Formulation Feedback and Recommendations

Thank you for sharing your silicone-based physical sunscreen formula and questions. It's great that you are experimenting and refining your formulation based on your observations. Here is some feedback and recommendations based on your questions:

1. Dry Skin Feeling

Physical sunscreens like Titanium Dioxide and Zinc Oxide, especially at higher concentrations, can sometimes contribute to a dry feeling on the skin. Your formula is also primarily silicone-based, which provides a smooth, non-greasy feel but doesn't offer significant hydration like oils or emollients would. To counteract the dryness and prevent a "crepey" appearance, consider incorporating emollients that are compatible with your silicone base. Adding a small percentage (e.g., 1-5%) of a light, non-volatile emollient or a silicone fluid with conditioning properties can help improve skin feel and reduce dryness without compromising the silicone texture. Examples of compatible emollients include certain types of Dimethicone with different viscosities or specific silicone esters.

2. Powder Amount and Texture

An 11% total concentration of physical sunscreen powders (6% Titanium Dioxide and 5% Zinc Oxide) is a moderate amount. The texture issue you are experiencing (not as smooth as liquid sunscreen formulas) is likely related to the dispersion of these powders in your silicone base and the overall composition with high silicone and film former content. Even with "EasyDisperse" powders, proper and thorough mixing is crucial to ensure the particles are evenly distributed and minimize grittiness or unevenness. Using a high-shear mixer can help. Additionally, the interaction between the powder particles, the various silicones (Silicone Gel, Cyclomethicone, Dimethicone), and the film formers (Silicone Film, Silicone Film) can significantly impact the final texture. Adjusting the ratios of the different silicone components or incorporating ingredients specifically designed to improve powder dispersion and texture in silicone systems might be beneficial.

3. SPF and PA Values

Determining the exact SPF and PA values for your specific formula requires in-vivo or in-vitro testing of the final product. Formulations are complex, and the interaction between ingredients, including the dispersion quality of the UV filters, significantly affects the final protection level. While 6% Titanium Dioxide and 5% Zinc Oxide are common concentrations in physical sunscreens, providing a precise SPF and PA value without testing is not possible. Generally, these concentrations can contribute to a broad spectrum SPF, but the exact level depends on the particle size, coating, and the overall formula matrix.

4. Silicone Film Performance

You noted that 20% Silicone Film (Soft Film, Low Viscosity, Cyclopentasiloxane Base) still rubs off, while 10% Vinyl Film adhered better but was uneven. The Silicone Film you used is described as providing a soft film. If you need better rub-off resistance, consider using a film former that creates a harder or longer-wearing film, provided it is compatible with your silicone base. For instance, Silicone Film (Powder, Hard Film, Comfort-Wear) (Trimethylsiloxysilicate) or Hydrogenated Polycyclopentadiene (Oil Film), which is soluble in silicone, are designed for longer wear and might offer better adhesion. You may need to experiment with different types and concentrations to achieve the desired balance of wear, feel, and evenness.

5. Mixing Film Formers

Mixing different film formers like Silicone Film, Oil Film (Hydrogenated Polycyclopentadiene), and Vinyl Film is possible in a silicone base, but it requires careful consideration of compatibility. Silicone Film and Hydrogenated Polycyclopentadiene are generally soluble in silicone. However, Vinyl Film (VP/Hexadecene Copolymer) is described as oil-soluble, which means it may not be directly compatible with a silicone base without a suitable blender or emulsifier to bridge the oil and silicone phases. Always perform small-scale tests to check for compatibility, stability, and desired film properties when mixing different film formers.

6. Using Oil Film / Vinyl Film in Silicone Base

As mentioned, Hydrogenated Polycyclopentadiene (Oil Film) is soluble in silicone and can be used in a silicone base. Vinyl Film (VP/Hexadecene Copolymer), being oil-soluble, is less likely to be directly compatible with a silicone base. While it might be possible to incorporate it using a suitable blender, it is generally recommended to use film formers that are soluble in your base for better stability and ease of formulation. For a silicone base, silicone-soluble film formers are typically the most straightforward choice.

7. Cleaning Equipment

Silicone-based formulations and film formers can indeed be difficult to clean from glassware with just soap and water. Silicones and many film formers are non-polar and hydrophobic. To effectively clean your equipment, first try rinsing with a non-polar solvent that can dissolve the silicone and film former residue. Isododecane or other similar cosmetic-grade solvents are often effective for this purpose. After dissolving the bulk of the residue with a solvent, you can then wash the glassware with detergent and water as usual. Ensure you work in a well-ventilated area when using solvents and follow proper safety precautions.