JP7294604B2 - Evaluation method of "softness" in spherical particles for cosmetics - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for objectively evaluating "softness", which is one of the tactile characteristics of spherical particles for cosmetics.

Skin care and make-up cosmetics are generally used by a user picking them up from a container and applying them directly to the skin. Since cosmetics are used every day, it can be said that not only the functionality but also the feeling of use (tactile sensation) is an extremely important factor in the cosmetic effect.

Methods for evaluating the texture characteristics of cosmetics and raw materials are roughly divided into sensory evaluation and physical property evaluation. While sensory evaluation has the advantage of being an evaluation that is close to actual use, since it is a subjective evaluation, it is greatly affected by individual differences due to the evaluator's sensibility, as well as skin condition and psychological state. . Therefore, there has been a strong demand for techniques such as evaluation methods and display methods based on objective indices using physical equipment, without relying solely on sensory evaluation.

Spherical particles used in cosmetics have various materials and particle sizes. In particular, particles controlled to have an average particle size of 5 to 20 μm are widely used for the purpose of improving feel.

The tactile properties of the spherical particles include "softness" as well as rollability, and due to these favorable tactile properties, they have been applied to various formulations of cosmetics (for example, Patent Documents 1 and 2). Among them, in powder cosmetics in which powder accounts for the majority of the constituent components, it is positioned as an important raw material because the texture characteristics of the raw material powder are likely to be directly reflected in the formulation characteristics.

Synthetic polymers such as silicone elastomers, which are one type of spherical particles, have rubber elasticity and are known to have high "softness" as touch characteristics. Since the "softness" of these powders is considered to correlate with the durometer hardness described in JIS K6253, it is expected that the lower the hardness of the raw material itself, the softer the powder.

On the other hand, "softness" includes not only material-specific softness (chemical structure, elasticity, plasticity, etc.), but also softness characterized by morphological control (hollowness, porosity, etc.). In recent years, technology for developing functional materials has become more sophisticated, and powders with controlled shapes such as hollow and porous powders have been disclosed (for example, Patent Documents 3 to 5). These hollow or porous particles have been found to have a higher "softness" as a tactile property than solid or non-porous particles of the same constituents, respectively. However, since the softness imparted by such morphology control is a characteristic of a single particle, durometer hardness measurement with a wide measurement target area (in the order of square millimeters) could not be applied.

Spherical particles for cosmetics that are applied for the purpose of imparting "softness", including morphology control powders, often exhibit plastic deformation behavior. In other words, if a uniform and objective "softness" evaluation technology for spherical particles that exhibit plastic deformation behavior is established, it will be possible to expand the scope of application to a wider variety of particles than before. In addition to improving efficiency, it can also be used as a development guideline for new functional powders.

Objective and comprehensive "softness" evaluation technology that can be applied to various spherical particles and uses the same index not only improves work efficiency in the development of cosmetic formulations, but is also beneficial in the development of new functional powders. can be a useful tool.

Based on these backgrounds, an objective index of "softness" using physical equipment is required.

JP-A-1995-258026 JP-A-1996-310915 JP 2014-058441 A JP-A-1985-197746 JP-A-1994-136175

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for objectively evaluating the "softness" of cosmetic spherical particles using a physical instrument.

Under such circumstances, the inventors of the present application conducted intensive studies to solve the above problems, and found that the apparent Young's modulus E _y obtained from specific microparticle crushing force measurement and sensory evaluation of spherical particles for cosmetics The inventors have found that there is a high correlation between the obtained "softness" and have completed the present invention.

That is, in the present invention, the "softness", which is one factor of the feel characteristics of spherical particles for cosmetics, is analyzed by a specific analysis method in microparticle crushing force measurement (by applying an external force to plastically deformed particles, strain ε The stress-strain curve obtained by plotting the corresponding stress σ is represented by two straight lines with different slopes, the intersection of which is the yield point (ε _y , σ _y ) and the straight line connecting the origin as the apparent Young's modulus _Ey ), which is associated with the apparent Young's modulus _Ey as a physical property index.

