Weibold Academy: Lower ash, lower performance? The hidden carbonaceous residue problem in recovered carbon black

Weibold Academy article series discusses periodically the practical developments and scientific research findings in the end-of-life tire (ELT) recycling and pyrolysis industry.

Claus Lamer

These articles are reviews by Claus Lamer – the senior pyrolysis consultant at Weibold. The reviews aim to give industry entrepreneurs, project initiators, investors, and the public a better insight into a rapidly growing circular economy. At the same time, this article series should stimulate discussion.

For completeness, we would like to emphasize that these articles are no legal advice from Weibold or the author. Please refer to the responsible authorities and specialist lawyers for legally binding statements.

Abstract

Ash content remains one of the most widely used quality indicators for recovered carbon black (rCB). Recently, research has challenged the assumption that lower ash necessarily means better performance in rubber compounds. Carbonaceous residues formed during tire pyrolysis can dilute the measured ash percentage while adversely affecting the surface properties and reinforcing behavior of the recovered carbon black.

This article examines how these secondary carbon deposits form, why conventional quality measurements may fail to reveal their presence, and what pyrolysis operators can do to limit their formation. It also discusses the limitations of downstream upgrading technologies and the implications for rCB specifications, process optimization, and commercial acceptance.

The central conclusion is that rCB quality cannot be judged by ash content alone. Effective process control and verified application performance are essential to producing a commercially competitive material.

Introduction: When a better number means a worse product

Ash content has long been one of the first parameters considered when evaluating recovered carbon black. The reasoning seems straightforward: commercial furnace carbon black contains very little inorganic material, whereas rCB retains the minerals originally incorporated into the tire. Reducing these impurities should therefore bring the recovered material closer to its conventional counterpart. There is considerable truth in this reasoning. Mineral content influences filler concentration, density, compound formulation, and, depending on the application, reinforcing performance.

But there is another side to the story.

During pyrolysis, rubber polymers decompose into volatile hydrocarbons and a carbon-rich solid residue. Under certain operating conditions, some of these decomposition products undergo secondary reactions and form additional carbonaceous deposits on the recovered carbon black.

These deposits increase the carbon content of the solid product. Consequently, the measured ash percentage can decrease even though the actual amount of inorganic material has not necessarily decreased.

What appears to be an improvement in the laboratory may therefore conceal a deterioration in product quality.

A study published by Norris and colleagues in October 2025 provides particularly convincing experimental evidence of this problem [1].

The ash paradox: What recent research reveals

The researchers compared recovered carbon black produced from passenger-car tires under different pyrolysis conditions.

One process was designed to limit the formation of carbonaceous residues. The other deliberately promoted their formation. Both materials subsequently underwent the same finishing treatment.

The results were striking.

Source: Norris et al. (2025), Tables 2 and 3 [1]. Selected experimental results, reproduced here as a condensed comparison with attribution.

The high-residue process produced substantially more solid material while reducing the measured ash percentage by approximately 39% on a relative basis.

At first glance, these figures might appear attractive to a producer seeking higher rCB yield and lower ash content.

However, tensile strength in the tested rubber compound declined by approximately 20%. Dispersion also deteriorated.

The two samples exhibited similarly low volatile matter contents. Therefore, the difference in reinforcing performance could not be attributed solely to residual volatile hydrocarbons.

The findings support an important conclusion: additional carbon formed during pyrolysis is not equivalent to the reinforcing carbon black originally incorporated into the tire.

This experiment has limits. The findings relate to the particular feedstock, processes, and rubber formulation investigated. They do not establish that every reduction in ash content is undesirable, nor do they provide a universal relationship between carbonaceous residue content and reinforcement. Moreover, a complete inorganic mass balance would be required to determine precisely how much of the observed ash reduction resulted from dilution rather than differences in inorganic recovery.

Nevertheless, the practical message is clear: an apparently favorable ash specification can be misleading when the condition of the recovered carbon black surface is ignored.

How carbonaceous residues form

The formation of additional carbon during tire pyrolysis is not new.

Research by Pantea and colleagues more than two decades ago already demonstrated that carbonaceous deposits form on recovered carbon black surfaces and between carbon black particles. Their experiments also revealed differences between atmospheric-pressure and vacuum pyrolysis [2].

The underlying mechanisms are complex, but two principal pathways are particularly relevant.

First, incompletely decomposed rubber-derived material remaining in contact with the solid phase may undergo further thermal decomposition, dehydrogenation, and carbonization.

Second, hydrocarbon vapors released during pyrolysis can undergo secondary cracking, condensation, and polymerization. Some of the resulting carbon-rich material may deposit on hot solid surfaces.

Both pathways can alter the properties of the original carbon black aggregates.

