Research on the intrinsic mechanism of the darkening of liquid foundation

Abstract

Background

To investigate the intrinsic mechanism that causes the darkening of liquid foundations.

Materials and method

A total of 36 commercial liquid foundations were firstly studied for preliminary screening of influencing factors. A basic liquid foundation was developed for controlling variables to study the influence of each single factor. These samples were evenly spread on the standard opacity charts with the thickness of 100 μm and applied onto human inner forearm skin with the dosage of 2 mg/cm². The discoloration of each sample was continuously recorded using spectrophotometers and reported in the CIE 1976 L*a*b* color space for at least 120 min, and ΔE was calculated to describe the severity of darkening.

Results

One hundred twenty‐minute ΔE of all commercial foundations was highly negatively correlated with their 120‐min ΔITA° (R² = 0.88, p < 0.01). A strong positive correlation was found between the severity of darkening and the volatilization of the basic foundations (R² = 0.83, p < 0.01). And the darkening of silicone‐based basic foundations using pigment coating with silicon is weaker than those without silicon (p < 0.05).

Conclusion

The process of the discoloration of liquid foundation is accompanied by the decrease of ITA° and manifested as darkening. The volatilization rate of the product and the coating method of the pigments used in the formula can noticeably affect the darkening of the liquid foundation.

1. INTRODUCTION

Base makeup products are a fundamental part of daily makeup among a large number of makeup products. Base makeup mainly includes liquid foundation, creamy foundation, concealer, BB/CC cream, cushion, etc. Consumers use base makeup products to even out skin tone, cover pores, blemishes, wrinkles, and brighten the complexion. ¹ In the Chinese market, foundation is still the core demand of consumers among the face makeup category, accounting for 78% of the total sale volume in the complete range in 2020. ²

Additionally, consumers' expectations for foundations are raising continuously, and their needs are becoming more and more complex. A survey conducted by Mintel in April 2021 on 2988 female Internet users who aged 18–49 years old and have used facial makeup products over the past 6 months indicates that 41% of the users thought that they would be more displeased if their foundations are prone to darkening in shade, and 45% of the users claimed interest in antidarkening performance of the products when selecting foundations. ² The market is also paying more and more attention to the problem of darkening, which is being mentioned in the promotion of several newly launched foundations. According to the data released by Tmall in 2021, 16 of the 49 foundations put on the market in 2019 claimed antidarkening, accounting for 32.7%, 14 of the 35 foundations put on the market in 2020 claimed antidarkening, accounting for 40%, and 21 of the 33 foundations put on the market in 2021 claimed anti‐darkening, accounting for 63.6%. It was revealed that the proportion of new foundation products claiming antidarkening has significantly increased from 2019 to 2021.

Yan et al. studied on how to test and characterize the darkening of liquid foundations. ³ This study continuously recorded the discolorations of the four liquid foundation products within 24 h by applying the liquid foundation uniformly on human skin and on a standard opacity chart and measured the color change with a spectrophotometer. It was found that the discoloring process was accompanied by the decrease of L* and h* (L* and h* of CIE L*C*h*). Dt (the discoloration amount after a passage of time) and TΔE (the time until a critical discoloration) were finally defined as the parameter for the degree of darkening of the liquid foundation. Huang et al. published a study in 2021 who studied liquid foundations with different treatments of titanium dioxide and different types of powder filler from three aspects involving sebum, sweat, and light. ⁴ The difference in lightness of the liquid foundations before and after intervention was measured using similar methods to compare the influence of each factor on the darkening. The final experiment showed that sebum, sweat, and light would all affect the darkening of the liquid foundations.

However, the mechanisms of the intrinsic darkening of liquid foundation have still remained elusive. In this study, we suggested a method to make scientific inquiry into the darkening problem quantitatively. And we firstly evaluated the darkening severity of 36 commercially available liquid foundations with different technologies and complaints of darkening at different levels to screen for the potential influencing factors. A basic liquid foundation was then developed, and it was adjusted by the addition of different solvents and pigments with different coating methods to further explore and verify the intrinsic mechanism of the darkening through multi‐dimensional measurement and analysis.

