The use of a polymeric emulsifier such as acrylates/C10-30 alkyl acrylate crosspolymer provides excellent emulsification and stability performance in drip-resistant broad spectrum low viscosity sun care formulations, according to Lubrizol researchers.

With growing health and skin aging concerns related to UV rays there is increasing consumer awareness of the need for sunscreen. By 2014, the global sun care market is projected to reach nearly $8 billion. Sun protection, the largest of three key segments in the global sun care market (sun protection, after sun and self-tanning), accounts for 65.4% of the market’s total value.¹ Sun care is a very segmented market. Consumers seek new products that deliver multifunctional benefits such as excellent stability and photostability, pleasant sensory, high/broad SPF, ease of use, water resistance, moisturization or anti-aging. With this long list of consumer desires, the demand for cost-effective products and the pressures from regulatory institutions, designing formulations that meet these requirements is a challenge that every sun care product manufacturer is facing.

Sun protection is the most important requirement in sun care. High levels of multiple organic sunscreens are typically used to achieve high/broad SPF values in formulations but doing so can present product stability challenges. Formulating options are often limited to creams and lotions that exhibit heavy and unpleasant sensory properties, and are difficult to spread on the skin. Formulating stable, low viscosity products containing high levels of sunscreens using traditional surfactants can require using prohibitively high levels of emulsifiers. Product aesthetics can be poor, tending to drip and run, making uniform coverage and protection challenging. High levels of emulsifiers may also create sensitivity issues and are suspected to enhance the penetration of sunscreen actives into the skin.^2,3

Because the emulsifying system and its HLB are adjusted to obtain optimal stability according to the polarity of the oil phase and the desired emulsion characteristics, any changes to the composition of the formulation would require an adjustment in the emulsifier type or dosage. In addition, upon application of a surfactant-based emulsion to the skin, the resulting film tends to rewet with subsequent re-emulsification of the oily sunscreens due to the presence of residual surfactant. This reduces resistance to water and sweat.

Consumers seek formulations having lighter sensory without sacrificing performance. The use of a polymeric emulsifier such as acrylates/C10-30 alkyl acrylate crosspolymer (Pemulen TR-2 polymeric emulsifier from Lubrizol Advanced Materials, Inc.) provides excellent emulsification and stability performance in drip-resistant broad spectrum low viscosity sun care formulations such as sprays and lotions that will uniformly cover the skin, while offering a refreshing initial sensory and mildness.

Emulsification Properties
By design, acrylates/C10-30 alkyl acrylate crosspolymer has emulsion stabilization capabilities. The stabilization provided by acrylates/C10-30 alkyl acrylate crosspolymer and some traditional emulsifiers was tested in a simple screening formula (Table 1) containing oily organic and inorganic UV filters. The UV filters dosage was selected to achieve a calculated SPF 15 and provide UVA protection.

Results from the experiments show that a low dosage (0.15wt%) of acrylates/C10-30 alkyl acrylate crosspolymer can stabilize this sunscreen emulsion having a calculated SPF 15. They also validate that the inclusion of a small amount of micronized/treated titanium dioxide in the formulations stabilized by acrylates/C10-30 alkyl acrylate crosspolymer is possible.

To further confirm the emulsion stabilization capability of acrylates/C10-30 alkyl acrylate crosspolymer, the polymer was tested with and without stearic acid in a screening formulation with higher levels of UV filters to meet a calculated SPF of 30 with UVA protection (Table 2). The performances of the polymeric emulsifiers at 0.20wt%. use level were also tested against some traditional emulsifiers.







































The results in Table 2 substantiate the ability of acrylates/C10-30 alkyl acrylate crosspolymer to effectively stabilize the sunscreen formulation with higher levels of oily UV filters (calculated at SPF 30). However, the combination of acrylates/C10-30 alkyl acrylate crosspolymer and a small amount of traditional emulsifier such as stearic acid, allows further stability across a more broad temperature range, which enhances freeze/thaw stability.

