Polyethylene terephthalate resin supplied under the Yisheng trade designation is a linear aromatic polyester produced by continuous esterification of purified terephthalic acid with monoethylene glycol at a PTA:MEG molar ratio of 1.15–1.20, followed by continuous polycondensation under progressive vacuum with an antimony-based catalyst. Bottle-grade output is subsequently advanced by solid-state polymerization to increase molecular weight, reduce carboxyl end-group concentration, and strip residual acetaldehyde. Published data for Yisheng-specific grade designations is limited; therefore, property boundaries reported here are drawn from industry technical bulletins for continuous polycondensation PET assets of equivalent melt-phase intrinsic viscosity. The pelletised resin is routinely characterised by intrinsic viscosity according to ASTM D4603-21 using a 0.50 g dL⁻¹ solution in 60/40 phenol/1,1,2,2-tetrachloroethane at 30°C; bottle-grade intrinsic viscosity after solid-state polymerization is typically 0.80–0.85 dL g⁻¹, while carboxyl end-group concentration by titration is 15–30 mmol kg⁻¹ and diethylene glycol content by gas chromatography is 1.0–1.5 wt%. Residual moisture at packing is specified below 50 ppm; when moisture rises above 0.004 wt%, hydrolytic chain scission during plasticating becomes measurable as intrinsic viscosity loss. The melt volume-flow rate determined by ISO 1133-1:2022 at 285°C under 2.16 kg is 22–34 cm³/10 min for bottle-grade material, although this value is sensitive to pre-drying and thermal equilibration. The crystalline melting peak under ISO 11357-3 at 10 K min⁻¹ is 245–252°C, the glass transition temperature is 78–82°C, and the cold crystallization peak in quenched film appears between 135–150°C. These values provide the boundary conditions for injection stretch blow moulding, sheet extrusion, and fibre spinning operations.

What Limits Acetaldehyde Generation in Bottle-Grade PET during Injection Stretch Blow Moulding?

Acetaldehyde concentration in the preform is the controlling quality parameter for still water and sensitive beverage packing because free acetaldehyde migrates into the packaged liquid and contributes off-taste at threshold concentrations above 10–20 µg L⁻¹, depending on product matrix. In bottle-grade Yisheng-type PET, the pellet acetaldehyde target is ≤1.0 µg g⁻¹ when measured by headspace gas chromatography according to ASTM F2013, but thermal and shear history during plastication can raise preform acetaldehyde to 3–8 µg g⁻¹ if melt residence time exceeds 5 min at 285°C. The rate of acetaldehyde formation follows an Arrhenius dependence with an activation energy of approximately 150–170 kJ mol⁻¹ in the molten state; therefore, a barrel-temperature reduction of 5°C can decrease acetaldehyde generation by 15–25% for equivalent screw speed. Production-scale desiccant dryers with dew-point capability below -40°C are fitted with aftercoolers and four-bed molecular sieve columns; residual moisture must be held below 30 ppm for intrinsic viscosity above 0.82 dL g⁻¹ because hydrolysis regenerates hydroxyl end groups that autocatalyse further chain scission and generate acetaldehyde precursors. Hot-runner systems on 96-cavity preform tools typically operate at 280–295°C, with gate thermocouple variation held within ±2°C; increasing hot-half temperature toward 300°C reduces melt viscosity but accelerates thermal degradation and raises free acetaldehyde by 2–4 µg g⁻¹. Preform acetaldehyde is further influenced by screw design: low-shear barrier screws with 24:1–25:1 L/D and compression ratios of 2.2–2.8 produce less frictional heat than high-compression general-purpose screws, and injection moulding machines with hydraulic accumulator-driven injection achieve volumetric filling of thin-wall preforms at fill times below 0.8 s to limit residence time. The interaction of residual diethylene glycol with acetaldehyde is also relevant; diethylene glycol concentrations above 1.5 wt% increase amorphous free volume and reduce the glass transition temperature, facilitating acetaldehyde diffusion to the preform surface. Bottle-grade resin specifications therefore combine pellet acetaldehyde ≤1.0 µg g⁻¹, diethylene glycol ≤1.4 wt%, and intrinsic viscosity 0.80–0.85 dL g⁻¹ as interdependent constraints rather than independent targets.

