Sunscreen Formulation Troubleshooting

It appears you are creating a sunscreen formulation using both chemical (from SPF Protect Ultra II) and physical (ZnO, TiO2) UV filters. The separation you are experiencing is a common challenge when formulating with high oil phases and solid powders like zinc oxide and titanium dioxide.

1. Cause of Separation and How to Fix It

The most likely cause of the separation ("like milk curdling") is the improper dispersion and incorporation of the Zinc Oxide and Titanium Dioxide powders. Physical sunscreen powders like ZnO and TiO2 tend to clump together and are difficult to disperse evenly in a formulation, especially if added at the wrong stage or without sufficient mixing energy.

In your procedure, you are adding the powders (ZnO, TiO2) after the cream has already formed by combining the oil and water phases and adding the Satin Cream Maker. This is generally too late in the process for physical sunscreens. They need to be thoroughly dispersed in one of the phases before emulsification occurs to ensure they are evenly distributed throughout the final product and to provide effective UV protection without aggregation, which leads to instability and separation.

Recommended Mixing Procedure to Prevent Separation:

1. Prepare the Powder Dispersion: Disperse the Zinc Oxide and Titanium Dioxide powders thoroughly in a portion of your oil phase ingredients (LipidSoft Lite is a good candidate for this). This dispersion step is critical. You may need high shear mixing (like a homogenizer or a high-speed mixer) to ensure the powders are finely and evenly dispersed without clumps. Heat this mixture to 85°C along with the rest of your oil phase ingredients (SPF Protect Ultra II, Polyethylene Wax, Milk Lotion Maker).
2. Prepare the Water Phase: Heat the water phase (H2O) to the same temperature (85°C).
3. Emulsification: While mixing the water phase vigorously (using a homogenizer or high-speed mixer), slowly add the heated oil phase (containing the dispersed powders and Milk Lotion Maker) to the water phase. Continue high-speed mixing for at least 15-20 minutes while the emulsion cools down. This helps create a stable emulsion and ensures the oil phase (with the dispersed powders) is well incorporated into the water phase.
4. Add Temperature-Sensitive Ingredients: Once the emulsion has cooled down to below 60°C, add the Satin Cream Maker and Phenoxyethanol SA. Mix until fully incorporated and the desired texture is achieved. Be careful not to mix Satin Cream Maker at excessively high speeds (>2500 rpm) as this can damage the polymer structure.

By dispersing the powders in the oil phase before emulsification and using proper mixing technique during the oil-in-water addition and cooling, you should achieve a much more stable and homogenous product.

2. Oil Phase Amount

Your total oil phase is approximately 36% (15% SPF Protect Ultra II + 15% LipidSoft Lite + 3% Polyethylene Wax + 3% Milk Lotion Maker). This is a relatively high oil load.

• LipidSoft Lite is an emollient that provides a light feel and can help disperse powders. Reducing its amount might affect the texture and the ease of powder dispersion.
• SPF Protect Ultra II is a blend of organic UV filters and emollients. It contributes significantly to the oil phase and is essential for achieving your desired SPF level. It contains emollients but its primary role is UV protection, not necessarily dispersing physical filters.

Milk Lotion Maker is described as being able to bind up to 40% oil, which suggests it can handle your oil load. However, Satin Cream Maker is recommended for formulas with no more than 25% oil. Using both together in this formulation with 36% oil might be contributing to instability, especially with the powders.

While 36% oil is high, it's not necessarily excessive for a sunscreen, particularly one containing both chemical and physical filters which often require a substantial oil phase for solubility and dispersion. The key is having an emulsifier system and mixing method robust enough to handle this oil load and the solid particles.

Before reducing LipidSoft Lite, try correcting the mixing procedure, especially the powder dispersion step. If separation persists, you might need to adjust the emulsifier system or slightly reduce the total oil phase.

3. Alternative Emulsifiers/Thickeners

You asked about using Tribehenin, Butter Cream Maker, or Cetearyl Alcohol to replace Milk Lotion Maker (6) and Satin Cream Maker (7).

• Tribehenin is a soft wax and a thickener/texturizer, not an emulsifier. It melts in the oil phase and helps build viscosity and improve payoff in stick products, but it cannot emulsify oil and water.
• Cetearyl Alcohol is a fatty alcohol used as a thickener and emollient. It is also not a primary emulsifier and needs to be used with an emulsifier to help stabilize and thicken the emulsion. It is mixed in the oil phase with heat.
• Butter Cream Maker is an emulsifier that can handle up to 30% oil and has good electrolyte resistance. It is mixed in the oil phase with heat. While it can create thick textures, especially with Cetyl Alcohol or Beeswax, your formula's oil phase is 36%, slightly exceeding its stated capacity.

Replacing both Milk Lotion Maker and Satin Cream Maker with a single alternative might be challenging given the high oil load and the presence of physical filters.

• If you want to simplify, you could potentially try increasing the percentage of Milk Lotion Maker (up to 5%) as it is designed for higher oil content (up to 40%) and is mixed in the oil phase with heat, fitting your initial heating steps. However, you would still need to ensure proper powder dispersion.
• Using Butter Cream Maker could be an option, but you might need to adjust the oil phase slightly or use it in combination with another ingredient to handle the 36% oil and the powders effectively. It is also mixed in the oil phase with heat.

Given that Milk Lotion Maker is already in your formula and is suitable for high oil loads, and a sample formula for SPF Protect Ultra II uses both Milk Lotion Maker and Satin Cream Maker, the issue is more likely the mixing method and powder dispersion rather than the emulsifiers themselves. It is recommended to first perfect the mixing technique with your current ingredients before exploring alternative emulsifiers. If you still face issues, consider slightly increasing Milk Lotion Maker or exploring emulsifiers specifically designed for high oil content and physical sunscreen dispersion.