Purified terephthalic acid produced at the Indorama Ventures (Portugal) PTA complex in Sines is supplied as a single high-purity aromatic dicarboxylic acid monomer rather than a multi-grade polymer series. The product is identified by CAS Registry Number 100-21-0, molecular formula C₈H₆O₄, and molecular weight 166.13 g/mol. It appears as a free-flowing white crystalline powder with a theoretical acid value of 675.4 mg KOH/g, corresponding to its two para-substituted carboxyl groups. Because purified terephthalic acid is a monomer intermediate, the relevant product identifier is not a melt-flow grade but the lot-specific certificate of analysis number, silo identification, and production date. The material is intended principally for direct esterification with ethylene glycol in continuous polyethylene terephthalate polymerisation, with secondary use in polybutylene terephthalate, polyester staple fibre, filament, film, and selected non-polymer applications requiring a dibasic acid of controlled purity.

The Sines complex operates a continuous oxidation and purification train in which p-xylene is oxidised in acetic acid using a cobalt/manganese/bromine catalyst system. The resulting crude terephthalic acid is dissolved in water at elevated temperature and passed over a fixed-bed palladium catalyst under hydrogen pressure to reduce 4-carboxybenzaldehyde. Crystallisation, filtration, and drying then produce a powder with controlled residual moisture and particle size distribution. This route is conventional for high-purity PTA, but the operation of the hydrogenation, crystallisation, and screening sections determines the final 4-carboxybenzaldehyde level and particle size distribution observed in downstream polyester plants.

What limits direct substitution in continuous PET melt-phase polymerisation?

Substitution limits arise primarily from trace monofunctional and aldehyde impurities rather than from nominal assay. 4-carboxybenzaldehyde is the principal impurity carried from the crude-acid hydrogenation step; in continuous PET lines operating with esterification temperatures of 240–265 °C and polycondensation finish temperatures of 275–290 °C, residual 4-carboxybenzaldehyde can enter the polymer chain and form conjugated colour bodies that increase b* and reduce resin thermal stability. The Sines product is released with 4-carboxybenzaldehyde at or below 25 mg/kg. Packaging-grade bottle resin lines often tighten internal acceptance to 15 mg/kg when downstream preforms are measured for yellowness under ISO 11664-4. p-Toluic acid is monofunctional and acts as a chain terminator during polycondensation. A release value of ≤150 mg/kg prevents measurable intrinsic-viscosity loss in high-IV bottle resin polymerised to 0.80–0.84 dL/g. Ash above 10 mg/kg can interfere with antimony trioxide or titanium alkoxide catalyst selectivity, and free moisture above 0.5 wt% alters the solids fraction in the EG/PTA paste feed.

Table 1 is not a sales specification; it is the representative release envelope used to qualify the Sines train for continuous PET polymerisation. Exact contractual limits may be tighter.

Because p-toluic acid is the hydrogenation product of 4-carboxybenzaldehyde, the two impurities are inversely influenced by hydrogenation severity. Insufficient hydrogenation leaves excess 4-carboxybenzaldehyde; excessive severity can raise p-toluic acid and reduce overall purity. The Portugal train balances these species to keep both within the specified envelope while maintaining the low ash and moisture values required for continuous polymerisation.

The downstream polymer line forms a paste from PTA and ethylene glycol. Because the acid is not soluble in ethylene glycol at mixing temperature, the slurry is a high-solids dispersion that must be transferred by progressive-cavity or gear pumps into the esterification reactor. The usual EG/PTA molar ratio lies between 1.1:1 and 1.6:1; higher ratios suppress oligomer formation at the reactor inlet, while lower ratios reduce recovered ethylene glycol. The esterification reactor is operated at 0.1–0.3 MPa and 240–265 °C; the reaction is autocatalytic, producing water and bis(2-hydroxyethyl) terephthalate. Water is removed through a reflux column, and the residence time is typically 2–4 h through the esterification and pre-polycondensation stages. The finisher is then operated under progressively reduced pressure to below 1 mbar to drive polycondensation. The controlled median particle size of the Sines product reduces dissolution-limited conversion and excessive fines. A high fraction below 10 µm increases dust generation in the paste preparation hall and can create local ethylene-glycol-rich regions; a coarse fraction above 250 µm can delay conversion in short-residence esterifiers.

Process-control laboratories characterise the PET output by intrinsic viscosity in 60/40 wt% phenol/1,1,2,2-tetrachloroethane at 30 °C according to ASTM D4603. Carboxyl end-group concentration is determined by potentiometric titration in o-cresol; diethylene glycol content is measured by gas chromatography after methanolysis. These cross-checks allow the polymer plant to distinguish between PTA-related shifts and changes in catalyst dosage or polycondensation vacuum.

