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Photoinitiator System Based on Thioxanthone and Tertiary Amines

Release time:

2024-08-10 23:24

Photoinitiator System Based on Thioxanthone and Tertiary Amines
Photoinitiator System Based on Thioxanthone and Tertiary Amines

A prominent feature of hydrogen-abstraction-type photoinitiators is that they typically need to be used in conjunction with a hydrogen donor (co-initiator) in order to effectively initiate polymerization reactions. This mechanism is known as “hydrogen-abstraction reaction,” in which the photoinitiator, after absorbing light energy, becomes sufficiently reactive to abstract a hydrogen atom from another molecule (the hydrogen donor), thereby generating free radicals. The thioxanthone/tertiary amine photoinitiator system represents an important class of hydrogen-abstraction-type photoinitiators. After absorbing ultraviolet light, thioxanthone undergoes an electronic transition from its ground state to an excited state and becomes sufficiently reactive to abstract a hydrogen atom from a tertiary amine molecule.

I. Reaction Principle

In the thioxanthone/tertiary amine photoinitiator system, thioxanthone acts as a hydrogen-abstraction-type photoinitiator, while the tertiary amine serves as a hydrogen donor (co-initiator). When thioxanthone absorbs ultraviolet light, it undergoes an electronic transition from its ground state to an excited state, becoming sufficiently reactive to abstract a hydrogen atom from a tertiary amine molecule. This process generates a thioxanthone radical and a positively charged tertiary amine ion. The thioxanthone radical is highly reactive and can rapidly react with unsaturated bonds in monomer or oligomer molecules, thereby initiating the growth of the polymerization chain.

II. Thioxanthone

Thioxanthone is a pale yellow powder with low solubility in most solvents, which to some extent limits its direct applications. However, by introducing substituents (such as chlorine or isopropyl groups), it is possible to obtain derivatives with improved solubility, such as 2-chlorothioxanthone (CTX) and isopropylthioxanthone (ITX).

III. Advantages of Thioxanthone

The thioxanthone/tertiary amine photoinitiator system plays a crucial role in photopolymerization technology. In particular, thioxanthone, as a hydrogen-abstraction-type photoinitiator, possesses a series of unique advantages.

1. Strong UV absorption capability

First, the maximum absorption wavelength range of thioxanthone is between 380 and 420 nm. This characteristic enables it to efficiently absorb and utilize common ultraviolet light sources, particularly those emitting at wavelengths of 365 nm and 405 nm. In contrast, although other types of photoinitiators, such as benzophenone, also possess certain light-absorbing capabilities, their efficiency within the 380–420 nm wavelength range is generally lower than that of thioxanthone. This means that in applications requiring efficient utilization of light sources within this wavelength range, thioxanthone can initiate polymerization reactions more rapidly and thoroughly, thereby enhancing both production efficiency and product quality.

2. Highly efficient photoinitiator activity

Second, thioxanthone exhibits a high extinction coefficient—on the order of 10²—which means that under identical illumination conditions, thioxanthone can absorb more photons and convert them into active species capable of initiating polymerization reactions. This high extinction coefficient not only enhances the efficiency of photoinitiation but also helps reduce both the duration and intensity of light exposure required during the photocuring process, thereby lowering energy consumption and production costs.

IV. Tertiary Amines

A tertiary amine is a class of compounds in which three alkyl or aryl groups are attached to a nitrogen atom. These compounds exhibit strong basicity and can serve as hydrogen donors. In photopolymerization reactions, they act as co-initiators when used in combination with hydrogen-abstraction-type photoinitiators such as thioxanthone.

V. The Role of Tertiary Amines

In the thioxanthone/tertiary amine photoinitiator system, thioxanthone is excited to a high-energy state upon illumination and subsequently abstracts a hydrogen atom from a neighboring tertiary amine molecule, generating a free radical. These free radicals then serve as active centers that initiate the polymerization reaction, driving chain-growth polymerization among monomer molecules. Meanwhile, the tertiary amine molecule acts as a hydrogen atom donor and plays a crucial role in the reaction process: not only does it provide hydrogen atoms to sustain the photoinitiation reaction, but it also modulates the pH of the system through its basic properties, thereby influencing both the rate of the polymerization reaction and the performance of the resulting polymer.

1. Hydrogen donor function

After thioxanthone absorbs ultraviolet light energy, it undergoes a transition from the ground state to the excited state. At this point, the excited-state thioxanthone molecule reacts with a tertiary amine molecule via a hydrogen-abstraction reaction, stripping a hydrogen atom from the tertiary amine to form a positively charged tertiary amine ion and a thioxanthone radical. This step is crucial in initiating the photopolymerization reaction.

2. Improve initiation efficiency

The thiophenanthrone radical generated by the hydrogen-abstraction reaction is highly reactive and can rapidly react with unsaturated bonds in monomer or oligomer molecules, thereby initiating the growth of the polymer chain. The presence of a tertiary amine significantly enhances the efficiency of this process.

3. Adjust the reaction rate

The concentration of tertiary amines affects the rate of hydrogen-abstraction reactions, which in turn influences the overall rate of the photopolymerization reaction. By adjusting the amount of tertiary amine used, it is possible to control, to some extent, the progression of the polymerization reaction and the properties of the resulting product.

4. Synergistic effect

Thioxanthone and tertiary amines exhibit excellent synergistic effects in photopolymerization reactions. Thioxanthone is responsible for absorbing ultraviolet light and initiating excitation, while the tertiary amine provides hydrogen atoms to generate active free radicals; together, these two components promote the progression of the polymerization reaction.

5. Impact on Stability

The addition of tertiary amines can also enhance the stability of the photoinitiator system to some extent, reducing the occurrence of undesirable reactions such as photodegradation, thereby helping to produce better-performing photocurable products.

VI. Conclusion

In summary, the thioxanthone/tertiary amine photoinitiator system is an efficient photopolymerization system based on thioxanthone-based photoinitiators and tertiary amine co-initiators. Thioxanthone and tertiary amines exhibit excellent synergistic effects in this photoinitiator system. This synergy not only enhances the efficiency of photoinitiation but also makes the system more stable and reliable. As an efficient, environmentally friendly, and energy-saving photocuring technology, this system holds great potential for wide-ranging applications across multiple fields.

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