How to Increase SPF from 15 to 50

Increasing the SPF of a sunscreen formula from 15 to 50 requires adjusting the concentration and types of UV filters used. Based on the ingredients in your formula, here's a guide on how you might approach this, along with an example of an adjusted formula.

First, it's important to note that achieving a specific SPF value requires laboratory testing (in-vivo or in-vitro). Formulation is complex, and simply increasing percentages doesn't guarantee a linear increase in SPF due to interactions between ingredients and how well the UV filters are dispersed and form a film on the skin.

In your original formula, you are using Titanium Dioxide, BMTE, and Homosalate as UV filters.

• Titanium Dioxide (easy mix): Based on the product description for Titanium Dioxide 200nm EasyDisperse™ (which matches the "easy mix" type), this particular grade is specified for make-up products for coverage and whiteness, not specifically for UV protection. To achieve a high SPF, you should use a grade of Titanium Dioxide that is designed for sunscreen applications, preferably a transparent or ultra-fine particle size type to minimize whitening, such as Titanium Dioxide Transparent (15nm, Dimethicone Coated) or Titanium Dioxide Transparent (15nm, Water-Dispersion).
• Homosalate: This is a UVB filter. Your formula uses 7%, which contributes to the current SPF 15. The maximum allowed concentration is typically 10% in many regions. Increasing Homosalate up to the maximum can help boost the SPF.
• BMTE (Tinosorb S): This is a highly effective broad-spectrum filter covering both UVA and UVB, and it is very photostable. Your formula uses 1.2%, which is relatively low considering its effectiveness. Increasing the concentration of BMTE is a good strategy to significantly increase both SPF and UVA protection.

To reach SPF 50, you will likely need a higher total concentration of effective UV filters and potentially a better combination of filters to cover the full spectrum (UVA and UVB).

Also, please note that the percentages in your original formula add up to more than 100% (102.7%). The percentages in any formula should sum to 100%. The example formula below has been adjusted to sum to 100%.

Here is an example of how you could modify your formula as a starting point to aim for SPF 50. This involves replacing the current Titanium Dioxide with a sunscreen-grade type and increasing the concentrations of Homosalate and BMTE.

Example Adjusted Sunscreen Formula (Guideline for SPF 50)

• Titanium Dioxide Transparent (15nm, Dimethicone Coated): 8.00%
• BMTE: 6.00%
• Homosalate: 10.00% (Maximum allowed concentration)
• Caprylic/capric triglyceride: 8.31%
• Stearin acid: 2.49%
• Glyceryl Stearate: 1.25%
• Cetus Alcohol: 0.42%
• Yellow Iron Oxide (easy mix): 0.83%
• Red Iron Oxide (easy mix): 0.25%
• Black Iron Oxide (easy mix): 0.17%
• DI Water: 50.46%
• Butylene Glycol: 4.15%
• Triethanolamine: 0.83%
• Xanthan Gum: 0.21%
• LipidSoft: 1.66%
• mineral oil: 1.66%
• Zinc PCA: 0.83%
• Water Guard: 2.49%
• Total: 100.00%

Key Changes and Considerations:

• Titanium Dioxide: Replaced the original Titanium Dioxide (likely for makeup) with Titanium Dioxide Transparent (15nm, Dimethicone Coated), a type designed for sun protection that offers broad-spectrum coverage (UVB and short UVA) with minimal whitening. The percentage is increased from 3% to 8%. You could also consider Titanium Dioxide Transparent (15nm, Water-Dispersion) if you find it easier to incorporate into your water phase, adjusting the percentage based on its active TiO2 content (ID 10795 contains 28% TiO2).
• BMTE: Increased from 1.2% to 6%. This significantly boosts both SPF and UVA protection due to BMTE's high efficacy and stability.
• Homosalate: Increased from 7% to 10%, reaching the typical maximum allowed concentration for additional UVB protection.
• Other Ingredients: The percentages of other ingredients have been proportionally reduced to accommodate the higher concentration of UV filters and ensure the formula sums to 100%.
• Formulation Process: Ensure the UV filters, especially the powder Titanium Dioxide and BMTE, are properly dispersed in the oil phase before emulsifying with the water phase. Proper dispersion is critical for achieving the stated SPF.
• Testing: This adjusted formula is a theoretical guideline. You must perform SPF testing on the final product to confirm that it achieves SPF 50. Adjustments may be needed based on the test results.

This revised formula provides a higher concentration of effective, sunscreen-grade UV filters, which is necessary to increase the SPF from 15 to 50. Remember that fine-tuning and testing are essential steps in sunscreen formulation.