July 22, 2022 Longchang Chemical

Selection of photoinitiators in UV coating formulations

Free radical photoinitiators are divided into two main categories according to the mechanism of action of the photoinitiator to produce active radicals, namely cleavage type free radical photoinitiators (also known as type I photoinitiators), and hydrogen capture type free radical photoinitiators (also known as type II photoinitiators). Commonly used cleavage type photoinitiators are mostly aryl alkyl ketones from the structural point of view, the common grades are 184, 2959, 651, 907, 369, 1173, 819, TPO, MBF, 754, etc. Commonly used hydrogen-capturing photoinitiators are mostly benzophenones or heterocyclic aromatic ketones from the structural point of view, with common grades such as BP, ITX and 2-EA. The common co-initiators are mainly reactive amines and tertiary amine benzoates. This article will combine the performance of photoinitiators and use cases to briefly explain the selection of photoinitiators in the formulation of light-curing (UV) coatings.

 

First, the absorption spectrum of the photoinitiator and light source emission spectrum matching principle.

 

Commercially available light sources are mercury lamps, LED lamps, induction lamps and metal halide lamps. In the selection of photoinitiators, according to the emission spectrum of the light source to choose a larger absorption of the spectrum of initiators.

 

Application examples. In nail polish formulations, common nail lamp tubes are fluorescent lamps and LED lamps. Fluorescent lamps have an emission spectrum of 370-420nm and LED lamps have an emission spectrum of around 365nm/395nm. Both lamp emission spectra belong to the long wavelength region and require the selection of initiators that absorb light at longer wavelengths. As Table 1 shows the absorption peaks of various common photoinitiators, if you need to achieve the desired initiation effect, you should choose a photoinitiator with an absorption peak above 365nm, such as TPO, 819, etc. In the actual test, all photoinitiators TPO and 819 curing effect is good, and the predicted effect is consistent.

 

Second, coloured systems deep curing photoinitiator selection.

 

In the coloured system, especially the dark system, the pigment itself will absorb part of the UV energy, resulting in UV light can not penetrate the paint film, the deep layer of the photoinitiator can not absorb enough energy to trigger the polymerisation, and ultimately cause deep curing poor. The lighter the film, the lower the adhesion, the more serious the wrinkling of the surface, affecting the physical and chemical properties of the paint film. In the light curing process, the longer the wavelength of ultraviolet light, the stronger its penetration and the easier it is to reach the deeper layers of the film, while short waves are not easy to reach the deeper layers of the film. This makes it difficult to initiate polymerisation or incomplete curing in the deeper layers of the paint film if there is no long-wave photoinitiator to absorb the energy from the long waves. In coloured systems, therefore, the choice of a deep photoinitiator is essential. Referring to Table 1 above, long-wave photoinitiators such as TPO/819/651 can be chosen to work better with short-wave photoinitiators such as 184/1173.

 

As in the UV single coat colour paint, the black system is prone to poor adhesion and the phenomenon of paint falling off from the 100 grid. After adding 1.5% of 819 to the formulation, the adhesion of the paint film increased significantly, indicating that 819 plays a facilitating role for deep curing. In addition, in the black/white system, 907/ITX+184 compound and 369/ITX+184 compound, the effect is outstanding.

 

Third, there are requirements for yellowing light curing system photoinitiator selection.

 

In the varnish and white system, yellowing resistance is an important indicator of the performance of the paint film, in addition to the selection of good yellowing resistance of the resin, monomer, photoinitiators bring yellowing problems also need to pay attention to. The presence of such a substituent in the conjugate structure of the photoinitiator as N-dimethylamine will result in a higher tendency to irradiate yellowing; similarly the presence of such a substituent in the structure of the active amine will also lead to increased yellowing.

Table 2 on the left shows the comparison of the yellowing index of various photoinitiator formulations after solidification with pentaerythritol triacrylate propoxide as the main body and no photoinitiator as a blank reference. As can be seen from the table, 184, 1173, 754, MBF are less yellowing photoinitiators, for the main choice of varnish and white system formulations.

 

Fourth, the solubility performance in the active thinner and oligomer.

Good solubility is an important prerequisite for the selection of photoinitiators in the formulation, the better the compatibility, the more stable the formulation system. As shown in the figure on the left is the solubility of some photoinitiators in common solvents and monomers.

