Formulation design of pasty facial-washes consisting of lowconcentration fatty acids and their effectiveness

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1 Formulation design of pasty facial-washes consisting of lowconcentration fatty acids and their effectiveness Ippei Tanaka 1, Emiko Hayashi 1, Takayuki Kado 1, Yukihiro Kembo 1, Cynthia Qi 2, and Toshio Horikoshi 3 1 Department of Research & Development, Iwase Cosfa Co., Ltd., Osaka, Japan, 2 Department of Personal & Home Care, Lubrizol Specialty Chemicals Manufacturing (Shanghai) Co., Ltd., Shanghai, China, and 3 Department of Personal & Home Care, Lubrizol Japan Ltd., Tokyo, Japan Abstract Pasty facial-washes are formulated with higher amounts of fatty acid salts and anionic surfactants than those of other washing products, and are associated with higher risks of skin irritation and rough skin. In the present study, we investigated the benefits of facial washes with lower fatty acid contents and formulation of pasty facial-washes with lower fatty acid contents. The results obtained demonstrated that washing with 18% PM significantly prevented reductions in moisture content more than 30% PM. In order to formulate pasty facial-washes with an 18% fatty- acid content, appropriate thickening polymers were screened. We found that a particular acrylates/c10-30 alkyl acrylate crosspolymer with higher thickening efficiency and salt tolerance thickened the system more efficiently among the polymers studied. A pasty facial-wash with an 18% fatty acid content thickened by this polymer significantly prevented reductions in the moisture content of the stratum corneum more than that with a 30% fatty acid content and provided sufficient sensory profiles. These results demonstrate the potential of pasty facial-washes with lower fatty acid contents thickened by this polymer as a new generation of soap-based facial-washes providing several benefits such as mildness to skin, environment friendliness, and cost effectiveness. 1. Introduction Various facial-wash products have been developed to date including solid soaps, pastes, powders, and pump foams. Consumers in Japan desire a refreshing feeling after washing [1], and, thus, the most popular facial-washes are pasty facial-washes containing fatty acid salts as a main surfactant, whereas facial-washes containing synthetic anionic surfactants are popular in Europe and America. Since the crystallization of fatty acid salts is an important factor influencing viscosity, marketed pasty facial-washes have to contain between 25 and 35% fatty acids [2]. Therefore, the total amounts of surfactants in pasty facial-washes are generally higher than those in other cleansing products such as body washes and shampoos (10-20%). Fatty acid salts have been categorized into a high skin-irritation group [3][4]; however, the findings of a consumer survey revealed that the number of consumers who dilute pasty facial-washes with an insufficient amount of water before use is small [5]. Therefore, pasty facial-washes are associated with higher risks of skin irritation and rough skin. Although reducing the amounts of fatty acid salts during the formulation of facial-washes reduces the risks of skin irritation and rough skin, these changes are associated with other issues including the failure to maintain a pasty appearance and deterioration of the original sensory texture. With the aim of designing an ideal facial-wash, this study used potassium myristate 1

2 (PM) as a fatty acid salt in a simplified soap system, and investigated the effects of washing with a PM solution on the moisture content of the stratum corneum (SC), and attempted to confirm the efficacy of reductions in fatty acid salt in washes on human skin. After screening thickening polymers suitable for maintaining the pasty appearance of facial-washes, even with reduced fatty acid salt contents, the formulation of pasty facial-washes containing an 18% fatty acid content was examined in terms of mildness to skin and sensory attributes as a pasty facial-wash. 2. Materials and Methods 2-1. Materials Thickening polymer Carbomer (CB) (Carbopol 980 Polymer), acrylates copolymer (AC) (Carbopol Aqua SF-1 Polymer), acrylates/beheneth-25 methacrylate copolymer (ABM) (Novethix L-10 Polymer), and 6 other acrylates/c10-30 alkyl acrylate crosspolymers (AACs); Carbopol SC-200 Polymer (AAC-1), Carbopol SC-500 Polymer (AAC-2), Carbopol ETD 2020 Polymer (AAC-3), Carbopol Ultrez 20 Polymer (AAC-4), Carbopol Ultrez 21 Polymer (AAC-5), and Pemulen TR-1 Polymer (AAC-6), were obtained from Lubrizol, Wickliffe, OH, USA. The chemical structures of all thickening polymers are shown in Figure 1. AC is supplied as a 30% aqueous solution. Figure 1: Chemical structures of thickening polymers used in the present study (A) In the case of - Carbomer, R 1 =H, R 2 =H - Acrylates copolymer, R 1 =H or CH 3, R 2 =C 1 -C 4 alkyl - Acrylates/C10-30 alkyl acrylate crosspolymer, R 1 =H or CH 3, R 2 =C 10 -C 30 alkyl. (B) Acrylates/beheneth-25 methacrylate copolymer, R=C 1 to C 5, R =H or CH 3, R =C 22 H Experimental methods All panelists who participated in human panel tests provided informed consent before testing. 2

