When EG/PTA Molar Feed Ratio Drops Below 1.12 in Continuous Polyester Fiber Polymerization
In continuous direct esterification for PET staple and filament, the primary set point is not feed purity alone but the EG/PTA molar ratio, because free glycol in the esterifier is consumed by side reactions that create diethylene glycol and oligomer carboxyl end groups. Fiber-grade melt lines using Sinopec Yangzi PTA typically run EG/PTA molar ratios between 1.15 and 1.25; the slurry is prepared at 60–68 wt% PTA in MEG. Compliance for the resulting fiber is governed by REACH Regulation (EC) No 1907/2006, Oeko-Tex Standard 100 Annex 4 limits for extractable antimony, GB/T 14464-2017 for polyester staple fibre, and ISO 1133-1:2022 for melt volume-flow rate if chip-like intermediate is characterized. Esterification is carried out in a train of agitated horizontal reactors at 260–270 °C and 0.15–0.35 MPa gauge, with first-stage conversion of 88–93%; antimony trioxide is added at 150–250 mg/kg Sb and a phosphorus stabilizer at a P/Sb molar ratio of 0.6–1.0. After esterification, the pre-polycondensation stage drops pressure to 10–20 kPa, and the finisher operates below 0.5 mbar to reach an intrinsic viscosity of 0.63–0.68 dL/g by ASTM D4603. On production-scale lines, the main failure mode is feed-ratio oscillation: when EG/PTA falls below 1.12, carboxyl end groups remain above 35 mmol/kg at the finisher inlet, forcing vacuum depth beyond design and reducing melt throughput; when the ratio exceeds 1.25, DEG formation accelerates through the vent system and can exceed 1.3 wt%, which broadens the melt crystallization exotherm and causes spin-pack pressure fluctuations above ±5 bar. The terminal product types are semi-dull and full-dull polyester staple fibre, partially oriented yarn, fully drawn yarn, and drawn textured yarn for apparel, nonwovens, and automotive textiles.
Solid-State Polycondensation of Food-Grade PET Under Acetaldehyde and Molecular Weight Boundaries
Bottle-grade PET resin based on PTA is differentiated from fiber-grade material by co-monomer ratio, melt-phase molecular weight ceiling, and the solid-state polycondensation stage that removes acetaldehyde and raises intrinsic viscosity without uncontrolled degradation. Typical bottle formulations use isophthalic acid as a co-monomer at 1.5–2.5 mol% of total diacids, with PTA supplying the remaining 97.5–98.5 mol%; the total glycol/diacid molar ratio is held at 1.10–1.20. Food-contact compliance is defined by FDA 21 CFR 177.1630, EU Regulation (EU) No 10/2011 with a specific migration limit for terephthalic acid of 7.5 mg/kg and for monoethylene glycol of 30 mg/kg, and China GB 4806.6-2016 for food-contact plastic materials and articles. Melt-phase polymerization is generally run to an IV of 0.60–0.65 dL/g, after which amorphous pellets are crystallized and introduced into vertical moving-bed SSP reactors at 205–215 °C under counter-current nitrogen flow of 0.3–0.5 Nm³/h per tonne. Residence time of 12–20 h raises the IV to 0.80–0.84 dL/g for carbonated soft drink bottles and 0.76–0.78 dL/g for still water; acetaldehyde drops from 50–100 ppm in melt-phase resin to below 1 ppm by ASTM F2013. The operational boundary is that SSP cannot fully correct high DEG content above 1.5 wt%, because DEG defects remain in the chain and reduce thermal crystallization capacity; therefore melt-phase DEG control in the esterifier is critical. Terminal product types include carbonated soft drink and beer bottles, mineral water bottles, hot-fill containers, and edible oil containers.
| Parameter | Melt-phase pellet | After SSP | Test method |
|---|
| Intrinsic viscosity | 0.60–0.65 dL/g | 0.80–0.84 dL/g | ASTM D4603 |
| Acetaldehyde content | 50–100 ppm | <1 ppm | ASTM F2013 |
| Carboxyl end groups | 25–35 mmol/kg | <15 mmol/kg | ASTM D7409 |
| DEG content | 1.2–1.5 wt% | 1.2–1.5 wt% | Internal HPLC method |
What Links 4-CBA Carryover to Pin-Hole Formation in High-Speed Biaxially Oriented Polyester Film Casting?
