The value of titanium dioxide is best realized when it is well dispersed. The
pigment particles tend to stick to each other during manufacture and storage,
resulting in aggregates and agglomerates many times the desired particle size.
Various techniques can be used for reducing these aggregates and agglomerates
to an acceptable size at a reasonable cost.
The principles of dispersion can be summarized by offering some guidelines
for achieving improved quality or more rapid dispersion by the three dispersion
processes used in the plastics industry:
Guide to Quality Dispersion:
By Dry Blending
Select polymer
- Choose small particle size.
- Choose a hard surface rather than soft, when possible. Select
a polymer that's chemically compatible with the pigment under
existing processing conditions.
- Choose rheological compatibility with the base polymer (especially
in concentrate preparation) to minimize let-down difficulties
due to difference in melt flow.
Select pigment and additive loadings to
provide proper flow under operating conditions.
Use a short, intensive mix cycle for most pigments.
Encourage subsequent melt pressing of these dispersions
at high shear.
By Melt Shear Processing
Select polymer.
- Provide good wetting of both pigment and grind surface.
- Select a polymer that has a small viscosity change with temperature.
- Choose a polymer that's chemically compatible with the pigment
under existing processing conditions.
- Choose rheological compatibility with the base polymer (especially
in concentrate preparation) to minimize let-down difficulties
due to differences in melt flow.
Select pigment loading to develop high
shear during operation.
Pre-blend dry ingredients (see "By Dry Blending"
above).
Where possible, flux resin by shear rather than by applying
external heat (e.g., Banbury mixer).
In Liquid Systems
Select vehicle.
- Provide good wetting of both pigment and grind surfaces.
- Choose tack to suit method (i.e., low tack in processes dependent
on impact between particles, high tack in those processes dependent
on shear through liquid layer).
- Choose a polymer that's chemically compatible with the pigment
under existing processing conditions.
Select pigment and additive loadings to
provide proper flow for equipment use.
Select let-down procedure to avoid seeding, gelling, or
poor mix-in (e.g., viscosity adjustment with thinner, high-shear
mixing, stepwise reduction).
Need advice to improve dispersion of TiO2?
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