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Ross Guide to Nanomixing
A White Paper Prepared By
Charles Ross & Son Company
Ross White Paper: Solutions to Batch Mixing Issues Page 2 of 13
Ross Guide to Nanomixing
Abstract
This white paper presents an overview of mixing technologies employed in the dispersion of
nano-sized solid particles into liquid. As major developments continue to unfold in the multi-
disciplinary field of nanotechnology, new mixing applications and processing challenges arise.
The aim of this paper is to provide practical information on the efficient use of specialty mixing
equipment for the preparation of low, medium and high viscosity nanodispersions.
Introduction
Research into new-generation nano-sized materials has been expanding and uncovering
interesting potential uses. More and more manufacturers are reacting and applying
nanotechnology into their existing products and this trend is expected to continue. From coatings
and cosmetics to fabrics and fuel cells, a growing number of consumer products are being
spiked, so to speak, with doses of nanoparticles to enhance material properties or provide new
functionalities.
Nanomaterials are structures the size of 100 nanometers or smaller in at least one dimension. To
put that scale into context, consider that human hair is approximately 80,000 nanometers in
diameter. Or imagine that a nanoparticle is the size of a football – this makes a virus the size of a
person!
It is precisely due to their size and structure that nanomaterials exhibit novel physical, chemical,
and biological properties. The increase in surface area to volume ratio translates to exponentially
more atoms at or near the surface of the particle, providing more sites for bonding or reaction
with surrounding materials. Put simply, particles in the nanoscale behave very differently than
larger particles of the same substance.
Indeed, the deepening interest in nanomaterials is justified by the broad range of applications.
For instance, the incorporation of nanofillers into a polymer matrix makes the compound even
stronger, harder or chemically resistant than today’s engineered composites. Carbon nanotubes
added to aircraft components, electronics and sports equipment give rise to enhanced electrical
and thermal conductivity, as well as excellent mechanical load bearing capacity. Nanopigments
increase transparency, gloss, smoothness as well as resistance to oxidation and UV radiation.
Metallic nanoparticles (zinc, silver and gold) are utilized for their electrical and magnetic
properties. These particles also exhibit catalytic behavior making them suitable for antibacterial
applications.
Ross White Paper: Solutions to Batch Mixing Issues Page 3 of 13
Saw-tooth disperser Rotor/stator mixer
The far-reaching contributions of nanomaterials to commercial products have the potential to
reshape the way we design, manufacture and consume goods and technologies. At the heart of
these changes is the mixing operation where the challenge of dispersing nanoparticles into a
liquid vehicle is gaining more ground in mainstream processes than ever before.
Low-Viscosity Nanodispersions
The dispersion of nanomaterials into a low-viscosity liquid typically involves a pre-mix stage to
combine the raw materials followed by a “polishing” step. Due to attractive forces between the
individual nanoparticles, wetting them out (i.e., adding the solids into the liquid vehicle) only
disperses agglomerates of the particles. High shear forces are often necessary to break up groups
of these agglomerates. How aggressive those shear forces need to be vary from one formulation
to another. Some nanoclays, for example, readily disperse in a polymer liquid vehicle when
mixed using a saw-tooth disperser or a rotor/stator device. In these cases, the powder wet-out
stage and polishing step are accomplished using a single mixer.
Running at tip speeds of around 5,000
ft/min, a saw-tooth disc-style disperser
creates vigorous flow within the mix
vessel and imparts greater shear
compared to other open-blade mixing
devices such as propellers or turbines. It
generates a vortex into which solids can
be added for quick incorporation into the
batch.
In comparison, a rotor/stator mixer
typically features a four-blade rotor
running within a close tolerance fixed stator at tip speeds in the range of 3,000-4,000 ft/min. This
type of device creates mechanical and hydraulic shear by continuously drawing product
components into the rotor and expelling them radially through the openings in the stator. Due to
the differential speed and close tolerance between the rotor and stator, this mixer design delivers
greater shear and faster deagglomeration compared to devices with an open-blade geometry,
even those that run at higher tip speeds.