If high-speed BOPET lines exceed 300 m/min, the correlation between PTA-derived 4-carboxybenzaldehyde and pin-hole formation is dominated by thermal degradation and gel nucleation rather than by simple diffusion. Film-grade polyester formulations generally run an EG/PTA molar ratio of 1.18–1.25. Slip and antiblock additives are introduced as amorphous silica at 500–1,200 ppm with a median particle size of 1.5–3.0 μm; capacitor-grade film reduces inorganic loading below 100 ppm. Relevant compliance includes FDA 21 CFR 177.1630 for food-contact films, EU Regulation (EU) No 10/2011 for overall migration, ASTM D882 and ISO 527-3 for tensile properties, and IEC 60674 series for electrical insulation film. Melt extrusion is performed through a slot die at 280–285 °C, followed by electrostatic pinning to a chill roll held at 25–35 °C. Longitudinal stretching occurs at 75–85 °C with a draw ratio of 3.2–3.5; transverse stretching in a tenter frame occurs at 95–110 °C with a draw ratio of 3.8–4.2; heat setting is conducted at 200–225 °C. In high-speed lines, 4-CBA levels above 25 mg/kg in PTA increase polymer color b* and generate gel-like particles that produce pin-holes during transverse stretching. Production experience shows that transverse temperature variation exceeding ±2 °C across the tenter creates thickness variability above 2% and bowing. Terminal product types include flexible food packaging, metallized barrier films, capacitor dielectric films, and photovoltaic backsheet films.
PBT Melt Viscosity Fluctuations Trace Back to THF Generation in the Esterification Stage
Direct esterification of PTA with 1,4-butanediol cannot be optimized by PET-line residence time alone because BDO dehydration to tetrahydrofuran reduces effective diol concentration and shifts melt viscosity. PBT compounding lines typically set BDO/PTA molar ratio at 1.40–1.60, with tetrabutyl titanate catalyst at 50–200 ppm Ti and triphenyl phosphite stabilizer at 0.05–0.15 wt%. Compliance for PBT compounds in electrical and electronic applications includes RoHS Directive 2011/65/EU, REACH Regulation (EC) No 1907/2006, UL 94 V-0 where flame retarded, ISO 7792-1:2014 for designation, and ASTM D638/ASTM D790 for tensile and flexural properties. Esterification is conducted in agitated reactors at 220–240 °C and 0.05–0.15 MPa, followed by polycondensation at 245–260 °C under vacuum below 100 Pa. THF generation consumes 2–3 mol% of the BDO feed, and the overhead recovery system must separate BDO, THF, and water; unstable reflux can shift the effective BDO/PTA ratio enough to change melt viscosity by more than 10%. Glass-fibre-reinforced compounds are produced on co-rotating twin-screw extruders with L/D 40:1 and barrel temperatures of 250–270 °C, using glass fibre loading up to 30 wt%. Injection molding is typically performed with melt temperature 250–265 °C and mould temperature 60–80 °C. An operational limitation is that amine-based stabilizers or flame retardants can accelerate hydrolysis and should be excluded from PBT formulations. Terminal product types include automotive connectors, relay sockets, brush holders, and electronic sensor housings.