 

Fifth, the choice of UV-LED photoinitiator.

UV-LED light source is a fast-growing curing equipment in recent years, because of its energy-saving and environmentally friendly, do not damage the substrate is very popular, so in UV-LED curing in the choice of photoinitiators are also being paid more and more attention to the use. UV-LED curing formulations in the selection of photoinitiators should also be combined with the above-mentioned principles, first of all, to select the absorption peak and light source emission spectrum matching the photoinitiator.

UV-LED light source emission spectrum between 360-405nm, in 365nm, 375nm, 385nm, 395nm, 405nm at the highest intensity, these belong to the long-wave region, should give priority to the use of long-wave photoinitiators. Through further testing, several photoinitiators with the highest absorption rates were found at 365nm, 385nm and 395nm respectively. In terms of efficacy, DETX and EMK are suitable photoinitiators for UV-LED light sources.

UV Photoinitiator Same series products

Product name CAS NO. Chemical name
Sinocure® TPO 75980-60-8 Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide
Sinocure® TPO-L 84434-11-7 Ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate
Sinocure® 819/920 162881-26-7 Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide
Sinocure® 819 DW 162881-26-7 Irgacure 819 DW
Sinocure® ITX 5495-84-1 2-Isopropylthioxanthone
Sinocure® DETX 82799-44-8 2,4-Diethyl-9H-thioxanthen-9-one
Sinocure® BDK/651 24650-42-8 2,2-Dimethoxy-2-phenylacetophenone
Sinocure® 907 71868-10-5 2-Methyl-4′-(methylthio)-2-morpholinopropiophenone
Sinocure® 184 947-19-3 1-Hydroxycyclohexyl phenyl ketone
Sinocure® MBF 15206-55-0 Methyl benzoylformate
Sinocure® 150 163702-01-0 Benzene, (1-methylethenyl)-, homopolymer,ar-(2-hydroxy-2-methyl-1-oxopropyl) derivs
Sinocure® 160 71868-15-0 Difunctional alpha hydroxy ketone
Sinocure® 1173 7473-98-5 2-Hydroxy-2-methylpropiophenone
Sinocure® EMK 90-93-7 4,4′-Bis(diethylamino) benzophenone
Sinocure® PBZ 2128-93-0 4-Benzoylbiphenyl
Sinocure® OMBB/MBB 606-28-0 Methyl 2-benzoylbenzoate
Sinocure® 784/FMT 125051-32-3 BIS(2,6-DIFLUORO-3-(1-HYDROPYRROL-1-YL)PHENYL)TITANOCENE
Sinocure® BP 119-61-9 Benzophenone
Sinocure® 754 211510-16-6 Benzeneacetic acid, alpha-oxo-, Oxydi-2,1-ethanediyl ester
Sinocure® CBP 134-85-0 4-Chlorobenzophenone
Sinocure® MBP 134-84-9 4-Methylbenzophenone
Sinocure® EHA 21245-02-3 2-Ethylhexyl 4-dimethylaminobenzoate
Sinocure® DMB 2208-05-1 2-(Dimethylamino)ethyl benzoate
Sinocure® EDB 10287-53-3 Ethyl 4-dimethylaminobenzoate
Sinocure® 250 344562-80-7 (4-Methylphenyl) [4-(2-methylpropyl)phenyl] iodoniumhexafluorophosphate
Sinocure® 369 119313-12-1 2-Benzyl-2-(dimethylamino)-4′-morpholinobutyrophenone
Sinocure® 379 119344-86-4 1-Butanone, 2-(dimethylamino)-2-(4-methylphenyl)methyl-1-4-(4-morpholinyl)phenyl-
Sinocure® 938 61358-25-6 Bis(4-tert-butylphenyl)iodonium hexafluorophosphate
Sinocure® 6992 MX 75482-18-7 & 74227-35-3 Cationic Photoinitiator UVI-6992
Sinocure® 6992 68156-13-8 Diphenyl(4-phenylthio)phenylsufonium hexafluorophosphate
Sinocure® 6993-S 71449-78-0 & 89452-37-9 Mixed type triarylsulfonium hexafluoroantimonate salts
Sinocure® 6993-P 71449-78-0 4-Thiophenyl phenyl diphenyl sulfonium hexafluoroantimonate
Sinocure® 1206 Photoinitiator APi-1206

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