Phthalic Anhydride (PA) Flakes: Primary Feedstock for PVC Plasticizers & Alkyd are produced by fixed-bed oxidation of o-xylene in multi-tubular reactors charged with a supported vanadia–titania catalyst. The reactor gas is partially condensed in switch condensers, where crude PA desublimates as a solid; the crude material is then thermally pretreated, distilled under vacuum, and flaked on a chilled drum. Flake solidification point for high-purity material is normally 130.8–131.2 °C, and bulk density is typically 0.65 kg/L to 0.80 kg/L. The flake form is hygroscopic: exposed surfaces hydrolyse to phthalic acid and can raise the acid number of downstream batches. Storage is therefore conducted under dry air or nitrogen, and silo discharge is designed for mass flow with hopper half-angles greater than 60°. Two downstream chemistries dominate demand: phthalate ester plasticizers made by esterification with 2-ethylhexanol, isononanol, or isodecanol, and alkyd resins formed by reaction with polyols and fatty acid derivatives. The difunctionality of PA contributes a rigid aromatic diester segment that raises glass transition temperature in both plasticized PVC and cured alkyd films.

When Reactor Overhead Decanters Retain Phthalic Acid Monobutyl Ester

In dioctyl phthalate (DOP) and diisononyl phthalate (DINP) production, a 25 m³ glass-lined or 316L esterification kettle is charged with flake PA and excess alcohol at a molar ratio of 2.2:1 to 2.6:1. The first esterification to the acid ester is rapid and exothermic; the second esterification is equilibrium-limited and requires a catalyst. Tetrabutyl titanate or dibutyltin oxide is dosed at 0.05 wt% to 0.15 wt% of the organic charge after the initial monoester phase, and the batch is heated to 210–230 °C while water of reaction is removed. Reduced pressure or excess alcohol azeotropy strips the water; if n-butanol is used as a codistillation aid, the overhead decanter can retain phthalic acid monobutyl ester as a separate liquid phase. This retained monoester alters the alcohol return ratio and can raise the final acid number unless the decanter temperature is maintained above the freezing point of the organic phase and the settled layer is drained. The neutralized plasticizer is then steam-stripped at 2–5 kPa and 160–190 °C to remove excess alcohol and low-boiling impurities. Plate filtration with activated carbon reduces colour and trace solids; release testing includes an acid value of ≤0.07 mg KOH/g, a colour of ≤25 APHA by ASTM D1209, and an ester value within specification by ASTM D1045 protocols. Overhead pressure swings larger than ±5 kPa during vacuum stripping disturb separator level control and can carry phthalate droplets into the liquid ring pump; demister pads and cooled knock-out vessels are therefore installed before the vacuum source. The temperature window is narrow: excessive heat accelerates ether formation and colour drift, while insufficient heat leaves unreacted monoester that consumes downstream stabilizer.

Plasticizer colour after heat ageing is screened by heating a 100 mL sample at 180 °C for 48 h and remeasuring APHA colour by ASTM D1209. Colour rise above 30 APHA after ageing is commonly rejected for high-clarity flexible PVC end-uses because it signals residual alcohol, catalyst fragments, or thermal oxidation products. Moisture in the alcohol feed should stay below 0.1% to avoid hydrolysis of the titanium catalyst and the development of free phthalic acid. In an integrated 100 kt/year plasticizer line, continuous esterification is conducted in two or three stirred reactors in series rather than a single batch kettle. The flake melt vessel operates at 140–150 °C, and the melt filter uses 50 µm stainless mesh to remove insoluble residues before the esterifier. Flash steam stripping under vacuum removes unreacted alcohol and water; recovered alcohol is returned to the reactor after drying through a molecular sieve bed.

Alkyd reactor charging sequences exploit the difunctionality of PA in ways that differ from plasticizer esterification. In a 20 m³ stainless alkyd reactor, a monoglyceride precursor is first formed by alcoholysis of soybean oil or tall oil fatty acids with glycerol at 235–245 °C under lithium hydroxide or calcium oxide at 0.01–0.05 wt%. PA flakes are then metered into the reactor at a controlled rate; the aromatic anhydride reacts preferentially with primary hydroxyl groups of glycerol and pentaerythritol. If free glycerol remains before PA addition, rapid chain growth narrows the gel point and creates batch-to-batch viscosity variation. Operators therefore confirm the alcoholysis endpoint by methanol solubility or turbidity before the first PA charge. For a medium-oil alkyd containing 22 wt% PA on solids, the cook is continued to an acid value below 10 mg KOH/g according to DIN EN ISO 2114 and a resin viscosity of 2.5–5.0 Pa·s at 23 °C by ISO 3219 after dilution to 60% solids in white spirit. Xylene at 3–5 wt% of reactor charge is commonly used as azeotrope to accelerate water removal; the water-xylene distillate is condensed and the organic layer is returned. PA-derived alkyds develop hardness through the aromatic diester segment, but exterior durability is lower than isophthalic-based resins; exterior long-oil formulations may replace a portion of PA with isophthalic acid only where coating specifications require greater UV resistance. The cook does not require high-shear dispersion, but the final let-down must avoid moisture above 0.05% because residual water hydrolyses aromatic ester linkages during storage and lowers the final molecular weight. Premature addition of PA before monoglyceride formation is complete causes free glycerol to react with the anhydride faster than the polyesterification pathway can control, producing a high-viscosity mass that approaches the gel point even at acid values above 8 mg KOH/g.

