Key takeaways
- PET (polyethylene terephthalate) is specified by intrinsic viscosity (IV), measured in dL/g — the polyester equivalent of melt flow index — not by MFI.
- Bottle grade (IV ~0.74–0.84) is the high-IV, solid-stated resin for carbonated and water bottles; fibre and film grades sit lower and serve different lines entirely.
- PET is hygroscopic: it must be dried to very low moisture before processing or it hydrolyses, IV drops, and parts go brittle — packaging and drying are part of the spec.
- rPET (recycled PET) is the fastest-moving segment, driven by EU PPWR and brand recycled-content commitments; food-grade rPET needs an approved decontamination process.
PET trips up buyers who arrive from the polyolefin world, because almost nothing transfers. You do not specify PET by melt flow index; you specify it by intrinsic viscosity. It is not forgiving about moisture the way polyethylene is; it must arrive dry and stay dry. And its grades — bottle, fibre, film — are far less interchangeable than the families within polyethylene or polypropylene.
Intrinsic viscosity (IV), measured in deciliters per gram (dL/g), is the proxy for PET's molecular weight — and therefore its strength, its stretch behaviour, and how hard it is to process. Higher IV means longer chains: more mechanical strength, more melt strength for blowing a bottle, and a more demanding dry-and-process window. When a PET RFQ omits IV, it is as incomplete as a polyethylene RFQ with no MFI and no comonomer.
| Grade | Typical IV (dL/g) | End use | Note |
|---|---|---|---|
| Bottle / preform | 0.74–0.84 | Water, CSD, and hot-fill bottles via stretch-blow | Solid-state polymerised; highest IV |
| Fibre (textile) | 0.62–0.67 | Polyester staple and filament for textiles | Lower IV, spun not blown |
| Film / BOPET | 0.58–0.64 | Packaging and technical film | Tuned for biaxial orientation |
| rPET (food-grade) | matched to use | Recycled-content bottles and trays | Needs approved decontamination process |
The single most expensive PET mistake is processing it wet. PET is hygroscopic, and at melt temperature absorbed water drives hydrolysis that cleaves the chains, drops the IV, and leaves you with brittle, hazy, low-strength parts. Bottle grade has to be dried to very low residual moisture — typically below about 50 ppm — in a desiccant dryer before it reaches the screw. That is why for PET the packaging, liner, and moisture-as-shipped are part of the technical spec, not logistics trivia, and why the Certificate of Analysis should report moisture.
Recycled PET is the fastest-moving part of the PET market, and the driver is regulation and brand commitment rather than price. The EU Packaging and Packaging Waste Regulation sets mandatory recycled-content thresholds for plastic packaging, and major brand owners have their own targets ahead of the law. The catch is food contact: only rPET produced through an approved decontamination process — assessed by EFSA in the EU or covered by an FDA letter of non-objection in the US — is food-safe. Confirm the approval and the recycled-content percentage on the paperwork; do not infer them.
- Grade family (bottle/preform, fibre, film) and end-application
- Intrinsic viscosity target and tolerance, in dL/g
- Colour (clear, light-blue tint), and any UV or fast-reheat additive for preforms
- Moisture as shipped and packaging with a moisture barrier
- For rPET: recycled-content percentage and the food-contact approval reference
- Incoterm and port, and whether a pre-shipment inspection will confirm grade and moisture
PET rewards precision. Pin the IV, treat moisture as a spec line rather than a handling note, and verify any food-contact or recycled-content claim against real documentation. Do that and PET sourcing is as predictable as any other family — the discipline is the same one behind a well-written RFQ.
Frequently asked
Questions on the desk
What is intrinsic viscosity in PET?
Intrinsic viscosity (IV), reported in deciliters per gram (dL/g), is the standard measure of a PET resin's molecular weight and therefore its strength and processing behaviour. It is the polyester world's equivalent of melt flow index for polyolefins — higher IV means longer chains, more strength, and a more demanding process. Bottle grade runs roughly 0.74–0.84 dL/g; fibre and film grades sit lower.
Why does PET have to be dried before processing?
PET is hygroscopic — it absorbs atmospheric moisture — and at melt temperature that moisture drives hydrolysis, which cuts the polymer chains and drops the intrinsic viscosity. The result is weak, brittle, hazy parts. Bottle-grade PET must be dried to very low residual moisture (typically below ~50 ppm) in a desiccant dryer before it reaches the screw. This is why PET packaging and moisture-as-shipped on the CoA matter more than for most resins.
What is the difference between bottle, fibre, and film grade PET?
Bottle grade is high-IV, solid-state-polymerised resin engineered for stretch-blow moulding into bottles and preforms. Fibre grade is lower-IV resin spun into polyester staple and filament for textiles. Film grade (including BOPET) is tuned for biaxial orientation into packaging and technical film. They are the same polymer family but are not interchangeable across lines.
Is recycled PET (rPET) food-safe?
rPET can be food-safe, but only when it is produced through a decontamination process that regulators have approved for food contact — for example processes assessed by EFSA in the EU or covered by an FDA letter of non-objection in the US. Mechanically recycled rPET from a non-approved stream is not automatically food-grade. Always confirm the food-contact approval and recycled-content percentage on the documentation.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.