In direct melt-to-yarn conversion of PTA-based polyester, Hainan Yisheng Petrochemical purified terephthalic acid is specified by non-fibre contaminants that survive continuous oxidation and transfer into melt colour, carboxyl end-group balance, and spinneret pack life. Textile filament lines maintain the ethylene glycol-to-PTA molar feed ratio between 1.10:1 and 1.15:1. The excess glycol compensates for distillation losses during esterification while limiting diethylene glycol formation to 1.0–1.5 wt%. The first esterification reactor operates at 265–270 °C under 0.10–0.30 MPa absolute, and the second-stage reactor reduces carboxyl end-groups to below 25 mmol/kg before polycondensation. Antimony trioxide catalyst is metered at 200–300 ppm Sb on polymer mass, and 0.30 wt% TiO₂ is dispersed when semi-dull fibre is required. Polycondensation proceeds at 285–290 °C and 0.2–0.5 kPa absolute until intrinsic viscosity reaches 0.62–0.68 dL/g by ISO 1628-5 or ASTM D4603. Esterification conversion must exceed 97 % before vacuum application because residual carboxyl end-groups above 30 mmol/kg accelerate hydrolytic chain scission during chip drying and melt conveying. The filtered melt passes through 20–40 µm sintered metal candles before reaching spin beams. In high-speed partially oriented yarn winding at 2,800–3,200 m/min, a CIE b* colour below 1.5 and consistent melt viscosity are required to avoid winding tension variation and downstream dye uptake shifts. Terminal products include drawn textured yarn, fully drawn filament, and staple fibre for cotton-blend spun yarn. Textile processing compliance is assessed through OEKO-TEX Standard 100 and ZDHC MRSL, while the polymer itself is registered under REACH.

What Acetaldehyde Reversion Mechanisms Constrain Food-Contact Bottle Resin Blow Moulding?

For bottle-grade production, PTA is esterified with monoethylene glycol and purified isophthalic acid. The isophthalic acid is metered at 1.5–3.0 mol% of total acid to slow spontaneous crystallisation during preform cooling. Diethylene glycol is targeted at 1.2–1.4 wt%. Levels below this window produce stiff preforms with poor stretch uniformity, whereas higher levels depress glass transition and increase acetaldehyde precursor. Melt polycondensation is stopped at an intrinsic viscosity of 0.60–0.65 dL/g, after which the amorphous chip enters a solid-state polycondensation vessel operating at 210–220 °C under 0.10–0.50 kPa absolute or with heated nitrogen at 0.5–1.0 m/s superficial velocity. SSP increases intrinsic viscosity to 0.80–0.84 dL/g over 12–20 h and strips acetaldehyde, but injection moulding at 270–290 °C regenerates acetaldehyde by thermal oxidation and ester interchain reactions. Headspace acetaldehyde in preforms for carbonated soft drinks is measured by ASTM F2013 and must remain below 4 µg/L, requiring low residual acetaldehyde precursor, controlled injection barrel temperatures, and short residence time. The reciprocating screw compression ratio is held near 2.5:1, and hot-runner temperatures are kept below 280 °C to avoid local degradation. Preforms are reheat-stretch-blow moulded at 90–110 °C surface temperature with longitudinal stretch 2.8–3.2× and hoop stretch 2.5–3.0×. Food-contact compliance is established under FDA 21 CFR 177.1630 and Commission Regulation (EU) No 10/2011. Migration testing follows EN 1186. Oxygen barrier is measured by ASTM D3985 for barrier-grade containers. Terminal products include carbonated soft drink bottles, still water bottles, and hot-fill containers for pasteurised beverages.

Biaxially oriented polyester film conversion places tighter ceilings on cyclic trimer content than bottle or fibre production because oligomers sublimate from the cast sheet and condense on cooler stenter clip assemblies. Film-grade PTA is esterified with ethylene glycol at a molar ratio of 1.12:1 to 1.18:1, and polycondensation is stopped at an intrinsic viscosity of 0.62–0.66 dL/g. Diethylene glycol is held at 0.8–1.5 wt% and carboxyl end-groups below 30 mmol/kg to limit hydrolysis during extrusion. The melt is extruded through a slot die at 275–285 °C onto a polished chill roll maintained at 20–30 °C. Electrostatic pinning at 7–10 kV prevents air entrapment. The cast sheet enters a sequential linear motor stenter where machine-direction draw is 3.0–4.0× at 85–95 °C, transverse-direction draw is 3.5–4.5× at 100–120 °C, and heat-setting is applied at 220–230 °C with 2–5 % relaxation. Cyclic trimer, predominantly cyclic tri(ethylene terephthalate), is present at 1.5–2.5 wt% in amorphous cast film and blooms to the surface during orientation and heat-setting. Deposits on stenter clips, chain rails, and exhaust ducting increase transverse drag force and can release back onto film edges as specks unless removed by hot ethylene glycol or aqueous alkaline wash every 150–250 h of line operation. Operators monitor oligomer deposition via differential pressure across exhaust filters and stenter chain current. Mechanical properties are tested by ASTM D882, and oxygen permeability by ASTM D3985 at 23 °C and 50 % RH. Compliance for food packaging is assessed under EU Regulation No 10/2011 and FDA 21 CFR 177.1630. Terminal products include printed flexible packaging laminates, capacitor dielectric films, and photovoltaic backsheet base films where oligomer haze below 0.5 % is required.

