Top 10 sun care formulation development mistakes

1. Suboptimal UV filter selection

Often a formulator will continue to work with the filters and combinations they are familiar with, rather than addressing the fact that a new brief may require a different approach. As a result, their UV filter selection could be suboptimal which will make it challenging to meet the desired requirements. The first step in solar formulation should be to review the UV filters and ensure you have made the right selection.

UV filters can be chemical (organic) or physical (mineral, usually Titanium Dioxide (TiO2) and Zinc Oxide (ZnO)). Each formulation can use a single broad-spectrum UV filter or a combination or multiple UV filters to achieve the desired SPF and meet regional or global regulatory requirements. Combinations can be organic only, mineral only, or a combination, sometimes referred to as hybrid systems.

Chemical filters can provide a less whitening formulation but are often narrow spectrum and therefore require a cocktail of different filters to meet regulatory requirements in terms of protection. It should be noted that some organic UV filters, such as Avobenzone can also be photo-unstable, meaning they degrade in light, which is one of the reasons for the recommendations to reapply sunscreens every two hours. Photostability can be improved by combining with other UV filters (such as Octocrylene or Bis-Ethylhexyloxyphenol Methoxyphenyl Triazone) or with triplet state quenchers.

TiO2 has traditionally provided mainly UVB attenuation but there are now many TiO2 products in the market, such as the Solaveil™SpeXtra range, that use a larger TiO2 particle size to provide enhanced UVA attenuation and can therefore be used as a single active to meet broad spectrum SPF and UVA requirements, but these are often more whitening due to the larger particle size. Smaller nano-grade particle sizes of TiO2, such as the Solaveil Clarus range, provide much greater transparency on skin, but a more narrow-spectrum protection primarily in the UVB region. There are intermediate particle sizes, such as Solaveil Harmony that offer a balance of extra UVA with lower levels of whitening.

To find out more about these two product families, and the other product families in the Solaveil range check out our handy Solaveil range infographic on the right hand side.

In order to meet regulatory requirements for UVAPF, nano TiO2 can be combined with ZnO or organic filters. In Europe, the UVA protection is required to be at least 1/3rd of the labelled SPF. You would be forgiven for thinking that as the required SPF is much higher than the required UVAPF, and TiO2 attributes more UVB protection (and therefore more towards SPF than UVAPF) than ZnO, which contributes highly to UVA attenuation, more TiO2 is needed than ZnO. However, as ZnO also attenuates in the UVB region as well as UVA, this is rarely the case. Our Solaveil Clarus range TiO2 and Clarus range ZnO can be combined in a 9:1 ratio (ZnO:TiO2) to meet requirements for UVA protection.

Although performance is always chassis dependent, to determine the level of UV filter required to reach the desired SPF, theoretical calculations are the best starting point. Our Solaveil Calculator can help to avoid this common formulation development mistake and determine the % w/w of a Solaveil product that is required to reached a specified SPF or the SPF that can be achieved through combinations of Solaveil UV filters. The SPF is additive, so the SPF of a combination of filters can easily be calculated. However, the critical wavelength and UVAPF of combinations cannot currently be calculated with this tool, only for single ingredients. Access our Solaveil calculator using the link on the right. 

2. Combining inorganics with Avobenzone

Combining inorganic and organic filters is a great way to cost-effectively achieve high SPF formulations but can lead to issues, so is one common formulation development mistake. Titanium Dioxide is often use as a broad-spectrum photostable filter that can be built upon with the addition of organic filters. Combining UV filter types often produces a more lightweight formulation than mineral-only systems, but with higher photostability and a less greasy sensory than organic-only systems and allows for higher overall loadings of UV filters without reaching individual inclusion limits. Combining mineral and organic UV filters can also provide synergies, with performance greater than the additive performance of the ingredients combined. Bethan Spruce, Formulation Scientist at Croda wrote an article about this winning combination, which you can read here. However, some organic UV filters, such as Avobenzone, can form coloured complexes with TiO2. This will not impact performance but will lead to yellowing of the formulation over time.

To minimise this interaction, organic and inorganic filters can be added to separate phases of the formulation, and chelating agents (such as Tetrasodium EDTA) can be added to the oil phase with the organic filters. Biphasal distribution of actives has also been shown to improve performance and stability of sun care formulations when compared to use of actives in the oil- or water-phase alone.

There is some research showing that ZnO can degrade Avobenzone, reducing efficacy, so to prevent this interaction, we would recommend using a coated ZnO where Avobenzone is also used in the formulation, such as Solaveil MZP7, Solaveil MZP8, or Solaveil MZ7-100.

Note: there are regulatory restrictions for combining TiO2 and ZnO with Avobenzone in USA.

