Why emulsions and foaming formulas fail stability testing

In order for a cosmetic formula to be commercially successful, it needs to sustain a minimum two-year shelf life.

While every cosmetic formula needs to be considered based on its individual components, packaging, storage requirements and consumer use, read on to find out the most common reasons which cause emulsified and foaming cosmetic formulas to fail their expected shelf-life requirements. Watch an overview of how to conduct stability testing of cosmetic formulas in this video.

Unacceptable changes to oil-in-water emulsions can include colour and/or aroma changes, separation and waxy/grainy appearance changes. Watch a summary of why emulsion formulas might fail stability testing in this video.

Here is a summary of when and why those changes may occur.

• Colour and/or aroma changes:
  • if colour change is accompanied with aroma changes, it is most likely oxidation. Add more antioxidant to the next formula and/or reduce oxidisable materials (plant oils and essential oils). Check also the quality of the antioxidant raw material – antioxidants have a shelf life too.
    
  • microbial contamination usually appears as odd coloured black, green or red/pink spots in a product. Check if it is microbial contamination by conducting microbial testing – if results come back within limits, it is not a contamination issue. If results come back showing contamination, check preservative compatibility, input amount and quality of preservative.
    
  • acid environments can turn an emulsion pink while alkaline environments can turn an emulsion yellow/olive/green – if this happens a chelating agent is required.
     
• Separation:
  
  • separation into water and oil or cream phases usually means there is too much oil, insufficient emulsifier and/or insufficient stabiliser. To fix this issue, check and adjust the lipid, emulsifier and polymer content to improve lipid:emulsifier/stabiliser ratio.
    
  • make sure any gums or polymers used to aid stabilisation have been added in a suitably compatible environment and using the correct method.
    
  • if there is also a significant viscosity and/or pH drop, there are other issues occurring, possibly oxidation or incompatibilities - check the formula for possible oxidation or incompatibility issues (especially if actives, excess electrolytes or acids/bases are present).
    
    
• Waxy and/or grainy appearance:
  
  • a waxy or grainy appearance generally shows that oil and water phases have been combined when one (or both) was below the melting point of the waxes. If this occurs during manufacture, you can reheat if no heat sensitive materials have been added. Make sure future production batches ensure both phases reach at least the melting point of waxes before combining, and ensure glossy emulsification before cooling starts.
    
  • if crystallization occurs at low temperatures, there is too much ionic emulsifier: non-ionic emulsifier ratio. Reduce the ionic emulsifier and/or increase the non-ionic emulsifier content.
    
  • it may also mean insufficient mixing when the phases were combined. If this is the case, try increase mixing time (and shear speed if suitable) to ensure very small droplet size especially as batch size increases. 

Unacceptable changes to water-in-oil emulsions are similar to oil-in-water emulsions.

The reason for colour changes and a waxy or grainy appearance could be the same as oil-in-water emulsions, however reasons for separation are very different. 

• Separation:
  
  • separation in water-in-oil emulsions typically occurs when too much water is used, insufficient low HLB emulsifier has been used, or insufficient oil compatible stabilising agent has been used. To fix these issues, try checking and adjusting water input, emulsifier input and stabilising agent.
    
  • only low HLB emulsifiers should be used in these types of emulsions. Only oil compatible thickeners should be used, and may be the reason the formula fails. Check all ingredients carefully, and make sure they are suited to water-in-oil formulas specifically!  

Foaming or solubilised cosmetic formulas can undergo aroma and/or colour changes, a powdery or lipid layer forming on top, a gummy layer forming, a drop in viscosity over time, and/or flocculation or sedimentation of particles or pigments. Watch a summary of why foaming formulas might fail stability testing in this video.

Here are common reasons why:

• Colour and/or aroma changes:
  
  • reasons why these changes may occur in oil-in-water formulations apply to foaming formulas too. Revisit that section and see if any of those reasons are relevant.
    
  • chelating agents may be needed in foaming formulas to help prevent colour changes over time. This is especially relevant where visible precipitation forms in the foaming product.
    
    
• Powdery or lipid layer forms on top:
  
  • a white powdery looking layer or oily layer can form on top of a foaming formula when too much lipid has been used compared to the surfactant input.
    
  • lipid input should be limited to 1% maximum, and should be mixed with a suitable input of surfactant or solubiliser within the formula before it is mixed with the water portion.
    
    
• Gummy layer forms:
  
  • gums are a common way to stabilise natural surfactant formulas, however, method is crucial in order for them to effectively hydrate and stabilise the formula.
    
  • gums and polymers should generally be added after the surfactants and water content have been mixed, unless supplier instructions indicate otherwise.
  • check the correct method and final pH environment have been used to ensure full hydration of the gum/polymer to avoid the formation of a gummy layer.
    
    
• Drop in viscosity over time:
  
  • a common cause of viscosity dropping in a surfactant formula is a drop in pH because of oxidation. Check what might be causing the oxidation!
    
  • it could also be any actives or essential oils used, as these are another common cause. You may need to remove the active ingredients or essential oils completely if this is the case, or at the very least, try using different actives or fragrances.
    
  • if salt has been used to thicken the product, any changes to pH will have a big impact on viscosity.
    
  • you may need to add a suitable gum or polymer where the pH has not changed and that is not a cause. 
     
• Flocculation or sedimentation of particles/pigments:
  
  • if you are adding particles or pigments to a foaming formula, you will normally need a gum or polymer to help stabilise the finished product.
    
  • flocculation or sedimentation of particles or pigments would therefore be related to incompatibly of the gum or thickener, insufficient input, or incorrect processing. Try adding more gum or polymer to help improve stability.
    
  • revisit the technical data of the gum or polymer carefully to ensure the correct method of addition has been used, or add after all water and surfactants have been combined (unless supplier information specifically states otherwise).

As you can see, the reasons for instability can be many and varied. Start by checking the changes against this list to narrow your focus and conduct additional testing to identify the cause for the best formulation solution. 

Happy formulating! 

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