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Introduction to Photoinitiators and Their Applications

Release time:

2016-01-19 09:32

Introduction to Photoinitiators and Their Applications

I. Encyclopedia of Photoinitiators

1. Definition: A photoinitiator, also known as a photosensitizer or a light-curing agent, is a compound used in light-curing systems. It absorbs energy at specific wavelengths in the ultraviolet (250–420 nm) or visible (400–800 nm) regions of the spectrum, generating free radicals, cations, or other reactive species that initiate the polymerization, crosslinking, and curing of monomers. Any substance that, upon exposure to light, generates free radicals and subsequently triggers polymerization is collectively referred to as a photoinitiator.

2. Principle

The initiator molecules exhibit a certain light-absorbing capability in the ultraviolet region (250–420 nm) or the visible region (400–800 nm). After directly or indirectly absorbing light energy, the initiator molecules undergo a transition from the ground state to an excited singlet state and then, via intersystem crossing, to an excited triplet state. Following chemical reactions—either unimolecular or bimolecular—in the excited singlet or triplet states, active species capable of initiating monomer polymerization are generated. These active species can be free radicals, cations, anions, or other types of reactive intermediates. According to their initiation mechanisms, photoinitiators can be classified into free-radical polymerization photoinitiators and cationic photoinitiators; among these, free-radical polymerization photoinitiators are the most widely used. Upon exposure to light, certain monomers absorb photons and enter an excited state: M + hv → M*. The excited active molecule then undergoes homolytic cleavage to generate free radicals: M* → R· + R'·, which subsequently initiate monomer polymerization and lead to the formation of polymers.

II. Classification of Photoinitiators

1. Classification

According to the photolysis mechanism:

(1) Photoinitiators are classified as follows: a. Free-radical polymerization photoinitiators; b. Cationic polymerization photoinitiators.

(2) Free-radical polymerization photoinitiators can be further classified into: a. Cleavage-type photoinitiators; b. Hydrogen-abstraction-type photoinitiators.

2. Definition of Type-I Photoinitiators

A so-called "cleavage-type photoinitiator" is a type of initiator molecule that, upon absorbing light energy, transitions to an excited singlet state and then undergoes intersystem crossing to an excited triplet state. In either its excited singlet or triplet state, the molecular structure becomes unstable, causing weak bonds within the molecule to undergo homolytic cleavage, thereby generating primary active free radicals that initiate polymerization.

After absorbing light energy, the photoinitiator molecule transitions from its ground state to an excited state, initiating a Norrish I reaction. In this process, the covalent bond between the carbonyl group and the adjacent carbon atom elongates, weakens, and eventually breaks, generating primary free radicals: X-Y------(X...Y)-X·+Y·

(In the above equation, the two primary free radicals generated can be identical or different.)

3. Classification of Type-I Photoinitiators

(1) Benzoin and its derivatives; (2) Benzoyl compounds; (3) Alkyl ketones; (4) Acyl phosphine oxides; (5) Methyl formate benzoate; (6) Esterified oxime ketone compounds; (7) Aryl peroxy ester compounds; (8) Organic sulfur-containing photoinitiators.

4. Hydrogen-abstraction photoinitiators

Free radicals are formed through hydrogen abstraction reactions, such as with BP.

Hydrogen-abstraction reaction mechanism: X-------X·--------XH·+R· (In this equation, X and RH can be identical.)

(The excited-state photoinitiator molecule abstracts a hydrogen atom from a hydrogen atom donor, such as an active monomer or a low-molecular-weight prepolymer, thereby becoming an active free radical and initiating the polymerization reaction.)

5. Classification of Hydrogen-Abstraction Photoinitiators

(1) Active amine; (2) Benzophenone/tertiary amine system; (3) Thioxanthone/tertiary amine system; (4) Anthraquinone/tertiary amine system; (5) Camphorquinone/tertiary amine system.

