Sunscreen Formulation: Zinc Oxide, Titanium Dioxide, Stability & Water Resistance

Question

I have a sunscreen formulation containing:

Zinc Oxide 35nm Liquid at 10%
Titanium Dioxide 15nm Liquid at 25%

I would like to know:

Is the concentration of Zinc Oxide 35nm Liquid at 10% suitable when used with Titanium Dioxide 15nm Liquid at 25%?
How can I ensure the stability of this formula, especially with the high concentration of sunscreens?
What ingredients can I add to make the formula water-resistant, and what percentages should I use for different levels of water resistance (sweat resistance vs. swimming)?
Are there any specific mixing instructions for these recommended additives?

MySkinRecipes Expert · Accepted Answer

Regarding your adjusted sunscreen formula with 10% Zinc Oxide 35nm Liquid, the staff confirmed that this percentage is acceptable and sufficient when combined with 25% Titanium Dioxide 15nm Liquid. Using a higher percentage of Zinc Oxide is possible but might make the formula stickier.

To ensure the stability of the formula due to the high concentration of sunscreens, the staff recommends adding 3-4% Milk Lotion Maker to help emulsify and prevent separation.

For water resistance, you have a few options depending on the desired level:

For good sweat resistance during the day: Add 1-2% Polyethylene Wax or 3-5% WaterGuard™ Lite.
For resistance allowing swimming: Add 5-6% WaterGuard™ Ultra.

Please note that Milk Lotion Maker and Polyethylene Wax require heating during mixing to dissolve properly, while WaterGuard™ products do not.