PET intrinsic viscosity grades: fiber, film, bottle and high-IV strapping chips

IV by end use

Higher IV means longer chains, more melt strength and more strength in the finished product, but also a stiffer melt and more energy to process. Each end use settles where its process and product meet. The windows below match our PET and PBT grade finder, with single-grade examples from producer data sheets.

Typical published windows. Producers measure IV by different methods (next section), so match the window and the grade family, not just the number.

How IV is measured, and why two labs disagree

IV is a solution test. The lab dissolves a weighed amount of PET in a solvent, times the solution through a glass capillary viscometer, and compares it with the pure solvent. The ratio of the two flow times is the relative viscosity, and everything else is calculated from it.

• ASTM D4603 (current edition D4603-26). Inherent viscosity of PET at 0.50% concentration in 60/40 phenol/1,1,2,2-tetrachloroethane, in a glass capillary viscometer (Type 1B Ubbelohde), with flow times taken at 30°C / 86°F. Inherent viscosity is the natural log of relative viscosity divided by concentration, in dL/g, and IV is often calculated from it. ASTM says results from differential viscometry by D5225 are comparable, and highly crystalline PET that will not dissolve in this solvent needs a different procedure.
• ISO 1628-5. Viscosity number (VN) of thermoplastic polyesters, including PET, PBT, PCT and PEN, in ml/g. VN is relative viscosity minus one, divided by concentration. ASTM notes that the ISO method offers a choice of solvents (the ASTM solvent is one of them), uses a Type 1C rather than 1B Ubbelohde viscometer, and reports VN rather than inherent viscosity. The current ISO edition dates from 1998. A revision (ISO/DIS 1628-5) closed its draft ballot in September 2026 and has not yet been published.
• Producer methods. Sheets we checked use 60/40 phenol/dichlorobenzene at 25°C / 77°F in an Ubbelohde viscometer, or China’s GB/T 14190 method (one sheet lists a 1:1 phenol-based solvent at 25°C / 77°F). Some producers will test to ASTM D4603 on request.
• What that means for you. The same lot can read slightly differently by different methods. Ask which method is behind the IV on every COA, compare suppliers on the same one, and agree a referee method before a dispute, not after.

Units box: dL/g, ml/g and the 78 ml/g tariff line

• IV and inherent viscosity: dL/g. Deciliters per gram, also written dl/g. 1 dL/g = 100 ml/g.
• Viscosity number: ml/g. Also written cm³/g. Used by ISO 1628-5 and on many European sheets.
• Do not divide a VN by 100 and call it IV. VN is measured at a set concentration and uses relative viscosity minus one, so for the same polymer it reads higher than 100 × IV. A VN of 78 ml/g means shorter chains than an IV of 0.78 dL/g.
• The U.S. tariff line is written in ml/g. HTS subheading 3907.61.00 covers PET “having a viscosity number of 78 ml/g or higher”, with a statistical split at 88 ml/g. PET below 78 ml/g falls under 3907.69.00 (other), which has its own statistical line for 70 to under 78 ml/g. Classification turns on VN, not IV.
• Trade-remedy scopes are written in dL/g. The U.S. antidumping orders continued in April 2022 cover PET resin with an IV of at least 0.70 and not more than 0.88 dL/g. Same product, two units.
• Melt flow is rarely used for PET. Buy and check PET on IV, by a stated method.

Solid-state polymerization (SSP) and high-IV chips

Melt-phase polymerization makes amorphous chips, often near fiber IV. To go higher, most plants crystallize the chips and then hold them hot, below the melting point, under nitrogen or vacuum for hours. The chains keep linking in the solid, and the gas sweeps out ethylene glycol, water and acetaldehyde. On the sheets we checked, copolymer bottle grades melt at about 240–250°C / 464–482°F and homopolymer grades at about 261°C / 502°F.

SSP raises IV, lowers acetaldehyde and ships crystallized chips that can go straight into a hot dryer. Some newer plants reach bottle IV in the melt and remove acetaldehyde afterward (melt-to-resin), so not every bottle chip is solid-stated. Strapping, industrial yarn, refillable jugs and CPET need SSP-level IV, and recyclers use SSP to bring flake back up to strapping or bottle IV. For a high-IV grade, ask for the IV spread lot to lot as well as the average, since strength-critical lines feel the spread.

Drying to about 50 ppm, and IV drop in processing

PET chips can leave the plant with up to a few tenths of a percent moisture; the sheets we checked allow 0.20–0.3% when packed. Bottle, sheet and fiber lines dry to about 50 ppm (0.005%), often at about 160–170°C / 320–338°F for 4–6 hours with air at a dew point of about −40°C / −40°F. Check the producer’s maximum dryer temperature. Amorphous chips and regrind soften and stick at about 70–80°C / 158–176°F, so they go through a crystallizer first. See the resin drying chart.

