Melt Viscosity Cascades in Continuous Textile Polyester Polycondensation

Continuous textile-grade polyester production using PTA from Yisheng New Material (Ningbo) places the esterification kettle at the centre of melt viscosity control. The slurry charge is prepared at a PTA-to-EG molar ratio of 1:1.15, with the excess ethylene glycol serving both as esterification reaction medium and as a boundary-layer diluent that suppresses premature polycondensation in the recirculating transfer line. Antimony trioxide is metered into the slurry as 150–250 ppm Sb relative to terephthalic acid; TiO₂ delustrant is introduced at 0.30–0.50 wt% only where semi-dull or full-dull staple and filament are required. The first esterification stage operates at 260–270 °C and 0.15–0.25 MPa, reducing the acid value to 18–25 mg KOH/g before the second esterification stage raises monomer conversion to 96–98%. Operators monitor diethylene glycol formation because DEG content in the range 1.2–1.8 wt% shifts dye uptake and depresses the crystalline melting point of the spun yarn. At the pre-polycondensation step the pressure is reduced stepwise to 1.0–2.0 kPa, and final polycondensation in a disc ring reactor at 280–290 °C under 50–100 Pa absolute yields a melt with intrinsic viscosity 0.62–0.68 dL/g measured according to ASTM D4603-18. The spinning beam is held at 292–298 °C; pressure drop across the filter packs is kept below 14 MPa to avoid gel particle breakthrough. Filament take-up speed is set at 3,000–3,500 m/min for partially oriented yarn, while staple production runs through a tow drawing train with a draw ratio of 3.2–3.6 and a crimper load of 150–200 kg/h per line. Continuous lines exhibit batch-to-batch variance in carboxyl end-group concentration when the esterification pressure controller drifts by more than ±0.02 MPa; the resulting acid value scatter can shift final polycondensation torque by ±5% and may require a melt-phase IV adjustment of ±0.02 dL/g. Vacuum pumps should be maintained at a suction pressure below 40 Pa to avoid cyclic oligomer deposition on the vapour line. Terminal products include polyester staple fibre, POY, FDY, air-textured yarn, and spunbond nonwoven. Textile compliance is anchored to OEKO-TEX Standard 100 Class I, ZDHC MRSL conformance for spin finish and dyestuff auxiliaries, and REACH Annex XVII substance restrictions; yarn and fibre laboratories quantify antimony migration and tensile values according to ASTM D2256-21 for continuous filament yarn.

Acetaldehyde management in bottle-grade PET begins at the esterification stage, not at the preform injection moulder. The PTA:EG molar ratio is held close to 1:1.15, but the formulation is deliberately modified by allowing 1.2–1.5 wt% DEG and by adding isophthalic acid at 2.0–3.0 wt% to disrupt crystallinity during stretch blow moulding. Antimony catalyst is maintained at 200–250 ppm; a higher antimony level accelerates acetaldehyde formation during melt processing, while a lower level forces a longer final polycondensation residence time and increases thermal history. Continuous melt polymerisation is followed by solid-state polycondensation at 210–220 °C under a nitrogen stream with a dew point below −30 °C; the SSP reactor raises intrinsic viscosity from 0.62–0.65 dL/g to 0.80–0.85 dL/g measured by ASTM D4603-18, reducing acetaldehyde to ≤10 ppm for mineral water preforms and ≤6 ppm for carbonated soft drink grades. Preform injection uses a screw with L/D 24:1 and a barrel profile of 270–285 °C; shear heating should not exceed 5 °C above set point because acetaldehyde concentration rises by 2–4 ppm per pass above 285 °C. Amorphous pellets must be predried at 160–170 °C for 4–6 h to below 50 ppm moisture; residual water above that level hydrolyses the melt and depresses IV by 0.02–0.04 dL/g in the barrel. Finished article types are PET preforms, blow-moulded water bottles, carbonated soft drink bottles, edible oil containers, and hot-fill containers. Food-contact compliance rests on FDA 21 CFR 177.1630, EU No 10/2011 with specific migration limits for antimony and DEG, China GB 4806.7-2016, and Japan JHOSPA.

