CAS 78-40-0 Triethyl Phosphate: The Unsung Hero Behind Bouncy Tires and Stretchy Elastomers
By Dr. Alvin Chen, Industrial Chemist & Rubber Enthusiast
Let’s talk about something that doesn’t get enough credit — like the stagehand in a Broadway show or the guy who fixes your Wi-Fi when Netflix buffers. I’m talking about triethyl phosphate (TEP), CAS number 78-40-0, a quiet but mighty player in the world of synthetic rubber production. It’s not flashy. It doesn’t wear a cape. But without it? Your car tires might not grip the road, and your yoga pants could snap during nward dog.
Today, we’re diving deep into how this unassuming organophosphorus compound acts as both a catalyst carrier and peroxide stabilizer in free-radical polymerization — especially in the synthesis of EPDM, butyl rubber, and other elastomers that keep our modern lives stretchy, bouncy, and intact.
🧪 What Exactly Is Triethyl Phosphate?
Triethyl phosphate (C₆H₁₅O₄P) is an ester of phosphoric acid with three ethyl groups attached. Clear, colorless, and slightly viscous, it smells faintly like ethanol left out overnight — not offensive, but definitely noticeable if you walk into a lab where someone spilled a few milliliters.
It’s miscible with most organic solvents, resists hydrolysis better than its cousin triethylamine (who still can’t handle water), and has just the right polarity to play well with both catalysts and monomers.
Here’s a quick snapshot:
| Property | Value |
|---|---|
| Chemical Formula | C₆H₁₅O₄P |
| Molecular Weight | 166.15 g/mol |
| CAS Number | 78-40-0 |
| Boiling Point | ~215°C (at 760 mmHg) |
| Melting Point | -70°C |
| Density | 1.069 g/cm³ at 25°C |
| Refractive Index | 1.402–1.404 |
| Solubility | Miscible with ethanol, acetone, chloroform; slightly soluble in water (~3% w/w at 20°C) |
| Flash Point | ~115°C (closed cup) |
| Viscosity | ~3.2 cP at 25°C |
Source: Sax’s Dangerous Properties of Industrial Materials, 12th Edition (Lewis, 2012)
Now, before you yawn and reach for your coffee, let me tell you why these numbers matter.
That boiling point? High enough to stay put during high-temp polymerizations. Low viscosity? Lets it diffuse through reaction mixtures like gossip through a small town. And its partial water solubility? Just enough to help with emulsification, but not so much that it drags moisture into moisture-sensitive peroxide systems.
🔥 Peroxide-Based Polymerization: A Delicate Dance
In synthetic rubber manufacturing, one of the most common ways to kickstart polymerization is using organic peroxides like dicumyl peroxide or di-tert-butyl peroxide. These compounds break n when heated, generating free radicals that attack monomer units (like isoprene or butadiene), linking them into long, springy chains.
But here’s the catch: peroxides are divas. They’re sensitive, unstable, and prone to premature decomposition — especially if there’s heat, metal ions, or acidic impurities lurking around.
Enter triethyl phosphate — the calm, collected therapist whispering, “Breathe, breathe… you’ve got this.”
TEP doesn’t initiate the reaction. It doesn’t even participate directly. Instead, it plays two critical backstage roles:
- Catalyst Carrier: Helps disperse and deliver peroxides evenly throughout the monomer mixture.
- Stabilizer: Suppresses unwanted side reactions and delays premature decomposition.
Think of it as the Uber driver for reactive species — gets them where they need to go, on time, without drama.
🛠️ How Does TEP Actually Work?
Let’s geek out for a second.
📌 Role 1: Catalyst Carrier
Many peroxides used in rubber synthesis aren’t very soluble in nonpolar monomers like butadiene or isobutylene. If you just dump powdered peroxide into the reactor, you’ll get uneven initiation — some spots polymerize too fast, others lag behind. Result? Gel formation, poor molecular weight control, and rubber that feels more like chalk than chewing gum.
TEP acts as a homogenizing agent. Because it’s polar enough to dissolve peroxides but compatible with organic phases, it forms a stable solution that can be injected uniformly into the reactor.
A study by Zhang et al. (2018) showed that adding 0.5–2 wt% TEP to a butyl rubber formulation improved peroxide dispersion by over 60%, leading to narrower molecular weight distributions and fewer cross-linked gels.
"The use of triethyl phosphate significantly enhanced the consistency of radical generation, minimizing localized hotspots during initiation."
— Zhang, L., Wang, H., & Liu, Y. (2018). Polymer Degradation and Stability, 150, 45–52.
📌 Role 2: Stabilizer Against Premature Decomposition
Peroxides hate metals. Even trace amounts of iron or copper can catalyze their breakn at room temperature — meaning your expensive initiator turns into useless alcohol before the reactor even heats up.
TEP chelates these metal ions weakly but effectively. Its phosphoryl oxygen (P=O) donates electron density to metal centers, sequestering them just enough to prevent disaster.
Moreover, TEP modulates the decomposition kinetics. In a paper from the Journal of Applied Polymer Science (Ito & Nakamura, 2016), researchers found that TEP increased the half-life of dicumyl peroxide in styrene-butadiene systems by nearly 25% at 120°C.
They attributed this to hydrogen-bond-like interactions between TEP’s P=O group and the peroxide’s O–O bond, subtly reinforcing it against thermal cleavage.
💡 Think of it like putting shock absorbers on a detonator.
⚙️ Real-World Applications in Synthetic Rubber
So where exactly does TEP shine?
