Photoinitiator is the key component of light-curing coatings, which plays a decisive role in the light-curing rate of light-curing coatings. Photoinitiator is a kind of substance that can absorb radiation energy, undergo chemical change by excitation, and produce active intermediates (free radicals or cations) with the ability to initiate polymerization. In light-curing coatings, photoinitiator content than oligomer and active diluent is much lower, generally in 3% to 5% or so, not more than 7% to 10%. In practice, the photoinitiator itself or its photochemical reaction products should not have adverse effects on the chemical and physical mechanical properties of the cured coating. Photoinitiators can be divided into ultraviolet photoinitiators (ultraviolet region 200 ~ 400nm) and visible photoinitiators (visible region 400 ~ 700nm) due to the absorption of different radiation energy. Photoinitiators can be categorized into free radical photoinitiators and cationic photoinitiators due to the different active intermediates generated. Free radical-type photoinitiators can be divided into two categories: cleavage-type photoinitiators and hydrogen-capturing photoinitiators due to the different mechanisms of generating free radicals. At present, the light curing technology is mainly for ultraviolet light curing, the photoinitiator used for ultraviolet light photoinitiator. Visible photoinitiators are sensitive to sunlight and common lighting sources, and are limited in production and use, and are only applied in a few fields such as dentistry and printing plate making. In recent years, with the increasing application of light curing technology, photoinitiators have added some new categories, such as hybrid photoinitiators, water-based photoinitiators, macromolecule photoinitiators and so on. In the light curing system, some photoinitiators are used together with other auxiliary components to promote the generation of active intermediates such as free radicals or cations to improve the photoinitiation efficiency. These auxiliary components consist of photosensitizers and sensitizers. Photosensitizer is the molecule can absorb light energy to jump to the excited state, through the energy transfer to the photoinitiator, photoinitiator accepts the energy from the ground state to jump to the excited state, itself undergoes a chemical change, the production of active intermediates, thus triggering the polymerization reaction, while the photosensitizer will be the energy transfer to the photoinitiator, and then return to the initial inactive state of its own, its chemical properties have not changed. Sensitizer itself does not absorb light energy, and does not initiate polymerization, but in the photoinitiating process, synergistic photoinitiators and participate in the photochemical reaction, which improves the initiation efficiency of the photoinitiator, also known as co-initiator. Tertiary amines, which are hydrogen donors for hydrogen-capturing photoinitiators, are sensitizers.
The following factors should be considered for the selection of photoinitiators.
1) The absorption spectrum of the photoinitiator matches the emission spectrum of the light source. At present, the light source for light curing is mainly high-pressure mercury lamps, in which the emission spectrum of 365nm, 313nm, 302nm, 254nm spectrum is very useful, many photoinitiators in the above wavelengths have a greater absorption. Photoinitiator molecules on the absorption of light, can be reflected by the molar extinction coefficient at this wavelength.
2)The photoinitiator has high efficiency, i.e., it has a high quantum yield of producing active intermediates (radicals or cations), and at the same time, the produced active intermediates have high reactivity.
3)For colored systems, due to the addition of pigments, all have different absorption in the ultraviolet region, therefore, it is necessary to use the photoinitiator whose pigments are least affected by ultraviolet absorption.
4)Good solubility in active diluent and oligomer.
5)Small odor and low toxicity, especially the photolysis product of photoinitiator should be low odor and low toxicity.
6)Not easy to volatilize and migrate.
7)There should be no yellowing after light curing, which is especially important for white, light-colored and colorless systems; it should also not cause polymer degradation during aging.
8) Good thermal and storage stability.
9) Easy to synthesize, low cost and cheap.
1.Cleavage-type free radical photoinitiators
Free radical photoinitiators according to the photoinitiator to generate active radicals of different mechanisms, mainly divided into two categories: cleavage-type free radical photoinitiators, also known as PI-1-type photoinitiators; hydrogen-capturing free radical photoinitiators, also known as PI-2-type photoinitiators.
The so-called cleavage-type free radical photoinitiator refers to the photoinitiator molecules absorb light energy after the leap to the excited single-line state, through the intersystem leap to the excited triple-line state, in its excited single-line state or excited triple-line state, the molecular structure is an unstable state, where the weak bond will occur homogeneous cleavage, resulting in the production of the primary active free radicals, triggering the polymerization of oligomers and reactive diluents crosslinking.
