Substitution of dried crystalline pellet feedstock with amorphous chip stock on a monoaxially oriented polyester strapping line introduces a set of thermal and rheological boundary conditions that are not encountered when the same extruder is supplied with crystallized resin. The amorphous chip morphology retains a glass transition temperature of approximately 78 °C and a crystallization exotherm maximum in the range 140 °C to 160 °C when heated at 10 °C/min in differential scanning calorimetry according to ISO 11357-3:2018. Because the chips soften and become tacky above the glass transition, direct feeding into a desiccant hopper maintained at 170 °C causes particle agglomeration, bridging, and eventual feed throat blockage. A production-scale single-screw extruder with L/D of 30:1 to 36:1, a vacuum vented barrel, and a gear pump assisted die group is typically used for PET strapping; this equipment imposes a narrow processing window because amorphous chips have lower thermal conductivity in the solid conveying zone and a broader melting range than pre-crystallized pellet stock. The strapping line downstream of the die consists of a chilled water quench tank, a reheating and conditioning zone, a machine direction orientation unit with multiple godet stands, an embossing station, and a winder. Published data for the specific substitution of amorphous chip stock in this exact configuration is limited; however, the unit operations governing crystallization and drying are sufficiently well characterized in polyester processing to establish operational boundaries.

What Defines the Pre-Crystallization Boundary for Amorphous Chip Feedstock?

For amorphous chip feedstock to be processed on a strapping line, pre-crystallization is required before the desiccant drying step because untreated amorphous material cannot withstand the drying temperatures used for crystalline PET. The upper temperature limit for pre-crystallization is determined by the onset of tackiness at the glass transition and the rate of heat transfer into the chip. In a fluidized bed crystallizer operating with an inlet air temperature of 120 °C to 140 °C, amorphous chips reach a crystallinity of 25% to 35% within 30 min to 90 min depending on chip weight, thickness, and air velocity. The residence time must be sufficient to shift the glass transition upward as crystallinity increases; a partially crystallized chip with 30% crystallinity can then be transferred to a desiccant dryer at 170 °C without agglomeration. If the crystallizer discharge temperature falls below 130 °C, only surface crystallization may occur, leaving a tacky amorphous core that collapses during subsequent drying and blocks the hopper discharge. Conversely, if the air temperature exceeds 150 °C, the chips begin to stick before crystallization is complete, forming clumps that sinter into a mass adhered to the crystallizer distributor plate. The acceptable crystallizer air temperature window is therefore approximately ±10 °C around the set point for a chip thickness of 3 mm to 5 mm, with a tighter window required for recycled amorphous flake or regrind containing fines.

At residual moisture levels above 30 ppm, the hydrolytic degradation of PET in the extruder becomes measurable as a loss in intrinsic viscosity when tested according to ASTM D4603-18. Residual moisture in amorphous chip feedstock after pre-crystallization and desiccant drying must be maintained below 30 ppm when measured by ISO 15512:2019 Karl Fischer coulometry. Crystalline pellets intended for the same strapping line are commonly dried at 160 °C to 180 °C for 4 h to 6 h with a desiccant bed dew point of -40 °C or lower, yielding final moisture levels below 30 ppm. Amorphous chips require a two-stage profile because the initial crystallinity is below 10%. The first stage in a hot-air or infrared crystallizer raises the bed temperature to 120 °C to 140 °C for 30 min to 90 min; the second stage in a desiccant dryer raises the material to 170 °C for an additional 4 h to 6 h. If the throughput of the strapping line rises above the rated crystallizer capacity, the chip residence time in the first stage drops and the subsequent drying stage can remove water only from the surface, not from the core. The resulting hydrolytic degradation in the extruder is measured as a loss in intrinsic viscosity of 0.03 dL/g or more; the magnitude depends on the residual moisture and the melt residence time. In such cases, the oriented strap tensile strength drops disproportionately because molecular weight loss in the extruder reduces the number of tie molecules that can be oriented in the machine direction.

When the Melt Temperature Setpoint is Locked at 270 °C Without Adjustment for Chip Morphology

Amorphous chip feedstock changes the heat flux demands in the extruder because the resin enters the plastication zone with a lower degree of crystallinity and a different particle surface area. The latent heat of fusion for semicrystalline PET is approximately 40 J/g to 50 J/g; amorphous chip stock must supply this enthalpy during extrusion in addition to the sensible heat required to raise the polymer from feed temperature to melt temperature. If the barrel zone temperature profile is not adjusted, the plastication length shifts downstream, and solids can reach the metering zone. In a single-screw extruder with L/D 36:1 and a barrier screw, the compression ratio is typically 2.8:1 to 3.5:1; this geometry is designed for pre-crystallized pellet feed. With amorphous chips, the apparent bulk density is lower and the feed section conveying efficiency decreases, resulting in lower discharge pressure at the gear pump inlet. The melt temperature setpoint that produces 0.80 dL/g intrinsic viscosity retention in crystalline pellet feed may produce a higher carboxyl end group concentration in amorphous chip feed if the chip has not been fully crystallized. Published data for this exact substitution is limited, but the response can be inferred from the accepted PET hydrolysis model, in which ester bond scission is accelerated by dissolved water at melt temperatures above 260 °C.

