Market data for polyethylene terephthalate bottle resin have registered periods in which virgin PET drops below rPET: defending your recycled sourcing requires separating spot-price inversion from polymer qualification. PET is a condensation polymer of terephthalic acid and monoethylene glycol, with an aromatic repeat unit that produces a glass transition temperature near 78°C and a crystalline melting range of 245–255°C when measured by differential scanning calorimetry per ISO 11357-3. Bottle-grade intrinsic viscosity is typically 0.80–0.84 dL/g when measured in 60/40 phenol/tetrachloroethane at 30°C according to ASTM D4603-18. Recycled PET carries the same repeat unit but exhibits chain scission, additive carryover, and occasional polymer cross-contamination from PVC, PLA, or polyolefin caps. The price inversion does not remove those physical factors.

The comparison in Table 1 summarizes typical published ranges rather than universal specifications. Lot-specific rPET variation is wider than virgin PET variation because feedstock composition, washing efficacy, and thermal history differ by source. A low virgin spot price should not displace a qualified rPET lot unless the substitution is tested against the same preform, sheet, or fibre performance requirements.

The glass transition near 78°C and cold crystallization near 130–150°C determine drying and crystallizer set points. Amorphous rPET flake introduced directly into a drying hopper at 170°C can soften and agglomerate before crystallizing. Mechanical recyclers therefore pass flake through a stirred crystallizer at 160–190°C before the main drying stage. Batch-to-batch crystallinity is checked by DSC per ISO 11357-3 or by density per ASTM D792, because crystalline PET has higher density than amorphous PET.

How Does Intrinsic Viscosity Loss Reshape High-Speed Preform Molding?

Injection molding of PET preforms on production-scale reciprocating screw machines with L/D ratios of 20:1 to 24:1 exposes the melt to shear heating and residence time in hot runner manifolds at 280–295°C. A measured intrinsic viscosity reduction from 0.82 dL/g to 0.74 dL/g changes the melt pressure response at fixed injection speed. On a 32-cavity preform mold the pressure difference can reach several MPa depending on gate diameter and runner balance. The lower molecular weight melt exhibits reduced melt elasticity, which increases the incidence of gate stringing and uneven preform wall thickness after stretch blow molding. Process capability is typically maintained only if the rPET lot remains within a supplier-defined IV band of ±0.02 dL/g around the qualified value.

Acetaldehyde generation follows melt temperature and residence time. When rPET feedstock enters the dryer with acetaldehyde above 3 ppm, the preform molder must either lower barrel zone temperatures or reduce screw speed, both of which affect cycle time. Vacuum venting or solid-state polymerization upstream is the normal corrective path. Blending with virgin resin dilutes acetaldehyde but does not remove it. The relevant test for bottle preforms is ASTM F2013-10, and carbonated soft drink and water specifications commonly require acetaldehyde below 2–3 ppm.

Under sustained moisture ingress, hydrolysis of PET proceeds as random ester-bond scission and becomes autocatalytic. Residual moisture above 0.005 wt% during plastication can reduce intrinsic viscosity by 0.03–0.06 dL/g in a single-screw sheet line, with the exact loss controlled by screw residence time distribution. Desiccant dryers delivering air at -40°C dew point are standard for both virgin and rPET. However, rPET flake with high fines surface area may require longer crystallizer residence at 160–180°C to avoid sticking before the main drying hopper. At ambient relative humidity above 60%, unsealed rPET regains surface moisture within minutes, so hopper loading must be closed-loop or blanketed with dry air.

When Virgin Resin Prices Invert, Solid-State Polymerization Still Governs rPET Quality

Solid-state polymerization is the primary route to raise intrinsic viscosity and strip volatile contaminants from post-consumer flake. The flake is crystallized at 160–190°C and then held under vacuum or inert nitrogen at 200–220°C for 12–20 h. Transesterification and polycondensation in the solid state increase molecular weight without dissolving the particle, but nonvolatile contaminants such as titanium dioxide, glass fragments, and silicone label residues remain. A low virgin price does not remove the need to specify post-SSP intrinsic viscosity, because batch-to-batch variation in washed feedstock can produce post-SSP IV values from 0.70 dL/g to 0.82 dL/g in the same line.

Published data for the kinetic constants of SSP in mixed-source rPET is limited because feedstock contamination and catalyst residues shift reaction rates. Processors therefore rely on reactor residence time and DSC to monitor crystallinity before solid-state processing. If the flake is not sufficiently crystallized, particles sinter and block the reactor. The melting temperature after SSP is typically 245–255°C, and the intrinsic viscosity is verified per ASTM D4603-18 on pellets or flake after cooling.

For food-contact sheet thermoforming, the lower melt strength of mechanically recycled PET appears at draw ratios above 3:1. Twin-screw extruders or single-screw extruders with L/D 30:1 and vacuum venting are used, with melt temperatures held below 290°C to limit acetaldehyde generation. A drop in intrinsic viscosity below 0.68 dL/g causes observable sheet sag in the thermoforming oven and irregular plug-assist thinning. Optical haze increases with gel particles and with barrier polymer remnants such as EVOH or polyamide. Filtration through 20–40 µm sintered metal or depth media removes coarse particles but not dissolved degradation products. Haze is measured according to ASTM D1003-21, and oxygen transmission for packaging is measured according to ASTM D3985.

Contaminant Carryover, Solid-State Polymerization, and Barrier Test Suites

Defending recycled sourcing when virgin PET drops below rPET is an evidentiary process. A certificate of analysis is insufficient unless it references recognized traceability and test methods. Recycled content claims fall under ISO 14021:2016, clause 5.7, which requires that the claim be based on total mass and be verified through documented traceability. EN 15343:2007 sets traceability criteria for plastics recyclers, and ISO 22095:2020 defines chain-of-custody models including identity preservation, controlled blending, and mass balance. Food-contact rPET in the European Union requires compliance with Regulation EU 2022/1616; in the United States, the applicable framework is 21 CFR 174.5 together with case-specific no-objection letters.

Batch-level testing should include intrinsic viscosity per ASTM D4603-18, acetaldehyde per ASTM F2013-10, moisture per ISO 15512:2019, and melting parameters per ISO 11357-3. Odor and migration testing may also be required for food-contact applications. Published data for specific regulatory migration limits is limited to the intended food simulant and process conditions, so the converter must refer to the EU positive list and FDA conditions of use for the particular package format.

In staple fibre production from rPET, intrinsic viscosity requirements can be lower, near 0.62–0.68 dL/g, and extrusion through spinnerets at 280–285°C is less sensitive to molecular weight than preform molding. However, PVC particles above 30 ppm can block spinneret holes and produce filament breaks. Filtration at 20 µm is common, and particle size distribution is measured per ISO 13320. A virgin price inversion does not eliminate this constraint. Fibre lines drawing from bottle-grade rPET sources must still reject loads with high polyolefin or PVC contamination, because downstream denier variation and spinning breaks increase with contaminant concentration.