Introduction
Rheology refers to the how materials flow, the more vicious (thick) a product is the more resistant it is to flow. For example think of the difference between juice and clotted cream, because juice is less vicious than clotted cream it will flow more easily than clotted cream will. When force is applied to a product this is known as ‘shear’ for example if you tip a container of clotted cream upside down it will not flow out of the container, however if you scoop it out with a spoon the force of spoon will cause it move this is an example of using shear to use or move a product. Watch the video below to see an example of how we use shear force in cosmetics.
What is a rheology modifier
A rheology modifier in basic terms is a material used to thicken or thin a formula. Remember the resistance to flow, the more viscous a product is the more resistant it is to flow. Sometimes resistance to flow in a cosmetic product is desired such as a cream and sometimes it is less desired for example a toner.
Why is this relevant in cosmetics products?
Different cosmetic products have different viscosities. For example consider the difference between a cream and lotion. Generally speaking the main difference between the two is the viscosity. So then if that is the only difference why do we have both creams and lotions. Great question! The main reason is consumer perception, we perceive a cream to be more moisturising than a lotion, although strictly speaking this isn’t true but consumers believe it is and there’s no changing that.
Another reason rheology is important in cosmetic formulation is the consumer experience. For example, we expect our shower gels to have a gel like viscosity not because it cleans us better but how annoying would it be if our shower gels flowed as freely as water - think of all the waste. Whilst technically it would wash us just as well, consumers would be a lot less inclined to buy the product because it would decrease the overall experience of using it.
Now with that said shear is another important aspect to consider. Whilst a cream must be thick to meet consumer expectations it must easily rub into the skin. Think about cocoa butter, it’s seldom used as a sole moisturiser because it requires extreme force to apply. So thinking back to our cream example whilst it must be viscous or resistant to flow, it shouldn’t be so viscous that the shear force of our fingers couldn’t rub it into our skin as that would decrease the consumer experience. Take a moment to think about all the cosmetic products you use - toothpaste, moisturiser, shampoo, conditioner, shower gels etc, consider their viscosity what do you expect from the experience of using these products you will then see how important rheology is in cosmetics and why it matters.
The rheology of a product also impacts the type of packing we use, for instance why wouldn’t we put a shower gel in a wide neck jar? Whilst shower gel isn’t as free flowing as water is, it is free flowing enough to make using it in a wide neck jar a pain and decrease the consumer experience. Why wouldn’t we put a cream is a serum bottle? Because it would be a pain to get out of the bottle and reduce the consumer experience.
Different types of rheological modifiers in cosmetic formulation
Xanthan Gum - This is one of the most common and versatile rheological modifiers if you look at the ingredients list you will find it listed on many products. Xanthan gum has no significant benefit to the skin, it does not greatly increase the effectiveness of a formula it does however greatly impact the consumer experience. It’s cheap, easily available and a natural material.
Guar Gum - Guar gum and its derivatives is another extremely common rheological modifier. It’s worth noting that whilst guar gum is a natural ingredient many of its derivatives on the market are synthetically produced to enhance its properties.
Emulsifiers - Whenever you mix oil and water you need to use an emulsifier. There are different types of emulsifiers used for different purposes depending on what type of formula you are trying to achieve. Emulsifiers such as BTMS 50 or Olivem 1000 tend to increase the viscosity of a product these are commonly used in conditioners and creams where a vicious finish is desired. Polysorbate 20 is also an emulsifier these are commonly found in toners, or room sprays where we want a small amount of oil to suspend in a large amount of water the finished viscosity tends to be quite thin and free flowing as is desired in products such as toners or room sprays.
How do we use rheology modifiers
Usually in cosmetic formulation when we use a rheology modifier we tend to use either xantham gum or guar gum and its derivatives. When using these gums we do not add it straight into the water phase we first make a slurry by mixing a humectant such as glycerin with the gum, before adding it to the water phase. This makes it a lot easier to disperse. Try adding 1g of xantham gum to distilled water and you will see it sits in small clumps at the top as opposed to dissolving in the water. Now try mixing 1g of xanthan gum with 5g of vegetable glycerin, make sure its throughly mixed through and the xanthan gum has fully combined with the glycerin. Then add this mixture to 94g of distilled water and you will see how it forms a gel like consistency. This is probably the most common method used to change a products rheology - turning liquid water into gel.
Conclusion
In this blog we have introduced what rheology is and why it is important in cosmetic formulation. We have given some examples of rheological modifiers and how we use them. We have explained the concept of shear and how it relates to rheology. I hope this blog has answered your questions on rheology in cosmetic formulation, why and when it is relevant and how we modify the rheology of our cosmetic products.