For an extruder configured with a barrier screw and a vacuum vent, the plastication behavior of amorphous chip stock requires a more aggressive feed section temperature profile and a longer compression zone than crystalline pellet stock. A barrier screw with an intensive mixing section and a downstream vacuum vent port is preferred for amorphous chip feed because the higher initial moisture load and the potential for trapped volatiles in the chip core require more devolatilization capacity. The vacuum vent is typically located in barrel zone 6 of a 9-zone extruder and is held at -0.08 MPa to -0.09 MPa relative to atmosphere during processing. The gear pump between the extruder and the slot die is set to maintain a die inlet pressure of 80 bar to 150 bar, with the exact value determined by die width, lip gap, and melt viscosity. Screen packs with nominal filtration ratings of 25 µm to 50 µm are installed before the gear pump to remove char particles and partially unmelted amorphous fragments; break plates with 200 µm to 400 µm hole diameters are typical. If the screen pressure before the gear pump exceeds 200 bar, the extruder speed is reduced and the screen pack is changed; elevated screen pressure is an early indication that amorphous chip feed is not melting completely or that degradation has generated gel particles. The screw speed on a 120 mm diameter single-screw extruder producing oriented strapping is usually controlled to keep the melt residence time between 4 min and 8 min; longer residence times at 270 °C cause measurable molecular weight loss.

Die Lip Pressure Fluctuations Across the Slot Width

Die lip pressure transducers installed in the adapter and die body provide the first indication of melt uniformity degradation when amorphous chip feed is used. Uniform melt delivery to the slot die is a stricter requirement when amorphous chip feed is used because the melt pool from a partially crystalline precursor retains local viscosity gradients. The slot die width for polyester strapping is typically 200 mm to 600 mm, with a lip gap of 0.8 mm to 2.0 mm; the die is fitted with deckle rods to adjust the edge flow. Die lip pressure fluctuations are measured with melt pressure transducers mounted in the die adapter and are typically controlled within ±1 bar at a setpoint of 100 bar to 120 bar for crystalline pellet feed. With amorphous chip feed that has not been fully homogenized, pressure fluctuations can increase to ±3 bar to ±5 bar, producing transverse thickness variation in the quenched sheet. A thickness variation greater than ±2% across the sheet width translates into uneven draw ratios in the orientation section and can initiate edge fibrillation during stretching. Melt temperature variation at the die exit is monitored with an infrared pyrometer or a thermocouple rake; a fluctuation of more than ±5 °C across the die width indicates poor mixing, excessive screw wear, or a cold feed stream from the amorphous chip feeder.

In the orientation section, the quenched sheet is reheated to a temperature between the glass transition and the crystallization onset, typically 85 °C to 100 °C for PET strapping, and then drawn between godet sets. Machine direction orientation converts the quenched amorphous sheet into a high-tenacity strap by stretching at controlled temperatures and draw ratios. The draw ratio is defined as the linear speed ratio of the second godet set to the first and is typically 5:1 to 8:1 for high-tenacity strapping; some formulations operate up to 10:1. The strain rate in the orientation gap is controlled by the godet spacing and the speed differential; a gap of 20 mm to 50 mm with a speed differential of 100 m/min to 300 m/min produces localized necking and strain-induced crystallization. The resulting strap develops a machine direction tensile strength of 400 MPa to 700 MPa when tested according to ISO 527-2:2012, depending on the draw ratio, the intrinsic viscosity retention, and the moisture level. If the feed resin retains too little molecular weight, the sheet fails before reaching the target draw ratio or shows surface fibrillation and brittle fracture at the embossing station.

Thermal Degradation Pathways and Carboxyl End Group Accumulation in Amorphous Feedstock

Thermal and hydrolytic degradation in PET proceed by ester bond scission to generate carboxyl end groups and vinyl ester intermediates; acetaldehyde is a principal volatile byproduct. The carboxyl end group concentration can be tracked by titration according to ASTM D7409-15, which specifies a phenol/chloroform solvent system and potentiometric titration. For PET strapping feed, carboxyl end group values are commonly controlled below 25 meq/kg to 35 meq/kg after drying and below 40 meq/kg after extrusion. Amorphous chip feed that is insufficiently crystallized and dried retains dissolved water above 50 ppm; at melt temperatures above 260 °C, hydrolytic chain scission can raise carboxyl end group concentration by 10 meq/kg to 30 meq/kg within 5 min residence time. The intrinsic viscosity loss follows a first-order relationship with water concentration and time, but the exact rate coefficient depends on melt temperature, catalyst residues, and comonomer content. If the intrinsic viscosity drops from 0.85 dL/g to 0.75 dL/g during plastication, the oriented strap loses not only tensile strength but also splitting resistance under tension. The carbonyl index from infrared spectroscopy can be used as a supplementary indicator of oxidative degradation, but the carboxyl end group titration remains the primary specification for polyester strapping resin.

Finished polyester strapping is tested for width, thickness, break load, elongation, and longitudinal splitting resistance according to ASTM D3950-17 or equivalent national specifications. A strap of 16 mm width and 0.8 mm thickness produced from a draw ratio of 7:1 typically exhibits a break load of 4.0 kN to 5.5 kN, corresponding to a tensile strength of 300 MPa to 500 MPa; the exact values are determined by the feed IV and the orientation temperature. Embossing rollers operate at 40 °C to 80 °C and apply a pattern that reduces the contact area and improves strapping tool seal performance, but the embossing depth must not exceed 0.05 mm to avoid creating stress concentrations. Winding tension is maintained between 5 N and 20 N depending on strap width to prevent blocking and telescoping on the spool. The specification for moisture content of the finished strap is not directly set, but the residual shrinkage measured by immersion in hot water at 85 °C for 10 min is typically 2% to 6% in the machine direction for polyester strapping. A higher shrinkage value indicates that the annealing and relaxation step after orientation was insufficient; amorphous chip feed with lower molecular weight retention generally requires a longer relaxation zone to achieve the same shrinkage limit.