DOP, a di-ester of phthalic anhydride and 2-ethylhexanol, is one of the most cost-effective plasticizers with strong plasticizing performance. Therefore, DOP (Dioctyl Phthalate) is a most used plasticizer“. BASTONE is the leading DOP manufacturer and supplier in China, with years of experience as a reliable producer and exporter of plasticizers.
DOP Production Process
DOP Producer continuous prodution process
Phthalic anhydride reacts with 2-ethylhexanol in a molar ratio of 1:1.6 at 120°C for monoesterification. The reaction proceeds continuously into a diesterification reactor.
At 130–150°C and 7.9 kPa, with sulfuric acid catalyst (0.5% of total feed), the reaction continues to form the diester (DOP).
The crude ester product is first washed with water at 50°C using 1/5 of the total volume, followed by neutralization with 2%–3% sulfuric acid at 60–70°C. Then, it is further neutralized at a 1:1 ratio with water at 70–80°C, followed by a final hot water wash.
Then, 0.1% activated carbon is added and the mixture is deacidified and decolored at 150°C and 3.9 kPa under vacuum. Finally, the product is filtered to obtain purified DOP.
Reaction Chemistry: Why the 1:1.6 Molar Ratio and 130–150°C Window
The manufacturing process for DOP is governed by an equilibrium reaction, and every parameter in the procedure above exists to push that equilibrium toward the diester. Phthalic anhydride opens to the half-ester (mono-ester) almost instantly at 120°C — that is the easy step. The hard step is forcing the second esterification, where the mono-ester’s free carboxylic acid must react with another 2-ethylhexanol molecule against rising concentrations of water.
The 1:1.6 molar excess of 2-EH supplies the driving force; without it, the reaction stalls at roughly 85–90% diester conversion. Temperature does the rest.
Below 130°C, the diesterification runs too slow for continuous operation. Above 150°C, side reactions accelerate — ether formation between two 2-EH molecules, oxidative coloration, and trace olefin byproducts that later show up as elevated chroma. The 130–150°C window is the kinetic sweet spot.
Combined with the 7.9 kPa vacuum, water vapor is continuously stripped from the reactor, dragging the equilibrium further toward the diester per Le Chatelier’s principle. Sulfuric acid at 0.5% feed is chosen over titanate or organotin catalysts for one reason: cost-per-conversion at commodity scale.
Challenges in the DOP production process cluster around five pressure points the parameters above mitigate:
- Thermal control — the 130–150°C window is non-negotiable: conversion stalls below, ether and color bodies form above.
- Side reactions — suppressed by capping temperature and venting 2-EH vapor at the condenser before it can dimerize.
- Catalyst poisoning — sulfuric acid tolerates wet feed cheaply, while titanate demands upstream drying; this is the cost lever.
- Byproduct removal — the 7.9 kPa vacuum continuously strips reaction water, dragging equilibrium toward the diester per Le Chatelier’s principle.
- Environmental compliance — 2-EH vapor capture, sulfuric-acid neutralization, and effluent treatment are non-optional capex, not afterthoughts.
Titanate gives cleaner color but costs roughly 30 times more per kilogram and demands tighter water control upstream. For commodity DOP grades, the post-reaction wash sequence neutralizes residual sulfuric acid effectively enough that the catalyst’s color penalty is recovered downstream.
Quality Specifications and the Process Steps That Control Them
| Appearance | Transparent oily liquid |
|---|---|
| Chroma | ≤30 |
| Purity | ≥99.5% |
| Density(20℃) | 0.982-0.988 g/cm3 |
| Flash Point | ≥196℃ |
| Acidity | ≤0.030% |
| Water | ≤0.10% |
Each line in this specification table maps back to a specific step in the production sequence, and reading the spec sheet without that mapping is how procurement teams end up qualifying a marginal supplier.
Purity ≥99.5% is the deepest indicator of process control. It depends on two things: the molar excess of 2-EH (without it, residual mono-ester drags purity below 99%) and the depth of diesterification at 130–150°C (premature reactor pull-through leaves unreacted intermediate). A producer running 1:1.4 ratio at 125°C will not hit ≥99.5% — and the certificate of analysis will show it.
Chroma ≤30 (Pt-Co scale) is controlled almost entirely by the activated carbon decolorization step at 150°C and 3.9 kPa. Color bodies in crude DOP come from oxidized 2-EH and trace iron leached from carbon-steel piping; activated carbon adsorbs both. A facility skipping this step or running carbon below 0.1% loading will ship product with chroma 60–100 — visually yellow, immediately rejected for clear-PVC applications.
Acidity ≤0.030% is the residual sulfuric acid neutralization signal. The 2–3% sulfuric acid wash at 60–70°C followed by water rinse at 70–80°C is calibrated to bring acid value down by roughly two orders of magnitude.
Skip the second wash and acidity lands at 0.1–0.2%, accelerating PVC stabilizer consumption downstream. Water content ≤0.10% comes from the same vacuum drying that performs decolorization — 3.9 kPa at 150°C strips both color bodies and dissolved water in one step.