Purified Terephthalic Acid (PTA)
The largest-volume aromatic intermediate in the world, and it is bought on one impurity. Everything about PTA purification exists to strip 4-CBA out of crude terephthalic acid so it can be polymerised into PET.
Molecular Structure
What is Purified Terephthalic Acid (PTA)?
Terephthalic acid is a benzene ring with a carboxyl group at each of the para positions. That straight, rigid, linear geometry is what makes polyester work: esterified with ethylene glycol, it produces PET chains that pack into a stiff, strong, semi-crystalline polymer, which is why a two-litre bottle can be thin-walled and still hold pressure.
It is made by liquid-phase air oxidation of p-xylene over a cobalt-manganese-bromide catalyst in acetic acid. The crude product precipitates out and carries a partially oxidised intermediate, 4-carboxybenzaldehyde, along with it. Because terephthalic acid neither melts nor dissolves usefully, it cannot be distilled or recrystallised in the ordinary way. The purified grade is produced by redissolving crude TPA in hot water under pressure and hydrogenating over palladium on carbon, which converts 4-CBA to p-toluic acid that stays in the mother liquor on crystallisation.
The word purified in the name is the entire commercial distinction. Crude terephthalic acid and purified terephthalic acid are the same molecule at very similar assay; what separates them is the 4-CBA figure, and that figure is what determines whether the material can be polymerised to fibre and bottle grade PET at all.
4-CBA is the specification that matters. It carries a single carboxyl group where the monomer needs two, so it caps a growing polyester chain and stops it. At the parts-per-million level it depresses intrinsic viscosity, adds colour and causes filament breaks — which is why PTA is quoted on 4-CBA content before anyone looks at the assay.
Quick Reference
Key Physical & Chemical Properties
Structure & Bonding
Two carboxyl groups in the para positions of a benzene ring. The linear, rigid geometry is what gives polyester its stiffness and dimensional stability — and the reason a partially oxidised impurity carrying only one carboxyl is so destructive to the polymer.
Skeletal structure — para-dicarboxylic acid on a benzene ring
Structural Identity
- IUPAC NameBenzene-1,4-dicarboxylic acid
- Common namesPTA, terephthalic acid, TPA
- Molecular FormulaC₈H₆O₄
- Molecular Weight166.13 g/mol
- Functional groupsTwo aromatic carboxylic acids
- SubstitutionPara (1,4) — linear geometry
- Critical impurity4-CBA — a chain terminator
- Made fromp-Xylene, Co/Mn/Br air oxidation
- Purified byHydrogenation over Pd/C, then crystallisation
- CAS Number100-21-0
- InChI KeyKKEYFWRCBNTPAC-UHFFFAOYSA-N
Product Specifications
Chelora supplies Purified Terephthalic Acid (PTA) in standard and custom grades. Contact us for specification sheets tailored to your process.
| Chemical name | Purified terephthalic acid |
|---|---|
| CAS Number | 100-21-0 |
| Molecular formula | C₈H₆O₄ |
| Molecular weight | 166.13 g/mol |
| Purity (min) | ≥ 99.90 wt% |
| 4-Carboxybenzaldehyde (max) | ≤ 25 ppm wt |
| p-Toluic acid (max) | ≤ 150 ppm wt |
| Acid number | 674 – 678 mg KOH/g |
| Colour, b* (max) | ≤ 1.5 |
| Transmittance at 340 nm (min) | ≥ 96% |
| Moisture (max) | ≤ 0.20 wt% |
| Ash (max) | ≤ 10 ppm wt |
| Iron (max) | ≤ 2.0 ppm wt |
| Cobalt (max) | ≤ 2.0 ppm wt |
| Manganese (max) | ≤ 2.0 ppm wt |
| Titanium (max) | ≤ 1.0 ppm wt |
| Median particle size | ≈ 100 µm |
| Bulk density | ≈ 800 – 900 kg/m³ |
| Flash point | 260 °C |
| Autoignition temperature | ≈ 496 °C |
| Dust explosion class | St 1 — combustible dust |
| Transport classification | Not regulated as dangerous goods |
Downstream Applications & Derivatives
Key derivative chains and industrial uses of Purified Terephthalic Acid (PTA).