The present invention provides an objective evaluation method for "softness" that can be applied to various spherical particles for cosmetics. It can also be used as a development guideline for new functional powders.

Overview of microparticle crushing force measurement device Graph showing correlation between apparent Young's modulus and softness of spherical particles for cosmetics

Hereinafter, a method for evaluating the feel of use of the spherical particles for cosmetics of the present invention will be described based on preferred embodiments thereof. According to the present invention, it is possible to objectively evaluate "softness", which is one of the feeling of use when spherical particles for cosmetics are applied to the skin. Here, the "softness" of the spherical particles for cosmetics refers to the sense of thickness of the powder and the feeling of good touch on the skin.

The evaluation method of the "softness" of the spherical particles for cosmetics of the present invention is carried out by performing microparticle crushing force measurement using a single particle of the powder to be evaluated, and plotting the stress σ corresponding to the strain ε. The "softness" of the cosmetic spherical particles is evaluated from the apparent Young's modulus _Ey calculated as the slope of the straight line connecting the yield point and the origin in the obtained stress-strain curve.

In an embodiment of the present invention, the apparent Young's modulus _Ey of the cosmetic spherical particles obtained from microparticle crushing force measurement according to the following procedure is evaluated as a physical property index.
(1) Sprinkle the spherical particles on the sample table and adjust the initial position so that the single particles are contained within the tip of the flat indenter while observing with an optical microscope.
(2) The sample table is operated at a constant speed in the direction of compression to apply an external force to the spherical particles, and the stress σ corresponding to the particle strain ε is continuously detected. Measurements are continued until the particles are completely collapsed.
(3) In the obtained stress-strain curve, the intersection of two straight lines with different slopes is defined as the yield point (ε _y , σ _y ), and the slope of the straight line with the origin is calculated as the apparent Young's modulus E _y .

The apparent Young's modulus _Ey of the cosmetic spherical particles obtained according to the above procedure is relatively compared, and the smaller the value, the higher the "softness".

In the measurement method of the present invention, the target spherical particles for cosmetics are particles exhibiting plastic deformation behavior. In the present invention, plasticity refers to the property of not returning to its original shape even if the load is removed after a certain stress (yield stress) is exceeded when an external force is applied. Conversely, the property of returning to its original shape is called elasticity, but in the measurement method of the present invention, particles exhibiting elastic deformation behavior are excluded.

In the measurement method of the present invention, the target particle diameter of the cosmetic spherical particles is 1 to 30 μm. Since the object of the present invention is to evaluate "softness" as a touch characteristic, the particle size range of spherical particles generally applied to powder cosmetics and the like for the purpose of improving touch is specified.

In the measurement method of the present invention, the shape of the target spherical particles for cosmetics is not limited to true spheres, and includes substantially spherical shapes. Practically, in applying microparticle crushing force measurement, it suffices if the particles have a sphericity to the extent that there is no problem.

In the evaluation of cosmetic spherical particles using a physical instrument, the tip diameter of the flat indenter used is not particularly limited, but is preferably φ30 μm or more. If the tip diameter of the planar indenter is smaller than φ30 μm, the particles protrude from the tip of the planar indenter during compression, and pressure cannot be applied to the entire particle, which may interfere with measurement.

In the evaluation of cosmetic spherical particles using physical instruments, the spring constant of the precision spring attached to the flat indenter used is not particularly limited, but is preferably 10 kN/m or less. If the spring constant is greater than 10 kN/m, it may not be possible to accurately detect strain on the low stress side.

In the evaluation of cosmetic spherical particles using a physical instrument, the moving speed of the sample table defined in procedure (2) is not particularly limited, but is preferably 1 μm/sec or less. If the moving speed is more than 1 μm/sec, the yield point may be reached in a short time, and a sufficient number of plots may not be obtained, resulting in a decrease in measurement accuracy.

In the evaluation of cosmetic spherical particles using a physical instrument, the data loading speed of stress and strain is not particularly limited, but is preferably in the range of 1 to 500 msec. If the data acquisition speed is 500 msec or more, a sufficient number of plots may not be obtained. A sufficient number of plots can be obtained if the data acquisition speed is 1 msec or more.

