Which PET Plastic for Which Job: Bottle, Film, Fibre or Sheet Grade

Polyethylene terephthalate is not a single melt-processing polymer; it is a family of grades differentiated before pelletisation by intrinsic viscosity, diethylene glycol content, carboxyl end group concentration, copolymer content, and solid-state polymerisation history. Intrinsic viscosity measured in accordance with ASTM D4603-18 or ISO 1628-1:2022 remains the primary grade-selection parameter because it correlates with melt strength, stretch ratio, and final tensile orientation. Density of the amorphous pellet is determined by ASTM D792-20; melt flow rate by ISO 1133-1:2022; and thermal crystallisation behaviour by differential scanning calorimetry under ISO 11357-3:2018. A resin with an intrinsic viscosity of 0.82 dL/g that performs in a carbonated soft-drink preform will not transfer unchanged to a biaxially oriented PET film line because extruder die pressure and melt temperature influence gel formation. Likewise, a textile staple fibre grade at 0.60 dL/g produces a preform with insufficient sidewall mechanical strength. Grade selection is therefore a converter-side decision based on the processing route and the final article thickness.

In carbonated soft-drink and water packaging, the grade is specified by intrinsic viscosity and comonomer level rather than by a single supplier code. Preform injection moulding lines using 48- to 72-cavity hot-runner tools with clamp forces of 4500–6000 kN require a melt with sufficient viscosity to hold gate dimensions during packing but not so high that the hot-runner system generates acetaldehyde above the 2 ppm preform limit commonly used for carbonated beverages. Bottle-grade PET typically spans 0.76–0.84 dL/g, with carbonated soft-drink grades at 0.80–0.84 dL/g and water grades at 0.76–0.80 dL/g. The comonomer is isophthalic acid at 2–3 wt%, which reduces the rate of spherulitic crystallisation enough to keep the preform transparent but does not prevent strain-induced crystallisation during stretch-blow moulding.

Solid-state polymerisation is the standard route to raise melt-phase PET from an intrinsic viscosity near 0.62–0.65 dL/g to the bottle-grade range, while simultaneously stripping residual acetaldehyde to below 1 ppm in the pellet. The preform injection process then runs at barrel set temperatures of 270–295 °C, hot-runner tip temperatures of 280–290 °C, and melt residence times below 5 min. Production-scale failure data show that a residence time above 5 min at 285 °C increases acetaldehyde generation sharply when the resin has carboxyl end groups above 20 meq/kg, producing preforms that are unacceptable for beverage contact.

The injection screw geometry also affects acetaldehyde formation. Multi-cavity preform systems typically use barrier screws with L/D ratios of 20:1–24:1 and compression ratios of 2.5:1–3.0:1; the hot-runner manifold is balanced to less than ±2 °C cavity-to-cavity temperature deviation. Preform cycle times range from 12–20 s depending on wall thickness and machine clamp force. Mold cooling circuits are set at 8–12 °C to freeze the gate region rapidly and prevent spherulitic crystallisation. If the gate is too warm, the preform develops a visible crystalline zone that raises haze after blow moulding and reduces elongation under ASTM D638-14.

Stretch-blow moulding uses axial and hoop stretching to create sidewall crystallinity; total stretch ratios in the sidewall are typically 10–16 for standard carbonated soft-drink containers. Internal carbonation pressure of 3.0–4.0 bar at 25 °C imposes a hoop stress that is met by sidewall orientation, not by increased wall thickness alone. Food-contact compliance remains anchored to FDA 21 CFR 177.1630 and EU No 10/2011. The operational boundary is therefore not one dimensional: a resin with the correct IV but with excess DEG or high cyclic oligomer content will create preforms that exhibit haze, colour shifts, or mould deposits.

Film-Grade Extrusion and the Coextruded Die Gap Constraint

Biaxially oriented PET film selects a lower intrinsic viscosity grade, typically 0.58–0.65 dL/g, because the flat-die extrusion process must limit die pressure while maintaining a stable cast sheet for subsequent orientation. A film line running a die gap of 0.8–1.2 mm with a die width of 2–5 m and a chill roll temperature of 25–65 °C will show edge bead instability and die lip deposit if bottle-grade resin is substituted. The lower molecular weight of film-grade PET reduces melt viscosity at 280 °C, which is necessary to prevent gel formation and excessive oligomer volatilisation at the die lip. Film-grade formulations typically contain silica antiblock at 500–2000 ppm and low diethylene glycol content because DEG residuals lower melting point and alter haze. Haze is measured under ASTM D1003-21; tensile properties are measured by ASTM D882-18 rather than ASTM D638-14 because film thickness below 250 µm responds differently to gauge variation.

