A Retailer Asks You to Prove the Code-1 PETE Mark on Your Bottle

Polyethylene terephthalate bottle resin is classified as resin identification code 1 under ASTM D7611/D7611M-21. The code is a material identification marker, not a statement of recycled content or a collection guarantee. A molded bottle carries the numeral 1 inside the triangular arrow symbol, with PETE or PET as the permitted abbreviation. For global generic marking, ISO 11469:2016 specifies the symbols >PET< and >PETE< when the material is polyethylene terephthalate. A retailer request to prove the Code-1 PETE mark therefore requires two parallel demonstrations: polymer identity on the finished article and correct use of the coding system on the bottle geometry.

The proof package normally includes a lot traceability certificate from the preform molder, resin supplier data sheets, a visual report for the molded code, and instrumental identification from the bottle wall. Because visual inspection cannot distinguish PET from PETG or some copolyester formulations, the analytical file should include Fourier-transform infrared spectroscopy with attenuated total reflectance, differential scanning calorimetry, density measurement, and intrinsic viscosity determination. The retailer request may also require evidence that the mark survives normal filling and distribution conditions without becoming illegible, but the core technical issue is whether the bottle is correctly identified as Code-1 PETE.

PET is a polyester formed from terephthalic acid or dimethyl terephthalate and ethylene glycol. Commercial bottle grades are copolymers with small percentages of isophthalic acid or diethylene glycol to modify crystallisation. The chain structure gives a strong ester carbonyl absorption near 1713 cm⁻¹ and para-substituted aromatic ring absorptions that distinguish it from polycarbonate and polystyrene. The FTIR spectrum should be obtained from both the amorphous neck finish and the strain-hardened sidewall because orientation does not change the functional group assignments but can shift the intensity of some skeletal bands. Marking legibility is verified at the mold inspection stage because the triangular symbol can be distorted by poor venting, worn cavity inserts, or shrink variation. A retailer audit may request a dimensional check of the triangle height and line thickness, but ASTM D7611/D7611M-21 does not mandate a single minimum size for every bottle configuration; practicality of molding and readability govern the actual cavity dimensions.

Differential scanning calorimetry per ISO 11357-3:2018 on a bottle sidewall specimen heated at 10 °C/min under nitrogen typically shows a glass transition near 70–80 °C, a cold crystallization exotherm between 130 °C and 145 °C, and a melting endotherm peak between 245 °C and 255 °C. Polycarbonate shows a glass transition near 145–150 °C and no PET melting endotherm. PETG remains amorphous and lacks the sharp melting peak. The thermal profile therefore separates Code-1 PETE from clear confusable resins.

Density measured by ISO 1183-1:2019 or ASTM D1505 places bottle-grade PET between 1.33 g/cm³ and 1.40 g/cm³, with oriented sidewall material at the higher end. Intrinsic viscosity by ASTM D4603-18 in a 60/40 phenol/1,1,2,2-tetrachloroethane solvent system is typically 0.72–0.84 dL/g for carbonated soft drink and water bottles. These values support the identification but are not sufficient alone; they must be combined with FTIR and DSC to prevent false positives from filled or multi-layer structures. For multi-layer bottles or surface-coated articles, the analytical sample must isolate the structural layer. Surface energy treatments such as corona or plasma do not change the FTIR spectrum enough to misidentify PET, but a surface coating may hide the carbonyl band. Microtoming to a depth of 50–100 µm or physical delamination should be performed before spectral acquisition.

What Instrumental Evidence Distinguishes Polyethylene Terephthalate from PETG and Polycarbonate?

The analytical separation of clear bottle resins begins with infrared spectroscopy. PET has a carbonyl stretching band near 1713 cm⁻¹, a strong complex of C–O stretching bands near 1240 cm⁻¹ and 1090 cm⁻¹, and a para-substituted aromatic out-of-plane band near 724 cm⁻¹. Polycarbonate has a carbonate carbonyl near 1775 cm⁻¹, while PETG has an ester carbonyl near 1710 cm⁻¹ but can be distinguished by the absence of the crystalline melting endotherm in differential scanning calorimetry. Oriented polystyrene has no carbonyl band and shows aromatic overtone patterns.

A laboratory report submitted to a retailer should measure the bottle wall rather than a pellet, because multi-layer bottles, barrier coatings, and colourant masterbatches can interfere. If the bottle wall is coated with an oxygen barrier or scuff-resistant layer, the outer surface should be microtomed or lightly abraded before FTIR analysis to expose the base polymer. The identification is confirmed when the FTIR spectrum corresponds to a reference PET spectrum collected under the same conditions and the thermal properties fall within the expected ranges.

Differential scanning calorimetry of oriented PET is not a purity test. The exact cold crystallization temperature depends on molecular weight, comonomer content, orientation, and moisture. A bottle wall that has been exposed to hot fill above 85 °C may already show annealing effects, shifting the cold crystallization peak and altering the melting endotherm shape without changing polymer identity.

Preform molding operations for Code-1 bottle resin are bounded by moisture control and acetaldehyde generation. Hydrolytic degradation proceeds rapidly when pellet moisture exceeds 50 ppm at melt temperatures above 270 °C. A desiccant dryer set point of 160–170 °C with a dew point below −40 °C is used to achieve residual moisture below 50 ppm. On production-scale injection molding machines with barrel L/D ratios of 20:1 to 24:1 and barrier screws, the melt temperature is usually held between 270 °C and 290 °C. If the preform molder operates the hot runner above 295 °C or extends resin residence time beyond 5 min, intrinsic viscosity loss and acetaldehyde formation become measurable. Production batch records commonly show melt pressure at the screw tip of 80–140 MPa and cooling water temperatures of 8–12 °C for preform molds. A preform molder aiming to hold acetaldehyde below 5 µg/g may select melt temperature 275–285 °C rather than 285–295 °C, accepting slightly higher injection pressure to reduce thermal degradation.

