Light shielding phenomenon of photoinitiator & two-photon photoinitiator
Due to the absorption and reflection of the pigment, the local ultraviolet light is shielded, so that the photoinitiator can not effectively absorb the light rays, and the curing conjunctiva rate of the ink is greatly reduced or the cured conjunctiva does not occur. The problem of pigments interfering with photoinitiator excitation involves the selection of a photoinitiator for the ink system, ie, a photoinitiator that minimizes the effects of light shielding.
Through experiments, it can be found that when the absorption spectrum of the photoinitiator is completely covered by the absorption spectrum of a certain pigment, the coloring system cannot absorb light effectively, resulting in difficulty in curing the ink. Studies have also found that many pigments have a relatively weak light-transmitting window in their absorption spectrum.
If these light-transmitting windows are used, the photoinitiator that matches them is used as the ink component, which can eliminate or reduce the effect of ink curing. The absorption spectra of Y, M, C tri-color inks can be seen in the effective UV range of 320~420nm. They have different degrees of weak absorption, and the absorption wavelength range is relatively large, so they have higher absorbance. The use of thioxanthone photosensitizers can obtain stronger light-absorbing ability. Due to its excellent photo-initiating properties, it is often used as an initiator for display performance of high-grade colored systems.
Two-photon initiated polymerization appears to be relatively late in many applications of two-photon absorbing materials, but it has attracted much attention!
The traditional single-photon absorption induced photopolymerization is usually performed under the excitation of ultraviolet light, and the photon energy is large. Generally, the polymerization reaction takes place at the place where the light passes through, and it belongs to the whole or surface polymerization, and the spatial resolution is very low. However, two-photon absorption refers to the process of exciting the sample with a light source that absorbs wavelengths nearly twice as fast as the sample, so that it directly absorbs two photons through an imaginary intermediate state and transitions to a high-energy excited state.
Two-photon absorption-induced polymerization has two distinct features:
- The two-photon-polymerized radiation source is a near-infrared laser, and its photon energy is far lower than the photon energy that causes conventional single-photon polymerization. Therefore, the linear absorption and Rayleigh scattering are relatively small, and the penetration of the laser in the medium is high. Photon aggregation technology has the ability to spatially select “spot” aggregation, and is a technology that can truly perform three-dimensional micro-machining.
- Under strong laser radiation, when the electron transitions from the ground state to the excited state, its transition probability is proportional to the square of the incident light intensity. Therefore, when the incident light beam is tightly focused, the polymerization occurs only at the wavelength of the incident light (3rd power). Within the small volume of the λ3) range, no change occurs in other regions, and there are significant differences in the physicochemical properties between the polymerized and unpolymerized portions (fluorescence, solubility, refractive index, etc.).