Ross Guide to Nanomixing - Industrial Mixers and Blenders ... · Ross Guide to Nanomixing ... and...

Ross Guide to Nanomixing

A White Paper Prepared By

Charles Ross & Son Company

Ross White Paper: Solutions to Batch Mixing Issues 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.


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.


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.


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.


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.


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.


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.


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 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.


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 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.


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.

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