Paraxylene-derived bottle grade polyethylene terephthalate is traditionally produced by oxidation of paraxylene to purified terephthalic acid, direct esterification or esterification with monoethylene glycol, and continuous melt polycondensation under vacuum at 270–285 °C in the presence of antimony, titanium, or germanium catalysts. Melt-phase polymerisation is terminated at an intrinsic viscosity of 0.60–0.65 dL/g because higher molecular weight raises melt viscosity into a regime where agitator torque, reactor heat transfer, and vapour removal become limiting and where thermal degradation generates acetaldehyde and vinyl ester end groups. Solid state polymerisation subsequently increases intrinsic viscosity to 0.80–0.86 dL/g by heating crystallised pellets below their melting point under vacuum or nitrogen flow, removing ethylene glycol and water as condensation by-products while retaining the solid morphology necessary for pneumatic conveying, drying, and injection moulding of preforms. Intrinsic viscosity is measured according to ASTM D4603-18 by dissolving PET in a 60:40 phenol/1,1,2,2-tetrachloroethane solvent at 30 °C and calculating the limiting viscosity number with the Billmeyer relation; the same property is reported as viscosity number under ISO 1628-5:1998 with minor procedural differences. Industrial bottle resin specifications also impose limits on acetaldehyde, colour, diethylene glycol, and carboxylic end groups because these parameters influence preform processing and packaged beverage sensory quality.

During the transition from melt-phase pelletisation to solid state polymerisation, amorphous pellets are first subjected to controlled crystallisation at 120–180 °C to achieve a crystalline fraction sufficient for preventing particle adhesion and collapse in the SSP reactor. If the pellets bypass this step or crystallise too slowly, agglomeration occurs in the SSP vessel at temperatures above 200 °C, creating lumps that disrupt gravity flow and nitrogen distribution. The SSP reaction itself is a heterogeneous process in which end-group diffusion through amorphous domains, by-product diffusion through crystalline and amorphous regions, and chemical esterification/transesterification kinetics are coupled; at lower temperatures the rate is controlled by by-product diffusion, while at higher temperatures chemical reaction and degradation compete. Published engineering design data for continuous SSP processes indicate that a bed temperature spread of ±2 °C is maintained across heating zones because the window between acceptable reaction rate and pellet sticking is narrow.

Why Is Solid State Polymerisation Required After Melt Phase Polycondensation?

Melt-phase polycondensation of bis(2-hydroxyethyl) terephthalate or terephthalic acid/ethylene glycol oligomers is an equilibrium-limited esterification-transesterification sequence. Removing ethylene glycol from a highly viscous melt at 270–285 °C requires surface renewal and vacuum levels below 1.0 mbar in finishing reactors; as intrinsic viscosity rises from 0.45 dL/g to 0.65 dL/g, the melt index decreases to values below 10 g/10 min at 285 °C under 2.16 kg load, measured by ISO 1133-1:2022, and agitator power becomes non-linear with viscosity. Further polycondensation in the melt also accelerates thermal degradation through beta-scission and acetaldehyde formation; acetaldehyde levels may exceed 100 µg/g in melt pellets if residence time is not tightly managed. Solid state polymerisation circumvents this by operating at 200–240 °C, below the crystalline melting point of 245–255 °C, so molecular mobility is restricted to amorphous regions and the solid pellet retains its geometry while by-products are removed by gas-phase mass transfer. Because the by-products ethylene glycol and water must migrate through a semicrystalline matrix, pellet size, crystallinity, and nitrogen dew point become process variables. Industrial SSP lines for bottle resin typically accept amorphous pellets of 0.60–0.65 dL/g and deliver bottle-grade product at 0.80–0.86 dL/g; the exact target depends on bottle category, preform wall thickness, and blow moulding orientation.

When Carbonated Soft Drink Preforms Demand Elevated IV and Reduced Acetaldehyde

Carbonated soft drink preforms are injection moulded from SSP PET and then stretch blow moulded into containers that must sustain internal pressure, top load, and environmental stress cracking. The hoop stress in a pressurised bottle scales with diameter and pressure and inversely with wall thickness; higher intrinsic viscosity increases molecular weight and orientation-induced strengthening, and is therefore specified at 0.82–0.86 dL/g for many carbonated soft drink bottle resins. Still water bottles, which require lower pressure resistance but high clarity and light weight, are often produced at 0.78–0.82 dL/g to reduce melt viscosity and preform injection pressures. Acetaldehyde in the bottle is limited because it migrates into the beverage and affects flavour; preform acetaldehyde measured by headspace gas chromatography according to ASTM D4526 is commonly controlled below 5 µg/g, while bottle wall acetaldehyde limits may be lower. The reduction in acetaldehyde during SSP occurs because the nitrogen purge strips acetaldehyde from pellets and because the aldehyde-generating vinyl ester end groups are partially consumed or passivated. However, acetaldehyde can reform during injection moulding if barrel residence time exceeds 5 min, barrel temperatures exceed 285 °C, or moisture is not reduced below 30 ppm; for this reason SSP pellets are dried at 160–180 °C for 4–6 h in desiccant dryers with a dew point of -40 °C before processing.