Cleavage-type radical photoinitiators from the structural point of view, mostly aryl alkyl ketone compounds, mainly benzene coupling and its derivatives, benzene coupling acyl and its derivatives, acetophenone and its derivatives, a-hydroxy alkyl acetophenone, a-amino alkyl acetophenone, acyl phosphine oxides and so on.
- Hydrogen-capturing free radical photoinitiators
Hydrogen-capturing type photoinitiator refers to the photoinitiator molecules absorb light energy, through the excitation and inter-system tampering to the excited trilinear state, and the assistant initiator-hydrogen donor bimolecular role, the electron transfer to produce reactive radicals, triggering the polymerization and crosslinking of oligomers and active diluent polymerization.
Hydrogen-capturing photoinitiators are benzophenone or heterocyclic aromatic ketone compounds from the structural point of view, mainly benzophenone and its derivatives, thioredoxins, anthraquinones and so on.
With the hydrogen-capture photoinitiator with the co-initiator – hydrogen donor is mainly tertiary amine compounds, such as aliphatic tertiary amine, ethanolamine tertiary amine, tertiary amine benzoate, active amine (with acryloyloxy, can participate in the polymerization and crosslinking of tertiary amine) and so on.
- Cationic photoinitiators
Cationic photoinitiator is another very important class of photoinitiators, it absorbs light energy to the excited state, the molecules undergo photolysis reaction, resulting in super-strong acid that is, super-strong protonate or Lewis acid, which triggers cationic polymerization of cationic oligomers and active diluents. Cationic photopolymerization of oligomers and active diluents are mainly epoxy compounds and vinyl ether, in addition to lactone, acetal, cyclic ether and so on. Cationic light curing has the following characteristics compared with free radical light curing. ① cationic light curing is not affected by oxygen; and free radical light curing is sensitive to oxygen, oxygen barrier polymerization occurs. ② cation light curing sensitive to water vapor, alkali substances, resulting in blocking polymerization; and free radical light curing is not sensitive to water vapor, alkali substances. ③ Cationic light curing volume contraction is small, conducive to the adhesion of the substrate; and free radical light curing volume contraction is large. Cationic light curing rate is slow, increase the temperature is conducive to improve the rate of light curing; while the free radical light curing rate is fast, the temperature has little effect. ⑤ Cationic light curing of the active intermediate super acid is chemically stable, due to the positive charge will not occur coupling and disappear, in the chain termination will also produce new super acid, so after the light stops, can still continue to trigger polymerization and cross-linking, post-curing, long life, suitable for thick coatings and colored coatings light curing; and free radical light curing due to free radicals can easily be coupled to lose the triggering activity, once the light stops, the light curing will stop immediately. The light curing stops immediately without post-curing.
- Large molecule photoinitiators
Currently used photoinitiators are organic small molecules, in the use of the following problems. ① Compatibility Some solid photoinitiators have poor solubility in oligomers and reactive diluents, and need to be heated to dissolve. When placed in a low temperature, the initiator may precipitate, thus affecting the use. ② Migration The residual photoinitiators and photolysis products will migrate to the surface of the coating after curing, which will affect the appearance and performance of the coating, and may also cause toxicity and yellowing. ③ Odor Some photoinitiators are volatile, and the photolysis products of most photoinitiators have different degrees of odor, thus affecting their use in hygiene and food packaging materials. In order to overcome the above disadvantages of small molecule photoinitiators, people designed a large molecule photoinitiator or polymerizable photoinitiators.
- Water-based photoinitiators
Water-based light-curing coatings is the latest development of light-curing coatings in a field, it is water as a diluent, instead of active diluent to dilute the oligomer viscosity adjustment, no active diluent skin irritation and odor, inexpensive, non-flammable and non-explosive and safe. Water-based light-curing coatings are composed of water-based oligomers and water-based photoinitiators, which requires water-based photoinitiators in water-based oligomers in good compatibility, high photoactivity in aqueous media, high initiation efficiency, as well as other photoinitiators required by the low volatility, non-toxic, tasteless, colorless and so on. Water-based photoinitiators can be divided into two categories: water-dispersible and water-soluble. At present, most of the photoinitiators used in conventional light-curing coatings are oil-soluble, insoluble in water or very small solubility, which is not suitable for water-based light-curing coatings, so in recent years, the research and development of water-based photoinitiators has become a hot topic, and has made promising progress. Many water-based photoinitiators are introduced into the structure of the original oil-soluble photoinitiators anionic, cationic or hydrophilic nonionic groups, making them water-soluble.