PET Plastic Data Sheet Decoded: Melting Point, Density, IV and Tg Side by Side

Polyethylene terephthalate (PET) resin data sheets commonly report four reference values: melting point, density, intrinsic viscosity (IV), and glass transition temperature (Tg). The data sheet format encourages side-by-side comparison, but each value is a function of solid-state polymerization, thermal history, moisture, and morphology. Melting point measured at 10 °C/min by ISO 11357-3:2018 is not an equilibrium melting temperature; it is the endothermic peak temperature of the crystalline fraction. Density spans the amorphous limit near 1.335 g/cm³ and the crystalline unit cell limit near 1.455 g/cm³. IV measured by ASTM D4603-18 reflects the degree of polymerization and controls melt strength and extensional viscosity. Tg measured by ISO 11357-2:2020 shifts from approximately 67 °C for fast-cooled amorphous specimens to 78–80 °C for semi-crystalline structures.

Solid-state polymerization is the side-by-side link between IV and density. Amorphous PET pellets exiting melt polymerization have IV near 0.55–0.60 dL/g and density near 1.33 g/cm³. After solid-state polymerization at 200–220 °C under nitrogen or vacuum for 12–24 h, IV rises to 0.80–0.84 dL/g while pellet density increases to 1.39–1.41 g/cm³ because chain extension and lamellar crystallization occur simultaneously. This densification is why high-IV bottle-grade pellets exhibit less blocking tendency during storage than amorphous fiber-grade pellets. The solid-state process also changes the first-heat differential scanning calorimetry signal: the melting endotherm becomes sharper and may shift upward by several degrees because of lamellar thickening. The combination of IV, density, and melting point associated with solid-state polymerization cannot be generated by melt compounding alone.

Moisture in PET is a processing variable that directly couples IV, density, and Tg. At 23 °C and 50 % RH, undried pellets can absorb approximately 0.25 % water by mass. In the melt, water hydrolyzes the ester linkage, reducing chain length and generating carboxylic acid end groups. A desiccant-wheel dryer delivering air at a dew point of −40 °C is typically set to 160–170 °C for 4–6 h; the target pellet moisture is below 50 ppm (0.005 %) as determined by ISO 15512:2019 or Karl Fischer titration. At moisture levels above this ceiling and melt temperatures of 280 °C, measurable IV reduction occurs within residence times of 3–5 min. Pre-drying is therefore required when ambient relative humidity exceeds 60 %.

Why does intrinsic viscosity govern melt-phase processing?

In PET, IV is not only a molecular weight indicator; it is a process-control parameter linking melt rheology, preform filling, and strain-hardening response. Bottle-grade resin after solid-state polymerization is usually specified at 0.80–0.84 dL/g. At that IV, barrel melt temperatures of 270–285 °C yield a viscosity low enough to fill a multi-cavity preform mold but high enough to avoid excessive melt instability. For high-tenacity industrial yarn, IV values in the range 0.85–1.00 dL/g are selected to maintain filament tension during drawing; for extrusion-coated film and sheet, IV values of 0.60–0.70 dL/g provide the lower melt strength required for leveling. The relationship is not linear: a drop of 0.02 dL/g near the low-IV boundary can produce visible melt edge instability, while the same change near bottle-grade IV may be acceptable in non-beverage applications.

During stretch blow molding, the practical lower boundary is set by biaxial strain hardening. At IV below 0.75 dL/g, preforms can tear or form thin regions on rotary stretch-blow machines running at 1,800–2,200 bottles/cavity/h. At IV above 0.88 dL/g, melt pressure increases in the injection step; a 72-cavity hot-runner preform tool may require injection pressures above 1,200 bar, approaching the limit of standard preform molding clamp systems and increasing shear heating at the gates. Production facilities monitor IV as a gate-keeping test before material enters the drying hopper, using ASTM D4603-18 with 60/40 phenol/tetrachloroethane solvent at 30 °C.

Because density reflects crystallinity and orientation, it is used as a quality proxy rather than a simple weight parameter. Amorphous PET has a density of approximately 1.335 g/cm³; perfect crystalline PET has a density of 1.455 g/cm³. Commercial bottle sidewalls produced with stretch ratios of 2.7–3.2 axial and 3.0–3.6 hoop, followed by hot-fill heat-setting, show density values near 1.36–1.38 g/cm³ by ISO 1183-1:2019 gradient-column method. The same measurement can be converted to fractional crystallinity using the two-phase density expression; an increase from 1.34 g/cm³ to 1.37 g/cm³ corresponds to a crystallinity change of roughly 25 percentage points. That shift fixes the dimensional stability of the article: lower-density regions shrink more in hot-fill environments, and sidewall density is therefore sampled alongside Tg to release preform lots for hot-fill lines.