3. Formulating with ZnO in O/W systems

ZnO is becoming an increasingly popular active, due to its naturality, mildness and transparency. In particular, our latest launch, Solaveil MicNo, also has the advantage of being micron sized, so also COSMOS approved. Below is an image of the unique platelet structure of Solaveil MicNo. You can also click here to find out more about our Solaveil MicNo COSMOS approved dispersion. However, ZnO formulations can be tricky to formulate. As ZnO is partially soluble in water, when ZnO is used in the oil phase of O/W emulsions, it has a tendency to migrate into the water phase. This can cause an increase in pH, agglomeration of ZnO (leading to reduced efficacy) and emulsion destabilisation.

It is best to formulate ZnO in the oil phase of W/O formulations, but a number of techniques can be utilised to improve stability when formulating O/W emulsions with ZnO:

• ZnO selection:
  - Coated ZnO formats keep Zn2+ migration to a minimum
  - Use a dispersion of ZnO where the dispersant system acts as an ‘in situ’ coating

• Emulsifier selection:
  - Add liquid hydrophobic (W/O) emulsifier as a co-emulsifier at 0.5-1.5% to strengthen the interface and prevent migration
  - Include anionic surfactant at 0.2% to stabilise liquid crystal structures in presence of electrolyte
•  
•  Emollient selection:
  - Use high polarity oils for uncoated ZnO powder, or low polarity oils for coated ZnO powder and ZnO dispersions
  - Add propylene glycol to the oil phase (3-5%)

• Thickener selection:
  - Add Xanthan Gum (0.1-0.4%) and Magnesium Aluminium Silicate (0.5-1.25%) to help redisperse any migrated ZnO, stabilise the system and improve efficacy (glycols in the water phase can also help this)
  - If using carbomers partially pre-neutralise prior to addition

• Formulation advice:
  - Buffer the formulation to pH 6.5-7.5 – it is recommended to add acid to the water phase prior to emulsification
  - Addition of chelating agents can help complex Zn2+ ions and reduce their interactions
  - Add ZnO last to a hot oil phase after other ingredients are dissolved and/or melted – post-addition is not possible in O/W formats
  - Maintain pH>6.0 after mixing oil and water. Final pH can be lowered with Citric acid but should only be done once (repeated adjustments should be avoided).

4. Emulsifier selection and compatibility

Emulsions, whether lotions, creams or sprays, are the most common form for sunscreens for multiple reasons. Emulsions provide a wide range of elegant sensories despite inclusion of heavy and oily ingredients, and their rheology allows for decent film formation on the skin, improving efficacy. The multiple phases of an emulsion allow for incompatible ingredients to be delivered in the same formulation and the high-water content can reduce cost.

The O/W emulsion format can provide a lighter sensory, usually preferred by the consumer and tend to be less whitening than W/O systems. Alternatively, W/O systems can offer higher efficacy and water resistance but typically have a heavier sensory. The emulsifier system selected can have a huge impact on both sensory and efficacy. It should be noted that cationic and anionic emulsifiers can be difficult to formulate with inorganic dispersions, and non-ionic or, as a minimum, combined ionic and non-ionic systems are recommended.

Croda offers a wide range of emulsifiers and our emulsifier selection guide is a handy tool to help you in your formulation development - find this on the right of the page. 

5. Insufficient oil for wetting powders and emulsification

If using powdered mineral UV filters, it is important to consider the level of oil to be used to avoid this common formulation development problem. To achieve a stable formulation, sufficient oil is required for both wetting and suspending of the lipophilic UV filter powder, as well as for emulsification, where applicable, and other requirements, such as solubilising organic filters in combination systems.

It should be noted that there are large differences in oil absorbency seen between different TiO2 powders. Oil absorbency correlates primarily with particle size: the greater the particle size, the lower the oil absorption. However, the coating also plays a role. Therefore, some TiO2 powders will require more oil to be wetted than others. You can quickly test the oil absorbance and amount of oil required for wetting by combining the desired oil dropwise, with a known quantity of metal oxide powder. More information can be found about oil absorbency in our working with powders emollient compatibility presentation which you can access on the right hand side of this page.

6. Insufficient solubilisation of solid organics

Solid organic UV filters can be difficult to solubilise. Selecting the right emollient will ensure maximum SPF efficacy whilst preventing crystallisation. Make sure to take note of the melting point of the UV filter in the relevant datasheet – filters such as Avobenzone require heating to greater than 83°C to ensure full solubilisation and maximum efficacy within the formulation.

Different emollients provide differing levels of solubilisation for each solid organic UV filter. But in general, increasing the concentration of polar oils in the formulation can improve the solubility of solid organic UV filters and therefore improve efficacy.

