Comparison of Standards for Carbon Black Content Determination Methods
Carbon black is one of the most commonly used additives for UV resistance and weathering stabilization in polyolefin materials. Its content not only directly affects the performance of pipes and fittings, but also determines the comparability of test data and the consistency of compliance when product standards reference specific test methods. With GB/T 13021 being upgraded from the earlier single-method framework to the 2023 edition and becoming identical to ISO 6964:2019, China’s testing and quality-control system has entered a new stage in terms of method selection, key parameter settings, and mutual recognition of results with international standards. To facilitate an accurate understanding of the differences among standards, this paper takes GB/T 13021-2023 as the main line, compares the key differences among ISO 6964:2019, ASTM D 1603-20, and ASTM D 4218-20, and discusses how these differences affect test execution and data interpretation.
Comparison of the current status of domestic and international standards and their method coverage
Comparison of applicable scope and target materials
Carbon black content is a critical quality-control indicator for the weathering resistance, UV aging resistance, and long-term service stability of polyolefin materials. Therefore, different standard systems have established their own test methods around the same core objective: how to separate the polymer matrix, retain carbon black, and quantify it under controlled atmospheres and temperature programs.
ISO 6964:2019 focuses on polyolefin pipes and fittings, and its standard design is naturally oriented toward material evaluation and product-standard referencing in the piping industry.
ASTM D 1603-20 is positioned as a more general material test method for determining carbon black content in olefin plastics. Its broader coverage also means that its test conditions emphasize general applicability rather than alignment with a specific product system.
ASTM D 4218-20 is centered on the muffle-furnace technique and applies to the determination of carbon black content in polyethylene compounds. Its logic is to treat “muffle-furnace determination” as an independent specialized method system, enabling rapid implementation in laboratories that already have muffle-furnace equipment.
By comparison, GB/T 13021-2023 explicitly targets polyolefin pipes and fittings. Its scope is highly consistent with ISO 6964:2019, and it enhances adaptability to different laboratory equipment conditions through a multi-method framework. This allows it to serve as a practical, comparable, and referenceable unified testing framework within China’s pipe-and-fitting testing system.
Comparison of method coverage
In terms of method coverage, ISO 6964:2019 includes the tube-furnace method, the muffle-furnace method, and thermogravimetric analysis (TGA), forming a relatively complete “pyrolysis + calcination” testing system. In contrast, the ASTM system shows a more separated structure: ASTM D 1603-20 mainly corresponds to the tube-furnace method, while ASTM D 4218-20 mainly corresponds to the muffle-furnace method, with each method managed under a different standard document. GB/T 13021-2023 adopts a three-method framework consisting of Method A (tube-furnace calcination), Method B (high-temperature tube furnace or microwave muffle furnace), and Method C (thermogravimetric analysis, TGA). This framework is broadly aligned with ISO 6964:2019. By organizing multiple technical routes under a single national standard, GB/T 13021-2023 enables testing implementation under different equipment conditions within one standard number, and it also provides a clearer institutional basis for cross-method data comparison.
Standard iteration and evolution of technical routes
From the perspective of technical evolution, the older version of the national standard was dominated by a single method, which is insufficient to meet current needs in testing organizations and enterprises for equipment diversity, improved efficiency, data comparability, and arbitration consistency. The key changes in GB/T 13021-2023 are twofold: it standardizes the pyrolysis and calcination processes through more modern atmosphere control and program control logic, and it introduces mature methods such as the muffle-furnace method and TGA, so that the standard is no longer limited to a single device and a single procedure. More importantly, GB/T 13021-2023 is identical to ISO 6964:2019 (IDT). This means its core methodology is consistent with international standards in both principles and procedural framework, providing stronger support for domestic and international data mutual recognition, export compliance communication, and the international interpretability of third-party test reports. It also enables comparative studies to focus more on detailed clause differences and engineering executability differences under a shared framework, rather than remaining at a purely conceptual level of method principles.
Carbon black content is a critical quality-control indicator for the weathering resistance, UV aging resistance, and long-term service stability of polyolefin materials. Therefore, different standard systems have established their own test methods around the same core objective: how to separate the polymer matrix, retain carbon black, and quantify it under controlled atmospheres and temperature programs.
