What Standard Governs PET Resin testing in China?
PET resin testing in China is governed by GB/T 17931-2018 Poly(ethylene terephthalate) (PET) Resin for Bottles (replacing GB/T 17931-2003). The standard covers both homopolymer PET (from pure terephthalic acid and ethylene glycol) and copolymer PET (with isophthalic acid / IPA as a comonomer to slow crystallisation for bottle-blowing). It classifies the resin into food-packaging and non-food-packaging applications, and into three quality grades: 优等品 (premium), 一等品 (first grade), 合格品 (qualified).
The acetaldehyde (AA) content requirement is mandatory — it was a强制性条款 in the 2003 edition and remains the most safety-critical parameter in the 2018 revision. Acetaldehyde is a residual by-product of the esterification/polycondensation process, and in bottle-grade PET it must not exceed ≤ 1.0 µg/g (ppm) for food-packaging premium grade, because AA migrates into the bottled beverage and imparts an off-taste (especially in mineral water, where the taste threshold is very low). The other parameters (intrinsic viscosity, colour, moisture) are推荐性 but are the standard commercial acceptance criteria for PET resin trade.
The test methods invoked are GB/T 14190 (PET test methods — covers intrinsic viscosity, colour, DEG, etc.), SH/T 1817-2017 (residual acetaldehyde by headspace gas chromatography), and GB/T 14190 for moisture (Karl Fischer). The standard also invokes the GB 4806 food-contact framework for food-packaging-grade resin.
For international reference, ASTM D4603 (intrinsic viscosity of PET by dilute-solution viscometry) and ISO 1628-5 (PET specific — determination of intrinsic viscosity) are the primary IV standards; ASTM D2016 / E1443 cover colour measurement; ISO 16000 / headspace GC methods cover acetaldehyde.
What Is Intrinsic Viscosity and Why Is It the Headline Parameter?
Intrinsic viscosity (IV) is the single most important quality parameter of PET resin — it is the proxy for the average molecular weight of the polymer, which determines the mechanical strength, gas-barrier performance, and bottle-blowing behaviour of the final container.
What it measures: IV is measured by dissolving the PET resin in a phenol/1,1,2,2-tetrachloroethane (50:50) mixed solvent at 25 °C and measuring the solution's flow time through an Ubbelohde viscometer (capillary viscometer) at defined concentration. The intrinsic viscosity is the extrapolation of reduced viscosity to zero concentration — the inherent resistance to flow of the polymer chains in solution, independent of concentration. It is reported in dL/g.
Typical values for bottle-grade PET: 0.76–0.84 dL/g, with premium food-packaging grade typically specified at 0.80 ± 0.02 dL/g. Higher IV = longer chains = higher molecular weight = stronger bottle. The IV is what the PET processor specifies when ordering resin, because it determines the wall thickness, top-load strength, and CO₂ retention of the blown bottle.
How IV affects processing: during bottle blowing (injection stretch blow moulding), the PET preform is heated above its glass transition temperature (~75 °C) and stretched and blown into the bottle mould. The polymer chains orient and crystallise during stretching, giving the bottle its strength and barrier. An IV that is too low produces a weak, thin-walled bottle with poor gas barrier (CO₂ escapes from carbonated drinks); an IV that is too high makes the resin difficult to process (high melt viscosity, incomplete filling of the preform mould).
Why IV changes in service: IV degrades with moisture — PET is a condensation polymer, and water hydrolyses the ester linkages, breaking chains and lowering IV. This is why PET resin must be dried to < 0.005 % (50 ppm) moisture before melt processing — even trace moisture in the extruder drops the IV, and a 0.02 dL/g IV drop from processing moisture is enough to move the resin out of specification. The moisture sensitivity of IV is the single most important quality-control parameter in PET processing.
What Is the Acetaldehyde Test and Why Is It Mandatory?
Acetaldehyde (AA, CH₃CHO) is a residual by-product formed during the esterification and polycondensation of PET. It remains trapped in the resin pellets after solid-state polycondensation (SSP), and during bottle processing (melt extrusion at ~280 °C), additional AA is generated by thermal degradation of the polymer. The residual AA in the finished bottle migrates into the beverage.
