Recycled plastic pyrolysis converts waste polymers into oil, gas, and a carbon-rich solid-phase product—pyrolysis char or recovered carbon black—whose commercial value depends directly on verified purity and performance. Professional testing of proximate and ultimate composition, heavy metals, halogens, PAHs, and carbon black properties confirms whether the solid residue can be sold as a filler, pigment, or fuel and whether it satisfies REACH, end-of-waste, and international waste-shipment requirements. This guide explains the sample types, testing items, analytical methods, and applicable ISO, ASTM, EN, and GB standards used by accredited third-party laboratories for solid-phase pyrolysis product inspection.
Technical Overview
Pyrolysis is an advanced chemical recycling route in which waste plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), PET, and mixed packaging streams are thermally decomposed at approximately 400–700 °C in an oxygen-free or oxygen-limited reactor. The process generates three product streams: condensable pyrolysis oil, non-condensable syngas, and a solid-phase residue commonly described as pyrolysis char, carbonaceous solid residue, or—after milling, demineralization, and pelletizing—recovered carbon black (rCB).
The solid fraction typically represents 5–40 % of the output mass, depending strongly on feedstock composition. It combines fixed carbon derived from carbon black pigments and polymer coke with inorganic ash originating from calcium carbonate, talc, glass fibers, titanium dioxide pigments, and catalyst residues. Because recycled feedstocks frequently contain lead and cadmium stabilizers, zinc oxide, antimony trioxide, brominated flame retardants, and chlorine-bearing PVC residues, these contaminants become concentrated in the char along with polycyclic aromatic hydrocarbons (PAHs) formed during thermal decomposition.
Since post-consumer plastic feedstocks are highly heterogeneous, solid product quality can vary significantly from batch to batch. Accredited third-party laboratories therefore perform systematic characterization of the solid phase to support product qualification, end-of-waste and by-product status determinations, transboundary waste movement documentation, buyer specifications, and reactor process optimization.
Testing Samples
Representative sampling and correct sample conditioning are prerequisites for meaningful results. Char samples are typically homogenized, divided by riffle splitting or cone-and-quartering, sealed in airtight containers, and protected from moisture pickup and oxidation during transport. Typical submitted sample types include the following.
Raw Pyrolysis Char and Bottom Residue
As-received solid discharged from the reactor, collected from drums or big bags, usually 200–500 g per batch. This sample type is used for proximate and ultimate analysis, calorific value, contaminants, and ash characterization. Sampling of bulk carbonaceous materials can follow practices such as ASTM D1900 or ASTM D1799, adapted to char matrices.
Recovered Carbon Black (rCB)
Milled, demineralized, or pelletized grades intended to substitute virgin carbon black in rubber, plastics, coatings, and inks. These samples undergo performance-oriented testing, including surface area, structure, tint strength, and pellet quality, in addition to purity screening for ash, PAHs, heavy metals, and extractables.
Pelletized Char and Solid Recovered Fuel (SRF)
Char that has been blended, densified, or pelletized for use as a solid recovered fuel or industrial kiln co-processing feedstock. Sample preparation follows EN 15413, with subsequent testing aligned to the EN 15357 and EN 15400 series framework for solid recovered fuels.
Process Control and Intermediate Samples
Cyclone catch, filter dust, scrubber sludge, spent bed material, and ash from gas polishing or combustion steps. Characterizing these intermediates identifies where contaminants accumulate, supports mass balance calculations, and verifies that the saleable char fraction is not cross-contaminated by downstream residues.
Testing Items
A complete inspection program for solid-phase pyrolysis products covers composition, contaminants, application performance, and environmental behavior. Core testing items include the following.
- Proximate analysis: moisture, volatile matter, ash content, and fixed carbon.
- Ultimate analysis: total carbon, hydrogen, nitrogen, sulfur, and oxygen (calculated by difference).
- Calorific value: gross calorific value (higher heating value) and net calorific value (lower heating value) for fuel applications.
- Trace elements and heavy metals: Pb, Cd, Hg, As, Cr, Ni, Cu, Zn, Sb, Sn, Ba, Co, Mo, Se, Tl, V, Mn, and Be.
