Recycled PET (rPET) purity testing is a critical quality control step for verifying the safety, consistency, and regulatory compliance of post-consumer and post-industrial recycled polyethylene terephthalate used in packaging, textiles, and technical applications. This guide explains the key purity indicators, sample types, analytical methods, and international standards—such as ISO, ASTM, EN, and GB—used to identify polymer contaminants, degradation products, heavy metals, and volatile impurities in recycled PET materials.
Technical Overview
Recycled polyethylene terephthalate (rPET) is produced by reclaiming post-consumer bottles, trays, and production scrap through sorting, grinding, hot washing, and reprocessing. Because the feedstock stream is inherently variable, rPET may carry impurities that are absent or tightly controlled in virgin resin. Purity testing therefore focuses on detecting and quantifying foreign polymers, degradation products, chemical residues, inorganic contaminants, and volatiles that can compromise processability, mechanical performance, optical quality, or consumer safety.
Typical impurity categories in rPET include other plastics such as PVC, PE, PP, PS, PA, PC, and PLA; thermo-oxidative and hydrolytic degradation products that reduce molecular weight; chemical contaminants absorbed during the first use cycle, such as flavorings, detergents, or fuel components; catalyst residues, primarily antimony; colorants, adhesives, and label fragments; and moisture, ash, and inorganic fines.
Purity requirements are defined by a combination of material specifications, buyer contracts, and regulatory frameworks. EN 15346 provides the characterization framework for PET recyclates in Europe, while food-contact use in China is governed by GB 4806.7 and related national standards. European food-contact plastics are regulated under Regulation (EU) No 10/2011, and US requirements for PET in food contact are addressed in 21 CFR 177.1630. Accredited third-party laboratories routinely test rPET against these frameworks to support supplier qualification, lot release, and recycling-process approval.
Testing Samples
Hot-Washed rPET Flakes
Washed flake is the most common sample form and represents the core commodity of the mechanical recycling industry. Testing of flakes focuses on foreign-particle content (PVC, polyolefins, metals, paper, colored PET), moisture, bulk color, and intrinsic viscosity. Bottle-grade flake specifications frequently require PVC contamination below 50–100 mg/kg and polyolefin contamination below 100–300 mg/kg, depending on the end use.
rPET Pellets and Granules
Regranulated rPET, typically produced with melt filtration and, for high grades, solid-state polycondensation (SSP), is tested for intrinsic viscosity restoration, color stability, acetaldehyde content, black-speck counts, and thermal behavior. Pellet samples allow assessment of extrusion-induced degradation and the effectiveness of filtration and decontamination steps.
Food-Contact Grade rPET Resin
So-called "super-clean" resin intended for bottle-to-bottle or tray applications undergoes the most demanding purity testing, including migration studies with food simulants, surrogate contaminant challenge testing of the recycling process, acetaldehyde and antimony determination, and verification that contaminants remain below regulatory limits under defined time–temperature conditions.
Fiber, Filament, and Sheet Grade rPET
Lower-viscosity streams used for staple fiber, POY/FDY filament, nonwovens, strapping, and thermoforming sheets are tested against application-specific viscosity windows (commonly 0.55–0.68 dL/g for fiber grades), color tolerances, and contamination limits. Although food-contact regulations do not apply, purity still determines spinning stability, dye uptake, and product appearance.
Finished rPET Articles
Preforms, bottles, trays, films, and injection-molded parts made from recycled PET are tested to confirm compliance of the final article, including overall and specific migration, residual acetaldehyde, color, and mechanical performance.