6. Cationic Photoinitiator

Cationic photoinitiators represent another highly important class of photoinitiators, including diazonium salts, diaryliodonium salts, triarylsulfonium salts, alkylsulfonium salts, iron aryl complexes, sulfonyloxy ketones, and triarylsiloxides. Their fundamental mechanism involves photoactivation, which promotes molecules into an excited state, triggering a series of decomposition reactions that ultimately generate ultrastrong proton acids—also known as Brønsted acids. These proton acids serve as active species for cationic polymerization, initiating the polymerization of epoxides, vinyl ethers, lactones, acetals, cyclic ethers, and other compounds.

7. Classification of Cationic Photoinitiators

(1) Diazonium salts; (2) Diaryliodonium salts; (3) Triarylsulfonium salts; (4) Alkylsulfonium salts; (5) Iron-arene salts; (6) Sulfonylethynyl ketones; (7) Triarylsiloxides

III. Characteristics and Applications of Common Photoinitiators

1. Characteristics of Ideal Photoinitiators

(1) Inexpensive and easy to synthesize; (2) The photoinitiator and its photolytic products should be non-toxic and odorless; (3) Good stability for long-term storage; (4) The absorption spectrum must match the emission band of the radiation source; (5) High intersystem crossing efficiency; (6) High initiation efficiency.

2. Introduction to Common Photoinitiators

(1) Benzoyl compounds, 651; (2) Methyl benzoate, MBF; (3) Active amines, EDB, amines; (4) Thioxanthone compounds, ITX, CTX, DETX; (5) TPO, TPO-L, 819, acyl phosphine oxides; (6) Alkyl ketones, 1173, 184, 2959, 907, 369; (7) BP, PBZ, MBZ, benzophenone compounds.

3, 184

Chinese name: 1-Hydroxy-cyclohexyl-phenyl ketone		Structural formula
English name: 1-Hydroxycyclohexyl phenyl ketone
Molecular formula: C13H16O2
Molecular weight: 204.3
CAS Number: 947-19-3
Appearance: White crystals	Melting point: 44–48℃	Absorption wavelengths: 244, 280, 330 nm
Product Overview: 184 is a highly efficient, Type I free-radical, non-yellowing photoinitiator used for UV polymerization of monofunctional or polyfunctional acrylic monomers and oligomers. It finds wide application in copy coatings, plastic coatings, wood coatings, adhesives, lithographic printing inks, screen-printing inks, flexographic printing inks, and electronic products. Disadvantages: 1. When stored improperly or with low purity, it tends to cake into a hard, solid mass, affecting its usability. 2. After photolysis, the benzoyl radicals generated partially abstract chlorine atoms, forming benzaldehyde, which has an unpleasant odor. Furthermore, the photolysis products of HCPK also include cyclohexanone, a compound with a distinct odor.

4. 1173

Chinese name: 2-Hydroxy-methylphenylpropan-1-one		Structural formula
English name: 2-Hydroxy-2-Methyl-Phenyl-Propane-1-one
Molecular formula: C10O2H12
Molecular weight: 164.2
CAS Number: 7473-98-5
Appearance: Pale yellow, transparent liquid	Melting point: 4℃	Absorption wavelengths: 244, 278, 322 nm
Product Overview: 1173 is a high-efficiency, non-yellowing UV photoinitiator. It features low odor, no yellowing, and excellent color stability, making it particularly suitable for UV-curable systems based on unsaturated polyester resins and polyfunctional monomers. It can be easily compounded with other photoinitiators. The recommended addition level is 1–4%. Disadvantages: Residual 1173 and odor issues remain.

5. TPO

Chinese name: 2,4,6-trimethylbenzoyl-diphenyl phosphine oxide		Structural formula
English name: 2,4,6-trimethylbenzoyldiphenylphosphine oxide
Molecular formula: C22H21O2P
Molecular weight: 348.4
CAS Number: 75980-60-8
Appearance: Pale yellow solid	Melting point: 91–94℃	Absorption wavelengths: 273, 370 nm
Product Overview: TPO is a highly efficient free-radical (Type I) photoinitiator, particularly well-suited for colored systems and applications involving thick coating films. Due to its broad absorption spectrum, TPO can be widely used in various coatings. Thanks to its excellent absorption performance, it is especially ideal for screen-printing inks, offset printing inks, flexographic inks, and wood coatings. When used in combination with 184, it is also suitable for adhesive products. The dosage of this product should be determined based on actual experimental results; the recommended addition level is 0.5% to 4%. Disadvantages: Insufficient resistance to gas-induced polymerization; the surface dries while the interior remains damp. Often compounded with agents such as 1173 and 184 to ensure both surface and internal drying.

