UV Adhesive Photoiniators Chemical Deep Dive

If you've ever looked at the ingredient list on a UV lash adhesive and felt a little overwhelmed, you're not alone. There’s a lot of confusion in the lash industry about the different chemicals used in UV-cure glues—especially when it comes to photoinitiators. These are the ingredients that react with UV light to harden the adhesive, and not all photoinitiators are created equal. Some are faster, some are safer, and some are facing stricter regulations that lash artists should be aware of.

Let’s break it down clearly so you know what you're working with—and why we choose the photoinitiators we do.

What Are Photoinitiators?

UV-cure lash adhesives rely on photoinitiators—special molecules that absorb UV light and generate the free radicals needed to kick off polymerization (the hardening process).

The three main photoinitiators you’ll most often see on ingredient labels are:

• TPO – Ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (CAS 84434-11-7)
• BAPO – Phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (CAS 162881-26-7)
• TMO – (2,4,6-trimethyl-benzoyl)bis(p-tolyl)phosphine oxide (CAS 270586-78-2)

How They Work

All three are known as Type I phosphine-oxide photoinitiators. When exposed to UVA light (365–405 nm), they split into two highly reactive radicals that attack the acrylate groups in the adhesive, starting the curing reaction.

While they all do the same basic job, their performance and safety profiles are a little different. Here’s a simple comparison:

A Closer Look at Each Photoinitiator

TPO

TPO has been the “go-to” photoinitiator for many lash adhesives because it cures fast under standard UV lamps and keeps adhesives crystal clear.

However, recent toxicology research raised concerns about reproductive toxicity. As a result, the EU officially classified TPO as Reproductive Toxicity 1B (H360FD) and added it to the REACH Candidate List in January 2025. Because of this, many brands are starting to move away from TPO.

Industry statement (FIDE 2024)

BAPO

BAPO is known for its incredible efficiency. It produces four radicals per UV photon absorbed, making it excellent for curing adhesives with pigments or fillers.

The downside? Studies suggest that BAPO may have higher cytotoxicity compared to TPO, and regulatory agencies are considering labeling it with the same Reprotoxic 1B classification in the near future.

MilliporeSigma SDS (2024)

TMO (Our Choice)

TMO is a new-generation photoinitiator developed by modifying the TPO structure. Adding extra methyl groups helps maintain strong UV absorption while significantly reducing bioavailability and lowering reproductive toxicity in animal testing.

As of April 2025, TMO has no official CMR classification in the EU and is not on the SVHC list. Because of this, it’s considered a safer and more future-proof choice for cosmetic-grade adhesives.

LIFT-Chem Technical Note (2023)

Why We Formulate With TMO

• Safety first – No CMR classification. Lower reproductive toxicity risk compared to TPO or BAPO.
• Excellent LED compatibility – Fast, deep cure under the 395 nm LED lamps used in our Halo UV Lash System.
• Color stability – Keeps adhesives water-clear with minimal yellowing over time.
• Low odor and easy handling – Better breathing for artists and smoother production processing.

Final Takeaways

If you’re using a UV adhesive that lists TPO or BAPO, it’s a good idea to stay updated on the latest regulations and safety findings. Switching to TMO-based adhesives is a smart, future-proof move that keeps your retention high while minimizing health risks for you and your clients.

Always check the latest Safety Data Sheet (SDS) and follow local regulations when handling UV-cure products.