Continuous melt-phase polymerisation of the MAK Germany-distributed Sohar Petrochemicals purified terephthalic acid (PTA) with ethylene glycol (EG) is the primary route to bottle-grade polyethylene terephthalate (PET). In a five-reactor train comprising a paste mixer at 70–90 °C, a primary esterifier at 255–270 °C and 0.15–0.35 MPa, a secondary esterifier at 265–275 °C, a pre-polycondensation vessel at 270–280 °C and 30–100 mbar, and a high-vacuum finisher at 280–290 °C and 1–3 mbar, the PTA/EG molar feed is maintained between 1:1.08 and 1:1.20 to offset glycol losses while limiting diethylene glycol (DEG) formation. Melt intrinsic viscosity at pelletisation is controlled between 0.60 dL/g and 0.65 dL/g by ASTM D4603-18; the base resin is then solid-state polymerised to bottle-grade IV of 0.76–0.84 dL/g, a range that balances drop-impact strength, environmental stress-crack resistance, and preform reheat behaviour.

The melt-phase operating window narrows when PTA impurity levels rise. 4-carboxybenzaldehyde (4-CBA), controlled at ≤25 mg/kg under ASTM D7884-20, can form conjugated aldehyde adducts that shift CIE-Lab b* above 1.5 by ASTM E313-15e1 and act as chain terminators when aldehyde functional groups survive esterification. p-Toluic acid, maintained at ≤150 mg/kg, is a monofunctional aromatic acid that terminates chain propagation and reduces the degree of polymerisation at constant finisher vacuum; the practical consequence in continuous PET lines is a measurable loss in melt IV measured by ASTM D4603-18 unless finisher residence time or vacuum is adjusted. Ash is controlled at ≤15 mg/kg because iron, cobalt, and molybdenum residues are known to accelerate acetaldehyde formation during solid-state processing and to promote thermo-oxidative degradation during preform injection moulding; the relevant PTA specification framework is ASTM D8062-23.

Solid-state polycondensation (SSP) is the critical conversion step where pellet crystallinity, gas dew point, and surface chemistry determine final bottle performance. Amorphous pellets are first crystallised in a pre-crystalliser at 150–170 °C for 10–15 min to a crystallinity of 36–40 %; the pellets then pass through a nitrogen-purged SSP reactor at 205–220 °C with nitrogen inlet dew point no higher than −40 °C and total residence time between 12 h and 20 h. Bottle resin specifications add acetaldehyde limits of ≤3.0 mg/L by ASTM F2013-10 for water and carbonated soft drink preforms; acetaldehyde is generated from vinyl ester end groups and residual DEG, both influenced by PTA esterification conditions. Pellet size distribution is screen-controlled to prevent SSP bed channeling, and static catalyst deactivation with phosphoric acid is often conducted upstream to reduce catalytic depolymerisation at high temperature.

Powder handling and slurry formation represent a separate processing boundary. Slurry preparation requires rapid wetting of PTA crystals; production-scale continuous PET units typically operate at 68–72 wt% solids with a median particle diameter near 120–140 µm and a fines fraction below 45 µm not exceeding 8 wt%. When fines exceed 10 wt%, paste mixer torque rises and downstream filter pack replacement intervals shorten in units equipped with rotary positive-displacement slurry pumps rated above 2.5 MPa; however, published data correlating such operational events specifically to the Sohar PTA grade is limited. PTA storage hoppers and screw feeders must be moisture-controlled because the powder can pick up surface water above 0.2 wt% under humid ambient conditions, leading to paste mixer bridging and erratic molar feed ratio.

Why Does Film-Grade PET Demand Tighter DEG and Carboxyl End-Group Limits Than Bottle Resin?

In biaxially oriented PET film processing, the melt-phase polymer is extruded through flat dies and chill-cast at 15–25 °C, then biaxially stretched at machine-direction ratios of 2.8:1–3.6:1 and transverse-direction ratios of 3.2:1–4.0:1. Film-grade PET typically specifies DEG at ≤1.2 wt%, IV between 0.62 dL/g and 0.65 dL/g by ASTM D4603-18, and carboxyl end-group concentration at ≤30 meq/kg by ASTM D7409-15; these limits reduce plane-strain orientation defects and maintain hydrolytic stability during metallisation, lamination, and long-term outdoor exposure. The Sohar PTA grade with 4-CBA at ≤25 mg/kg supports film haze control because aldehyde-derived colour bodies are detectable by ISO 14782-1999 haze measurement; p-toluic acid at ≤150 mg/kg limits chain stopper variability that can shift film IV and affect die-lip deposit formation.

