In UV curable coatings application, photoinitiators currently account for a large proportion and can be used to produce various varieties with unique characteristics and different uses.Light curing paint is mainly used in wood, paper, plastic coating, glass, metal materials, liquid crystal decorative plate and other materials.
The main characteristics are:
- Excellent chemical resistance, especially alkali resistance.
- The paint film has strong adhesion, especially to metals.
- Has good heat resistance and electrical insulation.
- The paint film has good color retention.
In actual production, the use of free radical photoinitiators to produce free radicals, cationic photoinitiators to produce cations are very few.
Selection of initiators
(1) The absorption spectrum of the photoinitiator matches the emission spectrum of the light source
In the selection of photoinitiator, according to the light source emission spectrum to select a larger absorption of the spectrum of the initiator.
The following table is the absorption peak of various common photoinitiators, if you need to achieve the ideal initiation effect,it better to choose the absorption peak above 365nm photoinitiator, such as TPO, 819, 784.Although the absorption peak wavelength is longer, its own price is too high.So market use less. In the actual test, TPO and 819 of all photoinitiators have the best effect, which is consistent with the predicted effect.
| Name | Absorption peak /nm |
| 184 | 240-250,320-335 |
| 651 | 330-340 |
| 369 | 325-335 |
| 907 | 320-325 |
| 819 | 360-365,405 |
| 1173 | 265-280 |
| 2959 | 275-285,320-330 |
| TPO | 350-400 |
| MBF | 225,335 |
| 784 | 380-390,460-480 |
(2) Selection of photoinitiator for deep curing of colored system
In the colored system, especially in the dark system, the pigment itself will absorb part of the ultraviolet energy, resulting in the ultraviolet light can not penetrate the film, and the deep photoinitiator can not absorb enough energy to trigger polymerization, and eventually cause poor deep curing. Light adhesion decreases, serious will cause surface wrinkling, affecting the appearance of the film and physical and chemical properties.
In ultraviolet light, the longer the wavelength, the stronger the penetration, the easier.It is to reach the deep layer of the paint film, while the short wave is not easy to reach the deep layer of the paint film. As a result, if there is no long-wave photoinitiator in the deep layer of the film to absorb the energy brought by this part of the long wave, it is difficult to initiate polymerization. Therefore, deep photoinitiator is essential in colored system. According to the above table, the combination of TPO/819/651 long wave photoinitiator and 184/1173 short wave photoinitiator can be selected, and the effect is better.
Case: In UV single paint, the black system is prone to poor adhesion and paint loss. After adding 1.5% of 819 in the formula, the adhesion of the paint film increased significantly, indicating that 819 promoted the deep curing.
In addition, in the black/white system, 907/ITX+184 combination, 369/ITX+184 combination, the effect is outstanding.
(3) Selection of photoinitiators for yellowing systems
In some varnishes and white systems, yellowing resistance is an important indicator to examine the performance of paint film, in addition to the selection of resin and monomer with good yellowing resistance, the yellowing of photoinitiator should also be avoided as far as possible. If there are substituents such as N-dimethylamino in the conjugated structure of photoinitiator, the tendency of irradiation yellowing is generally higher, and the presence of such substituents in the active amine structure will also lead to increased yellowing.
The following table shows the yellow index of each photoinitiator with pentaerythritol triacrylate as the main body and no photoinitiator as a blank reference.
| Name | △E |
| 184 | 0.13 |
| 651 | 0.31 |
| 1173 | 0.2 |
| 754 | 0.08 |
| MBF | 0.10 |
| TPO | 0.38 |
| 2959 | 0.14 |
| 907 | 1.01 |
| 819 | 5.51 |
| 369 | 8.56 |
| BP | 0.25 |
| BMS | 0.42 |
As can be seen from the above table, 184, 1173, 754, MBF are less yellow photoinitiators, which are the best choice for the formulation of varnish and white system.
(4) It has good solubility in active diluents and oligomers
Good solubility is an important prerequisite for adding photoinitiator to the system, the better the compatibility, the more stable the system formula.
The following table shows the solubility of some photoinitiators in common solvents and monomers.
| Name | EA | IBOA | TPGDA | TMPTA | PETA | HDDA |
| 184 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 |
| 1173 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 |
| 2959 | 20 | 5 | 5 | 5 | 5 | |
| 651 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 |
| 369 | 17 | 10 | 6 | 5 | 5 | 10 |
| 819 | 15 | 5 | 5 | 5 | 5 | 5 |
| TPO | 50 | 15 | 16 | 14 | 10 | 22 |
| MBF | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 | ＞50 |
| 单位 | g/100g | |||||
(5) Other performance
In the selection of photoinitiator, try to choose small odor, low toxicity, good thermal stability, not volatile migration. The selected photoinitiator ingredients comply with local laws and regulations.
Conclusion
In summary, the use of light curing technology to obtain hardened paint film and ink has the following advantages: energy saving, environmental protection, high quality and high efficiency. The selection of photoinitiator is not an independent work, but is compatible with the entire system and even the construction process. It is necessary to refer to the performance requirements of light source, other components of the system, and photocuring products, and choose an economical and efficient photoinitiator after comprehensive consideration.
