Polyethylene terephthalate (PET) is a saturated thermoplastic polyester obtained by melt-phase polycondensation of ethylene glycol with purified terephthalic acid, followed by solid-state polymerization when high intrinsic viscosity is required. A trade customer demands a BPA-free certificate for PET. This is a documentary and analytical obligation rather than an uncertainty about the polymer’s native composition: bisphenol A is not a reactant, comonomer, catalyst, or required processing additive in conventional PET synthesis. For bottle-grade resins, intrinsic viscosity is controlled between 0.79 dL/g and 0.84 dL/g after solid-state polymerization, with melt peak temperatures typically 245 °C to 265 °C under differential scanning calorimetry according to ISO 11357-3:2018. The certificate nevertheless requires raw-material audit data, migration test records, and batch-level traceability because downstream brand programs treat bisphenol A as a controlled substance across all food-contact polymers.

What Structural Evidence Excludes BPA from the PET Polymer Backbone?

Bisphenol A is a diphenol used as the carbonate-forming monomer in polycarbonate. PET is produced by esterification or transesterification, and its repeat unit is C₁₀H₈O₄ with a molar mass of 192.17 g/mol. The polymer chain contains ester linkages, not carbonate linkages, and does not carry the isopropylidene bridge between aromatic rings that is characteristic of bisphenol A. Density of solid-state polymerized bottle-grade PET is approximately 1.39 g/cm³ to 1.40 g/cm³ at 23 °C, but density alone is not diagnostic for bisphenol A. Compositional verification therefore rests on three limbs: monomer purchasing records for purified terephthalic acid and ethylene glycol, formulation review confirming the absence of bisphenol A-containing additives, and residual BPA testing on the finished resin. Under FDA 21 CFR §177.1630, bisphenol A is not listed among the permitted reactants for polyethylene phthalate polymers. Under the European plastic food-contact framework, BPA is not a monomer for PET. Published data for virgin PET generally indicate non-detectable BPA when liquid chromatography–tandem mass spectrometry methods with appropriate detection limits are used.

Solid-state polymerization raises the intrinsic viscosity of amorphous precursor pellets by keeping the polymer below its melt point while removing ethylene glycol and water under vacuum or inert-gas flow. Commercial solid-state reactors operate at 205 °C to 215 °C with nitrogen purge dew point below -40 °C. The temperature window is narrow because excursions above approximately 220 °C produce pellet surface sintering, agglomeration, and discharge bridging in continuous moving-bed or agitated column reactors. Pellet residence time is adjusted to raise intrinsic viscosity from approximately 0.62 dL/g to 0.82 dL/g for bottle-grade resin. This processing step does not generate bisphenol A, but it does influence acetaldehyde content, surface hydrolysis, and black-speck contamination from pellet fines. PET must be pre-dried before melt processing at 140 °C to 160 °C for 4 h to 6 h in desiccant-bed dryers with air dew point below -40 °C. Retained moisture above approximately 50 ppm hydrolyzes ester bonds during extrusion or injection molding and lowers intrinsic viscosity. A BPA-free certificate does not substitute for drying; it addresses bisphenol A, not hydrolysis or acetaldehyde control.

Specific Migration Limits and the Documentary Burden for a BPA-Free Certificate

The European framework for plastic food-contact materials is Regulation (EU) No 10/2011, as amended by Regulation (EU) 2018/213, which establishes a specific migration limit for bisphenol A of 0.05 mg/kg in food or food simulant. The limit applies to migration from the final article, not to the intrinsic polymer composition. For PET, a BPA-free certificate is therefore a conformity document that may combine a declaration of no intentional use of bisphenol A, raw-material certificates for purified terephthalic acid and ethylene glycol, a chromatographic non-detect report, and batch identification. Under Article 15 of Regulation (EU) No 10/2011, the declaration of compliance accompanies plastic materials and articles at intermediate and final stages, and supporting documents must be made available to enforcement authorities. A trade customer demanding a BPA-free certificate for PET frequently requests the underlying analytical report rather than only a signed declaration, because private certification schemes require verifiable evidence. The use of recycled PET does not alter the absence of BPA monomer in the primary polymer, but it may require additional conformity testing depending on feedstock source and collection-stream contamination.

Injection stretch-blow molding of PET preforms is performed on reciprocating-screw injection molding machines with barrel temperatures typically 270 °C to 290 °C. Hot runner and gate temperatures are maintained near 280 °C to 295 °C. Melt residence time at these temperatures is limited because acetaldehyde is generated by thermal degradation of the ester chain. For water-contact applications, preform acetaldehyde limits are frequently set by customers below 4 mg/kg. The melt viscosity of bottle-grade PET is highly shear-thinning; apparent viscosity declines from values above 200 Pa·s at low shear to below 50 Pa·s under injection molding shear rates. The processing window is therefore governed by residence time, temperature, and moisture content. These parameters must be controlled independently of the BPA-free certificate because they affect preform color, molecular weight, and acetaldehyde, not bisphenol A content.

When Recycled PET Feedstock Introduces Conformity Risk for BPA-Free Documentation

Recycled polyethylene terephthalate flake from mixed household collection may contain trace polycarbonate packaging that survives automated sorting. Clear polycarbonate can be misclassified as clear PET when label debris, coatings, or bottle design reduce optical contrast. Density contrast between PET at 1.39 g/cm³ and polycarbonate at 1.20 g/cm³ permits sink-float separation when the separation medium is maintained near 1.30 g/cm³, but separation efficiency depends on flake size, surface tension, detergent residue, and particle geometry. BPA in recycled PET pellet can therefore arise from polycarbonate contamination in the feedstock rather than from PET polymerization chemistry. Published data for this specific configuration is limited because contamination is bale-dependent and varies by collection region. A BPA-free certificate for recycled PET should be supported by flake input audits, wash-line sampling, density separation records, and LC-MS/MS data on the final pellet. The certificate should also state whether the resin is virgin or recycled, because the evidentiary burden for recycled material is heavier.

Establishing Detection Limits for BPA in Food Simulants

Analytical verification of BPA-free PET typically uses specific migration testing in food simulants assigned under Regulation (EU) No 10/2011. Simulant A is 10% v/v ethanol and simulant B is 3% w/v acetic acid, representing aqueous and acidic food types respectively. Migration exposure time and temperature follow the conditions of the intended application. Liquid chromatography with tandem mass spectrometry is the preferred determinative technique because it can distinguish bisphenol A from structurally related diphenols. Method validation under EN 13130-1:2004 requires recovery, repeatability, and limit of quantification to be established for the specific polymer and simulant. A reporting limit below 0.01 mg/kg is generally selected to ensure that the method can demonstrate clear compliance with the 0.05 mg/kg specific migration limit. The certificate should include method design, simulant type, exposure conditions, reporting limit, batch number, and sample surface area. A certificate that omits these data may satisfy a purchase-order condition but does not fully meet the analytical traceability expectations of a private compliance audit.

The BPA-free certificate for PET is product-specific and should contain the resin grade, production date, batch identification, a statement that bisphenol A was not intentionally used, and the analytical result with method reporting limit. The applicable regulatory references may include FDA 21 CFR §177.1630, Regulation (EU) No 10/2011, and Regulation (EU) 2018/213. There is no ISO standard for the certificate format itself, so private schemes differ in layout and required attachments. The certificate does not certify that the article is free from acetaldehyde, color shift, or hydrolysis; it is limited to the bisphenol A requirement stated by the customer.