PET Bottle Resin
Polymerised with mono ethylene glycol to bottle-grade PET, then solid-state polymerised to raise intrinsic viscosity for carbonated drinks, water, edible oil and hot-fill containers.
Polyester Fibre
Staple fibre and filament yarn for apparel, home textiles, nonwovens and industrial fabric. Polyester is the highest-volume textile fibre in the world, and PTA is its acid half.
PET Film & Sheet
Biaxially oriented PET film for packaging, electrical insulation, imaging, solar backsheets and flexible laminates, plus APET and PETG sheet for thermoforming.
PBT Engineering Resin
With 1,4-butanediol rather than MEG, giving polybutylene terephthalate — a fast-crystallising engineering thermoplastic for connectors, housings and automotive electrical parts.
Polyester Resins & Coatings
A rigid acid component in saturated polyester resins for powder coatings and coil coatings, and in unsaturated polyester where hardness and weathering resistance are needed.
Recycled Polyester Chains
The reference material for rPET quality benchmarking, and the product recovered when PET is chemically depolymerised by hydrolysis back to its acid and glycol.
Why source Purified Terephthalic Acid (PTA) through Chelora Petrochem?
Verified Origin
Full origin certification and asset-backed supply chain documentation — no grey-market supply.
Grade Flexibility
Fibre and bottle grades supplied. We specify and certify against 4-CBA, p-toluic acid, b* colour and catalyst metals, because those are the figures that decide whether your polycondensation runs to target intrinsic viscosity.
Full Documentation
CoA, SDS, REACH compliance, food-contact declarations for bottle-grade chains, and destination-market certificates with every shipment.
Logistics Support
Experienced in bulk solids logistics — jumbo bags, bulk silo trucks and containerised bulk with dust control and moisture protection, for Indian and international delivery.
Storage & Handling Guidelines
Always refer to the full SDS before handling. The following is a summary of key requirements.
- Store in a cool, dry, well-ventilated area. PTA is not strongly hygroscopic but surface moisture carries straight into the esterification stage and upsets the water balance in the reactor.
- Manage dust as an explosion hazard. PTA dust is combustible with a St 1 dust explosion class — earth and bond all conveying, control accumulation on beams and ledges, and assess explosion protection on silos, filters and dust collectors.
- Use dense-phase or low-velocity conveying where possible. High-velocity dilute-phase transfer generates fines and attrition, which changes bulk density and flow behaviour as well as raising dust load.
- Keep the material away from ignition sources and hot surfaces anywhere a dust cloud can form, including at bag tipping stations and silo fill points.
- Use stainless steel for wetted parts in slurry and solution service — dissolved iron drives colour in the finished polymer and shows up directly as b* failure.
- Segregate grades. Fibre-grade and bottle-grade material must not share silos, conveying lines or fill points, since bottle grade is the tighter specification and cross-contamination is not recoverable.
- Protect jumbo bags from ground moisture and direct sunlight, and rotate stock rather than holding it through a humid season.
- Control airborne dust by extraction and enclosure rather than by respirators. PTA dust is a respiratory irritant at concentrations well below any explosion risk.
- Provide eyewash and washing facilities at handling points, and label every container clearly.
Hazard Summary
Classification: PTA has no harmonised CLP classification. Self-classification commonly covers Eye Irrit. 2 and STOT SE 3 for respiratory irritation from dust. Acute toxicity is low by all routes, and PTA is used throughout the food packaging chain.
Dust explosion is the governing hazard: PTA is a fine organic solid handled in enormous volumes, and its dust forms explosible clouds at realistic concentrations. The St 1 classification means an explosion is credible and protective measures are required. This, not toxicity, is what drives the handling regime and the plant design.
Respiratory and eye irritation: dust irritates the eyes and upper airways on direct exposure. Control at source with enclosure and local extraction.