2. MATERIALS AND METHODS

2.1. Liquid foundation samples

Thirty‐six liquid foundations from different brands around the world with complaints of darkening at different levels and different technologies including formula systems, filming forming agents, pigment coating methods, solvents, etc. were selected for firstly exploring the potential influencing factors.

A basic liquid foundation was developed in the present study to evaluate the effect of single variable by addition of different solvents, solvent mixtures or pigments with different coating methods. The basic liquid foundation is consisting of water phase, oil phase, pigments, and preservative. Table 1 shows the formula structure of the basic liquid foundation.

TABLE 1.

Formula structure of the basic liquid foundation

Phase	Ingredient	Wt%
Water phase	AQUA	23.00
	SODIUM CHLORIDE	1.00
	GLYCERIN	8.00
Oil phase	EMULSIFIER	8.50
	SOLVENT	48.70
Pigments	TITANIUM DIOXIDE	8.80
	CI 77492	0.85
	CI 77491	0.28
	CI 77499	0.07
Others	PRESERVATIVE	0.80

2.2. Testing on standard opacity charts

2.2.1. Sample preparation

LENETA opacity chart shown in Figure 1A was selected as the substrate to study the darkening of different foundations over a period of time in the study. The laboratory temperature and humidity were controlled at 25°C ± 1 and 45% ± 5 respectively during the test. A four‐sided applicator with one side of thickness of 100 μm was used to evenly apply a 100‐μm thickness product on the opacity chart (Applicator: AICE SZQ Four‐Sided Applicator, 25, 50, 75, 100 μm, Figure 1B). The thickness of 100 μm enabled less influence from the subtract and ensured the repeatability of each test. One of the prepared samples is shown in Figure 1C. A spectrophotometer (HunterLab Agera, Hunter Associates Laboratory, Inc., Reston, VA, USA; Figure 1D) was used to record the darkening process of each sample.

FIGURE 1.

Materials for testing discoloration of liquid foundation on standard opacity chart. (A) LENETA Opacity chart; (B) AICE SZQ four‐sided applicator; (C) completed sample preparation; (D) HunterLab Agera spectrophotometer

2.2.2. Measurement and parameters

Measurement started immediately after sample preparation and resulted in colorimetric data (L*, a*, b* in CIE1976 L*a*b* color space), corresponding color simulations and spectrum data at range of 350 to 780 nm with the spectrophotometer. Before every measurement, the light source was configured at standard illuminant (D65) with viewing angle of 2°. Three measurements were happened for each sample, and their average value was calculated. For the colorimetric data, L* indicates the lightness, a* explains greenness‐redness, and b* explains blueness‐yellowness. ⁵

2.2.3. Data analysis

Through the colorimetric data, the ITA° (Individual Typology Angle) at each time point and the color difference versus baseline for each sample at each time point can be calculated using the equations below ⁶ , ⁷ :

$${\mathrm{ITA}}^{\circ }=\frac{{\tan }^{-1}\left[\left(L^{\ast }-50\right)/b^{\ast }\right]\times 180}{\pi }$$

$$\mathrm{\Delta }\mathrm{E}=\sqrt{{\left(\mathrm{\Delta }{\mathrm{L}}^{\ast }\right)}^2+{\left(\mathrm{\Delta }{\mathrm{a}}^{\ast }\right)}^2+{(\mathrm{\Delta }{\mathrm{b}}^{\ast })}^2},$$

where ΔL*, Δa*, and Δb* were the differences in the values of L*, a*, and b* at baseline (T0, immediately after sample preparation) and after a period of time. Table 1 shows the test data of discoloration of a commercial foundation (a long‐wear, long‐lasting moisturizing liquid foundation for dry skin) before and after 120 min. For the convenience of display, data at some time points were omitted. Figure 2A represents the color simulation of the foundation before and after 120 min, with the left side showing the color at baseline, and the right side illustrating the color at 120 min. It could be seen that the 120‐min darkening of this liquid foundation was relatively noticeable. Figure 2B shows the specific spectrum corresponding to the colors before and after 120 min, in which the red curve is the spectrum corresponding to the color at T0, and the black curve represents the spectrum corresponding to the color at 120 min. After calculation according to data in Table 2, Figure 3A depicts the variation trend of ITA° in 120 min for this commercial foundation, and Figure 3B shows the variation trend of ΔE within 120 min.