Most nanoparticle dispersions, however, require more aggressive mixing than can be
accomplished in conventional dispersers or four-blade rotor/stators. The pre-mix may be
prepared using these machines and then fed into more shear-intense equipment such as a high-
pressure homogenizer, colloid mill or a media mill. Over the years, way before the surge of
nanoapplications, these high energy devices have been the most commonly used equipment for
extremely fine dispersion requirements. The newest alternative to these traditional processes
come in the form of ultra-high shear mixers (UHSM’s) which also prove to be effective for
nanodispersions.
Ross White Paper: Solutions to Batch Mixing Issues Page 4 of 13
PreMax Batch Ultra-High Shear Mixer
with “Delta” rotor/stator
(US Patent No. 6,000,840).
Ross Series 700 Inline Ultra-High Shear
Mixer (production model)
PreMax Batch Ultra-High Shear Mixer
The Ross PreMax is a top-entering batch ultra-high
shear mixer equipped with the patented “Delta”
rotor/stator assembly. Turning at 5,000 ft/min, the
rotor is specially contoured for high pumping capacity
and shear intensity. Product is drawn from above and
below the mix chamber then expelled radially through
the stator slots at high velocity. This generates upper
and lower vortexes allowing for extremely efficient
powder additions and rapid turnover rates.
Depending on the formulation, the PreMax can
produce results comparable to one or two passes
through a media mill. Hence, for a nanoapplication
that requires media milling, the PreMax can reduce the
number of mill passes or eliminate milling entirely.
One-pot processing in the PreMax (where raw
materials are combined and brought to the final
dispersion in a single vessel) eliminates transfer steps
and simplifies clean-up, dramatically cutting overall
cycle time while reducing cost.
Series 700 Inline Ultra-High Shear Mixers
Ross Series 700 Mixers are inline devices available in three ultra-high shear rotor/stator designs
namely X-Series, QuadSlot and MegaShear (see next page for detailed descriptions of each
design). Running at tip speeds over 11,000 fpm, a Series 700 mixer is capable of far greater
flowrates compared to a similarly-sized high pressure homogenizer or colloid mill.
Proven applications include “de-bundling” of
carbon nanotubes strands dispersed in resin,
disintegration of agglomerates of nanoceramic
particles and preparation of superior quality nano-
pigment dispersions. Compared to high pressure
homogenizers, media mills and colloid mills,
Series 700 mixers are easier to clean and disinfect
in place. Based on user experiences, the shorter
cleaning time equates not only to a faster
changeover procedure but also to longer intervals
between cleaning cycles (longer production runs).
In addition, a comparably-sized inline ultra-high
shear mixer costs less than a high pressure
homogenizer while being less sensitive to
clogging and changes in viscosity.
Ross White Paper: Solutions to Batch Mixing Issues Page 5 of 13
Laboratory Model Ross Ultra-High
Shear Mixer with recirculation hopper
Rotor/Stators for Series 700 Inline Ultra-High Shear Mixers
X-Series
The X-Series head (US Patent No. 5,632,596) consists of
concentric rows of intermeshing teeth. The product enters at the
center of the stator and moves outward through channels in the
rotor/stator teeth. The combination of extremely close
tolerances and very high tip speeds subjects the product to
intense shear in every pass through the rotor/stator. The gap
between adjacent surfaces of the rotor and stator can be
adjusted to fine-tune shear level and flow rate.
QuadSlot
The QuadSlot mixing head is a multi-stage rotor/stator with a
fixed clearance. It imparts very high shear levels to process
fluids and produces even higher flow rates than an X-Series set.
Series 700 mixers can handle viscosities up to 10,000 cP
without external pumping assistance. An auxiliary pump
extends the allowable viscosity to around 200,000 cP.
MegaShear
The MegaShear head (US Patent No. 6,241,472) is the most
aggressive rotor/stator in terms of shear and throughput. High
velocity pumping vanes force the product into semi-cylindrical
grooves, splitting it into different streams which collide at high
frequency before exiting the mix chamber.
APPLICATION SNAPSHOT
At a production facility making high-performance
plastics, a 20HP Ross MegaShear Ultra-High Shear
Mixer is being used to disperse carbon nanotubes
into a polyol vehicle. The resin-nanotube premix is
fed to the mixer and allowed to recirculate. The
mixture progressively becomes more viscous and
glossier after each pass. Temperature also climbs
which is another indication of the intense shear
imparted to the product. When the finished sample is
viewed under the microscope, the nanotube
aggregates appear to be “de-bundled” and
exfoliated. Yield is maximized when the nanotubes
are in this well-dispersed state.