6. TPO-L

Chinese name: Ethyl 2,4,6-trimethylbenzoylphosphonate		Structural formula
English name: 2,4,6-Trimethylbenzoyl di-phenylphosphinate
Molecular formula: C18H21O3P
Molecular weight: 316
CAS Number: 84434-11-7
Appearance: Yellow liquid	Absorption wavelength: 299, 366 nm
Product Overview: TPO-L is a liquid photoinitiator suitable for formulations requiring low yellowing and low odor. Its photoinitiation activity is slightly lower than that of TPO. Due to its relatively broad absorption spectrum, TPO-L can also be used to cure white coatings containing pigments. TPO-L is often used in combination with other photoinitiators, such as 184, 1173, and benzophenone. The recommended usage concentration for TPO-L is 0.3% to 5%.

7,819

Chinese name: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide		Structural formula
English name: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide
Molecular formula: C26H27O3P
Molecular weight: 418.46
CAS Number: 162881-26-7
Appearance: Yellow powder	Melting point: 127–131℃	Absorption wavelengths: 295, 370 nm
Product Overview: 819 has been proven through testing to be suitable for UV-curable clear and colored coatings, including applications on wood, paper, metal, plastics, optical fibers, printing inks, and prepreg systems. In opaque white and colored furniture coatings, even a very low addition level of 819 can deliver outstanding curing performance and excellent resistance to yellowing. Moreover, 819’s superior absorption properties make it ideal for deep-cure applications as well. 819 can be used in combination with other photoinitiators, such as IRGACURE 184 or IRGACURE 651.

8, 907

Chinese name: 2-Methyl-2-(4-morpholinyl)-1-[4-(methylthio)phenyl]-1-propanone		Structural formula
English name: 2-Methyl-1-[4-(methylthio)phenyl]-2-morpholinopropanone-1
Molecular formula: C16H14OS
Molecular weight: 254.08
CAS Number: 71868-10-5
Appearance: White powder crystals	Melting point: 73–76℃	Absorption wavelengths: 231, 307 nm
Product Overview: This product works exceptionally well when used in conjunction with ITX and is particularly effective for deep curing in colored systems. 907 is suitable for applications in adhesives, composites, lithographic printing inks, flexographic printing inks, offset screen printing inks, and varnishes. It is also widely used in the electronics industry (e.g., as photoresists and solder mask inks) and in printing plates. The recommended addition level is 2-6%. Disadvantages: Prone to yellowing and unsuitable for white and clear-coat systems; photodegradation produces an unpleasant odor.

9, 369

Chinese name: 2-Phenylbenzyl-2-dimethylamino-1-(4-morpholinobenzylphenyl)butanone		Structural formula
English name: 2-Benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone
Molecular formula: C23H30N2O2
Molecular weight: 366.5
CAS Number: 119313-12-1
Appearance: Pale yellow powder	Melting point: 110–119℃	Absorption wavelengths: 232, 323 nm
Product Introduction: 369 is a highly efficient photoinitiator with a wide light-sensitivity range and excellent UV absorption. It features low migration and minimal odor, making it particularly well-suited for rapid curing in dark-colored systems. Disadvantages: After irradiation, yellowing is relatively severe, and there is a tendency for oxygen-induced polymerization inhibition; therefore, it needs to be used in combination with agents such as 1173.

10. MBF

Chinese name: Methyl benzoate formate		Structural formula
English name: Methyl Benzoylformate
Molecular formula: C9H8O3
Molecular weight: 164.16
CAS Number: 15206-55-0
Appearance: Yellowish-brown liquid	Boiling point: 246–250℃	Absorption wavelengths: 255, 325 nm
Product Introduction: This is a highly efficient liquid photoinitiator suitable for initiating the polymerization of certain unsaturated resins under UV light irradiation. Its photoinitiation activity is slightly lower than that of 1173.

