Quantitative Assessment of Pyrolysis Components in Recycled Plastics: Py-GC/MS and Thermogravimetric

Quantitative assessment of pyrolysis components in recycled plastics uses thermogravimetric analysis (TGA) and calibrated pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) to determine, at mg/kg levels, the thermal degradation products, monomer and oligomer yields, additive residues, and contaminants released when recyclates are heated. This testing is essential for recyclate qualification, contamination screening, thermal stability verification, and chemical recycling feedstock evaluation, and is performed by accredited third-party laboratories in accordance with ISO, ASTM, GB, and JIS standards.

Technical Overview

Recycled plastics derived from post-consumer and post-industrial streams carry an accumulated thermo-oxidative history, residual and degraded additives, and adventitious contaminants that are released or transformed whenever the material is heated, whether during melt reprocessing, end-use service, or intentional pyrolytic conversion. Quantitative assessment of pyrolysis components establishes a molecular-level mass balance of these decomposition products, spanning light gases, condensable hydrocarbons, monomers and oligomers, additive fragments, heteroatom-bearing species, and inorganic residues.

The evaluation integrates three complementary analytical platforms. Thermogravimetric analysis (TGA) provides whole-sample, step-resolved mass-loss quantification under controlled atmospheres. Hyphenated evolved-gas techniques, namely TGA-FTIR and TGA-MS, track functional-group signatures and targeted species in real time as they evolve. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), operated with isotope-dilution internal standards and multi-point calibration, delivers compound-specific quantification down to sub-mg/kg levels. Together, these methods generate the compositional fingerprints, degradation kinetic parameters, and gas/oil/wax/char yield distributions required for recyclate qualification, contamination screening, and chemical recycling feedstock evaluation by accredited third-party laboratories.

Testing Samples

Post-Consumer Recycled (PCR) Resins

Recycled PET flakes and pellets, rHDPE and rPP from bottle and rigid-packaging streams, rLDPE and rLLDPE film recyclates, rPS, rABS, and rPA. The typical objective is to characterize the degradation-product profile and confirm a safe thermal processing window for subsequent extrusion or injection molding.

Post-Industrial Regrind and Production Scrap

Sprues, runners, start-up purge material, off-spec parts, and in-house regrind from multiple prior heat histories. Analysis focuses on oxidation products formed during previous processing, such as lactones and ketones in polyolefins and acetaldehyde or oligomers in PET, which indicate the remaining stabilization margin.

Commingled and Mixed Plastic Streams

Mixed plastic waste, rejects from optical sorting lines, and agglomerated flakes from multi-material packaging. Multi-step TGA resolves the principal polymer fractions, while Py-GC/MS marker compounds quantify minor constituents and cross-contaminants, for example PVC in PET streams, EVOH barrier layers in polyolefins, or PC in ABS regrind.

Chemical Recycling Feedstocks and Intermediate Products

Shredded waste polyolefins destined for pyrolysis plants, together with derived pyrolysis oils, waxes, and solid chars. Testing quantifies the gas/liquid/wax/char yield split and identifies halogen-, nitrogen-, and sulfur-bearing products that constrain downstream upgrading and catalyst life.

Testing Items

Quantitative reporting covers the following measurands, expressed in mg/kg or wt% of original sample mass where applicable:

• Thermal stability parameters: onset decomposition temperature (T5%), T10%, T50%, DTG peak temperature (Tmax), and residual mass at defined endpoints.
• Step-resolved compositional fractions: moisture and light volatiles below 150 °C, semi-volatile additive fraction from 150–300 °C, main-chain polymer decomposition from 300–550 °C, carbon black and inorganic filler, and final ash.
• Light pyrolysis gas composition: C1–C4 hydrocarbons, CO and CO2, hydrogen chloride, hydrogen cyanide, and sulfur-containing gases by TGA-MS.
• Monomer and monomer-precursor yields: styrene and α-methylstyrene (PS, ABS), methyl methacrylate (PMMA), ε-caprolactam (PA6), ethylene and propylene (PE, PP), vinyl chloride (PVC), bisphenol A and phenol (PC, epoxy), acrylonitrile (ABS, SAN), and terephthalate esters with benzoic acid (PET).
• Oligomer distributions: PET cyclic trimer and related cyclic oligomers, PS dimer and trimer, and PA cyclic oligomers.
• Additive-derived decomposition products: phthalate and adipate plasticizers, hindered phenol antioxidants and their quinone methide oxidation products, phosphite and phosphate antioxidants, hindered amine light stabilizer fragments, slip agents such as oleamide and erucamide, stearates, and silicone oil.
• Contaminant markers: chlorinated aromatics and HCl evolution (PVC, PVDC traces), fluoropolymer fragments (PTFE), cellulosic furan markers, and mineral-oil saturated and aromatic hydrocarbon signatures.
• Oxygenated degradation products: formaldehyde, acetaldehyde, higher aldehydes, ketones, carboxylic acids, lactones, and alcohols.
• Aromatic by-products: benzene, toluene, ethylbenzene, xylenes, styrene, naphthalene, and polycyclic aromatic hydrocarbons up to four rings.
• Chemical recycling product yields: gas, naphtha-range liquid, wax, and char fractions in wt%, with mass-balance closure checks.
• Apparent kinetic parameters: activation energy and pre-exponential factor derived from multiple-heating-rate experiments using Kissinger, Ozawa–Flynn–Wall, and Friedman isoconversional methods.

