Photoinitiators | Migration and Toxicity

Hello everyone! I’m a star employee at CHROMÉCLAIR, a brand of hema free gel polish.Today, I’ll organize some information about the migration and toxicity of photoinitiators, hoping it will be helpful to you.

The molecular weight of photoinitiators typically ranges between 100 and 300. For example, 1173 has a molecular weight of 164.2, TPO has 348.4, and 819 has 418.5, all classified as low-molecular-weight organic compounds.

Under incomplete light exposure, these photoinitiator molecules may remain in the cured material, forming potential migration substances. Furthermore, the generation of free radicals by photoinitiators is predominantly achieved through cleavage reactions. These free radicals may form lower molecular weight compounds upon final quenching.

These small-molecule products not only contribute to migration concerns but may also generate toxic substances. Both residual photoinitiators and the small molecules produced during reactions pose potential migration risks in light-cured products, potentially adversely affecting users.

The most significant incident occurred in 2005 when the photoinitiator ITX was detected in Nestlé infant formula in Italy. Globally, the European Union maintains the strongest focus on the health impacts of chemicals and enforces the most stringent regulations.

Toxicological Classification and Restrictions on TPO Use

With the widespread use of the photoinitiator TPO, regulatory oversight has intensified. Under the EU’s CLP (Classification, Labelling, and Packaging) Regulation, TPO was initially classified as a Category 2 (H361) reproductive toxicant, meaning “suspected human reproductive toxicant, but the available evidence is not sufficient to conclusively classify it as Category 1.”

TPO (MAPO)

In June 2020, Sweden, a Nordic country, proposed revising the classification to Category 1B (H360DF) and adding it as a skin irritant (H317). Category 1B denotes “presumed human reproductive toxicant,” a conclusion based on substantial evidence from animal studies.

The EU’s Classification and Labelling Harmonization (CLH) Process

Regulations also categorize reproductive toxicants into Class 1, which is further subdivided into 1A and 1B. 1A denotes “known to be toxic to reproduction in humans,” based on substantial human evidence, while 1B denotes “presumed to be toxic to reproduction in humans,” based on substantial animal testing evidence. Category 1A substances are prohibited, while Category 1B substances are likely to be banned or restricted. Previously, Sweden, a Nordic country, also proposed modifying the classification to 1B (H360DF) and adding it as a skin irritant (H317).

In autumn 2021, the EU’s Risk Assessment Committee (RAC) agreed to update TPO’s classification. The European Commission (EC) is currently reviewing this classification, a process expected to take 0.5-1.5 years according to regulations. If approved by the EC, this classification will be added to Annex VI of the EU CLP Regulation via an ATP and become legally binding. In January 2023, Sweden announced its intention to propose TPO for inclusion in the SVHC (Substances of Very High Concern) Candidate List.

According to the ECHA website, TPO was added to the Candidate List of Substances of Very High Concern for Authorization on June 14, 2023. This means TPO is now banned or restricted in many applications, such as food contact materials.

TPO Alternative Options

Among phosphine oxide photoinitiators with good absorption in the UVA band, two other commonly used alternatives besides TPO are TPO-L and 819 (BAPO). However, TPO-L and 819 can only partially replace TPO in certain applications and cannot fully substitute it.

TPO-L possesses a structure similar to TPO, but one benzene ring in its molecule is substituted with an ethoxy group, resulting in lower toxicity. However, TPO-L exhibits significantly lower initiation efficiency compared to TPO.

Another phosphine oxide photoinitiator is 819 (BABO), which can be viewed as TPO with one benzene ring substituted by a 2,4,6-trimethylbenzoyl group, effectively possessing two 2,4,6-trimethylbenzoyl groups. While 819 exhibits higher initiation efficiency than TPO, it suffers from severe yellowing issues, making it unsuitable for applications with color requirements.

The Emergence of TMO: The Ultimate TPO Replacement

In this context, the advent of the novel photoinitiator TMO fundamentally resolves all challenges previously associated with TPO.

TMO

TMO, fully known as (2,4,6-trimethylbenzoyl)bis(p-tolyl)phosphine oxide, CAS 270586-78-2. Structurally, TMO introduces a methyl group to each of the two benzene rings based on TPO, significantly reducing TPO’s biotoxicity. Experimental findings indicate that TMO exhibits slightly superior initiation efficiency compared to TPO, while also demonstrating no yellowing and lower migration.

This photoinitiator is utilized in coating formulations, offering excellent solubility with oligomers and reactive diluents, and is benzene-free. TMO is suitable for rapid UV curing of high-quality coatings, inks, and adhesives on various substrates. It appears as a pale yellow crystalline powder with a characteristic aromatic odor, is stable, does not readily decompose at room temperature, and is soluble in various organic solvents.

The synthesis route for TMO is considered environmentally friendly, yielding high-purity products with high yields. In terms of photopolymerization kinetics, TMO demonstrates outstanding performance, exhibiting superior light absorption properties, photopolymerization activity, and deep cure penetration compared to traditional TPO photoinitiators. When blended with 1-hydroxycyclohexylbenzophenone (184), TMO enhances photosensitivity and deep-curing characteristics.

Double Bond Conversion Rate Curve of TMO and TPO Initiating TMPTA

TMO will serve as an ideal substitute for TPO and has already achieved mass production. Furthermore, TMO has obtained the EU REACH registration certificate, enabling its sale in Europe—the region with the strictest chemical regulations.

Photoinitiator TMO has been tested for use in printing inks and adhesives on wood, metal, plastic, paper, and optical fiber surfaces. It exhibits excellent solubility in common UV formulations (e.g., acrylate, unsaturated polyester systems). It can be used alone or blended with other photoinitiators.

‌TMO initiator synthesis is achieved using di(p-tolyl)phosphine oxide and 2,4,6-trimethylbenzoyl chloride as raw materials. This synthesis method is environmentally friendly, efficient, and yields a high-purity product with excellent yield. By testing its photophysical and chemical properties, applying it to ink systems, studying its photopolymerization kinetics, establishing a curing kinetic model, and comparing it with 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO), Results indicate that TMO exhibits superior light absorption properties, photopolymerization activity, and deep curing performance compared to TPO. Furthermore, co-formulating TMO with 1-hydroxycyclohexylbenzophenone (184) enhances TMO’s photosensitivity and deep curing characteristics.

 

I hope this article helps you understand photoinitiators more easily!

CHROMÉCLAIR offers Base coats, Top coats, solid color gel polish without HEMA, and hema free cat eye gel polish.

Their website also features nail art tutorials, such as:

How to Do the polka dot bow nail art at Home?

Halloween Nail Art Tutorial: Cyber Ghost