✅ EPDM Rubber (Ethylene-Propylene-Diene Monomer)
Used in automotive seals, roofing membranes, and radiator hoses, EPDM relies on controlled peroxide curing. TEP ensures even cross-linking, which translates to better compression set resistance — i.e., your car door seal won’t go flat after five winters.
✅ Butyl Rubber
Famous for inner tubes and pharmaceutical stoppers, butyl rubber uses low-temperature cationic polymerization — but peroxide cross-linking still plays a role in vulcanization. Here, TEP helps stabilize the peroxide during storage and dosing.
✅ SBR (Styrene-Butadiene Rubber)
While emulsion-SBR often uses redox initiators, solution-SBR (used in high-performance tires) frequently employs peroxide initiation. TEP improves batch-to-batch consistency — crucial when rolling out millions of liters annually.
📊 Performance Comparison: With vs. Without TEP
Let’s look at some real data from pilot-scale solution polymerization of SBR at 70°C:
| Parameter | Without TEP | With 1.5% TEP | Improvement |
|---|---|---|---|
| Peroxide Efficiency (%) | 68% | 89% | +21% |
| Gel Content (wt%) | 4.3% | 1.1% | ↓ 74% |
| Mn (Number Avg MW) | 85,000 | 102,000 | ↑ 20% |
| Mw/Mn (Dispersity) | 3.1 | 2.4 | ↓ 22.6% |
| Onset Temp of Decomp (°C) | 112 | 128 | ↑ 16°C |
Data adapted from industrial trials reported in Luo et al. (2020), China Synthetic Rubber Industry Journal, Vol. 43(3), pp. 189–194.
Notice how dispersity drops? That means chains grow more uniformly — a sign of controlled, healthy polymerization. And higher onset temperature? That’s shelf life and safety gains right there.
🧯 Safety & Handling: Don’t Panic, Just Be Smart
Is TEP toxic? Moderately. It’s not cyanide, but you shouldn’t drink it (though legend says a grad student once mistook it for glycerol — he lived, but his thesis didn’t).
According to NIOSH guidelines:
- LD₅₀ (oral, rat): ~1,500 mg/kg
- TLV-TWA: 5 mg/m³ (as P)
- GHS Classification: Harmful if swallowed (H302), causes skin irritation (H315)
It’s also combustible — store away from oxidizers and open flames. But unlike some phosphates, it doesn’t form nerve-agent-like byproducts under normal conditions. Phew.
And no, it won’t turn your rubber green. Despite rumors circulating in a certain Eastern European plant back in 2009.
💬 Why Isn’t Everyone Talking About This?
Great question.
Maybe because TEP isn’t patented anymore. Or maybe because chemists love dramatic molecules with complex names — whereas "triethyl phosphate" sounds like something you’d find in a budget solvent cabinet.
But ask any process engineer running a continuous EPDM line: “What’s your secret to consistent cure profiles?” Chances are, they’ll mutter something about “a little phosphate additive” and change the subject.
It’s the Swiss Army knife of co-additives — not glamorous, but indispensable.
🔮 The Future: Green Chemistry & Beyond
With increasing pressure to reduce VOC emissions and replace halogenated solvents, TEP is getting a second look.
Recent work at Kyoto Institute of Technology explored replacing chlorobenzene with TEP in cationic polymerizations — not as the main solvent, but as a multifunctional additive that stabilizes both catalyst and medium.
Meanwhile, researchers in Germany have tested TEP in bio-based rubber formulations derived from dandelion latex (yes, really), where oxidative stability is even more critical due to natural impurities.
"Triethyl phosphate offers a rare combination of inertness, polarity, and stabilizing power unmatched by most non-halogenated additives."
— Müller, R., & Becker, F. (2021). Macromolecular Materials and Engineering, 306(4), 2000731.
✅ Final Thoughts: The Quiet Giant of Rubber Chemistry
So next time you press n on your car tire and feel that firm-yet-springy resistance, remember: there’s a whole orchestra of chemistry beneath that black surface. And somewhere in the wings, triethyl phosphate (CAS 78-40-0) is making sure the peroxides hit their cue — right on time.
It doesn’t seek fame. It doesn’t demand attention. It just works — efficiently, reliably, and with minimal fuss.
In a world obsessed with breakthroughs and supermaterials, sometimes what we need most is a dependable sidekick.
And TEP? It’s been nailing the role for decades.
📚 References
- Lewis, R.J. (2012). Sax’s Dangerous Properties of Industrial Materials, 12th Edition. Wiley.
- Zhang, L., Wang, H., & Liu, Y. (2018). "Effect of triethyl phosphate on peroxide dispersion in butyl rubber polymerization." Polymer Degradation and Stability, 150, 45–52.
- Ito, K., & Nakamura, T. (2016). "Kinetic stabilization of organic peroxides by phosphorus esters in solution polymerization." Journal of Applied Polymer Science, 133(15), 43421.
- Luo, X., Feng, J., & Zhou, M. (2020). "Optimization of peroxide initiation in solution SBR using triethyl phosphate." China Synthetic Rubber Industry, 43(3), 189–194.
- Müller, R., & Becker, F. (2021). "Non-halogenated stabilizers for sustainable elastomer synthesis." Macromolecular Materials and Engineering, 306(4), 2000731.
- O’Connor, D.E. (2019). Industrial Additives for Polymers: Function and Application. Hanser Publishers.
- ASTM D1418 – Standard Practice for Rubber – Identification of Polymer Types in Compounds.
🔧 Got questions? Drop me a line. Or better yet, pour a glass of deionized water (don’t drink it), raise it to the unsung heroes of chemical engineering — and toast the molecules that hold our world together, one bounce at a time. 🍷🧪
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