An overview of the photoinitiators
In the light curing system, including UV glue, UV coating, UV ink, etc., accept or absorb the outside energy after chemical changes, decomposition into free radicals or cations, thereby causing polymerization reaction.
Photoinitiator refers to any substance that can produce free radicals and further cause polymerization by light.
After some monomers are illuminated, they absorb photons to form an excited state M* :
M+hv→M*;The excited active molecules generate free radicals through homolysis: M*→R·+R ‘·, which leads to monomer polymerization and polymer formation.
The principle of photoinitiators
The initiator molecule has a certain absorbance ability in the ultraviolet region (250~400 nm) or visible region (400~800 nm). After absorbing light energy directly or indirectly, the initiator molecule transitions from the ground state to the excited singlet state and through the system to the excited three-line state.
After the singlet or triplet state is excited to undergo monomolecular or bimolecular chemical action, the active fragments that can trigger monomer polymerization are generated, which can be free radicals, cations, anions, etc.According to different initiation mechanism, photoinitiators can be divided into free radical polymerization photoinitiators and cationic photoinitiators, among which free radical polymerization photoinitiators are the most widely used.
The characteristics of photoinitiators
The ideal photoinitiator should have the following advantages:
- Cheap and easy tosynthesize;
- Photoinitiator and its photolysis products should be non-toxic andtasteless;
- Good stability, easy to store for a longtime;
- The absorption spectrum of the photoinitiator must match the emission band of the radiation source and have a high molar extinctioncoefficient;
- Because most of the photoinitiator molecules absorb light energy and then jump to theexcitation
singlet state, the intersystem channeling jumps to the excitation three-line state, so the intersystem channeling jump efficiency of the initiator is high.
- High initiation
Common photoinitiators and their characteristics
1.907(2-methyl-1 -(4-methylmercapto phenyl) -2-merlin-1-acetone),a light beige solid, has a fast curing Studies show that at 313nm wavelength, the molar extinction coefficient of 907 is 2 orders of magnitude larger than that of 1173, and the number of active free radicals generated is much larger, so the initiation efficiency is very high. As the photolysis product of
907 is a sulfur-containing compound, that is, p-methylmercapto benzaldehyde, which has an obvious odor and poor yellow ing resistance, it cannot be used in transparent photocuring adhesives and its application is limited.
2. 1173(2-hydroxy-2-methyl-1-phenylacetone), colorless transparent liquid and 184(1-hydroxy-cyclohexyl benophenone), white solid, these two photoinitiators are currently the most widely used in the domestic light curing industry of the initiator, with its exce llent photoinduced performance, excellent thermal stability and other comprehensive balance performance and widely popular.There is no -H in the benzophenone group in the molecular of the two initiators, and they have good thermal stability. There is no substituted benzyl group structure during photolysis, and they have good yellowing resistance. Both of them are initiators
with moderate curing rate.The fly in the ointment is that benzaldehyde in 1173 photolysis products has bad odor and high volatility, benzaldehyde and cyclohexanone in 184 photolysis products have bad odor, and the two initiators are seriously affected by the oxygen in the air, poor surface dryness, the use of alone is limited.
3. TPO(2,4,6-(trimethylbenzoyl) diphenyl phosphine oxide), a beige solid, has a wide absorption range, with an effective absorption peak of 350-400nm, up to about 420nm, and its absorption peak is longer than that of conventional initiator. Two free radicals, 2,4,6 monomethylbenzoyl and phosphonyl, can be generated by light, which can trigger polymerization, so the photocuring speed is faster. Meanwhile, it also has the photobleaching effect, which is suitable for deep curing of thick film and the coating does not change yellow, and has the characteristics of low volatilization.
4.BP(benzophenone)white crystal, hydrogen capture initiator, because of its low cost, so it is widely However, the photoinitiator activity is poor and the yellowing is poor when used in large quantities, so it is often used with cracking photoinitiator.At the same time, BP also has the function of anti-oxygen and anti-accumulation, and the surface anti-oxygen effect is better.
5. EDAB (4-dimethylamino-ethyl benzoate), white solid and ITx(isopropyl thio-allione), yellow solid, generally mixed, high activity, but because of their yellow, affect the transmittance.
6.819: Phenylbis(2,4,6-trimethylbenzoyl)phosphineoxide, English name: phenylbis (2,4,6-trimethylbenzoyl) Phosphineoxide, molecular formula: C26H27O3P, molecular weight:46,
CAS No. : 162881-26-7, appearance: yellow powder, density: 1.19g/cm3, melting point: 127 ~ 131℃, boiling point: ≥168℃. The 819 photoinitiator has the widest absorption wavelength among these photoinitiators, which is even wider than TPO.
The initiation efficiency is also very high. After dissolution in the system, there will be a relatively liters of yellow, and the color will basically disappear after single curing.