FIGURE 2.

The discoloration of a commercial foundation in 120 min recorded by a spectrophotometer. (A) The color simulations of the foundation at 0 min (the left side) and at 120 min (the right side). (B) The spectrum corresponding to the color at 0 min (red curve) and at 120 min (black curve)

TABLE 2.

Discoloration date of a commercial liquid foundation in 120 min

T/min	L*	a*	b*
0	80.95	11.59	20.63
12	79.78	12.67	21.73
24	79.68	12.93	21.95
36	79.63	13.02	22.03
48	79.63	13.07	22.11
60	79.61	13.12	22.16
72	79.62	13.17	22.23
84	79.59	13.20	22.28
96	79.58	13.24	22.33
108	79.56	13.26	22.38
120	79.53	13.29	22.43

FIGURE 3.

The variation trend of ΔITA° within 120 min (A) and the variation trend of ΔE within 120 min (B) of basic foundation number: 1#

2.3. Testing method for the volatility of samples

The design of volatility test mainly referred to gravimetric method. ⁸ Specifically, a filter paper (Titan¢11 cm) was cut into a size of 2 * 2 cm², put into a disposable culture dish, and then, exposed to the constant temperature of 31°C for 30 min. The weight of the filter paper and the culture dish was measured and recorded (filter paper and culture dish). After that 0.2 g of the sample was taken and spread evenly on the filter paper, and the overall weight (sample, filter paper, and culture dish) was initially measured at 0 min. The test sample (sample, filter paper, and culture dish) was put into a 31°C incubator, and the weight of the sample dish was recorded every 15 min. After 60 min, the time interval of weighing was extended to 30 min, and after 120 min, the time interval was extended to 60 min. The 240‐min data were finally fetched. The decrease at each time point was calculated, and the scatter diagram for the percentage of weight loss was drawn. Two measurements were taken for each sample, and the arithmetic mean was calculated. Figure 4 shows the volatility curve of a basic foundation (number: 1#).

FIGURE 4.

Volatility curve of a basic foundation (number: 1#) in 240 min

2.4. Testing on human forearm skin

The laboratory temperature and humidity were controlled at 25°C ± 1 and 45% ± 5. Four healthy female participants were enrolled in the study, and their inner forearm skin was chosen as the test area because the sebum secretion on forearm skin is much slower than it on facial skin. Testing on inner forearm skin can help to reduce the impact from oil and better simulate the darkness of the liquid foundation intrinsically. Before each test, all participants would firstly wash their forearm skin with a cleanser, then stayed in the laboratory for 30 min to balance their skin condition. Products were applied onto the inner area of forearm skin at the size of 4 × 4 cm² with the dosage of 2 mg/cm². A photo of one basic foundation (number: 1#) tested on human forearm skin is shown in Figure 5A. Immediately after the product was evenly applied, another portable spectrophotometer (HunterLab Miniscan EZ, Hunter Associates Laboratory, Inc., Reston, VA, USA; Figure 5B) was used to test and record the colorimetric data of the liquid foundation at 0 min (immediately after foundation application), 60 min, and 120 min. Besides, three measurements for each sample were carried out, and average value of ΔE versus T0 for all participants was calculated. The data of discoloration of the basic foundation on one participant's forearm skin within 120 min are presented in Table 3.

FIGURE 5.

One basic foundation sample tested on human arm (A); a spectrophotometer used to test the discoloration of foundations on the human arm (B)

TABLE 3.