3 Changes in the moisture content of SC and transepidermal water loss (TEWL) in human skin washed with neutralized potassium myristate (PM) solution Aqueous solutions of 1.8 and 3.0% myristic acid neutralized by 70% with potassium hydroxide were prepared, named PM-1.8% and PM-3.0%, and heated at 40 C. Fifty grams of the warmed solution was placed into a mixer (IFM-600DG) (Iwatani, Tokyo, Japan) and mixed for 5 s. The foam obtained was used in subsequent experiments. In order to the moisture content of SC and TEWL in human skin, 4 panelists were asked to wash their left and right forearms with water and acclimate to specific conditions at a temperature of C and relative humidity of 40-55% for 20 min. A rectangular area (size: 4 x 6 cm) of the medial part of the forearm was used as the test area. Prior to sample application, the moisture content of SC and TEWL were measured with a moisture meter (ASA-M2) (Asahibiomed, Tokyo) and TEWL monitor (VAPO SCAN AS-VT100RS) (Asahibiomed), respectively. A total of 0.5 g of the sample foam was then applied to both skin areas and rubbed with two fingers for 45 s in order to wash the skin. These areas were then rinsed with 100 ml water. Washing with sample foam and rinsing were repeated 6 times with an interval of 10 min, and the panelists were asked to acclimate to the same conditions described above for 20 min. The moisture content of SC and TEWL were measured again. Water used in this study was Japanese pharmacopoeial purified water (Syowa Seiyaku, Osaka) unless otherwise described Effects of thickening polymers on the viscosity of 18% PM solution Soap solution composed of 18% myristic acid neutralized to 70% by potassium hydroxide was thickened by thickening polymers with different concentrations (0.3, 0.5, and 0.7% as total solids). Since potassium hydroxide contained in the soap solutions was also consumed to neutralize polymers, additional potassium hydroxide was added to the solution to maintain a neutralizing rate of 70%. The viscosities of the test solutions were measured at 25 C with a rotary viscometer (TVR 10) (Toki Sangyo, Tokyo) Thickening effects and salt tolerance of AACs In order to compare the thickening effects of the 6 AACs, the viscosities of 0.3% AAC solutions were measured at ph 7 and 25 C. In order to compare salt tolerances, NaCl powder was gradually added to 0.3% AAC solution (ph 7) at 25 C while monitoring the viscosity of the solution until it reached 50% of the initial viscosity. The amount of NaCl used is shown as the salt tolerance value Effects of washing with low fatty acid pasty facial-washes on the moisture content of SC A low fatty acid pasty facial-wash, shown in Table 1, and a conventional facial-wash were both diluted 10-fold with water and heated at 40 C, and 50 g of the warmed solution was placed into the mixer and mixed for 5 s. The foam obtained was applied to the skin areas of 6 panelists. Measurements were performed after washing 6 times by the same procedure described in Table 1 Formulations of soap-based pasty facial-washes with different fatty-acid contents 3

4 Type of facial-wash Conventional Conventional with low FA Low-fatty-acid Material* Facial-wash A with 30% FA** w/o AAC-1*** Facial-wash B with 18% FA w/o AAC-1 Facial-wash C with 18% FA with AAC-1 1 AAC Lauric acid Myristic acid Stearic acid Glyceryl stearate Potassium hydroxide Glycerin Tetrasodium EDTA Phenoxyethanol Polyquaternium Water up to up to up to *The names of materials were shown as INCI names. **FA indicates fatty acid. ***AAC-1, Carbopol SC- 200, one of acrylates/c10-30 alkyl acrylate crosspolymers (AACs) Evaluation of the sensory profile of the low fatty acid pasty facial-wash Twenty female panelists in their 20s ~ 60s were asked to use Facial-wash C (Table 1), a low fatty acid pasty facial wash, as usual once or twice, and a questionnaire survey was conducted in order to evaluate the sensory profile of the facial-wash. Nine evaluation items were easiness of foaming, foam volume, foam elasticity, comprehensive evaluation of foaming, refreshing feeling, moisture-rich feeling, no skin tightness, no irritation, and comprehensive evaluation. Panelists were asked to evaluate 9 evaluation items using a scale of one to five points; 1 (unsatisfactory) ~ 3 (fair) ~ 5 (satisfactory). The averaged points of 9 evaluation items were calculated and plotted on a radar chart. 3.Results and Discussion 3-1. Effects of washing with neutralized PM solution on the moisture content of SC and TEWL Changes in the moisture content of SC and TEWL in the skin areas washed 6 times with PM-1.8% and PM-3.0% were shown in Figure 2, and the moisture content and TEWL values were relative values to those obtained before washing. Washing with PM- 1.8% was found to significantly suppress reductions in the moisture content of SC more than that with PM-3.0% (Figure 2A). A previous study reported that when pasty facialwashes were foamed with water before use, the final dilution ratio was approximately 10 [5]. These findings support facial-washes with 18% PM suppressing reductions in the content than of SC moisture more than 30% PM, indicating the superiority of decreasing the amounts of fatty acids in pasty facial-washes. TEWL measurements revealed no significant differences in TEWL values of the skin between PM-1.8% and PM-3.0% (Figure 2B). 4

