Polyethylene terephthalate (PET) film-grade selection for biaxially oriented processes is governed primarily by the relationship between initial sheet crystallinity and the temperature–strain-rate window in which deformation remains homogeneous. Differential scanning calorimetry per ASTM D3418-21 at 10 °C/min places the glass transition of film-grade PET at 78–80 °C, the cold crystallization exotherm peak at 140–160 °C, and the melting endotherm at 245–255 °C. The stretch window lies between the glass transition and cold crystallization but is further constrained by cast-sheet crystallinity—commonly <5% by density or enthalpy ratio—before machine-direction draw. A grade with rapid crystallization kinetics, such as low-diethylene-glycol homopolymer PET, develops enough crystallinity during preheating to raise yield stress beyond the ductile limit; orientation becomes localized and web fracture occurs. Film-grade resins are therefore formulated with intrinsic viscosity in the range 0.60–0.65 dL/g by ASTM D4603-18 and with comonomers that retard crystallization without suppressing final heat-set crystallinity. For direct food-contact film, the resin must satisfy FDA 21 CFR 177.1630 and EU 10/2011; these regulatory constraints do not substitute for crystallinity control.

Quiescent DSC thermograms are not sufficient for line selection because cast sheet carries a thermal history that shifts crystallization kinetics. The measured cold crystallization peak under ASTM D3418-21 moves with heating rate: at 5 °C/min, Tcc may appear at 130–150 °C; at 20 °C/min, it may shift to 150–170 °C. A film grade with a high quiescent Tcc but rapid shear-induced crystallinity can still fail during preheat. Cast-sheet density by ASTM D1505-18 and web temperature profile are used as release criteria before orientation.

What Happens When Cast-Sheet Crystallinity Exceeds the Stretch Envelope?

A cast sheet that leaves the quench unit with excessive crystallinity carries nucleation history that cannot be erased by reheating. Density measurement by ASTM D1505-18 is used to compute fractional crystallinity from amorphous-phase density 1.333–1.335 g/cm³ and crystalline-phase density 1.455–1.457 g/cm³. Oriented film after heat setting normally reaches 1.390–1.400 g/cm³, corresponding to crystallinity near 40–50%. Cast sheet above 1.340–1.345 g/cm³ is considered near the upper stretching limit because spherulitic order raises yield stress and reduces the strain-rate range for homogeneous deformation. At the same preheat temperature of 100 °C, a sheet with 3% crystallinity draws uniformly, whereas a sheet with 8% crystallinity may show periodic necking and transverse-direction draw breaks at draw ratios above 3.0; published data for this specific configuration is limited but commercial BOPET lines report edge cracking when web density gradients exceed 0.005 g/cm³.

Cold crystallization onset occurs near 130–140 °C; preheating must therefore raise the sheet to 95–115 °C without allowing dwell time to enter the crystallization region. Infrared preheat tunnels with zone temperatures from 120 °C to 180 °C provide rapid surface heating, while roll-contact preheat units of 1,200–2,000 mm width are set to 85–105 °C to limit quench reversal. A sheet with initial crystallinity near 0% may have a usable dwell window of 20–30 s at 105 °C before measurable cold crystallization; nucleated sheet with 5% crystallinity may enter cold crystallization in less than 10 s. This nonlinear dependence is the central reason that crystallinity defines the practical stretch window rather than the glass-transition temperature alone.

Crystallinity Control Through Copolymer Content and Extrusion Quench

Comonomer selection is the primary resin variable used to set crystallization rate. Substitution of 2 mol% terephthalic acid with isophthalic acid disrupts chain regularity and shifts the cold crystallization peak upward by approximately 5–10 °C under ASTM D3418-21. Diethylene glycol generated during esterification at 1.0–1.5 mol% produces a similar but smaller retardation, and high-clarity grades may use higher comonomer concentrations to maintain an amorphous cast sheet. Carboxyl end-group concentration is kept below 30 mmol/kg because hydrolytic degradation during extrusion reduces melt viscosity and increases oligomer deposits. Excessive comonomer reduces maximum crystallinity after heat setting. A grade above 5 mol% comonomer may fail to reach 35% crystallinity at 230 °C, limiting tensile strength and dimensional stability; such grades are acceptable only when low shrink or optical clarity outweighs mechanical requirements.