The carbonated soft drink preform is injection-moulded from bottle-grade material with an intrinsic viscosity high enough to support pressure resistance at carbonation levels of 4.0–4.5 vol CO₂; the preform is then reheated and blown at a stretch ratio of 2.2–2.5 axial and 2.8–3.3 hoop, producing strain-induced crystallinity up to 25–30% in the sidewall when measured by density gradient or wide-angle X-ray scattering. Drying is performed at 160–170°C for 4–6 h in a desiccant dryer with an inlet air dew point below -40°C; maintenance of a moisture content below 30 ppm prevents intrinsic viscosity loss greater than 0.02 dL g⁻¹ during plastication. Barrel temperature profiles on production machines are set with the feed zone at 260–270°C, compression at 270–285°C, metering at 275–285°C, and injection nozzle at 280–290°C; melt temperature measured by an immersion pyrometer should not exceed 295°C for water-grade resin to hold preform acetaldehyde below 5 µg g⁻¹. The injection moulding machine clamp force for 96-cavity preform moulds is specified at 300–450 t, with mould cooling at 8–12°C and core cooling at 6–10°C to freeze the gate sufficiently before ejection. Preform design controls the critical thickness distribution: neck finish specifications follow PCO 1881 or PCO 1810 geometries, and gate vestige height is maintained below 0.25 mm to prevent stress concentration at the injection point. Batch release testing on preforms typically includes top-load strength, burst pressure, acetaldehyde, and drop impact at 5°C; top-load strength of 300–450 N at 23°C is typical for 30 g preforms after blowing into 500 mL containers. Injection stretch blow moulding machines with a five-station rotary configuration reheat preforms to a surface temperature of 105–115°C measured by pyrometer; overheating above 120°C causes pearlescence and spherulitic crystallization, while underheating below 100°C produces whisker haze and insufficient hoop orientation. Published data for Yisheng-specific preform performance is limited, but the above processing envelope is representative of bottle-grade PET of equivalent intrinsic viscosity and pellet acetaldehyde.

Thermal Degradation Pathways in Continuous Polycondensation and Solid-State Polymerization

The primary non-oxidative degradation routes in PET are thermal homolysis, β-scission of the hydroxyethyl ester end group, and hydrolysis; the first produces carboxyl end groups and vinyl ester species, the second yields acetaldehyde and a carboxyl end group, and the third requires water and regenerates two hydroxyl-carboxyl pairs from one ester linkage. During continuous polycondensation, the melt polymerization stage is conducted at 270–285°C under progressive vacuum from 100 kPa to 0.1–0.3 kPa, while catalyst, stabilizer, and batching of monoethylene glycol control side reactions. Solid-state polymerization follows at 205–220°C in a nitrogen-purged continuous reactor with bed residence times of 12–24 h; the solid-state reactor shifts the equilibrium toward chain extension because free condensation water is removed and the temperature remains below the crystalline melting onset. The typical progression is shown in Table 1.

Batch-to-batch control on continuous solid-state polymerization lines is achieved by in-line melt viscometry in the melt phase and near-infrared measurement of carboxyl end-group concentration in the solid phase; excursions greater than ±0.010 dL g⁻¹ from target trigger automatic adjustment of bed depth and nitrogen flow. The activation energy for acetaldehyde formation during melt processing is 150–170 kJ mol⁻¹, while the activation energy for solid-state polycondensation is reported between 120 and 140 kJ mol⁻¹; the lower barrier in the solid state reflects the dominance of end-group diffusion and condensation rather than macromolecular mobility. Degradation during plastication is minimised by maintaining melt temperature below 295°C and residence time below 4–6 min; at temperatures above 310°C, acetaldehyde generation accelerates and gel-like crosslinks can form from vinyl ester addition, causing black specks and pressure fluctuations in the melt filter. Oxygen from inadequately purged extrusion hoppers or leaking vacuum seals accelerates thermo-oxidative degradation, producing quinoid chromophores and yellowing; the b* colour coordinate measured by ASTM D6290 on pellets may increase from -1.0 to +4.0 when oxygen exposure exceeds 10 ppm during high-temperature processing. These degradation pathways define the operational boundaries for injection moulding, sheet extrusion, and film-grade applications.

Sheet extrusion of Yisheng PET resin for amorphous transparent food trays requires a single-screw extruder with 30:1 L/D, a barrier screw, melt pump, and vacuum vent, while drying below 30 ppm moisture at 160°C for 4–6 h is necessary to prevent intrinsic viscosity loss, bubble defects, and increased crystallization haze.