11. BDK

Chinese name: Benzoin dimethyl ether		Structural formula
English name: Benzil Dimethyl Ketal
Molecular formula: C16H16O3
Molecular weight: 256.30
CAS Number: 24650-42-8
Appearance: White crystals	Melting point: 64–68℃	Absorption wavelength: 250 nm
Product Overview: BDK is applicable to colored systems and is widely used in various UV-curing systems. It exhibits strong absorption performance in UV ink systems such as PCB inks and clear-coat systems. The recommended addition level is 2-5%. Disadvantage: Prone to yellowing.

12, 2959

Chinese name: 2-Hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone		Structural formula
English name: 2-Hydroxy-4'-（2-hydroxyethoxy）-2-methylpropiophenone
Molecular formula: C12H16O4
Molecular weight: 224.1
CAS Number: 106797-53-9
Appearance: White powdery crystals	Melting point: 86–89℃	Absorption wavelength: 276 nm
Product Overview: The only FDA-approved system that allows the use of photoinitiators, featuring low odor, low volatility, minimal yellowing, and active hydroxyl ethoxy end-group hydroxyls capable of participating in reactions. It is suitable for water-based UV-curable systems, with a solubility of 1.7% in pure water (solubility in 1173 is 0.14%). It can be used in adhesives that do not come into direct contact with food. It has a high melting point and is also suitable for UV-curable powder coatings. Disadvantage: Dissolvability is not ideal.

13. DEAP

Chinese name: a, a-diethoxyacetophenone		Structural formula
English name: 2,2-diethoxy-1-phenylethanone
Molecular formula: C12H16O3
Molecular weight: 208.11
CAS Number: 6175-45-7
Appearance: Light yellow, transparent liquid	Absorption wavelengths: 210, 250 nm
Product Overview: Primarily used for varnishes applied to wood, plastics, and metals, this product exhibits excellent compatibility with oligomers and reactive diluents.

14,784

Chinese name: Bis(1-(2,4-difluorophenyl)-3-pyrrolyl)dimethyltitanium		Structural formula
English name: Bis(1-(2,4-difluorophenyl)-3-pyrrolyl) titanocene
Molecular formula: C30H22F4N2Ti
Molecular weight: 534
CAS Number: /
Appearance: Yellow-orange powder	Melting point: 160–170℃
Product Introduction: UV coatings, UV inks, UV adhesives.

15. BP

Chinese name: Benzophenone		Structural formula
English name: Benzophenone
Molecular formula: C13H10O
Molecular weight: 182.22
CAS Number: 119-61-9
Appearance: White flaky solid	Melting point: 47–49℃	Absorption wavelength: 250 nm
Product Overview: This is a highly efficient, free-radical (Type II) solid photoinitiator, primarily used in combination with tertiary amine co-initiators for the UV curing of unsaturated prepolymers (such as acrylates). It also serves as an intermediate in the synthesis of organic pigments, pharmaceuticals, fragrances, and pesticides.

16. ITX

Chinese name: Isopropylthioxanthone		Structural formula
English name: 2-isopropylthioxanthone
Molecular formula: C16H14OS
Molecular weight: 254.08
CAS Number: 5495-84-1
Appearance: Pale yellow solid	Melting point: 70-76℃	Absorption wavelengths: 258, 382 nm
Product Introduction: ITX is a highly efficient free-radical (Type II) photoinitiator that acts as a sensitizer when used in conjunction with anionic photoinitiators. Product Application: Works exceptionally well when used in combination with 907; suitable for layer-wise curing of corresponding resins along with tertiary amine co-initiators. ITX is a highly efficient photoinitiator for transparent or colored UV-curable screen-printing inks, copy coatings, offset printing inks, flexographic printing inks, electronic products, wood coatings, adhesives, and photoresists. It is typically used in conjunction with the amine synergist EDB, and the recommended dosage is 0.2–2%.