Bulk powder flow and the role of particle size distribution

Bulk handling of the Sines PTA is governed by the same particle size distribution, surface moisture, and bulk density that affect esterification. The material is discharged from pressure-transfer tankers into silos and then metered by loss-in-weight screw feeders into the paste mixer. A Jenike shear cell test according to ASTM D6128 is used to determine cohesive strength and the boundary between mass flow and funnel flow; mass-flow silos with smooth stainless steel liners and rotary valves are normally specified for continuous polymer plants. The powder is a combustible dust. Area classification is conducted under IEC 80079-10-2:2016, and dust hazard analyses follow NFPA 652. Explosion severity data should be measured on the actual lot in a 20-L sphere according to ASTM E1226; published data for this specific configuration is limited, but combustible dust behaviour of aromatic carboxylic acids is strongly influenced by fines and moisture. The controlled D50 of 120–160 µm and the coarse/fine balance are therefore part of the plant’s dust-safety envelope as well as a polymerising convenience.

When lot-to-lot consistency enters the esterification mass balance

Continuous polymerisation lines are more sensitive to small analytical shifts than batch lines because catalyst feed, residence time, and vacuum are normally held constant. If p-toluic acid shifts from 60 mg/kg to 140 mg/kg within the allowed limit, a finisher controlling to a fixed final intrinsic viscosity may require a proportional adjustment in catalyst dosage to hold carboxyl end-group concentration within target. If the adjustment is not made, the observed failure mode is not batch rejection but a slow increase in final PET carboxyl end groups beyond 30–35 meq/kg, which then depresses solid-state polycondensation rates and increases injection moulding degradation. The Sines train controls this by continuous sampling and release testing of each silo before transfer; the certificate of analysis reports lot-specific 4-carboxybenzaldehyde, p-toluic acid, b*, and moisture values. A cross-check against intrinsic viscosity of test PET polymerised at the customer site or in a laboratory reactor is used when a line is qualified. Published data for the Portuguese product under all possible line configurations is limited; a material substitution trial should compare the same analytical variables using the same methods and a fixed paste solids ratio.

Outside bottle-grade PET, the Sines PTA is used in polyester staple fibre and continuous filament. Fibre lines running high draw ratios require stable melt viscosity and low gel formation; the specification for p-toluic acid and 4-carboxybenzaldehyde supports these requirements because monofunctional impurities reduce number-average molecular weight. In polybutylene terephthalate production, the acid is esterified with 1,4-butanediol, and titanium alkoxide catalysts are common; low ash and low moisture are critical because moisture hydrolyses titanium catalysts and shifts the tetrahydrofuran side reaction. In thin-film applications, colour b* and ash are tightened further to avoid visible defects and specks. In each case, the acid is converted in the polymerisation reactor rather than compounded into a resin; the relevant processing parameters are slurry solids, esterification temperature, and vacuum profile.

Source-to-source substitution is governed by analytical profile, not nominal assay

Chemically, PTA from compliant sources is the same aromatic dicarboxylic acid, and direct substitution is normally possible after confirming silo cleanliness and feed-system compatibility. Practical differences in polymer plants arise from the trace impurity profile, particle size distribution, bulk density, and lot-to-lot consistency. The Sines material is positioned as a European integrated monomer source; the main operating distinctions are the ≤25 mg/kg 4-carboxybenzaldehyde release value, the 120–160 µm median particle size band, and the ≤0.5 wt% moisture limit compatible with paste feed without in-line drying under normal humidity. Sources supplying the same purity but a higher fine fraction may require more extensive dust abatement and explosion protection. Sources with lower p-toluic acid may allow a broader catalyst window in high-IV bottle resin, while sources with wider particle size distribution may demand tighter paste-mixer control. Published comparative data for this specific product is limited; the correct evaluation is a controlled silo-to-silo trial measuring final PET intrinsic viscosity, carboxyl end groups, b*, and diethylene glycol content under identical conditions.

For food-contact packaging, compliance is established at the finished PET polymer or article level, not by direct addition of PTA to food. The monomer is handled under REACH and CLP obligations as an industrial chemical. Indirect food-contact suitability for PET polymers derived from the monomer is typically assessed under 21 CFR 177.1630 or EU 10/2011 at the finished article stage. The safety data sheet identifies combustible dust precautions, neutralisation risks from contact with strong bases, and spill containment requirements. Storage in sealed silos or bulk bags with low-humidity venting is recommended; at ambient relative humidity above 60%, the powder may take up surface moisture and require pre-drying or hopper purge before continuous paste preparation. Bulk tanker unloading should be designed with inert-gas blanketing where local dust-zone classification requires it.