Testing Methods

Sample Preparation and Conditioning

Bulk samples are homogenized and cryogenically milled to below 500 µm to ensure representativeness, then conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with ISO 291. Typical intakes are 5–10 mg for TGA and 50–200 µg for the Py-GC/MS sample cup, with all determinations performed in replicate.

Thermogravimetric Analysis (TGA)

Experiments follow ISO 11358-1 and its national adoption GB/T 33047.1, with additional compositional analysis per ASTM E1131, rapid degradation screening per ASTM D3850, and thermal stability determination per ASTM E2550. A standard program ramps from ambient to 600 °C at 10 °C/min under 50 mL/min nitrogen, then switches to air to 900 °C to oxidize char and reveal inorganic ash. Multi-step and multi-rate programs are applied when deconvolution of overlapping polymer fractions or kinetic analysis is required.

Evolved Gas Analysis (TGA-FTIR and TGA-MS)

Heated transfer lines maintained near 280 °C carry evolved species directly to a rapid-scan FTIR spectrometer for time-resolved functional-group profiling and library matching, and/or to a quadrupole mass spectrometer operating in electron ionization with selected-ion monitoring for targeted species such as HCl, CO2, styrene, caprolactam, and C1–C4 fragments. Semi-quantitative results are converted to calibrated values using external gas standards and gravimetric cross-checks.

Quantitative Pyrolysis-GC/MS

A micro-furnace or Curie-point pyrolyzer subjects the weighed micro-sample to flash pyrolysis at 500–700 °C for approximately 20 seconds under helium. Products are separated on a 30 m capillary column and detected by MS in full-scan and SIM modes. Quantification uses isotopically labeled internal standards added prior to pyrolysis, with at least five calibration levels per target analyte, determined relative response factors, and calibration linearity of R² ≥ 0.995. For condensation polymers such as PET, PC, and PA, thermochemolysis with tetramethylammonium hydroxide is applied to convert acidic and hydroxylated products into volatile methyl derivatives suitable for reproducible quantification.

Data Treatment and Quality Assurance

Results are reported in mg/kg for individual compounds and wt% for mass fractions, with mass-balance closure within 95–105% across gas, condensable, and residue fractions. Quality controls include method blanks, certified reference materials where available, replicate precision with relative standard deviations typically below 10% for major components, and documented measurement uncertainty. Laboratories operate under quality management systems aligned with ISO/IEC 17025 requirements.

Applications

Quantitative pyrolysis component data support decision-making across the recycled plastics value chain:

• Recyclate supplier qualification and incoming quality control, verifying thermal processing windows and additive retention across lots.
• Contamination screening of recycled streams for sensitive applications, including detection of PVC in PET and brominated flame retardant residues in electronic-waste-derived polymers.
• Support for food-contact recycling process validation, where degradation-product profiling supplements conventional migration and challenge testing evidence.
• Automotive interior air quality programs, providing root-cause identification for odor and fogging complaints linked to recyclate incorporation in cabin components.
• Chemical recycling development, including feedstock specification, product yield optimization, and heteroatom contaminant monitoring for pyrolysis oils and waxes.
• Failure analysis and research, such as tracking degradation across multiple re-extrusion cycles, evaluating stabilizer and compatibilizer performance in recycled blends, and supplying kinetic inputs for process simulation.
• Documentation for recycled-content certification schemes and ESG reporting, demonstrating the chemical quality and safety of recyclate incorporated into new products.

FAQ

What sample forms and minimum quantities are accepted?

Pellets, flakes, regrind, film, fiber, foam, and shredded material can all be analyzed. A minimum of 10–20 g is recommended to allow homogenization, replicate determinations, and retention of an archive portion.

What is the difference between TGA and Py-GC/MS results?

TGA quantifies how much mass is lost at each temperature and therefore provides bulk fraction yields and thermal stability parameters, while Py-GC/MS identifies which specific molecules are produced and quantifies individual compounds at mg/kg levels. The two methods are complementary and are normally commissioned together for a complete mass balance.

Can the testing distinguish recycled from virgin resin?

Pyrolysis analysis does not directly measure recycled content percentage. However, degradation-product fingerprints, oxidized species, additive depletion markers, and contaminant signatures provide strong indicators of prior heat history and waste-stream origin, which are used to corroborate recyclate documentation.

What detection limits can be expected?

For targeted compounds analyzed by Py-GC/MS in SIM mode with isotope-dilution calibration, method detection limits are typically in the range of 0.1–1 mg/kg depending on the analyte, while bulk TGA fractions are quantified to approximately ±0.5 wt%.

Is the analysis destructive?

Yes. Each determination consumes milligram-to-sub-milligram quantities through irreversible thermal decomposition, which is why homogenization of the submitted bulk sample is critical to representativeness.

What is the typical turnaround time?

Standard single-matrix evaluations are generally completed within 5–10 working days after sample receipt, with kinetic studies involving multiple heating rates or multi-stream comparative programs requiring additional time for method development and replicate runs.