Why it matters: acetaldehyde has a very low taste threshold in water — consumers can detect AA at concentrations as low as ~20–40 µg/L in mineral water, perceiving it as a sweet, fruity off-taste. For bottled mineral water (the largest PET bottle application by volume), AA contamination above this threshold ruins the product. This is why the AA content in bottle-grade PET resin is a mandatory parameter under GB/T 17931, with a premium-grade ceiling of ≤ 1.0 µg/g (ppm).
Test method (SH/T 1817-2017, headspace GC): the PET resin sample is placed in a sealed headspace vial and heated to a defined temperature (typically 150 °C) for a defined equilibration time. The AA that volatilises into the headspace gas is sampled and injected into a gas chromatograph with an FID detector. The peak area is compared to an AA standard calibration, and the AA content is reported in µg/g.
The processing effect: AA in the finished bottle is a combination of (a) the residual AA in the resin as shipped and (b) the AA generated during melt processing. A resin with 0.8 µg/g AA can produce a bottle with 2–4 µg/g after melt processing at high temperature. This is why the resin specification is tight — the processor needs headroom for the processing-generated AA. A resin at the 1.0 µg/g ceiling leaves no processing margin.
What Are the Colour and Clarity Requirements?
PET resin colour is measured on the CIE L*a*b* scale — L* is lightness (higher = whiter/brighter), b* is yellowness (lower/positive = more yellow). For bottle-grade PET:
- L* ≥ 80 (brightness — a low L* makes the bottle look grey)
- b* ≤ 1.0 for premium grade (preferably ≤ 0.5 — a high b* makes the bottle look yellow)
A b* value exceeding 3 produces a visibly yellowish preform and bottle — a quality defect that is immediately visible to the consumer. The yellowing comes from thermal degradation during polycondensation (over-long reaction time, over-high temperature) or from contamination.
Haze / clarity is also specified for bottle-grade PET, because PET bottles for water and beverages must be crystal-clear. Haze in the resin (from crystallinity, contamination, or additive incompatibility) produces cloudy bottles that fail the visual-quality acceptance at the beverage filler.
Colour and haze are measured on a pressed or injection-moulded plaque of the resin, not on the pellet itself — the pellet geometry affects light scattering and gives a false reading. The test plaque preparation is specified in GB/T 14190.
What Are Moisture and DEG Requirements?
Moisture content: PET resin is hygroscopic — it absorbs moisture from the atmosphere, and the moisture must be removed by drying before processing (typically 4–6 hours at 160–180 °C in a dehumidifying dryer, to < 50 ppm). The as-shipped moisture is tested per GB/T 14190 (Karl Fischer titration) and is typically specified at ≤ 0.4 % for the resin as packaged. Moisture above this level indicates inadequate packaging, storage, or transport, and the resin will require extended drying before processing.
Diethylene glycol (DEG) content: DEG is a by-product of the ethylene glycol self-condensation side reaction during PET polymerisation. It becomes incorporated into the polymer chain as a random copolymer. DEG affects the melting point (lowers it) and the thermal stability of the resin. For bottle-grade PET, DEG is typically controlled at ≤ 1.4 % (as a weight percentage of the polymer). The test is by GC after hydrolysis or by NMR.
Isophthalic acid (IPA) content: for copolymer PET (the most common bottle-grade resin), IPA is added as a comonomer (typically 1–3 mol %) to slow the crystallisation rate, making the resin amorphous enough for bottle blowing. The IPA content is verified by hydrolysis and HPLC or NMR. An IPA content outside specification changes the crystallisation kinetics and produces bottles with haze or uneven wall thickness.
How Does the GB Framework Map to International PET Standards?
| Scope | China (GB/T) | International (ISO / ASTM) |
|---|---|---|
| Bottle-grade PET product | GB/T 17931-2018 | ISO 1628-5 (IV), ASTM D4603 (IV) |
| Intrinsic viscosity | GB/T 14190 (Ubbelohde) | ISO 1628-5 / ASTM D4603 |
| Acetaldehyde | SH/T 1817-2017 (headspace GC) | ASTM F2013 (headspace GC) |
| Colour (L*a*b*) | GB/T 14190 | ASTM D6290 / ISO 11664 |
| Moisture | GB/T 14190 (Karl Fischer) | ASTM D7609 (Karl Fischer) |
| DEG content | GB/T 14190 (GC) | ASTM D7334 |
| Food-contact compliance | GB 4806.7 (plastic) | EU 10/2011, FDA 21 CFR 177.1630 |
The IV values are aligned across frameworks (ASTM D4603 and ISO 1628-5 give closely correlated results, though solvent and temperature differences can produce small discrepancies). The AA ceiling is industry-driven rather than standard-specific — most global bottle-grade specifications cite ≤ 1.0 ppm regardless of the product standard used.