The glass transition temperature of semi-crystalline PET falls near 78–80 °C, but the value is not fixed. Fast-cooled amorphous sheet can show a Tg closer to 67 °C; heat-set film and fiber with high crystallinity and orientation can show Tg values up to 82 °C by ISO 11357-2:2020. In continuous sheet thermoforming, the polymer surface must remain below Tg to avoid blocking; in hot-fill bottle production, heat-setting above Tg raises effective dimensional stability. Tg is therefore compared side-by-side with density because both are functions of the same crystalline fraction.

When bottle-grade resin is dried for high-cavitation preform molding

When bottle-grade resin is dried for high-cavitation preform molding, the allowable processing window narrows. Barrel temperature settings are typically 270–285 °C, with the compression zone configured to prevent local melt temperatures above 300 °C. Above 300 °C, thermal degradation and acetaldehyde generation accelerate; acetaldehyde measured by ASTM F2013-10 in preforms can exceed 8 ppm, a common sensory specification limit for water packaging. Shot residence time in the barrel should remain below 5 min; beyond this, IV can decrease by 0.02 dL/g or more, shifting the preform out of the 0.80–0.84 dL/g process window. Mold water temperatures are often set at 5–10 °C for cold preform molding, ensuring demolding occurs below Tg. If surface temperature at robotic transfer exceeds approximately 70 °C, the preform may deform in the take-out end-of-arm tool.

High-cavitation tools impose their own residence-time distribution. In a 72-cavity hot-runner mold, the center cavities may fill with lower shear heating than edge cavities, producing density and IV gradients across the tool. The resulting preform set can exhibit a coefficient of variation in weight greater than 0.5 %, which translates to wall-thickness variation after stretch blow molding. Production-scale troubleshooting therefore relies on comparing preform weight distribution, gate-pressure curves, and IV retention from feed pellets to molded preforms rather than relying only on data-sheet values. Published data for this specific configuration is limited, but the troubleshooting sequence is consistent across high-output preform molding platforms.

Thermal profiling of melting point and recrystallization exotherms

Melting point is the most frequently quoted PET thermal property, yet it is also the most misinterpreted. A first-heat DSC scan on bottle-grade resin at 10 °C/min typically shows a glass transition near 78–80 °C, a cold-crystallization exotherm near 140–160 °C, and a melting endotherm from 245–260 °C. The melting peak depends on lamellar thickness and comonomer content; diethylene glycol comonomer lowers the peak, and prolonged solid-state polymerization can raise the peak by thickening the crystalline lamellae. The practical processing implication is that barrel setpoints must exceed the melting peak by roughly 20–30 °C to complete crystal melting and avoid unmelted material at the gate tip. For a resin with a melting peak of 255 °C, a melt temperature of 275–285 °C is therefore not excessive; it is the minimum thermal driving force required for homogenization. Melt residence times must then be controlled to limit thermal degradation, especially in slow-moving hot-runner channels.

Low-IV recycled feedstocks in extrusion coating

Low-IV recycled PET streams with 0.50–0.60 dL/g create characteristic failures during extrusion coating and sheet. The reduced melt tenacity becomes visible as neck-in and edge waviness at melt temperatures of 260–280 °C. On a single-screw extruder with 24:1 to 30:1 L/D and no melt pump, screw speeds above 100 rpm can generate viscous heat that overshoots the barrel setpoint; localized IV reduction then produces melt-pressure variation at the die. A melt-pressure fluctuation greater than ±5 bar is often the first measurable sign of viscosity heterogeneity rather than a change in output. The use of such low-IV material is therefore limited to applications where lower melt strength is acceptable, such as strapping or non-critical sheet, and where the line is equipped with a continuous melt filter and vacuum venting to extract volatiles and degraded low-molecular-weight fractions.

Contamination with polyvinyl chloride in recycled feedstock introduces an additional limitation. Trace PVC can release hydrochloric acid during extrusion, accelerating ester hydrolysis and corroding screw and barrel surfaces. The feedstock specification for low-IV recycled PET therefore often includes a maximum PVC concentration below 50 ppm or verification by density separation in the wash line. Published data for this specific configuration is limited, so incoming flake is routinely tested for residual moisture and IV before blending with virgin resin.