Sponsored by Charles Ross & Son Company
Regulatory requirements in the past few decades have prompted the coatings industry to refine conventional low-solid, solvent-based formulations and develop more waterborne systems, high-solids coatings, energy-curable inks and other low-VOC technologies. Along with this important environmentally-responsible shift, the changing needs of modern businesses and consumers ushered in exciting developments in formulation and functionalities.
Now faced with a growing demand for specialty and high value-added coatings, manufacturers are taking a closer look at their processing methods, particularly the mixing and milling steps. Coatings producers are striving in earnest to improve performance and compliance while managing costs.
Infused with thermochromic, fluorescent, color-shifting, rust-inhibiting or other novel properties, many modern paints, inks and coatings contain unique combinations of innovative raw materials. But fundamentally, they remain formulations that rely on good mixing. The degree of mixing and dispersion invariably affects a variety of characteristics, such as:
- adhesion properties
- curing rate
It makes good business sense to optimize one’s mixing procedures. Even traditional products and well-established processes can benefit from a strategic reassessment of the mixing operation.
As illustrated in the following sections, manufacturers employ various dispersion tools in the production of paints, inks and coatings. Some of these technologies are relatively new solutions to age-old processing issues. Mixer selection is based on a number of factors including:
- viscosity profile
- shear requirement
- order of addition
High Speed Dispersers
The High Speed Disperser, also sometimes called High Speed Dissolver, is a standard workhorse in the coatings industries. An economical and relatively simple piece of mixing equipment, its primary purpose is to incorporate powders into liquid and break down loose agglomerates to produce an acceptable level of dispersion prior to milling.
Running at tip speeds up to around 5,000 ft/min, the open disc blade of the High Speed Disperser creates vigorous turbulent flow within a low viscosity batch. It also generates a characteristic vortex into which dry ingredients can be added for quick wet-out. As the batch thickens or increases in volume, blade speed is adjusted to maintain the vortex and rate of material turnover.
While many manufacturers still rely on High Speed Dispersers for pre-mix operations, a good number have started to integrate more powerful mixers that go beyond simple powder wet-out. The rationale behind this is if a pre-mixer is able to disperse agglomerates as close as possible to the desired specifications – in other words, mimic the early stages of milling – it can reduce the number of passes through the mill and even allow the use of smaller grinding media for milling finer solids.
High Shear Mixers
High Shear Mixers are rotor/stator agitators used for more challenging solid-liquid dispersions and emulsions which High Speed Dispersers cannot adequately process. This type of mixing device typically features a four-blade rotor turning at high speeds within a stationary stator. As the blades rotate, materials are continuously drawn into the mixing head and expelled at high velocity through the openings of the stator. The resulting hydraulic shear promotes fast homogenization, deagglomeration and emulsification. Typical rotor tip speeds are between 3,000 and 4,000 ft/min. High Shear Mixers are available in both batch and inline (continuous) designs.
Ultra-High Shear Mixers
Some new rotor/stator technologies now enable coatings manufacturers to wet out powders while also accomplishing some level of grinding and deagglomeration right in the same mixing vessel.
The Ross PreMax Ultra-High Shear Mixer is a top-entering batch mixer equipped with the patented “Delta” rotor/stator (US Patent No. 6,000,840). Supplied with a wear-resistant Stellite bushing, the rotor turns at tip speeds up to 5,000 ft/min and is uniquely contoured for high pumping capacity. Product is expelled radially through the stator slots at high velocity while new material is continuously being drawn from above and below the mix chamber. This generates upper and lower vortexes allowing for extremely efficient powder additions and rapid turnover rates.
High Shear Mixers for Powder Injection
When large amounts of powders need to be added quickly into liquid or when hard-to-disperse solids take too long to completely incorporate, a High Shear Mixer with built-in sub-surface powder induction capabilities is worth considering.
Ross Inline SLIM: The liquid stream (1) enters the mixer and immediately encounters the powder injection (2) at the high shear zone of the rotor/stator assembly. The resulting dispersion (3) is expelled centrifugally through the stator openings at high velocity.
The PreMax is typically used as a stand-alone unit and does not require supplemental agitation for products up to 50,000 cP. For more viscous products, it can be used in combination with an anchor sweep or other type of low-speed agitator.
Also available are Inline Ultra-High Shear Mixers which are capable of even more intense deagglomeration than the batch-style PreMax. Examples are the Ross X-Series, MegaShear and QuadSlot, all designed to run at tip speeds over 11,000 ft/min. These mixers cost considerably less than a high pressure homogenizer while being less sensitive to clogging and changes in viscosity. In almost all applications, an Inline Ultra-High Shear Mixer delivers greater particle size reduction and throughput compared to a conventional colloid mill.
Equipped with two or more independently-driven agitators working in tandem, Multi-Shaft Mixers are robust systems that deliver both high shear agitation and laminar bulk flow within a wide viscosity range: from water-like to several hundred thousand centipoise.
The simplest design is the Dual-Shaft Mixer which features a low-speed anchor and a high-speed saw-tooth disperser blade. The wings of the anchor agitator usually include adjustable scrapers for wiping the vessel bottom and sidewalls. This allows for tighter temperature control in addition to enhanced product turnover.
Another typical configuration is the Triple-Shaft Mixer which includes an additional rotor/stator assembly. This configuration is popular for formulations wherein the final particle size distribution is critical. As discussed earlier, using a saw-tooth blade to incorporate powders into liquid results in acceptable levels of dispersion, but applying a more shear-intensive mechanism such as rotor/stator mixing typically results in a finer and more uniform particle size distribution.
Specialty paints, inks and coatings that undergo very high viscosity peaks (above 1 million cP) are better prepared in Planetary Dispersers and Double Planetary Mixers. These machines feature two or more blades which rotate on their respective axes while revolving around the mix vessel. In other words, all agitators continually advance into the batch and contact fresh product all the time.
Combining slow-speed planetary agitation with an orbiting high speed disperser, the Ross PowerMix Planetary Disperser (US Patent No. 4,697,929) quickly incorporates powder additions into a thick liquid base. Each agitator is independently controlled so flow patterns and shear rates are easily fine-tuned with every change in batch rheology. Since the disperser is constantly moving through product, the PowerMix is able to deliver shear to high viscosity materials with minimal heat build-up.
For very challenging formulations, a Planetary Dual Disperser may be utilized. This mixer consists of two disperser shafts (each having two saw-tooth blades) and two planetary stirrers.
The classic Double Planetary Mixer (DPM), on the other hand, is ideal for melting and kneading semi-solid materials or highly viscous pastes. While not considered a high speed mixer, the DPM is equipped with two identical stirrers which impart increasing levels of shear as the batch gains considerable viscosity. A common processing technique in this very robust machine is mostly high viscosity mixing to ensure satisfactory solids dispersion (from 2 million cP up to around 6 million cP), followed by a let-down step towards the end of the cycle.
|MIXER DESIGN/CONFIGURATION||TYPICAL VISCOSITY RANGE|
|High Shear Mixer||Up to 20,000 cP|
|High Speed Disperser||Up to 50,000 cP|
|Multi-Shaft Mixer||Up to 500,000 cP|
|PowerMix Planetary Disperser||Up to 2 million cP|
|Double Planetary Mixer with Rectangular Blades||Up to 3 million cP|
|Double Planetary Mixer with Helical HV Blades||Up to 6 million cP|
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.