Acrylates/C10-30 alkyl acrylate crosspolymer also influences the rheological characteristics of formulations, as shown by some comparative tests conducted between a formulation containing acrylates/C10-30 alkyl acrylate crosspolymer or a commercial benchmark.⁴ Compared to the commercial sample, acrylates/C10-30 alkyl acrylate crosspolymer provides better vertical cling. The drip rate of the formulation with acrylates/C10-30 alkyl acrylate crosspolymer is more controlled. This will provide the user with more time to apply the product evenly on skin. This phenomenon is demonstrated by Figure 1.

Figure 1: Acrylates/C10-30 Alkyl Acrylate Crosspolymer vs. Commercial Sample

A Boost to UV Protection
The key to achieving high SPF efficacy is to ensure an even distribution of sunscreens on the skin. Combining oil-soluble filters with water-soluble filters provides biphasal distribution of sunscreens and ensures the most even coverage possible.

Phenylbenzimidazole sulfonic acid (PBSA) is an effective photostable UVB absorber that forms water-soluble salts with the addition of a base.

To demonstrate the high electrolyte tolerance of acrylates/C10-30 alkyl acrylate crosspolymer and its compatibility with PBSA, acrylates/C10-30 alkyl acrylate crosspolymer was tested in screening formulations containing 3.5 wt% of this water-soluble sunscreen (Table 3). A solution of PBSA neutralized to pH 7.2-7.5 with triethanolamine was added to the aqueous phase of the formulations.

Synergistic effects are often observed when organic filters are combined with inorganic filters.
Scattering of UV light by particles of mineral UV blocker in a sunscreen film increases path length and, hence, UV absorbance.

Titanium dioxide is a physical sunscreen protecting against UVB and short UVA light. It has high refractive index, strong UV light absorbing capabilities and has long been used for its synergistic SPF boosting properties when combined with organic UV filters. To show the capability of acrylates/C10-30 alkyl acrylate crosspolymer to emulsify and stabilize sunscreen lotions with micronized/treated titanium dioxide, oil- and water-dispersible grades of this inorganic sunscreen were also added to the formulations prepared with acrylates/C10-30 alkyl acrylate crosspolymer (Table 3).
































The addition of xanthan gum was also tested (B3 formulation) as it plays a complementary role to acrylates/C10-30 alkyl acrylate crosspolymer by providing high salt compatibility. This electrolyte stability enhancement allows for good thickening and suspending performance in formulations with high level of electrolytes while providing sensory benefits such as lubricity and slipperiness.

The experimental results substantiate the ability of acrylates/C10-30 alkyl acrylate crosspolymer to stabilize a sunscreen formulation with high levels of oil-soluble, water-soluble and inorganic UV filters. The combination of acrylates/C10-30 alkyl acrylate crosspolymer and a small amount of xanthan gum increases the resistance to salt giving a higher viscosity formulation with a long-lasting lubricious skin feel.

These formulations also show the versatility in use of acrylates/C10-30 alkyl acrylate crosspolymer that has high temperature stability. The sunscreen lotions were prepared using a hot process (75-80°C) to disperse solid UV filters and homogenized with an Ultra-Turrax homogenizer for 15 seconds at 6,000rpm to decrease the oil droplets size down to 2-5µm.

Conclusion
Polymeric emulsifiers such as acrylates/C10-30 alkyl acrylate crosspolymer offer efficient and robust sunscreen emulsion stabilization performance, making stable sun care formulations with low levels of emulsifier highly possible, even at low viscosity or in the presence of challenging UV filters.

Acrylates/C10-30 alkyl acrylate crosspolymer is an excellent option for making light sunscreen lotions delivering high, broad and photostable UV protection.

More info: www.lubrizol.com/PersonalCare/Formulations/Sunscreens.html

References

1. Datamonitor Global Industry Guide (Oct. 2010)
2. Sarunyoo Songkro, (2009 May – June), An overview of skin penetration enhancers: penetration enhancing activity, skin irritation potential and mechanism of action, Songklanakarin Journal of Science and Technology 31(3), 299-321.
3. Heather AE Benson, Vikram Sarveiya, Stacey Risk and al., (2005), Influence of anatomical site and topical formulation on skin penetration of sunscreens, Therapeutics and Clinical Risk Management: I(3), 209-218.
4. Commercial sample or benchmark is 250ml Ultra Trigger Spray Sunscreen SPF 30+ (The Cancer Council Australia)