Continuous Polymerisation Parameter Windows in Staple Fibre Production

Purified terephthalic acid from the Mailiao plant of Formosa Chemicals & Fibre is charged as a dry bulk solid into continuous slurry preparation vessels where ethylene glycol is mixed at a molar ratio of approximately 1.10–1.20 MEG:PTA. The slurry is delivered to an esterification train operating at 240–265 °C and 0.15–0.30 MPa absolute pressure. Water of esterification is removed through reflux columns. The oligomer is then transferred to polycondensation reactors under vacuum below 2 mbar at 280–295 °C. Antimony trioxide at 200–300 mg/kg Sb is commonly used as a polycondensation catalyst. The melt is circulated through finishers with surface-renewal agitator elements. Intrinsic viscosity for staple fibre spinning is typically held between 0.62 dL/g and 0.68 dL/g when measured according to ASTM D4603. The Mailiao PTA stream is assessed against the control bands in Table 1 before release into high-speed spinning.

Residual 4-carboxybenzaldehyde acts as a monofunctional chain terminator during melt condensation. Elevated carryover from oxidation raises carboxyl end-group concentration and broadens molecular weight distribution. A control band of ≤25 mg/kg limits the drop in melt-phase intrinsic viscosity to a tolerable range for staple machines. Batch-to-batch variation in 4-CBA can reduce spinline stability because low molecular weight fractions depress melt strength. In continuous lines, the resulting melt pressure excursions are observed as drawpoint drift in quench chambers. Ash content above 6 mg/kg accelerates filter pack pressure rise in spin packs. A filter pack reaching 200 bar differential can force line stoppage before scheduled pack replacement. For Mailiao PTA, the low b* value keeps polyester whiteness above the threshold for optical staple. The material is held in silos with dried air to keep moisture below 0.40 wt%. High-moisture PTA hydrolyses in the feed system and releases terephthalic acid dust at transfer points.

Staple fibre production uses spinnerets with 3,000–50,000 holes, quench air at 18–24 °C and 60–65% RH, draw ratios of 2.5–3.5, crimp densities of 10–14 crimps per 25 mm, and heat setting at 120–150 °C. Melt temperature at the die is restricted to 285–295 °C to avoid thermal degradation. PTA purity influences gel formation at the die because low-molecular-weight fractions and inorganic residues accumulate at the spinneret face. Spinneret wiping frequency and monomer evolution are operational indicators of PTA consistency. The melt flow rate of dried chip is monitored by ISO 1133-1. Tensile properties of the finished staple are validated by ASTM D2256. Yarn breakage during drawing is correlated with the carboxyl end-group concentration in the melt. A narrow PTA quality distribution from the Mailiao plant therefore reduces the frequency of drawline breaks and package rejects.

In bottle-grade polyethylene terephthalate synthesis, the Mailiao PTA stream is combined with MEG, isophthalic acid at 1.5–2.5 wt%, and diethylene glycol controlled to 1.0–1.5 wt% in the amorphous melt. The comonomer addition shifts the glass transition and slows crystallisation. Melt-phase resin exits the finisher at 0.60–0.63 dL/g intrinsic viscosity and is pelletised through underwater cutting. Amorphous pellets are crystallised in an agitated crystalliser at 140–180 °C to prevent sticking in solid-state polycondensation. The SSP reactor is operated at 200–225 °C under vacuum below 0.5 mbar or with nitrogen at a dew point below -60 °C. After 12–18 h of residence, intrinsic viscosity rises to 0.80–0.84 dL/g.

Residual acetaldehyde in the final preform must be controlled below 1.0 ppm for water and soft-drink packaging, measured by ASTM F2013. PTA quality affects acetaldehyde because carbonyl impurities and carboxyl end groups accelerate ester bond scission during injection moulding. Bottle polymer produced from this route is compliant with FDA 21 CFR 177.1630 and EU Regulation 10/2011 when polymer-grade PTA with low heavy-metal content is used. Ash below 6 mg/kg reduces filter clogging in 20–40 µm melt filters. Preform injection is performed on machines with clamp forces from 350 t to 600 t, barrel temperatures of 270–290 °C, and screw L/D ratios from 20:1 to 24:1. Drying at 160–170 °C for 4–6 h brings pellet moisture below 30 ppm. Higher moisture causes hydrolysis and a measurable intrinsic viscosity drop during plastication.

