Qualitative Analysis of Pyrolysis Products from Waste Polyethylene

Waste polyethylene pyrolysis converts post-consumer plastics into oils, waxes, and fuel gases through thermal cracking in an oxygen-free environment, and the commercial value of these products depends directly on their chemical composition. Qualitative analysis of waste polyethylene pyrolysis products identifies the specific hydrocarbon species, functional groups, and trace contaminants present in each product fraction. This article explains the sampling strategy, testing items, analytical techniques, and international standards used by accredited third-party laboratories to qualitatively characterize waste polyethylene pyrolysis products.

Technical Overview

Pyrolysis is a leading chemical recycling route for waste polyethylene (PE), including post-consumer HDPE containers, LDPE packaging films, and LLDPE agricultural mulch films. When polyethylene is heated to approximately 400–600°C in an oxygen-free reactor, the random scission of carbon–carbon bonds along the polymer backbone generates a homologous series of hydrocarbons. The primary products typically include normal alkanes, terminal alkenes (1-alkenes, or alpha-olefins), and alpha,omega-dienes, while secondary reactions such as cyclization, aromatization, and cross-linking become more significant at higher temperatures or longer vapor residence times.

The output of a waste PE pyrolysis process is conventionally divided into four fractions, each requiring a different qualitative analytical approach:

• Non-condensable gas: hydrogen, methane, and C2–C4 hydrocarbons
• Pyrolysis oil (condensate): mainly C5–C20 aliphatic and aromatic hydrocarbons
• Pyrolysis wax: primarily C20–C50+ linear alkanes, alkenes, and dienes
• Solid residue (char): carbonaceous material mixed with inorganic fillers, pigments, and metals

Qualitative analysis answers the fundamental question of what compounds are present in each fraction. It establishes the compound families, confirms the absence or presence of contaminants such as chlorinated species from PVC carryover, and provides the structural foundation upon which any subsequent quantitative analysis is built. The product profile is strongly influenced by pyrolysis temperature, heating rate, residence time, reactor pressure, catalyst use, and feedstock purity, so qualitative characterization is essential for both process development and product qualification.

Testing Samples

Waste Polyethylene Feedstock

Representative feedstock samples are collected before pyrolysis to verify polymer identity and screen for contamination. Typical samples include baled post-consumer films, rigid HDPE packaging, and agricultural films that may carry soil, inks, or residues. Feedstock pre-characterization by FTIR and TGA confirms that the material is predominantly polyethylene and detects foreign polymers such as PVC or PET that would introduce chlorine or oxygen into the product slate.

Pyrolysis Oil and Condensate

The liquid condensate, usually an amber to dark-brown liquid, spans the gasoline, kerosene, and diesel carbon ranges. Samples are collected in sealed amber vials with minimal headspace to preserve volatile components and prevent oxidation. Dissolved waxes and entrained water are noted during sampling because they affect subsequent chromatographic preparation.

Pyrolysis Wax

The wax fraction is solid or semi-solid at room temperature and consists mainly of long-chain hydrocarbons. Because of its high molecular weight, wax must be dissolved in a suitable solvent such as carbon disulfide or n-hexane, or analyzed on a high-temperature chromatographic column, to release its constituents for identification.

Non-Condensable Gas

Permanent gas exiting the condensation train is collected in inert gas bags or evacuated cylinders. Qualitative gas screening identifies hydrogen, light paraffins, and light olefins, which indicates the extent of secondary cracking within the process.

Solid Residue and Char

The solid fraction contains carbonized organic matter together with inorganic pigments, fillers, and metal contaminants from the original plastic articles. It is characterized to assess its suitability for use as recovered carbon black or as a solid fuel.

Testing Items

Qualitative analysis of waste polyethylene pyrolysis products typically covers the following testing items, tailored to each product fraction:

• Identification of individual hydrocarbon species, including n-alkanes, 1-alkenes, and alpha,omega-dienes in the oil and wax fractions
• Classification of hydrocarbon families: paraffins, olefins, dienes, naphthenes (cycloalkanes), and aromatics
• Identification of aromatic marker compounds, such as benzene, toluene, xylenes, styrene traces, and polycyclic aromatic hydrocarbons (PAHs) formed at elevated temperatures
• Screening for heteroatom-containing compounds, including sulfur-, nitrogen-, oxygen-, and chlorine-bearing species
• Functional group determination by infrared spectroscopy, confirming unsaturation, carbonyl oxidation products, and aromatic substitution patterns
• Polymer identification and contaminant screening in the feedstock, for example residual PVC, adhesives, or additives
• Qualitative gas composition screening for hydrogen and C1–C4 hydrocarbons
• Ash and inorganic residue identification in the char fraction

Testing Methods

Gas Chromatography–Mass Spectrometry (GC-MS)

GC-MS is the primary technique for qualitative analysis of pyrolysis oil and dissolved wax. The sample is injected onto a non-polar capillary column (typically a 5%-phenyl methylpolysiloxane phase, 30 m × 0.25 mm × 0.25 µm), and separated compounds are ionized by 70 eV electron impact ionization. Mass spectra are matched against reference libraries such as the NIST spectral database, with retention index confirmation used to distinguish overlapping homologous series. A typical oven program ramps from 40°C to 300°C, while heavier wax samples may require high-temperature columns capable of eluting compounds up to approximately C60. Method frameworks can reference ASTM D5769 for GC-MS determination of aromatics and ASTM D8071 for hydrocarbon group-type analysis by GC with vacuum ultraviolet detection.

