Phthalic Anhydride (PA)
The plasticiser anhydride. About half of it becomes phthalate esters for flexible PVC, and the regulatory pressure on those esters is the single biggest thing shaping demand for it.
Molecular Structure
What is Phthalic Anhydride (PA)?
Phthalic anhydride is a benzene ring fused to a five-membered anhydride ring. The aromatic core is rigid and contributes hardness and weathering resistance to whatever it goes into; the anhydride ring opens readily with alcohols to form esters and with glycols to build polyester chains. Almost every use turns on that ester-forming step.
It is made by catalytic oxidation of o-xylene over vanadium pentoxide on titania in a fixed-bed reactor, which accounts for the large majority of world capacity. Naphthalene oxidation remains in use where coal tar naphthalene is locally available and competitively priced.
Roughly half goes into phthalate plasticisers for flexible PVC. The rest is divided between unsaturated polyester resins, alkyd resins for decorative and industrial coatings, and smaller volumes into phthalocyanine pigments, dyes and PVC heat stabilisers.
Phthalate regulation is the demand story here. DEHP and several other low-molecular-weight phthalates are classified as reproductive toxicants and restricted in the EU and elsewhere, and the market has shifted towards higher phthalates such as DINP and DIDP and towards non-phthalate plasticisers entirely. Confirm the regulatory position for your destination market and end use before committing to a plasticiser chain.
Quick Reference
Key Physical & Chemical Properties
Structure & Bonding
A benzene ring fused to a five-membered cyclic anhydride. The aromatic ring provides rigidity and thermal stability in the finished ester or resin; the anhydride ring is the reactive part, opening with alcohols and glycols to form the ester linkage.
Skeletal structure — benzene ring fused to a cyclic anhydride
Structural Identity
- IUPAC Name2-Benzofuran-1,3-dione
- Common namesPhthalic anhydride, PA, PAN
- Molecular FormulaC₈H₄O₃
- Molecular Weight148.12 g/mol
- Functional groupFused cyclic anhydride
- Ring systemBenzene fused to five-membered anhydride
- ReactivityAcylating agent; opens with alcohols and glycols
- Key impurityMaleic anhydride — sets transport class
- Made fromo-Xylene oxidation over V₂O₅/TiO₂
- CAS Number85-44-9
- InChI KeyLGRLWUINFJPLSH-UHFFFAOYSA-N
Product Specifications
Chelora supplies Phthalic Anhydride (PA) in standard and custom grades. Contact us for specification sheets tailored to your process.
| Chemical name | Phthalic anhydride |
|---|---|
| CAS Number | 85-44-9 |
| Molecular formula | C₈H₄O₃ |
| Molecular weight | 148.12 g/mol |
| Purity (min) | ≥ 99.80 wt% |
| Solidification point (min) | ≥ 130.8 °C |
| Maleic anhydride (max) | ≤ 0.05 wt% |
| Phthalic acid (max) | ≤ 0.10 wt% |
| Colour, molten (max) | ≤ 20 APHA |
| Colour after heat stability test (max) | ≤ 40 APHA |
| 1,4-Naphthoquinone (max) | ≤ 20 ppm wt |
| Benzoic acid (max) | ≤ 0.05 wt% |
| Iron (max) | ≤ 2.0 ppm wt |
| Ash (max) | ≤ 0.05 wt% |
| Sublimation residue (max) | ≤ 0.10 wt% |
| Recommended molten storage | 140 – 160 °C under nitrogen |
| Flash point | 152 °C (closed cup) |
| Autoignition temperature | 570 °C |
| Transport classification | UN 2214, Class 8, PG III when maleic anhydride exceeds 0.05% |
Downstream Applications & Derivatives
Key derivative chains and industrial uses of Phthalic Anhydride (PA).
Phthalate Plasticisers
The largest outlet. Esterified with alcohols to give DOP/DEHP, DINP and DIDP for flexible PVC — cable, flooring, coated fabric and hose. The segment most affected by regulatory change.
Alkyd Resins
Condensed with glycerol or pentaerythritol and fatty acids to give alkyds — still the backbone of solvent-borne decorative and industrial coatings for their gloss and flow.