Flake Feedstock Assay, Moisture Pickup, and Molten Transfer Lines

PA flake released for plasticizer or alkyd use is screened for impurity levels that affect colour and catalysis. Because PA is classified under CLP as H302, H315, H317, H318, H334 and H335, manual dumping stations require local exhaust ventilation and dust-tight connections. Table 1 lists representative acceptance limits for high-purity flake. Moisture pickup at storage humidity above 60% RH produces surface phthalic acid, which raises acid number and can bridge hoppers; hoppers with 60° cone angles and vibratory bin activators reduce bridging failures. Dense-phase pneumatic transfer of flakes should use nitrogen or air with a pressure dew point below −40 °C, and conveying line velocity should remain above saltation velocity but below 20 m/s to limit flake attrition. Molten PA is transferred in traced 316L lines at 150–160 °C; sustained line temperatures above 170 °C increase sublimation and colour-body formation. If free water enters a molten line, hydrolysis to phthalic acid creates a slurry phase that accelerates carbon steel corrosion; therefore the unloading system includes a heated filter and a water-detection interlock.

Representative high-purity PA flake acceptance limits

Parameter	Limit or range	Test basis /instrument
Purity	≥99.8% (m/m)	GC after derivatization
Maleic anhydride	≤0.05% (m/m)	HPLC or GC
Solidification point	130.8–131.2 °C	Cooling curve
Melt colour	≤20 APHA	ASTM D1209 on molten sample
Ash	≤0.01% (m/m)	Sulfated ash method
Iron	≤2 mg/kg	ICP-OES
Water	≤0.05% (m/m)	Karl Fischer titration

Maleic anhydride residue is particularly relevant because it can form coloured polycondensates at alkyd cook temperatures above 220 °C. Phthalide and 1,4-naphthoquinone are also measured when downstream colour is critical; phthalide can open to anthraquinone-type chromophores under acidic catalyst conditions.

During final plasticizer finishing, volatility loss according to ISO 176:2005 is used to screen permanence in PVC, and migration kinetics in polymer matrices are ranked through compatibility tests such as ASTM D3291. DOP and DINP differ mainly in alcohol branching and molecular weight: DOP has a density near 0.984 g/cm³ at 20 °C, while branched DINP is typically 0.973 g/cm³ at the same temperature. Viscosity at 25 °C measured by ISO 3219 generally lies between 45 mPa·s and 90 mPa·s, with branched isononyl esters at the higher end. The solvating strength of the aromatic phthalate ester controls PVC dry-blend absorption in heated mixers; formulation trials with suspension PVC of K-value 70 at 50 phr plasticizer loading are designed around torque rise and final dry-blend density rather than a single fixed time. Because plasticizer loss from flexible PVC is diffusion-controlled, alcohol chain length and branching shift both extraction resistance and low-temperature flexibility. DINP normally shows lower volatility than DOP under identical test conditions, but the low-temperature flex point is less favourable; the balance is formulation-specific and must be verified with ASTM D1043. Phthalate plasticizer selection under REACH and RoHS Directive 2011/65/EU must follow specific ester restrictions: DIBP, DBP, BBP and DEHP are restricted under RoHS 2015/863 at 0.1% (m/m) per homogeneous material, while DINP and DIDP have separate article-specific restrictions. The PA feedstock itself is not the regulatory trigger; the final ester and end-use material control compliance.

What Limits the Use of PA Flakes in Short-Oil Alkyd Resins at 240 °C?

Short-oil alkyds with high PA content are cooked at 240–250 °C to reduce acid value and develop molecular weight, but the operating window is constrained by two competing failure modes: sublimation of unreacted PA and premature gelation. PA sublimes from the reacting mass at cook temperature and deposits on cooler partial condenser surfaces, manway undersides, and vapor piping. This deposit reduces condenser duty and can block the water of reaction outlet unless the vapor line is maintained above 160 °C or washed with hot xylene reflux. At 240 °C, the esterification rate is high; once the acid value falls below 5 mg KOH/g, viscosity growth accelerates and gelation can occur within minutes if sampling is infrequent. Production reactors therefore control the endpoint by agitator torque monitoring, acid value sampling every 10–15 min, and immediate cooling to 150 °C when the viscosity target is reached. Table 2 lists representative endpoint windows for two PA-modified alkyd types.

Additives containing primary or secondary amines are not charged during the cook because amine salts of residual phthalic acid precipitate and shift the gel point unpredictably. Moisture ingress above 0.05% in the thinning solvent after cook is an additional boundary condition: it hydrolyses the aromatic ester and lowers the final molecular weight. Published data for the exact gel-time shift caused by a ±5 °C deviation in a specific resin factory can be limited, but production records consistently show that endpoint viscosity doubling times shorten sharply as the cook temperature approaches 250 °C. The final alkyd may be cut in aromatic solvent after cooling below 150 °C; cutting at higher temperature accelerates solvent loss and can leave a high-viscosity heel in the mixing vessel.

Representative PA-modified alkyd endpoint windows

Resin type	PA content	Acid value	Viscosity	Reference method
Short-oil baking alkyd, 60% solids in xylene	30% on solids	≤10 mg KOH/g	2.5–5.0 Pa·s at 23 °C	DIN EN ISO 2114, ISO 3219
Long-oil air-drying alkyd, 70% solids in white spirit	18% on solids	≤8 mg KOH/g	5–10 Pa·s at 23 °C	DIN EN ISO 2114, ISO 3219