The apparent Young's modulus E _y can be measured using a displacement control microparticle crushing force measuring device as shown in FIG. Commercially available products include microparticle crushing force measuring device NS-A100 (manufactured by Nanoseeds Co., Ltd.). The indoor temperature and relative humidity when microparticle crushing force measurement is performed are preferably 10 to 35° C. and 30 to 65% RH, considering the environment when cosmetics are used.

EXAMPLES The present invention will be described in more detail below with reference to examples. It should be noted that these do not limit the present invention in any way.

(Example 1)
[Evaluation of the physical properties]
For cosmetic spherical particles A to E shown in Table 1, microparticle crushing force measurement was performed as follows to obtain apparent Young's modulus E _y . For each sample, 7 measurements were performed on different particles each time, the upper and lower 2 measurements were excluded, and the average value of the 5 measurements was calculated as the apparent Young's modulus E _y .
Measuring device: Microparticle crushing force measuring device (NS-A100 manufactured by Nanoseeds Co., Ltd.)
Measurement conditions: Room temperature 25°C, relative humidity 50% RH
Flat indenter tip diameter φ50μm
Spring constant 6.18kN/m
Sample table moving speed 0.2 μm/sec
Data acquisition speed 100msec
[sensory evaluation]
Regarding the "softness" of the cosmetic spherical particles A to E shown in Table 1, a use test was conducted by 7 specialist panels, and each panelist evaluated and scored on a scale of 5 according to the following absolute evaluation, and each sample was evaluated. The average value was calculated as the sensory evaluation value from the total score of all panel members.
1: very low 2: low 3: normal 4: high 5: very high

FIG. 2 is a graph plotting the apparent Young's modulus _Ey obtained from the fine particle crushing force measurement described in Table 1 and the "softness" obtained from the sensory evaluation for each sample. When a regression line was drawn for each plot, a coefficient of determination R ² =0.89 was found, indicating a very high correlation. In particular, cellulose with different particle morphology (B: porous, C: solid), or polymethyl methacrylate (D: non-crosslinked, E: crosslinked) with different chemical structures can be used to accurately detect differences in physical properties. The fact that it is caught in the figure proves the high measurement accuracy.

In the sensory evaluation results of spherical particles A to E for cosmetics, 3.5 points (because it is rounded to 4 points, it can be judged that the softness is high.) The plastic deformation generally recognized as a soft touch. Among spherical particles exhibiting behavior, it can be said that they are particularly soft spherical particles. That is, when the "softness" score of 3.5 is substituted for the linear function of the regression line, the Young's modulus is calculated to be approximately 100 MPa. Therefore, among spherical particles exhibiting plastic deformation behavior, those having a Young's modulus of 100 MPa or less can be judged to be particularly soft spherical particles.

The evaluation method of the present invention can evaluate the "softness", which is one factor of the feel characteristics of spherical particles for cosmetics, by associating the apparent Young's modulus _Ey obtained from microparticle crushing force measurement as a physical property index. be. If the evaluation method of the present invention is used, it can be used not only to improve the working efficiency of cosmetic formulation development, but also as a development guideline for novel functional powders.

Claims (1)

For spherical particles for cosmetics (limited to spherical particles exhibiting plastic deformation behavior), after obtaining the apparent Young's modulus E _y from the microparticle crushing force measurement according to the following procedures (1) to (3) , the apparent A method of relatively comparing Young's modulus E _y of E and judging that the smaller the value , the higher the "softness", which is one factor of the feel characteristics .
(1) Sprinkle the spherical particles on the sample table and adjust the initial position so that the single particles are contained within the tip of the flat indenter while observing with an optical microscope.
(2) The sample table is operated at a constant speed in the direction of compression to apply an external force to the spherical particles, and the stress σ corresponding to the particle strain ε is continuously detected. Measurements are continued until the particles are completely collapsed.
(3) In the obtained stress-strain curve, the intersection of two straight lines having different slopes that can be detected before and after plastic deformation is the yield point (ε _y , σ _y ), and the slope of the straight line with the origin is the apparent Young's modulus E Calculate as _y .

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