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Oct 290 min read
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When it comes to cosmetic formulation, one of the most crucial steps in achieving a stable and well-blended product is the mixing process. While the choice of ingredients, emulsifiers, and preservatives is important, how you mix your formulation can make or break the final product. In cosmetic chemistry, we often refer to two types of mixing methods: high shear and low shear. Understanding the difference between these techniques, and knowing when to apply them, is critical for formulators who want to create stable emulsions, creams, lotions, and other products.
In this blog post, we will delve into the differences between high shear and low shear mixing, explain when and why you need each method, and highlight special considerations, such as working with shear-sensitive materials like carbomer.
What Is Shear?
Before diving into high shear and low shear mixing, it’s essential to understand the concept of shear. In simple terms, shear refers to the force applied to a substance that causes it to deform or move. In mixing, shear is the force that breaks down materials, allowing them to blend or mix together more thoroughly.
Low shear applies gentle force, slowly moving and combining ingredients without disrupting their molecular structures too much.
High shear uses intense force to break down and disperse materials at a microscopic level, ensuring even blending of all components.
Both methods have their place in cosmetic formulation, but knowing which one to use is key to achieving the desired product texture, stability, and consistency.
High Shear Mixing
High shear mixing involves using powerful equipment, such as a high-shear mixer, homogenizer, or rotor-stator device, to apply intense force and speed to a formulation. This process is crucial for creating fine dispersions or stable emulsions, where the oil and water phases need to be combined in a uniform and consistent manner.
When to Use High Shear Mixing
High shear mixing is essential when working with formulations that contain oil and water, as it is necessary to break down the oil droplets into smaller sizes. This creates a stable emulsion, where tiny droplets of one phase (usually oil) are dispersed evenly throughout another phase (usually water). The more finely dispersed the oil droplets, the more stable and smooth the final emulsion will be.
Emulsions: Whether you’re making an oil-in-water (O/W) or water-in-oil (W/O) emulsion, high shear mixing is typically required to create the right droplet size and stability. Without high shear, the oil and water phases may separate, leading to an unstable product that breaks down over time.
Dispersions: High shear mixing is also used to disperse solid particles or pigments into a liquid. This is common in color cosmetics, like foundations and liquid lipsticks, where pigments need to be evenly distributed for consistent color payoff.
Benefits of High Shear Mixing
Enhanced Stability: High shear ensures that the oil droplets are reduced to a microscopic size, which prevents separation and enhances the overall stability of the emulsion.
Smooth Texture: The fine particle size achieved through high shear mixing results in a smoother, more elegant texture in creams, lotions, and emulsions.
Better Performance: Active ingredients that need to be dispersed uniformly for optimal performance benefit from high shear mixing.
Example: Creating Emulsions with High Shear
Let’s take the example of creating an oil-in-water (O/W) emulsion, such as a basic lotion or cream. The water phase typically contains water-soluble ingredients like humectants and stabilizers, while the oil phase contains oils, emulsifiers, and sometimes waxes. To create a stable emulsion:
Both the water and oil phases are heated to the correct temperature, often around 70°C.
Once the phases are combined, a high shear mixer or homogenizer is used to break down the oil phase into fine droplets, which are dispersed throughout the water phase.
The high shear process allows the emulsifier to coat the oil droplets, preventing them from coalescing and separating.
Low Shear Mixing
In contrast to high shear, low shear mixing involves gentle stirring or folding of ingredients. This method is used when delicate ingredients need to be incorporated into a formulation without disrupting their structure or causing degradation.
When to Use Low Shear Mixing
Low shear mixing is typically used when adding heat-sensitive or shear-sensitive materials that could be damaged by high-intensity mixing. Some ingredients, especially polymers and gelling agents, require low shear to maintain their functionality and stability. If exposed to high shear forces, these ingredients can break down or lose their ability to perform as intended.
Shear-sensitive ingredients: For example, carbomers, a group of thickening agents used in gels and creams, are highly shear-sensitive. When exposed to high shear, carbomers can lose their viscosity or thickening power, resulting in a product that does not have the desired consistency. Therefore, they should only be incorporated using low shear mixing.
Heat-sensitive ingredients: Ingredients like Vitamin C, retinol, or certain botanical extracts may also be sensitive to heat or intense mixing. After the primary emulsion is formed with high shear, these ingredients can be added during the cool-down phase using low shear mixing to prevent degradation.
Fragrances and Essential Oils: Adding fragrances or essential oils at the end of the formulation process also requires low shear mixing. These volatile compounds can evaporate or break down if exposed to high shear or high temperatures.
Benefits of Low Shear Mixing
Preserves Sensitive Ingredients: Low shear mixing ensures that delicate or shear-sensitive ingredients retain their efficacy and stability.
Prevents Product Breakdown: Using low shear prevents the risk of destabilizing the formulation, especially when incorporating ingredients like thickening agents or sensitive actives.
Example: Adding Carbomer with Low Shear
Let’s say you’re formulating a facial gel that uses carbomer as the primary thickening agent. Carbomer is a polymer that creates clear gels and provides excellent thickening properties.
However, it is highly sensitive to shear forces.
After preparing the base of the gel, you would add the carbomer during the cool-down phase using low shear stirring, ensuring it disperses evenly without breaking down the polymer structure.
Once the carbomer is fully incorporated, a neutralizing agent, like triethanolamine (TEA), would be added to adjust the pH and allow the carbomer to thicken the product.
If high shear mixing were used at this stage, the carbomer could lose its thickening ability, resulting in a thin, watery product.
When to Combine High and Low Shear Mixing
Many formulations require a combination of both high shear and low shear mixing. The high shear process is typically used at the beginning stages to create a stable emulsion, while low shear is employed during the cool-down phase to add sensitive ingredients like active compounds, preservatives, or fragrances.
For example, when making an emulsion, the oil and water phases are typically combined with high shear to ensure the stability of the emulsion. Once the emulsion is formed, and the formulation cools down to around 40°C, low shear mixing is used to add heat-sensitive ingredients such as vitamins, preservatives, or botanical extracts.
Final Thoughts: Understanding the Importance of Shear
The difference between high shear and low shear mixing is not just about the speed or force of mixing—it’s about understanding the science behind your formulation and how each ingredient behaves under different conditions. High shear mixing is crucial for creating stable emulsions and dispersing solid particles, while low shear is essential for preserving the integrity of sensitive ingredients.
As a cosmetic formulator, knowing when to apply high shear or low shear mixing will ensure that your products not only look and feel great but also maintain their stability and effectiveness over time. Whether you're working with emulsions, gels, or creams, always consider the specific requirements of your ingredients to achieve the best possible results.
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