Image contains a TGA chart by Norris et al. (2025) in Polymers 2025, 17(21), 2913
Why lowering the ash percentage can actually reduce the quality of recovered Carbon Black and why inorganic content, carbonaceous residues and rubber performance need to be considered together.
Dear colleagues and friends of recovered Carbon Black,
Are we sometimes optimising recovered Carbon Black for the laboratory result rather than for the customer?
Ash is probably one of the most discussed parameters in our industry. Customers set limits (e.g tyre companies like Michelin, Bridgestone, Pirelli), producers struggle to meet them and technology developers invest in methods to reduce inorganic content (e.g. BASF and RCB Nanotechnologies GmbH).
The motivation to limit ash content can stem from misconceptions. First that rCB with less than 20% ash can be covered by the REACH registration of furnace carbon black (it cannot) and second that ash is the only difference between rCB and furnace carbon black (it is not. rCB is a mixture of prime carbon blacks and also contains carbonaceous residues / coke).
But what if lowering the ash number makes the product worse?
1. An important presentation at rCB Europe 2024
At rCB Europe in Amsterdam on 6 November 2024 [1], Chris Norris , Mark Murfitt and Antonio López Cerdán of Murfitts Industries presented research under the remarkably simple title "Ash".
One of their opening questions was whether targeting low ash is actually the route to success.
Their conclusion was equally direct:
“Ash content is a poor predictor of in-rubber properties.”
The researchers compared recovered Carbon Black manufactured using different feedstocks and process conditions.
In one experiment, they used the same passenger-car tyre feedstock but altered the pyrolysis conditions to encourage deposition of carbonaceous residues.
The reported ash content fell from 30% to 16%.
An impressive reduction!
Except that the SBR compound performed considerably worse.
The tensile strength decreased from 19.7 to 12.8 MPa, a reduction of approximately 35%.
The test used a 60 phr filler loading, while the materials were milled to approximately the same particle-size top cut.
Lower ash. Lower performance.
This was not a comparison between two different tyre feedstocks. It was the result of different process conditions applied to the same feedstock.
That makes the finding particularly interesting.
2. The peer-reviewed research confirms the problem
One year later, Chris Norris , Antonio López Cerdán , Peter Eaton , Richard Moon and Mark Murfitt published their findings in Polymers under the title "The Importance of Feedstock and Process Control on the Composition of Recovered Carbon Black" [2].
The paper was published on 31 October 2025 and provides a much more detailed examination of feedstock composition, pyrolysis conditions, morphology and in-rubber performance.
The peer-reviewed paper reports the following results:
Source: Norris et al. (2025), Tables 2 and 3. Break stress measured in the paper's SBR-based ASTM D3191 formulation.
The ash results in the peer-reviewed paper are slightly different from those presented at the 2024 conference, reflecting the subsequently reported analytical dataset.
Now compare the two car-tyre-derived materials.
Ash declined from 30.8% to 18.7%, while break stress fell by approximately 20%.
Even more remarkable: Mi360HP and HCR had almost identical ash contents, but the HCR compound delivered approximately 36% lower break stress.
Same ash content. Very different performance.
Chris Norris and his co-authors concluded [2]:
“Carbonaceous residue is more detrimental to rCB performance than ash content.”
This is a very important distinction for anyone designing, operating or financing an industrial tyre-pyrolysis process.
3. Ash reduction is not necessarily inorganic removal
In a previous LinkedIn discussion, I distinguished two approaches:
A. Ash dilution through carbonaceous residue deposition.
B. Feedstock selection and controlled pyrolysis with minimal coking.
I clearly favour the second approach, although genuine inorganic-removal technologies may also create value where they are technically and commercially justified. [3]
Why?
Imagine 100 kg of recovered Carbon Black containing 20 kg of inorganic material.
The ash content is 20%.
Now imagine depositing another 25 kg of carbonaceous material during pyrolysis.
The total mass increases to 125 kg.
The 20 kg of inorganic material are still there.
But the ash concentration is now only 16%.
No inorganic material has been removed. We have simply changed the denominator.