Rigid polyisocyanurate foam producers select polyester polyol hydroxyl number and acid value before they consider aromatic PTA content. These aromatic polyester polyols are synthesized by direct esterification of PTA with diethylene glycol and often a recycled PET glycolysate. A typical formulation uses PTA at 20–30 wt% of the total charge, diethylene glycol at 45–60 wt%, and recycled PET glycolysate up to 20 wt%; the finished polyol is specified with a hydroxyl number of 200–350 mg KOH/g, an acid value of 1.0–3.0 mg KOH/g, and a viscosity of 2,000–5,000 mPa·s at 25 °C. The relevant standards for the foam product are EN 13165 for factory-made rigid polyurethane and polyisocyanurate boards, ASTM D1622 for core density, ASTM E84 for surface burning characteristics, and REACH Regulation (EC) No 1907/2006. The esterification process is run at 220–240 °C under nitrogen with a packed column to separate water and excess DEG; vacuum of 20–40 kPa is applied near the endpoint to drive the acid value down. The recovered DEG is often recycled to the next batch after drying. In foam production, the polyol is mixed with polymeric MDI at an isocyanate index of 2.5–3.0, n-pentane as blowing agent at 8–12 wt%, and potassium octoate trimerization catalyst at 1.0–3.0 parts per hundred polyol. The main operational constraint is residual water: if water in the polyol exceeds 0.1 wt%, foam cell coarsening occurs and compressive strength drops below the EN 13165 classification threshold. Terminal product types include PIR insulation boards for roofing, HVAC duct panels, and pipe insulation for low-temperature industrial lines.
Substituting phthalic anhydride with PTA in unsaturated polyester resin synthesis shifts the condensation from a homogeneous anhydride opening to a heterogeneous solid–liquid esterification, which changes first-stage reactor fouling patterns and final styrene compatibility. UPR formulations typically replace 15–35 mol% of phthalic anhydride with PTA in the saturated acid component, while maleic anhydride remains at 45–55 mol% of total dibasic acid; the total dibasic acid to propylene glycol molar ratio is kept at 1.0:1.05–1.10. Compliance for PTA-based unsaturated polyester laminates is tied to ASTM D790 and ASTM D638 for mechanical properties, ISO 75 for heat deflection temperature, and REACH Regulation (EC) No 1907/2006 for component registration. The synthesis is a two-stage melt condensation at 180–220 °C with xylene azeotropic distillation to remove water; endpoint acid values of 15–30 mg KOH/g are typical before styrene dilution. The resin is then dissolved in styrene monomer at 35–40 wt%, and cure in pultrusion is initiated with methyl ethyl ketone peroxide at 1.0–1.5 wt% and cobalt octoate at 0.2–0.5 wt%, with die temperatures between 130–150 °C. On production resin kettles with anchor agitators at 20–40 rpm, PTA fines can accumulate in the xylene decanter if the slurry is not pre-dispersed, reducing water removal and lengthening cook time. Terminal product types include glass-fibre-reinforced panels, pultruded structural profiles, cast marble, and polymer concrete.
For thermosetting powder coatings, the use of PTA in saturated polyester resin synthesis places acid value and glass transition temperature in a narrow processing band because both parameters determine extrusion behavior and final coating levelling. Typical super-durable and standard durable polyester resins for TGIC curing use PTA at 25–45 wt% of total monomer charge, neopentyl glycol at 35–50 wt%, trimethylolpropane at 0.5–2.0 mol%, isophthalic acid up to 5 wt%, and butylstannoic acid catalyst at 0.05–0.15 wt%. The resin is esterified at 230–250 °C under vacuum down to 50 mbar until the acid value reaches 30–36 mg KOH/g for TGIC systems and 20–28 mg KOH/g for hydroxyalkylamide systems. Compliance for finished powder coatings includes Qualicoat Class 1 and Class 2 specifications for architectural aluminum, ASTM D3359 for coating adhesion, ISO 1519 for bend resistance, and REACH Regulation (EC) No 1907/2006. The molten resin is cast onto a cooling belt, crushed, and then blended with TGIC at 7.5 wt% or hydroxyalkylamide at 5–7 wt%, plus flow agents and fillers. Extrusion compounding is performed in a twin-screw extruder with barrel temperatures of 90–110 °C, and curing is performed at 180–200 °C for 10–15 min. The main operational limitation is PTA-induced high glass transition above 70 °C, which can reduce flow and levelling if the polyester acid value and branching are not balanced. Terminal product types include architectural aluminum window profiles, automotive underbody parts, and general metal furniture.
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Sinopec Yangzi Petrochemical PTA is a purified terephthalic acid supplied as a free-flowing white crystalline powder for direct esterification in melt-phase polyester processes. The product is not differentiated by discrete model numbers; it is designated polyester-grade PTA and is sold against lot certificates aligned with GB/T 32685-2016. Its molecular formula is C8H6O4, relative molecular mass 166.13 g/mol, and theoretical acid value 675.4 mg KOH/g. Bulk delivery may be by railcar, bulk tanker, or 1000 kg flexible intermediate bulk container. The product is intended for conversion into polyethylene terephthalate, polybutylene terephthalate, polyester polyols, and related copolyesters.
