Determination of Solid Content in Pyrolysis Products of Waste Polyamide (PA)

Solid content determination is a critical quality-control parameter in the chemical recycling of waste polyamide (PA, nylon) via pyrolysis, as it quantifies the non-volatile residue—char fines, glass fibers, mineral fillers, and catalyst residues—present in pyrolysis oils, condensates, and solid residues. Accurate solid content data supports process optimization, downstream filtration design, product specification compliance, and yield accounting. This article presents the sample types, testing items, standard-referenced test methods, and practical considerations for determining solid content in waste PA pyrolysis products in accredited third-party laboratories and in-house QC facilities.

Technical Overview

Pyrolysis has become an increasingly important route for the chemical recycling of waste polyamide materials, including nylon 6 (PA6), nylon 66 (PA66), PA11/PA12, copolymers, textile blends, and glass-fiber or mineral-filled engineering grades. Thermal degradation of PA under oxygen-free or oxygen-limited conditions produces a complex product stream: condensable organics (pyrolysis oil or condensate, rich in caprolactam for PA6 and in cyclopentanone, nitriles, and amine fragments for PA66), non-condensable gases, and a solid residue consisting of carbonaceous char combined with inorganic fillers such as glass fiber, calcium carbonate, and montmorillonite.

Solid content is a key quality index for all of these fractions. In pyrolysis oils and condensates, suspended solids originate from entrained char aerosols, incomplete hot-gas filtration, catalyst fines, corrosion products, and released glass fragments; elevated solids cause nozzle fouling, pump and valve wear, downstream catalyst poisoning, and failure of product specifications. For the solid residue, solid content is typically interpreted as total non-volatile matter, that is, the mass fraction remaining after removal of moisture and volatiles, which directly determines recovered char, glass fiber, and filler yields.

Operationally, solid content is defined as the mass fraction of non-volatile, non-removed material obtained under prescribed drying, extraction, or ignition conditions. It must be clearly distinguished from related indices—moisture content, volatile matter, sediment (insoluble suspended solids), and ash (inorganic residue after ignition)—and professional reporting should always state the temperature, atmosphere, and method basis used.

Testing Samples

Pyrolysis Oil and Organic Condensate

Liquid organics collected from condenser trains, ranging from water-clear monomer-rich distillates to dark, char-laden oils. The parameter of interest is usually suspended char fines with particle sizes below approximately 100 micrometers, together with total non-volatile matter.

Aqueous Condensate Phase

Water-containing condensate derived from feedstock moisture, reaction water, or steam-assisted pyrolysis, carrying water-soluble caprolactam and polar organics. These samples require moisture correction so that solids can be reported on a dry basis.

Slurry, Wax, and Semi-Solid Fractions

High-boiling, waxy, or sludge-like fractions with elevated oligomer and char concentrations, often sampled from condenser walls, transfer lines, or bottoms streams.

Solid Char and Inorganic Residue

Reactor or drum bottom residue containing carbonaceous char, glass fiber (for example, from 30% glass-filled PA66), and mineral fillers. Testing focuses on loss on drying, volatile/combustible residue, and glass fiber or ash yield.

Feedstock and Intermediate Streams

Shredded waste carpet, fishing nets, textiles, and automotive plastic streams, as well as pre-pyrolysis dried flakes, may be characterized with the same techniques to establish baselines and mass balances.

Testing Items

Typical test items requested for waste PA pyrolysis products include:

• Total solid content (non-volatile matter) at 105 °C or another specified drying temperature
• Suspended (insoluble) solids and sediment content by membrane or glass-fiber filtration
• Ash content and glass fiber / mineral filler residue by muffle-furnace incineration
• Moisture (water) content by Karl Fischer titration or azeotropic distillation
• Volatile matter and compositional profile by thermogravimetric analysis (TGA)
• Loss on drying for solid char and recovered filler products
• Carbon residue, as supporting information for char characterization

Testing Methods

Gravimetric Drying Method (Non-Volatile Matter)

A weighed portion of sample, typically 1 to 5 grams, is dispersed in a preconditioned weighing dish, optionally diluted with a suitable solvent to form an even film, dried at 105 ± 2 °C (or another agreed temperature) in a forced-ventilation oven, cooled in a desiccator, and reweighed. Drying is repeated to constant mass, generally to a change of no more than 0.5 mg or 0.1% between successive weighings. Solid content is calculated as residue mass divided by initial sample mass, expressed as a mass percentage. Reference methods include ISO 3251, ASTM D2369, GB/T 1725, and GB/T 6284, adapted for pyrolysis products.

Temperature selection deserves attention: caprolactam melts at approximately 69 °C and boils at approximately 267 °C, so it is substantially retained at 105 °C, meaning the method captures char and inorganic fines as solids while retaining dissolved organics. For condensates containing lighter amines and water, results should be interpreted together with Karl Fischer water data.

Vacuum and Reduced-Temperature Drying

For heat-sensitive condensates or samples prone to slow oxidation or sublimation losses, drying at 60 to 80 °C under vacuum in a vacuum oven is preferred. This variant minimizes loss of semi-volatile monomer and oligomer species while still removing water and light volatiles, and it is commonly used when solid content must be decoupled from monomer yield studies.

