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Photoinitiator system of benzophenone and tertiary amine

Release time:

2024-08-10 17:24

Photoinitiator system of benzophenone and tertiary amine
Photoinitiator system of benzophenone and tertiary amine

Benzophenone is a common hydrogen-abstraction-type photoinitiator that appears as colorless or slightly yellowish crystals. When exposed to ultraviolet light, benzophenone molecules absorb energy from the ground state and transition to an excited state. However, due to the two benzene rings in its molecule, which create a relatively large free volume, it experiences significant steric hindrance and thus cannot directly initiate polymerization. To trigger the polymerization reaction, benzophenone must abstract hydrogen atoms from a hydrogen donor (such as a tertiary amine), transferring the absorbed energy to the hydrogen donor, thereby generating free radicals and initiating polymerization.

I. Ultraviolet Absorption Properties of Benzophenone

The main absorption wavelength of benzophenone lies in the ultraviolet region, with its maximum absorption peak occurring at around 340 nm. This characteristic enables benzophenone to effectively absorb ultraviolet light emitted by sources such as medium-pressure mercury lamps. Benzophenone-based UV absorbers exhibit some absorption capacity across UV-A, UV-B, and UV-C wavelengths, but they primarily absorb UV-A and UV-B bands.

II. Reaction Mechanism

Under irradiation by ultraviolet or visible light, benzophenone absorbs light energy and transitions from its ground state to an excited singlet state. Subsequently, through an intersystem crossing process, benzophenone further transitions to an excited triplet state. In the excited triplet state, benzophenone interacts with tertiary amines containing α-H atoms (such as triethylamine), forming an excited-state complex. Within this excited-state complex, an electron-transfer reaction occurs.

After benzophenone accepts an electron, it forms a relatively stable benzophenol radical while simultaneously generating a highly reactive amine cation. The resulting benzophenol radical remains relatively stable due to the conjugation effect of the two benzene rings. Meanwhile, the amine cation, having increased acidity, quickly loses a proton, transforming into a highly reactive amine alkyl radical.

The generated aminealkyl radicals exhibit strong addition reactivity toward vinyl monomers (such as styrene, acrylates, and others), serving as the primary active species responsible for initiating polymerization. The ultimate fate of diphenylmethanol radicals can take several paths: two identical radicals may undergo disproportionation, yielding diphenyl ketone and diphenylmethanol. Alternatively, they could undergo dimerization to form tetraphenyl pinacol. Yet another possibility is that they act as chain-transfer agents, combining with growing polymer chains to terminate the polymerization reaction.

III. Tertiary Amines

Tertiary amines are a class of organic compounds characterized by a basic nitrogen atom. This basic nitrogen atom endows tertiary amines with unique properties in chemical reactions, particularly in light-initiated systems.

In the benzophenone/tertiary amine photoinitiator system, benzophenone acts as a photosensitizer and can be excited to an excited state upon illumination. When benzophenone is in the excited state, its electronic configuration becomes unstable, making it prone to reacting with other molecules. At this point, the tertiary amine, serving as a hydrogen donor, reacts with the excited-state benzophenone.

During this reaction process, the nitrogen atom of the tertiary amine loses an electron, which may be transferred to the excited-state benzophenone, causing the benzophenone to form a negative ion. Meanwhile, the tertiary amine itself, having lost an electron, becomes a positive ion and carries an unpaired electron on its nitrogen atom; this state is referred to as an exciplex. The nitrogen atom in the exciplex further undergoes a reaction, generating an amine alkyl radical.

This amine alkyl radical exhibits high reactivity; it is highly unstable and readily reacts with other molecules. In light-initiated polymerization reactions, this radical can trigger polymerization between monomer molecules, leading to the formation of polymeric compounds.

IV. Characteristics of the Benzophenone/Tertiary Amine Photoinitiator System

1. Efficient collaboration

The tertiary amine, acting as a hydrogen donor, reacts with excited-state benzophenone upon illumination, generating highly reactive amine alkyl radicals. These radicals can efficiently initiate polymerization reactions, thereby enhancing the efficiency and quality of light-curing processes. The combined use of benzophenone and a tertiary amine produces a remarkable synergistic effect, enabling this system to exhibit outstanding performance in light-curing reactions.

2. Antioxidant and polymerization-inhibiting function

The antioxidant and anti-polymerization function of tertiary amines in the benzophenone/tertiary amine photoinitiator system is a highly significant feature. During the photocuring process, oxygen is a common interfering factor that can terminate free-radical chain polymerization by reacting with free radicals, thereby reducing the curing rate and degree of cure and even leading to incomplete curing. This phenomenon is known as “oxygen inhibition.”

As a component of this system, tertiary amines not only act as hydrogen donors, reacting with the excited-state benzophenone to generate free radicals, but also counteract oxygen-induced polymerization inhibition through their unique chemical properties. Tertiary amines can capture and stabilize free radicals, thereby reducing their likelihood of reacting with oxygen. Moreover, tertiary amines may also consume oxygen in the system by forming nitrogen oxides, further mitigating the effects of oxygen-induced polymerization inhibition. Consequently, the presence of tertiary amines can effectively suppress the polymerization-inhibiting effect of oxygen, enhancing both the stability and efficiency of the photocuring reaction.

V. Conclusion

As a typical representative of hydrogen-abstraction-type photoinitiators, the benzophenone/tertiary amine photoinitiation system occupies an important position in light-curing technology. By perfectly combining benzophenone as a photosensitizer with a tertiary amine as a co-initiator, this system exhibits remarkable synergistic effects. Such synergy not only significantly enhances the photoinitiation efficiency, leading to faster and more stable free-radical generation, but also facilitates the smooth progression of the polymerization reaction, ensuring superior performance of the cured products and demonstrating broad application prospects across multiple fields.

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