Nanotechnology and Nanomaterials: Nanometer Titanium Dioxide in Sunscreen Cosmetics

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About 5% of the rays radiated by the sun have ultraviolet rays with a wavelength ≤400 nm. Ultraviolet rays in sunlight can be divided into: long-wave ultraviolet rays with a wavelength of 320 nm~400 nm, called A-type ultraviolet rays (UVA); Medium-wave ultraviolet rays with a wavelength of 290 nm to 320 nm are called B-type ultraviolet rays (UVB) and short-wave ultraviolet rays with a wavelength of 200 nm to 290 nm are called C-type ultraviolet rays.

Due to its short wavelength and high energy, ultraviolet rays have great destructive power, which can damage people’s skin, cause inflammation or sunburn, and seriously produce skin cancer. UVB is the main factor causing skin inflammation and sunburn.

1. the principle of shielding ultraviolet rays with nano TiO2

TiO _ 2 is an N-type semiconductor. The crystal form of nano-TiO _ 2 used in sunscreen cosmetics is generally rutile, and its forbidden band width is 3.0 eV When UV rays with wavelength less than 400nm irradiate TiO _ 2, electrons on valence band can absorb UV rays and be excited to the conduction band, and electron-hole pairs are generated at the same time, so TiO _ 2 has the function of absorbing UV rays. With small particle size and numerous fractions,This greatly increases the probability of blocking or intercepting ultraviolet rays.

2. Characteristics of nano-TiO2 in sunscreen cosmetics

2.1

High UV shielding efficiency

The ultraviolet shielding ability of sunscreen cosmetics is expressed by the sun protection factor (SPF value), and the higher the SPF value, the better the sunscreen effect. The ratio of the energy required to produce the lowest detectable erythema for skin coated with sunscreen products to the energy required to produce erythema of the same degree for skin without sunscreen products.

As nano-TiO2 absorbs and scatters ultraviolet rays, it is regarded as the most ideal physical sunscreen at home and abroad. In general, the ability of nano-TiO2 to shield UVB is 3-4 times that of nano-ZnO.

2.2

Suitable particle size range

The ultraviolet shielding ability of nano-TiO2 is determined by its absorption ability and scattering ability. The smaller the original particle size of nano-TiO2, the stronger the ultraviolet absorption ability. According to Rayleigh’s law of light scattering, there is an optimal original particle size for the maximum scattering ability of nano-TiO2 to ultraviolet rays with different wavelengths. Experiments also show that the longer the wavelength of ultraviolet rays,The shielding ability of nano-TiO 2 depends more on its scattering ability; The shorter the wavelength, the more its shielding depends on its absorption ability.

2.3

Excellent dispersibility and transparency

The original particle size of nano-TiO2 is below 100 nm, far less than the wavelength of visible light. Theoretically, nano-TiO2 can transmit visible light when it is completely dispersed, so it is transparent. Because of the transparency of nano-TiO2, it will not cover the skin when added into sunscreen cosmetics. Therefore, it can show natural skin beauty.Transparency is one of the important indexes of nano-TiO2 in sunscreen cosmetics. In fact, nano-TiO 2 is transparent but not completely transparent in sunscreen cosmetics, because nano-TiO2 has small particles, large specific surface area and extremely high surface energy, and it is easy to form aggregates, thus affecting the dispersibility and transparency of products.

2.4

Good weather resistance

Nano-TiO 2 for sunscreen cosmetics requires certain weather resistance (especially light resistance). Because nano-TiO2 has small particles and high activity, it will generate electron-hole pairs after absorbing ultraviolet rays, and some electron-hole pairs will migrate to the surface, resulting in atomic oxygen and hydroxyl radicals in the water adsorbed on the surface of nano-TiO2, which has strong oxidation ability.It will cause discoloration of products and odor due to decomposition of spices. Therefore, one or more transparent isolation layers, such as silica, alumina and zirconia, must be coated on the surface of nano-TiO2 to inhibit its photochemical activity.

3. Types and development trends of nano-TiO2

3.1

Nano-TiO2 powder

The nano-TiO2 products are sold in the form of solid powder, which can be divided into hydrophilic powder and lipophilic powder according to the surface properties of nano-TiO2. Hydrophilic powder is used in water-based cosmetics, while lipophilic powder is used in oil-based cosmetics. Hydrophilic powders are generally obtained by inorganic surface treatment.Most of these foreign nano-TiO2 powders have undergone special surface treatment according to their application fields.

3.2

Skin color nano TiO2

Because nano-TiO2 particles are fine and easy to scatter blue light with shorter wavelength in visible light, when added into sunscreen cosmetics, the skin will show blue tone and look unhealthy. In order to match skin color, red pigments such as iron oxide are often added to cosmetic formulas in the early stage. However, due to the difference in density and wettability between nano-TiO2 _ 2 and iron oxide,Floating colors often occur.

4. Production status of nano-TiO2 in China

Small-scale research on nano-TiO2 _ 2 in China is very active, and the theoretical research level has reached the world advanced level, but the applied research and engineering research are relatively backward, and many research results cannot be transformed into industrial products. The industrial production of nano-TiO2 in China began in 1997, more than 10 years later than Japan.

There are two reasons that restrict the quality and market competitiveness of nano-TiO2 products in China:

① Applied technology research lags behind

The application technology research needs to solve the problems of adding process and effect evaluation of nano-TiO2 in composite system. The application research of nano-TiO2 in many fields has not been fully developed, and the research in some fields, such as sunscreen cosmetics, still needs to be deepened.Due to the lag of applied technology research, China’s nano-TiO2 _ 2 products can not form serial brands to meet the special requirements of different fields.

② The surface treatment technology of nano-TiO2 needs further study

Surface treatment includes inorganic surface treatment and organic surface treatment. Surface treatment technology is composed of surface treatment agent formula, surface treatment technology and surface treatment equipment.

5. Concluding remarks

The transparency, ultraviolet shielding performance, dispersibility and light resistance of nano-TiO2 in sunscreen cosmetics are important technical indexes to judge its quality, and the synthesis process and surface treatment method of nano-TiO2 are the key to determine these technical indexes.