Membrane Filtration and Extraction Method (Suspended Solids)

The sample is diluted or dissolved in an appropriate solvent, such as tetrahydrofuran, toluene, or water depending on the phase, and filtered through a preconditioned, pre-weighed glass-fiber or membrane filter with a nominal pore size of 0.45 or 0.7 micrometers. The filter is dried to constant mass and weighed; the mass gain divided by the sample mass gives the suspended solids content. The practice is adapted from ASTM D473, ASTM D4807, and ISO 11923, originally developed for sediment in petroleum products and suspended solids determination.

Ash and Filler Determination

Ignition in a muffle furnace combusts the organic matrix and char, leaving the inorganic ash, which for glass-filled PA residues corresponds to recovered glass fiber plus mineral filler. Typical temperatures range from 550 to 850 °C depending on filler chemistry, with 625 °C commonly applied to glass-fiber residues to avoid fiber softening; carbonate-filled materials require careful temperature control to avoid decomposition artifacts. Reference methods include the ISO 3451 series (including parts for glass-fiber-reinforced plastics), ASTM D2584, and GB/T 9345.

Thermogravimetric Analysis (TGA)

Small specimens of 5 to 20 milligrams are heated under nitrogen to approximately 600 °C, after which the atmosphere is switched to air or oxygen to burn off carbonaceous char. A single run resolves moisture, light volatiles and caprolactam, polymer-derived decomposition residue, and final ash, making TGA an efficient screening and cross-checking tool. Reference methods include ASTM E1131 and ISO 11358.

Moisture Correction

Water in condensates biases solids-by-difference calculations. Karl Fischer titration per ASTM E203, ISO 760, or GB/T 6283, or azeotropic distillation per ASTM D95 and ISO 3733, quantifies water so that solid content can be reported on a consistent dry or as-received basis.

Quality Control and Reporting

• Perform duplicate determinations with agreement typically within 0.3 to 0.5 percentage points absolute
• Run blanks for filters, dishes, and solvents used in dilution
• Apply the constant-mass criterion before final weighing
• Report the result basis (as-received or dry), drying temperature, solvent, filter pore size, and any pre-treatment such as phase separation or homogenization

Applications

• Chemical recycling and depolymerization plants: monitoring char carryover and filtration performance in caprolactam and monomer recovery circuits
• Pyrolysis oil qualification: verifying sediment and solids levels against fuel and feedstock acceptance limits for pumps, injectors, and downstream refining units
• Glass fiber and filler recovery: quantifying inorganic yield from pyrolyzed glass-filled PA6 and PA66 for reuse in reinforcement applications
• Process research and development: screening pyrolysis temperature, residence time, heating rate, and catalyst effects on solid product distribution
• Mass and energy balance studies in pilot and commercial units, where accurate solid yields are essential for techno-economic assessment
• Feedstock intake quality control for waste carpet, fishing net, textile, and automotive plastic streams
• Environmental and residue classification: determining combustible carbon versus inert mineral content in pyrolysis residues for disposal or utilization decisions

FAQ

What does solid content mean for a waste PA pyrolysis product?

It is the mass fraction of material that remains non-volatile under the specified test conditions, including suspended char fines, glass fragments, mineral fillers, catalyst residues, and dissolved non-volatile species such as cyclic oligomers. The definition is method-dependent, so the drying temperature and procedure must always accompany the result.

How is solid content different from ash content?

Solid content is measured by low-temperature drying and includes all non-volatile organic and inorganic matter, whereas ash content is the inorganic residue remaining after complete ignition at high temperature. Thermogravimetric analysis bridges the two by separately quantifying combustible char and inert ash in a single experiment.

Will caprolactam evaporate during 105 °C drying?

Loss is minor because caprolactam boils at approximately 267 °C, but it has measurable vapor pressure at 105 °C, and prolonged drying or strong air flow can cause slight sublimation. Standardized drying times, vacuum drying at lower temperatures, or TGA-based protocols are recommended when monomer retention must be preserved.

How is moisture-rich condensate handled?

The sample is homogenized, and water is determined independently by Karl Fischer titration or distillation. Solid content is then corrected and reported on a dry basis, which prevents water from being miscounted as volatile organic loss.

How much sample is required?

Typically 1 to 5 grams per determination for liquids and semi-solids, and 2 to 10 grams for solid char, with duplicates recommended. Larger volumes of heterogeneous condensate should be homogenized and subsampled to improve representativeness.

Which standards are commonly referenced?

Frequently cited documents include ISO 3251, ASTM D2369, GB/T 1725, and GB/T 6284 for non-volatile matter; ASTM D473, ASTM D4807, and ISO 11923 for sediment and suspended solids; the ISO 3451 series, ASTM D2584, and GB/T 9345 for ash; ASTM E1131 and ISO 11358 for TGA; and ASTM E203, ISO 760, ASTM D95, and ISO 3733 for moisture. Accredited third-party laboratories select and adapt these methods according to sample matrix and client specification.

What is the typical turnaround time?

Conventional gravimetric and ash determinations are generally completed within three to seven working days, while TGA and Karl Fischer measurements can often be delivered faster. Complex multi-item characterization packages may require additional time for method validation and duplicate confirmation.