Deposits may cover accessible surfaces, fill spaces between primary particles, or contribute to the formation of larger, more strongly bound agglomerates. These changes can affect dispersion and the filler-rubber matrix interaction.

The difficulty lies in distinguishing the desired rubber decomposition from the undesirable secondary reactions that follow.

Insufficient thermal treatment can leave organic residues. Excessive thermal exposure under unfavorable conditions can promote additional carbon formation.

Neither minimum temperature nor maximum temperature is, by itself, a reliable indicator of optimum rCB quality.

Process conditions matter more than a single temperature

Pyrolysis operators commonly discuss reactor temperature and residence time when comparing technologies. Both are important, but neither tells the full story.

Carbonaceous residue formation depends on interacting variables, including the temperature the solids actually experience, the duration of thermal exposure, the concentration of reactive hydrocarbon vapors, pressure, and gas-solid contact conditions.

Reactor geometry and vapor removal arrangements can therefore be as relevant as the nominal operating temperature. # For example, reducing the residence time of reactive vapors in contact with hot carbon-rich solids may help limit secondary deposition. Improved vapor extraction, controlled gas flow, and avoiding stagnant zones are potential engineering approaches worth investigating.

Earlier experiments have also shown that reduced-pressure pyrolysis can suppress carbonaceous deposition under certain conditions [2].

This does not mean that vacuum operation is necessarily the best solution for a commercial plant. Vacuum systems introduce their own engineering, safety, energy, maintenance, and economic considerations.

Similarly, results from laboratory or flash-pyrolysis experiments cannot be directly transferred to large rotary kilns, screw reactors, or other continuous technologies.

The appropriate operating window must be established for the specific reactor configuration and feedstock.

From a process-engineering perspective, the objective is to achieve sufficiently complete rubber decomposition while minimizing subsequent reactions that damage the recovered filler.

That requires attention to the complete reaction environment, not merely the temperature displayed on the operator's control screen.

Can downstream upgrading solve the problem?

The rCB industry has developed and investigated several approaches to improving the quality of recovered carbon black.

These include intensive milling, thermal post-treatment, chemical demineralization, and physical or chemical activation.

Each serves a different purpose.

• Milling reduces agglomerate size and can improve dispersion. However, it does not necessarily restore the original aggregate surface characteristics after secondary carbon deposition.
• Chemical demineralization removes selected inorganic constituents and can significantly reduce ash content. It does not, by itself, establish that carbonaceous surface deposits have been removed.
• Thermal post-treatment can reduce residual organic contamination, but its effectiveness depends on the nature of the deposits and the treatment atmosphere. Refractory carbon cannot be assumed to disappear simply because the material is heated again.
• Steam and CO₂ activation offer further possibilities for changing carbon surface properties and porosity. However, these treatments consume carbon and can affect both unwanted deposits and the original recovered filler.

A recent scientific review by Silva and colleagues examines these approaches and emphasizes the continuing challenges of balancing material improvements, carbon losses, processing costs, and industrial scalability [3].

This is especially important when evaluating technologies that promise substantial improvements in surface area or ash content.

Such improvements may be technically real without necessarily delivering equivalent improvements in rubber reinforcement.

For commercially oriented rCB production, the key question is not whether an upgrading treatment changes a laboratory parameter, but whether it produces a better, economically viable material for the intended application.

Measuring carbonaceous residues: The missing quality parameter

One reason carbonaceous residues receive less attention than ash is the difficulty of measuring them reliably.

Established analytical methods can determine important characteristics of recovered carbon black.

• Thermogravimetric analysis (TGA), for example, provides information about organic residues, overall carbon content, and inorganic ash. Toluene transmittance measurements help assess extractable organic contamination.
• Surface-area measurements, particle-size analysis, and other conventional tests provide additional information about the material's physical characteristics.

However, a fundamental analytical problem remains.

Both the carbon black originally incorporated into the tire and the additional refractory carbon formed during pyrolysis can contribute to the measured carbon fraction.

Conventional compositional analysis cannot reliably distinguish these two components by measuring only fixed carbon or total carbon content.

Advanced techniques, including electron microscopy and surface-sensitive spectroscopic methods, can provide valuable evidence of morphological changes and carbonaceous deposits.

Yet these methods are not equivalent to a universally validated routine test that quantifies the mass fraction of secondary carbonaceous residues in every commercial rCB product.

This analytical limitation deserves more attention from researchers and standards organizations alike. The current ASTM framework also reflects the importance of application-specific testing. ASTM D8632-26 provides a classification system for rCB used in rubber products, while ASTM D8466-26 addresses the use and limitations of conventional carbon black characterization methods when applied to rCB [4, 5].

Classification and routine quality-control measurements remain valuable. They should not, however, be confused with direct proof of reinforcing performance.

For demanding rubber applications, representative compounding and performance tests are indispensable.