Extrusion quench parameters are equally deterministic. On a cast film line with a single-screw extruder of 30:1 L/D and barrier screw, melt temperature is held at 270–285 °C. The melt is cast onto a quench roll maintained at 25–35 °C; roll-velocity differential below 1% reduces axial molecular orientation in the amorphous sheet. Electrostatic pinning and an air knife are used to maintain contact between the melt and quench roll, because any intermittent air gap increases localized crystallinity. Filtration through 20–40 µm screens removes gelled PET that would act as nucleation sites. Pre-drying to <50 ppm moisture at 170 °C for 4–6 h is required because hydrolysis at melt temperature reduces intrinsic viscosity and narrows the stretch window. Insufficient quench at the edges produces density increases greater than 0.005 g/cm³, and the resulting high-crystallinity trim cannot be recycled at levels above 30% without measurable cold crystallization peak depression.

Intrinsic viscosity is not a direct stretch-window variable but sets molecular weight distribution and entanglement density required for strain hardening. A higher-IV grade of 0.68 dL/g increases melt strength and final tensile modulus but also raises backpressure and thermal-degradation risk; a lower-IV grade of 0.58 dL/g may process at lower temperature but fails to develop uniform biaxial orientation at high draw ratios. Tensile properties of oriented PET film are measured by ASTM D882-18; commercial data sheets for balanced BOPET film of 12 µm thickness report tensile strength in the range 180–260 MPa in both machine and transverse directions and elongation at break above 80%. Property values depend on heat-set crystallinity; tensile strength rises with density while elongation decreases.

When Sequential Stretching Demands a Wider Processing Window

On a sequential biaxial orientation line, the machine-direction draw unit uses heated rolls at 90–105 °C and a draw gap of 2–4 mm. The cast sheet is drawn 3.0–3.8× in MD before entering the transverse-direction orienter. Because MD drawing induces orientation-induced crystallization, the TDO preheat zone must compensate with 100–120 °C air temperature to maintain ductility. Transverse draw ratio is 3.5–4.5×, and heat setting in the TDO at 220–240 °C for 5–10 s increases crystallinity to 40–50%. Residual shrinkage at 150 °C is controlled to <2.0% MD and <1.5% TD by ASTM D1204-14.

Strain-rate dependence is amplified by initial crystallinity. Industrial BOPET drawing operates at approximate extension rates of 1–10 s⁻¹; at the low end below 0.1 s⁻¹, molecular relaxation reduces orientation, while above 10 s⁻¹, extension-induced crystallization can raise stress enough to cause draw-induced voids. Machine-direction stretching at 3.5× can increase crystallinity from 3% to 15–20% even before TDO; this orientation-induced crystallinity lowers transverse drawability. Film-grade PET with slow cold crystallization permits the sheet to remain in the TDO preheat zone for 8–15 s without entering the high-crystallinity regime. Faster crystallizing homopolymer grades require preheat zones no longer than 4–6 s, which constrains maximum line width and throughput. When cast-sheet crystallinity is already above 5%, TDO web breaks increase and line speed may be reduced until stable operation is recovered.

In high-shrink PET film production, cast-sheet crystallinity is kept below 3% and MD/TD stretching is performed at 85–95 °C, followed by attenuated heat setting at 120–150 °C. The lower heat-set temperature leaves crystallinity at 20–30% and preserves free volume. Shrinkage at 100 °C can exceed 40% in machine and transverse directions when measured by ASTM D2732 or ASTM D1204; the exact value depends on comonomer content and the draw thermal profile. Optical film grades for display applications use low catalyst residue and require haze below 2.0% at 50 µm thickness per ASTM D1003-13, with total luminous transmittance above 88%. If spherulites nucleate before stretching, subsequent orientation cannot reduce their scattering contribution; the web is downgraded or recycled. For such products, crystallinity functions as a process variable that defines the usable stretch window before any orientation is applied.