5 Figure 2: Effects of neutralized potassium myristate (PM) on the moisture content of the stratum corneum (SC) and transepidermal water loss (TEWL) The relative moisture content of SC and TEWL values were calculated by dividing with the initial values before washing. (A) Changes in the moisture content of SC with PM-1.8% and PM-3.0% (1.8% and 3.0% PM solutions). (B) Changes in TEWL with PM-1.8% and PM-3.0% (1.8% and 3.0% PM solutions). The bars and vertical lines on the bars show the mean values and standard deviations (SD) (n = 4), respectively. Statistical analyses were performed using the Student s t-test, the asterisk (*) indicates a P value less than 0.05 (p < 0.05), and n.s. indicates not significant Thickening effects of thickening polymers on PM aqueous solution Since the superiority of decreasing the amounts of fatty acids in pasty facial-washes was indicated above, adequate thickening polymers with the ability to maintain the viscosities of final products in spite of reductions in the amounts of fatty acids were screened. Six commercially available AACs were selected as candidate thickening polymers. Figure 3 shows the properties of each polymer in a scatter graph of the viscosities of 0.3% neutralized AAC solutions and their salt tolerance values, which were the amounts of NaCl that resulted in the viscosities of polymers being 50% of the initial viscosity. The viscosities and salt tolerance values of the polymers examined varied, and may be attributed to their molecular weights, crosslinking degrees, and monomer ratios. An aqueous solution of 18% myristic acid neutralized by 70% with potassium hydroxide were prepared and named PM-18%. Figure 4 shows the thickening effects of 9 thickening polymers on PM-18%. ACCs exhibited stronger thickening effects than other acrylates types of polymers such as CB (Carbomer), AC (Acrylates Copolymer), and ABM (Acrylates/beheneth-25 methacrylate copolymer). Among the ACCs studied, AAC-1 exerted the strongest thickening effect. As shown in Figure 3, the viscosity of AAC-1 was high in neutralized aqueous solution, while its salt tolerance was moderate. Since PM is an electrolyte that functions in a similar manner to NaCl, the high salt tolerance of AAC-1 may be related to the most efficient thickening performance in PM solution. These results indicate that the properties of high salt tolerance and high efficient thickening of water solution are key properties for thickening polymers appropriate for thickening low fatty acid pasty facial-washes. 5

6 Figure 3: Scatter graph of viscosities of 0.3% neutralized acrylates/c10-30 alkyl acrylate crosspolymer (AAC) solutions and their salt tolerance values The names and chemical structures of AACs are provided in the text and Figure 1. *The salt tolerance value was defined by the amount of NaCl resulting in the viscosity of the polymer being 50% of the initial viscosity. Figure 4: Effects of thickening polymers on 18% potassium myristate (PM) aqueous solution The black bars and bars marked with horizontal lines and skew lines show the viscosities of 18% PM aqueous solutions mixed with 0.30, 0.50, and 0.70 w% of total solid of thickening polymers, respectively. The percentages show the final concentrations of the polymers. Thickening polymers used in these measurements were carbomer (CB), acrylates copolymer (AC), acrylates/beheneth-25 methacrylate copolymer (ABM), and 6 acrylates/c10-30 alkyl acrylate crosspolymer (AAC-1~-6), the names and chemical structures of which are provided in the text and Figure 1. AC is supplied as a 30% aqueous solution Preparation of the low fatty acid pasty facial wash with acrylates/c10-30 alkyl acrylate crosspolymer (AAC-1) Since AAC-1 showed the highest viscosity in 18% PM solution, we investigated 6