17. DETX

Chinese name: 2,4-Diethylthioxanthone		Structural formula
English name: 2,4-diethylthioxanthone
Molecular formula: C17H16OS
Molecular weight: 268
CAS Number: 82799-44-8
Appearance: Pale yellow powder	Melting point: 70–75℃	Absorption wavelengths: 261, 385 nm
Product Overview: Used in transparent or colored UV-curable inks, adhesives, coatings, and serves as a catalyst when combined with anionic initiators for photopolymerization.

18. PBZ

Chinese name: 4-Phenylbenzophenone		Structural formula
English name: 4-Benzoylbiphenyl
Molecular formula: C19H14O
Molecular weight: 258.31
CAS Number: 2128-93-0
Appearance: White or off-white powder	Melting point: 99–101℃	Absorption wavelength: 248 nm
Product Overview: PBZ is an initiator with a long-wave-absorbing molecular structure, primarily used as a photoinitiator in colored UV-curable formulations. PBZ is also employed as an intermediate in the synthesis of bifonazole, a pharmaceutical compound.

19. EDB

Chinese name: Ethyl 4-dimethylaminobenzoate		Structural formula
English name: Ethyl 4-dimethylaminobenzoate
Molecular formula: C11H150O2N
Molecular weight: 193.24
CAS Number: 10287-53-3
Appearance: White solid	Melting point: 61–64℃	Absorption wavelength: 308 nm
Product Overview: EDB is a highly effective amine co-initiator that, when used in conjunction with free-radical (Type II) photoinitiators, is suitable for UV polymerization of monomers and oligomers with either mono- or multifunctional groups. EDB is a solid amine synergist primarily used in offset printing inks, adhesives, silk-screen printing inks, solder mask inks, and other related products. It is typically used in combination with ITX or 907, and the recommended addition level is 2–5%.

20. EHA

Chinese name: Isooctyl p-dimethylaminobenzoate		Structural formula
English name: 2-ethylhexyl 4-(dimethylamino)benzoate
Molecular formula: C17H27NO2
Molecular weight: 277.4
CAS Number: 21245-02-3
Appearance: Pale yellow liquid	Absorption wavelength: 310 nm
Product Introduction: EHA is a highly efficient amine promoter that, when used in conjunction with free-radical (Type II) photoinitiators, is primarily employed in the UV-induced polymerization of single or multiple monomers.

21. EMK

Chinese name: Tetraethylmethylketone		Structural formula
English name: 4,4'-Bis(diethylamino)benzophenone
Molecular formula: C21H28N2O
Molecular weight: 324.66
CAS Number: 90-93-7
Appearance: Pale yellow crystals	Melting point: 93-95℃	Absorption wavelengths: 248, 374 nm
Product Overview: Used as a dye intermediate, UV-curable coatings, and inks.

22. CBZ

Chinese name: 4-Chlorobenzophenone		Structural formula
English name: 4-Chlorobenzophenone
Molecular formula: C13H9ClO
Molecular weight: 216.67
CAS Number: 134-85-0
Appearance: White flaky solid	Absorption wavelengths: 204, 258 nm
Product Overview: Used as an intermediate for pharmaceuticals and agrochemicals, as well as for UV-curable coatings and inks.

23. MBZ

Chinese name: 4-Methylbenzophenone		Structural formula
English name: 4-Methylbenzophenone
Molecular formula: C14H12O
Molecular weight: 196.24
CAS Number: 134-84-9
Appearance: White flaky solid	Absorption wavelength: 260 nm
Product Overview: This is a highly efficient Type II free-radical photoinitiator, primarily used for UV polymerization of corresponding resins in conjunction with tertiary amine co-initiators. It finds applications in clear coats, plastic coatings, wood coatings, adhesives, offset printing inks, screen-printing inks, flexographic inks, and electronic products.