Because no discrete model variants exist, procurement specifications typically define the product by upper limits for 4-carboxybenzaldehyde, p-toluic acid, ash, moisture, and Hunter b* color. The term Yangzi Petrochemical PTA therefore refers to a single polyester-grade stream rather than a range of performance models. Published data for Yangzi-specific lot-to-lot standard deviation is limited; supplied certificates should be checked for exact lot values.
What Limits 4-Carboxybenzaldehyde Tolerance in Continuous PET Polymerization?
The dominant quality boundary in continuous polyethylene terephthalate polymerization is 4-carboxybenzaldehyde (4-CBA), an oxidation intermediate that retains one aldehyde functionality and can interrupt chain extension. In polyester-grade PTA the 4-CBA ceiling is 25 mg/kg, whereas crude terephthalic acid may contain 2000–4000 mg/kg. The lower limit is not arbitrary; during direct esterification at 260–270 °C and 0.2–0.4 MPa gauge, residual 4-CBA can form conjugated color bodies and act as a monofunctional chain stopper. Operators using five-reactor continuous PET trains with final polycondensation pressure below 100 Pa report that 4-CBA excursions above the specification can force an increase in vacuum or catalyst addition to maintain target intrinsic viscosity. In a slurry process using a high-shear mixer at 1450 rpm and ethylene glycol-to-PTA mole ratio 1.10–1.20, the PTA must wet uniformly within 20–40 min; agglomerates with elevated p-toluic acid can persist as undissolved fines. HPLC determination of 4-CBA and p-toluic acid follows GB/T 32685-2016; the acidimetric assay confirms acid value near 675.4 mg KOH/g.
p-Toluic acid, the monofunctional oxidation intermediate, is controlled at ≤ 150 mg/kg because elevated concentrations reduce the effective end-group balance in PET. In a batch autoclave synthesizing PET from PTA and ethylene glycol with antimony trioxide catalyst at 280 °C, off-spec p-toluic acid can require an increase in vacuum duration to reach the same intrinsic viscosity. Ash content matters because metal residues can catalyze acetaldehyde formation; total ash is therefore specified at ≤ 15 mg/kg. Batch-to-batch variation in ash may be measured by combustion at 850 °C followed by gravimetric analysis under GB/T 32685-2016.
Commercial lot certification for polyester-grade PTA typically includes the parameters shown in Table 1. The values are representative specification ceilings for polyester-grade material, not necessarily the tighter internal control limits applied by the producing site.
| Parameter |
Unit |
Specification envelope |
Test method |
| Acidimetric purity |
% |
≥ 99.5 |
GB/T 32685-2016 |
| 4-Carboxybenzaldehyde |
mg/kg |
≤ 25 |
GB/T 32685-2016 |
| p-Toluic acid |
mg/kg |
≤ 150 |
GB/T 32685-2016 |
| Ash |
mg/kg |
≤ 15 |
GB/T 32685-2016 |
| Moisture, as received |
% (m/m) |
≤ 0.2 |
GB/T 6283-2008 |
| Hunter b* color |
— |
≤ 1.2 |
GB/T 30921-2014 |
| Median particle size D50 |
µm |
110–150 |
ISO 13320:2020 |
| Bulk density |
g/cm³ |
0.85–1.05 |
ISO 60:1977 |
The direct esterification route for polyethylene terephthalate consumes the product as an acid monomer. Slurry preparation is performed at 60–80 °C in agitated tanks with paddle or high-shear impellers; the slurry is then heated through a paste heater and fed to the first esterification reactor. In bottle-grade PET lines targeting intrinsic viscosity of 0.80 dL/g measured by ASTM D4603, low ash and low 4-CBA reduce the risk of degraded acetaldehyde generation and darkening during solid-state polycondensation. Fiber-grade PET lines targeting 0.62–0.68 dL/g are similarly sensitive to p-toluic acid because it can alter dye uptake and spin pack filtration. In polybutylene terephthalate production using 1,4-butanediol, esterification at 220–240 °C with titanium catalysts requires low carboxylic impurities to avoid off-stoichiometry and butanediol loss to tetrahydrofuran. The product may also be used for polyester polyols in high-solids coatings, where control of acid value and color is specified by ASTM D4662 or equivalent.