Unsaturated Polyester
The orthophthalic acid component in general-purpose UPR for FRP laminates, gel coats, cultured marble and construction panels.
Phthalocyanine Pigments
Precursor via phthalonitrile to copper phthalocyanine blue and green — among the most widely used organic pigments in inks, coatings and plastics.
Polyester Polyols
Aromatic polyester polyols for rigid polyurethane and polyisocyanurate insulation foam, where the aromatic ring contributes fire performance and dimensional stability.
Dyes & Fine Chemicals
Anthraquinone dye intermediates, phenolphthalein, saccharin routes, PVC heat stabilisers and speciality esters for lubricants and repellents.
Why source Phthalic Anhydride (PA) through Chelora Petrochem?
Verified Origin
Full origin certification and asset-backed supply chain documentation — no grey-market supply.
Grade Flexibility
Flake, briquette and molten supplied. We certify maleic anhydride content on every lot, since it governs both the resin colour you will get and the transport classification the cargo travels under.
Full Documentation
CoA, SDS, transport documentation with the maleic anhydride declaration, REACH compliance, respiratory sensitiser communication, and destination-market regulatory confirmation with every shipment.
Logistics Support
Experienced in both bagged and heated bulk logistics — bagged flake and briquettes, and insulated, nitrogen-blanketed ISO tanks for molten delivery at 140–160 °C, Indian and international.
Storage & Handling Guidelines
Always refer to the full SDS before handling. The following is a summary of key requirements.
- Hold molten material at 140–160 °C under a dry nitrogen pad. Below about 131 °C it solidifies; well above 160 °C colour development and sublimation both accelerate.
- Manage sublimation deliberately. Phthalic anhydride sublimes from molten storage and recondenses as needle-like crystals in vents, relief lines and flame arrestors — inspect and clear them on a defined schedule rather than on failure.
- Trace-heat and insulate every line, valve and pump on molten service, including dead legs. A frozen line on a 131 °C material is difficult to clear and dangerous to heat blind.
- Keep moisture out. Phthalic anhydride hydrolyses to phthalic acid, which raises the acid figure, changes melting behaviour and is not recoverable.
- Prevent sensitisation rather than manage exposure. Phthalic anhydride is a respiratory sensitiser, so enclose transfer points and extract at source, and treat reported respiratory symptoms as a containment failure to investigate.
- Control flake dust as both an irritant and a combustible dust — ground and bond conveying, control accumulation, and assess protection on enclosed equipment.
- Use stainless steel for wetted parts in molten service; iron pickup drives colour and shows directly on the heat stability test.
- Segregate from water, strong bases, amines, alcohols, oxidisers and nitric acid. Reaction with bases and amines is exothermic.
- Provide safety showers and eyewash at handling points, treat molten material as a serious thermal burn hazard, and declare the maleic anhydride content correctly on shipping documents.
Hazard Summary
Classification: Resp. Sens. 1 (H334), Skin Sens. 1 (H317), Eye Dam. 1 (H318), Skin Irrit. 2 (H315), STOT SE 3 (H335), Acute Tox. 4 oral (H302).
Respiratory sensitisation: phthalic anhydride is an established occupational asthmagen, in the same category of concern as maleic anhydride. Once sensitised, a worker can react to concentrations well below the general irritation threshold, and the effect is usually permanent. Control by containment and extraction with the aim of preventing sensitisation, rather than by working to an airborne limit.
Thermal burns: molten phthalic anhydride is handled at 140–160 °C. A splash at that temperature causes severe burns, made worse by the material solidifying on contact with skin. Specify splash protection for hot liquid, not for chemical contact alone.
Sublimation and vent blockage: the solid sublimes from the melt and recondenses as needles in cooler parts of the system. Blocked vents and relief paths are a recognised failure mode on phthalic anhydride tanks and demand a specific inspection regime.
Transport classification depends on purity: phthalic anhydride is regulated as UN 2214, Class 8, packing group III when it contains more than 0.05% maleic anhydride, and is generally unregulated below that. The declaration must reflect the actual analysis of the lot, not the nominal grade.