Discoloration data of a basic foundation (number: 1#) on human forearm skin in 120 min

Time	0 min			60 min			120 min
Parameters	L*	a*	b*	L*	a*	b*	L*	a*	b*
Repeat 1	76.34	9.89	21.08	75.1	10.22	20.73	74.59	9.85	19.66
Repeat 2	76.62	10.39	21.11	75.54	10.3	20.37	75.48	10.42	20.27
Repeat 3	74.16	8.94	20.16	74.25	9.61	19.68	74.66	9.87	19.68

3. RESULTS

3.1. Darkening of commercial foundations and data analysis

The discoloration curve of the 36 commercially available products tested on the standard optical chart compared with the baseline within 120 min is shown in Figure 6. As shown in the figure, the rate and degree of discoloration of the products varied noticeably. Among them, the 120‐min ΔE (ΔE vs. T0 at 120 min) of the product with the least discoloration was approximately 1. This discoloration is almost indistinguishable by human eye, since it's lower than 2.3 which is the just noticeable difference in color. ⁹ , ¹⁰ , ¹¹ The 120‐min ΔE of the liquid foundation with the largest discoloration was almost 8, which obviously exceeded 2.3, indicating that the severity of discoloration among products differs greatly. It can also be seen from Figure 6 that even products with a similar discoloration at 120 min showed a significant difference in the discoloration rate.

FIGURE 6.

120‐min discoloration curve of 36 commercially available liquid foundations

Comparison of 120‐min ΔE and 120‐min ΔITA° (difference of ITA° between 120 min and 0 min) of the commercial foundations is illustrated in Figure 7. As illustrated in the figure, there is a very high negative correlation between data of 120‐min ΔITA° and data of 120‐min ΔE (R² = 0.88, p < 0.01), indicating that the discolorations of the liquid foundations led to a decrease in ITA°, further demonstrating that the liquid foundation became duller. Therefore, ΔE was considered as the key parameter to compare the darkening difference of the liquid foundations in the present study.

FIGURE 7.

Comparison of 120‐min ΔE and 120‐min ΔITA° among 36 commercial liquid foundations

The present study further compared the data of ΔE at 5 min, 10 min, 60 min, and 120 min of all commercial foundations with their difference in formula including system categories, film forming agents, pigment coating methods, and solvents. The correlation is shown in Table 4, in which * represents P < .05, and ** indicates p < 0.01. As shown in Table 4, the darkening of liquid foundations was likely to be affected by film‐forming agents, pigment coating methods, and solvents.

TABLE 4.

Correlation analysis of products’ darkening and formula technologies at 5, 10, 60, and 120 min

Discoloration	Formula system	Film forming agent	Pigment coating with/without silicon	Formula with/without D5	Formula with/without isododecane
5‐min ΔE	−0.091	.323*	−0.154	−0.25	0.059
10‐min ΔE	0.035	0.239	−0.277	−.344*	0.094
60‐min ΔE	−0.11	.309*	−0.215	−0.116	0.077
120‐min ΔE	−0.085	.360*	−0.316*	−0.154	0.174

The formulation systems of all commercially used products were classified into water‐in‐silicone (silicone oil), water‐in‐silicone/D5 (Cyclopentasiloxane), hybrid, and water‐in‐oil (non‐silicone oil) systems. There were merely three products belonging to the oil‐in‐water system, the statistical comparison was infeasible, thus, the oil‐in‐water system was not herein discussed. The differences in the 120‐min ΔE among the systems are shown in Figure 8. The data showed that there was no statistically significant difference among the different formulation systems.

FIGURE 8.