The crystallisation half-time of amorphous bottle resin is sensitive to chain-end chemistry. A higher carboxyl end-group count from PTA side reactions accelerates nucleation but limits molecular weight build in SSP. Control of 4-CBA at ≤25 mg/kg and p-toluic acid at ≤150 mg/kg narrows variation in pellet crystallinity entering the SSP reactor. Batch-to-batch shifts in PTA particle size distribution change slurry viscosity in the paste mixing vessel. The Mailiao PTA D50 band of 100–150 µm allows reproducible slurry feed without excessive pump wear. Melt-phase carboxyl end-group concentration is typically maintained below 30 mmol/kg before SSP. This boundary prevents excessive acetic acid and acetaldehyde regeneration during preform reheating.

Does High Ash in PTA Affect Die-Line Haze in Thin-Gauge Polyester Film?

Biaxially oriented polyethylene terephthalate film production begins with extrusion of melt-phase resin at 275–285 °C through a slot die onto a chilled casting drum held at 30–40 °C. The sheet is then stretched in the machine direction at 75–95 °C with a draw ratio of 3.0–4.0 and in the transverse direction at 90–120 °C with a draw ratio of 3.0–4.0. Heat setting at 220–240 °C locks in dimensional stability. For thin capacitor and packaging grades, optical haze is measured by ASTM D1003 and must remain below 2%. Inorganic residues from PTA, especially ash above 6 mg/kg, contribute to gel formation at the die lip and die-line haze. The die-line deposits periodically slough into the cast sheet, producing optical defects that cannot be corrected downstream.

High ash in Mailiao PTA affects melt filtration downstream of the extruder. Continuous screen changers are set to 20–40 µm nominal retention. A pressure differential above 80 bar signals premature screen blinding from coarse ash or agglomerated additive. b* colour above 1.0 shifts film tint away from neutral and forces higher addition of optical brighteners. Capacitor-grade film requires low dielectric loss and high breakdown strength. Particulate contamination from PTA reduces film purity and increases electrical defect density. Electrostatic pinning is applied to the cast web to maintain chill-roll contact. Variation in oligomer content from PTA impurities shifts the pinning voltage required to stabilise the film at high casting speed.

Raw PTA moisture must be held below 0.40 wt%. Moisture from silo transfer enters the extruder feed throat and hydrolyses the melt during residence at 275–285 °C. The resulting drop in intrinsic viscosity below 0.60 dL/g lowers tensile strength and causes film breaks in the orientation zone. Tensile properties of the oriented film are validated by ASTM D882. Thermal shrinkage is tested at 150 °C for 30 min according to ASTM D1204. Film producers therefore set a maximum residual moisture limit and use desiccant dryers when ambient relative humidity exceeds 60%. The Mailiao PTA moisture band supports stable extrusion without requiring secondary drying in lines with closed silo transfer.

For rigid polyurethane and polyisocyanurate foam formulations, PTA from the Mailiao plant can be used as the aromatic monomer in polyester polyol production. A typical cook with diethylene glycol and phthalic anhydride is run at 200–230 °C until the acid number falls below 2 mg KOH/g. The hydroxyl value is controlled between 150 mg KOH/g and 350 mg KOH/g depending on the target foam cell structure. Viscosity at 25 °C ranges from 2000 mPa·s to 8000 mPa·s. PTA raises the aromatic content of the polyol, which increases char yield and improves compressive strength in PIR board. Reactivity with polymeric MDI is influenced by the ratio of primary to secondary hydroxyl groups. If the PTA charge is not fully digested, residual terephthalic acid crystals act as filler and raise viscosity.

