Phthalic anhydride is an aromatic dicarboxylic anhydride with CAS number 85-44-9, EINECS 201-607-5, molecular formula C₈H₄O₃, and molecular weight 148.12 g/mol. Commercial production is dominated by gas-phase oxidation of o-xylene in fixed-bed multitubular reactors charged with a vanadium pentoxide/titanium dioxide catalyst. Reactor hot-spot control is maintained near 380–420 °C, and the inlet hydrocarbon concentration is held below the lower explosive limit to prevent deflagration. Modern catalyst trains yield approximately 110–114 kg PA per 100 kg of o-xylene, corresponding to a molar selectivity near 80–82 mol%. The product is sold as white flakes or as a molten liquid in insulated tank containers, with a solidification point of 130.8 °C, density 1.53 g/cm³ at 20 °C, and closed-cup flash point 152 °C. Its saturated aromatic ring structure distinguishes it from maleic anhydride: PA imparts rigidity to polyester chains but contains no ethylenic unsaturation for free-radical crosslinking.

Commercial multitubular PA reactors typically use tube lengths of 3–5 m and internal diameters of 25–30 mm, with molten salt heat removal to manage the strong exotherm. Inlet o-xylene concentration is controlled at 0.8–0.9 vol% against a lower explosive limit of approximately 1.1 vol% in air. Operating practice varies across production lines, and published data for specific reactor configurations is limited; therefore, the stated ranges should be treated as representative industrial boundaries rather than universal design limits.

When is phthalic anhydride selected over maleic anhydride in unsaturated polyester resin formulation?

In unsaturated polyester resin manufacture, PA is selected where lower exotherm and reduced crosslink density are required. A representative orthophthalic resin is condensed from 1.0 mol PA, 1.0 mol maleic anhydride, and 2.1–2.2 mol propylene glycol at 180–220 °C under inert gas to an acid number below 25 mg KOH/g according to ISO 2114. The saturated aromatic diacid increases tensile modulus and heat deflection temperature relative to fully aliphatic maleate resins, while the maleic anhydride provides the fumarate unsaturation required for styrene copolymerization. Unfilled castings from commercial orthophthalic resins typically show tensile strength in the range 40–65 MPa when tested under ISO 527-2; published data for specific catalyst and inhibitor packages is limited. PA-based orthophthalic resins are less hydrolytically stable than isophthalic acid-based resins, and aggressive aqueous immersion service is therefore assigned to isophthalic or terephthalic backbones rather than PA.

Alkyd resin synthesis requires controlled anhydride-to-polyol ratios.

In solvent-process alkyds, PA is reacted with pentaerythritol or glycerol and fatty acids at 230–250 °C. Xylene azeotropic distillation removes water and shifts the esterification equilibrium; acid value for short-oil alkyds is typically reduced to <10 mg KOH/g, while viscosity reaches 3–10 Pa·s at 25 °C when cut to 60% solids in mineral spirits. Because PA is a difunctional aromatic anhydride, it increases coating hardness and shortens dry-to-touch time relative to long-chain aliphatic dibasic acids. Accelerated weathering under ISO 11507 shows a higher yellowing tendency for PA-containing alkyds compared with isophthalic-based controls; this limitation is relevant in white or pastel topcoats. Formulators generally qualify each lot through Gardner color according to ASTM D1544 and through viscosity stability rather than relying solely on supplier certificates, because published data for specific resin-grade PA sources under xenon-arc exposure is limited.

Esterification of PA with 2-ethylhexanol to produce bis(2-ethylhexyl) phthalate remains a volume application in regions outside REACH Annex XVII restrictions on phthalate plasticizers in articles. The reaction is acid-catalyzed at 150–200 °C, followed by vacuum stripping to <0.1% moisture and activated carbon filtration to obtain ester color below 25 APHA. DOP derived from PA typically exhibits density 0.984 g/cm³ at 20 °C, dynamic viscosity 56 mPa·s at 25 °C, and volume resistivity 2.5 × 10¹¹ Ω·cm when measured under IEC 60247. PA-based phthalate plasticizers differ from trimellitate esters in volatility: trimellitic anhydride-derived plasticizers are selected for high-temperature cable applications where diisodecyl phthalate would exceed volatility limits, but the trifunctional aromatic anhydride requires more complex esterification and higher raw-material cost. Published data for specific PA-based plasticizer performance under ISO 176 activated carbon volatility is limited; compounders usually screen migration by weight loss after 24 h at 70 °C.

Commercial Flake and Molten Grade Specifications

Commercial PA is supplied as flake packed in 25 kg multi-wall paper/polyethylene valve bags or 500–1000 kg FIBCs, and as molten material delivered in insulated tank containers trace-heated to 150–170 °C. Grade designations are supplier-specific; typical certificates state PA 99.8 or “Phthalic Anhydride Flake” rather than a standardized model. The table lists typical supply specifications for standard technical-grade material.

Specifications vary across plants; maleic anhydride content is controlled because it introduces ethylenic unsaturation that can alter polyester gel times. Naphthoquinone and phthalide are additional trace impurities monitored in refined PA for color-sensitive alkyd resins, but published analytical limits for these species are not uniformly available across supplier certificates.

Differentiation in downstream process selection is often driven by melting point, anhydride functionality, and volatility. The following table compares PA with maleic, trimellitic, and pyromellitic anhydrides commonly encountered in polyester and epoxy systems.

Phthalic anhydride differs from trimellitic and pyromellitic anhydrides primarily in functionality and thermal stability: the difunctional structure keeps polyester branching low and melt viscosity manageable, whereas trifunctional and tetrafunctional anhydrides produce branched or crosslinked networks with higher heat resistance but narrower processing windows. Compared with maleic anhydride, PA is less volatile and less water-sensitive, but its higher melting point requires flake melting or molten storage infrastructure.

If molten handling infrastructure is available, transfer temperature must still remain below 170 °C

Molten PA feed is used in continuous polyester and alkyd lines where flake handling creates dust exposure and batch-to-batch charge variation. Transfer from insulated tank containers to day tanks is carried out with low-pressure steam or thermal oil tracing at 150–170 °C. Below 131 °C, solidification begins and can block jacketed transfer lines; above 200 °C, prolonged holding raises color and free-acid content, and above 230 °C thermal decomposition becomes measurable. Closed nitrogen blanketing is required because contact with atmospheric moisture at molten temperatures hydrolyzes the anhydride to phthalic acid, increasing free acidity and causing insoluble deposits in feed filters. Carbon steel is acceptable for dry molten PA service where trace iron pick-up is tolerable; 316L stainless steel is specified for color-sensitive resin systems. Published data for continuous molten PA feed in specific twin-screw reactor configurations is limited, but plant-scale experience indicates that melt-charging reduces charge time by 30–50% relative to flake induction, depending on bag size, hopper design, and reactor temperature.

Transport classification for solid PA is UN 2214, Class 8, Packing Group III; molten PA is carried as an elevated-temperature material and requires additional operational controls. REACH-registered product requires safety data sheet documentation, and downstream phthalate ester manufacturers must verify article-specific restriction obligations under Annex XVII. Dust from flake PA forms airborne respirable particulate and should be controlled through local exhaust ventilation; process areas where bag dumping occurs are typically fitted with high-efficiency particulate filtration meeting EN 1822 filter classes. Contact with hot molten PA causes thermal burns; cold flake is hygroscopic and must be stored in sealed bags at ambient relative humidity below 60% to limit caking and free-acid development. Published data for specific storage interval effects at different humidity levels is limited; warehouse practice is to consume opened bags within 48 h or re-seal under nitrogen.