This is a simplified mass-balance illustration, not a measurement from the Murfitts study.
The important point is that ash can be reduced in at least two fundamentally different ways: by actually removing inorganic matter or by increasing the proportion of other material.
The first may improve the product. The second may damage it.
4. What does TGA actually tell us?
Thermogravimetric analysis measures how the mass of a sample changes during a controlled temperature and atmosphere programme.
The ASTM standard D8474-23 provides a method for compositional analysis of recovered Carbon Black by TGA [4].
Depending on the analysis programme, it can determine volatile or organic fractions, overall carbon content and the remaining inorganic residue.
The distinction between fixed carbon and ash is particularly useful.
But a fundamental limitation remains:
The fixed-carbon fraction contains both the original recovered Carbon Black and carbonaceous material deposited during pyrolysis.
A conventional TGA composition result cannot directly distinguish between these carbon components.
That means the TGA may report a lower ash percentage without revealing that additional carbonaceous residue has formed on the recovered filler surfaces.
In 2025, I commented on Chris Norris ' research [5]:
“The real bad boy is carbonaceous residues from rubber polymer carbonisation.”
My point was that these residues can mask the surfaces of recovered Carbon Black aggregates and interfere with the filler–rubber interactions that provide reinforcement.
The Murfitts study supports this interpretation through several complementary techniques, including surface-area measurement, electron microscopy, dispersion analysis and rubber testing.
For example, the published BET surface area of the milled car-derived materials declined from 85.8 m²/g for Mi360+ to 75.0 m²/g for HCR.
The study also reported inferior dispersion of the HCR material and reduced filler–polymer interaction.
Importantly, BET surface area alone does not establish why performance changed. The microscopy and rubber tests provide the additional evidence.
The missing measurement
The research highlights a significant analytical challenge.
We have methods for determining ash. We have methods for assessing volatile residues and extractables.
But quantifying carbonaceous residues separately from the original Carbon Black remains much more difficult.
A reliable, standardised method for doing this would be a valuable development for our industry.
5. Does this mean ash does not matter?
Absolutely not.
The same study demonstrated that the composition of the tyre feedstock influences the inorganic fraction and the performance of recovered Carbon Black.
The truck-tyre-derived material contained less inorganic matter, particularly silica, and demonstrated better reinforcement in the tested formulation.
But this result needs interpretation.
Truck and passenger-car tyres contain different Carbon Black grades and rubber formulations. Therefore, the difference in compound performance cannot be attributed exclusively to ash content.
Moreover, the inorganic fraction is not simply an inert, homogeneous contaminant.
Chris Norris made an interesting distinction in his Amsterdam presentation [1]:
“rCB contains inorganic components not ash!”
The main constituents include silica and zinc-containing compounds, but their relative proportions, chemical forms and distribution vary.
The researchers also demonstrated that a coupling agent could improve reinforcement in their higher-silica recovered Carbon Black compound.
Their observation was that [1]:
“Maintaining the surface activity of the recovered silica component increases compounding options.”
In other words, instead of removing silica, we might sometimes be better advised to make use of it.
This is a technically interesting alternative to purification, particularly when the additional CAPEX, OPEX, energy consumption, yield losses and environmental impacts of ash reduction are considered, that could make ash-reduced rCB less attractive.
There is no universal answer.
The right solution depends on the intended application, formulation, process economics and sustainability performance.
6. Consistency is the other big issue
I would add another consideration to the ash discussion.
Consistency can be more important than achieving the lowest possible number.
An average ash content of 15% sounds attractive.
But what if individual production batches vary considerably in inorganic content and composition?
How should a rubber compounder accommodate such a material?
At various ETRA conferences and at rCB Asia 2026, I described my approach as [6]:
“The Starbucks strategy. Make the same coffee (or rCB) everywhere.”
The statement was deliberately simple.
Industrial customers need predictable materials. They must be able to manufacture their products consistently, not merely obtain good results from an exceptional sample.
In a recent discussion about ash leaching, Irene Yurovska made a similar point [7]:
“The trick is ash consistency in terms of concentration and composition.”