Comparison of darkening (120‐min ΔE) of foundations with different formulation systems

The solvents contained in all above commercial foundations were classified into isododecane, D5, D5/isododecane, and other rarely applied solvents. D5/isododecane were applied to five liquid foundations, isododecane was used in eight foundations, and D5 was applied to 17 foundations. After comparing the degree of darkening of foundations containing different types of solvents, as shown in Figure 9, 5‐min ΔE and 10‐min ΔE of liquid foundations using isododecane were significantly greater than those of foundations using D5/isododecane (p < 0.05); the 10‐min ΔE of the foundations using isododecane was also significantly higher than that of the liquid foundation products using only D5 (p < 0.05). Although no significant difference was found at other time points, the ΔE of the products containing isododecane was also higher than those of the products containing other solvents, which indicated that the use of isododecane as a solvent could exacerbate the darkening of liquid foundation products. On the basis of the above‐mentioned findings, the present study further compared the 120‐min darkening between the liquid foundations with isododecane and the liquid foundations without isododecane, as shown in Figure 10. It can be seen from the figure that the 120‐min ΔE of the liquid foundation products containing isododecane is significantly greater than that of the liquid foundation products without isododecane (p < 0.05).

FIGURE 9.

Comparison of the darkening of foundations containing different volatile solvents

FIGURE 10.

Comparison of darkening at 120 min between foundations with and without isododecane

According to Table 4, there is also an evident correlation between the liquid foundations applied pigments with different coating methods and their severity of darkening. Due to the wide variety of pigment coating methods, ¹² the present study further divided the pigment coating methods into two categories: pigment coating with silicon and pigment coating without silicon. For the 36 commercial foundations, there were 17 products applied pigment coating with silicon and 18 products applied pigment coating without silicon. ΔE at 5 min, 10 min, 60 min, and 120 min between the two types of liquid foundation products were analyzed and compared in Figure 11. It can be seen that the 120‐min ΔE of the products applying pigment coating without silicon were significantly higher than those of the products applying pigment coating with silicon (p < 0.05). This indicated that different pigment coating methods could also have a certain influence on the darkening of the liquid foundations.

FIGURE 11.

Comparison of darkening between foundations applying pigment coating with silicon and foundations applying pigment coating without silicon

3.2. The effect of solvents on darkening of liquid foundations

The present study compared the volatilities of isododecane and D5, and it was found that the volatilization rate of isododecane was higher than that of D5 as shown in Figure 12. Thus, a hypothesis was generated: when the proportions of both solvents were close to equivalence, the higher volatilization rate of the solvent accelerates the increase of pigment concentration in the base, thereby resulting in the faster darkening. If the same volatile solvent is used, when the concentration of the volatile solvent in the product is higher, the final pigment concentration in the liquid foundation will be higher, and further lead to more severe darkening. The hypothetical model is illustrated in Figure 13.

FIGURE 12.

Comparison of volatility between isododecane and Cyclopentasiloxane

FIGURE 13.

Hypothesis on the high correlation between volatility and darkening of liquid foundations

In order to verify this hypothesis, 10 different basic foundations were developed by addition of different solvents or solvent mixtures. The formulas of these basic foundations are shown in Table 5. This study characterized the volatility and darkening of each basic liquid foundation in 240 min and analyzed the correlation between volatility and degree of discoloration. The volatilization curves of the foundations are shown in Figure 14A, and the darkening curves are illustrated in Figure 14B. The correlation between the 240‐min ΔE and the 240‐min volatilization (weight loss %) was R² = 0.83 (p < 0.01), and the mean correlation between darkening trend and volatilization trend in 240 min of each sample was R² = 0.75 (p < 0.01 for all samples). It indicated a high positive correlation between the volatilization rate and the rate of darkening of the liquid foundations. Meanwhile, the volatile percentage of the liquid foundation within a certain period of time and the darkening (ΔE) in the same period of time were also highly correlated. Figure 15 shows the volatilization curve of all the solvents used in above basic foundations. It can be seen from the figure that the volatilization curves of different solvents are significantly different.

TABLE 5.