Extruders in film lines typically use barrier screws with L/D ratios of 30:1–36:1 and melt temperatures of 275–285 °C to avoid excessive shear heating and surficial oligomer formation. Die-lip fouling is monitored as an increase in lip deposit mass per square metre of die width per hour; production records from thin-film lines show that deposit formation accelerates when PTA fines below 45 µm exceed 8 wt%, because unwetted fine crystals pass into the melt as local carboxyl-rich domains. The flat-die gap is typically set to 0.5–1.5 mm, and the cast film is pinned electrostatically; moisture in the pellet feed above 50 ppm can cause bubble defects and pinholes during tentering.

Across polyester staple fibre and filament spinning operations, PTA is introduced into continuous slurry systems at 68–72 wt% solids and esterified at 255–270 °C before polycondensation to staple-fibre IV targets of 0.55–0.64 dL/g and filament IV targets of 0.63–0.68 dL/g by ASTM D4603-18. Spin packs with hole diameters of 0.15–0.35 mm and filter media rated at 15–25 µm deliver melt to multi-end draw lines; after spinning, filaments are drawn at ratios of 2.5:1–4.0:1, heat-set at 160–180 °C, and crimped or textured to final linear densities between 0.7 dtex and 16.0 dtex by ISO 1973-2021. PTA quality affects spinning continuity less through colour than through pack pressure rise: ash at ≤15 mg/kg and p-toluic acid at ≤150 mg/kg limit hard insoluble residues and chain stopper-induced IV drift that increase spinneret pressure and break rates. Low 4-CBA at ≤25 mg/kg is still required because heat-setting at 160–180 °C can develop yellow chromophores on tow if aldehyde impurities oxidise.

When Direct Esterification of PTA with 1,4-Butanediol Replaces DMT Routes, Tetrahydrofuran Selectivity Shifts

Direct esterification of PTA with 1,4-butanediol (BDO) to polybutylene terephthalate (PBT) is run at 220–240 °C and atmospheric pressure, with titanium tetrabutoxide or tetraisopropyl titanate catalyst added at 30–80 mg/kg Ti. Esterification endpoints are typically defined by a clear melt and acid value below 5 mg KOH/g; the prepolymer then enters a high-vacuum finisher at 245–255 °C and 0.5–1.5 mbar to achieve injection-moulding intrinsic viscosities of 0.70–1.10 dL/g by ISO 1628-5. Tetrahydrofuran (THF) formation is the dominant side reaction and increases sharply above 240 °C when unconverted acid end groups remain; the direct PTA route therefore requires lower esterification temperatures than dimethyl terephthalate transesterification routes.

PBT processors monitor BDO/PTA feed ratio between 1.20:1 and 1.60:1 to compensate for THF losses; ratios above 1.80:1 increase THF yield and reduce esterification selectivity. Impurity effects are magnified because BDO is more reactive than EG and forms ether-linked species. p-Toluic acid at ≤150 mg/kg terminates chain propagation, lowering notched impact strength measured by ISO 180-A and increasing melt flow rate by ISO 1133-1:2022; 4-CBA at ≤25 mg/kg can form aldehyde-acetal by-products that shift yellowness index above 2.0 by ASTM E313-15e1. Hydrolytic stability is tested at 85 °C and 85 % relative humidity for 1,000 h; tensile strength retention by ISO 527-2 is often specified above 80 % for automotive connector grades. Injection moulding lines use barrel temperatures of 240–260 °C, mould temperatures of 60–90 °C, and injection pressures of 80–120 MPa for thin-wall connectors below 1.5 mm; dimensional stability is constrained by the PBT crystallisation rate and PTA-derived oligomer uniformity.

Published data for the Sohar PTA grade in PBT direct esterification is limited; however, the measured purity parameters fall within the typical direct-esterification window for continuous PBT plants. Operators should pre-dry PBT pellets to moisture below 0.02 wt% before moulding because hydrolytic degradation at processing temperatures reduces melt viscosity and impact performance. Hot-air dryers above 120 °C are avoided for long hold times because BDO-derived THF and aldehyde impurities produce yellowing.

Rigid polyisocyanurate and polyurethane foam systems employ aromatic polyester polyols synthesised from PTA, diethylene glycol, and process-derived glycol bottoms. Aromatic ester content raises foam compressive strength and char yield; formulations are controlled to hydroxyl numbers of 200–500 mg KOH/g by ASTM D4274-21, acid numbers below 2.0 mg KOH/g by ASTM D4662-20, and Brookfield viscosities of 1,000–5,000 mPa·s at 25 °C. Esterification proceeds at 200–240 °C with a tin or titanium esterification catalyst until a clear-cell distillate endpoint, followed by vacuum stripping to reduce free glycol and moisture below 0.10 wt% by ISO 760-1978. The PTA purity profile matters because p-toluic acid reduces average hydroxyl functionality and can delay foam gel time in blended systems, while ash above 15 mg/kg contributes to filter clogging in high-pressure foam machines with nozzle diameters of 0.5–1.5 mm and inline filters rated at 40–100 µm.