Ross White Paper: Solutions to Batch Mixing Issues Page 6 of 13
First generation eductor-based powder injection system
Sub-surface Powder Injection
Aside from a shear level standpoint, the use of rotor/stators has also evolved in recent years to
include the capability for sub-surface powder injection. Earlier versions of this technology
operated based on the venturi principle: a pump (A) accelerates liquid into an eductor (B)
creating a vacuum; powder fed through an overhead tube is drawn by this vacuum into the
eductor where it joins the liquid flow; finally, a rotor/stator device (C) mixes the powder and
liquid, and propels the flow downstream.
The theory is sound but in practice balancing the performance of the pump, eductor and mixer is
often difficult. In many applications, downtime is quite high due to routine clogging. The system
is temperamental and requires a lot of operator experience and attention to operate successfully.
With three separate devices in series, clean-up and maintenance are intensive.
Newer rotor/stator designs such as the Ross Solids/Liquid Injection Manifold (SLIM) addresses
the above limitations by utilizing a specially designed rotor/stator assembly that executes the
functions of both the pump and the eductor. The ported rotor generates an intense vacuum which
draws powders right into the high shear zone of the mix chamber, where they are dispersed
instantly with the liquid stream. An auxiliary pump and eductor are not required. Consequently,
operation is simpler and more straightforward. The SLIM technology simplifies material
handling, accelerates dispersion, reduces dusting and improves operator safety.
Using the SLIM, solids are combined with the liquid sub-surface and immediately subjected to
intense shear. Solids meet the liquid at precisely the point where flow is most turbulent,
preventing agglomerates from being formed. Dispersion is virtually instantaneous and complete.
Ross White Paper: Solutions to Batch Mixing Issues Page 7 of 13
Batch SLIM. In the above
mixer configuration, powders
are loaded into a hopper. As
soon as the rotor reaches
operating speed, the hopper
valve is opened and the
solids are quickly drawn into
the batch via the vacuum
generated by the rotor.
Medium-Viscosity Nanodispersions
Higher loadings of nanoparticles and other fillers result in a premix of substantial viscosity,
rendering single-shaft devices inadequate. In addition, when the starting liquid vehicle is already
viscous, it is easier to disperse nano-sized powders using multiple independently-driven agitators
working in tandem.
For applications above 50,000 cP, multi-agitator mixers are more efficient at producing an
acceptable flow pattern within the mix vessel. In this type of mixing system, a low-speed anchor
agitator typically complements one or two high speed devices, such as a saw-tooth disperser
blade and/or a rotor/stator assembly. Multi-shaft mixers are fairly robust and can process
formulations that are several hundred thousand centipoise. Temperature is easily controlled
through use of a jacketed mix vessel and scrapers attached to the anchor agitator.
Aside from the improved capability of multi-shaft mixers over single-shaft devices from a
viscosity and heat transfer standpoint, another design advantage is that they are closed systems
and can offer benefits in vacuum mixing. When processed under vacuum, certain applications
develop higher densities and achieve better tensile properties as a result of improved shearing
and contact of the different components. Mixing under vacuum also eliminates unwanted air
voids in the finished product. With other formulations, vacuum mixing keeps entrapped oxygen
to a minimum, ensuring longer shelf life and improved stability.
Inline (Continuous) SLIM. The liquid stream (blue) enters the mixer and
immediately encounters the powder addition. Drawn into the mixer by a
powerful vacuum, the powder (yellow) is injected through the ported rotor
directly into the high shear zone. The resulting dispersion (green) is expelled
centrifugally through the stator openings and pumped at high velocity.
In lieu of the feed hopper, a “hose & wand” attachment (inset) may be
used to draw extremely dusty powders straight from within the bulk bag or
container.
Ross White Paper: Solutions to Batch Mixing Issues Page 8 of 13
A sample procedure is to load the liquid phase first into the mix vessel and then pull vacuum to
release any entrapped air. Running the anchor slowly during vacuum application assists in
breaking up air pockets that may be within the batch. This helps to control the liquid level and
prevent product from rising to the mixer cover. After this step, the nanopowders are loaded on
top of the liquid and full vacuum is again established before any agitators are turned on. The
vacuum environment will cause powder agglomerates to explode. After the batch is mixed and
returned to atmospheric pressure, liquids fill interstitial spaces between solid particles and the
result is a void-free mixture.