Across continuous SSP reactor systems, hopper-fed crystallised pellets pass through a preheater, a reactor, and a cooler under a counter-current nitrogen flow with oxygen content below 10 ppm by volume and dew point below -40 °C. The nitrogen serves both as heat transfer medium and as carrier for ethylene glycol, water, acetaldehyde, and low-molecular-weight esters. Pellet residence time in the reactor is adjusted by rotary discharge valves and bed level sensors; typical total SSP residence times for bottle PET are 8–20 h, with the final 4–8 h contributing more to acetaldehyde stripping than to intrinsic viscosity increase. The pellets enter the reactor at 190–205 °C and leave at 225–240 °C; the upper limit is set by sticking tendency, which increases if the copolymer comonomer content is high or if the pellet surface contains oligomers. Reactor vessels are constructed of stainless steel and use internal flow distributors to avoid channelling; bed pressure drop is monitored to detect agglomeration. Scale-up from batch vacuum tumble dryers to continuous nitrogen-swept vessels introduces residence time distribution broadening, which can be quantified by tracer pellets and modelled with axial dispersion; published data for specific packet configurations is limited, but plant observations generally correlate poor pellet crystallinity and high reactor temperature with lump formation and production loss.

Thermal Oxidation of Paraxylene to Purified Terephthalic Acid and Its SSP-Relevant Trace Components

Paraxylene is oxidised to crude terephthalic acid in acetic acid using air and a cobalt-manganese-bromide catalyst system at 175–225 °C and 1.5–3.0 MPa; the intermediate 4-carboxybenzaldehyde (4-CBA) is hydrogenated to p-toluic acid during purification because residual 4-CBA acts as a chain stopper and colour precursor in PET. Industrial purified terephthalic acid specifications relevant to SSP include 4-CBA at or below 25 mg/kg, p-toluic acid at or below 150 mg/kg, ash below 15 mg/kg, and b-value below 1.0; cobalt and manganese residues are controlled to avoid accelerated oxidative degradation during SSP and preform moulding. Trace levels of p-toluic acid and monofunctional impurities lower the attainable molecular weight because they terminate chain growth during polycondensation; their influence becomes more visible after SSP when target intrinsic viscosity values exceed 0.82 dL/g. Paraxylene feedstock with high content of ortho- or meta-xylene isomers can generate isophthalic or orthophthalic co-monomers that broaden the processing window but reduce crystallinity and may require lower SSP temperatures to avoid sticking. Thus the SSP rate of a given PET is not solely a function of reactor temperature and time; it is also conditioned by oxidation catalyst residues, organic impurities, comonomer type, and the thermal history of the melt-phase pellet.

Controlling Particulate Agglomeration and Preheating in Continuous SSP Reactors

Preventing agglomeration in continuous SSP begins in the crystalliser, where amorphous pellets are heated above the glass transition temperature of 78 °C and agitated at 120–180 °C to induce cold crystallisation. Without sufficient crystallinity, pellets heated above 200 °C soften and bind into fused clusters; this is particularly observed for high-diethylene glycol copolymers and for pellets containing residual ethylene glycol. Crystalliser designs include stirred vessels, vibrating spiral elevators, and fluidised beds; the fluidised bed offers high heat transfer but requires particle size uniformity to avoid elutriation. After crystallisation, pellets enter a preheater where they are brought to within 20–40 °C of the reactor setpoint; rapid heating can induce surface melting because the amorphous portions of the pellet soften before the crystalline network can stabilise the geometry. The SSP reactor itself often operates as a moving bed with an L/D ratio selected to provide plug flow; bed heights are maintained by level sensors, and rotary valves at the discharge control the average residence time. Nitrogen is introduced at the reactor bottom and withdrawn at the top; oxygen ingress through flanges and rotary valves is a common field problem that oxidises pellet surfaces and increases colour. On production-scale continuous SSP lines, differential pressure transmitters across the bed and outlet gas analyzers for oxygen, water, and acetaldehyde are used to detect and control deviations before agglomeration causes blockages.

During injection moulding of SSP PET into preforms on reciprocating-screw equipment with 24:1 or 25:1 L/D high-compression screws, barrel temperature profiles are typically set between 260 °C and 285 °C, and the melt is injected into multi-cavity tools with hot runner valve gates. Higher intrinsic viscosity increases melt pressure and shear heating; thus preform moulders sometimes raise barrel temperatures or reduce screw speed, but excessive thermal energy regenerates acetaldehyde and lowers intrinsic viscosity through hydrolysis if drying is inadequate. Mold cooling water temperature is held at 8–12 °C to freeze the preform gate and wall, while cycle times for 28 mm neck finishes and 12–18 g preforms may range from 10 s to 18 s depending on cavitation and machine clamp force. Injection moulding machine clamp force is determined by cavity pressure and projected area; a 96-cavity preform tool may require a clamp force above 4000 kN, whereas smaller 32-cavity systems operate around 1500 kN. The use of SSP resin with narrow intrinsic viscosity distribution reduces cavity-to-cavity variation in preform weight and acetaldehyde; production-scale measurements of preform weight variation are often held to ±0.5 % by adjusting hot runner nozzle temperatures and valve gate timing.