Basic liquid foundations (number: 1# ‐ 10#) containing different solvents/solvent mixtures

	Formula number		1#	2#	3#	4#	5#	6#	7#	8#	9#	10#
Phase	Number	Ingredient	Wt%	Wt%	Wt%	Wt%	Wt%	Wt%	Wt%	Wt%	Wt%	Wt%
Water phase	∖	∖	32.00	32.00	32.00	32.00	32.00	32.00	32.00	32.00	32.00	32.00
Emulsifier	∖	∖	8.50	8.50	8.50	8.50	8.50	8.50	8.50	8.50	8.50	8.50
Slovents	1	Cyclopentasiloxane	31.70		23.70	48.70
	2	Isododecane		31.70	8.00
	3	Aqua										17.00
	4	Phenyl trimethicone	17.00	17.00	17.00		48.70					31.70
	5	Methyl trimethicone						48.70
	6	Caprylyl methicone							48.70
	7	Dimethicone 1cst								48.70
	8	Isononyl isononanoate									48.70
Pigments	∖	∖	10.00	10.00	10.00	10.00	10.00	10.00	10.00	10.00	10.00	10.00
Preservative	∖	∖	0.80	0.80	0.80	0.80	0.80	0.80	0.80	0.80	0.80	0.80
			100.00	100.00	100.00	100.00	100.00	100.00	100.00	100.00	100.00	100.00

FIGURE 14.

Volatilization curve of each basic formula (A), discoloration curve of each basic formula (B)

FIGURE 15.

Volatilization curves of each solvent involved in the present study

In order to further verify the influence from solvents on darkening of foundations, and considering avoiding influences from sebum and sweat, the inner forearm was selected for darkening evaluation on human skin. The 120‐min ΔE of each basic liquid foundation on the arm skin and on the standard opacity chart were compared. The scatter diagram is shown in Figure 16. It can be seen that the 120‐min ΔE of each product on the arm is highly correlated with the 120‐min ΔE on the opacity chart with R² = 0.71 (p < 0.01). The data further support the finding that more volatilization of a liquid foundation leads to more pronounced darkening of the product.

FIGURE 16.

Scatter diagram of the distribution of 120‐min ΔE of each basic foundation with different solvents on the human forearm and on the opacity chart

3.3. The effect of pigment coating methods on the darkening of liquid foundations

In order to verify the influences of different pigment coating methods on the darkening, same dosage of six types of pigment with different coating methods was added to a silicone‐based basic liquid foundation in the present study. The pigment used in each basic foundation and the corresponding pigment coating method is shown in Table 6. The discoloration trends of the six foundations are displayed in Figure 17. After comprehensively comparing the coating methods of the pigment used in the basic foundations, it can be seen that in products with the same formula, the darkening and discoloration between products using different pigment coating methods are quite different.

TABLE 6.

Pigment with different coating methods used in basic formulas (number: 11# ‐ 16#)

Formula number	Pigment coating method	Whether containing silicon	120‐min ΔE
11#	Aluminum hydroxide and dimethicone crosspolymer	Yes	0.96
12#	Aluminum hydroxide and titanium triisostearoylisopropoxide	No	1.50
13#	Aluminum hydroxide and polymethylsilane	Yes	0.66
14#	Aluminum hydroxide and myristic acid	No	1.39
15#	Aluminum hydroxide and disodium stearoyl glutamate	No	1.21
16#	Aluminum hydroxide and dimethicone and Titanium triisostearoylisopropoxide	Yes	1.08

FIGURE 17.

Trend of darkening of six basic foundations using pigment with different coating methods

The four participants further applied these six liquid basic foundations using pigments with different coating methods by turns on their forearm, and the darkening of each sample on each participant was recorded. Comparison of 120‐min ΔE between two groups of basic foundations on human forearm (foundations with pigment coating with silicon and foundations with pigment coating without silicon) is shown in Figure 18. The result indicated that silicone‐based formulas applying pigment coating with silicon have relatively lower darkening than that of the formulas applying pigment coating without silicon (p < 0.05).

FIGURE 18.