In continuous PET polycondensation, the finisher is often a disc-ring or comb-type reactor operated below 100 Pa absolute pressure. Impurities that survive esterification can accelerate thermal degradation and force higher catalyst loadings. For this reason, total metal content and ash are controlled tightly in polyester-grade PTA. Final polymer compliance for food-contact use is governed by FDA 21 CFR 177.1630 and Regulation (EU) No 10/2011; the monomer itself is not a direct food-contact article. In bottle-grade production, pellets are solid-state polymerized at 210–220 °C under vacuum or inert gas to raise intrinsic viscosity from 0.60 dL/g to 0.80–0.84 dL/g. Impurities from PTA can reduce the solid-state rate by consuming end groups; low 4-CBA and p-toluic acid preserve the end-group balance.
Silo Flow and Pneumatic Conveying Boundaries
Bulk handling of PTA is governed by particle size distribution, moisture uptake, and attrition behavior of the crystalline powder. Typical polyester-grade PTA shows a median particle size near 120 µm and a fines fraction below 10% through 45 µm. In carbon steel silos with 60° cone half-angle and aeration pads, discharge is generally uniform; however, fines segregation can occur if the silo is filled from a single central point without a distributor. Dense-phase pneumatic conveying systems are normally operated with pickup velocities between 18 m/s and 28 m/s. Longer lines above 100 m may require booster air and pressure control to avoid plugging. PTA dust is combustible; grounding and bonding should follow NFPA 654, and dust collection should be designed for explosion severity class St 1 as determined by ASTM E1226. Moisture pickup in open hoppers at relative humidity above 60% can increase surface moisture; closed-loop conveying with a dew point below -20 °C is preferred. If the product is exposed to humid air for more than 24 h, pre-drying at 110 °C for 2–4 h may be required before feeding to esterification.
PTA is an organic acid and should not be blended with strong bases or amines during storage, as neutralization may alter esterification stoichiometry and generate heat. Avoid contact with copper alloys in long-term storage because acidic moisture can corrode and introduce metal residues that degrade polyester color.
When Medium-Purity Terephthalic Acid Substitutes for Polyester-Grade PTA in Slurry Esterification
Substitution in continuous direct esterification is constrained by impurity differences. Medium-purity terephthalic acid can carry 4-CBA at 200–400 mg/kg and p-toluic acid at 500–1000 mg/kg, figures that are acceptable for some plasticizer or low-molecular-weight polyester applications but not for high-clarity bottle resin or high-tenacity fiber. Crude terephthalic acid contains 2000–4000 mg/kg 4-CBA and requires hydrogenation and crystallization before polyester use. Recycled terephthalic acid derived from chemical recycling may show variable trace-metal loads from catalyst residues; published data for this specific configuration is limited. The Yangzi Petrochemical PTA stream differs from these alternative materials by operating within the low-impurity specification envelope in Table 1, allowing direct slurry esterification without an extra purification step.
| Grade |
4-CBA (mg/kg) |
p-Toluic acid (mg/kg) |
Direct polyester use |
| Yangzi polyester-grade PTA |
≤ 25 |
≤ 150 |
Direct esterification |
| Medium-purity terephthalic acid |
200–400 |
500–1000 |
Limited to plasticizers and low-molecular-weight polyesters |
| Crude terephthalic acid |
2000–4000 |
3000–6000 |
No; requires purification |
| Recycled terephthalic acid |
Variable |
Variable |
Requires filtration and trace-metal control |
In direct comparison with other purified terephthalic acid sources, the distinguishing specification features are not only 4-CBA and p-toluic acid ceilings but also particle size stability and low ash. Purchasers should request lot certificates for ash, b* color, and moisture because these values can shift with storage and transport conditions. The product is not manufactured as an electronic-grade or pharmaceutical-grade material.