• Water cuts chains in the melt. Wet PET hydrolyzes at melt temperature: IV drops, acetaldehyde and carboxyl end groups rise, and the product can look fine and still be weak. See brittle parts that look fine.
• Some drop happens even when dry. Heat and shear in the extruder break chains too. Measure IV on the chips and on the preform, strap or yarn, and track the difference shift to shift.
• A bigger drop than usual points to the process. Look at dryer dew point and residence time first, then melt temperature and residence. A new lot of resin is often blamed for a dryer problem.
• High-IV chips lose more. They are processed hotter, so industrial yarn ends up below the chip IV. Build that loss into the chip IV you specify.
• Over-drying yellows. Long, hot drying in air raises b*. Follow the producer’s time and temperature.

Acetaldehyde, color b* and carboxyl end groups

• Acetaldehyde (AA). A by-product of melt degradation that can taint the taste of water. Bottle chip sheets we checked list 1 ppm maximum. Preform AA also depends on melt temperature, residence time and shear, so specify both the chip limit and your preform target. Fiber, film and strapping lines rarely specify AA.
• Color L* and b*. CIE L* is lightness and b* runs from blue (negative) to yellow (positive). One producer's bottle grades list b* of −1.5 ± 1.5 (CIE Lab), and its high-IV homopolymer grades list 3 ± 2 to 3 ± 3 by a different method. Other bottle sheets we read run from about −3.5 to about +1.2. Crystalline chips, amorphous chips and ground samples read differently, so compare b* only on the same form and method.
• Carboxyl end groups (CEG). Acid ends in meq/kg, which speed hydrolysis. Some bottle sheets list about 30 meq/kg maximum, one semi-dull fiber chip 38 maximum, and one tire-cord chip 15 ± 4. Ask for low CEG for industrial yarn, strapping and outdoor products.
• Also on fiber and bottle sheets. DEG content and TiO2 (fiber), crystallinity (one bottle grade lists 50% minimum), dust or fines (100 ppm maximum on several sheets), chip weight and melting point.
• Moisture as packed. A COA value, not a molding value. Dry every lot. See what a COA is.

Recycled PET and IV

Bottle flake often arrives at about 0.70–0.80 dL/g, food-contact rPET pellets that have been decontaminated and often solid-stated at about 0.75–0.84, and green or mixed-color flake for fiber and strapping at about 0.65–0.78. Strapping lines build IV back up with SSP or a chain extender. Expect a wider IV spread lot to lot than virgin, more moisture as delivered (washed flake can reach about 1%), and limits on PVC and other polymers in ppm, since one PVC chip can burn in the melt or break a fiber. For food contact, ask for the recycler’s FDA letter of no objection and its conditions of use. See recycled resin and prime vs. off-grade.

Buying PET by IV: packaging, imports and the RFQ checklist

Bottle, sheet and fiber PET mostly moves by hopper railcar (about 180,000–210,000 lb) and bulk truck (about 40,000–50,000 lb). Imported chips often come in bulk bags of about 1,000–1,250 kg. Gaylords suit trials, smaller sheet and strapping lines, and monofilament extruders, and cost more per pound than bulk. We quote case by case through our supply partners, from 22+ partner warehouse locations, starting at one gaylord. Every prime lot from our suppliers ships with a COA.

Imported PET can carry trade duties. A Department of Commerce notice in the Federal Register of April 13, 2022 (87 FR 21871) continued antidumping duty orders on PET resin with an IV of 0.70–0.88 dL/g from Canada, China, India and Oman, and a notice of April 11, 2022 continued countervailing duty orders on PET resin from China and India. Commerce was still publishing annual reviews of the Oman order in 2026, so the orders were in force at our last check. Orders change and other tariffs may apply, so check current status with the ITA and your customs broker before you count on an import price. PET prices follow PTA and MEG, imports and the beverage season; see the PET and PBT page and how resin is priced.

• End use and process. Fiber, film, sheet, bottle, strapping or yarn, and the line it runs on.
• IV window and method. Target, tolerance and the test: ASTM D4603, ISO 1628-5 (as VN in ml/g) or the producer’s method.
• Polymer type and form. Homopolymer or copolymer, crystallized or amorphous chips.
• Quality limits. AA maximum (bottles), L* and b* with the method, carboxyl end groups, DEG, TiO2 level for fiber, and additives such as reheat or antiblock.
• Recycled content. rPET share, IV range, PVC limit, and the FDA letter if food contact.
• Packaging and moisture. Railcar, bulk truck, bulk bag or lined gaylord, and moisture as packed.
• Documents. A lot COA with IV and its method, the producer’s FDA letter for food use (virgin PET food-contact letters often cite 21 CFR 177.1630), and country of origin.
• Quantity and ship-to. This order, yearly use, ship-to city, and whether imported resin is acceptable. See the RFQ package page.

Common questions

If your grade is not listed, tell us what you run. We source commodity, engineering and specialty resins.