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Mixing of materials in the laboratory or for smaller production volumes
is traditionally done by hand, or the use of some sort of mechanical
blade, impellor or paddle to agitate the mix. These are fine in their
way, being largely inexpensive, manual or simple powered tool processes,
and are adequate for many mixing tasks.
There are challenges when the materials involved include large
differentials in viscosity, when one or more component is a very high
viscosity, or when one or more components is a powder or other solid – in
high technology applications, these could include organic materials,
oils, aqueous solutions, clays, pastes, precious metal powders or nano-
tubes. Critical applications may demand a guaranteed completely
homogenous mix with no entrapped air, and a process which is both
reliable and repeatable. Where materials are expensive or rare, process
time and material wastage are also important factors, and mixing may be
required in smaller volumes (i.e. less than 5 grams).
Mechanical mixing techniques can be prone to introducing air into the
material, which would be unacceptable for many applications, for example,
an optical encapsulant. A separate and additional de-aeration process
would then be needed - equipment to centrifuge or vacuum the air out of
the mix. Usually vacuum degassing is a manual process which is time-
consuming, messy and can result in foaming, leading to overspill of the
material.
Clearly there ought to be a better way - and indeed an alternative
approach is available using the principles of simultaneous rotation and
revolution (planetary) motions, where the mixer equipment rotates the
container holding the material and at the same time revolves the
container. By simultaneously revolving and rotating, a strong whirlpool-
type flow is generated in the materials themselves, stirring and
dispersing them. By carefully altering the rotation and revolution
speeds, and, crucially, the ratio between these movement speeds, flow
currents are generated in the materials which are alone enough to
completely mix the materials, without the need for any invasive
mechanical stirrer. There is no air induced by this procedure; indeed,
previously entrapped air in the materials is removed.
At Intertronics we have found that the Thinky planetary centrifugal
mixers are capable of stirring with forces of up to 400g, which allows
them to mix and disperse even high viscosity materials such as oil-based
clay. This is made possible by programmable control of the various mixing
gyrations. For example, the revolution speed can be adjusted to stir and
disperse according to the material’s viscosity and mass. For materials
that are sensitive to increases in temperature, the revolution speed can
be lowered thus regulating the forces applied to the material. For high
viscosity materials, the revolution speed can be increased thus applying
maximum mixing force to the material, improving the quality of the mixing.
By altering the relative rotation and revolution motions, more
centrifugal forces are induced into the materials, and degassing or de-
aerating is achieved.
An excellent example of using the planetary rotation process with a
difficult to mix, fragile material is the mixing of alginic acid and
water. Normally it is kneaded by hand, but this process is not only messy
and very time consuming it also creates a major issue regarding the
enfolding of air bubbles.
Alginic acid (Alginate) is a polysaccharide found in brown algae, red
algae and coral weed - it is a common additive for food or elsewhere
where it is used as a thickener/gelling agent/stabiliser or in cosmetics,
dentistry, medical treatments, paper/textiles, fermentation and even in
fibrous form as surgical thread. The pure alginate is fibrous, granular
or powdery and insoluble in water which naturally complicates the mixing
process.
In a simple back to back test a traditional manual mix of alginate and
water in a 300ml container took 90 seconds. When solidified it was
sectioned and showed excessive variation and many large air bubbles. A
parallel process was carried out with the Thinky ARE250. The alginate and
water in their container were put into the machine which ran for
approximately 50 seconds. The result was a perfectly uniform mix
consistency and no air bubbles, with complete batch repeatability.
The mixing of fine powders into thin liquids is prone to agglomeration,
due to moisture and air entrapment effects. These may be overcome by the
use of this type of equipment. A simple barium titanate ceramic/water mix
can be seen to be mixed in two minutes with no agglomeration observed at
x1000 magnification. Mixing carbon nano-fibres in epoxy has also proved
problematical by traditional methods, largely because of the tangling
or “wiring” of the fibres – however tests have shown that gentle
progressive mixing generated by the planetary rotational motion enables
the fibres to slide over each other in the substrate epoxy, so producing
a homogeneous dispersion in a 15 minute operation.
Other test examples include nano-particles of carbon black in silicone,
two part silicones, two part epoxies and optical application adhesives,
potting compounds or sealants where inadequate mixing or the presence of
air bubbles would lead to unacceptable visual distortions or other
performance problems.
Traditional methods have served for many years but mixers built on the
rotation and revolution “planetary” principle are proving themselves
excellent performers in fields as demanding as cosmetics and
pharmaceuticals, where quantities as small as 5 grams can be handled for
colour adjustment and sample making in a mere 30 seconds. Fortunately the
principal is capable of scaling up and quantities up to 5kg can be mixed
even up to 1 million cP viscosity.
Further information regarding Intertronics’ products can be found at
www.intertronics.co.uk.
Plastics News International Magazine - December 2008
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