In pour-in-place refrigeration foam, polyol water content is kept below 0.10 wt% to control blowing reaction stoichiometry and cell size; the aromatic ester from PTA also raises the blended polyol viscosity, which is managed by partial substitution of diethylene glycol with phthalic anhydride or recycled PET glycolyzate in the cook. Storage stability of PTA-derived polyester polyols is evaluated by accelerated ageing at 50 °C for 7 days; phase separation or sediment formation above 0.1 vol% indicates residual ash or unreacted PTA fines. The Sohar grade’s low fines fraction supports filtration consistency, although published data is limited for this specific polyol configuration.

Powder Coating Resin Synthesis Uses PTA to Set Acid Value and Glass Transition

Carboxyl-functional polyester resins for powder coatings are synthesised by melt esterification of PTA with neopentyl glycol, trimethylolpropane, and isophthalic acid. The PTA-based aromatic diacid raises glass transition temperature and hardness; resin producers target acid values of 20–70 mg KOH/g, cone-plate melt viscosities of 2,000–6,000 Pa·s at 200 °C, and glass transition temperatures of 50–70 °C by ASTM D3418-15. The cook is run at 240–260 °C to a clear-pellet stage, followed by vacuum polycondensation to strip excess diol and water; mass temperature is held below 260 °C in high-PTA recipes to limit terephthalate oligomer colour generation. After cooling and grinding, the resin is dry-blended with TGIC or β-hydroxyalkylamide curatives and extruded at 80–110 °C in twin-screw extruders with L/D ratios of 32:1.

Resin b* is specified below 2.0 by CIE Lab measurement for white and light pastel coatings; 4-CBA at ≤25 mg/kg is a key input control because aldehyde oxidation products can form coloured quinoid structures during the cook. p-Toluic acid at ≤150 mg/kg acts as a chain stopper, reducing resin average functionality and potentially causing under-cure in TGIC systems if acid value drift is not compensated. Powder coating lines also use moisture and gel time specifications from ISO 8130-6:2021; resin powders with glass transition below 50 °C cold-block in storage, while those with melt viscosity above 6,000 Pa·s produce orange-peel defects in electrostatic spray application.

Dioctyl terephthalate production esterifies PTA with 2-ethylhexanol at 180–220 °C using a titanium alkoxide catalyst, followed by neutralisation, steam stripping under vacuum, and filtration to a final ester content above 99.5 % by gas chromatography, acid value below 0.05 mg KOH/g, and APHA colour below 30 by ASTM D1209-05; low 4-CBA and ash inputs in the PTA feed keep the finished plasticiser suitable for phthalate-free PVC wire and cable compounds processed at 170–190 °C.

Terephthaloyl Chloride Precursor Specifications in Para-Aramid Manufacture

Terephthaloyl chloride is produced from PTA by chlorination with thionyl chloride or phosgene in refluxing chlorobenzene or excess thionyl chloride at 80–120 °C using a dimethylformamide catalyst. The PTA feed must have moisture below 0.1 wt% by ISO 760-1978 and ash below 15 mg/kg; metal chlorides accelerate acid chloride hydrolysis and colour formation, while p-toluic acid at ≤150 mg/kg forms the corresponding p-toluoyl chloride that acts as a chain terminator in para-aramid polymerisation. 4-CBA at ≤25 mg/kg can generate aldehyde-containing acid chlorides that disrupt stoichiometric balance and lightfastness of the final fibre. Distillation or recrystallisation yields terephthaloyl chloride with purity above 99.9 % and hydrolyzable chloride below 0.05 %; the product is stored under dry nitrogen because moisture uptake above 0.02 wt% causes hydrolysis to terephthalic acid and hydrogen chloride.

Para-aramid spinning lines polymerise terephthaloyl chloride with p-phenylenediamine in amide solvents at 0–5 °C to suppress side reactions; monomer purity variation shifts inherent viscosity and spin dope pressure in spinnerets with hole diameters below 0.1 mm. The Sohar PTA grade’s low-moisture and low-ash profile aligns with precursor requirements reported in aramid process literature, although published data for this specific chlorination configuration is limited. Filter screens upstream of the spinneret are typically rated at 5–10 µm to remove gel particles formed from hydrolysed acid chloride; the resulting para-aramid fibre is cold-drawn and heat-treated under nitrogen at 300–400 °C to develop tensile modulus above 70 GPa by ASTM D7269-19.