4 - Chlorobenzophenone has a unique molecular structure that enables it to interact with ultraviolet light in a specific way. The molecule contains conjugated π - electron systems, primarily due to the presence of the two phenyl rings and the carbonyl group (). These conjugated systems are responsible for its ability to absorb UV light.
Once 4 - Chlorobenzophenone is in an excited state due to UV light absorption, it undergoes a series of chemical transformations to generate reactive species. In the case of 4 - Chlorobenzophenone, which is a type of free - radical photoinitiator, it typically generates free radicals.
The excited 4 - Chlorobenzophenone molecule can undergo a Norrish type I reaction. In this reaction, the carbon - carbon bond adjacent to the carbonyl group ( bond) is cleaved. The energy from the UV - induced excitation weakens this bond, causing it to break homolytically. As a result, two free radicals are formed: a benzoyl radical () and a 4 - chlorophenyl radical (). These free radicals are highly reactive species with unpaired electrons, and they are crucial for initiating the polymerization process in the UV - curable coating. The formation of these free radicals is a rapid process that occurs within a very short time after the absorption of UV light by 4 - Chlorobenzophenone.
The free radicals generated from 4 - Chlorobenzophenone play a central role in initiating the polymerization of the monomers and pre - polymers present in the UV - curable coating. Monomers in the coating typically contain carbon - carbon double bonds ().
The free radicals, such as the benzoyl radical and 4 - chlorophenyl radical, react with the double bonds of the monomers. The unpaired electron of the free radical attacks one of the carbon atoms in the double bond. This addition reaction forms a new covalent bond, and the unpaired electron is transferred to the other carbon atom of the double bond, creating a new free radical at the end of the monomer unit. For example, if the monomer is an acrylate monomer (, where is an organic group), the benzoyl radical () can add to the double bond as follows: . This new free - radical - terminated monomer can then react with another monomer molecule, continuing the chain - growth polymerization process.
Cost - Effectiveness
When considering the cost - effectiveness of 4 - Chlorobenzophenone compared to other common photoinitiators, it stands out in several aspects. In the market, the price of 4 - Chlorobenzophenone is relatively competitive. For example, compared to some high - performance but more expensive photoinitiators like 2,4,6 - Trimethylbenzoyl diphenylphosphine oxide (TPO), which is often used in applications requiring high - quality curing and is priced at a relatively high level per unit mass, 4 - Chlorobenzophenone offers a more cost - friendly option.
The cost - effectiveness is not only about the raw material price. Since 4 - Chlorobenzophenone can be used in relatively lower concentrations in UV - curable coating formulations while still achieving satisfactory curing results, it further reduces the overall cost. In many standard UV - curable coating applications, a concentration of 0.5% - 5% of 4 - Chlorobenzophenone by weight of the coating formulation is sufficient. This means that for large - scale production of coatings, the amount of photoinitiator required is relatively small, leading to significant cost savings over time. In contrast, some other photoinitiators may need to be used in higher concentrations to achieve the same level of curing efficiency, thus increasing the overall cost of the coating formulation.
Physical Properties
The use of 4 - Chlorobenzophenone as a photoinitiator imparts several desirable physical properties to the cured UV - curable coatings. One of the notable advantages is the improvement in hardness. Hardness is crucial for coatings as it determines the coating's resistance to scratching, abrasion, and wear. Coatings cured with 4 - Chlorobenzophenone often exhibit a higher hardness value compared to those cured with some other photoinitiators.
In a hardness - testing experiment using the pencil - hardness method, a UV - curable coating formulated with 4 - Chlorobenzophenone achieved a hardness of 3H, while a similar coating formulation with a different photoinitiator (e.g., a lower - performance benzophenone - based derivative) only reached a hardness of 2H. This higher hardness makes the coated products more suitable for applications where they are likely to come into contact with abrasive surfaces, such as in the automotive industry where car bodies are exposed to various environmental factors and potential scratches.
Environmental and Safety Aspects
4 - Chlorobenzophenone has the advantage of low volatility and a relatively low - odor profile, which is highly beneficial for both the environment and the health of users. Low volatility means that during the coating application and curing process, fewer harmful vapors are released into the air.
In a closed - environment coating process, such as in a paint - spraying booth, the use of 4 - Chlorobenzophenone can significantly reduce the concentration of volatile organic compounds (VOCs) in the air. This not only helps to meet environmental regulations regarding air quality but also creates a healthier working environment for the operators. Workers are less likely to be exposed to potentially harmful fumes, reducing the risk of respiratory problems and other health issues associated with VOC exposure.
Toxicity and Safety
When it comes to toxicity, 4 - Chlorobenzophenone has been studied, and its safety profile shows certain advantages compared to some other photoinitiators. According to available toxicology data, 4 - Chlorobenzophenone has a relatively low acute toxicity. In acute - toxicity tests, it has been found that the LD50 (the dose required to kill 50% of a test population) values for 4 - Chlorobenzophenone are relatively high, indicating that a relatively large amount of the substance would need to be ingested or otherwise exposed to cause severe harm.
In terms of skin and eye irritation, proper handling of 4 - Chlorobenzophenone, following safety guidelines such as wearing appropriate personal protective equipment (PPE) like gloves and safety glasses, can minimize the risk of adverse effects. When compared to some photoinitiators that may be more irritating to the skin or eyes, 4 - Chlorobenzophenone poses a lower risk. In industrial settings, the use of 4 - Chlorobenzophenone can be managed with standard safety protocols. For example, in a coating manufacturing plant, workers are trained to handle 4 - Chlorobenzophenone - containing formulations with care, and the plant is equipped with proper ventilation systems to prevent the accumulation of any potential fumes or dust. This ensures that the use of 4 - Chlorobenzophenone in the production of UV - curable coatings can be carried out safely, with minimal risk to the health of the workers and the environment.
Conclusion
4 - Chlorobenzophenone plays a pivotal role as a photoinitiator in UV - curable coatings. Its unique chemical structure enables it to absorb UV light effectively, generating reactive free radicals that initiate the polymerization process, leading to the formation of a durable and high - performance coating.
The advantages of 4 - Chlorobenzophenone over other photoinitiators are numerous. It offers cost - effectiveness, with a competitive price and the ability to be used in relatively low concentrations. Its curing efficiency is remarkable, featuring fast curing speeds and good depth of cure. The resulting coatings exhibit excellent physical and chemical properties, including high hardness, flexibility, wear resistance, and chemical resistance. Additionally, its low volatility, low - odor profile, and relatively low toxicity make it a more environmentally friendly and user - safe option.
