day |