Testing Items
Depending on the sample type, grade, and intended application, a purity testing program typically combines the following item groups:
Polymer Purity and Foreign-Matter Contamination
- Identification and quantification of non-PET polymers: PVC, PE, PP, PS, PA, PC, PMMA, PLA, ABS
- Colored PET flakes, opaque particles, and label or glue residues
- Metal particles, rubber, paper, and other visible contaminants
- Black-speck and gel counts in pellets and finished articles
Intrinsic Viscosity and Molecular Structure
- Intrinsic viscosity (IV), with bottle-grade resin typically specified at 0.72–0.85 dL/g
- Molecular weight distribution and weight-average molecular weight
- Carboxyl end-group concentration as an indicator of hydrolytic or thermal degradation
- Diethylene glycol (DEG) content incorporated in the polymer chain
Thermal and Rheological Properties
- Melting temperature, melting enthalpy, and crystallization behavior
- Glass transition temperature and cold crystallization peaks
- Melt flow rate / melt volume flow rate for process control
- Thermal stability and degradation onset
Color and Optical Properties
- CIELAB color coordinates (L*, a*, b*) and yellowness index
- Haze and transparency of films, sheets, and bottles
- Color consistency between production lots
Chemical Contaminants and Volatiles
- Acetaldehyde, typically limited to 1–3 mg/kg for water-bottle-grade resin
- Volatile organic compounds, including limonene and absorbed fragrance or solvent residues
- Oligomers and monomeric species such as terephthalic and isophthalic acid, and ethylene glycol
Elemental Impurities and Heavy Metals
- Antimony, lead, cadmium, mercury, chromium(VI), arsenic, nickel, cobalt, and zinc
- Compliance with packaging heavy-metal limits, such as the cumulative 100 mg/kg ceiling of the European Packaging Directive 94/62/EC
Food-Contact Safety and Migration
- Overall migration into food simulants under standardized time–temperature conditions
- Specific migration of antimony, terephthalic acid, primary aromatic amines, and other regulated substances per Regulation (EU) No 10/2011 and GB 4806.7
- Process challenge tests demonstrating decontamination efficiency of the recycling technology
Moisture, Ash, and Residues
- Moisture content, with washed flake commonly specified at or below 0.5–1.0% and processing-grade pellets requiring tight drying control to prevent IV loss
- Ash content and inorganic residue, indicating fillers, catalyst residues, and mineral contamination
- Density as a screening parameter for gross polymer mixtures
Testing Methods
FTIR and NIR Spectroscopic Identification
Fourier-transform infrared spectroscopy, performed in accordance with ASTM D5477, is the primary method for confirming polymer identity and identifying isolated contaminant particles. Near-infrared (NIR) techniques support rapid sorting verification and screening of large flake volumes for non-PET fractions, while hyperspectral imaging extends detection to mid-scale contamination mapping.
Differential Scanning Calorimetry (DSC)
DSC, conducted under ISO 11357 or ASTM D3418, measures melting behavior, glass transition, and crystallization of rPET. It is also a powerful purity tool: characteristic transitions reveal PVC, PE (melting near 130 °C), PP (near 165 °C), PLA, PA, and PC at low concentration levels, and shifts in melting enthalpy or the appearance of multiple melting peaks indicate processing history and contamination.
Thermogravimetric Analysis (TGA)
TGA quantifies ash content, inorganic residues, and filler levels, and distinguishes polymers by decomposition temperature. It complements ash testing performed under ASTM D5630 for routine quality control.
Headspace GC-MS for Volatiles and Acetaldehyde
Static headspace gas chromatography–mass spectrometry identifies and quantifies volatile organic contaminants. Acetaldehyde, the key bottle-grade parameter, is determined under ASTM F2013. GC-MS also detects absorbed limonene, toluene, and other surrogate contaminants used in recycling-process validation studies.
HPLC for Glycols, Acids, and Oligomers
High-performance liquid chromatography separates and quantifies diethylene glycol, terephthalic and isophthalic acid, ethylene glycol, and cyclic oligomers, supporting both formulation verification and degradation assessment.
ICP-OES, ICP-MS, and AAS for Elemental Analysis
Inductively coupled plasma optical emission spectrometry and mass spectrometry, applied in accordance with ISO 17294-2, deliver multi-element screening of antimony and heavy metals down to sub-mg/kg levels. Atomic absorption spectrophotometry serves as a targeted confirmatory technique for single elements such as antimony.
Solution Viscometry and GPC
Intrinsic viscosity is measured by dilute-solution viscometry under ISO 1628-5 or ASTM D4603, using phenol-based solvent systems. Gel permeation chromatography (size-exclusion chromatography) under ISO 16014 provides the full molecular weight distribution, revealing chain scission and broadening that a single IV value cannot capture.