Commercial consequences for pyrolysis operators

The implications extend well beyond laboratory analysis.

A producer may invest in additional equipment to reduce ash content, increase surface area, or improve apparent carbon purity. Such investments are justified when they produce a marketable product that meets customer requirements.

But without appropriate performance testing, the commercial benefit remains uncertain.

The same applies to reactor optimization.

Increasing the yield of the carbon-rich solid fraction may appear advantageous in a preliminary mass balance. If that additional yield includes unwanted carbonaceous deposits, the resulting material may become less attractive to higher-value rubber markets.

There may also be consequences for tire pyrolysis oil (TPO) and gas yields. Carbon retained in the solid phase through secondary reactions is not available in the same form for recovery in the liquid or gaseous products.

The precise economic trade-off depends on the individual process and cannot be determined from rCB yield alone.

For plant operators and project developers, this suggests a more comprehensive approach to quality optimization:

• Establish representative feedstock and operating-condition records.
• Evaluate ash, organic contamination, morphology, and surface characteristics together.
• Investigate process conditions that may encourage secondary carbon deposition.
• Validate proposed improvements through standardized, application-relevant rubber compound testing.
• Assess any upgrading investment against product yield, utility consumption, operating costs, and achievable market value.

The objective should be consistent production of a qualified filler, not optimizing an isolated laboratory number.

Conclusion: Quality must be proven in the application

The presence of carbonaceous deposits in recovered carbon black has been recognized for many years. Recent research adds stronger experimental evidence that these deposits can damage reinforcing performance more than ash percentage alone might suggest.

This does not make ash content irrelevant. Mineral composition remains important, and demineralization can be valuable for selected applications.

However, it challenges the industry to reconsider how it defines and assesses rCB quality.

A lower ash percentage does not necessarily mean that more useful carbon black has been recovered. A higher surface area does not automatically guarantee better reinforcement. And a higher solid yield is not necessarily an economic advantage.

For industry, the next step is to better understand and measure secondary carbonaceous residues while placing greater weight on feedstock control, reactor conditions, and verified application performance.

The most valuable rCB is not necessarily the material with the lowest ash content. It is the material that consistently delivers the required performance, economically.

Key references

[1] Norris, C., Lopez-Cerdan, A., Eaton, P., Moon, R., and Murfitt, M. (2025).
The Importance of Feedstock and Process Control on the Composition of Recovered Carbon Black. Polymers, 17(21), 2913. Published October 31, 2025.
https://doi.org/10.3390/polym17212913

[2] Pantea, D., Darmstadt, H., Kaliaguine, S., and Roy, C. (2003).
Heat-treatment of carbon blacks obtained by pyrolysis of used tires. Effect on the surface chemistry, porosity and electrical conductivity. Journal of Analytical and Applied Pyrolysis, 67(1), 55–76.
https://doi.org/10.1016/S0165-2370(02)00017-7

[3] Silva, C. M. C., et al. (2026).
Recovered carbon black: A comprehensive review of activation, demineralization, and incorporation in rubber matrices. Carbon Resources Conversion, 9(1), 100334. https://doi.org/10.1016/j.crcon.2025.100334

[4] ASTM International (2026).
ASTM D8632-26 – Standard Classification for Recovered Carbon Black—rCB Used in Rubber Products.
https://doi.org/10.1520/D8632-26

[5] ASTM International (2026).
ASTM D8466-26 – Standard Guide for Recovered Carbon Black—Carbon Black Test Methods for Testing rCB.
https://doi.org/10.1520/D8466-26

[6] ASTM International (2023).
ASTM D8474-23 – Standard Test Method for Recovered Carbon Black (rCB)—Compositional Analysis by Thermogravimetry (TGA).
https://doi.org/10.1520/D8474-23

[7] Lamer, C. / Weibold Academy (2021).
Upgrading Recovered Carbon Black from End-of-Life Tires. Published September 30, 2021.
https://weibold.com/upgrading-recovered-carbon-black-from-endoflife-tires

About Weibold

Weibold provides independent technical, commercial, and strategic consulting services to the international tire recycling and pyrolysis industry.

Our work includes technology assessment, plant performance evaluation, process optimization, product quality qualification, market research, and developing commercially viable rCB and TPO value chains.

Companies seeking to improve rCB quality, evaluate upgrading technologies, or establish technically credible product specifications are welcome to contact Weibold at consulting@weibold.com.

Source and copyright note: This article is an original editorial analysis of the referenced literature. Experimental data from Norris et al. are presented in a condensed, attributed comparison; no third-party figures or images have been reproduced. ASTM standards are identified by reference and are not reproduced.

Copyright: ©Robert Weibold GmbH. This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license CC BY 4.0. You must give appropriate credit, provide a link to the license and this article, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.