| Plane Colloidal Mill |
Mixers for Polymers -
Asphalt Emulsion Blends |
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The rate and quality of dispersion block copolymers in bitumen depend on
many parameters including e.g. polymer particle morphology and bitumen grade
and constitution. Most decisive, particularly in determining the dispersion
rate, is the effectiveness of the mixer involved. To get good quality
products the temperature should not exceed 185°C and the blending time
should be as short as possible consistent with there being sufficient time
to dissolve the polymer completely in the bitumen. Before defining what
criteria a good mixer should be judged by, this technical bulletin describes
the basic mechanism involved in dispersion.
When the polymer is added to the hot bitumen, the latter immediately starts
to penetrate into the polymer particles. Under the influence of the shear of
the mixer the swollen polymer becomes mobile and disperses in the bitumen.
From this general description three parameters can be identified which
mainly govern the dispersion rate of a particular copolymer.
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model
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Mini Plant Model 9 liters Capacity
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- The polymer particle size at
the stage of bitumen penetration: the smaller the particle size, the larger
the surface area, and the more rapidly, the polymer will be penetrated by
the bitumen.
- The shear exerted upon the particles after they have swollen sufficiently
for the molecules to become mobile. This effect however, is less important
than the particle size.
- The temperature since the diffusion rate of the penetration bitumen; which
increases as the temperature rises, determines when shear becomes effective.
The first condition to be satisfied when blending bitumens and polymers is
that the polymer particles should be small. This can be done by grinding the
granules into powder before their addition to the bitumen, or they can be
disintegrated after addition to the bitumen by using mixers in which the
bitumen acts as a lubricant in a "wet" grinding process. |
In the first method finely powdered polymer is
added to the bitumen, taking care to avoid lump formation, and the modest
shearing action or a paddle mixer may blend the two components together within a
reasonably short time, depending on the type of bitumen being used. The
additional shear provided by a high-speed mixer would increase the dispersion
rate. The second method of grinding the polymer in the bitumen saves the cost of
grinding the granules separately.
The machines most suited to disintegrate and
disperse solid materials into a liquid are those fitted with rotor and stator
systems. The rate and degree of disintegration, in such machines is mainly
governed by:
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gap-wid th |
rotation speed |
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the diameter of the rotor |
number of teeth on the
rotor and stator |
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the angle at which the
teeth are positioned |
the flow rate |
A study of inmersion mixers and miners with
serrated rotors and stators, has shown that the latter are capable if giving the
best results. This is due to their higher rotation speed and much greater number
of teeth, which together give a higher frequency of cutting action on the
granules.
It has also been found that the elapsed time between polymer addition and actual
grinding (pre-blending time ) is important. Long pre-blending times soften the
polymer granules and may make them less easy to cut. It depends on the internal
construction of the rotor and stator whether soft or hard granules are preferred
for optimum disintegration. After "wet" grinding the swollen particles, the
process continues with further penetration of the polymer. The shear in the
mixer, which eventually mobilises the polymer molecules, is directly related to
the velocity of rotation and to the viscosity of the bitumen, and is inversely
proportional to the gap-width.
On the other hand the flow rate rises as the gap-width is increased and
therefore, after desintegration, the dispersion process seems to become less
dependent on the gapwidth. This hypothesis is strongly supported by production
trials, where gap-width had no influence on total blending time. Thus overall
operatiou may be best when a small gap is used to start with followed by a wider
gap later. An additional benefit is that this would use less lower is the later
stages.
From the above, the ideal
mixer would be one which grinds the polymer granules very finely and
provides high-shear followed hy high flow rates after grinding. The
machinery should be powerful and soundly constructed and the grinding
tools wear-resistant.
Mixers with toothed rotors and stators are most likely to satisfy these
demands and can be recommended for blending polymer and bitumen. The
greatly increased throughput rates and the ability to use granular forms
of plymer more than compensate for their higher cost when compared with
the paddle mixers traditionally used by the industry. |
Schematic
representation of rotor/stator mixer
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