7. Insufficient dispersion of UV filter

Mineral filters are available in both powder and dispersion formats. Dispersions are designed to keep the metal oxide particles evenly dispersed throughout the formulation and can therefore improve SPF performance and stability compared to powders. However, the use of powders provides much more formulation versatility in terms of achieving aesthetics and claims such as natural. Insufficient dispersion of mineral UV filters can lead to reduced SPF performance, instability, poor rheology and increased whitening on the skin.

If using dispersions, addition the dispersion to the relevant phase under overhead stirring can help to ensure even distribution of filters and a homogenous formulation. Although not necessary, shaking or mixing of the dispersion prior to addition can help and homogenisation following either addition (for single phase formulations) or emulsification (for emulsions) can improve both dispersion of the filters and droplet size for emulsions, aiding stability and SPF performance.

You can watch one of our formulators using a Solaveil dispersion to make a sun care emulsion in the video below. 

As well as being wetted as mentioned in point 6, when using powders, the powder must be dispersed throughout the oil phase. Mechanical energy, such as high shear mixing or homogenisation, can break agglomerates and help to evenly distribute the powder. This energy input is key to producing a stable and efficacious system and can be done both after addition of the powder to the oil, before emulsification, as well as during/after emulsification, where applicable. In order to stabilise this dispersion of powder, dispersing agents should be added to provide a combination of electrostatic repulsion and steric hindrance to prevent agglomeration and provide long term stability.

8. Insufficient hydration of rheology modifiers

Optimising the rheology of sun care formulations is vital for film formation and achieving an even distribution of actives on the skin. The product must spread easily and recover after spreading to form an even film.

The use of hydrocolloids in the water phase, such as Xanthan Gum and Magnesium Aluminium Silicate can help to stabilise the emulsion as well as aiding dispersion of UV filters and improve SPF efficacy. However, hydrocolloids require significant hydration times to prevent the “fish eye” effect seen when thickeners are not fully hydrated. Dispersing the thickener in glycerine before slowing adding water to this premix can help ensure hydration is optimised. Lastly, it is essential to leave the water phase to hydrate whilst stirring prior to continuing with the next step of the manufacturing process.

9. Not considering microscopy

Instability issues can cause huge variations in the efficacy of a sun care formulation. Not considering microscopy is a common formulation development mistake as it is a vital tool to identify instability or formulation issues that can impact performance, particularly those that cannot be identified by the naked eye.

For mineral formulations, TiO2 and ZnO will appear under light microscopy as black particles or as swollen transparent particles. The smaller and more evenly dispersed the inorganic filters, the higher the chances of achieving stability and optimal efficacy. Agglomeration of the filters is a sign an incompatibility, which will require reformulation, or a sign of insufficient dispersion, which can be easily solved with either additional energy input during homogenisation or additional emollients to wet the powder.

For organic or combination systems, to obtain maximum efficacy, solid organic UV filters should be sufficiently solubilised in the oil phase and should not crystalise throughout the shelf life. It is important to check the microscopy of the formulation to ensure this. Crystals of solid UV filters (due to poor solubilisation or crystallisation) can be seen under polarised light as ‘spikey’ coloured crystals. Solubilisation can be improved by altering the emollients in the formulation and by ensuring the melting point temperature of the solid organic UV filter is reached in the manufacturing process.

10. Not adding by regulatory requirements

Regulatory requirements for sunscreens vary across the globe and products should be formulated based on the regulations of the region(s) in which they will be sold. Most regions regulate through a strict list of approved ingredients, ingredients outside of which cannot be used. All regions have maximum inclusion limits for UV filters and some regions have restrictions on certain combinations of UV filters (for example, TiO2 cannot be used in combination with Avobenzone in USA). It is vital to research the regulatory requirements for the relevant region with respect to filter approval, SPF testing requirements and claims.

There are increasing regulatory pressures on organic filters for environmental reasons, with some being banned, particularly in regions wishing to protect coral reefs. In addition, organic sunscreens are under some scrutiny in term of skin penetration, the FDA state that while they have not identified safety concerns with organics, existing evidence suggests that these are, or may be, absorbed through the skin, and data about the consequences of this absorption are missing. Therefore, TiO2 and ZnO, are a safe bet, they are approved worldwide, not subject to any bans, and are generally recognised as safe and effective (GRASE category 1) because they largely do not penetrate the skin.

So, that concludes our top 10 formulation development mistakes in sun care. If you avoid all these mistakes, you can ensure your formulation is high performing, safe, stable and feels and looks good on the skin. If you would like any more information on the topics discussed in this blog, or support in your sun care developments, don’t hesitate to contact us where our sales representative can put you in touch with one of our formulation experts in your area.