I agree.
A manufacturer should therefore consider both the average ash content and its variability, together with the composition of the inorganic fraction.
The next step is proper statistical process control.
For me, process capability is becoming increasingly important as rCB production moves from small-scale operations to a reliable industrial supply.
7. What should we optimise instead?
I would approach recovered Carbon Black product development from the customer's application backwards.
First, establish which rubber-compound properties matter, tensile performance, stiffness, dynamic behaviour, dispersion, ageing, permeability or other application-specific characteristics.
Next, determine which material and process variables influence those properties.
Then establish process controls that reliably maintain the required characteristics.
Only after this would I assess whether additional treatment, including ash removal, creates enough value to justify the additional costs and environmental impacts.
This is not an argument against ash reduction.
It is an argument against reducing ash as an objective in itself.
In my April 2026 newsletter, discussing purification research and application development, I wrote [8]:
“Technology alone does not create value, but technology with application development does!”
In my view, the greatest value of ash-reduction technologies for many existing recovered Carbon Black applications lies in achieving consistent ash content and composition, rather than simply reducing the ash content level.
Advanced ash-removal technologies, such as those being developed by BASF and RCB Nanotechnologies GmbH (both currently at TRL 4-5), could also unlock new, higher-value colouring applications, particularly in industrial coatings and plastics masterbatches, where low inorganic content, colour performance and dispersibility are critical. I will be following the industrial development of both technologies closely.
8. My conclusion
The work by Chris Norris and his colleagues is an important reminder that recovered Carbon Black is a complex, engineered material.
It contains carbon black aggregates, inorganic compounds and carbonaceous and organic residues.
Its performance is influenced by the interaction of all these components, not by a single analytical number. No single product analysis can predict its application performance.
The ash content remains important. The feedstock selection remains important. Proper pyrolysis and post-processing conditions remain important.
But ultimately, we need to measure whether the material performs in its intended application and whether the producer can deliver that application performance consistently.
My position, which I recently presented at rCB Asia 2026, is simple [9]:
“rCB is when it works.”
Not when the ash content is low.
My question for the rCB community
What has proven more challenging in your experience? Reducing inorganic content? Preventing carbonaceous residue formation? Or controlling product consistency?
And would you accept an rCB with 25% ash content if it consistently provided the performance your application requires?
I would be especially interested in hearing from compound developers, tyre manufacturers, rCB producers and equipment suppliers.
This discussion continues in our recovered Carbon Black community at rCB Europe in Amsterdam, 4–5 November 2026. I look forward to exchanging views with you.
References and source quotations
[1] Norris, C.; Murfitt, M.; López Cerdán, A. Ash. Presentation at Smithers recovered Carbon Black Conference, Amsterdam, 6 November 2024.
[2] Norris, C.; Lopez-Cerdan, A.; Eaton, P.; Moon, R.; Murfitt, M. (2025). The Importance of Feedstock and Process Control on the Composition of Recovered Carbon Black. Polymers, 17(21), 2913. https://doi.org/10.3390/polym17212913.
[3] Martin von Wolfersdorff . There are two ways of managing inorganic ("ash") content in rCB. LinkedIn discussion.
[4] ASTM International. ASTM D8474-23: Standard Test Method for Recovered Carbon Black (rCB)—Compositional Analysis by Thermogravimetry (TGA). ASTM standard. For the separate muffle-furnace ash determination, see ASTM D8621-25e1.
[5] Martin von Wolfersdorff. You think rCB ash content is bad? Think again. LinkedIn post discussing Norris et al..
[6] Martin von Wolfersdorff. (2026). rCB Asia 2026: The State of the rCB Industry. LinkedIn article.
[7] Irene Yurovska . Comment on Martin von Wolfersdorff's LinkedIn discussion Do you choose (A) ash leaching; (B) consistent ash; (C) ash does not matter? Discussion.
[8] Martin von Wolfersdorff. (2026). rCB News April 2026. Section on BASF purification and application development. LinkedIn newsletter.
[9] Martin von Wolfersdorff (2026). rCB is when it works. LinkedIn discussion.
