Comparison Of Chemical Sunscreens And Physical Sunscreens

The ultraviolet absorber is generally an organic compound with aromatic structure or  chromophore structure. The main mechanism of the action of UV absorber is using a closed conjugate system and relying on the π-π* transition to absorb photons, when the two π molecular orbitals are close enough, two degenerate molecular orbitals are generated, one with high energy and one with low energy. In general, the absorber is photo-stable and the energy after absorbing photons can generate resonance quantum (mainly) through molecular resonance or release by fluorescence and phosphorescence, and the reversible isomerization of the molecule changes the absorber from an excited state to steady state. 

The molecular structure of DHHB has perfect resonance and hydrogen ion transfer effect. When DHHB molecules absorb ultraviolet light, the energy is absorbed through the change of benzene ring structure and adjacent hydrogen ions in the molecule and the whole molecular structure will be temporarily changed. After the molecule slowly releases the energy, the molecular structure of DHHB returns to its original state.

Physical sunscreen scattering diagram

Titanium dioxide and zinc oxide are two approved physical sunscreen materials. The use of these physical sunscreens in sunscreens is primarily through absorption to attenuate the UV rays and is superimposed by some scattering. As small particle crystals (10-100 nm), these materials are semiconductors having a high band gap energy between the valence band and the conduction band. The band gap of the bulk crystal is in the range of energy corresponding to a wavelength between 380 and 420 nm. The smaller the primary particles, the higher the band gap energy, and the absorption of ultraviolet light by elevating electrons from the valence to the conduction band.

Both of these components are essentially semiconductors. Taking titanium dioxide as an example, after being excited by ultraviolet rays, it is an N-type semiconductor. When ultraviolet light with a wavelength of less than 400 nm is irradiated, the electrons on the interlayer absorb ultraviolet rays and are excited to generate electron-holes, so that titanium dioxide has a function of absorbing ultraviolet rays.

Then, when is the physical sunscreen of nano-titanium dioxide, when it is absorbed in the sunscreen product, when does it act as a scattering effect? This is related to its particle size. Experiments have shown that the longer the wavelength of ultraviolet light, the shielding of nano-titanium dioxide depends on its absorption. So, it is conceivable that in the UVB band, titanium dioxide is mainly absorption, while in the UVA band, nano titanium dioxide is mainly scattering. Therefore, to achieve a wide range of UV protection, both absorption and scattering are required, so there is an optimum primary particle size of titanium dioxide, not as small as possible. Considering the agglomeration of nano-titanium dioxide, it is generally believed that the optimum particle size of the ultraviolet-shielding titanium dioxide is 20-50 nm. Of course, particle size is only an indicator of the physical sunscreen required in the development of sunscreen products and many technical indicators need to be considered.