Hydroxyl number is measured by ASTM D4274. Acid number is measured by ASTM D4662. Water content in the polyol is held below 0.10 wt% by ASTM E203 to prevent side reactions with isocyanate. The production vessel is a glass-lined or stainless steel reactor with an overhead partial condenser and a Dean-Stark trap for water removal. Finished polyols are filtered through 25 µm bag filters before loading. PTA purity affects polyol colour because metal residues from crude oxidation can promote darkening during storage and foam processing. Coarse PTA fractions require longer digestion or finer milling before the esterification step. Aromatic polyester polyols made from PTA are compatible with pentane and hydrofluoroolefin blowing agents. Boardstock lamination lines using these polyols require stable viscosity to maintain uniform fibre wet-out and foam rise profiles.

When 1,4-Butanediol Is Esterified for Polybutylene Terephthalate Moulding Grades

Polybutylene terephthalate polymerisation from PTA and 1,4-butanediol is conducted in two stages. In the esterification stage, the Mailiao PTA is slurried with 1.4–1.8 mol of 1,4-butanediol per mole PTA because the diol dehydrates to tetrahydrofuran under acid catalysis. Esterification is maintained at 190–230 °C. The overhead system recovers tetrahydrofuran, water, and butanediol. Titanium tetrabutoxide at 50–150 mg/kg Ti is used as polycondensation catalyst. The second stage is run at 245–260 °C under vacuum below 1 mbar. Intrinsic viscosity for injection moulding grades is brought to 0.80–1.10 dL/g, measured by ISO 1628-5 or ASTM D4603.

PBT granules from this route require pre-drying at 110–130 °C for 4–6 h to reduce moisture below 0.02%. Hydrolysis in the injection barrel causes a reduction in molecular weight. Moulding is performed at melt temperatures of 240–260 °C and mould temperatures of 60–90 °C. The material is used in electrical connectors and automotive brackets. Tensile strength is measured by ISO 527-2, flexural modulus by ISO 178, and flame retardance by UL 94 V-0 for glass-filled grades. PTA purity influences the colour of PBT. Low b* PTA allows natural and pale-colour moulding grades without excessive TiO2 masking.

Glass-filled PBT compounds are produced on co-rotating twin-screw extruders with L/D ratios from 40:1 to 48:1. Side-fed glass fibre at 20–30 wt% is dispersed under high shear. Melt temperature is limited to 270 °C to avoid butanediol degradation and fumaric acid formation. Free 1,4-butanediol remaining from incomplete esterification volatilises at the die and condenses on the cooling strand. A vacuum vent at -0.8 bar gauge is maintained in the second decompression zone to strip residual tetrahydrofuran. Moisture exposure above 0.02% in the feed hopper leads to bubble formation in moulded parts. PTA ash and fine particles are filtered through melt screens upstream of the die to protect dimensional tolerances in small connectors.

Polytrimethylene Terephthalate Spinning Crystallinity and Crimp Stability

PTT is produced from Mailiao PTA and 1,3-propanediol in a direct esterification and polycondensation sequence. The mole ratio is held between 1.10 and 1.30 diol:PTA. Esterification at 225–245 °C is followed by polycondensation at 250–270 °C with a titanium catalyst such as tetrabutyl titanate at 30–100 mg/kg Ti. The target intrinsic viscosity for fibre extrusion is 0.80–1.00 dL/g. PTT crystallises more slowly than PET and has a melting point near 228 °C. The PTA purity requirements for PTT are similarly stringent because residual 4-CBA and p-toluic acid terminate growing chains and broaden the molecular weight distribution.

PTT fibres are used in carpet and apparel applications requiring elastic recovery. Spinning temperatures are typically 245–260 °C at the die. Quench air is set to 16–20 °C at 60–70% RH. Draw ratios are lower than PET, commonly 1.5–2.5, because PTT crystallinity develops rapidly under stress. Crimp stability after heat setting at 150–170 °C is evaluated by measuring crimp take-up. PTA ash above 6 mg/kg shortens spinneret life and raises pack pressure in PTT spin lines. Moisture in PTT pellets is held below 0.02% and measured by ASTM E203. Tensile properties of oriented yarn are tested by ASTM D2256. Cross-section uniformity in trilobal carpet yarn is monitored because PTT melt rheology responds strongly to small changes in carboxyl end-group concentration.