When Post-Consumer PET Flake Replaces Virgin Resin in Extrusion-Grade Formulations

Replacement of virgin Yisheng PET resin with post-consumer bottle flake in extrusion-grade formulations introduces a series of competing constraints. The flake after hot-wash and caustic cleaning typically retains a bulk density of 0.30–0.40 g cm⁻³ before decontamination, and the intrinsic viscosity of clear bottle flake is commonly 0.70–0.80 dL g⁻¹, already 0.03–0.08 dL g⁻¹ below the bottle-grade virgin precursor because of accumulated hydrolytic degradation during consumer use and washing. In a vacuum-vented twin-screw extruder with 40:1 L/D, the melt-phase intrinsic viscosity loss in undried flake can exceed 0.10 dL g⁻¹; therefore, crystallized flake is dried at 150–170°C for 6–8 h and processed at melt temperatures below 285°C with a vacuum vent of ≤0.05 kPa to strip residual acetaldehyde and water. A melt filtration system is necessary: 20–40 µm depth filtration removes aluminium and paper fines, while 50–60 µm continuous screen-changers are used for sheet; gel counts are specified below 10 particles m⁻² larger than 0.5 mm in the cast sheet to avoid pinholes during thermoforming. The food-contact suitability of rPET is evaluated against FDA 21 CFR 177.1630 and the applicable food-contact notification, or in the European market against Regulation (EU) 10/2011 Annex I with a specific migration limit for antimony of 40 µg kg⁻¹ simulant. Because Yisheng supply-chain-specific flake quality data is limited, feedstock evaluation protocols must separately measure polyvinyl chloride contamination below 50 ppm, polyolefin content below 100 ppm, and paper label residue below 20 ppm before blending; PVC above 50 ppm generates hydrochloric acid during extrusion and accelerates iron-catalysed degradation, causing black specks and odour.

For food-contact bottle-grade applications, compliance is established under 21 CFR 177.1630 in the United States and Regulation (EU) 10/2011 in the European Union; the petitioned specification generally limits total non-volatile extractives to 0.5 mg in⁻² under specified food simulants and temperature conditions. The global compliance matrix for Yisheng-type PET resin in packaging typically includes the entries in Table 2.

The antimony catalyst residue in continuous-process PET is typically 150–250 mg kg⁻¹; migration testing according to EN 1186-1 and EN 13130 is required because antimony trioxide is classified under Regulation (EC) No 1272/2008 as a suspected carcinogen by inhalation. In practice, the specific migration of antimony from bottle wall into 3% acetic acid at 40°C/10 days is below 1–5 µg kg⁻¹ for well-polymerised PET, well below the EU specific migration limit of 40 µg kg⁻¹. For non-food industrial applications, REACH registration under EC 1907/2006 requires substance identity and the absence of substances of very high concern above 0.1 wt%; RoHS 2011/65/EU applies to electrical and electronic applications and imposes maximum concentration values of 1000 mg kg⁻¹ for lead, 100 mg kg⁻¹ for cadmium, and 1000 mg kg⁻¹ for mercury and hexavalent chromium in homogeneous materials. Yisheng PET resin does not contain halogenated flame retardants by design, but halogen content must be verified by IEC 61249-2-21 where electronics-grade flame-retardant claims are made.

Linear Film Grade Melt Rheology and Extrusion Die Design

Extrusion coating and cast film operations require a narrower molecular weight distribution and lower melt temperature sensitivity than bottle-grade resins because draw resonance and neck-in are governed by extensional viscosity and apparent shear viscosity at film die temperatures of 275–285°C. The melt volume-flow rate measured at 285°C/2.16 kg is 28–38 cm³/10 min for film-grade material, and the apparent shear viscosity at 100 s⁻¹ is 150–250 Pa s in the same temperature range; at 1000 s⁻¹, the viscosity falls to 30–60 Pa s, indicating highly shear-thinning behaviour with a power-law index of 0.5–0.7. Die design must account for high normal stresses: coat-hanger manifolds are designed to deliver less than 5% variation in transverse melt delivery, and lip gaps are maintained at 0.4–0.8 mm for cast film thicknesses of 12–50 µm; neck-in from a 1000 mm die at a 100 mm air gap may reach 80–120 mm per side for low-viscosity PET, requiring edge trim and pinning systems at the chill roll. The chill roll surface is held at 15–25°C to quench amorphous film, and electrostatic edge pinning or vacuum boxes are required to secure the web at speeds above 150 m min⁻¹. For gauge control, an automated die with thermal bolt spacings of 25 mm and beta-ray gauge measurement of thickness maintains 2 σ gauge variation below ±1.5%. Draw resonance is suppressed by keeping draw ratios below 10:1 or by increasing melt temperature; however, increases above 290°C raise acetaldehyde and degrade optical clarity. Published data for Yisheng-specific film-grade rheology is limited; the processing boundaries above align with general-purpose film-grade PET technical bulletins.