By Bob Wilcox
Fluid viscosity is a measure of resistance to deformation. Deformation can be initiated by actions such as stirring or mixing. When applied to liquids it is often referred to as thickness or resistance to flow. Contrasting examples of viscosity are provided by water and molasses just out of the refrigerator. At the highest extreme glass has sometimes been called a liquid because it lacks crystalline structure. But let’s not go there because you folks won’t be mixing glass in your lab, but instead be mixing in glass in your lab.
Viscosity is measured in centipoise or pascal units. While the science may be interesting the important point for the cosmetic scientist to deal with is, well, dealing with it. This is especially true when blending ingredients that have higher viscosities or that develop higher viscosities during processing as you add ingredients to the mix.
Viscosity and Stirring Tool Options
Three examples of lab and pilot plant mixing tools are magnetic stirrers, overhead stirrers and homogenizers. A brief look at these options may help you decide which one — or which ones as the case may be — is best for your operations.
The magnetic stirrer is a great mixing tool because it has only one moving part — a motorized rotating magnet in its base that causes a free bar magnet in the beaker or flask to rotate. The maximum stirring speed depends on the make and model. Those available from CAT Scientific have controllable speeds to 1600 rpm and can accommodate flasks to 10 liters in volume. They are also equipped with timers. Hotplate magnetic stirrers can heat your samples. The maximum temperature depends on the model but can reach 500?C (932?F).
The great thing about magnetic stirrers is that they can run unattended for hours, allowing researchers to devote attention to other matters. For long-term stirring these units can be fitted with a platinum temperature probe to provide more accurate temperature measurement. When engaged the probe actually takes over the operation of the equipment. It will initiate a shutdown in the event of a malfunction.
The weakness of these stirrers is that there is no mechanical connection between the drive magnet and the stirring magnet. Because of this they are not the best choice for highly viscous materials or those that become viscous during the stirring operation. These work best for thin cosmetic formulations.
Programmable overhead stirrers, in contrast, do provide a mechanical connection between the drive motor and the stirring tools, which are a range of paddles, impellers and blade configurations attached at the bottom of the stirring shaft. The configurations and geometries are designed to perform different types of stirring action. Because of the direct connection they deliver much higher torque than magnetic mixers and are better able to accommodate higher viscosity cosmetic blends. Their ability to do this is specified in their Ncm rating, or Newton centimeters. A higher number usually equates to more power to process higher viscosity samples but it also can indicate limitations in stirring speed. As examples:
- The CAT R80D-PC overhead stirrer for dynamic viscosity has a torque rating of 80 Ncm over its entire speed range of 20 to 700 RPM.
- The 2,000 RPM R100C unit, in contrast, has a maximum torque of 150 Ncm but is limited to 20 minutes of operation at that value.
- Our most powerful stirrer, the R100CT, has a 600 Ncm rating but only stirs to 500 RPM.
These units are equipped with a self-test function to protect the motor against overload and overheating should the sample viscosity rise beyond the equipment’s design capability. They are also capable of long-term unmonitored mixing but do not have the provision to heat the media. These are most frequently used by cosmetic formulators.
Our third example is the lab homogenizer, a tool capable of high speed mixing over a range of viscosities. Units are specified by drive motor power expressed in watts. But don’t equate watts with mixing speed. For example, our 125-watt homogenizer has a speed range of 15,000 to 35,000 rpm while our most powerful 1700-watt unit homogenizes at 2,500 to 23,000 rpm.
Mixing is done in what is called the homogenizer generator. It consists of the motor-powered rotor designed to complement the stationary stator. The rotor is attached to a drive shaft using a special wrench. The stator is screwed onto a stainless steel tube that houses the rotor shaft. The assembly is attached to the drive motor housing where the rotor shaft locks into position and the stator shaft is affixed by a tightening a knob on the housing.
Both components have sharply machined teeth designed to process viscous, low viscosity or fine samples. The complete unit is clamped to a support rod to position the generator at the proper depth and angle in the beaker or flask.
In operation the generator draws product in through the open stator base where the rotor violently propels it against and through the slots in the stator to perform the mixing action. Generator configurations can easily be changed in a few minutes.
Hopefully, one or more of these equipment options will be able to handle viscosity issues in your cosmetics lab. We’ve looked at each of them in more detail in previous Chemists Corner articles. If viscosity is not an issue then you can probably settle on the magnetic mixer. But processing creams and ointments probably requires more muscle. Check into the options available and match performance characteristics against your requirements.
Bob Wilcox has represented CAT Scientific’s family of liquid metering, homogenizing, magnetic stirring, and related laboratory equipment since 2002. In addition to heading the sales function in the US he is chief technician for the CAT equipment service organization.