Polyethylene terephthalate (PET) is a semicrystalline thermoplastic polyester obtained by melt polycondensation of monoethylene glycol with terephthalic acid or dimethyl terephthalate. The repeating unit is [-OCH₂CH₂OCOC₆H₄CO-]_n, and the resin is supplied in amorphous chip, crystallised chip, film, sheet, and fibre forms. PET’s Historical Trade Names: Dacron, Terylene, Trelene and Mylar are product-form designations encountered in early polyester patents, textile specifications, and film procurement documents; they are not distinct polymer chemistries but identify specific fibre, filament, and biaxially oriented film product lines built on the same backbone. Intrinsic viscosity, measured according to ISO 1628-5:2015 or ASTM D4603-18, is the primary molecular-weight proxy used to separate these grades.

Industrial production of PET proceeds via esterification of terephthalic acid with ethylene glycol at 250–270 °C, followed by polycondensation at 275–290 °C under vacuum below 1 mbar with an antimony or titanium catalyst. Phosphorus-based stabilisers are added to control catalyst activity, and the polymer is pelletised under nitrogen. This process yields amorphous chip with intrinsic viscosity near 0.60–0.65 dL/g before downstream solid-state polymerisation. The dimethyl terephthalate route, used in older Dacron and Terylene fibre lines, employs transesterification with methanol recovery and requires ester-interchange catalysts; modern PTA-based continuous plants predominate for bottle-grade resin.

What Distinguishes Dacron Fibre from Mylar Film at the Molecular Level?

Fibre and film grades differ principally in average molar mass, diethylene glycol (DEG) content, and orientation state. Dacron-type staple fibre and Terylene-type filament are typically produced from resins with intrinsic viscosity in the range 0.55–0.65 dL/g for apparel, while high-tenacity technical yarn can require 0.85–1.00 dL/g. Mylar-type biaxially oriented film is produced from resin with intrinsic viscosity near 0.58–0.68 dL/g and is stretched sequentially or simultaneously above the glass transition temperature. The key structural differentiator is not composition but morphology: fibre spinning relies on uniaxial drawing at draw ratios of 3:1 to 6:1, whereas film orientation balances machine-direction and transverse-direction draw ratios.

Continuous filament spinning of PET for Dacron-type yarns is performed on spin beams operating at 290–300 °C melt temperature and winding speeds of 2500–5000 m/min. Draw ratios of 1.4:1 to 1.8:1 in partially oriented yarn are common, while fully drawn yarns may use 3:1 to 6:1 total draw. Spin-pack filtration media are specified with absolute retention of 20–40 µm to remove gels and catalyst residues; carboxyl end-group content above 30 mmol/kg is associated with shorter pack life, filament breaks, and spin finish deposition problems. Dacron and Terylene trademarks historically signified licensed process formats, whereas Trelene appeared in certain European staple-fibre specifications; these names do not indicate a different intrinsic viscosity range or crystallinity limit.

Diethylene glycol (DEG) is formed as a side product during esterification and acts as a comonomer that disrupts crystallinity. Fibre grades may tolerate 1.0–1.5 mol% DEG to regulate dye uptake and tensile elongation, but bottle-grade preform resin is often controlled to 0.7–1.2 mol% because excess DEG reduces thermal stability and increases oxygen permeability. A shift of 0.5 mol% DEG produces a measurable depression in differential scanning calorimetry melting point under ASTM D3418-21, narrowing the reheat-stretch blow-moulding window. For film-grade resins used in Mylar-type structures, DEG is kept at the lower end because biaxial orientation already lowers crystalline order; excessive comonomer produces haze and dimensional instability during heat setting.

The principal thermal transitions under ASTM D3418-21 are a glass transition near 78 °C, a cold-crystallisation exotherm from 130–150 °C, and a melting endotherm from 250–260 °C. These transitions are modulated by DEG and orientation; preforms are heated above the glass transition but below the cold-crystallisation onset for stretch blow moulding.

