What is shear, and the equipment to use.
To understand what rheology modifiers are, you first need to understand what shear is.
Rheology is often defined as “the science of how things flow” and is a requirement of personal care that they flow in the right way. When a product is thicker, the more resistant the flow the product will be.
In the cosmetic industry, you will hear the word “shear” quite so very often, so familiarising yourself with this word is a must. Many raw materials require high shear to disperse properly in a formulation.
Shear is the force required to cause movement within a product.
Think of a jar of cream, if you rub your finger over the surface of the cream, the cream will move, this is due to shear being applied.
When using rheology modifiers, it is best to contact your supplier for the MSDS as this will normally include the type of shear needed for your modifier of choice. Method of use for any raw material is just as important as the formula itself. When processing your modifier, if you use an incorrect method, you risk your formula becoming unstable. Processing your modifier incorrectly will cause the material to become irreversibly damaged, losing any chance of building viscosity.
When it comes to shear, you need to ensure you are using the correct equipment to be able to carry out low, medium, and high shear.
1. Low shear: The mixer needed is a Paddle or Propeller- rpm speed- 0-2,000rpm
2. Medium: The mixer needed is Planetary- rpm speed 0-2,000
3. Medium-High: The mixer needed is an Impeller- rpm speed 0-2,000
4. Very High: The mixer needed for high shear is the Homogeniser- rpm speed 3,000- 25,000
When it comes to your formulation method remember, it is not the speed that makes mixing low, medium, or high shear, it’s the type of stirrer you use that will determine the shear needed for the modifier you chose.
Although speed being used may help build your emulsion, it is not what builds the viscosity in your rheology modifier, in fact some rheology modifiers become irreversibly damaged if introduced to high shear, an example being Polymers. The blades in a homogeniser will break certain modifier particles too small rendering them damaged.
Another method you may see low, medium, or high shear, though not so common, is by the speed of the mixer. Knowing the speed, can help determine which mixer is needed for your materials.
The speed of the mixer is usually measured as RPM (Revolutions Per Minute), for example: some Xanthan Gums require to be stirred at 1,500rpm, this tells us that the gum needs to be mixed at low-medium shear.
As formulators it is always best to refer to low, medium, or high shear while writing your formulations, especially when preparing for a manufacturer as you will never know the kinds of equipment they have on hand.
Another incorrect method for stirring is “stir for 5 minutes”. It is best to never use this method, as when a formulation is handed over to a manufacturer, it is safe to say that their batch sizes will be larger, therefor what may take 5 minutes to stir in a lab batch will take longer in a scale up batch due to the increase of raw materials.
What is a Rheology Modifier?
You have now heard me mention rheology modifier quite a bit, but what is a Rheology Modifier?
When you hear the word Rheology modifier in cosmetics, it is often referred to your thickeners. This all includes your water-based thickeners and your oil-based thickeners, which we are just about to get into detail with.
Rheology modifiers help to build viscosity within your formula as well as stabilising it. The more % used of a modifier, the thicker your formulation will become. When working with your natural gums, the higher % used, your formulation will become more gel like. In some formulations this is not ideal, so please chose your rheology modifiers carefully.
We all know that when we add oil into a cup of water, it separates and floats to the top, that is because the water likes to stick together, and the oil likes to stick together.
Oils have their own specific charge, without a rheology modifier there is nothing to stop the oil from drawing together. Your rheology modifier helps to build barriers around the oils to stop them from joining and floating back to the top of the water phase in your formula and keeps your oils dispersed. Although rheology modifiers play a huge part within building a stable formulation, you cannot use them solely, you will need an emulsifier.