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Emulsions are the most commonly used carrier systems for cosmetic applications. Lotions, creams, moisturizers, make-up, hair conditioners, sunscreens and similar products require a stable and high-quality emulsion to achieve the desired effectiveness, consistency, texture, appearance, shelf-life and many other properties.
Industry-wide, High Shear Mixers and Multi-Shaft Mixers are employed in the production of emulsions for such various end uses. These machines are available in different configurations and price points so proper mixer selection is critical. By investing in a versatile, high-throughput mixing strategy capable of handling a wide range of viscosities and shear requirements, cosmetic manufacturers gain increased flexibility to respond to ever-changing consumer tastes and market trends.
High Shear Mixers
High Shear Mixers, also called rotor/stator mixers, are commonly used in the production of low- to medium-viscosity emulsions up to around 10,000 cps. The conventional rotor/stator mixer achieves its high shear characteristics through the interaction between a rotor running at tip speeds in the range of 3,000 – 4,000 ft/minute and a fixed stator featuring vertical slots, square openings, round holes or a fine screen (Figure 1).
The immiscible phases are drawn into the mix chamber and expelled at high velocity through the stator openings. The differential speed and close tolerance between the rotor and stator generate high levels of hydraulic shear which, in conjunction with chemical surfactants or emulsifiers, create the emulsion.
Droplets of the dispersed phase are reduced in size with every pass through the rotor/stator but with diminishing effects. The greatest extent of droplet size reduction occurs within the first few passes where the distribution becomes narrower and tighter. Past this stage of sharp decrease in droplet size, the emulsion hovers at an equilibrium state despite subsequent mixing. It is useful to accurately determine this equilibrium point to avoid unnecessary over-heating and over-processing.
Multi Shaft Mixers
Products that undergo viscosity changes peaking well over 50,000 cP are typically batched in Multi-Shaft Mixers. These machines are equipped with two or more independently-driven agitators working in tandem to deliver a combination of high shear agitation and laminar bulk flow.
A Triple-Shaft Mixer featuring a low-speed anchor, a rotor/stator assembly and a saw-tooth disperser blade is particularly ideal for creating highly-filled cosmetic emulsions. The wings of the anchor agitator normally include adjustable scrapers for wiping the vessel bottom and sidewalls. This allows for tighter temperature control in addition to enhanced product turnover. The saw-tooth disperser generates a vortex in the liquid surface for quick incorporation of solid ingredients and, along with the anchor, continues to agitate the batch when it becomes too thick to flow through the rotor/stator.
Processing tip: Mix under vacuum
Multi-Shaft Mixers are often built for vacuum operation, typically supplied with a built-in vacuum pump. A vacuum environment accomplishes a number of mixing goals including improved aesthetics (imagine a smooth cream or clear gel with no air bubbles) and removal of oxygen to prolong shelf-life, prevent decomposition of sensitive ingredients or thwart unwanted chemical reactions and microbial growth. Confirm the benefits of vacuum mixing on your particular application. If practical, visit the laboratory of a reputable mixer manufacturer and test-run a Multi-Shaft Mixer using your own raw materials, replicating your process environment as closely as possible.
Consider Inline Ultra-High Shear Mixing
Every emulsion has a shear threshold in which the optimal droplet size and characteristics will develop. Insufficient shear produces droplets that are too large; they coalesce rather quickly and eventually separate from the continuous phase. On the other hand, excessive shear can irreversibly damage the product.
Of course, apart from mixing intensity, several other factors affect emulsion stability such as the type and concentration of surfactant, order of addition, processing temperature, etc. But when dealing with challenging emulsions that conventional rotor/stator mixers cannot successfully produce, manufacturers are compelled to switch to more aggressive, higher energy devices such as colloid mills and high pressure homogenizers. The downside is that these are comparatively expensive machines and commonly associated with a number of issues including:
- low throughput
- tendency to clog
- time-consuming clean-up
- intensive maintenance
New developments in rotor/stator technology offer a number of practical alternatives to traditional emulsifying equipment. Inline Ultra-High Shear Mixers are capable of producing a wide range of very fine emulsions within a single-pass. Equipped with special rotor/stator generators designed to run at tip speeds over 11,000 ft/min, Ultra-High Shear Mixers are high-throughput machines that are just as easy to operate and maintain as conventional rotor/stators mixers.
High tip speeds and complex, turbulent mixing patterns enable Ultra-High Shear Mixers to impart extremely high levels of hydraulic and mechanical shear without sacrificing throughput and even allow manufacturers to potentially reduce surfactant levels, reaping significant savings in raw material costs.
In many cosmetic applications, Ultra-High Shear Mixers deliver comparable, if not better, emulsification results compared to colloid mills and high pressure homogenizers. Ultra-High Shear Mixers are available in laboratory (3” diameter rotor) and production sizes (up to 15” diameter rotor). Sanitary models are also available for processing sensitive applications with provisions for CIP/SIP (cleaning and sterilization in place).
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