Amorphous PET chip entering a melt-processing line at a residual moisture above 50 ppm hydrolyses in the extruder or injection barrel before melting is complete. Hopper-dryer conditions of 160–180 °C for 4–6 h with air dew point ≤ −40 °C are standard for resin initially below 0.25% moisture. Crystallised chip may require slightly higher temperatures, but exceeding 190 °C risks stickiness and bridging in the dryer cone. The measured moisture before processing is commonly checked by Karl Fischer titration according to ISO 15512:2019; shipment of undried resin directly to a twin-screw extruder with closed venting is not considered acceptable for intrinsic viscosity-sensitive bottle or film lines.

Solid-State Polymerisation Kinetics and the Carboxyl End-Group Ceiling

Solid-state polymerisation (SSP) is used to raise the intrinsic viscosity of amorphous melt-phase resin from 0.60 dL/g to bottle-grade or technical-yarn values while simultaneously reducing residual acetaldehyde. Batch vacuum tumble dryers and continuous nitrogen-fluidised-bed SSP reactors operate at 205–220 °C, below the melt temperature but above the onset of chain mobility. Under vacuum of 1–10 mbar or dry nitrogen, polycondensation and esterification reactions continue; diffusion of ethylene glycol and water from the chip controls the rate. The reaction is terminated or slowed when the target intrinsic viscosity is reached because excessive carboxyl end-group formation creates melt-processing instability. Bottle-grade resins are frequently specified with carboxyl end groups below 25 mmol/kg by ASTM D7409-15. The SSP process also reduces residual acetaldehyde from melt-phase values of 50–150 µg/g to below 1 µg/g in dried chip, depending on temperature and residence time. Extended SSP at high temperature can raise intrinsic viscosity beyond 0.90 dL/g but also shifts molar mass distribution and increases the risk of gel particles during subsequent injection moulding.

Reheat stretch blow moulding of bottle preforms requires a surface temperature of 100–110 °C measured by pyrometry. Axial stretch ratios of 2.5:1 to 3.0:1 and hoop stretch ratios of 3.0:1 to 3.5:1 produce a total planar stretch ratio of 8:1 to 12:1. Preform temperature homogeneity must be controlled within ±2 °C because warmer bands yield pearlescent sidewall crystallisation and colder bands produce stress-whitened necks. Blowing pressures are typically 25–40 bar. The same molecular weight that gives 0.80 dL/g bottle-grade intrinsic viscosity also raises melt viscosity and preform injection pressure; only the solid-state route decouples melt-phase viscosity from final bottle strength.

When Acetaldehyde Regeneration Exceeds 8 ppm in Bottle Preforms

Acetaldehyde (AA) is produced by thermal degradation of PET and is a sensory contaminant in water and carbonated soft drinks. A widely applied preform ceiling is 8 µg/g measured by static headspace gas chromatography according to ASTM F2013-10; mineral water and sensitive fruit-juice applications may impose an internal specification of ≤ 4 µg/g. AA regeneration accelerates above 270 °C, meaning preform injection moulders must balance melt viscosity, screw recovery time, and barrel residence time against acetaldehyde generation. When moulding a 28 mm PET water-bottle preform, hot runner temperatures of 280–295 °C and total residence times below 2 min are typical, but hot runner dead spots and periodic machine stoppages can produce local concentrations above the bulk value. Low-acetaldehyde bottle resins are produced by melt-phase process modifications and solid-state finishing rather than by neutralising chemistry alone; a preform specification of 8 µg/g is therefore an integrated pellet, drying, screw, and hot-runner outcome. Acetaldehyde scavengers are used when the forming envelope is tight, but they do not eliminate the need for low-shear screw geometry and valve-gated hot runners.