Karl Fischer Titration and Moisture Analysis
Water content is determined by Karl Fischer titration under ISO 15512 or by rapid moisture analyzers. Accurate moisture data are essential because residual water drives hydrolytic IV loss during melt processing.
Colorimetry and Haze Measurement
Instrumental color measurement under ASTM E313 and ISO 11664 quantifies L*, a*, b* values and yellowness index of flakes, pellets, and articles, while hazemeters evaluate optical clarity of films and bottles against specification limits.
Melt Flow and Density Testing
Melt flow rate is determined under ISO 1133-1 or ASTM D1238, and density under ISO 1183-1, ASTM D792, or ASTM D1505. These routine physical indicators screen for gross contamination, batch consistency, and processing suitability.
Migration Testing for Food-Contact Compliance
Finished articles and test plaques are exposed to standardized food simulants under time–temperature conditions reflecting intended use, then analyzed for overall migration and specific migrants. Compliance is assessed against Regulation (EU) No 10/2011 (for example, the 0.04 mg/kg antimony specific migration limit and 10 mg/dm² overall migration limit) and against GB 4806.7 for the Chinese market.
Applications
rPET purity testing supports decision-making across the entire recycling and converting value chain:
- Bottle-to-bottle recycling: verifying flake and resin purity for direct food-contact use, including decontamination process validation and lot release
- Food packaging compliance: demonstrating conformity with European, US, and Chinese food-contact regulations for trays, containers, films, and preforms
- Polyester fiber and textile production: controlling viscosity windows, color, and contaminants that affect spinning performance and fabric quality
- Sheet, film, and strapping manufacturing: monitoring contamination levels, thermal properties, and optical quality for converting stability
- Automotive and engineering components: qualifying rPET grades where mechanical reliability and batch traceability are required
- Supplier qualification and recycled-content claims: providing objective laboratory evidence for incoming material inspection, quality disputes, and recycled-content certification schemes
FAQ
Why is PVC considered the most critical contaminant in recycled PET?
PVC releases hydrogen chloride at PET processing temperatures, which catalyzes chain scission, causes severe yellowing, and corrodes processing equipment. Even concentrations of a few tens of mg/kg can degrade batch quality, which is why bottle-grade specifications commonly limit PVC to below 50–100 mg/kg.
What intrinsic viscosity is required for bottle-grade rPET?
Bottle-grade applications generally require an intrinsic viscosity of approximately 0.72–0.85 dL/g. Recycled flakes typically measure 0.70–0.78 dL/g after washing, and solid-state polycondensation is used to restore or upgrade the IV toward the virgin-resin window.
Is purity testing mandatory for food-contact recycled PET?
Yes. Recycled PET intended for direct food contact must be produced by an approved or otherwise validated recycling process and must comply with applicable food-contact regulations, such as Regulation (EU) No 10/2011, US requirements under 21 CFR 177.1630, and GB 4806.7. Independent testing at accredited third-party laboratories provides the analytical evidence required for process approval and market surveillance.
How are non-PET plastics detected and quantified?
Manual and automated sorting checks quantify visible foreign flakes in a defined sample mass, while FTIR (ASTM D5477) identifies individual suspect particles. DSC (ISO 11357, ASTM D3418) detects characteristic thermal transitions of polyolefins, PVC, PA, PLA, and PC, and TGA or density separation can further verify polymer mixtures.
What does a typical rPET purity testing program include?
A standard program for washed flakes usually covers intrinsic viscosity, color (L*, a*, b*), moisture, ash, PVC and polyolefin contamination, DSC purity screening, and heavy metals. Food-grade programs additionally include acetaldehyde by ASTM F2013, antimony and elemental screening by ICP-based methods, and full migration testing.
How much sample is needed and how long does testing take?
Sample requirements depend on the scope: contamination counts and IV testing may require 100–500 g of flake, while full migration programs may need several kilograms of resin or finished articles. Typical turnaround at accredited third-party laboratories ranges from five to ten working days for routine purity panels, with migration studies requiring additional exposure time.