ISO 6964:2019 focuses on polyolefin pipes and fittings, and its standard design is naturally oriented toward material evaluation and product-standard referencing in the piping industry.
ASTM D 1603-20 is positioned as a more general material test method for determining carbon black content in olefin plastics. Its broader coverage also means that its test conditions emphasize general applicability rather than alignment with a specific product system.
ASTM D 4218-20 is centered on the muffle-furnace technique and applies to the determination of carbon black content in polyethylene compounds. Its logic is to treat “muffle-furnace determination” as an independent specialized method system, enabling rapid implementation in laboratories that already have muffle-furnace equipment.
By comparison, GB/T 13021-2023 explicitly targets polyolefin pipes and fittings. Its scope is highly consistent with ISO 6964:2019, and it enhances adaptability to different laboratory equipment conditions through a multi-method framework. This allows it to serve as a practical, comparable, and referenceable unified testing framework within China’s pipe-and-fitting testing system.
Comparison of method coverage
In terms of method coverage, ISO 6964:2019 includes the tube-furnace method, the muffle-furnace method, and thermogravimetric analysis (TGA), forming a relatively complete “pyrolysis + calcination” testing system. In contrast, the ASTM system shows a more separated structure: ASTM D 1603-20 mainly corresponds to the tube-furnace method, while ASTM D 4218-20 mainly corresponds to the muffle-furnace method, with each method managed under a different standard document. GB/T 13021-2023 adopts a three-method framework consisting of Method A (tube-furnace calcination), Method B (high-temperature tube furnace or microwave muffle furnace), and Method C (thermogravimetric analysis, TGA). This framework is broadly aligned with ISO 6964:2019. By organizing multiple technical routes under a single national standard, GB/T 13021-2023 enables testing implementation under different equipment conditions within one standard number, and it also provides a clearer institutional basis for cross-method data comparison.
Standard iteration and evolution of technical routes
From the perspective of technical evolution, the older version of the national standard was dominated by a single method, which is insufficient to meet current needs in testing organizations and enterprises for equipment diversity, improved efficiency, data comparability, and arbitration consistency. The key changes in GB/T 13021-2023 are twofold: it standardizes the pyrolysis and calcination processes through more modern atmosphere control and program control logic, and it introduces mature methods such as the muffle-furnace method and TGA, so that the standard is no longer limited to a single device and a single procedure. More importantly, GB/T 13021-2023 is identical to ISO 6964:2019 (IDT). This means its core methodology is consistent with international standards in both principles and procedural framework, providing stronger support for domestic and international data mutual recognition, export compliance communication, and the international interpretability of third-party test reports. It also enables comparative studies to focus more on detailed clause differences and engineering executability differences under a shared framework, rather than remaining at a purely conceptual level of method principles.
Comparison of the overall framework for carbon black content determination (method classification perspective)
Positioning and consistency of the tube-furnace method across standards
The core concept of the tube-furnace method is to pyrolyze the polymer under an inert atmosphere (typically nitrogen) so that the matrix decomposes while preventing carbon black from being oxidized during the pyrolysis stage. The residue is then calcined at high temperature under an oxidizing atmosphere so that the carbon black is completely oxidized and removed. The carbon black content is finally calculated based on the mass difference of residues at different stages. Method A in GB/T 13021-2023 and the tube-furnace method in ISO 6964:2019 belong to the same methodological system, and ASTM D 1603-20 follows the same overall principle. Therefore, these three standards are highly consistent in their methodological backbone. The main differences lie in executable requirements such as nitrogen purification and deoxygenation, sample insertion and structural design, pyrolysis temperature–time windows, and gas flow settings. These differences largely determine the stability, repeatability, and comparability of the method across different laboratories.