Our Testing Capabilities
Beijing ZKGX Research provides PET resin testing against GB/T 17931-2018 and the ASTM / ISO reference framework.
Polymer quality (GB/T 17931):
- Intrinsic viscosity (GB/T 14190 / ASTM D4603, Ubbelohde viscometer, phenol/TCE solvent)
- Acetaldehyde content (SH/T 1817-2017, headspace GC, ≤ 1.0 µg/g)
- Colour L*a*b* and haze (on injection-moulded plaque)
- Moisture content (Karl Fischer)
- DEG content (GC after hydrolysis)
- IPA content (for copolymer PET, HPLC/NMR)
Food-contact compliance (GB 4806.7):
- Overall migration and specific migration
- Antimony (Sb) — residual catalyst residue (ICP-MS)
- Heavy metals and NIAS screening (for rPET)
Thermal and physical:
- Melting point (DSC, GB/T 19466)
- Density
- Crystallinity (DSC)
If you need a GB/T 17931 PET resin batch-release report for food-packaging-grade or non-food-grade resin, an intrinsic-viscosity verification, an acetaldehyde compliance test, a colour/haze qualification, or a food-contact migration test for PET resin destined for bottle, film, or fibre applications — contact our laboratory with the resin grade (bottle / film / fibre), homopolymer or copolymer, declared IV, and applicable standard, and we will scope the test plan.
FAQ
What is intrinsic viscosity and why is it the most important PET test?
IV measures the average molecular weight of the PET polymer by dissolving it in a mixed solvent and measuring flow time through a capillary viscometer. Higher IV = longer chains = stronger bottle. Bottle-grade PET is typically 0.76–0.84 dL/g. IV degrades with moisture during processing, so it must be controlled both in the resin as shipped and during drying and melt processing.
Why is acetaldehyde a mandatory parameter?
Because AA migrates into the bottled beverage and imparts a sweet, fruity off-taste detectable at 20–40 µg/L in mineral water. The mandatory ceiling of ≤ 1.0 µg/g in the resin ensures that even after the additional AA generated during melt processing, the finished bottle stays below the taste threshold. A resin at the ceiling leaves no processing margin — this is why premium resin is specified well below 1.0 µg/g.
How does moisture affect PET processing?
PET is a condensation polymer, and water hydrolyses the ester linkages at processing temperature (~280 °C), breaking chains and lowering IV. Even trace moisture (100–200 ppm) can drop the IV by 0.02 dL/g during extrusion, moving the resin out of specification. This is why PET must be dried to < 50 ppm moisture before processing — the moisture test verifies the as-shipped condition, but the processor must re-dry if the resin has absorbed moisture in storage.
What is the difference between homopolymer and copolymer PET?
Homopolymer PET contains only ethylene glycol and terephthalic acid repeat units. Copolymer PET contains 1–3 mol % isophthalic acid (IPA) as a comonomer, which disrupts the regular chain packing and slows crystallisation. Copolymer PET is the standard bottle-grade resin because the slow crystallisation allows the preform to be clear and amorphous before bottle blowing — homopolymer PET crystallises too quickly and produces hazy bottles.
How is rPET tested differently from virgin PET?
Virgin PET is tested for its inherent quality parameters (IV, AA, colour, DEG, moisture) per GB/T 17931. Recycled PET (rPET) is tested for the same quality parameters plus additional safety parameters: residual contaminants from the recycling stream (non-intentionally added substances / NIAS), heavy metals (Pb, Cd, Cr, Sb accumulation from the recycling loop), and migration compliance for food-contact use. rPET for food-contact bottles must pass a challenge test (EFSA or FDA recycling process certification) that demonstrates the recycling process removes contaminants to safe levels.