Comparison of darkening on human forearm between basic foundations applying pigment coating with silicon and pigment coating without silicon

The hypothesis behind this phenomenon is described below: the pigment coating with silicon has a better dispersity in the silicone‐based foundation formula, while the pigment coating without silicon has a relatively poor dispersity and is easier to agglomerate, resulting in more darkening.

4. DISCUSSION

In the China's market, consumers mainly take the words “oxidation” and “darkening” to describe the discoloration of liquid foundations. However, the statement about the oxidation of liquid foundation has not been scientifically confirmed. The basic ingredients in liquid foundation are water, grease, silicone oil, emulsifier, film forming agent, and pigment, ¹³ of which water is naturally not prone to oxidation, and most of the grease used in liquid foundation is saturated synthetic grease, which does not involve oxidation problem. Silicone oil is highly stable, ¹⁴ and it is also difficult to oxidize in the formula of liquid foundation and at normal temperature. The pigment in the liquid foundation is made from inorganic pigments, such as titanium dioxide and iron oxides (ferric oxide, ferric oxide monohydrate, and ferrosoferric oxide), which are all in their oxidized forms and difficult to be oxidized further. Some scholars concentrated on the measurement of the darkening of liquid foundation and extrinsic factors influencing the degree of darkening. While in the present study, it was attempted for the first time, to investigate the intrinsic mechanism of the darkening of liquid foundation products.

However, there are some limitations of this study that deserve further study. Firstly, the present study only investigated the darkening of different liquid foundations on the standard opacity chart and tested the reliability of the hypotheses on the inner forearm skin in a well‐controlled experimental environment with less impact from external factors such as sebum secretion. While the actual darkening on users’ facial skin would be much more complex due to different skin types, external environment, and the user's skin conditions, such as sweating, oiliness, use of premakeup products, etc. Different users may also have different perceptions of the darkening of liquid foundation. The study by a spectrophotometer is relatively objective, and the actual subjective experience also requires further research.

Secondly, this study selected isododecane and Cyclopentasiloxane as the solvent classification criteria for making comparison before proposing the hypotheses, because these two solvents are commonly used in commercial products. Hence, only the volatility and darkening of the liquid foundation products were compared, which does not necessarily indicate that liquid foundation products using isododecane are more prone to darkening. The final darkening performance of a product is more complex and should be determined by several factors.

Thirdly, a trend was found that pigment coating with silicon has a relatively less darkening versus using pigment coating without silicon. However, only six pigment coating methods were involved in this study. A more solid conclusion will still require further assessment.

Fourthly, according to the analysis of the correlation between the darkening of the 36 commercially available liquid foundations, a correlation between the darkening and the film forming agent was found, which was not investigated in this study in detail.

Lastly, none of the 36 commercially available liquid foundations involved in this study belongs to pure oil‐in‐water system products, and the sample size of water‐in‐oil system products is relatively small. The darkening mechanism of these two types of liquid foundations requires further study.

5. CONCLUSIONS

This study quantitatively analyzed the discoloration of foundations with different solvents and pigments with different coating methods by adjusting the formula in the same basic liquid foundation and verified the accuracy on the human inner forearm skin. Finally, the following conclusions are drawn:

1. There is a strong positive correlation between the rate of darkening and the volatility of different products. The faster the volatilization is, the faster the darkening of the liquid foundation product. The more volatile the liquid foundation product is, the stronger the darkening.

2. In the formula of liquid foundation containing silicone, the darkening degree of product using pigment coating with silicon is weaker than that of the pigment coating without silicon, that is, applying pigment coating with silicon in the silicon‐containing foundation products can help reduce the severity of darkening to some extent.

CONFLICT OF INTEREST

The authors declare that they have no competing interests.

FUNDING INFORMATION

The authors received no specific funding for this work.

Chen G, Tan Y, Wang S, Yu J, Yang C. Research on the intrinsic mechanism of the darkening of liquid foundation. Skin Res Technol. 2023;29:1–11. 10.1111/srt.13236

DATA AVAILABILITY STATEMENT

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.