October 14, 2024
TPO is a chemical compound known as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, which absorbs ultraviolet (UV) light and breaks down into reactive molecules (free radicals). Due to these properties, TPO is classified as a photoinitiator.
The photoinitiators (such as TPO) play a crucial role in UV gels and hybrid nail polishes: these compounds absorb light and create active centers (free radicals), which initiate the polymerization reaction. The process of free-radical photoinitiated polymerization involves three distinct stages: initiation (starting the polymerization reaction), propagation (growth of the polymer chain), and termination (completion of the polymerization process) [1]. The market for UV-cured nail products would not exist without the presence of photoinitiators in formulations. They initiate the polymerization of double bonds found in functional groups (acrylic or methacrylic).
UV and UV-LED nail polishes are products that under the influence of ultraviolet light undergo a chemical reaction (polymerization), most commonly cross-linking process (hardening). The UV/UV-LED nail polish market has been growing for many years, as well as the market for ultraviolet light sources. Previously, mercury lamps emitting wide-spectrum ultraviolet light were common, but UV-LED lamps have largely replaced them in the nail styling market due to their numerous advantages. Depending on the manufacturer, UV-LED lamps emit light at specific wavelengths ranging from 365-385 nm and 395-405 nm. Lamps emitting light at 365 nm have become the most popular due to their lower cost, better curing efficiency, increased surface gloss, and ability to cure hard gels [2]. Therefore, the selection of appropriate photoinitiators that absorb UV light at this wavelength is crucial for properly curing gels and hybrid polishes. Until now, the TPO photoinitiator has been highly effective in this application, with maximum absorption in a wide range of 350-410 nm (three absorption maxima at 365 nm, 380 nm, and 395 nm - these values were determined in a model TPO/ethanol system, but they may slightly differ in a specific product (e.g., by 3 nm) due to shifts in absorption maxima caused by the overall properties of the given formulation [3]).
The discussions on the potential health risks of TPO began several years ago. On March 27, 2014, the European Commission's - Scientific Committee on Consumer Safety (SCCS) issued the following opinion on TPO in cosmetic products: "TPO is safe when used as a nail modeling product at concentrations of up to 5.0%. However, TPO is considered a moderate skin sensitizer. Other potential uses of trimethylbenzoyldiphenylphosphine oxide in cosmetic products cannot be assessed without additional documentation" [4]. Further research has been conducted on this topic. In 2021, the Risk Assessment Committee (RAC) of the European Chemicals Agency (ECHA) adopted an opinion of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide on September 16, 2021. This ultimately led to the European Commission's adoption of Delegated Regulation 2024/197 of October 19, 2023, amending Regulation (EC) No 1272/2008 on the harmonized classification and labeling of certain substances. TPO (CAS No: 75980-60-8) was classified as a reproductive toxicant (1B) and skin sensitizer (1B), labeled as H360Fd and H317. The regulation will come into force on September 1, 2025. Therefore, due to the classification of the TPO photoinitiator as a substance harmful to fertility, it cannot be used in cosmetic products from September 2025 onward [5, 6].
The withdrawal of the TPO photoinitiator presents a significant challenge for manufacturers of UV-cured gels and nail polishes, because many products on the market still contain TPO. Replacing this photoinitiator with other permissible alternatives requires considerable costs and time for conducting numerous tests on modified formulations, as well as updating labels on packaging to reflect the new ingredients. The replacement of TPO may also involve modifying the entire formulations (not only in terms of photoinitiators but also other ingredients) to achieve comparable properties to previously available products. Some products will be withdrawal completely if it becomes impossible to achieve the required final properties.
The introduction of new legal regulations regarding the use of certain chemical substances in cosmetic products will undoubtedly enhance the safety of customers using new products that comply with these regulations. The removing TPO from UV and UV-LED gels and polishes will provide greater protection for consumers' health. Prohibition of the use of TPO will also cause manufacturers to develop a deeper awareness of continuously seeking and using safer raw materials in their products to be prepared for inevitable changes. Producers of photoinitiators are also faced with the challenge of increasing the production scale of widely used photoinitiators that serve as alternatives to TPO or seeking entirely new compounds to initiate the photopolymerization process.
[1] H. S. Lawrence, PhD thesis "Factors influencing the efficiency of photoinitiation in radiation curable ink formulations" (1997), 11-12.
[2] L. Pavlovic, "The science of UV cure nail gels: A short review," Nail Division Coty, Inc.
[3] X. Shi, X. Gu, Y. Zhang, L. Dong, Q. Meng, "Preparation of an aqueous photoinitiator through supramolecular assembly oriented towards 3D printing hydrogel without VOC emission" J. Sol-Gel Sci. Technol., 105(2) (2023), 1-10.
[4] Scientific Committee on Consumer Safety (SCCS), Opinion on Trimethylbenzoyl diphenylphosphine oxide [TPO] (2014), 1-31.
[5] Commission Delegated Regulation (EU) 2024/197 of October 19, 2023, 1-19.
[6] European Chemicals Agency (ECHA), Support document for identification of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide as a substance of very high concern because of its toxic for reproduction (article 57c) properties (2023), 1-8.