Pyrolysis-GC/MS (Py-GC/MS)

In pyrolysis-GC/MS, a microgram-scale polymer sample is rapidly heated in a micro-furnace or Curie-point pyrolyzer directly coupled to the GC-MS system. This technique serves two purposes: it confirms the identity of the feedstock polymer through its characteristic pyrogram, and it reveals the primary scission products before extensive secondary reactions occur. Single-shot and double-shot (evolved gas then pyrolysis) modes further allow additives, plasticizers, and surface contaminants to be distinguished from the polymer degradation pattern itself.

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR provides rapid functional group screening of all fractions. The spectrum of pyrolysis products shows characteristic C–H stretching bands near 2920 and 2850 cm⁻¹, terminal olefin =C–H stretching near 3070 cm⁻¹, C=C stretching near 1640 cm⁻¹, and out-of-plane olefinic bands near 990 and 910 cm⁻¹. Aromatic content is indicated by bands near 3030 cm⁻¹, 1600 cm⁻¹, and 700–900 cm⁻¹, while carbonyl absorption near 1700 cm⁻¹ signals oxidative degradation. Spectral acquisition and interpretation follow general practices such as ASTM E1252 and GB/T 6040.

Nuclear Magnetic Resonance Spectroscopy (NMR)

¹H and ¹³C NMR spectroscopy complement chromatographic results by confirming the degree and type of unsaturation, the ratio of terminal to internal double bonds, and the extent of chain branching. NMR is particularly valuable for verifying diene content in waxes where GC resolution of high-carbon-number homologues becomes limited.

Elemental and Contaminant Screening

Although primarily compositional rather than structural, elemental screening is integral to a complete qualitative picture. Carbon, hydrogen, and nitrogen are determined by instrumental combustion analysis in accordance with ASTM D5291; sulfur is measured by ultraviolet fluorescence per ASTM D5453 or ISO 20846; and chlorine, a critical indicator of PVC contamination, is determined by combustion ion chromatography. Metals and inorganic residues in the char fraction are identified by ICP-OES or ICP-MS and by X-ray fluorescence screening.

Supporting Techniques

• Thermogravimetric analysis (TGA) characterizes feedstock decomposition behavior and residual ash
• Differential scanning calorimetry (DSC) confirms the melting range and crystallinity of wax fractions
• Simulated distillation per ASTM D2887 and ASTM D6352 provides the boiling range distribution that anchors the qualitative carbon number assignments
• GC-MS library matching is cross-checked with authentic standards where definitive species confirmation is contractually required

All analyses are performed under quality management systems conforming to ISO/IEC 17025, and gas chromatographic operations follow general method conventions consistent with GB/T 9722 and related national standards.

Applications

Qualitative analysis of waste polyethylene pyrolysis products supports a wide range of industrial and research applications:

• Chemical recycling process development, where product fingerprints guide optimization of temperature, residence time, and catalyst selection
• Qualification of pyrolysis oil as a steam cracker feedstock, where the balance of paraffins, olefins, and aromatics determines ethylene and propylene yield potential
• Refinery co-processing studies and alternative fuel development based on naphtha-like or diesel-like condensates
• Quality control of commercial pyrolysis waxes for use in coatings, adhesives, and polymer processing
• Regulatory compliance and safety documentation, including contaminant screening for chlorinated and sulfur-containing species under frameworks such as REACH and national waste-to-chemical regulations
• Academic and industrial research on plastic degradation mechanisms, kinetic modeling, and upcycling pathways

FAQ

What compounds are typically identified in waste polyethylene pyrolysis oil? The oil is dominated by homologous series of n-alkanes, 1-alkenes, and alpha,omega-dienes, accompanied by smaller amounts of branched alkanes, cycloalkanes, and aromatics. Aromatic content, including benzene, toluene, and PAHs, increases with higher pyrolysis temperature and longer vapor residence time.

Why perform qualitative analysis before quantitative analysis? Qualitative analysis establishes which compounds and compound families are actually present, defines the calibration scope, and flags unexpected contaminants. Attempting quantification without a verified qualitative profile risks systematic error, since co-eluting homologues and unidentified peaks are common in pyrolysis products.

What is the difference between GC-MS and Py-GC/MS in this context? GC-MS analyzes the already-generated liquid or dissolved wax products collected from the pyrolysis plant, whereas Py-GC/MS performs micro-scale pyrolysis of the feedstock inside the instrument and analyzes the evolved products immediately. Py-GC/MS therefore reflects primary decomposition chemistry and is ideal for feedstock identification and contaminant screening.

How are chlorine and other contaminants detected? Chlorine originating from PVC contamination is quantified by combustion ion chromatography, and chlorinated organic species can be specifically located in the GC-MS chromatogram by their characteristic isotope patterns. Sulfur and nitrogen heteroatom species are screened by ultraviolet fluorescence, chemiluminescence, or sulfur- and nitrogen-selective GC detectors.

Can wax fractions be fully characterized by GC-MS? Wax fractions up to approximately C60 can be profiled using high-temperature columns and suitable dissolution solvents. Beyond that range, complementary techniques such as high-temperature simulated distillation, NMR, and DSC are used to confirm the overall chain-length and unsaturation profile.

Which standards govern this testing? There is no single standard covering pyrolysis product qualitative analysis end to end; instead, testing programs combine GC-MS and FTIR conventions (ASTM E1252, GB/T 6040, GB/T 9722), elemental methods (ASTM D5291, ASTM D5453, ISO 20846), hydrocarbon-type methods (ASTM D5769, ASTM D8071), and simulated distillation methods (ASTM D2887, ASTM D6352), all executed under ISO/IEC 17025 laboratory quality systems by accredited third-party laboratories.