28 SKIN PROTECTION
The solvent systems consisted of a combination of 5% EHS and either 15% Dibutyl Adipate (reference) or 15% Tributyl Citrate. Consequently the maximum usage rate of EHT differed depending on solubility and/or regulatory limits. SPF, SPF rating and filter efficiency (i.e. SPF divided by total filter concentration) were determined in silico (
http://www.sunscreensimulator.basf.com). Table 2 gives an overview of the solubility values
of BEMT, EHT and DHHB in standard emollients and citrate esters. The table reveals the exceptional solvent power of Tributyl Citrate for EHT of up to 35%. This is more than twice as high as that of Dibutyl Adipate, the standard emollient ranked second in solvent power at 16%. A formulation with Tributyl Citrate thus allows for a higher loading with EHT and thus a higher SPF. Both Tributyl and Triethyl Citrate showed excellent solvent power of 29% for DHHB, which is in line with the solvent power of the widely used emollient Dibutyl Adipate. The example calculation illustrates that
the increased solvent power for EHT positively affects SPF efficiency: Table 3 shows the maximum solubility values of BEMT, EHT and DHHB in a formulation with 5% EHS and 15% emollient (benchmark system with Dibutyl Adipate vs. alternative system with Tributyl Citrate). In the benchmark system, the use of EHT is limited to 3% due to solubility. In the alternative system, the use of EHT can be increased to the limit of 5% imposed by regulatory restrictions. The other components of the UV filter combination were kept equal between the two systems. The use of Tributyl Citrate enables a slightly higher amount of EHT, a UVB filter with exceptionally high absorption efficiency,4
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
290 310 330 350 Wavelenght [nm]
Figure 1: Comparison of extinction between benchmark system and alternative system. An increased extinction in the UVB range is observed in the alternative system with higher EHT content.
TABLE 3: MAXIMUM SOLUBILITY AND REGULATORY LIMIT OF DIFFERENT UV FILTERS IN BENCHMARK SYSTEM (15% DIBUTYL ADIPATE + 5% EHS) VERSUS ALTERNATIVE SYSTEM (15% TRIBUTYL CITRATE + 5% EHS). USAGE RATE REFERS TO UV FILTER COMBINATION USED TO DETERMINE PERFORMANCE IN SILICO. SOLUBILITY WAS CALCULATED ACCORDING TO THE FOLLOWING FORMULA: AMOUNT SOLVENT A* FILTER SOLUBILITY IN SOLVENT A/(100-FILTER SOLUBILITY IN SOLVENT A) + AMOUNT SOLVENT B* FILTER SOLUBILITY IN SOLVENT B/(100-FILTER SOLUBILITY IN SOLVENT B).
UV filter
BEMT EHT
DHHB EHS
which results in a significant increase
in extinction in the UVB range (Figure 1). Consequently, the SPF increases from 23.7 to 31.9, leading to an increase in the SPF rating from 20 to 30. The increase in filter efficiency (i.e. SPF/filter concentration) from 1.73 to 2.03 is of direct economic benefit. In addition, greater solvent power of the
emollient gives the formulator flexibility regarding the exact composition of the oil phase to optimise skin feel and product aesthetics. When using Tributyl Citrate, the oil phase can be lowered overall and still offer sufficient solvent power for UV filters, thus achieving a lighter, less greasy skin feel. Furthermore, co-emollients can be chosen based on their sensory properties instead of having to focus on good solvent power.
35 30 25 20 15 10 5 0
Figure 2: SPF and UVA-PF determined in vitro in different test formulations. C12-15 Alkyl Benzoate
Triethyl Citrate PERSONAL CARE June 2021 Tributyl Citrate
Max. solubility [%] in benchmark system
3.0 3.0 9.3 -
Max. solubility [%] in alternative system
1.7
8.2 8.8 -
Influence of emollients on SPF and UVA-PF Another important aspect is the influence of the solvent matrix on UV absorbance and SPF. This has been described in the scientific literature both for technical solvents5
for sunscreen formulations.6
and for emollients widely used It was demonstrated
that emollients can shift the wavelength of maximum absorbance (λmax) and the specific extinction at the maximum absorbance E (1,1)max.6 To investigate a possible influence of the
emollient on the SPF, three o/w test emulsions with a calculated SPF of 25 and UVA-PF of 16 were prepared (Table 4). They differed only in the emollient system. To ensure full solubility of the filter platform, 10% Dibutyl Adipate was used in all test formulations, combined with either 15% C12-15 Alkyl Benzoate (reference), Tributyl Citrate or Triethyl Citrate.
SPF ■ UVA-PF ■ Calculated SPF Calculated UVA-PF
Max. [%] permitted*
10.0 5.0
10.0 5.0
*according to Annex VI of the Regulation (EC) No 1223/2009 on Cosmetic Products
SPF and UVA-PF were determined in vitro based on ISO 24443:2012 with slight adaptations. An amount of 27.6 mg of sunscreen formulation was applied to roughened PMMA plates (4.8 cm x 4.8 cm, Type SB 6, Helioscreen, Paris, France). The sunscreen formulation was rubbed with a saturated latex gloved finger so that it was distributed evenly on the PMMA plate. After an equilibration time of 30 minutes, 5 transmittance measurements per plate were performed using a UV Transmittance Analyzer UV 2000S (Labsphere, North Sutton, USA). In vitro SPF and UVA-PF before irradiation (UVA-PF0) were calculated from the transmission data. The plates were then irradiated with a dose calculated using the ISO 24443 spreadsheet by means of an Atlas Suntest CPS+ (Atlas MTT, Linsengericht, Germany). Transmission measurements were taken again and post-irradiation UVA-PF calculated. The SPF and UVA-PF values given in Figure 2 represent the mean results from three plates (5 measurements per plate). Analysing the test emulsions, all of the
combinations achieved an SPF of ≥ 25 (Fig 2). No negative deviation from the previously calculated SPF was observed when formulating with citrate esters. Looking at the UVA-PF, only the formulation with C12-15 Alkyl Benzoate showed a slightly negative deviation from the calculated value of 16.1. The formulation with Tributyl Citrate matches the calculated UVA-PF exactly. Interestingly, when using Triethyl Citrate, an increase in UVA-PF by almost 40% compared to the calculated value was obtained. This may be explained by a higher polarity of Triethyl
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Usage rate [%] in benchmark system
1.3
3.0 5.4 4.0
Usage rate [%]
in alternative system 1.3
5.0 5.4 4.0
370
Benchmark system ■ Alternative system ■
390
Protection Factor
Extinction
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