Dust: flake and briquette handling generates combustible dust that also irritates the eyes and airways. Ground and bond conveying and control accumulation.
Downstream regulatory exposure: the largest use is phthalate plasticisers, several of which are classified as reproductive toxicants and restricted in the EU and other markets. That is a commercial and compliance consideration for the ester, not a property of the anhydride itself, but it should inform any long-term supply position.
PPE minimum: chemical goggles and face shield, gloves and coveralls, heat-resistant protection for molten service, and respiratory protection where containment cannot be assured.
Consult the full SDS for exposure limits, health surveillance guidance, spill containment and emergency response before any use.
Frequently Asked Questions
Technical and commercial questions about Phthalic Anhydride (PA) sourcing and specifications.
What is phthalic anhydride used for?
Around half goes into phthalate plasticisers for flexible PVC — cable insulation, flooring, coated fabric and hose. Alkyd resins for decorative and industrial coatings are the next largest use, followed by orthophthalic unsaturated polyester resin for FRP laminates and gel coats. Smaller volumes feed copper phthalocyanine pigments, aromatic polyester polyols for rigid insulation foam, anthraquinone dye intermediates and PVC heat stabilisers.
Why does maleic anhydride content determine the transport classification?
Because maleic anhydride is the corrosive component. Phthalic anhydride carries a small amount of it as a residue from o-xylene oxidation, and transport regulations treat the material as UN 2214, Class 8 packing group III when that residue exceeds 0.05%, and generally as unregulated below it. The same figure is also a quality specification, because maleic anhydride carried into a resin cook contributes unwanted unsaturation and colour. It is unusual for a single trace analysis to govern both the dangerous goods declaration and the product performance, and it means the shipping paperwork has to follow the actual lot analysis rather than the grade name.
Why is phthalic anhydride a respiratory sensitiser?
Acid anhydrides as a class are reactive enough to bind to proteins in the respiratory tract, and the modified protein can then be recognised by the immune system as foreign. That is the mechanism behind occupational asthma from phthalic and maleic anhydride, and from trimellitic anhydride, which is a well-studied example. The practical consequence is that the dose-response logic behind ordinary exposure limits does not apply once someone is sensitised: they may react to traces. Health surveillance and containment are the appropriate controls, and any new respiratory symptom in a worker handling the material deserves investigation rather than reassurance.
How are phthalate regulations affecting demand?
Substantially, and the direction is settled even if the pace varies. DEHP — the ester usually sold as DOP — is classified in the EU as a reproductive toxicant, is a REACH SVHC subject to authorisation, and is restricted in toys and childcare articles in many jurisdictions. Demand has moved towards higher-molecular-weight phthalates such as DINP and DIDP, which face lighter restrictions, and increasingly towards non-phthalate plasticisers including DOTP and citrates, which do not use phthalic anhydride at all. Anyone taking a long position in the plasticiser chain should check the position in their destination market and for their specific end use rather than relying on a general statement.
What is the difference between the o-xylene and naphthalene routes?
Both are catalytic air oxidations over vanadium-based catalysts, and both give the same product. o-Xylene is the dominant feedstock worldwide because it is a petrochemical stream available wherever a reformer and xylene splitter run, and it gives a higher yield per tonne of feed. Naphthalene, recovered from coal tar, is used where coal chemistry is well established and the feedstock is locally cheap. The naphthalene route tends to give a slightly different impurity profile, notably in naphthoquinone content, which is why that figure appears on the specification.
What documentation does Chelora provide?
Every shipment includes a Certificate of Analysis covering purity, solidification point, maleic anhydride content, phthalic acid, colour in the melt and after the heat stability test, naphthoquinone, benzoic acid, iron, ash and sublimation residue; a Safety Data Sheet to GHS and REACH format; transport documentation with the maleic anhydride declaration determining whether UN 2214 applies; the respiratory sensitiser communication downstream users are entitled to; an origin certificate; and destination-market regulatory confirmation for the declared end use.
Request a quote or specification sheet
Talk to Chelora's sourcing team about Phthalic Anhydride (PA) grade, volume, logistics, documentation, and lead times. We respond within one business day.
