SKIN PROTECTION
27
Citrate esters in sunscreen formulations
Dr Teresa Berninger, Carolin Stern - Jungbunzlauer, Germany Stanislaw Krus, Dr Ulrich Issberner, Jochen Giesinger – BASF, Germany
In sunscreen formulations, several components act synergistically to provide effective protection to the exposed skin. Alongside the UV filters themselves, emollients are particularly relevant to both effectiveness and consumer acceptance of the product. This is due to their influence on the solubility of solid organic UV filters, dispersibility of inorganic UV filters and skin feel of sunscreen products. With the specific challenges posed by sunscreen
formulation in mind, we will take a closer look at the use of citrate esters as emollients in this context. Citrate esters are clear, odourless, oily liquids and have a long history of safe use as emollients in various personal care products. Jungbunzlauer’s CITROFOL® citrate esters are obtained by acidic esterification of citric acid, which is produced by fermentation of carbohydrate raw materials like corn. Among Jungbunzlauer’s portfolio of citrate esters, Triethyl Citrate (CITROFOL® AI) and Tributyl Citrate (CITROFOL® BI) are the most relevant derivatives for use in sunscreen formulation. Both come with a guaranteed non-GMO status. As shown in Table 1, both are regarded as medium spreading emollients, but differ with regard to other physico-chemical properties. The fundamental parameter for our
investigations was the solvent power of Triethyl Citrate and Tributyl Citrate in relation to selected standard organic UV filters. We also assessed the influence of both citrate esters on sun protection factor (SPF) and UVA protection factor (UVA- PF) in vitro. Their sensory profile was evaluated and compared to a benchmark emollient in a sunscreen matrix. The dispersibility of TiO2
and
ZnO in Triethyl Citrate was investigated with a specific focus on certified natural formulations. Based on the results, we outline the benefits of using citrate esters as emollients with regard to cost-in-use and formulation flexibility.
Formulating sunscreens with organic UV filters Emollients function as solubilisers of solid organic UV filters such as the widely used Bis-Ethylhexyloxyphenol Methoxyphenyl Triazine (BEMT), Diethylamino Hydroxybenzoyl Hexyl Benzoate (DHHB) and Ethylhexyl Triazone (EHT), but their solvent power differs significantly for the various UV filters. The choice of emollient and its usage rate therefore correlate directly to maximum applicable UV filter concentration and resulting SPF. In addition to emollients, oily-liquid UV
filters such as Ethylhexyl Salicylate (EHS) and Octocrylene are often incorporated into sunscreens to improve solvent power. However, this approach is not always feasible. Restrictions may arise from regulatory constraints. For example, in the EU, EHS is allowed only up to 5% in formulations according to Annex VI of the Regulation (EC) No 1223/2009 on Cosmetic Products. Octocrylene, in contrast, may be incorporated up to 10%, but has come under scrutiny as an endocrine disruptor with potentially negative effects on human health and the environment.1,2
Furthermore, the absorbance
spectrum of the oily-liquid filter with the best solvent power may not be the most suitable fit to provide efficient broad spectrum coverage, resulting in a loss of filter efficiency and increased costs. Last but not least, oily-liquid UV filters do not optimise product haptics. They frequently leave a greasy skin feel, hamper spreadability and absorption and thus limit consumer acceptance. When using emollients as solvents it is
important to consider their potential effect on the SPF. While a potential increase in absorbance can be exploited to improve the performance of sunscreens at the same usage rate of UV
TABLE 1: OVERVIEW OF PROPERTIES OF CITRATE ESTERS. CITROFOL® AI Triethyl Citrate
INCI
CAS registry number ISO 16128a COSMOS
Interfacial tension [mN/m]b Spreading value [mm2
own data /10 min]b
Solubility in water at 25°C [g/100 ml]b Viscosity at 20°C [mPas]b aNatural Origin Index; b
77-93-0 NOI = 1 Yes
32.0 660 5.8 37
CITROFOL® BI Tributyl Citrate
77-94-1
NOI = 0.39 No
28.5 531
Insoluble 43
filter, resulting in an attractive cost benefit, this must be weighed against any potential reduction in the UV light absorbance. The choice of solvents is therefore influenced
by considerations affecting targeted SPF, regulatory and certification aspects, costs and skin feel.
Solubility and efficient use of organic UV filters Ethylhexyl Triazone (UVINUL® T150-EHT), Diethylamino Hydroxybenzoyl Hexyl Benzoate (UVINUL® A plus − DHHB) and Bis- Ethylhexyloxyphenol Methoxyphenyl Triazine (Tinosorb® S – BEMT) were selected as representatives of solid organic UV filters. Their solubility at 25 °C in Triethyl Citrate and Tributyl Citrate was determined according to the method described by Herzog et al.3
For in silico analysis,
examples of UV filter combinations based on two different solvent systems were defined.
TABLE 2: MAXIMUM SOLUBILITY OF UV FILTERS BEMT, EHT AND DHHB IN DIFFERENT SOLVENTS.
Solvent INCI
Caprylic/Capric Triglyceride Dibutyl Adipate Tributyl Citrate Triethyl Citrate
Dicaprylyl Carbonate C12-15 Alkyl Benzoate
EHS (UV filter, max. usage rate 5%)
aSohn et al., 2020; b own data
Solubility BEMT [%] Solubility EHT [%] 5a
6a
10a 3b 1b
9a 12a 20b 16a
35b 11b 6a 4a
4b
Solubility DHHB [%] 14a 31a
29b 29b 18a
22a 34b
www.personalcaremagazine.com
June 2021 PERSONAL CARE
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