Quality is the commercial risk: PTA's real exposure is economic rather than toxicological. Moisture pickup, iron contamination from the wrong metallurgy, and cross-contamination between fibre and bottle grades will all show up as intrinsic viscosity and colour failures in the polymer, discovered on the spinning line rather than in the warehouse.
Fire: a combustible solid. It sublimes rather than melting at atmospheric pressure, and combustion produces carbon monoxide and carbon dioxide. Fires in bulk solids storage are difficult to reach and should be planned for.
Environmental: low aquatic toxicity and readily biodegradable, but large releases impose a substantial oxygen demand. Contain spills and recover as solid rather than washing to drain.
PPE minimum: safety goggles, gloves, coveralls, and dust protection where enclosure alone does not control the exposure.
Consult the full SDS for exposure limits, spill containment, emergency response and disposal before any use.
Frequently Asked Questions
Technical and commercial questions about Purified Terephthalic Acid (PTA) sourcing and specifications.
What is PTA used for?
Effectively all of it goes into polyester. The two largest outlets are PET resin for bottles and packaging, and polyester fibre — staple and filament — for apparel, home textiles and industrial fabric. Beyond those, PTA goes into biaxially oriented PET film, PBT engineering resin when reacted with butanediol instead of glycol, and saturated polyester resins for powder and coil coatings. It is the highest-volume aromatic intermediate produced anywhere.
What is 4-CBA and why is it the key specification?
4-Carboxybenzaldehyde is p-xylene that only made it part of the way through oxidation — one methyl group became a carboxyl, the other stopped at an aldehyde. That leaves it monofunctional where the polymerisation needs two reactive ends, so it attaches to a growing polyester chain and terminates it. The effect on molecular weight is out of all proportion to the concentration, and the aldehyde group also generates colour. A few hundred parts per million is enough to make material unusable for fibre or bottle resin, which is why purified grade is held at 25 ppm or below.
What is the difference between crude TPA and purified TPA?
Chemically, very little — the assay figures are close. The difference is the 4-CBA content and the purification step that removes it. Crude terephthalic acid straight from the oxidation reactor carries a few thousand parts per million of 4-CBA and cannot be polymerised to fibre or bottle grade. Purified grade is produced by redissolving the crude in hot pressurised water and hydrogenating over palladium on carbon, converting 4-CBA to p-toluic acid, which is soluble enough to stay behind in the mother liquor when the PTA recrystallises. That purification step is what the P in PTA refers to.
Why does PTA need such tight control on metals?
Because the polycondensation is catalysed, and metals interfere with it. Cobalt and manganese are carried over from the oxidation catalyst, iron comes from equipment corrosion, and titanium can come from either. All of them promote thermal degradation during melt polymerisation and solid-state polymerisation, which shows up as yellowing and as acetaldehyde generation in bottle resin. Because PET is processed hot and for a long time, even single-digit parts per million of the wrong metal has a visible effect in the finished bottle.
Why is PTA dust treated as an explosion hazard?
Because it is a fine, dry, combustible organic solid moved in very large quantities through enclosed conveying and storage. Any material of that description will form an explosible dust cloud given a suitable concentration and an ignition source, and PTA is classed St 1. The consequence is that PTA plants and receiving terminals are designed around grounding and bonding, dust accumulation control, and explosion protection on silos, mills and dust collectors — the same regime applied to flour or sugar, and for the same reason.
What documentation does Chelora provide?
Every shipment includes a Certificate of Analysis covering purity, 4-CBA, p-toluic acid, acid number, b* colour, transmittance at 340 nm, moisture, ash, and iron, cobalt, manganese and titanium content; a Safety Data Sheet to GHS and REACH format; confirmation that the material is not regulated as dangerous goods; an origin certificate; and food-contact declarations or destination-market certificates where the packaging chain requires them.
Request a quote or specification sheet
Talk to Chelora's sourcing team about Purified Terephthalic Acid (PTA) grade, volume, logistics, documentation, and lead times. We respond within one business day.