On a production injection-moulding machine, the preform process uses a reciprocating screw with an L/D of 25:1 to 32:1, a shut-off nozzle, and valve-gated hot runner. Barrel temperatures are profiled from 250 °C at the feed zone to 280–295 °C at the metering zone; melt temperature is usually measured by an infrared probe at nozzle exit. Injection pressures of 800–1200 bar and clamp force of 150–300 t are typical for multi-cavity water-bottle preforms, although the exact values depend on cavity count, preform weight, and runner balance. Cooling time of 8–15 s is common for a 24 g water-bottle preform; insufficient cooling produces gate crystallinity and dimensional ovality after demoulding.

Controlling Intrinsic Viscosity Loss During Melt Processing

Melt processing degrades PET by hydrolytic, thermal, and thermo-oxidative mechanisms. Hydrolysis is the dominant route below 300 °C, and the rate is controlled by drying rather than by antioxidant packages. Thermal decomposition becomes measurable at melt temperatures above 290 °C; acetaldehyde, carbon dioxide, and vinyl ester species are formed. Thermo-oxidative degradation at barrel surfaces can be limited by low oxygen ingress, but hopper-dryer air locks and nitrogen blanketing of the feed hopper are practical countermeasures. Compounding extruders used for PET engineering grades are typically co-rotating twin-screw machines with 32:1 to 48:1 L/D and vacuum venting at −0.08 MPa to remove moisture, acetaldehyde, and low-molecular-weight volatiles. Intrinsic viscosity loss of 0.02–0.05 dL/g during compounding is considered acceptable for non-bottle applications; for bottle preforms, melt-path residence above 4 min at 295 °C may reduce intrinsic viscosity below 0.72 dL/g and compromise top-load performance. Melt-volume-flow rate is measured according to ISO 1133-1:2022, while melting and crystallisation transitions are compared by ASTM D3418-21 differential scanning calorimetry.

Nucleating agents in C-PET sheet and bottles are used to alter crystallisation kinetics. Talc at 0.1–0.3 wt% reduces the isothermal crystallisation half-time, as measured by ASTM D3418-21, and shifts the cold-crystallisation peak. Without nucleant, PET crystallises slowly; this is why amorphous bottle preforms remain clear after rapid cooling. Excessive nucleant causes mould deposits and reduces notched impact resistance.

Barrier properties of oriented PET are governed by crystallinity and free volume. Oxygen permeability of biaxially oriented film is typically 0.04–0.06 cm³·mm·m^-2·day^-1·atm^-1 at 23 °C and 0% RH. Carbon dioxide permeability is approximately 3:1 to 4:1 times the oxygen permeability. Orientation increases barrier by reducing free volume, while DEG and plasticising migrants raise permeability. This is why bottle walls and film webs are specified by both intrinsic viscosity and DEG, and why compliance testing must be conducted on finished articles rather than on resin alone.

Food-contact compliance for PET is defined by national and regional positive lists. Specific migration testing under Commission Regulation (EU) No 10/2011 uses food simulants such as 3% w/v acetic acid and 10% v/v ethanol, with test conditions selected from Annex III. The table below summarises the principal entries; it is not a substitute for the full text.

For biaxially oriented film manufacture, the sequential stretching operation preheats the cast sheet to 95–110 °C for machine-direction draw, then 110–120 °C for transverse-direction draw, with draw ratios of 3:1 to 5:1. Film properties are controlled by heat setting at 180–220 °C to stabilise shrinkage. Thermoformed C-PET trays crystallise at 150–170 °C in the mould, and the final crystallinity is generally between 25% and 35%. C-PET mould residence time of 6–12 s is used for 0.5 mm sheet gauge. The sheet formulation includes a crystallisation promoter and may include talc or another inorganic nucleant at 0.1–0.3 wt%. Legacy film specifications still reference Mylar, while Dacron, Terylene and Trelene continue to appear in fibre purchase descriptions, even though the underlying intrinsic viscosity and DEG specifications are unified under the standards cited above.