APPLICATION SNAPSHOT
A sunscreen product containing
nano-sized zinc oxide is
batched in a Ross VersaMix
Triple-Shaft Mixer. The solvents
and dispersant gel are first
combined and heated under full
vacuum. Once the proper
temperature is reached, the
nano zinc oxide powders are
added in increments. Ross VersaMix Triple-Shaft Mixer and X-Series Ultra-High Shear Mixer
When the formulation is complete, it is discharged from the VersaMix and transferred to the
Ross X-Series Ultra-High Shear Mixer. A true single pass through the X-Series brings down the
median particle size to the desired submicron level.
Some sample configurations of Ross Multi-Shaft Mixers.
Ross White Paper: Solutions to Batch Mixing Issues Page 9 of 13
High-Viscosity Nanodispersions
As product viscosity climbs to a million centipoise and higher, a multi-agitator mixing system
will eventually fail to produce adequate flow. This can be characterized by the anchor agitator
simply carving a path through the batch (instead of pushing product away from the walls) or by
localized overheating near the disperser and rotor/stator assemblies. At this point, agitators with
a fixed axis of rotation no longer suffice and a move to a planetary mixer is recommended. The
agitators of a planetary mixer rotate and travel through the mix vessel, passing through every
point within the batch and not just along the periphery. Batch components are constantly
recombined and physically moved from one part of the vessel to another until a homogenous
state is achieved.
Double planetary mixers (DPM)
Double planetary mixers move material by rotating two identical blades on their own axes as
they orbit on a common axis. In 36 revolutions, or just about a minute of mix time, the blades
have contacted virtually every point of the batch. The DPM’s powerful and thorough agitation
mechanism is widely proven for various high viscosity applications such as adhesives, sealants,
battery and fuel cell pastes, electronic inks, pharmaceuticals, dental compounds and composites.
Most traditional double planetary mixers utilize blades with a rectangular open paddle design.
Others feature finger blades for handling special applications such as delicate fibers. Both of
these stirrer designs are ideal for processing mixtures up to approximately 3 million cP.
Perhaps the most important engineering innovation in planetary mixers in recent decades was the
introduction of High Viscosity or “HV” blades which extended the DPM’s viscosity range to
around 6 million cP. Each HV blade consists of a pair of precisely-angled helical flights that
extend to the bottom, maintaining a close tolerance to the vessel surface. Unlike the rectangular
stirrers, HV blades pass each other in a slicing motion, enabling them to move through a very
viscous batch with less resistance. The absence of horizontal crossbars on the HV blades also
allows the agitators to be lifted easily out of a viscous batch.
Rectangular Blades Finger Blades High Viscosity Blades
(US Patent No. 6,652,137)
Ross White Paper: Solutions to Batch Mixing Issues Page 10 of 13
Ross PowerMix Planetary Disperser
(PDM Model) US Patent No. 4,697,929
Ross Planetary Dual Disperser
(PDDM Model)
The slope and spiral of the HV blade are critical to its performance. This design enables the
blade to push the product forward, inward and downward, ensuring that batch materials do not
climb up into the gearbox area. The HV blade’s unique mixing action especially benefits any
application that contains a very small amount of a nano-sized component because product
migration can easily result in an off-spec batch or lead to inconsistent quality from one lot to the
next. Based on user experiences, HV blades outperform rectangular and finger blades in terms of
mixing efficiency, operational range and cleanability.
Hybrid planetary mixers
Newer planetary mixer designs feature one or two stirrer blades (similar to those in a DPM)
supplemented by one or two high speed disperser shafts which also revolve on their own axes
while orbiting the mix vessel. These “hybrid” planetary mixers, also called planetary dispersers,
are ideal for processing viscous applications that require high speed, high shear mixing.
The classic double planetary mixer is a relative low speed device that relies on high viscosity to
impart shear to the product. By comparison, a planetary disperser’s shearing mechanism does not
rely on product being viscous. Due to its high speed disperser blade(s), the hybrid planetary
mixer can properly shear batch material during any “viscosity stage” of the mix cycle, from
water-like to up to 2.5 million cP.
