Effective mixing technology: COSSMA

Not just the ingredients and the formulation are decisive for a good product. The manufacturing process also influences the quality. Ulf Sieckmann explains how a special technique can be used to achieve a high sun protection factor with little use of materials.

The perfect dispersion of titanium dioxide (TiO2) in high viscous liquid phases in the shortest possible time is the key for highest process efficiency and maximum quality standards.

A special homogenisation technology enables very effective process times and quality advantages in sunscreen manufacturing processes.

The advantages are

higher SPF with less titanium dioxide
higher stability and longer shelf lives
shorter process times
large scale production
perfect particle size distribution
shorter cleaning times
100% process repeatability
safer and controlled raw material handling
right-first-time scaling

The combination of a rotor-to-rotorhomogeniser¹, coaxial agitator system, programmable vacuum induction system and secondary circuit temperature control in vacuum-mixing- and homogenising systems provide full control over all process steps with 100 % process repeatability, independent from operator experience and influence.

Relation of total energy input and operating diagram of homogeniser systems. figures: symex

Key-technology

The core is a rotor-rotor-homogeniser with independent control of shear and flow, providing a shear rate range from 0 to 120,000 1/s (0-60 m/s) at all flow rates. While rotor-stator homogenisers, mills and high-pressure homogenisers can only handle shear and flow in mutual dependence, this technology can provide high shear at low flow as well as low (or even no) shear at high flow rates by just setting two parameters in the control system. This technology allows to set the optimal shear-flow combination for a very fast particle size reduction (down to primary particle size) which opens the maximum specific surface of the TiO2 in the product.

Achieving the maximum level of specific surface means the most efficient utilisation of the TiO2 in the product and results in a higher SPF with less raw material. At the same time, the optimal shear-flow-combination also ensures the optimal (homogeneous) droplet size distribution of the carrier emulsion, which is key to highest stability and shelf life. Process times of less than four hours for sunscreen products in 10 ton. scale, including product discharge and cleaning are easily achievable with all process steps fully automated (even manual steps) and documented.

The principle

While the rotor stator principle can only turn the rotor to generate a shear rate in the gap between the rotor and the stator, this system can also turn the “dynamic stator”, in fact in both directions. Two independently VFD driven motors allow independent rpm and direction of rotation of both the rotor and the “dynamic stator”. The rotor with pump blades and shear teeth turns in one direction, only as it is optimised for an infinitely variable and smooth flow curve. The “dynamic stator” is equipped with shear teeth, but can rotate in both directions, the same as the rotor or counter the rotor.

The use of water-cooled motors even allows the “dynamic stator” to “turn” with 0 rpm so that the rotor-rotor-homogeniser can represent the rotor-stator-principle. This option makes the system fully scalable to all traditional homogenising systems.

These are the typical operation modes of the rotor-to-rotor homogeniser

High flow – no shear

Rotor and dynamic stator turn simultaneously in the same direction. There’s 0 m/s speed difference bet-ween rotor and dynamic stator result in 0 1/s shear rate. It is often used for very shear sensitive products that require an extensive mixing and for extreme fast product discharge without impact on the product properties.

High flow – high shear

Rotor and dynamic stator turn in opposite directions. As the dynamic stator can achieve the same rpm as the rotor, but into the opposite direction, the technology can reach double the shear rate as a traditional rotor-stator-system. 60 m/s differential speed between the two rotors result in shear rates of 120,000 1/s on standard rotors. Special rotors for higher shear demands can achieve shear rates of 240,000 1/s.

This mode represents the efficiency of mills and high-pressure homogenisers.

Combination between the extremes

The independently driven rotors with variable speeds allow the combination of any flow and shear rate that is best for the process and product. 5 m/s shear speed at maximum flow rate to quickly disperse a highly shear sensitive gel (carbomer) or 45 m/s shear speed at minimum flow rate to extend the residence time of the particle in the shear gap for maximum grind impact are both just a question of two parameters in the control system.

External recirculation system

Highest flexibility does not only mean that all product types can be manufactured in the same system. Flexibility also means that small and large batches can be made in one system size.

The rotor-to-rotor homogeniser is integrated into an external product recirculation system that works according to the “First-In-First-Out-Principle”. This not only allows the true scaling by batch-turnover (and not time), but also guarantees the manufacturing of batch sizes from 15 % to 100 % with identical process steps and product properties.

While the long loop returns the product from the homogeniser back into the mixing system above product level, the short loop ends with a special flow nozzle in the bottom of the mixing system.

The short loop product returns under product level with a special flow pattern avoids vortexes, splashing and aeration of the product even at lowest filling volumes. A simple switch from short loop to long loop during the process always enables the best access to the optimal settings for the individual process steps.

Influences of energy input to droplet size distribution curves. figures: symex

Raw material merging

The vacuum induction system for powders and liquids is located between the mixing system vessel bottom and the homogeniser head. Programmable (sequencing) valves open and close automatically in product specific intervals and merge the optimal amount of material into the homogeniser head for an instantaneous dispersion. This principle works for powders and liquids. A highly hydrating powder can be inducted and can be dispersed immediately before it hydrates and forms the typical clumps that usually take time long time deagglomerate and dwell.

A special hot-cold-induction port even allows the phasing of a hot oil phase into a cold aqueous phase during emulsification. The special transfer system with a controlled transfer rate guarantees an emulsification before solidification when the hot oil phase meets the cold aqueous phase.

The hot-cold-emulsification is not only a significant process time reducer, but also a significant contribution to a high-level of sustainability.

CIP – SIP

Fully automated CIP and SIP processes with no manual operator interference and lowest water/detergent consumption enable the highest efficiency and availability for production.

Various individual process steps and individual cleaning parameters allow the optimal creation of cleaning recipes for each individual product. Typical (complete) CIP-times range between 45 and 60 minutes for “hard-to-clean-products” and consume water amounts of about 15 % of the vessel volume.

The systems are available from 4 to 20,000 litre batch size with 100 % scalability and in all executions from standard to full-pharma. A large variety of technology and size options allow a fully customised solution for all purposes, locations, and applications.

This homogenising technology offers a lot of new solutions in terms of manufacturing process time and product quality for many cosmetic products – not only sun care.