Phthalic Anhydride CAS 85-44-9

What is Phthalic Anhydride?

Phthalic anhydride is the cyclic anhydride of phthalic acid, C₈H₄O₃, a white crystalline solid melting at about 131 °C with a sharp, characteristic odour. It is traded as flake or as molten liquid in heated tanks.

It is made by catalytic air oxidation of ortho-xylene over vanadium pentoxide, a route that long ago displaced naphthalene oxidation on yield and product quality.

Plasticisers are the largest outlet, through esterification with 2-ethylhexanol and other alcohols. Unsaturated polyester resins and alkyd paint resins take most of the rest.

PAN sublimes readily, which causes crystalline deposits in vent lines and headspaces, and it is a respiratory sensitiser, which means exposure control matters as much as containment.

Why Phthalic Anhydride specifically

• A reactive aromatic anhydride. Opens with alcohols to esters and with glycols to polyester backbones in a single step.
• The plasticiser route. Esterification gives DOP, DINP and the wider phthalate family.
• Rigidity in resins. The aromatic ring gives alkyds and polyesters hardness and gloss retention.
• Low cost at scale. Large single-train plants on ortho-xylene keep the whole plasticiser chain economical.

How Phthalic Anhydride is Manufactured

Phthalic anhydride is made by vapour-phase oxidation of ortho-xylene, with naphthalene oxidation surviving only on older lines.

• Ortho-xylene feed. Phthalic-grade OX with low sulphur and nitrogen is vaporised and mixed with a large excess of air.
• Catalytic oxidation. The mixture passes over supported vanadium pentoxide at about 350 to 400 °C in a molten-salt-cooled tubular reactor.
• Heat recovery. The reaction is strongly exothermic; the salt loop removes heat and raises steam.
• Switch condensers. Crude PAN is desublimed directly from the gas onto finned condensers, then melted off.
• Thermal and chemical treatment. The crude melt is heat soaked to convert by-products before distillation.
• Distillation and forming. Vacuum distillation gives the pure anhydride, which is flaked or kept molten for dispatch.

Typical Commercial Specification

Figures below are representative of commercial phthalic anhydride. Tell us the application and the form you can receive and we will confirm the matching sheet.

Physical & Chemical Properties

Behaviour in the process

PAN's sublimation and moisture sensitivity drive the practical handling rules:

• Sublimes readily. Crystalline deposits form in vent lines, headspaces and any cool surface, blocking them over time.
• Hydrolyses to phthalic acid. Moisture converts anhydride to acid, raising melting point and causing caking.
• Melts at 131 °C. Molten handling needs well-traced systems at higher temperature than most molten products.
• Respiratory sensitiser. Dust and vapour can sensitise workers, after which very low exposures provoke a reaction.

PAN Grades and Alternative Anhydrides

Phthalic anhydride is bought by form and colour grade, with the downstream reaction deciding both.

Stated plainly: in unsaturated polyester, PAN and maleic anhydride are partners rather than alternatives — one gives the backbone, the other the crosslink site. Isophthalic acid replaces PAN where corrosion resistance justifies the higher cost.

Industries We Supply Phthalic Anhydride To

PAN reaches plasticisers, composites, coatings and specialty chemicals.

Where Phthalic Anhydride is Used

Plasticisers take about half of PAN output, with unsaturated polyester and alkyd resins taking most of the remainder.

In plasticiser manufacture

PAN is esterified with two moles of alcohol — 2-ethylhexanol for DOP, isononanol for DINP — under acid or titanate catalysis with water removal. Colour is the critical quality variable: the reaction runs hot for hours, and any colour precursor in the anhydride develops into visible yellowness in the finished plasticiser.

Handling molten PAN

Molten phthalic anhydride is held around 140 to 150 °C under nitrogen in traced, lagged systems. Sublimation is the operational nuisance: vapour recrystallises in vents, instrument lines and tank headspaces, and blocked relief paths on a molten PAN tank are a genuine safety concern rather than a housekeeping one.

Quality Control & Testing

Every lot is released against a batch-specific Certificate of Analysis, with molten colour and heat test colour reported because they predict performance in the esterification or resin cook.

• Phthalic anhydride purity and solidification point — confirming identity and bulk quality.
• Colour, molten and after heat test — the practical predictors of plasticiser and resin appearance.
• Free acid content — the direct indicator of moisture exposure in transit or storage.
• Maleic anhydride content — reported as the characteristic process impurity.
• Iron and ash — measured in ppm because metals catalyse colour development.
• Sublimation residue — confirming freedom from heavy contamination.

Common Process Problems & Likely Causes

When phthalic anhydride causes trouble, it is usually moisture, colour, sublimation or dust. This table is the shortcut our technical desk works through before anyone opens a complaint file.

