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Dispersion of AEROSIL® fumed silica

A good dispersion of AEROSIL® fumed silica is very necessary, as it doesn’t exist as a primary particle. Right after synthesis, it forms aggregates and agglomerates. To be able to build up a network with the base oil to show optimal performance the agglomerates have to be broken up and the dispersed aggregates have to be homogeneously distributed in the system. With insufficient dispersion, the system can separate or particles could settle down.
Consequence of Inadequate Dispersion:
  • The final products exhibit specks and/or other surface flaws.
  • The amount of AEROSIL® fumed silica used does not achieve the desired thixotropic effect.
  • The thickened resins exhibit poor storage stability:
  • sedimentation of AEROSIL® fumed silica may occur
  • viscosity deteriorates the longer the product is stored.
  • Oil separates under pressure and long storage
The Right Dispersing Equipment
The dispersing equipment, which is recommended to be used, depends on the viscosity of the system. When the viscosity gets too high, a Dissolver or Rotor/Stator system cannot get enough shear force into the system. The other way round an Extruder or a Kneader will not work for liquid systems.
Relationship for proper mixing: dimension of impeller and vessel
Dispersion
Diameter of the disc: D
Diameter of the container: 2 D - 3 D


Height to which the container is filled with mill vase: approx. 2 D 


Height of the stirring disc over the bottom  of the container: 0.5 D - 1.0 D 



Suggested Tip Speed:  8 - 10 m/s 



Tip Speed (m/s) = [(D in mm) x (pi) x (rpm)]/ 60 x 1000

 Importance of blade: vessel size ratio. 1 + 2 = axial material stream, 3 + 4 = radial material stream 

 
Examples for Dispersion parameters in lab scale:
A) three roll mill:
  • gap of 0,05 mm
  • torque of 12,5 Nm
  • three passages.
B) planetary dissolver
  • 10 min. of dispersion
  • disc speed of 2000 rpm
  • scraper speed of 100 rpm
  • The peripheral speed of the dissolver disc should be at least ~8 m/s or higher.
 
  • Regarding the addition order of the ingredients we recommend to add the silica to the base oil first and disperse it and add other additives afterwards, as the silica needs some time and energy to be dispersed. Other additives approximately might be shear sensitive and therefore should be added afterwards.
  • If the viscosity of the grease is so high, that the homogenization of other additives isn't possible afterwards, than of course these must be added before the silica.
  • For the usage of silica generally no temperature is needed, but if other additives need to be added at a certain temperature, this is possiblle.
    For very high consistency greases it could make sense to heat the formulation up, as to have a lower viscosity and therefore more movement in the system. Please be aware that with low consistency greases it is possible that the shear force gets too low, when viscosity drops due to temperature rising.