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TRIMETHYLBENZOYL DIPHENYLPHOSPHINE OXIDE.

Avoid CAS 75980-60-8 / SKIN CONDITIONING

Trimethylbenzoyl Diphenylphosphine Oxide, or TPO, is a photoinitiator primarily employed in UV-curable formulations such as gel nail polishes. It facilitates rapid polymerization upon UV light exposure, causing products to cure and harden, with the ingredient largely consumed in the process.

Summary

Photoinitiator Nail Sculpting Polymerization Initiator

Science

Mechanism of Action

When exposed to ultraviolet (UV) light, Trimethylbenzoyl Diphenylphosphine Oxide (TPO) acts as a photoinitiator by undergoing rapid cleavage from a triplet excited state. This generates two highly reactive free radical fragments—a trimethylbenzoyl radical and a diphenylphosphinoyl radical—which then initiate the polymerization of monomers. This process results in the curing and hardening of the product, with TPO largely consumed and its minimal residual amounts becoming trapped within the hardened polymer matrix.

Research

Clinical Evidence

Medium confidence
Effective range 0.25–5%
Optimal

5%

Key findings

  1. 01 Products containing 0.25-3.65% TPO applied to nails over 6 weeks showed no irritation in a use assay, though this was not a standard Human Repeat Insult Patch Test (HRIPT).
  2. 02 In artificial nails treated with gel polishes containing 3% TPO, residual amounts were nearly undetectable (less than 0.2% of total TPO) after curing, indicating substantial consumption during polymerization.
  3. 03 TPO is currently used and anticipated at concentrations between 0.5% and 5.0% in nail gels for polymerization in artificial nail systems, and up to 4% in nail polish and enamel formulations.

Transparency

Dusting Analysis

Not commonly dusted

Trimethylbenzoyl Diphenylphosphine Oxide functions as a reactive photoinitiator that is predominantly consumed during the UV-induced polymerization process to cure nail products. It does not offer direct skin benefits nor is it typically included in formulations for 'dusting' purposes, as its primary role involves chemical transformation within the product matrix.

The Formula

Formulation

Solubility
Oil
Optimal pH 6 – 7
0 7 14

Stability

TPO is sensitive to light and should be stored in dark, cool, and dry conditions for a shelf life of at least two years. It is readily consumed during the polymerization process in UV-curable systems.

Safety

Safety Profile

CIR Status
Safe with restrictions
Max tested
4%
Pregnancy
Caution
Sensitization risk Low

The CIR expert panel concluded TPO is safe for cosmetic use, provided it is formulated to prevent skin sensitization, deeming it to have low sensitization potential. While the SCCS initially considered it safe in UV-curable nail formulations up to 5% with no direct skin contact, the EU has reclassified TPO as a Category 1B reproductive toxicant (CMR 1B) by ECHA due to animal studies showing adverse effects on fertility and reproduction. This classification will lead to its prohibition in EU cosmetic products from September 1, 2025. There are currently no regulations banning its use in the US.

Your Skin

Skin Compatibility

No Normal
No Dry
No Oily
No Sensitive
Irritancy Moderate
Comedogenicity Unknown

Our Assessment

Verdict

Avoid

Despite its functional role in UV-curable nail systems where it is largely consumed, Trimethylbenzoyl Diphenylphosphine Oxide presents significant reproductive toxicity concerns (CMR 1B classification in the EU leading to a ban) and offers no direct benefits for precision skincare applications.

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