Packaging, Storage & Shipping

• Standard packing. 25 kg bags and jumbo bags for flake; heated ISO tank containers and road tankers for molten product.
• Temperature control. Hold molten PAN at 140 to 150 °C. Below 131 °C it solidifies and plugs lines.
• Moisture exclusion. Water converts anhydride to phthalic acid, causing caking and raising melting point.
• Nitrogen blanketing. Standard for molten storage to exclude moisture and limit colour development.
• Sublimation management. Trace or regularly clean vent lines and relief paths; blocked vents are a safety issue.
• Material compatibility. Stainless steel preferred; carbon steel corrodes once free acid is present.
• Shelf life. Typically 12 months for sealed flake in dry storage; molten should be used promptly.
• Transport classification. UN 2214, phthalic anhydride, Class 8 corrosive, packing group III.

Hazards & Safe Handling

Phthalic anhydride is corrosive and a respiratory sensitiser. Dust and vapour exposure are the main concerns, and sensitised workers react at very low concentrations.

• Respiratory sensitisation. A recognised respiratory sensitiser; once sensitised, workers can develop asthma at very low exposure levels.
• Corrosivity. Causes skin burns and serious eye damage as dust, vapour and molten liquid.
• Occupational exposure. Limits are low, commonly around 1 ppm as an 8-hour average; confirm the limit in your jurisdiction.
• Combustible dust. Flake handling generates explosible dust; use earthed, enclosed conveying with extraction.
• Hot liquid. Molten PAN above 140 °C causes severe combined thermal and chemical burns.
• Water reactivity. Reacts with water to phthalic acid with heat release; keep systems dry.
• PPE. Chemical-resistant gloves, goggles and face shield, with respiratory protection for dust or vapour exposure.
• Environment. Harmful to aquatic life; contain spills and prevent entry to drains.

Regulatory & Compliance Notes

Phthalic anhydride is regulated as a corrosive respiratory sensitiser, with downstream phthalate restrictions affecting its customers. The notes below are orientation rather than legal advice.

• GHS classification. Acute toxicity Category 4, serious eye damage Category 1, respiratory and skin sensitisation Category 1 and STOT SE Category 3.
• Health surveillance. Respiratory sensitisation means medical surveillance is commonly required for exposed workers.
• Dangerous goods transport. UN 2214, Class 8 corrosive, PG III. ADR, IMDG, IATA and DOT rules apply, with elevated temperature declarations for molten.
• Downstream phthalate regulation. Several phthalate plasticisers made from PAN are restricted in toys, food contact and medical devices, which affects your customers rather than the anhydride.
• Dust explosion regulation. Flake handling falls under ATEX, NFPA or equivalent combustible dust requirements.
• REACH and inventories. Registered under EU REACH and listed on the major chemical inventories.

We Supply Phthalic Anhydride Worldwide

Chelora Chemicals is a supplier and export partner, not a single-plant trader. We ship Phthalic Anhydride from India and from partner sources to customers across Asia, Europe, the Middle East, Africa, and North and South America, handling the export documentation, transport paperwork and freight so the material arrives ready to use.

• Incoterms. EXW, FOB, CIF, CFR and DAP quoted to suit your purchasing terms and freight arrangements.
• Ports. Regular shipments through Nhava Sheva (JNPT) and Mundra, with other ports arranged where routing and cargo rules allow.
• Documentation. Commercial invoice, packing list, certificate of origin, COA, SDS and the transport declaration issued as a complete set.
• Flexible volumes. From a pallet of bagged flake up to repeat bulk shipments on a supply contract.
• Supply continuity. Because we are independent of any one producer, a plant turnaround or a shipping disruption at one source does not have to become your stock-out.

Buying Phthalic Anhydride from Chelora

We are an independent sourcing and supply partner. For phthalic anhydride that means settling three things at enquiry: flake or molten supply, the colour and heat test grade your resin or plasticiser requires, and the moisture exclusion and vent design your storage can maintain.

• Documentation on every lot. Batch-specific COA (purity, solidification point, colour, acid number), plus SDS and the full transport paperwork.
• Analysis before commitment. Representative analysis shared ahead of a commercial order so your lab can qualify the material.
• Quantities. From a pallet of bagged flake through to full bulk shipments on contract supply.
• Grade matching. Flake or molten PAN to the purity, colour, heat test and free acid limits your esterification or resin process requires.
• Inspection. Independent third-party inspection available at loading when the contract requires it.
• Related products. Ortho-Xylene, Maleic Anhydride, Dioctyl Phthalate and other feedstocks alongside Phthalic Anhydride from one supplier.

How ordering works

• Send the enquiry. Product, grade or purity, quantity, destination port and the Incoterm you buy on. If you have a spec sheet, attach it — it saves a round trip.
• We quote. Price, lead time, packing format and a sample COA, normally within one working day.
• Qualify the supply. For first orders or critical applications, we share the analysis and handling documentation so your technical and safety teams can sign off before you commit.
• Confirm the order. Proforma invoice issued; payment terms agreed against the volume and the relationship.
• Allocation and QC. Material allocated and released against a batch-specific COA, with an independent inspection where the contract calls for it.
• Ship and document. Filled, labelled and loaded with the full export document set, and tracked through to arrival.

Related Products

Buyers of phthalic anhydride often source related resin and plasticiser inputs alongside it — ortho-xylene, maleic anhydride and plasticiser alcohols are commonly bought on the same programme.