- Halogens: total chlorine, bromine, and fluorine, plus soluble chloride, relevant to PVC and flame-retardant residues.
- PAHs: the 16 US EPA priority PAHs, benzo[a]pyrene, and the REACH-restricted PAH suite for rubber and plastic articles.
- Carbon black performance: nitrogen surface area (NSA and STSA), oil absorption number (OAN), crushed oil absorption number (COAN), tint strength, iodine adsorption number, pour density, and pellet crush strength.
- Physical properties: particle size distribution, sieve residue, bulk density, and pH of aqueous extract.
- Residual organics: solvent extractables, total oil content, and toluene extract transmittance as a purity indicator.
- Leaching behavior: regulatory leaching tests for waste acceptance and environmental release assessment.
- Morphology and phase composition: scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray fluorescence (XRF) screening.
Testing Methods
Methods are selected according to sample matrix, intended application, and regulatory context. The principal analytical techniques and their governing standards are summarized below.
Proximate and Ultimate Analysis
Moisture, volatile matter, ash, and fixed carbon are determined by furnace and thermogravimetric procedures in accordance with ASTM D3173, ASTM D3174, ASTM D3175, ASTM D7582, and, for refuse-derived matrices, ASTM D5142. Carbon, hydrogen, nitrogen, and sulfur are measured by combustion-based elemental analyzers per ASTM D5373, ISO 12902, and EN 15407, with oxygen reported by difference. Compositional profiles across temperature stages can be supplemented by thermogravimetric analysis under ASTM E1131.
Calorific Value Determination
Gross and net calorific values are measured by adiabatic or isoperibolic bomb calorimetry following ISO 1928, ASTM D5865, and EN 15400. Results establish the energy value of char destined for solid fuel or cement kiln co-processing and support energy balance audits of the recycling plant.
Metals and Ash Chemistry
After microwave-assisted acid digestion under EPA Method 3052 or aqua-regia digestion under EN 13657, trace elements are quantified by ICP-OES or ICP-MS in line with EPA Methods 6010D and 6020B, and the trace-element framework of EN 15411 for solid recovered fuels. Bulk ash composition (Ca, Si, Ti, Al, Mg, Fe, Na, K) is determined by X-ray fluorescence using procedures analogous to ASTM D4326, and ash fusibility behavior, important for combustion applications, follows ISO 540 or ASTM D1857.
Halogens and Sulfur
Total chlorine, bromine, and fluorine are determined by oxygen-bomb or automated combustion followed by ion chromatography in accordance with EN 14582, with anion quantification per ISO 10304. Adsorbable organic halogens (AOX) may be assessed under ISO 9562. Total sulfur is measured by high-temperature combustion methods per ASTM D4239 and EN 15408. Elevated halogen levels typically indicate PVC or halogenated flame-retardant carry-over and directly restrict several end uses.
PAHs and Residual Organics
PAHs are extracted by ultrasonic, Soxhlet, or pressurized liquid extraction and quantified by GC-MS following EPA Method 8270E or ISO 18287, or by HPLC under EPA Method 8310. Compliance checks against REACH Annex XVII are standard, since eight carcinogenic PAHs, including benzo[a]pyrene, are restricted to 1 mg/kg per substance in rubber and plastic articles, tightened to 0.5 mg/kg for toys and prolonged skin-contact articles. Residual oil content is determined gravimetrically after solvent extraction, and the transmittance of the toluene extract is measured under ASTM D1618 as an indicator of extractable contamination.
Carbon Black Performance Testing
For recovered carbon black intended to replace virgin grades, surface area is measured by nitrogen adsorption per ASTM D6556 (NSA and STSA) and ISO 4652. Structure is characterized by the oil absorption number under ASTM D2414 and the crushed oil absorption number under ASTM D3493. Tinting strength is evaluated per ASTM D3265, iodine adsorption number per ASTM D1510 and ISO 1304, heating loss (moisture) per ASTM D1509, ash per ASTM D1506 and ISO 1138, sieve residue per ASTM D1514, pour density per ASTM D1513, and individual pellet crush strength per ASTM D5230. Chinese national carbon black methods of the GB/T 3780 series are applied where domestic market compliance is required.