Polybutylene Terephthalate Moulding Compound: Moisture Limits and Comparative Tracking Index Retention

Polybutylene terephthalate is produced from PTA and 1,4-butanediol by molten esterification at 220–250 °C with tetrabutyl titanate at 50–150 ppm Ti. The butanediol-to-PTA molar ratio is held between 1.20:1 and 1.30:1 because excess butanediol promotes tetrahydrofuran formation. Tetrahydrofuran is recovered from the overhead stream and monitored by gas chromatography. Elevated THF concentration indicates over-temperature excursions in the esterifier. Polycondensation proceeds at 250–260 °C under 0.1–0.5 kPa vacuum until intrinsic viscosity reaches 0.85–1.20 dL/g as determined by ASTM D4603 in phenol/tetrachloroethane 60/40. PBT pellets are dried to 0.02 % moisture or below at 120 °C for 3–4 h in desiccant dryers before compounding. Glass fibre reinforcement at 30 wt% is fed downstream in a co-rotating twin-screw extruder with L/D 40:1. The fibres are side-fed after the melt seal, and melt temperature is kept below 270 °C to avoid polymer degradation. Flame-retarded PBT compounds for connectors are formulated with brominated flame retardant and antimony trioxide to achieve UL 94 V-0 at 0.8 mm. Comparative tracking index is measured by IEC 60112 and is typically above 400 V for unfilled grades but can drop to 250–300 V when glass and flame retardant packages are added. Injection moulding uses barrel temperatures of 245–265 °C, mould temperatures of 60–90 °C, and anisotropic shrinkage of 0.8–1.2 % in flow direction and 1.0–1.5 % transverse. Terminal parts include automotive sensor housings, relay sockets, lamp sockets, and electrical connectors where heat deflection temperature is validated by ISO 75-2 method A at 1.80 MPa. Published data for specific Hainan Yisheng PTA-derived PBT molecular weight distribution are limited. Compounders use PTA with low p-toluic acid because monofunctional impurities cap chain ends and reduce impact strength measured by ISO 179-1/1eA.

Because CHDM suppresses spherulitic crystallisation, PTA-based amorphous sheet is produced with a glycol mixture in which 1,4-cyclohexanedimethanol comprises 30–50 mol% of total glycol. At the lower bound, haze and impact still resemble crystalline PET behaviour, while at the upper bound melt strength drops and calendering becomes difficult. Esterification is carried out at 230–250 °C with a glycol-to-acid molar ratio of 1.05:1 to 1.15:1, and polycondensation is stopped at an intrinsic viscosity of 0.70–0.80 dL/g. The amorphous pellets require closed-loop desiccant drying at 65–70 °C for 4–6 h to below 0.01 % moisture because hydrolytic degradation in the extruder increases yellowness and extrudate thickness variation. Sheet extrusion uses a single-screw extruder with a barrier screw and a polished three-roll stack at 220–250 °C. Die lip opening is set to 1.2–1.5× final sheet thickness to accommodate die swell and roll draw. Coextruded cap layers with higher CHDM content are used for chemical resistance and gloss. Thermoforming is performed on plug-assisted formers at 90–110 °C surface temperature. Food-contact suitability is assessed under EU Regulation No 10/2011, and medical packaging is evaluated by ISO 10993-5 for cytotoxicity after gamma or ethylene oxide sterilisation. Terminal products include clear medical device trays, retail display clamshells, and printed card overlays. The absence of a sharp melting endotherm is confirmed by ISO 11357-1 differential scanning calorimetry.

When Purified Terephthalic Acid Replaces Phthalic Anhydride in Rigid Polyol Transesterification

Aromatic polyester polyols for rigid polyurethane and polyisocyanurate foam are synthesized by transesterifying PTA with diethylene glycol. PTA loads of 20–35 wt% of the total polyol charge are typical because higher levels raise viscosity beyond pumpable handling at ambient temperature. The reaction is run at 200–220 °C with overhead water removal until the acid value drops below 2 mg KOH/g. Titanium-based catalysts are used at 20–60 ppm Ti, and vacuum is applied only in the final stage to avoid excessive DEG loss. The hydroxyl number is targeted between 250 mg KOH/g and 350 mg KOH/g as measured by ASTM D4274. Brookfield viscosity at 25 °C is controlled between 5,000 mPa·s and 20,000 mPa·s by ASTM D2196. This aromatic polyester polyol is blended with blowing agent, surfactants, and polymeric MDI in a high-pressure foam machine. In rigid polyisocyanurate panel lamination, higher terephthalate content increases char yield and reduces smoke release in ASTM E84 Steiner tunnel testing, but published data for Hainan Yisheng PTA in this specific polyol formulation are limited. The main operational boundary is storage stability of low-viscosity formulations: free diethylene glycol above 12 wt% causes phase separation and viscosity drift at 15 °C. Terminal products include metal-faced sandwich panels for cold-storage enclosures, continuous-line insulation board, and pour-in-place foam for pipe supports. Compressive strength is measured by ASTM D1621 at 10 % deformation, and density by ASTM D1622.