After quenching, the amorphous sheet is stretched sequentially in the machine direction at 95–115 °C to draw ratios of 2.5–3.5 and in the transverse direction at 105–125 °C to draw ratios of 3.0–4.0. Heat-setting is then conducted at 200–230 °C. The moisture limit is stricter than bottle processing; hydrolysis at 280 °C causes measurable intrinsic viscosity loss when pellet moisture exceeds 100 ppm, and film curl, flatness defects, and reduced tear strength follow. Oxygen transmission rate for 12 µm BOPET is commonly 100–150 cm³/(m²·day·bar) at 23 °C and 0% RH measured by ASTM D3985-17, which is why metallised BOPET is used for high-barrier laminates. Food-contact film grades are assessed under EU No 10/2011 and FDA 21 CFR 177.1630.

Coextruded film structures are used when the surface coefficient of friction, seal initiation, or antistatic performance must be separated from the core. In a two-layer or three-layer die, the skin layers may contain silica concentrations up to 5000 ppm while the core remains below 300 ppm to preserve optical clarity. The die gap constraint arises because total flow must remain within the die design viscosity range; changing the core to a high-IV bottle resin alters layer encapsulation defects and raises die head pressure. Because PET melt viscosity scales approximately with the 3.4 power of weight-average molecular weight, raising intrinsic viscosity from 0.60 dL/g to 0.80 dL/g generates a substantial increase in die pressure. The film grade is therefore selected by flat-die rheology, not by final film tear strength alone.

When an Intrinsic Viscosity of 0.63 dL/g Is No Longer Sufficient in Technical Filament Spinning

Textile staple fibre and nonwoven PET are melt-spun from resin with an intrinsic viscosity of 0.55–0.63 dL/g; lower molecular weight is acceptable because spun yarn is drawn to final orientation in a subsequent step. The spin pack assembly uses spinneret holes of 0.25–0.50 mm diameter and final filtration of 20–40 µm to remove agglomerated TiO₂. Delustrant addition is typically 0.3–0.5 wt% TiO₂, but the same additive creates pack pressure and spinneret wear on prolonged runs. Spinning temperatures are held at 280–295 °C, and quench air velocity in the blow box is generally 0.3–0.6 m/s for round-hole spinnerets; published data for all spinneret configurations is limited because spin pack geometry is proprietary.

When the required yarn tenacity moves into technical rope, tyre cord, or geotextile territory, the grade must change. High-tenacity technical filament is spun from solid-state PET with intrinsic viscosity 0.85–0.95 dL/g, which increases spin pack backpressure and requires melt temperatures of 290–310 °C. The high-IV melt does not behave like bottle-grade because shear heating in the spin pack can create local temperature gradients; the spinneret pack pressure may reach 250 bar for fine-hole multijet tooling. After spinning, high-tenacity yarn is drawn at draw ratios of 5.0–6.5 and heat-set at 220–240 °C; tenacity measured per ASTM D3822-20 can reach 0.80–0.90 N/tex. A textile staple grade at 0.63 dL/g cannot survive the same draw ratio and will show filament breaks and lower tenacity below the technical specification.

The process boundary in fibre manufacture is not only molecular weight. Spinneret hole count, hole geometry, filtering, and quench symmetry determine whether a given IV can be spun without melt fracture, drips, or broken filaments. A grade that runs acceptably through a 1000-hole staple spinneret may be unsuitable in a 300-hole technical filament pack because the higher molecular weight resin develops excessive spin-line stress. Conversely, a low-IV fibre grade cannot be used in a bottle preform tool because the melt lacks the extensional stress response needed for blow-moulded sidewall orientation.

Amorphous PET sheet for thermoforming uses a separate grade envelope with intrinsic viscosity 0.72–0.80 dL/g, a sheet thickness range of 0.25–1.5 mm, and a melt temperature at the die entry of 270–290 °C. Production lines may use twin-screw extruders with L/D 32:1–36:1 and vacuum venting, or single-screw barrier machines with L/D 30:1–34:1, to devolatilise residues from post-consumer rPET flake. Sheet manufacturers frequently blend 20–50 wt% washed rPET flake; the mix must be pre-dried because flake moisture above 300 ppm at extrusion temperature can hydrolyse the matrix and reduce melt viscosity. The resulting sheet is thermoformed at a surface temperature of 110–150 °C with plug-assisted forming and aluminium tooling. If the intrinsic viscosity drops below 0.70 dL/g, sheet sag increases and areal draw ratios greater than 3:1 produce local thinning at the corners of deep trays. The sheet process is distinct from film because thickness above 250 µm changes heat transfer and orientation; it is also distinct from bottle conversion because the sheet-grade resin has not been solid-state polymerised for high sidewall orientation and internal pressure resistance.