The preform intrinsic viscosity drop should be limited to ≤0.02 dL/g against the incoming resin certificate. Higher losses indicate hydrolytic or thermal degradation and can reduce bottle burst strength. Acetaldehyde residue in water-bottle preforms is often specified at ≤5 µg/g when measured by headspace gas chromatography per ASTM D4509-96. Acetaldehyde forms from intramolecular degradation of PET ester groups and from traces of polycondensation catalyst residues. The rate is sensitive to melt temperature, screw shear, hot-runner stagnation, and gate geometry. At the gate, preform molecular orientation is absent, leaving the injection point amorphous. When the preform is reheated, the neck finish is protected by cooling neck jaws to prevent crystallisation below the neck support ring. A gate crystallinity haze defect occurs when the preform injection speed is too low or the cooling water temperature is above 15 °C. The resulting preform can exhibit an enlarged crystalline region near the gate, which may survive stretch blow molding as a brittle spot.

Injection stretch blow molding follows with preform surface reheating to 95–115 °C, blowing air or stretch rod pressure of 2.5–4.0 MPa, and mold temperature of 5–15 °C. The process induces biaxial orientation and strain-induced crystallinity in the bottle sidewall, improving drop impact and carbon dioxide barrier performance. At relative humidity above 60%, preforms stored outside conditioning hoppers for more than 4 h can reabsorb moisture and produce haze or inconsistent stretch during blowing. That operational boundary is especially relevant in tropical bottling plants where preform conditioning is shared across multiple lines.

For food-contact compliance, Code-1 PETE bottles in the United States are evaluated under 21 CFR 177.1630, which covers polyethylene terephthalate polymers and copolymers used in contact with food. A resin supplier certificate should state that the terephthalic acid, ethylene glycol, and comonomer composition meet the applicable extraction and end-use conditions. The retailer may also require a declaration of compliance with (EU) No 10/2011 and its amendments for articles placed on the European market. Specific migration limits include ethylene glycol at 30 mg/kg, terephthalic acid at 7.5 mg/kg, isophthalic acid at 5 mg/kg, and antimony at 0.04 mg/kg in food simulant. Compliance is demonstrated by migration testing using the intended food simulant and time-temperature conditions assigned to the packaging application. Overall migration under (EU) No 10/2011 is limited to 10 mg/dm² for plastic materials and articles. Testing uses food simulants assigned under (EU) No 10/2011, such as simulant A, B, C, D1, D2, or E depending on food type, and the contact time-temperature conditions in the regulation. A Code-1 PETE water bottle tested with cold water may not be compliant for hot-fill or high-alcohol beverages without additional migration data.

Acetaldehyde is not regulated as a plastic monomer under most food-contact frameworks, but it is a sensory contaminant in water and carbonated beverages. The Code-1 PETE proof request may therefore include a preform acetaldehyde certificate even when the regulatory file is otherwise complete. Antimony catalyst residues are also monitored because ethylene glycol residues and antimony trioxide used in PET polycondensation can migrate at trace levels under aggressive hot-fill or long-term storage conditions.

The migration kinetic assumption behind these limits is that polyethylene terephthalate has low diffusivity at ambient and cold-fill temperatures, but the effective diffusivity increases as the glass transition is approached. Hot-fill and retort applications require separate end-use testing because the polymer matrix becomes more mobile above 70 °C, and antimony migration can rise when the bottle is held at elevated temperature for prolonged periods. Multi-layer or recycled PET streams must be assessed separately when post-consumer recyclate is incorporated under food-contact authorisations.

When Recycled Flake Is Screened by Sink-Float Separation and NIR Sortation

When a bottle is returned to a material recovery facility, the Code-1 mark enables manual and near-infrared optical sortation. Near-infrared sensors identify PET by the characteristic overtone absorptions of the ester and aromatic groups, not by reading the printed numeral alone. The code supports manual quality assurance and complements automated sorting, but a bale of Code-1 bottles may still contain PVC, PETG, polycarbonate, or multi-layer barrier bottles unless sorting capacity is sufficient. After grinding, washed PET flake is separated from polyolefin caps and labels by sink-float tanks using water density around 1.0 g/cm³; PET flake sinks while polyolefins float. Polycarbonate and PVC with densities above water require additional separation or spectroscopic ejection.

Bottle-grade recycled PET flake is characterised by intrinsic viscosity, moisture content, residual label, and colour. Solid-state polymerisation of recycled PET flake is conducted in vacuum tumble driers or moving-bed reactors at temperatures below the melting point, commonly 200–220 °C, to increase intrinsic viscosity by post-condensation. The process also reduces residual moisture and acetaldehyde. Flake quality, pellet crystallinity, and nitrogen purge rates control the reaction rate, and the final intrinsic viscosity is verified by ASTM D4603-18 before bottle preform use. The Code-1 PETE mark does not distinguish virgin from recycled resin; post-consumer recycled PET used in food-contact bottles must meet the relevant regulatory authorisation and migration testing. Published data for specific retailer audit files is limited because resin and bottle converter certificates are confidential to each supply chain.