Thermogravimetric Analysis (TGA) Testing of Recycled Materials

Thermogravimetric Analysis (TGA) testing of recycled materials is an essential quality control and characterization technique used to evaluate the composition, thermal stability, and filler content of recycled plastics, rubbers, and polymer compounds. By continuously monitoring mass loss as a sample is heated under controlled atmospheric conditions, TGA reveals moisture, volatiles, polymer decomposition behavior, carbon black, and inorganic residue with high precision. This article explains the principles, sample types, testing items, methodologies, standards, and practical applications of TGA testing for recycled materials in modern manufacturing and supply chains.

Technical Overview

Thermogravimetric Analysis (TGA) is a thermal characterization technique that measures the mass of a specimen as a function of temperature or time while it is subjected to a controlled temperature program in a defined atmosphere. For recycled materials—commonly referred to as regrind, reprocessed resin, or reclaimed polymer—TGA provides a powerful fingerprint of material composition and thermal integrity that is often difficult to obtain by other means.

Recycled feedstocks frequently contain moisture, processing residues, degraded polymer fractions, mineral fillers, glass fiber, flame retardants, and carbon black. Because these constituents volatilize, decompose, or oxidize at characteristic temperatures, a single TGA experiment can quantify them and flag contamination or formulation drift between lots. This makes TGA one of the most widely used tools for incoming inspection of recycled resins and for validating supplier declarations of recycled content and filler loading.

Typical TGA testing is performed in accordance with recognized standards such as ISO 11358, ASTM E1131, ASTM E2550, and ASTM D6370, ensuring that results are reproducible and comparable across accredited third-party laboratories and internal quality departments.

Testing Samples

Recycled Thermoplastic Pellets and Regrind

Pelletized or regrind forms of recycled polyethylene (rPE), polypropylene (rPP), ABS, polycarbonate (PC), polyamide (PA), polystyrene (PS), PET, and PVC are the most common sample categories. Small representative specimens, typically 5 to 20 milligrams, are cut or taken directly from the lot to ensure homogeneity.

Filled and Reinforced Recycled Compounds

Recycled compounds containing talc, calcium carbonate, glass fiber, or other inorganic reinforcements are routinely analyzed to verify filler loading against specification. The residual ash at the end of the test directly corresponds to the inorganic fraction.

Recycled Rubber and Elastomeric Materials

Crumb rubber from end-of-life tires and recycled elastomer compounds are analyzed for oil content, polymer fraction, carbon black content, and inorganic residue using a dedicated multi-step TGA method.

Post-Consumer Flakes and Mixed Plastic Waste

PET bottle flakes, mixed packaging streams, and other post-consumer recyclates can be screened for volatile contamination, multi-polymer composition, and non-polymeric impurities before compounding or direct conversion.

Testing Items

TGA testing of recycled materials typically reports the following quantitative parameters:

• Moisture and volatile content: mass loss measured from ambient temperature to approximately 150 °C, indicating absorbed water and low-boiling residues.
• Decomposition onset temperature: the temperature at which significant polymer degradation begins, a key indicator of thermal stability.
• Temperature at 5% mass loss (T5%) and other defined mass-loss milestones used for lot-to-lot comparison.
• Maximum rate decomposition temperature: derived from the derivative thermogravimetric (DTG) curve to identify polymer type and degradation kinetics.
• Polymer fraction: the total organic mass lost during pyrolysis under inert atmosphere.
• Carbon black or soot content: determined by switching from inert to oxidative gas and measuring the combustion step.
• Ash and inorganic filler residue: the non-combustible mass remaining at the final test temperature, reflecting mineral filler or glass fiber content.
• Contamination indicators: abnormal decomposition steps or unexpected residues that reveal blending, adulteration, or cross-contamination with foreign polymers.

Testing Methods

The general procedure for TGA testing of recycled materials follows internationally recognized methodologies and consists of the following steps:

• Sample preparation: representative material is selected, homogenized if necessary, and a specimen of approximately 5 to 20 mg is loaded into an open alumina or platinum crucible.
• Inert heating stage: the furnace is purged with high-purity nitrogen, and the sample is heated from ambient temperature to a setpoint such as 600 °C at a controlled heating rate, commonly 10 to 20 °C/min, following ISO 11358 and ASTM E1131 principles.
• Gas switching stage: at the chosen switching temperature, the atmosphere is changed to air or oxygen to combust carbonaceous residue and carbon black, enabling separate quantification of carbon black from inorganic ash.
• Final oxidation stage: the temperature is raised to a final setpoint, typically 700 to 900 °C, until a constant residual mass is achieved.
• Data analysis: the mass-loss curve and its derivative (DTG) are evaluated to determine step temperatures, mass percentages for each component, and residual ash content, with results reported against applicable standards such as ISO 11358, ASTM E1131, ASTM E2550, ASTM D6370, and related national specifications.

Test atmosphere, heating rate, specimen mass, and switching temperature must be documented in the report, as these parameters materially influence results. For thermal stability assessment alone, a single-scan method under nitrogen in accordance with ASTM E2550 may be used.

Applications

TGA testing of recycled materials supports a broad range of industrial and regulatory objectives:

• Incoming quality control: verifying that purchased recycled resin matches declared specifications for filler content, volatiles, and thermal stability before production.
• Supplier qualification and comparison: benchmarking multiple recyclate suppliers on ash content, degradation onset, and composition consistency.
• Recycled content verification: confirming that compound formulations contain the declared fractions of recycled polymer and additives.
• Process optimization: detecting thermal degradation caused by excessive reprocessing temperatures or repeated extrusion cycles.
• Failure analysis: investigating field failures or processing defects linked to contamination, moisture, or filler underloading in recycled feedstock.
• Research and development: characterizing new recycled compounds, stabilizer packages, and compatibilized blends during formulation development.
• Regulatory and customer documentation: generating objective test data for material data sheets, environmental claims, and buyer specifications.

FAQ

How much sample is required for TGA testing?

A single test typically requires only 5 to 20 milligrams; however, laboratories usually request several grams of the recycled material so that representative subsampling can be performed to account for heterogeneity.

Can TGA determine the recycled content of a plastic part?

TGA alone does not directly measure the percentage of recycled polymer versus virgin polymer of the same chemistry. It quantifies composition in terms of volatiles, polymer fraction, carbon black, and inorganic fillers. Deviations from a reference virgin formulation, such as altered ash content or shifted decomposition behavior, provide strong indirect evidence of recycled content or blending.

What does a high ash residue indicate?

A high residual mass at the end of the test indicates an elevated inorganic fraction, which may reflect added mineral fillers, glass fiber, pigment, or inorganic contamination. Comparing the measured residue against the expected formulation quickly identifies under- or over-loading.

Why are both nitrogen and air atmospheres used?

Heating under inert nitrogen pyrolyzes the organic polymer without burning carbon black, allowing polymer and carbonaceous components to be separated. Switching to air or oxygen then combusts the remaining carbon, so that carbon black and true inorganic ash can be individually quantified.

What is a typical turnaround time?

A single TGA run is completed within one to two hours. Including sample preparation, calibration verification, and reporting, results from accredited third-party laboratories are generally available within three to five working days.

Which standards govern this testing?

Commonly applied standards include ISO 11358 for thermogravimetry of polymers, ASTM E1131 for compositional analysis by TGA, ASTM E2550 for thermal stability determination, and ASTM D6370 for compositional analysis of rubber, along with corresponding national and regional standards where applicable.