24. OMBB

Chinese name: Methyl benzoate benzoylate		Structural formula
English name: methyl 2-benzoylbenzoate
Molecular formula: C15H12O3
Molecular weight: 240.26
CAS Number: 606-28-0
Appearance: White crystals	Melting point: 50–54℃	Absorption wavelengths: 204, 254 nm
Product Overview: Low migration, low odor, and excellent safety; high initiation activity; suitable for applications with stringent odor requirements.

IV. Principles for Selecting Photoinitiators

1. Principles for Selecting Photoinitiators

(1) Select an appropriately active photoinitiator based on the types of prepolymer and monomer.

(2) It exhibits excellent solubility and reactivity, requires a small dosage, and boasts high initiation efficiency.

(3) It must exhibit a certain degree of thermal stability, remaining non-decomposing below 85℃ and demonstrating long-term storage stability.

(4) It is best to use a combination of several photoinitiators, which can initiate curing across different wavelength ranges and offer faster curing speeds than using a single photoinitiator.

(5) The photoinitiator is used in combination with the amine promoter EDAB;

(6) Low odor, non-toxic, and does not cause environmental pollution;

(7) Inexpensive and readily available, with low costs.

V. Future Development Directions of Photoinitiators

1. Future development directions for photoinitiators

(1) Hybrid type; (2) Water-based type; (3) Visible-light type; (4) Macromolecular type; (5) Dual-curing.

2. Free-radical-cation hybrid photoinitiator

Free-radical photopolymerization systems cure rapidly but exhibit significant shrinkage. In contrast, cationic photopolymerization features minimal volume shrinkage and strong adhesion; its curing process is not inhibited by oxygen, the reaction is less likely to terminate, and it boasts excellent “post-curing” capability, making it well-suited for thick-film photopolymerization—but its curing speed is relatively slow. By combining free-radical and cationic photoinitiators into a hybrid system, we can leverage the advantages of both: free-radical polymerization can trigger cationic polymerization, thus offsetting their respective drawbacks and achieving synergistic effects. Moreover, using a combination of two or more photoinitiators can yield even more satisfactory results.

3. Visible-light initiator

Fluorinated diphenyl titanocene (Irgacure 784) and bis(pentafluorophenyl) titanocene exhibit outstanding photoinitiating activity, storage stability, and low toxicity. Their absorption wavelengths have been extended up to 500 nm, with significant absorption in the visible light region, making them particularly effective for visible-light-initiated polymerization and curing of acrylates. Moreover, due to the photobleaching effect of titanocenes under illumination, these materials exhibit a low yellowing index in cured films; they also demonstrate excellent depth-of-cure performance, facilitating thorough curing even in thick films. In acrylate systems, the photoinitiator activity of fluorinated diphenyl titanocene at a dosage of 0.2% is 2 to 6 times higher than that of 2% Irgacure 651.

4. Water-based photoinitiator

By introducing ammonium or sulfonate functional groups into conventional photoinitiators, these initiators can be made water-soluble, giving rise to waterborne photoinitiators. The main types include ketone-based compounds, such as benzophenone derivatives, thioxanthone derivatives, alkylbenzophenone derivatives, and benzoyl peroxide derivatives.

5. Macromolecular Photoinitiator

By introducing conventional photoinitiators into macromolecular chains, we obtain macromolecular photoinitiators, which exhibit excellent compatibility with resins, do not migrate after curing, are不易 volatile, and thus reduce odor. Macromolecular photoinitiators can be classified into four types: side-chain cleavage type, main-chain cleavage type, side-chain hydrogen-abstraction type, and main-chain hydrogen-abstraction type. Among these categories, the side-chain cleavage type macromolecular photoinitiator is one of the most successful.

6. Dual curing

This involves combining light curing with other curing methods, which complement each other and highlight their respective advantages. The resulting system boasts rapid curing at low temperatures and exceptional stability, effectively preventing uncured material from separating and yielding cured products with excellent mechanical properties and dimensional stability. Developing dual-curing systems that integrate light curing with other curing methods has proven highly effective in overcoming the weaknesses of light-curable adhesives, expanding their application scope, and enhancing their competitiveness. Other curing methods include thermal curing, moisture curing, oxidative curing, and anaerobic curing.

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