Ross Guide to Nanomixing

Transcription

1 Ross Guide to Nanomixing A White Paper Prepared By Charles Ross & Son Company

2 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 multidisciplinary 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 2 of 13

3 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 Saw-tooth disperser 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 highpressure 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 3 of 13

4 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. PreMax Batch Ultra-High Shear Mixer with Delta rotor/stator (US Patent No. 6,000,840). 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 nanopigment 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 Series 700 Inline Ultra-High Shear Mixer (production model) Ross White Paper: Solutions to Batch Mixing Issues Page 4 of 13

5 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. 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. MegaShear 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. Laboratory Model Ross Ultra-High Shear Mixer with recirculation hopper Ross White Paper: Solutions to Batch Mixing Issues Page 5 of 13

6 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. First generation eductor-based powder injection system 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 6 of 13

7 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. 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. 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. Ross White Paper: Solutions to Batch Mixing Issues Page 7 of 13

8 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. Some sample configurations of Ross Multi-Shaft Mixers. 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. Ross White Paper: Solutions to Batch Mixing Issues Page 8 of 13

9 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 9 of 13

10 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 PowerMix Planetary Disperser (PDM Model) US Patent No. 4,697,929 Ross Planetary Dual Disperser (PDDM Model) Ross White Paper: Solutions to Batch Mixing Issues Page 10 of 13

11 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 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 takeoff 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. 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. Ross White Paper: Solutions to Batch Mixing Issues Page 11 of 13

12 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 Double Planetary Mixer and Discharge System mounted on a common bench. Ross White Paper: Solutions to Batch Mixing Issues Page 12 of 13

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. Ross White Paper: Solutions to Batch Mixing Issues Page 13 of 13