Commercial polyethylene terephthalate is frequently specified by the comparative statement “PET Density of 1.33-1.38 g/cm3: Amorphous and Crystalline Compared” because the bulk density of a finished article is not an intrinsic constant. Amorphous PET quenched from the melt exhibits a density of 1.333 g/cm³ to 1.335 g/cm³ at 25 °C, while the ideal triclinic crystalline phase calculated from unit-cell parameters reaches approximately 1.455 g/cm³. The practical specification range of 1.33 g/cm³ to 1.38 g/cm³ therefore corresponds to semi-crystalline states with mass-fraction crystallinity values from near zero to roughly 40%. This distinction matters because transparency, gas permeability, tensile modulus, and thermal shrinkage all shift as amorphous PET densifies into the crystalline arrangement.

The practical density values are obtained on conditioned specimens according to ASTM D1505-18 or ISO 1183-1:2019, with sample conditioning commonly performed at 23 °C and 50% RH per ISO 291 or ASTM D618. Moisture absorption is low in PET, but trapped bubbles, surface wetting, and specimen orientation produce measurable density shifts. A density change of 0.005 g/cm³ is therefore analytically significant when correlating density to crystalline fraction.

Why does the same PET formulation generate densities from 1.33 g/cm³ to 1.38 g/cm³?

The density shift is not primarily a formulation variable; it is a free-volume and chain-packing response to thermal history. Amorphous PET retains a random coil state with excess free volume, while crystalline PET packs chains into a triclinic unit cell with dimensions reported as a = 0.456 nm, b = 0.594 nm, c = 1.075 nm, α = 98.5°, β = 118°, and γ = 112°. The crystalline density derived from this cell is approximately 1.455 g/cm³. Differential scanning calorimetry under nitrogen at 10 K/min typically places the glass transition of amorphous PET at 76 °C to 80 °C, the cold crystallization exotherm at 130 °C to 140 °C, and the melting endotherm at 245 °C to 255 °C. These transitions define the processing windows in which density is controlled.

Cooling from the melt at 10 K/min produces a melt-crystallization exotherm frequently observed between 190 °C and 210 °C, yielding a semi-crystalline structure with density above 1.34 g/cm³. In contrast, cooling rates above 100 K/s, typical of chilled injection molding, suppress long-range order and freeze in the amorphous state at 1.333 g/cm³ to 1.335 g/cm³. The practical density range of 1.33 g/cm³ to 1.38 g/cm³ is therefore a process history indicator rather than a single polymer constant.

Density-gradient column, pycnometer, and unit-cell X-ray validation

Density-gradient column testing per ASTM D1505-18 is the preferred quality-control method for small PET specimens because it can resolve density differences on the order of 0.001 g/cm³ when the column is properly calibrated. Liquid pycnometer and immersion methods per ISO 1183-1:2019 Method A are applied to void-free molded plaques and pellets. The density-gradient method is specified separately in ISO 1183-2:2019. Wide-angle X-ray scattering complements these measurements by resolving the crystalline unit cell directly; it also reveals that density-based crystallinity values can deviate when the rigid amorphous fraction is substantial.

The mass-fraction crystallinity from density is calculated from the two-phase equation:

X_c = 100 × ρ_c(ρ − ρ_a) / [ρ(ρ_c − ρ_a)]

Using ρ_a = 1.333 g/cm³ and ρ_c = 1.455 g/cm³, a bulk density of 1.380 g/cm³ yields X_c = 40.6%; a bulk density of 1.370 g/cm³ yields X_c = 32.2%; and a bulk density of 1.350 g/cm³ yields X_c = 15.0%. These values are valid only for unmodified homopolymer PET with no voids, additives, or rigid amorphous phase contributions.

The relationship between measured bulk density and calculated crystallinity is nonlinear because the denominator includes the measured density of the specimen. Table 2 reports selected values using the two-phase density equation with ρ_a = 1.333 g/cm³ and ρ_c = 1.455 g/cm³.

In injection molding of PET preforms, the process window is deliberately biased toward the amorphous side to preserve optical clarity and uniform reheating before stretch blow molding. Melt temperatures of 270 °C to 290 °C and chilled mold temperatures of 5 °C to 15 °C are common on production-scale injection machines. Cooling rates at the preform skin can exceed 100 K/s, producing a preform density of 1.335 g/cm³ to 1.340 g/cm³. If cooling is unbalanced or cycle time extends into the high-crystallization-rate regime, spherulitic haze develops and the preform density rises measurably. Screw designs with L/D of 24:1 to 25:1 and compression ratios near 2.5:1 are used to manage residence time and avoid excessive acetaldehyde generation. Pre-drying at 160 °C to 170 °C for 4 h to 6 h in a desiccant dryer to below 50 ppm moisture is mandatory; residual moisture above 0.005% causes hydrolytic degradation and poor density reproducibility.

When strain-induced crystallization increases sidewall density in reheat stretch blow molding

Reheat stretch blow molding transforms the amorphous preform into a biaxially oriented container by reheating to 100 °C to 120 °C and stretching at planar extension ratios generally between 9× and 12×. Chain orientation during stretching produces strain-induced crystallization without long thermal residence in the spherulite-growth regime. The sidewall density measured by ASTM D1505-18 increases from approximately 1.335 g/cm³ in the preform to 1.368 g/cm³ to 1.375 g/cm³ in the bottle. This density range corresponds to mass-fraction crystallinity near 28% to 32% under the two-phase model. The oriented sidewall remains optically transparent because the strain-induced crystals are small and oriented, unlike slowly grown spherulites that scatter light.

Heat-set variants for hot-fill bottles hold the oriented sidewall in a mold at 130 °C to 160 °C for several seconds. This increases sidewall density to 1.380 g/cm³ to 1.395 g/cm³ and reduces shrinkage at 85 °C fill temperatures. The density increase is accompanied by higher thermal stability but can reduce clarity if the heat-setting is excessive or nonuniform.

Solid-state polymerization of bottle-grade PET pellets is conducted at 210 °C to 220 °C under vacuum or inert gas for 12 h to 24 h; intrinsic viscosity rises from approximately 0.60 dL/g to 0.80 dL/g, and pellet density rises to 1.400 g/cm³ to 1.410 g/cm³. The crystallinity increase locks in pellet geometry and prevents sintering in downstream drying hoppers. Such pellets are outside the 1.33 g/cm³ to 1.38 g/cm³ range typically specified for molded articles, demonstrating that density specifications must be tied to the processing stage.

Density-derived crystallinity breaks down when rigid amorphous fraction and voids coexist

Density-based crystallinity becomes unreliable when the amorphous phase no longer has a fixed density. Isophthalic acid or cyclohexanedimethanol comonomers increase free volume or alter chain packing; bottle-grade copolyesters containing 2 wt% to 4 wt% isophthalic acid may show an amorphous density shifted from homopolymer PET. In such systems, a density reading of 1.340 g/cm³ may not represent the same crystallinity as in unmodified PET. Similarly, oriented films with microvoids from cavitation, nucleated systems with inorganic pigments, or recycled flake with residual fillers can overstate the crystalline fraction when density alone is used. The rigid amorphous fraction in oriented PET also contributes to density without being truly crystalline. Wide-angle X-ray scattering and ISO 11357-3 DSC are therefore used to validate density-derived crystallinity in such systems.