Shear levels and flow patterns in a hybrid planetary mixer are easily fine-tuned because the
agitators are individually-controlled. Nano-sized powders and other solid components are
quickly incorporated into the bulk material and stubborn agglomerates are dispersed regardless
of product flow characteristics.
Ross White Paper: Solutions to Batch Mixing Issues Page 11 of 13
In a three roll mill, each adjacent roll rotates at progressively higher
speeds. For example, the feed roll may rotate at 30 rpm, the center
roll at 90 rpm and the apron roll at 270 rpm.
A typical application of hybrid planetary mixers is the preparation of viscous epoxy resin
formulations. Raw materials include resin, hardener, curing agent, catalyst, additives and fillers.
Filler materials range in particle size from nano to conventional micron size. The combination of
high speed mixing and thorough agitation in a hybrid planetary mixer ensures complete reaction
and a homogenous mixture. In addition, short cycle times and low mixing temperatures are
readily achieved in this equipment. This helps to maximize the pot life of the formulation.
It is worth noting that many filled resin formulations can also be prepared in multi-shaft mixers.
Product rheology is a main factor in mixer selection and the starting, maximum and final
viscosities must be considered in detail. In most cases, the best way to observe these different
viscosity profiles and how the product behaves at certain temperatures and shear levels is to
perform simulations on one or more mixing systems. Mixer evaluations utilizing your own raw
materials allow you to not only confirm end product quality but also analyze process efficiencies.
Three roll milling of medium- and high-viscosity nanodispersions
After being batched in a multi-shaft mixer or planetary mixer, the product may be further
processed in a three roll mill for polishing the dispersion.
A three roll mill is composed of three horizontally positioned rolls rotating in opposite directions
and at different speeds. The shear forces between adjacent rolls generate the dispersion. Gap
settings in the range of 0.001” are common.
The material to be milled
is placed between the
feed and center rolls and
gets transferred from the
center roll to the apron
roll by adhesion. Milled
material is scraped from
the apron roll by a take-
off knife. The cycle can
be repeated to improve
dispersion or until
equilibrium particle size
is reached.
The three roll mill is a
classic technology with
inherently low throughput
and requires a skilled
operator but it remains to be
one of the best methods for
preparing very fine particle
dispersions.
Ross White Paper: Solutions to Batch Mixing Issues Page 12 of 13
Ross Double Planetary Mixer and Discharge
System mounted on a common bench.
APPLICATION SNAPSHOT A vacuum-rated Ross Double Planetary Mixer with HV blades was evaluated for the
preparation of a paste premix containing micro- and nano-sized silver particles. The test batch
began with the loading of solvents and a partial amount of silver powders into the mix vessel.
Vacuum was established prior to starting the HV blades. Once the initial powders appeared
to be wetted out, the mixer was stopped and opened in order to add the remaining silver
powders on top of the batch. Vacuum was again pulled before resuming mixing. The HV
blades were very effective in keeping all the raw materials within the mixing zone at all times,
ensuring no loss of product.
The finished premix was then transferred to a three-roll mill for the polishing step. The final
milled product was a smooth, glossy and void-free paste.
A Ross Discharge System was used to
empty the premix after being processed in
the double planetary mixer; the same
discharge system was used to load the
final product into syringes.
In a discharge system, a stainless steel
platen is hydraulically lowered into the
mix vessel. An O-ring on the platen rides
against the vessel walls, literally wiping
them clean. Product is forced out through
the top of the platen or through the
discharge valve at the bottom of the
vessel. Various size and style adaptors
allow direct discharge into syringes,
cartridges and tubes.
Ross White Paper: Solutions to Batch Mixing Issues Page 13 of 13
Conclusion
A seemingly minor change in formulation, like the addition of a nano-sized component, can
present a processing challenge that calls for careful reevaluation of mixing methods and
equipment. If practical, perform actual testing on a variety of mixer designs using your own raw
materials and simulating conditions as close to your actual operation as possible. Reputable
suppliers offer a range of mixer evaluation services at their facility. Renting equipment to run
trials on your process floor or laboratory is another option. Through these programs, you can
choose a mixing technology that is completely scalable and best fits your particular
nanoapplication.