Photoinitiators, also known as photosensitizers or photocuring agents, are a type of energy that can absorb a certain wavelength in the ultraviolet region (250-420nm) or visible light region (400-800nm) to generate free radicals, cations, etc., thereby initiating monomers. Polymeric cross-linked cured compounds. The initiator molecule has a certain ability to absorb light in the ultraviolet region (250~400 nm) or visible light region (400~800 nm). After directly or indirectly absorbing light energy, the initiator molecule transitions from the ground state to the excited singlet state, and the intersystem Jump to the excited triplet state; after the excited singlet state or triplet state undergoes unimolecular or bimolecular chemical action, active fragments that can initiate the polymerization of monomers are generated, and these active fragments can be free radicals, cations, anions, etc. According to different initiation mechanisms, photoinitiators can be divided into free radical polymerization photoinitiators and cationic photoinitiators, among which free radical polymerization photoinitiators are the most widely used.
In the light-curing system, including UV glue, UV coating, UV ink, etc., after receiving or absorbing external energy, it undergoes a chemical change and decomposes into free radicals or cations, thereby triggering a polymerization reaction.
All substances that can generate free radicals and further initiate polymerization by light are collectively referred to as photoinitiators. After some monomers are illuminated, they absorb photons to form an excited state M*: M+hv→M*; the excited active molecules undergo homolysis to generate free radicals: M*→R + R′, which in turn initiates monomer polymerization, generate macromolecules.
Radiation curing technology is a new energy-saving and environmentally friendly new technology. Ultraviolet light (UV) and electron beam (EB), infrared light, visible light, laser, chemical fluorescence and other radiation light curing are fully in line with the “5E” characteristics: Efficient (high efficiency), Enabling (practical), Economical (economical), Energy Saving (energy saving), Environmental Friendly (environmentally friendly), so it is known as “green technology”. Photoinitiator is one of the important components of photocurable adhesives, which plays a decisive role in curing rate. After the photoinitiator is irradiated by ultraviolet light, it absorbs the energy of the light and splits into two active free radicals, which triggers the chain polymerization of the photosensitive resin and the active diluent to cross-link and cure the adhesive. It is characterized by fast speed, environmental protection and energy saving.
An ideal photoinitiator should have the following advantages:
(1) Cheap and simple to synthesize;
(2) The photoinitiator and its photolysis products should be non-toxic and tasteless;
(3) Good stability, easy to store for a long time;
(4) The absorption spectrum of the photoinitiator must match the emission band of the radiation light source, and has a high molar extinction coefficient;
(5) Since most photoinitiator molecules transition to the excited singlet state after absorbing the light energy, and then jump to the excited triplet state through the intersystem, therefore, the intersystem transition efficiency of the initiator is high;
(6) Higher initiation efficiency.
The development direction of photoinitiators focuses on hybrid, visible light, water-based, macromolecular, etc., as well as the use of dual curing methods to achieve icing on the cake.
- Radical-cationic hybrid photoinitiator
The free radical R&D system cures quickly, but shrinks more. However, during cationic light curing, the volume shrinkage is small, the adhesion is strong, the curing process is not inhibited by oxygen, the reaction is not easy to terminate, and the “post-curing” ability is strong, which is suitable for light curing of thick films, but the curing speed is slow. Combining the advantages of the two, the free radical and the cationic photoinitiator are formulated into a hybrid system, which can not only generate cationic polymerization of free radical polymerization tourists, but also promote strengths and avoid weaknesses, and has a synergistic effect. Compatible use of two or more photoinitiators can achieve more satisfactory results.
- Visible light initiator
Fluorinated diphenyl titanocene (Irgacure 784) and bis(pentafluorophenyl) titanocene have outstanding photoinitiated activity, storage stability, and low toxicity, and their absorption wavelengths have been extended to 500 nm, with larger absorption in the visible region. Absorbing, visible light-initiated polymerization curing of acrylates is particularly effective. Due to the photobleaching effect of titanocene light, the yellowing index of the film is small; and the deep curing is good, which is conducive to the complete curing of the thick film. Fluorinated diphenyltitanocene photoinitiator is active. In acrylate system, the photoinitiation efficiency of 0.2% dosage is 2~6 times higher than that of 2% Irgacure651.
- Water-based photoinitiator (WSP)
An ammonium salt or a sulfonate functional group is introduced into the common photoinitiator to make it compatible with water to make a water-based photoinitiator. The main types are aromatic ketones, including benzophenone derivatives, thioxanthone derivatives, alkyl aryl ketone derivatives, benzil derivatives, etc.
- Macromolecular photoinitiator
The common photoinitiator is introduced into the macromolecular chain to become the macromolecular photoinitiator, which has good compatibility with the resin, does not migrate after curing, is not easy to volatilize, and reduces the odor. Macromolecular photoinitiators can be divided into four types: side chain cleavage type, main chain cleavage type, side chain hydrogen abstraction type and main chain hydrogen abstraction type. kind.
- Double curing
That is, the combination of light curing and other curing methods complements each other and has outstanding advantages. It has low temperature rapid curing and excellent stability, which can avoid separation of uncured products and obtain cured products with excellent mechanical properties and dimensional stability. The development of a dual-curing system shared by light-curing and other curing methods is effective in overcoming the weaknesses of light-curing adhesives, expanding the scope of application and improving competitiveness. Other curing methods heat curing, moisture curing, oxidative curing, anaerobic curing, etc.