PET Resin COA Review: The Six Numbers a Buyer Should Check

A certificate of analysis for polyethylene terephthalate resin is not a pass-or-fail label; it is a set of method-dependent process indicators that determine whether a lot will survive drying, melting, forming, and downstream storage. A buyer should check six numbers on every COA before silo transfer: intrinsic viscosity, carboxyl end group content, diethylene glycol, peak melting temperature, CIE b*, and residual acetaldehyde. Each value interacts with the same polyester backbone. Thermal, hydrolytic, and oxidative degradation often shift several values simultaneously, so isolated acceptance checks can miss substitution, wet resin, or solid-state polymerization drift.

Application-specific acceptance windows vary by supplier and conversion line. The values below are not universal material specifications, but they represent ranges frequently observed on commercial COAs when the same test methods are applied.

What an Intrinsic Viscosity Reading Cannot Reveal About Melt Stability

Intrinsic viscosity is measured by dilute-solution viscometry according to ASTM D4603-18 using a 60/40 phenol/1,1,2,2-tetrachloroethane solvent mixture at 30 °C. A vacuum-dried resin is dissolved at a concentration of 0.5 g/dL, and the reported value is expressed in dL/g. A stretch blow molding grade commonly carries an IV between 0.80 dL/g and 0.84 dL/g; textile filament grades fall between 0.63 dL/g and 0.68 dL/g; film and sheet grades are often specified from 0.64 dL/g to 0.70 dL/g. The value is a viscosity-average molecular weight proxy, but it does not detect low-molecular-weight tails, cyclic oligomers, gel particles, or contamination by other condensation polymers. A lot reported at 0.82 dL/g can still contain gel species that pass pellet inspection and then raise melt filtration pressure at the extruder screen pack.

On a 350-metric-ton reciprocating-screw injection molding machine with a 24:1 L/D plasticizing unit and preform tooling, a grade shift from 0.80 dL/g to 0.84 dL/g increases melt viscosity at constant screw speed. Barrel set-points within the 270 °C to 290 °C window must be adjusted to maintain preform fill. If the adjustment is delayed, the high-IV lot may produce short shots in cavities at the end of the hot manifold and increase shear heating near the gate. Pre-drying must also be fixed before lot acceptance: a desiccant dryer at 160 °C to 180 °C for 4 h to 6 h with a dew point at or below -40 °C is standard for keeping residual moisture below 50 mg/kg. At ambient relative humidity above 60%, pellet and regrind can re-absorb moisture before the feed throat, producing hydrolysis-driven IV loss that the pellet COA cannot show.

On the same certificate, carboxyl end group content is determined by potentiometric titration following ASTM D7409-15 and reported in mmol/kg. Stretch blow molding and film grades are commonly accepted at ≤35 mmol/kg; fiber conversion may require ≤25 mmol/kg because carboxyl end groups promote hydrolytic chain scission during melt extrusion. CEG is not a simple purity marker. It records the balance between esterification, thermal degradation, and solid-state polymerization. A lot with 38 mmol/kg to 42 mmol/kg may still meet intrinsic viscosity but can generate more acetaldehyde and lose melt strength more rapidly after drying at 170 °C. When the downstream molding line uses a hot-runner manifold above 290 °C, borderline CEG values multiply acidolysis and discoloration risk. A buyer should compare CEG against the last three accepted lots from the same reactor line rather than against the supplier’s generic maximum only.

Diethylene Glycol Co-Monomer Drift and Crystallisation Rate

Diethylene glycol is a side-product of ethylene glycol etherification during esterification and is incorporated into the PET chain as a co-monomer. It is measured after depolymerisation by gas chromatography, commonly following ASTM D4604, and reported in wt%. Bottle resin typically carries 1.0 wt% to 1.5 wt%; elevated values depress peak melting and broaden the cold-crystallisation window. The co-monomer drift changes the cooling profile in an injection-oriented preform because the polymer remains in the melt state longer, delaying gate freeze and increasing cycle time on thick-wall preforms. A high DEG value can be detected by differential scanning calorimetry in accordance with ISO 11357-3:2018 before molding. Buyers should not compare DEG values across suppliers without confirming the same sample preparation: improper grinding can expose pellet surfaces unevenly and skew gas chromatographic results.

Color data on a PET COA are expressed in CIE L*, a*, and b* coordinates following measurement of dried pellets under D65 illumination and a 10° observer per ASTM D6290-19. The b* value is the yellow-blue axis; a clear bottle resin with b* below 1.0 is usually suitable for preform molding. Values above 1.5 indicate thermal degradation, antimony catalyst residue, insufficient polycondensation vacuum, or contamination from colored flake. A shift of 0.5 in b* between lots can appear as visible yellow tint in a high-cavitation preform sidewall. Pellet surface moisture can scatter reflectance and shift the reading, so sample drying must follow the same protocol used for the calibration plaque.

When Residual Acetaldehyde Limits Carbonated-Soft-Drink Compliance

Residual acetaldehyde is measured on resin pellets by automated headspace gas chromatography with flame ionisation detection per ASTM F2013-10(2016) and reported in mg/kg. For carbonated soft drink preforms, a pellet COA value of ≤3 mg/kg is a common acceptance ceiling; still water and mineral water packaging often uses ≤5 mg/kg. The pellet value is not the final preform value. Injection molding with barrel temperatures from 280 °C to 290 °C and high screw recovery time can add 1 mg/kg to 4 mg/kg of acetaldehyde before the preform cools. Because acetaldehyde migrates from the container wall into water and has an organoleptic threshold commonly reported between 20 µg/L and 50 µg/L, a lot that meets the pellet limit can still fail sensory testing if the hot-runner residence time exceeds the established processing window. Buyers should review the COA value together with injection molding hold-up time and barrel purge records, not as an isolated defect indicator.

Peak Melting Temperature as a Purity Indicator

Peak melting temperature from differential scanning calorimetry functions as a fingerprint of comonomer content and contamination. A heat-cool-heat cycle at 10 °C/min according to ISO 11357-3:2018 or ASTM D3418-15 should be used because first-heating values are affected by prior thermal history. Bottle-grade PET with low DEG often shows a second-heating peak between 242 °C and 248 °C. A value near 255 °C suggests lower comonomer content or a different polyester grade, while a value below 235 °C may indicate high DEG, isophthalic acid comonomer, or contamination with a lower-melting polyester. The cooling exotherm should also be examined; a substituted resin can show a similar melting peak but a shifted crystallisation onset. In practice, a buyer should require both the heating trace and the cooling trace at the same scanning rate before accepting a new lot.