TPO 819 1173 - Radical Photoinitiators for UV LED Curing

If you formulate UV-curable coatings, inks, or 3D printing resins, you've probably used at least one of these three: Sinocure 1173, TPO, or 819. They've been around for years, but choosing the right one – or the right blend – still depends on your light source, film thickness, and color requirements.

Let's go through what actually matters.

1. Quick reference table

2. Structure vs. performance

2.1 1173 – The classic α‑hydroxy ketone

1173 is the go‑to α‑hydroxy ketone. Simple structure: one benzoyl group attached to a tertiary carbon with a hydroxyl group.

What that gives you:

• Liquid form – easy to mix, great compatibility

• High initiation efficiency

• Very low yellowing

• Low cost

The trade‑offs:

• Absorbs mostly at 331 nm – not a good match for LEDs

• Small molecule → migration risk

• Decomposition creates trace formaldehyde and acetone → mild odor

2.2 TPO – Single‑acyl phosphine oxide

TPO contains one benzoyl and one diphenylphosphine oxide group. The phosphorus atom shifts absorption to longer wavelengths.

Why formulators like it:

• Long‑wave absorption (380–393 nm) – perfect for 385/395 nm LEDs

• Photobleaching effect – helps deep cure

• Low volatility, low odor

Downsides:

• Powder form – needs heating to dissolve

• Slightly more yellowing than 1173

2.3 819 – Bis‑acyl phosphine oxide

819 has two benzoyl groups and one phosphine oxide group. That means up to four radicals per molecule.

Strengths:

• Dual absorption peaks at 370 nm and 405 nm – wide coverage

• Strong photobleaching – great for thick sections

• Fastest cure in white and heavily pigmented systems

Weaknesses:

• Noticeable yellowing (ΔE ≈ 5.5)

• Lower solubility

• Higher price

3. Absorption and light source matching

This is where the three really separate.

3.1 1173 – Mercury lamp specialist

1173 absorbs at 245, 280, and 331 nm. That overlaps well with mercury lamp emission lines (254, 313, 365 nm).

On a 365 nm LED? Partial overlap, but efficiency drops. On 385 or 395 nm LED? Almost no activation.

Verdict: Great for mercury lamps. Not for LED.

3.2 TPO – The LED workhorse

Absorption peaks at 380 and 393 nm. That's a direct match for 385 nm and 395 nm LEDs.

This is why TPO became the default photoinitiator for the LED era. It works, it's affordable, and it smells clean.

Verdict: Best all‑round choice for 385/395 nm LED systems.

3.3 819 – Wide‑band specialist

Two peaks: 370 nm and 405 nm. That covers 365 nm LEDs, 405 nm LEDs, and even some visible‑light systems.

The long‑wave absorption also helps penetrate pigments and fillers – essential for white coatings and thick films.

Verdict: Excellent for thick, white, or heavily filled systems.

3.4 Light source compatibility

4. Curing characteristics and application positioning

4.1 1173 – Low‑yellowing choice for clear coats

Key advantages:

• Very low yellowing (ΔE ≈ 0.2) – performs well in clear and light‑colored systems

• Liquid form – easy to add, good compatibility

• Low cost – high value for money

Main limitations:

• Odor from decomposition products – not suitable for food packaging or sensitive applications

• Poor LED compatibility – cannot be used effectively with LED lamps

• Only moderate surface cure – sensitive to oxygen inhibition

Where to use it:

• Wood clear coats, plastic clear coats, metal gloss coats

• Paper overprint varnishes

• General coatings where yellowing must be minimized

Typical dosage: 1–4%

4.2 TPO – The all‑rounder for the LED era

Key advantages:

• Matches 385/395 nm LEDs perfectly

• Photobleaching effect – helps deep curing

• Low odor – good for applications with strict smell requirements

• Balanced cost and performance

Main limitations:

• Needs heating to dissolve – may crystallize out at low temperatures

• Slightly higher yellowing than 1173, but still acceptable for most uses

Where to use it:

• LED‑curable coatings and inks

• 3D printing resins

• Pigmented systems (often blended with ITX)

• Applications where low odor is important

Typical dosage: 1–3%

4.3 819 – Specialist for white and thick systems

Key advantages:

• Dual absorption peaks (370 nm and 405 nm) – covers 365–405 nm range

• Strong photobleaching – absorption decreases as reaction proceeds, allowing light to penetrate deeper

• Excellent in white systems – cures effectively even with titanium dioxide

• High initiation efficiency – two acyl groups generate up to four radicals per molecule

Main limitations:

• Noticeable yellowing (ΔE ≈ 5.51) – not suitable for light‑colored clear coats

• Lower solubility – check solubility in your monomer system

• Higher cost

Where to use it:

• White coatings and white inks

• Thick films (>50 µm)

• Outdoor coatings requiring weather resistance (used with UV absorbers)

• Glass‑fiber reinforced composites

Typical dosage:

• Clear systems: 0.1–0.2%

• White systems: 0.5–1%

5. Blending strategies – because one PI rarely does it all

In real formulations, you almost never use just one. Here's what works.

5.1 1173 + TPO/819 – Surface + deep cure

5.2 TPO + 819 – The LED duo

5.3 Real‑world blend examples

Example 1 – White UV coating
TPO 1% + 819 0.5% + 184 1%
→ TPO and 819 provide long‑wave penetration, 184 handles surface cure.

Example 2 – Clear thick film coating
819 0.2% + 184 2%
→ Strong photobleaching from 819 ensures deep cure.

Example 3 – LED 3D printing resin
TPO 2% + 819 1%
→ Covers 385–405 nm LEDs, fast cure throughout.

6. Bottom line – three tools, different jobs

Think of them this way:

• 1173 – The low‑yellowing specialist for mercury‑lamp clear coats. Still useful, but not for LED.

• TPO – The all‑rounder for LED curing. Good balance of performance, cost, and odor.

• 819 – The heavy lifter for white, thick, or highly filled systems. Expensive, but sometimes the only real option.

No single photoinitiator does everything. That's why we blend. And that's why knowing their differences – not just reading spec sheets – still matters.

Have a specific system in mind? We can help you pick the right starting blend. 

Drop us a line at info@sinocurechem.com.