What Limits Casting Drum Speeds in PTA-Derived BOPET Film?

The casting drum speed on a PTA-derived BOPET line is limited by two competing defects: pinning instability and quench non-uniformity. The base resin is a homopolyester with a PTA:EG ratio of 1:1.15, an intrinsic viscosity of 0.60–0.65 dL/g, and silica antiblock at 0.1–0.5 wt%; silica aggregates above 10 μm are rejected by a 20 μm filtration element at the die inlet. Extrusion takes place at 280–290 °C, and the flat die lip gap is set to 1.5–2.0 mm to allow for draw-down. The cast roll is held at 20–40 °C, and electrostatic pinning uses a wire electrode at 7–10 kV; insufficient pinning causes air pockets and non-uniform crystallisation, while excessive voltage produces pinholes at the film edges. Machine-direction draw ratio is 3.0–3.8, transverse draw ratio is 3.5–4.0, and stretching is carried out at 100–120 °C before heat-setting at 210–230 °C. On production lines running above 200 m/min cast speed, the limiting failure mode is not machine-direction draw but transverse orientation breakage caused by shoulder edge temperature falling below 90 °C. Edge bead must be trimmed and recycled at a ratio not exceeding 15 wt%, because oxidised edge regrind raises carboxyl end-group concentration and shifts electrostatic pinning behaviour. Polyester flake must be dried to 50 ppm moisture at 170–180 °C for 4–6 h; otherwise hydrolysis during extrusion reduces die-lip melt strength and causes shoulder-in instability. Converted film grades are BOPET films for flexible food packaging, lidding, release liners, and electrical insulation; gauge variation must remain within ±2% across the line width. Food-contact compliance follows FDA 21 CFR 177.1630 and EU No 10/2011; tensile and elongation are verified according to ASTM D882-18; electrical-grade film is additionally tested by IEC 60674-3-2. Published data for line speeds above 250 m/min with this specific configuration is limited.

Unlike PET polyester chemistry, direct esterification of PTA with 1,4-butanediol in PBT production must contend with acid-catalysed dehydration of the diol to tetrahydrofuran. The PTA:BDO molar ratio is set between 1:1.2 and 1:1.4; the excess BDO is recovered from the overhead stream by distillation and returned to the slurry tank, while THF is separated as a by-product. Tetra-n-butyl titanate or tetraisopropyl titanate is added at 50–150 ppm Ti, and the esterification stage operates at 225–250 °C at atmospheric or slight positive pressure until the acid value falls below 10 mg KOH/g. Vacuum finishing at 250–260 °C and 60–100 Pa absolute raises intrinsic viscosity to 0.90–1.10 dL/g; melt flow is commonly validated by ISO 1133-1:2022 at 250 °C with 2.16 kg load, producing values between 15 and 35 g/10 min for unfilled material. For engineering plastic compounds, 30 wt% chopped glass fibre is melt-compounded in a co-rotating twin-screw extruder with L/D 40:1, barrel temperature 240–260 °C, and screw speed 350–450 rpm; the screw profile includes two kneading blocks downstream of the glass-fibre side feeder to limit fibre attrition below 200 μm weight-average length. Injection moulding is performed at a melt temperature of 250–270 °C, mould temperature 80–110 °C, and clamp force requirements based on a projected area of 0.35–0.55 t/cm². One operating boundary is the THF formation floor: when BDO concentration is allowed to fall below the stoichiometric point during slurry feed fluctuations, THF yield rises and the final PBT batch shows lighter coloration and lower melt viscosity. Vacuum finishing must be interrupted if the polymer cooler jacket exceeds 260 °C, because thermal release of THF from ester end groups creates voiding during moulding. Terminal products include automotive connectors, relay housings, coil bobbins, and electrical enclosures requiring UL 94 V-0 performance at 0.8 mm wall thickness. Compliance pathways include UL 94 V-0, IEC 60695-2-11 and IEC 60695-2-12 glow-wire tests, RoHS Directive 2011/65/EU, and REACH Annex XVII.