7 whether AAC-1 performs well, even in 18% total solid fatty acid solution composed of different fatty acid chain lengths with a combination ratio that is widely used in commercial pasty-facial washes (C12:C14:C18=1:4:1). Three different formulations of facial washes were prepared according to the formulae shown in Table 1. The appearance of each formulation is shown in Figure 5. Facial-wash A is a conventional pasty facial-wash with a 30% total fatty acid content without a polymer and showed a firm pasty appearance (Figure 5-A). When the fatty acid content was reduced to 18% without a polymer, a liquid appearance was observed and may have been due to the insufficient crystallization of fatty acid salts (Facial-wash B, Figure 5-B). On the other hand, a facial-wash with an 18% total fatty acid content was thickened with 0.5% total solid of AAC-1, and it showed a firm pasty appearance without liquidity (Facial-wash C, Figure 5-C). The stability of facial-wash C was confirmed at 40 C for 3 months without separation or a significant reduction in viscosity. These results suggest that AAC-1 compensates for the insufficient crystallization of fatty acids due to its stronger thickening efficiency, while resulted in the pasty appearance being maintained with long-term stability. Figure 5: Appearances of prepared pasty facial-washes Refer to Table 1 for information on each formulation Effects of washing with the low fatty acid pasty facial-wash on the moisture content of SC After Facial-washes A (30% of fatty acid without polymer) and C (18% of fatty acid with 0.5% total solid of AAC-1) were diluted 10-fold with water, the skin of the inner forearm was washed with these facial-washes 6 times. As shown in Figure 6, Facialwash C significantly prevented reductions in the moisture content of SC more than Facial-wash A. This result corresponded to the results shown in Figure 2, confirming the efficacy of the lower fatty acid content on the moisture content of SC in prepared facialwashes. 7

8 Figure 6: Effects of washing with pasty facial-washes on the moisture content of the stratum corneum (SC) The relative moisture content value of SC was calculated by dividing with the initial values before washing. Bars and vertical lines on the bars show the mean values and standard deviations (SD) (n = 6), respectively. Statistical analyses were performed using the Student s t-test, the asterisk (*) indicates a P value of less than 0.05 (p < 0.05), and n.s. indicates not significant Evaluation of the sensory profile of the low fatty acid pasty facial-wash In order to confirm whether the sensory profile of low fatty acid pasty facial washes meets consumers expectations, 20 female panelists were asked to use Facial-wash C in their usual manner once or twice for several days, and at the end of the use test, a questionnaire survey was conducted in order to evaluate the sensory profile of the facial-wash (Fig. 7). Although decreases in the amounts of fatty acid salts were expected to adversely affect foam volume and foaminess, the sensory scores of Facialwash C in Easiness of foaming, Foam volume, and Foam elasticity were more than 3, with a score greater than 3 in Comprehensive evaluation of foaming. Other sensory parameters including No skin tightness after use and No irritation also obtained a score greater than 3, indicating that Facial-wash C is mild on the skin. These results indicate that higher amounts of fatty acid salts in facial-washes are not necessary to maintain sensory profiles for consumer satisfaction if the viscosity of a facial-wash remains high due to the use of a thickening polymer. 4. Conclusion Current pasty facial-wash products on the market contain significant amounts of fatty acid salts as a main surfactant, which are known to be more irritating to the skin than other surfactants used for wash products. The results of the present study clearly demonstrated that (1) a facial-wash with a lower fatty acid content (18%) was thickened by a particular acrylates/c10-30 alkyl acrylate crosspolymer, which has higher thickening efficiency and salt tolerance, (2) a facial-wash with lower fatty acid level thickened by this polymer contributed to the prevention of decreases in the moisture content of skin caused by washes, and (3) its sensory profiles were accepted by consumers as a facial-wash in spite of the low fatty acid content. 8

9 With these new results, facial-washes with lower fatty acid contents thickened by a polymer may be more widely used in the market as a new generation of soap-based pasty facial-washes. Figure 7: Radar chart of the sensory profile of the low fatty acid pasty facial-wash Panelists (n = 20) were asked to evaluate 9 sensory parameters on a scale of one to five; 1 (unsatisfactory) ~ 3 (fair) ~ 5 (satisfactory). The average individual scores of each sensory parameter were calculated and plotted on the radar chart. References [1] E. Oshimura, Y. Imori, Refreshed feeling of body wash shampoo, Fragrance Journal, 2, (1995) [2] H. Yoshihara, D. Kaneko, Developments and applications of new amino acid based surfactants for facial cleansing, Fragrance Journal, 7, (1996) [3] K. Deguchi, M. Arisawa, A. Ishida, K. Okamoto, G. Imokawa, Evaluation of Anionic Surfactants ; Usefulness of Monoalkyl Phosphates in Skin Cleaner Formulations, J. Soc. Cosmet. Chem. Jpn., 15(2), (1981) [4] T. Kurosawki, G. Imokawa, A. Ishida, Yukagaku, The Development of Monoalkyl Phosphate as a Low Skin Irritating Anionic Surfactant, 36(9), (1987) [5] C. Sato, How to the Use of Facial Cleansers Affects the skin: Facial Washing with an Ideal Lathering for Tenderness to the Skin, J. Soc. Comet. Chem. Jpn., 47(2), (2013) 9