Polyethylene terephthalate is the most recycled polymer in the world by volume and the one with the most developed grading vocabulary. Where PVC has K-value and polyethylene has melt flow index, PET has intrinsic viscosity — and it is the number that decides what a given lot can be made into. This is a plain-English reference for buying and selling PET, virgin or recycled, flake or pellet.
What is intrinsic viscosity in PET?
Intrinsic viscosity, written IV and quoted in dl/g, is a measure of the polymer's molecular weight derived from how it behaves in solution. Higher IV means longer chains, more strength and more melt viscosity. It is the single most important number on a PET offer because it maps directly onto end use:
- Fibre grade — roughly 0.62 to 0.67. Staple fibre, filament, non-woven.
- Bottle grade — roughly 0.72 to 0.84. The familiar carbonated and still drinks bottle range.
- Sheet and thermoforming grade — roughly 0.70 to 0.80. Trays, punnets, clamshells.
- Strapping and engineering grade — roughly 0.85 to 1.0 and above, where strength matters most.
IV falls during processing, and it falls fast if the material is wet. That is why recycled PET is often solid-state polymerised to build IV back up, and why a recycled lot's IV needs to be measured rather than assumed from what it used to be.
Why does moisture matter so much in PET?
PET hydrolyses. If it is processed with moisture present, water attacks the polymer chain and IV drops permanently — the material is degraded, not merely damp. This is why PET must be dried to very low moisture before extrusion, and why moisture content is a legitimate contractual point on any recycled PET lot rather than a technicality. Flake stored outdoors or baled wet is a genuine risk, not a cosmetic one.
How is recycled PET traded?
- Bottle bales — baled post-consumer bottles, sorted by colour. Clear or natural sits well above light blue, which sits above mixed and green. Yield after sorting and washing is what the buyer is really paying for.
- Hot-washed flake — bottles ground, washed and separated from caps and labels. Priced on colour, PVC and polyolefin contamination, moisture and IV.
- rPET pellet — flake extruded and pelletised, sometimes solid-state polymerised to restore IV. The most finished form and the highest priced.
- Food-grade rPET — material produced through an approved decontamination process with the corresponding regulatory approval. This commands a substantial premium and is not a claim to accept without documentation.
One contamination point outranks all others: PVC. Even small amounts of PVC in a PET stream degrade it badly on processing and cause yellowing and black specks. A PET offer that cannot speak to PVC contamination is an offer with a hole in it.
What drives the price of PET?
- Intrinsic viscosity and whether it suits the buyer's process without further building.
- Colour — clear and natural above light blue above mixed above green, on yield and end-use flexibility.
- Food-contact approval, which can be worth more than every technical property combined where it applies.
- Contamination, especially PVC, polyolefins, adhesive, and residual label and cap material.
- Moisture and how the material has been stored.
- Form — pellet above washed flake above bales, reflecting the processing already done.
- Quantity, location and incoterm, with bales carrying disproportionate freight cost.
These move independently and the recycled market in particular is volatile, so a fixed list dates quickly. PET is best priced against live offers of the same form, colour and IV.
How does Prime Polymers handle PET?
PET listings state form, grade, condition, quantity, location, incoterm and the seller's own asking price per tonne, so bales, flake and pellet are compared as the different products they are. Prices are shown exactly as sellers enter them — never computed, adjusted or benchmarked by us. Browsing is free and open without an account, and technical documents such as a COA sit behind a free account. If nothing on the board fits your IV and colour requirement, a Wanted post puts it in front of sellers. Prime Polymers is operated by ARC Polymers Ltd, a UK polymer trading company.
Frequently asked questions
- What IV is bottle-grade PET?
- Bottle grade typically runs about 0.72 to 0.84 dl/g. Fibre grade sits lower at roughly 0.62 to 0.67, sheet and thermoforming around 0.70 to 0.80, and strapping higher at 0.85 and above. IV falls during processing, so a recycled lot should be measured rather than assumed.
- What is the difference between rPET flake and rPET pellet?
- Flake is washed, ground bottle material. Pellet is flake that has been extruded and pelletised, often with solid-state polymerisation to restore intrinsic viscosity. Pellet is more processed, more consistent and higher priced; flake is cheaper and needs more work at the buyer's end.
- Why is PVC contamination a problem in PET?
- PVC degrades at PET processing temperatures and releases acid, which attacks the PET chain. Even small quantities cause yellowing, black specks and IV loss. It is the single most damaging contaminant in a PET stream, and any serious offer should address it.
- Is food-grade rPET different from ordinary rPET?
- Yes. Food-grade rPET is produced through a decontamination process with regulatory approval for food contact, and carries documentation to prove it. It commands a substantial premium over ordinary recycled PET, and the approval should be evidenced rather than asserted.
- How can I see what PET is trading at?
- Compare live offers of the same form, colour and IV rather than relying on a dated figure. On Prime Polymers each PET listing shows the seller's asking price per tonne next to grade, quantity, location and incoterm. If nothing matches, post a Wanted requirement and let sellers approach you.