Differentiated implementation of the muffle-furnace method in ISO and ASTM
The muffle-furnace method is also based on calculating carbon black content from the mass difference between the pyrolysis and calcination stages. However, its key risk is a stronger dependence on an oxygen-deficient environment during pyrolysis. In particular, air ingress during pyrolysis and the furnace chamber size can significantly affect oxygen consumption and thus the stability of the oxygen-deficient condition. ISO 6964:2019 incorporates the muffle-furnace method into its standard system and further divides it into two routes: the conventional muffle furnace and the microwave muffle furnace. It controls the pyrolysis process through programmed heating and holding steps, and then applies a higher temperature during the subsequent calcination stage to ensure complete oxidation of carbon black. In contrast, ASTM D 4218-20 adopts a short-time pyrolysis strategy at 600–610°C followed by calcination within the same temperature range, placing greater emphasis on speed and operational practicality. It quantifies carbon black mass loss through a defined weighing rhythm. Method B in GB/T 13021-2023 includes both a high-temperature tube-furnace route and a microwave muffle-furnace route. Its organizational structure is closer to ISO’s multi-route system, but attention must still be paid to the sources of differences from ASTM D 4218-20 in specific parameters and weighing rules, in order to avoid result bias caused by improper method selection or execution.
Alignment of the thermogravimetric analysis method with ISO TGA provisions
The TGA method replaces repeated removal and weighing with a continuous mass-change curve. It measures stage-wise mass losses during pyrolysis and calcination through programmed heating and controlled atmosphere switching. ISO 6964:2019 introduces TGA as a newly added method and specifies key conditions such as sample mass range, heating rate, temperature plateaus, and atmosphere switching parameters. Method C in GB/T 13021-2023 is aligned with the ISO framework and offers advantages in automation, energy efficiency, and reduced smoke generation. However, because the sample mass is typically at the milligram level, sample representativeness and control of inter-sample variability become critical issues for practical engineering implementation.
The core concept of the tube-furnace method is to pyrolyze the polymer under an inert atmosphere (typically nitrogen) so that the matrix decomposes while preventing carbon black from being oxidized during the pyrolysis stage. The residue is then calcined at high temperature under an oxidizing atmosphere so that the carbon black is completely oxidized and removed. The carbon black content is finally calculated based on the mass difference of residues at different stages. Method A in GB/T 13021-2023 and the tube-furnace method in ISO 6964:2019 belong to the same methodological system, and ASTM D 1603-20 follows the same overall principle. Therefore, these three standards are highly consistent in their methodological backbone. The main differences lie in executable requirements such as nitrogen purification and deoxygenation, sample insertion and structural design, pyrolysis temperature–time windows, and gas flow settings. These differences largely determine the stability, repeatability, and comparability of the method across different laboratories.
Differentiated implementation of the muffle-furnace method in ISO and ASTM
The muffle-furnace method is also based on calculating carbon black content from the mass difference between the pyrolysis and calcination stages. However, its key risk is a stronger dependence on an oxygen-deficient environment during pyrolysis. In particular, air ingress during pyrolysis and the furnace chamber size can significantly affect oxygen consumption and thus the stability of the oxygen-deficient condition. ISO 6964:2019 incorporates the muffle-furnace method into its standard system and further divides it into two routes: the conventional muffle furnace and the microwave muffle furnace. It controls the pyrolysis process through programmed heating and holding steps, and then applies a higher temperature during the subsequent calcination stage to ensure complete oxidation of carbon black. In contrast, ASTM D 4218-20 adopts a short-time pyrolysis strategy at 600–610°C followed by calcination within the same temperature range, placing greater emphasis on speed and operational practicality. It quantifies carbon black mass loss through a defined weighing rhythm. Method B in GB/T 13021-2023 includes both a high-temperature tube-furnace route and a microwave muffle-furnace route. Its organizational structure is closer to ISO’s multi-route system, but attention must still be paid to the sources of differences from ASTM D 4218-20 in specific parameters and weighing rules, in order to avoid result bias caused by improper method selection or execution.
Alignment of the thermogravimetric analysis method with ISO TGA provisions
The TGA method replaces repeated removal and weighing with a continuous mass-change curve. It measures stage-wise mass losses during pyrolysis and calcination through programmed heating and controlled atmosphere switching. ISO 6964:2019 introduces TGA as a newly added method and specifies key conditions such as sample mass range, heating rate, temperature plateaus, and atmosphere switching parameters. Method C in GB/T 13021-2023 is aligned with the ISO framework and offers advantages in automation, energy efficiency, and reduced smoke generation. However, because the sample mass is typically at the milligram level, sample representativeness and control of inter-sample variability become critical issues for practical engineering implementation.
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