Aromatic Polyester Polyol Acid Number Drift During PIR Panel Foaming

When a PIR panel line raises isocyanate index above 250, the aromatic polyester polyol's acid number becomes the limiting analytical parameter. The polyol is synthesised from PTA and diethylene glycol, with a small triol content to control branching; the PTA:DEG molar ratio is set at 1:1.25–1.35 and the excess DEG is recovered through a rectification column. The hydroxyl value is maintained at 220–260 mg KOH/g, the acid value is held below 2.0 mg KOH/g, and the water content is kept below 0.10 wt% because water competes with the blowing agent and creates urea density gradients. Esterification is run in a batch reactor at 200–230 °C, initially at atmospheric pressure with a rectification column to return DEG, then under vacuum at 40–80 kPa absolute until the acid value target is reached. On a production-scale line, the viscosity at 25 °C is typically 1,500–3,500 mPa·s; higher aromatic content increases viscosity and slows down the subsequent mixing step. The formulated polyol blend is metered through a high-pressure polyurethane machine with mix head pressure of 120–150 bar and component temperature held at 23–28 °C; isocyanate index is set between 180 and 300 for polyisocyanurate ring formation. Acid number drift above 2.5 mg KOH/g consumes amine catalyst and creates a slow cream time, which in continuous lamination appears as an adhesion weakness between the PIR core and the metal facing. The final manufactured forms are rigid polyisocyanurate insulation boards for construction, metal-faced sandwich panels, and pipe insulation sections. Fire performance is assessed under EN 13501-1 and ASTM E84; REACH and raw material VOC emission data are required for CE marking of construction products.

Powder coating resins formulated with PTA require a narrow acid value window because the final network density is a direct function of carboxyl functionality. For TGIC-cured systems the resin acid value is set at 30–35 mg KOH/g; for hydroxyalkylamide systems the acid value is set at 20–25 mg KOH/g. The polyester backbone is produced by melt polymerisation of PTA, neopentyl glycol, and trimethylolpropane, with PTA charged at 30–50 wt%, NPG at 30–40 wt%, and TMP at 1–5 wt% of the resin charge. Esterification is carried out at 235–245 °C under nitrogen, followed by vacuum stripping at 50–80 kPa absolute until the acid value and melt viscosity targets are reached; the resin is then cooled on a flaking belt and ground to a D50 of 30–40 μm. The ground resin is premixed with 7.0–7.5 wt% TGIC or the stoichiometric equivalent of hydroxyalkylamide, and melt-extruded in a twin-screw extruder at 90–110 °C; after cooling and grinding, the powder is applied electrostatically and cured at 200 °C for 10 min. The operating boundary is the resin glass transition temperature: a resin Tg below 55 °C tends to sinter during bulk storage in warm climates, while a Tg above 65 °C can reduce flow and levelling at the cure temperature. Article categories served by this chemistry include architectural aluminium profile coatings and general industrial finishes where Qualicoat Class 1 or Class 2, AAMA 2604, and RoHS Directive 2011/65/EU apply.

When CHDM Replaces Monoethylene Glycol in Copolyester Sheet Extrusion

Substitution of 30–35 mol% of monoethylene glycol with 1,4-cyclohexanedimethanol suppresses crystallinity in PTA-based copolyester sheet. The PTA-to-total glycol molar ratio is held at 1:1.10–1.15; the CHDM content is controlled by diol recovery after esterification, and the resulting copolyester remains amorphous after cooling. Melt polymerisation is carried out at 275–300 °C under vacuum, with intrinsic viscosity between 0.70 and 0.80 dL/g; excessive residence time above 300 °C produces thermal degradation that shifts the b value and increases haze in extruded sheet. Sheet extrusion runs at 230–260 °C through a polished roll stack at 40–60 °C; the sheet is converted into thermoformed medical trays and shrink-sleeve labels. The copolyester must be dried to below 200 ppm moisture at 60–70 °C for 4–6 h because higher temperatures can cause pellet agglomeration in the dryer hopper. Food packaging compliance is evaluated under FDA 21 CFR 177.1315 and EU No 10/2011; medical packaging validation follows ISO 11607-1.