Physical and Morphological Characterization
Particle size distribution is determined by laser diffraction under ISO 13320, supported by dry sieve analysis for coarse fractions. SEM-EDS reveals particle morphology, inclusions, and elemental distribution; XRD identifies crystalline phases such as anatase and rutile TiO2, calcium carbonate, or quartz; and Raman spectroscopy (ID/IG ratio) assesses the degree of carbon ordering and graphitization, which correlates with reinforcement performance.
Leaching and Environmental Compliance
Leaching behavior is assessed using the toxicity characteristic leaching procedure of EPA Method 1311 or the compliance leaching tests of the EN 12457 series, enabling comparison against landfill acceptance criteria and supporting end-of-waste or by-product status assessments under the EU Waste Framework Directive. These data are also central to documentation for transboundary waste shipments regulated under the Basel Convention.
Applications
Verified test data unlock commercial and regulatory value for the solid phase of plastic pyrolysis plants. Typical applications supported by inspection programs include the following.
- Rubber compounding filler for tires, hoses, belts, and molded technical goods, where rCB substitution rates are governed by surface area and structure results.
- Pigment and colorant for plastics, masterbatches, inks, and coatings, qualified through tint strength, sieve residue, and PAH compliance testing.
- Solid recovered fuel and cement kiln co-processing feedstock, qualified by calorific value, chlorine, sulfur, and heavy metal data under the EN 15400 series.
- Asphalt and bitumen modification, supported by purity and softening-point-relevant compositional screening.
- Precursor for activated carbon and industrial adsorbents, where fixed carbon content, ash chemistry, and porosity development potential are decisive.
- Regulatory documentation for end-of-waste determinations, REACH compliance, and international waste shipment permits.
- Process research and development, including catalyst evaluation, residence-time optimization, and mass-and-energy balance verification.
By matching the testing scope to the target application, producers avoid costly downgrades, rejected shipments, and non-compliance penalties while maximizing the market value of the recovered solid.
FAQ
The following questions address the most common technical and practical concerns regarding inspection of solid-phase products from recycled plastic pyrolysis.
What is the solid-phase product of plastic pyrolysis actually made of?
It is a heterogeneous mixture of fixed carbon, residual adsorbed hydrocarbons, and inorganic ash. The ash fraction concentrates mineral fillers, pigments, and metal residues from the feedstock, while the carbon fraction originates from carbon black pigments and polymer coking. Composition is highly feedstock-dependent, which is why batch-by-batch verification is recommended for commercial release.
Why is heavy metal testing so important for pyrolysis char?
Metals such as lead, cadmium, chromium, antimony, and zinc are widely present in legacy plastic formulations as stabilizers, pigments, catalysts, and flame-retardant synergists. These elements thermally concentrate in the solid residue. Heavy metal data determine whether the char can enter fuel, filler, or consumer-contact applications, and they are mandatory for waste classification and leaching compliance.
Can pyrolysis char directly replace virgin carbon black?
Not on a one-to-one basis. Most recovered carbon black contains significantly higher ash than virgin grades, which lowers reinforcement efficiency. Typical substitution rates in non-critical rubber applications range from 10 % to 30 %, but acceptance always depends on measured surface area, structure, tint strength, pellet quality, and contaminant levels rather than visual appearance.
How much sample is required and how should it be packaged?
A minimum of 200 g of homogenized char is generally sufficient for a full characterization package, while 500 g is recommended when performance testing, leaching, and PAH analysis are all required. Samples should be sealed in airtight, solvent-compatible containers with minimal headspace, labeled with batch number and production date, and accompanied by a completed chain-of-custody form.
What are typical turnaround times?
Proximate, ultimate, calorific value, and halogen analyses are typically completed within five to seven working days. Metals, PAHs, leaching tests, and full carbon black performance packages generally require seven to ten working days, depending on digestion, extraction, and instrumental scheduling.
Which PAH and contaminant limits should producers plan for?
For rubber and plastic article applications, the REACH Annex XVII limit of 1 mg/kg per restricted PAH, and 0.5 mg/kg for toys and prolonged skin-contact articles, is the key benchmark. For fuel uses, chlorine and sulfur limits under the EN 15400 series framework and plant-specific heavy metal thresholds usually govern acceptance.