How to substitute tpo photoinitiator?

The ECHA has officially announced that diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, also known as photoinitiator TPO, has been included in the 29th batch of the Candidate List of Substances of Very High Concern (SVHC). This brings the total number of substances on the SVHC Candidate List to 235. This means that companies are given significant responsibility for the chemicals on the list. They are required to do their utmost to manage risks and to provide their customers and consumers with detailed information on the safe use of these chemicals. This is because these substances will most likely be included in the authorization list at some point in the future. Once a substance is included, it will be banned unless the company concerned successfully applies to the European Commission for authorization to continue using it.
Let’s look at the basic information on the photoinitiator TPO first. Its chemical name is diphenyl (2,4,6-trimethylbenzoyl)phosphine oxide, also known as photoinitiator TPO, with EC number 278-355-8 and CAS number 7598 0 – 60 – 8, is listed for reasons of reproductive toxicity (Article 57 (c)), and is commonly used in a wide range of areas such as inks and toners, coating products, photopolymer, adhesives and sealants, as well as fillers, gypsum modelling clay and many more.

Looking back on the development of light curing, it is a very distinctive technology. Light curing mainly refers to the process of curing monomers, oligomers or polymer substrates under the action of light, which plays a key role in the film-forming process. Its high efficiency, adaptability, economy, energy conservation and environmental friendliness have made it a key technology in modern industry. Light curing can be broadly divided into two types: traditional mercury lamp curing and the emerging UV LED curing. Traditional mercury lamps, if not disposed of properly after use, can cause serious environmental pollution, which is one of the main reasons for their phase-out. UV LED curing is gradually emerging in the field of curing equipment due to its many advantages, such as being more energy-efficient, ready to switch on and off, and compact in size. It is poised to replace traditional mercury lamp curing and become the mainstream light source.

In a light curing formulation system, the photoinitiator accounts for only about 2% – 5% of the total, which may seem insignificant, but it actually plays an indispensable role. Due to the special requirements of the photopolymerization reaction, photoinitiators need to absorb ultraviolet light to generate free radicals, which in turn initiate the polymerization reaction and ultimately cause the product to cure. Traditional photoinitiators such as 1173 and 184 have a maximum absorption wavelength in the short-wavelength UVC region, so they are more suitable for curing with traditional mercury lamps. UV LEDs, on the other hand, mainly focus on specific wavelengths such as 365nm, 385nm, 395nm and 405nm. Among these wavelengths, phosphine oxide photoinitiators exhibit relatively strong absorption capabilities. Photoinitiator TPO is a typical representative and is widely used in the field of UV LEDs. TPO not only has the excellent characteristics of high induction efficiency and low yellowing, but is also relatively affordable. However, in the past few years, with the strong growth momentum of UV LED curing technology, the global supply of TPO has been extremely tight, and it has become extremely difficult to obtain a single product. Fortunately, in recent years, due to the continuous expansion of production scale by domestic mainstream photoinitiator manufacturers, coupled with the gradual entry of new manufacturers, the tight supply of TPO has been greatly eased, and the price has gradually returned to normal levels. The stable supply of TPO has also strongly promoted the further development of UV LED technology.

Let’s take a closer look at the toxicity classification and restricted use of TPO. Photoinitiators are mostly small organic molecules. When the light conditions are not sufficient, these photoinitiator molecules can remain inside the cured product, thus forming potential migration substances. In addition, in most cases, the process of photoinitiators producing free radicals is achieved by breaking chemical bonds. After these free radicals are eventually quenched, they may form compounds with a lower molecular weight. These small molecule products not only pose a migration problem, but may also produce toxic substances, which undoubtedly pose a potential threat to human health and environmental safety. With the increasing use of photoinitiator TPO, regulatory efforts against it have also continued to intensify. According to the EU’s CLP (Classification, Labeling and Packaging) regulations, TPO was initially classified as a Category 2 (H361) reproductive toxicant, which is also known as a “suspected human reproductive toxicant”. In June 2020, the Nordic country of Sweden proposed a change in classification to 1B (H360DF) based on evidence obtained from extensive animal experiments, and also added the classification of skin irritant (H317) (1B indicates “presumed human reproductive toxicant”). In the fall of 2021, the EU Risk Assessment Committee (RAC) agreed to update the classification of TPO. Once approved by the European Commission, the classification will be added to Annex VI of the EU CLP Regulation through an ATP and become legally binding. In January 2023, Sweden issued another notice of intent to propose that TPO be included in the SVHC (Substances of Very High Concern) list, and comments on the proposal were due by April 3, 2023. As of now, TPO has been officially included in the 29th batch of the Candidate List of Substances of Very High Concern (SVHC).

In terms of exploring alternatives to the photoinitiator TPO, in addition to TPO, there are two commonly used photoinitiators in the category of phosphine oxide photoinitiators with strong ultraviolet light absorption: Photoinitiator TPO – L and Photoinitiator 819 (BAPO). The molecular structure of TPO – L is similar to that of TPO, but its toxicity is relatively low because one of the benzene rings in the molecule is replaced by an ethoxy group. However, it also has a significant drawback: the initiation efficiency of TPO-L is much lower than that of TPO. The other phosphine oxide photoinitiator 819 (BABO) can be understood as the product of replacing the benzene ring in TPO with a 2,4,6-trimethylbenzoyl substituted with two 2,4,6-trimethylbenzoyl groups. 819 has a higher starting efficiency than TPO, but it has a serious yellowing problem, which means it cannot be used in applications where color is critical. In summary, TPO-L and 819 can only replace TPO in some specific applications, but they cannot completely replace it.

Fortunately, a new alternative to TPO has emerged: Photoinitiator TMO. The full name of Photoinitiator TMO is (2,4,6-trimethylbenzoyl) bis(4-methylphenyl)phosphine oxide, and its CAS number is 270586-78-2. Judging from the molecular structure, Photoinitiator TMO has introduced a methyl group to each of the two benzene rings of TPO. It is this slight structural change that has greatly reduced the biotoxicity of TPO. Extensive experimental verification has found that the starting efficiency of Photoinitiator TMO is even slightly higher than that of TPO, and it has the excellent characteristics of no yellowing and low migration. At present, Photoinitiator TMO has been successfully mass-produced and has successfully obtained the EU REACH registration certificate, which means that it can be successfully sold in the European market, where chemical control is the strictest. The emergence of this new photoinitiator undoubtedly provides new ideas and direction for the photopolymer industry in dealing with the material selection dilemma after TPO was included in the SVHC candidate list. In the future, with the continuous advancement of technology and in-depth research, there may be more innovations and breakthroughs in the field of photoinitiators. We will wait and see.

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